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Bridging the g-C3N4 interlayers for enhanced photocatalysis Ting Xiong, Wanglai Cen, Yuxin Zhang, and Fan Dong ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02922 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 7, 2016
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Bridging the g-C3N4 interlayers for enhanced photocatalysis
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Ting Xiong a,†, Wanglai Cen b,†, Yuxin Zhang c, Fan Dong a,*
5
a
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Environment and Resources, Chongqing Technology and Business University, Chongqing
7
400067, China.
8 9 10 11
Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of
b
Institute of New Energy and Low Carbon Technology, Sichuan University, Chengdu 610065, China.
12
c
13
Science of Micro/Nano-Devices and System Technology, Chongqing University, Chongqing
14
400044, China.
College of Materials Science and Engineering, National Key Laboratory of Fundamental
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ABSTRACT: Graphitic carbon nitride (g-C3N4) has been widely investigated and applied in
3
photocatalysis and catalysis, but its performance is still unsatisfactory. Here, we demonstrated
4
that K-doped g-C3N4 with unique electronic structure possessed highly enhanced visible light
5
photocatalytic performance for NO removal, which was superior to Na-doped g-C3N4. DFT
6
calculations revealed that K or Na doping can narrow the bandgap of g-C3N4. K atoms,
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intercalated into the g-C3N4 interlayer via bridging the layers, could decrease the electronic
8
localization and extend the π conjugated system, while Na atoms tended to be doped into the
9
CN planes and increased the in-planar electron density. Based on theoretical calculation
10
results, we synthesized K-doped g-C3N4 and Na-doped g-C3N4 by a facile thermal
11
polymerization method. Consistent with the theoretical prediction, it was found that K was
12
intercalated into space between the g-C3N4 layers. The K-intercalated g-C3N4 sample showed
13
increased visible light absorption, efficient separation of charge carriers and strong oxidation
14
capability, benefiting from the narrowed band gap, extended π conjugated systems and
15
positive-shifted valence band position, respectively. Despite that the Na-doped g-C3N4
16
exhibited narrowed bandgap, the high recombination rate of carriers resulted in the reduced
17
photocatalytic performance. Our discovery provides a promising route to manipulate the
18
photocatalytic activity simply by introducing K atoms in the interlayer and gains a deep
19
understanding of doping chemistry with congeners. The present work could provide new
20
insights into the mechanistic understanding and the design of electronically optimized layered
21
photocatalysts for enhanced solar energy conversion.
22 23
KEYWORDS: K-intercalated g-C3N4; bridging K atoms; charge redistribution and transfer;
24
thermal polymerization; visible light photocatalysis.
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1. INTRODUCTION
2
Recently, beyond conventional semiconductors, two-dimensional (2D) materials are of
3
particular interests in consideration of their fantastic, unusual properties and potential
4
applications in various fields.1-3 Typically, graphene, graphitic carbon nitride, hexagonal
5
boron nitride and transition metal dichalcogenides with remarkable properties have been
6
successfully applied in optical and electronic devices, energy generation and storage,
7
environmental remediation, hybrid materials as well as chemical sensors.4-8
8
Among the various available 2D layered compounds, g-C3N4, consisting of tris-triazine
9
units connected with planar amino groups in each layer and weak van der Waals force
10
between layers (Figure 1), has triggered extensive investigation due to the suitable bandgap,
11
low cost, ease of preparation, good stability, and environmental friendly feature, thus leading
12
to multifunctional application for photocatalytic degradation of pollutants, photocatalytic
13
hydrogen generation, carbon dioxide reduction as well as supercapacitors.9-14 Nevertheless,
14
the pure g-C3N4 has been restricted by the low photocatalysis efficiency mainly for its
15
marginal visible light absorption and fast charge recombination.15,16 In order to improve the
16
photocatalytic performance of g-C3N4, various modification strategies have been
17
employed,17-25 and doping is known to considerably broaden the light responsive range and
18
enhance the charge separation of semiconductors. Doping of g-C3N4 with S, F, C or B via
19
substituting for lattice atoms has been applied to modify its texture and electronic structure
20
for improving the photocatalytic performance.26-29 Also, transitional metals (Fe3+, Mn3+, Co3+,
21
Ni3+, and Cu2+) have been incorporated into the framework (nitrogen pores) of g-C3N4 to
22
enhance the performance in benzene hydroxylation and styrene epoxidation reaction.30-32 As
23
can be seen, the externally doped atoms either substitute for the lattice atoms or exist in the
24
in-planar caves of g-C3N4.
