Kaiyou Zhang

Introducing kalium into copper sulfide for the enhancement of thermoelectric properties

Advanced thermoelectric technology offers the potential to convert waste heat into useful electricity, and a mechanism of transmission-free methods for solid state cooling. A low thermal conductivity is a prerequisite for obtaining high efficiency thermoelectric materials. It is a challenge to achieve low thermal conductivity without simultaneously destroying the electric conductivity, for which a ‘phonon glass/liquid–electron crystal’ is proposed. To realize the phonon glass–electron crystal, a host–guest cage crystal system is considered, while to realize the phonon liquid–electron crystal, superionic conductivity is needed. Here we report a novel material, a KCu7−xS4 nanowire, which exhibits enhanced thermoelectric properties compared to the traditional chalcogenide Cu7S4 nanostructure. The presence of K ions not only forms a clathrate and a superionic fluid structure, which provides the phonon glass and liquid–electron crystal, but also adjusts the product to give a nanowire-like morphology. A low thermal conductivity and large Seebeck coefficient can be achieved when the nanowires are pressed into a bulk material. Higher electrical conductivity is also obtained below 420 K. In addition, the numerous grain boundaries, Cu deficiency and the orientated nanowires further increase the thermoelectric properties. The results indicate a new strategy to obtain high efficiency thermoelectric materials by introducing kalium into copper chalcogenides to form a new crystal structure with ‘phonon glass and liquid–electron crystal’ properties.

Three-dimensional Ag{sub 2}O/WO{sub 3}·0.33H{sub 2}O heterostructures for improving photocatalytic activity

Three-dimensional Ag2O/WO3·0.33H2O heterostructures were fabricated by loading Ag2O nanoparticles on WO3·0.33H2O 3D networks via a simple chemical precipitation method. The Ag2O/WO3·0.33H2O heterostructures exhibited much enhanced photocatalytic activity for the degradation of methylene blue (MB) under simulated solar light irradiation. The optimal molar ratio of Ag2O and WO3·0.33H2O is 1:2. The outstanding photocatalytic activity of the Ag2O/WO3·0.33H2O can be attributed to its large surface area of the three-dimensional networks, the enhanced sunlight absorption and the prevention of electrons–holes combination from the heterostructures. The experiment result demonstrates that wide band gap semiconductor (WO3·0.33H2O) modified by narrow band gap metal oxide (Ag2O) with 3D architecture will be an effective route to enhance its photocatalytic activity.

Synthesis and thermoelectric properties of Bi{sub 2}O{sub 2}Se nanosheets

Graphical abstract: Sheet-like Bi{sub 2}O{sub 2}Se is synthesized by composite-molten-salt (CMS) method. The film made from the as-synthesized Bi{sub 2}O{sub 2}Se nanosheets show very low thermal conductivity and high electrical conductivity. - Highlights: • Bi{sub 2}O{sub 2}Se nanosheets are synthesized by the composite-molten-salt approach. • Thermoelectric properties of a film consisting of Bi{sub 2}O{sub 2}Se are investigated. • The film shows very low thermal conductivity and high electrical conductivity. - Abstract: The pure tetragonal bismuth oxiselenide nanosheets are synthesized by the composite-molten-salt approach at 200 °C. Field emission scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy are used to characterize the structure, morphology, and composition of the sample. The results show that the thickness of the nanosheets is about 120 nm. The sample for thermoelectric property measurements is prepared using cold pressing method. Thermoelectric properties, including electrical conductivity, Seebeck coefficient and thermal conductivity, are measured from room temperature to 470 K. The results show that the sample exhibits n-type semiconductor behavior with low thermal conductivity (κ{sub 300} = 0.346 Wm{sup −1} K{sup −1}, and κ{sub 470} = 0.458 Wm{sup −1} K{sup −1}). Although the obtained figure of merit is small, it would be enhanced with increase in temperature.