Search

Recently, there has been an increased interest in quaternary clathrate systems as promising thermoelectric materials. Because of their increased complexity, however, the chemical ordering in the host framework of quaternary clathrates has not yet been comprehensively analyzed. Here, we have synthesized a prototypical quaternary type-I clathrate Ba8AlxGa16-xGe30 by Czochralski and flux methods, and we employed a combination of X-ray and neutron diffraction along with atomic scale simulations to investigate chemical ordering in this material. We show that the site occupancy factors of trivalent elements at the 6c site differ, depending on the synthesis method, which can be attributed to the level of equilibration. The flux-grown samples are consistent with the simulated high-temperature disordered configuration, while the degree of ordering for the Czochralski sample lies between the ground state and the high-temperature state. Moreover, we demonstrate that the atomic displacement parameters of the Ba atoms in the larger tetrakaidecahedral cages are related to chemical ordering. Specifically, Ba atoms are either displaced toward the periphery or localized at the cage centers. Consequently, this study reveals key relationships between the chemical ordering in the quaternary clathrates Ba8AlxGa16-xGe30 and the structural properties, thereby offering new perspectives on designing these materials and optimizing their thermoelectric properties.

It has been recently demonstrated that the pseudoternary Ba8AlxGayGe46-x-y clathrate undergoes an order-disorder transition with increasing temperature that can be observed via site occupation factors (SOFs) and manifests itself, e.g., in electrical transport properties. Here, we generalize this result and analyze the characteristics of this order-disorder transition in the pseudobinary clathrates Ba8GaxGe46-x, Ba8GaxSi46-x, Ba8AlxGe46-x, and Ba8AlxSi46-x. To this end, we employ atomistic simulations that combine alloy cluster expansions trained against density functional theory calculations with Wang-Landau and ensemble Monte Carlo simulations. The simulations show that all four systems studied here display order-disorder transitions for at least some composition range. Based on an extensive literature survey, we also provide evidence for signatures of the transition in earlier experimental studies that to the best of our knowledge have hitherto not been related to such transitions. The predicted transition temperatures are lower for Ba8GaxGe46-x and Ba8GaxSi46-x than for Ba8AlxGe46-x and Ba8AlxSi46-x, although it appears that the simulations underestimate the transition temperatures for Ga-containing systems compared to the experiment. This nonetheless provides a sensible explanation for why the experimentally determined Al SOFs agree better with the simulated higherature disordered configurations, while the Ga SOFs more closely agree with the simulated ground-state configurations. As a result of stronger interactions, the SOFs vary substantially, especially near the stoichiometric 16:30 composition, providing an indication of why it has proved difficult to synthesize Ba8AlxGe46-x and Ba8AlxSi46-x samples at this ratio. The present study thereby yields detailed atomic-scale insights into the ordering in inorganic clathrates that, given the connection to transport properties established earlier, are not only useful from a fundamental perspective but also relevant for applications.

Compared to traditional electric double-layer capacitors, redox-enhanced electrochemical capacitors (redox-ECs) show increased energy density and steadier power output thanks to the use of redox-active electrolytes. The aim of this study is to understand the electrochemical mechanisms of the aqueous pentyl viologen/bromide dual redox system at the interface of an ordered mesoporous carbon (CMK-8) and improve the device performance. Cells with CMK-8 carbon electrodes were investigated in several configurations using different charging rates and potential windows. The pentyl viologen electrochemistry shows a mixed behavior between solution-based diffusion and adsorption phenomena, with the reversible formation of an adsorbed layer. The extension of the voltage window allows for full reduction of the viologen molecules during charge and a consequent increase in the specific discharge energy delivered by the cell. Investigation of the mechanism indicates that a 1.5 V charging voltage with a 0.5 A g-1 charging rate and fast discharge rate produces the best overall performance.

Inorganic clathrates have been extensively investigated owing to their unique and intriguing atomic structure as well as their potential as thermoelectric materials. The connection between the chemical ordering and the physical properties has, however, remained elusive. Here, this relation is uncovered through a combination of first-principles calculations, atomistic simulations, and experimental measurements of thermodynamic as well as electrical transport properties. This approach is, specifically, used to reveal the existence of an order-disorder transition in the quaternary clathrate series Ba8AlxGa16-xGe30. The results, furthermore, demonstrate that this phenomenon is responsible for the discontinuity in the heat capacity that has been observed previously. Moreover, the unusual temperature dependence of both Seebeck coefficient and electrical conductivity can be fully explained by the alterations of the band structure brought about by the phase transformation. It is finally argued that the phenomenology described here is not limited to this particular material but should be present in a wide range of inorganic clathrates and could even be observed in other materials that exhibit chemical ordering on at least one sublattice.

CO2 can be enzymatically reduced to methanol in a cascade reaction involving three enzymes: formate-, formaldehyde- and alcohol dehydrogenase (FateDH, FaldDH, ADH). We report an improvement in the yield of this reaction by co-immobilizing the three dehydrogenases in siliceous mesostructured cellular foams (MCF). This material consists of large mesopores suitable for the co-immobilization of these comparatively large enzymes. To improve the interaction between the enzymes and support, the host silica material was functionalized with mercaptopropyl groups (MCF-MP). The enzymes were fluorescently labelled to independently monitor their uptake and spatial distribution into the particle. The three dehydrogenases were co-immobilized using two sequential methods. In the first one, the enzymes were immobilized according to the reaction order (FateDH → FaldDH → ADH) and secondly in order of increasing enzyme size (FateDH → ADH → FaldDH). Two protein loadings were also tested: 50 and 150 mgenzymes gsupport−1. We could observe a 4.5-fold higher methanol yield in comparison to enzymes free in solution when the enzymes were immobilized in order of size and with a loading of 50 mgenzymes gsupport−1. The results of this work show that by using MCF-MP, a simple method of immobilization can be applied to significantly increase the activity of the enzymes for the cascade reaction.

A novel template-free colloidal assembly method that combines colloidal zeolite (silicalite-1) suspensions in a water-in-oil emulsion with an evaporation-induced assembly process has been developed for preparing hierarchical micro-/mesoporous zeolite microspheres (MZMs). Such particles have an interconnected mesoporosity and large mesopore diameters (25-40 nm) combined with 5.5 Å diameter micropores of the zeolite nanoparticles. The method developed has the advantages of employing mild synthesis conditions, a short preparation time, and not requiring the use of a mesoporogen template or post-treatment methods. The method provides a new range of micro-/mesoporous zeolites with tunable mesoporosity dictated by the size of the zeolite nanoparticles. It also offers the possibility of combining several zeolite particle sizes or optionally adding amorphous silica nanoparticles to tune the mesopore size distribution further. It should be generally applicable to other types of colloidal zeolite suspensions (e.g. ZSM-5, zeolite A, beta) and represents a new route amenable for cost-effective scale-up.

