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Surface processes in the detection of reducing gases with SnO2-based devices
Sensors and Actuators, 18 (1989) 71 - 113
71
SURFACE PROCESSES IN THE DETECTION WITH SnO,-BASED DEVICES
OF REDUCING
GASES
D. KOHL 2. Physikalisches Institut der Rheinisch-Westfcilischen Tempkrgmben 55. D-51 00 Aachen (F.R.G.) (Received June 16,1988;
Technischen
Hochschule
Aachen,
accepted September 15,1988)
Abstract Single-crystal faces, thin evaporated and sputtered films and sintered specimens of Sn02 were investigated in UHV. After exposure to hydrogen, water, amine, carbon monoxide, methane, acetic acid and ethanol, a sensitive mass spectrometer recorded the desorbing species in thermal desorption spectroscopy and served for reactive scattering investigations. The surface conductivity was observed at the same time. The combination of the different results with published infrared spectroscopic data allows some conclusions to be drawn on reaction mechanisms, surface binding and the origin of conductance variations. It will be shown that even such a simple molecule as methane undergoes many reaction steps, some of which affect the observed conductance change. The investigation of more complex molecules, such as ethanol and acetic acid, supports the understanding of the methane reactions; the decomposition of methane on SnOz is accompanied by synthesis reactions yielding these products. Finally, some consequences and possible improvements of gas sensors are discussed.
1. Introduction Gas sensors based on semiconducting oxides can be used for the detection of combustible and noxious gases in air. Their advantages are high sensitivity, simple design, low weight and cost. Thin oxide films on silicon substrates with integrated low-power heating elements also fulfil the requirements of mass production [ 11. The range of applications is limited by selectivity and stability. An improvement of these properties cannot be achieved simply by trial and error; but requires a better understanding of the surface processes connected with the conductance changes. This report proposes some concepts for the interpretation of sensor properties. The concepts are derived to some extent from our own experimental studies and also from the literature on the spectroscopy of adsorbates and on heterogeneous catalysis employing oxides. 0250-6874/89/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
72
The type of oxygen available at the surface is crucial for the operating behaviour. An understanding of the role of hydrogen and water is a precondition for interpreting sensor properties. Water is a usual constituent of the ambient air. Hydrogen and water are also formed during the decomposition of hydrogen-containing molecules. Carbon monoxide undergoes different reactions in the presence and absence of humidity. Methane represents a simple hydrocarbon of practical interest. Ethunol and its oxidation product acetic acid show some analogous features in surface reactions. Arsine is investigated and compared with other hydrides. Only a fraction of the spectroscopic methods successfully applied to the investigation of metal surfaces are transferable to oxide surfaces. For example, strong surface phonon excitations prevent the identification of adsorbate vibrations on the SnO, surface. On metal surfaces adsorbate coverages of one monolayer are quite common. Typical coverages on oxides are often found in the range 10 -3 to 10e5 of a monolayer (ML). Chemisorbed oxygen cannot exceed a coverage of 10s3 monolayers for electrostatic reasons [2]. Less sensitive methods, e.g., photoemission or Auger spectroscopy, cannot be used in many cases. Infrared spectroscopy is a powerful tool to investigate the presence and stability of intermediate surface products, but its application is limited to polycrystalline samples with a large effective area including internal surfaces. Thermal desorption spectroscopy (TDS) performed with a muss spectrometer ~II an ultrahigh vacuum (UHV) apparatus gives access to coverages below lo-’ ML. Also small amounts of one gas in the presence of another are detectable down to a limit of 10m5 to lo-’ ML. However, nondesorbing species can only be detected if they can be removed by reaction with another gas. An example is the removal of a carbon deposit in the presence of oxygen or hydrogen. An extension of the mass spectrometer technique, the reactive scattering of a molecular beam at the sensor surface at working temperature, has shown promising results. The two mass spectrometric methods are applied in our own experiments described in Section 3. Up to now the description of surface reactions has been based mainly on the combination of mass spectrometric and infrared data. Since this basis is not as wide as in the case of metals, the proposed reaction paths contain some speculative elements. This may be justified if the proposed reactions stimulate ideas to improve the performance of sensor devices. The discussion is mostly confined to SnO,, which dominates in research and in applications. Sintered layers and thin films are in practical use. Because of the polycrystalline structure, an interpretation of the observed conductance changes is often difficult. By also studying the surface of single crystals under well-defined conditions, one might try to achieve a better separation of parameters influencing the properties of gas sensors [ 31. In gases escaping from combustion processes, 02, CO, COZ, HZS, SOZ, NO and NO2 usually have to be monitored together; in smog situations SO*, CO and NO2 are relevant for an alarm release. Therefore, some reactions including two of these gases are included in the discussion.
73
2. Electronic properties of tin dioxide Bulk properties SnOz is a fairly ionic semiconductor with a wide band gap of about 3.6 eV [4]. At the edge of the conduction band the calculated density of states, derived from atomic Sn 5s states, is low 151. Correspondingly the measured Hail mobilities of bulk electrons in single crystals are reasonably high, 80 and 200 cm*/V s at 500 K and 300 K, respectively [6]. The edge of the valence band emerges from 0 2p orbitals, which are directed perpendicular to the Sn-0 bond (lone-pair orbit&) [ 51. This geometry is extremely unfavourable for current transport by holes in the valence band. At temperatures above about 800 K, the concentration of oxygen vacancies and the conductance depend on the oxygen pressure [ 71: 0
a (Po*Y”
(1)
The experimental value m = 6 corresponds to a doubly-ionized oxygen vacancy ( Vo)*+. This vacancy acts as a donor with an energy level 150 meV below the conduction band. The conductivity of Sn02 can be increased by some substitutional impurities, fluorine on an oxygen site or antimony on a tin site, which also act as shallow donors [ 4 1. Surface properties Near room temperature the oxygen vacancies are frozen in and isothermal conductance changes of an Sn02 device are due to chemisorption. The band gap of a clean Sn02 (110) face is free of intrinsic surface states [8]. The extraction and injection of electrons by extrinsic surface acceptors or surface donors, respectively, is connected with the variation of a spacecharge layer. The electron concentration near the semiconductor surface varies with the density and occupancy of surface acceptors and donors. In a gas sensor this density of surface states depends on surface reactions with ambient gases. The left and the middle columns of Fig. 1 demonstrate the two types of space-charge layers occurring at the surface under the assumption of a homogeneous donor concentration in the bulk. Oxygen or hydrogen are chosen as examples of gases producing surface acceptors and donors, respectively . Depletion layer If oxygen is chemisorbed, the positive charge of the double layer resides in ionized bulk donors, causing a band bending of up to about 1 eV. A simple calculation of such a Schottky depletion layer gives a depth of lo+ to 1o-5 cm. Further examples of surface acceptors are methoxy and formate groups [9], discussed in Section 3.5. With growing surface density of ionosorbed oxygen the band bending of the depletion layer increases, lifting the level of surface acceptors above the Fermi level (Fig. 1, left
S-2 Depletion
(b)
(0)
Surface acceptors
surface
Accumulation
!&face donors
Bulk donors
IllSTANCE
Inhomogeneous
P FROM
SURFACE
Fig. 1. Space-charge layers on the surface of an n-type semiconductor with a wide band gap. (a) Distribution of charges; (b) band scheme near conduction band edge; (c) concentration n(z) of electrons in the conduction band. EC = conduction band edge; EF = Fermi level; ED = donor level; V, = surface potential; S = surface states by adsorption of oxygen or atomic hydrogen, acceptors or donors, respectively; nb = electron concentration within the bulk. For the case of an inhomogeneous donor distribution below the surface, a depletion or accumulation can also be additionally induced by surface states, e.g., due to adsorbed species.
column: EC - Es = 1 eV). By this process the coverage with negativelycharged species is limited to IO I2 - 1013 cme2 (Weisz limitation) [ 21. Accumulation layer In the case of prevailing ionized surface donors, e.g., adsorbed hydrogen atoms, electrons are injected forming a negative space-charge layer. This accumulation layer induces the opposite band bending to a depth of about 10m6 cm (Fig. 1). The oxygen atom in the molecules of water, formaldehyde or ethanol and the nitrogen atom in ammonia can bond to surface tin atoms via their lone-pair orbit&. Since this type of orbital is occupied by two electrons, these species act as surface donors [9]. In the bulk of Sn02 and ZnO, several donor and acceptor levels can be simultaneously present, taking up higher carrier concentrations [4, 7,10, ll]. Oxygen vacancies Not only adsorbed oxygen can be removed by surface treatments, but also surface lattice oxygen. Exposure to hydrogen and hydrogen-containing gases, such as methane, at temperatures above 350 K produces surface oxygen vacancies. They are well known as donors, for example, on Ti02 (110) faces [8]. Munnix and Schmeits [S] showed by band-structure calcula-
7s
tions that surface oxygen vacancies on SnOz (110) faces do not act as donors. A study [ 12 ] comparing the ultraviolet photoemission spectra (UPS) of TiOz (110) and SnOz (110) demonstrated that oxygen vacancies created by argon ion bombardment produce new surface states in the upper part of the bulk band gap of TiO 2, but not of SnOz. After the bombardment the reduced surface tin is present as Sn(I1) [13]. Conductivity measurements [14] indicate that these surface defects on Sn02 do not act as donor states, in contrast both with bulk oxygen deficiency and with sputtered TiOz surfaces. After exposure of a stoichiometric Sn02 (101) face or a sputtered SnO, film to methane in UHV, considerable amounts of water desorb at temperatures near 350 K leaving surface oxygen vacancies, but the conductivity does not increase below 550 K (see Fig. 9). So the theoretical prediction that surface oxygen vacancies on SnO, do not act as surface donors is supported by UPS and by conductance measurements after reducing argon bombardment, and also by conductance measurements after exposure to methane. Annealing of an argon-bombarded SnOz (110) face in UHV increases the conductance till a plateau is reached at 550 K /14]. Surface oxygen vacancies may diffuse towards the bulk, where they are able to act as donors as explained above. Oxygen vacancies left after water formation at 670 to 770 K (see Section 3.2) increase the conductance. Again a migration towards bulk sites may ‘activate’ the vacancies as donors. Since the vacancies have to cross only a few lattice planes, the temperature of 550 K seems to be sufficient for ‘activation’. This situation is sketched in Fig. 1, right column. In reality, adsorbed acceptors and donors and inhomogeneously distributed ‘subsurface’ donors appear together. Influence of gases on the conductance by surface reactions For conductance changes at the surface of monocrystalline samples, the effect of geometry can be eliminated by the introduction of ‘surface conductivity’. A detailed explanation is given elsewhere [ 15 1. The processes listed below alter the conductance of n-type oxides: (a) Adsorbates acting as acceptors or donors extract or inject electrons from or into the space-charge layer. The conductance decreases or increases correspondingly. In the presence of oxygen, only acceptors with a high electron affinity are effective because of the Weisz limit. (b) Surface oxygen vacancies can migrate a small distance into the bulk to become active as donors. Hydrogen, methane and carbon monoxide remove lattice oxygen from the surface. (c) Besides oxygen, carbon dioxide and acetic acid also refill surface oxygen vacancies. The conductance decreases by the diffusion of bulk vacancies to the surface. (d) Reducing gases may also react with adsorbed oxygen ions. The decrease of oxygen acceptors causes a conductance increase. A more detailed discussion can be found elsewhere 1161. For sintered samples the action of barriers separating domains of agglomerates, each comprising a rather large number of crystallites, is instead
76
proposed as an explanation [17]. Surface acceptors at the grain boundaries induce depletion layers. The conductance of the sample is determined by the electron concentration n, at the grain boundary. ng decreases exponentially with growing barrier height E, - Er, which increases with the density of ‘negativelycharged species at the grain boundary. The net surface charge is diminished by reaction with the adsorbed oxygen or by the production of adsorbed donors near the grain boundary, which flatten the barrier. 3. Surface reactions 3.1. Experimental
Preparation of samples The SnOZ crystals were grown from the vapour phase. They exhibit mainly (101) and (110) surfaces of the rutile structure containing equal numbers of Sn and 0 atoms. The crystallographic orientation was controlled by X-ray diffractometry. Thin films were prepared by vapour deposition of metallic tin on amorphous quartz substrates (at 430 K in a vacuum of 10-j Pa, oxidation at 830 K for 90 min) or borosilicate glass substrates (at 670 K, oxidation at 990 K for 60 min). These films consist of Sn02 as checked by X-ray diffractometry [ 181. Scanning electron micrographs show that the layers on borosilicate glass consist of interconnected spheres. The diameter of the spheres increases with the film thickness (0.2 pm for 25 nm and 2 pm for 280 nm films). The films on borosilicate glass were used in acetic acid experiments and the films on quartz glass in arsine experiments. Sputtered films were also studied [ 191. They consisted of a porous alumina substrate, 3 mm square, bearing interdigital conductance electrodes (1 by 1 mm, length-to-width ratio 267) surrounded by an Au heating strip and a Pt thermoresistor below a 100 nm sputtered SnOZ layer. Auger spectroscopy was used to control the cleanliness of the single-crystal faces and films. Sintered specimens were prepared by drying and sintering (1170 K, 1 h) a 0.5 mm thick layer of water/SnOz paste on an alumina platelet. These samples do not contain any stabilizing admixtures, for example SiOZ, frequently used to obtain mechanically stable sensors. No catalytic active metals were added. In two experimental methods a highly-sensitive mass spectrometer was used: In thermal desorption spectroscopy (TDS) a sample is exposed to a certain gas dose and after pump down, is heated at a rate of about 1 to 50 K per second. The desorbing products are monitored by the mass spectrometer in ultrahigh vacuum. In the present studies corrections are made for dissociation within the mass spectrometer by ionization (cracking patterns). Usually all reaction products are also offered to the sample in separate TDS runs. By comparison, some products are found to desorb immediately after reaction, while others are stored in a chemisorbed state before desorption. Products desorbing together are characteristic of a com-
77
mon preceding surface reaction step. The reaction order of the slowest reaction step can be derived from the shift of the peak temperature of the desorption maximum with heating rate. In reactive scattering experiments a gas beam is directed onto the surface continuously or in a chopped mode. The desorbing molecules are monitored by the mass spectrometer. The observation of rates at different temperatures gives kinetic information. Slow degradation of catalytic properties is discernible; in particular, poisoning of the surface by nondesorbing products can be investigated. Such products can be desorbed by a heat pulse or by offering a reactive gas. During reactive scattering experiments the conductance can be monitored simultaneously, so the effect of surface reactions changing the density of surface donors or acceptors is observed. The initial state of the surface largely influences the results and must be carefully controlled. A decrease in the available density of lattice oxygen atoms allows a conclusion to be reached on whether lattice oxygen or adsorbed oxygen participates in a certain reaction step. Gas-mixing system and electrical measurements The samples were exposed to various gases at a normal pressure of 1 bar in a stainless steel reactor (volume 18 cm3). Mixtures of the reducing gas with artificial air (80% Nz and 20% 02, Linde) were adjusted by means of precise flowmeters. Regulated electrical valves maintained a constant gas flow of 40 l/h. A small pump drew a fraction of the gas flow, 10 l/h, into the reactor. It was possible to switch between the mixture and pure air within 0.2 s. The sensor conductance was measured by the four-probe technique using Keithley 602 electrometers for current and voltage. Conductance and temperature as a function of time were registered by a small computer. Also during some of the reactive scattering measurements in UHV, the conductance was recorded [ 201. 3.2. Exposure to oxygen, hydrogen and water For the oxides the overall stoichiometry has a decisive influence on the surface conductivity. Oxygen vacancies left after surface reactions can diffuse towards the bulk, increasing the conductivity near the surface. Adsorbed oxygen ions act as surface acceptors, binding electrons and diminishing the surface conductivity. Figure 2 shows the various species of oxygen relevant to surface reactions. The energy difference between doubly-charged 02- in a lattice site and in an adsorbed state is estimated to be about 2000 kJ/mole or 20 eV, the difference between 302 BPsand +(O, &amounts to 140 kJ/mole or 1.5 eV [21]. The Figure does not show the energy barriers retarding transitions between different states. At room temperature the equlibrium of the (Oz.,,)coverage with gaseous O2 is approached slowly, although the reaction is exothermic. On SnOp films reaction (2) takes place with increasing temperature, as concluded from EPR studies 1223 : (0 2ads)-+
e-t--,
2(oads)-
(21
s E z
I
I
ELEClROPillIC -0(C-C-BONO RUPTURE)
I I I I
NUCLEOPHlllC ~OEHYORO6ENATIONI
GAS
:%JTION-
to”
CHEMI -
SORPTION
STkELATTICE
Fig. 2. Energy diagram of various oxygen species in the gas phase, adsorbed at the surface and bound within the lattice of a binary metal oxide. After Bielanski and Haber [21].
Above approximately 450 K (O,,& ions are found as the prevailing species. At constant oxygen coverage, the transition causes an increase of surface charge density with corresponding variations of band bending and surface conductivity. From conductance measurements it is concluded that the transition takes place slowly. A fast temperature change of sensors is usually followed by a creeping change of the conductance. There are two reasons for this: the oxygen coverage is adjusting to a new equilibrium and the adsorbed oxygen can turn into another species. But diffusion processes may also cause a slow response. The reactivity of (O,,,)is high and exceeds that of (O,&-. But the coverage by negatively-charged species is limited to 10m2 to 10e3 ML by space-charge effects (Weisz limitation, see Section 2). (O,,,)2- is unstable and has to be stabilized by the Madelung potential of the lattice, i.e., on a lattice site. With respect to oxidation reactions, the adsorbed (O,,,,)and (O,,,)species and also exposed oxygen atoms on step sites are classified as ‘electrophilic’ reactants, which preferentially attack the C=C double bond of adsorbates abstracting electrons, whereas the ‘nucleophilic’ 02- ions bound within the lattice at the surface react with activated hydrogen or dehydrogenated hydrides and hydrocarbons [ 231. Reactive oxygen on a step site is sketched in Fig. 13(g). Activation means excitation of a bond and, as a possible consequence, ionization, dissociation or formation of radicals such as CH3. Surface oxygen vacancies are produced by heating in vacuum, by chemical reduction (see below) or by photolysis with band-gap light. In the oxide layer of gas sensors, oxygen vacancies may also be induced by the
oxidation of reducing gases. At the operation temperature no fast healing process takes place, only increased oxygen chemisorption due to the higher charge density (see Section 2). In this way the surface of the oxide is modified during use by an increased donor concentration at the surface being compensated by ionosorbed oxygen [16]. This process may be responsible for some drift phenomena in gas sensors. In the present paper it will be shown that many oxidation reactions occur on an oxide surface in the absence of any ambient oxygen. Also a second oxide, e.g., Sbz04, present in separated clusters at the surface can adsorb and activate oxygen from the gas phase [ 241. Hydrogen The reactions of the oxide surface with hydrogen are important not only for the detection of gaseous hydrogen. Hydrocarbons or hydrides-can dissociate either on the SnOZ surface or on the surface of deposited noble metal clusters. Subsequently the hydrogen atoms migrate to adsorption or reaction sites on the Sn02 surface. The energy for migration of hydrogen along the surface is low; an estimation is given at the end of Section 3.6. Two processes will be considered for the increase of conductivity. In any case a dissociation of the hydrogen molecule is required; adsorbed Hz molecules do not change the conductance. At moderate temperatures adsorbed hydrogen atoms act as donors. These donors provide additional electrons and induce an accumulation layer:
H+ad-
OatW
+
e-
(3)
The oxygen atoms remain within the lattice during thermal desorption and molecular hydrogen appears in a TDS spectrum after exposure to atomic hydrogen [ 161. Similar spectra are found after exposure to molecular hydrogen, if the sample temperature is raised to 470 K for dissociation [ 251. The conductance increase of a (110) face in vacuum is reversible by moderate heating. (110) faces and sputtered layers kept at 500 K in air show a reversible response to admixtures of 65 to 1000 ppm hydrogen [ 261. At higher temperature a reduction of the lattice oxygen occurs: 2H
+ at
-
HZ%ls
+
vo
(4)
By comparison with infrared spectra [27], the weak water desorption at 390/410 K can be assigned to adsorbed water molecules, whereas the main water desorption above 650 K arises from the condensation of surface hydroxyl groups [ 161. By removal of oxygen from surface lattice sites vacancies are produced, which diffuse into lattice planes below the surface, acting there as subsurface donors. In vacuum this increase of conductance is not reversible at moderate temperatures. The initial conductance can be restored by exposure to oxygen or another oxidizing gas at sufficiently high temperatures. The highest conductance ratio in air with and without hydrogen admixture is found at temperatures between 600 and 670 K for various specimens
80
(whiskers with a rough surface [3], evaporated films of small gram size (see Fig. 5(b)), sputtered films [28] and sintered specimens without additions [28]). Water vapuur After exposuring oxidized (110) crystal faces to water at 300 K Semancik et al. 1291 found features in their UPS spectra that they attributed to undissociated water molecules besides dissociated ones. A single water desorption peak at 480 K found in thermal desorption spectroscopy [ 16 ] may correspond to the undissociated state identified in UPS by Semancik. A hydrogenation reaction of an adsorbed gas can involve protons from a hydroxyl group or from an adsorbed water molecule. A proton from a water molecule (strong Br$nsted acid) is more easily available than the tightly-bound proton from an OH group (weak Brgnsted acid). A preparation to stabilize the undissociated state of water and also the existence of hydroxonium ions, H30+, has been successfully applied to polycrystdline SnO, samples 1391. Therefore, this preparation is named as proton&ion in the literature [30, p. 2181. An application, the enhancement of NH3 adsorption and the formation of the positively charged surface species (NH,)+ by proton uptake, is described in detail in Section 3.3. The increase of conductance by exposure to water vapour can be described as a sequence of dissociation and reduction processes. Yamazoe et al. 1251 find on sintered samples containing stabilizers, which can facilitate dissociation, a second desorption maximum at 670 K. This high-temperature desorption maximum increases in height when the exposure temperature is raised. A comparison with the i.r. spectra after hydrogen exposure shows that these samples also dissociate hydrogen very effectively. If there are two types of hydroxyl groups, a rooted one including lattice oxygen (a rooted surface intermediate includes one or more surface oxygen atoms) and another one bound to lattice tin, two equations can be written:
H20 + Swat + (Aat -+
(HO-Snl,,)
+ (OlatH)+ + e-
(5)
or H2O
+ Snht
+ 0~
----+
2(HO-Snl,,)
+ V,
(6)
The oxygen vacancy on the right-hand side of eqn. (6) has to move towards the bulk in order to become effective as a donor (compare Section 2). Two types of water desorption occur if the system is activated by heating of the substrate, offering of atomic hydrogen or compounds containing hydrogen, or by the presence of a noble metal (oxide) admixture in the sintered specimens: (I) Water including lattice oxygen desorbs with a low-temperature TDS maximum near 410 K, presumably from a molecular state [ 27 1, after exposure to: atomic hydrogen on a (101) face [31); atomic hydrogen on a sintered specimen (weak) [ 16 3 ;
molecular hydrogen on a sintered specimen with Pd [ 321; methane on a (101) face, see Section 3.5; acetic acid on a (101) face, see Section 3.7; acetic acid on an evaporated film, see Section 3.7 ; ethanol on a (110) face, see Section 3.6. (2) A second high-temperature TIM maximum between 670 and 770 K, resulting from a condensation of hydroxyl groups [ 27,291, is observed after exposure to : atomic hydrogen on a sintered specimen [ 161; molecular hydrogen on a sintered specimen at 470 K [25] ; molecular hydrogen on a sintered specimen with Pd [32]; water on a sintered specimen at 300 K or 860 K [14]; ethanol on a (110) face, compare Section 3.6. 3.3. Exposure to hydrides An m-sine sensor seems desirable, since this highly poisonous gas is applied in the manufacture of GaAs semiconductor components. Furthermore, amine escapes during some metallurgical processes. Figure 3 shows the results of reactive scattering on a thin (40 run) evaporated film of SnOz [33]. Above 600 K the backscatteringrate of AsHs decreases and at the same time As and Hz0 appear as the products of decomposition and oxidation. No oxidation products of arsenic are found and no arsenic is detected by Auger electron spectroscopy on the oxide surface after exposure. AsH3 (tetragonal pyramidal structure) cannot form hydrogen
-
350
continuous exposure to I.L.t0’6 AsH3 molecules eni*+’
450
550
TEMPERATURE
650
750
(K)
Fig. 3. Thin evaporated film. Desorption flux of reactively scattered products as a function of temperature during continuous exposure to AsH3. After Mokwa et 02. [ 331.
