Hierarchical Porous O-Doped g-C3N4 with Enhanced Photocatalytic CO2 Reduction Activity

Article abstract of DOI:10.1002/smll.201603938

Artificial photosynthesis of hydrocarbon fuels by utilizing solar energy and CO₂ is considered as a potential route for solving ever-increasing energy crisis and greenhouse effect. Herein, hierarchical porous O-doped graphitic carbon nitride (g-C₃N₄) nanotubes (OCN-Tube) are prepared via successive thermal oxidation exfoliation and curling-condensation of bulk g-C₃N₄. The as-prepared OCN-Tube exhibits hierarchically porous structures, which consist of interconnected multiwalled nanotubes with uniform diameters of 20–30 nm. The hierarchical OCN-Tube shows excellent photocatalytic CO₂ reduction performance under visible light, with methanol evolution rate of 0.88 μmol g^?1 h^?1, which is five times higher than bulk g-C₃N₄ (0.17 μmol g^?1 h^?1). The enhanced photocatalytic activity of OCN-Tube is ascribed to the hierarchical nanotube structure and O-doping effect. The hierarchical nanotube structure endows OCN-Tube with higher specific surface area, greater light utilization efficiency, and improved molecular diffusion kinetics, due to the more exposed active edges and multiple light reflection/scattering channels. The O-doping optimizes the band structure of g-C₃N₄, resulting in narrower bandgap, greater CO₂ affinity, and uptake capacity as well as higher separation efficiency of photogenerated charge carriers. This work provides a novel strategy to design hierarchical g-C₃N₄ nanostructures, which can be used as promising photocatalyst for solar energy conversion.

