Structure and Electrochemical Properties of Species Formed as a

The study of the interaction of transition metal ions with the titanium dioxide surface as well as of the properties of systems obtained as a result o...
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J. Phys. Chem. B 1999, 103, 1308-1315

Structure and Electrochemical Properties of Species Formed as a Result of Cu(II) Ion Adsorption onto TiO2 Nanoparticles S. K. Poznyak,*,† V. I. Pergushov,‡ A. I. Kokorin,§ A. I. Kulak,| and C. W. Schla1 pfer⊥ Institute of Physico-Chemical Problems, Belarusian State UniVersity, Leningradskaya St. 14, Minsk 220080, Belarus, Department of Chemistry, Moscow State UniVersity, Moscow 119899, Russia, Institute of Chemical Physics, Russian Academy of Sciences, Kosygin St. 4, Moscow 117334, Russia, Institute of General and Inorganic Chemistry, Belarusian Academy of Sciences, SurganoVa St. 9, Minsk 220072, Belarus, and Institute of Inorganic Chemistry, UniVersity of Fribourg, Fribourg CH-1700, Switzerland ReceiVed: October 13, 1998; In Final Form: December 14, 1998

Sol-gel-derived nanostructured TiO2 thin films and powders treated with Cu2+-containing solutions have been studied using electrochemical methods, ESR, XPS, and electroreflection spectroscopies. Analysis of the voltammograms in combination with ESR data has allowed us to reveal at least four types of copper species formed on the TiO2 surface after the adsorption of aqueous Cu2+ ions. These are monovalent copper ions, magnetically isolated Cu(II) ions, Cu(II) ions forming specific areas (“associates”) with a high local concentration and strong interaction between the ions, and formally diamagnetic copper hydroxide species. The last type dominates when adsorbing Cu(II) ions from solutions with pH > 5 and its fraction increase with increasing the Cu2+ ion concentration in solution. The presence of Cu(I) ions at the TiO2 surface was proved independently by XPS measurements. The appearance of them can be associated with the partial reduction of adsorbed Cu2+ ions by electrons of the TiO2 matrix. The Cu(II) ions bound to the TiO2 surface give rise to the electroactive surface states within the band gap of the oxide. Their energy position has been determined by the electrolyte electroreflectance method. These surface states are located ca. 1.1 eV below the conduction band edge.

Introduction The study of the interaction of transition metal ions with the titanium dioxide surface as well as of the properties of systems obtained as a result of this interaction is of interest from several viewpoints. First, high chemical stability and high ion-exchange capacity of hydrous titanium dioxide allow us to consider this material as a promising inorganic ion exchanger and sorbent.1 Second, titanium dioxide modified with ions and small particles of various metals can be used as an efficient catalyst and photocatalyst for a number of different reactions.2 There are many studies devoted to the study of TiO2 systems involving ions, complexes, and small particles of noble metals such as platinum, palladium, and ruthenium.3 Considerably less attention has been given to the processes of the interaction of TiO2 matrix with ions of common transition metals such as copper. The Cu(II) ion is an important representative for the cation adsorption in the natural environment. The behavior of Cu2+ ions at the TiO2-solution interface has been inferred mainly from the adsorption isotherms and from surface charge and potential determinations.4-6 It has been established that the Cu(II) ions are specifically adsorbed on the TiO2 surface by a true ion-exchange mechanism, involving a direct first-sphere complexation of the ions with the surface groups of TiO2.4,6 Important information on the structure of surface copper * Corresponding author. E-mail: [email protected]. † Belarusian State University. ‡ Moscow State University. § Russian Academy of Sciences. | Belarusian Academy of Sciences. ⊥ University of Fribourg.

