Design Strategies to Enable the Efficient Use of Sodium Metal Anodes in High-Energy Batteries.

Sodium-based batteries have attracted considerable attention and are recognized as ideal candidates for large-scale and low-cost energy storage. Sodium (Na) metal anodes are considered as one of the most promising anodes for next-generation, high-energy, Na-based batteries owing to their high theoretical specific capacity (1166 mA h g-1 ) and low standard electrode potential. Herein, an overview of the recent developments in Na metal anodes for high-energy batteries is provided. The high reactivity and large volume expansion of Na metal anodes during charge and discharge make the electrode/electrolyte interphase unstable, leading to the formation of Na dendrites, short cycle life, and safety issues. Design strategies to enable the efficient use of Na metal anodes are elucidated, including liquid electrolyte engineering, electrode/electrolyte interface optimization, sophisticated electrode construction, and solid electrolyte engineering. Finally, the remaining challenges and future research directions are identified. It is hoped that this progress report will shape a consistent view of this field and provide inspiration for future research to improve Na metal anodes and enable the development of high-energy sodium batteries.

Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes.

In conventional intercalation cathodes, alkali metal ions can move in and out of a layered material with the charge being compensated for by reversible reduction and oxidation of the transition metal ions. If the cathode material used in a lithium-ion or sodium-ion battery is alkali-rich, this can increase the battery's energy density by storing charge on the oxide and the transition metal ions, rather than on the transition metal alone1-10. There is a high voltage associated with oxidation of O2- during the first charge, but this is not recovered on discharge, resulting in reduced energy density11. Displacement of transition metal ions into the alkali metal layers has been proposed to explain the first-cycle voltage loss (hysteresis)9,12-16. By comparing two closely related intercalation cathodes, Na0.75[Li0.25Mn0.75]O2 and Na0.6[Li0.2Mn0.8]O2, here we show that the first-cycle voltage hysteresis is determined by the superstructure in the cathode, specifically the local ordering of lithium and transition metal ions in the transition metal layers. The honeycomb superstructure of Na0.75[Li0.25Mn0.75]O2, present in almost all oxygen-redox compounds, is lost on charging, driven in part by formation of molecular O2 inside the solid. The O2 molecules are cleaved on discharge, reforming O2-, but the manganese ions have migrated within the plane, changing the coordination around O2- and lowering the voltage on discharge. The ribbon superstructure in Na0.6[Li0.2Mn0.8]O2 inhibits manganese disorder and hence O2 formation, suppressing hysteresis and promoting stable electron holes on O2- that are revealed by X-ray absorption spectroscopy. The results show that voltage hysteresis can be avoided in oxygen-redox cathodes by forming materials with a ribbon superstructure in the transition metal layers that suppresses migration of the transition metal.

Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2.

The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2p orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li+-O(2p)-Li+ interactions). Na2/3[Mg0.28Mn0.72]O2 exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg2+ resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na2/3[Mg0.28Mn0.72]O2 does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg2+ remains in Na2/3[Mg0.28Mn0.72]O2, which stabilizes oxygen.

Comparative Study of Potential Applications of Graphene, MoS2, and Other Two-Dimensional Materials in Energy Devices, Sensors, and Related Areas.

Novel properties of graphene have been well documented, whereas the importance of nanosheets of MoS2 and other chalcogenides is increasingly being recognized over the last two to three years. Borocarbonitrides, BxCyNz, with insulating BN and conducting graphene on either side are new materials whose properties have been attracting attention. These two-dimensional (2D) materials contain certain common features. Thus, graphene, MoS2, and borocarbonitrides have all been used in supercapacitor applications, oxygen reduction reactions (ORRs), and lithium-ion batteries. It is instructive, therefore, to make a comparative study of some of the important properties of these layered materials. In this article, we discuss properties related to energy devices at length. We examine the hydrogen evolution reaction facilitated by graphene, MoS2, and related materials. We also discuss gas and radiation sensors based on graphene and MoS2 as well as gas storage properties of graphene and borocarbonitrides. The article should be useful in making a judicious choice of which 2D material to use for a particular application.

