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Charge-Transfer-Induced Surface-Enhanced Raman Scattering on Ag-TiO2 Nanocomposites Libin Yang,†,‡ Xin Jiang,‡ Weidong Ruan,† Jingxiu Yang,† Bing Zhao,*,† Weiqing Xu,† and John R. Lombardi*,§ State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China, College of Chemistry and Pharmacology, Jiamusi UniVersity, Jiamusi 154007, People’s Republic of China, and Department of Chemistry, The City College of New York, New York, New York 10031 ReceiVed: April 19, 2009; ReVised Manuscript ReceiVed: August 4, 2009
In this paper, a series of silver-deposited TiO2 (Ag-TiO2) nanoparticles (NPs) with a varying content of Ag were prepared by a photoreduction method and were attempted to serve as SERS-active substrates for the first time. SERS signals of 4-MBA molecules adsorbed on Ag-TiO2 NPs were further enhanced considerably relative to those enhancements on pure TiO2 NPs. The surface-deposited Ag on TiO2 can inject additional electrons into molecules adsorbed on the TiO2 surface through the conduction band of TiO2 NPs because of plasmon resonance absorption of Ag under incident visible laser, besides the intrinsic TiO2-to-molecule chargetransfer (CT) contribution. The two contributions mentioned are responsible for the whole SERS intensity of the molecules adsorbed on Ag-TiO2 NPs. This work is valuable in developing nanosized TiO2 used as a promising, nontoxic and biologically compatible SERS-active substrate as well as in studying the CT mechanism between Ag and TiO2 for potential photoelectrochemical applications. Introduction Research on semiconductor-based surface-enhanced Raman scattering (SERS) has attracted increasing attention recently due to its broad application prospects, despite the fact that the enhancement capacity of semiconductor-based SERS substrates is still considerably weaker than that of metals. This undertaking not only develops novel SERS-active substrates, which previously had been restricted to studies on noble metals (Ag, Au, and Cu) and the transition metals (Pt, Pd, Ru, Rh, Fe, Co, and Ni),1-5 but also broadens the applicability of Raman spectroscopy as a SERS technology to investigate the adsorption problems on semiconductor surfaces. Nanoscale titanium dioxide (TiO2) is one of the most widely investigated semiconductor oxide materials in many fields, such as environmental cleanup,6 photocatalysts,7-9 and solar cells, etc.,10-13 due to its excellent performances. Just owing to its important applications in the above fields, increasing interest is focusing on SERS studies based on TiO2 since the high sensitivity and resolution of SERS make it ideal for extracting detailed information as to the nature of molecular interactions with semiconductor particles. These include the adsorption of molecules on the surface of nanosized TiO2 and charge-transfer behaviors between them,14-16 which are very important for a deep understanding of the photochemical and photophysical phenomena on the surface of nanosized TiO2 and for optimizing the performance of the materials. Runge et al.17 studied SERS spectra of viologen dye derivatized on a film of anatase TiO2 by the method of addition of a silver colloid solution. Song et al.18 also detected SERS signals of 4-mercaptopyridine adsorbed on the surface of TiO2 nanofibers coated with Ag nanoparticles. * Corresponding authors. Fax: +86-431-85193421 (B.Z.). E-mail:
[email protected] (B.Z.),
[email protected] (J.R.L.). † Jilin University. ‡ Jiamusi University. § The City College of New York.
