The Preparation And Visible-Light Photocatalytic Properties Of TiO₂/Porous Bulk Narrow Gap Semiconductor Composites

Semiconductor photocatalysis has attracted a wide attention for its outstanding performance on environmental purification and energy production. TiO2 is considered as one of the best photocatalysts due to its non-toxic, excellent chemical stability, suitable band location and low price. However, the photocatalytic activity of TiO2 is limited by its poor optical response and low separation efficiency of photo-generated carriers. To extend the wavelength response range and improve the separation of photon-generated carriers, thereby increasing the quantum yield of TiO2, has always been a hot topic in this area.In this paper, we report simple wet-chemical methods to fabricate porous InVO4 microspheres and porous bulk g-C3N4 with large size as well as the in situ growth of TiO2 nanoparticles on their surface. Coupling TiO2 with other narrow gap semiconductors of matching band potentials can not only extend the absorption wavelength range to visible region, but also inhibit the recombination of photo-generated electron-hole pairs. On this basis, the further introduction of porous structure in the matrix material can develop the composite material with a series of special physicochemical properties, such as high specific surface area, more surface reactive sites and excellent light absorption characteristics. Moreover, the as-prepared porous composites with large size combined the advantages of easy separation property and high specific surface area, which are all quite important for practical applications. The main research contents and results are as follows:(1) Preparation and characterization of porous InVO4 microspheresOrthorhombic InVO4 nanocrystals were prepared via a combination of coprecipitation and hydrothermal method. The effects of pH value and V/In molar ratio on the phase of InVO4 were investigated. Porous InVO4 microspheres were then successfully synthesized by a hydrothermal method in the presence of cetyltrimethyl-ammonium bromide (CTAB). Moreover, various techniques were adopted to analyse the influence of CTAB content on structure, mophology and properties of InVO4 microspheres, and the formation mechanism was also discussed. The results show that the pure orthorhombic phase of InVO4 can be obtained when the pH value and the V/In molar ratio are 4 and 2, respectively. The porous microspheres with diameter size about 2-5 μm are assembled by numerous InVO4 nanocrystals. Oswald ripening and self-assembly aggregation are presumed to play a key role in the formation of these porous InV04 microspheres. The cationic CTAB can bind the InVO4 nanoparticles together through electrostatic interaction and promote the process of self-assembly aggregation. As the CTAB content increases, the band gap of InVO4 samples decrease first and then increase, while the separation efficiency of photo-generated carriers is improved significantly. Generally, when the concentration of CTAB solution is 1 wt%, the porous InV04 microspheres exhibit best photocatalytic activity. The degradation ratio of Rhodamine B after visible-light irradiation for 10 h is 77%, and the corresponding degradation rate k is 0.156 h-1, which is about 2.14 times as high as that of the InV04 sample prepared without CTAB.(2) Preparation and characterization of Ti02/porous InVO4 microspheres compositesPorous InVO4 microspheres with TiO2 nanoparticles uniformly loaded on the surface were successfully synthesized via a combination of sol-gel and hydrothermal method. Various techniques were adopted to analyse the influence of Ti/In molar ratio on structure, mophology and properties of as-prepared samples, and the reasons why the composites achieved an enhanced photocatalytic activity were also explained. The results show that when introducing tetrabutyl titanate (TBT) as the precursor of TiO2, TBT may be firstly adsorbed on the InVO4 porous microspheres via capillary action, thereby inducing the in-situ formation of nuclei on the InVO4 porous microspheres, and subsequently growing into nanocrystals during the hydrothermal process. However, the excessive TBT will lead to the fast formation of dispersive TiO2 nanocrystals by homogeneous nucleation and growth in the liquid phase. The uniformity and porosity of the composite were then significantly deteriorated. When the Ti/In mole ratio reaches 1/4, HRTEM images indicate that these two components of TiO2 and InVO4 have formed a close interface, which is favorable for the highly efficient interparticle electron transfer. Moreover, the composites exhibit an appropriate porous structure and visible-light response. Therefore, the TiO2/porous InVO4 microspheres composites achieve the best photocatalytic activity when the Ti/In mole ratio reaches 1/4, and the corresponding degradation rate k is 0.271 h-1, which is about 3.3 times as high as that of pure InVO4 without modification.(3) Preparation and characterization of TiO2/porous bulk g-C3N4 compositesA network structured porous g-C3N4 with TiO2 nanoparticles uniformly loaded on the surface was fabricated via a combination of sol-gel and hydrothermal method. Various techniques were adopted to study the influence of Ti/g-C3N4 molar ratio on structure, mophology and properties of as-prepared samples. The formation mechanism of porous structure in the g-C3N4 matrix was prosposed, and the reasons why the composites achieved an enhanced photocatalytic activity were also explained. The results show that the formation of porous structure in the g-C3N4 matrix is mainly related to the protonation effect. The intense protonation of base functionalities (-C-N-) occurs in the presence of H+ions generated from the hydrolysis of Ti(SO4)2 under hydrothermal conditions, leading to the formation of network porous structure on the surface of g-C3N4. Meanwhile, the g-C3N4 is favorable to the adsorption of [Ti(OH)2(OH2)4]2+species on the formed g-C3N4 surface defects of terminal amino groups or bridging nitrogens via hydrogen bonding force, thus providing the nucleating sites for the growth of TiO2 nanocrystals, and in such a way the uniformly TiO2/porous bulk g-C3N4 composites can be obtained. When the Ti/g-C3N4 molar ratio reaches 1/3, the composites exhibit a network porous structure. The specific surface area and pore volume are 21.21 m2/g and 0.111 cm3/g, which is 6.0 and 2.1 times as high as that of pure g-C3N4, respectively. The network porous structure in the composites largely improves the adsorption capacity for the target pollutants, and the TiO2/g-C3N4 nanojunction effectively promotes the separation of photo-g-enerated carriers. As a result, the TiO2/porous bulk g-C3N4 composites achieve the best visible-light photocatalytic activity, and the corresponding degradation rate is about as 12.0 times high as that of pure g-C3N4 without modification.(4) Preparation and characterization of three-dimensional porous bulk g-C3N4 with large specific surface areaA novel and facile template-free method was presented to fabricate three-dimensional (3D) porous bulk g-C3N4 with interconnected framework. Various techniques were adopted to analyse the structure and physicochemical properties of as-prepared samples. The reasons why the composites achieved an enhanced photocatalytic activity were also discussed. The results show that the first-step protonation of melamine can decrease the polymerization activity energy, and induce porous and loosened structure in the g-C3N4. On this basis, more protons can diffuse into the inner layers during the successive second-step protonation, leading to the formation of 3D interconnected framework with abundant nanopores. The specific surface area and pore volume of as-obtained g-C3N4are 108.3 m2/g and 0.714 cm3/g, which is 30.6 and 13.6 times as high as that of the original one, respectively. In addition, the marginal increase in the band gap (ca.0.16 eV) of three-dimensional porous bulk g-C3N4 may improve the redox ability of charge carriers generated in the g-C3N4. Furthermore, time-resolved fluorescence decay spectra (TR-PL), electron paramagnetic resonance (EPR) and photoelectrochemical measurements all indicate that this 3D porous bulk g-C3N4 shows much more efficient photo-generated carrier transfer and separation than that of the original one. This may be related to the electronic band structure and 3D porous interconnected framework of the as-obtained g-C3N4. As a result, the visible-light photocatalytic activity of g-C3N4 is significantly enhanced, and the degradation rate of methyl orange (MO) over the 3D porous bulk is about 6.21 times as high as that over the original one.