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Strained ruthenium complexes are potent light-activated anticancer agents.
Nanoscale
Advances
PAPER
Cite this: Nanoscale Adv., 2020, 2,
5362
Evaluating the chemical exfoliation of lithium
cobalt oxide using UV-Vis spectroscopy†
Kevin Pachuta,
*a Emily Pentzer
bc
and Alp Sehirlioglu
*a
Two-dimensional materials have been at the forefront of chemistry and materials science research for the
past decade owing to promising applications across many fields. Improvements in exfoliation processes
continually give access to new two-dimensional material compositions, demanding a deeper
understanding of the defect structure and exfoliation mechanisms. Chemical exfoliation processes allow
for both the fabrication of new, and the production of industrial-scale quantities of two-dimensional
materials. For this reason, we report a rapid, efficient, and simple method for evaluating the exfoliation
behavior of protonated lithium cobalt oxide. Using a two-step chemical exfoliation method, first by
proton–cation exchange, then by treatment with an organo-ammonium hydroxide, the exfoliation yield
of lithium cobalt oxide was quantified with a quick and powerful technique, ultraviolet-vis spectroscopy
characterization. This method provided an in-depth analysis of the exfoliation of lithium cobalt oxide
confirming and discovering many key aspects of its soft-chemical exfoliation relating to layered
transition metal oxides. It was determined that the exfoliation yield has a strong dependence on multiple
factors, such as the concentration of protons in the powder, the presence of water and hydroxide
groups in solution, and the ionic radius and concentration of the intercalating cation. Both
morphological changes occurring as a function of reaction conditions and the two-dimensional nature
Received 8th September 2020
Accepted 15th October 2020
of the final sheets were revealed through scanning electron microscopy and atomic force microscopy.
Relative proton concentration of acid-treated lithium cobalt oxide was determined as a function of acid
concentration using time of flight secondary ion mass spectrometry after deuterated acid treatment.
DOI: 10.1039/d0na00755b
These experiments led to an improved understanding of the soft-chemical exfoliation of lithium cobalt
rsc.li/nanoscale-advances
oxide and can be applied to many layered transition metal oxides.
Introduction
Interest in two-dimensional (2D) materials has grown steadily
in the chemistry and materials science communities over the
past decade, but in fact, these materials have been utilized for
millennia as dyes and other additives.1 Notable reports in the
scientic literature came as early as 1954 with studies of the
swelling and exfoliation of layered micas and clays,2 and have
extended to present-day for the exfoliation of many other
layered materials, such as graphite, layered transition metal
dichalcogenides, layered metal oxides, layered double hydroxides, and layered transition metal carbides and nitrides.1,3–7
Today 2D materials are commonly produced via top-down
exfoliation1,5,6 and bottom-up synthesis8,9 methods in which
the starting material is a bulk layered material or atomic
precursor, respectively. Top-down exfoliation methods provide
a
Department of Materials Science and Engineering, Case Western Reserve University,
USA. E-mail: kgp22@case.edu
b
c
Department of Materials Science and Engineering, Texas A&M University, USA
Department of Chemistry, Texas A&M University, USA
† Electronic supplementary
10.1039/d0na00755b
information
(ESI)
5362 | Nanoscale Adv., 2020, 2, 5362–5374
available.
See
DOI:
the capability for high-throughput, large monolayer sheets and
are mainly limited by the size of the bulk crystal.10 Generally,
layered materials can be classied as either exhibiting interlayer
bonding that is van der Waals (graphite, hexagonal boron
nitride, black phosphorus, etc.) or coulombic (transition metal
oxides, transition metal hydroxides, etc.). While many layered
materials held together via van der Waals interactions can be
readily mechanically exfoliated via the scotch-tape method,11
those held together by the stronger interplanar forces of
coulombic attraction must undergo chemical treatment to
realize their exfoliation. To date, the large-scale production of
2D materials can be a signicant limitation in their integration
into devices and applications. The underlying mechanisms for
producing 2D materials must evolve to develop a deeper
understanding as they are developed for next-generation
devices and technologies.
With many layered materials available for exfoliation, the
properties of 2D materials can oen be guided by their layered
precursors. For instance, layered materials exhibit a broad
range of material properties related to the chemical composition, out-of-plane bonding, in-plane coordination, oxidation
state, and doping concentrations.1 This wide variety of available
This journal is © The Royal Society of Chemistry 2020
Paper
material properties gives rise to a substantial amount of selectivity when exfoliating layered materials to form 2D materials
for specic applications. Although some properties are inherent
to the bulk layered material, many aspects of the material's
physical, electronic, and chemical properties may be vastly
different for the nanosheet compared to the bulk layered
precursor. For example, 2D materials have tremendous anisotropy leading to an exponential increase in surface area-tovolume ratios allowing for efficient catalysts and sensors.1 As
well, 2D materials exhibit interesting thermal, electronic, and
mechanical properties nding applications in transistors, solar
cells, batteries, supercapacitors, and more.1 Multiple 2D materials can be combined as building blocks to form unique heterostructures creating hybrid ultra-thin devices,1,5,7 making 2D
materials attractive for many advanced applications.
