| Summary: | 碩士 === 國立清華大學 === 化學系所 === 105 === The significant difference between photocatalytically inactive Cu2O cubes and the high photocatalytic performance of Cu2O rhombic dodecahedra (RD) in methyl orange (MO) photodegradation has been demonstrated previously, and we have further introduced various chemical scavengers to probe the facet-dependent photocatalytic mechanisms of Cu2O cubes, octahedra, and RD in order to reveal the contribution of different catalytic species in photodegradation of MO. With the addition of electron scavenger, the conversions of MO are 1%, 13%, and 65% after 90 min of irradiation using Cu2O cubes, octahedra and RD as photocatalysts, respectively. With the addition of hole scavenger under same conditions, the conversions of MO are 3%, 69%, and 97% using Cu2O cubes, octahedra and RD respectively as photocatalysts. Cubes remain inactive with and without the introduction of electron and hole scavengers, matching out previous assumption that the photogenerated electrons and holes do not reach the surface of Cu2O cubes. RD preserve relatively independent activity upon introduction of electron and hole scavengers, implying that both photogenerated electrons and holes could efficiently produce radicals, thus maintaining its activity when scavengers capture either electrons or holes. Interestingly, the photocatalytic activity of Cu2O octahedra is significantly quenched by electron scavenger but not hole scavenger, suggesting their activity mostly comes from photogenerated electrons. This could be interpreted as photogenerated electrons travel to surface to yield radical species, while holes are largely unavailable for photooxidation reaction.
Based on these experimental results, a modified band diagram with different degrees of band bending for Cu2O crystals is constructed to explain the photocatalytic performance of differently exposed crystal facets. Ultraviolet photoelectron spectroscopy (UPS) was also used to analyze the energy level at the surface. However, it was not able to give sufficient degrees of surface bend bending to explain why Cu2O cubes should be photocatalytically inactive. Semiconductor facet effects have largely been missed because the measured band energies do not explain or predict extremely different photocatalytic ability exhibited by the same material with different exposing crystal facets.
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