Photocatalysis in an evanescent field: An: in situ approach to studying photocatalytic performance by tracing interfacial refractive index changes and kinetics

Article abstract of DOI:10.1039/c8ta08705a

An in situ strategy to understand the photocatalytic performance of Cu₂O-Ag@rGO nanocomposites through interfacial photocatalysis on an optical microfiber has been proposed and experimentally demonstrated. The photocatalysts were conjugated on the surface of an optical microfiber immersed in an organic solution, forming an interface consisting of photocatalysts and organic molecules between the silica microfiber and the solution, which was within the evanescent field of the optical microfiber. In the photocatalytic process, the decomposition of organics changed the refractive index (RI) of the interface. The interaction between the evanescent wave and the microfiber interface modulated its optical spectrum. Therefore, the optical microfiber monitored the photocatalytic performance of the photocatalysts in real-time and in situ. The results reveal that the photodegradation efficiency of Cu₂O-Ag@rGO was almost twice that of Cu₂O, while the degradation rate constant was more than five times larger, because the coupling between the silver nanoparticles and reduced graphene oxide (rGO) enhanced the photocatalytic performance of Cu₂O. By taking advantage of its miniature size and mechanical flexibility, the microfiber can directly probe a real matrix to study the local photocatalytic performance. This strategy is suitable for the evaluation of most photocatalysts and might provide a new perspective to assess the photocatalytic performance of the catalysts.

Full text of DOI:10.1039/c8ta08705a

Journal of
Materials Chemistry A
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PAPER
Photocatalysis in an evanescent field: an in situ
approach to studying photocatalytic performance
by tracing interfacial refractive index changes and
kinetics†
Cite this: J. Mater. Chem. A, 2018, 6,
0513
2
Yunyun Huang,
Aoxiang Xiao, Guanhua Hou, Hongtao Li, Tuan Guo
and Bai-Ou Guan*
2
An in situ strategy to understand the photocatalytic performance of Cu O–Ag@rGO nanocomposites
through interfacial photocatalysis on an optical microfiber has been proposed and experimentally
demonstrated. The photocatalysts were conjugated on the surface of an optical microfiber immersed in
an organic solution, forming an interface consisting of photocatalysts and organic molecules between
the silica microfiber and the solution, which was within the evanescent field of the optical microfiber. In
the photocatalytic process, the decomposition of organics changed the refractive index (RI) of the
interface. The interaction between the evanescent wave and the microfiber interface modulated its
optical spectrum. Therefore, the optical microfiber monitored the photocatalytic performance of the
photocatalysts in real-time and in situ. The results reveal that the photodegradation efficiency of Cu
Ag@rGO was almost twice that of Cu O, while the degradation rate constant was more than five times
larger, because the coupling between the silver nanoparticles and reduced graphene oxide (rGO)
enhanced the photocatalytic performance of Cu O. By taking advantage of its miniature size and
2
O–
2
Received 7th September 2018
Accepted 30th September 2018
2
mechanical flexibility, the microfiber can directly probe a real matrix to study the local photocatalytic
performance. This strategy is suitable for the evaluation of most photocatalysts and might provide a new
perspective to assess the photocatalytic performance of the catalysts.
DOI: 10.1039/c8ta08705a
rsc.li/materials-a
1
5
Ti O have been reported to photodegrade the pollutants in
2
1
Introduction
16
17,18
water, their toxicity or ultraviolet light requirement
Renewable energy source exploitation is one of the greatest led to hindrances in their practical application. Cu
have
2
O has
challenges to addressing the increasing energy demand of our gained a lot of attention as an environmentally friendly p-type
12
society and the environmental issues associated with the use of semiconductor with a band gap of 2.0–2.2 eV. It is nontoxic,
1
fossil energy. Recently, some exciting studies have been con- abundant, relatively cheap, and easy to fabricate, and most
2
–4
ducted and reported. Solar energy is the largest source avail- importantly, exhibits excellent photocatalytic activity under
able to us and can be converted into chemical energy stored visible light.
