Shape-Controlled Metal-Semiconductor Heterostructured Nanocrystals for Photocatalytic Hydrogen Generation

Abstract

Colloidal heterostructured nanocrystals have recently gained keen attention due to the synergistic properties arising from different compositions within a single nanocrystal. Particularly, colloidal metal-semiconductor heterostructured nanocrystals have been extensively studied from the development of new synthetic chemistry, characterizations, carrier dynamics and various applications such as self-assembly, photocatalysis, bioimaging, electrical devices, and so forth. Furthermore, complex heterostructures consisting of multicomponents of two or more semiconductors and metals were reported to precisely engineer the morphologies as well as electronic properties of each component to achieve high-performance heterostructured nanomaterials and devices. Among many applications with the metal-semiconductor hybrid nanomaterials, the use of these heterostructured hybrid nanomaterials for photocatalysis has gained much attention due to increased interest in exploring new sources of renewable energy. Novel metals such as Au or Pt were typically employed as cocatalysts when combined with semiconductor nanomaterials with light-absorbing properties to generate excitons. Proper bandgap engineering of these multicomponent hybrid nanocrystals enabled the increase in photocatalytic hydrogen generation efficiencies by splitting water with electrons and holes generated. More specifically, platinum-incorporated cadmium chalcogenide semiconductor nanocrystals have been studied to improve the efficiency of photocatalytic hydrogen generation reaction. Several research groups have already reported on the incorporation of Pt onto cadmium chalcogenide semiconductor nanocrystals with different synthetic mechanisms as well as with different morphologies. Tipping of Pt nanocrystals at the ends of CdSe or CdS nanorods and tetrapods has been introduced, followed by the decoration of Pt nanocrystals by the photodeposition of Pt precursors under irradiation with certain wavelength of light. These synthetic efforts enabled to design new heterostructured nanocrystals and to study the carrier dynamics between Pt metal cocatalysts and CdSe or CdS semiconductor photocatalysts. Since metallic Pt nanocrystals act as reaction sites for the hydrogen evolution by combining with electrons generated from semiconductor nanocrystals, studies on the effect of Pt nanocrystals for the photocatalytic hydrogen generation reaction have also been reported by many research groups. Therefore, the surface properties of semiconductor nanocrystals combined with structural properties of metallic nanocrystals are important issues to be considered for the rational design of new metal-semiconductor heterostructured nanocrystal photocatalysts. A key criterion in the design of efficient photocatalysts lies in the reliable synthesis of semiconductor nanocrystals with different shape. In general, controlling the surface energy of certain crystal facets by either using proper choices of strongly or weakly binding ligands, or controlling monomer concentration, is the commonly used approach. Alkylphosphonic acids were the typical ligands used to suppress the growth rate of specific crystal facets. Our group has previously reported on the synthesis of well-defined CdSe tetrapods by using alkyl halides and also by keeping the arm growth stage within the kinetic growth regime. As a result, each of these semiconductor nanocrystals prepared with different synthetic schemes has resulted in different surface chemistry and topography, which then need to be considered for further rational design of metal-semiconductor heterostructured nanocrystals. This thesis demonstrates the systematic study on the influence of each component in metal-semiconductor heterostructured nanocrystals for photocatalytic hydrogen generation application. Methodologies for the shape control of semiconductor nanocrystals followed by incorporation methods of metal nanocrystals onto semiconductor nanocrystals with different surface chemistry have been established. Morphological effects of individual metal and semiconductor nanocrystals on the final performance of photocatalytic hydrogen generation have been systematically studied. It is believed that this systematic study will guide to rational design of high-performance photocatalysts for photocatalytic water splitting applications.