carbon vacancy-induced enhancement of the visible light-driven...

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Carbon Vacancy-Induced Enhancement of the Visible Light-driven

Photocatalytic Oxidation of NO over g-C3N4 Nanosheets

Yuhan Lia,b, Wingkei Hoa,*, Kangle Lvb,*, Bicheng Zhuc, Shun Cheng Leed

aDepartment of Science and Environmental Studies, The Education University of

Hong Kong, Tai Po, N.T., Hong Kong, P.R. China

bKey Laboratory of Catalysis and Materials Science of the State Ethnic Affairs

Commission and Ministry of Education, Hubei Province, College of Resources and

Environmental Science, South-Central University for Nationalities, Wuhan 430074,

P.R. China

cState Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, Wuhan, China

d Department of Civil and Environmental Engineering, The Hong Kong Polytechnic

University, Hung Hom, Hong Kong

Tel.: +852-2948 8255; Fax: +852-2948 7726.

E-mail: [email protected] (W.K. Ho)

E-mail: [email protected] (K.L. Lv)

mailto:[email protected]

mailto:[email protected]


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Abstract

g-C3N4 (gCN) with nitrogen vacancy has been extensively investigated and applied

in (photo)catalysis. Engineering the carbon vacancy in gCN is of great importance,

but it remains a challenging task. In this work, we report for the first time the

fabrication of gCN with carbon vacancy (Cv-gCN) via thermal treatment of pristine

gCN in CO2 atmosphere. The photocatalytic performance of Cv-gCN is evaluated on

the basis of NO oxidization under visible light irradiation ( > 400 nm) in a continual

reactor. The successful formation of carbon vacancy in gCN is confirmed through

electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS).

The photocatalytic oxidation removal rate of NO over Cv-gCN is 59.0%, which is two

times higher than that over pristine gCN (24.2%). The results of the quenching

experiment show that superoxide radicals (O2•-) act as the main reactive oxygen

species, which is responsible for the oxidation of NO. The enlarged BET surface areas

and negatively shifted conduction band (CB) potential enhance the photocatalytic

activity of Cv-gCN, which facilitates the efficient electron transfer from the CB of

Cv-gCN to the surface adsorbed oxygen, resulting in the formation of O2•- that can

oxidize NO.

Keywords: Carbon vacancy; g-C3N4; Thermal etching; CO2; NO oxidation.


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1. Introduction

Extensive use of fossil fuels remarkably contributes to environmental

contamination, which results in air quality deterioration and atmospheric

pollution problems, such as greenhouse effect, ozone layer depletion, and acid

rain [1]. Thus, efforts have been devoted for the development of sustainable and

efficient air purification methods. One such method is semiconductor

photocatalysis, which has been considered as a promising strategy and has

achieved progress in recent years [2-6]. Among semiconductor photocatalysts

[7-15], g-C3N4 (gCN) is an intriguing photocatalyst stimulated by visible light

because of its appropriate band position, which can satisfy the thermodynamic

requirements for various potential photocatalytic applications, such as hydrogen

evolution [16], organic pollutant degradation [17-20], air purification [21], and

CO2 reduction [22, 23]. However, the photocatalytic performance of pristine

gCN fails to meet the expected results because of its low specific surface area

and rapid photo-generated electron–hole pair recombination. In practical

applications, strategies with high-efficiency must be developed to increase the

surface area of gCN and improve the separation/transport of photogenerated

carriers.

Although vacancies are common defects in photocatalysts, they function as

mediators that provide new mechanisms that promote the separation efficiency

of carriers and engineer the electronic structure of photocatalysts. Vacancies

have also been found to enhance photocatalytic efficiencies. As their name

implies, vacancies can be introduced when the atoms escape from a lattice and

subsequently expose on the surface of materials. In general, these vacancies can

be further divided into two types, namely, anionic and cationic vacancies. The


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following anionic vacancies are of great importance: O-vacancies in oxide

photocatalysts, such as TiO2 [24], In2O3 [25], SrTiO3 [26], CeO2 [27], BiPO4

[28], γ-Bi2O3 [29], La(OH)3 [30], NiCo2O4 [31], BiOI [32], HfO2 [33], ZnO

[34], SnO2 [35], WO3 [36], and Fe2O3 [37], S-vacancies in sulfide

photocatalysts, such as CdS [38] and ZnS [39], and N-vacancies in nitride

photocatalysts, including gCN [40, 41]. These anionic vacancies provide an

optimum platform for high light-harvesting capacity, rapid photo-induced

charge carrier separation or transport, and evident photocatalytic performance.

