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Influence of oxygen assisted recombination on persistent photocurrent characteristics of ZnO polycrystalline thin films prepared by RF magnetron sputtering
(2020-07-01)
Worasawat, Suchada
;
Neo, Yoichiro
;
Hatanaka, Yoshinori
;
Pecharapa, Wisanu
;
Mimura, Hidenori
Decay characteristics in persistent photocurrent (PPC) of zinc oxide (ZnO) polycrystalline films highly aligned in the hexagonal c-axis direction (0002) deposited by radio frequency magnetron sputtering were investigated. Long-term decay of the photocurrent after turning off UV light illumination has been known as a PPC, which affected the performance of optical and electrical devices. Compared to the case of argon ambient, the decay characteristics were strongly influenced by the oxygen ambient. Our phenomenological model proposed could derive the recombination coefficient from an average waiting time that electrons/holes encounter the oxygen molecules attacking the surface of the ZnO polycrystalline films. The decay time response resulted is explained as a modified hyperbolics with the time constant τ and dispersion parameter β. Our experimental results are adapted to the theoretical equation. Using the average β value, τ dependence on the oxygen volume rate is in agreement with the theoretical one and presented a dispersive process on the surface reaction of oxygen gas. These investigations are useful for applications such as photoconductive sensors, gas sensors and a photocatalytic electrode.
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Persistent photocurrent characteristics of ZnO polycrystalline films prepared by RF magnetron sputtering
(2019-01-01)
Worasawat, Suchada
;
Tasaki, Katsuyosi
;
Neo, Yoichiro
;
Pecharapa, Wisanu
;
Hatanaka, Yoshinori
We study zinc oxide (ZnO) polycrystalline films highly aligned in the hexagonal c-axis direction 0002 deposited by RF magnetron sputtering. Photoconductive decay current characteristics are investigated. The decay current shows a long decay tail called persistent current. This temporal feature is analyzed with regard to the inhomogeneous amorphous semiconductor structure. We conclude that the photo-excited carriers of electrons and holes recombine between spatially separated states near the thermo-activated conduction band edge and those near the valence band edge. Although the activation energy of the recombination process is dependent on the ultraviolet irradiation intensity, under weak values an activation potential of about 0.15 eV is obtained, which clarifies the carrier transport in this polycrystalline system, and may give rise to future applications of this system, e.g., catalytic processes.
1
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Growth of ZnO nano-rods and its photoconductive characteristics on the photo-catalytic properties
(2019-01-01)
Worasawat, Suchada
;
Tasaki, Katsuyoshi
;
Hatanaka, Yoshinori
;
Neo, Yoichiro
;
Mimura, Hidenori
Zinc oxide (ZnO) nano-rods has widely been investigated as a promising material for photo-catalysis, due to its high photocatalytic activity for the ambient gas molecules. Well-aligned ZnO nano-rods arrays were obtained by the two steps growth. Firstly, seeding layer deposition by RF magnetron sputtering, and secondly, nano-rods growth by hydrothermal processes were investigated. Moreover, after the samples were annealed at 450 ºC, these samples were compared in crystalline characteristics and photo-electronic characteristics. The morphology of grown ZnO nano-rods were observed as the hexagonal nano-rods and XRD shows a (002) orientation in wurtzite crystals structure. Annealing processes shows a remarkable improvement of crystallinity. PL measurements showed a steep band edge emission and broad defect states emission and also fairly improvement in crystalline properties by the annealing process. The photoconductivity measurement could lead to investigate the photo-catalytic phenomena on the surface of ZnO nano-rods against the environmental gas molecules. The annealed samples showed a higher photo-conductivity and surface activity for the oxygen gas molecules comparing to the as grown samples.
1
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Synthesis and characterization of ZnO nanorods by hydrothermal method
(2018-01-01)
Worasawat, Suchada
;
Masuzawa, Tomoaki
;
Hatanaka, Yoshinori
;
Neo, Yoichiro
;
Mimura, Hidenori
Hydrogen generation through solar-driven photoelectrolysis of water is a promising technology for clean power supply. Zinc oxide (ZnO) is often applied to a photoelectrochemical (PEC) cells for water splitting, because ZnO has a favorable band edge that straddles the redox potential of photoelectrolysis in water. In addition, formation of ZnO nanostructure is attracting much attention, since the efficiency of PEC cell largely depends on a surface area of contacts. Here we report a modified hydrothermal method to prepare ZnOnanorods, whose structures can be controlled by changing preparation parameters. Our method includes seeding process by simple dip-coating technique, followed by post annealing process and hydrothermal growth process of ZnO nanostructures. The morphology of the grown ZnO depended on the concentration of the solvent used to form the seed layer formation, post-annealing temperature, and growth temperature. At the seed layer solvent concentration of 0.2 M, the ZnOnanorods were needle-like. At 0.9 M, hexagonal ZnOnanorods were obtained. We have successfully prepared hexagonalZnOnanorods by varying seed layer concentration at 0.9M, annealed at 150 ºC for 2 h and grown at 120 ºC for 1 h.

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