Charge Transfer in Au Nanoparticle–Nonpolar ZnO Photocatalysts

Aug 5, 2014 - Center for Condensed Matter Sciences, National Taiwan University, Taipei .... Lixin Song , Yang Su , Yunfeng Guo , Lingnan Wu , Tao Zhan...
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Charge Transfer in Au Nanoparticle−Nonpolar ZnO Photocatalysts Illustrated by Surface-Potential-Derived Three-Dimensional Band Diagram Wan-Hsien Lin,†,‡ Jih-Jen Wu,*,† Mitch M. C. Chou,§ Yu-Ming Chang,∥ and Masahiro Yoshimura‡ †

Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Promotion Center for Global Materials Research (PCGMR), Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan § Department of Materials Science and Optoelectronic Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan ∥ Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: Au nanoparticle (NP)-enhanced activity of a semiconductor in ultraviolet (UV) photocatalysis is generally observed. However, the photoinduced charge transfer behavior and the beneficial role of Au NPs in promoting photocatalytic reactions remain controversial. In the present work, the surface potentials (SPs) of Au NP-nonpolar ZnO composites in the dark and under UV irradiation were measured using Kelvin probe force microscopy (KPFM). On the basis of the KPFM results, the surface photovoltages (SPVs) of Au NP-ZnO composites were obtained by calculating the difference between the SP values acquired under UV irradiation and in the dark. Three-dimensional band diagrams of the Au NP-ZnO photocatalysts after equilibrium in the dark and under UV irradiation were thus constructed. Accordingly, charge transfer between three fundamental interfaces, namely Au NP/ZnO, ZnO/solution, and Au NP/solution, in the tested photocatalytic system is clearly described. With the positive SPV values of photocatalysts, the excess holes in the photocatalyst under steady-state UV irradiation are likely the major contributor in the present work. Furthermore, the SPV values of the Au NP-ZnO photocatalysts, as an indication of average excess carrier concentration, show a systematic correlation with photocatalytic activity. This result suggests that the SPV value of a photocatalyst could be a reasonable index for the evaluation of photocatalytic activity.

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O2 + e−CB → •O−2

INTRODUCTION In photocatalytic processes, oxidation and reduction reactions may occur on the surfaces of semiconductors once the energy potentials of the photoinduced charge carriers are appropriate. TiO2, a well-known ultraviolet (UV) photocatalyst, has been extensively employed in photocatalysis.1,2 ZnO has a band edge position similar to TiO2 and is thus a promising alternative.3 Several groups have investigated the mechanisms as well as the intermediate species in UV degradation of organic compounds with TiO2.1,4 Methyl orange (MO), a typical organic azo-dye, is usually utilized as a pollutant in photocatalysis. The mechanisms of photodegrading MO solution with ZnO are proposed in eqs 1−5.5 Hydroxyl radicals (•OH) and superoxide radicals (•O−2 ), active species created at the interface between photocatalysts and water, have been recognized to play crucial roles in the photodegradation process of organic pollutants.4,6 ZnO + hv → e−CB + h+VB

(1)

H 2O + h+VB → •OH + H+

(2)

−

OH +

h+VB

→ •OH

•OH/•O−2 + MO → SO24 − + NO−3 + NH+4 + CO2 + H 2

(5)

Many strategies have been used to enhance the photocatalytic activity of ZnO. For example, the surface area of ZnO photocatalysts has been increased by varying the particle size and/or geometric structures.7−10 Loading metal particles on ZnO11−15 and the fabrication of oxide-ZnO composites16−18 have also attracted a lot of attention. The orientationdependent photoactivity of ZnO nanocrystals caused by the different chemical reactivities of polar (Zn-/O-polar) and nonpolar (a-/m-plane) ZnO surfaces has been extensively investigated.7,19,20 Among Zn-, O-polar, and nonpolar m-plane ZnO surfaces, nonpolar m-plane ZnO single crystals show the highest activity in degrading MO aqueous solution [MO(aq)].21 Noble metal nanoparticles (NPs), used for improving the photocatalytic activity of ZnO photocatalysts,13,14,22,23 are Received: May 19, 2014 Revised: July 1, 2014 Published: August 5, 2014

