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Electrical and spectroscopic properties of Ti0.2 Sn0.8O2 solid solution for gas sensing
Thin Solid Films 517 (2009) 6176–6183
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electrical and spectroscopic properties of Ti0.2 Sn0.8O2 solid solution for gas sensing M.C. Carotta a,⁎, S. Gherardi a, V. Guidi a, C. Malagù a, G. Martinelli a, B. Vendemiati a, M. Sacerdoti b, G. Ghiotti c, S. Morandi c a b c
CNR-INFM and Dipartimento di Fisica, Università di Ferrara, 44100 Ferrara, Italy Dipartimento di Scienze della Terra, Università di Ferrara, 44100 Ferrara, Italy Dipartimento di Chimica IFM, Università di Torino, 10125 Torino, Italy
a r t i c l e
i n f o
Available online 5 April 2009 Keywords: Nanomaterials Semiconductor oxides Solid solutions Chemoresistive gas sensors
a b s t r a c t In this work we report the synthesis, microstructure, electric and spectroscopic properties, and sensing performances of TixSn1 − xO2 (x = 0.1, 0.2, 0.3, 0.5, 0.7, 0.9) nano-powders and of SnO2 and TiO2 reference samples, prepared via sol-gel route starting from metal-organic precursors working in hydro-alcoholic media. Actually, the attention is particularly focused on properties of the sample with x = 0.2, in comparison with ones of the other solid solutions and of the single oxides. Indeed, this solid solution showed a borderline behaviour between that of the solid solution with x = 0.1 and that of the other solid solutions with x ≥ 0.3. An abrupt change in the structural, electrical and spectroscopic properties has been observed, passing from sample with x = 0.1, showing a behaviour very similar to that of SnO2, to one x = 0.3 showing a behaviour very similar to that of TiO2. The borderline properties of the mixed oxide with x = 0.2 represent the expected continuous transition among the two behaviours. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Among the metal oxides, tin dioxide and titanium dioxide, due to their chemical and electrical properties, are particularly appealing both for basic research and for a wide variety of possible applications [1,2]. Tin dioxide is the most common material in gas sensing, but it is widely used as transparent conductor and in heterogeneous catalysis; titanium dioxide is used as a photocatalyst, in solar cells, as an optical coating, in gas sensing, etc. Tin dioxide and titanium dioxide are both wide-gap semiconductors, showing several similarities in structural as well as in electronic properties. However, they exhibit also some peculiar differences, such as electrical conductivity and gas sensing behaviour. For both materials, the n-type behaviour is due to stoichiometric defects, generally oxygen vacancies acting as electronic donor levels, their energetic positions being much more deep inside the band gap for TiO2 [3,4]. On the other hand, there is not a general agreement on what causes the donor levels in TiO2, some authors assigning donor levels, 0.8 eV deep, at Ti3+ ions rather than at oxygen vacancies [2,5]. A part from this controversy, the observed much smaller conductivity of TiO2 with respect to that of SnO2 is reasonably imputable to the different energetic position of the donor levels in the two semiconductors [6]. The gas sensing mechanism in all polycrystalline n-type semiconductors is generally ascribed to the Schottky barrier formation at gas-semiconductor interface, leading to a negative surface charge ⁎ Corresponding author. Tel.: +39 0532974230; fax: +39 0532974325. E-mail address: [email protected] (M.C. Carotta). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.04.002
accumulation, typically O− ions. The variation of the height of the intergranular barrier is the result of surface chemical reactions with environmental gases leading to electrical conductance modifications. The two materials differ in gas sensing behaviour, being TiO2 much less reactive versus reducing agents than SnO2, however the mechanism of sensing seems to be common. The operation temperature of the two materials are very similar, in the range of 400–600 °C, which are typical of surface interaction mechanism, and not bulk one. Some of the authors of this paper discussed this point in [7] considering the different role played by the Fermi level pinning in the two semiconductors. Usually, mixed metal oxides and solid solutions have been considered for the superior performances shown in comparison to the single oxides [8–12]. In this context, we attempted to join advantages of the better characteristics of both materials: high gas sensitivity for SnO2 and lower influence by humidity for TiO2, and to overcome their disadvantages: poor selectivity for SnO2 and high resistivity and the polymorphic transition with temperature accompanied by exaggerated grain growth for TiO2 [13]. With respect to the structural properties, SnO2 and TiO2 exhibit rutile crystalline structure, space group P42/mnm. For this reason, they would easily form solid solution. Actually, these compounds show a miscibility gap with a reported critical temperature, Tc, ranging from 1300 to about 1500 °C, above which the solid solution is formed [14,15]. Indeed, as described in a previous paper, we could clearly observe a spinodal decomposition in tin-rich and titanium-rich oxide phases, only for the composition with x = 0.7 when calcined at high temperature (850 or 1050 °C) [16].
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To this aim, TixSn1 − xO2 (0.1 ≤ x ≤ 0.9, step 0.2) nanopowders were synthesised and characterized in an other paper [17]. We observed an abrupt change in electrical and spectroscopic behaviour passing from the solid solution with x = 0.1 to the other solutions (starting from x = 0.3), the first one showing a behaviour very similar to that of pure SnO2 while all the others to that of pure titania. To state more precisely the Ti molar content for which this transition occurs, this work has been particularly devoted to the synthesis and investigation of the composition with x = 0.2 not previously investigated. Electron microscopy, X-ray diffraction (XRD) and specific surface area measurements were adopted to analyse the morphology, the crystalline structure and the mean grain radius of the powders. Diffuse reflectance UV–Vis-NIR and absorbance FT-IR spectroscopies were used to investigate bulk and surface electronic properties, and surface chemistry under different atmospheres. Finally, electrical measurements (conductance, surface barrier potential behaviour and gas sensing properties) were performed. 2. Experimental 2.1. Powder synthesis and film preparation Each Tix Sn1 − xO2 solid-solution was obtained via sol-gel route starting from the stoichiometric solution of Sn(II)-ethylhexanoate (95% Aldrich) and Ti-butoxide (97% Aldrich) in hydro-alcoholic media. As an example, Ti0.2 Sn0.8O2 solution ([Sn2+]:[Ti4+] = 4:1) was synthesised mixing 2.7 mmol of Ti-butoxide together with 10.8 mmol of Sn(II)ethylhexanoate. Both the Ti4+ and Sn2+ sources were used without further purification. Diluted HNO3 was added dropwise to hydrolyse the metal-organic molecules. For each stoichiometry, the whole process was carried out by maintaining the solution under soft stirring at the temperature of 50 °C. The resulting colloids were washed, dried and crunched yielding the white hydrous precursors of the solid solutions. The irreversible conversion precursor/binary oxide was gained by calcining at 550 °C for 2 h. The completion of the reaction was previously checked by a TG/DTA analysis through a Netzsch STA 409 equipment. In order to study the structural and the morphological evolution of the nanopowders with temperature, each precursor was further calcined at 650, 850 or 1050 °C for 2 h. (Ti,Sn)O2 binary oxides will be hereinafter labelled as [ST-x × 100]T where T is the calcination temperature. The powders were used to deposit sensing layers by means of screen printing technology onto miniaturized alumina substrates each one provided with a heater element and comb-type gold electrodes for electrical measurements. The layers were fired for 1 h at 650, 750 or 850 °C in order to study the influence of the temperature on the sensing behaviour of each film. Moreover, to compare [ST-x × 100] with pure SnO2 and pure TiO2 powders were synthesised and thick films were realized with the same preparation parameters.
