J. Phys. Chem. 1992,96, 7718-7724
7718
will react preferably with surface cus oxygen and produce hydroxyls and carbonate species, and correspondingly species I failed to be detected for the methane adsorption on the contaminated CeOz as shown in Figure 5c. The four C-H bonds of C H 4 are equivalent and arranged so that their respective axes define a regular tetrahedron; i.e., methane belongs to the Tdpoint group. The Tdsymmetry of CH, is reduced to lower symmetry, such as C3, when C H 4 is strongly adsorbed on surface. Accordingly, the four C-H bonds become unequivalent and one of them may be weakened, ready for cleavage. For species I as depicted in Figure 6 the strong chemical interaction causes a structural distortion of CH4and the Tdsymmetry possibly turns into C, symmetry, and the mode a t 2917 cm-'therefore becomes infrared active. Species I1 exhibits a similar spectrum as that of free CHI, indicating that the adsorbed methane still keeps the identity of free CHI owing to the relative weak interaction between
oxygen and CH4. Acknowledgment. We gratefully acknowledge the Natural Science Foundation of China (NSFC) for support of this research. Registry No. Ce02, 1306-38-3; CH4, 74-82-8; C02, 124-38-9; H20, 7732-18-5.
Refereaces pad Notes (1) Saillard, J.-Y.; Hoffmann, R.1.Am. Chem. SOC.1984, 106, 2006. (2) Ceyer, S. T. Lungmuir 1990, 6, 82. (3) Zaera, F. Catal. Lett. 1991, 11, 95. (4) Anderson, A. B.; Maloney, J. J. J. Phys. Chem. 1988, 92. 809. (5) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis. Studies in Surface Science and Caialysis; Elsevier Science Publishers: Amsterdam, 1989; Vol. 45. (6) Kiselev, V. F.; Krylov, 0. V. Adsorpiion and Catalysis on Transition Metals and Their Oxides; Springer-Verlag: New York, 1989.
(7) Busca, G.;Marchetti, L.; Xerlia, T.; Girelli, A,; Sorlino, M.; Lorenzelli, V. 8th Int. Congr. Catal., Tokyo 1984, 111-299. (8) Eedohelyi, A.; Solymosi, F. J. Catal. 1990, 123, 31. (9) Centi, G.; Trifiro, F. Chem. Rev. 1988, 88, 55. (IO) Lee, J. S.;Oyama, S.T. Catal. Rev.-Sei. Eng. 1988, 30, 249. (1 1) Hutchings, G.J.; Scurrell, M. S.; Woodhouse, J. R.Chem. Soc. Reu. 1989, 18, 251. (12) Pokier, M.; Breault, R.Appl. Carol. 1991, 71, 103. (13) Parkyns, N. D.; Brown, M. J. Catal. Today 1991,8, 305. (14) Li, C.; Domen, K.; Maruya, K.; Onishi, T. J . Chem. Soc., Chem. Commun. 1988, 1541. (15) Li, C.; Domen, K.; Maruya, K.; Onishi, T. J . Am. Chem. Soc. 1989, 1 1 1 , 7683. (16) Li, C.; Xin, Q.; Guo, X.-X. (a) Chin. J . Mol. Caral. 1991, 5, 193; (b) Catal. Lett. 1992, 12, 297. (17) Li, C.; Xin, Q.; Guo, X.-X.; Onishi, T. Symp. 10th Inr. Congr. Catal.,
Budapest, in press. (18) Li. C.; Sakata, Y.;Arai, T.;Domen, K.; Maruya. K.: Onishi. T. J . Chem. SOC.,Faraday Trans. 1 1989, 85, 929. (19) Zaki, M. I.; Knozinger, H. Spectrochim. Acta 1987, 43A, 1455. (20) Sheppard, N.; Yates, D. J. C. Proc. R.Soc. London 1956, A238,69. (21) Shimanouchi,T. 'Table of Molecular Vibrational Frequencies Consolidated Volume l", NSRDS-NBS 39, Nat. Stand. Ref. Data Ser.,Nat. Bur. Stand. (US.), 1972, 44. (22) Kung, M. C.; Kung, H. H. Catal. Reu.