Ultra-trace simultaneous detection of Hg(II), Cd(II), and Cu(II) and mechanism based on Co/CoO/Co₃O₄ Z-type heterojunctions

Ultra-trace electrochemical determination of various heavy metal ions (HMIs) in water environment is of vital significance. Z-type heterojunction modified electrode materials have attracted vital interest in the electrochemical detection of HMIs because of its valence change and electron transfer capability. Herein, a facile and novel strategy is proposed to synthesize Co/CoO/Co₃O₄ polyvalent cobalt heterojunction via modified hydrothermal followed microwave carbon bath process. The strong valence change cycle of polyvalent cobalt (Co⁰, Co²⁺ and Co³⁺) plays a significant role in the electrochemical detection for Cd(II), Cu(II) and Hg(II) compared with other cobalt valence states according to the experiment characterizations and density functional theory (DFT) calculation. Co/CoO/Co₃O₄ exhibits the sensitivity of 4.31 μA/µM for Cd(II), 19.34 μA/µM for Cu(II) and 13.25 μA/µM for Hg(II), which are successfully applied in the real water environment and maintain excellent stability and anti-interference. The results of DFT calculations show that the adsorption energies of Co/CoO/Co₃O₄ heterojunction for Cu(II), Hg(II) and Cd(II) are − 1.147, − 1.016 and − 0.999 eV, respectively, showing an excellent adsorption capacity for HMIs. This design not only successfully constructs Z-type heterostructures with cobalt multivalence change cycle, but also demonstrates low cost and high efficient electrochemical sensors for simultaneous detection of HMIs.