12 research outputs found
New Basic Insight into Reductive Functionalization Sequences of Single Walled Carbon Nanotubes (SWCNTs)
The
reactivity of reduced single walled carbon nanotubes (SWCNTs)
(carbon nanotubides), prepared under strict inert conditions in a
glovebox with respect to the covalent functionalization with hexyl
iodide and subsequent exposure to ambient conditions (air, moisture),
was systematically investigated by Raman, absorption, fluorescence,
and IR spectroscopy as well as by TG/MS measurements. We have discovered
that the alkylation does not lead to a complete discharging of the
tubes since follow-up reactions with moisture still take place leading
to mixed functionalized carbon nanotube derivatives containing H-
and OH-addends (but no carboxylates) next to the hexyl groups. This
was confirmed by the exposure of carbon nanotubides to ambient conditions.
The degree of hexylation determined both under strict inert (ic) and
ambient (ac) conditions increases with an increasing K:C ratio of
the reduced SWCNT starting material. The presence of OH-groups covalently
attached to the nanotubes was also confirmed by postfunctionalization
reactions with 2-thiophenecarbonyl chloride, leading to the corresponding
esters. Control experiments with KO<sub>2</sub> give rise to the formation
of the same oxygen functionalities. These combined findings allowed
for the suggestions of a plausible reaction mechanism, describing
all the observed reactions on the SWCNTs side walls. The amount of
subsequent side reactions after the treatment of reduced SWCNTs with
electrophiles is strongly influenced by the reduction potential of
the electrophile, which is responsible for the extent of reoxidation.
Incomplete quenching of negative charges allows stronger oxidants/electrophile
(e.g., O<sub>2</sub>) to perform follow-up reactions
Polyhydrogenated Graphene: Excited State Dynamics in Photo- and Electroactive Two-Dimensional Domains
Understanding
the phenomenon of intense photoluminescence in carbon
materials such as hydrogenated graphene, graphene nanoribbons, and
so forth is at the forefront of investigations. In this study, six
different types of hydrogenated graphene (phG) produced from different
starting materials were fully characterized in terms of structure
and optical spectroscopy. Comprehensive photoluminescence lifetime
analyses of phGs were conducted by combining time-correlated single-photon
counting with steady-state fluorescence spectroscopy and femtosecond
transient absorption spectroscopy. The conclusion drawn from these
assays is that graphene islands with diameters in the range from 1.1
to 1.75 nm reveal band gap photoluminescence between 450 and 800 nm.
As a complement, phGs were implemented in hybrids with water-soluble
electron accepting perylenediimides (PDIs). By virtue of mutual Ď-stacking
and charge transfer interactions with graphene islands, PDIs assisted
in stabilizing aqueous dispersion of phG. Implicit in these ground
state interactions is the formation of 300 ps lived charge separated
states once photoexcited
Topology-Driven Reductive Silylation of Synthetic Carbon Allotropes
Herein, the combined application
of characterization tools, such as Raman spectroscopy, thermal gravimetric
analysis coupled with mass spectrometry, and optical and atomic force
microscopy, confirms the reductive silylation of synthetic carbon
allotropes as a new covalent functionalization strategy for the formation
of heteroatomâcarbon bonds. In particular, our study gives
interesting insights into the topology-driven retrofunctionalization
of nanotubide and graphenide derivatives
Scanning-Raman-Microscopy for the Statistical Analysis of Covalently Functionalized Graphene
We report on the introduction of a systematic method for the quantitative and reliable characterization of covalently functionalized graphene based on Scanning-Raman-Microscopy (SRM). This allows for recording and analyzing several thousands of Raman spectra per sample and straightforward display of various Raman properties and their correlations with each other in histograms or coded 2D-plots. In this way, information about the functionalization efficiency of a given reaction, the reproducibility of the statistical analysis, and the sample homogeneity can be easily deduced. Based on geometric considerations, we were also able to provide, for the first time, a correlation between the mean defect distance of densely packed point defects and the Raman <i>I</i><sub>D</sub>/<i>I</i><sub>G</sub> ratio directly obtained from the statistical analysis. This proved to be the prerequisite for determining the degree of functionalization, termed θ. As model compounds, we have studied a series of arylated graphenes (GPh) for which we have developed new synthetic procedures. Both graphite and graphene grown by chemical vapor deposition (CVD) were used as starting materials. The best route toward GPh consisted of the initial reduction of graphite with a Na/K alloy in 1,2-dimethoxyethane (DME) as it yields the highest overall homogeneity of products reflected in the widths of the Raman <i>I</i><sub>D</sub>/<i>I</i><sub>G</sub> histograms. The Raman results correlate nicely with parallel thermogravimetric analysis (TGA) coupled with mass spectrometry (MS) studies
