Defects Promote Ultrafast Charge Separation in Graphitic Carbon Nitride for Enhanced Visible-Light-Driven CO2 Reduction Activity

Article abstract of DOI:10.1002/chem.201805923

Fundamental photocatalytic limitations of solar CO₂ reduction remain due to low efficiency, serious charge recombination, and short lifetime of catalysts. Herein, two-dimensional graphitic carbon nitride nanosheets with nitrogen vacancies (g-C₃N_x) located at both three-coordinate N atoms and uncondensed terminal NH_x species were prepared by one-step tartaric acid-assistant thermal polymerization of dicyandiamide. Transient absorption spectra revealed that the defects in g-C₃N₄ act as trapped states of charges to result in prolonged lifetimes of photoexcited charge carriers. Time-resolved photoluminescence spectroscopy revealed that the faster decay of charges is due to the decreased interlayer stacking distance in g-C₃N_x in favor of hopping transition and mobility of charge carriers to the surface of the material. Owing to the synergic virtues of strong visible-light absorption, large surface area, and efficient charge separation, the g-C₃N_x nanosheets with negligible loss after 15 h of photocatalysis exhibited a CO evolution rate of 56.9 μmol g^?1 h^?1 under visible-light irradiation, which is roughly eight times higher than that of pristine g-C₃N₄. This work presents the role of defects in modulating light absorption and charge separation, which opens an avenue to robust solar-energy conversion performance.

Full text of DOI:10.1002/chem.201805923

A Journal of
Accepted Article
Title: Defects Promoting Ultrafast Charge Separation in Graphitic
Carbon Nitride Enhanced Visible-Light-Driven CO2 Reduction
Activity
Authors: Hainan Shi, Saran Long, Jungang Hou, Lu Ye, Yanwei Sun,
Wenjun Ni, Chunshan Song, Keyan Li, Gagik G. Gurzadyan,
and Xinwen Guo
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To be cited as: Chem. Eur. J. 10.1002/chem.201805923
Link to VoR: http://dx.doi.org/10.1002/chem.201805923
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Defects Promoting Ultrafast Charge Separation in Graphitic
Carbon Nitride Enhanced Visible-Light-Driven CO2 Reduction
Activity
Hainan Shi,[a] Saran Long,[a] Jungang Hou,*[a], Lu Ye,[a] Yanwei Sun,[a] Wenjun Ni,[a] Chunshan Song,[a,b]
Keyan Li,*[a], Gagik G.Gurzadyan[a] and Xinwen Guo*[a]
Dedication (optional)
Abstract: The fundamental photocatalytic limitations for solar CO2
reduction remain due to low efficiency, serious charge recombination
and short lifetime of catalysts. Herein, as a prototype, two-dimensional
graphitic carbon nitride nanosheets with nitrogen vacancies (g-C3Nx)
located at both tricoordinated N atoms and uncondensed terminal NHx
species were prepared by one-step tartaric acid assistant thermal
polymerization of dicyandiamide. Transient absorption spectra results
revealed that the defects in the g-C3N4 acted as the trapped states of
charges resulting into the prolonged lifetimes of charge carrier
photoexcited. Time-resolved photoluminescence spectra analysis
ensured that the faster decay of charges was due to the decreased
layer stacking distance in the g-C3Nx in favor of hopping transition and
mobility of charge carriers to surface of materials. Benefiting from the
synergic virtues of strong visible-light absorption, large surface area
as well as efficient charge separation, the g-C3Nx nanosheets with
negligible loss after 15 h photocatalysis exhibited CO evolution rate of
56.9 μmol g-1 h-1, roughly 8 times higher than that of pristine g-C3N4
under visible light irradiation. This work presents the role of defects in
modulating light absorption, charge separation, opening an avenue
enabling robust solar energy conversion performances.
since Inoue et al.[3] reported the photoelectrocatalytic reduction of
CO2 conducted by semiconductors in aqueous solution, the
photocatalytic conversion of CO2 into fuels has attracted much
attention.[4] However, the CO2 reduction reaction itself is a much
complex process, involving multiple electrons and protons
coupling resulting into several different reaction pathways, and
thus leading to many possible final products.[2c, 5] In addition, most
of materials applied to CO2 reduction require a large over-
potential, which gives rise to the limit of materials choice and the
low turnover rate of CO2 reduction product. Therefore, it is of great
importance to design a high-performance CO2 photoreducion
catalyst to cater for technological development.
