A Heterogeneous Photocatalytic Hydrogen Evolution Dyad: [(tpy

Article abstract of DOI:10.1002/chem.201601789

The development of an artificial heterogeneous dyad by covalently anchoring a hydrogen-evolving molecule catalyst to a semiconductor photosensitizer through a bridging ligand is highly challenging. Herein, we adopt the inorganic–organic hybrid CdS–DETA NSs (DETA=diethylenetriamine, NSs=nanosheets) as initial matrix to successfully construct an imine bond (-CH=N-) linked heterogeneous dyad [CdS?N=CH?Ni] through the condensation reaction between the amino groups of CdS–DETA and the aldehyde group of the water reduction molecular catalyst, [(tpy-CHO)₂Ni]Cl₂(tpy=terpyridine). The [CdS?N=CH?Ni] enables a turnover number (TON) of about 43 815 versus Ni catalysts and an initial turnover frequency (TOF) of approximately 0.47 s^?1in 26 h under visible-light irradiation (λ>420 nm). The apparent quantum yield (AQY) reaches (9.9±0.8) % at 420 nm. Under optical conditions, the [CdS?N=CH?Ni] can achieve a considerable amount of hydrogen production, 507.1±27 μmol H₂for 6 h, which is 1.27 times that generated from the mechanically mixed system of CdS–DETA NSs and [(tpy-CH=NR)₂Ni]Cl₂(III) under otherwise identical conditions. Furthermore, its TON value based on Ni species is also higher than that of the mixed system of CdS–DETA and III.

Full text of DOI:10.1002/chem.201601789

DOI: 10.1002/chem.201601789
Full Paper
&
Photochemistry
A Heterogeneous Photocatalytic Hydrogen Evolution Dyad:
[(tpy**-CHO)2Ni]Cl2 Covalently Anchored to CdS–Amine Inorganic–
Organic Hybrid Nanosheets
Weiwei Zhao+,[a, c] Yi Huang+,[a] Yang Liu,[b] Liming Cao,[a] Fang Zhang,[a] Yamei Guo,[a] and
Bin Zhang*[a]
Abstract: The development of an artificial heterogeneous
dyad by covalently anchoring a hydrogen-evolving molecule
catalyst to a semiconductor photosensitizer through a bridg-
ing ligand is highly challenging. Herein, we adopt the inor-
ganic–organic hybrid CdS–DETA NSs (DETA=diethylenetria-
mine, NSs=nanosheets) as initial matrix to successfully con-
struct an imine bond (-CH=N-) linked heterogeneous dyad
[CdSꢀN=CHꢀNi] through the condensation reaction between
the amino groups of CdS–DETA and the aldehyde group of
the water reduction molecular catalyst, [(tpy-CHO)2Ni]Cl2
(tpy=terpyridine). The [CdSꢀN=CHꢀNi] enables a turnover
number (TON) of about 43815 versus Ni catalysts and an ini-
tial turnover frequency (TOF) of approximately 0.47 sꢀ1 in
26 h under visible-light irradiation (l>420 nm). The appar-
ent quantum yield (AQY) reaches (9.9ꢁ0.8)% at 420 nm.
Under optical conditions, the [CdSꢀN=CHꢀNi] can achieve
a considerable amount of hydrogen production, 507.1ꢁ
27 mmol H2 for 6 h, which is 1.27 times that generated from
the mechanically mixed system of CdS–DETA NSs and [(tpy-
CH=NR)2Ni]Cl2 (III) under otherwise identical conditions. Fur-
thermore, its TON value based on Ni species is also higher
than that of the mixed system of CdS–DETA and III.
and low catalytic turnover numbers (TON).[4] Adopting alterna-
tive inorganic semiconductor PSs with larger absorption cross-
sections over a broad spectral range, appropriate band gap,
and excellent photostability can substantially conquer these
shortcomings.[5] Recently, a series of heterogeneous artificial
hybrid systems have been constructed through coordinative
linking complex catalysts to inorganic semiconductor PSs.[6] For
example, Chen and co-workers successfully designed a quan-
tum dot (QD)/cobaloxime hybrid through robustly anchoring
the cobaloxime catalyst molecules on CdSe/ZnS core/shell QDs
through a phosphonate linkage, which showed a faster charge
separation rate coupling with slower recombination rate.[6a]
Wu’s group reported a CdSe/Fe2S2(CO)6 hybrid by interface-di-
rected assembly of a simple [FeFe]-H2ase mimic onto CdSe
QDs, displaying improved performance towards the photocata-
lytic H2 evolution reaction (PHER).[6b] Nevertheless, these inter-
face-directed surface affinity interactions between semiconduc-
tor PSs and molecular catalysts are relatively weak, which will
inevitably lead to the dissociation of complex catalysts, and
thus further reduce the catalytic activity. The application of co-
valent linkage methods in such a heterogeneous system may
be an avenue for exploration. However, to the best of our
knowledge, it is still a big challenge to covalently anchor mo-
lecular catalysts onto semiconductor PSs for a heterogeneous
dyad.
