Multi-functional F-doped NiTiO3 structures with high stability were prepared by a template-free hydrothermal method. A multi-technique approach including several physicochemical techniques was employed to describe the morphology, electronic properties, and structural characters. The F-doped NiTiO3 catalyst showed a much higher CO2 to CH4 photocatalytic conversion rate (2.6 μmol g−1 h−1) than pristine NiTiO3 (1.6 μmol g−1 h−1), highlighting the key role of F impurities in suppressing the charge carriers recombination. In addition to the higher efficiency and reusability than Pt/C reference electrode toward electrocatalytic water splitting, a lower Tafel slope was recorded for the F-doped structure (81.2 mV dec−1). This implies significant improvement in the reaction kinetics, arising from the synergistic effects between the F sites and bulk constituents to supply more desired active electronic states. The capability of the structures for the biomedical wastewater (containing the Caco-2 cancer cells) treatment was also explored. According to several biochemical assays, the fluorinated material shows a promising performance against the tumor cells at its IC50 concentration, 0.15 μg/mL, in a typical hospital wastewater. Our results pave the way for developing novel multi-functional photoactive titanate perovskites aiming at future clean energy transition. 

Multi-functional fluorinated NiTiO3 perovskites for CO2 photocatalytic reduction, electrocatalytic water splitting, and biomedical waste management

We carried out detailed high-pressure Raman scattering studies on lead-free perovskite piezoceramics KNbO3 (KNO) with a smaller pressure interval so that the successive phase transition pressures are precisely determined (PO→T: 8.2 GPa; PT→C: 10.9 GPa). The pressure-quenched phase does not return to the ambient orthorhombic phase completely. In this work, the pressure-induced phase transition sequence is discussed in detail. Also, based on the pressure-dependent Raman spectra of KNO, we have discovered clear evidence of a pressure-induced Fano resonance. It reveals the role played by the Raman-active continuum and discrete energy states in the condensed matter system under high pressure and enables the high-pressure Fano resonances engineering in arbitrary geometries [q∈(−∞, 0)]. These findings highlight the opportunity for designing and tuning the wave transmission and optical switch properties of lead-free piezoceramics KNO via high pressure.

https://pubs.aip.org/aip/apl/article-pdf/doi/10.1063/5.0143105/17945022/232901_1_5.0143105.pdf

Pressure-induced successive phase transitions and Fano resonance engineering in lead-free piezoceramics KNbO3

Defect chemistry and optimized dielectric properties of Ni1-δO ceramics via doping donor ions

Ilmenite-type NiTiO3 has been studied by Raman spectroscopy under hydrostatic and non-hydrostatic pressure. The rhombohedral structure does not undergo phase transitions up to 35 GPa but evidence of it is found under non-hydrostatic pressure at 45 GPa. The pressure evolution of different modes will be discussed in detail. Our findings not only provide direct experimental evidence of difference in the lattice of NiTiO3 between hydrostatic and non-hydrostatic conditions, but also offer a new insight to understand the phase stability region of ilmenite minerals in the deep earth. 

Lattice dynamics of NiTiO3 under high pressure: Raman evidence under two pressure-transmitting mediums

Titanium cobalt oxide $\mathrm{CoTi}{\mathrm{O}}_{3}$ (CTO) with ilmenite structure has attracted considerable attention in functional materials owing to their multiferroic and chemical stability characteristics similar to $\mathrm{FeTi}{\mathrm{O}}_{3}$ and perovskites. However, information about structural stability and lattice deformation of CTO under high pressure remains unclear. Here, we present a unique pressure gauge through a quantitative investigation on the vibrational properties of compressed CTO by in situ high-pressure Raman experiments up to 40 GPa. Neither phase transitions nor amorphization has been observed in such pressure range, demonstrating good lattice stability of CTO. These results have been validated by both the Jahn-Teller effect and Fermi resonance. More importantly, we have attempted to elucidate the evolution of Raman signals for a pressure gauge based on the synergistic effect of plasmon-resonance field enhancement. Our findings not only provide direct experimental evidence that CTO stays phase stable up to 40 GPa, but also offer insight to understanding the high-pressure Raman behaviors for other ilmenite based functional materials. 

Mineral pressure gauge based on lattice stability and plasmonic enhancement of cobalt titanate under high pressure

Phase Transition Toward High Symmetry Above 40 GPA in Zero-Strain Li2TiO3

As a typical layered oxide material, Li2TiO3 has attracted considerable attention in the energy revolution and military industries, owing to its lithium-rich and "zero-strain" characteristics. However, its phase-transition behavior under high pressure remains unclear. Herein, we report a second-order phase transition from the monoclinic phase to a higher-symmetry phase in nano-polycrystalline Li2TiO3 at 43 GPa by in situ high-pressure Raman experiments and first-principles calculations at 300 K. As validated by the experiments and calculations, the distortion of layered oxide-TiO6 in Li2TiO3 is crucial in the phase transition. By modulating the gap between octahedral-TiO6 layers, we propose a potential Li2TiO3 structural model for improving the electrochemical performance of lithium-ion batteries. Our findings suggest that, based on its high-pressure phase, Li2TiO3 is a promising candidate for layered cathode materials and solid tritium breeding materials for lithium-ion batteries.

Wenming Qi

Papers

Lattice dynamics of NiTiO3 under high pressure: Raman evidence under two pressure-transmitting mediums

Mineral pressure gauge based on lattice stability and plasmonic enhancement of cobalt titanate under high pressure

Pressure-induced successive phase transitions and Fano resonance engineering in lead-free piezoceramics KNbO3

Phase Transition Toward High Symmetry Above 40 GPA in Zero-Strain Li2TiO3

Pressure-induced phase transition toward high symmetry in zero-strain Li2TiO3.