Mesoporous silica (MPS) particles are promising materials for the immobilization of a variety of enzymes due to the possibility of tailoring their morphology and surface properties. A better unders...

In the synthesis of iron-chelating ordered mesoporous nitrogen-functionalized carbon (Fe–OMC) catalysts, the removal of the silica template is a crucial and critical step. The silica removal method used today for the Fe–OMCs is based on the very toxic hydrofluoric acid (HF), which usage should be limited where possible. Therefore, the aim of this study is to establish a mild (user-friendly), yet efficient, etch method based on dilute sodium hydroxide (NaOH), which does not impair the catalytic performance of the catalyst, to replace the currently used HF method. A comparison between catalysts etched with HF versus NaOH was performed in order to gain understanding how the two etch methods influence the final catalysts in terms of electrochemical, structural and catalytic properties. The NaOH etch was found to successfully remove the silica template, and interestingly, also improve the catalytic performance. The improved activity is explained by a carbon activation process occurring in the catalyst treated with the NaOH etch. With these findings, we show that it is possible to remove the silica in a more user-friendly way and simultaneously increase the catalysts’ performance by activation of the carbon.

A two-step procedure for the surface functionalization of carbon materials has been developed. In the first step, mesoporous carbon (CMK3), carbon black (XC-72R Vulcan), and activated carbon (AC) are bromomethylated efficiently under mild conditions using commercially available reagents, resulting in reproducible surface bromine loadings, the concentrations of which correlate to the carbon's surface area. The resulting bromomethylated materials display excellent stability over the course of months when stored under ambient conditions. In the second step, substitution reactions with a variety of nucleophiles proceed efficiently. Example nucleophiles include azide, amines, ammonia and iodide, and exhibit high conversion yields. To demonstrate the application of this two-step functionalization method, bromomethylated CMK3, Br-CMK3, was reacted with ethylenediamine (EN) to form EN-CMK3, which was used as the conductive host for the sulfur cathode in lithium–sulfur (Li–S) batteries. Impregnation of EN-CMK3 with a lithium polysulfide-containing electrolyte with either lithium nitrate (LiNO3) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the supporting electrolyte increases the battery performance relative to pristine CMK3. With LiNO3, the surface-bonded EN allows for increased sulfur use and results in higher capacities of ca. 300 mA h g−1; with LiTFSI, the EN groups attenuate the polysulfide shuttle (LiPS shuttle) and the initial charging efficiency (ICE) is increased substantially from 3% to 73%. These results provide a proof-of-principle of the versatility of bromomethylated carbons as a useful starting material for a variety of functional materials.

A promising strategy to improve the rate performance of Li-ion batteries is to enhance and facilitate the insertion of Li ions into nanostructured oxides like TiO2. In this work, we present a systematic study of pentavalent-doped anatase TiO2 materials for third-generation high-rate Li-ion batteries. Mesoporous niobium-doped anatase beads (Nb-doped TiO2) with different Nb5+ doping (n-type) concentrations (0.1, 1.0, and 10% at.) were synthesized via an improved template approach followed by hydrothermal treatment. The formation of intrinsic n-type defects and oxygen vacancies under RT conditions gives rise to a metallic-type conduction due to a shift of the Fermi energy level. The increase in the metallic character, confirmed by electrochemical impedance spectroscopy, enhances the performance of the anatase bead electrodes in terms of rate capability and provides higher capacities both at low and fast charging rates. The experimental data were supported by density functional theory (DFT) calculations showing how a different n-type doping can be correlated to the same electrochemical effect on the final device. The Nb-doped TiO2 electrode materials exhibit an improved cycling stability at all the doping concentrations by overcoming the capacity fade shown in the case of pure TiO2 beads. The 0.1% Nb-doped TiO2-based electrodes exhibit the highest reversible capacities of 180 mAh g(-1) at 1C (330 mA g(-1)) after 500 cycles and 110 mAh g(-1) at 10C (3300 mA g(-1)) after 1000 cycles. Our experimental and computational results highlight the possibility of using n-type doped TiO2 materials as anodes in high-rate Li-ion batteries.

Designing carbonaceous materials with heightened attention to the structural properties such as porosity, and to the functionalization of the surface, is a growing topic in the lithium-ion batteries (LIBs) field. Using a mesoporous silica KIT-6 hard template, mesoporous carbons belonging to the OMCs (ordered mesoporous carbons) family, namely 3D cubic CMK-8 and N-CMK-8 were synthesized and thoroughly structurally characterized. XPS analysis confirmed the successful introduction of nitrogen, highlighting the nature of the different nitrogen atoms incorporated in the structure. The work aims at evaluating the electrochemical performance of N-doped ordered mesoporous carbons as an anode in LIBs, underlining the effect of the nitrogen functionalization. The N-CMK-8 electrode reveals higher reversible capacity, better cycling stability, and rate capability, as compared to the CMK-8 electrode. Coupling the 3D channel network with the functional N-doping increased the reversible capacity to ~1000 mAh&middot, g&minus, 1 for the N-CMK-8 from ~450 mAh&middot, 1 for the undoped CMK-8 electrode. A full Li-ion cell was built using N-CMK-8 as an anode, commercial LiFePO4, a cathode, and LP30 commercial electrolyte, showing stable performance for 100 cycles. The combination of nitrogen functionalization and ordered porosity is promising for the development of high performing functional anodes.

Ordered mesoporous titania, prepared via low-temperature spray deposition, was examined as an anode material for lithium ion batteries. The material exhibits an exceptionally high electrochemical capacity of 680 mA h g−1 during the first discharge, which rapidly decreases over the following cycles. The capacity stabilizes at around 170 mA h g−1 after 50 cycles and the material delivers 83 mA h g−1 at high charge/discharge rates (10C). A combination of electrochemical and structural characterization techniques were used to study the charge/discharge behavior of the material and the origin of the irreversible capacity. To determine the effect of cycling on the structure of the material, X-ray absorption spectroscopy (XAS) and energy filtered TEM were carried out on pristine and cycled samples in intercalated and deintercalated states. Titanium K-edge XAS measurements showed that intercalated lithium affects the NEXAFS region. By comparing peak intensity ratios, we propose a method to quantify the amount of lithium inserted into the titania structure and to differentiate between lithium bound in close proximity to titanium, and lithium bound further away from titanium. Additionally, we suggest that the irreversible loss in capacity is due to the formation of phases that are stable, and thereby electrochemically inactive, over the electrochemical cycling conditions applied.