25
For layered materials, intercalation provides the materials with promising properties. For
26
example,
calcium-intercalated
bilayer
graphene
C6CaC6
27
superconductivity since the interlayer electrons participate in the transport properties.33 FeCl3
28
intercalated few-layer graphene has a square resistance lower than ITO.34 Also, it is found
29
that Pt-intercalated layered KCa2Nb3O10 shows efficient photocatalytic activity for water
30
splitting.35 Given that g-C3N4 has a layered structure with interlayer galleries that could allow 3 ACS Paragon Plus Environment
on
silicon
possesses
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doping of heteroatom within the interlayer, it is a feasible route to synthesize intercalated
2
g-C3N4 compound to achieve enhanced photocatalytic performance. Yan et al. prepared
3
intercalated carbon nitride photocatalysts for hydrogen production, which is unstable in
4
aqueous solution.36 Hence, it still remains a challenge to fabricate stable and superior
5
intercalated g-C3N4 compound.
6
Recently, several literatures reported that the introduction of K atoms can improve the
7
photocatalytic performance of g-C3N4.37,38 However, the position of the introduced K atoms
8
is not clear and the essential evidence for the improved photocatalytic performance has not
9
been revealed clearly. Furthermore, whether other congener alkaline elements, for instance
10
the Na atoms with similar electronic structure, possess similar promotion effects, are of great
11
interest to be uncovered. Herein, we performed DFT calculations to investigate the electronic
12
and band structures of K/Na-doped g-C3N4. It is amazing to find that the bandgap of g-C3N4
13
could be narrowed either by K or Na doping, but they exerted different influence on the
14
electronic structure and photocatalytic activity of g-C3N4. The K atoms tended to exist in the
15
g-C3N4 interlayer and extended the π conjugated systems, while Na atoms were doped into
16
the conjugated plane and increased the electron density in the CN layers. The structural
17
differences may lead to the different photocatalytic performance. Aimed to this point, we
18
prepared K/Na-doped g-C3N4 by a simple thermal polymerization method. As expected, the
19
K-intercalated g-C3N4 exhibited high photocatalytic activity toward the removal of NO.
20
However, Na-doped g-C3N4 showed low photocatalytic activity. A series of characterizations
21
indicated that the K-intercalated g-C3N4 possessed narrowed band gap and strong VB holes
22
oxidation. The intercalated K atoms benefited the transfer and separation of charge carriers.
23
However, it was not the way for Na, although the Na doped g-C3N4 also exhibited narrowed
24
band gap. The present work could provide new insights into the effects of alkali metal doping
25
on g-C3N4 as well as the design of intercalated photocatalysts with highly efficient
26
visible-light-driven activity for air purification.
27
2. EXPERIMENTAL
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2.1 Materials and synthesis
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All chemicals used in this study were analytical grade and were used without further
30
treatment. In a typical synthesis procedure, 10 g of thiourea, and a known amount (3 wt %) 4 ACS Paragon Plus Environment
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(relative to the experimentally obtained g-C3N4) of X (X = Na, K) coming from X (X = Na,
2
K)Br were dissolved in 30 mL water in an alumina crucible. The obtained solution was then
3
dried at 80 °C overnight to get the solid precursors. The solid composite precursors were
4
placed in a semi-closed alumina crucible with a cover. The crucible were heated to 550 °C at
5
a heating rate of 15 °C·min-1 in a muffle furnace and maintained for 2 h. After the thermal
6
treatment, the crucible was cooled down to room temperature in the muffle furnace. The
7
resulted samples were collected for further use. The pristine g-C3N4 was prepared without
8
adding XBr and labeled as CN. The Na-doped g-C3N4 was labeled as CN-Na3. The K-doped
9
g-C3N4 with different weight ratio of K to g-C3N4 (1, 3, 5 and 10 wt %) was labeled as
10
CN-K1, CN-K3, CN-K5 and CN-K10, respectively.
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2.2 Characterization
12
The crystal phases of the sample were analyzed by X-ray diffraction (XRD) with Cu Kα
13
radiation (model D/max RA, Rigaku Co., Japan). Scanning electron microscopy (SEM;
14
model JSM-6490, JEOL, Japan) was used to characterize the morphology of the obtained
15
products. The morphology and structure of the samples were examined by transmission
16
electron microscopy (TEM; JEM-2010, Japan). X-ray photoelectron spectroscopy (XPS) with
17
Al Kα X-rays (Thermo ESCALAB 250, USA) was used to investigate the surface properties.
