Edge activation of an inert polymeric carbon nitride matrix with boosted absorption kinetics and near-infrared response for efficient photocatalytic CO2reduction

Article abstract of DOI:10.1039/d0ta03870a

The reduction of CO2 into C1 feedstocks (e.g., CO) by utilizing solar energy has attracted increasing attention for the efficient production of renewable energy. However, a significant challenge in the reduction of CO2 is achieving high conversion efficiency due to the high CO dissociation energy of CO2 and difficultly in accessing the surface of photocatalysts. Herein, we fabricated a polymeric carbon nitride (PCN) catalyst with hydroxyethyl groups grafted on its edge via a facile bottom-up strategy, facilitating the efficient surface absorption of CO2 and lowering the CO2 transformation energy barrier; this was accompanied with exceptional extended optical absorption ability to the near-infrared region and increase in the density of states at the Fermi level. Thus, concentrated CO2 molecules could contact the surface of PCN and be easily activated; this resulted in an excellent CO production rate of up to 209.24 μmol h-1 g-1 in the modified PCN (i.e., 39.5-fold increase compared to that of pristine PCN) and a selectivity of 98.5% under white LED illumination, exceeding that of most PCN-based energy conversion systems reported to date. Notably, this PCN matrix also exhibited photocatalytic activity for the production of CO in the near-infrared region from 780 to 850 nm. These results pave the way for the development of structured photocatalysts with easy accessibility for CO2 and broadband spectral response for the efficient photocatalytic reduction of CO2. This journal is

Full text of DOI:10.1039/d0ta03870a

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Journal of
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Materials for energy and sustainability
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W. Tao, H. Zhu, L. Zhong, F. Wang, Z. Ren, Y. Lei, C. Liu and X. Peng, J. Mater. Chem. A, 2020, DOI:
10.1039/D0TA03870A.
Volume
Number 12
28 March 2018
Pages 4883-5230
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DOI: 10.1039/D0TA03870A
Edge Activation of Inert Polymeric Carbon Nitride Matrix with
Boosted Absorption Kinetic and Near-infrared Response for
Efficient Photocatalytic CO2 Reduction
Qiong Liua,b*, Zhongxin Chenb, Weijian Taoc, Haiming Zhuc, Linxin Zhonga, Fuxian
Wangd, Ren Zoua, Yongqian Leid, Cuibo Liub, Xinwen Penga*
aState Key Laboratory of Pulp and Paper Engineering, South China University of
Technology, Guangzhou 510641, China
bDepartment of Chemistry and Centre for Advanced 2D Materials (CA2DM)
National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
cCentre for Chemistry of High-Performance & Novel Materials, Department of
Chemistry, Zhejiang University
dGuangdong Provincial Key Laboratory of Emergency Test for Dangerous Chemicals,
Guangdong Institute of Analysis, Guangzhou 510070, China
Corresponding Author
*E-mail: chmv231@nus.edu.sg (Q. Liu); fexwpeng@scut.edu.cn (X. Peng).
1

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Abstract: Reduction of CO2 into C1 feedstocks (e.g. CO) by utilizing solar energy
has attracted increasing attention for the efficient production of renewable energy.
While, a significant challenge in CO2 reduction is achieving with high conversion
efficiency due to the high C=O dissociation energy of CO2 and difficultly inaccessible
to the surface of photocatalyst. In the present contribution, we fabricate a polymeric
carbon nitride (PCN) catalyst with hydroxyethyl group grafted in the edge via a facile
bottom-up strategy, facilitating an efficient surface absorption of CO2, lowering the
CO2 transformation energy barrier, accompanying with an exceptional extended optical
absorption ability to near-infrared region as well as the increased of state density along
the Fermi level. Thus, concentrated CO2 molecules are able to contact the surface of
PCN and easily activated, enables an excellent CO production rate up to 209.24 μmolh-
1g-1 in the modified PCN (i.e. 39.5 folds’ increase than pristine PCN) and a selectivity
of 98.5% under the white LED light illumination, exceeding most of the PCN-based
energy conversion system reported to date. Notably, this PCN matrix also exhibits
photocatalytic CO production activity at the near-infrared region from 780 to 850 nm.
These results pave the way to develop structured photocatalysts with easy accessibility
of CO2 and broadband spectral response for efficient photocatalytic CO2 reduction.
Keywords: polymeric carbon nitride, photocatalytic CO2 reduction, hydroxyethyl group,
edge CO2 activation, near-infrared response
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1. Introduction
Conversion of CO2 into valuable chemical fuels by solar-driven photocatalytic
strategy is one of the most attractive routes for addressing the problem of global
warming and the production of renewable energy 1-4. Although many photocatalysts
have been applied for CO2 conversion in the previous decades, including metal oxides
13
(TiO2, WO3, ZnO) 5-9, metal chalcogenides (CdS) 10-12, layered double hydroxides ,
15
16,
Metal–Organic Framework14,
and layered Bi-based semiconductors
17. The
photocatalytic CO2 reduction efficiency remains undesired owing to the high activation
energy barriers of CO2 (i.e. high C=O dissociation energy of 750 kJmol-1 in CO2
molecule) and relative low selectivity due to multiple electron transfer steps to produce
a variety of products as well as the sluggish kinetics for the CO2 absorption and mass
transfer. This requires favorable CO2 absorption, easy activation, and sufficient excited
18, 19
electron and proton transfers to facilitate subsequent CO2 reduction processes
.
