Crystal engineering using electrochemical approach : mechanism and applications

Abstract

Research into crystal engineering is stimulated by the fascinating size- and shape-dependent properties of nano/micro-crystals, which bridge the atoms and bulk solids and direct unique applications in catalysis, sensing, imaging, biomedicine and so on. A tremendous amount of scientific efforts have been made to exploit a versatile method for the shape-controlled synthesis of crystals. This dissertation focus on the electrochemical synthesis of well-defined crystals from the viewpoint of crystallographic control. The research work herein is divided into two parts: (1) the exploitation of a novel electrochemical approach to synthesize metal/metal oxide nano/micro-crystals with various shapes; (2) the development of the crystals with designed structures for efficient utilization in catalysis and surface-enhanced Raman scattering (SERS). At one end, a new electrochemical approach, named as cyclic scanning electrodeposition (CSE) method, is developed to convert thin metal films into nano/micro-crystals in specific electrolytes consisting of different anionic capping agents. A set of experimental parameters including the pre-deposition of self-assembled monolayers, the potential waveform, and the composition of electrolyte have been studied to manipulate the crystal shape. Notably, the versatility of the CSE has been demonstrated to produce anisotropic crystals (e.g., nanoplate, nanorod, dendrite) and polyhedrons (e.g., decahedron, icosahedron, octahedron). The underlying mechanisms governing the shape evolution of the crystals are proposed on the basis of observations from electron microscopy and electrochemical analysis. Also, the synthetic protocols of crystallographic control have been proven to be reproducible and controllable in the syntheses of metals (e.g., Cu, Ag) and metal oxides (e.g., Cu2O, Ag2O). The CSE method opens up a promising route to tailor the crystal shape in a methodical fashion and explores the great adaptability of electrochemical synthesis for various compositions of materials. At the other end, the crystal engineering of Ag and Cu2O nano/micro-crystals affords the distinct arrangements of surface atoms and unique properties, which are demonstrated in three representative applications: electro-oxidation of glucose, photoelectrochemical hydrogen production, and SERS. Firstly, the three-dimensional flower-like Ag nanocrystals exhibit superior catalytic activity for the electro-oxidation of glucose with one order of magnitude improvement in detection limit as well as the highest sensitivity achieved to date. The nanoporous Ag flowers are also fabricated on Ni foil via CSE to improve the figure of merit of glucose sensor application associated with the selectivity to interferences. Secondly, the Cu2O concave octahedrons enclosed by high-index facets are abundant in low-coordinated atoms, by which an enhanced catalytic activity is delivered in photoelectrochemical hydrogen production. Thirdly, the Cu2O nanoplates display a remarkable SERS activity and an appreciable reproducibility, offering an opportunity to launch a systematic investigation on the SERS mechanism of Cu2O. The design of Cu2O nanocrystals not only provides the prospect of optimizing a new type of SERS-active substrate but also shed light on the understanding of SERS mechanism in metal oxides.