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Graphene Sheets Grafted Ag@AgCl Hybrid with Enhanced Plasmonic Photocatalytic Activity under Visible Light Hui Zhang, Xinfei Fan, Xie Quan,* Shuo Chen, and Hongtao Yu Key Laboratory of Industrial Ecology and Environment Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China
bS Supporting Information ABSTRACT: Interfacing photocatalyst with graphene sheet gives rise to an extraordinary modification to the properties of the resulting hybrids. Graphene sheet grafted Ag@AgCl composite is fabricated by photoreducing AgCl/graphene oxide (GO) hybrids prepared by deposition-precipitation method. The microscopic analysis and Raman scattering reveal the direct interface between Ag nanocrystal and graphene sheet, which manipulates the electronic structures of Ag@AgCl. UVvis absorption spectra of Ag@AgCl/reduced GO (RGO) hybrids exhibit strong absorbance in the visible region due to the surface plasmon resonance (SPR) absorption of Ag nanocrystal. In situ assembled Ag@AgCl/RGO plasmonic photocatalyst exhibits remarkable photocatalytic activity. Compared with bare Ag@AgCl nanoparticle, a 4-fold enhancement in the photodegradation rate toward rhodamine B is observed over Ag@AgCl/RGO hybrids under visible light irradiation. The large enhancement of photocatalytic activity was attributed to the effective charge transfer from plasmon-excited Ag nanocrystal to RGO, which suppress the charge recombination during photocatalytic process. This work could provide new insights into the fabrication of high performance plasmonic photocatalyst and facilitate their practical application in environmental issues.
’ INTRODUCTION The growing concerns about energy and environmental problems have stimulated intense research on solar energy utilization.13 Artificial photosynthesis systems, for example, photoelectrochemical water splitting, photocatalysis, and photovoltaic cells, are highly desirable because of abundant sunlight resource and less of carbon footprint. Various strategies are explored to broaden photocatalysts response to visible light, such as elemental substituted UV-active photocatalysts,4,5 dye-/nanocrystalsensitized heterojunction catalyst,69 and composite metal oxides visible photocatalysts.10,11 However, absorption toward visible light and photocatalytic activity are still “bottleneck” for these photocatalyst to meet the requirement of practical application. Recently, the surface plasmon resonance of metal nanocrystal (such as Au and Ag) has found to be excited under visible light and exhibits photocatalytic activity in the whole solar spectral region with good stability.1215 Plasmon-induced Ag@AgX (Cl, Br, I) photocatalyst has been synthesized by ion-exchange or direct precipitation reaction followed by reduction.1619 Nevertheless, the manipulation of photocatalyst structure and morphology is still difficult. Several micrometers sized Ag@AgX particle cause the plasmon-induced electronhole pairs recombined before they arrive at the photocatalyst surface. The high charge carrier recombination rate results in the loss in the efficiency of plasmonic photocatalytic system. In exploring approaches to modulate the photon and carrier transport in photocatalysis system for high photoactivity, graphener 2011 American Chemical Society
based nanocomposite system has been harnessed recently in optical, electronic, and catalytic devices2022 not just for its high carrier mobility, but also for sp2 hybridized two-dimensional carbon sheet as an atomic thickness substrate for functional nanomaterials grown.2325 Graphene oxide (GO) is an attractive precursor to graphene after its reduction. The solution-dispersibility of GO could make them participate in building block assembly process to form new hybrids. Reduced graphene oxide (RGO) in the hybrids could also extract the excited charge carriers to modulate photocatalytic activity. Moreover, recent progress in graphene sheet supported metal nanocrystal system revealed that graphene would modulate the electronic structure of metal nanocrystal and further modify its electrocatalytic activities.26,27 Therefore, it is expected that graphene sheet grafted Ag@AgX nanoparticle potentially provide novel optoelectronic and plasmonic photocatalytic properties. However, no work related to graphene based plasmonic photocatalyst has been reported. Hence, the development of graphene based plasmon-induced photocatalysis system is of fundamental and practical significance. Herein, graphene sheet grafted Ag@AgCl composite is designed for high efficiency photocatalytic application. A facile method for growing Ag@AgCl nanoparticles on RGO sheet is Received: January 24, 2011 Accepted: May 19, 2011 Revised: May 4, 2011 Published: June 10, 2011 5731
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Scheme 1. Schematic Representation of Graphene Sheet Grafted Ag@AgCl Hybrids
developed by photoreducing AgCl/GO hybrids prepared by deposition-precipitation method. RGO sheets function as an in-solution substrate for Ag@AgCl nanoparticles in situ growth. The obtained Ag@AgCl/RGO photocatalyst exhibits remarkably enhanced plasmonic photocatalytic activity toward organic contaminant under visible light, which is ascribed to the synergistic effect of strong surface plasmonic resonance (SPR) of Ag nanocrystal and a graphene sheet network for fast carrier transfer between the active materials in the hybrids. Furthermore, the prepared Ag@AgCl/RGO photocatalyst displays good cycling stability under both visible light and simulated sunlight.
