Construction of novel Ag/HKUST-1/g-C3N4 towards enhanced photocatalytic activity for the degradation of pollutants under visible light

Article abstract of DOI:10.1039/c9ra08489d

A novel Ag/metal-organic framework/graphitic carbon nitride (Ag/HKUST-1/g-C₃N₄, AHC) photocatalyst was prepared via an in situ growth strategy and photo-deposition technique for environmental remediation. The as-obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), N₂ adsorption-desorption isotherm measurement, UV-vis diffuse reflection spectroscopy (UV-vis DRS), and photoluminescence (PL) spectroscopy. The results indicated that the hybrids have large surface area, mesoporous structure and enhanced visible-light absorption. The as-prepared hybrid samples exhibited considerable improvement in photocatalytic activity and stability for rhodamine B (RhB) degradation under visible light irradiation (λ > 420 nm). In addition, they also have good adsorption properties. Compared to the pure g-C₃N₄ and Ag/g-C₃N₄, the 5% AHC photocatalyst showed superior photocatalytic activity. Moreover, 5% AHC exhibits good photocatalytic activity even after four cycles. Additionally, the active species trapping and electron spin resonance (ESR) experiments indicated that h⁺ and ·OH were the main active species.

Full text of DOI:10.1039/c9ra08489d

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Construction of novel Ag/HKUST-1/g-C3N4
towards enhanced photocatalytic activity for the
Cite this: RSC Adv., 2019, 9, 41591
degradation of pollutants under visible light†
ab
a
*
Yu Qiao,
Qian Han,a Dong Li,d Hongji Li,a Bing Wei,c Guangbo Che,
a
b
*
Wei Jiang and Weisheng Guan
A novel Ag/metal–organic framework/graphitic carbon nitride (Ag/HKUST-1/g-C3N4, AHC) photocatalyst
was prepared via an in situ growth strategy and photo-deposition technique for environmental
remediation. The as-obtained samples were characterized by X-ray diffraction (XRD), transmission
electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy
(XPS), N2 adsorption–desorption isotherm measurement, UV-vis diffuse reflection spectroscopy (UV-vis
DRS), and photoluminescence (PL) spectroscopy. The results indicated that the hybrids have large
surface area, mesoporous structure and enhanced visible-light absorption. The as-prepared hybrid
samples exhibited considerable improvement in photocatalytic activity and stability for rhodamine B
(RhB) degradation under visible light irradiation (l > 420 nm). In addition, they also have good adsorption
properties. Compared to the pure g-C3N4 and Ag/g-C3N4, the 5% AHC photocatalyst showed superior
photocatalytic activity. Moreover, 5% AHC exhibits good photocatalytic activity even after four cycles.
Additionally, the active species trapping and electron spin resonance (ESR) experiments indicated that h+
and $OH were the main active species.
Received 17th October 2019
Accepted 20th November 2019
DOI: 10.1039/c9ra08489d
rsc.li/rsc-advances
semiconductor photocatalysts, and the respective advantages
are brought into play. The low-concentration and highly toxic
Introduction
pollutants in wastewater are adsorbed on the surface of the
photocatalysts, and then decomposed into H2O, CO2 and non-
toxic inorganic acids.
With environmental degradation becoming increasingly prom-
inent throughout the world, the effective control of environ-
mental pollutants and government intervention have become
urgent to address this issue. During the past decade, a variety of
strategies have been employed to solve this problem.1–3 The
semiconductor photocatalytic technique using the renewable
solar energy to degrade organic pollutants to non-toxic products
has received word-wide attention.4 Simultaneously, visible-
light-driven photocatalysts have attracted considerable atten-
tion because sunlight consists of about 50% visible light and
only about 4% ultraviolet (UV) light. Additionally, nano-
materials with adsorptive properties can also increase the rate
of removal of pollutants in water. For this purpose, materials
with good adsorptive properties are combined with the
Graphitic carbon nitride (g-C3N4), a metal-free polymeric
semiconductor, has attracted extensive attention due to its good
thermochemical stability, electronic properties, optical charac-
teristics and suitable band gap (around 2.70 eV).5–7 To date, the
recent improvements with regards to g-C3N4 have been pub-
lished in a large number of articles. For example, the different
morphologies of g-C3N4, such as g-C3N4 quantum dots (QDs)
and two-dimensionally ultrathin g-C3N4 nanosheets, were
prepared successfully. In addition, g-C3N4 as a novel visible-
light-driven photocatalyst has been reported for hydrogen
evolution and pollutant degradation.8–12 However, as a single-
phase catalyst, g-C3N4 is restricted by its high recombination
rate of photoinduced electron–hole pairs, leading to its poor
quantum efficiency and low photocatalytic activity. In order to
improve the photocatalytic performance of g-C3N4, numerous
methods have been employed, such as doping of metal/
nonmetal elements13,14 and coupling with other semi-
conductors.15,16 Hong et al. reported a novel direct solid-state Z-
scheme V2O5/g-C3N4 heterojunction obtained by a facile in situ
growth strategy for the rst time. The photocatalytic perfor-
mance was evaluated by the degradation of RhB and TC under
visible light irradiation (l > 420 nm).17 Zhu et al. synthesized
aKey Laboratory of Preparation and Application of Environmental Friendly Materials
(Jilin Normal University), Ministry of Education, Changchun 130103, P. R. China.