Among catalysts, graphitic carbon nitride (g-C3N4) has been
extensively explored as
a
heterogeneous metal-free
semiconductor photocatalyst due to its earth-abundance,
nontoxicity, facile synthesis, good stability, as well as a suitable
bandgap.[6] However, the pristine g-C3N4 suffers from the low
surface area, rapid recombination rate of photoexcited charge
carriers and unsatisfied visible light absoprtion, leading to the poor
photocatalytic activity. Great efforts have been made to solve
these problems, such as doping with heteroatoms (metal[7] or non-
metal[8] atom), introducing defects,[9] decorating with function
groups,[10] constructing ordered porous structure,[11] and
controlling morphology.[12] It is reported that the intrinsic defects
that usually generated a mid-gap state as a band tail state can
play the role of the trapping sites for photoexcited carriers,
consequently boosting the photocatalytic activity.[13] Recently,
mamy strategies of bulding nitrogen vacancies in g-C3N4 have
been widely investigated for enhancing the photocatalytic activity
including controlling the polycondensation temperature,[14]
Introduction
Over-consumption of finite fossil fuels not only breaks out energy
crisis but also brings out environmental issue related to CO2
atmosphere progressive increase, both of which will impose
restriction on society sustainable development and human
survival in the long run.[1] CO2 converted into chemicals driven by
solar energy provides an attractive and promising way to address
global energy demands and alleviate climate change.[2] Ever
hydrothermal
treatment,[15]
alkali-assisted
thermal
polymerization[16] and hydrogen reduction.[17] Acid-treament of g-
C3N4 or its precusor such as using HNO3,[18] HCl,[9a] H2SO4[19] and
[20]
H3PO4 is also an efficient method to produce defects. Tartaric
[a] H. Shi, Dr. S. Long, Dr. J. Hou, Dr. L. Ye, Y. Sun, W. Ni, Dr. C. Song,
Dr. K. Li, Dr. G. Gurzadyan, Dr. X. Guo
acid (TA) is a weak acid without heteroatom except C, H and O
atoms. In addition, it will react with amino in the precusor of g-
C3N4 to form supermolecule in the aqueous solution. Meanwhile,
TA will completely decompose into CO2 and H2O in high
temperature, which might lead to defects forming during the
thermal treament. Inspired of these merits and defect engineering,
it is desirable to explore a facile approach to produce defective g-
C3N4 using TA to enhance photocatalytic activity. In addition, it is
of great interest to investigate the truth of the charge separation
dynamics of the nitrogen defects in the g-C3N4 and relationship
between charge seapariton and photoreduction CO2 activity,
State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for
Energy Research
School of Chemical Engineering
Dalian University of TechnologyDalian 116024, P. R. China.
Dalian 116024, P. R. China.
E-mail: guoxw@dlut.edu.cn; jhou@dlut.edu.cn; keyanli@dlut.edu.cn
[b] Dr. C. Song Department
EMS Energy Institute, PSU-DUT Joint Center for Energy Research and
Department of Energy & Mineral Engineering, Pennsylvania State
university
University Park, Pennsylvania 16802, United States
Supporting information for this article is given via a link at the end of the
document.
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10.1002/chem.201805923
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presenting in-depth understanding of the photocatalytic
mechanism.
from the formation of nitrogen vacancies or the substitution of C
or N with O in the structure of tri-s-triazine motifs.[4] Remarkably,
the slight shift of 27.4o peak to higher 2 θ increases with TA usage
as shown in Figure 1b, indicating the smaller layered interfacial
stacking distance. Furthermore, this peak gradually weakens with
TA amount, demonstrating the loss of ordered structures within
the framework caused by defects generated during the thermal-
polymerization process. This can be further explained by the
results of XRD patterns and FT-IR spectra of solid mixture of TA
and D (Figure S1 and S2, Supporting Information). The strongest
peak located at 26.3o weakens and the weaker peaks at 26.7o and
28.2o became stronger in XRD patterns, while the positions of
main diffraction peaks in XRD patterns and FT-IR spectra remains
unchanged. These phenomena implie that TA interacts with D
through hydrogen bond and ionic bond. The interaction by
hydrogen bond between both precursors finally results in the
decrease of layered interfacial distance. Furthermore, the weight
loss speed of 0.05TA-D is slower than D after 300oC in the TG
curves, which demonstrates that the interaction between TA and
D alleviates the sublimation and accelerates condensation
process thus resulting in structure change (Figure S3).
