An efficient catalyst based on a water-soluble cobalt(II) complex of S,S′-bis(2-pyridylmethyl)-1,2-thiobenzene for electrochemical- and photochemical-driven hydrogen evolution

Article abstract of DOI:10.1002/aoc.5390

The reaction of S,S′-bis(2-pyridylmethyl)-1,2-thiobenzene and CoCl₂ affords a water-soluble cobalt(II) complex, [(bptb)CoCl₂], which has been characterized using various methods. Under blue light, together with CdS nanorods as a photosensitizer and ascorbic acid as a sacrificial electron donor, [(bptb)CoCl₂] can catalyze hydrogen generation from water and can work for 90 h. Under optimal conditions, this photocatalytic system achieves a turnover number (TON) of 22 900 moles of H₂ per mole of catalyst during 60 h of irradiation, and the highest apparent quantum yield is ca 26.63% at 469 nm. Moreover, [(bptb)CoCl₂] exhibits much higher activity than [(bpte)CoCl₂] (bpte = S,S′-bis(2-pyridylmethyl)-1,2-thioethane; TON = 6740 moles of H₂ per mole of catalyst during 60 h of irradiation), indicating that bptb can constitute a better catalyst for hydrogen production than bpte. This result can be attributed to the electronic properties of the ligands (bptb and bpte). The introduction of phenyl makes the electron distribution more uniform in the cobalt complex, allowing easier formation of the Co(III)–H species, further promoting the formation of hydrogen.

Full text of DOI:10.1002/aoc.5390

Received: 2 September 2019
DOI: 10.1002/aoc.5390
Revised: 29 October 2019
Accepted: 6 November 2019
F U L L P A P E R
An efficient catalyst based on a water-soluble cobalt(II)
0
complex of S,S -bis(2-pyridylmethyl)-1,2-thiobenzene for
electrochemical- and photochemical-driven hydrogen
evolution
Zhen-Lang Xie | Wen-Xing Jiang | Nan-Shu Wang | Shu-Zhong Zhan
College of Chemistry and Chemical
0
The reaction of S,S -bis(2-pyridylmethyl)-1,2-thiobenzene and CoCl affords a
Engineering, South China University of
Technology, Guangzhou, 510640, China
2
water-soluble cobalt(II) complex, [(bptb)CoCl ], which has been characterized
2
using various methods. Under blue light, together with CdS nanorods as a pho-
Correspondence
Wen-Xing Jiang or Shu-Zhong Zhan,
College of Chemistry and Chemical
Engineering, South China University of
Technology, Guangzhou 510640, China.
Email: shzhzhan@scut.edu.cn;
jiangwenxing1230@qq.com
tosensitizer and ascorbic acid as a sacrificial electron donor, [(bptb)CoCl ] can
2
catalyze hydrogen generation from water and can work for 90 h. Under opti-
mal conditions, this photocatalytic system achieves a turnover number (TON)
of 22 900 moles of H per mole of catalyst during 60 h of irradiation, and the
2
highest apparent quantum yield is ca 26.63% at 469 nm. Moreover, [(bptb)
0
CoCl ] exhibits much higher activity than [(bpte)CoCl ] (bpte = S,S -bis(2-
Funding information
2
2
National Science Foundation of China,
Grant/Award Numbers: 21875074,
pyridylmethyl)-1,2-thioethane; TON = 6740 moles of H per mole of catalyst
2
during 60 h of irradiation), indicating that bptb can constitute a better catalyst
for hydrogen production than bpte. This result can be attributed to the elec-
tronic properties of the ligands (bptb and bpte). The introduction of phenyl
makes the electron distribution more uniform in the cobalt complex, allowing
easier formation of the Co(III)–H species, further promoting the formation of
hydrogen.
21271073; South China University of
Technology
K E Y W O R D S
cobalt complexes, electro- or photocatalyst, hydrogen evolution, molecular structures,
modification of ligand
1
| INTRODUCTION
hydrogen production, a challenge for overcoming this
issue would be to develop an efficient and stable catalytic
In order to reduce environmental contamination caused
by the burning of fossil fuels, many synthetic systems
have been developed to afford renewable and environ-
mentally friendly alternative energy, such as hydrogen.