’ EXPERIMENTAL SECTION Ag@AgCl/RGO Photocatalyst Preparation. GO was prepared
by a modified Hummers et al. method.28,29 The Ag@AgCl/RGO composites were prepared by deposition-precipitation reaction followed by photoreduction method. In a typical synthesis procedure, AgNO3 (0.21 g) in ammonia (2.3 mL, 25 wt % NH3) was diluted to 50 mL by deionized water. GO aqueous solution (3.5 mL, with concentration of 0.8 g L1) was added to the above solution and stirred for 0.5 h. Next, concentrated HCl (1.5 mL) was added to the mixture solution and intensively stirred for 24 h to form homogeneous suspension. The above solution was mixed with 50 mL ethanol and irradiated with filtered light (λ > 400 nm) from a 300 W Xe arc lamp. Then the resulting precipitate was centrifuged, washed with deionized water and dried in air. Following this procedure, different amounts of graphene oxide were added to prepare Ag@AgCl/RGO photocatalyst. In the control experiment, Ag@AgCl was prepared by a similar method without GO. Structural and Optical Properties Characterization. X-ray diffractometer (XRD) patterns were obtained on a Shimadzu LabX XRD-6000. The transmission electron microscopy (TEM) was performed on an FEI-Tecnai G2 20 and scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800. X-rays photoelectron spectroscopy (XPS) was performed with a VG ESCALAB 250 spectrometer using a nonmonochromatized Al KR X-ray source (1486.6 eV). The Raman spectra was carried out using a Renishaw Micro-Raman system 2000 spectrometer with HeNe laser excitation (wavelength 623.8 nm). The UVvis absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer. Transform infrared spectrophotometer (FTIR) was performed using IR Prestige-21 instrument. Evaluation of Photocatalytic Activity of Ag@AgCl/RGO Hybrids. The photocatalytic performance of the samples was evaluated by the degradation of rhodamine B (RhB), which were carried out with the photocatalyst (0.05 g) suspended in RhB solution (50 mL, with concentration of 10 mg L1). The light source was a 350 W Xe arc lamp equipped with a UV cutoff filter
(λ > 400 nm) and the illumination intensity was 100 mW cm2. Prior to irradiation, the suspension was stirred in the dark for 30 min to ensure that the RhB was adsorbed to saturation on the surface of catalyst. At certain time intervals of visible light irradiation, reaction solution was collected and centrifuged to remove photocatalyst particles for analysis. The concentration of RhB dye was monitored by UVvis spectroscopy through recording the absorbance of the characteristic peak of RhB at 554 nm. The stability of Ag@AgCl/RGO hybrids under sunlight was evaluated by the degradation of Rhodamine B (RhB) under simulated AM1.5 sunlight with one-sun irradiation (Oriel, 300W model, 91160).
