Publications by authors named "Zachary D Hood"

32 Publications

High-Entropy 2D Carbide MXenes: TiVNbMoC and TiVCrMoC.

ACS Nano 2021 Jun 15. Epub 2021 Jun 15.

Department of Mechanical and Energy Engineering, Purdue School of Engineering and Technology, Indiana University-Purdue University Indianapolis, Indianapolis, Indiana 46202, United States.

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are a fast-growing family of 2D materials. MXenes 2D flakes have + 1 ( = 1-4) atomic layers of transition metals interleaved by carbon/nitrogen layers, but to-date remain limited in composition to one or two transition metals. In this study, by implementing four transition metals, we report the synthesis of multi-principal-element high-entropy MCT MXenes. Specifically, we introduce two high-entropy MXenes, TiVNbMoCT and TiVCrMoCT, as well as their precursor TiVNbMoAlC and TiVCrMoAlC high-entropy MAX phases. We used a combination of real and reciprocal space characterization (X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, and scanning transmission electron microscopy) to establish the structure, phase purity, and equimolar distribution of the four transition metals in high-entropy MAX and MXene phases. We use first-principles calculations to compute the formation energies and explore synthesizability of these high-entropy MAX phases. We also show that when three transition metals are used instead of four, under similar synthesis conditions to those of the four-transition-metal MAX phase, two different MAX phases can be formed (.., no pure single-phase forms). This finding indicates the importance of configurational entropy in stabilizing the desired single-phase high-entropy MAX over multiphases of MAX, which is essential for the synthesis of phase-pure high-entropy MXenes. The synthesis of high-entropy MXenes significantly expands the compositional variety of the MXene family to further tune their properties, including electronic, magnetic, electrochemical, catalytic, high temperature stability, and mechanical behavior.
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http://dx.doi.org/10.1021/acsnano.1c02775DOI Listing
June 2021

Atomistic insights into the nucleation and growth of platinum on palladium nanocrystals.

Nat Commun 2021 Jun 2;12(1):3215. Epub 2021 Jun 2.

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA.

Despite the large number of reports on colloidal nanocrystals, very little is known about the mechanistic details in terms of nucleation and growth at the atomistic level. Taking bimetallic core-shell nanocrystals as an example, here we integrate in situ liquid-cell transmission electron microscopy with first-principles calculations to shed light on the atomistic details involved in the nucleation and growth of Pt on Pd cubic seeds. We elucidate the roles played by key synthesis parameters, including capping agent and precursor concentration, in controlling the nucleation site, diffusion path, and growth pattern of the Pt atoms. When the faces of a cubic seed are capped by Br, Pt atoms preferentially nucleate from corners and then diffuse to edges and faces for the creation of a uniform shell. The diffusion does not occur until the Pt deposited at the corner has reached a threshold thickness. At a high concentration of the precursor, self-nucleation takes place and the Pt clusters then randomly attach to the surface of a seed for the formation of a non-uniform shell. These atomistic insights offer a general guideline for the rational synthesis of nanocrystals with diverse compositions, structures, shapes, and related properties.
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http://dx.doi.org/10.1038/s41467-021-23290-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8173021PMC
June 2021

Local electronic structure variation resulting in Li 'filament' formation within solid electrolytes.

Nat Mater 2021 May 31. Epub 2021 May 31.

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA.

Solid electrolytes hold great promise for enabling the use of Li metal anodes. The main problem is that during cycling, Li can infiltrate along grain boundaries and cause short circuits, resulting in potentially catastrophic battery failure. At present, this phenomenon is not well understood. Here, through electron microscopy measurements on a representative system, LiLaZrO, we discover that Li infiltration in solid oxide electrolytes is strongly associated with local electronic band structure. About half of the LiLaZrO grain boundaries were found to have a reduced bandgap, around 1-3 eV, making them potential channels for leakage current. Instead of combining with electrons at the cathode, Li ions are hence prematurely reduced by electrons at grain boundaries, forming local Li filaments. The eventual interconnection of these filaments results in a short circuit. Our discovery reveals that the grain-boundary electronic conductivity must be a primary concern for optimization in future solid-state battery design.
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http://dx.doi.org/10.1038/s41563-021-01019-xDOI Listing
May 2021

Kinetically Controlled Synthesis of Rhodium Nanocrystals with Different Shapes and a Comparison Study of Their Thermal and Catalytic Properties.

J Am Chem Soc 2021 04 14;143(16):6293-6302. Epub 2021 Apr 14.

School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.

We report the synthesis of Rh nanocrystals with different shapes by controlling the kinetics involved in the growth of preformed Rh cubic seeds. Specifically, Rh nanocrystals with cubic, cuboctahedral, and octahedral shapes can all be obtained from the same cubic seeds under suitable reduction kinetics for the precursor. The success of such a synthesis also relies on the use of a halide-free precursor to avoid oxidative etching, as well as the involvement of a sufficiently high temperature to remove Br ions from the seeds while ensuring adequate surface diffusion. The availability of Rh nanocrystals with cubic and octahedral shapes allows for an evaluation of the facet dependences of their thermal and catalytic properties. The data from electron microscopy studies indicate that the cubic and octahedral Rh nanocrystals can keep their original shapes up to 700 and 500 °C, respectively. When tested as catalysts for hydrazine decomposition, the octahedral nanocrystals exhibit almost 4-fold enhancement in terms of H selectivity relative to the cubic counterpart. As for ethanol oxidation, the order is reversed, with the cubic nanocrystals being about three times more active than the octahedral sample.
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http://dx.doi.org/10.1021/jacs.1c02734DOI Listing
April 2021

Elucidating Interfacial Stability between Lithium Metal Anode and Li Phosphorus Oxynitride via Electron Microscopy.

Nano Lett 2021 Jan 18;21(1):151-157. Epub 2020 Dec 18.

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.

