- Email: [email protected]
XPS studies of active elements surface of gas sensors based on WO3−x nanoparticles
Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 172–175
XPS studies of active elements surface of gas sensors based on WO3−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 WO3−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 WO3 ·(OH2 )n -phase on the surface of nanoparticles. A correlation between value x of nonstoichiometric WO3−x oxide matrix and contents of OH-groups in it was revealed. It was noticed, that the increase of number of W5+ -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; WO3−x
1. Introduction
2. Experimental
Nanodisperse nonstoichiometric WO3−x oxides are promising materials and are used for producing of gas sensors active elements [1–8]. In the most cases the range of their work temperatures 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 example [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 molecular hydrogen and rare gases at room temperature. Earlier the effect of sensitivity to rare gases at physical adsorption at room temperature was detected for TiO2 polydisperse samples [11]. Electrical resistance changes at that were completely reversible (R/R ∼ 0.3). That effect increased with the rise of rutile imperfection. 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 WO3−x surface before and after temperature influence at contact with air to reveal their connection with sensor characteristics of synthesized nanopowders.
Three series of WO3, WO2.9 and WO2.72 nanopowders were synthesized at atmospheric pressure by method of electric explosion 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 conditions was chosen for synthesis, injected energy was E = 1.9·Es , where Es is the sublimation energy of metal, diameter of tungsten 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 WO3−x was determined by XPS-method by means of ratio between W5+ 4fand W6+ 4f-states. Active elements of gas sensors were prepared from WO3 , WO2.9 and WO2.72 -nanopowders on Al2 O3 -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 installation 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 WO3−x oxides was examined in annealed and non-annealed active elements. The electronic structure of the surface of WO3−x was explored by method of X-ray photoelectron spectroscopy (XPS)
∗
Corresponding author. E-mail address: [email protected] (A.M. Korduban).
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 WO3−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 W4flevel were decomposed into peak couples with parameters of spin–orbit separation Ep (4f5/2 − 4f7/2 ) = 2.1 eV and intensities ratio I4f5/2 /I4f7/2 = 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 H2 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 represents the results of decomposition of W4f-line in the annealed and non-annealed samples. The maxima of peak couples correspond to W4f7/2 - and W4f5/2 -levels of tungsten atoms for W5+ -states of oxide (comps. c-c , Ep W4f7/2 = 34.8 eV) [15]
Fig. 2. Peak synthesis for W4f-level XPS-spectrum of tungsten atoms.
and W6+ -states of oxide (comps. d-d Ep W4f7/2 = 35.7 eV) and hydroxide (comps.e-e , Ep W4f7/2 = 36.1 eV) [15,16], where Ep – peak energy. According to the XPS data in the non-annealed sample of stoichiometric WO3 the surface of nanoparticles consists of WO3 -phase (comps. d-d , Fig. 2-1) and WO3 ·(OH2 )n -phase formed after contact with air (comps. e-e’, Fig. 2-1). The results of decomposition of W4f-line for nonstoichiometric oxides WO2.9 and WO2.72 indicate essential differences in comparison with stoichiometric WO3 . 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 nanoparticles’ surface with water vapor during contact with air right up to formation of a shell of several WO3 ·(OH2 )n -monolayers. In the sample WO2.72 the contribution of W5+ -states from nonstoichiometric oxide phase can be seen (comps. c-c , Fig. 2-3). In the sample WO2.9 a signal from nonstoichiometric oxide phase is
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Table 1 Components, peak energies (Ep ) and relative intensities of the peaks (I) for WO3 , WO2.9 and WO2.72 samples Components, peak energies
WO3 at 293 K, I (%)
WO2.9 at 293 K, I (%)
WO2.72 at 293 K, I (%)
WO3 at 553 K, I (%)
WO2.9 at 553 K, I (%)
WO2.72 at 553 K, I (%)
c, W5+ , Ep W4f7/2 = 34.8 eV d, W6+ , Ep W4f7/2 = 35.7 eV e, W6+ , Ep W4f7/2 = 36.1 eV f, O2− , Ep = 530.6 eV g, OH, Ep = 531.1 eV h, O− , Ep = 532 eV m, H2 O, Ep = 533.1 eV
– 57.5 42.5 48.5 17.0 18.6 15.9
– 14.3 85.7 29.4 27.5 26.8 16.3
12.1 31.0 56.9 19.8 30.5 28.1 21.6
– 100 – 81.4 – 10.8 7.8
9.1 90.9 – 39.6 23.0 28.5 8.9
14.2 85.8 – 18.6 31.1 38.1 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 W5+ -states. The structure of W4f-line for stoichiometric and nonstoichiometric oxides after anneal (553 K) also differs essentially.
