Gram-scale synthesis of aligned C3N4-polypyrrole heterojunction aerogels with tunable band structures as efficient visible and near infrared light-driven metal-free photocatalysts

Article abstract of DOI:10.1039/c7ta08389k

The assembly of aligned porous materials from simple building blocks is of widespread interest for engineering materials with enhanced and synergistic properties. To date, however, how to develop 3D heterojunction aerogels with aligned porosity based on 2D semiconductor materials and 1D conducting polymers for solar energy conversion in the visible and near-infrared (NIR) light region remains a significant challenge. Here a new class of gram-scale 3D aligned heterojunction aerogels of polypyrrole (PPy)/C₃N₄ nanosheets (NSs) were designed and synthesized by directional freezing of polypyrrole (PPy)/polyvinyl alcohol (PVA) and C₃N₄ NS aqueous suspension. The synthesis of aligned C₃N₄-PPy heterojunction aerogels can be achieved on a large scale. The formed aerogel expresses stable and uniform dispersion of the two building blocks, long-range channel aligned structures along the whole monolithic sample, and additional special complementary optical properties between PPy and C₃N₄ NSs. Based on the above unique structure and optical properties, this novel metal-free heterojunction aerogel exhibits excellent photocatalytic activity and long-term stability for direct arylation of heteroaromatics under visible and near infrared (NIR) light irradiation at room temperature, far exceeding those of the single- and two-component systems. Our work therefore not only provides a new approach to obtain aligned heterojunction aerogels based on metal free semiconductors but also paves a way to develop gram-scale aerogels as a new type of highly efficient visible and NIR light induced heterogeneous photocatalyst.

Full text of DOI:10.1039/c7ta08389k

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Journal of Materials Chemsitry A
ARTICLE
Gram-Scale
Synthesis
of
Aligned
C3N4-Polypyrrole
Heterojuction Aerogel with Tunable Band Structures as an
Efficient Visible and Near Infrared Light -Driven Metal-Free
Photocatalyst
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Jian Liu,a Hao Wang, a Jinwei Bai, a Tianrong Li, a Yanmei Yang, a Yong Peng, *,b Baodui Wang*, a
www.rsc.org/
The assembly of aligned porous materials from simple building blocks is of widespread interest for
engineering materials with enhanced and synergistic properties. To date, however, how to develop 3D
heterojuction aerogels with aligned porosity based on 2D semiconductor materials and 1D conducting
polymer for solar energy conversion in the visible and near-infrared (NIR) light region still remains a
significant challenge. Here a new class of gram-scale 3D aligned heterojuction aerogels of polypyrrole
(PPy)/C3N4 nanosheets (NSs) were designed and synthesized by directional freezing of polypyrrole
(PPy)/polyvinyl alcohol (PVA) and C3N4 NSs aqueous suspension. The synthesis of aligned C3N4-PPy
heterojuction aerogel can be achieved at large scale. The formed aerogel expresses stable and uniform
dispersion of the two building blocks, long-range channel aligned structures along the whole monolithic
sample, and additional special complementary optical properties between PPy and C3N4 NSs. Based on
the above unique structure and optical property, this novel metal-free heterojunction aerogel exhibits
excellent photocatalytic activity and long-term stability for direct arylation of heteroaromatics under
visible and near infrared (NIR) light irradiation at room temperature, far exceeding those of the single-
and two-component systems. Our work therefore not only provides a new approach to obtain aligned
heterojuction aerogels based on metal free semiconductors but also paves a way to develop gram-scale
aerogel as a new type of highly efficient visible and NIR light induced heterogeneous photocatalyst.
such technique has little use to construct aligned heterostructures
based on 2D materials with optical activity and conducting polymer
which are of considerable interest in photocatalysis.
1. Introduction
The assembly of functional materials with aligned aerogels from
simple building blocks has gained extensive attention in applications
such as electrochemical energy storage, solid phase supports for
catalysis, membrane separation, bio-engineering, etc..1-4 Especially,
aligned channel structures in porous materials are fundamentally
important for gaining a better understanding of self-assembly
processes in biological systems and mimicking natural aligned
tissues. Directional freezing-driven 0D nanoparticles (NPs) assembly
has been successfully applied to align the building blocks in the form
of 2D oriented flat patterns or further 3D porous structures.5-7
Recently, composite structures of various kinds of array, such as
polymer-inorganic nanocomposites,8 porous microfibres and
biaxially aligned networks,9 aligned gold microwires and microwire
networks,6 were synthesized using this simple technique. However,
In principle, multifunctional 3D architectures based on 2D
materials, especially graphene and its analogues, provide new
possibilities in electronics, catalysis, and energy conversion and
storage.10,11 The chemical activity of such architectures and their
hybrids exhibits unprecedented flexibility, which is dependent on the
ability to control the electronic states of the crystal surface by
changing the work function,12,13 the formation of monolithic
structures based on different 2D crystals, and the construction of
nanoreactors in the space among different 2D crystals.14 As a widely
investigated 2D catalyst, it was found that graphitic C3N4 (g-C3N4)
presents promise in photo- and electro-catalysis as well as
conventional heterogeneous catalysis, due to its unique electronic
structure and rich Lewis basic functions.15 Generally, combining g-
C3N4 with other materials can further enhance the photocatalytic
performance of g-C3N4 owing to the synergistic effect of different
active sites or the increase of the active sites.16 g-C3N4 with the band
gap of ~2.7 eV shows an intrinsic semiconductor-like absorption in
the short-wavelength region of the visible spectrum. In addition, the
most g-C3N4 based catalysts were found to be very efficient under
visible light, but not under near-infrared (NIR)-light.17 The ability to
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
Province and State Key Laboratory of Applied Organic Chemistry Lanzhou
University Gansu, Lanzhou, 730000 (P.R. China) Lanzhou University, China. *E-
mail: wangbd@lzu.edu.cn.
