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Heterostructures of gold-nanoparticle-decorated reduced-graphene-oxide (rGO)-wrapped ZnO hollow spheres (Au/rGO/ZnO) are synthesized using tetra-n-butylammonium bromide as a mediating agent. The structure of amorphous ZnO hollow spheres is found to be transformed from nanosheet- to nanoparticle-assembled hollow spheres (nPAHS) upon annealing at 500 °C. The ZnO nPAHS hybrids with Au/rGO are characterized using various techniques, including photoluminescence, steady-state absorbance, time-resolved photoluminescence, and photocatalysis. The charge-transfer time of ZnO nPAHS is found to be 87 ps, which is much shorter than that of a nanorod (128 ps), nanoparticle (150 ps), and nanowall (990 ps) due to its unique structure. The Au/rGO/ZnO hybrid shows a higher charge-transfer efficiency of 68.0% in comparison with rGO/ZnO (40.3%) and previously reported ZnO hybrids. The photocatalytic activities of the samples are evaluated by photodegrading methylene blue under black-light irradiation. The Au/rGO/ZnO exhibits excellent photocatalytic efficiency due to reduced electron-hole recombination, fast electron-transfer rate, and high charge-transfer efficiency.

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The photocatalytic performance of a multidimensional heterojunction composed of decorated Ag nanoparticles on ZnO nanorods vertically grown on a chemically converted graphene (CCG) was investigated. The combined heterojunction helps to improve photocatalytic activity by increasing light absorption, preventing photoinduced electron–hole recombination, and providing a carrier pathway for the giant π-conjugated system of CCG. A significant finding was that the low work function value of Ag (−4.74 eV) at the (111) surface makes possible the transfer of electrons from CCG to Ag. Consequently, the degree of photodegradation by Ag/ZnO/CCG is much higher than the sum of photodegradations by Ag/ZnO and ZnO/CCG samples, which indicates that the combination of metal, CCG, and semiconductor provides enhanced photocatalytic activity through the double transference of electrons.

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