Cu2(OH)PO4/reduced graphene oxide nanocomposites for enhanced photocatalytic degradation of 2,4-dichlorophenol under infrared light irradiation

Article abstract of DOI:10.1039/c7ra12684k

Sparked by the growing environmental crises, photocatalytic degradation of chlorophenols with inexhaustible solar energy is expected to be converted into actual applications. Here, we report the preparation of the nanocomposite of Cu₂(OH)PO₄ and reduced graphene oxide (Cu₂(OH)PO₄/rGO) through a one-step hydrothermal method and examined its infrared-light photocatalytic activity in the degradation of 2,4-dichlorophenol (2,4-DCP). As evidenced by the absorption spectra and the degradation of 2,4-DCP, Cu₂(OH)PO₄/rGO exhibited enhanced infrared light-driven photocatalytic activity compared to pure Cu₂(OH)PO₄ and was very stable even after repeated cycling. More importantly, the introduction of hydrogen peroxide (H₂O₂) could combine the photocatalytic and photo-Fenton effects into one reaction system and maximize the infrared light photocatalytic efficiency. Typically, the rate constant of Cu₂(OH)PO₄/rGO and H₂O₂ was more than 6.25 times higher than that of only Cu₂(OH)PO₄/rGO, and almost 10 times greater than the value for pure Cu₂(OH)PO₄. Further, a plausible mechanism for the enhanced photocatalytic properties of Cu₂(OH)PO₄/rGO has been discussed. These findings may help the development of novel hybrid photocatalysts with enhanced infrared light photocatalytic activity for applications in the treatment of chlorophenol-contaminated wastewater.

Full text of DOI:10.1039/c7ra12684k

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Cu (OH)PO /reduced graphene oxide
2
4
nanocomposites for enhanced photocatalytic
degradation of 2,4-dichlorophenol under infrared
light irradiation†
Cite this: RSC Adv., 2018, 8, 3611
Chenyang Zhang, Zhen Du, Ruyi Zhou, Peng Xu, Xinghua Dong, Yanyan Fu,*f
ab
b
b
d
bc
e
a
bc
bc
Qing Wang, Chunjian Su,* Liang Yan
*
and Zhanjun Gu
*
Sparked by the growing environmental crises, photocatalytic degradation of chlorophenols with
inexhaustible solar energy is expected to be converted into actual applications. Here, we report the
2 4 2 4
preparation of the nanocomposite of Cu (OH)PO and reduced graphene oxide (Cu (OH)PO /rGO)
through a one-step hydrothermal method and examined its infrared-light photocatalytic activity in the
degradation of 2,4-dichlorophenol (2,4-DCP). As evidenced by the absorption spectra and the
degradation of 2,4-DCP, Cu
activity compared to pure Cu
importantly, the introduction of hydrogen peroxide (H
Fenton effects into one reaction system and maximize the infrared light photocatalytic efficiency.
Typically, the rate constant of Cu (OH)PO /rGO and H was more than 6.25 times higher than that of
only Cu (OH)PO /rGO, and almost 10 times greater than the value for pure Cu (OH)PO . Further,
a plausible mechanism for the enhanced photocatalytic properties of Cu (OH)PO /rGO has been
2
(OH)PO
4
/rGO exhibited enhanced infrared light-driven photocatalytic
and was very stable even after repeated cycling. More
) could combine the photocatalytic and photo-
2
(OH)PO
4
2 2
O
2
4
2 2
O
2
4
2
4
Received 22nd November 2017
Accepted 5th January 2018
2
4
discussed. These findings may help the development of novel hybrid photocatalysts with enhanced
infrared light photocatalytic activity for applications in the treatment of chlorophenol-contaminated
wastewater.
DOI: 10.1039/c7ra12684k
rsc.li/rsc-advances
accumulate in the human body, and subsequently cause
Introduction
disturbances in the structure of cellular bilayer phospholipids,
9
Pesticides are chemical substances widely used in horticulture, nally causing carcinogenic effects. Therefore, a large number
1–4
forestry and public health – and, of course, in agriculture of methods have been developed to remove chlorophenols from
10
11
where the unwanted pests that carry or transmit diseases can be water, including adsorption, biological degradation and
5
12
repelled and killed. Over the past decades, tremendous prob- electrochemical degradation. However, adsorption merely
lems, such as water contamination and overall ecological concentrates chlorophenols, but does not degrade them into
degradation, have been caused by the misuse and over-use of less toxic compounds. Biological treatment suffers from the
6
–8
pesticides.
For example, chlorophenols readily bio- drawbacks of slow reaction rate and the need for strict control of
suitable pH and temperature. For these reasons, an effective
technique needs to be proposed for the removal of chlor-
ophenols from different water systems.
a
College of Mechanical and Electronic Engineering, Shandong University of Science
and Technology, Qingdao 266590, P. R. China. E-mail: suchunjian2008@163.com
b
Alternatively, semiconductor nanomaterials have been
CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute
of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China. emerging as efficient photocatalysts for the degradation of
1
3
E-mail: yanliang@ihep.ac.cn; zjgu@ihep.ac.cn
chlorophenols, such as TiO
2
in the ultraviolet range (<400 nm)
c
14
University of Chinese Academy of Sciences, Beijing 101408, P. R. China
and Ag PO in the visible range (400–800 nm). For the opti-
3
4
d
CAS Key Laboratory of Standardization and Measurement for Nanotechnology,
mized use of solar energy, efficient and stable photocatalysts
that are capable of harvesting infrared light, which accounts for
ca. 50% of solar energy, are required. Much effort has been
under way so far to tentatively seek the efficient photocatalysts,
National Center for Nanoscience and Technology, Beijing 100190, P. R. China
e
School of Material Science and Engineering, Shandong University of Science and
Technology, Qingdao 266590, P. R. China
f
State Key Lab of Transducer Technology, Shanghai Institute of Microsystem and
16
Information Technology, Chinese Academy of Sciences, Changning Road 865, including Bi
2
WO
6
(ref. 15) and WS
2
,
for the degradation of
Shanghai 200050, P. R. China. E-mail: fuyy@mail.sim.ac.cn
organic pollutants, but not chlorophenols, under infrared irra-
diation. The limitation of photocatalysts for pesticide
†
Electronic supplementary information (ESI) available: Fig. S1–S16 and Table S1.
