Microwave-Assisted Coprecipitation Synthesis and Local Structural Investigation on NiO, β-Ni(OH)₂/Co₃O₄ Nanosheets, and Co₃O₄ Nanorods Using X-ray Absorption Spectroscopy at Co–Ni K-edge and Synchrotron X-ray Diffraction

Abstract

Developing the most straightforward, cheapest, and eco-friendly approaches for synthesizing nanostructures with well-defined morphology having the highest possible surface area to volume ratio is challenging for design and process. In the present work, nanosheets of NiO and β-Ni(OH) 2 /Co 3 O 4 , and nanorods of Co 3 O 4 have been synthesized at a large scale via the microwave-assisted chemical coprecipitation method under low temperature and atmospheric pressure. X-ray absorption spectroscopy (XAS) measurements, which comprises both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques, have been carried out at Co and Ni K-edges to probe the electronic structure of the samples. Also, the local atomic structural, chemical bonding, morphological, and optical properties of the sample were systematically investigated using XAS, synchrotron X-ray diffraction (SXRD), Raman spectroscopy, FTIR, transmission electron microscopy (TEM), and UV–visible spectroscopy. The normalized XANES spectra of the β-Ni(OH) 2 /Co 3 O 4 nanosheets show the presence of Ni 2+ and a mixed oxidation state of Co. The disorder factor decreases from β-Ni(OH) 2 /Co 3 O 4 to Co 3 O 4 with increasing Co–O bond length. The SXRD pattern analyzed using Rietveld refinement reveals that NiO has a face-centered cubic phase, Co 3 O 4 has the standard spinal structure, and β-Ni(OH) 2 /Co 3 O 4 has a mixed phase of hexagonal and cubic structures. TEM images revealed the formation of nanosheets for NiO and β-Ni(OH) 2 /Co 3 O 4 samples and nanorods for Co 3 O 4 samples. FTIR and Raman spectra show the formation of β-Ni(OH) 2 /Co 3 O 4 , which reveals the fingerprints of Ni–O and Co–O.

Structural evolution of a water oxidation catalyst by incorporation of high-valent vanadium from the electrolyte solution

“…2d). 62–64 The similar R -space profiles of Ni-AC-1 and VNi-AC-1 suggest both the active catalysts have analogous atomic structure (Fig. S19 & Table S3†).…”

“…S19 & Table S3†). 62–64 However, the shortening of Ni–O peak in VNi-AC-1 has been observed compared to the Ni-AC-1, which is accompanied by the increase in the oxidation state of Ni (Ni 3+ ) in VNi-AC-1 (Fig. 2d).…”

“…2c). 62–64 The low spin Ni 3+ in VNi-AC-1 has e g 1 configuration, which leads to the Jahn–Teller distortion in the catalyst structure. This results in the optimum adsorption and desorption of the oxygen intermediates to improve the catalytic performance.…”

“…The fitting results of EXAFS analysis indicate that the CN of Ni–O (5.14) in VNi-AC-1 was lower than the CN of Ni–O (5.69) in Ni-AC-1, suggesting structural defects as well as the presence of a higher amount of high valent Ni 3+ in VNi-AC-1 induced by the incorporation of high valent V ions. 62–64…”

“…S19†). 62–64 Further, the charge state of Ni decreases owing to the charge redistribution from the V site to the Ni site through the Ni–O–V bond. As V K-edge shows that the V is incorporated in the form of high valent V ions in VNi-AC-1, the charge state can still be enhanced owing the presence of high valent V ions.…”