The Journal of
Physical Chemistry
0 Copyright, 1982, by the American Chemical Society
VOLUME 86, NUMBER 22
OCTOBER 28, 1982
LETTERS Electrochemical and Surface Characteristics of the Photocatalytic Platinum Deposits on TIO, M. Koudelka, J. Sanchez, and J. Augustynskl' Gpartement de Chlmle Minhle, Analytlqw et Appllq&, In Flnal Form: August 1 1 . 1982)
Unlversn6 de Genbve, 121 1 Geneva 4, Switzerland (Received: May 19, 1982;
The polycrystalline TiOz (anatase) samples, partially or completely covered with a platinum deposit formed by two slightly different modifications of a photocatalytic method, have been examined by means of cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). It is shown that the initial stage of the photocatalytic Pt deposition from a solution containing hexachloroplatinate and acetate ions involves strong irreversible adsorption of Pt(1V) species. The limiting steady-state coverage by the adsorbed Pt(1V) species is attained only after several hours and is not significantly affected by the illumination of the TiOz surface. The presence of a product or an intermediate of the decarboxylation of acetate (most likely C02),adsorbed on the surface of Ti02, is suggested to be the reason for the observed blocking of the Pt photodeposition from the PtCb2-/CH3C0; solution. The influence of the adsorption properties of TiOzon its behavior as a photocatalyst is emphasized.
Early studies of the photoinduced deposition of metals on n-type semiconductors (particularly TiOJ considered the possible application of such reactions to a photographic process.' The reduction of metal ions from the solution, under illumination of the semiconductor with the light of a suitable wavelength, has been recognized to proceed through a combination of a photoanodic reaction (such as oxygen evolution from water), due to positive holes in the valence band of the semiconductor, and of a cathodic reaction involving electrons from the conduction band. Recently we undertook a comparative study of Ti02supported noble metal (essentially platinum) catalysts,2 (1) Mdlers, F.; Tolle, H. J.; Memming, R. J.Electrochem. SOC.1974, 121, 1160, and references therein. 0022-3654/82/2086-4277$0 1.2510
prepared by different known methods such as thermal decomposition of a H2PtCI, solution followed by a suitable heat treatment,3 electrodeposition, and light-induced (photocatalytic) deposition. We intended to correlate electrochemical characteristics of these materials with the results of surface analyses by X-ray induced photoelectron spectroscopy (XPS or ESCA). In order to render the comparison meaningful, various types of platinum deposits (2) The irradiation of platinized Ti02 with UV light has been claimed to lead, for example, to the splitting of water into H2and 02:Bulatov, A. V.; Khidekel, M. L. Zzu. Akad. Nauk SSSR, Ser. Khim. 1976, No. 8, 1902. (3) Koudelka, M.; Monnier, A.; Augustynski, J. Paper No 357 presented at the 161st Meeting of the Electrochemical Society, Montreal, May 9-14, 1982; Extended Abstracts p 586.
0 1982 American Chemical Society
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were all formed onto identical TiOz supports consisting of an about 10-pm thick anatase film. We describe here a series of observations regarding (i) the process of photocatalytic deposition of platinum and (ii) some particular properties of Pt-Ti02 surfaces.
