Preparation Of TiO₂ Hierarchical Nanoarrays And Their Applications In Photoelectrochemical UV Detector

Ultraviolet（UV） light plays an important role in industrial production and our daily life. At the same time, UV light can cause harm to human body. So it has important significance to detect the UV light. In recent years, UV sensors have drawn extensive attentions due to their potential application in civil and military fields, such as chemical/biological analysis, environmental monitoring, flame detection, astronomical studies, remote control and memory storage. The long response time, surroundings-dependent behavior and power supply requirement of conventional photoconductive UV sensors limit their application. Additionally, the fabrication of the traditional UV sensor involves many technical problems, such as semiconductor doping, p-n junction, ohmic junction and schottky junction. These processes lead to the complex manufacturing process, high material requirement and high production cost. And these problems have seriously restricted the development of the traditional UV sensors, and it can only be used in some special industries. Photoelectrochemical（PEC） UV sensor is a new type of liquid junction photovoltaic UV sensing device. It has attracted much attention of researchers due to its simple production process, low production costs and considerable detection performance. Generally, high performance PEC UV sensors should have a high UV light utilization, active and continuous photocarrier generation and efficient electron transport. At present, the study of PEC UV sensor is still in the initial stage. The working mechanism is not completely clear, the sensing performance of PEC UV sensors remains to be improved, including responsivity, response spectrum, response time and materials synthesis. In view of the above problems, this paper has carried on the following aspects of research based on TiO₂ materials.TiO₂ is one of the most potential materials in the PEC UV sensors. Nano arrays composed of one dimensional TiO₂ nanostructures can provide excellent carrier transport properties. With using the proper precursors and solutions, we have successfully synthesized anatase and rutile TiO₂ nanoarray structures by hydrothermal methods, respectively. Through the exploration and analysis of various experimental conditions, we have realized the controllable growth of the nanoarrays with a variety of crystal forms and morphologies. Firstly, hierarchical chain-like TiO₂ nanoarrays are successfully synthesized on FTO glass by a facile one-pot hydrothermal method. The novel nanostructure arrays are composed of nanowire trunks and epitaxial nanobranches, and covered by interconnected nanosheets. The responsivity of PEC UV sensor prepared by the simple and facile method has achieved considerable performance. The responsivity and response time is 0.11 A W-1 and 20-40 ms respectively. Secondly, we have successfully synthesized high-quality monocrystalline rutile TiO₂ nanowire arrays. The TiO₂ nanowire array has a high aspect ratio of 150, high surface density of 90-110/μm2 and good dispersion property.To further improve the UV absorption and specific surface area of photoanodes, we have demonstrated a hierarchical dendriform nanostructure based on monocrystalline rutile TiO₂ nanowire array. Dense nanowires and nanobranches form a compact interlaced film, which can provide good light scattering and large specific surface area. The short-circuit current density value of the dendriform TiO₂ nanowire array based UV sensor is 176% and 46% higher than that of the bare TiO₂ NW film and TiO₂ nanocrystalline film. The incident power conversion efficiency can reach 14.5%. Moreover, the UV sensor exhibits a high on/off ratio of 1903.One-dimensional nanostructures which have a high aspect ratio and efficient electron transport networks provide a promising scheme to fabricate fast-response and super-sensitive PEC UV sensors. In this part, we demonstrated a UV sensor based on vertically aligned TiO₂ nanowire arrays which have been interconnected with Zn O nanosheets. Three-dimensional TiO₂/Zn O hierarchical nanostructures are composed of high-aspect-ratio TiO₂ nanwire skeletons and external Zn O nanosheet conducting channels. Benefited from the fast transport paths of photoelectrons provided by the Zn O nanosheets, the PEC cell exhibits a super sensitivity and a fast responsivity. The rising and decay time of the sensor are reduced from 15.5, 32.5 and 8.0 ms to 5.5, 13.5 and 4.0 ms.The conventional sandwich-like device structure use FTO glass as substrate. The filtration effect of FTO on UV light severely limits the performance of the device. We prepare the polymer nanowires hollow grid via electrospinning technology. Then the magnetron sputtering is taken out to fabricate the Ag nanowire grid using polymer grid as a tamplate. Finally, a UV transparent conductive substrate is obtained by transfer the Ag nanowire grid to the quartz glass. The detection spectrum of the PEC UV sensors is successfully broadened by using the Ag nanowire based UV transparent conductive substrate.