Decoration of TiO2 Nanotubes with Metal Nanoparticles Using

Article pubs.acs.org/Langmuir

Decoration of TiO2 Nanotubes with Metal Nanoparticles Using Polyoxometalate as a UV-Switchable Reducing Agent for Enhanced Visible and Solar Light Photocatalysis Andrew Pearson,†,‡,⊥ Haidong Zheng,§ Kourosh Kalantar-zadeh,§ Suresh K. Bhargava,*,‡,⊥ and Vipul Bansal*,†,‡,⊥ †

NanoBiotechnology Research Lab (NBRL), ‡Centre for Advanced Materials & Industrial Chemistry (CAMIC), ⊥School of Applied Sciences, and §School of Electrical and Computer Engineering, RMIT University, GPO Box 2476V, Melbourne, VIC, 3001 Australia ABSTRACT: We present the employment of the Keggin ion 12-phosphotungstic acid as a UV-switchable reducing agent for the decoration of Au, Ag, Pt, and Cu nanoparticles onto the surface of TiO2 nanotubes synthesized by electrochemical anodization. The synthesized composites were studied using SEM, GADDS XRD, and EDX, and the photocatalytic activity of the composites was examined by measuring the photodegradation of the organic dye “Congo red” under simulated solar light. Decoration with metal nanoparticles was observed to enhance the activity of the photocatalytic process by upward of 100% with respect to unmodified TiO2 nanotubes.

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INTRODUCTION Titania (TiO2) is a widely used semiconductor material which has received intense scrutiny for a range of applications due to its interesting chemical and physical properties such as its wide band gap, wide availability, low cost, nontoxicity, and biocompatibility. Titania, particularly its anatase form, is widely known to enhance catalytic activity due to a strong interaction between the active phase and the support;1 however, the application of titania is often limited due to problems associated with the charge-recombination (electron−hole recombination) phenomenon inherent to semiconductor materials and a large band gap of 3.2 eV,2 which requires its exposure to ultraviolet radiation for potential photocatalysis applications. TiO2 nanotubes have been demonstrated to possess greatly enhanced specific surface area, which can significantly increase the photocatalytic performance of the material due to an increase in the number of reaction sites.3 Up until recently there have been several routes to synthesize TiO2 nanotubes including, layer-by-layer,4 atomic deposition,5 chemical vapor deposition,6 hydrothermal reaction,7 and electrochemical anodization.8 Of these techniques, electrochemical anodization provides a convenient and easily reproducible method for the synthesis of TiO2 nanotubes. To reduce the charge-recombination phenomenon on TiO2 nanotubes, efforts have been made to introduce metal nanoparticles into the TiO2 matrix, which can be achieved by various chemical and photodeposition techniques, whereby the deposition of noble metal nanoparticles is observed to increase the photocatalytic activity of the system.9−12 To this end, TiO2 nanotubes decorated with Pt,13−15 Ag and Au,16,17 NiO,18 CdS,19 WO3,20 and Ir/Co21 have previously been explored for the enhancement of © 2012 American Chemical Society

photocatalytic activity. Unfortunately, many of the techniques described above suffer from extreme reaction conditions such as high temperatures and pressures to be viable. Recently, we reported the facile synthesis of a variety of metal-decorated TiO2 nanoparticles using a novel photoirradiation approach, wherein polyoxometalate (POM) molecules sandwiched between TiO2 and metal nanoparticles were employed as UV-switchable agents to control the metal loading onto TiO2 surface.22,23 Notably, Keggin ions of polyoxometalate molecules have recently attracted significant attention in catalysis due to their interesting electron and proton transfer and storage characteristics and high thermal stability.24 Therefore, we observed that a TiO2/POM/metal nanocomposite essentially acted as a cocatalytic system, wherein not only the photocatalytic efficiency of this system is significantly improved, this composite also showed good activity for visible light photocatalysis. In the current article, we describe the decoration of titania nanotubes synthesized by electrochemical anodization, with gold, silver, platinum, and copper nanoparticles, by using the Keggin ions of 12-phosphotungstic acid (PTA) as a UVswitchable reducing agent with inherent photochemical properties. PTA was chosen as a cocatalyst because it has been recognized as the strongest POM, and its ability to act as a UVswitchable reducing agent for the synthesis of metal nanoparticles has been well documented.25−27 We also report the photocatalytic activity of these composite photococatalyst Received: August 22, 2012 Revised: September 11, 2012 Published: September 18, 2012 14470

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spectroscopy. A similar approach was followed while excitation with 253 nm UV lamp for 15 min and 575 nm lamp for 30 min.

materials under simulated solar light and compare the influence of different metals on photocatalytic efficiency.

