Tin oxide nanomaterials: Active centers and gas sensor properties

Tin oxide nanomaterials: Active centers and gas sensor properties

7

Alexander Gaskov, Marina Rumyantseva, Artem Marikutsa Chemistry Department, Moscow State University, Moscow, Russia Chapter outline 7.1 Introduction  163 7.2 Structure of nanocrystalline SnO2  165 7.3 Types of active centers on the surface of nanostructured SnO2  169 7.4 Acid-base centers  170 7.4.1 Influence of Synthesis Conditions and Microstructure Parameters on Concentration of Acid Centers of Different Types  172

7.5 Chemisorbed oxygen  173 7.5.1 Influence of Synthesis Conditions and Microstructure Parameters on Concentration and Predominant Form of Chemisorbed Oxygen  175

7.6 Hydrate-hydroxyl layer  177 7.6.1 Influence of Synthesis Conditions and Microstructure Parameters on HHL  178

7.7 Paramagnetic centers  179 7.7.1 Relationships Between the Parameters of SnO2 Microstructure and the Type and Concentration of Paramagnetic Centers  183

7.8 Effect of doping impurities on active sites of nanocrystalline SnO2  185 7.9 Effect of catalytic additives on the active sites of nanocrystalline SnO2  187 7.9.1 Active Sites Formed by Catalytic Modifiers  188 7.9.2 Effect of Modifiers on the Active Sites of SnO2  190

7.10 Oxygen exchange of nanocrystalline SnO2  194 7.10.1 Effect of Tin Dioxide Microstructure on Oxygen Exchange  195 7.10.2 Effect of Catalytic Modifiers on Oxygen Exchange  198

7.11 Active sites and gas sensor properties of nanocrystalline SnO2  201 7.11.1 The Role of Nature of Active Sites  204 7.11.2 Influence of Concentration of Active Sites  209

7.12 Conclusion  211 References  212

7.1 Introduction Interest in tin dioxide as a material for heterogeneous catalysis and chemical sensors stimulated the development of nanostructured SnO2 technology and the extensive investigation of its chemical features. Nanostructured SnO2 with a characteristic particle size less than 20 nm and specific surface area of 100–200 m2/g is of particular interest for gas sensing and catalysis. High surface-to-volume ratio for nanoparticles Tin Oxide Materials: Synthesis, Properties, and Applications. https://doi.org/10.1016/B978-0-12-815924-8.00007-4 © 2020 Elsevier Inc. All rights reserved.

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Tin Oxide Materials: Synthesis, Properties, and Applications

s­ignificantly changes chemical behavior and increases the efficiency of nanostructured materials in heterogeneous processes in comparison with bulk materials [1, 2]. The high adsorption and reactivity performances of nanostructured tin dioxide are mainly due to the nature and concentration of the surface active sites, especially the coordinately unsaturated tin cations Sncus and oxygen vacancies VO. In an atmosphere of pure air, an adsorption layer is formed on the surface of nanoparticles. It consists of adsorbed water molecules H2O, hydroxyl groups OH and chemisorbed oxygen in the forms of molecular O2, O2−, and/or atomic species O−, O2− [3]. Chemisorption of gas molecules occurs along with the charge transfer between a surface site and an adsorbate which induces the effect of electron energy band bending in a subsurface layer (with the thickness close to Debay length) [4]. Chemisorbed oxygen can interact with the components of the gas phase. Such chemical reactions change the charge distribution between the surface and the bulk of semiconductor and cause an increase or a decrease of conductivity which is measured as a sensor signal. For the synthesis of SnO2 nanocrystals with controlled crystallite size, the “­bottom-up” and “top-down” approaches have been developed [1]. “Bottom-up” concept includes a broad group of methods such as chemical precipitation, sol-gel, freeze-drying, hydrothermal, green synthesis, aerosol combustion and flame-spray pyrolysis, atomic layer deposition (ALD), electrospinning, chemical vapor deposition (CVD), laser deposition, and reactive magnetron sputtering. “Top-down” methods include high-energy milling and chemical etching methods. The size and morphology of nanocrystals are the important factors determining the reactivity of nanocrystalline tin dioxide. Nanocrystalline SnO2 with various morphologies can be obtained: quasi one-dimensional (1D) nanowires, nanofibers, nanorods, nanotubes, two-dimensional (2D) nanoplates, three-dimensional (3D) nanospheres, and dendrites. Poor selectivity is the most critical feature of functional materials based on nanocrystalline tin dioxide. Improvement of specificity of the chemical reactivity of tin dioxide has been achieved via surface modification with clusters or nanoparticles of noble metals (Pd, Pt, Au), noble metal oxides (RuO2, Rh2O3, PdOx, PtOy), or transition metal oxides (La2O3, CeO2, V2O5, Cr2O3, Mn2O3, Fe2O3, Co3O4, NiO, CuO, Ag2O) [2, 5, 6]. The reasonable choice of modifiers is complicated by complex and incompletely understood roles of catalytic clusters in the gas-solid interactions. The modifiers are usually distributed between the surface and the bulk of nanocrystals. The additives affect the size and degree of agglomeration of SnO2 crystallites and significantly alter the surface composition and the concentration of adsorption centers. Selectivity of adsorption and surface reactions of modified SnO2 is the result of the synergetic effect of a number of factors: the formation of acid or basic centers, the activation of chemisorbed molecules, and in some cases of the spillover effect. In view of the wide variety and high concentration of structural defects on the surface, it is not completely clear which of surface sites are actually “active,” that is, directly react with gas molecules and what are the mechanisms of these reactions. The choice of a modifier can be justified by knowledge on acid-base behavior, Sanderson electronegativity and optical basicity scales [7–9]. In most works, the choice of a modifier was carried out by the “trial and error” method without analyzing the correlation of the physical-chemical properties of the modifier, nanostructure parameters, and

Tin oxide nanomaterials: Active centers and gas sensor properties165

f­ unctional properties of the materials. The main difficulties in the experimental study of the reactivity of nanocrystalline SnO2 are due to the chemical lability of the surface centers and of the corresponding electronic states. The studies should be performed under controlled conditions in the presence of target molecules with using of in situ or operando techniques. In this section, the data are systematized on the effect of catalytic modifiers on the composition, microstructure, surface active sites, and the gas sensor performances of nanocrystalline tin dioxide.

7.2 Structure of nanocrystalline SnO2 Bulk crystal structure of SnO2 depends on the type of polymorphic phase. Tin dioxide with tetragonal rutile structure (space group P42/mnm) occurring naturally as the cassiterite mineral is commonly available and stable [1]. The unit cell contains six atoms: two tin atoms and four oxygen atoms (Fig. 7.1). Each tin cation is surrounded by six oxygen anions located at the apexes of regular octahedron, and each oxygen is surrounded by three tin atoms located at the apex of an equatorial triangle. The parameters of the unit cell are a = b = 4.7374 Å and c = 3.1864 Å [2]. The ionic radii of O2− and Sn4 + are equal to 1.40 and 0.71 Å, respectively [10]. The structure of tin dioxide is represented by 3D periodic slabs of three types, forming structural blocks with nominal composition {(O2−)(Sn2O24+)(O2−)}. The ideal stoichiometric surface could be obtained by cleavage of the structure along the boundaries of the blocks which leads to the complete correspondence of the atomic coordination (bond lengths, angles, etc.) on the surface with that in the bulk (Fig. 7.2). The stoichiometric surface is that with the ratio of the number of oxygen and tin ions [O2−]:[Sn4+] = 2. The most stable surface is that possessing lower free energy and is formed by the (110) crystal plane of SnO2 [1, 2]. On the ideal (110) surface, fivefold (Sn4+5c) and sixfold coordinated (Sn4+6c) tin cations are present in equal amounts. The outermost surface layer is formed by bridging oxygen anions (O2−b) which form rows in the [001] direction and occupy the bridge positions between Sn4+6c cations

Fig. 7.1  Unit cell of SnO2 with tetragonal rutile structure. Adapted with permission from Das S, Jayaraman V. SnO2: A comprehensive review on structures and gas sensors. Progr Mater Sci 2014;66:112–255.

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Fig. 7.2  The structure of the stoichiometric (left) and reduced (right) surfaces of single crystal SnO2. With permission from Batzill M, Diebold U. The surface and materials science of tin oxide. Progr Surf Sci 2005;79:47–154.

(Fig.  7.3A). The stoichiometric SnO2 (110) surface is nonpolar and bears no net charge [1]. The formation of a stoichiometric surface was studied on the (110) plane of SnO2 single crystals grown from vapor using different techniques: low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ultraviolet (UV) spectroscopy, and ion scattering spectroscopy. The formation of a stoichiometric surface was observed after annealing a cleaved single crystal in oxygen at pO2 > 1 Torr and T = 700 K. The criterion for the stoichiometric surface formation was the absence of peaks in the SnO2 band gap in the UV spectra [11–14]. The real stoichiometric surface of tin dioxide undergoes a certain change in the ionic coordination at the surface and in the near-surface region in comparison with the bulk SnO2 structure. Such a surface relaxation does not affect the translational symmetry but leads to a decrease in the surface energy [15]. The structural characteristic of a real surface is especially important for understanding the physical properties and reactivity of nanocrystalline tin dioxide. The estimated ratio of the number of surface (NS) and bulk atoms (NV) is Ns 4 ( D / 2) r3 8r = × = 2 3 Nv r ( D / 2) D 2

(7.1)

where D is particle diameter and r is average atomic radius. For SnO2 nanocrystal with average diameter D = 3–5 nm, the ratio reaches 30% implying that the surface

Tin oxide nanomaterials: Active centers and gas sensor properties167

Fig. 7.3  Model of stoichiometric (A) and partially reduced (B) SnO2 (110) surface showing the vacancies of bridging oxygen (1) and in-plane oxygen (2).

contribution to the crystal structure is noticeable. The feature of the real SnO2 surface is the possibility of a reversible transition of tin cations from the electronic state Sn+4 (stoichiometric composition) to Sn+2 (oxygen deficiency) depending on conditions. Such change leads to distortion of the crystal structure and to the formation of oxygen vacancies VO in the near-surface layer. Two types of oxygen vacancies VO are distinguished: vacancies of bridging oxygen anions [Fig. 7.3B, (1)] and vacancies of in-plane anions [Fig. 7.3B, (2)]. Since the formation of such defects on the surface formally leads to a decrease in the oxidation state of tin cations, such surface is called reduced. The surface reduction leads to reconstruction of near-surface layers via the formation of ordered oxygen-deficient superstructures [2, 16]. Oxygen vacancies influence the semiconductor behavior of SnO2 via the formation of donor electronic states in the band gap above valence band maximum, with the more prominent effect being due to the bridging oxygen vacancies [1]. For bulk materials, the formation of atomic defects and deformation of the near-­ surface layers does not affect the results of structure analysis. However, for nanoscaled materials, a significant impact of the surface leads to broadening of X-ray diffraction (XRD) spectra (Fig.  7.4) and Raman spectra (Fig.  7.5). Experimentally, the influence of the grains size (dXRD) and the specific surface area (SBET) on the structure of SnO2 nanocrystals was studied by the Raman spectroscopy [17]. The Raman spectra of SnO2 nanocrystals obtained at different annealing temperatures and differing in the crystallite size and the specific surface area are shown in Fig. 7.5. The peculiarity of Raman spectra of nanocrystalline SnO2 is the broad feature between 400 and 700 cm−1 referred to as the band of surface modes. For large SnO2 crystals of in this region, the

Fig. 7.4  (A) XRD spectra of nanocrystalline SnO2 annealed at different temperature. Asterisks mark the reference peak positions of cassiterite phase. (B) Particle size (dXRD) and specific surface area (SBET) in relation to annealing temperature of nanocrystalline tin dioxide.

dXRD = 3–6 nm

dXRD = 10–12 nm

dXRD = 16–20 nm dXRD = 26–35 nm A1g

I B2g

Eg 400

600

800 400

600

800 400

600

800 400

600

800

Raman shift (cm–1)

Fig. 7.5  Raman spectra of nanocrystalline SnO2 obtained at different annealing temperature: 300°C (A), 500°C (B), 700°C (C), and 1000°C (D). With permission from Rumyantseva MN, Gaskov AM, Rosman N, Pagnier T, Morante JR. Raman surface vibration modes in nanocrystalline SnO2: correlation with gas sensor performances. Chem Mater 2005;17:893–901.

Tin oxide nanomaterials: Active centers and gas sensor properties169

bulk modes were detected. The origin of surface modes has been ascribed to the loss of long-range periodicity in nanocrystals of low particle size of 4–9 nm or due to the existence of a lattice distortion in the near-surface layer. The spectrum of sample of grain size dXRD = 26 nm annealed at 1000°C is similar to the spectrum of polycrystalline SnO2. Three of the four modes active in the Raman spectra of single-crystal tin dioxide are present: A1g (629 cm−1), Eg (476 cm−1), and B2g (772 cm−1).

7.3 Types of active centers on the surface of nanostructured SnO2 The active center is defined as a local area on the surface, which has specific chemical properties. Individual atoms on the surface, groups of atoms with various compositions, adsorbed molecules and their derivatives, as well as point defects or extended dislocations of the surface structure can act as active centers [18, 19]. These sites play the key role in adsorption and reactivity of solids due to the presence of dangling bonds and unsaturated coordination of the surface atoms. Active centers act as actuators of chemical information in sensor materials. On the one hand, surface sites interact with ambient-phase molecules. On the other hand, these sites are in electronic contact with the semiconductor bulk. Taking into account the structure of a metal oxide, the following types of intrinsic surface sites can exist on the surface: coordinately unsaturated metal cations and oxygen anions as well as atomic defects (cation and anion vacancies, interstitial atoms). Due to contact with atmosphere, two more types of sites are formed on the surface: chemisorbed oxygen species and hydrate-hydroxyl layer (HHL), including all kinds of molecular and dissociated derivatives of water adsorption. Modification by catalytic clusters of noble metals or oxides is focused on the formation of extrinsic active centers. Schematically, the types of active sites on the surface of modified tin dioxide are summarized in Fig. 7.6.

Extrinsic

Intrinsic Paramagnetic

M

Lewis Broensted

Oxidative

O

–

O 2,chem O –2

Reductive ·OH Vo

Sn4+ cus

OH

Sn-OH

Modifier

Acid sites

Hydratehydroxyl layer

H2O

Fig. 7.6  Types of active sites on the surface of modified SnO2. With permission from Marikutsa AV, Vorobyeva NA, Rumyantseva MN, Gaskov AM. Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO2 and gas sensitivity, Rus Chem Bul 2017;66(10):1728–64.

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Depending on the chemical reactivity, the active sites can be roughly divided into acid-base (acid) and oxidation-reduction (redox) ones. The centers of Lewis acidity are coordinately unsaturated metal cations, the basic sites are surface oxygen anions O2−. Broensted acid sites possessing mobile protons are chemisorbed OH-groups or protonated water molecules. The hydroxyl species as well as chemisorbed oxygen are typical oxidation sites on the surface of metal oxide sensors and catalysts. The type and concentration of active sites on the surface of tin dioxide are determined by the synthesis conditions and the content of bulk dopants and surface modifiers. In the following sections, these relationships are analyzed for nanocrystalline SnO2 synthesized by precipitation from aqueous solutions with subsequent annealing at T = 300–1000°C [20–23]. In all cases, the samples obtained contained only one crystalline phase of SnO2 (cassiterite). With increasing annealing temperature, the average size of the SnO2 crystallites increases, and the specific surface area, on the contrary, decreases (Fig. 7.4).

7.4 Acid-base centers On the surface of tin dioxide, there are two types of acid centers: Lewis (strong) and Broensted (weak) centers. Lewis centers are distinguished by their ability to be donors (base centers) or acceptors (acid sites) of an electron pair when interacting with gas molecules. Broensted acid-base centers interact with molecules through the exchange of a proton (H+). Broensted acid is a site which donates a proton to chemisorbed molecules. On the contrary, Broensted base is a proton acceptor in the interaction. Lewis centers on the SnO2 surface are represented by: (1) Acid centers—coordinately unsaturated tin cations Sn4+5c with coordination number 5 (on the stoichiometric surface) and Sn2+4с with coordination number 4 (on the reduced surface) [24–26] and (2) Base centers—surface oxygen anions O2−.

