Simultaneous Activation–Exfoliation–Reassembly to Form Layered Carbon with Hierarchical Pores

Article abstract of DOI:10.1002/cctc.201700137

Currently, the production of hierarchical 2 D layered carbon is mostly limited by low yields, laborious processing, and the high tendency for reaggregation. Hence, it is still a challenge to attain stable porous carbon structures with a well-preserved layered structure and hierarchical pores. Here, we demonstrate a facile and simultaneous activation–exfoliation–reassembly technique to perform structural modification and pore tailoring of carbonaceous matrices conveniently. Various pore creations can be realized concurrently from in-plane micro-/mesopores by chemical activation, expansion of intersheet mesopores, and interconnectivity of the layered structure to form 3 D macropores. Finally, a proof-of-concept demonstration of the combination of hierarchical macro-/micro-/mesoporous layered carbon structure and TiO₂ nanosheets shows promising adsorption and catalytic properties.

Full text of DOI:10.1002/cctc.201700137

Accepted Article
Title: Simultaneous Activation-Exfoliation-Reassembly to Formation of
Layered Carbon with Hierarchical Pores
Authors: Wei Li Ong, Judith Hui Min Wong, Serene Wen Ling Ng, Hui
Ru Tan, and Ghim Wei Ho
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201700137
Link to VoR: http://dx.doi.org/10.1002/cctc.201700137
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10.1002/cctc.201700137
ChemCatChem
Simultaneous Activation-Exfoliation-Reassembly to Formation of
Layered Carbon with Hierarchical Pores
Wei Li Ong,a Judith Hui Min Wong,a Serene Wen Ling Ng,a Hui Ru Tan,c and Ghim Wei Ho*abc
a
Department of Electrical and Computer Engineering, National University of Singapore, 4
Engineering Drive 3, Singapore 117583
b
Engineering Science Programme, National University of Singapore, 9 Engineering Drive 1,
Singapore 117575
c Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and
Research), 3 Research Link, Singapore 117602
*Corresponding author: A/Prof. Ghim Wei Ho
Email: elehgw@nus.edu.sg
Keywords: Layered carbon, activation-exfoliation, hierarchical pores, TiO2 nanosheets, sorbent-
catalytic properties
Abstract
Currently, the production of hierarchical 2D layered carbon is mostly limited by low yield,
laborious processing and high inclination of re-aggregation. Hence, it is still a challenge to attain
stable porous carbon structure with well-preserved layered structure and hierarchical pores. Here,
we demonstrate a facile and simultaneous activation-exfoliation-reassembly technique to
conveniently perform structural modification and pore tailoring of carbonaceous matrix. Various
pores creation can be realized concurrently from in-plane micro/mesopores by chemical activation,
expansion of intersheet mesopores and interconnectivity of layered structure to form 3D
macropores. Finally, a proof-of-concept demonstration of the combination of hierarchical
macro/micro/mesoporous layered carbon structure and TiO2 nanosheets shows promising
adsorption and catalytic properties.
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Introduction
Carbon layered materials have recently gained much attention due to the emergence of novel
properties when a few-layer thick sheets are constructed which behave vastly different from the
bulk multilayered carbon.[1] Carbon layered materials specifically graphene and its derivatives
have been extensively studied and developed for various potential applications such as catalysts,
sorbents, energy storage and conversion etc.[2-4] Essentially, the two-dimensional (2D) layered
materials constitutes a better alternative to particulate/bulk structure as they possess a host of
attributes which include shortened charge transport dimension, high surface area, abundant
exposed sites and optimized contact interfaces.[5] At the same time, the stability of the layered
structures along with their exceptional topotactic interactions and reactivity offer intriguing
opportunities for manipulation and hybridization towards desirable functional nanocomposite.
Carbon shows relatively stronger in-plane chemical bonds than out-of plane direction, which allow
it to be exfoliated into 2D layered structures. Taking advantage of the intrinsic 2D configuration
of carbon, one of the most common methods to fabricate the layered sheets is by mechanical
exfoliation of bulk graphite to cleave, delaminate and split apart the crystal layers.[6] Liquid
exfoliation has also been achieved in various surfactant and intercalator media follow by long
duration ultrasonication.[7] Besides, bottom-up approaches i.e. vapor−solid and chemical vapor
deposition (CVD) methods have also been demonstrated to produce few-layer carbon materials
mostly limited on substrates.[8] Additionally, it is noted that graphene/carbon layered structure
shows high tendency to reaggregate or self-stacking during the processing steps, which results in
a substantial loss of active surface area and hence leading to impoverished physicochemical
performance. Consequently, it is still a challenge to fabricate stable/robust carbon layered structure
with well-preserved hierarchical pores on a large scale.