Li phosphorus oxynitride (LiPON) is one of a very few solid electrolytes that have demonstrated high stability against Li metal and extended cyclability with high Coulombic efficiency for all solid-state batteries (ASSBs). However, theoretical calculations show that LiPON reacts with Li metal. Here, we utilize electron microscopy to observe the dynamic evolutions at the LiPON-Li interface upon contacting and under biasing. We reveal that a thin interface layer (∼60 nm) develops at the LiPON-Li interface upon contact. This layer is composed of conductive binary compounds that show a unique spatial distribution that warrants an electrochemical stability of the interface, serving as an effective passivation layer. Our results explicate the excellent cyclability of LiPON and reconcile the existing debates regarding the stability of the LiPON-Li interface, demonstrating that, though glassy solid electrolytes may not have a perfect initial electrochemical window with Li metal, they may excel in future applications for ASSBs.
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http://dx.doi.org/10.1021/acs.nanolett.0c03438DOI Listing
January 2021

Toward Controlling Filament Size and Location for Resistive Switches via Nanoparticle Exsolution at Oxide Interfaces.

Small 2020 Oct 17;16(41):e2003224. Epub 2020 Sep 17.

Electrochemical Materials, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Av., Cambridge, MA, 02139, USA.

Memristive devices are among the most prominent candidates for future computer memory storage and neuromorphic computing. Though promising, the major hurdle for their industrial fabrication is their device-to-device and cycle-to-cycle variability. These occur due to the random nature of nanoionic conductive filaments, whose rupture and formation govern device operation. Changes in filament location, shape, and chemical composition cause cycle-to-cycle variability. This challenge is tackled by spatially confining conductive filaments with Ni nanoparticles. Ni nanoparticles are integrated on the bottom La Sr Ti Ni O electrode by an exsolution method, in which, at high temperatures under reducing conditions, Ni cations migrate to the perovskite surface, generating metallic nanoparticles. This fabrication method offers fine control over particle size and density and ensures strong particle anchorage in the bottom electrode, preventing movement and agglomeration. In devices based on amorphous SrTiO , it is demonstrated that as the exsolved Ni nanoparticle diameter increases up to ≈50 nm, the ratio between the ON and OFF resistance states increases from single units to 180 and the variability of the low resistance state reaches values below 5%. Exsolution is applied for the first time to engineer solid-solid interfaces extending its realm of application to electronic devices.
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http://dx.doi.org/10.1002/smll.202003224DOI Listing
October 2020

Scalable neutral HO electrosynthesis by platinum diphosphide nanocrystals by regulating oxygen reduction reaction pathways.

Nat Commun 2020 Aug 6;11(1):3928. Epub 2020 Aug 6.

Department of Chemistry, Wake Forest University, Winston-Salem, NC, 27106, USA.

Despite progress in small scale electrocatalytic production of hydrogen peroxide (HO) using a rotating ring-disk electrode, further work is needed to develop a non-toxic, selective, and stable O-to-HO electrocatalyst for realizing continuous on-site production of neutral hydrogen peroxide. We report ultrasmall and monodisperse colloidal PtP nanocrystals that achieve HO production at near zero-overpotential with near unity HO selectivity at 0.27 V vs. RHE. Density functional theory calculations indicate that P promotes hydrogenation of OOH* to HO by weakening the Pt-OOH* bond and suppressing the dissociative OOH* to O* pathway. Atomic layer deposition of AlO prevents NC aggregation and enables application in a polymer electrolyte membrane fuel cell (PEMFC) with a maximum r(HO) of 2.26 mmol h cm and a current efficiency of 78.8% even at a high current density of 150 mA cm. Catalyst stability enables an accumulated neutral HO concentration in 600 mL of 3.0 wt% (pH = 6.6).
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http://dx.doi.org/10.1038/s41467-020-17584-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7411044PMC
August 2020

Oxygen Exchange in Dual-Phase LaSrMnO-CeO Composites for Solar Thermochemical Fuel Production.

ACS Appl Mater Interfaces 2020 Jul 7;12(29):32622-32632. Epub 2020 Jul 7.

Electrochemical Materials Laboratory, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.

Increasing the capacity and kinetics of oxygen exchange in solid oxides is important to improve the performance of numerous energy-related materials, especially those for the solar-to-fuel technology. Dual-phase metal oxide composites of LaSrMnO-%CeO, with = 0, 5, 10, 20, 50, and 100, have been experimentally investigated for oxygen exchange and CO splitting thermochemical redox reactions. The prepared metal oxide powders were tested in a temperature range from 1000 to 1400 °C under isothermal and two-step cycling conditions relevant for solar thermochemical fuel production. We reveal synergetic oxygen exchange of the dual-phase composite LaSrMnO-CeO compared to its individual components. The enhanced oxygen exchange in the composite has a beneficial effect on the rate of oxygen release and the total CO produced by CO splitting, while it has an adverse effect on the maximum rate of CO evolution. Raman and XRD analyses are used to shed light on the relative oxygen content during thermochemical cycling. Based on the relative oxygen content in both phases, we discuss possible mechanisms that can explain the observed behavior. Overall, the presented findings highlight the beneficial effects of dual-phase composites in enhancing the oxygen exchange capacity of redox materials for renewable fuel production.
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http://dx.doi.org/10.1021/acsami.0c04276DOI Listing
July 2020

Effects of Surface Terminations of 2D BiWO on Photocatalytic Hydrogen Evolution from Water Splitting.

ACS Appl Mater Interfaces 2020 Apr 15;12(17):20067-20074. Epub 2020 Apr 15.

Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States.