Broensted centers are acidic bridging OH-groups formed via the protonation of bridging anions O2−b (Fig.  7.7) and terminal OH-groups that originate from water molecules dissociation and can be both acidic and basic Broensted sites. H O

H

H

O O M

H

H

O M

M

H O

O M

M

M

Fig. 7.7  Scheme of dissociative adsorption of water molecules on the surface of a metal oxide with the formation of bridging and terminal hydroxyls. With permission from Marikutsa AV, Vorobyeva NA, Rumyantseva MN, Gaskov AM. Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO2 and gas sensitivity, Rus Chem Bul 2017;66(10):1728–64.

Tin oxide nanomaterials: Active centers and gas sensor properties171

The experimental methods of investigation of the oxidation state and coordination environment of surface cations and anions include X-ray photoelectron spectroscopy (XPS) [27], electron paramagnetic resonance (EPR) [28], and Mossbauer spectroscopy [29]. The reactivity of acid centers can be probed using base molecules (ammonia, amines, and pyridine) under in situ conditions by the method of spectrophotometric titration in the presence of indicators [30, 31], thermal desorption spectrometry (TDS) [32], and temperature-programmed desorption (TPD) [30]. The indicator-based methods evaluate the strength of acid-base sites using Hammett parameter (H0) and thermal desorption methods—by the temperature at which the probe molecules desorb from the surface. Ammonia NH3 is a convenient probe molecule in the study of the acid properties of the surface of nanocrystalline tin dioxide by TPD. The thermodynamic condition for desorption is ∆Gdes = ∆H des – T ∆Sdes < 0

(7.2)

where enthalpy of desorption is always ΔHdes > 0 (endothermic process) and ΔSdes > 0, since during desorption the entropy of the system increases due to an increase in the concentration of gas-phase molecules. The enthalpy of desorption is determined by the binding energy of molecules with adsorption centers. Since the entropy factor depends little on the type of adsorption sites, the temperature T ≥ ΔHdes/ΔSdes at which desorption of NH3 is possible can be considered a characteristic of the strength of acid sites. The concentrations of acid sites of different strengths can be determined using the model [33], which assumes that each NH3 molecule is desorbed from a single acid site. The fourfold coordinated cations Sn2+4c on a reduced SnO2 surface can only coexist with oxygen vacancies in the nearest environment. However, under normal conditions, the oxygen deficiency in tin dioxide is low (10−4–10−3 at%). Thus, the experimental observation of Sn2+4c is possible only after reducing treatment, for example, annealing in vacuum, heating in a reducing gas, or etching with argon plasma [29]. Tin dioxide treated in vacuum at 450°C showed an infrared (IR) absorption peak of the Sn2+-O2 bond at ν = 1060 cm−1 after contact with oxygen [19]. The adsorption properties of surface cations Sn2+4c and Sn4+5c were established with respect to both the acceptor molecules: oxygen [34, 35], nitrogen dioxide [36], and donor molecules: CO [37], H2O [38], and NH3 [24, 39]. It is assumed that the adsorption capacity of Sn2+4c centers is higher than that of Sn4+5c resulting in higher binding energy to adsorbates [24, 34, 40, 41]. The Lewis acidity of surface tin cations is due the presence of unoccupied electron orbitals. It favors the chemisorption of gas molecules having lone electron pairs: ammonia [24, 39], water, ethanol [39, 42], and oxygen [34]. The ability of tin cations to be in two oxidation states determines the redox properties of the SnO2 surface [1, 19]. Reduction of Sn4+ to Sn2+ can be directly from the chemisorption of donor molecules or indirectly due to trapping the electrons released by lattice anions while the oxygen vacancy formation [43]. In turn, Sn2+ cations exhibit reducing behavior determining the high adsorption capacity of the oxygen-deficient SnO2 surface to the acceptor gas molecules [34, 36].

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7.4.1 Influence of synthesis conditions and microstructure parameters on concentration of acid centers of different types Surface acidity of nanostructured SnO2 with different grain size was investigated by TPD of ammonia. The TPD profiles (Fig. 7.8) show the temperature dependence of NH3 desorption rate from the surface of SnO2 preliminarily saturated with ammonia. Desorption of NH3 from weak (Broensted) acid sites (OH-groups) takes place in the low-temperature range (T < 200°C) [39, 44, 45]. At higher temperature (T > 200°C),

Fig. 7.8  Profiles of NH3 TPD from the surface of nanocrystalline SnO2 with different microstructure parameters. With permission from Marikutsa AV, Vorobyeva NA, Rumyantseva MN, Gaskov AM. Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO2 and gas sensitivity, Rus Chem Bul 2017;66(10):1728–64.

Tin oxide nanomaterials: Active centers and gas sensor properties173

ammonia molecules are desorbed from Lewis acid sites—coordinately unsaturated tin cations. The Lewis acid centers with different strength were distinguished: medium (NH3 desorption at T = 200–400°C) and strong (NH3 desorption at T > 400°С) [39]. It is assumed that on a reduced SnO2 surface, the sites with medium strength are Sn4+ cations and strong centers are Sn2+ ones [24]. However, for nanocrystalline SnO2 in air, the presence of Sn2+ cations available for NH3 adsorption is unlikely, since these electron-donor sites would be blocked by chemisorbed oxygen. Thus, the presence of Lewis sites with different acid strength suggested by the wide NH3 desorption peaks at T = 300–600°C (Fig. 7.8) can be explained by differences in the local coordination of Sn4+ cations on the surface of SnO2 nanocrystals. No effect on the concentration of Broensted sites was observed with the change of average crystallites (Table 7.1). With the increase of SnO2 particle size, the total number of Lewis centers decreased and the portion of medium strength acid centers increased. The reason for this might be the decline of the amount of coordinately unsaturated cations following the coarsening of SnO2 microstructure. The number of defects and of strongly acidic Sn4+ cations with low coordination numbers decreases with the rise in the degree of the oxide crystallinity [39, 44]. On the contrary, the impact of “regular” surface Sn4+ cations having an almost saturated coordination environment and weaker acidity increases with the growth of SnO2 crystallites.

7.5 Chemisorbed oxygen Chemisorbed oxygen participates in the redox processes on the surface of tin dioxide. Different species are considered when studying oxygen chemisorption on the oxide surface: (a) physically adsorbed molecules O2,phys held by van der Waals force; (b) chemisorbed molecules O2,chem bound to tin cations by a covalent bond through local charge transfer; and (c) ionic species of molecular O2−, O22− and atomic O−, O2− oxygen.

The type of chemisorbed oxygen species on SnO2 surface has been established to depend on temperature: (1) at T < − 70°C, the physical adsorption is predominant [46]; (2) in the range from room temperature to 150–200°C molecular chemisorption takes place yielding uncharged species O2,chem [34] and anionic ones O2− [2, 47]; (3) at T = 200–400°C, ionosorption with the formation of O− and O2− [2, 34]; and (4) at T > 400°С—complete ionization of the chemisorbed atomic species to O2− [25].

Oxygen chemisorption on SnO2 (110) surface was modeled by first-principles calculations within the density functional theory (DFT). The experimental investigation was carried out on the nanostructured tin dioxide by spectral methods: electron paramagnetic resonance (EPR) [48], infrared (IR) spectroscopy [19], XPS [49], thermal desorption [50], and by electrical conductivity [34, 49, 51] and the work function measurements [34, 52].

174

Table 7.1  The concentration of acid sites on the surface of SnO2 with different annealing temperature and particle size, doped by Sb and modified by catalytic additives of PdOx and RuOy (1 wt%) Acid sites concentration, 10−6 mol/m2 Lewis dXRD, nm

Modifier

Broensted (weak)

Medium

Strong

Total

3–6 3–5 3–6 3–5 3–5 3–5 2–3 3–6 10–12

– PdOx

500

– – 0.5 1.0 2.0 4.0 8.0 – –

700

–

16–20

1000

–

26–35

0.6 ± 0.2 0.9 ± 0.2 1.2 ± 0.2 1.0 ± 0.2 0.8 ± 0.2 1.0 ± 0.2 1.2 ± 0.2 0.6 ± 0.2 0.5 ± 0.2 0.8 ± 0.2 0.5 ± 0.2 0.5 ± 0.2 0.9 ± 0.2 0.6 ± 0.2 0.6 ± 0.3 0.9 ± 0.4 0.6 ± 0.3

1.3 ± 0.2 2.7 ± 0.4 3.1 ± 0.3 2.9 ± 0.3 2.9 ± 0.3 3.3 ± 0.3 3.4 ± 0.4 2.1 ± 0.2 0.9 ± 0.2 2.1 ± 0.3 1.3 ± 0.2 1.0 ± 0.2 1.3 ± 0.2 1.3 ± 0.2 0.5 ± 0.3 1.1 ± 0.5 0.8 ± 0.4

1.1 ± 0.3 0.5 ± 0.1 0.6 ± 0.2 0.1 0.1 0.1 0.6 ± 0.1 – 0.6 ± 0.2 – – 0.2 ± 0.1 – – 0.2 ± 0.1 – –

2.4 ± 0.5 3.2 ± 0.5 3.7 ± 0.5 3.0 ± 0.4 3.0 ± 0.4 3.4 ± 0.4 4.0 ± 0.5 2.1 ± 0.2 1.5 ± 0.4 2.1 ± 0.3 1.3 ± 0.2 1.2 ± 0.3 1.3 ± 0.2 1.3 ± 0.1 0.7 ± 0.4 1.1 ± 0.5 0.8 ± 0.4

Тanneal, С 300

RuOy – PdOx RuOy – PdOx RuOy – PdOx RuOy

Tin Oxide Materials: Synthesis, Properties, and Applications

Sb content, at%

о

Tin oxide nanomaterials: Active centers and gas sensor properties175

DFT predicts that on the stoichiometric SnO2 (110) surface, the adsorption of oxygen is endothermic with the binding energy of Sn4+5c-O2 − 0.02 eV [53]. On the ­oxygen-deficient surface, the adsorption is energetically favorable due to charge transfer from Sn2+ cations either directly in the case of binding to Sn2+4c, or mediated by the coordination of O2 on neighboring Sn4+5c cations [46]. The modeling indicated the side-on adsorbed peroxide ions O22− coordinated by both oxygen atoms to Sn2+4c cations as the most stable species [35, 39, 46, 49]. Ionic O2− form end-on coordinated to a cation and inclined to the surface was suggested as a favorable adsorbate with a lower binding energy [40]. Despite the calculations predict low activation barriers for the dissociation of O22− and higher binding energies of atomic ionosorbates than of the molecular ones, the concentration of surface O- ions is expected to be low because of instability and high electron affinity [35, 46]. However, spectroscopic studies disproved the occurrence of O22− species on the surface of SnO2 [35]. On the contrary, the presence of O2− anions at temperatures below 160°C has been shown by EPR [48, 54] and IR spectroscopy [19].

7.5.1 Influence of synthesis conditions and microstructure parameters on concentration and predominant form of chemisorbed oxygen Chemisorbed oxygen is important for gas sensing because of oxidation activity to the reducing gases (CO, H2, NH3, H2S, CH4, etc.). Atomic O− is considered as the strongest oxidative oxygen species [40, 49]. A proper method for studying chemisorbed oxygen on the surface of nanocrystalline oxides is temperature-programmed reduction (TPR) by hydrogen. The temperature at which hydrogen is consumed for the reduction of various oxidation sites is a characteristic of the oxidative activity of the surface. Evaluation of the amount of H2 consumed in definite temperature ranges allowed to quantify the concentration of chemisorbed oxygen on the surface of nanocrystalline SnO2 [55, 56]. The main H2 consumption peak at temperature T = 400–800°C in the TPR profiles (Fig.  7.9) is due to bulk reduction of SnO2 to metallic tin [55]: SnO2(TB) + 2 H2( ) = Sn( ) + 2 H2O( )

(7.3)

The amount of H2 consumed in this temperature range is independent of the samples grain size, but the reduction maxima shift to higher temperature with the increase of SnO2 particle size. This is due to kinetic difficulties in the interaction of hydrogen gas with the atoms in the solid bulk. The broad shoulder at the beginning of the high-temperature peaks indicates on the formation of intermediate tin oxides: Sn2O3, Sn3O4, and SnO. Hydrogen consumption at 100–300°C was observed only for highly dispersed materials with particle size less than 20–25 nm and specific surface area higher than 20 m2/g. This peak decreased with a decrease of surface area (Fig.  7.9). Thus, at low temperature peaks were ascribed to the reduction of oxidation centers on the surface of SnO2. They can include chemisorbed oxygen molecules O2,chem, ionized forms of O2−, O− and OH-groups. The concentration of

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Tin Oxide Materials: Synthesis, Properties, and Applications

Fig. 7.9  TPR profiles of nanocrystalline SnO2 with different microstructure parameters. With permission from Marikutsa AV, Vorobyeva NA, Rumyantseva MN, Gaskov AM. Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO2 and gas sensitivity, Rus Chem Bul 2017;66(10):1728–64.

oxidation centers (Table 7.2) was evaluated under the assumption that chemisorbed oxygen molecules are the p­ redominant species interacting with hydrogen according to the equation: O2,chem + 2 H 2 → 2 H 2 O

(7.4)

This assumption is based on the fact that at room temperature, diatomic oxygen forms predominate on the surface of SnO2 [35, 47, 57]. At the same time, the EPR method proved that the concentration of ionic adsorbates O2− on the SnO2 surface was not higher than ~ 1.5 × 10−11 mol/m2 which is six orders of magnitude less than the total concentration of oxidation centers (~ 10 μmol/m2). As shown in Table 7.2, the concentration of chemisorbed oxygen is significantly reduced when the crystallite size increases.

Tin oxide nanomaterials: Active centers and gas sensor properties177

Table 7.2  Concentration of oxidative sites O2,chem on the surface of nanocrystalline SnO2 with different annealing temperature and particle size, doped by Sb and modified by catalytic additives of PdOx and RuOy (1 wt%) Тanneal, оС

Sb content, at. %

dXRD, nm

Modifier

300

– – 1.0 1.5 2.0 3.0 4.0 6.0 7.0 – –

3–6 3–6 3–6 3–5 3–6 3–5 3–4 2–4 2–3 3–6 10–12

– Pd

500

Ru – Pd Ru

Oxidative sites concentration, 10−6 mol/m2 11.8 ± 0.4 13.6 ± 0.5 11.8 ± 0.5 14.2 ± 0.5 11.2 ± 0.5 9.6 ± 0.5 14.8 ± 0.5 13.5 ± 0.5 13.4 ± 0.5 14.9 ± 0.5 7.6 ± 0.6 8.8 ± 0.7 12.2 ± 0.5

7.6 Hydrate-hydroxyl layer HHL is formed on the surface of oxides due to water molecules adsorption. It consists of: (1) molecular adsorbed H2O and its derivatives H3O+, H5O2+ [58] and (2) dissociated hydrated species including isolated OH-groups and hydrogen-bound OH…OH associates [49].

By first-principles DFT studies, the binding energy of H2O and OH-groups to different adsorption sites on the model SnO2 (110) surface was estimated [59]. The experimental studies include IR spectroscopy for determining the type and estimating the amount of adsorbed hydrated species [58, 60]. Thermodesorption with mass spectrometry [25] and TPD of water [38] allow for quantitative analysis of the concentrations of hydrated species with distinct binding energies. According to these studies, hydroxyls are the predominant species in HHL of polycrystalline SnO2. Molecular H2O adsorption with hydrogen bonding to anions O2− at the surface was observed at low temperature. Heating above 200°C leads to water molecules desorption or dissociation into OH-groups [47]. Two types of hydroxyl groups are distinguished on the SnO2 (110) surface: terminal (OHt) and bridging ones (OHb) [60, 61]. The former are dissociation residuals of H2O molecules bound to Lewis acid sites Sn4+5c. Bridging hydroxyls groups are derived from bridging anions O2− on the SnO2 surface. Adsorption of water on SnO2 yielding terminal and bridging OH-groups [25, 60] and increasing the electrical conductivity of semiconductor is described by the following equation [62]: H 2 O + Sn 4 + + O2 − = ( Sn − OH ) + OH − + e − 4+

(7.5)

where (Sn-OH)4+ denotes terminal hydroxyl, (OH) −—bridging hydroxyl, and e− is an electron.

