Study On Fabrication,Tunable Microstructure And Photocatalysis Of Surface-dispersed H-titanate Nanosheets Heterojunctions

At present,environmental pollution and energy shortage are global problems in the world.Photocatalysis using semiconductor materials is a promising technology to effectively address environmental pollution with solar energy.Photocatalysts can be directly used to degrade pollutants in air and water by sunlight and generate hydrogen as an ideal fuel.However,with either photocatalytic degradation of pollutants or hydrogen production by water decomposition,there are still some challenges in practical application,such as low efficiency of photogenerated electron-hole pair separation,low yield of charge transfer and transport,narrow light response range that limits effective use of sunlight,poor reaction selectivity,and difficulty with recovery and regeneration of the photocatalysts.Heterojunction photocatalysts can effectively address some of the limitations of the single-phase semiconductor photocatalysts,such as light response and charge separation,and show promise for applications in photocatalysis.The structure and property of the surface/interface of heterojunctions play a critical role in the separation and transport of photogenerated carriers.Thus,control of the surface/interface of heterojunctions can greatly promote the separation and migration of photogenerated electron-hole pairs and is advantageous to further improve the photocatalytic performance of heterojunction catalysts.This work focuses on constructing efficient heterojunction photocatalysts by controlling the surface/interface of heterojunctions with combined theoretical and experimental approaches.Specifically,TiO2-based nanosheets(TNSs)with surface area and thinner thickness have been studied as model photocatalytic materials to gain a deeper understanding of the relationship between structure and properties.Based on the mechanism of growth of TNSs by hydrothermal method,we design and fabricate high efficiency nanoscale heterojunction photocatalysts by carefully controlling their microstructures.The control of the constructed TNSs heterojunction interface involves three aspects:the contact area of the heterojunction interface,the separation efficiency of photogenerated carriers,and the rate of migration to the surface of catalyst.The major points are summarized as follows.Firstly,a facile one-pot approach for MnxOy(Mn = Mn2+ Mn3+)loaded TiO2-based nanosheets(Mn-TNSs)with surface enrichment of MnxOy nanoparticles via a simple hydrothermal route is reported.The as-synthesized samples showed clearly sheet-like structure,with high specific surface area(200-300 m2/g)and small thickness(ca.4 nm).The small MnxOy nanoparticles with uniform size(1-2 nm)were highly dispersed on the surface of the TNSs.Loaded MnxOy not only have influenced the crystal structure and surface area of the TNSs,but also resulted in considerable enhancement of visible-light absorption and red-shift for TNSs.The results of X-ray photoelectron spectroscopy showed that Mn2+ and Ma3+ co-exist in Mn-TNSs.The loading of MnxOy on TNSs resulted in changes in binding energies of Ti and O.The concentration of Ti3+ increased gradually with the increasing amount of loaded MnxOy.The photo-reactivity of the samples were evaluated by measuring the formation of photo-induced hydroxyl radical(·OH)using coumarin(COU)as a probe molecule and photocatalytic degradation of Rhodamine B(RhB)and dichromate(Cr(VI))under visible-light irradiation.It has been found that the Mn-TNSs photocatalysts showed better visible-light photocatalytic activity for aqueous RhB or Cr(VI)solution.The photocatalytic activity gradually increased with the increasing content of MnxOy initially,and then decreased after attaining the maximum at an optimal content(2.5 at.