82
1
I
Exposure
to arsine
ASgas
H HH
(a)
\a[ ..
lb)
c
3,
-
,Sn, ,Sn, 0000000
HZOgos
(cl --jr
H ‘I’ H ,Sn,l,Sn,l,SnJ
/ 9, 0
,Sn, ,o
1 9, 0
vacancy
Exposure
to ammonia
Ammonium ion hydroxyhted surface
on
Ammonium protonated surface
ion
on
I
Fig. 4. Hydrides on SnOz. (a) Arsine adsorbing covalently; (b) arsine dissociated to arsenic and hydrogen; (c) desorption of arsenic atoms and water leaving a vacancy; (d) hydrogen-bonded ammonia; (e) formation of an ammonium ion from NH3 and OH groups on an untreated SnOg surface; (f) formation of an ammonium and chlorine ion pair from NH3 and Hz0 on an SnO? surface pretreated with HCl.
bridge bonds, because the charge density in the p orbitals of species with higher atomic numbers (As : 2 = 33) is too low [ 341. The As atom of ASH, may be mainly coordinatively bound to a surface tin atom, since the electronegativity difference is not large (As: 2.20, Sn: 1.72 [35, p. 95]), see Fig. 4(a). Then the surface-bound AsHs can dissociate, Fig. 4(b), because ASH, is only metastable with a low binding energy for hydrogen, EAS_n = 59 kcal/mole (for comparison, I&,-n = 111 kcal/mole). Under vacuum the oxygen for the Hz0 formation must come from a reduction of the oxide surface, Fig. 4(c). This catalytic combustion corresponds to the known combustion of arsine in an oxygen-deficient atmosphere or with a cold flame (Marsh probe, [36, p. 4221): 4AsHs + 302 -
4As + 6Hz0
(7)
On the contrary, combustion in the presence of an oxygen excess forms AszOs, which is absent in the combustion with lattice oxygen. If the 40 nm thick evaporated film is exposed to air with an arsine admixture, a sensitivity maximum becomes observable above 600 K, see Fig. 5(a) [37]. Both decomposition products, hydrogen and arsenic, act as
83 I
.
I
I
SnOp thin film
500 600 700 TEMPERATURE (K)
LOO
Fig. 5. Sensitivity of SnOz thin films exposed to (a) AsH3 in air and (b) Hz in air. films are made by deposition of tin on quartz substrates and subsequent oxidation, Section 3.1. The sensitivities to AsH3 and Hz vary linearly with the concentration. sheet conductances in air at 600 K are 6.4 x 10m8 and 6.5 x 10e4 A/V for the 40 400 nm films, respectively. After Mokwa [37].
The see The and
donors. The complicated surface states related to arsenic ions are discussed by Morrison [38, p. 1911. The film is less sensitive to hydrogen by about a factor of 10, compare Fig. 5(b). AsHs dissociates on the 40 nm film consisting of small interconnected spheres (see Section 3.1.). However, on a 400 nm film with a smoother surface, only a low-temperature conductance maximum near 430 K is visible in Fig. 5(a). Undissociated arsine can adsorb via its arsenic lone-pair orbital filled with two electrons. Part of this charge can be transferred into the space-charge layer and increases the conductance. Above 430 K desorption of AsHs molecules decreases the conductance. At 430 K the selectivity against hydrogen reaches a value of 100, compare Fig. 5(b). Environmental air without intended addition of hydrogen contains about 1 ppm hydrogen. Therefore, a sufficient selectivity against hydrogen is a prerequisite for hydride (and other) sensors operating in the sub-ppm range. Two other hydrides with elements of the fifth group and tetragonal pyramidal structure, ammonia and phosphine, are of interest in sensor applications. Ammonia sensors can be usefully applied in the removal of NOz by NHs injection in order to monitor and minimize an NH, excess (> 5 ppm desirable) : 6NOz + 8NHs -
7N, + 12Hz0
(8)
This extraction technique is already applied to the smoke gas of industrial plants and has been discussed for the exhaust gas of large engines. Control of the maximum tolerable concentration in a workroom, 50 ppm in the presence of hydrocarbons, is of interest, too. On polycrystalline SnO, the more stable ammonia molecule (EN-u = 93 kcal/mole) is weakly hydrogen [34, p_ 55]), and readily bonded to an oxygen atom (&__H-~ = 5 k&mole desorbed upon evacuation at room temperature [39], compare Fig. 4(d).
84
In the presence of hydroxyl groups, e.g., after admission of water vapour, a second adsorption state, stable up to 495 K, is detectable [39]. An (NH4)+-like species is formed including the proton of the hydroxyl group. This ion is kept at the surface by a hydrogen bond to the oxygen atom of the former hydroxyl group (Brgnsted acid site). The ammonia ion on the surface is sketched in Fig. 4(e). The fourth hydrogen atom positioned straight behind the nitrogen atom is not visible. An Sn02 thick film kept at 400 “C shows a linear conductance increase between 0.1 ppm and 10 ppm /40]. Evaporated SnOz films (thickness: 60 nm) at 300 “C require 700 ppm NHs for the same conductance increase as found with 100 ppm Hz [41]. The lower sensitivity for the hydride may be related to the firm binding state in Fig. 4(e), preventing fast decomposition and water formation. The dissociation of NH, on some oxides (CaO, MgO, SrO) was found to need a pair of exposed metal and oxygen atoms, where the ion pair (NH,)- (amino group) and H+ is irreversibly formed [42]. CaO and MgO are an admixture in some sintered SnOl samples as stabilizers (compare Section 4). A reaction of the released hydrogen with adsorbed or lattice oxygen can increase the conductance. The described adsorption state connected with the uptake of a proton can be further stabilized by a protonation pretreatment: the surface is exposed to HCl vapour. Chloride ions bound to tin atoms emarge the poaitive charge not only at the underlying tin atom but also in the neighbouring tin atoms. As a result, the lone pair orbital of a water molecule is more effectively drawn onto a neighbouring tin site. The strengthening of the oxygen-tin bond weakens the oxygen-hydrogen bond, but the water molecule does not dissociate. So a strong Br@nsted acid site with an easily available proton is formed (protonation). Again an NH4+ ion-like species with a proton transfer from the adsorbed water is formed, compare Fig. 4(f). The lowering of the bending vibration of the NH4+ ion by 60 cm-’ indicates that the interaction of the proton with the remaining OH group is weaker. Such an ammonium ion is more stable, because it is additionally bound by electrostatic interaction to the chloride ions adsorbed on the surface. TDS curves of ammonia from en A120s substrate, pretreated with HF, are shifted by 120 K to higher temperatures [ 38, p. 2541. Too much HCl reverses the effect. The surface becomes hydrophobic if many tin sites are already occupied. Removal of the chloride ions is possible by wetting the surface with water or ammonia solution [30, p. 2191. Furthermore, the sensitivity of an Sn02 sensor to other gases can be changed in the presence of NH3. NHs can poison the surface Brgnsted acid centres (OH groups or molecularly adsorbed water). These centres are also active in the dehydration of alcohols and cracking of hydrocarbons [38, p. 3321, compare Section 4. The metastable phosphine (energy of formation = -1.3 k&/mole), used in the molecular beam epitaxy of GaP and InP, is not only very poisonous but also explosively inflammable at 150 “C in air. Its behaviour on SnOz has not been investigated up to now. Hydrogen bonding is as unlikely as in
the case of AsH3, because the atomic number is too high (2 = 15) [34]. The P atom may be coordinatively bound to a surface tin atom, since the electronegativity difference of P (2.06) and Sn (1.72) is small. Pure PHs is commercially fabricated from phosphonium chloride in a liquid base with the equilibrium on the right-hand side: PH4+ + OH- -
PH, + Hz0
(8)
However, on the surface PH4+ ions can be formed in analogy to the (NH,+)+ ions already discussed. The reason is that the addition of a proton to the hydride on the surface needs the binding energy Ee_n or EN-n respectively and the rehybridization energy to reach an sp3 configuration. The sum of both is nearly equal for gaseous PH, and NH3. In solution only the (NHJ’ ion is stabilized. PH, does not form a (PH,)’ ion in solution because the solvation energy of NH3 is about 20 kcaljmole higher [35, p. 4741. So the additional adsorption states for ammonia, shown in Figs. 4(e) and (f), should also exist for phosphine. Since AsH3 does not form an (AsHa)’ ion, an HCl preexposure could enhance the AsH3/PH3 selectivity. A sensitive and selective sensor for hydrogen sulphide is of interest, too. HzS in small concentrations is not only poisonous to humans but is also known as a catalyst poison contained in coalderived synthesis gases with concentrations up to some tenths of a percent 136, p. 287,431. However, a sub-ppm concentration destroys the function of copper-based catalysts for methanol formation from synthesis gas 1441; poisoning mechanisms are discussed by Brand et al. [45]. Hydrogen bridge bonding is ruled out for H2S adsorption because the atomic number of sulphur (16) is too high. The strong ionic binding of the sulphur (electronegativity 2.44) to the oxide metal atom weakens the sulphur-hydrogen bond (Es-n = 81 kcal/mole for the gaseous molecule) and facilitates dissociation to an S2- or an SH- ion and two or one H+ ions. The hydrogen donors already cause a conductance maximum at 370 K, see Fig. 6
1461.
A thin Sn02 film pretreated in SO2 is able to detect 5 ppm H2S (sensitivity maximum at 370 K) in the presence of 1000 ppm benzene (sensitivity maximum at 670 K) [46, Fig. 11.14]. An Sn02 thin film doped with ZnO is able to detect 0.01 ppm HPS in air at an operating temperature of 480 K [47], but the film needs more than 100 ppm H2 for the same resistance change. An admixture of metals to SnO, with a larger electronegativity difference to sulphur than Sn, for example Zn (1.66) or Ag (1.42), enhances the conductance increase by a factor of five for 10 ppm H2S in air. Zn and Ag are weak electron acceptors and weak Lewis acids. Smaller electronegativity differences occur if P (2.06) or Sb (1.82) is added. These metals are stronger Lewis acids with a weakened ionic bond to sulphur, causing less H2S dissociation. The addition of P or Sb decreases the sensitivity be a factor of five [47].
86
0
Loo
Bw
600
TEMPERATURE ( K) Fig. 6. Conductance as a function of temperature of a sintered SnOz sample in air and in air with admixtures of 40 ppm H$3, 1000 ppm SO2 and 500 ppm HZ. After Pijolat [461.
On SnOz sintered samples with additions of Zr&, the relative conductance increase after admission of HzS shows a maximum near 330 K. In the presence of water an additional broad maximum appears near 450 K, which can be suppressed by a hydrophobic coating (trimethyl silica) [ 481. An additional binding state of the basic NH3 molecule in the presence of hydroxyl groups was discussed above. Even the weakly acid HzS molecule seems to form this type of hydrogen bond. Prolonged exposure to HzS of a sputtered SnOl film at 200 “C resulted in a slow response [49]. From Ru (001) faces it is known that a 0.1 ML sulphur coverage, built up by HzS dissociation, reduces the lateral hydrogen mobility by one order of magnitude [ 451. One sulphur atom sterically blocks its nearest neighbour (and by electrostatic repulsion its next-nearest neighbour) sites for hydrogen diffusion. At 200 “C on Sn03 residual sulphur may accumulate and obstruct a fast reaction of hydrogen with oxygen. The suppression of hydrogen surface diffusion by sulphur deposits also counter. acts spillover from noble metal deposits and interferes in the decomposition of hydrocarbons. Besides HzS, smoke gas also contains SO1 (acid rain), which is known as a bleaching agent. Powdered PbO is reduced to PbS04 in a strongly exothermic process [36, p. 2973. The conductance of SnOz increases, because the filled lone-pair sulphur orbital undergoes bonds to two neighbouring lattice oxygen atoms acting as donors (Fig. 6). In effect a doublerooted pyramidal (SO#ion is formed. This picture was verified by XPS and i.r. measurements [46,26]. At 800 K the conductance change is essentially reversible. However, in a TDS experiment a small SO desorption maximum also appears between 1000 and 1200 K. An SO* pretreatment makes Sn02 sensitive to benzene [ 461. 3.4. Exposure to carbon monoxide CO detection is of interest to prevent intoxication from incomplete combustion in heating appliances or from exhaust gas of gasoline engines.
a7
Combustion also produces NO and NO2 in a mixture sometimes designated as NO,. Therefore, the interaction of CO with these species is also discussed. Singie crystals
TDS CO desorbs from a (101) face mainly as CO; only a small fraction forms COz by consuming lattice oxygen [ 201: In
co
+
Olat
-
(101
COzg*ar
No COz desorption maximum appears up to 800 K. Conductivity measurements are not known. Under sensing conditions adsorbed oxygen and OH groups are present. Therefore, in a recent UHV experiment [50) a (101) face kept at 570 K was exposed at the same time to a CO beam (12 ML/s), an O2 beam (2 ML/s) and to a background pressure of H3 adjusted to corre(H&O) but no methanol spond to 0.02 ML/s. Besides COz, formaldehyde leaves the surface. In the technical synthesis of methanol from CO and Hz over ZnO-Cu catalysts at high pressure, the intermediates formate (Fig. 7(e)) and methoxy (CH@--) are sequentially formed [511. In TDS/XPS studies on ZnO single crystals, surface formate decomposed to gaseous formaldehyde [52]. In analogy, the following reactions may apply to CO in the presence of O2 and H, on SnOz (101): 0
ads + Hads-
co,ds
01)
OHad,
(fOl!lIldX3)
+ OH,,,-HCOO,d,
(W
On more reduced faces (previous or simultaneous hydrogen types of formyl groups can exist:
(u) ,Sn
* t--0 Sn, Sn ‘d,O
IbI
‘=I H 0 ,Sn,
\I 5:
,Sn, ,Sn, 0 0
exposure)
(d)
tel 0*
,Sn,
P
,&$Sn, 0 0
p f
3,
two
‘9
Sn 0
Sn, ‘0’
Fig. 7; SnOs after exposure to carbon monoxide. Proposed surface intermediates: (a) unidentate carbonate after extraction of a lattice oxygen [68]; (b) carboxylate [59]; (c) formyl group formed in the presence of hydrogen or water [53]; [d) rooted formyl group formed in the presence of hydrogen or water 1541; (e) formate group [51,56]. Coadsorptioh of CO and NO on stepped or polycrystalline SnOl surfaces ((f) - (h)) and on a flat SnOz surface ((i) and (k)) [68].
88
cHo,d, (-
CO,d, + Hada+ toads
+ Hada
+ Olat
Fig. 7(c))
HCO-Oi,,
__+
(13)
(see Fig. 7(d))
(‘141
The formyl groups (c) and (d) are known from i.r. investigations on CuOZnO [53] and on MgO [54], respectively. Formyl (c) can react with adsorbed oxygen or lattice oxygen atoms exposed on step sites to formate. Formyl (d), a rooted formate containing one lattice oxygen atom, can extract this atom leaving a vacancy, because surface lattice oxygen is removable from SnOz at low temperature [55] : HCO-O1,t
-
HCOOtis + V,
(15)
Formate is known to be stable up to 500 K on polycrystalline SnOz [56]. A quantum chemical study for ZnO shows that formate becomes negative upon adsorption; so that a substantial stabilization up to 600 K on ZnO (0001) results from the interaction with the positive end of the ZnO dipole [9]. Above 500 K, formate on Sn02 can react to give formaldehyde, compare ref. 52: HCOO,d, + OHad, -
H2COgas
+ 2oads
(16)
Formate on ZnO also acts as the key intermediate reaction [ 9, 521: + w&J
C%* In
analogy,
HCooads
co2
-
in the water gas shift
gas + HZ gas
formate on SnOz can decompose, + Hads
-
Cogas
+ Hads
+
co2
+ H2Ogas
compare Section 3.7.: (17)
Or HCOOada
gas + H2 gas
(18)
Formate intermediates are also essential in the formation of methane and methanol from CO and CO2 with H2 and Hz0 on metal catalysts, for example rhodium [ 573. As will be shown below, a hydrogen-containing intermediate is decisive for the CO sensitivity of Sn02 sensor devices in the atmosphere.
Sintered samples and thin films After CO exposure, mainly CO2 with a TDS maximum near 410 K leaves the surface of a polycrystalline sample, compare Fig. 8(a). Additional sites not present on the (101) face contribute to reaction (10). These sites are possibly connected with oxygen in a more exposed position. An intermediate, C02-Vo+, is proposed to exist on ZnO [ 581 and may precede the desorption of COz, compare Fig. 7(a). As discussed in Section 3.2., oxygen vacancies formed at low temperature do not contribute to conductance. Sintered samples in dry air do not exhibit a conductance increase if 1000 ppm CO are added. Simultaneous i.r. measurements [59] verified a carboxylate species, stable up to 570 K, compare Fig. 7(b). In the same tem-
89
1fP
$,& 5 5
1cP
G
2 166 %
u ro-’ 300
500 600 100 TEMPERATURE
700 IK)
800
Fig. 8. (a) Sintered sample of Sn02 without admixtures. Thermal desorption spectra with a heating rate of 9 K/s after exposure to CO. After Jacobs [87 1. (b) An initially wet porous pellet of SnOz exposed to an alternating gas atmosphere; 15 min in dry air, 15 min in dry air with an admixture of 1% CO. The temperature is increased (full curve) and decreased (dotted curve). After McAleer et OZ.[17].
perature range thin films in UHV do not exhibit any conductance change up to 10m4 Pa CO [60]. These films were prepared by vapour deposition of 100 nm tin and oxidation at 970 K in air for 45 h. Figure 8(b) shows conductance-temperature profiles for an initially wet pellet subjected to an atmosphere alternating between dry air with and without 1% CO admixture [17]. First the temperature is raised from room temperature to 800 K and then lowered to room temperature again. As discussed in Section 3.2., OH groups leave the surface mainly between 670 K and 770 K as water. Therefore, OH groups are absent while the temperature decreases. Since the vacancies left by the low-temperature COz formation (eqn. (lo), Figs. 7(a) and 8) do not contribute to the conductance, only the decay of the carboxylate (Fig. 7(b)) causes the CO sensitivity above 570 K by consumption of adsorbed oxygen ions (dotted curve in Fig. 8(b)): CO + Oads __f
CO* ,&carboxylate)
-+
CO* gas
(19)
On the wet (hydroxylated) pellet an additional conductance mechanism working below 570 K is due to the presence of OH groups on the surface (Fig. 8(b), straight curve). A hydroxylated polycrystalline sample heated in a 1000 ppm CO/air mixture exhibits a conductance maximum at 490 K, absent on the non-hydroxylated sample [ 591. The CO sensitivity of sintered Sn02 devices pretreated with sulfur dioxide increases with the partial pressure of water [61]. Since an operating temperature of 600 K was chosen, probably both conductance mechanisms are superimposed. The reactive scattering experiment with primary fluxes of CO, O2 and H,, repeated on
90
sputtered films, revealed mainly COz besides some formaldehyde. The nonrooted formate of eqn. (12) can decay according to eqn. (18). Since formate on sintered SnO, samples decays near 500 K [56 1, the conductance maximum at 490 K can be caused by the consumption of oxygen from adsorbed OH groups. Carbonate groups, Sn-CO2 detected on sintered SnOz by i.r. spectroscopy at lower temperatures [ 27 ] , seem to have no effect on conductance. Pd, Pt and Ir admixtures in sintered samples decrease the sensitivity (factor of two) and shift the conductance maximum to higher temperatures (50 - 150 K); the mechanisms are not known [62]. In some cases, e.g., in automobile exhaust, CO and NO are present at the same time. Nitrogen monoxide in the absence of CO does not change the conductance of a sputtered SnOz film [63] or a sintered SnOz specimen [64,65]. However, traces of NO reduce the CO sensitivity of the sintered SnO, specimen drastically [64, 651, so a simple superposition of conductance mechanisms is ruled out. This behaviour is not surprising, since polycrystalline SnOz is a (weak) catalyst for the oxidation of CO by NO, which activity can be enhanced by addition of PdO, CuO and other transition metal oxides [60,66,67 1. A reaction of NO and CO at metal-lattice oxygen pairs with the oxygen in an exposed position, verified by i.r., ESR and isotope exchange on MgO [ 681, may apply on polycrystalline or stepped Sn02, too: CO,,, + NO,,, + 02- -
(CNOs)“-
(26)
The formation of this intermediate needs only a marginal activation energy, see Figs. 7(f) and (g). The corresponding species (CN0,)2- on the flat surface of a single crystal, see Fig. 7(i), requires a highly-activated disproportionation (slow reaction) : 2CO,,, + NO,,, + 02- -
(C0,)2-
+ 2(CN0s)2-
(21)
The intermediate of reaction (20) decomposes to nitrogen and carbonate, which is presumably not active in conductance, as mentioned above: 2(CN0s)2-
-
2(C0~)~-
+
N2gas
(22)
The further decomposition of the carbonate is discussed by Harrison et al. 159,271. It should be noted that in the hot exhaust environment NO can be oxidized to N02. Nitrogen dioxide is a strong acceptor on Sn02, presumably with a surface state energetically below the state of adsorbed oxygen. Therefore, NO2 decreases the surface conductance of Sn02 reversibly [63, 691. A small irreversible part of the decrease has to be attributed to the reduction of adsorbed NO2 to NO by the oxidation of donors, i.e., the refilling of oxygen vacancies with oxygen atoms or the oxidation of tin atoms [70]. The latter effect plays a more prominent role for high partial pressures of NO2 and/or low O2 partial pressures; it predominates under UHV conditions [so]. Small admixtures of NO2 to CO in air can strongly reduce the signal of an SnO,-based CO sensor 1711. A coal absorption filter is recommended to suppress this interference. If NO and NO* are extracted by NHs from
91
combustion gases (compare Section 3.2.), adsorbed NH1 may also interact with CO, resulting in a formamide intermediate known from i.r. spectra of CaO and MgO (Fig. 8(k)) [42]. Nonequilibrium effects are essential in a CO sensor using a cyclic variation of temperature [72]. The SnOz thick film contains admixtures of 50% a-A1203 and 0.4% Pd. After 10 s at 570 K, the device is cooled down to 350 K within another 10 s. Immediately before the next heating the conductance is sampled within 0.5 s. This type of sensor mainly uses the carboxylate decay mechanism and benefits from the increasing sensitivity of the barrier conductance mechanism at low temperature. But at 100 ppm CO, a humidity-dependent conductance contribution remains visible (formate reaction). Non-equilibrium conductance measurements with similar cyclic temperature variations verify that adsorbed O- ions play an essential role (compare eqn. (19)) [73]. An increase of the film thickness allows the more reactive species to be suppressed (compare Section 3.3.). 30 ppm NO2 reduce the signal of 100 ppm CO by one order of magnitude on 30 pm thick films, but do not change the signal of films with a thickness exceeding 300 ~.cm. The sensitivity of 30 pm films to CO and ethanol is equal, but a 300 pm film is less sensitive to ethanol by a factor of 20. 3.5. Expusure to methane Decomposition of methane on a single crystal (101) face Figure Q(a) shows a TDS spectrum recorded after exposure to methane [74]. Significant amounts of CH4 are not desorbing at low temperatures. Hydrogen bonding as discussed in Section 3.3. should not occur, because it is only observed for carbon atoms with an increased Zeffective by 7rbonding (e.g., in HCN) or involved in bonds to strongly electronegative atoms (e.g., in CHCls) [34]. An adsorption of undecomposed methane via two of its hydrogen atoms at two adjacent oxygen lattice atoms and subsequent
TEMPERATURE (K)
Fig. 9. Thermal desorption spectra of (a) a single crystal, and (b) sputtered layer of Sn02. Heating rate 13.2 K/s. Before each run the oxygen pretreatment was applied to remove contaminants from the as-grown surface and to establish a stoichiometric surface: 30 min at 870 K and 15 min at 670 K in 0.25 Pa of 02, finally 5 min at 670 K during pump down. After Thoren [33] and Zacheja 11031.
reactions with ethane and ethylene are only discussed for basic oxides strong enough to abstract these two hydrogen atoms from the adsorbate [ 75, p. 10). Because Sn& is only a weak basic oxide, it is assumed that most of the methane dissociates to a methyl group and hydrogen: CH4
gas -
CH3ad,
(23).
