Influence of Metal Ions on the Charge Recombination Processes

Chem. B , 2004, 108 (30), pp 11054–11061. DOI: 10.1021/jp0484128. Publication Date (Web): June 18, 2004. Copyright © 2004 American Chemical Society...
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11054

J. Phys. Chem. B 2004, 108, 11054-11061

Influence of Metal Ions on the Charge Recombination Processes during TiO2 Photocatalytic One-Electron Oxidation Reactions Takashi Tachikawa, Sachiko Tojo, Mamoru Fujitsuka, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan ReceiVed: April 10, 2004; In Final Form: May 10, 2004

The influence of inorganic salt, such as NaClO4, LiClO4, and Mg(ClO4)2, on the charge recombination process during the TiO2 photocatalytic one-electron oxidation reaction of stilbene derivatives has been investigated by time-resolved diffuse reflectance (TDR) spectroscopy. In the case of trans-4-methoxystilbene (MtSt), a one-electron oxidation process in the submicrosecond time domain was clearly observed. This is the first example of a direct observation of an electron transfer (ET) from the organic compound adsorbed on the TiO2 surface to the photogenerated holes in TiO2. The average lifetime (τ) of the MtSt radical cation (MtSt•+), which is generated from the one-electron oxidation reaction by the photogenerated holes at the TiO2 surface, significantly increased with the addition of metal ions, especially, Mg2+, in Ar-saturated acetonitrile (MeCN). On the other hand, a slight enhancement was observed for the stilbenemethanol radical cation (StM•+), although τ of StM•+ generated from the photosensitized electron transfer reaction with 9,10-dicyanoanthracene (DCA) in the excited singlet state significantly increased with the addition of metal ions. These results clearly suggest that the adsorption dynamics of substrates (S), such as MtSt and StM, and metal ions on the TiO2 surface is important for the back electron transfer (BET) process between S•+ and the photogenerated electrons in the TiO2 particles. To clarify the influence of metal ions on the decay kinetics of S•+, the driving force dependence of BET rate constants (kBET) was examined in terms of Marcus theory. It was found that a significant increase in τ of S•+ with the addition of metal ions results from suppression of the BET process between S•+ and the photogenerated electrons due to the desorption of S•+ from the TiO2 surface.

Introduction TiO2 has been extensively used in the field of heterogeneous photooxidation catalysis for environmental cleanup because TiO2 is nontoxic, stable, and inexpensive.1-6 Typically, the reaction processes are initiated by the band-gap excitation of the TiO2 particles with UV irradiation to generate the oxidizing species, for example, the OH radicals derived from the oxidation of water adsorbed on the surface.5,6 The reactivity and lifetime of these oxidizing species have been considered to play an important role in controlling the overall kinetics of the oxidative processes. The photocatalytic efficiency of TiO2 also depends on the degree of charge separation of the photogenerated electron and hole pairs at the surface of TiO2 particles.7,8 To enhance the charge separation, suitable metals have been deposited onto the surface of TiO2.9-11 These methods have been used with the aim of enhancing the photocatalytic activity both in gas-solid and in liquid-solid systems. There have been a great number of studies involving the influence of the deposition and adsorption of various ions on the performance of dye-sensitized solar cells as well as the photocatalytic activity.12-16 It has been known that several cations existing in the electrolyte solution as the countercation of I- and I3- play important roles in the high-energy conversion efficiency. For example, the interaction of Li+ with the TiO2 surface enhances the electron transfer from the adsorbed sensitized dye to the conduction band in TiO2 and also the electron transfer from I- to the oxidized dye, leading to a high photocurrent. These effects generally increased in the order of * Address correspondence to this author. Phone: +81-6-6879-8495. Fax: +81-6-6879-8499. E-mail: [email protected].