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Terminal OHt groups on the surface of SnO2 are regarded as isolated groups traced by the separate narrow vibration peaks in the high-frequency region of IR spectra (Table 7.3) [58, 60, 63]. In contrast, bridging hydroxyl groups form the s­ o-called families consisting of series of bridging surface anions O2− bound with delocalized protons (xO2−b…yH +) [38]. Or in another interpretation, the associates of bridged hydroxyls bonded by hydrogen bonds (OHb…OHb) [64]. Because of indefinite composition and delocalized character, the families of bridging hydroxyls cause wide absorption bands shifted toward lower frequencies in IR spectra (Table 7.3) [60, 63]. On the surface of nanocrystalline tin dioxide with large specific surface area, the composition of HHL is more diverse than on single crystals. This causes the appearance of wide bands in the IR spectra of nanocrystalline SnO2 in the wave number range 3700–2000 and 1700–1600 cm−1. Comparing the O-H vibration frequencies (Table 7.3) suggests that in the families (OHb…OHb) protons are bound to oxygen more loosely than in terminal OHter groups. Thus, bridged hydroxyl groups are stronger Boensted acids [60]. The acidity of the OH-groups facilitates ammonia adsorption [39, 44]. Besides, surface hydroxyls possess oxidation activity in interaction with CO [25, 62, 65]. The oxidation behavior of hydroxyls was reported to be more pronounced than that of ionic chemisorbed oxygen [66]. Bridging hydroxyls can oxidize H2 enriching the families (OHb…OHb) with protonated species on the surface of SnO2 [60].

7.6.1 Influence of synthesis conditions and microstructure parameters on HHL Adsorbed water molecules can undergo a number of transformations on nanocrystalline SnO2 surface yielding various hydrated species. Eq. (7.5) illustrates the formation of Broensted acid sites (bridging OH-groups) and paramagnetic ∙ OH centers. The most common data on the qualitative content of HHL can be obtained by IR absorption spectroscopy. The spectra of nanocrystalline SnO2 samples with different microstructure parameters are shown in Fig. 7.10. To compare the intensities of the bands of surface species (3600–1000 cm−1), the IR spectra are normalized at the peaks of lattice Sn-O-Sn vibrations (630 cm−1). Comparing the observed IR bands with the reference Table 7.3  The vibration wave numbers of hydrated species on the surface of nanocrystalline tin dioxide in the IR spectra Hydrated species

Wave number, cm−1

Vibration

Reference

OHt

3630, 3662–3700, 3728–3780 3555, 3524, 3483 3400–3200 3400–3200 1620 3300–3150, 2650–2470 1700–1670 3000–2850, 2250–2200 1700–1660

ν(O-H)

[58, 60, 63]

ν(O-H) ν(O-H) ν(O-H) δ ν(O-H) δ ν(O-H) δ

[58, 63] [25, 58, 60, 63] [25, 60]

OHb OHb…OHb H2O H3O+ H5O2+

[58] [58]

Tin oxide nanomaterials: Active centers and gas sensor properties179

Fig. 7.10  IR absorption spectra of nanocrystalline SnO2 with different microstructure parameters. With permission from Marikutsa AV, Rumyantseva MN, Konstantinova EA, Shatalova TB, Gaskov AM. Active sites on nanocrystalline tin dioxide surface: effect of palladium and ruthenium oxides clusters. J Phys Chem C 2014;118:21541–9.

data in Table 7.1, it can be concluded that the HHL of tin dioxide includes dissociated hydrated species: terminal OH-groups, isolated bridging OH-groups, families of bridging hydroxyls OH…OH bound by hydrogen bonds, as well as molecular species: H2O, H3O+, and H5O2+ [20, 67]. The overlapped IR peaks make it difficult to distinguish the impacts from various adsorbates in the HHL of nanocrystalline tin dioxide. Thermodesorption spectroscopy allows to estimate the total contents of dissociated and molecular hydrated species, since they differ in the bond strength with the surface and have different desorption temperature [38]. Fig. 7.11 shows the thermal analysis of nanocrystalline SnO2 with different grain size. Comparison with the data of mass spectrometry suggests that in the low-temperature range (50–200°C), the mass loss is due to desorption of molecular hydrated species, while at higher temperature (250–600°C) desorption of OH-groups bound with oxide surface by stronger covalent bonds [67]. Calculation of the concentrations of molecular (H2O) and dissociated (OH) species from the mass loss confirmed that the latter were predominant on the surface of nanocrystalline SnO2 (Table 7.4). The total content of OH-groups on the surface of highly dispersed SnO2 with a particle size of 3–6 nm and a specific surface area of 90–100 m2/g was about 10 μmol/m2 in which not more than 10% of hydroxyls exhibited Broensted acidity and only a tiny part (~ 10−4%) were the paramagnetic centers ∙ OH.

7.7 Paramagnetic centers Paramagnetic active centers on the surface of gas-sensitive oxides are of particular interest due to the presence of unpaired electrons and chemical reactivity. These sites

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3 100

2

m (%)

99

-H2O

98

-OH 1

97 0

100

200

300

400

500

600

700

800

T (°C)

Fig. 7.11  Thermal analysis of nanocrystalline SnO2 with different microstructure parameters: (1) dXRD = 3–6 nm, SBET = 90–100 m2/g, (2) dXRD = 10–12 nm, SBET = 20–25 m2/g, and (3) dXRD = 16–20 nm, SBET = 5–10 m2/g. The ranges corresponding to desorption of molecular and dissociated hydrated species are demarked. With permission from Marikutsa AV, Rumyantseva MN, Konstantinova EA, Shatalova TB, Gaskov AM. Active sites on nanocrystalline tin dioxide surface: effect of palladium and ruthenium oxides clusters. J Phys Chem C 2014;118:21541–9. Table 7.4  Concentration of hydrated species on the surface of nanocrystalline SnO2 annealed at 300 0C (dXRD = 3–6 nm) and modified by catalytic additives PdOx and RuOy (1 wt%) Hydrated species concentration, 10−6 mol/m2 Modifier

Dissociated OHsurf

Molecular H2Osurf

– PdOx RuOy

8.5–8.8 14.0–14.5 12.0–12.4

3.0–3.5 4.1–4.7 3.5–4.0

can significantly affect the interaction with gas molecules, but because of lability they occur in a small concentration and are difficult for experimental observation. DFT calculations suggest the formation of such paramagnetic centers as partially ionized vacancies of oxygen [35, 40] and ionic species of chemisorbed oxygen [49] on the SnO2 (110) surface. The experimental method of direct research of paramagnetic centers is EPR spectroscopy. However, interpretation of EPR data can be ambiguous. For example, the ERP signal with g = 1.95–1.96 in the spectra of tin dioxide has been attributed to oxygen vacancies VO− [68] (Table 7.5), but in earlier works it was referred to the cations Sn3+ [28]. Indirect results can be obtained by other experimental methods: electrical conduction measurements [43, 70], impedance [71], and luminescence spectroscopy [72]. For single crystal SnO2, the following paramagnetic sites are typical: ionosorbed forms of oxygen (O2−, O−) and single-charged oxygen vacancies (VO−). Oxygen ­vacancies can be found both on the surface and in the volume of the oxide. As surface sites, ­single-charged oxygen vacancies can be the adsorption centers for acceptor molecules [35, 71]. In the presence of air, oxygen is adsorbed yielding ionic chemisorbed species O2−:

Tin oxide nanomaterials: Active centers and gas sensor properties181

Table 7.5  Types of paramagnetic centers in polycrystalline SnO2 and corresponding g-factors Paramagnetic center −

O2 O− (existence was not proven) ∙OH VO− Sn3+

(

VO− + O2 = VO − O2−

)

g-Factor

Reference

g1=2.0210, g2=2.0030, g3=1.9833 g=2.002 g1=2.0021, g2=g3=2.0009 g=1.9812 g=1.98

[54] [69] [54] [54, 68] [28]

(7.6)

In general, the description of these paramagnetic centers in single-crystal tin dioxide is based on first-principles methods using the oxygen-deficient SnO2 (110) model surface. Filling the surface vacancy by the O atom is one of the pathways to dissociative adsorption of oxygen on defect pairs Sn2+4c-VO or Sn4+5c-VO2− [35, 43]. Oxygen adsorption stimulates the diffusion of lattice oxygen vacancies and the consequent long-term drift of electric conductivity of SnO2 [70, 73]. According to the DFT calculations, the presence of CO or H2O vapor in air raises the concentration of oxygen vacancies on the surface [70]. Modeling the NO2 interaction with the reduced SnO2 surface (110) shows that the vacancies of bridging oxygen contribute largely to charge transfer to the adsorbate and provide higher adsorption energy than the vacancies of in-plane oxygen [36, 72]. Investigation of nanocrystalline tin dioxide by EPR showed that the predominant type of paramagnetic sites on the surface is molecular ionic chemisorbed oxygen O2− [54]. Besides, paramagnetic hydroxyl groups were found on the surface of SnO2 (Fig. 7.12). The concentration of ∙ OH centers correlates with the relative humidity of air (Fig. 7.13). This indicates on equilibrium with adsorbed water molecules:

Fig. 7.12  Experimental EPR spectrum of nanocrystalline tin dioxide (1) and simulated signals of the paramagnetic centers O2− (2) and ∙OH (3). With permission from Konstantinova EA, Pentegov IS, Marikutsa AV, Rumyantseva MN, Gaskov AM, Kashkarov PK. EPR study of nanocrystalline tin dioxide. Phys Stat Sol C 2011;8:1957–60.

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Fig. 7.13  (A) ERP spectra of nanocrystalline tin dioxide exposed to different relative humidity: 100% (1), 20% (2), and 0% (3). (B) Concentration of ∙OH centers on the SnO2 surface in relation to relative humidity. With permission from Konstantinova EA, Pentegov IS, Marikutsa AV, Rumyantseva MN, Gaskov AM, Kashkarov PK. EPR study of nanocrystalline tin dioxide. Phys Stat Sol C 2011;8:1957–60.

H 2 O + O2 − ↔ OH ⋅ +OH − + e −

(7.7)

The electron release in this process agrees with the increase of electrical conductivity of tin dioxide with increasing humidity [25, 47]. Within the model of Heiland and Kohl [74], the conductivity increase is due to the formation of electron-deficient adsorbate (Sn-OH) as a result of the dissociative adsorption of water on the SnO2 surface. In the EPR spectra of nanocrystalline tin dioxide, the signals of single-charged oxygen vacancies VO− were detected, but only at liquid helium temperature (5 K) [75]. Temperature affects the frequency of phonon oscillations of the solid lattice that limits the lifetime of an excited state to spin-lattice relaxation. The lifetime of the excited spin (Δτ) is inversely proportional to the width of the EPR signal (ΔH), according to the uncertainty principle [76]:

Tin oxide nanomaterials: Active centers and gas sensor properties183

(7.8)

∆E ⋅ ∆τ = g ⋅ µ B ⋅ ∆H ⋅ ∆τ ≥ 

Given a constant concentration of spin centers the signal broadening causes the intensity decrease. Thus, the signals of paramagnetic centers in the bulk of a solid have large broadening depending on temperature. The EPR spectra of nanocrystalline SnO2 recorded at T = 5 K exhibit a low-intensity broad signal of vacancies VO− along with narrow and intense signals of O2− and ∙ OH (Fig. 7.14). Thus, the paramagnetic centers VO− are in the bulk of SnO2 nanoparticles, while the latter ones related to adsorbed oxygen and water species are at the surface. When used in sensors, SnO2 is heated to 200°С–450°С. According to the ERP, at these temperatures the surface sites O2− and ∙ OH persisted on the surface of nanocrystalline SnO2 and no new signals appeared, but the concentrations could not be estimated due to strong peaks broadening. There is no direct experimental data on the chemical activity of paramagnetic centers on the surface of nanocrystalline SnO2 upon interaction with gases. The centers O2− are characterized by oxidative properties, as well as other species of chemisorbed oxygen. The hydroxyl centers ∙ OH likely behave as oxidative sites, at least in interaction with reducing gases H2 or CO [62, 69].

7.7.1 Relationships between the parameters of SnO2 microstructure and the type and concentration of paramagnetic centers The paramagnetic centers OH ∙ and O2− on the surface of nanocrystalline tin dioxide were detected as a complex signal in the range H = 3350–3360 G in EPR spectra, its intensity increasing with a decrease of particle size and an increase of the specific surface area (Fig.  7.15). The total concentration of paramagnetic surface sites

O2–, OH.

VO

g = 1.9812

IEPR 3240

.

3280

3320

3360

3400

3440

3480

H/G

Fig. 7.14  EPR spectrum of nanocrystalline tin dioxide registered at 5 K.

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Tin Oxide Materials: Synthesis, Properties, and Applications

1

IEPR

2

3

3320

3360

3340

3380

H/G

Fig. 7.15  EPR spectra of nanocrystalline tin dioxide with different microstructure parameters: (1) dXRD=3–6 nm, SBET = 90–100 m2/g, (2) dXRD=16–20 nm, SBET=5–10 m2/g, and (3) dXRD=35–50 nm, SBET=1–5 m2/g. With permission from Konstantinova EA, Pentegov IS, Marikutsa AV, Rumyantseva MN, Gaskov AM, Kashkarov PK. EPR study of nanocrystalline tin dioxide. Phys Stat Sol C 2011;8:1957–60.

increases by two orders of magnitude with a decrease of SnO2 particles size from 35–50 to 3–6 nm and a corresponding increase of the specific surface area from 1–5 to 100–110 m2/g (Table 7.6). On the surface of highly dispersed SnO2 (particle size 3–6 nm), the concentrations of paramagnetic sites are n(O2−) = 1.3 × 10−11 mol/m2 and n(∙ OH) = 2.5 × 10−12 mol/m2. The equivalent coverage of SnO2 by ionic adsorbates O2− is ~ 10−6 monolayers assuming the end-on orientation to the surface and the radius as in the ion O− (1.76 Å). It is below the Weisz limit for coating the semiconductor with charged adsorbates (10−2–10−3 monolayer [77]). The concentration of oxygen vacancies VO− in this material was estimated as 2 × 1016 g−1.

Table 7.6  Concentration of paramagnetic centers on the surface of nanocrystalline tin dioxide annealed at 300°C (dXRD = 3–6 nm) and modified by catalytic additives PdOx and RuOy (1 wt%) Paramagnetic center concentration, 10−6 mol/m2 Modifier

ОН∙

O2−

VO−

– Pd Ru

2.6 × 10−6 4.9 × 10−6 2.1 × 10−6

1.4 × 10−5 8.7 × 10−6 1.5 × 10−5

3.4 × 10−4 – –

Tin oxide nanomaterials: Active centers and gas sensor properties185

7.8 Effect of doping impurities on active sites of nanocrystalline SnO2 Doping impurities are introduced into tin dioxide to modify the electronic properties of the material. Usually, cations at a higher oxidation state than Sn4+ are used, provided that the ionic radii are close. Doping with fluoride-anions is also of use for producing transparent conducting films. As a cationic dopant to SnO2 antimony is most widely used [2, 77]. When the dopant content is within the solubility limits in the oxide bulk the electric conductivity correlates with the impurity concentration and the surface properties are unaffected. At higher dopant content, the additive is distributed between the surface and the bulk of the oxide grains. In this case, the surface properties depend on the concentration of impurity and the bulk semiconductor parameters do not change. Here, we analyze the properties of nanocrystalline SnO2(Sb) depending on the concentration of antimony in the range 0–10 at%. The effect of the dopant content on the materials microstructure parameters is summarized in Fig.  7.16. For the samples obtained at the same annealing temperature, the grain size decreases with increasing the Sb content. This may be due to segregation of the additive on the surface of SnO2 grains which inhibits the particle growth [44]. The solubility of antimony in nanocrystalline tin dioxide has been estimated to 3–6 at% from the conductivity dependence on the dopant concentration [78–81]. Methods for structure analysis are not effective to unveil the distribution of Sb in Tanneal = 700°C Tanneal = 500°C Tanneal = 300°C

20

dXRD (nm)

15

10

5

0 0

2

4

6

8

10

[Sb]:[Sn+Sb] (at. %)

Fig. 7.16  The effect of annealing temperature and Sb content on the grain size dXRD of nanocrystalline SnO2(Sb). With permission from Marikutsa AV, Vorobyeva NA, Rumyantseva MN, Gaskov AM. Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO2 and gas sensitivity, Rus Chem Bul 2017;66(10):1728–64.

