Interfacial Electron Transfer Control And The Photocatalytic Properties Of Heterojunction Photocatalysts

Photocatalysis is considered as a promising technology in pollution control as it could mineralize organic pollutants at a high degree under mild reaction conditions, with only light as the energy source. However, this process always suffers from low efficiency arised from high charge carrier recombination efficiencies and limited light absorption ranges of the photocatalysts. Heterojunction construction is an effective way to address this problem as the available spectra for heterojunctions are expanded, and the separation of electrons and holes is enhanced under the driving of the built-in electric field. Nevertheless, influenced by the contact area and the energy potential barrier, the charge carrier separation efficiency is still limited. Moreover, the reduction abilities of electrons and the oxidation abilities of holes in heterojunctions are weakened than their counterpart single-component photocatalysts. Therefore, in this work, we tried the improvement of the interfacial contact area, introduction of narrow band gap semiconductors, selection of the contact facet, and introduction of the proper electron transfer mediator to further improve the photocatalytic performance of heterojunctions. Detailed work and results are as follows:（1） Gr wrapped ZnIn2S4 （ZnIn2S4@Gr） heterojunction was prepared by electrostatic self-assembly method. Morphology of the photocatalyst was observed using an scanning electron microscope, and result indicated the Gr layer wrapped the ZnIn2S4 microspheres entirely, which ensure their high contact area than that of ZnIn2S4-Gr. The photocatalytic performance of ZnIn2S4@Gr was evaluated by phenol degradation. As a result, the kinetic constant of phenol degradation on ZnIn2S4@Gr was 3.03 h-1, which was 8.4-fold and 1.5-fold higher than those on ZnIn2S4 and ZnIn2S4-Gr, respectively. Mechanism analysis indicated·OH generated from photogenerated electrons was the main active species for phenol degradation.（2） CuPc NW@TiO₂ heterojunction was prepared by electrophoretic deposition of CuPc nanowire array followed by chemical vapor deposition of TiO₂. The light absorption property and the charge carrier separation performance were investigated by ultraviolet-visible diffuse reflectance spectrum （DRS） and photocurrent test, respectively. Results showed that CuPc NW@TiO₂ could utilize light with λ< 800 nm; and the photocurrent of CuPc NW@TiO₂ under visible light irradiation （-15μA-cm-2 at-0.8 V （vs SCE）） is 2.1 times higher than that of CuPc nanowires （-7 μA·cm-2）. The photocatalytic performance of CuPc NW@TiO₂ was evaluated by photoelectrocatalytic degradation of 4-dichlorophenol. Result showed the kinetic constant of 2,4-dichlorophenol degradation on CuPc NW@TiO₂ heterojunction was 2.28 h-1, which was 1.4-fold higher than that of CuPc nanowire array （1.58 h-1）.（3） By controlling the seeding and growth processes of TiO₂ nanoparticles, BiVO4-110-TiO₂ heterojunction （TiO₂ grown on the{110} facet of BiVO4） and BiVO4-010-TiO₂ heteroj unction （TiO₂ grown on the{010} facet of BiVO4） were obtained successfully. Photocurrent test was conducted to study the charge carrier separation property, result of which indicated BiVO4-110-TiO₂ possessed better charge separation performance than that of BiVO4-010-TiO₂. Photocatalytic degradation of RhB and 4-nonylphenol on the heteroj unctions were performed to evaluate their photocatalytic capabilities. As a result, the degradation rates of RhB and 4-nonylphenol on the former were 1.19 h-1 and 3.07 h-1, respectively, which was about 1.3 times higher than those on BiVO4-010-TiO₂. Moreover, the photoreduction site as well as the charge carrier lifetime of the heterojunctions were tested, and results proved the superior performance of BiVO4-110-TiO₂ was attributed to the more fluent electron transfer to TiO₂ from the{110} facet of BiVO4 than from the{010} facet of BiVO4.（4） With Ag, Cu, and Au as the electron mediators between WO3 and gC3N4, the WO3-metal-gC3N4 heterojunctions were prepared. The OH production test and the photocurrent test were conducted to verify the charge carrier transfer mechanism and charge carrier separation property, respectively. Results indicated only Cu, with proper Fermi level, was a favorable charge carrier mediator to promote the electron transfer between WO3 and gC3N4 via the Z-scheme mechanism, and the photocurrent of WO3-Cu-gC3N4 （11.5 μA·cm-2） was 3.5 times and 2.3 times higher than those of WO3 （3.3 μA·cm-2） and WO3-gC3N4 （5.0 μA·cm-2）, respectively. Result of the photocatalytic test indicated the degradation rate of 4-nonylphenol on WO3-Cu-gC3N4 was 0.78 h-1,11.6 times higher than that on WO3-gC3N4.（5） BiVO4-BiFeO3-CuInS₂ heteroj unction was prepared by spin-coating method. Effects of direction and thickness of the polarization field on electron transfer were studied. Result indicated the electron transfer at the interface coincided with Z-scheme mechanism when the direction of polarization field was from CuInS₂ to BiVO4. And the optimal thickness of BiFeO3 layer was 70 nm. The photocatalytic performance of BiVO4-BiFeO3-CuInS₂ was evaluated by degradation of p-nitrophenol and 2,4-dichlorophenol. The results indicated the photocatalytic degradation rate of p-nitrophenol on BiVO4-BiFeO3-CuInS₂ （1.19 h-1） was 2.8 and 16.9 times higher than those on CuInS₂ and BiVO4-CuInS₂, respectively; and the degradation rate of 2,4-dichlorophenol on the former was 1.6 and 3.4 times of the latter, respectively.