Molecular cobalt salophen catalyst-integrated BiVO4 as stable and robust photoanodes for photoelectrochemical water splitting

Article abstract of DOI:10.1039/c8ta01304g

Photoelectrochemical (PEC) water splitting is a promising method for the conversion and storage of solar energy. A combination of catalysts with photoelectrodes is generally required for the development of active photoanodes in PEC devices. In this work,

Full text of DOI:10.1039/c8ta01304g

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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Molecular Cobalt Salophen Catalyst-Integrated BiVO4 as Stable
and Robust Photoanodes for Photoelectrochemical Water
Splitting
Received 00th January 20xx,
Accepted 00th January 20xx
Yidan Liu, a Yi Jiang,*a Fei Li, b Fengshou Yu, b Wenchao Jiang a and Lixin Xia*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Photoelectrochemical (PEC) water splitting is a promising method for the conversion and storage of solar energy. A
combination of catalysts with photoelectrodes is generally required in the development of active photoanodes in PEC
devices. In this work, we present two BiVO4 photoanodes modified with cobalt salophen (Co(salophen)) complexes for PEC
water oxidation. The resulting photoanodes show significantly enhanced PEC performance. Under simulated sunlight
illumination (AM 1.5G, 100 mW/cm2), high photocurrents of 3.89 mA/cm2 and 4.27 mA/cm2 were obtained for Co1/BiVO4
and Co2/BiVO4 respectively at 1.23 V (vs. reversible hydrogen electrode (RHE)) in a neutral solution, an almost three-fold
enhancement over that of the unmodified BiVO4. Intensity-modulated photocurrent spectroscopy (IMPS) analysises show
that the Co(salophen) complexes, not only accelerate the water oxidation reaction, but also reduce the surface
recombination. The half-cell solar energy conversion efficiencies for Co1/BiVO4 and Co2/BiVO4 were 1.09% and 1.18% at
0.7 V, respectively. Due to their hydrophobic nature, the Co(salophen) complexes can bind strongly to the surface of
BiVO4. When the Co2 complex featuring four hydrophobic tert-butyl groups in salophen ligand was anchored to BiVO4, an
extremely stable photocurrent of more than 3.5 mA/cm2 at 1.23 V vs. RHE is sustained for at least 3 h without decay. Such
a stable and robust photoanode based on molecular WOC surpass those attained by most of the state-of-the-art
heterogeneous catalysts.
sluggish water oxidation kinetics. An efficient way to overcome
this problem is integrating BiVO4 electrodes with a water
oxidation catalyst (WOC).11-17
Introduction
Photoelectrochemical (PEC) water splitting to generate
hydrogen using sunlight offers an attractive route to achieve
the conversion and storage of solar energy.1-3 In a PEC device,
the oxygen evolution reaction (OER) in the photoanode as the
key half reaction is generally considered as the bottleneck, due
to its multiple electron and proton processes which are
thermodynamically demanding.4-6 Therefore, one of the
barriers needed to be overcome prior to a widespread use of
this technology is development of stable and efficient
photoanodes.7,8 BiVO4 has been considered a promising
candidate due to its appropriate bandgap of 2.4 eV to harvest
visible light, and has other merits including abundance, low
cost, nontoxicity, and suitable valence band energy for water
oxidation.9,10 However, the PEC performance of unmodified
BiVO4 has been limited by its poor charge transport and
Previous studies have revealed that the immobilisation of
inorganic WOCs such as Co-Pi11,12 and FeOOH/NiOOH13 can
improve the PEC performance of BiVO4. However, these
heterogeneous WOCs on the electrode surface have been
generally
prepared
through
electrodeposition
or
photodeposition, and in this process the thickness of the
catalyst film must be controlled carefully because of its
significant influence on light absorption and charge
transport.18,19 Apart from heterogeneous WOCs, molecular
WOCs have recently attracted attention due to their high
activity, structural tuneability, and facility in mechanistic
studies.20-23
However, there are relatively few reports on the
combination of BiVO4 photoanodes with a molecular co-
catalyst.24-26 One such recent study by Li and co-workers
researched a hybrid photoanode by combining BiVO4 film with
a molecular co-cubane catalyst, which gives a high PEC
performance.26 Though in their work, an alkaline condition (pH
9) was required, which is not considered environmentally
friendly. Therefore, a molecular catalyst-based photoanode
with high activity and stability remains a prerequisite for
efficient PEC water splitting under mild and environmentally
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of carbon, nitrogen and cobalt by EDX spectroscopy of (d)
Co1/BiVO4 electrode and (e) Co2/BiVO4 electrode.
friendly conditions. Given this, our research reports on two
hybrid photoanodes, fabricated by combining molecular
Co(salophen) complexes onto
a porous BiVO4 surface.
