Microwave-Based Synthesis of Functional Morphological Variants and Carbon Nanotube-Based Composites of VS₄ for Electrochemical Applications

Abstract

A novel facile, fast, and efficient microwave-assisted method was developed to synthesize a number of diverse nanostructured motifs (ranging from nanorods to nanoflowers) of VS 4 along with its associated composite heterostructures, VS 4 / multi-walled carbon nanotube (MWNT; i.e., multi-walled carbon nanotubes). In particular, we have probed and correlated the effects of a number of specific experimental variables, including primarily precursor, solvent, temperature, and time. We noted that nanorods formed more readily with VO(acac) 2 as the vanadium precursor and n-methyl-2-pyrrolidone (NMP) as a polar reaction solvent. By contrast, we determined that hierarchical three-dimensional (3D) nanoflower-like assemblies, ranging in size from 100 to 200 nm in average diameter, could be controllably synthesized by using Na 3 VO 4 as the vanadium precursor and an aqueous water: polar solvent mixture as the reaction medium. We also observed that VS 4 disintegrates, when in the presence of either air, solution, or a combination of these environments, and established that the extent of VS 4 nanorod decomposition could be almost fully prevented by storage under nitrogen. From an application's perspective, our VS 4 is electrochemically active and shows behavior, consistent with the literature. In particular, as compared with pristine VS 4 nanorods alone, we observed enhanced electrochemical activity with (i) 3D hierarchical flower-like motifs, (ii) unique necklace-like VS 4 nanorod−MWNT composites, and (iii) samples in which as-prepared VS 4 nanorods had been annealed. Moreover, we found that the rational application of specific physical and chemical processing treatments, such as (i) thermal annealing to improve crystallinity, (ii) the addition of MWNTs to form conductive composites, and (iii) the evolution of morphology from onedimensional (1D) nanorods to more complex 3D nanoflowers, was favorable to the resulting electrochemical performance with respect to increasing stability and reversibility.

Microwave-assisted synthesis of iron sulfide motifs for electrochemical applications

“…In particular, Fe 2+ peaks were observed at 709.5 and 723.2 eV for the 2p 3/2 and 2p 1/2 signals, respectively, whereas Fe 3+ manifested itself as peaks present at 712.0 and 725.7 eV for the 2p 3/2 and 2p 1/2 data, respectively. Furthermore, the C 1s region exhibited peaks, indicative of C-C and C-O bonds at 284.4 and 286.5 eV respectively, comparable to previously published literature on metal sulfide/MWNT composites (figure S8(C)) [43].…”

“…In particular, as mentioned, this material is limited by an undesirable volume expansion that occurs under realistic cycling conditions. Previously, in a similar study, our group mitigated for this effect through the microwave-assisted addition of conductive MWNTs in an effort to boost the intrinsic electrochemical behavior of vanadium sulfide motifs [43].…”

“…Therefore, by analogy, we hypothesized a similar outcome in terms of improving the reversibility and conductivity of our as-produced FeS 2 materials through a simple addition of conductive MWNTs into the reaction medium, prior to metal precursor and sulfur precursor addition. As with our previously published strategy on vanadium sulfides [43], an in-situ method was initially tried. In particular, we added in 1 mg of MWNTs to the mixed solvent solution, before placing in iron and sulfur-containing precursors.…”

“…To account in part for these issues, Fe 3 S 4 has previously been combined with reduced graphene oxide (rGO) with the resulting composite evincing good performance as an LIB anode [8]. By analogy, herein, our strategy of improving upon Fe 3 S 4 's intrinsic conductivity has been to mix these sulfides with either multi-walled carbon nanotube (MWNT) or graphene additives, an idea which has been earlier shown to not only increase stability but also enhance overall performance [36,43].…”

“…As precedence for this approach, our group has previously generated a monometallic sulfide-MWNT composite, incorporating VS 4 , using a microwave-assisted method. When tested within an LIB anode battery configuration, it was noted that the addition of MWNTs to the VS 4 nanorod motif led to both a greater conductivity and an improved stability of the as-prepared product [43]. To date, to the best of our knowledge, no previous motifs consisting of Fe 3 S 4 coupled with MWNTs within an LIB anode have been reported, whereas a prior study of a FeS 2 /MWNT composite emphasized its superior performance as compared with both FeS 2 /rGO and pristine FeS 2 counterparts [44].…”