Chemical exfoliation methods allow for the exfoliation of
many layered materials not typically used with traditional
mechanical exfoliation methods. These methods, such as the
liquid phase intercalation of large alkylammonium cations were
rst explored for the swelling of layered clays, smectites, and
oxides.12–14 As early as 1985, the intercalation and exfoliation
behavior of layered a-zirconium phosphate was determined to
be dependent on the identity and concentration of various
intercalating alkylamines.15,16 Investigations of protonated
layered oxides in the 1990s demonstrated acid–base neutralization reactions in the interlayer and were extended to a variety
of layered oxides. This was demonstrated notably with the
exfoliation of a layered perovskite,17 layered titanate,18 layered
birnessite,19 and layered lithium cobalt oxide (LCO),20 among
others. These studies led to the discovery of 100-fold swelling of
layered titanate giving an improved insight into the intercalation and swelling mechanisms of many layered oxides.21 Additionally, recent theoretical studies have increased our
understanding of the exfoliation mechanisms of layered materials.22,23 Though the processes underlying intercalation, exfoliation, and osmotic swelling are generally understood,
quantitative measurements of the exfoliation yield have been
elusive. Swelling of protonated layered oxide using multiple
intercalates has been explored,19,21,24–26 yet only a few studies
have related exfoliation yield to ionic size,25 with the general
acceptance that this process is colligative in nature.5 Generally,
intercalation effects – which describe the extent of the
Nanoscale Advances
equilibrium reaction – are quantied through the measurement
of intercalated cations in the structure in the bulk (via chemical
analysis or titration).19,21 While these measurements prove
useful for osmotic swelling studies, exfoliation yield is not easily
deduced from these data. Furthermore, even though the
swelling and exfoliation of each layered oxide are generally
similar, these materials have slightly different intrinsic properties linked to exfoliation, such as reactivity to acids, acidexchange potential, interlayer spacing, interlayer cation identity and site occupancy, and inter- and intra-layer bonding.
Variations in these intrinsic properties can consequently alter
the ability of the layered structure to exfoliate under a given set
of conditions, as is the case for LCO.
As described in Fig. 1, a layered TMO (typically an alternatively layered structure of transition metal and alkali metal
octahedra) can be exfoliated to individual TMO nanosheets
using a two-step chemical exfoliation method.19,20,27,28 Step one
is described by the layered TMO reacting with a protic acid
replacing the alkali ions in the interstices of the TMO layers
with protons. This can be thought of as two concurrent
processes: alkali cation extraction and proton replacement in
the interlayer. While alkali cation extraction is incomplete for
some layered TMOs, such as LCO, as proven by elemental
analysis,20,27 acid treatment of commonly exfoliated TMOs such
as CsxTi2x/4,x/4O4 where x ¼ 0.7, Cs0.8[Ti1.2Fe0.8]O4,
K0.45MnO2, K[Ca2Nan3NbnO3n1], and KCa2Nb3O10 all undergo
complete alkali cation extraction as described in Table 1.17,21,29–31
However, for layered oxides that undergo complete alkali cation
extraction, complete proton replacement does not necessarily
always follow. For example, while complete (1 : 1) proton
replacement is observed in acid-treated CsxTi2x/4,x/4O4 where
x ¼ 0.7, Cs0.8[Ti1.2Fe0.8]O4, K[Ca2Nan3NbnO3n1], and KCa2Nb3O10, proton replacement is 0.3 : 1 for K0.45MnO2.21,29–31
Electron transfer between the TMO host layers (e.g. MO2) forms
as a result of this incomplete cation exchange reaction. Changes
in valence are most commonly associated with the transition
metal20,27,30 but are also known to occur for oxygen.32–34 Although
changes in stoichiometry and valence are occurring, proton
replacement is the key driving force for step two, intercalation,
exfoliation, and osmotic swelling in the powder.
Step two for the exfoliation of TMO nanosheets takes the
protonated powder produced aer step one and reacts it with
Fig. 1 A schematic of the two-step soft-chemical exfoliation process to obtain atomically thin TMO nanosheets from bulk TMOs in which
nanosheets are held together by coulombic interactions. Step one uses a protic acid (H+) to replace alkali ions with protons in the interstices of
the layered structure, producing a “protonated powder”. The second step uses a hydroxide bearing a bulky cation [i.e., a quaternary amine
(NR4OH)] to promote osmotic swelling and exfoliation of individual TMO nanosheets.
This journal is © The Royal Society of Chemistry 2020
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Table 1
Paper
The extent of alkali metal extraction and proton replacement, as reported17,21,27,29–31
Layered TMO
Alkali metal extraction (%)
Proton replacement (%)
Resulting stoichiometry
LiCoO2
K0.45MnO2
CsxTi2x/4,x/4O4 (x ¼ 0.7)
Cs0.8[Ti1.2Fe0.8]O4
K[Ca2Nan3NbnO3n1]
KCa2Nb3O10
63
100
100
100
100
100
38
29
100
100
100
100
Li0.37H0.24Co0.96,0.04O21.97
H0.13MnO2
H0.7Ti1.83,0.17O4
H0.8[Ti1.2Fe0.8]O4
H[Ca2Nan3NbnO3n1]
HCa2Nb3O10
a hydroxide bearing bulky cation (typically a quaternary
ammonium hydroxide, NR4OH, where R ¼ methyl or butyl),
where the bulky cation intercalates into the layered host and
promotes the swelling and exfoliation of individual TMO layers
(i.e., nanosheets). This can be understood rst by the reaction of
protons located in the interlayer with hydroxide (OH) groups
dissociated from the bulky organo-ammonium cations in
solution, which produces water. Osmotic pressure and charge
balancing of the structure results in an inux of water and bulky
cations into the interlayer, thereby promoting swelling of the
structure and exfoliation of the TMO layers. Osmotic pressure in
the solution is subsequently governed by the [NR4+]/[H+] ratio in
the reaction, where proton concentration is the concentration
of protons in the powder and NR4+ concentration is the
concentration of bulky cations in solution.5,20,21,28,30,35 Previous
reports, including the exfoliation of LCO, used only one proton
concentration. Therefore, the dependence on proton concentration in the system, and consequently the [NR4+]/[H+] ratio,
have not been fully revealed. Furthermore, proton concentration determined via indirect measurements such as iodometric
titration and thermogravimetric analysis (TGA) may allow for
discrepancies in the true proton concentration in the powder as
compared to the reported values. Therefore, changes in alkali
cation and proton concentration in the powder (i.e., different
acid treatments) may have considerable effects on the ability for
TMOs to swell and exfoliate.
