Transformative Inorganic Nanocrystals during Cation Exchange Reaction

概要

Inorganic colloidal nanocrystals (NCs) are usually defined as the nanometer-sized (1–100 nm) inorganic particles coated with surfactants. The fine-tuning of size, shape, composition, phase, and surfactants can control the electric, optical, magnetic, and chemical properties of colloidal NCs. These features make colloidal NCs attractive and promising building blocks for advanced materials and devices, which are not achieved by the bulk counterparts. Tremendous efforts have been made to develop versatile synthesis methods of colloidal NCs based on chemistry. However, the synthesis of NCs with desired size, shape, composition and crystal structure is still a significant challenge. Access to the sophisticated nanoscale structures is essential to develop highly functional colloidal NCs. Besides the direct chemical synthetic methods, various strategies have been developed to prepare colloidal NCs with complicated structures. Post-synthetic treatment of NCs is the effective methodology, such as ion exchange reaction, galvanic replacement, pressure-induced transformation, and temperature-induced phase transformation. Among them, ion exchange reaction has been widely regarded as a facile and efficient way to obtain well-designed NCs to build novel electronic, magnetic, and photonic devices. Objectives of this doctoral thesis are to study the mechanism of crystal structure transformation and morphology evolution process of ionic NCs during the cation exchange reaction and develop the novel environmentally-friendly cocatalyst for photocatalytic overall water splitting.

Chapter 2. Crystal Structure Transformation of Ionic Nanocrystals in Cation Exchange Reactions
Ion exchange reactions are a promising route for overcoming current limits imposed by direct synthetic routes to NCs and for increasing the library of available crystal structures. Generally, incoming cations expel the original cations and preserve the robust anion sublattice because the smaller cations diffuse much faster than larger anions and the overall morphology and crystal system of a NC are retained. In particular, crystal structure transformations derived from anion sublattice deformation through cation exchange without changing the overall host NC morphology are rare. In chapter 2, determinants of the crystal structure transformation of ionic NCs during the cation exchange reaction was investigated. It was discovered the height of hexagonal-prism Cu1.8S NCs with a distorted hcp S2− sublattice determines the final crystal phase of the cation-exchanged products with Co2+ (CoS with hcp S2–, and/or Co9S8 with ccp S2–) through chemical synthesis and computational modeling. Thermodynamic instability of exposed planes drives reconstruction of anion frameworks under mild reaction conditions. Other incoming cations (Mn2+, Zn2+, and Ni2+) modulate crystal structure transformation during the cation exchange reactions by various factors, such as volume, thermodynamic stability, and coordination environment.

Chapter 3. Morphological Evolution of Cu1.8S-MnS Heterostructured Nanoplatelets during Cations Exchange Reaction
It has been believed that the cation exchange reactions hardly transform the shape of host NCs because the original anion framework is quite stable. Some studies reported the cation exchange-induced shape transformation of host ionic NCs, but the shape transformation process and the stability of the atypical intermediates during the cation exchange reaction have not yet thoroughly investigated. In chapter 3, a unique morphological evolution process of shape memory behavior during the cation exchange of Cu1.8S NPLs with Mn2+ was demonstrated. The hexagonal Cu1.8S NPLs evolved to incomplete bow-shaped Cu1.8S-MnS NPLs and then grew back to complete hexagonal MnS NPLs. We found the Cl– from the Mn precursor plays an important role in the decomposition of Cu1.8S. This shape memory behavior was also proved for the Mn cation exchange with rod-shaped and spherical host Cu1.8S NCs. This discovery gives a new aspect to the morphological change during the cation exchange reaction and paves a way to synthesize more complicated heterostructured NCs.

Chapter 4. Self-activated Rh–Zr Mixed Oxide as a Nonhazardous Cocatalyst for Photocatalytic Hydrogen Evolution
Platinum group metals are widely used for the hydrogen evolution reaction; they reduce H2O because of the metals' high activity and stability. However, these metals also catalyze backward reactions, such as O2 photoreduction and H2O formation, from evolved H2 and O2. In chapter 4, RhZrOx was found to be an effective, robust and environmentally friendly cocatalyst. Zr and Rh precursors with a certain raio (Zr/Rh = 5 wt/wt%) formed RhZrOx cocatalyst particles on STOA exhibited a higher photocatalytic water-splitting activity (31×) than a RhOx cocatalyst. The dissociation of Cl– ions from preformed Rh–Cl–Zr–O solid led to formation of the active phase of RhZrOx. Additional CoOx loading as an oxygen evolution cocatalyst further improved the activity by 120%, resulting in an apparent quantum yield of 33 (±4)% at 365 nm and a long durability of 60 h. Our discovery could help scale up photocatalytic hydrogen production.