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Feb 4, 2015 - Hybrid Au/ZnO Hexagonal Pyramid Nanostructures: Preferred. Growth on the Apexes of the Basal Plane than on the Tip. Mingli Yue,. †...
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Hybrid Au/ZnO Hexagonal Pyramid Nanostructures: Preferred Growth on the Apexes of the Basal Plane than on the Tip Mingli Yue,† Ming Yang,*,† Dan Zhang,‡ Di Xiang,† Ying Hou,*,† and Jiecai Han*,§ †

Key Laboratory of Microsystems and Micronanostructures Manufacturing, ‡School of Chemical Engineering and Technology, and Center for Composite Materials and Structures, Harbin Institute of Technology, 2 Yikuang Street, Harbin 150080, P. R. China

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S Supporting Information *

ABSTRACT: Nanoscale materials having size- and shape-dependent interactions with light provide flexible opportunities for harvesting solar energy. Photocatalysts based on semiconductor nanoparticles (NPs) have been the most effective materials for the conversion of light into chemical energy, the efficiency of which can be further enhanced by the incorporation of metallic NPs forming hybrid nanostructures. The structural parameters of not only constituent components but also the resultant hybrid nanostructures are critical for the optimization of photocatalytic performance of composite catalysts. Here we demonstrated the successful size control over ZnO hexagonal pyramids (HPs) for the first time. The smallest HPs showing the best photocatalytic properties were used for further Au attachment. Interestingly, we found that most of the Au NPs preferred to grow on the apexes of the basal plane. Very occasionally, Au NPs at the tip of ZnO HPs can be observed. The role of light in promoting the reduction of gold salt by sodium citrate was also revealed. Quantum mechanical calculations were used to explain the site-specific growth of Au on the surface of ZnO HPs. Enhanced degradation rates over organic dyes were found for Au/ZnO hybrids under both UV and visible light irradiation.

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INTRODUCTION Size- and shape-controlled synthesis of nanoparticles (NPs) represents a foundation for the development of nanotechnology with an essential role for the design of functional devices based on nanoscale building blocks.1−3 This is also quite relevant for the catalyst design which has been shown to be important for both the catalytic reactivity and the selectivity exemplified by metallic NPs such as Pt,4 Au,5 and Ag6 and semiconductor NPs including TiO27 and ZnO.8 Generally, nanospheres with smaller sizes can have more surface reactive sites and are preferred as photocatalysts. However, the symmetric shape determines a higher electron−hole recombination rate in smaller nanospheres,9 potentially diminishing the advantage of higher surface area. The use of NPs with anisotropic shapes may improve the lifetime of charge carriers due to their less localized nature and longer free transport channels.7 A well-established bottom-up approach has allowed the emergence of NPs with various nonspherical shapes including rods,10,11 tetrapods,12 octapods,13 and cubes,14 just to name a few. Geometric parameters such as length for rods can be controlled usually through the variations of reactant concentrations or relative ratios of precursors and surfactants, thereby achieving size-dependent catalytic properties.15 However, for the synthesis of some particularly shaped nanocrystals such as hexagonal pyramids (HPs),16−18 it still remains a constant challenge to control their dimensions because typical strategies for size control may result in the shape variation due to the restrictive formation conditions required. Also, the size and © XXXX American Chemical Society

shape of NPs are usually interdependent. For example, it was found that for TiO219 and Pt20 NPs, the evolution of different shapes is accompanied by the size increase. The establishment of experimental systems leading to different sized NPs with unique shapes such as HPs will be therefore fundamentally important for the better understanding of size- and shaperelated properties, making possible the advent of novel physical and chemical characteristics. An alternative way to further enrich the properties of NPs is the combination of different materials on one entity. The resulting hybrid structures can possess multiple functionalities with a usually observed synergetic effect.21,22 One particularly interesting combination of materials is that of a metal and a semiconductor forming either a Schottky barrier or Omhic contact depending on the difference between the interfacial conduction band edge of semiconductors and the Fermi level of metals. In hybrid semiconductor−metal (S−M) nanostructures, the interesting properties of individual components such as size-dependent emission23 for semiconductor and plasmonic absorption24 and fluorescence25 for metal NPs can be adjusted according to the way they connect. One notable example is the exciton−plasmon interactions, which can result in either quenching or enhancement of the emission from semiconductors according to the relative positions of the two Received: December 17, 2014 Revised: January 26, 2015