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n­ anocrystalline SnO2 due to the close ionic radii of Sb5+ and Sn4+ (0.65 and 0.69 Å, respectively [10]). Thus, the cationic substitution does not affect unit cell parameters of SnO2. Due to proximate atomic numbers of Sn and Sb, it is impossible to visualize the impurity in SnO2(Sb) by Z-contrast using electron microscopy. According to the Mossbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge spectroscopy (XANES), antimony at low concentration is incorporated into the bulk of SnO2 as Sb5+, but when its percentage exceeds solubility (3–6 at%) the dopant occurs at the surface in the form of Sb3+ [82]. The segregation of additives on SnO2 influences the acid-base surface properties, in particular, the chemisorption of water molecules from atmosphere. In nanocrystalline SnO2(Sb) raising the antimony concentration in the range 1–8 at% resulted in the decrease of surface acidity [83]. An approach to quantitative ranking of metal oxides according to their Lewis acidity is based on the concept of the so-called optical basicity (Λ) [7]. Accordance to it, the Lewis acidity of the cations in an oxide is inversely proportional to their optical basicity. The calculated values of optical basicity for cations Sb5+ and Sn4+ in octahedral oxygen environment are calculated as Λ = 0.985 and 0.870, respectively [84]. Due to lower positive charge density on Sb5+ cations than on Sn4+ on the surface of doped tin dioxide the chemisorption of water and concentration of hydrated species are lowered. This was evidenced by the decrease of IR absorption bands of stretching vibrations of OH-groups and deformation vibrations of H2O molecules in the IR spectra of SnO2(Sb) (Fig. 7.17). However, the concentrations of acid sites estimated from the TPD of ammonia vary insignificantly within the error limits (Table 7.3). n(OH)

d(H2O)

80 [Sb] [Sb+Sn] (at.%)

70

T (%)

0 1 60

2 3 4 6

50

7 40 4000 3600 3200 2800 2400 2000 1600 1200

800

400

–1

Wavenumber (cm )

Fig. 7.17  IR spectra of nanocrystalline SnO2 doped with Sb. With permission from Marikutsa AV, Vorobyeva NA, Rumyantseva MN, Gaskov AM. Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO2 and gas sensitivity, Rus Chem Bul 2017;66(10):1728–64.

Tin oxide nanomaterials: Active centers and gas sensor properties187

The concentration of chemisorbed oxygen on the surface of nanocrystalline SnO2(Sb) doped with 0.5–8 at% of antimony was in the range 10–15 mol/m2 (Table 7.2) with no significant dependence on the dopant concentration.

7.9 Effect of catalytic additives on the active sites of nanocrystalline SnO2 The concept of chemical modification consists in creating new active centers on the semiconductor surface with specific adsorptive behavior and/or reactivity to the target gases. Numerous studies have shown that modifying the surface of semiconductor oxides improves their sensor properties: sensitivity, selectivity, response dynamics, and stability [47]. As a result of a systematic study of the sensor sensitivity of nanocrystalline SnO2 modified by various additives (noble metals and transition metal oxides), an approach to the selection of modifiers has been developed [8, 9, 44]. It relies on the fact that the modifier reactivity should be complementary to the chemical behavior of the target gas. For example, to selectively increase the adsorption of gas molecules with Lewis basicity (NH3, amines), the sensor could be modified by acidic additives (MoO3, WO3, V2O5) [9, 44]. Unlike acid-base additives, the role of catalytic additives is the facilitation of the redox reaction of target molecules with the sensor surface, rather than an increase of the gas adsorption. Catalytic effect consists in lowering the activation energy of the gas-solid interaction responsible for the sensor signal formation. It results in not only an increase of sensor sensitivity, but also in a decrease in the operating temperature of sensors [85–89]. The choice of selective modifiers is based on the insights into the mechanisms of heterogeneous catalysts action [90]. The activity and specificity of an oxidation catalyst depends on the adsorption energy of the reducing gas and oxygen molecules as well as the binding energies to the intermediate substances and reaction products. Transition elements of the 8th, 9th, and 10th groups have the proper adsorption energy to various molecules due to the electronic structure [91]. The metals at the beginning of the transition series (the less occupied d-shell) as well as the s-, p-, and f-elements have strong adsorption capacity to gases (to oxygen—up to the formation of oxides) due to the vacant orbitals [90]. On the contrary, the elements with completed d-shell (group 11) are less prone to gas adsorption and the catalytic activity is due to the formation of electron vacancies in the d-orbitals, in particular, in case of nanostructured metals [91, 92]. A proper modifier for specific detection of a particular reducing gas can be selected by considering the dependence of the catalytic activity of metals in the oxidation reaction of the target gas on the binding energy with adsorbates. For example, Pd and Pt are the optimal catalysts for CO oxidation, since the adsorption energy of oxygen (340–360 kJ/mol) is comparable to that of CO [93, 94]. Palladium and platinum are also effective modifiers for improving the sensor properties to CO [87, 95–98]. Gold is of interest for catalysts and sensors modification, but the activity is due to nano-size effect rather than adsorption energies [88, 99, 100]. Other catalytically active metals for CO oxidation, such as Rh, Ru, and Ni, are less widely used for sensors modification, since under the operating conditions they form stable oxide phases with less prominent catalytic behavior [93, 101]. An

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example of the correspondence between the catalytic properties of the modifier and the sensor properties of the modified oxide are the “metal oxide-Ru-NH3” systems. On the one hand, ruthenium-based catalysts are active and selective catalysts for the synthesis, decomposition, and oxidation of ammonia [102, 103]. This is due to the optimal Ru-N bond energy which enables the low-activation barrier transformations of chemisorbed NH3 to intermediates (NH2-, NH =, N ≡) and final oxidation products (N2, NOx) [104, 105]. On the other hand, the Ru-modified SnO2 has excellent sensitivity and selectivity to NH3 [21, 106, 107]. It was shown that the oxidation of ammonia to nitrogen oxides catalyzed by the modifier plays a key role in the formation of the sensor signal [20]: RuO2,s NH 3,g + O − surf = NOsurf + OH surf + e − ↓ O − surf NO2,g + e −

(7.9)

Oxygen does not inhibit the catalyst, since the surface of RuO2 formed in air exposes coordinately unsaturated Ru4+ cations available for NH3 molecules binding and thus has similar catalytic behavior as metallic Ru [108, 109].

7.9.1 Active sites formed by catalytic modifiers Here, we consider the effect of catalytic modifiers PdOx and RuOy (1 wt%) on the type and concentration of active sites on the surface of nanocrystalline SnO2. The modification was carried out by tin dioxide impregnation by ethanol solutions of Pd2+ and Ru3+ acetylacetonates followed by heat treatment in air [21, 55]. The active sites created by catalytic additives are regarded as clusters or nanoparticles of noble metals (noble metal oxides) attached to the surface of the semiconductor matrix. This was experimentally confirmed by the study of nanocomposites by the methods of transmission (TEM) and scanning transmission electron microscopy (STEM) [21, 55, 88, 96]. To distinguish the clusters by contrast the atomic numbers of the modifier and the cations of the semiconductor oxide should differ significantly (e.g., SnO2 modified by Au in Fig. 7.18A). Otherwise, the unambiguous distinction of particle sizes of the modifier clusters and of metal oxide support is favorable for the microscopy investigation (Fig. 7.18B). On the contrary, when nanocrystalline SnO2 with particle size less than 10 nm is modified by noble metals of the same row in Periodic table (Ru, Rh, Pd) the detection of additives by electron microscopy is difficult [110]. To study the elements distribution in such nanocomposites, the energy-dispersive X-ray spectral analysis (EDX mapping) is informative. For example, in Fig. 7.18C and D, the EDX maps of nanocomposites SnO2/PdOx and SnO2/RuOy (1 wt% of additives) are shown and the clusters of modifiers with the size 1–5 nm are distinguishable on the network of 3–6 nm sized SnO2 particles [21, 55]. In this case, the catalytic clusters cover the agglomerates of SnO2 nanoparticles the sizes of which can reach from several tens to hundreds of nanometers.

Tin oxide nanomaterials: Active centers and gas sensor properties189

Fig. 7.18  High-resolution TEM images of nanocrystalline SnO2 (dXRD = 3–6 nm) modified by 1 wt% Au (A) and of nanocrystalline SnO2 (dXRD = 16–20 nm) modified by 1 wt% PdOx (B). STEM images of nanocomposites SnO2/PdOx (C) and SnO2/RuOy (d) based on nanocrystalline tin dioxide (dXRD=3–6 nm, 1 wt% of noble metals), the insets show EDX maps of the selected areas (dark is Sn L signal and bright is L signals of noble metals). Adapted with permission from Marikutsa A, Krivetskiy V, Yashina L, Rumyantseva M, Konstantinova E, Ponzoni A, et al. Catalytic impact of RuOx clusters to high ammonia sensitivity of tin dioxide. Sens Actuators B 2012;175:186–193; Rumyantseva MN, Gaskov АМ. Chemical modification of nanocrystalline metal oxides: effect of the real structure and surface chemistry on the sensor properties. Russ Chem Bull, Int Ed 2008;57:1106–25; Frolov DD, Kotovshchikov YN, Morozov IV, Boltalin AI, Fedorova AA, Marikutsa AV, et al. Oxygen exchange on nanocrystalline tin dioxide modified by palladium. J Solid State Chem 2012;186:1–8; Marikutsa A, Rumyantseva M, Frolov D, Morozov I, Boltalin A, Fedorova A, et al. Role of PdOx and RuOy clusters in oxygen exchange between nanocrystalline tin dioxide and the gas phase. J Phys Chem C 2013;117:23858–67.

Describing the active sites formed by modifiers, it is important to determine the oxidation state of metals. It depends on the nature of the additive. For noble metals, the affinity to oxygen decreases in the row: Ru, Rh, Pd, Pt, and Au. This is due to the decrease of metal-oxygen bond energy, of the crystal lattice energy, of the thermal stability, and of the absolute value of oxide formation enthalpy. Ruthenium and rhodium on the surface of nanocrystalline oxides are in the oxidized state in the form of RuO2 (ΔfH = − 150 ÷ − 200 kJ/mol) and Rh2O3 (ΔfH ≈ − 100 ÷ − 150 kJ/mol), respectively [107, 111]. EPR has shown that ruthenium in SnO2/RuOy (1 wt% Ru) nanocomposites obtained by impregnation with Ru(acac)3 could be in a mixed oxidation state with a

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part of Ru3+ in RuO2 clusters [22, 67]. Metallic gold is present even in the form of small clusters (1–3 nm) on the surface of sensor materials [112, 113], since the oxide Au2O3 is an endothermic compound [111]. Palladium and platinum can be found both in the metallic state and in the form of oxides PdO, PtO, or PtO2 (ΔfH ≈ − 50 ÷ − 100 kJ/mol [96, 97]). The oxidation state in this case depends on the nanocomposite synthesis procedure, size, and morphology of the modifier and ambient conditions: temperature and gas-phase composition. Studies of palladium deposited on SnO2 showed that at low concentrations (0.1–0.2 wt%) Pd was distributed at an atomic level over the support which is favorable for oxidation by chemisorbed oxygen to PdO [2, 114]. With increasing Pd content to 1–3 wt%, it formed 3D clusters on the SnO2 surface [2] which were oxidized in the presence of oxygen on heating to 200°C. As a result, mixed PdOx clusters were formed containing Pd2+ as the main form (x = 0.7–0.8) along with zero-valent palladium [19, 114]. Pd2+ cations can substitute Sn4+ in the SnO2 lattice [2], but the ionic diffusion into the subsurface layers of SnO2 required heating to not less than 400°C [115]. A detailed study of the composition of materials based on nanostructured SnO2/PdOx (1 wt%) by XPS, EPR, XANES, and EXAFS revealed that PdOx clusters consisting of amorphous PdO also contain Pd0 atoms and a small fraction of Pd3+ cations likely stabilized at the cluster-support interface [22, 67]. A further increase in the amount of deposited palladium leads to percolation of metal clusters, the formation of continuous Pd layers, and finally a film of Sn-Pd solid solution (2–5 monolayers thickness) [2]. Similarly, the particle size of platinum affects its oxidation state on the surface of nanocrystalline SnO2: it was shown that clusters smaller than 3–4 nm consist of PtO2 and larger ones are zero-valent Pt0 [116]. According to the XPS data, in nanocomposites containing 1 wt% of platinum obtained by impregnation with Pt(acac)2 the additive was in the form of PtO typical for small Pt clusters (less than 2 nm).

7.9.2 Effect of modifiers on the active sites of SnO2 Besides the creation of new active sites, surface modification affects the intrinsic active centers of a semiconductor oxide. This influences the interaction of nanocomposites with gases and operating conditions of gas sensors. Here, we consider effects of palladium and ruthenium oxides (1 wt%) on the active sites of nanocrystalline tin dioxide. From TPR measurements, the shift of the low-temperature hydrogen absorption peak toward low temperatures was observed. It suggests an increase of the oxidative activity of surface sites in catalytically modified SnO2 (Fig. 7.19). The quantitative effect of modifiers is the increase of chemisorbed oxygen concentration on the surface of SnO2 brought about by palladium and, to a greater extent, by ruthenium oxide (Table 7.2). The concentration of chemisorbed oxygen on modified SnO2 by three to five times exceeds the surface coverage by PdOx or RuOy indicating that the these additives not only form the new oxidation sites themselves, but also contribute to an accumulation of chemisorbed oxygen. According to the TPD of ammonia, the catalytic additives affect the surface acidity of SnO2. Modification with palladium oxide leads to a systematic increase in the concentration of Broensted acidic OH-centers on the surface of materials with different

Tin oxide nanomaterials: Active centers and gas sensor properties191

Fig. 7.19  TPR profiles of nanocrystalline SnO2 modified by 1 wt% of platinum group metal oxides and gold. With permission from Marikutsa AV, Vorobyeva NA, Rumyantseva MN, Gaskov AM. Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO2 and gas sensitivity, Rus Chem Bul 2017;66(10):1728–64.

microstructure parameters (Table 7.1). The NH3 desorption peaks from the Lewis acid sites were observed at lower temperatures than in case of pristine SnO2. The TPD signals were in the temperature range corresponding to the medium-strength acid centers (Fig. 7.20). This indicates on weakening the Lewis surface acidity of SnO2 due to the presence of PdOx or RuOy, as was proved using an independent acidity evaluation technique [23]. Yet, desorption of catalytic ammonia oxidation products could be a prominent by-process during the TPD experiments with catalytically modified tin dioxide [21, 67]. Surface modification affected paramagnetic donor sites of nanocrystalline tin dioxide so that the signal of oxygen vacancies VO− was not detected in the EPR spectra. It was ascribed to complete ionization of single-charged vacancies caused by ­electron-acceptor action of the PdOx and RuOy clusters or by the promotion of oxygen chemisorption [75]. The relative concentrations of the surface O2− and ∙ OH centers (Table 7.6) were unaffected by the modification by ruthenium oxide. However, on the surface of SnO2/PdOx, the concentration of hydroxyl centers was twice higher and that of oxygen O2− was 1.5 times lower in comparison to SnO2. This effect was enhanced by the hydration of SnO2/PdOx so that the EPR spectrum of the material held in humid air did not exhibit the O2− signal (Fig. 7.21). PdOx clusters likely favor the transformation of oxygen sites to hydroxyls in the presence of water vapor [54, 113]. Catalytic additives brought about changes of the composition of HHL of nanocrystalline SnO2 [19, 65]. The concentration of molecular and dissociated water adsorbates increased (Table 7.4), in agreement with IR spectroscopy data (Fig. 7.22) and thermal analysis (Fig. 7.23). IR spectra (Fig. 7.22) indicate changes in the composition of the OHgroups: the modification of SnO2 by palladium or ruthenium oxide resulted in increased impact of hydrogen-bound bridging hydroxyls OH…OH (Fig. 7.22 and Table 7.3). This may be due to higher proton mobility of OH-groups on modified surfaces.

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Fig. 7.20  Profiles of ammonia TPD from the surface of nanocrystalline SnO2 with different microstructure parameters and catalytic modifiers (1 wt% of additives). With permission from Marikutsa AV, Rumyantseva MN, Konstantinova EA, Shatalova TB, Gaskov AM. Active sites on nanocrystalline tin dioxide surface: effect of palladium and ruthenium oxides clusters. J Phys Chem C 2014;118:21541–9.

