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Sodium tantalate, NaTaO 3 , nanomaterials are highly potent photocatalysts for hydrogen production from H 2 O. Proper interfacing of nano-NaTaO 3 with finely dispersed nano-NiO can produce an n−p type-II heterojunction {NaTaO 3 /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 {NaTaO 3 /NiO} heterojunctions, with specific emphasis on the NaTaO 3 nanosize and interfacing with ultrafine NiO nanoparticles. The FSP process allows the single-step synthesis of very small NaTaO 3 (<15 nm), with ultrafine NiO (<3 nm) finely dispersed on NaTaO 3 . Utilizing the versatility of FSP, we analyze the thermodynamics of nanosized NaTaO 3 perovskite gas-phase formation in flames. A library of large/small nano-NaTaO 3 and Ta 2 O 5 was synthesized, employing different NiO deposition methods. The double-nozzle FSP-made {12 nm NaTaO 3 /NiO} achieved benchmark photocatalytic H 2 production >10.000 μmol g −1 h −1 from a H 2 O/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 {NaTaO 3 /NiO} is inherently related to the small NaTaO 3 nanosize that allows critical migration of photoinduced electron/hole pairs to the particle surface, outcompeting recombination.

ABO3 perovskites offer versatile photoactive nano-templates that can be optimized towards specific technologies, either by means of doping or via heterojunction engineering. SrTiO3 is a well-studied perovskite photocatalyst, with a highly reducing conduction-band edge. Herein we present a Double-Nozzle Flame Spray Pyrolysis (DN-FSP) technology for the synthesis of high crystallinity SrTiO3 nanoparticles with controlled La-doping in tandem with SrTiO3/CuO-heterojunction formation. So-produced La:SrTiO3/CuO nanocatalysts were optimized for photocatalysis of H2O/CH3OH mixtures by varying the La-doping level in the range from 0.25 to 0.9%. We find that, in absence of CuO, the 0.9La:SrTiO3 material achieved maximal efficient photocatalytic H2 production, i.e., 12 mmol g−1 h−1. Introduction of CuO on La:SrTiO3 enhanced selective production of methane CH4. The optimized 0.25La:SrTiO3/0.5%CuO catalyst achieved photocatalytic CH4 production of 1.5 mmol g−1 h−1. Based on XRD, XRF, XPS, BET, and UV-Vis/DRS data, we discuss the photophysical basis of these trends and attribute them to the effect of La atoms in the SrTiO3 lattice regarding the H2-production, plus the effect of interfacial CuO on the promotion of CH4 production. Technology-wise this work is among the first to exemplify the potential of DN-FSP for scalable production of complex nanomaterials such as La:SrTiO3/CuO with a diligent control of doping and heterojunction in a single-step synthesis.

Bi-Fe oxides are stable materials with potential photocatalytic activity under solar light photons. So far, however the photocatalytic potential of pure-phase nanosized mullite-Bi2Fe4O9 has not been studied. Usually, synthesis of pure-phase nanosized mullite-Bi2Fe4O9 is hampered by co-formation with perovskite BiFeO3. Herein we demonstrate that pure-phase mullite-Bi2Fe4O9 nanoparticles prepared by Flame Spray Pyrolysis (FSP) technology are highly efficient O2 production photocatalysts, achieving >1500 μmol g−1h−1. This outperforms all -so far reported- O2 production Bi-Fe-O photocatalysts. We present an FSP-based process for production of a versatile Bi-Fe-O platform, that can be easily optimized to obtain 100% mullite-Bi2Fe4O9 or 100% perovskite-BiFeO3 or their heterojunctions. The phase-evolution of the Bi-Fe-O materials has been studied by XPS, Raman, and EPR spectroscopies. Short post-FSP annealing process impacts the photoactivity of the BiFeO3 and Bi2Fe4O9 in distinct ways. Fe2+ centers in BiFeO3 can improve dramatically its O2 production efficiency, while solid-melt formation in Bi2Fe4O9 is a limiting factor.