ARTICLE pubs.acs.org/JPCC
Solar Photoconversion Using Graphene/TiO2 Composites: Nanographene Shell on TiO2 Core versus TiO2 Nanoparticles on Graphene Sheet Hyoung-il Kim, Gun-hee Moon, Damian Monllor-Satoca, Yiseul Park, and Wonyong Choi* School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea
bS Supporting Information ABSTRACT: Size controlled nanographene oxides (NGOs; 320 nm), and the photoelectrochemical behaviors were characterized using the electrode coated with the composite photocatalyst. The rates of H2 production and photocurrent generation were higher with r-NGOT than r-LGOT, which indicates that the presence of r-GO shell on the surface of TiO2 facilitates the interfacial electron transfer. The direct contact between r-NGO and TiO2 is maximized in r-NGOT by retarding the charge recombination and accelerating the electron transfer. The geometry of the core/shell structure should be effective in the design of a graphene/TiO2 composite for solar conversion applications.
’ INTRODUCTION Titanium dioxide (TiO2) as a heterogeneous photocatalyst has been widely investigated during the past decades because of its outstanding advantages such as low cost, nontoxicity, strong photo-oxidizing power, and stability. However, TiO2 suffers from the limited absorption of solar light due to its large bandgap (∼3 eV) and low photocatalytic activity because of the fast recombination of charge carriers.1 A variety of strategies have been employed to solve these problems such as doping with various elements,2,3 coupling with different bandgap semiconductors,4,5 and loading noble metals.6 In particular, the combination of TiO2 nanoparticles (NPs) with carbon nanomaterials such as carbon nanotubes,7 graphite oxide,8 and graphene911 has been proposed as a suitable method for increasing the photocatalytic activity. Since graphene coupled with TiO2 showed excellent electron withdrawing and storing ability,9 the composites of grapheneTiO2 have received much attention in many photofunctional applications such as the photocatalytic purification of water and air, water splitting, solar cell, and bacterial inactivation.1220 Graphene which is a two-dimensional carbon sheet with one atom thickness layer has many unique properties such as high charge carrier mobility,21 large surface area,22 high transparency,23 and high flexibility.24 When graphene oxide (GO) is prepared r 2011 American Chemical Society
from graphite by chemical oxidation and the following sonication, the lateral sizes of GO sheets (precursor of reduced GO: r-GO) widely vary from micrometer to nanometer size25,26 and the different sizes or shapes of graphene sheets induce dissimilar physical properties.27,28 Therefore, many approaches have been tried to make size-controlled GO sheets with uniform size distribution.2931 Nanometer-sized ( 320 nm). The filtered light was focused onto a 50-mL Pyrex reactor with a quartz window. The evolved hydrogen was collected in the headspace of the closed reactor and analyzed by a gas chromatograph (GC, HP6890N) with a thermal conductivity detector and Ar used as carrier gas. An incident light intensity was measured using an optical power meter (Newport 1830-C) equipped with a silicon diode detector and was determined to be about 205 mW/cm2 in the wavelength range 320420 nm. Photoelectrochemical Measurements. Photoelectrochemical (PEC) measurements were carried out in a conventional three-electrode system connected to a potentiostat (Gamry, Reference 600). A Pt plate (1 1 cm2), a graphite rod, and a Ag/AgCl electrode were used as working, counter, and reference electrodes, respectively. The photocurrent was collected on an inert electrode (Pt) through electron shuttles (using a reversible redox couple of Fe3+/Fe2+) in the aqueous suspension of photocatalysts under UV light irradiation (λ > 320 nm).38 The electrolyte solution was continuously purged with N2 during the measurement, and the Pt electrode was biased with an applied potential +0.7 VAg/AgCl. Photogenerated electrons transfer from the suspended photocatalyst particles to the Pt electrode via the electron 1536
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Figure 2. (a) Absorption coefficient (α) spectra and (b) the variation of absorption maxima of LGO, NGO-100, NGO-200, and NGO-300 in its aqueous solutions. Figure 1. As synthesized LGO and NGOs. Tapping-mode AFM images (top) and the height cross-sectional profiles (middle) of (a) LGO, (b) NGO-100, (c) NGO-200, and (d) NGO-300. The size distribution histograms (bottom) were obtained by counting (frequency) from the AFM images. The average sizes of LGO, NGO-100, NGO-200, and NGO-300 are 520 ((480), 164 ((128), 59 ((47), and 36 ((27) nm, respectively.
