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New Insights into the Structure of Nanoporous
Carbons from NMR, Raman, and Pair
Distribution Function Analysis
Alexander C. Forse
a
, Céline Merlet
a
, Phoebe K. Allan
a,c
, Elizabeth K. Humphreys
a
, John M. Griffin
a
, Mesut
Aslan
d
, Marco Zeiger
d,e
, Volker Presser
d,e
, Yury Gogotsi
f
, and Clare P. Grey
a,b*
a
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.
b
Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA.
c
Gonville and Caius College, Trinity Street, Cambridge, CB2 1TA, UK.
d
INM - Leibniz-Institute for New Materials, 66123 Saarbrücken, Germany.
e
Department of Materials Science and Engineering, Saarland University, 66123 Saarbrücken, Germany.
f
Department of Materials Science and Engineering and A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, PA
19104, USA.
*Corresponding author. Email: cpg27@cam.ac.uk
Abstract
The structural characterisation of nanoporous carbons is a challenging task as they generally lack
long-range order and can exhibit diverse local structures. Such characterisation represents an
important step towards understanding and improving the properties and functionality of porous
carbons, yet few experimental techniques have been developed for this purpose. Here we demonstrate
the application of nuclear magnetic resonance (NMR) spectroscopy and pair distribution function
(PDF) analysis as new tools to probe the local structures of porous carbons, alongside more
conventional Raman spectroscopy. Together, the PDFs and the Raman spectra allow the local
chemical bonding to be probed, with the bonding becoming more ordered for carbide-derived
carbons (CDCs) synthesised at higher temperatures. The ring currents induced in the NMR
experiment (and thus the observed NMR chemical shifts for adsorbed species) are strongly dependent
on the size of the aromatic carbon domains. We exploit this property and use computer simulations to
show that the carbon domain size increases with the temperature used in the carbon synthesis. The
techniques developed here are applicable to a wide range of porous carbons, and offer new insights
into the structures of CDCs (conventional and vacuum-annealed) and coconut shell-derived activated
carbons.
Introduction
Nanoporous carbons are an important class of materials used in a range of applications including
capacitive energy storage, gas storage, water treatment, and catalysis.
1–3
In each case, the
nanoporosity and high specific surface areas (typically > 1500 m
2
g
-1
), achieved by activating
carbonaceous precursors, are exploited to store molecules or ions. In principle, carbon structures can
be engineered for a given application, though characterisation of the highly disordered structures
poses a significant challenge. The challenges in determining local- and long-range structure make it
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extremely difficult to establish structure-function correlations beyond those simply derived from
surface area and pore-size distributions.
The structures of carbon materials
4,5
have been actively researched since the pioneering X-ray
diffraction studies of Franklin.
6,7
She distinguished between graphitizing and non-graphitizing
carbons, the former transforming into graphite upon heating to high temperature, and the latter
showing no such transformation at temperatures as high as 3000 °C.
7
For nanoporous carbons,
analysis of the broad Bragg peaks is generally of limited use due to the long range disordered
structures of these materials. However, inclusion of the diffuse scattering in the analysis allows the
extraction of a pair distribution function (PDF), which is a weighted histogram of atom-to-atom
distances showing the likelihood of finding an atom-pair separated by a certain distance.
8
PDF
studies show that porous carbons often exhibit a high degree of local ordering, with a propensity for
hexagonal carbon rings in which the carbon atoms are sp
2
hybridised.
9–11
Correlations in the PDFs
typically extend over tens of Angstroms, suggesting that there is local order on this length scale.
10
Transmission electron microscopy (TEM) images corroborate these concepts and generally show
curved carbon sheets arranged in a disordered fashion.
9,12–14
The sheet curvature is thought to arise
from the presence of non-hexagonal carbon rings, observed in experimental
13
and modelling
studies.