See DOI: 10.1039/c7ra12684k
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 3611–3618 | 3611

View Article Online
RSC Advances
Paper
the generated electrons at CuO
infrared light irradiation (forming Cu sites) can be transferred
4
(OH) trigonal bipyramids under
III
I
20,21
to the neighboring CuO (OH) octahedra (forming Cu sites).
4
2
III
Subsequently, the produced Cu sites are responsible for
oxidizing chlorophenols (Fig. 1, route 1). Nevertheless, Cu (OH)
PO , as an infrared-activated photocatalyst, suffers from the fast
2
4
recombination of photogenerated electron–hole pairs. To
overcome this limitation, graphene with the high-surface area
and electrical conductivity should act as an avenue for driving
photogenerated carriers away from the surface of Cu (OH)
2
22,23
III
PO4,
facilitating more efficient generation of Cu sites
which are applied to degrade chlorophenols and, as a result,
II
become Cu sites (Fig. 1, route 2). Moreover, to fully use the
advantage of this photocatalyst, we use hydrogen peroxide
2 4
Scheme 1 Schematic illustration of the preparation of Cu (OH)PO /
rGO.
(H
2 2
O ) as an electron acceptor to react irreversibly with the
I
produced Cu sites (that is, photo-Fenton reaction, similar to
Cu -induced Fenton reaction ) to further enhance the separa-
I
24
tion efficiency of photogenerated electron–hole pairs; while the
produced Cu can effectively promote the generation of highly
photocatalytic degradation under infrared light is essentially
due to the insufficient photocatalytic activity that results from
charge-carrier recombination as well as the low-photon energy
of infrared light, and the inhibition of charge transfer because
of the mismatched band energy alignment between each other.
To overcome the above limitation, graphitic carbon nitride
coupled with upconversion nanoparticles can extend the
activity towards the infrared region for the photodegradation of
chlorophenols. However, the efficiency of this photocatalyst is
rather low due to the narrow absorption band of light at 980 nm.
Therefore, infrared light responsive photocatalysts for the
degradation of chlorophenols are still being actively pursued.
Herein, we synthesized the infrared-light active nano-
composites composing of copper hydroxide phosphate
I
active hydroxyl radicals (HOc) which are capable of oxidizing
II
chlorophenols, and return to Cu sites. More importantly, these
two processes can complete the full photocatalytic circle and be
occurred repeatedly, resulting in the combination of the pho-
tocatalytic and photo-Fenton effects in one reaction system and

nally maximizing the photocatalytic activity of Cu
2 4
(OH)PO /
17
rGO for the mineralization of chlorophenols to CO and H O. As
2
2
expected, our results show that, compared to pure Cu (OH)PO ,
2
4
the as-prepared Cu (OH)PO /rGO exhibits remarkably
2
4
enhanced photocatalytic activity for the degradation of 2,4-
dichlorophenol (2,4-DCP, a typical type of chlorophenols) under
infrared light irradiation (>800 nm). Typically, the photo-
catalytic rate constant of Cu
2
(OH)PO
4
2
/rGO and H
2 2
O is almost
(
Cu (OH)PO ) and reduced graphene oxide (rGO) by a one-step
2 4
10 times higher than that of only Cu
(OH)PO under the same
4
hydrothermal method (Cu
PO , which consists of CuO
2
(OH)PO
4
/rGO) (Scheme 1). Cu
2
octahedron and CuO
2
4
(OH)
(OH)
condition. In addition, there is no appreciable loss of photo-
4
4
(OH)
catalytic activity aer repeated cycles, and the morphology and
trigonal bipyramid, has been considered as a promising pho-
tocatalyst for the degradation of organic pollutants under
structure of Cu (OH)PO /rGO remain nearly unchanged.
2
4
18,19
Finally, mechanism of enhanced photocatalysis under infrared
light is further proposed and discussed in detailed.
visible light.
With the presence of the distorted polyhedrons
in the crystal structure, Cu (OH)PO is even responsive to
2
4
infrared light and hence displays photocatalytic activity in the
20
infrared range. In previous studies, it has been reported that
Experimental
Materials and chemicals
Graphite (99%) and 2,4-dichlorophenol (2,4-DCP, 97%) were
purchased from Alfa Aesar. Chemical reagents including diso-
dium hydrogen phosphate dodecahydrate (Na
2 4 2
HPO $12H O,
9
8%), copper(II) nitrate trihydrate (Cu(NO ) $3H O, 99%) and
3
2
2
sodium hydroxide (NaOH, 96%) were obtained from Aladdin
Chemical Co. Potassium permanganate (KMnO 99%),
hydrogen peroxide (H , 30 wt%) and concentrated sulfuric
acid (H SO , 98 wt%) were obtained from Beijing Chemical Co.
4
,
2 2
O
2
4
Terephthalic acid (TA, 99%) was purchased from Sigma-Aldrich.
HUVECs (human umbilical vein endothelial cells) and cell
counting kit-8 (CCK-8) were acquired from Wuhan Boster Bio-
logical Technology Ltd (Wuhan, China). Dulbecco's Modied
Eagle Medium (DMEM), fetal bovine serum (FBS) and Peni-
cillin–Streptomycin Solution were purchased from Gibco
(Shanghai, china). All of the reagents were analytical grade and
Fig. 1 Schematic diagram for the photocatalytic mechanism of pure
Cu (OH)PO and Cu (OH)PO /rGO under infrared light irradiation.
2 4 2 4
3612 | RSC Adv., 2018, 8, 3611–3618
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
were used without further purication. In addition, deionized was rstly stirred in the dark for 100 min to achieve an
water was used in the whole experimental process.
adsorption/desorption equilibrium between the photocatalyst
and 2,4-DCP. Aerwards, an appropriate amount of H O2
2
Synthesis of nanocomposites with different mass ratios of
aqueous solution was added into the obtained mixture prior to
photo-irradiation if necessary. Then, the mixture was irradiated
by infrared light (>800 nm) using a 300 W xenon lamp installed
an 800 nm cut-off lter, where the transmission spectrum of the
cut-off lter was shown in Fig. S1.† Then, 1.5 mL of mixture was
collected at varied irradiation time, centrifuged, and nally
analyzed by a UV-3900 UV-vis spectrophotometer to determine
Cu (OH)PO to graphene oxide
2
4
A series of nanocomposites of Cu (OH)PO4 and rGO with
2
various mass ratios of graphene oxide (GO) were synthesized by
20
a simple one-step hydrothermal method (Cu
Briey, 4 mL of Cu(NO solution (1.0 M) and an appropriate
amount of GO dispersion (2.0 mg mL , ESI†) were mixed into
0 mL of deionized water under constantly stirring for 15 min.
