Syntheses, Crystal Structures, and Electronic Structures of Quaternary Group IV-Selenide Semiconductors.

Early transition-metal chalcogenides have garnered recent attention for their optoelectronic properties for solar energy conversion. Herein, the first Zr-/Hf-chalcogenides with a main group cation, Ba9Hf3Sn2Se19 (1) and Ba8Zr2SnSe13(Se2) (2), have been synthesized. The structure of 1 is formed from isolated SnSe44- tetrahedra and distorted HfSe6 octahedra. The latter condense via face-sharing trimeric motifs that are further vertex-bridged into chains of 1∞[Hf(1)2Hf(2)Se11]10-. The structure of 2 is comprised of SnSe44- tetrahedra, Se22- dimers, and face-sharing dimers of distorted ZrSe6 octahedra. These represent the first reported examples of Hf-/Zr-chalcogenides exhibiting face-sharing octahedra with relatively short Hf-Hf and Zr-Zr distances. Their preparation in high purity is inhibited by their low thermodynamic stability, with calculations showing small calculated ΔUdec values of +7 and +9 meV atom-1 for 1 and 2, respectively. Diffuse reflectance measurements confirm the semiconducting nature of 1 with an indirect band gap of ∼1.4(1) eV. Electronic structure calculations show that the band gap absorptions arise from transitions between predominantly Se-4p valence bands and mixed Hf-5d/Sn-5p or Zr-4d/Sn-5p conduction bands. Optical absorption coefficients were calculated to be more than ∼105 cm-1 at greater than 1.8 eV. Thus, promising optical properties are demonstrated for solar energy conversion within these synthetically challenging chemical systems.

Switching Lead for Tin in PbHfO3 : Noncubic Structure of SnHfO3.

The removal of lead from commercialized perovskite-oxide-based piezoceramics has been a recent major topic in materials research owing to legislation in many countries. In this regard, Sn(II)-perovskite oxides have garnered keen interest due to their predicted large spontaneous electric polarizations and isoelectronic nature for substitution of Pb(II) cations. However, they have not been considered synthesizable owing to their high metastability. Herein, the perovskite lead hafnate, i.e., PbHfO3 in space group Pbam, is shown to react with SnClF at a low temperature of 300 °C, and resulting in the first complete Sn(II)-for-Pb(II) substitution, i.e. SnHfO3 . During this topotactic transformation, a high purity and crystallinity is conserved with Pbam symmetry, as confirmed by X-ray and electron diffraction, elemental analysis, and 119 Sn Mössbauer spectroscopy. In situ diffraction shows SnHfO3 also possesses reversible phase transformations and is potentially polar between ≈130-200 °C. This so-called 'de-leadification' is thus shown to represent a highly useful strategy to fully remove lead from perovskite-oxide-based piezoceramics and opening the door to new explorations of polar and antipolar Sn(II)-oxide materials.

Well-Defined Iron Sites in Crystalline Carbon Nitride.

Carbon nitride materials can be hosts for transition metal sites, but Mössbauer studies on iron complexes in carbon nitrides have always shown a mixture of environments and oxidation states. Here we describe the synthesis and characterization of a crystalline carbon nitride with stoichiometric iron sites that all have the same environment. The material (formula C6N9H2Fe0.4Li1.2Cl, abbreviated PTI/FeCl2) is derived from reacting poly(triazine imide)·LiCl (PTI/LiCl) with a low-melting FeCl2/KCl flux, followed by anaerobic rinsing with methanol. X-ray diffraction, X-ray absorption and Mössbauer spectroscopies, and SQUID magnetometry indicate that there are tetrahedral high-spin iron(II) sites throughout the material, all having the same geometry. The material is active for electrocatalytic nitrate reduction to ammonia, with a production rate of ca. 0.1 mmol cm-2 h-1 and Faradaic efficiency of ca. 80% at -0.80 V vs RHE.

Capturing Metastable Oxide Semiconductors for Applications in Solar Energy Conversion.