Noteworthy, surface modification affects the concentration of different active centers in distinct ways. Ruthenium oxide contributes more to the surface sites enrichment by chemisorbed oxygen. PdOx clusters exhibit a hydration effect, that is, increasing the concentration of surface hydrated species: molecular-adsorbed water and OH-groups, Broensted acid sites and paramagnetic centers ∙ OH (Tables 7.1, 7.4, and 7.6). The

Tin oxide nanomaterials: Active centers and gas sensor properties193

Fig. 7.21  EPR spectra of nanocrystalline SnO2 (1), SnO2/PdOx (2), and SnO2/PdOx exposed to 100% relative humidity in air (3). With permission from Marikutsa AV, Rumyantseva MN, Gaskov AM, Konstantinova EA, Grishina DA, Deygen DM. CO and NH3 sensor properties and paramagnetic centers of nanocrystalline SnO2 modified by Pd and Ru. Thin Solid Films 2011;520:904–8.

Fig. 7.22  IR absorption spectra of nanocrystalline tin dioxide modified by 1 wt% of noble metal oxides. With permission from Marikutsa AV, Rumyantseva MN, Konstantinova EA, Shatalova TB, Gaskov AM. Active sites on nanocrystalline tin dioxide surface: effect of palladium and ruthenium oxides clusters. J Phys Chem C 2014;118:21541–9.

194

Tin Oxide Materials: Synthesis, Properties, and Applications – H2O

– OH

100 SnO2 SnO2/PdOx

99 m (%)

SnO2/RuOy 98

97

96 0

100

200

300

400

500

600

700

800

T (°C)

Fig. 7.23  Thermal analysis of nanocrystalline tin dioxide modified by 1 wt% of noble metal oxides. With permission from Marikutsa AV, Rumyantseva MN, Konstantinova EA, Shatalova TB, Gaskov AM. Active sites on nanocrystalline tin dioxide surface: effect of palladium and ruthenium oxides clusters. J Phys Chem C 2014;118:21541–9.

increased chemisorption of H2O in the presence of palladium oxide is likely due to electronic interaction with the semiconductor oxide. At the concentration of 1 wt%, the clusters PdOx consisted of Pd2+ oxide (about 70% of the total palladium content) and of Pd0. The work function of PdO is 6.0 eV [116]. These two states could form a redox pair PdO/Pd which redox potential corresponds to lowest unoccupied electron energy level about ~ 5.5 eV below the vacuum level [117]. Since work function of tin dioxide is less (4.8 eV), a depleted layer is formed in the region close to the contact with PdOx clusters (Fig. 7.24A). The electron-deficient character of the boundary region promotes the adsorption and dissociation of donor H2O which [according to Eq. (7.7)] is responsible for the formation of OH-groups and paramagnetic ∙ OH centers on the SnO2/PdOx surface. The smaller hydration effect of ruthenium oxide is due to weaker electron-acceptor action: the work function of RuO2 is 5.0–5.1 eV [118] and the contact with SnO2 produces less pronounced electronic depletion (Fig. 7.24B) than at the SnO2/PdOx interface.

7.10 Oxygen exchange of nanocrystalline SnO2 Various types of oxygen can participate in the processes of interaction of nanocrystalline tin dioxide with gases during sensor response formation, that is, chemisorbed oxygen, lattice anions from the near-surface layer and from the bulk of SnO2 particles. An effective tool to study the mechanisms of such interactions is ­temperature-programmed exchange of oxygen isotopes 18O/16O under dynamic conditions [119]. Experimental measurements are carried out using gas mixture of molecular oxygen isotopes

Tin oxide nanomaterials: Active centers and gas sensor properties195 E0

E0

+++ ++ ++

EC

E (RuO2/Ru3+)

EF E (PdO/Pd)

EF

+

EC

EF (RuO2)

ED

EF (PdO)

ED

EV EV SnO2

PdOx

SnO2

RuOy

Fig. 7.24  Scheme of the formation of depleted layer in SnO2 in contact with PdOx clusters (A), and with RuOy clusters (B). E0 is vacuum level, EC, EF, ED, and EV indicate the levels of conduction band, Fermi level, donor states, and valence band of SnO2, short-dashed lines show the levels corresponding to the work function of the PdO and RuO2 and the potentials of the PdO/Pd and RuO2/Ru3+ redox pairs. With permission from Marikutsa AV, Rumyantseva MN, Konstantinova EA, Shatalova TB, Gaskov AM. Active sites on nanocrystalline tin dioxide surface: effect of palladium and ruthenium oxides clusters. J Phys Chem C 2014;118:21541–9. 16

O2,  16O18O, 18O2 with a known composition. It is passed through a heated chamber with the oxide and the outlet gas is simultaneously analyzed by mass spectrometry (Fig. 7.25). The molar fractions of isotope oxygen molecules and the calculated atomic fraction of 18O as the functions of temperature are thus obtained. Simulation of the dependencies obtained allows determining the mechanism and the kinetic parameters of oxygen exchange [55, 56, 119].

7.10.1 Effect of tin dioxide microstructure on oxygen exchange Fig. 7.26 shows the temperature dependences of the molar fractions of molecular oxygen isotopes 16O2 (f32), 16O18O (f34), and 18O2 (f36) in a carrier gas passed through nanocrystalline SnO2 with different microstructure parameters. The inclination of fi(T) curves from the initial plateaus indicates the start of oxygen exchange between the O2 gas and SnO2. For highly dispersed SnO2 with particle size 3–6 nm, it started at T ≈ 430°C and for larger SnO2 particles 35–50 nm—at T ≈ 520°C which indicates the lower activity of the coarser-crystalline sample. The temperature dependences of 16O18O molecular fraction f34(T) and of 18O atomic fraction f18(T) in the outlet gas flow are shown in Fig. 7.27A and B. The decrease of 18 O atomic fraction at the beginning of oxygen exchange implies that heteroexchange takes place with the constituent oxygen of SnO2. Simulating the dependences f18(T) and f34(T) (Fig. 7.26) showed that on pristine SnO2 the limiting process was so-called

7

3

18

H2 O 11

4 9

1 Ar

2 5

16

O2

10

6 8

Fig. 7.25  Scheme of the experimental setup for measurements of temperature-programmed oxygen isotopic exchange: (1) argon gas bottle, (2) oxygen 16O2 gas bottle, (3) electrolytic source of 18O2, (4) mass-flow controller, (5) sample powder in a reactor, (6) furnace, (7) thermocouple, (8) temperature programmer, (9) gas flow divider, (10) mass-spectrometer, and (11) exhaust.

Fig. 7.26  Temperature dependence of the molar fractions of 16O2 (f32), 18O16O (f34), and 18O2 (f36) molecules in the gas passed through tin dioxide with various microstructure parameters: dXRD=3–6 nm, SBET=90–100 m2/g (A), dXRD=26–35 nm, SBET=1–5 m2/g (B). With permission from Frolov DD, Kotovshchikov YN, Morozov IV, Boltalin AI, Fedorova AA, Marikutsa AV, et al. Oxygen exchange on nanocrystalline tin dioxide modified by palladium. J Solid State Chem 2012;186:1–8.

Tin oxide nanomaterials: Active centers and gas sensor properties197

“simple heteroexchange” of each gas molecule with single oxygen atom of the oxide crystal structure [55]: 18

O2,g + 16 Os ↔

16

O 18Og + 16Os ↔ 16O2, g + 18Os

16

O 18 Og + 18 Os

(7.10)

The total amount of exchangeable oxygen estimated from the f18(T) curves increases from 4 to 100 at% when decreasing the average particle size of SnO2 from 26–35 to 3–6 nm [55, 56]. The kinetic parameters determined within the simple ­heteroexchange model are summarized in Table 7.7.

Fig. 7.27  Experimental (points) and simulated (line) temperature dependences of the atomic fraction 18O (f18) and the molecular fraction 18O16O (f34) in the gas after interaction with tin dioxide with particle size dXRD=3–6 nm (A) and dXRD=26–35 nm (B). The dashed line indicates the exchange of surface oxygen. With permission from Frolov DD, Kotovshchikov YN, Morozov IV, Boltalin AI, Fedorova AA, Marikutsa AV, et al. Oxygen exchange on nanocrystalline tin dioxide modified by palladium. J Solid State Chem 2012;186:1–8.

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Tin Oxide Materials: Synthesis, Properties, and Applications

Table 7.7  Kinetic parameters of oxygen exchange of tin dioxide with different particle size and surface modifiers [55, 56]

Parameter Temperature of exchange start, °С Simple heteroexchange rate (at T = 300°C)/m−2s−1 Complex heteroexchange rate (at T = 300°C)/m−2s−1 Diffusion coefficient (at T = 300°C)/m2s−1 Activation energy of simple heteroexchange, kJ/mol Activation energy of complex heteroexchange, kJ/mol Activation energy of diffusion, kJ/mol Fraction of exchanged oxygen in the oxide, at%

SnO2 (dXRD = 26–35 nm)

SnO2 (dXRD = 3–6 nm)

SnO2/PdOx (dXRD = 3–6 nm)

SnO2/RuOy (dXRD = 3–6 nm)

520

430

310

200

–

1.3×1012

2.6×1012

2.6×1012

–

–

5.7×1014

3.6×1017

–

6.1×10−24

1.9×10−22

3×10−21

–

130

130

130

–

–

110

130

–

80

80

60

4

60

54

46

The total amount of exchangeable oxygen in highly dispersed SnO2 was estimated to (7 ± 0.6)×1021 atoms per gram which corresponds to the stoichiometric oxygen content in SnO2 (8.0 × 1021 atom/g). Thus all constituent oxygen of nanostructured SnO2 could be substituted by O atoms from the gas phase O2 molecules. It should be noted that tin dioxide has usually been considered as the material with rigid cation-anion lattice and the insignificant mobility of the oxygen anions is implicit in the description of sensor response formation within the oxygen vacancy model [2]. The low-temperature peak (Fig. 7.27A, dotted line) corresponding to oxygen with high activity in the heteroexchange was distinguished in the f18(T) curve of highly dispersed SnO2. Its amount was calculated as (1.6 ± 0.4)×1021 atom/g consistent with the estimated c­ oncentration of lattice oxygen in the surface layer of SnO2 (95 m2/g × 1.4 × 1019 atom/m2 ≈ 1.4 × 1021 atom/g). Thus, at the beginning of exchange, oxygen is first replaced at the surface of the material, while at higher temperatures the main process is the replacement of anions in the bulk of nanoparticles. The fraction of surface atoms in the total amount of exchangeable oxygen is 1.6 × 1021/8.0 × 1021  = 0.2.

7.10.2 Effect of catalytic modifiers on oxygen exchange Investigation of oxygen exchange of nanocrystalline SnO2 modified by PdOx and RuOy suggested that the catalytic clusters promote the exchange process and the additive of

Tin oxide nanomaterials: Active centers and gas sensor properties199

ruthenium oxide exhibited a comparatively stronger effect [56]. This was noticed first by the decrease of the starting temperature of exchange to 310°C for SnO2/PdOx and to 200°C for SnO2/RuOy (compared to 430°C for unmodified SnO2). The decrease in the atomic fraction of 18O with the beginning of exchange indicates (Fig. 7.28) that heteroexchange takes place as well as in the case of SnO2. The modifiers also affect the exchange of surface oxygen. The corresponding peak on the f18(T) curves was distinguished as a low-temperature shoulder in the curve of SnO2/PdOx and as a clearly resolved peak on the curve of SnO2/RuOy (Fig.  7.28).

Fig. 7.28  The experimental (points) and calculated (line) temperature dependences of the atomic fraction 18O (f18) and the molecular fraction 18O16O (f34) in the gas after interaction with nanocrystalline tin dioxide (d XRD = 3–6 nm), unmodified and modified by 1 wt% of palladium and ruthenium oxides. The dashed line indicates the exchange of surface oxygen. With permission from Marikutsa A, Rumyantseva M, Frolov D, Morozov I, Boltalin A, Fedorova A, et al. Role of PdOx and RuOy clusters in oxygen exchange between nanocrystalline tin dioxide and the gas phase. J Phys Chem C 2013;117:23858–67.

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Tin Oxide Materials: Synthesis, Properties, and Applications

This implies that the mobility of surface oxygen increased in the row SnO2 < SnO2/ PdOx < SnO2/RuOy. Noteworthy, in the same row increased the concentration of chemisorbed oxygen on the materials surface (Table 7.2). Modeling the dependences f18(T) and f34(T) on the basis of the experimental (Fig. 7.28) showed that oxygen exchange proceeded via two ways: the “simple heteroexchange” (as on pristine SnO2) and “complex heteroexchange” in which each gas molecule substitutes two oxygen atoms of the oxide [56]: 18

O2,g + 2 16 Os ↔

16

O2,g + 2 18 Os

18

O2,g + 2 16 Os ↔

16

O 18 Og + 16 Os + 18 Os

16

O Og + 2 Os ↔ O2, g + Os + Os 18

16

16

16

(7.11)

18

The kinetic parameters for both mechanisms are summarized in Table  7.7. Comparing the heteroexchange parameters for different materials suggests that the “simple heteroexchange” was due to the modifier-free areas of SnO2 surface. The “complex heteroexchange” characterized by much higher exchange and diffusion rates and having lower activation energy was due to the presence of catalytic additives. Simple heteroexchange is known to proceed via the Riedel-Eley mechanism. The complex heteroexchange starts with oxygen molecules dissociation on an oxide surface followed by the replacement of lattice anions by adsorbed O atoms. It is often realized through the spillover effect on noble metal clusters (Pt, Pd) deposited on an oxide support [119–121]. Assuming the same effect on the deposited PdOx and RuOy on the surface of SnO2, the process of heteroexchange can be expressed by the scheme in Fig. 7.29. Taking the dissociation of O2 as the rate-limiting stage

O2 g

O2 g

simple heteroexchange

complex heteroexchange

2 Osurf

2 Osurf

MOx O

O

Sn O

O Sn

O

O

O Sn

O

O

O

O Sn

O Sn

O

Sn O

O

Sn

Sn O

O

O

O

Sn O

O Sn

O

O diffusion

O

O Sn

Fig. 7.29  The effect of catalytic modifiers (clusters of palladium or ruthenium oxides) on the mechanism of oxygen exchange of tin dioxide with a gas phase. With permission from Marikutsa A, Rumyantseva M, Frolov D, Morozov I, Boltalin A, Fedorova A, et al. Role of PdOx and RuOy clusters in oxygen exchange between nanocrystalline tin dioxide and the gas phase. J Phys Chem C 2013;117:23858–67.

Tin oxide nanomaterials: Active centers and gas sensor properties201

because of high O-O bond energy (493 kJ/mol in gas phase [90]), this processes is equivalent to the mechanism of complex heteroexchange (Eq. 7.11). Thus, the effect of deposited PdOx and RuOy clusters on the interaction of SnO2 with O2 involves the formation of sites for dissociative adsorption and rapid migration of atomic oxygen to the surface. The study of isotope exchange showed that the surface modification of nanocrystalline SnO2 favors the interaction of the material with oxygen via the spillover mechanism. It is likely the origin of increasing concentration of chemisorbed oxygen on the surface of tin dioxide modified by PdOx and RuOy. Moreover, the chemisorbed oxygen content and activity in oxygen exchange increased in the sequence SnO2 < SnO2/ PdOx < SnO2/RuOy. Adsorption and dissociation of molecular oxygen on the catalytic clusters should be consistent with the higher affinity of the noble metal oxides to oxygen. RuOy clusters that consisted mainly of RuO2 contributed to the increase of chemisorbed oxygen concentration (Table 7.2) and to activation of oxygen exchange (Table 7.7) notably more, as opposed to PdOx. This correlates with the lower enthalpy of RuO2 formation and higher metal-­oxygen bond energy (ΔfH = − 150 ÷ − 200 kJ/mol) than those of PdO (ΔfH = − 50 ÷ − 100 kJ/mol) [110].

