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1. Introduction
Recently, the photoelectrochemical (PEC) conversion of CO2 into methanol and photoelectrolysis of water into H2 gas have received a great deal of attention from the scientific community [1–5]. These reactions are popularly called as artificial photosynthesis, which can transform solar light into chemical energy. These reactions require thermodynamic energy inputs of 1.23 and 1.5 eV, respectively [4–9]. Greater energy inputs are required to make up the losses due to band bending (necessary in order to separate charge at the semiconductor surface), resistance losses, and overvoltage potentials. The most frequently studied material for the photoelectrode is TiO2 [6, 7]. Given its indirect bandgap transition, the TiO2 in the form of anatase phase is the most preferred one among its three crystal modifications (i.e., anatase, rutile, and brookite) for PEC applications. Despite its high bandgap of 3 eV, TiO2 is the favored material owing to its high corrosion resistance. The maximum value obtained for the photovoltage of a PEC cell equipped with a TiO2 photoanode is ~0.7–0.9 eV [10]. This implies that the single-electrode TiO2-based PEC cell requires some amount of an external bias voltage for performing artificial photosynthesis reactions [1–10]. For the first time, Nozik [11, 12] successfully photooxidized water without employing any external bias voltage by utilizing simultaneously n-type TiO2 and p-GaP as photoanode and photocathode, respectively. As p-GaP undergoes photocorrosion; there has been a continuous quest for stable oxide-based p-type semiconducting materials for PEC applications [8]. Given its superior corrosion resistance and most promising flat band potentials suitable for artificial photosynthesis reactions, more focus was made on TiO2 to convert it from n-type conducting to p-type, and to reduce its bandgap energy and recombination of its photogenerated electron-hole pairs [13–20]. The p-type conducting behavior was occasionally observed for TiO2 after doping with certain metal ions such as Fe3+, Co3+, Ni2+, Cu1+ [8, 13–20]. The change of semiconducting behavior has been attributed to the heterounions formed between n-type TiO2 and p-type metal oxide dopant [18]. Among various transition metal ion dopants, Ni2+ appears to be a more efficient dopant for TiO2 as it has improved the photocatalytic activity of certain semiconductor photocatalysts [21, 22]....