bKey Laboratory for Magnetism and Magnetic Materials of the Ministry of Educatio
n, Lanzhou University, Lanzhou, 730000 P.R. China*E-mail: pengy@lzu.edu.cn.
Electronic Supplementary Information (ESI) available: [Experimental details,
synthesis and characterization]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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develop aligned heterostructures based on g-C3N4 for photocatalysis were used without further purification. The H2O durViniegw AthrtieclewOhnolinlee
DOI: 10.1039/C7TA08389K
under the visible-NIR light provides new opportunities.
experimental process was deionized H2O.
Recently, intergating semiconductor nanomaterials with
conducting polymer (CPy) has been considered to be an effective
a
2.2. Measurements
method to promote charge carrier separation and improve visible- The morphology of the samples was investigated by field-emission
19
light activity.18,
Among well known π-conjugated conducting scanning electron microscope (FE-SEM, FEI, Sirion 200) and
polymers, polypyrrole (PPy) has attracted much attention because of transmission electron microscope (TEM, JEOL, JEM-2100, 200kV).
its easy preparation, high electrical conductivity and large surface X-ray powder diffraction (XRD) patterns of the nanomaterials were
area.20-22 Tremendous efforts have been devoted to create recorded on a Bruker AXS D8-Advanced diffractometer with Cu Kα
heterojunction materials containing PPy layer and inorganic layer, radiation (λ = 1.5418Å). N2 adsorption-desorption isotherms were
o
polymer intercalation in 2D nanomaterials, and assembly 2D measured at 77 K after heating the samples at 120 C for 8 h to
nanomaterials on multi-dimensional polymers.23-25 Despite numerous remove any moisture and solvent molecules presented in the pores
efforts, there is still only limited information on the synthesis of with Micromeritics TriStar II 3020 analyzer. The Brunauer-Emmett-
aligned heterostructures based on g-C3N4 and PPy.
Teller (BET) method was utilized to calculate the specific surface
On the other hand, arylated heteroarenes are widely used in areas (SBET). By using the Barrett-Joyner-Halenda (BJH) model, the
biomedical applications, peptide mimetics and drugs.26 pore volumes and pore size distributions were derived from the
Photocatalysis has provided a valuable alternative to the direct adsorption branches of isotherms. X-ray photoelectron spectroscopy
arylation of heteroarenes by C−H bond activation without requiring (XPS) measurements were performed on a PHI-5702 multifunctional
preactivation of the heteroarenes.27-29 Nowadays, direct arylation of spectrometer using Al Kα radiation. The XPS profiles were fitted by
heteroaromatics generally use metal nanomaterials/metal complexes the Gaussian–Lorentzian method after background subtraction using
as photoredox catalysts. However, metallic catalysts suffer from Shirley’s method.32 Raman spectra were collected using a confocal
corrosion or aggregation in photocatalytic reaction so as to limit their microprobe Raman system (Renishaw, RM2000). FT-IR spectra
practical applications. Elegantly, König reported that Eosin Y can be were recorded on a Nicolet FT-170SX spectrometer. UV-vis-NIR
directly used as homogeneous photoredox catalyst under visible diffuse reflectance spectra were carried out on an Agilent Cary 5000
light,30 while they have not considered the reusability. Therefore, UV-Vis-NIR Spectrophotometer. Pd contents are determined by
inexpensive metal-free alternative heterogeneous photocatalysts for inductively coupled plasma-atomic emission spectrometry (ICP-AES,
rapid and direct arylation of heteroaromatics under sufficient Varian VISTA-MPX). Photocalysis was performed using a white
irradiation are desirable.