-Sci. Eng. 1985, 27, 425. (23) Feng, Y.; Niiranen, J.; Gutman, D. J. Phys. Chem. 1991,95,6558, 6564. (24) Zhang, X.; Ungar, R. K.; Lambert, R. M. J . Chem. SOC.,Chem. Commun. 1989,473. (25) Ito, T.; Lunsford, J. H. Nature (London) 1985, 314, 721. (26) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J . Am. Chem. SOC. 1985, 107, 5062. (27) Driscoll, D. J.; Lunsford, J. H. J . Phys. Chem. 1983, 87, 301. (28) Otsuka, K.; Jinno, K. Inorg. Chim. Acta 1986, 121, 237. (29) Otsuka, K.; Said, A. A.; Jinno, K.; Komatsu, T. Chem. Lett. 1987, 77. (30) Keller, G.E.; Bhasin, M. M. J . Catal. 1982, 73, 9. (31) Gaffney, A. M.; Jones, C. A.; Lfonard, J. J.; Sofranko, J. A. J. Caral. 1988, 114, 422. (32) Che, M.; Tench, A. J. Adu. Catal. 1982, 31, 77.
Influence of the Electrolytic Medium Composition on the Structural Evoiutlon of Thin Eiectrochromlc MOO, Films Probed by X-ray Absorption Spectroscopy Daniel Guay,**fGrard Tourillon, LURE, Bdtiment 209 D, Universiti Paris-Sud, Orsay 91 405, France
Guylaine Laperrik, and Daniel Wlanger Dipartement de Chimie, Universiti du Quibec 6 Montrial, C.P. 8888, Succursale A , Montrial, Quibec, Canada H3C 3P8 (Received: February 3, 1992; In Final Form: April 30, 1992) We have performed an X-ray absorption spectroscopy study at the Mo K edge of Moo3thin films to determine the effect of the composition of the electrolyte on the structural modifications occurring in the layer during the electrocoloration process. Both Li+ and H+cations containing solutions were studied. The M a 3 layers were obtained by thermal decomposition of electrodeposited MoS3 and the structure of the resulting material was confirmed by X-ray absorption spectroscopy. Upon Li+ (H+) incorporation into the M a 3 material, a 5% (2.3%) increase of the shortest Mo-Mo separation distance is observed. This structural deformation occurs in a direction perpendicular to the layered structure of the material. In potential sweep experiments, the structural modification induced by the incorporation of the cations within the M a 3 material is not totally reversible, and a buildup of chemical disordering is observed with the number of sweeps. This chemical disorder affects both the first 0 and the second Mo coordination shell and is more important when Li+ rather than H+cations are incorporated into the Moo3 material. The perturbing effect of H+and Li+ is best understood if the cations are located near the bridging 0 atoms within the M a 3 layer rather than between the M a 3 sheets of the layered compound. The cation-specific structural deformation and chemical disorder induced into the Moo3 material are responsible for the specific deterioration of the electrochemical and electrochromic properties of the layer in potential sweep experiments. This effect most probably arises from a variation of the diffusivity of the cation according to the extent of structural deformation and disorder in the material.
Introduction Transition-metal oxides make u p a t&nologially ~ ~ t e rlie i ~ l ~ and class
important change
wo3, v,oS
*To whom correspondence should be sent. 'Permanent address: INRS-Encrgieet Mat&iaux, 1650 MontQ Ste-Julie, C.P. 1020 Varennes, Qu&ec, Canada J3X 1S2.