complexes, including some evidence of inhomogeneous distribution of Cu2+ ions over the TiO2 surface, has been obtained from the ESR measurements.5,6 Data on the electronic interaction between Cu(II) ions and TiO2 support are few in number and scattered. It has been noted that the titania-supported copper catalysts exhibit hydrogen sorption at enhanced temperatures in excess of that required for complete reduction of Cu(II) compared to the silicasupported copper materials.7 This effect was assigned to the anomalous metal-support interactions in the Cu-TiO2 system. Chen et al. have shown that copper supported on TiO2 has a CO hydrogenation activity less than that of bulk copper because of the electronic charge redistribution from the support to Cu.8 The cupric ions have been revealed to increase markedly the photocatalytic activity of TiO2 slurry systems.9 This increase is observed at low Cu(II) concentrations (10-6-10-5 M) in solution, and high metal ion concentrations have a detrimental effect. In the previous paper, we have described the marked effect of adsorbed Cu2+ ions on the photoelectrochemical behavior of TiO2 electrodes that manifests itself as a significant decrease in the quantum efficiency of photoelectrochemical processes in the short-wavelength region of the action spectra.10 This effect was associated with efficient capturing of photoelectrons, generated by light in TiO2, by adsorbed cupric ions.10-12 In the present paper, electrochemical methods in combination with electroreflection, ESR, and XPS spectroscopies are applied as a tool for studying the adsorption processes at nanostructured TiO2 contacting aqueous Cu(II) ion solutions, the state of the adsorbed copper species, and the electronic interactions in the adsorbed copper-TiO2 systems.

10.1021/jp9840580 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/06/1999

Cu(II) Adsorption onto TiO2

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Experimental Section Electrode Preparation. Nanocrystalline microporous TiO2 film electrodes were produced by spin-coating chemically polished titanium plates by a colloidal TiO2 solution. Titanium dioxide sol was prepared as described in detail in our previous paper.10 Briefly, the hydrous titanium dioxide precipitate was obtained by titration of TiCl4 in aqueous HCl solution with NH4OH solution at 0 °C. Then the precipitate was washed and ultrasonically treated to obtain an opalescent stable TiO2 sol with a concentration of ca. 80 g/L. The procedure of TiO2 sol deposition onto titanium plates followed by heating at 160 °C for 20 min was repeated until the desired number of coatings was deposited. After the final coating, a sintering step was performed at 200 °C for 1 h in air. A minimum temperature was chosen for final thermal treatment after which the TiO2 films were no longer redissolved in water but its high adsorption capability is retained. The TiO2 films obtained consist of anatase crystallites as determined by X-ray diffraction method. The average diameter of the crystallites estimated from the half-widths of the diffraction peaks was 4-5 nm. The results of TEM measurements support the validity of these data. The specific surface areas of the sol-gel-derived TiO2 films and xerogel powders were ascertained by lowtemperature nitrogen adsorption using the BET method. Its value is about 240 m2/g for samples heated at 200 °C. Preparation of Copper-Modified TiO2 Samples. Adsorption of Cu2+ ions onto TiO2 nanocrystalline electrodes was carried out by immersing the electrodes into Cu(ClO4)2 solutions at a constant ionic strength (0.1 M) that was fixed with NaClO4. The pH of solution was adjusted to the desired value with small amounts of HClO4 or NaOH and corrected periodically in the course of the adsorption. After 24 h of equilibration at a temperature of 20 ( 1 °C, the electrodes were removed from solution and then immersed in distilled water for 10 min to remove unbound Cu2+ ions. For ESR measurements, TiO2 powders, obtained by drying TiO2 sol with the following heat treatment of xerogel at 200 °C for 1 h, were used. The powder was washed repeatedly with doubly distilled water to remove residual amounts of acid used for stabilization of the TiO2 sol and then dried at room temperature. The treatment of the powder samples with Cu2+containing solutions was carried out under the same conditions as the electrodes. The suspension was occasionally shaken to attain equilibrium. Then the powder was separated from solution by centrifugation and washed twice with doubly distilled water. Apparatus and Chemicals. Electrochemical measurements were performed in a two-compartment cell equipped with an optical quality window. The working electrode compartment of the cell was separated from the counter electrode compartment by a fine porous glass partition. A conventional potentiostat with programmer controlled a standard three-electrode cell with a platinum counter electrode and a saturated Ag|AgCl|KCl(sat.) electrode as the reference electrode (+0.201 V vs SHE). All potentials are reported vs Ag|AgCl electrode. The measurements of the optical reflectivity of the working electrode and of the electroreflectance signal were carried out in situ using a stabilized 450 W tungsten-halogen lamp with a high-intensity grating monochromator. The light incidence on the electrode surface was close to normal. The reflected light was directed to a photomultiplier, the output of which was measured as a voltage drop over a 100 kΩ high-precision resistor by a sensitive high-resistance voltmeter and by a lockin amplifier in order to obtain the constant and the modulated reflectance signals, respectively. The reflectance modulation was

