Nanopowders of chromium doped TiO2 for gas sensors

Sensors and Actuators B 175 (2012) 163–172

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Nanopowders of chromium doped TiO2 for gas sensors B. Lyson-Sypien a,∗ , A. Czapla a , M. Lubecka a , P. Gwizdz a , K. Schneider a , K. Zakrzewska a , K. Michalow b , T. Graule b , A. Reszka c , M. Rekas d , A. Lacz d , M. Radecka d a

Faculty of Electrical Engineering, Automatics, Computer Science and Electronics, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Cracow, Poland EMPA, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for High Performance Ceramics, Uberlandstrasse 129, 8600 Duebendorf, Switzerland Institute of Physics, Polish Academy of Science, al. Lotników 32/46, 02-668 Warsaw, Poland d Faculty of Materials Science and Ceramics, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Cracow, Poland b c

a r t i c l e

i n f o

Article history: Available online 24 February 2012 Keywords: TiO2 nanopowders Cr dopant Gas sensors Hydrogen detection Flame spray synthesis

a b s t r a c t Nanocrystalline powders of TiO2 and TiO2 :Cr (0.1–10 at.% Cr) obtained by flame spray synthesis (FSS), are used as starting materials for preparation of gas sensors. Characterization of nanopowders is carried out by thermogravimetry (TG), Brunauer–Emmett–Teller (BET), adsorption isotherms, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Gas sensing materials are calcined at 400 ◦ C in a form of tablets, the morphology of which is similar to that of starting materials. The mass loss of nanopowders upon heating, as determined from its temperature profile in TG, is correlated with the specific surface area (SSA) obtained from BET measurements. High SSA exceeding 100 m2 /g is inherently related to the enhanced decomposition of organic residua below 400 ◦ C. XRD diffraction patterns indicate small crystallite sizes (6–27 nm) and the presence of both polymorphic forms: anatase and rutile, independently of the form of nanomaterials. SEM images demonstrate agglomeration of crystallites into spherical grains. Gas sensing characteristics of TiO2 :Cr nanosensors upon interaction with H2 are recorded in a self-assembled experimental system. Detection of hydrogen is carried out over the concentration range of 50–3000 ppm at the temperatures extending from 200 to 400 ◦ C. It is demonstrated that nanomaterials based on TiO2 :Cr are attractive for ultimate sensor applications due to a substantial decrease in the operating temperature down to 210–250 ◦ C. At a certain level of doping (of about 5 at.%) a reversal of the sensor response from that of n-type to that of p-type semiconductor is seen. This effect can be accounted for by the acceptor-type substitutional defects CrTi built into TiO2 lattice. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In 1991, Yamazoe [1] demonstrated an important improvement in performance of metal oxide semiconducting gas sensors with a nanometric crystallite size. Since then substantial efforts have been made to produce and test nanostructured materials for gas sensing [2] with the ultimate aim to increase their sensitivity and selectivity, decrease the response time and temperature of operation while still preserving long term stability and reproducibility. Different forms of nanomaterials were proposed, among which the most attractive seem to be nanotubes [3–5], nanobelts [6,7] and nanowires [8,9]. Nanostructures offer not only a decreased particle size, but also an increased specific surface area, which results in many new physical and chemical phenomena that are impossible to encounter at the micrometer scale. The mechanism of gas sensing involves adsorption and desorption processes at the grain boundaries, sur-

∗ Corresponding author. Tel.: +48 12 617 29 01; fax: +48 12 633 23 98. E-mail address: Barbara.Lyson@fis.agh.edu.pl (B. Lyson-Sypien). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.02.051

faces or interfaces. Therefore, smaller particle size, by its favorable impact on the specific surface area, i.e., an increased surface-tovolume ratio, is expected to enhance the density of centers active for chemisorption [2,10]. When the average size of a building block of a nanomaterial becomes comparable with the width of the depletion region, the sensitivity and kinetics of response to gases are improved. Furthermore, nanosensors can work at lower temperatures as compared with their microcrystalline counterparts. This is particularly important for reduction of both power consumption and cost of operation of any sensor system. Titanium dioxide is a well known and extensively examined gas sensing material. Resistive gas sensors based on TiO2 emerged thanks to promising features such as: reversible and large changes in the electrical resistance along with the exceptional chemical stability of titania [11–13]. Early studies brought up the subject of TiO2 bulk sensor working over high temperature range [14,15] and governed by thermodynamics of point defects. Since that time an extensive effort has been made to improve TiO2 based gas sensor’s parameters, i.e., to decrease the operating temperature [3,16], response and recovery times [16,17], costs of operation and to lower the detection limit