Experimental Section The Ti02films were prepared by thermal decomposition of a titanium tetrachloride solution on 99.8% titanium (Kobe Steel Ltd.) substrates. The deposits were carried out either on the cross section (-0.28 cm2)of Ti rods or on the entire surface (-7 cm2) of Ti cylinders. The solution of TiC1, (puriss from Fluka) in a mixture of methanol and ethanol was applied onto the substrates, layer by layer, and decomposed in air a t -400 "C. In order to increase the conductivity of the film, the Ti02 covered samples were finally heated for 1 h in argon at 550 "C. Such TiOz films consisted of about 10-pm a n a t a ~ e .The ~ activation treatment increased the carrier density Nd of TiOz to greater than 1019carriers ~ m -as ~ determined , from capacitance measurements with the Mott-Schottky relationship. Surface analyses were carried out on a Varian IEE-15 photoelectron spectrometer with Mg K L YX-rays ~ , ~ at 1253.6 eV. Binding energies were referred to the C 1s electron peak, due to residual pump oil on the sample surface, taken at 285 eV. Basic pressure in the sample chamber of the spectrometer was lo-' torr. The deconvolution of the multiple signals was performed graphically with Gaussian peak shapes. The relative photoelectron line sensitivities referred to the F 1s signa15p6served as a basis for the calculation of the relative concentrations of different species. The concentrations obtained in this way were reproducible within 20%. Before being introduced to the sample chamber of the spectrometer the platinized TiOz films were carefully washed with twice distilled water and then kept in a desiccator for at least 15 h. All solutions employed were made up with analytical grade chemicals and twice distilled water. For most experiments, a calomel reference electrode in 0.1 M KC1 solution was used. Potentials on this scale are referred to as E (V vs. 0.1 NCE). Argon under 1 atm was continuously bubbled through the solutions except for the duration of a cyclic voltammetry experiment. All measurements were effect,ed at 40 "C. Results and Discussion So that the possible influence of the photoanodic counterreaction (taking place during the process of the photocatalytic Pt deposition) on the properties of the Pt-TiOz catalyst could be evaluated, TiOz samples were platinized by two modifications of the photocatalytic method recently described in the literature.'^^ Thus, the part of the surface of titanium samples covered with the TiOz film was exposed to (i) a 1.1 X lo-, M K2PtC14solution or (ii) a solution prepared according t o ref 7, using 0.1 M H2PtC1, + 0.1 M HC1 neutralized with Na2C03and then brought to pH -4 by addition of CH3COOH,and was illuminated for 3.5 h with the full output of a 350-W high-pressure mercury lamp. XPS analyses of the samples platinized in the K2PtC14 solution revealed the presence of a metallic Pt deposit having the thickness of at least several tens of angstroms (4)Stalder, C.;Augustynski, J. J.Electrochem. SOC.1979,126,2007. (5)Wagner, C. D. Anal. Chem. 1972,44,1050. (6)Berthou, H.; Jerrgensen, C. K. Anal. Chem. 1975,47,487. (7)Kraeutler, B.;Bard, A. J. J. Am. Chem. SOC.1978,100, 4317. ( 8 ) Lehn, J.-M.; Sauvage, J.-P.; Ziessel, R. N o w . J.Chim. 1980,4,623.
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Figure 1. The platinum 4f electron spectra for the TiO, film irradiated for 3.5 h in a PtC1,2-/CH,C02- solution before (curve a) and after a sequence of cyclic potential sweeps in 0.5 M KCI shown in Figure 2a (curve b).
as indicated by the absence of the Ti 2p photoelectron signal from the spectra. The position of the corresponding Pt 4f'I2 electron peak, at 71.4 eV, was slightly higher than the binding energy (BE) values of metallic platinum reported in the literatureBand was close to the Pt 4 f'12 BE equal to 71.3 eV we have obtained for a galvanic Pt deposit on Ti02 Rather unexpectedly, the deposits formed onto TiO, in the PtC&2-/CH3C0, solution under the conditions described above did not contain perceptible quantities of metallic platinum. As a matter of fact, photoelectron spectra of the Pt 4f region, recorded for the latter samples, consisted typically of a broad signal extending between -72 and -80 eV. The deconvolution of such a signal is shown in Figure la; two Pt 4F12peaks, at 73.0 and at 75.2 eV, are obtained. The comparison with the previously reported binding energy values for various platinum compounds'O indicates clearly that the above-mentioned peaks have to be assigned, in order of increasing BE, to divalent, Pt(II), and tetravalent, Pt(IV), species. In particular, the latter binding energies are in a reasonable agreement with the values measured by Katrib" for K2PtC14,73.2 eV, and for K2PtC1,, 75.7 eV (the values corresponding to the Pt 4FI2 lines). If the results of XPS analyses do not allow a direct identification of the nature of Pt(I1) and Pt(1V) species adsorbed on or incorporated into the surface of TiOz samples, an indication in this sense is obtained from the presence of chlorine species.l2 The amount of chlorides accompanying the Pt species was much less than it would be expected in the case of adsorption of PtCh2- or (9) See, for example: (a) Kim, K. S.; Winograd, N.; Davis, R. E. J.Am. Chem. SOC.1971,93,6296. (b) Allen, G.C.; Tucker, P. M.; Capon, A.; Parsons, R. J. Electroanal. Chem. 1974,50, 335. (c) Norton, P.R. J. Catal. 1975,36,211. (10)(a) Riggs, W. M. In "Electron Spectroscopy"; Shirley, D. A., Ed.; North Holland: Amsterdam, 1972;p 713. (b) Nefedov, V. I.; Sacharova, I. A.; Porai-Koshiz, M. A. Dokl. Akad. Nauk SSSR 1972,205,119. (11)Katrib, A. J.Electron Spectrosc. Relat. Phenom. 1980,18,275. (12)The measured C1 2p3/*binding energy, 198.8 eV, is close to the valueslOJ1reported for C1 species in KzPtCll and K2PtC&.
