Ultra-thin nanosheet assemblies of graphitic carbon nitride for enhanced photocatalytic CO2 reduction

Article abstract of DOI:10.1039/c6ta08310b

A two-dimensional layered polymeric photocatalyst, graphitic carbon nitride (g-C₃N₄), is becoming the rising star in the field of solar-to-fuel conversion. However, the performance of commonly prepared g-C₃N₄ is usually very weak because of the high recombination rate of photogenerated charge carriers and a small amount of surface active sites. Here we demonstrate simultaneous texture modification and surface functionalization of g-C₃N₄via a stepwise NH₃-mediated thermal exfoliation approach. The resulting g-C₃N₄ photocatalyst possesses a hierarchical structure obtained by the assembly of amine-functionalized ultrathin nanosheets and thus exhibits remarkably enhanced light harvesting, a high redox ability of charge carriers, increased CO₂ adsorption and a larger amount of surface active sites, as well as improved charge carrier transfer and separation. Therefore the aforementioned hierarchical g-C₃N₄ consisting of amine-functionalized ultra-thin nanosheets shows much better performance for photocatalytic CO₂ reduction than unmodified conventional g-C₃N₄ photocatalysts.

Full text of DOI:10.1039/c6ta08310b

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Ultra-thin nanosheet assemblies of graphitic carbon nitride for
enhanced photocatalytic CO2 reduction
Pengfei Xia,a Bicheng Zhu,a Jiaguo Yu,*a Shaowen Cao*a and Mietek Jaroniec*b
Received 00th January 20xx,
Accepted 00th January 20xx
A two-dimensional layered polymeric photocatalyst, graphitic carbon nitride (g-C3N4), has becoming the rising star in the
field of solar-to-fuels conversion. However, the performance of commonly prepared g-C3N4 is usually very weak because of
the high recombination rate of photogenerated charge carriers and a small amount of surface active sites. Here we
demonstrate a simultaneous texture modification and surface functionalization of g-C3N4 via a stepwise NH3-mediated
thermal exfoliation approach. The resulting g-C3N4 photocatalyst possesses a hierarchical structure obtained by assembly
of amine-functionalized ultrathin nanosheets, and thus exhibits a remarkably enhanced light harvesting, high redox ability
of charge carriers, increased CO2 adsorption and larger amount of surface reactive sites, as well as improved charge carrier
transfer and separation. Therefore the aforementioned hierarchical g-C3N4 consisting of amine-functionalized ultra-thin
nanosheets shows much better performance for photocatalytic CO2 reduction than the unmodified conventional g-C3N4
photocatalysts.
DOI: 10.1039/x0xx00000x
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also increase the specific surface area.10-12 Moreover, their
Introduction
surface amine-functionalization can promote the CO2
An increasing consumption of fossil fuels over several past
decades continues to deepen the worldwide energy crisis
accompanied by green-house effect.1,2 Since CO2 is one of the
main green-house gases, its direct photocatalytic conversion
into hydrocarbon fuels such as CH3OH and CH4 is a very
promising way to address the energy shortage and global
warming.3,4 However, the photocatalytic reduction of CO2 by
using semiconductor photocatalysts is a complex process,
which exhibits quite low efficiency due to various
indeterminacy issues.5 It is known that the efficient photon
utilization, fast charge carrier migration and separation,
effective mass transfer, abundant surface adsorption and
active sites are crucial factors for achieving high-performance
photocatalytic CO2 reduction. Hence, there is a great interest
in the development of rational strategies for the synthesis of
semiconductor photocatalysts with the above-mentioned
properties to enhance the CO2 photoreduction.6-8 For instance,
the development of hierarchical structures can enlarge the
specific surface area and improve the overall porous structure,
which facilitate the molecular diffusion and transfer and
strengthen the light harvesting.9 Design of photocatalysts with
ultra-small size can lead to efficient charge carrier transfer and
separation, enhance the redox ability of charge carriers, and
adsorption through the base-acid interactions.13,14