Then, 2 mL of Na HPO solution (1.0 M) was added to the above
2
(OH)PO/rGO).
3 2
)
ꢀ1
the concentration of 2,4-DCP in the absence of H O2 or by
2
2
a Multi TOC Analyzer (2100, Analytik Jena AG Corporation) to
2
4
detect total organic carbon (TOC) in the presence of H
photodegradation of 2,4-DCP by other Cu (OH)PO/rGO were
also performed under the similar condition. In addition, the
stability of Cu (OH)PO/rGO was studied by a separated photo-
2 2
O . The
mixture. Aer stirring for another 1 h, the pH value of the ob-
tained mixture was adjusted to ꢁ7.00 through gradually adding
NaOH aqueous solution. The resulting suspension was then
transferred into a 45 mL sealed teon-scaled autoclave and kept
2
2
ꢂ
degradation experiment which repeatedly re-employed the used
samples for the next cycle under the identical conditions. Aer
at 120 C for 6 h. Aer naturally cooling to room temperature,
the products were collected by centrifuging, washed with
deionized water several times, and nally dried overnight in
a freezer dryer for further use. According to the mass ratios of
Cu (OH)PO and GO in the original mixtures, nanocomposites
each photocatalytic process, Cu (OH)PO/rGO was recovered by
2
centrifugation, washed with deionized water, and then dried in
the freezer dryer under vacuum for 24 h before until the
subsequent reaction cycle.
2
4
were labelled as 1 : 0.001, 1 : 0.002, 1 : 0.005, 1 : 0.01, 1 : 0.02,
: 0.05 and 1 : 0.1, respectively.
1
Detection of hydroxyl radical
Characterization
The generation of HOc by nanocomposites under infrared light
irradiation was evaluated using terephthalic acid (TA). The
concentration of hydroxyl radical (HOc) is determined via
monitoring the uorescence of 2-hydroxy terephthalic acid
Transmission electron microscopy (TEM) images were ob-
tained on a JEM-2100 microscope at an acceleration voltage of
25
2
00 kV. Morphologies of samples were characterized using
scanning electron microscopes (FE-SEM, S-4800, an accelera-
tion voltage of 10 kV, Hitachi High-technologies, Japan) with
an energy-dispersive X-ray (EDX) acceleration voltage analyzer.
X-ray diffraction (XRD) patterns was obtained from a Bruker
D8 Advance X-ray diffractometer (Bruker, USA) with Cu-Ka
(
TAOH, the maximum uorescence peak at 435 nm) which is
formed from the reaction of HOc with TA. Taking sample
: 0.005 as an example, ve groups were obtained: group I (TA
and infrared light); group II (TA, H and infrared light);
group III (sample 1 : 0.005 and infrared light); group IV (TA,
sample 1 : 0.005 and infrared light); group V (TA, H
1
2 2
O
ꢂ
ꢀ
1
˚
radiation (l ¼ 1.5406 A) at a scanning rate of 10 min and
2 2
O ,
ꢂ
ꢂ
a scanning range from 10 to 90 . X-ray photoelectron spec-
troscopy (XPS) measurement was carried out with an
ESCALab220i-XL spectrometer using a twin-anode Al-Ka X-ray
source (1486.6 eV). Micro-Raman spectra were achieved from
sample 1 : 0.005 and infrared light). The nal working
concentrations were 50 mg mL , 100 mM and 500 mM for
sample 1 : 0.005, H O and TA, respectively. Aer infrared
2 2
light irradiation, the changes at the 435 nm uorescence
ꢀ1
a
Raman Spectroscope (Renishaw inVia plus, United
emission peak were recorded.
Kingdom) under ambient conditions with 514 nm excitation
from an argon ion laser. Ultraviolet-visible (UV-vis) data were
acquired with a U-3900 spectrophotometer (Hitachi, Ltd., Cytotoxicity assay for Cu (OH)PO /rGO nanocomposite
2
4
Japan). Ultraviolet-visible-infrared (UV-vis-IR) diffuse reec-
tance spectra were recorded at room temperature on an Agi-
lent Cary 500 UV-vis-IR Spectrometer equipped with an
The cytotoxicity of Cu
2
(OH)PO
4
/rGO nanocomposites with
1
: 0.005 ratio to HUVECs (human umbilical vein endothelial
cells) was assessed by using the CCK-8 assay. HUVECs were
integrating sphere using BaSO
4
as a reference. The photo-
3
placed in 96-well plates at a density of 8 ꢃ 10 cells per well,
luminescence spectra were obtained using a Horiba Jobin
Yvon FluoroLog3 spectrometer.
where HUVECs were maintained in the DMEM medium with
ꢂ
10% FBS, 1% penicillin/streptomycin for 24 h at 37 C in 5%
CO
2
. Then, the cells were incubated with different concentra-
Photocatalytic performance measurement
ꢀ1
tions of sample 1 : 0.005 (5, 10, 20, 40, 60, 80, and 90 mg mL ).
The photocatalytic activity of Cu
the degradation of 2,4-DCP in the presence and absence of H
2
(OH)PO
4
/rGO was evaluated by Aer incubation for another 24 h, the cell medium was removed
and replaced with 100 mL of fresh culture medium containing
2 2
O
under infrared light irradiation. In addition, the photocatalytic 10 mL of CCK-8 solution per well for detecting the cell viability.
activity of Cu (OH)PO and sample 1 : 0.005 to 2,4-DCP was Aer 1 h of incubation, cell viability on HUVECs was assayed by
2
4
explored under visible light irradiation. Typically, 60 mL measuring the absorbance at 450 nm using a microplate reader
ꢀ
1
mixture of sample 1 : 0.005 (60 mg) and 2,4-DCP (30 mg mL
)
(Thermo Scientic, Multiscan MNK3).