Full text of DOI:10.1002/smll.201603938

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Carbon Nitride Nanotubes
Hierarchical Porous O-Doped g-C3N4 with Enhanced
Photocatalytic CO2 Reduction Activity
Junwei Fu, Bicheng Zhu, Chuanjia Jiang, Bei Cheng, Wei You, and Jiaguo Yu*
Artificial photosynthesis of hydrocarbon fuels by utilizing solar energy and CO2
is considered as a potential route for solving ever-increasing energy crisis and
greenhouse effect. Herein, hierarchical porous O-doped graphitic carbon nitride
(g-C3N4) nanotubes (OCN-Tube) are prepared via successive thermal oxidation
exfoliation and curling-condensation of bulk g-C3N4. The as-prepared OCN-
Tube exhibits hierarchically porous structures, which consist of interconnected
multiwalled nanotubes with uniform diameters of 20–30 nm. The hierarchical OCN-
Tube shows excellent photocatalytic CO2 reduction performance under visible light,
with methanol evolution rate of 0.88 µmol g−1 h−1, which is five times higher than
bulk g-C3N4 (0.17 µmol g−1 h−1). The enhanced photocatalytic activity of OCN-
Tube is ascribed to the hierarchical nanotube structure and O-doping effect. The
hierarchical nanotube structure endows OCN-Tube with higher specific surface area,
greater light utilization efficiency, and improved molecular diffusion kinetics, due to
the more exposed active edges and multiple light reflection/scattering channels. The
O-doping optimizes the band structure of g-C3N4, resulting in narrower bandgap,
greater CO2 affinity, and uptake capacity as well as higher separation efficiency
of photogenerated charge carriers. This work provides a novel strategy to design
hierarchical g-C3N4 nanostructures, which can be used as promising photocatalyst
for solar energy conversion.
in solving the increasing energy demand and environmental
1. Introduction
crisis.[1] Since Inoue et al.[2] reported the photoelectrocata-
Converting solar energy directly into value-added chemical lytic reduction of CO2 by semiconductor in aqueous, the
energy has attracted worldwide attention due to its potential photocatalytic conversion of CO2 and H2O into hydrocarbon
fuels by semiconductors has been extensively studied.[3–14]
Among the various semiconductors investigated, metal-free
graphitic carbon nitride (g-C3N4) has played an increas-
J. W. Fu, B. C. Zhu, Dr. C. J. Jiang, Prof. B. Cheng,
Prof. W. You, Prof. J. G. Yu
ingly significant role in photocatalytic solar-to-fuels conver-
sion systems, due to its low cost, nontoxicity, good stability,
visible-light response, and proper electronic band struc-
ture.[15–18] However, bulk g-C3N4, commonly synthesized by
the thermal polycondensation of carbon/nitrogen-containing
organic precursors, still suffers from poor specific surface
area, unsatisfactory visible light utilization and rapid recom-
bination of photogenerated charge carriers. As a result, the
photocatalytic activity of conventional g-C3N4 photocatalysts
is still not high enough for practical applications. Tremendous
efforts have been made to optimize the nanostructure and
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
Prof. J. G. Yu
Department of Physics
Faculty of Science
King Abdulaziz University
Jeddah 21589, Saudi Arabia
DOI: 10.1002/smll.201603938
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surface chemical state of g-C3N4 mate-
rials for enhancing the photocatalytic
activity, e.g., coupling with other semicon-
ductors/metals,[19–24] self-optimizing the
crystal structure,[25,26] doping with hetero-
atoms,[27–29] and constructing various nano
structures.[30,31,32,33,34a]
Recently, two dimensional (2D) lay-
ered structure nanosheets have exhibited
great potential in photocatalysis due to
their unique structural properties.[35–37]
For example, 2D materials typically have
high specific surface area, which provides
more surface reactive sites. Moreover,
short bulk diffusion/transfer distance
reduces the recombination probability
of photogenerated charge carriers.[38]
Considering the layered structure char-
acteristic of bulk g-C3N4 (similar to
graphite), delamination into nanosheets
is a promising strategy for nanostruc-
Figure 1. a) Schematic of the experimental process of OCN-Tube in tube furnace. b) Illustration
ture engineering of g-C3N4 materials. As of the formation process of hierarchical O-doped g-C3N4 nanotubes (OCN-Tube). (C, N, and
O atoms are, respectively, represented by gray, blue, and red spheres in the atomic model.)
reported, 2D g-C3N4 nanosheets with
unique surface and electronic properties
can be obtained by breaking the hydrogen bonds between with a long quartz tube as the main reactor. As illustrated in
the layers.[17,38] Efficient tactics such as liquid-phase sonica- Figure 1, under the high-temperature oxidation process in
tion exfoliation,[30,39] in situ ion intercalation[40] and thermal airflow, bulk g-C3N4 can be easily exfoliated to form g-C3N4
oxidation thinning[38] have been applied for obtaining few- nanosheets. For minimizing the surface energy, these formed
layered g-C3N4 nanosheets. However, due to their high g-C3N4 nanosheets tend to bend and curl.With the help of the
surface energy, the exfoliated g-C3N4 nanosheets will seri- airflow, g-C3N4 nanosheets can self-assemble into hierarchical
ously aggregate during application processes, which is unfa- one dimensional (1D) nanotube structure and condense on
vorable to the photocatalytic reactions.[32] As limited by this the cold wall of the quartz tube. The obtained hierarchical 1D
severe disadvantage of thin nanosheets, there is still much nanotube structure is composed of curled g-C3N4 nanosheets,
room to optimize the physicochemical properties of g-C3N4 thus keeping the structure merits of nanosheets. Meanwhile,
nanosheets and maintain their high photocatalytic activity.
O heteroatom doping into the g-C3N4 nanosheets can occur
Assembly of individual nanosheets into hierarchical by substituting C or N atoms during the high-temperature
nanostructures is an ideal way to solve the uncontrolled oxidation process in airflow.[28,43,44] Nonmetal heteroatom
aggregation problems.[41] Generally, hierarchical nanostruc- doping is considered as an effective method to distinctly tune
ture with abundant porous networks and more exposure sur- the optical, electronic, and other physicochemical properties
face can greatly enhance light harvesting, and facilitate the of g-C3N4.[27,28,43–45] As a result, the obtained hierarchical
diffusion kinetics of reactants and products.[42] For example, O-doped g-C3N4 nanotubes (OCN-Tube) exhibited signifi-
Wang and co-workers[32] constructed g-C3N4 nanospheres cantly higher photocatalytic CO2 reduction performance
