Synthesis Of π-Conjugated Materials-BiVO₄ Composites And Their Photocatalytic Properties

In photocatalytic degradation process,photocarriers can be easily combined and lose effective concentrations and activities,resulting in problems such as incomplete degradation and low efficiency.Therefore,in this study,on basis of crystal growth controlling and ions doping,the crystal phase,morphology,crystal size,surface defects and active crystal surface of BiVO₄ were improved to favor the interfacial activity.The π-conjugated g-C₃N₄ and GO were used with BiVO₄ to construct heterojunctions,promoting the carriers’ generation-separation efficiencies,playing the important conductivity and adsorption together,leading to wholly enhanced target pollutant removal efficiencies.The results are shown as follows:（1）The effects of pH and Gd-doping on the formation and transformation of BiVO₄ and the effect of BiVO₄ crystal morphology on the photocatalytic activity were studied.In the strong alkaline environment,the V ions complex will change,inducing VO₄ tetrahedron and BiO6 octahedral distortions and forming fibrous Bi₁₇V₃O₃₃/Bi_1.7V₈O₁₆.With Gd3+ addition,the fiber surfaces will dissolve and recrystallize,with the near-surface Bi3+ being substituted by Gd3+ and the Bi_1.7V₈O₁₆ being transformed into Bi₁₇V₃O₃₃.In the near neutral condition,the fibrous Bi₁₇V₃O₃₃/Bi_1.7V₈O₁₆ has significant UV absorption,and the photocatalytic efficiency is increased to three times as compared with that before modification.（2）The growth mechanism and mineralization efficiency of g-C₃N₄/tz-BiVO₄ heterojunctions were studied.Gd3+ can induce the transformation from ms-BiVO₄ into tz-BiVO₄,while the steric effect of g-C₃N₄ slows down the ions transport and reduces the tz-BiVO₄ crystal size.Through continuous polycondensation,g-C₃N₄ can coat on the tz-BiVO₄ surfaces,promoting high-efficient interfacial charge transition.The electrons at the conduction band of g-C₃N₄ can be injected into the conduction band of tz-BiVO₄ to react with OH-to produce the ·OH groups.The holes at the valence band of tz-BiVO₄ can be transferred to the valence band of g-C₃N₄,reacting with dissolved oxygen to forming O2·-groups.After degradation under UV-light for 20 min,the degradation rate is 94% and the mineralization rate is 75%.After cycling tests for 5 times,the degradation rate decreased by only 7%,showing stable using potential.（3）The outstanding adsorption property of GO/tz-BiVO₄ composite was studied.By extending low temperature oxidation time,ultrasonic and acid-Base treatments,the oxidation degree of GO prepared by a Hummers method is increased,with its sheet size reduced.Through adopting the ultrasonic stirring method,the tz-BiVO₄ in GO/tz-BiVO₄ composite is well-crystallized.The GO layers uniformly distributed on the surface of tz-BiVO₄ can help the tz-BiVO₄’ surface electrons’ export,instead of combining and lose activities,and can absorb a large amount of RhB molecules,providing active sites.Finally,the GO/tz-BiVO₄ show high performances of the adsorption properties with strong stability.（4）According to the above study of g-C₃N₄/tz-BiVO₄ and GO/tz-BiVO₄,by pre-preparing（010）crystal plane exposed ms-BiVO₄,the g-C₃N₄/GO/BiVO₄ photocatalyst was synthesized after simple ultrasonic stirring treatments.Among the solution system,GO plays a solubilizing effect on g-C₃N₄,while g-C₃N₄ has the steric effect to decrease the grain size of（010）BiVO₄.The GO and g-C₃N₄ can form p-n heterojunction and BiVO₄ and GO can form n-n heterojunction.Therefore,the electrons are transfered to the surfaces of BiVO₄ and holes are transfered to the surfaces of g-C₃N₄.Thus,the adsorption properties and conductivities of the samples are both sufficient.After dark adsorption for 30 min,RhB adsorption rate reaches 50%,and after another 30 min visible light irradiation,the degradation rate is more than 90%.