Graphitic carbon nitride (g-C3N4),
a polymeric-type
semiconductor composed of earth-abundant nitrogen and
carbon elements, has been recognized as a fascinating
photocatalyst for a variety of applications due to several
advantages such as low cost, non-toxicity, excellent thermal
and chemical stability, and appropriate band structure.15,16
Especially, g-C3N4 has a great potential for CO2 photoreduction,
because it possesses a sufficiently negative conduction band
(CB) position to satisfy the thermodynamic requirement of
most photocatalytic CO2 reduction reactions.9,17 However, the
photocatalytic efficiency of CO2 reduction in the presence of
commonly prepared g-C3N4 materials is very weak because of
the high recombination rate for photogenerated charge
carriers and a small amount of surface active sites for CO2
adsorption, which is mainly caused by the stacking of
polymeric layers.18,19 Herein, we aim to improve the
photocatalytic CO2-reduction performance over g-C3N4 by
simultaneous modification of its texture and surface
functionalization. Namely, the hierarchical g-C3N4 consisting of
amine-functionalized ultra-thin nanosheets, are prepared via a
stepwise NH3-mediated thermal exfoliation of dicyandiamide-
derived g-C3N4. The texture- and surface-modified g-C3N4
photocatalyst exhibits a remarkably high CO2 photoreduction
performance due to the formation of hierarchical structure,
greatly reduced thickness of nanosheets and their amine-
functionalization, which essentially enhance light harvesting,
reinforce the redox ability of charge carriers, increase the CO2
adsorption and the amount of surface reactive sites, as well as
facilitate the charge carrier transfer and separation.
a.State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, P. R. China
E-mail: jiaguoyu@yahoo.com; swcao@whut.edu.cn.
b.Department of Chemistry and Biochemistry, Kent State University, OH 44242, USA
E-mail: jaroniec@kent.edu.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Experimental
Preparation of bulk g-C3N4
The in-situ FT-IR spectra were automatically recorVdieewdAortincleaOnnilinne-
situ diffuse reflectance infrared Fourier transform
DOI: 10.1039/C6TA08310B
spectroscopy (Nicolet iS50, TMO, US). The as-prepared sample
was degassed for 6 h at 150 °C prior to measurement. Then
the degassed sample was carefully sealed in a specimen
chamber of the equipment without any illumination. After that,
this specimen chamber with the sample was firstly purged with
nitrogen for 1 h to remove the air. Then the flowing gas
mixtures consisting of CO2 and vapour were purged with a rate
of 20 mL/min for another hour. Subsequently, the light source
from a 100 W LED lamp was offered till the end of the test.
All chemical reagents (analytical grade, SINOPHARM, CHINA)
were purchased and used without any further purification. In
order to prepare bulk g-C3N4,
a crucible with 10 g
dicyandiamide was placed in a muffle furnace. Then the
programmed temperature was set up to 550 °C for 3 h with a
ramp rate of 5 °C/min. After heat treatment, the light yellow
powder was obtained and marked as bulk-CN.
Synthesis of g-C3N4 nanosheets
Graphic carbon nitride nanosheets were prepared by directly
exfoliating the bulk g-C3N4 in the NH3 atmosphere. In detail, 1
g of g-C3N4 was thoroughly ground and was covered in a
ceramic boat, which was then placed in a tube furnace. After
that the sample was exposed to flowing ammonia (15 mL/min)
to remove the air prior to calcination. The temperature
program was set to 580 °C for 3 h with a heating rate of
20 °C/min. Ammonia was continuously flowing during the
entire heat treatment. After rapid cooling to room
temperature, the above temperature program was repeated
for another two times. Finally, the obtained samples were
carefully collected and labeled as NS-CN.
DFT calculations
Density function theory (DFT) calculations were performed
using the CASTEP Package. The Perdew−Burke−Ernzerhof (PBE)
of the generalized gradient approximation (GGA) was used as
the exchange-correlation function. An energy cutoff of 450 eV
and the Monkhorst–Pack k-point mesh of 4 × 4 × 1 were used
to perform geometry optimizations. The energy and force
convergence criterions were set as 1.0 × 10–5 eV/atom and
0.03 eV/Å, respectively.