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550 °C in vacuum and in dry oxygen, then cooled down at RT before the treatments in CO at increasing temperatures up to 500 °C. Diffuse reflectance UV–Vis-NIR spectra were run at RT on a Varian Cary 5 spectrophotometer, working in the range of wavenumbers 53,000– 4000 cm− 1. UV–Vis-NIR spectra are reported in the figures as raw data; to employ a quantity equivalent to the absorbance used in the absorption spectroscopy, in the ordinate scale we used the KubelkaMunk function [f (R∞) = (1 − R∞)2 / 2 R∞, where R∞ = the reflectance of an ‘infinitely thick’ layer of the sample] [19]. For FT-IR analysis, the powders were compressed in self-supporting disks and placed in a commercial heatable stainless steel IR cell (Aabspec) allowing in situ thermal treatments up to 600 °C and simultaneously spectra recording. The disks were treated in situ, first at 550 °C in vacuum and in dry oxygen, then cooled down at pre-set temperatures (from RT up to 500 °C) before the interaction with CO at the same temperatures. Absorption FT-IR spectra were run on a Perkin-Elmer System 2000 FT-IR spectrophotometer equipped with a Hg–Cd–Te cryodetector, working in the range of wavenumbers 7800– 580 cm− 1. FT-IR spectra are reported in the figures either as raw data or as difference spectra (where the subtrahend spectrum is that of the sample out-gassed and oxidized at 550 °C and then cooled in O2 at the temperature chosen for the interaction with CO). 2.3. Film characterization The morphology of all the films was observed using scanning electron microscopy (SEM, model EVO 40, Carl Zeiss). The flow-through technique was used maintaining a constant flow of 0.5 l/min to carry out conductance measurements in dry or wet air or in mixtures of air and test gases. Dynamic responses of sensing films were obtained with methane and carbon monoxide in dry or wet air, by varying the operating temperature from 400 to 650 °C. Moreover, temperature-stimulated conductance measurements, which consist in measuring conductance as a function of time after a fast temperature variation, were also performed to determine the energy barrier, e.g., the difference in energy between the conduction band bottom at surface and that in the bulk, as a function of the temperature (25–650 °C), with a procedure described elsewhere [20,21].
2.2. Powders characterization X-ray diffraction (XRD) analysis measurements were performed using a Philips PW 1830 vertical diffractometer with Bregg–Brentano geometry (Cu Kα radiation, 40 kV, 30 mA) provided with a graphite monochromator along the diffracted beam. Diffraction patterns were collected in the range 10–120° (2θ) with steps of 0.02° and 10 s of dwell time. XRD data were elaborated using a Rietveld analysis program FullProf (release 2006) [18]. The microstructure of the powders was observed by transmission electron microscopy (TEM, model H-800, Hitachi, at an accelerating voltage of 200 kV). For UV–Vis-NIR analysis, powders were placed in a quartz cell, allowing thermal treatments up to 800 °C, but spectra recording only at room temperature (RT). The powders were treated in situ, first at
Fig. 1. X-ray diffraction pattern of ST-20 powder compared with those of ST-10 and ST30, all calcined at 850 °C. The Bragg's reflections are compared with the positions of the same peaks related to pure SnO2 (solid line) and pure TiO2 (dashed line).
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3. Results and discussion 3.1. Structural and morphological characterization X-ray diffraction analysis at room temperature was carried out to recognize the crystalline phase of all [ST-x × 100]T powders calcined at the above mentioned temperatures. ST-20 solid solution, as all the other examined compositions, exhibited a rutile-like single phase (space group P42/m 21/n 2/m) [17]. In Fig. 1, the diffractogram of ST20 powder was compared with those of ST-10 and ST-30, all calcined at 850 °C. Focusing on the XRD Bragg's reflection of the (211) planes, it has been possible to observe that the ST-20 peak position lies between those of ST-10 and ST-30, allowing us to indirectly verify that the synthesis of the material correctly occurred. Moreover, in the same figure, the position of the solid solution reflexes was compared with those of the two single oxides. In the rutile unit cell the metal central atom is surrounded by six oxygen atoms, four of these lay on (110) plane (basal oxygens) and two on [110] direction (apical oxygens). The M–O basal distances are greater than the apical ones in SnO2, the opposite in TiO2. In Fig. 2, the ratio between apical and basal M–O distances (ABMO) versus the Ti molar ratio in the solid solutions is reported. It is very interesting to note that the basal distances become equal to the apical ones if the content of titanium is corresponding to x = 0.2 (ST-20). On the other hand, for x N 0.2 the apical distances become greater than the basal ones. In the following, both in electrical and spectroscopic characterizations, it will be clearly evident the border-line behaviour of the ST20 sample. Moreover, ABMO ratio greatly increases from 1, corresponding with ST-20, to 1.065 in ST-70, the composition for which the spinodal decomposition is manifestly evident. For 0.7 b x ≤ 1, ABMO ratio quickly decreases down to 1.01 corresponding to pure TiO2. Taking into account of the ABMO trend versus Ti molar ratio, we can assume it as a parameter useful to evaluate the probability of occurrence of spinodal decomposition. Fig. 3 shows the TEM micrographs of [ST-20]550 and [ST-50]550. The powder was made of agglomerated nanoparticles whose size, evaluated through XRD analysis using Scherrer's equation, was 5 nm and 4.5 nm, respectively. The crystallite size of the same powders calcined at 1050 °C were 36 and 23 nm, respectively, showing that, in spite of very high calcination temperature, they didn't suffer from exaggerated grain coalescence. Indeed, Table 1 highlights that the powders with medium stoichiometry between the two pure oxides SnO2 and TiO2, in particular ST-50 particles, remained nanosized. On
Fig. 3. TEM micrographs of [ST-20]550 (A) and [ST-50]550 (B) powders.
ST-20 films, a morphological characterization as a function of firing temperature (650, 750 and 850 °C) was also carried out (see Fig. 4). It can be observed that the morphology of the sensing layers remain unchanged; indeed, the usual increase of average dimension of the grains with temperature is practically undetectable. 3.2. Electrical characterization Fig. 5 shows the Arrhenius plot of the various samples performed in dry air. ST-20 film was compared with ST-10 and ST-30, all fired at 650 °C. Moreover, in the same figure, the conductances of the two single oxides were reported, for comparison. First of all, the borderline behaviour of ST-20 film must be highlighted. Indeed, while the trend of the conductance versus temperature of ST-10 is very similar to that of pure SnO2, at the same time, the shape of ST-30 is practically equal to that of pure TiO2. Moreover, also the films obtained from the other chemical compositions (0.3 b x ≤ 0.9), not shown here, behaved as ST30 [17]. The sigma shape of the Arrhenius plot of Sn — rich samples is due to surface reactions not visible in Ti — rich samples in the same range of temperature. No other phenomena, such as anomalous resistance increase with temperature observed in Sn — rich samples in [22], were revealed.
Table 1 Crystallite sizes of the powders calcined at 1050 °C, estimated through XRD analysis using Scherrer's equation.
Fig. 2. Ratio between apical and basal M–O distances (ABMO) versus the Ti molar ratio.
Sample
SnO2
ST-10
ST-20
ST-30
ST-50
ST-70
ST-90
TiO2
Crystallite size (nm)
70
53
36
31
23
29
74
101
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The second observation regards the magnitude of the conductance: at a temperature of 500 °C, the conductance of pure SnO2 is of about four order of magnitude greater than that of TiO2; in ST-10, the solid solution with the minimum content of titanium, it is more than one order of magnitude lower, in ST-20 get down to three orders of
Fig. 5. Conductance vs. temperature in dry air of ST-10, ST-20 and ST-30 compared with those of the single oxides, all films fired at 650 °C.
magnitude and finally, the conductance of ST-30 is almost equal to that of TiO2 as well as for all the other solid solutions, differently from what is observed in the literature (see for example [10]) where the maximum of resistance in a similar range of temperature is found for x ≅ 0.7. Again, ST-20 is characterized by a border-line behaviour between SnO2-like and TiO2-like ones. Moreover, because of ST-30, with respect to conductance, behaves as pure TiO2, we can deduce that a small content of titanium (x N 0.2) in the solid solution is sufficient to significantly lower the deepness in the gap of the donor levels from those shallow of pure SnO2 (mono-ionized oxygen vacancies, 0.03– 0.15 eV under the bottom of the conduction band) to that of pure TiO2 (about 1.0 eV under the bottom of the conduction band). Actually, this fact could also support the idea that the donor level in TiO2 are not ascribable to oxygen vacancies, but at Ti3+ defects, as confirmed by
Fig. 4. SEM micrographs of [ST-20]650, [ST-20]750, and [ST-20]850 thick films.
Fig. 6. Energy barrier dependence on temperature in dry air for the same samples as that in Fig. 5.