Herein we demonstrate a cost and process effective method to simultaneously activate-exfoliate-
reassemble carbon nanospheres into thin layered hierarchical porous structures. Compared to other
preparation routes, the activation-exfoliation-reassembly approach conveniently fulfills the
formation of thin layered planar sheets and hierarchical pores with large specific surface area. As
such, the proposed technique is attractive due to its easy procedure, mild conditions, high
reproducibility and controllability process. Other than the formation of primary in-plane
micro/mesopores introduced by chemical activation, the expansion of interlayer galleries between
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the lamellar structures leads to spontaneous exfoliation which then creates an additional level of
hierarchy mesopores. Finally, the congruent exfoliation-reassembly of the carbon layered structure
affords a three-dimensional (3D) interconnected macroporous framework. A proof-of-concept
demonstration shows the constructed hierarchical porous carbon structure with well-assembled
TiO2 nanosheets (NSs) warrants efficient transport of reactant throughout the carbonaceous matrix,
leading to enhanced adsorption and catalytic performance.
Results and Discussion
Carbon spheres (CSs) were obtained from a simple hydrothermal treatment of glucose, and
subsequently subjected to potassium hydroxide (KOH) treatment with CS: KOH mass ratios of 1:2
(AC-1), 1:4 (AC-2) and 1:6 (AC-3), during which the CSs undergo a simultaneous process of
activation, exfoliation and reassembly, resulting in a transformation of the CSs into a 3D
interconnected macroporous framework as illustrated in Scheme 1.
Activation
Exfoliation
Reassembly
Scheme 1. Schematic diagram illustrating the morphological evolution of CSs through the
processes of activation, exfoliation and reassembly of thin carbon planar sheets.
The pristine CSs have a smooth surface and uniform diameter of about 250 nm, and no visible
pores were observed in the corresponding TEM images (Figure S1 in the Supporting Information).
When these nano-sized CSs were subjected to KOH activation treatment, exfoliation and peeling
of the outer layers were observed (Figure 1a,b). With increased amounts of KOH, the CSs were
disintegrated by the exfoliation process and monolithic fragments with irregular shapes and “swiss
cheese”-like macropores[9] of varying sizes were observed (Figure 1c). The monolith is formed by
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a reassembly of the carbon sheets into a 3D macro/micro/mesoporous carbonaceous matrix (Figure
1d). The textural characteristics of these porous carbon structures were investigated by BET
measurements. The N2 adsorption–desorption isotherm measured at 77 K is shown in Figure 1e,
and the corresponding pore size distribution is shown in Figure 1f. The microstructural properties
including specific surface area, pore volume, and average pore diameter are summarized in Table
1. The isotherms of the pristine CSs can be identified as type III according to the IUPAC
classification. The adsorbed amount was very low and close to zero at relatively low pressures,
suggesting low porosity of the solid pristine CSs. The total pore and micropore volume of the CSs
were 0.03187 and 0.0119 cm3 g-1, respectively, which are in agreement with the predicted values
for CSs without pore structures. After KOH treatment, AC-1 and AC-2 exhibited Type II isotherm
while AC-3 exhibited Type I isotherm. From the isotherms, AC-1 has the lowest rate of adsorption
at low pressures which indicates that it has the least number of micropores amongst the AC
samples. In comparison, a rapid saturation at low pressures is observed in AC-2 and AC-3
indicating the existence of large amounts of micropores.[10] AC-1 has a surface area of 1115 m2 g-
1
which is more than 100 times larger than that of pristine CSs (10.33 m2 g-1), and this can be
attributed to the presence of mesopores and micropores (Vt = 0.898 cm3 g-1, Vmicro = 0.714 cm3 g-
