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

Abstract

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.

Industrial-Scale Engineering of Nano {RuO2/TiO2} for Photocatalytic Water Splitting: The Distinct Role of {(Rutile)TiO2–(Rutile)RuO2} Interfacing

“…FSP Setups for Engineering of RuO 2 /TiO 2 Nanointerfaces. Two distinct FSP configurations (single-nozzle or double-nozzle) 40,44 were applied (Figure 1): in the singlenozzle-FSP (SN-FSP) configuration, all precursor solutions, i.e., Ti and Ru precursors, were combined in in the same solvent mixture. Using a syringe pump (KDS 100 legacy syringe pump, KDScientific), the liquid precursor solution comprising Ti and Ru atoms was sprayed via a capillary tube, at a rate of P = 5 mL min 1 .…”

“…In the DN-FSP configuration, TiO 2 and RuO 2 nanoparticles were formed separately using two FSP nozzles, code-named Nozzle-1 and Nozzle-2, respectively (refer to Figure D). After a series of optimizations, ,, an asymmetrical DN-FSP setup was employed (see Figure B,D), where the two nozzles were arranged at specific intersection distances and angles. Specifically, Nozzle-2, dedicated to the synthesis of RuO 2 , was positioned at an angle of β = 30° and a distance of 64 cm from the particle-collection glass-fiber filter.…”

“…To date, FSP has been extensively utilized in the production of most known metal oxides. 36,37 42,43 NaTaO 3 /NiO, 44 and PdO/Pd 0 /TiO 2 45 have been produced in one step. In the case of RuO 2 , due to the high volatility of Ru-atoms at elevated temperatures (T > 800 °C), such as those occurring in a typical FSP flame, 35 the formation of highly volatile oxides RuO 3 and RuO 4 rather than RuO 2 could be favored.…”

“…To date, FSP has been extensively utilized in the production of most known metal oxides. , Several research groups have exemplified the used advanced FSP configurations to produce more complex nanoconfigurations for catalytic technologies. Pertinent examples include the single-nozzle FSP (SN-FSP), double-nozzle FSP (DN-FSP), or sequential-deposition FSP, where catalytic heterostructures of Pt/TiO 2 , − Pt/Ba/Al 2 O 3 , Co/Al 2 O 3 , , NaTaO 3 /NiO, and PdO/Pd 0 /TiO 2 have been produced in one step. In the case of RuO 2 , due to the high volatility of Ru-atoms at elevated temperatures ( T > 800 °C), such as those occurring in a typical FSP flame, the formation of highly volatile oxides RuO 3 and RuO 4 rather than RuO 2 could be favored .…”

Flame Spray Pyrolysis Synthesis of Vo-Rich Nano-SrTiO3-x

“…In this context, flame spray pyrolysis (FSP) is a versatile technology for the engineering of multifunctional nanostructures and nanodevices [ 20 ] with controllable characteristics (size, phase, crystallinity), and can provide nanoparticles at large quantities. Recently, we have demonstrated that FSP can be successfully employed to synthesize highly-photoactive perovskite materials, e.g., BiFeO 3 [ 21 ], NaTaO 3 [ 22 ]. Herein, we show that FSP-made SrTiO 3 with controlled Vos is a novel approach towards enhanced photoactivity.…”