Carbon quantum dot sensitized Pt@Bi2WO6/FTO electrodes for enhanced photoelectro-catalytic activity of methanol oxidation

Article abstract of DOI:10.1039/c7ra01867c

Carbon quantum dot sensitized Pt@Bi₂WO₆/FTO electrodes (simplified as CQDs-Pt@Bi₂WO₆/FTO) were successfully prepared by loading platinum particles onto Bi₂WO₆ nanoplates via a photo-deposition method and sensitized carbon quantum dots (CQDs) via a dip-coating method. The photoelectro-catalytic properties of the Pt@Bi₂WO₆/FTO and CQDs-Pt@Bi₂WO₆/FTO electrodes for methanol oxidation were investigated. The results indicated that the CQDs-Pt@Bi₂WO₆/FTO electrode shows higher photoelectro-catalytic activity and better stability than that of the Pt@Bi₂WO₆/FTO electrode. The higher photoelectro-catalytic performance for methanol oxidation was attributed due to the special synergetic effects between the photocatalytic and electrocatalytic process under solar light irradiation. More importantly, the introduction of CQDs broaden the photoresponse range of the Bi₂WO₆ material and improves the mobility of the photocarriers. Meanwhile, the doped CQDs act as preferential adsorption sites for the intermediate carbonaceous species during methanol oxidation. This not only alleviates CO poisoning towards the Pt particles, but also improves the efficiency of methanol oxidation by constructing a new type of CQDs-Pt electrocatalyst. The composite material, which combines the dual function of photocatalysis and electrocatalysis will be a promising candidate for new photoelectro-catalytic fuel cells.

Full text of DOI:10.1039/c7ra01867c

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Carbon quantum dot sensitized Pt@Bi2WO6/FTO
electrodes for enhanced photoelectro-catalytic
activity of methanol oxidation
Cite this: RSC Adv., 2017, 7, 26943
ab
Ping Niua and Zhefei Zhaoa
*
Huajun Zheng,
Carbon quantum dot sensitized Pt@Bi2WO6/FTO electrodes (simplified as CQDs-Pt@Bi2WO6/FTO) were
successfully prepared by loading platinum particles onto Bi2WO6 nanoplates via a photo-deposition
method and sensitized carbon quantum dots (CQDs) via a dip-coating method. The photoelectro-
catalytic properties of the Pt@Bi2WO6/FTO and CQDs-Pt@Bi2WO6/FTO electrodes for methanol
oxidation were investigated. The results indicated that the CQDs-Pt@Bi2WO6/FTO electrode shows
higher photoelectro-catalytic activity and better stability than that of the Pt@Bi2WO6/FTO electrode. The
higher photoelectro-catalytic performance for methanol oxidation was attributed due to the special
synergetic effects between the photocatalytic and electrocatalytic process under solar light irradiation.
More importantly, the introduction of CQDs broaden the photoresponse range of the Bi2WO6 material
and improves the mobility of the photocarriers. Meanwhile, the doped CQDs act as preferential
adsorption sites for the intermediate carbonaceous species during methanol oxidation. This not only
alleviates CO poisoning towards the Pt particles, but also improves the efficiency of methanol oxidation
by constructing a new type of CQDs-Pt electrocatalyst. The composite material, which combines the
dual function of photocatalysis and electrocatalysis will be a promising candidate for new photoelectro-
catalytic fuel cells.
Received 14th February 2017
Accepted 4th May 2017
DOI: 10.1039/c7ra01867c
rsc.li/rsc-advances
Pt nanoparticles doped on TiO2 nanotubes (Pt-NPs/TNTs) were
fabricated for alternative anode materials. The Pt-NPs/TNTs
1. Introduction
electrodes signicantly enhanced the electrocatalytic perfor-
mance and stability for methanol oxidation. The oxidation peak
currents on the Pt-NPs/TNTs electrodes are several times larger
than those on a platinum electrode and the photocatalytic
properties of the TiO2 make the Pt-NPs/TNTs electrode reusable
aer a short UV treatment.12 From the point of view of their
nature, the anode materials for methanol oxidation reaction
should use a photocatalyst with a strong oxidation ability as
a substrate. As known, bismuth tungstate (Bi2WO6) is one of the
simplest members of the Aurivillius compounds with a special
layered structure, which can respond to visible-light up to
460 nm.13 The more attractive merits of Bi2WO6 are its strong
oxidation ability with the lower valence band potential (3.2 eV
vs. NHE). In fact, Bi2WO6 materials comprised of various
complicated 3D hierarchical nanostructures have been synthe-
sized and used as the substrate for noble metals. Zhang et al.
