Synthesis, Micro Structure And Visible-light Photocatalytic Performance Of Ag/N Modified TiO₂-based Nanomaterials

One of the key points of photocatalysis technology is to achieve high-performance photocatalysts. TiO2-derived photocatalysts have been attracting the worldwide attentions due to their low cost, low toxicity, and outstanding physical, chemical properties. TiO2-derived photocatalysts have been widely researched for several decades for applications in different fields including environmental depollution and energy conversion. However, there are still some disadvantages for TiO2-derived photocatalysts, hindering their practical applications, such as low utilization of sunlight, poor quantum efficiency and lack of photocatalytic stability. Therefore, it is essential to develop TiO2-derived photocatalysts with higher photocatalytic activity and stability. This research work is pertinently progressing from two aspects:one is to improve visible-light photocatalytic activity of TiO2-derived materials with Ag loading and/or N doping; the other is to enhance the photocatalytic stability of these photocatalysts with Ag, N co-modification.In this research work, series of Ag-loaded, N-doped and Ag-N co-modified TiO2-derived photocatalysts have been successfully synthesized. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (DRS), fluorescence spectroscopy (FL), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and N2adsorption-desorption analysis are employed to characterize the as-prepared materials. Rhodamine B (RhB) and benzene are used as the degradation objects to evaluate the liquid-phase and gas-phase photocatalytic activities of the as-prepared samples. Some achievements have been made in this research work.Protonated anatase/titanate nanosheets with a high specific surface area of378m2/g were synthesized through an alkaline hydrothermal method with subsequent acid washing. In situ Raman, XRD and other techniques are employed to investigate the structural transformation influenced by thermal treatment. During the thermal treatment, dehydration of the nanosheets resulted in the complete transformation from titanate phase to anatase phase and the destroying of the laminated structure. The anatase phase has not been found to grow at temperatures below300℃. Increasing the temperature can lead to crystallinity improvement and the decrease of the surface area and the pore volume. These conversions influenced liquid-phase and gaseous-phase photocatalytic activity of the materials in different ways. With temperature increasing, the liquid-phase photocatalytic activity for RhB of the materials exhibited a downward trend, while the gaseous-phase photocatalytic activity for benzene firstly increased and then decreased when the temperature exceeded450℃.The aforementioned protonatcd anatase/titanate nanosheets are used as raw material to prcapare N-doped TiO2with high specific surface area. The as-prepared N-doped TiO2shows high visible-light photocatalytic activity for benzene. Influences of calcination temperature and mole feed ratio on the structure and properties of N-doped TiO2are also studied. It is found that the optimal calcination temperature is450℃. Lower temperature would lead to incomplete decomposition of urea, while higher temperature would hinder N dopant remaining in the TiO2lattice because of the excessive thermal vibration. N doping content will increase with the increasing mole feed ratio. However, the excess urea make no contribution to increase the N doping content when the mole feed ratio is above2:1. It is also found that the photocatalytic stability of N-doped TiO2is unsatisfactory because the photocatalytic activity of N-doped TiO2gradually decreases during the cycle experiment. That is because the N dopants in the TiO2lattice would be oxidized by the photogenerated holes. The photocatalytic activity of N-doped TiO2will decrease with the loss of N dopants. Therefore, it is necessary to improve the stability of the N dopants in the TiO2lattice.A facile one-pot hydrothermal method is proposed for synthesis of layered protonated titanate nanoshects (LPTNs) loaded with highly dispersed Ag nanoparticles, and the formation mechanism of this unique structure is also proposed. The as-synthesized samples are confirmed to be H2T2O5·H2O structure with mesoporous of3-4nm in diameter and high surface area of about200m2/g. The Ag nanoparticles loaded on the nanosheets are4-6nm in diameter and mainly exist in the form of zero-valence, which shifts the absorption edges toward longer wavelengths and enhances the visible-light absorption for the nanosheets due to the SPR of Ag. Ag loading remarkably enhances both the liquid-phase and gas-phase photoeatalytic activities of the titanate nanosheets under visible-light irradiation. An alternative possible mechanism for the enhancement of the visible-light photocalalylic activity is proposed. Moreover, the photocatalytic activity increases gradually with increasing Ag loading content first, and then decreases after maximizing at the optimal Ag/Ti molar ratio (2.87mol.%for photocatalytic degradation of RhB and1.57mol.%for photocatalytic mineralization of benzene, respectively). It can be attributed to that the excess Ag may aggregate to act as the recombination site and prevent light absorption of the LPNTs host by covering its surface. Therefore, the optimal Ag loading content should be explored for achieving the highest photocatalytic activity.Various contents of Ag nanoparticles were successfully introduced into the N-doped TiO2photocatalysts via a hydrothermal procedure. The photocatalysts were uniform particles and the adherent Ag mainly existed in the form of zero-valence. The existence of Ag restrained the escape of N dopants from the oxide during hydrothermal procedure. Such stabilization may be attributable to an electron transfer from the Ag5s orbitals towards the2p orbitals of the implanted N. The dependence of the photocatalytic activity on Ag content was also investigated by degradation of RhB under visible light irradiation. The photocatalytic activity increases gradually with increasing Ag content first, and then decreases after maximizing at the optimal Ag/Ti molar ratio of0.92mol.%. Therefore, the photocatalytic activity of Ag loaded N-doped TiO2photocatalysts can be adjusted by the Ag content and the optimal Ag loading content should be explored for achieving the highest photocatalytic activity.