’ RESULTS AND DISCUSSION Structure and Properties Characterization of Ag@AgCl/ RGO Hybrid. The strategy for the fabrication of graphene sheet
grafted Ag@AgCl hybrids is shown in Scheme 1, by a two-step procedure. Chemically derived GO, with a thickness of ∼1.2 nm and size distribution on the order of micrometers (Figure 1a), carries abundant functional groups such as epoxide, hydroxyl, carbonyl, and carboxyl on the sheet surface.30 In GO suspension, these functional groups are leveraged to stabilize Ag ion and subsequently grown to AgCl particles via the seeding growth mechanism. Under the irradiation, Agþ on the surface of the AgCl/GO composites particle is reduced to Ag0 species, simultaneously with the reduction of the adjacent GO to RGO. X-ray diffraction (XRD) patterns of Ag@AgCl/RGO hybrids (Supporting Information (SI) Figure S1) show little diffraction peaks of Ag0 species, which may attribute to the ultrafine particle size and its high dispersion on the hybrid surface. Typical morphology of the prepared Ag@AgCl/RGO hybrids is shown in Figure 1c. Compared with the Ag@AgCl particle prepared without addition of GO (Figure 1b), Ag@AgCl/RGO hybrids exhibit more uniform and smaller size with diameter in the range of 0.41.1 μm. SEM images of Ag@AgCl particles formed with different contents of GO were shown in SI Figure S2. It is found that the size of the hybrids gradually decreases with the GO contents increased from 0.22 wt % to 4.32 wt %, though the effect of RGO contents on the size of Ag@AgCl particles become weaker at a high RGO content. This phenomenon may arise from the surfactant effect of GO for controlled growth of AgCl particle. GO involves the AgCl growth process, which affect the morphology and size of the resultant particles. The wrinkles appearing on the surface of the particles could illustrate the existence of graphene sheet. TEM image (Figure 1d) of Ag@AgCl/RGO hybrids further shows evidence of Ag@AgCl particle growth on the surface of the RGO. Size distribution of the Ag nanocrystal is difficult to measure because AgCl is decomposed under the high 5732
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Figure 3. (a) UVvis diffuse-reflectance spectra of the samples. (b) Raman spectra for AgCl/GO and Ag@AgCl/RGO hybrids.
Figure 1. (a) Typical AFM images of GO sheet and their height profiles. (b) SEM image of the as-prepared Ag@AgCl particle. (c) SEM and (d) TEM images of Ag@AgCl/RGO hybrids.
Figure 2. C 1s XPS spectra of (a) AgCl/GO and (b) Ag@AgCl/RGO hybrids. Four types of carbon are observed at 284.5 eV (C—C), 286.6 eV (C—O), 287.8 eV (CdO), and 289.0 eV (OdC—O).
energy electron beam. Furthermore, a successful reduction of GO to RGO by light irradiation is verified by X-ray photoelectron spectroscopic (XPS) data, according to the diminished C—O, CdO, C(O)O group contents31 and the increased C/O atomic ratio (from 2.2 to 4.4) after light irradiation (Figure 2). The IR spectrum of RGO also shows that the typical absorption bands of oxy-functional groups, such as νOH at 3403 and 1396 cm1, νCdO at 1728 cm1, νCO at 1061 cm1 decreased significantly (SI Figure S3), indicating the GO has been reduced to RGO. The change in the absorption spectra of samples after the irradiation is shown in Figure 3a. In AgCl/GO samples, the absorption edge of AgCl is not shifted, but the absorption toward the visible light is enhanced. This suggests that GO sheets are grafted onto the surface of AgCl nanoparticle. Under the visible light irradiation process, the reduced Ag0 species nucleate on AgCl nanoparticle surface and form dispersed Ag nanograins. The obtained Ag@AgCl and Ag@AgCl/RGO samples exhibited strong absorbance in the visible region due to the strong surface plasmon resonance (SPR) absorption of Ag nanocrystal.1619 Accordingly, the suspension color was also changed from light brown to lilac gray during the irradiation process, which indicates the generation of Ag nanocrystal.
