Pt Nanoparticle-Decorated TiO2 Nanofibers with Plasmon

Article pubs.acs.org/JPCC

Au/Pt Nanoparticle-Decorated TiO2 Nanofibers with PlasmonEnhanced Photocatalytic Activities for Solar-to-Fuel Conversion Zhenyi Zhang,†,‡ Zheng Wang,† Shao-Wen Cao,† and Can Xue*,† †

Solar Fuels Laboratory, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore ‡ School of Physics and Materials Engineering, Dalian Nationalities University, Dalian 116600, China S Supporting Information *

ABSTRACT: We present the fabrication of TiO2 nanofibers codecorated with Au and Pt nanoparticles through facile electrospinning. The Au and Pt nanoparticles with sizes of 5−12 nm are well-dispersed in the TiO2 nanofibers as evidenced by electron microscopic analyses. The present design of Au/Pt codecoration in the TiO2 nanofibers leads to remarkably enhanced photocatalytic activities on both hydrogen generation and CO2 reduction. This great enhancement is attributed to the synergy of electron-sink function of Pt and surface plasmon resonance (SPR) of Au nanoparticles, which significantly improves charge separation of photoexcited TiO2. Our studies demonstrate that through rational design of composite nanostructures one can harvest visible light through the SPR effect to enhance the photocatalytic activities of semiconductors initiated by UV-light to more effectively utilize the whole solar spectrum for energy conversion.

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INTRODUCTION The effective conversion of solar energy into chemical fuels through photocatalytic water splitting and carbon dioxide (CO2) reduction using a high-performance nanostructural photocatalyst has been considered as one of the most promising strategies to obtain renewable energy sources for the sustainable future.1−3 In the past decades, titanium dioxide (TiO2) as a prototype photocatalyst has attracted the greatest attention in the fields of environmental remediation and solar fuel production owing to its high stability, nontoxicity, wide availability, and low cost.4−7 Nevertheless, the applications of pure TiO2 for solar fuel production are often restricted by several main disadvantages: (1) due to the wide band gap (Eg: ∼3.2 eV), TiO2 can only absorb UV light which accounts for only ∼4% of the solar irradiation;6,8,9 (2) rapid recombination of photogenerated charge carriers leads to low quantum yield of TiO2;10 and (3) large overpotential for hydrogen (H2) production and CO2 reduction.3,11,12 Researchers have recognized that loading TiO2 photocatalysts with metal cocatalysts is an effective method to promote the quantum yield and lower the overpotential of TiO2 because the metal cocatalysts can work as an electron-sink and active reaction sites for H 2 production and CO 2 reduction.13−17 Among all metal cocatalysts, platinum (Pt) has been most widely used because of its small work function and low overpotential for solar fuel production.12,18−20 However, the loading of small Pt nanoparticles (NPs) usually could not improve the solar-harvesting capability of TiO2 photocatalysts since Pt NPs lack visible-light absorption.21−24 © 2013 American Chemical Society

As is well-known, nanostructures of some noble metals, such as gold (Au) and silver (Ag), can strongly absorb visible light due to the surface plasmon resonance (SPR) derived from the collective coherent oscillation of surface electrons.25−27 Thereby, the plasmonic metal NPs have gained considerable interest in extending the photoresponse range of wide bandgap semiconductors.28−30 For example, coupling Au NPs with TiO2 has been widely employed to form visible-light-active photocatalysts with benefit from a strong SPR effect and high chemical stability of Au NPs.31−33 In the Au/TiO2 composite nanostructure, the Au NPs can facilitate visible-light absorption through the SPR effect to enhance the photocatalytic activity of TiO2 and serve as the electron sink to retard the recombination of photogenerated charge carriers of TiO2.34−36 When the Au/ TiO 2 nanocomposites were used for photocatalytic H 2 production, the Au NPs could also act as the active sites for hydrogen evolution reaction (HER).37,38 However, as for the role of electron-sink and HER catalyst, in general Au has much lower activity than Pt.18,20,35,36 Thus, to achieve high efficiency for photocatalytic H2 production, the coloading of Pt NPs into Au/TiO 2 composite structures would be desirable by combining the advantages of both SPR effect of Au NPs and activation effect of Pt NPs for HER. Herein, in this work, we report the successful fabrication of Au/Pt nanoparticle-decorated TiO2 (denoted as Au/Pt/TiO2) Received: September 17, 2013 Revised: November 13, 2013 Published: November 13, 2013 25939