Visible-light-induced generation of H2 by nanocomposites of few-layer TiS2 and TaS2 with CdS nanoparticles.

Graphene analogues of TaS2 and TiS2 (3-4 layers), prepared by Li intercalation followed by exfoliation in water, were characterized. Nanocomposites of CdS with few-layer TiS2 and TaS2 were employed for the visible-light-induced H2 evolution reaction (HER). Benzyl alcohol was used as the sacrificial electron donor, which was oxidized to benzaldehyde during the reaction. Few-layer TiS2 is a semiconductor with a band gap of 0.7 eV, and its nanocomposite with CdS showed an activity of 1000 µmol h(-1) g(-1). The nanocomposite of few-layer TaS2, in contrast, gave rise to higher activity of 2320 µmol h(-1) g(-1), which was attributed to the metallic nature of few-layer TaS2. The amount of hydrogen evolved after 20 and 16 h for the CdS/TiS2 and CdS/TaS2 nanocomposites was 14,833 and 28,132 µmol, respectively, with turnover frequencies of 0.24 and 0.57 h(-1), respectively.

Highly effective visible-light-induced H(2) generation by single-layer 1T-MoS(2) and a nanocomposite of few-layer 2H-MoS(2) with heavily nitrogenated graphene.

Two sorts of MoS2 : A single-layer, metallic form of MoS2 (1T-MoS2 ) and a nanocomposite of a second form of MoS2 (few-layer 2H-MoS2 ) with heavily nitrogenated reduced graphene oxide (NRGO; N content ca. 15 %) show outstanding performance in the production of H2 under visible-light illumination.

Graphene analogues of inorganic layered materials.

The discovery of graphene has created a great sensation in chemistry, physics, materials science, and related areas. The unusual properties of graphene have aroused interest in other layered materials, such as molybdenum sulfide and boron nitride. In the last few years, single- as well as few-layer as well as chalcogenides and other inorganic materials have been prepared and characterized by a variety of methods. These materials possess interesting properties, and some have potential applications. This Review provides an up-to-date account of these emerging two-dimensional nanomaterials. Not only are the synthesis and characterization covered, but also important aspects such as spectroscopic and optical properties, magnetic and electrical properties, as well as applications. Salient features of the composites formed from the layered inorganic structures with graphene and polymers are presented along with a brief description of borocarbonitrides.

Synthesis, characterization, photocatalysis, and varied properties of TiO2 cosubstituted with nitrogen and fluorine.

TiO2 (anatase) codoped with nitrogen and fluorine, synthesized by a simple solid state route, using urea and ammonium fluoride as sources of nitrogen and fluorine, respectively, as well as by decomposition of (NH4)2TiF6 for comparison, has been characterized by various techniques. XPS analysis shows the composition to be TiO1.7N0.18F0.12 for urea-based method (N, F-TiO2-urea) and TiO1.9N0.04F0.06 for complex decomposition method (N, F-TiO2-complex). Both the materials are defect-free as revealed by photoluminescence spectroscopy. Thus, N, F-TiO2-urea exhibits smaller defect-induced magnetization compared to the nitrogen-doped sample. Cosubstitution of N and F is accompanied with an enhancement of the absorption of light in the visible region giving rise to yellow color and with a band gap of ∼2.2 eV in the case of N, F-TiO2-urea. It exhibits enhanced photocatalytic activity and also significant hydrogen evolution (400 µmol/g) on interaction with visible light in the absence of any cocatalyst, which is much higher compared to N, F-TiO2-complex and N-TiO2. First-principles calculations show significant local distortions on codoping TiO2 with N and F and a lowering of energy by 1.93 eV per N, F pair. With virtual negative and positive charges on nitrogen and fluorine, respectively, the dopants prefer pairwise clustering. Our calculations predict a reduction in the band gap in TiO2 cosubstituted with nitrogen and fluorine. The calculated band structure shows that nitrogen 2p states form a separate subband just above the valence band which is enhanced on incorporation of fluorine. Our calculations also indicate anomalous Born effective charges in N, F-TiO2 and predict enhanced photocatalytic activity on codoping of TiO2 by N and F.

