Tuning of intrinsic antiferromagnetic to ferromagnetic ordering in microporous α-MnO2 by inducing tensile strain

Tuning the structural stability and spin-glass behavior in α-MnO2 nanotubes by Sn ion doping

A series of α-Mn_1-xSn_xO₂ was synthesized by a simple hydrothermal method to shed light on the effect of substitution. Powder X-ray diffraction and scanning electron microscopy indicated that the particle size, crystal structure and morphology of the samples did not change with an increase of the Sn content. Sn, Mn, O and K elements were all uniformly distributed in the particles, which was observed using energy-dispersive X-ray spectroscopy. However, thermogravimetric analysis showed that the structural stability increased, and an increase of the Mn oxidation state from 3.8+ to nearly 4.0+ was observed by X-ray absorption spectroscopy. Besides, ¹¹⁹Sn Mössbauer spectroscopy revealed that the Sn ions are all 4+ and incorporate into the lattice by replacing the Mn ions. The DC and AC magnetic susceptibility measurements down to 2 K exhibited a spin-glass phenomenon, and the freezing temperature, T_f, decreased from 44 K to 30.5 K with increasing Sn content. This indicates that increased disorder by nonmagnetic substitution results in the enhancement of the frustration in the lattice. Meanwhile, with doping of Sn⁴⁺ ions, the Curie-Weiss temperature increased, indicating enhanced antiferromagnetic interaction. Although the mixed valence of Mn³⁺ and Mn⁴⁺ almost disappeared, the reduction of charge disorder did not lead to the magnetic ordering in the sample. Since the Sn⁴⁺ ions are diamagnetic and have the same magnetic effect as cation vacancies in the lattice, so it is reasonable to believe that the spin-glass transition in α-MnO₂ results from the cation vacancies rather than the mixture of Mn³⁺ and Mn⁴⁺.

Uncovering the origin of enhanced field emission properties of rGO-MnO2 heterostructures: a synergistic experimental and computational investigation

The unique structural merits of heterostructured nanomaterials including the electronic interaction, interfacial bonding and synergistic effects make them attractive for fabricating highly efficient optoelectronic devices. Herein, we report the synthesis of MnO2 nanorods and a rGO/MnO2 nano-heterostructure using low-cost hydrothermal and modified Hummers' methods, respectively. Detailed characterization and confirmation of the structural and morphological properties are done via X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM). Compared to the isolated MnO2 nanorods, the rGO/MnO2 nano-heterostructure exhibits impressive field emission (FE) performance in terms of the low turn-on field of 1.4 V μm-1 for an emission current density of 10 μA cm-2 and a high current density of 600 μA cm-2 at a relatively very low applied electric field of 3.1 V μm-1. The isolated MnO2 nanorods display a high turn-on field of 7.1 for an emission current density of 10 μA cm-2 and a low current density of 221 μA cm-2 at an applied field of 8.1 V μm-1. Besides the superior FE characteristics of the rGO/MnO2 nano-heterostructure, the emission current remains quite stable over the continuous 2 h period of measurement. The improvement of the FE characteristics of the rGO/MnO2 nano-heterostructure can be ascribed to the nanometric features and the lower work function (6.01 and 6.12 eV for the rGO with 8% and 16% oxygen content) compared to the isolated α-MnO2(100) surface (Φ = 7.22 eV) as predicted from complementary first-principles electronic structure calculations based on density functional theory (DFT) methods. These results suggest that an appropriate coupling of rGO with MnO2 nanorods would have a synergistic effect of lowering the electronic work function, resulting in a beneficial tuning of the FE characteristics.