In this study, the emulsification performance of functionalized colloidal silica is explored with the aim to achieve phase inversion of particle-stabilized (Pickering) emulsion systems. An increased understanding of inversion conditions can facilitate surfactant-free emulsion fabrication and expand its use in industrial applications. Phase inversion was achieved by adjusting the temperature but without changing the composition of the emulsion formulation. Silica nanoparticles modified with hydrophobic propyl groups and hydrophilic methyl poly(ethylene)glycol (mPEG) groups are used as emulsifiers, enabling control of the wettability of the particles and exploration of phase inversion phenomena, the latter due to the thermoresponsiveness of the attached PEG chains. The phase inversion conditions as well as the reversibility of the emulsion systems were examined at varying electrolyte concentrations and pH values of the suspensions. Transitional phase inversions, from oil-in-water and water-in-oil and back, were observed in functionalized silica particle-stabilized butanol emulsions at distinct temperatures. The phase inversion temperature was affected by electrolyte concentration and pH conditions due to salting-out effects, PEG-silica interactions, and the effects of the particle surface charge. Investigations of phase inversion conditions, temperature, and hysteresis effects in Pickering emulsions can improve the theoretical understanding of these phenomena and facilitate the implementation of low-energy emulsion preparation.

Lithium‐sulfur (Li‐S) batteries are of great interest due to their potentially high energy density, but the low electronic conductivity of both the sulfur (S 8 ) cathode active material and the final discharge product lithium sulfide (Li 2 S) require the use of a conductive host. Usually made of relatively hydrophobic carbon, such hosts are typically ill‐suited to retain polar discharge products such as the intermediate lithium polysulfides (LiPs) and the final Li 2 S. Herein, we propose a route to increase the sulfur utilization by functionalizing the surface of ordered mesoporous carbon CMK3 with polar groups. These derivatized CMK3 materials are made using a simple two‐step procedure of bromomethylation and subsequent nucleophilic substitution with amine or amide nucleophiles. We demonstrate that, compared to the unfunctionalized control, these modified CMK3 surfaces have considerably larger binding energies with LiPs and Li 2 S, which are proposed to aid the electrochemical conversion between S 8 and Li 2 S by keeping the LiPs species in close proximity to the carbon surface during Li‐S battery cycling. As a result, the functionalized cathodes exhibit significantly improved specific capacities relative to their unmodified precursor.

A low-temperature spray deposition synthesis was developed to prepare locally hexagonally ordered mesoporous titania films with polycrystalline anatase pore walls in an evaporation-induced self-assembly process. The titania film preparation procedure is conducted completely at temperatures below 50 °C. The effects of spray time, film thickness, synthesis time prior to spray deposition, and aging time at high relative humidity after deposition on the atomic arrangement and the mesoorder of the mesoporous titania were studied. We find the crystallite size to depend on both the synthesis time and aging time of the films, where longer times result in larger crystallites. Using the photocatalytic activity of titania, the structure-directing agent is removed with UV radiation at 43–46 °C. The capability of the prepared films to remove the polymer template increased with longer synthesis and aging times due to the increased crystallinity, which increases the photocatalytic efficiency of the titania films. However, with increasingly longer times, the crystallites grow too large for the mesoorder of the pores to be maintained. This work shows that a scalable spray coating method can be used to prepare locally ordered mesoporous polycrystalline titania films by judiciously tuning the synthesis parameters.

By combining two enzymes, formate dehydrogenase (FateDH) and formaldehyde dehydrogenase (FaldDH), it is possible to drive the thermodynamically unfavorable conversion of CO2 to formaldehyde. For this purpose, the enzymes were coimmobilized in siliceous mesostructured cellular foams (MCFs). A high degree of adsorption of both enzymes was achieved by coimmobilizing the enzymes sequentially, i.e., first FateDH and then FaldDH. The highest conversion rate was obtained with an enzyme mass ratio of 1:15 (FateDH/FaldDH). Using MCF functionalized with mercaptopropyl groups (MCF-MP), the activity increased ∼4 times compared to the enzymes free in solution. To probe the distance between the two enzymes, they were separately labeled with either Cy3 or Cy5 dyes and studied with Forster resonance energy transfer (FRET). An increased energy transfer was observed when the enzymes were coimmobilized in MCF-MP, suggesting that the two enzymes are in close proximity, resulting in higher conversion of CO2 to formaldehyde.

Formaldehyde dehydrogenase (FaldDH) is used as a catalyst to reduce formate to formaldehyde in a cascade reaction to convert CO2 to methanol. This enzyme, however, has low activity and is sensitive to substrate/product concentration and pH. To improve the performance of FaldDH, it can be immobilized through physical adsorption in siliceous mesostructured cellular foams (MCF), which physical properties are suitable for the immobilization of large molecules as FaldDH (molecular size of 8.6 × 8.6 × 19 nm). In this work two MCF materials were synthesized: MCF1 with a pore size of 26.8 nm and window size of 10.5 nm; and MCF2 with a pore size of 32.9 nm and window size of 13.0 nm. The surfaces of the materials were functionalized with octyl, mercaptopropyl or chloromethyl groups. FaldDH was successfully immobilized inside all the materials, yielding enzyme loadings of about 300 mg g‐1 in MCF1 and more than 750 mg g−1 in MCF2. However, the enzyme was inactive upon immobilization on MCF1, whereas on MCF2 the enzyme retained its catalytic activity presumably owing to the larger pores of this material and the need for the enzyme to undergo configurational changes during the reaction. Using MCF2 functionalized with mercaptopropyl groups the activity of FaldDH was enhanced beyond that of the free enzyme. Additionally, low leakage of the enzyme from the MCF2 was observed during the reactions. Thus, tailored MCF is a highly attractive material for employment of the FaldDH enzyme.

