8 research outputs found
Molecular Dynamics Study of Carbon Nanotubes/Polyamide Reverse Osmosis Membranes: Polymerization, Structure, and Hydration
Carbon
nanotubes/polyamide (PA) nanocomposite thin films have become very
attractive as reverse osmosis (RO) membranes. In this work, we used
molecular dynamics to simulate the influence of single walled carbon
nanotubes (SWCNTs) in the polyamide molecular structure as a model
case of a carbon nanotubes/polyamide nanocomposite RO membrane. It
was found that the addition of SWCNTs decreases the pore size of the
composite membrane and increases the Na and Cl ion rejection. Analysis
of the radial distribution function of water confined in the pores
of the membranes shows that SWCNT+PA nanocomposite membranes also
exhibit smaller clusters of water molecules within the membrane, thus
suggesting a dense membrane structure (SWCNT+PA composite membranes
were 3.9% denser than bare PA). The results provide new insights into
the fabrication of novel membranes reinforced with tubular structures
for enhanced desalination performance
Three-Dimensional Nitrogen-Doped Multiwall Carbon Nanotube Sponges with Tunable Properties
A three-dimensional (3D) nitrogen-doped
multiwall carbon nanotube (N-MWCNT) sponge possessing junctions induced
by both nitrogen and sulfur was synthesized by chemical vapor deposition
(CVD). The formation of āelbowā junctions as well as
āweldedā junctions, which are attributed to the synergistic
effect of the nitrogen dopant and the sulfur promoter, plays a critically
important role in the formation of 3D nanotube sponges. To the best
of our knowledge, this is the first report showing the synthesis of
macroscale 3D N-MWCNT sponges. Most importantly, the diameter of N-MWCNT
can be simply controlled by varying the concentration of sulfur, which
in turn controls both the spongeās mechanical and its electrical
properties. It was experimentally shown that, with increasing diameter
of N-MWCNT, the elastic modulus of the sponge increased while the
electrical conductivity decreased. The mechanical behaviors of the
sponges have also been quantitatively analyzed by employing strain
energy function modeling
Formation of Nitrogen-Doped Graphene Nanoribbons <i>via</i> Chemical Unzipping
In this work, we carried out chemical oxidation studies of nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNTs) using potassium permanganate in order to obtain nitrogen-doped graphene nanoribbons. Reaction parameters such as oxidation reaction, reaction time, the oxidizer to nanotube mass ratio, and the temperature were varied, and their effect was carefully analyzed. The presence of nitrogen atoms makes CNx-MWCNTs more reactive toward oxidation when compared to undoped multiwalled carbon nanotubes (MWCNTs). High-resolution transmission electron microscopy studies indicate that the oxidation of the graphitic layers within CNx-MWCNTs results in the unzipping of large diameter nanotubes and the formation of a disordered oxidized carbon coating on small diameter nanotubes. The nitrogen content within unzipped CNx-MWCNTs decreased as a function of the oxidation time, temperature, and oxidizer concentration. By controlling the degree of oxidation, the N atomic % could be reduced from 1.56% in pristine CNx-MWCNTs down to 0.31 atom % in nitrogen-doped oxidized graphene nanoribbons. A comparative thermogravimetric analysis reveals a lower thermal stability of the (unzipped) oxidized CNx-MWCNTs when compared to MWCNT samples. The oxidized graphene nanoribbons were chemically and thermally reduced and yielded nitrogen-doped graphene nanoribbons (N-GNRs). The thermal reduction at relatively low temperature (300 Ā°C) results in graphene nanoribbons with 0.37 atom % of nitrogen. This method represents a novel route to preparation of bulk quantities of nitrogen-doped unzipped carbon nanotubes, which is able to control the doping level in the resulting reduced GNR samples. Finally, the electrochemical properties of these materials were evaluated
Effective Antiscaling Performance of Reverse-Osmosis Membranes Made of Carbon Nanotubes and Polyamide Nanocomposites
The antiscaling properties of multiwalled
carbon nanotube (MWCNT)āpolyamide
(PA) nanocomposite reverse-osmosis (RO) desalination membranes (MWCNTāPA
membranes) were studied. An aqueous solution of calcium chloride (CaCl<sub>2</sub>) and sodium bicarbonate (NaHCO<sub>3</sub>) was used to precipitate
in situ calcium carbonate (CaCO<sub>3</sub>) to emulate scaling. The
MWCNT contents of the studied nanocomposite membranes prepared by
interfacial polymerization ranged from 0 wt % (plain PA) to 25 wt
%. The inorganic antiscaling performances were compared for the MWCNTāPA
membranes to laboratory-made plain and commercial PA-based RO membranes.
The scaling process on the membrane surface was monitored by fluorescence
microscopy after labeling the scale with a fluorescent dye. The deposited
scale on the MWCNTāPA membrane was less abundant and more easily
detached by the shear stress under cross-flow compared to other membranes.