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RESULTS AND DISCUSSION Scheme 1 describes the series of reactions that were undertaken to facilitate the decoration of TiO2 nanotube−PTA cocatalytic

EXPERIMENTAL SECTION

Materials. Titanium foil (99.5% purity), ammonium fluoride (NH4F, AR grade), potassium tetrabromoaurate (KAuBr4·3H2O, AR grade), chloroplatinic acid (H2PtCl6·2H2O, AR grade), cupric chloride (CuCl2, AR grade), and silver sulfate (Ag2SO4, AR grade) were obtained from Sigma-Aldrich. 12-Phosphotungstic acid hydrate (H3PW12O40·3H2O, AR grade) was obtained from Scharlau Chemie, and propan-2-ol (isopropanol, LR grade) was obtained from BDH Chemicals. All chemicals were used as received. Fabrication of TiO2 Nanotube Arrays. TiO2 nanotube arrays were fabricated by electrochemical anodization of titanium foil in a conventional anode (target sample)−cathode (platinum plate) system at room temperature using a facile protocol previously reported by us.8 The electrolyte solution was composed of 0.5% (w/v) NH4F in ethylene glycol with the addition of 3% (v/v) deionized H2O, and the anodization voltage was kept constant at 60 V. Upon the completion of anodization, samples were washed using acetone and isopropanol and dried under N2. Anatase TiO2 was obtained by annealing the anodized samples in a standard laboratory furnace at 450 °C for 60 min in ambient air, with a ramp-up and ramp-down rate of 2 °C min−1. PTA Functionalization of TiO2 Nanotube Arrays. Four 5 mm × 5 mm squares of TiO2 nanotube arrays on Ti foil were cut from the bulk electrochemically anodized films and immersed overnight in a solution containing 10 mL of 1 × 10−2 M phosphotungstic acid (PTA). The films were then removed from the PTA solution and washed three times with deionized water to remove any unbound PTA molecules. In further experiments, these samples are referred to as TiO2-PTA nanotubes. Photochemical Deposition of Metal Nanoparticles onto TiO2 Nanotube Arrays. To decorate the modified nanotubes with metal nanoparticles, the ability of PTA to act as a UV-switchable reducing agent was exploited. In a typical experiment, four 5 mm × 5 mm squares of PTA-functionalized nanotube arrays were placed in separate quartz tubes and immersed in 1 mL of isopropanol and 4 mL of deionized water before purging with N2 gas for 15 min and photoexciting the solution for 2 h using a UV lamp (λex 253 nm) to allow TiO2 nanotube-bound PTA molecules to get reduced. 9 mL of 1 × 10−5 M of an appropriate metal salt (KAuBr4, Ag2SO4 H2PtCl6, or CuCl2) was then added to the reduced TiO2-PTA nanotubes and allowed to mature for 2 h. After 2 h, the films were removed from solution and washed three times with Milli-Q water. The metaldecorated nanotube samples prepared using this approach have been referred to as TiO2-PTA-Au, TiO2-PTA-Ag, TiO2-PTA-Pt, and TiO2PTA-Cu nanotubes, respectively. The experiment was performed simultaneously for all four metal salts to ensure the reaction conditions were as similar as possible. Materials Characterization. All materials were examined at various stages of synthesis using scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and X-ray diffraction (XRD). The samples were examined under SEM without any additional preparation on a FEI Nova NanoSEM coupled with an EDX Si(Li) X-ray detector. X-ray diffraction was performed on a Bruker AXS D8 Discover with a general area detector diffraction system (GADDS). Photocatalytic Degradation of Congo Red Dye. The photocatalytic ability of the composites was investigated by immersing a 5 mm × 5 mm square of metal decorated nanotubes in a solution of the organic dye Congo red (CR) (5 mL, 5 × 10−5 M) and recording the intensity of the characteristic absorption maximum after a period of 30 min exposure to simulated solar light. The photocatalytic ability of TiO2 and TiO2-PTA nanotubes were also recorded to provide comparison. An Abet Technologies LS-150 Series 150 W Xe arc lamp source was used with the sample placed in a quartz vial at the focal point, 10 cm from the source with slow mechanical stirring to promote mixing of the solution. After 30 min of irradiation, the composites were removed and the remaining solutions were examined by UV−vis