To resolve the limitations of low efficiency, serious charge
recombination and short lifetime of catalysts, two-dimensional
graphitic carbon nitride nanosheets with nitrogen vacancies (g-
C3Nx) located at both tricoordinated N atoms and uncondensed
terminal NHx species were prepeard by one-step tartaric acid
assistant thermal polymerization of dicyandiamide. Transient
absorption spectra results revealed the defects in the g-C3N4
result in the prolonged photoexcited charge carrier lifetimes and
acted as the trapped states of charges to promote charge
separation. Time-resolved photo-luminescence spectra analysis
ensured the faster decay of charges was due to the decreased
layer stacking distance in g-C3Nx in favor of hopping transition and
mobility of charge carriers to surface of materials. Benefiting from
the synergic virtues of enhanced visible light absorption,
abundant active sites and efficient charge separation and transfer
as well as prolonged charge lifetimes, defective g-C3Nx
nanosheets with negligible loss after 15 h photocatalysis exhibited
CO formation rate of 56.9 μmol g-1 h-1, roughly 8 times higher than
that of pristine g-C3N4 under visible light irradiation. This work
provides insight into design and preparation of high performance
g-C3N4 based photocatalysts.
Scheme 1. Illustration of synthesis of DCN-x by hydrogen bond induced
supramolecular assembly. (C, N, O and H atoms are indicated by gray, blue,
red and purple spheres, respectively.)
Figure 1. (a, b) XRD patterns, (c) FT-IR spectra of DCN and DCN-x. i-vi
represents DCN, DCN-0.01, DCN-0.025, DCN-0.05, DCN-0.075 and DCN-0.1.
(d) High-resolution C1s, N1s XPS spectra of DCN and DCN-x. I-IV represent
DCN, DCN-0.01, DCN-0.05 and DCN-0.1.
Results and Discussion
Two-dimensional graphitic carbon nitride nanosheets with
nitrogen vacancies (g-C3Nx) located at both tricoordinated N
atoms and uncondensed terminal NHx species were prepared by
one-step tartaric acid (TA) assistant thermal polymerization of
dicyandiamide (D) (Scheme 1). The crystal structure of pristine g-
C3N4 (DCN) and a series of DCN-x samples were characterized
by XRD patterns and FT-IR spectroscopy. As shown in Figure 1a,
DCN exhibits two distinct diffraction peaks at 13.1o and 27.4o,
which are attributed to in-plane packing of tri-s-triazine motifs (100
plane) and layered interfacial stacking of the conjugated tri-s-
triazine structure (002 plane), respectively.[21] DCN-x samples
show the similar XRD patterns with DCN, suggesting that DCN-x
have the same structure with g-C3N4. Furthermore, it is observed
that the (100) peak gradually becomes broadened when TA was
introduced into the reaction system (Figure 1b), resulting from the
less ordered stacking of tri-s-triazine motifs.[22] This may arise
Based on the analysis of FT-IR spectra for the bonding
structures of DCN-x, the main molecular structure of g-C3Nx was
almost unchanged through TA assistant synthesis of g-C3N4. As
shown in Figure 1c, the peaks located at 810 cm‒1 and 1200-1800
cm‒1 are ascribed to breathing vibration of heptazine rings out-
plane and skeletal vibrations of aromatic N— CN heterorings,
respectively. The bands appeared at 3000-3500 cm‒ region
1
correspond to N—H stretching vibration.[23] To further investigate
the chemical state and surface elemental composition of DCN-x,
XPS and elemental analysis were performed. The very small O1s
peaks in the XPS survey spectra are due to the surface adsorbed
oxygen-containing species such as water (Figure S4). The C1s
XPS spectra for the pristine g-C3N4 and DCN-x contain three
kinds of C species located at 284.6, 286.0 and 288.0 eV,
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corresponding to the sp2 hybrid orbital of adventitious carbon C—
C/C=C, C—NHx bond on the edges of heptazine units, and the sp2
hybrid orbital of N=C—N species in the s-triazine ring of g-C3N4,
respectively (Figure 1d). Remarkably, the area of the peak located
at 286.0 eV for DCN-0.05 is much smaller compared with other
samples, which indicates the content of C—NHx species in DCN-
0.05 is lower. The N1s XPS spectra (Figure 1d) for the DCN and
DCN-x show three peaks at 398.4, 399.7 and 401.0 eV, assigned
to bicoordinated N atom (N—(C)2), tricoordinated N atom (N—(C)3)
and surface amino groups (N— Hx), respectively. The ratio of
N(2C)/N(3C) and N(2C)/NHx of DCN-x calculated by XPS N1s spectra
(Table S1) increases compared with DCN, indicating the
formation of nitrogen vacancies via the departure of N(3C) and NHx
species in the process of TA and D co-polymerization.[24]
Therefore, the DCN-x are rich in N defects loacted at both
tricoordinated N atoms and uncondensed terminal NHx species.