Two-dimensional (2D)-structured photocatalysts with atomically thin layers not only have the potential to enhance hydrogen generation efficiency but also allow more direct investigations of the effects of surface terminations on photocatalytic activity. Taking 2D BiWO as a model, we found that the configuration of bilayer BiO sandwiched by alternating WO layers enabled the thermodynamic driving potential for photocatalytic hydrogen evolution. Without Pt deposition, the H generation efficiency can reach to 56.9 μmol/g/h by 2D BiWO as compared with no activity of BiWO nanocrystals under simulated solar light. This configuration is easily functionalized by adsorption of Cl/Br to form Bi-Cl/Bi-Br bonds, which leads to the decrease of recombination in photogenerated charge carriers and narrower band gaps. This work highlights an effective way to design photocatalysts with efficient hydrogen evolution by tuning the surface terminations.
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http://dx.doi.org/10.1021/acsami.0c01802DOI Listing
April 2020

Lithium-Battery Anode Gains Additional Functionality for Neuromorphic Computing through Metal-Insulator Phase Separation.

Adv Mater 2020 Mar 20;32(9):e1907465. Epub 2020 Jan 20.

Electrochemical Materials, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Av., 02139, Cambridge, MA, USA.

Specialized hardware for neural networks requires materials with tunable symmetry, retention, and speed at low power consumption. The study proposes lithium titanates, originally developed as Li-ion battery anode materials, as promising candidates for memristive-based neuromorphic computing hardware. By using ex- and in operando spectroscopy to monitor the lithium filling and emptying of structural positions during electrochemical measurements, the study also investigates the controlled formation of a metallic phase (Li Ti O ) percolating through an insulating medium (Li Ti O ) with no volume changes under voltage bias, thereby controlling the spatially averaged conductivity of the film device. A theoretical model to explain the observed hysteretic switching behavior based on electrochemical nonequilibrium thermodynamics is presented, in which the metal-insulator transition results from electrically driven phase separation of Li Ti O and Li Ti O . Ability of highly lithiated phase of Li Ti O for Deep Neural Network applications is reported, given the large retentions and symmetry, and opportunity for the low lithiated phase of Li Ti O toward Spiking Neural Network applications, due to the shorter retention and large resistance changes. The findings pave the way for lithium oxides to enable thin-film memristive devices with adjustable symmetry and retention.
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http://dx.doi.org/10.1002/adma.201907465DOI Listing
March 2020

Colloidal silver diphosphide (AgP) nanocrystals as low overpotential catalysts for CO reduction to tunable syngas.

Nat Commun 2019 12 16;10(1):5724. Epub 2019 Dec 16.

Department of Chemistry, Wake Forest University, Winston-Salem, NC, 27106, USA.

Production of syngas with tunable CO/H ratio from renewable resources is an ideal way to provide a carbon-neutral feedstock for liquid fuel production. Ag is a benchmark electrocatalysts for CO-to-CO conversion but high overpotential limits the efficiency. We synthesize AgP nanocrystals (NCs) with a greater than 3-fold reduction in overpotential for electrochemical CO-to-CO reduction compared to Ag and greatly enhanced stability. Density functional theory calculations reveal a significant energy barrier decrease in the formate intermediate formation step. In situ X-ray absorption spectroscopy (XAS) shows that a maximum Faradaic efficiency is achieved at an average silver valence state of +1.08 in AgP NCs. A photocathode consisting of a np-Si wafer coated with ultrathin AlO and AgP NCs achieves an onset potential of 0.2 V vs. RHE for CO production and a partial photocurrent density for CO at -0.11 V vs. RHE (j) of -3.2 mA cm.
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http://dx.doi.org/10.1038/s41467-019-13388-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6915715PMC
December 2019

Synthesis of CaO Nanocrystals and Their Spherical Aggregates with Uniform Sizes for Use as a Biodegradable Bacteriostatic Agent.

Small 2019 09 22;15(36):e1902118. Epub 2019 Jul 22.

The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, 30332, GA, USA.

As a solid precursor to O and hydrogen peroxide (H O ), calcium peroxide (CaO ) has found widespread use in applications related to disinfection and contaminant degradation. The lack of uniform nanoparticles, however, greatly limits the potential use of this material in other applications related to medicine. Here, a new route to the facile synthesis of CaO nanocrystals and their spherical aggregates with uniform, controllable sizes is reported. The synthesis involves the reaction between CaCl and H O to generate CaO primary nanocrystals of 2-15 nm in size in ethanol, followed by their aggregation into uniform, spherical particles with the aid of poly(vinyl pyrrolidone) (PVP). The average diameter of the spherical aggregates can be easily tuned in the range of 15-100 nm by varying the concentrations of CaCl and/or PVP. For the spherical aggregates with a smaller size, they release H O and O more quickly when exposed to water, resulting in superior antimicrobial activity. This study not only demonstrates a new route to the synthesis of uniform CaO nanocrystals and their spherical aggregates but also offers a promising bacteriostatic agent with biodegradability.
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http://dx.doi.org/10.1002/smll.201902118DOI Listing
September 2019

Ruthenium Nanoframes in the Face-Centered Cubic Phase: Facile Synthesis and Their Enhanced Catalytic Performance.

ACS Nano 2019 Jun 3;13(6):7241-7251. Epub 2019 Jun 3.

School of Chemistry and Biochemistry , Georgia Institute of Technology , Atlanta , Georgia 30332 , United States.

Owing to their highly open structure and a large number of low-coordination sites on the surface, noble-metal nanoframes are intriguing for catalytic applications. Here, we demonstrate the rational synthesis of Ru cuboctahedral nanoframes with enhanced catalytic performance toward hydrazine decomposition. The synthesis starts from Pd nanocubes, which quickly undergo truncation at the corners as a consequence of oxidative etching caused by Br ions. Afterward, the galvanic replacement reaction between Pd and Ru(III) ions dominates, leading to the selective deposition of Ru atoms on the corners and edges and thereby the fabrication of [email protected] core-frame cuboctahedra. Significantly, the deposited Ru atoms are crystallized in a face-centered cubic (fcc) phase instead of the hexagonal close-packed (hcp) structure typical of bulk Ru. Upon the removal of Pd remaining in the core via chemical etching, we obtain Ru cuboctahedral nanoframes. By varying the amount of the Ru(III) precursor, the ridge thickness of the nanoframes can be tuned from a few atomic layers up to 10. Both the frame structure and fcc crystal phase of the Ru cuboctahedral nanoframes can be well preserved up to 300 °C. When compared with hcp-Ru nanoparticles, the fcc-Ru nanoframes displayed substantial enhancement in terms of H selectivity toward hydrazine decomposition. This work offers the opportunity to engineer both the morphology and crystal phase of Ru nanocrystals for catalytic applications.
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http://dx.doi.org/10.1021/acsnano.9b02890DOI Listing
June 2019

Reversibly tuning the surface state of Ag via the assistance of photocatalysis in Ag/BiOCl.

