Geometrical Configuration of Cobalt Cations in Spinel Oxides to Promote Photothermal Conversion Efficiency

Spinel oxides are important photothermal conversion material. ZnCo 2 O 4, Co 3 O 4, and CoAl 2 O 4 were used as model spinel systems to investigate the influence of cation geometric configuration on photothermal conversion. In these systems, all of the octahedral sites (Oh) are occupied by Co 3+ while all of the tetrahedral sites (Td) are occupied by Co 2+ . Experimental results indicate that Co 3+ -Oh (ZnCo 2 O 4 ) exhibits a higher photothermal conversion efficiency (85.7%) than Co 2+ -Td (74.0%) under a light intensity of 94 W/m 2 . The superior performance of Co 3+ -Oh over Co 2+ -Td is attributed to the reduced bandgap, which enhances nonradiative transitions and electron–phonon coupling, thereby improving photothermal conversion efficiency. Furthermore, subsequent Ni doping into the Co 3 O 4 framework promotes the migration of Co ions into octahedral sites, further improving the photothermal conversion efficiency to 86.5%, which surpasses the commercial material Pyromark 2500. This work elucidates the structure–property relationship of spinel-based photothermal materials and identifies the optimal geometric configuration.