Engineering Directional Charge Carrier Transport Using Ferroelectric Polarization for Enhanced Photoelectrochemical Water Oxidation.

Introducing ferroelectric polarization has shown great potential to facilitate interfacial charge separation in photoelectrochemical (PEC) water splitting. However, unambiguous evidence of the actual influence of spontaneous ferroelectric polarization, as compared to heterojunction formation, on electron extraction and PEC water splitting is still lacking to date. Herein, core-shell BaTiO3/TiO2 nanostructures are designed as photoanodes based on paraelectric cubic and ferroelectric tetragonal phases BaTiO3 (BTO) perovskite. The cubic and tetragonal crystalline phases are stabilized using selected elaboration methods. Compared to the paraelectric cubic (c-BTO), the ferroelectric tetragonal (t-BTO) leads to a favorable ferroelectric polarization, enhancing directional charge separation and as a consequence to increased photocurrent up to a factor of 1.95. More interestingly, the charge separation efficiency can be tuned by applying positive or negative polarization, with the highest charge separation obtained for the positive one. When loading Ni(OH)2 as a cocatalyst on the t-BTO@TiO2 photoanode, the Ni(OH)2/TiO2/t-BTO exhibits a high performance and superior stability toward PEC water oxidation with a photocurrent almost 6.7 times that of the reference SiO2@TiO2. The proposed facilitation may open an avenue to engineer charge separation and transport for high-performance PEC water oxidation.

Chemical and Structural Evolution of Rb2-2xK2xTi2O5 Layered Titanates When Exposed to Ambient Air.

In this report, we combine thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy to investigate the chemical and structural evolution upon exposure of alkali titanates Rb2Ti2O5 and K2Ti2O5 and mixed compound Rb2-2xK2xTi2O5, for which 0 ≤ x ≤ 1, to the ambient atmosphere. We show that a complete solid solution exists between the potassium and rubidium end-members and that mixed compounds exhibit an intermediate behavior. When exposed to the ambient atmosphere, all compounds degrade, with the progressive formation of first hydrates and then bicarbonates and a much quicker degradation of the rubidium titanate than of the potassium titanate. On the contrary, we show that upon hydration in the absence of CO2, the crystal structure of K2Ti2O5 is more quickly impacted than that of Rb2Ti2O5. These observations should be useful for understanding the evolution of the functional properties of M2Ti2O5 alkali titanates upon aging in the ambient atmosphere, as well as for the optimization of their hydration-dependent properties.

High thermoelectric properties of n-type AgBiSe2.

We report on the thermoelectric (TE) performance of intrinsic n-type AgBiSe2, a Pb-free material with more earth-abundant and cheaper elements than intrinsic p-type homologous AgSbTe2. Pb doping changes n-type AgBiSe2 to p-type but leads to poor electrical transport properties. Nb doping enhances the TE properties of n-type AgBiSe2 by increasing the carrier concentration. As a result of the intrinsically low thermal conductivity (0.7 W m(-1) K(-1)), low electrical resistivity (5.2 mΩ cm), and high absolute Seebeck coefficient (-218 µV/K), the TE figure of merit (ZT) at 773 K is significantly increased from 0.5 for solid-state-synthesized pristine AgBiSe2 to 1 for Ag0.96Nb0.04BiSe2, which makes it a promising n-type candidate for medium-temperature TE applications.

Remarkable enhancement in thermoelectric performance of BiCuSeO by Cu deficiencies.

A significant enhancement of thermoelectric performance in layered oxyselenides BiCuSeO was achieved. The electrical conductivity and Seebeck coefficient of BiCu(1-x)SeO (x = 0-0.1) indicate that the carriers were introduced in the (Cu(2)Se(2))(2-) layer by Cu deficiencies. The maximum of electrical conductivity is 3 × 10(3) S m(-1) for Bicu(0.975)Seo at 650 °C, much larger than 470 S m(-1) for pristine BiCuSeO. Featured with very low thermal conductivity (∼0.5 W m(-1) K(-1)) and a large Seebeck coefficient (+273 µV K(-1)), ZT at 650 °C is significantly increased from 0.50 for pristine BiCuSeO to 0.81 for BiCu(0.975)SeO by introducing Cu deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.

BaCu2Se2 based compounds as promising thermoelectric materials.

In the past few years, many studies have been devoted to the thermoelectric properties of copper selenides and sulfides, and several families of materials have been developed with promising performances. In this paper, we report on the synthesis and thermoelectric properties of Na-doped BaCu2Se2 from 20 K to 773 K. By Na doping at the Ba site, the electrical conductivity can be increased by 2 orders of magnitude, and the power factor can reach 8.2 µW cm(-1) K(-2) at 773 K. Combined with a low thermal conductivity of 0.65 W m(-1) K(-1), a ZT of 1.0 has been obtained for Ba0.925Na0.075Cu2Se2 at 773 K, which is the highest value reported in this family to date. However, BaCu2Se2 volatilizes at 473 K, so a protective coating is necessary for its application. Besides, we studied the thermal expansion coefficient of BaCu2Se2 in this paper.

Structure and Transport Properties of the BiCuSeO-BiCuSO Solid Solution.

In this paper, we report on the crystal structure and the electrical and thermal transport properties of the BiCuSe1-xSxO series. From the evolution of the structural parameters with the substitution rate, we can confidently conclude that a complete solid solution exists between the BiCuSeO and BiCuSO end members, without any miscibility gap. However, the decrease of the stability of the materials when increasing the sulfur fraction, with a simultaneous volatilization, makes it difficult to obtain S-rich samples in a single phase. The band gap of the materials linearly increases between 0.8 eV for BiCuSeO and 1.1 eV in BiCuSO, and the covalent character of the Cu-Ch (Ch = chalcogen element, namely S or Se here) bond slightly decreases when increasing the sulfur fraction. The thermal conductivity of the end members is nearly the same, but a significant decrease is observed for the samples belonging to the solid solution, which can be explained by point defect scattering due to atomic mass and radii fluctuations between Se and S. When increasing the sulfur fraction, the electrical resistivity of the samples strongly increases, which could be linked to an evolution of the energy of formation of copper vacancies, which act as acceptor dopants in these materials.