Solution combustion synthesis of Ni-based hybrid metal oxides for oxygen evolution reaction in alkaline medium

Gd3+-substituted BiFeO3 perovskite nanoparticles: facile synthesis, characterization, and applications in heterogeneous catalysis

We present the combustion-based synthesis of BiFeO₃ (BFO) and Gd:BiFeO₃ perovskite nanoparticles. XRD analysis demonstrates that the undoped BFO (x = 0) perovskite sample shows a single perovskite phase with a rhombohedral structure. However, increase in the Gd³⁺ content from x = 0.05 and 0.15 to 0.25 led to the occurrence of a structural phase transformation from rhombohedral (BiFeO₃) to orthorhombic (Bi₂Fe₄O₉). With an increase in the Gd-dopant the average crystallite size of rhombohedral structures increased from 16 to 23 nm. The perovskite samples were examined using XPS, which confirmed the presence of Bi³⁺, Gd³⁺, Fe²⁺, and O²⁺ ions. FT-IR spectroscopy indicated the existence of elemental functional groups in the synthesized perovskite nanoparticles. Furthermore, the direct band gap measured by DRS reduced from 2.16 to 2.0 eV as the Gd concentration increased. The nanoparticles of the BFO perovskite had an uneven shape, a tendency to agglomerate, and fused grains with defined grain boundaries. At ambient temperature, both the undoped and Gd:BFO perovskite nanoparticles exhibit a ferromagnetic characteristic. It was found that the BET surface area of the undoped and Gd-doped BFO perovskite nanoparticles varied progressively from 4.38 to 33.52 m² g^-1. The catalytic oxidation studies conducted in a batch reactor under air conditions revealed that the synthesized catalysts, in particular, Gd:BFO (x = 0.25), exhibited higher conversion and selectivity efficiencies for glycerol (con. 100% and sel. 99.5%, respectively).

MOF-Derived Cobalt@Mesoporous Carbon as Electrocatalysts for Oxygen Evolution Reaction: Impact of Organic Linker

Water electrolysis has attracted scientists' attention as a green route for energy generation. However, the sluggish kinetics of oxygen evolution reaction (OER) remarkably increases the reaction overpotential. In this work, we developed Co-based nanomaterials as cost-effective, highly efficient catalysts for OER. In this regard, different Co-based metal-organic frameworks (MOFs) were synthesized using different organic linkers. After annealing under inert atmosphere, the corresponding Co-embedded mesoporous carbon (Co/MC) materials were produced. Among them, Co/MC synthesized using 2-methyl imidazole (Co/NMC-2MeIM) expressed the highest surface area (412 m2/g) compared to its counterparts. Furthermore, it expressed a higher degree of defects as depicted by Raman spectra. Co/NMC-2MeIM exhibited the best catalytic performance toward OER in alkaline medium. It afforded an overpotential of 292 mV at a current density of 10 mA cm-2 and a Tafel slope of 99.2 mV dec-1. The superior electrocatalytic performance of Co/NMC-2MeIM is attributed to its high content of Co3+ on the surface, high surface area, and enhanced electrical conductivity induced by nitrogen doping. Furthermore, its high content of pyridinic-N and high degree of defects remarkably enhance the charge transfer between the adsorbed oxygen species and the active sites. These results may pave the avenue toward further investigation of metal/carbon materials in a wide range of electrocatalytic applications.

Insight into nanocrystal synthesis: from precursor decomposition to combustion

Nanotechnology-based synthesis of nanoscale materials has appealed to the attention of scientists in the modern scientific community. In the bottom-up approach, atoms start to aggregate/agglomerate and form nuclei within the minimum and maximum supersaturation range. Once nuclei are generated above the critical-free energy/radius, the growth is initiated by obeying the LaMar model with a slight extra simple growth by diffusion advancement. The in situ real-time liquid phase analysis using STEM, AFM, and XAS techniques is used to control precursor decomposition to the nanocrystal formation process and should be a non-stoppable technique. Solution combustion synthesis (SCS) is a time-/energy-efficient self-sustained process that produces mass-/ion transport active porous materials. SCS also permits the synthesis of evenly distributed-doped and hybrid-nanomaterials, which are beneficial in tuning crucial properties of the materials. The growth and development of nanocrystals, dehydrating the sol in the presence of a surfactant or/and fuel results in combustion once it arrives at the ignition temperature. Besides, the kinetic and thermodynamics controlled architecture-directing agent-assisted SCS offers colloidal nanocrystal framework formation, which is currently highly applicable for energy devices. This short review provides insightful information that adds to the existing nanocrystal synthesis process and solution combustion synthesis and recommends future directions in the field.

The LaMar model visualizes the process of nanocrystal formation. The solution combustion synthesis approach is a noble methodology resulting in highly stable and ordered porous nanomaterials.