Preparation And Photoelectric Properties Study Of Nanocrystalline TiO₂ Thin Film

The properties of nanomaterials are associated with the nanostructures, which will be greatly influenced by the technology and method of synthesizing nanomaterials. At present, the development tendency of nanomaterials science and technology focuses on the study of controllable process, including the shape, dimension, surface and microstructure of materials, so that their unique properties could find their corresponding applications in the industry.In the present work, nano-structured titanium dioxide (TiO₂) thin films have been prepared on metal substrates using a layer-by-layer dip-coating method. The metal substrates include Ti-sputtered nickel plate and bare nickel plate. The phase structure, elemental composition and morphologies of preparing samples were characterized by means of X-ray powder diffraction (XRD), Energy dispersive X-ray (EDX) spectrometry and field-emission scanning electron microscopy (FESEM).Precursor solution for TiO₂ thin films were prepared by hydrolysis of tetrabutyl titanate. The chemical composition of Ti(OC₄H₉)₄ : CH₃COCH₂COCH₃ : H₂O in the alkoxide solution was 1 : 0.3 : 1 in molar ratio. The TiO₂ colloidal solution was stirred vigorously and ultrasonicated for 10 min prior to dip-coating to achieve a consistent homogeneous mixture. the metal substrate were dip-coated in the TiO₂ colloidal solution with a withdrawing speed of 3.0 cm/min. After deposition, each layer was dried in a muffle furnace at 300 oC for 20 min in air to remove organics. The corresponding XRD patterns of the as-coated on Ti-sputtered Ni and heat treated thin films shows when the TiO₂ thin films were coated on the Ti-sputtered Ni plate and annealed at 400℃in air for 2 h, in addition to the diffraction peaks coming from the Ni and TiO₂, another one strong diffraction peak is observed at 2theta values of 34.75°,which can be indexed as (0 0 4) diffraction peak of the hexagonal phase Ti. However, when heated up to 450℃for 2 h without any additional pretreatment, the diffraction peak of Ti disappear. The intensity of the Ni diffraction peaks dramatically decreases when the sintered temperature increases from 400℃to 500℃. The FESEM micrographs of samples formed on the Ti-sputtered (～500nm) nickel foil at different annealing temperatures for 2 h shows at the lowest temperature of 400℃, a nanoparticulate thin film forms on the metal substrate. The diameter of these particles is around 100 nm, the surface morphology of the as-deposited gel thin film by dip-coated method, which reveals that relatively dense and smooth TiO₂ thin film electrode. Increasing the temperature to 450℃, the particle size further increases to about 150 nm and a relatively less porous surface with grain-cluster is found. Further increasing the sintered temperature to 500℃, we could see the partical film disappearance, the surface morphology of nanocrystalline titanium dioxide thin film has already changed to a porous structure.The typical dark I-V response for series of TiO₂ deposited on Ti-sputtered (～500nm) nickel foils electrodes at different annealing temperatures shows an asymmetric Schottky barrier response is observed. It is noted that The TiO₂ thin film electrode prepared with the annealing temperature of 450℃and 400℃show good rectification behaviour, which is need for functioning as a photovoltaic device. While, the TiO₂ electrode prepared at 450℃shows a better rectification than that of 400℃under dark conditions. However, further increasing the sintered temperature to 500℃, the TiO₂ thin film electrode shows quasisymmetric rather than diode-like behaviour. However, the dark current versus voltage curves for the thin films prepared on the different metal substrate shows that without Ti coated on the Ni plate does not have rectification behaviour, the result indicates that the Schottky junction does not form between TiO₂ thin film and nickel substrate.The photocurrent density versus applied potential (versus Ag/AgCl) plots of TiO₂/Ti/Ni samples prepared at different annealing temperatures shows that under the illumination of simulated solar light, a photocurrent density of 1.42 mA/cm² at 0 V is recorded for the sample annealed at 400℃. Increasing the temperature to 450℃the photocurrent density is 1.75 mA/cm² at 0 V. The sample annealed at 500℃shows the lowest photocurrent density among all the samples investigated. To investigate the effect of metal substrate on photovoltaic properties, the TiO₂ thin film electrodes were fabricated on Ti-sputtered nickel foil and nickel substrate at same sintered tempertature. The photocurrent-potential characteristics of the TiO₂/Ti/Ni and TiO₂/Ni samples shows that the photocurrent density can be dramatically enhanced by using Ti-sputtered nickel foil as the substrate, comparing with bare nickel foil. The photocurrent produced by using Ti-sputtered nickel foil as the substrate reachs 1.75 mA/cm², which is more than 10 times of that by using bare nickel foil as the substrate.