Study On The Modification And Properties Of Bi₄Ti₃O₁₂-based Photocatalysts

Environmental pollution and energy crisis are two of the most serious problems facing humanity in the twenty-first century,Semiconductor Photocatalysts provide a clean and feasible solution to these two problems.TiO₂ is the representative of semiconductor photocatalyst,which is stable,cheap and non-toxic.However,TiO₂ has a large band gap（3.0³.2eV）,which can only be excited by UV light,and its utilization ratio of solar light is low.In addition,it has a high recombination rate of photogenerated carriers and low quantum efficiency.Therefore,it is an inevitable trend to develop a new type of high efficiency and non-TiO₂ visible light photocatalyst in the field of photocatalysis.Bi₄Ti₃O₁₂ is a new type of visible light photocatalyst,which is easy to absorb visible light because of its narrow band gap.However,the photogenerated carriers are easy to be recombination and the photocatalytic properties still need to be improved.To further improve the performance of Bi₄Ti₃O₁₂ photocatalyst,need to solve two key problems: one is how to further suppress the recombination of photogenerated charge carriers;the two is to further broaden the scope of the light absorption edge of the catalyst,so that it can make full use of visible light.To solve the above problems,the following work is done in this paper.（1）Novel chromium-modified Bi₄Ti₃O₁₂photocatalyst（Bi4Ti3-xCrxO12,x=0–0.5）was developed by a facile sol-gel hydrothermal process with low reaction temperature.The XRD,SEM,TEM,and XPS analyses indicated the formation of the single-crystalline,rectangular,and {001} facet-exposed Bi₄Ti_3-XCr_XO₁₂ nanosheets.Their average side lengths and thickness progressively decreased with increase in the Cr content,and the minimal side length and thickness for Bi4Ti2.6Cr0.4O12 are 95nm and 15 nm,respectively.With more addition of Cr content,sample showed significant particle agglomeration even though some of small nanosheets were observed.The UV–vis study revealed a reduction in the band gap of Bi₄Ti_3-XCr_XO₁₂ and enhanced photoabsorption in the visible light region,compared to the calcined Bi₄Ti₃O₁₂.The photolumi-nescence and electrochemical impedance spectroscopy results displayed a decreased recombination of photogenerated charges in the Cr-modified Bi₄Ti₃O₁₂ compared to Cr-free one.The photocatalysts were optimized for both maximum hydrogen generation from alcohol-water mixture and methyl orange（MO）degradation in an aqueous solution under visible light irradiation.As a result,addition of Cr effectively improves photocatalytic performance of Bi₄Ti₃O₁₂,Bi₄Ti_3-XCr_XO₁₂,the optimal photocatalyst,showed highly stable reusability as a hydrogen generator.The photocatalyst reached as much as 117μ molg-1h-1 hydrogen evolution rate under 240 min of irradiation,which was ～2.85 times that of the Bi₄Ti₃O₁₂ nanosheet.The hydrogen yields of the Bi₄Ti_2.6Cr_0.4O₁₂ using various alcohol-water mixtures decreased in he order: isopropanol > ethanol > methanol.The optimal photocatalyst also showed up to 0.01003min-1 of average apparent rate constant in the MO degradation,about 3 times that of the Cr-free nanosheet.The improved photocatalytic performance of the Cr-modified Bi₄Ti₃O₁₂ is attributed to its strong absorptionin visible light region,small nanosheet size,exposed {001} facets as well as the low recombination rateor the high separation efficiency for photogenerated electron-hole pairs.（2）Firstly,the Gd doped Bi₄Ti₃O₁₂ nanosheets(Bi_3.15Gd_0.85Ti₃O₁₂)were synthesized by sol-gel hydrothermal method,and then the nano Pt particles were loaded on the nanosheets by solution dipping method.The results of Uv-vis and PL spectra show that the surface plasmon resonance effect of Pt can effectively reduce the band gap of Bi_3.15Gd_0.85Ti₃O₁₂ and enhance the absorption of visible ligh;the Schottky barrier produced by the interface between Pt particles and Bi_3.15Gd_0.85Ti₃O₁₂ nanosheets can promote the separation of photogenerated electrons and holes.Thus,the visible light photocatalytic performance of Bi_3.15Gd_0.85Ti₃O₁₂ is greatly improved.（3）Nd doped Bi₄Ti₃O₁₂ nanosheets(Bi_3.15Nd_0.85Ti₃O₁₂)were prepared by sol gel hydrothermal method and then heated treatment with H2/Ar gas.In addition,the effects of heat treatment temperature and holding time on the photocatalytic properties of the samples were studied.The experimental results show that the photocatalytic activity of the samples under the conditions of 250℃-3h is the best.After heat treatment in hydrogen atmosphere,oxygen vacancy formation on the surface of Bi_3.15Nd_0.85Ti₃O₁₂ nano film,surface oxygen vacancy can improve the top of valence band,reduce the band gap of Bi_3.15Nd_0.85Ti₃O₁₂,broaden the range of visible light absorption,and effectively restrain the recombination of photogenerated electron-hole.Thus,improve the photocatalytic performance.