Molecular cobalt-salen complexes as novel cocatalysts for highly efficient photocatalytic hydrogen production over a CdS nanorod photosensitizer under visible light

Article abstract of DOI:10.1039/c5ta03515e

An efficient photocatalytic system is highly demanded for the production of hydrogen fuel through water splitting. Herein, we report an artificial photocatalytic system made of low-cost materials for high-performance H₂ production from water. The new system contains semiconductors (CdS nanorods) as the photosensitizer, a cobalt-salen complex as the H₂ evolution cocatalyst, and Na₂S and Na₂SO₃ as sacrificial electron donors. Under optimal conditions, the highest hydrogen evolution turnover number reached 64 700 after 37 hours and the rate was 106 μmol h^-1 mg^-1, which is much higher than when using CdS NRs and also is among the best for photocatalytic systems using molecular cocatalysts for H₂ production. The highest apparent quantum yield (AQY) was ～29% at 420 nm. Steady state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) decay spectra revealed that the system allows effective electron transfer from the excited CdS NRs to the cobalt-salen complex for highly efficient H₂ production.

Full text of DOI:10.1039/c5ta03515e

View Article Online
View Journal
Journal of
Materials Chemistry A
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: H. Chen, Z. Sun, S.
Ye, D. Lu and P. Du, J. Mater. Chem. A, 2015, DOI: 10.1039/C5TA03515E.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/materialsA

Page 1 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
Molecular Cobalt-Salen Complexes as Novel Cocatalysts for Highly
Efficient Photocatalytic Hydrogen Production over CdS Nanorods
Photosensitizer under Visible Light
Haiyan Chen, Zijun Sun, Sheng Ye, Dapeng Lu, Pingwu Du*
CAS Key Laboratory of Materials for Energy Conversion, Department of Materials
Science and Engineering, iChEM (Collaborative Innovation Center of Chemistry for
Energy Materials), University of Science and Technology of China (USTC), Hefei,
230026, China
*To whom correspondence should be addressed
E-mail: dupingwu@ustc.edu.cn
Tel/Fax: 86-551-63606207
1

Journal of Materials Chemistry A
Page 2 of 32
View Article Online
DOI: 10.1039/C5TA03515E
Abstract. An efficient photocatalytic system is highly demanded for the production of
hydrogen fuel through water splitting. Herein we report an artificial photocatalytic
system made of low-cost materials for high-performance H2 production in water. The
new system contains semiconductor (CdS nanorods) as the photosensitizer, a
cobalt-salen complex as H2 evolution cocatalyst, and Na2S and Na2SO3 as sacrificial
electron donors. Under optimal conditions, the highest hydrogen evolution turnover
number reached 64700 after 37 hours and the rate was 106 µmol·h-1·mg-1, which is
much higher than when using CdS NRs and also is among the best for photocatalytic
systems using molecular cocatalysts for H2 production. The highest apparent quantum
yield (AQY) was ~29 % at 420 nm. Steady state photoluminescence (PL) spectra and
time-resolved photoluminescence (TRPL) decay spectra revealed that the system
allows effective electron transfer from the excited CdS NRs to the cobalt-salen
complex for highly efficient H2 production.
Keywords. Photocatalysis; Hydrogen Production; Molecular Catalyst; Cobalt-Salen
Complex; Visible Light
2

Page 3 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
1. Introduction
Efficient Hydrogen (H2) production via water splitting is believed to be an ideal
pathway to store solar energy for a future supply of clean energy.1-2 In nature, three
typical iron-based hydrogenases ([FeFe] hydrogenase,3-4 [FeNi] hydrogenase,3 and
[Fe] only hydrogenase5) serve as efficient catalysts to highly promote proton
reduction for H2 production. To mimic the function of the natural process, many kinds
of molecular catalysts have been reported in the literature for photocatalytic H2
production, such as cobalt (II) complexes (such as cobaloximes,6 cobalt dithiolenes,7
cobalt polypyridyl complexes.8), nickel (II) complexes (such as nickel phosphines9-10
and nickel pyridinethiolate complexes11), and iron (II) complexes (such as iron
pentacarbonyl12 and iron carbonyl phosphines13). In a typical reaction system, these
molecular catalysts are combined with a molecular photosensitizer and a sacrificial
electron donor. The well-known molecular photosensitizers include ruthenium (II)
trisbipyridyl complexes,14-15 platinum terpyridyl complexes,16-17 iridium (II)
polypyridyl complexes,18 and organic dyes.19 However, only a specific wavelength or
narrow region of visible light can be absorbed by these photosensitizers.20 In addition,
these organic photosensitizers can easily suffer degradation and instability under light
illumination.9,20-22
Besides molecular photosensitizers, semiconductors have also been extensively
studied as heterogeneous photocatalysts for H2 production since Honda and Fujishima
reported the use of TiO2 semiconductor for photoelectrochemical water splitting in
1972.23 Semiconductors have a broad and continuous spectral absorption range for
3