Introduction
Photocatalytic water reduction into clean and non-carbon-con-
taining hydrogen (H2) fuel is considered as a promising way to
meet long-term global energy demands.[1] Photoinduced elec-
tron transfer (PET) from an excited photosensitizer (PS) to
proton reduction molecular catalysts has been extensively re-
searched because of its relevance to photocatalytic H2 evolu-
tion.[2] Considerable efforts have been devoted to elevating the
mobility of charge carriers by conjugating a light-harvesting
material with a proton reduction catalyst.[3] Covalently linked
molecular dyads or triads feature a forward charge transfer
and increased H2 production, but the transition-metal-complex
photosensitizers in the homogeneous photocatalytic system
suffer from a narrow spectral absorption, long-term instability,
[a] Dr. W. Zhao,+ Y. Huang,+ L. Cao, F. Zhang, Dr. Y. Guo, Prof. B. Zhang
Department of Chemistry, School of Science
Tianjin University, and Collaborative Innovation Center of
Chemical Science and Engineering (Tianjin), Tianjin 300072 (P. R. China)
E-mail: bzhang@tju.edu.cn
[b] Y. Liu
Analysis and Testing Center of Tianjin University
Tianjin University, Tianjin 300072 (P. R. China)
[c] Dr. W. Zhao+
Department of Chemistry, School of Science
Tianjin University of Science & Technology, Tianjin 300457 (P. R. China)
[+] These authors contributed equally to this work.
[**] tpy=Terpyridine.
The CdS semiconductor, with a broad optical absorption,
narrow band gap, and fast charge separation characteristics, is
considered an excellent PS model for structure- or morpholo-
gy-dependent photocatalytic performance.[7] Some organic
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201601789.
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molecules with terminal multi-functional groups have sparked
intense interest because of their improved functionalization in
organic synthesis. For example, the diethylenetriamine (DETA)
molecule contains two amino functional groups (-NH2), which
have the ability to undergo a condensation reaction with other
functional groups, such as aldehyde groups (-CHO),[8] carboxyl-
ic groups (-COOH),[9] or acyl chlorides (-CO-Cl).[10] Inorganic–or-
ganic hybrid CdS–diethylenetriamine (CdS–DETA) nanomateri-
als are composed of multi-atomic layers of (CdS)n and organic
DETA molecules,[11] and thus possess the intrinsic properties of
the inorganic CdS component and the functionality of amino
functional groups.[12] These attractive characteristics render
them promising candidates as an ideal host platform for the
construction of a covalently linked heterogeneous photocata-
lytic hybrid system through a direct condensation reaction be-
tween the exposed amino-terminal groups on the surface of
the CdS NSs and the terminal groups of proton reduction mo-
lecular complex catalysts.
Results and Discussion
Synthesis and characterization
The inorganic–organic hybrid CdS–DETA NSs are fabricated ac-
cording to a solvothermal method reported in our previous
work.[11] The obtained samples are purified with water and al-
cohols to remove the free ions and physically adsorbed DETA
molecules on their surface. The morphology and structure are
first determined by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). Typical SEM (Fig-
ure 1a) and TEM (Figure 1b) images clearly show that the
Herein, we first synthesize the molecular complex [(tpy-
CHO)2Ni]Cl2 (I; tpy=terpyridine) with aldehyde-terminal groups
(-CHO) and earth-abundant Ni element[13] as a molecular cata-
lyst. Then, the aldehyde-terminal groups of I were used to per-
form a condensation reaction with the exposed amino groups
of CdS–DETA NSs, resulting in its chemically stable grafting
onto the CdS–DETA through an imine linkage (-N=CH-)
(Scheme 1). The process affords a new-formed [CdSꢀN=CHꢀNi]
Scheme 1. Schematic illustration of the formation and photoinduced elec-
tron transfer process of the [CdSꢀN=CHꢀNi] dyad.
dyad. The photocatalytic performance of [CdSꢀN=CHꢀNi] for
H2 production is evaluated in the presence of sacrificial re-
agents (triethylamine, TEA) under visible-light irradiation (l>
420 nm). The system shows exceptional TONs (based on the
amount of surface complexes) and an initial TOF of up to
43815 and 0.47 sꢀ1 for 26 h, respectively. This work may pro-
vide a prototype for constructing various covalently coupled
heterogeneous hybrid systems.
Figure 1. a) SEM, b) TEM images, and c) N1s XPS spectrum of the inorganic–
organic hybrid CdS–DETA NSs. d) Single-crystal X-ray structure of I. e and
f) Raman spectra of I and II. g) Raman spectra of CdS–DETA NSs and the
[CdSꢀN=CHꢀNi] dyad. h) FTIR spectra of I, II, and IV. i) FTIR spectra of CdS–
DETA and [CdS-N=C-Ni]. j) Band deconvolution analysis in the 1500–
1725 cmꢀ1 spectral region.
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CdS–DETA NSs possess a lateral size distribution of 300–
800 nm and a thickness of about 30 nm. Powder X-ray diffrac-
tion (PXRD, Figure S1a in the Supporting Information), energy-
dispersive X-ray spectroscopy (EDX, Figure S1b in the Support-
ing Information), and X-ray photoelectron spectroscopy (XPS,
Figure S2 in the Supporting Information) indicate that the ob-
tained nanosheets are inorganic–organic hybrids and com-
posed of Cd, S, C, and N. The N1s XPS spectrum (Figure 1c) is
fitted into two distinct peaks. The binding energies at 394.5
and 392.7 eV could be assigned to NꢀCd and NꢀH bonds, re-
spectively,[14] and thus indicate that abundant amino groups
exist on the surface of the CdS–DETA NSs.
