Rodolfo Bezerra da Silva

Green synthesis has emerged as a sustainable alternative for preparing nanostructured electrocatalysts using renewable biological resources. In this work, NiO nanoparticles were synthesized using an ethanolic extract of Sargassum sp. as a biogenic mediator, and the influence of calcination temperature (300, 500, and 700 °C) on their physicochemical and electrocatalytic properties was systematically investigated. X-ray diffraction confirmed the formation of NiO cubic phase and showed an increase in crystallite size with increasing calcination temperature. FTIR, Raman, XPS, and EPR analyses revealed temperature-dependent changes in surface composition and defect chemistry, including mixed Ni2+/Ni3+ species and oxygen-vacancy-related centers. EPR signals at g ≈ 2.0005–2.0006 are assigned to F+-type centers. Electrochemical measurements in 1.0 mol L–1 KOH showed that NiO-300 exhibited the highest OER activity, requiring an overpotential of 352 mV to reach 10 mA cm–2, compared with 368 and 356 mV for NiO-500 and NiO-700, respectively. Although NiO-300 displayed the lowest estimated electrochemically active surface area among the samples, it consistently showed the lowest charge-transfer resistance, indicating more favorable interfacial kinetics. Its enhanced performance is therefore associated with the combined effects of defect-related surface species, oxygen-vacancy-related centers, and improved charge-transfer dynamics. Chronopotentiometric measurements at 10 and 100 mA cm–2 further demonstrated the excellent electrochemical stability of NiO-300, while post-OER XRD and Raman analyses confirmed the preservation of its crystalline framework after prolonged operation.

We report a temperature-dependent Raman scattering study of the hybrid organic–inorganic azide perovskite of formula [(CH3)2NH2][Mn(N3)3]([DMA][Mn(N3)3]). The Raman spectra reveal distinct shifts in phonon frequencies, particularly in the νsCNC and νs(ν1)N3- modes, demonstrating that spin-phonon interactions play a critical role in their thermal and magnetic responses. Differential scanning calorimetry confirmed a first-order structural phase transition, from an orthorhombic high-temperature HT(α) phase, belonging to the Cmca space group, to a monoclinic low-temperature LT(β) phase, with P21 symmetry. The associated entropy change was found to be |∆S| ~37.2 J kg-1 K-1, and the barocaloric (BC) coefficient (|δTt/δP|) was ~2.94 K kbar-1, estimated by using the Clausius-Clapeyron method. Room-temperature FTIR reflectivity spectra revealed the polar phonon modes and their corresponding damping coefficients, showing minimal contributions from the DMA+ cation to the intrinsic dielectric constant (~4.3). These results provide new insights into the coupling between lattice dynamics, spin interactions, and barocaloric behavior in azide-based hybrid perovskites.