Microwave synthesis of molybdenene from MoS2.

Dirac materials are characterized by the emergence of massless quasiparticles in their low-energy excitation spectrum that obey the Dirac Hamiltonian. Known examples of Dirac materials are topological insulators, d-wave superconductors, graphene, and Weyl and Dirac semimetals, representing a striking range of fundamental properties with potential disruptive applications. However, none of the Dirac materials identified so far shows metallic character. Here, we present evidence for the formation of free-standing molybdenene, a two-dimensional material composed of only Mo atoms. Using MoS2 as a precursor, we induced electric-field-assisted molybdenene growth under microwave irradiation. We observe the formation of millimetre-long whiskers following screw-dislocation growth, consisting of weakly bonded molybdenene sheets, which, upon exfoliation, show metallic character, with an electrical conductivity of ~940 S m-1. Molybdenene when hybridized with two-dimensional h-BN or MoS2, fetch tunable optical and electronic properties. As a proof of principle, we also demonstrate applications of molybdenene as a surface-enhanced Raman spectroscopy platform for molecular sensing, as a substrate for electron imaging and as a scanning probe microscope cantilever.

Dual Single-Atomic Co-Mn Sites in Metal-Organic-Framework-Derived N-Doped Nanoporous Carbon for Electrochemical Oxygen Reduction.

Synthesizing dual single-atom catalysts (DSACs) with atomically isolated metal pairs is a challenging task but can be an effective way to enhance the performance for electrochemical oxygen reduction reaction (ORR). Herein, well-defined DSACs of Co-Mn, stabilized in N-doped porous carbon polyhedra (named CoMn/NC), are synthesized using high-temperature pyrolysis of a Co/Mn-doped zeolitic imidazolate framework. The atomically isolated Co-Mn site in CoMn/NC is recognized by combining microscopic as well as spectroscopic techniques. CoMn/NC exhibited excellent ORR activities in alkaline (E1/2 = 0.89 V) as well as in acidic (E1/2 = 0.82 V) electrolytes with long-term durability and enhanced methanol tolerance. Density functional theory (DFT) suggests that the Co-Mn site is efficiently activating the O-O bond via bridging adsorption, decisive for the 4e- oxygen reduction process. Though the Co-Mn sites favor O2 activation via the dissociative ORR mechanism, stronger adsorption of the intermediates in the dissociative path degrades the overall ORR activity. Our DFT studies conclude that the ORR on an Co-Mn site mainly occurs via bridging side-on O2 adsorption following thermodynamically and kinetically favorable associative mechanistic pathways with a lower overpotential and activation barrier. CoMn/NC performed excellently as a cathode in a proton exchange membrane (PEM) fuel cell and rechargeable Zn-air battery with high peak power densities of 970 and 176 mW cm-2, respectively. This work provides the guidelines for the rational design and synthesis of nonprecious DSACs for enhancing the ORR activity as well as the robustness of DSACs and suggests a design of multifunctional robust electrocatalysts for energy storage and conversion devices.

Electrocatalytic Reduction of Nitrogen to Ammonia Using Tiara-like Phenylethanethiolated Nickel Cluster.

The fixing of N2 to NH3 is challenging due to the inertness of the N≡N bond. Commercially, ammonia production depends on the energy-consuming Haber-Bosch (H-B) process, which emits CO2 while using fossil fuels as the sources of hydrogen and energy. An alternative method for NH3 production is the electrochemical nitrogen reduction reaction (NRR) process as it is powered by renewable energy sources. Here, we report a tiara-like nickel-thiolate cluster, [Ni6 (PET)12 ] (where, PET=2-phenylethanethiol)] as an efficient electro-catalyst for the electrochemical NRR at ambient conditions. Ammonia (NH3 : 16.2±0.8 µg h-1 cm-2 ) was the only nitrogenous product over the potential of -2.3 V vs. Fc + /Fc with a Faradaic efficiency of 25%±1.7. Based on theoretical calculations, NRR by [Ni6 (PET)12 ] proceeds through both the distal and alternating pathways with an onset potential of -1.84 V vs. RHE (i.e., -2.46 V vs. Fc + /Fc ) which corroborates with the experimental findings.

