Hydrogen production on a hybrid photocatalytic system composed of ultrathin CdS nanosheets and a molecular nickel complex

Article abstract of DOI:10.1002/chem.201406642

The production of clean and renewable hydrogen through water splitting by using solar energy has received much attention due to the increasing global energy demand. We report an economic and artificial photosynthetic system free of noble metals, consisting of ultrathin CdS nanosheets as a photosensitizer and nickelbased complex as a molecular catalyst. Emission quenching and flash photolysis studies reveal that this hybrid system allows for effective electron transfer from the excited CdS nanosheets to the nickel-based complex to generate reduced intermediate species for efficient hydrogen evolution. Notably, the unique morphological and structural features of the ultrathin CdS nanosheets contribute to the highly efficient photocatalytic performance. As a consequence, the resulting system shows exceptional activity and stability for photocatalytic hydrogen evolution in aqueous solution with a turnover number (TON) of about 28 000 versus catalyst and a lifetime of over 90 h under visible light irradiation.

Full text of DOI:10.1002/chem.201406642

DOI: 10.1002/chem.201406642
Communication
&
Artificial Photosynthesis
Hydrogen Production on a Hybrid Photocatalytic System
Composed of Ultrathin CdS Nanosheets and a Molecular Nickel
Complex
[a]
[a]
[a]
[b]
[a]
You Xu, Xuguang Yin, Yi Huang, Pingwu Du, and Bin Zhang*
photocatalytic systems with both high activity and long-term
stability still remains a significant challenge.
Abstract: The production of clean and renewable hydro-
gen through water splitting by using solar energy has re-
ceived much attention due to the increasing global
energy demand. We report an economic and artificial pho-
tosynthetic system free of noble metals, consisting of ul-
trathin CdS nanosheets as a photosensitizer and nickel-
based complex as a molecular catalyst. Emission quench-
ing and flash photolysis studies reveal that this hybrid
system allows for effective electron transfer from the excit-
ed CdS nanosheets to the nickel-based complex to gener-
ate reduced intermediate species for efficient hydrogen
evolution. Notably, the unique morphological and struc-
tural features of the ultrathin CdS nanosheets contribute
to the highly efficient photocatalytic performance. As
a consequence, the resulting system shows exceptional
activity and stability for photocatalytic hydrogen evolution
in aqueous solution with a turnover number (TON) of
about 28000 versus catalyst and a lifetime of over 90 h
under visible light irradiation.
In comparison with traditional organic or organometallic
photosensitizers, colloidal semiconductor nanocrystals are
more promising light-harvesting materials because of their
[4,8]
broad spectral absorption and good photostability.
Among
them, two-dimensional (2D) semiconductor nanosheets (NSs)
with ultrathin thickness have sparked intense interest because
of their novel electronic structures, distinctive physicochemical
properties, and high specific surface areas, as compared with
[9–11]
conventional nanocrystallites and bulk materials.
These
semiconductor materials possess tunable absorption spectra
and high molar absorptivity, and thus they can continuously
absorb multiple photons even after electrons or holes are ac-
cumulated on the semiconductors. These attractive characteris-
tics render them excellent candidates to fulfil multiple func-
tionalities required in artificial photosynthetic systems as pho-
tosensitizers, charge accumulation sites, and electron donors
when coupled with redox catalysts for H generation. Despite
2
fundamental progress made in the development of heteroge-
nization of molecular catalysts on the surfaces of semiconduc-
[
8,12–26]
tors with various morphologies,
such as nanoparticles
and quantum dots
[12]
[13–15]
[16]
(
NPs),
nanorods,
8,17–22]
nanowires,
[
Solar-driven production of hydrogen (H ) from water by using
(QDs),
the solar-driven production of H2 from artificial
2
earth-abundant materials is considered as one of the promis-
photocatalytic systems, composed of ultrathin semiconductor
NSs and molecular catalysts, is still in its infancy.
[
1,2]
ing means to provide clean fuels in a postfossil age. Inspired
by natural photocatalysis, many attempts have been made in
mimicking the basic principles of nature’s masterpiece and de-
Herein, we employed ultrathin 2D CdS NSs with a thickness
of about 4 nm, stabilized by l-cysteine (l-Cys-CdS NSs), as
a photosensitizer, Ni-based complex (Ni complex 1) as a molec-
ular catalyst, and triethylamine (TEA) as the sacrificial electron
donor, and developed a robust and efficient photocatalytic
[3–5]
signing artificial photosynthetic systems for H generation.