However, these SERS enhancements of molecules adsorbed on TiO2 were all ascribed to the contribution of the long-range electromagnetic effect of the surface-modified Ag NPs, instead of the contribution of nanoscale TiO2, which did not availably utilize characteristics of nanoscale TiO2 itself. In a recent publication,16 we have observed direct SERS for 4-mercaptobenzoic acid (4-MBA) adsorbed on TiO2 NPs, which was attributed to the dominant contribution of the TiO2-to-molecule charge-transfer (CT) mechanism. Currently, however, the study of the synergetic contribution of incorporated Ag and TiO2 in Ag-TiO2 nanocomposites to SERS has not been investigated. Accordingly, in this work, Ag-TiO2 nanocomposites with a varying content of Ag were prepared by a photoreduction method and were attempted to employ as SERS-active substrates for the first time for the above-mentioned extensive purposes, and also, we proposed a new CT-induced SERS mechanism involved in the synergetic contribution of incorporated Ag and TiO2 in Ag-TiO2 nanocomposites. Furthermore, the purpose of the present work also exists in the fact that Ag-TiO2 nanoparticle composites have been widely investigated over the past several decades for improving the photochemical and photoelectrochemical performances of TiO2 NPs19-22 and using as a biologically compatible material.23 These performances usually are influenced heavily by the interaction and the CT behavior between Ag NPs and TiO2. Therefore, the explorations of these behaviors are very important for optimizing the performance of the materials. The present SERS study on Ag-TiO2 nanocomposites is expected to be valuable for revealing those behaviors. Experimental Section Chemicals. 4-Mercaptobenzoic acid (4-MBA) was purchased from Acros Organics Chemical Co. and used as received without further purification. The other chemicals were all analytical
10.1021/jp903600r CCC: $40.75 2009 American Chemical Society Published on Web 08/18/2009
Surface-Enhanced Raman Scattering on Ag-TiO2 grade without further purification, too. Triply distilled water was used in all experiments. Sample Preparation. The synthesis of anatase crystal TiO2 NPs in this work is similar to that described in previous papers,16 employing a sol-hydrothermal method. First, a mixed solution of 5 mL of tetrabutyl titanate and 5 mL of anhydrous ethanol was added dropwise into another mixed solution, consisting of 20 mL of anhydrous ethanol, 5 mL of water, and 1 mL of 70% nitric acid, at room temperature under rough stirring to carry out hydrolysis. Subsequently, the yellowish transparent sol was obtained by continuously stirring for 1 h. The as-prepared sol was kept at 160 °C for 6 h in a stainless-steel vessel, then cooled to room temperature. The sol-hydrothermal production was dried at 60 °C for 24 h. Finally, anatase crystal TiO2 NPs were obtained by calcining the sol-hydrothermal production at 450 °C for 2 h. Silver-deposited TiO2 (Ag-TiO2) nanocomposites were prepared by a simple method of photoreduction of silver nitrate (AgNO3) using TiO2 NPs (as photocatalysts) under UV light.24 Briefly, 0.1 g of as-prepared TiO2 NPs was dispersed into 20 mL of AgNO3 aqueous solution with different concentrations (10-4, 10-3, 10-2, and 0.1 M). The suspension was stirred for 15 min in the absence of light to keep the reactive system uniform and adsorption equilibrium and then irradiated with a high-pressure mercury lamp (125 W) for 30 min under magnetic stirring. The resulting Ag-TiO2 nanocomposites were obtained by centrifugal separation, rinsed with purified water several times, and finally dried at room temperature in the dark. TiO2 and Ag-TiO2 NPs surface-modified by 4-MBA molecules were obtained as follows: 20 mg of TiO2 or Ag-TiO2 NPs was dissolved in 10 mL of 4-MBA (1 × 10-3 M) ethanol solution, and the mixture was stirred for 2 h. The precipitate was then centrifuged and rinsed with purified water once more. TiO2 and Ag-TiO2 NPs modified by 4-MBA were obtained. The silver colloid used in this work was prepared by the aqueous reduction of silver nitrate (10-3 M, 200 mL) with trisodium citrate (1%, 4 mL) using a method of Lee and Meisel.25 The SERS spectrum of 4-MBA adsorbed on the Ag colloid was measured in solution. Sample Characterization. The crystal structure of TiO2 and Ag-TiO2 samples was determined by X-ray diffraction using a Siemens D5005 X-ray powder diffractometer with a Cu KR radiation source at 40 kV and 30 mA. The surface morphology of the sample was measured on a Hitachi H-8100 transmission electron microscope (TEM) operated at an acceleration voltage of 200 kV. The element composition and chemical state of samples were analyzed by X-ray photoelectron spectroscopy (XPS) with a VG ESCALAB MK II X-ray photoelectron energy spectrometer. The UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded on a Shimadzu UV-3600 UV-vis spectrophotometer. Raman spectra were obtained by using a Renishaw Raman system model 1000 spectrometer. The 514.5 nm radiation from a 20 mW air-cooled argon ion laser was used as exciting source. Data acquisition was the result of three 30 s accumulations for 4-MBA molecules adsorbed on TiO2 and Ag-TiO2 NPs at room temperature. Replicate measurements of each sample were made at least three times to verify that the spectra were reproduceable. Results and Discussion Measurements of XRD, TEM, and XPS. Figure 1 shows the XRD patterns of pure TiO2 NPs and Ag-TiO2 samples obtained by irradiating different concentration AgNO3 suspensions with UV light. The XRD peaks at 2θ ) 25.4° (101) and
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Figure 1. XRD patterns of pure TiO2 and different Ag-TiO2 nanoparticles.