According to the results of decomposition of W4f-line the surface of stoichiometric tungsten oxide is formed only of W6+ states of WO3 -phase (comps. d-d , Fig. 2-4). A signal from WO3 ·(OH2 )n is absent (comps. e-e , Fig. 2-4). The results of decomposition of W4f-line of the annealed samples WO2.9 and WO2.72 (Fig. 2-5, 2-6) show that the surface of nanoparticles is formed of nonstoichiometric phase WO3−x (W5+ -states, comp. c-c and W6+ -states, comps. d-d ). A signal from WO·(OH2 )n was not fixed. Fig. 3 shows the results of decomposition into peaks of O1s-line for the annealed and non-annealed tungsten oxide samples. The peaks with maxima Ep O1s = 530.6 eV correspond to O1s-levels of oxygen atoms O2− 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 MOH and double OH–M–OH [17]. Before anneal OH-groups (Ep O1s = 531.1 eV) [16] and groups C O (Ep O1s = 531.2 eV), C–O–C, C–O–H (Ep O1s = 532.5–532.8 eV) [18] are present on the surface of nanoparticles, thus the structure of components 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 CO2 on catalytically active oxide surface. After anneal the maxima of peaks in region 531.1 eV correspond at most to single and double OHgroups (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 H2 O molecules in region Ep = 533.1 eV is presented by component m [20,21]. In the non-annealed sample of stoichiometric WO3 (Fig. 3-1) the main signal is given by lattice oxygen atoms O2− (comp. f) also components g, h and the contribution from H2 O molecules (comp. m) are present. In the non-annealed samples of nonstoichiometric WO2.9 and WO2.72 (Fig. 3-2, 3-3) the contributions of comps. g, h and H2 O molecules (comp. m) into O1s-line Table 2 Ratios of OH/O2− , O− /O2− , H2 O/O2− for annealed WO3 , WO2.9 and WO2.72 samples
Fig. 3. Peak synthesis for O1s-level XPS-spectrum of oxygen atoms.
Ratios
WO3 at 553 K
WO2.9 at 553 K
WO2.72 at 553 K
OH/O2−
0 0.1 0.1
0.6 0.7 0.2
1.7 2.1 0.7
O− /O2− H2 O/O2−
A.P. Shpak et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 172–175
is more considerable and comparable to the contribution from O2− -states. After anneal of the stoichiometric WO3 sample the contribution into O1s-line from lattice oxygen atoms O2− dominates (Fig. 3-4, comp. f). A signal in region Ep = 532 eV may correspond 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 OHgroups and H2 O on the surface of WO2.9 and WO2.72 remains high after anneal and it is proportional to the nonstoichiometry value x of WO3−x oxides (comps. g, m, Fig. 3-5 and 3-6; OH/O2− , H2 O/O2− 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− /O2− in Table 2) rises. In all the samples the decrease of contribution of comp. m can be seen, that can be connected mainly with destruction of WO3 ·(OH2 )n -phase. Thus the contents of OH-groups and H2 O in the nonstoichiometric WO3−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. Therefore synthesis of oxide nanoparticles with enlarged contents of M(n−y)+ -cations relatively to Mn+ -cations allows to form an exceed of OH− -anions which determines catalytic and electrochemical 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 W5+ -cation should be in the centre of octahedron of five O2− -ions and one OH− -anion. The formula of such nonstoichiometric compound can be written as Wx 5+ W1−x 6+ O3−x (OH)x (1), where x are the hydroxyl groups that filled oxygen vacancies in the lattice. However high value of ratio OH/O2− (Table 2) indicates the oxide-hydroxide compound Wx 5+ W1−x 6+ O3−x−y ·(OH)x+2y (2), where 2y are two hydroxyl groups which replaced O2− -ion in WO6 -octahedron due to reaction O2− + H2 O → 2OH− . On the whole in the nonstoichiometric tungsten oxides a transition Wx 5+ W1−x 6+ O3−0.5x → Wx 5+ W1−x 6+ O3−x ·(OH)x → WO3 ·(OH2 )n as a result of hydration in water vapors is observed with formation of hydrate shell particles on the surface. After anneal on air a transition WO3 ·(OH2 )n → Wx 5+ W1−x 6+ O3−x−y ·(OH)x+2y is observed. Sensor response R/R for 3% H2 concentration in air at room temperature equals 2 for active element based on WO2.9 and 2.2 for active element based on WO2.72 . Electrical resistance changes at that are completely reversible. After a month the signal becomes two times lower. For active elements based on stoichiometric WO3 nanopowders sensor response R/R = 0.