Journal of Materials Chemistry A
Page 4 of 32
View Article Online
DOI: 10.1039/C5TA03515E
effective collection of sunlight, rendering them quite promising for photocatalytic H2
production from water.24-27 Many heterogeneous photocatalysts have much better
photoresponse than TiO2 in the visible region for H2 evolution reaction (HER), such
as other metal oxides (Fe2O3,28 Cu2O,29 etc.), metal oxynitrides,30-31 and metal
sulfides.19,32-34 Among them, materials incorporating CdS have attracted much
attention because of its low band gap (~2.4 eV), high photocatalytic activity, and low
cost.4,35-38 To prevent the recombination of photogenerated electron-hole pairs,
loading a cocatalyst on CdS semiconductor photocatalysts is an effective approach to
improve the photocatalytic activity,19,27,33,39-40 resulting in photocatalysts such as
CdS-Pt,41 CdS-Ni,42 CdS-NiO,43 CdS-Cd,37 and CdS-MoS2.44 A few publications
report molecular catalysts that are combination with CdS for photocatalytic H2
production, such as cobaloximes33 and ruthenium carbonyl complexes.45
In this paper, we report for the first time that cobalt-salen complexes can serve as
novel low-cost and efficient catalysts in an artificial photocatalytic system for H2
production using CdS nanorods (CdS NRs) as the photosensitizer. Under visible light
irradiation (λ > 420 nm), the highest turnover number (TON) achieved was ~64700
for H2 production after 37 hours of visible light irradiation in aqueous solution. The
highest rate of hydrogen production based on cobalt-salen complex 6 was 106
µmol·h-1·mg-1 and the highest apparent quantum yield is ~29% at 420 nm.
2. Experimental Details
2.1. Materials
4

Page 5 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
All the chemical reagents were purchased from Aldrich or Alfa and used without
further purification, including ethylene diamine (99.0%), salicylic aldehyde (98.0%),
o-phenylenediamine
(99.0%),
1,2-hexamethylene
diamine
(98.0%),
3,5-di-tert-butylsalicylaldehyde (98.0%), cobalt acetate tetrahydrate (Co(OAc)2·4H2O,
99.5%), cadmium chloride hemi(pentahydrate) (CdCl2·2.5H2O, 99.0%), thiourea
(CH4N2S, 99.0%), ethylenediamine (H2NCH2CH2NH2, 99.0%), ethanol (EtOH,
99.7%), acetonitrile (C2H3N, 99.7%), sodium sulfide nonahydrate (Na2S·9H2O, 99%),
anhydrous sodium sulfate (Na2SO3, 97.0%), sodium sulfate (Na2SO4, 99.0%),
triethanolamine (TEOA, 99%), N, N-Dimethylformamide (DMF, 99.5%), and glacial
acetic acid (HOAc, 99.0%). 5% Nafion solution (Dupont company product) was
purchased from Shanghai Geshi Energy Technology Co. Ltd. CdS NRs were prepared
according to a reported method.46 The aqueous solutions were freshly prepared with
Millipore water (resistivity: ~18 Mꢀ·cm).
2.2. Synthesis of cobalt(III)-salen complexes 1-6
The salen ligands were synthesized according to a method reported in the
literature.47-50 Co(II)-salen complexes, the precursor of Co(III)-salen complex, were
synthesized from salen ligands and cobalt(II) acetate in ethanol according to the
procedure described.47-48,50-51 Co(III)-salen complexes 1-6 were prepared from the
Co(II)-salen complexes by treatment with HOAc in dichloromethane: 0.5 mL acetic
acid was added to the Co(II)-salen complex (200 mg) in dichloromethane (15 mL).
After stirring the mixture for 50 min, the solvent of dichloromethane was removed
with a rotovap and the excess acetic acid was removed under vacuum.52-57 The
5

Journal of Materials Chemistry A
Page 6 of 32
View Article Online
DOI: 10.1039/C5TA03515E
resulting solids were recrystallized in ethanol to obtain final products. The molecular
structures of the Co(III)-salen complexes 1-6 are shown in Scheme 1.
2.3. Preparation of CdS NRs
CdS NRs were prepared according to a modified method.46 Typically, 20.25 mmol
CdCl2·2.5H2O (4.62 g) and 60.75 mmol CH4N2S (4.62 g) were added in 60 mL
ethylenediamine in a 100 mL Teflon-lined stainless steel autoclave without stirring.
Then the autoclave was sealed and maintained at 160 ºC for ~50 h in a muffle furnace
and allowed to cool down to room temperature (RT). The obtained yellow powder
was washed with Millipore water and ethanol three times each to remove impurities.
Finally, the powder was dried under vacuum at RT for overnight.
2.4. Photocatalytic H2 evolution
The photocatalytic reactions were carried out in a 50 mL flask, except for the
experiment on long-term hydrogen production (in which a 250 mL reaction flask was
used). Generally, the reaction system contained 20 mL Millipore water, 1.0 mg CdS
NRs, different amounts of cobalt-salen complexes, Na2S, and Na2SO3. The reaction
mixture was stirred for 10 min in the dark and degassed by bubbling with high purity
N2 for 15 min to remove air. A 300 W Xe-lamp equipped with a long-pass cut-off
filter (λ > 420 nm) was used for visible light irradiation. The amounts of hydrogen
produced during photocatalysis were determined by a gas chromatography (GC)
equipped with a TCD detector using methane as the internal standard.16,19
2.5. Photoelectrochemical measurements
6

