Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.

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Complex thermoelectric materials.

TiO2 photocatalysis and related surface phenomena

Spurred by recent progress in materials chemistry and drug delivery, stimuli-responsive devices that deliver a drug in spatial-, temporal- and dosage-controlled fashions have become possible. Implementation of such devices requires the use of biocompatible materials that are susceptible to a specific physical incitement or that, in response to a specific stimulus, undergo a protonation, a hydrolytic cleavage or a (supra)molecular conformational change. In this Review, we discuss recent advances in the design of nanoscale stimuli-responsive systems that are able to control drug biodistribution in response to specific stimuli, either exogenous (variations in temperature, magnetic field, ultrasound intensity, light or electric pulses) or endogenous (changes in pH, enzyme concentration or redox gradients).

Stimuli-responsive nanocarriers for drug delivery

Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.

https://link.springer.com/content/pdf/10.1007%2Fs10853-010-5113-0.pdf

Review of the anatase to rutile phase transformation

Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

New and Old Concepts in Thermoelectric Materials

Nanocomposites of ultra-thin copper oxide nanosheets (CuO NSs) and single-wall carbon nanotubes (SWCNTs) were produced and studied for their thermoelectric (TE) properties. Incorporating a small amount of SWCNTs into the matrix of CuO NSs enhanced the TE properties. This might be due to good band alignment between the two phases with large contact areas. The nanocomposite [CuO]99.9[SWCNT]0.1 showed a rapid increase with the increasing temperature in both the Seebeck coefficient and power factor. At 673 K, they reached 882 μV/K and 2500 μW/mK2, respectively. The phonon and charge transport properties were attributed to the CuO NS/SWCNT interfaces. A small module of 2 p-n pairs based on CuO/SWCNT nanocomposites (p-type) and SnO2 nanoparticles (n-type) was constructed and worked in a temperature range of 573–673 K with good stability and reusability. The measured output power was ˜1200 μW, which can power small electronic devices.

Nanocomposites of CuO/SWCNT: Promising thermoelectric materials for mid-temperature thermoelectric generators

We studied the direct micropatterning of a lanthanum-based thin film on a template of self-assembled monolayers in an aqueous solution at 80 °C. The template composed of silanol and octadecyl areas was prepared by UV-modified octadecyltrichlorosilane SAMs through a photomask. The amorphous La 2 O(CO 3 ) 2 · x H 2 O thin films were selectively deposited in the silanol regions. Crystallized La 2 O 3 was obtained after heating at 800 °C in air.

Micropatterning of lanthanum-based oxide thin film on self-assembled monolayers

Formation and characterization of thin films of indium hydroxide, In(OH)3⋅nH2O were carried out. The indium hydroxide sample was prepared in the gel form via the reaction of indium nitrate solution with ammonium hydroxide solution. After the precipitated gel was extracted using a centrifugal separator, the gel was dispersed in deionized water for several days to prepare a colloidal solution. Films dip-coated on glass substrates were heated at several temperatures. Thin films were characterized by means of XRD, SEM, AFM, thickness measurement, TG-DTA and conductivity measurement. Although only one weak peak of the (200) plane was observed in XRD patterns, the thin films could be regarded as almost amorphous. Surfaces of all the samples were found to be smooth with roughness Rmax=7nm, and the thickness of each film was about 40nm. Depending on both the heat-treatment temperature of thin films and the relative humidity, the conductivity varied in the range of 10-7-10-3S/cm at room temperature. In addition, the conductivity measured along the surface of the sample was on the order of 10-6S/cm which was comparable with the value measured in the direction perpendicular to the surface. These results suggested that the carriers of conduction were protons based on water incorporated in the crystallographic channels and grain boundaries of the thin films.

Formation and Characterization of Indium Hydroxide Films

A misfit layer sulfide (BiS)1.2(TiS2)2 with a natural superlattice structure has been shown to be a promising thermoelectric material, but its high carrier concentration should be reduced so as to further optimize the thermoelectric performance. However, ordinary acceptor doping has not succeeded because of the non-parabolic band structure. In this paper, we have successfully doped chromium ions into the Ti sites, which can maintain or even enhance the high effective mass of electrons so as to effectively improve ZT value. X-ray diffraction analysis, coupled with X-ray photoelectron spectroscopy, shows that chromium has been substituted into titanium sites in TiS2 layers and confirms its ionic state. The chromium doping has successfully reduced the carrier concentration with the subsequent reduction of electrical conductivity. Unlike other acceptor dopants (alkaline earth metals), chromium also enhances Seebeck coefficient and the effective mass, which can possibly be attributed to the formation of additional resonant states near Fermi level. Though the power factor does not improve, the significant reduction in the electronic part of the thermal conductivity leads to a measurable improvement in ZT.

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Thermoelectric performance enhancement of (BiS)1.2(TiS2)2 misfit layer sulfide by chromium doping

High thermoelectric figure of merit semiconducting ceramics of n-type iron disilicide with modified local compositions have been developed. Sintering and annealing of the composite powder composed of iron disilicide and precipitated cobalt (II) hydroxide resulted in the dissolution of excess Co and oxygen into the iron disilicide phase. Excess Co segregated to the grain boundary region, while interstitially incorporated oxygen was distributed homogeneously in the microstructure. The maximum figure of merit achieved was 5.2 × 10−4/K at 673 K.

Kunihito Koumoto

Papers

Nanocomposites of CuO/SWCNT: Promising thermoelectric materials for mid-temperature thermoelectric generators

Micropatterning of lanthanum-based oxide thin film on self-assembled monolayers

Formation and Characterization of Indium Hydroxide Films

Thermoelectric performance enhancement of (BiS)1.2(TiS2)2 misfit layer sulfide by chromium doping

Improvement in Thermoelectric Characteristics of n-type Iron Disilicide by Local Composition Modification