Nanotechnology 2019 Jul 15;30(30):305601. Epub 2019 Apr 15.

Electron Microscopy Center of Chongqing University, College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, People's Republic of China.

Silver (Ag) nanoparticles can be spontaneously oxidized and present in different oxidized surface phases. The impact of oxidation induced photo absorption property and related photocatalytic activity are still unclear in Ag-decorated semiconductor photocatalysts. Herein, Ag-decorated BiOCl with the metallic Ag to oxidized Ag were employed to investigate the effect of surface state of Ag on relative photocatalyst properties. A redshift of localized surface plasmon resonance was observed as the Ag oxidized to Ag and a reversible manipulation was realized in UV light-driven photocatalysis. It is found that the Ag/BiOCl presents higher photocatalytic activity than Ag/BiOCl, but this difference is gradually decreasing under UV light irradiation compared with visible light irradiation. A controlled experiment suggests that the reduction of Ag under UV light reduced the difference between Ag/BiOCl and Ag/BiOCl. The possible mechanism for electron transport and the conversion between Ag and Ag via the assistance of the photoelectric effect from BiOCl has been elucidated. This photocatalytic reaction assisted reversible tuning the surface state of Ag/BiOCl will open up the possibility of rationally designing Ag-decorated semiconductors for light harvesting.
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http://dx.doi.org/10.1088/1361-6528/ab192eDOI Listing
July 2019

Ru Octahedral Nanocrystals with a Face-Centered Cubic Structure, {111} Facets, Thermal Stability up to 400 °C, and Enhanced Catalytic Activity.

J Am Chem Soc 2019 May 22;141(17):7028-7036. Epub 2019 Apr 22.

School of Chemistry and Biochemistry , Georgia Institute of Technology , Atlanta , Georgia 30332 , United States.

Ruthenium nanocrystals with both a face-centered cubic ( fcc) structure and well-controlled facets are attractive catalytic materials for various reactions. Here we report a simple method for the synthesis of Ru octahedral nanocrystals with an fcc structure and an edge length of 9 nm. The success of this synthesis relies on the use of 4.5 nm Rh cubes as seeds to facilitate the heterogeneous nucleation and overgrowth of Ru atoms. We choose Rh because it can resist oxidative etching under the harsh conditions for Ru overgrowth, it can be readily prepared as nanocubes with edge lengths less than 5 nm, and its atoms have a size close to that of Ru atoms. During the seed-mediated growth, the atomic packing of Ru overlayers follows an fcc lattice, in contrast to the conventional hexagonal close-packed ( hcp) lattice associated with bulk Ru. The final product takes an octahedral shape, with the surface enclosed by {111} facets. Our in situ measurements suggest that both the octahedral shape and the fcc crystal structure can be well preserved up to 400 °C, which is more than 100 °C higher than what was reported for Ru octahedral nanocages. When utilized as catalysts, the Ru octahedral nanocrystals exhibited 4.4-fold enhancement in terms of specific activity toward oxygen evolution relative to hcp-Ru nanoparticles. We also demonstrate that Ru{111} facets are more active than Ru{100} facets in catalyzing the oxygen evolution reaction. Altogether, this work offers an effective method for the synthesis of Ru nanocrystals with an fcc structure and well-defined {111} facets, as well as enhanced thermal stability and catalytic activity. We believe these nanocrystals will find use in various catalytic applications.
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http://dx.doi.org/10.1021/jacs.9b01640DOI Listing
May 2019

2D/2D heterojunction of TiC/g-CN nanosheets for enhanced photocatalytic hydrogen evolution.

Nanoscale 2019 Apr;11(17):8138-8149

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China.

Photocatalytic hydrogen evolution from water has received enormous attention due to its ability to address a number of global environmental and energy-related issues. Here, we synthesize 2D/2D Ti3C2/g-C3N4 composites by electrostatic self-assembly technique and demonstrate their use as photocatalysts for hydrogen evolution under visible light irradiation. The optimized Ti3C2/g-C3N4 composite exhibited a 10 times higher photocatalytic hydrogen evolution performance (72.3 μmol h-1 gcat-1) than that of pristine g-C3N4 (7.1 μmol h-1 gcat-1). Such enhanced photocatalytic performance was due to the formation of 2D/2D heterojunctions in the Ti3C2/g-C3N4 composites. The intimate contact between the monolayer Ti3C2 and g-C3N4 nanosheets promotes the separation of photogenerated charge carriers at the Ti3C2/g-C3N4 interface. Furthermore, the ultrahigh conductivity of Ti3C2 and the Schottky junction formed between g-C3N4/MXene interfaces facilitate the photoinduced electron transfer and suppress the recombination with photogenerated holes. This work demonstrates that the 2D/2D Ti3C2/g-C3N4 composites are promising photocatalysts thanks to the ultrathin MXenes as efficient co-catalysts for photocatalytic hydrogen production.
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http://dx.doi.org/10.1039/c9nr00168aDOI Listing
April 2019

Photothermal transformation of Au-Ag nanocages under pulsed laser irradiation.

Nanoscale 2019 Feb;11(6):3013-3020

School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.