+ Hads
The existence of the reverse direction of eqn. (23) is verified by methane formation after exposure to deuterated acetic acid (see Section 3.7.). Two adjacent methyl groups can combine to form a rooted ethoxy-like species : CH3 + CHs + Olat -
CH3CH2%t
(24)
+ Heda
This oxidative coupling was first proposed by Lunsford and is extensively discussed in refs. 76 and 77. In the chemistry of catalysts the formation of CzH4 (and also C2Hs, not investigated here) from adjacently adsorbed methyl groups is a process with a yield of up to 15% on oxides of Sn, Pb, Sb, Te, Mn, and Cd. An ethoxy-like species on SnOz is also discussed as an intermediate after ethane or ethylene exposure; unfortunately all characteristic i.r. bands of this species were obscured by hydroxy stretching modes and strong absorptions of the oxide itself [ 551. The low-temperature peak of mass 28 in Figs, 9 and 10 is accompanied by a proportional mass-26 signal and has therefore to be ascribed to ethylene. In a dehydration process ethylene is formed and the remaining hydrogen desorbs as water: CHsCH@lat
+ Hads -
H2Olat
+ C2H4gas
HlLOges
+ C2H4
gas +
Vo
A contribution to the desorbing ethylene flux from a recombination surface methylene groups cannot be ruled out [ 781: 2CH3
ads __f
2CH2
ads + 2Hed,
-
GH4gas
+ 2&d,
(25) of two (26)
The formation of methyl groups is not rate limiting for the ethylene desorption, because the water signal in Fig. 9 appears at lower temperature. The hump in the CO2 desorption near 450 K may arise from an oxidation of nondesorbing C2H4 as discussed in ref. 76. The ethoxy-like species can be converted to an acetate-like species [ 551. For details compare the corresponding reaction of the ethoxy group in Section 3.6.: CH&H@l,t
+ Clat -
CH3CQatOlet
+ 2Hads
(27)
In infrared studies on polycrystalhne Sn02 samples, acetate was found to be the most stable carbon-containing intermediate [ 551. The decomposition of surface acetate starts near 580 K and is completed at 750 K. CH4, CO2 and a species with mass 28 desorb in this temperature range, showing a common maximum between 700 and 740 K, compare Fig. 9. In this temperature range no mass-26 signal appears, therefore mass 28 has to be identified with
93
El
$y
Ethoxylike
7 H-C-H
species
+H
,Sn,l ,Sn .(),S”‘ *
I
I 1 v
t . .. . . . . *. . . . . . . . . .
;
H\~/H C
C2H4
P??!!!??
A
,
L-l Acetatelike
species
i ...........................
::.................. CHL desorbingi &. .:
. . . . .. . . .. . .*.. . . *. .. .. . . .. . . . .. .. .. ... . . .
i CO, CO2 desorbing i : . . . . .. ... ... .. . . . .. . .. . ... . . .. . .. .. . ... . Fig. 10. Decomposition of methane on SnO2 under UHV conditions (absence of gaseous or adsorbed oxygen). The surface intermediates and also the accumulated number of hydrogen atoms are indicated. Water and small amountr of hydrogen are also found deaorbing. The transition from the ethoxy-like species to the acetate-lie specie6 ie more explicitly discuwed in the corresponding ca8e of ethoxide decay in Section 3.6.
CO, compare the discussion of Fig. 10. After the subsequent dehydrogenation steps described in eqns. (23), (24) and (27), enough hydrogen is available to feed the reaction CH301Jhat
+ =&ids
-
CH,,,
+ HC%Jhat
(28)
At the temperature of the acetate decay the formate-like species, HCOletOlst, can be assumed to be unstable and decomposes further according to eqns. (29) and (30) below. A formate intermediate on polycrystalline SnOz samples exists only up to 500 K, as is known from changes of infrared spectra with temperature [ 561. Rooted formate-like species including lattice oxygen are also known from i.r. measurements at ZnO after exposure to methanol [ 791: HColat~lat
-
C”2gas
+ Hads
+ 2v0
(29)
94
or HC%,%l,
-
CC,,, + %,H
+ Vo
(30)
This set of equations describes an analogy to the water gas shift reaction, with the HCO1,,O1,, intermediate responsible for the CO/CO2 balance. The reactions (29) and (30) produce oxygen vacancies on the surface with a maximal rate between 700 and 740 K. A survey of the methane decay reactions is shown in Fig. 10. CzH4 already desorbs near 400 K. CH4, CO and CO2 show a common desorption maximum, because the acetate-like species is more stable at high temperatures than the formate-like species. All reaction steps release hydrogen atoms. Only the step from acetate to formate (eqn. (28)) needs hydrogen atoms to form methane. Most of the hydrogen leaves the surface as water, consuming lattice oxygen. The saturation value of the water desorption flux in Fig. 9(a) is reached near 450 K, the temperature where the low-temperature water formation (compare Section 3.2.) occurs. At higher temperature the OlatHads groups formed in eqn. (30) can recombine with Hads to form water. However, the formation of methane in reaction (28) reduces the amount of hydrogen feeding the water formation. The flux of desorbing water decreases correspondingly. Even in the absence of reducing gases, the oxygen/tin ratio at the surface shows a broad minimum around 550 K, as was shown by Auger spectroscopy (SO]. Thus a competing reaction without 0 let consumption is favoured around 550 K and also consumes a small fraction of the available adsorbed hydrogen: H ads
+ Hads
=
H2
gas
(31)
The reactions (29) and (30) produce oxygen vacancies, increasing the conductivity. The surface lattice oxygen supply is exhausted by consecutive TDS runs in v&u&n. Therefore, the mass-28 contribution in the hightemperature rar$e consisting of CO decreases, whereas the low-temperature peak produced by ethylene desorption persists, Fig. 11(a). Also the CO2 desorption is reduced. Only an oxygen pretreatment restores the original height of the CO and CO1 peaks (see Fig. 9). On other oxides, and under other reaction conditions further processes are discussed [ 751. For example, in the presence of gaseous oxygen, methyl groups can react directly to give formate with subsequent desorption of formaldehyde, CO and H,. This reaction is excluded under UHV conditions, where only lattice oxygen is available, but it may be effective under sensor working conditions. Conductance measurements on distinct single crystal faces are not known. However, whiskers (thin needle-shaped single crystals) show a conductivity increase on addition of 1% methane to air at temperatures above 720 K [ 811 .-Methane is a very stable mole&e in the gas bhase. This fact is often made responsible for the finding that the sensitivity of SnOzbased devices (conductance change) reaches a maximum 100 to 150 K higher than for most other gases (ethanol, hydrogen, . ._). However, water and
96
‘jO0
500
700
300
TEMPERATURE
500
700
900
[Kl
Fig. 11. Thermal desorption spectra (TDS) of mass-28 and maw44 species after an exposure to 6000 Langmuir CH4. After exposures (1) and (2) the surface was not restored by a standard pretreatment in oxygen as applied before the f&t exposure, Fig. 9. After Thoren [33] and Zacheja [ 103 1.
ethane already leave the Sn02 surface below 350 K, because the first surface reaction requires only the activation energy to break only one of the four C-H bonds. The reaction path yielding ethylene, predominating below 600 K, does not change the conductance. Only the generation of oxygen vacancies starting near 700 K in the reaction path via ethoxy/formate-like species evokes a conductance increase. The low-temperature water formation consuming lattice oxygen is not reflected in a conductance increase. During continuous exposure to methane at temperatures well below 700 K, the stable acetate-like species may even block sites for further methane adsorption and prevent an increase of conductance. Decomposition of methane on a sputtered film To compare the results of the single crystal with those of a commercial SnO, device, the TDS runs were repeated with a sputtered layer on a porous A1203 substrate, Fig. 9(b). The methane ‘desorption signal’ arises mainly from a diffusion out of the substrate through the Sn02 layer. An auxiliary measurement after exposure of a bare substrate to oxygen resulted in a similar slope of the ‘desorption signal’. The water formation begins at the same temperature as on the (101) face. Ethylene starts to desorb at slightfy lower temperature and increases faster; the less well-defined sputtered surface may expose more reactive sites. More reactive sites also enhance the oxidation of nondesorbing CzH4 markedly. The hump in the CO2 desorption is much larger and shifted from 450 to 400 K. Hydrogen molecules do not desorb; the more reactive surface seems to convert the hydrogen more effectively to water. The maxima of CO and CO2 appear at higher temperature. Two reasons are possible for this. The acetate may be more tightly bound on sites not present at the (101) face. On the other hand, the CH4 dissolved in the substrate can diffuse back at elevated temperatures
96
and occupy sites needed for the formation of the acetate-like species. Thus the leading flank of the maximum may shift to higher temperatures. Both cases can only be distinguished using layers on substrates like quartz glass. Such a substrate would also facilitate the detection of methane formation during the acetate/formate transition. Besides SnC&based elements cr-FezOrbased elements are also commercially offered for methane detection. In contrast on this n-type semiconductor methane acts at 430 “C as a donor molecule. Only methane is found in desorption. However, ethanol and isobutane decompose at the same temperature [ 82 1. Conductance measurements on sputtered Sn02 films during methane exposure do not exist. However, a sintered specimen in a flow of air shows a maximum of the relative conductance increase between 730 and 750 K upon addition of 100 ppm of methane [83,623. In this temperature range the consumption of methane and the desorption of a proportional amount of carbon dioxide were also found to become observable [83]. Other decomposition products were not investigated. The selectivity of a sintered Sn02 film depends on its thickness (see Section 3.2. for corresponding effects of AsH3, H2S and CO/NO2 sensors). The sensitivity ratios at 720 K for 3000 ppm of the gases CHI, Hz, CO or ethanol in air are favourable for methane detection at a thickness of 500 pm (4.0/0.3/0.06/0.3), but unfavourable at a thickness of 50 pm (1.2/7.0/0.5/ 4.8) [83]. 3.6. Exposure to gaseous ethanol Single crystals After exposure of a (110) face to ethanol the desorption spectra in Figs. 12(c) and (d) show the desorption of ethanol besides the decomposition products acetaldehyde, ethylene and water [ 841. The ethanol molecule
ik”
5: 3
0 300
LOO 500
600
TEMPERATURE
700
(K)
800
300
LOO
500
600
TEMPERATURE
700
800
(K)
Fig. 12. Thermal desorption spectra (solid curves) and sheet conductance (dotted curve). Standard pretreatment (Fig. 9). Heating rate 9 K/s in the TDS runs. (a) and (b) sintered sample, (c) and (d) single crystal. After Jacobs et al. [84].
2%
1
Ethoxy species Ethylene desorbing
Acetaldehyde adsorbed
H3$ (b) (4
HJ:
“:I+
-
,Sn$nb,
‘S”‘6
I
~ (dl
-
,=n,
P “‘00
Acetaldehyde desorbing
No hydrogen desoorption
350K
P 0
sn
I?
n,
Ethoxy group with Sn-0 bond and
C-H-O
bond
Ethanol adsorbing
Fig. 13. Surface intermediates after exposure to ethanol. Sn03 (110) face: (a) molecular adsorption; (b) after dissociation (dehydrogenation); (c) after a second dehydrogenation; (d) desorption of acetaldehyde. Polycrystalline or stepped SnOz sample: (e) ethoxy group with an additional hydrogen bond; (f) desorption of hydrogen and acetaldehyde; (g) double bond of adsorbed acetaldehyde attacked by electrophilic oxygen; (h) acetate group at a step.
may be weakly bound to the surface, e.g., via the oxygen atom of its OH group, Fig. 13(a), The TDS maximum at 360 K is near the maximum of acetic acid at 350 K, where the same type of binding is possible. CH3CH20H,a,
-
CH&H20H,d,
(32)
The extended high-temperature tail of the ethanol desorption arises from a reversible dissociation to an ethoxy group (Fig. 13(b)) and adsorbed hydrogen (dehydrogenation) : CHBCH,OH,d, -
CH3CH20~d~
+ Hads
(33)
The hydrogen from reaction (33) can react to form water and desorb with a maximum at 400 K, Fig. 12(d). For sintered specimens the reversible dissociation has been verified by experiments with deuterated ethanol (see below). The desorption of ethylene and water near 400 K and between 660 K and 750 K can be formally regarded as a dehydration of ethanol: CH,CH,OH -
C2H4
gas + H2°gas
(34)
The reverse reaction is well known from industrial ethanol production over Cu/ZnO catalysts. Since ethylene adsorbed on polycrystalline Sn02 is known
to form ethoxy groups [ 551, ethylene may be evolved by decomposition the ethoxy group of eqn. (33): CH&H,%,
+
Hads -
H2°gas
+ C2H4
gas
of (35)
The difference from the methane case (eqn. (25)) consists in the fact that no lattice oxygen is required. The desorption spectrum also contains acetaldehyde, a dehydrogenation product of the ethoxy group: CH3CHZoads
-
Hads
+
CH3CHOgas (acetaldehyde)
(36)
The ethoxy groups are the common supply of three simultaneous reactions: (33) reverse, (35) and (36). The maxima of water (and also ethylene) desorption coincide with the maxima generally found at 400 K and 670 770 K after exposure to hydrogen-containing species (Section 3.2.). Since the ethoxy species and water or hydroxyl groups are bound via an oxygen lone pair orbital to the surface, the breaking of this bond may be rate limiting in both cases. The minimum in acetaldehyde desorption reflects the water/ethylene maximum near 400 K, Figs. 12(c) and (d). Acetaldehyde finally stops desorbing when the high-temperature water/ethylene desorption starts. The ethanol desorption according to eqn. (33) reverse is roughly proportional to the water formation between 400 K and 550 K, pointing to a hydrogen/water limitation in this range. Hydrogen bound in OlatH groups seems not to be available for recombination with the ethoxy species to form ethanol. The condensation of OH groups to water above 550 K consumes lattice oxygen, increasing the sheet conductance at higher temperatures, Carbon monoxide and carbon dioxide are absent in the desorption. The sequence of the first two bond scissions in the ethanol decomposition is not oxide specific. On Ni (ill), bond activation was verified in TDS using deuterium and 13C isotopic labels and AES lineshape analysis [85] : (1) O-H with H, desorption between 300 and 350 K, compare eqn. (33); (2) C-H of the methylene group CH2, rate limiting for acetaldehyde desorption (compare eqn. (36)) and CH4 desorption (compare eqn. (28)) above 260 K; (3) C-C not rate limiting for CH4 desorption, see above; (4) C-H of the methyl group CH, and carbidic carbon deposit. The CO bond does not break and CO desorbs from an adsorption state at 430 K. Sn02 (110) and Ni (110) differ in the rate-limitation mechanism for acetaldehyde desorption (for polycrystalline SnOz, see below). Furthermore, the C-C bond remains intact on the clean (110) Sn02 face, preventing CO or CO2 desorption. To break the C-C bond on SnOz faces active oxygen is necessary, as will be discussed below. Heating of SnO, single crystals with (110) faces exposed to ethanol leads to a conductance maximum (dotted curve in Fig. 12(c)) nearly coinciding with the ethanol desorption maximum [84]. Molecular ethanol bound via its oxygen lone-pair orbital (filled with two electrons) to a surface tin
99
atom can act as a surface donor. The right flank of the conductance maximum will contain contributions from hydrogen donors of the dissociated molecule. Since methoxy on ZnO is an acceptor [9], it is likely that ethoxy on SnOa will also be an acceptor, partly reducing the conductance increase caused by the hydrogen donor. The water formation near 400 K consumes the hydrogen donors; the vacancies from this low-temperature process are not active in conductance (compare Section 3.2.). At high temperatures, above 560 K, vacanciesleft after condensation of hydroxyl groups and water desorption can act as subsurface donors. The absolute and the relative conductance increases of SnOz (110) faces kept at 600 K in air with admixtures of 85 to 950 ppm ethanol reveal a nearly linear response to the ethanol concentration [ 861. Sintered specimens The main desorption maxima of ethanol, acetaldehyde and ethylene in Figs. 12(a) and (b) are found between 460 and 480 K. Smaller fluxes of these gases appear as shoulders between 350 K and 380 K, indicating that a minor fraction of the adsorbed ethanol adsorbs and reacts in the same way as on the (110) face. At edge sites on the polycrystalline surface (Fig. 13(e)) ethoxy can be more tightly fixed by an additional hydrogen bond of the methylene hydrogen to an exposed lattice oxygen. The observed shift of the TDS maxima corresponds to about 8 kcal/mole, typical for hydrogen bridge bonding. Only on the polycrystalline sample at 480 K does a desorption maximum of hydrogen also appear. After adsorption of water on a polycrystalline sample, a dominant water desorption maximum at 480 K also appears [ 871. Water is able to form an additional hydrogen bond at edge sites, too. This water desorption maximum is not observed on a single-crystal (101) face. The adsorption of ammonia on hydroxylated SnOz is also attributed to hydrogen bridge bonding, as discussed in Section 3.3. In contrast to the finding on the (110) face, CO and COz also desorb from the polycrystalline samples above 600 K. The C-C bond can be broken by the following mechanism: the acetaldehyde formed in eqn. (36) contains a double bond, Fig. 13(g). An exposed electrophilic oxygen can attack the double bond and form a carboxylate group (acetate) (Fig. 13(h)): CH&H%,+
oexposed-
CHaCOO (acetate)
(37)
It has been shown that formaldehyde and adsorbed oxygen ions on copper react in a similar way to the process for a formate group [ 881. Various ketones adsorbing with their carbonyl group on polycrystalline SnO, are attacked by OH groups at the double bond and also form carboxylate groups, e.g., acetone reacts on hydroxylated SnOz to give acetate [SS]. A schematic survey of these reactions is sketched in Fig. 17. Acetate can decompose via eqns. (28), (29) and (30) to give CO and COz. The acetate after ethanol exposure contains only one (exposed) lattice oxygen atom, whereas acetate after methane exposure contains two. The second maximum
100
of water desorption appears at nearly the same temperature (750 K) as on the (110) face. Again the vacancies left after water desorption can act as donors. After exposure of sintered specimens to deuterated ethanol, &HsOD (EtOD), EtOH is desorbed in addition to EtOD [87]. The desorption maximum of EtOD coincides with the desorption maximum of EtOH after adsorption of EtOH. The EtOH desorption includes at least three steps: (1) Adsorption of an EtOD molecule and formation of an ethoxy group losing its D atom according to eqn. (33). (2) Adsorption of another EtOD molecule somewhere on the surface losing a D atom and an H atom, forming acetaldehyde in eqns. (33) and (36). (3) The adsorbed hydrogen released in reaction (36) migrates to the ethoxy group and recombines to give EtOH (eqn. (33) reverse). The TDS maxima of acetaldehyde and EtOH appear together. Since the acetaldehyde formation may be rate limiting for the EtOH formation, only an upper limit for the activation energy of hydrogen migration is obtainable from the observed temperature difference of 30 K for the two ethanol desorption maxima. It should be mentioned that hydrogen migration can be quenched by blocking of sites (compare Section 3.3.) or by trapping at carbon deposits, forming lateral C-H bonds [45]. 3.7, Exposure to gaseous acetic acid Single crystals In some biosensors for pesticides or for the control of biochemical processes, an acetic acid detector is required [ 901. Acetic acid can adsorb on SnOa (101) faces as a molecule or dissociatively as acetate, compare Figs. 14(a) and (b), respectively: CHsCOOH,,,
H3C
L=O
-
CH3COOad,
H3C I
Acetic acid 4
p’?
,4n ‘o’ Sf-kO, SfxO, 0
Rooted acetate
3-k
Acetate
J%
HO ,Sn,
(38)
+ Hads
H
-Sa,S=.o,Sn,o,Sn$,
[b)
(al
C/CH3
5YH , 9 i sn,J,Sv, 0 O/ vacancy ICI
fiH2 F 0
K@tW desorbing
,Sn.!~3k.o,Sn,~.Sn
‘d Ml
Fig. 14. Surface intermediates after exposure of SnOl to acetic acid: (a) molecular adsorption; (b) dissociative adsorption (dehydrogenation); (c) dissociative adsorption at a defective Sn02 surface; (d) keten desorption.
101
The equilibrium can be on either side of eqn. (38), depending on the experimental conditions. During continuous exposure to 1016 molecules/cm2s in a reactive-scatteringexperiment with linearly increasing temperature, the acetic acid does not find enough time and/or sites to decompose [74]. It adsorbs only weakly, e.g., via the oxygen of the hydroxyl group (desorption maximum at 350 K). However, after an initial exposure to 10” molecules/ cm2 with subsequent linear heating in UHV and correspondingly lower coverages, the acid decomposes according to eqn. (38). The decomposition becomes apparent in second-order desorption (TDS maximum at 430 K) and is further proved by a low-temperature desorption of water. This water desorption is not observed during continuous exposure. Hydrogen released in eqn. (38) is mobile and can recombine with another acetate, as was shown in an H-D exchange experiment similar to the EtOD experiment described in Section 3.6. However, a considerable part of the mobile H reacts with lattice oxygen and forms water. The further decomposition of the surface acetate starts with the reaction CH&OO,,,
f 2H,,
--+
CH4,,, + HCOOad, (formate)
(39)
The TDS spectra of mass 28 (CO or C,H,) and of CO2 look very similar to each other after acetic acid and methane exposure, but are clearly distinct after CO exposure. Therefore, it s&ms reasonable that the decay reactions of methane via a formate intermediate are also applicable for the acetate decay, eqn. (39). However, it should be kept in mind that the formate of eqn. (28) includes lattice oxygen, while the formate from eqn. (39) does not. Photoemission experiments (UPS) on ZnO (1010) faces after exposure to formic acid revealed a formate species [91]. No low-temperature methane desorption was found after methane exposure. Therefore the methane formed from acetic acid at temperatures above 400 K must be formed during the acetate decay in eqn. (39). Enough hydrogen is available, as indicated by the water desorption. The increase of electrical conductance at a sample temperature of 473 K may arise from adsorbed hydrogen acting as donor
WI-
Above about 650 K acetate can fill a vacancy with one of its oxygen atoms (singly-rooted acetate), compare Fig. 14(c): CHsCOOH,,, + V, -
Hads + ‘0~’
+
CH3Coads
(49)
Filling of a vacancy with oxygen is also possible (Section 2.2.) and with COs, but not with CO, as was shown in an XPS/UPS study on ZnO (OOOi) [51]. The singly-rooted acetate with its higher binding energy causes an increase of the acetic acid desorption again during continuous exposure at high temperatures. A fraction of the rooted acetate forms keten, observed in desorption, thereby leaving one oxygen atom to fill the vacancy, compare Fig. 14(d): CHsCO-‘Olat’ -
HzC=C=Opa, + HPdr + ‘Olat’
(41)
102
The formate from reaction (39) contains two oxygen atoms of the initial adsorbate. It decomposes into CO and CO2 without the V. formation of eqns. (29) and (30). The singly-rooted acetate from eqn. (40) can undergo an analogous reaction, delivering a formate including one lattice oxygen atom : Cl&CO-‘Olat’
CH 2 ads
-
+ HCOO1at
(42)
If reactions (29) and (30) are responsible for the similarity in the CO and CO, desorption behaviour of methane and acetic acid, the rate-limiting step is the decomposition of formate. The type of incorporated oxygen, Olat or 0 ads, has apparently negligible influence on the reaction temperatures. During continuous exposure to acetic acid in a reactive scattering experiment, the COz and the CO desorption fluxes decay. The conductance increases irreversibly. The common reason may be a depletion of lattice oxygen by high-temperature water formation, which is only partly compensated for by reaction (40). The desorption of water in the high-temperature regime is not quenched by continuous exposure because the OH groups are products of reaction (30). Thin evaporated films Acetic acid increases the surface conductivity at 573 K, presumably by the effect of hydrogen donors, Fig. 15. At 623 K and 673 K the conductance responds with a transient minimum to acetic acid addition to air. The minimum possibly reflects the filling of oxygen vacancies by acetate
3.6 1
’ 0
I ’
’ 20
Time
’
’ LO
1
’ 60
J
(4
Fig. 15. Conductance g as a function of time during exposure of an SnOg thin film to acetic acid in air for various temperatures. After Schnakenberg et al. [IS I.