E-mail: guangboche@jlnu.edu.cn; Tel: +86-434-3295100
bSchool of Environmental Science and Engineering, Chang'an University, Xi'an
710054, P. R. China. E-mail: guanweisheng@263.net
cInstitute of Green Chemistry & Chemical Technology, Jiangsu University, Zhenjiang
212013, P. R. China
dSchool of Materials Science and Engineering, Liaoning Technical University, Fuxin
123000, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra08489d
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a highly dispersed Ag/Fe3O4/g-C3N4 composite photocatalyst by experiments. Furthermore, the stability of AHC composite
means of the selective photo-deposition technique. It exhibited photocatalyst was tested by the circulation experiments.
a signicantly improved photocatalytic activity and stability for
degrading TC, besides retaining the recyclable magnetic prop-
Experimental section
Materials and methods
erty.18 Recent studies have shown that noble metal loading
proved to be an effective way of increasing the photocatalytic
activity. This is because the localized surface plasmon reso-
nance (SPR) of a noble metal can enhance the localized electric
elds by several distinct orders of magnitude at the noble metal/
g-C3N4 photocatalyst interface. Noble metal nanoparticles can
serve as antennae to capture photoinduced electrons from the
conduction band (CB) of g-C3N4, thus decreasing the recombi-
nation of the electron–hole pairs and further improving the
photocatalytic activity. Ag, as a low-cost and widely used noble
metal, has been employed to synthesize composite photo-
catalysts, such as Ag@AgCl, Ag/AgX/GO and Ag/g-C3N4.
However, there are still some disadvantages that restrict the
application of Ag/g-C3N4 in environmental remediation,19
particularly its low surface area.20 Therefore, in order to further
enlarge the adsorption capacity and improve the stability, it is
necessary to modify Ag/g-C3N4 using porous materials.
AgNO3, Cu(NO3)2, trimesinic acid (H3BTC), melamine, ethanol,
acetic acid, polyethylene glycol 800 (PEG 800), triethanolamine
(TEOA), isopropanol (IPA), and 5,5-dimethyl-1-pyrroline N-oxide
(DMPO) were analytical grade agents and purchased from
Aladdin (China). RhB was analytically pure and used without
further purication. All of the phase compositions and crystal
structures of the prepared samples were determined by the
powder X-ray diffraction (XRD) method using Cu Ka radiation (l
˚
¼ 1.54178 A) (D/MAX-2500 diffractometer, Rigaku, Japan) with
a Cu Ka radiation source (k ¼ 1.54056) over the 2q range of 5–
90ꢀ at a scanning rate of 10ꢀ minꢁ1. The morphologies and
structure details of the as-prepared samples were obtained
using scanning electron microscopy (SEM, JSM-6510) and
transmission electron microscopy (TEM, JEM-2100F). X-ray
photoelectron spectroscopy (XPS) maps were obtained by
a Thermo ESCALAB 250X (America) electron spectrometer using
a 150 W Al Ka X-ray source. The specic surface area of these
samples was determined from the nitrogen absorption–
desorption data and Brunauer–Emmett–Teller (BET) measure-
ments (ASAP2020, Micromeritics, USA). The UV-vis diffused
reectance spectra (DRS) of the samples were obtained from
a UV-vis spectrophotometer (UV-2450, Shimadzu, Japan); BaSO4
was used as a reectance standard. The photoluminescence
(PL) spectra for the solid samples were obtained on a F4500
(Hitachi, Japan) photoluminescence detector. The photocurrent
and electrochemical impedance spectroscopy (EIS) measure-
ments were conducted using a CHI660C (Chenhua Instruments
Co. Shanghai China) electrochemical workstation. The electron
spin resonance (ESR) analysis was conducted with a Bruker EPR
A 300-10/12 spectrometer.
As important porous materials, metal–organic frameworks
(MOFs) have the advantages of adjustable pore size and pore
surface, ultra-low density, ultra-high specic surface area, as
well as insolubility in common solvents among other charac-
teristics, compared with traditional pore materials, such as
zeolites, mesoporous silica and activated carbon.21–23 Conse-
quently, much attention has been paid to the potential appli-
cations of MOFs in gas capture and storage, chemical
separation, sensor devices, drug delivery and catalysis.24–29
Wang et al. reported the synthesis of g-C3N4/Ti-benzenedi-
carboxylate (MIL-125(Ti)) heterostructure photocatalysts. By
optimizing the compositions, an environment-friendly
composite was found to show excellent photocatalytic activity
towards RhB dye degradation under visible-light irradiation.
