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Notes Electrodeposition of Hedgehog-Shaped Gold Crystallites on TiO2 Surface and Their Behavior in Anodic Oxidation of Oxalic Acid Jerzy Haber,* Paweł Nowak, and Paweł Z˙ urek Polish Academy of Sciences, Institute of Catalysis and Surface Chemistry, ul. Niezapominajek 8, 30239 Krako´ w, Poland Received September 17, 2002. In Final Form: November 1, 2002
Introduction Deposition of noble metals on TiO2/Ti substrates has been extensively studied in relation to dimensionally stable anodes; it, however, concerned rather the platinum group metals (see ref 1 and references therein). The interest in the deposition of gold on titania exploded after the discovery that fine particles of gold exhibit surprisingly high catalytic activity for CO oxidation even at temperatures as low as 200 K (see the review of Haruta and Date´2 on catalysis on Au nanoparticles). For catalytic purposes, gold is deposited on titania usually by the deposition-precipitation method;3,4 for the surface-science investigations gold has been deposited on rutile monocrystals by Au evaporation at the high-vacuum conditions.5-8 Formation of two-dimensional and then threedimensional (at higher coverages) islands at the (110) rutile surface was observed in these studies. Gold nanoparticles deposited on the surface of titania showed catalytic activity in the photocatalytic decomposition of organic contaminations in water.9 For that purpose gold was photodeposited on titania in situ. In the present paper the electrodeposition of hedgehog-shaped gold crystallites of highly developed surface area at the TiO2 surface, prepared directly on the surface of Ti foil, is described. The possibility of using such obtained gold electrodes in electrocatalysis was tested in the reaction of oxalic acid anodic oxidation.
Figure 1. First cathodic-direction voltammetric half-cycle at the sweep rate of 10 mV s-1 for the TiO2/Ti (dashed line) and vanadium doped TiO2/Ti electrode (full line) in the solution containing 0.1 mol dm-3 HAuCl4 and 0.5 mol dm-3 H2SO4 as well as for a vanadium doped TiO2/Ti electrode in 0.5 mol dm-3 H2SO4 (dotted line). The current for the TiO2/Ti electrode (undoped) in 0.5 mol dm-3 H2SO4 was very low (the curve was practically indistinguishable from the potential axis).
Experimental Section In view of the possible practical application, commercial Ti foil, used in industry for the construction of dimensionally stable anodes, was used as the substrate. Rectangular electrodes of dimensions 1.5 × 2 cm2 were cut from the 1 mm thick Ti foil and oxidized in air at the temperature 823 K during 48 h. The obtained TiO2 layers had the thickness of about 1 µm and were composed * Corresponding author. Fax: (+4812)425 19 23. E-mail:
[email protected]. (1) Mahe´, E.; Devilliers, D. Electrochim. Acta 2000, 46, 629. (2) Haruta, M.; Date´, M. Appl. Catal. A 2001, 222, 427. (3) Boccuzzi, F.; Chiorino, A.; Manzoli, M. Surf. Sci. 2000, 454-456, 942. (4) Sakurai, H.; Ueda, A.; Kobayashi, T.; Haruta, M. Chem. Commun. 1997, 3, 271. (5) Zhang, L.; Cosandey, F.; Persaud, R.; Madey, Th. E. Surf. Sci. 1999, 439, 73. (6) Parker, S. C.; Grant, A. W.; Bondzie, V. A.; Campbell, C. T. Surf. Sci. 1999, 441, 10. (7) Kolmakov, A.; Goodman, D. W. Surf. Sci. 2001, 490, L597. (8) Spiridis, N.; Haber, J.; Korecki, J. Vacuum 2001, 63, 99. (9) Dobosz, A.; Sobczyn´ski, A. Monatsh. Chem. 2001, 132, 1037.