103
species, thus cancelling subsurface donors (compare Section 2) Later in the decomposition process of the singly-rooted acetate, the formate extracts oxygen out of the surface according to reactions (29) and (30), producing new donors. Therefore the conductance increases again. At higher temperatures the decomposition of the singly-rooted acetate proceeds faster and the minimum becomes narrower on the time scale. Finally at 723 K the decomposition of the singly-rooted acetate dominates and a minimum is no longer detectable. The conductance of sintered specimens behaves similarly; the initial slope of conductance is proportional to the acetic acid concentration in the range 50 to 700 ppm [18]. SnOz thin-film devices, exposed to acetic acid in air, show after some time an oscillatory conductance behaviour. Stable behaviour can only be restored by transient heating in air to 870 K [18]. In a reactive scattering experiment with the film kept at 585 K (below the TDS maxima of the formate products), only water desorbs as decomposition product, compare Fig. 16(a). According to a sum equation elementary carbon must be deposited: CHsCOOH,,,
-
2H20
+ 2Cdeposited
(43)
A keten desorption according to eqn. (41) is missing on thin films. On the rough surface of thin evaporated films, the highly reactive keten can react at its carbon-carbon double bond with exposed oxygen atoms at step sites. A similar reaction was discussed in Section 3.6. for the decay of acetaldehyde. This type of electrophilic attack by oxygen, observed on oxidized copper and silver surfaces [SS], is known to result in carbon deposition on the surface. In the presence of air at sufficiently high temperature, it is possible to remove deposited carbon by the formation of CO or CO2 [ 881. The curves in Figs. 16(b) and (c), recorded at substrate temperatures of 673 K and 725 K, also show products of the acetate/formate decay described in Section 3.5.: CO and CO2 appear, too.
UJ
01
z
2
3 TIME
(minutesl
Fig. 16. Fluxes of reactively scattered products from an evaporated film at three different temperatures during exposure to acetic acid. The signals of the decomposition products are corrected for cracking contributions from the acetic acid. After Schnakenberg et al. 118 I.
104
4. Common reaction steps in hydrocarbon decomposition The reactions of hydrocarbons on SnOz, discussed in the preceding Sections, have some steps in common. A scheme is given in Fig. 17. It is completed with decay patterns of some hydrocarbons of interest, where spectroscopic data of intermediates are available from the literature. Results from infrared investigations on polycrystalline Sn02 samples are used. They were obtained after exposure to methanol [ 921, ethane [ 55 ] , ethylene [ 553, propylene [93], acetic acid [94], formic acid [ 563, acetaldehyde [92], acetone [ 921 and other ketones [ 95,891, carbon monoxide [ 27,59,94,96], carbon dioxide [27,94,96, 971, and water [27,39]. Molecular adsorption can already raise the conductance, if the adsorbate is a Lewis base making electronic charge available to an accumulation layer. Molecules with a lone-pair orbital occupied by two electrons adsorbed at a tin site, e.g., alcohols, act as surface donors. Acetic acid can also adsorb this way. Oxygen lattice atoms provide sites for ethylene; the adsorbed molecule together with the lattice oxygen atom can be regarded as a rooted ethoxy group. It is well known that the ability of metal surfaces to abstract hydrogen from an adsorbate is enhanced by an oxidation pretreatment. Adjacent metal/oxygen sites provide Lewis acid/base pairs; the oxygen atom takes up the hydrogen atom after dissociation. By this mechanism the tin-oxygen pairs at the surface of SnOz are already efficient in dehydrogenation of methane, alcohols and acetic acid at moderate temperature. Hydrogen atoms ..
. . . . . . .. . ... . ..._...................................
_. . . .._ . .. .. .*. .. . . . . . . . . . . .. . . . . . .. . . . .. .
Mqlecules adsorbing
\I/
Alkox y surface groups Carbonyle surface groups
(..........,.._.._.......
P
: Methoxy--..---I i . ......_....__.._..._ . ..! .. . . . .._._.... . .... .. . . .... . ..._.._._...._... !.!X.XY.:l..!
‘C’ ,--i]l
I
I......*
$;
~ . . . ..,,.. .. . . . . . . . . .. . . ..... .... . . .. .. .... . . . j /Formaldehyde1 1Acetaldehyde
: .. . . \ ‘-Oxygen
. .. . . . ..
. ..
. . . . . . .. . .
. .. .. ... . . ....... . . .. .... .. . .
. .i . . .. . . i
Keten
1 Acetone . . . . . . . . . . . . . .. . . .. . . . . . . . . . . .._. . ..I . . . . . . .. . . .
attacks double
. . . . . . l .. . . . . . . . . . . . .. . . . . . . .
1
Oxygen attacks double
. . .... . ..( . .. . .. .. . . . . . , ,.. .. .. . .. Acetate
..... ................... .. ..... ............. ...............................*..Products desor bing
~.COi__O,~~, Hz0 (total oxidation) . . . . . . . . ..I...............................
.. ... . . . .. . . ....*. .. .. .. .*... . . . ... . . ....... *.. .......
Fig, 17. Schematic representation of common reaction steps for various carbon-containing molecules on SnOz. See Section 4 for explanation.
105
already act as mobile surface donors at 400 K. Recombination and desorption is also observed. Exposed or incompletely coordinated tin atoms are more effective in dehydrogenation [ 971; compare also the behaviour of evaporated SnO, films on exposure to arsine (Section 3.3.). In the presence of water vapour a second type of acid/base pair is active, where hydroxyl groups take up the hydrogen atom’/971 (compare Sections 3.2. and 3.3.). . Besides dissociative adsorption, associative adsorption including the uptake of hydrogen is also possible. Propylene in the presence of water vapour is attacked by the proton of an adsorbed hydroxyl group at its double bond (electrophilic addition). The resulting intermediate is an isopropoxy group [93]. Silica, often regarded as an inert mechanical stabilizer of sintered SnOz sensor devices, binds the oxygen atom of hydroxyl groups more tightly. Therefore, the hydroxyl group becomes more reactive towards propylene [93]. That means a hydroxyl group adsorbed on a Si atom is a stronger BrOnsted acid with a more easily available proton, compare Section 3.2. Both dissociative and associative adsorption result in the formation of alkoxy groups bound to tin sites, compare Fig. 17. Neighbouring methyl groups form an ethoxy group by oxidative coupling at a lattice oxygen atom. In a further dehydrogenation step, carbonyl surface groups containing a double bond are formed, see Fig. 17. The same class of intermediates appears after exposure to acetaldehyde (formaldehyde was not investigated), acetone and other ketones [92, 96, 891. On a single-crystal (110) face acetaldehyde does not react further and desorbs (Fig. 13(d)). Electrophilic oxygen species (exposed lattice oxygen atoms on polycrystalline samples, adsorbed oxygen ions or the oxygen atoms of hydroxyl groups) can attack the carbonyl double bond. The resulting surface species are acetate or for-mate, as verified by infrared spectroscopy. Acrylates are found after adsorption of various ketones [SS]. The relatively stable acetate can decay to fonnate above 580 K. The formate species is also decisive for the low-temperature sensitivity of SnOl towards CO in the presence of water. Finally formate decomposes into the products of total oxidation, CO, CO2 and H,O. Two acetate groups can combine in an acetic anhydride (two acetate groups share a common oxygen atom) and a chemisorbed oxygen atom. The typical decomposition product of the anhydride is keten [W]. After exposure to ethanol or acetic acid on single crystals, keten is observed in desorption. On polycrystalline films again an electrophilic attack of exposed oxygen at the keten double bond occurs. The resulting deposition of carbon spoils the sensor function if the carbon is not periodically removed by heat treatment in oxygen or hydrogen.
5. Possible improvements of gas sensors The existence of common or analogous intermediates for several gases may discourage the hope for selectivity. However, the scheme in Fig. 17 can
106
also be used as a guideline to avoid detours and to concentrate modifications of the sensor. Some ideas are listed below.
on reasonable
Operating temperature and thermal pretreatments The relative abundance of adsorbed (0,) and O- ions (electrophilic attack) depends on the operating temperature. Since the equilibrium between both species is approached slowly after a variation of temperature, the abundance ratio of the oxygen ions is shifted by cycling the temperature in the appropriate range. The concentration of hydroxyl groups taking part in some reactions (CO, propylene, NH,) is also varied in a temperaturecycled mode. At low temperature only the adsorption of donors and acceptors changes the conductance. Oxygen vacancies left in many surface reactions need elevated temperatures to migrate below the surface and become active as donors. Conversely, NO2 and acetic acid annihilate surface vacancies and at elevated temperatures reduce the concentration of subsurface donors and thereby the conductance. The irreversible loss of donors is undesirable for prereduced NO2 sensors (limitation of lifespan), but it can be used for quantiative acetic acid measurements. Examples have been given that exposed oxygen and tin atoms are essential for some partial reactions. The carbon deposition problem caused by electrophilic oxygen has already been mentioned. Therefore, the grain size in sintered specimens and evaporated layers is a parameter that has to be stabilized for constant sensitivity and selectivity by annealing pretreatments 1321 or admixtures of salts that change the ionic conductivity at the sintering temperature (containing, e.g., K, Ca, or Mg [ 981). Reduced surface Surface sites are made more negatively charged by neighbouring oxygen vacancies. The bond energy for donor molecules, e.g., HZ, HzO, NH3, methanol, is lowered and desorption of water and methanol facilitated [30, pp. 174 and 1461. Conversely the bond energy for acceptor molecules increases with the density of oxygen vacancies. Then oxygen should be adsorbed preferentially at oxygen vacancies if the temperature is not sufficiently high for reincorporation. It is also proposed that alcohols dissociate mainly at vacancy sites 1381. Treatment with gases NH3 can poison the surface Br#nsted acid centres (OH groups or molecularly adsorbed water). Thereby the nucleophilic attack of carbonyls can be prevented. Small sulphur deposits remaining (e-g., acetaldehyde) after H2S treatment can suppress the hydrogen diffusion and may affect hydrocarbon decomposition (compare Section 3.3.). The effect of sulphur on hydrogen spillover from metal deposits has not yet been investigated. SOZ is known to enhance the benzene sensitivity of Sn02 [ 461 (see Section 3.3.).
107
A pretreatment with HCl, HF or HBr (protonation) facilitates hydrogenation. The bond of species with an oxygen lone-pair orbital (methanol, ethanol) is strengthened, as verified on Si&, A120s and TiOz using electron paramagnetic resonance [30, p. 1461. Water from dehydration reactions (e.g., formate decay) can be kept on the surface. Therefore, these centres are also active in the dehydration of alcohols and cracking of hydrocarbons [38, p. 3321, see Section 3.3.
Admixtures and deposits Specimens of pure SnO, are often brittle after sintering below 1000 “C. A frequently used stabilizer, SiOz, can provide additional adsorption sites, thus changing the selectivity. The adsorption of propylene has been given as an example. Infrared investigations also exist for water [39], ethane [55], ethylene [55], formic acid [56], COz [97] and ammonia [39]. The admixture of metal oxides by addition of salts before sintering or by cosputtering means a percentage of tin sites at the surface is occupied by foreign metal atoms. If simple covalent/ionic adsorption is decisive for the sensor signal, the electronegativity of the foreign metal can rule the sensitivity, as was shown for HIS in Section 3.3. Surface species on Pd sites of polycrystalline SnOz samples with Pd admixture have been investigated with i.r. spectroscopy after exposure to ethane [55], ethylene [55], propylene [93], formic acid [56], water [39], COz [97] and ammonia [39]. In a comparative i.r. study of CO, NO, and CO/NO, admixtures of Cr, Mn, Fe, Co, Ni and Cu oxides to Sn02 [60] are used. A conductance study with CO exposure uses admixtures of Pd, Pt and Ir (oxides) [62]. Examples were given in Sections 3.4. and 3.5. that the selectivity between gases of low and high reactivity changes markedly with film thickness, if the device has buried electrodes. An active catalyst deployed only on the outer edge of a thick polycrystalline device enhances combustion in the outer region and suppresses sensitivity for gases of high reactivity. This leads to a diminished response at higher temperature, depending on electrode geometry [98]. The effect of precombustion of gases reacting faster than methane is also successfully applied in a device with a porous Pt catalyst, kept at 400 “C to oxidize CO, Hz and higher hydrocarbons but not CH4, in front of a sintered SnOn sample kept at 400 “C [99]. Foreign metals or their oxides can also be deposited (e.g., by electron evaporation) on single-crystal faces or thin films. Usually clusters are formed, leaving part of the SnOz surface open for reactions (Auger control) [3,100]. Then the catalytic properties of the foreign metal play a role, too. Frequently a dehydrogenation is already possible at room temperature; ethanol on Ni is given as an example in Section 3.6. Hydrogen atoms can spill over to the substrate and increase the conductance. A quantitative investigation of hydrogen spillover with a local infrared probe exists for Pt on SiOz [ 1011. An electronic effect of metal deposits is also postulated [98, 1021.
108
6. Conclusions Sensors have a ‘life cycle’ consisting of preparation, activation, operation with deactivation and, possibly, regeneration. Thus understanding the performance in terms of reaction and conductance mechanisms is only a part of the total understanding of a sensor. Given that the performance of an existing sensor will have been optimized by trial and error, it is probably the case that the benefits of surface characterization come from also understanding and modifying the deactivation and regeneration processes. The reported observations and proposed models may be a stimulation to investigate reactions on other oxidic materials, e.g., ZnO, Fe203 or perovskites, which are not treated here.
Acknowledgements Thorough discussions with G, Heiland are gratefully acknowledged. This work was supported by the Bundesminister fur Forschung Technologie.
und
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30 31 32
33 34
R. G. Egdeii, S. Eriksen and W. R. Fiavell, A spectroscopic study of electron and ion beam reduction of Sn02(110), Surface Sci,, 192 (1987) 265 - 274. E. de Fresart, J. DarviBe and J. M. GiIies, Influence of the surface reconstruction on the work function and surface conductance of (llO)Sn02, Appl. Surface Sci., 1 l/12 (1982) 637 - 651. G. Heiiand and H. Liith, Adsorption on oxides, in D. A. King and D. P. Woodruff (eds.), The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 3B, Elsevier, Amsterdam, 1983, p. 137. G. HeiIand and D. Kohl, Physical and chemical aspects of oxidic semiconductor gas sensors, in T. Seiyama (ed.), Chemical Sensor Technology, Vol. 1, Kodansha/ EIsevier, Tokyo/Amsterdam, 1988, to be published. J. F. McAIeer, P. T. Moseley, J. 0. W. Norris and D. E. Williams, Tin oxide gas sensors: Part 1. - Aspects of the surface chemistry revealed by electrical conductance variations, J. Chem. Sot. Faruday Trans. I, 83 (1987) 1323 - 1346. U. Schnakenberg, W. Thoren, D. Kohl, J. Woitok and G. Heiland, Oberfliichenreaktionen an SnO&&hichten verschiedener Priiparation beim Nachweis von Essigsiiuredampf, NTG-Fachberichte ‘Sensoren Technologie und Anwendung’, Vol. 93, VDEVerlag, Berlin, 1986, pp. 73 - 81. Driiger AG, Liibeck, kindly supplied the sputtered films. W. Thoren, D. Kohl and G. Heiland, Kinetic studies on the decomposition of CHsCOOH and CHsCOOD on SnO2 single crystals, Surface Sci., 162 (1986) 402 410. A. Bielanski and J. Haber, Oxygen in catalysis on transition metal oxides, Catal. Rev. Sci. Eng., 19 (1979) 1 - 41. S.-C. Chang, Oxygen chemisorption on tin oxide: correlation between electrical conductivity and EPR measurements, J. Vuc. Sci. Technol., 17 (1980) 366 - 369. J. Haber, Catalysis and surface chemistry of oxides, Proc. 8th Znt. Congr. on Catalysis, Berlin, F. R.G., July 1984, Vol. I, Veriag Chemie, Weinheim, pp. 85 - 112. P. Ruiz and B. Dehnon, Selective oxidation of hydrocarbons, Proc. Properties and Uses of Inorganic Tin Chemicals, Brussels, Belgium, Oct. 1986, pp. 1 - 29. N. Yamazoe, J. Fuchigami, M. Kishikawa and T. Seiyama, Interactions of tin oxide surface with Oa, Hz0 and Hz, Surface Sci_, 86 (1979) 335 - 344. M. L. Burke and R. J. Madix, SO2 structure and reactivity on clean and sulfur modified Pd(lOO), Surface Sci., 194 (1988) 223 - 244. E. W. Thornton and P. G. Harrison, Tin oxide surfaces: Part 1. - Surface hydroxyl groups and the chemisorption of carbon dioxide and carbon monoxide on tin(IV) oxide, J. Chem. Sot. Faraday Trans., 71 (1975) 461- 472. U. Dibbern, G. Kiirsten and P. WiIlich, Gas sensitivity, sputter conditions and stoichimetry of pure tin oxide layers, Proc. 2nd Int. Meet. on Chem. Sensors, Bordeaux, France, July 7 - 10, 1986, pp. 127 - 130. S. Semancik and D. F. Cox, Fundamentai characterization of clean and gas-dosed tin oxide, Proc. 2nd Int. Meet, on Chem. Seneo?q Bordeaux, France, July 7 - 10, 1986, pp. 226 - 229 and Sensom and Actuators, 12 (1987) 101 - 106. V. F. Kiselev and 0. V. Krylov, Electron Phenomena in Adsorption and Catalysis, Springer, Berlin, 1st edn., 1987. G. HeiIand and D. KohI, Problems and possibilities of oxidic and organic semiconductor gas sensors, Sensors und Actuators, 8 (1985) 227 - 233. Y. Nakamura, S. Yasunaga, N. Yamaxoe and T. Seiyama, Stabilization of SnOz gas sensor sensitivity, ?+oc. 2nd Int. Meet. on Chem. Sensors, Bordeaux, Frunce, July 7 - 10. 1986. pp. 163 - 166. W. Mokwa, D. Kohl and G. Heiiand, An SnOz thin film for sensing amine, Sensors and Actuators. 8 (1985) 101 - 108. M. B. Ormerod, The Architecture and Properties of Matter -An Approach Through Models, E. Arnold, London, 1970, p. 55.
110 35 F. A. Cotton and G. Wilkinson, Anorgansiche Chemie, Verlag Chemie (Interscience Publishers), Weinheim, 1970, p. 95. 36 A. F. Ho&man and E. Wiberg, Lehrbuch der onorganischen Chemie, Walter de Gruyter, Berlin, 1960. 37 W. Mokwa, Der Nachweis von Hydriden mit SnO,-Diinnfilmen unterschiedlicher Schichtdicke, Proc. VDZ-Berichte 677: Sensoren, Technologk und Anwendung. Bad Nuuheim, F.R.G., March 1988, VDI Verlag, Diisseldorf, 1988, pp. 391 - 394. 38 S. R. Morrison, The of Surfaces, Plenum Press, New York, 1977. 39 P. G. Harrison and B. M. Maunders, Tin oxide surfaces: Part 12. - A comparison of the nature of tin(IV) oxide, tin(IV) oxidesilica and tin(IV) oxide-palladium oxide: Surface hydroxyl groups and ammonia adsorption, J. Chem. Sot. Famday Xmns., 80 (1984) 1341- 1356. 40 A. Ikegsmi, M. Kaneyasu and K. Yamada, NTG-Fachberichte ‘Sensoren Xechnologie und Anwendung’, VDE-Verlag, Berlin, Vol. 93, 1986, p. 122. 41 Y. K. Fang and J. J. Lee, personal communication. 42 E. Garrone and F. S. Stone, UV diffuse reflectance spectra of ammonia adsorbed on alkaline earth oxides and analogy with charge-transfer-to-solvent spectra, in M. Che and G. C. Bond (eds.), Adsorption and Catulysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, pp. 97 - 107. 43 H. Kniizinger, Photo-adsorption of oxygen on acid and basic Ti02 surfaces, in M. Che and G. C. Bond (eds.), Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, pp. 109 - 111. 44 K. Weissermel and H. J. Arpe, industrklle organ&he Chemie, Verlag Chemie, Weinheim, 1978, pp. 22 - 31. 45 J. J. Brand, A. A. Deckert and S. M. George, Surface diffusion of hydrogen on sulfurcovered Ru(OOf ) surfaces studied using laser-induced thermal desorption, Surface Sci., 194 (1988) 467 - 474. 46 C. Pijolat, Etude des propri&& physico-chimiques et des propri&es Qectriques du dioxyde d’&ain en fonction de l’atmosphke gaxeuse envlronnante. Application ii la detection s&ctive des gsz, Thesis, Grenoble, 1986. 47 T. Nakahara, K. Takahata and S. Matsuura, High sensitive Sn02 gas sensor: deteotion of volatile sulfides, in T. Seiyama (ed.), Chemical Sensor Technology, Vol. I, Kodansha/Elsevier, Tokyo/Amsterdam, 1988, to be publiihed. 48 S. Kanefusa, M. Nitta and M. Haradome, H2S detection by ZrOzdoped SnOz, IEEE Trans. Electron Devices, ED-36 (1988) 65 - 69. 49 J. Lagois, personal communication. 50 V. Sommer, personal communication. 51 C. T. Au, W. Hirsch and W. Hirschwald, Adsorption of carbon monoxide and carbon dioxide on annealed and defect zinc oxide (0, 0, 0, -1) surfaces studied by photoelectron spectroscopy (KPS and UPS), Surfwe Sci., 297 (1988) 391 - 401. 52 J. M. Vohs and M. A. Barteau, Conversion of methanol, formaldehyde and formic acid on the polar faces of zinc oxide, Surface Sci., X76 (1986) 91 - 114. 53 G. Ghiotti, F. Boccuzzi and A. Chiorino, Surface characterization of Cu/ZnO catalysts: IR VIS, UV study of CO chemisorption, in M. Che and G. C. Bond (eds.), Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, pp. 235 246. 54 G. W. Wang and H. Hattori, Reaction of adsorbed carbon monoxide with hydrogen on magnesium oxide, J. Chem. Sot. Famday Trans. I, 80 (1984) 1039 - 1047. 55 P. G. Harrison and B. Maunders, Tin oxide surfaces: Part 14. - Infrared study of the adsorption of ethane and ethene on tin(IV) oxide, tin(IV) oxide-silica and tin(IV) oxide-palladium oxide, J. Chem. Sot. Faraday Trans. X, 81 (1985) 1311- 1327. 56 P. G. Harrison and B. Maunders, Tin oxide surfaces: Part 16. - Infrared study of the adsorption of formic acid, acry& acid and acrolein on tin(IV) oxide, tin(IV) oxide silica and tin(IV) oxide-palladium oxide, J. Chem. SOC. Faraday Trans. Z, 8Z (1985) 1345 - 1355.