Mg2+ > Li+ > Na+ and are attributed to the charge density of the metal ions which are found to be potential-determining.15 Some evidence suggested that the adsorption of metal ions is responsible for the positive shift of the flat band potential at low concentrations, while intercalation close to the electrode surface may be important at higher concentrations.12 Recently, from another point of view, Mizuno et al. reported that the TiO2 photooxygenation process of 1,2-diarylcyclopropanes is greatly accelerated by the addition of Mg(ClO4)2 in acetonitrile (MeCN).17 They suggested that the enhanced efficiency of this process is due to the suppression of back electron transfer from the photogenerated electrons in TiO2 to the substrates and the interaction of Mg2+ with ionic species such as the photogenerated electrons in TiO2 and O2•-. However, the generation and deactivation processes of organic radical cations during such photocatalytic reactions are still unclear. It is important for the development of synthetic organic photochemistry with use of TiO2 to clarify the effects of metal ions on the reaction dynamics of organic radical cations during TiO2 photocatalytic reactions. Time-resolved diffuse reflectance (TDR) spectroscopy is a powerful tool for the investigation of photocatalysis under various conditions.18-21 Fox and co-workers reported that many oxidation reactions appear to occur by a direct electron transfer from various compounds to the photoexcited TiO2 powder in MeCN.18 Recently, we studied the one-electron oxidation of several aromatic compounds adsorbed on the surface of the TiO2 powder slurried in MeCN by TDR spectroscopy and concluded that the -OH and -COOH groups play an important role in the adsorption on the surface of TiO2 and the efficiency of the

10.1021/jp0484128 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/18/2004

Influence of Metal Ions on Charge Recombination one-electron oxidation of aromatic compounds.21a-c We also demonstrated new reaction processes for the one-electron oxidation of the nonadsorbed S in terms of cascade two-step hole transfer reactions.21e,f In the present study, we investigated the influence of several salts, such as sodium perchlorate (NaClO4), lithium perchlorate (LiClO4), and magnesium perchlorate (Mg(ClO4)2), on the charge transfer and recombination processes during the TiO2 photocatalytic one-electron oxidation reactions. To examine the effects of the -OH group on the adsorption and reaction dynamics, we used trans-4-methoxystilbene (MtSt) and stilbenemethanol (StM) as substrates (S). First, we studied the influence of the salts on the adsorption of S on the TiO2 surface slurried in MeCN using steady-state UV-vis absorption and ζ-potential measurements. Next, we investigated the influence of salts on the formation and decay kinetics of radical cations of S (S•+) generated from the TiO2 photocatalytic one-electron oxidation reactions using TDR spectroscopy. For comparison, we examined the influence of salts on the decay kinetics of S•+ in the bulk MeCN solutions. The relationship between the reaction and adsorption dynamics of S•+ is also discussed. Experimental Section Materials. The TiO2 powder (P25, Japan Aerosil) was a mixture of rutile (20%) and anatase (80%) with a BET surface area of 50 m2 g-1 and an average primary particle size of 21 nm. MtSt was synthesized by the Wittig reaction of the corresponding substituted benzaldehyde and benzyltriphenylphosphonium chloride with sodium ethoxide in absolute ethanol at room temperature according to literature procedures22 and was recrystallized from ethanol before use. StM (Aldrich) was recrystallized from ethanol before use. 9,10-Dicyanoanthracene (DCA) (Aldrich) and 10-methylacridinium ion (AcrH+) were used as a photosensitizer. NaClO4 (Nakarai Tesque), LiClO4 (Wako), and Mg(ClO4)2 (Wako) were used without further purification. Fresh MeCN (Nakarai Tesque, spectral grade) was used as the solvent without further purification. Instrumentation. The steady-state UV-vis absorption spectra were measured by UV-vis-NIR spectrophotometers (Shimadzu, UV-3100) at room temperature. The sample solutions containing TiO2 powder (20 g dm-3) were sonicated for 10 min, and the TiO2 particles in suspension were then completely removed by centrifugation (10 000 rpm, 10 min), using a highspeed microcentrifuge (Hitachi, himac CF16RX) at 22 °C for the UV absorption measurements. All procedures for the sample preparation were performed with shielding from UV light.21 The TDR measurements were performed with use of the third harmonic generation (355 nm, 5 ns full width at half-maximum) from a Q-switched Nd3+:YAG laser (Continuum, Surelite II10) for the excitation operated with temporal control by a delay generator (Stanford Research Systems, DG535).21 In these experiments, the spot irradiated on the quartz cell with a thickness of 2 mm was approximately 1 cm2. The reflected analyzing light from a pulsed 450-W Xe-arc lamp (Ushio, XBO450) was collected by a focusing lens and directed through a grating monochromator (Nikon, G250) to a silicon avalanche photodiode detector (Hamamatsu Photonics, S5343). The transient signals were recorded by a digitizer (Tektronix, TDS 580D). The reported signals are averages of 4-20 events. Laser flash photolysis measurements with transmission detection were performed with use of the third harmonic generation (355 nm, 5 ns full width at half-maximum) of a Q-switched Nd3+:YAG laser (Continuum, Surelite II-10) as the excitation source. A silicon avalanche photodiode detector (Hamamatsu

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Figure 1. Mg(ClO4)2 concentration dependence of the amount of MtSt (solid circles) and StM (open circles) adsorbed on the TiO2 surface and of the ζ-potential (solid triangles) of the TiO2 particles in MeCN at room temperature. The solid lines are visual guides.