Figure 1. Cyclic voltammograms for untreated (-) and coppermodified (- - -) nanostructured TiO2 electrodes. The copper(II) adsorption was carried out in 0.01 M Cu2+ solution (pH ) 4.3). Electrolyte was 0.1 M Ar-saturated acetic buffer solution with pH ) 4.4. Potential sweep rate was 5 mV s-1.

accomplished with a low-frequency (21 Hz) variable amplitude sinusoidal ac voltage from a Robotron signal generator connected to the potentiostat. The XPS measurements were performed in an X-ray photoelectron spectrometer ES-2401 using the Mg KR X-ray beam (1253.6 eV). The operation pressure in the spectrometer was ca. 10-10 Torr. The binding energy scale was calibrated with the C 1s line at 284.6 eV. The ESR spectra were measured with Varian E-4 and Varian E-3 X-band spectrometers in quartz tubes 4 mm in diameter at 298 and 77 K. The magnetic field was graduated by the spectra of Mn2+ in the MgO matrix and by the DPPH signal with g0 ) 2.0036. The spin Hamiltonian parameters g|, g⊥, and A| have been calculated from the ESR spectra according to recommendations of Zhidomirov et al.13 The amount of the adsorbed Cu2+ ions has been estimated by double integration of the EPR spectrum of the sample and its comparison with that for the reference (single crystal of CuSO4‚ 5H2O with a known number of spins). Analytical-grade reagents and doubly distilled water were used in the electrolyte preparation. Most of the electrochemical measurements were carried out in 0.1 M acetic buffer solutions that were deoxygenated thoroughly with purified argon. The low concentrations of Cu(II) in solutions were determined by the AES method using an atomic emission spectrometer Spectroflame Modula with an inductively coupled plasma excitation source. Results and Discussion Electrochemical Behavior of Untreated Nanocrystalline TiO2 Electrodes. A typical cyclic voltammogram for untreated TiO2 film electrodes in Ar-saturated acetic buffer solution with pH ) 4.4 is presented in Figure 1. A sharp cathodic current peak is observed at potentials slightly positive of the expected flat band potential (Efb ) -0.59 V vs Ag|AgCl) calculated using the relationship Efb ) -0.13 - (0.059)pH (V vs SHE) found for TiO2 anatase colloids.14 This peak grows and is shifted in the negative direction with increasing pH of the solution. The value of ∂Ep/∂(pH) is ca. 60 mV and coincides well with the ∂Efb/∂(pH). Similar cathodic peaks with the coupled anodic

