Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 172–175 XPS studies of active elements surface of gas sensors based on WO 3−x nanoparticles A.P. Shpak, A.M. Korduban ∗ , M.M. Medvedskij, V.O. Kandyba G.V. Kurdyumov Institute of Physics of Metals, National Academy of Sciences, Ukraine, Boulevard Akad. Vernadskogo 36 03142 Kyiv, Ukraine Available online 26 January 2007 Abstract Nanodisperse WO 3−x oxides (x ≥ 0) were synthesized by method of electrical explosion of wires. The structure of core levels W4f and O1s was explored by XPS-method. Exposure of oxides to air at 293 K leads to the formation of WO 3 ·(OH 2 ) n -phase on the surface of nanoparticles. A correlation between value x of nonstoichiometric WO 3−x oxide matrix and contents of OH-groups in it was revealed. It was noticed, that the increase of number of W 5+ -states at nanoparticles synthesis leads to rise of the sensor response to molecular hydrogen at room temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; XPS spectroscopy; Nonstoichiometric oxides; WO 3−x 1. Introduction Nanodisperse nonstoichiometric WO 3−x oxides are promis- ing materials and are used for producing of gas sensors active elements [1–8]. In the most cases the range of their work temper- atures is 520–620 K. But during exploitation of active elements at relatively high work temperatures the degradation processes are initiated in the nanoparticles of semiconductor oxides. There are works, which are dealing with room temperatures, for exam- ple [9–10]. For such work conditions important is the exploration of the state of sensor’s surface after long contact with air at room temperature and influence of short-time anneal. We synthesized nanodisperse nonstoichiometric tungsten oxides, which demonstrated high sensor response to molecu- lar hydrogen and rare gases at room temperature. Earlier the effect of sensitivity to rare gases at physical adsorption at room temperature was detected for TiO 2 polydisperse samples [11]. Electrical resistance changes at that were completely reversible (R/R ∼ 0.3). That effect increased with the rise of rutile imper- fection. Further investigations in that field could be prospective. The goal of present work was the exploration of features of core W4f- and O1s-levels of WO 3−x surface before and after temperature influence at contact with air to reveal their connec- tion with sensor characteristics of synthesized nanopowders. ∗ Corresponding author. E-mail address: akord@imp.kiev.ua (A.M. Korduban). 2. Experimental Three series of WO 3, WO 2.9 and WO 2.72 nanopowders were synthesized at atmospheric pressure by method of electric explo- sion of wires (EEW) at different proportions between argon and oxygen. Synthesis of nanoparticles by EEW-method [12,13] belongs to high-energy methods of synthesis and allows to obtain nanoparticles with high presence of defects on the surface and unusual charge states of ions. An explosion in coordinated con- ditions was chosen for synthesis, injected energy was E = 1.9·E s , where E s is the sublimation energy of metal, diameter of tung- sten wire was 0.6 mm. According to the TEM data (Fig. 1) the mean size of nanoparticles is 10–35 nm. The nanoparticles have spherical shape and normal size distribution; their agglomeration is almost absent. The value x in the nanopowders WO 3−x was determined by XPS-method by means of ratio between W 5+ 4f- and W 6+ 4f-states. Active elements of gas sensors were prepared from WO 3 ,WO 2.9 and WO 2.72 -nanopowders on Al 2 O 3 -substrate (10 × 10 mm) with two electric contacts and slot (100 ␮m width) by method of sputtering