Pulsed laser irradiation has emerged as an effective means to photothermally transform plasmonic nanostructures after their use in different biomedical applications. However, the ability to predict the products after photothermal transformation requires extensive ex situ studies. Here, we report a systematic study of the photothermal transformation of Au-Ag nanocages with a localized surface plasmon resonance at ca. 750 nm under pulsed laser irradiation at different fluences and a pulse duration of 5 ns. At biologically relevant laser energies, the pulsed laser transforms Au-Ag nanocages into pseudo-spherical, solid nanoparticles. The solid nanoparticles contained similar numbers of Au and Ag atoms to the parent Au-Ag nanocages. At increased laser fluences (>16 mJ cm-2) and number of pulses (>150), the average diameter of the resulting pseudo-spherical particles increased due to the involvement of Ostwald ripening and/or attachment-based growth. The changes in optical properties as a result of the transformation were validated using simulations based on the discrete dipole approximation method, where the spectral profiles and peak positions of the initial and final states matched well with the experimentally derived data. The results may have implications for the future use of Au-Ag nanocages in biomedicine, catalysis, and sensing.
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http://dx.doi.org/10.1039/c8nr10002kDOI Listing
February 2019

Direct in Situ Observation and Analysis of the Formation of Palladium Nanocrystals with High-Index Facets.

Nano Lett 2018 11 5;18(11):7004-7013. Epub 2018 Oct 5.

The Center for Nanophase Materials Sciences , Oak Ridge National Laboratory , Oak Ridge , Tennessee 37831 , United States.

Synthesizing concave-structured nanoparticles (NP) with high-index surfaces offers a viable method to significantly enhance the catalytic activity of NPs. Current approaches for fabricating concave NPs, however, are limited. Exploring novel synthesis methods requires a thorough understanding of the competing mechanisms that contribute to the evolution of surface structures during NP growth. Here, by tracking the evolution of Pd nanocubes into concave NPs at atomic scale using in situ liquid cell transmission electron microscopy, our study reveals that concave-structured Pd NPs can be formed by the cointroduction of surface capping agents and halogen ions. These two chemicals jointly create a new surface energy landscape of Pd NPs, leading to the morphological transformation. In particular, Pd atoms dissociate from the {100} surfaces with the aid of Cl ions and preferentially redeposit to the corners and edges of the nanocubes when the capping agent polyvinylpyrrolidone is introduced, resulting in the formation of concave Pd nanocubes with distinctive high-index facets. Our work not only demonstrates a potential route for synthesizing NPs with well-defined high-index facets but also reveals the detailed atomic-scale kinetics during their formation, providing insight for future predictive synthesis.
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http://dx.doi.org/10.1021/acs.nanolett.8b02953DOI Listing
November 2018

Enabling Complete Ligand Exchange on the Surface of Gold Nanocrystals through the Deposition and Then Etching of Silver.

J Am Chem Soc 2018 09 13;140(38):11898-11901. Epub 2018 Sep 13.

School of Chemistry and Biochemistry , Georgia Institute of Technology , Atlanta , Georgia 30332 , United States.

We report an indirect method for the effective replacement of ligands on the surface of Au nanocrystals with different morphologies. The method involves the deposition of an ultrathin layer of Ag to remove a strong capping agent such as cetyltrimethylammonium chloride (CTAC), followed by selective etching of the Ag layer in the presence of citrate ions as a stabilizer. Using multiple characterization techniques, we confirm that the surface of the Au nanocrystals is covered by citrate ions after the indirect ligand exchange process, and there is essentially no aggregation during the entire process. We also demonstrate that this method is effective in suppressing the toxicity of Au nanospheres by completely replacing the initially used CTAC with citrate.
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http://dx.doi.org/10.1021/jacs.8b06464DOI Listing
September 2018

Vacuum-Assisted Low-Temperature Synthesis of Reduced Graphene Oxide Thin-Film Electrodes for High-Performance Transparent and Flexible All-Solid-State Supercapacitors.

ACS Appl Mater Interfaces 2018 Apr 21;10(13):11008-11017. Epub 2018 Mar 21.

Oak Ridge National Laboratory , Oak Ridge , Tennessee 37831 , United States.

Simple and easily integrated design of flexible and transparent electrode materials affixed to polymer-based substrates hold great promise to have a revolutionary impact on the functionality and performance of energy storage devices for many future consumer electronics. Among these applications are touch sensors, roll-up displays, photovoltaic cells, health monitors, wireless sensors, and wearable communication devices. Here, we report an environmentally friendly, simple, and versatile approach to produce optically transparent and mechanically flexible all-solid-state supercapacitor devices. These supercapacitors were constructed on tin-doped indium oxide coated polyethylene terephthalate substrates by intercalation of a polymer-based gel electrolyte between two reduced graphene oxide (rGO) thin-film electrodes. The rGO electrodes were fabricated simply by drop-casting of graphene oxide (GO) films, followed by a novel low-temperature (≤250 °C) vacuum-assisted annealing approach for the in situ reduction of GO to rGO. A trade-off between the optical transparency and electrochemical performance is determined by the concentration of the GO in the initial dispersion, whereby the highest capacitance (∼650 μF cm) occurs at a relatively lower optical transmittance (24%). Notably, the all-solid-state supercapacitors demonstrated excellent mechanical flexibility with a capacity retention rate above 90% under various bending angles and cycles. These attributes underscore the potential of the present approach to provide a path toward the realization of thin-film-based supercapacitors as flexible and transparent energy storage devices for a variety of practical applications.
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http://dx.doi.org/10.1021/acsami.8b01938DOI Listing
April 2018

One-Step Synthesis of Nb O /C/Nb C (MXene) Composites and Their Use as Photocatalysts for Hydrogen Evolution.

ChemSusChem 2018 02 29;11(4):688-699. Epub 2018 Jan 29.

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831, USA.