1310 J. Phys. Chem. B, Vol. 103, No. 8, 1999 counterpeaks have been observed previously for single-crystal and polycrystalline TiO2 electrodes at potentials near the Efb.15 These peaks have not been shown to be associated with any added redox pair and, therefore, inherent in the TiO2-aqueous electrolyte interface. The appearance of them has been assigned to the surface states located below the conduction band edge.15b-d In addition, it has been established that the peak of the irreversible reduction of oxygen occurs in the same potential region where the surface states undergo a reversible electron transfer.15c,d On the basis of this and some other facts, a hypothesis has been proposed that the surface states mediate the oxygen reduction at the TiO2 surface. 15c,d In our case, the cathodic peak has no a coupled anodic counterpeak. Moreover, the peak amplitude drops somewhat in going from the first potential scan to the second and successive scans. If the potential sweep is stopped after several scans and the potential is kept a constant at E g 0 V, the peak increases after restarting the sweep. These facts permit us to suggest that the cathodic peak observed for the untreated nanocrystalline TiO2 electrodes is associated with the reduction of adsorbed oxygen, traces of which can occur in the Ar-saturated electrolyte. Electrochemical Behavior of Copper-Modified TiO2 Electrodes. After treating the TiO2 electrodes with Cu2+-containing solutions, the cyclic voltammograms are changed (Figure 1). The cathodic peak (B) coinciding reasonably closely in position with the peak for untreated electrodes increases significantly. In addition, a cathodic prepeak (A) and a peak (C) can be observed in the more positive and more negative potential region, respectively. The only small anodic counterpeak appears at potentials close to 0 V after the cathodic sweep. These observations indicate that the adsorbed cupric ions can be reduced electrochemically at the nanostructured TiO2 electrodes and that the products of the reduction are reoxidized partially in the anodic potential cycle. To elucidate whether the adsorption of Cu2+ ions and its subsequent electroreduction take place over the bulk of the TiO2 film or only in the uppermost layers near the electrode-solution interface, we studied the effect of the film thickness on the electrochemical behavior of the electrodes after the Cu2+ adsorption. Figure 2 shows the dependence of the charge corresponding to all the cathodic peaks on a number of deposited TiO2 layers. A linear rise in the cathodic charge occurs in the range of layers from 1 to 4, and when the film thickness increases further, some departure from linearity in the increasing direction is observed. This departure may be related to the reduction of excess Cu2+ ions occluded by the film bulk owing to the slowed rate of cupric ion diffusion in the nanopores of thick TiO2 films. Most experiments in the present work were performed at the TiO2 electrodes with an optimum thickness involving four layers. Thus, these results permit us to conclude that the nanostructured TiO2 film electrodes are very permeable to the electrolyte and that the adsorbed cupric ions can be reduced electrochemically over the thickness of the film up to the substrate. Figure 3 shows the negative-going branches of the current vs potential curves obtained in deaerated Cu2+-free acetic buffer electrolytes with pH ) 4.4 (Figure 3a) and pH ) 6 (Figure 3b) for nanostructured TiO2 electrodes, previously equilibrated in solutions containing Cu2+ ions with various concentrations (Ccu) ranging from 10-5 to 10-1 mol/L. At pH ) 4.4, prepeak A and peak B grow markedly with increasing the Ccu and peak C appears only at high cupric ion concentrations Ccu g 10-2 M (Figure 3a). However, the peak C is less than peak B even at Ccu ) 10-1 M. At pH ) 6, peak C is predominant and peak B

Poznyak et al.

Figure 2. Dependence of the total cathodic charge corresponding to peaks A-C at the current vs potential curves for copper-modified TiO2 film electrodes with various thickness of the oxide film. The copper(II) adsorption was carried out in 3 × 10-4 M Cu2+ (pH ) 6.0). Electrolyte was 0.1 M Ar-saturated acetic buffer solution with pH ) 6.0. Potential sweep rate was 5 mV s-1.

converts to its shoulder at Ccu > 10-5 M (Figure 3b). It should be noted that the cathodic peaks B and C and prepeak A decrease significantly and are shifted to the more negative potentials with decreasing pH of the solution. These data indicate that the copper ion adsorption at the TiO2 surface is weakened essentially with a lowering of the pH, in accordance with the results of Ludwig and Schindler.4 However, the Cu2+ adsorption takes place even below the point of zero charge of titanium dioxide, which is about 5.7 for our TiO2 samples. The charge vs log(Ccu) plots calculated from the curves shown in Figure 3 are presented in Figure 4. The charges corresponding to peaks B and C were estimated after subtraction of the background of monotonically increasing cathodic current. If we assume that all adsorbed Cu2+ ions are completely reduced to Cu(0) in the region of main peaks B and C and prepeak A, we may estimate the amount of adsorbed cupric ions by the cathodic charge. To support this assumption, we performed the following measurements. Immediately after cathodic peak C, the electrode was removed from the electrochemical cell under conditions of negative polarization and was dipped into a 0.1 M solution of HNO3 to wash the copper from the electrode surface. Then the copper content in this solution was determined by the AES method. These experiments have shown that the amount of copper washed off the electrode coincides well (within a range of 10%) with the amount calculated from the total cathodic charge (peaks A-C), assuming that the two-electron transfer occurs. ESR Measurement Data and Its Comparison with the Electrochemical Results. Additional information on the nature of copper complexes formed on the TiO2 surface and their relative amount was obtained using the ESR technique. Figure 5 presents typical ESR spectra of Cu2+ ions adsorbed onto TiO2 nanostructured samples from solutions containing different concentrations of Cu(ClO4)2 at pH ) 4.3 and 6.0. At low copper (II) concentrations, the ESR spectrum is typical for isolated mononuclear slightly distorted octahedral Cu(II) complexes with spin Hamiltonian parameters A| ) 12.7 ( 0.3 mT, g| ) 2.358 ( 0.005, and g⊥ ) 2.075 ( 0.005 at 77 K, which are in a good agreement with those obtained in refs 5, 6, 10, and 16. Some