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The Journal of Physical Chemistry C components.26 Interesting collective properties of such hybrid structures may be further achieved based on the self-assembly process thanks to their colloidal nature.27 The collective interactions of hybrid S−M nanostructures with electromagnetic radiation make them ideal materials as photocatalyst.28 Typically, the light-induced charge carriers in semiconductor NPs serve as active species for redox reactions, which however are labile due to their rapid recombination.29 The attachment of noble metals with great capacity for accommodating electrons has been proposed to increase the photocatalytic efficiency by spatially separating the photogenerated electrons and holes.30,31 Extensive studies have been devoted to the synthesis of composite catalysts based on semiconductors such as TiO2,32−35 CdS,36−39 and ZnO.40−43 Besides the traditional enhancement mechanism, recent studies also highlighted the importance of hot electron injections into the conduction band of semiconductors in accelerating the redox reactions.44−48 This is made possible by the strong interactions of metal NPs with visible light due to the presence of free electrons, known as localized surface plasmon. Such effect also allows the extension of the absorption edge of semiconductor photocatalysts from UV to visible light, important for better utilization of solar energy.49 Similar ideas have been used to improve the efficiency of photovoltaics through strengthening light trapping by scattering from metal NPs.50 To further improve the catalytic efficiency of hybrid S−M nanostructures, the semiconductor part of the hybrids should be tailored for optimal light absorption better with exposed crystal planes showing high catalytic activity. Engineering the nanoscale connection with variable metallic nanostructures and understanding the formation mechanism of hybrids will be also quite useful for addressing the critical photocatalytic parameters. Efforts toward the resolution of nontrivial interfacing of materials with different compositions and crystal structures have been successful in the past years and resulted in a diverse spectrum of material combinations.51 Both nonspecific30,32 and site-specific growth36,40 of metallic phases on the body of the semiconductors have been achieved, which is very useful for revealing the charge transfer mechanism.44 In this contribution, we started from the geometric control over the semiconductor part. For the first time, we achieved the size-controlled synthesis of ZnO hexagonal pyramids (HPs) in a methanol−water cosolvent system showing size-dependent properties. Au/ZnO hybrids were further synthesized aiming at the enhancement of photocatalytic properties. It was found that Au NPs preferably grew on the apexes of the basal plane with much less common deposition at the tip. Our observation also indicated the role of light in promoting the reduction of gold salt by sodium citrate at room temperature. Possible mechanisms responsible for the specific growth of Au NPs were proposed according to the quantum mechanical calculations. The superior photocatalytic properties of the hybrids under both UV and visible light irradiation were demonstrated.

cooled naturally to room temperature. The obtained product was first purified by centrifugation and then washed 3 times with methanol before drying in the air. For the synthesis of ZnO nanorods, the same procedure was used except that 2.8 g of KOH was added. Synthesis of Au/ZnO Hybrids. For the preparation of Au/ ZnO hybrids, 0.05 g of ZnO hexagonal pyramids was dispersed into 25 mL of 3 mM sodium citrate solution by sonication for 10 min, forming a homogeneous mixture. After that, 75 mL of 0.5 mM HAuCl4 aqueous solution was added and stirred at room temperature for 48 h under natural light or in the dark. The resulting product was first purified by centrifugation and then washed 3 times with DI water before drying in the air. Characterization. To study the morphologies of different samples, one drop from the ZnO or Au/ZnO dispersions was placed on a holey carbon support. The specimens were then allowed to dry at room temperature and then can be used for TEM observation (HRTEM, FEI TECNAI). Dried samples in the powder form were used for obtaining XRD patterns (XRD, D/max-rb). A U4100 UV− vis−NIR spectrometer (Hitachi, Japan) was used to characterize the light absorption properties of different solution samples. The fluorescence spectra were measured using a Fluoromax-4 fluorescence spectrophotometer (HORIBA Jobin Yvon, Japan) with an excitation wavelength at 360 nm. The photocatalytic activities of different samples were measured by the degradation of Rhodamine B (RhB, 5 mg/L) under UV and visible light. A 500 W mercury lamp was used as the UV source, and a 500 W metal halide lamp with a UVcutoff filter (>400 nm) was used as the visible light source. In a typical experiment, 10 mg of dried samples was added into 50 mL of an aqueous solution of RhB. Before photoirradiation, the suspensions were stirred for 1 h in the dark to establish an adsorption−desorption equilibrium between the photocatalyst and the organic dyes. The mixture was then transferred into a quartz tube (60 mL) which was placed 5 cm away from the light source. After the reaction mixture was irradiated under continuous stirring for a given time, about 3 mL of the suspension was taken out and centrifuged to get a clear solution. The degradation of RhB was monitored by probing the intensity of its absorption peak at 550 nm. The degradation performance was evaluated by calculating C/C0 (C is the concentration of RhB after irradiation for a given time, and C0 is the initial concentration of RhB), which can be derived from the absorbance according to the Lambert−Beer law. Quantum Mechanical Calculations. The calculation was performed using a semiempirical parameter model 3 (PM3) under a single-point energy mode. More detailed information about the calculations can be found in the Results and Discussions.

