Paul A. Maggard

Alignment of acentric MoO3F33− anions in a polar material: (Ag3MoO3F3)(Ag3MoO4)Cl

(Ag 3 MoO 3 F 3 )(Ag 3 MoO 4 )Cl was synthesized by hydra(solvato)thermal methods and characterized by single-crystal X-ray diffraction (P3m1. No. 156, Z = 1, a = 7.4488(6) A, c = 5.9190(7) A). The transparent colorless crystals are comprised of chains of distorted fac-MoO 3 F 3- 3 octahedra and MoO 2- 4 tetrahedra anions, as suggested by the formulas Ag 3 MoO 3 F 3 and Ag 3 MoO + 4 , and are connected through Ag + cations in a polar alignment along the c-axis. One Cl anion per formula unit serves as a charge balance and connects the two types of chains in a staggered fashion, offset by ∼1/2 x c. In MoO 2- 4 the Mo atom displaces towards a single oxide vertex, and in MoO 3 F 3- 3 , the Mo displaces towards the three oxide ligands. The ordered oxide-fluoride ligands on the MoO 3 F 3- 3 anion is important to prevent local inversion centers, while the polar organization is directed by the Cl - anion and interchain dipole-dipole interactions. The dipole moments of MoO 3 F 3- 3 and MoO 2- 4 align in the negative c-axis direction, to give a polar structure with no cancellation of the individual moments. The direction and magnitude of the dipole moments for MoO 3 F 3- 3 and MoO 2- 4 were calculated from bond valence analyses and are 6.1 and 1.9 debye (10 -18 esu cm) respectively, compared to 4.4 debye for polar NbO 6 octahedra in LiNbO 3 , and 4.5 debye for polar TiO 6 octahedra in KTiOPO 4 (KTP).

CuNb3O8: A p-Type Semiconducting Metal Oxide Photoelectrode.

A new p-type CuNb3O8 polycrystalline photoelectrode was investigated and was determined to have indirect and direct bandgap sizes of 1.26 and 1.47 eV, respectively. The p-type polycrystalline film could be prepared on fluorine-doped tin oxide glass and yielded a cathodic photocurrent under visible-light irradiation (λ > 420 nm) with incident photon-to-current efficiencies of up to ∼6-7% and concomitant hydrogen evolution. A Mott-Schottky analysis yielded a flat band potential of +0.35 V versus RHE (pH = 6.3) and a calculated p-type dopant concentration of ∼7.2 × 10(15) cm(-3). The conduction band energies are found to be negative enough for the reduction of water under visible light irradiation. A hole mobility of ∼145 cm(2)/V·s was obtained from J(I)-V(2) measurements using the Mott-Gurney relation, which is ∼50% higher than that typically found for p-type Cu2O. DFT-based electronic structure calculations were used to probe the atomic and structural origins of the band gap transitions and carrier mobility. Thus, a new p-type semiconductor is discovered for potential applications in solar energy conversion.

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

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 metal-oxides has led to the stabilization of phases with 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 ion-exchange reactions). Thus, flux preparations of metal-oxide photocatalysts assist in the growth and optimization of their particles in order to understand and tune the photocatalytic reaction rates at their surfaces.

Crystal chemistry, band engineering, and photocatalytic activity of the LiNb3O8-CuNb3O8 solid solution.

A new solid solution has been prepared in the system LiNb3O8-CuNb3O8, and the impacts of chemical composition and crystal structure have been investigated for the resulting band gap sizes and photocatalytic activities for water reduction to hydrogen under visible light. All members of the solid solution were synthesized by solid-state methods within evacuated fused-silica vessels, and their phase purities were confirmed via powder X-ray diffraction techniques (space group P2(1)/a, a = 15.264(5)-15.367(1) Å, b = 5.031(3)-5.070(1) Å, c = 7.456(1)-7.536(8) Å, and β = 107.35(1)-107.14(8)°, for 0 ≤ x ≤ 1). Rietveld refinements were carried out for the x = 0.09, 0.50, and 0.70 members of the solid solution, which reveal the prevailing isostructurality of the continuous solid solution. The structure consists of chains of (Li/Cu)O6 and NbO6 octahedra. The optical band gap size across the solid solution exhibits a significant red-shift from ∼3.89 eV (direct) to ∼1.45 eV and ∼1.27 eV (direct and indirect) with increasing Cu(I) content, consistent with the change in sample color from white to dark brown to black. Electronic structure calculations based on density-functional theory methods reveal the rapid formation of a new Cu 3d(10)-based valence band that emerges higher in energy than the O 2p band. While the pure LiNb3O8 is a highly active UV-photocatalyst for water reduction, the Li(1-x)Cu(x)Nb3O8 solid is shown to be photocatalytically active under visible-light irradiation for water reduction to hydrogen.