Moreover, N defects formation in the DCN-x was further
comfirmed by elemental analysis (Table S1).[16, 24b, 25] Combined
with the results of XPS and elemental analysis, DCN-0.05 with the
highest ratio of N(2C)/N(3C) and N(2C)/N-Hx shows the highest
concentration of the surface N defects. And the total N defects in
the DCN-x increase with increasing of TA amount.
homogenously distributed (Figure 2c,d). Based on above-
mentioned analysis, the morphology change from bulk to
nanosheets could be attributed to the release of gas by TA
decomposition during the thermal-polymerization resulting in the
volume expansion of the catalyst, thus leading to the increase of
specific surface area and pore volume.
Table 1. Specific surface areas, pore volume, band gap derived from the
transformed Kubelka–Munk function and photocatalytic performance of the
samples.
Figure 2. (a) TEM image. (b) HAADF-STEM image. (c, d) elemental mapping
for of DCN-0.05.
Yield
Sel.
(%)
SBET
(m2 g-1)
Vtotal
(cm3 g-1)
Eg
(μmol g-1)
Sample
(eV)
CO
H2
CO
It is rather remarkable that the color of DCN-x changed from
yellow to deep brown with the increase of TA amount (Figure S7).
After the optical analysis by UV-vis diffuse reflectance
spectroscopy (Figure 3a), the strong absorption tail in the visible
light region is observed for the DCN-x. The band gap structures
by transformed Kubelka–Munk function are shown in Figure 3b
and in Table 1. The variation of band gap for DCN-x is narrowed
with increasing of TA amount. The second band gaps are also
observed in the DCN-x and gradually narrowed from 2.02 eV to
1.70 eV. The apparently decreased band gaps could be ascribed
to the nitrogen vacancies, which not only have enhanced the
visible light absorption but also introduced a defect level and
modified the electron structure of the g-C3N4.[24b] To further
elaborate the band gap of DCN-x, the valance band (VB) position
was collected by XPS spectra (Figure S8). As shown in Figure S8,
all DCN-x exhibit similar VB maximum compared with DCN (VB ≈
1.50 eV). The contact potential difference between the samples
and the analyzer was estimated to be ≈ 1.66 V versus normal
hydrogen electrode (NHE) using the formula ENHE/V = Φ + 1.50
eV − 4.44 (ENHE: potential of normal hydrogen electrode; Φ of 4.60
eV: the electron work function of the analyzer).[15-16, 26] The result
suggests that the VB position has not been affected by the
nitrogen vacancies in the framework of g-C3N4. However, the
conduction band (CB) has an obvious decrease on the basis of
the narrow band gap. The gradual narrowing second band gap is
contributed to N defects in DCN-x, i.e. it is directly proportional to
N defects content. The proposal of CB, VB and defect level
positions in detail is drawn in the Figure 3c. It can be seen that
DCN
10.0
14.4
19.3
29.5
16.8
15.9
0.037
0.046
0.059
0.076
0.051
0.043
2.70
2.60
2.56
2.49
2.40
2.24
35.5
5.9
85.7
88.9
83.9
84.7
85.5
88.1
DCN-0.01
DCN-0.025
DCN-0.05
DCN-0.075
DCN-0.1
240.2
251.1
284.7
186.1
119.3
30.1
48.3
51.6
31.5
16.1
It is well known that the specific surface area and porosity
volume of the photocatalysts are crucial factors for the
photocatalytic reaction performance. N2 adsorption-desorption
isotherms of the pristine g-C3N4 and g-C3Nx exhibit the type IV-H3
type hysteresis loops, indicating the existence of the mesopores
(Figure S5). Obviously, DCN has the lowest N2 adsorption volume
while DCN-0.05 displays the highest one. As listed in Table 1, the
specific surface area and pore volume of DCN-x samples are
larger than that of DCN (10.0 m2 g-1, 0.037 cm3 g-1), and increase
progressively from DCN-0.01 to DCN-0.05 and then decrease.