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[17,18]. Additional requirements include: improved thermal stability [19], increased selectivity and sensitivity towards different agents, e.g., H2 [20,21], alcohol [16,22], CO [23,24], NH3 [25,26], organic vapors [27], NO2 [28] and humidity [29,30]. There are different approaches to the problem of modification of TiO2 necessary for fabrication of efficient gas sensors. Among many others, the most exploited for the moment seems to be synthesis of nanostructured TiO2 in a form of nanopowders [31,32], nanotubes [4,33,5], nanowires [34,35] and nanofibers [36,37]. More traditional methods consist in loading TiO2 with metal additives such as: Pt [19,27,38], La [22], Cu [19,23], Nb [19,28,29], Pd [19,33,36], Ag [17,39]. Modification of the surface of TiO2 nanostructures [17,40] becomes a new promising technology. It is generally accepted that basic parameters characterizing gas sensor performance such as sensitivity, selectivity and response time of TiO2 -based sensors can be optimized by choosing an appropriate type and level of doping [41–44]. Many attempts have been undertaken in the past to exploit the influence of a trivalent Cr dopant acting as an acceptor type impurity on the electrical and gas sensing properties of TiO2 [45–48]. It has been demonstrated that doping improves the response time and sensitivity [45,46]. Moreover, it enables to decrease the initial resistivity (baseline) of the sensor thus widens the signal detection range [45,47,48]. Furthermore, it has been observed that Cr additive can change the type of conductivity from n to p [47–51], which is particularly interesting for detection of oxidizing gases. Incorporation of Cr is accompanied by formation of additional defects like oxygen vacancies VO and titanium interstitials Tii and leads to a shift of n → p transition towards lower oxygen partial pressure [50,51]. Owing to that, not only the detection range of the sensor can be extended but also the baseline resistance is lowered, as well. Therefore, the operation temperature can be reduced. Chromium forms several oxides among which the best-known and the most important are chromium (VI) oxide CrO3 , chromium (III) oxide Cr2 O3 and hydrated chromium dioxide CrO2 ·nH2 O. At temperatures below 400–500 ◦ C various oxygen-rich phases exist, and the compositions of oxide phases in the Cr2 O3 –CrO3 region are reported to be CrO2.65 (beta-oxide), CrO2.44 (gamma oxide), Cr6 O15 , Cr5 O15 , CrO2 [52–54]. Cr2 O3 is the only solid chromium oxide which is stable at high temperatures above 500 ◦ C. Despite of lot of effort spent on the studies of the mechanism of incorporation of Cr into TiO2 , this problem remains unresolved. It is usually assumed that Cr3+ substitutes for Ti up to 5 at.% [48]. According to the phase diagrams of Cr2 O3 –TiO2 [55] up to 10 at.% of Cr can be accommodated in TiO2 without any visible precipitation of new phases. In nanopowders, this solubility limit can be extended to higher values. However, some authors [56,57] claim that Cr(VI) can be present in TiO2 along with Cr(III). Our previous papers concentrated on titanium dioxide thin films and microcrystalline ceramics for gas sensing applications [43,58]. Nanopowders doped with chromium were applied as photocatalytic materials [59,60]. The analysis of structure and morphology as well as the mechanism of Cr incorporation into TiO2 nanopowders obtained by flame spray synthesis (FSS) has been performed in [59,60]. FSS method has been applied as an attractive technology because it provides crystalline powders in a single step due to a high temperature in the flame [61]. The ultimate aim of the present work is to fabricate sensitive hydrogen nanosensors from these TiO2 :Cr nanopowders and to determine the suitable conditions for sensor preparation. The processing of sensors from nanopowders should be controlled especially because it involves calcinations at elevated temperature that can have detrimental effect on the nanostructure.

Fig. 1. Specific surface area SSA and concentration  of Cr dopant as a function of nprec for TiO2 :Cr nanopowders the sum of precursors’ mole number per minute obtained by FSS.

The motivation for this study is to decrease the temperature of hydrogen detection as well as to investigate the size effect with respect to gas sensing properties. The effect of chromium concentration and crystallite size on the sensor dynamic responses to hydrogen is discussed. 2. Experimental Nanopowders of TiO2 :Cr with up to 10 at.% Cr were obtained by flame spray synthesis (FSS) as described in detail in [59,60]. Titanium tetra-isopropoxide TTIP and a solution of chromium acetyloacetonate CHAA in m-xylene were used as precursors of Ti and Cr, respectively. The total feed flow rate FRTotal of precursors was kept constant during the synthesis process. In order to increase chromium concentration in TiO2 nanoparticles, the feed flow rate FRdopant of Cr precursor was increased and FRTTIP of the main precursor of titanium TTIP was diminished. This resulted in a simultaneous increase in the Cr concentration in TiO2 and, as shown in Fig. 1, an enhanced specific surface area, SSA. Specific surface area, SSA, of nanopowders was determined from nitrogen adsorption BET (Brunauer–Emmett–Teller) isotherms obtained with a Beckman-Coulter SA3100. Powders were annealed under nitrogen atmosphere at 180 ◦ C for 2 h in order to desorb water from their surface. As self-contained nanopowders cannot be used directly in gas sensing experiments, further processing of the starting materials into tablets by calcinations was necessary. In order to establish an appropriate temperature of the subsequent calcinations, thermogravimetry (TG) measurements were carried out on SDT 2960 TA INSTRUMENTS apparatus. The nanopowder samples of mass around 20 mg were placed in the standard platinum crucibles and heated at a rate of 10 ◦ C min−1 . The measurements were carried out under dynamic conditions (the flow of 100 cm3 min−1 ) in synthetic air atmospheres (<15 ppm H2 O). The volatile products of decomposition were analyzed by a quadrupole mass spectrometer (THERMOSTAR QMD 300 BALZERS) connected on-line to SDT 2960 system by a quartz capillary heated up to 200 ◦ C. Then, the nanosensors were prepared in a form of circular tablets of 7 mm diameter and 1 mm thickness. Nanopowders have undergone calcinations at a pressure of 25 MPa and temperature of 400 ◦ C following the results of DTA–TG. Planar silver electrodes were applied 5 mm apart. In order to understand the influence of the processing of tablets on the properties of the material used for gas