ConspectusMany small bandgap semiconductors have been discovered or predicted to exist beyond the edges of stability, that is, accessible only as metastable solids that are thermodynamically unstable. In many cases, these metastable semiconductors have been revealed to have technologically promising properties for solar energy conversion, such as in photocatalysis or in photovoltaics. This Account presents a review of research results selected from my group and others in recent years on these semiconductors. Notably, these include the chemical systems of mixed-metal oxides (i.e., M'MOx; M = Ti(IV), Nb(V), or Ta(V) cation; M' = Ag(I), Cu(I), Sn(II), Pb(II), or Bi(III) cation), which have diverse structure types and compositions. High photocatalytic activities have been found for the light-driven reduction or oxidation of water as p- or n-type photoelectrodes, respectively, or as suspended powders in aqueous solutions. These have exhibited new combinations of favorable semiconductor properties, such as deep visible-light absorption, near-optimal band edge energies, defect tolerance, and functional carrier mobilities and charge separation. As described herein, this set of properties is inextricably linked to their metastable nature, that is, the crystalline structures and compositions needed for these characteristics lead naturally to thermodynamic instabilities.This Account focuses on current research efforts that have begun unlocking the potential of these semiconductors via new recent advances in (1) synthetic approaches that enable their preparation and (2) the understanding of structure-property relationships discovered at the precipices of stability that lead to the improved semiconductor properties. For example, low-temperature reactions have been developed to facilitate greater kinetic control, such as with the use of molten salts, and have been a key factor in preparing many of these semiconductors. As a result, a plethora of promising new mixed-metal oxide systems have been uncovered that exhibit band gaps spanning the range of photon energies from ∼1.3 to >3.0 eV. Especially relevant for visible-light applications are the Cu(I)- and Sn(II)-containing semiconductors. For example, n-type Sn(II)-titanates and p-type Cu(I)-niobates can be synthesized by flux methods and exhibit some of the smallest known visible-light band gaps that also maintain suitable conduction and valence band edges for driving the water-splitting half reactions. Kinetic stabilization of these metastable semiconductors against thermally driven phase segregation is increased with the formation of solid solutions for both the M and M' cation sites, leading to effective strategies to more finely tune their band gaps, band edge energies, and photoelectrochemical properties. Many unique and useful relationships are emerging between the synthesis and structures of metastable semiconductors and their physical properties, leading to more efficient solar energy conversion.

Renaissance of Topotactic Ion-Exchange for Functional Solids with Close Packed Structures.

Recently, many new, complex, functional oxides have been discovered with the surprising use of topotactic ion-exchange reactions on close-packed structures, such as found for wurtzite, rutile, perovskite, and other structure types. Despite a lack of apparent cation-diffusion pathways in these structure types, synthetic low-temperature transformations are possible with the interdiffusion and exchange of functional cations possessing ns2 stereoactive lone pairs (e. g., Sn(II)) or unpaired ndx electrons (e. g., Co(II)), targeting new and favorable modulations of their electronic, magnetic, or catalytic properties. This enables a synergistic blending of new functionality to an underlying three-dimensional connectivity, i. e., [-M-O-M-O-]n , that is maintained during the transformation. In many cases, this tactic represents the only known pathway to prepare thermodynamically unstable solids that otherwise would commonly decompose by phase segregation, such as that recently applied to the discovery of many new small bandgap semiconductors.

Structure, Stability, and Photocatalytic Activity of a Layered Perovskite Niobate after Flux-Mediated Sn(II) Exchange.