Mapping Charge Transport by Electroluminescence in Chirality-Selected Carbon Nanotube Networks
We demonstrate random network single-walled carbon nanotube (SWNT) field-effect transistors (FETs) in bottom contact/top gate geometry with only five different semiconducting nanotube species that were selected by dispersion with poly(9,9-dioctylÂfluorene) in toluene. These FETs are highly ambipolar with balanced hole and electron mobilities and emit near-infrared light with narrow peak widths (<40 meV) and good efficiency. We spatially resolve the electroluminescence from the channel region during a gate voltage sweep and can thus trace charge transport paths through the SWNT thin film. A shift of emission intensity to large diameter nanotubes and gate-voltage-dependent photoluminescence quenching of the different nanotube species indicates excitation transfer within the network and preferential charge accumulation on small band gap nanotubes. Apart from applications as near-infrared emitters with selectable emission wavelengths and narrow line widths, these devices will help to understand and model charge transport in realistic carbon nanotube networks
Effect of Polymer Molecular Weight and Solution Parameters on Selective Dispersion of Single-Walled Carbon Nanotubes
The selective dispersion of single-walled carbon nanotube
species
(n,m) with conjugated polymers such as polyÂ(9,9-dioctylfluorene) (PFO)
and polyÂ(9,9-dioctylfluorene-<i>co</i>-benzothiadiazole)
(F8BT) in organic solvents depends not only on the type of solvent
but also on the molecular weight of the polymer. We find an increasing
amount of nanotubes and altered selectivities for dispersions with
higher molecular weight polymers. Including the effects of different
aromatic solvents, we propose that solution viscosity is one of the
factors influencing the apparent selectivity by changing the reaggregation
rate of the single-walled carbon nanotubes (SWNT). The type of solvent,
polymer molecular weight, concentration, and viscosity should thus
be taken into account when screening for new polymers for selective
SWNT dispersion
Direct Covalent Coupling of Porphyrins to Graphene
Grapheneâporphyrin
nanohybrid materials with a direct covalent
linkage between the graphene carbon network and the functional porphyrin
unit have been successfully synthesized via a one-pot reductive diazotation
approach. A graphiteâpotassium intercalation compound (KC<sub>8</sub>) was dispersed in THF, and different isolated porphyrinâdiazonium
salts were added. The direct covalent binding and the detailed characterization
of the functional hybrid material were carried out by Raman spectroscopy,
TG-MS, UV/vis, and fluorescence spectroscopy. LDI-ToF mass spectrometry
was introduced as a new versatile and sensitive tool to investigate
covalently functionalized graphene derivatives and to establish the
composition of the respective nanohybrid materials
Unifying Principles of the Reductive Covalent Graphene Functionalization
Covalently functionalized
graphene derivatives were synthesized
via benchmark reductive routes using graphite intercalation compounds
(GICs), in particular KC<sub>8</sub>. We have compared the graphene
arylation and alkylation of the GIC using 4-<i>tert</i>-butylphenyldiazonium
and bisÂ(4-(<i>tert</i>-butyl)Âphenyl)Âiodonium salts, as well
as phenyl iodide, <i>n</i>-hexyl iodide, and <i>n</i>-dodecyl iodide, as electrophiles in model reactions. We have put
a particular focus on the evaluation of the degree of addition and
the bulk functionalization homogeneity (<i>H</i><sub>bulk</sub>). For this purpose, we have employed statistical Raman spectroscopy
(SRS), and a forefront characterization tool using thermogravimetric
analysis coupled with FT-IR, gas chromatography, and mass spectrometry
(TGA/FT-IR/GC/MS). The present study unambiguously shows that the
graphene functionalization using alkyl iodides leads to the best results,
in terms of both the degree of addition and the <i>H</i><sub>bulk</sub>. Moreover, we have identified the reversible character
of the covalent addition chemistry, even at temperatures below 200
°C. The thermally induced addend cleavage proceeds homolytically,
which allows for the detection of dimeric cleavage products by TGA/FT-IR/GC/MS.
