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.

The Ag(I) and Bi(III) tantalates

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.

Layered Dion−Jacobson phases RbLaNb 2 O 7 and RbA 2 Nb 3 O 10 (A = Ca, Sr) and the Ruddlesden−Popper phase Rb 2 La 2 Ti 3 O 10 were prepared by solid-state methods at a reaction time of 50 h and a temperature of 1100 °C. The products were silver-exchanged within a AgNO 3 flux at a reaction time of 24 h and a temperature of 250 °C. Substitution of silver cations into the interlayer spacing of the layered structures is found to decrease the optical bandgap sizes on average by ∼0.5 to ∼1.0 eV. The products were found by scanning electron microscopy (SEM) to exhibit irregularly shaped platelet morphologies with an average size of ∼1−5 μm across their lateral dimensions and stepped edges ranging from ∼20 to ∼300 nm in height. Significant increases in photocatalytic hydrogen production rates for all silver-exchanged products were observed. The silver-exchanged RbA 2 Nb 3 O 10 layered structures exhibited the highest photocatalytic hydrogen formation rates under ultraviolet and visible irradiation (∼13,616 μmol H 2 •g −1 •h −1 ). These rates were 10 times higher than prior to silver exchange (∼1,418 μmol H 2 •g −1 •h −1 ). However, photocatalytic activity under only visible light irradiation is not observed. It is also found that the silver cations located at the surfaces are reduced to Ag(s) after prolonged UV and visible light exposure in solution, which functions to increase their activity under UV irradiation. Electronic-structure calculations based on density functional theory show that the highest-energy valence band states are composed of Ag 4d-orbital and O 2p-orbital contributions within the interlayer spacing of the structure. The lowest-energy conduction band states arise from the Nb/Ti d-orbital and O 2p-orbital contributions that are confined to the twodimensional niobate/titanate sheets within the structures and along which the excited-electrons can preferentially migrate.

Pd-containing alloys are promising materials for catalysis. Yet, the relationship of the structure−property performance strongly depends on their chemical composition, which is currently not fully resolved. Herein, we present a physical vapor deposition methodology for developing Pd x Au 1−x alloys with fine control over the chemical composition. We establish direct correlations between the composition and these materials' structural and electronic properties with its catalytic activity in an ethanol (EtOH) oxidation reaction. By combining X-ray diffraction (XRD) and X-ray photelectron spectroscopy (XPS) measurements, we validate that the Pd content within both bulk and surface compositions can be finely controlled in an ultrathin-film regime. Catalytic oxidation of EtOH on the Pd x Au 1−x electrodes presents the largest forward-sweeping current density for x = 0.73 at ∼135 mA cm −2 , with the lowest onset potential and largest peak activity of 639 A g Pd −1 observed for x = 0.58. Density functional theory (DFT) calculations and XPS measurements demonstrate that the valence band of the alloys is completely dominated by Pd particularly near the Fermi level, regardless of its chemical composition. Moreover, DFT provides key insights into the Pd x Au 1−x ligand effect, with relevant chemisorption activity descriptors probed for a large number of surface arrangements. These results demonstrate that alloys can outperform pure metals in catalytic processes, with fine control of the chemical composition being a powerful tuning knob for the electronic properties and, therefore, the catalytic activity of ultrathin Pd x Au 1−x catalysts. Our highthroughput experimental methodology, in connection with DFT calculations, provides a unique foundation for further materials' discovery, including machine-learning predictions for novel alloys, the development of Pd-alloyed membranes for the purification of reformate gases, binder-free ultrathin electrocatalysts for fuel cells, and room temperature lithography-based development of nanostructures for optically driven processes.

Titanium dioxide (TiO2) semiconductor photocatalysts were photosensitized to the visible spectrum with gold nanospheres (AuNSs) and gold nanorods (AuNRs) to study the ethanol photo-oxidation cycle, with an emphasis toward driving carbon–carbon (C–C) bond cleavage at low temperatures. The photocatalysts exhibited a localized surface plasmon resonance (SPR) that was harnessed to drive the complete photo-oxidation of formic acid (FA) and ethanol (EtOH) via augmented carrier generation/separation and photothermal conversion. Contributions of transverse and longitudinal localized SPR modes were decoupled by irradiating AuNSs–TiO2 and AuNRs–TiO2 with targeted wavelength ranges to probe their effects on plasmonically assisted photocatalytic oxidation of FA and EtOH. Photocatalytic performance was assessed by monitoring the yield of gaseous products during photo-oxidation experiments using a gas chromatography–mass spectrometry–multiple headspace extraction (GC–MS–MHE) analysis method. The complete oxidation of EtOH to CO2 under visible-light irradiation was confirmed by GC–MS–MHE for both AuNSs and AuNRs on TiO2 at room temperature. Photothermal and local field enhancements were found to aid in selectively cleaving the C–C bond in EtOH to form FA, while FA was further oxidized to CO2 by plasmon-induced electron transfer mechanisms. Under visible-light (>420 nm) irradiation, carrier generation/separation, and photothermal conversion was achieved, resulting in the photogenerated “hot” holes driving the photo-oxidation primarily on the gold nanoparticles. Specifically, plasmonic enhancement by AuNR–TiO2 enhances EtOH oxidation, providing a method to selectively cleave C–C bonds.