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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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