Importance of trivalency and the e(g)(1) configuration in the photocatalytic oxidation of water by Mn and Co oxides.

Prompted by the early results on the catalytic activity of LiMn2O4 and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn2O3, LaMnO3, and MgMn2O4 are found to be very good catalysts with turnover frequencies of 5 × 10(-4) s(-1), 4.8 × 10(-4) s(-1), and 0.8 × 10(-4) s(-1), respectively. Among the cobalt oxides, Li2Co2O4 and LaCoO3--especially the latter--exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10(-4) s(-1) and 1.4 × 10(-3) s(-1), respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co(3+) in the Co oxides is in the intermediate t2g(5)e(g)(1) state whereas Mn(3+) is in the t2g(3e(g)(1) state. The presence of the e(g)(1) electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.

Strategies for the synthesis of graphene, graphene nanoribbons, nanoscrolls and related materials.

Single-layer graphene (SLG), the 3.4 Å thick two-dimensional sheet of sp(2) carbon atoms, was first prepared in 2004 by mechanical exfoliation of graphite crystals using the scotch tape technique. Since then, SLG has been prepared by other physical methods such as laser irradiation or ultrasonication of graphite in liquid media. Chemical methods of synthesis of SLG are more commonly used; the most popular involves preparation of single-layer graphene oxide followed by reduction with a stable reagent, often assisted by microwave heating. This method yields single-layer reduced graphene oxide. Other methods for preparing SLG include chemical vapour deposition over surfaces of transition metals such as Ni and Cu. Large-area SLG has also been prepared by epitaxial growth over SiC. Few-layer graphene (FLG) is prepared by several methods; arc discharge of graphite in hydrogen atmosphere being the most convenient. Several other methods for preparing FLG include exfoliation of graphite oxide by rapid heating, ultrasonication or laser irradiation of graphite in liquid media, reduction of few-layer graphene oxide, alkali metal intercalation followed by exfoliation. Graphene nanoribbons, which are rectangular strips of graphene, are best prepared by the unzipping of carbon nanotubes by chemical oxidation or laser irradiation. Many graphene analogues of inorganic materials such as MoS(2), MoSe(2) and BN have been prepared by mechanical exfoliation, ultrasonication and by chemical methods involving high-temperature or hydrothermal reactions and intercalation of alkali metals followed by exfoliation. Scrolls of graphene are prepared by potassium intercalation in graphite or by microwave irradiation of graphite immersed in liquid nitrogen.

Ferromagnetism exhibited by nanoparticles of noble metals.

Gold nanoparticles with average diameters in the range 2.5-15 nm, prepared at the organic/aqueous interface by using tetrakis(hydroxymethyl)phosphonium chloride (THPC) as reducing agent, exhibit ferromagnetism whereby the saturation magnetization M(S) increases with decreasing diameter and varies linearly with the fraction of surface atoms. The value of M(S) is higher when the particles are present as a film instead of as a sol. Capping with strongly interacting ligands such as alkane thiols results in a higher M(S) value, which varies with the strength of the metal-sulfur bond. Ferromagnetism is also found in Pt and Ag nanoparticles prepared as sols, and the M(S) values vary as Pt>Au>Ag. A careful study of the temperature variation of the magnetization of Au nanoparticles, along with certain other observations, suggests that small bare nanoparticles of noble metals could indeed possess ferromagnetism, albeit weak, which is accentuated in the presence of capping agents, specially alkane thiols which form strong metal-sulfur bonds.

Tunable fluorescence in chromophore-functionalized nanodiamond induced by energy transfer.