shuttle. The PEC performances were also investigated using the photoelectrode on which photocatalysts were deposited. The PEC reactor contained the photoelectrode, a coiled Pt wire, and a Ag/AgCl electrode as working, counter, and reference electrodes, respectively, which were immersed in aqueous electrolyte of 0.1 M HClO4. The photoelectrodes were fabricated by following the method as described previously.10 The paste of catalyst in ethanol (0.15 g/mL) was spread on an FTO glass (Pilkington, TEC8) and then calcined at 450 °C under Ar flow. The final photoelectrodes have an average film thickness of around 8 μm and an active geometric area of about 0.8 cm2. The incident photon-to-current conversion efficiencies (IPCEs) were obtained using a SAP301 grating monochromator (Newport, Oriel 77250) in the wavelength range from 300 to 420 nm. The photoelectrode was biased with a potential of +0.2 VAg/AgCl during the IPCE measurements.
’ RESULTS AND DISCUSSION Characterization of NGOs and r-NGOTs. The prepared NGOs were characterized by atomic force microscopy (AFM), of which images (shown in Figure 1) apparently exhibit the different sizes of LGO, NGO-100, NGO-200, and NGO-300. The size of GO sheets decreased with increasing the amount of the oxidant (KMnO4) used in the preparation. The thickness of GO is about 0.9 nm which confirms the single layered GO sheet,39,40 but the thickness of NGOs varies in the range of
0.82.0 nm, which indicates that some of the NGO sheets are stacked as double layers. The bottom parts in Figure 1 display the size distribution histograms of LGO and NGOs obtained from AFM images. As compared with the parent GO (LGO) which has a wide distribution of size (520 ((480) nm), the average sizes of NGO-100, NGO-200, and NGO-300 are successively reduced to 164 ((128), 59 ((47), and 36 ((27) nm, respectively. In addition, all NGOs show much smaller average sizes and narrower size distributions compared to those for LGO. Absorption coefficient (α) spectra of LGO and NGOs were measured using a UVvisible spectrophotometer and compared in Figure 2a. All LGO and NGOs show the absorption coefficient maximum around 5.6 eV (≈ 222 nm) which is ascribed to π f π* transition of CdC in the two-dimensional matrix of GO.4143 However, both the absorption maximum position and the absorption edge region slightly vary with the oxidation of GO. Both show the blue shift as the degree of oxidation is enhanced, which is attributed to the reduced size of the π conjugation domain on the GO sheet.44,45 When the delocalized π electrons are confined within a smaller domain, the quantum confinement effect should be more prominent with increasing the HOMO LUMO gap. The main absorption peaks of LGO, NGO-100, NGO-200, and NGO-300 correspond to 5.33 (∼233 nm), 5.62 (∼221 nm), 5.69 (∼218 nm), and 5.75 (∼216 nm) eV, respectively, as shown in Figure 2b. The results of UVvisible absorption measurements are in good agreement with the AFM images and size distribution histograms in that the higher degree of oxidation is related to the smaller size of NGO. Among NGOs, NGO-300 was chosen as a model substrate for making a nanosized graphene oxide coated TiO2 (NGOT) because it has the smallest size and highest degree of oxidation state. The various NGOTs which contain different amounts of NGO were prepared using NGO-300 and were further reduced by the photocatalytic reduction process under UV irradiation. 1537
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The Journal of Physical Chemistry C The optical absorption of r-NGOTs with various content of NGO (06 wt %) was measured with diffuse reflectance UV/visible absorption spectroscopy (DRUVS) and compared in Figure 3. The addition of r-NGO onto the surface of TiO2 induced the change of the optical property of TiO2. All r-NGOTs show a broad absorption in the whole visible range. The visible absorption background of r-NGOTs increased with increasing the NGO content because more r-NGOs are formed on TiO2 surface. The inset shows the change of absorption spectra before and after the photoreduction of NGOT-0.7. The absorption background in the visible range was much enhanced after photoreduction because of the restoration of the π-electron conjugation within GO sheets.46 The addition of r-NGO to TiO2 also affected the surface charge of TiO2. The zeta (ζ) potential of r-NGOT is clearly shifted to the negative direction compared with bare TiO2 NPs because of the acidic functional groups present on r-NGO.47 As a result, the zero-point charge (ZPC) of r-NGOT is also located at low pH compared with bare TiO2 (pH 5.7 vs pH 6.6, see the Supporting Information, Figure S1). Figure 4a displays the FT-IR spectra of LGO, NGO, and r-NGOT. LGO and NGO exhibit a broad IR peak centered at 3421 cm1 corresponding to the OH stretching vibration, 1728 and 1056 cm1 to the CdO and CO stretching vibration of COOH groups, and 1620 cm1 to the stretching vibration of CdC. The intensity ratio of peaks at 1728 and 1620 cm1 (CdO/CdC) in
Figure 3. Diffuse reflectance UV/vis spectra of r-NGOT-6, r-NGOT-4, r-NGOT-2, r-NGOT-0.7, r-NGOT-0.4, and TiO2. (In r-NGOT-X, X indicates “wt % of initial NGO” content.) The inset compares the diffuse reflectance UV/vis spectra of NGOT-0.7 before and after the photocatalytic reduction.