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4. Conclusions Thus in the nonstoichiometric tungsten oxides the contents of OH-groups is proportional to the nonstoichiometry value x of the oxide matrix WO3−x . Exposure to air at 293 K of nonstoichiometric nanodisperse tungsten oxides leads to the formation of WO3 ·(OH2 )n -phase on the surface of nanoparticles. The sensor response is observed only in oxides with high contents of W5+ -states. Fragment W5+ -OH− responds for electron and proton component of conductivity. We suppose that resistance change is caused by the process of protonic and electronic conductivity blocking at H2 adsorption by the surface of Wx 5+ W1−x 6+ O3−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. Yamazoe, Sens. Actuators B 66 (2000) 74. [5] C.N. Xu, N. Miura, Y. Ishida, K. Matsuda, N. Yamazoe, Sens. Actuators B 65 (2000) 163. [6] E. Llobet, G. Molas, P. Molin`as, J. Calderer, X. Vilanova, J. Brezmes, J.E. Sueiras, J. Electrochem. Soc. 147 (2000) 776. [7] G. Sberveglieri, L. Depero, S. Groppelli, P. Nelli, Sens. Actuators B 26–27 (1995) 89. [8] S. Moulzolf, S. Ding, R. Lad, Sens. Actuators B 77 (2001) 375. [9] J. Solis, S. Saukko, L. Kish, C. Granqvist, V. Lantto, Sens. Actuators B 77 (2001) 316. [10] J. Shieh, H.M. Feng, M.H. Hon, H.Y. Juang, Sens. Actuators B86 (2002) 75. [11] V.F. Kiselev, O.V. Krylov, Electron Phenomena in Adsorption and Catalysis in Semiconductors and Dielectrics, Nauka, Moscow, 1979 (in Russian). [12] S.A. Pikuz, T.A. Shelkovenko, D.B. Sinars, J.B. Greenly, Y.S. Dimant, D.A. Hammer, Phys. Rev. Lett. 83 (1999) 4313. [13] P. Sen, J. Ghosh, A. Abdullan, P. Kumar, Chem. Sci Proc. Indian Acad. Sci. Vol. 115, 5–6, New Delhi, India, October–December, 2003, p. 499. [14] D. Briggs, M.P. Seach, Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, John Wiley & Sons, Chichester—New York, 1983. [15] P. Charton, L. Gengembre, P. Armand, J. Solid State Chem. 168 (2002) 175. [16] J.C. Dupin, D. Gonbeau, P. Vinatier, A. Levasseur, Phys. Chem. Chem. Phys. 2 (2000) 1319. [17] V.F. Kiselev, S.N. Kozlov, A.V. Zoteev, Fundamentals of Physics of Solids’ Surface, Moscow University Publishers, Moscow, 1999 (in Russian). [18] S.J. Kerber, J.J. Bruckner, S. Seal, S. Hardcastle, T.L. Barr, J. Vac. Sci. Technol. A14 (1996) 1314. [19] Y. Stoh, Surface 2 (1987) 68 (in Russian). [20] C.D. Wagner, J.F. Moulder, L.E. Davis, W.M. Riggs, Handbook of X-ray Photoelectron Spectroscopy, Perking-Elmer Corp., New York, 1979. [21] V.I. Nefedov, X-Ray Spectroscopy of Chemical Compunds, Chemistry, Moscow, 1984 (in Russian). [22] N.D. Ivanova, E.I. Boldyrev, O.A. Stadnik, N.E. Vlasenko, Nanosyst. Nanomater. Nanotechnol. 4 (2004) 1185 (in Ukrainian).