Cobalt(II) salophen complexes are easily synthesised and have
been well studied in many catalytic fields.27,28 Recently, a
Co(salophen) complex was shown to be an effective water
oxidation catalyst either in a homogeneous system or on the
electrode surface.29-32
Characterisation of Photoanodes
The BiVO4 thin films on the FTO-coated glass substrates were
fabricated by a two-step procedure described previously by
Choi and co-workers (see the experimental section for details)
and the X-ray diffraction peaks are essentially consistent with
that reported previously (Figure S1).13 The assembled
Co(salophen)/BiVO4 electrodes were facilely prepared by
soaking the BiVO4 electrode in a solution of Co(salophen)
complex in acetonitrile at room temperature, the as-prepared
electrodes are referred to as Co1/BiVO4 and Co2/BiVO4. Due to
their hydrophobic nature, the Co(salophen) complexes are
sufficiently stable when assembled on the surface of BiVO4.
Static contact angle measurements (Figure S2) demonstrate
that the hydrophobic nature of the electrodes are enhanced
In this work, two Co(salophen) complexes (Scheme 1) were
directly and physically absorbed onto porous BiVO4 electrodes
via hydrophobic interaction. Compared with the conventional
loading methods of molecular catalysts, this facile method
avoids the high cost by using Nafion33-35 as the adhesive and
the complicated synthetic process required in the introduction
of a -PO3H2 or -COOH binding group.36-39 The hydrophobic
property of the Co(salophen) complex provides a strong
binding interaction to the electrode surface rather than
dissolving into the solution. The as prepared hybrid
photoanodes exhibit excellent PEC activity and stability at
neutral pH, which is even higher than those attained by most
of the state-of-the-art heterogeneous catalysts. To the best of
our knowledge, such a stable and robust hybrid photoanode,
based on molecular catalyst in neutral condition, is yet to be
reported.
o
after the modification of Co(salophen) complexes (37.1 for
Co1/BiVO4, 47.3 o for Co2/BiVO4).
Scanning electron microscopy (SEM) was used to observe
the morphology of the as prepared electrodes. The bare BiVO4
film shows the appearance of a three-dimensional (3D)
network, composed of multiple worm-like nanoparticles
(Figure 1a). This nanoporous structure guarantees the
adsorption of the molecular catalyst. After the modification
with Co(salophen) complexes, the electrodes are retained, and
no aggregation or new features are observed (Figures 1b and
c). The elemental mapping images indicate that Co is uniformly
dispersed on the surface of BiVO4 (Figures 1d and e). The Co 2p
spectra in X-ray photoelectron spectroscopy (XPS) show two
characteristic peaks at 781 eV and 796 eV, which is belong to
the Co 2p3/2 and Co 2p1/2 spin-orbital of Co (II) in Co1 and
Co2 (Figures S3). Raman spectroscopy also confirms that
Cobalt complexes have been successfully modified on the
surface of BiVO4 as the characteristic signal of Cobalt
complexes are observed completely (Figure 2). According to
the results of inductively coupled plasma spectrometry (ICP),
the amount of Co bound to Co1/BiVO4 and Co2/BiVO4 were
calculated to be 44.9 nmol/cm2 and 39.7 nmol/cm2,
respectively. The difference in the catalyst loading amount is
due mainly to steric hindrance, for which Co2 has a larger
molecular volume than Co1. With this extent of loading,
Co(salophen) complexes have no impact on the light
absorption of the original electrode, as is shown in the UV-Vis
spectra (Figure S4). This indicates that the molecular catalyst
cannot block or reflect the relevant photons in the visible
range, which is one of the greatest advantages of using a
molecular catalyst over inorganic one.
