Simple but Effective Way To Enhance Photoelectrochemical Solar-Water-Splitting Performance of ZnO Nanorod Arrays: Charge-Trapping Zn(OH)₂ Annihilation and Oxygen Vacancy Generation by Vacuum Annealing

Abstract

This study presents an effective and the simplest method to substantially improve the photoelectrochemical water-splitting ability of hydrothermally grown ZnO nanorod arrays (NRAs). In the hydrothermal growth of ZnO NRAs, unwanted Zn(OH) species are formed, which act as trapping sites of photoexcited charges. We found that those inherent charge-trapping sites could be annihilated by the desorption of the hydroxyl groups upon vacuum annealing above 200 °C, which resulted in an enhancement of the charge-separation efficiency and photocurrent density. Another drastic increase in the photocurrent density occurred when ZnO NRAs were treated with annealing at higher temperature (700 °C), which can be attributed to the introduced oxygen vacancies acting as shallow donors in the ZnO crystal lattice. The removal of the charge-trapping Zn(OH) and the generation of oxygen vacancies were confirmed by photoluminescence (PL) and XPS analyses. The ZnO NRAs treated by this simple method yield a photocurrent density of 600 μA/cm at 1.23 V under 1 sun illumination, which is 20 times higher than that obtained from as-grown ZnO NRAs. This study presents a highly efficient way of increasing the bulk electric conductivity and photoelectrochemical activity of metal oxide nanorods without requiring the introduction of any extrinsic dopants.

Highly efficient UV-sensing properties of Sb-doped ZnO nanorod arrays synthesized by a facile, single-step hydrothermal reaction

“…44,46 According to this equation, the slope of Mott Schottky plot is inversely proportional to the charge-carrier density. In Fig.…”

Photocatalytic Water Oxidation on ZnO: A Review

“…The increase in charge separation efficiency may be attributed to the role of oxygen vacancies as electron acceptors, and interstitial oxygen as a shallow trap for photogenerated holes [21], preventing the recombination of the photogenerated electron-hole species. Zn(OH) 2 species act as deep trapping sites of photoexcited charges, resulting in poor photocatalytic activity [22]. The reader is also referred to a comprehensive review on the influence of doping and native defects on the electrical conductivity and optical properties of ZnO [10].…”

Enhancing Durability and Photoelectrochemical Performance of the Earth Abundant Ni–Mo/TiO2/CdS/CIGS Photocathode under Various pH Conditions

“…Until now,n umerous studies have mainly focused on the metal oxide photoanode materialsb ecause they are relatively stable, and various earthabundant materials are available, such as ZnO, [1,2] TiO 2 , [3] WO 3 , [4,5] and BiVO 4 . In general, aP EC cell consistso fap-typep hotocathode or an n-type photoanode combined with ac ounter electrode.…”

“…In general, aP EC cell consistso fap-typep hotocathode or an n-type photoanode combined with ac ounter electrode. Until now,n umerous studies have mainly focused on the metal oxide photoanode materialsb ecause they are relatively stable, and various earthabundant materials are available, such as ZnO, [1,2] TiO 2 , [3] WO 3 , [4,5] and BiVO 4 . [6,7] Compared to photoanode materials, however,p hotocathode materials have not been widely studied.…”

“…[9][10][11] As an alternative, CIGS has recently occupiedt he position as the best photocathode candidate. CIGSi sap-type I-III-VI 2 semiconductor that has ac hemical formula of Cu(In,Ga)(S,Se) 2 .I t has ac halcopyritec rystal structure and special band gap characteristics that can be tuned from 1.02 to 1.7 eV by adjusting the elemental composition ratio of In/Ga. [12] Additionally,i th as Cu(In,Ga)(S,Se) 2 (CIGS) is ap romising photocathode material owing to its high absorption coefficient, adjustable band gap, and suitable band edge for the hydrogen evolution reaction (HER).…”