Facet-controlled Synthesis Of TiO₂and BiOX（X=Cl, Br And I） And Their Photocatalytic Activity

Semiconductor photocatalytic technology plays a very important role in solving the energy crisis and environmental protection which are the world problems. And the enhanced photocatalytic activity of photocatalys is the ultimate goal of the field. In this case, we carried out the photocatalytic activity of TiO2and BiOX (X=Cl, Br and I) with active facts exposure. The main contents and conclusion are as follows:1. Anatase TiO2crystals with{101}-{001} and{010}-{001} two facets coexistence were synthesized by (NH4)2TiF6acting as titanium and fluorine sources and the photocatalytic activity of the samples were investigated. The replacement of{101} by{010} facets inhibits the photocatalytic activity of anatase TiO2. The mechanism study demonstrated that photoinduced charge transfer properties between{101} facets and{001} facets resulted in the efficient charge separation. Replacing{101} by{010}, the charge transfer mechanism was altered and the photoinduced electron-hole pairs cannot be separated well. The photocatalytic activity of it has thus been inhibited.2. Anatase TiO2nanocrystals with{101},{001} or{010} single facets90%exposing were controllable synthesized from potassium titanate without fluorine. The liquid phase photocatalytic activity order of anatase TiO2facets for·OH production is{001}>{101}>{010}. But the gaseous phase photocatalytic activity order for of anatase TiO2facets for photoreduction of CO2to CH4is{010}>{101}>{001}. DRS, PL and ATR-IR spectra analysis showed that the photoactivity order of the gas phase for the photoreduction of CO2to CH4mainly depends on the CO2molecule adsorption property on the different exposed facet, and the separation efficiency of photo-generated carriers determines the photoactivity order for the·OH production in liquid phase.3.3D TiO2nanobipyramids (TiO2-NP) and1D TiO2nanoblets (TiO2-NB) were synthesized potassium titanate K2Ti6O13. The as-synthesized3D TiO2nanobipyramids and1D TiO2nanoblets all show high{101} facet exposed percentages and were used to investigate the shape effect of TiO2with the same facets exposure.3D TiO2nanobipyramids (101) showed higher photocatalytic activity than1D TiO2nanoblets (101). The reasons were discussed using ROS (reactive oxygen species) production, time-resolved photoluminescence spectra, XPS (X-ray photoelectron spectroscopy) spectra and ATR-IR (attenuated total reflectance infrared spectroscopy) spectra. The postulated mechanism suggests that3D TiO2 nanobipyramids has more step edges which results in more-OH/H2O molecules adsorption and longer life times of photoinduced carriers than1D TiO2nanoblets.4. BiOCl nanosheets (BiOCl NSs) were synthesized by hydrolyzing a hierarchical flowerlike molecular precursor (Bin(Tu)xCl3n,Tu=thiourea). The{001} facets percentage was controlled from71%to87%, by adjusting the feed ratios of BiCl3:Tu in molecular precursors. The relationship between the percentage of{001} facets and the photoactivity of BiOCl was investigated and a positive correlation was found. The mechanism study showed that the high oxygen atom density in{001} facets is considered the fundamental cause for the increase of the UV-induced oxygen vacancies in the crystal lattice, that enhances, consequently, the photoactivity of BiOCl.5. Bismuth complexes, Bin(Tu)xCl3n, were found to be photosensitizers for the first time. BiOCl nanosheets with inner Bin(Tu)xCl3n were synthesized The as-synthesized BiOCl displayed high visible light (λ≥420nm) photocatalytic activity which was112and13times higher than P25and BiOCl without Bin(Tu)xCl3n, for degrading RhB, respectively. The investigation of the photocatalytic mechanism demonstrated that Bin(Tu)xCl3n sensitized the BiOCl and resulted in unusually high visible light photocatalytic activity. The superoxide radical was the main active species in the photodegradation process. Furthermore, the concentration of the superoxide radical was quantized by the molecular probe nitroblue tetrazolium.6. Black BiOCl with oxygen vacancies was prepared by UV light irradiation with Ar blowing. The as-prepared black BiOCl sample showed20times higher visible light photocatalytic activity than white BiOCl for RhB degradation. The trapping experiment showed that the superoxide radical (O2·-) and holes (h+) were the main active species in aqueous solution under visible light irradiation.7. Highly symmetrical BiOI single-crystal nanosheets (BiOI SCNs) with dominant exposed{001} facets (up to95%) have been synthesized by annealing Bil3. The thickness and the{001} facets percentage of BiOI SCNs can be tuned by changing the annealing temperature. BiOI SCNs exhibit higher photoactivity (about7times) than irregular BiOI for degradation of Rhodamine B (RhB) dye under visible light irradiation. The{001} facets are the reactive facets of BiOI. The origin of{001} facets-dependent photoactivity is due to an improvement of the separation efficiency of photo-induced electrons and holes.8. BiOI thin film (BiOI TF) was prepared via a low temperature chemical vapor transport (CVT) route for the first time. As-synthesized BiOI thin film was composed of high symmetrical BiOI nanosheets with dominant exposed{001} facets. It displayed better photocatalytic activity, durability and selectivity than benchmark P25TiO2thin film and the origin come from the layered structure and good photoelectrochemical performance, CVT immobilization, the100%terminal oxygen atoms of{001} facets, respectively. At end, the photocatalytic mechanism with O2·-production was studied.9. Ag/AgX/BiOX (X=Cl, Br) three-component visible-light-driven (VLD) photocatalysts were synthesized by a low-temperature chemical bath method. The Ag/AgX/BiOX composites showed enhanced VLD photocatalytic activity for the degradation of rhodamine B, which was much higher than Ag/AgX and BiOX. The photocatalytic mechanisms were analyzed by active species trapping and superoxide radical quantification experiments. It revealed that metallic Ag played a different role for Ag/AgX/BiOX VLD photocatalysts, surface plasmon resonance for Ag/AgCl/BiOCl, and the Z-scheme bridge for Ag/AgBr/BiOBr.10. BiOBr-g-C3N4inorganic-organic composite photocatalysts were synthesized by a one-step chemical bath method at low temperature The BiOBr-g-C3N4composite showed much higher visible-light-driven (VLD) photocatalytic activity than pure g-C3N4and BiOBr for rhodamine B (RhB) degradation. The active species trapping and quantification experiments indicated that the photoinduced charges transfer between g-C3N4and BiOBr resulted in the O2·-generation at a higher conduction band of BiOBr.