An efficient and highly selective photocatalytic conversion of carbon dioxide (CO 2 ) into valuable chemicals such as carbon monoxide (CO) using water (H 2 O) as an electron donor has been much attractive and deeply desired, which requires the development of advanced photocatalysts based on a functional design. As the Ag-loaded calcium titanate (CaTiO 3 , CTO) photocatalyst showed a high selectivity to CO 2 reduction in aqueous solution and the Ag-loaded gallium oxide (Ga 2 O 3 ) photocatalyst showed a higher activity for both H 2 O splitting and CO 2 reduction, herein, a series of composite photocatalyst samples consisting of Ga 2 O 3 and CTO were simply fabricated by calcination of the physical mixtures, followed by loading of a Ag cocatalyst with a photodeposition method. The optimized sample with the Ag cocatalyst exhibited both a high CO formation rate of 56.9 μmol h −1 (higher than that of Ag/Ga 2 O 3 ) and a high selectivity of 95.0% (comparable to Ag/CTO) in the photocatalytic CO 2 reduction with H 2 O. In this composite photocatalyst, most of the electrons generated in the photoexcited Ga 2 O 3 part migrated to the minor CTO particles to contribute to the selective CO 2 reduction reaction, which was evidenced by the selective photodeposition of Ag species on the CTO part. The selective CO formation originates from the property of Ag-loaded CTO photocatalyst as the active part in the composite photocatalyst. The Ga 2 O 3 part functions as an antenna to receive the light and donate the photoexcited electrons to the much decorated Ag/CTO part, where the concentrated electrons would promote CO 2 reduction with high efficiency.

Heterogeneous photocatalytic CO2 reduction with water has attracted great attention. Although the addition of Ag nanoparticles (NPs) as a cocatalyst on a semiconductor photocatalyst has been known to improve both the photocatalytic activity and reaction selectivity for CO2 reduction, the addition of Pr as Pr6O11 species on the surface of an Ag-loaded CaTiO3 (CTO) photocatalyst further improved the photocatalytic activity. The different calcination temperatures for the sample preparation changed the state of Pr species, and it strongly influenced the photocatalytic performance. The Pr6O11 species was found to be loaded between the CTO surface and Ag NPs on the Ag/Pr/CTO photocatalyst, proposing that it improves the electron migration from the CTO photocatalyst to Ag NPs via the Pr6O11 layer.

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