The signicantly reduced electron–hole recombination rate of
photocatalysts was not only caused by the sufficient surface
contact of g-C3N4 and MIL-125(Ti), but also induced by the
synergic effect between g-C3N4 and MIL-125(Ti).30 He et al.
manifested the photocatalytic hydrogen production efficiency
Photocatalysts preparation
Synthesis of g-C3N4 sheets. The metal-free bulk g-C3N4 was
of CdS nanoparticles embedded on MIL-101 under visible-light synthesized by the thermal treatment of melamine, according to
irradiation, which was superior to that of bare CdS due to the a previous report.33 Typically, 10 g melamine was put into a 50
large specic surface area, more active adsorption sites, pho- mL alumina crucible with a cover, and then heated in a muffle
ꢁ1
ꢀ
ꢀ
tocatalytic reaction centers and good visible-light harvesting furnace at a rate of 2.3 C min and kept for 4.0 h at 550 C.
capability.31 Liang et al. used a simple self-assembly strategy for Aer being cooled to room temperature, the resulting products
increasing the interfacial contact to prepare MIL-53(Fe)-reduced were collected and milled into powder in an agate mortar for
graphene oxide nanocomposites.32 Therefore, combining MOFs further use.
with semiconductor photocatalysts can improve the photo-
catalytic efficiency.
Synthesis of AHC. The HKUST-1/g-C3N4 composites were
synthesized according to a modied method.34 First, a certain
Herein, we report a facile process for the preparation of amount of g-C3N4 was dispersed into ethanol with the aid of
plasmonic AHC photocatalysts. First, the metal-free bulk g-C3N4 ultrasonication. Then, H3BTC (0.1 mmol) was added into
was synthesized by the thermal treatment of melamine. Second, a mixture of g-C3N4/ethanol (30 mL) and sonicated for 30 min to
we prepared HKUST-1/g-C3N4 composites by a facile in situ afford
a homogeneous solution. Second, copper acetate
growth strategy. Finally, the AHC composite photocatalysts were (Cu(OAc)2$H2O; 0.15 mmol) was dissolved in acetic acid solu-
successfully synthesized through a photo-deposition technique. tion (HAc: 3.0 mL; H2O: 30 mL) to form another solution.
In addition, the possible photocatalytic mechanism is tenta- Subsequently, the solutions obtained in the rst step and the
tively proposed based on the active species trapping and ESR second step were mixed and stirred for an hour at high speed.
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The products were collected by centrifugation at 11 000 rpm for experiments, 1.0 mM TEOA and 1.0 mM IPA were respectively
3.0 min, washed with ethanol several times, and nally dried in employed as the scavengers for h+ and $OH. The method is
ꢀ
an oven at 60 C for 10 h.
similar to the former photocatalytic activity test. Furthermore,
A certain amount of the as-prepared HKUST-1/g-C3N4 was theꢁESR technique was further used to discover the existence of
mixed with 200 mL of deionized water by ultrasonication for 30 $O2 and $OH radicals in the photocatalytic reaction system
min. Then, 1.0 mL of 5% polyethylene glycol (PEG) 800 solution under visible light (l > 420 nm). The samples for ESR
was added and the dispersion was stirred for another 10 measurements were prepared as follows: 10 mg of each of the
min.35,36 For deposition of silver on the surface of HKUST-1/g- samples was dissolved in 0.5 mL methanol (DMPO–$O2ꢁ) or 0.5
C3N4, a photo-deposition technique was applied as follows. mL deionized water (DMPO–$OH) and then, 45 mL DMPO was
Initially, 5.0 mL of AgNO3 solution (0.015 mol Lꢁ1) was added to added to the solution, followed by ultrasonic dispersion for 5.0
the dispersion. Then, the suspension was transferred to a water- min.
cooled reactor (250 mL) and irradiated under a PLS-SXE300 Xe
lamp with 250 W illumination intensity for 60 min. The theo-
retical value of the Ag loading amount was 4.0 wt%. Subse-
quently, 5% AHC, 6% AHC, and 7% AHC were synthesized.
Results and discussion
Structure and morphology analysis
The XRD patterns of the as-prepared g-C3N4, 4% AHC, 5% AHC,
6% AHC, 7% AHC, HKUST-1 and Ag (PDF#87-0718) are shown
in Fig. 1. As can be seen, pure g-C3N4 shows one diffraction peak
ascribed to the (100) plane at about 2q ¼ 13.1ꢀ, corresponding to
the characteristic interlayer structural packing. Another
diffraction peak ascribed to the (002) plane at 27.2ꢀ corresponds
to the interplanar stacking peaks of the aromatic systems.37,38
Compared with pure g-C3N4, the 4% AHC, 5% AHC, 6% AHC
and 7% AHC samples exhibit two additional peaks at 38.2ꢀ and
44.4ꢀ, which are indexed to the (111) and (200) planes of Ag,
respectively. Such observations indicate that the Ag NPs were
successfully loaded on g-C3N4 aer irradiation.39 Nevertheless,
the diffraction peaks of HKUST-1 were not detected in the
samples of AHC, which may be due to its low loading content
and high dispersity.