Page 7 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
Analyses of the photoelectrochemical properties of CdS NRs and CdS
NRs/cobalt-salen complexes were performed in a standard three-electrode system at
room temperature using CHI602E potentiastat (purchased from Shanghai Chenhua
Instrument Co., Ltd.). FTO plates were ultrasonicated and washed with Millipore
water and ethanol three times each. The suspension of CdS NRs containing Nafion
was dropped using a pipette onto the conductive surface of an FTO plate within an
area of 1 cm2 to form the CdS/FTO electrode (working electrode). Then, a
cobalt-salen complex in acetonitrile was dropped using a pipette onto the CdS/FTO
electrode and dried in air. The ratio of CdS NRs and cobalt-salen complex is the same
as that used in the photocatalytic system. An Ag/AgCl electrode (3 M KCl, 0.21 V vs.
NHE) was used as the reference electrode and Pt wire as the counter electrode. The
photocurrents were obtained in 0.5 M Na2SO4 solution at an applied potential of 0 V
under visible light irradiation (λ > 420 nm).
2.6. Apparent quantum yield (AQY)
The AQY was measured under the optimal condition for photocatalysis. A 300 W Xe
lamp equipped with a monochromatic band-pass filter (λ = 420 nm 5 nm) was used
for visible light irradiation. The value of AQY was calculated according to equation (1)
and the turnover number (TON) was calculated according to equation (2):26,58-59
number of reacted electrons
ꢀ ꢁ
AQY % =
× 100
number of incident photons
number of evolved Hꢂ molecules × 2
=
× 100 (1)
number of incident photons
number of product molecules
number of evolved Hꢂ molecules
TON =
=
(2)
number of cocatalyst molecules number of cocatalyst molecules
7

Journal of Materials Chemistry A
Page 8 of 32
View Article Online
DOI: 10.1039/C5TA03515E
2.7. Physical characterization
The crystalline diffraction patterns of CdS NRs were analyzed by powder X-ray
diffraction (XRD, D/max-TTR III) using graphite monochromatized Cu Kα radiation
of 1.54178 Å, operating at 40 kV and 200 mA. The scanning rate was 5° min−1 from
20° to 80° (2θ). Ultraviolet-visible (UV-vis) diffuse reflection spectra were
characterized by using an UV-visible spectrophotometer (SOLID 3700 UV−vis
spectrometer). Transmission Electron Microscopy (TEM) images were obtained using
a JEM-2010 electron microscope, operated at an acceleration voltage of 200 kV. The
steady state photoluminescence (PL) spectra were recorded using a PerkinElmer LS
55 fluorescence spectrometer and time-resolved photoluminescence (TRPL) decay
spectra tests were performed by PicoHarp300 spectrometer at room temperature (RT).
The chemical compositions and valence states of the photocatalysts were probed with
an ESCALAB 250 X-ray photoelectron spectroscopy (XPS) instrument.
3. Results and discussion
The CdS NRs material was successfully synthesized by a modified solvothermal
method.46 The morphology of the obtained product was investigated by TEM, which
confirmed the formation of nanorods shape with lengths of 0.5~2 ꢁm and diameters of
40-90 nm, as shown in Figure 1a. The powder XRD diffraction pattern of the CdS
NRs was also collected (Figure 1b), in which all the different peaks of CdS NRs are
well indexed as hexagonal CdS (JCPDS No.77-2306).19,38 The molecular structures of
8

Page 9 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
cobalt-salen complexes 1-6 are displayed in scheme 1. Figure S1 shows the UV-vis
diffuse reflectance spectra of pure CdS NRs, complex 6, and mixed complex 6/CdS
NRs samples. Pure CdS material has good absorption in the visible region with a band
gap at ~2.4 eV. When mixing complex 6 with CdS NRs, the band gap of CdS remains
unchanged.
When mixing a cobalt-salen complex with CdS NRs, the TEM image of the
composite is similar to the image of pure CdS NRs (Figure S2). No obvious
cobalt-salen complex could be observed probably because of its low content in the
composite. Figure 1b shows XRD pattern of complex 6/CdS composite, in which all
the different peaks are also well matched with the pattern of hexagonal CdS (JCPDS
No.77-2306), indicating that the loading cobalt-salen complex does not affect the
crystallinity of CdS NRs.
The photocatalytic activities of H2 production of CdS NRs and CdS/cobalt-salen
complexes 1-6 were studied in aqueous solution, as shown in Figure 2a. The solution
contained 1.0 mg CdS photosensitizer, a cobalt-salen complex (1
×
10-5 M), Na2S
(0.25 M), Na2SO3 (0.35 M), and 20 mL Millipore water. All the complexes can
promote H2 production under the same conditions, indicating that cobalt-salen
complexes are active cocatalysts for proton reduction. Pure CdS NRs showed low
photocatalytic activity (~12 ꢁmol·h-1·mg-1). Complexes 1, 3, and 5 showed slight
enhancement for H2 production. Impressively, the photocatalytic activities of 2, 4, and
6 are much higher than pure CdS NRs, yielding H2 evolution rates of ~37, ~51, and
~73 µmol·h-1·mg-1, respectively. It is not very clear why complex 6 has the highest
9

Journal of Materials Chemistry A
Page 10 of 32
View Article Online
DOI: 10.1039/C5TA03515E
activity to promote photocatalytic H2 production in the present system. Comparing the
structural differences, complexes 1, 3, and 5 have bulky tert-butyl groups but
complexes 2, 4, and 6 are more planar. The bulky tert-butyl groups may significantly
decrease the water solubility, causing aggregation of 1, 3, and 5 in aqueous solution.
However, due to their good solubility in alkaline solution, complexes 2, 4, and 6
probably have better contact with CdS NRs than the complexes with tert-butyl groups,
which will help transport the photogenerated electrons. The combination of complex 6
and CdS NRs may be highly preferable, resulting in the highest H2 evolution rate in
the present photocatalytic system.
It was found that the photocatalytic activities are dependent on the
concentrations of cobalt-salen complexes and the sacrificial donors. Figure 2b shows
the H2 evolution rates in the complex 6/CdS NRs system with different concentrations
of complex 6. With the initial increasing concentration of complex 6, an improvement
of photocatalytic activity was clearly observed. When its concentration reached more
than 0.015 mM, the H2 evolution rate decreased. The optimal rate was achieved at a
concentration of 0.015 mM complex 6 (106 µmol·h-1·mg-1), which is ~9 times higher
than pure CdS NRs without a cobalt-salen complex. A further increase in
concentration of complex 6 resulted in a decrease of the rate for photocatalytic H2
evolution, probably a result of the fact that more cobalt-salen molecules per unit area
can shield the visible light absorption of CdS NRs and lead to a decrease of photons
passing through the photocatalysts. A comparison of Co2+ and complex 6 was further
explored. When complex 6 in the system was replaced by Co2+ with the same
10