dx.doi.org/10.1021/jp409311x | J. Phys. Chem. C 2013, 117, 25939−25947

The Journal of Physical Chemistry C

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Photocatalytic H2 Production. The photocatalytic H2 production tests were performed in a 40 mL quartz reactor. Typically, 5 mg of the as-electrospun NF photocatalysts was suspended in 10 mL of aqueous solution of L-ascorbic acid (0.1 M, pH = 4 adjusted by 1 M NaOH). This suspension was sealed in the quartz reactor by a rubber plug and then purged with argon gas for half an hour to drive away the residual air. Subsequently, the reactor was exposed under a 300 W Xe lamp (MAX-302, Asahi Spectra Co. Ltd.). The gas product composition from the upper space above the liquid in the quartz reactor was periodically analyzed by an Agilent 7890A gas chromatograph (GC) equipped with a thermal conductivity detector (TCD). Photocatalytic CO2 Conversion. The photocatalytic CO2 reduction tests were carried out in a 90 mL gastight reactor with a quartz plate at the top and sampling ports at two sides. In a typical process, 5 mg of the as-electrospun NF photocatalysts was loaded on a 2 cm × 2 cm glass substrate with 0.1 mL of deionized water. After that, the above glass with the NF photocatalysts was sealed in the reactor. Before the irradiation, high purity CO2 was bubbled through deionized water and then flowed into the reactor for 30 min to remove the residual air. Finally, the reactor loading with the aselectrospun NFs was exposed under a 500 W Xe lamp (66902, Newport Corp.). The gas products were periodically analyzed quantitatively by an Agilent 7890A GC equipped with a TCD.

nanofibers (NFs) through facile electrospinning with tunable molar ratio of Au/Pt to TiO2. The electrospun nanofibrous materials have obtained extensive applications in photovoltaics, gas sensors, and photocatalysis owing to the three-dimensional (3D) open structure, large surface areas, and high porosity.39−42 Researchers have demonstrated that the electrospun TiO2 NFs could act as better photocatalysts compared to the conventional TiO2 particles due to the enhanced charge carrier lifetime and transport rate and light-scattering behavior.43,44 In our studies, we demonstrate that the codecoration of Au and Pt NPs (5−12 nm) in the electrospun TiO2 NFs enables much higher photocatalytic activities than the Au/TiO2 and Pt/TiO2 NFs for not only proton reduction to H2 but also CO2 reduction. It is believed that the cooperation of the Au SPR effect with electron-sink and activation roles of Pt NPs contributes to the improved the photocatalytic performance of Au/Pt/TiO2 NFs toward efficient solar fuel production.

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EXPERIMENTAL SECTION Fabrication of Au/Pt/TiO2 Composite Nanofibers (NFs). The Au/Pt/TiO2 NFs were fabricated through an electrospinning method with postcalcination. In a typical procedure, tetrabutyl titanate (Ti(OC4H9)4) (2.0 mL) and a certain amount of HAuCl4 were added to a mixture solution containing 2 mL of acetic acid and 5 mL of ethanol under vigorous stirring. Then 0.4 g of poly(vinyl pyrrolidone) (PVP) powder (Mw = 1 300 000) was dissolved in the this solution followed by vigorous stirring for 6 h. Subsequently, a certain amount of H2PtCl6·6H2O was added to this solution which was then kept vigorously stirring for another 3 h. Afterward this mixture solution of PVP/HAuCl4/H2PtCl6/Ti(OC4H9)4 was transferred into a plastic syringe with a 23-gauge needle made of stainless steel for electrospinning. The distance between the needle tip and the collector was about 15 cm, and the feeding rate was 2.0 mL·h−1. The dense web of the electrospun NFs of the PVP/HAuCl4/H2PtCl6/Ti(OC4H9)4 composite was fabricated at an applied electric voltage of 15 KV between the needle tip and the collector. Finally, the above composite NFs were calcined in air at 500 °C with a rising rate of 2 °C·min−1 and kept for 2 h at the required temperature. Thus, the finally formed Au/Pt/TiO2 composite NFs are denoted as Au(1−x)/ Ptx/TiO2 NFs in which x is the molar concentration (x%) of Pt to Ti in the precursor solution. In this work, Au1/TiO2, Au0.75/ Pt0.25/TiO2, Au0.5/Pt0.5/TiO2, Au0.25/Pt0.75/TiO2, and Pt1/TiO2 NFs were fabricated to investigate the optimal content ratio of Au to Pt for solar fuels production. Meanwhile, the pure TiO2, Pt0.25/TiO2, Pt0.5/TiO2, and Pt0.75/TiO2 NFs were also fabricated as the control samples for comparison. Characterization. X-ray diffraction (XRD) measurements were carried out by using the Shimadzu XRD-600 X-ray diffractometer with a Cu Kα line of 0.1541 nm. X-ray photoelectron spectroscopy (XPS) measurement was performed on a Thermo Scientific Theta probe XPS with a monochromatized Al Kα (hν = 1486.7 eV) source. Field emission scanning electron microscopy (FESEM; JSM-7600F) and transmission electron microscopy (TEM; JEOL JEM2100) were used to characterize the morphologies and structures of the products. Energy-dispersive X-ray (EDX) spectroscopy being attached to SEM and TEM was used to analyze the composition of products. UV−vis diffuse reflectance spectroscopy (DRS) of the products was recorded on a Lambda 750 UV/vis/NIR spectrophotometer (Perkin-Elmer, USA).

