The photoreduction of selenite and selenate on the surface of few layer black phosphorus and a UiO-66 p–n junction heterostructure

Abstract

The increase in anthropogenic activities has led to an increase in selenium-based anions in water bodies, posing health concerns for humans and animals. The strict regulations governing the levels of selenium-based anions in water necessitate that new and more efficient removal techniques be developed. This document reports on the formation of a heterojunction between the University of Oslo (UiO-66) and few layer black phosphorus (FL-BP) at various ratios. Changes in the peak position and intensity were observed in the case of the heterostructures relative to virgin UiO-66 and FL-BP when the materials were characterized using X-ray diffraction, laser Raman spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. This showed that the heterostructures had been formed; also, transmission electron showed that the UiO-66 nanocubes were deposited on top of the FL-BP mat-like structure. Compositing UiO-66 with FL-BP yielded materials with bandgaps ranging between 1.85 and 1.89 eV, much narrower than UiO-66 that has a bandgap of 3.26 eV. Photoluminescence and electrochemical impedance spectroscopy showed that the heterojunctions had a much lower charge carrier recombination rate compared to that of the pure materials. This was because of the higher effectual charge transfer between UiO-66 and FL-BP through the formation of the p–n junction as was shown through the Mott–Schottky plots. The materials were then tested as photocatalysts for the photoreduction of SeO₃²⁻ and SeO₄²⁻ under conditions optimized using UiO-66. Light ranging between 200 and 800 nm was used as a source of energy. The heterostructures showed superior photocatalytic activity to the virgin materials. Also, the heterojunction that consisted of a FL : BP : UiO-66 ratio of 1 : 40 labeled as FL/U(40) outperformed all the tested photocatalysts. These observations were consistent with the electron/hole recombination rate measurements and light-harvesting characteristics of the materials. It was also observed that FL/U(40) could be used over 5 cycles with minimal loss in photocatalytic activity.