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 3611–3618 | 3613

View Article Online
RSC Advances
Paper
Results and discussion
XRD patterns of pure Cu
2
(OH)PO
4
and Cu
2 4
(OH)PO /rGO with
different mass ratios of Cu
2
(OH)PO
4
and GO nanosheets with
the thickness of ꢁ1.2 nm (Fig. S2†) are displayed and compared
in Fig. 2. According to JCPDS card no. 360404, all the diffraction
2
peaks are associated with the orthorhombic phase of Cu (OH)
26
PO4. Moreover, it is clear that the position of the diffraction
peaks of Cu (OH)PO /rGO basically keep unchanged, which
2
4
indicates that the crystalline structure of Cu (OH)PO is not
2
4
affected aer the introduction of GO. Additionally, no stacking-
ꢂ
related (002) diffraction peaks of graphene (at ꢁ26 for graphite Fig. 3 SEM images of pure Cu (OH)PO4 (a) and Cu (OH)PO /rGO
2
2
4
ꢂ
and ꢁ13 for graphite oxide) are detected, suggesting that the nanocomposites with 1 : 0.001 (b), 1 : 0.002 (c), 1 : 0.005 (d), 1 : 0.01
e),1 : 0.02 (f), 1 : 0.05 (g) and 1 : 0.1 (h) ratios. Inset: enlarged SEM
images of pure Cu (OH)PO (a) and Cu (OH)PO /rGO.
(
dispersion of graphene is probably close to the single-sheet level
27
2
4
2
4
in all the nanocomposites.
The morphology of pure Cu (OH)PO and Cu (OH)PO /rGO
2
4
2
4
were characterized by SEM and TEM measurement. As shown in
Fig. 3, pure Cu (OH)PO are ellipsoid-shaped with an average
and Cu (OH)PO /rGO are displayed in Fig. S4a.† From Raman
2
4
2
4
spectra of pure Cu (OH)PO and Cu (OH)PO /rGO, vibration
2
ꢀ
4
2
4
length of 3–4 mm and an aspect ratio of ꢁ3, and there are
1
peak at ꢁ972.5 cm
is the characteristics of Cu (OH)PO .
2 4
ravines on their surface that are short and narrow. In contrast,
Raman spectra of rGO exhibits two characteristic peaks corre-
2 4
with the addition of GO, Cu (OH)PO are tightly encapsulated
by (rGO) nanosheets, which is consistent with the TEM images
ꢀ
1
sponding to D band at around 1350.4 cm
disorder and defect) and G band at about 1599.6 cm
(involving rst order scattering of the tangential stretching
(involving the
ꢀ
1
shown in Fig. S3.† This indicates the presence of strong van der
28
Waals force between graphene and Cu (OH)PO . In addition,
2
4
29,30
phonon mode), respectively.
2
In comparison to rGO, Cu (OH)
on increasing the mass ratio of GO, more Cu (OH)PO is
2
4
PO /rGO also shows the typical features of graphene with the
4
encapsulated by graphene, exhibiting more obviously crinkled
and rough textures on the surface of Cu (OH)PO /rGO.
Remarkably, graphene nanosheets not only are adsorbed onto
the surface of Cu (OH)PO /rGO tightly, but also are connected
presence of D band and G band, indicating the successful
combination of Cu (OH)PO with rGO. Interestingly, it can be
2
4
2
4
obviously found from Fig. S4b† that, on progressively increasing
the proportion of GO, there is signicant red-shi in the G
2
4
or even overlapped between the adjacent microcrystals,
building interconnected conductive pathways for electron
transfer.
ꢀ
1
band; while the D band rstly blue-shis to 1361.14 cm from
samples 1 : 0.001 to 1 : 0.005, and then red-shis to
ꢀ
1
1
351.95 cm from samples 1 : 0.005 to 1 : 0.1. This indicates
The structural and chemical information of Cu (OH)PO /
2
4
the presence of van der Waals interaction alone with charge
transfer between rGO and Cu (OH)PO , and the interaction is
rGO was further studied using Raman spectroscopy and XPS
measurement. Raman spectra of GO, rGO, pure Cu (OH)PO
2
4
2
4
31
the strongest for sample 1 : 0.005. Moreover, the D/G intensity
ratios of Cu (OH)PO /rGO are larger than that of GO (I /I
.806), which suggests a decrease in the average size of the sp
domains upon reduction of GO, as well as an increase of edge
2
4
D
G
¼
2
0
32
planes and the degree of disorder. The full-scale XPS spectra of
Cu (OH)PO /rGO shown in Fig. 4a and S5† reveal the presence
2
4
of P, O, and Cu elements, which is consistent with the EDS
results shown in Fig. S6.† For the XPS spectra of pure Cu (OH)
2
PO , two main peaks are observed at about 936.61 and
4
9
55.86 eV, which can be attributed to Cu 2p3/2 and Cu 2p1/2 of
33
copper ions, respectively (Fig. 4b). More importantly, the peak
position of Cu 2p3/2 in the Cu (OH)PO /rGO rstly decreases
from 935.76 eV to 935.46 eV as the mass ratio increases from
: 0.001 to 1 : 0.005, and then increases to 936.56 eV on further
2
4
1
increasing the proportion of GO. This may be due to the
screening effect via charge transfer between rGO and pure
34
Cu (OH)PO . With low mass ratio of GO, the electron of rGO
2
4
2 4
can effectively transfer to pure Cu (OH)PO , causing the shi of
both Cu 2p3/2 and Cu 2p1/2 towards lower binding energy;
whereas, above the mass ratio of 1 : 0.005, rGO begins to
aggregate, thereby leading to inhibit electron transfer. There-
fore, we predict that sample 1 : 0.005 may be the optimal mass
2 4 2 4
Fig. 2 XRD patterns of pure Cu (OH)PO and Cu (OH)PO /rGO.
3614 | RSC Adv., 2018, 8, 3611–3618
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
ratio of GO reaches up to 1 : 0.01, the absorption peak in the
infrared region gradually decreases because that the excessive
graphene is capable of shielding the infrared light and inhibits
41,42
the photo absorption by Cu (OH)PO .