LED lamp (30 W). 1H-NMR and 13C-NMR spectrums were gathered
In this study, we demonstrate that C3N4 nanosheets (NSs) can be on a JEOL ESC 400M instrument. The morphology of the samples
assembled with conductor polymer, and thereby preparing aligned was investigated by field-emission scanning electron microscope
heterojuction aerogel by directional freezing of polypyrrole (FE-SEM, FEI, Sirion 200) and transmission electron microscope
(PPy)/polyvinyl alcohol (PVA) composites and C3N4 NSs. The (TEM, JEOL, JEM-2100, 200kV). X-ray powder diffraction (XRD)
resultant heterojunction aerogel (denote as CNPVPy) has several patterns of the nanomaterials were recorded on a Bruker AXS D8-
remarkable features, including (i) long-range channel aligned Advanced diffractometer with Cu Kα radiation (λ = 1.5418Å). X-ray
structures along the whole monolithic sample, enabling mass photoelectron spectroscopy (XPS) measurements were performed on
transport and efficiently contact between the reaction substrates and a PHI-5702 multifunctional spectrometer using Al Kα radiation. The
catalysts, (ii) broad absorption from UV to NIR-range, (iii) high XPS profiles were fitted by the Gaussian–Lorentzian method after
stability in solution, (iv) efficient charge separation,and (v) tunable background subtraction using Shirley’s method. Raman spectra were
band structure by changing the mass ratio of C3N4 and PPy. We collected using a confocal microprobe Raman system (Renishaw,
demonstrate that CNPVPy macrosturctures are highly efficient under RM2000). FT-IR spectra were recorded on a Nicolet FT-170SX
visible as well as NIR light for the direct arylation of spectrometer. UV-vis-NIR diffuse reflectance spectra were carried
heteroaromatics at 25 0C. Interestingly, we find that CNPVPy out on an Agilent Cary 5000 UV-Vis-NIR Spectrophotometer.
1
heterojuction aerogel exhibits high photocatalytic activity, far Photocalysis was performed using a Xenon lamp (HSX-F300). H-
exceeding those of the single- and mixed-component systems, as a NMR and 13C-NMR spectrums were gathered on a JEOL ESC 400M
result of the interaction between C3N4 and PPy to form p-n junction instrument.
thus to cause the enhanced charge separation. Furthermore, this
2.3. Synthesis of polypyrrole polyvinyl alcohol nanocomposites
photocatalyst is very stable in repeated cycling. To the best of our
(PPy/PVA NCs)
knowledge, this finding constituted the first experimental evidence
of the photocatalytic activity of aligned metal-free heterojuction
aerogel under visible and NIR light for synthesis of fine chemicals.
PVA (3 g) was dissolved in H2O (60 ml) at 100 oC, and then cooled
to room temperature. FeCl3 6H2O (3.78 g) was added into the
.
aqueous PVA solution. After 1 h stirring, pyrrole (420 μl) was
o
introduced dropwise into the PVA/FeCl3 aqueous solution at 0 C,
2. Experimental
and equilibrated for 4 h. After completion of polymerization, the
resulting dark green seriflux was purified through the dialysis
(MWCO = 300 kDa) in H2O for 1 week to remove excess FeCl3,
oligomers, and contaminants. The final dark green colloidal
dispersion (PPy/PVA, 24 mg/mL) was collected as stock solution.
2.1. Chemicals
Pyrrole, urea, and catalytic substrate reagents were purchased from
Alfa Aesar. Polyvinyl alcohol (PVA, 99% hydrolyzed, MW =
170000) and urea were obtained from Tianjin Med. All chemicals
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2.4. Formation of aligned CNPVPy heterojuction aerogels
3. Results and discussion
View Article Online
DOI: 10.1039/C7TA08389K
Different volumes of C3N4 suspension synthesized according to
previous work with modification31 were added into PPy/PVA
suspension (1 ml), then additional H2O was added until 3 ml volum,
and the mixtures were ultrasonicated for 30 min at r.t.. To produce
aligned self-assembly composites (CNPVPy) between C3N4 and
PPy/PVA, the mixtures were slowly put into liquid nitrogen, and
then freeze-dried using a Heto Lyolab 3000 freeze-dryer. (We
denoted the final architecture CNPVPyx, x is the ratio of PPy/PVA
weight to C3N4 weight)
3.1. Preparation and Characterization of CNPVPy heterojuction
aerogel
Scheme 1 and Scheme S1 showed the structural evolution of
CNPVPy heterojuction aerogel from 2D C3N4 NSs and PPy/PVA
composites. Cooper’s work has suggested that the PVA plays a
crucial role as a structuring agent to produce 2D/3D aligned porous
materials.6 In order to obtain aligned porous structure, we first
prepared PPy/PVA aqueous suspension with negative zeta potential
(-7.13 mV) by one-step oxidation of pyrrole using ferric ions in PVA
solution.32 C3N4 NSs aqueous suspension with positive zeta potential
(+18.9 mV) was deliberately prepared according to the literature
with little modification.17,33 The above two aqueous suspensions
were mixed thoroughly and then the directional freezing
arrangement generation aligned CNPVPy heterojuction aerogel. In
addition, by adjusting the mass ratio of PPy/PVA and C3N4, we can
get a series of assemblies containing different amounts of C3N4.
To form orderly aligned self-assembly composites (CNPVPx)
between C3N4 and PVA, the same proportion of materials were used
except using PVA solution to replace PPy/PVA suspension.
2.5. Synthesis of aryldiazonium salts
The aryldiazonium salts were prepared according to the literature
with slight modification.27 The corresponding aniline (0.02 mol) was
dissolved in a mixture of ultrapure water (8 ml) and 50% fluoroboric
acid (7 ml). After the temperature fell below zero, 3 mL sodium
nitrite solution (1.38 g) was added slowly. The mixture was stirred
for 40 minutes in ice bath, and the resulting precipitate was collected
and washed several times with CH2Cl2. The resulting solid was
recrystallized by acetone and CH2Cl2.