Results and discussion
Scheme 1. Structures of the Co(salophen) complexes.
Figure 1. SEM images of (a) BiVO4 electrode, (b) Co1/BiVO4
electrode and (c) Co2/BiVO4 electrode; the elemental mapping
2 | J. Name., 2012, 00, 1-3
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Figure 2. Raman spectra of BiVO4 electrode, Co(salophen)
powders, and Co(salophen)/BiVO4 photoelectrodes.
PEC Performance
The PEC performance of the resulting photoanodes was
measured in a 0.1 M phosphate buffer solution (pH 7), using
simulated sunlight (AM 1.5, 100 mW/cm2), in a standard three-
electrode system with the sample photoanodes as the working
electrodes, a platinum wire as the counter electrode, and
Ag/AgCl as the reference electrode. First, the electrocatalytic
activities for water oxidation of the Co(salophen) complexes
on the bare FTO substrate were investigated. The
photocurrent-potential (J-V) curves determined by linear
sweep voltammetry (LSV) are shown in Figure 3a. Both the two
complexes show catalytic activity for water oxidation with a
similar onset potential of 1.85 V versus the reversible
hydrogen electrode (RHE, all further potentials are reported
against RHE unless otherwise stated), which implies an
overpotential of 620 mV for Co1 and Co2, respectively. Co2
exhibits higher electrocatalytic activity than Co1, but neither of
them is high efficient in electrocatalytic water oxidation,
compared to other WOCs. However, under simulated sunlight
illumination, the Co(salophen)-modified samples exhibit
considerably enhanced photocurrents (Figure 3b). High
photocurrents of 3.89 mA/cm2 and 4.27 mA/cm2 are obtained
for Co1/BiVO4 and Co2/BiVO4 respectively, at an applied
potential of 1.23 V. While for the bare BiVO4, the photocurrent
at the same potential is only 1.48 mA/cm2. Moreover,
compared to the bare BiVO4, the onset potential is cathodically
shifted by more than 200 mV, and the potential at a
photocurrent density of 1 mA/cm2 is cathodically shifted by
600 mV for Co(salophen)-modified BiVO4. The remarkable
reduction in onset potential and increase in photocurrent
demonstrates that Co(salophen) complexes act as efficient
cocatalysts on the surface of the photoanodes in PEC water
splitting.
Figure 3. (a) J-V curves of Co1/FTO, Co2/FTO in comparison
with the bare FTO under dark condition; (b) J-V curves of
Co1/BiVO4 and Co2/BiVO4 in comparison with the bare BiVO4
under AM 1.5G simulated sunlight irradiation.
Based on the LSV results in Figure 3b, the applied bias
photon-to-current efficiency (ABPE) for Co1/BiVO4 was
calculated, with a maximum value of 1.09% at 0.7 V. The
maximum ABPE for Co2/BiVO4 was calculated to be 1.18% at
0.7 V, about eight-fold enhancement over that of unmodified
BiVO4 (Figure S5). Figure S6 shows the incident photon-to-
current conversion efficiencies (IPCE) on bare and
Co(salophen)-modified BiVO4 photoanodes at different
wavelengths measured under a bias potential of 1.23 V. With
the increasing wavelength, the IPCE value of each sample
remains steady before 460 nm after which it decreases, which
is consistent with their light absorption. Co(salophen)-modified
BiVO4 photoanodes show higher IPCE values than the bare
BiVO4. At 430 nm, the IPCE values are around 86% and 89%,
for Co1/BiVO4 and Co2/BiVO4, respectively.