their light-absorbing properties under an externally applied electric fieldi-' can be in nonemissive d i s p l a ~ . ~Moreover, .~ hey have attracted a lot of attention as positive electrodes for ambient temperature secondary lithium batteries? Molybdenum trioxide has been prepared by several methods: thel'n'lal evaporation? sPutk&? andimtion? spray PYrol@%'o colloidal sol-gel method,l'J2 chemical vapor deposition,13J4 and
0022-36S4/92/2096-7718$03.00/00 1992 American Chemical Society
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The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7719
electrodepositi~n,~~-~~ More rccently, thin films of Moo3 have been obtained by the thermal oxidation of an electrochemically deposited molybdenum trisulfide layer.18 It is well-known that the kinetics of the electrochromic process is influenced by the method of preparation of the electr0des.97'~~~ It is therefore of interest to characterize the electrochromic properties of Moo3 layers formed by the thermal oxidation of MoS3and to correlate these properties with the composition and structure of the material. Ex situ X-ray diffraction has been used to get information about the changes in composition and structure of the electrochromic material after c l e c t r o c o l o r a t i ~ n . ~More ~ ~ ~ ~recently, special electrochemicalcells have been developed for in situ X-ray diffraction measurements. W03 and M a 3 layers have been studied with such a technique, in which the sample stays continuously in an electrochemically controlled environment while the data acquisition step proceeds.26 Molybdenum trioxide has an orthorhombic structure made of layers stacked in the direction of the 6 axis and the (Om) reflectionsdominate the X-ray diffraction pattern. Upon cathodic polarization, the film is immediately colored dark blue. However, this fast electrocoloration process does not give rise to any modification of the X-ray diffraction To observe a change in the crystallite structure of the Moo3 material, the films have to be polarized for a period of time exceeding 0.5 h. In such conditions, new peaks were observed on the low 28 side of the (OM)) reflections, and they were attributed to the formation of an orthorhombic phase of H,Mo03, with x = 0.34.26 While it is of interest for their use as positive electrodes in ambient-temperature lithium batteries to study the change in composition and structure occurring in Mo03 layers after prolonged charging and discharging periods (exceeding say 0.5 h), such studies are less relevant if one is concerned by the structural modifications occurring at the very beginning of the electrocoloration process. As evidenced previously, the X-ray diffraction technique is not suited for this ~urpase,~ and we decide to address this question by using X-ray absorption spectroscopy. X-ray absorption spectroscopy is a well-suited technique to characterize both the electronic and structural properties of a material. As opposed to X-ray diffraction where the signal originates from a coherent scattering due to an ordered arrangement of atoms, X-ray absorption spectKwcopy probes the local structure and dots not rely on any long-range order effect. It is therefore particularly useful in giving electronic and structural information on processes involving amorphous materials. We used X-ray absorption spectroscopy to characterize the electronic and structural modifications occurring in thermally oxidized MaS3layers after electrmloration. We were particularly interested in studying the effect of the composition of the electrolyte on the structural modiication ocmrring in the M a 3 layer during the electrocoloration process, and we have correlated the evolution of the electrochemical and electrochromic properties of the electrode with such structural changes.
Expedmtntnl Section The molybdenum trioxide thin films were synthesized in two steps. A molybdenum trisulfide thin film was electrodeposited at a potential of 0.6 V vs a saturated calomel reference electrode (SCE) on a tin oxide covered glass electrode from a 10 mM aqueous ammonium tetrathiomolybdate s ~ l u t i o n . ~ The ~*~~ thickness of the molybdenum sulfide film can be controlled by varying the deposition time. This electrodeposited film was then heated in air at temperatures ranging from 400 to 500 OC. Electrochemical characterization of the Moo3 thin films was done in a 1 M LiClO,/propylene carbonate (PC) medium or a 1 M LiCIO,, 0.014 M HClO,, 2.0% (v/v) H20/PC electrolytic medium. A platinum electrode was used as the counter electrode in a one-compartment electrochemical cell with a saturated calomel reference electrode. A PAR Model 273 potentiostat/ galvanostat was used for the cyclic voltammetry experiments. Thermogravimetric analysis (TGA) was performed in air, at a temperature variation rate of 10 'C/min, using a Mettler an-
300 500 700 TEMPERATURE, 'C
Figure 1. Thermogravimetric curve of MoS3 annealed in air at a temperature variation rate of 10 "C min-I.