Cu(II) Adsorption onto TiO2

Figure 3. Dependence of the dark cathodic current on the electrode potential for copper-modified TiO2 film electrodes that were equilibrated in solutions with different concentrations of Cu2+ ions at pH ) 4.3 (a) and 6.0 (b). Electrolyte was 0.1 M Ar-saturated acetic buffer solution with pH ) 4.4 (a) and 6.0 (b). Potential sweep rate was 5 mV s-1.

noticeable broadening of the ESR lines corresponding to the parallel orientation of Cu(II) complexes in an external magnetic field, which is observed even for the most magnetically diluted samples, in our opinion, is indicative of the existence of several paramagnetic centers with similar but not identical structure, i.e., with slightly different values of g and A tensors. The increase of Cu(II) concentration results initially in a trivial increase of the spectrum amplitude. Beyond certain concentrations, depending on the pH value, a more complicated spectrum appears (Figure 5, curves 2 and 3). A computer analysis showed that this spectrum is a superposition of two different ones: the initial multiplate spectrum of the isolated adsorbed copper(II) ions and the anisotropic singlet signal (dashed and dotted lines in Figure 5, respectively). The relative contribution of both types to a resulting spectrum depends on the pH value and the Cu2+ concentration in solution used for TiO2 treatment. Such broad anisotropic singlet spectra are typical of Cu(II) complexes with strong magnetic dipole-dipole and spin-exchange interaction. They were observed at various carriers treated with metal ions5,17 and were related to the formation of specific areas with a very high local concentration of paramagnetic centers, which were called “clusters” or “associates”. The average distance between the nearest-neighboring Cu(II) ions in “associates” on the surface

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Figure 4. Dependence of the total cathodic charge and the charge corresponding to prepeak A, calculated from the current vs potential curves shown in Figure 3, on the logarithm of the Cu2+ concentration in solutions with pH ) 4.3 (a) and 6.0 (b) that were used for copper (II) adsorption onto the TiO2 electrode surface.

of our TiO2 samples has been estimated according to refs 17c, 17d, and 18 and ranges from 0.6 to 0.9 nm. It is important to note that in such superimposed spectra as shown in Figure 5 (curves 2 and 3) the individual spectrum shapes of a multiplate and of a single line are not changing with their varying relative amplitude. It follows that the superimposed spectra can be used for calculation of individual amounts of isolated Cu(II) ions and “associates” at any ratio of them in the sample. The estimated total amount of paramagnetic Cu2+ ions, [Cu2+ads]par, existing on the TiO2 surface and the amount of Cu2+ ions forming the “associates”, [Cu2+ads]as, are presented in Figure 6 for two different pH values. This figure also shows the total amount of adsorbed copper, [Cu2+ads]tot, which was estimated from the electrochemical data (using the charge of the cathodic peaks). Analysis of these results reveals that at pH ) 4.3, the [Cu2+ads]par coincides very closely with the [Cu2+ads]tot for Cu2+ concentrations in solution up to 10-2 M. The adsorbed copper(II) species formed under these conditions are electrochemically reduced mainly in the region of peak B. At Ccu ) 0.1 M and pH ) 4.3, the [Cu2+ads]par constitutes ca. 60% of the [Cu2+ads]tot and peak C appears on the j-E plots (Figure 3a). Characteristically, the copper amount estimated from the charge of peak C agrees well with the difference between [Cu2+ads]tot and