48° (200) are generally identified as the characteristic diffraction peaks of the anatase crystal phase of TiO2.26 As shown, all Ag-TiO2 samples exhibit the usual pure anatase phase, just like original TiO2, with similar peak intensities and shapes. Even at the highest concentration (0.1 M) of the AgNO3 solution used for depositing Ag, there are no new diffraction peaks related to Ag species appearing, which might be due to the low amount and the amorphous state of Ag.22,27 Furthermore, no shifts in the positions of diffraction peaks are observed, indicating that Ag is deposited on the surface of TiO2 NPs through the formation of Ag-TiO2 nanocomposites. This could be supported by the TEM and XPS measurements. The representative TEM images of pure TiO2 and different Ag-TiO2 samples are demonstrated in Figure 2. Silver deposits with wide size distributions were located on the surface of the individual TiO2 crystallites. The polydisperse and agglomerates of Ag should be attributed to its rapid overgrowth on the original TiO2 nanoparticles in the process of UV light irradiation.24,28 It can be seen from Figure 2 that the size (about 7-8 nm) and shape of the TiO2 crystallites were unchanged after Ag deposition. Moreover, the amount of Ag deposition in the Ag-TiO2 nanocomposites also increased gradually with the increase of the concentration of the AgNO3 solution used for depositing Ag, which could be further confirmed by the UV-vis DRS measurement described below. The TEM results show that the silver particles are strongly anchored onto TiO2, but the XRD results give no information about the silver species. To further identify the elemental composition and chemical state of Ag-TiO2 nanocomposites, XPS measurements were carried out. Figure 3 shows the XPS spectra of TiO2 and Ag-TiO2 samples. It can be seen that the Ti, O, and Ag elements exist on the surface of the Ag-TiO2 samples. The surface-deposited Ag does not induce a change of the chemical states of Ti and O in Ag-TiO2 nanocomposites compared with that of pure TiO2. The Ag 3d spectrum of Ag-TiO2 consists of two individual peaks at 368.2 and 374.3 eV that can be attributed to Ag 3d5/2 and Ag 3d3/2 binding energies, respectively, which indicates that Ag deposited on the surface of TiO2 NPs is in the state of metal Ag(0).27,29,30 In other words, no silver oxide species were found on the surface of TiO2 NPs. This is also further supported by O 1s spectra (Figure 3C), in which the deposition of Ag on TiO2 does not bring any change of O 1s binding energy compared to that of pure TiO2 (529.7 eV), illustrating the absence of any silver oxide species. Measurements of UV-vis DRS. UV-vis spectra were carried out to investigate the effect of deposited Ag on optical properties of unmodified samples and those surface-modified by 4-MBA molecules. Figure 4 shows the UV-vis DRS spectra
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Figure 2. Representative TEM images of TiO2 (A) and different Ag-TiO2 samples obtained by irradiating 10-4 M (B), 10-3 M (C), 10-2 M (D), and 0.1 M (E) AgNO3 suspensions with UV light.