7.11 Active sites and gas sensor properties of nanocrystalline SnO2 Gas sensitivity is based on the electronic phenomena on the surface and in the bulk of a semiconductor caused by chemisorption of the gas components. The term chemisorption means that the adsorbed molecules or atoms are in electronic interaction with the material, that is, charge transfer occurs between the localized adsorbate-related surface states and the delocalized electronic states of the semiconductor [4]. In terms of band theory, the charge redistribution results in bands bending and the formation of a charge-depletion or a charge-accumulation layer in the subsurface region of a solid (Fig. 7.30). This leads to a corresponding change of electrical conductance of a semiconductor as a whole that is measured as a sensor response. For semiconductor metal oxides, the formation of sensor response has been described by two different models: the ionosorption model and the oxygen vacancies model. The ionosorption model addresses the electronic effects due to the chemisorption of oxygen or other electron-­acceptor molecules (ozone, nitrogen oxides) as well as desorption of these adsorbates in the presence of electron-donor molecules (reducing gases) like H2, CO, hydrocarbons in ambient air. The oxygen vacancies model is applied to n-type semiconductor metal oxides and attributes the sensing phenomena to the changes of oxygen deficiency in the oxides as a result of interaction with gases. Sensor signal S is defined as a ratio of electric conduction (G) or resistance (R) in air to that in the presence of a target gas: = S G= Rair / Rgas gas / Gair

(7.12)

202

Tin Oxide Materials: Synthesis, Properties, and Applications Evac Dc

DF Evac

F

eDVs

F

c

–

EC EF

eDVs

–

–

–

ED

O2–

ESS EV

(A)

(B)

Md+

d– O2,chem

Fig. 7.30  Scheme of the band structure of an n-type semiconductor under no chemisorption (A) and under the chemisorption of an electron-acceptor molecule like O2 (B). O2− is ionic chemisorbed species, O2,chem is neutral chemisorbed species, EVAC is vacuum level, EC, EF, ED, and EV are the levels of conduction band, Fermi level, donor states level, and valence band of SnO2, ESS is the level of surface states formed by ionic chemisorbed species, χ and Φ are electron affinity and work function of SnO2, respectively, and eΔVs—surface potential barrier height. Adapted with permission from Oprea A, Barsan N, Weimar U. Work function changes in gas sensitive materials: Fundamentals and applications. Sens Actuators B 2009;142:470–93.

Within the ionosorption model, the formation of sensor signal is attributed to the changes of localized surface charge density and surface potential barrier height of the material during the following processes [2]: (a) Chemisorption of oxygen from the air:

β (7.13) O2, g + α e − ↔ Oβα,−surf 2 where O2,g is the oxygen molecule in the gas phase; e−—electron with energy sufficient to overcome surface potential barrier created by the localized negative surface charge, Oa−b,surf— chemisorbed oxygen species: α = 1 or 2 for singly and doubly charged species, respectively, and β = 1 or 2 for atomic and molecular species, respectively. (b) Interaction of the reducing gas R (e.g., H2, CO, NH3) with chemisorbed oxygen: β Rg + Oαβ ,−surf → β RO g + α e −

(7.14)

where ROg is the product of oxidation of the reducing gas R (e.g., H2O, CO2, N2). c) Chemisorption of an oxidizing gas (e.g., NO2): NO2, g + e − ↔ NO2−, surf

(7.15)

The chemisorption of oxygen from the gas phase on the surface of n-type semiconductors like tin dioxide results in electron depletion layer in near-surface region

Tin oxide nanomaterials: Active centers and gas sensor properties203

(within the Debay length) and the potential barrier at the surface (Fig. 7.30) [122]. Thus, the charge transport across the particles as well as the surface conduction decrease in the presence of chemisorbed oxygen. Interaction with reducing gases (CO, NH3, H2) decreases the concentration of chemisorbed oxygen and the associated negative surface charge leading to the decrease of the depletion layer thickness and lowering the surface potential barrier height. Thus, charge carrier concentration and conductivity of the material increase. Chemisorption of an oxidizing gas having higher electron affinity (e.g., NO2) than oxygen increases the electronic depletion in the semiconductor and decreases its conductivity. Fig. 7.31 shows the dynamical response of a sensor based on nanocrystalline SnO2 to reducing and oxidizing gases in air. The oxygen vacancies model [123] describes sensor response formation of SnO2 in an inert atmosphere in the presence of reducing gases, for example, CO [28], CH4 [66], and H2 [40]. It is assumed that the lattice oxygen anions on the oxide surface directly interact with the molecules of reducing gases which can be expressed as O 2 − + R → VO2 − + RO ↔ VO− + e − + RO ↔ VO + 2 e − + RO

(7.16)

where R is the reducing gas molecule and RO is the oxidation product molecule. Chemical activity of the lattice anions O2− has rarely been experimentally confirmed. For example, the formation of Sn2+ cations accompanying ionized oxygen vacancies was observed by the Mossbauer spectroscopy of nanocrystalline SnO2 exposed to CO (1 vol%) in nitrogen at an elevated temperature [29]. There are IR data that signal of carbonate groups appear in the spectra of tin dioxide interacting with CO at 300°C. The appearance of carbonate groups was independent of the oxygen content in the gas phase and was not followed by the release of CO2. The formation of such carbonates could be due to the interaction of CO with lattice oxygen on the surface of SnO2 [124]:

Fig. 7.31  Dynamic resistance response of nanocrystalline SnO2-based sensor to reducing gas NH3 with the concentration 20 parts per million (ppm) in air and to oxidizing gas NO2 (1 ppm) at T=150°C.

204

Tin Oxide Materials: Synthesis, Properties, and Applications

(7.17)

CO + O 2 − → CO32 −

Of particular importance to study the mechanisms of sensor materials interaction with gases are the in situ investigation techniques. Information on adsorption sites, chemisorption of gas molecules, and their transformations on the surface could be obtained by Raman spectroscopy [16], Fourrier transform InfraRed (FTIR) and diffuse reflection IR spectroscopy [125, 126], and gas chromatography and mass spectrometry [9] directly in the process of interaction of a nanocrystalline tin dioxide with gas phase. Additional data can be gained measuring the electronic parameters, for example, work function by the Kelvin probe method or synchronous measurements of electric conduction and work function. According to the band model (Fig. 7.30), reversible chemisorption leads to changes in the position of the Fermi level relative to the vacuum level (Φ). This may be impacted by two modes of chemisorption. Ionosorption is most common for acceptor molecules (O2, oxidizing gases like NO2) and involves charge transfer from delocalized electronic states of the semiconductor (conduction band or donor level) and yields negative surface charge and surface potential barrier with the height eVs that determines the electrical resistance (R). Local chemisorption yields neutral adsorbates interacting with localized surface states of the material, for example, via ion-dipole, dipole-dipole, or van der Waals forces. The latter affects the dipolar momentum at the surface which is related to the electron affinity parameter (χ) [52]: ∆Φ = −eVs + ∆χ = kT ln

Rair + ∆χ Rgas

(7.18)

The symbol Δ indicates the change of the electronic parameter due to interaction with the gas, Rair and Rgas are the resistance values in air and in the presence of target gas, respectively. The value Δχ is proportional to the change of dipolar chemisorbates concentration on the surface. Thus, simultaneous measurements of resistance and work function are valuable for the study of local interactions of gas molecules with the active sites on the semiconductor surface [126]. Impedance spectroscopy is inevitable to evaluate the changes of ionic (e.g., protonic) conduction along with the electronic ones in the processes of gas-solid interaction [20].

7.11.1 The role of nature of active sites The oxidation processes of CO and NH3 on the surface of unmodified tin dioxide at different temperatures can be represented by the following schemes [83]: 2 COg + O2−, surf → CO3−,surf + COg → 2 CO2, g + e −

at 150°C < T < 200°C (7.19)

− COg + Osurf → CO2−, surf → CO2, g + e − at T > 200°C − ↓ +Osurf CO32,−surf

(7.20)

Tin oxide nanomaterials: Active centers and gas sensor properties205

(7.21)

4 ( NH3 − HO )surf + 6O2−, surf → 2 N 2, g + 4OH surf + 6 H 2 Osurf + 3e − at 150°C < T < 200°C

(

2 NH3 − Sn 4 +

)

surf

(

− + 6Osurf → N 2, g + 2 HO − Sn 4 +

)

surf

+ 6e − at T > 200°C (7.22)

Formation of intermediate and resultant carbonate species on the surface of SnO2 was observed by IR spectroscopy [65, 124, 125]. In interaction with NH3 prior to oxidation, the molecules are chemisorbed on the acid sites, that is, OH-groups and surface cations [126]. That ammonia oxidation on the unmodified surface is completed at the stage of N2 evolvement agrees with the concerns in the works [86, 87, 104] and with the analysis of gas-phase products of SnO2–NH3 interaction [21]. The modification of nanocrystalline SnO2 by catalytic additives PdOx and RuOy improves the sensitivity and selectivity of sensors to CO and NH3, respectively [20, 21, 111, 127]. The catalytic effect was also inferred from the shift of maximum sensor signal to lower temperature (Figs. 7.32 and 7.33). The data of simultaneous work function and conductance measurements indicated on specific surface reactivity of SnO2/ PdOx to CO and of SnO2/RuOy to NH3. From the results of in situ diffuse reflectance IR spectroscopy, it was concluded that the specific gas-solid interactions were provided by the catalytic clusters on tin dioxide surface [126]. Evolution of carbonyl vibrational peaks was observed only in the interaction of CO with SnO2/PdOx (Fig. 7.34). The positions of C-O peaks indicated on CO bound to Pd atoms in linear (2090 cm−1) and bridging (1910–1840 cm−1) conformations. The redshift relative to the gas-phase molecule (2143 cm−1) or physically adsorbed CO (2138 cm−1) is due to π-dative interaction with palladium weakening the C-O bond [37]. At raised 100

SnO2

S (arb.units)

SnO2/PdOx SnO2/RuOy 10

1 0

25

50

100

150 T (°C)

200

250

Fig. 7.32  Sensor signals to 50 ppm CO of nanocrystalline SnO2 depending on the modifier (1 wt%) and temperature. With permission from Marikutsa A, Rumyantseva M, Gaskov A. Selectivity of catalytically modified tin dioxide to CO and NH3 gas mixtures. Chemosensors 2015;3:241–52.

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SnO2

100

SnO2/PdOx

S (arb.units)

SnO2/RuOy

10

1 0

25

50

100

150

200

250

T (°C)

Fig. 7.33  Sensor signals to 50 ppm NH3 of nanocrystalline SnO2 depending on the modifier (1 wt%) and temperature. With permission from Marikutsa A, Rumyantseva M, Gaskov A. Selectivity of catalytically modified tin dioxide to CO and NH3 gas mixtures. Chemosensors 2015;3:241–52.

n(Pd-C=O)

n(Pd-C=O) Pd

IR absorbance (arb.units)

SnO2/PdOx SnO2 SnO2/RuOy SnO2/PtO SnO2/Au

4000 3600 3200 2800 2400 2000 1600 1200 Wavenumber (cm–1)

Fig. 7.34  Diffuse reflectance IR spectra of nanocrystalline tin dioxide modified by various catalytic additives exposed to CO (100 ppm) at room temperature. With permission from Marikutsa A, Rumyantseva M, Gaskov A. Specific interaction of PdOxand RuOy-modified tin dioxide with CO and NH3 gases: kelvin probe and DRIFT studies. J Phys Chem C 2015;119:24342–50.

Tin oxide nanomaterials: Active centers and gas sensor properties207

temperature (200°C), no spectral features in the presence of CO were detected. The specific chemisorption of CO on PdOx clusters was observed only at T < 150°C. The Pd sites for CO chemisorption were likely originated from partial reduction of PdOx, as confirmed by the increase of Pd0:Pd2+ relative amounts in the presence of CO observed by XPS [20]. The oxidation of CO on SnO2/PdOx at low temperature (25°C–150°C) involved surface OH-groups, as shown by IR spectroscopy and impedance measurements [20, 126]. Thus, the role of PdOx clusters in the specific interaction of SnO2/PdOx – CO interaction at T < 150°C was recognized as follows [20, 126]: (1) Oxidation of CO molecules by PdOx clusters which are being partially reduced and the fraction of Pd0 atoms increased: PdOx + CO → Pd +

1+ x CO2 2

(7.23)

(2) Specific chemisorption of CO molecules on Pd0 which is essentially the catalytic effect, since it weakens the interatomic C-O bond and facilitates further conversion of the chemisorbed molecule: Pd 0 + CO ↔ Pd δ + − COδ −

(7.24)

(3) Modification by PdOx increases the concentration of reactive hydroxyl species (paramagnetic ∙ OH centers, hydrogen-bound OH…OH groups) (Tables 7.4 and 7.6) which participate in the catalytic oxidation of chemisorbed CO molecules: CO + OH → CO2 + H + + e −

(7.25)

The high sensitivity and selectivity of the sensor response of SnO2/RuOy to NH3 at an elevated temperature (150–200°C) is also explained by selective enhancement of the material surface reactivity under these conditions [21, 126]. At room temperature, ammonia is adsorbed on surface OH-groups and Sn4+ cations traced by the evolution of corresponding NH4+ and NH3 bending vibrational peaks in the spectra of IR diffuse reflectance (Fig.  7.35A). On heating to T = 200°C only the adsorption on cation (Lewis) acid sites remains (Fig. 7.35B), since acidity of Broensted acid sites is insufficient to retain adsorbed NH3 at this temperature, according to the TPD (Fig. 7.8). Characteristic of the specific catalytic effect of RuOy clusters in the reaction of ammonia oxidation (Eq. 7.9) is the appearance of the N-O stretching vibrational peak related to nitrosyl groups bound to ruthenium cations (1870 cm−1) [126]. The evolution of nitrogen dioxide NO2 from ammonia oxidation on the surface of SnO2/RuOy was shown in the analysis of the gaseous products of the interaction [21]. In addition, modification by RuOy promoted the chemisorbed oxygen on tin dioxide surface that is favorable for the oxidation of target gas molecules. This was inferred from the increased concentration of chemisorbed oxygen on the SnO2/RuOy (Table 7.2) and from the acceleration of oxygen exchange with the gas phase (Table  7.7). Since the latter involves dissociative adsorption of O2

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d(NH3)as

n(O-H) n(N-H)

d(NH4+)

d(NH4+) d(NH3)sym

IR absorbance (arb.units)

SnO2/RuOy

SnO2/PdOx

SnO2 4000

3500

(A)

3000

2500

2000

1500

1000

Wavenumber (cm–1)

n(O-H) n(N-H)

n(Ru-N=O)

d(NH3)as

d(NH3)sym

IR absorbance (arb.units)

SnO2/RuOy

4000

(B)

SnO2/PdOx

SnO2

3500

3000

2500

2000

1500

1000

Wavenumber (cm–1)

Fig. 7.35  Diffuse reflectance IR spectra of nanocrystalline materials SnO2, SnO2/PdOx, and SnO2/RuOy exposed to NH3 (100 ppm) at room temperature (A) and at T = 200°C (B). With permission from Marikutsa A, Rumyantseva M, Gaskov A. Specific interaction of PdOxand RuOy-modified tin dioxide with CO and NH3 gases: kelvin probe and DRIFT studies. J Phys Chem C 2015;119:24342–50.

and the spillover effect (Fig. 7.29) and the ­beginning of oxygen exchange (200°C) coincided with the temperature favorable for catalytic oxidation and gas sensing of NH3, it can be assumed that the oxidation reaction involves reactive atomic species of chemisorbed oxygen.

Tin oxide nanomaterials: Active centers and gas sensor properties209

7.11.2 Influence of concentration of active sites Revealing the correlations between active sites concentration and the sensor signals to certain gases is a guideline to establish the sensing mechanism and to optimize the composition and microstructure parameters of the sensor materials. The experimental results obtained to date agree that the chemical interaction (reception) during the formation of SnO2 sensor response to any kind of oxidizing or reducing gas includes chemisorption and redox interaction of the gas molecules with the material surface. The sensitivity of tin dioxide modified by palladium oxide in case of room temperature CO detection decreases with the introduction of the doping impurity Sb5+. In Fig. 7.36, the dependence of sensor signals on dopant concentration in SnO2(Sb)/ PdOx is compared with the content of OH-groups estimated by the relative intensities

S(CO) (arb.units)

25 20 15 10 5

(A)

0

A(OH) (arb.units)

0.7

0.6

0.5

0.4

(B)

0

2

4

6

8

[Sb]/[Sb+Sn] (at.%)

Fig. 7.36  Sensor signal of nanocrystalline SnO2(Sb)/PdOx materials to 5 ppm CO at room temperature (A) and the normalized intensity of IR absorption of OH-groups at ν = 3400 cm−1 (B) in relation to the concentration of antimony. With permission from Marikutsa A, Rumyantseva M, Gaskov A. Effect of n-type doping of SnO2 and ZnO on surface sites and gas sensing behavior. Procedia Eng 2016;468:1082–5.