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RESULTS AND DISCUSSIONS Size-Controlled Synthesis of ZnO HPs. ZnO as an important wide band gap semiconductor has been the focus of fundamental and industrial research.52 Intriguing applications based on its optical/electrical properties have been demonstrated including ultraviolet lasers,53 nanogenerators,54 and bioimaging.55 ZnO has also been extensively studied as photocatalysts and an alternative to TiO2.56−62 Due to their polar crystal structures, anisotropic growth can be easily achieved resulting in nanostructures such as nanorods,10 nanowires,53 and nanotubes.63 For hexagonal pyramids (HPs), however, the syntheses were typically carried out by employing long-chain organic molecules as stabilizers.16,18

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EXPERIMENTAL SECTION Synthesis of ZnO Hexagonal Pyramids with Different Sizes. In a typical reaction, 5.5 g of Zn(Ac)2·H2O and 0.2 g of KOH were first added into different solvents (50 mL of pure methanol; 45 mL of methanol and 5 mL of water; 40 mL of methanol and 10 mL of water). The mixed solution was then stirred and heated to reflux for 48 h. The reaction was then B

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Figure 1. TEM images and size distributions of ZnO hexagonal pyramids obtained at different solvent conditions: (a−c) 50 mL of methanol, (d−f) 5 mL of water and 45 mL of methanol, and (g−i) 10 mL of water and 40 mL of methanol. (c, f, and i) Dotted lines are Gaussian fitting curves.

Figure 2. (a) XRD patterns, (b) UV−vis spectra, and (d) photodegradation curves of RhB under UV irradiation of ZnO HPs with different sizes. (c) UV−vis spectra of RhB after different UV irradiation time in the presence of 15 nm ZnO HPs.

These thick organic coatings can prevent the direct contact with solvent species, potentially affecting their effective use as photocatalyst. Furthermore, the current synthesis strategies faced the difficulties in obtaining size-controlled ZnO HPs. The interdependent nature of shape- and size-control parameters

makes the dimensional manipulation over HPs very challenging. Here we employed a dissolution−recrystallization process which was coupled with the disassembly of NPs17 to achieve the size control over ZnO HPs for the first time. The reaction C