create the [CdSꢀN=CHꢀNi] dyad through a facile covalent link-
age method under mild solvothermal conditions. The obtained
[CdSꢀN=CHꢀNi] samples were precipitated with water and eth-
anol for several times to completely remove the physically ad-
sorbed molecules I. To further demonstrate the successful co-
valent grafting between CdS–DETA and I, Raman and FTIR
spectra have been adopted to analyze the CdS–DETA and
[CdSꢀN=CHꢀNi], respectively. It can be seen from Figure 1g
that there are three similar peaks at around 300 cmꢀ1
,
600 cmꢀ1, and 900 cmꢀ1 for CdS–DETA and the [CdSꢀN=CHꢀ
Ni] dyad, respectively, which correspond to the typical Raman
feature of CdS components.[21] As for [CdSꢀN=CHꢀNi], the
Complex I is synthesized by the reaction of NiCl2·6H2O with
4’-([2,2’:6’,2’’-terpyridin]-4’-yl)-[1,1’-biphenyl]-4-carbaldehyde by
using a modified procedure.[15] The molecule structure is deter-
mined by single-crystal X-ray analysis (Figure 1d), demonstrat-
ing that I contains aromatic aldehyde-terminal groups. X-ray
crystallographic data has been deposited with the Cambridge
Crystal Data Centre (CCDC) as supplementary publication num-
bers 1016346–1016346 (Table S1 in the Supporting Informa-
tion). To prove the feasibility of the condensation reaction be-
tween I and DETA, we performed an experiment adding I and
DETA into methanol solution and reacting for 10 h. Then,
Raman spectroscopy and Fourier transform infrared spectros-
copy (FTIR) were employed to make a comparison between I
and the as-obtained complex [(tpy-CH=N)2Ni]Cl2 (II). It can be
seen from Raman spectroscopy, shown in Figure 1e, that there
other bands at around 1357.5 cmꢀ1
,
1472.8 cmꢀ1, and
1605.5 cmꢀ1 are close to that observed in I (Figure 1e), indicat-
ing I exists in the [CdSꢀN=CHꢀNi] dyad. Moreover, slight band
shifts relative to I suggest a changed chemical environment of
the grafted complex, demonstrating that I has covalently anch-
ored on the surface of CdS–DETA NSs.[6b] Further evidence for
the covalent linkage between CdS–DETA and I comes from
FTIR spectra. Figure 1i shows the FTIR spectra of CdS–DETA
and [CdSꢀN=CHꢀNi]. Compared with the FTIR spectra of CdS–
DETA, the characteristic bands of [CdSꢀN=CHꢀNi] at around
631 cmꢀ1, 1035.8 cmꢀ1, and 1657.7 cmꢀ1 are similar to that of
the free complex I (Figure 1h). However, note that a slight
band shift also occurs. This further implies that the chemical
environment of I experiences a change, confirming that I has
covalently anchored on the surface of CdS–DETA NSs.[6b] To fur-
ther understand the structure of [CdSꢀN=CHꢀNi], deconvolu-
tion infrared band analysis (Figure 1j) was performed in the
spectral region of 1500–1715 cmꢀ1. The band at 1587 cmꢀ1 can
be ascribed to the characteristic bands of CdS–DETA (Fig-
ure 1i). Whereas the band at 1660.3 cmꢀ1 can be separated
into two typical absorption bands at 1650 cmꢀ1 and
1696 cmꢀ1. The band at 1696 cmꢀ1 can be assigned to the
characteristic C=O stretching mode of I (Figure 1h), suggesting
the presence of an aldehyde group in [CdSꢀN=CHꢀNi]. The
other band at 1650 cmꢀ1 can be attributed to the characteristic
C=N stretching mode of II (Figure 1h), indicating that I has
been grafted on CdS-DETA NSs by an imine (-C=N-) linkage.
The highly rigid conjugated structure of I makes it difficult for
it to lie on the surface of the CdS–DETA NSs after one aldehyde
group of I has undergone a condensation reaction with the
amine groups in CdS–DETA, and thus another aldehyde group
will be exposed in the solution. In this case, the percentage of
aldehyde groups and imine groups should be the same in the
[CdSꢀN=CHꢀNi] products. Actually, because some DETA mole-
cules can be stripped from CdS-DETA when stirred in methanol
solution,[22] and then can quickly react with the exposed alde-
hyde groups of I/CdS-DETA, partially grafted complex mole-
cules are found in the form of II (Scheme 1). Additionally, as
shown in Figure 1j, the presence of unreacted aldehyde
groups in [CdSꢀN=CHꢀNi] illustrates that only a tiny amount of
DETA molecules are stripped from CdS–DETA, thus indicating
that the CdS–DETA NSs are of good stability.
are several similar peaks at around 895.8 cmꢀ1, 1004.4 cmꢀ1
,
1148.5 cmꢀ1, 1242.6 cmꢀ1, 1353.4 cmꢀ1 in the 500–1400 cmꢀ1
region, which are mainly attributed to fingerprint vibrations of
benzene and pyridine.[16] The characteristic bands at
1543.8 cmꢀ1 and 1596.6 cmꢀ1 of I can be assigned to the
stretch vibrations of C=O and C=C/C=N (Figure 1 f).[17] It is
noteworthy that, after its reaction with DETA, the band at
1543.8 cmꢀ1, characteristic of n(C=O), disappears, but the inten-
sity of the n(C=C/C=N) peak at around 1596.6 cmꢀ1 increases.
These results indicate that a new imine group (-C=N-) is
formed in II through the condensation reaction between an al-
dehyde group of I and amino group of DETA. The FTIR spec-
trum in Figure 1 h shows strong vibration modes in the 1740–
400 cmꢀ1 spectral range. The peaks in the range 990–650 cmꢀ1
are mainly attributed to the characteristic bending vibration
frequencies of aromatic groups.[18] The peak at around
1696 cmꢀ1 of I (Figure 1 h ) is ascribed to stretching vibrations
of aldehyde groups (O=CH-),[19] which disappears after the re-
action between I and DETA. A new peak at 1650 cmꢀ1 of II
(Figure 1 h) is assignable to the stretching vibrations of imine
groups (-C=N-),[20] and further confirms the successful conden-
sation reaction between I and DETA. High-resolution mass
spectrometry of II (Figure S3 in the Supporting Information)
shows that the estimated value [Mꢀ2ClꢀH]+ =1053.2666 is
consistent with the theoretical value [Mꢀ2ClꢀH]+ =
1053.4417, also demonstrating that the aromatic aldehyde
group of I can perform a condensation reaction with amine
groups of DETA.