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and W. Ho, Effect of adsorbate proximity on surface reactions: Synthesis and decomposition of the formate intermediate in UHV from coadsorbed CO, HzO, and 0 on Rh(lOO), J. Chem. Phys., 87 (1987) 1376 - 1391. W. GSpel, Initial steps of interface formation: surface states and thermodynamics, J. Vat. Sci. Technol., 16 (1979) 1229 - 1235. P. G. Harrison and M. J. Willett, Tin(IV) oxide gas sensors: surface chemistry and electrical conduction effects, Proc. Eurosensors Z, Cambridge, 1987, p. 39. K. D. Schierbaum, Elektrische und spektroskopische Untersuchungen an Dunnschicht-SnOz-Gassensoren, Dr. The&, Tiibingen, F.R.G., 1987. G. Tournier, C. Pijolat and R. La Lauze, CO detection in town environment, Proc. EurosensorsZ. Cambridge, Sept. 1987, pp. 162 - 163. S. J. Gentry and ‘I. A. Jones, The role of catalysis in solid-state gas sensors, Sensors und Actuators, 10 (1986) 141 - 163. J. Lagois, H. P. Oepen and W. Hagen, HaIbleitergassensoren zur quantitativen Messung nitroser Gase, NTG-Fachberichte ‘Sensoren Technologie und Anwendung’, Vol. 93, VDE-Verlag, Berlin, 1986, pp. 93 - 96. V. Lantto, P. Romppainen and S. Leppivuori, Investigative study on effect of pure and mixed gas systems on some resistive type metal oxide sensor under different experimental conditions, Proc. 2nd Int. Meet. on Chem. Sensors, Bordeaux, France, July 7 - 10, 1986, pp. 186 - 190. P. Romppainen, H. Torvela, J. Viiiintinen and S. Leppiivuori, Effect of CH4, SO2 and NO on the CO response of an SnOz-based thick film gas sensor in combustion gases,
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P. G. Harrison the adsorption ion-exchanged
67
P. G. Harrison and E. W. Thornton, Tin oxide surfaces: Part 8. - Infrared study of formation of a surface isocyanate species on SnOz-0.55 CuO during catalysis of the oxidation of carbon monoxide by nitric oxide, J. Chem. Sot. Faraday Trans. I, 74 (1978) 2604 - 2616. E. Garrone and E. Giameilo, The interaction of CO and NO at the surface of MgO: An IR and ESR study, in M. Che and G. C. Bond (eds.), Adsorption and Catalysis on Oxide Swrfuces, EIsevier, Amsterdam, 1985, pp. 226 - 233. sensors, IEEE Trans. Electron Devices, S.-C. Chang, Thin-film semiconductor NO, ED-26 (1979) 1875 - 1880. S. Chang, personal communication. H. Laurs, personal communication N. Murakami, K. Takahata and T. Seiyama, Selective detection of CO by SnOz gas sensor using periodic temperature change, Proc. Transducers ‘87, Tokyo+, Japan, June 1987, pp. 618 - 621. V. Lannto and P. Romppainen, Electrical studies on the reactions of CO with different oxygen species on SnOz surfaces, Surface Sci., I92 (1987) 243 - 264. W. Thoren, Reaktionen auf Halbleiteroxiden, Reaktive Streuung und Thermodesorptionsspektroskopie: CH&OOH/CO/CH., auf SnOz- und ZnO-Diinnschichten und Einkristalien, Dr. Thesis, Aachen, F.R.G., 1985. J. A. S. P. Carreiro and M. Baerns, Catalytic conversion of methane by oxidative coupling to C2+ hydrocarbons, React. Kin&. Catal. Lett., (1988) in press. W. Hinsen, W. Bytyn and M. Baerns, Oxidative dehydrogenation and coupling of methane, Proc. 8th Int. Congr. of Catalysis, Berlin, July 1984, Vol. 3, Verlag Chemie, Weinheim, pp. 581 - 592. W. Bytyn and M. Baerns, Supported PbO catalyst for the oxidative coupling of methane - The effect of surface acidity of the support on C2+ selectivity, Appl. Catal., 28 (1986) 199 - 207. J. Carreiro, personal communication.
74 (1978)
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69 70 71 72
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and E. W. Thornton, Tin oxide surfaces: Part 9. - Infrared study of of CO, NO and CO + NO mixtures on tin(IV) oxide gels containing Cr”’ , Mn**, Fe'rr , Colr , Ni” and CulI, J. Chem. Sot. Faraday Trans. Z.
- 2713.
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79 K. Tamaru, Dynamic Heterogeneow Catalysis, Academic Press, London, 1978, pp. 115 - 124. 80 S. Semancik and D. F. Cox, Fundamental characterization of clean and gas-dosed tin oxide, Sensors and Actuators, 12 (1987) 101 - 106. 81 M. Egashira, T. Matsumoto, Y. Shimizu and H. Iwanaga, Ar+ sputtering effect on gas sensing characteristics of tin dioxide whiskers, Proc. Transducers 87, Tokyo, Japan, June 1987, pp. 622 - 625. 82 Y. Nakatani and M. Matsuoka, personal communication. 83 N. Komori, S. Sakai and K. Komatsu, Sintered SaO2 sensor for methane, in T. Seiyama, K. Fueki, J. Shiokawa and S. Suzuki (eds.), Proc. Int. Meet. on Chemical SeMors, Fukuoka, Japan, Sept. 1983, Kodansha/EIsevier, Tokyo/Amsterdam, 1984, pp. 57 61. 84 H. Jacobs, W. Mokwa, D. Kohl and G. HeiIand, Characterization of structure and reactivity of ZnO and SnOz supported Pd catalysts, Vacuum, 33 (1983) 869 - 871. 85 S. M. Gates, J. N. Russel and J. T. Yates, Bond activation sequence observed in the chemisorption and surface reaction of ethanol on Ni( 111 ), Surface Sci., I 71 (1986) 111 - 134. 86 U. Brittger, personal communication. 87 H. Jacobs, Reaktionen auf HaIbleiteroxiden mit geringer MetaUbedeckung: &HsOH/ H2/H20 auf Pd-SnO2 und Pd-ZnO, Aachen, Dr. The&, Aachen, F.R.G., 1983. 88 B. Bowker and R. J. Madix, The adsorption and oxidation of acetic acid and acetaldehyde on Cu (1 lo), Appl. Surface Sci., 8 (1981) 299 - 317. 89 P. G. Harrison and B. M. Maunders, Tin oxide surfaces: Part 11. - Infrared study of the chemisorption of ketones on tin(IV) oxide, .!. Chem. Sot. Faraday Tmns., 80
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90 K. Camman, Dacr Arbeiten mit ionenseZektiuen Elektroden, Springer Verlag, Berlin, 1977, p. 75. 91 H. Liith, G. W. Rubloff and W. D. Grobmann, Chemisorption and decomposition reactions of oxygen containing organic molecules on clean Pd surfaces studied by UV photoemission, Surfoce Sci., 63 (1977) 325 - 338. 92 P. G. Harrison and E. W. Thornton, Tin oxide surfaces: Part 2. - Infrared study of the adsorption of pyridine and ammonia on tin(IV) oxide, d. Chem. Sot. Faraday Tmns., 71 (1975) 1013 - 1020. 93 P. G. Harrison and B. M. Maunders, Tin oxide surfaces: Part 15. - Infrared study of the adsorption of propene on tin(IV) oxide, tin(IV) oxide-silica and tin(IV) oxidepalladium oxide, d. Chem. Sot. Famday Trans., 81 (1985) 1329 - 1343. 94 P. G. Harrison and E. W. Thornton, Tin oxide surfaces: Part V. - An infrared study of the reactions of methylchlorosihanes with the surface of tin(IV) oxide, 3. Chem. Sot. Faraday Trans., 72 (1976) 1310 - 1316. 95 P. G. Harrison and E. W. Thornton, Tin oxide surfaces: Part 7. - An infrared study of the chemisorption and oxidation of organic Lewis base molecules on tin(IV) oxide, J. Chem. Sot. Faraday Trans., 72 (1976) 2484 - 2491. 96 P. G. Harrison and E. W. Thornton, Tin oxide surfaces: Part 4. - Infrared study of the adsorption of oxygen and carbon monoxide + oxygen mixtures on tin(IV) oxide, and the adsorption of carbon dioxide on ammonia-pretreated tin( IV) oxide, J. Ckem. Sot. Famday Trans., 74 (1978) 2597 - 2603. 97 P. G. Harrison and B. M. Maunders, Tin oxide surfaces: Part 13. - A comparison of the nature of tin(IV) oxide, tin(IV) oxide-palladium oxide and tin(IV) oxidtsihca: An infrared study of the adsorption of carbon dioxide, J. Chem. Sot. Famday Trans., 80 (1984) 1357 - 1365. 98 J. F. McAleer, P. T. Moseley, J. 0. W. Norris, D. E. Williams and B. C. Tofield, Tin dioxide gas sensors: Part 2. - The role of surface additives, J. Chem. Sot. Famday Trans. I, 84 (2) (1988) 441- 467. 99, E. M. Logothetis, M. D. Hurley, W. J. Kaiser and Y. C. Yao, Selective methane sensors, Proc. 2nd Int. Meet. on Chem. Senaore. Bordeaux, France, July 7 - 10, 1986, pp. 175 - 178.
113 100 R. Huck, D. Kohl and G. Heiland, ??oc. Int. Symp. on Trend8 and New Applications in Thin Films, Vol. 2, Strasburg, March 1987, pp. 675 - 679. 101 W. C. Conner, J. F. Cevalloa-Candau, N. Shah and V. Haenael, Hydrogen spillover and surface diffusion: Spillover from a point source, in S. J. Teichner and J. E. Germain (eds.), Spillover of Adsorbed Species, Studies in Surface Science and Catalysis, Vol. 17, Elaevier, Amsterdam, 1983, pp. 31 - 44. 102 N. Yamazoe, Y. Kurakowa and T. Seiyama, Effects of additives on semiconductor gas sensors, Sensors and Actuators, 4 (1983) 283 - 289. 103 J. Zacheja, Dipl. Thesis, 1988.
Dieter Kohl received the Dr. rer. nat. from the RWTH, Aachen in 1972. Since 1982 he has worked as a lecturer at the 2. Physikalisches Institut of the RWTH Aachen. His research interests are electronic and chemical properties of compound semiconductors like ZnO, SnOp or GaAs.
71
SURFACE PROCESSES IN THE DETECTION WITH SnO,-BASED DEVICES
OF REDUCING
GASES
D. KOHL 2. Physikalisches Institut der Rheinisch-Westfcilischen Tempkrgmben 55. D-51 00 Aachen (F.R.G.) (Received June 16,1988;
Technischen
Hochschule
Aachen,
accepted September 15,1988)
Abstract Single-crystal faces, thin evaporated and sputtered films and sintered specimens of Sn02 were investigated in UHV. After exposure to hydrogen, water, amine, carbon monoxide, methane, acetic acid and ethanol, a sensitive mass spectrometer recorded the desorbing species in thermal desorption spectroscopy and served for reactive scattering investigations. The surface conductivity was observed at the same time. The combination of the different results with published infrared spectroscopic data allows some conclusions to be drawn on reaction mechanisms, surface binding and the origin of conductance variations. It will be shown that even such a simple molecule as methane undergoes many reaction steps, some of which affect the observed conductance change. The investigation of more complex molecules, such as ethanol and acetic acid, supports the understanding of the methane reactions; the decomposition of methane on SnOz is accompanied by synthesis reactions yielding these products. Finally, some consequences and possible improvements of gas sensors are discussed.
1. Introduction Gas sensors based on semiconducting oxides can be used for the detection of combustible and noxious gases in air. Their advantages are high sensitivity, simple design, low weight and cost. Thin oxide films on silicon substrates with integrated low-power heating elements also fulfil the requirements of mass production [ 11. The range of applications is limited by selectivity and stability. An improvement of these properties cannot be achieved simply by trial and error; but requires a better understanding of the surface processes connected with the conductance changes. This report proposes some concepts for the interpretation of sensor properties. The concepts are derived to some extent from our own experimental studies and also from the literature on the spectroscopy of adsorbates and on heterogeneous catalysis employing oxides. 0250-6874/89/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
72
The type of oxygen available at the surface is crucial for the operating behaviour. An understanding of the role of hydrogen and water is a precondition for interpreting sensor properties. Water is a usual constituent of the ambient air. Hydrogen and water are also formed during the decomposition of hydrogen-containing molecules. Carbon monoxide undergoes different reactions in the presence and absence of humidity. Methane represents a simple hydrocarbon of practical interest. Ethunol and its oxidation product acetic acid show some analogous features in surface reactions. Arsine is investigated and compared with other hydrides. Only a fraction of the spectroscopic methods successfully applied to the investigation of metal surfaces are transferable to oxide surfaces. For example, strong surface phonon excitations prevent the identification of adsorbate vibrations on the SnO, surface. On metal surfaces adsorbate coverages of one monolayer are quite common. Typical coverages on oxides are often found in the range 10 -3 to 10e5 of a monolayer (ML). Chemisorbed oxygen cannot exceed a coverage of 10s3 monolayers for electrostatic reasons [2]. Less sensitive methods, e.g., photoemission or Auger spectroscopy, cannot be used in many cases. Infrared spectroscopy is a powerful tool to investigate the presence and stability of intermediate surface products, but its application is limited to polycrystalline samples with a large effective area including internal surfaces. Thermal desorption spectroscopy (TDS) performed with a muss spectrometer ~II an ultrahigh vacuum (UHV) apparatus gives access to coverages below lo-’ ML. Also small amounts of one gas in the presence of another are detectable down to a limit of 10m5 to lo-’ ML. However, nondesorbing species can only be detected if they can be removed by reaction with another gas. An example is the removal of a carbon deposit in the presence of oxygen or hydrogen. An extension of the mass spectrometer technique, the reactive scattering of a molecular beam at the sensor surface at working temperature, has shown promising results. The two mass spectrometric methods are applied in our own experiments described in Section 3. Up to now the description of surface reactions has been based mainly on the combination of mass spectrometric and infrared data. Since this basis is not as wide as in the case of metals, the proposed reaction paths contain some speculative elements. This may be justified if the proposed reactions stimulate ideas to improve the performance of sensor devices. The discussion is mostly confined to SnO,, which dominates in research and in applications. Sintered layers and thin films are in practical use. Because of the polycrystalline structure, an interpretation of the observed conductance changes is often difficult. By also studying the surface of single crystals under well-defined conditions, one might try to achieve a better separation of parameters influencing the properties of gas sensors [ 31. In gases escaping from combustion processes, 02, CO, COZ, HZS, SOZ, NO and NO2 usually have to be monitored together; in smog situations SO*, CO and NO2 are relevant for an alarm release. Therefore, some reactions including two of these gases are included in the discussion.
73
2. Electronic properties of tin dioxide Bulk properties SnOz is a fairly ionic semiconductor with a wide band gap of about 3.6 eV [4]. At the edge of the conduction band the calculated density of states, derived from atomic Sn 5s states, is low 151. Correspondingly the measured Hail mobilities of bulk electrons in single crystals are reasonably high, 80 and 200 cm*/V s at 500 K and 300 K, respectively [6]. The edge of the valence band emerges from 0 2p orbitals, which are directed perpendicular to the Sn-0 bond (lone-pair orbit&) [ 51. This geometry is extremely unfavourable for current transport by holes in the valence band. At temperatures above about 800 K, the concentration of oxygen vacancies and the conductance depend on the oxygen pressure [ 71: 0
a (Po*Y”
(1)
The experimental value m = 6 corresponds to a doubly-ionized oxygen vacancy ( Vo)*+. This vacancy acts as a donor with an energy level 150 meV below the conduction band. The conductivity of Sn02 can be increased by some substitutional impurities, fluorine on an oxygen site or antimony on a tin site, which also act as shallow donors [ 4 1. Surface properties Near room temperature the oxygen vacancies are frozen in and isothermal conductance changes of an Sn02 device are due to chemisorption. The band gap of a clean Sn02 (110) face is free of intrinsic surface states [8]. The extraction and injection of electrons by extrinsic surface acceptors or surface donors, respectively, is connected with the variation of a spacecharge layer. The electron concentration near the semiconductor surface varies with the density and occupancy of surface acceptors and donors. In a gas sensor this density of surface states depends on surface reactions with ambient gases. The left and the middle columns of Fig. 1 demonstrate the two types of space-charge layers occurring at the surface under the assumption of a homogeneous donor concentration in the bulk. Oxygen or hydrogen are chosen as examples of gases producing surface acceptors and donors, respectively . Depletion layer If oxygen is chemisorbed, the positive charge of the double layer resides in ionized bulk donors, causing a band bending of up to about 1 eV. A simple calculation of such a Schottky depletion layer gives a depth of lo+ to 1o-5 cm. Further examples of surface acceptors are methoxy and formate groups [9], discussed in Section 3.5. With growing surface density of ionosorbed oxygen the band bending of the depletion layer increases, lifting the level of surface acceptors above the Fermi level (Fig. 1, left
S-2 Depletion
(b)
(0)
Surface acceptors
surface
Accumulation
!&face donors
Bulk donors
IllSTANCE
Inhomogeneous
P FROM
SURFACE
Fig. 1. Space-charge layers on the surface of an n-type semiconductor with a wide band gap. (a) Distribution of charges; (b) band scheme near conduction band edge; (c) concentration n(z) of electrons in the conduction band. EC = conduction band edge; EF = Fermi level; ED = donor level; V, = surface potential; S = surface states by adsorption of oxygen or atomic hydrogen, acceptors or donors, respectively; nb = electron concentration within the bulk. For the case of an inhomogeneous donor distribution below the surface, a depletion or accumulation can also be additionally induced by surface states, e.g., due to adsorbed species.
column: EC - Es = 1 eV). By this process the coverage with negativelycharged species is limited to IO I2 - 1013 cme2 (Weisz limitation) [ 21. Accumulation layer In the case of prevailing ionized surface donors, e.g., adsorbed hydrogen atoms, electrons are injected forming a negative space-charge layer. This accumulation layer induces the opposite band bending to a depth of about 10m6 cm (Fig. 1). The oxygen atom in the molecules of water, formaldehyde or ethanol and the nitrogen atom in ammonia can bond to surface tin atoms via their lone-pair orbit&. Since this type of orbital is occupied by two electrons, these species act as surface donors [9]. In the bulk of Sn02 and ZnO, several donor and acceptor levels can be simultaneously present, taking up higher carrier concentrations [4, 7,10, ll]. Oxygen vacancies Not only adsorbed oxygen can be removed by surface treatments, but also surface lattice oxygen. Exposure to hydrogen and hydrogen-containing gases, such as methane, at temperatures above 350 K produces surface oxygen vacancies. They are well known as donors, for example, on Ti02 (110) faces [8]. Munnix and Schmeits [S] showed by band-structure calcula-
7s
tions that surface oxygen vacancies on SnOz (110) faces do not act as donors. A study [ 12 ] comparing the ultraviolet photoemission spectra (UPS) of TiOz (110) and SnOz (110) demonstrated that oxygen vacancies created by argon ion bombardment produce new surface states in the upper part of the bulk band gap of TiO 2, but not of SnOz. After the bombardment the reduced surface tin is present as Sn(I1) [13]. Conductivity measurements [14] indicate that these surface defects on Sn02 do not act as donor states, in contrast both with bulk oxygen deficiency and with sputtered TiOz surfaces. After exposure of a stoichiometric Sn02 (101) face or a sputtered SnO, film to methane in UHV, considerable amounts of water desorb at temperatures near 350 K leaving surface oxygen vacancies, but the conductivity does not increase below 550 K (see Fig. 9). So the theoretical prediction that surface oxygen vacancies on SnO, do not act as surface donors is supported by UPS and by conductance measurements after reducing argon bombardment, and also by conductance measurements after exposure to methane. Annealing of an argon-bombarded SnOz (110) face in UHV increases the conductance till a plateau is reached at 550 K /14]. Surface oxygen vacancies may diffuse towards the bulk, where they are able to act as donors as explained above. Oxygen vacancies left after water formation at 670 to 770 K (see Section 3.2) increase the conductance. Again a migration towards bulk sites may ‘activate’ the vacancies as donors. Since the vacancies have to cross only a few lattice planes, the temperature of 550 K seems to be sufficient for ‘activation’. This situation is sketched in Fig. 1, right column. In reality, adsorbed acceptors and donors and inhomogeneously distributed ‘subsurface’ donors appear together. Influence of gases on the conductance by surface reactions For conductance changes at the surface of monocrystalline samples, the effect of geometry can be eliminated by the introduction of ‘surface conductivity’. A detailed explanation is given elsewhere [ 15 1. The processes listed below alter the conductance of n-type oxides: (a) Adsorbates acting as acceptors or donors extract or inject electrons from or into the space-charge layer. The conductance decreases or increases correspondingly. In the presence of oxygen, only acceptors with a high electron affinity are effective because of the Weisz limit. (b) Surface oxygen vacancies can migrate a small distance into the bulk to become active as donors. Hydrogen, methane and carbon monoxide remove lattice oxygen from the surface. (c) Besides oxygen, carbon dioxide and acetic acid also refill surface oxygen vacancies. The conductance decreases by the diffusion of bulk vacancies to the surface. (d) Reducing gases may also react with adsorbed oxygen ions. The decrease of oxygen acceptors causes a conductance increase. A more detailed discussion can be found elsewhere 1161. For sintered samples the action of barriers separating domains of agglomerates, each comprising a rather large number of crystallites, is instead
76
proposed as an explanation [17]. Surface acceptors at the grain boundaries induce depletion layers. The conductance of the sample is determined by the electron concentration n, at the grain boundary. ng decreases exponentially with growing barrier height E, - Er, which increases with the density of ‘negativelycharged species at the grain boundary. The net surface charge is diminished by reaction with the adsorbed oxygen or by the production of adsorbed donors near the grain boundary, which flatten the barrier. 3. Surface reactions 3.1. Experimental
Preparation of samples The SnOZ crystals were grown from the vapour phase. They exhibit mainly (101) and (110) surfaces of the rutile structure containing equal numbers of Sn and 0 atoms. The crystallographic orientation was controlled by X-ray diffractometry. Thin films were prepared by vapour deposition of metallic tin on amorphous quartz substrates (at 430 K in a vacuum of 10-j Pa, oxidation at 830 K for 90 min) or borosilicate glass substrates (at 670 K, oxidation at 990 K for 60 min). These films consist of Sn02 as checked by X-ray diffractometry [ 181. Scanning electron micrographs show that the layers on borosilicate glass consist of interconnected spheres. The diameter of the spheres increases with the film thickness (0.2 pm for 25 nm and 2 pm for 280 nm films). The films on borosilicate glass were used in acetic acid experiments and the films on quartz glass in arsine experiments. Sputtered films were also studied [ 191. They consisted of a porous alumina substrate, 3 mm square, bearing interdigital conductance electrodes (1 by 1 mm, length-to-width ratio 267) surrounded by an Au heating strip and a Pt thermoresistor below a 100 nm sputtered SnOZ layer. Auger spectroscopy was used to control the cleanliness of the single-crystal faces and films. Sintered specimens were prepared by drying and sintering (1170 K, 1 h) a 0.5 mm thick layer of water/SnOz paste on an alumina platelet. These samples do not contain any stabilizing admixtures, for example SiOZ, frequently used to obtain mechanically stable sensors. No catalytic active metals were added. In two experimental methods a highly-sensitive mass spectrometer was used: In thermal desorption spectroscopy (TDS) a sample is exposed to a certain gas dose and after pump down, is heated at a rate of about 1 to 50 K per second. The desorbing products are monitored by the mass spectrometer in ultrahigh vacuum. In the present studies corrections are made for dissociation within the mass spectrometer by ionization (cracking patterns). Usually all reaction products are also offered to the sample in separate TDS runs. By comparison, some products are found to desorb immediately after reaction, while others are stored in a chemisorbed state before desorption. Products desorbing together are characteristic of a com-
77
mon preceding surface reaction step. The reaction order of the slowest reaction step can be derived from the shift of the peak temperature of the desorption maximum with heating rate. In reactive scattering experiments a gas beam is directed onto the surface continuously or in a chopped mode. The desorbing molecules are monitored by the mass spectrometer. The observation of rates at different temperatures gives kinetic information. Slow degradation of catalytic properties is discernible; in particular, poisoning of the surface by nondesorbing products can be investigated. Such products can be desorbed by a heat pulse or by offering a reactive gas. During reactive scattering experiments the conductance can be monitored simultaneously, so the effect of surface reactions changing the density of surface donors or acceptors is observed. The initial state of the surface largely influences the results and must be carefully controlled. A decrease in the available density of lattice oxygen atoms allows a conclusion to be reached on whether lattice oxygen or adsorbed oxygen participates in a certain reaction step. Gas-mixing system and electrical measurements The samples were exposed to various gases at a normal pressure of 1 bar in a stainless steel reactor (volume 18 cm3). Mixtures of the reducing gas with artificial air (80% Nz and 20% 02, Linde) were adjusted by means of precise flowmeters. Regulated electrical valves maintained a constant gas flow of 40 l/h. A small pump drew a fraction of the gas flow, 10 l/h, into the reactor. It was possible to switch between the mixture and pure air within 0.2 s. The sensor conductance was measured by the four-probe technique using Keithley 602 electrometers for current and voltage. Conductance and temperature as a function of time were registered by a small computer. Also during some of the reactive scattering measurements in UHV, the conductance was recorded [ 201. 3.2. Exposure to oxygen, hydrogen and water For the oxides the overall stoichiometry has a decisive influence on the surface conductivity. Oxygen vacancies left after surface reactions can diffuse towards the bulk, increasing the conductivity near the surface. Adsorbed oxygen ions act as surface acceptors, binding electrons and diminishing the surface conductivity. Figure 2 shows the various species of oxygen relevant to surface reactions. The energy difference between doubly-charged 02- in a lattice site and in an adsorbed state is estimated to be about 2000 kJ/mole or 20 eV, the difference between 302 BPsand +(O, &amounts to 140 kJ/mole or 1.5 eV [21]. The Figure does not show the energy barriers retarding transitions between different states. At room temperature the equlibrium of the (Oz.,,)coverage with gaseous O2 is approached slowly, although the reaction is exothermic. On SnOp films reaction (2) takes place with increasing temperature, as concluded from EPR studies 1223 : (0 2ads)-+
e-t--,
2(oads)-
(21
s E z
I
I
ELEClROPillIC -0(C-C-BONO RUPTURE)
I I I I
NUCLEOPHlllC ~OEHYORO6ENATIONI
GAS
:%JTION-
to”
CHEMI -
SORPTION
STkELATTICE
Fig. 2. Energy diagram of various oxygen species in the gas phase, adsorbed at the surface and bound within the lattice of a binary metal oxide. After Bielanski and Haber [21].
