A charge
mismatch between transition-metal-ion dopants and metal
oxide nanoparticles (MO NPs) within an engineered complex engenders
a significant number of oxygen vacancies (VO) on the surface
of the MO NP construct. To elucidate in-depth the mechanism of this
tendency, Co ions with different charge states (Co3+ and
Co2+) were doped into ZnO NPs, and their atomic structural
changes were correlated with their photocatalytic efficiency. We ascertained
that the increase of the Zn–O bond distances was distinctly
affected by Co3+-ion doping, and, subsequently, the number
of VO was noticeably increased. We further investigated
the mechanistic pathways of the photocatalytic oxidation of 2,5-hydroxymethylfurfural
(HMF), which have been widely investigated as biomass derivatives
because of their potential use as precursors for the synthesis of
sustainable alternatives to petrochemical substances. To identify
the reaction products in each oxidation step, selective oxidation
products obtained from HMF in the presence of pristine ZnO NPs, Co3+- and Co2+-ion-doped ZnO NPs were evaluated. We
confirmed that Co3+-ion-doped ZnO NPs can efficiently and
selectively oxidize HMF with a good conversion rate (∼40%)
by converting HMF to 2,5-furandicarboxylic acid (FDCA). The present
study demonstrates the feasibility of improving the production efficiency
of FDCA (an alternative energy material) by using enhanced photocatalytic
MO NPs with the help of the charge mismatch between MO and metal-ion
dopants.