A new strategy to incorporate the Sn(II) cation and its stereoactive lone pair into the structure of a photocatalytic oxide has been achieved by leveraging the asymmetric coordination environments within the (111)-oriented perovskite-type layers of Ba5Nb4O15. This layered perovskite represents one of the few known photocatalysts capable of efficiently splitting water, but its activity is restricted to ultraviolet radiation owing to its large band gap. By reacting this layered niobate at 350 °C for 24 h within a low-melting SnCl2/SnF2 salt, the new (Ba1-xSnx)Nb4O15 (x = 0-0.5; P3̅m1; a = 5.79650(5) Å, c = 11.79288(8) Å; Z = 2) has been prepared in high purity with up to ∼50% Sn(II) cations. Statistical disordering of the Sn(II) cations was probed by neutron diffraction Rietveld refinements and found to occur predominantly over the asymmetric cation sites, Ba2 and Ba3, for the 40% Sn(II) composition of x = 0.4. An increasing Sn(II) amount significantly red-shifts the band gap (Eg) from 0% Sn for x = 0 (3.78 eV; ultraviolet, indirect) to 40% Sn for x = 0.4 (Eg = 2.35 eV; visible, indirect), as found by UV-vis diffuse reflectance. Density functional theory calculations show an increasing metastability, i.e., a thermodynamic instability toward decomposition to the simpler oxides SnO, Nb2O5, and SnNb2O6. A synthetic limit of ∼50% Sn(II) cations can be kinetically stabilized under these reaction conditions. For the highest Sn(II) amounts, photocatalytic rates are observed for the production of molecular oxygen from water of up to ∼77 µmol O2 h-1 g-1 (visible irradiation) and ∼159 µmol O2 h-1 g-1 (UV-vis irradiation), with apparent quantum yields of ∼0.35 and 0.52%, respectively. By comparison, pure Ba5Nb4O15 exhibits no measurable photocatalytic activity under visible-light irradiation. Electronic structure calculations show that the decreased band gap stems from the introduction of the Sn(II) cations and the formation of a higher-energy valence band arising from the filled 5s2 valence orbitals. Thus, visible-light bandgap excitation occurs from electronic transitions predominantly involving the Sn(II) (5s2) to Nb(V) (4d0) cations. This study demonstrates the new and powerful utility of low-temperature Sn(II)-exchange reactions to sensitize layer-type oxide photocatalysts to the visible region of the solar spectrum, which is facilitated by exploiting their asymmetric cation environments.

A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode.

A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon heating in air, yielding CuO as nano-sized islands. However, the bulk crystalline structure was maintained, with up to 50% Cu(I)-vacancies and a concomitant oxidation of the Cu(I) to Cu(II) cations within the structure. Thermogravimetric and magnetic susceptibility measurements showed the formation of increasing amounts of Cu(II) cations, according to the following reaction: Cu3Ta7O19 + x/2 O2 → Cu(3-x)Ta7O19 + x CuO (surface) (x = 0 to ~0.8). With minor amounts of surface oxidation, the cathodic photocurrents of the polycrystalline films increase significantly, from <0.1 mA cm-2 up to >0.5 mA cm-2, under visible-light irradiation (pH = 6.3; irradiant powder density of ~500 mW cm-2) at an applied bias of -0.6 V vs. SCE. Electronic structure calculations revealed that its defect tolerance arises from the antibonding nature of its valence band edge, with the formation of defect states in resonance with the valence band, rather than as mid-gap states that function as recombination centers. Thus, the metastable Cu(I)-containing semiconductor was demonstrated to possess a high defect tolerance, which facilitates its high cathodic photocurrents.

Physical Properties of Molecules and Condensed Materials Governed by Onsite Repulsion, Spin-Orbit Coupling and Polarizability of Their Constituent Atoms.

The onsite repulsion, spin-orbit coupling and polarizability of elements and their ions play important roles in controlling the physical properties of molecules and condensed materials. In celebration of the 150th birthday of the periodic table this year, we briefly review how these parameters affect the physical properties and are interrelated.

Impact of Nb(V) Substitution on the Structure and Optical and Photoelectrochemical Properties of the Cu5(Ta1- xNb x)11O30 Solid Solution.

A family of solid solutions, Cu5(Ta1- xNb x)11O30 (0 ≤ x ≤ 0.4), was investigated as p-type semiconductors for their band gaps and energies and for their activity for the reduction of water to molecular hydrogen. Compositions from 0 to 40 mol % niobium were prepared in high purity by solid-state methods, accompanied by only very small increases in the lattice parameters of ∼0.05% and with the niobium and tantalum cations disordered over the same atomic sites. However, an increasing niobium content causes a significant decrease in the bandgap size from ∼2.58 to ∼2.05 eV owing to the decreasing conduction band energies. Linear-sweep voltammetry showed an increase in cathodic photocurrents with niobium content and applied negative potential of up to -0.6 mA/cm2 (pH ∼7.3; AM 1.5 G light filter with an irradiation intensity of ∼100 mW/cm2). The cathodic photocurrents could be partially stabilized by heating the polycrystalline films in air at 550 °C for 1 h to produce surface nanoislands of CuO or using protecting layers of aluminum-doped zinc oxide and titania. Aqueous suspensions of the Cu5(Ta1- xNb x)11O30 powders were also found to be active for hydrogen production under visible-light irradiation in a 20% aqueous methanol solution with the highest apparent quantum yields for the 10% and 20% Nb-substituted samples. Electronic structure calculations show that the increased photocurrents and hydroen evolution activities of the solid solutions arise near the percolation threshold of the niobate/tantalate framework wherein the Nb cations establish an extended -O-Nb-O-Nb-O- diffusion pathway for the minority carriers. The latter also reveals a novel pathway for enhancing charge separation as a function of the niobium-oxide connectivity. Thus, these results illustrate the advantages of using solid solutions to achieve the smaller bandgap sizes and band energies that are needed for solar-driven photocatalytic reactions.