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Flgure 2. Cyclic voltammograms at 50 mV.s-' for a TO, film electrode covered with a piatinum deposit formed photocatalyticaiiy in 1.1 x 1o4 M K m I , solution: curves a and b obtained, respectively, in deaerated 0.5 M KCI and 0.5 M KHC03.
PtC142-ions and corresponded to a Cl/Pt ratio < 1. This suggests that PtCs2-ions from the solution undergo partial or complete hydrolysis, probably after being adsorbed on the Ti02 surface. Consequently, the deposit obtained under illumination from the PtC162-/CH3C02-solution consisted, in fact, of platinum oxychloride or/and oxide species. The amount of Pt(I1) and Pt(1V) detected on the Ti02 surface depended on the duration of the experiment and ranged from about 8 at. % l3 for a 1.5-h deposit to -25 at. % for the 3.5-h deposit. A similar quantity (i.e., -25 at. %) of platinum species of the same nature was observed following a simple 15-h exposure at -60 OC (in the absence of illumination) of the TiOz surface to the PtC&2-/CH3C02solution. This indicates clearly that the eventual photocatalytic Pt deposition is preceded by a strong adsorption of Pt(1V) species on the TiOz surface. The above-described results might appear at variance with the original report of Kraeutler and Bard' who have found the deposit obtained from the PtC1,2-/CH3C02solution to consist exclusively of metallic platinum. In reality, the observed difference is most likely due to the fact that the latter authors used as a substrate fine Ti02 (anatase) powder and subjected it to very strong illumination (originated from a Hg-Xe lamp operated at 1600 W). An important consequence of using the powder substrate is that, during the Pt photodeposition, the entire surface of TiOz particles is never illuminated simultaneously. Therefore, one can expect a different distribution of the photoanodic and cathodic sites on TiOz particles than in the case of a uniformly illuminated TiOz film. Additional information about the possible reasons for blocking the Pt photodeposition from the solution containing acetate ions was obtained from cyclic voltammetry experiments. For comparison purposes in Figure 2 are (13) Relative concentrations given with respect to Ti(1V) (from TiOJ
taken
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100 atom %.
Flgure 3. Cyclic voltammetry i vs. E profiles at 50 mV.s-' in Ar-saturated 0.5 M KCI solution for a TiO, film electrode after being illuminated for 3.5 h in a PtC1,2'/CH3C02- solution (curves a) and for the same electrode after the cathodic reduction of adsorbed Pt(I1) and R(1V) species (curves b).