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Figure 5. ESR spectra of Cu(II) ions adsorbed onto TiO2 nanostructured samples from solutions containing 10-4 M (1, 2) and 10-3 M Cu2+ (3) at pH ) 4.3 (1) and 6.0 (2, 3). Dotted and dashed lines represent two additive spectra forming spectrum 3 as a superposition. T ) 77 K.

[Cu2+ads]par. These facts indicate that there exist formally diamagnetic copper(II) species on the TiO2 surface that cannot be recorded by the ESR method and that are electrochemically reduced at more negative potentials than the adsorbed paramagnetic Cu(II) ions. These species seem to have copper hydroxide nature (in copper hydroxide, all the spins are coupled and it is formally diamagnetic). In neutral and alkaline solutions, the redox potential of the Cu(OH)2/Cu0 pair is more negative than that of Cu2+/Cu0 pair.19 As can be seen in Figure 6, the fraction of formally diamagnetic copper (II) species and that of “associates” increase with increasing Cu2+ concentration in solution and pH. At pH ) 6, the former constitutes most of the adsorbed copper (84% at Ccu ) 10-3 M) and, correspondingly, cathodic peak C is predominant in the current-potential curves. From the data obtained, it follows that the treatment of the nanostructured TiO2 samples with 10-2 M Cu2+ solution at pH ) 6 results in the accumulation of about 5% copper in the TiO2 film. Considering the specific area of the oxide and taking the radius of the hydrated Cu2+ ions to be 3.42 × 10-10 m,5 adsorption of this copper amount must cover the TiO2 surface with ca. 0.9 monolayer of hydrated cupric ions. It is important to note that at such high levels of the surface coverage, there exists a marked amount of magnetically isolated Cu2+ ions on the surface that is indicative of the strongly inhomogeneous distribution of Cu2+ ions over the TiO2 surface. Identification of Adsorbed Cu+ Ions at the TiO2 Surface. The charge consumed in the region of prepeak A is about 10 times smaller than that corresponding to peaks B and C (Figure 4). Moreover, species responsible for the appearance of this prepeak are reduced at potentials about 200 mV more positive than those producing peak B. We have suggested that these species could be monovalent Cu+ ions attached to the TiO2 surface. This assumption is strengthened by the fact that the redox potential of the Cu+/Cu0 pair is ca. 190 mV more positive than that of Cu2+/Cu0 pair.19 To obtain more weighty evidence of the presence of Cu(I) ions at the TiO2 surface, we performed XPS studies of the nanostructured TiO2 films treated with Cu2+-containing solution. Figure 7 shows a typical Cu 2p3/2 core level spectrum for these samples. This spectrum can be decomposed into two compo-

Figure 6. Dependence of the total amount of the adsorbed Cu(II) species (9), measured electrochemically by the charge of cathodic peaks B and C, and the total amount of adsorbed paramagnetic Cu2+ ions (O) and Cu2+ ions forming the “associates” with high local concentration (×), measured by ESR, on logarithm of the Cu2+ concentration in solutions with pH ) 4.3 (a) and 6.0 (b) that were used for copper(II) adsorption onto the nanostructured TiO2 samples.

nents (using Lorentzian functions) with binding energies of 932.9 and 935.0 eV. A binding energy of 935.0 eV is in good agreement with the values previously reported for divalent copper compounds.20 The Cu 2p3/2 binding energy for monovalent copper compounds is very close to that for metallic copper and ranges from 932.5 to 933.1 eV according to the different studies.20 The second lower energy peak of the spectrum, shown in Figure 7, falls well within this range. The proximity of the binding energies for Cu(0) and Cu(I) compounds does not permit us to assign unambiguously the peak with an energy of 932.9 eV to Cu+ ions. However, this assignment is more justified, since atomic copper is very reactive and its presence on the TiO2 surface in dark without external polarization is unlikely. The appearance of monovalent copper ions at the surface of the TiO2 nanoparticles after their treatment with Cu2+-containing solutions may be associated with the partial reduction of adsorbed Cu2+ ions by electrons of the conduction band of the TiO2 matrix by analogy with the formation of adsorbed O2species at the titanium dioxide surface. In this case, the surface concentration of copper(I) species should be proportional to the content of Ti(III) donor defects in the TiO2. To verify this

