Enhanced Thermoelectric Performance in the SrTi0.85Nb0.15O3 Oxide Nanocomposite with Fe2O3-Functionalized Graphene.

Doped SrTiO3 is considered one of the potential thermoelectric (TE) candidates but its TE figure of merit, ZT needs to be improved for practical application of electricity generation from high-grade waste-heat. In the present work, enhanced TE performance has been realized for SrTi0.85Nb0.15O3 (STN) perovskite adopting the strategy of composite formation with Fe2O3-functionalized graphene (FGR). We have achieved a maximum electrical conductivity of 1.4 × 105 S m-1 for 1 wt % FGR added to STN, which is around 1185% larger than that of pristine STN. The presence of FGR in the STN matrix acts as a mobility booster of electrons, overcoming the effect of Anderson localization of electrons, which impedes the electron transport in STN. This is evident from the order of magnitude increase in weighted mobility of STN after FGR addition. Furthermore, the incorporation of FGR causes about a 34% decrease in the lattice thermal conductivity. The Debye-Callaway model demonstrates that the phonon-phonon Umklapp scattering is primarily responsible for reduced thermal conductivity. The presence of FGR sheets along the grain boundaries of STN, Fe2O3 nanoparticles, and lattice imperfections gives rise to the glass-like temperature-independent phonon mean-free-path, especially above Debye temperature. The maximum ZT ∼ 0.57 has been obtained at 947 K for the 1 wt % FGR sample, which is around 420% higher than that of pristine STN. Furthermore, we have fabricated a prototype of a four-legged n-type TE module, demonstrating one of the highest power outputs of 18 mW among reported oxide thermoelectrics.

Designing rare earth-free high entropy oxides with a tungsten bronze structure for thermoelectric applications.

In recent years, forming high entropy oxides has emerged as one of the promising approaches to designing oxide thermoelectrics. Entropy engineering is an excellent strategy to improve thermoelectric performance by minimizing the thermal conductivity arising from enhanced multi-phonon scattering. In the present work, we have successfully synthesized a rare-earth-free single phase solid solution of novel high entropy niobate (Sr0.2Ba0.2Li0.2K0.2Na0.2)Nb2O6, with a tungsten bronze structure. This is the first report on the thermoelectric properties of high entropy tungsten bronze-type structures. We have obtained a maximum Seebeck coefficient of -370 µV K-1 at 1150 K, which is the highest among tungsten bronze-type oxide thermoelectrics. The minimum thermal conductivity of 0.8 W m-1 K-1 is obtained at 330 K, which is so far the lowest reported value among rare-earth-free high entropy oxide thermoelectrics. This synergistic combination of large Seebeck and record low thermal conductivity gives rise to a maximum ZT of 0.23 which is so far the highest among rare-earth free high entropy oxide-based thermoelectrics.

Enhanced Thermoelectric Performance of Rare-Earth-Free n-Type Oxide Perovskite Composite with Graphene Analogous 2D MXene.

Here, the first experimental demonstration on the effect of incorporating new generation 2D material, MXene, on the thermoelectric performance of rare-earth-free oxide perovskite is reported. The charge localization phenomenon is predominant in the electron transport of doped SrTiO3 perovskites, which deters from achieving a higher thermoelectric power factor in these oxides. In this work, it is shown that incorporating Ti3 C2 Tx MXene in a matrix of SrTi0.85 Nb0.15 O3 (STN) facilitates the delocalization of electrons resulting in better than single-crystal-like electron mobility in polycrystalline composites. A 1851% increase in electrical conductivity and a 1000% enhancement in power factor are attained. Besides, anharmonicity caused by MXene in the STN matrix has led to enhanced Umklapp scattering giving rise to lower lattice thermal conductivity. Hence, 700% ZT enhancement is achieved in this composite. Further, a prototype of thermoelectric generator (TEG) using only n-type STN + MXene is fabricated and a power output of 38 mW is obtained, which is higher than the reported values for oxide TEG.

Enhanced Thermoelectric Performance in Oxide Composites of La and Nb Codoped SrTiO3 by Using Graphite as the Electron Mobility Booster.

