(Invited) New Synthesis Approaches to Nanostructured Complex Functional Metal Oxides

Metal oxide materials can display unique electronic, structural, and optical properties when scaled down to nanometer dimensions and find utility in many applications such as ferroelectrics and piezoelectrics, superconductors, dielectrics, heterogeneous catalysis, photocatalysis, fuel cells, and batteries.  To this end, the ability to fabricate or synthesize functional oxides in a variety of nanostructured geometries is important. In this presentation, three example systems will be presented detailing new synthetic approaches to obtaining metal oxides with advanced nanostructured morphologies. The effect of the nanostructured morphology on the structure and properties of the materials will also be discussed.

Recently, there has been much interest in developing synthesis methods to obtain nanostructures with higher order geometries such as branching or hyperbranching. The ability to synthesize metal oxides in hierarchical structures can allow for new properties or phenomena not observable in bulk morphologies. In the first example, the solution-phase synthesis of cubic perovskite potassium lanthanum titanate into orthogonal nanostructures with hyperbranched and hexapod morphology will be discussed. A facile hydrothermal reaction without using a template, catalyst, substrate, or structure-directing agent was employed. The properties of the hyperbranched materials as water splitting photocatalysts for hydrogen generation will be discussed.

In the second example, our efforts towards understanding the materials chemistry of nanostructured solid state Li-ion conductors, which are attractive as safer alternatives to liquid organic electrolytes, will be presented. Our group has developed electrospinning methods for synthesizing titanate and zirconate-based Li-ion conducting ceramics into nanowire morphologies. Some of the improved properties we have observed in these solid electrolyte nanowires, such as improved sintering characteristics, phase stability, and Li ionic conductivity, will be discussed.

Finally, our efforts towards the synthesis of functional 2D nanosheets comprising Li-ion battery cathode materials such as LiCoO₂ and LiNi_1/3Mn_1/3Co_1/3O₂ will be presented. 2D materials have attracted a great deal of attention for their unique electrical and magnetic properties, but may also play important roles in energy storage applications. Many conventional battery materials have layered structures, and hence can be readily exfoliated into 2D nanosheet materials.  The high surface area and short ionic diffusion distances in the 2D nanosheets may improve the charging/discharging rates and result in more lithium insertion or surface adsorption. Furthermore, hybrid electrode materials comprised of layers of different cathode materials may be possible by reassembling different nanosheets. However, the synthesis of Li-rich materials into 2D nanosheets is challenging due to the strong interlayer electrostatic interactions and binding by protons during osmotic swelling processes, which can block Li sites. Our modified osmotic swelling approach circumvents these problems and allows for the synthesis of 2D nanosheets from Li-rich materials that can be restacked and display the expected electrochemical properties of bulk counterparts. Our work is a firm step forward for improving understanding of osmotic swelling processes for the synthesis of nanosheets from complex metal oxides as well as the design and fabrication of high performance hybrid electrodes for energy storage applications.

References:

T. Yang, Z.D. Gordon, C.K. Chan, Cryst. Growth Des., 13 , 3901-3907 (2013).

T. Yang, Y. Li, C.K. Chan, J. Power Sources, 287, 164-169 (2015).