Meanwhile, cationic vacancies have been extensively investigated because of

their positive effects on photocatalytic reactivity, which are comparable to or

even better than those of anionic vacancies. Ti vacancies in anatase have been

achieved via a sol-gel method [42]. Since then, the cationic vacancies of

photocatalysts have been widely explored. Enamul et al. [43] synthesized ZnO

single crystal with Zn vacancy through UV radiation. Guan et al. [44] then

developed Bi vacancies in ultrathin BiOCl nanosheets to promote the

photodegradation of rhodamine B under simulated solar irradiation. Wang et al.

[45] investigated the surface Bi vacancies of Bi6S2O15 core/shell nanowires

with enhanced photocatalytic decomposition of methylene blue. Meanwhile,

Savariraj et al. [46] demonstrated that knit-coir-mat-like CuS thin films with Cu

vacancies exhibit comparable electrochemical and photovoltaic performances.

In addition to the widely investigated monovacancies, Bi/Cu dual vacancies of

BiCuSeO have been developed, and their electrical conductivity has been

significantly increased [47].

On the basis of the strong influence of the characteristics of vacancies, such

as electronic structure, charge carrier separation/transport, and superior


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performance, researchers suggested that the photocatalytic performances of

gCN can also be tuned by introducing carbon vacancies into gCN. By etching

gCN in NH3 atmosphere, Liang et al. [48] successfully fabricated gCN

nanosheets with surface carbon vacancies (Cv-gCNs); in their work, the

photocatalytic hydrogen production rate of Cv-gCN increased nearly 20 times in

comparison with that of bulk gCN. Li et al. [49] prepared Cv-gCN by thermally

treating gCN under high purity argon gas flow and found that the presence of

carbon vacancies not only enhances photocatalytic H2O2 production, but also

changes the H2O2 generation pathway from a two-step single-electron indirect

reduction to a one-step two-electron direct reduction. Therefore, searching

novel methods for synthesizing gCN with carbon vacancies and applying such

methods to address environmental issues is of great importance.

In the present work, gCN nanosheets with surface carbon vacancies

(Cv-gCN) were fabricated through a double thermal etching method performed

under CO2 gas flow. As expected, the introduction of carbon vacancies

effectively enhanced the photocatalytic activity of gCN in photocatalytic

oxidation of NO. The structure and photocatalytic performance of Cv-gCN were

then systematically studied.

2. Experimental

2.1 Synthesis

All chemicals were of analytical grade and used without further purification.

Samples were synthesized through simple thermal pyrolysis in a muffle

furnace. Pristine g-C3N4 (gCN) was obtained by heating four crucibles (each

containing 10 g of thiourea) at 550 °C for 2 h with a heating rate of


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15 °C·min−1. The obtained fulvous solid product was mixed and ground into

powder in an agate mortar. Cv-gCN was also synthesized at 550 °C for 2 h by

calcining six crucibles, of which four contained 0.5 g of the as-prepared gCN

powder each and two contained 10.0 g of sodium bicarbonate each to produce

CO2 gas atmosphere. The gas produced during calcination was absorbed by

0.05 M dilute alkaline solution before emission.

2.2 Characterization

The phase structures of the samples were investigated with an X-ray

diffractometer (XRD; D/max RA, Japan). The morphological characteristics

and microstructures of the samples were characterized through scanning

electron microscopy (SEM; JEOL JSM-6490, Japan) and transmission electron

microscopy (TEM; JEM-2010, Japan). Nitrogen adsorption–desorption

isotherms were obtained in a nitrogen adsorption apparatus (ASAP 2020,

USA). All the samples were degassed at 150 °C prior to the measurements to

investigate the surface areas and pore size distributions of the samples. The

samples embedded in the KBr pellets were subjected to Fourier transform

infrared (FT-IR) spectroscopy in a Nicolet Nexus spectrometer to detect the

functional groups on the sample surface. The surface chemical composition was

investigated, and valence band (VB) was probed through X-ray photoelectron

spectroscopy (XPS; Thermo ESCALAB 250, USA) with Al Kα X-ray (hν =

1486.6 eV) operated at 150 W. The shift of the binding energy attributed to

relative surface charging was corrected with the C 1s level at 284.8 eV as an

internal standard. The optical properties of the samples were obtained in a scan

UV-Vis spectrophotometer (UV-Vis diffuse reflectance spectra [DRS];


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UV-2450, Shimadzu, Japan) equipped with an integrating sphere assembly, and

BaSO4 was used as the reflectance sample.