15–17
Nuclear magnetic resonance (NMR) spectroscopy of adsorbate molecules is emerging as an advanced
method to characterise the structure of carbon nanomaterials,
18–24
as well as to characterise gas
storage
18,25,26
and energy storage
27–31
systems in situ. In NMR spectroscopy, molecules or ions
adsorbed inside carbon nanopores give rise to a spectral feature that is distinct from that of non-
adsorbed ones. This arises as the delocalized carbon π electrons circulate in the presence of an
applied magnetic field, inducing a local magnetic field that shields nearby nuclei (referred to as a ring
current effect). The chemical shifts observed for different adsorbed species are generally very similar
for a given carbon,
23,32–34
suggesting that the ring current shift is nucleus-independent to a first
approximation. This has motivated the use of nucleus-independent chemical shift (NICS) calculations
to rationalise the chemical shifts observed for adsorbates.
20,22,23,35,36
When different carbon structures
are studied experimentally, the chemical shifts observed for a given adsorbed molecule can vary
dramatically.
19,20,22,32
Our recent NICS calculations on model carbon fragments, in combination with
lattice-simulations, suggested that the carbon pore size and the size of the hexagonally bonded carbon
fragments in which ring currents are present can each have significant effects on the shifts observed
for adsorbed species.
20,24
Today, nanoporous carbons are most commonly characterised by gas sorption experiments,
37
which
allow a determination of the carbon pore size distribution, and Raman spectroscopy, which provides
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a measure of disorder in the carbon bonding network.
38–40
Based on the large Raman cross-section of
sp
2
-hybridized carbon, structural analysis of small amounts of material is possible. Incompletely
graphitized carbons show two prominent spectral features: the G-mode between 1580 and 1590 cm
–1
indicative of sp
2
-hybridized carbon (bond stretching of sp
2
-hybridized carbon atoms in rings and
chains coming from the zone centre E
2g
mode) and the D-mode between 1330 and 1342 cm
–1
which
represents the breathing mode of the six-fold carbon rings, but is only Raman active in the presence
of defects, such as edges or vacancies.
39
While the D/G peak area ratio can be used as a quantitative
measure for the crystallite size in nano-crystalline graphites,
41,42
for disordered porous carbons, the
D- and G-band widths are commonly used as a measure of disorder in the carbon-carbon bonding
network.
12,43,44
Given the complexity and variety of nanoporous carbon structures, there is considerable scope for the
development of characterisation tools that can provide more accurate and complete structural
information. Here, we investigate the structures of a series of titanium carbide-derived carbons (TiC-
CDCs) with a range of methods to compare and contrast the information that can be obtained. These
materials were chosen for study because it is possible to control the porosity and degree of ordering
by varying the synthesis temperature,
12
and also by introducing additional vacuum annealing steps.
14
Pair distribution function analysis offers a powerful probe of local chemical structure, providing
information which is complementary to that obtained from Raman analysis. These experiments show
that the chemical bonding of TiC-CDCs becomes more uniform for materials synthesised at higher
temperatures. The chemical shifts observed for adsorbates in NMR are shown to be particularly
sensitive to the sizes of the carbon domains over which ring currents are present, and therefore offer a
further probe of the local carbon structure. With the NMR approach, variations of the carbon pore
size must be taken into account when interpreting the data. We demonstrate a convenient way to do
this using a lattice simulation method that allows the size of the carbon domains to be estimated. The
carbon domains in which ring currents are present are shown to increase in size with the maximum
temperature used in the carbon synthesis. The application of the approach to commercial activated
carbons is then demonstrated.