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of O-H vibrational peaks in IR absorbance spectra. Thus, one of the reasons for lower sensitivity under the influence of antimony could be the decrease of the concentration of surface OH-groups which play the role of oxidation centers in the reaction with CO (Eq. 7.25). However, electrophysical factors such as increased electronic conduction and descended surface potential barriers could also contribute to the drop of electrical response in the presence of target gas [83]. Chemisorption and oxidation phenomena are of utmost significance for NH3 sensing. An interesting example is the effect of active sites concentration on the sensor properties of catalytically modified tin dioxide. In contrast to interaction with CO which is chemisorbed and catalytically oxidized on the same sites (i.e., PdOx clusters) of the modified SnO2 surface, the chemisorption of ammonia molecules is determined by acid sites and oxidation—by RuOy clusters and chemisorbed oxygen. The challenging task was recently implemented to optimize the sensor material composition via independent variation of surface acidity and the catalyst content so as to reach higher sensitivity and selectivity to NH3 [23]. The catalyst loading was determined by synthesis conditions of SnO2/RuOy, while to control the number of acid sites the additive of sulfate groups was introduced to tin dioxide. Dependences of surface acidity, oxidizing activity and sensor signals to NH3 on the content of RuOy and SO42− in the nanocomposites were analyzed (Fig. 7.37). It was concluded that the proper sensitivity was provided by SnO2 modified by equivalent amounts (1–3 wt%) of RuOy and

Fig. 7.37  Sensor signals of sulfated SnO2/RuOy to 20 ppm NH3 at 150–200°C in relation to the concentrations of RuOy and SO42−. With permission from Marikutsa A, Sukhanova A, Rumyantseva M, Gaskov A. Acidic and catalytic co-functionalization for tuning the sensitivity of sulfated tin oxide modified by ruthenium oxide to ammonia gas. Sens Actuators B 2018;255:3523–32.

Tin oxide nanomaterials: Active centers and gas sensor properties211

SO42− enabling the balance between NH3 chemisorption and catalytic oxidation. On the contrary, increasing the concentration of strong acid sites (SO42−) inhibited the sensitivity to NH3 because of too strong binding of the molecules making them inactive for further oxidation. In its turn, the catalyst loading exceeding 3 wt% RuOy was detrimental for surface acidity and probably prevented the interaction of NH3 with the active sites of SnO2.

7.12 Conclusion The surface of nanocrystalline tin dioxide contains a variety of active sites: coordinately unsaturated cations and anions, oxygen vacancies, chemisorbed oxygen, and hydroxyl groups. These sites determine adsorptive properties and surface reactivity. The diverse nature of the active sites is the reason for low selectivity of pristine tin dioxide in the reactions with ambient gas molecules. Using the complex of investigation techniques, the following types of active sites were observed on the oxide surface: acidic Lewis centers (surface cations Sn4+), chemisorbed oxygen in neutral O2,chem and ionized O−2 states, oxygen vacancies VO, chemisorbed water molecules and hydroxyl groups including Broensted acid sites and paramagnetic centers ∙ OH. The concentration of active sites on the surface of nanostructured tin oxide can be controlled by varying the synthesis conditions. With the decrease of crystallites size and the increase of specific surface area, the concentration of surface adsorption centers increase. The introduction of doping impurities into SnO2 nanocrystals also affects the concentration of active sites and acid-base properties of the surface. An efficient tool to improve selectivity is the modification of material surface by immobilization of the clusters of noble metals Au, Pt, Pd, Ru, or noble metal oxides. It was shown that sensing behavior (sensitivity, selectivity, detection temperature) to CO and NH3 was improved due to modification by palladium and ruthenium oxides, respectively. In the former case, the high sensitivity to CO was reached at room temperature which is of great interest for the fabrication of wireless carbon monoxide sensors. High sensitivity of nanostructured SnO2/PdOx materials to trace concentrations of CO in air is due to the specific chemisorption of CO molecules on Pd atoms followed by weakening the C-O bond. The selective catalytic action of ruthenium oxide during the oxidation of NH3 on the surface of SnO2/RuOy nanocomposites is efficient at raised temperature (150–200°C). Effect of catalytic additives on the intrinsic active centers of tin dioxide is an important factor determining the proper conditions of gas-solid interaction and gas sensing. Modification with palladium and ruthenium oxides has different effects on the concentration and reactivity of the active sites of nanocrystalline SnO2. It was shown that modification by PdOx resulted in increased concentration of hydroxyl groups, which are the centers of CO oxidation at room temperature. Modification by ruthenium oxide promoted the interaction of supporting tin dioxide with oxygen which resulted in the increment of chemisorbed oxygen concentration and thus contributed to catalytic oxidation of ammonia on the surface of the modified tin dioxide.

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References [1] Das S, Jayaraman V. SnO2: A comprehensive review on structures and gas sensors. Progr Mater Sci 2014;66:112–255. [2] Batzill M, Diebold U. The surface and materials science of tin oxide. Progr Surf Sci 2005;79:47–154. [3] Marikutsa AV, Vorobyeva NA, Rumyantseva MN, Gaskov AM. Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO2 and gas sensitivity. Rus Chem Bul 2017;66(10):1728–64. [4] Wolkenstein F. Electronic processes on semiconductor surfaces during chemisorption. New York: Springer; 1991. [5] Cheng JP, Wang J, Li QQ, Liu HG, Li Y. A review of recent developments in tin dioxide composites for gas sensing application. J Ind Eng Chem 2016;44:1–22. [6] Korotcenkov  G, Cho  BK. Metal oxide composites in conductometric gas sensors: achievements and challenges. Sens Actuators B 2017;244:182–210. [7] Bordes-Richard E, Courtine P. Optical basicity: a scale of acidity/basicity of solids and its application to oxidation catalysis. In: Fierro JLS, editor. Metal oxides: chemistry and application. Boca Raton: Taylor & Francis; 2006. p. 319–52. [8] Krivetskiy V, Rumyantseva M, Gaskov A. Design, synthesis and application of metal oxide-based sensing elements: a chemical principles approach in metal oxide nanomaterials for chemical sensors. In: Carpenter MA, et al., editors. Integrated analytical systems. New York: Springer; 2013. p. 69–115. [9] Krivetskiy VV, Rumyantseva MN, Gaskov AM. Chemical modification of nanocrystalline tin dioxide for selective gas sensors. Russ Chem Rev 2013;82:917–41. [10] Shannon  RD. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr 1976;A32:751–67. [11] Thiel B, Helbig R. Growth of SnO2 single crystal by vapour phase reaction method. J Cryst Growth 1976;32:259–64. [12] Cox DF, Fryberger TB, Semancik S. Surface reconstruction of surface deficient SnO2 (110). Surf Sci 1989;224:121–42. [13] Cox DF, Fryberger TB. Preferential isotopic labeling of lattice oxygen positions on the SnO2 (110) surface. Surf Sci 1990;227:105–8. [14] Cox DF, Fryberger TB, Semancik S. Oxygen vacancies and defect electronic states on the SnO2 (110)-1×1 surface. Phys Rev B 1988;38:2072–83. [15] Manassidis I, Goniakowski J, Kantrovich LN, Gillan MJ. The structure of the stoichiometric and reduced SnO2 surface. Surf Sci 1995;339:258–71. [16] Batzill M, Diebold U. Surface studies of gas sensing metal oxides. Phys Chem Chem Phys 2007;9:2307–18. [17] Rumyantseva MN, Gaskov AM, Rosman N, Pagnier T, Morante JR. Raman surface vibration modes in nanocrystalline SnO2: correlation with gas sensor performances. Chem Mater 2005;17:893–901. [18] Morrison SR. The chemical physics of surfaces. first ed. New York: Springer; 1990. [19] Davydov  A. Molecular spectroscopy of oxide catalyst surfaces. Chichester: Wiley; 2003. [20] Marikutsa AV, Rumyantseva MN, Yashina LV, Gaskov AM. Role of surface hydroxyl groups in promoting room temperature CO sensing by Pd-modified nanocrystalline SnO2. J Solid State Chem 2010;183:2389–99. [21] Marikutsa A, Krivetskiy V, Yashina L, Rumyantseva M, Konstantinova E, Ponzoni A, Comini E, Abakumov A, Gaskov A. Catalytic impact of RuOx clusters to high ammonia sensitivity of tin dioxide. Sens Actuators B 2012;175:186–93.

Tin oxide nanomaterials: Active centers and gas sensor properties213

[22] Marikutsa  AV, Rumyantseva  MN, Gaskov  AM, Konstantinova  EA, Grishina  DA, Deygen DM. CO and NH3 sensor properties and paramagnetic centers of nanocrystalline SnO2 modified by Pd and Ru. Thin Solid Films 2011;520:904–8. [23] Marikutsa  A, Sukhanova  A, Rumyantseva  M, Gaskov  A. Acidic and catalytic co-­ functionalization for tuning the sensitivity of sulfated tin oxide modified by ruthenium oxide to ammonia gas. Sens Actuators B 2018;255:3523–32. [24] Abee  MW, Cox  DF. NH3 chemisorption on stoichiometric and oxygen deficient SnO2(110) surfaces. Surf Sci 2002;520:65–77. [25] McAleer JF, Moseley PT, Norris JOW, Williams DE. Tin dioxide gas sensors. Part 1— Aspects of the surface chemistry revealed by electrical conductance variations. J Chem Soc Faraday Trans 1987;83:1323–46. [26] Zhang Y, Kolmakov A, Lilach Y, Moskovits M. Electronic control of chemistry and catalysis at the surface of an individual tin oxide nanowire. J Phys Chem B 2005;109:1923–9. [27] Gaggiotti G, Galdikas A, Kacilius S, Mattogno G, Setkus A. Surface chemistry of tin oxide based gas sensors. J Appl Phys 1994;76:4467–71. [28] Ivanovskaya MI, Branitskiy GA, Orlik DR, Malchenko SI, Vrublevskiy AI. Nature of paramagnetic centers in tin dioxide. Rus J Inorg Chem 1992;37:1147–52. [29] Safonova  O, Bezverkhy  I, Fabrichnyi  P, Rumyantseva  M, Gaskov  A. Mechanism of sensing CO in nitrogen by nanocrystalline SnO2 and SnO2(Pd) studied by Mössbauer spectroscopy and conductance measurements. J Mater Chem 2002;12:1174–8. [30] Shali NB, Sugunan S. Influence of transition metals on the surface acidic properties of titania prepared by sol-gel route. Mater Res Bull 2007;42:1777–83. [31] Noei  H, Qiu  H, Wang  Y, Loeffler  E, Woell  C, Muhler  M. The identification of hydroxyl groups on ZnO nanoparticles by infrared spectroscopy. Phys Chem Chem Phys 2008;10:7092–9. [32] Rodriguez-Gonzalez V, Gomez R, Moscosa-Santillan M, Amouroux J. Synthesis, characterization, and catalytic activity in the n-heptane conversion over Pt/In-Al2O3 sol-gel prepared catalysts. J Sol-Gel Sci Technol 2007;42:165–71. [33] Yushchenko  VV. Calculation of the acidity spectra of catalysts from temperature-­ programmed ammonia desorption data. Russ J Phys Chem A 1997;71:547–50. [34] Sahm T, Gurlo A, Barsan N, Weimar U. Basics of oxygen and SnO2 interaction; work function change and conductivity measurements. Sens Actuators B Chem Chem 2006;118:78–83. [35] Gurlo A. Interplay between O2 and SnO2: oxygen ionosorption and spectroscopic evidence for adsorbed oxygen. Chem Phys Chem 2006;7:2041–52. [36] Prades JD, Cirera A, Morante JR, Pruneda JM, Ordejon P. Ab initio study of NOx compounds adsorption on SnO2 surface. Sens Actuators B Chem Chem 2007;126:62–7. [37] Hadjiivanov KI, Vayssilov GN. Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule. Adv Catal 2002;47:307–511. [38] Egashira  M, Nakashima  M, Kawasumi  S, Seiyama  T. Temperature programmed desorption study of water adsorbed on metal oxides. 2. Tin oxide surfaces. J Phys Chem 1981;155:4125–30. [39] Kovalenko  VV, Zhukova  AA, Rumyantseva  MN, Gaskov  AM, Yushchenko  VV, Ivanova  II, Pagnier  T. Surface chemistry of nanocrystalline SnO2: Effect of thermal treatment and additives. Sens Actuators B Chem Chem 2007;126:52–5. [40] Yamaguchi  Y, Tabata  K, Suzuki  E. Density functional theory calculations for the interaction of oxygen with reduced M/SnO2(110) (M = Pd, Pt) surfaces. Surf Sci 2003;526:149–58. [41] Jina S, Kwon K, Park C, Chang H. The oxygen reduction electrocatalytic activity of intermetallic compound of palladium-tin supported on tin oxide-carbon composite. Catal Today 2011;164:176–80.

214

Tin Oxide Materials: Synthesis, Properties, and Applications

[42] Neri G, Bonavita A, Micali G, Rizzo G, Pinna N, Niederberger M, Ba J. Effect of the chemical composition on the sensing properties of In2O3-SnO2 nanoparticles synthesized by a non-aqueous method. Sens Actuators B Chem Chem 2008;130:222–30. [43] Williams DE, Pratt KFE. Classification of reactive sites on the surface of polycrystalline tin dioxide. J Chem Soc Faraday Trans 1998;94:3493–500. [44] Rumyantseva MN, Gaskov АМ. Chemical modification of nanocrystalline metal oxides: effect of the real structure and surface chemistry on the sensor properties. Russ Chem Bull, Int Ed 2008;57:1106–25. [45] Vorobyeva N, Rumyantseva M, Filatova D, Konstantinova E, Grishina D, Abakumov A, Turner S, Gaskov A. Nanocrystalline ZnO(Ga): Paramagnetic centers, surface acidity and gas sensor properties. Sens Actuators B 2013;182:555–64. [46] Habgood M, Harrison N. An ab initio study of oxygen adsorption on tin dioxide. Surf Sci 2008;602:1072–9. [47] Gurlo A, Barsan N, Weimar U. Gas Sensors Based on Semiconducting Metal Oxides. In: Fierro JLS, editor. Metal oxides: chemistry and application. Boca Raton: Taylor & Francis; 2006. p. 683–738. [48] Yamazoe N, Fuchigami J, Kishikawa M, Seiyama T. Interactions of tin oxide surface with oxygen, water, and hydrogen. Surf Sci 1979;86:335–44. [49] Tabata K, Kawabe T, Yamaguchi Y, Nagasawa Y. Chemisorbed oxygen species over the (110) face of SnO2. Catal Surv Asia 2003;7:251–9. [50] Shen  GL, Casanova  R, Thornton  G. Interaction of O2 with SnO2(110)1×1 and 4 ×1. Vacuum 1992;43:1129–35. [51] Rumyantseva  MN, Makeeva  EA, Badalyan  SM, Zhukova  AA, Gaskov  AM. Nanocrystalline SnO2 and In2O3 as materials for gas sensors: The relationship between microstructure and oxygen chemisorption. Thin Solid Films 2009;518:1283–8. [52] Oprea  A, Barsan  N, Weimar  U. Work function changes in gas sensitive materials: Fundamentals and applications. Sens Actuators B 2009;142:470–93. [53] Zhu  Z, Deka  RC, Chutia  A, Sahnoun  R, Tsuboi  H, Koyama  M, Hatakeyama  N, Endou  A, Takaba  H, Del Carpio  CA, Kubo  M. Enhanced gas-sensing behaviour of Ru-doped SnO2 surface: a periodic density functional approach. J Phys Chem Solid 2009;70:1248–55. [54] Konstantinova  EA, Pentegov  IS, Marikutsa  AV, Rumyantseva  MN, Gaskov  AM, Kashkarov PK. EPR study of nanocrystalline tin dioxide. Phys Stat Sol C 2011;8:1957–60. [55] Frolov DD, Kotovshchikov YN, Morozov IV, Boltalin AI, Fedorova AA, Marikutsa AV, Rumyantseva  MN, Gaskov  AM, Sadovskaya  EM, Abakumov  AM. Oxygen exchange on nanocrystalline tin dioxide modified by palladium. J Solid State Chem 2012;186:1–8. [56] Marikutsa A, Rumyantseva M, Frolov D, Morozov I, Boltalin A, Fedorova A, Petukhov I, Yashina L, Konstantinova E, Sadovskaya E, Abakumov A, Zubavichus Y, Gaskov A. Role of PdOx and RuOy clusters in oxygen exchange between nanocrystalline tin dioxide and the gas phase. J Phys Chem C 2013;117:23858–67. [57] Chon H, Pajares J. Hall effect studies of oxygen chemisorption on zinc oxide. J Catal 1969;14:257–60. [58] Harbeck S, Szatvanyi A, Barsan N, Weimar U, Hoffmann V. DRIFT studies of thick film un-doped and Pd-doped SnO2 sensors: temperature changes effect and CO detection mechanism in the presence of water vapor. Thin Solid Films 2003;436:76–83. [59] Bandura AV, Kubicki JD, Sofo JO. Comparisons of multilayer H2O adsorption onto the (110) surfaces of α-TiO2 and SnO2 as calculated with density functional theory. J Phys Chem B 2008;112:11616–24.