18
The UV-vis diffuse-reflectance spectrometry (DRS) spectra were obtained for the dry-pressed
19
disk samples using a Scan UV-vis spectrophotometer (TU-1901, China) equipped with an
20
integrating sphere assembly, using 100% BaSO4 as the reflectance sample. Nitrogen
21
adsorption-desorption isotherms were obtained on a nitrogen adsorption apparatus (ASAP
22
2020, USA) with all samples degassed at 150 °C for 12 h prior to measurements. The
23
photoluminescence spectra were measured with a fluorescence spectrophotometer (F-7000,
24
Japan) using a Xe lamp as excitation source with optical filters. Steady and time-resolved
25
fluorescence emission spectra were recorded at room temperature with a fluorescence
26
spectrophotometer
27
thermogravimetric-differential scanning calorimetry analysis (TG-DSC: NETZSCHSTA 409
28
PC/PG), 20 mg dry sample was sealed in an Al2O3 crucible with a lid and scanned at a rate of
29
20 °C·min−1. The photocurrent response and electrochemical impedance spectra
30
measurements were conducted in a three electrode system on a CH 660D electrochemical
31
work station, Platinum wire was used as the counter electrode. Sturated calomel electrodes
32
were used as the reference electrodes. The as-prepared samples film electrodes on ITO served 5
(Edinburgh
Instruments,
FLSP-920).
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the
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as the working electrode. All the potentials quoted here are with respect to saturated calomel
2
electrode. For photoelectrochemical test, the working electrode was irradiated from the
3
as-prepared sample films side under a 300 W Xe lamp (PerfectLight Co. Ltd,
4
PLS-SXE-300UV). Incident visible-light was obtained by utilizing a 420 nm cutoff filter. The
5
photocurrent-time dependence of as-prepared sample films at open circuit potential (OCV)
6
was measured in 0.5 M Na2SO4 under chopped illumination with 60s light on/off cycles.
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2.3 DFT Calculations
8
DFT calculations were carried out using the “Vienna ab initio simulation package”
9
(VASP5.3).39 The Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional was used
10
within the spin polarized generalized gradient approximation (GGA).40 A plane-wave basis
11
set was employed within the framework of the projector augmented-wave (PAW) method.41
12
In order to get accurate results, the cut-off was set to 450 eV. A Gaussian smearing was used
13
with a smearing width of 0.2 eV. Geometry relaxations were carried out until the residual
14
forces on each ion were smaller than 0.02 eV/Å. We firstly relaxed a 1×1×2 supercell of bulk
15
g-C3N4. Then, a single K or Na atom was introduced in interstitial or substitutional doping
16
ways. The most stable relaxed configurations are the in plane doping pattern for Na, and the
17
interlayer bridging pattern for K according to our testing calculations (see the supporting
18
information). The most stable relaxed configuration was denoted as CN-K2 (CN-Na2). It
19
results in a concentration of 3 wt% Na for CN-Na2 or 5 wt% K for CN-K2. The electronic
20
structures were calculated based on the fully relaxed lattice parameters and ionic positions. In
21
all calculations, k-points were sampled in 5×5×3 Monkhorst-Pack grid. The van der Waals
22
(vdW) correction was included by using the DFT-D2 approach.42
23
2.4 Evaluation of visible light photocatalytic activity
24
The photocatalytic activity was investigated by removal of NO at ppb levels in a
25
continuous flow reactor at ambient temperature. The volume of the rectangular reactor, made
26
of polymeric glass and covered with Saint-Glass, was 4.5 L (30 cm × 15 cm × 10 cm). A 150
27
W commercial tungsten halogen lamp was vertically placed outside the reactor. A UV cutoff
28
filter (420 nm) was adopted to remove UV light in the light beam. For each photocatalytic
29
activity test, 0.20 g of the as-prepared sample was dispersed in distilled water (50 mL) in a
30
beaker via ultrasonic treatment for 10 min and then coated onto two glass dishes (12.0 cm in 6 ACS Paragon Plus Environment
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diameter). The coated dish was pretreated at 60 °C to remove water in the suspension and
2
then cooled to room temperature before photocatalytic test.
