Fabrication of Au/Graphene-Wrapped ZnO-Nanoparticle-Assembled Hollow Spheres with Effective Photoinduced Charge Transfer for Photocatalysis Nguyen Tri Khoa, † Soon Wook Kim, † Dae-Hwang Yoo, † Shinuk Cho, † Eui Jung Kim, ‡ and Sung Hong Hahn* ,† † Department of Physics and Energy Harvest-Storage Research Center and ‡ Department of Chemical Engineering, University of Ulsan, Ulsan 680-749, South Korea * S Supporting Information ABSTRACT: Heterostructures of gold-nanoparticle-decora- ted reduced-graphene-oxide (rGO)-wrapped ZnO hollow spheres (Au/rGO/ZnO) are synthesized using tetra-n- butylammonium bromide as a mediating agent. The structure of amorphous ZnO hollow spheres is found to be transformed from nanosheet- to nanoparticle-assembled hollow spheres (nPAHS) upon annealing at 500 °C. The ZnO nPAHS hybrids with Au/rGO are characterized using various techniques, including photoluminescence, steady-state absorbance, time- resolved photoluminescence, and photocatalysis. The charge- transfer time of ZnO nPAHS is found to be 87 ps, which is much shorter than that of a nanorod (128 ps), nanoparticle (150 ps), and nanowall (990 ps) due to its unique structure. The Au/rGO/ZnO hybrid shows a higher charge-transfer efficiency of 68.0% in comparison with rGO/ZnO (40.3%) and previously reported ZnO hybrids. The photocatalytic activities of the samples are evaluated by photodegrading methylene blue under black-light irradiation. The Au/rGO/ZnO exhibits excellent photocatalytic efficiency due to reduced electron-hole recombination, fast electron-transfer rate, and high charge-transfer efficiency. KEYWORDS: graphene oxide, zinc oxide, gold nanoparticle, hierarchical aggregate, photoinduced reactivity, charge transfer ■ INTRODUCTION In recent years, heterostructural photocatalysts have greatly attracted attention in the photodegradation of organic pollutants, such as methylene blue, methyl orange, rhodamine, and phenol. 1-6 The principle of photocatalytic cells is based on the absorption of irradiated photons to create electron-hole pairs, which are separated and diffused across interfaces to react with adsorbed substances on the surface. The excited electron- hole pairs can recombine at the valence band edge or can be captured at trap sites (defects, vacancies). 7 Therefore, the enhancement of photoinduced charge transfer is crucial to improve the photocatalytic performance. There are several ways to improve charge transfer in a semiconductor. For example, metal nanoparticle, metal oxide, or carbon material decorated on the semiconductor surface facilitates electron transfer from the semiconductor to the metal via Schottky contact, serves as the cocatalyst material, or easily separates electrons from semiconductor surface, respectively. 8-12 Semiconductor photo- catalysts, including TiO 2 , ZnO, CdS, and ZnS, have been known as excellent materials for the photoinduced redox reaction due to their specific electronic structure (the filled valence band and empty conduction band) and have been used for the degradation of many hazardous compounds in water and air. 13 In particular, ZnO, a wide band gap semiconductor, has been widely researched as a photocatalyst in sustainable solar energy harvesting areas due to its abundant morphologies, easy synthesis, low cost, and nontoxicity. 14 Graphene, a two- dimensional honeycomb structure, has been known as an excellent material for improving the optoelectrical and electrochemical activities of hybrid materials. 15-18 Recently, reduced graphene oxide (rGO) based composite photocatalysts have gained increasing attention because they improve the charge transfer at the interface and have good adsorption capabilities for organic pollutants on their surfaces. Xu’s research group has claimed that a graphene-based nano- composite holds a great potential in the development of multifunctional photocatalytic performance. 19-27 This group reported that the enhancement of the photocatalytic activity of graphene-semiconductor results from the strengthened interfacial contact and optimized interfacial composition. Su and co-workers have fabricated rGO/ZnO hollow spheres to investigate the suppression of charge-carrier recombination for enhancing photocurrent and photocatalytic activities. 28 How- ever, to the best of our knowledge, no studies have been reported on the photoinduced properties of Au/rGO/ZnO Received: October 16, 2014 Accepted: January 28, 2015 Published: January 28, 2015 Research Article www.acsami.org © 2015 American Chemical Society 3524 DOI: 10.1021/acsami.5b00152 ACS Appl. Mater. Interfaces 2015, 7, 3524-3531