Page 11 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
concentration, the system did produce H2, but only with lower photocatalytic activity
than complex 6 (Figure S3).
The photocatalytic activities of H2 evolution over different amounts of sacrificial
electron donors are shown in Table 1. The concentration ratio of Na2S:Na2SO3 was
fixed at 5:7. With increasing concentrations of the sacrificial electron donors from
0.25 M:0.35 M to 1.25 M:1.75 M (Na2S:Na2SO3), the catalytic turnover numbers
(TONs) based on complex 6 increased from ~1200 to ~6360 after 3 hours of visible
light irradiation, corresponding to the amounts of H2 from 355 µmol to 1872 µmol.
When the sacrificial reagents were oversaturated in aqueous solution with a
concentration of 1.5 M Na2S and 2.1 M Na2SO3, the amount of evolved H2 decreased
(1777 µmol, TONs = 6040). During photocatalysis, protons are reduced to produce H2
by photogenerated reducing species accompanied by oxidation of sacrificial electron
donors. A higher concentration of sacrificial electron donors can oxidize more
photogenerated holes to prevent the recombination of electron-hole pairs on CdS NRs
semiconductor under visible light, and subsequently increase the photocatalytic
activity for H2 production. In an oversaturated solution, a large number of the solid
crystals may also shield the visible light absorption of CdS NRs, resulting in a lower
H2 evolution rate.
Figure 3a shows the apparent quantum yields (AQYs) of the photocatalytic
system for H2 production. The reaction system was irradiated for 10 hours under
monochromic light with a band-pass filter (λ = 420 nm 5 nm). Initially, the AQY
was ~25% in the first hour. After the first hour, the AQYs increased. The average
11

Journal of Materials Chemistry A
Page 12 of 32
View Article Online
DOI: 10.1039/C5TA03515E
value of AQY was calculated to be ~29% after 10 hours of irradiation. Such a high
AQY is comparable with or even higher than those in the reported studies based on
molecular cocatalysts, such as, cobalt dithiolenes (AQY ~24%),60 cobalt bipyridyl
complexes (AQY ~13%),61 and cobaloxime (AQY ~9%).33
To test the photocatalytic stability during H2 production, a long-term
photocatalytic experiment was performed in a solution containing 1.0 mg CdS NRs,
0.006 mM complex 6, 1.25 M Na2S, and 1.75 M Na2SO3 (Figure 3b). The result
demonstrates good stability for H2 production for more than 37 hours, with a nearly
linear relationship between the reaction time and the amount of H2, indicating good
stability for this system. The highest TON was achieved at 64700, corresponding to a
TOF number of ~1748 h-1.
For photocatalytic H2 production systems based on a metal complex, one may
wonder if the metal complex actually decomposed to form metal(0) as the real
catalyst for proton reduction. Therefore, more physical characterizations (such as
TEM and XPS) were performed after long-term visible light irradiation. After 10
hours of photocatalysis, the reaction solution was centrifuged to collect CdS NRs,
followed by washing with acetonitrile and water three times each to remove molecular
cobalt complexes and inorganic salts. The recovered CdS NRs material was examined
by TEM (Figure S4a), which still showed morphology and shape similar to the CdS
sample before photocatalytic reaction. No obvious cobalt(0) nanoparticles were
observed from the TEM image. These results indicate that the cobalt-salen complex
did not decompose to form cobalt(0) in 10 hours, but the observation does not
12

Page 13 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
guarantee no cobalt(0) formation after a much longer time of irradiation. The EDX
spectrum of CdS NRs after reaction is shown in Figure S4b, confirming no cobalt
species deposited on CdS NRs during 10 hours of photocatalysis. Similar result was
previously reported that there was no Co(0) species on the surface of CdS after
short-term photocatalysis.33
The photocatalyst samples were further investigated by X-ray photoelectron
spectroscopy (XPS) (Figure 4). Figure 4a shows the survey spectra of the samples.
The black plot is the spectrum of a mixed complex 6/CdS NRs sample and the red
plot is for the sample washed by acetonitrile and water after photocatalysis. Both
survey spectra are quite similar, with the presence of Cd, S, O, and C elements. The
difference lies in that the mixed sample clearly shows the presence of a Co peak.
Before photocatalysis, the high resolution XPS spectra of S 2p and Cd 3d are
exhibited in Figures 4b and 4c. Two main peaks are located at 162.52 and 161.31 eV,
which can be attributed to S 2p in CdS.19,62-64 Figure 4c demonstrates two obvious Cd
3d peaks located at 404.42 eV and 411.15 eV, which are consistent with the Cd
character in CdS materials.62 After photocatalysis, both S 2s and Cd 3d peaks only
have slight changes of binding energies but not significant. The broadening of the
peaks in Cd 3d and S 2s before photocatalysis could be due to the presence of
cobalt-salen complexes (poor conductivity for XPS measurement). The high
resolution XPS spectrum of Co 2p in a mixed complex 6/CdS NRs sample is shown in
Figure 4d. Before irradiation (black plot), two appreciable Co 2p peaks are observed
at 780.3 eV and 794.5 eV, indicating the presence of a highly oxidized state of cobalt.
13