To explore the interface charge transport behaviour of the
as-prepared photoelectrodes, the photocurrents were
measured in the presence of sodium sulfite (Na2SO3) to
eliminate the holes injection barrier (Figure S7). The
photocurrent density arising from PEC water oxidation can be
described as:
JH2O =Jabs × ηsep × ηinj,
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where: Jabs is the photocurrent density when the absorbed on BiVO4 have shown that when Co-Pi was deposited on the BiVO4,
photons completely convert into the current, ηsep is the charge it passivates recombination centers rather than works as water
separation efficiency of the photogenerated holes that refer to oxidation catalyst.44-46 Figure 5a shows typical IMPS responses of
the bulk recombination, and ηinj is the charge injection Co1/BiVO4, Co2/BiVO4 and bare BiVO4 in the complex plane. It can
efficiency of the surface reaching holes into the electrolyte.11,13 be shown that the IMPS response is expressed as a function of
With Na2SO3 as a hole scavenger, the surface recombination is frequency. The frequency at the maximum imaginary part
eliminated, so ηinj = 1, and the photocurrent density can be corresponds to the sum of the charge transfer (ktrans
)
and
described as:
recombination (krec) rate constants (ktrans + krec = 2πfmax). The low
JNa2SO3 = Jabs × ηsep
.
frequency intercept in normalized form, at which the imaginary part
The bare BiVO4 generates
a
significantly enhanced equals 0 (Im(jphoto/jh) = 0), corresponds to the charge transfer
photocurrent in the presence of Na2SO3 because of the efficiency ( ktrans/(ktrans + krec)). Thus, the key parameters krec and
negligible surface recombination. Co1/BiVO4 and Co2/BiVO4 ktrans at different applied bias potentials are readily obtained.40 The
show similar photocurrents to the bare BiVO4. The relative electron transfer time (τd) can be estimated from frequency at the
charge separation efficiencies of Co1/BiVO4 and Co2/BiVO4 to minimum imaginary part (τd = 1/2πfmin).47
BiVO4 are shown in Figure S8, which obtained by dividing
As shown in Figure 5b, both Co1/BiVO4 and Co2/BiVO4 exhibit
JNa2SO3 of Co1/BiVO4 and Co2/BiVO4 by JNa2SO3 of BiVO4. The higher values of ktran compared to the bare BiVO4 over the entire
approximate separation efficiencies for each photoanode potential range, indicating that both Co(salophen) complexes
indicate that the modification of Co (salophen) cannot alter perform as OER catalysts on the surface of the photoanodes. The
the bulk recombination. The ηinj for each photoelectrode can krec values for each photoanode are shown in Figure 5c. Lower
be calculated by JNa2SO3/JH2O. It can be seen from Figure 4 that values of krec are observed for both Co1/BiVO4 and Co2/BiVO4.
Co(salophen)-modified BiVO4 exhibit higher ηinj than the bare Additionally, Co2/BiVO4 exhibits a slightly increase in ktrans and
BiVO4 over the whole potential range. The ηinj at 1.23 V for decrease in krec than Co1/BiVO4, which is in accordance with its high
Co1/BiVO4 and Co2/BiVO4 can reach 89.3% and 92.4%, photocurrent. These results demonstrate that Co(salophen)
respectively, whereas that obtained by the bare BiVO4 is only complexes not only accelerate the water oxidation reaction as OER
58.0%. These results confirm that Co(salophen) complexes can catalysts, but also reduce the surface recombination, which is in
effectively restrain the surface recombination and promote contrast to the mere passivation action of Co-Pi as reported.
hole injection into the electrolyte. But it is not known from this
The charge transfer efficiencies, defined as ktrans/(ktrans + krec), are
experiment whether the ηinj improvement stems from an obtained (Figure S9 ). The enhanced values for Co1/BiVO4 and
absolute increase in surface charge transfer (Co(salophen) Co2/BiVO4 are consistent with the ηinj as mentioned above. The
complex works as an OER catalyst), an absolute decrease in transfer times of the photoanodes are shown in Figure 5d. Each
surface recombination (Co(salophen) complex works as a sample exhibits
a decreasing τd with the increase of the bias
surface passivation cocatalyst), or both.
potential. The electron lifetimes of Co1/BiVO4 and Co2/BiVO4 are
much shorter (less than half) than that of the bare BiVO4, again
supporting their superior charge transfer abilities.
Figure 4. Hole injection efficiencies of Co1/BiVO4 and
Co2/BiVO4 in comparison with the bare BiVO4.