1). The BET surface area of AC-2 and AC-3 are 1533 m2 g-1 (Vt = 0.796 cm3 g-1, Vmicro = 0.681
cm3 g-1) and 2107 m2 g-1 (Vt = 0.945 cm3 g-1, Vmicro = 0.877 cm3 g-1) respectively. The high surface
area observed in AC-3 can be attributed to the formation of large amounts of pores due to the
vigorous etching at a high concentration of KOH. In addition to the BET surface area analysis, the
QSDFT method was also used to analyze the distribution of the micropores and mesopores. It can
be observed from Figure 1f that the AC samples exhibit multimodal distribution in both the
micropore and mesopore domains. The micropores of AC-2 have a peak pore size of ~0.39 nm,
and with increasing amounts of KOH, larger pore size with a wider pore size distribution was
observed due to the pore widening effect caused by KOH inside the existing pores.[11] This effect
is observed in the micropores of AC-3 which has a peak pore size of ~0.51 nm. These results
indicate that KOH activation of CSs can produce carbonaceous structures with highly developed
porosity, and also significant increases in surface area and pore volume. Besides the textural
characteristics, surface properties of the carbon structures before and after KOH activation were
also analyzed with FTIR (Figure 1g). The broad peak at 3400 cm-1 is associated with –OH
stretching vibration of hydroxyl functional groups and the intensity of the peak decreases with
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increasing amounts of KOH activation. This could be due to dehydration which occurs during the
activation process. The peak at 1160 cm-1 corresponds to the C-OH bending vibrations.[12, 13] The
band at 1640 cm-1 is assigned to the C=O stretching of a non-aromatic carboxylic moiety, which
might be derived from partial dehydrogenation of glucose during the hydrothermal reaction. A
weak vibration was observed at 1380 cm-1 which is consistent with the presence of O=C-O groups,
and the intensity of this band decreases with amounts of KOH activation.
(a)
(c)
200 nm
500 nm
(b)
(d)
100 nm
200 nm
(f)0.5
AC-2 (g)
AC-3
O-H
600
(e)
C=O
COO–
C-OH
0.4
0.3
0.2
0.1
0.0
CSs
AC-1
CSs
400
200
0
AC-1
AC-2
AC-3
AC-2
AC-1
CSs
AC-3
5
10
15
20
0.0 0.2 0.4 0.6 0.8 1.0
Relative pressure (P/P0)
4000
3000
Wavenumber (cm-1)
2000
1000
Pore radius (Å)
Figure 1. Low and high magnification SEM images of (a,b) AC-2, and (c,d) AC-3 depicting the
exfoliation and reassembly of thin carbon planar sheets upon KOH activation treatment. (e) N2
adsorption-desorption isotherms measured at 77 K, (f) corresponding pore size distributions
obtained from QSDFT, and (g) FTIR spectra detailing the surface properties of CSs and AC.
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Table 1. Specific surface areas, pore volumes, and pore sizes of pristine and activated carbon
spheres.
[a]
[b]
Sample
Smicro
SBET
Vmicro
Vmeso
Vt[c]
Ratiomicro Average pore
[m2 g-1] [m2 g-1] [cm3 g-1] [cm3 g-1] [cm3 g-1] [%]
size [nm]
CSs
6.343
1065
1450
1657
10.33
1115
1533
2107
163.9
94.93
6.35
0.012
0.714
0.681
0.877
0.146
0
0.019
0.184
0.116
0.067
0.011
0.295
0.036
0.032
0.898
0.796
0.945
0.156
0.295
0.036
40.63
79.51
85.43
92.91
92.95
0
6.91
AC-1
AC-2
AC-3
1.61
1.28
0.89
TiO2-AC 162.1
TiO2 NSs 0
1.93
17.51
11.59
A-TiO2
0
0
0
[a] Surface area was calculated using the BET method at P/P0 = 0.01–0.1. [b] Evaluated by
Quenched Solid State Functional Theory (QSDFT) method. [c] Total pore volume at P/P0 = 0.99.
The morphological features were further characterized by TEM as shown in Figure 2. No visible
pores were observed in the TEM images of the pristine CSs (Figure S1). After KOH activation,
exfoliation and peeling of the outer layers of AC-2 was clearly observed (Figure 2a), and the
spherical structure appears porous. The high-magnification TEM image from the selected area of
Figure 2a reveals the presence of a few-layer porous nanosheets on the exterior of the spherical
structure (Figure 2b) and the HRTEM image suggests the presence of graphitic carbon structure.
