Synthesis And Visible-light Photocatalytic Activities Of P-doped TiO₂ And Its Loading On Various SiO₂ Porous Supports

TiO₂ photocatalytic degradation (PCD) method is one of the most promising methods in waste treatment, due to their high PCD rate of various organic compounds. However, the well-known highly photo-reactive anatase TiO₂ responds only under UV-light irradiation. In order to make better use of solar energy, many attempts have been made to sensitize TiO₂ into visible-light region. Since Asahi et al. (2001) first reported the visible-light photocatalytic activity of N-doped TiO₂, many researches have been carried out in order to dope TiO₂ with inorganic elements. It has been found that, comparing with other inorganic elements, phosphorus can significantly increase the specific surface area of TiO₂ and prevent the anatase-to-rutile phase transformation, resulting in the enhancement of photocatalytic activity of TiO₂. However, the mechanism of P doping has been subjected to a considerably smaller number of studies.Considering the difficulties in recycling process of fine powders, TiO₂ must be fixed on inert and ideal supports in order to be further used in large scale. Among various host materials, the porous silicates seem attractive candidates due to their high surface areas and chemical stabilities. Previous works have found that even the photo-inert porous SiO₂ supports also improved the photocatalytic activities of loaded TiO₂ to some extent, and the reasons for such enhancement are still not clear. Moreover, most of the researches only focus on a single type of SiO₂ support. While studies on the effects of porous structures, specific areas, wall components, and crystalline degrees of various SiO₂ supports on the photocatalytic activities of the loaded photocatalysts are very rear.In this article, by employing H3PO4 and Ti(OC4H9)4, P-doped TiO₂ nanoparticles with various phosphorus contents were synthesized by sol-gel method. The samples were calcinated at different temperatures and charactered by XRD, FTIR, Raman, UV-vis, XPS, and ICP methods, so that the effects of doping amount and the calcination temperatures on the crystalline structures, the crystal sizes, the specific areas, the bonding conditions, the dispersion of P, the adsorption edges, and the visible-light photocatalytic activities could be inspected. The results show that P species hinders the particle growth of anatase leading to remarkable increase of anatase-to-rutile phase transformation temperature. When the P content is very low, P species introduces oxygen into TiO₂ lattice and hence causes red shift of adsorption band edge of anatase, leading to the increased visible-light photocatalytic activity of P-doped TiO₂. While when the P content is very high, P species acts as the interface phase between TiO₂ clusters and strongly retards the crystal growth of anatase, resulting in the widened band gap of P-doped TiO₂. When calcined over 900 oC, a new titanyl phosphate, Ti5O4(PO4)4, was observed in P-doped TiO₂. One the basic of the above phenomena, a possible mechanism is also proposed to explain the formation of the two phases during the sol-gel process. In P-doped TiO₂, P species is likely to have two different states. One state is named as"separated state", whose P content is very low so that the P species is surrounded by TiO₂. The other state is named as"congregated state", whose P:Ti ratio is high enough to make the TiO₂ clusters isolated by P species. Both of the two phases hinder the crystal growth of anatase. But only the"separated state"shows visible-light photocatalytic activities and causes red-shift ofλg.In order to prevent the hydrolyzation speed of Ti(OC4H9)4, acetic acid was employed in the sol-gel process of P-doped TiO₂. The amount effects of acetic acid on the structures and visible-light photocatalytic activities of P-doped TiO₂ are examined carefully. Moreover, on the basic of our proposed doping mechanism of P species in TiO₂, reasons for the enhancement of visible-light photocatalytic activity caused by acetic acid were also deduced. The experimental results show that the addition of acetic acid leads to the lattice expansion, the increased crystal size, and slight decrease of anatase-to-rutile phase transformation temperature. It suggests that acetic acid improves the formation of"separated state"of P species in P-doped TiO₂, resulting in the red-shift ofλg and the decay of"congregated state". With the help of acetic acid, P-doped TiO₂ could achieve very high visible-light photocatalytic activities under relatively low calcination temperatures.Natural SiO₂ minerals with perfect crystalline structures, such as quartz and montmorillonite, were employed as the pore-wall materials for the mesopores, in order to get various mesoporous SiO₂ supports with different crystalline degrees. Thus, the complicated crystallization process of mesoporous SiO₂ walls could be omitted. And mesoporous SiO₂ supports with various crystalline degrees could be easily synthesized by the similar synthetic method of MCM-41. The resultant compound mesoporous minerals were charactered by low-angle XRD, FTIR, TEM, and N2 adsorption/desorption isotherms, in order to check the porous structure, surface morphology, and hydrothermal stabilities of the samples. The compound mesoporous quartz exhibits mesoporous structure and its specific area is up to 600 m2/g or so. Although the vibrations of Si-O bonds vary after synthesis, the hydrothermal stabilities of mesoporous quartz are still no better than common MCM-41. Comparing with mesoporous quartz, the mesoporous montmorillonite displays high specific surface areas and extraordinary hydrothermal stabilities. Three types of silica resources, TEOS, Na2SiO3, and nano-scaled white silica, were employed during synthesis in order to reveal the effects of the type of silica resource on the specific surface areas and hydrothermal stabilities of mesoporous montmorillonite. In the best conditions, the specific surface areas of resultant mesoporous montmorillonite is up to 770 m2/g, and it keeps at 580 m2/g or so even after boiled for 10 days. In addition, natural macroporous diatomite is combined with mesoporous silica, resulting in a macro- and meso-porous hierarchical structure. The resultant mesoporous diatomite is composed of amorphous SiO₂, and its specific surface area is up to 860¹040 m2/g which is similar to the specific surface area of ordinary MCM-41. Moreover, these mesoporous diatomites display superior hydrothermal stabilities than ordinary MCM-41.Ten types of porous SiO₂ are employed as the supports of P-doped TiO₂, such as mesoporous quartz, mesoporous diatomite, mesoporous montmorillonite, MCM-41, SBA-15, dry gel of SiO₂, wet gel of SiO₂, montmorillonite, mordenite zeolite, and diatomite. These SiO₂ supports have various specific areas, porous structures, and crystalline degrees. During synthesis, the resultant SiO₂ supports are immersed in the sol of P-doped TiO₂ for loading, and the ratio between P-doped TiO₂ and compound photocatalyst is 25 wt%. The compound photocatalysts were charactered by XRD, FTIR, Raman, and UV-vis spectra. The results suggest that the porous structures of various SiO₂ supports are remained after loading of P-doped TiO₂, and both of the particle sizes and the crystal growth of anatase are strongly restricted by porous SiO₂ supports. Comparing with unloaded P-doped TiO₂, the Ti-O vibrations of anatase in compound photocatalysts reduce sharply, and only PT-MCM displays Ti-O-Si vibration at 960 cm-1. All of the compound photocatalysts show superior visible-light photocatalytic activities when the calcination temperature is around 500 600 oC. And PT-S displays the best photocatalytic activity, indicating the advantage of wet gel material. The specific areas, the surface charges, the porous structures, and the crystalline degrees of the pore walls show remarkable effects on the crystal sizes and photocatalytic activities of loaded P-doped TiO₂. If theλg of porous SiO₂ support is larger than P-doped TiO₂, the visible-light photocatalytic activity of resultant compound photocatalysts would drop sharply. Moreover, high crystalline degree of porous SiO₂ support benefits the UV-light photocatalytic activity of P-doped TiO₂.