Effect of asymmetric power distribution in bipolar reactive sputtering on the optoelectronic and microstructure properties of titanium dioxide for solar water splitting

Abstract

Titanium dioxide is one of the most commonly used materials for solar water splitting, because of its excellent corrosion resistance. However, its optoelectronic properties are not sufficient to achieve high solar to hydrogen conversion. It is known that the choice of deposition method has a great influence on the optoelectronic and structural properties of titanium dioxide.Here we combine two different sputtering configurations and vary the process parameters to obtain thin films of black TiOx. Bipolar magnetron sputtering from two metallic titanium targets is used in combination with an oxygen partial pressure control via a closed-loop residual gas monitoring system. The power distribution between the two targets in the bipolar sputtering mode is thereby varied from sample to sample, while the oxygen partial pressure remains constant. This causes one target to shift into the oxidic regime, while the other target shifts into the metallic regime. Because the metallic target has a higher sputter yield the process leads to off-stochiometric samples. These samples exhibit a broad-band absorption and an increased photoelectrochemical activity (up to five times) compared to samples without the modification of the power distribution. The results are discussed based on the optoelectronic and microstructural analysis of the thin films.