With intense studies on photocatalysts, porous layered materials with a high surface
area via some modifications may be the candidates for CO2 reduction 20. In this case,
21-24
visible-light-driven polymeric carbon nitride (PCN) has drawn massive attention
due to its layered structure, chemical stability, and low cost, while the CO2 reduction
rate of pristine PCN is far from satisfactory.
Pioneering studies have revealed improved CO2 photocatalytic efficiency by
25-27
combining PCN with other semiconductors
to facilitate the separation of photo-
induced charge carriers. The incorporation of modulated complex cocatalysts such as
iron quaterpyridine, cobalt phthalocyanine or binuclear ruthenium (II) complex in the
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PCN matrix can achieve similar elevated activity 28-31. Grafted a hydrophobic DpOoIl:y10m.10e3r9/D0TA03870A
in the PCN surface to overcome the mass transfer limitation of CO2, then proceeding
the enhanced CO2 reduction activity and selectivity 32. However, attempts on basal
plane activation of the PCN matrix remain scarce, where the lack of sufficient active
sites on the surface of the catalyst and altered redox ability hampers long-term
applications of the above PCN-coupled systems. The introduction of surface defects
becomes increasingly important because it helps to tune the chemical and electronic
structures of the PCN matrix, to generate more active sites and lower the activation
energy of CO2 33. Generating carbon vacancies in the PCN matrix provides long-lived
photo-generated charges and enhanced reaction kinetics and this can be achieved via a
steam-etching approach 34 however, this requires rigorous conditions and breakage of
long-range order. On the other hand, when considering the surface termination of
incomplete cyclization of heptazines, residual NH2/NH may also serve as the real active
sites, which leads to difficulty to activate CO2 and poor transport of charge carriers 35-
38. The replacement of terminal amino groups by cyano groups in a solvothermal
process can lead to enhanced activity towards CO2 reduction 39. Nonetheless, such
modification only allows a weak optical response absorption region (< 500 nm) and
insufficient selectivity, severely inhibiting the broadband, solar spectrum-utilization
efficiency for the target product 40, 41. More importantly, although the improved activity
is generally achieved by the reported works, but little has been done to optimize the
CO2 absorption kinetics and reduce the CO2 transformation energy barriers. Although
rarely reported, it is evidenced that introducing coordination molecule linkers in the
4

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edge of the material structural unit (e.g. the homogeneous transition photoredox
catalysts 42 or metal-organic frameworks 43) is able to change the rate-determining step,
modulate the intrinsic affinity and engineer the electronic structure. Bearing the
covalent multi-step polymerization processes from cyanamide derivatives during the
synthesis of PCN, allowing nucleophilic addition reaction with some electron-rich
motifs. These observations inspire that some functional organic linkers can be
introduced in the structural units in PCN via a facile bottom-up strategy to modulate
the reaction kinetics for CO2 conversion, but not yet achieved.
Bearing these considerations in mind, we elaborately designed hydroxyethyl-
functionalized one dimension PCN nanostructures by applying an organic amine
catalyst (e.g., N-acetylethanolamine, NA) into the hydrothermal pretreatment synthetic
procedure and then the calcination process. Combining a set of systematic experimental
analyses and density functional theory (DFT) calculations, we identify that the
replacement of terminal amino groups by hydroxyethyl groups caused by the addition
of NA drastically facilitates the absorption capability of CO2 molecule and reduce the
CO2 transformation energy barrier. And the existence of hydroxyethyl groups enhances
the optical absorption capability of the PCN matrix up to the near-infrared region due
to the optimized electronic structure and promotes the photo-generated charge
separation. This led to a remarkable CO production rate of 209.42 μmolh-1g-1 (~ 39.5
fold increase compared to that of pristine PCN) together with a selectivity of 98.5% for
the NA-modified PCN catalyst and an apparent quantum efficiency of 5.26% at λ=420
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nm for photocatalytic CO2 reduction, together with a extended response activity among
the near-infrared region.
2. Experimental Section
2.1. Sample synthesis
The HCNT-NA was prepared in the following process, briefly, 3 g of
dicyandiamide (DCDA) was added into a 15 mL mixture solution containing 5 mL
deionized water and 10 mL propanol, then followed added 0.15 mL N-
acetylethanolamine (NA) and stirred for 30 min. Then, the mixture was transferred into
a 25 mL stainless steel autoclave (Teflon-lined) and heated at 180 °C for 10 h 44. When
natural cooled down, the obtained precipitate was washed by water and ethanol several
times, followed dried at 80 °C for 6 h in a vacuum oven. Finally, the intermediate solids
were heated to 550 °C at a ramp rate of 10 °C/min and kept for 4 h at the static air
atmosphere by using Nabertherm GmbH muffle furnace to obtain polymeric carbon
nitride (PCN) sample, named as HCNT-NA. And the g-C3N4 samples via different
addition content of NA, such as 0.05, 0.10, 0.15 and 0.20 mL were also prepared,
donated as HCNT-NA1, HCNT-NA2, HCNT-NA3, and HCNT-NA4. This process for
chemical structure variation of synthesizing the HCNT-NA is illustrated in Scheme S1
and S2. For comparison, HCN-NA or HCNT was prepared without added propanol or
NA under the above same conditions. And a bulk PCN sample (denoted BCN) was
synthesized from DCDA by the directly calcined in a muffle furnace at 550 °C/4 h with
an elevated rate of 10 °C min-1, according to the previous reports, the chemical structure
variation along with the elevated temperature is illustrated in Scheme S3.
