From 0D to 2D: N-doped carbon nanosheets for detection of alcohol-based chemical vapours.

The application of N-doped carbon nanosheets, with and without embedded carbon dots, as active materials for the room temperature chemoresistive detection of methanol and/or ethanol is presented. The new carbons were made by converting 0D N-doped carbon dots (NCDs) to 2D nitrogen-doped carbon nanosheets by heat treatment (200-700 °C). The nanosheets exhibited a lateral size of ∼3 µm and a thickness of ∼12 nm at the highest annealing temperature. Both Raman and TEM analyses showed morphological transitions of the dots to the sheets, whilst XPS analysis revealed transformation of the N-bonding states with increasing temperature. PDF analysis confirmed the presence of defective carbon sheets. Room temperature screening of the chemical vapours of two alcohols (methanol and ethanol), revealed that the structure and the type of N-configuration influenced the detection of the chemical vapours. For instance, the lateral size of the nanosheets and the high charge density N-configurations promoted detection of both methanol and ethanol vapours at good sensitivity (-16.8 × 10-5 ppm-1 EtOH and 1.2 × 10-5 ppm-1 MeOH) and low LoD (∼44 ppmEtOH and ∼30.3 ppmMeOH) values. The study showed that the composite nature as well as the large basal area of the carbon nanosheets enabled generation of adequate defective sites that facilitated easy adsorption of the VOC analyte molecules, thereby eliminating the need to use conducting polymers or the formation of porous molecular frameworks for the alcohol detection.

The Transformation of 0-D Carbon Dots into 1-, 2- and 3-D Carbon Allotropes: A Minireview.

Carbon dots (CDs) represent a relatively new type of carbon allotrope with a 0-D structure and with nanoparticle sizes < 10 nm. A large number of research articles have been published on the synthesis, characteristics, mechanisms and applications of this carbon allotrope. Many of these articles have also shown that CDs can be synthesized from "bottom-up" and "top-down" methods. The "top-down" methods are dominated by the breaking down of large carbon structures such as fullerene, graphene, carbon black and carbon nanotubes into the CDs. What is less known is that CDs also have the potential to be used as carbon substrates for the synthesis of larger carbon structures such as 1-D carbon nanotubes, 2-D or 3-D graphene-based nanosheets and 3-D porous carbon frameworks. Herein, we present a review of the synthesis strategies used to convert the 0-D carbons into these higher-dimensional carbons. The methods involve the use of catalysts or thermal procedures to generate the larger structures. The surface functional groups on the CDs, typically containing nitrogen and oxygen, appear to be important in the process of creating the larger carbon structures that typically are formed via the generation of covalent bonds. The CD building blocks can also 'aggregate' to form so called supra-CDs. The mechanism for the formation of the structures made from CDs, the physical properties of the CDs and their applications (for example in energy devices and as reagents for use in medicinal fields) will also be discussed. We hope that this review will serve to provide valuable insights into this area of CD research and a novel viewpoint on the exploration of CDs.

Platinum Nanocatalysts Supported on Defective Hollow Carbon Spheres: Oxygen Reduction Reaction Durability Studies.

The durability and long-term applicability of catalysts are critical parameters for the commercialization and adoption of fuel cells. Even though a few studies have been conducted on hollow carbon spheres (HCSs) as supports for Pt in oxygen reduction reactions (ORR) catalysis, in-depth durability studies have not been conducted thus far. In this study, Pt/HCSs and Pt/nitrogen-doped HCSs (Pt/NHCSs) were prepared using a reflux deposition technique. Small Pt particles were formed with deposition on the outside of the shell and inside the pores of the shell. The new catalysts demonstrated high activity (>380 µA cm-2 and 240 mA g-1) surpassing the commercial Pt/C by more than 10%. The catalysts demonstrated excellent durability compared to a commercial Pt/C in load cycling, experiencing less than 50% changes in the mass-specific activity (MA) and surface area-specific activity (SA). In stop-start durability cycling, the new materials demonstrated high stability with more than 50% retention of electrochemical active surface areas (ECSAs). The results can be rationalised by the high BET surface areas coupled with an array of meso and micropores that led to Pt confinement. Further, pair distribution function (PDF) analysis of the catalysts confirmed that the nitrogen and oxygen functional groups, as well as the shell curvature/roughness provided defects and nucleation sites for the deposition of the small Pt nanoparticles. The balance between graphitic and diamond-like carbon was critical for the electronic conductivity and to provide strong Pt-support anchoring.

Defect-Engineered ß-MnO2-δ Precursors Control the Structure-Property Relationships in High-Voltage Spinel LiMn1.5Ni0.5O4-δ.

This study examines the role of defects in structure-property relationships in spinel LiMn1.5Ni0.5O4 (LMNO) cathode materials, especially in terms of Mn3+ content, degree of disorder, and impurity phase, without the use of the traditional high-temperature annealing (≥700 °C used for making disordered LMNO). Two different phases of LMNO (i.e., highly P4332-ordered and highly Fd3̅m-disordered) have been prepared from two different ß-MnO2-δ precursors obtained from an argon-rich atmosphere (ß-MnO2-δ (Ar)) and a hydrogen-rich atmosphere [ß-MnO2-δ (H2)]. The LMNO samples and their corresponding ß-MnO2-δ precursors are thoroughly characterized using different techniques including high-resolution transmission electron microscopy, field-emission scanning electron microscopy, Raman spectroscopy, powder neutron diffraction, X-ray photoelectron spectroscopy, synchrotron X-ray diffraction, X-ray absorption near-edge spectroscopy, and electrochemistry. LMNO from ß-MnO2-δ (H2) exhibits higher defects (oxygen vacancy content) than the one from the ß-MnO2-δ (Ar). For the first time, defective ß-MnO2-δ has been adopted as precursors for LMNO cathode materials with controlled oxygen vacancy, disordered phase, Mn3+ content, and impurity contents without the need for conventional methods of doping with metal ions, high synthetic temperature, use of organic compounds, postannealing, microwave, or modification of the temperature-cooling profiles. The results show that the oxygen vacancy changes concurrently with the degree of disorder and Mn3+ content, and the best electrochemical performance is only obtained at 850 °C for LMNO-(Ar). The findings in this work present unique opportunities that allow the use of ß-MnO2-δ as viable precursors for manipulating the structure-property relationships in LMNO spinel materials for potential development of high-performance high-voltage lithium-ion batteries.