Cu(II) Adsorption onto TiO2

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Figure 7. Cu 2p3/2 core emission spectrum of nanostructured TiO2 films treated with 10-3 M Cu2+ solution (pH ) 6.0).

Figure 9. Electroreflection spectra of untreated (a) and copper-modified (b) TiO2 film electrodes under negative polarization (the electrode potentials are shown in the figure) in 0.1 M acetic buffer solution (pH ) 6). The copper(II) adsorption was carried out in 10-3 M Cu2+ solution (pH ) 6.0). Modulating voltage is 300 mV peak to peak, and frequency is 21 Hz. Figure 8. Cathodic current vs potential curves for copper-modified initial TiO2 electrode and TiO2 electrode reduced electrochemically at a potential of -1.4 V for 10 min just before the Cu2+ adsorption. The copper adsorption was carried out in 0.01 M Cu2+ solution (pH ) 6). Electrolyte was 0.1 M Ar-saturated acetic buffer solution with pH ) 6.0. Potential sweep rate was 5 mV s-1.

hypothesis, we have compared the potentiodynamic current vs potential curves for two copper-modified TiO2 electrodes, one of which was reduced electrochemically at a potential of -1.4 V for 10 min just before the Cu2+ adsorption at its surface to increase the concentration of Ti3+ species in the oxide. As can be seen from Figure 8, the preliminary cathodic reduction of the electrode results in the marked (by a factor of about 2) increase in the prepeak area, whereas peaks B and C are not changed noticeably. These results are evidence in favor of the above-stated assumption that the nature of prepeak A is associated with the reduction of Cu+ species. Electrolyte Electroreflection Measurements. As noted above, cupric ions are attached firmly to the TiO2 surface with the formation of copper complexes involving titanium dioxide surface groups. In principle, this process may result in the appearance of surface states at the copper-modified TiO2

films. To throw light on this problem, we have studied nanostructured TiO2 film electrodes modified by Cu2+ ions using the electrolyte electroreflection (EER) method. This method has been previously used successfully to investigate the band structure,21 to determine flat band potentials,22 and to identify surface states23-25 for a number of semiconductor electrodes including TiO2. Figure 9a shows the EER spectra for untreated nanocrystalline TiO2 electrodes under conditions of cathodic polarization in an indifferent acetic buffer solution. We have analyzed mainly a sub-band-gap spectral region (hω < 3.2 eV) in which the effect of surface states is most probable. The shape of the EER spectrum in this spectral region is essentially featureless and independent of the applied dc bias for untreated electrodes. The marked ∆R/R signal appears in the sub-band-gap region at potentials more negative than -0.3 V, grows significantly with increasing E in the negative direction, reaches its peak at E ) -0.7 V, and then begins to fall (Figures 9a and 10). The similar featureless EER spectra have been previously observed at photon energies hω < Eg for single-crystal, polycrystalline, and nanocrystalline TiO2 electrodes and have been associated with intrinsic surface states of titanium dioxide.24-26

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Poznyak et al. -0.2 to -0.4 V (Figures 10 and 11). Just in this potential region, the filling of the surface states with electrons is modulated most strongly by the Fermi level. Since ∆R/R < 0 in the spectrum, the increase in absorption takes place at positive half-wave of the potential. This electroabsorption may be associated with the light-induced transfer of electrons from the TiO2 valence band onto the levels of surface states in the oxide band gap. These surface states formed by the adsorbed Cu2+ ions are located ca. 1.1 eV below the conduction band edge and are rather “deep”. Conclusions

Figure 10. Dependence of the electroreflectance signal on the electrode potential for untreated and copper-modified nanocrystalline TiO2 electrodes. The copper adsorption was carried out in 10-3 M Cu2+ solution (pH ) 6.0). Photon energy is 2.1 eV. Electrolyte is 0.1 M acetic buffer with pH ) 6. Modulating voltage is 300 mV peak to peak, and frequency is 21 Hz.