(3) © 2014 American Chemical Society

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maintained at 650 °C and 200 Torr, respectively. More details associated with the pretreatment of substrates and the growth of ZnO epilayers have been described elsewhere.33 Different amounts of Au NPs were deposited on ZnO (referred to as Au−ZnO composite) for 5 s by sputtering (EMI TECH, K 550X) with sputter currents of 10, 30, and 50 mA, with corresponding Au−ZnO composites coded as 10Au−ZnO, 30Au−ZnO, and 50Au−ZnO, respectively. (Au)−ZnO is used to denote ZnO with/without sputtered Au NPs. The photodegradation of MO solution was conducted in air under 365 nm UV irradiation with an intensity of 150 mW/ cm2. The temperature of the whole quartz reactor containing the MO solution was kept at 27 °C using a water-cooling circulation system. The initial concentration (C0) and volume of MO solution were 15 ppm and 30 mL, respectively. The total photodegradation period for each sample was 240 min. A calibration curve of the absorbance at 465 nm was constructed with the concentration of MO solution in the range of 0 and 50 ppm.11 The evolution of the MO concentration during photodegradation under UV irradiation was determined using the calibration curve. The crystal structures of Au NPs and ZnO films as well as their orientation relationship were examined using transmission electron microscopy techniques (TEM, FEI E.O Tecnai F20 G2MAT S-TWIN Field Emission Gun Transmission Electron Microscope)―high-resolution TEM (HRTEM) and nano beam diffraction (NBD). The coverage of Au NPs on ZnO films was evaluated from cross sections employing high-angle annular dark-field scanning TEM (HAADF-STEM, JEOL-2100F CS STEM). The cross-sectional TEM specimens were prepared using a focused ion beam (FIB, SMI 3050). The SPs of (Au)−ZnO composites were acquired by KPFM (Veeco diInnova) in N2 atmosphere in a light-shielded box. KPFM measurements were performed in lift mode, in which the surface topography is recorded along a forward line profile and a backward scan is acquired along the same line profile at a chosen lift height from the sample surface.34 PtIr-coated silicon cantilevers (Olympus, OSCM-PIT) with a nominal resonance frequency of 75 kHz were used as conductive probes. In order to rule out the degradation of the conductive tip, calibration was performed by scanning a reference sample, fresh highly oriented pyrolytic graphite (HOPG), before and after each measurement. Hence, the SP values of (Au)-ZnO samples measured in the dark and under UV illumination by KPFM in this study are modified as SP = SP(Au)‑ZnO − SPHOPG. The SP value was acquired by performing KPFM measurements on each sample at more than five positions both in the dark and under UV illumination (SPECTROLINE, ENF-260C, 365 nm, 6 W). Accordingly, the determined SP values represents are the mean values of each sample, with the deviation denoted by an error bar. KPFM data were processed using the free software WsxM (version 5.0).35 The acquisition of the average SP values of (Au)−ZnO samples is described in the Supporting Information (SI) and Figure S1 therein. SPV is the photoinduced change in SP value. The SPV values of all (Au)−ZnO composites were obtained by calculating the difference between the average SPs acquired under UV irradiation and in the dark. On the basis of the KPFM results, the work functions of (Au)− ZnO composites could be estimated from the definition of contact potential difference (CPD)36 and the work function of HOPG (4.65 eV) in air.37 Details of surface work function calculations are described in the SI.

much different from their bulk counterparts in terms of electrical and optical properties because of their small size. Generally, noble metal NPs dramatically enhance photocatalytic efficiency via the following mechanisms. (1) Metal NPs act as electron traps, leaving more holes at the ZnO surface for photocatalytic reactions.14 (2) The surface plasmon resonance (SPR) effect dominates the optical properties of Au and Ag NPs.22,23 (3) The formation of a Schottky barrier13 or an Ohmic contact24 at the junction of metal NPs and ZnO leads to more efficient charge separation. Despite the strong positive effect of SPR on photocatalysis efficiency, resonance only occurs if the incident light source is at the plasmon frequency of Au NPs, typically in the visible range (∼530 nm).25 In general, a Schottky barrier is expected to be established at the interface between n-type ZnO and Au26,27 because the work function of Au metal (5.1 eV)28 is higher than that of n-type ZnO (∼4.25 eV). 29 Nevertheless, we have previously demonstrated that an Ohmic contact forms at the junction of ZnO nanosheets and Au NPs when equilibrium is achieved in the dark using Kelvin probe force microscopy (KPFM).24 The electron transfer channel of the Ohmic contact facilitates charge separation. Discrepancy between the formation of a Schottky barrier or an Ohmic contact may arise from various work function combinations of ZnO and Au species. For instance, the work functions of Zn-, O-polar, and nonpolar m-plane ZnO surfaces have been reported to be in the range of ∼3.6 and 4.95 eV.30 Theoretical calculations show that the electronic structure of Au depends closely on its particle size and shape.31,32 Our previous work found an Au particle size effect on the photodegradation of MO(aq) using Au−ZnO nanocomposites as UV photocatalysts.11 A dramatically enhanced photoactivity of Au−ZnO nanocomposites was observed when the Au particle size was reduced to 5 nm.11 Both theoretical31,32 and experimental11 results imply that the work function of Au NPs can deviate from that (5.1 eV) of Au metal. To explore the role of Au NPs in the enhancement of the photoactivity of ZnO under UV irradiation, KPFM is applied in the present work. Instead of Au NP-ZnO nanostructures with high roughness, Au NP−ZnO film composites were employed for practicable and fundamental study by KPFM measurements. Nonpolar m-plane ZnO epitaxial films were selected as the matrix of Au−ZnO composites because of their highest chemical activity in bleaching MO(aq) compared to that of other ZnO surfaces.21 The surface potentials (SPs) of the Au NP/m-plane ZnO epilayer composites in the dark and under UV irradiation were measured using KPFM. The surface photovoltage (SPV) values of Au NP−ZnO composites were obtained by calculating the difference between the SP values under UV irradiation and in the dark. Three-dimensional band diagrams associated with the three fundamental interfaces, namely Au NP/ZnO, ZnO/MO(aq), and Au NP/MO(aq) are proposed. The three-dimensional band diagrams give a comprehensive understanding of the charge transfer in the Au NP-enhanced m-plane ZnO photocatalyst. On the basis of our results, the SPV values of the photocatalysts show a strong correlation with their corresponding photocatalytic activity.