Electrochemical and photochemical methods for hydro-
gen evolution from water have been explored as cost-
system established using abundant and low-cost mate-
[
7,8]
rials.
It is apparent that hydrogenase enzymes based
on iron or nickel complexes can efficiently catalyze both
[
9,10]
the production and oxidation of hydrogen.
However,
it is impossible to obtain large amounts for practical
uses, due to limitations outside of their native
[
1–3]
[9,10]
effective ways of producing a carbon-neutral fuel.
environment.
The most viable method for large-scale growth in carbon-
free energy is the light-driven splitting of water into its
To mimic this natural process, much effort has been
focused on the design and development of molecular cat-
alysts for electrochemical- and photochemical-driven
[4–6]
constituent elements.
To improve the efficiency for
Appl Organometal Chem. 2019;e5390.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 9
https://doi.org/10.1002/aoc.5390

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XIE ET AL.
[
41]
water reduction to form hydrogen, and several
method,
the ligand bptb was prepared (Figures S3 and
[11–13]
[14–17]
[18]
nickel,
cobalt
and iron
complexes have
S4). The cobalt(II) complex [(bptb)CoCl ] was obtained
2
been designed as photocatalysts for hydrogen evolution.
A typical system contains a molecular catalyst, a sacrifi-
cial electron donor and a molecular photosensitizer, such
by the reaction of CoCl and bptb (Scheme 1). The result
is consistent with the structural and electrospray ioniza-
tion (ESI) MS analyses.
2
as the ruthenium(II) trisbipyridyl complex Ru(bpy) Cl2
X-ray crystallography was used to determine the
3
(having strong absorption in the visible region). However,
molecular structure of [(bptb)CoCl ] in the solid state,
2
this molecular photosensitizer is unstable during the
with the results listed in Tables S1 and S2 and shown in
Figure 1. The cobalt atom is connected to two nitrogen
atoms and two sulfur atoms from bptb and to two chlo-
[19–22]
photocatalytic process.
Based on that CdS has a nar-
row band gap (with E of 2.4 eV) and is stable during irra-
g
−
diation, CdS materials have been selected as
photosensitizers for the conversion of solar energy into
rine atoms; the two Cl ligands are in mutually cis posi-
tions. The bond lengths of Co–N are 2.157(5) and 2.174
(5) Å, and the Co–S distances are 2.5142(17) and 2.532
(2) Å. The distances between Co and Cl are 2.3623
(17) and 2.365(2) Å. As shown in Figure S5, the ESI-MS
[
23,24]
chemical energy under visible-light irradiation.
Moreover, the potential of its conduction band is more
negative than the reduction potential of hydrogen proton
+
(
H /H ), making it more appropriate for H2
spectrum of [(bptb)CoCl ] in methanol showed one peak
2
2
[25–27]
generation.
However, the catalytic efficiency of CdS
at m/z 419.9742, which is assigned to the species [(bptb)
+
itself is very low due to rapid charge recombination of
CoCl] . Powder X-ray diffraction was used to determine
[28]
photogenerated electrons.
To stop this recombination
the purity of the [(bptb)CoCl ] sample, with the results
2
and improve photocatalytic efficiency, it is necessary to
shown in Figure S6. According to Figure S6, the peak
positions of experimental patterns are in good agreement
with those of the simulated ones, indicating that the
[29,30]
load a co-catalyst on CdS.
These considerations have
led to the development of co-catalysts employing more
abundant metal compounds, and several transition metal
complexes have been developed as co-catalysts for hydro-
[(bptb)CoCl ] sample is of good purity.
2
[31–36]
gen generation.
Based on the consideration that transition metal com-
plexes supported by sulfur-containing ligands can act as
catalysts for photochemical-driven hydrogen evolu-
2.2 | Electrocatalytic behavior of [(bptb)
CoCl2]
[37,38]
tion,
a
cobalt-based catalyst, [(bpte)CoCl2]
Considering that cobalt complexes can serve as electro-
0
(bpte = S,S -bis(2-pyridylmethyl)-1,2-thioethane), has
catalysts for hydrogen generation via an unstable hydride
[39]
[16,17,42]
been designed in our laboratory.