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Fig. 2. Schematic diagram of a gas sensing experimental set-up.

sensors, i.e., the morphology, crystallinity and phase composition standard characterization methods were employed. Dimensions of crystallites were analyzed on the basis of XRD patterns recorded with the help of X’Pert MPD Philips diffractometer both for nanopowders and sensing tablets. The crystallite size was calculated from XRD patterns using Scherrer method [62] as: DXRD =

0.9  cos 

(1)

where  is X-rays wavelength,  denotes a full width at half maximum (FWHM),  is the diffraction angle. Scanning electron microscopy (SEM) of nanopowders and tablets was performed with a Hitachi SU-70 apparatus. Dynamic changes in the electrical resistance R/R0 upon hydrogen exposure were detected over low-to-medium concentration range of 50–3000 ppm at 200–400 ◦ C in a custom-made system described in detail elsewhere [63]. Here, the schematic diagram of the experimental set-up is presented in Fig. 2. Prior to all measurements, the standardization procedure was applied for each sensor. The sensing tablets were heated up to 400 ◦ C for 18 h in a synthetic air atmosphere. The temperature increase was kept constant at 2◦ /min. Gas sensors placed in a measuring chamber were not powered continuously but a multiplexer was used to switch between the measured signals connected to the Hewlett Packard HP34401 multimeter. The multimeter measured sequentially 5 signals: voltage drop at the Pt–RhPt thermocouple, and two sensor resistances at two different current polarizations R+ and R− . The sensor signal was calculated as: (R+ + R− )/2. The time between switching was programmed and its value could be chosen within the range from 10 s to 1 min. Air, at the flow rate of 120 sccm, was used as a reference gas. Sensor response, S was calculated as S=

R0 − R R0

(2)

where R represented the electrical resistance upon interaction with gas and R0 denoted the electrical resistance in the reference gas (air). 3. Results Flame spray synthesis (FSS) provided us with the nanocrystalline powders of undoped TiO2 and Cr-doped TiO2 with a large specific surface area SSA over the range of 37–160 m2 /g and a small crystallite size of 6–27 nm (see Table 1). The increase in SSA with higher chromium concentration is determined by the synthesis parameters. As can be seen in Fig. 1 there is a correlation between the precursor’s mole number and

Fig. 3. Mass loss determined from thermogravimetric TG measurements as a function of temperature (a) for FSS-made and commercial (Evonic Degussa and Sigma–Aldrich) nanopowders of TiO2 with different specific surface area SSA and (b) for FSS-made TiO2 :Cr nanopowders with different concentrations of Cr; specification is given in Table 1.

SSA. Namely, higher precursor’s mole number per time unit is related to lower SSA, because higher concentration of precursor in the flame leads to the higher supersaturation and as a result to the creation of bigger particles. Results of the thermogravimetric measurements concerning the mass loss upon heating are shown in Fig. 3. Undoped TiO2 nanopowders obtained by flame spray synthesis (FSS) are compared with commercial nanopowders: P25 from Evonic Degussa and TiO2 rutile from Sigma–Aldrich in Fig. 3a. As it can be seen, the mass loss that starts at about 30–40 ◦ C and continues up to 400 ◦ C is more pronounced for nanopowders of an enhanced specific surface area (SSA) and thus smaller particle size. This applies to both undoped and Cr doped TiO2 nanopowders. As the nanopowders are synthesized from organic precursors, the mass loss observed below 400 ◦ C is due to the combustion of organic residua. Mass spectra indicate emission of CO2 and H2 O, only, upon heating. For undoped TiO2 and for nanopowders with low Cr content, up to 0.5 at.% the loss of mass is of the order of 5% and has been completed at 400 ◦ C. At higher Cr concentration much smaller loss of mass continues above 400 ◦ C and for TiO2 :5 at.% Cr it amounts to about 0.64% over the temperature range of 400 ◦ C and 800 ◦ C. The reasons of this instable behavior are difficult to verify experimentally. We took into account the possible conversion