Photocatalytic experiments
The photocatalytic activities of the as-prepared samples were
investigated by the degradation of RhB under visible light
irradiation. The typical photocatalytic experiment was per-
formed in a Pyrex reactor with reuxed water at room temper-
ature and illuminated using a 250 W xenon lamp with a 420 nm
cut off lter. In detail, 50 mg of the powered samples was
dispersed in the 100 mL of 0.5 mg Lꢁ1 pollutant aqueous
solutions. In order to ensure the adsorption equilibrium, the
suspension was continuously stirred for 45 min in the dark
before irradiation. The concentration changes of the pollutant
aqueous solutions were monitored by measuring the UV-vis
absorption of the suspensions at 15 min intervals. During
irradiation, 5.0 mL of the suspension was taken out and
centrifuged (10 000 rpm, 2.0 min) to remove the photocatalyst
before measurements. Then, the supernatant was monitored by
a UV-vis spectrophotometer at the characteristic absorption of
RhB at 553 nm.
The morphologies and microstructures of g-C3N4 and 5%
AHC composite photocatalyst were further investigated by TEM
and SEM (Fig. 2). As shown in Fig. 2(a), it could be observed that
g-C3N4 displays two dimensional and overlapped lamellar
structures, whereas as shown in Fig. 2(b), for 5% AHC, irregular
Ag NPs and HKUST-1 particles, which covered with bits of g-
Photoelectrochemical measurements
The photocurrent and electrochemical impedance spectra tests
were performed using a CHI660C electrochemical workstation
in a standard three-electrode conguration, with the as-
prepared samples as the working electrodes, a Pt foil as the
counter electrode and an Ag/AgCl electrode as a reference
electrode. A 0.5 mol Lꢁ1 Na2SO4 aqueous solution was used as
the electrolyte. The working electrodes were prepared as the
follows: 0.3 g of each sample, 0.03 mL oleic acid and 0.01 g of
polyvinyl pyrrolidone (PVP) were dissolved in 3.0 mL of ethanol
to form a suspension, which was then spin coated onto a 15 mm
ꢂ 20 mm indium tin oxide (ITO) conducting glass electrode by
ꢀ
using a drop-casting method and annealed at 500 C in air for
2.0 h. The visible light source was provided by a 300 W xenon
lamp with a 420 nm cut off lter was used to lter out UV light (l
< 420 nm).
Active species trapping and ESR experiments
It is well known that the holes (h+), superoxide radical ($O2
)
ꢁ
and hydroxyl radicals ($OH) were the major reactive species for
Fig. 1 XRD patterns of the as-prepared g-C3N4, 4% AHC, 5% AHC, 6%
the photocatalytic oxidation. In the active species trapping AHC, 7% AHC and HKUST-1 composites.
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Fig. 2 TEM images of (a) pure g-C3N4; (b) 5% AHC; (c) SEM image of 5% AHC.
C3N4 nanosheets, were found to be embedded in the wrinkling HKUST-1 and 5% AHC samples are shown in Fig. 3(a). The
g-C3N4 lamellar structure. The SEM image of 5% AHC (Fig. 2(c)) spectra indicated that C and N exist on the surface of g-C3N4,
and its EDS elemental mappings (Fig. S1†) further conrm that Cu, C and O exist on the surface of HKUST-1, and Ag, C, N and O
the 5% AHC composite photocatalyst was composed of Ag, Cu, exist in 5% AHC. The doublet peaks of Ag 3d5/2 and Ag 3d3/2
C, N and O elements. Thus, the composite obtained by were located at about 366.08 eV and 372 eV, respectively, which
combining Ag, HKUST-1 and g-C3N4 was successfully were very close to the binding energy values of Ag (Fig. 3(b)).40
fabricated.
The Cu 2p XPS spectra for HKUST-1 and 5% AHC are shown in
Herein, based on the above experimental results, it could be Fig. 3(c). For HKUST-1, the peaks of Cu 2p3/2 and Cu 2p1/2
demonstrated that the AHC composite photocatalysts were located at 934 eV and 953.6 eV, respectively. However, when
successfully synthesized, which were expected to facilitate the HKUST-1 combined with g-C3N4, the Cu 2p3/2 peak of 5% AHC
transfer of photo-generated charge carriers.
displayed a slight shi. Furthermore, the well-known ‘‘shake-up
satellite bands’’ (at the 937–946 eV regions) were clearly
observed, which was indicative of the paramagnetic chemical
Chemical states analysis
state of Cu2+ 41 Correspondingly, for HKUST-1, the peaks of O 1s
.
XPS spectra were employed to analyze the detailed chemical
status of the samples. The spectra were recorded to analyze the
surface chemical compositions of g-C3N4, HKUST-1 and 5%
AHC composite photocatalyst. The XPS survey spectra of g-C3N4,
are located at 530.5 eV and 531.5 eV, respectively. However, one
of the O 1s peaks of 5% AHC also exhibited a slight shi to the
lower binding energy at about 530.4 eV (Fig. 3(d)). Fig. 3(e)
Fig. 3 XPS spectra of pure g-C3N4, HKUST-1 and 5% AHC samples: (a) survey spectrum; (b) Ag 3d; (c) Cu 2p; (d) O 1s; (e) C 1s and (f) N 1s.