have reported that Ag nanoparticles can be rmly anchored on
a Bi2WO6 heterostructure via a photo-deposition method. The
decoration of Ag effectively broadened the visible-light response
of Bi2WO6 and inhibited the recombination of the photo-
generated electron–hole pairs. The synergistic effect between Ag
and Bi2WO6 resulted in an optimum photodegradation effi-
ciency with a rate constant 3.1-folds greater than Bi2WO6.13
Direct methanol fuel cells (DMFCs), as an environmentally
friendly energy conversion technology, have attracted more and
more attention in recent decades.1–5 The key factor of the
industrial application of DMFCs is a catalyst with high elec-
trocatalytic activity and a long-duration of stability.6 To date,
platinum-based electrodes, as one of the best catalytic mate-
rials, have been widely used in the electrocatalytic oxidation of
methanol. However, the major limitations of platinum-based
materials are that they are expensive and easily poisoned via
strong CO adsorption on its active sites.7 One effective strategy
to reduce the amount of precious metals in the catalysts and
improve their performance is the development of an innovative
type of photoelectrocatalyst, which loads less precious metal
(such as Pt, Au, Ru and Ag) particles onto photocatalysts. The
photocatalytic oxidation of methanol takes place on the pho-
tocatalyst under solar light illumination and the oxidation
reaction will be simultaneously electrocatalysed by the precious
metal particles. Recently, more photoelectrocatalysts have been
developed as anode materials used for methanol oxidation.8–11
aDepartment of Applied Chemistry, Zhejiang University of Technology, Hangzhou
310032, China. E-mail: zhenghj@zjut.edu.cn
bState Key Laboratory Breeding Base of Green Chemistry Synthesis Technology,
Zhejiang University of Technology, Hangzhou 310032, China
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Methanol is oxidized to various intermediate carbonaceous purchased from Yinkou Opvtech Co., Ltd. Deionized Water was
species (such as CO, –CHO, –COOH and CH4) according to obtained from a Hitech-K ow water purication system.
various oxidation process and reaction pathways.14 The inter-
mediate carbonaceous species (mainly CO) are easily adsorbed
on the electrode surface, blocking the active sites and signi-
cantly slows the overall reaction. Therefore, an important
method to improve the catalytic stability is to alleviate CO
poisoning towards the platinum-based electrocatalyst. One
solution is to reduce the intermediate production by controlling
the oxidation process. When more holes or OHc with high
activity gather around the active sites on the surface of the
anode in the electrocatalytic oxidation of methanol, the OHc
facilitates the local methanol concentration to the neighbor
noble metal1,9 and then can oxidize intermediate CO,2 including
the methanol molecules. Another solution is the construction of
a Pt–M bimetallic catalyst with a second metal M (M ¼ Ru, Co,
Ni, Fe, Mn), which allows the oxidation of CO at a lower
potential than with Pt. Pt–Ru alloys in particular have been
shown to be especially successful in the prevention of Pt catalyst
poisoning.15
2.2 Preparation of the Bi2WO6/FTO sample
The Bi2WO6/FTO samples were prepared via a hydrothermal
reaction from unred WO3/FTO samples. The WO3/FTO
samples were prefabricated according to a method, which has
been reported in our previous work.18,27 In a typical run, 4 mmol
of Bi(NO3)3$5H2O was dissolved into 50 mL of 5% dilute nitric
acid and was stirred until the solution became transparent.
Then, the transparent liquid was added into an 80 mL Teon-
lined autoclave and the WO3/FTO was vertically placed into it.
The ꢂautoclave was sealed in a stainless steel tank and heated at
160 C for 24 h. Subsequently, the reactor was allowed to cool
down to room temperature naturally. The resulting samples
were taken out and washed with deionized water and absolute
ethyl alcohol, and then dried in air for 0.5 h. Subsequently,
thermal treatment of the Bi2WO6/FTO was cꢀa1rried out at 400 ꢂC
ꢂ
in air for 4 h at a ramping rate of 2 C min
.
