Elucidating, understanding, and correlating the (photo)electrochemical and physicochemical properties of HKUST-1 through an experimental and computational approach

Abstract

HKUST-1 MOF and HKUST-1-based compounds have been employed to develop active photocatalytic materials; however, a deep understanding of several fundamental physicochemical properties to explain the photocatalytic activity of these materials is limited. Several physicochemical properties of HKUST-1 were studied through experimental and computational approaches to determine its (photo)electrochemical (PEC) properties and applications. These properties were correlated with the structural features of the material and the photocatalytic activity towards the hydrogen evolution reaction. A meticulous characterization confirmed the synthesis of HKUST-1 using a slow evaporation methodology. XRD measurements revealed a large crystallite size (≈123 nm). Meanwhile, UV-vis spectroscopy, electrochemical measurements, and DFT calculations allowed us to identify three fundamental issues that limit the photocatalytic activity of HKUST-1: (i) a low relative permittivity of the semiconductor (ε_r), (ii) a wide bandgap value, and (iii) a low reduction potential for the conduction band (CB). The issues mentioned above are related to high porosity, a small conjugated system, and the presence of open metal sites in the secondary building units (SBU) that constitute the CB, respectively. Instability problems are also related to the decrement of the observed open metal sites. Regarding PEC measurements, it was determined that HKUST-1 is an n-type semiconductor suitable for photocatalytic reduction or PEC oxidation processes, however, it presents recombination issues. Determining the relative permittivity for HKUST-1 through computational tools provides an accurate methodology to calculate the density of charge carriers (N_D) through the Mott–Schottky equation. The measurements and analysis presented in this work point out the main issues responsible for low photocatalytic hydrogen evolution and suggest the employment of HKUST-1 in PEC oxidation processes.