Oxygen Vacancy Enhanced Gas Sensing Performance of CeO₂/Graphene Heterostructure at Room Temperature

Oxygen vacancies (O_v) as the active sites have significant influences on the enhanced gas sensing performance of metal oxide. In this paper, hydrothermal process is adopted to fabricate the composites of graphene and CeO₂ nanoparticles, in which H₂O₂ (L(+)-ascorbic acid (AA)) as the oxidant (reducing agent) is adopted to reduce (increase) the concentration of Ce³⁺ ions. It is found that the sensitivity of the composites to NO₂ is increased gradually, as the concentration of Ce³⁺ is increased from 14.6% to 50.7%, but decreases if the concentration of Ce³⁺ is higher than 50.7%. First-principles calculations illustrates that CeO₂ becomes metallic at the Ce³⁺ concentration of < 50.7%, the chemical potential of electrons on surface decreases and the fermi level shifts upwards, resulting in reduced Schottky barrier height (SBH) at the CeO₂/Graphene interface, enhanced interfacial charge transfer and high gas sensing performance. However, deep energy level is induced at the Ce³⁺ concentration of > 50.7%, and CeO₂ behaves as a semiconductor, the fermi level is pinned at the interface. As a result, the density of free electrons is reduced, leading to increased SBH and poor gas sensing response.