in argon stream. Active elements were stored on air at room temperature during 14 days. Before instal- lation into the XPS-spectrometer’s camera a half of them was annealed on air during 45 min at T = 553 K. The surface of nanodisperse WO 3−x oxides was examined in annealed and non-annealed active elements. The electronic structure of the surface of WO 3−x was explored by method of X-ray photoelectron spectroscopy (XPS) 0368-2048/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2006.12.059 A.P. Shpak et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 172–175 173 Fig. 1. TEM micrograph of WO 3−x nanoparticles. by spectrometer ES-2404 with PHOIBOS-100 SPECS energy analyzer (E Mg K␣ = 1253.6 eV, P = 300 W). The spectrometer is equipped with the ion gun IQE-11/35 and the flood gun FG 15/40 for sample charge neutralization. The spectra of W4f- level were decomposed into peak couples with parameters of spin–orbit separation E p (4f 5/2 − 4f 7/2 ) = 2.1 eV and intensi- ties ratio I 4f5/2 /I 4f7/2 = 0.77, full width on half maximum height (FWHM) was 1.2 eV. The spectra of O1s-level were decomposed into separate peaks, FWHM = 1.4 eV. The decomposition was carried out by Gauss–Newton method. The area of peaks was determined after subtraction of background by Shirley method [14]. Sensor response (R/R, R-resistance change) to H 2 was measured by means of system, which consists of measuring camera, multimeter Keithley-2010 and PC. 3. Results and discussion The XPS-spectra of W4f- and O1s-levels of the samples and the results of their decomposition into peaks are shown in Figs. 2 and 3 correspondingly and in Tables 1 and 2. Fig. 2 rep- resents the results of decomposition of W4f-line in the annealed and non-annealed samples. The maxima of peak couples cor- respond to W4f 7/2 - and W4f 5/2 -levels of tungsten atoms for W 5+ -states of oxide (comps. c-c  , E p W4f 7/2 = 34.8 eV) [15] Fig. 2. Peak synthesis for W4f-level XPS-spectrum of tungsten atoms. and W 6+ -states of oxide (comps. d-d  E p W4f 7/2 = 35.7 eV) and hydroxide (comps.e-e  , E p W4f 7/2 = 36.1 eV) [15,16], where E p – peak energy. According to the XPS data in the non-annealed sample of stoichiometric WO 3 the surface of nanoparticles consists of WO 3 -phase (comps. d-d  , Fig. 2-1) and WO 3 ·(OH 2 ) n -phase formed after contact with air (comps. e-e’, Fig. 2-1). The results of decomposition of W4f-line for nonstoichiometric oxides WO 2.9 and WO 2.72 indicate essential differences in compari- son with stoichiometric WO 3 . The spectra of the non-annealed samples show domination of the hydroxide phase (comps. e-e  , Fig. 2-2, 2-3), this indicates more active interaction of nanopar- ticles’ surface with water vapor during contact with air right up to formation of a shell of several WO 3 ·(OH 2 ) n -monolayers. In the sample WO 2.72 the contribution of W 5+ -states from nonstoi- chiometric oxide phase can be seen (comps. c-c  , Fig. 2-3). In the sample WO 2.9 a signal from nonstoichiometric oxide phase is 174 A.P. Shpak et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 172–175 Table 1 Components, peak energies (E p ) and relative intensities of the peaks (I) for WO 3 ,WO 2.9 and WO 2.72 samples Components, peak energies WO 3 at 293 K, I (%) WO 2.9 at 293 K, I (%) WO 2.72 at 293 K, I (%) WO 3 at 553 K, I (%) WO 2.9 at 553 K, I (%) WO 2.72 at 553 K, I (%) c, W 5+ , E p W4f 7/2 = 34.8 eV – – 12.1 – 9.1 14.2 d, W 6+ , E p W4f 7/2 = 35.7 eV 57.5 14.3 31.0 100 90.9 85.8 e, W 6+ , E p W4f 7/2 = 36.1 eV 42.5 85.7 56.9 – – – f, O 2− , E p = 530.6 eV 48.5 29.4 19.8 81.4 39.6 18.6 g, OH, E p = 531.1 eV 