When increased amounts of KOH was used (AC-3), a porous network of thin carbon sheets was
observed (Figure 2d) and the marked area in Figure 2d actually consists of sparsely distributed thin
carbon nanosheets (Figure 2e). These nanosheets are filled with micropores and mesopores as
indicated by the SEM image (Figure 1d) and QSDFT analysis (Figure 1f). Similarly, the ordered
arrangement of the lattice fringes in Figure 2f also suggests the presence of graphitic carbon. The
XRD pattern of AC-3 (Figure S2) also reveals the presence of the typical (002) and (100) peaks
from graphitic carbon.
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(d)
(e)
(f)
(a)
(b)
(c)
50 nm
50 nm
20 nm
5 nm
20 nm
5 nm
Figure 2. TEM and HRTEM images of (a-c) AC-2 and (d-f) AC-3.
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During the activation process, the CSs react with KOH and undergoes etching through a series of
chemical reactions listed below:[14]
4 KOH + C → K2CO3 + K2O + H2
2KOH → K2O + H2O
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
C + H2O (steam) → CO + H2
CO + H2O → CO2 + H2
CO2 + K2O → K2CO3
K2O + H2 → 2K + H2O
K2O + C → 2K + CO
K2CO3 + 2C →2K + 3CO
In equations 1, 3, 7 and 8, carbon undergoes etching which results in the formation of exfoliated
sheets and also primary in-plane micropores and mesopores. The exfoliation could be attributed to
the infusion of KOH into the CSs which breaks down the structural integrity and causes the peeling
of sheets.[15] It is also possible that the metallic K penetrates into the carbon lattices, causing an
expansion of the lattices by the intercalated metallic K, and subsequently the intercalated K were
removed from the carbon matrix.[16] This expansion of the interlayer galleries between the lamellar
structures leads to spontaneous exfoliation which in turn creates an additional level of hierarchy
mesopores. When the amount of KOH is increased, vigorous etching of the core and surface of
CSs occurs simultaneously, while the interconnections between the CSs remained, resulting in a
morphological transformation of the spheres into nanosheets that are readily cross-linked due to
the poly condensation effect.[15, 17] This congruent exfoliation-reassembly of the carbon layered
structure produces a 3D interconnected macroporous framework.
As a demonstration of the functional properties of the porous carbon structures, the adsorption
performance in both methyl orange (MO) aqueous solutions and volatile organic compound (VOC)
vapors were investigated. The MO adsorption properties of the CSs were significantly enhanced
after activation (Figure 3a). The pristine CSs did not adsorb any MO at all (not shown) while AC-
2 performed the best and managed to fully adsorb the MO molecules in 90 min. AC-1 did not
perform as well and adsorbed only 90 % of MO after 120 min, while AC-3 achieved complete
adsorption but only after 120 min. A similar trend was also observed in the adsorption of ethanol
vapor as shown in Figure 3b. AC-2 exhibited the fastest rate of adsorption followed by AC-3 then
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AC-1. AC-2 was able to remove 80% of the ethanol in 120 min. The adsorption of all 3 VOCs
investigated are summarized in Figure 3c. It was observed that in both methanol and formaldehyde,
AC-2 was the second best performing sample instead. This could be due to the fact that the
individual AC samples possess differing affinity towards different VOC molecules.
AC-1
AC-2
AC-3
AC-1
AC-2
AC-3
(a)1.0
0.8
(b)100
80
60
40
20
0
0.6
0.4
0.2
0.0
0
30
60
90
120
0
30
60
90
120
Time (min)
Time (min)
(c)
AC-1
AC-2
AC-3
100
80
60
40
20
0
Ethanol
Methanol
Formaldehyde
Figure 3. Adsorption kinetics of time evolution (a) MO, (b) ethanol and (c) various VOCs
adsorption in the presence of AC.