Journal of Materials Chemistry A
Page 14 of 32
View Article Online
DOI: 10.1039/C5TA03515E
In contrast, no apparent Co 2p peaks can be seen in the sample after irradiation,
further confirming no metallic cobalt formation during photocatalytic H2 production
over a period of 10 hours.
To gain more insights into the photocatalytic reaction mechanism, the
photophysical properties were studied by both steady state photoluminescence (PL)
spectra and time-resolved photoluminescence (TRPL) decay spectra (Figure 5). Under
an excitation wavelength of 405 nm, pure CdS NRs show an intense luminescence
band centered at ~506 nm and a weak luminescence band at ~712 nm (Figure 5a),
which is consistent with the previous reports.45,65 The intense luminescent band can be
assigned to near-band-edge emission and the weak band may be attributed to defects
of the CdS NRs surface. The luminescence intensities of pure CdS NRs are readily
quenched by adding cobalt-salen complex 6. This phenomenon probably results from
efficient electron transfer in the complex 6/CdS NRs hybrid system from CdS to
cobalt complex, leading to the spatial separation of electrons and holes. Therefore, the
cobalt-salen complex could serve as an electron accepter and transporter, which will
prevent fast recombination of electron-hole pairs upon irradiation. The fast electron
transfer process can be further confirmed by TRPL spectra, as shown in Figure 5b.
The PL intensity of the complex 6/CdS NRs composite system decayed much faster
than that of pure CdS, indicating that the composite system has shorter PL lifetimes
and better charge transfer capability to facilitate photocatalytic H2 production. The
data of complex 6/CdS NRs can fit to a biexponential decay kinetics, F(t) = A1*exp(-
t/τ1) + A2*exp(- t/τ2), where A1, and A2 are preexponentials related to the
14

Page 15 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
concentration of emitting species, τ1, and τ2 are the corresponding lifetimes, t is the
measurement time, and F(t) is the photon counts at time t. The PL lifetimes of
complex 6/CdS NRs are ~59 ps and ~360 ps, which are significantly shorter than for
pure CdS NRs (~500 ps and ~7700 ps).
Measurements of the photocurrent responses of CdS NRs and complex 6/CdS
NRs were performed in 0.5 M Na2SO4 under chopped visible light irradiation (λ >
420 nm), as shown in Figure S5. When no light irradiation existed, both samples
showed very low dark current densities (j < 10-6 mA/cm2). In contrast, much higher
photocurrent was obtained under irradiation, indicating good photoresponse to visible
light. Meanwhile, the photocurrent response for complex 6/CdS NRs was higher than
that of pure CdS NRs under the same conditions, indicating that cobalt-salen complex
can highly enhance transport of the photoexcited charge carriers under visible light
illumination.
UV-vis absorption spectra of the reaction solution during photocatalysis were
studied to examine the reaction mechanism in detail. Inspired by previous studies,16-17
a Pt chromophore (platinum terpyridyl phenylacetylide complex) was chosen as the
photosensitizer. The solution contained 1.6
× ×
10-2 M triethanolamine (TEOA), 1.1
10-5 M Pt chromophore and 3.0 10-4 M cobalt-salen complex 6 in 20 mL DMF (5%
×
water). The results are shown in Figure 6. Prior to photocatalysis, the color of the
solution was light yellow and the absorption spectrum exhibited two peaks located at
334 nm and 403 nm (black plot), which is in agreement with previous study.66 After
one hour of irradiation, the solution color had turned to dark yellow and the
15

Journal of Materials Chemistry A
Page 16 of 32
View Article Online
DOI: 10.1039/C5TA03515E
absorption spectrum has had significant changes (red plot). New absorption bands are
clearly seen. One absorption band maximized at 500 nm can be attributed to the
formation of Co(II) species.16,66 Another new broad absorption band centered at ~800
nm is consistent with Co(I) species absorption.67-68
Based on all of the above results, a reaction mechanism for photocatalytic H2
production based on cobalt-salen complex/CdS NRs is proposed in Scheme 2. Upon
visible light irradiation, CdS NRs absorb photons and generate electron-hole pairs.
The holes can be scavenged by the Na2S/Na2SO3 electron donors. The decomposition
mechanism of these donors has been well studied in the literature.32,69-70 The
photogenerated electrons are powerful electron reducing reagents, which are capable
of reducing Co(III) ions in a cobalt-salen complex to generate Co(II) species. The
conversion of Co(II) to Co(I) is probably achieved by further electron transfer from
the photosensitizer to Co(II) after the initial reduction of complex 6. The Co(III)
hydride generated from Co(I) is probably the key reaction intermediate for H2
production. Subsequently, our proposed mechanism for the H2-forming contains a
bimolecular reaction of two Co(III) hydrides to produce H2 + 2Co(II) and a
monometallic reaction of one Co(III) hydride and protons to produce H2 + Co(II). The
regenerated Co(II) is probably the dominant Co species in the system during
photolysis. A nearly linear relationship (Figure 7) can be obtained from Figure 2b
when the concentrations of cobalt-salen complex 6 increase from zero to 0.015 mM,
probably indicating that the H2 generation process is a monometallic process. A
schematic illustration of the proposed reaction mechanism is shown in Scheme 3.
16

