4 research outputs found
A “Nanopore Lithography” Strategy for Synthesizing Hierarchically Micro/Mesoporous Carbons from ZIF-8/Graphene Oxide Hybrids for Electrochemical Energy Storage
Porous carbons derived from
metal-organic frameworks (MOFs) are promising materials for a number
of energy- and environment-related applications, but their almost
exclusively microporous texture can be an obstacle to their performance
in practical uses. Here, we introduce a novel strategy for the generation
of very uniform mesoporosity in a prototypical MOF, namely, zeolitic
imidazolate framework-8 (ZIF-8). The process, referred to as “nanopore
lithography”, makes use of graphene oxide (GO) nanosheets enclosing
ZIF-8 particles as masks or templates for the transfer of mesoporous
texture to the latter. Upon controlled carbonization and activation,
nanopores created in the GO envelope serve as selective entry points
for localized etching of carbonized ZIF-8, so that such nanopores
are replicated in the MOF-derived carbonaceous structure. The resulting
porous carbons are dominated by uniform mesopores ∼3–4
nm in width and possess specific surface areas of ∼1300–1400
m<sup>2</sup> g<sup>–1</sup>. Furthermore, we investigate and
discuss the specific experimental conditions that afford the mesopore-templating
action of the GO nanosheets. Electrochemical characterization revealed
an improved capacitance as well as a faster, more reversible charge/discharge
kinetics for the ZIF-8-derived porous carbons obtained through nanopore
lithography, relative to those for their counterparts with standard
activation (no GO templating), thus indicating the potential practical
advantage of the present approach in capacitive energy storage applications
Investigating the Dispersion Behavior in Solvents, Biocompatibility, and Use as Support for Highly Efficient Metal Catalysts of Exfoliated Graphitic Carbon Nitride
The
liquid-phase exfoliation of graphitic carbon nitride (g-C<sub>3</sub>N<sub>4</sub>) to afford colloidal dispersions of two-dimensional
flakes constitutes an attractive route to facilitate the processing
and implementation of this novel material toward different technological
applications, but quantitative knowledge about its dispersibility
in solvents is lacking. Here, we investigate the dispersion behavior
of exfoliated g-C<sub>3</sub>N<sub>4</sub> in a wide range of solvents
and evaluate the obtained results on the basis of solvent surface
energy and Hildebrand/Hansen solubility parameters. Estimates of the
three Hansen parameters for exfoliated g-C<sub>3</sub>N<sub>4</sub> from the experimentally derived data yielded δ<sub>D</sub> ≈ 17.8 MPa<sup>1/2</sup>, δ<sub>P</sub> ≈ 10.8
MPa<sup>1/2</sup>, and δ<sub>H</sub> ≈ 15.4 MPa<sup>1/2</sup>. The relatively high δ<sub>H</sub> value suggested that, contrary
to the case of other two-dimensional materials (e.g., graphene or
transition metal dichalcogenides), hydrogen-bonding plays a substantial
role in the efficient interaction, and thus dispersibility, of exfoliated
g-C<sub>3</sub>N<sub>4</sub> with solvents. Such an outcome was attributed
to a high density of primary and/or secondary amines in the material,
the presence of which was associated with incomplete condensation
of the structure. Furthermore, cell proliferation tests carried out
on thin films of exfoliated g-C<sub>3</sub>N<sub>4</sub> using murine
fibroblasts suggested that this material is highly biocompatible and
noncytotoxic. Finally, the exfoliated g-C<sub>3</sub>N<sub>4</sub> flakes were used as supports in the synthesis of Pd nanoparticles,
and the resulting hybrids exhibited an exceptional catalytic activity
in the reduction of nitroarenes
Achieving Extremely Concentrated Aqueous Dispersions of Graphene Flakes and Catalytically Efficient Graphene-Metal Nanoparticle Hybrids with Flavin Mononucleotide as a High-Performance Stabilizer
The
stable dispersion of graphene flakes in an aqueous medium is highly
desirable for the development of materials based on this two-dimensional
carbon structure, but current production protocols that make use of
a number of surfactants typically suffer from limitations regarding
graphene concentration or the amount of surfactant required to colloidally
stabilize the sheets. Here, we demonstrate that an innocuous and readily
available derivative of vitamin B<sub>2</sub>, namely the sodium salt
of flavin mononucleotide (FMNS), is a highly efficient dispersant
in the preparation of aqueous dispersions of defect-free, few-layer
graphene flakes. Most notably, graphene concentrations in water as
high as ∼50 mg mL<sup>–1</sup> using low amounts of
FMNS (FMNS/graphene mass ratios of about 0.04) could be attained,
which facilitated the formation of free-standing graphene films displaying
high electrical conductivity (∼52000 S m<sup>–1</sup>) without the need of carrying out thermal annealing or other types
of post-treatment. The excellent performance of FMNS as a graphene
dispersant could be attributed to the combined effect of strong adsorption
on the sheets through the isoalloxazine moiety of the molecule and
efficient colloidal stabilization provided by its negatively charged
phosphate group. The FMNS-stabilized graphene sheets could be decorated
with nanoparticles of several noble metals (Ag, Pd, and Pt), and the
resulting hybrids exhibited a high catalytic activity in the reduction
of nitroarenes and electroreduction of oxygen. Overall, the present
results should expedite the processing and implementation of graphene
in, e.g., conductive inks, composites, and hybrid materials with practical
utility in a wide range of applications
Electrochemical Exfoliation of Graphite in Aqueous Sodium Halide Electrolytes toward Low Oxygen Content Graphene for Energy and Environmental Applications
Graphene
and graphene-based
materials have shown great promise in many technological applications,
but their large-scale production and processing by simple and cost-effective
means still constitute significant issues in the path of their widespread
implementation. Here, we investigate a straightforward method for
the preparation of a ready-to-use and low oxygen content graphene
material that is based on electrochemical (anodic) delamination of
graphite in aqueous medium with sodium halides as the electrolyte.
Contrary to previous conflicting reports on the ability of halide
anions to act as efficient exfoliating electrolytes in electrochemical
graphene exfoliation, we show that proper choice of both graphite
electrode (e.g., graphite foil) and sodium halide concentration readily
leads to the generation of large quantities of single-/few-layer graphene
nanosheets possessing a degree of oxidation (O/C ratio down to ∼0.06)
lower than that typical of anodically exfoliated graphenes obtained
with commonly used electrolytes. The halide anions are thought to
play a role in mitigating the oxidation of the graphene lattice during
exfoliation, which is also discussed and rationalized. The as-exfoliated
graphene materials exhibited a three-dimensional morphology that was
suitable for their practical use without the need to resort to any
kind of postproduction processing. When tested as dye adsorbents,
they outperformed many previously reported graphene-based materials
(e.g., they adsorbed ∼920 mg g<sup>–1</sup> for methyl
orange) and were useful sorbents for oils and nonpolar organic solvents.
Supercapacitor cells assembled directly from the as-exfoliated products
delivered energy and power density values (up to 15.3 Wh kg<sup>–1</sup> and 3220 W kg<sup>–1</sup>, respectively) competitive with
those of many other graphene-based devices but with the additional
advantage of extreme simplicity of preparation