Photocatalytic CO2 reduction
All samples were degassed for 6 h at 150 °C prior to
measurement. In a typical experiment, a 300 W Xe lamp (XD
350, CHIAN) was employed as the light source. 100 mg of
degassed nanosheets and 0.084 g of NaHCO3 were separately
placed into a reactor, which was a two-neck Pyrex container
with two parallel planes. Then the reactor was sealed with the
rubber plug. After that, this reaction system was purged with
nitrogen for 1 h to thoroughly remove air. CO2 was introduced
by a simple reaction between H2SO4 and NaHCO3 in the closed
container. The H2SO4 was elaborately injected into the special
groove on one neck of the container to react with NaHCO3. 1
mL of resulting gas was sampled at an interval of 1 h
irradiation, and then detected by a gas chromatograph (GC-
14C, SHIMADZU, JAPAN).
Characterization
X-ray diffraction (XRD) patterns were obtained by a D/Max-RB
X-ray diffractometer (Rigaku, Japan). Fourier transform
infrared spectra (FTIR) were measured on an IR Affinity-1 FTIR
spectrometer (Shimadzu, Japan). TEM images were obtained
on a transmission electron microscope (TEM, JEM-2100F, JEOL,
JAPAN) with an accelerating voltage of 200 kV. FESEM images
were recorded on
a field emission scanning electron
microscope (FESEM, JSM-7500, JAPAN) with an accelerating
voltage of 15 kV. Chemical states of elements were measured
on an X-ray photoelectron spectroscopy (XPS) equipped with
an ultra-high vacuum VG ESCALAB 210 electron spectrometer.
The binding energy was calibrated with the C 1s peak at 284.8
eV before analysis. The thickness of as-prepared nanosheets
was evaluated by an atomic force microscope (AFM) (SPM-
9500, SHIMADZU, JAPAN). UV–visible diffuse reflectance
spectra were obtained on a UV–visible spectrophotometer
(UV-2550, Shimadzu, Japan), using BaSO4 as the reference
standard. N2 adsorption-desorption isotherms were measured
on a Micromeritics ASAP 3020 equipment (USA), which was
also used to conduct CO2 adsorption measurements at room
temperature. All as-synthesized samples were degassed at
180 °C for 5 h prior to adsorption measurements. Specific
surface area was determined by the multipoint Brunauer–
Results and Discussion
The XRD patterns of dicyandiamide-derived g-C3N4 (bulk-CN)
and g-C3N4 ultra-thin nanosheets (NS-CN) are shown in Fig. 1.
The XRD pattern of the bulk-CN exhibits the typical (100) and
(002) diffraction peaks at 13.2° and 27.3°, respectively. The
former corresponds to the repeated structural packing of tri-s-
triazine heterocycles in the conjugated planes, and the latter
can be ascribed to the regular graphitic-like interlayer
stacking.20,21 Consistently, the pattern of NS-CN shows the (100)
and (002) peaks at the same positions, indicating the
preservation of the molecular structure of graphitic carbon
nitride. However, the peak intensities are much weaker than
those of the bulk-CN and moreover, a broad peak in the range
of 20°~30° corresponding to the amorphous phase appears.
These results reveal a decrease in the crystallinity of graphitic
carbon nitride after NH3-mediated thermal exfoliation, which
leads to the formation of long-range ordered ultra-thin
nanosheets and short-range ordered graphitic molecular
fragments (related to the amorphous peak).22,23
Emmett–Teller
photoluminescence (TRPL) spectra were recorded on a FLS920
fluorescence lifetime spectrophotometer (Edinburgh,
(BET)
method.
Time-resolved
Instruments, UK). Electrochemical measurements were carried
out on an electrochemical workstation (CHI66E, CHINA), using
the Na2SO4 solution as the electrolyte. The elemental analysis
was performed on an elemental analyzer (Vario EL cube,
Elementar, Germany).
In-situ FT-IR measurement
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carriers from the interior to the surface of g-C3N4, resulting in
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higher charge separation efficiency and sDmOaI:ll1e0r.1r0e3c9o/Cm6TbAin08a3ti1o0nB
of photogenerated charge carriers. Moreover, ultrathin g-C3N4
has larger band gap, which infers the formation of
photogenerated charge carriers with stronger redox ability.24,25
All these features of ultrathin g-C3N4 will enhance its
photocatalytic activity toward CO2 reduction.