2.3 Photocatalytic activity evaluation

The photocatalytic activity of the photocatalyst was evaluated in terms of the

oxidation of NO and NO2 at ppb levels in a continuous flow reactor at ambient

temperature. Composed of stainless steel and covered with quartz glass, the

rectangular reactor had a volume of 4.5 L (30 cm × 15 cm × 10 cm; L × W ×

H). A visible LED lamp (150 W) with a cut-off filter (>400 nm) was used as

the light source. Approximately 0.2 g of the as-prepared photocatalyst was

added to 30 mL of H2O. The resulting mixture was ultrasonicated for 30 min,

which was then then coated onto a dish in diameter of 11.5 cm. The coated dish

was heated at 70 °C to ensure the complete evaporation of water and then

cooled to room temperature prior to photocatalytic testing. NO and NO2 gases

were acquired from a compressed gas cylinder containing 50 ppm of NO and

NO2 (N2 balance) in accordance with the traceable standards recommended by

the National Institute of Stands and Technology. The initial NO and NO2

concentrations were diluted to approximately 600 ppb with an air stream

supplied by a zero-air generator (Advanced Pollution Instrumentation, A

Teledyne Technologies Company, Model 701). The gas streams were

completely premixed in a gas blender, and the flow rate was controlled with a

mass flow controller at 1.0 L·min−1. After the adsorption-desorption

equilibrium was achieved, the lamp was turned on. The NO and NO2

concentrations were continuously measured with a chemiluminescence NOx

analyzer (Advanced Pollution Instrumentation, A Teledyne Technologies


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Company, Model T200), which monitors the concentration of NO, NO2, and

NOx (NOx represents NO + NO2) with a sampling rate of 1.0 L·min−1.

The removal ratio (η) of NO or NO2 was calculated according to equation 1:

η (%) = (1 − C/C0) × 100% (1)

where C and C0 are NO or NO2 concentration in the outlet steam and feeding

stream, respectively.

2.4 Active species trapping

Active species trapping experiments were performed to investigate the

photocatalytic oxidation mechanism of NO. Potassium iodide (KI), potassium

dichromate (K2Cr2O7), and tert-butyl alcohol (TBA) were selected as

scavengers of the photo-generated holes, electrons, and hydroxide radicals

(∙OH), respectively. Specifically, 0.2 g of photocatalyst containing 0.002 g of

KI, 0.002 g of K2Cr2O7, or 1 mL of TBA was dispersed in 30 mL of H2O to

obtain different samples under the same conditions. Each sample dish was used

to evaluate the photocatalytic activity under visible light irradiation.

2.5 Photoelectrochemical experiments

The g-C3N4 samples were deposited on a transparent conductive FTO glass

substrate through electrophoretic deposition. The working electrode was

prepared via the following process: First, 10 mg of g-C3N4 sample was

dispersed in 25 mL of 0.2 mg·mL−1 I2/acetone solution under ultrasonic

treatment for 2 min. A two-electrode process was used to deposit the samples at

an applied potential of 20 V for 3 min. The FTO glass substrates with a coated

area of approximately 1.5 cm × 2.5 cm were used for both electrodes. The

deposited electrode was dried at 200 °C for 30 min to evaporate the I2 residues.


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Photoelectrochemical measurements were performed in a standard

three-electrode cell containing a saturated potassium chloride-silver chloride

electrode (Ag/AgCl) as a reference electrode, a platinum foil (1.0 cm × 1.0 cm)

as a counter electrode, and a gCN sample as a working electrode. The

electrolyte was 0.1 M Na2SO4. Linear sweeps and transient photocurrent were

determined in a CHI 660D electrochemical workstation. A 300 W Xe arc lamp

coupled with an AM 1.5 G global filter (100 mW·cm−2) was used as a radiation

source. A UV cut-off filter and a UV-Vis cut-off filter were used to simulate

visible light and IR light, respectively. AC impedance was obtained in the same

configuration at a cathodic bias of 20 V versus Ag/AgCl at a frequency range of

106-0.1 Hz.

