Template-free large-scale synthesis of g-C3N4 microtubesfor enhanced visible light-driven photocatalytic H2 production

Chao Zhou1, Run Shi1,2, Lu Shang1, Li-Zhu Wu1, Chen-Ho Tung1, and Tierui Zhang1,2 ()

1 Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese

Academy of Sciences, Beijing 100190, China 2 University of Chinese Academy of Sciences, Beijing 100049, China

Received: 27 October 2017

Revised: 1 January 2018

Accepted: 23 January 2018

© Tsinghua University Press

and Springer-Verlag GmbH

Germany, part of Springer

Nature 2018

KEYWORDS

graphitic carbon nitride

(g-C3N4)

microtubes,

photocatalytic,

water splitting,

visible light response

ABSTRACT

A template-free hydrothermal-assisted thermal polymerization method has been

developed for the large-scale synthesis of one-dimensional (1D) graphitic carbon

nitride (g-C3N4) microtubes. The g-C3N4 microtubes were obtained by simple thermal

polymerization of melamine-cyanuric acid complex microrods under N2 atmosphere,

which were synthesized by hydrothermal treatment of melamine solution at

180 °C for 24 h. The as-obtained g-C3N4 microtubes exhibited a large surface

area and a unique one-dimensional tubular structure, which provided abundant

active sites for proton reduction and also facilitated the electron transfer processes.

As such, the g-C3N4 microtubes showed enhanced photocatalytic H2 production

activity in lactic acid aqueous solutions under visible light irradiation ( ≥ 420 nm),

which was ~ 3.1 times higher than that of bulk g-C3N4 prepared by direct thermal

polymerization of the melamine precursor under the same calcination conditions.

The development of efficient visible light-driven photocatalysts

with earth-abundant and inexpensive elements is of great

interest because of their practical applications in converting

renewable solar energy into storable chemical energy

[1]. Graphitic carbon nitride (g-C3N4), as a metal-free

and visible light-active photocatalyst, has attracted intensive

interest in the field of water splitting [2–7], pollutant

degradation [8], CO2 reduction [9], and other important

catalytic reactions [10]. It has been extensively reported

that the structures of g-C3N4 play a crucial role in its

physicochemical properties and thus affect its catalytic

performances. To date, several different morphologies

of g-C3N4 photocatalysts, including nanosheets [11, 12],

nanoribbons [13], hollow spheres [14], nanopowders [15],

nanorods [16], nanofibers [17], and nanotubes [18–21] have

been successfully synthesized. In particular, one-dimensional

(1D) g-C3N4 nanostructures have recently received

considerable attention because their unique 1D structure

facilitates the electron transfer processes and leads to

improved photocatalytic performance.

Since the discovery of carbon nanotubes, a variety

of materials with 1D tubular structures have attracted

Address correspondence to tierui@mail.ipc.ac.cn

Nano Research 2018, 11(6): 3462–3468 https://doi.org/10.1007/s12274-018-2003-2

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3463 Nano Res. 2018, 11(6): 3462–3468

intense attention in the fields of gas storage, gas sensing,

catalysis, and photovoltaics [22]. Recently, g-C3N4 nanotubes

have generated a lot of interest because of their unique

structural, electronic, and optical properties, which have

been extensively investigated by theoretical studies

[23–26]. To date, several wet-chemical routes have been

developed to prepare tubular g-C3N4 materials. Zou

et al. reported the synthesis of millimeter-long tubular

g-C3N4 based on a 1D fibrous self-assembly of protonated

melamine in glycol mediated with nitric acid aqueous

solution, which was subsequently thermalized into the

nanotubular form via a plausible rolling-up mechanism

[27]. Cao and coworkers developed a catalytic-assembly

solvothermal route to prepare carbon nitride nanotubes

by the reaction of cyanuric chloride with sodium using

NiCl2 as the catalyst precursor [28]. However, most

synthetic processes for the preparation of g-C3N4 tubular

structures require either extensive use of various organic

additives or high pressure. In addition, the introduced

organic additives are usually difficult to remove completely,

and the residues may deteriorate the performances of

the g-C3N4 tubes. Therefore, it is still a great challenge

to develop a facile and yet effective synthetic method

for 1D g-C3N4 tubular structures.