2
4
2 4
To evaluate the photocatalytic properties of Cu (OH)PO /rGO
under infrared light (l > 800 nm), 2,4-DCP, which is fairly stable
under solar light irradiation, is used as a model pollutant. To
minimize the loss of 2,4-DCP by evaporation, the temperature of
ꢂ
the solution was maintained at 20–25 C during infrared light
irradiation. Fig. 5a and S10a† shows time proles of C /C under
t
0
infrared light irradiation in the presence of Cu (OH)PO /rGO,
2
4
where C is the concentration of 2,4-DCP at the irradiation time
t
0
of t and C is the concentration in the adsorption equilibrium of
the photocatalysts before photo-irradiation. The results show
that almost no photolysis is observed without photocatalysts
aer 6 h of infrared light irradiation. 2,4-DCP is slightly
degraded in the presence of Cu
2 4
(OH)PO ; while the degradation
is remarkably accelerated with Cu (OH)PO /rGO. On progres-
2
4
Fig. 4 (a) Full-scale XPS spectra of GO and sample 1 : 0.005. (b) Cu 2p sively increasing the proportion of GO, the degradation rate
XPS spectra of pure Cu (OH)PO and Cu (OH)PO
2
4
2
4
/rGO. (c) C 1s XPS initially increases for Cu (OH)PO /rGO, and then declines.
2 4
spectrum of GO. (d) C 1s XPS spectrum of sample 1 : 0.005.
Among all the nanocomposites, sample 1 : 0.005 shows the
highest degradation rate (87.1%) aer irradiation with infrared
light for 6 h, which is rather higher than that of pure Cu
2
(OH)
ratio for the photocatalytic degradation of chlorophenols.
Furthermore, the C 1s XPS spectra of GO and Cu (OH)PO /rGO
2 4
are shown in the Fig. 4c and d. For GO, three different peaks
PO (66.4%). This enhancement clearly indicates effective
4
recombination suppression, which can be attributed to the van
der Waal heterojunction between Cu (OH)PO and rGO. This is
2
4
located at 284.70, 286.78 and 287.84 eV can be obtained, cor-
consistent with the results of uorescence spectra of pure
35,36
responding to C]C, C–OH, and C]O bonds, respectively.
4
There is also three peaks at the same position for Cu (OH)PO /
Cu (OH)PO , 1 : 0.001, 1 : 0.005 and 1 : 0.1, which is shown in
2
4
2
Fig. S11.† However, from samples 1 : 0.01 to 1 : 0.1, the degra-
dation rate decreases to 78.5% aer 6 h of infrared light
rGO (ESI, Fig. S7†); while the peak areas of C–OH and C]O
signicantly decrease compared to GO, suggesting the oxygen
functional groups are removed during the hydrothermal treat-
ment. This is good agreement with the result obtained by the
Raman spectroscopy. These demonstrate that graphene is
successfully combined with Cu
reduced effectively during the hydrothermal reaction. There-
fore, this structure of Cu (OH)PO /rGO can enhance their
photocatalytic activity under infrared light via promoting
2 4
(OH)PO and simultaneously is
2
4
37,38
charge separation of photocarriers.
The optical properties of pure Cu (OH)PO and Cu (OH)PO /
2
4
2
4
rGO are investigated by UV-vis-NIR diffuse reectance spec-
troscopy (DRS). As shown in Fig. S8a,† pure Cu (OH)PO
2
4
displays a strong absorption in the infrared region, which can
be further tted with four Gaussian peaks centered at 662, 777,
965 and 1237 nm (1.55 eV, 1.32 eV, 1.06 eV and 0.88 eV,
respectively, Fig. S8b†). According to the previous report, the
2
absorption peak at ꢁ777 nm is mainly attributed to the E –B
g
1g
II
transition for Cu sites that exists in axially elongated
CuO
4 2
(OH) octahedra, and the absorption peaks located at
ꢁ
662, 965, and 1237 nm are largely associated with d–d transi-
Fig. 5 (a) Photodegradation of 2,4-DCP over pure Cu
Cu (OH)PO /rGO nanocomposites with 1 : 0.001, 1 : 0.005 and
: 0.01 ratios with infrared light irradiation at 20–25 C. (b) UV-vis
2 4
(OH)PO and
II
tions for Cu sites that exist in axially compressed CuO
4
(OH)
2
4
20,39,40
trigonal bipyramids.
