Mn-Doping in NiO Nanoparticles: Defects-Modifications and Associated Effects Investigated Through Positron Annihilation Spectroscopy

Abstract

Manganese-doped nickel oxide (Ni1-xMnxO) nanoparticulate samples with x in the range 0 (undoped sample) to 0.35 were synthesized by sol-gel method involving chemical reactions between the solutions of nickel nitrate hexahydrate and manganese acetate tetrahydrate. The nanocrystallites obtained after annealing of the precipitates for different durations were characterized by X-ray diffraction and high resolution transmission electron microscopy. The samples showed high degree of purity with no secondary phase up to 35 at.% (x = 0.35) of Mn-doping. At the initial doping concentrations, the crystallite sizes increased due to vacancy type defects being recombined with some of the doped Mn2+ ions. However, substitution-induced strain soon overtook the crystallite dynamics and the sizes rapidly started reducing again as an indirect consequence of the necessity to accommodate majority of the doped cations on the surfaces of the nanocrystallites. There was conspicuous changes in the lattice parameter too which could again be attributed to the strain and charge effects. The average sizes of the crystallites were obtained in the range 5.5 nm to 13.1 nm for the different samples. UV-Vis absorption studies indicated the formation of excitonic states in NiO on Mn-doping. The band gap energy (Eg) derived from the optical absorption spectra showed a continuous increase with increase of Mn-doping of the samples. Positron lifetime and Doppler broadening spectroscopic studies were carried out on those samples to characterize the vacancy type defects and defect clusters/complexes. There were also indications to suggest positron annihilation at the crystallite surfaces owing to their sizes of nanometer order. Positron lifetimes decreased upon increase of Mn-doping. The coincidence Doppler broadened ratio curves indicated definite shifts of the prominent oxygen-electron-annihilation peak and the variation of the lineshape parameter S also indicated clearly the effects of Mn-doping.

Hierarchical Manganese–Nickel Sulfide Nanosheet Arrays as an Advanced Electrode for All-Solid-State Asymmetric Supercapacitors

“…The XRD pattern of Mn–Ni–O NAs (1:2) is also presented in Figure S6 for comparison. Mn–Ni–O NAs (1:2) showed diffraction peaks at 2θ angles of 37.0, 42.9, 62.3, 74.7, and 78.6°, matching well with the (111), (200), (220), (311), and (222) planes of cubic (NiO) 0.75 (MnO) 0.25 (JCPDS card no.78-0425), respectively. , The Raman spectra of Mn–Ni–S NAs (1:2) and Mn–Ni–O NAs (1:2) are displayed in Figure S7. Two obvious characteristic Raman bands at 560 and 1100 cm –1 , detected on the Raman spectrum of Mn–Ni–S NAs (1:2), were ascribed to the optical band and high-frequency band of NiS, respectively .…”

Diverse Spectroscopic Studies and First-Principles Investigations of the Zinc Vacancy Mediated Ferromagnetism in Mn-Doped ZnO Nanoparticles

“…The ratio curves generated from the CDB spectra of all the samples following the procedure described by Asoka-Kumar et al and using annealed single crystalline Al (of purity 99.999%) as the reference material are shown in Figure . The characteristic peak at the electron momentum p L = 9.8 × 10 –3 m o c is due to positron annihilation by the 2p electrons of oxygen environment surrounding the cation (Zn) monovacancies and is usually observed in wide band gap metal oxide semiconductors. ,,− In other words, positrons are mainly trapped in vacancies generated by the absence of Zn 2+ ions or its clusters. A report that discusses the annihilation of positrons at the negatively charged Zn 2+ vacancies in ZnO confirms it with the corresponding CDB spectra of CdO, NiO and Zn metal .…”

Annealing temperature dependency of structural, optical and electrical characteristics of manganese-doped nickel oxide nanosheet array films for humidity sensing applications

“…12 In addition, the performance of several other applications of NiO, such as electrochromic devices 13 and perovskite solar cells, 14 was also improved by doping. Various metals have been investigated as dopants, including Al, 4,15 Zn, 13 Li, 14 Sn 16 and Mn, 17,18 to improve the properties of NiO. In particular, the combination of Mn with NiO has attracted significant attention due to its distinctive magnetic properties.…”