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EXPERIMENTAL SECTION Nonpolar m-plane ZnO epilayers (abbreviated as ZnO hereafter) were grown on 1 × 1 cm2 (100) LiGaO2 singlecrystal substrates using the metal−organic chemical vapor deposition method.33 The temperature and pressure were 19815

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RESULTS AND DISCUSSION Formation of nonpolar m-plane ZnO epitaxial film on the (100) LiGaO2 substrate has been examined by TEM characterization, as shown in Figure S2 (SI). The photocatalytic activities of (Au)-ZnO composites were determined from the photodegradation of MO solutions under 365 nm UV irradiation. Figure 1 shows the photodegradation kinetics of MO in the

are identified to be the d-spacing values of the (200) and (111) planes of cubic Au (JCPDS file, no. 001−1174), respectively. With a d-spacing of ∼0.28 nm, the lattice planes parallel to the LiGaO2 substrate are assigned to the ZnO (101̅0) planes of the wurtzite structure. The interfacial correlation between Au NPs and m-plane ZnO epilayers was further confirmed by NBD, as shown in Figure 2(b). In addition to ZnO (101̅0) diffraction patterns (dotted circles in Figure 2(b)), another two sets of diffraction spots are associated with the (200) and (111) planes of cubic Au (solid and dashed circles in Figure 2(b), respectively). No specific epitaxial relationship exists between the sputtered Au NPs and m-plane ZnO epilayers. In photocatalysis, it has been recognized that free radicals, • OH and •O−2 , generated at/near the interface between ZnO and MO solution contribute to the degradation of MO molecules.6 The ZnO surfaces directly in contact with the MO solution are thus essential in the generation of the free radicals for photocatalytic reactions. Accordingly, high Au NP coverage on the surface of the ZnO epilayer results in the decrease of the exposed ZnO surfaces and thus shows detrimental effect on the photoactivity of Au−ZnO composite. However, to precisely estimate the exposed surface area of ZnO to MO solution in the Au−ZnO composite after loading Au NPs is impracticable. HAADF-STEM was thus applied to the three Au−ZnO composites to evaluate their Au NP coverages. Figure 3

Figure 1. Photocatalytic kinetics of MO degradation by (Au)−ZnO photocatalysts under 365 nm UV irradiation for 240 min.

presence of Au−ZnO composites compared to that of pristine ZnO. The concentration of MO solution remains nearly unchanged in the absence of photocatalysts under UV irradiation and in the presence of ZnO without irradiation as shown in Figure 1. Similar results have been demonstrated in our previous work.11,24 These results indicate that for the photolysis of MO, the catalysis of MO by Au/ZnO nanostructures without the assistance of illumination as well as the adsorption of MO molecules on the nonpolar ZnO photocatalysts can be neglected. Accordingly, the bleaching of MO solution in this work, as shown in Figure 1, is attributed to the degradation of MO via photocatalysis. The MO concentration after a 240 min reaction (C240) decreased to 88% of the initial concentration (C0) in the presence of pristine ZnO. The C240 value decreased to 0.6C0, 0.75C0, and 0.77C0 when 10Au−, 30Au−, and 50Au−ZnO composites were employed as photocatalysts, respectively. Therefore, the photocatalytic activities of (Au)−ZnO composites show the order: 10Au−ZnO > 30Au−ZnO ≈ 50Au−ZnO > ZnO. TEM measurements were performed to examine the crystal structures of Au NPs and m-plane ZnO as well as the microstructural relationship between them. An HRTEM micrograph of the 50Au−ZnO sample is shown in Figure 2(a). Two sets of apparent lattice spacings, 0.203 and 0.235 nm,

Figure 3. Cross-sectional HAADF-STEM images of Au−ZnO composites. (a) 10Au−ZnO, (b) 30Au−ZnO, and (c) 50Au−ZnO. The scale bars in the insets of (a)−(c) are 4 nm.

shows HAADF-STEM cross-sectional images of the three Au− ZnO composites. Au NPs are easily distinguished from ZnO due to the remarkable contrast caused by their different average atomic numbers. For 10Au−ZnO, a few Au NPs are randomly dispersed on the surface of the ZnO epilayer, as indicated by the arrows in Figure 3(a). Compared to 10Au−ZnO, Figure 3(b) reveals that the amount of Au NPs on the surface of ZnO increased for 30Au−ZnO. The insets of Figures 3(a) and 3(b) show that the average diameters of the Au NPs are ∼2.5 nm for both 10Au−ZnO and 30Au−ZnO composites. The diameter of the Au NPs is ∼3.6 nm for 50Au−ZnO composite, as shown in the inset of Figure 3(c), which is slightly larger than those of 10Au−ZnO and 30Au−ZnO. The larger Au particle size for 50Au−ZnO composite may be attributed to (1) the higher sputtering current used for deposition, (2) the coalescence of Au NPs, or (3) the superimposition of 2.5 nm Au NPs with an unobvious grain boundary between two Au NPs in the HAADF-STEM images. Despite the different Au particle sizes of 30Au−ZnO and 50Au−ZnO (2.5 and 3.6 nm, respectively), these two Au−ZnO composites present similar photocatalytic activities, as shown in Figure 1. However, with the same Au particle size (2.5 nm), 10Au−ZnO and 30Au−ZnO present much different performance in photodegradation of MO solution. Accordingly, it is believed that the size-dependent effect of Au NPs on the