Figure 3. XPS spectra of TiO2 and Ag-TiO2 samples: (A) Ti 2p, (B) Ag 3d, and (C) O 1s.
of pure TiO2 and different Ag-TiO2 NPs. All samples exhibit a wide optical absorption in the range below 410 nm, which can be attributed to the band-band electron transition of the TiO2 nanocrystals according to its band gap energy of ca. 3.2 eV. The tailing at 410-490 nm corresponds to the optical absorption related to the surface state of TiO2 NPs. Compared to bare TiO2, the optical absorption related to the surface state of TiO2 is enhanced in the spectra of Ag-TiO2 NPs, and a broad absorption covering the range of 490-800 nm with a summit
at about 540 nm appears, which should be ascribed to the localized surface plasma resonance (LSPR) effect of surfacedeposited Ag(0). The LSPR absorption is relatively weak in the range of low concentration of the AgNO3 aqueous solution used for deposition. With the increase in concentration of the AgNO3 aqueous solution, the LSPR absorbance at around 540 nm increases, which is due to the increase of the Ag loading amount and aggregation degree. This is consistent with TEM characterization results. Figure 5 shows UV-vis DRS spectra
Surface-Enhanced Raman Scattering on Ag-TiO2
Figure 4. UV-vis DRS spectra of pure TiO2 and different Ag-TiO2 NPs.
Figure 5. UV-vis DRS spectra of TiO2 and Ag-TiO2 surfacemodified by 4-MBA molecules as well as unmodified TiO2 and the mode (inset) of the charge transfer between the adsorbed molecule and TiO2 that contributed to SERS.
of TiO2 and Ag-TiO2 surface-modified by 4-MBA molecules as well as unmodified TiO2. On comparison of 4-MBA-modified TiO2 with unmodified TiO2, the photoabsorption threshold of the TiO2 band-band transition for the 4-MBA-modified TiO2 sample exhibits a slight blue shift, and the optical absorption (about 410-490 nm) corresponding to the surface state of TiO2 is also enhanced. Moreover, its photoresponse is extended to about 530 nm. These changes can be attributed the interaction between the adsorbed molecules and the TiO2 substrate due to the formation of a charge-transfer (CT) complex, as shown in Figure 5 (inset).15,16,31 Similar changes of optical adsorption have been previously reported by Rajh et al.32 in CT complexes of enediol molecules with TiO2 NPs. After adsorption of 4-MBA on Ag-TiO2 samples, however, it is more interesting that the optical absorption (about 410-530 nm) can be further enhanced remarkably, and with the increase in the amount of Ag loading, the absorbance increases, indicating that deposited Ag can improve optical absorption, corresponding to the CT complex of the molecule with TiO2 NPs as well as relating to the surface state of TiO2 NPs. This can foreshadow a consequent higher SERS enhancement for the adsorbed molecules in the Ag-TiO2 system due to the contribution of deposited Ag. SERS Spectra of 4-MBA on Different TiO2 Substrates and Enhanced Mechanism Analysis. Figure 6 shows the SERS spectra of 4-MBA adsorbed on pure TiO2, Ag-TiO2 NPs, and the Ag colloid. Raman peaks (1594, 1148, 1182, and 1078 cm-1) are characteristic SERS signals of the 4-MBA molecule on the TiO2 substrates. The strong bands at about 1594 and 1078 cm-1 are assigned to ν8a (a1) and ν12 (a1) aromatic ring characteristic
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Figure 6. SERS spectra of 4-MBA adsorbed on pure TiO2, Ag-TiO2 NPs, and the Ag colloid.