3
The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of
4
NO (N2 balance). The initial concentration of NO was diluted to about 600 ppb by the air
5
stream. The desired relative humidity level of the NO flow was controlled at 50% by passing
6
the zero air streams through a humidification chamber. The gas streams were premixed
7
completely by a gas blender, and the flow rates of the air stream and NO were controlled at
8
2.4 L·min-1 and 15 mL·min-1 by a mass flow controller, respectively. After the
9
adsorption-desorption equilibrium was achieved, the lamp was turned on. The concentration
10
of NO was continuously measured by a chemiluminescence NO analyzer (Thermo
11
Environmental Instruments Inc., 42i-TL), which monitors NO with a sampling rate of 1.0
12
L·min-1. The removal ratio (η) of NO was calculated as η (%) = (1-C/C0) ×100%, where C
13
and C0 are concentrations of NO in the outlet steam and the feeding stream, respectively.
14
3. RESULTS AND DISCUSSION
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3.1 DFT calculation and electronic structure
16
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1 2
Figure 1. The calculated crystal structures of CN (a), CN-K2 (b) and CN-Na2 (c). (d) and (e)
3
are the topviews of the doped layer in (b) and (c), correspondingly. Selected distances are
4
marked in pm.
5
DFT calculation was first utilized to predict the crystal structure of g-C3N4 without and
6
with K or Na atoms doping. The optimal g-C3N4 crystal structure is shown in Figure 1a.
7
Related lattice parameters were collected in Table 1. The in-planar distance of the nitride
8
pores at the center of adjacent heptazine units and the interlayer distance of the g-C3N4 are
9
determined to be ca. 7.11 and 3.14 Å, respectively, in agreement with the other theoretical
10
results.43,44 Based on the optimal crystal structure of g-C3N4, we tested both the in cave
11
doping and interlayer bridging sites for Na and K. Related results have been provided in the
12
supporting information (Table S1-S2 and Figure S1-S4). The most stable relaxed
13
configuration is the in cave doping pattern for Na, and the interlayer bridging pattern for K.
14
The optimized crystal structures of Na doped g-C3N4 (CN-Na2) and K intercalated g-C3N4
15
(CN-K2) are shown in Figure 1b and 1d, Figure 1c and 1e, respectively. Obviously, after
16
structural optimization, the Na atoms are found to locate in the π-conjugated planes, while K
17
atoms are present in the g-C3N4 interlayer. Moreover, Na doping almost does not change the
18
crystal structure, but the crystal cell becomes slim after K doping. Such different influence
19
induced by Na and K doping mainly originates from the size of the two atoms. Na atoms with
20
small radius of 1.86 Å can enter the cave of the planes, 45 and then exert little influence on the
21
crystal structure. K atoms with relatively large radius of 2.32 Å are inclined to intercalate into
22
g-C3N4 interlayer, which are expected to form a bridge for charge transfer.45
23
Table 1. Comparison of the calculated crystal structures over CN, CN-Na2 and CN-K2. a (Å)
b (Å)
c (Å)
α (o)
β (o)
γ (o)
CN
7.11
7.11
12.26
90.00
90.00
120.00
CN-Na2
6.98
7.00
12.56
90.00
89.94
120.14
CN-K2
6.98
7.02
13.18
90.00
91.90
120.24
Parameters
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CN43
7.16
6.96
12.35
90.00
90.00
120.00
CN44
7.13
6.92
12.35
90.00
90.00
120.00
1 2
Figure 2. The charge difference distribution of (a) CN-K2 and (b) CN-Na2, and electronic
3
location function (ELF) analysis of (c) CN-K2 and (d) CN-Na2. In (a) and (b), the yellow
4
region denotes charge depletion while blue region denotes accumulation. The isosurface is
5
0.02 e/Å3. Refer to Figure S5 in the supporting information for full views.
6
Then we analyzed the differences in spatial charge distribution and ELF of CN-K2 and
7
CN-Na2 as shown in Figure 2. Although Na and K atoms are congeners, they exert distinct
8
influence on the charge distribution property. All C-N bonds are sp2 orbital hybridization and
9
the lone electrons in the pz orbital form a big π-bond in g-C3N4.9 In the host g-C3N4, valence
10
electrons are preferable to locate at N atoms for its stronger eletrophilicity than C atom,
11
which results in the formation of N anion. When K and Na are introduced, the outermost
12
electrons are transferred to g-C3N4 and the Na/K presents as cation. Subsequently, there is
13
static coulomb interaction between Na/K and N nearest to them. K atoms lead to the drastic
14
rearrangement of the g-C3N4 layer. The sp2 orbital planes lose electrons and the pz orbital
15
planes obtain electrons in CN-K2 (Figure 2a and Figure S5a). The charge of the upper layer 9 ACS Paragon Plus Environment
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N (C) atoms and the lower layer C (N) atoms close to the interbedded K atoms increases,
2
forming a funnel-like three dimensional structure. ELF is defined as a dimensionless quantity
3
and takes values in the range between 0 and 1, where ELF = 1 corresponds to the perfect
4
localization and ELF = 0.5 is for the uniform electron gas. It can be used as an indicator of
5
covalent bond between two atoms.46 The ELF of CN-K2 is displayed in Figure 2c, the
6
minimum value of ELF between K-N and K-C are quite small (420 nm, [Na2SO4] = 0.5 M).