Together with CdS
intermediate,
we first investigated the electro-
nanorods (NRs) and ascorbic acid, [(bptb)CoCl ] exhibits
chemical behaviors of this cobalt complex. Cyclic
2
photocatalytic activity for hydrogen evolution with a
voltammograms (CVs) of CoCl , the ligand bptb and the
2
−
1
turnover number (TON) of 6740 mol H (mol cat) dur-
cobalt complex [(bptb)CoCl ] were measured in CH CN
2
2
3
ing 60 h of irradiation. To further improve the catalytic
with 0.10 M [(n-Bu) N]ClO as the supporting electrolyte,
4 4
efficiency for hydrogen production, we have developed a
with the results shown in Figure S7 and Figure 2. As
0
new catalyst based on [(bptb)CoCl ] (bptb = S,S -bis(2-
shown in Figure 2a, [(bptb)CoCl ] displayed one reduc-
2
2
pyridylmethyl)-1,2-thiobenzene). Compared with [(bpte)
CoCl ], [(bptb)CoCl ] shows much higher efficiency for
tion wave at −1.21 V versus Ag/AgNO , which is
assigned to the Co couple.
3
II/I
2
2
hydrogen production. Herein we report the synthesis,
characterization and catalytic behavior of [(bptb)CoCl2].
2
2
| RESULTS AND DISCUSSION
.1 | General characterization of CdS
NRs and [(bptb)CoCl2]
[40]
Based on a reported procedure,
CdS NRs were
obtained, with characterization using transmission elec-
tron microscopy, scanning electron microscopy (SEM)
and powder X-ray diffraction as shown in Figures S1 and
S2 (JCPDS no. 41-1049). According to a reported
SCHEME 1 Synthesis of ligand bptb and complex [(bptb)
CoCl ]
2

DESIGN OF CATALYST FOR HYDROGEN EVOLUTION BASED ON METAL COMPLEX
3 of 9
cycle for H evolution from acid mediated by [(bptb)
2
CoCl ] is depicted in Scheme 2. First, a Co(I) species,
2
+
[
[
(bptb)Co] , was obtained via a one-electron reduction of
2+
+
(bptb)Co] . Addition of hydrogen proton (H ) led a
III
III
2+
Co –H species, [H–Co (bptb)] , to be formed. Next,
one-electron reduction of the Co –H species led to the
formation of a Co –H species, [H–Co (bptb)] . Further
addition of hydrogen proton to [H–Co (bptb)] afforded
III
II
II
+
II
+
a H molecule.
2
Next, bulk electrolysis was used to investigate the
electrocatalytic efficiency of [(bptb)CoCl ], with the
2
results shown in Figure S9a. Upon addition of [(bptb)
CoCl2], under −1.45
V
versus Ag/AgNO3, the
electrocatalytic system gave 65 mC of charge during
2
min of electrolysis, with accompanying production of a
FIGURE 1 X-ray crystal structure of cobalt complex [(bptb)
CoCl ]
2
Next, employing acetic acid as proton source, the cat-
alytic activity of [(bptb)CoCl ] was investigated, giving
2
the results shown in Figure 2b. Upon addition of various
concentrations of acetic acid from 0.0 to 10.72 mM,
voltammetric currents emerging at −1.21 V versus Ag/
AgNO exhibited a systematic increase. This rise in cur-
3
rent can be assigned to the catalytic evolution of
[16,17]
dihydrogen from acetic acid.
Moreover, the current
strength at −1.21 V decreased with the addition of
triethylamine (TEA) (Figure S8). This can be attributed
to the neutralization of hydrogen proton in [(bptb)Co–
2+
[43]
H] by TEA.
This result indicates that the reduction
of Co(II) to Co(I) and protonation are responsible for
hydrogen generation in this electrocatalytic system. Addi-
tionally, upon addition of 5.36 mM acetic acid, a new
reduction wave was observed near −1.83 V versus Ag/
AgNO . Further addition of acetic acid resulted in an
3
increase in the current, consistent with a catalytic
[44]
process.