Surface Kinetics by IMPS
To understand the real role of Co(salophen) complexes on the
photoanodes, intensity modulated photocurrent spectroscopy
(IMPS) measurements were employed. This technique has been
previously applied to study the charge transfer and surface
Figure 5. (a) Intensity-modulated photocurrent spectroscopy
responses of bare BiVO4, Co1/BiVO4 and Co2/BiVO4 at 1.11 V;
(b) Charge transfer rate constants (ktrans), (c) Charge
recombination rate constants (krec), (d) Charge transfer times
recombination kinetics of photoanodes.40-44 Several recent reports of different photoanodes extracted from IMPS analysis.
4 | J. Name., 2012, 00, 1-3
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ARTICLE
In order to accurately assess the effect of the molecular
Co(salophen) catalysts on actual oxygen production, the
evolved O2 in headspace was quantified by gas
chromatography. With a four hour photoelectrolysis, oxygen
The PEC stability of the photoelectrode is of great importance
for a PEC device, especially for a molecular assembled device.
In order to check the stability of the photoanodes during PEC
water oxidation, a long-term photolysis at 1.23 V under visible-
light irradiation was conducted (Figure 6). The unmodified
BiVO4 shows a low initial photocurrent density of no more than
1.48 mA/cm2 and then a rapid decline in photocurrent to less
than 0.86 mA/cm2 within three hours. The decline in
photocurrent caused by photocorrosion can be restrained by
formed at rates of 6.25 ꢀmol/h, 24.77 ꢀmol/h and 27.78
mol/h on BiVO4, Co1/BiVO4 and Co2/BiVO4, respectively.
ꢀ
Additionally, the stoichiometric ratio of evolved oxygen to
hydrogen detected on the counter electrodes was close to 1:2.
The faradaic efficiencies of the electrodes were obtained by
comparing the actual amount of detected O2 with the
theoretical amount of O2, which was calculated from the
generated photocurrent by assuming that all charges were
used for 4e- oxidation of water. As shown in Figure S11,
Co1/BiVO4 and Co2/BiVO4 exhibit a significant increase in
faradaic efficiency (88% and 91%, respectively) relative to the
bare BiVO4 (74%), indicating that by introducing Co(salophen)
complexes, more surface arriving holes are utilised through a
four-electron transfer process. These results demonstrate that
the modification of Co(salophen) complexes on BiVO4
photoanodes as high-performance OER catalysts cannot only
increase the photocurrent, but also selectively improve the
kinetics of the O2 evolution reaction.
Co(salophen) complexes. Co1/BiVO4 shows
a
strong
photocurrent followed by a slightly decay. For Co2/BiVO4, a
stable photocurrent of more than 3.5 mA/cm2 is sustained for
at least 3 h without decay. Such stable PEC performance is very
rare for molecular catalyst-based photoanodes, even
surpassing that in most heterogeneous catalyst-based
photoanodes.
T
he high PEC stability of Co1/BiVO4 and Co2/BiVO4 is mainly
attributed to the hydrophobic nature of the Co(salophen)
complexes. To prove this point, PEC experiments were carried
out after soaking the Co(salophen)-modified photoanodes in
different solutions overnight. As shown in Figure S10, after
soaking treatment in water, the J-V curve is almost unchanged
compared with the untreated one. After soaking treatment in
ethanol, the J-V curve exhibits a slightly decrease. While after
soaking treatment in dichloromethane, the J-V curve decreases
significantly, indicating a serious detachment of Co(salophen)
complex. Therefore, the hydrophobic nature of the
Co(salophen) complexes enables them to maintain on the
electrode rather than dissolving into water, which gives rise to
a high PEC stability. Co2/BiVO4 exhibits a more stable
photocurrent compared with Co1/BiVO4. The tert-butyl group
in Co2 improves its hydrophobicity, so that it is more difficult
to desorb from the electrode. This is evidenced by ICP
measurements. After three hours of photoelectrolysis, the
amount of Co on Co1/BiVO4 decreased by 43.4% (from 44.9
nmol/cm2 to 25.4 nmol/cm2), while that on Co2/BiVO4
decreased only by 8.3% (from 39.7 nmol/cm2 to 36.4
Real catalyst or catalyst precursor
nmol/cm2). These results are indicative of
combination of the catalysts and BiVO4.
a perfect
Figure 7. (a) LSV curves of Co2/BiVO4 in 0.1 M phosphate
buffer solution with and without bpy (0.1 mM) under AM 1.5G
simulated sunlight irradiation; (b) UV-Vis spectra of Co2 in
solution before and after 3 h irradiation; (c) SEM images of
Co2/BiVO4 after 3 h PEC measurement; (d) Raman spectra of
Co2/BiVO4 photoelectrode before and after
measurement.