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displays two strong peaks at 283.03 eV and 287.03 eV, which technique. As shown in Fig. 5(a), it is clear that pure g-C3N4
were assigned to C 1s, which should originate from HKUST-1. shows absorption wavelengths from UV to visible range up to
The binding energy of C 1s in the 5% AHC composite photo- 460 nm, and its band gap is around 2.70 eV, which agrees well
catalyst also exhibited a slight shi to the low energy region with the previous reports.44,45 Compared with that of pure g-
compared to that of HKUST-1. These binding energy shis in C3N4, the absorption edge of the 5% Ag/g-C3N4 composite
the spectra could be attributed to the presence of strong inter- photocatalyst shows a systematic slight red shi. This may be
actions between g-C3N4 and HKUST-1. In the N 1s spectrum attributed to the surface plasmon resonance (SPR) effect of Ag
(Fig. 3(f)), two peaks at 396.7 eV and 399.3 eV are separately species.46 Furthermore, HKUST-1 displays an absorption band
ascribed to sp2 N bonded to two carbon atoms (C]N–C) and lower than about 460 nm, resulting in a low visible light utili-
tertiary nitrogen (N–(C)3).42,43 The XPS results validate the zation efficiency. However, as the wavelength increases, the
coexistence of Ag, g-C3N4 and HKUST-1 in the 5% AHC response of HKUST-1 to the visible light becomes stronger.
composite photocatalyst. In addition, it is also concluded that Therefore, the 5% AHC composite photocatalyst has a strong
there are strong interfacial interactions between g-C3N4 and absorption under visible light irradiation, suggesting that it can
HKUST-1, which could facilitate the rapid transfer of photoin- efficiently utilize the visible light for the degradation of organic
duced charge and improve the photocatalytic performance of dyes pollutants. Furthermore, in order to conrm the relative
5% AHC.
band gap position of the samples, the band gap energy (Eg) of
sample was calculated by the following formula based on the
DRS results (1):
Specic surface area and optical properties
ahn ¼ A(hn ꢁ Eg)n/2
(1)
The N2 adsorption–desorption isotherms of pure g-C3N4 and
5% AHC are shown in Fig. 4. It can be seen that the isotherms
are similar and all of them are classical type IV, suggesting the
presence of abundant mesopores and macropores. The BET
surface area of pure g-C3N4 was found to be 10.4 m2 gꢁ1, while
that of 5% AHC was 27.4 m2 gꢁ1, which was higher than that of
pure g-C3N4. Correspondingly, the total pore volume of 5% AHC
was 0.12 cm3 gꢁ1, which was higher than that of pure g-C3N4
(0.08 cm3 gꢁ1). The pore size distribution (PSD) of 5% AHC was
calculated via nonlocal density functional theory (NLDFT) (the
inset in Fig. 4). The 5% AHC composite photocatalyst exhibits
a dominant pore width of about 20 nm and several smaller
pores in the pore-size range of 60–100 nm. The PSD curves agree
with the shape of the nitrogen isotherms (Fig. 4) and suggest
that the polymers are predominantly mesoporous and
macropores.
where a is the absorption coefficient, n is the optical frequency,
Eg is the band gap energy, and A is a constant. As shown in
Fig. 5(b), the band gap energy of g-C3N4, HKUST-1, 5% Ag/g-
C3N4 and 5% AHC can be estimated to be 2.71 eV, 2.89 eV, 2.63
eV and 2.11 eV, respectively. Similarly, the band gap energy of g-
C3N4, HKUST-1, 5% Ag/g-C3N4 and 5% AHC can also be inferred
from a plot of (ahv)2 versus energy (hv).
The transfer and recombination processes of electron–hole
pairs in the interfaces of the composite photocatalysts are one
of most important factors that affect the photocatalysis activity.
PL analysis was used to estimate the photoinduced charge
carrier separation and transfer ability of the as-prepared
samples. It is well acknowledged that the higher PL intensity
indicates the fast recombination of the charge carriers, result-
ing in lower photocatalytic activity. Fig. 6 presents the PL
spectra of g-C3N4 and AHC with an excitation wavelength of 325
nm.47 Pure g-C3N4 has a strong peak at around 466 nm in the PL
spectrum, which can be attributed to the band gap recombi-
nation of electron–hole pairs.48 However, the quenching
phenomenon exists in the PL spectrum of AHC, which is related
to good electrical conductivity and induced SPR effect of the Ag
species. Therefore, AHC inhibits a more effective recombina-
tion of photogenerated charge carriers to improve photo-
catalytic activity. Moreover, the PL intensity of the 5% AHC
composite photocatalyst possessed the lowest PL intensity,
suggesting its high photocatalytic activity.
