Photocatalytic Performance of a Ag/ZnO/CCG Multidimensional Heterostructure Prepared by a Solution-Based Method Dae-Hwang Yoo, † Tran Viet Cuong, ‡ Van Hoang Luan, ‡ Nguyen Tri Khoa, † Eui Jung Kim, ‡ Seung Hyun Hur,* ,‡ and Sung Hong Hahn* ,† † Department of Physics and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea ‡ School of Chemical Engineering and Bioengineering, University of Ulsan, Ulsan 680-749, Republic of Korea * S Supporting Information ABSTRACT: The photocatalytic performance of a multidimensional heterojunction composed of decorated Ag nanoparticles on ZnO nanorods vertically grown on a chemically converted graphene (CCG) was investigated. The combined heterojunction helps to improve photocatalytic activity by increasing light absorption, preventing photo- induced electron-hole recombination, and providing a carrier pathway for the giant π-conjugated system of CCG. A significant finding was that the low work function value of Ag (-4.74 eV) at the (111) surface makes possible the transfer of electrons from CCG to Ag. Consequently, the degree of photodegradation by Ag/ZnO/CCG is much higher than the sum of photodegradations by Ag/ZnO and ZnO/CCG samples, which indicates that the combination of metal, CCG, and semiconductor provides enhanced photocatalytic activity through the double transference of electrons. A. INTRODUCTION Metal oxide semiconductors, such as TiO 2 and ZnO, are promising materials for the degradation of organic pollutants and the dissolution of contaminants by light irradiation due to their high chemical stability, low toxicity, high oxidation capacity, and ready availability. 1-3 Of the common oxide semiconductor materials, ZnO, with a wide direct band gap of 3.37 eV, 4 has been a material of considerable interest for photocatalysis applications. 5 When a photon with an energy higher than the band-gap energy (E g ) of the semiconductor is incident to the semiconductor, photoinduced electron-hole pairs are created; an electron in the valence band (VB) is excited into the conduction band, leaving a positive hole in the VB. Photo- generated holes and electrons play a very important role in pollutant degradation and photocatalytic disinfection. However, photoinduced electrons and holes can also recombine easily to reduce the photocatalytic activity of a semiconductor. To overcome this fast recombination process, considerable research has been carried out utilizing a combination of semiconductors and other materials, such as Fe, Pt, and Ag. 6,7 Recently, there has been increased interest in fabricating block heterojunctions with compositions and/or new structures that can modulate the properties of the materials and can be applied to diverse areas, such as electronic devices 8,9 and photocatalysts. 10-12 The metal/semiconductor is one of the most popular types of heterostructures, which has been extensively studied because of its excellent catalytic activity. For example, a Ag/ZnO heterostructure photocatalyst with high catalytic activity has been reported. 13 Ag nanoparticles and oxygen vacancy defects on the surface of ZnO nanocrystals help to suppress electron-hole recombination, thus enhancing the photocatalytic activity. In heterostructures, a different dimen- sional structure of the composite materials can result in a difference in the photocatalytic performance. In particular, particles with a nanosized and 0-dimensional structure are most useful for absorbing light and trapping photoinduced electrons. 14,15 Graphene, with a 2-dimensional honeycomb structure, has attracted considerable attention due to its outstanding mechanical, thermal, optical, and electrical properties. Func- tionalized graphene-based semiconductor photocatalysts have attracted interest due to their good electron conductivity, large specific surface area, and high adsorption. 16-18 Numerous attempts have been made to combine graphene with semi- conductor photocatalysts to enhance photocatalytic perform- ance. 19-22 In our previous studies, it was found that graphene oxide (GO) on TiO 2 film helps to enhance the photocatalytic activity of TiO 2 by providing π-π stacking interactions between methylene blue (MB) and the aromatic regions of GO and by electron transfer of photoinduced electrons. 23 It is well known that significant improvement in photocatalytic performance can be achieved with a vertical ZnO nanostructure because of its intrinsic high absorption efficiency. We Received: October 25, 2011 Revised: February 27, 2012 Published: March 5, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 7180 dx.doi.org/10.1021/jp210216w | J. Phys. Chem. C 2012, 116, 7180-7184