DCN-0.05 shows the largest specific surface area (29.5 m2 g-1)
and pore volume (0.076 cm3 g-1), which are both larger than those
of DCN, respectively. Surface area and pore volume increased
may be concerned with the morphology change. Scanning
electron microscopy (SEM) images of DCN-0.05 (Figure S6a)
display a curly sheet-like structure in smaller sizes, even no longer
bulk-shaped as DCN (Figure S6c, d). Furthermore, transmission
electron microscopy (TEM) and high-angle annular dark field
STEM (HAADF-STEM) images further confirm that DCN-0.05
possesses the curly nanosheets shape (Figure S6b and Figure
2b). To conduct the component, the energy dispersive X-ray (EDX) the defect levels of DCN-x are below the CB with gradual lower
elemental mapping images reveal that C and N elements are position. Furthermore, the location of defect levels for the DCN-
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0.075 and DCN-0.1 is −0.06 and −0.04 eV, respectively, and
below the potential of CO2 reduced to CO (−0.07 eV), which is
disadvantageous to photocatalytic reaction for CO2 to CO.
The formation of nitrogen vacancies in the DCN-x samples may
also influence the separation efficiency of charge. As shown in
Figure 3d, the steady photoluminescence (PL) spectrum for
pristine g-C3N4 shows a high intensity fluorescence signal at 400-
580 nm. The gradual decrease of signal intensity for the nitrogen
deficient g-C3N4 indicates that the recombination of the
photoinduced electrons and holes is substantially suppressed.
Especially, the lowest intensity for the DCN-0.1, implies the
superior separation efficiency of charge carriers.
obvious decrease is observed in three cycles for CO evolution
rate of DCN-0.05 (Figure 4c). From XRD patterns and FT-IR
spectra of used catalyst (Figure S10), the basic framework of g-
C3N4 still remains for the used photocatalyst. SEM images of
used photocatalyst also disclose that used catalyst still maintains
nannsheets-like morphology (Figure S11). The results above
demonstrate the high stability of the catalysts.
To analyze the reasons of the enhanced photocatalytic
activities of DCN-x, the EPR and photoelectrochemical
mesurements were explored. DCN-0.05 as well as DCN display a
paramagnetic absorption signal with a g value of 2.003, which is
assigned to the unpaired electron localized at the carbon atom in
a typical heptazine of g-C3N4 (Figure S12).[24b, 27] The stronger
peak signal of DCN-0.05 is attributed the formation of nitrogen
vacancies, leading to the increased amount of unpaired electrons
as the active sites for solar CO2 reduction.[28] Transient
photocurrent measurements of DCN and DCN-0.05 show a
sensitive photocurrent response during on/off visible light
irradiation cycles (Figure S13). Compared with DCN, the higher
photocurrent response for DCN-0.05 indicates more efficient
charge carrier separation and transportation rate in deficient g-
C3N4. From the EIS tests (Figure S14), a smaller arc radius for
DCN-0.05 is observed, further indicating the faster charge
transportation and better charge carrier separation for DCN-x
compared with DCN.
Figure 3. (a) UV-Vis diffuses reflectance spectra. (b) Plots of transformed
Kubelka–Munk function versus photon energy. (c) Band structure alignments
and (d) steady PL spectra for DCN and DCN-x.
Based on unique physicochemical characteristics of defective
g-C3Nx nanosheets, it is essential to evaluate the photocatalytic
CO2 reduction performance. At the initial stage, no products were
detected without either photocatalyst or light source and even
without the introduction of CO2 under light irradiation As shown in
Figure 4a and Figure S9, all of DCN-x exhibit excellent
photocatalytic CO2 reduction activity compared with DCN. The
photocatalytic activity firstly increased and then deceased rapidly
from DCN-0.05 to DCN-0.1. It is noted that the superior
photocatalyic activity of the DCN-0.05 could be attributed to its
larger specific surface area and pore volume, enhanced light
absorption and better charge separation efficiency caused by N
defects. However, the deceased performance for the DCN-0.075
and DCN-0.1 can be explained by the position for defect levels
(−0.06 and −0.04 eV) lower than the potencial (−0.07 eV) of the
photocatalytic CO2 into CO. Besides, the CO evolution rate, 284.7
μmol g-1 on DCN-0.05 with the selectivity of approximate 90% was
nearly eight times higher than that of the pristine g-C3N4 (35.5
μmol g-1), corresponding a small amounts of H2 products (Figure
4b and Table 1) under 5 h visible-light irradiation. For the practical
application, the stability is a pivot role for the photocatalyst. No
Figure 4. (a, b) CO evolution and CO and H2 distribution for DCN and DCN-x.