The optical properties of the as-prepared g-C3N4, HKUST-1,
5% Ag/g-C3N4 (the method of synthesis was in the ESI), and 5%
AHC composite photocatalyst were measured via the UV-vis DRS
Photocatalytic activity
The photocatalytic activities of the as-prepared samples were
evaluated by the degradation of RhB under visible light irradia-
tion (l > 420 nm). As shown in Fig. 7(a), the concentration of RhB
could reach adsorption–desorption equilibrium aer reaction
under dark for 45 min. It can be seen that the adsorption of 5%
AHC was signicantly better than that of pure g-C3N4 and 5% Ag/
g-C3N4. Hence, more RhB molecules can be adsorbed, which was
benecial for improving their visible light activities.
Fig. 4 N2 adsorption–desorption isotherms of the as-prepared pure
g-C3N4 and 5% AHC samples (closed symbols: desorption; open
symbols: adsorption; inset: pore size of 5% AHC).
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Fig. 5 (a) UV-vis diffuse reflectance spectra of pure g-C3N4, HKUST-1 and 5% AHC; (b) the plots of (ahv)2 versus hv for the band gap energies of
pure g-C3N4, HKUST-1 and 5% AHC.
The photocatalytic activities of the AHC composite photo-
Furthermore, for the photocatalysts, the durability and
catalysts with different Ag contents as well as the blank, pure g- stability are also crucially important for their practical appli-
C3N4 and 5% Ag/g-C3N4 were tested under visible light irradia- cations. Aer each cycling test, the samples were collected and
tion. As shown in Fig. 7(b) and (c), the blank test without washed with distilled water and absolute ethanol three times
a catalyst reveals that the photolysis of RhB occurs very slowly, and then, the as-obtained samples were dried in vacuum at 80
which can be neglected. As shown in Fig. 7(b), when the content ꢀC for 12 h for further cycling. The recycling capability of 5%
of Ag species increases to 5.0 wt%, the photocatalytic activity of AHC was veried by performing a four-run test for the degra-
AHC is optimal and the degradation rate of RhB reaches 87.4% dation of RhB. As shown in Fig. 8, no signicant inactivation of
while undergoing a photocatalytic reaction for 90 min. In the photocatalysts was observed, revealing that the 5% AHC
particular, with respect to the introduction of Ag species, Ag NPs composite photocatalyst possesses high stability for its practical
deposited on the surface of HKUST-1/g-C3N4 not only induce application. Moreover, in order to further testify the reusability
a plasmon resonance effect to widen light absorption ability, and stability of the catalysts, the XRD pattern of the 5% AHC
but also export photogenerated electrons on the HKUST-1/g- composite photocatalyst aer 4th run was obtained (Fig. S2†).
C3N4, which more efficiently improve the separation ability of There was no apparent change in intensity aer the photo-
the electron–hole pairs, thus resulting in the higher photo- catalytic reaction, which once again proved the stability of the
catalytic activity. However, the excess Ag species will lead to the catalyst.
reduction of photocatalytic activity owing to the shielding effect
that weakens light harvesting ability of AHC. Fig. 7(c) indicates
that the pure g-C3N4 sample exhibits low photocatalytic
Photocatalytic activity photocurrents and EIS analyses
To further understand the separation and recombination of
electron–hole pairs in the composite photocatalysts, the
photocurrents over all the as-prepared samples have been
investigated. Generally, the corresponding relationship was that
performance for the degradation of RhB under visible light.
Furthermore, using 5% Ag/g-C3N4, 70% RhB was degraded.
However, the as-synthesized 5% AHC composite photocatalyst
can efficiently enhance the photocatalytic activity compared
with the g-C3N4 and 5% Ag/g-C3N4. This is mainly because
HKUST-1 not only has good adsorption performance, but can
also be used as a medium for the rapid transfer of photo-
generated charges.
At the same time, the kinetic behavior of photocatalytic
degradation using the composite photocatalysts was further
investigated, and the results are shown in Fig. 7(d) and (e); all of
the plots t well with the pseudo-rst-order correlation:
ln(C0/C) ¼ kt
where C is the concentration of RhB remaining in the solution
at irradiation time t, C0 is the initial concentration at t ¼ 0, and
k is the apparent rate constant for degradation. The order of the
RhB degradation rate for the as-prepared photocatalysts was 5%
AHC (0.021 minꢁ1) > 4% AHC (0.016 minꢁ1) > 6% AHC (0.015
minꢁ1) > 5% Ag/g-C3N4 (0.014 minꢁ1) > 7% AHC (0.012 minꢁ1) >
pure g-C3N4 (0.007 minꢁ1) > blank (0.00095 minꢁ1).
Fig. 6 Photoluminescence (PL) spectra of pure g-C3N4 and AHC.
