Fe-doping induced surface Fe2+/Fe3+ cycle and activated redox-inert TiO2 for enhanced Hg(II) electrochemical sensing: An efficient strategy to strengthen the redox activity.

Recently, Fe-based metal oxide with a variable-valence ability (i.e., the Fe2+/Fe3+ cycle) can participate in the redox of target heavy metal ions (HMIs) and enhance the electrochemical signal, which have attracted significant attention. However, it has not yet been proved whether iron-based metal oxides with variable-valence ability can activate the variable valence behavior of inert metal oxides (i.e., TiO2) and enable them to participate in the redox of target HMIs. Herein, we develop an efficient Fe-doped strategy to activate TiO2 nanoparticles for the electrochemical detection of Hg(II). TiO2 nanoparticles with the 5% Fe-doped content (FT5) possess the best detection sensitivity of 400.63 μA μM-1 cm-2 for Hg(II), which is dramatically higher than that of pure TiO2. The synergistic effects of enhanced adsorption by OVs and promoted redox activity by surface Fe2+/Fe3+ and Ti3+/Ti4+ cycle help FT5 to obtain an excellent electrochemical detection performance of Hg(II). In detail, Fe doping tune the concentration of oxygen vacancies (OVs) in TiO2 nanoparticles, which contributes to improving the adsorption ability of Hg(II). The exposed OVs on the surface of Fe-doped TiO2 nanoparticles form numerous hydroxyl groups (-OH) in water, and the hydroxyl groups can bond with Hg(II), tremendously accelerating the capture of Hg(II). Upon successfully obtaining OVs, the Ti3+ species are created in TiO2, achieving the activation of TiO2. Moreover, it is found that large amount surface Fe2+/Fe3+ and Ti3+/Ti4+ cycle on FT5 can accelerated the redox of Hg(II) and then favor to electrochemical detection performance. This study emphasizes that doping transition metal elements with variable valence states can control OVs concentration and successfully activate inert metal oxides.