Jiangying Peng

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.

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.

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.

Study on lattice dynamics of filled skutterudites InxYbyCo4Sb12

As a promising class of thermoelectric materials, skutterudites are featured by the naturally formed oversize cages in its crystal lattice. Cage-filling by guest atoms has thus become an important approach to reducing the lattice thermal conductivity and optimizing the thermoelectric performance. To probe the impact of filler atoms on lattice dynamics, we herein reported specific heatmeasurements in two single-filled skutterudite samples In0.2Co4Sb12 and Yb0.2Co4Sb12, and a double-filled skutterudite sample In0.2Yb0.2Co4Sb12. The low temperature specific heat data was analyzed in the context of combined electronic specific heat (the Sommerfeld term), the Debye mode (long wavelength acoustic phonon modes), and the Einstein modes (localized vibration modes). We found that the specific heat difference between the single and double-filled samples can be well accounted by one extra Einstein mode, as expected from the extra filler atom and confirmed by the results of inelastic neutron scatteringmeasurements. Interestingly, two Einstein modes, in addition to the Sommerfeld term and the Debye term, are needed to satisfactorily account for the specific heat of the single-filled sample. The Einstein mode with lower frequency has the frequency close to the low-lying mode reported in La, Ce, Tl single-filled skutterudites, this mode is largely unaffected when the second type of filler atoms is introduced. The frequency of this mode has been verified by inelastic neutron scatteringmeasurement. The other Einstein mode with higher frequency may be originated from the motion of Sb atoms.

Phase Transformation and Synthesis of Ni Substituted CoSb3 Skutterudite Synthesis during Solid State Reaction

Starting from elemental Co (99.9%), Ni (99.9%) and Sb (99.99%) powders, Co4-xNixSb12 powder mixture with different Ni concentration were subjected to mechanical alloying by using a planetary ball mill. Phase transformation during mechanical alloying process of Co-Ni-Sb system was studied in this paper. Ni substituted skutterudite could be synthesized by solid state reaction. With assistance of hot pressing for two hours, the as mechanically alloyed low Ni concentration powders could be completely transformed into skutterudite compound. The lattice parameter of Ni substituted skutterudite compound complies well with Vegard law when x is not more than 0.2. When Ni concentration is larger than the maximum solubility, NiSb2 compound forms and single phase skutterudite compound can not be obtained.

Effect of sintering temperature on formation and thermoelectric properties of La0.4Ni0.4Co3.6Sb12 skutterudite by mechanical alloying and hot pressing

In this work, La0.4Ni0.4Co3.6Sb12 filled skutterudite was prepared successfully by mechanical alloying (MA) and subsequent hot pressing. With increasing hot press (HP) temperature, the content of the filled skutterudite phase increases and its composition tends to the nominal composition; a single phase filled skutterudite compound can be obtained when the HP temperature is higher than 550 °C. The Seebeck coefficient and electrical resistivity of the hot pressed sample increased while its thermal conductivity decreased and the resultant figure of merit increased with the increase in the HP temperature. A fractograph observation shows that there are two nanocrystalline microstructures in the hot pressed filled skutterudite sample—one is the prime skutterudite phase formed by MA with a relatively large size, about 300 nm, and the other is the skutterudite produced during the HP process which has an average grain size about 80 nm.

Epitaxial growth of successive CdSe ultrathin films and quantum dot layers on TiO2 nanorod arrays for photo-electrochemical cells

In this work, successive cadmium selenide (CdSe) ultrathin films and quantum dot layers were successfully deposited on TiO2 nanorod arrays by the electrochemical atomic layer epitaxy method (ECALE). The underpotential deposition (UPD) processes of the successive CdSe films and quantum dot layers were recorded in detail. The photo-electrochemical properties of the CdSe coated TiO2 nanorod array electrodes were also investigated, and the maximum current density reached 14.6 mA cm−2 under one sun (AM 1.5G, 100 mW cm−2). Using the ECALE method to grow a buffer layer between quantum dots and their supporting material will be useful for other energy-providing materials.

Hierarchical double-layered SnO2 film as a photoanode for dye-sensitized solar cells

In this paper, a self-assembling double-layered film consisting of SnO2 nanosheet arrays as an underlayer and hierarchical SnO2 microspheres as an overlayer was fabricated via a facile hydrothermal process. The effect of experimental parameters, such as seed layer, acetylacetone, and NH4F on the morphology of the SnO2 hierarchical double-layered film was investigated. We disclose that these hierarchical structures are the consequence of the simultaneous processes of growing and recrystallization on the FTO seeded surface and in solution. A formation mechanism was proposed to understand the growth process. The reflectance spectra of SnO2 double-layered films were also examined.The novel SnO2-based film shows an enhanced light-to-electricity conversion efficiency compared with a simple composite of SnO2 nanosheet arrays and microspheres due to their self-assembling capability and favorable nanostructures.

Enhancement of thermoelectric properties of Ce0.9Fe3.75Ni0.25Sb12 p-type skutterudite by tellurium addition

Recently, research interests in p-type skutterudites are focused on multi-filling in the intrinsic void sites of Fe4−xMxSb12. The four-membered antimony rings, another important structural feature of p-type skutterudites, seem to be overlooked. In this study, Te has been employed to substitute Sb in single-filled p-type Ce0.9Fe3.75Ni0.25Sb12 skutterudite and a systematic investigation has been carried out into the doping effect of Te on the microstructure and thermoelectric properties of this material. With an increase of Te doping, the electrical resistivity decreases due to the increase of hole concentration and the gradual decrease of mobility; while the density of state effective mass increases rapidly with the increase of hole concentration due to the intrinsic multiple bands effects, thus the Seebeck coefficient increases slightly. In addition, Te doping on Sb sites changes the valance electron balance and results in the formation of microscale γ-Ce and nanoscale CeTe2 compound. These microscale and nanoscale precipitates work together with the atomic scale distortion of Sb4 rings by the substitution of Te for Sb, could scatter phonons with a wide range of frequency thus result in a significant reduction of the lattice thermal conductivity. Therefore the thermoelectric performance has been enhanced greatly, and a maximum ZT value of 1.0 at 773 K was obtained for the Ce0.9Fe3.75Ni0.25Sb11.9Te0.1 sample.