Above approximately 450 K (O,,& ions are found as the prevailing species. At constant oxygen coverage, the transition causes an increase of surface charge density with corresponding variations of band bending and surface conductivity. From conductance measurements it is concluded that the transition takes place slowly. A fast temperature change of sensors is usually followed by a creeping change of the conductance. There are two reasons for this: the oxygen coverage is adjusting to a new equilibrium and the adsorbed oxygen can turn into another species. But diffusion processes may also cause a slow response. The reactivity of (O,,,)is high and exceeds that of (O,&-. But the coverage by negatively-charged species is limited to 10m2 to 10e3 ML by space-charge effects (Weisz limitation, see Section 2). (O,,,)2- is unstable and has to be stabilized by the Madelung potential of the lattice, i.e., on a lattice site. With respect to oxidation reactions, the adsorbed (O,,,,)and (O,,,)species and also exposed oxygen atoms on step sites are classified as ‘electrophilic’ reactants, which preferentially attack the C=C double bond of adsorbates abstracting electrons, whereas the ‘nucleophilic’ 02- ions bound within the lattice at the surface react with activated hydrogen or dehydrogenated hydrides and hydrocarbons [ 231. Reactive oxygen on a step site is sketched in Fig. 13(g). Activation means excitation of a bond and, as a possible consequence, ionization, dissociation or formation of radicals such as CH3. Surface oxygen vacancies are produced by heating in vacuum, by chemical reduction (see below) or by photolysis with band-gap light. In the oxide layer of gas sensors, oxygen vacancies may also be induced by the
oxidation of reducing gases. At the operation temperature no fast healing process takes place, only increased oxygen chemisorption due to the higher charge density (see Section 2). In this way the surface of the oxide is modified during use by an increased donor concentration at the surface being compensated by ionosorbed oxygen [16]. This process may be responsible for some drift phenomena in gas sensors. In the present paper it will be shown that many oxidation reactions occur on an oxide surface in the absence of any ambient oxygen. Also a second oxide, e.g., Sbz04, present in separated clusters at the surface can adsorb and activate oxygen from the gas phase [ 241. Hydrogen The reactions of the oxide surface with hydrogen are important not only for the detection of gaseous hydrogen. Hydrocarbons or hydrides-can dissociate either on the SnOZ surface or on the surface of deposited noble metal clusters. Subsequently the hydrogen atoms migrate to adsorption or reaction sites on the Sn02 surface. The energy for migration of hydrogen along the surface is low; an estimation is given at the end of Section 3.6. Two processes will be considered for the increase of conductivity. In any case a dissociation of the hydrogen molecule is required; adsorbed Hz molecules do not change the conductance. At moderate temperatures adsorbed hydrogen atoms act as donors. These donors provide additional electrons and induce an accumulation layer:
H+ad-
OatW
+
e-
(3)
The oxygen atoms remain within the lattice during thermal desorption and molecular hydrogen appears in a TDS spectrum after exposure to atomic hydrogen [ 161. Similar spectra are found after exposure to molecular hydrogen, if the sample temperature is raised to 470 K for dissociation [ 251. The conductance increase of a (110) face in vacuum is reversible by moderate heating. (110) faces and sputtered layers kept at 500 K in air show a reversible response to admixtures of 65 to 1000 ppm hydrogen [ 261. At higher temperature a reduction of the lattice oxygen occurs: 2H
+ at
-
HZ%ls
+
vo
(4)
By comparison with infrared spectra [27], the weak water desorption at 390/410 K can be assigned to adsorbed water molecules, whereas the main water desorption above 650 K arises from the condensation of surface hydroxyl groups [ 161. By removal of oxygen from surface lattice sites vacancies are produced, which diffuse into lattice planes below the surface, acting there as subsurface donors. In vacuum this increase of conductance is not reversible at moderate temperatures. The initial conductance can be restored by exposure to oxygen or another oxidizing gas at sufficiently high temperatures. The highest conductance ratio in air with and without hydrogen admixture is found at temperatures between 600 and 670 K for various specimens
80
(whiskers with a rough surface [3], evaporated films of small gram size (see Fig. 5(b)), sputtered films [28] and sintered specimens without additions [28]). Water vapuur After exposuring oxidized (110) crystal faces to water at 300 K Semancik et al. 1291 found features in their UPS spectra that they attributed to undissociated water molecules besides dissociated ones. A single water desorption peak at 480 K found in thermal desorption spectroscopy [ 16 ] may correspond to the undissociated state identified in UPS by Semancik. A hydrogenation reaction of an adsorbed gas can involve protons from a hydroxyl group or from an adsorbed water molecule. A proton from a water molecule (strong Br$nsted acid) is more easily available than the tightly-bound proton from an OH group (weak Brgnsted acid). A preparation to stabilize the undissociated state of water and also the existence of hydroxonium ions, H30+, has been successfully applied to polycrystdline SnO, samples 1391. Therefore, this preparation is named as proton&ion in the literature [30, p. 2181. An application, the enhancement of NH3 adsorption and the formation of the positively charged surface species (NH,)+ by proton uptake, is described in detail in Section 3.3. The increase of conductance by exposure to water vapour can be described as a sequence of dissociation and reduction processes. Yamazoe et al. 1251 find on sintered samples containing stabilizers, which can facilitate dissociation, a second desorption maximum at 670 K. This high-temperature desorption maximum increases in height when the exposure temperature is raised. A comparison with the i.r. spectra after hydrogen exposure shows that these samples also dissociate hydrogen very effectively. If there are two types of hydroxyl groups, a rooted one including lattice oxygen (a rooted surface intermediate includes one or more surface oxygen atoms) and another one bound to lattice tin, two equations can be written:
H20 + Swat + (Aat -+
(HO-Snl,,)
+ (OlatH)+ + e-
(5)
or H2O
+ Snht
+ 0~
----+
2(HO-Snl,,)
+ V,
(6)
The oxygen vacancy on the right-hand side of eqn. (6) has to move towards the bulk in order to become effective as a donor (compare Section 2). Two types of water desorption occur if the system is activated by heating of the substrate, offering of atomic hydrogen or compounds containing hydrogen, or by the presence of a noble metal (oxide) admixture in the sintered specimens: (I) Water including lattice oxygen desorbs with a low-temperature TDS maximum near 410 K, presumably from a molecular state [ 27 1, after exposure to: atomic hydrogen on a (101) face [31); atomic hydrogen on a sintered specimen (weak) [ 16 3 ;
molecular hydrogen on a sintered specimen with Pd [ 321; methane on a (101) face, see Section 3.5; acetic acid on a (101) face, see Section 3.7; acetic acid on an evaporated film, see Section 3.7 ; ethanol on a (110) face, see Section 3.6. (2) A second high-temperature TIM maximum between 670 and 770 K, resulting from a condensation of hydroxyl groups [ 27,291, is observed after exposure to : atomic hydrogen on a sintered specimen [ 161; molecular hydrogen on a sintered specimen at 470 K [25] ; molecular hydrogen on a sintered specimen with Pd [32]; water on a sintered specimen at 300 K or 860 K [14]; ethanol on a (110) face, compare Section 3.6. 3.3. Exposure to hydrides An m-sine sensor seems desirable, since this highly poisonous gas is applied in the manufacture of GaAs semiconductor components. Furthermore, amine escapes during some metallurgical processes. Figure 3 shows the results of reactive scattering on a thin (40 run) evaporated film of SnOz [33]. Above 600 K the backscatteringrate of AsHs decreases and at the same time As and Hz0 appear as the products of decomposition and oxidation. No oxidation products of arsenic are found and no arsenic is detected by Auger electron spectroscopy on the oxide surface after exposure. AsH3 (tetragonal pyramidal structure) cannot form hydrogen
-
350
continuous exposure to I.L.t0’6 AsH3 molecules eni*+’
450
550
TEMPERATURE
650
750
(K)
Fig. 3. Thin evaporated film. Desorption flux of reactively scattered products as a function of temperature during continuous exposure to AsH3. After Mokwa et 02. [ 331.
82
1
I
Exposure
to arsine
ASgas
H HH
(a)
\a[ ..
lb)
c
3,
-
,Sn, ,Sn, 0000000
HZOgos
(cl --jr
H ‘I’ H ,Sn,l,Sn,l,SnJ
/ 9, 0
,Sn, ,o
1 9, 0
vacancy
Exposure
to ammonia
Ammonium ion hydroxyhted surface
on
Ammonium protonated surface
ion
on
I
Fig. 4. Hydrides on SnOz. (a) Arsine adsorbing covalently; (b) arsine dissociated to arsenic and hydrogen; (c) desorption of arsenic atoms and water leaving a vacancy; (d) hydrogen-bonded ammonia; (e) formation of an ammonium ion from NH3 and OH groups on an untreated SnOg surface; (f) formation of an ammonium and chlorine ion pair from NH3 and Hz0 on an SnO? surface pretreated with HCl.
bridge bonds, because the charge density in the p orbitals of species with higher atomic numbers (As : 2 = 33) is too low [ 341. The As atom of ASH, may be mainly coordinatively bound to a surface tin atom, since the electronegativity difference is not large (As: 2.20, Sn: 1.72 [35, p. 95]), see Fig. 4(a). Then the surface-bound AsHs can dissociate, Fig. 4(b), because ASH, is only metastable with a low binding energy for hydrogen, EAS_n = 59 kcal/mole (for comparison, I&,-n = 111 kcal/mole). Under vacuum the oxygen for the Hz0 formation must come from a reduction of the oxide surface, Fig. 4(c). This catalytic combustion corresponds to the known combustion of arsine in an oxygen-deficient atmosphere or with a cold flame (Marsh probe, [36, p. 4221): 4AsHs + 302 -
4As + 6Hz0
(7)
On the contrary, combustion in the presence of an oxygen excess forms AszOs, which is absent in the combustion with lattice oxygen. If the 40 nm thick evaporated film is exposed to air with an arsine admixture, a sensitivity maximum becomes observable above 600 K, see Fig. 5(a) [37]. Both decomposition products, hydrogen and arsenic, act as
83 I
.
I
I
SnOp thin film
500 600 700 TEMPERATURE (K)
LOO
Fig. 5. Sensitivity of SnOz thin films exposed to (a) AsH3 in air and (b) Hz in air. films are made by deposition of tin on quartz substrates and subsequent oxidation, Section 3.1. The sensitivities to AsH3 and Hz vary linearly with the concentration. sheet conductances in air at 600 K are 6.4 x 10m8 and 6.5 x 10e4 A/V for the 40 400 nm films, respectively. After Mokwa [37].
The see The and
donors. The complicated surface states related to arsenic ions are discussed by Morrison [38, p. 1911. The film is less sensitive to hydrogen by about a factor of 10, compare Fig. 5(b). AsHs dissociates on the 40 nm film consisting of small interconnected spheres (see Section 3.1.). However, on a 400 nm film with a smoother surface, only a low-temperature conductance maximum near 430 K is visible in Fig. 5(a). Undissociated arsine can adsorb via its arsenic lone-pair orbital filled with two electrons. Part of this charge can be transferred into the space-charge layer and increases the conductance. Above 430 K desorption of AsHs molecules decreases the conductance. At 430 K the selectivity against hydrogen reaches a value of 100, compare Fig. 5(b). Environmental air without intended addition of hydrogen contains about 1 ppm hydrogen. Therefore, a sufficient selectivity against hydrogen is a prerequisite for hydride (and other) sensors operating in the sub-ppm range. Two other hydrides with elements of the fifth group and tetragonal pyramidal structure, ammonia and phosphine, are of interest in sensor applications. Ammonia sensors can be usefully applied in the removal of NOz by NHs injection in order to monitor and minimize an NH, excess (> 5 ppm desirable) : 6NOz + 8NHs -
7N, + 12Hz0
(8)
This extraction technique is already applied to the smoke gas of industrial plants and has been discussed for the exhaust gas of large engines. Control of the maximum tolerable concentration in a workroom, 50 ppm in the presence of hydrocarbons, is of interest, too. On polycrystalline SnO, the more stable ammonia molecule (EN-u = 93 kcal/mole) is weakly hydrogen [34, p_ 55]), and readily bonded to an oxygen atom (&__H-~ = 5 k&mole desorbed upon evacuation at room temperature [39], compare Fig. 4(d).
84
In the presence of hydroxyl groups, e.g., after admission of water vapour, a second adsorption state, stable up to 495 K, is detectable [39]. An (NH4)+-like species is formed including the proton of the hydroxyl group. This ion is kept at the surface by a hydrogen bond to the oxygen atom of the former hydroxyl group (Brgnsted acid site). The ammonia ion on the surface is sketched in Fig. 4(e). The fourth hydrogen atom positioned straight behind the nitrogen atom is not visible. An Sn02 thick film kept at 400 “C shows a linear conductance increase between 0.1 ppm and 10 ppm /40]. Evaporated SnOz films (thickness: 60 nm) at 300 “C require 700 ppm NHs for the same conductance increase as found with 100 ppm Hz [41]. The lower sensitivity for the hydride may be related to the firm binding state in Fig. 4(e), preventing fast decomposition and water formation. The dissociation of NH, on some oxides (CaO, MgO, SrO) was found to need a pair of exposed metal and oxygen atoms, where the ion pair (NH,)- (amino group) and H+ is irreversibly formed [42]. CaO and MgO are an admixture in some sintered SnOl samples as stabilizers (compare Section 4). A reaction of the released hydrogen with adsorbed or lattice oxygen can increase the conductance. The described adsorption state connected with the uptake of a proton can be further stabilized by a protonation pretreatment: the surface is exposed to HCl vapour. Chloride ions bound to tin atoms emarge the poaitive charge not only at the underlying tin atom but also in the neighbouring tin atoms. As a result, the lone pair orbital of a water molecule is more effectively drawn onto a neighbouring tin site. The strengthening of the oxygen-tin bond weakens the oxygen-hydrogen bond, but the water molecule does not dissociate. So a strong Br@nsted acid site with an easily available proton is formed (protonation). Again an NH4+ ion-like species with a proton transfer from the adsorbed water is formed, compare Fig. 4(f). The lowering of the bending vibration of the NH4+ ion by 60 cm-’ indicates that the interaction of the proton with the remaining OH group is weaker. Such an ammonium ion is more stable, because it is additionally bound by electrostatic interaction to the chloride ions adsorbed on the surface. TDS curves of ammonia from en A120s substrate, pretreated with HF, are shifted by 120 K to higher temperatures [ 38, p. 2541. Too much HCl reverses the effect. The surface becomes hydrophobic if many tin sites are already occupied. Removal of the chloride ions is possible by wetting the surface with water or ammonia solution [30, p. 2191. Furthermore, the sensitivity of an Sn02 sensor to other gases can be changed in the presence of NH3. NHs can poison the surface Brgnsted acid centres (OH groups or molecularly adsorbed water). These centres are also active in the dehydration of alcohols and cracking of hydrocarbons [38, p. 3321, compare Section 4. The metastable phosphine (energy of formation = -1.3 k&/mole), used in the molecular beam epitaxy of GaP and InP, is not only very poisonous but also explosively inflammable at 150 “C in air. Its behaviour on SnOz has not been investigated up to now. Hydrogen bonding is as unlikely as in
the case of AsH3, because the atomic number is too high (2 = 15) [34]. The P atom may be coordinatively bound to a surface tin atom, since the electronegativity difference of P (2.06) and Sn (1.72) is small. Pure PHs is commercially fabricated from phosphonium chloride in a liquid base with the equilibrium on the right-hand side: PH4+ + OH- -
PH, + Hz0
(8)
However, on the surface PH4+ ions can be formed in analogy to the (NH,+)+ ions already discussed. The reason is that the addition of a proton to the hydride on the surface needs the binding energy Ee_n or EN-n respectively and the rehybridization energy to reach an sp3 configuration. The sum of both is nearly equal for gaseous PH, and NH3. In solution only the (NHJ’ ion is stabilized. PH, does not form a (PH,)’ ion in solution because the solvation energy of NH3 is about 20 kcaljmole higher [35, p. 4741. So the additional adsorption states for ammonia, shown in Figs. 4(e) and (f), should also exist for phosphine. Since AsH3 does not form an (AsHa)’ ion, an HCl preexposure could enhance the AsH3/PH3 selectivity. A sensitive and selective sensor for hydrogen sulphide is of interest, too. HzS in small concentrations is not only poisonous to humans but is also known as a catalyst poison contained in coalderived synthesis gases with concentrations up to some tenths of a percent 136, p. 287,431. However, a sub-ppm concentration destroys the function of copper-based catalysts for methanol formation from synthesis gas 1441; poisoning mechanisms are discussed by Brand et al. [45]. Hydrogen bridge bonding is ruled out for H2S adsorption because the atomic number of sulphur (16) is too high. The strong ionic binding of the sulphur (electronegativity 2.44) to the oxide metal atom weakens the sulphur-hydrogen bond (Es-n = 81 kcal/mole for the gaseous molecule) and facilitates dissociation to an S2- or an SH- ion and two or one H+ ions. The hydrogen donors already cause a conductance maximum at 370 K, see Fig. 6
1461.
A thin Sn02 film pretreated in SO2 is able to detect 5 ppm H2S (sensitivity maximum at 370 K) in the presence of 1000 ppm benzene (sensitivity maximum at 670 K) [46, Fig. 11.14]. An Sn02 thin film doped with ZnO is able to detect 0.01 ppm HPS in air at an operating temperature of 480 K [47], but the film needs more than 100 ppm H2 for the same resistance change. An admixture of metals to SnO, with a larger electronegativity difference to sulphur than Sn, for example Zn (1.66) or Ag (1.42), enhances the conductance increase by a factor of five for 10 ppm H2S in air. Zn and Ag are weak electron acceptors and weak Lewis acids. Smaller electronegativity differences occur if P (2.06) or Sb (1.82) is added. These metals are stronger Lewis acids with a weakened ionic bond to sulphur, causing less H2S dissociation. The addition of P or Sb decreases the sensitivity be a factor of five [47].
86
0
Loo
Bw
600
TEMPERATURE ( K) Fig. 6. Conductance as a function of temperature of a sintered SnOz sample in air and in air with admixtures of 40 ppm H$3, 1000 ppm SO2 and 500 ppm HZ. After Pijolat [461.
On SnOz sintered samples with additions of Zr&, the relative conductance increase after admission of HzS shows a maximum near 330 K. In the presence of water an additional broad maximum appears near 450 K, which can be suppressed by a hydrophobic coating (trimethyl silica) [ 481. An additional binding state of the basic NH3 molecule in the presence of hydroxyl groups was discussed above. Even the weakly acid HzS molecule seems to form this type of hydrogen bond. Prolonged exposure to HzS of a sputtered SnOl film at 200 “C resulted in a slow response [49]. From Ru (001) faces it is known that a 0.1 ML sulphur coverage, built up by HzS dissociation, reduces the lateral hydrogen mobility by one order of magnitude [ 451. One sulphur atom sterically blocks its nearest neighbour (and by electrostatic repulsion its next-nearest neighbour) sites for hydrogen diffusion. At 200 “C on Sn03 residual sulphur may accumulate and obstruct a fast reaction of hydrogen with oxygen. The suppression of hydrogen surface diffusion by sulphur deposits also counter. acts spillover from noble metal deposits and interferes in the decomposition of hydrocarbons. Besides HzS, smoke gas also contains SO1 (acid rain), which is known as a bleaching agent. Powdered PbO is reduced to PbS04 in a strongly exothermic process [36, p. 2973. The conductance of SnOz increases, because the filled lone-pair sulphur orbital undergoes bonds to two neighbouring lattice oxygen atoms acting as donors (Fig. 6). In effect a doublerooted pyramidal (SO#ion is formed. This picture was verified by XPS and i.r. measurements [46,26]. At 800 K the conductance change is essentially reversible. However, in a TDS experiment a small SO desorption maximum also appears between 1000 and 1200 K. An SO* pretreatment makes Sn02 sensitive to benzene [ 461. 3.4. Exposure to carbon monoxide CO detection is of interest to prevent intoxication from incomplete combustion in heating appliances or from exhaust gas of gasoline engines.
a7
Combustion also produces NO and NO2 in a mixture sometimes designated as NO,. Therefore, the interaction of CO with these species is also discussed. Singie crystals
TDS CO desorbs from a (101) face mainly as CO; only a small fraction forms COz by consuming lattice oxygen [ 201: In
co
+
Olat
-
(101
COzg*ar
No COz desorption maximum appears up to 800 K. Conductivity measurements are not known. Under sensing conditions adsorbed oxygen and OH groups are present. Therefore, in a recent UHV experiment [50) a (101) face kept at 570 K was exposed at the same time to a CO beam (12 ML/s), an O2 beam (2 ML/s) and to a background pressure of H3 adjusted to corre(H&O) but no methanol spond to 0.02 ML/s. Besides COz, formaldehyde leaves the surface. In the technical synthesis of methanol from CO and Hz over ZnO-Cu catalysts at high pressure, the intermediates formate (Fig. 7(e)) and methoxy (CH@--) are sequentially formed [511. In TDS/XPS studies on ZnO single crystals, surface formate decomposed to gaseous formaldehyde [52]. In analogy, the following reactions may apply to CO in the presence of O2 and H, on SnOz (101): 0
ads + Hads-
co,ds
01)
OHad,
(fOl!lIldX3)
+ OH,,,-HCOO,d,
(W
On more reduced faces (previous or simultaneous hydrogen types of formyl groups can exist:
(u) ,Sn
* t--0 Sn, Sn ‘d,O
IbI
‘=I H 0 ,Sn,
\I 5:
,Sn, ,Sn, 0 0
exposure)
(d)
tel 0*
,Sn,
P
,&$Sn, 0 0
p f
3,
two
‘9
Sn 0
Sn, ‘0’
Fig. 7; SnOs after exposure to carbon monoxide. Proposed surface intermediates: (a) unidentate carbonate after extraction of a lattice oxygen [68]; (b) carboxylate [59]; (c) formyl group formed in the presence of hydrogen or water [53]; [d) rooted formyl group formed in the presence of hydrogen or water 1541; (e) formate group [51,56]. Coadsorptioh of CO and NO on stepped or polycrystalline SnOl surfaces ((f) - (h)) and on a flat SnOz surface ((i) and (k)) [68].