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Fig. 7 (a) The dark adsorption–desorption equilibrium of RhB in catalysts; (b and c) the photocatalytic activities of as-prepared samples for RhB
degradation under visible-light (l > 420 nm); (d and e) the first-kinetic of the photocatalytic degradation of RhB.
the higher photocurrent implies the higher electron–hole transfer rate occurring at the contact interface between the
separation efficiency, thus leading to higher photocatalytic working electrode and electrolyte solution. The smaller radius
activity.49 It can be clearly observed from Fig. 9(a) that rapid and of the Nyquist arc indicates lower charge transfer resistance.52
stable photocurrent responses are detected in all the as- As shown in Fig. 9(b), all the AHC samples feature signicantly
prepared samples as electrodes under visible light illumination. smaller radius than pure g-C3N4. All the above results indicated
Clearly, the 5% AHC composite photocatalyst presents the that introducing Ag had a positive effect on the separation and
higher photocurrent response intensity than other composites, transfer of photogenerated charge carriers. Moreover, the arc
which indicates that it has the lowest electron–hole recombi- radius of the 5% AHC composite photocatalyst was the smallest,
nation rate. This conclusion can be conrmed by PL measure- demonstrating the fast interfacial charge transfer property of
ments. In addition, the separation of photogenerated charge 5% AHC.
carriers can be illustrated by the EIS.50,51 EIS was performed to
explore the process of their charge transfer resistance. The
radius of the arc in the EIS Nyquist plot represented the charge
HPLC-MS spectra analysis
In order to investigate the intermediates and pathways in depth
during the degradation of RhB, HPLC-MS experiments were
performed. As shown in Fig. 10, RhB was depicted with a frag-
ment ion at m/z 443, which is in good consistency with depro-
tonated RhB. As shown in Fig. 11, on progression of the
reaction, RhB is resolved into small molecules with m/z ¼ 443,
m/z ¼ 415, m/z ¼ 387, and m/z ¼ 274. The rest of the fragments
may be derived from other complex intermediate products.
Finally, with the progress of photocatalytic reaction, the inter-
+
mediate products would be degraded to CO2, H2O, and NH4
and some other small inorganic molecular materials.
Photocatalytic mechanism
In order to explore the photocatalytic mechanism of RhB,
different active species trapping experiments were conducted.
The interaction between the capturer and photocatalyst played
a dominant role in impacting the degradation efficiency of
organic molecules.18 Therefore, a series of radical trapping
experiments were performed using TEOA (a quencher of h+) and
Fig. 8 The four reaction cycles of 5% AHC photocatalyst for the
degradation of RhB.
IPA (a quencher of $OH) over the 5% AHC composite
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Fig. 9 (a) Transient photocurrent response of the as-prepared samples; (b) electrochemical impedance spectroscopy of as-prepared samples.
photocatalyst.53–55 As shown in Fig. 12 and S3,† for the 5% AHC no ESR signals were observed when the reaction was performed
sample (87.4%), it can be seen that when TEOA was added, the in the dark. As expected, for the 5% AHC sample, the four
photocatalytic degradation rate signicantly decreased (8%), characteristic peaks of the DMPO–$OH adducts were observed
indicating that h+ was the predominant active species. When (Fig. 13(a)), indicating that the $OH radicals were produced in
IPA was added into reaction solution, the degradation rate of 5% AHC reaction systems. In addition, weak characteristic
ꢁ
RhB was also inhibited (51%), suggesting that $OH also played peaks of DMPO–$O2 adducts were observed (Fig. 13(b)), indi-
an important role in the photocatalytic degradation process.
cating that $O2ꢁ radicals were produced in the 5% AHC reaction
ꢁ
The ESR teꢁchnique was further used to detect the presence of system. However, the number of $O2 radicals were less than
$OH and $O2 radicals in the 5% AHC photocatalytic reaction the number of $OH radicals.
systems under visible light. As can be seen from Fig. 11a and b,
Fig. 10 m/z of degraded RhB over the 5% AHC composite photocatalyst: (a) the initial RhB solution; (b) degradation of RhB after 30 min; (c)
degradation of RhB after 60 min; (d) degradation of RhB after 90 min.
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Fig. 11 Degradation process of RhB with its intermediate products over 5% AHC.
Thus, the inuence order of the activated species in the $OH can oxidize RhB (eqn (8) and (9)). Moreover, the EVB value
ꢁ
process of RhB photodegradation was h+ > $OH > $O2
.