Herein, 1D g-C3N4 microtubes were successfully synthesized

on a large scale via a template-free hydrothermal-assisted

thermal polymerization method using melamine as a

precursor. The formation mechanism of g-C3N4 microtubes

has also been proposed. The as-obtained g-C3N4 microtubes

showed enhanced visible light-driven photocatalytic

H2 production activity compared with bulk g-C3N4

prepared by the direct thermal polymerization of the

melamine precursor under the same calcination conditions.

Scheme 1 shows the schematic process for the synthesis

of g-C3N4 microtubes. Briefly, 0.5 g of melamine powder

was dissolved in 30 mL of pure water under stirring

at 90 °C for 10 min until a clear solution was obtained.

The as-obtained melamine clear solution was then

transferred into a 50 mL Teflon stainless autoclave and

heated at 180 °C for 24 h in an oven. The collected

white melamine-cyanuric acid (MCA) precursor complex

was dried and then heated at 550 °C for 4 h with a heating

rate of 5 °Cmin–1 under N2 atmosphere. After cooling

to room temperature, the final yellow products were

collected for further characterization. For comparison,

bulk g-C3N4 was also prepared by the same pyrolysis

method using commercial melamine powder as a precursor

(Fig. S1 in the Electronic Supplementary Material (ESM)).

Further details of the synthetic procedure are described

in the ESM.

The crystal structures of MCA complex microrods

and g-C3N4 microtubes were verified by X-ray diffraction

(XRD). As shown in Fig. 1(a), the melamine precursor was

transformed into a new phase, MCA complex, after

hydrothermal treatment (180 °C, 24 h). Thomas and

coworkers reported that the MCA complex, formed by

precipitation from equimolecular mixtures of melamine

and cyanuric acid in dimethyl sulfoxide, could be further

Scheme 1 Schematic illustration for the synthesis of 1D g-C3N4 microtubes.

Figure 1 (a) XRD patterns of the MCA complex microrods, bulk g-C3N4, and g-C3N4 microtubes. (b) FT-IR spectra of melamine, cyanuric acid, and MCA complex microrods.

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3464 Nano Res. 2018, 11(6): 3462–3468

transformed into mesoporous g-C3N4 hollow spheres

after pyrolysis under N2 at 550 °C [29]. This demonstrated

that the MCA complex was a desirable precursor for

the preparation of g-C3N4 with different morphologies.

In our work, g-C3N4 microtubes were obtained by the

thermal polycondensation of the MCA complex microrods

under N2 at 550 °C. As shown in Fig. 1(a), two distinct

diffraction peaks were observed at ~ 13.1° and ~ 27.5° for

both g-C3N4 microtubes and bulk g-C3N4, proving their

graphite-like structure. The peak at 13.1° was indexed

as the (100) plane corresponding to the interlayer structural

packing motif of the tri-s-triazine units, while the latter

peak was assigned to the (002) plane, a characteristic

of the interlayer stacking of conjugated aromatic systems

[30–33]. Fourier transform infrared (FT-IR) spectra further

confirmed the formation of the MCA complex (Fig. 1(b)).

While the triazine ring vibration of melamine in the MCA

complex was shifted to a lower wavenumber from 814

to 770 cm–1, the C=O stretching vibration of cyanuric acid

was shifted to a higher wavenumber from 1,694 to 1,733 cm–1,

indicating the presence of hydrogen-bonding interactions

between melamine and cyanuric acid [29, 33, 34]. The

FT-IR spectrum of g-C3N4 microtubes was also consistent

with that of bulk g-C3N4, demonstrating the successful

transformation of the MCA complex microrods (Fig. S2

in the ESM). Several characteristic bands in the fingerprint

region (1,200–1,650 cm–1) correspond to the typical stretching

modes of CN heterocycles of g-C3N4. The typical band

at 801 cm–1 was assigned to the out-of-plane bending of

the s-triazine units [29, 33].

To reveal the formation mechanism of the MCA complex,

the effect of hydrothermal reaction time at 180 °C on

the phase structures of the final products was investigated.

As shown in Fig. S3 in the ESM, the XRD pattern of the

sample obtained at 180 °C for 12 h was distinctly different

from that of the melamine precursor and a coexistent

phase corresponding to the MCA complex was evident.

After 24 h, the pristine MCA complex was obtained.