4
Figure S8 shows the XRD pattern of CN, CN-Na3 and CN-K3. Notably, compared with
5
the pure g-C3N4, the peaks position for Na-doped g-C3N4 keep unchanged, whereas the peaks
6
position experience a shift for K-doped g-C3N4. This experimental result confirms the
7
theoretical result that K is intercalated g-C3N4 and changes the layer distance but Na is doped
8
into g-C3N4 and has no obvious influence on the layer distance. The photocurrent response
9
and electrochemical impedance spectra measurements were carried out to distinguish the
10
variations of photoelectric response after Na or K doping. In contrast to CN, CN-K3 exhibits
11
an improved photocurrent, but CN-Na3 shows a decreased photocurrent under visible light
12
irradiation (Figure 5a). The increased photocurrent suggests more efficient separation of
13
photogenerated electron/hole pairs.47 In addition, EIS Nyquist analysis were conducted as
14
shown in Figure 5b. The diameter of the arc radius on the Nyquist plots of the K-intercalated
15
g-C3N4 is smaller than that of the bare g-C3N4, whose diameter is smaller than that of
16
Na-doped g-C3N4. Figure S9 displays that the PL intensity of the three samples follows the
17
order of and CN-K3 < CN < CN-Na3. It turns out that introducing K atoms into g-C3N4 can
18
facilitate the separation and transfer efficiency of photogenerated carriers, whereas
19
introducing Na atoms plays an opposite role.
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1 2
Figure 6. The ns-level time-resolved fluorescence spectra monitored under 420 nm excitation
3
at room temperature for CN, CN-Na3 and CN-K3.
4
The ns-level time-resolved fluorescence decay spectra were further employed to
5
investigate the charge transfer dynamics over CN, CN-Na3 and CN-K3 (Figure 6). The
6
curves can be fitted well to a biexponential decay function, with all the fitting parameters
7
summarized in Table 2. The short lifetime (τ1) is 10.5, 6.3 and 14.0 ns, and the long lifetime
8
(τ2) is 85.6, 80.6 and 90.3 ns for CN, CN-Na3 and CN-K3, respectively. Obviously, in
9
contrast to CN, the lifetime of charge carriers is prolonged for CN-K3 while that for CN-Na3
10
is shortened, elucidating that K doping can enhance the charge transfer and separation of
11
g-C3N4 while the Na doping could not do so. The prolonged lifetime of charge carriers can
12
increase their probability to participate in photocatalytic reactions before recombination.48
13
These results combined with the analysis revealed by the photocurrent responses, EIS spectra
14
and PL spectra confirm the conclusion that the fast transfer and separation of charge carriers
15
are responsible for the high photocatalytic activity of CN-K3 while the high recombination of
16
carriers results in low activity for CN-Na3.
17
Table 2. Kinetic parameters of the fitting decay parameters of CN, CN-Na3 and CN-K3. Relative Samples
Parameters
Life time (ns)
χ2
Percentage (%) τ1
10.5
54.3
τ2
85.6
45.7
τ1
6.3
70.9
CN CN-Na3
1.041
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1.056
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τ2
80.6
29.1
τ1
14.0
49.5
τ2
90.3
50.5
CN-K3
1
1.065
3.4 Phase Structure and Chemical Composition
2 3 4
Figure 7. The XRD of g-C3N4 and the K-intercalated g-C3N4 samples with different K doping content (a) and the enlarged profile of the (002) diffraction region (b).