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Table 2 The band gap values (Eg/eV) calculated by the diffuse reflectance UV–Vis spectra of the powders treated in O2 at 550 °C. SnO2
ST-10 ST-20 ST-30 ST-50 ST-70 ST-90 TiO2
Band gap Eg/eV Direct 3.66a 3.72 Indirect 2.36 2.77
3.75 3.03
3.40 2.99
3.26 –
3.15 –
2.87b 2.70b
– 2.97c
a
In good agreement with data by literature for single crystal and thin films [1,21–23]. b Actually, both the linearization of I2 (I = Kubelka–Munk values) and I1/2 vs. the energy hν gave very bad results. c In good agreement with data by literature [2].
spectroscopic measurements. Indeed, if the defects were oxygen vacancies for all the mixed oxides, a continuous decreasing of the deepness of the energy level (and therefore a continuous decreasing of the conductance) should occur with the increasing of titanium content in the solid solution, which is not observed. Concerning height and trend vs. temperature of the surface barrier potential, Fig. 6 compares ST-20 with ST-10 and ST-30, in turn compared with the single oxides SnO2 and TiO2. Three observations can be made: i) as previously observed [23], titania films feature energy barriers vs temperature much higher than SnO2, where almost three activation energies are exhibited, differently from TiO2, in which only two regions of barrier are present; ii) ST-10 behaves similarly to SnO2 with an increased barrier height corresponding to the decreased conductivity; iii) ST-30 behaves like titania (as trend vs. temperature), but the height of the energy barrier was determined by the size of the nanoparticles. Indeed, the grain radius of ST-30 is smaller than Λ (the depletion layer width) and the complete band bending cannot be fully developed. Consequently, the potential does not vanish on the centre of the nano-grain, and a partial flattening of the band bending takes place [24]. ST-20 exhibited a borderline behaviour between those SnO2-like and TiO2-like. In fact, the trend vs. temperature is quite similar to titania, but the height of the barrier did not increase
correspondingly to the conductance decreasing because of the above mentioned nano-grain size phenomenon. 3.3. Spectroscopic characterization Diffuse reflectance UV–Vis-NIR spectra of the ST-20 sample treated in O2 at 550 °C has been recorded and the energy gap (Eg) determined as already done [17] for the two pure oxides and the other [ST-x × 100] (see Table 2). In particular the band gaps were obtained by plotting I2 or I1/2 (where I = Kubelka–Munk values) versus the energy hν of the radiation and extrapolating the linear part of the plot at I = 0. As known [25], the extrapolation gives the direct or the indirect allowed band gap, respectively. For SnO2 [1,26–28] and for [ST-x × 100] samples with x = 0.1, 0.2, 0.3 both direct and indirect gaps are found. Only a direct gap for ones with x = 0.5, 0.7 and, conversely, both direct and indirect gaps for sample with x = 0.9 are found. Eventually only an indirect gap as expected, for TiO2 rutile [2]. It appears evident that only for [ST-x × 100] sample with 0.3 ≤ x ≤ 0.7 on increasing the Ti content the band gap decreases from the value of SnO2 to that of rutile TiO2, as theoretically predicted for ternary compounds TixSn1 − xO2. We did not take in account ST-90 sample because both the linearization of I2 (I = Kubelka–Munk values) and I1/2 vs. the energy hν were very poor. Absorbance FT-IR spectra in the medium IR (MIR) region of the materials treated in O2 at 550 °C (as already reported for solid solution with x ≠ 0.2 [17]), are best consistent with the formation of solid solutions. Actually, they reveal a progressive upward shift of the absorption edge of their skeletal vibration modes (LO absorption) passing from pure SnO2 (~780 cm− 1) to pure TiO2 (~960 cm− 1). However, for the sample with the minor amount of Ti (ST-10) the shift is very pronounced, while for further Ti addition it is much less pronounced and almost linearly proportional to the Ti addition, so that the absorption edges of the all mixed oxides are closer to absorption edge of pure TiO2 than of pure SnO2 one.
Fig. 7. (A) Difference absorbance FT-IR spectra (curves b–a) in the MIR region showing the changes induced by the interaction with CO at RT with SnO2 and ST-10; (B) FT-IR absorbance spectra in the region of the absorption edge of the skeletal vibrations for ST-20, and ST-30: curves a, in oxygen at 200 °C and curves b, after interaction with CO at 200 °C (note: the ST-20 sample spectra have been shifted up along the absorbance scale for sake of clarity); (C) Difference absorbance FT-IR spectra (curve b–a) in the MIR region showing the changes induced by the interaction with CO at 400 °C with ST-20.
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As has been reported in the previous paragraph, electric characterization in dry air at different temperatures reveals for ST-20 sample a borderline behaviour between ST-10 and ST-30. Absorbance FT-IR spectra of different materials run in oxygen at temperature between 500 °C and RT did not suggest any information about this behaviour. However, some information could be achieved increasing the number of stoichiometric defects by treating the sample under CO atmosphere at increasing temperature. For SnO2 and ST-10 samples, the effect on the FT-IR spectra, passing from oxygen to CO at RT is a pronounced absorbance increase in the MIR region. To better appreciate these absorbance changes, in Fig. 7A the results are reported as difference spectra (curves b–a), the subtrahend spectrum (a) being that of the sample in oxygen. A very broad absorption appears for both materials, with maximum at 1500 cm− 1 and 2500 cm− 1 for SnO2 and ST-10, respectively. Actually, this is what always happens for CO interaction at RT with nanometric tin oxide [29,30], irrespective the preparation method: a broad absorption appears with maximum in the range 1200–1500 cm− 1 (0.15–0.18 eV), near the ionisation energy of mono-ionized oxygen vacancies, VO˙ [27], therefore assigned to the photo-ionisation of such levels, repopulated or created during the reductive phenomenon. By analogy, we propose to assign the absorption observed for ST-10 sample to similar electronic transition underlining that the value of its maximum at 2500 cm− 1 (0.30 eV) reveals the presence of levels deeper in the gap. Increasing interaction temperature at 100 °C, for both samples the broad absorption increases of about 10 times; for higher temperatures the samples completely absorb MIR radiation. For [ST-x × 100] samples with 0.2 ≤ x ≤ 0.9, no changes in the MIR spectra were observed by interaction with CO at RT or 100 °C. Increasing the interaction temperatures, samples with 0.3 ≤ x ≤ 0.9 do not show any electronic absorption, but an erosion of the absorption edge related to the skeletal vibration modes. For each sample, the erosion extent sensibly increases with the temperature up to 400 °C, but, for each temperature, it decreases by increasing Ti content, as reported in [17]. It is worth noticing that the presence of a sharp peak at 1013 cm− 1 in the spectra in oxygen, almost completely
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eroded by CO interaction already at 200 °C, whose intensity decreases increasing the Ti content (actually, it is absent on ST-90 sample), assigned to defective, strongly reactive Ti = O groups isolated in a surface environment where the SnO2 prevails [17]. Accordingly, the peak at 1013 cm− 1 is restored by a subsequent O2 interaction at high temperature. The erosion of the metal-oxygen vibration modes on the high frequency side of the skeletal LO absorption can be related to the loss of surface oxygen during the reducing treatment in CO. This explanation has been confirmed by the fact that the edge comes back to the original position after subsequent O2 interaction at 500 °C. However, the electrons released during these reduction treatments are not trapped at levels with an energy depth in the band gap of the order of the MIR radiation. Furthermore, the observed erosion decrease with increasing Ti content can be interpreted as a decrease in the sample reducibility increasing Ti content, according with the fact that no erosion is observed in the MIR spectra of TiO2, irrespective of the temperature chosen for the CO treatment. The story is different for the sample with x = 0.2. Actually, for CO interaction at T b 350 °C, it shows the same behaviour of the other ones, i.e. the erosion of the absorption edge related to the skeletal vibration modes (even if less pronounced than for ST-30 sample) and of the sharp pick at 1013 cm− 1, also observed on this sample. This is shown in Fig. 7B where a comparison between the behaviours of ST-20 and ST-30 samples at 200 °C is reported. At variance, for T ≥ 350 °C a sensible absorbance increase in the MIR region is observed passing from oxygen to CO atmosphere. The result obtained at T = 400 °C is reported in Fig. 7C as difference spectrum (curve b–a), the subtrahend spectrum (a) being that of the sample in oxygen. A very broad absorption appears, with maximum at about 2250 cm− 1, very similar to that already seen after CO interaction at RT with ST-10. Keeping attention at the absorbance scale in the Fig. 7A and C, it is evident that the absorbance changes are an order of magnitude lower for ST-20 sample. Actually, the comparison should be made for the same interaction temperature. This is not possible, because the ST-10 sample at 400 °C in CO absorbs completely the MIR radiation, indicating that the absorbance changes passing from ST-10 to ST-20
Fig. 8. UV–Vis-NIR spectra recorded at RT of ST-10 (A), ST-20 (B), ST-30 (C), ST-50 (D) calcined at 550 °C (a) and after treatment in CO at 400 °C (b).
