Hydrogen Treatment of Metal Oxides: Methods, Mechanisms, and Implications

Manipulating and tailoring the properties of metal oxides toward specific applications is key to achieving current economic and technological goals. Different forms of hydrogen treatment, either during synthesis or in a postprocess treatment, can have wide-ranging effects on a metal oxide material. Hydrogen has the potential to reduce the material by inducing oxygen vacancies at the surface and in bulk but also can be integrated as a substitutional or interstitial dopant. Reports of improved material, optoelectronic, and magnetic properties after such treatments have spurred a large response from the scientific community. The ability of hydrogen treatment to tune these properties has cast a wide net for possible applications, many especially relevant to current initiatives toward sustainable energy generation and storage, and electronic components. This research reached a height in the 2010s with studies exploring the fundamental chemistry of the underlying mechanisms or optimizing the process of inducing these observed changes. Interest in hydrogen treatment has waned in recent years and this review aims to reinvigorate the field by summarizing some of the profound effects that hydrogen treatment has on metal oxides, as well as some of the unanswered questions of how hydrogen treatment achieves these effects.