Enhancing photocatalytic CO2 reduction by coating an ultrathin Al2O3 layer on oxygen deficient TiO2 nanorods through atomic layer deposition

Abstract In this work, anatase nanorods (ANR) of TiO 2 with active facet {100} as the major facet were successfully synthesized, and reducing the ANR by NaBH 4 led to the formation of gray colored oxygen deficient TiO 2-x (ReANR). On the surface of ReANR, a thin layer of Al 2 O 3 was deposited using atomic layer deposition (ALD), and the thickness of Al 2 O 3 varied by the number of ALD cycles (1, 2, 5, 10, 50, 100, or 200). The growth rate of Al 2 O 3 was determined to be 0.25 A per cycle based on high-resolution TEM analysis, and the XRD result showed the amorphous structure of Al 2 O 3 . All the synthesized photocatalysts (ANR, ReANR, and Al 2 O 3 coated ReANR) were tested for CO 2 photocatalytic reduction in the presence of water vapor, with CO detected as the major reduction product and CH 4 as the minor product. Compared with ANR, ReANR had more than 50% higher CO production and more than ten times higher CH 4 production due to the oxygen vacancies that possibly enhanced CO 2 adsorption and activation. By applying less than 5 cycles of ALD, the Al 2 O 3 coated ReANR had enhanced overall production of CO and CH 4 than uncoated ReANR, with 2 cycles being the optimum, about 40% higher overall production than ReANR. Whereas, both CO and CH 4 production decreased with increasing number of ALD cycles when more than 5 cycles were applied. Photoluminescence (PL) analysis showed an ultrathin layer of Al 2 O 3 (2 cycles of ALD) coating on the ReANR was able to reduce the charge carrier recombination rate, likely because of the passivation of surface states. On the other hand, a relatively thick layer of Al 2 O 3 may act as an insulation layer to prohibit electron migration to the catalyst surface. This work gives valuable insights on the application of ALD coating on photocatalysts to promote CO 2 photoreduction to fuels.