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To illustrate the versatile of this modulated synthesis methods, severDaOlI:o10t.h10e3r9/D0TA03870A
additives were selected including Ethylene glycol (EG), Ethylenediamine (EDA),
Ethanolamine (EOA), Diethanolamine (DOA), Triethanolamine (TOA) to react with
the precursor, typically, 0.10 mL additive was added into 25 mL aqueous suspension
containing 3 g DCDA to dissolve completely, then hydrothermally treated at 180 °C
for 10 h. After dried the obtained intermediate solids, continuously heated them at
550 °C 4 h with a 10 °C/min rising rate. Resultant CN-based samples denoted as HCN-
EG, HCN-EDA, HCN-EOA, HCN-DOA, and HCN-TOA, respectively. The molecule
of selected additives together with the N-acetylethanolamine (NA) is shown in Scheme
S4. Similar to the synthesis process of HCNT-NA, we also select three additives
including EOA, DOA, TOA (0.15 mL) to prepare HCNT-EOA, HCNT-DOA, and
HCNT-TOA, respectively.
2.2. General information for Characterization
X-ray diffraction (XRD) patterns were collected on a Bruker D8 advance
diffractometer by using Cu Kα1 radiation. X-ray photoelectron spectroscopy (XPS) and
valence band-XPS spectra were monitored by a Kratos Axis Ulra DLD spectrometer
with charge neutralization and equipped with a monochromatic Mg Kα X-ray source,
samples were pressed onto a silicon foil and referenced with the adventitious carbon C
1s peak at 284.90 eV. Fourier transform infrared (FT-IR) spectra were obtained by a
13
Bruker Vector 33. Magic Angle Spinning solid-state C nuclear magnetic resonance
(NMR) spectra were recorded on an Agilent VNMRS 600 MHz magnetic resonance
spectrometer. CNHO elemental analyses were performed on an Elementar Vario
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MICRO analyzer. The morphologies and texturization of the samples were investigated
by applying a Hitachi SU8220 scanning electron microscope (SEM) and a JEM-2100
JEOL transmission electron microscope (TEM, 200 kV). The roughness of the surface
over the working electrode was measured by Bruker Multimode 8. Surface areas were
conducted according to Brunauer-Emmett-Teller (BET) theory from the N2 absorption-
desorption isotherms by using a Micromeritics ASAP 2020 apparatus at 77 K and CO2
absorption curve at 298 K. Diffuse reflectance UV-Vis spectra (DRS) were recorded
on a Shimadzu U-3010 spectrophotometer, referenced to BaSO4. Static
photoluminescence (PL) emission spectra (375 nm excitation) were acquired with a
Hitachi F-4600 fluorescence spectrometer at room temperature.
For femtosecond transient absorption spectroscopy (TA), the fundamental output
from Yb: KGW laser (1030 nm, 220 fs Gaussian fit, 100 kHz, Light conversion Ltd)
was separated to multiple light beams. One was introduced to a non-collinear optical
parametric amplifier and a second harmonic generation crystal BBO to generate a
certain wavelength for pump beam (here we use 400 nm). The other was focused on a
BBO crystal to generate 515 nm pulses and then focused on a YAG crystal to generation
continuum blue light as a probe beam. The temporal delay between them is controlled
via a motorized delay stage. The pump and probe pulses overlapped spatially in the
sample and the transmitted probe light was collected by a linear array detector.
In situ infrared (IR) spectrum was performed on Thermo Fisher Nicolet iS 50 with
an in situ Harrick IR cell. The in situ FTIR experiment was performed on a Bruker
Tensor II FTIR spectrometer. Firstly, the samples were sealed in the specimen chamber,
8

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then, the 40 mL/min pure CO2 was followed into the specimen chamber which was
carrying 10% TOA/acetonitrile aqueous solution with a certain amount of [Co(bpy)2]
Cl2·6H2O (bpy=2’2-bipyridine) to simulate the CO2 photoreduction reaction condition.
After 30 min later, the specimen chamber loaded samples were irradiated from the SiO2
window with a Xe lamp for 1 h and the infrared absorbance spectra were recorded for
every 10 minutes.
2.3. Photoelectrochemical measurements
Electrochemical measurements were carried out on a CHI660E electrochemical
workstation equipped with a standard three-electrode system, Pt plate as the counter
electrode and saturated Ag/AgCl (3.5M KCl) as the reference electrode. The prepared
of working electrode was as follows: firstly, using acetone and ethanol to clean the F-
doped thin oxide (FTO) glass, the obtained samples (10 mg) was added into 1 mL
ethanol solution with 20 μL of 5% Nafion, sonicated for 30 min to form the
homogeneous slurry. Then transferred onto the FTO glass by dipping to achieve a 1*1
cm2 active region, followed by heating at 150 °C for 2 h. Supporting electrolytes was
0.5 M Na2SO4 aqueous solution (pH 6.8), 0.2 M NaH2PO4 and Na2HPO4 served as the
buffer solution. A 300 W Xe-lamp (Beijing ProfectLight, Microsolar300) with a 420
nm cutoff filter was applied as the incident visible light source. The photocurrent-time
response (I-t) was conducted in the dark and under the light irradiation by chopping
from the 300 W Xe lamp at a bias potential of -0.2 V vs. Ag/AgCl. Moreover, the I-t
curves by using the different monochromatic wavelengths light source of 420 nm, 450
nm, 535 nm, 550 nm, 600 nm, 730 nm, 765 nm and 810 nm were also measured.
