Junyou Yang

Coaxial heterogeneous structure of TiO2 nanotube arrays with CdS as a superthin coating synthesized via modified electrochemical atomic layer deposition.

We report the fabrication and characterization of CdS/TiO(2) nanotube-array coaxial heterogeneous structures. Such structures may potentially be applied in various photocatalytic fields, such as water photocatalytic decomposition and toxic pollutant photocatalytic degradation. Thin films of CdS are conformally deposited onto TiO(2) nanotubes using a modified method of electrochemical atomic layer deposition. We propose that such nanostructured electrodes can overcome the poor absorption and high charge-carrier recombination observed with nanoparticulate films. The practical electrochemical deposition technique promotes the deposition of CdS onto the TiO(2) tube walls while minimizing deposition at the tube entrances, thus preventing pore clogging. The coaxial heterogeneous structure prepared by the new electrochemical process significantly enhances CdS/TiO(2) and CdS/electrolyte contact areas and reduces the distance that holes and electrons must travel to reach the electrolyte or underlying conducting substrate. This results in enhanced photon absorption and photocurrent generation. The detailed synthesis process and the surface morphology, structure, elemental analysis, and photoelectrochemical properties of the resulting films with the CdS/TiO(2) nanotube-array coaxial heterogeneous structure are discussed. In comparison with a pure TiO(2) nanotube array, a 5-fold enhancement in photoactivity was observed using the coaxial heterogeneous structure. This methodology may be useful in designing multijunction semiconductor materials for coating of highly structured substrates.

Multiple heteroatom induced carrier engineering and hierarchical nanostructures for high thermoelectric performance of polycrystalline In4Se2.5

In this paper, different atom combinations of Pb, I and Cu have been doped into the In4Se2.5 matrix and a systematic investigation has been carried out for the synergistic effect of multiple heteroatoms on the microstructure and thermoelectric properties of polycrystalline In4Se2.5. By this approach, the electron and phonon transport properties are rationally regulated and the electrical conductivity increases greatly due to the multiple doping, which result in a simultaneous increase of carrier concentration and mobility. The Seebeck coefficient also remains at a relatively high level in a high temperature range due to the energy-dependent electron scattering at the metal nanoparticle–matrix interfaces. In addition, the lattice thermal conductivity is also greatly reduced because of the wide frequency phonon scattering by the point defects and hierarchical metal nanoparticles combined with the phonon–phonon interactions. Consequently, an enhancement of the ZT with a maximum of 1.4 (723 K) has been achieved in the multiple doped In4Se2.5 sample.

Enhancement of thermoelectric properties of Yb-filled skutterudites by an Ni-Induced “core–shell” structure

Since the lattice thermal conductivity of n-type multi-filled skutterudites have been reduced below 1 W (mK−1), the development of new strategies that can further enhance the power factor while maintaining the low thermal conductivity is highly desired. In this paper, we conducted a pioneering work by introducing a “core–shell” microstructure into Yb single-filled skutterudite thermoelectric materials to realise this purpose. The “core–shell” structure formed by the thermal diffusion of well dispersed Ni nanoparticles in the Yb0.2Co4Sb12 powder during hot pressing is composed of the normal “core” grains surrounded by Ni-rich nanograin “shells”. The electrical resistivity is greatly reduced due to the increase in both carrier concentration and mobility. However, the Seebeck coefficient first increases due to the increased density of states at the Fermi energy and then decreases gradually. As a consequence, the power factor is remarkably increased for the samples with the addition of Ni nanoparticles. In addition, the lattice thermal conductivity is also reduced by the extra phonon scattering introduced by the “core–shell” microstructure. The concomitant effects enable a maximum ZT of 1.07 for the 0.2 wt% Ni sample at 723 K.

A study of Yb0.2Co4Sb12–AgSbTe2 nanocomposites: simultaneous enhancement of all three thermoelectric properties

The single-filled skutterudite Yb0.2Co4Sb12 has been long known as a promising bulk thermoelectric material. In this work, we adopted a melting–milling–hot pressing procedure to prepare nanocomposites that consist of a micrometer-grained Yb0.2Co4Sb12 matrix and well-dispersed AgSbTe2 nanoinclusions on the matrix grain boundaries. Different weight percentages of AgSbTe2 inclusions were added to optimize the thermoelectric performance. We found that the addition of AgSbTe2 nanoinclusions systematically and simultaneously optimized the otherwise adversely inter-dependent electrical conductivity, Seebeck coefficient and thermal conductivity. In particular, the significantly enhanced carrier mobility led to a ∼3-fold reduction of the electrical resistivity. Meanwhile the absolute value of Seebeck coefficient was enhanced via the energy filtering effect at the matrix–nanoinclusion interfaces. Moreover there is a topological crossover of the AgSbTe2 inclusions from isolated nanoparticles to a nano-plating or nano-coating between 6 wt% and 8 wt% of nanoinclusions. Above the crossover, further addition of nanoinclusions degraded the Seebeck coefficient and the electrical conductivity. Meanwhile, the addition of nanoinclusions generally reduced the lattice thermal conductivity. As a result, the power factor of the 6 wt% sample was ∼7 times larger than that of the nanoinclusion-free sample, yielding a room temperature figure of merit ZT ∼ 0.51.

Electrochemical aspects and structure characterization of VA-VIA compound semiconductor Bi2Te3/Sb2Te3 superlattice thin films via electrochemical atomic layer epitaxy.