Hydrogen production through facile photocatalytic water splitting is regarded as a promising strategy to solve global energy problems. Transition-metal carbides (MXenes) have recently drawn attention as potential co-catalyst candidates for photocatalysts. Here, we report niobium pentoxide/carbon/niobium carbide (MXene) hybrid materials (Nb O /C/Nb C) as photocatalysts for hydrogen evolution from water splitting. The Nb O /C/Nb C composites were synthesized by one-step CO oxidation of Nb CT . Nb O grew homogeneously on Nb C after mild oxidation, during which some amorphous carbon was also formed. With an optimized oxidation time of 1.0 h, Nb O /C/Nb C showed the highest hydrogen generation rate (7.81 μmol h  g ), a value that was four times higher than that of pure Nb O . The enhanced performance of Nb O /C/Nb C was attributed to intimate contact between Nb O and conductive Nb C and the separation of photogenerated charge carriers at the Nb O /Nb C interface; the results presented herein show that transition-metal carbide are promising co-catalysts for photocatalytic hydrogen production.
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http://dx.doi.org/10.1002/cssc.201702317DOI Listing
February 2018

Self-Assembled Framework Formed During Lithiation of SnS Nanoplates Revealed by in Situ Electron Microscopy.

Acc Chem Res 2017 07 6;50(7):1513-1520. Epub 2017 Jul 6.

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.

Lithium-ion batteries (LIBs) commercially dominate portable energy storage and have been extended to hybrid/electric vehicles by utilizing electrode materials with enhanced energy density. However, the energy density and cycling life of LIBs must extend beyond the current reach of commercial electrodes to meet the performance requirements for transportation applications. Carbon-based anodes, serving as the main negative electrodes in LIBs, have an intrinsic capacity limitation due to the intercalation mechanism. Some nanostructured carbon materials offer very interesting reversible capacities and can be considered as future anode materials. However, their fabrication processes are often complicated and expensive. Theoretically, using a lithium metal anode is the best way of delivering high energy density due to its largest theoretical capacity of more than 3800 mAh g; however, lithium metal is highly reactive with liquid electrolytes. Alternative anodes are being explored, including other lithium-reactive metals, such as Si, Ge, Zn, V, and so forth. These metals react reversibly with a large amount of Li per formula unit to form lithium-metal alloys, rendering these materials promising candidates for next-generation LIBs with high energy density. Though, most of these pure metallic anodes experience large volume changes during lithiation and delithiation processes that often results in cracking of the anode material and a loss electrical contact between the particles. Nanosized metal sulfides were recently found to possess better cycling stability and larger reversible capacities over pure metals. Further improvements and developments of metal sulfide-based anodes rely on a fundamental understanding of their electrochemical cycling mechanisms. Not only must the specific electrochemical reactions be correctly identified, but also the microstructural evolution upon electrochemical cycling, which often dictates the cyclability and stability of nanomaterials in batteries, must be clearly understood. Probing these dynamic evolution processes, i.e. the lithiation reactions and morphology evolutions, are often challenging. It requires both high-resolution chemical analysis and microstructural identification. In situ transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS) has recently been raised as one of the most powerful techniques for monitoring electrochemical processes in anode materials for LIBs. In this work, we focus on elucidating the origin of the structural stability of SnS during electrochemical cycling by revealing the microstructural evolution of SnS upon lithiation using in situ TEM. Crystalline SnS was observed to undergo a two-step reaction after the initial lithium intercalation: (1) irreversible formation of metallic tin and amorphous lithium sulfide and (2) reversible transformation of metallic tin to Li-Sn alloys, which is determined to be the rate-determining step. More interestingly, it was discovered that a self-assembled composite framework formed during the irreversible conversion reaction, which has not been previously reported. Crystalline Sn nanoparticles are well arranged within an amorphous LiS "matrix" in this self-assembled framework. This nanoscale framework confines the locations of individual Sn nanoparticles and prevents particle agglomeration during the subsequent cycling processes, therefore providing desired structural tolerance and warranting a sufficientelectron pathway. Our results not only explain the outstanding cycling stability of SnS over metallic tin anodes, but also provide important mechanistic insights into the design of high-performance electrodes for next-generation LIBs through the integration of a unique nanoframework.
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http://dx.doi.org/10.1021/acs.accounts.7b00086DOI Listing
July 2017

Understanding the Thermal Stability of Palladium-Platinum Core-Shell Nanocrystals by In Situ Transmission Electron Microscopy and Density Functional Theory.

ACS Nano 2017 05 12;11(5):4571-4581. Epub 2017 May 12.

Department of Chemical and Biological Engineering, University of Wisconsin-Madison , Madison, Wisconsin 53706, United States.

Core-shell nanocrystals offer many advantages for heterogeneous catalysis, including precise control over both the surface structure and composition, as well as reduction in loading for rare and costly metals. Although many catalytic processes are operated at elevated temperatures, the adverse impacts of heating on the shape and structure of core-shell nanocrystals are yet to be understood. In this work, we used ex situ heating experiments to demonstrate that [email protected] core-shell nanoscale cubes and octahedra are promising for catalytic applications at temperatures up to 400 °C. We also used in situ transmission electron microscopy to monitor the thermal stability of the core-shell nanocrystals in real time. Our results demonstrate a facet dependence for the thermal stability in terms of shape and composition. Specifically, the cubes enclosed by {100} facets readily deform shape at a temperature 300 °C lower than that of the octahedral counterparts enclosed by {111} facets. A reversed trend is observed for composition, as alloying between the Pd core and the Pt shell of an octahedron occurs at a temperature 200 °C lower than that for the cubic counterpart. Density functional theory calculations provide atomic-level explanations for the experimentally observed behaviors, demonstrating that the barriers for edge reconstruction determine the relative ease of shape deformation for cubes compared to octahedra. The opposite trend for alloying of the core-shell structure can be attributed to a higher propensity for subsurface Pt vacancy formation in octahedra than in cubes.
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http://dx.doi.org/10.1021/acsnano.6b08692DOI Listing
May 2017

Hydroxyl-Dependent Evolution of Oxygen Vacancies Enables the Regeneration of BiOCl Photocatalyst.

ACS Appl Mater Interfaces 2017 May 8;9(19):16620-16626. Epub 2017 May 8.

Division of Physical Science and Engineering, King Abdullah University of Science and Technology (KAUST) , Thuwal 23955-6900, Saudi Arabia.