Figure 2. Energetic situation at the TiO2 (rutile)/aqueous solutions of Cu2+ and Au3+ ions (pH ) 0) interface. ΦCu and ΦAu are the Fermi energies of the respective metals. E°cs and E°vs are the conduction band edge and valence band edge of rutile. mainly of a rutile crystallographic modification of TiO2. More detailed information on the properties of the Ti foil used as well as the procedure of preparation and the properties of the oxide layer will be published elsewhere.10 In part of the experiments, TiO2 layers were doped with vanadium.10 The doping was performed in the following way. The surface of the TiO2/Ti electrode was covered with the paste obtained by wetting V2O5 with water, heated in the stream of air to the temperature 1023 K, kept at that temperature for 2 min, and removed quickly from the oven. Vanadium pentoxide melted at that temperature, (10) Haber, J.; Nowak, P.; Z˙ urek, P. In preparation.
10.1021/la020793y CCC: $25.00 © 2003 American Chemical Society Published on Web 12/11/2002
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Figure 3. Representative SEM pictures of the gold crystallites deposited at the TiO2/Ti and vanadium doped TiO2/Ti electrodes from the solution containing 0.1 mol dm-3 HAuCl4 and 0.5 mol dm-3 H2SO4 at galvanostatic conditions (I ) 50 µA cm-2). The nominal surface coverage by gold ) 80 monolayers. covering the surface of the electrode with the thin compact layer. The bulk V2O5 was removed from the surface by dissolving in 0.5 mol dm-3 H2SO4, so that only strongly bound V ions incorporated into the TiO2 surface layer remained. The amount of vanadium at the electrode surface corresponded to the surface coverage of approximately four monolayers per geometric area. Note, however, that in the case of a polycrystalline surface, which must have the surface roughness factor much higher than 1, the real surface coverage was probably well below one monolayer. Gold was deposited at the TiO2/Ti or vanadium doped TiO2/Ti electrodes galvanostatically at the current density 50 µA cm-2, from the solution containing 0.1 mol dm-3 HAuCl4 and 0.5 mol dm-3 H2SO4. All reagents used in the experiments were of analytical reagent grade purity. A typical electrochemical cell in the three-electrode configuration with investigated titanium foil as a working electrode, a mercury-mercurous sulfate/0.5 mol dm-3 sulfuric acid electrode as reference electrode, and a piece of Pt foil as a counter electrode was used in electrochemical experiments. All potentials in this paper are quoted versus a standard hydrogen electrode, assuming that the mercurymercurous sulfate electrode has the potential +0.682 V. All measurements were performed in the dark, at the temperature 25 °C, using Electrochemical Interface 1286 (a digital potentiostat) produced by Schlumberger-Solartron. 0.5 mol dm-3 H2SO4 was used as a base electrolyte. Water purified by catalytic pyrodistillation11 was used to prepare the solutions. Before experiments, the solutions were bubbled with argon to remove dissolved oxygen.
Results and Discussion Figure 1 presents the first voltammetric half-cycle of the TiO2/Ti and vanadium doped TiO2/Ti electrodes in 0.1 mol dm-3 HAuCl4 solution and in pure base electrolyte. It is to be seen that the current of gold electrodeposition is at least an order of magnitude higher in the case of the vanadium doped electrode in comparison with the undoped one. In our earlier works it was showed that doping of both rutile monocrystal12-15 and polycrystalline TiO2 electrodes10 by vanadium creates surface states in the band gap of the rutile semiconductor at the energies 0.43 and 0.74 eV below the conduction band edge of rutile in aqueous solution. These surface states may mediate the charge exchange between the rutile surface and redox couples in the solution, facilitating the occurrence of electrochemical reactions. Some metals may easily be deposited at the surface of TiO2 by electrolysis. For (11) Conway, B. E.; Angerstein-Kozlowska, H.; Sharp, B. E.; Criddle, E. E. Anal. Chem. 1973, 45, 1331. (12) Haber, J.; Nowak, P. Catal. Lett. 1994, 27, 369. (13) Haber, J.; Nowak, P. Langmuir 1995, 11, 1024. (14) Haber, J.; Nowak, P. Top. Catal. 1999, 8, 199. (15) Haber, J.; Nowak, P. Top. Catal. 2002, 20, 75.