Photonics, S5343) equipped with a monochromator (Nikon, G250) was used to detect probe light from a 450-W Xe lamp. Cyclic voltammograms were obtained with a conventional three-electrode system (BAS, CV-50W) in MeCN solution at room temperature. A platinum electrode was used as the working electrode and an Ag/AgNO3 electrode was used as the reference electrode. ζ-potentials were obtained from ζ-potential measurements (ZetaProbe, BEL Japan, Inc.) for TiO2 suspensions at room temperature. Results and Discussion Adsorption of Substrates and Metal Ions on the TiO2 Surface. The concentrations of the adsorbates in MeCN containing TiO2 powder (20 g dm-3) after reaching adsorption equilibrium were determined from the absorbance observed by steady-state UV absorption experiments. The Mg(ClO4)2 concentration dependence of the amounts (nad) of MtSt (solid circles) and StM (open circles) adsorbed on the TiO2 surface is shown in Figure 1. The nad values are normalized for comparison. Incidentally, the nad values of 3.3 × 10-6 and 4.2 × 10-5 mol g-1 were determined for MtSt (4 mM) and StM (2 mM), respectively. As shown in the figure, the nad values significantly decreased with increasing Mg(ClO4)2 concentration, suggesting that S desorbs from the TiO2 surface on the addition of Mg(ClO4)2. In particular, MtSt desorbs easily from the surface on the addition of Mg(ClO4)2 compared with StM. From Langmuir adsorption isotherms,21,23,24 we determined the adsorption equilibrium constants (Kad) of ∼5 and ∼300 M-1 for MtSt and StM, respectively, in the absence of Mg(ClO4)2, indicating that StM is strongly adsorbed on the TiO2 surface compared with MtSt. Therefore, the observed difference in the amount of S desorbed from the TiO2 surface is mainly due to the difference in the binding strength onto the TiO2 surface. Electrophoretic measurements were carried out to clarify the effects of the addition of Mg2+ on the change in the TiO2 particles’ surface charge. Figure 1 also shows the Mg2+ concentration dependence on ζ-potential (solid triangles) of the TiO2 particles suspended in MeCN in the absence of S. The ζ-potential increased with increasing Mg2+ concentration. This result clearly indicates that Mg2+ is adsorbed on the TiO2 surface, although we cannot directly observe the amount of Mg2+ adsorbed on the TiO2 surface. The addition of a concentration of 0.1 mM Mg2+ increased the ζ-potential of the TiO2 particle from 11.6 to 37.1 mV. Because metal ions are hard to solvate in aprotic solvents such as MeCN, its adsorption on the TiO2 should occur easily in MeCN. In fact, it was reported that the flat band potential of TiO2 was independent of the Li+ concentration in protic solvents because of the

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Tachikawa et al. SCHEME 1: Photosensitized Electron Transfer from S to DCA in the Excited Singlet State (1DCA*) in the Presence of a Salt (M+X-)

SCHEME 2: Photosensitized Electron Transfer from BP to AcrH+ in the Excited Singlet State (1AcrH+*) and the Subsequent Electron Transfer from S to BP•+ in the Presence of a Salt (M+X-)

Figure 2. (A) Time-resolved absorption spectra attributed to S•+ observed 300 ns after the laser flash during the 355-nm laser photolysis of DCA (0.01 mM) in the presence of MtSt (10 mM) (solid circles) or StM (10 mM) (open circles) and Mg(ClO4)2 (5 mM) in Ar-saturated MeCN at room temperature. (B) The influence of Mg(ClO4) on the lifetimes of S•+. The upper two panels depict the time traces observed at 485 nm during the 355-nm laser flash photolysis of DCA (0.01 mM) in the presence of MtSt (10 mM) (a) or StM (10 mM) (b) and in the absence (i) or presence (ii) of Mg(ClO4)2 (5 mM) in Ar-saturated MeCN at room temperature. The lower two panels depict the time traces observed at 485 nm during the 355-nm laser flash photolysis of AcrH+ (0.05 mM) in the presence of BP (20 mM) and MtSt (1 mM) (c) or StM (1 mM) (d) and in the absence (i) or presence (ii) of Mg(ClO4)2 (5 mM) in Ar-saturated MeCN at room temperature.