3 h PEC
For molecular WOC, a notable question is whether the
complex is a real homogeneous catalyst or only a function as a
precursor of the heterogeneous active species.48-51 To exclude
the possibility that the enhancement in PEC performance was
due to the Co2+ or to formed heterogeneous CoOx from the
dissociation of molecular catalyst, the following experiments
were conducted. First, the control experiments were
Figure 6. I-t curves of BiVO4 (black), Co1/BiVO4 (blue) and
Co2/BiVO4 (red) at 1.23 V vs. RHE.
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conducted by using the BiVO4 electrode soaked in a solution deionised water, acetone, and ethanol, respectively, at least
containing only cobalt ions or salophen ligand, and no increase three times (5 min).
in photocurrent density can be observed (Figure S12),
suggesting that the Co(salophen) complexes themselves are
Synthesis of Co(salophen) complexes
necessary to improve the PEC performance. Second, a certain
Co(salophen) complexes were synthesised according to a
method previously reported.29,48 First, the salophen ligand
was synthesised by adding o-phenylenediamine (5 mmol) and
salicyl-aldehyde or 3,5-di-tert-butylsalicylaldehyde (10 mmol)
into ethanol (25 mL) at room temperature (RT), and then the
mixture of the solutions was refluxed for six hours. The
precipitated Schiff-base ligand was filtered, giving the desired
product, salophen ligand. The corresponding ligand and
amount of 2,2’-bipyridine (bpy) was added into the electrolyte
as a chelating agent to clarify the contribution from the probable
free cobalt ions released from the Co(salophen)
complexes.26,52,53 As shown in Figures 7a and S13, the presence
of bpy has no influence on the photocurrent. Though it cannot
completely exclude the existence of the released cobalt ions, it
can verify that the improved photocurrent is not owing to free
cobalt ions. Moreover, Co2+/BiVO4 exhibits unchanged i-t curve
compared with BiVO4 (Figure S14), indicating that even trace
amounts of Co2+ released from the Co(salophen) complexes, it
will not form sufficient CoOx which can lead to an enhanced
photocurrent, which rules out the possibility that the improved
photocurrent is due to formed CoOx nanoparticles by Co2+
during the photoelectrolysis. Third, UV-vis measurements for
Co(salophen) complexes in solution showed that the
complexes were stable from degradation under illumination
(Figures 7b and S15). Fourth, if CoOx or Co-Pi form on the
electrode, there is usually an obvious increase or decline in
photocurrent.54-57 However, in this work, the photocurrent is
Co(OAc)2·4H2O (molar ratio 1:1.2) were added to ethanol (30
mL). Then the mixture was refluxed for two hours. During
refluxing, the colour of the reaction solution changed as
opposed to the starting ligand. After cooling to RT, the
precipitate was filtered with a Buchner funnel and washed
three times each with deionised water and ethanol. The
obtained precipitate was dried in a vacuum oven for 10 hours
at RT.
Preparation of BiVO4 electrodes
The BiVO4 thin film was prepared by modification of a
method reported by Choi and co-workers.13 Adding Bi(NO3)3
very stable. After
a three hour photoelectrolysis, no
∙
morphology change and no obvious new nanoparticles are
found on the surface of BiVO4 from the SEM images (Figures 7c
and S16), indicating that the cobalt complexes did not
decompose to form heterogeneous cobalt oxide materials.
Fifth, the raman spectra revealed that the signal of the
5H2O (0.04 M) and KI (0.4 M) to a solution of HNO3 (50 mL, pH
= 1.7). This solution was then mixed with absolute ethanol (20
mL) containing 0.23 M p-benzoquinone, and was vigorously
stirred for twenty minutes. A typical three-electrode cell was
used for electrodeposition with a FTO working electrode, an
Ag/AgCl (3 M KCl) reference electrode, and a platinum counter
Co(salophen)
complexes
remain
unchanged
after
photoelectrolysis (Figures 7d and S17), confirming the
molecular integrity of the Co(salophen) catalyst on the
electrode. Therefore, based on the above results, we speculate
that the real catalyst is the molecular Co(salophen) complex
itself. On the other hand, this work provides a simple and
efficient way to introduce a WOC onto a semiconductor
photoanode. Even if the molecular catalyst had partly, or
completely changed to other active species, such
enhancement and stability of photocurrent are still
noteworthy in PEC water oxidation.
electrode.
