The relationship between the contact type in metal-semiconductor junctions and their photocatalytic efficiencies is investigated. Two metal-semiconductor junctions, silver on zinc oxide (Ag/ZnO) and platinum on zinc oxide (Pt/ZnO) serve as model system for Ohmic and Schottky metal-semiconductor contact, respectively. Ag/ZnO, with Ohmic contact, exhibits a higher photocatalytic efficiency than Pt/ZnO, with Schottky contact. The direction of electric fields within the semiconductor is found to play a crucial role in the separation of photogenerated charges, and thus strongly influences the photocatalytic efficiency.

In this study, we report on an insight into the influential factors of the conversion efficiency of ZnO-based dye-sensitized solar cells. With ZnO nanoflowers as the photoelectric anode and dye N719 as the sensitizer, we found with the addition of N719 that the surfaces of ZnO nanoflowers were slightly etched at the beginning and gradually destroyed with time. On the basis of these observations, a series of experiments were further carried out to distinguish the influence of dye-induced aggregation from that of dye-induced etching on the solar-to-electric conversion efficiency. SEM observation reveals that there were no obvious dye/Zn(2+) aggregations on any of the samples. XRD results indicate that there was no new phase formed during the dye-sensitizing process. I-V measurements reveal clearly that the efficiency of ZnO-based DSSCs was inversely proportional to the etching level of ZnO surfaces. We concluded that the dye-induced etching of the ZnO anode may be an assignable cause that results in the low efficiency of ZnO-based DSSCs. The etching of ZnO may lead to low surface absorption efficiency of the dye, low electron mobility, and a high surface recombination ratio of photocarriers. Therefore, we suggest that special attention should be paid to protecting the surface structure of the ZnO anode during the dye-sensitizing process.

Incorporation of new functional components into a three-dimensional graphene (3DG) framework improves the performance of supercapacitors based on 3DG as electrodes by tailoring the framework’s structure and properties. In this work, graphene quantum dots (GQDs) were incorporated into 3DG via one-step hydrothermal treatment of GQDs and graphene oxide (GO). By simply adjusting the GQDs/GO feeding ratio by weight, various GQDs/3DG composites were formed. The maximum feeding ratio was 80%, and the prepared composites possessed saturated GQDs loading on the 3DG framework, whereas composites obtained with a GQDs/GO feeding ratio of 40% as electrodes exhibited optimal specific capacitance of 242 F·g−1 for supercapacitors, an increase of 22% compared with that of pure 3DG electrodes (198 F·g−1). This improved performance was mainly due to better electrical conductivity and larger surface area for GQDs/3DG composites with moderate GQDs content. The fabricated GQDs/3DG composites as electrodes for supercapacitors revealed high electrochemical stability. Their capacitance kept 93% of the initial value after 10,000 charge-discharge cycles.