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Electrochemical impedance spectroscopy (EIS) was conducted with an ACDvOIo: 1l0ta.1g03e9/D0TA03870A
amplitude of 10 mV at -0.2 V vs. Ag/AgCl over the frequency range from 10 kHz to
0.01 Hz in the dark. The Mott-Schottky plots were carried out by using the frequency
of the AC potential at 3000, 4000 and 5000 Hz. The carrier concentration was
calculated from the (1)2:
1
2
푘푇
푒
=
훷푆퐶 ―
(1)
(
)
퐶2푆퐶 휀0휀푟푒푁퐷퐴2
Where e is the elemental charge: 1.6×10-19 C, A is the active area of work electrode, ε0
is the permittivity of vacuum: 8.85×10-14 F/cm, ε is the relative permittivity of the
material, the value is 5.5 for C3N4. The value of kT/e is too low; it is generally ignored.
2.4. Evaluation of photocatalytic CO2 reduction
Photocatalytic CO2 reduction was monitored by using a Beijing ProfectLight
PCX-50C photochemical reaction device equipped with a 5 W white LED lamp.
Typically, 50 mg catalyst, 1μmol CoCl2, 15 mg 2,2-bipyridine was added in 3 mL 10%
TOA/acetonitrile solution with 0.5 mL H2O and ultrasonically dispersed for 15 min.
The reactor was degassed three times to vent air and filled into the high pure CO2 gas
(99.999%). Then the reactor was illuminated under the white LED lamp. The gaseous
components (CO, CH4) were monitored by an FID detector equipped with a mechanized
by gas chromatography and the generation of H2 was simultaneously detected by the
TCD detector. Note that the role of the mechanizer was used to convert CO to CH4 for
FID analysis. The liquid products were also detected by the FID detector after reaction
by injected 10 μL sample, but no product was observed in this work. The reaction
temperature was controlled at 26 ℃ by an liquid cooling system. The isotopic label
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experiments reaction is conducted with the same condition mentioned above for
13
illuminating 4 h under the white LED lamp, the generated CO gas was investigated
by an Agilent gas chromatography-mass spectrometry (GC-MS, 7890 A, and 5975 C).
For recycling tests. 50 mg catalyst, 1μmol CoCl2, 15 mg 2,2-bipyridine was added
in 3 mL 10% TOA/acetonitrile solution with 0.5 mL H2O, purged with pure CO2, and
irradiated with white LED lamp. After 4 h, the catalyst was collected by centrifugation
at 10,000 rpm for 5 min, washed with acetonitrile and ethanol, after dried, redispersed
in fresh TOA/acetonitrile solution with the sample condition, irradiated 4 h again for
subsequent 3 recycling runs.
2.5. Calculation of apparent quantum efficiency (AQE)
The apparent quantum efficiency (AQE) for CO evolution was evaluated by using
the ProfectLight PCX50B under the illumination of a 5 W LED lamp. The different
monochromatic wavelength light was set at 385 nm, 420nm, 465 nm, 500 nm, 550 nm,
595 nm, 630 nm, 730 nm, 785 nm, 810, and 840 nm. Note that the latter three kinds of
monochromatic light source of 785 nm, 810, and 840 nm belong to the near-infrared
region. The amount of generated CO was collected after the light irradiation lasted for
four hours. AQE under the different wavelengths were calculated by the following
equation:
2 × 푡ℎ푒 푒푣표푙푣푒푑 퐻2 푚표푙푒푐푢푙푒푠 푛푢푚푏푒푟(푁푒)
AQE =
× 100% (2)
푡ℎ푒 푖푛푐푖푑푒푛푡 푝ℎ표푡표푠 푛푢푚푏푒푟 (푁푝)
The number of CO molecules can be calculated from the moles of CO of which the
generation rate of CO under 420 nm is 5.646 μmolh-1 and Avogadro’s number (6.022
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× 1023). Thus, the received electron number of the products: 2×5.D6O4I:61×0.110039- /D0TA03870A
6/3600=3.114×10-9 mols-1.
The energy of single-photon at λ = 420 nm is calculated to be 4.736 × 10-19 J,
according to hc/λ, h and c is the Planck constant and the speed of light. The light
intensity was measured to be 3.44 mW/cm2 by calibrating with a light power meter (PL-
MW2000). The diameter of the illumination window of the reactor was 2.5 cm, then
the irradiation area was calculated to be 4.91 cm2. Thus, the total incident photos
number can be calculated to be 5.924×10-8 mols-1. Thus, the AQE=3.114×10-9 mols-
1/5.924×10-8 mols-1 ×100%=5.26%.
2.6. Calculation of turnover number (TON)
The calculation of turnover number (TON) over Co(bpy)2/HCNT-NA was using
the following equation:
푚표푙푒푠 표푓 푒푣표푙푣푒푑 푝푟표푑푢푐푡푠 푔푎푠
푚표푙푒푠 표푓 푎푐푡푖푣푒 푐표푚푝표푛푒푛푡푠 표푛 푝ℎ표푡표푐푎푡푎푙푦푠푡푠
TON =
(3)
Moles of evolved products gas: herein, only the CO and H2 were detected as the
products of CO2 reduction. By using the 50 mg Co(bpy)2/HCNT-NA catalyst, the
photocatalytic activity could relatively maintain stable during 8 h. After the reaction of
8 h, the amount of CO and H2 were detected to be 83.70 μmol and 0.26 μmol,
respectively. Moles of active components on photocatalysts: the addition of CoCl2 is
1μmol for 50 mg HCNT-NA. Thus, the TON= (83.70+0.26) μmol/1μmol=83.96, the
TON is obviously larger than, demonstrating the catalytic nature of this CO2 reduction
reaction by using HCNT-NA.