According to literature precedents
observations and analysis described above, a catalytic
[45–47]
and the
SCHEME 2 Possible electrocatalytic mechanism for proton
reduction to hydrogen by [(bptb)CoCl ]
2
FIGURE 2 (a) CV of [(bptb)CoCl
1.69 mM) in CH CN with 0.10 M [n-
2
]
(
3
Bu
4
N]ClO
4
solution at a glassy carbon
−1
electrode and a scan rate of 100 mV s
.
(
b) CVs of a solution of [(bptb)CoCl
1.69 mM) with varying amounts of
2
]
(
acetic acid. Conditions: 0.10 M [n-Bu
ClO as supporting electrolyte; scan rate,
00 mV s− ; glassy carbon working
electrode (1 mm in diameter); Pt counter
electrode; Ag/AgNO reference electrode
4
N]
4
1
1
3

4
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XIE ET AL.
gas, which was confirmed as H using gas chromatogra-
Overpotential = Applied potential−EðpHÞ
Applied potential−ð−0:059pHÞ
2
ð3Þ
phy. However, a charge of only 8 mC was obtained with-
out the cobalt complex (Figure S9b), indicating that this
cobalt complex does indeed serve as an electrocatalyst for
=
[48]
[49]
hydrogen evolution. Equations (1)
and (2)
were
used to calculate the overpotential (OP) and turnover fre-
quency (TOF), with the results shown in Figure S10. For
instance, at an OP of 941.6 mV, the cobalt complex could
2.3 | Stability and durability of [(bptb)
CoCl ] in electrocatalytic system
2
catalyze hydrogen evolution with a TOF of 35.2 mol H
2
(
mol catalyst)− h . Notably, [(bptb)CoCl ] shows higher
1
−1
To investigate the stability of [(bptb)CoCl ] in the cata-
2
2
efficiency for hydrogen production than [(bpte)CoCl ]
lytic system, a long-time electrolysis was carried out, giv-
ing the results shown in Figure S15. Under −1.45 V
versus Ag/AgCl, a 30 h controlled-potential electrolysis of
the electrocatalytic system with 6.1 μM [(bptb)CoCl2]
could afford 1496 C of charge, with an accompanying pH
2
−
1
−1 [39]
(
TOF of 25.6 mol H (mol catalyst) h ),
indicating
2
that bptb can constitute a catalyst of higher activity for
hydrogen evolution than bpte. This result can be attrib-
uted to the electronic properties of the ligands (bptb and
bpte). The introduction of phenyl makes the electron dis-
tribution more uniform in the cobalt complex, allowing
increase from 7.0 to 10.1. This result can be attributed to
−
the accumulation of OH
by water reduction:
III
−
−
easier formation of the Co –H species, further promot-
2H O + 2e ! H + 2OH . Moreover, when the pH of
2
2
[50–52]
ing the formation of hydrogen.
the solution was adjusted back to 7.0, this catalytic
behavior can be recovered and the reaction repeated sev-
ꢀ
Overpotential = Applied potential−E HA
eral times. Additionally, in the presence of [(bptb)CoCl
the electrocatalytic current can remain stable for at least
2 h (Figure S16). The results indicate that [(bptb)CoCl2]
is stable during electrolysis.
2
],
Â
Ã
ꢀ
+
=
Applied potential – E
−ð2:303RT=FÞpKaHA
H
7
ð1Þ
ð2Þ
TOF = ΔC=ðF × n × n × tÞ
1
2
2
.4 | Heterogeneous photocatalytic
Next, the electrocatalytic behavior of this water-solu-
ble cobalt complex was characterized in aqueous media,
with the results listed in Figure S11. Upon decreasing pH
values from 7.0 to 5.5, the strength of the reduction wave
system with [(bptb)CoCl ] for H2
generation
2
To investigate the photocatalytic behavior of the cobalt
complex, a photocatalytic system containing ascorbic acid
(H A) as an electron donor, [(bptb)CoCl ] as a catalyst
and CdS NRs as a photosensitizer (Figure S17) was
designed. To assemble an optimal photocatalytic system
for hydrogen evolution, a series of experiments were car-
ried out. First, we looked for the impact of the pH of
media on the photocatalytic activity of H production,
with the results listed in Figure S18. During 2 h of photol-
ysis, the highest catalytic activity for hydrogen evolution
of [(bptb)CoCl ] increased and the onset of the catalytic
2
wave was shifted to higher potentials, which are attrib-
2
2
[53]
uted to a catalytic process.
