C. Torres1*, R. Carrasco², J. García-Alonso2, G. C. Vásquez2, and D. Maestre2 

1 Inter American University of Puerto Rico, 

2 Departamento de Física de Materiales, Facultad de CC. Físicas, Universidad Complutense de Madrid, 28040, Madrid, Spain

Transition metal oxides, including Mn and Ni oxides, are gaining increasing interest in many areas of research, such as sensors, catalysts, electronic devices, and energy storage, among others [1], based on their interesting and tailored properties. NiO has a wide bandgap around 3.6–4.3 eV, exhibits excellent durability, low toxicity, and is one of the few p-type oxides [1]. Mn-based oxides (e.g: MnO, MnO2, Mn3O4), exhibit wide bandgaps and variable properties depending on the corresponding cationic valence [2]. Besides, Ni-Mn based oxides, such as NiMn2O4, exhibit characteristic properties based on the multivalence and variable lattice positions of the cations, being considered a go-to material in diverse applications, such as thermistors, catalyst, and electrodes in Li-ion batteries [3], which improvement requires further research.

In this research, the samples were synthesized following a vapor-solid method under a controlled Ar or N2 flow using diverse Mn- and Ni-based precursors. Thermal treatments were carried out between 850 and 1400 C, in order to shed light to the stability of the spinel NiMn2O4 and the formation of Ni- and Mn- oxides at the micro- and nanoscale. The samples were characterized by SEM (Scanning Electron Microscopy), EDS (Energy Dispersive x-ray Spectroscopy), Photoluminescence (PL), and micro-Raman Spectroscopy. Treatments at low temperature promote the formation of spinel NiMn2O4, as confirmed by EDS and Raman, in combination with NiO, leading to n-p heterostructures at the surface of the treated pellets. Mn3O4 is observed as well at low temperature, as NiMn2O4 undergoes partial decomposition at temperatures near 900 C [3]. The stability of this nickel manganite has been analyzed at variable temperatures up to 1400 C. Micro- and nanostructures based on Mn-oxides, mainly Mn3O4, in forms of rods, wires, plates and hierarchical structures with variable sizes have been obtained by a vapor-solid method using different precursor materials and thermal treatments.

Figure 1. Microrods show thinner nanowires with submicrometric widths, growing from their lateral surfaces.

Further studies on the spinel oxide formation and stability, as well as the study of the diverse Mn-based oxide nano- and microstructures will lead to improved applications based on these oxides.

References

[1] M. Taeño, D. Maestre, and A. Cremades. Nanophotonics, 2021, 10 (7), 1785-1799.

[2] J.E. Post, D.A. McKeown, P.J. Heaney. American Mineralogist, 2020, 105, 1175-1190.

[3] J. Dinger, T. Friedich, T. Reimann, and J. Töpfer. J, Am. Cer. Soc., 2023, 106, 1834-1847.