The presence of the Ni-based complex in [CdSꢀN=CHꢀNi] is
also confirmed by EDX analysis (Figure S4a in the Supporting
Information). In comparison with the EDX spectrum of individ-
Inspired by the above results, the as-prepared inorganic–or-
ganic hybrid CdS–DETA NSs are then hybridized with I to
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ual CdS–DETA NSs (Figure S1b in the Supporting Information),
a clear Ni signal appears in the [CdSꢀN=CHꢀNi] spectrum, fur-
ther proving the successful covalent grafting of the Ni-based
complex on CdS–DETA. Inductively coupled plasma-mass spec-
troscopy (ICP-MS) determines the amount of anchored com-
plex I as 0.7% (w/w) in [CdSꢀN=CHꢀNi].
search has shown that the VBM of anatase TiO2 is located at
+2.64 V versus NHE at pH 7.[25] Thus, the VBM of CdS–DETA is
calculated to be 1.71 V versus NHE at pH 7 and the conduction
band minimum (CBM) of CdS–DETA is ꢀ0.75 V versus NHE at
pH 7 (Figure 2b). The above analysis was further verified by
a dark electrochemical scan and Mott–Schottky curve of CdS–
DETA NSs (Figure S6 in the Supporting Information). The reduc-
tion potential (Ered) of I and II obtained from cyclic voltammo-
grams are ꢀ0.5 V (Figure 2e) and ꢀ0.45 V (Figure 2 f), respec-
tively, which are all negative compared to the CBM of CdS–
DETA. Thus, the PET process from the conduction band of
CdS–DETA to the Ni-based catalytic center of I or II are both
thermodynamically favorable (Scheme 1).
Free-energy change of PET
The free-energy change of PET from excited CdS–DETA NSs to
Ni-based catalysts is then estimated. The UV/Vis absorption
spectrum (Figure 2a) indicates CdS–DETA NSs have a favorable
Photocatalytic hydrogen evolution
The [CdSꢀN=CHꢀNi] dyad and sacrificial agents are added into
a Pyrex reactor to construct a H2 evolution system. The H2 evo-
lution reaction is performed under visible-light irradiation (l>
420 nm) with a 300 W Xenon lamp. Because [CdSꢀN=CHꢀNi] is
prone to decompose in acidic solution,[8a] the initial system is
set at neutral or alkaline conditions. Control experiments show
that the efficiency of light-driven H2 production is highly de-
pendent on the solution components, the pH value of the so-
lution, and the concentration of the sacrificial donor (TEA).
Figure 3a shows that the H2 evolution efficiency is depen-
dent on solution components in the photocatalytic system.
Only a negligible amount of hydrogen is produced in pure
water. When we replaced the pure water with H2O/EtOH or
H2O/TEA mixed solvent, the efficiency of H2 evolution is im-
proved greatly. More encouragingly, significantly higher activity
is achieved in the mixed solution of H2O/ethanol with TEA as
sacrificial agent. This can be accounted for by the fact that the
as-synthesized Ni-based catalyst is more soluble in the H2O/
EtOH mixture (Figure S7 in the Supporting Information) and
TEA also possesses better solubility in the H2O/EtOH mixed sol-
vent than in pure water.[5a,26] Figure 3b looks at the influence
of pH on the H2 production system. To guarantee the same
concentration of TEA throughout the control experiments, we
adjusted the pH value with 1m HCl and 1m NaOH prior to visi-
ble-light irradiation. pH 12.2 is observed to give both the high-
est initial activity and the maximum TON of the system. This
pH-dependent effect may be involved in the following aspects.
Higher pH values can result in lower proton concentration in
solution, easier dissociation of the covalent imine bond (-CH=
N-), and more thermodynamically unfavorable H2 genera-
tion.[13b,27] Protonation of TEA at lower pH values from 12.0 to
9.0 makes it unable to function as an electron donor, and TEA+
decomposition becomes less facile, simultaneously.[13b,27b] Thus,
an appropriate pH is of big significance to obtain a maximal H2
production performance. It is demonstrated from the results
that the concentration of TEA is closely related to the rate of
H2 evolution at the optimal pH value of 12.2. As shown in Fig-
ure 3c, the H2 evolution activity is significantly improved as
the concentration of TEA increases from 1.6% to 6.3%. Further
increasing the concentration of TEA to 9.1% leads to a de-
creased TON value. These results suggest that a concentration
Figure 2.
a) UV/Vis diffuse reflectance spectrum and transformed Kubel-
ka–Munk spectrum of CdS–DETA NSs. b) Energy diagram and scheme illus-
trating the photocatalytic process for H2 evolution in the [CdSꢀN=CHꢀNi]
dyad. XPS valence band spectrum of the CdS–DETA NSs (c) and TiO2 (d)
Cyclic voltammograms of I (e) and II (f).
