Effect of Cu-doping on the visible photocatalytic performance of Cu<sub>2</sub>O immersion-plated on silicon nanoporous pillar array

Abstract

In the past decades, semiconductor photocatalytic degradation was considered as one of the primary means to deal with the organic pollutants in water, because its final products, water and carbon dioxide, are harmless to environment. Owing to the high photocatalytic activity and corrosion stability, titanium dioxide (TiO2) has been widely studied and much promising progress has been made in the past years. However, TiO2 is a wide bandgap semiconductor (~3.2 eV), which decides that the generation of the active holes needed for photocatalysis can only be realized under the ultraviolet illumination accounting for only ~5% of the total solar energy. Considering that the solar energy distributed in the visible region is over 45%, the exploration on high-efficiency visible photocatalysts is of special significance. Actually, great efforts have been made to expand the active light wavelength of TiO2 from ultraviolet to visible regime, such as through doping or co-doping techniques or modifying semiconductor surfaces with other substances. Although the results from theoretical calculations is rather encouraging, the actual effectiveness through either element doping or surface modifying on tuning the energy band structure is far from satisfactory in experiments. Furthermore, the unstability of the semiconductors after doping or co-doping treatment is still a problem to be solved. Cuprous oxide (Cu2O) is a p-type and direct bandgap (2.17 eV) semiconductor material, which imply that it can directly utilize the visible light regime accounting for 45% of the solar energy. As a visible light photocatalyst, Cu2O has been studied and exhibited given photocatalytic activity. Meanwhile, the photocatalyst existed in nanopowder form would not be helpful for realizing cyclic utilization. So, preparing a high-efficiency, easy to be recycled photocatalyst under the visible light irradiation is of great importance. In the previous study, we reported the preparation and characterization of silicon nanoporous pillar array (Si-NPA), a silicon hierarchical structure featured by an array of regularly aligned, quasi-identical and highly nanoporous silicon pillars with strong light absorption across the whole visible region. These structural and optical properties imply that Si-NPA might be an effective photocatalyst to degrade organic pollutants under visible light irradiation. Its large specific surface area could provide sufficient space or active sites for pollutant adsorption, its strong visible region light absorption would be beneficial to fully utilize the sunlight energy, and its solid-state form would be helpful for realizing cyclic utilization compared with nanopowder photocatalysts. Considering the special structure and physical properties of Si-NPA, this work selected Si-NPA as substrates and the samples were obtained with different preparing parameters. Cu2O/Si-NPA, Cu2O:Cu/Si-NPA (with Cu2O as the main component), Cu:Cu2O/Si-NPA (with Cu as the main component) and Cu/Si-NPA were prepared by an immersion-plating method. The surface morphologies, microstructures and visible photocatalytic degradation performances to methyl orange (MO) were characterized. It is disclosed that for samples specified by 1.8 cm×2.0 cm, a photocatalytic degradation rate of ~53.6% to MO for Cu2O:Cu/Si-NPA was achieved under visible light (400–800 nm) with an optical power density of 10 mW/cm2, which was about 20% higher than Cu2O/Si-NPA. The efficiency promotion is attributed to the Cu-doping in Cu2O, which would be beneficial to decrease the probability of the photon-generated electrons captured by defects or recombined with holes and enhance their transport process from internal to material surface. Such an enhancement would produce the reactive oxygen species with a concentration that higher than pure Cu2O. These results might provide a promising way for further promoting the visible photocatalytic degradation rate of Cu2O to organic pollutant.