Cation-Doped Nanocarbons for Enhanced Perfluoroalkyl Substance Removal: Exotic Bottom-Up Solution Plasma Synthesis and Characterization.

Perfluorinated alkyl substances (PFAS), such as perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), are pervasive organic contaminants that are widespread in aquatic environments, posing significant health risks to humans and wildlife. Due to their persistent nature, urgent removal is necessary. Conventional adsorbents are inefficient at removing PFOS and PFOA, highlighting the need for alternative materials. Herein, we present a synthetic method for quaternary ammonium cation-doped carbon nanoparticles (QACNs) using a solution plasma process for the efficient removal of PFOS and PFOA. QACN is formed simultaneously through a one-step discharge of nonequilibrium plasma at the interface of benzene and pyridinium chloride. The resulting material exhibited a high surface electrical charge and enhanced hydrophilicity as well as an amorphous structure of a nonporous nature, involving nanoparticles with an undefined shape. The obtained adsorbent demonstrated high adsorption efficiency and stability, adsorbing 998.45 and 889.37 mg g-1 of PFOS and PFOA, respectively, exceeding the efficiencies of conventional carbon-based adsorbents (80.89-313.15 mg g-1). The adsorption performance was dependent on the adsorbent dosage, pH of the solution, and the coexisting ionic species. Adsorption studies, including adsorption kinetics, isotherms, and thermodynamics, revealed that PFOS and PFOA were chemisorbed to the QACN surface, forming multilayers endothermically and spontaneously. Experimental and computational analyses revealed that adsorption primarily occurs via electronic interactions between the PFAS active sites and the quaternary ammonium group in the carbon framework. The slightly lower adsorption potential of the PFOS and PFOA fluorocarbon chains on the adsorbent was elucidated. Furthermore, the dispersibility of the adsorbent in solution significantly affected the adsorption performance. These findings highlight the potential of the novel synthetic method proposed in this study, offering a pathway for the development of highly effective carbon adsorbents for environmental remediation.

Molecular dynamics of liquid-electrode interface by integrating Coulomb interaction into universal neural network potential.

Computational understanding of the liquid-electrode interface faces challenges in efficiently incorporating reactive force fields and electrostatic potentials within reasonable computational costs. Although universal neural network potentials (UNNPs), representing pretrained machine learning interatomic potentials, are emerging, current UNNP models lack explicit treatment of Coulomb potentials, and methods for integrating additional charges on the electrode remain to be established. We propose a method to analyze liquid-electrode interfaces by integrating a UNNP, known as the preferred potential, with Coulomb potentials using the ONIOM method. This approach extends the applicability of UNNPs to electrode-liquid interface systems. Through molecular dynamics simulations of graphene-water and graphene oxide (GO)-water interfaces, we demonstrate the effectiveness of our method. Our findings emphasize the necessity of incorporating long-range Coulomb potentials into the water potential to accurately describe water polarization at the interface. Furthermore, we observe that functional groups on the GO electrode influence both polarization and capacitance.

One-dimensional van der Waals heterostructures: Growth mechanism and handedness correlation revealed by nondestructive TEM.

We recently synthesized one-dimensional (1D) van der Waals heterostructures in which different atomic layers (e.g., boron nitride or molybdenum disulfide) seamlessly wrap around a single-walled carbon nanotube (SWCNT) and form a coaxial, crystalized heteronanotube. The growth process of 1D heterostructure is unconventional-different crystals need to nucleate on a highly curved surface and extend nanotubes shell by shell-so understanding the formation mechanism is of fundamental research interest. In this work, we perform a follow-up and comprehensive study on the structural details and formation mechanism of chemical vapor deposition (CVD)-synthesized 1D heterostructures. Edge structures, nucleation sites, and crystal epitaxial relationships are clearly revealed using transmission electron microscopy (TEM). This is achieved by the direct synthesis of heteronanotubes on a CVD-compatible Si/SiO2 TEM grid, which enabled a transfer-free and nondestructive access to many intrinsic structural details. In particular, we have distinguished different-shaped boron nitride nanotube (BNNT) edges, which are confirmed by electron diffraction at the same location to be strictly associated with its own chiral angle and polarity. We also demonstrate the importance of surface cleanness and isolation for the formation of perfect 1D heterostructures. Furthermore, we elucidate the handedness correlation between the SWCNT template and BNNT crystals. This work not only provides an in-depth understanding of this 1D heterostructure material group but also, in a more general perspective, serves as an interesting investigation on crystal growth on highly curved (radius of a couple of nanometers) atomic substrates.