Here we report a rapid, efficient, and simple method for
evaluating the exfoliation yield of TMOs using UV-Vis spectroscopy, using the preparation of cobalt oxide nanosheets
(CONs) from bulk LCO powder as an example. Due to the
inherent absorption proles of numerous 2D materials, UV-Vis
spectroscopy can be implemented as a way to observe exfoliation, and may also provide insight into the intrinsic properties
of the exfoliated material, such as the band-gap, solvent effects,
surface functional groups, lateral size, thickness, vacancies, etc.
Using UV-Vis absorbance as the dening characterization
technique for exfoliation, we performed a systematic study of
multiple exfoliation conditions (acid concentration, tetraalkylammonium identity, and tetraalkylammonium concentration)
to establish their impact on exfoliation yield. Combining these
results with measured relative proton concentration values of
multiple acid-treated LCO powders, exfoliation of LCO was
examined as a function of relative proton concentration, the
cation radius of NR4OH, and concentration of reagents. The
wide range of reaction conditions investigated for the
5364 | Nanoscale Adv., 2020, 2, 5362–5374
exfoliation of LCO can be used as a guide for the exfoliation of
other layered oxides. Specically, fundamental relationships
between the layered host and the exfoliation conditions are
conrmed updating known mechanisms for the two-step sochemical exfoliation of LCO and other layered TMOs.
Experimental
Reaction conditions
The exfoliation of LCO was studied using various two-step sochemical reactions. Step one, separate batches of LCO (MTI
corp.) were treated with hydrochloric acid (HCl) (36.5–38 wt%,
Fisher Scientic) at four different concentrations (0.1 M, 0.3 M,
1 M, and 3 M HCl). The reaction proceeded for 24 hours with
10 mg LCO per mL HCl at room temperature and pressure in
a sealed round bottom ask (RBF) mixing at 400 rpm. Each of
these powders was washed with copious amounts of deionized
water to remove any remaining acid and salts formed from the
reaction. Each powder was allowed to dry in air overnight then
stored in a capped glass vial until its use. In step two, the acidtreated powders were treated separately at four initial concentrations of tetraalkylammonium hydroxide aqueous solutions
(0.014 M, 0.044 M, 0.14 M, and 0.44 M NR4OH, where R ¼ H,
methyl, ethyl, butyl). These exfoliation reagents studied are
ammonium hydroxide (28.4 wt%, Fisher Sci.), tetramethylammonium hydroxide (25 wt%, Alfa Aesar), tetraethylammonium hydroxide (35 wt%, Alfa Aesar), and tetrabutylammonium
hydroxide (40 wt%, Alfa Aesar). Additional concentrations at
0.0044 M NR4OH, 0.025 M NR4OH, 0.075 M NR4OH, 0.25 M
NR4OH were tested for certain acid-treated powders if exfoliation was expected in these regions of interest. Due to the
inherent etching nature of the exfoliation reagents chosen,
plastic containers were used to prevent the dissolution of the
glassware (dissolution of cations from the glassware into the
solution will cause the colloidal suspension of CONs to
agglomerate). As a result, these powders were reacted for 24
hours in plastic centrifuge tubes at 10 mg protonated LCO
per mL NR4OH under room temperature and pressure with
vigorous stirring (1000 rpm). Aer the reaction was complete
the solutions were centrifuged for a total of 20 minutes at 4500
relative centrifugal force (rcf) (or 5000 rpm using an Eppendorf
A5810). In total, 95 experimental exfoliation conditions were
examined using UV-Vis spectroscopy, spanning a wide range of
HCl (0.1–3 M, 30) and NR4OH (0.0044–0.44 M, 100)
concentrations, as summarized in Fig. 2. Additional reactions
This journal is © The Royal Society of Chemistry 2020
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Fig. 2 A schematic of the experimental space used to investigate the
dependence of proton concentration, NR4OH concentration, and
NR4+ ionic radius on the exfoliation yield of LCO.
were completed in water, tetramethylammonium chloride
(synthesized through acid–base neutralization of TMAOH and
HCl), and TMAOH in methanol (25 wt%, Arcos Organics).
Characterization
To analyze the effective concentration of 2D CONs exfoliated via
each reaction condition, UV-Vis spectroscopy was utilized to
measure the absorbance of each sample aer centrifugation. By
capitalizing on the intrinsic absorbance of suspended 2D CONs
in an aqueous solution, the exfoliation yield from each reaction
condition was established. Absorbance measurements of each
sample were completed using an integrating sphere on a Cary
6000i UV-Vis-NIR Spectrophotometer. Using this experimental
set-up, any scattering of light due to the presence of larger
particles was collected by the detector, thus, removing scattering artifacts from these measurements. Absorbance data
were collected from 1075 nm to 260 nm and the absorbance of
each reagent solution the nanosheets were suspended in was
subtracted. Any absorbance below 260 nm was ignored due to
reagent absorption in this region (ESI Section Spectra analysis,
Fig. S1†). Samples with high concentrations of dispersed CONs
were diluted such that the measured absorbance was less than
1.5 A.U. to avoid saturation of the detector. The recorded UV-Vis
data were processed and analyzed with R.36 To compare exfoliation yield for multiple reaction conditions, the absorbance of
each diluted sample was scaled according to their dilution
factor. The absorbance for each sample at 400 nm was used to
determine the relative exfoliation under each condition aer
a background value was subtracted. This background value was
due to the absorbance of unexfoliated LCO particles suspended
in solution (Fig. S3†). To properly subtract the baseline absorbance across the entire spectrum, the absorbance at 1075 nm
was used due to negligible absorption from the exfoliated CONs
in this region. Optical images of selected reactions were taken
and are shown in the ESI Section Spectra analysis, Fig. S2.† UVVis measurements of solutions of CONs were completed aer 30
This journal is © The Royal Society of Chemistry 2020
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and 60 days of storage in sealed centrifuge tubes (ambient
temperature and pressure). To prevent the measurement of
loose sedimentation as a result of aging, aged CONs samples
were centrifuged for 5 minutes at 4200 rcf before transferring to
a cleaned UV-Vis cuvette. UV-Vis absorbance was measured for
as received LCO powder using the same instrument set-up. To
prevent sedimentation, LCO powder was suspended in a solution of polyethylene glycol; a blank solution of PEG was subtracted from the measurement.
