| Solar energy is among the most promising sustainable energy sources that has gained increasing attention of the science and technology world under the context of fossil fuels shortage and environmental deterioration.Emerged at the right moment,semiconducting photocatalysis is a trustworthy approach to convert solar light to chemical energy.The past few decades have witnessed tremendous efforts in harvesting solar light for diversified useful purposes like environmental control,water splitting,fuels production and nitrogen fixation,via the semiconductors-based photocatalytic approach.An overall photocatalytic process typically involves four key steps,i.e.light absorption,charge generation,separation and consumption.To be specific,upon solar light excitation across the bandgap,photoexcited electrons are promoted to the conduction band(CB),leaving positively charged holes in the valence band(VB).Charge carriers,who possess sufficient time to be separated,may migrate to the surface to participate in redox reactions.At the semiconductor surface,electrons can reduce electron acceptors,such as O2,H2O,CO2 and N2,to generate reactive oxygen species,hydrogen,methane and ammonia,respectively.In turn,surface holes can recombine with typical donor species,causing the oxidation of H2O to O2 or mineralization of dissolved organic compounds.Only when these three indispensable and complementary key steps are fully completed can a desirable photocatalytic efficiency be achieved.Unfortunately,far from practical application,state-of-the-art photocatalytic techniques exert dissatisfactory quantum yield.Many researches assume that poor absorption of semiconductors to visible light,which accounts for about 47%of solar irradiance,and robust charge carriers recombination within several picoseconds,are the culprits of the poor solar energy conversion efficiency.However,the confluence of modern material characterization techniques and electronic-structure computations is providing solid evidences that on real photocatalysts,surface defects matter more as compare with the bulk structure and composition,corresponding to the heterogeneous catalytic nature of semiconductor-based photocatalysis.It has been documented that a variety of intrinsic physical properties of semiconductors,including electrical conductivity,carrier diffusion kinetics,thermoelectric power and optical absorption are determined by surface defects even in infinitesimal concentration.More recently,accumulative data uncovers that photocatalytic reactivity and selectivity can be remarkably modified by the presence of defects,oxygen vacancies(OVs)mainly.OVs are the most prevalent and widely-studied anion defects identified so far with a relative low formation energy on oxide surfaces.OVs of coordinating unsaturated character not only facilitate substrate adsorption by providing more dangling bonds,but also modulate the electronic states of substrate by activating unreactive chemical bonds via localized back electron donation,fatefully leading to distinct reaction pathways.In the light of this,an in-depth scenario depicting the interplay between OVs and the kinetics,energetics and mechanisms of photocatalytic processes remains great expectation.Although great efforts have been devoted to illuminate the inherent functionality of OVs in photocatalysis at the surface molecular level,research in this area is still in its infancy,leaving enormous room for further advancing.Bismuth oxychloride(BiOGl)is a typical "V-VI-VII ternary oxide material,which recently has attracted considerable attention in the area of photocatalysis due to its non-toxicity,earth abundance and chemical stability.Besides,due to its layered structure,BiOCl easily anisotropically crystallizes into single-crystalline 2D nanosheets,offering high fraction of exposed surface.Different from other semiconductors,BiOCl can easily generate OVs under oxygen-deficient conditions or during photocatalysis under solar light due to the surface long Bi-O bond of low energy.These merits make 2D BiOCl an ideal platform for providing atomic-level insights into the influence of OVs on photocatalytic processes.Therefore,the aim of this doctoral dissertation is to provide molecular level mechanistic insight into the influence of OVs on the photocatalytic processes.The understanding of OVs chemistry presented in this doctoral dissertation can help to underpin and advance the fundamental theories of photocatalysis,but also offer new perspectives and guidelines for the rational design of catalysts with satisfactory performance.The main contents of this doctoral dissertation are summarized as follows:1.Photocatalytic molecular oxygen activation is related to the surface chemistry of the oxide semiconductors,thus highly depend on the interaction between molecular oxygen and oxide surface.We developed a facile solvothermal method to confine OVs of different concentrations on the(001)surface of BiOCl and clarified the intrinsic roles of OVs for enhanced photocatalytic molecular oxygen activation.Presence of OVs could induce the formation of localized electronic states beneath the CB of BiOCl,endowing BiOCl certain visible light absorption capability.Besides,theoretical and experimental results revealed OVs on BiOCl(001)surface selectively activated molecular oxygen to superoxide radicals(·O2-),which could be further transformed into other reactive oxygen species via the ·O2-?O22-?H2O2?