Application Of Sulfur-resistant [Mo₃S₁₃]^2- Nanoclusters In Photocatalytic Degradation Of Organic Pollutants And Photocatalytic Hydrogen Evolution

The growing global environmental contamination and energy crisis especially in developing country have attracted worldwide attention as threat for sustainable development of human society.Photocatalysis is one of the most promising potential technology candidates for organic pollutants degradation and hydrogen generation due to its low cost,high efficiency and utilization of solar energy.Unfortunately,the classical and most widely used photocatalysts of TiO₂ can only utilize the UV light,which covers only 4%of the whole solar spectrum.Although many visible light harvesting catalysts have been developed to extend their activities to the visible light,the commercial application of photocatalysis is limited due to the low efficiency,poor stability and high cost.According to the extensive investigations,the development of a more reliable and lowcost photocatalyst that can be activated by visible and solar light,or both,should be explored further for potential application in wastewater treatment and hydrogen generation.Recently,molybdenum sulfide materials（MoS_x）have emerged as a promising cocatalyst due to its high activity,earth-abundant composition,low cost,and especially sulfur resistance.In order to overcome the above disadvantages of photocatalysts,in this dissertation,we combined the active[Mo₃S₁₃]^2-nanoclusters with photocatalysts such as BiWO₆,BiOBr,CdTe QDs and CdTe/CdS QDs to facilely assemble the composite photocatalysts for wastewater treatment and H₂ evolution with enhanced efficiency and photostability.Mainly works were carried out.The research results are as follows.We have prepared the novel photocatalyst of Mo₃S₁₃^2-/Bi₂WO₆nanocomposite.The thiomolybdate[Mo₃S₁₃]^2-nanoclusters were facilely deposited onto hydrothermally synthesized Bi₂WO₆ nanoparticles.The Mo₃S₁₃^2-/Bi₂WO₆ composites have the wider absorption of visible light compared with pure Bi₂WO₆.Our experiments show that[Mo₃S₁₃]^2-nanoclusters are an excellant photocatalysts’cocatalysts,which is comparable to Pt.The results indicate that the novel nanocomposite exhibits unprecedented photocatalytic activity for the decolorization of RhB and MB.The improvement of Mo₃S₁₃^2-/Bi₂WO₆ photocatalyst under visible light is ascribed to the enhanced charge separation efficiency of photogenerated h⁺/e^-pairs,which is promoted by the effective charges transfer between Bi₂WO₆ nanoparticles and the co-catalysts of[Mo₃S₁₃]^2-clusters.Mo₃S₁₃^2-/Bi₂WO₆ nanocomposite with high photocatalytic activities,stability and recyclability is a promising candidate for the potential environmental remediation especially in wastewater treatment.The trapping experiments demonstrate that h⁺and·O^2-should be the main active species in the visible light photocatalytic degradation of RhB and MB process,while·OH played a little role for the RhB photodegradation process.The photocatalytic mechanism results indicate that[Mo₃S₁₃]^2-clusters,as“platinum-like”cocatalyst,promote the chemical reaction owing to the fast surface electron transfer rate.The photogenerated electrons can easily transfer to the oxygen molecules（O₂）adsorbed on the surface of the Bi₂WO₆.The trapping and releasing process of electron can effectively reduce the h⁺and e^-recombination,which contribute to dyes degradation by the h⁺leaving in the valence band.In addition,the released electrons react with O₂ to form another active species（·O₂^-）for dyes degradation.To further widen the absorption of visible light of the nanocomposites,BiOBr has been selected as the photocatalysts for coupling with active[Mo₃S₁₃]^2-nanoclusters.Wepreparedthe[Mo₃S₁₃]^2-/BiOBr