shown i vs. E profiles recorded for TiOz electrodes platinized in the K2PtCl, solution. The behavior of this type of electrode appears very similar to that of bulk platinum, particularly with regard to hydrogen adsorption/desorption. Thus, when chloride ions are present in the solution, the deposition of hydrogen atoms is partially blocked by the competitive adsorption of the C1- anion14as shown by a significant decrease of the charge under the i vs. E curve (Figure 2a) in comparison with a similar curve recorded in KHC03 solution (Figure 2b). Cyclic voltammograms analogous to those in Figure 2, obtained for a Ti02 electrode which has been previously illuminated for 3.5 h in the PtCb2-/CH3C02-solution and then carefully washed with twice distilled water and transferred to a 0.5 M KC1 solution, are shown in Figure 3a. Repetitive cycling of the potential of this electrode between 0 V (vs. 0.1 NCE) and various cathodic limits results in a rather complex picture characterized by an important reduction/oxidation charge, appearing already in the potential region preceding the hydrogen formation. XPS analyses of several electrodes of this type, after being submitted to such a cyclic voltammetry experiment (5-10 cycles), revealed the presence of metallic platinum (the Pt 4f'I2 signal at 71.1 eV, Figure 1b) and disappearance of the chloride species from the surface. This indicates that the adsorbed Pt(1V) and Pt(I1) species undergo, as expected, a complete cathodic reduction. However, as shown in Figure 3b, the electrochemical behavior of the "reduced" Pt deposit does not undergo substantial changes except for a decrease of the current densities. The similarity of shapes of the i vs. E curves in parts a and b of Figure 3 suggests that some other reduction reaction takes place in the region of potentials in which reduction of the Pt(IV) and Pt(I1) species is occurring. This additional reaction is very likely to involve a product or an intermediate of (14) Angerstein-Kozlowska, H.; Conway, B. E.; Barnett, B.; Mozota,
J. J. Electroanal. Chem. 1979, 100, 417.
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the photoinduced decarboxylation of acetate. As a matter of fact, TiOz powders (both platinized and without platinum) has been to induce the photodecompostion of CH3C02-ions into carbon dioxide, methane, and small amounts of ethane and hydrogen. The decarboxylation of acetate plays also the role of anodic counterreaction in the process of photoinduced Pt deposition on TiOz powder suspended in the PtC&2-/CH3COz-s ~ l u t i o n .Absorption ~ of a photon of energy greater than the band gap of TiOz (-3.2 eV far anatase) leads to the excitation of an electron from the valence band into the conduction band and hence to the creation of the electron-hole pair. Positive holes, h+, in the valence band or trapped on the surface of the semiconductor, will tend to react with acetate ions: CHzC02- + h+ CH,. + C02 (1) +
2CH3. C2H6 (2) while the most easily reducible gpecies from the solution (e.g., Pt(1V) and Pt(I1)) are expected to act as acceptors for electrons in the conduction band. However, the above reaction scheme is likely to be modified due to the appearance of C02 on the surface of TiOz. In fact, carbon dioxide has recently been shown to adsorb irreversibly on Ti02and to undergo reduction at potentials significantly -+
(15) Kraeqtler, B.; Bard, A. J. J . Am. Chem. SOC.1978, 100, 5985.
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more positive than the flat-band potential of the semic ~ n d u c t o r The . ~ products of the latter reaction, adsorbed on the surface of Ti02,can be readily oxidized, restoring C02(ads).Thus, under illumination with light of moderate intensity, carbon dioxide adsorbed on Ti02 can act as recombination centers for positive holes and electrons, leading to the slowing down or even to the suppression of other photoinduced reactions. This is supported by the observation that the rate of the photocatalytic decarboxylation of acetic acid, in the presence of a suspension of platinized Ti02,decreases drastically with a decrease of light intensity.15 Increase of the rate of decarboxylation for high light intensities, allowing the deposition of platinum from the PtC&2-/CH3C0z-solution, is caused most likely by the photodesorption of COz from the surface of Ti02. The fact that the photoinduced Pt deposition from the K2PtC14solution occurs under much less drastical conditions of illumination is to be ascribed to the irreversibility of the corresponding photoanodic counterreaction (oxygen evolution from water). As shown by the results of surface analyses by XPS, the platinum photodeposition is preceded by a strong adsorption of Pt(I1) and Pt(1V) species on the surface of Ti02.
Acknowledgment. The support of this research by the Swiss National Science Foundation is gratefully acknowledged.
