Preparation Of TiO₂ Nanoparticle Composite And Its Photodetective Characteristics

In recent years,the environmental issues have been paid special attentions.It is necessary to prepare more convenient,sensitive and efficient ultraviolet photodetectors（UV PDs）in order to better monitor pollutant emissions,detect alarm forests and oilfield fires.At present,commercial ultraviolet light detectors mostly use semiconductor materials such as Si and GaAs,but the ultraviolet light selectivity is poor.Therefore,wide bandgap semiconductor materials with excellent ultraviolet light selectivity have become the hotspot of current research.As an important n-type semiconductor material,titanium oxide（TiO₂）has a band gap of 3.0³.2 eV with excellent chemical-physical stability and optical properties.TiO₂ only absorb the UV light which wavelength below³80 nm with excellent UV selectivity,so it is an excellent candidate for visible blind-UV PDs.TiO₂ NPs have the high photoactive area,which is good for absorbing oxygen molecules to produce a large amount of initial photoelectrons.However,the low electron transport rate of TiO₂ and the interparticle gap make the light responsivity low and the photoreactivity rate slow.Therefore,two methods to improve the photodetective characteristics of UV PDs:MgO blocking layer unidirectional transmission of photoelectrons and SnO₂ NR array directional transmission of photoelectrons.In this paper,TiO₂ NPs with branching structure were firstly prepared by hydrothermal method.The effects of the reaction conditions such as hydrothermal reaction time and precursor concentration on the thickness of TiO₂ NPs layer were investigated,and the photodetective characteristics were explored.When 0.2 M TiCl₄ was hydrolyzed for 36 h,the prepared TiO₂ NPs based UV PDs had a rise time of 0.5 s,a recovery time of 0.3 s,a photoreactivity of 10.18 A W^-1 and a photosensitivity of 2449.This is because TiO₂ NPs have a large number of multi-branched structures to provide a large specific surface area,which generates a large amount of initial photoelectrons when irradiated with ultraviolet light,but the lower migration rate of TiO₂ leads to easy recombination of electron-holes,resulting in low light responsiveness and photoresponse rate.In order to improve the photodetective characteristics of TiO₂ NPs based UV PDs,MgO layer was successfully fabricated on the surface of FTO by hydrolysis method and MgO/TiO₂NPs based UV PDs was prepared.The UV photodetector of MgO/TiO₂ NPs showed the excellent responsivity(29.97 A W^-1)and quick response rate（rise and decay time were 0.02 s and 0.05 s,respectively）.In addition,MgO/TiO₂ NPs based on UV photodetector showed significant selectively of UV light in 365 nm,blinded to visible light,and has high detection sensitivity.Compared with the UV PDs based TiO₂ NPs,the photo-response performance of the MgO/TiO₂ NPs composite structure was significantly improved,the photocurrent was increased by 2.75 times,and the photosensitivity was increased by an order of magnitude.SnO₂ nanorods（NRs）array was prepared by hydrothermal on the FTO glass to provide a directional transmission channel for photoelectrons in order to improve the photodetective characteristics of UV PDs based TiO₂ NPs.Heterojunction is formed to accelerate electrons transmission rate.The present SnO₂ NRs/TiO₂ NPs based UV PDs simultaneously exhibited a short response time and a high photoresponsivity,which are less than 0.2 s（rise and decay time）and 21.6 A W^-1 respectively,which was much higher than the commercial values(0.1-0.2 A W^-1).Compared with the UV PDs based TiO₂ NPs,the photocurrent of SnO₂NRs/TiO₂ NPs composite structure was about 2 times,and the response speed was increased by 1 time.