Design, Synthesis And Application Of One-Dimensional Chalcogenide Heteronanostructures

Semiconducting nanomaterials are particular attractive for energy conversion and storage due to the quantum confinement effects dictating their unique optical and electronic properties. Advanced synthesis strategies have been developed during the past decades to control size, shape and composition of single nanocrystals. However, it is often difficult to simultaneously achieve sufficient efficiency, stability and low cost in luminescent and energy conversion devices with a single material component. The heteronanostructures with multiple integrated functional components could combine the advantages of different components and often even obtain synergetic properties exceeding the functionality of individual components. In particular, chalcogenide-based heterostructures are attracting significant attention, owing to the unique physical and chemical properties decided by their energy band gaps. Design rational synthesis approaches offering high degree of control over compositon, size and morphology of novel heteronanostructures and thus achieving high conversion efficiency and long-term performance stability for applications correlated to structural architectures will be highly desired, but remains a challenge.The present dissertation will focus on the design, synthsis of unique one-dimensional (ID) chalcogenide heteronanostructures for the expanding applications in energy conversion. We designed the controllable preparation strategies to synthesize1D chalcogenide heteronanostructures with various fine structures, which can be further used as excellent template materials for preparing other novel and complex hybrid architectures through a series of chemical transformations. We studied heterogenous growth mechanisms of novel heteronanostructures for developing a facile and general method to prepare more novel heterostructures. We investigated the band gap structure simulations, detailed charge carrier behavior and unique optoelectronic properties of the prepared hybrid nanostructures. The main results can be summarized as follows:1. A seed mediated colloidal solution-phase growth method was developed for preparing binary chalcogenide heteronanostructures (Cu2S-PbS and Ag2S-PbS). Ionic semiconductors (Cu1.94S, Ag2S) were prepared as seeds for the reason that cations in these semiconductors behave virtually like a "fluid" in the high temperature reaction solution, endowing them to catalyze the nucleation and growth of other chalcogenide semiconductors on their surface. Unique Cu2S-PbS heteronanostructures have good photothermal conversion effect due to the synergistic effect. Using tiny Ag2S nanocrystals as catalysts can prepare ultrathin ZnS nanorods/nanowires. Chloride ions introduced in the reaction induced the controlled morphology transition from straight to kinking.2. A novel strategy was developed for fabricating ternary semiconductor-semiconductor-metal heteronanorods. Basing on previously prepared ZnS nanorods, we constructed unique1D ternary heteronanorods-[S1-(S2/M)]-S1-[S1-(S2/M)]-S1-, with segmented node sheaths S2decorated by M (SI:ZnS; S2:CdS; M:Au, Pd, Pt) through the chemical transformation strategy. The ternary hybrids were prepared by the heterogenous growth of binary multi-node sheath-[ZnS-CdS]-ZnS-[ZnS-CdS]-ZnS-heteronanorods, which were transformed from single component ZnS nanorods via sequential cation exchange. Compared to traditional core-shell or simple multiple hybrids, the structural characteristic of1D multi-node sheath gives rise to increased availability for light absorption and continuity for charge transport. More importantly, selective growth of metal on the semiconductor with smaller band gap (CdS node sheath) results in the type-Ⅰ-to-type-Ⅱ conversion due to Fermi-level alignment (first-principles simulations). This ensures the delivery of photo-generated electrons from the CdS node sheath not only to the metal surface but also to the exposed ZnS stem, forming two electron-rich active centers. The charge-separation efficacy in this unique ternary nanosystem has been verified by ultrafast spectroscopic characterizations and demonstrated by performance improvement of optical-to-electrical conversion.3. A post-synthetic processing technology was developed for producing heteronanostructures with more functions. To achive more functionalities of fabricated novel and complex heteronanostructures, we investigated the post-synthetic modification of binary multi-node sheath ZnS-CdS heteronanorods, including structural reconstruction, Mn doping and SiO2encapsulation. Binary multi-tetraheron sheath ZnS-CdS heteronanorods were synthesized with dodecanethiol surfactant via regrowth process (structural reconstruction), which further transformed to ternary ZnS-(CdS/Au) with Au only being grown on the vertexs and edges of CdS tetrahedron sheaths to improve the performance of optical-to-electrical conversion. Mn:ZnS-CdS heteronanorods were prepared by the tactics of first doping and then transformation. Doping Mn in binary ZnS-CdS changed the antimaganetic nanomaterial to be paramagnetic and obtained better performance in optoelectronic application. Selective coating ZnS-CdS@SiO2and entire coating (ZnS-CdS)@Si02heteronanowires were fabricated through the ligand exchange and reverse microemulsion method. The silica-encapsulated binary multi-node sheath heteronanowires brings the compatibility of hydrophobic heteronanostructures in hydrophilic solvents, and improves the safety in applications on account of the silica protection.4. A synthetic technique was designed and developed for synthesizing ternary chalcogenide heteronanorods with full-spectrum absorption. ZnS-CdS heteronanostructures exhibit good absorption in ultraviolet and visable light. For further exploit solar energy to reach the almost full-spectrum absorption (UV+vis+NIR), we designed and synthesized ternary multi-node sheath chalcogenide heteronanorods (e.g. ZnS-CdS-Cu2S, ZnS-CdS-Ag2S, ZnS-CdS-PbS) by the cation exchange procedure in binary ZnS-CdS heteronanorods so as to incorporate NIR absorption component (Cu2S, Ag2S, PbS). ZnS-CdS-Cu2S was chosen as a case study for optical-to-electrical conversion. Cu2S-CdS in ternary system forms P-N junction and results in the construction of type-Ⅱ heterojunction. This band gap structure promotes the delivery of photo-generated electrons from the conduction bands of ZnS and Cu2S to that of CdS, while holes transfer to the valence band of CU2S. This result ensures the electron-hole separation, that was further demonstrated by photocatalytic test. Quaternary ZnS-CdS-(Cu2S/Au) heteronanostructures were constructed by the post-synthetic modification of ternary ZnS-CdS-Cu2S. This unique quaternary complicated heteronanorod provides a special model for studying the heteronanostructures.