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Results and Discussion
1. Probing the structure of TiC-CDCs
Figure 1 Characterisation of TiC-CDC samples. a)
19
F MAS NMR (7.1 T) spectra of TiC-CDCs soaked with
NEt
4
BF
4
/dACN (1.5 M) electrolyte. An example Δδ value is shown for TiC-CDC-1000. The MAS rate was 5 kHz, and
spinning sidebands are marked by asterisks. The spectrum of neat electrolyte is shown for comparison. b) Pore size
distributions measured by N
2
gas sorption (see Supporting Information for the sorption isotherms). c) Raman spectra,
with the deconvoluted full-width at half-maximum intensity (FWHM) values for the D-band shown in the inset. Full
details of the fitted parameters are given in the Supporting Information. d) X-ray PDFs. The inset shows the region
between 1 and 6 Å with the assignments to various C-C correlations indicated.
A range of experiments were carried out to characterize the structures of a series of TiC-CDCs,
prepared by etching of titanium carbide in chlorine at different temperatures (a sample synthesised at
X °C is referred to as TiC-CDC-X, e.g., TiC-CDC-600).
19
F magic angle spinning (MAS) NMR
spectra of TiC-CDCs soaked with tetraethylammonium tetrafluoroborate (NEt
4
BF
4
) in deuterated
acetonitrile (D
3
CCN, referred to as dACN here) (1.5 M) are shown in Figure 1a. In each case, a
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resonance arising from ions nearby carbon surfaces inside carbon nanopores (referred to as “in-pore”)
is observed, as well as an “ex-pore” resonance arising from ions in large spaces between the carbon
particles (see introduction). Here, we define the Δδ value as the chemical shift difference between the
in-pore and the neat electrolyte resonances (see Figure 1a). Measured Δδ values are –2.4, –3.9 and –
5.5 ppm for TiC-CDC-600, -800 and -1000, respectively, the magnitude of the Δδ value giving a
measure of the strength of the ring currents for each carbon. Our choice to report
19
F Δδ values for
BF
4
–
is somewhat arbitrary, and values from
1
H NMR spectra of the NEt
4
+
cations reveal similar
results, with the Δδ values being nucleus-independent to a first approximation (see Supporting
Information). We note that the in-pore line widths also show changes with synthesis temperature,
which likely arise from a combination of changes of in-pore ionic diffusion rates, and differences in
the profile of adsorption sites (with different distributions of ring current shifts) between the
carbons.
45
Here, we focus our discussion on the Δδ values, which offer information about the carbon structures.
Our previous NICS calculations for a range of model carbon fragments showed that carbons with
smaller pores give rise to Δδ values of greater magnitude (as the ring current effects from each pore
wall are additive) while there is also an effect from carbon ordering, whereby carbons with larger
domain sizes (domains of hexagonally bonded carbon in which ring currents are established) also
give rise to Δδ values of greater magnitude.
20,24
In the spectra presented here, the Δδ values increase
in magnitude as the CDC synthesis temperature is increased, as in previous work.
19,20
This increase
occurs despite small increases in the carbon pore size, with average pore size values of 8.2, 9.1, and
9.3 Å measured by N
2
sorption for TiC-CDC-600, -800, and -1000, respectively (Figure 1b). Instead,
this suggests that increases in the sizes of the carbon domains over which the ring currents act
dominate the variation of the Δδ values.
Raman spectra of these carbons (Figure 1c) show D- and G-bands typical for disordered porous
carbons. As observed previously,
12,43,44
a decrease of the D-band (a feature associated with defects or
disorder in the carbon sheets
1,39
) full-width at half-maximum intensity (FWHM) is observed as the
chlorine-treatment temperature is increased. This indicates that the carbon-carbon bonding becomes
more uniform and less defective as the synthesis temperature is increased.
X-ray PDFs of the carbons are consistent with hexagonally bonded sp
2
-hybridised carbon (Figure
1d), with correlations extending to distances of approximately 20 Å. The loss of the correlations at
large distances arises from disorder in the carbon sheets, with a combination of sheet-curvature and
termination of the carbon sheets by either hydrogen atoms or functional groups likely responsible.
The correlations extend to slightly larger distances for TiC-CDCs produced at higher temperatures,
suggesting that the carbon sheets are larger and/or less curved. The FWHMs for the first three peaks