Tin oxide nanomaterials: Active centers and gas sensor properties215

[60] Pavelko RG, Daly H, Hardacre C, Vasiliev AA, Llobet E. Interaction of water, hydrogen and their mixtures with SnO2 based materials: the role of surface hydroxyl groups in detection mechanisms. Phys Chem Chem Phys 2010;12:2639–47. [61] Koziej  D, Barsan  N, Weimar  U, Szuber  J, Shimanoe  K, Yamazoe  N. Water-oxygen interplay on tin dioxide surface: Implication on gas sensing. Chem Phys Lett 2005;410:321–3. [62] Barsan N, Weimar U. Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO2 sensors in the presence of humidity. J Phys Condens Matter 2003;15:813–39. [63] Helwig A, Muller G, Sberveglieri G, Eickhoff M. On the low-temperature response of semiconductor gas sensors. J Sensors 2009;(2009). 620720(17 pp.). [64] Rout CS, Hegde M, Govindaraj A, Rao CN. Ammonia sensors based on metal oxide nanostructures. Nanotechnology 2007;18(9):205504. [65] Koziej D, Barsan N, Shimanoe K, Yamazoe N, Szuber J, Weimar U. Spectroscopic insights into CO sensing of undoped and palladium doped tin dioxide sensors derived from hydrothermally treated tin oxide sol. Sens Actuators B 2006;118:98–104. [66] Schmid  W, Barsan  N, Weimar  U. Sensing of hydrocarbons and CO in low oxygen conditions with tin dioxide sensors: possible conversion paths. Sens Actuators B 2004;103:362–8. [67] Marikutsa AV, Rumyantseva MN, Konstantinova EA, Shatalova TB, Gaskov AM. Active sites on nanocrystalline tin dioxide surface: effect of palladium and ruthenium oxides clusters. J Phys Chem C 2014;118:21541–9. [68] Canevali C, Chiodini N, Morazzoni F, Scotti R. Electron paramagnetic resonance characterization of ruthenium dispersed tin oxide obtained by sol-gel and impregnation methods. J Mater Chem 2000;10:773–8. [69] Chang S-C. Oxygen chemisorption on tin oxide: Correlation between electrical conductivity and EPR measurements. J Vac Sci Technol A 1980;17:366–9. [70] Lopez N, Prades JD, Hernandez-Ramirez F, Morante JR, Mathur S. Bidimensional versus tridimensional oxygen vacancy diffusion in SnO2-x under different gas environments. Phys Chem Chem Phys 2010;12:2401–6. [71] Maier J, Gopel W. Investigation of the bulk defect chemistry of polycrystalline tin(IV) oxide. J Solid State Chem 1988;72:293–302. [72] Epifani M, Prades JD, Comini E, Pellicer E, Avella M, Siciliano P, Faglia G, Cirera A, Scotti R, Morazzoni F, Morante JR. The role of surface oxygen vacancies in the NO2 sensing properties of SnO2 nanocrystals, J Phys Chem C 112 (2008) 19540–19546. [73] Hernandez-Ramirez  F, Prades  JD, Tarancon  A, Barth  S, Casals  O, Jimenez-Diaz  R, Pellicer E, Rodriguez J, Morante JR, Juli MA, Mathur S, Romano-Rodriguez A. Insight into the role of oxygen diffusion in the sensing mechanisms of SnO2 nanowires. Adv Funct Mater 2008;18:2990–4. [74] Heiland G, Kohl D. Physical and chemical aspects of oxidic semiconductor gas sensors. In: Seiyama T, editor. Chemical sensor technology. Tokyo: Kodansha; 1988. p. 15–38. [75] Konstantinova  EA, Marikutsa  AV, Rumyantseva  MN, Deygen  DM, Zabotnov  SV, Martyshov MN, Vorontsov AS, Forsh PA, Kashkarov PK. Study into the influence of molecular environments and modification of semiconductor nanocrystals of stannic oxides with various metal admixtures on the nature and properties of their defects. Sci Rev 2013;9:297–302. [76] Wertz JE, Bolton JR. Electron spin resonance. New York: McGraw Hill Book Company; 1972. [77] Park CO, Akbar SA. Ceramics for chemical sensing. J Mater Sci 2003;38:4611–37.

216

Tin Oxide Materials: Synthesis, Properties, and Applications

[78] Liu TJ, Jin ZG, Feng LR, Wang T. Conducting antimony-doped tin oxide films derived from stannous oxalate by aqueous sol–gel method. Appl Surf Sci 2008;254:6547–53. [79] Noonuruk R, Vittayakorn N, Mekprasart W, Sritharathikhun J, Pecharapa W. Sb-doped SnO2 nanoparticles synthesized by sonochemical-assisted precipitation process. J Nanosci Nanotechnol 2015;15:2564–73. [80] Liu J, Liu X, Zhai Z, Jin G, Jiang Q, Zhao Y, Luoa C, Quan L. Evaluation of depletion layer width and gas-sensing properties of antimony-doped tin oxide thin film sensors. Sens Actuators B 2015;220:1354–60. [81] Yao P. Effects of Sb doping level on the properties of Ti/SnO2-Sb electrodes prepared using ultrasonic spray pyrolysis. Desalination 2011;267:170–4. [82] Rockenberger J, zum Felde U, Tischer M, Troeger L, Haase M, Weller H. Near edge X-ray absorption fine structure measurements (XANES) and extended X-ray absorption fine structure measurements (EXAFS) of the valence state and coordination of antimony in doped nanocrystalline SnO2. J Chem Phys 2000;112:4296–304. [83] Marikutsa A, Rumyantseva M, Gaskov A. Effect of n-type doping of SnO2 and ZnO on surface sites and gas sensing behavior. Procedia Eng 2016;468:1082–5. [84] Bordes-Richard E. Multicomponent oxides in selective oxidation of alkanes theoretical acidity versus selectivity. Top Catal 2008;50:82–9. [85] Cabot A, Arbiol J, Morante JR, Weimar U, Barsan N, Gopel W. Analysis of the noble metal catalytic additives introduced by impregnation of as obtained SnO2 sol–gel nanocrystals for gas sensors. Sens Actuators B 2000;70:87–100. [86] Aroutiounian V. Metal oxide hydrogen, oxygen, and carbon monoxide sensors for hydrogen setups and cells. Int J Hydrogen Energy 2007;32:1145–58. [87] Ramgir NS, Hwang YK, Jhung SH, Mulla IS, Chang J-S. Effect of Pt concentration on the physicochemical properties and CO sensing activity of mesostructured SnO2. Sens Actuators B 2006;114:275–82. [88] Bahrami B, Khodadadi A, Kazemeini M, Mortazavi Y. Enhanced CO sensitivity and selectivity of gold nanoparticles-doped SnO2 sensor in presence of propane and methane. Sens Actuators B 2008;133:352–6. [89] Yamazoe N, Kurokawa Y, Seiyama T. Effects of additives on semiconductor gas sensors. Sens Actuators 1983;4:283–9. [90] Bond GC, Louis C, Thompson DT. Catalysis by gold. London: Imperial College Press; 2006. [91] Haruta  M. Catalysis of gold nanoparticles deposited on metal oxides. CATTECH 2002;6:102–15. [92] Santra AK, Goodman DW. Oxide-supported metal clusters: models for heterogeneous catalysts. J Phys Condens Matter 2002;14:R31–62. [93] Savchenko  VI, Boreskov  GK, Kalinkin  AV, Salanov  AN. On the state of oxygen on metals surfaces and catalytic activity in the reaction of oxidation of carbon oxide. Kinet Catal 1983;24:983–90. [94] Over H, Muhler M. Catalytic CO oxidation over ruthenium – bridging the pressure gap. Prog Surf Sci 2003;72:3–17. [95] Rella  R, Siciliano  P, Capone  S, Epifani  M, Vasanelli  L, Licciulli  A. Air quality monitoring by means of sol-fel integrated tin oxide thin films. Sens Actuators B 1999;58:283–8. [96] Belmonte JC, Manzano J, Arbiol J, Cirera A, Puigcorbe J, Vila A, Sabate N, Gracia I, Cane  C, Morante  JR. Micromachined twin gas sensor for CO and O2 quantification based on catalytically modified nano-SnO2. Sens Actuators B 2006;114:881–92.

Tin oxide nanomaterials: Active centers and gas sensor properties217

[97] Korotcenkov  G, Brinzari  V, Boris  Y, Ivanov  M, Schwank  J, Morante  J. Influence of surface Pd doping on gas sensing characteristics of SnO2 thin films deposited by spray pyrolysis. Thin Solid Films 2003;436:119–26. [98] Dolbec R, El Khakania MA. Sub-ppm sensitivity towards carbon monoxide by means of pulsed laser deposited SnO2:Pt based sensors. Appl Phys Lett 2007;90(3):173114. [99] Manjula P, Arunkumar S, Manorama SV. Au/SnO2 an excellent material for room temperature carbon monoxide sensing. Sens Actuators B 2011;152:168–75. [100] Ramgir  NS, Hwang  YK, Jhung  SH, Kim  HK, Hwang  J-S, Mulla  IS, Chang  J-S. CO sensor derived from mesostructured Au-doped SnO2 thin film. Appl Surf Sci 2006;252:4298–305. [101] Santra AK, Goodman DW. Catalytic oxidation of CO by platinum group metals: from ultrahigh vacuum to elevated pressures. Electrochim Acta 2002;47:3595–609. [102] Yin SF, Xu BQ, Zhou XP, Au CT. A mini-review on ammonia decomposition catalysts for on-site generation of hydrogen for fuel cell applications. Appl Catal A 2004;277:1–9. [103] Lorenzut  B, Montini  T, Pavel  CC, Comotti  M, Vizza  F, Bianchini  C, Fornasiero  P. Embedded Ru@ZrO2 Catalysts for H2 Production by Ammonia Decomposition. ChemCatChem 2010;2:1096–106. [104] Schloegl R. Catalytic Synthesis of Ammonia—A “Never-Ending Story”? Angew Chem Int Ed 2003;42:2004–8. [105] Jones  G, Bligaard  T, Abild-Pedersen  F, Norskov  JK. Using scaling relations to understand trends in the catalytic activity of transition metals. J Phys Condens Matter 2008;20(9):064239. [106] Krivetskiy VV, Ponzoni A, Comini E, Badalyan SM, Rumyantseva MN, Gaskov AM. Materials based on modified SnO2 for selective gas sensors. Inorg Mater 2010;46:1100–5. [107] Wagh MS, Jain GH, Patil DR, Patil SA, Patil LA. Modified zinc oxide thick film resistors as NH3 gas sensor. Sens Actuators B 2006;115:128–33. [108] Wang Y, Jacobi K, Scholne W-D, Ertl G. Catalytic oxidation of ammonia on RuO2(110) surfaces: mechanism and selectivity. J Phys Chem B 2005;109:7883–93. [109] Salomonsson  A, Petoral  RM, Uvdal  K, Aulin  C, Per-Olov  K, Ojama  L, Strand  M, Sanati M, Lloyd Spetz A. Nanocrystalline ruthenium oxide and ruthenium in sensing applications—an experimental and theoretical study. J Nanopart Res 2006;8:899–910. [110] Safonova OV, Delabouglise G, Chenevier B, Gaskov AM, Labeau M. CO and NO2 gas sensitivity of nanocrystalline tin dioxide thin films doped with Pd, Ru and Rh. Mater Sci Eng C 2002;21:105–11. [111] Ashcroft  SJ, Schwarzmann  E. Standard enthalpy of formation of crystalline gold(III) oxide. J Chem Soc Faraday Trans 1972;68:1360–1. [112] Wang S, Zhao Y, Huang J, Wang Y, Ren H, Wu S, Zhang S, Huang W. Low-temperature CO gas sensors based on Au/SnO2 thick film. Appl Surf Sci 2007;253:3057–61. [113] Neri G, Bonavita A, Milone C, Galvagno S. Role of the Au oxidation state in the CO sensing mechanism of Au/iron oxide-based gas sensors. Sens Actuators B 2003;93:402–8. [114] Koziej  D, Hubner  M, Barsan  N, Weimar  U, Sikorazc  M, Grunwaldt  J-D. Operando X-ray absorption spectroscopy studies on Pd-SnO2 based sensors. Phys Chem Chem Phys 2009;11:8620–5. [115] Schierbaum KD, Kirner UK, Geiger JF, Gopel W. Schottky-barrier and conductivity gas sensors based upon Pd/SnO2 and Pt/TiO2. Sens Actuators B 1991;4:87–94. [116] Gaidi  M, Hazeman  JL, Matko  I, Chenevier  B, Rumyantseva  MN, Gaskov  AM, Labeau M. Role of Pt aggregates in Pt/SnO2 thin films used as gas sensors: investigation of the catalytic effect. J Electrochem Soc 2000;147:3131–9.

218

Tin Oxide Materials: Synthesis, Properties, and Applications

[117] Matsushima  S, Teraoka  Y, Miura  N, Yamazoe  N. Electronic interaction between metal additives and tin dioxide in tin dioxide-based gas sensors. Jpn J Appl Chem 1988;27:1798–802. [118] Yun  D-J, Lee  S, Yong  K, Rhee  S-W. In situ ultraviolet photoemission spectroscopy measurement of the pentacene-RuO2/Ti contact energy structure. Appl Phys Lett 2010;97(3):073303. [119] Sadovskaya  EM, Ivanova  YA, Pinaeva  LG, Grasso  G, Kuznetsova  TG, van Veen  A, Sadykov VA, Mirodatos C. Kinetics of oxygen exchange over CeO2−ZrO2 fluorite-based catalysts. J Phys Chem A 2007;111:4498–505. [120] Martin  D, Duprez  D. MobEE. 1. Isotopic exchange of 18O2 with 16O of SiO2, Al2O3, ZrO2, MgO, CeO2, and CeO2-Al2O3. Activation by noble metals. correlation with oxide basicity. J Phys Chem 1996;100:9429–38. [121] Bedrane S, Descorme C, Duprez D. 16O/18O isotopic exchange: A powerful tool to investigate oxygen activation on M/CexZr1-xO2 catalysts. Appl Catal A 2005;289:90–6. [122] Rumyantseva MN, Safonova OV, Boulova MN, Ryabova LI, Gaskov AM. Dopants in nanocrystalline tin dioxide. Russ Chem Bull, Int Ed 2003;52:1217–38. [123] Vasiliev  RB, Dorofeev  SG, Rumyantseva  MN, Ryabova  LI, Gaskov  AM. Impedance spectroscopy of the ultradisperse SnO2 ceramic with variable grain size. Semiconductors 2006;40:104–7. [124] Baraton  MI. Spectroscopic study of the gas detection mechanism by semiconductor chemical sensors. In: Proceedings of the NATO advanced study institute on sensors for environment, health and security: advanced materials and technologies, Vichy. 2007. p. 31–45. [125] Koziej D, Thomas K, Barsan N, Thibault-Starzyk F, Weimar U. Influence of annealing temperature on the CO sensing mechanism for tin dioxide based sensors-Operando studies. Catal Today 2007;126:211–8. [126] Marikutsa  A, Rumyantseva  M, Gaskov  A. Specific interaction of PdOx- and RuOymodified tin dioxide with CO and NH3 gases: kelvin probe and DRIFT studies. J Phys Chem C 2015;119:24342–50. [127] Marikutsa A, Rumyantseva M, Gaskov A. Selectivity of catalytically modified tin dioxide to CO and NH3 gas mixtures. Chemosensors 2015;3:241–52.