Cathodic
deposition
was
performed
potentiostatically at -0.1 V vs Ag/AgCl at RT with deposition
times of five minutes. The BiOI electrode was then obtained
after rinsing with deionised water and air-drying. 0.2 mL of a
dimethyl sulfoxide (DMSO) solution containing 0.2 M vanadyl
acetylacetonate (VO(acac)2) was placed on the BiOI electrode
(1 cm × 1 cm) and heated in a muffle furnace at varying
temperatures for two hours in air to convert BiOI to BiVO4.
BiVO4 electrodes showing the highest photocurrent were
obtained when BiOI electrodes were annealed at 450 oC
o
(ramping rate = 2 C/min) for two hours. Excess V2O5 present
Experimental
in the BiVO4 electrodes was removed by soaking in 1 M NaOH
solution for 45 min with gentle stirring. The resulting pure
BiVO4 electrodes were rinsed with deionised water and dried
at RT.
Chemicals and Reagents
All chemical reagents including Bi(NO3)3
p-benzoquinone, o-phenylenediamine, salicylic aldehyde, 3,5-
di-tert-butylsalicylaldehyde, and Co(OAc)2 4H2O were
·5H2O, VO(acac)2,
Fabrication of Co(salophen)/FTO electrode
·
The Co(salophen) complexes (0.5 mM) were dissolved in
purchased from Aldrich or Aladdin. All other reagents were
commercially sourced and used without further purification.
acetonitrile with 5% Nafion. After ultrasound for 20 min, 60 ꢀL
of the solution was casted onto the FTO, and drying in the air.
The fluorine-doped tin oxide (FTO, surface resistivity of 8-12
Ω
sq-1 glass plates were purchased from Zhuhai Kaivo Electronic
Components Co., Ltd. Prior to the electrochemical
experiments, the FTO plates were cleaned by ultrasonication in
Fabrication of Co(salophen)/BiVO4 electrode
6 | J. Name., 2012, 00, 1-3
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The assembled Co(salophen)/BiVO4 electrode was finally
obtained by soaking the BiVO4 electrode in an acetonitrile
solution containing 0.5 mM Co(salophen) catalyst at RT for 10
hours in the dark, followed by rinsing with excess acetonitrile
and drying with N2. With the same way, we obtained the
Co2+/BiVO4 electrode by soaking the BiVO4 electrode in an
acetonitrile solution containing 0.5 mM cobalt acetate.
The applied bias photon-to-current efficiency (ABPE) was
calculated fromthe J-V curve using the equation:
J ×(1.23−Vbia
)
ABPE(%)=
×100%
Pin
where: J is the photocurrent density, Vbias is the applied bias,
and Pin is the incident illumination power density (AM 1.5 G,
100 mW/ cm2).
Incident photon-to-current efficiency (IPCE) at each
wavelength was measured at 1.23 V vs. RHE using
Characterisation
monochromatic light illumination from a 300 W Xe arc lamp
equipped with a CEAULIGHT CEL-IS151 monochromator. The
IPCE values have been calculated using the equation:
1240 × J
The purity and crystal structure of BiVO4 electrodes were
examined by powder X-ray diffraction (Bruker Smart APEX II).