within the chemical bonds of reduced compounds such as
However, it has been found that electron–hole pairs easily
hydrogen, methanol, and methane. Due to its various appli- recombine in Cu O, leading to a very low quantum efficiency
cations in the decontamination and disinfection of water and and limiting its promising photocatalytic applications. For
5
2
19
6
7
8
air, hydrogen production, solar energy cells, photochemical this reason, a great deal of effort has been made to design and
9
10
synthesis, photopolymerization, and targeted brain cancer fabricate suitable Cu
2
O-based composite/hybrid materials with
photocatalysis, which generates photoelectrons a high charge separation efficiency to improve the photo-
under optical illumination and yields free radicals as active catalytic activity, and a group of new materials has been devel-
11
therapy,
12
20–22
fragments, has attracted extensive interest in recent years.
Although a series of photocatalysts such as CdS, ZnO, and oxide (rGO) hetero-structure and Cu
oped.
It is believed that the binary Cu
2
O/reduced graphene
O/noble metal (Au, Ag, Pt,
etc.) can effectively promote the separation and transfer of
photoexcited carriers. Ternary noble metal–Cu O/rGO compos-
13
14
23
2
22
2
Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications,
Institute of Photonics Technology, Jinan University, Guangzhou 210632, China.
E-mail: tguanbo@jnu.edu.cn; Tel: +86-20-85222046
24
ites have further been synthesized. Deep insights into the
photocatalytic performance of these materials may be of great
†
Electronic supplementary information (ESI) available: The schematic importance in the evaluation of the material properties and
illustration of silica optical microber fabrication, SEM-EDS elemental
guidance of the composite design. Nevertheless, to the best of
mappings and XPS survey spectra. See DOI: 10.1039/c8ta08705a
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our knowledge, until now, an understanding of the catalytic dispersive spectrometer (EDS, Oxford). The morphology of the
25
mechanism and relevant theoretical basis is lagging. There as-prepared microber was characterized using a eld emission
have been few studies on the photocatalytic process and eval- scanning electron microscope (SEM, ULTRA 55, ZEISS,
uation of the photocatalytic performance of photocatalysts, BRUKER). X-ray diffraction (XRD) patterns were obtained with
beyond gas chromatography (GC) and UV-vis absorption spec- a PANalytical B. V. (Empyrean) diffractometer using Cu Ka
troscopy. GC is usually employed to identify the photocatalytic radiation. Fourier transform infrared (FT-IR) spectroscopy
26
degradation products coupled with mass spectrometry. It can data were measured on a Bruker-Vertex 80v spectrometer. X-ray
measure the conversion of substrates and the selectivity of photoelectron spectroscopy (XPS) was performed using
27,28
products quantitatively.
However, it requires large and a Thermo Fisher Scientic K-ALPHA+ system with Mono Al Ka
expensive equipment and could not record the local photo- (1486.68 eV, 12 kV). UV-vis spectra were obtained using a Shi-
catalytic process in real-time and in situ. UV-vis spectrometry is madzu UV-2550 UV-Vis-NIR spectrophotometer. Raman spectra
one of the most common methods to measure the effect of were obtained with a micro Raman spectrometer (Thermo
22,29
organic degradation.
It can evaluate the photocatalytic Fisher Scientic, DXR, excited by a 532 nm laser line).
performance of photocatalysts through the UV-vis absorption
spectrum. But the absorption spectrum requires a higher
sample dosage, and it also could not record the local photo-
catalytic process in real-time and in situ.
2
.3 Preparation of Cu
2 2 2
O–Ag@rGO, Cu O@rGO and Cu O
nanocomposites
Herein, we develop an alternative approach to understand The Cu O@rGO nanocomposites were prepared through a two-
2
12
the in situ photocatalytic performance of photocatalytic mate- step in situ reduction approach. In a typical procedure, the GO
ꢀ1
rials through interfacial photocatalysis on an optical microber. dispersion (0.5 mg mL , 30 mL) and glucose solution (1.15 g,
Because of the strong evanescent-eld–matter interaction on its 30 mL) were added to a 40 mL copper acetate solution (1.265 g).