XPS studies of active elements surface of gas sensors based on WO3−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 WO3−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 WO3 ·(OH2 )n -phase on the surface of nanoparticles. A correlation between value x of nonstoichiometric WO3−x oxide matrix and contents of OH-groups in it was revealed. It was noticed, that the increase of number of W5+ -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; WO3−x
1. Introduction
2. Experimental
Nanodisperse nonstoichiometric WO3−x oxides are promising materials and are used for producing of gas sensors active elements [1–8]. In the most cases the range of their work temperatures 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 example [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 molecular hydrogen and rare gases at room temperature. Earlier the effect of sensitivity to rare gases at physical adsorption at room temperature was detected for TiO2 polydisperse samples [11]. Electrical resistance changes at that were completely reversible (R/R ∼ 0.3). That effect increased with the rise of rutile imperfection. 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 WO3−x surface before and after temperature influence at contact with air to reveal their connection with sensor characteristics of synthesized nanopowders.
Three series of WO3, WO2.9 and WO2.72 nanopowders were synthesized at atmospheric pressure by method of electric explosion 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 conditions was chosen for synthesis, injected energy was E = 1.9·Es , where Es is the sublimation energy of metal, diameter of tungsten 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 WO3−x was determined by XPS-method by means of ratio between W5+ 4fand W6+ 4f-states. Active elements of gas sensors were prepared from WO3 , WO2.9 and WO2.72 -nanopowders on Al2 O3 -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 installation 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 WO3−x oxides was examined in annealed and non-annealed active elements. The electronic structure of the surface of WO3−x was explored by method of X-ray photoelectron spectroscopy (XPS)
∗
Corresponding author. E-mail address: [email protected] (A.M. Korduban).
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 WO3−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 W4flevel were decomposed into peak couples with parameters of spin–orbit separation Ep (4f5/2 − 4f7/2 ) = 2.1 eV and intensities ratio I4f5/2 /I4f7/2 = 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 H2 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 represents the results of decomposition of W4f-line in the annealed and non-annealed samples. The maxima of peak couples correspond to W4f7/2 - and W4f5/2 -levels of tungsten atoms for W5+ -states of oxide (comps. c-c , Ep W4f7/2 = 34.8 eV) [15]
Fig. 2. Peak synthesis for W4f-level XPS-spectrum of tungsten atoms.
and W6+ -states of oxide (comps. d-d Ep W4f7/2 = 35.7 eV) and hydroxide (comps.e-e , Ep W4f7/2 = 36.1 eV) [15,16], where Ep – peak energy. According to the XPS data in the non-annealed sample of stoichiometric WO3 the surface of nanoparticles consists of WO3 -phase (comps. d-d , Fig. 2-1) and WO3 ·(OH2 )n -phase formed after contact with air (comps. e-e’, Fig. 2-1). The results of decomposition of W4f-line for nonstoichiometric oxides WO2.9 and WO2.72 indicate essential differences in comparison with stoichiometric WO3 . 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 nanoparticles’ surface with water vapor during contact with air right up to formation of a shell of several WO3 ·(OH2 )n -monolayers. In the sample WO2.72 the contribution of W5+ -states from nonstoichiometric oxide phase can be seen (comps. c-c , Fig. 2-3). In the sample WO2.9 a signal from nonstoichiometric oxide phase is
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Table 1 Components, peak energies (Ep ) and relative intensities of the peaks (I) for WO3 , WO2.9 and WO2.72 samples Components, peak energies
WO3 at 293 K, I (%)
WO2.9 at 293 K, I (%)
WO2.72 at 293 K, I (%)
WO3 at 553 K, I (%)
WO2.9 at 553 K, I (%)
WO2.72 at 553 K, I (%)
c, W5+ , Ep W4f7/2 = 34.8 eV d, W6+ , Ep W4f7/2 = 35.7 eV e, W6+ , Ep W4f7/2 = 36.1 eV f, O2− , Ep = 530.6 eV g, OH, Ep = 531.1 eV h, O− , Ep = 532 eV m, H2 O, Ep = 533.1 eV
– 57.5 42.5 48.5 17.0 18.6 15.9
– 14.3 85.7 29.4 27.5 26.8 16.3
12.1 31.0 56.9 19.8 30.5 28.1 21.6
– 100 – 81.4 – 10.8 7.8
9.1 90.9 – 39.6 23.0 28.5 8.9
14.2 85.8 – 18.6 31.1 38.1 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 W5+ -states. The structure of W4f-line for stoichiometric and nonstoichiometric oxides after anneal (553 K) also differs essentially.