alyzer. X-ray photoelectron spectra were obtained with a VG, Escalab MKII spectrometer equipped with an hemispherical analyzer and a twin anode (Mg and Al). The Mg anode (Ka X-rays at 1253.8 eV) was used in these experiments. The UVvisible spectra were obtained with a Hewlett-Packard 8452A rapid scan spectrometer equipped with the HP89531A M S - W W/vis operating software. Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)measurements were performed at L U R E + h a y using the synchrotron radiation issuing from the DCI storage ring (1.85 GeV, 250 mA). The X-rays were monochromatized by a Si (331) channel-cut single crystal. The incident beam was collimated by slits, and its intensity was measured with an ionization chamber. Powders and thin foil reference compounds (Moo3, Na2Mo04,MoS2, and Mo) were recorded in the transmission mode, using a second ionization chamber after the sample to measure the transmitted photon flux. The absorption spectra of the thin Moo3 films deposited on Sn02 substrates were obtained by recording the total electron yield of the sample at He atmospheric pressure.29 Spectra for both the oxidized and reduced films were recorded after withdrawing the sample from the electrolyte at the end of an anodic or cathodic potential sweep. On the time scale of our measurement, which is of a few hours, no detectable change in the films was observed. A complete description of the analysis procedure of the EXAFS signal is beyond the scope of this paper and can be found in ref 30 and in references therein. In brief, the analysis procedure involves a background subtraction to isolate the characteristic EXAFS oscillations of the absorption coefficient. A Fourier transformation of the k3-weighted oscillations yields a radial structure function (RSF)curve, which givea the approximate radial position of the coordination shells surrounding the absorbing element. The position of the coordination shell found in the RSF curve differs from the actual one by a phase shift factor. To obtain quantitative structural information, each peak of the RSF curve is sorted out by a window function and back-transformed in the k space to yield the characteristic EXAFS oscillations originating from a particular coordination shell. On the basis of known reference compounds, a fitting procedure is then applied, to yield the type, number, and distance of the neighbors. Na2Mo04and MoS2 were used as reference compounds to extract the backscattering amplitude and phase shift functions of the Mo-O and Mo-Mo pairs of atoms, respectively. In N a 2 M d 4 , each Mo atom is surrounded by four 0 atoms at a distance of 1.97 A.3' MoS2 is a lamellar compound, with an in-plane Mo-Mo separation distance of 3.16 A.3l In this compound, the oscillation amplitude shows a strong polarization-dependent effect, and adequate backscattering amplitude and phase shift functions have been obtained with the electric field vector of the incident radiation parallel to the plane of the MaS2lamellar sheets. A Moo3 powder reference compound and a Mo metallic foil were used to check the adequacy of both the Mo-O and Mo-Mo backscattering amplitude and phase shift functions.
ResulQ and Discussion " h d ~ tOf h MaS, to MOO? c-Ady&. Figure 1 shows the thermogravimetric curve of a molybdenum
7720 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992
Guay et al.