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RESULTS AND DISCUSSION The crystallographic structure and phase purity of the aselectrospun products were identified by X-ray diffraction (XRD) patterns. As shown in Figure 1A, the diffraction peaks

Figure 1. XRD patterns of the as-electrospun NFs: (a) TiO2, (b) Au1/ TiO2, (c) Au0.75/Pt0.25/TiO2, (d) Au0.5/Pt0.5/TiO2, (e) Au0.25/Pt0.75/ TiO2, and (f) Pt1/TiO2.

for all the products present similar profiles and can be perfectly indexed as the tetragonal anatase TiO2 (JCPDS, no. 21-1272). As calculated by the Debye−Scherrer formula, the average crystallite size of TiO2 is 18 nm or so for all products. The enlarged view of XRD patterns (from 35° to 55°) presented in Figure 1B reveals the presence of cubic Au (JCPDS, no. 040784) and/or Pt (JCPDS, no. 04-0802) diffraction peaks for the Au/Pt/TiO2 NFs. These peaks are rather broad, indicating that the sizes of the metal nanoparticles (NPs) are very small. Note that when the metal content is below 0.5% the diffraction 25940



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Figure 2. SEM images of the as-electrospun NFs: (A) TiO2, (B) Au1/TiO2, (C) Au0.75/Pt0.25/TiO2, (D) Au0.5/Pt0.5/TiO2, (E) Au0.25/Pt0.75/TiO2, and (F) Pt1/TiO2. The scale bar is 1 μm.

Figure 3. Typical TEM images of the as-electrospun NFs at low and high magnification: (A, E) TiO2; (B, F) Au1/TiO2; (C, G) Pt1/TiO2; (D, H) Au0.25/Pt0.75/TiO2; (I−L) the HRTEM images from the corresponding square marks in the images (B−D).