visible absorption capability with a direct band gap of about
2.46 eV, measured from the transformed Kubelka–Munk spec-
trum.[23] The CdS–DETA NSs exhibit enhanced visible-light ab-
sorption intensity compared with CdS nanoparticles (Figure S5
in the Supporting Information). With covalently anchored I on
the surface, the [CdSꢀN=CHꢀNi] dyad has a similar absorption
intensity as CdS–DETA NSs (Figure S5 in the Supporting Infor-
mation). XPS valence band spectrum of CdS–DETA (Figure 2c)
confirms its energy of valence band maximum (VBM) as
1.08 eV, which is 0.93 eV smaller than the 2.01 eV of anatase
TiO2 (Figure 2d).[24] This indicates that CdS–DETA possesses
a smaller VBM than anatase TiO2 by approximately 0.93 V rela-
tive to a normal hydrogen electrode (vs. NHE). Previous re-
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Figure 3. a) Photocatalytic H2 evolution of the [CdSꢀN=CHꢀNi] dyad (0.94 mm) in different solvents (A–D). A: H2O (30 mL); B: H2O/EtOH=15 mL/15 mL; C:
H2O/TEA=30 mL/2 mL; D: H2O/EtOH/TEA 15 mL:15 mL:2 mL. b) Photocatalytic H2 production at different pH values in H2O/EtOH (1:1, v/v, 30 mL) solution;
[CdSꢀN=CHꢀNi] (0.94 mm); TEA (6.3%, v/v, 2 mL). c) The dependence of H2 evolution on the concentration of TEA from 1.4% to 9.1% (v/v) in H2O/EtOH (1:1,
v/v, 30 mL) solution; [CdSꢀN=CHꢀNi] (0.94 mm). d) Time course of H2 evolution under optimal conditions: [CdSꢀN=CHꢀNi] (0.94 mm), EtOH/H2O (1:1, v/v,
30 mL) solution, TEA (6.3%, v/v, 2 mL), pH 12.2.
of 6.3% (v/v) is optimal for PHER in such systems. The blank
experiments in the dark or when illuminated with visible light
in the absence of the catalysts yield no detectable amount of
H2, and thus indicating that the light and catalyst are both es-
sential for H2 generation. Accordingly, optimized standard con-
ditions giving the highest TON versus grafted Ni-catalyst are
adopted, including 30 mL of H2O/EtOH solution (1/1, v/v),
0.94 mm of [CdSꢀN=CHꢀNi], 2 mL of TEA (6.3%, v/v) at
pH 12.2. Irrespective of H2 dissolution in the solution, the total
amount of H2 reached 1.3 mmol within 26 h of irradiation, as
shown in Figure 3d. An exceptional TON (based on the
amount of surface complexes) is achieved at 43815, and an ini-
tial TOF is determined up to 0.47 sꢀ1. The apparent quantum
yield (AQY) of [CdSꢀN=CHꢀNi] is determined to be (9.9ꢁ0.8)%
Figure 3d, the H2 evolution rate gradually declines from
98.7 mmolhꢀ1 at 1 h to 29.3 mmolhꢀ1 at 26 h. This is mainly due
to the dissociation of partially anchored Ni-based catalysts
from [CdSꢀN=CHꢀNi] as a result of the external DETA molecule
extraction from CdS–DETA NSs.[22] In addition, further improv-
ing the catalytic performance of [CdSꢀN=CHꢀNi] by absorbing
more molecular catalysts is limited because the vast majority
of the surface is occupied and the active sites are saturated by
chemical bonds. The understanding of the formation process
and structural properties of the [CdSꢀN=CHꢀNi] encourages us
to design and construct multiple covalently linked heterogene-
ous photocatalytic dyads with greatly improved activity and
stability. Of particular interest is to explore desirable covalently
immobilized heterogeneous dyads between a diverse range of
at 420 nm (Table 1). Under the same conditions, the [CdSꢀN= representative inorganic–organic hybrid semiconductors [e.g.,
CHꢀNi] dyad exhibits the highest activity compared to the
other inorganic CdS-based photocatalysts (Figure S8 and Fig-
ure S9 in the Supporting Information). Additionally, the catalyt-
ic activity is also comparable with that of other similar photo-
catalytic systems (Table S3 in the Supporting Information). In
metal oxides,[28] selenides,[29] tellurides,[30] fluorescent metal–or-
ganic framework (MOFs)[31]] and noble-metal-free biomimetic
molecular catalysts containing nickel bis(diphosphine) com-
plexes NiP,[1c] [FeFe]-H2ase mimics,[3a] and so on.
The [CdSꢀN=CHꢀNi] dyad after photocatalysis for 26 h was
investigated by SEM (Figure S10a in the Supporting Informa-
tion) and XRD (Figure S10b in the Supporting Information).
This shows that the dyad does not experience any significant
change during the photocatalytic process, and thus demon-
strates their good morphological and structural stability. XPS
spectra (Figure S11 in the Supporting Information) show that
the characteristic peak of NiII is retained and no Ni0 signals can
be detected after photocatalysis, indicating that the grafted
Ni-based complex has good photostability under light irradia-
tion. EDX analysis (Figure S4b in the Supporting Information)
still clearly exhibits a clear Ni element signal, further proving
Table 1. Control experiments of the photocatalytic H2 evolution.[a]
Photocatalytic system
n(H2) [mmol]
TON (vs. Ni catalyst)
AQY [%]
CdS–DETA
[CdSꢀN=CHꢀNi]
CdS–DETA/III
133.95ꢁ15
507.1ꢁ27
399.5ꢁ20
–
16736ꢁ900
13185ꢁ900
9.9ꢁ0.8
5.7ꢁ0.2
[a] All the systems were kept under identical conditions containing 30 mL
of H2O/EtOH solution (1:1, v/v), CdS–DETA (0.94 mm), 2 mL of TEA (6.3%,
v/v), pH 12.2, and irradiated for 6 h by a 300 W Xenon lamp; the concen-
tration of III is fixed at 0.95 mm.
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a firm covalent anchor of the Ni-based complex on the CdS–
DETA NSs.
To investigate the necessity of the aromatic aldehyde group
of I in preparing the [CdSꢀN=CHꢀNi] dyad, we synthesized
a new complex [(tpy-CH=NR)2Ni]Cl2 (III), the structure of which
is similar to I, but the aldehyde-terminal groups (-CHO) are
protected by N-ethylethylenediamine (Scheme S3 in the Sup-
porting Information). In this case, the chemical reaction be-
tween III and CdS–DETA can be ruled out. After CdS–DETA and
III are stirred in methanol for 12 h, the products are cleaned by
water and ethanol several times. Under optimal conditions, an
equal amount of H2 product to that of individual CdS–DETA
NSs is collected. Accordingly, we can infer that the aldehyde-
terminal groups (-CHO) in complex I are indispensable to build
the [CdSꢀN=CHꢀNi] dyad.