Metastable Cu(I)-niobate semiconductor with a low-temperature, nanoparticle-mediated synthesis.

A nanoparticle synthetic strategy for the preparation of a new metastable Cu(I)-niobate is described, and that involves multipored Li₃NbO₄ nanoparticles as a precursor. A hydrothermal reaction of HNbO₃ and LiOH·H₂O in PEG200 and water at ∼180 °C yields ∼15-40 nm Li₃NbO₄ particles. These particles are subsequently used in a solvothermal copper(I)-exchange reaction with excess CuCl at 150 °C. Heating these products within the used CuCl flux (mp = 430 °C) to 450 °C for 30 min yields ∼4-12 nm Cu₂Nb₈O₂₁ crystalline nanoparticles, and for a heating time of 24 h yields μm-sized, rod-shaped crystals. The new structure was characterized by single-crystal X-ray diffraction to have a condensed network consisting of NbO₇ polyhedra and chains of elongated CuO₄ tetrahedra. The compound thermally decomposes starting at ∼250 °C and higher temperatures, depending on the particle sizes, owing to the loss of the weakly coordinated Cu(I) cations from the structure and a concurrent disproportionation reaction at its surfaces. Thus, conventional solid-state reactions involving higher temperatures and bulk reagents have proven unsatisfactory for its synthesis. The measured bandgap size is ∼1.43-1.65 eV (indirect) and shows a dependence on the particle sizes. Electronic structure calculations based on density functional theory show that the bandgap transition results from the excitation of electrons at the band edges between filled Cu(I) 3d¹⁰-orbitals and empty Nb(V) 4d⁰-orbitals, respectively. The p-type nature of the Cu₂Nb₈O₂₁ particles was confirmed in photoelectrochemical measurements on polycrystalline films that show a strong photocathodic current under visible-light irradiation in aqueous solutions. These results demonstrate the general utility of reactive nanoscale precursors in the synthetic discovery of new Cu(I)-based semiconducting oxides and which also show promise for use in solar energy conversion applications.

Copper-organic/octamolybdates: structures, bandgap sizes, and photocatalytic activities.

The structures, optical bandgap sizes, and photocatalytic activities are described for three copper-octamolybdate hybrid solids prepared using hydrothermal methods, [Cu(pda)]4[β-Mo8O26] (I; pda = pyridazine), [Cu(en)2]2[γ-Mo8O26] (II; en = ethylenediamine), and [Cu(o-phen)2]2[α-Mo8O26] (III; o-phen = o-phenanthroline). The structure of I consists of a [Cu(pda)]4(4+) tetramer that bridges to neighboring [β-Mo8O26](4-) octamolybdate clusters to form two-dimensional layers that stack along the a axis. The previously reported structures of II and III are constructed from [Cu2(en)4Mo8O26] and [Cu2(o-phen)4Mo8O26] clusters. The optical bandgap sizes were measured by UV-vis diffuse reflectance techniques to be ∼1.8 eV for I, ∼3.1 eV for II, and ∼3.0 eV for III. Electronic structure calculations show that the smaller bandgap size of I originates primarily from an electronic transition between the valence and conduction band edges comprised of filled 3d(10) orbitals on Cu(I) and empty 4d(0) orbitals on Mo(VI). Both II and III contain Cu(II) and exhibit larger bandgap sizes. Accordingly, aqueous suspensions of I exhibit visible-light photocatalytic activity for the production of oxygen at a rate of ∼90 μmol O2 g(-1) h(-1) (10 mg samples; radiant power density of ∼1 W/cm(2)) and a turnover frequency per calculated surface [Mo8O26](4-) cluster of ∼36 h(-1). Under combined ultraviolet and visible-light irradiation, I also exhibits photocatalytic activity for hydrogen production in 20% aqueous methanol of ∼316 μmol H2 g(-1) h(-1). By contrast, II decomposed during the photocatalysis measurements. The molecular [Cu2(o-phen)4(α-Mo8O26)] clusters of III dissolve into the aqueous methanol solution under ultraviolet irradiation and exhibit homogeneous photocatalytic rates for hydrogen production of up to ∼8670 μmol H2·g(-1) h(-1) and a turnover frequency of 17 h(-1). The clusters of III can be precipitated out by evaporation and redispersed into solution with no apparent decrease in photocatalytic activity. During the photocatalysis measurements, the dissolution of the clusters in III is found to occur with the reduction of Cu(II) to Cu(I), followed by subsequent detachment from the octamolybdate cluster. The lower turnover frequency, but higher photocatalytic rate, of III arises from the net contribution of all dissolved [Cu2(o-phen)4(α-Mo8O26)] clusters, compared to only the surface clusters for the heterogeneous photocatalysis of I.

