Efficient Co@Co3O4 core-shell catalysts for photocatalytic water oxidation and its behaviors in two different photocatalytic systems

Abstract

Co@Co 3 O 4 photocatalyst was prepared in situ to investigate its water splitting performance and mechanism in two photocatalytic systems. • Photocatalytic activity of catalyst 1 in two systems were compared for the first time. • Catalyst 1 exhibited excellent photocatalytic performances. • Core-shell structure of Catalyst 1 can promote light capture and electron transmission. The photocatalytic performances of water oxidation were usually carried out in two different systems, photosensitizer and non-photosensitizer systems. There is few report about the same catalyst used in two systems and therefore it is of great significant to compare its role of the same catalyst in two systems and explore its different reaction mechanisms. In this work, first 4 kinds of metallic Co microparticles were obtained by different reduction methods through hydrothermal processes, and Co@Co 3 O 4 core-shell microparticles (1–4) were obtained from these metallic Co microparticles oxidized in air or in the reacting solution in situ. The core-shell structure of 1 was characterized by a series of analytical techniques. 1–4 exhibited excellent activities and stabilities in the [Ru(bpy) 3 ] 2+ /S 2 O 8 2− /light system when they were used as catalysts for the photocatalytic water oxidation. The maximum O 2 evolution of 1 after 20 min’s illumination was 98.2 μmol, the O 2 yield was 65.5%, the initial turnover frequency was 6.6 × 10 −3 , the initial quantum efficiency ( Φ QY initial ) was 15.0% higher than Co (8.3%), CoO (11.7%), Co 2 O 3 (1.2%), Co 3 O 4 (2.8%) and 5 (2.2%). Even after the sixth run, the catalytic activity of recovered 1 still remained 85.1% of initial activity. In addition, the photocatalytic performances of 1 in the [Ru(bpy) 3 ] 2+ /S 2 O 8 2− /light system and S 2 O 8 2− /light system were compared for the first time. In the non-photosensitizer system, 1 shows bifunctional roles and acts as optical absorption center and active catalytic site, and oxygen evolution rate is lower and it takes longer time. In the photosensitizer system, 1 only acts as a catalyst, the photosensitizer enhances the light absorption and promotes water oxidation reaction with higher O 2 yield and QE, meanwhile the photosensitizer brings the defect of high cost and instability into the system. Based on the results the two different reaction mechanisms were deeply discussed.