Photoelectrochemical UV Detectors Based On Nanostructured KNbO₃

As part of the solar spectrum,ultraviolet light has been closely related to human production and life.With the functions of sterilization,health care,detection and anti-counterfeiting,UV light has brought us many conveniences.However,excessive UV exposure can cause damage to cellular tissues and endanger human health.With a wavelength range of 10 to 400 nm,UV light cannot be observed by the human eye.For this reason,UV detection has been of great interest.At present,UV detectors are divided into three types:thermal,photodetective and optoelectronic.The thermal detector is based on the temperature change of the device after absorbing UV light to achieve UV detection,but other wavelengths of electromagnetic waves will also cause changes in temperature,so the wavelength selectivity of such detectors is not high.Photodetection is based on the analysis of data obtained from a single exposure to achieve the detection of UV light,but the UV detector based on the photodetection principle generally has low sensitivity and poor response.Photoelectric UV detectors are mainly based on the internal photoelectric effect to achieve the conversion between optical and electrical signals.In addition,the detection of electrical signals is very convenient,fast and easy to realize digital,and photoelectric detector has the advantages of high sensitivity,rapid response and selectivity.At present,photodetectors have the broadest range of applications and much of the research work revolves around them.As an emerging UV detector structure in recent years,the photochemical UV detectors have received much attention because of their fast response,low manufacturing cost,low environmental impact and self-energy supply.Photoelectrochemical UV detectors generally consist of a photoanode,an electrolyte and a counter electrode.The photoanode is used as the working electrode and it is an important part of the device to achieve light absorption and photoelectric conversion.For UV detectors,the photoanode material is mainly a wide band semiconductor,and the band width of KNb O₃ is 3.2 e V.The wide-band gap makes it possible to avoid the responsibility to the visible region and achieve visible light blind detection.At the same time,KNb O₃ is also a typical chalcogenide structured semiconductor with excellent light absorption,good carrier mobility and long carrier diffusion length,making it an ideal material with UV detection potential.In this work,KNb O₃ nanostructures are used as photoanode material to enhance the UV absorption capability,improve the efficiency of photogenerated carrier separation and the carrier lifetime,so as to enhance the UV detection capability by improving the material microscopic morphology and introducing heterojunctions.The main research work of the thesis is as follows:（1）Cubic KNb O₃ nanoparticles were synthesized under different conditions using hydrothermal methods,which were applied to photoanodes of photo-chemical type UV detectors and tested for UV detection performance.It was found that the concentration of the KOH solution,the hydrothermal time and the hydrothermal temperature affected the microscopic morphology and the crystal quality of the KNb O₃ nanoparticles.In turn,the difference of microscopic morphology and crystal quality will make the UV detection performance of the material change.When the KNb O₃ nanoparticles obtained by hydrothermal treatment at 260°C for 16 h with a KOH solution at a concentration of 6 M,the encapsulated devices had the best UV detection performance with the short-circuit current density of 44μA cm^-2 and the response decline time of 74 and 28 ms,respectively.And the devices also had good input-output characteristics and visible light blindness.For the reason that the excellent crystal quality facilitates the carrier transport in the photoanode.And the smaller particle size allows for better contact between the photoanode and the electrolyte,increasing the number of ion exchange sites and enabling the detector to operate with better photogenerated carrier transport and exchange capabilities.（2）To improve the photoresponse performance of UV detector devices by constructing KNb O₃-based heterojunctions.The Ti O₂/KNb O₃ heterojunctions were formed by coating KNb O₃ nanofilms through the chemical bath deposition method,and the results showed that the Ti O₂/KNb O₃ nanocomposite films with the immersion time of 60 min present the best UV detection capability when the Ti Cl₄ solution concentration is constant.Compared with the pure KNb O₃ and Ti O₂ UV detectors,the photoelectric performance of the Ti O₂/KNb O₃ composite photoanode UV detector was greatly improved:Under the UV illumination at a wavelength of 365 nm and an intensity of 35 m W cm^-2,the short-circuit current density of the device reached 419.7μA cm^-2,which was 9.5 times higher than that of the pure KNb O₃ UV detector(J_sc=44μA cm^-2)and 3 times that of the Ti O₂ UV detector prepared under the same conditions(J_sc=138.6μA cm^-2).Moreover,there are improvements in the photogenerated carrier lifetime,response time and decline time of the devices.Benefiting from the Ti O₂coating,a heterojunction is formed at the interface of the two semiconductors,which generates a built-in electric field that can well promote the separation of photogenerated carriers and suppress the compounding of photogenerated electron-hole pairs,thus improving the detection performance of the UV detector.