Iron ion-chelating ordered mesoporous carbon (Fe-OMC) catalysts are a class of non-precious metal catalysts for the oxygen reduction reaction in fuel cells. Here, the performance of Fe-OMC catalysts is studied in relation to variations in conditions employed during their synthesis. Specifically, we studied the effects of (i) the hydration state of the iron salt precursor, (ii) the aging time after mixing of the precursor solution and (iii) the relative amount of iron to N/C-precursor used during synthesis. The formation of complexes between the iron ion and the N/C-precursor was investigated by NMR spectroscopy and it was shown that a complex is formed with the amine group of the N/C-precursor being closest to the iron ion. All three parameters studied (i, ii, and iii) were found to have a significant impact on the complex formation between the iron ion and the N/C-precursor as well as on the performance of the resulting catalyst materials.

We examine the influence of precursor functional groups on the formation and electrocatalytic performance of iron ion-chelating ordered mesoporous carbon (Fe-OMC) fuel cell catalysts. First, we study whether the active sites in these catalysts consist of Fe–Nx or Fe–Ox chelates. To verify this, catalysts were prepared from two different molecular precursors (furfurylamine and furfuryl alcohol) and the functional groups’ (−NH2 vs −OH) role in the formation of iron ion-chelating active sites in the catalysts was established. From electrochemical tests and EPR spectroscopy, conspicuously different behaviors were obtained for the catalyst prepared from furfurylamine compared to that prepared from furfuryl alcohol. It was unambiguously established that the amine group is central to the formation of electrocatalytically active sites in Fe-OMC catalysts and that these are of the Fe–Nx-OMC type. Additional Fe-OMC catalysts were prepared with the purpose to determine the influence of the two precursors on the form...

Development of non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells with high activity and durability and with optimal water management properties is of outmost technological importance and highly challenging. Here we study the possibilities offered through judicious selection of small molecular precursors used for the formation of ordered mesoporous carbon-based non-precious metal ORR catalysts. By combining two complementary precursors, we present a one-pot synthesis that leads to a composite material consisting of transition metal ion-chelating ordered mesoporous carbon and multi-walled carbon nanotubes (TM-OMC/CNT). The resulting composite materials show high specific surface areas and a carbon structure that exhibits graphitic signatures. The synthesis procedure allows for tuning of the carbon structure, the surface area, the pore volume and the ratio of the two components of the composite. The TM-OMC/CNT composites were processed into membrane electrode assemblies and evaluated in single cell fuel cell measurements where they showed a combination of good ORR activity and very high durability.

Mg2Si1-xSnx alloys with nominal values of x [0.03:0.18] were synthesized at 780 °C by solid-state reaction from Mg2Si and Mg2Sn and subsequently annealed at either 680 or 580 °C. Their microstructure was investigated by X-ray diffraction using the Rietveld method. Depending on the treatment temperature and the nominal composition, the solid solutions split into different Si- and/or Sn-rich Mg2Si1-xSnx phases. Traces of spinodal decomposition were observed for the samples with a low Sn content independent of treatment temperature due to the limited diffusion kinetics when entering the miscibility gap. A similar effect was observed when applying a higher cooling rate to the samples with higher Sn concentration. In this case, the samples experience thermodynamic spinodal decomposition being located in the spinodal region sufficiently long time at higher temperatures. Samples treated in the miscibility gap showed an agreement of the Si-rich binodal line with calculated phase diagrams. However, the Sn-rich binodal line stays undefined, perhaps due to grain boundary pinning of diffusing atoms. The study elucidates the possibility of tailoring the microstructure of magnesium silicide-stannide alloys utilizing merely judiciously designed heat treatment protocols. A particular attention is brought to spinodal decomposition, which has the potential to reduce the lattice thermal conductivity.

Temperature-dependent phase-separation, clouding, has been observed in suspensions of silica nanoparticles surface-functionalized with methyl-poly(ethylene glycol) silane. Interparticle interactions and conformational changes of the grafted poly(ethylene glycol) chains influence the observed cloud points, and can be controlled by electrolyte concentration and pH. These findings open new routes to tailoring properties of Pickering emulsions.

Evolution of the polymorph selectivity of titanium dioxide was studied under acidic and low-temperature synthesis conditions. Short synthesis times resulted in high relative amounts of the rutile phase, and long synthesis times resulted in high relative amounts of the brookite and anatase phases. The effect of titania precursor concentration was investigated and found to have a large impact on the polymorph selectivity. As the reaction proceeds with time, changes in the chemical environment, caused in particular by the gradually decreasing titania precursor concentration, are therefore likely the cause of the change in polymorph selectivity observed.

Microporous (2 in the eminent battle against global warming and climate change. The processability of conventional zeolite powders is, however, poor, which limits their implementation in many applications, such as in gas filtration industrial systems. In this work, we overcome this issue through the preparation of hybrid foams using mesoporous/macroporous supporting materials based on the strong network properties of gelatin/nanocellulose, which can support ultrahigh loadings of silicalite-1, used as a model sorbent nanomaterial. We achieved up to 90 wt % of zeolite content and a microporous/mesoporous/macroporous hybrid material. The application of hybrid foams for selective CO 2 sorption exhibits a linear relationship between the zeolite content and CO 2 adsorption capacity and high selectivity over N 2, where the gelatin/nanocellulose foam efficiently supports the zeolite crystals without apparently blocking their pores.

Type-I inorganic clathrates are promising high temperature thermoelectric materials. They are known for their intrinsic low thermal conductivity, but a moderate power factor leaves room for further improvement. In this paper, a new route for improving the power factor by enhanced carrier mobility achieved via modulation doping is reported. A series of clathrates with composition Ba-8(AlxGa1-x)(16)Ge-30 are synthesized through ball milling and spark plasma sintering of mixtures of Ba8Al16Ge30 and Ba8Ga16Ge30. Among the materials with x = 0.20, 0.23, and 0.25, it is found that the electrical conductivity is significantly enhanced with increasing x, while the Seebeck coefficient decreases slightly. It is further revealed that the carrier mobility of the sintered sample x = 0.25 is greatly increased, reaching a value that exceeds that for a single crystal. Electron microscopy analysis reveals that the material consists of a heterostructure and is composed of a Ga-rich clathrate matrix phase and Al particles, suggesting that the power factor enhancement is due to modulation doping. As a result, the highest power factor is achieved for Ba-8(Al0.25Ga0.75)(16)Ge-30, with a value of 1.89 mW m(-1) K-2 at 800 degrees C. Consequently, the maximum zT of sample x = 0.25 reaches 0.93 at 800 degrees C.