Page 17 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
4. Conclusions
In summary, we have studied the cobalt-salen complex as a highly efficient and
inexpensive molecular catalyst for photocatalytic H2 production in aqueous solution.
Our system combines CdS NRs as the photosensitizer, a cobalt-salen complex as the
catalyst, and Na2S and Na2SO3 as the sacrificial electron donors. Under visible light
irradiation (λ > 420 nm), photocatalytic H2 production was successfully achieved with
a highest TON of ~64700 under optimal conditions. The average AQY was ~29 %
after 10 hours of irradiation with 420 nm monochromic light. The efficient interfacial
electron transfer from photoexcited CdS NRs to the cobalt-salen complex is proposed
as the reason for the high photocatalytic activity.
Supporting Information Available. Experimental details are available including
UV-vis spectra, comparison of hydrogen evolution rates of Co2+ and complex 6, EDX
spectrum and TEM image of photocatalyst after reaction, and current immediate
effect data. This material is available free of charge via the Internet at
http://www.rsc.org.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of
China (21271166, 21473170), the Fundamental Research Funds for the Central
Universities (WK3430000001, WK2060140015, WK2060190026), the Program for
17

Journal of Materials Chemistry A
Page 18 of 32
View Article Online
DOI: 10.1039/C5TA03515E
New Century Excellent Talents in University (NCET), and the Thousand Young
Talents Program. We appreciate the kind help from Prof. X. Y. Wang at Nanjing
University for assistance with the photoluminescence (PL) spectra and the
time-resolved photoluminescence (TRPL) spectra.
References
(1)
Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
15729-15735.
(2)
(3)
Du, P. W.; Eisenberg, R. Energy Environ. Sci. 2012, 5, 6012-6021.
Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y. Chem. Rev.
2007, 107, 4273-4303.
(4)
(5)
(6)
Li, Y. X.; Tang, L. F.; Peng, S. Q.; Li, Z. C.; Lu, G. X. CrystEngComm 2012,
14, 6974-6982.
Shima, S.; Pilak, O.; Vogt, S.; Schick, M.; Stagni, M. S.; Meyer-Klaucke, W.;
Warkentin, E.; Thauer, R. K.; Ermler, U. Science 2008, 321, 572-575.
Lazarides, T.; Sazanovich, I. V.; Simaan, A. J.; Kafentzi, M. C.; Delor, M.;
Mekmouche, Y.; Faure, B.; Reglier, M.; Weinstein, J. A.; Coutsolelos, A. G.;
Tron, T. J. Am. Chem. Soc. 2013, 135, 3095-3103.
(7)
(8)
(9)
McNamara, W. R.; Han, Z. J.; Alperin, P. J.; Brennessel, W. W.; Holland, P. L.;
Eisenberg, R. J. Am. Chem. Soc. 2011, 133, 15368-15371.
Guttentag, M.; Rodenberg, A.; Bachmann, C.; Senn, A.; Hamm, P.; Alberto, R.
Dalton Trans. 2013, 42, 334-337.
McLaughlin, M. P.; McCormick, T. M.; Eisenberg, R.; Holland, P. L. Chem.
Commun. 2011, 47, 7989-7991.
(10) Gan, L.; Groy, T. L.; Tarakeshwar, P.; Mazinani, S. K. S.; Shearer, J.; Mujica,
V.; Jones, A. K. J. Am. Chem. Soc. 2015, 137, 1109-1115.
(11) Das, A.; Han, Z. J.; Brennessel, W. W.; Holland, P. L.; Eisenberg, R. ACS
Catal. 2015, 5, 1397-1406.
(12) Frankel, E. N.; Emken, E. A.; Davison, V. L. J. Org. Chem. 1965, 30, 2739-&.
(13) Gartner, F.; Boddien, A.; Barsch, E.; Fumino, K.; Losse, S.; Junge, H.;
Hollmann, D.; Bruckner, A.; Ludwig, R.; Beller, M. Chem. Eur. J. 2011, 17,
6425-6436.
(14) Wang, M.; Na, Y.; Gorlov, M.; Sun, L. C. Dalton Trans. 2009, 6458-6467.
(15) Pullen, S.; Fei, H. H.; Orthaber, A.; Cohen, S. M.; Ott, S. J. Am. Chem. Soc.
2013, 135, 16997-17003.
(16) Du, P. W.; Knowles, K.; Eisenberg, R. J. Am. Chem. Soc. 2008, 130,
18