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Table 1 Results of the quantitative analysis of XRD diffraction patterns of Cr-doped TiO2 nanopowders. at.% Cr

Type of sample

Specific surface area SSA (m2 /g)

Crystallite size from XRD (nm)

Lattice parameters (nm)

Anatase (A)

Rutile (R)

a

c

0.3784 (A) 0.4606 (R) 0.3785 (A) 0.4598 (R) 0.3785 (A) 0.4597 (R) 0.3787 (A) 0.4594 (R) 0.3787 (A) 0.4595 (R) 0.3784 (A) 0.4597 (R) 0.3786 (A) 0.4595 (R) 0.3781 (A) 0.4596 (R) 0.3779 (A) 0.4595 (R) 0.3784 (A) 0.4593 (R)

0.9512 (A) 0.2956 (R) 0.9511 (A) 0.2955 (R) 0.9507 (A) 0.2953 (R) 0.9508 (A) 0.2959 (R) 0.9508 (A) 0.2960 (R) 0.9511 (A) 0.2951(R) 0.9507 (A) 0.2954 (R) 0.9499 (A) 0.2950 (R) 0.9514 (A) 0.2953 (R) 0.9510 (A) 0.2958 (R)

0

As prepared

37.5

26.8

13.6

0.1

As prepared

48.4

21.2

9.6

0.2

As prepared

47.6

22.7

11.2

0.2

Annealed 450 ◦ C

–

22.3

18.7

0.2

Annealed 800 ◦ C

–

32.3

30.2

0.5

As prepared

72.2

15.7

13.0

1

As prepared

87.1

13.8

9.3

5

As prepared

126.6

9.1

7.5

10

As prepared

160.7

6.0

6.5

22.0

19.4

10

◦

Annealed 800 C

–

Table 2 Calculated mass loss due to oxygen removal according to (3) and (4) reactions given in the text; calculation were performed for TiO2 :5 at.%Cr. Reaction

Mass of O2 (g) (for 1 mol Ti0.95 Cr0.05 )

Mass variation (%)

Reaction (3) Reaction (4)

0.3999 1.2015

0.4995 1.4857

of Cr(IV) or Cr(VI) to Cr(III) with oxygen loss above 400–450 ◦ C according to the following reactions: 2CrO2 → Cr2 O3 + 1/2O2

(3)

2CrO3 → Cr2 O3 + 3/2O2

(4)

Theoretically predicted values of mass loss due to oxygen removal are given in Table 2 for TiO2 :5 at.% Cr. Reaction (3) seems to be quite probable at higher temperatures as the calculated mass loss agrees with the one experimentally observed. On the other hand, the presence of Cr(IV) in TiO2 cannot be excluded on the basis of XPS measurements as shown in our previous paper [59]. Fig. 4 shows comparison between X-ray diffraction patterns of nanopowders and calcined tablets. Two polymorphic forms of TiO2 , i.e., rutile and anatase are clearly seen. The fraction of anatase largely dominates over rutile, but the rutile amount increases progressively with the addition of Cr, as shown before [59,60]. Up to 10 at.% Cr none of the secondary phases, neither chromium oxide nor chromium titanates have been identified. This is compatible with the reported solubility limit of Cr3+ in TiO2 and the fact that the ionic radii of Cr3+ and Ti4+ are very similar [64]. The lattice constants of TiO2 remain almost unaffected upon variation in Cr concentration, as shown in Table 1, indicating substitutional incorporation of chromium at titanium lattice sites. As can be seen in Fig. 4, apparently there is no big difference between XRD patterns of nanopowders and tablets. The crystallite size does not change during material processing at 400 ◦ C. However, small differences in the intensities of X-ray diffraction peaks between powders and tablets remain and can be attributed to the decomposition of organic residua during calcination up to 400 ◦ C as identified by TG measurements (Fig. 3). In order to study evolution of the crystallographic structure of nanopowders upon post-synthesis heat treatment, in Fig. 5 we have demonstrated XRD patterns for two nanopowder samples of TiO2 doped with 0.2 at.% Cr (Fig. 5a) and 10 at.% Cr (Fig. 5b) annealed at

Fig. 4. Comparison of XRD patterns for TiO2 -based nanopowders and gas sensors tablets calcined from nanopowders at the pressure of 25 MPa at the temperature of 400 ◦ C. P25 Evonic Degussa powder with the SSA = 50 m2 /g, well-known for its excellent photocatalytic properties, is given as a reference; A – anatase; R – rutile.

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Fig. 5. XRD patterns of Cr-doped TiO2 nanopowders annealed at 450 ◦ C and 800 ◦ C: (a) 0.2 at.% Cr and (b) 10 at.% Cr; A – anatase; R – rutile.