2
In this context, recent studies on various homogeneous artifi-
[
6]
cial photosynthetic systems have shown high activity. Howev-
er, most of these homogeneous systems typically suffer from
short lifetimes, mainly because of decomposition of photosen-
system for H evolution from aqueous solution at room tem-
2
perature. Under optimal conditions, this hybrid photocatalytic
system generates a turnover number (TON) of over 28000 for
[
7]
sitizer molecules. Thus, the search for robust H -evolving
2
H (with respect to the catalyst) with a lifetime of over 90 h
2
[
a] Y. Xu, X. Yin, Y. Huang, Prof. Dr. B. Zhang
Department of Chemistry, School of Science
Tianjin University and Collaborative Innovation Center of Chemical Science
and Engineering (Tianjin)
under visible light irradiation (l>420 nm). Notably, the unique
morphological and structural features of the l-Cys-CdS NSs
with ultrathin thickness contribute to the high photocatalytic
activity and long-term stability.
Tianjin 300072 (China)
E-mail: bzhang@tju.edu.cn
The l-Cys-CdS NSs were prepared according to the previous-
[27]
[
b] Prof. Dr. P. Du
ly reported procedures and used directly (see Experimental
Section and Figure S1 in the Supporting Information). The l-
Cys-CdS NSs were selected as the photosensitizer, owing to
their broad visible absorption ability, aqueous dispersion,
simple preparation, suitable conduction band (CB) potential for
proton reduction, and economical advantage over noble-
Department of Materials Science and Engineering
CAS Key Laboratory of Materials for Energy Conversion
University of Science and Technology of China
Hefei 230026 (China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406642.
Chem. Eur. J. 2015, 21, 1 – 6
1
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The activity of light-driven H2 production was
found to depend on the pH value of the solution,
the concentration of the photosensitizer (l-Cys-CdS
NSs), and the sacrificial electron donor (TEA). As
shown in Figure 1a, a maximal rate for H generation
2
was achieved at pH 12.5, while lower amounts of H2
were observed at both lower and higher pH values.
The decrease in H -evolving activity at higher pH
2
value is probably a result of the lower proton con-
[28,29]
centration in solution.
Simultaneously, H genera-
2
tion would become thermodynamically unfavorable
[28]
with increasing pH values. In contrast, lowering the
pH value also leads to a slower H evolution rate,
2
which may be due to the fact that the protonation of
TEA weakens its ability to function as an electron
+
donor, and the decomposition of TEA becomes less
[5,28]
facile under such conditions.
Moreover, the solu-
bility of Ni complex 1 in aqueous solution is poor at
lower pH values, which is unfavorable for the contact
between the photosensitizer and the molecular cata-
lyst. The concentration of l-Cys-CdS NSs also affects
the activity of H production. At a fixed concentration
2
of Ni complex 1 and TEA at pH 12.5, the optimal con-
centration of photosensitizer was found to be
À1
0
.045 mgmL . As shown in Figure 1b, the H -evolv-
2
ing rate was improved by increasing the concentra-
À1
tion of l-Cys-CdS NSs from 0.011 to 0.045 mgmL .
2
Scheme 1. Schematic illustration of the photocatalytic process for H evolution in the ar-
tificial photocatalytic system comprising l-Cys-CdS ultrathin NSs as the photosensitizer
and Ni complexes as the molecular catalyst.
When the concentration of l-Cys-CdS NSs was in-
À1
creased to 0.056 mgmL , no clear increase in H pro-
2
duction can be observed. However, increasing the
À1
concentration of l-Cys-CdS NSs to 0.18 mgmL re-
sulted in a lower rate of H evolution. Aggregation of
2
metal-based photosensitizers. The Ni complexes 1–4 were syn-
thesized by the reaction of NiCl ·6H O with various ligands
CdS photosensitizer can be clearly observed after only 5 h of ir-
radiation (results not shown here). The results may be attribut-
ed to the light-filter effect at a high concentration of photo-
sensitizer, as reported in other hybrid photocatalytic sys-
2
2
(
Scheme 1 and Experimental Section in the Supporting Infor-
mation). The solid-state molecular structure of Ni complexes 1,
, and 3 was determined by single-crystal X-ray crystallography
Figure S2 and Table S1–S3 in the Supporting Information). Ni
[18]
2
(
tems. Moreover, the concentration of TEA also has an influ-
ence on the rate of H production, with an optimal concentra-
2
complex 4 was identified through the MALDI-TOF mass spec-
trometry (Figure S3 in the Supporting Information). We first se-
lected Ni complex 1 as the model molecular catalyst to couple
tion of 4.7% (v/v; Figure S5 in the Supporting Information). In
the presence of l-Cys-CdS NSs, Ni complex 1, and TEA, photo-
catalytic H production can also be implemented in pure water
2
with l-Cys-CdS NSs for the photocatalytic H production. Of
under the same irradiation conditions. However, the rate of H2
2
particular interest is that the as-obtained Ni complex 1 has
a good solubility in alkaline aqueous solution, making it possi-
ble for the construction of artificial photocatalytic system that
can operate in aqueous solution.
