Ultrasensitive detection of Hg(II) by small-sized Mn3O4 loaded on g-C3N4 nanosheets: Heterojunction facilitates electron transfer and Mn(II)/Mn(III)/Mn(IV) cycle

Transition metal oxides are widely used in electrochemical detection because of the promotion of redox of heavy metal ions (HMIs) by valence change behavior. However, it is challenging to favorably promote the valence change to achieve the improvement of detection sensitivity. Herein, a Mn3O4/g-C3N4 composite (named as MOCN) with small-sized of Mn3O4 and high proportion of Mn(II) and Mn(III) was prepared, which reveals an excellent performance on detecting mercury ion (Hg(II)). It is discovered that Mn3O4 becomes small in size and well disperses on g-C3N4, which solves the adverse effect of agglomeration and also lead to a good conductivity. And g-C3N4 can provide more adsorption sites to enhance the adsorption on Hg(II). Heterojunction is proved to form in MO-CN and thus accelerates electrons to flow from g-C3N4 to Mn3O4. This results in transforming Mn(IV) to Mn(II) and Mn(III) in Mn3O4, thereby promoting the cycle of Mn(II)/Mn(III)/Mn(IV) and furthermore facilitating the redox of Hg(II). Simultaneously, the obtained sensitivity (473.43 mu A mu M- 1 cm-2) and limit of detection (LOD, 0.003 mu M) are as expected. The nanocomposites and heterojunction based on transition metal oxide and 2D nanomaterials is promising to boost the detection of HMIs.

Automated summary

A Mn3O4/g-C3N4 composite with small-sized Mn3O4 particles and high proportion of Mn(II) and Mn(III) was prepared, showing excellent performance in detecting mercury ions. The dispersion of Mn3O4 on g-C3N4 solves the agglomeration issue and improves the conductivity. g-C3N4 provides more adsorption sites, while the heterojunction formed in the composite accelerates electron transfer and promotes the redox of mercury ions.