Fig. 6. Comparison of morphology for TiO2 -based nanopowders (SEM images) and for gas sensors tablets prepared from nanopowders at the pressure of 25 MPa at the annealing temperature of 400 ◦ C. The influence of the increasing amount of Cr can be seen.

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Fig. 7. Dynamic changes in the electrical resistance for TiO2 and TiO2 :5 at.% Cr nanosensors upon exposure to 500, 1100 and 3000 ppm H2 at a constant temperature of 350 ◦ C.

Fig. 9. Gas sensor response S as a function of temperature at a constant hydrogen concentration of (a) 200 ppm and (b) 1100 ppm.

Fig. 8. Absolute value of the gas sensor response |S| as a function of hydrogen concentration at a constant temperature of (a) 250 ◦ C and (b) 350 ◦ C.

450 ◦ C and 800 ◦ C. As can be noticed, in the case of samples heated at 450 ◦ C anatase polymorphic form predominates. However, the contribution from rutile appears and increases upon annealing at 800 ◦ C. Rutile amount is more significant for TiO2 :10 at.% Cr than for TiO2 :0.2 at.% Cr at 800 ◦ C which is related to the increased density of vacancies accompanying Cr doping. Higher density of oxygen vacancies is believed to enhance the anatase to rutile transformation [65]. SEM images of the nanopowders and tablets of TiO2 and TiO2 :Cr are shown in Fig. 6. Nanopowders particle morphology is typical for flame spray synthesis. Large, spherical grains of the size up to 40 nm are formed for pure TiO2 and at low Cr concentration (0.2 at.%). These large agglomerates are composed of much smaller crystallites in agreement with X-ray diffraction analysis. The size of crystallites decreases systematically with the increasing Cr composition (see Table 1). This is a consequence of a dilution effect described in detail in [66] and explained in Fig. 1. It is possible that Cr prevents an excessive agglomeration. Fig. 7 presents changes of the electrical resistance R of undoped TiO2 and TiO2 doped with 5 at.% Cr sensor materials upon interaction with the reducing gas H2 as a function of time at a constant temperature of 350 ◦ C. Large and reproducible responses of the sensors follow the step changes 0–500 ppm–0, 0–1100 ppm–0, 0–3000 ppm–0 in H2 concentration. As we can see, there is a systematic decrease in the electrical resistance R for undoped TiO2 upon exposure to H2 , whereas for the TiO2 :5 at.% Cr the same parameter R increases upon interaction with H2 . Such changes in

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Fig. 10. Repeatability of the dynamic responses in the electrical resistance R upon exposure to H2 for (a) undoped TiO2 (SSA = 37.5 m2 /g) at 350 ◦ C; (b) undoped TiO2 (SSA = 106.8 m2 /g) at 350 ◦ C; (c) TiO2 :0.2 at.% Cr (SSA = 47.6 m2 /g) at 300 ◦ C.

R upon adsorption of hydrogen indicate that undoped TiO2 and TiO2 doped with up to 1 at.% Cr behave as n-type semiconductors, whereas starting from 5 at.% Cr the samples exhibit p-type conductivity. Moreover, as one can see in Fig. 7, incorporation of 5 at.% Cr into TiO2 causes a decrease by two orders of magnitude in the baseline electrical resistance R0 of the material in air, as compared with undoped TiO2 . In order to compare the sensors sensitivity the absolute value |S| is presented in Fig. 8 because of the observed sign reversal of the sensor response S defined by Eq. (2) when approaching 5 at.% Cr. At 250 ◦ C the best sensor response is obtained at 5 at.% of Cr, whereas at higher temperatures of 350 ◦ C undoped TiO2 is slightly better than TiO2 :5 at.% of Cr. This is due to the fact that at 250 ◦ C the baseline resistance of undoped TiO2 is too high and the signal is not measurable. The responses are nonlinear and tend to saturate at high hydrogen concentrations. Fig. 9 demonstrates the response S of the TiO2 :Cr nanosensors as a function the operating temperature for a given change in the hydrogen concentration (from 0 to 200 ppm and from 0 to 1100 ppm). As can be noticed, with the decreasing temperature the sensor responses have a tendency to increase what is especially important from the point of view of power consumption. For undoped TiO2 the lowest temperature at which the baseline resistance could be measured was 300 ◦ C. This relatively high operating temperature for the undoped TiO2 can be lowered to 210–250 ◦ C by adding Cr. Moreover, Cr incorporation enables the extension of the temperature range under which the sensors work. The optimum operating temperature depends on the sensor composition. Taking into account the relatively low operating temperature the