Recently, carbon quantum dots (CQDs) have been intro-
duced to semiconductor materials, including CQDs/TiO2,16
2.3 Preparation of the CQDs-Pt@Bi2WO6/FTO sample
CQDs/WO3,17,18 CQDs/Fe3O4,19 CQDs/BiOX,20–22 CQDs/Bi2WO6,23 The Pt@Bi2WO6/FTO sample was prepared via a photo-
CQDs/Bi2MoO6 (ref. 24) and CQDs/BiVO4,25 due to their excel- deposition method. The as-prepared Bi2WO6/FTO sample was
lent electron transfer ability. Chang et al. have fabricated immersed in 0.5 mM H2PtCl6 and was irradiated with UV and
Cu2ꢀxS/carbon nanodots (C dots) electrodes for direct methanol visible light for 1 h. Subsequently, the Pt@Bi2WO6/FTO sample
fuel cells and the EIS results revealed that it has a lower charge- was completed aer washing with deionized water and ethanol.
transfer resistance.26 Moreover, CQDs can act as the preferential The obtained lms were dried overnight at 60 C.
adsorption sites for carbon monoxide to achieve the transfer of The CQDs-Pt@Bi2WO6/FTO samples were prepared via a dip-
ꢂ
the CO adsorption sites, reducing the adsorption of CO on Pt. coating method. The CQDs were synthetized according to
Then, the CO adsorbed on CQDs will be oxidized by OHc,2 which a literature procedure followed by a thermal treatment.28 The as-
is produced via Bi2WO6 photocatalysis. Therefore, the CQDs-Pt prepared Pt@Bi2WO6/FTO sample was immersed in a specied
catalyst will be benecial in reducing the incidence of CO concentration of CQDs aqueous solution for 1 h. Finally, the
poisoning towards the Pt particles and improve the efficiency of CQDs-Pt@Bi2WO6/FTO samples were obtained by washing with
methanol oxidation.
deionized water and dried in air.
In this paper, we demonstrate our strategy for designing
a novel bifunctional catalyst (CQDs-Pt@Bi2WO6/FTO), which is
based on Bi2WO6 nanoplates as a photocatalyst and platinum
particles as an electrocatalyst. By doping CQDs into the
Pt@Bi2WO6 nanoplates, not only do they increase the photo-
catalytic activity of Bi2WO6 and improve the utilization of solar
energy, but also the sensitized CQDs are the preferential
absorption sites for the intermediate carbonaceous species,
which can alleviate CO poisoning towards the Pt particles.
Therefore, the CQDs-Pt@Bi2WO6/FTO electrode reveals excel-
lent catalytic activity and better stability, and is a potential
candidate for photoelectro-catalytic cells.
2.4 Photocatalyst characterization
The morphology, microstructure and chemical composition of
the samples were observed using transmission electron
microscopy (TEM, JEOL, JEM-2010) and scanning electron
microscopy (SEM, Hitachi S+4800) with the elemental mapping
images and EDX. Powder X-ray diffraction (XRD, Bruker D8 with
Cu-Ka radiation) experiments were carried out to analyze the
crystallographic structural information of the samples. X-ray
photoelectron spectroscopy (XPS, Thermo Fisher Scientic,
USA) was performed using an ESCALab MKII spectrometer with
Al Ka (1.4866 keV) as the X-ray source. The UV-vis diffuse
reectance spectra of the samples were recorded on a UV2700
type spectrophotometer.