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The Journal of Physical Chemistry C proceeded in a simple polar solvent (methanol) in which the formation of ZnO NPs is typically a result of the hydrolysis process of zinc precursors.64 Importantly, the shape of ZnO NPs in the current synthesis is determined by the relative ratio of the reactants, i.e., the presence of an excess amount of acetate groups is the critical factor for the formation of HPs. For example, we found that if the mole ratio of KOH to Zn(Ac)2 is increased to 2:1, ZnO nanorods will be obtained instead of HPs (Supporting Information, Figure S1). The shape independence on the solvent compositions allowed us to vary the amounts of water to control the sizes of ZnO HPs. We note that such procedure may not be effective in other systems because the relative amounts of different solvents are also essential for shape control.16,18 It was found that the amount of water indeed has a dramatic effect on the synthesis. Typically, the addition of more water will produce HPs with larger sizes (Figure 1). ZnO HPs with a side length of ca. 15 nm can be obtained in pure methanol (Figure 1a−c). Larger sizes can be achieved by a simple variation of the volume ratio of methanol to water (Figure 1d−i). For example, the addition of 5 or 10 mL of water can lead to pyramids with side lengths of ca. 80 (Figure 1d−f) or ca. 170 nm (Figure 1g−i), respectively. The size increase of HPs is probably due to a faster growth of crystalline ZnO phases in the presence of water, which can compete with the disassembly of smaller HPs.17 XRD patterns of all these samples can be assigned to the hexagonal phase of ZnO (Figure 2a). The UV−vis spectra showed a continuous downshift of absorption onsets along with the size increase (Figure 2b). The photocatalytic properties were investigated by the study of photodegradation of Rhodamine B (RhB), which can be monitored by absorption spectra (Figure 2c). Importantly, the catalytic properties of these HPs were found to be size dependent (Figure 2d). The superior photocatalytic properties of 15 nm ZnO HPs can be largely attributed to their larger surface areas. For smaller HPs, more active sites are available for the generation of free radicals by the reaction with electrons and holes.62 Smaller HPs may also have a longer exciton lifetime, which can result in a lower recombination rate.9 Synthesis of Hybrid Nanostructures and Proposed Formation Mechanism. The smallest ZnO HPs with the best photocatalytic performance will be used as substrates for the synthesis of Au/ZnO hybrid nanostructures. Compared with previous studies,42,65,66 ZnO HPs used in this work for attaching metallic NPs are much smaller with no organic ligands coating the surface. A citrate reduction process, which was commonly used for the synthesis of Au NPs,67 was employed to deposit Au on the surface of ZnO HPs. We performed time-dependent UV−vis spectroscopy to monitor the formation process of Au NPs at room temperature. It was found that the plasmonic absorption from metallic phases was gradually established along with the proceeding of the reaction (Figure 3e). After 48 h, a well-defined absorption peak around 540 nm can be observed (Figure 3e). Interestingly, we found that Au NPs preferred to grow at the apexes of basal plane (Figure 3a−c). It was rare to find hybrid nanostructures consisting of Au NPs at the tips of the HPs (Figure 3d). Au NPs with a diameter of ca. 3−4 nm are distinguished from ZnO HPs clearly due to the different contrast (Figure 3b−d). The crystal lattices in HRTEM images are in accord with Au (111) and ZnO (001) planar distances, respectively (Figure 3c). The XRD pattern further confirmed the presence of both ZnO and Au in the hybrid nanostructures without any detectable

Figure 3. TEM images of Au/ZnO hybrids showing (a−c) dominant products with Au at the apexes of the basal plane and (d) much less common observed structures with Au at the tip of HPs. (e) Timedependent UV−vis spectra for the formation of Au/ZnO hybrids, and (f) XRD pattern of Au/ZnO hybrids.

impurities (Figure 3f). We occasionally found that light has an important influence on the formation of Au NPs in the presence of ZnO HPs. Under dark condition, no plasmonic absorption of Au NPs can be identified even after a 48 h reaction (Figures S2 and S3, Supporting Information), which indicated that the reduction of gold salts was assisted by light. The photolysis mechanism may first involve the dissociation of AuCl4 in water to generate AuCl3 and a chlorine atom; the subsequent donation of hydrogen from sodium citrate can generate strongly reducing radicals, which can initiate the formation of Au NPs.68 The much slower formation of Au NPs compared with previous work68 is due to the weak UV emission from natural light. We note that the absorption of light by ZnO is also possible, which can potentially result in the generation of charge carriers that are active for the reduction of Au3+. The latter mechanism may be used to explain the much less common growth of Au NPs at the tips of HPs (vide inf ra). We used quantum mechanical calculations to understand better the reasons for the site-specific growth of Au NPs. We first built several ZnO HP atomistic structures (Zn69O106H839+, Zn105O 153 H11115+, Zn102 O150H10913+, Zn95O143O 105 9+, and Zn83O131H1004+) with different degrees of truncations. The atomistic model of Zn69O106H839+ is a complete HP, and others are HPs with different truncations. In these structures, the exposed crystal surfaces with hexagonal packing are assumed to be O terminated, and H atoms are added to form an outmost OH layer. The established atomistic structure is reasonable considering the aqueous system involved in our experiments and consistent with the results from the previous studies.69 The net formal charge of the model is calculated from the atomic stoichiometry by associating H, Zn, and O with a charge of +1, D

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Figure 4. Atomistic models in ball and wire mode and electrostatic potential mappings for (a1, a2, a3) Zn69O106H839+, (b1, b2, b3) Zn105O153H11115+, and (c1, c2, c3) Zn102O150H10913+, respectively. Negative electrostatic potential is represented by colors toward the red, while positive electrostatic potential is represented by colors toward the blue.