Figure 2. (a) HRTEM image and (b) NBD pattern of interface between Au NPs and ZnO film of 50Au−ZnO sample. The incident beam direction is parallel to hexagonal ZnO [0001] and cubic Au [011̅]. 19816

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fluctuation during measurements.40 In the dark, ZnO has the highest SP value (0.27 V) among all samples, as shown in Figure 4(a). For comparison, an 84 nm-thick Au NP layer (Figure S3 of the SI) was prepared via sputtering to serve as an Au reference in this study. KPFM measurements were performed on the Au NP layer, giving an average SP value of −0.46 V. On the basis of the average SPs of pristine ZnO and the Au NP layer, the proposed band diagram of ZnO and Au NPs in the dark before contact formation is sketched in Figure 5(a), where EF,ZnO indicates the Fermi level of pristine ZnO. Compared to ZnO, the SP of 10Au−ZnO in the dark reduces to −0.01 V, i.e., the Fermi level of 10Au−ZnO is higher. This observation indicates that electrons transfer from Au NPs to ZnO after contact formation in order to align the Fermi levels of Au NPs and the ZnO epilayer in 10Au−ZnO sample.24 An Ohmic contact is thus established at the junction of Au and ZnO. A systematic decrease of the SP values of 30Au− and 50Au−ZnO in the dark can be observed in Figure 4(a), suggesting that more electrons transfer from Au NPs to the ZnO epilayer for Fermi level alignment at equilibrium when the amount of Au NPs on the ZnO epilayer is higher. A typical band diagram of the Au−ZnO composites after formation of Ohmic contact is shown in Figure 5(b), where EF,Au‑ZnO indicates the Fermi level of Au−ZnO composites after equilibrium in the dark. For comparison, Au NPs were sputtered on fresh HOPG with a sputtering current of 10 mA for 5 s (coded as 10Au-HOPG). An average SP value of −0.16 V was obtained for 10Au-HOPG in the dark, indicating that electrons transfer from Au NPs to HOPG in order to achieve equilibrium. Figure 5(a) also demonstrates the relative positions of HOPG and the Au NP layer before Fermi level alignment. This result again confirms that the work function of the Au NP layer in this study is smaller than that of Au bulk counterparts (5.1 eV) (taking 4.65 eV as the work function of HOPG).37 Figure 4(a) also shows the SPs of (Au)−ZnO samples measured under 365 nm UV illumination. With steady-state UV light shining on a semiconductor, some photoelectron−hole pairs recombine and some are separated by either a Schottky barrier or an Ohmic contact in the vicinity of the surface. The Kelvin probe method is traditionally adopted to measure the photoinduced minority carrier properties of semiconductors.41 The measured SP reflects the steady-state concentration of photogenerated carriers near the surface of (Au)−ZnO, which is the net effect of generation, separation, and recombination of photocarriers in semiconductors. For ZnO sample, a Schottky barrier formed at the interface between ZnO and air is

photocatalytic activity of Au−ZnO composites is hardly dominative in this work. It should be noted that for 50Au− ZnO, the ZnO surfaces were not completely covered by Au NPs, as shown in Figure 3(c). On the basis of the HAADFSTEM images, the bare ZnO surface areas of the three Au− ZnO composites, for direct absorption of UV irradiation and in contact with MO solution, are in the order: 10Au−ZnO > 30Au−ZnO > 50Au−ZnO. However, it has been reported that Au NPs with particle sizes of 5−50 nm show a sharp absorption band in the visible range of 520−530 nm, whereas those with particle sizes 30Au−ZnO ≈ 50Au−ZnO > ZnO, which is consistent with the photocatalytic results of Figure 1. In addition to the interface of Au NP/ZnO, another two fundamental interfaces, namely ZnO/MO(aq) and Au/MO(aq), are involved when using Au−ZnO composites as the photocatalysts for the degradation of MO solution. Figure 6(a) shows the band diagram related to the ZnO/MO(aq) interfaces of the (Au)−ZnO composites before they make contact with MO(aq). The energy potential of MO(aq) was simply represented by the standard redox potential of H2O because of the low concentrations (15 ppm) used. When Au− ZnO composite makes contact with MO solution, its Fermi level (EF,Au‑ZnO) tends to align with the standard redox potential of H2O [+1.23 V, vs NHE at 25 °C, which equals 0 V (4.5 eV)]46 until equilibrium is achieved. Accordingly, a Schottky barrier forms at the junction on the Au−ZnO composite side, as shown in Figure 6(b). When UV light illuminates the Au− ZnO composite, the electric field triggers the initial dissociation of photogenerated electron−hole pairs in the vicinity of the interface, creating free electrons/holes. As shown in Figure 6(c), the photoelectrons after dissociation at/near the interface are forced to move toward the bulk of the Au−ZnO composite, while the holes move toward the surface due to the electric field. The energy levels shown in Figure 6(c) are based on information in the literature,46,47 where the potentials of OH−/•OH and H2O/OH−, H+ are +2.3146 and +1.83 V47 (vs NHE at 25 °C), respectively. Due to the suitability of the energy level for charge transfer, the photogenerated holes that swept to the surface may react with the adsorbed H2O and/or 19818