2. Experimental
2.1 Materials and reagents
2.5 Photocatalytic(PC) and photoelectron-catalytic(PEC)
measurement
Ethanol, nitric acid, hydrochloric acid, citric acid, sodium
hydroxide, chloroplatinic acid (H2PtCl6), sodium tungstate
(Na2WO4$2H2O) and bismuth nitrate (Bi(NO3)3$5H2O) were The photocurrent responses were investigated using a three-
purchased from Aladdin. All the reagents were of analytical electrode system using an electrochemical workstation (CHI
grade and were used without any further purication. Fluorine- 760D), wherein the modied Bi2WO6/FTO electrode, platinum
doped tin oxide (FTO) conducting glass (15 ꢁ 50 mm) was plate (1 cm ꢁ 1 cm) and saturated calomel electrode (SCE)
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served as the working electrode, the counter electrode and the was vertically grow on the FTO glass. When the photo-
reference electrode, respectively. A mixed solution of 1.0 M KOH deposition reaction and dip-coating steps were carried out in
and 0.5 M CH3OH was used as the electrolyte. Before the turn, the platinum particles were successfully deposited on the
experiments, all the solutions were purged with nitrogen for surface of the Bi2WO6/FTO nanoplates, which can be observed
30 min in order to exclude O2. Electrochemical impedance in the magnied SEM images. As shown in Fig. 1c and d, the
spectroscopy (EIS) was recorded between 0.1 and 105 Hz with an morphology of the Bi2WO6/FTO nanoplates did not change,
AC voltage amplitude of 5.0 mV. All PEC measurements were however, the surface becomes rough. Direct evidence of the
under a solar light simulator with a 150 W Xenon arc lamp deposited Pt comes from the element mapping spectrum
(Beijing, Perfect Labsolar 300). The light intensity was measured (elements distribution of Pt, Bi, W, O and C) of the CQDs-
to be 100 mW cmꢀ2. Amperometric I–t curves were obtained at Pt@Bi2WO6/FTO sample (Fig. 1e). It was found that the Pt
0 V (vs. SCE) with light on-off switches of 20 s.
elements were evenly distributed on the surface of the nano-
plates, which conrms that the Pt particles were photo-
deposited on the surface of the nanoplates. This is also sup-
ported by the energy dispersive X-ray spectroscopy (EDX) results
and the content of the Pt, Bi, W, O and C elements were
measured in Fig. 1f, respectively.
More interestingly, the C element from the mapping spec-
trum and its contents from the EDX spectrum provided some
information on the CQDs adsorbed on the Pt@Bi2WO6/FTO
sample. It is difficult to observe any change in the morphology
of the original substance because a small quantity of CQDs is
loaded via simple electrostatic adsorption. However, evidence of
their existence on the surface of the Bi2WO6 nanoplates can be
further provided by the high-resolution transmission electron
microscopy (HRTEM) images. The HRTEM image in Fig. 2
shows the uniform decoration of the CQDs throughout the
surface of the Bi2WO6 nanoplates and the lattice spacing of the
CQDs (Fig. 2b) is about 0.321 nm, which can be ascribed to the
(002) crystallographic planes of the CQDs. These results are
consistent with the observations previously reported.30
3. Results and discussion
3.1 Structure and morphology
The surface morphologies of the as-prepared samples were
characterized by SEM. Fig. 1 depicts the SEM images of the
Bi2WO6/FTO and CQDs-Pt@Bi2WO6/FTO samples. The growth
mechanism of the WO3 nanoplates on surface of the FTO glass
has been reported in our previous study.19,28 In this work, WO3
can be converted to Bi2WO6 because the unred WO3 lm
contains water molecules and the (Bi2O2)2+ units can replace the
water molecules during the hydrothermal process.29 Fig. 1a
shows the SEM images of the uniform Bi2WO6 nanoplates with
a thickness of 20–30 nm. From the cross-sectional SEM image of
the Bi2WO6/FTO sample, as shown in Fig. 1b, it can be clearly
seen that the Bi2WO6 layer with the thickness around 2–2.5 mm
The surface compositions and chemical states of CQDs-
Pt@Bi2WO6/FTO were analysed by XPS and the results are dis-
played in Fig. 3. The typical survey scans in Fig. 3a display the
ve elements of Bi, W, O, C and Pt. The trace amount of C was
more likely to come from the carbon quantum dots. Fig. 3b–f
shows the high resolution XPS spectra of Bi 4f, W 4f, O 1s, C 1s
and Pt 4f, respectively. The peaks in Fig. 3b at 164.4 and
159.1 eV correspond to the Bi 4f5/2 and Bi 4f7/2 orbitals of Bi3+
.