Synthesis, Characterization, and Antimicrobial Efficacy of Photomicrobicidal Cellulose Paper.

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.

Coexisting Bi and Se surface terminations of cleaved Bi2Se3 single crystals

Evidence for the coexistence of both Bi and Se terminations of the topological insulator Bi2Se3 is presented that is connected with details of sample storage and cleaving procedures. X-ray photoelectron spectroscopy of the Bi 4f core levels show a lower binding energy component indicative of metallic Bi near the sample surface. Single crystals stored and cleaved in high vacuum predominantly show the usual Se surface termination while those stored in air for long periods of time have a high probability for Bi termination. The different terminations have very different electronic structures as measured by angle resolved photoelectron spectroscopy. Our photoemission studies show the Se-terminated electronic structure can be recovered after annealing at 400 °C.

CuNb1−xTaxO3 (x ≤ 0.25) solid solutions: impact of Ta(V) substitution and Cu(I) deficiency on their structure, photocatalytic, and photoelectrochemical properties

Solid solutions of Cu(I)-containing oxide p-type semiconductors provide key opportunities to probe the fundamental relationships between chemical compositions and crystal structures, bandgap sizes, band energies, and photoelectrochemical properties. Members of the CuNb1−xTaxO3 (0 < x ≤ 0.25) solid solution have been synthesized via high temperature solid-state methods. The structure of CuNbO3 was found to be Cu-deficient Cu0.965NbO3 after heating in air at 250 °C for 3 hours, i.e., under similar conditions as those used to prepare it as a polycrystalline film. Powder X-ray diffraction techniques confirmed the purity of each composition up to x ≤ 0.25 and the lattice parameters were refined as the molar ratio of Nb(V) and Ta(V) was varied (a = 9.499 to 9.506 A, b = 8.439 to 8.451 A, c = 6.768 to 6.781 A and β = 90.847 to 90.694°). An increase in the amount of Ta(V) yielded a small blue shift of the bandgap size from ∼1.89 eV to ∼1.97 eV for CuNb1−xTaxO3 from x = 0 to 0.25. Polycrystalline films of each member of the CuNb1−xTaxO3 solid solutions produced relatively comparable p-type photocurrents of up to −0.5 mA cm−2, while the stability of the cathodic photocurrent also remained similar with increasing Ta(V) content. Mott–Schottky analysis of CuNb1−xTaxO3 showed that the conduction band edge of −1.5 (vs. SHE) provides a sufficient overpotential (∼800 mV) to drive the reduction of water to hydrogen gas at the surface. The capability of the solid solutions to drive hydrogen production was confirmed through suspended particle photocatalysis. Further characterization of the CuNb0.91Ta0.09O3 composition included scanning electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analyses. These data show that Cu(I) is oxidized to Cu(II) as CuNb1−xTaxO3 is heated in air. Thus, the formation of Cu(II) rich regions at the surface, together with the Ta(V) content, are found to play important roles in the stability and magnitude of the cathodic photocurrents produced under visible-light irradiation. Importantly, these results demonstrate that solid solution compositions can be used in films for solar energy conversion, notwithstanding their inherent atomic disorder.

Substitutional chemistry in Mn5Si3-type scandium–main group compounds and the formation of quasibinary phases

Abstract The compositions Sc5M3−xM′x (M=Al or Ga; M′=Sn, Sb or Te) were prepared by high-temperature solid-state techniques, and their Mn5Si3-type structures were either identified by powder X-ray means or determined by single crystal X-ray diffraction (hexagonal P63/mcm (No. 193), Z=2). Each Al or Ga (M) atom type exhibits mixed occupancy with Sn, Sb, or Te (M′) over different composition regions. For systems annealed at 1100–1575°C, single crystal data indicate that the phase widths of Sc5M3−xM′x extend over the ranges x=1.38(6)–2.25(2), 0.83(1)–0.96(1), 0–2.25(6), and 0–1.25(3) for Al/Sb, Al/Te, Ga/Sb, and Ga/Te, respectively. Powder X-ray data on the Sn systems shows phase width ranges of x=∼1.2–3.0 with Al and 0.0–3.0 with Ga, respectively. No interstitials in the Mn5Si3-type host were seen. The series of mixed compounds illustrate regular effects of substitution of larger and electron-richer M′ atoms in the flexible structure. The lattice constant trends follow Vegard’s law, with natural increases of a (b) and V with increasing x, but with small and irregular changes in c. Physical property measurements show that many of the compounds display metallic characteristics, with positive temperature-dependent resistivity and Pauli-like paramagnetism. The structure of Sc5Sb3 is reassigned to the Y5Bi3-type (Pnma).