We report on sulfur-functionalized ordered mesoporous carbons aimed for lithium-sulfur battery electrode applications with improved charge capacity retention. The carbons were obtained by a hard-template strategy using a mixture of furfuryl alcohol and furfuryl mercaptan. For the application as electrode material in lithium-sulfur batteries, the carbons were additionally loaded with sulfur following a traditional melt-diffusion approach. It was found that the sulfur interacts stronger with the sulfur-functionalized carbon matrix than with the non-functionalized material. Electrodes showed very high capacity in the second discharge-charge cycle amounting to approximately 1500, 1200 and 1400 mAh/g (sulfur) for carbon materials with no, medium and high degrees of sulfur functionalization, respectively. More importantly, the sulfur-functionalization of the carbon was found to increase the capacity retention after 50 discharge-charge cycles by 8 and 5% for the carbons with medium and high degrees of sulfur-functionalization, respectively, compared to carbon with no sulfur-functionalization. We attribute this significant improvement to the presence of covalently bound sulfur groups at the internal surface of the functionalized carbon providing efficient anchoring sites for catenation to the sulfur loaded into the pores of the carbons and provide experimental support for this in the form of results from cyclic voltammetry and X-ray photoelectron spectroscopy.

Transition metal ion-chelating ordered mesoporous carbons (TM-OMCs) were studied as polymer electrolyte membrane fuel cell cathode catalysts. The active site structure of the TM-OMCs was studied by X-ray absorption spectroscopy in combination with variations of a range of synthesis variables of the TM-OMCs. The variations were found to have significant influence on the catalyst structure both in the mesoscale and on the atomic local structure allowing for detailed conclusions on the nature of the active sites. The main active site was found to be FeNx chelates. An additional highly active site was found and proposed to be a FeNx-dioxygen site. It was further found that the catalytic activity could be increased threefold by acid washing and subsequent heat treatment of the as-synthesized TM-OMC materials.

The oxygen reduction reaction (ORR) is usually catalyzed by precious metals. As the kinetics of the reaction are sluggish comparatively large amounts of precious metal are needed to achieve satisfactory reaction rates. This results in high cost of technologies utilizing the ORR, like low temperature fuel cells. Recent years have seen tremendous research efforts in the development of non-precious metal catalysts (NPMCs) with a wide range of newly developed materials resulting in improved catalyst materials, an increased understanding of the ORR mechanism on NPMC materials and better knowledge of the active site structure. Here we summarize the developments from 2011 and onwards with a special focus on carbon-based NPMCs developed for use in acid environments. We include explicit comparisons of PEMFC measurement results in all referenced studies and detailed information on the physical characterization methods used in various publications.

Mesoporous N- and Fe,N-carbon materials, prepared by polymerization of small molecule precursors and subsequent pyrolysis, exhibit high and accessible surface areas, high electrical conductivities, and high electrocatalytic activities that are comparable to more expensive activated-carbon-supported precious-metal (Pt) fuel cell electrocatalysts. Their favorable properties are due to a combination of factors, including high nitrogen contents that increase the number of active moieties, selective formation of certain types of N moieties at surface sites, relatively large (3 nm) mesopores to promote facile diffusion, and sufficient electron conductivity to surface N environments where the reactions occur. The resulting materials exhibit electrocatalytic properties that are comparable to a standard catalyst containing 20 wt% Pt supported on activated carbon. This is evident in Figure 1a, which shows current-voltage plots associated with the reduction of oxygen for 20 wt% Pt-C and for various N- and Fe,N-mesoporous carbon materials that were prepared identically except for the use of different mesostructured porogen templates. These results show that the oxygen reduction activities of the mesoporous N-carbons are significantly higher when a ZnCl2/NaCl salt mixture is used to produce mesoporosity, compared to mesoporous silica. In addition, the incorporation of a small amount (0.5 wt%) of Fe in the mesoporous N-carbon greatly improves oxygen reduction reactivity, which is shown to be superior to the commercial 20 wt% Pt-C. Similarly favorable activities and trends are observed for these materials as electrocatalysts for sulfur reduction. While the inclusion of N heteroatoms has previously been demonstrated to improve the reduction activities of carbon-based electrocatalysts,1 the atomic-level origins of such properties have remained elusive. This has been due to the multicomponent and non-stoichiometric compositions of these porous graphitic solids, their complicated distributions of disordered and ordered regions, and their paramagnetic and/or conductive properties. Nevertheless, advanced solid-state NMR techniques, in combination with Raman spectroscopy, X-ray diffraction, electron microscopy, and DFT modeling, yield detailed new insights on the atomic environments and interactions of N heteroatoms in mesoporous N-carbon electrocatalysts.2 Most importantly, two-dimensional (2D) 13C-15N correlation NMR spectra resolve distinct signals that enable the types and distributions of different N heteroatom environments to be established and correlated with electrocatalytic properties. For example, as shown in Figure 1b, the distributions of correlated of 15N and 13C signals in the 2D contour-plot spectrum of the salt-templated mesoporous N-carbon material manifest at least four distinct types of N-heteroatom environments: pyrrolic (purple), graphitic (blue), edge/isolated pyridinic (yellow), and pyrazinic/pyridinic (red) moieties. Green-shaded regions indicate correlated signals that arise from atomically proximate (15N{1H} CP-MAS NMR spectra enable surface N species to be selectively distinguished from interior moieties (e.g., graphitic N) by the formers’ interactions with adsorbed water. In combination, these powerful multi-nuclear NMR analyses unambiguously establish that certain types of N-carbon moieties are more important to electrocatalytic performance than others. Incorporation of non-precious transition metals, such as Fe, significantly increases electrocatalytic activity.3,4 This is similarly the case for mesoporous N-carbon materials investigated here that were synthesized under otherwise identical conditions, except for the addition of a small (0.5 wt%) Fe (Figure 1a). The important role(s) of Fe, however, are challenging to establish, due to its generally dilute loading and its paramagnetic (and possibly ferromagnetic) properties. Nevertheless, 57Fe Mössbauer spectra, solid-state 15N NMR spectra, and 15N spin-lattice (T 1) relaxation time analysesresolve 15N signals from 15N species that are proximate to paramagnetic Fe heteroatoms, from which 15N-Fe distances are estimated. These analyses provide detailed new insights into the types, atomic environments, and distributions of 15N heteroatoms in mesoporous N- and Fe,N-carbon materials, which, until now, have been infeasible to distinguish by scattering or other spectroscopic techniques. Understanding the roles of these moieties in the catalytic reduction of oxygen or sulfur is expected to yield new design criteria for syntheses of high performance non-precious-metal electrocatalysts for diverse fuel cell and battery applications. References (1) Gong, K.; et al. Science 2009, 323, 760–764. (2) Becwar, S. M.; et al. submitted. (3) Donghun, K.; et al. ACS Appl. Mater. Interfaces 2018, 10, 25337−25349. (3) Al-Zoubi, T.; et al. J. Am. Chem. Soc. 2020, 142, 5477–5481. Figure 1