Page 19 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
12576-12577.
(17) Du, P. W.; Schneider, J.; Luo, G. G.; Brennessel, W. W.; Eisenberg, R. Inorg.
Chem. 2009, 48, 4952-4962.
(18) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.; Rohl, R.; Pascal, R. A.;
Malliaras, G. G.; Bernhard, S. Chem. Mater. 2005, 17, 5712-5719.
(19) Yan, Z. P.; Yu, X. X.; Han, A.; Xu, P.; Du, P. W. J. Phys. Chem. C 2014, 118,
22896-22903.
(20) Probst, B.; Kolano, C.; Hamm, P.; Alberto, R. Inorg. Chem. 2009, 48,
1836-1843.
(21) Eckenhoff, W. T.; Eisenberg, R. Dalton Trans. 2012, 41, 13004-13021.
(22) Eckenhoff, W. T.; Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2014, 53,
9860-9869.
(23) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38.
(24) Maeda, K.; Teramura, K.; Lu, D. L.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K.
Nature 2006, 440, 295-295.
(25) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851-7861.
(26) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253-278.
(27) Yan, H. J.; Yang, J. H.; Ma, G. J.; Wu, G. P.; Zong, X.; Lei, Z. B.; Shi, J. Y.; Li,
C. J. Catal. 2009, 266, 165-168.
(28) Preethi, V.; Kanmani, S. Int. J. Hydrogen Energy 2014, 39, 1613-1622.
(29) Barreca, D.; Fornasiero, P.; Gasparotto, A.; Gombac, V.; Maccato, C.; Montini,
T.; Tondello, E. ChemSusChem 2009, 2, 230-233.
(30) Hara, M.; Nunoshige, J.; Takata, T.; Kondo, J. N.; Domen, K. Chem. Commun.
2003, 3000-3001.
(31) Hara, M.; Takata, T.; Kondo, J. N.; Domen, K. Catal. Today 2004, 90,
313-317.
(32) Bao, N. Z.; Shen, L. M.; Takata, T.; Domen, K. Chem. Mater. 2008, 20,
110-117.
(33) Wen, F. Y.; Yang, J. H.; Zong, X.; Ma, B. J.; Wang, D. G.; Li, C. J. Catal. 2011,
281, 318-324.
(34) Xie, Y. P.; Yu, Z. B.; Liu, G.; Ma, X. L.; Cheng, H. M. Energy Environ. Sci.
2014, 7, 1895-1901.
(35) Nasr, C.; Hotchandani, S.; Kim, W. Y.; Schmehl, R. H.; Kamat, P. V. J. Phys.
Chem. B 1997, 101, 7480-7487.
(36) Peng, T. Y.; Li, K.; Zeng, P.; Zhang, Q. G.; Zhang, X. G. J. Phys. Chem. C
2012, 116, 22720-22726.
(37) Wang, Q. Z.; Li, J. J.; Bai, Y.; Lian, J. H.; Huang, H. H.; Li, Z. M.; Lei, Z. Q.;
Shangguan, W. F. Green Chem. 2014, 16, 2728-2735.
(38) Sun, Z.; Yue, Q.; Li, J.; Xu, J.; Zheng, H.; Du, P. J. Mater. Chem. A 2015, 3,
10243-10247.
(39) Wen, F. Y.; Li, C. Acc. Chem. Res. 2013, 46, 2355-2364.
(40) Marschall, R. Adv. Funct. Mater. 2014, 24, 2421-2440.
(41) Yao, W. F.; Song, X. L.; Huang, C. P.; Xu, Q. J.; Wu, Q. Catal. Today 2013,
199, 42-47.
19

Journal of Materials Chemistry A
Page 20 of 32
View Article Online
DOI: 10.1039/C5TA03515E
(42) Wang, H.; Chen, W.; Zhang, J.; Huang, C. P.; Mao, L. Q. Int. J. Hydrogen
Energy 2015, 40, 340-345.
(43) Khan, Z.; Khannam, M.; Vinothkumar, N.; De, M.; Qureshi, M. J. Mater.
Chem. 2012, 22, 12090-12095.
(44) Zong, X.; Yan, H. J.; Wu, G. P.; Ma, G. J.; Wen, F. Y.; Wang, L.; Li, C. J. Am.
Chem. Soc. 2008, 130, 7176-7177.
(45) Liu, S. Q.; Chen, Z.; Zhang, N.; Tang, Z. R.; Xu, Y. J. J. Phys. Chem. C 2013,
117, 8251-8261.
(46) Jang, J. S.; Joshi, U. A.; Lee, J. S. J. Phys. Chem. C 2007, 111, 13280-13287.
(47) Rhodes, B.; Rowling, S.; Tidswell, P.; Woodward, S.; Brown, S. M. J. Mol.
Catal. A: Chem. 1997, 116, 375-384.
(48) Felicio, R. C.; Cavalheiro, E. T. G.; Dockal, E. R. Polyhedron 2001, 20,
261-268.
(49) Abdulghani, A. J.; Khaleel, A. M. N. Bioorg. Chem. App. 2013.
(50) Fraile, J. M.; Garcia, N.; Herrerias, C. I.; Mayoral, J. A. Catal. Sci. Technol.
2013, 3, 436-443.
(51) Dzygiel, P.; Reeve, T. B.; Piarulli, U.; Krupicka, M.; Tvaroska, I.; Gennari, C.
Eur. J. Org. Chem. 2008, 1253-1264.
(52) Jain, S.; Venkatasubbaiah, K.; Jones, C. W.; Davis, R. J. J. Mol. Catal. A:
Chem. 2010, 316, 8-15.
(53) Fan, G. Z.; Zhao, H. T.; Duan, Z. X.; Fang, T.; Wan, M. H.; He, L. N. Catal.
Sci. Technol. 2011, 1, 1138-1141.
(54) Fast, A.; Esfandiari, N. M.; Blum, S. A. ACS Catal. 2013, 3, 2150-2153.
(55) Peretti, K. L.; Ajiro, H.; Cohen, C. T.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 2005, 127, 11566-11567.
(56) Shyu, S. G.; Tseng, C. K.; Chang, C. K.; Chen, H. P.; Liu, H. C.; Twu, J. J.
Chin. Chem. Soc. 2012, 59, 443-451.
(57) Zhang, P.; Yang, M.; Lu, X. P. Asian J. Chem.
2007, 19, 2083-2092.
(58) Cao, S.; Wang, C. J.; Lv, X. J.; Chen, Y.; Fu, W. F. Appl. Catal., B 2015, 162,
381-391.
(59) Martin, D. J.; Qiu, K. P.; Shevlin, S. A.; Handoko, A. D.; Chen, X. W.; Guo, Z.
X.; Tang, J. W. Angew. Chem. Int. Ed. 2014, 53, 9240-9245.
(60) Das, A.; Han, Z. J.; Haghighi, M. G.; Eisenberg, R. Proc. Natl. Acad. Sci.
U.S.A. 2013, 110, 16716-16723.
(61) Eckenhoff, W. T.; McNamara, W. R.; Du, P. W.; Eisenberg, R. Biochim.
Biophys. Acta, Bioenerg. 2013, 1827, 958-973.
(62) Chen, Y. T.; Ding, J. B.; Guo, Y.; Kong, L. B.; Li, H. L. Mater. Chem. Phys.
2003, 77, 734-737.
(63) Yang, Z. H.; Chu, H. B.; Jin, Z.; Zhou, W. W.; Li, Y. Chin. Sci. Bull. 2010, 55,
921-926.
(64) Hsu, S. H.; Hung, S. F.; Chien, S. H. J. Power Sources 2013, 233, 236-243.
(65) Xu, D.; Liu, Z. P.; Liang, J. B.; Qian, Y. T. J. Phys. Chem. B 2005, 109,
14344-14349.
(66) Liao, C. M.; Hsu, C. C.; Wang, F. S.; Wayland, B. B.; Peng, C. H. Polym.
20