The structural change and the interaction effect of Ag@AgCl particle with the RGO surface after photoreduction were probed by Raman spectra analysis of AgCl/GO and Ag@AgCl/RGO hybrids (Figure 3b). Two feature peaks of GO are observed, where G mode arises from the first order scattering of the E2g phonon of sp2 C atoms (at ∼1340 cm1)32 and D mode is a defect peak due to intervalley scattering (at ∼1590 cm1). Interestingly, these bands intensity is significantly enhanced approximately 4.2 fold after photoreduction. The observed Raman signal enhancement is similar to the surface-enhanced Raman scattering (SERS) effect previously reported.33,34 The strong Raman enhancement in the present system can be attributed to SPR field induced by Ag nanocrystal in Ag@AgCl/RGO hybrid. It reveals the Ag nanocrystals are closely adjacent to graphene sheet, which could greatly influence electronic structure in plasmonic photocatalytic system. In additional, an increased D/G intensity ratio is observed compared to that of GO, indicating more graphene domains formed on RGO sheet by removal of oxy-functional groups on GO and a partially ordered crystal structure of RGO sheet. This enhances the conductivity of RGO sheet and thus facilitating the charge transfer. The formation of Ag@AgCl/RGO hybrid by photoreduction can be explained based on the following proposed mechanism. The first step involves the formation of AgCl particle on GO via electrostatic force between Agþ and negatively charged groups on GO. Then, under visible light irradiation, AgCl would decompose and generate Ag0 species by photochemical reaction. The generated Ag atoms tend to aggregate to form Ag nanocrystal and deposit on AgCl particle surface. Subsequently, the generated Ag nanocrystals are excited by visible light. The plasmon-induced electrons in Ag nanocrystal are injected into the adjacent GO sheets and reduce certain functional groups, while the holes are scavenged by ethanol. A similar photoreduction mechanism has been observed in the TiO2/GO system.35 During this process, the generation of Ag nanocrystal and the photoreduction of GO occur simultaneously. With prolonged irradiation time, more Ag nanocrystals form and further promote the reduction of GO. Thus, the direct interaction between Ag nanocrystal and RGO sheet occurs, which is consistent with the Raman spectra results. Such a procedure explores a photoreduction method to fabricate graphene based plasmonic photocatalytic system. Evaluation of Photocatalytic Activity. To investigate the plasmonic photocatalytic activity of the Ag@AgCl/RGO hybrids, the photodegradation of RhB dye was carried out under visible light irradiation. The normalized temporal concentration changes of RhB during the photodegradation process are shown in Figure 4. It is noted here that no more than 20% of RhB molecules are adsorbed on the Ag@AgCl/RGO hybrids when reaching the equilibrium adsorption state under the dark reaction. Under visible light irradiation, a sudden drop of the RhB 5733
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Figure 4. Variation of RhB concentration as a function of irradiation time (with the time of light on set as 0) under visible light (λ > 400 nm) with different photocatalysts. C is the RhB concentration at time t, and C0 that in the initial solution.
concentration occurs over the Ag@AgCl/RGO hybrids, even with a little amount of RGO (0.22 wt %) in the photocatalyst, which was apparently caused by photocatalytic oxidation process. An approximate 95% of the dyes is removed over Ag@AgCl/ RGO hybrids within 16 min, whereas only 55% of the dyes are degraded for bare Ag@AgCl catalyst during the same time duration. It demonstrates that graphene sheet grafted Ag@AgCl hybrids have much higher photocatalytic activity than bare Ag@AgCl under visible light. An exponential decay of the RhB concentration with the irradiation time is evident, indicating the photodegradation process follows a pseudo-first-order reaction. For Ag@AgCl/RGO hybrids photocatalyst with RGO contents 0.22 wt %, 0.44 wt %, and 1.56 wt %, the corresponding degradation rate constant (kRhB) are estimated to be 0.233 min1, 0.273 min1, and 0.306 min1, respectively, which is up to 4-fold faster than that over the bare Ag@AgCl (about 0.060 min1). The photocatalytic activity of bare Ag@AgCl prepared by the reported ion-exchange method16 with a similar size to Ag@AgCl/RGO hybrids was performed. Although the smaller-sized bare Ag@AgCl catalysts have enhanced photocatalytic efficiency, kRhB of smaller-sized bare Ag@AgCl (0.083 min1) is still much slower than that of Ag@AgCl/RGO hybrids, suggesting the great enhanced photocatalytic activity of Ag@AgCl/RGO hybrids was ascribed to the introduction of graphene. In the Ag@AgCl/RGO system, although Ag@AgCl/ RGO (4.32 wt %) exhibits a similar size to Ag@AgCl/RGO (1.56 wt %) hybrids, its photodegradation rate (0.193 min1) is much lower than that of Ag@AgCl/RGO (1.56 wt %) hybrids (0.306 min1). This indicates that the major factor to enhanced photodegradation efficiency is not the size effect, but the efficient separation of photogenerated charge carriers. The evolution absorption spectra of RhB solution during irradiation process (SI Figure S4) reveal different degradation pathways over Ag@AgCl and Ag@AgCl/RGO. During the irradiation, over bare Ag@AgCl, the characteristic absorption peak of RhB at 554 nm decreased gradually, indicating the cleavage of the conjugated chromophore structure of RhB, which mainly caused by reactive oxygen species (ROS) in solution, as reported by Zhao et al.36 With the presence of Ag@AgCl/RGO hybrids, a gradually hypsochromic shift of maximum absorbance wavelength of RhB occurs, corresponding to a stepwise N-deethylation of RhB to yield mono-, di-, tri-, tetra-deethylated rhodamine species during the photodegradation process. This N-deethylation reaction of RhB was usually observed as a surface reaction induced by ROS on catalyst surface.36,37 Although the exact
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Figure 5. Degradation kinetics of RhB over Ag@AgCl/RGO hybrids (1.56 wt % RGO) photocatalyst under visible light (λ > 400 nm) for five successive photocatalytic experiments.