2.6 Theoretical calculation.

Density functional theory (DFT) calculation was carried out by using CASTEP

package based on the plane-wave-pseudo-potential method. The exchange-correlation

function adopted the Perdew-Burke-Ernzerhof (PBE) of the generalized gradient

approximation (GGA). The ultrasoft pseudo-potential described the interaction

between valence electrons and the ionic core. An energy cutoff of 450 eV and

Monkhorst-Pack k-point meshes of 4×4×1 for Brillouin zone sampling were used.

The convergence tolerance of energy, maximum force and maximum displacement

were less than 1.0×105 eV/atom, 0.03 eV/Å and 0.001 Å , respectively.

3. Results and discussion

3.1 Morphology and phase structure

Fig. 1 compares the SEM and TEM images of pristine gCN and Cv-gCN samples.

The representative morphological characteristics of an intact CN with bulk layers can


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clearly be observed in Fig. 1a, which shows that the thick layers are aggregated and

linked to one another. The corresponding TEM image (Fig. 1b) reveals that gCN is

composed of randomly aggregated thick slabs. After the calcination of gCN in the

CO2 atmosphere, the obtained Cv-gCN sample decreased in size and exhibited a loose

layered morphology (Fig. 1c), which was due to the heat-induced exfoliation of bulk

gCN (Fig. 1d).

The microstructure of the photocatalyst was further investigated by EDS elemental

mapping. Fig. S1 shows that both C and N are homogeneously distributed in these

samples.

Fig. 2 shows that both samples exhibit similar XRD patterns. The strong

peak of the XRD patterns at 27.5° (d = 0.324 nm) corresponded to the (002)

interlayer diffraction of the gCN graphitic-like structure, and the weak peak of

the XRD patterns at a low angle of approximately 13.0° (d = 0.682 nm)

belonged to the (001) peak of the in-plane structural motif. The peak intensity

of Cv-gCN was much smaller than that of pristine gCN, indicating that the

crystallization of gCN was affected by the introduction of carbon vacancy.

Further observation showed that the (002) diffraction peak of Cv-gCN shifted

toward a higher 2θ value from 27.59° to 27.77° in comparison with that of

pristine gCN (inset of Fig. 2). This outcome implied that the gallery distance

between the basic layers of gCN decreased because of carbon vacancy.

3.2 FTIR analysis

Fig. 3 compares the FT-IR spectra of the photocatalysts. The featured vibration

modes attributed to the representative gCN can be clearly observed for both samples,

reflecting the high thermal stability of gCN. Several strong bands in the region of

1,200-1,700 cm-1 can be ascribed to the stretching vibration of the gCN heterocycles,


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and the sharp bands at 808 cm-1 are the characteristic breathing mode of tri-s-triazine

motifs [21,50,51]. The broad bands located at 2,900-3,400 cm-1 can be attributed to

the residual N-H components and the O-H bands from the uncondensed amino groups

and absorbed H2O molecules. However, compared with gCN, the Cv-gCN sample

showed an improved absorption at 2,900-3,400 cm-1, possibly due to the exfoliation of

gCN and the formation of amino groups, accompanying by the appearance of carbon

vacancies in Cv-gCN (around 3182 cm-1). Since carbon vacancies are bonded with

abundant unsaturated N atoms, which would be compensated by adsorbed hydrogen

atoms and thus generate some amino groups. Therefore, it is understandable that the

amino groups located at around 3182 cm-1 can be found in FTIR spectrum of Cv-gCN

sample.