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are surely more than one order of magnitude. In any case these experiments clearly show the borderline behaviour of the ST-20 sample. Looking at the UV–Vis-NIR spectra after treatments in CO at increasing temperature, it is possible to describe the changes induced by the reduction grouping the samples as follows: i) for SnO2 (not reported for sake of brevity), ST-10, ST-20 and ST-30 a change in the shape of the VB-CB absorption edge is observed, in particular a broadening of the absorption edge increasing with increasing the reduction temperature. However, differently from ST-30, SnO2 and ST10 (already by interaction at RT) and for ST-20 (only by interaction at T N 200 °C) on the low frequency side of the spectra a steep increase in the Kubelka–Munk values is visible, due to the tile of the broad absorption observed in the MIR region. This can be seen in Fig. 8, sections A, B and C, for ST-10, ST-20 and ST-30, respectively, where the situation before and after interaction with CO at 400 °C is reported; ii) for the other mixed oxides (0.5 ≤ x ≤ 0.9) no broadening of the absorption edge is observed, but an absorption near to the VB-CB edge, at about 21,000 cm− 1 (2.6 eV) appears, starting from 200 °C and increasing with increasing the reduction temperature. This can be seen in Fig. 8, section D where the situation before and after interaction with CO at 400 °C is reported. It is also worth of note that the intensity of this absorption is lower for the sample with the higher Ti content and absent on TiO2, irrespective of the reduction temperature as reported in a previous paper [17]. Furthermore, only for the ST-90, an absorption centered at 8000 cm− 1 also appears after treatment in CO at T ≥ 300 °C. This is the unique feature present on TiO2 spectra after interaction with CO at T ≥ 300 °C even if with intensity lower than that of the ST-90 sample [17]. The broadening of the VB-CB absorption edge increasing with increasing the reduction temperature for SnO2, ST-10, ST-20 and ST-30 mixed oxides is ascribable to a stoichiometry seriously perturbed by oxygen loss, causing a strong modification in the density of the states at the bottom of the conduction band and inducing the presence of a defect states distribution in the band gap [1,17]. As for the absorptions observed for rutile TiO2 and the other mixed oxides, the broad band at 8000 cm− 1 can be easily related to the presence of Ti3+ ions and assigned to a metal–metal charge transfer transition between Ti3+ and Ti4+ ions, also known as polaronic transition. Conversely, the assignment of the absorption at 21,000 cm− 1 (2.6 eV) is not straightforward. Its assignment has been already discussed in a previous paper [17] and related to defects present in regions of high tin dilution: either empty oxygen vacancies bridging between a Sn and a Ti ion (able to trap electrons photoionized by electromagnetic
Fig. 9. Responses to 50 ppm of CO in dry air of the solid solutions ST-10, ST-20 and ST-30 compared with those of the two single oxides.
Fig. 10. Response to 50 ppm of CO and 500 ppm of CH4 in dry air at the working temperature of 550 °C of all films versus the Ti molar ratio.
radiation with energy of about 2.6 eV) or Sn2+ ions with electronic levels very deep in the band gap, able to be photoionized by electromagnetic radiation with energy of about 2.6 eV. These results, in any case, reinforce the results obtained by absorbance FT-IR spectroscopy and confirm the border line behavior of the ST-20 sample. However, taking into account the spectra in the UV–Vis region, the border line behavior is also a prerogative of the ST30 sample. Actually, differently from the sample with higher Ti content, it shows, in common with the samples at lower Ti content, a broadening of the VB-CB absorption edge increasing with increasing the reduction temperature. All the other spectroscopic features in the MIR being those of the samples at higher Ti content. 3.4. Gas sensing properties Fig. 9 shows the responses defined as Ggas/Gair (ratio between the conductance in presence of the target gas and the conductance in air) of the solid solutions ST-10, ST-20 and ST-30 compared with the two single oxides. The dynamic conductance variations were obtained with 50 ppm of CO in dry air by varying the working temperature from 450 to 650 °C. A first observation is concerning the difference of the response curve vs. temperature for ST-30 sample with respect to all the other sensors in the figure. Indeed, while the ST-30 response versus temperature exhibited a bell-shaped curve, the responses of all the other sensors increased with the decreasing of temperature. Moreover, all the other solid solutions (not shown here) with 0.3 ≤ x ≤ 0.9 exhibited the same bell-shaped behaviour. This is a clear indication that these new materials exhibit different characteristics from the single oxides. A second observation concerns the intensity of the responses. Indeed, Fig. 9 highlights that ST-20 and ST-30 are much more sensitive than the single oxides and ST-10. Actually, the gas sensing performances of all solid solutions with 0.2 ≤ x ≤ 0.7 are clearly better than the single oxides. In addition, it must be noted that pure titania samples, coherently with all the other films, were fired at 650 °C, temperature for which titania is in anatase form and nanosized. To have pure titania in rutile phase like all the other samples, the firing temperature must be higher than 800 °C. In this case, the gas response should been much lower than that in Fig. 9, because of the phase transformation from anatase to rutile is accompanied by a dramatic grain growth [13]. In Fig. 10 the responses of all the solid solutions with respect to 50 ppm of carbon monoxide and 500 ppm of methane in dry air at 550 °C of working temperature are shown. It is clearly evident,
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particularly with carbon monoxide, that the materials can be divided in two groups: the first consisting of the single oxides and the solid solutions close to them (ST-10 and ST-90), and the second of all the others. Taking into account the IR spectroscopy measurements under CO atmosphere, there is evidence of distinction between the compositions with x ≤ 0.2 and x ≥ 0.3. Indeed, for x ≤ 0.2 the effect of CO interaction is a very broad absorption with maximum value in the range 1200–1500 cm− 1 (0.15–0.18 eV). For x ≥ 0.3 an erosion of the absorption edge related to the skeletal vibration modes is observed, whose extent is increasing with temperature and, at each temperature, decreasing by increasing the Ti content in the solid solution. Thereby, this transition can be related to the improved performances exhibited as gas sensors, but further investigations are needed. Finally, using wet air as carrier gas, it turned out that the solid solutions with x ≥ 0.2 resulted completely unaffected by the water vapour when the working temperature was 650 °C.