TiO2 nanosheets (NSs) were then hydrothermally grown on AC-2 to form a TiO2-AC composite
material as a proof-of-concept demonstration. From the SEM image (Figure 4a), each of the AC
nanospheres was observed to be uniformly covered by well-assembled TiO2 NSs forming
hierarchical flower-like nanostructures. The TEM image (Figure 4b) and EDX elemental maps
(Figure 4c) reveal the core-shell structure of the composite where the AC core is fully encompassed
by a radial assembly of 2D TiO2 NSs on the outer surface. The fairly transparent appearance of the
TiO2 NSs in Figure 4d suggests the ultrathin nature of the 2D sheets. These 2D NSs possess both
physical and chemical advantages over nanoparticles such as high surface area to volume ratio,
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extensive interfacial contact area and easily accessible reaction sites. The HRTEM image of the
TiO2 NSs in Figure 4e shows clear lattice fringes indicating good crystallinity of the TiO2 NSs,
and the lattice spacing of 0.35 nm corresponds to the (101) plane of anatase TiO2. The XRD
diffraction pattern obtained from the TiO2-AC composite is the same as that of the pristine TiO2
NSs (Figure 4f). The peaks at 25.34, 37.91, 48.10, 54.14, 55.17, 62.73, 68.88, 70.29 and 75.13○
can be indexed to the (101), (004), (200), (105), (211), (204), (116), (220) and (215) crystal planes
of anatase TiO2 (JCPDS card no. 21-1272), indicating that the TiO2 NSs in the TiO2-AC composite
is also in the anatase phase.
Figure 4. (a) SEM, (b) TEM images, and (c) elemental mapping of TiO2-AC composite. (d) High
magnification TEM and (e) HRTEM of TiO2 NSs in the TiO2-AC composite. (f) XRD pattern of
pristine TiO2 NSs and TiO2-AC composite.
BET measurement was carried out on TiO2-AC and the corresponding isotherm and pore size
distributions are plotted in Figure S3, with the microstructural details listed in Table 1. The
structural properties of commercial anatase TiO2 (A-TiO2) nanoparticles and pristine TiO2 NSs
were also included for comparison. The TiO2-AC composite exhibits a Type I isotherm which
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suggests the presence of large amounts of micropores due to the AC-2 at the core of the hierarchical
structure. The composite has a surface area of 162.1 m2 g-1 which is higher than both the A-TiO2
(6.35 m2 g-1) and TiO2 NSs (94.93 m2 g-1) due to the presence of AC-2, suggesting better adsorption
capacity. However, the surface area is lower than that of AC-2 possibly due to a partial coverage
or blockage of the pores during the synthesis of TiO2.[18] The UV-vis absorption spectra of the
various samples are shown in Figure S4. The TiO2-AC composite exhibited a marked increase in
absorption in the visible region without any observable shift in the bandgap. This indicates that the
presence of a carbon core greatly enhances the absorption of visible wavelengths, which can aid
in the enhancement of photocatalytic activity through the generation of more electron-hole pairs
under UV-visible light illumination.
The samples were then tested in the photodegradation of MO (Figure 5), which is akin to
demonstrating its capability in wastewater treatment. When the TiO2-AC composite is illuminated
by solar irradiation, the photogenerated electrons will react with dissolved oxygen molecules to
•-
form oxygen peroxide radicals O2 , while the photogenerated holes react with H2O to produce
hydroxyl radicals OH• (Figure S5). These reactive oxidizing radicals will then mineralize the
organic pollutants into CO2, H2O and other mineralization products.[19, 20] Prior to illumination, the
mixture was magnetically stirred in the dark for 60 min to attain adsorption-desorption equilibrium
between the composite material and the MO solution. From the plot in Figure 5a, it can be seen
that a significant amount of MO was adsorbed by TiO2-AC composite in the dark, while little or
no adsorption was observed for A-TiO2 and TiO2 NSs. This is due to the presence of AC, which
is a very strong absorbent, in the core of the composite material. Upon illumination, the TiO2-AC
composite performed the best and was able to completely degrade MO in 120 min. This
enhancement in degradation performance can be attributed to the high surface area of the TiO2
NSs and the high adsorption capability of AC. The adsorption of MO by the AC core aids in
concentrating the MO molecules for faster degradation by TiO2. The photodegradation activity
was further analyzed by studying the pseudo-first order kinetics of the various photocatalysts as
shown in Figure S6, and the rate constants, k, are summarized in Table S1. The constant k of A-
TiO2 is the lowest at 0.0317 min-1, while TiO2-AC exhibited the highest k constant at 0.0422 min-
1, indicating its enhanced dye degradation capability. The degradation results clearly show that the
photodegradation of MO improved with the presence of AC in the core of the TiO2 NSs. AC has
a well-developed pore structure, large surface area and strong adsorption capacity. In the
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composite, AC acts as a center where the organic pollutant molecules are adsorbed and
concentrated before transferring to illuminated TiO2 NSs for degradation, resulting in higher rates
of reaction and in turn, higher rates of MO degradation. The increased absorption of light also
resulted in the production of more electron-hole pairs which helps in enhancing the degradation
rate. The long-time stability of the composite material is of high importance for actual application,
hence the degradation activity of TiO2-AC was examined in three successive MO degradation
reactions. As shown in Figure 5b, the MO solution with a concentration of 0.03 mM was degraded
in 60 min and the performance was consistent over the 3 cycles, indicating a stable catalytic
activity for TiO2-AC.