selective solvation of Li+ by the protic molecules such as water and ethanol but dependent on it in aprotic solvents.12 Therefore, the observed decrease in nad is mainly attributable to the replacement between S and Mg2+ on the adsorption sites or/ and the desorption of S due to the neutralization of the negatively charged TiO2 surface resulting from the adsorption of Mg2+. Influence of Metal Ions on the Decay Kinetics of S•+ in the Bulk MeCN Solutions. To examine the influence of metal ions on the decay kinetics of S•+, we observed the transient absorption spectra and decay kinetics during the 355-nm laser photolysis of DCA in the presence of S and Mg(ClO4)2 and in the absence of TiO2 in Ar-saturated MeCN at room temperature. Figure 2A shows the time-resolved absorption spectra attributed to S•+ observed at 300 ns after the laser flash during the 355-nm laser photolysis of DCA (0.01 mM) in the presence of MtSt (10 mM) (solid circles) or StM (10 mM) (open circles) and Mg(ClO4)2 (5 mM) in Ar-saturated MeCN solutions at room temperature. The photosensitized electron-transfer reaction scheme is indicated in Scheme 1. As shown in Figure 2A, the transient absorption bands with a maximum around 485 nm appeared after the laser flash. These absorption bands are assigned to the MtSt radical cation (MtSt•+) and the StM radical cation (StM•+) as already reported.25 Similar absorption spectra attributed to S•+ were also observed in the absence of Mg(ClO4)2. Figure 2B shows the influence of Mg(ClO4)2 on the lifetimes of S•+. The upper two panels in Figure 2B depict the time traces observed at 485 nm during the 355-nm laser flash photolysis of DCA (0.01 mM) in the presence of MtSt (10 mM) (a) or

StM (10 mM) (b) and in the absence (i) and presence (ii) of Mg(ClO4)2 (5 mM) in Ar-saturated MeCN solutions at room temperature (see Scheme 1). The lower two panels in Figure 2B depict the time traces observed at 485 nm after the laser flash during the 355-nm laser flash photolysis of AcrH+ (0.05 mM) in the presence of biphenyl (BP) (20 mM) and MtSt (1 mM) (c) or StM (1 mM) (d) and in the absence (i) and presence (ii) of Mg(ClO4)2 (5 mM) in Ar-saturated MeCN solutions (see Scheme 2).26,27 As shown in panels a and b, significant increases in the lifetimes of S•+ were observed in the presence of Mg(ClO4)2. On the other hand, almost the same lifetimes were observed in the absence and presence of Mg(ClO4)2 for the photosensitized electron-transfer reaction between AcrH+*, BP, and S (panels c and d). As is well-known, the photogenerated DCA radical anion (DCA•-) interacts with metal cations in MeCN, but no influence of metal ions on the AcrH+-sensitized photoinduced electron-transfer reaction is expected.28,29 As shown in Scheme 1, therefore, the significant increase in the lifetimes of S•+ is mainly due to the suppression of the back electron-transfer (BET) process of S•+ with DCA•- that reacts with Mg2+, while AcrH• does not interact with Mg2+. The present experimental results also suggest that ClO4- hardly affects the decay kinetics of S•+ in the present time regime. One-Electron Oxidation of S Adsorbed on the TiO2 Surface. We assumed that the transient signals observed in the absence of S are attributable to species deeply trapped in TiO2 which cannot react with S on the surface in the present time period (several tens of ns ∼ ms after excitation).21 The absorption values attributed to S (% absorption) were obtained by subtracting the absorption value observed in the absence of S from that observed in the presence of S. Here, percent of absorption (% abs) is given by eq 1, where R and R0 represent

% abs )

R0 - R × 100 R0

(1)

the intensities of the diffuse reflected monitor light with and without excitation, respectively.18,21,30 Figure 3 shows the TDR spectra obtained during the laser photolysis of TiO2 with 355-nm light in the presence of MtSt (4 mM) (A) or StM (2 mM) (B) and Mg(ClO4)2 (0.5 mM) in Ar-saturated MeCN solutions at room temperature. As shown in Figure 3A, a transient absorption band with a maximum around 485 nm appeared after the laser flash. This absorption band is assigned to MtSt•+ as shown in Figure 2A. Similar absorption spectrum attributed to MtSt•+ was observed in the absence of Mg(ClO4)2. The transient absorption spectrum attributed to StM•+ was also observed during the laser photolysis

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Figure 4. The percent of absorption (% abs) at 485 nm versus time after the laser flash during the 355-nm laser flash photolysis of TiO2 powder in the presence of MtSt (4 mM) and Mg(ClO4)2 (0 (a), 0.05 (b), and 0.1 (c) mM). The solid lines represent nonlinear least-squares curve fits based on eqs 4 and 5.