Scheme 1. Schematic Representation of the Formation of TiO2−PTA Cocatalytic Materials Decorated with Metal Nanoparticles of Copper, Silver, Platinum, and Golda

a

The process involves the modification of the TiO2 nanotube surface by functionalizing with PTA [(PW12O40)3−] ions (step 1), followed by UV-irradiation of the composite for 2 h in a N2 atmosphere (step 2). The reduced PTA ions [(PW12O40)4−] are referred to as PTA*, reflecting that the excited PTA molecule is able to locally reduce the selected metal ions (step 3: CuCl2, H2PtCl6, Ag2SO4, or KAuBr4) to form metal nanoparticles of the respective metal salt directly onto the nanotube surface. The colour change in the composite material is notably due to the surface plasmon resonance of as-formed noble metal nanoparticles decorating the composite surface.

composites with nanoparticles of copper, silver, platinum, or gold. Step 1 involves the functionalization of TiO2 nanotubes formed through an electrochemical anodization process with the heteropolyacid Keggin ion PTA [(PW12O40)3−], which results in the formation of a TiO2−PTA composite material. Proceeding to step 2, the now functionalized TiO2-PTA composite is exposed to UV radiation in the presence of isopropanol and a nitrogen atmosphere for a period of 2 h, which after irradiation appears to have a slight blue tinge indicating the reduction of the PTA molecules bound to the nanotube surface to become [(PW12O40)4−], which are denoted as PTA* in Scheme 1. Finally, step 3 is split into four independent processes whereby metal salts of CuCl2, Ag2SO4, H2PtCl6, or KAuBr4 are introduced to the TiO2-PTA* composites to facilitate the decoration of the composites with the respective noble metal nanoparticles. After introduction of the metal salt, the solution is allowed to mature for 2 h where the reduced state of the surface bound PTA (PTA*) acts as a highly localized reducing agent to reduce the metal salt to metal nanoparticles directly onto the nanotube surface. Displayed in Figure 1 are top-down and side-view scanning electron microscopy (SEM) images of the TiO2 nanotubes synthesized by an electrochemical anodization process. The synthesized nanotubes are ca. 5 μm in length, with quasispherical openings of ca. 100 nm diameter, and the walls of the nanotubes are well-defined with a thickness of ca. 10 nm. It must be noted that as the TiO2 nanotubes are grown out of a piece of Ti foil, the synthesized nanotubes are capped at one end, which has significant ramifications for metal nanoparticle decoration and will be discussed later. Modification of the 14471

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Figure 1. SEM images of as-synthesized TiO2 nanotubes.

nanotube surface with PTA had little notable effect on the morphology of the nanotubes, and as such the data are not displayed here for conciseness; confirmation of PTA modification of the TiO2 surface was achieved by performing EDX analysis (Figure 2), which showed a distinct energy signature at 1.7 keV, which corresponds to the W Mα line.

Figure 3. SEM images of (a) TiO2-PTA-Cu, (b) TiO2-PTA-Ag, (c) TiO2-PTA-Pt, and (d) TiO2-PTA-Au.

Ag) is evident from Figure 3b, which depicts the deposition of a large number of 10−20 nm Ag nanoparticles, once again deposited onto the outer walls and around the openings of the nanotubes. Decoration with platinum nanoparticles (TiO2PTA-Pt) produced results similar to those for silver nanoparticles, in which a large number of 10−20 nm nanoparticles were observed deposited onto the outside and around the rim of the nanotubes (Figure 3c). Lastly, decoration with gold nanoparticles (TiO2-PTA-Au) resulted in the deposition of larger 20−30 nm nanoparticles predominantly around the openings but also found decorating the sides of the nanotubes (Figure 3d). Interestingly, it was observed that with all the four metals no metal nanoparticles were observed inside the nanotubes themselves, which is due to the capping of the nanotubes at another end caused by the method of synthesis. As the nanotubes are capped, there exists a pocket of air within the nanotube, which is difficult to be dislodged by a solution, and as a result, metal ions (and most likely PTA molecules also) could not enter the nanotubes, thereby leading to no metal nanoparticle decoration within the nanotubes. Further, EDX analysis (Figure 2) was employed to confirm the presence of each noble metal in the respective composites as well as to semiquantitatively assess the elemental composition of composites to determine the degree of metal nanoparticle loading (% w/w) onto TiO2 nanotubes. By observing the area under the W Mα energy lines in the EDX spectrum relative to an internal standard, semiquantitative analysis was performed on multiple sample areas and averaged using an in-built EDX software suite. On the basis of EDX analysis, it was determined that 6% W was present in the TiO2PTA nanotube composite. After decoration of TiO2-PTA nanotubes with metal nanoparticles, a characteristic energy line for the respective metal nanoparticle was observed in respective samples. TiO2-PTA-Cu (Figure 3a and Figure 2A, curve e) displays an energy signature at 0.93 keV corresponding to the Cu Lα line where 5% W and 4.5% Cu were observed in the TiO2-PTA-Cu composite; TiO2-PTA-Ag (Figure 3b and Figure