This paper concerns the electrochemical atom-by-atom growth of VA-VIA compound semiconductor thin film superlattice structures using electrochemical atomic layer epitaxy. The combination of the Bi2Te3 and Sb2Te3 programs and Bi2Te3/Sb2Te3 thin film superlattice with 18 periods, where each period involved 21 cycles of Bi2Te3 followed by 21 cycles of Sb2Te3, is reported here. According to the angular distance between the satellite and the Bragg peak, a period of 23 nm for the superlattice was indicated from the X-ray diffraction (XRD) spectrum. An overall composition of Bi 0.25Sb0.16Te0.58, suggesting the 2:3 stoichiometric ratio of total content of Bi and Sb to Te, as expected for the format of the Bi2Te3/Sb2Te3 compound, was further verified by energy dispersive X-ray quantitative analysis. Both field-emission scanning electron microscopy and XRD data indicated the deposit grows by a complex mechanism involving some 3D nucleation and growth in parallel with underpotential deposition. The optical band gap of the deposited superlattice film was determined as 0.15 eV by Fourier transform infrared spectroscopy and depicts an allowed direct type of transition. Raman spectrum observation with annealed and unannealed superlattice sample showed that the LIF mode has presented, suggesting a perfect AB/CB bonding in the superlattice interface.

Ternary CuSbSe2 chalcostibite: facile synthesis, electronic-structure and thermoelectric performance enhancement

A single phase CuSbSe2 polycrystalline chalcostibite compound has been facilely synthesized through mechanical alloying for the first time, and the phase evolution has been revealed in detail. The large Seebeck coefficient and poor electrical conductivity of this compound are ascribed to the existing Cu–Se ionic bonding and heavy band at the VBM. The active lone-pair s2 electrons in Sb3+ ions are likely to be responsible for the experimentally observed low thermal conductivity in the CuSbSe2 compound. By introducing narrow band-gap Cu3SbSe4 into the CuSbSe2 matrix, the carrier concentration and mobility can be tuned effectively; thus the power factor has been improved remarkably. As a consequence, the figure of merit (ZT) is increased by a factor of 1.6 at 623 K, from 0.25 (matrix) to 0.41.

Multiple effects of Bi doping in enhancing the thermoelectric properties of SnTe

We studied the effect of doping with Bi on the thermoelectric properties of SnTe-based materials. Doping with Bi reduced the density of holes and increased the Seebeck coefficient over a wide temperature range as a result of modulation of the carrier concentration and an increase in the density of states effective mass. The lattice thermal conductivity was also greatly reduced as a result of the wide frequency range of phonon scattering by multiscale architectures derived from Bi doping. A maximum ZT value of c. 1.1 at 873 K was obtained in Sn0.94Bi0.06Te, an enhancement of 165% compared with the undoped sample.

Ion size and image effect on electrokinetic flows.

Electrokinetic phenomena play a major role in microfluidic systems, and such a role becomes even more significant in nanofluidic systems due to the increase of the surface-to-volume ratio. Description of the electric double layer (EDL) at a solid-liquid interface is the key to understand and utilize electrokinetic phenomena. However, the traditional Gouy-Chapman (GC) theory for the EDL, which has been successfully used in many microfluidic applications, does not include some important characteristics such as ion size and image effect. These characteristics are indeed important in nanofluidics. This paper explores the impacts of ion size and the image effect on micro- and nanoscale electrokinetic flows. An advanced theory, the modified Poisson-Boltzmann (MPB) theory proposed by Outhwaite and his co-workers,1,26 is adopted to describe the EDL. Electrokinetic flows in micro- and nanochannels are reinvestigated. The results show that ion size has significant effects on electrokinetic flows in nanosystems in terms of both the flow field and the streaming potential, while the image effect only significantly affects the streaming potential.

A simultaneous increase in the ZT and the corresponding critical temperature of p-type Bi0.4Sb1.6Te3 by a combined strategy of dual nanoinclusions and carrier engineering

By means of β-Zn4Sb3 addition and thermal decomposition, dual nanoinclusions of Zn and ZnSb were introduced and Zn atoms were doped into p-type Bi0.4Sb1.6Te3 successfully. Due to the increase of hole concentration by Zn doping and band structure optimization, the bipolar conduction was suppressed and the intrinsic excitation shifts to higher temperature. The power factors of the samples were slightly improved, while the lattice thermal conductivities of the samples were greatly reduced due to the extra phonon scattering introduced by the dual nanoinclusions. As a result, the critical temperature corresponding to the maximum ZT value was greatly increased to 423 K, which is about 120 K higher when compared with the conventional Bi2Te3-based materials. A maximum ZT of 1.44 was achieved for the sample with 1.5 wt% β-Zn4Sb3 at 423 K, which is also the highest ZT value ever reported at such a high temperature for p-type Bi2Te3-based materials. This work is of importance to expand the application of Bi2Te3-based materials for low grade waste-heat recovery.

Evaluation of Ca3Co2O6 as cathode material for high-performance solid-oxide fuel cell

A cobalt-based thermoelectric compound Ca3Co2O6 (CCO) has been developed as new cathode material with superior performance for intermediate-temperature (IT) solid-oxide fuel cell (SOFC). Systematic evaluation has been carried out. Measurement of thermal expansion coefficient (TEC), thermal-stress (σ) and interfacial shearing stress (τ) with the electrolyte show that CCO matches well with several commonly-used IT electrolytes. Maximum power density as high as 1.47 W cm−2 is attained at 800°C, and an additional thermoelectric voltage of 11.7 mV is detected. The superior electrochemical performance, thermoelectric effect, and comparable thermal and mechanical behaviors with the electrolytes make CCO to be a promising cathode material for SOFC.