Indium is a soft and malleable post-transition metal, it is also a critical element in a wide range of electronic components. Its unique electrical properties make it the perfect fit for thin-film liquid crystal displays and transparent electrodes, for example. Researchers from the Republic of Korea have recently coupled this element with graphitic nanoplatelets, forming indium-carbon bonds through a mechanically induced reaction and opening up a new type of catalytic material. [Jeon, I.-Y. et al., Mater Today Adv (2020); DOI: 10.1016/j.mtadv.2019.100030]

Much effort has been exerted in attempting to functionalize graphitic nanoplatelets and graphene (monolayer graphite, itself). Given the relative inertness of these carbon materials it has proven difficult. Various elements - boron, nitrogen, sulfur, phosphorus, antimony, iodine, and selenium - and various combinations of those elements have been tested. Several useful experimental materials have been formed including ?ame retardants, energy conversion and energy storage materials. Adding indium to the repertoire would benefit an even wider range of applications.

Now, the Korean team has used the mechanochemical ball-milling of graphite to form graphitic nanoplatelets with broken edges with which indium can react to form indium-carbon bonds in a rather useful manner. The team used aqua regia and hydrochloric acid to preclude the presence of any free-standing indium metal from the system. The distribution of the indium atoms as revealed by atomic-resolution transmission electron microscopy (AR-TEM) and scanning TEM is evidence of the formation of In-C bonds rather than the generation of indium clusters at the edges of the nanoplatelets. Additional evidence came from X-ray photoelectron spectroscopy (XPS). This is the first time, the teams says, that indium-carbon bonds have been formed using a solid state mechanochemical reaction between graphite and indium beads.

The team has now tested their novel product in the context of catalytic activity towards the oxygen reduction reaction (ORR) and found it to be on a par with commercial platinum on graphite catalysts. Moreover the material has excellent durability and tolerance to impurities, such as methanol and carbon monoxide in alkaline solution, which also bodes well for applications.