88
cHo,d, (-
CO,d, + Hada+ toads
+ Hada
+ Olat
Fig. 7(c))
HCO-Oi,,
__+
(13)
(see Fig. 7(d))
(‘141
The formyl groups (c) and (d) are known from i.r. investigations on CuOZnO [53] and on MgO [54], respectively. Formyl (c) can react with adsorbed oxygen or lattice oxygen atoms exposed on step sites to formate. Formyl (d), a rooted formate containing one lattice oxygen atom, can extract this atom leaving a vacancy, because surface lattice oxygen is removable from SnOz at low temperature [55] : HCO-O1,t
-
HCOOtis + V,
(15)
Formate is known to be stable up to 500 K on polycrystalline SnOz [56]. A quantum chemical study for ZnO shows that formate becomes negative upon adsorption; so that a substantial stabilization up to 600 K on ZnO (0001) results from the interaction with the positive end of the ZnO dipole [9]. Above 500 K, formate on Sn02 can react to give formaldehyde, compare ref. 52: HCOO,d, + OHad, -
H2COgas
+ 2oads
(16)
Formate on ZnO also acts as the key intermediate reaction [ 9, 521: + w&J
C%* In
analogy,
HCooads
co2
-
in the water gas shift
gas + HZ gas
formate on SnOz can decompose, + Hads
-
Cogas
+ Hads
+
co2
+ H2Ogas
compare Section 3.7.: (17)
Or HCOOada
gas + H2 gas
(18)
Formate intermediates are also essential in the formation of methane and methanol from CO and CO2 with H2 and Hz0 on metal catalysts, for example rhodium [ 573. As will be shown below, a hydrogen-containing intermediate is decisive for the CO sensitivity of Sn02 sensor devices in the atmosphere.
Sintered samples and thin films After CO exposure, mainly CO2 with a TDS maximum near 410 K leaves the surface of a polycrystalline sample, compare Fig. 8(a). Additional sites not present on the (101) face contribute to reaction (10). These sites are possibly connected with oxygen in a more exposed position. An intermediate, C02-Vo+, is proposed to exist on ZnO [ 581 and may precede the desorption of COz, compare Fig. 7(a). As discussed in Section 3.2., oxygen vacancies formed at low temperature do not contribute to conductance. Sintered samples in dry air do not exhibit a conductance increase if 1000 ppm CO are added. Simultaneous i.r. measurements [59] verified a carboxylate species, stable up to 570 K, compare Fig. 7(b). In the same tem-
89
1fP
$,& 5 5
1cP
G
2 166 %
u ro-’ 300
500 600 100 TEMPERATURE
700 IK)
800
Fig. 8. (a) Sintered sample of Sn02 without admixtures. Thermal desorption spectra with a heating rate of 9 K/s after exposure to CO. After Jacobs [87 1. (b) An initially wet porous pellet of SnOz exposed to an alternating gas atmosphere; 15 min in dry air, 15 min in dry air with an admixture of 1% CO. The temperature is increased (full curve) and decreased (dotted curve). After McAleer et OZ.[17].
perature range thin films in UHV do not exhibit any conductance change up to 10m4 Pa CO [60]. These films were prepared by vapour deposition of 100 nm tin and oxidation at 970 K in air for 45 h. Figure 8(b) shows conductance-temperature profiles for an initially wet pellet subjected to an atmosphere alternating between dry air with and without 1% CO admixture [17]. First the temperature is raised from room temperature to 800 K and then lowered to room temperature again. As discussed in Section 3.2., OH groups leave the surface mainly between 670 K and 770 K as water. Therefore, OH groups are absent while the temperature decreases. Since the vacancies left by the low-temperature COz formation (eqn. (lo), Figs. 7(a) and 8) do not contribute to the conductance, only the decay of the carboxylate (Fig. 7(b)) causes the CO sensitivity above 570 K by consumption of adsorbed oxygen ions (dotted curve in Fig. 8(b)): CO + Oads __f
CO* ,&carboxylate)
-+
CO* gas
(19)
On the wet (hydroxylated) pellet an additional conductance mechanism working below 570 K is due to the presence of OH groups on the surface (Fig. 8(b), straight curve). A hydroxylated polycrystalline sample heated in a 1000 ppm CO/air mixture exhibits a conductance maximum at 490 K, absent on the non-hydroxylated sample [ 591. The CO sensitivity of sintered Sn02 devices pretreated with sulfur dioxide increases with the partial pressure of water [61]. Since an operating temperature of 600 K was chosen, probably both conductance mechanisms are superimposed. The reactive scattering experiment with primary fluxes of CO, O2 and H,, repeated on
90
sputtered films, revealed mainly COz besides some formaldehyde. The nonrooted formate of eqn. (12) can decay according to eqn. (18). Since formate on sintered SnO, samples decays near 500 K [56 1, the conductance maximum at 490 K can be caused by the consumption of oxygen from adsorbed OH groups. Carbonate groups, Sn-CO2 detected on sintered SnOz by i.r. spectroscopy at lower temperatures [ 27 ] , seem to have no effect on conductance. Pd, Pt and Ir admixtures in sintered samples decrease the sensitivity (factor of two) and shift the conductance maximum to higher temperatures (50 - 150 K); the mechanisms are not known [62]. In some cases, e.g., in automobile exhaust, CO and NO are present at the same time. Nitrogen monoxide in the absence of CO does not change the conductance of a sputtered SnOz film [63] or a sintered SnOz specimen [64,65]. However, traces of NO reduce the CO sensitivity of the sintered SnO, specimen drastically [64, 651, so a simple superposition of conductance mechanisms is ruled out. This behaviour is not surprising, since polycrystalline SnOz is a (weak) catalyst for the oxidation of CO by NO, which activity can be enhanced by addition of PdO, CuO and other transition metal oxides [60,66,67 1. A reaction of NO and CO at metal-lattice oxygen pairs with the oxygen in an exposed position, verified by i.r., ESR and isotope exchange on MgO [ 681, may apply on polycrystalline or stepped Sn02, too: CO,,, + NO,,, + 02- -
(CNOs)“-
(26)
The formation of this intermediate needs only a marginal activation energy, see Figs. 7(f) and (g). The corresponding species (CN0,)2- on the flat surface of a single crystal, see Fig. 7(i), requires a highly-activated disproportionation (slow reaction) : 2CO,,, + NO,,, + 02- -
(C0,)2-
+ 2(CN0s)2-
(21)
The intermediate of reaction (20) decomposes to nitrogen and carbonate, which is presumably not active in conductance, as mentioned above: 2(CN0s)2-
-
2(C0~)~-
+
N2gas
(22)
The further decomposition of the carbonate is discussed by Harrison et al. 159,271. It should be noted that in the hot exhaust environment NO can be oxidized to N02. Nitrogen dioxide is a strong acceptor on Sn02, presumably with a surface state energetically below the state of adsorbed oxygen. Therefore, NO2 decreases the surface conductance of Sn02 reversibly [63, 691. A small irreversible part of the decrease has to be attributed to the reduction of adsorbed NO2 to NO by the oxidation of donors, i.e., the refilling of oxygen vacancies with oxygen atoms or the oxidation of tin atoms [70]. The latter effect plays a more prominent role for high partial pressures of NO2 and/or low O2 partial pressures; it predominates under UHV conditions [so]. Small admixtures of NO2 to CO in air can strongly reduce the signal of an SnO,-based CO sensor 1711. A coal absorption filter is recommended to suppress this interference. If NO and NO* are extracted by NHs from
91
combustion gases (compare Section 3.2.), adsorbed NH1 may also interact with CO, resulting in a formamide intermediate known from i.r. spectra of CaO and MgO (Fig. 8(k)) [42]. Nonequilibrium effects are essential in a CO sensor using a cyclic variation of temperature [72]. The SnOz thick film contains admixtures of 50% a-A1203 and 0.4% Pd. After 10 s at 570 K, the device is cooled down to 350 K within another 10 s. Immediately before the next heating the conductance is sampled within 0.5 s. This type of sensor mainly uses the carboxylate decay mechanism and benefits from the increasing sensitivity of the barrier conductance mechanism at low temperature. But at 100 ppm CO, a humidity-dependent conductance contribution remains visible (formate reaction). Non-equilibrium conductance measurements with similar cyclic temperature variations verify that adsorbed O- ions play an essential role (compare eqn. (19)) [73]. An increase of the film thickness allows the more reactive species to be suppressed (compare Section 3.3.). 30 ppm NO2 reduce the signal of 100 ppm CO by one order of magnitude on 30 pm thick films, but do not change the signal of films with a thickness exceeding 300 ~.cm. The sensitivity of 30 pm films to CO and ethanol is equal, but a 300 pm film is less sensitive to ethanol by a factor of 20. 3.5. Expusure to methane Decomposition of methane on a single crystal (101) face Figure Q(a) shows a TDS spectrum recorded after exposure to methane [74]. Significant amounts of CH4 are not desorbing at low temperatures. Hydrogen bonding as discussed in Section 3.3. should not occur, because it is only observed for carbon atoms with an increased Zeffective by 7rbonding (e.g., in HCN) or involved in bonds to strongly electronegative atoms (e.g., in CHCls) [34]. An adsorption of undecomposed methane via two of its hydrogen atoms at two adjacent oxygen lattice atoms and subsequent
TEMPERATURE (K)
Fig. 9. Thermal desorption spectra of (a) a single crystal, and (b) sputtered layer of Sn02. Heating rate 13.2 K/s. Before each run the oxygen pretreatment was applied to remove contaminants from the as-grown surface and to establish a stoichiometric surface: 30 min at 870 K and 15 min at 670 K in 0.25 Pa of 02, finally 5 min at 670 K during pump down. After Thoren [33] and Zacheja 11031.
reactions with ethane and ethylene are only discussed for basic oxides strong enough to abstract these two hydrogen atoms from the adsorbate [ 75, p. 10). Because Sn& is only a weak basic oxide, it is assumed that most of the methane dissociates to a methyl group and hydrogen: CH4
gas -
CH3ad,
(23).
+ Hads
The existence of the reverse direction of eqn. (23) is verified by methane formation after exposure to deuterated acetic acid (see Section 3.7.). Two adjacent methyl groups can combine to form a rooted ethoxy-like species : CH3 + CHs + Olat -
CH3CH2%t
(24)
+ Heda
This oxidative coupling was first proposed by Lunsford and is extensively discussed in refs. 76 and 77. In the chemistry of catalysts the formation of CzH4 (and also C2Hs, not investigated here) from adjacently adsorbed methyl groups is a process with a yield of up to 15% on oxides of Sn, Pb, Sb, Te, Mn, and Cd. An ethoxy-like species on SnOz is also discussed as an intermediate after ethane or ethylene exposure; unfortunately all characteristic i.r. bands of this species were obscured by hydroxy stretching modes and strong absorptions of the oxide itself [ 551. The low-temperature peak of mass 28 in Figs, 9 and 10 is accompanied by a proportional mass-26 signal and has therefore to be ascribed to ethylene. In a dehydration process ethylene is formed and the remaining hydrogen desorbs as water: CHsCH@lat
+ Hads -
H2Olat
+ C2H4gas
HlLOges
+ C2H4
gas +
Vo
A contribution to the desorbing ethylene flux from a recombination surface methylene groups cannot be ruled out [ 781: 2CH3
ads __f
2CH2
ads + 2Hed,
-
GH4gas
+ 2&d,
(25) of two (26)
The formation of methyl groups is not rate limiting for the ethylene desorption, because the water signal in Fig. 9 appears at lower temperature. The hump in the CO2 desorption near 450 K may arise from an oxidation of nondesorbing C2H4 as discussed in ref. 76. The ethoxy-like species can be converted to an acetate-like species [ 551. For details compare the corresponding reaction of the ethoxy group in Section 3.6.: CH&H@l,t
+ Clat -
CH3CQatOlet
+ 2Hads
(27)
In infrared studies on polycrystalhne Sn02 samples, acetate was found to be the most stable carbon-containing intermediate [ 551. The decomposition of surface acetate starts near 580 K and is completed at 750 K. CH4, CO2 and a species with mass 28 desorb in this temperature range, showing a common maximum between 700 and 740 K, compare Fig. 9. In this temperature range no mass-26 signal appears, therefore mass 28 has to be identified with
93
El
$y
Ethoxylike
7 H-C-H
species
+H
,Sn,l ,Sn .(),S”‘ *
I
I 1 v
t . .. . . . . *. . . . . . . . . .
;
H\~/H C
C2H4
P??!!!??
A
,
L-l Acetatelike
species
i ...........................
::.................. CHL desorbingi &. .:
. . . . .. . . .. . .*.. . . *. .. .. . . .. . . . .. .. .. ... . . .
i CO, CO2 desorbing i : . . . . .. ... ... .. . . . .. . .. . ... . . .. . .. .. . ... . Fig. 10. Decomposition of methane on SnO2 under UHV conditions (absence of gaseous or adsorbed oxygen). The surface intermediates and also the accumulated number of hydrogen atoms are indicated. Water and small amountr of hydrogen are also found deaorbing. The transition from the ethoxy-like species to the acetate-lie specie6 ie more explicitly discuwed in the corresponding ca8e of ethoxide decay in Section 3.6.
CO, compare the discussion of Fig. 10. After the subsequent dehydrogenation steps described in eqns. (23), (24) and (27), enough hydrogen is available to feed the reaction CH301Jhat
+ =&ids
-
CH,,,
+ HC%Jhat
(28)
At the temperature of the acetate decay the formate-like species, HCOletOlst, can be assumed to be unstable and decomposes further according to eqns. (29) and (30) below. A formate intermediate on polycrystalline SnOz samples exists only up to 500 K, as is known from changes of infrared spectra with temperature [ 561. Rooted formate-like species including lattice oxygen are also known from i.r. measurements at ZnO after exposure to methanol [ 791: HColat~lat
-
C”2gas
+ Hads
+ 2v0
(29)
94
or HC%,%l,
-
CC,,, + %,H
+ Vo
(30)
This set of equations describes an analogy to the water gas shift reaction, with the HCO1,,O1,, intermediate responsible for the CO/CO2 balance. The reactions (29) and (30) produce oxygen vacancies on the surface with a maximal rate between 700 and 740 K. A survey of the methane decay reactions is shown in Fig. 10. CzH4 already desorbs near 400 K. CH4, CO and CO2 show a common desorption maximum, because the acetate-like species is more stable at high temperatures than the formate-like species. All reaction steps release hydrogen atoms. Only the step from acetate to formate (eqn. (28)) needs hydrogen atoms to form methane. Most of the hydrogen leaves the surface as water, consuming lattice oxygen. The saturation value of the water desorption flux in Fig. 9(a) is reached near 450 K, the temperature where the low-temperature water formation (compare Section 3.2.) occurs. At higher temperature the OlatHads groups formed in eqn. (30) can recombine with Hads to form water. However, the formation of methane in reaction (28) reduces the amount of hydrogen feeding the water formation. The flux of desorbing water decreases correspondingly. Even in the absence of reducing gases, the oxygen/tin ratio at the surface shows a broad minimum around 550 K, as was shown by Auger spectroscopy (SO]. Thus a competing reaction without 0 let consumption is favoured around 550 K and also consumes a small fraction of the available adsorbed hydrogen: H ads
+ Hads
=
H2
gas
(31)
The reactions (29) and (30) produce oxygen vacancies, increasing the conductivity. The surface lattice oxygen supply is exhausted by consecutive TDS runs in v&u&n. Therefore, the mass-28 contribution in the hightemperature rar$e consisting of CO decreases, whereas the low-temperature peak produced by ethylene desorption persists, Fig. 11(a). Also the CO2 desorption is reduced. Only an oxygen pretreatment restores the original height of the CO and CO1 peaks (see Fig. 9). On other oxides, and under other reaction conditions further processes are discussed [ 751. For example, in the presence of gaseous oxygen, methyl groups can react directly to give formate with subsequent desorption of formaldehyde, CO and H,. This reaction is excluded under UHV conditions, where only lattice oxygen is available, but it may be effective under sensor working conditions. Conductance measurements on distinct single crystal faces are not known. However, whiskers (thin needle-shaped single crystals) show a conductivity increase on addition of 1% methane to air at temperatures above 720 K [ 811 .-Methane is a very stable mole&e in the gas bhase. This fact is often made responsible for the finding that the sensitivity of SnOzbased devices (conductance change) reaches a maximum 100 to 150 K higher than for most other gases (ethanol, hydrogen, . ._). However, water and
96
‘jO0
500
700
300
TEMPERATURE
500
700
900
[Kl
Fig. 11. Thermal desorption spectra (TDS) of mass-28 and maw44 species after an exposure to 6000 Langmuir CH4. After exposures (1) and (2) the surface was not restored by a standard pretreatment in oxygen as applied before the f&t exposure, Fig. 9. After Thoren [33] and Zacheja [ 103 1.
ethane already leave the Sn02 surface below 350 K, because the first surface reaction requires only the activation energy to break only one of the four C-H bonds. The reaction path yielding ethylene, predominating below 600 K, does not change the conductance. Only the generation of oxygen vacancies starting near 700 K in the reaction path via ethoxy/formate-like species evokes a conductance increase. The low-temperature water formation consuming lattice oxygen is not reflected in a conductance increase. During continuous exposure to methane at temperatures well below 700 K, the stable acetate-like species may even block sites for further methane adsorption and prevent an increase of conductance. Decomposition of methane on a sputtered film To compare the results of the single crystal with those of a commercial SnO, device, the TDS runs were repeated with a sputtered layer on a porous A1203 substrate, Fig. 9(b). The methane ‘desorption signal’ arises mainly from a diffusion out of the substrate through the Sn02 layer. An auxiliary measurement after exposure of a bare substrate to oxygen resulted in a similar slope of the ‘desorption signal’. The water formation begins at the same temperature as on the (101) face. Ethylene starts to desorb at slightfy lower temperature and increases faster; the less well-defined sputtered surface may expose more reactive sites. More reactive sites also enhance the oxidation of nondesorbing CzH4 markedly. The hump in the CO2 desorption is much larger and shifted from 450 to 400 K. Hydrogen molecules do not desorb; the more reactive surface seems to convert the hydrogen more effectively to water. The maxima of CO and CO2 appear at higher temperature. Two reasons are possible for this. The acetate may be more tightly bound on sites not present at the (101) face. On the other hand, the CH4 dissolved in the substrate can diffuse back at elevated temperatures
96
and occupy sites needed for the formation of the acetate-like species. Thus the leading flank of the maximum may shift to higher temperatures. Both cases can only be distinguished using layers on substrates like quartz glass. Such a substrate would also facilitate the detection of methane formation during the acetate/formate transition. Besides SnC&based elements cr-FezOrbased elements are also commercially offered for methane detection. In contrast on this n-type semiconductor methane acts at 430 “C as a donor molecule. Only methane is found in desorption. However, ethanol and isobutane decompose at the same temperature [ 82 1. Conductance measurements on sputtered Sn02 films during methane exposure do not exist. However, a sintered specimen in a flow of air shows a maximum of the relative conductance increase between 730 and 750 K upon addition of 100 ppm of methane [83,623. In this temperature range the consumption of methane and the desorption of a proportional amount of carbon dioxide were also found to become observable [83]. Other decomposition products were not investigated. The selectivity of a sintered Sn02 film depends on its thickness (see Section 3.2. for corresponding effects of AsH3, H2S and CO/NO2 sensors). The sensitivity ratios at 720 K for 3000 ppm of the gases CHI, Hz, CO or ethanol in air are favourable for methane detection at a thickness of 500 pm (4.0/0.3/0.06/0.3), but unfavourable at a thickness of 50 pm (1.2/7.0/0.5/ 4.8) [83]. 3.6. Exposure to gaseous ethanol Single crystals After exposure of a (110) face to ethanol the desorption spectra in Figs. 12(c) and (d) show the desorption of ethanol besides the decomposition products acetaldehyde, ethylene and water [ 841. The ethanol molecule
ik”
5: 3
0 300
LOO 500
600
TEMPERATURE
700
(K)
800
300
LOO
500
600
TEMPERATURE
700
800
(K)
Fig. 12. Thermal desorption spectra (solid curves) and sheet conductance (dotted curve). Standard pretreatment (Fig. 9). Heating rate 9 K/s in the TDS runs. (a) and (b) sintered sample, (c) and (d) single crystal. After Jacobs et al. [84].