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NGO is larger than that of LGO, which indicates that NGO is more oxidized than LGO. As for r-NGOT, the broad band below 1000 cm1 corresponds to the TiOTi stretching vibration10 and the peak at 1728 cm1 (CdO stretching vibration) vanishes because the oxygen functional groups on NGO are removed during the photoreduction process on TiO2. The X-ray photoelectron spectroscopy (XPS) results in Figure 4b also confirm that NGO has a higher carbon oxidation state than LGO and the oxygen containing groups (CdO and CO at 287.1 and 286.5 eV, respectively) on r-NGOT are considerably reduced after the photocatalytic reduction. The intensity ratio of CO and CC peaks (at 286.5 and 284.6 eV) in NGO is higher than that of LGO, which is consistent with FT-IR spectra in that NGO contains more oxygen functional groups than LGO. The transmission electron microscopy (TEM) analysis was conducted with two r-NGOTs which have different NGO contents as shown in Figure 5 (panels ad for r-NGOT-0.7 and panels eh for r-NGOT-4). The existence of carbon on r-NGOT was confirmed by the electron energy loss spectroscopy (EELS) mapping as shown in Figure 5, panels b and f. The TiO2 particles in r-NGOT-0.7 and r-NGOT-4 are covered with carbon, and the carbon content on r-NGOT-4 is much higher than that of r-NGOT0.7. The TEM images of r-NGOT-0.7 (Figure 5, panels c and d) clearly show the presence of carbon layers surrounding TiO2. The interlayer d-spacing of carbon layers on r-NGOT-0.7 is about 0.5 nm, which is larger than the d-spacing of pristine graphite (∼0.34 nm)48 but smaller than that of graphene oxide (∼1 nm).39,40 This implies that the oxygen containing functional groups on NGO were partially removed when it was transformed into r-NGOT. The TEM images also clearly show that the carbon layers on TiO2 are thicker with the higher NGO content (Figure 5, panels c and d vs panels g and h). Photocatalytic Activity of r-NGOT. The photocatalytic activity of r-NGOT was evaluated for the hydrogen evolution under UV light irradiation (λ > 320 nm). Figure 6, panels a and c, shows the effect of NGO content in r-NGOT (panel a without Pt and panel c with Pt cocatalyst) on the photocatalytic hydrogen production. The loading of NGO on TiO2 markedly enhanced the production of H2 (for both r-NGOT and Pt/r-NGOT) and the loading effect was maximized at 0.7 wt %. However, the NGOenhanced effect is reduced with increasing the loading of NGO (i.e., as the thickness of NGO layers increases), which should be ascribed to the light shielding by the NGO layer on TiO2.49 The opacity of graphene material linearly increases with the number of graphene sheets.23 However, even r-NGOT-6, which has the highest NGO loading (6 wt %) with a few layers of graphenes,
Figure 4. (a) FT-IR transmittance spectra of LGO, NGO, and r-NGOT-0.7. (b) The high resolution C 1s XPS spectra of NGO, LGO, and r-NGOT-0.7. 1538
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Figure 5. TEM images of r-NGOTs. (a) The bright field TEM image, (b) EELS mapping (corresponding to panel a), and (c) the low-resolution and (d) high-resolution TEM images of r-NGOT-0.7. (e) The bright field TEM image, (f) EELS mapping (corresponding to panel e), and (g) the low-resolution and (h) high-resolution TEM images of r-NGOT-4.
