P-doped WO<sub>3</sub> semiconductor with enhanced conduction band on highly efficient photoelectrocatalytic reduction of CO<sub>2</sub>

Abstract

Global warming is considered a serious climate problem caused by excessive emissions of carbon dioxide. Studies on catalytic conversion of CO2 are significant in terms of reducing CO2 concentration in the atmosphere, thus remain popular in the field. The photoelectrocatalytic (PEC) reduction of CO2 is an environment-friendly and efficient method to convert excessive CO2 into chemicals. With such conversion, the greenhouse effect can be alleviated through the storage and transformation of solar energy into chemical energy. WO3 is an n-type semiconductor with wide band gap and low conduction band position. Therefore, the use of WO3 is rare for photocatalytic water splitting and CO2 reduction. Although some techniques are used to improve the ability of WO3 in photocatalytic water splitting, the light quantum efficiency (AQE) is still less than TiO2. In current study, a facile method is explored to enhance the conduction band position of semiconductors, thereby improving the photoelectrocatalytic reduction of CO2. To the best of our knowledge, this is the first paper includes novel P-doped WO3 catalysts being designed, prepared, and used in the reduction of CO2 to carbon-based chemicals in water. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), and electron paramagnetic resonance (EPR) experiments were applied to confirm the formation of new catalysts. The photoelectric properties of new catalysts were characterized by UV-vis, linear sweep voltammetry (LSV), Mott-Schottky, and electrochemical impedance spectroscopy (EIS) experiments. We have developed a new type of hybrid semiconductor (P-doped WO3 vertical bar) as catalytic materials by introducing the vaporized phosphide (NaH2PO2) onto the surface of WO3 substrate under the flow of argon gas in a tube furnace. The PEC cell of P-doped WO3 vertical bar SCE vertical bar BiVO4 was equipped with a trielectrode system and powered by a chemical workstation, in which, p-doped WO3 vertical bar was used as photocathodes in photoelectrocatalytic reduction of CO2. The PEC cell was fulfilled with an electrolyte containing 1 mmol/L photosensitizer Eosin Y and 0.1 mol/L KHCO3, and bubbled with pure CO2 in 30 min to saturation, the experiment of CO2 reduction in water was carried out with a proper voltage under irradiation of simulated sun light (300W Xe lamp) to generate methanol, ethanol, acetone, etc. Our results indicates that the conduction band position of p-doped WO3 increases from -0.13 to -0.54 V along with the increase of P-doped quantity, augmenting its ability to reduce CO2. It can be seen that p-doped WO3 with oxygen vacancies has higher activity than pure WO3, in which, the highest activity is the catalyst 15-P, approaching to 0.4% AQE equal to natural plant, yielding methanol in a rate of 20.8 mu mol/(L h cm(2)) and acidic acid at a rate of 4.7 mu mol/(L h cm(2)). The influence of oxygen vacancies on the reaction is positive and verified by the quenching experiments of H2O2. The more oxidation time, the less oxygen vacancies. The EPR signal of 15-P catalyst treated by 2 h oxidation was reduced to 10%. The catalytic activity of CO2 reduction was decreased to 50%. In summary, we designed and prepared a new type of p-doped WO3 hybrid semiconductor material as efficient catalysts for photoelectrocatalytic reduction of CO2, resulting carbon-based chemicals. P-doped WO3 improves the conduction band position of the semiconductors, increases oxygen vacancies and defect sites. Therfore, the activity and efficiency of catalytic reduction of CO2 can be improved. The low voltage is favoured to ethanol production, and high voltage is favoured to acid production. The experimental results have guiding significance for the development of new hybrid semiconductor materials and their photoelectrocatalytic reduction of CO2.