2.6.General procedure for direct arylation of heteroaromatics
catalyzed by CNPVPy
CNPVPy (5 mg), heteroaromatics
1 (1 ml), aryldiazonium
Scheme 1. Synthetic route and proposed formation mechanism of CNPVPy
tetrafluoroborate 2 (0.1 mmol), and EtOH (2 ml) were added into a
10 ml glass tube. The reactor was evacuated and refilled with N2,
and then stirred under visible light or NIR light irradiation (distance
10 cm) for 1h. After reaction finished, the catalyst was filtered and
separated, and ethanol was washed 3 times. The filtrate was
collected and diluted to 50 ml. The yield was detected by GC. The
products were characterized by 1H-NMR and 13C-NMR.
For recycling test, CNPVPy was isolated by filtration, soaked with
EtOH (3 × 5 ml), and dried at ambient condition for next cycle.
Another heteroaromatics 1 (1 ml), aryldiazonium tetrafluoroborate 2
(0.1 mmol), and EtOH (2 ml) in 10 ml glass tube were used.
heterojuction aerogel.
The formation of CNPVPy heterojuction aerogel from the C3N4
and PPy/PVA was confirmed by FT-IR spectra. As shown in Figure
1a and Figure S2, the C3N4 spectrum contained several main bands
range from 1205 cm-1 to 1640 cm-1, which are belonging to the
characteristic vibrational modes of CN heterocycles.17 While the
bands at 1240 and 1320 cm-1 are corresponding to the stretching
vibrations of C−N−C (partial condensation) and the connected units
of N−(C)3 (full condensation), respectively. A FT-IR spectrum of
PPy/PVA showed pyrrole ring stretching band at 1550 cm-1, =C−H
in-plane vibration band at 1315 and 1185 cm-1, conjugated C−N
stretching band at 1450 cm-1, and =C−H out-of-plane vibration band
at 915 cm-1.32 After formation of CNPVPy aerogel, the C3N4 and
PPy signals appeared simultaneously. But with the increase of
PPy/PVA component, the C3N4 signals gradually weakened, while
PPy signals gradually became strong.
In Figure 1b, XRD powder analysis of C3N4 revealed a weak (100)
diffraction peak at 13.5° for the inter-planar structural packing, and a
sharp (002) diffraction peak at 27.9° for interlayer stacking of
aromatic systems.17,33 In addition, a broad signal at 22.4° belongs to
amorphous carbon, which is similar to that of PPy/PVA. After
formation of CNPVPy macrosturctures, the characteristic peaks
belonging to C3N4 are weakened, while the broad signals for
amorphous carbon are enhanced (Figure S3). Similarly, X-ray
photoelectron spectroscopy (XPS) depth analysis indicated the
existence of N, C, and O species in CNPVPy (Figure 1c). We also
discovered that the intensity of O and C in the CNPVPy increased
and the intensity of N decreased with the increase of PPy/PVA
component. In addition, the high-resolution XPS survey spectra of
C1s, N1s, and O1s also confirmed that CNPVPy has the same
2.7. The apparent quantum efficiency (AQE) calculation
Under visible light irradiation (450 ± 10 nm), the catalytic
suspension was irradiated by a 300 W Xe lamp equipped with 450 ±
10 nm band-pass filter for 1h. During the reaction with only mono-
substituted product in the initial stage of reaction, which is
reasonable for parallel calculation. The average intensity of
irradiation was found to be 0.443 W and 0.265 W before and after
the reaction flask by a power meter (Newport; 843-R). So, total
absorb light energy ΔE = (0.443 – 0.265) × 3600 = 640.8 J,
Irradiation for 1h, n = 95% × 0.1 mmol = 0.095 mmol,
energy per photon E0 = hc/λ = 6.63 × 10-34 × 3 × 108/(450 × 10-9) =
4.42 ×10-19 J, molar of photons np =ΔE/(E0 × NA) = 640.8/(4.42 ×10-
19 × 6.02 × 1023) = 24.083 ×10-4 mol = 2.4083 mmol, AQE(initial) = n/
np = (0.095/2.4083) × 100% = 3.94%. With the same method, we can
calculate the initial AQE 0.95% and 5.15% for UV light irradiation
(300 ± 10 nm) and NIR light irradiation (900 ± 10 nm), respectively.