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peaks could not be observed due to the weak signal beyond the XRD detection limit. The scanning electron microscopy (SEM) images (Figure 2A−F) reveal that all of the as-electrospun products have continuous fibrous structures with diameters of 130−200 nm (Figure S1, Supporting Information). These NFs have lengths of several micrometers and are aligned in random orientations and interweaved. Researchers have reported that the TiO2 NFs in this size range exhibited very strong light-scattering behavior for enhancing the photocatalytic H2 production.44 Moreover, the energy-dispersive X-ray (EDX) spectra from the corresponding SEM images (Figure S2, Supporting Information) confirm that the element contents of Au and/or Pt in the aselectrospun Au/Pt/TiO2 NFs are very close to the theoretical value in the precursor solution, suggesting nearly no mass loss during the preparation process. Transmission electron microscopy (TEM) images (Figure 3A−H) indicate that the embedded spherical metal NPs with the uniform size and well-dispersed distribution can be clearly distinguished from the NF matrix due to their high electron density. The mean sizes of the metal NPs for the Au1/TiO2 and Pt1/TiO2 NFs are about 7.85 and 5.64 nm (Figure S3, Supporting Information), respectively, which are a little smaller than the corresponding values of the Au0.75/Pt0.25/TiO2 (9.08 nm), Au0.5/Pt0.5/TiO2 (9.31 nm), and Au0.25/Pt0.75/TiO2 NFs (9.18 nm) (Figure S4, Supporting Information). The larger size of the metal NPs for the Au/Pt/TiO2 NFs might be caused by the local aggregation between Au and Pt NPs. Figure 3I−L shows the high-resolution (HR) TEM images from the corresponding color squares of Figure 3B−D, in which the interplanar distances of 0.234 and 0.224 nm correspond well to the lattice spacings of the Au(111) and Pt(111) planes, respectively. These observations suggest that both metallic Au and Pt NPs coexist in the Au/Pt/TiO2 NFs. More detailed information regarding the chemical states of the as-electrospun metal/TiO2 NFs was investigated by X-ray photoelectron spectroscopy (XPS) analysis. Figure 4A presents the Ti 2p core-level spectra of the Au1/TiO2, Pt1/TiO2, and Au0.25/Pt0.75/TiO2 NFs, in which all the above products display two symmetric peaks. The peak centered at around 458.8 eV corresponds to the Ti 2p3/2, and another one centered at 464.5 eV is assigned to Ti 2p1/2.45 The observed spin−orbit splitting between the Ti 2p3/2 and Ti 2p1/2 is 5.7 eV, which is in excellent agreement with the values of the Ti4+ state in TiO2.46 Figure 4B shows the O 1s signal could be fitted into two symmetric peaks. The peaks located at 530.1 and 531.2 eV are ascribed to the crystal lattice oxygen (Ti−O) and surface hydroxyl groups (Ti−OH), respectively.45,46 These results indicate that the chemical states for Ti and O were not changed after Au/Pt decoration. The Au 4f and Pt 4f signals are shown in Figure 4C and D, respectively. The presented Au 4f7/2 and Au 4f5/2 peaks are located at 83.4 and 87.2 eV, respectively, which well agree with the values of the metallic Au0 state.47 The Pt 4f signal of the Pt1/TiO2 NFs shown in Figure 4D could be fitted into six symmetric peaks, suggesting the existence of three states of Pt species. The Pt 4f7/2 peak at 70.6 eV and Pt 4f5/2 peak at 73.9 eV are attributed to the metallic Pt0, while the binding energy peaks found at 72.5, 74.7, 76.1, and 77.8 eV are originated from 4f7/2 of Pt2+, 4f7/2 of Pt4+, 4f5/2 of Pt2+, and 4f5/2 of Pt4+, respectively.48−50 The presence of Pt2+ and Pt4+ states might be attributed to the formation of a Pt−O bond driven by the oxygen chemisorption on the surface of Pt nanostructures, which is in agreement with the literature.48−50 According to the

Figure 4. XPS spectra (A) Ti 2p; (B) O 1s; (C) Au 4f; and (D) Pt 4f of the as-electrospun NFs: (a) Au1/TiO2; (b) Pt1/TiO2; (c) Au0.25/ Pt0.75/TiO2.