Oxidation of Benzyl Alcohol Using Cobalt Oxide Supported Inside and Outside Hollow Carbon Spheres.

Cobalt oxide nanoparticles (6 nm) supported both inside and outside of hollow carbon spheres (HCSs) were synthesized by using two different polymer templates. The oxidation of benzyl alcohol was used as a model reaction to evaluate the catalysts. PXRD studies indicated that the Co oxidation state varied for the different catalysts due to reduction of the Co by the carbon, and a metal oxidation step prior to the benzyl alcohol oxidation enhanced the catalytic activity. The metal loading influenced the catalytic efficiency, and the activity decreased with increasing metal loading, possibly due to pore filling effects. The catalysts showed similar activity and selectivity (to benzaldehyde) whether placed inside or outside the HCS (63 % selectivity at 50 % conversion). No poisoning was observed due to product build up in the HCS.

Performance of a NiFe2O4@Co Core-Shell Fischer-Tropsch Catalyst: Effect of Low Temperature Reduction.

In situ TEM gas-cell imaging and spectroscopy with in situ XRD have been applied to reveal morphological changes in NiFe2O4@Co3O4 core-shell nanoparticles in hydrogen. The core-shell structure is retained upon reduction under mild conditions (180 °C for 1 h), resulting in a partially reduced shell. The core-shell structure was retained after exposing these reduced NiFe2O4@Co3O4 core-shell nanoparticles to Fischer-Tropsch conditions at 230 °C and 20 bar. Slightly harsher reduction (230 °C, 2 h) resulted in restructuring of the NiFe2O4@Co3O4 core-shell nanoparticles to form cobalt islands in addition to partially reduced NiFe2O4. NiFe2O4 underwent further transformation upon exposure to Fischer-Tropsch conditions, resulting in the formation of iron carbide and nickel/iron-nickel alloy. The turnover frequency in the Fischer-Tropsch synthesis over NiFe2O4@Co3O4 core-shell nanoparticles reduced in hydrogen at 180 °C for 1 h was estimated to be less than 0.02 s-1 (cobalt-time yield of 8.40 µmol.g-1.s-1) with a C5+ selectivity of 38 C-%. The low turnover frequency under these conditions in relation to the turnover frequency obtained with unsupported cobalt is attributed to the strain in the catalytically active cobalt.

Tris(tert-butyl isocyanide)bis[tris(4-methoxyphenyl)phosphine]cobalt(I) perchlorate dichloromethane disolvate and tris(tert-butyl isocyanide)bis[tris(4-methoxyphenyl)phosphine]cobalt(II) bis(perchlorate) dichloromethane disolvate: modification of a trigonal-bipyramidal structure with change of metal oxidation state.

The title complexes, [Co(C(5)H(9)N)(3)(C(21)H(21)O(3)P)(2)]ClO(4).2CH(2)Cl(2), (I), and [Co(C(5)H(9)N)(3)(C(21)H(21)O(3)P)(2)](ClO(4))(2).2CH(2)Cl(2), (II), respectively, crystallize in the hexagonal space group P6(3)/m and the monoclinic space group P2(1)/n, respectively. The cation of complex (I) has D(3h) site symmetry around the Co atom and the overall symmetry is C(3h). Complex (II) is best described as having a distorted trigonal-bipyramidal coordination, with a Co site symmetry of C(s). Compounds (I) and (II) form an analogous pair of five-coordinate Co(I) and Co(II) complexes with the same ligands, making it possible to establish (i) if the Co site coordination for both complexes is indeed trigonal-bipyramidal, as initially assumed, and (ii) if significant structural differences occur when the oxidation state of the metal is changed.

Diaquatetrakis(tert-butyl isocyanide)cobalt(II) bis(perchlorate): an example of cobalt(II) coordinated by only four alkyl isocyanide ligands.

The title compound, [Co(C5H9N)4(H2O)2](ClO4)2, crystallizes in the monoclinic space group C2/m. The cation has space-group-imposed 2/m symmetry, while the perchlorate ion is disordered about a mirror plane. The two slightly non-equivalent Co-C bonds [1.900 (3) and 1.911 (3) A] form a rectangular plane, with a C-Co-C bond angle of 86.83 (11) degrees, and the linear O-Co-O C2 axis is perpendicular to this plane. The C[triple-bond]N bond lengths are 1.141 (4) A and the Co-C[triple-bond]N and C[triple-bond]N-C angles average 175.5 (4) degrees. The perchlorate counter-ions are hydrogen bonded to the water molecules. The title compound is the first example of four alkyl isocyanide ligands coordinating Co(II) upon initial reaction of Co(ClO4)(2).6H2O/EtOH with alkyl isocyanide. In all other known examples, five alkyl isocyanide molecules are coordinated, as in [(RNC)5Co-Co(CNR)5](ClO4)4 (R = Me, Et, CHMe2, CH2Ph, C4H9-n or C6H11) or [Co(CNC8H17-t)5](ClO4)2. This complex, therefore, is unique and somewhat unexpected.