The presence, thickness, and lateral dimensions of the
exfoliated nanosheets were conrmed using atomic force
microscopy (AFM) (Park Nanosystems NX10) on selected
samples aer the solution was drop-cast onto freshly cleaved
mica substrates. To visualize the powders before and aer each
reaction (isolated pellet from centrifugation), scanning electron
microscopy (SEM) (FEI Helios Nanolab 650) was utilized. To
determine the lattice parameters of the powders aer HCl
treatment, X-ray diffraction (XRD) (Bruker D8 Advance) was
completed. Time of Flight Secondary Ion-Mass Spectroscopy
(ToF-SIMS) (ToF.SIMS 5, IONTOF) was used to determine the
concentration of protons in the interlayer structure of LCO
(more details in ESI Section Acid treated powders, Fig. S10†).
This was accomplished by measuring deuterium intensity in
LCO powders aer reaction with 0.1 M, 0.3 M, 1 M, and 3 M
deuterium chloride (diluted in deuterium oxide) solutions.
Reactions were carried out using the same reaction parameters
as before (10 mg LCO per mL acid solution, RBF, constant
stirring) but were kept in under inert atmosphere during the
reaction and in storage (N2 purge, to prevent deuterium–proton
exchange). To analyze the bulk concentrations of elemental
species in the treated LCO powders, inductively coupled plasma
mass spectroscopy (ICP-MS) was completed using a PerkinElmer NexION 300D.
Results and discussion
Exfoliation of LiCoO2 (LCO) using “standard” conditions
The exfoliation of LCO was accomplished using a two-step wetchemical exfoliation method rst reported by Kim et al.20 and
improved in our recent work.27,37 Aer the second step of the
reaction was completed, centrifugation of the resulting slurry
allowed for the separation of larger particles (pellet) from
colloidally stabilized nanosheet suspensions (supernatant).
SEM characterization of the centrifuged pellet showed significant morphological changes under known exfoliation conditions with tetramethylammonium hydroxide (TMAOH) (i.e.
3 M HCl and 0.044 M TMAOH treatment, 1 M HCl and 0.14 M
TMAOH treatment). For instance, swelling and plasticity of the
layered structure occurred as compared to LCO powder Fig. 3A,
and acid-treated LCO Fig. 3B(i)–E(i). This is attributed to the
inuence of intercalated bulky cations in between the layers of
cobalt-oxide octahedra (Fig. 3D(ii) and E(ii)). Other reaction
conditions, such as reaction with ammonium hydroxide
(NH4OH), displayed no signicant changes in structure or
morphology (Fig. 3B(i and ii)). For a given concentration of
TMAOH, the most drastic changes were observed for powders
that were exposed to higher concentration acid in step one. For
Nanoscale Adv., 2020, 2, 5362–5374 | 5365
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Paper
Fig. 3 SEM micrographs of as-received and treated LCO powders. Selected powders shown are (A) LCO as-received, (B(i)) 0.1 M HCl treated
LCO, (B(ii)) 0.1 M HCl + 0.014 M NH4OH treated LCO, (C(i)) 0.3 M HCl treated LCO, (C(ii)) 0.3 M HCl + 0.014 M NH4OH treated LCO, (D(i)) 1 M HCl
treated LCO, (D(ii)) 1 M HCl + 0.14 M TMAOH treated LCO, (E(i)) 3 M HCl treated LCO, (E(ii)) 3 M HCl + 0.044 M TMAOH treated LCO. All powders
were reacted for 1 day under constant stirring. SEM micrographs highlighted in red are after acid treatment (step one). SEM micrographs
highlighted in blue are after acid and amine treatment (step two).
example, as shown in Fig. 3D(ii), swelling of 1 M HCl treated
powder with TMAOH led to the substantial separation of the
layered structure. Fig. 3E(ii) demonstrates the complete exfoliation of large plate-like particles from LCO and is indicative
of the importance of using the appropriate acid-treatment
conditions en route to the exfoliation of LCO and layered
TMOs.
UV-Vis characterization of the supernatant shows the characteristic absorption prole of the exfoliated CONs dispersed in
solution. Fig. 4A shows the difference between the absorption of
LCO powder, and exfoliated CONs dispersed in solution. A
broad featureless absorption was observed for LCO powder,
while three distinct absorbance bands were observed for the
supernatant. The stark difference in absorption proles
between these two materials is due to the presence of 2D CONs
in solution, as veried using AFM (Fig. 4B). The broad nature of
the absorption peaks of the nanosheets indicates a distribution
of thicknesses and lateral sizes. It is assumed that any absorbance is due to 2D CONs in solution and not remaining bulk
particles in solution. Currently, these data cannot provide
quantitative statistical representations of the particle size,
shape, thickness, or defect concentration, but these analyses are
currently under investigation in our lab.
Impact of acid treatment of LCO
The rst step in the exfoliation procedure is completed via acid
treatment of LCO with aqueous HCl (Fig. 3B(i)–E(i)). As discussed in previous work, the determination of lithium, cobalt,
and oxygen content in the treated powder can be quite difficult
and has been reported for only one treatment condition (1 M
HCl treated LCO).27 Unless neutron diffraction is readily available, the most meaningful way to report the lithium content in
the sample is through the Li/Co ratio of the powder via inductively coupled plasma-mass spectrometry (ICP-MS) analysis.
However, stoichiometry composition based on ICP-MS characterization can be misleading due to cobalt vacancies derived
from acid treatment,27 known stability of lithium and cobalt
intermixing, and oxygen vacancies,38 or changes in bonding
iconicity aer delithiation.32–34 For the acid-treated powders
investigated, the Li/Co ratio decreased logarithmically as
Fig. 4 (A) UV-Vis absorbance spectra of a solution of CONs (blue) and lithium cobalt oxide powder (orange). CONs (prepared from 1 day 3 M HCl
and 1 day 0.044 M TMAOH treatment) were suspended in 0.044 M TMAOH and were separated from bulk powder via centrifugation. Lithium
cobalt oxide was suspended in polyethylene glycol to prevent sedimentation. (B) AFM image and (C) line scan height profile of the same solution
of CONs drop cast onto a clean mica substrate.