Figure 3. Time-resolved diffuse reflectance spectra attributed to S•+ observed after the laser flash during the 355-nm laser photolysis of TiO2 powder in the presence of MtSt (4 mM) (A) or StM (2 mM) (B) and Mg(ClO4)2 (0.5 mM) in Ar-saturated MeCN at room temperature.

of TiO2 with 355-nm light in the presence of StM (2 mM) and Mg(ClO4)2 (0.5 mM) as shown in Figure 3B, although the spectral shapes were broader than that obtained in the bulk MeCN solution as shown in Figure 2A. The spectral shape and absorption maximum significantly depend on the electronic interaction between S•+ and the TiO2 surface.21 For example, the spectral feature observed for the radical cation of 4-hydroxybiphenyl adsorbed on the TiO2 surface indicates the significant broadening and blue shift in the absorption band compared with that obtained in the bulk MeCN solution, while that observed for the radical cation of 4-cyano-4′-hydroxybiphenyl adsorbed on the TiO2 surface is similar to that in the bulk MeCN solution.21b Therefore, the broadening of spectra observed for StM•+ clearly indicates the strong electronic interaction between StM•+ and the TiO2 surface compared with that between MtSt•+ and the TiO2 surface. As mentioned in the Introduction, TiO2 particles generate the valence band hole (hVB+) and the conduction band electron (eCB-) during the band gap excitation (eq 2). The fast charge hν

TiO2 98 hVB+ + eCB-

(2)

recombination and transfer kinetics of these photogenerated carriers have been studied in detail by several groups.7,8,19 Although most charge carriers quickly recombine, a minority that trapped at the surface of the particles will be available for reactions with adsorbates such as a surface-bound OH- and O2. A trapped positive hole, such as a surface-bound OH radical, typically oxidizes an organic molecule and thus induces its further oxidative degradation. In the present systems, we assume that the hVB+ or the shallow trapped positive holes (htr+) are the main oxidizing species as given by eq 3, where hVB+ has a

hVB+(htr+) + S f S•+

(3)

flat band potential of the valence band (EVB) of +2.1 V vs NHE.31 Since the oxidation potentials (Eox) of MtSt and StM were +1.36 and +1.62 V vs NHE, respectively, it is possible that hVB+ directly oxidizes S.

Interestingly, we observed a rise in the transient signal attributed to MtSt•+ as shown in Figure 4. To our knowledge, this is the first example of a direct observation of the electron transfer (ET) from the organic compound adsorbed on the TiO2 surface to the photogenerated holes in TiO2. The formation of S•+ in the submicrosecond time domain was also observed for other stilbene derivatives such as trans-4,4′-dimethoxystilbene and trans-4,4′-dimethylstilbene, although the ET rates are different. In the present experiments, it is possible to extract the relative change in the S•+ concentration from the initial signal intensity (% abst)0) due to the fact that the signal intensity was found to be approximately proportional to the transient concentrations, although we do not know the absolute concentration of S•+. The linearity of the actual absorptions can be satisfied only when the percent of absorption is below 10% as suggested elsewhere.30 As we previously reported,21a-c the % abst)0 values increased with increasing nad at the present S concentration, suggesting that S•+ is generated by a bimolecular reaction with the photogenerated oxidizing species on the TiO2 surface.6 Additionally, MtSt would be oxidized by htr+ because hVB+ is trapped within less than the laser duration. According to eq 3, therefore, we obtained a system of coupled linear first-order differential equations for the concentrations of the htr+ and MtSt•+ as follows,

d[htr+] ) -(kETnad + kr)[htr+] dt

(4)

d[MtSt•+] ) kETnad[htr+] - kd[MtSt•+] dt

(5)

where kET is the ET rate from MtSt adsorbed on the TiO2 surface to htr+, and kr-1 and kd-1 are the lifetimes of htr+ and MtSt•+, respectively. The nonlinear least-squares curve fits shown in Figure 4 were performed by numerically solving the above equations by the Runge-Kutta method. Using the kr-1 value of 0.7 µs, which is estimated from the decay rate of the transient species such as trapped holes observed at 485 nm during the laser photolysis of TiO2 with 355-nm light in the absence of S, the kET values of (8.0 ( 0.4) × 106, (4.4 ( 0.4) × 106, and (2.8 ( 0.3) × 106 s-1 were determined at [Mg2+] of 0, 0.05, and 0.1 mM, respectively. The decay kinetics of S•+ was examined by using a stretched exponential function as discussed in the next section. On the other hand, StM•+ was generated within the laser duration (5 ns) as shown in the next section. In the case of a low nad (2.5 × 10-5 mol g-1) of StM (0.1 mM), we cannot observe a rise in the transient signal attributed to StM•+, although

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Tachikawa et al.

