Synthesis And Modification Of TiO₂ Nanosheets Array Film And Study On Photoelectrochemical Property

The energy shortage and the pollution caused by traditional fossil energy are urging us to seek new energy. Solar energy is a green and renewable energy. Research about the solar energy is expected to solve the problems facing the human society. The utilization of solar energy is research hotspot in the energy field recently. Photoelectrochemical technique is a kind of ideal clean energy technology and environmental pollution control technology driven by the solar energy. Functional nanomaterials with the excellent photoelectric properties play an important role in the photoelectrochemical techniques. Among them, titanium dioxide(Ti O2) is a kind of ideal photoelectric functional material, which has excellent carrier transport properties, high stability and friendly to environment. It can be widely used in the fields of photoelectrochemical solar cell, photocatalysis, and optoelectronic devices. Anatase phase Ti O2 exposed the(001) facets has attracted extensive attention due to its high photoreactivity. However, the intrinsic narrow light response range of Ti O2 and the low quantum yield limit the photochemicalactivity of(001) facets dominant Ti O2 and its application. It is necessary to modify the Ti O2 exposed(001) facets for expanding its practical application. Herein,(001) facets dominant Ti O2 nanosheets array film was prepared by hydrothermal method on transparent conduction glass. We proposed a possible growth mechanism by studying on the morphology evolution under different reaction conditions. The Ti O2 nanosheets array film was sensitized by quantum dots or modified by narrow band gap semiconductor compound to optimize the photoelectrochemical system. It was aimed at exploring light response properties and photoelectrochemical properties of photoelectrode with high activity(001) facets dominant Ti O2 nanosheets. The main contents of this paper are as follows:(1) With butyl titanate as titanium source, dilute hydrochloric acid as hydrolysis controlling agent and ammonium titanate as crystal controlling agent, orderly Ti O2 array films were prepared on transparent conductive fluorine-doped tin oxide glass substrates by the hydrothermal method. The morphology, crystal structure and optical properties of Ti O2 arrays films were characterized. The Ti O2 array film has a fishing net shape and composed of the(001) facets dominant Ti O2 nanosheets. The proportion of the exposed(001) facets is about 85% by the calculation. The photoelectrochemical property of the(001) facets dominant Ti O2 nanosheets array film at 0V vs Ag/Ag Cl electrode was studied by using three electrode systems under simulated sunlight irradiation. The photocurrent density and photoconversion efficiency of the Ti O2 nanosheets array film are 0.48 m A cm-2 and 0.2%. In addition, the effects of reaction time, reaction temperature, morphology controlling agent concentration and titanium source concentration on the morphology of Ti O2 films were also investigated. The possible growth mechanism of(001) facets dominant Ti O2 nanosheets was proposed.(2) Successive ionic layer adsorption and reaction(SILAR) method was used to load Cd S quantum dots in situ on the surface of the Ti O2 nanosheets array films. The morphology, interfacial structure and optical absorption properties of the composite films were studied. When the number of deposition times for Cd S quantum dots is 7, the photoelectrochemical performance of the composite film system is optimal with the photocurrent density of 3.24 m A cm-2 and photoconversion efficiency of 1.59%. If continue to increase the deposition times of Cd S quantum dots, it is found that photocurrent density has a decreasing tendency. It is attributed that agglomeration happened between Cd S nanoparticles and they become the carrier recombination centers. The Pb S quantum dots was deposited on the Ti O2 nanosheets array film after 7 times Cd S quantum dots loading by SILAR. The Ti O2 nanosheets film was co-modified by Cd S and Pb S quantum dots. When the deposition of Pb S quantum dots is 5 times, the optimal photocurrent density and photoconversion efficiency of the composite film are 6.12 m A cm-2 and 2.68%, respectively. It indicated that the quantum dots co-sensitization is significant for improving photoelectrochemical performance of Ti O2 nanosheets array film.(3) Hierarchical rutile/anatase Ti O2 heterojunction films were prepared by two step hydrothermal method. Rutile phase Ti O2 was situate on the upper layer to increase specific surface area for loading more quantum dots. The bottom layer was anatase phase Ti O2 nanosheets array film, providing a direct transport channel for photogenerated carrier. Heterojunction formed between the rutile phase and anatase phase promoted the separation of photogenerated charge carriers. The crystal structure, morphology and interfacial structure of rutile/anatase film prepared by different hydrothermal conditions were characterized. The optical and photoelectrochemical properties were also investigated. The optimal photocurrent density of the rutile/anatase film was 0.90 m A cm-2. 7 times Cd S and 5 times Pb S quantum dots were loaded on the surface of rutile/anatase heterojunction film by the SILAR method. The photocurrent density of the quantum dots co-sensitized heterojunction film was increased to 7.72 m A cm-2 and photoconversion efficiency was 3.67%。(4) Narrow band gap semiconductor α-Fe2O3 was prepared on Ti O2 nanosheets array film by a simple chemical bath deposition and thermal burning method. Ferric nitrate in aqueous solution was used as precursor of chemical bath deposition for preparing the α-Fe2O3/Ti O2 heterojunction films. The heterojunction films under different chemical bath time were obtained. The morphology and crystal structure were characterized. With chemical bath deposition time increasing, the morphology of α-Fe2O3 changed correspondingly. The optical and photoelectrochemical properties were also investigated. When the chemical bath deposition time was 90 min, the optimal photocurrent density is 2.51 m A cm-2 and the photoconversion efficiency is 1.25%, which is six times higher than that of pure Ti O2 nanosheets array film photoanode. It is attributted that α-Fe2O3 semiconductor greatly broadened the light response range of the modificated film. Under visible light irradiation, α-Fe2O3 as the absorbing layer is excited to generate carriers. The photogenerated carriers can separate fast under the effect of electric field at the heterojunction formed between α-Fe2O3 and Ti O2, resulting in improvement of the photoelectrochemical performance of the α-Fe2O3/Ti O2 heterojunction film.