5366 | Nanoscale Adv., 2020, 2, 5362–5374
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a function of HCl concentration, as determined by ICP-MS
(Table 2 and ESI Section Acid treated powders, Fig. S8†).
However, at concentrated HCl solutions (12 M), LCO
completely dissolved aer 24 hours. Increasing the time for acid
treatment in each diluted condition increased the Li removal
only asymptotically, reaching at or near the limit at the times
considered.39 Thus, competing effects of dissolution and ionic
replacement associated with acid treatment limit the full ionic
replacement of Li-ions with protons in LCO. Therefore, the acid
treatment of LCO must be tuned appropriately to achieve high
degrees of proton replacement together with a maximal yield of
protonated powder (i.e., to prevent the dissolution of the
powder and resulting nanosheets). Due to the drastic stoichiometric changes occurring in the powder, XRD was utilized to
determine if any crystallographic changes occurred as a result of
acid treatment. No major changes in structural symmetry are
reported, but an increase in the c-lattice parameter was
observed for each acid treatment (XRD plots are shown in ESI
Section Acid treated powders, Fig. S9† and Table 2) and is
attributed to a decreased coulombic attraction between the
cobalt oxide layers and the interstitial Li-ion. This suggests
weaker bonding between protons and cobalt oxide sheets. The
minor decrease in the a-lattice parameter upon treatment with
more concentrated acid is attributed to the tighter bonding
between cobalt and oxygen in the in-plane (orthogonal to 001)
direction and relates to changing iconicity of cobalt–oxygen
bonding.
Proton replacement was further investigated by ToF-SIMS
analysis of LCO treated with 0.1 M, 0.3 M, 1 M, and 3 M DCl
in D2O (referred to as deuterated power samples). The relative
concentrations of deuterium (i.e., protons) examined as a function of DCl treatment were compared with the Li/Co ratios
determined by ICP-MS. Measurements of pristine LCO powder
showed very low deuterium intensity, attributed to noise within
the measurement; this value was subtracted as a baseline from
each acid-treated sample. Analysis of each fragment was
completed using MCs+ and MCs2+ clusters (where M ¼ matrix
components 2H, 7Li, and 59Co) to reduce the matrix and surface
Li/Co ratio (ICP-MS), and the a- and c-lattice spacing (XRD
and Rietveld refinement) are shown for LCO as received, 0.1 M HCl
treated, 0.3 M HCl treated, 1 M HCl treated, and 3 M HCl treated. D/Co
(ToF-SIMS) data are shown for LCO as received, 0.1 M DCl treated,
0.3 M DCl treated, 1 M DCl treated, and 3 M DCl treated. All powders
were reacted for 1 day under constant stirring
Table 2
LiCoO2 treatment
Li/Co
ICP-MS
a-Lattice
(Å) XRD
c-Lattice
(Å) XRD
D/Co
ToF-SIMSa
As received
0.1 M HCl
0.3 M HCl
1 M HCl
3 M HCl
1.04
0.77
0.56
0.42
0.37
2.819
2.813
2.811
2.809
2.812
14.07
14.23
14.38
14.38
14.40
—
1.37
0.85
2.12
7.42
a
Deuterium powder samples. The measured D/Co ratios in the powder
are greater than 1, therefore not permitted through charge balance in
the powder, the sensitivity of deuterium may need to be corrected for
absolute concentrations to be determined.
This journal is © The Royal Society of Chemistry 2020
effects typically encountered in ToF-SIMS measurements when
pure matrix fragments are used.40–43 Since the Li/Co ratio is
known for each sample from ICP-MS analysis, sensitivity factors
were used to correct the measured CsLi+/CsCo+ ratios of each
sample. As shown in Table 2, deuterium (D) – which is assumed
to have the same chemical potential and undergo the same
reactions with LCO as protons – was measured at signicantly
higher intensities in acid-treated powders compared to
untreated LCO. Relative deuterium concentration was
compared for each sample by monitoring the D/Co ratio. At
higher acid concentrations, higher D/Co ratios were measured,
directly proving the presence of protons in the interlayer. For
instance, the measured D/Co ratio for 3 M DCl treated LCO is
approximately three times that of 0.1 M DCl, proving that
increased concentration of acid treatment imparts both more
alkali cation extraction (ICP-MS) and proton replacement (ToFSIMS) in the powder. Normalized ToF-SIMS plots for each
matrix component compared to total counts for each sample are
shown in ESI Section Acid treated powders, Fig. S10† with a full
table and description of the calculations in ESI Section Acid
treated powders, Tables S1 and S2.† As reported for the acidtreatment of layered TMOs, hydrated H+ (i.e., hydronium ions)
could be present in the interlayer or as surface adsorbates aer
acid treatment.12,21 To mitigate these effects, the treated powder
was dried and stored under N2 to eliminate adsorbed ambient
water. Additionally, XRD measurements of LCO and acidtreated LCO suggest the presence of water in the interlayer is
minimal. Since the diameter of a water molecule (2.75 Å) is
more than half the interlayer spacing (4.68 Å, 001 direction dspacing) of untreated LCO, interlayer water molecules are expected to cause a substantial increase in the measured (003) dspacing, as observed with HCa2Nb3O10.31 It was found that the
interlayer expansion of LCO is only 3% aer treatment with
even the highest acid concentration, interlayer hydration is
likely not occurring. Additional XRD measurements of wet acidtreated LCO stored in DI water (to prevent any drying, and
therefore expulsion of water from interlayer) show no signicant changes in lattice parameters, again conrming that the
interstices of the layered structure aer acid treatment do not
contain water molecules (ESI Section Acid treated powders,
Table S3†).