5
As the K-intercalation could significantly enhance the photocatalytic activity of g-C3N4,
6
we prepared a series of K-intercalated g-C3N4 photocatalysts by tuning the amount of K to
7
further optimize the photocatalyst. Figure 7a depicts the X-ray diffraction (XRD) patterns of
8
the samples. Two peaks at ca. 13.1° (indexed as (100) diffraction planes associated with the
9
in-plane repeated unites) and 27.7° (corresponding to the (002) peak of the periodic graphitic
10
stacking of the conjugated aromatic system) can be observed in the g-C3N4 and
11
K-intercalated g-C3N4 samples, reflecting the existence of graphitic-like layer structures.9
12
Also noted is the downshift of the (002) diffraction peaks in K-intercalated g-C3N4 compared
13
with pure g-C3N4 with the increase of K doping content (Figure 7b). This could be attributed
14
to the incorporation of K atoms between the CN layers. K atoms existing in g-C3N4 interlayer
15
are bonded with the adjacent C or N atoms, which will then induce the expansion of the
16
crystal lattice for g-C3N4. This is confirmed by the theoretic result that obvious change of
17
crystal structure is observed for K-intercalated g-C3N4.
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2 3
Figure 8. XPS spectra of CN and CN-K5, (a) C 1s, (b) N 1s, (c) K 2p, (d) Br 3d.
4
The compositions and chemical states of CN and CN-K5 were further tested by X-ray
5
photoelectron spectroscopy (XPS) for comparison. In Figure 8a, the two bonding states of
6
carbon species are evidenced with the C 1s binding energies of around 284.8 and 288.2 eV.
7
The former is assigned as the surface adventitious carbon or sp2 C–C bonds, and the latter is
8
attributed to the sp2-bonded carbon in N-containing aromatic rings.49 The high resolution N1s
9
XPS spectra of the two samples are shown in Figure 8b. The predominant peaks at a binding
10
energy of ca. 398.5 eV can be ascribed to the sp2 hybridized nitrogen involved in triazine
11
rings (C–N=C). Whereas the other two peaks with binding energies of 401.1 and 404.1 eV
12
are assigned to the N-H groups and the charging effects about the π-excitations, separately.50
13
Furthermore, the K element with binding energy located at 293 and 295.7 eV corresponding
14
to the K 2p3/2 and K2p1/2 peaks (Figure 8c) is observed in CN-K5, suggesting the presence of
15
the K species in the form of K-N and K-C bonds.51,52 The atomic concentration of the doped
16
K is determined to be 4.3% by XPS, close to the theoretic doped amount. The Br element is 16 ACS Paragon Plus Environment
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not doped into g-C3N4 as it is not detected in CN-K5 (Figure 8d). Notably, there are obvious
2
chemical shifts in the predominant N 1s and C1s peaks in relative to the pure g-C3N4. The
3
observed shifts for N 1s and C 1s binding energy further indicate the strong effects existing
4
between K and C/N atoms, which agree well with the theoretic analysis. These results
5
coupled with XRD analysis demonstrate the successful synthesis of K-intercalated g-C3N4.
6
3.5 Thermo stability
7 8
Figure 9. TG (a) and DSC (b) of the CN and CN-K3 samples.
9
In order to understand the thermal properties of CN and CN-K3, the thermogravimetric
10
(TG) and differential scanning calorimetry (DSC) analysis were carried out. The detected
11
range of temperature was from room temperature to 1000 °C at a heating rate of 20 °C·min-1.
12
In the case of TG, the weight loss of less than 10% at temperatures of up to 200 °C is
13
associated with the removal of physically adsorbed water. While a drastic weight loss is
14
observed in the range of 530–780 °C, which can be attributed to the loss in
15
tri-s-triazine-based units or other advanced condensates (Figure 9a).53 Correspondingly, the
16
DSC thermogram also witnesses the endothermic and exothermic stages in the same ranges
17
as those obtained in the TG thermogram (Figure 9b). It is noted that the major endothermic
18
peak at 743 °C experiences a downshift for CN-K3 in comparison with the pure g-C3N4,
19
implying that the stability of g-C3N4 decreases slightly after K doping. This further
20
demonstrates that K atoms are incorporated into the interlayer and create enlarged interlayer
21
distance, and consequently decrease the stability. 17 ACS Paragon Plus Environment
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3.6 Morphological Structure
(b)
(a)
2
(d)
(c)
3
N
C
(e)
K
4
Figure 10. SEM and TEM images of CN (a) and CN-K5 (b), FESEM-EDX elemental
5
mapping of CN-K5 (c, d and e).
