###### 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 (Co\({}_{3}\)O\({}_{4}\)) and microflowers with nanowire petals (Co\({}_{3}\)O\({}_{4}\)/CoO) were produced at 450 \({}^{\circ}\)C and 550 \({}^{\circ}\)C, respectively. Evaluation of the PEC performance of the samples in KOH (pH 13), Na\({}_{2}\)SO\({}_{4}\) (pH 6.7), and H\({}_{2}\)SO\({}_{4}\) (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 Co\({}_{9}\)O\({}_{4}\) (450 \({}^{\circ}\)C) in H\({}_{2}\)SO\({}_{4}\) (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 H\({}_{2}\)SO\({}_{4}\) (pH 1). The present study showed that Co\({}_{9}\)O\({}_{4}\) is photoactive under acidic conditions, which is encouraging for HER compared with the mixed-phase Co\({}_{3}\)O\({}_{4}\)/CoO. cobalt oxide; mixed-phase; photoelectrochemical; water splitting Article Synthesis of Cobalt Oxide on FTO by Hydrothermal Method for Photoelectrochemical Water Splitting Application  
[PERSON], [PERSON], [PERSON], L.J., and [PERSON]  
1 Fuel Cell Institute, Universit Kebangsan Malaysia, Bangi 43600, Malaysia; [EMAIL_ADDRESS] (S.N.F.M. ); [EMAIL_ADDRESS] (M.N.I.S. ); [PERSON]_ADDRESS] (L.J.M. ); [PERSON]MAIL_ADDRESS]my (M.B.K.) [PERSON], [PERSON], L.J., and [PERSON]  
1 Fuel Cell Institute, Universit Kebangsan Malaysia, Bangi 43600, Malaysia; [EMAIL_ADDRESS]  
water oxidation potential. Furthermore, a stable and highly photoactive photoelectrode is required to achieve an efficient PEC performance [6]. TiO\({}_{2}\)[7; 8], Cu\({}_{2}\)O [9], WO\({}_{3}\)[9], Fe\({}_{2}\)O\({}_{3}\)[10], and perovskites [11] are among the semiconductor materials that have been extensively investigated as photoelectrodes, because these materials are highly abundant and require simple fabrication techniques and low-cost preparation processes. However, recently, cobalt oxide itself has also received considerable attention as a photocatalyst material. Cobalt oxide is attractive because of its suitable optical band gap of \(\sim\)2.00 eV, which falls within the favorable band gap range for visible light absorption [9; 12; 13; 14; 15]. Co\({}_{3}\)O\({}_{4}\) and CoO are reportedly the most stable phases of cobalt oxide and are the most investigated materials for PEC water-splitting applications [13]. [PERSON] et al. (2017) reported that photocurrent densities produced by Co\({}_{3}\)O\({}_{4}\) for PEC performance at \(-\)0.4 V bias vs. Ag/AgCl could be achieved at \(-\)0.50 mA cm\({}^{-2}\), approximately [9]. [PERSON] et al. (2016) revealed that CoO can cleave water molecules without the addition of cocatalysts or sacrificial reagents. The photocatalytic process achieved \(\sim\)5% efficiency of solar to hydrogen, which is the highest efficiency for a single material. Cobalt oxide can be synthesized using various methods through a chemical or physical route [16]. The chemical route includes the sol-gel method [17], hydrothermal method [12], solvothermal method [18], cathodic reduction [19], and chemical vapor deposition (CVD) [20]. Meanwhile, a physical approach includes sputtering and physical vapor deposition (PVD) [19; 21]. Cobalt (II) salts, such as cobalt (II) chloride hexahydrate, cobalt acetate tetrahydrate, and cobalt (II) nitrate hexahydrate are most used as starting materials [22; 23].