Cu (OH)PO Cu (OH)PO
On the introduction of GO into
ꢂ
1
2
4
,
2
4
/rGO exhibits enhanced optical absorption spectra the aqueous solution of 2,4-DCP with the irradia-
tion time. Inset: enlarged spectra ranges from 260 nm to 310 nm. (c)
Plots of ln(C /C) versus time for pure Cu (OH)PO and Cu (OH)PO /
0 2 4 2 4
rGO nanocomposites with 1 : 0.001, 1 : 0.005 and 1 : 0.01 ratios. (d)
Photodegradation efficiency of 2,4-DCP for sample 1 : 0.005 in the
2 4
absorption in infrared region over pure Cu (OH)PO , with red-
shi in the maximum absorption peak. (Fig. S8c, d and S9†)
Note that the color of Cu (OH)PO /rGO becomes much darker
2
4
with the increasing mass ratio of GO. Remarkably, when mass
presence of 2.4 mL of H
O
2 2
under infrared light irradiation.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 3611–3618 | 3615

View Article Online
RSC Advances
Paper
irradiation. This is largely attributed to the fact that the exces-
Furthermore, the introduction of H
2
O
2
as a sacricial elec-
sive graphene shields Cu (OH)PO , preventing absorption and tron acceptor can maximize the photocatalytic efficiency for the
2
4
blocking electron transfer between Cu (OH)PO and graphene, degradation of 2,4-DCP with Cu (OH)PO /rGO through the
2
4
2
4
as demonstrated by the results of XPS, Raman spectrum and combination of the photocatalytic and photo-Fenton effects
DRS measurements. It is worth noting that these values are still into one reaction system. Taking sample 1 : 0.005 as an
higher than that of pure Cu
ation of the absorption spectra of 2,4-DCP with reaction time in improved on the increase of the amount of H
the presence of sample 1 : 0.005. shown in Fig. 5d, when 2.4 mL H is added, the photo-
2
(OH)PO
4
. Fig. 5b shows the evalu- example, it is found that the photocatalytic activity is gradually
2 2
O
(Fig. S12†). As
2 2
O
The pseudo-rst order kinetic model is then used for the degradation efficiency of 2,4-DCP increases from 33.6% to
determination of the photocatalytic degradation rate constant 74.1% aer 2 h of infrared light irradiation. According to the
ꢀ
1
43,44
(
k, h ) which is expressed by eqn (1):
kinetic curves in Fig. S13,† the photocatalytic rate constant of
sample 1 : 0.005 and 2.4 mL H O is more than 6.25 times
2
2
ln(C
0
/C
t
) ¼ kt,
(1)
higher than the corresponding value for only sample 1 : 0.005,
and almost 10 times greater than the value for pure Cu (OH)
PO . These results conrm that the addition of H as
a sacricial acceptor can remarkably enhance the photocatalytic
efficiency of Cu (OH)PO /rGO for the degradation of 2,4-DCP
2
Fig. 5c and S10b† depict the ln(C
Cu (OH)PO and Cu (OH)PO /rGO. Pure Cu (OH)PO presents
an apparent photocatalytic rate constant of 0.190 h under the
irradiation of infrared light. However, the photocatalytic rate
0
/C
t
) versus t for pure
4
2 2
O
2
4
2
4
2
4
ꢀ
1
2
4
under infrared light irradiation. In addition, it is found from
Fig. S14† that the photocatalytic properties of sample 1 : 0.005
is not much better than pure Cu (OH)PO under visible light
constants of Cu
of pure Cu (OH)PO
Cu (OH)PO /rGO with various mass ratios, sample 1 : 0.005
shows the highest photocatalytic rate constant, which is 1.72
times than that of pure Cu (OH)PO . The photocatalytic rate
2
(OH)PO
4
/rGO for 2,4-DCP are higher than that
2
4
under the same condition. Among these
2
4
2
4
irradiation. At the same time, sample 1 : 0.005 shows the higher
degradation rate (87.1%) aer irradiation with infrared light for
2
4
6
h relative to the degradation rate (54.5%) with visible light
constant of other samples is summarized in Table S1.†
The above results clearly demonstrate that graphene plays
a signicant role in the enhanced infrared light photocatalytic
activity. A tentative photocatalytic mechanism of Cu (OH)PO /
2 4
rGO for degradation of 2,4-DCP under infrared light irradiation
is proposed and schematically illustrated in Fig. 1. Similar to
(OH)PO , copper atoms of Cu (OH)PO /rGO have two
2 4 2 4
nonequivalent crystallographic sites, including octahedral sites
rGO-OCT Cu ) and trigonal bipyramidal sites (rGO-TBP Cu ).
In this system, the distorted polyhedrons can lead to a net
dipole moment in such units, facilitating electron transfer from
rGO-TBP Cu to neighboring rGO-OCT Cu . Beneting from the
strong absorption in infrared region, Cu (OH)PO /rGO can be
excited by infrared light and therefore generate electrons
forming rGO-OCT Cu sites at the CuO
holes (forming rGO-TBP Cu sites at the CuO
bipyramids). Then, by taking advantage of the high-specic area
surface and electrical conductivity, graphene nanosheets can
provide more sites to absorb 2,4-DCP and promote photo-
generated holes transfer from rGO-TBP Cu to 2,4-DCP, which
is responsible for degrading 2,4-DCP. In parallel, rGO-OCT Cu
irradiation, which it is possible that the redox potential of 2,4-
DCP is not match the band position of Cu
sponding to visible light.
2 4
(OH)PO corre-
The mechanism of photocatalytic degradation of 2,4-DCP by
Cu (OH)PO /rGO with H O as the sacricial electron acceptor
2
4
2 2
is proposed as follows:
20
pure Cu
I
rGO-OCT Cu + H
II
ꢀ
2
O
2
/ rGO-OCT Cu + HOc + OH (6)
II
II
(
+
ꢀ
H + OH / H O
(7)
(8)
2
+
ꢀ
II
II
2
2 2
,4-DCP + HOc / H O + CO + H + Cl
2
4
It is generally accepted that H O is a better sacricial elec-
2
2
45
I
tron acceptor than O . Therefore, in the presence of H O ,
2 2 2
(
4
(OH)
2
octahedra) and
(OH) trigonal
I
III
rGO-OCT Cu produced under infrared light irradiation can
react irreversibly with H to (a) further improve the electron–
4
2 2
O
hole separation and (b) the effective generation of powerful
reactive species HOc radicals via the photo-Fenton reaction,
which can further oxidize 2,4-DCP during the photocatalytic
III
46
II
I
degradation reaction, nally returning to Cu sites. In addi-
tion to reaction (4), these two processes complete the full pho-
tocatalytic circle and are occurred repeatedly, nally
maximizing the photocatalytic efficiency for the mineralization
may drive the half-reaction of oxygen reduction. The main
processes in the photocatalytic degradation of 2,4-DCP could be
summarized as follows:
of 2,4-DCP to CO
these Cu (OH)PO
catalyst. As shown in Fig. S15,† it can be easily seen that sample
(3) 1 : 0.005 is capable of catalyzing H efficiently to generate
2
2
and H O. This is the unique advantage of
II
III
ꢀ
/rGO as a novel infrared-light-active photo-
rGO-TBP Cu + hv / rGO-TBP Cu + e
(2)
2
4
II
ꢀ
I
rGO-OCT Cu + e / rGO-OCT Cu
2 2
O
HOc only when irradiated with infrared light. The amount of the
generated HOc for sample 1 : 0.005 under infrared light irradi-
ation is over 7 times than the control groups. Remarkably, there
is no generation of HOc without the irradiation of infrared light.