To further study the hierarchical structure of NS-CN, the N2
adsorption–desorption isotherms were obtained and shown in
Fig. 3. Both bulk-CN and NS-CN possess type IV nitrogen
adsorption–desorption isotherms according to BDDT
(Brunauer–Deming–Demin–Teller) classification, indicating
that
the
multilayer
adsorption
and
capillary
condensation/evaporation take place during nitrogen
adsorption–desorption process.26,27 Besides, the shape of both
hysteresis loops is H3 type according to the International
Union of Pure and Applied Chemistry (IUPAC) classification,
revealing that these layered materials have the slit-shaped
pores due to aggregation of flake-like particles.26-28
Importantly, NS-CN shows much higher N2 adsorption capacity
than the bulk-CN in the full range of relative pressures (P/P0).
This is because of the existence of larger amount of mesopores
in the NS-CN sample. The pore size distribution (PSD) curves
(inset in Fig. 3) show that despite the presence of macropores
in both samples, a much more intensive peak appeared on the
PSD curve at the pore sizes of about 3 nm for NS-CN as
compared to that for the bulk-CN. The specific surface area,
total pore volume and average pore size of NS-CN are 116
m2/g, 0.35 cm3/g, and 14.5 nm, respectively, which all are
greatly improved as compared to those of bulk-CN (15 m2/g,
0.11 cm3/g, and 23.6 nm, respectively). These results confirm
the formation of hierarchical structures during assembly of
ultrathin nanosheets, which consequently increases the
specific surface area and facilitates the mass diffusion and
transfer due to the presence of interconnected porous
network, as well as improves the light harvesting through light-
scattering effects.9,29
Fig. 1 XRD patterns of the bulk-CN and NS-CN samples.
The morphology of NS-CN was investigated by FESEM
imaging. Interestingly, hierarchical structures assembled by
ultra-thin nanosheets are visible on the FESEM images (Fig. 2a
and b), while the bulk-CN is composed of aggregated particles
(Fig. 2c). A nanosheet-type structure of NS-CN was further
probed by TEM and AFM analysis. As shown in Fig. 2d, the
nanosheets domains are thin and nearly transparent,
indicating their ultra-thin thickness. The AFM image in Fig. 2e
confirms that the thickness of these nanosheets is ~3 nm,
corresponding to 9-10 layers of conjugated planes of graphitic
carbon nitride. The results demonstrate that the NH3-
mediated thermal treatment of the bulk g-C3N4 led to its
efficient exfoliation into ultrathin nanosheets. The formation
of ultrathin nanosheets is illustrated in Fig. 2f. As a small and
polar molecule, NH3 can intercalate into the interplanar space
of the bulk g-C3N4 during heating process, and consequently
break the weak van der Waals bonds between the graphitic
layers of carbon nitride. Then efficient exfoliation of the bulk g-
C3N4 into ultrathin nanosheets could be achieved by the
thermal shock of NH3 gas. Such ultrathin structure can not only
increase the specific surface area and the amount of surface
active sites, but also shorten the diffusion distance of charge
Fig. 3 N2 adsorption–desorption isotherms measured for the
bulk-CN and NS-CN, and (b) the corresponding pore-size
distribution curves obtained by the BJH method using
adsorption branch data.
Fig. 2 (a,b) FESEM images of NS-CN. (c) FESEM image of the
bulk-CN. (d) TEM and e) AFM images of NS-CN. (f) Illustration
of the formation process of NS-CN photocatalysts.
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ascribed to adsorbed H2O or CO2 on the surface.39 It is
noteworthy that all the peaks in the high-resolution spectra of
C and N elements show no shift, suggesting that the main
molecular structure of g-C3N4 on the basis of tri-s-triazine was
not affected during NH3-mediated thermal exfoliation process.
However, the estimated molar ratios of C/N in the bulk-CN and
NS-CN according to the XPS analysis are 0.73 and 0.65,
respectively. The signal intensity of fitted peaks (inset, Fig 5c)
of amino nitrogen atoms on NS-CN sample is higher than that
of bulk-CN, implying that the NS-CN sample processes more
marginal amino groups than bulk-CN. Furthermore, the
elemental analysis confirms that the content of hydrogen
element on the surface of NS-CN and bulk-CN is 1.177% and
0.861%, respectively. Besides, the content of nitrogen for NS-
CN and bulk-CN is 63.74% and 63.51%, respectively, based on
the elemental analysis. An increase in the H and N contents in
NS-CN can be attributed to the introduction of amino groups
as discussed above.