Figure 5. The percent of absorption at 485 nm versus time after a laser flash during the 355-nm laser flash photolysis of TiO2 powder in the presence of MtSt (4 mM) and NaClO4 (a), LiClO4 (b), and Mg(ClO4)2 (c) in Ar-saturated MeCN. Solid lines indicate the results fitted by the stretched exponential function.

Figure 6. The percent of absorption at 485 nm versus time after a laser flash during the 355-nm laser flash photolysis of TiO2 powder in the presence of StM (2 mM) and NaClO4 (a), LiClO4 (b), and Mg(ClO4)2 (c) in an Ar-saturated MeCN. Solid lines indicate the results fitted by the stretched exponential function.

the % abst)0 was significantly decreased compared with that observed at [StM] ) 2 mM. In a recent study, we found a high efficiency of the one-electron oxidation of 4-(methylthio)phenylmethanol adsorbed on the TiO2 surface compared with that of 4-methyl-p-tolylsulfide at the same nad values.21a This result suggests that the -OH group plays an important role in the efficiency of the one-electron oxidation of S. Therefore, the observed slow MtSt•+ formation is mainly due to the weak interaction with the TiO2 surface. To clarify the one-electron oxidation mechanisms of S adsorbed on the TiO2 surface, however, we need further investigation of the influence of metal ions on the interactions between S and TiO2, the migration and trapping of the photogenerated charge carriers,16 the flat band potentials,12 and so on. Such studies are now underway. Influence of Metal Ions on the Decay Kinetics of S•+ Adsorbed on the TiO2 Surface. The influence of metal ions on the decay kinetics of S•+ was studied in the presence of a fixed concentration of S. Figure 5 shows the time traces observed at 485 nm during the laser photolysis of TiO2 with the 355-nm light in the presence of MtSt (4 mM) and several concentrations of NaClO4 (a), LiClO4 (b), and Mg(ClO4)2 (c). In particular, the lifetimes of MtSt•+ significantly increased with increasing Mg(ClO4)2 concentration as shown in Figure 5c. We assumed that the observed biexponential decay is assigned to heterogeneous BET kinetics. Similar nonexponential BET kinetics have been reported for other systems.18,21 The observed time traces were well-reproduced by using a stretched exponential function as given by eq 6,32,33 where τ is the average

resulting in a broad distribution of detrapping times of the photogenerated electrons.32,33 On the other hand, in the case of StM, no significant increase in τ was observed on the addition of salts except those obtained at high Mg(ClO4)2 concentrations (>0.5 mM) as shown in Figure 6c. In the bulk Ar-saturated MeCN solutions, the τ values obtained for MtSt•+ and StM•+ significantly increased with the addition of metal ions as shown in Figure 2B. As mentioned above, the observed increases in τ are mainly attributable to the interaction between DCA•- and Mg2+, because almost the same decay rates were observed during the laser photolysis of AcrH+ with the 355-nm light in the absence and presence of Mg(ClO4)2 as shown in Figure 2B. These experimental results clearly indicate that ClO4- does not react with S•+, while metal ions do react with DCA•- in the bulk MeCN solutions. Therefore, the significant increase in τ observed for MtSt•+ is attributable to the adsorption of metal ions on the TiO2 surface. In the case of StM•+, this effect became small due to the strong adsorption of StM on the TiO2 surface compared with that of MtSt. As shown in Figure 7, the increase in τ of S•+ strongly depended on the metal ions. In the case of MtSt, especially, a remarkable difference in the increase in τ was clearly observed. The observed τ values increased with increasing [Metal ion], and this enhancement effect on τ increased in the order of Mg2+ . Li+ > Na+. For example, as illustrated by the dotted lines in Figure 7, the τ values observed for MtSt•+ increased twice at [Metal ion] of 2.3, 0.17, and 0.02 mM for Na+, Li+, and Mg2+, respectively, compared with those (1/τ ) 0.56 and 0.91 × 106 s-1 for MtSt•+ and StM•+, respectively) obtained in the absence of metal ions. These values were tentatively found to correlate with the charge density of the metal ions, which follows the sequence Mg2+ (1.7 eÅ-3) > Li+ (1.1 eÅ-3) > Na+ (0.2 eÅ-3).34 As is well-known, metal ion dopants influence the photoreactivity of TiO2 by acting as electron traps.10 A recent study of Ohtani and co-workers points out that inclusion of transition metal ions such as Fe3+, Mo5+, and V4+ decreased the photocatalytic activity of TiO2 under UV irradiation.10c