Photoinduced oxygen vacancies (OVs) are widely investigated as a vital point defect in wide-band-gap semiconductors. Still, the formation mechanism of OVs remains unclear in various materials. To elucidate the formation mechanism of photoinduced OVs in bismuth oxychloride (BiOCl), we synthesized two surface hydroxyl discrete samples in light of the discovery of the significant variance of hydroxyl groups before and after UV light exposure. It is noted that OVs can be obtained easily after UV light irradiation in the sample with surface hydroxyl groups, while variable changes were observed in samples without surface hydroxyls. Density functional theory (DFT) calculations reveal that the binding energy of Bi-O is drastically influenced by surficial hydroxyl groups, which is intensely correlated to the formation of photoinduced OVs. Moreover, DFT calculations reveal that the adsorbed water molecules are energetically favored to dissociate into separate hydroxyl groups at the OV sites via proton transfer to a neighboring bridging oxygen atom, forming two bridging hydroxyl groups per initial oxygen vacancy. This result is consistent with the experimental observation that the disappearance of photoinduced OVs and the recovery of hydroxyl groups on the surface of BiOCl after exposed to a HO(g)-rich atmosphere, and finally enables the regeneration of BiOCl photocatalyst. Here, we introduce new insights that the evolution of photoinduced OVs is dependent on surface hydroxyl groups, which will lead to the regeneration of active sites in semiconductors. This work is useful for controllable designs of defective semiconductors for applications in photocatalysis and photovoltaics.
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http://dx.doi.org/10.1021/acsami.7b01701DOI Listing
May 2017

Interfaces in Heterogeneous Catalysts: Advancing Mechanistic Understanding through Atomic-Scale Measurements.

Acc Chem Res 2017 04 16;50(4):787-795. Epub 2017 Feb 16.

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States.

Developing novel catalysts with high efficiency and selectivity is critical for enabling future clean energy conversion technologies. Interfaces in catalyst systems have long been considered the most critical factor in controlling catalytic reaction mechanisms. Interfaces include not only the catalyst surface but also interfaces within catalyst particles and those formed by constructing heterogeneous catalysts. The atomic and electronic structures of catalytic surfaces govern the kinetics of binding and release of reactant molecules from surface atoms. Interfaces within catalysts are introduced to enhance the intrinsic activity and stability of the catalyst by tuning the surface atomic and chemical structures. Examples include interfaces between the core and shell, twin or domain boundaries, or phase boundaries within single catalyst particles. In supported catalyst nanoparticles (NPs), the interface between the metallic NP and support serves as a critical tuning factor for enhancing catalytic activity. Surface electronic structure can be indirectly tuned and catalytically active sites can be increased through the use of supporting oxides. Tuning interfaces in catalyst systems has been identified as an important strategy in the design of novel catalysts. However, the governing principle of how interfaces contribute to catalyst behavior, especially in terms of interactions with intermediates and their stability during electrochemical operation, are largely unknown. This is mainly due to the evolving nature of such interfaces. Small changes in the structural and chemical configuration of these interfaces may result in altering the catalytic performance. These interfacial arrangements evolve continuously during synthesis, processing, use, and even static operation. A technique that can probe the local atomic and electronic interfacial structures with high precision while monitoring the dynamic interfacial behavior in situ is essential for elucidating the role of interfaces and providing deeper insight for fine-tuning and optimizing catalyst properties. Scanning transmission electron microscopy (STEM) has long been a primary characterization technique used for studying nanomaterials because of its exceptional imaging resolution and simultaneous chemical analysis. Over the past decade, advances in STEM, that is, the commercialization of both aberration correctors and monochromators, have significantly improved the spatial and energy resolution. Imaging atomic structures with subangstrom resolution and identifying chemical species with single-atom sensitivity are now routine for STEM. These advancements have greatly benefitted catalytic research. For example, the roles of lattice strain and surface elemental distribution and their effect on catalytic stability and reactivity have been well documented in bimetallic catalysts. In addition, three-dimensional atomic structures revealed by STEM tomography have been integrated in theoretical modeling for predictive catalyst NP design. Recent developments in stable electronic and mechanical devices have opened opportunities to monitor the evolution of catalysts in operando under synthesis and reaction conditions; high-speed direct electron detectors have achieved sub-millisecond time resolutions and allow for rapid structural and chemical changes to be captured. Investigations of catalysts using these latest microscopy techniques have provided new insights into atomic-level catalytic mechanisms. Further integration of new microscopy methods is expected to provide multidimensional descriptions of interfaces under relevant synthesis and reaction conditions. In this Account, we discuss recent insights on understanding catalyst activity, selectivity, and stability using advanced STEM techniques, with an emphasis on how critical interfaces dictate the performance of precious metal-based heterogeneous catalysts. The role of extended interfacial structures, including those between core and shell, between separate phases and twinned grains, between the catalyst surface and gas, and between metal and support are discussed. We also provide an outlook on how emerging electron microscopy techniques, such as vibrational spectroscopy and electron ptychography, will impact future catalysis research.
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http://dx.doi.org/10.1021/acs.accounts.6b00596DOI Listing
April 2017

Facile synthesis of [email protected] core-sheath nanowires with greatly improved stability against oxidation.

Chem Commun (Camb) 2017 Feb;53(12):1965-1968

School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, USA. and The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, USA.

We report a facile synthesis of [email protected] core-sheath nanowires through the conformal deposition of Au atoms onto the surface of pre-synthesized Ag nanowires. The resulting [email protected] nanowires showed morphology and optical properties almost identical to the pristine Ag nanowires, but with greatly improved stability under different corrosive environments.
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http://dx.doi.org/10.1039/c6cc09878aDOI Listing
February 2017

Synthesis and Characterization of Pt-Ag Alloy Nanocages with Enhanced Activity and Durability toward Oxygen Reduction.

Nano Lett 2016 10 26;16(10):6644-6649. Epub 2016 Sep 26.