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chemical bond as PPy/PVA and C3N4 (Figure S4 and S5). We firstly
explored the absorption of PVA in Figure S6 and found no
interferential signal peaks, suggesting that PVA just acted as guiding
role matrix. The UV-vis-NIR spectra of C3N4, PPy/PVA, and
CNPVPy were compared in Figure 1d. C3N4 expressed strong
absorption in the UV-vis range (300~700 nm), and PPy/PVA
expressed obvious absorption at NIR range (600~1050 nm). After
formation of CNPVPy, the absorption intensity in the whole UV-vis-
NIR range was enhanced. Also, the absorption intensity increased
with the increase of PPy/PVA content in CNPVPy heterojuction
aerogel (Figure S7a). In Figure S7b, the absorption edge is
determined by the intersection point of the tangent of the curves. For
bare C3N4 and PPy/PVA, the absorption edge is 460 nm and 546 nm,
respectively. With the increase of PPy/PVA content to C3N4
suspension, the absorption edge gradually occured red shift,
indicating that the Eg of the macrosturctures gradually decrease. It is
reasonable that the Eg of each component will be almost constant
when the heterojunction between two components form.17 That is,
tunable band structure by changing the mass ratio of C3N4 and PPy
can be realized, and the red shift of the Eg for CNPVPyx
demonstrated the intrinsic heterojunction in this macrostructures.
Especially, the absorption edge at ~535 nm of CNPVPy20 is almost
the same of PPy/PVA (546 nm), which encouraged us to use
CNPVPy20 as enhanced photocatalyst.
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DOI: 10.1039/C7TA08389K
Figure 2. Typical TEM images of (a) C3N4, (b) PPy/PVA, (c) CNPVPy20 and (d)
the corresponding high-magnification images of blue range in c. (e) HAADF
image and the corresponding STEM-EDS elemental mapping images of (f) carbon,
(g) nitrogen, (h) oxide in CNPVPy20 composites.
The morphology and hierarchical porous structure of the aligned
CNPVPy heterojuction aerogel were investigated by SEM. Typically,
C3N4 exhibits stacked nanosheets in Figure 3a and S10. The
PPy/PVA exhibits aligned 3D macroscopic structures after
directional freezing (Figure 3b and S12). Direct freezing of the
mixed colloidal solution of PPy/PVA and C3N4 followed by freeze-
drying resulted in CNPVPy macrostructures (Figure 3c~3i), which
showed long-range channel aligned structures along the whole
monolithic sample that was similar to the PVA (Figure S11) and
PPy/PVA materials (Figure 3b and S12). Compared with the smooth
surface of PVA materials (Figure S11), the surface of CNPVPy was
rougher because of the doped C3N4 nanosheets and PPy
macrospheres in PVA matrix. From lateral view of CNPVPy
aerogels, the SEM image of such composite material exhibited
numerous unique interconnected honeycomb structures with an
average pore size of 2~5 μm (Figure 3d). Also, we explore the effect
of the doping amount of C3N4 nanosheets on the morphology of the
formed CNPVPy heterojuction aerogel. As can be seen in Figure 4
Figure 1. (a) FT-IR, (b) XRD, (c) wide scan XPS, and (d) UV-vis-NIR spectra of
C3N4, PPy/PVA, CNPVPy3.5, and CNPVPy20. (i, ii, iii, and iv represent C3N4,
PPy/PVA, CNPVPy3.5, and CNPVPy20, respectively).
The microstructures of the formed CNPVPy heterojuction aerogel
were further confirmed by TEM, EDX, high angle annular dark field
scanning transmission electron microscopy (STEM-HAADF) image,
and the element mapping images. Figure 2a and S8 showed that
C3N4 exhibited nanoscale sheets approximately 200 nm. In Figure 2b
and S9, the mean diameter of PPy microspheres was uniformly at
~110 nm. After the formation of CNPVPy, PPy microspheres and
C3N4 nanosheets were uniformly distributed in PVA matrix, and the
C3N4 distributed around the PPy microspheres and closely contacted
with PPy (Figure 2c~d). The STEM-HAADF image and element
mapping images indicated that O, N and C uniformly dispersed in
the CNPVPy heterojuction aerogel (Figure 2e~h).
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and S13, under the low doping of C3N4, aligned 3D macrostructurs unoccupied molecular orbital (LUMO) levels of the PPy was
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DOI: 10.1039/C7TA08389K
were generated (Figure 4). However, when the amount of C3N4 energetically lower than that of conduction band (CB) of C3N4,
increased, the structure became fluffier, and a large number of electron transfers easily from PPy to C3N4. The coupling between n-
detached fragments presented (Figure S13), indicating that PVA C3N4 and p-PPy semiconductors is believed to build up an internal
matrix play an important role to form aligned macrostructures.
electric field to form p-n junction, causing the enhanced charge
separation and thus greatly suppressing e/h+ recombination rate.30,35
As a result, the aforementioned synergism could help to quickly
separate charge and effectively retard the recombination between
photo-generated e/h+ pairs, so as to enhance photocatalytic activity
of CNPVPy.
Figure 3. Typical SEM image of (a) C3N4, (b) PPy/PVA, (c) simulated image of
CNPVPy20, (e) the photographs of CNPVPy20, (d) vertical view and (f) lateral
view of CNPVPy20, (g, h, i) High-magnification images of red range i, ii, iii in f.
The insets are the corresponding enlarged images of blue ranges.
Figure 4. SEM images of orderly aligned self-assembly composites CNPVPy. (a,
b) CNPVPy30, (c, d) CNPVPy25, (e, f) CNPVPy15, (g, h) CNPVPy10, (i, j)
CNPVPy8, (k, l) CNPVPy5, (m, n) CNPVPy3.5, (o, p) CNPVPy2.