example, copper may be electrodeposited at the TiO2/Ti cathode with very low overpotential.16 However, the energetic situation at the interface rutile/aqueous solution of Cu2+ ions is completely different from the situation at the interface rutile/aqueous solution of Au3+ ions. The standard potential of the redox couple Cu0/Cu2+ is situated just below the conduction band edge of rutile, and the cathodic reaction of copper electrodeposition may easily occur directly via the conduction band. In the case of gold, the difference between the conduction band edge of rutile and the standard potential of the Au0/Au3+ redox couple is 1.52 V. The redox potential calculated for the couple Au0/Au3+, taking into account the conditions of the experiment and the stability constant of the AuCl4complex, is +1.41 V, and on the electronic energy scale, the energy state of this redox couple is situated exactly in the middle of the energy band gap of semiconducting TiO2, where no occupied electron energy states are available. So, much more probable is the charge transfer through surface states, especially that one of the surface states created by vanadium at the TiO2/aqueous solution interface and situated just in the middle between the conduction band edge energy of TiO2 and the energy state of the Au0/Au3+ redox couple (see Figure 2). Undoped TiO2 is always an n-type semiconductor with the flat-band potential located close to conduction band edge, and at the potentials anodic to the flat-band potential, the depletion layer is formed at the surface with no electrons available for the cathodic reaction. So, the low current of Au deposition on an undoped TiO2 electrode may be unequivocally ascribed to the unfavorable matching of the redox potentials of the Au0/Au3+ redox couple and the conduction band of TiO2. Not only was the current at potentiostatic conditions lower in the case of the undoped TiO2/Ti electrode but also the gold crystallites at that electrode were much less uniformly and less densely distributed in comparison to the case of electrode doped with vanadium (see Figure 3). Evidently the vanadium doped TiO2 surface offers better conditions for the nucleation of gold crystallites, leading to a higher number of smaller crystallites. The crystallites of gold had a strongly developed surface, with the shape of crystallites resembled a hedgehog (see Figure 4). The shape of crystallites was similar in the case of both doped and undoped electrodes, but the crystallites on the vanadium doped surface were much more uniform in shape and in dimensions. (16) Delplancke, J.-L.; Sun, M.; O’Keffe, J.; Winand, R. Hydrometallurgy 1989, 23, 47.
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Figure 4. Gold crystals deposited at the surface of a vanadium doped TiO2/Ti electrode from the solution containing 0.1 mol dm-3 HAuCl4 and 0.5 mol dm-3 H2SO4 at galvanostatic conditions (I ) 50 µA cm-2).
Figure 5. Cyclic voltammogram in 0.5 mol dm-3 H2SO4 of the TiO2/Ti electrode doped with vanadium and covered by gold crystallites. The nominal surface coverage by gold ) 80 monolayers. Potential sweep rate 10 mV s-1.
Figure 5 shows the cyclic voltammogram of the TiO2/Ti (doped with vanadium), covered with gold crystallites, in 0.5 mol dm-3 H2SO4. Both the redox transformations of surface vanadium as well as formation and reduction of the gold oxide are well visible. Comparison of the charge of gold oxide formation/reduction (assuming that the gold oxidation is restricted to a monolayer coverage) with the charge consumed in the deposition of gold at the surface leads to the conclusion that approximately one gold atom of 90 deposited is exposed at the surface. Dividing this number by the proportion of the surface atoms to the total number of atoms in a gold sphere of the same radius (about 2 µm), one may calculate the surface roughness factor of the crystallites to be about 150. In the above calculations the stoichiometry of the layer of hydrated gold oxide monolayer formed at the gold surface had to be assumed. However, the formation of the monolayer oxide species at the gold surface and the bulk oxidation of gold are not well resolved from each other and the number of electrons per one surface gold atom depends on such factors as potential sweep rate, the composition of electrolyte, and the anodic potential limit. To avoid that problem, the number of electrons per one surface gold atom in the reaction of gold oxidation was established by experimentally tracing the cyclic voltammograms on a bright gold
Figure 6. Cyclic voltammograms in 0.5 mol dm-3 H2SO4 (dashed line) and in 0.5 mol dm-3 H2SO4 plus 0.01 mol dm-3 oxalic acid (full line) at the potential sweep rate 10 mV s-1 for (a) a massive gold electrode, (b) gold deposited on an undoped TiO2/Ti electrode, and (c) gold deposited on a TiO2/Ti electrode doped with vanadium. The nominal surface coverage by gold ) 80 monolayers.
