Article pubs.acs.org/IECR
Process Intensification in the Production of Photocatalysts for Solar Hydrogen Generation Chia-Ying Chiang,† Ming-Hui Chang,‡ Hwai-Shen Liu,‡ Clifford Y. Tai,‡ and Sheryl Ehrman*,† †
Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland, United States Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
‡
ABSTRACT: CuO nanoparticles with diameter in the 20−30 nm range were prepared by a spinning disk reactor at the rate of 16.9 kg/day based on the process intensification concept. The intense environment, that is, vigorous mixing and fast reaction at the same time, created within an initially crystal-free liquid film moving over the disk, generated very high supersaturation, and consequently small and fairly uniform crystals were formed via precipitation. These CuO nanoparticles were spin coated and sintered at 450 °C for 1 h to form a thin film electrode for use in a photoelectrochemical cell. The cell had a 1.20% solar to hydrogen conversion efficiency which is the highest among intrinsic CuO photoelectrochemical (PEC) cells reported to date in the literature. The bandgap of CuO films sintered at 450 °C for 1 h was 1.68 eV. The charge carrier density was 9.0 × 1020 cm−3 and the conduction and valence band edges were located at −3.54 and −5.22 eV, respectively. Furthermore, a detailed comparison based on the preparation method, particle size, bandgap, porosity, and conductivity of the films in the CuO PEC cell literature is also reported in this study.
1. INTRODUCTION People are eager to find ways to solve the energy crisis with systems that absorb the abundant but intermittent solar energy and transform it into a renewable energy such as hydrogen. On the basis of widely used commercial photovoltaic electricity generation systems coupled with electrolysis, solar to hydrogen conversion efficiency has been demonstrated to be around 8%.1−3 However, compared to the required potential for water splitting, that is, 1.23 V, electric potentials as high as 1.9 V are needed for a commercial electrolyzer to operate at its optimal condition, which limits the overall energy conversion efficiency.1 Thus a more straightforward method to generate hydrogen by water splitting is based on a photoelectrochemical (PEC) cell, where the photon generated electron−hole pair can perform the water splitting directly, eliminating the electrolysis step. Economical and environmental concerns, low toxicity earth abundant metal oxide photocatalysts, such as TiO2,4,5 Fe2O36−9 and CuO10−16 are of high interest. Among them, the small bandgap of CuO, that is, 1.4−1.7 eV, is very attractive because CuO can absorb about half of the solar irradiation while the wide band gap TiO2 and Fe2O3 having band gaps of 3.0 and 2.3 eV can only use
