| Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms densely packed in a honeycomb crystal lattice. Owing to its exceptional charge transport, thermal, optical, and mechanical properties, graphene has attracted vast interests in the fields of physics, chemistry, biotechnology, and materials science. At present, graphene and its derivatives are being studied in nearly every field of science and engineering. Recent progress has shown that the graphene-based materials have broad application prospects in fields such as electronic and optoelectronic devices, chemical sensors, nanocomposites and energy storage.Zinc oxide is a wide band gap semiconductor (3.37 eV), having a large exciton binding energy (60 meV). ZnO nanomaterials have wide application foreground in all kinds of fields due to its unique performance, which is different from bulk ZnO. Particularly, ZnO nanoarrays materials become the hotspot researches in current national academia, because of its simple preparation process and higher specific surface. ZnO nanoarrays can be used as photocatalyst, nanolaser, field emitter, and it also can be used as electrode materials for sensors, solar batteries, fuel cells, lithium-ion batteries, and other fields. But in the process of light excitation, the disadvantages both of low utilization rate of solar energy, low quantum efficiency and serious light corrosion limit its application in the field of photocatalysis and photoelectric conversion.Based on the above analysis, in order to improve the photocatalytic and photoelectric conversion performances of ZnO nanoarrays, in this paper, graphene and ZnO nanoarrays were combined. And graphene/ZnO nanoarrays composites were prepared by low-temperature solution method and in situ reduction method, respectively. Hence we obtained the graphene/ZnO composite on different substrate (Fe-Co-Ni alloy and FTO glass) by using low temperature liquid method and in situ reduction method, respectively. At the same time, the prepared graphene/ZnO nanoarrays composites were applied for degradation of organic pollutants and the dye sensitized solar cell. In addition, we investigated the influence of the experimental parameters on the photocatalytic and photoelectric conversion properties of graphene/ZnO nanoarrays composite.The results are as following:(1) The graphite oxide was prepared from natural flake graphite by the improved Hummers method. The graphene was subsequently prepared by thermal reduction. The results show that a portion of the carbon atom combine with oxygen bond to forming oxygen-containing functional group of C=O, C-OH,-COOH and C-O-C, the layer spacing increased to 9.7 nm. The graphite oxide (001) diffraction peak completely disappears after thermal reduction and a dispersion diffraction peak appears at 2θ=25°and graphene still contains part of the oxygen-containing functional group, which indicates the success of the reduction of graphite oxide and graphene was obtained. The thickness of the graphene oxide sheets is around 1.1～1.3 nm, the result is consistent with the thickness of the characteristic single-layer graphene oxide. Based on improved Hummers method, graphite oxide with different oxidation degree was prepared by changing the process parameters. Oxidation time without any obvious impact on characteristic structure of graphite oxide, when oxidation time more than 24 h, further extending the oxidation time without any obvious impact on oxidation degree of graphite oxide; The ratio of oxidant is key factor influencing the characteristic structure of graphite oxide. And with the amount of KMnO4 in the oxidant increased, graphite oxide oxidation degree increased. When the KMnO4to the mixed acid ratio is 1:20, the oxidation degree of prepared graphite oxide is appropriate.(2) ZnO nanorods arrays (ZNRs) were prepared from nitrate and ammonia on Fe-Co-Ni alloy substrate by low-temperature solution method. The graphene/ZnO nanorods arrays (RGO/ZNRs) with high photocatalytic performance were subsequently prepared by wet chemical method. The results show that the diffraction intensity of ZnO decreases with the increase in the amount of graphene, due to the shielding effect of graphene on the ZnO surface with increasing the amount of graphene in the composite. Owing to the formation of Zn-O-C bond, the band gap of pure ZnO has been slightly reduced from 3.3 eV to 3.0 eV. As a result of the extended the light absorption range and the enhanced visible light absorption, a more efficient utilization of the solar spectrum could be achieved. The amount of graphene was an important factor affecting the photocatalytic activity of RGO/ZNRs and the highest activity was obtained at an optimum amount of graphene around 2 mg/ml. With the concentration of graphene increases, the photocatalytic performance of the composites first increases and then decreases, when the graphene concentration is 2 mg/mL, content of graphene in composites reach the optimal value and the photocatalytic performance is the best,97.5% of MO is degraded by RGO/ZNRs composite within 2 h irradiation.(3) Graphene oxide/ZnO nanowire arrays (GO/ZNWs) were prepared by spin coating, and the graphene/ZnO nanorod arrays (RGO/ZNWs) was subsequently prepared by in situ thermal reduction. The morphology of the composite was characterized by SEM. The as-prepared RGO/ZNWs were used to assemble photoelectrode for application in dye-sensitized solar cells (DSSCs). The DSSCs was characterized by EIS and current-voltage (J-V) characteristics. The results suggest that surface coverage of RGO nanosheets on ZNWs can be manipulated by optimizing dropping cycles. DSSCs based on RGO-modified ZNWs show significantly higher energy conversion efficiencies than DSSC consisting of bare ZNWs. Such enhancement is ascribed to the graphene significantly reduces the contact resistance between ZnO photoanode and Pt counter electrode, and enhance the electronic conductivity of photo-anodes, and beneficial for the optimization of photoelectric conversion efficiency of DSSCs. Introduction of graphene can accelerate the electronic transmission rate and prolong the electron lifetime, thus inhibiting the recombination of relectron-hole pairs and benefit to optimize the photoelectric conversion efficiency of the cells. In addition, the content of graphene in anode has a great influence on the photoelectric conversion efficiency, when GO coating cycles is 2 times, short circuit current density (Jsc) 8.41 mA·cm-2, the open circuit voltage (Voc) is 0.71 V, the fill factor (FF) is 48.7% and the photoelectric conversion efficiency reaches the maximum value (η=2.59%), compared to the bare ZNWs anode dye-sensitised cells, the energy conversion efficiency of RGO/ZNWs has been extremely improved as large as 1.9 times. |
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