To further understand the electrocatalytic activity of
[
(bptb)CoCl ] in aqueous media, bulk electrolysis was
2
carried out in buffer, with the results plotted in
Figure S12. Under −1.45 V versus Ag/AgCl, only 236 mC
of charge was provided from a neutral buffer during
2
2
min of electrolysis (Figure S12a). Surprisingly, the
introduction of [(bptb)CoCl ] resulted in 2172 mC of
charge under identical conditions (Figure S12b).
According to Figure S13a, electrolysis for 1 h can afford
was found at pH 4.5, with a TON of 3390 mol H
(mol catalyst) .
2
2
−
1
Next, a series of photocatalytic systems containing
0.10 M H A, [(bptb)CoCl ] (0.010 mM) and various
2
4.9 ml of H with a Faradaic efficiency of 96% (Fig-
2
2
2
[54]
ure S13b). According to Equations (2) and (3),
TOFs
amounts of CdS NRs were designed to study the impact
of the amount of photosensitizer on photocatalytic H2
generation, giving the results shown in Figure S19. Dur-
ing 2 h of irradiation, upon increasing the amount of CdS
NRs, the TON increased until a highest value of 3960 mol
and OPs were estimated, giving the results shown in
Figure S14. For example, at an OP of 837.6 mV, [(bptb)
CoCl ] can electrocatalyze hydrogen evolution with a
2
−
1
−1
TOF of 1200 mol H (mol catalyst) h . Similar to that
2
−
1
−1
in organic media, [(bptb)CoCl ] also shows much higher
H (mol catalyst) was reached at 0.14 mg ml . Then,
2
2
efficiency for hydrogen production from water than
as the concentration of CdS NRs was further increased,
[
(
(bpte)CoCl ]
(TOF
of
243.9
mol
H2
the TON decreased (Figure S19). Next, photocatalytic sys-
2
mol catalyst)− h ).
1
−1 [39]
tems containing CdS NRs (0.14 mg ml ), [(bptb)CoCl ]
−1
2

DESIGN OF CATALYST FOR HYDROGEN EVOLUTION BASED ON METAL COMPLEX
5 of 9
(
0.010 mM) and various amounts of H A were designed
TABLE 1 Photocatalytic activities of [(bptb)CoCl
2
] and [(bpte)
2
a
to find the optimum amount of H A, giving the results
2
CoCl ]
2
listed in Figure S20. During 2 h of irradiation, the TON
b
c
Component
TON
AQY (%)
increased with increasing concentration of H A until a
2
[
(bptb)CoCl
(bpte)CoCl
2
]
22 900
6 740
24.10
7.10
highest value of 6520 mol H2 (mol catalyst)− was
1
[
2
]
reached at 0.28 M. After that, the TON decreased when
aAn optimal three-component system, containing 0.14 mg ml−1 CdS NRs,
.28 M ascorbic acid and [(bptb)CoCl ] or [(bpte)CoCl ] (0.010 mM).
the concentration of H A was further increased.
2
0
2
2
Based on these observations and analyses, an optimal
three-component system containing 0.14 mg ml− CdS
NRs, 0.28 M H A and [(bptb)CoCl ] (0.010 mM) was
b
1
During 60 h of photolysis.
During 10 h photolysis.
c
2
2
assembled. According to Figure 3, upon light irradiation,
H generation started immediately and the rate increased
2
sharply and could last for about 60 h. Then the H gener-
2
ation build-up slightly increased until H2 formation
ceased after about 90 h. This photocatalytic system used
for 60 h of irradiation can afford hydrogen with a TON of
−
1
2
2 900 mol H (mol catalyst) . Similar to the results for
2
electrocatalytic activity, under the same conditions,
[
(bptb)CoCl ] also exhibits much higher photocatalytic
2
[39]
activity than [(bpte)CoCl ] (Table 1).