Zeta potentials measured for the bulk-CN and NS-CN
samples in aqueous solution under different pH values reveal
the variation of surface charges, as shown in Fig. 6. Normally,
there are primary (CNH2), secondary (C2NH) and tertiary (C3N)
amino groups in the structure of g-C3N4. The CNH2 and C2NH
amino groups exist in the marginal part of the g-C3N4
framework due to an incomplete condensation of melem,
while the C3N amino groups exist in the conjugated plane
composed of tri-s-triazine heterocyclic rings. Particularly, the
CNH2 and C2NH groups can accept either hydroxyl ions or
protons in the aqueous solution under different pH values.
However, the C3N groups are regarded as only the proton
Fig. 4 FTIR spectra for the bulk-CN and NS-CN samples.
FT-IR spectra (Fig. 4) were measured to investigate the
surface properties of the bulk-CN and NS-CN samples. Both
spectra display a sharp peak at 810 cm–1 that corresponds to
the characteristic breathing mode of tri-s-triazine heterocycles,
revealing that the tri-s-triazine-based structure was not broken
during the NH3-mediated thermal exfoliation process.30 The
peaks in the region of 1200–1700 cm–1 originate from different
vibration modes of C–N bond within the tri-s-triazine rings.11,31
Moreover, the broad absorption band from 3000 to 3400 cm–1
can be ascribed to various vibration modes of N–H bond,31,32
implying that the existence of amino groups on the surface of
both g-C3N4 samples. Note that the transmissivity of this N–H
related band on the IR spectrum (inset) of Bulk-CN and NS-CN
is 55% and 43%, respectively, indicating that more amino
groups are introduced onto the surface of g-C3N4 during the
NH3-mediated thermal treatment. The amino groups are
expected to enhance the CO2 adsorption ability through the
base-acid interactions.13,14,33
The chemical states of surface elements were analyzed by
XPS. The full survey spectra in Fig. 5a indicate the existence of
C, N and O elements in both bulk-CN and NS-CN samples. The
high-resolution spectra of C 1s shown in Fig. 5b can be fitted
into two peaks at 284.8 and 287.8 eV. The former corresponds
to sp2 C–C bonding from surface adventitious carbon34,35 and
the latter is attributed to the characteristic N=C–N2 bonding in
the molecular structure of g-C3N4.36 Fig. 5c shows the high-
resolution spectra of N 1s spectra. The peaks at binding
energies of 398.2 and 399.7 eV are assigned to sp2-hybridized
nitrogen atoms (C=N–C) and bridging nitrogen atoms ((C)3–N),
respectively.37 The peak centered at 401.2 eV is an indicative
of N–H bonding, and the weak peak at 404.1 eV is due to the
charging effects caused by π-excitations.38 Additionally, a
single peak is also observed at 532.3 eV (Fig. 5d), which is
Fig. 5 (a) Survey XPS spectra of the bulk-CN and NS-CN, high-
resolution XPS spectra of the C1s (b), N1s (c), and O1s (d)
recorded for the bulk-CN and NS-CN.
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Fig. 6 Zeta potential curves obtained for the bulk-CN and NS-
CN samples.
Fig. 8 The optimized configurations of a single CO2 adsorbed
on the surface of (a) bulk-CN and (b) NS-CN based on DFT
calculations.
