Flux-mediated crystal growth of metal oxides: synthetic tunability of particle morphologies, sizes, and surface features for photocatalysis research

Abstract

Molten-salt reactions can be used to prepare single-crystal metal-oxide particles with morphologies and sizes that can be varied from the nanoscale to the microscale, subsequently enabling a growing number of novel investigations into their photocatalytic activities. Crystal growth using flux-mediated methods facilitates finer synthetic manipulation over particle characteristics. The synthetic flexibility that flux synthesis affords for the growth of metaloxides has led to the stabilization of phases limited stability, the discovery of new compositions, and access to alternate crystal morphologies and sizes that exhibit significant changes in photocatalytic activities at their surfaces, such as for the reduction of water to hydrogen in aqueous solutions. This approach has significantly impacted the current understanding of the optical and photocatalytic properties of metal-oxides, such as the dependence of band gap energies on the structure and chemical composition (i.e., obtained from flux-mediated ionexchange reactions). Thus, flux preparations of metal-oxide photocatalysts assist in the growth 2 and optimization of their particles in order to understand and tune the photocatalytic reaction rates at their surfaces.

Fabrication of well-shaped Sr2KTa5O15 nanorods with a tetragonal tungsten bronze structure by a flux method for artificial photosynthesis

“…The formation rate of CO increased and that of H 2 dramatically suppressed over Sr 2 KTa 5 O 15 fabricated by the flux method, as compared to that prepared by the SSR method. Flux method as a simple and cost-effective way to fabricate well-shaped and well-crystallized particles has been widely investigated [25,32,33]. Sr 2 KTa 5 O 15 nanorods fabricated in KCl flux in a lower calcination temperature and shorter time showed the highest photocatalytic activity and good selectivity toward CO evolution comparing with that of samples fabricated using SSR method.…”

NH3-Assisted Flux-Mediated Direct Growth of LaTiO2N Crystallites for Visible-Light-Induced Water Splitting

“…[35][36][37] The crystalline phases formed were identified using X-ray diffraction (XRD, MiniflexII, Rigaku)…”

Structural and electronic investigations of PbTa4O11 and BiTa7O19 constructed from α-U3O8 types of layers

“…Flux-synthetic methods have been shown to allow for facile ion-exchange of the interlayer A-site cations within micronsized particles, while simultaneously conserving the initial particle morphologies. [25][26][27][28] Previous investigations of the optical properties and the photocatalytic activities of the ternary tantalates within this structural family by the Maggard group have included the fluxprepared Na 2 Ta phases showed reduced bandgap sizes and significant enhancement in the water reduction and oxidation photocatalytic activities. 1,11 For example, the lowest-energy band gap transitions were found to be red-shifted in comparison to Na 2 Ta 4 O 11 .…”

“…1 Molten fluxes also act to kinetically stabilize new phases that cannot be directly synthesized using traditional solid-state methods. 25 Flux-mediated exchange reactions utilize pre-synthesized metal oxides with a desired parent structure that are reacted within a low-temperature molten-salt flux. The molten salt can take on the role of a reagent by exchanging its cations with the A-site cations in the parent structure.…”

“…Particles above the critical radius for Ostwald ripening will not re-crystallize within the molten flux into welldefined morphologies 25,[46][47][48][49]. The growth of micron-sized particles from nanoparticles into welldefined crystal morphologies has been previously reported by our group for the flux synthesis of metal oxides, such as Cu 2 Nb 8 O 21 5,25,27,50. …”