Figure 4. Total turnover numbers of H2 containing CdS–DETA (0.94 mm), TEA
(6.3%, v/v, 2 mL) in H2O/EtOH (1:1, v/v, 30 mL) at pH 12.2 upon irradiation
with visible light (l>420 nm) with varying amounts of III.
DETA/III with an improved amount of III (Figure 4). This is
mainly due to the under-utilization of non-attached Ni-based
proton reduction catalysts in the energy transfer process.[32]
The strong covalent interaction and intimate contact between
the excited PS and the anchored Ni-based catalysts are be-
lieved to be crucial for full utilization of the H2 evolution cata-
lysts. Overall, this highlights the critical superiority of the
chemisorbed dyad over physically adsorbed hybrids towards
PHER.[33]
Next, we also sought to determine whether or not the
active site was on the CdS surface or in the solution during H2
evolution. Figure S12 in the Supporting Information shows
that, after irradiation of the [CdSꢀN=CHꢀNi] dyad H2 genera-
tion system for 6 h, the [CdSꢀN=CHꢀNi] dyads were separated
from the solution by centrifugation and washed three times
with water. The separated [CdSꢀN=CHꢀNi] dyads were re-dis-
persed in a fresh solution of H2O/ethanol with TEA as sacrificial
agent. It was found to have approximately the same activity
for H2 generation. However, when the pure CdS–DETA NSs
were dispersed in the above separated solution, the activity is
not increased compared with pure CdS–DETA NSs. In addition,
to prove the integrity of the [CdSꢀN=CHꢀNi] dyad for PHER,
To gain insight into the photocatalytic H2 production, we
carried out a flash photolysis investigation to characterize the
PET process (Figure S14 in the Supporting Information). Upon
a laser pulse of the CdS–DETA NSs by 355 nm light, no charac-
teristic absorption signal was detected. However, for the [CdSꢀ
FTIR spectra were measured to characterize the [CdSꢀN=CHꢀ N=CHꢀNi] dyad, a set of characteristic peaks centered at
Ni] samples after photocatalysis for 26 h. Before the measure-
ments, the dyad was first separated by centrifugation and
washed with water and ethanol six times to adequately
remove the physically adsorbed organic molecules. The FTIR
spectrum (Figure S13 in the Supporting Information) is consis-
tent with that before photocatalysis (Figure 1h). These results
suggest that the dyad remained intact during the irradiation
and the water reduction reaction mainly occurs at the surface
of the active sites. The enhanced photocatalytic performance
benefits from the integrated system. In addition, control ex-
periments were designed to demonstrate the superiority and
advantages of the chemical adsorption, induced by covalently
linked interactions, between semiconductor CdS PS and com-
plex catalyst I in the heterogeneous [CdSꢀN=CHꢀNi] dyad. A
corresponding simple mixed multicomponent counterpart of
CdS–DETA/III, with the identical component concentrations ref-
erenced to [CdSꢀN=CHꢀNi], is constructed. H2 production is
evaluated under the same conditions. With visible-light irradia-
tion (l>420 nm) for 6 h, the [CdSꢀN=CHꢀNi] dyad produces
507.1ꢁ27 mmol H2, which is 1.27 times that generated from
the simple mixed system of CdS–DETA/III (Table 1). The im-
proved catalytic performance can be ascribed to the linear and
rigid bridges between PS and molecular catalysts, which can
shorten the separation between PS and Ni-based catalysts and
contribute to a promoted electron transfer from the PS* to the
Ni-based catalytic site. Furthermore, the TON value based on
Ni-species in the [CdSꢀN=CHꢀNi] dyad is also superior to that
in any mechanically mixed multicomponent systems of CdS–
309 nm, 509 nm, 570 nm, and 720 nm emerge with a lifetime
of 3.6 ms at 550 nm (Figure S15 in the Supporting Information).
These absorption bands may originate from the active NiI inter-
mediate species generated by the reduction of the NiII com-
plex by the photogenerated electrons, as reported in the litera-
ture.[13b,c,34]
On the basis of the above experimental measures and spec-
troscopic analyses, we propose a reasonable mechanism for
the photochemical H2 evolution reaction catalyzed by the
[CdSꢀN=CHꢀNi] dyad. As shown in Scheme 1, CdS–DETA ab-
sorbs visible light efficiently to form the reduced electron and
oxidized holes. The excited electron in the conduction band
(CB) of the CdS–DETA is transferred to the grafted Ni-based
complexes, leaving the oxidized holes in the valence band (VB)
of the CdS–DETA. As a consequence, electron injection to Ni-
based complexes leads to the formation of the NiI intermediate
species. The active reduced NiI center is considered to play
a critical role in electron transfer to a proton in a catalytic
cycle for H2 production. Many other NiI species have been pro-
posed as important intermediates in photocatalytic H2 evolu-
tion by their corresponding Ni-based complexes. Simultane-
ously, the remaining holes in CdS–DETA are subsequently con-
sumed by the sacrificial electron donor TEA.[13b]
Conclusion
A covalently linked heterogeneous [CdSꢀN=CHꢀNi] dyad, em-
ploying imine (-CH=N-) as a linkage, has been constructed
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uct (450 mg, 75%). The structure of complex I was determined by
the signal-crystal X-ray diffraction (Table S1 in the Supporting Infor-
mation).