·OH pathway.These reactive oxygen species endowed BiOCl superior reactivity for chlorophenols removal like pentachlorphenol,because selective generation of ·O2-could accelerate the dechlorination of pentachlorphenol,which favored the subsequent benzene ring cleavage toward mineralization.These findings shed light on the importance of OVs for efficient molecular oxygen activation,and provide new insight for manipulating reactive oxygen species for certain pollutants removal.2.Water chemistry on oxide surfaces is of great significance for environmental control and water splitting,and a central issue is to understand water adsorption mode and related reactivity.We systematically studied the adsorption of water on BiOCl surfaces in a combined theoretical and experimental approach,and evaluated the capability of water molecules in different adsorption states for the trapping of holes.More than just enhancing the adsorption of water on oxide surfaces,we demonstrated that OVs also offered a possibility of activating water toward thermodynamically enhanced photocatalytic water oxidation,while the water activation state were strongly dependent on the structures of OVs.OVs of the BiOCl(010)surface with dissociatively adsorbed water exhibited much higher photoreactivity toward oxidation in comparison to those of the BiOCl(010)surface with molecularly adsorbed water,generating more water-oxidized species such as ·OH,H2O2,and O2.It was proposed that OVs of the BiOCl(010)surface could facilitate the barrierless O-H bond breaking of the first proton removal reaction(H2O + h+ ? ·OH + H+),and also allow more electron transfer from the OVs to dissociatively adsorbed water,leading to a higher water activation level.These findings highlight the indispensable role of OVs for water activation toward oxidation and open up new avenues for the rational design of new water oxidation photocatalysts.3.H2O2 is a very important intermediate during either the O2 activation or water oxidation on the OVs of BiOCl.Meanwhile,as one of most important reactive oxygen species,transportation and transformation of H2O2 on photocatalysts surface is of considerable importance in the area of environmental catalysis.In this study,we theoretically and experimentally studied the adsorption and reaction of H2O2 on the OVs of BiOCl surfaces.H2O2 was mainly molecularly adsorbed on defect-free BiOCl surfaces via hydrogen bond or Lewis-acid interactions.However,on the OVs,H2O2 could be spontaneously dissociated with a-OH group being pinned at the OV,and another-OH fragment being formed,while the existing form of ·OH was closely related to the delicate structure of OVs.Generated-OH tended to diffuse away from BiOCl(001)surface,preferring to oxidize dissolved substrate with high reactivity in solution,while on BiOCl(010)surface,·OH chose to stay on the surface,reacting with strongly-adsorbed substrate with high priority.The interaction scheme between OVs and H2O2 is a new but very important reaction path for the consumption of H2O2 during photocatalysis,which also allows the selective control ·OH existing form for targeted catalytic reactivity.4.Photocatalytic nitrogen fixation is a very promising technique to overcome the energy-consuming limitation of industrial ammonia synthesis,while its further development is remarkably restricted by the poor reaction efficiency and elusive reaction mechanism.In this study,we first drew inspiration from biological nitrogen fixation,introducing the OVs on BiOBr surface,and then studied the influences of OVs for molecular nitrogen adsorption,activation and fixation.OVs with abundant localized electrons could efficiently activate the N-N triple bond of molecular nitrogen to enhance its thermodynamics toward reduction.Besides,we further demonstrated distinct structures of OVs on different BiOCl surfaces strongly determined the N2 fixation pathways by influencing both the adsorption structure and the activation level of N2.The fixation of "terminal end-on bound N2" on the OVs of BiOCl(001)surface followed an asymmetric distal mode by selectively generating NH3 via N2 ? N-NH3 ? 2NH3,while the reduction of "side-on bridging N2"on the OVs of BiOCl(010)surface was fixed in a symmetric alternating mode to produce N2H4 as the main intermediate via N2? N2H2 ? N2H4.Moreover,protons from water assisted the interfacial electron transfer during photocatalytic N2 fixation processes due to efficient N2 activation by OVs.These findings clarifies the contributions of OVs for enhanced N2 activation/fixation,which are crucial for the ultimate establishment of a truly catalytic system for solar ammonia synthesis.5.Photocatalysis is a promising technology to use clean and abundant O2 in the air for selective oxidation of alcohols,while the both the conversion efficiency and selectivity are seriously impeded by poor interaction of O2 with semiconductor surfaces and strongly-oxidative photoinduced holes.In this study,via combining the characteristics of OVs for enhanced O2 activation and hot holes of mild oxidizing capability within plasmonic Au,we for the first time synthesized a Au/BiOCl nanocomposite.The OVs were first demonstrated to facilitate the separation of plasmonic hot carriers through trapping hot electrons.Besides,OVs could also work cooperatively with nanosized Au for efficient benzyl alcohol oxidation with high selectivity.During the process of benzyl alcohol oxidation,OVs on BiOCl facilitated the trapping and transfer of plasmonic hot electrons to adsorbed O2,producing ·O2-radicals,while plasmonic hot holes remaining on the Au surface mildly oxidized benzyl alcohol to corresponding carbon-centered radicals.Concerted ring addition of these two radical species on the BiOCl surface highly favored the production of benzaldehyde along with an oxygen atom transfer from O2 to the product.The results and understanding acquired in this study reveal the dual role of OVs for efficient plasmonic hot carriers separation and O2 activation,and will contribute to the development of more active and/or selective plasmonic catalysts for a wide variety of organic transformations. |
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