17.0 27.5 30.5 – 23.0 31.1 h, O − , E p = 532 eV 18.6 26.8 28.1 10.8 28.5 38.1 m, H 2 O, E p = 533.1 eV 15.9 16.3 21.6 7.8 8.9 12.2 Precision, I ±1.2 too much reduced due to the absorption by the hydroxide shell so we didn’t succeed to distinguish the contribution of W 5+ -states. The structure of W4f-line for stoichiometric and nonstoi- chiometric oxides after anneal (553 K) also differs essentially. Fig. 3. Peak synthesis for O1s-level XPS-spectrum of oxygen atoms. According to the results of decomposition of W4f-line the surface of stoichiometric tungsten oxide is formed only of W 6+ - states of WO 3 -phase (comps. d-d  , Fig. 2-4). A signal from WO 3 ·(OH 2 ) n is absent (comps. e-e  , Fig. 2-4). The results of decomposition of W4f-line of the annealed samples WO 2.9 and WO 2.72 (Fig. 2-5, 2-6) show that the surface of nanoparticles is formed of nonstoichiometric phase WO 3−x (W 5+ -states, comp. c-c  and W 6+ -states, comps. d-d  ). A signal from WO·(OH 2 ) n was not fixed. Fig. 3 shows the results of decomposition into peaks of O1s-line for the annealed and non-annealed tungsten oxide sam- ples. The peaks with maxima E p O1s = 530.6 eV correspond to O1s-levels of oxygen atoms O 2− in the lattice (comp. f) [16]. Free oxide surfaces contacting with the atmosphere are always hydrated, i.e. contain water molecules and hydroxyl groups. There are two types of OH-groups on the surface: single M- OH and double OH–M–OH [17]. Before anneal OH-groups (E p O1s = 531.1 eV) [16] and groups C O(E p O1s = 531.2 eV), C–O–C, C–O–H (E p O1s = 532.5–532.8 eV) [18] are present on the surface of nanoparticles, thus the structure of compo- nents g, h (Fig. 3) in the initial samples is very complex. The presence of C O, C–O–C, C–O–H groups may be caused by oxidation-reduction and acid-base transitions of CO 2 on catalyt- ically active oxide surface. After anneal the maxima of peaks in region 531.1 eV correspond at most to single and double OH- groups (comp. g), in region 532 eV the maxima of peaks belong to weakly adsorbed species and O − oxygen states (comp. h) [16,19]. Signal from H 2 O molecules in region E p = 533.1 eV is presented by component m [20,21]. In the non-annealed sample of stoichiometric WO 3 (Fig. 3-1) the main signal is given by lattice oxygen atoms O 2− (comp. f) also components g, h and the contribution from H 2 O molecules (comp. m) are present. In the non-annealed samples of nonstoi- chiometric WO 2.9 and WO 2.72 (Fig. 3-2, 3-3) the contributions of comps. g, h and H 2 O molecules (comp. m) into O1s-line Table 2 Ratios of OH/O 2− ,O − /O 2− ,H 2 O/O 2− for annealed WO 3 ,WO 2.9 and WO 2.72 samples Ratios WO 3 at 553 K WO 2.9 at 553 K WO 2.72 at 553 K OH/O 2− 0 0.6 1.7 O − /O 2− 0.1 0.7 2.1 H 2 O/O 2− 0.1 0.2 0.7 A.P. Shpak et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 172–175 175 is more considerable and comparable to the contribution from O 2− -states. After anneal of the stoichiometric WO 3 sample the contri- bution into O1s-line from lattice oxygen atoms O 2− dominates (Fig. 3-4, comp. f). A signal in region E p = 532 eV may cor- respond to O − oxygen states and particularly to remnants of adsorbed species (comp. h) and it is much decreased. The main difference is the complete absence of OH-groups (comp. g) on the surface. On the contrary the relative contribution of OH- groups and H 2 O on the surface of WO 2.9 and WO 2.72 remains high after anneal and it is proportional to the nonstoichiome- try value x of WO 3−x oxides (comps. g, m, Fig. 3-5 and 3-6; OH/O 2− ,H 2 O/O 2− in Table 2). Thus, after anneal OH-groups (comp. g) are present