6
The typical SEM and TEM images of CN and CN-K5 are shown in Figure 10. Figure 10a
7
shows that g-C3N4 sample is composed of irregularly curved layers. These layers are packed
8
in a random way, resulting in porous structure. Figure 10b illustrates that the CN-K5, similar
9
to g-C3N4, is also composed of curved layers, which implies that introduction of K atoms into
10
the crystal structure does not influence the morphology. Besides, elemental mapping based
11
CN-K5 was performed. It reveals that there appears a distribution of C, N, and K across the
12
bulk structure (Figure 10c, 10d and 10e). The K signal is detected in the entire structure,
13
verifying the homogenous dispersion of K element in g-C3N4.
14
3.7 Light Absorption and Band Structure
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(d)
2 3
Figure 11. UV-vis spectra (a) and the estimated band gaps (b) of g-C3N4 and the
4
K-intercalated g-C3N4 samples, VB XPS (c) and schematic illustration of the band gap
5
structure of CN and CN-K5 (d).
6
The light absorption property of these samples was investigated as shown in Figure 11a. In
7
contrast to the pure g-C3N4, the K-intercalated g-C3N4 samples show enhanced visible
8
absorption, and the absorption edges of K-intercalated g-C3N4 samples undergo red shifts.
9
Evidently, CN-K5 exhibits the strongest visible light absorption. The band gaps of the
10
as-synthesized CN, CN-K1, CN-K3, CN-K5 and CN-K10 (Eg) samples estimated from the
11
intercept of the tangents to the plots of (αhν)1/2 νs. photoenergy (Figure 11b) are 2.45, 2.34,
12
2.19, 2.15 and 2.15 eV, respectively, confirming that the bandgap can be narrowed by
13
introducing K atoms into g-C3N4 interlayers.54 Estimated from Figure 11c, the VB edges of
14
g-C3N4 and CN-K5 are estimated to be 1.58 and 1.86, respectively.55,56 According to the
15
bandgap and the VB position of the two samples, we can draw the bandgap structures as 19 ACS Paragon Plus Environment
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displayed in Figure 11d. It is obviously that the CB and VB position down-shift over CN-K5.
2
These facts that K-intercalated g-C3N4 samples possess narrowed band gap and enhanced
3
oxidization ability of VB holes relative to g-C3N4 are in line with the DOS results. These
4
experimentally determine that band structures are well consistent with the DFT calculation
5
results (Figure 3).
6
3.8 Carriers separation and transfer
7 8
Figure 12. PL spectra of the as-prepared samples.
9
To further understand the transfer and recombination processes of photoexcited charge
10
carriers in these K-intercalated g-C3N4 samples, PL spectra were measured. As displayed in
11
Figure 12, the K-intercalated g-C3N4 samples give similar PL spectra to pristine g-C3N4. But
12
the PL emission intensity over K-intercalated C3N4 is decreased with respect to g-C3N4, and
13
the intensity decreases with the increasing K content. Namely, the recombination of the
14
photoexcited charge carriers is effectively inhibited by introduction of K atoms into the
15
g-C3N4 interlayer. Additionally, the BET specific surface areas (SBET) of CN, CN-K1, CN-K3,
16
CN-K5 and CN-K10 products are determined to be 27, 18, 14, 11 and 8 m2·g−1, respectively.
17
It can be seen that the K-intercalated g-C3N4 samples show slightly lowered SBET in contrast
18
to the pure g-C3N4.
19
Table 3. The SBET, Band gap, interlayer distance, NO removal ratio and k values of all 20 ACS Paragon Plus Environment
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2
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K-intercalated g-C3N4 samples Sample
SBET /m2·g
Band gap /eV
Interlayer distance /nm
η(NO) /%
k value /min−1
CN
27
2.45
0.322
22.4
0.0939
CN-K1
18
2.34
0.323
33.5
0.1195
CN-K3
14
2.19
0.324
34.9
0.1138
CN-K5
11
2.15
0.325
36.8
0.1061
CN-K10
8
2.15
0.326
27.0
0.1082
3.9 Photocatalytic activity and stability
3 4
Figure 13. Comparison of the visible photocatalytic performance over the g-C3N4 and
5
K-intercalated g-C3N4 (a) and the cycling test of CN-K5 (b).
