Browsing by Author "Naresh, Gollapally"

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Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O Polyhedra Arising from Tunable Interfacial Band Alignment
(2019-01-23)
Naresh, Gollapally
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Hsieh, Pei-Lun
;
Meena, Vandana
;
Lee, Shih-Kuang
;
Chiu, Yi-Hsuan
ZnS particles were grown over Cu2O cubes, octahedra, and rhombic dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, Cu2O cubes stay photocatalytically inactive. ZnS-decorated Cu2O octahedra show enhanced photocatalytic activity, resulting from better charge carrier separation upon photo excitation. Surprisingly, Cu2O rhombic dodecahedra give greatly suppressed photocatalytic activity after ZnS deposition. Electron paramagnetic resonance spectra agree with these experimental observations. Time-resolved photoluminescence profiles provide charge-transfer insights. The decrease in the photocatalytic activity is attributed to an unfavorable band alignment caused by significant band bending within the Cu2O(110)/ZnS(200) plane interface. A modified Cu2O-ZnS band diagram is presented. Density functional theory calculations generating plane -specific band energy diagrams of Cu2O and ZnS match well with the experimental results, showing that charge transfer across the Cu2O(110)/ZnS(200) plane interface would not happen. This example further illustrates that the actual photocatalysis outcome for semiconductor heterojunctions cannot be assumed because interfacial charge transfer is strongly facet-dependent.
Shape-Tunable SrTiO3 Crystals Revealing Facet-Dependent Optical and Photocatalytic Properties
(2019-06-06)
Hsieh, Pei-Lun
;
Naresh, Gollapally
;
Huang, Yu-Sheng
;
Tsao, Chun-Wen
;
Hsu, Yung-Jung
SrTiO3 cubes with tunable sizes of 160-290 nm have been synthesized by mixing TiCl4, SrCl2, and LiOH in pure ethanol or a water/ethanol mixed solution at just 70 degrees C for 3 h. Replacing water/ethanol with water/hexanol and water/ethylene glycol, and fine tuning the amounts of other reagents, resulted in the formation of edge-truncated cubes and {100}-truncated rhombic dodecahedra, respectively. X-ray diffraction and transmission electron microscopy characterization, supported by Rietveld refinement analysis, have revealed shape-dependent tuning in lattice parameters. The cubes display slight size-related optical band shifts, and they show clearly more blue-shifted light absorption than the other particles exposing significant {110} faces. The {100}-truncated rhombic dodecahedra are far more efficient than cubes at photodegradation of methylene blue and photocatalyzed hydrogen evolution from water in the presence of methanol. The photocatalytic activity variation should arise from different degrees of surface band bending for the {100} and {110} faces of SrTiO3, suggesting surface facet control as a strategy for enhancing photocatalyzed hydrogen production.