evolution is much lower than in EtOH/H O (1:1, v/v), which is
2
probably due to the poor solubility of TEA in pure water (Fig-
ure S6 and S7 in the Supporting Information).
We found the morphology of CdS-based photosensitizer
was also an important factor that governs the activity of pho-
The photocatalytic H2 evolution experiments were carried
out in an aqueous ethanol solution (EtOH/H O of 1:1, v/v)) in
tocatalytic H evolution. Figure 2a shows the photocatalytic ac-
2
2
À5
the presence of Ni complex 1 (3.25ꢁ10 m), l-Cys-CdS NSs
tivities on various CdS-based photosensitizers. The morpholo-
gies of CdS-DETA hybrid NSs, CdS-NS-based aggregates, l-Cys-
CdS QDs, as well as CdS NPs were shown in Figure S8 (see the
Supporting Information). It was found that l-Cys-CdS NSs ex-
hibited the highest activity compared with other CdS-based
photosensitizers (Figure 2a), indicating that shape effects on
À1
(
0.045 mgmL ), and TEA (4.7%, v/v) under visible light irradia-
tion (l>420 nm) at room temperature. The generated H was
2
characterized by means of online gas chromatography (GC,
Agilent 7890A) with methane as the internal standard. Control
experiments indicated that l-Cys-CdS NSs, Ni complex 1, and
TEA are all essential for efficient H production. The absence of
the H evolution activities exist in this hybrid photocatalytic
2
2
any of the three components results in no significant H gener-
system. We hypothesize that the catalytic system functions
through light absorption by the CdS photosensitizer, subse-
2
ation (Figure S4 in the Supporting Information).
&
&
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Communication
Figure 2. a) Photocatalytic H
2
production at pH 12.5 in the presence of vari-
ous photosensitizers (A–E) in mixed EtOH-H
Ni complex 1 (3.25ꢁ10 m), photosensitizer (0.045 mgmL ), and TEA
2
O (1:1, v/v) solution containing
À5
À1
(
4.7%, v/v): A: CdS NPs; B: l-Cys CdS QDs; C: CdS-DETA hybrid NSs; D: CdS
NS-based aggregates; E: l-Cys-CdS NSs. b) Photocatalytic H
2
evolution under
À5
Figure 1. a) Photocatalytic H
2
production in EtOH/H
2
O (1:1, v/v) solution at
the optimized conditions containing Ni complex 1 (3.25ꢁ10 m), l-Cys-CdS
À5
À1
different pH values containing Ni complex 1 (3.25ꢁ10 m), l-Cys-CdS NSs
NSs (0.045 mgmL ) and TEA (4.7%, v/v) in EtOH/H O (1:1, v/v) at pH 12.5.
2
À1
(
0.045 mgmL ), and TEA (4.7%, v/v). b) Photocatalytic H
2
production at ini-
The inset is the comparison of the H evolution activities in the presence of
2
2
+
tial pH 12.5 in EtOH/H
the l-Cys-CdS NSs photosensitizer containing Ni complex 1 (3.25ꢁ10 m)
2
O (1:1, v/v) solution as a function of concentration of
the same concentration of Ni complex 1 and Ni .
À5
and TEA (4.7%, v/v).
mation). As shown in the inset of Figure 2b, when the same
2
+
concentration of Ni ions were added into the reaction solu-
quent electron transfer from CdS to the molecular catalyst, and
proton reduction by the molecular catalyst. The exceptional
enhancement in the system comprising l-Cys-CdS NSs may be
attributed to their unique morphological and structural fea-
tures. Firstly, l-Cys-CdS NSs could offer a high specific surface
area and large fraction of uncoordinated surface atoms for har-
vesting more photons and absorbing the catalyst mole-
tion to replace the Ni complex 1, the photocatalytic activity of
Ni ions was fairly lower than the Ni complex 1 catalyst. The
2
+
results further indicated that the high photocatalytic H -evolu-
2
tion activity of Ni complex 1 can be only attributed to the effi-
cient electron transfer form l-Cys-CdS NSs to Ni complex 1, in
which the proton is reduced to H . The quantum efficiency
2
(QE) of this photocatalytic system for H production is 11.5%,
2
[
27,30,31]
cules.