The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University , Atlanta, Georgia 30332, United States.

Engineering the elemental composition of metal nanocrystals offers an effective strategy for the development of catalysts or electrocatalysts with greatly enhanced activity. Herein, we report the synthesis of Pt-Ag alloy nanocages with an outer edge length of 18 nm and a wall thickness of about 3 nm. Such nanocages with a composition of PtAg could be readily prepared in one step through the galvanic replacement reaction between Ag nanocubes and a Pt(II) precursor. After 10 000 cycles of potential cycling in the range of 0.60-1.0 V as in an accelerated durability test, the composition of the nanocages changed to PtAg, together with a specific activity of 1.23 mA cm toward oxygen reduction, which was 3.3 times that of a state-of-the-art commercial Pt/C catalyst (0.37 mA cm) prior to durability testing. Density functional theory calculations attributed the increased activity to the stabilization of the transition state for breaking the O-O bond in molecular oxygen. Even after 30 000 cycles of potential cycling, the mass activity of the nanocages only dropped from 0.64 to 0.33 A mg, which was still about two times that of the pristine Pt/C catalyst (0.19 A mg).
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http://dx.doi.org/10.1021/acs.nanolett.6b03395DOI Listing
October 2016

Quantitative Analysis of the Reduction Kinetics Responsible for the One-Pot Synthesis of Pd-Pt Bimetallic Nanocrystals with Different Structures.

J Am Chem Soc 2016 09 12;138(37):12263-70. Epub 2016 Sep 12.

The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University , Atlanta, Georgia 30332, United States.

We report a quantitative understanding of the reduction kinetics responsible for the formation of Pd-Pt bimetallic nanocrystals with two distinctive structures. The syntheses involve the use of KBr to manipulate the reaction kinetics by influencing the redox potentials of metal precursor ions via ligand exchange. In the absence of KBr, the ratio between the initial reduction rates of PdCl4(2-) and PtCl4(2-) was about 10.0, leading to the formation of [email protected] octahedra with a core-shell structure. In the presence of 63 mM KBr, the products became Pd-Pt alloy nanocrystals. In this case, the ratio between the initial reduction rates of the two precursors dropped to 2.4 because of ligand exchange and, thus, the formation of PdBr4(2-) and PtBr4(2-). The alloy nanocrystals took a cubic shape owing to the selective capping effect of Br(-) ions toward the {100} facets. Relative to the alloy nanocubes, the [email protected] core-shell octahedra showed substantial enhancement in both catalytic activity and durability toward the oxygen reduction reaction (ORR). Specifically, the specific (1.51 mA cm(-2)) and mass (1.05 A mg(-1) Pt) activities of the core-shell octahedra were enhanced by about four- and three-fold relative to the alloy nanocubes (0.39 mA cm(-2) and 0.34 A mg(-1) Pt, respectively). Even after 20000 cycles of accelerated durability test, the core-shell octahedra still exhibited a mass activity of 0.68 A mg(-1) Pt, twice that of a pristine commercial Pt/C catalyst.
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http://dx.doi.org/10.1021/jacs.6b07213DOI Listing
September 2016

A Visible-Light-Active Heterojunction with Enhanced Photocatalytic Hydrogen Generation.

ChemSusChem 2016 07 10;9(14):1869-79. Epub 2016 Jun 10.

Department of Chemistry, Wake Forest University, Winston Salem, NC, 27109, USA.

A visible-light-active carbon nitride (CN)/strontium pyroniobate (SNO) heterojunction photocatalyst was fabricated by deposition of CN over hydrothermally synthesized SNO nanoplates by a simple thermal decomposition process. The microscopic study revealed that nanosheets of CN were anchored to the surface of SNO resulting in an intimate contact between the two semiconductors. Diffuse reflectance UV/Vis spectra show that the resulting CN/SNO heterojunction possesses intense absorption in the visible region. The structural and spectral properties endowed the CN/SNO heterojunction with remarkably enhanced photocatalytic activity. Specifically, the photocatalytic hydrogen evolution rate per mole of CN was found to be 11 times higher for the CN/SNO composite compared to pristine CN. The results clearly show that the composite photocatalyst not only extends the light absorption range of SNO but also restricts photogenerated charge-carrier recombination, resulting in significant enhancement in photocatalytic activity compared to pristine CN. The relative band positions of the composite allow the photogenerated electrons in the conduction band of CN to migrate to that of SNO. This kind of charge migration and separation leads to the reduction in the overall recombination rate of photogenerated charge carriers, which is regarded as one of the key factors for the enhanced activity. A plausible mechanism for the enhanced photocatalytic activity of the heterostructured composite is proposed based on observed activity, photoluminescence, time-resolved fluorescence emission decay, electrochemical impedance spectroscopy, and band position calculations.
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http://dx.doi.org/10.1002/cssc.201600424DOI Listing
July 2016

An Air-Stable Na3 SbS4 Superionic Conductor Prepared by a Rapid and Economic Synthetic Procedure.

Angew Chem Int Ed Engl 2016 07 1;55(30):8551-5. Epub 2016 Jun 1.

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA.

All-solid-state sodium batteries, using solid electrolyte and abundant sodium resources, show great promise for safe, low-cost, and large-scale energy storage applications. The exploration of novel solid electrolytes is critical for the room temperature operation of all-solid-state Na batteries. An ideal solid electrolyte must have high ionic conductivity, hold outstanding chemical and electrochemical stability, and employ low-cost synthetic methods. Achieving the combination of these properties is a grand challenge for the synthesis of sulfide-based solid electrolytes. Design of the solid electrolyte Na3 SbS4 is described, realizing excellent air stability and an economic synthesis based on hard and soft acid and base (HSAB) theory. This new solid electrolyte also exhibits a remarkably high ionic conductivity of 1 mS cm(-1) at 25 °C and ideal compatibility with a metallic sodium anode.
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http://dx.doi.org/10.1002/anie.201601546DOI Listing
July 2016
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