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.

Manganese-Vanadate Hybrids: Impact of Organic Ligands on Their Structures, Thermal Stabilities, Optical Properties, and Photocatalytic Activities.

Manganese(II)-vanadate(V)/organic hybrids were prepared in high purity using four different N-donor organic ligands (2,6:2',2″-terpyridine = terpy, 2,2'-bipyrimidine = bpym, o-phenanthroline = o-phen, and 4,4'-bipyridine = 4,4'-bpy), and their crystalline structures, thermal stabilities, optical properties, photocatalytic activities and electronic structures were investigated as a function of the organic ligand. Hydrothermal reactions were employed that targeted a 1:2 molar ratio of Mn(II)/V(V), yielding four hybrid solids with the compositions of Mn(terpy)V2O6·H2O (I), Mn2(bpym)V4O12·0.6H2O (II), Mn(H2O)(o-phen)V2O6 (III), and Mn(4,4'-bpy)V2O6·1.16H2O (IV). The inorganic component within these hybrid compounds, that is, [MnV2O6], forms infinite chains in I and layers in II, III, and IV. In each case, the organic ligand preferentially coordinates to the Mn(II) cations within their respective structures, either as chelating and three-coordinate (mer isomer in I) or two-coordinate (cis isomers in II and III), or as bridging and two coordinate (trans isomer in IV). The terminating ligands in I (terpy) and III (o-phen) yield nonbridged "MnV2O6" chains and layers, respectively, while the bridging ligands in II (bpym) and IV (4,4'-bpy) result in three-dimensional, pillared hybrid networks. The coordination number of the ligand, that is, two- or three-coordinate, has the predominant effect on the dimensionality of the inorganic component, while the connectivity of the combined metal-oxide/organic network is determined by the chelating versus bridging ligand coordination modes. Each hybrid compound decomposes into crystalline MnV2O6 upon heating in air with specific surface areas from ∼7 m(2)/g for III to ∼41 m(2)/g for IV, depending on the extent of structural collapse as the lattice water is removed. All hybrid compounds exhibit visible-light bandgap sizes from ∼1.7 to ∼2.0 eV, decreasing with the increased dimensionality of the [MnV2O6] network in the order of I > II ≈ III > IV. These bandgap sizes are smaller by ∼0.1-0.4 eV in comparison to related vanadate hybrids, owing to the addition of the higher-energy 3d orbital contributions from the Mn(II) cations. Each compound also exhibits temperature-dependent photocatalytic activities for hydrogen production under visible-light irradiation in 20% methanol solutions, with threshold temperatures of ∼30 °C for III, ∼36 °C for I, and ∼40 °C for II, IV, and V4O10(o-phen)2. Hydrogen production rates are ∼142 µmol H2 g(-1)·h(-1), ∼673 µmol H2 g(-1)·h(-1), ∼91 µmol H2 g(-1)·h(-1), and ∼218 µmol H2 g(-1)·h(-1) at 40 °C, for I, II, III, and IV, respectively, increasing with the oxide/organic network connectivity. In contrast, the related V4O10(o-phen)2 exhibits a much lower photocatalytic rate of ∼36 H2 g(-1)·h(-1). Electronic structure calculations based on density-functional theory methods show that the valence band edges are primarily derived from the half-filled Mn 3d(5) orbitals in each, while the conduction band edges are primarily comprised of contributions from the empty V 3d(0) orbitals in I and II and from ligand π* orbitals in III. Thus, the coordinating organic ligands are shown to significantly affect the local and extended structural features, which has elucidated the underlying relationships to their photocatalytic activities, visible-light bandgap sizes, electronic structures, and thermal stabilities.

