The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

Hydrogen has been hailed as a clean and sustainable alternative to finite fossil fuels in many energy systems. Water splitting is an important method for hydrogen production in high purity and large quantities. To accelerate the hydrogen evolution reaction (HER) rate, it is highly necessary to develop high efficiency catalysts and to select a proper electrolyte. Herein, the performances of non-noble metal-based carbon composites under various pH values (acid, alkaline and neutral media) for HER in terms of catalyst synthesis, structure and molecular design are systematically discussed. A detailed analysis of the structure-activity-pH correlations in the HER process gives an insight on the origin of the pH-dependence for HER, and provide guidance for future HER mechanism studies on non-noble metal-based carbon composites. Furthermore, this Review gives a fresh impetus to rational design of high-performance noble-metal-free composites catalysts and guide researchers to employ the established electrocatalysts in proper water electrolysis technologies.

Non-Noble Metal-based Carbon Composites in Hydrogen Evolution Reaction: Fundamentals to Applications.

In this Review, we aim to provide an updated summary of the research related to hollow micro- and nanostructures, covering both their synthesis and their applications. After a brief introduction to the definition and classification of the hollow micro-/nanostructures, we discuss various synthetic strategies that can be grouped into three major categories, including hard templating, soft templating, and self-templating synthesis. For both hard and soft templating strategies, we focus on how different types of templates are generated and then used for creating hollow structures. At the end of each section, the structural and morphological control over the product is discussed. For the self-templating strategy, we survey a number of unconventional synthetic methods, such as surface-protected etching, Ostwald ripening, the Kirkendall effect, and galvanic replacement. We then discuss the unique properties and niche applications of the hollow structures in diverse fields, including micro-/nanocontainers and rea...

Synthesis, Properties, and Applications of Hollow Micro-/Nanostructures

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS-based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water-splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS-based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water-splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.

https://espace.library.uq.edu.au/view/UQ:df1181f/UQdf1181f_OA.pdf

Nanoarchitectonics for Transition-Metal-Sulfide-Based Electrocatalysts for Water Splitting.

Replacing the rare and precious platinum (Pt) electrocatalysts with earth-abundant materials for promoting the oxygen reduction reaction (ORR) at the cathode of fuel cells is of great interest in developing high-performance sustainable energy devices. However, the challenging issues associated with non-Pt materials are still their low intrinsic catalytic activity, limited active sites, and the poor mass transport properties. Recent advances in material sciences and nanotechnology enable rational design of new earth-abundant materials with optimized composition and fine nanostructure, providing new opportunities for enhancing ORR performance at the molecular level. This Review highlights recent breakthroughs in engineering nanocatalysts based on the earth-abundant materials for boosting ORR.

Earth-Abundant Nanomaterials for Oxygen Reduction.

Ammonia (NH3) is pivotal to the fertilizer industry and one of the most commonly produced chemicals1. The direct use of atmospheric nitrogen (N2) had been challenging, owing to its large bond energy (945 kilojoules per mole)2,3, until the development of the Haber–Bosch process. Subsequently, many strategies have been explored to reduce the activation barrier of the N≡N bond and make the process more efficient. These include using alkali and alkaline earth metal oxides as promoters to boost the performance of traditional iron- and ruthenium-based catalysts4–6 via electron transfer from the promoters to the antibonding bonds of N2 through transition metals7,8. An electride support further lowers the activation barrier because its low work function and high electron density enhance electron transfer to transition metals9,10. This strategy has facilitated ammonia synthesis from N2 dissociation11 and enabled catalytic operation under mild conditions; however, it requires the use of ruthenium, which is expensive. Alternatively, it has been shown that nitrides containing surface nitrogen vacancies can activate N2 (refs. 12–15). Here we report that nickel-loaded lanthanum nitride (LaN) enables stable and highly efficient ammonia synthesis, owing to a dual-site mechanism that avoids commonly encountered scaling relations. Kinetic and isotope-labelling experiments, as well as density functional theory calculations, confirm that nitrogen vacancies are generated on LaN with low formation energy, and efficiently bind and activate N2. In addition, the nickel metal loaded onto the nitride dissociates H2. The use of distinct sites for activating the two reactants, and the synergy between them, results in the nickel-loaded LaN catalyst exhibiting an activity that far exceeds that of more conventional cobalt- and nickel-based catalysts, and that is comparable to that of ruthenium-based catalysts. Our results illustrate the potential of using vacancy sites in reaction cycles, and introduce a design concept for catalysts for ammonia synthesis, using naturally abundant elements. Ammonia is synthesized using a dual-site approach, whereby nitrogen vacancies on LaN activate N2, which then reacts with hydrogen atoms produced over the Ni metal to give ammonia.