6
The photocatalytic performances of g-C3N4 and K-intercalated g-C3N4 samples have been
7
tested toward the removal of NO under visible light irradiation. Previous researches reveal
8
that NO could not be photolyzed without photocatalysts under visible light irradiation.57 As
9
can be seen from Figure 13a, for all samples, the NO removal ratios reach maximum in 5 min
10
and then decrease and finally reach a stable value with the following irradiation time. The
11
reduced activity may be caused by the accumulated reaction intermediates and products
12
generated on the surface during photocatalytic reaction. With the presence of pure g-C3N4,
13
NO removal ratio of 16% can be achieved after 30 min irradiation. Introducing K atoms into 21 ACS Paragon Plus Environment
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g-C3N4 leads to enhanced visible photocatalytic performance. It is found that all the
2
K-intercalated g-C3N4 samples display boosted activity and high reaction rate constants k
3
(Figure S10), which are superior to g-C3N4 and the physically mixed g-C3N4 and KBr
4
(summarized in Table 3). CN-K5 possesses the highest NO removal ratio of 37%, indicating
5
that there is an optimal K doping content to improve the activity of g-C3N4. The high visible
6
photocatalytic activity is ascribed to the intercalated K atoms. For one hand, K atoms
7
introduced into the g-C3N4 interlayer not only narrow the band gap and enhance visible light
8
absorption, but also downshift the VB position to improve the holes’ oxidization capability.
9
On the other hand, K atoms located at the g-C3N4 interlayer, which can function as delivery
10
channels for charge carries and then prolong the lifetime of photogenerated holes and
11
electrons. Based on previous study, ·O2−, h+ and ·OH are found to be the active species
12
involved with NO removal reactions.12 However, excess K doping may create nonradiative
13
recombination sites and decrease the photocatalytic activity.58,59 In addition, the catalytic
14
stability of CN-K5 was evaluated by six consecutive tests (Figure 13b). After two consecutive
15
tests, CN-K5 shows slightly decreased photocatalytic performance. Later, the NO removal
16
ratio keeps constant without noticeable deactivation, and exceeds the photocatalytic
17
performance of g-C3N4.
18
On the basis of all the results, we could gain a deeper understanding on the alkaline metals
19
doping and their differences. K and Na doping can narrow the band gap of g-C3N4. Meantime,
20
the band edges experience downshifts. Importantly, the congener alkaline metals doping exert
21
different effects on the electronic structures. For example, K with large size tends to be doped
22
into the interlayer while Na is doped into the CN layers. The intercalated K bonded with
23
atoms at the adjacent layers could form the delivery channels, increase the electronic
24
delocalization and extend the π conjugated systems, which will contribute to the transfer and
25
separation of charge carriers. Nevertheless, Na doping increases the in-planar electron density
26
and then increases the recombination of the charge carriers. In addition, K possesses stronger
27
metallicity than Na atoms. Na atoms combine with in-planar N atoms with ionic bonds,
28
whereas the interbedded K atoms are ionically and covalently combined with the N and C
29
atoms at the adjacent layers. This is the first report to enhance the charge separation via
30
bridging the layers with alkaline metal. This work is essential for better understanding the 22 ACS Paragon Plus Environment
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congener element doping-activity relationships as well as for the mechanistic understanding
2
of photocatalysis and rational design of potent photocatalysts.
3 4
4. CONCLUSION
5
We revealed the effects of K and Na doping on the electronic structure and photocatalytic
6
activity of g-C3N4 with a combined experimental and theoretical approach. The band gap of
7
g-C3N4 was narrowed by introducing K atoms or Na atoms. The K-doped g-C3N4 showed
8
enhanced visible light absorption and strong oxidation ability. K atoms intercalated into the
9
g-C3N4 interlayer served as delivery channel via bridging the layers, which is beneficial to the
10
charge carriers separation and transfer between adjacent layers. However, the Na atoms
11
existing in the caves of CN plane increased the in-planar electron density and caused high
12
carriers recombination. As a result, K-intercalated g-C3N4 exhibited high visible
13
photocatalytic performance and Na-doped g-C3N4 showed decreased activity for NO removal
14
in comparison with g-C3N4. The present work has provided new insights into the
15
understanding of photocatalysis mechanism and doping chemistry. Our present finding opens
16
a new avenue for synthesis of efficient intercalated photocatalysts for applications.
17 18
AUTHOR INFORMATIONS
19
Corresponding Author
20
† These two authors contributed equally to this work.
21
*E-mail:
[email protected] (F. Dong). Tel.: +86 23 62769785 605, Fax: +86 23 62769785
22
605.
23
Notes
24
The authors declare no competing financial interest.
25
ACKNOWLEDGEMENTS
26
This research is financially supported by the National Natural Science Foundation of China 23 ACS Paragon Plus Environment
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1
(51478070, 51508356, 51108487).
2
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Graphical Abstract g-C3N4
K-intercalated g-C3N4
K intercalation
Bridging the layers Charge redistribution Tuning band structure Enhanced photocatalysis
4
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