Based on the above analysis, a proposed formation

mechanism of the MCA complex during hydrothermal

process has been described in Scheme S1 in the ESM.

During the hydrothermal reaction, the melamine precursor

is partially hydrolyzed and gradually yields the intermediate

cyanuric acid (Reaction 1). Then, melamine reacts with

the in situ formed cyanuric acid, quickly generating the

MCA complex (Reaction 2), as demonstrated by XRD

and FT-IR analysis (Fig. 1). As Reaction 2 was noticeably

faster than Reaction 1, it was difficult to detect the

intermediate cyanuric acid during the hydrothermal

process. It has been reported that the precipitation of

the MCA complex occurs immediately, i.e., as soon as

melamine and cyanuric acid aqueous solutions are mixed

together [29, 33]. Fu and co-workers also reported that

the MCA precursor can be prepared by the self-assembly

of melamine with cyanuric acid from in situ hydrolysis

of melamine under phosphorous acid-assisted hydrothermal

conditions [35].

The morphologies of the as-obtained MCA complex

and the derived g-C3N4 were detected by field emission

scanning electron microscopy (FE-SEM) and transmission

electron microscopy (TEM). As shown in Figs. 2(a) and

2(b), MCA complex microrods were obtained on a large

scale. The length and diameter of the MCA microrods

were estimated to be ~ 20–50 μm and ~ 1–2 μm, respectively.

After heating at 550 °C for 4 h under N2 atmosphere,

the MCA microrods were transformed into 1D g-C3N4

microtubes. As shown in Figs. 2(c) and 2(d) and Fig. S4

in the ESM, the g-C3N4 microtubes were several micrometers

long. The wall thickness of these microtubes was typically

50 nm and their outer diameters was ~ 1–2 μm.

The formation mechanisms of the MCA complex

microrods and g-C3N4 microtubes can be described as

follows. During the hydrothermal process, a hexagonal

plate-like morphology was formed first (Fig. S5(a) in

the ESM), reflecting the molecular structure of the MCA

complex. Subsequently, with increasing hydrothermal

reaction temperature, hexagonal microrods could be

observed in the crystal morphology (Figs. S5(b) and S6

Figure 2 FE-SEM images of ((a), (b)) MCA complex microrods, and ((c), (d)) 1D g-C3N4 microtubes.

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3465 Nano Res. 2018, 11(6): 3462–3468

in the ESM). The water molecules played an important

role in the transformation of the plates into rods by guiding

the growth of the MCA precursor in the longitudinal

direction via competition for hydrogen bonding. Consequently,

the plates self-assembled into rods [35]. In the case of

the MCA complex hexagonal microrods, a higher defect

density probably existed at the center of the crystal

because of rapid crystallization at the early stages of the

hydrothermal reaction. Therefore, the triazine molecules

in the inner part underwent quick sublimation during

thermal polycondensation, and the rods gradually

extended outward to form tube-like structures [33–35].

The specific surface area and photoabsorption ability

of photocatalysts are the two key factors that determine

their catalytic performances. It is known that mesoporous

materials with high surface-to-volume ratios are usually

good candidates for photocatalysis [36–38]. The mesoporous

structure of the g-C3N4 microtubes was first analyzed

by the Brunauer–Emmett–Teller (BET) gas adsorption

method. The N2 adsorption/desorption isotherms are

shown in Fig. 3(a). Two well-defined steps were observed

in the isotherms. The g-C3N4 microtubes exhibited typical

type-IV curves with a well-defined hysteresis loop in

the relative pressure (P/P0) range of 0.5–0.9, suggesting

the existence of mesopores. The specific surface areas

of the g-C3N4 microtubes and bulk g-C3N4 were estimated

to be 12.8 and 5.6 m2·g–1, respectively. Consequently, the

g-C3N4 microtubes exhibited enhanced H2 production

activity compared with bulk g-C3N4. The optical properties

of the g-C3N4 samples were also investigated by UV–Vis

diffuse reflectance spectroscopy. The g-C3N4 microtubes

showed a very similar light absorption performance

compared to bulk g-C3N4 (Fig. 3(b)). Both samples showed

an absorption edge at ~ 460 nm, which was assigned to

the electronic transition from the valance band maximum

to the conduction band minimum upon light excitation

[39]. According to the absorption edge values (Fig. S7

in the ESM), the band gaps of the g-C3N4 microtubes and

bulk g-C3N4 were estimated to be 2.73 and 2.70 eV, respectively.