reason for this difference is not clear, a possible explanation is the adsorbed RhB molecule stand by directly binding its two diethylamino groups with the catalyst surface, as observed by Bai et al. through a scanning tunneling microscopy method.37 This configuration makes ROS on catalyst surface attack diethylamino groups of RhB easier than that on conjugated chromophore structure. For Ag@AgCl/RGO hybrids, the significantly strengthened interaction with RhB induced a promoted N-deethylation reaction whereas for Ag@AgCl, ROS in solution govern the photodegradation reaction. In order to eliminate self-photosensitized effect, the photodegradation of 2,4-dichlorophenol (2,4-DCP) over Ag@AgCl/ RGO composite under visible light irradiation is performed (SI Figure S5). A greatly enhanced photocatalytic activity toward 2,4-DCP is also observed due to the existence of RGO, which is similar to that of RhB. Furthermore, an increasing photocatalytic activity toward 2,4-DCP is also displayed with increased RGO content, suggesting the contribution of RGO to the activity of the resultant plasmonic photocatalyst. The stability of plasmonic photocatalyst Ag@AgCl/RGO hybrids was further investigated by recycling the photocatalyst for RhB degradation under both visible light (Figure 5) and simulated sunlight (SI Figure S6). The photocatalytic activity does not significantly decrease during the five consecutive recycle experiments. It is interesting to note that the simulated sunlight could drive enhanced efficiency toward RhB over Ag@AgCl/RGO hybrids than that under visible light, which might be ascribed to the broad-spectra of solar light, especially included UV light. Importantly, the plasmonic photocatalytic activity of Ag@AgCl/ RGO hybrids could be recovered well during the successive experiments. The XRD pattern of the Ag@AgCl/RGO hybrids after recycle experiments exhibits almost identical character to the as-prepared sample (SI Figure S1). The typical SEM images of Ag@AgCl/RGO hybrids after photocatalytic reaction clearly showed that the morphology keeps almost same to the freshly prepared sample (SI Figure S7). These results indicate Ag@AgCl/ RGO hybrids are stable under solar light, which are promising in practical photocatalysis application. Photocatalytic Mechanism. Based on the physical and structure characterization, the enhanced plasmonic photocatalytic activity of Ag@AgCl/RGO hybrids should be ascribed to the efficient charge separation and transfer. When subjected to visible light irradiation, both Ag/AgCl and Ag@AgCl/RGO hybrids are excited due to the localized SPR of Ag nanocrystal and generate electronhole pairs in Ag nanocrystal. Because the surface of AgCl particles is usually terminated by Cl ion and therefore negatively charged, free electrons in metallic Ag nanoparticle 5734
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due to enhanced charge separation and transportation from plasmon-excited Ag nanocrystal. With the unique structure, Ag@AgCl/RGO hybrid shows up to 4-fold enhancement in photodegradation rate than bare Ag@AgCl toward RhB under visible light irradiation. This study will motivate new developments in plasmonic photocatalyst methodology and promote their practical application in environmental issues and photovoltaic cell.
’ ASSOCIATED CONTENT
bS Figure 6. Schematic drawing of the photocatalytic induction process in the Ag@AgCl/RGO hybrids.