Hybrid materials comprising diamond nanoparticles (ND) and oligo(phenylenevinylenes) (OPVs) have been synthesized by the covalent linking of acid-functionalized ND and OPV-amine. Chromophore-functionalized ND particles with long alkyl and π-conjugated groups are readily dispersed in various organic solvents without any precipitation after several hours. A careful study of the properties of the hybrid materials revealed an aggregation-induced energy transfer from the blue fluorescent nanodiamonds to green emitting OPVs. At very low concentrations the hybrid emits in the blue region, but as the concentration is increased a gradual transition from blue to green emission occurs. Competitive processes such as aggregation-induced enhanced emission and self-absorption have been ruled out and a molecular picture of the phenomenon is proposed. This strategy can open a plethora of new avenues for fluorescent nanodiamonds in optoelectronics and light harvesting apart from bio-imaging.

Temperature effects on the Raman spectra of graphenes: dependence on the number of layers and doping.

Temperature effects on the various features in the Raman spectra of several graphene samples and graphene nanoribbons have been investigated over the temperature range 77-573 K. The temperature coefficient of the G and 2D band frequencies are found to depend on the number of layers, the former decreasing with the increase in the number of layers. The number of layers also affects the temperature coefficients of the FWHMs of these bands. Doping of graphene affects these Raman features significantly. The defect-related bands D and D(') bands are not sensitive to the number of layers or doping. We can understand the observed temperature effects on the basis of electron-phonon coupling, thermal expansion and anharmonic phonon-phonon interactions.

Chemical storage of hydrogen in few-layer graphene.

Birch reduction of few-layer graphene samples gives rise to hydrogenated samples containing up to 5 wt % of hydrogen. Spectroscopic studies reveal the presence of sp(3) C-H bonds in the hydrogenated graphenes. They, however, decompose readily on heating to 500 °C or on irradiation with UV or laser radiation releasing all the hydrogen, thereby demonstrating the possible use of few-layer graphene for chemical storage of hydrogen. First-principles calculations throw light on the mechanism of dehydrogenation that appears to involve a significant reconstruction and relaxation of the lattice.

Photoluminescence, white light emitting properties and related aspects of ZnO nanoparticles admixed with graphene and GaN.

ZnO nanoparticles exhibit a broad band centred around 530 nm in the photoluminescence (PL) spectrum due to the presence of oxygen vacancies. Composites of ZnO nanoparticles with graphenes show marked changes in the PL spectrum with broad bands covering the entire visible region, making them candidates for solid state lighting, while graphene prepared by arc discharge of graphite in a hydrogen atmosphere (HG) containing 2-3 layers as well as boron-doped (BHG) and nitrogen-doped (NHG) samples of HG give white light when admixed with ZnO nanoparticles; excellent results are obtained with the addition of just 7 wt% of BHG to the ZnO nanoparticles. Mixtures of ZnO and GaN nanoparticles also exhibit white light emission. The quantum yields of these ZnO nanoparticle based white light sources are in the 4-6% range. Photoconductivity characteristics of ZnO nanoparticles are affected by the addition of even a small amount of graphene (<0.5 wt%).

Extraordinary synergy in the mechanical properties of polymer matrix composites reinforced with 2 nanocarbons.

One of the applications of nanomaterials is as reinforcements in composites, wherein small additions of nanomaterials lead to large enhancements in mechanical properties. There have been extensive studies in the literature on composites where a polymer matrix is reinforced by a single nanomaterial such as carbon nanotubes. In this article, we examine the significant synergistic effects observed when 2 different types of nanocarbons are incorporated in a polymer matrix. Thus, binary combinations of nanodiamond, few-layer graphene, and single-walled nanotubes have been used to reinforce polyvinyl alcohol. The mechanical properties of the resulting composites, evaluated by the nanoindentation technique, show extraordinary synergy, improving the stiffness and hardness by as much as 400% compared to those obtained with single nanocarbon reinforcements. These results suggest a way of designing advanced materials with extraordinary mechanical properties by incorporating small amounts of 2 nanomaterials such as graphene plus nanodiamond or nanodiamond plus carbon nanotube.