| The population growth and industrial development have caused serious fossil fuels shortage and increasingly environmental deterioration.The exploration and utilization of clean and safe renewable energy sources are regarded as the key to alleviating these problems.Solar energy is the cheapest and abundant renewable energy.However,the conversion of solar energy into storable energy is still a challenge.Photocatalysis could achieve the direct storage and conversion of solar energy with the semiconductor materials.Since 1972,photocatalysis has been widely applied in water splitting,CO2 reduction,sewage treatment,indoor air purification,seawater utilization,nitrogen fixation and organic synthesis.Photocatalytic efficiency of semiconductors is concurrently restricted by the photon absorption,the production,separation and transfer of electron-hole pairs,and the surface redox reaction.Regulating the behavior of carriers is regarded as the crucial approachs to enhance the photocatalytic performance,which can affect the concentration of carriers and redox ability of free electrons/holes,but also restrict the thermodynamic/dynamic process and reaction pathways of photocatalytic reaction.The common regulation strategies for charge carriers include the construction of heterojunction,surface sensitization,surface modification with metal oxides and regulation of internal electric field.However,the current understanding and regulation of carrier processes are based on the weak interplay of electron-hole in semiconductors.As for the semiconductors with poor confinemenrt effect,the electrons and holes can break through the Coulomb interaction easily,then separated successfully and migrate to surface smoothly.However,in the semiconductors with strong confinement effect,the generated electron and hole would be bound by the Coulomb interaction to form an electroneutral particle-’exciton’,which is independent in space but interacted together.For most photocatalytic reactions,their catalytic processes are dominated by charge carriers.Thus,the production of exciton would restrict the conversion of photon to free electrons and holes severely,resulting in the decreased photocatalytic efficiency.Nowdays,two-dimensional layered materials have widely applied in the fields of energy storage and conversion.Compared with the bulk materials in photocatalysis,two-dimensional layered materials have drawn more attentations due to their appropriate aspect ratio and larger specific surface area,which are beneficial to the diffusion of charge carriers in bulk,and regulation of facets and surface defects.However,the robust confinement effect and poor shielding effect in two-dimensional semiconductors would induce the free electrons and holes to form bound excitons before the successful separation,deteriorating the carriers-dominated photocatalytic performances.Hence,the key for enhancing the photocatalytic efficiency of two-dimensional semiconductors is to tune their bulk structure,promoting the bound excitons to dissociate into free electrons and holes.BiOCl is a typical Ⅴ-Ⅵ-Ⅶ two-dimensional layered metal oxychloride with non-toxicity,earth abundance,optical and chemical stability,moderate energy band structure and adjustable electronic structure,which have been regarded as one of the optimum model materials for exploring the relationship of structure and photocatalytic performance.The alternating arrangement of[Bi2O2]layers and[Cl2]layers endows BiOCl with robust confinement effect,which can also be served as an ideal model to investigate the excitonic effect in two-dimensional layered materials.Thus,we regulate the layered structure of BiOCl nanosheets via the inventive molten salt,syngas-synthesis-like reaction-driven gas-phase exfoliation,ammonium oxalate mediated gas-phase exfoliation and surface peroxidation.We investigated the separation and migration process of charge carriers for BiOCl nanosheets via the regulation of excitonic effects,developing new strategies for manipulating the excitonic process in two-dimensional materials,deepening the comprehensive understanding of structure-activity relationship between the structure and excitonic effect of photocatalysts,and finally achieving the regulation and optimization of excitonic process in the carrier-involved photocatalytic reactions.The details of this dissertation are summaried as follows:1.We successfully synthesized homogeneously B-doped BiOCl nanosheets via an inventive molten salt strategy for achieving photocatalytic CO2 reduction with