best candidates for commercially used sensors in terms of possible costs are TiO2 :5 at.% Cr and TiO2 :10 at.% Cr. In order to investigate stability of the gas sensing performance of synthesized TiO2 nanomaterials the hydrogen sensing measurements were repeated intentionally. Fig. 10a and b demonstrates dynamic changes in the electrical resistance for undoped TiO2 with different specific surface areas: SSA = 37.5 m2 /g (Fig. 10a) and SSA = 106.8 m2 /g (Fig. 10b) upon interaction with H2 at 350 ◦ C recorded in two consecutive experiments performed after one year (Fig. 10a) and one month (Fig. 10b). As it can be seen for both samples the curves remain in good agreement as far as dynamics of the sensor response is concerned. However, the decrease in the initial electrical resistance value (base line) after one year (Fig. 10a) and one month (Fig. 10b) cannot be neglected. The same type of measurement was also implemented for TiO2 :0.2 at.% Cr (SSA = 47.6 m2 /g). Fig. 10c presents dynamic changes in the electrical resistance of the sample upon exposure to H2 at 300 ◦ C observed in two experiments performed after two weeks. In this case both dynamics of the sensor response and the base line remain in good conformity being a proof of satisfying short term stability of the sample.

4. Discussion Theoretical models of the gas sensing mechanism and the influence of the grain size on the metal-oxide sensor performance take into account the relation between grain size D and Debye length D which corresponds to the width of depletion or accumulation near surface layer formed by ions chemisorbed at the surface [67,68].

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Fig. 11. The relation between crystallite size DXRD and double Debye length 2D as a function of the concentration of ionic defects Nd calculated at 298 K and 600 K.

For the purposes of this work, the Debye length D was calculated assuming that Cr dopant is completely ionized, Cr ions substitute for Ti ions in TiO2 lattice and there is no segregation of dopants. The following formula was used:



D =

εε0 kT e2 Nd

(5)

where ε denotes the dielectric constant, ε0 is the permittivity of vacuum, k is the Boltzmann constant, e is an elementary charge, T denotes temperature while Nd represents the concentration of ionic defects related to the concentration of chromium dopant introduced into TiO2 lattice. The concentration of ionic defects Nd was calculated following the formula: Nd =

 · NA p · M 100

(6)

where  is the density of anatase, NA denotes Avogadro constant, M is molecular mass of TiO2 , whereas p stands for atomic percentage of ionic defects. Fig. 11 illustrates the ratio of the crystallite size DXRD and double Debye length 2D as a function of the concentration of ionic defects Nd for temperatures 298 K and 600 K (left vertical and bottom horizontal scales). The crystallite size DXRD as a function of Cr concentration (right vertical and top horizontal scales) is also presented. The crystallite size DXRD for pure TiO2 sample is comparable to 2D which means that the relatively large crystallite is almost fully affected by the interaction with gas. As we enhance the amount of chromium dopant the two effects, namely the increase of concentration of ionic defects which leads to the decrease in the Debye length as well as the decrease of crystallite size due to dilution effect overlap and as a result the ratio DXRD /2D increases. This means that in the case of TiO2 doped with Cr the near surface layer affected by the interaction with gas is comparatively thin when confronted with the crystallite size. Chromium dopant affects the electronic structure of TiO2 and forms localized acceptor levels in the forbidden band gap. Although, as far as we consider both pure TiO2 as well as TiO2 lightly doped with Cr (up to 1 at.% Cr) the gas sensing mechanism is governed •• by doubly ionized oxygen vacancies VO which are formed spontaneously according to the reaction: 1  OO ↔ O2(g) + V•• O + 2e 2

(7)

Fig. 12. Diagram of the electronic structure of nanomaterials in contact with the gas phase for (a) undoped TiO2 and (b) TiO2 doped with Cr; D is Debye length; Eg – band gap energy, EV – valence band edge; EC – conduction band edge; EF – Fermi energy, E – electron energy; x – distance in real space.

The influence of trivalent chromium dopant becomes predominant as the concentration of additive increases (starting from TiO2 :5 at.% Cr). Cr ions incorporate into TiO2 structure substitutionally according to reaction: Cr2 O3 +

1 O2 → 2CrTi + 4OO + 2h• 2

(8)

which modifies the concentration of electrons and electron holes and leads to transfer from n-type to p-type conductivity. Fig. 12 presents the band diagram proposed for two discussed limits of the Debye length. Fermi level EF is placed below the bottom of the conduction band EC (x), which comes as a consequence of the positive ionic defects resulting from the oxygen nonstoichiometry in TiO2 . For the crystallite diameter exceeding the twice the Debye length 2D , the interior of crystallites exhibits an n-type behavior, but the near-surface layers may show a p-type conductivity due to the band bending that stems from oxygen chemisorptions (Fig. 12a). Taking into consideration that mobilities of electrons and electron holes in TiO2 assume comparable values, the following criteria on n- and p-type conductivity across the grain (0 < x < rg , where rg is the grain radius) can be given: n-type : EC (x) − EF < EF − EV (x) p-type : EC (x) − EF > EF − EV (x) Fermi level EF of TiO2 with smaller Debye length at higher chromium content, is demonstrated in Fig. 12b. Fermi level EF is located just above the maximum of the valence band edge EV (x)