of g-C3N4 (+1.57 eV) was lower than the standard redox potential
Therefore, according to the above results of the active species of $OH/OHꢁ (+1.99 eV), and photogenerated holes (h+) le on
trapping experiments and ESR analysis, we can makꢁe the VB of g-C3N4 could not react with H2O or OHꢁ to generate
a conclusion that the h+ and $OH play major roles and $O2
the active oxidative species $OH. Hence, h+ on the VB of g-C3N4
plays a secondary role in the 5% AHC photocatalytic degra- would be consumed by directly decomposing RhB (eqn (10)).56,57
dation reaction. The major routes of the photocatalytic degradation of RhB
Combined with the above discussion, the possible photo- under visible-light irradiation are proposed as follows:
catalytic mechanism for the degradation of RhB with 5% AHC
composite photocatalyst is proposed, as shown in Fig. 14. First,
g-C3N4 + hn / g-C3N4 (eꢁ + h+)
(2)
under visible-light irradiation, g-C3N4 can be excited and photo-
induced electrons and holes are generated (eqn (2)). Electrons
are rapidly transferred from the g-C3N4 conduction band to the
HKUST-1 surface. Due to this, the organic bridging ligands of
HKUST-1 can serve as antennae to harvest visible light and then
effectively transfer the energy to the inorganic copper-oxygen
cluster parts. Subsequently, the photogenerated electrons on
HKUST-1 rapidly migrate to the Ag surface. Some of the
photoelectrons on the g-C3N4 conduction band indirectly
transfer to Ag NPs via HKUST-1 and serve as terminal electron
acceptors, thus prolonging the photogenerated electron lifetime
and facilitating the charge separation in the entire photo-
catalytic system. This result has been conrmed by the PL
spectra, photocurrent test and EIS analyses. Second, the pho-
togenerated electrons react with O2 absorbed on the surface of
Ag NPs to produce the active species $O2ꢁ (eqn (3)) because the
ECB value of g-C3N4 (ꢁ1.13 eV) was more negative than the
potential of O2/$O2ꢁ (ꢁ0.33 eV). Then, $O2ꢁ may also react with
H+ to further produce $OH (eqn (4)–(7)) based on the results of
ꢁ
Fig. 12 Photodegradation rates of RhB over 5% AHC in the present of
different scavengers.
active species trapping and ESR analysis. Thus, both $O2 and
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ꢁ
Fig. 13 ESR spectra of the 5% AHC sample for (a) DMPO–$OH and (b) DMPO–$O2
.
Fig. 14 Possible mechanism for the photo-induced charge transfer route of 5% AHC photocatalyst.
O2 + eꢁ / $O2
(3)
(4)
ꢁ
Conclusions
ꢁ
$O2 + 2H+ + eꢁ / H2O2
H2O2 + eꢁ / $OH + OHꢁ
OHꢁ + h+ / $OH
In summary, in situ growth strategy and photo-deposition tech-
nique were utilized to fabricate the 5% AHC composite photo-
catalyst. The 5% AHC composite photocatalyst exhibited superior
photocatalytic performance in the degradation of RhB compared
with pure g-C3N4 and other percentages of AHC. The enhanced
photocatalytic activity under visible light irradiation can be
ascribed to the adsorption and superior electron transport prop-
erty of HKUST-1 and surface SPR of Ag, which greatly promoted
the efficient separation of the photoexcited electron–hole pairs as
well as depressed the recombination of the charge carriers.
Finally, the present study provides the feasibility of designing new
composite photocatalysts via a facile synthetic method and gives
a new direction to design other high-efficiency composite semi-
conductor photocatalysts for practical applications.
(5)
(6)
H2O + h+ / $OH + H+
(7)
ꢁ
$O2 + RhB / OX. prod.
(8)
$OH + RhB / OX. prod.
h+ + RhB / OX. prod.
(9)
(10)
41600 | RSC Adv., 2019, 9, 41591–41602
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18 Z. Zhu, Z. Y. Lu, D. D. Wang, X. Tang, Y. S. Yan, W. D. Shi,
Y. S. Wang, N. L. Gao, X. Yao and H. J. Dong, Appl. Catal.,
B, 2016, 182, 115–122.
Conflicts of interest
There are no conicts to declare.
19 X. J. Bai, S. C. Yan, J. J. Wang, L. Wang, W. J. Jiang, S. L. Wu,
C. P. Sun and Y. F. Zhu, J. Mater. Chem. A, 2014, 2, 17521–
17529.
20 Y. Wang, J. S. Zhang, X. C. Wang, M. Antonietti and H. R. Li,
Angew. Chem., Int. Ed., 2010, 49, 3356–3359.
21 A. Dailly and E. Poirier, Energy Environ. Sci., 2011, 4, 3527–
3543.
Acknowledgements
This study was supported by the National Natural Science
Foundation (No. 21905107, 21902060, 21576112, 21607051),
Natural Science Foundation Project of Jilin Province
22 F. J. Ma, S. X. Liu, C. Y. Sun, D. D. Liang, G. J. Ren, F. Wei,
Y. G. Chen and Z. M. Su, J. Am. Chem. Soc., 2011, 133,
4178–4181.
23 H. K. Chae, D. Y. Siberio-Perez, J. Kim, Y. Go, M. Eddaoudi,
A. J. Matzger, M. O'Keeffe and O. M. Yaghi, Nature, 2004, 427,
523–527.
(20180623042TC,
20180101181JC,
20150623024TC-19,
20170520147JH), the Project of Human Resources and Social
Security Department of Jilin Province (2017956), and the Project
of Education Department of Jilin Province (JJKH20180775KJ,
JJKH20191009KJ).
24 Y. Qiao, Y. F. Ma, W. Jiang, X. Y. Wang, W. S. Guan, G. B. Che,
W. K. Li and F. Qin, CrystEngComm, 2018, 20, 7782–7794.
25 J. D. Xiao, Q. C. Shang, Y. J. Xiong, Q. Zhang, Y. Luo and
S. H. Yu, Angew. Chem., Int. Ed., 2016, 128, 9389–9393.
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