Facet-Tailored Oxide Catalysts for ORR in Alkaline Media

Recent breakthroughs indicate that some oxides such as spinels and perovskites could be used as promising electrocatalysts for ORR (one significant reaction in fuel cells) in alkaline environments to replace scarce noble metals such as Pt. By taking advantage of crystal shape-controlled synthesis, this study centralized on two fronts: facet-dependent effectiveness and structure-dependent effectiveness. The research activities include (1) design and synthesis of state-of-the-art oxide electrocatalysts such as Mn-based spinels with particle shape control and (2) investigation of the correlation between the ORR activity/durability and surface feature of the selected facet-tailored oxide(s).   

Further reading materials:

ACS Catal.,  12 (21) 13663 - 13670 (2022).   10.1021/acscatal.2c03275

Single-Atoms Catalysts (SACs) and Single-Atom Alloy (SAA) Catalysts

Single-atom catalysts (SACs) have become a frontier in catalysis as an attractive technique with exceptional performance, offering a promising platform for improving many key electrocatalytic and catalytic reactions, such as small organic molecule oxidation and ORR in fuel cells, OER and HER in water electrolysis, eCO2RR, water-gas shift (WGS) reactions (e.g., CO + H₂O → CO₂ + H₂), and hydrogenation reactions. The supported SACs contain isolated individual atoms dispersed on, and/or coordinated with, surface atoms of appropriate supports, which not only maximize the atomic efficiency of metals but also provide an alternative strategy to tune the activity and selectivity of electrocatalytic and catalytic reactions.

In recent years, single-atom alloy (SAA) catalysts have also been shown to be powerful for a variety of catalytic reactions, such as selective hydrogenation reactions, dehydrogenation reactions, oxidation reactions, hydrogenolysis, and coupling reactions. The creation of SAA catalysts is based on the deposition of isolated reactive metal adatoms into host metal surfaces (of a relatively inert metal). The catalytic performance of SAA catalysts strongly depends on metal-support interactions and their composition and structure.

Further reading materials:

Small Struct.  2 2000051 (2021).    10.1002/sstr.202000051

Synthesis of 2D Topological Insulators

Topological insulators (TIs), such as Bi₂Te₃ and Bi₂Se₃, are newly discovered quantum materials. TIs are bulk insulators for which strong spin-orbit interaction inverts the orbital character of the conduction and valence bands at the band edges. Two-dimensional TIs are a remarkable class of layered materials, exhibiting unique symmetry-protected helical metallic edge states with an insulating interior. The realization of various exotic quantum phenomena like the quantum anomalous Hall effect (QAHE), which is expected to be important for quantum computing, is rapidly transforming into the development of realistic materials that can be used to build novel quantum devices.

Using this insight, we are applying our wet-chemical synthesis technique to the development of Bi₂Te₃-based 2D TIs such as tetradymite-type compounds with improved critical temperature for QAHE, creating a new starting point in the effort to exploit QAHE via collaboration using a unique natural heterostructure of intralayer ferromagnetic and interlayer antiferromagnetic planes intergrowing with layers of TI materials in the absence of an external magnetic field.

Further reading materials:

J. Phys. Chem. Lett.  8(8), 1905-1919 (2017).    10.1021/acs.jpclett.7b00222                 

J. Am. Chem. Soc., 127(28), 10112 - 10116 (2005).    10.1021/ja052286j

Non-Spherical Assembly

Supercrystals consist of ordered arrays of nanocrystals in 3D or 2D, presenting periodicity in terms of the positions and chemical compositions of the building blocks. Supercrystals can be achieved by assembling high-quality nanocrystals that are synthesized via a wet-chemical approach. A replacement of the traditional spherical nanocrystals with non-spherical building blocks could offer some unique characteristics due to the anisotropic nature when the orientations of the building blocks are also introduced into the assembly. Meanwhile, the packing density of the non-spherical nanocrystal-based supercrystals will be different from that of traditional packing efficiencies with spherical units. These novel supercrystals could exhibit not only the building block size- and shape-dependent properties but also new collective properties that emerge from the optical and electronic interactions among the non-spherical nanocrystals. Furthermore, a successful fabrication and understanding of the superstructures of supercrystals assembled from non-spherical NCs could result in a rational design to build up novel devices with specific nano-architecture and meet the demands of various novel applications. Superstructures of assemblies containing numerous non-spherical building block systems such as In₂O₃, PbS, PbSe, PbTe, Pt₃Ni, Pt₃Cu_2, and Pt have been studied/reviewed.

Further reading materials:

NanoToday 5 (5) 390-411,(2010).   10.1016/j.nantod.2010.08.011

Pressure-Induced Phase Transition

The property of materials could vary greatly under pressure. For example, we have uncovered that the semiconductor PbTe has a high-pressure-tuned metastable structure that can be retained at ambient conditions. This showed, for the first time, a reversal of a so-called Hall-Petch relation, relating the structural stability to particle size (10.1021/nl203409s), raising a possibility that PbTe semiconductor materials could someday serve a host of useful technological applications, such as thermo-electronics, energy conversion, etc. We also focused the pressure-dependent study on hybrid perovskites, a class of current state-of-the-art photovoltaic materials. We observed pressure-induced crystallographic transitions and band-gap tuning of MAPbI₃ (MA = methylammonium) through collaboration with research groups from NTU and CalTech (10.1002/adma.201705017; 10.1002/anie.201601788). A later publication also discussed the case of FAPbI₃ (10.1021/jacs.8b09316). Since the pressure-induced band-gap variation significantly affects the power conversion efficiency, an application of modest pressure could be sufficient to initiate phase changes and bandgap adjustments if the hybrid perovskite compositions are "pre-adjusted" chemically. Such a combined chemical-pressure strategy may prove valuable in the design of new perovskites for photovoltaic applications.

Further reading materials:

Nano Lett., 2011, 11(12), 5531-5536, 2011.   10.1021/nl203409s

Nanoscale, 8 (9), 5214-5218, (2016).   10.1039/C5NR08291A  

Nano. Lett. 13 (8) 3729-3735, (2013).   10.1021/nl4016705

Angew. Chem. Int. Ed. 55 (22) 6540-6544, (2016).   10.1002/anie.201601788

Adv. Mater. 30(2), 1705017, (2017).   10.1002/adma.201705017

J. Am. Chem. Soc.  140(42), 13952-13957 (2018).   10.1021/jacs.8b09316

J. Am. Chem. Soc. 141 (3) 1235 - 1241, (2019).   10.1021/jacs.8b07765