2
A photocatalytic system was irradiated for 10 h under
monochromic light with a bandpass filter (λ = 469 nm) to
determine the apparent quantum yields (AQYs), with the
[
55]
results shown in Figure 4. According to Equation (4),
the AQYs of the photocatalytic system for H generation
2
2 2
FIGURE 4 Photocatalytic H production using [(bptb)CoCl ]
were estimated. According to the data listed in Figure 4,
under visible light (λ = 469 nm) and AQY of [(bptb)CoCl ] under
2
during the first 2 h of irradiation, the AQY was ca
monochromatic light (λ = 469 nm). The reaction system contained
0.14 mg ml− CdS NRs, 0.010 mM [(bptb)CoCl
1
] and 0.28 M H
A
2
3.45%, and then it increased to 26.20% in 6 h of irradia-
2
2
(
pH 4.5)
tion. The average value of AQY was ca 24.10% during
1
0 h of photolysis. Compared with [(bpte)CoCl ] (AQY of
2ηH2NAhc
2
AQY ð%Þ =
× 100
ð4Þ
[39]
ca 7.1%)
(Table 1), [(bptb)CoCl ] has a much higher
t λIA
irr
2
quantum yield.
To find the factors responsible for the photocatalytic
H evolution in this system, any two of the three compo-
2
nents (H A, CdS NRs or [(bptb)CoCl ]) were mixed to see
2
2
if H could be afforded. As shown in Figure S21, during
2
2
h of irradiation, the combination of [(bptb)CoCl ] with
2
CdS NRs only gave 0.20 μmol H , the mixture of H A and
2
2
CdS NRs resulted in a slight increase in H production
2
(
3.61 μmol) and 1.25 μmol H was obtained when H A
2
2
and [(bptb)CoCl ] were mixed. Therefore, the combina-
2
tion of [(bptb)CoCl ], H A and CdS NRs is essential for
2
2
the photocatalytic activity.
The effect of the morphology (crystallinity) of a pho-
tosensitizer such as CdS on the photocatalytic efficiency
in a heterogeneous environment is a general concern. To
look for the effect of the crystallinity of CdS on the photo-
catalytic activity, we also investigated the catalytic behav-
ior of the photocatalytic system with CdS clusters
(
Figure S22) as a photosensitizer, giving the results
FIGURE 3 Hydrogen evolution kinetics obtained upon
shown in Figure S23. Compared with CdS clusters, CdS
NRs exhibited much better photocatalytic activity. For
example, under the same conditions, during 2 h of
continuous visible irradiation (λ = 469 nm) of a pH 4.5 buffer
solution containing 0.14 mg ml− CdS NRs, 0.28 M H
1
A and [(bptb)
2
2
CoCl ] (0.01 mM)

6
of 9
XIE ET AL.
irradiation, CdS clusters only provided 1.5 μmol H , and
strength of peak at 266 nm decreased over a photolysis
period of 90 h (Figure S28), indicating that the amount of
2
CdS NRs could afford 282.2 μmol (Figure S23). This can
be attributed to the smaller size causing a larger surface
area and thus more photocatalytic sites (nanosize effects),
and the CdS NRs having a much larger surface area than
CdS clusters.
H A decreases after photolysis under visible light. More-
2
over, after 70 h of photolysis, adjustments of H A back to
2
the original 0.28 M and the solution pH back to the origi-
nal value of 4.5 can recover the photocatalytic activity
under further illumination (Figure S29). These results
suggest that although some components of the photo-
catalytic system may deactivate during photolysis, the
2
.5 | Investigation of stability and
durability of photocatalytic system
decomposition of H A is the primary reason for the even-
2
tual cessation of hydrogen evolution after illumination.
To investigate the stability and durability of the photo-
catalytic system with the cobalt complex, the photocur-
rent response versus time of [(bptb)CoCl ]/CdS NRs was
measured, with the results shown in Figure S24. It was
2.6 | Study of photocatalytic mechanism
2
found that the photocurrent response of [(bptb)CoCl ]/
To understand the photocatalytic process of [(bptb)
2
CdS NRs was reversible and stable. Moreover, the current
can reproducibly increase under each irradiation cycle
and recover rapidly in the dark (Figure S24). The results
CoCl ], several measurements were carried out. Com-
pared with CdS NRs, upon introduction of [(bptb)CoCl2]
to CdS NRs, a red shift of the absorption onset and reduc-
2
suggest that [(bptb)CoCl ] and CdS NRs are stable during
tion of E of CdS NRs were found (Figure S30), indicating
2
g
irradiation. Additionally, after a 90 h photolysis period,
the pH increased by 1.1 units (from 4.5 to 5.6; Figure S25),
that the range and ability of visible light absorption of
CdS NRs are improved.