%)of MnxOy in TNSs for degradation of aqueous RhB or Cr(VI)solution,respectively.The cyclic tests that performed five times demonstrated high stability and recyclable usability of the photocatalysts.A possible alternate mechanism for the enhancement of the photocatalytic activity under visible-light irradiation was also proposed.Secondly,for the large number of transition metal oxides(MxOy),due to different element valence,structure,and atomic radius,MrnxOy,CoxOy,VxOy loaded TNSs maybe achieve different properties and photocatalytic effects.In this chapter,the surface-dispersed CoxOy/TNSs and VxOy/TNSs heterojunctions are fabricated by one-pot hydrothermal route,and the structural characterization and photocatalytic performance of as-synthesized samples were investigated systematically.It can be found that different MxOy have different regulation effect on the transformation from 3D P25 nanoparticles to 2D-TNSs and different catalytic effects.As-synthesized 1.0%VxOy/TNSs(1.0%-V-TNSs)and 2.5%CoxOy/TNSs(2.5%-Co-TNSs)heterojunctions exhibit the optimal effect on the photo-induced carrier separation and visible-light-driven photocatalysis.In addition,regeneration and utilization of MxOy/TNSs are also studies systematically.During the whole recycle-regeneration process,the structure evolution process(raw-intermediate-recovery)for MxOy/TNSs heterojunctions in each process are obtained,and found that the Cr(VI)removal efficiency of 2.5%-Co-TNSs was 81.57%,and the removal efficiency of phenol was 83.24%after 7 cycles of regeneration in Cr(VI)-phenol coexistence system,indicating that the surface-dispersed MxOy/TNSs heterojunction has excellent regeneration and utilization.The mechanism of the enhanced visible-light-driven photocatalytic activity for MxOy/TNSs heterojunction were studied and explained by the positron annihilation lifetime spectroscopy(PALS)and density functional theory(DFT)calculations.Thirdly,the surface-dispersion Pt/TNSs(0D-2D Schottky barrier heterostructures)and CoxOy/Pt/TNSs(Z-type ternary heterojunction)were fabricated by combining hydrothermal method with photoreduction method.The structure,photoelectric properties and photocatalytic decomposition of water hydrogen production of as-synthesized samples are studied systematically.Initially,by changing the content of Pt0,the apparent quantum yield of Pt/TNSs obtained maximum(11.48%)when the deposited Pt0 content was 0.25 wt.%.Appropriate amount of functional 0D-2D Schottky barrier heterogeneous structure formation is a key factor to improve the photocatalytic activity.Additionally,combined with CoxOy/TNSs heterojunction materials in the previous chapter,by depositing Pt0 in the CoxOy/TNSs heterojunction,it can be found that the Pt0 and CoxOy nanoparticles showed synergistic effect in the CoxOy/Pt/TNSs compositions.When Pt0 and CoxOy content was respectively 0.25 wt.%and 2.5 at.%,the apparent quantum yield of CoxOy/Pt/TNSs ternary heterojunction has about 25.65%.The enhanced photocatalytic hydrogen production efficiency attributed to the formation of Z-type heterojunctions,which further improved the separation and transfer of photogenerated electron-hole pairs.This novel structure provides a large enough and close contact interface,which is more conducive to the rapid separation of photoelectric charge.The apparent enhancement of the photocurrent response observed in the CoxOy/Pt/TNSs composites demonstrates the effective transfer of charge between CoxOy,Pt0 and TNSs.This work creates new possibilities for designing efficient photocatalysts through interface engineering.Fourthly,using the special 4f5d energy level structure of rare earth metal oxide(TbtOy)as well as TbxOy itself is also a photocatalyst and Tb ions have variable valence(+3 and +4 valence),the surface-dispersed TbxOy/TNSs heterojunctions(Tb-TNSs)are constructed by a simple one-pot hydrothermal route.The microstructure,surface chemical state and photoelectric properties of TbxOy/TNSs heterojunctions are studied systematically.By the structural characteristics of the TNSs(large specific surface area and small-size thickness),the surface-dispersedTbxOy nanoparticles were loaded on the surface of TNSs.The rich 4f5d energy level structure of TbxOy is introduced into the TbxOy/TNSs heterojunction interface to expand the light absorption capacity of TNSs by controlling the proportion of added TbxOy to regulate the transformation from P25 nanoparticles(3D)to TNSs(2D).Appropriate amount of TbxOy/TNSs heterojunction(1.0%-Tb-TNSs)can not only enhance the light absorption capacity,but also can improve the separation efficiency of photo-generated electron-hole pairs and the utilization of samples.Besides,the as-synthesized Tb-TNSs samples show excellent photocatalytic activity for the removal of Cr(VI)under visible light irradiation.Initially the photocatalytic activity increased with the content of TbxOy,and then decreased after attaining the maximum at an optimal content(1.0 at.