3.3 BET surface areas and pore structure

Nitrogen adsorption-desorption isotherms were measured to obtain additional

information about the microstructure of the photocatalyst (Fig. 4). It can be seen that

the adsorption of nitrogen for gCN sample in the relative lower pressure (P/P0=0-0.3)

is much smaller than that of Cv-gCN, indicating the low BET surface area of the

former. The BET surface area of Cv-gCN (147 m2g−1) was 5.4 times higher than that

of gCN (27 m2g−1). Cv-gCN revealed type IV isotherms with a hysteresis loop at a

high relative pressure between 0.6 and 1.0, thereby suggesting the presence of

mesopores (2-50 nm) and macropores (>50 nm) [52,53]. The shapes of the hysteresis

loop were of type H3, and thus narrow slit-shaped pores that are generally associated

with plate-like particles were present. Such characteristic agrees well with their

sheet-like morphology (Figs. 1c and 1d). The Cv-gCN had a larger pore volume (0.75

cm3g−1) than the precursor gCN (0.14 cm3g−1). The large BET surface area can


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provide a considerable number of reactive sites, and a large pore volume facilitates

the diffusion of gas, thus benefiting the photocatalytic oxidation of exhaust gas.

3.3 XPS and EPR

XPS was used to confirm the formation of carbon vacancies in g-C3N4. According

to the XPS survey spectra shown in Fig. 5a, both samples contain C, N and O

elements, and no obvious peak ascribed to sulfur species was identified. This outcome

implied that the S element was nearly removed completely during calcination. The

detected element contents are presented in Table 1, from which the atom ratios of

carbon to nitrogen, RC:N, were calculated to be 0.84 and 0.77 for pristine gCN and

Cv-gCN samples, respectively. The decreased carbon content suggested the formation

of carbon vacancy in the Cv-gCN sample.

The detailed chemical states of the as-prepared samples are revealed in

high-resolution XPS (Figs. 5b-5d).

Two main carbon species, corresponding to the binding energy of 288.4 and 284.8

eV, are illustrated in Fig. 5b. The former peak typically belongs to the sp2-bonded

carbon of the N=C-N coordination [54]; the latter can be attributed to the sp2-bonded

carbon of C-C because of the residual carbon from the samples and the adventitious

hydrocarbon from XPS instruments. In the case of Cv-gCN, the C 1s peak at 288.4 eV

of N=C-N shifts toward a higher binding energy with approximately 0.16 eV as the

C/N ratio decreases (Table 1) because of the redistribution of extra electrons left by

the missed carbon atoms. Therefore, the electron density around the carbon of the

N=C-N coordination decreases.

The high-resolution XPS spectrum of N 1s region was shown in Fig. 5c. The

main two peaks located at around 398.8 and 401.2 eV can be ascribed to the sp2

C-N-C and C-N3 groups [55], respectively. However, for the Cv-gCN, there is a new


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peak centered at 399.9 eV, which can be ascribed to the formation of amino groups

(such as -NH2) due to the deletion of tertiary carbon. This fact confirmed the

generation of carbon vacancies in Cv-gCN.

The high-resolution XPS spectrum of O 1s was also performed (Fig. 5d). The O 1s

spectrum can be curve-fitted into three peaks with binding energies of around 530.9,

532.3 and 534.0 eV, which correspond to C=O, C-O and surface adsorbed H2O

molecules, respectively.

Electron paramagnetic resonance (EPR) was also employed to verify the formation

of carbon vacancies. Fig. 6 shows that both samples exhibited a major signal at

g = 2.0043, which represents a characteristic of unpaired electrons on paramagnetic

aromatic carbon atoms. The intensity of EPR signal does not relate to vacancy defects

directly. However, in the presented study, the EPR signal centered at g = 2.0043 stems

from the unpaired electrons on the carbon atoms of the aromatic rings. The much

weaker EPR signal indicates the less of carbon amount in the Cv-gCN when compared

with pristine gCN. The result was consistent with the documented literature [49].

Therefore, carbon vacancies were successfully introduced into gCN after heat

treatment at CO2 atmosphere.

3.4 Optical properties and band structures

UV-Vis absorption spectrum was used to evaluate the light-harvesting ability of the

photocatalyst. Fig. 7 shows that both gCN and Cv-gCN exhibited similar light

absorption spectra. The onset of the absorption spectrum for the pristine gCN started

from 462 nm, corresponding to a band gap of approximately 2.68 eV, which is similar

to the reports in the literature [56-58]. Although the shape of the DRS spectrum curve

for Cv-gCN was similar to that of pristine gCN, the sharp decrease in the DRS

spectrum was blue-shifted for 25 nm. The band gap of Cv-gCN was calculated to be


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2.84 eV, which is 0.16 eV larger than that of the pristine gCN. The increased bandgap

of Cv-gCN can also confirm by DFT calculation [59] (Fig. 8), which is possibly due to

the formation of carbon vacancy.