vibrations, respectively. Other weak bands at about 1148 (ν15, b2) and 1182 cm-1(ν9, a1) are attributed to the C-H deformation modes. They are consistent with those previously reported for 4-MBA adsorbed onto ZnO nanocrystals33 as well as TiO2,16 which has been attributed to the dominant contribution of the TiO2-to-molecule CT mechanism related to the surface-state energy level (ESS) of TiO2 NPs (as shown in Figure 5, inset). Comparing curves b-d with curve a in Figure 6, it can be clearly seen that the frequencies of all Raman bands do not display any changes as 4-MBA adsorbed on the Ag-TiO2 nanocomposites. This illustrates that the SERS signals of 4-MBA adsorbed on the Ag-TiO2 (curves b-d) also resulted from the TiO2-to-molecule CT contribution, just like pure TiO2 (curve a), instead of the direct contribution from surfacedeposited Ag due to the low amount of loading and aggregation degree as well as the small size of Ag on TiO2 (as shown in Figure 2) that cannot generate sufficiently strong LSPR to contribute to SERS (such low Ag loading amount and aggregation degree as well as small Ag particle size do not induce SERS enhancement from the electromagnetic contribution). This can also be confirmed by the fact that, when the concentration of the AgNO3 aqueous solution used for Ag deposition increased up to 0.1 M, the frequency of the Raman band assigned to the ν8a aromatic ring characteristic vibration shifts to 1588 cm-1 (curve e), very close to that (1587 cm-1) on the Ag colloid (curve f), which can be ascribed to the contribution of electromagnetic (EM) enhancement of surface-deposited Ag(0) due to the increase of the loading amount and aggregation degree of Ag on TiO2, besides the TiO2-to-molecule CT contribution. This can be more clearly indicated by the result of the multipeak fit using the Lorentzian line shape in Figure 7. It is more interesting in the present work, however, that the SERS signals of 4-MBA adsorbed on the Ag-TiO2 nanocomposites (curves b-d) exhibit a higher enhancement as compared with the native enhancement of 4-MBA adsorbed on the pure TiO2 NPs. With the increase of the concentration of the AgNO3 aqueous solution used for Ag deposition, that is, more than 10-4 M, it is well-known that the fraction of surface coverage of TiO2 NPs should be increased and the corresponding adsorption quantity of 4-MBA adsorbed directly on TiO2 NPs should be decreased. However, this does not result in a decrease in intensity of 4-MBA SERS signals contributed from the TiO2to-molecule CT in Ag-TiO2 nanocomposites, and higher SERS intensity still remains even when the concentration of the AgNO3 aqueous solution used for Ag deposition is as high as 10-2 M. However, when the concentration of the AgNO3 solution continues to increase to 0.1 M, the intensity of 4-MBA SERS signals from the contribution of the TiO2-to-molecule CT
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Figure 7. Magnified SERS spectra of 4-MBA adsorbed on pure TiO2, Ag-TiO2 NPs, and the Ag colloid between 1700 and 1450 cm-1. (The dashed curve is the result of the multipeak fit of curve e using the Lorentzian line shape.)
decreases due to the excessive increase of the fraction of surface coverage of TiO2 NPs, and a new 4-MBA SERS signal derived from the EM contribution of surface-deposited Ag(0) appears simultaneously. These indicate that an appropriate amount of surface-deposited Ag can obviously enhance the SERS effect of TiO2 NPs to surface-adsorbed molecules induced by the TiO2to-molecule CT. It is obvious that the observed remarkable Raman enhancement of 4-MBA on the Ag-TiO2 nanocomposites, higher than that on pure TiO2 NPs, must be related to the incorporated Ag and the interaction between Ag and TiO2 NPs. It is well-known that the work function of Ag is larger than that of TiO2; when a Ag particle is in contact with an n-type semiconductor particle such as TiO2, the charge distribution is readjusted for equilibration of the Fermi level between the two particles, which results in the elevation of the Ag Fermi level and the formation of a Schottky barrier (depletion layer) at the junction between the two materials.34,35 Recently, influences of surface-deposited Ag on photoelectrochemical properties of Ag-TiO2 composites were