(c) Time courses of CO evolution of DCN-0.05. (d) Time-resolved
photoluminescence decay spectra of DCN and DCN-0.05 probed at 500 nm by
time-correlated single-photon counting. The samples were excited by incident
light of 400 nm from a picosecond pulsed light emitting diode.
To further clarify the in-depth understanding of charge
separation dynamics, time-resolved photoluminescence (tr-PL)
and transient absorption (TA) spectroscopies were investigated
systematically. The fluorescence lifetime of charge carriers by
time-correlated single photon counting (TCSPC) has been shown
in Figure 4d. Both spectra show multi-exponential decays and the
overall decay of DCN-0.05 is faster than that of DCN.
Table 2 Decay lifetimes of tr-PL, fs-TA and ns-TA spectra and their relative amplitude of photoexcited charge carriers in the g-C3N4 and nitrogen-deficient g-C3N4.
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[b]
[c]
Method
Sample
λ[a], nm
τ1
A1
τ2
A2
τ3
A3
tr-PL
DCN
500
500
0.30 ns
0.22 ns
0.31
0.49
1.14 ns
0.90 ns
0.47
0.40
4.00 ns
3.40 ns
–
0.22
0.11
–
λex = 400 nm
DCN-0.05
-0.53
-0.54
-0.47
-0.46
fs-TA
DCN
500
500
8.5 ± 2.0 ps
28 ± 8 ps
2152 ± 500 ps
3900 ± 1200 ps
λex = 355 nm
DCN-0.05
–
–
–
–
ns-TA
DCN
430
430
120 ± 1 ns
85 ± 0.6 ns
0.96
0.91
7.8 ± 0.2 μs
9.2 ± 0.2 μs
0.04
0.09
λex = 350 nm
DCN-0.05
–
–
[a] λ represents probed wavelength. [b] τ is referred to decay lifetime. [c] A represents amplitude of decay.
Deconvolution of the decay spectra gives the three lifetimes of
charge carries as listed in Table 2. The three lifetimes of charge
carriers are attributed to free polarons hopping transport of singlet
excitions in intraplanar, interplanar or intrachain with following
with the electron-hole recombination.[29] The population of short-
lived electron-hole pairs (less than 1 ns) in the defective g-C3N4 is
dominant resulting from the rapid recombination of photoinduced
charge carriers, which is in accordance with the literatures
results.[14, 30] Compared with DCN, the amplitude of short lifetimes
in DCN-0.05 increases from 0.78 to 0.89, and the amplitude of the
long-lived charges decreases from 0.22 to 0.11. As is well-known,
PL spectra presents the information about recombination of free
charges lay near band edges rather than the recombination
caused by deeply trapped charges.[31] Besides, previous reports
elucidated that the decay lifetimes of g-C3N4 shortened with the
decrease of stacking distance between two adjacent layers,
leading to enhanced photocatalytic activity.[30, 31b, 32] The reports
above are in accordance with our tr-PL results and photocatalytic
CO2 reduction activity. Therefore, the PL results can be explained
that the faster decay is attibuted the decreased layered stacking
distance caused by defects in DCN-x. The decrease of layers
stacking distance of DCN-x is in favor of hopping rate and mobility
of electrons and holes resulting into enhanced catalytic activity.[29]
In order to analyze the mobility of charges, the transient
absorption (TA) spectra were performed over six orders of
magnitude in time. As shown in Figure 5a, b, the fs-TA
measurements reveal a ground state bleaching signal in 400-600
nm region for both materials, which is assigned to the stimulated
emission.[29-30] The decay curves for DCN and DCN-0.05 are
shown in Figure 5c, d, multi-exponential decays fitting was made
to yield two lifetimes spanning as listed in Table 2. The decay
kinetics profiles of fs-TA spectra probed at 500 nm for DCN
exhibits a short-lived (8.5 ps, －0.53) and a long-lived (2152 ps,
－0.47) lifetime of charges, which are due to the photogenerated
electron-hole recombination and nonradiative relaxation,
respectively. Importantly, a longer lifetime of charges (28 ps and
3900 ps) was observed for DCN-0.05 (the amplitude is almost
unchanged), suggesting that defect states in g-C3N4 act as the
initial trapped states of charges, thus leading to enhanced charge
separation and transport.[33]
Figure 5. (a, b) fs-TA spectra. (c, d) the decay curves and fitting curves of DCN
and DCN-0.05 dispersions (2 mg/mL aqueous) probed at 500 nm with 350 nm
excitation. (e, f) ns-TA spectra of DCN and DCN-0.05 dispersions (0.1 mg/mL
aqueous) probed at 430 nm with 355 nm excitation.