+2, and −2, respectively. To calculate the electrostatic potential distribution, the Merck molecular force field (MMFF) was used to optimize the geometric model to achieve an energyminimized configuration. The equilibrium structure will be used to calculate the electrostatic potential mapping using the semiempirical parameter model 3 (PM3) under a single-point energy mode. Interestingly, the calculations showed that for the first three structures of Zn 69 O 106 H 83 9+ (Figure 4a), Zn105O153H11115+ (Figure 4b), and Zn102O150H10913+ (Figure 4c), the parts with the most negative electrostatic potential reside at the apexes of the basal plane (Figure 4). When ZnO HP is further truncated, the most negative parts will move to the apexes of the top plane, illustrating the importance of anisotropy in determining the surface properties of ZnO HPs (Figure S4, Supporting Information). This result also indicates that a possible way to achieve site-specific growth of Au is to vary the extent of truncation. To link the results from the calculation with the experimental observation, it is helpful to first note that ZnO HPs synthesized in this work are nearly complete ones with only small truncations. The first three atomistic models therefore more reasonably reflect the experimental conditions. Using the calculation results based on the first three structures, the reaction mechanism may be described as follows: first, more Au3+ will be present around the apexes of basal plane due to their more negative electrostatic potential; second, sodium citrate will reduce Au3+, which is triggered by light forming Au NPs; third, once Au nanoclusters have deposited on one of the apexes, further crystal growth will continuously occur at this site. During the synthesis, heterogeneous nucleation is more

favorable due to the lower interfacial energy and can be further promoted by the slow reduction of Au3+. However, the above-mentioned mechanism could not explain the growth of Au NPs at the tips of ZnO HPs. Even if the probability of finding such hybrid structure is very low, its presence has been confirmed by TEM observation (Figure 3d). We therefore seek a way to explain its formation. As mentioned before, it is also possible for ZnO HPs to absorb light, which may result in an excited state with active species for the reduction of Au3+. Our attention was then focused on locating the lowest unoccupied molecular orbitals (LUMOs) because electrons will be promoted to LUMOs from the highest occupied molecular orbitals (HOMOs) under excitation. We found that for a perfect HP (Zn69O106H839+) the most important LUMOs would stay at the tips of HPs (Figure 5a). However, for a truncated one (Zn105O153H11115+), LUMOs will move to the apexes of the basal plane (Figure 5b). We therefore speculated that an excited state of ZnO is involved during the growth of Au NPs at the tip. The photogenerated electrons more likely located at the tips of ZnO HPs can react with Au3+, resulting in the deposition of Au at this site. According to the experimental observation, this mechanism is much less important than the first one. The possible reasons could include the following: first, the presence of excited ZnO is very limited due to the low absorption efficiency of light; second, the recombination of photogenerated electrons and holes will reduce the probability for the reduction of Au3+; third, the concentration of Au3+ around the tip is much lower than that around the basal plane due to the distribution of electrostatic potential; fourth, most of the HPs are incomplete ones with small truncations. E

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into the conduction band (CB) of ZnO HPs (path 2 in Figure 7d). Free radicals such as •O2 may form by reaction between the injected electrons and the surface-absorbed O2 (Figure 7d). These active oxygen radicals are more likely responsible for the oxidation of RhB under visible light, different from the photocatalytic degradation of organics under UV irradiation where free radicals such as •OH may play a more important role.70 Photoluminescence spectra with an excitation wavelength of 360 nm were used to understand better the interactions between ZnO and RhB. The measurement showed that the addition of RhB initially caused quenching of the exciton emission72 (Figure 7a), which may be understood by considering the transition of photoexcited electrons from ZnO to the ground state of RhB. However, further increase of the concentrations of RhB resulted in the emission enhancement (Figure 7a). Such result indicates that electrons are transferred from the excited state of RhB to the CB of ZnO (path 2 in Figure 7d), which can then recombine with holes at the VB generated by the excitation. Upon the attachment of Au, electron transfer to the Au part directly from S* (path 3 in Figure 7d) or via the CB of ZnO (path 2 and 4 in Figure 7d) is possible after excitation (path 1 in Figure 7d) due to its more negative Fermi energy level.30 The charge transfer from ZnO to Au can be evidenced by the strong quenching of the emission after the attachment of Au (Figure 7c). This process can further promote the electron transfer from excited dyes to ZnO by regulation of the electron population in the CB. As a result, a more obvious emission enhancement in the presence of RhB was observed for the hybrids (Figure 7b). The higher photodegradation rate (Figure 6b) can be therefore attributed to the better charge-carrier separation, which can generate more active oxygen species. At present it remains a challenge to selectively attach Au on the tip or at the apexes of basal plane, which prevents direct comparison of the relative photocatalytic properties of two

Figure 5. Atomistic models in ball and wire mode showing the positions of LUMOs for (a1, a2) Zn69O106H839+ and (b1, b2) Zn105O153H11115+, respectively.