through the condensation reaction between the amino groups
of inorganic–organic hybrid CdS–DETA and the aldehyde
group of molecular catalyst I. Under optimal conditions, this
system is capable of producing H2 with a TONꢂ43815 versus
Ni catalysts and an initial turnover frequency (TOF) of ~0.47 sꢀ1
with visible-light irradiation for 26 h. The AQY is (9.9ꢁ0.8)% at
420 nm. The photocatalytic performance after 6 h is 1.27 times
higher than that of the reference simple mixed multicompo-
nent system CdS–DETA/III, and the TON value based on Ni-spe-
cies is also larger than that of CdS–DETA/III with an increased
amount of III. The superior catalytic performance of this dyad
can be attributed to the strong covalent interaction and inti-
mate contact between the excited PS and the anchored Ni-
based catalysts, which not only efficiently promote the photo-
excited electron transfer but also achieve full utilization of
anchored molecular catalysts. Additionally, the dyad possesses
good morphological and structural stability. As the first exam-
ple of fabricating covalently linked heterogeneous dyads
[CdSꢀN=CHꢀNi] with inorganic–organic hybrid CdS–DETA as
a model platform, this work provides a new perspective for
building other composites between a diverse range of repre-
sentative inorganic–organic hybrid semiconductors [e.g., metal
oxides, selenides, tellurides, fluorescent metal–organic frame-
work (MOFs)] and noble-metal-free biomimetic molecular cata-
lysts containing nickel bis(diphosphine) complex NiP, [FeFe]-
H2ase mimics, and so on. Further studies are in progress to
adjust the structural modifications, control the PET distance,
and better understand the electron transfer kinetics for
PHER.[2a,35]
Synthesis of complex II
The synthetic route for II is shown in Scheme S2 in the Supporting
Information. Complex I (500 mg, 0.5 mmol) was dissolved in meth-
anol (50 mL) to form a homogeneous solution. Then, DETA (52 mg,
0.5 mmol) was added dropwise and the solution was heated at
reflux for 10 h. After cooling to room temperature, the products
were collected by concentration under reduced pressure. Complex
II was confirmed by the high-resolution mass spectrometry (Fig-
ure S3 in the Supporting Information).
Synthesis of complex III
The synthetic route for III is shown in Scheme S3 in the Supporting
Information. Complex I (500 mg, 0.5 mmol) was dissolved in meth-
anol (30 mL) to form a homogeneous solution. Then, N-ethylethyle-
nediamine (88 mg, 1.0 mmol) was added dropwise and the solu-
tion was heated at reflux for 10 h. After cooling to room tempera-
ture, the products were collected by concentration under reduced
pressure.
Synthesis of complex IV
The synthetic route for IV is shown in Scheme S4 in the Supporting
Information. 4’-(4-Bromophenyl)-2,2’:6’,2’’-terpyridine (L1, 550 mg,
1.2 mmol) and NiCl2·6H2O (140 mg, 0.6 mmol) were added to
methanol (50 mL) and heated at reflux for 10 h. After cooling to
room temperature, the solid was collected by filtration and recrys-
tallized from methanol to give the product.
Preparation of the [CdSꢀN=CHꢀNi] dyad
Experimental Section
CdS–DETA nanosheets (25 mg) and I (15 mg) were added into
methanol (10 mL), and then vigorously stirred for 12 h at room
temperature. A long reaction time was conducive for the conden-
sation reaction between a primary amine of the CdS–DETA nano-
sheets and an aldehyde group of I. Then, the samples were precipi-
tated with water and ethanol six times to adequately remove the
physically adsorbed complex I. The purified [CdSꢀN=CHꢀNi] hy-
brids were dried in a vacuum oven at 608C for 6 h.
Materials
All chemicals were of analytical grade from commercial suppliers
and were used as received without further purification.
Synthesis of inorganic–organic hybrid CdS–DETA nano-
sheets
The CdS–DETA nanosheets were prepared according to the report-
ed reference.[11] In a typical synthesis procedure, CdCl2·2.5H2O
(0.0732 g), and S powder (0.064 g) were added into DETA solution
(12 mL). The mixture was vigorously stirred for 30 min to form a ho-
mogeneous suspension. Then, the mixed solution was transferred
into a 20 mL Teflon-lined stainless-steel autoclave and kept at 808C
for 48 h. After cooling to room temperature, the light-yellow pre-
cipitate was collected and washed with ethanol and distilled water
three times, respectively. The final product was dried in a vacuum
oven at 608C for 6 h.
Characterization
Scanning electron microscopy (SEM) and energy-dispersive X-ray
spectroscopic (EDX) analysis were measured with a Hitachi S-4800
scanning electron microscope (SEM, 5 kV) equipped with a Thermo
Scientific energy-dispersion X-ray fluorescence analyzer. Transmis-
sion electron microscopy (TEM) was carried out with a JEOL-2100F
system equipped with EDAX Genesis XM2. Specimens for TEM
measurements were prepared by drop-casting a droplet of ethanol
suspension onto a copper grid and drying in air. X-ray diffraction
patterns (XRD) were recorded with a Bruker D8 Focus Diffraction
System using a CuKa source (l=0.154178 nm). CCDC 1016346–
1016346 contain the supplementary crystallographic data for this
paper. These data are provided free of charge by The Cambridge
Crystallographic Data Centre. UV/Vis spectroscopy was measured
on a Shimadzu UV-2550 ultraviolet/visible diffuse reflectance spec-
trophotometer, employing BaSO4 as the internal reflectance stan-
dard. Fourier transform infrared spectroscopy (FTIR) was recorded
on a MAGNA-IR 750 (Nicolet Instrument Co) FTIR spectrometer.
