UC Riverside

All nanomaterials share a common feature of large surface-to-volume ratio, making their surfaces the dominant player in many physical and chemical processes. By means of surface engineering such as ligands bonding, defects engineering and incorporation, the intrinsic energy band gap, surface wettability and adhesion of the nanostructures have been regulated, which is vital for their application in catalysis, optical device and biomedicine. Herein, I summarized my five-years research work on the fabrication of functional nanostructures by surface engineering.

Through bonding of surface ligands, large mechanical deformations and changes to

overall particle morphology during chemical transformations have been overcome. Specifically, via stabilization with strong coordinating capping ligands, we demonstrate the

effectiveness of this method by transforming β-FeOOH nanorods into magnetic

Fe3O4 nanorods, which are known to be difficult to produce directly. The surface-

protected conversion strategy is believed to represent a general self-templating method for nanocrystal synthesis, as confirmed by applying it to the chemical conversion of nanostructures of other morphologies (spheres, rods, cubes, and plates) and compositions (hydroxides, oxides, and metal organic frameworks). In addition to chemical transformation, the method was found effective to create oxygen vacancies, which provide more active sites and promote faster exchange of intermediates and electrons. The as prepared cobalt oxide nanoplates manifest oxygen evolution reaction (OER) overpotential as low as 306 mV at 10 mA/cm2 in 1 M KOH, which is superior to the values of most reported Co-based electrocatalysts.

Surface defect engineering is also an effective mean to fabricate functional materials. We discovered that metal sulfides (MoS2, WS2, Cr2S3, CoS, PbS or ZnS) could serve as excellent co-catalysts to greatly increase the efficiency of H2O2 decomposition and significantly decrease the required dosage of H2O2 and Fe2+ in AOPs. The unsaturated S atoms on the surface of metal sulfides can capture protons in the solution to form H2S, and expose metallic active sites with reductive property to accelerate the rate-limiting step of Fe3+/Fe2+ conversion. This discovery is expected to drive great advances in the use of AOPs for large-scale practical applications such as environmental remediation.

The ability to engineer surface patches holds great importance for functionalization of nanostructures. We demonstrate shape switching of patchy particles via fine-tuning of the spreading coefficient upon post-treatment process. When heated above glass transition temperature, the polymer patches would melt and reconfigure themselves according to the new established spreading coefficient, which holds potentials for many new applications in optics, catalysis and self-assembly.

Moreover, functional nanostructures of noble metal were prepared by surface dewetting. This study reports a novel confined-space thermal dewetting strategy for the fabrication of Au nanocups with tunable diameter, height, and size of cup opening. With strong scattering in near infrared, the Au nanocups exhibit superior efficiency as contrast agents for spectral-domain optical coherence tomography imaging. This confined-space thermal dewetting strategy is scalable and general, and can be potentially extended to the synthesis of novel anisotropic nanostructures of various compositions that are difficult to produce by conventional wet chemical or physical methods, thus opening up opportunities for many new applications.

At last, we developed a colloidal system with bistability based on the surface adhesion. In a dynamical colloidal dispersion, a bistable system has two stable equilibrium states with local minima of potential energy separated by a local maximum. Herein, we prepared amine-functionalized superparamagnetic particles, which can switch between assembled state and dispersion state upon external stimuli. Since the energy barrier between the two stable states is determined by electrostatic potential, which highly depends on surface charges, pH dependent bistability is achieved by managing [H+] related protonation of amine groups.