Study of electrodeposition on titanium dioxide and its potential applications

Abstract

It is commonly understood that it is impossible to electroplate on insulating materials. Hence, electrodeposition has always been carried out on conductive substrates. However, a recent study [1] regarding the mechanism of low resistance characteristics on an insulating material postulated the existence of conductive filaments constructed by oxygen-vacancies assisting the change of resistance in the titanium dioxide, TiO2, layer. By taking advantage of the conductive elements during application of voltage, we investigate the possibility of electroplating on polycrystalline TiO2. These conductive filaments would allow electron flow and hence can be the electrons conducting paths during electrodeposition. This feasibility study has been carried out for both direct and pulsed current for Au onto fluorine doped tin oxide (FTO) and planar-TiO2 substrates. We identified and studied the two main factors influencing the nucleation density and the size dispersity of the electrodeposits: (1) substrate conditions, i.e. conductive and non-conductive substrates, which has the major influence on the electron concentration and transportation, and (2) electrodeposition process parameters for example frequency and duty cycle dependency. It was found that the nucleation and growth of Au islands are two competitive processes, especially for FTO substrate. The electrodeposits start from nucleation stage; as the Au islands grow, small hemispherical Au islands, and worm-like structure connections is observed. The optimum electrodeposition on FTO substrate is proposed to be carried out at a pulse frequency of 100 Hz with the duty cycle of 1/16 as this condition promotes a surface coverage of 85.7% in 8 s. For electrodeposition study on planar-TiO2, our research interest remains on the Au deposits formed via conductive filaments through bulk TiO2. The nucleation density on planar-TiO2 substrate is three orders of magnitude lower than FTO substrate due to the substrate’s nature in terms of the availability of electron transportation path and electron concentration. Most notably, we have successfully electroplated Au islands on the planar-TiO2 substrates, by using electrons supply through conductive filaments, base on the FTO as the back contact. Each pulse during electrodeposition encourages new conductive filaments formation with lateral growth, which act as the electron transportation paths. We have discovered that conductive filaments can reliably populate an insulator surface with reduction sites sufficiently to nucleate electrodeposits. This work will enable a new class of heterogeneous material combination for novel devices.