| Environmental pollution and greenhouse effect have directly or indirectly caused increasing casualties and economic losses every year.In the past two years,there have been frequent occurrences of extreme weather,and meteorological disasters are showing signs of global havoc.Some researchers try to develop a variety of ways to alleviate or systematacially curb environmental pollution,and strive to clean up the earth’s problems;some researchers are working on clean energy sources to try to cool the planet.As a kind of important functional materials,semiconductor plays an important role in all aspects of modern industry.In the 1950s,Bell LABS in the United States accidentally discovered that doped Si material had better photosensitivity,and then prepared the world’s first solar cell,opening a new era of human use of solar energy.Subsequently,Fujishima and Honda found that TiO2 single crystal can decompose water as a photoanode,and semiconductor as a photocatalyst is considered as a green strategy to deal with environmental pollution.Therefore,researchers focus a lot of energy on the development of semiconductor materials with efficient photoelectric conversion characteristics.Among them,oxide semiconductor has great advantages,such as good weather resistance,excellent photocorrosion resistance,non-toxic,low cost and etc.,which has always attracted the attention of many researchers.However,classical oxide semiconductor,such as,ZnO,TiO2 and etc.,are wide band gap semiconductor,which can only absorb part of violet light or ultraviolet light.However,the energy distribution of sunlight is about 7%in the ultraviolet region,50%in the visible region,and 43%in the infrared region,which is one of the main reasons for the low utilization of sunlight in wide band gap semiconductors.The main factors to affect photoelectric conversion are as follows:(1)the absorption capacity of sunlight,(2)the separation efficiency of photo-generated electron and hole pair,(3)the migration rate of photo-generated carriers in bulk phase,(4)surface,and the recombination rate of photo-generated carriers.To solve the above problem,researchers develop a variety of methods,for example,doped semiconductor,photosensitization and other means to expand the scope of light absorption,and deposition of the noble metal nanoparticles on the surface of semiconductors to improve the separation efficiency of electronic-hole pair.Moreover,building the p-n junction or preparing the ferroelectric composite semiconductor can accelerate photo-generated carrier migration in partical bulk or on the surface.However,due to the limitations of energy band engineering and the problems of energy band matching interface regulation of composite photocatalyst,it is necessary to develop novel single-phase high efficiency photoelectric conversion materials.In order to solve the above problems,we developed a novel narrow-band gap oxide ferroelectric semiconductor(β-CuGaO2),which has an excellent light absorption band gap and a large spontaneous polarization value,that is,β-CuGaO2 has the potential to obtain large open circuit voltage and short circuit current at the same time.These results indicate that β-CuGaO2 is a kind of high efficiency single-phase photoelectric conversion material.Therefore,this paper is mainly divided into the following parts:(1)In order to develop clean energy,the ferroelectric photovoltaic properties ofβ-CuGaO2 were studied with solar cell.Firstly,β-CuGaO2 nanocrystals were sintered into sheets by SPS technique.Secondly,the sample piece was polished and thinned to less than 0.1 mm.Then,according to the band structure,appropriate electron transport layer or hole transport layer materials are selected,such as SnO2,ZnO,etc.Furthermore,the upper back electrode(Ag)and the upper top electrode(ITO)were respectively steamed and sputtered.However,the photovoltaic device assembled by the sample sheet has no obvious photovoltage,indicating that the device is probably in a state of complete short-circuit.Through AFM analysis,we know that the sample surface is rough and there are some gullies,indicating that the density of the sample is not enough.Therefore,we believe that the low density of the sample may lead to the formation of a connected network of electrode components through the gully,resulting in a short circuit of the device.Because β-CuGaO2 is a typical metastable material,the phase transition temperature in Ar gas is only 510℃,which is not suitable for the preparation of compact sheets by high temperature sintering.In the final analysis,the sintering of the sample density sheet depends on the electrostatic attraction between the nanoparticles,so as to increase the grain size,and the magnetic analysis shows that β-CuGaO2 has a strong diamagnetization,which may be the reason for the low density of β-CuGaO2 using SPS.(2)To treat environmental sewage as final goal and degrade organic pollutant as the method,the photoelectric conversion efficiency of β-CuGaO2 was comparatively studied through the degradation reaction in dark state and light,which explored the advantage of ferroelectric material with narrow band gap semiconductor and a large polarization in the photoelectric conversion advantage.Firstly,the effect of ferroelectric domain orientation on degradation reaction was investigated by pre-polarization.Secondly,the effect of photo-electric coupling on degradation reaction was investigated by combining pre-polarization and light irrradiation.Then,based on photoelectric coupling,another ultrasonic effect(mechanical force)was introduced to explore the influence of mechanical-photo-electrical coupling on degradation reaction.The results show that adjusting the orientation of ferroelectric domain(pre-polarization)has a significant positive effect on degradation reaction.Moreover,adjusting the ferroelectric domain orientation and introducing light can accelerate the degradation rate more obviously than light alone.Ultrasonic vibration(mechanical force)is added to photo-electric coupling.The mechanical-photoelectric coupling still shows the characteristics of highly efficient degradation material for high concentration of organic pollution,demonstrating the potential ofβ-CuGaO2 as all-weather degradation catalyst,providing a more environmentally friendly choice for the development of river pollutant treatment under natural conditions.(3)Considering that β-CuGaO2 is a ternary derivative of ZnO,β-CuGa1-xZnxO2 functional ceramics were prepared by means of unequal doping of Zn2+ at Ga-site.The effects of Zn2+doping at Ga-site on the thermal stability and photoelectric properties of β-CuGa1-xZnxO2 were studied.The results of thermal stability show that the phase transition temperature of β-CuGa1-xZnxO2 decreases with low concentration doping,while the phase transition temperature of β-CuGa1-xZnxO2 rises slightly with high concentration doping,which provides a direction for improving the thermal stability of β-CuGaO2 series functional ceramics.In addition,due to the difference of atomic radius,the unequal doping of Zn2+in Ga3+reduces the lattice distortion.The doping of Zn2+ has a significant adverse effect on spontaneous polarization and optical absorption,and the deep energy level of Zn2+is not enough to compensate for this adverse effect.Therefore,β-CuGa1-xZnxO2 is not conducive to the occurrence of photoelectric reaction.The above statements are supported by first-principles studies and organic degradation experiments.(4)Considering the thermal stability research in(3),the effect of ZnO on the thermal stability and photoelectric properties of β-CuGaO2-xZnO functional ceramics was studied by preparing β-CuGaO2-xZnO solid solution.The thermal stability analysis shows that the phase transition temperature of β-CuGaO2-xZnO solid solution increases gradually with the increase of x,and reaches the highest value when x=0.5.While x>0.5,the phase transition temperature gradually decreases.It is possible that β-CuGaO2 is Pna21 space group of orthonormal system,wurtzite ZnO is P63mc space group of hexagonal system,and α-CuGaO2 is P63mmc space group of hexagonal system.These results indicate that when the content of ZnO reaches a certain level,solutes in the solid solution of ZnO become solvents,which is more conducive to promote the evolution of the solute β-CuGaO2 in the orthogonal-phase to α-CuGaO2 in the hexagonal phase,to reduce the lattice mismatch of the whole material and reduce the energy of the system.In addition,the photo-degradation of β-CuGaO2-xZnO exhibits a similar trend to the phase transition temperature,increasing first and then decreasing,reaching the peak at x=0.5,but the degradation rate is obviously higher than that of the bulk phase β-CuGaO2,which is the result of the internal polarization electric field and photo-induced semiconductor junction effect. |
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