Synthesis of Graphene-ZnO-Au Nanocomposites for Efficient Photocatalytic Reduction of Nitrobenzene Prathik Roy, † Arun Prakash Periasamy, † Chi-Te Liang, ‡ and Huan-Tsung Chang* ,† † Department of Chemistry, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 106, Taiwan ‡ Department of Physics, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 106, Taiwan * S Supporting Information ABSTRACT: A simple hydrothermal method of preparing highly photocatalytic graphene-ZnO-Au nanocomposites (G- ZnO-Au NCs) has been developed. Zinc acetate and graphene oxide are reduced by catechin to form graphene-zinc oxide nanospheres (G-ZnO NSs; average diameter of (45.3 ± 3.7) nm) in the presence of ethylenediamine (EDA) as a stabilizing agent and gold nanorods (Au NRs) at 300 °C for 2 h. Then Au NRs are deposited onto as-formed G-ZnO NSs to form G- ZnO-Au NCs. Upon ultraviolet light activation, G-ZnO-Au NCs (4 mg mL -1 ) in methanol generates electron-hole pairs. Methanol (hydroxyl group) assists in trapping holes, enabling photogenerated electrons to catalyze reduction of nitro- benzene (NB) to aniline with a yield of 97.8% during a reaction course of 140 min. The efficiency of G-ZnO-Au NCs is 3.5- and 4.5-fold higher than those provided by commercial TiO 2 and ZnO NSs, respectively. Surface assisted laser desorption/ionization mass spectrometry has been for the first time applied to detect the intermediates (nitrosobenzene and phenylhydroxylamine) and major product (aniline) of NB through photoelectrocatalytic or photocatalytic reactions. The result reveals that the reduction of NB to aniline is through nitrosobenzene to phenylhydroxylamine in the photoelectrocatalytic reaction, while via nitrosobenzene directly in the photocatalytic reaction. G- ZnO-Au NC photocatalyst holds great potential in removal of organic pollutants like NB and in the production of aniline. ■ INTRODUCTION Nitrobenzene (NB) is an attractive chemical in the industry as it can be used for the manufacture of aniline, soaps, and metal polishes and as a solvent for the preparation of cellulose ester from cellulose acetate. 1 It is however carcinogenic and genotoxic to human beings, causing diseases such as methemoglobinaemia. 1 A permissible limit of 17 ppm (13.8 μM) for NB in lakes and streams has been set by the U.S. Environmental Protection Agency (EPA). The exposure limit of NB to human beings is 5 mg m -3 for an 8h-work day. 1 NB has a high Hammet constant (0.71) and is one of the most difficult aromatic compounds to be oxidized. 2 This has prompted many researchers to focus on the reduction of NB using microbial, chemical, and catalytic methods. 3-9 Anaerobic reduction with microbial cultures often takes an extremely long time extending to more than a week. In addition, it is difficult to handle high concentrations of NB. Chemical reduction using inexpensive iron(0) is a common technique; however, inactive ferric oxide forming on its surface through corrosion is problematic. 10 In addition, its use is limited to neutral conditions. Catalytic hydrogenation is efficient in the treatment of wastewater, but it is expensive. Alternatively, electrochemical techniques are efficient in the reduction of NB, but the production of hazardous byproducts (e.g., phenylhydroxyl- amine and nitrophenol) is a concern. 7 Nanoparticles (NPs) such as TiO 2 , 4,11 ZnO, 12 and CdS 13 having great surface areas and high surface energies have emerged as useful photocatalysts for the degradation of various hazardous compounds such as NB and phenol. 4,11,13 Surface modification of photocatalysts such as TiO 2 with amino acids or organic acids has been found effective to provide better electron transfer between TiO 2 NPs and NB, leading to greater photoreduction of NB. 14,15 Semiconductor NPs however only provide high activity (generate electron-hole pairs) under UV light irradiation. In order to improve their efficiency in the visible region, surface modification of the photocatalysts with metal or metal oxides such as B-Ni, Fe 3+ , and VO 3 - has also been demonstra- ted. 16-18 However, transition metal doping commonly suffers from the formation of a discrete level in the forbidden band of the photocatalyst, resulting in low-mobility of electrons and holes in the dopant level and thus limiting their activity enhancement. 19 Recently, NPs decorated onto the surface of graphene have become interesting catalysts for the degradation of various compounds and production of hydrogen by taking Received: January 28, 2013 Revised: May 1, 2013 Accepted: May 23, 2013 Published: May 23, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 6688 dx.doi.org/10.1021/es400422k | Environ. Sci. Technol. 2013, 47, 6688-6695