The Role Of Tungstate Crystal Phase Inhibitors In The Hydrothermal Synthesis Of M-BiVO₄ With Highly Photocatalytic Activity

The monoclinic BiVO₄?m-BiVO₄?is one of the most attractive water oxidation photocatalytic semiconductors because it has the advantages of a narrow band?2.4 e V?,non-toxic,low price,stable chemical and light properties.Further improving its activity is the key to the application.Under the hydrothermal conditions of p H = 2,180 °C and 24 h,BiVO₄ was synthesized by hydrothermal method using Bi?NO3?3·5H2O and NH4VO3 as raw materials,Na₂WO₄ and H₃PW₁₂O₄₀ were employed as crystal phase transfer inhibitors,constructing high-activity m-BiVO₄ photocatalyst with homogeneous structures and expose high-energy active crystal planes.The main content is as follows:Adding a small amount of Na₂WO₄ during the synthesis process will not change the crystal phase and introduce foreign ions into the crystal lattice,but it can form a disordered layer with high oxygen holes coated on the surface of m-BiVO₄ photocatalyst.The photocatalytic O2-evolution activity of as-prepared m-BiVO₄ samples has been investigated in 200 m L of aqueous Ag NO3 solution?0.05 M?under visible light irradiation?? ? 420 nm?.The average rate of the m-BiVO₄ sample obtained by adding 500 ?mol L-1 Na₂WO₄ solution is 722.8 mmol h-1 g-1 and the average rate is 21.2 times that of m-BiVO₄-0 sample.The electrochemical water-oxidation performance of m-BiVO₄ samples show that the sample behaves with the current density of 12.8 ?A cm-2,which is 8.8 times as high as that of m-BiVO₄-0 sample.The m-BiVO₄-5000 sample is attributed to a synergetic effect of homojuctions between the disordered amorphous layers on the surface and the coated monoclinic crystal particles as well as oxygen vacancies.As a comparison,increasing the hydrothermal temperature to 240 ? resulted in a monoclinic m-BiVO₄-5000 sample.W ions doped into the lattice sites of the sample,but m-BiVO₄-5000 sample did not show a high photocatalytic activity,which 1.8 times of the m-BiVO₄-0 sample.Employing as H₃PW₁₂O₄₀ as a crystal phase inhibitor,W-doped BiVO₄ samples with exposed?111?facets were synthesized.It is found that the tetragonal phase content of the generated BiVO₄ samples increased with the amount of H₃PW₁₂O₄₀ increasing.It is indicated that H₃PW₁₂O₄₀ have an inhibitory effect on the transformation of BiVO₄ from the tetragonal phase to the monoclinic phase.At the same time,it was found that the morphology of the monoclinic BiVO₄ samples were changed,the samples showed the?111?crystal face,and the I?010?/ I?110?crystal face were reduced.For the samples,the photocatalytic oxygen evolution and the electrochemical water-oxidation performances were studied.It was found that the introduction of H₃PW₁₂O₄₀ in the hydrothermal synthesis can significantly increase the photocatalytic activity of monoclinic phase BiVO₄,which is related to the formation of the?111?crystal face.It is indicated that the?111?crystal face in the monoclinic phase is also the active centers of O2 generation.