The absorption of the longer-lived charges was further
measured by ns-TA spectroscopy. As shown in Figure 5e, f, ns-
TA spectra for DCN present a broad positive absorption in the
visible region peaking at 430 nm. Similar positive features are also
observed in the DCN-0.05 ns-TA spectra, and the band is red-
shifted to 460 nm and a new band at 580 nm appears. Currently,
transient absorption following up to the microsecond time scale
has been measured (Figure S15). The decay curves were fitted
by two-exponential decay (Figure S16 and Table 2). A short-lived
(120 ns, 0.96) and long-lived (7.8 μs, 0.04) lifetimes of charge
carriers were obtained for DCN due to geminate recombination
and bimolecular recombination, respectively. A shorter lifetime
(85 ns, 0.91) and a longer lifetime (9.2 μs, 0.09) were also deteced
in DCN-0.05. The population of short-lived charge carriers is
dominant for both materials. Obviously, DCN-0.05 has a faster
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decay lifetime along with the amplitude of short lifetime decreases
and the amplitude of long lifetime increases compared with DCN.
This result further indicates that the decreased stacking distance
is responsible for shortening of the lifetime of charges. Meanwhile,
defects in the g-C3N4 could act as the trapped states of electrons
or holes, which slow down the bimolecular recombination
relaxation resulting in prolonging the lifetime of charge carriers.
activity in CO2 reduction driven by visible light. This work presents
the crucial role of defect sites in modulating light absorption,
charge separation and lifetime as well as solar CO2 reduction
efficiency, opening an avenue enabling efficient and stable solar
fuel production.
Experimental Section
Fabrication of g-C3N4 and g-C3Nx. Bulk g-C3N4 was prepared by thermal-
polymerization of dicyandiamide. In detail, 5 g of dicyandiamide was
calcined at 550oC in a muffle furnace for 3 h with a heating rate of 5oC min-
1. Nitrogen deficient g-C3N4 photocatalysts were synthesized as follows:
different amount of tartaric acid (TA) (0.01-0.1g) were dissolved in 10 mL
deionized water, and then 5 g dicyandiamide was added into the tartaric
acid aqueous solution with stirring. The above mixture was dried in an oven
at 90oC overnight. The resulting solid mixture was then calcined in a muffle
furnace at 550oC for 3 h with a heating rate of 5oC min-1. The products
Figure 6. Schematic of dynamics process of electrons and holes
photogenerated in the g-C3Nx for enhanced CO evolution driven by solar
irradiation
were denoted as DCN-x (x
= 0.01, 0.025, 0.05, 0.075 and 0.1
corresponding to the mass (g) of tartaric acid, and DCN represents the
pristine g-C3N4 synthesized by dicyandiamide).
Characterization. X-ray diffraction (XRD) patterns were recorded on a
Smart Lab 9 X-ray diffractometer (Rigaku Corporation). Fourier transform
infrared (FT-IR) spectra were measured using an EQUINOX55 FT-IR
spectrometer. Thermogravimetric Analysis (TG) results were received on
a SDT Q600 apparatus from 298 K to1073 K with a heating rate of 10
K/min in air. X-ray photoelectron spectroscopy (XPS) spectra were
recorded on a Thermo VG ESCALAB250 instrument. Elemental analysis
(EA) was studied on a Vario EL cube instrument (Elementar, Germany).
The nitrogen adsorption isotherms were measured with a Quantachrome
autosorb-iQ2 gas adsorption analyzer at 77 K. Scanning electron
microscopy (SEM) images were obtained on a Hitachi SU8220 instrument
with an acceleration voltage of 5 kV. Transmission electron microscopy
(TEM) images were taken on a Tecnai G220 S-twin instrument (FEI
Company). UV-Vis spectra were taken on a JASCO V-550 spectrometer.
Photoluminescence (PL) spectra were acquired on a JASCO FP-6200
spectrofluorometer with the excitation wavelength at 310 nm. The electron
paramagnetic resonance (EPR) was carried out at room temperature on
an A220-9.5/12 (Bruker). Time-resolved photoluminescence (tr-PL)
measurements were carried out at room temperature by the time-
correlated single photon counting (TCSPC) technique (PicoQuant
PicoHarp 300). By use of deconvolution/fit program (PicoQuant FluFit) the
time resolution was reached down to 10 ps. The second harmonic of a
titanium−sapphire laser (Mai Thai DeepSee) at 350 nm (150 fs, 80 MHz)
was used as the excitation source.
