Morphological control of hydrothermally synthesized cobalt oxide particles using poly(vinyl pyrrolidone)
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The hydrothermal synthesis of Co3O4 particles using cobalt nitrate as the precursor and citric acid as a weak chelating agent was modulated using poly(vinyl pyrrolidone) (PVP) to control the shape and size of the resulting particles. The molar ratio of PVP to the cobalt in the precursor solution was key to controlling the particle morphology. In the absence of PVP, large truncated cube-like particles with a bimodal size distribution (200 and 1450 nm) are formed. When the molar ratio of PVP/Co is less than ≈ 0.5, the particle size is generally invariant. As the molar ratio of PVP/Co is increased from 0.5 to nearly 2, the cubes with rounded edges are formed, but the hydrodynamic diameter decreases from nearly 1.4 μm to 800 nm at a molar ratio of 1.8. At PVP/Co > 2, there is a significant decrease in the particle size down to 142 nm at PVP/Co = 3.5 and the shape becomes spherical, but the particles remain crystalline, characteristic of Co3O4. The size of Co3O4 particles was also determined using scanning electron microscopy. This size is consistent with the particle size obtained from dynamic light scattering. A competitive growth mechanism between free cobalt ions and PVP-bound cobalt ions in the solution was proposed to explain the influence of PVP on the size and shape of the Co3O4 particles. Due to heavy metal specificity for the binding with PVP and the crystal space group for metal oxides, the evolution in particle shape is not universal with no apparent change in the shape obtained over a similar PVP concentration range for iron oxide.
KeywordsCobalt oxide Hydrothermal synthesis Shape control
We want to thank Thomas Quick for his help with the XRD measurements. This work was partially supported by the National Science Foundation under Grant No. CBET-1510612. MLB acknowledges support from the W. Gerald Austen Endowed Chair in Polymer Science and Polymer Engineering via the Knight Foundation. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for the partial support of this research under award no. 53739-ND7.
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Conflict of interest
The authors declare that they have no conflict of interest.