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

Abstract

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.