Besides MO, the samples were also investigated in the degradation of VOCs, namely ethanol,
methanol, and formaldehyde (which can possibly exist as indoor air pollutants) as shown in Figure
5c and S8. Prior to illumination, the samples were also left in the dark for 120 min to attain
adsorption-desorption equilibrium between the composite material and the VOC vapor (Figure
S7). Upon illumination with UV light, the TiO2-AC composite was able to photocatalytically
degrade all 3 types of VOCs completely in the shortest time amongst the samples tested. Similar
to the MO degradation results, the excellent performance can be attributed to the adsorption
properties of the AC core which has high surface area and abundant active adsorption sites, aiding
in the adsorption and concentration of the VOC vapors which are then photocatalytically degraded
by the TiO2 NSs. The degradation kinetics is shown in Figure S8 and the pseudo-first order rate
constants, k, of the various photocatalysts are summarized in Table S2. The TiO2-AC composite
has the highest k constant compared to A-TiO2 and TiO2 NSs for all the VOCs tested. The
repeatability of the TiO2-AC composite was also tested in repeated degradation of ethanol vapor
with a concentration of 300 ppm (Figure 5d). TiO2-AC could consistently degrade ethanol to
completion in 30 min over the 3 cycles of testing. This shows that the composite material is stable
over prolonged and repeated use in the photodegradation of VOCs.
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0.8
0.6
0.4
0.2
0.0
Adsorption
Degradation
A-TiO2
(a)1.0
(b)
TiO2 NSs
TiO2-AC
0.8
0.6
0.4
0.2
0.0
-60 -30 0 30 60 90 120 150
Time (min)
0
30 60 90 120 150 180 210
Time (min)
A-TiO2
(d)100
100
(c)
TiO2 NSs
TiO2-AC
80
60
40
20
0
80
60
40
20
0
0
30 60 90 120 150 180
Time (min)
0
30
60
90
120
Time (min)
Figure 5. Degradation kinetics of time evolution (a) MO and (c) ethanol photodegradation in the
presence of A-TiO2, TiO2 NSs, and TiO2-AC composite. Repeated cycling of photocatalytic
degradation of (b) MO and (d) ethanol in the presence of TiO2-AC composite.
Conclusions
Carbon nanospheres derived from hydrothermally treated glucose solution were successfully
transformed into thin layered planar sheets and hierarchical pores with large specific surface area
through a cost and process effective process of simultaneous activate-exfoliate-reassemble. It
possesses a hierarchical porous structure consisting of micro/meso/macropores. TiO2 NSs were
then grown on the hierarchical porous carbon structure as a proof-of-concept demonstration of a
composite material that facilitates efficient transport of reactants throughout the carbonaceous
matrix, leading to enhanced adsorption and photocatalytic degradation performance of aqueous
and gaseous organic pollutant compounds. This monolithically integrated dual functionalities
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material system could possibly serve as a promising sustainable environmental remediation
solution in the future.
Experimental Section
Synthesis of carbon spheres (CSs) and KOH activation
1.89 g of D(+)-glucose (Sigma Aldrich) was dissolved in 35 mL of deionized (DI) water and placed
in a 50 mL Teflon-lined stainless steel autoclave at 190 °C for 4 h. After cooling to room
temperature, the brown mixture was collected and washed three times using DI water via
centrifuging. The precipitate was then dried at 80 °C for 6 h and the brown solid obtained was CSs.
The CSs were then subjected to potassium hydroxide (KOH, Acros Organics) activation treatment
with CS: KOH mass ratios of 1:2, 1:4 and 1:6 and labeled as AC-1, AC-2 and AC-3 respectively.
0.1 g of CSs was mixed with the corresponding amount of KOH in an agate mortar and pestle, and
subsequently annealed under flowing nitrogen gas at 600 °C for 2 h with a ramping rate of 20 °C
min-1. The resultant product was washed repeatedly with DI water until the pH reaches 7, and then
dried at 70 °C for 5 h.