ST-20 shows a behaviour TiO2-like and, at higher temperature, SnO2like, thereby confirming the same trend shown both in structural and electrical characteristics. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
4. Conclusions Among the various synthesised Tix Sn1 − xO2 solid solutions, the stoichiometry with x = 0.2 (ST-20) exhibited a borderline behaviour between those SnO2-like and TiO2-like, for all examined properties. Indeed, with regard to the structural characteristics, ABMO (ratio between apical and basal M–O distances versus the Ti molar ratio) is equal to 1, separating the materials with x ≤ 0.2 (SnO2-like behaviour) for which ABMO is lower than 1 from those with x N 0.2 (TiO2-like behaviour) for which, vice versa, ABMO is higher than 1. From the electrical point of view, the materials with x b 0.2 exhibit conductivity much higher than the compositions with x ≥ 0.2, the two classes of materials showing also different shapes of the conductance curve vs. temperature. FT-IR spectra recorded under CO atmosphere at RT, for the materials with x b 0.2 exhibited a very broad absorption in the MIR region, related to the photoionization of VO˙, repopulated or created following the reductive treatment, as shown by SnO2. On the contrary, samples with 0.2 ≤ x ≤ 0.9 and TiO2 do not show changes in the MIR spectra. Increasing the interaction temperature, sample with x = 0.2 exhibited the increase of the absorption related to the photoionization of VO˙, while samples with 0.3 ≤ x ≤ 0.9 exhibit a completely different phenomenon consisting in an erosion of the absorption edge related to the skeletal M–O vibration modes. In summary, at low temperature
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Electrical and spectroscopic properties of Ti0.2 Sn0.8O2 solid solution for gas sensing M.C. Carotta a,⁎, S. Gherardi a, V. Guidi a, C. Malagù a, G. Martinelli a, B. Vendemiati a, M. Sacerdoti b, G. Ghiotti c, S. Morandi c a b c
CNR-INFM and Dipartimento di Fisica, Università di Ferrara, 44100 Ferrara, Italy Dipartimento di Scienze della Terra, Università di Ferrara, 44100 Ferrara, Italy Dipartimento di Chimica IFM, Università di Torino, 10125 Torino, Italy
a r t i c l e
i n f o
Available online 5 April 2009 Keywords: Nanomaterials Semiconductor oxides Solid solutions Chemoresistive gas sensors
a b s t r a c t In this work we report the synthesis, microstructure, electric and spectroscopic properties, and sensing performances of TixSn1 − xO2 (x = 0.1, 0.2, 0.3, 0.5, 0.7, 0.9) nano-powders and of SnO2 and TiO2 reference samples, prepared via sol-gel route starting from metal-organic precursors working in hydro-alcoholic media. Actually, the attention is particularly focused on properties of the sample with x = 0.2, in comparison with ones of the other solid solutions and of the single oxides. Indeed, this solid solution showed a borderline behaviour between that of the solid solution with x = 0.1 and that of the other solid solutions with x ≥ 0.3. An abrupt change in the structural, electrical and spectroscopic properties has been observed, passing from sample with x = 0.1, showing a behaviour very similar to that of SnO2, to one x = 0.3 showing a behaviour very similar to that of TiO2. The borderline properties of the mixed oxide with x = 0.2 represent the expected continuous transition among the two behaviours. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Among the metal oxides, tin dioxide and titanium dioxide, due to their chemical and electrical properties, are particularly appealing both for basic research and for a wide variety of possible applications [1,2]. Tin dioxide is the most common material in gas sensing, but it is widely used as transparent conductor and in heterogeneous catalysis; titanium dioxide is used as a photocatalyst, in solar cells, as an optical coating, in gas sensing, etc. Tin dioxide and titanium dioxide are both wide-gap semiconductors, showing several similarities in structural as well as in electronic properties. However, they exhibit also some peculiar differences, such as electrical conductivity and gas sensing behaviour. For both materials, the n-type behaviour is due to stoichiometric defects, generally oxygen vacancies acting as electronic donor levels, their energetic positions being much more deep inside the band gap for TiO2 [3,4]. On the other hand, there is not a general agreement on what causes the donor levels in TiO2, some authors assigning donor levels, 0.8 eV deep, at Ti3+ ions rather than at oxygen vacancies [2,5]. A part from this controversy, the observed much smaller conductivity of TiO2 with respect to that of SnO2 is reasonably imputable to the different energetic position of the donor levels in the two semiconductors [6]. The gas sensing mechanism in all polycrystalline n-type semiconductors is generally ascribed to the Schottky barrier formation at gas-semiconductor interface, leading to a negative surface charge ⁎ Corresponding author. Tel.: +39 0532974230; fax: +39 0532974325. E-mail address: [email protected] (M.C. Carotta). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.04.002
accumulation, typically O− ions. The variation of the height of the intergranular barrier is the result of surface chemical reactions with environmental gases leading to electrical conductance modifications. The two materials differ in gas sensing behaviour, being TiO2 much less reactive versus reducing agents than SnO2, however the mechanism of sensing seems to be common. The operation temperature of the two materials are very similar, in the range of 400–600 °C, which are typical of surface interaction mechanism, and not bulk one. Some of the authors of this paper discussed this point in [7] considering the different role played by the Fermi level pinning in the two semiconductors. Usually, mixed metal oxides and solid solutions have been considered for the superior performances shown in comparison to the single oxides [8–12]. In this context, we attempted to join advantages of the better characteristics of both materials: high gas sensitivity for SnO2 and lower influence by humidity for TiO2, and to overcome their disadvantages: poor selectivity for SnO2 and high resistivity and the polymorphic transition with temperature accompanied by exaggerated grain growth for TiO2 [13]. With respect to the structural properties, SnO2 and TiO2 exhibit rutile crystalline structure, space group P42/mnm. For this reason, they would easily form solid solution. Actually, these compounds show a miscibility gap with a reported critical temperature, Tc, ranging from 1300 to about 1500 °C, above which the solid solution is formed [14,15]. Indeed, as described in a previous paper, we could clearly observe a spinodal decomposition in tin-rich and titanium-rich oxide phases, only for the composition with x = 0.7 when calcined at high temperature (850 or 1050 °C) [16].
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To this aim, TixSn1 − xO2 (0.1 ≤ x ≤ 0.9, step 0.2) nanopowders were synthesised and characterized in an other paper [17]. We observed an abrupt change in electrical and spectroscopic behaviour passing from the solid solution with x = 0.1 to the other solutions (starting from x = 0.3), the first one showing a behaviour very similar to that of pure SnO2 while all the others to that of pure titania. To state more precisely the Ti molar content for which this transition occurs, this work has been particularly devoted to the synthesis and investigation of the composition with x = 0.2 not previously investigated. Electron microscopy, X-ray diffraction (XRD) and specific surface area measurements were adopted to analyse the morphology, the crystalline structure and the mean grain radius of the powders. Diffuse reflectance UV–Vis-NIR and absorbance FT-IR spectroscopies were used to investigate bulk and surface electronic properties, and surface chemistry under different atmospheres. Finally, electrical measurements (conductance, surface barrier potential behaviour and gas sensing properties) were performed. 2. Experimental 2.1. Powder synthesis and film preparation Each Tix Sn1 − xO2 solid-solution was obtained via sol-gel route starting from the stoichiometric solution of Sn(II)-ethylhexanoate (95% Aldrich) and Ti-butoxide (97% Aldrich) in hydro-alcoholic media. As an example, Ti0.2 Sn0.8O2 solution ([Sn2+]:[Ti4+] = 4:1) was synthesised mixing 2.7 mmol of Ti-butoxide together with 10.8 mmol of Sn(II)ethylhexanoate. Both the Ti4+ and Sn2+ sources were used without further purification. Diluted HNO3 was added dropwise to hydrolyse the metal-organic molecules. For each stoichiometry, the whole process was carried out by maintaining the solution under soft stirring at the temperature of 50 °C. The resulting colloids were washed, dried and crunched yielding the white hydrous precursors of the solid solutions. The irreversible conversion precursor/binary oxide was gained by calcining at 550 °C for 2 h. The completion of the reaction was previously checked by a TG/DTA analysis through a Netzsch STA 409 equipment. In order to study the structural and the morphological evolution of the nanopowders with temperature, each precursor was further calcined at 650, 850 or 1050 °C for 2 h. (Ti,Sn)O2 binary oxides will be hereinafter labelled as [ST-x × 100]T where T is the calcination temperature. The powders were used to deposit sensing layers by means of screen printing technology onto miniaturized alumina substrates each one provided with a heater element and comb-type gold electrodes for electrical measurements. The layers were fired for 1 h at 650, 750 or 850 °C in order to study the influence of the temperature on the sensing behaviour of each film. Moreover, to compare [ST-x × 100] with pure SnO2 and pure TiO2 powders were synthesised and thick films were realized with the same preparation parameters.