According to the XPS results (Fig. 9A), the VB edge potential of gCN and Cv-gCN

are 2.18 and 2.08 V, respectively. Hence, the CB minimum energy for the pristine

gCN and that for Cv-gCN are -0.5 and -0.76 V, respectively. The Mott-Schottky

measurements were also implemented to determine the flat-band potentials of both

gCN and Cv-gCN, as shown in Fig. 9B. In the case of n-type semiconductor, the CB

values is quite close to their corresponding flat-band potential. Therefore, the result of

Mott-Schottky is accordance with the estimated CB values according to equation 2:

Eg=Ec-Ev (2)

The band structures of the two samples are compared in Scheme 1. It can be seen

that the introduction of carbon vacancy into the lattice of gCN results in a negatively

shifted CB position. The negatively shifted CB potential of Cv-gCN should facilitates

the photo-generated electron transfer from CB to oxygen, leading to the formation of

super oxide radicals (O2•-), which are important reactive oxygen species (ROSs) for

NO oxidation.

Charge separation and migration significantly influence photocatalytic

performance. Therefore, we compared the photocurrent responses of the pristine gCN

and Cv-gCN. Fig. 10A shows that the photocurrent of gCN is extremely low (less than

0.15 mAcm−2), which is ascribed to the quick recombination rate of photo-generated

electron–hole pairs. After the introduction of carbon vacancy, the photocurrent of

Cv-gCN sharply increased to 0.5-0.6 mAcm−2, thus indicating the delayed

recombination of carriers. The efficient separation of carriers was further confirmed

with electrochemical impedance spectroscopy (EIS) shown in Fig. 10B. The arc


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radius on the EIS Nyquist plot of Cv-gCN is smaller than that of the pristine gCN. The

radius of the arc on the EIS spectra reflects the reaction rate occurring at the surface

of the electrode [60]; the result suggests that an effective separation of photogenerated

electron–hole pairs and a rapid interfacial charge transfer occurred after the

introduction of carbon vacancy into gCN.

3.5 Visible light photocatalytic oxidation of NO

As a common urban air pollutant in indoor environments, NO exhibits low

solubility and reactivity; this pollutant also causes severe respiratory diseases,

including DNA strand breaks and/or base alterations that are potentially mutagenic

[61]. Therefore, this work aimed to harness environmentally friendly and economical

photocatalysis technology to remove NO in air. The photocatalytic performance of

g-C3N4 was evaluated via photocatalytic oxidation of NO under visible light

irradiation (λ > 400 nm).

The direct photolysis of NO can be neglected in the absence of either photocatalyst

or light irradiation (not shown here) [62]. In the presence of g-C3N4, however, a sharp

decrease in the concentration of NO was observed at the beginning of the visible light

irradiation; the concentration latter reaches a relatively stable value (Fig. 11A). After

irradiation for 15 min, the concentration of NO remained almost unchanged in the

continuous reactor. At that time, the removal rate of NO over Cv-gCN reached as high

as 59.0%, which is two times higher than that over pristine gCN (removal rate of only

24.2%). We also monitored the concentration of in situ-generated NO2 (Fig. 11B) and

found that the NO2 concentration in the outlet gas over gCN (81 ppb) was much

higher than that over Cv-gCN (only 19 ppb). The experiment results indicate that in

comparison with pristine gCN, g-CN with carbon vacancy (Cv-gCN) not only exhibits

higher photocatalytic activity in the oxidation of NO but also possesses the potential


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to oxidize NO2. These results were also confirmed via the photocatalytic oxidation of

NO2 under similar identical conditions (Fig. S2). After irradiation under visible light

for 15 min, 24.1% of the NO2 was photocatalytic oxidized over pristine gCN while

50.1% of NO2 was removed over Cv-gCN.

3.6 Radical-trapping experiment

To accounted for the photocatalytic oxidation mechanism of NO over Cv-gCN, we

performed trapping experiments using KI, K2Cr2O7, and TBA as hole, electron, and

•OH radical scavengers, respectively. Fig. 12 presents the minimal effect of the

addition of KI and TBA on the oxidation of NO. This result indicates that holes or

•OH radicals were not the main ROSs involved in the oxidation of NO over g-CN.