Methyl orange (MO) adsorption by AC
5 mg of AC was dispersed in 10 mL of 0.24 mM aqueous MO solution by sonication, and then
magnetically stirred in the dark. At regular time intervals of 15 min, the mixture was centrifuged,
extracted and passed through a 0.2 μm syringe filter (Thermo Scientific), before analyzing the
concentration of the MO solution using a UV-vis-NIR spectrophotometer. The maximum
absorbance peak value at 462.5 nm was used to plot the amount of MO adsorbed and thus,
determine the adsorption activity of the AC.
Volatile organic compound (VOC) adsorption by AC
1 mg of AC was coated on a 1.5 x 2.5 cm glass substrate which was then placed into a quartz vial
reactor. The reactor was filled with a VOC-Argon vapor mixture of 2000 ppm. The VOCs tested
were ethanol, methanol and formaldehyde. 100 µL gas samples were extracted at regular time
intervals using a gas-tight syringe and injected into a GCMS (Shimadzu GCMS-QP2010 Ultra) to
determine the concentration of VOC remaining in the reactor.
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10.1002/cctc.201700137
ChemCatChem
Synthesis of TiO2 NSs on AC (TiO2-AC)
5 mg of AC was dispersed in 20 mL of pure ethanol (Merck) by ultrasonication for 5 min. 40 µL
of titanium tetraisopropoxide (TTIP, Sigma Aldrich) was added to the solution and stirred for 5
min, followed by the addition of 200 µL of diethylenetetramine (DETA, Alfa Aesar) and 10 min
of stirring. The solution was heated at 200 °C for 20 h in a Teflon-lined stainless steel autoclave.
Once cooled, the product was washed with ethanol by centrifuging several times and dried at 80
°C. Finally, the dried product was annealed at 450 °C for 2 h with a heating rate of 2 °C min-1.
This composite material is denoted as TiO2-AC.
Photocatalytic degradation of aqueous MO
10 mg of the powdered photocatalyst was dispersed in 20 mL of 0.06 mM aqueous MO solution
in the quartz vial reactor by sonication. The mixture was then magnetically stirred in the dark for
60 min to achieve complete adsorption-desorption equilibrium, before irradiation with a 160 mW
light emitting diode (LED) of wavelength 365 nm. The concentration of MO was determined using
a UV-vis-NIR spectrophotometer and the maximum absorbance peak value at 462.5 nm was used
to plot the amount of MO degraded and thus, determine the photodegradation activity of the TiO2-
AC composite.
Photocatalytic degradation of VOCs
1 mg of the powdered photocatalyst was coated on a 1.5 x 2.5 cm glass substrate which was then
placed into a quartz vial reactor. The reactor was filled with VOC vapor of 2000 ppm
concentration. The VOCs tested were ethanol, methanol and formaldehyde. The mixture was left
in the dark for 2 h to achieve complete adsorption-desorption equilibrium before it was illuminated
with a 160 mW light emitting diode (LED) of wavelength 365 nm. 100 µL gas samples were
extracted at regular time intervals using a gas-tight syringe and injected into a GCMS (Shimadzu
GCMS-QP2010 Ultra) to determine the concentration of VOC remaining in the reactor.
Materials characterization
The morphology of the nanostructures were characterized with scanning electron microscopy
(SEM, JEOL FEG JSM 7001F) operated at 15 kV. The elements present in the nanostructures were
analyzed using energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments) and also
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700137
ChemCatChem
scanning transmission electron microscopy (STEM, JEOL 2100 TEM). The crystalline structures
of the TiO2-AC composite were analyzed using X-ray diffraction (XRD, D5005 Bruker X-ray
diffractometer equipped with graphite-monochromated Cu Kα radiation at λ = 1.541 Å) and
transmission electron microscopy (TEM, JEOL 2100 TEM). Brunauer–Emmett–Teller (BET,
Quantachrome Nova 2200e) measurements were conducted with nitrogen (N2) as the adsorbate at
liquid nitrogen temperature. The specific surface area (SBET) was calculated according to the
multiple-point Brunauer-Emmett-Teller (BET) method at relative pressure P/P0=0.01-0.1. Porosity
distributions were calculated by the Quenched Solid Density Functional Theory (QSDFT) and
Barrett– Joyner–Halenda (BJH) method. The total pore volume was obtained from the volume of
N2 adsorbed at a relative pressure of P/P0 ≈ 0.99. Absorption spectra of the samples were measured
with a UV-VIS-NIR spectrophotometer (UV-VIS, Shimadzu UV-3600). Fourier transform
infrared (FTIR) spectra of the products were recorded on a Shimadzu IRPrestige-21 FT-IR
spectrophotometer. The concentration of VOCs was measured by gas chromatography mass
spectrometry (Shimadzu GCMS-QP2010 Ultra).