Synthesis of complex I
Complex
I was synthesized by using a modified reported
method.[15] The synthetic route for I is shown in Scheme S1 in the
Supporting Information. 4’-([2,2’:6’,2’’-Terpyridin]-4’-yl)-[1,1’-biphen-
yl]-4-carbaldehyde (L2, 500 mg, 1.2 mmol) and NiCl2·6H2O (140 mg,
0.6 mmol) were added to methanol (50 mL) and heated at reflux
for 10 h. After cooling to room temperature, the solid was collect-
ed by filtration and recrystallized from methanol to give the prod-
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Raman spectroscopy was recorded with a DXR Raman Microscope
under excitation with 532 nm laser light. Thermogravimetric analy-
sis (TGA) was measured on a Mettler Toledo TGA/DSC 1 STARe
system equipped with a gas controller (GC 200) and a recirculating
The turnover number (TON) for photocatalytic H2 evolution was
calculated by using the following equation:[36]
number of product molecules
TON¼
1
stage cooling bath set at 228C (Julabo). H NMR and 13C NMR spec-
number of catalyst molecules
ð1Þ
number of evolved H2 molecules
¼
tra were recorded on Varian Mercury Plus 400 instruments at
400 MHz (1H NMR) or 100 MHz (13C NMR). The elemental chemical
analysis of complexes II, III, and IV was conducted on an Elementar
Vario Micro Cube (Elementar, Germany). X-ray photoelectron spec-
troscopy (XPS) analysis was performed on a PerkinElmer PHI-1600
spectrometer with AlKa (1486.6 eV) irradiation. It was used to inves-
tigate the surface components of the materials. For the valence
band determination study, UVPS scans were collected in the
energy range 30 to ꢀ5 eV with 0.1 eV step size. Transient absorp-
tion spectroscopy was recorded on an Edinburgh Analytical Instru-
ments F900 at room temperature. In the transient absorption spec-
troscopy experiments, the samples were purged with argon for
30 min. Excitation was provided by using a Nd:YAG laser (third har-
monic, 10 ns) at 355 nm. Electrochemical measurements were per-
formed by using an electrochemical workstation (CHI 660D, CH In-
struments, Austin, TX). A three-electrode system was used for the
measurement. A 3 mm glass carbon electrode, Pt wire, and saturat-
ed calomel electrode (SCE) (Tianjin Incole Union Technology Co.,
Ltd) were used as the working electrode, counter electrode, and
reference electrode, respectively. nBu4NPF6 (0.1m) in N,N-dimethyl-
formamide (DMF) was used as the electrolyte in the cyclic voltam-
metric measurements. The electrolyte solution was degassed by
bubbling with argon for 30 min before measurement. The working
electrode was polished with a 0.05 mm alumina paste and sonicat-
ed for 15 min before use. Linear sweep measurements (the aque-
ous electrolyte contained 1m KCl and 0.1m Na2SO3, pH 9.2) and
Mott–Schottky curve (the aqueous electrolyte contained 0.1m
phosphate buffer, pH 7) of CdS–DETA nanosheets were taken in
the dark with CdS–DETA nanosheets (details of the sample prepa-
ration are given in the Supporting Information), Pt wire, and Hg/
HgO as the working, counter, and reference electrodes, respective-
number of ½ðtpyꢀCHOÞ2NiꢃCl2 molecules
The turnover frequency (TOF) for photocatalytic H2 evolution was
calculated by using the following equation:[36]
TON
reaction time
TOF ðsꢀ1Þ ¼
sꢀ1
ð2Þ
The apparent quantum yield (AQY) measurements were performed
with a solar light simulator equipped with a narrow band pass
filter to provide a monochromatic light source at l=420ꢁ10 nm
(I=5.03 mWcmꢀ2). The AQY was then calculated by using the fol-
lowing equation:[4f,37]
number of reacted electrons
number of incident photons
AQY ð%Þ¼
ꢄ 100
ð3Þ
number of evolved H2 molecules ꢄ 2
¼
ꢄ 100
number of incident photons
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (No. 21422104 and 21101112),
Key Project of Natural Science Foundation of Tianjin City (No.
16JCZDJC30600), and the Youth Innovation Foundation of
Tianjin University of Science & Technology (No. 2015LG15).
1
ly. The scan speed was 10 mVsꢀ1. H and 13C NMR spectra were re-
Keywords: heterogeneous catalysts
· hybrid materials ·
corded on Varian Mercury Plus 400 instruments at 400 MHz
(1H NMR) and 100 MHz (13C NMR). Chemical shifts are reported in
ppm down field from internal Me4Si. Multiplicity is indicated as fol-
lows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet),
dd (doublet of doublets), br (broad). Coupling constants are report-
ed in Hertz (Hz). Mass spectra were determined on a Brꢁker Auto-
flex tof/tof III MALDI-TOF mass spectrometer.
hydrogen production · photocatalysis · water splitting
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Received: April 15, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
9
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
&
Photochemistry
W. Zhao, Y. Huang, Y. Liu, L. Cao,
F. Zhang, Y. Guo, B. Zhang*
&& – &&
A Heterogeneous Photocatalytic
Hydrogen Evolution Dyad: [(tpy-
CHO)2Ni]Cl2 Covalently Anchored to
CdS–Amine Inorganic–Organic Hybrid
Nanosheets
Water splitting: A covalently linked het-
erogeneous [CdSꢀN=CHꢀNi] dyad, em-
ploying an imine (ꢀCH=Nꢀ) linkage, was
constructed through the condensation
reaction between the amino groups of
the inorganic–organic hybrid CdS–DETA
(DETA=diethylenetriamine) and the al-
dehyde group of molecular catalyst
[(tpy-CHO)2Ni]Cl2 (tpy=terpyridine; see
scheme).
&
&
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
10
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!