Inherent insulating nature of oxides makes it challenging for use in thermoelectric applications that warrant reasonable electrical conductivity. In the present work, we have used graphite (G) to improve the electron transport in La0.07Sr0.93Ti0.93Nb0.07O3 (LSTN) by making composites. Graphite acts as the electron momentum booster in the LSTN matrix, which otherwise suffers from Anderson localization of electrons, causing an order of magnitude increase in weighted mobility and electrical conductivity. As a result, the thermoelectric power factor increases more than 6 times due to graphite incorporation in LSTN. Furthermore, the lattice thermal conductivity is suppressed due to enhanced Umklapp scattering, as derived from the Debye-Callaway model. Hence, we have recorded ∼423% increment in the figure of merit (ZT) in LSTN + G composites. The maximum ZT obtained is 0.68 at 980 K for the LSTN with 1 wt % graphite composite. Furthermore, we have fabricated a four-legged n-type thermoelectric power generator demonstrating a milliwatt level power output, which hitherto remained unattainable for oxide thermoelectrics.

Synthesis of Template-Free Iron Oxyhydroxide Nanorods for Sunlight-Driven Photo-Fenton Catalysis.

Designing a photocatalyst with high efficiency using semiconductor materials emerges as a promising approach for the treatment of wastewater. At the same time, it is very essential to develop nondestructive, green, and sustainable techniques for the degradation of refractory pollutants. Here, we have demonstrated a facile route to prepare iron oxyhydroxide nanorods (ß-FeOOH) without employment of any templating agent via a light-driven solution chemistry pathway and explored the as-prepared nanorods as the photo-Fenton catalyst under solar light irradiation. The photocatalytic experiments were performed toward the degradation of the aqueous solution of two different pollutants, namely, methylene blue and rhodamine B dyes. We have illustrated the effect of pH of the solution together with the concentration of H2O2 during the degradation process and optimized the solution pH as well as the H2O2 concentration. The superb photocatalytic efficiency of ß-FeOOH is attributed to the generation of reactive oxygen species in the presence of solar light, and these photo-produced reactive oxygen species assist the degradation process. The excellent photocatalytic efficacy and sustainability of ß-FeOOH nanorods along with their effortless synthesis approach point to a cost-effective and environmentally benign pathway in fabricating a highly active photocatalyst for the degradation of organic dyes.

MnO2 flowery nanocomposites for efficient and fast removal of mercury(ii) from aqueous solution: a facile strategy and mechanistic interpretation.

We report the synthesis of MnO2 flowery nanocomposites consisting of MnO2 nanoflowers grown over the surface of clay nanomaterials using an easy and green approach. The MnO2 nanocomposites were explored as a cost-effective nanoadsorbent for mercury removal from aqueous solution and they demonstrated excellent efficiency towards mercury uptake. Monolayer molecular adsorption of Hg(ii) was attained over the surface of the MnO2 nanocomposites and the experimental data acquired in the kinetic study demonstrated that the Hg(ii) adsorption kinetics proceeded via a pseudo-second-order kinetic model. pH dependent adsorption study revealed that their sorption capacity increases until pH 7.0 and then gradually decreases with increasing pH. Apart from the experimental study, we have provided a mechanistic interpretation to illustrate the mechanism of kinetics and thermodynamics during Hg(ii) adsorption. Theoretical understanding along with experimental results indicates a spontaneous and highly favorable Hg(ii) uptake up to 50 °C, representing endothermicity of the adsorption process and then exothermicity above 50 °C, resulting in reduced sorption capacity. The exceptional adsorption performance of the MnO2 nanocomposites may be attributed to their negative surfaces, which facilitated the binding of positively charged Hg(ii) ions through electrostatic interaction. Hence, MnO2 nanocomposites proved to be an effective and inexpensive nanoadsorbent for the removal of Hg(ii) from aqueous solution and may hold a promise for wastewater treatment.

Ultra-small intermetallic NiZn nanoparticles: a non-precious metal catalyst for efficient electrocatalysis.