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[ß-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 [ß-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.

Coordination of copper within a crystalline carbon nitride and its catalytic reduction of CO2.

Inherently disordered structures of carbon nitrides have hindered an atomic level tunability and understanding of their catalytic reactivity. Starting from a crystalline carbon nitride, poly(triazine imide) or PTI/LiCl, the coordination of copper cations to its intralayer N-triazine groups was investigated using molten salt reactions. The reaction of PTI/LiCl within CuCl or eutectic KCl/CuCl2 molten salt mixtures at 280 to 450 °C could be used to yield three partially disordered and ordered structures, wherein the Cu cations are found to coordinate within the intralayer cavities. Local structural differences and the copper content, i.e., whether full or partial occupancy of the intralayer cavity occurs, were found to be dependent on the reaction temperature and Cu-containing salt. Crystallites of Cu-coordinated PTI were also found to electrophoretically deposit from aqueous particle suspensions onto either graphite or FTO electrodes. As a result, electrocatalytic current densities for the reduction of CO2 and H2O reached as high as ∼10 to 50 mA cm-2, and remained stable for >2 days. Selectivity for the reduction of CO2 to CO vs. H2 increases for thinner crystals as well as for when two Cu cations coordinate within the intralayer cavities of PTI. Mechanistic calculations have also revealed the electrocatalytic activity for CO2 reduction requires a smaller thermodynamic driving force with two neighboring Cu atoms per cavity as compared to a single Cu atom. These results thus establish a useful synthetic pathway to metal-coordination in a crystalline carbon nitride and show great potential for mediating stable CO2 reduction at sizable current densities.

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 ß = 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.

Ethanol Upgrading to n-Butanol Using Transition-Metal-Incorporated Poly(triazine)imide Frameworks.

The upgrading of ethanol to n-butanol was performed using a molecular catalyst integrated into a carbon nitride support, one of the first examples of a supported molecular catalyst performing the Guerbet process. Initial studies using crystalline poly(triazine)imide (PTI) with lithium or transition-metal cations imbedded in the support together with a base as the catalyst system did not produce any significant amounts of n-butanol. However, when using the catalyst material formed by treatment of PTI-LiCl with [(Cp*)IrCl2]2 (Cp* = pentamethylcyclopentadienyl) along with sodium hydroxide, a 59% selectivity for butanol (13% yield) was obtained at 145 °C. This PTI-(Cp*)Ir material exhibited distinct UV-vis absorption features and powder X-ray diffractions which differ from those of the parent PTI-LiCl and [(Cp*)IrCl2]2. The PTI-(Cp*)Ir material was found to have a metal loading of 27% iridium per empirical unit of the framework. Along with the formation of n-butanol from the Guerbet reaction, the presence of higher chain alcohols was also observed.

Unveiling the complex configurational landscape of the intralayer cavities in a crystalline carbon nitride.

The in-depth understanding of the reported photoelectrochemical properties of the layered carbon nitride, poly(triazine imide)/LiCl (PTI/LiCl), has been limited by the apparent disorder of the Li/H atoms within its framework. To understand and resolve the current structural ambiguities, an optimized one-step flux synthesis (470 °C, 36 h, LiCl/KCl flux) was used to prepare PTI/LiCl and deuterated-PTI/LiCl in high purity. Its structure was characterized by a combination of neutron/X-ray diffraction and transmission electron microscopy. The range of possible Li/H atomic configurations was enumerated for the first time and, combined with total energy calculations, reveals a more complex energetic landscape than previously considered. Experimental data were fitted against all possible structural models, exhibiting the most consistency with a new orthorhombic model (Sp. Grp. Ama2) that also has the lowest total energy. In addition, a new Cu(i)-containing PTI (PTI/CuCl) was prepared with the more strongly scattering Cu(i) cations in place of Li, and most closely matching with the partially-disorder structure in Cmc21. Thus, a complex configurational landscape of PTI is revealed to consist of a number of ordered crystalline structures that are new potential synthetic targets, such as with the use of metal-exchange reactions.

Circumventing thermodynamics to synthesize highly metastable perovskites: nano eggshells of SnHfO3.

