Atomic layer deposition enabled MgO surface coating on porous TiO2 for improved CO2 photoreduction

In this work, we have prepared porous TiO2 with mixed anatase-rutile phases from a metal-organic framework MIL-125 and employed atomic layer deposition (ALD) method to coat an ultrathin MgO overlayer on porous-TiO2. The CO2 photoreduction performance of porous-TiO2 was more than 4 times higher than that of commercial Degussa P25. The MgO coating on porous-TiO2 by ALD in the range of 1 to 100 atomic layers all improved CO2 photoreduction performance, with 5 layers of MgO being the optimal, delivering 4 times higher CO production than that by pristine porous-TiO2 and 21 times higher than P25. It is demonstrated that the uniform dispersion of MgO on TiO2 surface led to increased concentration of surface Ti3+ species and Mg bonded hydroxyl groups, which are active sites for CO2 adsorption and photoreduction. The ALD layer also served to passivate the TiO2 surface states and hinder surface electron-hole recombination. In addition, a conventional wet-impregnation (WI) method was applied to incorporate MgO on porous-TiO2 to serve as a comparison. The WI modified MgO/TiO2 samples were more active than pristine porous-TiO2 but less active than ALD modified MgO/TiO2, because the ALD samples presented significantly higher amounts of surface Ti3+ than WI samples due to higher and more uniform MgO surface coverage by ALD coating. Findings from this original work on ALD modification to enhance photocatalytic performance may be extended to other photocatalyst systems and other catalytic reactions.