According to the results of decomposition of W4f-line the surface of stoichiometric tungsten oxide is formed only of W6+ states of WO3 -phase (comps. d-d , Fig. 2-4). A signal from WO3 ·(OH2 )n is absent (comps. e-e , Fig. 2-4). The results of decomposition of W4f-line of the annealed samples WO2.9 and WO2.72 (Fig. 2-5, 2-6) show that the surface of nanoparticles is formed of nonstoichiometric phase WO3−x (W5+ -states, comp. c-c and W6+ -states, comps. d-d ). A signal from WO·(OH2 )n was not fixed. Fig. 3 shows the results of decomposition into peaks of O1s-line for the annealed and non-annealed tungsten oxide samples. The peaks with maxima Ep O1s = 530.6 eV correspond to O1s-levels of oxygen atoms O2− 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 MOH and double OH–M–OH [17]. Before anneal OH-groups (Ep O1s = 531.1 eV) [16] and groups C O (Ep O1s = 531.2 eV), C–O–C, C–O–H (Ep O1s = 532.5–532.8 eV) [18] are present on the surface of nanoparticles, thus the structure of components 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 CO2 on catalytically active oxide surface. After anneal the maxima of peaks in region 531.1 eV correspond at most to single and double OHgroups (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 H2 O molecules in region Ep = 533.1 eV is presented by component m [20,21]. In the non-annealed sample of stoichiometric WO3 (Fig. 3-1) the main signal is given by lattice oxygen atoms O2− (comp. f) also components g, h and the contribution from H2 O molecules (comp. m) are present. In the non-annealed samples of nonstoichiometric WO2.9 and WO2.72 (Fig. 3-2, 3-3) the contributions of comps. g, h and H2 O molecules (comp. m) into O1s-line Table 2 Ratios of OH/O2− , O− /O2− , H2 O/O2− for annealed WO3 , WO2.9 and WO2.72 samples
Fig. 3. Peak synthesis for O1s-level XPS-spectrum of oxygen atoms.
Ratios
WO3 at 553 K
WO2.9 at 553 K
WO2.72 at 553 K
OH/O2−
0 0.1 0.1
0.6 0.7 0.2
1.7 2.1 0.7
O− /O2− H2 O/O2−
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is more considerable and comparable to the contribution from O2− -states. After anneal of the stoichiometric WO3 sample the contribution into O1s-line from lattice oxygen atoms O2− dominates (Fig. 3-4, comp. f). A signal in region Ep = 532 eV may correspond 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 OHgroups and H2 O on the surface of WO2.9 and WO2.72 remains high after anneal and it is proportional to the nonstoichiometry value x of WO3−x oxides (comps. g, m, Fig. 3-5 and 3-6; OH/O2− , H2 O/O2− 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− /O2− in Table 2) rises. In all the samples the decrease of contribution of comp. m can be seen, that can be connected mainly with destruction of WO3 ·(OH2 )n -phase. Thus the contents of OH-groups and H2 O in the nonstoichiometric WO3−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. Therefore synthesis of oxide nanoparticles with enlarged contents of M(n−y)+ -cations relatively to Mn+ -cations allows to form an exceed of OH− -anions which determines catalytic and electrochemical 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 W5+ -cation should be in the centre of octahedron of five O2− -ions and one OH− -anion. The formula of such nonstoichiometric compound can be written as Wx 5+ W1−x 6+ O3−x (OH)x (1), where x are the hydroxyl groups that filled oxygen vacancies in the lattice. However high value of ratio OH/O2− (Table 2) indicates the oxide-hydroxide compound Wx 5+ W1−x 6+ O3−x−y ·(OH)x+2y (2), where 2y are two hydroxyl groups which replaced O2− -ion in WO6 -octahedron due to reaction O2− + H2 O → 2OH− . On the whole in the nonstoichiometric tungsten oxides a transition Wx 5+ W1−x 6+ O3−0.5x → Wx 5+ W1−x 6+ O3−x ·(OH)x → WO3 ·(OH2 )n as a result of hydration in water vapors is observed with formation of hydrate shell particles on the surface. After anneal on air a transition WO3 ·(OH2 )n → Wx 5+ W1−x 6+ O3−x−y ·(OH)x+2y is observed. Sensor response R/R for 3% H2 concentration in air at room temperature equals 2 for active element based on WO2.9 and 2.2 for active element based on WO2.72 . Electrical resistance changes at that are completely reversible. After a month the signal becomes two times lower. For active elements based on stoichiometric WO3 nanopowders sensor response R/R = 0.
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