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due to the acceptor-type substitutional defects CrTi formed according to Eq. (8). In this case, the material exhibits p-type conductivity across the whole grain. 5. Conclusions The results obtained in the course of these studies allow to formulate the following conclusions: • Crystalline nanopowders of mostly anatase TiO2 , grown by flame spray synthesis, constituting starting materials for gas sensors, are characterized by large specific surface area and small crystallite size depending on Cr doping. • The mass loss of nanopowders upon heating up to 400 ◦ C is higher for more developed specific surface area SSA. • Calcination temperature of 400 ◦ C is suitable for gas sensor processing from nanopowders. • Morphology of gas sensing materials calcined at 400 ◦ C in a form of tablets is similar to that of starting nanomaterials. • Nanosensors obtained from TiO2 :Cr nanopowders show promising dynamic characteristics in response to hydrogen. The sensor responses are large and reproducible. • The electrical resistance R decreases upon hydrogen exposure up to 1 at.% Cr while the reversed effect is observed at 5 at.%. • The sensor performance clearly improves with a decrease in the operating temperature. Incorporation of 1 at.% and 5 at.% of Cr into TiO2 allows to attain the measurable baseline resistance over the temperature range of 210–250 ◦ C thus proving high sensor sensitivity to hydrogen. • Electronic structure of TiO2 is modified by doping with chromium. The point defect structure of TiO2 :Cr is proposed. • The n–p transition depends on the dopant concentration and crystallite size of nanopowders. Acknowledgements This work is supported by the Polish Ministry of Science and Higher Education (2009–2012) grant no. N N507 466537. One of the authors (B.L.) acknowledges the Statutory Project for Science for 2012 at the Department of Electronics AGH–UST. References [1] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sens. Actuators B Chem. 5 (1991) 7–19. [2] E. Comini, Metal oxide nano-crystals for gas sensing, Anal. Chim. Acta 568 (2006) 28–40. [3] O.K. Varghese, D. Gong, M. Paulose, K.G. Ong, C.A. Grimes, Hydrogen sensing using titania nanotubes, Sens. Actuators B Chem. 93 (2003) 338–344. [4] K.E. La Flamme, C.A. Grimes, Non-carbon nanotubes hydrogen sensors based on TiO2 , in: F.J. Arregui (Ed.), Sensors Based on Nanostructed Materials, Springer, 2009, pp. 29–57, and references herein. [5] M.-H. Seo, M. Yuasa, T. Kida, J.-S. Huh, K. Shimanoe, N. Yamazoe, Gas sensing characteristics and porosity control of nanostructured films composed of TiO2 nanotubes, Sens. Actuators B Chem. 137 (2009) 513–520. [6] E. Comini, G. Faglia, G. Sberveglieri, Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts, Appl. Phys. Lett. 81 (2002) 1869–1871. [7] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles, Nano Lett. 5 (2005) 667–673. [8] E. Comini, G. Faglia, M. Ferroni, A. Ponzoni, A. Vomiero, G. Sberveglieri, Metal oxide nanowires: preparation and application in gas sensing, J. Mol. Catal. A Chem. 305 (2009) 170–177. [9] R. Yoshida, Y. Suzuki, S. Yoshikawa, Syntheses of TiO2 (B) nanowires and TiO2 anatase nanowires by hydrothermal and post-heat treatments, J. Solid State Chem. 178 (2005) 2179–2185. [10] S.A. Hooker, Nanotechnology advantages applied to gas sensors development, in: The Nanoparticles 2002 Conference Proceedings, Norwalk, CT, USA, Business Communications Co., Inc., 2002, pp. 1–7. [11] W. Göpel, T.A. Jones, M. Kleitz, J. Lundsröm, T. Seiyama (Hrsg), Chemical and biochemical sensors, in: W. Göpel, K.D. Schierbaum (Eds.), Sensors, vol. 2, Verlag VCH, 1991.

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Biographies B. Lyson-Sypien received her master degree in physics from the Pedagogical University of Kraków, Poland in 2008. Since 2008 she has been a Ph.D. student at Faculty of Physics and Applied Computer Science at AGH University of Science and Technology, Kraków, Poland. Her scientific research concern nanomaterials for resistive gas sensors. A. Czapla received his Ph.D. degree in physics from the AGH University of Science and Technology, Kraków, Poland in 1973. Being employed as a member of staff at the Faculty of Electrical Engineering, Automatics, Computer Science and Electronics at the AGH University of Science and Technology, he has been involved in the research concerning gas sensing characteristics and optical properties of mixed oxide thin films and nanopowders. M. Lubecka received her Ph.D. degree in physics from the AGH University of Science and Technology, Kraków, Poland in 1971. She has worked as a senior research