Figure 6. Photocatalytic production of hydrogen in the aqueous suspension of (a and b) r-NGOTs and (c and d) Pt (0.05 wt %)/r-NGOTs. Hydrogen production rate as a function of the content of NGO in (a) r-NGOT and (c) Pt (0.05 wt %)/r-NGOTs. (b and d) Time profiles of H2 production with various photocatalysts. The experimental conditions are [catalyst] = 0.5 g/L, [methanol]0 = 10 vol %, N2-purged, and λ > 320 nm.
exhibited measurable photocatalytic activity, which implies that the presence of NGO layers on the surface of TiO2 does not totally block the light penetration. The time profiles of hydrogen production are compared among various photocatalysts in Figure 6, panels b and d. Compared with bare TiO2, the loading of NGO enhanced the photocatalytic activity and the subsequent reduction of NGO to r-NGO further increased the activity. r-NGO coating on TiO2 assists the photocatalysis by withdrawing electrons and subsequently retarding the charge pair recombination. Earlier studies have also
explained that the enhanced photocatalytic activity of the TiO2 graphene composite is mainly attributed to the inhibition of charge recombination.911 The fact that r-NGOT is more active than NGOT for the photocatalytic hydrogen production should be ascribed to the fact that r-GO is a better conductor and a stronger electron acceptor compared to GO.50 In this respect, the role of r-NGO is similarly compared with that of Pt deposited on TiO2 photocatalyst. However, the hydrogen production activity of r-NGOT is negligibly small compared with Pt/TiO2 as shown in Figure 6d, which implies that the catalytic role of r-NGO in H2 1539
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Figure 7. (a) Time-dependent profiles of Fe3+-mediated photocurrent density collected on a Pt electrode in UV light-irradiated suspension of TiO2 and r-NGOT-0.7. The experimental conditions were [catalyst] = 1 g/L, [Fe3+] = 0.1 mM, [LiClO4] = 0.1 M, pHi 1.8, Pt electrode held at +0.7 VAg/AgCl, continuously N2-purged, and λ > 320 nm. (b) Linear sweep voltammograms at a scan rate of 2 mV/s (with chopping every 10 s, range between 0.8 and +0.7 VAg/AgCl), (c) IPCE values as a function of wavelength (at the applied voltage of +0.2 VAg/AgCl), and (d) time profiles of the open-circuit potential (Eoc) decay (after UV irradiation was turned off) of TiO2 and r-NGOT-0.7 electrodes. All PEC measurements (LSV, IPCE, and Eoc decay) were conducted in an electrolyte solution of [HClO4] = 0.1 M under continuous N2 purging.
production is insignificant. However, it is noted that the coexistence of r-NGO along with Pt on TiO2 shows a significant synergic effect in the production of H2. The hydrogen production rate with Pt/r-NGOT-0.7 (50 μmol/h) was almost twice as high as that with Pt/TiO2 (27 μmol/h). The enhanced production of hydrogen on Pt/r-NGOT can be explained by the electron withdrawing/storing ability and the high conductivity of r-NGO which can enhance the charge separation and the subsequent electron transfer on the surface of photocatalyst. The presence of r-NGO (as a good conductor) on the surface of TiO2 should facilitate the electron transfer from the TiO2 conduction band to Pt sites.51,52 In addition, the NGO layer with various functional groups should provide plenty of nucleation sites for the deposition of Pt and enhance the dispersion of Pt NPs with preventing their aggregation.53 Photoelectrochemical Activity of r-NGOT. To evaluate photoelectrochemical (PEC) properties of r-NGOT, various measurements were conducted in a three-electrode system (with r-NGOT0.7). Figure 7a shows the time profile of the photocurrent collected in a N2-saturated catalyst suspension through Fe3+/Fe2+ electron shuttles under UV light (λ > 320 nm). After turning on the light, the photocurrent collected in the r-NGOT suspension rapidly increased and then reached a saturation value, whereas the bare TiO2 slurry system showed a very slow increase in the photocurrent. The
saturated photocurrent density obtained with r-NGOT (108 μA/ cm2 at 2000 s) was about 5 times higher than that of TiO2 (22 μA/ cm2). This clearly indicates that the presence of r-NGO on TiO2 accelerates the interfacial electron transfer under irradiation. The linear sweep voltammogram (LSV; Figure 7b) also shows the enhanced PEC property of r-NGOT compared to bare TiO2. Although the photocurrent onset potential was the same between r-NGOT and TiO2, the photo-oxidation current was much enhanced with the r-NGOT electrode. The incident photon-tocurrent conversion efficiency (IPCE; eq 1) of r-NGOT and TiO2 electrodes were measured as a function of the incident light wavelength. IPCE ¼ 1240 IðAÞ=ðλðnmÞ Pi ðWÞÞ 100%
ð1Þ
Figure 7c compares IPCE spectra of r-NGOT and TiO2. Although r-NGOT exhibits a broad absorption background in the visible region (see Figure 3), the r-NGOT electrode does not show any photocurrent generation under visible light irradiation. Nevertheless, the r-NGOT electrode showed a higher efficiency of solar light conversion in the whole UV region than the bare TiO2 electrode. All of the above PEC results indicate that the 1540
