Fabrication of Graphene-based Composite Materials and Their Photo-functional Applications for Energy and Environment

Abstract

Graphene has many unique properties including high conductivity, high surface area, high thermal conductivity, etc., so it has been actively investigated as not only an electron mediator but also a co-catalyst in photocatalytic systems. In general, noble metals are widely utilized as a cocatalyst since the photogenerated electrons are trapped on the metals as a result of Schottky barrier so the recombination of charge carriers can be effectively retarded. However, the high material cost limits its practical applications. In this regards, reduced graphene oxide (rGO) is one of the candidates to replace noble metals in photocatalytic systems in that it can be prepared by simple and cheap procedure but still has high conductivity and high specific surface area. In the first and second topics, we studied the role of rGO as a cocatalyst and an electron mediator in TiO2-based photocatalytic systems for arsenic oxidation and the production of H2O2. In the third topic, visible light activity of graphene oxide (GO) was observed, which was applied for the growth of metal nanoparticles on GO by only irradiation. 1. The preoxidation of arsenite [As(III)] to arsenate [As(V)] is usually needed for efficient removal of arsenic from water, and TiO2-based photocatalytic oxidation has been investigated as an environmentally benign process for this purpose. The oxidation efficiency can be markedly enhanced when using platinum (Pt) as a cocatalyst, but its expensive material cost hinders its practical application. Herein, we prepared the rGO hybridized with TiO2 as a low-cost alternative to Pt and achieved a highly enhanced activity for the photocatalytic oxidation of As(III), which is comparable to that of Pt/TiO2. While either superoxide or hydroxyl radical can be involved as a main oxidant depending on the experimental condition, this study shows that not only superoxide but also hydroxyl radicals (or hole) can be directly involved in As(III) photo-oxidation when rGO is present as a cocatalyst. The photocatalytic activity, charge transfer characteristics, and arsenic oxidation mechanism observed with rGO-loaded TiO2 are very similar to those of Pt/TiO2. The nanocomposite of rGO/TiO2 that consists of earth-abundant elements only is proposed as a practical environmental photocatalyst for pretreating As(III)-contaminated water. 2. To develop not only organic-electron-donor-free but also noble-metal-free TiO2-based photocatalytic system for the generation of H2O2, in situ formation of cobalt phosphate (CoPi) was achieved on the rGO/TiO2 composite. TiO2 hybridized with rGO as a cocatalyst enhanced the formation of H2O2 even comparable to noble metals (Au, Ag, and Pt) loaded on TiO2 in the presence of 2-propanol as an electron donor. Since the photocatalytic production of H2O2 was largely limited by the rapid degradation of H2O2 on TiO2, the surface of TiO2 was adsorbed by phosphate to inhibit the adsorption of H2O2 and its subsequent decomposition on the TiO2 surface, which consequently enhanced the H2O2 production up to a millimolar level. Further addition of cobalt ions in the suspension of rGO/TiO2 adsorbed with phosphate induced the production of H2O2 even in the absence of organic electron donors as CoPi that functioned as a water oxidation catalyst was in situ generated. The photocatalytic system consisting of earth-abundant elements only (Ti, O, C, Co, and P) can be proposed as an eco-friendly method for the solar production of H2O2. 3. In the presence of silver or gold ions, visible light irradiation (> 420 nm) induces the formation of metal nanoparticles on graphene (GO) sheets without the need of any chemical reducing reagents. GO sheets serve as not only a good substrate for dispersion of metal nanoparticles but also a self-reactive material itself for the photo-induced reduction of metal ions.