2.7. DFT calculations
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DFT calculations were performed in the Vienna ab initio simulation package
(VASP). A spin-polarized GGA PBE functional, all-electron plane-wave basis sets with
an energy cutoff of 400 eV, and a projector augmented wave (PAW) method was
adopted. g-C3N4 is simulated using a surface model of p (2 × 2) unit cell periodicity. A
(3 × 3 × 1) Monkhorst-Pack mesh was used for the Brillouin-zone integrations to be
sampled. 15 Å vacuum layer was added to avoid the interaction between adjacent layers.
The electronic density of states (DOS) of (2 × 2) supercells were calculated using a
higher 9 × 9 × 1 k-point mesh. The conjugate gradient algorithm was used in the
optimization. The convergence threshold was set 1*10-4 eV in total energy and 0.05 eV/
Å in force on each atom. The adsorption energy change (ΔEabs) was determined as
follows: ΔEabs = Etotal- Esur- Emol
where Etotal is the total energy for the adsorption state, Esur is the energy of pure surface,
Emol is the energy of the molecule.
The free energy change (ΔG) for adsorptions were determined as follows:
ΔG = Etotal-Esur+ΔEZPE-TΔS
where Etotal is the total energy for the adsorption state, Esur is the energy of the pure
surface, ΔEZPE is the zero-point energy change and ΔS is the entropy change.
3. Results and discussion
3.1. Material synthesis and characterization
The synthesis of PCN-based photocatalyst involves two steps: a hydrothermal
pretreatment to form a melamine-cyanaurate complex, followed by thermal
condensation to obtain NA-modified holey PCN microtubes (HCNT-NA, Fig. 1a).
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Hydroxyethyl groups were grafted at the edge of heptazine motifs through in-situ
replacement of NH/NH2 groups by nucleophilic addition with the precursor during
hydrothermal pretreatment in a water/propanol mixture (tentative reaction mechanism
is illustrated in Scheme S1, supporting information). We also synthesized a series of
control samples as illustrated in Scheme S2 & S3, such as bulk PCN (BCN), holey PCN
microtubes (HCNT) and NA-modified holey PCN nanosheets (HCN-NA).
Detailed characterizations on the structural change of pristine and modified PCN
have been provided in Fig. 1b & c, Fig. S1, and Table S1. All PCN samples show
similar two diffraction peaks (Fig. S1), reflecting the periodic packing of in-plane
heptazine motifs and the interlayer stacking of lamellar structure, indexed to (100) and
(002) lattice face, respectively 45, 46. Chemically tailored PCN (HCNT-NA) exhibits a
slight left shift for both (002) peak and (100) peak than the BCN counterpart due to
distorted heptazine motifs and enlarged layered spacing by the grafted hydroxyethyl
group and formation of 1D tube structure 47. Much higher BET specific surface area of
HCNT-NA (73.3 m2/g versus 9.5 m2/g of BCN) is verified by the N2 adsorption-
desorption isotherm (Fig. S2) due to the introduction of well-defined mesopores 48.
The successful grafting of hydroxyethyl on the PCN matrix is confirmed by solid-
state 13C magic angle spinning (MAS) NMR in Fig. 1b. HCNT-NA shows two
characteristic peaks at 154.9 and 164.8 ppm from the C-(N)3 (1) and NH2-C(N)2 (2) in
heptazine motifs, respectively, and possess slight left shift than BCN 49, 50. The peak
ratio (I1/I2) gradually increases with the incorporation of NA due to the replacement of
NH2-C(N)2 by the hydroxyethyl group (see detailed model structure in the diagram of
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Fig. S5). This is reflected in two additional peaks at 136.4 and 189.5 ppm inDHOIC: 10N.1T03-9/D0TA03870A
51, 52
NA from the C-C-OH (3) and aromatic ring-C-C (4) in the hydroxyethyl group
,
this could be confirmed in previous work which successfully introduced diethylamino
groups in heptazine derivatives 53, 54
.
Further evidence of edge-functionalization is provided by high-resolution XPS N1s
spectra in Fig. 1c & S4, where certain peak shifts of the C-N=C, N-(C)3 and C-N-H
species can be observed in HCNT-NA, HCN-NA relative to those in BCN 55. It is
recognized that the C-N heterocycles have a strong polarity, where hydroxyethyl groups
serve as electron donors to enhance the π-conjugated, electron-rich system, resulting in
the positive shifts in escape energy of surface electrons (binding energy in XPS) 56, 57
.
A similar observation can be found in the XPS C1s spectra in Fig. S3. The binding
energy of N-C=N in HCNT-NA, indexed to the in-plane hybridized carbon of the C-N
heterocycles, is slightly higher than that of BCN 45, 58. Although we also acquired the
FTIR spectra in Fig. S1, the hydroxyethyl group is hard to distinguish from the
overlapping –NH2/OH groups in 3200-3500 cm-1 and the C-N heterocycles in 1200-
1700 cm-1 21, 46. A higher C/N ratio as well as a higher O percentage in HCNT-NA from
elementary analysis in Table S1 also confirms the installation of hydroxyethyl groups
on the PCN matrix.
