Flux‐mediated synthesis and photocatalytic activity of NaNbO₃ particles

Abstract

Using molten‐salt synthetic techniques, NaNbO3 (Space group Pbcm; No. 57) was prepared in high purity at a reaction time of 12 hours and a temperature of 900°C. All NaNbO3 products were prepared from stoichiometric ratios of Nb2O5 and Na2CO3 together with the addition of a salt flux introduced at a 10:1 molar ratio of salt to NaNbO3, that is, using the Na2SO4, NaF, NaCl, and NaBr salts. A solid‐state synthesis was performed in the absence of a molten salt to serve as a control. The reaction products were all found to be phase pure through powder X‐ray diffraction, for example, with refined lattice constants of a = 5.512(5) Å, b = 5.567(3) Å, and c = 15.516(8) Å from the Na2SO4 salt reaction. The products were characterized using UV‐Vis diffuse reflectance spectroscopy to have a bandgap size of ~3.5 eV. The particles sizes were analyzed by scanning electron microscopy (SEM) and found to be dependent upon the flux type used, from ~<1 μm to >10 μm in length, with overall surface areas that could be varied from 0.66 m2/g (for NaF) to 1.55 m2/g (for NaBr). Cubic‐shaped particle morphologies were observed for the metal halide salts with the set of exposed (100)/(010)/(001) crystal facets, while a truncated octahedral morphology formed in the sodium sulfate salt reaction with predominantly the set of (110)/(101)/(011) crystal facets. The products were found to be photocatalytically active for hydrogen production under UV‐Vis irradiation, with the aid of a 1 wt% Pt surface cocatalyst. The platinized NaNbO3 particles were suspended in an aqueous 20% methanol solution and irradiated by UV‐Vis light (λ > 230 nm). After 6 hours of irradiation, the average total hydrogen production varied with the particle morphologies and sizes, with 753 µmol for Na2SO4, 334 µmol for NaF, 290 µmol for NaCl, 81 µmol for NaBr, and 249 µmol for the solid‐state synthesized NaNbO3. These trends show a clear relationship to particle sizes, with smaller particles showing higher photocatalytic activity in the order of NaF > NaCl > NaBr. Furthermore, the particle morphologies obtained from the Na2SO4 flux showed even higher photocatalytic activity, though having a relatively similar overall surface area, owing to the higher activity of the (110) crystal facets. The apparent quantum yield (100 mW/cm2, λ = 230 to 350 nm, pH = 7) was measured to be 3.7% for NaNbO3 prepared using the NaF flux, but this was doubled to 6.8% when prepared using the Na2SO4 flux. Thus, these results demonstrate the powerful utility of flux synthetic techniques to control particle sizes and to expose higher‐activity crystal facets to boost their photocatalytic activities for molecular hydrogen production.

“Soft” Alkali Bromide and Iodide Fluxes for Crystal Growth

“…The ABI fluxes have also been used to control the crystal morphology of several oxides such as NaNd(MoO 4 ) 2 (Liu et al, 2016 ), KEu(MoO 4 ) 2 (Wu et al, 2013 ), Li 2 NiPO 4 F (Yamada et al, 2018 ), and NaNbO 3 (Hamilton et al, 2020 ). In each case the identity of the flux or the amount of flux played a significant role in the crystal morphology.…”

“…A variety of fluxes were used for the synthesis of NaNbO 3 in order to determine the influence on particle size and morphology as well as photocatalytic activity. Out of the fluxes used (Na 2 SO 4 , NaF, NaCl, and NaBr), NaBr produced the largest particles with the surface area of the particles was nearly twice as large as the next leading flux; however, photocatalytic activity was much higher in particles grown using Na 2 SO 4 (Hamilton et al, 2020 ).…”

Studies on structural, dielectric and optical properties of (Ba0.95Ca0.05)1−x(Ti0.8Sn0.2)1−xNaxNbxO3 lead-free ceramics

“…16 In addition, NaNbO 3 perovskite is an important photocatalysis semiconductor with a band gap energy of 3.4 eV. 17,18 As a result, NN is also a promising candidate in many areas of optical applications involving energy storage, optical waveguides, ultrafast optical switching, nonvolatile ferroelectric memories and semiconductor photoelectrodes. 19 Moreover, in the eld of photocatalysis, it is used for the separation of water, purifying the air and the decomposition of dyes.…”

“…It exhibits a phase transition at T C around 913 K with a maximum real permittivity of 1750 at T C . 22 NaNbO 3 (NN) is a anti-ferroelectric at room temperature 17 and it transforms to ferroelectric substance, if mixed with a small amount of KNbO 3 , 7 LiNbO 3 , 23 BaSnO 3 , 24 and SrSnO 3 . 25 However, only several studies have been conducted on the ceramics formed BaTiO 3 –NaNbO 3 system with the aim to examine the effect of cation substitution in A and B sites, including significant changes in the dielectric and structural properties.…”

Molten-Salt Flux Mediated Synthesis of ZnO-SnO2 Composites: Effects of Surface Areas and Crystallinities on Photocatalytic Activity

“…In a flux synthesis, a selected inorganic salt is made molten by being heated above its melting point, which acts as a solvent to dissolve the solid reactants in the synthesis and for crystallization of the products. Compared to other synthetic methods, the flux synthesis has several advantages, such as shortening the reaction time and lowering the reaction temperature by improving the diffusion rate of the reactants, facilitating crystal growth with improved morphology and particle size homogeneity, and manipulating particle sizes and morphologies [36][37][38][39][40][41][42][43] . Therefore, the molten salt flux-mediated method provides a straight-forward way to synthesize ZnO-SnO2 composites with the possibility to tune the crystallite sizes and crystallinities of the composite by varying synthetic conditions such as the temperature.…”