The peaks at 37.2 and 35.0 eV correspond to the W 4f5/2 and W
4f7/2 orbitals of W6+, as shown in Fig. 3c. The convoluted peaks
at 530.2 and 532.6 eV of O 1s in Fig. 3d were consistent with the
different chemical environments of the oxygen element in Bi–O
Fig. 1 SEM images of the as-prepared samples: Bi2WO6/FTO (a), (b)
and CQDs-Pt@Bi2WO6/FTO (c), (d). The elemental mapping image (e) Fig. 2 (a) TEM and (b) HRTEM images of the as-prepared sample:
and EDX spectrum (f) for CQDs-Pt@Bi2WO6/FTO.
CQDs@Bi2WO6/FTO.
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samples were almost identical to Bi2WO6/FTO, which suggests
the composites do not change the crystal structure of Bi2WO6.
The results also indicate that the CQDs and Pt contents in
CQDs-Pt@Bi2WO6/FTO samples were very rare.
3.2 Optical absorption properties
The optical absorption properties of the Bi2WO6/FTO, CQDs-
Bi2WO6/FTO, Pt@Bi2WO6/FTO and CQDs-Pt@Bi2WO6/FTO
samples were investigated using UV-vis spectrophotometry and
the results are shown in Fig. 5a. The UV-vis diffuse reectance
spectrum of the Bi2WO6/FTO sample showed the absorption
edge at ꢃ450 nm. When it was sensitized by the CQDs, a small
red-shi was observed in the absorption edge and the absorp-
tion edge of the CQDs@Bi2WO6/FTO sample increases to
470 nm. Similar results were observed in the Pt@Bi2WO6/FTO
sample aer doped CQDs. The photo-response range of the
Pt@Bi2WO6/FTO sample was around 500 nm and the range of
the CQDs-Pt@Bi2WO6/FTO sample was up to 510 nm. Similarly,
because the presence of the metallic Pt particles contributes to
the enhanced absorption in the visible light region, the range of
light absorption for the Pt@Bi2WO6/FTO sample was wider than
Fig. 3 The XPS spectra of the CQDs-Pt@Bi2WO6/FTO: (a) Full scan, (b)
Bi 4f, (c) W 4f, (d) O 1s, (e) C 1s and (f) Pt 4f.
and W–O.31 The peak at 531.4 eV corresponds to the hydroxyl
groups on the surface of Bi2WO6. Fig. 3e shows the C 1s spectra
of CQDs-Pt@Bi2WO6/FTO. By using a Gaussian tting, the C 1s
spectra can be deconvoluted into three peaks (284.6, 286.26 and
288.38 eV), which were ascribed to the C–C, C–O and C]O
bonds, respectively.23 The high-resolution Pt 4f spectrum can be
deconvoluted into four peaks as shown in Fig. 3f. The strong
signals, two peaks at a low binding energy of 74.85 eV and a high
binding energy at 78.5 eV, are because of the Pt4+ species, sug-
gesting that Pt in the CQDs-Pt@Bi2WO6/FTO was mainly in the
form of Pt(OH)4. The relatively weak peaks located at 73.1 and
76.5 eV can be signed to the Pt2+ chemical state.32,33
Fig. 4 displays the XRD patterns of the Bi2WO6/FTO, CQDs-
Bi2WO6/FTO and CQDs-Pt@Bi2WO6/FTO samples. For Bi2WO6/
FTO sample, the peaks at 28.3ꢂ, 32.9ꢂ, 47.2ꢂ, 55.8ꢂ, 76.0ꢂ and
78.5ꢂ correspond to the (131), (002), (202), (331), (210) and (204)
planes of orthorhombic Bi2WO6 (JCPDS: 39-0256). The peaks at
26.6ꢂ, 33.8ꢂ, 51.8ꢂ, 61.7ꢂ, 65.7ꢂ correspond to the (110), (101),
(211), (310) and (301) planes of FTO (JCPDS: 46-1088). No peaks
indicating impurities are observed, demonstrating that the WO3
have been completely translated into Bi2WO6 and the nal
samples have good crystallinity and purity. Aer the deposition
of the Pt nanoparticles and CQDs doping, the main peaks of
both the CQDs@Bi2WO6/FTO and CQDs-Pt@Bi2WO6/FTO
Fig. 5 The UV-vis diffuse reflectance spectra (a) and Tauc plots of the
Fig. 4 The XRD patterns of Bi2WO6/FTO, CQDs-Bi2WO6/FTO and corresponding absorption spectra (b) of Bi2WO6/FTO, CQDs-Bi2WO6/
CQDs-Pt@Bi2WO6/FTO.