Intermetallics are long-range-ordered alloys traditionally synthesized by annealing nanoparticles of a random alloy, which results in the sintering of the nanoparticles and leads to the formation of polydispersed samples. It thus remains a challenge to achieve a monodispersion of tiny intermetallics. In the current work, ultra-small monodisperse intermetallic NiZn nanoparticles were synthesized based on a low-temperature solution chemistry route involving the chemical conversion of metal nanoparticles into an ordered alloy using an organometallic zinc precursor. During the transformation of single metal nanoparticles into the corresponding alloy, the particles retained their morphology. The resulting ordered alloy made up of earth-abundant materials demonstrated high electrocatalytic performance for the oxygen evolution reaction (OER) with a low overpotential of 283 mV at a current density of 10 mA cm-2 and a small Tafel slope of 73 mV dec-1, along with excellent stability and durability. The prepared intermetallic NiZn exhibited outstanding OER efficacy, better than those of a Ni0.7Zn0.3 alloy, pure Ni nanoparticles and even state-of-the art RuO2. The atomic ordering as well as the modification of the electronic structure of Ni upon becoming alloyed with Zn, together with an atomic-scale synergistic effect produced from Ni and Zn, led to the enhanced intrinsic catalytic activity. The present findings point to a general route to produce nanoscale tiny alloys and also provide excellent electrocatalysts having exceptional energy conversion efficiency.

Model-Free Estimation of Energy-Transfer Timescales in a Closely Emitting CdSe/ZnS Quantum Dot and Rhodamine†6G FRET Couple.

Analysis of Förster resonance energy transfer (FRET) in nanoparticles is often complicated by a number of factors, such as close emission band positions of the donor and acceptor, the presence of simultaneous photoinduced electron transfer, fluorescence blinking, and natural excited-state decay processes. To address these concerns, mostly from materials chemists and biologists, herein, a state-of-the-art FRET analysis method that utilizes the bright green emission of a CdSe/ZnS core/shell quantum dot (QD) is proposed. The uniqueness of this model-free FRET analysis demonstrates the potential of these QDs to be part of an efficient FRET assay in molecular ruler applications. Molecular distance calculations relying on the proposed FRET analysis complement nicely with the expected donor-to-acceptor separation distances; the QD, as a photoluminescent marker, is electrostatically attached to a compatible fluorescent dye, rhodamine 6G. The beneficial aspects of the model-free FRET analysis provide many possibilities, including the use of low cytotoxicity QD-based FRET assays as a next-generation molecular ruler for the accurate estimation of distances inside a biological system.

Clay-Based Nanocomposites as Recyclable Adsorbent toward Hg(II) Capture: Experimental and Theoretical Understanding.

Here, we report the development of inorganic-organic hybrid nanocomposites through selective modification of the negative outer surfaces of halloysite nanoclays with two different organosilanes having primary or secondary amine sites to be explored them as novel and cost-effective adsorbents for the extraction of toxic inorganic contaminants from aqueous solution. They possess excellent selectivity for the adsorption of mercury, which shows monolayer molecular adsorption over the nanocomposites. The adsorption kinetics of Hg(II) is very fast and follows pseudo-second-order model compared to pseudo-first-order model. A combined experimental and theoretical study demonstrated that Hg(II) uptake by these nanocomposites is highly favorable and spontaneous up to 40 °C, and beyond this temperature, the uptake capacity gradually reduced. Temperature-dependent adsorption study exhibits endothermicity at low temperature (≤40 °C) and exothermicity beyond 40 °C. pH-dependent adsorption study showed their high uptake capacity until pH 7, which reduced at alkaline pH. All of the nanocomposites hold excellent adsorption capacity even at low concentration of adsorbate, along with multicycle sorption capability. The outstanding adsorption capacity as well as the easy synthetic route to achieve these nanocomposites may attract researchers to develop low-cost adsorbents to capture toxic metals, which in turn regulate the permissible limit of these toxic metals in drinking water.

Halloysite nanotubes with immobilized silver nanoparticles for anti-bacterial application.

Halloysite nanotubes (HNTs) with immobilized silver (Ag) nanoparticles (NPs) were prepared by methods of wet chemistry and were characterized by using the transmission electron microscopy, x-ray diffraction, optical spectroscopy and experiments with E. coli bacteria in-vitro. It was found that Ag NPs with almost perfect crystalline structure and sizes from ∼9nm were mainly attached over the external surface of HNTs. The optical absorption measurement revealed a broad plasmonic resonance in the region of 400-600nm for HNTs with Ag NPs. The later samples exhibit bactericidal effect, which is more pronounced under illumination. A role of the plasmonic excitation of Ag NPs for their bioactive properties is discussed. The obtained results show that Ag NPs-decorated HNTs are promising agents for the antibacterial treatment.

