Band gap reduction of titania thin films using graphene nanosheets

Titania possesses a band gap of ~3.2 eV, whereas the work function of graphene ~4.5 eV exists near the lowest unoccupied molecular orbital (LUMO) of titania. The photoexcitation of electrons in titania (TiO2) is possible only under the ultraviolet radiation. If such conversion is required in visible light range, band gap has to be lowered below 3.1 eV. This research work hence aims at lowering the band gap of titania thin films by incorporation of graphene nanosheets. Graphene was synthesized using modified Hummer׳s method. Oxidation of graphite was confirmed by X-ray diffraction (XRD) revealing a peak at 11.8° with interplanar distance of 7.5 Å and energy dispersive X-ray spectroscopy (EDS) analysis. Atomic force microscopy (AFM) analysis revealed exfoliation of single layer graphene oxide nanosheets having thickness of 0.6–1.0 nm. Fourier transform infrared spectroscopy (FT-IR) analysis confirmed the reduction of graphene oxide by revealing removal of carbonyl group (CO). Titania sol was obtained using hydrolysis and condensation of titanium tetraisopropoxide. The TiO2–graphene composite was prepared by liquid phase dispersion of graphene oxide into titania sol and reduction of deposited films was carried out by exposing hydrazine hydrate vapors. In the second method, nanoparticles of reduced graphene oxide were dispersed in titania sol; thin films were deposited on indium–tin oxide (ITO) coated glass using spin coating and subsequently heat treated. A scanning electron microscope (SEM) was used to reveal 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 highest occupied molecular orbital (HOMO) and 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.