ꢁ
surface, the microber could capture the surface refractive The mixture was stirred for 4 h at 25 C. Then, a sodium
index (RI) changes and translate them into a signicant wave- hydroxide solution was added dropwise to the abovementioned
30
length shi in the interferometric fringe. In this work, pho- dispersion under continuous stirring to adjust the pH to
tocatalysts were conjugated on the surface of an optical approximately 8.0. Aerwards, another 80 mL of glucose solu-
microber immersed in an organic solution. In the photo- tion (1.15 g) was added, and the dispersion was stirred contin-
ꢁ
catalytic process, a number of organic molecules aggregated on uously at 90 C for 1 h. Finally, the Cu O@rGO nanocomposites
2
the microber surface, forming an interface with photocatalysts were washed, separated by centrifugation and then dried in
ꢁ
and organics. This interface overlapped with the evanescent a vacuum oven at 60 C for 12 h.

eld of the optical microber. The interaction between the
evanescent wave and the photocatalytic interface on the following the above approach without adding a GO dispersion.
microber modulates its optical spectrum. Therefore, the The Cu O–Ag@rGO nanocomposites were prepared by
2
The synthesis of Cu O nanoparticles was performed
2
optical microber can monitor the photocatalytic performance adsorbing the aminated Cu
of photocatalysts in situ and in real time. Taking advantage of its
2
O onto the Ag@rGO nanosheets.
miniature size and mechanical exibility, the microber can
directly probe a real matrix to study the local photocatalytic
performance. In this method, the volume of the organic solu-
2
.4 Relationship between the surface RI change and the
evanescent wave of the optical microber
tion can be reduced to a few hundred microliters. This strategy The optical microber in this work was tapered from
is suitable for the evaluation of most photocatalysts and might a commercial single mode silica ber supplied by CorActive
provide a new perspective to assess the photocatalytic perfor- High-Tech Inc. (UVS-INT-PREMIUM, 100536). The microber
mance of the catalysts.
structure was fabricated following the procedure described
31
previously (Fig. S1†). With a 5 mm-width ame scanning
across it, the ber was slowly stretched using two linear stages.
The moving speed of the ame and the stages mainly deter-
mined the nal geometrical parameters, including the ber
2
Experimental section
2.1 Reagents and materials
32
diameter and transition region length. Therefore, a microber
with a uniform region of 7.56 mm diameter and 0.6 cm length
was fabricated (Fig. 1a and b).
All chemicals and solvents supplied by Aladdin were of analyt-
ical grade and were used without further purication. The
silver-decorated reduced graphene oxide nanosheet (Ag@rGO)
dispersion and GO dispersion were obtained from Nanjing
XFNANO Materials Tech Co., Ltd. (China). The concentration of
This geometry allowed the coupling and recombination of
modes in the microber. When optical signals entered the
tapered down region, the fundamental core mode excited
a fundamental mode (HE11) and a higher order mode (HE12).
Mode beating was mainly between the HE11 and the HE12
modes (Fig. 1c). The interference between the HE11 and HE12
ꢀ1
phenol solutions for the photocatalytic process was 200 mg L
.
2.2 Characterization
The morphology of Cu O–Ag@rGO, Cu O@rGO and Cu O modes created an interferometric fringe in the optical micro-
2
2
2
nanoparticles was observed using a transmission electron ber transmission spectrum (Fig. 1d). Surface RI is of para-
microscope (TEM) on a JEM-2100 equipped with an energy mount signicance, as it could modulate the transmitting light
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microber, as follows: the silica microber was cleaned in
a bath with piranha solution which consisted of 30% H O and
2
2
concentrated H SO at a volume ratio of 1 : 3 for 30 min to
2
4
generate reactive hydroxyl groups.
