Unraveling Janus-like behavior of copper phosphide for selective production of reactive oxygen species: Singlet oxygen versus hydroxyl radical

Abstract

The selective control of reactive oxygen species (ROS) and its mechanistic understanding are crucial to effectively abate aquatic organic pollutants in advanced oxidation processes (AOPs). The ROS are generated by single or multiple reaction steps and mainly depend on an electron and/or energy transfer achieved either by an external energy input (e.g., heating, applying electricity, light irradiation, etc.) or a spontaneous process stimulated under certain experimental conditions (△Go < 0). In this study, we demonstrated the Janus-like behavior of copper phosphide (CuxP) for the selective production of ROS under different conditions. The self-dissolution of CuxP at pH 3 in the dark resulted in the formation of hydroxyl radical (∙OH) via a sequential reaction step (i.e., dissolved oxygen (O2) → hydrogen peroxide (H2O2) → ∙OH), while visible light irradiation (λ ≥ 420 nm) at pH 7 led to the production of singlet oxygen (1O2) via an energy transfer from the photo-excited CuxP. CuxP underwent gradual loss of the oxidation activity with the dissolution of Cu and P species during the repetitive reactions at pH 3 in the dark, whereas it maintained the activity without the dissolution at pH 7 under visible light irradiation. The Janus-like behavior of CuxP enables its application not only as a solid oxidative reagent under acidic and dark conditions, but also as a photosensitizer under neutral and light conditions for the treatment of specific organic pollutants vulnerable to the radical and non-radical attacks. The photo-switching control for a selective ROS formation provides a new insight on water treatment by these species and paves the way for the development of highly efficient AOP systems.