Secondly, the band gap of these l-Cys-CdS NSs is
which was obtained under optimized conditions at 420 nm.
larger than that of other CdS-based photosensitizers because
of the quantum size effect (Figure S9 and S10, and Table S4 in
the Supporting Information), which may lead to a larger shift
of the CB and the increase in reducing power for proton re-
duction. Additionally, the ultrathin thickness shortens the trans-
fer distance and facilitates fast transfer of the photogenerated
The re-addition of Ni complex 1 restarted H production, while
2
the re-adding of CdS or TEA could not regenerate H under ir-
2
radiation (Figure S12 in the Supporting Information). These re-
sults indicate that the deactivation of the system is mainly due
to the decomposition of the Ni complex 1 and that the CdS
photosensitizer is still active. After over 100 h of irradiation, the
CdS photosensitizer was separated from the solution by centri-
fugation and washed several times. Scanning electron micro-
scope (SEM) and energy-dispersive X-ray spectroscopy (EDX)
analysis results suggested that the sheet-like morphology can
be retained and no Ni signals can be detected in the EDX spec-
trum (Figure S13 in the Supporting Information). The unusual
longevity may be attributed to the use of semiconductor as
the photosensitizer, as reported in some other hybrid photo-
[
30,31]
charge carriers on the surface of the photosensitizer.
Considering all above experimental trials, we carried out the
photocatalytic reaction under the optimized condition, that is,
EtOH/H O solution (1:1, v/v, 42 mL) containing Ni complex
2
À5
À1
1
(3.25ꢁ10 m), l-Cys-CdS NSs (0.045 mgmL ), and TEA
(
4.7%, v/v) at pH 12.5. The system continuously produces H at
2
a relatively constant rate for over 90 h, as shown in Figure 2b.
A TON (based on Ni complex 1) of about 28000 is achieved
À1
[8]
and the initial TOF is determined to be about 311 h . Both the
synthetic systems. When other Ni complexes with similar
photocatalytic activity and stability are remarkably superior to
those of the homogeneous catalytic system containing fluores-
cein as the photosensitizer (Figure S11 in the Supporting Infor-
structures to 1 are employed as molecular catalysts, such as Ni
complex 2, 3, and 4, high photocatalytic activity for H evolu-
2
tion can be also obtained (Scheme 1 and Figure S14 and S15
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2
+
in the Supporting Information), demonstrating the general ap-
active dye methyl viologen (MV ) and obtained an approxi-
mate value of À0.77 V (vs. NHE, pH 7; Figure S17 in the Sup-
porting Information). The reductive potential of À0.53 V (vs.
NHE) of the Ni complex 1 was taken from cyclic voltammo-
grams (Figure S18 and Table S5 in the Supporting Information).
The results implied that the electron transfer from excited l-
Cys-CdS NSs to Ni complex 1 is thermodynamically favorable
(Figure S19 in the Supporting Information). Moreover, when
the electron donor TEA was added together with Ni complex
1, the PL intensity of l-Cys-CdS NSs further dropped, indicating
that the presence of TEA can consume the photogenerated
holes on excited l-Cys-CdS NSs. The electron transfer from l-
Cys-CdS NSs to complex 2, 3, or 4 is also thermodynamically
favourable. There is no significant change in the potentials for
plicability of this hybrid photocatalytic system for H produc-
2
tion.
The photophysical properties of this artificial photocatalytic
system were further investigated to understand the reaction
mechanism. Steady-state photoluminescence (PL) spectra were
measured to explore the charge transfer pathways in this
hybrid photocatalytic system. As shown in Figure 3a, the emis-
sion spectrum of l-Cys-CdS NSs shows a strong excitonic emis-
sion peak centered at around 430 nm in the EtOH/H O (1:1,
2
v/v) solution. With the addition of Ni complex 1 into the l-Cys-
CdS NSs solution, the intensity of this excitonic emission band
immediately showed a significant decrease. In view of the
small spectroscopic overlap of absorption of Ni complex 1 and
the emission of l-Cys-CdS NSs (Figure 3a), the energy transfer
between excited l-Cys-CdS NSs to Ni complex 1 would be neg-
ligible if it occurs, so the emission quenching may be attribut-
ed to the photoinduced electron transfer from l-Cys-CdS NSs
to Ni complex 1. When the same concentration of Ni complex
II
I
the Ni /Ni couple among Ni complexes 1–4 (Figure S18, S20
and Table S5 in the Supporting Information). Interestingly,
however, these four complexes, 1–4, exhibit different activities
for H2 production. Ni complex 4 clearly shows lower PL
quenching efficiency for l-Cys-CdS NNs and smaller H -gener-
2
1
was added into EtOH/H O (1:1, v/v) solution of other CdS-
ating rate than other three complexes, which may be attribut-
ed to its poor solubility in this photocatalytic system (Fig-
ure S15 and S21 in the Supporting Information).