Hydrogen sulphide in exhaled breath: a potential biomarker for small intestinal bacterial overgrowth in IBS.

There is a pressing need to develop a novel early-detection strategy for the precise evolution of small intestinal bacterial overgrowth (SIBO) in irritable bowel syndrome (IBS) patients. The current method based on a hydrogen breath test (HBT) for the detection of SIBO is highly controversial. HBT has many limitations and drawbacks. It often fails to indentify SIBO when IBS individuals have 'non-hydrogen-producing' colonic bacteria. Here, we show that hydrogen sulphide (H2S) in exhaled breath is distinctly altered for diarrhea-predominant IBS individuals with positive and negative SIBO by the activity of intestinal sulphate-reducing bacteria. Subsequently, by analyzing the excretion kinetics of breath H2S, we found a missing link between breath H2S and SIBO when HBT often fails to diagnose SIBO. Moreover, breath H2S can track the precise evolution of SIBO, even after the eradication of bacterial overgrowth. Our findings suggest that the changes in H2S in the bacterial environment may contribute to the pathogenesis of SIBO and the breath H2S as a potential biomarker for non-invasive, rapid and precise assessment of SIBO without the endoscopy-based microbial culture of jejunal aspirates, and thus may open new perspectives into the pathophysiology of SIBO in IBS subjects.

Assessing Atmospheric CO2 Entrapped in Clay Nanotubes using Residual Gas Analyzer.

A residual gas analyzer (RGA) coupled with a high-vacuum chamber has been explored to measure atmospheric CO2 entrapped in aminosilane-modified clay nanotubes. Ambient CO2 uptake efficacy together with stability of these novel adsorbents composed of both primary and/or secondary amine sites has been demonstrated at standard ambient temperature and pressure. The unprecedented sensitivity and accuracy of the RGA-based mass spectrometry technique toward atmospheric CO2 measurement has been substantiated with a laser-based optical cavity-enhanced integrated cavity output spectroscopy. The adsorption kinetics of atmospheric CO2 on amine-functionalized clay nanotubes followed the fractional-order kinetic model compared to that of the pseudo-first-order or pseudo-second-order rate equations. The efficiency along with stability of these novel adsorbents has also been demonstrated by their repetitive use for CO2 capture in the oxidative environment. Our findings thus point to a fundamental study on the atmospheric CO2 adsorption by amine-loaded adsorbents using an easy handling and low-cost benchtop RGA-based mass spectrometer, opening a new strategy for CO2 capture and sequestering study.

Advances in nanoscale alloys and intermetallics: low temperature solution chemistry synthesis and application in catalysis.

Based on the bottom-up chemistry techniques, the size, shape, and composition controlled synthesis of nanoparticles can now be achieved uniformly, which is of great importance to the nanoscience community as well as in modern catalysis research. The low-temperature solution-phase synthesis approach represents one of the most attractive strategies and has been utilized to synthesize nanoscale metals, alloys and intermetallics, including a number of new metastable phases. This perspective will highlight the solution-based nanoparticle synthesis techniques, a low-temperature platform, for the synthesis of size and shape-tunable nanoscale transition metals, alloys, and intermetallics from the literature, keeping a focus on the utility of these nanomaterials in understanding the catalysis. For each solution-based nanoparticle synthesis technique, a comprehensive overview has been given for the reported nanoscale metals, alloys, and intermetallics, followed by critical comments. Finally, their enhanced catalytic activity and durability as novel catalysts have been discussed towards several hydrogenation/dehydrogenation reactions and also for different inorganic to organic reactions. Hence, the captivating advantages of this controllable low-temperature solution chemistry approach have several important implications and together with them this approach provides a promising route to the development of next-generation nanostructured metals, alloys, and intermetallics since they possess fascinating properties as well as outstanding catalytic activity.

Mechanisms linking metabolism of Helicobacter pylori to (18)O and (13)C-isotopes of human breath CO2.