A method for grafting lithium sulfonate (LiSO3) groups to carbon surfaces is developed and the resulting carbons are evaluated for their potential to reduce the lithium polysulfide (LiPS) shuttle in lithium–sulfur (Li–S) batteries, replacing the common electrolyte additive lithium nitrate (LiNO3). The LiSO3 groups are attached to the ordered mesoporous carbon (CMK3) surface via a three-step procedure to synthesize LiSO3-CMK3 by bromomethylation, sodium sulfite (Na2SO3) substitution, and cation exchange. As a comparison, ethylenediamine (EN)-substituted CMK3, EN-CMK3, is also synthesized and tested. When used as a cathode in Li–S batteries, the unfunctionalized CMK3 suffers from strong LiPS shuttling as evidenced by its low initial Coulombic efficiencies (ICEs, 75%). Postcycling analysis reveals the benefits of cathode surface functionalization on the lithium anode via an attenuated LiPS shuttle. When monitored at open circuit, the functionalized cathodes maintain their cell voltages much better than the CMK3 control and concurrent electrochemical impedance spectroscopy reveals their higher total cell resistance, which provides evidence for a reduced LiPS shuttle in the vicinity of both electrodes. Overall, such surface groups show promise as cathode-immobilized “lithium nitrate mimics.”.

Hybrid energy storage systems aim to achieve both high power and energy densities by combining supercapacitor-type and battery-type electrodes in tandem. The challenge is to find sustainable materials as fast charging negative electrodes, which are characterized by high capacity retention. In this study, mesoporous anatase beads are synthetized with tailored morphology to exploit fast surface redox reactions. The TiO2-based electrodes are properly paired with a commercial activated carbon cathode to form a Li-ion capacitor. The titania electrode exhibits high capacity and rate performance. The device shows extremely stable performance with an energy density of 27 mWh g-1 at a specific current of 2.5 A g−1 for 10,000 cycles. The remarkable stability is associated with a gradual shift of the potential during cycling as result of the formation of cubic LiTiO2 on the surface of the beads. This phenomenon renews the interest in using TiO2 as negative electrode for Li-ion capacitors.

Designing alloys with an accurate temperature-independent electrical response over a wide temperature range, specifically a low temperature coefficient of resistance (TCR), remains a big challenge from a material design point of view. More than a century after their discovery, Constantan (Cu–Ni) and Manganin (Cu–Mn–Ni) alloys remain the top choice for strain gauge applications and high-quality resistors up to 473–573 K. Here, an average TCR is demonstrated that is up to ≈800 times smaller in the temperature range 5–300 K and >800 times smaller than for any of these standard materials over a wide temperature range (5 K < T < 1200 K). This is achieved for selected compositions of AlxCoCrFeNi high-entropy alloys (HEAs), for which a strong correlation of the ultralow TCR is established with the underlying microstructure and its local composition. The exceptionally low electron–phonon coupling expected in these HEAs is crucial for developing novel devices, e.g., hot-electron detectors, high-Q resonant antennas, and materials in gravitational wave detectors.

Fluorescence spectroscopy of protein-bound molecular rotors Cy3 and Cy5 is used to monitor the effective viscosity inside the pores of two types of mesoporous silica (SBA-15 and MCF) with pore diameters between 8.9 and 33 nm. The ratio of the peak intensities is used to measure viscosity independently of solvent polarity, and the response of the lipase-bound dyes is calibrated using glycerol/water mixtures (no particles). The two dyes are either attached to the same protein or separate proteins in order to investigate potential effects of energy transfer (FRET) on the fluorescence properties, when using them as reporter dyes. The effective viscosity inside the pores at infinite protein dilution is one order of magnitude higher than in bulk water, and the effect of protein concentration on the measured viscosity indicates a stronger effect of protein–protein interactions in the pores than in similarly concentrated protein solutions without particles. In MCF-particles with octyl-groups covalently attached to the pore walls, a more efficient uptake of the lipase resulted in FRET between the protein-bound dyes even if the two dyes were attached to different proteins. In contrast to the unmodified particles the intensity-ratio method could therefore not be used to measure the viscosity, but the presence of FRET in itself indicates that octyl–protein interactions lead to a non-homogenous protein distribution in the pores. The dye labels also report a less polar pore environment as sensed by the proteins through a redshift in the dye emission. Both observations may help in understanding the higher efficiency of lipase immobilization in octyl-modified particles.

A facile low-temperature synthesis method of crystalline titanium dioxide nanoparticles has been developed, and the effect of temperature and acidity on polymorph selectivity has been systematically investigated. The crystal polymorph of titanium dioxide could be tuned by carefully controlling the condensation rate of titania. Conditions providing high condensation rate favoured the formation of the anatase and brookite phases, while lowering the condensation rate yielded higher rutile content. The reaction was found to proceed either in liquid state for reactions performed at low temperatures and high acidity or in a sol–gel manner for higher temperatures and lower acidity. The aggregation behaviour was found to be strongly affected by the employed synthesis conditions and by the crystal polymorph formed. The design of a facile low-temperature synthesis system with predictive control of polymorph selectivity is important for further development of titania-based materials for applications where type of polymorph is vital, such as photocatalytic and photonic.

Core-shell microcapsules consisting of a titanium dioxide shell and a hydrophobic solvent core have been prepared with diameters of a few micrometers and a narrow size distribution using a simple and fast airbrush technique. These microcapsules were prepared at room temperature in a single-step process in which an oil with a dissolved titanium alkoxide precursor was forced together with an aqueous solution, containing a surface-active polymer, through a narrow spray nozzle using a nitrogen gas propellant. Several different parameters of chemical, physical, and processing origin were investigated to find an optimal recipe. Two different alkanes, one ketone, and four alcohols were tested and evaluated as core materials, alone or together with the antifungal biocide 2-n-octyl-4-isothiazolin-3-one (OIT). Long-chain alcohols were found suitable as core oil due to their low solubility in water and surface activity. The addition of the surface-active polymers in the water phase was important in aiding the formation and stabilization of the titanium dioxide shell. An impressive loading of 50 wt% of the semi-hydrophobic OIT was possible to encapsulate using this simple and applicable procedure.

