Abstract
Cobalt oxide thin films were successfully grown directly on fluorine-doped tin oxide glass substrates through a simple, green, and low-cost hydrothermal method. An investigation into the physicochemical characteristics and photoelectrochemical (PEC) properties of the developed cobalt oxide thin film was comprehensively performed. At various annealing temperatures, different morphologies and crystal phases of cobalt oxide were observed. Microflowers (Co3O4) and microflowers with nanowire petals (Co3O4/CoO) were produced at 450 °C and 550 °C, respectively. Evaluation of the PEC performance of the samples in KOH (pH 13), Na2SO4 (pH 6.7), and H2SO4 (pH 1) revealed that the highest photocurrent −2.3 mA cm−2 generated at −0.5 V vs. reversible hydrogen electrode (RHE) was produced by Co3O4 (450 °C) in H2SO4 (pH 1). This photocurrent corresponded to an 8-fold enhancement compared with that achieved in neutral and basic electrolytes and was higher than the results reported by other studies. This promising photocurrent generation was due to the abundant source of protons, which was favorable for the hydrogen evolution reaction (HER) in H2SO4 (pH 1). The present study showed that Co3O4 is photoactive under acidic conditions, which is encouraging for HER compared with the mixed-phase Co3O4/CoO.
Highlights
Hydrogen (H2 ) is an ideal future energy carrier to replace fossil fuels
This work provided the necessary support for the direct growth of cobalt oxide on fluorine-doped tin oxide (FTO)
This work provided the necessary support for the direct growth of cobalt oxide on thin films, the effect of annealing on the phase transformation of cobalt oxide, and the effect
Summary
Hydrogen (H2 ) is an ideal future energy carrier to replace fossil fuels. H2 as fuel in a fuel cell system has been proven to be safe, clean, and environmentally friendly, and its only byproduct is pure water [1]. H2 can be extracted from water molecules through photoelectrochemical (PEC) water-splitting using direct solar energy, which has been actively studied globally [2,3]. Both components, water and sunlight, are abundant and available everywhere. PEC water-splitting requires a semiconductor material as a photoelectrode. This photoelectrode is exposed to sunlight and is the site for the initiation of the catalytic reaction. The n-type semiconductors, such as titanium dioxide (TiO2 ), act as a photoanode; the water molecule is oxidized to produce oxygen (O2 ) gas. P-type semiconductors, such as Cu2 O, act as a photocathode. The proton (H+ ) is reduced, and H2 is generated [4]
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