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550 °C in vacuum and in dry oxygen, then cooled down at RT before the treatments in CO at increasing temperatures up to 500 °C. Diffuse reflectance UV–Vis-NIR spectra were run at RT on a Varian Cary 5 spectrophotometer, working in the range of wavenumbers 53,000– 4000 cm− 1. UV–Vis-NIR spectra are reported in the figures as raw data; to employ a quantity equivalent to the absorbance used in the absorption spectroscopy, in the ordinate scale we used the KubelkaMunk function [f (R∞) = (1 − R∞)2 / 2 R∞, where R∞ = the reflectance of an ‘infinitely thick’ layer of the sample] [19]. For FT-IR analysis, the powders were compressed in self-supporting disks and placed in a commercial heatable stainless steel IR cell (Aabspec) allowing in situ thermal treatments up to 600 °C and simultaneously spectra recording. The disks were treated in situ, first at 550 °C in vacuum and in dry oxygen, then cooled down at pre-set temperatures (from RT up to 500 °C) before the interaction with CO at the same temperatures. Absorption FT-IR spectra were run on a Perkin-Elmer System 2000 FT-IR spectrophotometer equipped with a Hg–Cd–Te cryodetector, working in the range of wavenumbers 7800– 580 cm− 1. FT-IR spectra are reported in the figures either as raw data or as difference spectra (where the subtrahend spectrum is that of the sample out-gassed and oxidized at 550 °C and then cooled in O2 at the temperature chosen for the interaction with CO). 2.3. Film characterization The morphology of all the films was observed using scanning electron microscopy (SEM, model EVO 40, Carl Zeiss). The flow-through technique was used maintaining a constant flow of 0.5 l/min to carry out conductance measurements in dry or wet air or in mixtures of air and test gases. Dynamic responses of sensing films were obtained with methane and carbon monoxide in dry or wet air, by varying the operating temperature from 400 to 650 °C. Moreover, temperature-stimulated conductance measurements, which consist in measuring conductance as a function of time after a fast temperature variation, were also performed to determine the energy barrier, e.g., the difference in energy between the conduction band bottom at surface and that in the bulk, as a function of the temperature (25–650 °C), with a procedure described elsewhere [20,21].
2.2. Powders characterization X-ray diffraction (XRD) analysis measurements were performed using a Philips PW 1830 vertical diffractometer with Bregg–Brentano geometry (Cu Kα radiation, 40 kV, 30 mA) provided with a graphite monochromator along the diffracted beam. Diffraction patterns were collected in the range 10–120° (2θ) with steps of 0.02° and 10 s of dwell time. XRD data were elaborated using a Rietveld analysis program FullProf (release 2006) [18]. The microstructure of the powders was observed by transmission electron microscopy (TEM, model H-800, Hitachi, at an accelerating voltage of 200 kV). For UV–Vis-NIR analysis, powders were placed in a quartz cell, allowing thermal treatments up to 800 °C, but spectra recording only at room temperature (RT). The powders were treated in situ, first at
Fig. 1. X-ray diffraction pattern of ST-20 powder compared with those of ST-10 and ST30, all calcined at 850 °C. The Bragg's reflections are compared with the positions of the same peaks related to pure SnO2 (solid line) and pure TiO2 (dashed line).
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3. Results and discussion 3.1. Structural and morphological characterization X-ray diffraction analysis at room temperature was carried out to recognize the crystalline phase of all [ST-x × 100]T powders calcined at the above mentioned temperatures. ST-20 solid solution, as all the other examined compositions, exhibited a rutile-like single phase (space group P42/m 21/n 2/m) [17]. In Fig. 1, the diffractogram of ST20 powder was compared with those of ST-10 and ST-30, all calcined at 850 °C. Focusing on the XRD Bragg's reflection of the (211) planes, it has been possible to observe that the ST-20 peak position lies between those of ST-10 and ST-30, allowing us to indirectly verify that the synthesis of the material correctly occurred. Moreover, in the same figure, the position of the solid solution reflexes was compared with those of the two single oxides. In the rutile unit cell the metal central atom is surrounded by six oxygen atoms, four of these lay on (110) plane (basal oxygens) and two on [110] direction (apical oxygens). The M–O basal distances are greater than the apical ones in SnO2, the opposite in TiO2. In Fig. 2, the ratio between apical and basal M–O distances (ABMO) versus the Ti molar ratio in the solid solutions is reported. It is very interesting to note that the basal distances become equal to the apical ones if the content of titanium is corresponding to x = 0.2 (ST-20). On the other hand, for x N 0.2 the apical distances become greater than the basal ones. In the following, both in electrical and spectroscopic characterizations, it will be clearly evident the border-line behaviour of the ST20 sample. Moreover, ABMO ratio greatly increases from 1, corresponding with ST-20, to 1.065 in ST-70, the composition for which the spinodal decomposition is manifestly evident. For 0.7 b x ≤ 1, ABMO ratio quickly decreases down to 1.01 corresponding to pure TiO2. Taking into account of the ABMO trend versus Ti molar ratio, we can assume it as a parameter useful to evaluate the probability of occurrence of spinodal decomposition. Fig. 3 shows the TEM micrographs of [ST-20]550 and [ST-50]550. The powder was made of agglomerated nanoparticles whose size, evaluated through XRD analysis using Scherrer's equation, was 5 nm and 4.5 nm, respectively. The crystallite size of the same powders calcined at 1050 °C were 36 and 23 nm, respectively, showing that, in spite of very high calcination temperature, they didn't suffer from exaggerated grain coalescence. Indeed, Table 1 highlights that the powders with medium stoichiometry between the two pure oxides SnO2 and TiO2, in particular ST-50 particles, remained nanosized. On
Fig. 3. TEM micrographs of [ST-20]550 (A) and [ST-50]550 (B) powders.
ST-20 films, a morphological characterization as a function of firing temperature (650, 750 and 850 °C) was also carried out (see Fig. 4). It can be observed that the morphology of the sensing layers remain unchanged; indeed, the usual increase of average dimension of the grains with temperature is practically undetectable. 3.2. Electrical characterization Fig. 5 shows the Arrhenius plot of the various samples performed in dry air. ST-20 film was compared with ST-10 and ST-30, all fired at 650 °C. Moreover, in the same figure, the conductances of the two single oxides were reported, for comparison. First of all, the borderline behaviour of ST-20 film must be highlighted. Indeed, while the trend of the conductance versus temperature of ST-10 is very similar to that of pure SnO2, at the same time, the shape of ST-30 is practically equal to that of pure TiO2. Moreover, also the films obtained from the other chemical compositions (0.3 b x ≤ 0.9), not shown here, behaved as ST30 [17]. The sigma shape of the Arrhenius plot of Sn — rich samples is due to surface reactions not visible in Ti — rich samples in the same range of temperature. No other phenomena, such as anomalous resistance increase with temperature observed in Sn — rich samples in [22], were revealed.
Table 1 Crystallite sizes of the powders calcined at 1050 °C, estimated through XRD analysis using Scherrer's equation.
Fig. 2. Ratio between apical and basal M–O distances (ABMO) versus the Ti molar ratio.
Sample
SnO2
ST-10
ST-20
ST-30
ST-50
ST-70
ST-90
TiO2
Crystallite size (nm)
70
53
36
31
23
29
74
101
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The second observation regards the magnitude of the conductance: at a temperature of 500 °C, the conductance of pure SnO2 is of about four order of magnitude greater than that of TiO2; in ST-10, the solid solution with the minimum content of titanium, it is more than one order of magnitude lower, in ST-20 get down to three orders of
Fig. 5. Conductance vs. temperature in dry air of ST-10, ST-20 and ST-30 compared with those of the single oxides, all films fired at 650 °C.
magnitude and finally, the conductance of ST-30 is almost equal to that of TiO2 as well as for all the other solid solutions, differently from what is observed in the literature (see for example [10]) where the maximum of resistance in a similar range of temperature is found for x ≅ 0.7. Again, ST-20 is characterized by a border-line behaviour between SnO2-like and TiO2-like ones. Moreover, because of ST-30, with respect to conductance, behaves as pure TiO2, we can deduce that a small content of titanium (x N 0.2) in the solid solution is sufficient to significantly lower the deepness in the gap of the donor levels from those shallow of pure SnO2 (mono-ionized oxygen vacancies, 0.03– 0.15 eV under the bottom of the conduction band) to that of pure TiO2 (about 1.0 eV under the bottom of the conduction band). Actually, this fact could also support the idea that the donor level in TiO2 are not ascribable to oxygen vacancies, but at Ti3+ defects, as confirmed by
Fig. 4. SEM micrographs of [ST-20]650, [ST-20]750, and [ST-20]850 thick films.
Fig. 6. Energy barrier dependence on temperature in dry air for the same samples as that in Fig. 5.
