Preparation, Characterization And The Controllable Mechanism On Photocatalytic Activity Of Novel Photocatalysts

Environment and energy issues are still the bottle neck for the continuable development of human being. Semiconductor photocatalysis is an advanced technology that employing the electrons on conduction band and holes on valence band in photons excitated semiconductor, pollutants can be removed via oxidization or reduction and hydrogen can be obtained via water-splitting. As its essence is to convert solar energy into chemical energy, this technology may become an available approach to settle the environment and energy issues. The focus of semiconductor photocatalysis is to develop photocatalysts with wide wavelength light response, high efficiency and stability. To make the absorption spectrum of photocatalysts match with that of the solar's is the prerequisite for exploiting solar energy. High efficiency makes its application to be dynamically possible, and high stability ensures the technology to be cheap and feasible. In this paper, we focused on developing photocatalysts with such properties as wide wavelength light response, high efficiency and stability, and integrated the investigation techniques of material characterization, density functional theory (DFT) and photoelectrochemistry, to explore the controllable mechanism of photocatalytic activity.The relationship of the crystal structure and energy band structure of doped TiO₂ with photocatalytic performance has been investigated. Based on the DFT, the crystal structure and energy band structure of undoped, N-doped, I-doped and Pt-doped TiO₂ have been calculated by Castep code. The results showed that there are three new energy bands in the band gap of N-doped and I-doped TiO₂, and five ones in that of Pt-doped TiO₂. The new band is the origin of their visible light response. The analysis of the compositions of VB and CB showed that the dominant contribution to VB of I-doped TiO₂ is O_2p, mixed with some I_5p and Ti_3d, however the dominant contribution to CB is Ti_3d, as well as mixed with some O_2p and I_5p. The Pt_5d orbital in Pt-doped TiO₂ split into two parts which are involved in VB and CB, respectively. The band potentials of I-doped and Pt-doped TiO₂ shift downwards, which indicates that photogenerated holes have stronger oxidative power. The results of the dipole moment calculations showed the distortion of TiO₆ in doped TiO₂ is I-doped > Pt-doped> N-doped. Because the internal dipole moments promote the charge separation, Pt-doped and I-doped TiO₂ exhibited higher photocatalytic activity.The preparation, characterization and photocatalytic performance of CoOx modified BiVO₄ composite have been studied. The monoclinic BiVO₄ with micron size has been prepared by aqueous precipitation, and BiVO₄/CoOx composite has been prepared by impregnation method. The crystal, valence, morphology and optical properties have been characterized by XRD,XPS,DRS,BET,SEM,TEM,FTIR. XPS showed that Co in the composite calcined at 300 0C with 0.8wt% Cobalt content is present as Co₃O₄. DRS showed that band gap of BiVO₄ is 2.35 eV, and the optical absorption after CoOx modified extended to 800 nm. SEM and TEM showed that 20-50 nm Co₃O₄ particles dispersed on the surface of micron BiVO₄ particles. BET was improved from 0.74 m²g^-1 to 1.38 m²g^-1 after surface modification. Phenol degradation test showed the highest efficiency is observed when the sample calcined at 300 C with 0.8 wt % cobalt content. Phenol removal efficiency is 96% after 3 h irradiation. The composite photocatalyst exhibits good stability and precipitation performance in aqueous solution. The migration principle of photogenerated carriers in p-n heterojunction nanocomposite with three different kinds of energy band position has been studied. The VB and CB of p type semiconductor more anodic than those of n type one is not favorable for the separation of electrons and holes. The VB and CB of BiVO₄ and Co₃O₄ have been estimated by absolute electronegativity. Combined with the result of PL spectra, the origin of enhanced performance of the composite is the effective separation of charge carriers. Nanosheet of h-BiVO₄ have been prepared by hydrothermal method with SDS as morphology-directing template. TEM and HTEM showed that h-BiVO₄ is 100 nm nanosheet with a preferred (010) surface orientation. The phenol removal efficiency over the h-BiVO₄/Co₃O₄ composite is as high as 99% after 2 h irradiation.The photoelectrochemical properties of BiVO₄ and BiVO₄/Co₃O₄ composite electrode have been studied. The flatband potential of BiVO₄ and Co₃O₄ have been determined as -0.3 V vs. NHE(pH=7.0)and +0.54 V vs. NHE(pH=7.0)respectively. The result showed the VB and CB of p type semiconductor Co₃O₄ are more cathodic than that of n type BiVO₄. The pressed electrodes of BiVO₄,Co₃O₄ and BiVO₄/Co₃O₄ have been prepared by doctor blade method. The peroxide intermediates during water photoelectrochemical splitting on BiVO₄ electrode can serve as recombination center and decreased the efficiency, and even lead to the switch of photocurrent direction upon low bias and extending irradiation. By cyclic voltammogram reducing the peroxide species 80% photocurrent can be restored. By adding the holes scavenge KI or modified surface with Co₃O₄, the intermediates can be inhibited and the stability of the electrode can be improved. BiVO₄/Co₃O₄ electrode exhibited better photoelectrochemical performance than BiVO₄, and on the former electrode surface recombination of photogenerated electrons and holes inhibited. Adding reductive such as methanol and KI can not improve the efficiency, which indicates that electrons migrated from CB of BiVO₄ to ITO and produce enhanced anodic photocurrent, and simultaneously holes on the VB of Co₃O₄ can be consumed by oxidizing water. In alkaline electrolyte NaOH, the efficiency and stability of BiVO₄ electrode are improved. The Cyclic voltammogram after irradiation exhibited that peroxide intermediates are less in the alkaline, and other unknown intermediates produced, and the mechanism of water photooxidization by the holes is changed.The photocatalytic performance of composite and their mixtures have been studied. Ag, Pt, Cu, Ni, Ru modified BiVO₄ composites have been prepared. Their activity on phenol degradation under visible light irradiation is: BiVO₄/Co₃O₄>Ag-BiVO₄>BiVO₄/NiO_x>Pt-BiVO₄>BiVO₄/RuO₂>BiVO₄/CuO. The effect of electrolytes inhibiting the degradation of phenol over BiVO₄/Co₃O₄ is NO₃^--4^2-. Cl^- showed negligible effect on phenol removal, but the ring-opening process has been inhibited. Phenol removal efficiency increased with the pH value, and decreased with phenol initial concentration. The hydroxyl radical scavenges such as methanol or isopropanol did not affect the phenol removal, but inhibited the ring-opening process. Holes scavenge KI promoted the phenol photocatalytic degradation. The phenol removal can be improved by argon purging and adding with electron scavenge SF₆. The IPET (InterParticle Electron Transfer) on BiVO₄ and Co₃O₄ mixture have been found, the phenol removal efficiency is highest (as high as 41%) when the weight ratio of BiVO₄: Co₃O₄ is 4:1. The formation of salicylic acid upon irradiation of BiVO₄ and Co₃O₄ mixture in presence of benzoic acid established the presence of hydroxyl radicals.The hybrid n/p semiconductor electrode for light driven logic device has been investigated. Four kinds of hybrid n/p semiconductor pressed electrodes of BiVO₄/Co₃O₄,TiO₂/Co₃O₄,BiVO₄/CuO,TiO₂/CuO have been prepared by doctor blade method. By investigating their wavelength dependence photocurrents, it has been established that hybrid n/p semiconductor electrode used for XOR logic is a general approach. The prerequisite on photoelectrochemical properties for wavelength controlled switching of photocurrent direction is that VB and CB of n type semiconductor should be more anodic than those of p type one. Based on well-fitting photoelectrochemical, electrical and chemical properties, the satisfying switch behavior can be achieved.