2 2
The Cu O–Ag@rGO (or Cu O@rGO or Cu O) nano-
2
composites were surface-modied by adding them to a 10%
solution of 3-aminopropyl-triethoxysilane (APTES) in ethanol
with stirring for 30 min and were then thoroughly washed.
Then, the amino-group modied nanocomposites were
dispersed in ethanol.
Following that, the cleaned microber was immersed in the
nanocomposite dispersion for 60 min, then pulled out and
dried.
2.6 Experimental setup and optical conguration
The experimental setup permitted photocatalytic tracing in the
transmission mode as shown in Fig. 1e. The photocatalysts
2 2 2
(Cu O–Ag@rGO, Cu O@rGO, or Cu O nanocomposites here)
were immobilized on the optical microber surface as
mentioned in Section 2.5. Aer that, the microber was
immersed in an organic solution (as a typical toxic organic
34
pollutant, phenol is employed as the pollutant model in this
work because its photodegradation is of great signicance) to
record the in situ photocatalytic process. To maintain stability
during the photocatalytic process, a microchannel chip was
employed in the experiments, as shown in Fig. 1e. The sensor
was xed in the microchannel (width 200 mm and height 1.5
mm) with the help of UV-sensitive adhesive on both sides over
the sensing element, which was a centimetre in length (for
a total sensing volume of ꢂ1 mL). This microchannel chip
provided an airtight environment for the microber measure-
ment and eliminated potential environmental inuences (e.g.,
temperature uctuations) during measurement. The key to the
evaluation of the photocatalytic performance of the nano-
composites is the interfacial photocatalysis on the optical
Fig. 1 (a) Photo and (b) optical microscope photo of the microfiber. (c)
The composition of the reflective resonances and the transverse
electric field amplitude distribution of the HE12x mode of the silica microber, as shown in Fig. 1f. When the microber coated
microfiber. (d) The transmission spectrum of the silica microfiber
interferometer. (e) The optical setup for recording the in situ photo-
catalytic process. (f) Schematic illustration of the interfacial photo-
catalysis and (g) the corresponding wavelength shift.
with the photocatalysts was immersed in phenol solution, the
phenol molecules gathered at the microber surface, forming
an interface that included photocatalysts and organic molecules
between the microber surface and the solution. As the
organics (phenol is used here) in the interface between the
solution and the optical microber degraded in the photo-
catalytic process, the surface RI of the microber decreased
within the ber. Normally, the surface RI sensitivity of the
interferometer can be expressed by eqn (1).
33
correspondingly. According to eqn (1) in Section 2.4, this
surface RI change was captured by the optical microber
because of the interaction between the evanescent eld and the
interface. The HE12 mode was excited by the abruptly tapered
silica microber and interfered with the HE11 mode, creating an
interferometric fringe in the ber surface coating and over-
lapped with the photocatalytic interface, it captured the surface
RI change and translated it into a wavelength shi in the
interference fringe, as shown in Fig. 1g (the transmission dips
would blue-shi). Therefore, the microber monitored the
photocatalytic process in real-time on its surface.
ꢀ
ꢁ
dl
1
1
dDn
¼
l
(1)
dnsur
G
Dn dnsur
where G is the dispersion factor, which characterizes the effect
of variation of the index difference with wavelength. According
to eqn (1), when the surface RI increases, the transmission dips
will red-shi.