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Table 2 The band gap values (Eg/eV) calculated by the diffuse reflectance UV–Vis spectra of the powders treated in O2 at 550 °C. SnO2
ST-10 ST-20 ST-30 ST-50 ST-70 ST-90 TiO2
Band gap Eg/eV Direct 3.66a 3.72 Indirect 2.36 2.77
3.75 3.03
3.40 2.99
3.26 –
3.15 –
2.87b 2.70b
– 2.97c
a
In good agreement with data by literature for single crystal and thin films [1,21–23]. b Actually, both the linearization of I2 (I = Kubelka–Munk values) and I1/2 vs. the energy hν gave very bad results. c In good agreement with data by literature [2].
spectroscopic measurements. Indeed, if the defects were oxygen vacancies for all the mixed oxides, a continuous decreasing of the deepness of the energy level (and therefore a continuous decreasing of the conductance) should occur with the increasing of titanium content in the solid solution, which is not observed. Concerning height and trend vs. temperature of the surface barrier potential, Fig. 6 compares ST-20 with ST-10 and ST-30, in turn compared with the single oxides SnO2 and TiO2. Three observations can be made: i) as previously observed [23], titania films feature energy barriers vs temperature much higher than SnO2, where almost three activation energies are exhibited, differently from TiO2, in which only two regions of barrier are present; ii) ST-10 behaves similarly to SnO2 with an increased barrier height corresponding to the decreased conductivity; iii) ST-30 behaves like titania (as trend vs. temperature), but the height of the energy barrier was determined by the size of the nanoparticles. Indeed, the grain radius of ST-30 is smaller than Λ (the depletion layer width) and the complete band bending cannot be fully developed. Consequently, the potential does not vanish on the centre of the nano-grain, and a partial flattening of the band bending takes place [24]. ST-20 exhibited a borderline behaviour between those SnO2-like and TiO2-like. In fact, the trend vs. temperature is quite similar to titania, but the height of the barrier did not increase
correspondingly to the conductance decreasing because of the above mentioned nano-grain size phenomenon. 3.3. Spectroscopic characterization Diffuse reflectance UV–Vis-NIR spectra of the ST-20 sample treated in O2 at 550 °C has been recorded and the energy gap (Eg) determined as already done [17] for the two pure oxides and the other [ST-x × 100] (see Table 2). In particular the band gaps were obtained by plotting I2 or I1/2 (where I = Kubelka–Munk values) versus the energy hν of the radiation and extrapolating the linear part of the plot at I = 0. As known [25], the extrapolation gives the direct or the indirect allowed band gap, respectively. For SnO2 [1,26–28] and for [ST-x × 100] samples with x = 0.1, 0.2, 0.3 both direct and indirect gaps are found. Only a direct gap for ones with x = 0.5, 0.7 and, conversely, both direct and indirect gaps for sample with x = 0.9 are found. Eventually only an indirect gap as expected, for TiO2 rutile [2]. It appears evident that only for [ST-x × 100] sample with 0.3 ≤ x ≤ 0.7 on increasing the Ti content the band gap decreases from the value of SnO2 to that of rutile TiO2, as theoretically predicted for ternary compounds TixSn1 − xO2. We did not take in account ST-90 sample because both the linearization of I2 (I = Kubelka–Munk values) and I1/2 vs. the energy hν were very poor. Absorbance FT-IR spectra in the medium IR (MIR) region of the materials treated in O2 at 550 °C (as already reported for solid solution with x ≠ 0.2 [17]), are best consistent with the formation of solid solutions. Actually, they reveal a progressive upward shift of the absorption edge of their skeletal vibration modes (LO absorption) passing from pure SnO2 (~780 cm− 1) to pure TiO2 (~960 cm− 1). However, for the sample with the minor amount of Ti (ST-10) the shift is very pronounced, while for further Ti addition it is much less pronounced and almost linearly proportional to the Ti addition, so that the absorption edges of the all mixed oxides are closer to absorption edge of pure TiO2 than of pure SnO2 one.
Fig. 7. (A) Difference absorbance FT-IR spectra (curves b–a) in the MIR region showing the changes induced by the interaction with CO at RT with SnO2 and ST-10; (B) FT-IR absorbance spectra in the region of the absorption edge of the skeletal vibrations for ST-20, and ST-30: curves a, in oxygen at 200 °C and curves b, after interaction with CO at 200 °C (note: the ST-20 sample spectra have been shifted up along the absorbance scale for sake of clarity); (C) Difference absorbance FT-IR spectra (curve b–a) in the MIR region showing the changes induced by the interaction with CO at 400 °C with ST-20.
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As has been reported in the previous paragraph, electric characterization in dry air at different temperatures reveals for ST-20 sample a borderline behaviour between ST-10 and ST-30. Absorbance FT-IR spectra of different materials run in oxygen at temperature between 500 °C and RT did not suggest any information about this behaviour. However, some information could be achieved increasing the number of stoichiometric defects by treating the sample under CO atmosphere at increasing temperature. For SnO2 and ST-10 samples, the effect on the FT-IR spectra, passing from oxygen to CO at RT is a pronounced absorbance increase in the MIR region. To better appreciate these absorbance changes, in Fig. 7A the results are reported as difference spectra (curves b–a), the subtrahend spectrum (a) being that of the sample in oxygen. A very broad absorption appears for both materials, with maximum at 1500 cm− 1 and 2500 cm− 1 for SnO2 and ST-10, respectively. Actually, this is what always happens for CO interaction at RT with nanometric tin oxide [29,30], irrespective the preparation method: a broad absorption appears with maximum in the range 1200–1500 cm− 1 (0.15–0.18 eV), near the ionisation energy of mono-ionized oxygen vacancies, VO˙ [27], therefore assigned to the photo-ionisation of such levels, repopulated or created during the reductive phenomenon. By analogy, we propose to assign the absorption observed for ST-10 sample to similar electronic transition underlining that the value of its maximum at 2500 cm− 1 (0.30 eV) reveals the presence of levels deeper in the gap. Increasing interaction temperature at 100 °C, for both samples the broad absorption increases of about 10 times; for higher temperatures the samples completely absorb MIR radiation. For [ST-x × 100] samples with 0.2 ≤ x ≤ 0.9, no changes in the MIR spectra were observed by interaction with CO at RT or 100 °C. Increasing the interaction temperatures, samples with 0.3 ≤ x ≤ 0.9 do not show any electronic absorption, but an erosion of the absorption edge related to the skeletal vibration modes. For each sample, the erosion extent sensibly increases with the temperature up to 400 °C, but, for each temperature, it decreases by increasing Ti content, as reported in [17]. It is worth noticing that the presence of a sharp peak at 1013 cm− 1 in the spectra in oxygen, almost completely
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eroded by CO interaction already at 200 °C, whose intensity decreases increasing the Ti content (actually, it is absent on ST-90 sample), assigned to defective, strongly reactive Ti = O groups isolated in a surface environment where the SnO2 prevails [17]. Accordingly, the peak at 1013 cm− 1 is restored by a subsequent O2 interaction at high temperature. The erosion of the metal-oxygen vibration modes on the high frequency side of the skeletal LO absorption can be related to the loss of surface oxygen during the reducing treatment in CO. This explanation has been confirmed by the fact that the edge comes back to the original position after subsequent O2 interaction at 500 °C. However, the electrons released during these reduction treatments are not trapped at levels with an energy depth in the band gap of the order of the MIR radiation. Furthermore, the observed erosion decrease with increasing Ti content can be interpreted as a decrease in the sample reducibility increasing Ti content, according with the fact that no erosion is observed in the MIR spectra of TiO2, irrespective of the temperature chosen for the CO treatment. The story is different for the sample with x = 0.2. Actually, for CO interaction at T b 350 °C, it shows the same behaviour of the other ones, i.e. the erosion of the absorption edge related to the skeletal vibration modes (even if less pronounced than for ST-30 sample) and of the sharp pick at 1013 cm− 1, also observed on this sample. This is shown in Fig. 7B where a comparison between the behaviours of ST-20 and ST-30 samples at 200 °C is reported. At variance, for T ≥ 350 °C a sensible absorbance increase in the MIR region is observed passing from oxygen to CO atmosphere. The result obtained at T = 400 °C is reported in Fig. 7C as difference spectrum (curve b–a), the subtrahend spectrum (a) being that of the sample in oxygen. A very broad absorption appears, with maximum at about 2250 cm− 1, very similar to that already seen after CO interaction at RT with ST-10. Keeping attention at the absorbance scale in the Fig. 7A and C, it is evident that the absorbance changes are an order of magnitude lower for ST-20 sample. Actually, the comparison should be made for the same interaction temperature. This is not possible, because the ST-10 sample at 400 °C in CO absorbs completely the MIR radiation, indicating that the absorbance changes passing from ST-10 to ST-20
Fig. 8. UV–Vis-NIR spectra recorded at RT of ST-10 (A), ST-20 (B), ST-30 (C), ST-50 (D) calcined at 550 °C (a) and after treatment in CO at 400 °C (b).