These demonstrate that the infrared light photocatalytic activity
III
II
rGO-TBP Cu + 2,4-DCP / rGO-TBP Cu + H O
2
+
CO + H + Cl
ꢀ
+
(4)
(5)
2
I
rGO-OCT Cu + H + O
+
II
2
/ rGO-OCT Cu + H
2
O
3616 | RSC Adv., 2018, 8, 3611–3618
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
enhanced under infrared light irradiation. With an optimal
mass ratio of 0.5 wt% graphene oxide, the highest rate of 2,4-
DCP degradation is achieved due to the effective hybridization
between Cu (OH)PO . Moreover, the introduction of H O as
2
4
2 2
the sacricial electron acceptor can maximize the infrared light
photocatalytic activity via the combination of the photocatalytic
and photo-Fenton effects into one reaction system. In this
I
system, the reaction of H
2
O
2
with rGO-OCT Cu sites, as well as
the coupling of graphene with Cu
2
4
(OH)PO , can improve the
separation and transportation of photogenerated electrons and
holes; while the generated HOc via the photo-Fenton reaction
can further oxidize 2,4-DCP. These are the unique advantages of
Cu
Typically, the photocatalytic rate constant of Cu
and H
is ꢁ6.25 times higher than the corresponding value
for only Cu (OH)PO
/rGO, and ꢁ10 times greater than the value
for pure Cu (OH)PO . Moreover, Cu (OH)PO /rGO is very stable
2
(OH)PO
4
/rGO as a novel infrared-light-active photocatalyst.
2
(OH)PO /rGO
4
2 2
O
2
4
2
4
2
4
aer many photocatalytic cycles and can be reused without
signicant loss of photocatalytic activity. In addition, a possible
decomposition mechanism for the degradation of 2,4-DCP is
Fig. 6 (a) Cycles of the photocatalytic degradation of 2,4-DCP in the
presence of sample 1 : 0.005. (b) SEM image of sample 1 : 0.005 after
the photocatalytic reaction cycles. Inset: enlarged SEM image. (c and
d) The Cu 2p XPS spectra and XRD patterns of sample 1 : 0.005 after further proposed. This work may help the development of a new
the photocatalytic reaction cycles.
strategy to search stable and effective photocatalysts with high
infrared light photocatalytic activity and bring the promise to
fullment of actual applications in the treatment of non-
is further signicantly enhanced when the photocatalytic and biodegradable chlorophenols with lower costs and non-
photo-Fenton effects are combined in one reaction system.
Finally, the stability of Cu (OH)PO /rGO was evaluated using
secondary pollution to the environment.
2
4
cyclic experiments. Fig. 6a shows the degradation of 2,4-DCP for
four runs of reactions. The photocatalytic efficiency of sample
Conflicts of interest
1
: 0.005 does not decrease even aer several cycles. This indi-
There are no conicts to declare.
cates that sample 1 : 0.005 can be efficiently recycled and reused
for repeated cycles without appreciable loss of activity. SEM
images and XPS spectra of sample 1 : 0.005 before the photo- Acknowledgements
catalytic reaction and aer the photocatalytic reaction cycles are
This work was supported by the National Basic Research
Programs of China (2016YFA0201600 and 2015CB932104),
National Natural Science Foundation of China (No. 31571015,
presented in Fig. 6b and c, respectively. No differences have
been found, either in the morphology of sample 1 : 0.005 or in
their chemical structure aer photocatalytic degradation.
Furthermore, XRD patterns shown in Fig. 6d indicate that the
crystal structures of sample 1 : 0.005 remained the same aer
repeated photocatalytic cycling. At the same time, SEM images,
XPS spectra and XRD patterns shown in Fig. S16–S18,† respec-
tively arrested that other Cu (OH)PO /rGO also possess nice
11621505, 11435002 and 21320102003) and Youth Innovation
Promotion Association CAS (2013007 and 2015190).
Notes and references
2
4
photocatalytic stability. In addition, in order to evaluate the
biosecurity of Cu (OH)PO /rGO nanocomposites, the cytotox-
1 C. Gonçalves and M. F. Alpendurada, Talanta, 2005, 65,
1179–1189.
2
4
icity of sample 1 : 0.005 was assessed to HUVECs (human
umbilical vein endothelial cells) by the CCK-8 assay. It can be
seen from Fig. S19† that no obvious cytotoxicity is induced in
human umbilical vein endothelial cells, even at high concen-
2 J. Regueiro, M. Llompart, C. Garcia-Jares, J. C. Garcia-
Monteagudo and R. Cela, J. Chromatogr. A, 2008, 1190, 27–38.
3 V. K. Gupta, I. Ali, Suhas and V. K. Saini, J. Colloid Interface
Sci., 2006, 299, 556–563.
4 M. C. Alavanja, J. A. Hoppin and F. Kamel, Annu. Rev. Public
Health, 2004, 25, 155–197.
ꢀ1
trations up to 90 mg mL , which exhibits Cu (OH)PO /rGO
2
4
nanocomposites have good biosecurity.
5
Y. Xiang, M. Wang, X. Sun, D. Cai and Z. Wu, ACS Sustainable
Chem. Eng., 2014, 2, 918–924.
Conclusion
6
S. Malato and A. Ag u¨ era, in Pesticide Decontamination and
Detoxication, American Chemical Society, 2003, vol. 863,
ch. 9, pp. 113–126.
7 Z. Gerstl, A. Nasser and U. Mingelgrin, J. Agric. Food Chem.,
1998, 46, 3803–3809.
In summary, Cu (OH)PO /rGO nanocomposites with different
2
4
mass ratios of GO are successfully synthesized through a one-
step hydrothermal method. By coupling with graphene, the
photocatalytic activity of pure Cu (OH)PO is remarkably
2 4
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 3611–3618 | 3617

View Article Online
RSC Advances
Paper
8
9
M. G. Mogul, H. Akin, N. Hasirci, D. J. Trantolo, J. D. Gresser
and D. L. Wise, Resour., Conserv. Recycl., 1996, 16, 289–320.
U. D. Patel and S. Suresh, J. Hazard. Mater., 2007, 147, 431– 28 D. Pierucci, H. Henck, C. H. Naylor, H. Sediri, E. Lhuillier,
I. A. Aksay, R. K. Prud'Homme and L. C. Brinson, Nat.
Nanotechnol., 2008, 3, 327–331.
4
38.
0 H. Ding, X. Li, J. Wang, X. Zhang and C. Chen, J. Environ. Sci.,
016, 43, 187–198.
1 J. Hou, F. Liu, N. Wu, J. Ju and B. Yu, J. Nanobiotechnol., 2016,
4, 5.