acceptors because of the existence of lone pair electrons in sp2
hybrid-N atoms.34,35 The isoelectric points (IEP) of the bulk-CN
and NS-CN samples were measured to be 4.82 and 5.76,
respectively. Since the amount of the exposed internal C3N
groups after exfoliation of the bulk-CN into ultrathin
nanosheets is higher, the proton adsorption capacity of NS-CN
is higher as evidenced by larger IEP value. These more exposed
C3N groups with lone pair electrons of N are actually Lewis
base sites and can serve as adsorption sites of CO2
molecules.40
A higher affinity of a photocatalyst toward CO2 is beneficial
for surface catalytic reactions.19,41 Once CO2 molecules are
trapped by active sites on the surface of a photocatalyst, they
can be reduced by photoelectrons and converted into fuels
such as CH4, CH3OH and so on.17,42 As such, CO2 adsorption
curves were measured and shown in Fig. 7. As can be seen
from this figure the NS-CN sample shows a remarkably
enlarged CO2 adsorption, which is more than 4 times higher
than that measured for the bulk-CN. The much higher CO2
adsorption capacity of NS-CN in the pressure range studied can
be mainly attributed to the improved physical adsorption that
originates from the high specific area and well developed
porosity as well as chemical adsorption of CO2 molecules on
amine groups. In addition, the density functional theory (DFT)
calculations were employed to evaluate the adsorption energy
(Ea) of CO2 molecules on the photocatalysts studied. As defined
here, a more negative Ea value suggests stronger adsorption of
CO2 on the surface of a photocatalyst.43 The optimized
configurations of CO2 adsorbed on the surface of the bulk-CN
and NS-CN are shown in Fig. 8. Particularly, the calculated
adsorption energy (Ea) of the bulk-CN and NS-CN are –0.99 and
–1.37 eV, respectively. The results indicate that the NS-CN
sample shows higher adsorption affinity toward CO2 under the
identical conditions, which is beneficial for the photocatalytic
CO2 conversion.
Fig. 9a shows the UV–vis absorption spectra of the bulk-CN
and NS-CN. The absorption edge of the bulk-CN is ~455 nm,
which corresponds to the band gap of ~2.72 eV according to
the Kubelka-Munk function (inset of Fig. 9a). However, an
obvious blue shift of the absorption edge visible on the
spectrum of NS-CN, gives rise to an enlarged band gap of ~2.98
eV. This is due to the quantum size effect of the ultrathin
structure of NS-CN.44 An enhanced light absorption in the wide
range of 450-800 nm is also observed for NS-CN, caused by the
light-scattering effects of the hierarchical structures, which can
promote surface photocatalytic reactions. The Mott–Schottky
plots (Fig. 9b and c) and XPS valence band (VB) spectra (Fig. 9d)
are further measured to elucidate the CB and VB positions of
the bulk-CN and NS-CN. Both samples are n-type
semiconductors based on the Mott–Schottky plots. The flat-
band potentials are –1.16 and –1.25 V (vs. NHE, pH = 7) for the
bulk-CN and NS-CN, respectively. While their VB values from
XPS are determined to be 1.58 and 1.76 V, respectively.
Thereby the band structures can be well resolved as illustrated
in Fig. 9e. It is noteworthy that NS-CN possesses a more
negative CB and more positive VB as compared to those of
bulk-CN, indicating stronger reduction and oxidation ability of
photogenerated charge carriers.9,45
The electronic structures for the bulk-CN and NS-CN were
also investigated by the DFT calculations. The density of states
(DOS) (Fig. 10) show that their CB are comprised of N 2p and C
Fig. 7 CO2 adsorption isotherms measured on the bulk-CN and
NS-CN
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Fig. 10 Calculated band structures and density of states (DOS)
of the simulated (a) bulk-CN and (b) NS-CN models.
bulk-CN, respectively.
The in-situ FTIR spectra of NS-CN were analyzed to
understand the conversion process of photocatalytic CO2
reduction. As shown in Fig. 11b and c, the peaks at 1512, 1438,
1631, 1265 and 1048 cm–1 belong to the carbonate species.47,48
The peaks at 1286, 1326, 1386, 1532, 1558, 1641 and 1695
cm–1 are ascribed to formate.49-51 The peaks at 1419, 1458,
1607 and 1657 cm–1 refer to formaldehyde.48 Besides, the
peaks at 1732 and 1103 cm–1 are attributed to the vibration of
methoxyl groups, which originate from methanol.47 The
remaining peaks at 1716 and 1756 cm–1 are assigned to the
carboxylate and carbonyl groups, respectively.48,50 CH4 was not
detected due to its nonpolar nature and low affinity toward
the g-C3N4 surface. It is clear that prior to irradiation, primary
carbonate species were first detected on the surface of
photocatalyst in the presence of CO2 and water vapor in the
reaction system. After the irradiation of the monochromatic
light (365 nm), formate, formaldehyde and methanol appeared
and their amount was gradually increased accompanied by a
simultaneous decrease of carbonate species, although CH4
molecule could not be detected due to its nonpolar nature and
low affinity toward the g-C3N4 surface. The results indicate that
the photocatalytic conversion of CO2 into hydrocarbon fuels on
the g-C3N4 surface undergoes a multi-step process involving
intermediate products such as formic acid and formaldehyde,
and finally they are converted into CH4 and CH3OH.