sulfide sample heated in air at a temperature ranging from 135 to 700 O C . A small loss of weight is observed between 150 and 225 O C , which is then followed by an abrupt loss of weight around 400 OC. At about 475 OC, this decrease of weight levels off at about 34.2% of the initial weight of the compound. This process is then followed by a slow increase of weight for temperatures ranging from 475 to 675 O C . The total weight variation is 30.6%. The weight variation expected for a complete transformation of MoS, to MOO, is 25.1%. The discrepancy between the expected and experimental values stems from the fact that the starting material corresponds to the chemical formula MoS,.~and is made of a mixture of MoS3 and elemental In this case, the expected weight variation is 33.7%. The initial loss shown in Figure 1 between 150 and 225 OC can thus be explained by the removal of some elemental sulfur which is incorporated into MoS3 during its electrodeposition. The observed weight variation between 150 and 225 OC does not correspond to the total loss of the 0.8 atom of sulfur, and the remaining elemental sulfur should react above 400 O C , during the oxidation of the sulfide. The shape of the TGA curve between 400 and 700 OC clearly indicates that two different and consecutive chemical processes are occurring during the transformation. A similar two-step mechanism has already been observed in the oxidative thermal conversion of Sb2S3to Sb203and then to Sb2O4.,, The thermogravimetric curve is consistent with a reaction mechanism involving the rapid departure of the sulfur atoms and the formation of a Moo2 compound. It has recently been shown by X-ray diffraction analysis that M a 3 is readily converted to Mo02 when briefly annealed in air at 400 0C.34This process must then be followed by the slower oxidation of Moo2 to with a concomitant increase of weight. At the end of the transformation, the small difference between the actual weight variation and the expected one may be due to small departure from the M o S ~ . ~ stoichiometry and/or to the presence of water molecules in the starting material. In fact, elemental analysis of the starting material reveals that molybdenum and sulfur contribute to 95% of the total weight of the sample. No evidence of sulfur atom has been found by X-ray photoelectron spectroscopy in M a 3 films annealed in air at 400 O C for 30 min, the survey spectrum exhibiting only the characteristic Mo and 0 peaks, with some carbon coming from contamination of the surface. X-rayAbsorption Spectroscopy. Na2Mo04is characterized by a tetrahedral symmetry of the first coordination sphere around the Mo atom, with a Mo-0 separation distance of 1.97 A.31 On the other hand, MOO, crystallizes in an orthorhombic structure with cell dimensions u0 = 3.963 A, bo = 13.855 A, and co = 3.696 g1.35*36 The oxygen atoms are arranged in a distorted octahedron structure around the Mo ion, with the Mo-0 separation distance varying from 1.83 to 2.33 A. Figure 2A depicts the three-dimensional arrangement of these Moo6 octahedra. Molybdenum trioxide is a layered compound, with the oxygens which are not shared by the adjacent octahedra located between the layers. Figure 2B shows the sublattice formed by the Mo atoms within a specific layer of the compound. Each Mo atom has three pairs of nearest Mo neighbors at distances of 3.392,3.696,and 3.963 4, respectively. Curves A and B of Figure 3 depict the Mo K edge XANES spectra of Na2Mo04and MOO, powder reference compounds, respectively. The XANES spectrum of Na2Mo04is similar to a previously published result.,' The resonances appearing at the Mo K edge above the sharp increase in the absorption coefficient are due to transitions of the Mo 1s core electron to unoccupied states of p orbital character, while those occurring below it arise from transitions to empty valence states made of metal p states admixed with some d orbital character. The most distinctive feature of Na2Mo04and MOO3arises from the preedge region, where Na2Mo04shows a well-distinct peak located a few electronvolts below the shoulder observed in MOO,. In the pre-edge region, the intensity of a resonance is determined by the extent of admixing between states of p and d orbital character, which in turn is determined by the symmetry of the Mo environment. Assuming a perfect octahedral symmetry, the
A
Ib a
B
3.392
C
Figure 2. (A) View along the c axis of the three-dimensional arrangement of M a 6 octahedra in MOO,. (B) Representation of the sublattice formed by the molybdenum atoms within a given layer. Representative Mo-Mo distances are indicated. L
1.6
-
1.2
-
.
-
-
.
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-
.
.
.
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.
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-
-
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-
.
-
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-
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Figure 3. Mo K edge X-ray absorption near edge structure spectrum of Na2Mo04(A), MOO, powder reference compound (B), and MoS3 after annealing in air at 550 "C during 30 min.