Figure 2. EDX analysis of the synthesized TiO2-PTA-metal composites (a) Ti/TixO film, (b) TiO2 nanotubes, (c) PTA, (d) TiO2-PTA, (e) TiO2-PTA-Cu, (f) TiO2-PTA-Ag, (g) TiO2-PTA-Pt, and (h) TiO2-PTA-Au. Energy edges characteristic of Ti Kα and Ti Kβ are observed at 4.5 and 4.98 keV, respectively. The Ti Lα line is present as a small shoulder on the O Kα peak at ca. 0.56 keV. Peaks attributable to the W Mα line are observed at 1.81 keV in all samples containing PTA. Peaks attributable to Cu Lα are observed at 0.93 keV, Ag Lα at 3.35 keV, Pt Mα at 2.12 keV, and Au Mα at 2.29 keV in their respective samples. All spectra, except that of pure PTA, are normalized against the Ti Kα line to enable comparison.

Decoration of the TiO2 nanotubes with metal nanoparticles is evident from SEM images in Figure 3. Figure 3a shows TiO2 nanotubes decorated with copper nanoparticles (TiO2-PTACu), wherein the particles appear to be small, 3−5 nm in diameter, and quasi-spherical that are well dispersed and deposited on the outside and around the opening of the nanotubes. Decoration with silver nanoparticles (TiO2-PTA14472

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silver nanoparticles (TiO2-PTA-Ag, pattern e) results in an XRD feature at ca. 44° 2θ (marked with #), which is attributable to fcc silver (JCPDS - 03-065-8428). Decoration with platinum nanoparticles (TiO2-PTA-Pt, pattern f) results in a peak at 45° 2θ (marked with ∧), which correlates well with the literature for fcc platinum (JCPDS - 01-088-2343). Lastly, introduction of gold nanoparticles (TiO2-PTA-Au, pattern g) results in a similar pattern to that of TiO2-PTA-Ag, whereby a single peak is observed at 44° 2θ (marked with ∗), which correlates well with the literature for fcc gold (JCPDS - 03-0652870). Therefore, XRD investigations clearly support that different metals are present in metallic form (M0) in respective TiO2-PTA-M nanotube composites. As we previously mentioned, the majority of semiconductorbased materials suffer from the phenomenon of electron/hole recombination, which actively hampers their ability to perform efficient photocatalysis.28 In the photocatalytic process, electrons are promoted from the valence band across the band gap to the conduction band upon excitation by incident photons where there exists a very large driving force to recombine the newly generated electron and hole.28 When metal nanoparticles are deposited onto a semiconductor surface, a Schottky barrier is formed at the junction of the semiconductor and the metal due to the difference in their Fermi level positions.29,30 Because of the difference in Fermi energy levels between the metal and semiconductor, metal acts as an electron sink, and therefore electrons tend to migrate from the semiconductor to the metal until equilibrium of the Fermi levels is achieved. This results in suppression of the electron/hole recombination phenomenon, thereby allowing the hole to diffuse to the semiconductor surface where it can oxidize organic species.28 Metal nanoparticles are therefore able to play an integral part in catalyzing the charge-transfer processes,31,32 which can result in significant improvement in the photocatalytic performance of such composite nanomaterial systems.30,33 By immersing different TiO2-PTA-metal nanotube composites in separate solutions of the organic dye “Congo red” (CR) and irradiating the solution with simulated solar light, the effect of these nanocomposite materials on the photocatalytic degradation of the organic dye can be easily followed. Congo red being an organic molecule, its exposure to solar light may cause its breakdown, resulting in a reduction in color intensity. The solution of Congo red before exposure (the initial absorbance) is shown as the black curve in Figure 5. As a control experiment the same solution, without any added nanotube composite, when irradiated for a period of 30 min, the intensity of the red color is observed to slightly fade that corresponded with a decrease in the characteristic absorbance of the peak at ca. 500 nm by ca. 10% of its original value; i.e., 10% of the dye was degraded by solar light itself within 30 min. Photodegradation in the presence of an unmodified Ti/TixO foil resulted in 33% degradation of CR, while in comparison, introducing TiO2 nanotubes to the CR solution resulted in ca. 42% degradation, indicating the morphology and increased surface area of the nanotubes aid in the photocatalytic process. PTA is also a well-known photoactive material,25 and its effectiveness in degrading CR has been previously documented.21,22 The introduction of PTA in the TiO2-PTA composite is observed to increase the photocatalytic ability of the TiO2 nanotubes through a cocatalytic process to facilitate the degradation of ca. 49% of the CR. Furthermore, the decoration with metal nanoparticles is observed to have a

