Thermoelectric properties of new transition metal arsenides and antimonides

The main focus of this work is on exploratory investigation of thermoelectric (TE) materials. Thermoelectric devices are solid-state devices that convert thermal energy from a temperature gradient into electrical energy (Seebeck effect), or convert electrical energy into a temperature gradient (Peltier effect). Modifying existing materials and finding new materials with proper thermoelectric properties are the two approaches considered in this research. Good thermoelectric materials are usually narrow band gap semiconductors with large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. Early transition metal antimonides and arsenides, with unique structural features were chosen for finding high performance TE materials. During the investigation of group four antimonides, a series of new ternaries, ZrSiδSb2-δ, ZrGeδSb2-δ and HfGeδSb2-δ was developed. Single crystal X-ray diffraction was used for crystal structure determination, and energy depressives X-ray analysis (EDX) was used for compositional analysis. Metallic properties of these compounds were predicted by electronic structure calculations and confirmed by physical property measurements. It was revealed that Mo3Sb7 turns semiconducting by partial Sb/Te exchange. Similarly, isostructural Re3As7 was modified to become semiconducting by partial Ge/As exchange. Crystal structures were determined by single crystal X-ray and powder X-ray diffraction utilizing Rietveld method. Electronic structures were determined by using the LMTO method and confirmed the semiconducting properties of these ternary compounds. Physical property measurements showed exceptional TE properties for these compounds. It was also confirmed by the X-ray single crystal analysis that it is possible to intercalate different cations with the proper size into the existing cubic voids of the structure. The effect of cation intercalation on physical properties of these compounds were investigated and revealed the enhancement of transport properties as a result of this intercalation

From Yellow to Black: New Semiconducting Ba Chalcogeno-Germanates

The new germanates Ba 2 GeSe 4−δ Te δ (δ < 2.5) were prepared by reacting the elements under exclusion of air at 800 • C, followed by slow cooling to room temperature. These germanates form the Sr 2 GeS 4 type, monoclinic space group P2 1 /m, with lattice dimensions of a = 699.58(4), b = 709.38(4), c = 917.38(6) pm, β = 109.135(1) • , V = 430.11(4) · 10 6 pm 3 (Z = 2) for Ba 2 GeSe 4 . The structure contains isolated GeSe 4 tetrahedra. The oxidation states are assigned to be Ba II , Ge IV , and Se −II . The yellow color of this ortho-seleno-germanate is indicative of semiconducting behavior with an activation energy of 2.6 -3.0 eV, and the black appearance of the seleno-telluro-germanates points towards gaps < 1.7 eV. Electronic structure calculations based on the LMTO approximation resulted in smaller gaps of 1.7 -0.8 eV, a tendency that is typical for this calculation method

High-Efficiency Dye-Sensitized Solar Cell with Three-Dimensional Photoanode

Herein, we present a straightforward bottom-up synthesis of a high electron mobility and highly light scattering macroporous photoanode for dye-sensitized solar cells. The dense three-dimensional Al/ZnO, SnO2, or TiO2 host integrates a conformal passivation thin film to reduce recombination and a large surface-area mesoporous anatase guest for high dye loading. This novel photoanode is designed to improve the charge extraction resulting in higher fill factor and photovoltage for DSCs. An increase in photovoltage of up to 110 mV over state-of-the-art DSC is demonstrated

See-Through Dye-Sensitized Solar Cells: Photonic Reflectors for Tandem and Building Integrated Photovoltaics

See-through dye-sensitized solar cells with 1D photonic crystal Bragg reflector photoanodes show an increase in peak external quantum efficiency of 47% while still maintaining high fill factors, resulting in an almost 40% increase in power conversion efficiency. These photoanodes are ideally suited for tandem and building integrated photovoltaics

Enhanced Hematite Water Electrolysis Using a 3D Antimony-Doped Tin Oxide Electrode

We present herein an example of nanocrystalline antimony-doped tin oxide (nc-ATO) disordered macroporous “inverse opal” 3D electrodes as efficient charge-collecting support structures for the electrolysis of water using a hematite surface catalyst. The 3D macroporous structures were created <i>via</i> templating of polystyrene spheres, followed by infiltration of the desired precursor solution and annealing at high temperature. Using cyclic voltammetry and electrochemical impedance spectroscopy, it was determined that the use of this 3D transparent conducting oxide with a hematite surface catalyst allowed for a 7-fold increase in active surface area for water splitting with respect to its 2D planar counterpart. This ratio of surface areas was evaluated based on the presence of oxidized trap states on the hematite surface, as determined from the equivalent circuit analysis of the Nyquist plots. Furthermore, the presence of nc-ATO 2D and 3D “underlayer” structures with hematite deposited on top resulted in decreased charge transfer resistances and an increase in the number of available active surface sites at the semiconductor–liquid junction when compared to hematite films lacking any nc-ATO substructures. Finally, absorption, transmission, and reflectance spectra of all of the tested films were measured, suggesting the feasibility of using 3D disordered structures in photoelectrochemical reactions, due to the high absorption of photons by the surface catalyst material and trapping of light within the structure

Pd@H_<i>y</i>WO_3–<i>x</i> Nanowires Efficiently Catalyze the CO₂ Heterogeneous Reduction Reaction with a Pronounced Light Effect

The design of photocatalysts able to reduce CO₂ to value-added chemicals and fuels could enable a closed carbon circular economy. A common theme running through the design of photocatalysts for CO₂ reduction is the utilization of semiconductor materials with high-energy conduction bands able to generate highly reducing electrons. Far less explored in this respect are low-energy conduction band materials such as WO₃. Specifically, we focus attention on the use of Pd nanocrystal decorated WO₃ nanowires as a heretofore-unexplored photocatalyst for the hydrogenation of CO₂. Powder X-ray diffraction, thermogravimetric analysis, ultraviolet–visible-near infrared, and in situ X-ray photoelectron spectroscopy analytical techniques elucidate the hydrogen tungsten bronze, H_<i>y</i>WO_3–<i>x</i>, as the catalytically active species formed via the H₂ spillover effect by Pd. The existence in H_<i>y</i>WO_3–<i>x</i> of Brønsted acid hydroxyls OH, W­(V) sites, and oxygen vacancies (V_O) facilitate CO₂ capture and reduction reactions. Under solar irradiation, CO₂ reduction attains CO production rates as high as 3.0 mmol g_cat^–1 hr^–1 with a selectivity exceeding 99%. A combination of reaction kinetic studies and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements provide a valuable insight into thermochemical compared to photochemical surface reaction pathways, considered responsible for the hydrogenation of CO₂ by Pd@H_<i>y</i>WO_3–<i>x</i>