Figure 11. Electroreflection spectra of copper-modified nanocrystalline TiO2 film electrodes, which are represented in Figure 9b, after subtraction of the background ∆R/R signal from the TiO2 matrix.

After the adsorption of Cu2+ ions on the surface of nanostructured TiO2 electrodes, the ∆R/R spectra are changed markedly (Figure 9b). A distinct minimum of ∆R/R at hω ≈ 2.1 eV against a background of the monotonically growing electroabsorption signal appears in the sub-band-gap region. This minimum after subtraction of the background ∆R/R signal inherent in the TiO2 matrix is shown in Figure 11 for various potentials. Since the ∆R/R minimum appears in the EER spectra only after sorption of Cu2+ ions onto the TiO2 surface, it is reasonable to relate it to the surface states formed by surface copper complexes. As can be seen from Figure 11, the shape of the ∆R/R peak, its half-width, and the position are not changed appreciably with changing electrode potential, whereas its amplitude grows significantly with decreasing potential from

We have demonstrated that the electrochemical methods in combination with ESR spectroscopy allow us to obtain important information on the adsorption behavior of Cu(II) ions on the titanium dioxide surface and on the nature of species formed as a result of the adsorption. The Cu(II) ions adsorbed strongly at the surface of nanostructured TiO2 films can be reduced under cathodic polarization of the TiO2 electrodes to Cu(0). The amount of the adsorbed cupric ions can be determined from the charge consumed for the electroreduction. There are several peaks at the potentiodynamic current-potential curves that correspond to the different types of surface species formed on the Cu2+ adsorption at TiO2. These types are the following. (a) One type is individual Cu2+ ions coupled to the surface Ti(IV) ions via oxygen. They give an ESR spectrum that is characteristic of immobilized magnetically isolated Cu(II) ions. This type of surface species is dominant when Cu(II) ions are adsorbed from the dilute solutions and at lower values of the pH. (b) Another type is isolated ions of monovalent copper formed as a result of the partial reduction of the adsorbed Cu(II) ions by electrons of the TiO2 matrix. The surface concentration of these species averages ca. 10% of the total surface concentration of the adsorbed copper ions. (c) A third type is “associates” of Cu2+ ions with the strong magnetic dipole-dipole and spin-exchange interaction between them. The amount of these species increases with increasing Cu2+ concentration in solution and pH. However, the appearance of “associates” can be observed even at low levels of adsorption, which is indicative of the inhomogeneous distribution of Cu2+ ions over the TiO2 surface. (d) A fourth type is formally diamagnetic copper hydroxide particles. These particles dominate when adsorbing from the concentrated Cu2+ solutions and at higher pH (>5). At pH ) 6, they constitute most (80-90%) of the adsorbed copper. The copper ions adsorbed on the TiO2 form electroactive surface states within the band gap of the oxide, the energy position of which was determined by the electrolyte electroreflection method. Copper-induced surface states have been established to be located ca. 1.1 eV below the conduction band edge. Acknowledgment. The funding of this research by the INTAS (Grant No. 94-0266) is gratefully acknowledged. References and Notes (1) (a) Williams, W. J.; Gillam, A. N. Analyst 1978, 103, 1239. (b) Yamashita, H.; Ozawa, Y.; Nakajima, F.; Murata, T. Bull. Chem. Soc. Jpn. 1980, 53, 1. (c) Inoue, Y.; Yamazaki, H. Bull. Chem. Soc. Jpn. 1980, 53, 811. (d) Malati, M. A.; Smith, A. E. Powder Technol. 1979, 22, 279. (e) Bonsack, J. P. J. Colloid Interface Sci. 1973, 44, 430.

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