The band gap (Eg) of a semiconductor can be calculated from the
Tauc plot, the curve of converted (αhv)n vs hv from the UV-vis-NIR
diffuse reflectance spectra (Figure S14), in which h, v, and α are the
Planck constant, light frequency, and absorption coefficient,
respectively. n =1/2 is for an indirect band gap material and n = 2 is
for a direct band gap material. From Figure 5a, a standard
relationship of (αhv)2 = A(hν − Eg) was then used to draw the Tauc
plots for C3N4 NSs, PPy/PVA, and CNCPVPy20.17 Eg values of 2.24
eV and 2.77 eV for the individual PPy/PVA and C3N4, respectively,
were extracted by extrapolating a straight-line of the linear part of
the curve in the lowest energy area to (αhv)2 = 0. Using the same
method, the Eg values of PPy/PVA and C3N4 in the final
CNCPVPy20 were determined to be 2.09 eV and 2.69 eV, which
were close to that of individual PPy/PVA and C3N4. In order to
confirm the charge separation between C3N4 nanosheets and PPy in
CNPVPy heterojuction aerogel, photoluminescence (PL) spectrum
was performed because of its intrinsic origin from radiative
recombination of free charge carriers.34 Generally, suppression of e-
/h+ recombination leads to a photoluminescence quenching. As
shown in Figure 5b, CNPVPy20 has much lower PL intensity than
free C3N4 under the same C3N4 content. That is to say, charge
recombination process of CNPVPy20 can be greatly inhibited by
coupling with PPy as a result of e-/h+ redistribution driven by the
band offsets.
Figure 5. (a) The Tauc plots of C3N4 nanosheets, PPy/PVA, CNCPVPy3.5, and
CNCPVPy20. (b) PL spectra of different aqueous samples with the same
component proportion as CNPVPy20. (c) Energy band diagram of the CNPVPy20
nanojunction system.
3.2. The direct arylation of heteroaromatics under irradiation at
room temperature
Based on the strong absorption band of CNPVPy heterojuction
aerogel in the whole UV-vis-NIR range, We decided to utilize the
catalyst to study the direct arylation of heteroarenes between
diazonium salts and heteroarenes under the different wavelength
According to the estimated conduction, valence band edges, and
Eg values of PPy and C3N4 (Figure 5a and S15), we proposed a
mechanism schematic of charge separation for CNPVPy
heterojuction aerogel illustrated in Figure 5c. C3N4 and PPy
harvested light, and absorbed photon promoted electron from the
ground states into an excited states. Given that the lowest
o
light irradiation for 1 h at 25 ± 1 C using a thermostatic control
device to eliminate the effect of light on the temperature of the
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reaction system. As shown in Figure 6a, under the different applicability of CNPVPy, the method was subsequentVlyiewexAtrteicnledOednlintoe
DOI: 10.1039/C7TA08389K
irradiation wavelength within 1 h, the yields and apparent quantum other aryldiazonium salts derivatives, furan and thiophene. Table 1
efficiency (AQE) of CNPVPy20 to the direct arylation of between 4- showed the results of these arylations. Gratifyingly, the presence of
nitrobenzenediazonium tetrafluoroborate (1) and furan (2) are electron-withdrawing groups as well as electron-donating groups on
different. The highest yield (95%) with AQE (3.94%) at the the benzene ring could be proceeding smoothly in good yield,
wavelength of 450 ± 10 nm indicates that visible light contributed to especially for furan derivatives in yields up to 88% (3a-3h).
the reaction mostly. The NIR light could also enhance the However, the corresponding yields from thiophene were lower than
photocatalytic activity of CNPVPy, while only 27% and 10% yields that of furan, while they were still up to 77% (3i~3p). The
were obtained when illuminated under UV light and without light CNPVPy20 catalyst is preferable to that of other reported
illumination, respectively. The action and absorption spectra of photocatalysts.36-38
CNPVPy20 coincided in the range of 450 nm~900 nm, which was
thought that the photoinduced arylation was driven by the absorption
of CNPVPy20. So we further tested the transient photocurrent
density of C3N4, PPy, and CNPVPy20. In Figure 6b, the transient
photocurrents were measured during repeated ON/OFF illumination
cycles under visible light irradiation. The photocurrent of
CNPVPy20 showed 2 times and 6.6 times as high as that of C3N4
and PPy under visible light, respectively. The photocurrent
enhancement of the CNPVPy20 collectively confirmed the presence
of photoelectron transfer in photocatalysis, which could be attributed
to the synergetic effect of C3N4 and PPy.
Figure 7. Optimization of the catalyst using EtOH as solvent with white LED
lamp irradiation.
Table 1. Substrate scope of furan and thiophene with aryldiazonium salts.
Figure 6. (a) Action and absorption spectra of CNPVPy20 for C-H arylation of
heteroarenes reaction under different laser irradiations for 1 h at 25 ± 1 oC. The
corresponding wavelength captured for AQE calculation are located at 300 ± 10
nm, 450 ± 10 nm, 550 ± 10 nm, 650 ± 10 nm, 800 ± 10 nm, and 900 ± 10 nm.
