Highly Crystalline Nanosized NaTaO₃/NiO Heterojunctions Engineered by Double-Nozzle Flame Spray Pyrolysis for Solar-to-H₂ Conversion: Toward Industrial-Scale Synthesis

Sodium tantalate, NaTaO₃, nanomaterials are highly potent photocatalysts for hydrogen production from H₂O. Proper interfacing of nano-NaTaO₃ with finely dispersed nano-NiO can produce an n-p type-II heterojunction {NaTaO₃/NiO} with superior photocatalytic conversion efficiency. Making such nanomaterials widely applicable requires the establishment of an industrial-scale synthesis method, which would allow at least control of nanosize, composition, crystallinity, and interface. Herein, we have developed a scalable double-nozzle flame spray pyrolysis (DN-FSP) method, for one-step synthesis of highly crystalline {NaTaO₃/NiO} heterojunctions, with specific emphasis on the NaTaO₃ nanosize and interfacing with ultrafine NiO nanoparticles. The FSP process allows the single-step synthesis of very small NaTaO₃ (<15 nm), with ultrafine NiO (<3 nm) finely dispersed on NaTaO₃. Utilizing the versatility of FSP, we analyze the thermodynamics of nanosized NaTaO₃ perovskite gas-phase formation in flames. A library of large/small nano-NaTaO₃ and Ta₂O₅ was synthesized, employing different NiO deposition methods. The double-nozzle FSP-made {12 nm NaTaO₃/NiO} achieved benchmark photocatalytic H₂ production >10.000 μmol g^-1 h^-1 from a H₂O/methanol mixture, without implementation of any noble metal as a cocatalyst. This corresponds to a solar-to-hydrogen (STH) conversion efficiency of 0.89%, which is well above the average. The photocatalytic mechanism underlying this performance is discussed based on in situ monitoring of the photoinduced holes and electrons using electron paramagnetic resonance spectroscopy. Specifically, the carrier kinetics indicates that the superior STH conversion achieved by {NaTaO₃/NiO} is inherently related to the small NaTaO₃ nanosize that allows critical migration of photoinduced electron/hole pairs to the particle surface, outcompeting recombination.