Acknowledgements
This research is supported by the National Research Foundation (NRF), under Energy Innovation
Research Programme (EIRP) R-263-000-B82-279, managed on behalf of Building and
Construction Authority (BCA).
References
[1]
[2]
A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183-191.
S. L. Candelaria, Y. Shao, W. Zhou, X. Li, J. Xiao, J.-G. Zhang, Y. Wang, J. Liu, J. Li, G.
Cao, Nano Energy 2012, 1, 195-220.
[3]
[4]
[5]
E. Lam, J. H. T. Luong, ACS Catalysis 2014, 4, 3393-3410.
P. Hadi, J. Guo, J. Barford, G. McKay, Environ. Sci. Technol. 2016, 50, 5041-5049.
R. Mas-Balleste, C. Gomez-Navarro, J. Gomez-Herrero, F. Zamora, Nanoscale 2011, 3,
20-30.
[6]
M. Yi, Z. Shen, J. Mater. Chem. A 2015, 3, 11700-11715.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700137
ChemCatChem
[7]
K. R. Paton, E. Varrla, C. Backes, R. J. Smith, U. Khan, A. O’Neill, C. Boland, M. Lotya,
O. M. Istrate, P. King, T. Higgins, S. Barwich, P. May, P. Puczkarski, I. Ahmed, M.
Moebius, H. Pettersson, E. Long, J. Coelho, S. E. O’Brien, E. K. McGuire, B. M. Sanchez,
G. S. Duesberg, N. McEvoy, T. J. Pennycook, C. Downing, A. Crossley, V. Nicolosi, J. N.
Coleman, Nat. Mater. 2014, 13, 624-630.
[8]
[9]
Y. Zhang, L. Zhang, C. Zhou, Accounts of Chemical Research 2013, 46, 2329-2339.
L. Mao, Y. Zhang, Y. Hu, K. H. Ho, Q. Ke, H. Liu, Z. Hu, D. Zhao, J. Wang, RSC Adv.
2015, 5, 9307-9313.
[10] L. Xu, L. Guo, G. Hu, J. Chen, X. Hu, S. Wang, W. Dai, M. Fan, RSC Adv. 2015, 5, 37964-
37969.
[11] D. Lozano-Castelló, M. A. Lillo-Ródenas, D. Cazorla-Amorós, A. Linares-Solano, Carbon
2001, 39, 741-749.
[12] M. Sevilla, A. B. Fuertes, Chem. Eur. J. 2009, 15, 4195-4203.
[13] X. Sun, Y. Li, Angew. Chem. Int. Ed. 2004, 43, 597-601.
[14] Y. Guo, S. Yang, K. Yu, J. Zhao, Z. Wang, H. Xu, Mater. Chem. Phys. 2002, 74, 320-323.
[15] X. Zheng, W. Lv, Y. Tao, J. Shao, C. Zhang, D. Liu, J. Luo, D.-W. Wang, Q.-H. Yang,
Chem. Mater. 2014, 26, 6896-6903.
[16] J. Wang, S. Kaskel, J. Mater. Chem. 2012, 22, 23710-23725.
[17] X. Zheng, J. Luo, W. Lv, D.-W. Wang, Q.-H. Yang, Adv. Mater. 2015, 27, 5388-5395.
[18] B. Gao, P. S. Yap, T. M. Lim, T.-T. Lim, Chem. Eng. J. 2011, 171, 1098-1107.
[19] S. Helali, E. Puzenat, N. Perol, M.-J. Safi, C. Guillard, Appl. Catal. A: Gen 2011, 402, 201-
207.
[20] A. A. Vega, G. E. Imoberdorf, M. Mohseni, Appl. Catal. A: Gen 2011, 405, 120-128.
This article is protected by copyright. All rights reserved.