Note: all the reaction used a thermostatic control device to eliminate the effect of
light on the temperature of the reaction system. (b) Transient photocurrent density
versus time plotted for C3N4, PPy, and CNPVPy20 in 0.01 M Na2S electrolyte
under 400 nm~700 nm.
The activity of CNPVPy20 for the direct arylation of
heteroaromatics was then investigated under visible irradiation. For
our purpose, we first investigated the optimal condition for the
arylation between 4-nitrobenzenediazonium tetrafluoroborate (1) and
furan (2). The corresponding conversions were presented in Table S1.
Among the different solvents tested (Table S1 entries 1-8), ethanol is
considered the superior solvent for the photoreaction under 1 h
irradiation at r.t. (Table S1 entry 2). We also found that 12% yield
was obtained without irradiation (Table S1 entry 9), and the yield
was only 56% when the reaction temperature was 60 oC without
irradiation (Table S1 entry 10). Meanwhile, the photocatalytic
activities of C3N4, PPy/PVA, simply mixed C3N4 and PPy/PVA were
also carried out under visible light irradiation at 25 ± 1 oC. As shown
in Figure 7, the photocatalytic activity of CNPVPy20 is the highest
among the C3N4, PPy/PVA, simply mixed C3N4 and PPy/PVA. Also,
yields increased with increasing the ratio of PPy/PVA, which may be
caused from the Eg decrease.(Figure S7b) However, when the ratio
of PPy/PVA further increased beyond 20 (CNPVPy20), the yield
decreased, because of excess PVA surrounded C3N4-PPy junction
3.3. Robust test and recycling of the Catalyst
hindering effective contact with reactant. To illustrate the general
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The recyclability of CNPVPy20 was then evaluated because cyclical photocatalysts under irradiation. To investigate Vtihewe ArpticlaleuOsniblinlee
DOI: 10.1039/C7TA08389K
stability is an inevitable topic of photocatalyst in green system. The mechanism of this arylation in the presence of CNPVPy20 catalyst,
arylation between 4-nitrobenzenediazonium tetrafluoroborate (1) and the standard reactions with diazonium salt 1b (R = 4-Cl) were
furan (2) was performed in each cycle under the same reaction carried out and the azoether
4 was detected by UV-vis
conditions as described above except using the recovered catalyst. spectroscopy.36,38 As shown in Figure 8b, in the standard reaction
After each run, CNPVPy20 was isolated by filtration and soaked without CNPVPy20, an absorption maximum at 412 nm could be
with EtOH, and CNPVPy20 was directly used for next cycle. It was detected after 20 min and the intensity decreased slowly for more
demonstrated that the catalyst could be reused at least 8 times with than 40 min. When the reaction was conducted without furan, slight
above 90% yields (Figure S16). Furthermore, after the 8 times cycle, maximum was observed over a period of 15 min. These results
the used catalyst showed little morphology change (Figure 8a and indicated that diazonium salt reacted with the alcohol to form an
S17), indicating CNPVPy20 still keeps long-range pore alignment. intermediate azoether 4b, and furan as a base could help to form
Meanwhile, the peaks and element contents of C1s, N1s, and O1s are azoether 4b. It was hardly to detect absorption maximum over a
negligible change in high-resolution XPS spectra (Figure S18), period of 2 h in the standard reaction, which could be due to the
suggesting the stability of CNPVPy20 in this reaction system.
enhanced catalytic performance of CNPVPy20 to conver azoether 4a
to aryl radical 5b, which has no absorption peak at 450nm.
Based on above experiment results, we proposed a plausible
mechanism for the direct arylation of heteroaromatics (Figure 8c),
which is similar to that mediated by TiO2 and Fe3O4@Cu2-xS-MoS2
frameworks.36,38 First, diazonium salt 1 reacts with ethanol to
generate azoether 4 which then produces aryl radical 5 excited by
single electron transfer (SET) excitation from the excited state of
CNPVPy. The generated aryl radical 5 is easily reacted with
heteroaromatic 2 to form radical intermediate 6 which is further
oxidized by CNPVPy to form carbocation intermediate 7 by a radical
chain transfer mechanism. In another path, the radical intermediate 6
can be also oxidized by azoether 4 to generate aryl radical 5. And
finally, the intermediate 7 forms the final arylated heteroarenes 3.
Moreover, CNPVPy with aligned microstructures architectures
generate numerous hierarchical pore structures. In Figure S19 and
Table S2, the BET surface area and pore size of CNPVPy20 are
35.3874 cm2/g and 18.3170 nm, respectively. The pore structures
can be acted as nanoreactors to help the reaction substrates
efficiently contact with the surface of C3N4 nanosheets and PPy
39
macrospheres.38,
Hierarchical structures allow scattering and
multi-reflection of irradiated light sources and thus increasing
quantities of photogenerated electrons and holes functioning,
endowing with enhanced photocatalytic activity.40,41 Thus the
photocatalytic efficiency is enhanced by the unique structure of the
photocatalyst. (Figure 8d).