FTO, Pt@Bi2WO6/FTO and CQDs-Pt@Bi2WO6/FTO.
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that of the Bi2WO6/FTO sample. This phenomenon is highly
consistent with the previous reports.34
The band gap energies of the four type samples were calcu-
lated based on the Tauc plot:
ahn ¼ A (hn ꢀ Eg)n
where h is the Planck's constant, n is the frequency of the
vibration, a is the absorption coefficient, Eg is the band gap of
the semiconductor and n is a constant, which is 1/2 for a direct
transition or 2 for an indirect transition.35 For the Bi2WO6
semiconductor, n is equal to 1/2. The calculated Eg values of
Bi2WO6/FTO, CQDs-Bi2WO6/FTO, Pt@Bi2WO6/FTO and CQDs-
Pt@Bi2WO6/FTO sample were 2.90, 2.84, 2.57 and 2.55 eV,
respectively (Fig. 5b). Obviously, the band gap of the sample is
decreased aer being doped with the CQDs and the photo-
deposition of the Pt particles.
3.3 Methanol oxidation
The photocatalytic oxidation abilities towards methanol on the
Bi2WO6/FTO and CQDs-Bi2WO6/FTO electrodes were investi-
gated using the amperometric I–t curves (photocurrent
response) under light illumination conditions in 1 M KOH or
1 M KOH + 0.5 M CH3OH aqueous solution. There are two main
factors that may affect the photocurrent density. One is the
oxygen evolution reaction (OER) and the other is the methanol
oxidation reaction (MOR). The photocurrent density of the
electrode in 1 M KOH aqueous solution was attributed to the
OER, while that in 1 M KOH + 0.5 M CH3OH aqueous solution
was attributed to both the OER and MOR. As shown in Fig. 6,
the photocurrent density on both the Bi2WO6/FTO and CQDs-
Bi2WO6/FTO electrodes can be measured in two types of elec-
trolytes. This phenomenon can be explained by the valence
band potential of Bi2WO6 being relatively positive, which indi-
cates that Bi2WO6 has a stronger photocatalytic oxidation ability
for the OER and MOR. More importantly, there was a signicant
increase in the photocurrent density of the Bi2WO6/FTO elec-
trode upon CQDs doping (Fig. 6b), whether in the presence of
methanol or not. This may be due to the strong ability of the
CQDs to capture and transfer the photo-generated carriers.36
Furthermore, both the Bi2WO6/FTO and CQDs-Bi2WO6/FTO
electrodes in the absence of methanol are severely reduced with
an increase in time. On the contrary, the photocurrent densities
of both electrodes in 1 M KOH + 0.5 M CH3OH aqueous solution
show an increasing tendency and then reach a plateau. The total
photocurrent density of the Bi2WO6/FTO and CQDs-Bi2WO6/
FTO electrode reached 0.044 and 0.068 mA cmꢀ2, and the other
from the MOR are 0.032 and 0.048 mA cmꢀ2, respectively. This
conrmed the excellent photocatalytic properties for methanol
oxidation on the Bi2WO6/FTO and CQDs-Bi2WO6/FTO
electrodes.
Fig. 6 The I–t curves of the (a) Bi2WO6/FTO and (b) CQDs-Bi2WO6/
FTO samples; the measurements were conducted at 0 V (vs. SCE)
under a 150 W Xenon arc lamp.
a scan rate of 50 mV sꢀ1 with or without solar light, respectively.
Undoubtedly, the reaction on the electrodes without solar light
illumination is an electrochemical process, while both the
electrocatalytic and photocatalytic methanol oxidation
processes are under light irradiation. From Fig. 7a and b,
whether under solar light illumination or not, it is obvious that
the classical phenomena of electrocatalytic methanol oxidation
can be observed, that is, a double oxidation peak both in the
forward sweep and backward sweep. The forward peak current
density (If) and the backward peak current density (Ib) of the two
type of electrodes with and without solar light illumination are
listed in Table 1. Obviously, the Pt particles attached on the
Bi2WO6/FTO electrode play the main role in the electrochemical
methanol oxidation and the effect of Bi2WO6/FTO for methanol
oxidation should be in the photocatalytic oxidation and syner-
gistic effect for electro-oxidation.