UV-Vis spectroscopic analysis of reaction solutions
To quantify the effects of and relationship between step one and
step two, the impact of proton concentration, NR4+ concentration, and NR4+ ionic size on the exfoliation of LCO was evaluated by UV-Vis absorbance spectroscopy. Specically, the
supernatant obtained aer step two of the reaction was characterized. The exfoliation yield of each reaction condition was
able to be determined using the fundamental relationship
between measured absorbance and concentration (Beer–
Lambert law). In other words, the measured absorbance from
each reaction condition was directly related to the concentration of CONs in solution. Just under half of the reactions
exhibited characteristics relating to the presence of 2D CONs as
described by three broad absorbance peaks in the UV-Vis
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region, while the others showed no peaks and therefore indicated a lack of exfoliation. For more information regarding the
absorbance spectra of all samples, refer to ESI Section Spectra
analysis, Fig. S2.† For samples containing 2D CONs, the
absorbance at 400 nm was determined to be the most reliable
wavelength (when compared against 270 nm and 700 nm) for
consistency as a function of concentration with no overlapping
solvent peaks (water, NR4OH) (ESI Section Spectra analysis,
Fig. S1 and S3†). As shown in Fig. 5A, this wavelength is indicated by the vertical line on each spectrum, and Fig. 5B plots the
absorbance at 400 nm for each reaction as a function of acid
concentration (x-axis), and NR4+ concentration (y-axis) for
different NR4+ sizes (each panel) to elucidate trends in LCO
exfoliation behavior.
Paper
Signicance of interlayer protons
A signicant dependence of the exfoliation yield on proton
concentration in the protonated layered oxide is quantitatively
shown in Fig. 5B. In short, the conditions used to treat LCO with
protic acid (step 1) directly impact the conditions of step 2
needed for exfoliation. Exfoliation is not as prevalent in the
protonated powders tested with low proton concentration
(0.1 M HCl and 0.3 M HCl treated LCO), regardless of the NR4+
ionic size or concentration. For a given cation, increasing
proton concentration increased the exfoliation yield. When the
maximum exfoliation yield achievable is high (e.g., 3 M HCl and
0.044 M TMAOH treatment), exfoliation yield can be achieved
across many NR4+ concentrations (e.g., high exfoliation yield
3 M HCl treated LCO from 0.014–0.14 M TMAOH). While high
(A) UV-Vis absorbance spectra of solutions of exfoliated CONs organized by the ionic radius of the exfoliation reagent used, increasing left
to right. The color of each spectrum represents the concentration of the acid used in step one, signifying varying degrees of proton replacement.
A vertical line at 400 nm indicates the absorbance used to determine the degree of exfoliation. (B) The degree of exfoliation derived from the
spectra in (A) plotted in a 2D array with pre-treated acid powder (0.1 M HCl, 0.3 M HCl, 1 M HCl, 3 M HCl) as the x-axis, the concentration of the
exfoliation reagent (0.0044 M, 0.0075 M, 0.014 M, 0.025 M, 0.044 M, 0.075 M, 0.14 M, 0.25 M, 0.44 M) as the y-axis, and separated by the ionic
radius of the exfoliation reagent used, increasing left to right. A color scale at the bottom of the plot illustrates the variation of color corresponding to the degree of exfoliation. Adjusted absorbance is shown for the degree of exfoliation and accounts for sample dilution by multiplying
measured absorbance by the value of the dilution. All raw data, individual spectra analysis, and R code is published in an OSF project https://
osf.io/xkme6/ (DOI: 10.17605/OSF.IO/XKME6).
Fig. 5
5368 | Nanoscale Adv., 2020, 2, 5362–5374
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Paper
[H+] in the protonated powder is ideal to achieve the highest
exfoliation yield and is possible for many other layered TMOs
such as CsxTi2x/4,x/4O4 (x ¼ 0.7) K0.45MnO2, K[Ca2Nan3NbnO3n1], and KCa2Nb3O10, the solubility of LCO in acidic solutions creates an upper limit on the [H+] that can be obtained.
Therefore, for TMOs with less stability towards acid solutions, it
is crucial to determine a suitable treatment method that allows
for high alkali ion removal and high proton replacement, while
minimizing the amount of dissolution occurring. To illustrate
this point, reaction conditions with TMAOH, tetraethylammonium hydroxide (TEAOH), and tetrabutylammonium hydroxide
(TBAOH) all showed increased exfoliation yield as the concentration of acid treatment increased. Reaction with NH4OH
solutions proved insignicant for the exfoliation of protonated
LCO. For this study, it was determined that 3 M HCl treatment
provided the best conditions for step 1, providing a broader
range of concentrations useful for exfoliation in step 2.
Signicance of water and hydroxide ions in solution
The reliance of proton concentration in the powder for swelling
and exfoliation conrms the two-step chemical reactions in the
interlayer as suggested by others.5,21 To test this hypothesis,
known reaction conditions promoting exfoliation (step 1 ¼ 3 M
HCl; step 2 ¼ 0.044 TMAOH), were mimicked with tetramethylammonium chloride (TMACl), a salt with a non-basic anion.
The reaction of protonated LCO with TMACl under these
conditions produced a clear and colorless supernatant aer
centrifugation of the reacted slurry, indicating that no exfoliated occurred and conrming that a Brønsted base (OH) is
required for exfoliation.21 To determine if an aqueous reaction
environment is needed to supply the inux of water in the
interlayer and promote swelling and exfoliation, a methanolic
solution of TMAOH was used in place of the aqueous solution.