Next, photoluminescence spectra of the related com-
ponents were used to further look for the reason for
−
consistent with accumulation of OH via water reduc-
−
−
tion: 2H O + 2e ! H + 2OH . However, this catalytic
2
2
function could be recovered by 37% when the solution
pH was adjusted back to the original 4.5. This result can
photocatalytic hydrogen generation over [(bptb)CoCl ]-
2
modified CdS NRs, with the results shown in Figure S31.
With excitation at 450 nm, CdS NRs exhibited two emis-
sion peaks at 555 and 681 nm. However, with the immo-
be attributed to the decomposition of CdS NRs, H A or
2
[
(bptb)CoCl ] during photolysis.
2
Several physical or physiochemical methods were
bilization of [(bptb)CoCl ], the peak intensity of CdS NRs
2
used for further measurements and analysis. After 90 h of
at 675 nm decreased, showing a lower possibility of elec-
irradiation, CdS NRs were washed with CH CN and
tron–hole pair recombination due to the fast electron
3
[
59]
water, and then investigated using SEM. According to
Figure S26, SEM images were similar to those before irra-
diation, suggesting that CdS NRs are stable during 90 h
of photolysis.
transfer from CdS NRs to [(bptb)CoCl2].
Therefore, as
an electron acceptor loaded on the CdS NR surfaces,
[(bptb)CoCl ] can trap efficiently photogenerated elec-
2
+
trons and promote their combination with H to form
Next, X-ray photoelectron spectroscopy (XPS) mea-
surements and analysis were carried out, giving the
results shown in Figure S27. Before photocatalysis, two
main peaks were located at 160.83 and 162.01 eV
H . On the basis of the above electrochemical investiga-
2
tions, it is reasonable to speculate that a Co(I) species is
II
generated via the reduction of [(bptb)Co Cl ], which was
2
[
60]
found for previously reported samples.
The results are
(
Figure S27a), which can be attributed to S 2p in CdS
also in agreement with the orbital calculation described
in the following.
[56–58]
NRs.
Figure S27b demonstrated two obvious Cd 3d
peaks located at 404.84 and 411.57 eV, which are consis-
Gaussian B3LYP/6-31G was employed to carry out
[56]
tent with the Cd character in CdS NRs.
irradiation, the position and strength of both S 2s and Cd
d peaks remained almost constant (Figures S27a and
After 90 h of
orbital calculation for [(bptb)CoCl ], giving the results
2
shown in Figure 5. According to the calculated results,
the highest occupied molecular orbital (HOMO) and low-
est unoccupied molecular orbital (LUMO) of [(bptb)
3
S27b), indicating that CdS NRs are stable as a photosensi-
tizer during photocatalysis. Note that no XPS signals of
CoCl ] were at −4.513 and −1.517 eV, respectively. Con-
2
[61]
[
(bptb)CoCl ] were found, because it is soluble in aque-
sidering that the conduction band of CdS (−3.8 eV)
is
2
ous media.
From Figure S28, before irradiation, the three-compo-
smaller than ELUMO (−1.517 eV) of [(bptb)CoCl ], it is
energetically favorable for the photogenerated electrons
2
nent system afforded a main peak at 266 nm, which is
to transfer from CdS to [(bptb)CoCl2].
assigned to H A. The photochemical reduction catalyzed
Next, electrochemical impedance spectroscopy was
employed to explore charge transfer properties and
2
by [(bptb)CoCl ] resulted in no new absorption, but the
2

DESIGN OF CATALYST FOR HYDROGEN EVOLUTION BASED ON METAL COMPLEX
7 of 9
2
SCHEME 3 Possible mechanism for H production via the
photocatalytic system with [(bptb)CoCl
2
]
III
introduction of hydrogen proton to the Co –H species
gave dihydrogen and regenerated the starting cobalt
complex.