% abs ∝ exp(-(t/τ)R)

(6)

lifetime of S•+ and R is a heterogeneous parameter. In the case of MtSt, for example, τ values of 1.8 and 30 µs and R values of 0.47 ( 0.3 and 0.40 ( 0.3 were obtained in the absence and presence (0.1 mM) of Mg(ClO4)2, respectively. It seems that these remarkably small R values are mainly attributable to the inhomogeneous distribution of trap depths in the TiO2 particles,

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Figure 8. Mechanistic scheme for the back electron-transfer reaction from the conduction band electron (eCB-) to S•+ adsorbed on the TiO2 surface.

Figure 7. Metal ion concentration ([Metal ion]) dependence of the reciprocal of the average lifetime (τ) of S•+ observed for MtSt (A) and StM (B). Dotted lines indicate the half values of τ-1 obtained for S•+ in the absence of metal ions. The solid lines are visual guides.

The metal ion sites were shown to act as recombination centers for the photogenerated charge carriers. On the other hand, dopants with a closed-shell electronic configuration such as Li+, Mg2+, and A13+ have little effect on the observed photoreactivity as reported by Hoffmann et al.10a In the present systems, the effects of metal ions on the trap site of electrons on the TiO2 surface are not expected, because we observed no significant metal ion effects on the decay rate of % abs at 485 nm attributed to the trapped electron and hole in the absence of S.8c As reported elsewhere, the ζ-potential of the TiO2 colloid was also found to correlate with the charge density of the metal ions, which follows the sequence Mg2+ > Li+ > Na+ > K+.15 Therefore, we considered that the metal ion effects on τ mainly depend on the charge density of metal ions. To confirm the influence of metal ions on the decay kinetics of S•+, we examined the driving force dependence of the BET rate constant (kBET) from eCB- to S•+ adsorbed on the TiO2 surface in terms of the Marcus theory.35 The BET process is simply described in Figure 8, where electron transfer is allowed from the conduction edge to the surface-bound S•+ at the TiO2 surface. Assuming a random distribution of distances between the electrons in the TiO2 nanoparticle and the adsorbed S•+,36 kBET can be expressed as

kBET ) 2π [H ]2 p ab d

lSC 2/3 SC

1

(6/π)1/3 x4πΛkBT

[

exp -

]

(∆GBET + Λ)2 (7) 4ΛkBT

where p is Planck’s constant divided by 2π, kB is Boltzmann’s constant, T is the absolute temperature, Hab is the electronic coupling for a second-order reaction, lSC is the effective coupling length between S•+ and the nanoparticle, dSC is the density of the atoms that contribute to the density of states in the band concerned, ∆GBET is the free energy change for BET (∆GBET ) ECB - Eox, where ECB is the conduction band energy), and Λ is the total reorganization energy for ET ()λs + λv, where λv and λs are the internal and solvent reorganization energies,

Figure 9. TiO2 photocatalytic one-electron oxidation process of S adsorbed on the TiO2 surface (A), and the desorption and back electrontransfer (BET) processes of S•+ (B).

respectively). The λv depends on frequency of the vibrational mode associated with the electron transfer and λs is given by eq 8,36 where n and  are the refractive index and the dielectric

λs )

[(

)

(

)]

2 2 2 1 (∆e) 1 1 1 1 nTiO2 - n 1 TiO2 -  1 2 4π0 a n2  2r n 2 + n2 n2 TiO2 +   TiO2