As with the TMACl conditions, a clear and colorless supernatant
was produced aer centrifugation of the reaction slurry, again
indicating no exfoliation occurred. The size of methanol is
larger than water and has methanol has a lower polarity and
dielectric constant, which may prevent its diffusion to the
interlayer. These observations support that the neutralization of
H+ with OH to form water and an aqueous environment is
essential for the introduction of bulky cations and the promotion of exfoliation. Images and spectra of these reactions are
shown in ESI Section Spectra analysis, Fig. S3.†
Signicance of NR4+ concentration
Relationships between the NR4+ concentration in solution and
proton concentration in the protonated layered TMO were
investigated. As shown in Fig. 5B, the exfoliation yield of step
two has a strong dependence on the concentration of the
exfoliation reagent used in the reaction. For instance,
TMAOH, TEAOH, and TBAOH all showed an increase in
exfoliation yield at selective NR4+ concentration when varying
proton concentration in protonated LCO. This means for
a specic NR4+ size and proton concentration, the exfoliation
yield is maximized at a specic NR4+ concentration. This
reveals a [NR4+]/[H+] ratio (dependent on the NR4+ size) that is
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Nanoscale Advances
suitable for exfoliation, as commonly described in the literature. Although, as is the case with TMAOH, the [NR4+]/[H+]
relationship does not hold across multiple proton concentrations. In other words, if the [NR4+]/[H+] ratio was a true
indication of the exfoliation yield, lower proton concentration
in protonated LCO would require lower NR4+ concentration to
promote exfoliation. Alternatively, higher proton concentration in protonated LCO would require higher NR4+ concentration to promote exfoliation. However, our results showed
that the maximum exfoliation yield was obtained at the
following conditions 0.1/0.014 M HCl/TMAOH, 0.3/0.014 M
HCl/TMAOH, 1/0.14 M HCl/TMAOH, 3/0.044 M HCl/TMAOH.
Thus for protonated LCO, the [NR4+]/[H+] ratio is not a good
indicator of optimum exfoliation conditions since proton
concentration may vary within for various acid-treatment
conditions. This deviation from the preferred [NR4+]/[H+]
ratio may be a result of the changing concentrations of both
Li+ and H+ in the powder as well as other chemical or bonding
changes in the material as a result of different acid
treatments.
Signicance of NR4+ ionic size
The ionic size of the exfoliation reagent is yet another crucial
factor in the success of the exfoliation reaction. As shown in
Fig. 5B, TMAOH (r ¼ 3.2 Å) provided the best yield of exfoliation for every protonated LCO tested, with an order of
magnitude increase in the yield of exfoliated material in
comparison to NH4OH, TEAOH, and TBAOH. The results
shown in Fig. 5B conrm that specic reaction concentrations
dictate the swelling and subsequent exfoliation of protonated
LCO, but do not coincide with results of exfoliation for
multiple exfoliation reagents. H0.8[Ti1.2Fe0.8]O4, HxTi2x/4,x/
4O4 (x ¼ 0.7), H0.13MnO2, and HCa2Nb3O10 have been reported
to be exfoliated using reagents with large differences in cation
size, namely TMAOH and TBAOH (0.64 nm and 0.98 nm
respectively, TBA+ is 1.5 bigger than TMA+).19,21,24–26 The UVVis data (Fig. 5B) demonstrates that protonated LCO does
not exhibit similar swelling properties compared to these
other layered oxides. If this was the case, exfoliation of
protonated LCO with TMAOH and TEAOH would be expected
to display similar exfoliation yield, since both cations have
a similar ionic diameter (0.64 nm and 0.78 nm respectively,
TEA+ is 1.2 bigger than TMA+). Reactions of protonated LCO
with TMAOH provide a signicantly greater exfoliation yield
compared to reactions with TEAOH, conrming that protonated LCO exhibits different exfoliation behavior than other
layered oxides, contrary to previous reports.5 These ndings
elucidated a key aspect of exfoliation of protonated LCO which
is oen overlooked with other layered oxides – interlayer
spacing. By examining the interlayer spacing of LCO aer acid
treatment and comparing it to the ionic diameter of each
reagent used in the exfoliation reaction, a critical diameter of
the cation relative to the interlayer spacing is proposed for the
exfoliation of protonated LCO. For ionic diameters that were
considerably small (NH4+) compared to the interlayer spacing
of LCO, exfoliation did not proceed. Alternatively, for ionic
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diameters much larger (TBA+) than the interlayer spacing of
LCO, exfoliation also did not proceed. The ionic diameter of
the exfoliation reagent that proved most effective for LCO was
approximately 1.3 the size of the interlayer spacing. Other
layered oxides such as H0.8[Ti1.2Fe0.8]O4, HxTi2x/4,x/4O4 (x ¼
0.7), H0.13MnO2, H[Ca2Nan3NbnO3n1], and HCa2Nb3O10 have
much larger interlayer spacing than protonated LCO (0.89,21
0.94,18 0.73,30 1.6–2.8,14 1.44,31 and 0.48 (ref. 20 and 27) nm,
respectively), possibly reducing the effects the ionic diameter
has in relation to the interlayer spacing. For example, the
interplanar spacing in other protonated oxides is large enough
that water (diameter of 0.28 nm) and bulky cations can easily
diffuse into the layered host promoting exfoliation.
Conversely, for more compact layered oxides, such as LCO, the
diffusion of bulky cations and water molecules may not be as
favorable, thereby imposing stricter limits on the ionic size
needed for exfoliation in relation to the interlayer spacing.
Furthermore, the intercalation of these materials with smaller
cations such as Na+, K+, or NH4+ may not lead to a signicant
enough expansion of the layers to promote water intercalation,
swelling, and exfoliation.
Paper
Effects of aging
The exfoliation yield of protonated LCO was determined to be
dependent on the aging time between step 1 and step 2. Aer
the acid-treated powder was washed with DI water and dried
overnight in air, the powder was stored in a capped glass vial.