ultra-stability and high-selectivity.With B2O3 as both molten salt and doping precursor,this molten salt doping approach ensures boron(B)atom uniform dispersion from the surface into the bulk with dual functionalities.Bulk B doping mitigates the strong excitonic effect confined in 2D BiOCl by vastly reducing exciton binding energy,while the surface doped B atom reconstructs the BiOCl surface by extracting a lattice hydroxyl group,giving rise to an intimate B-oxygen vacancy(B-OV)associate.The exclusive B-OV associate enables spontaneous CO2 activation,suppresses competitive hydrogen evolution,and promotes the proton-coupled electron transfer(PCET)step by stabilizing*COOH for selective CO generation.As a result,the homogeneous B-doped BiOCl nanosheets exhibit a 98%selectivity for CO2-to-CO reduction under visible light,with an impressive rate of 83.64 μmol g-1 h-1 and an ultra-stability for long-term testing of 120 h.2.Here we report an efficient,pure water CO2-to-CO conversion photo-catalyzed by sub-3-nm-thick BiOCl nanosheets featuring van der Waals gaps(VDWGs)on the two-dimensional facets,a novel graphene-analogue motif distinct from the majority of previously reported nanosheets,which bear VDWGs on the lateral facets.Compared with bulk BiOCl,the VDWGs-rich atomic layers possess a weaker excitonic confinement power that devalues the exciton binding energy 3.8-fold to 36 meV and consequently yields a 50-fold enhancement in the bulk charge separation efficiency.Moreover,the VDWGs facilitate the formation of VDWG-Bi-Vo¨-Bi,a catalytically-active defective configuration that can accelerate the CO2-to-CO transformation via the synchronous optimization of CO2 activation,*COOH splitting,and*CO desorption.These improvements in both exciton-to-electron and CO2-to-CO conversions result in a PCR rate of 188.2 μmol g-1 h-1 under visible light in pure water without the involvement of co-catalyst,hole scavenger,or organic solvent These results suggest that increasing VDWG exposure could pave a new way to high-performance solar-fuel generation systems.3.We developed a novel visible-light-driven HClO production route from seawater(Bohai Sea,China)by homogeneously N-doped BiOCl ultrathin nanosheets with a spatial charge separation functionality.Due to facet heterojunction,the {110} facet,with a prominent Cl reservoir nature,favors hole trapping on lattice Cl,enabling one-step Cl2 evolution(Cliattice+Cl-+h+→ Cl2+Clvacancy).In formed Cl vacancy is readily replenished by dissociative Cl-owing to the van der Waals force in the confined layer(Clvacancy+Cl-+h+→ Cllattice).Disproportionation of Cl2 produces HClO(Cl2+H2O→ HClO+HCl).Meanwhile,in the presence of dissolved CO2,CO is concurrently evolved from the {001} facet by an intentionally-introduced N that serves as a Lewis base site for CO2 reduction.The importance of such a concurrent CO2 reduction along with HClO production(CO2+Cl-→ CO+ClO-)provides a possibility mitigate ocean acidification arisen from the uptake of superfluous atmospheric CO2.A superior visible-light-driven production rate of HClO(87.15 μmol h-1)and CO(95.45 μmol h-1)is achieved for the first time.The insights gained from this work offer a practical starting point for designing advanced photocatalytic processes for resource utilization of seawater in the chemicals industry.4.Based on the understanding of band structure and excitonic effect of BiOCl,we synthesized the van der Waals band gap-rich BiOCl ultrathin nanosheet via a hydrothermal method,which was then treated in hydrogen peroxide to construct the oxygen-rich surface structure.It was revealed that the oxygen-rich surface of BiOCl can weaken the contribution of halogen ions to excitonic effect by tuning the elemental ratio of surface chemical composition,promoting the bound exciton to dissociate into free charge carriers and further inhibiting the annihilation of high-concentation excitons.The BiOCl ultrathin nanosheet with oxygen-rich surface also contributes to the formation of valence band and enhanced the transfer rate of holes in the valence band.Especially,BiOCl nanosheets with the surface oxygen-rich structure can activate molecular oxygen into reactive oxygen species(·O2-and·O22-)via the charge carriers-dominated photocatalytic process.Meanwhile,the accumulated holes on catalyst surface can oxidize ·O2-and ·O22-into singlet oxygen(1O2)via the charge transfer pathway,which can directly oxidize the gasous NO into NO3-without the emission of high-toxicity NO2. |
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