Cycle voltammetry (CV) was employed to examine the elec-
trocatalytic and photoelectro-catalytic performance of the
Pt@Bi2WO6/FTO and CQDs-Pt@Bi2WO6/FTO electrodes for
methanol oxidation. The CV curves obtained for these elec-
trodes at the 100th cycle are shown in Fig. 7a and b, which were
carried out in an 1 M KOH + 0.5 M CH3OH aqueous solution at
In order to study the role of Bi2WO6, Pt/FTO was rstly
investigated using CV. For the Pt/FTO electrode, the forward
peak current density and the backward peak current density
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Fig. 7 The 100th cycle voltammetry curves of the (a) Pt@Bi2WO6/FTO and (b) CQDs-Pt@Bi2WO6/FTO electrodes compared to the Pt/FTO
electrode without light in 1.0 M KOH + 0.5 M CH3OH at a scan rate of 50 mV sꢀ1. (c) The chronoamperometry curves of the electrodes under
solar light in 1.0 M KOH + 0.5 M CH3OH at a potential of ꢀ0.2 V (vs. SCE) for 1200 s. The EIS of the (d) Pt@Bi2WO6/FTO and (b) CQDs-Pt@Bi2WO6/
FTO electrodes with or without solar light illumination.
Table 1 The electrocatalytic and photoelectro-catalytic parameters of the different electrodes
Pt@Bi2WO6/FTO
Electro-oxidation
CQDs-Pt@Bi2WO6/FTO
Electro-oxidation
Electrode
Photoelectro-oxidation
Photoelectro-oxidation
If (mA cmꢀ2
)
)
0.52
0.30
0.70
0.48
0.73
0.45
0.89
0.64
Ib (mA cmꢀ2
without light reaches 0.12 and 0.09 mA cmꢀ2, respectively. In a cooperative catalyst and a Pt support material to improve the
addition, the methanol oxidation peak appears accordingly at electrocatalytic performance.8 Under solar light irradiation, the
the potentials of ꢀ0.20 and ꢀ0.27 V (vs. SCE), respectively. For forward and backward peak current density for methanol
the Pt@Bi2WO6/FTO electrode, as shown in Fig. 7a, it can be oxidation remarkably increases to 0.70 and 0.48 mA cmꢀ2, and
perceived that the forward peak current density (0.52 mA cmꢀ2
)
the peak potentials also positive shi to ꢀ0.18 and ꢀ0.25 V. The
and the backward peak current density (0.30 mA cmꢀ2) without positive shi in the potential aer illumination is also consis-
light are ꢃ4.3 and 3.33 times higher than that from the Pt/FTO tent with the literature.10 The increased peak current density
electrode, respectively. In addition, the methanol oxidation (0.18 mA cmꢀ2) and the positive shied peak potential (0.01 V)
double peaks appear accordingly at the potentials of ꢀ0.19 and can be attributed to photocatalytic oxidation from the Bi2WO6
ꢀ0.26 V (vs. SCE), respectively, which is consistent with the nanoplates and their synergistic effect between electro-
literature.37 When compared with Pt/FTO, the potential is oxidation and photoelectro-oxidation. For the CQDs-Pt@Bi2-
positive (0.01 V). The enhanced peak current density and the WO6/FTO electrode, as shown in Fig. 7b, the two oxidation peak
positive potential were due to the fact that Bi2WO6 acts as current densities without light are 0.73 and 0.45 mA cmꢀ2, and
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their corresponding potentials are ꢀ0.16 and ꢀ0.25 V, respec-
tively. When compared with Pt/FTO, the forward peak current
density and the backward peak current density of the CQDs-
Pt@Bi2WO6/FTO electrode without light are ꢃ6.08 and 5.00
times higher and the corresponding potential is positive (0.04
V). Similarly, when receiving solar light irradiation, the
photoelectro-oxidation of methanol takes place, which brings
about further increased peak current densities and further
positively shied peak potentials. The highest forward and
backward peak current density reaches 0.89 and 0.64 mA cmꢀ2
,
and the peak potentials are positively shied to ꢀ0.15 and
ꢀ0.24 V, respectively. When compared with the oxidation peak
current densities and peak potentials of the two electrodes
under solar light irradiation, it clearly reveals that the CQDs
provide a promotion effect on the photoelectro-oxidation of
methanol. Just as the energy gap of the sample can be decreased
by CQDs doping, the take-off and peak potentials were visibly
positively shied. This further infers that the CQDs act as
electron acceptors to extract the photogenerated electrons from
the Bi2WO6 nanoplates and transfer to the Pt particles. What's
more, the lower peak potential and higher current density are
essentially practical from an economical point of view.