scientist at the Faculty of Electrical Engineering, Automatics, Computer Science and Electronics at the AGH University of Science and Technology. Her current research interests concern gas sensing and photocatalytic properties of TiO2 –SnO2 nanocrystalline ceramics. P. Gwizdz received his master degree in electronics in 2010 from the Faculty of Electrical Engineering, Automatics, Computer Science and Electronics, AGH University of Science and since that time he has been a Ph.D. student at the same faculty. His research interests include gas sensor arrays, pattern recognition and intelligent electronic sensors systems with a goal to construct an electronic nose based on nanosensors. K. Schneider received her Ph.D. in physics from the Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland in 2004 working on neutron transport. Currently she is a regular staff member at the Faculty of Electrical Engineering, Automatics, Computer Science and Electronics at AGH University of Science and Technology. Her research interests concern nanopowders and thin films for the photocatalytic applications. K. Zakrzewska received her Ph.D. in technical physics from AGH-University of Science and Technology, Kraków, Poland in 1986 working on transparent conductive oxides TCO. In 1986–1988 she was employed as a member of staff at the Center of Laser Studies at the University of Southern California. Since 1989, she has been given a permanent position at the Department of Electronics, AGH-University of Science and Technology. In 2004 she has presented her habilitation on the application of titanium dioxide thin films for gas sensors and photonic devices. Since 2007, she has been appointed a professor of AGH. K. Zakrzewska is currently a head of the research team that works actively in several scientific projects concerning thin film deposition, studies of the resulting thin film properties and gas sensing applications with the emphasis on the titanium compounds (oxides and nitrides). The technological efforts of this team concentrate at the reactive ion sputtering. K. Michalow received her master degree in materials science from AGH University of Science and Technology in 2005. She studied in the International PhD School Switzerland–Poland and obtained her Ph.D. degree in chemical science in the field of chemistry from AGH University of Science and Technology in 2010. Since 2009 she is employed as a Postdoc scientist in Laboratory for High Performance Ceramics at Empa Materials Science and Technology, Switzerland. Her main research interests cover synthesis and properties of doped titanium dioxide nanoparticles as well as application of nanoparticles in photocatalysis of the pollution in water and air. T. Graule received his Ph.D. in analytical chemistry from the University of Dortmund and MPI for Metals Research, Germany, in 1988. After conducting research in the field of nanotechnology at the Fraunhofer Institute for Applied Materials Research, Bremen, Germany (1988) and in ceramics technology at the Swiss Federal Institute of Technology (ETH) Zurich (1989–1994), he joined Katadyn Products Inc., Wallisellen, Switzerland, in 1995 as Head of Production and Development. In 1999, he became Head of the Laboratory for High Performance Ceramics at Empa Materials Science and Technology. A. Reszka received her B.Sc. degree in Applied Informatics (2006) and graduated in Physics (2010) at the Faculty of Mathematics and Natural Sciences, Cardinal S. Wyszynski University in Warsaw. Since 2009 she has been working in the Division of Solid State Spectroscopy, Institute of Physics Polish Academy of Sciences, focusing on scanning electron microscopy and cathodoluminescence spectroscopy of semiconducting nanostructures. M. Rekas received his PhD and habilitation degrees in physical chemistry from the AGH University of Science and Metallurgy, Krakow, Poland in 1972 and 1986, respectively. In 1983 he was a vesting scientist in the School of Mining in Saint-Etienne, France, where he worked on electrochemical gas sensors. In 1987 he joined the sensor group in the Max-Planck-Institute for Solid State Research in Stuttgart. Between 1994 and 2002 he worked at the Materials Division of the Australian Nuclear Science and Organisation (4 years) and then at the School of Materials Science and Engineering, the University of New South Wales in Sydney. He is currently full professor of chemistry at the Faculty of Materials Science and Ceramics, AGH University of Science and Metallurgy in Krakow, Poland. His research interests are the physics and chemistry of solids, particularly electrochemistry of solids, fuel cells and chemical gas sensors A. Lacz received her Ph.D. degree in chemistry form the AGH University of Science and Technology, Kraków, Poland in 2007. Since her masters studies she has been interested, and involved in application of the thermal analysis methods and the mass spectrometry, especially related to the thermal decomposition of solids. Currently she is a regular staff member of the Department of Inorganic Chemistry, AGH University of Science and Technology. M. Radecka received her Ph.D. and habilitation degree in chemistry from the AGH University of Science and Technology, Kraków, Poland in 1993 and 2004, respectively. She is currently full professor of chemistry at the Faculty of Materials Science and Ceramics, AGH University of Science and Metallurgy in Krakow, Poland. Her research interests include development of semiconducting gas sensors and photoelectrochemical cells, experimental studies of the electrical and structural properties of ceramic oxides and thin films as well as the electron transport mechanisms in oxide semiconductors. Her research works tend to understand the correlation between the defect structure of crystals and physico-chemical properties of nonstoichiometric compounds.