Hydrothermal Synthesis And Properties Of Lanthanides-doped BiVO₄ Photocatalysts

In recent years,in order to deal with the increasingly serious energy shortage and environmental pollution,semiconductor photocatalytic technology has been developed rapidly.With its redox reaction,not only can water be decomposed into oxygen and hydrogen,but it can also degrade organic contaminants in water.Due to its wide band gap,conventional titanium dioxide photocatalysts can only utilize ultraviolet light,which accounts for less than 5%of sunlight energy,therefore its application in the field of photocatalysis is greatly limited.Therefore,it is particularly important to develop a novel photocatalytic material with a suitable energy band structure.Bismuth vanadate?BiVO₄?has become a new kind of photocatalyst due to its advantages of environmental friendliness,low cost and suitable band structure,and has attracted widespread attention.However,the compounding rate of electron-holes exceeding 60%,poor surface adsorption and slow electron transfer rate also constrain its application in the field of photocatalysis.Therefore,BiVO₄ is usually modified by morphology control,doping,and compounding methods to improve its photocatalytic activity.The abundant energy levels and the special up-conversion luminescence of lanthanides can effectively cope with the above-mentioned problems of BiVO₄.In recent years,studies related to lanthanides-doped BiVO₄ have also attracted widespread attention.In view of the disadvantages of BiVO₄ and the special physicochemical properties of lanthanides,we have prepared a series of lanthanides-doped BiVO₄ using a simple hydrothermal method.By controlling the doping ratio,hydrothermal time and pH of precursor,BiVO₄ photocatalysts which could utilize a variety of photocatalytic mechanisms were prepared.The influence of lanthanides doping on the crystal phase of BiVO₄ has also been explored.The main work of this article is as follows:1.The Nd-doped BiVO₄ photocatalysts with heterojunction structure were prepared by a hydrothermal method.The effects of hydrothermal reaction time,doping ratio and pH on the structures and properties of BiVO₄ samples were systematically studied.The results showed that the crystal phase of Nd-doped BiVO₄ transformed from tetragonal zircon to monoclinic scheelite with the increase of hydrothermal time,and Nd doping could inhibite this phase transformation.Nd-doped BiVO₄ samples exhibited up-conversion luminescence properties,expanding the light absorption range.The pH of precursor also had significant influence on the crystalline structure and morphology of samples.In the above phase transformation,the two kinds of crystal phases could form a staggered band structure,which could effectively separate the photoelectron-hole pairs and have a significant promotion on photocatalytic performance.In addition to the heterojunction structure,the impurity level introduced by doping and the up-conversion luminescence also promoted the photocatalytic performance of BiVO₄ samples.2.Er-doped BiVO₄ photocatalysts with heterojunction structure were prepared by a hydrothermal method.The effects of hydrothermal reaction time and doping ratio on the structure,morphology and properties of samples were studied.It was found that Er doping could improve the thermodynamic stability of the tetragonal zircon phase of BiVO₄ and inhibit the phase transformation from tetragonal zircon to monoclinic scheelite.In addition,tests had shown that the sample might exhibit a heterostructured core-shell structure.This special morphology structure could more effectively separate the photoelectron-hole pairs,and could increase the migration rate of carrier to sample surface.The photocatalytic performance of the prepared sample was greatly improved.In addition to the mechanism of hoterojunction structure,doping and up-conversion luminescence,the special shell-core structure further enhanced the photocatalytic activity of the Er-doped BiVO₄ sample.3.The Nd,Er co-doped BiVO₄ samples with different crystal phases were prepared by a hydrothermal method.The effects of doping ratios on the structure and properties of the samples were systematically studied.The results showed that Nd and Er were doped into the lattice in the form of substituting Bi sites,which improved the thermodynamic stability of tetragonal zircon BiVO₄,leading to the phase change from monoclinic scheelite to tetragonal zircon phase with the increase of doping ratio.Lanthanides doping also introduced an upconversion luminescence mechanism for BiVO₄,expanding the light absorption range for samples.And lanthanides doping had little influence on the band gap and morphology of BiVO₄.The difference in the band gap and morphology were mainly caused by the change of crystal phase.Compared to the monoclinic scheelite BiVO₄ with narrow band gap,sample with heterojunction structure and tetragonal zircon as the main phase exhibited the strongest photocatalytic activity.Photoluminescence spectra showed that the heterojunction structure could effectively inhibit the recombination of photogenerated carriers.With the help of this multiple mechanism,the photocatalytic activity of tetragonal zircon BiVO₄ was effectively improved.