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.
Photocatalytic assembly of heterometallic nanoarchitectures via plasmonic hot electrons is demonstrated by liquid-phase, reductive photodeposition of platinum (Pt) onto gold nanorods (AuNR) under longitudinal surface plasmon (LSP) excitation. Nucleation of Pt 0 from PtCl 6 2−was initiated by plasmonic hot electrons at the Au surface. Sub-5 nm epitaxial overgrowth of Pt followed a core−shell morphology. Measured 6.5 longitudinal:transversal growth aspect ratio revealed longitudinal growth preferentiality that was consistent with the LSP dipole polarity. In situ spectroscopic monitoring of the photocatalytic growth process permitted real-time feedback into Au surface functionalization with PtCl 6 2− according to 16 nm red-shift in its Cl−Pt ligand-to-metal charge-transfer (L π MCT) band involving ligand π orbitals. Subsequent Pt 0 growth kinetics were tracked using the L π MCT band. Discrete dipole models elucidated evolving lightmatter interactions of Pt-decorated AuNR that were consistent with experimental characterization. These studies provide a foundational mechanistic understanding toward guided assembly of heterometallic nanoarchitectures at ambient conditions via plasmonic hot electrons.
Toward our goal of scalable, antimicrobial materials based on photodynamic inactivation, paper sheets comprised of photosensitizer-conjugated cellulose fibers were prepared using porphyrin and BODIPY photosensitizers, and characterized by spectroscopic (infrared, UV-vis diffuse reflectance, inductively coupled plasma optical emission) and physical (gel permeation chromatography, elemental, and thermal gravimetric analyses) methods. Antibacterial efficacy was evaluated against Staphylococcus aureus (ATCC-2913), vancomycin-resistant Enterococcus faecium (ATCC-2320), Acinetobacter baumannii (ATCC-19606), Pseudomonas aeruginosa (ATCC-9027), and Klebsiella pneumoniae (ATCC-2146). Our best results were achieved with a cationic porphyrin-paper conjugate, Por((+))-paper, with inactivation upon illumination (30 min, 65 ± 5 mW/cm(2), 400-700 nm) of all bacterial strains studied by 99.99+% (4 log units), regardless of taxonomic classification. Por((+))-paper also inactivated dengue-1 virus (>99.995%), influenza A (∼ 99.5%), and human adenovirus-5 (∼ 99%). These results demonstrate the potential of cellulose materials to serve as scalable scaffolds for anti-infective or self-sterilizing materials against both bacteria and viruses when employing a photodynamic inactivation mode of action.
The n-type Sn2TiO4 phase was synthesized using flux methods and found to have one of the smallest visible-light bandgap sizes known that also maintains suitable conduction and valence band energies for driving photocatalytic water-splitting reactions. The Sn2TiO4 phase was synthesized using either a SnCl2 flux or a SnCl2/SnF2 peritectic flux in a 2:1 flux-to-precursor ratio heated at 600 and 400 °C for 24 h, respectively. The two types of salt fluxes resulted in large rod-shaped particles at 600 °C and smaller tetragonal prism-shaped particles at 400 °C. Surface photovoltage spectroscopy measurements produced a negative photovoltage under illumination >1.50 eV, which confirmed electrons as the majority charge carriers and ∼1.50 eV as the effective band gap. Mott–Schottky measurements at pH 9.0 showed the conduction (−0.54 V vs NHE) and valence band (+1.01 V vs NHE) positions meet the critical thermodynamic requirements for total water splitting. The Sn2TiO4 particles were deposited and annealed as polycrystalline films on FTO slides, and exhibited photoanodic currents in aqueous solutions under visible-light irradiation. The Sn2TiO4 particles were also suspended in aqueous methanol solutions and irradiated with visible and ultraviolet light. The larger rod-shaped Sn2TiO4 particles had the higher rates of photocatalytic hydrogen production (∼11.6 μmol H2 h–1) in comparison to the smaller tetragonal prism-shaped Sn2TiO4 particles (∼3.4 μmol H2 h–1). Conversely, for photocatalytic oxygen production, the rates for the smaller tetragonal prism-shaped particles in aqueous AgNO3 solution were slightly higher (∼16.3 μmol O2 h–1) than for the larger rod-shaped particles (∼11.9 μmol O2 h–1). Apparent quantum yields of 0.995% and 0.0098% were measured for O2 and H2 production, respectively, under 435 nm light.
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