2
.5 Immobilization of Cu
over the microber surface
Electrostatic attraction was employed in surface functionaliza-
2 2 2
O–Ag@rGO, Cu O@rGO or Cu O
A broadband source (BBS) for the beam ranging from 1250–
tion to conjugate the nanoparticles to the surface of the silica 1650 nm was employed to excite the sensing microber, while
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an optical spectrum analyser (OSA) with a minimum wavelength and Cu
2
O nanocomposites were modied with amino groups by
resolution of 0.02 nm was utilized to monitor the transmission reaction with APTES (Fig. 4a), while the surface of the silica
spectra. The measurements were recorded continuously at microber was functionalized with hydroxyl groups through
a rate of one spectrum every 30 s. During the experiments, the reaction with piranha solution, and then, the aminated nano-
stability of the sensor was maintained by a microchannel chip. composites were attached to the hydroxylated surface of the
30
The photocatalytic process of the nanocomposites was triggered microber by electrostatic attraction. The FT-IR spectra in
by irradiation with a normal LED white light (KM-6667C, 2.2 W). Fig. 4b conrm the successful amino-functionalization of
Cu
2
O–Ag@rGO, Cu
2
ꢀ
O@rGO and Cu
2
O: the FT-IR absorption
1
ꢀ1
ꢀ1
peaks at 3373 cm , 1620 cm , 767 cm represent –NH
bonding and disclose the presence of APTES. The FT-IR
absorption peak at 1034 cm representing the Si–O–C bond
and the peak at 1117 cm
2
3
Results and discussion
41
ꢀ
1
3
.1 Fabrication of photocatalysts
Fig. 2 shows the structural characterization of the Cu
Ag@rGO and Cu O@rGO. Fig. 2a and b show the XRD pattern
and TEM image of GO. XRD patterns (Fig. 2a, c and e) of the
obtained products show strong Cu O peaks (JPPDS no. 05-0667)
with the presence of a weak carbon peak.
ꢀ
1
representing the Si–O–Si bond
2
O–
indicate the successful coating of APTES onto the nano-
composites through chemical bonding. The Raman spectra
2
42
ꢀ
1
taken from the microber surfaces show peaks at 1346 cm
2
ꢀ
1
34–36
and 1600 cm , indicating the successful functionalization of
the photocatalysts onto the microber, which is conrmed by
the EDS results in Fig. S4 and S5.† The peak located at
In the TEM image
22
in Fig. 2d, Cu O nanoparticles with a diameter of ꢂ10 nm are
2
observed well dispersing on the surfaces of the rGO nanosheets.
ꢀ
1
3
ꢂ1300 cm (D peak) belongs to the edge and sp defect carbon,
Fig. 2e shows the XRD pattern of the Cu
2
O–Ag@rGO hetero-
ꢀ1
while the peak located at ꢂ1600 cm (G peak) is attributed to
structure. In addition to the peaks of Cu
2
O, the characteristic
2
43,44
ꢁ
ꢁ
ꢁ
the ordered vibrational sp bonded carbon atoms.
defect density indicated by the relative peak intensity ratio of
the D peak to the G peak (I /I ) in Cu O@rGO is 1.04, while that
in Cu O–Ag@rGO is 1.12. This nding suggests the formation
of more sp defects in Cu
The
peaks of Ag are also detected at 38.2 , 44.4 and 64.7 , corre-
sponding to the Ag (JCPDS no. 04-0783) (111), (200) and (220)
24
D
G
2
planes, respectively. This result conrms the existence of Ag
and Cu O in Cu O–Ag@rGO nanocomposites. The surface
2
2
2
3
2
O–Ag@rGO.
morphology of the Cu O–Ag@rGO sample was visualized by the
2
Fig. 4d shows the transmission spectra recorded during the
TEM image shown in Fig. 2f. It shows that the Cu O–Ag@rGO
2
functionalization process of Cu O–Ag@rGO nanocomposites
2
2
sample is composed of Cu O nanoparticles with a diameter of
on the microber surface. A redshi of ꢂ15 nm was observed
aer 10 min-coating and tended to remain steady aerwards,
thus strengthening the successful functionalization of the
ꢂ10 nm and Ag nanoparticles with a diameter of 25–30 nm,
decorating the rGO nanosheets. The EDS spectrum in Fig. 2g
reveals that C, Cu and Ag coexist in the Cu
The corresponding elemental mapping of Cu
2
O–Ag@rGO sample.
O–Ag@rGO,
45
photocatalysts on the microber, which paved the way for
2
photocatalytic performance evaluation.
illustrated in Fig. 2h and S2,† further demonstrates that the Ag,
Cu and C elements are well-arranged over the Cu O–Ag@rGO
2
nanosheets.