%)for degradation of aqueous Cr(VI)solution.The photocatalytic reduction of Cr(VI)is also significantly promoted by the addition of phenol.The synergism between Cr(VI)reduction and degradation of phenol which is demonstrated by measuring the effect of multiple usage of Tb-TNSs on its photocatalytic efficiency.Desorbed Tb-TNSs(Tb-TNSs-Des)are easily regenerated and reused for Cr(VI)removal with excellent performance.A possible alternate mechanism for the enhancement of photocatalytic activity under visible light irradiation is also proposed.Fifthly,two-dimensional TiO2-based nanosheets(TNSs)co-modified by surface-enriched carbon dots(CDs)and Gd2O3 nanoparticles:(Gd-C-TNSs)were synthesized by two-step hydrothermal route using Gd(NO3)3 and glucose as the carbon and gadolinium precursor,respectively.The CDs(2-3 nm)and Gd2O3 nanoparticles(1-2 nm)were highly dispersed over the surface of the TNSs.The co-modification by Gd203 nanoparticles and CDs influenced the crystallinity,crystal structure,and surface area of the TNSs.The adsorption capacity of obtained TNSs is insufficient when treating organic pollutants with high concentration dye solution.The co-modification by Gd2O3 and CDs not only results in the enhancement of adsorption capacity,but also causes the improvement of photocatalytic activity.It is ascribed to the smaller band gap of CDs than that of TNSs:On the one hand,the co-modification of Gd2O3 and CDs extended the light response;on the other hand,appropriate amount of modification(0.5 at%Gd/Ti and 3.0 g/L of CDs)facilitates the effective separation of photo-induced electrons and holes.It was found that Gd-C-TNSs can effectively remove the organic pollutants from high concentration methylene blue,which exhibited excellent reusability in the cyclic experiments.Sixthly,TiO2-based nanorods(TNRs)were self-assembled on the large graphitic carbon nitride(g-C3N4)sheets via solvothermal-assisted route using 2D-TNSs and g-C3N4 as precursor.The results demonstrated that the effective anchoring of TNRs(side length of ca.200～300 nm)was highly dispersed to the surface of whole g-C3N4 sheets.The shift in the Ti 2p XPS core level spectrum indicated an increase in the net positive charge of the Ti ions,ensuring the formation of interface between TNRs and g-C3N4.The charge transferred from g-C3N4 sheets to TNRs,effectively prevented the recombination of excited charges,which was consistent with the significant quenching of PL.The extent of visible-light-sensitive photocatalytic activity was evaluated by the removal of potassium dichromate(Cr(VI))or the degradation of RhB.The photocatalytic removal of Cr(VI)using RhB was effectively improved.Synergistic effect between removal of Cr(VI)and degradation of RhB was revealed on multiple utilization of TNRs/g-C3N4 for PC activity.The effective suppression of recombination of photo-induced charges and the absorption of RhB was responsible for the enhancement of photocatalysis.An alternate mechanism for enhanced visible-light photocatalytic activity was also discussed.In addition,a Z-scheme g-C3N4/Ag/MoS2 ternary plasmonic photocatalyst in a flower-like architecture of diameter about 0.4-0.6 μm was successfully synthesized by a reliable and effective method.The as-synthesized g-C3N4/AglMoS2 photocatalyst showed excellent improvement for visible-light absorption and separation efficiency of photo-induced electron-hole pairs.The g-C3N4/Ag/MOS2 system exhibited optimum visible-light-induced photocatalytic activity in degrading organic Rhodamine B(RhB),which was 9.43-fold and 3.56-fold of Ag/MoS2 and g-C3N4/MoS2 systems,and 8.78-fold and 2.08-fold in the production of hydrogen(H2)from water,respectively.The excellent photocatalytic activities were attributed to two aspects:one is the synergetic effects of Ag,g-C3N4 and MoS2 nanophase structures in the g-C3N4/Ag/MoS2 composites;the other is the Z-scheme mechanism which assisted fast separation and slow recombination of photo-induced electron-hole pairs.