The gastric pathogen Helicobacter pylori utilize glucose during metabolism, but the underlying mechanisms linking to oxygen-18 ((18)O) and carbon-13 ((13)C)-isotopic fractionations of breath CO2 during glucose metabolism are poorly understood. Using the excretion dynamics of (18)O/(16)O and (13)C/(12)C-isotope ratios of breath CO2, we found that individuals with Helicobacter pylori infections exhibited significantly higher isotopic enrichments of (18)O in breath CO2 during the 2h-glucose metabolism regardless of the isotopic nature of the substrate, while no significant enrichments of (18)O in breath CO2 were manifested in individuals without the infections. In contrast, the (13)C-isotopic enrichments of breath CO2 were significantly higher in individuals with Helicobacter pylori compared to individuals without infections in response to (13)C-enriched glucose uptake, whereas a distinguishable change of breath (13)C/(12)C-isotope ratios was also evident when Helicobacter pylori utilize natural glucose. Moreover, monitoring the (18)O and (13)C-isotopic exchange in breath CO2 successfully diagnosed the eradications of Helicobacter pylori infections following a standard therapy. Our findings suggest that breath (12)C(18)O(16)O and (13)C(16)O(16)O can be used as potential molecular biomarkers to distinctively track the pathogenesis of Helicobacter pylori and also for eradication purposes and thus may open new perspectives into the pathogen's physiology along with isotope-specific non-invasive diagnosis of the infection.

A facile approach to fabricate halloysite/metal nanocomposites with preformed and in situ synthesized metal nanoparticles: a comparative study of their enhanced catalytic activity.

A widespread study has been carried out by the researcher during the last few decades to improve the properties and performance of halloysite-based inorganic-organic hybrid nanocomposites (NCs) because of their excellent structural features and characteristic properties. Here, we report the fabrication of halloysite/metal nanocomposites on a large-scale through the immobilization of metal precursors followed by reduction or direct loading of preformed metal nanoparticles (NPs) over the surface of aminosilane modified halloysite nanotubes (HNTs), which in turn develops environmentally benign and low-cost heterogeneous catalysts. Characterization by different physical methods authenticates the successful fabrication of four different HNTs/metal NCs by these two different synthetic approaches, having monodispersed spherical morphology of the metal NPs. The catalytic activity and recyclability of all the NCs have been evaluated considering the reduction of 4-nitrophenol using sodium borohydride as a model reaction, attributed to their almost comparable catalytic efficiency. However, a detailed kinetic study demonstrates the enhanced catalytic activity of in situ synthesized HNTs/Ag among the four NCs, owing to the absence of any capping materials over the surface of NPs. Activation energy, pre-exponential factor, and entropy of activation have been estimated for this reduction reaction. A comparison study of their catalytic activity has been carried out with the reported heterogeneous catalysts, indicating the higher activity of these NCs for the reduction of nitroaromatics. Nevertheless, such an outstanding catalytic efficiency was only observed for HNTs/Au and HNTs/Ag NCs, with no activity of HNTs or aminosilane modified HNTs.

Halloysite nanotubes capturing isotope selective atmospheric CO2.

With the aim to capture and subsequent selective trapping of CO2, a nanocomposite has been developed through selective modification of the outer surface of the halloysite nanotubes (HNTs) with an organosilane to make the nanocomposite a novel solid-phase adsorbent to adsorb CO2 from the atmosphere at standard ambient temperature and pressure. The preferential adsorption of three major abundant isotopes of CO2 ((12)C(16)O2, (13)C(16)O2, and (12)C(16)O(18)O) from the ambient air by amine functionalized HNTs has been explored using an optical cavity-enhanced integrated cavity output spectroscopy. CO2 adsorption/desorption cycling measurements demonstrate that the adsorbent can be regenerated at relatively low temperature and thus, recycled repeatedly to capture atmospheric CO2. The amine grafted halloysite shows excellent stability even in oxidative environments and has high efficacy of CO2 capture, introducing a new route to the adsorption of isotope selective atmospheric CO2.

Synthesis and modeling of hollow intermetallic Ni-Zn nanoparticles formed by the Kirkendall effect.