3
.3 In situ photocatalytic process recorded by the optical
The Cu
2
O–Ag@rGO was further characterized by XPS
O and rGO,
microbers
measurement to conrm the existence of Ag, Cu
and the results are shown in Fig. 3 and S3.† The peaks situated
at 284.8, 368.3, 532.2 and 933.2 eV are attributed to C 1s, Ag 3d,
O 1s and Cu 2p, respectively,
rGO, Ag and Cu O in the Cu O–Ag@rGO composites. In Fig. 3a,
the C 1s spectrum of Cu O–Ag@rGO shows three peaks at
approximately 284.5, 285.5 and 287.8 eV. The weak C]O peak
as well as the signicant decrease of the C–O peak indicates the
reduction of GO to rGO. The peaks of Ag 3d5/2 and Ag 3d3/2
2
Fig. 5a shows the optical wavelength shi in the dark envi-
ronment recorded by the microber functionalized with
Cu O–Ag@rGO, Cu O@rGO nanosheets and Cu O nano-
2 2 2
37,38
conrming the existence of
particles. In all these cases, redshis were recorded in the
optical transmission: a redshi of ꢂ0.20 nm was recorded by
2
2
2
2
the Cu O–Ag@rGO-functionalized microber, while redshis
of ꢂ0.18 nm and ꢂ0.10 nm were recorded by Cu
Cu O-functionalized microbers in the rst 60 min, respec-
tively, and then, the shis tended to become balanced
Fig. 5a). This apparent RI increase information on the
microber surface indicates that a certain number of organic
2
O@rGO and
24
2
,
with binding energies of 368.2 and 374.2 eV, respectively, shown
in Fig. 3b show that the spin energy separation of Ag 3d is
(
6.0 eV. This nding is consistent with the standard spin energy
4
5
molecules gathered at the microber surface, forming an
interface that included photocatalysts and organic molecules
between the microber surface and the solution. Fig. 5b shows
separation of metallic Ag, conrming the existence of metallic
39
Ag. The binding energies of Cu 2p3/2 and Cu 2p1/2 are slightly
shied to 933.2 and 953.1 eV, respectively (Fig. 3c). This shi is
40
the photocatalytic performance of Cu
nanosheets and Cu O nanoparticles recorded by the micro-
ber under white light irradiation. l here is the wavelength
shi recorded by the photocatalyst-modied microber
For conjugation to the surface of the silica microber, electro- between the organic pollutant solution and deionized water.
static attraction was employed in the surface functionalization, The relative wavelength shi (Dl/rl r) was employed to eval-
2 2
O–Ag@rGO, Cu O@rGO
2
due to an interaction between Cu O, Ag and rGO.
2

0
3.2 Functionalization of the optical microber
0
as shown in Fig. 4. The surfaces of Cu
2
O–Ag@rGO, Cu
2
O@rGO uate the photocatalytic performance. It was observed that
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Fig. 2 XRD patterns of (a) GO, (c) Cu
2
O@rGO and (e) Cu
2 2 2
O–Ag@rGO. TEM images of (b) GO, (d) Cu O@rGO and (f) Cu O–Ag@rGO. EDX
spectrum (g) and the corresponding mapping (h) of Cu
2
O–Ag@rGO (red: C; green: Ag; cyan: Cu).
without catalysts, no obvious photodegradation of the organic times) and Cu
pollutant solution was recorded in our experiments. When intrinsic reaction rates of the three photocatalysts. The
photocatalysts were employed, by contrast, obvious photo- degradation rate constants of the Cu O–Ag@rGO, Cu O@rGO
catalytic effects were recorded by the microber. As shown in and Cu O catalysts were 3.864, 0.738 and 0.702 h , respec-
Fig. 5b, Cu O–Ag@rGO nanocomposites photodegraded ca. tively, in the rst 60 min and tended to become balanced. This
5% of the phenol molecules within 180 min, while Cu2- shows that the degradation rate of Cu O–Ag@rGO was more
2
O nanoparticles (1.98 times). Fig. 5c shows the
2
2
1
ꢀ
2
2
8
2
O@rGO nanosheets photodegraded ca. 49% and Cu O nano- than ve times those of the other two photocatalysts. These
2
particles photodegraded ca. 43%. Thus, Cu O–Ag@rGO results further conrmed the superiority of Cu O–Ag@rGO
2
2
nanocomposites showed much improved photocatalytic nanocomposites, implying that the interaction of the inter-
3
6
2
activity when compared to the Cu O@rGO nanosheets (1.73 connected interface was crucial.
