Controlled fabrication of WO3⋅H2O nanocrystals via submerged photosynthesis of crystallites under ultraviolet and infrared irradiation

With the current trend toward ecofriendly industrial production, clean and simple submerged photosynthesis of crystallites (SPsC) is attracting attention for the fabrication of nanomaterials for a wide range of applications, such as sensing materials and solar cells. Although functional nanomaterials with high solar utilization rates such as heterojunctions and other element-doped structures have been successfully fabricated via SPsC, some of the major challenges associated with this method (e.g., long fabrication times) may be overcome by investigating the crystal formation mechanism in detail. Hence, to clarify the respective roles played by light and heat in crystal formation, tungstic acid crystal nucleation (seed formation) was investigated under ultraviolet and infrared irradiation. The results indicated that light and heat affect tungstic acid seed formation and crystal growth, respectively, with infrared heating significantly accelerating crystal growth. Given that the activation energy of crystallization for tungstic acid (68.7 ± 5.9 kJ) was the same under illumination and thermal conditions, we hypothesized that oxygen defects could be introduced into the crystal lattice using low-temperature SPsC. To this end, we successfully introduced oxygen defects into the tungstic acid crystal lattice via SPsC performed at temperatures of < 25°C, achieving high solar utilization performance without element doping. • Ultraviolet (UV) irradiation is a key controlling factor in seed crystal formation. • Tungstic acid crystal growth can be accelerated by infrared (IR) heating. • The fabricated crystals were orthorhombic, and the activation energy of crystallization was evaluated. • Oxygen defects can be introduced into the crystal lattice via submerged photosynthesis of crystallites at low temperatures.