Wet Chemical Synthesis of Amorphous Nanomaterials with Well-Defined Morphologies

ConspectusAmorphous nanomaterials, with unique structural features such as long-range atomic disorder and nanoscale particle or grain sizes, possess some advantageous properties for a number of materials applications. For example, amorphous vanadium oxide exhibits a record-high cycling stability for supercapacitors. Several synthetic strategies have been developed to produce amorphous nanomaterials, such as physical processing-based approaches including cutting, deposition, and spinning, as well as chemical syntheses by solution or solid state reactions. However, despite the rapid development of amorphous nanomaterials, their morphology is still irregular or primarily sphere-like. The limited morphology control is partially attributed to the lack of preferred growth direction for the amorphous materials. It will be interesting to know whether different morphologies of even amorphous nanomaterials can influence their properties as much as their crystalline counterparts.Wet chemical synthesis, which can usually be carried out under relatively facile reaction conditions, has achieved remarkable success for creating crystalline nanomaterials with well-defined shapes, such as one-dimensional (1D) nanowires, two-dimensional (2D) nanosheets, and three-dimensional (3D) complex structures. However, there are two main obstacles for shape control of amorphous nanomaterials using wet chemical strategies. First, it is difficult to form a stable amorphous state under wet chemical conditions. The amorphous state is metastable in solution according to classic nucleation theory, which is prone to phase transformation to form crystalline state. Second, common morphology control mechanisms for nanocrystals are ultimately relying on the intrinsic directionality of the lattices, which unfortunately is not relevant to the isotropic amorphous nanomaterials.In this Account, we describe how shape control of 1D, 2D, and 3D amorphous nanomaterials can be achieved in wet chemical synthesis to create well-defined morphologies, which are based on two main strategies: Blocking agents that can stabilize the amorphous state in solution, and morphology-tunable parameters that can induce directional growth. Discussion on the phase transfer blocking mechanisms includes lattice disordering, controlled hydrolysis, rapid reaction, and ionic exchange, and for morphology control through confinement it includes precursor transformation, templated reactions, interfacial etching, and spatial confinement.In addition, we present examples showing how these well-defined morphology leads to desirable properties in applications of mechanics, energy storage, catalysis, and optics, highlighting their structural-properties relationship. Finally, some perspectives are discussed regarding future research opportunities in the area of amorphous nanomaterials.