ZnO/(La, Sr) CoO 3 (ZnO/LSCO) core-shell composite nanorod arrays have been successfully synthesized by a sequential combination process of a hydrothermal synthesis followed by a pulsed laser deposition (PLD) process (or a colloidal deposition process). Compared to the colloidal deposition process, PLD produces a more uniform and efficient deposition of continuous and mesoporous LSCO thin films onto ZnO nanorod arrays. During the PLD process, the deposited film uniformity was found to be dependent on the nanorod diameter, array density, and thus specific surface area of the nanorod arrays, in addition to the PLD deposition parameters. Field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to investigate the surface morphologies and orientations of the composite nanorod arrays. With densely packed ZnO nanorod arrays as a unique support structure, the mesoporous LSCO thin film coated on top exhibited better photocatalytic properties than ZnO nanorod arrays and LSCO thin films deposited on flat Si substrates. With optimization of the structure, dimensionality, packing density, as well as the composition and interface structure, these unique composite nanoarchitectures could be a promising class of photocatalyst candidates for organic molecule degradation. Experimental ZnO nanorod array growthThe films of ZnO nanorod arrays were grown on substrates using the hydrothermal method. The substrates used in our experiments included (100) silicon wafer and glass solid substrates. Acetone was used to clean the substrates in an ultrasonic bath. Deposition of ZnO nanorod thin films was achieved using aqueous solutions of zinc acetate (ZnAc 2 , 0.025 mol dm À3 ) and

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The sequential hydrothermal process is used to synthesize ZnO nanostructures on Si substrates. The synthesized ZnO nanostructures are inspected by scanning electron microscope and transmission electron microscope. They present a morphology of two-dimensional structures, named nanoflakes. The ZnO nanoflakes have a thickness of tens of nanometers.The energy dispersive x-ray spectrum reveals their compositions of only Zn and O elements. In addition, its crystalline structures are investigated by high-resolution transmission electron microscope. The nanoflakes are then dispersed for another morphology measurement using atomic force microscope and their average thickness is determined. The dispersed nanoflakes are contacted with metal electrodes for electron transport measurements. Through the analysis of electrical and temperature dependences of resistivity, it is confirmed that the electron transport in ZnO nanoflakes agree well with the theory of Mott's two-dimensional variable range hopping. The nature of two-dimensional electron system in ZnO nanoflakes points to the application of this two-dimensional semiconductor as new channel materials for electronic devices.

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