nanocomposites via a facile hydrothermal method.The results indicate that[Mo₃S₁₃]^2-clusters were successfully deposited on the surface of BiOBr microspheres.The reflectance of the[Mo₃S₁₃]^2-/BiOBr composites is gradually red shifted compared with pure BiOBr due to the coupling of these brown[Mo₃S₁₃]^2-nanocluster onto BiOBr.We investigated the photodegradation of RhB and MB under visible light irradiation catalyzed by our[Mo₃S₁₃]^2-/BiOBr nanocomposites.This novel nanocomposite exhibits an admirable photocatalytic activity comparable to state of art Pt/BiOBr photocatalyst.The enhanced visible light photocatalytic activity of[Mo₃S₁₃]^2-/BiOBr composites is mainly contributed to the cocatalysts of[Mo₃S₁₃]^2-clusters,which effectively promote the photoinduced charges transfer and separation.[Mo₃S₁₃]^2-cluster is a promising Pt alternative cocatalyst for photocatalyst.Furthermore,sulfur resistance test demonstrated that[Mo₃S₁₃]^2-/BiOBr nanocomposite shows enhanced sulfur tolerant photocatalytic activities while the Pt/BiOBr would be total deactivated by Pt’s sulfur poison.The[Mo₃S₁₃]^2-/BiOBr composites exhibit only little decrease in photocatalytic activities after five cycles of photocatalytic decolorization MB.Therefore,the low cost,excellent photocatalytic active and photocatalytic stability of[Mo₃S₁₃]^2-/BiOBr nanocomposite enable it to be a promising candidate for the environmental remediation on sulfur containing water pollutant.The trapping experiments demonstrate that the decolorization of dye over[Mo₃S₁₃]^2-/BiOBr composite is dominated by the direct hole and the generated·O^2-radicals oxidation process.The analysis of photocatalytic mechanism indicate that the photogenerated electrons could easily migrate from the inner region to the surface to participate in the surface reaction to form active radicals（·OH and·O₂^-）.The surface-adsorbed O₂ as an electron acceptor could react with electrons to generate·O₂^-which could further transform to·OH via a series of reactions with H⁺.The deposited[Mo₃S₁₃]^2-clusters can act as electron traps to facilitate the photogenerated h⁺and e^-separation for BiOBr composite.The photogenerated electrons are trapped by O₂ to form active oxygen species like·O₂^-which could effectively oxidize dyes.On the other hand,photogenerated holes not only directly decompose the organic compounds but also accumulate at the valence band（VB）of BiOBr to react with H₂O to form hydroxyl radical·OH,which is the photo–oxidation process.CdTe QDs have a better capacity for visible light absorption compared with Bi-based photocatalysts（Bi₂WO₆ and BiOBr）.However,the surface of CdTe QDs contains large amount of sulfur-containing compounds as a stabilizer,which could poison the noble metal cocatalysts especially platinum.[Mo₃S₁₃]^2-clusters have emerged as a promising cocatalyst due to its high activity,earth-abundant composition,low cost,and especially sulfur resistance.Therefore we have reported a noval“artificial catalyst”,which combine the advantages of CdTe QDs and[Mo₃S₁₃]^2-nanoclusters to enhance the photocatalytic activity and stability for H₂ evolution.Our experiments show that the optimal conditions of[Mo₃S₁₃]^2--CdTe for photocatalytic H₂ evolution is in the presence of 20mL aqueous solution containing CdTe QDs(CdTe,6.5×10^-6 M),[Mo₃S₁₃]^2-nanoclusters(4.6×10^-6 M),and H₂A（20 mg/mL）present at pH 4.5.Irradiation of[Mo₃S₁₃]^2--CdTe QDs under the optimal condition would result in efficient H₂ evolution with TON of 2 653 with respect to the[Mo₃S₁₃]^2-clusters,respectively,for 12 h illumination（λ>420 nm）,which presents higher photocatalytic activity than that of Pt-CdTe QDs.