Ink-jet printing films of molybdates of alkaline earth metals with scheelite structure applied in the photocatalytic CO2 reduction

Abstract

The photocatalytic CO2 reduction is a clean and sustainable process to generate value-added products by using water, sunlight, and photocatalysts materials. In this work, it is proposed the use of thin films based on molybdates of alkaline earth metals of general formula AMoO4 (A = Ca, Sr, and Ba) with scheelite-structure as photocatalysts for the conversion of CO2. The films were deposited by ink-jet printing, which is a technique that offers high speed and low cost of fabrication, and provide the possibility to print over different substrates. The AMoO4 films were characterized by razing incidence X-ray diffraction (GZXD), Fourier-transform Infrared Spectroscopy (FTIR), Scanning Electronic Microscopy (SEM), Diffuse Reflectance Spectroscopy (DRS), profilometry, spectroscopic ellipsometry (SE), Electrochemical Impedance Spectroscopy (EIS), and zeta potential measurements. According to the characterization, it was confirmed the crystallization of the scheelite structure in the CaMoO4, SrMoO4, and BaMoO4 films. The effect of the type and size of the A cation affected several physical properties such as crystallite size, thickness, potential of the conduction band, zeta potential, and the affinity for CO2 adsorption, which it was reflected in the photocatalytic activity of each film. The main product of the photocatalytic CO2 reduction was CH3OH, although it was detected the formation of H2 as by-product of the reaction. The photocatalytic activity for methanol production increased as follows: CaMoO4>SrMoO4>BaMoO4, which it was related to a higher affinity for CO2 molecule, low crystallite size, low thickness, adequate potential of conduction band and a higher donor number. The highest solar fuel yield obtained was 18 μmol cm−2 h−1, which it was higher than previous reports using films of other semiconductor materials.