Sn(ii)-based perovskite oxides, being the subject of longstanding theoretical interest for the past two decades, have been synthesized for the first time in the form of nano eggshell particle morphologies. All past reported synthetic attempts have been unsuccessful owing to their metastable nature, i.e., by their thermodynamic instability towards decomposition to their constituent oxides. A new approach was discovered that finally provides an effective solution to surmounting this intractable synthetic barrier and which can be the key to unlocking the door to many other predicted metastable oxides. A low-melting KSn2Cl5 salt was utilized to achieve a soft topotactic exchange of Sn(ii) cations into a Ba-containing perovskite, i.e., BaHfO3 with particle sizes of ∼350 nm, at a low reaction temperature of 200 °C. The resulting particles exhibit nanoshell-over-nanoshell morphologies, i.e., with SnHfO3 forming as ∼20 nm thick shells over the surfaces of the BaHfO3 eggshell particles. Formation of the metastable SnHfO3 is found to be thermodynamically driven by the co-production of the highly stable BaCl2 and KCl side products. Despite this, total energy calculations show that Sn(ii) distorts from the A-site asymmetrically and randomly and the interdiffusion has a negligible impact on the energy of the system (i.e., layered vs. solid solution). Additionally, nano eggshell particle morphologies of BaHfO3 were found to yield highly pure SnHfO3 for the first time, thus circumventing the intrinsic ion-diffusion limits occurring at this low reaction temperature. In summary, these results demonstrate that the metastability of many theoretically predicted Sn(ii)-perovskites can be overcome by leveraging the high cohesive energies of the reactants, the exothermic formation of a stable salt side product, and a shortened diffusion pathway for the Sn(ii) cations.

Site-differentiated solid solution in (Na(1-x)Cu(x))2Ta4O11 and its electronic structure and optical properties.

The (Na(1-x)Cu(x))(2)Ta(4)O(11) (0 ≤ x ≤ 0.78) solid-solution was synthesized within evacuated fused-silica vessels and characterized by powder X-ray diffraction techniques (space group: R3c (#167), Z = 6, a = 6.2061(2)-6.2131(2) Å, c = 36.712(1)-36.861(1) Å, for x = 0.37, 0.57, and 0.78). The structure consists of single layers of TaO(7) pentagonal bipyramids as well as layers of isolated TaO(6) octahedra surrounded by Na(+) and Cu(+) cations. Full-profile Rietveld refinements revealed a site-differentiated substitution of Na(+) cations located in the 12c (Wyckoff) crystallographic site for Cu(+) cations in the 18d crystallographic site. This site differentiation is driven by the linear coordination geometry afforded at the Cu(+) site compared to the distorted seven-coordinate geometry of the Na(+) site. Compositions more Cu-rich than x ~ 0.78, that is, closer to "Cu(2)Ta(4)O(11)", could not be synthesized owing to the destabilizing Na(+)/Cu(+) vacancies that increase with x up to the highest attainable value of ~26%. The UV-vis diffuse reflectance spectra show a significant red-shift of the bandgap size from ~4.0 eV to ~2.65 eV with increasing Cu(+) content across the series. Electronic structure calculations using the TB-LMTO-ASA approach show that the reduction in bandgap size arises from the introduction of Cu 3d(10) orbitals and the formation of a new higher-energy valence band. A direct bandgap transition emerges at k = Γ that is derived from the filled Cu 3d(10) and the empty Ta 5d(0) orbitals, including a small amount of mixing with the O 2p orbitals. The resulting conduction and valence band energies are determined to favorably bracket the redox potentials for water reduction and oxidation, meeting the thermodynamic requirement for photocatalytic water-splitting reactions.

Ligand-mediated interconversion of multiply-interpenetrating frameworks in Cu(I)/Re(VII)-oxide hybrids.