Deciphering the Role of Substitution in Transition-Metal Phosphorous Trisulfide (100) Surface: A Highly Efficient and Durable Pt-free ORR Electrocatalyst.

The rational design and development of earth-abundant, cost-effective, environmentally benign, and highly robust oxygen reduction reaction (ORR) electrocatalysts can circumvent the obstacles associated with the large-scale commercialization of fuel cells. Here, using first-principles-based density functional theory (DFT), we have computationally screened the potential and feasibility of transition-metal phosphorous trisulfides (TMPS3 ) (100) surfaces as efficient ORR electrocatalyst in acidic fuel cell application. MnPS3 (100) surface emerges to be the best among TMPS3 surfaces with optimal O2 activation resulting in very low overpotential. The study reveals that ORR occurs on the MnPS3 surface via 4e reduction associative pathway where the kinetically rate-determining step (RDS) is the formation of O*+H2 O with an activation barrier of 0.66 eV. Additionally, high CO tolerance and easy desorption of H2 O make MnPS3 a robust catalyst. Substitution in half of the Mn sites of MnPS3 (100) surface with Co considerably enhances the ORR activity. Mn0.5 Co0.5 PS3 (100) surface exhibits an ultralow overpotential of 0.39 V vs RHE switching ORR pathway from associative to dissociative. Spontaneous dissociation of H2 O2 on Mn0.5 Co0.5 PS3 proves 4e reduction pathway excluding 2e one. Electronic structure analysis reveals that pristine MnPS3 (100) surface is a narrow band gap semiconductor which upon Co substitution transforms into a conducting metallic surface enhancing ORR activity. Besides, Mn0.5 Co0.5 PS3 (100) surface obtains the apex of the volcano plot due to its optimal position of the d-band center which further justifies the improved ORR activity. With Pt-like onset potential, facile H2 O desorption ability, and robust dynamic and thermal stability, this CO tolerant Mn0.5 Co0.5 PS3 catalyst can be a potential alternative to Pt with encouraging practical viability.

Epitaxial Orientation Angle Tuned Disk-on-Rod Nanoheterostructures for Boosting Charge Transfer.

Controlling the compositions of Se(VI) and Te(VI) ions in a 2D disk on 1D structures of Sb(V) chalcogenides, disk-on-rod heterostructures having three different epitaxial angles with different surface facets are reported. Te injection temperature determined the composition, ensuring heterostructure formation with trigonal Sb2SexTe3-x disks on orthorhombic Sb2Se3 rods having orientation angles 180°, 135°, and 90°. The growth kinetics of disks connected at one/two heads of parent rods is manipulated using an Se precursor as a limiting reagent. Theoretical calculations established the energy minimization of different orientations, their possible formation, and suitability in energy transfer applications. Electrochemical measurements were also in agreement with theoretical calculations. Hence, this is a case study of advanced modular synthesis of disk-on-rod nanostructures, leading a step further in nanocrystal engineering for more desirable complex structures and their charge transfer property.

Tuning intermediate adsorption in structurally ordered substituted PdCu3 intermetallic nanoparticles for enhanced ethanol oxidation reaction.

Co and Ni-substituted structurally ordered intermetallic PdCu3 nanoparticles (NPs) synthesized at low temperature exhibit remarkable enhancement of the ethanol electrooxidation (EOR) activity with improved durability. The first-principle calculations suggest that prompted generation of OH and CH3CO radicals in close proximity and shifting of the d-band center towards the Fermi level boost the EOR efficiency.

Formation of Metallic Polyferrocene Chains under Pressure.