The immobilisation of four feruloyl esterases (FAEs) (FaeA1, FaeA2, FaeB1, FaeB2) from the thermophilic fungus Myceliophthora thermophila C1 was studied and optimised via physical adsorption onto various mesoporous silica particles with pore diameters varying from 6.6 nm to 10.9 nm. Using crude enzyme preparations, enrichment of immobilised FAEs was observed, depending on pore diameter and protein size. The immobilised enzymes were successfully used for the synthesis of butyl ferulate through transesterification of methyl ferulate with 1-butanol. Although the highest butyl ferulate yields were obtained with free enzyme, the synthesis-to-hydrolysis ratio was higher when using immobilised enzymes. Over 90% of the initial activity was observed in a reusability experiment after nine reaction cycles, each lasting 24 h. Rinsing with solvent to remove water from the immobilised enzymes further improved their activity. This study demonstrates the suitability of immobilised crude enzyme preparations in the development of biocatalysts for esterification reactions.

Societies’ increasing need for energy storage makes it necessary to explore new concepts beyond the traditional lithium ion battery. A promising candidate is the lithium-sulfur technology with the potential to increase the energy density of the battery by a factor of 3–5. However, so far the many problems with the lithium-sulfur system have not been solved satisfactory. Here we report on a new approach utilizing a self-standing reduced graphene oxide based aerogel directly as electrodes, i.e. without further processing and without the addition of binder or conducting agents. We can thereby disrupt the common paradigm of “no battery without binder” and can pave the way to a lithium-sulfur battery with a high practical energy density. The aerogels are synthesized via a one-pot method and consist of more than 2/3 sulfur, contained inside a porous few-layered reduced graphene oxide matrix. By combining the graphene-based aerogel cathode with an electrolyte and a lithium metal anode, we demonstrate a lithium-sulfur cell with high areal capacity (more than 3 mAh/cm2 after 75 cycles), excellent capacity retention over 200 cycles and good sulfur utilization. Based on this performance we estimate that the energy density of this concept-cell can significantly exceed the Department of Energy (DEO) 2020-target set for transport applications.

Ordered mesoporous anatase films can be prepared at low temperature by evaporation-induced self-assembly of a microemulsion used to preform nanocrystals of titania. Here, we study the impact of storage time of the reaction solution on the structure and photocatalytic properties of such films. The formation of anatase crystallites was found to initiate and proceed during storage of the reaction solution prior to the preparation of the titania films as expected. However, in contrast to current understanding, we find that the main part of the anatase formation occurs during aging of the prepared films in a controlled relative humidity as a consequence of the rapidly increased concentration of precursor caused by the solvent evaporation during film preparation. Ordered mesoporous films were thus obtained for reaction solutions stored up to 48 h, and the crystallinity of the pore walls was found to increase with increased storage time. Interestingly, the anatase crystallite size was found to remain between 2 and 5 nm for all films prepared from reaction solutions stored up to 48 h showing that the increased crystallinity is due to an increased number density of crystallites. Films prepared from reaction solutions stored for longer times than 48 h were found to be mesoporous but with a disordered pore arrangement. These films contained crystallites with sizes up to 10 nm, apparently too large to arrange in a meso-ordered fashion in the liquid crystalline template. The photocatalytic activity of the films was evaluated for phenol degradation and found to be higher for films prepared from reaction solutions with longer storage times of the reaction solution ascribed to their larger degree of crystallinity and larger crystallite size. Interestingly, although the degree of mesoporosity is important in providing access to high specific surface area and facile transport of reactants and products to the catalyst surface, the degree of order of the mesopores was found to be of less importance for the photocatalytic performance. In addition, this work provides new generic insight into the formation mechanism of low-temperature surfactant-templated synthesis of ordered mesoporous and crystalline materials.

Alkaline sodium silicate solutions with SiO2:Na2O molar ratios in the range 4-10 are known to be colloidally unstable manifested in phase separation or gelation. The mechanistic understanding of this instability is generally poor. To improve this situation the microscopic structure of a series of solutions with ratios in the range 3.3-8.9 has been characterised using small-angle X-ray scattering, Dynamic light scattering, Fourier transformed infrared spectroscopy, and (29)Si Nuclear magnetic resonance spectroscopy to cover the relevant length scales related to silica clusters, aggregates, and particles present. In the starting solution, with ratio 3.3, there are silica present in three fractions. The main part is present as small silica clusters with a radius of 0.7 nm. There are also a significant portion of monomers/small oligomeric silica species as well as a minute amount of larger colloidal silica particles. At a higher SiO2:Na2O ratio, above approximately 4, smaller spherical colloidal particles are formed due to condensation reactions. However, as a result of a too high ionic strength the suspension is not stable and the particles aggregate to fractal structures with a size that depends on ratio and ageing time. At the highest SiO2:Na2O ratio, fractals are not formed because of the lower ionic strength and the smaller colloidal particles are stable in the solution. By carefully adding small amounts of NaCl to the high ratio solution it is possible to induce gelation of the solution confirming the hypothesis that the instability region is due to too high electrolyte concentration for the silica species present under those conditions.

A new concept for noble metal-free polymer electrolyte membrane fuel cell catalysts has been developed. The catalysts consist of chelated transition metal ions incorporated in a nitrogen-functionalized ordered mesoporous carbon matrix, which is evidenced by a combination of X-ray absorption fine structure analysis and high-resolution transmission electron microscopy. The ordered mesoporous carbon matrix of the catalyst offers an exceptionally high specific surface area and allows conceptually for a high degree of tuning, enabling controlled variability of, e.g., pore size and curvature and thickness of the pore walls of the catalysts. Single cell fuel cell tests of membrane electrode assemblies prepared with a cathode made of iron- or cobalt-based versions of the catalyst show high power densities, reaching up to one-third of a commercial Pt/C catalyst at 0.6 V.

Colloidal silica particles, functionalized with hydrophilic and hydrophobic groups, have been studied for utilization in particle-stabilized emulsions, so called Pickering emulsions. The amounts of attached groups have been characterized using NMR spectroscopy and elemental analysis. A range of particles were prepared, with sizes from around 13 to 70 nm in diameter. Hydrophilic functionalization of the silica sols was achieved by attaching methyl poly(ethylene glycol) (mPEG) silane to the silica particle surface. This provides a reduction of surface charge density, a pH dependent and controllable flocculation behavior and surface activity. The hydrophobic functionalization of the silica sols was accomplished by attaching organosilanes containing mainly propyl and methyl groups. The emulsification abilities were evaluated by preparing Pickering emulsions using particles, with varying degrees and combinations of surface functionalization, as stabilizers and comparing the obtained emulsion droplet size distributions. It was found that colloidal silica functionalized with hydrophobic groups produced emulsions with smaller droplets compared to using unmodified silica. The emulsification performance was further improved by the combination of both hydrophilic and hydrophobic groups. All particles having this heterogeneous modification were found to generate emulsions with high stability towards coalescence (from five weeks to 1.5 years).