2A, curve f) displays an energy signature at 3.35 keV corresponding to the Ag Lα line while 4% W and 5.5% Ag were observed in the TiO2-PTA-Ag composites; TiO2-PTA-Pt (Figure 3c and Figure 2A, curve g) displays an energy signature at 2.12 keV characteristic of a collection of Pt Mα lines while 4.5% W and 4.6% Pt were observed in the TiO2-PTA-Pt composite; and last, TiO2-PTA-Au (Figure 3d and Figure 2, curve h) displays an energy signature at 2.29 keV characteristic of a collection of Au Mα lines where 4.2% W and 6% Au were observed for TiO2-PTA-Au nanotube composite. The relative loading of the metal on the nanotube surface is of crucial importance for photocatalysis applications, as it has been shown that above the optimum metal loading the efficiency of the material for photocatalysis is observed to decrease, and as such care must be taken to ensure a quantity of metal, which provides optimum photocatalytic activity.28 XRD patterns of the synthesized nanotube composites are displayed in Figure 4, whereby additional information regarding

Figure 4. XRD patterns of (a) Ti/TixO, (b) TiO2 nanotubes, (c) TiO2-PTA, (d) TiO2-PTA-Cu, (e) TiO2-PTA-Ag, (f) TiO2-PTA-Pt, and (g) TiO2-PTA-Au. Peaks attributable to the corresponding metals are designated with special characters.

the crystallinity of the composite materials could be determined. Pattern a describes a predominantly Ti film with a surface oxide layer which has not undergone any modification with PTA; the peaks shown correlate closely with that expected from primitive hexagonal Ti (JCPDS - 00-044-1294) with some additional peaks attributable to TixO (where x = 3, 6) (JCPDS 01-073-1583 and 01-072-1471) at 20°, 27.5°, and 54° 2θ, respectively. In comparison, the as-synthesized TiO2 nanotubes (pattern b) show the peaks attributable to Ti as observed in pattern a, along with additional peaks observed at 25°, 48°, 54°, and 69° 2θ, which correlate closely with the diffraction pattern for anatase TiO2 (JCPDS - 01-070-6826). Pattern c depicts TiO2 nanotubes modified with PTA, wherein minor features attributable to PTA (JCPDS - 00-050-0662) can be observed at ca. 20°, 21°, and 34° 2θ. The presence of minor PTA features in TiO2-PTA suggests that PTA forms only a thin coating on TiO2 nanotubes. Further decoration of TiO2-PTA nanotube surface with copper nanoparticles (TiO2-PTA-Cu, pattern d) results in two peaks attributable to fcc copper (JCPDS - 01070-3039) at 43° and 50° 2θ (marked with @). Introduction of 14473

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Figure 5. (a) UV−vis spectra of the organic dye Congo red upon exposure to a range of photocatalysts for 30 min under simulated solar light conditions and (b) % photodegradation of Congo red expressed as reduction in the intensity of absorbance at 500 nm. Control in (b) represents the % photodegradation of Congo red in the absence of a photocatalyst but in the presence of simulated solar light for 30 min.