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Figure 7. Three-dimensional band diagrams of Au−ZnO composite in MO solution (a) in the dark and (b) under 365 nm UV irradiation.

OH− to further generate •OH radicals for the degradation of MO molecules. For the interface of Au/MO(aq) in the present photocatalytic system, the extreme case of 50Au−ZnO under UV irradiation is taken as an example. Since the highest coverage of Au NPs on the ZnO surface was obtained for the 50Au−ZnO sample, its average SP value under UV irradiation is likely close to the Fermi level of Au NPs after charge transfer. Figure S6(a) (SI) shows the energy band diagram associated with the interface of Au/MO(aq) for 50Au−ZnO. The average SP value-derived Fermi level of 50Au−ZnO under UV irradiation (E’F,50Au‑ZnO) is not appropriate for charge transfer between Au NPs and O2/•O2− and for the formation of •O−2 radicals. However, if the localized Fermi level of Au NPs is considered, it may be gradually lifted due to more photoelectrons transferred from ZnO and stored inside the Au NPs as shown in Figure S6(a) (SI). Accordingly, once the localized energy level of Au NP is higher than the potential of O2/•O2−, the generation of •O−2 radicals may possibly occur and lead to the degradation of MO(aq) as sketched in Figure S6(b) (SI). However, the positive SPV values of four (Au)−ZnO samples that observed in Figure 4(b), we believe that the excess holes in the photocatalysts play the principal role in the degradation of MO molecules in the tested photocatalytic system. HAADF-STEM images have confirmed that parts of the ZnO surface are exposed in the three Au−ZnO composites, particularly in the 50Au−ZnO sample with the highest Au NP coverage. Moreover, an Ohmic contact is established at the interface of Au NP/ZnO and a Schottky barrier forms at that of ZnO/MO(aq), as shown in Figure 5(b) and 6(b), respectively. The three-dimensional energy band diagrams in Figure 7 give a comprehensive view of the charge transfer between the three interfaces of the Au−ZnO composites and show how the photogenerated charges contribute to the photodegradation of MO(aq). Figure 7(a) shows that Au−ZnO composites are in equilibrium with MO(aq) in the dark, which includes the interfaces of Au-NP-covered (Au NP/ZnO) and uncovered (ZnO/MO(aq)) ZnO surfaces, as discussed. Due to the formation of an Ohmic contact at the interface of Au NP/

ZnO in the dark, Au NP becomes a sink for electrons, as shown in Figure 7(a). Accordingly, when UV light irradiates Au−ZnO composites, the photogenerated electron−hole pairs could be dissociated either by the Ohmic contact at the interface of Au NP/ZnO or by the Schottky barrier at the interface of ZnO/ MO(aq). If not recombined with holes, then the photoelectrons could be either trapped in Au NPs at the interface of Au NP/ ZnO or swept to the ZnO bulk via the Schottky barrier at the interface of ZnO/MO(aq), as shown in Figure 7(b). As for the holes generated at/near the interface of Au NP/ZnO, the Ohmic contact drives them toward the interior of ZnO. However, these excess holes could also move laterally to the vicinity of the Au-NP-uncovered ZnO surface. Together with the holes dissociated via the Schottky barrier at the interface of ZnO/MO(aq), the net photogenerated holes contribute to the generation of •OH radicals and the following degradation of MO solution. From this viewpoint, the exposed ZnO surface of Au−ZnO composites (with the order 10Au−ZnO > 30Au− ZnO > 50Au−ZnO, as shown in Figure 3) plays a crucial role in the photodegradation of MO solution. However, although pristine ZnO has a larger ZnO/MO(aq) interfacial area compared to those of Au−ZnO composites, its photocatalytic activity is inferior, which is mainly ascribed to inefficient charge separation. The Ohmic contact at the interface of Au NP/ZnO facilitates photoelectron transfer from ZnO to the Au NPs for efficient charge separation. The SPV values of Au−ZnO composites are therefore larger than that of the ZnO sample. Among the three Au−ZnO composites, 10Au−ZnO has the highest SPV and the largest ZnO/MO(aq) interfacial area, and thus shows the highest photocatalytic activity for MO photodegradation.