Vacancy-enabled N 2 activation for ammonia synthesis on an Ni-loaded catalyst

Activating high-energy multiple bonds using earth-abundant metals is one of the most significant challenges in catalysis. Here, we show that LaCoSi—a ternary intermetallic compound—is an efficient and stable catalyst for N2 activation to produce NH3. The ammonia synthesis is significantly promoted by shifting the reaction bottleneck from the sluggish N2 dissociation to NH
 x
 formation, which few catalysts have achieved. Theoretical calculations reveal that the negatively charged cobalt mediates electron transfer from lanthanum to the adsorbed N2, which further reduces the activation barrier of N2 dissociation. Most importantly, the specific LaCoSi geometric configuration stabilizes the N2 adsorption with a strong exothermic effect, which dramatically decreases the apparent energy barrier of N2 activation. Consequently, LaCoSi shows a superior activity (1,250 μmol g−1 h−1), with a 60-fold increase over the activity of supported cobalt catalysts under mild reaction conditions (400 °C, 0.1 MPa). Ammonia synthesis is an energy-intensive process due to the high activation barrier for N2 dissociation. Here, Hosono and co-workers show that the intermetallic compound LaCoSi can lower the energy requirement for N2 activation and shift the rate-determining step of the process to NH
 x
 formation under mild conditions.

Ternary intermetallic LaCoSi as a catalyst for N 2 activation

Effective integration of one-dimensional carbon nanofibers (CNF) and two-dimensional carbon sheets into three-dimensional (3D) conductive frameworks is essential for their practical applications as electrode materials. Herein, a novel “vein-leaf”-type 3D complex of carbon nanofibers with nitrogen-doped graphene (NG) was prepared through a simple thermal condensation of urea and bacterial cellulose. During the formation of the 3D complex CNF@NG, the graphene species was tethered to CNF via carbon–carbon bonds. Such an interconnected 3D network facilitates both the electron transfer and mass diffusion for electrochemical reactions.

Strongly Veined Carbon Nanoleaves as a Highly Efficient Metal‐Free Electrocatalyst

Mn+1AXn phases are a large family of compounds that have been limited, so far, to carbides and nitrides. Here we report the prediction of a compound, Ti2InB2, a stable boron-based ternary phase in the Ti-In-B system, using a computational structure search strategy. This predicted Ti2InB2 compound is successfully synthesized using a solid-state reaction route and its space group is confirmed as P
 $$\bar 6$$
 m2 (No. 187), which is in fact a hexagonal subgroup of P63/mmc (No. 194), the symmetry group of conventional Mn+1AXn phases. Moreover, a strategy for the synthesis of MXenes from Mn+1AXn phases is applied, and a layered boride, TiB, is obtained by the removal of the indium layer through dealloying of the parent Ti2InB2 at high temperature under a high vacuum. We theoretically demonstrate that the TiB single layer exhibits superior potential as an anode material for Li/Na ion batteries than conventional carbide MXenes such as Ti3C2. Two-dimensional materials are promising for electrochemical storage and conversion, but are somewhat limited in composition. Here the authors use a computational strategy to predict the existence of a layered boride material, which they synthesize and demonstrate prospective for use as an anode material.

/pdf/discovery-of-hexagonal-ternary-phase-ti2inb2-and-its-2h57ammmtt.pdf

Discovery of hexagonal ternary phase Ti2InB2 and its evolution to layered boride TiB

Due to the high cost of Pt-based materials for electrocatalysis, substitute catalysts composed of nonprecious metals have been in high demand. Herein, an ultrastable electrocatalyst with cobalt nanocrystals grown through the plane of graphene subunits of nitrogen-doped graphenes was synthesized via a one-step route. The catalyst has more positive onset and half-wave potential than Pt/C, high methanol crossover tolerance, and superior stability. It is the introduction of strongly bonded cobalt nanocrystals into the network of graphenes that modulate the electronic properties of the latter, resulting in the superb electrocatalytic performance.

Tian-Nan Ye

Papers

Vacancy-enabled N 2 activation for ammonia synthesis on an Ni-loaded catalyst

Ternary intermetallic LaCoSi as a catalyst for N 2 activation

Strongly Veined Carbon Nanoleaves as a Highly Efficient Metal‐Free Electrocatalyst

Discovery of hexagonal ternary phase Ti2InB2 and its evolution to layered boride TiB

Anchoring Cobalt Nanocrystals through the Plane of Graphene: Highly Integrated Electrocatalyst for Oxygen Reduction Reaction