Molecular dynamics simulations revealed that the attraction of Ca<sup>2+</sup> ions was hindered by the interfacial water layer formed
on the surface of the MWCNTāPA membrane. Together, our findings
revealed that the observed outstanding antiscaling performance of
MWCNTāPA membranes results from (i) a smooth surface morphology,
(ii) a low surface charge, and (iii) the formation of an interfacial
water layer. The MWCNTāPA membranes described herein are advantageous
for water treatment
Super-stretchable Graphene Oxide Macroscopic Fibers with Outstanding Knotability Fabricated by Dry Film Scrolling
Graphene oxide (GO) has recently become an attractive building block for fabricating graphene-based functional materials. GO films and fibers have been prepared mainly by vacuum filtration and wet spinning. These materials exhibit relatively high Youngās moduli but low toughness and a high tendency to tear or break. Here, we report an alternative method, using bar coating and drying of water/GO dispersions, for preparing large-area GO thin films (<i>e.g.</i>, 800ā1200 cm<sup>2</sup> or larger) with an outstanding mechanical behavior and excellent tear resistance. These dried films were subsequently scrolled to prepare GO fibers with extremely large elongation to fracture (up to 76%), high toughness (up to 17 J/m<sup>3</sup>), and attractive macroscopic properties, such as uniform circular cross section, smooth surface, and great knotability. This method is simple, and after thermal reduction of the GO material, it can render highly electrically conducting graphene-based fibers with values up to 416 S/cm at room temperature. In this context, GO fibers annealed at 2000 Ā°C were also successfully used as electron field emitters operating at low turn on voltages of <i>ca.</i> 0.48 V/Ī¼m and high current densities (5.3 A/cm<sup>2</sup>). Robust GO fibers and large-area films with fascinating architectures and outstanding mechanical and electrical properties were prepared with bar coating followed by dry film scrolling
Antiorganic Fouling and Low-Protein Adhesion on Reverse-Osmosis Membranes Made of Carbon Nanotubes and Polyamide Nanocomposite
We
demonstrate efficient antifouling and low protein adhesion of multiwalled
carbon nanotubes-polyamide nanocomposite (MWCNT-PA) reverse-osmosis
(RO) membranes by combining experimental and theoretical studies using
molecular dynamics (MD) simulations. Fluorescein isothiocyanate (FITC)-labeled
bovine serum albumin (FITC-BSA) was used for the fouling studies.
The fouling was observed in real time by using a crossflow system
coupled to a fluorescence microscope. Notably, it was observed that
BSA anchoring on the smooth MWCNT-PA membrane was considerably weaker
than that of other commercial/laboratory-made plain PA membranes.
The permeate flux reduction of the MWCNT-PA nanocomposite membranes
by the addition of FITC-BSA was 15% of its original value, whereas
those of laboratory-made plain PA and commercial membranes were much
larger at 34%ā50%. Computational MD simulations indicated that
the presence of MWCNT in PA results in weaker interactions between
the membrane surface and BSA molecule due to the formation of (i)
a stiffer PA structure resulting in lower conformity of the molecular
structure against BSA, (ii) a smoother surface morphology, and (iii)
an increased hydrophilicity involving the formation of an interfacial
water layer. These results are important for the design and development
of promising antiorganic fouling RO membranes for water treatment
Clean Nanotube Unzipping by Abrupt Thermal Expansion of Molecular Nitrogen: Graphene Nanoribbons with Atomically Smooth Edges
We report a novel physicochemical route to produce highly crystalline nitrogen-doped graphene nanoribbons. The technique consists of an abrupt N<sub>2</sub> gas expansion within the hollow core of nitrogen-doped multiwalled carbon nanotubes (CN<sub><i>x</i></sub>-MWNTs) when exposed to a fast thermal shock. The multiwalled nanotube unzipping mechanism is rationalized using molecular dynamics and density functional theory simulations, which highlight the importance of open-ended nanotubes in promoting the efficient introduction of N<sub>2</sub> molecules by capillary action within tubes and surface defects, thus triggering an efficient and atomically smooth unzipping. The so-produced nanoribbons could be few-layered (from graphene bilayer onward) and could exhibit both crystalline zigzag and armchair edges. In contrast to methods developed previously, our technique presents various advantages: (1) the tubes are not heavily oxidized; (2) the method yields sharp atomic edges within the resulting nanoribbons; (3) the technique could be scaled up for the bulk production of crystalline nanoribbons from available MWNT sources; and (4) this route could eventually be used to unzip other types of carbon nanotubes or intercalated layered materials such as BN, MoS<sub>2</sub>, WS<sub>2</sub>, <i>etc.</i
Clean Nanotube Unzipping by Abrupt Thermal Expansion of Molecular Nitrogen: Graphene Nanoribbons with Atomically Smooth Edges
We report a novel physicochemical route to produce highly crystalline nitrogen-doped graphene nanoribbons. The technique consists of an abrupt N<sub>2</sub> gas expansion within the hollow core of nitrogen-doped multiwalled carbon nanotubes (CN<sub><i>x</i></sub>-MWNTs) when exposed to a fast thermal shock. The multiwalled nanotube unzipping mechanism is rationalized using molecular dynamics and density functional theory simulations, which highlight the importance of open-ended nanotubes in promoting the efficient introduction of N<sub>2</sub> molecules by capillary action within tubes and surface defects, thus triggering an efficient and atomically smooth unzipping. The so-produced nanoribbons could be few-layered (from graphene bilayer onward) and could exhibit both crystalline zigzag and armchair edges. In contrast to methods developed previously, our technique presents various advantages: (1) the tubes are not heavily oxidized; (2) the method yields sharp atomic edges within the resulting nanoribbons; (3) the technique could be scaled up for the bulk production of crystalline nanoribbons from available MWNT sources; and (4) this route could eventually be used to unzip other types of carbon nanotubes or intercalated layered materials such as BN, MoS<sub>2</sub>, WS<sub>2</sub>, <i>etc.</i