A. Balan, J. E. Rault, Y. J. Dappe, F. Bertran, P. L. F `e vre,
A. T. C. Johnson and A. Ouerghi, Sci. Rep., 2016, 6, 26656.
29 S. Han, L. Hu, Z. Liang, S. Wageh, A. A. Al-Ghamdi, Y. Chen
and X. Fang, Adv. Funct. Mater., 2014, 24, 5719–5727.
30 E. J. Heller, Y. Yang, L. Kocia, W. Chen, S. Fang, M. Borunda
and E. Kaxiras, ACS Nano, 2016, 10, 2803–2818.
31 K. Singh, A. Ohlan, V. H. Pham, B. R. S. Varshney, J. Jang,
S. H. Hur, W. M. Choi, M. Kumar, S. K. Dhawan,
B.-S. Kong and J. S. Chung, Nanoscale, 2013, 5, 2411–2420.
1
1
1
1
1
1
1
1
1
1
2
2
1
2 J. De Coster, W. Vanherck, L. Appels and R. Dewil, J. Environ.
Manage., 2017, 190, 61–71.
3 C. Casta n˜ eda, F. Tzompantzi, R. G ´o mez and H. Rojas, J.
Chem. Technol. Biotechnol., 2016, 91, 2170–2178.
4 Y. Ren, Q. Zhao, X. Li, W. Xiong, M. Tade and L. Liu, J. 32 T. Som, G. V. Troppenz, R. Wendt, M. Wollgarten, J. Rappich,
Nanopart. Res., 2014, 16, 2532.
5 J. Tian, Y. Sang, G. Yu, H. Jiang, X. Mu and H. Liu, Adv.
Mater., 2013, 25, 5075–5080.
6 Y. Sang, Z. Zhao, M. Zhao, P. Hao, Y. Leng and H. Liu, Adv.
Mater., 2015, 27, 363–369.
7 M.-Z. Huang, B. Yuan, L. Dai and M.-L. Fu, J. Colloid Interface
Sci., 2015, 460, 264–272.
F. Emmerling and K. Rademann, ChemSusChem, 2014, 7,
854–865.
33 B. Bajaj, H. I. Joh, S. M. Jo, G. Kaur, A. Sharma, M. Tomar,
V. Gupta and S. Lee, J. Mater. Chem. B, 2016, 4, 229–236.
34 J. S. Corneille, J.-W. He and D. W. Goodman, Surf. Sci., 1994,
306, 269–278.
35 D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii,
Z. Sun, A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour,
ACS Nano, 2010, 4, 4806–4814.
8 L. Ji and R. Yu, Asia-Pacic Energy Equipment Engineering
Research Conference, Beijing, April 2015, pp. 224–228.
9 C. Chen, Y. Zhou, N. Wang, L. Cheng and H. Ding, RSC Adv., 36 T. Xing, Y. Zheng, L. H. Li, B. C. C. Cowie, D. Gunzelmann,
2
015, 5, 95523–95531.
S. Z. Qiao, S. Huang and Y. Chen, ACS Nano, 2014, 8,
6856–6862.
0 G. Wang, B. Huang, X. Ma, Z. Wang, X. Qin, X. Zhang, Y. Dai
and M.-H. Whangbo, Angew. Chem., Int. Ed., 2013, 52, 4810– 37 Y.-C. Yang, L. Xu, W.-Q. Huang, C.-Y. Luo, G.-F. Huang and
813. P. Peng, J. Phys. Chem. C, 2015, 119, 19095–19104.
1 Z. Li, Y. Dai, X. Ma, Y. Zhu and B. Huang, Phys. Chem. Chem. 38 X. Pan, M.-Q. Yang, Z.-R. Tang and Y.-J. Xu, J. Phys. Chem. C,
Phys., 2014, 16, 3267–3273. 2014, 118, 27325–27335.
2 L. Yan, Y. B. Zheng, F. Zhao, S. Li, X. Gao, B. Xu, P. S. Weiss 39 A. Lin, B. H. Kim, D. S. Moon, Y. Chung and W.-T. Han, Opt.
and Y. Zhao, Chem. Soc. Rev., 2012, 41, 97–114. Express, 2007, 15, 3665.
3 D. Deng, K. S. Novoselov, Q. Fu, N. Zheng, Z. Tian and X. Bao, 40 X. Peng, M. Li and C. K. Chan, J. Phys. Chem. C, 2015, 119,
Nat. Nanotechnol., 2016, 11, 218–230. 4684–4693.
4 A. D. Bokare and W. Choi, J. Hazard. Mater., 2014, 275, 121– 41 T. Xian, H. Yang, L. Di, J. Ma, H. Zhang and J. Dai, Nanoscale
35. Res. Lett., 2014, 9, 327.
5 Y. H. Kim, Y. J. Hong, K. Y. Baik, G. C. Kwon, J. J. Choi, 42 C. Han, M.-Q. Yang, N. Zhang and Y.-J. Xu, J. Mater. Chem. A,
4
2
2
2
2
2
1
G. S. Cho, H. S. Uhm, D. Y. Kim and E. H. Choi, Plasma
Chem. Plasma Process., 2014, 34, 457–472.
6 Y. Zhan, H. Li and Y. Chen, J. Hazard. Mater., 2010, 180, 481–
2014, 2, 19156–19166.
43 J. Sun, Y. Fu, P. Xiong, X. Sun, B. Xu and X. Wang, RSC Adv.,
2013, 3, 22490–22497.
2
2
4
85.
44 S. Das and S. Jana, Environ. Sci.: Nano, 2017, 4, 596–603.
7 T. Ramanathan, A. A. Abdala, S. Stankovich, D. A. Dikin, 45 C. C. Wong and W. Chu, Environ. Sci. Technol., 2003, 37,
M. Herrera Alonso, R. D. Piner, D. H. Adamson, 2310–2316.
H. C. Schniepp, X. Chen, R. S. Ruoff, S. T. Nguyen, 46 A. N. Pham, G. Xing, C. J. Miller and T. D. Waite, J. Catal.,
013, 301, 54–64.
2
3618 | RSC Adv., 2018, 8, 3611–3618
This journal is © The Royal Society of Chemistry 2018