Based on the above analysis, the energy-band configuration of
g-C3Nx is drawn in Figure 6. Briefly, upon visible light excitation,
hot electrons are generated, which will transfer from VB to CB of
g-C3Nx and hole left in the VB (<200 fs). The initial charge carriers
separate in ultrafast time scale (ps) in competition with geminate
recombination and further bimolecular recombination relaxation
(ns-μs) in the excited states. However, the N defects can act as
charges trapped states to prolong the lifetime of both short-lived
(ps) and long-lived (μs) charge carriers for further promoting
charges separation. Furthernmore, the free electrons go back to
the ground state (VB) resulting in photoluminescence emission
(ns). The decreased layer distance attributed to faster tr-PL decay
in favor of trapped electrons migrating to catalyst surface, then
reacting with CO2 molecular to produce CO. Thus, the defects act
as the crucial role in trapping and separating charges to enhance
photocatalytic CO2 reduction into CO activity.
Conclusions
In summary, we successfully prepared the two-dimensional
graphitic carbon nitride nanosheets with nitrogen vacancies
located at both tricoordinated N atoms and uncondensed terminal
NHx species by one-step tartaric acid assistant thermal
polymerization of dicyandiamide. Compared with pristine g-C3N4,
the obtained g-C3Nx possessed much larger specific area and
pore volume, decreased band gap, and enhanced visible light
absorption as well as charge separation efficiency. Transient
absorption spectra revealed that the defects in g-C3Nx acted as
the trapped states of charge resulting in the prolonged lifetimes of
charge carriers photoexcited. Time-resolved photoluminescence
spectra analysis ensured that the faster decay of charges was
attributed to the decreased layer stacking distance caused by
defects in g-C3Nx in favor of hopping transition and mobility of
charge carriers to surface of materials. These synergic features
of the g-C3Nx nanosheets contribute to the high photocatalytic
Transient Absorption Spectroscopy. The transient absorption (TA)
spectra were measured by optical femtosecond pump-probe spectroscopy
described in detail in Ref..[34] The output of a mode-locked Ti-sapphire
amplified laser system (Spitfire Ace, Spectra-Physics) with wavelength
800 nm, pulse-width 35 fs, repetition rate 1 kHz, average power 4 W was
split into two beams (10:1). The strong radiation was converted into UV-
VIS-IR in the range of 240-2400 nm by use of Optical Parametric Amplifier
(TOPAS, Light Conversion) with 355 nm excitation and used as a pump
beam. The weaker beam after passing a variable delay line (up to 6 ns)
was focused in a 3 mm thickness rotated CaF2 plate to produce a white
light continuum (WLC), which was used as a probe beam. Home-built
pump-probe setup was used for obtaining transient absorption spectra and
kinetics. The relative polarizations between pump and probe beam were
set to the magic angle (54.7°) to avoid rotational depolarization effects.
The entire setup was controlled by a PC with the help of LabView software
(National Instruments). All measurements were performed at room
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10.1002/chem.201805923
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temperature under aerated conditions. 2 mg/mL samples were dispersed
in aqueous solution and transferred to 1 mm path length cuvettes. The
experimental data were fitted to a multiexponential decay function
convoluted with the instrument response function. Overall time resolution
was about 20-30 fs.
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Table of Contents
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Hainan Shi,[a] Saran Long,[a] Jungang
Hou,*[a], Lu Ye,[a] Yanwei Sun,[a] Wenjun
Ni,[a] Chunshan Song,[a,b] Keyan Li,*[a],
Gagik G.Gurzadyan[a] and Xinwen
Guo*[a]
Rich nitrogen defects in g-C3N4 were achieved by one-step tartaric acid assistant
thermal polymerization of dicyandiamide. The defects in g-C3N4 act as the charge
trapped states to prolong the lifetime of charge carriers. The decreased layer
distance caused by nitrogen vacancies is in favor of charge transport and mobility
to surface of catalyst. Deficient g-C3N4 exhibits the enhanced photocatalytic CO2
reduction activity.
Page No.1 – Page No.8
Defects Promoting Ultrafast Charge
Separation in Graphitic Carbon
Nitride Enhanced Visible-Light-Driven
CO2 Reduction Activity
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