Engineering Co2+/Co3+ redox activity of Ni-mediated porous Co3O4 nanosheets for superior Hg(II) electrochemical sensing: Insight into the effect of valence change cycle and oxygen vacancy on electroanalysis

Transition metal oxides (TMOs) have been recognized as promising sensing materials for heavy metal ions (HMIs) detection, mainly ascribing to a strong adsorption of HMIs on surface oxygen vacancies (OVs) of TMOs. However, the possibly more prominent role of valence change induced redox activity of TMOs in HMIs detection has not received its deserved attention, thus severely hindering the further development of TMOs-based HMIs detection. Herein, a facile strategy is proposed to co-engineer the redox activity and surface OVs of porous Co3O4 nanosheets by Ni doping. As a result, a boosted redox activity and an enhanced adsorption have been simultaneously achieved by optimizing the Ni doping level. Specifically, the 5% (atomic ratio) Ni-doped Co3O4 nanosheets exhibited best electroanalytical performance towards toxic Hg(II). More importantly, it has been demonstrated that the valence change cycle of Co cations (Co2+/Co3+) plays a more important role in electrochemical detection, rather than the adsorption of Hg(II) on OVs. Namely, the reduction of Hg(II) to Hg(0) on Co3O4 nanosheets can be effectively promoted by Ni-doping facilitated valence change cycle of Co cations (Co2+/Co3+). This work provides a feasible way to develop TMOs-based sensing materials by energetically synergizing their redox activity and adsorption ability towards HMIs simultaneously.