Enhanced Photocatalytic Properties. Improved photocatalytic properties were found for Au/ZnO hybrids under both UV and visible light irradiation when compared with ZnO HPs (Figure 6a and 6b) using the photodegradation of Rhodamine B (RhB) as a probe (Figure 6c and 6d). The mechanism for such enhancement under UV irradiation has been well established and could be ascribed to the prevention of electron accumulation on the semiconductor part.70 The spatial chargecarrier separation limits the probability for the recombination of electrons and holes, which are responsible for the degradation of organic dyes through oxidation. Under visible light, the decolorization by ZnO HPs (Figure 6b) can be explained by the photosensitization effect,71 which involves the charge injection from the excited sensitizer (S*)

Figure 6. Photodegradation curves of RhB under (a) UV and (b) visible light irradiation. UV−vis spectra of RhB after different irradiation time in the presence of Au−ZnO hybrids using (c) UV and (d) visible light. F

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Figure 7. (a−c) Photoluminescence spectra of ZnO and Au−ZnO hybrids in water and different RhB solutions with an excitation wavelength of 360 nm. (d) Possible mechanism for the enhanced photocatalytic properties under visible light irradiation. Main charge transfer processes are indicated: (1) the transition of electrons from the ground to the excited state; electron transfer from the excited state of dyes to (2) the CB of ZnO and (3) the Femi level of Au; (4) electron transfer from the CB of ZnO to the Femi level of Au. The dotted line represents the Femi level at equilibrium.

degree of truncations may result in the site-selective growth of metallic phase and allows the investigation of structuredependent photocatalytic properties. A deeper understanding of the origin of the optical activity under visible light will be also quite useful for rationalizing the design of photocatalysts with a broad absorption band.

different hybrid structures. The main difficulty lies in the fact that it is hard to obtain a complete pyramid, which however seems critical for the deposition of Au on the tip. We believe the high surface energy associated with high curvature is responsible for the dissolution of the tip, forming more stable truncated pyramids. However, it may be still possible and interesting to make some predictions. As the electron transfer from ZnO to Au is the main mechanism for the photocatalytic enhancement, it could be reasonable to believe that an Au on the tip could fulfill a better task: for such hybrid structure, Au sits at the position where the most important LUMOs exist, which means the photogenerated electron (under UV irradiation) or electrons transferred from organic dyes (under visible light irradiation) will more readily transfer to Au, resulting in more effective inhibition of carrier recombination. However, there might be other factors influencing the ultimate performance. For example, the strain-mediated electron transfer process may be also important here.73 The variation of molecular orbitals after the attachment of Au may also need to be considered.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of ZnO nanorods obtained when the mole ratio of Zn(Ac)2 to KOH is 1:2; optical images of the reaction mixture after 48 h under natural light and dark condition; timedependent UV−vis spectra for the formation of Au/ZnO hybrids under dark condition; electrostatic potential mapping of further truncated ZnO pyramids. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

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*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

CONCLUSIONS In this work, the first example of size-controlled synthesis of ZnO HPs was demonstrated in a simple polar solvent system. The smallest HPs showing the best photocatalytic properties were further modified by Au NPs with site-specific growth mostly at the apexes of the basal plane and very occasionally at the tip. The role of light in promoting the reduction of gold ions by sodium citrate was also indicated. A good explanation for hybrid formation was given by our quantum mechanical calculations. Enhanced photocatalytic properties under both UV and visible light irradiation were observed for the hybrid structures, and the mechanisms for the improvement were discussed. We believe the presented work may be a guide to future research for controllable synthesis of semiconductor− metal nanohybrids. For example, a focus on the control of the

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.Y. is thankful for the financial support from the National Natural Science Foundation of China (Grant No. 21303032), China Postdoctoral Science Foundation (Grant No. 2014M550184), Heilongjiang Postdoctoral Science Foundation (Grant No. LBH-Q13074), HIT Young Talent Program (Grant No. AUGA5710050613), and Fundamental Research Funds for the Central Universities (Grant No. HIT. IBRSEM. A. 201406). Y.H. is thankful for the financial support from HIT 100-talent program (Grant No. AUGA5710006813) and G

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Fundamental Research Funds for the Central Universities (Grant No. HIT IBRSEM. A. 201405). J.H. is thankful for the financial support from the National Natural Science Foundation of China (Grant No. 11421091).

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DOI: 10.1021/jp512570b J. Phys. Chem. C XXXX, XXX, XXX−XXX