Fig. 9 (a) UV–vis absorption spectra. Mott–Schottky plots of (b)
bulk-CN and (c) NS-CN. (d) XPS valence band (VB) spectra and
(e) band structure of bulk-CN and NS-CN.
2p orbitals, while the VB mainly consist of N 2s and N 2p
orbitals. The calculated band gap values of the bulk-CN and
NS-CN are 1.07 and 1.18 eV, respectively, which are smaller
than the above-discussed experimental values due to the
limitation of the DFT calculations.16,46 However, this result is
still indicative and reveals that NS-CN has a relatively wide
band gap as compared to that of the bulk-CN material, which is
consistent with experimental results.
Photocatalytic CO2 reduction experiments were carried out
to evaluate the photocatalytic properties of the as-prepared
samples. As shown in Fig. 11a, a successful CO2 conversion into
CH4 and CH3OH occurred in the presence of g-C3N4
photocatalysts. Obviously, the generation rates of both CH4
and CH3OH for NS-CN are greatly enhanced, which are 10 and
5 times higher than the corresponding values obtained for the
According to the above-discussed results,
a possible
mechanism of photocatalytic CO2 reduction was proposed as
illustrated in Fig. 12. Under the irradiation of UV–visible light,
the g-C3N4 sample is excited and generates holes and electrons.
Fig. 11 (a) Photocatalytic CO2-reduction performance of the bulk-CN and NS-CN. (b, c) The in-situ FTIR spectra with different
wavenumber ranges of the NS-CN sample: (1) without CO2 gas and irradiation, (2) with flowing CO2 gas for 30 min without
irradiation, (3) irradiation for 30 min and (4) 60 min.
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resolved photoluminescence (TRPL) spectroscopy. As shown in
Fig. 13a, the photocurrent intensity of NS-CN is notably higher
than that of the bulk-CN with several on-off cycles of light
irradiation, indicating
a
more efficient electron-hole
pairtransfer and separation on the NS-CN photocatalyst. Note
that the current intensity of NS-CN shows a gradual increase as
the light is turned on, and gradual decrease as the light is
turned off. Such phenomenon is caused by the relaxation
effect during excitation and de-excitation process.39,58 For NS-
CN, the amount of surface delocalized π bonds is higher than
that for the bulk-CN due to the larger amount of exposed
aromatic heterocycles after exfoliation. A large portion of
photogenerated electrons in the conduction band of NS-CN
can be gradually accumulated because of these delocalized π
bonds. Therefore, the photocurrent is gradually increased due
to the photogenerated electrons stored in these delocalized π
bonds. On the other hand, when the light is turned off, the
stored electrons are gradually released from the surface
delocalized π bonds and then transferred to the working
electrode, leading to a gradual decrease of the photocurrent
to zero. Hence, the enrichment of photogenerated electrons in
the delocalized π bonds can reduce the recombination of
photogenerated electrons and holes, and consequently
facilitate the photocatalytic reduction of CO2 molecules and
formation of CH4 and CH3OH, which are eight-electron and six-
electron processes of CO2 photoreduction, respectively.
Fig. 12 The multi-electron process of CO2 phororeduction
Those holes can be utilized to produce plenty of H+ with water.
Meanwhile, the photo-generated electrons involve in a series
of photocatalytic reactions with a multi-electron process.4,17,52-
54
Briefly, the adsorbed CO2 on the surface is firstly reduced
into CHOOH species with 2e- and 2H+. Then these intermediate
products are subsequently reduced into HCOH, CH3OH and CH4
species step by step.55-57 Besides, the 2e- from g-C3N4 and 2H+
from the surrounding environment are required for each step
of the reaction. Finally, the CO2 molecules can be converted
into CH4 and CH3OH chemical fuel under the irradiation of the
light.
