Density functional theory study on the electronic properties of doped-cobalt oxide (CoO)

Cobalt oxide (CoO) has been widely studied for photocatalyst of water splitting and displaying a high-efficiency material. This paper reports a Density Functional Theory (DFT) study on the electronic properties of rock-salt CoO and analyzes effects of cations (Ni and Fe) and anions (N and F) dopants on the electronic properties. For this purpose, CASTEP software used for first principles plane-wave pseudo-potential calculations at different functional, i.e: GGA-PW91 and LDA. The electronic calculations of the CoO optimized structure showed a metallic structure if without considering spin-orbital interactions. After considering the spin-orbital interaction calculation, the CoO band structure possessed indirect and direct band gaps. The direct bandgap by GGA-PW91 calculation is 2.10 eV, it was agreed to the experimentally reported value of approximately 1.9-2.6 eV. Meanwhile, Ni, Fe, and F-doped CoO, demonstrating decreased CoO direct band gaps to 1.70 eV, 1.80 eV, and 1.73 eV, respectively. While N-doped CoO increased the CoO direct bandgap to 3.05 eV. All dopants shifted the conduction and valence bands position, where Ni-doped CoO band edges keep straddle to the redox potential of water splitting. Among other elements in this study, Ni is a more desirable dopant of CoO to enhance photoelectrochemical hydrogen production

Composite of titanium dioxide nanotube and cobalt sulfide for photoelectrochemical application

TiO2 nanotubes (NT) offer several advantages over other geometries for photoelectrochemical (PEC) applications. However, their performance in PEC water-splitting application has remained unsatisfactory due to its wide bandgap. To address this limitation, one approach is the incorporation of other materials as co-catalysts. Hence, in this study, a composite of TiO2 NT and cobalt sulfide (Cox Sy ) was successfully synthesized, and its potential as a photoelectrode for water molecules splitting was evaluated. The TiO2 NT was synthesized using electrochemical anodization of Ti foil, followed by annealing at 500 °C. Subsequently, Cox Sy was added to the TiO2 NT using hydrothermal method, and the composite was further annealed at 400 °C. Characterization technique, such as FESEM and XRD were employed to identify the morphological and phase structures, while UV-Vis reflectance spectroscopy was used for optical analysis. The efficiency of Cox Sy deposited on to TiO2 NT were evaluated by measuring the photocurrent generation. Remarkably, the sample of 60_Cox Sy /TiO2 NT exhibited photocurrent as high as 0.375 mA/ cm2 which is over sixfold higher than the bare TiO2 NT. The results reported in this study were higher than those reported previously

Experimental and theoretical study of Cu2 O photoelectrode and Cu2 O doped with Ag, Co, Ni and Zn metals for water splitting application

In the present study, cuprous oxide nanowire fabricated using wet chemical oxidation method was proven to produce high photoactive film for photoelectrochemical (PEC) water splitting. A relatively high photocurrent density of -5mA cm-2 at 0.6V vs Ag/AgCl was generated. The PEC performance is the reflection of intrinsic light absorption capacity at visible region which correspond to 2.0eV, an ideal band gap for PEC water splitting. Comparison with calculated data based on density functional theory using CASTEP shows that the band gap and light absorption capacity obtained from experimental work exhibited a close match. Hence, this study suggested that the preparation of Cu2 O thin film via wet chemical oxidation method obeyed the theoretical prediction. However, the Cu2 O is limited with poor stability in PEC condition attributed to the insufficient potential of its valence band to oxidize water. Therefore, an effort was directed to address the feasibility of shifting the valence band by modeling a doped Cu2 O with several dopants using DFT technique. The selected dopants were Ag, Co, Ni and Zn. Preliminary conclusion of this study indicated that doping could be used to tune the band gap of Cu2 O due to ionic radii of the dopant affected the shifting of band gap. In this study, Co showed more significant improvement of Cu2 O for photoelctrochemical water splitting process. However, to validate the simulation, further study should be carried out experimentally

Hydrogen production from water splitting using TiO2 /CoS composite photocatalyst

Photocatalytic water splitting reaction has been considered an ideal method for hydrogen generation. In this study, a composite of TiO2 /CoS photocatalyst prepared by hydrothermal synthesis method assisted by ball milling crushing process was used. The TiO2 /CoS composites prepared with three variation compositions of 90/10, 80/20, and 70/30 were named M-10, M-20, and M-30, respectively. Field-emission scanning electron microscopy images showed that the morphologies of the composites were porous and uniform of nanospheres. The X-ray diffraction and energy dispersive spectroscopy analyses confirmed the presence of CoS in the composites. Ultraviolet–visible absorption characterization demonstrated the smallest bandgap value of approximately 2.72 eV presented by sample M-30 with the photocurrent density of 0.32 mA cm−2 at 0.9 V vs. Ag/AgCl. The presence of CoS in this study could increase the PC hydrogen generation of TiO2 by nearly 2.5 times. The composites forming a p-n heterojunction between TiO2 and CoS could prevent electron–hole recombination and increase the overall photoactivity of TiO2.

Synthesis of Cobalt Oxide on FTO by Hydrothermal Method for Photoelectrochemical Water Splitting Application

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

A coral-like Mo2C/TiO2 photoelectrode for photoelectrochemical water splitting

Titanium dioxide (TiO2) is one of the most explored photoelectrode materials of water splitting for hydrogen generation. However, TiO2 has a bandgap of 3.2 eV, which restricts its energy absorption to UV light, and the photoexcited electrons and holes swiftly recombine. Thus, alteration of the band structure, such as by adding materials as cocatalysts, is needed. 2D molybdenum carbide (Mo2C) has been researched extensively as an excellent non-noble cocatalyst owing to its Pt-like H+ adsorption capacity and high conductivity. In this work, composites of TiO2 and Mo 2C with four different compositions were produced using the sol-gel method, and their photoelectrochemical activity for water splitting was assessed. The composites were spin-coated onto FTO conducting glass, and FESEM analysis indicated that TiO2nanoparticles are widely disseminated across Mo2C to form coral-like structures. Analysis via X-ray diffraction verified the existence of peaks composed of TiO2 and Mo2C. The sample containing 3% Mo2C had the greatest increase in photocurrent density, which was approximately 1.56 mA cm-2 at a potential of 1.0 V against Ag/AgCl (1.59 vs. RHE), which is five times that of bare TiO2. In addition, the composite’s onset potential moved to a lower potential. Our findings suggest that adding Mo2C increases the photoelectrochemical performance of the TiO2 photoelectrode. This work indicates the feasibility of employing Mo2C as a cocatalyst to improve the performance of TiO 2 for photoelectrochemical H2 production