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CONCLUSIONS In this work, Au−ZnO composites were prepared by sputtering Au NPs on m-plane ZnO epilayers. There was no epitaxial relationship between Au NPs and ZnO surfaces, as confirmed by HRTEM and NBD results. In order to investigate the effect of Au NPs on the photoactivity of the m-plane ZnO epilayer, the SPs of pristine ZnO and Au−ZnO composites in the dark 19819

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Activity of Au/TiO2 Thin Films for Azo-Dye Degradation. J. Catal. 2003, 220, 127−135. (2) Augugliaro, V.; Loddo, V.; Marci, G.; Palmisano, L.; LópezMuñoz, M. J. Photocatalytic Oxidation of Cyanides in Aqueous Titanium Dioxide Suspensions. J. Catal. 1997, 166, 272−283. (3) Daneshvar, N.; Salari, D.; Khataee, A. R. Photocatalytic Degradation of Azo Dye Acid Red 14 in Water on ZnO as an Alternative Catalyst to TiO2. J. Photochem. Photobiol. A 2004, 162, 317−322. (4) Li, W.; Li, D.; Lin, Y.; Wang, P.; Chen, W.; Fu, X.; Shao, Y. Evidence for the Active Species Involved in the Photodegradation Process of Methyl Orange on TiO2. J. Phys. Chem. C 2012, 116, 3552− 3560. (5) Xu, Y.; Xu, H.; Li, H.; Xia, J.; Liu, C.; Liu, L. Enhanced Photocatalytic Activity of New Photocatalyst Ag/AgCl/ZnO. J. Alloys Compd. 2011, 509, 3286−3292. (6) Zhong, J. B.; Li, J. Z.; He, X. Y.; Zeng, J.; Lu, Y.; Hu, W.; Lin, K. Improved Photocatalytic Performance of Pd-doped ZnO. Curr. Appl. Phys. 2012, 12, 998−1001. (7) Jang, E. S.; Won, J.-H.; Hwang, S.-J.; Choy, J.-H. Fine Tuning of the Face Orientation of ZnO Crystals to Optimize Their Photocatalytic Activity. Adv. Mater. 2006, 18, 3309−3312. (8) Lee, H. U.; Park, S. Y.; Lee, S. C.; Seo, J. H.; Son, B.; Kim, H.; Yun, H. J.; Lee, G. W.; Lee, S. M.; Nam, B.; et al. Highly Photocatalytic Performance of Flexible 3 Dimensional (3D) ZnO Nanocomposite. Appl. Catal., B 2014, 144, 83−89. (9) Chiu, W. S.; Khiew, P. S.; Cloke, M.; Isa, D.; Tan, T. K.; Radiman, S.; Abd-Shukor, R.; Abd Hamid, M. A.; Huang, N. M.; Lim, H. N.; et al. Photocatalytic Study of Two-Dimensional ZnO Nanopellets in the Decomposition of Methylene Blue. Chem. Eng. J. 2010, 158, 345−352. (10) Sun, T. J.; Qiu, J. S.; Liang, C. H. Controllable Fabrication and Photocatalytic Activity of ZnO Nanobelt Arrays. J. Phys. Chem. C 2008, 112, 715−721. (11) Wu, J.-J.; Tseng, C.-H. Photocatalytic Properties of nc-Au/ZnO Nanorod Composites. Appl. Catal., B 2006, 66, 51−57. (12) Xie, W.; Li, Y. Z.; Sun, W.; Huang, J. C.; Xie, H.; Zhao, X. J. Surface Modification of ZnO with Ag Improves Its Photocatalytic Efficiency and Photostability. J. Photochem. Photobiol. A 2010, 216, 149−155. (13) Zhang, Y. A.; Xu, J. Q.; Xu, P. C.; Zhu, Y. H.; Chen, X. D.; Yu, W. J. Decoration of ZnO Nanowires with Pt Nanoparticles and Their Improved Gas Sensing and Photocatalytic Performance. Nanotechnology 2010, 21, 285501 (7pp). (14) Subramanian, V.; Wolf, E. E.; Kamat, P. V. Green Emission to Probe Photoinduced Charging Events in ZnO−Au Nanoparticles. Charge Distribution and Fermi-Level Equilibration. J. Phys. Chem. B 2003, 107, 7479−7485. (15) Yuan, J. Q.; Choo, E. S. G.; Tang, X. S.; Sheng, Y.; Ding, J.; Xue, J. M. Synthesis of ZnO−Pt Nanoflowers and Their Photocatalytic Applications. Nanotechnology 2010, 21, 185606 (10pp). (16) Xie, Q. S.; Zhao, Y.; Guo, H. Z.; Lu, A. L.; Zhang, X. X.; Wang, L. S.; Chen, M. S.; Peng, D. L. Facile Preparation of Well-Dispersed CeO−ZnO Composite Hollow Microspheres with Enhanced Catalytic Activity for CO Oxidation. ACS Appl. Mater. Interfaces 2014, 6, 421− 428. (17) Chen, T. T.; Chang, I. C.; Yang, M. H.; Chiu, H. T.; Lee, C. Y. The Exceptional Photo-Catalytic Activity of ZnO/RGO Composite Via Metal and Oxygen Vacancies. Appl. Catal., B 2013, 142, 442−449. (18) Li, X. S.; Wang, Q. A.; Zhao, Y. B.; Wu, W.; Chen, J. F.; Meng, H. Green Synthesis and Photo-catalytic Performances for ZnOReduced Graphene Oxide Nanocomposites. J. Colloid Interface Sci. 2013, 411, 69−75. (19) McLaren, A.; Valdes-Solis, T.; Li, G.; Tsang, S. C. Shape and Size Effects of ZnO Nanocrystals on Photocatalytic Activity. J. Am. Chem. Soc. 2009, 131, 12540−12541. (20) Yang, J.; Wang, J.; Li, X.; Lang, J.; Liu, F.; Yang, L.; Zhai, H.; Gao, M.; Zhao, X. Effect of Polar and Non-Polar Surfaces of ZnO

