2020 journal article

Flux‐mediated synthesis and photocatalytic activity of NaNbO 3 particles

Journal of the American Ceramic Society, 103(1), 454–464.

co-author countries: United States of America 🇺🇸
author keywords: molten salt synthesis; niobium; niobate compounds; photocatalysis
Source: ORCID
Added: December 21, 2020

Abstract Using molten‐salt synthetic techniques, NaNbO 3 (Space group Pbcm ; No. 57) was prepared in high purity at a reaction time of 12 hours and a temperature of 900°C. All NaNbO 3 products were prepared from stoichiometric ratios of Nb 2 O 5 and Na 2 CO 3 together with the addition of a salt flux introduced at a 10:1 molar ratio of salt to NaNbO 3 , that is, using the Na 2 SO 4 , 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 Na 2 SO 4 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 m 2 /g (for NaF) to 1.55 m 2 /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 NaNbO 3 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 Na 2 SO 4 , 334 µmol for NaF, 290 µmol for NaCl, 81 µmol for NaBr, and 249 µmol for the solid‐state synthesized NaNbO 3 . 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 Na 2 SO 4 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/cm 2 , λ = 230 to 350 nm, pH = 7) was measured to be 3.7% for NaNbO 3 prepared using the NaF flux, but this was doubled to 6.8% when prepared using the Na 2 SO 4 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.