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Figure 8. (a) Photocatalytic production of hydrogen in the aqueous suspension of TiO2, r-LGOT, and r-NGOT in the absence (left panel) and the presence (right panel) of Pt (0.05 wt %). (b) Time profiles of H2 generation with Pt/TiO2, Pt/r-LGOT, and Pt/r-NGOT. [catalyst] = 0.5 g/L, [methanol]0 = 10 vol %, N2-purged, and λ > 320 nm. (c) Time-dependent profiles of Fe3+-mediated photocurrent collected on a Pt electrode in UV lightirradiated suspension of Pt/TiO2, Pt/r-LGOT, and Pt/r-NGOT. The experimental conditions were [catalyst] = 1 g/L, [Fe3+] = 0.1 mM, [LiClO4] = 0.1 M, pHi 1.8, Pt electrode held at +0.7 VAg/AgCl, continuously N2-purged, and λ > 320 nm. TEM images of (d) r-LGOT and (e) r-NGOT (the insets represent the schematic illustrations for r-LGOT and r-NGOT, respectively). All r-LGOT, Pt/r-LGOT, r-NGOT, and Pt/r-NGOT have the same initial GO content (0.7 wt %) and all Pt/TiO2, Pt/r-LGOT, and Pt/r-NGOT have the same Pt content (0.05 wt %).
combination of TiO2 and r-NGO, in a core/shell structure, enhances the PEC activity which is mainly ascribed to the retarded recombination of photogenerated charge carriers and the higher conductivity of the r-NGO shell.54 The slower charge recombination in r-NGOT was also observed in the open-circuit potential (Eoc) decay tests. Figure 7d shows the normalized Eoc time profiles obtained with the r-NGOT and TiO2 electrodes. Upon turning off the light, Eoc of r-NGOT much slowly decayed compared to that of TiO2, which indicates slower recombination in r-NGOT. Influence of the Lateral Size of r-GO for Photocatalytic Activity. In all the previous studies of r-GO and TiO2 composite, TiO2 NPs were dispersed on the larger graphene sheet (approximately micrometer size).911,17,18,55,56 The present study is clearly different from the previous ones in that the TiO2 and r-GO was organized into a core/shell structure (see Scheme 1). The structural difference between r-LGOT (TiO2 NPs on r-GO sheet) and r-NGOT (TiO2 core/r-GO shell) should influence their photocatalytic and PEC activities. In order to investigate the effect of the r-GO form in the composite, r-LGOT was prepared and its photocatalytic and PEC activities were compared with those of r-NGOT. The r-LGOT was prepared by the similar method of r-NGOT with maintaining the GO content the same to r-NGOT (0.7 wt % in this case). The original GO which has an average size of 520 ((480) nm (shown in Figure 1a) is denoted as LGO, while the NGO-300 used in the preparation of r-NGOT has an average size of 36 ((27) nm (shown in Figure 1d). The photocatalytic activities of r-NGOT and r-LGOT were compared for the hydrogen generation without and with Pt cocatalyst. As shown in Figure 8a, the photocatalytic hydrogen production
activities of r-LGOT and r-NGOT are a little different in the absence of Pt, but Pt/r-NGOT is markedly more active than Pt/r-LGOT. The time profiles of photocurrent collected through Fe3+/Fe2+ electron shuttles also show that Pt/r-NGOT generates the photocurrent more rapidly than Pt/r-LGOT as shown in Figure 8c. Figure 8, panels d and e, compare the TEM images of r-LGOT and r-NGOT. A larger improvement for the hydrogen and photocurrent generation with Pt/r-NGOT compared to Pt/r-LGOT can be explained as follows. (1) The photogenerated conduction band electrons in TiO2 can be more easily transferred to r-NGO than r-LGO because the TiO2 surface in r-NGOT is fully contacted by the r-GO shell whereas only a small fraction of TiO2 surface is in a direct contact with r-LGO sheet in r-LGOT (see Scheme 1). (2) The stronger synergy between Pt and r-NGOT should be ascribed to the fact that Pt was directly loaded on the core/shell composite. While Pt and TiO2 NPs in Pt/r-LGOT are separated on the GO sheet and the electron transfer from TiO2 to Pt requires a distance to travel, Pt on r-NGOT is in the contact with TiO2 NPs only through a thin graphene layer. Such a close contact between Pt and TiO2 through a conductive r-GO layer should facilitate the charge separation and the subsequent production of hydrogen. (3) The oxygen functional groups at the edges and on the GO plane can act as nucleation sites53 during the photodeposition of platinum. NGO that has higher oxygen content than LGO provides more nucleation sites than LGO, which induces the better dispersion of Pt NPs on the GO surface. Although more functional groups and more defect carbons in NGOs should reduce the conductivity, NGOs show 1541
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The Journal of Physical Chemistry C higher photocatalytic and PEC performances compared with LGOs when they are combined with TiO2 NPs in the core/shell structure.