3.2. Morphological characterization
Scanning and transmission electron microscopy (SEM & TEM) were used to
observe the morphology of catalysts. As shown in Fig. 1d, the intermediate compound
of HCNT-NA after hydrothermal treatment of dicyandiamide (DCDA) with NA has a
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well-ordered, pillar-like morphology with diameters of 2 ~ 3 μm, which is distinct from
the large, irregular particles from DCDA precursor in Fig. S6. This is attributed to the
transformation into a modified melamine-cyanaurate complex (see Fig. S7 & S8 for
XRD and FTIR) via liquid-liquid supramolecular self-assembling process during
hydrothermal treatment 59, 60. The modified PCN samples (HCNT-NA) can maintain its
1D hexagonal pillar structure (Fig. 1e) after calcination due to the thermal stability of
intermediate compounds, despite volume shrinkage by the release of NH3 and H2O.
SEM images at higher magnifications further reveal the packing of leaf-like
nanostructures to form the hexagonal entity (Fig. 1f), where vertically aligned
nanoarrays are observed on the surface (Fig. 1g), which will form the spatial potential
difference to boost the directional charge transfer and the absorption of CO2 61. Such a
unique mesoporous structure with hollow microchannels within the individual “leaf”
provides sufficient active sites for the activation of CO2 and facilitates the mass transfer
of reactants, 36, 61. Further insight on the microstructure was displayed in the TEM
images in Fig. 1h-1l. The 1D porous network is comprised of regularly packed 2D
nanosheets with a uniform dimension and a well-ordered high crystalline domain.
Bright diffraction patterns in corresponding selected area electron diffraction (SAED)
indicate relatively high crystallinity of HCNT-NA, where NA is critical for the
formation of high-quality crystals (see the difference of SAED patterns in HCN-NA
and HCNT in Fig. S10). Such ordered packing configuration is beneficial for the rapid
transfer of photo-induced carriers and the extended optical absorption 62-65. STEM-EDS
mapping also confirms the uniform distribution of C, N, and O in HCNT-NA
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throughout the selected area.
3.3. Optical absorption and band-gap structure
The optical absorption property of catalysts has been investigated by the UV-vis
diffuse reflectance spectra (DRS) in Fig. 2a. Owing to quantum confinement and higher
specific surface area, the modified PCN matrix exhibit a slight blue shift in the
absorption edge than BCN counterpart 34, 66. HCNT-NA shows the highest absorption
at λ > 460 nm with an extended absorption up to near-infrared region due to the
formation of an intermediate band in the n-π* state transition, increased density of band
tail states, as well as well redistributed localized states (vide infra), resulting in
excitations by photons with induced energy lower than the intrinsic band-band
transition. Similar results were found in the photoluminescence (PL) spectra and band
structure (Fig. S11 ~ S13, see the related discussions in section 4 of supporting
information). The profound extension in optical region response leads to a higher
concentration of excited electron to facilitate the CO2 conversion rate. XPS valence
band (VB) spectra and Mott–Schottky (M-S) measurements were conducted to
elucidate the change in band-edge position. As shown in Fig. 2b, the VB position to the
Fermi level shifts positively in HCN-NA (2.16 eV) and HCNT-NA (2.19 eV) than BCN
67
(1.89 eV) . For n-type semiconductors, the conduction band (CB) position can be
estimated from the flat potential (0.1-0.2 V lower than CB) by extrapolating the x-axis
intercept in the M-S plot 68, 69. As depicted in Fig. 2c, the flat potential of HCNT-NA is
obviously less negative than that of BCN (-1.20 versus -1.53 V vs. Ag/AgCl).
Importantly, HCNT-NA also has a lower slope in M-S plots, demonstrating its higher
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donor carrier density than BCN (1.88*1021 versus 7.38*1020, calculation process shown
below Fig. S15). Such a 2.55-fold increase in carrier donor density is beneficial for
64
subsequent photocatalytic reduction process . Based on the above results, the band
diagrams of HCNT-NA and BCN were proposed in Fig. 2d.
To reveal the critical role of hydroxyethyl group on the modification of electronic
structure, density functional theory (DFT) calculations on the total and partial density
of states (PDOS) of catalysts were performed in Fig. 2e & 2f (see DFT models in Fig.
S16). After installation of the hydroxyethyl group on the tri-s-triazine units, the CB and
VB positions of PCN-NA shifted downwards together with a smaller bandgap than
pristine PCN, in good accordance with the XPS-VB and M-S observations. The
appearance of an intermediate band from the C2p and N2p orbitals in PCN-NA agrees
with the extended absorption tail along the optical region 35, 70, and leading to a
significant shorten electron transportation distance for the photo-generated carriers 40.
Note that the formation of intermediate band states may be also arouse from the well-
ordered high-crystalline structure in HCNT-NA, but it is difficult to be quantitated 64.
The density of state along the Fermi level is also significantly increased which is
ascribed to the redistributed localized electronic states below the CBM, suggesting a
higher concentration of charge carriers 71. This a high density of localized states
associated with the band tails states further contribute to the continuum extension
optical region. Additionally, the transient-state photocurrent in Fig. S17 manifests a
high carrier separation efficiency in HCNT-NA due to the modulation in the electronic
structure after the installation of hydroxyethyl groups. Such extended absorption ability,
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intensive concentration of charge carrier and promoted charge separation endow
efficient activation of CO2 on HCNT-NA under visible light.