Accurate measurement of thermal conductivity is essential to determine the thermoelectric figure-of-merit, zT. Near the phase transition of Cu2Se at 410 K, the transport properties change rapidly with temperature, and there is a concurrent peak in measured heat capacity from differential scanning calorimetry (DSC). Interpreting the origin as a broad increase in heat capacity or as a transient resulted in a three-fold difference in the reported zT in two recent publications. To resolve this discrepancy, thermal effusivity was deduced from thermal conductivity and diffusivity measurements via the transient plane source (TPS) method and compared with that calculated from thermal diffusivity and the two interpretations of the DSC data for heat capacity. The comparison shows that the DSC measurement gave the heat capacity relevant for calculation of the thermal conductivity of Cu2Se. The thermal conductivity calculated this way follows the electronic contribution to thermal conductivity closely, and hence the main cause of the zT peak is concluded to be the enhanced Seebeck coefficient.

The current status of the development of composite thermoelectric materials with embedded nanoparticles is reviewed. An introduction is given to the suggested mechanisms of improving thermoelectric properties by inclusions of nanoparticles and to experimental methods used to prepare such composites. The progress made in the development of thermoelectric materials with embedded nanoparticles is then covered, grouping the studies according to the optimal temperature range of operation of the materials investigated. Most studies have been devoted to materials within the medium temperature range, followed by low temperature materials, whereas high temperature materials have not yet received much attention within this area. In the majority of the materials systems studied, reports of improved thermoelectric performance upon introduction of nanoparticles in bulk thermoelectrics are found. However, for continued progress in this area, there is a need for systematic experimental studies that unambiguously correlate the resulting physical effects of the nanoinclusions to the measured materials properties.

Alumina-supported In2O3, Ga2O3 and B2O3 were investigated as catalysts for lean NOx reduction with DME as reducing agent and compared to pure Al2O3 and In2O3. The In2O3-promoted alumina catalysts showed the highest NOx conversion at low temperatures, although with a narrow temperature window. Pure In2O3, on the other hand, was inactive for NOx reduction with DME. The Ga2O3- and B2O3-promoted alumina catalysts gave the highest NOx conversion at higher temperatures, showed a temperature window similar to pure alumina, but were less sensitive to H2O inhibition. Possible reasons for these observations are discussed.

The highly oxygenated hydrocarbon triethylene glycol dimethyl ether or triglyme (CH3O-(C2H4O-)(3)CH3) was found to efficiently reduce NOx under lean conditions over Ag/Al2O3, but gave a low NOx conversion over Cu-ZSM-5. Furthermore, triglyme showed an extraordinary promoting effect when added together with propene as reducing agent for NOx over Ag/Al2O3 at low temperature. This is most likely due to that triglyme promotes the activation of propene.

Thermoelectric thin films of the ternary compounds (BixSb1−x)2Te3 and Bi2(Te1−ySey)3 were synthesized using potentiostatic electrochemical deposition on gold-coated silicon substrates from aqueous acidic solutions at room temperature. The surface morphology, elemental composition, and crystal structure of the deposited films were studied and correlated with preparation conditions. The thermoelectric properties of (BixSb1−x)2Te3 and Bi2(Te1−ySey)3 films, i.e., Seebeck coefficient and electrical resistivity, were measured after transferring the films to a nonconductive epoxy support. (BixSb1−x)2Te3 thin films showed p-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively large negative potential with composition close to Bi0.5Sb1.5Te3. In addition, Bi2(Te1−ySey)3 thin films showed n-type semiconductivity, and the highest power factor was obtained for film deposited at a relatively small negative potential, having composition close to Bi2Te2.7Se0.3. In contrast to Bi2Te2.7Se0.3 thin films, an annealing treatment was required for Bi0.5Sb1.5Te3 thin films to achieve the same magnitude of power factor as Bi2Te2.7Se0.3. Therefore, Bi2Te2.7Se0.3 thin films appear to be good candidates for multilayer preparation using electrochemical deposition, but the morphology of the films must be further improved.

Commercial Cu-ZSM-5- and Ag/Al2O3-based lean NOx catalysts were evaluated in a synthetic exhaust gas bench with the fuels RME, B30, B15, Agrodiesel 15, GTL, NExBTL, and MK1 as reducing agents. The influence of reductant was larger for Ag/Al2O3, albeit moderate, whereas the Cu-zeolite showed the highest NOx conversion at lower temperature for all alternative fuels tested.

A new direct synthesis methodology for the preparation of mesoporous titania with hexagonal mesostructure and with walls of tunable crystallinity and crystallite size has been developed. As the crystallites grow they eventually reach a point where they are too big to fit within the pore walls, the mesoorder becomes distorted and is finally lost. It is run at an unprecedented low temperature of 40 °C without the need for an autoclave or high-temperature post-treatment procedures and is thus compatible with low temperature stable substrates. High-resolution transmission electron microscopy and electron diffraction reveal that the pore walls of the mesoordered material consists of anatase TiO2 nanoparticles. The method allows for a wide choice of synthesis time during which the anatase crystallites grow slowly within the range of 2−5 nm in size but needs to be kept such that the crystallites have not yet grown too large for them to be coassembled into an ordered mesostructure.

Bismuth telluride thin films have been synthesized by electrochemical deposition onto stainless steel substrates from acidic solutions. The influence of deposition variables on film composition, morphology and crystal orientation associated with the growth of the film was investigated by means of constant potential deposition and pulsed potential deposition. In-plane thermoelectric and transport properties of the electrodeposited films were measured. The carrier concentration of the electrodeposited films was found to be one order of magnitude larger than typically reported for optimized bulk bismuth telluride, which explains the unusually low Hall mobility and Seebeck coefficient values found for the electrodeposited films. Pulse deposited films showed slightly lower electrical resistivity and higher Seebeck coefficient due to the lower porosity and less preferred crystal orientation of the films compared to the continuously deposited films. Improvements of the film properties are necessary to make them viable for applications.