composed of nanosheets. This hierarchical nanostructure than bulk g-C3N4. The possible reaction products and inter-
can greatly prevent the aggregation of nanosheets and take mediates of photocatalytic CO2 reduction reactions were
advantage of the special structural merits of both nanosheets investigated by in situ Fourier transform infrared (FTIR)
and nanospheres. However, this method needs a complex spectroscopy, which is helpful for elucidating the mechanisms
template to control the morphology, which is not conducive of surface catalytic reactions.[46,47] Moreover, the combina-
to large-scale application. g-C3N4 nanosheets tend to curl for tion of self-assembly of hierarchical nanotube structure and
reducing the high surface energy.[30,38] Making full use of this O-doping into the g-C3N4 materials provides new insight
characteristic, rational design of self-assembled nanotube for designing and fabricating highly efficient g-C3N4-based
structure by orderly curling of g-C3N4 nanosheets will be an photocatalysts.
appealing strategy.
Inspired by the direct thermal oxidation “etching”
method for preparing g-C3N4 nanosheets,[38] we developed
a facile method to construct hierarchical g-C3N4 nanotubes
by simultaneous thermal oxidation exfoliation and curling-
condensation of bulk g-C3N4. This simple protocol involves
2. Results and Discussion
2.1. Morphology and Microstructure Characteristics
bulk g-C3N4 powders prepared by thermal polycondensa- The morphology and microstructure of OCN-Tube were
tion of melamine as the starting materials and a tube furnace studied by field-emission scanning electron microscopy
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cold wall of the quartz tube. With the help of continuous air-
flow, the g-C3N4 nanotubes condense and self-assemble into
hierarchical porous network structures. In order to further
demonstrate the formation mechanism of OCN-Tube, we
investigated the morphologies of condensation products at
different positions of the quartz tube. Interestingly, FESEM
images (shown in Figure S2a,b, Supporting Information) of
the product located near the heating center zone (Zone A
in Figure 1a) show agglomerates of nanosheets with curled
and rough edges. As for the product collected from Zone B,
porous network composed of curled nanosheets (Figure S2c,d,
Supporting Information) could be observed, and the curling
of nanosheets became more obvious. Clear hierarchical
porous network structure (OCN-Tube) composed of fully
curled nanosheets (multiwalled nanotubes) appeared in the
product of Zone C (Figure 2). Moreover, when the airflow
rate increased from 30 to 100 mL min−1, only agglomerates
of incompletely curled nanosheets could be found, which
was due to the overly fast condensation process at the high
Figure 2. a,b) Typical FESEM and c,d) TEM images of OCN-Tube (image
d is the enlarged view of the box part in image (c)).
(FESEM) and transmission electron microscope (TEM). airflow rate. These results indicate the gradual curling and
The typical FESEM image of bulk g-C3N4 (Figure S1, Sup- condensation process of the exfoliated g-C3N4 nanosheets.
porting Information) exhibited obvious solid agglomerates Sufficient time for the curling of exfoliated nanosheets is nec-
with irregular size, while the morphology of OCN-Tube essary for the formation of g-C3N4 multiwalled nanotubes.
changed distinctly: uniform porous network structure
without bulk agglomerate could be observed (Figure 2a). The
FESEM image with higher magnification (Figure 2b) shows 2.2. Crystal Structure and Surface Chemical State
that the elementary unit of the porous network structure is
1D nanostructure with uniform diameters of 20–30 nm. In X-ray diffraction (XRD) was used to study the crystal
order to further understand the microstructure of these 1D structure of OCN-Tube, and representative XRD patterns
nanostructures, the OCN-Tube sample was characterized by are shown in Figure 3a. The XRD pattern of bulk g-C3N4
TEM, as shown in Figure 2c. The 1D nanostructures exhib- exhibited two distinct diffraction peaks at around 13.2°
ited regular multiwalled tubular structure, which is similar to and 27.3°, which are attributed to the in-plane tri-s-triazine
the typical multiwalled carbon nanotubes. This special g-C3N4 motifs (i.e., the (100) planes) and the periodic stacking of
nanotube structure is also illustrated in Figure 1b. Further conjugated aromatic systems (i.e., the (002) planes), respec-
observation in Figure 2d demonstrates that the 1D tubular tively.[25,26,30,38,48,49] The OCN-Tube showed a similar XRD
nanostructures consist of thin nanosheets, and some isolated pattern with bulk g-C3N4, suggesting that OCN-Tube shares
curled nanosheets were also observed. These results provide the same core crystal structure with bulk g-C3N4. Remark-
direct evidence that the 1D nanostructures in OCN-Tube are ably, the (100) peak in OCN-Tube became weaker, which
nanotubes derived from curled nanosheets. This hierarchical is induced by the less ordered stacking of tri-s-triazine
porous network structure of OCN-Tube has many advan- motifs.[38] In this case, first, the substitution of C or N with
tages in photocatalysis. For example, this structure brings O changes the structure of tri-s-triazine motifs. Second, the
increased specific surface area and multiple light reflection/ exfoliated nanosheets from bulk g-C3N4 show decreased
scattering channels, resulting in more exposed active edges, planar size of the layers. Third, the curling of nanosheets
enhanced light harvesting and improved molecular diffusion influences the arrangement of in-plane structure. These three
kinetics, which are all beneficial for enhancing the photocata- factors resulted in the less ordered stacking of tri-s-triazine
lytic activity.[42]
The preparation process of OCN-
Tube with hierarchical nanotube struc-
ture is shown in Figure 1a. First, bulk
g-C3N4 is thermally exfoliated into g-C3N4
nanosheets by breaking the hydrogen
bonds between the layers.[25,38] Most of
the exfoliated g-C3N4 nanosheets will sus-
pend in the hot airflow.[34a,38] Then, the
nanosheets tend to curl and gradually turn
into nanotubes under the driving force
of airflow.[30] When these hot suspended
materials move to the cool zone of the
tubular furnace, they will deposit on the
Figure 3. a) XRD patterns and b) FTIR spectra of bulk g-C3N4 and OCN-Tube.
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motifs and less pronounced (100) peak. As shown in the inset indicating more abundant surface O atoms in OCN-Tube.
of Figure 3a, a slight negative shift of OCN-Tube (002) peak As displayed in the high-resolution O 1s spectra (Figure 4b),
can be observed. This phenomenon means a larger interlayer there are two kinds of O species in bulk g-C3N4, with