However, the oxidation of NO was severely retarded in the presence of K2Cr2O7, thus

suggesting the involvement of photo-generated electron in the oxidation of NO. The

VB potential of g-CN (2.08 V for Cv-gCN) was not sufficiently high to oxidize the

-OH group and form a reactive •OH radical (E0(–OH/•OH)= 2.4 V) [57,63-65]. Thus, the

minimal effect of TBA on the photocatalytic oxidation of NO is understandable. The

weak adsorption of NO on g-CN may also hamper oxidation because of the hole

radicals. Therefore, the photocatalytic oxidation of NO over Cv-gCN is achieved

mainly through super oxide radicals (O2•-) formed by photo-generated electrons and

adsorbed oxygen.

The introduction of carbon vacancies resulted in a negatively shifted CB

edge potential (Scheme 1 and Table 1) and thus facilitates the electron transfer

from the CB to oxygen. The photocatalytic activity of Cv-gCN was higher than

that of pristine gCN.

In practical applications, the stability of photocatalysts is of great

importance. Therefore, the stability of g-CN with carbon vacancies was


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evaluated in this study. As illustrated in Fig. 13, the Cv-gCN sample did not

show any evident loss of activity even after five successive cycles, thereby

demonstrating its satisfactory reusability in NO degradation.

4 Conclusions

Through the calcination of bulk g-C3N4 (gCN) in CO2 atmosphere, carbon

vacancy was successfully introduced into g-C3N4 (Cv-gCN). Carbon vacancy

resulted in a negatively shifted CB position of the g-C3N4 and therefore

facilitated the electron transfer from the CB of g-C3N4 to the adsorbed oxygen,

thereby forming superoxide radicals and enhancing the photocatalytic NO

oxidation. Meanwhile, the enlarged BET surface area and pore volume

improved the photocatalytic activity of Cv-gCN. Thus, carbon vacancy can

engineer the structural characteristics and physiochemical properties of g-C3N4

and provide a new type of functional material for photocatalysis and catalysis

applications.

Acknowledgments

This research was financially supported by the National Key Research and

Development Program of China (2016YFA0203000), and Internal Research Grant

(RG 11/2016-2017R), EdUHK. It was also supported by the National Natural

Science Foundation of China (51672312 & 21373275) and the Science and

Technology Program of Wuhan, China (2016010101010018).

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27

Table 1. Physical properties of the photocatalyst.

Sample

Nitrogen sorption Element contents (at.%)

RC:Nc

Band structure

SBET

(m2g−1)

PVa

(cm3g−1)

C

N

O

S

Egd

(eV)

VB

(V)

CB

(V)

gCN 27 0.14 44.79 53.31 0.9 1.0 0.84 2.68 2.18 −0.5

Cv-gCN 147 0.75 42.39 55.19 2.42 n.d.b 0.77 2.84 2.08 −0.76

apore volume.

bnot detected.

catom ratio of carbon to nitrogen.

dbandgap of the semiconductor photocatalyst.


28

Scheme 1. Comparison between the band structures of g-C3N4 before and after the

introduction of carbon vacancy.


29

Fig. 1. SEM and TEM images of gCN (a and b) and Cv-gCN (c and d).


30

Fig. 2. XRD patterns of the gCN and Cv-gCN samples (inset showing the enlarged

XRD patterns of the photocatalysts at (002) diffraction peak region).


31

http://time.qq.com/jieqi/mangzhong/20170605.htm?pgv_ref=aiotime

4000 3500 3000 2500 2000 1500 1000 500

Amino group

(a) gCN

T

ran

sm

itta

nc

e (

a.u

.)

Wavenumbers (cm-1)

(b) Cv-gCN

Fig. 3. Comparison of the FT-IR spectra of the as-prepared photocatalysts.

http://time.qq.com/jieqi/mangzhong/20170605.htm?pgv_ref=aiotime


32

0.0 0.2 0.4 0.6 0.8 1.0

0

100

200

300

400

500

600

Relative pressure (P/P0)

Vo

lum

e a

dso

rbed

(c

m3g

-1)

gCN

Cv-gCN

Vo

lum

e a

ds

orb

ed

(cm

3g

-1)

Fig. 4. N2 adsorption–desorption isotherms and corresponding pore size distribution

curves (inset) of gCN and Cv-gCN.