This powder was reacted using the identical conditions as
before (step 1 ¼ 3 M HCl; step 2 ¼ 0.044 M TMAOH), but aer 25
and 150 days of storage in the glass vial. As shown in ESI Section
Spectra analysis, Fig. S5† the UV-Vis absorbance of the supernatant collected aer 1 day of reaction and therefore exfoliated
CONs in solution is reduced by over half (56%) aer 25 days in
storage and by three quarters aer 150 days (75%). Though
the effects of powder aging corresponding to exfoliation yield
are drastic, the mechanism for aging is currently not clear. XRD
measurements (ESI Section Acid treated powders, Table S3†) do
not show supporting evidence for changes associated with
aging. Very small changes in the lattice spacing were observed
(0.1% for the c-lattice parameter) for the powder measured
aer approximately the same period (167 days) meaning no
signicant bonding or structural changes occurred within this
time frame. It can be speculated that the decrease in efficiency
(A–C) UV-Vis absorbance spectra of solutions of exfoliated CONs separated by the ionic radius of the exfoliation reagent used. The color
of each spectrum represents the concentration of the acid of pre-treated LCO signifying varying degrees of proton replacement in the structure.
A vertical line at 400 nm indicates this wavelength was taken as the value for the degree of exfoliation. Spectra are shown for as-prepared
samples (A), after 30 days (B), and after 60 days (C).
Fig. 6
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Paper
by aging may be due to drying or redistribution of Li+ and H+ in
the powder over time. This mechanism is suggested as the
protonation reaction starts at the surface and proceeds into the
powder, and it has been shown that Li is remaining in the
powders aer this rst step. However, currently, we do not have
a direct observation of changes in ion distributions with aging
time. Additional exfoliation parameters were considered such
as the time of the exfoliation reaction, protonated LCO loading,
and stirring rate and presented in ESI Section Spectra analysis,
Fig. S5.†
Tauc plot analysis of each exfoliation spectra was completed
to determine the optical band gap of the exfoliated nanosheets
and shown in ESI Section Spectra analysis, Fig. S6.† These
results were compared to the determined optical band gap of
the starting protonated LCO powders and discussed in the ESI
Section Spectra analysis, Fig. S7.† The optically determined
band gaps of each exfoliation condition were related to the
degree of acid treatment, with lower band gaps of exfoliated
Nanoscale Advances
CONs measured for higher acid pre-treatment (3 M HCl, 2.2
eV) compared to lower acid pre-treatment (0.1 M HCl, 2.4 eV).
Each solution of CONs was characterized aer 30 and 60 days;
notable decreases in peak width (FWHM) and blue shi in the
absorption maxima were observed, along with an increase in
the band gap. Changes in the absorption proles for the solutions upon standing can be seen in Fig. 6A–C with a signicant
drop in absorbance over time (Fig. 7A–C). A decrease in
absorption intensity is attributed to the settling of large particles (agglomerated nanosheets), the decrease in peak width is
attributed to the narrowing distribution of particle sizes in
solution over time, and the blue shi in peak maxima may point
to a decreased size of the particles remaining in the solution.
Conclusions
The exfoliation of LCO was examined as a function of conditions of both steps of exfoliation: proton concentration, NR4+
Fig. 7 (A–C) The degree of exfoliation for each spectrum was plotted in a 2D array with pre-treated acid powder (0.1 M HCl, 0.3 M HCl, 1 M HCl,
3 M HCl) as the x-axis, the concentration of the exfoliation reagent (0.0044 M, 0.0075 M, 0.014 M, 0.025 M, 0.044 M, 0.075 M, 0.14 M, 0.25 M,
0.44 M) as the y-axis, and organized by the ionic radius of the exfoliation reagent used. A color scale at the bottom of the plot shows the variation
of color corresponding to the degree of exfoliation. Adjusted absorbance is shown for the degree of exfoliation and accounts for sample dilution
by multiplying measured absorbance by the value of the dilution. The degree of exfoliation for each spectrum is shown as measured (A), after 30
days (B), and after 60 days (C).
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ionic size, NR4+ concentration, and protonated LCO sample
aging. Characterization of the supernatant by AFM and UV-Vis
absorbance measurements conrmed the presence of twodimensional CONs. Through the UV-Vis characterization and
analysis of multiple reaction conditions, a detailed exfoliation
landscape was produced for protonated LCO. Higher acid
concentrations used in step 1 yielded protonated powders that
gave increased exfoliation yield aer step 2, coinciding well with
known so-chemical exfoliation mechanisms. Specically, 3 M
HCl treatment provided the best acid reaction conditions to
promote the exfoliation of LCO. Tetraalkylammonium ionic size
was another crucial factor for the exfoliation yield, with tetramethylammonium hydroxide providing the most suitable
reaction conditions to promote exfoliation. These results
elucidated a critical ion size to the interlayer spacing ratio
(1.3 : 1) for the exfoliation of LCO. Additionally, the concentration of NR4+ in solution, the presence of OH groups, and an
aqueous reaction environment were essential for exfoliation to
occur. However, exfoliation did not occur for a constant [NR4+]/
[H+] ratio across multiple H+ concentrations in the powder,
challenging the suggestion of such a ratio based on studies that
use only one H+ concentration in protonated transition metal
oxides. Additionally, aging effects of protonated LCO should be
considered in any scaling of the process due to a signicantly
diminished exfoliation yield over time (56% aer 25 days,
75% aer 150 days). Finally, the analysis of CONs aging in
solution under ambient conditions revealed substantial
decreases in concentration (i.e., poor colloidal stability),
changes in absorption peak shape, and blue energy shis and
changes in the optically determined band gap.
Conflicts of interest
The authors declare no competing nancial interests.
Acknowledgements
This work was made possible through AFOSR FA 9550-18-10030. The authors would like to thank Will Huddleston (XRD),
Elahe Farghadany (SEM), Maria Escamilla (ICP-MS), Menhong
Wang (UV-Vis), Jiqi (R), Prof. Roger French (UV-Vis). Tanguy
Terlier (ToF-SIMS), MDI JADE (Rietveld Renement), Kathryn
Howard, Malcolm Kenney, and the Case Western Reserve
University Chemistry Department. ToF-SIMS analysis was
carried out with support provided by the National Science
Foundation CBET-1626418 using resources of the Shared
Equipment Authority at Rice University. ICP-MS was performed
in the Department of Chemistry at Texas A&M University.
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