investigated by Zhang and his coworkers.36 During the photocurrent spectra measurements of Ag(0)-TiO2, it was demonstrated that Ag(0) was photoexcited because of plasma resonance in the visible light region, and charge separation was accomplished by the transport of photoexcited electrons from Ag(0) to TiO2 with the simultaneous formation of Ag(I). A similar phenomenon of photoexcitation of Ag deposited in TiO2 NPs because of plasma resonance under the visible light, resulting in multicolor photochromism of Ag-TiO2 films, has also been reported by Fujishma37 and Tatsuma.38 Accordingly, a possible mechanism is suggested to interpret the observed higher enhancement of molecules on Ag-TiO2 NPs than on pure TiO2 NPs in this work (as shown in Figure 8): Deposited Ag on the TiO2 NPs’ surface was excited by incident visible laser, and photoexcited electrons were injected into the conduction band of TiO2 connecting with Ag and/or subsequently relaxed vibratingly onto the ESS of TiO2 and then transferred to the LUMO orbits of the molecules adsorbed on TiO2 NPs, which can enhance photoabsorption of the CT complex, as mentioned in UV-vis DRS. These additional electrons endow TiO2 with a considerable SERS effect to adsorbed molecules, besides the intrinsic TiO2-to-molecule CT. Moreover, it should also be mentioned that the formed Schottky barrier (depletion layer) in the Ag-TiO2 system may be favorable to the transport of photoexcited electrons from Ag to TiO2 according to the point of energy. However, an excess amount of Ag deposition on TiO2 NPs can bring on lower SERS intensity of the molecules adsorbed on TiO2, which is most likely due to the fact that, as the concentration of silver becomes
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Figure 8. Sketch map of the proposed mechanism that contributed to SERS in the Ag-TiO2 nanocomposites.
large, it results in one of the following effects: (i) it acts as a barrier preventing light absorption by TiO2, (ii) it prevents the molecules from contacting the TiO2 surface due to the larger fraction of surface coverage of TiO2 NPs, or (iii) the excess amount of silver itself may become a significant center for electron-hole recombination,21 resulting in lower efficiency of the TiO2-to-molecule CT. Conclusions In summary, TiO2-to-molecule CT-induced SERS for the molecules adsorbed on Ag-TiO2 NPs can be significantly improved by incorporating an appropriate amount of Ag on TiO2 NPs through a photoreduction method. The observed considerable SERS enhancement on Ag-TiO2 nanocomposites compared with that on pure TiO2 NPs can be attributed to the incorporation of silver, which can bring the additional electrons into the molecules adsorbed on the TiO2 surface through the conduction band of TiO2 NPs because of plasmon resonance absorption of Ag under incident visible laser, besides the intrinsic TiO2-to-molecule CT contribution. These are responsible for the whole SERS intensity of the molecules adsorbed on TiO2 in Ag-TiO2 nanocomposites. An excess amount of Ag deposition on TiO2 is not favorable to the TiO2-to-molecule CT and the resulting SERS for the adsorbed molecules. This work has significance not only in developing nanosized TiO2 used as a promising, nontoxic and biologically compatible SERS-active substrate but also in the study of the CT mechanism between Ag and TiO2 in potential photoelectrochemical applications. Acknowledgment. The research was supported by the National Natural Science Foundation (Grant Nos. 20773044 and 20873050) of the People’s Republic of China, the 111 project (B06009), the Scientific and Technical Research Project of Education Department of Heilongjiang Province (Grant No. 11541360), and the Project for Excellent Youth Scholars of Higher Education of Heilongjiang Province of China (1154G13). References and Notes (1) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 2002, 106, 9463– 9483. (2) Ren, B.; Lin, X. F.; Yang, Z. L.; Liu, G. K.; Aroca, R. F.; Mao, B. W.; Tian, Z. Q. J. Am. Chem. Soc. 2003, 125, 9598–9599. (3) Dieringer, J. A.; McFarland, A. D.; Shah, N. C.; Stuart, D. A.; Whitney, A. V.; Yonzon, C. R.; Young, M. A.; Zhang, X.; Van Duyne, R. P. Faraday Discuss. 2006, 132, 9–26.
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