The effect of pressure on the structural reorganization of ferrocene, Fc = (C5H5)2Fe, is studied using density functional theory (DFT) calculations assisted by evolutionary crystal structure prediction algorithms based on USPEX code. Pressure brings the individual molecules in close contact, and above 220 GPa, the 18-electron closed-shell molecular unit undergoes polymerization through the formation of quasi-one-dimensional (1D) chains, [(C5H5)2Fe]∞, termed as polyferrocene (p-Fc). Pressure induced polymerization (PIP) of Fc causes significant deviations from the 5-fold symmetry of the cyclopentadiene (Cp, C5H5 rings) and loss of planarity due to the onset of envelope-like distortions. This triggers distortions within the multidecker sandwich structures and σ(C-C) bond formation between the otherwise weak noncovalently interacting Cp rings in Fc crystals. Pressure gradually reduces the band gap of Fc, and for p-Fc, metallic states are found due to increased electronic coupling between the covalently linked Cp rings. Polyferrocene is significantly more rigid than ferrocene as evident from the 5-fold increase in its bulk modulus. Pressure dependent Raman spectra show a clear onset of polymerization in Fc at P = 220 GPa. Higher mechanical strength coupled with its metallicity makes p-Fc an interesting candidate for high pressure synthesis.

Designing C6N6/C2N van der Waals heterostructures for photogenerated charge carrier separation.

Photocatalytic water splitting mechanism boosted by two-dimensional catalyst materials has become the vibrant field of research toward clean energy initiative. Here, we propose a new two-dimensional (2D) van der Waals type-II heterostructure based on C6N6/C2N composites as an efficient photocatalyst. The structural, electronic and optical properties of the free-standing monolayer as well as their heterostructure have been investigated by first principles based density functional theory (DFT). The band edge positions of C6N6/C2N heterostructures satisfy the photocatalytic water splitting requirements. The potential drop at the interface of the heterostructure induces a large built-in electric field directed from the C6N6 layer to the C2N layer, thereby facilitating the charge transfer from C2N to C6N6 layer. The higher hole mobility as compared to that of electrons aids in separation of charge carriers and thus prohibiting the recombination of the photogenerated charge carriers by separating them in different layers. This is also reflected in the planar averaged charge density difference and partial charge density calculation. The wide band gap semiconductor (C6N6) in combination with a moderate band gap semiconductor (C2N) allows one to harness solar energy efficiently in the visible region for water splitting as also confirmed in the optical absorption spectra. The favorable band edge position with respect to the water redox potential makes it an ideal substrate for the visible light induced oxygen evolution reaction and the hydrogen evolution reaction.

Nickel-cobalt oxalate as an efficient non-precious electrocatalyst for an improved alkaline oxygen evolution reaction.

The quest for developing next-generation non-precious electrocatalysts has risen in recent times. Herein, we have designed and developed a low cost electrocatalyst by a ligand-assisted synthetic strategy in an aqueous medium. An oxalate ligand-assisted non-oxide electrocatalyst was developed by a simple wet-chemical technique for alkaline water oxidation application. The synthetic parameters for the preparation of nickel-cobalt oxalate (Ni2.5Co5C2O4) were optimized, such as the metal precursor (Ni/Co) ratio, oxalic acid amount, reaction temperature, and time. Microstructural analysis revealed a mesoporous block-like architecture for nickel-cobalt oxalate (Ni2.5Co5C2O4). The required overpotential of Ni2.5Co5C2O4 for the alkaline oxygen evolution reaction (OER) was found to be 330 mV for achieving 10 mA cmgeo -2, which is superior to that of NiC2O4, CoC2O4, NiCo2O4 and the state-of-the-art RuO2. The splendid performance of Ni2.5Co5C2O4 was further verified by its low charge transfer resistance, impressive stability performance, and 87% faradaic efficiency in alkaline medium (pH = 14). The improved electrochemical activity was further attributed to double layer capacitance (C dl), which indefinitely divulged the inferiority of NiCo2O4 compared to Ni2.5Co5C2O4 for the alkaline oxygen evolution reaction (OER). The obtained proton reaction order (ρ RHE) was about 0.80, thus indicating the proton decoupled electron transfer (PDET) mechanism for OER in alkaline medium. Post-catalytic investigation revealed the formation of a flake-like porous nanostructure, indicating distinct transformation in morphology during the alkaline OER process. Further, XPS analysis demonstrated complete oxidation of Ni2+ and Co2+ centres into Ni3+ and Co3+, respectively under high oxidation potential, thereby indicating active site formation throughout the microstructural network. Additionally, from BET-normalised LSV investigation, the intrinsic activity of Ni2.5Co5C2O4 was also found to be higher than that of NiCo2O4. Finally, Ni2.5Co5C2O4 delivered a TOF value of around 3.28 × 10-3 s-1, which is 5.56 fold that of NiCo2O4 for the alkaline OER process. This report highlights the unique benefit of Ni2.5Co5C2O4 over NiCo2O4 for the alkaline OER. The structure-catalytic property relationship was further elucidated using density functional theory (DFT) study. To the best of our knowledge, nickel-cobalt oxalate (Ni2.5Co5C2O4) was introduced for the first time as a non-precious non-oxide electrocatalyst for alkaline OER application.

