Two-dimensional metallic transition metal dichalcogenides are emerging as prototypes for uncovering fundamental physical phenomena, such as superconductivity and charge-density waves, as well as for engineering-related applications. However, the batch production of such envisioned transition metal dichalcogenides remains challenging, which has hindered the aforementioned explorations. Herein, we fabricate thickness-tunable tantalum disulfide flakes and centimetre-sized ultrathin films on an electrode material of gold foil via a facile chemical vapour deposition route. Through temperature-dependent Raman characterization, we observe the transition from nearly commensurate to commensurate charge-density wave phases with our ultrathin tantalum disulfide flakes. We have obtained high hydrogen evolution reaction efficiency with the as-grown tantalum disulfide flakes directly synthesized on gold foils comparable to traditional platinum catalysts. This work could promote further efforts for exploring new efficient catalysts in the large materials family of metallic transition metal dichalcogenides, as well as exploiting their applications towards more versatile applications.
There has been an argument on carbon nanotube (CNT) based gas detectors with a field-effect transistor (FET) geometry: do the response signals result from charge transfer between adsorbed gas molecules and the CNT channel and/or from the gas species induced Schottky barrier modulation at the CNT/metal contacts? To differentiate the sensing mechanisms, we employed three CNTFET structures, i.e., (1) the entire CNT channel and CNT/electrode contacts are accessible to NH(3) gas; (2) the CNT/electrode contacts are passivated with a Si(3)N(4) thin film, leaving the CNT channel open to the gas and, in contrast, (3) the CNT channel is covered with the film, while the contacts are open to the gas. We suggest that the Schottky barrier modulation at the contacts is the dominant mechanism from room temperature to 150 degrees C. At higher temperatures, the charge transfer process contributes to the response signals. There is a clear evidence that the adsorption of NH(3) on the CNT channel is facilitated by environmental oxygen.
Chitosan and its various neutral pH water-soluble derivatives were investigated for dispersing single-walled carbon nanotubes (SWNTs). Chitosan (CS) can produce good dispersion of SWNTs, but only in acidic pH condition. Our two novel derivatives, O-carboxymethylchitosan (OC) and OC modified by poly(ethylene glycol) at the −COOH position (OPEG), were able to produce highly effective debundling and dispersion of SWNTs in neutral pH aqueous solution. Atomic force microscopy (AFM), transmission electron microscopy (TEM), photoluminescence, UV−vis−NIR spetroscopy, and Raman spectroscopy confirmed that SWNTs are present as individual nanotubes in the dispersions. The solubilities of individually dispersed SWNTs in neutral water are 0.021 and 0.032 g/L for OC and OPEG, respectively, which are comparable to 0.038 g/L for SWNTs using CS in acetic acid. Further, OC and OPEG aqueous solutions (1 wt %) do not significantly lower the surface tensions (65−67 mN/m). From the Fourier transform infrared spectroscopic results, we conclude that the free electron pair in the pendant amine groups of OC and OPEG plays a vital role in finely dispersing the SWNTs; the −NH2 contributes to the adsorption of these two chitosan derivatives on the nanotubes. Quaternary ammonium chitosan (QC), with alkyl substitution at the protonated amine, was found to be unable to disperse SWNTs; possibly cation−π interaction with nanotubes is diminished due to steric hindrance.
Heterogeneous catalysis of formic acid dehydrogenation at room temperature is a promising tactic for safely storing and producing H 2 as an efficient energy carrier. Up to now, the catalysts for this purpose are largely developed based on trial and error. In this work, we demonstrate that a careful analysis of the formic acid dehydrogenation mechanism can shed light on rational design and facile synthesis of efficient Pd-based catalysts, that is, carbon black-supported fine Pd nanoparticles with adatoms of an sp metal (including but not limited to Bi). In fact, Pd@Bi/C with an optimal atomic ratio doubles the Pd mass activity of the Pd/C in terms of hydrogen production rate, specifically with a global turnover frequency of 4350 h −1 at 303 K in a mixed 1.1 M formic acid and 2.4 M sodium formate solution without engineering the catalyst support. Apparent kinetic measurement, in situ interfacial IR spectroscopy, and density functional theory calculation results further confirm that Bi adatoms favor the adsorption of the formate intermediate to facilitate the C−H bond cleavage and weaken the adsorption of H and CO on Pd sites, resulting in a prominently enhanced H 2 production performance.
Semiconducting single-walled carbon nanotubes (s-SWNTs) have emerged as a promising class of electronic materials, but the metallic (m)-SWNTs present in all as-synthesized nanotube samples must be removed for many applications. A high selectivity and high yield separation method has remained elusive. A separation process based on selective chemistry appears to be an attractive route since it is usually relatively simple, but more effective chemicals are needed. Here we demonstrate the first example of a new class of dual selective compounds based on polycyclic aromatic azo compounds, specifically Direct Blue 71 (I), for high-purity separation of s-SWNTs at high yield. Highly enriched (~93% purity) s-SWNTs are produced through the simple process of standing arc-discharge SWNTs with I followed by centrifugation. The s-SWNTs total yield is up to 41%, the highest yet reported for a solution-based separation technique that demonstrates applicability in actual transistors. 91% of transistor devices fabricated with these s-SWNTs exhibited on/off ratios of 10(3) to 10(5) with the best devices showing mobility as high as 21.8 cm(2)/V s with on/off ratio of 10(4). Raman and X-ray photoelectron spectroscopic shifts and ultraviolet-visible-near-infrared (UV-vis-NIR) show that I preferentially complexes with s-SWNTs and preferentially suspends them. Preferential reaction of naphthyl radicals (generated from I with ultrasonication) with m-SWNTs is confirmed by changes in the D-band in the Raman spectroscopy, matrix-assisted desorption-ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and molecular simulation results. The high selectivity of I stems from its unique dual action as both a selective dispersion agent and the generator of radicals which preferentially attack unwanted metallic species.
The methanol‐to‐olefins (MTO) process is becoming the most important non‐petrochemical route for the production of light olefins from coal or natural gas. Maximizing the generation of the target products, ethene and propene, and minimizing the production of byproducts and coke, are major considerations in the efficient utilization of the carbon resource of methanol. In the present work, the heterogeneous catalytic conversion of methanol was evaluated by performing simultaneous measurements of the volatile products generated in the gas phase and the confined coke deposition in the catalyst phase. Real‐time and complete reaction profiles were plotted to allow the comparison of carbon atom economy of methanol conversion over the catalyst SAPO‐34 at varied reaction temperatures. The difference in carbon atom economy was closely related with the coke formation in the SAPO‐34 catalyst. The confined coke compounds were determined. A new type of confined organics was found, and these accounted for the quick deactivation and low carbon atom economy under low‐reaction‐temperature conditions. Based on the carbon atom economy evaluation and coke species determination, optimized operating conditions for the MTO process are suggested; these conditions guarantee high conversion efficiency of methanol.
A facile and eco-friendly approach for the synthesis of mesoporecontaining ZSM-5 zeolite with small crystal size was proposed by subtly making use of a coke-deposited spent zeolite catalyst.When used in the methanol to propylene reaction again, the refabricated ZSM-5 catalyst exhibited a much higher propylene selectivity and a longer catalytic lifetime. † Electronic supplementary information (ESI) available: Experimental details on the synthesis and characterization of zeolite, physicochemical properties of the spent catalyst, pore size distribution curves and NH 3 -TPD profiles of the as-synthesized ZSM-5 zeolites. See
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