Preparation And Photocatalytic Properties Of Cu₂O Thin Films Via Chemical Bath Deposition

Solar energy is regarded as the future primary energy of mankind’s survival anddevelopment because of its unlimited and clean characteristic in all energy. Currently,solar cells are based on the wide band gap semiconductor materials. Although itsenvironmental pollution problems are solved, there are the high preparation cost andlow photoelectric conversion efficiency. For example, the most promising ZnOnanorod array is a high conductivity material and excited by the sunlight. The less than5%of UV light in the sunlight could be absorbed and used, while the43%of visiblelight are not used. Since Cu2O could be excited by the visible light, Cu2O/ZnO filmheterojunction is consisted of Cu2O and ZnO to improve the utilization of solar energy.These preparation methods of Cu2O and ZnO thin film could be divided into thevapor phase and liquid phase method. Compared to the vapor phase method, liquidphase method is remarkable for their cheap experimental setups, large productivity, andmild conditions. In liquid phase methods, the post treatment at high temperature forbinder removal and calcination is required during sol-gel process, and the flat substrateused in electrodeposition process has to be electrically conducting. However, chemicalbath deposition not only has advantages over the above-mentioned techniques, but is aprevalent low-temperature aqueous technique for directly depositing large-area films ofsemiconductors.In this paper, Cu2O thin film, ZnO thin film and Cu2O/ZnO film heterojunctionhave been directly prepared on the glass substrate by a one-step CBD method. After themorphology controls and formation mechanisms of Cu2O and ZnO thin film wereproposed, Cu2O/ZnO compound film has been obtained on ZnO nanorod array.Photocatalytic degradation of methyl orange in the aqueous suspension has beenemployed to evaluate the photocatalytic activity of Cu2O thin film, ZnO thin film andCu2O/ZnO compound film. The results of the study are as follows:1. For Cu2O thin film.(1) The pure Cu2O thin film has been obtained on the glasssubstrate with a mole ratio (CuSO4(0.30M): C6H5Na3O7: C6H12O6) of12:9:8at pH12.0.The bargain glucose and trisodium citrate were respectively as reductant and ascomplexant.(2) The reason of morphology transition from the octahedral micro-particleof monocrystal Cu2O film at0.05M to the sphere micro-particle of polycrystallineCu2O film more than0.10M was crystal growth mode change from thetwo-dimensional growth to the aggregation growth mode.(3) The self-produced gluconic acid and the formation of corrosion channels in the microspheres were the keyto the formation of the core–shell microstructure in the range of pH from12.3to13.2.(4) The improved efficiency of photogenerated electron-hole separation ofmicrospherical Cu2O film contributed to the formation of OH with the strong electronacceptance ability of H2O2. Moreover, the neutrality media was favorable to thephotodegradation of MO solution in the presence of Cu2O film and H2O2and stability ofCu2O film. The rate constant sof MO degradation declined with increasing the initialMO concentration according to the pseudo-first-order kinetics.2. For ZnO thin film.(1) ZnO thin films with the sheet-like, rose-like to flower-likestructures were directly synthesized on glass substrates via adjusting the contents ofTSC and NH4OH.(2) From the solution, it was difficult to fabricate the highly verticalZnO nanorod array.(3) After the glass substrate was dipped in a high concentration ofZn2+solution, ZnO nanorod array with the sheet structure was obtained in the precursorconsisted of Zn(NO3)2and NH4OH.(4) The photocatalytic activity of ZnO nanorodarray with the sheet structure was better than that of the nanorod ZnO thin film due tothe high specific surface area of ZnO nanorod array with the sheet structure.3. For Cu2O/ZnO compound film. When the reductant was the glucose, Cu2Onanoparticles were easy to form the microsphere, leading to the low effective contactarea between ZnO nanorod and Cu2O microsphere. C6H7NaO6was used as the reductant.Although ZnO thin film was partly dissolved, Cu2O nanoparticles were deposited on thesurface of ZnO nanorods to obtain the Cu2O/ZnO compound film. Beside, the improvedefficiency of photogenerated electron-hole separation of ZnO nanorods was the reasonthat the photogenerated hole in the valence band of ZnO was migrated into the valenceband of Cu2O under the UV light.