This dimerization points to a certain degree of regioselectivity,
leading to a low sheet homogeneity (<i>H</i><sub>sheet</sub>). Finally, we developed this concept by performing the reductive
alkylation reaction in monolayer CVD graphene films. This work provides
important insights into the understanding of basic principles of reductive
graphene functionalization and will serve as a guide in the design
of new graphene functionalization concepts
Fundamental Insights into the Reductive Covalent Cross-Linking of Single-Walled Carbon Nanotubes
Single-walled
carbon nanotubes (SWCNT) have been covalently cross-linked
via a reductive functionalization pathway, utilizing negatively charged
carbon nanotubides (KC<sub>4</sub>). We have compared the use of difunctional
linkers acting as molecular pillars between the nanotubes, namely, <i>p</i>-diiodobenzene, <i>p</i>-diiodobiphenyl, benzene-4,4â˛-bisÂ(diazonium),
and 1,1â˛-biphenyl-4,4â˛-bisÂ(diazonium) salts as electrophiles.
We have employed statistical Raman spectroscopy (SRS), a forefront
characterization tool consisting of thermogravimetric analysis coupled
with gas chromatography and mass spectrometry (TG-GC-MS) and aberration-corrected
high-resolution transmission electron microscopy imaging series at
80 kV to unambiguously demonstrate the covalent binding of the molecular
linkers. The present study shows that the SWCNT functionalization
using iodide derivatives leads to the best results in terms of bulk
functionalization homogeneity (<i>H</i><sub>bulk</sub>)
and degree of addition. Phenylene linkers yield the highest degree
of functionalization, whereas biphenylene units induce a higher surface
area with an increase in the thermal stability and an improved electrochemical
performance in the oxygen reduction reaction (ORR). This work illustrates
the importance of molecular engineering in the design of novel functional
materials and provides important insights into the understanding of
basic principles of reductive cross-linking of carbon nanotubes
Fundamental Insights into the Reductive Covalent Cross-Linking of Single-Walled Carbon Nanotubes
Single-walled
carbon nanotubes (SWCNT) have been covalently cross-linked
via a reductive functionalization pathway, utilizing negatively charged
carbon nanotubides (KC<sub>4</sub>). We have compared the use of difunctional
linkers acting as molecular pillars between the nanotubes, namely, <i>p</i>-diiodobenzene, <i>p</i>-diiodobiphenyl, benzene-4,4â˛-bisÂ(diazonium),
and 1,1â˛-biphenyl-4,4â˛-bisÂ(diazonium) salts as electrophiles.
We have employed statistical Raman spectroscopy (SRS), a forefront
characterization tool consisting of thermogravimetric analysis coupled
with gas chromatography and mass spectrometry (TG-GC-MS) and aberration-corrected
high-resolution transmission electron microscopy imaging series at
80 kV to unambiguously demonstrate the covalent binding of the molecular
linkers. The present study shows that the SWCNT functionalization
using iodide derivatives leads to the best results in terms of bulk
functionalization homogeneity (<i>H</i><sub>bulk</sub>)
and degree of addition. Phenylene linkers yield the highest degree
of functionalization, whereas biphenylene units induce a higher surface
area with an increase in the thermal stability and an improved electrochemical
performance in the oxygen reduction reaction (ORR). This work illustrates
the importance of molecular engineering in the design of novel functional
materials and provides important insights into the understanding of
basic principles of reductive cross-linking of carbon nanotubes