Scanning electron microscopy (SEM) images and energy-
dispersive X-ray analysis (EDX) spectra of the films were
obtained with a SU8000 Schottky field emission scanning
electron microscope (SFE-SEM) equipped with a Rontec EDX
system. UV-vis absorption spectra were obtained on a Lambda
35 UV-vis spectrophotometer, in which the sample electrode
was placed in the centre of an integrating sphere to measure
all light reflected and transmitted to accurately assess the
absorbance. FTO glass was used as the reference for these
absorption measurements. Element content was determined
by an inductively coupled plasma mass spectroscopy (ICP, PE-
43008300). Raman spectra were collected by raman
spectrometer (Renishaw inVia). X-ray photoelectron spectra
(XPS) measurements were carried out on the thermo scientific
IPCE (%) =
×100 %
λ
× Pin
where: J is the photocurrent density (mA/cm2), λ is the
incident light wavelength (nm), and Pin is the measured
irradiance (mW/cm2). λ is the incident light wavelength (nm),
and Pin is the measured irradiance (mW/cm2).
The evolved oxygen and hydrogen were measured by a Gas
Chromatograph 6890A using a thermal conductivity detector
and argon as the carrier gas. Prior to the measurement, the
electrolyte was degassed by bubbling with high purity Ar for 30
minutes. The faradaic efficiency was calculated by assuming
that all of the charges were caused by 4e-oxidation of water to
produce O2.
ESCALAB 250 instrument (150 W, spot size of 500
ꢀm and Al
Kα
radiation at 1486.6 eV) to obtain the surface elements.
Conclusions
In summary, two molecular WOC/semiconductor hybrid
photoanodes (Co1/BiVO4 and Co2/BiVO4) were prepared by
introducing Co(salophen) complexes onto BiVO4 via
hydrophobic interaction for PEC water oxidation in neutral
water. Co(salophen) complexes gave rise to a remarkable
enhancement in the PEC performance by the restraining the
surface recombination of the photoanodes. Under AM 1.5G
illumination, an impressive transient photocurrent of 4.27 mA
/cm2 was obtained at 1.23 V RHE for Co2/BiVO4, nearly three
times higher than the bare BiVO4. Studies on surface kinetics
by IMPS measurements demonstrate that Co(salophen)
complexes improve the PEC performance by simultaneously
accelerating the water oxidation reaction and reducing the
surface recombination. Additionally, an extremely stable
photocurrent of more than 3.5 mA/cm2 is sustained for at least
three hours without decay. The resulting photoanodes based
on molecular WOC achieved robust PEC activity and excellent
stability. These results suggest that the rational design of
molecular WOC/semiconductor photoanode hold great
promise for future applications in efficient solar-fuel
conversion.
Electrochemical and photoelectrochemical methods
Electrochemical experiments were performed in a three-
electrode electrochemical cell with a CHI 660E instrument
potentialstat (Shanghai Chenhua Instrument Co., Ltd.) at RT. In
a three-electrode electrochemical system, the photoanodes
were used as working electrodes, with an Ag/AgCl electrode (3
M KCl) as the reference electrode and a platinum wire as the
counter electrode. The photoelectrodes were irradiated using
a Xe lamp (CEL-HXF300C) equipped with an AM 1.5G-filtered
illumination, and the light intensity was calibrated to 100
mW/cm2. The light irradiation came from the behind the
photoelectrodes. J-V curves were obtained by linear sweep
voltammetry with a scan rate of 10 mV/s. All measured
potentials were converted to RHE (ERHE = EAg/AgCl + 0.197 V +
0.059 pH). Generally, the electrolyte was a 0.1 M phosphate
buffer solution.
IMPS measurements were conducted using a potentiostat
(IM6ex, Zahner Company) controlled by
electrochemical workstation. Intensity-modulated light was
provided by light-emitting diode (LED) that allowed
superimposition of sinusoidal modulation ( 10%) on a dc
a Zahner IMPS
a
～
illumination level. The wavelength of light was 426 nm with an
average intensity of 10 W m-2.The photocurrent as a function
of frequency (from 10 kHz to 100 mHz) was recorded at
different potentials. The same three-electrode configuration
and electrolyte were employed as given in the PEC
characterization section.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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DOI: 10.1039/C8TA01304G
ARTICLE
Journal Name
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This work was supported by the National Natural Science
Foundation of China (21401092, 21671089), the Shenyang
Natural Science Foundation of China (F16-103-4-00), Scientific
Research Fund of Liaoning Province (20170540409).
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8 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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DOI: 10.1039/C8TA01304G
Stable and robust photoanodes were assembled by introduce cobalt salophen complexes onto
BiVO4 electrode for PEC water oxidation.