electrode at identical conditions to those in the case of gold deposited on TiO2 (surface roughness factor of gold was assumed 1 in that case). Figure 6 presents the cyclic voltammograms for a massive gold electrode, gold deposited on an undoped TiO2/ Ti electrode, and gold deposited on a TiO2/Ti electrode doped with vanadium, in the solution of base electrolyte and in a solution containing oxalic acid. Oxalic acid is oxidized anodically according to the reaction
H2C2O4 f 2CO2 + 2H+ + 2e-
Notes
In the case of both doped and undoped TiO2/Ti electrodes, the same charge was passed during the galvanostatic deposition of gold, so the amount of electrodeposited gold should be in both cases similar (although the overpotential in the case of the undoped TiO2/Ti electrode was much higher and part of the charge was lost due to hydrogen evolution). It is to be seen that not only is the current of the oxalic acid oxidation in the case of the doped electrode much higher, which may be partially caused by the fact that more gold was deposited on the surface of the vanadium-doped TiO2/Ti electrode and that gold deposited on this electrode had a much better developed surface, but also the shapes of the voltammograms are different. In both the case of the massive gold electrode and gold deposited on an undoped TiO2/Ti electrode, the current drops down after the attainment of some potential, due to (most probably) the poisoning of the electrode surface by the adsorbed intermediate or adsorbed product of the electrode reaction. No such effect was observed in the case of gold deposited on a vanadium doped TiO2/Ti electrode. The probable explanation is the spillover of some oxygen species from the surface of vanadium-doped TiO2/Ti to the gold surface. Such a mechanism was invoked by Vayenas et al.17 to explain the phenomenon of the electrochemical activation of catalytic reactions. It was (17) Vayenas, C. G.; Jaksic, M. M.; Bebelis, S. I.; Neophytides, S. G. In Modern Aspects of Electrochemistry; Bockris, J. O’M., Conway, B. E., White, R. E., Eds.; Plenum Press: New York, 1996; Vol. 29, p 57.
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checked that the vanadium doped TiO2 electrode, reduced galvanostatically at conditions similar to those of the gold deposition (but with no HAuCl4 in solution), shows no activity in the reaction of oxalic acid oxidation. It was also checked that gold deposited on the bright gold surface behaves in a similar manner to that of gold deposited on the bright gold electrode. Conclusions Gold may be deposited electrochemically at the surface of a TiO2 covered Ti electrode, forming at the surface hedgehog-shaped crystallites of well developed surface area. Doping of the TiO2 layer by vanadium facilitates the gold electrodeposition, resulting in more uniform and denser distribution of crystallities. At the same time, gold electrodeposition on vanadium doped TiO2 occurs at lower overpotential in comparison with that of the undoped TiO2 layer. In the reaction of the anodic oxidation of oxalic acid, gold deposited at a vanadium doped TiO2 surface behaves in a different manner in comparison with the case of either a massive gold electrode or gold deposited at the undoped TiO2 electrode. In the latter case, the poisoning of the electrode surface by the products and/or intermediates of the electrode reaction occurs, whereas no such poisoning is observed in the case of gold deposited at the vanadium doped TiO2 electrode. LA020793Y