and under UV irradiation were measured using KPFM. From the SP values of HOPG, Au NP layer, and Au NP−HOPG composite, the work function of Au NPs was found to be smaller than that of Au metal. On the basis of the SP values of (Au)−ZnO samples, three-dimensional band diagrams of the interfaces of Au NP/ZnO, ZnO/MO(aq), and Au NP/MO(aq) were constructed. An Ohmic contact formed at the junction of Au NPs and the ZnO epilayer at equilibrium, which facilitated photoelectron transfer from the CB of ZnO to Au NPs and ensured effective photocarrier separation under illumination. However, the photoelectrons stored in Au NPs were suggested to show minor contribution to the formation of •O−2 . The excess holes, which were laterally transferred from Au-NPcovered ZnO and originally generated at Au-NP-uncovered ZnO, produced •OH on the surface, and thus mainly contributed to the degradation of MO molecules. The SPV values of the photocatalysts were determined from the modulation of SPs measured under illumination and in the dark. The positive SPV values of the Au NP/ZnO composites, which are associated with excess hole concentrations, show a systematic correlation with the photocatalytic activities. The results suggest that the SPV value of photocatalysts can be an effective index for evaluating photocatalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Acquisition of average surface potential values of (Au)− ZnO samples. (2) Cross-sectional transmission electron microscopy image of pristine m-plane ZnO film. (3) Crosssectional scanning electron microscopy image of Au NP layer. (4) Optical properties of (Au)−ZnO photocatalysts. (5) Typical topography and corresponding surface potential mappings of (Au)−ZnO samples. (6) Estimation of surface work function. These materials are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Headquarters of University Advancement at National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan, and from the Ministry of Science and Technology in Taiwan under Grant 102-2221-E-006-215-MY3 is gratefully acknowledged.

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ABBREVIATIONS NP, nanoparticle; UV, ultraviolet; KPFM, Kelvin probe force microscopy; SP, surface potential; SPV, surface photovoltage; MO, methyl orange; SPR, surface plasmon resonance; TEM, transmission electron microscopy; HRTEM, high-resolution TEM; NBD, nano beam diffraction; HAADF-STEM, Highangle annular dark-field scanning TEM; HOPG, highly oriented pyrolytic graphite; CPD, contact potential difference; SAED, selected area electron diffraction

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REFERENCES

(1) Arabatzis, I. M.; Stergiopoulos, T.; Andreeva, D.; Kitova, S.; Neophytides, S. G.; Falaras, P. Characterization and Photocatalytic 19820