Direct CO2 capture and conversion to fuels on magnesium nanoparticles under ambient conditions simply using water.

Converting CO2 directly from the air to fuel under ambient conditions is a huge challenge. Thus, there is an urgent need for CO2 conversion protocols working at room temperature and atmospheric pressure, preferentially without any external energy input. Herein, we employ magnesium (nanoparticles and bulk), an inexpensive and the eighth-most abundant element, to convert CO2 to methane, methanol and formic acid, using water as the sole hydrogen source. The conversion of CO2 (pure, as well as directly from the air) took place within a few minutes at 300 K and 1 bar, and no external (thermal, photo, or electric) energy was required. Hydrogen was, however, the predominant product as the reaction of water with magnesium was favored over the reaction of CO2 and water with magnesium. A unique cooperative action of Mg, basic magnesium carbonate, CO2, and water enabled this CO2 transformation. If any of the four components was missing, no CO2 conversion took place. The reaction intermediates and the reaction pathway were identified by 13CO2 isotopic labeling, powder X-ray diffraction (PXRD), nuclear magnetic resonance (NMR) and in situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and rationalized by density-functional theory (DFT) calculations. During CO2 conversion, Mg was converted to magnesium hydroxide and carbonate, which may be regenerated. Our low-temperature experiments also indicate the future prospect of using this CO2-to-fuel conversion process on the surface of Mars, where CO2, water (ice), and magnesium are abundant. Thus, even though the overall process is non-catalytic, it could serve as a step towards a sustainable CO2 utilization strategy as well as potentially being a first step towards a magnesium-driven civilization on Mars.

Correction: Citrate combustion synthesized Al-doped CaCu3Ti4O12 quadruple perovskite: synthesis, characterization and multifunctional properties.

Correction for 'Citrate combustion synthesized Al-doped CaCu3Ti4O12 quadruple perovskite: synthesis, characterization and multifunctional properties' by Kamalesh Pal et al., Phys. Chem. Chem. Phys., 2020, 22, 3499-3511, DOI: 10.1039/C9CP05005A.

Transition-Metal Phosphorus Trisulfides and its Vacancy Defects: Emergence of a New Class of Anode Material for Li-Ion Batteries.

In the search of suitable anode candidates with high specific capacity, favorable potential, and structural stability for lithium-ion batteries (LIBs), transition-metal phosphorus trisulfides (TMPS3 ) can be considered as one of the most promising alternatives to commercial graphite. Here, it was demonstrated that the limitations of commercial anode materials (i.e., low specific capacity, large volume change, and high lithium diffusion barrier as well as nucleation) can be circumvented by using TMPS3 monolayer surfaces. The study revealed that lithium binds strongly to TMPS3 monolayers (-2.31 eV) without any distortion of the surface, with Li@TMPS3 exhibiting enhanced stability compared with other 2D analogues (graphene, phosphorene, MXenes, transition-metal sulfides and phosphides). The binding energy of lithium was overwhelmingly enhanced with vacancy defects. The vacancy-mediated TMPS3 surfaces showed further amplification of Li binding energy from -2.03 to -2.32 eV and theoretical specific capacity of 441.65 to 484.34 mAh g-1 for MnPS3 surface. Most importantly, minimal change in volume (less than 2 %) after lithiation makes TMPS3 monolayers a very effective candidate for LIBs. Additionally, the ultralow lithium diffusion barrier (0.08 eV) compared with other existing commercial anode material proves the superiority of TMPS3 .

Defects in nanosilica catalytically convert CO2 to methane without any metal and ligand.