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are surely more than one order of magnitude. In any case these experiments clearly show the borderline behaviour of the ST-20 sample. Looking at the UV–Vis-NIR spectra after treatments in CO at increasing temperature, it is possible to describe the changes induced by the reduction grouping the samples as follows: i) for SnO2 (not reported for sake of brevity), ST-10, ST-20 and ST-30 a change in the shape of the VB-CB absorption edge is observed, in particular a broadening of the absorption edge increasing with increasing the reduction temperature. However, differently from ST-30, SnO2 and ST10 (already by interaction at RT) and for ST-20 (only by interaction at T N 200 °C) on the low frequency side of the spectra a steep increase in the Kubelka–Munk values is visible, due to the tile of the broad absorption observed in the MIR region. This can be seen in Fig. 8, sections A, B and C, for ST-10, ST-20 and ST-30, respectively, where the situation before and after interaction with CO at 400 °C is reported; ii) for the other mixed oxides (0.5 ≤ x ≤ 0.9) no broadening of the absorption edge is observed, but an absorption near to the VB-CB edge, at about 21,000 cm− 1 (2.6 eV) appears, starting from 200 °C and increasing with increasing the reduction temperature. This can be seen in Fig. 8, section D where the situation before and after interaction with CO at 400 °C is reported. It is also worth of note that the intensity of this absorption is lower for the sample with the higher Ti content and absent on TiO2, irrespective of the reduction temperature as reported in a previous paper [17]. Furthermore, only for the ST-90, an absorption centered at 8000 cm− 1 also appears after treatment in CO at T ≥ 300 °C. This is the unique feature present on TiO2 spectra after interaction with CO at T ≥ 300 °C even if with intensity lower than that of the ST-90 sample [17]. The broadening of the VB-CB absorption edge increasing with increasing the reduction temperature for SnO2, ST-10, ST-20 and ST-30 mixed oxides is ascribable to a stoichiometry seriously perturbed by oxygen loss, causing a strong modification in the density of the states at the bottom of the conduction band and inducing the presence of a defect states distribution in the band gap [1,17]. As for the absorptions observed for rutile TiO2 and the other mixed oxides, the broad band at 8000 cm− 1 can be easily related to the presence of Ti3+ ions and assigned to a metal–metal charge transfer transition between Ti3+ and Ti4+ ions, also known as polaronic transition. Conversely, the assignment of the absorption at 21,000 cm− 1 (2.6 eV) is not straightforward. Its assignment has been already discussed in a previous paper [17] and related to defects present in regions of high tin dilution: either empty oxygen vacancies bridging between a Sn and a Ti ion (able to trap electrons photoionized by electromagnetic
Fig. 9. Responses to 50 ppm of CO in dry air of the solid solutions ST-10, ST-20 and ST-30 compared with those of the two single oxides.
Fig. 10. Response to 50 ppm of CO and 500 ppm of CH4 in dry air at the working temperature of 550 °C of all films versus the Ti molar ratio.
radiation with energy of about 2.6 eV) or Sn2+ ions with electronic levels very deep in the band gap, able to be photoionized by electromagnetic radiation with energy of about 2.6 eV. These results, in any case, reinforce the results obtained by absorbance FT-IR spectroscopy and confirm the border line behavior of the ST-20 sample. However, taking into account the spectra in the UV–Vis region, the border line behavior is also a prerogative of the ST30 sample. Actually, differently from the sample with higher Ti content, it shows, in common with the samples at lower Ti content, a broadening of the VB-CB absorption edge increasing with increasing the reduction temperature. All the other spectroscopic features in the MIR being those of the samples at higher Ti content. 3.4. Gas sensing properties Fig. 9 shows the responses defined as Ggas/Gair (ratio between the conductance in presence of the target gas and the conductance in air) of the solid solutions ST-10, ST-20 and ST-30 compared with the two single oxides. The dynamic conductance variations were obtained with 50 ppm of CO in dry air by varying the working temperature from 450 to 650 °C. A first observation is concerning the difference of the response curve vs. temperature for ST-30 sample with respect to all the other sensors in the figure. Indeed, while the ST-30 response versus temperature exhibited a bell-shaped curve, the responses of all the other sensors increased with the decreasing of temperature. Moreover, all the other solid solutions (not shown here) with 0.3 ≤ x ≤ 0.9 exhibited the same bell-shaped behaviour. This is a clear indication that these new materials exhibit different characteristics from the single oxides. A second observation concerns the intensity of the responses. Indeed, Fig. 9 highlights that ST-20 and ST-30 are much more sensitive than the single oxides and ST-10. Actually, the gas sensing performances of all solid solutions with 0.2 ≤ x ≤ 0.7 are clearly better than the single oxides. In addition, it must be noted that pure titania samples, coherently with all the other films, were fired at 650 °C, temperature for which titania is in anatase form and nanosized. To have pure titania in rutile phase like all the other samples, the firing temperature must be higher than 800 °C. In this case, the gas response should been much lower than that in Fig. 9, because of the phase transformation from anatase to rutile is accompanied by a dramatic grain growth [13]. In Fig. 10 the responses of all the solid solutions with respect to 50 ppm of carbon monoxide and 500 ppm of methane in dry air at 550 °C of working temperature are shown. It is clearly evident,
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particularly with carbon monoxide, that the materials can be divided in two groups: the first consisting of the single oxides and the solid solutions close to them (ST-10 and ST-90), and the second of all the others. Taking into account the IR spectroscopy measurements under CO atmosphere, there is evidence of distinction between the compositions with x ≤ 0.2 and x ≥ 0.3. Indeed, for x ≤ 0.2 the effect of CO interaction is a very broad absorption with maximum value in the range 1200–1500 cm− 1 (0.15–0.18 eV). For x ≥ 0.3 an erosion of the absorption edge related to the skeletal vibration modes is observed, whose extent is increasing with temperature and, at each temperature, decreasing by increasing the Ti content in the solid solution. Thereby, this transition can be related to the improved performances exhibited as gas sensors, but further investigations are needed. Finally, using wet air as carrier gas, it turned out that the solid solutions with x ≥ 0.2 resulted completely unaffected by the water vapour when the working temperature was 650 °C.
ST-20 shows a behaviour TiO2-like and, at higher temperature, SnO2like, thereby confirming the same trend shown both in structural and electrical characteristics. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
4. Conclusions Among the various synthesised Tix Sn1 − xO2 solid solutions, the stoichiometry with x = 0.2 (ST-20) exhibited a borderline behaviour between those SnO2-like and TiO2-like, for all examined properties. Indeed, with regard to the structural characteristics, ABMO (ratio between apical and basal M–O distances versus the Ti molar ratio) is equal to 1, separating the materials with x ≤ 0.2 (SnO2-like behaviour) for which ABMO is lower than 1 from those with x N 0.2 (TiO2-like behaviour) for which, vice versa, ABMO is higher than 1. From the electrical point of view, the materials with x b 0.2 exhibit conductivity much higher than the compositions with x ≥ 0.2, the two classes of materials showing also different shapes of the conductance curve vs. temperature. FT-IR spectra recorded under CO atmosphere at RT, for the materials with x b 0.2 exhibited a very broad absorption in the MIR region, related to the photoionization of VO˙, repopulated or created following the reductive treatment, as shown by SnO2. On the contrary, samples with 0.2 ≤ x ≤ 0.9 and TiO2 do not show changes in the MIR spectra. Increasing the interaction temperature, sample with x = 0.2 exhibited the increase of the absorption related to the photoionization of VO˙, while samples with 0.3 ≤ x ≤ 0.9 exhibit a completely different phenomenon consisting in an erosion of the absorption edge related to the skeletal M–O vibration modes. In summary, at low temperature
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[14] [15] [16]
[17]
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
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