Question

Chemistry of reactions of tio2-graphene heterostructures

Answer 1

We study the structure of the photocatalytic graphene oxide–titanium dioxide (GO–TiO2) nanocomposites prepared by in situ sol–gel nucleation and growth of TiO2 on GO sheets. Fourier transform-infrared (FTIR) and X-ray photoelectron (XPS) spectra of these composites indicate that the GO sheets and the TiO2 nanoparticles interact through Ti–O–C bonds. This chemical interaction is strong enough to ensure mutual stabilization during thermal annealing, and thereby GO inhibits TiO2 crystallization. In addition, thermal reduction of GO nanoribbons anchored to TiO2 nanoparticles occurs at a higher temperature and with a lower released energy than in the pure GO powder. Understanding of the mutual-stabilization mechanisms is critical for the rational design of GO–TiO2 photocatalysts. 1. Introduction The breakthroughs of graphene research have been revolutionizing many research elds owing to the superior physical and chemical properties of graphene-based materials over a broad application spectrum.1–3 For instance, graphene–TiO2 heterostructures have opened a new direction in the development of heterogeneous photocatalysts for environmental applications.4–6 Nowadays, the benchmark material for photocatalytic application is TiO2, because it is inexpensive, chemically inert and has high photocatalytic activity in the abatement of organic pollutants.7 However, the TiO2 efficiency is limited by high rate of electron–hole pair recombination and its band-gap can only be used to exploit UV-light.8–10 In principle, the combination of TiO2 and graphene allows for superior photocatalytic properties, because graphene can potentially act as electron acceptor for hindering electron–hole recombination and it can increase the absorption range from UV to visible light of TiO2. 7,10,11 In addition, graphene can function as an absorbant,12 thus holding pollutants close to the TiO2 photocatalytic centers.2 TiO2–graphene photocatalysts have recently attracted considerable interest and different methods have been established for their fabrication. Such methods oen involve the synthesis or the deposition of TiO2 nanoparticles on water dispersed graphene oxide (GO) sheets. Indeed, the use of GO offers several advantages, because GO can be easily prepared by chemical oxidation and exfoliation of natural graphite,2,13 it can be easily dispersed in water, and it can subsequently be thermally or chemically reduced to graphene-like structures (rGO).12,14 Furthermore, in such heterostructures the interactions between GO functional groups and the surface of the nanoparticles are benecial for integrating the respective merits and to solve compatibility problems during synthesis and posttreatment, thus yielding composites with enhanced properties.15,16 TiO2–rGO composites prepared by hydrothermal and solvothermal processes exhibit good chemical bonding at the interface.8,17 However, these methods work for specic conditions and equipment, e.g., in the cases of high temperature, Teon autoclave or organic solvents.8,17,18 On the contrary, the sol–gel synthesis is simple, requires mild conditions, and makes it possible to obtain narrow size distributions in the nanometer range.19 In addition, controlling pH offers the possibility to exploit the strong electrostatic interaction between the negative charged GO sheets and the positively charged surface of TiO2 nanoparticles.11,19 For instance, Zhang et al. succeeded in synthesis of a GO–TiO2 intercalated composite by electrostatic attraction via a sol–gel process at 80 C.20 [TiO]2+ was introduced into GO interlayer exfoliated in 0.2 M NaOH, so that the nucleation and growth of TiO2 crystal occurred in situ. The photo-degradation of methyl orange solution under UV light of this GO–TiO2 composite within 15 minutes (87.2%) was stronger than that of the reference Degussa P25 powder (38.4%). The interaction between GO and TiO2 nanoparticles can also be used to prepare stacked graphene membranes with photocatalytic properties,21–24 where the