Active and stable metal-free heterogeneous catalysts for CO2 fixation are required to reduce the current high level of carbon dioxide in the atmosphere, which is driving climate change. In this work, we show that defects in nanosilica (E' centers, oxygen vacancies, and nonbridging oxygen hole centers) convert CO2 to methane with excellent productivity and selectivity. Neither metal nor complex organic ligands were required, and the defect alone acted as catalytic sites for carbon dioxide activation and hydrogen dissociation and their cooperative action converted CO2 to methane. Unlike metal catalysts, which become deactivated with time, the defect-containing nanosilica showed significantly better stability. Notably, the catalyst can be regenerated by simple heating in the air without the need for hydrogen gas. Surprisingly, the catalytic activity for methane production increased significantly after every regeneration cycle, reaching more than double the methane production rate after eight regeneration cycles. This activated catalyst remained stable for more than 200 h. Detailed understanding of the role of the various defect sites in terms of their concentrations and proximities as well as their cooperativity in activating CO2 and dissociating hydrogen to produce methane was achieved.

Citrate combustion synthesized Al-doped CaCu3Ti4O12 quadruple perovskite: synthesis, characterization and multifunctional properties.

The facile synthesis of the Al-doped CaCu3Ti4O12 quadruple perovskite, a well-known and vastly studied material for various technological applications, using the modified citrate combustion route along with structural, microstructural, and X-ray photoelectron spectroscopic (XPS) characterization and magnetic, dielectric and electrical properties has been investigated and reported here. The possible applications of the material as a Schottky barrier diode (SBD) in optoelectronic devices and as a catalyst in methanol steam reforming (MSR) reaction for hydrogen generation, hitherto unreported in the open literature, have also been explored. The compound is crystallized in the cubic body centered Im3[combining macron] space group and the particle size is found to be in nanodimension with rather narrow size distribution. The enhanced resistivity could be attributed to the grain boundary effect, and consequently, it exhibits better performance as a SBD compared to the undoped sample. Desired cationic composition with expected valence states within the probe range is confirmed by XPS analysis. A better catalytic activity towards MSR is noticed for the Al-doped CaCu3Ti4O12 compared to the undoped composition. These new findings, namely MSR activity and applicability in the Schottky device, have highlighted further the multifunctional nature of the material in energy related issues and would thus be of interest to the materials community searching for functional materials.

A semiconducting supramolecular Co(ii)-metallohydrogel: an efficient catalyst for single-pot aryl-S bond formation at room temperature.

A novel mechanically stable supramolecular Co(ii)-metallohydrogel has been synthesized. Cobalt(ii) nitrate hexahydrate and monoethanolamine, as a low molecular weight organic gelator, are used to get the gel. The mechanical stability of the supramolecular hydrogel was analyzed. The morphology of the supramolecular metallohydrogel was scrutinized. The semiconducting features of the metallohydrogel were studied. The conducting properties of the Co(ii)-metallohydrogel establish a Schottky barrier diode type nature. The catalytic nature of the Co(ii)-metallohydrogel based room temperature single pot aryl-S coupling reaction was explored. Most interestingly, the Co(ii)-metallohydrogel based catalytic aryl-S coupling reaction does not require any column-chromatographic purification protocol to get pure aryl-thioethers. Thus, through this work a semiconducting Schottky barrier diode application and catalytic role in the room temperature single pot aryl-S coupling reaction of a supramolecular Co(ii)-metallohydrogel have been explored.

Supramolecular Aggregate of Cadmium(II)-Based One-Dimensional Coordination Polymer for Device Fabrication and Sensor Application.

A novel mixed ligand one-dimensional coordination polymer (1D CP), {[Cd2(adc)2(4-nvp)6]·(MeOH)·(H2O)} n (1; H2adc = 9,10-anthracenedicarboxylic acid, and 4-nvp = 4-(1-naphthylvinyl)pyridine), has been synthesized and structurally characterized by single crystal X-ray crystallography. The 1D polymer undergoes supramolecular aggregation via hydrogen bonding, C-H···π, and π···π interactions. Interestingly, compound 1 shows increasing conductivity upon irradiation of light. Therefore, it has the potential to be used in optoelectronic devices. Moreover, the supramolecular assembly of 1 specifically detects Cr3+ cation in the presence of other competitive analytes. Most importantly, compound 1 exhibits fascinating turn-on Cr3+ sensing, which seems to be an ornament in the field of sensing application.