only in nonstoichiometric oxides. Also the contribution of O − oxygen states (comp. h, Fig. 3-5 and 3-6; O − /O 2− in Table 2) rises. In all the samples the decrease of con- tribution of comp. m can be seen, that can be connected mainly with destruction of WO 3 ·(OH 2 ) n -phase. Thus the contents of OH-groups and H 2 O in the nonstoichio- metric WO 3−x -oxides is proportional to the nonstoichiometry value x. Paper [22] shows direct link between catalytic and electrochemical activity of oxide-hydroxide compounds and ion component of conductivity, formed by hydroxyl groups. There- fore synthesis of oxide nanoparticles with enlarged contents of M (n−y)+ -cations relatively to M n+ -cations allows to form an exceed of OH − -anions which determines catalytic and electro- chemical activity of oxides. The increase of OH-groups contents in the nonstoichiometric tungsten oxides can be explained due to possibility of oxygen vacancy infill by OH-group. At that W 5+ -cation should be in the centre of octahedron of five O 2− -ions and one OH − -anion. The formula of such nonstoichiometric compound can be written as W x 5+ W 1−x 6+ O 3−x (OH) x (1), where x are the hydroxyl groups that filled oxygen vacancies in the lattice. However high value of ratio OH/O 2− (Table 2) indicates the oxide-hydroxide compound W x 5+ W 1−x 6+ O 3−x−y ·(OH) x+2y (2), where 2y are two hydroxyl groups which replaced O 2− -ion in WO 6 -octahedron due to reaction O 2− +H 2 O → 2OH − . On the whole in the nonstoichiometric tungsten oxides a tran- sition W x 5+ W 1−x 6+ O 3−0.5x → W x 5+ W 1−x 6+ O 3−x ·(OH) x → WO 3 ·(OH 2 ) n as a result of hydration in water vapors is observed with formation of hydrate shell parti- cles on the surface. After anneal on air a transition WO 3 ·(OH 2 ) n → W x 5+ W 1−x 6+ O 3−x−y ·(OH) x+2y is observed. Sensor response R/R for 3% H 2 concentration in air at room temperature equals 2 for active element based on WO 2.9 and 2.2 for active element based on WO 2.72 . Electrical resistance changes at that are completely reversible. After a month the signal becomes two times lower. For active elements based on stoichiometric WO 3 nanopowders sensor response R/R =0. 4. Conclusions Thus in the nonstoichiometric tungsten oxides the contents of OH-groups is proportional to the nonstoichiometry value x of the oxide matrix WO 3−x . Exposure to air at 293 K of nonstoichiometric nanodisperse tungsten oxides leads to the formation of WO 3 ·(OH 2 ) n -phase on the surface of nanoparti- cles. The sensor response is observed only in oxides with high contents of W 5+ -states. Fragment W 5+ -OH − responds for elec- tron and proton component of conductivity. We suppose that resistance change is caused by the process of protonic and elec- tronic conductivity blocking at H 2 adsorption by the surface of W x 5+ W 1−x 6+ O 3−x−y ·(OH) x+2y -phase. References [1] J. Solis, S. Saukko, L. Kish, C. Granqvist, V. Lantto, Thin Solid Films 391 (2001) 255. [2] H. Lin, C. Hsu, H. Yang, P. Lee, C. Yang, Sens. Actuators B 22 (1994) 63. [3] M. Ando, T. Tsuchida, S. Suto, T. Suzuki, C. Nakayama, N. Miura, N. Yamazoe, J. Ceram. Soc. Jpn. 104 (1996) 1112. [4] X. Wang, N. Miura, N. 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Nanomater. Nanotechnol. 4 (2004) 1185 (in Ukrainian). . Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 172–175 XPS studies of active elements surface of gas sensors based on WO 3−x nanoparticles A.P complete absence of OH-groups (comp. g) on the surface. On the contrary the relative contribution of OH- groups and H 2 O on the surface of WO 2.9 and WO 2.72 remains high