XPS spectra, the molar ratios of Pt0/Pt2+/Pt4+ states are estimated as 0.27:0.44:0.29 for the Pt1/TiO2 nanofibers and 0.33:0.40:0.27 for the Au0.25/Pt0.75/TiO2 nanofibers. We note that the content of the oxidation states of Pt in the Pt1/TiO2 nanofibers is slightly higher than that in the Au0.25/Pt0.75/TiO2 nanofibers, which might be attributed to the smaller size of Pt NPs in the Pt1/TiO2 nanofibers, leading to the larger ratio of surface oxidation states. It should be noted that the Au 4f(7/2, 5/2) and Pt 4f(7/2, 5/2) peak positions of the Au0.25/Pt0.75/ TiO2 NFs are nearly unchanged as compared to that of Au1/ TiO2 and Pt1/TiO2 NFs, which suggests no alloy formation for the metallic Au and Pt NPs in the TiO2 NFs. Figure 5 shows the UV−vis absorption spectra of the aselectrospun NFs, which are converted from the measured diffuse reflectance spectra by means of the Kubelka−Munk function. With regard to all products, the intense UV absorption band at wavelengths shorter than 400 nm can be assigned to the intrinsic bandgap absorption of anatase TiO2 (Eg: ∼3.2 eV).8,10 As observed in Figure 5, the TiO2 NFs display a very weak absorption band centered at around 420 nm, which might be attributed to the defects or impurity doping by the polymer template during the sintering process.22,41 In contrast to the pure TiO2 NFs, the Au1/TiO2 NFs exhibit obvious enhancement on light absorption in the visible region with a broad band centered at around 590 nm which can be caused by the surface plasmon resonance (SPR) of embedded Au NPs.23,34,37 However, the SPR absorption band is hardly observed in the spectrum of Pt1/TiO2 NFs (inset of Figure 5) because of the high imaginary part of the dielectric function of Pt.23,24 Note that this SPR peak wavelength is relatively red-shifted as compared to that of similar sized Au NPs due to the high refractive index of the anatase TiO2 NF 25942



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Figure 5. UV−vis absorption spectra of the as-electrospun NFs: (a) TiO2, (b) Au1/TiO2, (c) Au0.75/Pt0.25/TiO2, (d) Au0.5/Pt0.5/TiO2, (e) Au0.25/Pt0.75/TiO2, and (f) Pt1/TiO2. Figure 6. (A) Plots of photocatalytic H2 production amount versus UV−vis light irradiation time over 5 mg of different NFs. (B) H2 production rates of the different NFs (5 mg): (a) TiO2, (b) Au1/TiO2, (c) Au0.75/Pt0.25/TiO2, (d) Au0.5/Pt0.5/TiO2, (e) Au0.25/Pt0.75/TiO2, (f) Pt1/TiO2, (g) Pt0.25/TiO2, (h) Pt0.5/TiO2, and (i) Pt0.75/TiO2. (C) Cycling test of photocatalytic H2 production for the Au0.25/Pt0.75/TiO2 nanofibers.

matrix (n = 2.49).34,51 Interestingly, when Pt NPs are also present in the TiO2 NF matrix, the Au SPR peaks are blueshifted to 540 nm (see Figure 4c−e), which might be ascribed to the existence of more negative dielectric function of Pt NPs.23 The size changes of Au NPs in the Au/Pt/TiO2 NFs with different Au content may also be a reason for inducing the SPR band shift. In short, these Au/Pt/TiO2 NFs displayed strong visible-light absorption rising from the SPR effect of codecorated Au NPs that are capable of enhancing the photocatalytic activities of the NFs toward efficient solar-tofuel conversion. The photocatalytic hydrogen generation over different NFs was evaluated under UV−vis light irradiation in the presence of L-ascorbic acid (0.1 M, pH = 4.0) as the sacrificial reagent to quench the photogenerated holes. Control experiments in the absence of either light irradiation or TiO2-based NFs showed no H2 production. Figure 6A shows the H2 production plots over different NFs as a function of irradiation time. The pure TiO2 NFs have a very low H2 production rate (0.009 μmol·h−1) due to fast recombination of photogenerated charge carriers and large H2 evolution overpotential on TiO2 surfaces. In contrast, the Pt-decorated TiO2 NFs (Pt1/TiO2) exhibit much higher H2 production rate (8.289 μmol h−1) because the Pt NPs in NFs can act as electron sinks to suppress the charge recombination process and serve as active sites for H2 production by lowering the overpotential. Interestingly, when 25% of Pt is replaced by Au in the composite NFs, for example the sample Au0.25/Pt0.75/TiO2, the H2 production rate can be remarkably enhanced to 11.658 μmol h−1. However, as more Pt is replaced by Au, we observe the gradually decreased H2 production rate by using the Au0.5/Pt0.5/TiO2 NFs (9.274 μmol·h−1) and Au0.75/Pt0.25/TiO2 NFs (6.940 μmol·h−1), suggesting that the Pt NPs are the major contribution to the active sites for H2 production. We note that in the absence of Pt the Au1/TiO2 NFs show tremendously lower (at least 18 times) H2 production rate (0.391 μmol h−1), which further decreases with lower Au content in the NFs (e.g., Aux/TiO2, x < 1). This result indicates that the H2 generation on Au particles is fairly minor as