2
based photosensitizers, the emission was also quenched. How-
ever, the quenching efficiency for other CdS-based photosensi-
tizers is lower than that for l-Cys-CdS NSs (Figure S16 in the
Supporting Information). The direction of charge transfer be-
tween l-Cys-CdS NSs and Ni complex 1 can be further predict-
ed according to the energy band structure of CdS and the
redox potential of Ni complex 1. We measured the energy of
the CB of our l-Cys-CdS NSs by photoreduction of the redox-
A flash photolysis study was carried out to further verify the
photoinduced electron-transfer process. On laser excitation of
the l-Cys-CdS NSs with light of 355 nm wavelength, no charac-
teristic absorption signal could be detected (Figure S22 in the
Supporting Information). In contrast, when Ni complex 1 was
introduced, a strong absorption at around 370 nm and a broad
absorption at the range of 510~600 nm emerged immediately
with a lifetime of ~62 ns at 370 nm (Figure 3b). Although it is
difficult to determine its structure, it is reasonable to speculate
I
that a Ni intermediate species is generated through the reduc-
tion of Ni complex 1, as many other Ni complexes reported in
[32–34]
the literatures.
In light of the above experimental observation and spectro-
scopic analysis, we can propose a plausible mechanism for the
efficient photocatalytic H generation. As shown in Scheme 1,
2
the photoexcited electron in the CB of l-Cys-CdS NSs transfers
to the Ni complex 1, leading to the formation of a reactive re-
I
duced Ni center, which would play a critical role in subsequent
electron transfer to a proton to be involved in a catalytic cycle
I
for H production. Many other Ni species have also been pro-
2
posed as important intermediates in photocatalytic H produc-
2
[23,32–34]
tion by other Ni-based complexes.
The photogenerated
hole remaining in the l-Cys-CdS NSs after light irradiation is
subsequently coupled with electrons transferred from the sac-
rificial electron donor TEA, thus suppressing the recombination
of photogenerated charge carries and allowing the forward re-
actions. Further studies are needed to describe the mechanism
in full detail for the catalytic processes and to investigate the
exceptional stability of this complex catalyst in such reaction
conditions.
Figure 3.
a) UV-vis absorption spectrum of Ni complex 1 (left) in EtOH/
H
2
O (1:1, v/v) and emission spectra of the l-Cys-CdS NSs at pH 12.5 in the
In summary, a robust, inexpensive, and highly efficient heter-
ogeneous photocatalytic system, comprising l-Cys-CdS NSs
with a thickness of ~4 nm as the photosensitizer and a Ni-
based complex (Ni complex 1) as the molecular catalyst, has
absence (curve 1) and presence of Ni complex 1 (curve 2) or Ni complex 1+
TEA (curve 3), excited at 380 nm. b) Transient absorption spectrum of Ni
complex 1 and l-Cys-CdS NSs in EtOH/H O (1:1, v/v) at pH 12.5 upon laser-
2
pulsing (wavelength 355 nm). The inset of b) is the kinetic decay monitored
at 370 nm.
been achieved for H production in aqueous solution. Under
2
&
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Communication
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2
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2
[
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Keywords: artificial photosynthesis
·
electron transfer
·
hydrogen production · molecular catalysts · nanostructures
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Published online on && &&, 0000
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Hydrogen evolution: A robust, efficient,
and stable heterogeneous photosyn-
thetic system comprising 2D CdS nano-
sheets as photosensitizers and Ni com-
plex 1 (see scheme; TON=turnover
number, TEA=triethylamine) as a molec-
ular catalyst has been successfully con-
&
Artificial Photosynthesis
Y. Xu, X. Yin, Y. Huang, P. Du, B. Zhang*
& – &&
&
Hydrogen Production on a Hybrid
Photocatalytic System Composed of
Ultrathin CdS Nanosheets and
a Molecular Nickel Complex
structed for H production form aque-
2
ous solution under visible light irradia-
tion. The results pave a new route to
design artificial photosynthetic systems
for solar energy conversion applications.
&
&
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!