EIS Nyquist plots obtained for the bulk-CN and NS-CN are
The charge transfer dynamics taking place on the bulk-CN
and NS-CN was analysed by transient photocurrent response,
electrochemical impedance spectroscopy (EIS), and time-
shown in Fig. 13b. In the analog equivalent circuit (inset of Fig.
13b), Rctr represents the charge-transfer resistance of the
working electrode; Rsr represents the solution resistance
(Na2SO4); and CPE represents the constant phase element
between electrolyte solution and electrode. The size of the
semicircular arc in the Nyquist plots here reflects Rctr,59 which
is estimated as ~2.2 and ~6.4 KΩ for NS-CN and bulk-CN,
respectively. Namely, smaller semicircle radius implies lower
charge-transfer resistance and thus a faster charge-transfer
rate in NS-CN due to its ultrathin structure.
The TRPL spectra were further recorded to investigate the
lifetime of photogenerated charge carriers as shown in Fig. 13c.
The curves of both bulk-CN and NS-CN can be well fitted
through a bi-exponential function.60-62 The short lifetime, long
lifetime, and average lifetime are 2.8, 9.5, and 6.7 ns for NS-CN,
and 1.5, 6.3, and 3.4 ns for the bulk-CN, respectively. The
results indicate that the lifetimes of photogenerated charge
carriers in NS-CN have been remarkably prolonged as
compared to those obtained for the bulk-CN, which is due to
strong electron capture ability of the surface states in NS-CN.
This result combined with the transient photocurrent response
Fig. 13 (a) Transient photocurrent responses (λ = 420 nm), (b)
EIS Nyquist plots, (c) TRPL spectra of the bulk-CN and NS-CN.
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11 Q. H. Liang, Z. Li, Z. H. Huang, F. Y. Kang, Q. H. Yang, Adv.
and EIS analysis indicates that a significantly enhanced transfer
and separation efficiency can be achieved over the NS-CN
photocatalyst, which is one of the crucial factors to facilitate
the photocatalytic CO2 reduction.
View Article Online
Funct. Mater. 2015, 25, 6885.
DOI: 10.1039/C6TA08310B
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Conclusions
In summary, the essence for the enhancement of
photocatalytic CO2 reduction activity over hierarchical g-C3N4
assembled by amine-functionalized ultra-thin nanosheets can
be ascribed to several aspects. Firstly, the formation of
ultrathin nanosheets (~3 nm) endows g-C3N4 with (1) much
higher specific surface area (116 m2/g) and abundant surface
active sites, (2) more negative CB and more positive VB for
stronger redox ability of photogenerated charge carriers, (3)
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C3N4 for efficient charge transfer and separation. Secondly, the
formation of hierarchical structures further enlarges the
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facilitate the diffusion and adsorption of molecules, charge
transfer, as well as strengthens the light harvesting. Lastly,
amine-functionalization of g-C3N4 can remarkably promote the
CO2 adsorption through the base-acid interactions. As a result,
the photocatalytic generation rates of CH4 and CH3OH
increased by 10 times and 5 times, respectively, as compared
to those of the bulk-g-C3N4. Hence this work provides a
promising strategy to improve the photocatalytic activity of g-
C3N4 by simultaneous texture modification and surface
functionalization.
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29 L. T. Ma, H. Q. Fan, J. Wang, Y. W. Zhao, H. L.Tian, G. Z. Dong,
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We gratefully acknowledge the financial support from 973
program (2013CB632402), NSFC (51272199, 51320105001,
51372190, 21433007 and 51472191). Also, this work was
financially supported by the Natural Science Foundation of
Hubei Province of China (2015CFA001), the Fundamental
Research Funds for the Central Universities (WUT: 2015-III-034)
and Innovative Research Funds of SKLWUT (2015-ZD-1).
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Table of Contents
This work demonstrates the development of hierarchical ultra-thin g-C3N4 nanosheets
with enhanced photocatalytic CO2 reduction activity.