Two new copper(I)-rhenate(VII) hybrid solids, Cu(bpy)ReO(4) (I) and Cu(bpy)(2)ReO(4).0.5H(2)O (II) (bpy = 4,4'-bipyridine), with 2-fold and 4-fold interpenetrating networks, respectively, were prepared from hydrothermal reactions, and their structures characterized by single-crystal X-ray diffraction [I, Pbca (No. 61), Z = 8, a = 10.8513(3) A, b = 12.9419(4) A, c = 15.6976(5) A; II, P1 (No. 2), Z = 2, a = 11.8190(4) A, b = 12.6741(4) A, c = 13.7585(5) A, alpha = 85.8653(13) degrees, beta = 81.6197(13) degrees, gamma = 84.0945(11) degrees]. The structure of I contains 6(3) nets of neutral CuReO(4) layers that are pillared via bpy ligands on the Cu sites {CuO(3)N(2)} to yield a 2-fold interpenetrating pillared-layered network. Conversely, the structure of II consists of a 4-fold interpenetrating diamond-type network with tetrahedral {CuN(4)} coordination nodes that are bridged by bpy ligands, with both H(2)O and ReO(4)(-) within the pores. A surprising reversible structural interconversion between these two interpenetrating structures is possible via the insertion and removal of a single bpy ligand and (1/2)H(2)O per copper atom. The structural interconversion is accompanied by a change in color from yellow to red for I and II, respectively. Measured UV-vis diffuse reflectance spectra exhibit a significant red-shift in the absorption edge of approximately 0.3 eV, with the optical bandgap size decreasing from approximately 2.5 eV to approximately 2.2 eV for I and II, respectively. X-ray photoelectron spectra and electronic structure calculations indicate that the valence band derived from the Cu 3d and N 2p orbitals in II are pushed higher in energy compared to those in I because of the coordination of the additional bpy ligand. There is a much smaller change in the energy of the conduction band that is derived from the Re 5d orbitals. These results demonstrate that the ligand-mediated structural transformations of (d(0)/d(10))-hybrid solids represent a new and convenient low-temperature approach to modulate their optical bandgap sizes toward the visible wavelengths for use with solar energy.

Ligand-based modification of the structures and optical properties of new silver(I)-rhenate(VII) oxide/organic hybrid solids.

A new series of silver(I)-rhenate(VII) hybrids was systematically prepared under hydrothermal conditions from eight different N-donor organic ligands (isonicotinate = inca, pyrazine-2-carboxylate = pzc, 1,2,4-triazole = tro, pyridazine = pda, 4,4'-bipyridine = bpy, 1,2-bis(4-pyridyl)-ethane = dpa, 2,3-bis(2-pyridyl)pyrazine = bpp, and tetra-2-pyridinylpyrazine = tpp), and their resulting structures and optical properties were investigated. The reactions targeted a 1:1 molar ratio of Ag/Re, and new hybrid solids were prepared with the compositions Ag(bpp)ReO(4) (1), Ag(tpp)ReO(4) x H(2)O (2), Ag(Hinca)(2)ReO(4) x H(2)O (3), Ag(tro)ReO(4) (4), Ag(pda)ReO(4) x 1/2 H(2)O (5), Ag(Hpzc)ReO(4) (6), Ag(2)(Hpzc)(pzc)(H(2)O)ReO(4) (7), Ag(bpy)ReO(4) (8), and Ag(dpa)(2)ReO(4) (9). Hybrid solids 1, 2, and 3 each exhibit low-dimensional structures, consisting of [Ag(2)(bpp)(4)](2+) and [Ag(2)(Hinca)(4)](2+) dimers in 1 and 3, respectively, and [Ag(tpp)](n)(n+) chains in 2. Hybrid solids 4 and 5 contain a [Ag(tro)](+) chain and a [Ag(3)(pda)(3)](3+) cyclic trimer, respectively, that are both ReO(4)-bridged into layered structures. Both 6 and 8 consist of ligand-pillared "AgReO(4)" layers, while 7 is a Re-deficient analogue of 6 that contains ligand-pillared [Ag(2)(H(2)O)ReO(4)](+) layers where H(2)O replaces the missing ReO(4)(-) anion. The hybrid networks of 8 and 9 are interpenetrating, owing to the length of the bpy and dpa ligands, and consist of bpy-pillared "AgReO(4)" layers and ReO(4)-filled [Ag(dpa)(2)](+) diamond-type networks that are 2-fold and 6-fold interpenetrating, respectively. Their optical properties and thermal stabilities were investigated using UV-vis transmittance, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The measured properties were analyzed with respect to the varying structural modifications. The Ag-ReO(4) network dimensionalities, Ag coordination environments, and the ligand lengths and geometries are found to play important roles in the absorption coefficients, bandgap sizes, and whether the structure collapses softly to give condensed AgReO(4), respectively.