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2
Fig. 3 (a) C 1s, (b) Ag 3d and (c) Cu 2p XPS spectra of the Cu O–Ag@rGO composites.
Additionally, to avoid RI impact of the accumulation of some organic solution has been studied as shown in Fig. S6† (with light
photogenerated electrons on photocatalysts on the photocatalytic irradiation but without organic solution). It is observed that the
performance study of the microber, the optical wavelength shi interfacial RI change caused by the accumulation of some pho-
recorded by the photocatalyst-functionalized ber without togenerated electrons on the photocatalysts is too small for the
Fig. 4 Functionalization process of the optical microfiber. (a) Schematic process of the microfiber functionalization. (b) FTIR spectra of the
photocatalysts before and after amino modification. (c) Raman spectra of microfiber surfaces before and after photocatalyst functionalization. (d)
The transmission wavelength shift in the microfiber functionalization process.
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Fig. 5 The interfacial photocatalytic process recorded by the optical microfibers. (a) Optical wavelength shift recorded by the microfiber
functionalized with photocatalysts in a dark environment: an interface formed between the microfiber and the organic solution; (b) the relative
wavelength shift (Dl/rl
0
r) under visible-light irradiation; (c) plots of ln(rl
0
r/(rl
0
r ꢀ rDlr)) versus irradiation time with the fitting results included. l
0
:
wavelength shift recorded by the photocatalyst-modified microfiber from organic pollutant solution to deionized water.
optical microber to perceive (it might also contain the photo-
To explain the characterization mechanism of the optical
thermal effects). Therefore, this effect could hardly affect the microber, numerical mode simulation (COMSOL) soware was
record result of photocatalytic performance.
used to analyse the transverse electric eld amplitude distribu-
tions of the HE12 mode (because the HE12 mode spread into the
surface coating) as shown in Fig. 6c. It reveals that for a microber
with a diameter of 7.56 mm, most of the energy was concentrated
3.4 Photocatalytic performance characterization mechanism
The schematic illustration of the photocatalytic performance within the core, and only a few evanesced out as the evanescent
48
probing mechanism through the photocatalyst-functionalized eld. In this work, the penetration depth (the distance to which
microber is shown in Fig. 6a. When the microber functional- the evanescent eld extends beyond the microber surface) was
ized with photocatalyst nanocomposites was immersed in the ꢂ1 mm. Because the characterization of the photocatalytic process
organic pollutant solution, a certain number of organic mole- was based on the interaction between the evanescent eld and the
cules were adsorbed onto the microber surface, forming an interface materials, the microber only responded to the photo-
interface consisting of photocatalysts and organics molecules catalytic process within ꢂ1 mm of its surface, as shown in Fig. 6d.
between the silica microber and the organic solution. Due to the Such a thin interface layer of nanometres in size is suitable for
p–p interaction between rGO and organic molecules and the monitoring the performance of the photocatalysts in real-time, in
4
6,47
high specic surface area of rGO,
combine with the Cu O–Ag@rGO and Cu
than with the Cu
O surface. Aer 3 h of white light irradiation, delineated the photocatalytic performance improvement
a certain number of organic molecules in the interface were mechanism of the Cu O–Ag@rGO nanocomposites, as shown in
degraded to water and carbon dioxide, inducing a reduction of Fig. 7. The UV-vis absorption spectra in Fig. 7a show that the
the interfacial RI.
cooperation of rGO and Ag nanoparticles expanded the
Fig. 6b shows the sensitivity of the silica microber with the absorption range in visible light, which endowed the Cu O–
RI ranging from 1.3340 to 1.3440, which covers the RI modu- Ag@rGO nanocomposites with higher photocatalytic activity by
organic molecules would situ and in trace concentrations.