spacing in OCN-Tube, which is consistent with the looser binding energies at 531.9 eV (for O H) and 533.4 eV (for
[28]

stacking between layers in the exfoliated g-C3N4 nanosheets.
O N).
The former is attributed to the surface hydroxyl
The bonding structure of OCN-Tube was further con- groups and the latter is derived from the intermediates of
firmed by FTIR spectroscopy. In Figure 3b, the characteristic melamine thermal-polymerization.[28] However, an obvious
peaks located at 1200–1600 and 810 cm−1 can be attributed peak at lower binding energy (531.1 eV) appeared in the O

to skeletal vibrations of aromatic CN heterocycles and 1s spectrum of OCN-Tube, which is attributed to the C O
breathing vibration of triazine units, respectively.[25,30] These and N C O species in lattice.[28,44] This result confirms that
 
features further indicate that OCN-Tube maintains the O atoms are bonded with C atoms in the basic aromatic CN
core CN heterocycle structure similar to bulk g-C3N4. Com- heterocycles in OCN-Tube.
pared with bulk g-C3N4, two small peaks between 3000 and
As shown in high-resolution C 1s spectra (Figure 4c), the
3500 cm−1 became stronger,[38] suggesting more exposure of spectrum of bulk g-C3N4 is deconvoluted into three kinds


surface amino (N H) and hydroxyl (O H) groups in OCN- of C species. The peak at 284.8 eV originates from adventi-
3