The chronoamperometry curves of the CQDs-Pt@Bi2WO6/
FTO and Pt@Bi2WO6/FTO electrodes were measured in 1 M
KOH + 0.5 M CH3OH at a potential of ꢀ0.2 V (vs. SCE) for 1200 s
to verify the stability of the electrodes.38 As shown in Fig. 7c, the
current density of the two types of electrodes decays quickly
during the initial 200 s. De Dios and Salgueirino considered that
this is because the as-generated intermediate species (mainly
CO) are absorbed on the active sites of electrode during the
initial stage of the methanol oxidation reaction.39 As the
oxidation reactions occur, the current density continues to
decrease but the trend become slow and the CQDs-Pt@Bi2WO6/
FTO electrode shows a lower declining rate when compared
with the Pt@Bi2WO6/FTO electrode. This is more evidence that
the introduction of CQDs is benecial for improving the catalyst
tolerance towards CO.
Fig. 8 The current density produced by the different catalytic actions.
produced via photocatalysis, electrocatalysis and photoelectron-
catalysis on the two types of electrodes were summed up in
Fig. 8. The current density produced by photoelectron-catalysis
is greater than the sum of the current densities induced by
photocatalysis and electrocatalysis, respectively. Undoubtedly,
the increment of the current density may be attributed to
a synergistic effect between the photocatalytic and electro-
catalytic oxidation of methanol.
On the basis of the above experimental results and the litera-
ture,37,40 a schematic representation of photoelectro-catalytic
oxidation of methanol at the CQDs-Pt@Bi2WO6/FTO electrode
under light irradiation is given in Fig. 9. When the electrode is
exposed to solar light irradiation, the photogenerated electrons
(eꢀ) and holes (h+) are obtained in photocatalyst Bi2WO6. The
holes (h+) produced in the valence band of Bi2WO6 will migrate to
the Pt particles, then oxidize the methanol and produce CO2 and
H2O. The photoelectro-catalytic reaction of methanol oxidation on
the Pt particles and Bi2WO6 nanoplates is:
2Bi2WO6 + hn / Bi2WO6 (eꢀ) + Bi2WO6 (h+)
(1)
EIS was used to further evaluate the intrinsic behavior of the
anodic and photo-anodic processes. The charge transfer resis-
tance of the electrode can be deduced by calculating the
diameter of the primary semicircle. The Nyquist plots of EIS for
the Pt@Bi2WO6/FTO and CQDs-Pt@Bi2WO6/FTO electrodes are
shown in Fig. 7d. It was observed that the semicircle radius on
the Nyquist plots of the EIS obtained for the sensitized CQDs
electrode are much smaller than the other electrodes whether
they were under light irradiation or not, clearly indicating that
the incorporation of CQDs results in the improved conductivity
of the electrode. Furthermore, in contrast to the dark state, the
resistances of the two types of electrodes were reduced under
solar light irradiation, which means that the synergistic effects
between the electro-catalytic process and photoelectro-catalytic
process are benecial for the charge transfer in methanol
oxidation. Herein, it is reported that the CQDs-Pt@Bi2WO6/FTO
electrode has less impedance, which is consistent with the I–t
curve results.
Fig.
9 A schematic representation of the photoelectro-catalytic
In order to explore the synergistic effect of the photocatalytic
oxidation of methanol using the CQDs-Pt@Bi2WO6/FTO electrode
and electrocatalytic oxidation methanol, the current densities under solar light irradiation.
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Pt + xBi2WO6 (h+) + CH3OH + H2O /
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In the present work, we have synthesized Bi2WO6 nanoplates on
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The Pt particles distributed on the Bi2WO6 nanoplates produced
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Acknowledgements
The nancial support from Zhejiang scientic and technolog-
ical projects (No. 2009R50002-20) is gratefully acknowledged.
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