dramatic effect on the photocatalytic activity of the synthesized composites, wherein Cu, Ag, and Pt are observed to have a very similar effect on the photocatalytic activity, resulting in ca. 76%, 78%, and 80% degradation of CR for Cu, Ag, and Pt, respectively. TiO2-PTA-Au nanotube is observed to be the most photocatalytically active composite, resulting in ca. 89% degradation of the CR. It must be noted that the data shown here has been adjusted for equivalent weight amounts of these composite nanotube materials. The trend of photoactivity with different metal loading is similar to our previous study, where an increasing trend of photoactivity with an increasing nobility of metal onto nanoparticulate TiO2 was observed.21,22 Therefore, the outcomes of this study further strengthen the importance of the choice of noble metal for obtaining high photocatalytic performance of semiconductors. Furthermore, to assess whether these nanotube composite materials show any visible light photoactivity, the applicability of these materials toward the degradation of CR under UV (253 nm) and visible (575 nm) light was compared (Figure 6). As expected, all the nanotube-based TiO2-PTA-M nanotube composites demonstrated significant activity toward the degradation of CR under irradiation with UV light for 15 min (Figure 6Aplease note that 30 min of exposure resulted in complete degradation of CR). The activity trends were found similar to that observed for simulated solar light irradiation, except that the TiO2-PTA-Pt nanotube catalyst was found to be the slightly more photoactive than the TiO2-PTA-Au catalyst. This observation for nanotube composites is similar to that previously observed by us for TiO2 particle-based nanocomposites, wherein we noticed that increased photoactivity of Pt-decorated TiO2 in UV light was because of the absence of the surface plasmon resonance (SPR) features in the case of Pt.21,22 Conversely, when irradiated with visible light, the TiO2PTA-Au nanotube composite demonstrated the highest photoactivity (Figure 6B), which is likely be due to higher SPR absorbance of Au nanoparticles under visible light. The photocatalysis results shown herein therefore suggest that nanotube-based TiO2-PTA-M composites can be employed for the efficient photodegradation of organic dyes. The TiO2 nanotube array-based nanocomposites reported in this study provide an edge over nanoparticulate TiO2 in terms of either

Figure 6. Percentage photodegradation of CR expressed as reduction in the intensity of A500 on its exposure to different TiO2 nanotube based photocatalysts in the presence of (A) 253 nm UV light for 15 min and (B) 575 nm visible light for 30 min. Five bars within each labeled catalyst indicate activity of photocatalysts during reusability of catalyst for up to 5 cycles.

the ease of recovery of the catalyst platform for reusability or use of such system for continuous flow photocatalysis.

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CONCLUSION In summary, we have demonstrated the potential of a metal nanoparticle decorated TiO2 nanotube/Keggin ion cocatalytic material as an effective photocatalyst for the photoreduction of the organic dye “Congo red”. By depositing metal nanoparticles of gold, silver, platinum, and copper on the surface of TiO2 nanotubes using 12-phosphotungstic acid as a photocatalytic 14474

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(21) Khan, M. A.; Yang, O.-B. Catal. Today 2009, 146, 177−182. (22) Pearson, A.; Jani, H.; Kalantar-zadeh, K.; Bhargava, S. K.; Bansal, V. Langmuir 2011, 27, 6661−6667. (23) Pearson, A.; Bhargava, S. K.; Bansal, V. Langmuir 2011, 27, 9245−9252. (24) Pope, M. T.; Mueller, A. Angew. Chem., Int. Ed. 1991, 30, 34−48. (25) Keita, B.; Liu, T.; Nadjo, L. J. Mater. Chem. 2009, 19, 19−33. (26) Mandal, S.; Selvakannan, P.; Pasricha, R.; Sastry, M. J. Am. Chem. Soc. 2003, 125, 8440−8441. (27) Mandal, S.; Anandrao, B. M.; Sastry, M. J. Mater. Chem. 2004, 2868−2871. (28) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735−758. (29) Iliev, V.; Tomova, D.; Bilyarska, G.; Tyuliev, G. J. Mol. Catal. A: Chem. 2007, 32−38. (30) Jakob, M.; Levanon, H.; Kamat, P. V. Nano Lett. 2003, 3, 353− 358. (31) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (32) Subramanian, V.; Wolf, E. E.; Kamat, P. V. Langmuir 2003, 469−474. (33) Subramanian, V.; Wolf, E.; Kamat, P. V. J. Phys. Chem. B 2001, 11439−11446.

linker molecule and UV-switchable reducing agent, the effects of the charge-recombination phenomenon could be suppressed and the lifetime of the electron−hole pair increased, thereby enhancing the photocatalytic activity of the system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (V.B.), suresh.bhargava@ rmit.edu.au (S.K.B.); Ph +61 3 9925 2121; Fax +61 3 9925 3747. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.B. acknowledges the Australian Research Council (ARC), Commonwealth of Australia for award of an APD Fellowship and research support through the ARC Discovery (DP0988099; DP110105125), Linkage (LP100200859), and LIEF (LE0989615) grant schemes. Support of RMIT University to V.B. through award of Seed Grants, Incentive Capital Funding, and Emerging Researcher Grant is also acknowledged. V.B. also acknowledges the support of Ian Potter Foundation to establish a Multimode Spectrophotometry Facility at RMIT University, which was used in this study.

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