deposited on AgCl surface are polarized.16,19 Under polarization field provided by AgCl core, the plasmon-excited electrons move and accumulate on the surface of Ag nanocrystal, while holes to the surface of AgCl nanoparticle and oxidize RhB molecules. Nevertheless, the limitation imposed on the electron transport by Ag@AgCl structure hinders the progress in achieving higher carrier separation efficiency, and then higher photocatalytic efficiency. In graphene sheet grafted Ag@AgCl system, the electronic structure of Ag nanocrystal may be modulated by the interaction with graphene sheet.26,27 Under visible light irradiation, the plasmon-induced electrons of Ag nanocrystal transfer quickly to RGO by interface between Ag nanocrystal and RGO sheet, thus inhibiting the charge recombination and promoting the photocatalytic activity (Figure 6). Moreover, the excellent electronmobility of graphene sheet increases the charge transport rate and achieves enhanced charge separation subsequently accomplished. Indeed, the efficient electron transfer from the plasmon-induced Ag nanocrystal to RGO sheet also sustains the stability of Ag@AgCl/GO hybrids by keeping the plasmon-excited electrons away from AgCl, which avoids the electron being trapped by the Agþ in AgCl and thus inhibits the decomposition of AgCl during the photocatalytic process. Therefore, the stability and the high photocatalytic activity of Ag@AgCl/RGO hybrids are ensured. The enhanced charge separation ability of Ag@AgCl/ RGO system is demonstrated by the increased photocatalytic rate of the hybrids along with the increase of RGO content, which is attributed to the finite Ag nanocrystal/RGO interface for the charge transfer at low RGO content. Nevertheless, when the RGO content further increase to 4.32 wt %, the activity is suppressed (SI Figure S8) because the excess RGO could hinder the light absorption and cover up the active sites. In addition to the enhanced charge separation, the photocatalytic reaction also involves the transportation of contaminant molecule over catalyst surface. Because of the giant π-conjugation between RhB molecule and graphene domains, RGO sheet would extract RhB molecules from solution and then concentrate them near the catalyst surface. Previous reports have shown the catalyst on adsorbent supports could concentrate the pollutant.38 Under light irradiation, the concentrated contaminant molecules environment over catalyst surface speed them react with photogenerated active species, which promotes the photocatalytic degradation toward RhB molecules. In summary, graphene sheet grafted Ag@AgCl plasmonic photocatalyst with high activity have been successfully synthesized through a precipitation reaction followed by photoreduction. The assembled material consisting of Ag@AgCl nanoparticle anchored graphene sheet network exhibits excellent photocatalytic activity
Supporting Information. XRD, SEM, FT-IR spectra characterizations of graphene based Ag@AgCl composites; the photocatalytic activity of Ag@AgCl/RGO hybrid toward RhB and 2,4-DCP, the repeated photocatalytic experiments. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ86-411-84706140; fax: þ86-411-84706263; e-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Nature Science Foundation of China (No.20837001), National Basic Research Program of China (2011CB936002), and Program for Changjiang Scholars and Innovative Research Team in University (IRT0813). ’ REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37–38. (2) Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A. High-rate solar photocatalytic conversion of CO2 and water vapor to hydrocarbon fuels. Nano Lett. 2009, 9, 731–737. (3) Chiba, Y.; Islam, A.; Watanade, Y.; Komiya, R.; Koide, N.; Han, L. Dye-sensitized solar cells with conversion efficiency of 11.1%. Jpn. J. Appl. Phys. 2006, 45, L638–L640. (4) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visiblelight photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269–271. (5) Sakthivel, S.; Kisch, H. Daylight photocatalysis by carbonmodified titanium dioxide. Angew. Chem., Int. Ed. 2003, 42, 4908–4911. (6) Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. B.; Lee, J. S. Effects of surface anchoring groups (carboxylate vs phosphonate) in rutheniumcomplex-sensitized TiO2 on visible light reactivity in aqueous suspensions. J. Phys. Chem. B 2004, 108, 14093–14101. (7) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. Quantum dot solar cells. Harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films. J. Am. Chem. Soc. 2006, 128, 2385– 2393. (8) Lee, Y. L.; Lo, Y. S. Highly efficient quantum-dot-sensitized solar cell based on co-sensitization of CdS/CdSe. Adv. Funct. Mater. 2009, 19, 604–609. (9) Tada, H.; Mitsui, T.; Kiyonaga, T.; Akita, T.; Tanaka, K. Allsolid-state Z-scheme in CdS-Au-TiO2 three-component nanojunction system. Nat. Mater. 2006, 5, 782–786. (10) Amano, F.; Yamakata, A.; Nogami, K.; Osawa, M.; Ohtani, B. Visible light responsive pristine metal oxide photocatalyst: Enhancement of activity by crystallization under hydrothermal treatment. J. Am. Chem. Soc. 2008, 130, 17650–17651. (11) Kim, H. G.; Hwang, D. W.; Lee, J. S. An undoped, single-phase oxide photocatalyst working under visible light. J. Am. Chem. Soc. 2004, 126, 8912–8913. 5735
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