Study of Relaxation Time Distribution in TiO2-GO Nanocomposite Thin Films by DC Bias Dependent Impedance Spectroscopy

Abstract

\n Graphene oxide (GO), a unique 2D nanomaterial made of sp2 -hybridized carbon atoms, is one of the most promising candidates to enhance the optoelectronic properties of metal oxide semiconductors by decreasing recombination and increasing lifetime of photo-generated charge carriers of host matrix. Here we report the fabrication of highly transparent TiO2 -GO nanocomposite thin films by simple cost-effective sol-gel spin coating technique and demonstrate the modulation of optical and transport properties with reinforcement of little amount of GO sheets into host TiO 2 matrix. Films are characterized by microstructural and optical measurements to confirm the formation of highly transparent composite thin films with rutile phase of nanocrystalline TiO2 with GO sheets incorporated into the host. DC bias dependent impedance spectroscopic study in the frequency range 4 Hz-5 MHz is carried out at room temperature for different external bias voltages. The Nyquist plot(Z / vs Z // ) is drawn and fitted by suitable model electrical circuit comprised of several parallel RC circuits in series in order to take different contributions arising from core grains and grain boundaries. Relaxation times (τ = RC) arising out of such different portions are estimated under the application of small ac signal. Variation of relaxation times with external bias and it’s change after GO reinforcement confirms the grain boundary modification and it’s effect on charge transport in such materials. Also, the significant decrease of τ value attributed to modification of grain-boundary due to inclusion of GO into TiO2 host material. This in turn confirms that conducting GO network is capable of modifying the grain-boundary of pristine TiO2 and facilitates better charge transport through it. External dc bias variation of frequency dependent dielectric constant, loss factor and ac conductivity are also studied for the pristine TiO2 and TiO2 -GO nanocomposites for different concentrations of GO.