Lowering of Band Gap of Titania Thin films by Integration of Graphene Nanosheets

Abstract

Titania possesses a band gap of 3.2 eV, while work function of graphene 4.5 eV exists near LUMO of titania. The photoexcition of electrons in titania is only possible under ultraviolet radiations. In order to obtain such conversion in visible light range, band gap has to be lowered below 3.1 eV. This research work aims at lowering the band gap of titania thin films by incorporation of graphene nanosheets. Graphene was synthesized using modified Hummer’s method in which graphite flakes were oxidized using KMnO4 in an acidic solution. After washing of graphite oxide, exfoliation was carried out by sonication of graphite oxide dispersion, producing graphene oxide. Reduction of graphene oxide nanosheets was carried out by treating it with hydrazine hydrate resulting in reduced graphene oxide. Oxidation of graphite was confirmed by XRD revealing a peak at 11.8° with interplanar distance of 7.5 Å and EDS analysis showed almost complete oxygenation of carbon. AFM analysis revealed exfoliation of single layer graphene oxide nanosheets having thickness of 0.6 – 1.0 nm. FT-IR analysis confirmed reduction of graphene oxide by revealing removal of carbonyl group (C=O).

Titania sol was produced using hydrolysis and condensation of titanium tetraisopropoxide in aqueous acidic solution containing glacial acetic acid and hydrochloric acid. The TiO2-graphene composite was prepared by two routes. In one method, liquid phase dispersion of graphene oxide was added in titania sol and reduction of deposited films was done by exposing hydrazine hydrate vapors. In second method, nanoparticles of reduced graphene oxide were dispersed in titania sol and thin films were fabricated subsequently. Thin film fabrication was carried out using spin coating. Every film deposited was dried at 80°C for 5 min and annealed at 450°C for 50 min at a heating rate of 2°C min-1.

TiO2-graphene films characterized using SEM, revealed homogenous dispersion of graphene nanosheets among homogenously distributed titania nanoparticles of uniform size distribution (~ 30-50 nm). Surface roughness of about 16 nm was observed by AFM topographic images. The HOMO/LUMO levels were calculated by cyclic voltammetry and subsequent band gap was calculated which came out to be as low as 2.9 eV. Ti-O-C chemical bonding between titania and graphene sheets resulted in enhanced electron transport in the obtained composite films. These results favor the use of such films in Inverted Organic PVs as electron collecting layer and active layer in normal OPVs.