Tube. The surface amino and hydroxyl groups are conducive tious carbon (sp C C), while the ones at 286.2 eV (C N)
 
to CO2 capture for photocatalytic CO2 reduction reaction. and 288.0 eV (N C N) are ascribed to carbon at the two
Moreover, the more surface amino and hydroxyl groups different spatial locations in the basic aromatic CN heterocy-
mean more exposure edges and surfaces in OCN-Tube, which cles.[48] A new peak at high binding energy of 288.9 eV cor-
 
is consistent with the direct observation by FESEM. More- responding to N C O bonds appeared in the C 1s spectrum
over, it is worth noting that a new sharp peak at 2150 cm−1 of OCN-Tube, which is consistent with the assumption that
appeared in OCN-Tube. This peak is ascribed to the vibration O atoms are doped into the matrix of CN heterocycles by
[34]


The presence of C N bonding with C atoms. In order to study the true location of
of cyano terminal groups (C N).

indicates that the surface C N bond of the tri-s-triazine ring O atoms in the CN matrix, the N 1s spectra of bulk g-C3N4
will partly fracture and restructure during thermal treatment. and OCN-Tube (Figure 4d) are also deconvoluted. Both sam-
X-ray photoelectron spectroscopy (XPS) was performed ples showed peaks located at 401 eV, which are derived from
to investigate the surface elemental composition and chem- the N atom of the surface amino groups. The peaks at 399
ical states of OCN-Tube, and the corresponding spectra are and 398.5 eV (or 398 eV) can be ascribed to tricoordinated

 
shown in Figure 4. In the XPS survey spectra of bulk g-C3N4 N atoms (N (C)3) and dicoordinated N atoms (C N C) in
and OCN-Tube (Figure 4a), only O 1s, N 1s, and C 1s peaks the aromatic CN heterocycles, respectively.[30,48] Compared
could be observed. Interestingly, the intensity of O 1s peak with bulk g-C3N4, the ratio of dicoordinated to tricoordinated
of OCN-Tube was much stronger than that of bulk g-C3N4, N atoms in OCN-Tube deceased obviously. Moreover, the
binding energy of dicoordinated N atoms
shifted from 398.5 eV (bulk g-C3N4) to
398.0 eV (OCN-Tube). The lower binding
energy is attributed to the introduction of
O atoms with higher electronegativity in
the aromatic CN heterocycles (electron-
egativity: O = 3.5, N = 3.0, C = 2.5). It is
concluded that O atoms are preferentially
bonded to C atoms by substituting dico-
ordinated N atoms. In addition, the actual
percentages of C and N atoms in the sam-
ples were determined by elemental anal-
ysis. The atomic ratio of C and N increases
from pristine 0.65 (bulk g-C3N4) to 0.69
(OCN-Tube), and the atomic ratio of O
to C is calculated to be 0.08 (shown in
Table S1, Supporting Information). This
result is reasonable because some N atoms
are substituted by O atoms. Remarkably,
OCN-Tube exhibited
a new peak at
binding energy of 400.2 eV in Figure 4d.
Coupled with the results of FTIR analysis,
this peak was assigned to cyano terminal
[50]

groups C N.
During high-temperature

thermal treatment, a portion of the C N
bonds in aromatic CN heterocycles are
“cleaved” to reconstruct, and the formation
Figure 4. a) XPS survey spectra and b,c,d) high-resolution O 1s, C 1s, N 1s XPS spectra of
bulk g-C3N4 and OCN-Tube.
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