Page 21 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
Chem. 2013, 4, 3098-3104.
(67) Alleman, K. S.; Peters, D. G. J. Electroanal. Chem. 1998, 451, 121-128.
(68) Alleman, K. S.; Peters, D. G. J. Electroanal. Chem. 1999, 460, 207-213.
(69) Liu, H.; Yuan, J.; Shangguan, W. Energy Fuels 2006, 20, 2289-2292.
(70) Gomathisankar, P.; Hachisuka, K.; Katsumata, H.; Suzuki, T.; Funasaka, K.;
Kaneco, S. ACS Sustainable Chem. Eng. 2013, 1, 982-988.
Scheme 1. Molecular structures of cobalt-salen complexes 1-6.
21

Journal of Materials Chemistry A
Page 22 of 32
View Article Online
DOI: 10.1039/C5TA03515E
Figure 1. (a) TEM image of CdS NRs; (b) Powder XRD pattern of CdS NRs (red plot)
and cobalt-salen complex 6 mixed with CdS NRs (1.4 wt % of cobalt in the
composite). Blue plot is the standard XRD pattern, CdS-PDF#77-2306.
22

Page 23 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
Figure 2. (a) Comparison of H2 production rate of the cobalt-salen complexes 1-6
(0.01 mM) under visible light. The systems contained 1.0 mg CdS NRs, Na2S (0.25
M), and Na2SO3 (0.35 M) in 20 mL Millipore water. Pure CdS and complex 6 were
used for comparison. (b) Influence of the concentration of complex 6 on
photocatalytic H2 production in a system containing 1.0 mg CdS NRs, Na2S (0.25 M),
and Na2SO3 (0.35 M) in 20 mL Millipore water.
23

Journal of Materials Chemistry A
Page 24 of 32
View Article Online
DOI: 10.1039/C5TA03515E
Table 1. Influence of the concentrations of sacrificial electron donors on
photocatalytic H2 production in 3 hours under visible light irradiation (λ > 420 nm).
The system contained 1.0 mg CdS NRs, 0.015 mM complex 6, and 20 mL Millipore
water.
24

Page 25 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
Figure 3. (a) Photocatalytic H2 production of complex 6 under visible light (λ > 420
nm) and apparent quantum yield (AQY) of complex 6 under monochromatic light (λ
= 420 nm). The reaction system contained 1.0 mg CdS NRs, 0.015 mM complex 6,
1.25 M Na2S, 1.75 M Na2SO3 and 20 mL deionized water. (b) Long-term H2
production performance under visible light (λ > 420 nm) irradiation. The system
contained 1.0 mg CdS NRs, 0.006 mM complex 6, 1.25 M Na2S, 1.75 M Na2SO3, and
50 mL Millipore water in a 250 mL reaction flask.
25

Journal of Materials Chemistry A
Page 26 of 32
View Article Online
DOI: 10.1039/C5TA03515E
Figure 4. (a) XPS survey spectra of the photocatalyst samples. Black plot is for the
mixture of CdS NRs and complex 6. Red plot is the photocatalyst after H2 production
reaction, which was centrifuged and washed by acetonitrile and water after 10 hours
of irradiation. (b) High resolution spectra of S 2p. (c) High resolution spectra of Cd 3d.
(d) High resolution spectra of Co 2p.
26

Page 27 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
Figure 5. (a) Photoluminescence spectra and (b) time-resolved photoluminescence
decay spectra of CdS NRs (black plot) and complex 6/CdS NRs (red plot) excited at a
wavelength of 405 nm. The complex 6 and CdS NRs were dissolved in acetonitrile
and then dropped using a pipette onto a glass plate to form a film.
27

Journal of Materials Chemistry A
Page 28 of 32
View Article Online
DOI: 10.1039/C5TA03515E
Figure 6. (a) Color change and (b) UV-vis absorption spectra of a solution containing
1.6
10-2 M TEOA, 1.1 10-5 M Pt chromophore, and 3.0 10-4 M complex 6 in
×
×
×
DMF (5% water) under inert atmosphere. Black plot is from the solution without
irradiation and red plot is from the solution after photocatalysis for 60 min. After 1
hour illumination, the color of the solution had turned from light yellow to dark
yellow.
28

Page 29 of 32
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C5TA03515E
Scheme 2. Proposed mechanism of photocatalytic H2 production using complex 6 and
CdS NRs under visible light (λ > 420 nm).
29