O@rGO surfaces better
Combining the results obtained by the optical microber, we
2
2
2
2
2
49
lation range of the interface. Based on the corresponding linear making full use of sunlight. In the Cu
tting result, the silica microber shows an RI sensitivity of composites, Ag nanoparticles form a metal–semiconductor
814.5 nm per RIU. Since the photocatalytic interface is in the heterostructure with Cu O on rGO nanosheets. Wang and his
evanescent eld of the optical microber, the interfacial RI colleagues believed that a new Fermi energy (EfAg ¼ 4.30 eV)
2
O–Ag@rGO nano-

1
2
30
24
changes were the effective surface RI change of the microber.
This nding indicates that when the interfacial RI slightly photoexcited electrons can transfer from the conduction band
changes by ꢀ0.001, the microber records an obvious blueshi (CB) of Cu O to the surface of Ag nanoparticles due to the fact
of 1.81 nm in the optical transmission. Therefore, such that the CB of Cu O is higher than the new EfAg, as shown in
a microber is a sensitive approach to recording the photo- Fig. 7b. At the same time, aer the Schottky barrier forms
catalytic performance. In the case of Cu
O–Ag@rGO, the wave- between the Ag and Cu O metal–semiconductor interface, Ag
length shi in the in situ photocatalytic process was nanoparticles act as an electron trapper to suppress the
formed in this process. Upon irradiation with visible light, the
2
2
2
2
ꢀ
+
50,51
approximately 0.56 nm, corresponding to interfacial RI changes recombination of electrons (e ) and holes (h ).
This
of 3.09 ꢃ 10 in 180 min and 7.29 ꢃ 10 in the rst 50 min. improvement of the photocatalytic performance through the
Such slight interfacial RI changes and kinetics could be traced spatial separation of charge carriers for Cu O was caused by
by the microber, and thus, the photocatalytic process has been a vectorial charge transfer, as shown in Fig. 7c. On the other
ꢀ
4
ꢀ6
2
recorded.
hand, the rGO, with superior conductivity, can enhance the
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Fig. 6 (a) Schematic illustration of photocatalytic performance probing mechanism through the microfiber functionalized with Cu
2
O–Ag@rGO,
Cu O@rGO and Cu O. (b) The RI sensitivity of the silica microfiber. (c) The transverse electric field amplitude distributions of the HE12 mode of
2
2
the silica microfiber (the x-axis represents the distance from the microfiber core's centre). (d) Schematic of the interfacial photocatalysis inside
the evanescent field on the microfiber's surface. The lighter colours represent weaker evanescent field.
Fig. 7 (a) Absorption spectra of Cu
2
O–Ag@rGO, Cu
2
O@rGO and Cu
2
O nanocomposites. (b) The band structure of the Cu
O–Ag@rGO.
2
O–Ag junction and
Cu O–rGO junction. (c) Schematic mechanism of photodegradation of organics by Cu
2
2
20520 | J. Mater. Chem. A, 2018, 6, 20513–20522
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charge separation signicantly when the photoelectrons are Innovation Talents of Guangdong (No. 2014TQ01X539), and the
transferred from the Cu O to the rGO, which acts as an electron Fundamental Research Funds for the Central Universities (No.
2
44,45,52,53
transfer channel,
which is crucial for the electron- 21617420). We thank the AMERICAN JOURNAL EXPERTS for
dominated reduction reaction. It also provides more a specic editing the manuscript for grammar, clarity, and consistency
5
4,55
surface and broader absorption range
and improves the with a specic emphasis on improving the language of the text.
+
stability of Cu species and the oxygen defects, which can
absorb organics and increase the organics’ concentration
Notes and references
56
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