’ CONCLUSIONS We developed a new type of r-GO/TiO2 composites, which have a self-assembled core/shell structure with varying content of NGOs and tested their photocatalytic and PEC activities. All previous r-GO/TiO2 composites employed micrometer-sized sheets of graphene on which TiO2 NPs were loaded (referred as r-LGOT in the paper). Among the synthesized r-NGOT composites, r-NGOT-0.7 (containing 0.7 wt % of NGO) showed the highest activity in both photocatalytic and PEC measurements. The addition of r-NGOs onto TiO2 NPs enhanced the photocatalytic and PEC activities, which can be attributed to the retarded charge recombination and the enhanced electron transfer on the surface of r-NGOT. The activity comparison between r-NGOT and r-LGOT shows that r-NGOT with the core/shell structure has higher efficiency in both photocatalytic and PEC measurements than r-LGOT. Compared with r-LGOT, r-NGOT has more contact area between r-NGO and TiO2 NPs. The unique TiO2 core/ r-NGO shell structure on which Pt NPs are loaded enables the closer contact between Pt and TiO2 and facilitates the efficient electron transfer for higher photocatalytic and PEC activities. ’ ASSOCIATED CONTENT
bS
Supporting Information. pH-dependent zeta (ζ) potential of bare TiO2 and r-NGOT-0.7 in aqueous electrolyte (0.1 mM NaNO3) suspensions. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: +82-54-279-8299.
’ ACKNOWLEDGMENT This work was supported by KOSEF NRL program funded by the Korea government (MEST; No. R0A-2008-000-20068-0), the KOSEF EPB center (Grant No. R11-2008-052-02002), and the Korea Center for Artificial Photosynthesis (KCAP: Sogang Univ.) funded by MEST through NRF (2009-C1AAA001-20090093879). ’ REFERENCES (1) Friedmann, D.; Mendive, C.; Bahnemann, D. Appl. Catal. B: Environ. 2010, 99, 398. (2) Liu, G.; Wang, L.; Yang, H. G.; Cheng, H.-M.; Lu, G. Q. M. J. Mater. Chem. 2010, 20, 831. (3) Choi, J.; Park, H.; Hoffmann, M. R. J. Phys. Chem. C 2010, 114, 783. (4) Zhang, H.; Chen, G.; Bahnemann, D. W. J. Mater. Chem. 2009, 19, 5089. (5) Choy, J.-H.; Lee, H.-C.; Jung, H.; Hwang, S.-J. J. Mater. Chem. 2001, 11, 2232. (6) Sakthivel, S.; Shankar, M. V.; Palanichamy, M.; Arabindoo, B.; Bahnemann, D. W.; Murugesan, V. Water Res. 2004, 38, 3001. (7) Yu, J.; Ma, T.; Liu, S. Phys. Chem. Chem. Phys. 2011, 13, 3491. (8) Park, Y.; Kang, S.-H.; Choi, W. Phys. Chem. Chem. Phys. 2011, 13, 9425. (9) Williams, G.; Seger, B.; Kamat, P. V. ACS Nano 2008, 2, 1487.
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