3.4. Photocatalytic activity evaluation and mechanism analysis
We examined the photocatalytic efficiency of PCN-based catalysts for CO2
reduction under a 5 W white LED lamp with Co complex as cocatalyst and
triethanolamine (TOA) as the electron donor (reaction setup seen in Fig. S18). As
shown in Fig. 3a, BCN has an undesired CO production rate of 5.29 μmolh-1g-1 owing
to the inert surface for CO2 activation. Self-assembling into PCN microtubes led to a
moderate improvement to 43.91 μmolh-1g-1 for HCNT. Surprisingly, the incorporation
of NA into the PCN matrix can dramatically boost the photocatalytic CO2 reduction,
where 24.4 and 39.5 folds increases in the CO yields (129.08 & 209.24 μmolh-1g-1)
together with a selectivity of ~ 97.5% are observed for HCN-NA and HCNT-NA
relative to BCN. Considering their similar specific surface areas, edge activation of the
inert PCN matrix by NA functionalization should be the main cause for such
improvement. This is confirmed by strong correlations between NA content and CO2
activity in Fig. S19. Control experiments in Fig. S20-S23 reveal that the catalyst,
cocatalyst, TOA, light source, and CO2 gas are all essential for such a reduction reaction.
The PCN catalyst serve as the photocatalysts and the Co complex as the cocatalyst.
Under the illumination of white light, the excited electron from PCN would transfer to
the Co complex. The 13C isotopic experiments in the inset of Fig. 3b confirm the
13
13
generated CO gas at m/z=29 in a mass spectrum is originated from the used CO2.
HCNT-NA also shows excellent cycling stability for photocatalytic CO2 reduction,
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where no obvious change in the structure and morphology could be found in tDhOeI:X10R.10D39/D0TA03870A
and FTIR even after 4 cycles (Fig. S24 & S25).
Along with its excellent activity, HCNT-NA exhibits a remarkable apparent
quantum efficiency (AQE) at λ=420 nm (5.26%) in Fig. 3b (see the experimental
section 1.5 in supporting material), outperforming most of the state-of-art
heterogeneous photocatalysts for CO2 reduction in Table S2. The turnover numbers
(TONs) when HCNT-NA was used as the catalyst is calculated to be 83.96 for the
evolved products over 8 h (see the experimental section 1.6 in supporting material),
demonstrating the catalytic nature of this CO2 reduction reaction. CO production can
be extended to longer wavelengths (500 ~ 850 nm), whereas the detectable amount of
CO gas is generated even in the near-infrared region such as λ=785, 810 and 840 nm
using HCNT-NA. This is further verified by the observation of recycled transient
photocurrent response with the chopped light under near-infrared region (Fig. S26) and
time-dependent of CO product evolution at λ=785, λ=810, and λ=840 nm (Fig. S27)
over HCNT-NA. To the best of our knowledge, this is the first case using the PCN-
based photocatalysts system for the CO2 conversion in the near-infrared region.
Considering the redox potential of CO2/CO is as high as -0.53 V. vs NHE, the
photocatalytic activity in the near-infrared region may contribute to the formation of
inter-band excitation and the nonlinear two-photon responsive 43, 72
.
In order to illustrate the versatility of our functionalized-activation strategy, five
different additives such as ethylene glycol (EG), ethylenediamine (EDA), ethanolamine
(EOA), diethanolamine (DOA) and triethanolamine (TOA) were chosen (the molecule
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structure shown in Scheme S3) and reacted with the DCDA precursor. As expected, all
of them produce a much higher CO production than BCN in Fig. 3c, revealing the
universal applicability of in-situ C-N coupling for photocatalytic CO2 reduction.
Additionally, when considering the different molecular structures of EDA and EOA,
HCN-EDA (modified by ethyl) and HCN-EOA (modified by hydroxyethyl) delivered
a similar CO evolution rate. It suggests that the improved photocatalytic activity is
mainly determined by the grafting of the ethyl group, rather than the modification by
73
hydroxy group . While the grafted hydroxyethyl group of HCN-EOA has a slightly
enhanced activity than HCN-EDA which implies that the presence of hydroxy also
favors CO2 reduction.
We performed the CO2 absorption capacity test, in-situ FTIR measurements, and
density functional theory (DFT) calculations to understand the catalytic origin of our
catalyst. As shown in Fig. 3d, HCNT-NA shows the strongest absorption capability
toward CO2, which is five times larger than that of BCN (5.49 versus 0.95 cm3/g),
demonstrating a much enhanced CO2 absorption. Such preferred absorption of CO2 on
the surface of HCNT-NA is validated by the in-situ FTIR spectra in Fig. 3e. A series
of infrared peaks ascribed to carbonate species were observed under illumination.
2-
Specifically, the peaks at 1540, 1513 and 1462 cm-1 (m-CO3 ), 1337, 1635 cm-1 (b-
2-
CO3 ), 1209, 1406 and 1574 cm-1 (hydroxyl groups of carboxylic acid) are key
evidence of the presence of surface-absorbed COOH* species 74. Such COOH* species
is the critical intermediate during the surface activation of CO2, whose intensity keeps
constant after 10 mins due to a fast equilibrium between COOH* and the final product
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(CO). Meanwhile, the characteristic peak of CO at 2054 cm-1 gradually increaDsOeIs: 1w0.1i0t3h9/D0TA03870A
time, in good agreement with higher CO production under prolonged illumination (Fig.
S28).