Intermetallic Ni-Zn nanoparticles (NPs) were synthesized via the chemical conversion of nickel NPs using a zerovalent organometallic zinc precursor. After the injection of a diethylzinc solution, Ni NPs progressively transformed from a solid to a hollow Ni-Zn intermetallic structure with time. During the transformation of Ni NPs to intermetallic structures, they retained their overall spherical morphology. The growth mechanism for the solid-to-hollow nanoparticle transformation is ascribed to the nanoscale Kirkendall effect due to unequal diffusion rates of Ni and Zn. We develop a diffusion model for nonreactive, homogeneous, diffusion-controlled intermetallic hollow NP formation including moving boundaries at the interfaces of void-solid and solid-bulk solutions. Apparent diffusion coefficients for both metals and vacancy were evaluated from modeling the time-dependent growth of the void. The apparent diffusion coefficients obtained in this system compared favorably with results from measurement at grain boundaries in bulk Ni-Zn. This study represents the first combined experimental modeling of the formation of hollow nanostructures by the nanoscale Kirkendall effect.

miRNA-548c: a specific signature in circulating PBMCs from dilated cardiomyopathy patients.

High fidelity genome-wide expression analysis has strengthened the idea that microRNA (miRNA) signatures in peripheral blood mononuclear cells (PBMCs) can be potentially used to predict the pathology when anatomical samples are inaccessible like the heart. PBMCs from 48 non-failing controls and 44 patients with relatively stable chronic heart failure (ejection fraction of ≤ 40%) associated with dilated cardiomyopathy (DCM) were used for miRNA analysis. Genome-wide miRNA-microarray on PBMCs from chronic heart failure patients identified miRNA signature uniquely characterized by the downregulation of miRNA-548 family members. We have also independently validated downregulation of miRNA-548 family members (miRNA-548c & 548i) using real time-PCR in a large cohort of independent patient samples. Independent in silico Ingenuity Pathway Analysis (IPA) of miRNA-548 targets shows unique enrichment of signaling molecules and pathways associated with cardiovascular disease and hypertrophy. Consistent with specificity of miRNA changes with pathology, PBMCs from breast cancer patients showed no alterations in miRNA-548c expression compared to healthy controls. These studies suggest that miRNA-548 family signature in PBMCs can therefore be used to detect early heart failure. Our studies show that cognate networking of predicted miRNA-548 targets in heart failure can be used as a powerful ancillary tool to predict the ongoing pathology.

Seeded growth induced amorphous to crystalline transformation of niobium oxide nanostructures.

A novel high-temperature synthesis of niobium oxide nanostructures has been developed through an amorphous intermediate that crystallizes into anisotropic crystalline structures through a complex mechanism of nucleation-dissolution-selective growth induced by seeded growth. The amorphous materials formed by the thermolysis of niobium oleate transformed into crystalline platelets, wires or bundled wires after the injection of additional niobium oleate at different temperatures. The temperature of the solution during injection of the additional precursor determined the morphology of the formed crystalline structure. The time- and temperature-dependent evolution of the niobium oxide nanostructures demonstrates that amorphous materials progressively turned to crystalline materials. We tuned the size of wires and platelets by the consecutive injection of the precursor solution into the reaction mixture under isothermal conditions. With the sequential injection of the precursor solution, we demonstrate that the anisotropic growth of individual nanowires occurs exclusively along the ends, without the formation of any new nuclei. A mechanism for the transformation of well-defined platelets from wires has been proposed and is due to the exclusive loss of mass at the ends of the wires while growth of the platelets initially occurs along the (001) direction which is then replaced by the (110) face.

Layer-by-layer deposition of silver/gold nanoparticles for catalytic reduction of nitroaromatics.

A general method has been fabricated to achieve normal as well as inverted core-shell architectures of silver/gold through a layer-by-layer deposition technique on a commercial anion exchange resin. Electrostatic field force of the charged resin beads supports immobilization of anionic metal precursors [MX(n)]-, in turn deposition of silver/gold nanoparticles onto the solid resin matrix and reduction of 2-nitrobenzoic acid to obtain the corresponding amines through effective catalysis. The shell thickness has been tailored made by exploiting a new method of cyclic and repetitive deposition of the desired metal precursors. Thermodynamic parameters for the reduction reaction have been presented. Kinetic study reveals a comparative account of rates between the mono- and bi-metallic nanoparticles where silver stands to be a better catalyst for the reduction of nitroaromatics.