[Mo₃S₁₃]^2--CdTe can retain 85%of the initial H₂ evolution rate even after four cycle（48 h）without renewing the sacrificial agents,suggesting the good stability of the[Mo₃S₁₃]^2--CdTe.The mechanism for photocatalytic H₂ evolution by[Mo₃S₁₃]^2--CdTe also has been proposed.Firstly,the surface dangling bonds(Cd²⁺)would couple with[Mo₃S₁₃]^2-clusters to form the[Mo₃S₁₃]^2--CdTe before irradiation.Secondly,the e^-/h⁺pairs are generated upon photoexcitation of[Mo₃S₁₃]^2--CdTe under the irradiation of visible light（λ>420 nm）.Then[Mo₃S₁₃]^2-clusters would capture the electrons which are transferred from the surface of the excited CdTe QDs.Therefore the bridging S₂^2-and terminal S₂^2-atoms react with protons to generate H₂.In addition,the formed h⁺that remains in the[Mo₃S₁₃]^2--CdTe after the electron transfer is regenerated by the sacrificial electron donor of H₂A.In consequence,the catalytic cycle for the evolution of H₂ is achieved.Although CdTe QDs with tunable band gaps has become one of the promising candidates for efficient visible-light-driven H₂,low photostability limits their large scale application.In order to overcome the low photocorrosion resistance of CdTe QDs,most straightforward strategies are to construct the core/shell heterostructures to improve the photostability.CdTe/CdS core/shell QDs aqueous solution is formed by growing a controllable CdS shell on CdTe cores with a facile aqueous synthesis.For further facilitate charge separation,we combine the stable CdTe/CdS core/shell QDs with active[Mo₃S₁₃]^2-nanoclusters to facilely assemble the[Mo₃S₁₃]^2--CdTe/CdS photocatalyst for photocatalytic H₂evolution with enhanced efficiency and photostability.Our experiments show that the optimal conditions of[Mo₃S₁₃]^2--CdTe/CdS for photocatalytic H₂ evolution is in the presence of 20 mL aqueous solution containing CdTe/CdS QDs(CdTe,6.5×10^-6 M),[Mo₃S₁₃]^2-nanoclusters(4.6×10^-6 M),and H₂A（20 mg/mL）present at pH 2.5.According to the H₂ evolution of Pt-CdTe,Pt-CdTe/CdS,[Mo₃S₁₃]^2--CdTe,and[Mo₃S₁₃]^2--CdTe/CdS under the optimized conditions,we concluded that the sulfur ligand covered CdTe/CdS QDs are facilely decorated with the low cost sulfur tolerant[Mo₃S₁₃]^2-nanoclusters to exhibit enhanced visible-light photocatalytic H₂ generation as compared to that of the CdTe/CdS QDs catalyzed with classical cocatalysts of Pt.The photocatalytic H₂ evolutions of Pt-CdTe/CdS and[Mo₃S₁₃]^2--CdTe/CdS are improved comparing with the optimal system of Pt-CdTe and[Mo₃S₁₃]^2--CdTe.With the help of a stable CdS shell on the CdTe cores,the CdTe/CdS core/shell QDs exhibit better charge carrier separation and photostability due to the passivated surface.Irradiation of[Mo₃S₁₃]^2--CdTe/CdS QDs under the optimal condition would result in efficient H₂ evolution with TON of 12788 with respect to the[Mo₃S₁₃]^2-clusters,respectively,for 20 h illumination（λ>420 nm）.In addition,the potentialmechanismforphotocatalyticH₂evolutionby[Mo₃S₁₃]^2--CdTe/CdS has been proposed.Before irradiation,the surface dangling bonds(Cd²⁺)and sulfur vacancies on the surface of the CdTe/CdS QDs interact with[Mo₃S₁₃]^2-clusters to form the[Mo₃S₁₃]^2--CdTe/CdS QDs photocatalyst in situ.With the irradiation of visible light（λ>420 nm）,the e^-/h⁺pairs are formed upon photoexcitation of[Mo₃S₁₃]^2--CdTe/CdS QDs.Electrons transfer from the excited CdTe/CdS QDs are captured by[Mo₃S₁₃]^2-clusters.The bridging S₂^2-and terminal S₂^2-atoms from[Mo₃S₁₃]^2-clusters have the catalytic active for H₂evolution.Therefore the bridging S₂^2-and terminal S₂^2-atoms further reacts with protons to generate H₂.On the other hand,the formed h⁺that remains in the[Mo₃S₁₃]^2--CdTe/CdS QDs after the electron transfer is regenerated by the sacrificial electron donor of H₂A.In consequence,the catalytic cycle for the evolution of H₂ is achieved.