dx.doi.org/10.1021/jp504903z | J. Phys. Chem. C 2014, 118, 19814−19821

The Journal of Physical Chemistry C

Article

Nanostructures on Photocatalytic Properties. J. Alloys Compd. 2012, 528, 28−33. (21) Kislov, N.; Lahiri, J.; Verma, H.; Goswami, D. Y.; Stefanakos, E.; Batzill, M. Photocatalytic Degradation of Methyl Orange over Single Crystalline ZnO: Orientation Dependence of Photoactivity and Photostability of ZnO. Langmuir 2009, 25, 3310−3315. (22) Lee, M. K.; Kim, T. G.; Kim, W.; Sung, Y. M. Surface Plasmon Resonance (SPR) Electron and Energy Transfer in Noble Metal-Zinc Oxide Composite Nanocrystals. J. Phys. Chem. C 2008, 112, 10079− 10082. (23) Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods. J. Phys. Chem. B 1999, 103, 8410− 8426. (24) Sung, Y.-H.; Frolov, V. D.; Pimenov, S. M.; Wu, J.-J. Investigation of Charge Transfer in Au Nanoparticle−ZnO Nanosheet Composite Photocatalysts. Phys. Chem. Chem. Phys. 2012, 14, 14492− 14494. (25) Hou, W.; Cronin, S. B. A Review of Surface Plasmon Resonance-Enhanced Photocatalysis. Adv. Funct. Mater. 2013, 23, 1612−1619. (26) Coppa, B. J.; Fulton, C. C.; Kiesel, S. M.; Davis, R. F.; Pandarinath, C.; Burnette, J. E.; Nemanich, R. J.; Smith, D. J. Structural, Microstructural, and Electrical Properties of Gold Films and Schottky Contacts on Remote Plasma-cleaned, n-Type ZnO{0001} Surfaces. J. Appl. Phys. 2005, 97, 103517 (13pp). (27) Chen, Z. H.; Tang, Y. B.; Liu, C. P.; Leung, Y. H.; Yuan, G. D.; Chen, L. M.; Wang, Y. Q.; Bello, I.; Zapien, J. A.; Zhang, W. J.; et al. Vertically Aligned ZnO Nanorod Arrays Sensitized with Gold Nanoparticles for Schottky Barrier Photovoltaic Cells. J. Phys. Chem. C 2009, 113, 13433−13437. (28) Eastman, D. E. Photoelectric Work Functions of Transition, Rare-Earth, and Noble Metals. Phys. Rev. B 1970, 2, 1−2. (29) Romero, R.; López, M. C.; Leinen, D.; Martín, F.; RamosBarrado, J. R. Electrical Properties of the n-ZnO/c-Si Heterojunction Prepared by Chemical Spray Pyrolysis. Mater. Sci. Eng., B 2004, 110, 87−93. (30) Moormann, H.; Kohl, D.; Heiland, G. Work Function and Band Bending on Clean Cleaved Zinc-Oxide Surfaces. Surf. Sci. 1979, 80, 261−264. (31) Qian, H.; Zhu, M.; Wu, Z.; Jin, R. Quantum Sized Gold Nanoclusters with Atomic Precision. Acc. Chem. Res. 2012, 45, 1470− 1479. (32) Batista, R. J. C.; Mazzoni, M. S. C.; Chacham, H. First-principles Investigation of Electrochemical Properties of Gold Nanoparticles. Nanotechnology 2010, 21, 065705 (8pp). (33) Lin, W.-H.; Wu, J.-J.; Chou, M. M. C.; Chang, L. Growth and Characterization of Nonpolar (1010̅ ) Zn1‑xMgxO (0 ≤ x ≤ 0.113) Epitaxial Films: A Comparison of γ-LiAlO2 (100) and Sapphire (101̅0)Substrates. Cryst. Growth Des. 2009, 9, 3301−3306. (34) Palermo, V.; Palma, M.; Samorì, P. Electronic Characterization of Organic Thin Films by Kelvin Probe Force Microscopy. Adv. Mater. 2006, 18, 145−164. (35) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. Wsxm: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705 (8pp). (36) Zhang, W.; Chen, Y. S. Experimental Determination of Conduction and Valence Bands of Semiconductor Nanoparticles Using Kelvin Probe Force Microscopy. J. Nanopart. Res. 2013, 15, 1334 (7pp).. (37) Sommerhalter, C.; Matthes, T. W.; Glatzel, T.; Jäger-Waldau, A.; Lux-Steiner, M. C. High-Sensitivity Quantitative Kelvin Probe Microscopy by Noncontact Ultra-High-Vacuum Atomic Force Microscopy. Appl. Phys. Lett. 1999, 75, 286−288. (38) Kamat, P. V. Photophysical, Photochemical and Photocatalytic Aspects of Metal Nanoparticles. J. Phys. Chem. B 2002, 106, 7729− 7744.

(39) Henglein, A. Radiolytic Preparation of Ultrafine Colloidal Gold Particles in Aqueous Solution: Optical Spectrum, Controlled Growth, and Some Chemical Reactions. Langmuir 1999, 15, 6738−6744. (40) Chaney, J. A.; Pehrsson, P. E. Work Function Changes and Surface Chemistry of Oxygen, Hydrogen, and Carbon on Indium Tin Oxide. Appl. Surf. Sci. 2001, 180, 214−226. (41) Kronik, L.; Shapira, Y. Surface Photovoltage Phenomena: Theory, Experiment, and Applications. Surf. Sci. Rep. 1999, 37, 1−206. (42) Kronik, L.; Shapira, Y. Surface Photovoltage Spectroscopy of Semiconductor Structures: At the Crossroads of Physics, Chemistry and Electrical Engineering. Surf. Interface Anal. 2001, 31, 954−965. (43) Zhan, Z. B.; Wang, Y. H.; Lin, Z.; Zhang, J. Y.; Huang, F. Study of Interface Electric Field Affecting the Photocatalysis of ZnO. Chem. Commun. 2011, 47, 4517−4519. (44) Zhao, J.; Osterloh, F. E. Photochemical Charge Separation in Nanocrystal Photocatalyst Films: Insights from Surface Photovoltage Spectroscopy. J. Phys. Chem. Lett. 2014, 5, 782−786. (45) Streicher, F.; Sadewasser, S.; Lux-Steiner, M. C. Surface Photovoltage Spectroscopy in a Kelvin Probe Force Microscope under Ultrahigh Vacuum. Rev. Sci. Instrum. 2009, 80, 013907 (6pp). (46) Koppenol, W. H.; Stanbury, D. M.; Bounds, P. L. Electrode Potentials of Partially Reduced Oxygen Species, from Dioxygen to Water. Free Radical Bio. Med. 2010, 49, 317−322. (47) Chipman, D. M. Electron-Affinity of Hydroxyl Radical. J. Chem. Phys. 1986, 84, 1677−1682.

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