| Titanium dioxide(TiO2)has the advantages of high efficiency,non-toxicity,chemical stability and low-cost,enabling it to become a benchmarking photocatalyst which has been widely used in degradation of organic pollutants,electrocatalysis,solar cells and self-cleaning.As the photocatalyst performance is determined by the synergistic effect of spectral response range,photon conversion efficiency,effective separation of photogenic carriers and rapid migration,there are still some limitations in the photocatalytic process of TiO2:(1)The band gap energy(Eg)of TiO2 is about 3.2eV.Therefore,to excite electrons from the valence band to the conduction band requires sufficient energy photons(UV region,??387 nm),which limits the effective use of sunlight.(2)The photo-generated electron-hole pairs will recombine and vanish away in 10?100 ns,much shorter than the time for electrons to be captured and transfer(100ns?1 ms),so the majority of carriers cannot migrate to the surface of the catalyst,where the photocatalytic reaction occurs,consequently reducing the photocatalytic reactivity.Moreover,the photocatalytic degradation reaction is greatly affected by the TiO2 morphology,crystal size and exposed facets.These are problems that need to be solved and are worthy of further study in practical application.In order to further improve the photocatalytic performance of TiO2 and reveal the photocatalytic mechanism,we have modified Ti02 by doping and compositing methods.The main research contents and results are as follows:1.In order to reveal the effects of dopant average valence on the photocatalytic performance,Ta and Rh co-doped Ti0.96Ta0.04-xRhxO2(x=0,0.008,0.016,0.02,0.024,0.032,and 0.04)samples were crystallized at 450 ? and 800 ?,respectively;then the 800 ? crystallized x = 0.02 sample was subsequently reduced in the H2/N2 mixture gases at 600,700,800,and 900 ?.The valence of Rh and Ta is commonly +3 and +5 respectively,and the ionic radius of both Rh3+ and Ta5+ is 0.68 A,equal to that of Ti4+in TiO2.Therefore,with changing the concentration of Ta and Rh in Ti0.96Ta0.04-xRhxO2,the average ionic radius will keep constant nearly,while the average valence of Ta and Rh can be tuned in the region of +3?+5.Under different processing conditions,Ta remains the valence of+5,while the valence of Rh varies.Rh exhibits Rh3+ and Rh4+ in the 450 ? crystallized samples and is completely oxidized to Rh4+ in the 800 ?crystallized samples.In the reduced samples,Rh exists in the forms of Rh,Rh3+ and Rh4+.Among the 450 ? crystallized samples,the x=0.02 sample with the high ratio of Ta5+(94.4%)and Rh3+/Rh4+(0.95)exhibits the highest photocatalytic performance.In the case of 800 ? crystallized samples,Rh3+was completely oxidized to Rh4+,and the ratio of Ta5+ becomes smaller compared with all 450 ? crystallized and all reduced samples;correspondingly,these samples exhibit the worst photocatalytic performance.For the 800 ? reduced sample(x = 0.02)which has the optimal photocatalytic performance among all studied samples,the Rhn+/Rh4+ ratio has the highest value of 2.11.In brief,the photocatalytic performance of Ti0.96Ta0.04-xRhxO2 can be greatly improved by increasing the ratio of Rh3+/Rh4+ or controlling the average valence of doped Rh and Ta close to+4.2.Using vapor-thermal method to dope S element in(001)-facet exposed TiO2 by different S/Ti molar ratio,and at 180 ? and 250 ? respectively.The experimental data was characterized by XRD,TEM,FT-IR,XPS,UV-vis DRS,PL,BET and ESR.The doped sample at 180 ? showed little change in structure,morphology,composition(chemical state)and properties,indicating that this temperature is not sufficient to achieve S doping.When the doping temperature is 250 ?,the effect of S doping is as follows:(1)The crystal structure is distorted,and the ratio of unit cell parameters c/a varies with the degree of S doping.The value of c/a is maximum when the S/Ti molar ratio being 3.This is because the S element is different from the ionic radius of the replaced elements(O and Ti).(2)The morphology of the particles changes,and the particles aggregate,resulting in a decrease in specific surface area.(3)S-doping increases the adsorption coefficient Ae(the adsorption of pollutant MB on TiO2)from 0.9%to 68.5%.The adsorption coefficient is mainly determined by the number of oxygen vacancies.Moreover,the SO42-adsorbed on the surface of TiO2 particles and the pore size also have an effect on the adsorption coefficient.(4)S-doping increased the degradation rate(the degradation of MB on TiO2)from 6.9×10-2 Min-1 to 18.2×10-2 min-1,and the corresponding samples had higher number of ·OH and ·O2-free radicals.3.The(001)-facet exposed TiO2,i.e.(001)-TiO2,was compounded with CuO in water vapor environment at 250 ?.The CuO/(001)-TiO2 composite was synthesized by changing the Cu/Ti molar ratio(RCu/Ti).The experimental data was characterized by XRD,TEM,BET,photocatalytic degradation,UV-vis DRS,XPS,PL and ESR.Through the data analysis,the following conclusions can be drawn:(1)Through the vapor-thermal method,the close contact between CuO and(001)-TiO2 is realized and a heterojunction is formed,which causes the valence band of TiO2 to move up.It can improve separation efficiency of photogenerated carriers.(2)The band gap of CuO(1.7 eV)is smaller than TiO2(3.2 eV),which can promote the absorption of visible light by compositing.(3)In the composite,an appropriate amount of CuO promotes photocatalysis,but excessive CuO inhibits photocatalytic reaction.(4)The signal of hydroxyl radical ·IH and superoxide radical ·O2-is present in pure(001)-TiO2.The hydroxyl radical-OH signal was not detected in the composite,and the superoxide radical ·O2-played a major role in the catalytic reaction.(5)A small amount of Cu in the composite can promote the transmission of photogenic electrons thus improving the photocatalytic effect. |
PDF Full Text Request