FIGURE 5 Calculated molecular orbital energy levels of
[
(bptb)CoCl
2
] with the molecular orbital maps of HOMO
and LUMO
3
| CONCLUSIONS
separation efficiency of photogenerated charge carriers,
with the results shown in Figure 6. Compared with CdS
We have reported a new cobalt(II) complex, [(bptb)
CoCl ], obtained from the reaction of bptb and cobalt
2
NRs, the mixture of CdS NRs and [(bptb)CoCl ] showed
much smaller arc radius, indicating that the introduction
salt. This cobalt complex exhibits catalytic activity for
electrochemical- and photochemical-driven hydrogen
evolution from water. The photocatalytic system con-
2
of [(bptb)CoCl ] makes interfacial charge transfer faster,
2
and the separation efficiency of photogenerated charge
taining CdS NRs as a photosensitizer, [(bptb)CoCl ] as a
2
[8,62]
carriers much higher.
Combining these observation, analysis and literature
co-catalyst and H A as a sacrificial electron donor can
2
work for about 90 h. Moreover, the photocatalytic activi-
[63–65]
precedents,
we propose a possible mechanism for
ties depend on concentrations of CdS NRs and H A, the
2
photocatalytic H production. As outlined in Scheme 3,
under irradiation, the conduction band of CdS NRs pro-
vided electrons; then the transfer of the photoexcited
pH of media and the morphology of CdS. The studies
show that bptb can constitute a more active catalyst for
hydrogen evolution than bpte. This can be attributed to
the bptb ligand with phenyl making the low oxidation
2
electrons to [(bptb)CoCl ] afforded a reduced Co
2
+
(I) species, a highly reactive intermediate. The accumula-
state of cobalt (such as Co ) more stable than bpte ligand
tion of the Co(I) species will undergo oxidative addition
of a proton to form the Co hydride species. Further
with ethyl. These findings may offer a new chemical par-
adigm for the design of effective non-noble-metal cata-
lysts based on transition metal complexes via the
modification of ligands for hydrogen generation that are
highly active.
III
4
| EXPERIMENTAL
Physical measurements, X-ray crystallography methods
and equations for the calculations of TOF and TON are
provided in the supporting information. The CdS NRs
[
40]
were prepared using a reported method.
4.1 | Preparation of bptb
[
41]
According to a reported method,
the ligand bptb was
1
prepared. H NMR (600 MHz, chloroform-d, δ, ppm):
FIGURE 6 Electrochemical impedance spectroscopy Nyquist
plots of CdS NRs and CdS/[(bptb)CoCl
Fe(CN) electrolyte in the dark
2 3 6
] in 0.010 M K Fe(CN) /
8.51 (d, 2H), 7.54 (td, 2H), 7.27 (d, 2H), 7.23–7.20 (m,
2H), 7.11 (dd, 2H), 7.03 (dd, 2H), 4.25 (s, 4H). C NMR
13
K
4
6

8
of 9
XIE ET AL.
(
1
151 MHz, chloroform-d, δ, ppm): 157.28, 149.20, 136.62,
36.49, 129.99, 126.84, 123.16, 122.09, 39.76.
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.2 | Synthesis of cobalt complex [(bptb)
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2
2
0
0
.5 mmol) was added to a solution of ligand bptb (0.16 g,
.5 mmol) in 10 ml of ethanol and the mixture was
[
18] F. Gärtner, A. Boddien, E. Barsch, K. Fumino, S. Losse,
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crystals were obtained in 71% yield (0.20 g). Elemental
analysis: calcd for C H C CoN S (%): C, 47.59; H,
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16 l2
2 2
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[
20] W. T. Eckenhoff, R. Eisenberg, Dalton Trans. 2012, 41, 13004.
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ACKNOWLEDGMENTS
This work was supported by the National Science Foun-
dation of China (nos. 21271073 and 21875074) and the
Student Research Program (SRP) of South China Univer-
sity of Technology.
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ORCID
Shu-Zhong Zhan https://orcid.org/0000-0002-5101-
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