Highly stable and active low-temperature CO oxidation catalysts without noble metals are desirable to achieve a sustainable society. While zero-dimensional to three-dimensional Co3O4 nanoparticles show high catalytic activity, simple-structured nanocrystals easily self-aggregate and become sintered during catalytic reaction. Thus, complex three-dimensional nanostructures with high stability are of considerable interest. However, the controlled synthesis of complex nanoscale shapes remains a great challenge as no synthesis theory has been established. In this study, 100 nm raspberry-shaped nanoparticles composed of 7–8 nm Co3O4 nanoparticles were synthesized by hydrothermally treating cobalt glycolate solution with sodium sulfate. Surface single nanometer-scale structures with large surface areas of 89 m2·g−1 and abundant oxygen vacancies were produced. The sulfate ions functioned as bridging ligands to promote self-assembly and suppress particle growth. The Co3O4 nano-raspberry was highly stable under catalytic tests at 350 °C and achieved nearly 100% CO conversion at room temperature. The addition of bridging ligands is an effective method to control the formation of complex but ordered three-dimensional nanostructures that possessed extreme thermal and chemical stability and exhibited high performance.
Raspberry-shaped Co 3 O 4 nanoparticles has a great potential as a CO oxidation catalyst in a wide temperature range because of a high stability and a low-temperature oxidation activity. In this study, primary particle sizes, morphology and crystallite sizes were controlled by changing a synthesis time to enhance the CO oxidation activity and to reveal growth mechanism of the raspberry structure. The primary particle sizes increased while decreasing crystallite size, indicating crystal orientation and particle growth of Co 3 O 4 nanoparticles were occurred in multistage, and a single-crystal-like structure formed in the hydrothermal treatment for 3.0 h. Longtime hydrothermal treatment for 12.5 h caused decomposition of the crystallographic orientation and the raspberry structure. H 2 -temperature programmed reaction analysis indicated that crystal orientation among multiple Co 3 O 4 nanoparticles improved a mobility of bulk oxygen species, and our previous findings that 93% of CO conversion rate for the raspberry-shaped Co 3 O 4 nanoparticles was confirmed analytically by the high oxygen mobility in the early 3.0 h-hydrothermal treatment.
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