(8)

constant for the solvent (n ) 1.344 and  ) 35.94 for MeCN), respectively, and nTiO2 and TiO2 are n and  for anatase TiO2 (nTiO2 ) 2.5 and TiO2 ) 86 for anatase TiO2).37,38 Using a molecular radius (a) of 0.42 nm for stilbenes39 and a distance between electron donor and acceptor (r) of 1.5 nm40 and λv of 0.24 eV,41 we obtained Λ of 1.1 eV from eq 8. This Λ value is significant smaller than -∆GBET values of 2.2 and 2.7 eV calculated for MtSt•+ and StM•+, respectively, suggesting that kBET falls in the Marcus inverted region where ET rates should ultimately decrease with increasing thermodynamic driving force. It seems that the decay kinetics of S•+ observed in the microsecond time domain is mainly due to the slow BET process in the Marcus inverted region.42 According to the Marcus theory, the increase in τ with the increasing of [Metal ion] suggests that the apparent -∆GBET values increased with increasing [Metal ion], that is, ECB shifts to more negatiVe values. Fitzmaurice et al. reported that the additions of 1 mM NaClO4, LiClO4, and Mg(ClO4)2 lead to the slight positiVe shifts of 0.07, 0.06, and 0.02 V vs NHE in the flat band potential.12 However, the significant increase in τ cannot be interpreted by the shifts in the flat band potential at the TiO2 particles on the addition of metal ions. Therefore, we considered that the adsorption of metal ions on the TiO2 surface promotes the desorption process of S•+ from the TiO2 surface and suppresses the charge recombination process between S•+ and eCB- (etr-) on the TiO2 surface as shown in Figure 9. These results suggest that two deactivation processes of the surfacebound and free S•+ are involved in the decay kinetics of S•+ at high [Metal ion], although we assumed the stretched exponential BET dynamic to estimate the τ values.

11060 J. Phys. Chem. B, Vol. 108, No. 30, 2004 As recently reported, the TiO2 photooxygenation process of 1,2-diarylcyclopropanes (10 mM) is greatly accelerated by the addition of 2.5 mM Mg(ClO4)2 in MeCN.14 It is considered that the increased efficiency of the reactions is mainly due to the suppression of the BET process between S•+ and eCB- (etr-), where the adsorption of metal ions on the TiO2 surface is a key factor in increasing τ of S•+. The positive surface potential of the TiO2 particles induced by the adsorption of metal ions may also enhance the desorption process of S•+ from the TiO2 surface. On the other hand, in the case of the strongly adsorbed S, such as phenols and benzoic acids, no significant effects of metal ions on the reaction efficiency are expected due to the small amount of metal ions adsorbed on the TiO2 surface. Conclusions We have investigated the influence of several salts, such as NaClO4, LiClO4, and Mg(ClO4)2, on the charge transfer and recombination process reaction during the TiO2 photocatalytic one-electron oxidation reactions. For the one-electron oxidation processes of S adsorbed on the TiO2 surface, we observed a rise in the transient absorption attributed to MtSt•+ in the submicrosecond time domain. To our knowledge, this is the first example of a direct observation of HT from the photogenerated holes in TiO2 to the organic compound adsorbed on the TiO2 surface. The τ of MtSt•+, which is generated from the one-electron oxidation reaction by the photogenerated holes at the TiO2 surface, significantly increased on the addition of metal ions, especially Mg2+, in Ar-saturated MeCN. On the other hand, slight enhancement was observed for StM•+, although the lifetime of StM•+ generated from the electron transfer reaction with DCA significantly increased with the addition of metal ions. These results clearly suggest that the adsorption kinetics of S and metal ions on the TiO2 surface is important for the charge recombination process between S•+ and eCB- (etr-). To clarify the influences of metal ions on the flat band potential at the TiO2 particles, we examined the driving force dependence of kBET in terms of the Marcus theory. The observed τ-1 values decreased with increasing [Metal ion] in the Marcus inverted region, clearly suggesting that the significant increase in τ on the addition of metal ions results from the suppression of the charge recombination process between S•+ and eCB- (etr-) due to the desorption of the S•+ from the TiO2 surface. This suppression of the charge recombination process should be a key factor in the photocatalytic synthesis in aprotic solutions containing metal ions. Acknowledgment. We are grateful to Mr. K. Imai (BEL Japan, Inc.) for his help in measuring the ζ-potential. This work has been partly supported by a Grant-in-Aid for Scientific Research on Priority Area (417), 21st Century COE Research, and others from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. References and Notes (1) (a) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000, 1, 1. (b) Fujishima, A.; Hashimoto, K.; Watanabe T. Photocatalysis; BKC Inc: Tokyo, Japan, 1999. (2) (a) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. ReV. 1993, 417. (b) Mills, A.; Hunte, S. L. J. Photochem. Photobiol. A 1997, 108, 1. (3) (a) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (b) Fox, M. A. Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley: New York, 2001; Vol. 2, p 271. (4) Linsebigler, A. L.; Lu, G.; Yates, Y. T., Jr. Chem. ReV. 1995, 95, 735. (5) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69.

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