Chromium incorporation into TiO2 at high pressure

Structural, Magnetic, and Electronic Properties of the Cr1-xTixO2 Solid

Cr_1-xTi_xO₂ (0 ≤ x ≤ 1) solid solution was synthesized by a high-pressure and high-temperature method, whereafter systematic experimental and computational studies were conducted on the Cr_1-xTi_xO₂ system. The crystal structure of the samples where 0 ≤ x ≤ 0.4 and x = 1 was of a rutile structure (P4₂/mnm), while samples where 0.5 ≤ x ≤ 0.9 crystallized in a CaCl₂ structure (Pnnm). The structural transformation from rutile-type to CaCl₂-type structure should be due to the combined action of positive chemical pressure and physical pressure. The saturation magnetization of the Cr_1-xTi_xO₂ samples decreased linearly with the increase of x because Ti⁴⁺ is nonmagnetic. In addition, the Curie temperature of the Cr_1-xTi_xO₂ samples also decreased noticeably with the increase of x. When nonmagnetic Ti⁴⁺ randomly replaced Cr⁴⁺ and occupied its position, the net exchange coupling in Cr_1-xTi_xO₂ would decrease. When Ti⁴⁺ occupied the majority in the system, Cr⁴⁺ ions would be separated by nonmagnetic Ti⁴⁺ ions far enough for the long-range ferromagnetic order to weaken or even disappear with the increase of x, causing the Cr_1-xTi_xO₂ system to finally approach a paramagnetic state. Density functional theory calculations were performed for the Cr_1-xTi_xO₂ system, and the predicted trends of the magnetic properties agreed well with the experimental results. These calculations also showed that Cr_1-xTi_xO₂ was still half-metallic until x reached 0.6.

A symbiotic hetero-nanocomposite that stabilizes unprecedented CaCl2-type TiO2 for enhanced solar-driven hydrogen evolution reaction

Symbiotic hetero-nanocomposites prevail in many classes of minerals, functional substances and/or devices. However, design and development of a symbiotic hetero-nanocomposite that contains unachievable phases remain a significant challenge owing to the tedious formation conditions and the need for precise control over atomic nucleation in synthetic chemistry. Herein, we report a solution chemistry approach for a symbiotic hetero-nanocomposite that contains an unprecedented CaCl2-type titania phase inter-grown with rutile TiO2. CaCl2 structured TiO2, usually occurring when bulk rutile-TiO2 is compressed at an extreme pressure of several GPa, is identified to be a distorted structure with a tilt of adjacent ribbons of the c-axis of rutile. The structural specificity of the symbiotic CaCl2/rutile TiO2 hetero-nanocomposite was confirmed by Rietveld refinement, HRTEM, EXAFS, and Raman spectra, and the formation region (TiCl4 concentration vs. reaction temperature) was obtained by mapping the phase diagram. Due to the symbiotic relationship, this CaCl2-type TiO2 maintained a high stability via tight connection by edge dislocations with rutile TiO2, thus forming a CaCl2/rutile TiO2 heterojunction with a higher reduction capacity and enhanced charge separation efficiency. These merits endow symbiotic CaCl2/rutile TiO2 with a water splitting activity far superior to that of the commercial benchmark photocatalyst, P25 under simulated sunlight without the assistance of a cocatalyst. Our findings reported here may offer several useful understandings of the mechanical intergrowth process in functional symbiotic hetero-nanocomposites for super interfacial charge separation, where interfacial dislocation appears to be a universal cause.