Crystal Facets Controlling Photocatalytic CO2 Reduction

Title: Hollow Multi-Shelled Structures of Co3O4 Dodecahedron with Unique Crystal Orientation for Enhanced Photocatalytic CO2 Reduction

Authors: Li Wang, Jiawei Wan,Yasong Zhao, Nailiang Yang, Dan Wang

Year: 2019

Journal: Journal of American Chemical Society

All Figures used in this writing were taken from the article with due permission.
What are facets and why are they important in materials? Imagine a diamond that is not cut evenly. It would not appear shiny and attractive as it could not scatter light in all directions. Nowadays, most of the brilliant cut diamonds are cut to create 57-58 facets.
So, facets are essentially flat surfaces oriented to specific directions in a material. Let’s look along the 3D cartesian co-ordinates, we can try to understand the facets in a cube shown below-


Fig. 1 Various Facets in a cube (also known as Miller planes)

Depending on the distance of 6 facets from the origin (0,0,0) and its orientation, we have (100), (010), (001), (1̅00), (01̅0) and (001̅) – these 6 facets present. (Fig 1)
For materials that are applicable in various reactions, facets are useful handles to control interaction of materials with other chemicals. Having more facets that are properly aligned to interact with gases/liquids in those materials increase the activity of the material. A group of scientists in this work has developed a strategy to induce a high degree of similar facet orientation in it. Instead of combining metal in its atomic form, they have used a pre-formed metal-organic framework with metals as a template. One can imagine the organic parts in these frameworks are the bricks of a house while the metal atoms are the windows. All one needs to do is somehow get rid of the brick walls without disturbing the position of the windows. In case of these materials, it is straightforward to essentially burn the soft organic C, H, N and O atoms at elevated temperature without disrupting the pre-orientation of the metal atoms. This was the procedure for a material known as ZIF-67 containing Cobalt metals. Upon heating ZIF-67, they obtained a single facet Co3O4 of varying sizes (also known as shell number). The following scheme (Scheme 1) shows how they achieved various sized crystals of Co3O4, mostly possessing (111) facet. This has been verified with X-ray crystallography Since (111) facet gives rise to higher interaction with a gaseous substrate such as CO2, this newly developed technique is a superior catalyst. It is capable to form (111) facets almost exclusively in Co3O4, especially in the materials with higher shell number.
Researchers have discovered that these oxides are also hollow. Why are these hollow structures look interesting to researchers? Because a hollow material can absorb a high concentration of gases such as carbon dioxide, which is a greenhouse gas, and convert it to other useful chemicals. Therefore, it is desirable for the material to achieve high reactivity of towards a specific chemical (CO2 in this case, also known as a substrate) to convert it to one product instead of a mixture of products (selectivity).
Further experiments showed these Co3O4 are more reactive in reducing CO2 to CO under light irradiation more efficiently compared to oxides where mixed facets are present. To understand this

higher photocatalytic activity, they did impedance spectroscopy which is a routine


Scheme 2. Effect of Synthetic Conditions on Higher Number of Shell Formation

electrochemical technique for semiconductor oxides. The newly developed Co3O4 shows lower impedance or higher efficiency of electron transfer through (111) facets compared to mixed faceted oxides. The charge transfer is also retained over a longer span compared to the conventional Co3O4.
To summarize, this work has shown how to control the growth of a single facet in a material by employing a highly ordered metal-organic framework such as ZIF-67 to increase the desired reactivity, selectivity and longevity of that material.

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