Aryl-platform-based tetrapodal 2-iodo-imidazolium as an excellent halogen bond receptor in aqueous medium.

An acyclic tetrapodal receptor (L4+-I)(4PF6)4- comprised of four 2-iodo-imidazolium motifs showed moderate to strong binding of halides through halogen bonding interactions in organic and aqueous media, with these binding levels established by performing isothermal titration calorimetry studies. Importantly, single crystals suitable for X-ray diffraction studies were obtained from both water and an acetonitrile-water binary solvent mixture, and exhibited halogen bonding interactions in the solid state.

Electrochemical Stimuli-Driven Facile Metal-Free Hydrogen Evolution from Pyrene-Porphyrin-Based Crystalline Covalent Organic Framework.

A [2 + 2] Schiff base type condensation between 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAP) and 1,3,6,8-tetrakis (4-formylphenyl) pyrene (TFFPy) under solvothermal condition yields a crystalline, quasi-two-dimensional covalent organic framework (SB-PORPy-COF). The porphyrin and pyrene units are alternatively occupied in the vertex of 3D triclinic crystal having permanent microporosity with moderately high surface area (∼869 m2 g-1) and promising chemical stability. The AA stacking of the monolayers give a pyrene bridged conducting channel. SB-PORPy-COF has been exploited for metal free hydrogen production to understand the electrochemical behavior using the imine based docking site in acidic media. SB-PORPy-COF has shown the onset potential of 50 mV and the Tafel slope of 116 mV dec-1. We expect that the addendum of the imine based COF would not only enrich the structural variety but also help to understand the electrochemical behavior of these class of materials.

Facile Aqueous-Phase Synthesis of the PtAu/Bi2O3 Hybrid Catalyst for Efficient Electro-Oxidation of Ethanol.

In this work, we present a facile aqueous-phase synthesis of a hybrid catalyst consisting of PtAu alloy supported on Bi2O3 microspheres. Multistep reduction of HAuCl4 and K2PtCl4 salts on Bi2O3 and subsequent annealing lead to the formation of this hybrid catalyst. To the best of our knowledge, this is the first report of using Bi2O3 as a catalyst support in fuel cell applications. The material was characterized by powder X-ray diffraction and various microscopic techniques. This composite showed remarkable activity as well as stability toward the electro-oxidation of ethanol in comparison to commercially available Pt/C. The order of the reactivity was found to be commercial Pt/C (50.4 mA/m2mgPt-1) < Pt/Bi2O3(10) (108 mA/m2mgPt-1) < PtAu/Bi2O3(10) (459 mA/m2mgPt-1). The enhancement in the activity can be explained through cooperative effects, namely, ligand effects of gold and Bi2O3 support, which helps in removing carbon monoxide molecules to avoid the poisoning of the Pt active sites.

Electrochemical Dealloying of PdCu3 Nanoparticles to Achieve Pt-like Activity for the Hydrogen Evolution Reaction.

Manipulating the d-band center of the metal surface and hence optimizing the free energy of hydrogen adsorption (ΔGH ) close to the optimal adsorption energy (ΔGH =0) for hydrogen evolution reaction (HER), is an efficient strategy to enhance the activity for HER. Herein, we report a oleylamine-mediated (acting as the solvent, stabilizer, and reducing agent) strategy to synthesize intermetallic PdCu3 nanoparticles (NPs) without using any external reducing agent. Upon electrochemical cycling, PdCu3 transforms into Pd-rich PdCu (ΔGH =0.05 eV), exhibiting remarkably enhanced activity (with a current density of 25 mA cm-2 at ∼69 mV overpotential) as an alternative to Pt for HER. The first-principle calculation suggests that formation of low coordination number Pd active sites alters the d-band center and hence optimal adsorption of hydrogen, leading to enhanced activity. This finding may provide guidelines towards the design and development of Pt-free highly active and robust electrocatalysts.