Mo atom sits at an inversion center of the local environment, and no mixing is allowed between p and d orbital character. In this case, resonance in the pre-edge region arises mainly from quadrupole allowed transition of the Mo 1s core electron to unoccupied states of d orbital character and are very On the other hand, no inversion center is found in a tetrahedral symmetry like that of Na2MO04,and an electric-dipole transition to plike states admixed with some d-like states is allowed.'" This is clearly the case of Na2MO04,where a distinct pre-edge resonance is observed. MOO, falls just between those two limiting cases. Distortion from
Influence of the Electrolytic Medium Composition
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7721
CI
z
? m
LL:
5 4
L L W
n
3
L
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2
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..03
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a .n3
2
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DISTRNCE(R)
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Figure 4. Radial structure function of the k3-weightedEXAFS oscillations (4.0 < k < 14.1 A-I) of the as-deposited MoSl film (0)and after an annealing treatment in air at 550 OC during 30 min (-).
the pure octahedral symmetry removes the inversion center and allows for some admixture of p and d orbitals, although to a less extent than in the case of Na2Mo04,and a shoulder is observed in the pre-edge region at a few electronvolts above the characteristic peak of Na2Mo04. The pre-edge region of the XANES spectrum can therefore be used as a fingerprint of the site symmetry occupied by the Mo atom. Curve C of Figure 3 shows the Mo K edge XANES spectrum of a MoS, film annealed in air at 550 OC during 30 min. This spectrum is different from that of the MoS3 starting material, which does not show any feature in the pre-edge r e g i ~ n . ~In ~?~' fact, it is similar to that of the Moo3 powder reference compound, with the distinctive shoulder in the pre-edge region. Little change, if any, has been observed between the XANES spectra of MoS3 films annealed in air at temperatures ranging from 400 to 550 OC. These facts reveal that, upon annealing, MoS, is transformed to MOO, and that the symmetry of the Mo environment is not modified as the annealing temperature is increased. Figure 4 shows the radial structure function of the as-deposited MoS3 film and that of the film annealed in air at 550 OC for 30 min. The RSF of the as-deposited amorphous MOSS film is identical to the curve reported elsewhere.32 It shows one major peak centered at about 1.95 A, characteristic of the Mo-S separation distance, with no evidence of a second coordination shell at larger distances. The annealing treatment causes a dramatic change in the RSF curve of the amorphous M o S ~film: (i) the first peak of the RSF curve is shifted to shorter distances, from 1.95 to 1.45 A; (ii) the first peak becomes broader and two well-resolved maxima are clearly distinguishable; and (iii) a second coordination shell is appearing at about 3.3 A, which evidences the development of a long-range order in the material. After annealing, the RSF curve of the MoS, sample is quite similar to that of the Mo03 powder reference compound and both materials should have a similar composition and structure. In fact, all the samples annealed in air for 30 min at temperatures ranging from 400 to 550 OC show RSF curves which are quite similar to those of the MOO, powder reference compound. A more quantitative analysis of the RSF curve has been performed to identify the nature of the neighboring atoms and their distances to the Mo absorbing element. This is done by performing an inverse Fourier transformation of each individual peak of the RSF curve and by comparison with known reference compounds. Figure 5 shows the EXAFS signal originating from the first (A) and second (B) peaks of the RSF curve of the MoS, film annealed in air at 550 OC during 30 mins. In this figure, the dotted line stands for the best fitting curve obtained by using Mo-O (A) and Mo-Mo(B) absorber-scatterer pairs of atoms. For the first peak, a two-shell model was used, while a three-shell model was necessary for the second peak. In both cases, the quality of the fit is excellent, and the fitting parameters are listed in Table I.
-.a3
1
1
Figure 5. Inverse Fourier transformation (full line) of the first (A) (window function defined between 0.9 and 2.1 A) and second (B)(window function defined between 2.9 and 4.0 A) peaks in the radial structure function of the thermally treated MoS, sample. The dotted line stands for the best fitting curve obtained by using the M d (A) and Mc-Mo (B) absorberscatterer pairs of atoms and the fitting parameters listed in Table I.
TABLE I: Structural Parameters of the Thermally Annealed MoS3
Electrodes" coord shell
first second
atom 0 0 Mo Mo Mo
Nb 4.3 1.7 2.0 2.0 2.0
R,C A 1.94 2.16 3.41 3.70 3.92
Au2,dA2 0.007 -0.003 0.009 -0.001 0.01 1
"Thermal annealing in air at 550 OC during 30 min. bN: coordination number.