compared to that on Pt particles. Nevertheless, the Au/Pt-codecorated TiO2 NFs with an appropriate Au ratio still can exhibit remarkable enhancement on the H2 generation rate compared with the Pt1/TiO2 NFs, which suggests that the optical response (e.g., SPR) of Au NPs in the NFs must play a crucial role in the photocatalytic H2 generation. To further investigate the effect of Au NPs in the Au/Pt/ TiO2 NFs, we compared the photocatalytic activity of the samples with the same Pt content, for example Ptx/TiO2 NFs versus Au1−xPtx/TiO2 NFs (x = 0.25, 0.5, and 0.75), as shown in Figure 6B. Compared to the corresponding Ptx/TiO2 NFs (x = 0.25, 0.5, and 0.75), the Au0.75/Pt0.25/TiO2, Au0.5/Pt0.5/TiO2, and Au0.25/Pt0.75/TiO2 NFs show an enhancement factor of 2.4, 1.3, and 1.5, respectively, in the H2 production rate. Since the H2 production on Au sites is very low as aforementioned, this comparison demonstrates that codecoration of Au NPs into Pt/ TiO2 composite structures enables great enhancement on the photocatalytic efficiency through the plasmonic excitation of Au NPs in the composites. Furthermore, we have tested the stability and reusability of Au0.25/Pt0.75/TiO2 NFs for the photocatalytic H2 evolution through a cycling test. As shown in Figure 6C, the result indicates no obvious decrease of the photocatalytic activity after the three cycles, demonstrating the good stability of the Au0.25/Pt0.75/TiO2 NFs. To generalize the hypothesis of plasmon-enhanced photocatalytic activity from codecorated Au NPs, we employed a more challenging reaction, photoreduction of carbon dioxide (CO2) with water vapor, as a model system to evaluate the photocatalytic activities of the Au/Pt/TiO2 NFs. Photocatalytic CO2 conversion into hydrocarbon fuels has been considered as a promising way to solve the green-house and energy shortage problem.3,52−55 According to the literature,3,12,55 the mecha25943



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light irradiation, CO2 can be reduced to CH4 and CO in the presence of water vapor by using the as-electrospun NFs as the photocatalysts. The pure TiO2 NFs (Figure 7a) show poor CH4 production (0.03 μmol h−1) and high CO production (0.06 μmol h−1) after irradiation for 3 h. In comparison, the Pt1/TiO2 NFs (Figure 7d) display more than 10 times higher activity for CH4 production (0.42 μmol h−1) and slightly higher activity in CO production (0.08 μmol h−1) because the Pt NPs can act as an electron-sink to promote charge separation on TiO2 surfaces, while the Au1/TiO2 NFs (Figure 7b) showed relatively lower activity in CH4 production (0.31 μmol h−1) with higher CO production (0.20 μmol h−1). This is because the electronsink effect of Au NPs is lower than that of Pt NPs due to its higher work function of −5.1 eV (−5.65 eV for Pt), thus the multielectron reaction process would occur more easily on the Pt/TiO 2 . 3,12,16 Consistent with the observation in H 2 production, when some Pt in the NFs is replaced by Au, the Au0.25/Pt0.75/TiO2 NFs (Figure 7c) exhibit 1.35 times higher activity in CH4 production (0.57 μmol h−1) than the Pt1/TiO2 NFs. This observation further demonstrates that the codecorated Au NPs in the NFs provide additional enhancement on the photocatalytic activity of CO2 reduction through surface plasmon excitation. The possible principles of plasmon-enhanced photocatalytic activity over the Au/Pt/TiO2 NFs are illustrated as Figure 8. According to literature studies,56−58 the SPR effect of Au NPs in the NFs could contribute to the photocatalytic process through direct transfer of plasmon-excited “hot” electrons and/ or the strongly enhanced local electric field. Upon SPR excitation, the surface electrons on Au NPs may be excited to high-energy level and transfer into the conduction band (CB) of TiO2 through the Au−TiO2 interfaces for reduction reaction on TiO2 surfaces. The remaining holes on Au NPs are capable of implementing some oxidation reaction in the presence of appropriate electron donors, for instance ascorbic acid in the current study for H2 production. However, when we utilized the irradiation at 550 ± 20 nm, which directly excites the Au SPR, for the photocatalytic test of all Au/Pt/TiO2 NFs, no H2 evolution was observed, suggesting that the SPR-induced hot electrons of Au NPs have no contribution to the photocatalytic H2 production in the present study.