2%
1
Ethoxy species Ethylene desorbing
Acetaldehyde adsorbed
H3$ (b) (4
HJ:
“:I+
-
,Sn$nb,
‘S”‘6
I
~ (dl
-
,=n,
P “‘00
Acetaldehyde desorbing
No hydrogen desoorption
350K
P 0
sn
I?
n,
Ethoxy group with Sn-0 bond and
C-H-O
bond
Ethanol adsorbing
Fig. 13. Surface intermediates after exposure to ethanol. Sn03 (110) face: (a) molecular adsorption; (b) after dissociation (dehydrogenation); (c) after a second dehydrogenation; (d) desorption of acetaldehyde. Polycrystalline or stepped SnOz sample: (e) ethoxy group with an additional hydrogen bond; (f) desorption of hydrogen and acetaldehyde; (g) double bond of adsorbed acetaldehyde attacked by electrophilic oxygen; (h) acetate group at a step.
may be weakly bound to the surface, e.g., via the oxygen atom of its OH group, Fig. 13(a), The TDS maximum at 360 K is near the maximum of acetic acid at 350 K, where the same type of binding is possible. CH3CH20H,a,
-
CH&H20H,d,
(32)
The extended high-temperature tail of the ethanol desorption arises from a reversible dissociation to an ethoxy group (Fig. 13(b)) and adsorbed hydrogen (dehydrogenation) : CHBCH,OH,d, -
CH3CH20~d~
+ Hads
(33)
The hydrogen from reaction (33) can react to form water and desorb with a maximum at 400 K, Fig. 12(d). For sintered specimens the reversible dissociation has been verified by experiments with deuterated ethanol (see below). The desorption of ethylene and water near 400 K and between 660 K and 750 K can be formally regarded as a dehydration of ethanol: CH,CH,OH -
C2H4
gas + H2°gas
(34)
The reverse reaction is well known from industrial ethanol production over Cu/ZnO catalysts. Since ethylene adsorbed on polycrystalline Sn02 is known
to form ethoxy groups [ 551, ethylene may be evolved by decomposition the ethoxy group of eqn. (33): CH&H,%,
+
Hads -
H2°gas
+ C2H4
gas
of (35)
The difference from the methane case (eqn. (25)) consists in the fact that no lattice oxygen is required. The desorption spectrum also contains acetaldehyde, a dehydrogenation product of the ethoxy group: CH3CHZoads
-
Hads
+
CH3CHOgas (acetaldehyde)
(36)
The ethoxy groups are the common supply of three simultaneous reactions: (33) reverse, (35) and (36). The maxima of water (and also ethylene) desorption coincide with the maxima generally found at 400 K and 670 770 K after exposure to hydrogen-containing species (Section 3.2.). Since the ethoxy species and water or hydroxyl groups are bound via an oxygen lone pair orbital to the surface, the breaking of this bond may be rate limiting in both cases. The minimum in acetaldehyde desorption reflects the water/ethylene maximum near 400 K, Figs. 12(c) and (d). Acetaldehyde finally stops desorbing when the high-temperature water/ethylene desorption starts. The ethanol desorption according to eqn. (33) reverse is roughly proportional to the water formation between 400 K and 550 K, pointing to a hydrogen/water limitation in this range. Hydrogen bound in OlatH groups seems not to be available for recombination with the ethoxy species to form ethanol. The condensation of OH groups to water above 550 K consumes lattice oxygen, increasing the sheet conductance at higher temperatures, Carbon monoxide and carbon dioxide are absent in the desorption. The sequence of the first two bond scissions in the ethanol decomposition is not oxide specific. On Ni (ill), bond activation was verified in TDS using deuterium and 13C isotopic labels and AES lineshape analysis [85] : (1) O-H with H, desorption between 300 and 350 K, compare eqn. (33); (2) C-H of the methylene group CH2, rate limiting for acetaldehyde desorption (compare eqn. (36)) and CH4 desorption (compare eqn. (28)) above 260 K; (3) C-C not rate limiting for CH4 desorption, see above; (4) C-H of the methyl group CH, and carbidic carbon deposit. The CO bond does not break and CO desorbs from an adsorption state at 430 K. Sn02 (110) and Ni (110) differ in the rate-limitation mechanism for acetaldehyde desorption (for polycrystalline SnOz, see below). Furthermore, the C-C bond remains intact on the clean (110) Sn02 face, preventing CO or CO2 desorption. To break the C-C bond on SnOz faces active oxygen is necessary, as will be discussed below. Heating of SnO, single crystals with (110) faces exposed to ethanol leads to a conductance maximum (dotted curve in Fig. 12(c)) nearly coinciding with the ethanol desorption maximum [84]. Molecular ethanol bound via its oxygen lone-pair orbital (filled with two electrons) to a surface tin
99
atom can act as a surface donor. The right flank of the conductance maximum will contain contributions from hydrogen donors of the dissociated molecule. Since methoxy on ZnO is an acceptor [9], it is likely that ethoxy on SnOa will also be an acceptor, partly reducing the conductance increase caused by the hydrogen donor. The water formation near 400 K consumes the hydrogen donors; the vacancies from this low-temperature process are not active in conductance (compare Section 3.2.). At high temperatures, above 560 K, vacanciesleft after condensation of hydroxyl groups and water desorption can act as subsurface donors. The absolute and the relative conductance increases of SnOz (110) faces kept at 600 K in air with admixtures of 85 to 950 ppm ethanol reveal a nearly linear response to the ethanol concentration [ 861. Sintered specimens The main desorption maxima of ethanol, acetaldehyde and ethylene in Figs. 12(a) and (b) are found between 460 and 480 K. Smaller fluxes of these gases appear as shoulders between 350 K and 380 K, indicating that a minor fraction of the adsorbed ethanol adsorbs and reacts in the same way as on the (110) face. At edge sites on the polycrystalline surface (Fig. 13(e)) ethoxy can be more tightly fixed by an additional hydrogen bond of the methylene hydrogen to an exposed lattice oxygen. The observed shift of the TDS maxima corresponds to about 8 kcal/mole, typical for hydrogen bridge bonding. Only on the polycrystalline sample at 480 K does a desorption maximum of hydrogen also appear. After adsorption of water on a polycrystalline sample, a dominant water desorption maximum at 480 K also appears [ 871. Water is able to form an additional hydrogen bond at edge sites, too. This water desorption maximum is not observed on a single-crystal (101) face. The adsorption of ammonia on hydroxylated SnOz is also attributed to hydrogen bridge bonding, as discussed in Section 3.3. In contrast to the finding on the (110) face, CO and COz also desorb from the polycrystalline samples above 600 K. The C-C bond can be broken by the following mechanism: the acetaldehyde formed in eqn. (36) contains a double bond, Fig. 13(g). An exposed electrophilic oxygen can attack the double bond and form a carboxylate group (acetate) (Fig. 13(h)): CH&H%,+
oexposed-
CHaCOO (acetate)
(37)
It has been shown that formaldehyde and adsorbed oxygen ions on copper react in a similar way to the process for a formate group [ 881. Various ketones adsorbing with their carbonyl group on polycrystalline SnO, are attacked by OH groups at the double bond and also form carboxylate groups, e.g., acetone reacts on hydroxylated SnOz to give acetate [SS]. A schematic survey of these reactions is sketched in Fig. 17. Acetate can decompose via eqns. (28), (29) and (30) to give CO and COz. The acetate after ethanol exposure contains only one (exposed) lattice oxygen atom, whereas acetate after methane exposure contains two. The second maximum
100
of water desorption appears at nearly the same temperature (750 K) as on the (110) face. Again the vacancies left after water desorption can act as donors. After exposure of sintered specimens to deuterated ethanol, &HsOD (EtOD), EtOH is desorbed in addition to EtOD [87]. The desorption maximum of EtOD coincides with the desorption maximum of EtOH after adsorption of EtOH. The EtOH desorption includes at least three steps: (1) Adsorption of an EtOD molecule and formation of an ethoxy group losing its D atom according to eqn. (33). (2) Adsorption of another EtOD molecule somewhere on the surface losing a D atom and an H atom, forming acetaldehyde in eqns. (33) and (36). (3) The adsorbed hydrogen released in reaction (36) migrates to the ethoxy group and recombines to give EtOH (eqn. (33) reverse). The TDS maxima of acetaldehyde and EtOH appear together. Since the acetaldehyde formation may be rate limiting for the EtOH formation, only an upper limit for the activation energy of hydrogen migration is obtainable from the observed temperature difference of 30 K for the two ethanol desorption maxima. It should be mentioned that hydrogen migration can be quenched by blocking of sites (compare Section 3.3.) or by trapping at carbon deposits, forming lateral C-H bonds [45]. 3.7, Exposure to gaseous acetic acid Single crystals In some biosensors for pesticides or for the control of biochemical processes, an acetic acid detector is required [ 901. Acetic acid can adsorb on SnOa (101) faces as a molecule or dissociatively as acetate, compare Figs. 14(a) and (b), respectively: CHsCOOH,,,
H3C
L=O
-
CH3COOad,
H3C I
Acetic acid 4
p’?
,4n ‘o’ Sf-kO, SfxO, 0
Rooted acetate
3-k
Acetate
J%
HO ,Sn,
(38)
+ Hads
H
-Sa,S=.o,Sn,o,Sn$,
[b)
(al
C/CH3
5YH , 9 i sn,J,Sv, 0 O/ vacancy ICI
fiH2 F 0
K@tW desorbing
,Sn.!~3k.o,Sn,~.Sn
‘d Ml
Fig. 14. Surface intermediates after exposure of SnOl to acetic acid: (a) molecular adsorption; (b) dissociative adsorption (dehydrogenation); (c) dissociative adsorption at a defective Sn02 surface; (d) keten desorption.
101
The equilibrium can be on either side of eqn. (38), depending on the experimental conditions. During continuous exposure to 1016 molecules/cm2s in a reactive-scatteringexperiment with linearly increasing temperature, the acetic acid does not find enough time and/or sites to decompose [74]. It adsorbs only weakly, e.g., via the oxygen of the hydroxyl group (desorption maximum at 350 K). However, after an initial exposure to 10” molecules/ cm2 with subsequent linear heating in UHV and correspondingly lower coverages, the acid decomposes according to eqn. (38). The decomposition becomes apparent in second-order desorption (TDS maximum at 430 K) and is further proved by a low-temperature desorption of water. This water desorption is not observed during continuous exposure. Hydrogen released in eqn. (38) is mobile and can recombine with another acetate, as was shown in an H-D exchange experiment similar to the EtOD experiment described in Section 3.6. However, a considerable part of the mobile H reacts with lattice oxygen and forms water. The further decomposition of the surface acetate starts with the reaction CH&OO,,,
f 2H,,
--+
CH4,,, + HCOOad, (formate)
(39)
The TDS spectra of mass 28 (CO or C,H,) and of CO2 look very similar to each other after acetic acid and methane exposure, but are clearly distinct after CO exposure. Therefore, it s&ms reasonable that the decay reactions of methane via a formate intermediate are also applicable for the acetate decay, eqn. (39). However, it should be kept in mind that the formate of eqn. (28) includes lattice oxygen, while the formate from eqn. (39) does not. Photoemission experiments (UPS) on ZnO (1010) faces after exposure to formic acid revealed a formate species [91]. No low-temperature methane desorption was found after methane exposure. Therefore the methane formed from acetic acid at temperatures above 400 K must be formed during the acetate decay in eqn. (39). Enough hydrogen is available, as indicated by the water desorption. The increase of electrical conductance at a sample temperature of 473 K may arise from adsorbed hydrogen acting as donor
WI-
Above about 650 K acetate can fill a vacancy with one of its oxygen atoms (singly-rooted acetate), compare Fig. 14(c): CHsCOOH,,, + V, -
Hads + ‘0~’
+
CH3Coads
(49)
Filling of a vacancy with oxygen is also possible (Section 2.2.) and with COs, but not with CO, as was shown in an XPS/UPS study on ZnO (OOOi) [51]. The singly-rooted acetate with its higher binding energy causes an increase of the acetic acid desorption again during continuous exposure at high temperatures. A fraction of the rooted acetate forms keten, observed in desorption, thereby leaving one oxygen atom to fill the vacancy, compare Fig. 14(d): CHsCO-‘Olat’ -
HzC=C=Opa, + HPdr + ‘Olat’
(41)
102
The formate from reaction (39) contains two oxygen atoms of the initial adsorbate. It decomposes into CO and CO2 without the V. formation of eqns. (29) and (30). The singly-rooted acetate from eqn. (40) can undergo an analogous reaction, delivering a formate including one lattice oxygen atom : Cl&CO-‘Olat’
CH 2 ads
-
+ HCOO1at
(42)
If reactions (29) and (30) are responsible for the similarity in the CO and CO, desorption behaviour of methane and acetic acid, the rate-limiting step is the decomposition of formate. The type of incorporated oxygen, Olat or 0 ads, has apparently negligible influence on the reaction temperatures. During continuous exposure to acetic acid in a reactive scattering experiment, the COz and the CO desorption fluxes decay. The conductance increases irreversibly. The common reason may be a depletion of lattice oxygen by high-temperature water formation, which is only partly compensated for by reaction (40). The desorption of water in the high-temperature regime is not quenched by continuous exposure because the OH groups are products of reaction (30). Thin evaporated films Acetic acid increases the surface conductivity at 573 K, presumably by the effect of hydrogen donors, Fig. 15. At 623 K and 673 K the conductance responds with a transient minimum to acetic acid addition to air. The minimum possibly reflects the filling of oxygen vacancies by acetate
3.6 1
’ 0
I ’
’ 20
Time
’
’ LO
1
’ 60
J
(4
Fig. 15. Conductance g as a function of time during exposure of an SnOg thin film to acetic acid in air for various temperatures. After Schnakenberg et al. [IS I.
103
species, thus cancelling subsurface donors (compare Section 2) Later in the decomposition process of the singly-rooted acetate, the formate extracts oxygen out of the surface according to reactions (29) and (30), producing new donors. Therefore the conductance increases again. At higher temperatures the decomposition of the singly-rooted acetate proceeds faster and the minimum becomes narrower on the time scale. Finally at 723 K the decomposition of the singly-rooted acetate dominates and a minimum is no longer detectable. The conductance of sintered specimens behaves similarly; the initial slope of conductance is proportional to the acetic acid concentration in the range 50 to 700 ppm [18]. SnOz thin-film devices, exposed to acetic acid in air, show after some time an oscillatory conductance behaviour. Stable behaviour can only be restored by transient heating in air to 870 K [18]. In a reactive scattering experiment with the film kept at 585 K (below the TDS maxima of the formate products), only water desorbs as decomposition product, compare Fig. 16(a). According to a sum equation elementary carbon must be deposited: CHsCOOH,,,
-
2H20
+ 2Cdeposited
(43)
A keten desorption according to eqn. (41) is missing on thin films. On the rough surface of thin evaporated films, the highly reactive keten can react at its carbon-carbon double bond with exposed oxygen atoms at step sites. A similar reaction was discussed in Section 3.6. for the decay of acetaldehyde. This type of electrophilic attack by oxygen, observed on oxidized copper and silver surfaces [SS], is known to result in carbon deposition on the surface. In the presence of air at sufficiently high temperature, it is possible to remove deposited carbon by the formation of CO or CO2 [ 881. The curves in Figs. 16(b) and (c), recorded at substrate temperatures of 673 K and 725 K, also show products of the acetate/formate decay described in Section 3.5.: CO and CO2 appear, too.
UJ
01
z
2
3 TIME
(minutesl
Fig. 16. Fluxes of reactively scattered products from an evaporated film at three different temperatures during exposure to acetic acid. The signals of the decomposition products are corrected for cracking contributions from the acetic acid. After Schnakenberg et al. 118 I.
104
4. Common reaction steps in hydrocarbon decomposition The reactions of hydrocarbons on SnOz, discussed in the preceding Sections, have some steps in common. A scheme is given in Fig. 17. It is completed with decay patterns of some hydrocarbons of interest, where spectroscopic data of intermediates are available from the literature. Results from infrared investigations on polycrystalline Sn02 samples are used. They were obtained after exposure to methanol [ 921, ethane [ 55 ] , ethylene [ 553, propylene [93], acetic acid [94], formic acid [ 563, acetaldehyde [92], acetone [ 921 and other ketones [ 95,891, carbon monoxide [ 27,59,94,96], carbon dioxide [27,94,96, 971, and water [27,39]. Molecular adsorption can already raise the conductance, if the adsorbate is a Lewis base making electronic charge available to an accumulation layer. Molecules with a lone-pair orbital occupied by two electrons adsorbed at a tin site, e.g., alcohols, act as surface donors. Acetic acid can also adsorb this way. Oxygen lattice atoms provide sites for ethylene; the adsorbed molecule together with the lattice oxygen atom can be regarded as a rooted ethoxy group. It is well known that the ability of metal surfaces to abstract hydrogen from an adsorbate is enhanced by an oxidation pretreatment. Adjacent metal/oxygen sites provide Lewis acid/base pairs; the oxygen atom takes up the hydrogen atom after dissociation. By this mechanism the tin-oxygen pairs at the surface of SnOz are already efficient in dehydrogenation of methane, alcohols and acetic acid at moderate temperature. Hydrogen atoms ..
. . . . . . .. . ... . ..._...................................
_. . . .._ . .. .. .*. .. . . . . . . . . . . .. . . . . . .. . . . .. .
Mqlecules adsorbing
\I/
Alkox y surface groups Carbonyle surface groups
(..........,.._.._.......
P
: Methoxy--..---I i . ......_....__.._..._ . ..! .. . . . .._._.... . .... .. . . .... . ..._.._._...._... !.!X.XY.:l..!
‘C’ ,--i]l
I
I......*
$;
~ . . . ..,,.. .. . . . . . . . . .. . . ..... .... . . .. .. .... . . . j /Formaldehyde1 1Acetaldehyde
: .. . . \ ‘-Oxygen
. .. . . . ..
. ..
. . . . . . .. . .
. .. .. ... . . ....... . . .. .... .. . .
. .i . . .. . . i
Keten
1 Acetone . . . . . . . . . . . . . .. . . .. . . . . . . . . . . .._. . ..I . . . . . . .. . . .
attacks double
. . . . . . l .. . . . . . . . . . . . .. . . . . . . .
1
Oxygen attacks double
. . .... . ..( . .. . .. .. . . . . . , ,.. .. .. . .. Acetate
..... ................... .. ..... ............. ...............................*..Products desor bing
~.COi__O,~~, Hz0 (total oxidation) . . . . . . . . ..I...............................
.. ... . . . .. . . ....*. .. .. .. .*... . . . ... . . ....... *.. .......
Fig, 17. Schematic representation of common reaction steps for various carbon-containing molecules on SnOz. See Section 4 for explanation.
105
already act as mobile surface donors at 400 K. Recombination and desorption is also observed. Exposed or incompletely coordinated tin atoms are more effective in dehydrogenation [ 971; compare also the behaviour of evaporated SnO, films on exposure to arsine (Section 3.3.). In the presence of water vapour a second type of acid/base pair is active, where hydroxyl groups take up the hydrogen atom’/971 (compare Sections 3.2. and 3.3.). . Besides dissociative adsorption, associative adsorption including the uptake of hydrogen is also possible. Propylene in the presence of water vapour is attacked by the proton of an adsorbed hydroxyl group at its double bond (electrophilic addition). The resulting intermediate is an isopropoxy group [93]. Silica, often regarded as an inert mechanical stabilizer of sintered SnOz sensor devices, binds the oxygen atom of hydroxyl groups more tightly. Therefore, the hydroxyl group becomes more reactive towards propylene [93]. That means a hydroxyl group adsorbed on a Si atom is a stronger BrOnsted acid with a more easily available proton, compare Section 3.2. Both dissociative and associative adsorption result in the formation of alkoxy groups bound to tin sites, compare Fig. 17. Neighbouring methyl groups form an ethoxy group by oxidative coupling at a lattice oxygen atom. In a further dehydrogenation step, carbonyl surface groups containing a double bond are formed, see Fig. 17. The same class of intermediates appears after exposure to acetaldehyde (formaldehyde was not investigated), acetone and other ketones [92, 96, 891. On a single-crystal (110) face acetaldehyde does not react further and desorbs (Fig. 13(d)). Electrophilic oxygen species (exposed lattice oxygen atoms on polycrystalline samples, adsorbed oxygen ions or the oxygen atoms of hydroxyl groups) can attack the carbonyl double bond. The resulting surface species are acetate or for-mate, as verified by infrared spectroscopy. Acrylates are found after adsorption of various ketones [SS]. The relatively stable acetate can decay to fonnate above 580 K. The formate species is also decisive for the low-temperature sensitivity of SnOl towards CO in the presence of water. Finally formate decomposes into the products of total oxidation, CO, CO2 and H,O. Two acetate groups can combine in an acetic anhydride (two acetate groups share a common oxygen atom) and a chemisorbed oxygen atom. The typical decomposition product of the anhydride is keten [W]. After exposure to ethanol or acetic acid on single crystals, keten is observed in desorption. On polycrystalline films again an electrophilic attack of exposed oxygen at the keten double bond occurs. The resulting deposition of carbon spoils the sensor function if the carbon is not periodically removed by heat treatment in oxygen or hydrogen.
5. Possible improvements of gas sensors The existence of common or analogous intermediates for several gases may discourage the hope for selectivity. However, the scheme in Fig. 17 can
106
also be used as a guideline to avoid detours and to concentrate modifications of the sensor. Some ideas are listed below.
on reasonable
Operating temperature and thermal pretreatments The relative abundance of adsorbed (0,) and O- ions (electrophilic attack) depends on the operating temperature. Since the equilibrium between both species is approached slowly after a variation of temperature, the abundance ratio of the oxygen ions is shifted by cycling the temperature in the appropriate range. The concentration of hydroxyl groups taking part in some reactions (CO, propylene, NH,) is also varied in a temperaturecycled mode. At low temperature only the adsorption of donors and acceptors changes the conductance. Oxygen vacancies left in many surface reactions need elevated temperatures to migrate below the surface and become active as donors. Conversely, NO2 and acetic acid annihilate surface vacancies and at elevated temperatures reduce the concentration of subsurface donors and thereby the conductance. The irreversible loss of donors is undesirable for prereduced NO2 sensors (limitation of lifespan), but it can be used for quantiative acetic acid measurements. Examples have been given that exposed oxygen and tin atoms are essential for some partial reactions. The carbon deposition problem caused by electrophilic oxygen has already been mentioned. Therefore, the grain size in sintered specimens and evaporated layers is a parameter that has to be stabilized for constant sensitivity and selectivity by annealing pretreatments 1321 or admixtures of salts that change the ionic conductivity at the sintering temperature (containing, e.g., K, Ca, or Mg [ 981). Reduced surface Surface sites are made more negatively charged by neighbouring oxygen vacancies. The bond energy for donor molecules, e.g., HZ, HzO, NH3, methanol, is lowered and desorption of water and methanol facilitated [30, pp. 174 and 1461. Conversely the bond energy for acceptor molecules increases with the density of oxygen vacancies. Then oxygen should be adsorbed preferentially at oxygen vacancies if the temperature is not sufficiently high for reincorporation. It is also proposed that alcohols dissociate mainly at vacancy sites 1381. Treatment with gases NH3 can poison the surface Br#nsted acid centres (OH groups or molecularly adsorbed water). Thereby the nucleophilic attack of carbonyls can be prevented. Small sulphur deposits remaining (e-g., acetaldehyde) after H2S treatment can suppress the hydrogen diffusion and may affect hydrocarbon decomposition (compare Section 3.3.). The effect of sulphur on hydrogen spillover from metal deposits has not yet been investigated. SOZ is known to enhance the benzene sensitivity of Sn02 [ 461 (see Section 3.3.).
107
A pretreatment with HCl, HF or HBr (protonation) facilitates hydrogenation. The bond of species with an oxygen lone-pair orbital (methanol, ethanol) is strengthened, as verified on Si&, A120s and TiOz using electron paramagnetic resonance [30, p. 1461. Water from dehydration reactions (e.g., formate decay) can be kept on the surface. Therefore, these centres are also active in the dehydration of alcohols and cracking of hydrocarbons [38, p. 3321, see Section 3.3.
Admixtures and deposits Specimens of pure SnO, are often brittle after sintering below 1000 “C. A frequently used stabilizer, SiOz, can provide additional adsorption sites, thus changing the selectivity. The adsorption of propylene has been given as an example. Infrared investigations also exist for water [39], ethane [55], ethylene [55], formic acid [56], COz [97] and ammonia [39]. The admixture of metal oxides by addition of salts before sintering or by cosputtering means a percentage of tin sites at the surface is occupied by foreign metal atoms. If simple covalent/ionic adsorption is decisive for the sensor signal, the electronegativity of the foreign metal can rule the sensitivity, as was shown for HIS in Section 3.3. Surface species on Pd sites of polycrystalline SnOz samples with Pd admixture have been investigated with i.r. spectroscopy after exposure to ethane [55], ethylene [55], propylene [93], formic acid [56], water [39], COz [97] and ammonia [39]. In a comparative i.r. study of CO, NO, and CO/NO, admixtures of Cr, Mn, Fe, Co, Ni and Cu oxides to Sn02 [60] are used. A conductance study with CO exposure uses admixtures of Pd, Pt and Ir (oxides) [62]. Examples were given in Sections 3.4. and 3.5. that the selectivity between gases of low and high reactivity changes markedly with film thickness, if the device has buried electrodes. An active catalyst deployed only on the outer edge of a thick polycrystalline device enhances combustion in the outer region and suppresses sensitivity for gases of high reactivity. This leads to a diminished response at higher temperature, depending on electrode geometry [98]. The effect of precombustion of gases reacting faster than methane is also successfully applied in a device with a porous Pt catalyst, kept at 400 “C to oxidize CO, Hz and higher hydrocarbons but not CH4, in front of a sintered SnOn sample kept at 400 “C [99]. Foreign metals or their oxides can also be deposited (e.g., by electron evaporation) on single-crystal faces or thin films. Usually clusters are formed, leaving part of the SnOz surface open for reactions (Auger control) [3,100]. Then the catalytic properties of the foreign metal play a role, too. Frequently a dehydrogenation is already possible at room temperature; ethanol on Ni is given as an example in Section 3.6. Hydrogen atoms can spill over to the substrate and increase the conductance. A quantitative investigation of hydrogen spillover with a local infrared probe exists for Pt on SiOz [ 1011. An electronic effect of metal deposits is also postulated [98, 1021.
108
6. Conclusions Sensors have a ‘life cycle’ consisting of preparation, activation, operation with deactivation and, possibly, regeneration. Thus understanding the performance in terms of reaction and conductance mechanisms is only a part of the total understanding of a sensor. Given that the performance of an existing sensor will have been optimized by trial and error, it is probably the case that the benefits of surface characterization come from also understanding and modifying the deactivation and regeneration processes. The reported observations and proposed models may be a stimulation to investigate reactions on other oxidic materials, e.g., ZnO, Fe203 or perovskites, which are not treated here.
Acknowledgements Thorough discussions with G, Heiland are gratefully acknowledged. This work was supported by the Bundesminister fur Forschung Technologie.
und
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Dieter Kohl received the Dr. rer. nat. from the RWTH, Aachen in 1972. Since 1982 he has worked as a lecturer at the 2. Physikalisches Institut of the RWTH Aachen. His research interests are electronic and chemical properties of compound semiconductors like ZnO, SnOp or GaAs.