Photoluminescence from Single-Walled MoS2 Nanotubes Coaxially Grown on Boron Nitride Nanotubes.

Single-walled and multiwalled molybdenum disulfide (MoS2) nanotubes have been coaxially synthesized on small-diameter boron nitride nanotubes (BNNTs) that are obtained from removing single-walled carbon nanotubes (SWCNTs) in heteronanotubes of SWCNTs coated by BNNTs. The photoluminescence (PL) from single-walled MoS2 nanotubes supported by core BNNTs is observed in this work, which evidences the direct bandgap structure of single-walled MoS2 nanotubes with a diameter around 6-7 nm. The observation is consistent with our DFT results that the single-walled MoS2 nanotube changes from an indirect-gap to a direct-gap semiconductor when the diameter of a nanotube is more than around 5.2 nm. On the other hand, when there are SWCNTs inside the heteronanotubes of BNNTs and single-walled MoS2 nanotubes, the PL signal from MoS2 nanotubes is considerably quenched. The charge transfer and energy transfer between SWCNTs and single-walled MoS2 nanotubes were examined through characterizations by PL, X-ray photoelectron spectroscopy, and Raman spectroscopy. Moreover, the PL signal from multiwalled MoS2 nanotubes is significantly quenched. Single-walled and multiwalled MoS2 nanotubes exhibit different Raman features in both resonant and nonresonant Raman spectra.

One-dimensional van der Waals heterostructures.

We present the experimental synthesis of one-dimensional (1D) van der Waals heterostructures, a class of materials where different atomic layers are coaxially stacked. We demonstrate the growth of single-crystal layers of hexagonal boron nitride (BN) and molybdenum disulfide (MoS2) crystals on single-walled carbon nanotubes (SWCNTs). For the latter, larger-diameter nanotubes that overcome strain effect were more readily synthesized. We also report a 5-nanometer-diameter heterostructure consisting of an inner SWCNT, a middle three-layer BN nanotube, and an outer MoS2 nanotube. Electron diffraction verifies that all shells in the heterostructures are single crystals. This work suggests that all of the materials in the current 2D library could be rolled into their 1D counterparts and a plethora of function-designable 1D heterostructures could be realized.

Molecular Dynamics of Chirality Definable Growth of Single-Walled Carbon Nanotubes.

In order to achieve the chirality-specific growth of single-walled carbon nanotubes (SWCNTs), it is crucial to understand the growth mechanism. Even though many molecular dynamics (MD) simulations have been employed to analyze the SWCNT growth mechanism, it has been difficult to discuss the chirality determining kinetics because of the defects remaining on the SWCNTs grown in simulations. In this study, we demonstrate MD simulations of defect-free SWCNTs, that is, chirality definable SWCNTs, under the optimized carbon supply rate and temperature. The chiralities of the SWCNTs were assigned as (14,1), (15,2), and (9,0), indicating the preference of near-zigzag and pure-zigzag SWCNTs. The SWCNTs contained at least one complete row of defect-free walls consisting of only hexagons. The near-zigzag SWCNTs grew via a kink-running process, in which bond formation between a carbon atom at a kink and a neighboring carbon chain led to formation of a hexagon with a new kink at the SWCNT edge. Defects including pentagons and heptagons were sometimes formed but effectively healed into hexagons on metal surfaces. The pure-zigzag SWCNTs grew by the kink-running and the hexagon nucleation processes. In addition, chirality change events along SWCNTs with incorporation of pentagon-heptagon pair defects were observed in the MD simulations. Here, pentagons and heptagons were frequently observed as adjacent pairs, resulting in ( n, m) chirality changes by (±1,0), (0,±1), (1,-1), or (-1,1).