nism of CO2 photoreduction is the proton-assisted multielectron instead of a single electron transfer process. The gas products are strongly dependent on the number of protons and electrons. CO production needs only two protons and two electrons (CO2 + 2H+ + 2e− → CO + H2O), while CH4 can be produced directly through a reduction process involving eight electrons and eight protons (CO2 + 8H+ + 8e− → CH4 + 2H2O) or formed by the depletion of as-produced CO through a six-electron reaction (CO + 6H+ + 6e− → CH4 + H2O). Control experiments in the absence of CO2 gas showed no CH4 or CO production, indicating that the CH4 and CO products were generated from CO2 reduction rather than surface contaminants. As shown in Figure 7a−d, under UV−vis

Figure 7. Plots of CH4 and CO production amounts as a function of UV−vis light irradiation time over 5 mg of different NFs: (A) TiO2, (B) Au1/TiO2, (C) Au0.25/Pt0.75/TiO2, and (D) Pt1/TiO2.

Figure 8. Schematic diagram showing the photocatalytic process for H2 production and CO2 reduction on the Au/Pt/TiO2 NFs. 25944



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from the 973 Program (Grant No. 2012CB626801) and National Natural Science Foundation of China (Grant No. 11274057).

Therefore, a more possible mechanism of SPR effect may be attributed to the strongly enhanced localized electric field near the Au/TiO2 interface, where the electron−hole separation of photoexcited TiO2 may be promoted through plasmon− exciton coupling.25,28−33 This would allow a longer lifetime for photogenerated electrons to diffuse toward Pt NPs, which serve as electron-sink and active sites, for proton reduction to H2 or CO2 reduction to CH4. In short, the overall high photocatalytic activity of Au/Pt codecorated TiO2 NFs in fuel production can be attributed to the synergy between the SPR effect of Au NPs and the electron-sink effect of Pt NPs. Nevertheless, the photocatalytic reactions on Au NPs should not be fully excluded in our studies even though the Au NPs show higher work function and higher H2-production overpotential as compared to Pt NPs. In addition, compared to conventional bulk TiO2 structures, the unique structure of the electrospun TiO2 NF web with strong light-scattering features could enhance the light harvesting capability of the entire structure and increase the contact between the Au/Pt/TiO2 NFs and the reagent, which might also improve the photocatalytic activity of TiO2 for solar-to-fuel conversion.

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CONCLUSIONS In summary, by using a facile electrospinning technique, we have successfully fabricated Au/Pt nanoparticle-decorated TiO2 composite nanofibers with different molar ratios of Au and Pt. The Au/Pt/TiO2 NFs exhibit remarkably improved photocatalytic activities for not only hydrogen generation but also CO2 reduction as compared to the TiO2 NFs with only single metal decoration (Pt/TiO2 and Au/TiO2 NFs). These enhanced photocatalytic activities through codecoration of Au and Pt NPs are attributed to the synergy of electron-sink function of Pt NPs and Au SPR effect that improves charge separation of photoexcited TiO2. Our studies demonstrate that through rational design of composite nanostructures one can utilize a high-energy photon (blue or UV region) in the solar spectrum to generate charge carriers for photocatalytic reactions, and meanwhile, the visible light in the solar spectrum can be synergically used for SPR excitation to enhance the charge separation and photocatalytic efficiency as well. This provides a more effective way to harvest solar energy for fuel production.

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ASSOCIATED CONTENT

S Supporting Information *

The EDX results and size distribution histograms of the aselectrospun nanofibers with additional SEM and TEM images (Figure S1−S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +65 6790 6180. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from NTU seed funding for Solar Fuels Laboratory, MOE AcRF-Tier1 RG 44/ 11, MOE AcRF-Tier2 (MOE2012-T2-2-041, ARC 5/13), and CRP program (NRF-CRP5-2009-04) from the Singapore National Research Foundation. Z. Y. Zhang thanks the support 25945



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Pt Nanoparticle-Decorated TiO2 Nanofibers with Plasmon

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