Novel One-dimensional Semiconducting Heteronanostructures:design,Precise Synthesis,and Applications

With the booming industrialization and modernization,the environment and energy issues are lying at the central of our concerns.Solar-driven chemical conversion,a sustainable process that dissociate and associate chemical bonds via solar energy,has proven to be an ideal way to address above issues by producing clean and efficient fuels.To achieve the conversion towards specific chemicals,photocatalysts with high efficiency and high selectivity are essential.However,performances of most photocatalysts to date have been limited by shortcomings such as poor light-harvesting ability,low charge-separation efficiency,slow reaction rate,and poor reaction selectivity.One-dimensional colloidal heteronanostructures are attractive thanks to their tunable compositions and architectures,as well as unique electronic structure.However,the rational design and precise synthesis of novel one-dimensional heterostructured semicondutors to achieve high photocatalytic activity and selectivity remains challenging.This dissertation focuses on the design,colloidal synthesis and photocatalytic performances of one-dimensional heterostructured semiconductor nanorods/nano wires.Based on theoreticial design,we fabricated a spectrum of one-dimensional colloidal heterostructured photocatalysts that enables high-efficiency and high-selectivity solar-to-chemical conversion.Combined with time-resolved spectroscopy and photocatalytic reactions,we verified advantages of those design strategies.The main findings are summarized as follows:1.Axially segmented colloidal quantum-dots-in-nanowire(QDNWs)heterostructures were designed and fabricated to achieve facet-selective passivation of QDs.We took the view that epitaxially stacking QDs in nanowires can selectively passivate defective facets in QDs,avoiding non-radiative recombinations of photo-generated carriers at surface traps,while simultaneously enables efficient carrier transfer onto QD surface for redox reactions,thus largely enhancing photocatalytic acitivity.A synthetic strategy,termed pulsed axial epitaxy,was proposed to construct such QDNW architecture based on the catalyst-assisted growth.The incorporation energy differences of distinct semiconductor atoms into host catalysts determines their nucleation sequences at the catalyst-nanowire interface.Thus,we can dynamically switch the reaction precursors to alternately grow QDs and nanowires,thereby stacking multiple CdS QDs in ZnS nanowires.Such synthetic strategy is highly flexible and can precisely control the size,number,spacing and crystal phase of quantum dots.First-principles calculations and time-resolved spectroscopy analyses show that the selective passivation on(111)facets of CdS QDs by ZnS nanowires avoids the localization of charge carriers at surface traps and significantly increases the lifetime of photo-generated electrons.Consequently,the photocatalytic hydrogen production rate of the QDNWs is an order-of-magnitude higher than that of plain CdS quantum dots.The facet-selective passivation effect of QDNW architecture provides a new pathway to rationally design high-efficiency photocatalysts.2.A library of axial superlattice nanowire photocatalysts were designed and fabricated to address the shortcomings in conventional photocatalyst architectures.Segments with distinct compositions and structures are segmentedly along the nanowire to form axial superlattice nanowires,which are benificial to fully utilizing sunlight,flexibly designing band structures,and controlling axial length that fits carrier diffusion length,thus holds great promises in solar-driven chemical reactions.A programmable synthetic methodology was proposed to preciesely synthesize a series of colloidal axial superlattice nanowire photocatalysts with atomically sharp interfaces,offering opportunities for deep understanding in structure-property relationships.With selective cation/anion exchange reactions from CdS-ZnS QDNWs,different photocatalytic components were integrated into the superlattice nanowire architectures.As a result,more than 23 axial superlattice nanowire photocatalysts,including P-N junctions and semiconductor-cocatalyst hybrids,were constructed with precise control over their compositions,dimensions,interfaces,and periodicity.3.A spectrum of colloidal heteronanorods(HNRs)with regioselective magnetization were designed and fabricated to achieve optical activity(chirality).Compared with the convertional electric-dipole modulation strategy for chirality,we proposed that local magnetic fields at specific sites of nanostructures offer routes to achieve stable optical activity by modulting magnetic dipole moments,thereby avoiding problems such as poor environmental stability and poor conductivity of traditional chiral photocatalyst during asymmetric photosynthesis.Using a "double buffer-layer epitaxy" strategy,Fe3O4 magnetic nanoparticles were selectively grown at one tip of a spectrum of ZnxCd1-xS HNRs to achieve regioselective magnetization.Circular dichroism(CD)spectroscopy studies have shown that all HNRs have the same chirality.This work opens an exciting avenue toward engineering novel chiral nanomaterials for asymmetric photocatalysis.