To further reveal the influence of the hydroxyethyl group on the absorption and
activation of CO2, the Gibbs free energy involving the reduction pathways of CO2 to
CO via COOH* were calculated by DFT calculations in Fig. 3f. The results confirm
much lower absorption energy of CO2 on HCNT-NA than BCN (-0.292 versus -0.187
eV), demonstrating the facile activation of CO2 on HCNT-NA to form COO*. Detailed
models on each CO2 reduction intermediate are presented in Fig. S29. The activation
energy barrier for COOH* formation over HCNT-NA is slightly smaller than BCN
(0.853 versus 0.869 eV, Table S3), indicating that HCNT-NA is also kinetically more
favorable for the formation of COOH* than the BCN counterpart 75, 76. As shown in the
charge difference contours in Fig. 3g & 3h, the presence of hydroxyethyl group can
significantly enhance the charge density around adjacent nitrogen atoms in HCNT-NA,
of which the charge distribution is clearly more delocalized to interact with the
intermediates in CO2 reduction. This enables a closer C=O bond to catalyst surface for
subsequent electron transfer and generation of *COOH intermediate, thus promoting
the overall reduction process. We also calculated the carried charge (△q) to estimate
the electron transport direction, where the negative value reflects the number of
electrons transferred to the CO2 molecule 16. Apparently, HCNT-NA has a larger
absolute value (-0.391 e) than BCN (-0.129 e), inferring stronger electron transfer to
active CO2 molecules. Collectively, the existence of hydroxyethyl in the structural edge
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of HCNT-NA is in favor of CO2 adsorption, provides more excited electron to active
CO2 molecules and lowers the CO2 transformation energy barrier.
Finally, the carrier dynamics during the photocatalytic process were investigated
by femtosecond transient absorption (TA) spectroscopy in Fig. 4 to understand the
importance of the hydroxyethyl group on the movement of charge carriers. After the
excitation at 400 nm, both BCN and HCNT-NA display a ground state bleach signal
and an excited state absorption signal. The ground state bleach signal in HCNT-NA is
broader than BCN and further extends to 440 nm in Fig. 4c, inconsistent with the studies
on the band structure. The color contours of TA spectra display much stronger positive
absorption in HCNT-NA from its higher excited charge carrier density 77, 78. Since the
ground state bleaching originated from the state filling and reflects the excited state
carrier population, the carrier dynamics from this signal has been subtracted and plotted
in Fig. 4d. It is clearly seen that the lifetime of excited state carriers is substantially
prolonged by an order of magnitude (from 2.62 ps to 21.83 ps) with the modification
of the hydroxyethyl group. Such a prolonged period for carrier transportation to the
interface allows a better charge separation efficiency to provide sufficient surface
electrons to participate in the CO2 reduction.
In this regard, the experimental results can be well illustrated by the introduced of
hydroxyethyl groups at the edge and the simultaneous formed the porous 1D microtube
with hollow micro-channels as well as the accompanying high crystalline well-ordered
domain by using a feasible bottom-up strategy of that endured the NA motifs
nucleophilic addition and hydrothermal pretreatment process. On the basis of
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aforementioned experimental analyses and theoretical calculations, thisDOuI:n1i0q.1u03e9/D0TA03870A
structure in HCNT-NA enables the substantially improved photocatalytic CO2
conversion, as proposed following i) the existence of hydroxyethyl in the structural
edge is in favor of CO2 absorption and lowers the CO2 transformation energy barrier;
ii) The generated inter-band states, increased density of band tail and localized states
originated from the hydroxyethyl group and well-ordered mesocrystal structure leads
to continuously extended optical region from 450-850 nm; iii ) The lifetime of excited
state carriers is substantially prolonged than that of pristine PCN; iv ) the functionalized
structure accompanied with hierarchy porous 1D microtube can largely limit the
radiative recombination of photoinduced electron-hole pairs and facilitate the rapid
charge transfer. While, in general, when considering the involved three essential steps
in the semiconductor-based photocatalytic CO2 reduction reaction, namely: i) light
absorption, ii) charge carriers separation and transfer, and iii) surface absorption and
redox reaction, the enhanced optical absorption region, excellent carriers separation and
boosted CO2 absorption capacity in HCNT-NA, as a result, lead to a significantly
photocatalytic performance for CO2 reduction.
4. Conclusions
In conclusion, We have developed a covalent functionalization strategy to promote
the photocatalytic CO2 reduction on carbon nitride by activating the inert amino edge
with the hydroxyethyl group. As synthesized hydroxyethyl functionalized of one
dimensional, mesoporous matrix (HCNT-NA) exhibited 39.5 folds’ increase in the CO
production rate (209.24 μmolh-1g-1) than pristine BCN, together with a CO selectivity
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of 98.5% under the white LED light illumination. The hydroxyethyl group at the edge
of the CN matrix leads to the formation of an intermediate band in the energy gap and
a higher state density along the Fermi level as well as the well-ordered mesocrystal
structure, thus enabling stronger light-harvesting ability up to the near-infrared region.
The presence of the hydroxyethyl group also promotes the activation of CO2 by
decreasing its absorption energy barrier and activation energy. Meanwhile, the
prolonged period for carrier transportation to the interface allows a better charge
separation efficiency in HCNT-NA to provide sufficient surface electrons to participate
in the CO2 reduction. Our findings indicate the great potential for using the
hydroxyethyl functionalized PCN in visible-light-driven photocatalytic CO2 conversion
and also provide a feasible bottom-up strategy for simultaneously realizing the
activation of CO2 and wide-spectrum optical absorption of strong photoredox ability in
the photocatalysts.
Conflicts of interest
There are no conflicts to declare.
Acknowledgment
X.W. Peng thanks for the National Natural Science Foundation of China (31971614,
31430092, 21736003), Guangdong Natural Science Funds for Distinguished Young
Scholar (2016A030306027), Guangdong Natural Science Funds (2017A030313130),
Guangzhou science and technology funds (201904010078). F. Wang thanks the
National Natural Science Foundation of China (21707128, 61904167). The authors
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would like to thank Shiyanjia Lab (www.shiyanjia.com) for the support of DFT
calculation.
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