33

100 200 300 400 500 600 700 800

(a) Survey

Cv-gCNC

1s

N 1

s

O 1

s

Inte

ns

ity

(a

.u.)

Binding Energy (eV)

gCN

292 290 288 286 284 282

284.9 eV

284.8 eV

Cv-gCN

gCN

288.56 eV

288.4 eV

(b) C 1s

Binding energy (eV)

Inte

nsit

y (

a.u

.)

402 400 398 396

398.7 eV

Binding energy (eV)

Inte

ns

ity

(a

.u.)

(c) N 1s

gCN

Cv-gCN

399.9 eV

401.2 eV

398.8 eV

401.2 eV

536 535 534 533 532 531 530 529 528

534.0 eV

530.9 eV

531.2 eV533.6 eV

532.3 eV

gCN

Cv-gCN

532.3 eV

Binding energy (eV)

Inte

ns

ity

(a

.u.)

(d) O 1s

Fig. 5. Representative survey (a), high-resolution XPS spectra of C 1s (b), N 1s (c)

and O 1s (d) of gCN and Cv-gCN.


34

1.99 2.00 2.01 2.02

In

ten

sit

y (

a.u

.)g=2.0043

gCN

Cv-gCN

Value (g-Factor)

Fig. 6. EPR spectra of g-C3N4 samples.


35

Fig. 7. UV-Vis diffuse reflectance spectra (DRS) of pristine gCN (a) and Cv-gCN (b).


36

Fig. 8. Calculated band structure of gCN (a) and Cv-gCN (b).


37

10 8 6 4 2 0 -2

(a) VB

Cv-gCN

Binding Energy (eV)

Inte

ns

ity

(a

.u.)

gCN

-1.0 -0.5 0.0 0.5 1.0 1.5

(b) Mott-Schottky plots

-0.76 V

-0.5 V

C-2 (

F-2)

E (V / SCE)

gCN

Cv-gCN

Fig. 9. (a) VB XPS and (b) Mott-Schottky plots of gCN and Cv-gCN.


38

0 100 200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

off

(b)

Ph

oto

cu

rren

t (m

Acm

-2)

Time (S)

(a) gCN

(b) Cv-gCN

(a)

on (A)

0K 10K 20K 30K 40K 50K 60K 70K 80K0K

10K

20K

30K

40K

50K

Z'' (

oh

m)

Z' (ohm)

gCN

Cv-gCN

(B)

Fig. 10. Time-dependent photocurrent curves (A) and EIS Nyquist plots (B) of gCN

and Cv-gCN electrodes under visible light irradiation (λ > 420 nm; [Na2SO4] =

0.1 M).


39

0 5 10 15 20 25 3030

40

50

60

70

80

90

100

(b) Cv-gCN

(A)

C/C

0 %

Irradiation time (min)

(a) gCN

C/C

0 (

%)

0 5 10 15 20 25 300

20

40

60

80

100

120

(b) Cv-gCN

NO

2 e

vo

lved

(p

pb

)


(a) gCN

(B)

Fig. 11. Photocatalytic oxidation of NO (A), together with the corresponding NO2

formation profiles (B). The initial concentration of NO in continuous reactor: 600

ppb.


40

0 5 10 15 20 25 30

40

50

60

70

80

90

100

C/C

0 (

%)


Cv-gCN

Cv-gCN + TBA

Cv-gCN + KI

Cv-gCN + K2Cr2O7

Fig. 12. Photocatalytic oxidation profiles of NO over Cv-gCN in the presence of

different scavengers (the initial concentration of NO in the continuous reactor is 600

ppb).


41

0 30 60 90 120 150

40

50

60

70

80

90

100

5th4

th3

rd2

nd

C/C

0 (

%)


1st

Fig. 13. Stability of Cv-gCN in five consecutive runs for photocatalytic oxidation of

NO under visible light irradiation (the initial concentration of NO in the continuous

reactor is 600 ppb).

carbon vacancy-induced enhancement of the visible light-driven...

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