The photocatalytic activity of the as-obtained g-C3N4

samples was evaluated by photocatalytic H2 evolution

under visible light irradiation using lactic acid as a sacrificial

agent. As shown in Fig. 4(a), under visible light irradiation

( ≥ 420 nm) bulk g-C3N4 with 0.5 wt.% loading of Pt

Figure 3 (a) N2 adsorption–desorption isotherms and (b) ultraviolet–visible (UV–Vis) diffuse reflectance spectra of g-C3N4 microtubes and bulk g-C3N4.

Figure 4 (a) Photocatalytic H2 evolution over 0.5 wt.% Pt loaded g-C3N4 microtubes and bulk g-C3N4 under visible light irradiation (λ 420 nm) in lactic acid aqueous solutions. (b) PL spectra of g-C3N4 microtubes and bulk g-C3N4 under excitation at 380 nm. Catalyst, 50 mg; H2O, 16 mL; lactic acid, 4 mL; 300 W Xe lamp.

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3466 Nano Res. 2018, 11(6): 3462–3468

nanoparticles exhibited a relatively low H2 evolution rate

of ~ 16 μmol·h–1, primarily because of the large particle size

(Fig. S1 in the ESM) and low specific surface area

(~ 5.6 m2·g–1). On the other hand, g-C3N4 microtubes showed

enhanced photocatalytic H2 evolution rate of ~ 50 μmol·h–1,

which was ~ 3.1 times higher than that of bulk g-C3N4.

The higher catalytic activity of the g-C3N4 microtubes

is presumably because of their relatively larger specific

surface area and tubular-like structures that facilitate

the transport of photogenerated charge carriers [27, 35].

Photoluminescence spectroscopy (PL) is an effective

technique for detecting the recombination processes

of photogenerated charge carries. As shown in Fig. 4(b),

g-C3N4 microtubes exhibited a much weaker PL intensity

compared to bulk g-C3N4. This indicated that the radiative

recombination in the g-C3N4 microtubes was greatly

suppressed, which proved to be beneficial to the enhancement

of photocatalytic H2 production activity.

To demonstrate the applicability of g-C3N4 microtubes

in photocatalytic H2 production, a 15-h photocatalytic

recycling experiment was performed. After three consecutive

cycles, the g-C3N4 microtubes maintained an almost constant

photocatalytic H2 production rate without any noticeable

catalyst deactivation (Fig. S8 in the ESM). This demonstrated

that the g-C3N4 microtubes can be used as a good candidate

for photocatalytic applications.

In summary, 1D g-C3N4 microtubes were successfully

fabricated on a large scale via a facile and template-free

hydrothermal-assisted pyrolysis method. The g-C3N4

microtubes were obtained from high temperature treatment

of MCA complex microrods under N2 atmosphere, which

were easily synthesized from the hydrothermal treatment

of melamine. The photocatalytic H2 production tests

demonstrated that the H2 evolution rate of the as-obtained

g-C3N4 microtubes was ~ 3.1 times higher than that of bulk

g-C3N4 because of its relatively larger specific surface

area and improved separation efficiency of photogenerated

charge carriers. Our findings provide new opportunities

for rational design of g-C3N4 tubes with different morphologies

and their practical applications in several other fields.

Acknowledgements

This work was supported by the National Basic Research

Program of China (No. 2014CB239402), the National Key

Projects for Fundamental Research and Development

of China (Nos. 2016YFB0600901, 2017YFA0206904, and

2017YFA0206900), the National Natural Science Foundation

of China (Nos. 51772305, 51572270, U1662118, and 21401207),

the Strategic Priority Research Program of the Chinese

Academy of Sciences (No. XDB17000000), and the Youth

Innovation Promotion Association of the CAS.

Electronic Supplementary Material: Supplementary

material (detailed synthetic procedures for 1D MCA complex

microrods and 1D g-C3N4 microtubes, characterization,

FE-SEM and TEM images, XRD patterns, FT-IR spectra,

and proposed schematic illustration for MCA complex)

is available in the online version of this article at

https://doi.org/10.1007/s12274-018-2003-2.

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