4. Conclusion
In summary, we obtained gram-scale 3D aligned heterojuction
aerogel of of PPy/C3N4 NSs by directional freezing of PPy/PVC
composites and C3N4 NSs. In such structure, the PPy and C3N4
formed p-n heterojunctions, in which the PPy directly injected the
photogenerated electrons into the C3N4. In addition, tunable band
structure can be realized by changing the mass ratio of C3N4 and PPy.
Therefore, such constructed frameworks help to synergistically
promote the charge transfer abilities and retards the recombination of
electronhole pairs of PPy/C3N4. Because of its long-range channel
aligned structures along the whole monolithic sample and good light
capture both in visible range and NIR range, the synthesized 3D
aligned heterojuction aerogel of PPy/C3N4 NSs catalyst exhibited
unexpected photocatalytic activity and robust cycling for the direct
arylation of heteroaromatics at 25 0C, superior to the single- and
mixed-component systems, and recently reported photocatalysts. Our
Figure 8. (a) Typical SEM image of reused CNPVPy20 after 8th photocatalytic
cycles. (b) Reaction kinetic of formation and decomposition of azoether 4b
(R
= 4-Cl) via detection of absorption maximum at 412 nm. (c) Postulated
reaction mechanism of CNPVPy20 for photo-mediated direct C−H arylation of
heteroarenes at room temperature. (d) The scheme to illustrate the proposed
reaction pathway of tetrafluoroborate and heteroarenes in heterojunction aerogel.
3.4. Mechanistic Studies
Previous studies suggested that alcohol could react with the
diazonium salt to generate an intermediate azoether 4, which
exhinited a strong absorption at 412 nm, and such intermediate plays
an important role to form the target products in the presence of
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work thereby not only provides a new approach to prepare aligned 18 S. Ghosh, N. A. Kouame, L. Ramos, S. Remita,ViAew. ADrtiaczlezOi,nlAine.
Deniset-Besseau, P. Beaunier, F. Goubard, P. H. Aubert, H.
Remita, Nat. Mater., 2015, 14, 505-511.
19 X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M.
DOI: 10.1039/C7TA08389K
metal-free heterojunction aerogels based on the 2D semiconductor
materias and 1D conducting polymers, but also opens up a new field
for the development of metal free heterojuction aerogels as a new
type of highly efficient visible and NIR light induced heterogeneous
photocatalysts. The employment of 3D aligned heterojuction aerogel
of PPy/C3N4 NSs as a heterogeneous photocatalyst for other types of
reactions and the development of other 1D/2D heterojuction aerogels
as highly efficient visible and NIR light induced heterogeneous
photocatalysts are currently underway in our laboratory.
Carlsson, K. Domen, M. Antonietti, Nat. Mater., 2009, 8, 76-
80.
20 L. Zhang, P. Liu, Z. Su, Polym. Degrad. Stabil., 2006, 91
,
2213-2219.
21 D. Chowdhury, D. Paul, A. Chattopadhyay, Langmuir 2005,
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22 M. Sevilla, P.Valle‐Vigón, A. B. Fuertes. Adv. Funct. Mater.,
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23 H. Tang, J. Wang, H. Yin, H. Zhao, D. Wang, Z. Tang, Adv.
Mater., 2015, 27, 1117-1123.
24 L. Wang, J. Schindle, J. A. Thomas, C. R. Kannewurf, M. G.
Conflicts of interest
Kanatzidis, Chem. Mater., 1996, 7, 1753-1755.
25 G. Ma, H. Peng, J. Mu, H. Huang, X. Zhou, Z. Lei, J. Power
Sources 2013, 229, 72-78.
There are no conflicts to declare.
26 K. Muto, T. Hatakeyama, J. Yamaguchi, K. Itami, Chem. Sci.,
2015,
27 G.X. Li, C. A. Morales-Rivera, Y. Wang, F. Gao, G. He, P.
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Acknowledgements
7
This work was supported by the National Natural Science
Foundation of China (21671088, 21401091, 51631001, 21431002,
and 21501080). State key Project of Research and Development of
China (2017YFA0206301, 2016YFA0200102)
28 T. P. Yoon, M. A. Ischay, J. Du, Nat. Chem., 2010, 2, 527-
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DOI: 10.1039/C7TA08389K
Table of contents
Gram-Scale Synthesis of Aligned C3N4-Polypyrrole Heterojuction
Aerogel with Tunable Band Structures as an Efficient Visible and Near
Infrared Light -Driven Metal-Free Photocatalyst
Jian Liu,a Hao Wang, a Jinwei Bai, a Tianrong Li, a Yanmei Yang, a Yong Peng, *,b Baodui Wang*, a
Gram-scale 3D aligned heterojuction aerogel of PPy/C3N4 NSs with long-range pore
alignment over the whole length of the monolithic sample and a channel structure were
prepared. Such aerogel as metal free photocatalyst displayed excellent photocatalytic activity
for direct arylation of heteroaromatics under visible-near infrared (NIR) light irradiation at
room temperature.