Diana Dragoe

High-entropy oxides containing more than five different cations have emerged as platforms for diverse functionalities, including magnetism. We synthesized a high-entropy oxide with an ilmenite-type structure (Mg0.2Mn0.2Co0.2Ni0.2Zn0.2)TiO3 and revealed that it undergoes long-range antiferromagnetic ordering below TN ∼ 9.5 K despite strong cation disorder. Neutron diffraction measurements showed that the magnetic structure comprises ferromagnetic honeycomb planes that stack antiferromagnetically, analogous to those of the parent CoTiO3 and NiTiO3 ilmenites. Derivatives of the high-entropy ilmenite were obtained by substituting one of the divalent cations of (Mg0.2Mn0.2Co0.2Ni0.2Zn0.2)TiO3 with an equimolar amount of Li+ ions and trivalent cations. The antiferromagnetic order remains robust against aliovalent doping that introduces extra disorder owing to cations with different valence states. In contrast, when the cation ratio was tuned to a Mn-rich composition of (Mg0.2Mn0.3Co0.15Ni0.15Zn0.2)TiO3, spin-glass freezing was observed, most likely because of the enhanced competition between the in-plane magnetic interactions. This suggests that long-range magnetic ordering in high-entropy materials is formed by a subtle balance of competing interactions, and fine-tuning of constituent magnetic ions provides control over their magnetic ground states.

Transition-metal phosphides (TMPs) are emerging as versatile materials for diverse applications, yet their scalable fabrication remains a critical challenge, limiting industrial adoption. Here, we report the direct liquid injection chemical vapor deposition (DLI-CVD) of cobalt and nickel phosphide nanomaterials, both as thin films and nanowires, using a novel class of single-source precursors. These precursors, based on a dialkyldithiophosphinate (SPS) scaffold, were synthesized in high yields and large scales, enabling reproducible and scalable production. The cobalt and nickel complexes were isolated post sublimation and fully characterized via mass spectrometry, NMR spectroscopy (1H, 13C, and 31P for the diamagnetic [Ni(SPS)2]), X-ray crystallography, and thermogravimetry analysis (TGA), confirming their structural integrity and thermal stability. When deployed as precursors in DLI-CVD, they produce phase-pure phosphide nanomaterials with precise control over composition and morphology. By modulating the deposition temperature, we demonstrate the tunable growth of CoP and Ni2P as thin films or nanowires, as validated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). This morphological versatility unlocks opportunities for surface-sensitive applications, notably catalysis. Finally, we showcase their efficacy as electrocatalysts for the hydrogen evolution reaction (HER) by depositing the nanomaterials onto carbon substrates, which underscores their potential for energy-related technologies.

Lithium lanthanum titanium oxide (LLTO) was studied as an all-solid-state electrode exhibiting either pH-sensitive or reference behavior depending on processing conditions. LLTO ceramics were prepared by solid-state reaction and densified by spark plasma sintering (SPS) using different temperatures and dwell times. Electrochemical impedance spectroscopy shows that the bulk ionic conductivity remains close to 10-3 S.cm-1 at room temperature, while grain-boundary conductivity strongly depends on SPS conditions and controls the overall ionic transport. X-ray photoelectron spectroscopy reveals variations in surface hydroxylation without changes in the Ti4+ bulk valence. Densely sintered samples exhibit a pH-dependent response with slopes up to -26 mV.pH-1, whereas fine-grained ceramics display a stable, pH-independent potential of ~154 mV vs. SHE, over the pH range 4-13. The reference behavior remains stable under low oxygen partial pressure (~10-6 atm) and in sulfide-containing media (10-7-10-3 M). These results demonstrate that processing-controlled transport and surface chemistry enable switching between pH-sensitive and reference regimes in LLTO all-solid-state electrodes.

A bifunctional catalyst combining iron porphyrin for CO ₂ RR and viologen units to selectively divert O ₂ reduction was developed. While a dominant ORR over CO ₂ RR was observed in homogeneous conditions, the reactivity shifted once heterogenized onto MWCNTs, achieving a 62% FE(CO) under 5% O ₂ .

Titanium dioxide (TiO₂), a widely used semiconductor in photocatalysis owing to its adequate potential for water hydrolysis, chemical stability, low toxicity, and low cost. However, its efficiency is limited by fast charge-carrier recombination and poor visible light absorption. Coupling TiO₂ with a p-type semiconductor, such as nickel oxide (NiO), forming a p-n heterojunction, decreases the recombination of charge carriers and increases photocatalytic activity. In this work, the surface of TiO₂ modified with NiO nanoparticles (NPs) induced by radiolysis for photocatalytic hydrogen production was studied. The photocatalytic activity of NiO/TiO₂ was evaluated using methanol as a hole scavenger under UV–visible light. All modified samples presented superior photocatalytic activity compared to bare TiO₂. The dynamics of the charge carriers, a key electronic phenomenon in photocatalysis, was investigated by time-resolved microwave conductivity (TRMC). The results highlight the crucial role of Ni-based NPs modification in enhancing the separation of the charge carrier and activity under UV–visible irradiation. Furthermore, the results revealed that under visible irradiation, NiO-NPs inject electrons into the conduction band of titanium dioxide.