A photocatalytic system with a bis(thiosemicarbazonato)?nickel over CdS nanorods for hydrogen evolution from water under visible light

Article abstract of DOI:10.1016/j.inoche.2019.01.027

Diacetyl?2?(4?N?methyl?3?thiosemicarbazone)?3?(4?N?amino?3?thiosemicarbazone) (H₂ATSM) reacts with NiCl₂ to form a new nickel(II) complex, Ni-ATSM, a new co-catalyst for hydrogen production. Under photoirradiation with blue light (λ = 469 nm), together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H₂A) as a sacrificial electron donor, the nickel complex can photocatalyze hydrogen evolution with a turnover number (TON) of 11,500 mol H₂ per mol of catalyst (mol of cat^?1) during 80 h irradiation in a heterogeneous environment.

Full text of DOI:10.1016/j.inoche.2019.01.027

Inorganic Chemistry Communications 102 (2019) 5–9
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
A photocatalytic system with a bis(thiosemicarbazonato)‑nickel over CdS
nanorods for hydrogen evolution from water under visible light
⁎
Wen-Xing Jiang, Zhen-Lang Xie, Shu-Zhong Zhan
College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
G R A P H I C A L A B S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords:
Diacetyl‑2‑(4‑N‑methyl‑3‑thiosemicarbazone)‑3‑(4‑N‑amino‑3‑thiosemicarbazone) (H
2
ATSM) reacts with NiCl
2
Nickel(II) complex
CdS materials
to form a new nickel(II) complex, Ni-ATSM, a new co-catalyst for hydrogen production. Under photoirradiation
with blue light (λ = 469 nm), together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid
Hydrogen evolution
Photocatalytic system
(
H
2
A) as a sacrificial electron donor, the nickel complex can photocatalyze hydrogen evolution with a turnover
−1
number (TON) of 11,500 mol H
2
per mol of catalyst (mol of cat ) during 80 h irradiation in a heterogeneous
environment.
1
. Introduction
compounds are good choice. Compared to noble metals, such catalysts
would be of special advantage for costs, abundance and toxicity. Thus,
significant efforts have made to apply nickel [4], cobalt [5–7] and iron
[8] complexes as water reduction catalysts (WRC) in the photocatalytic
hydrogen generation from water. However, this homogeneous system
cannot work for a long time because of the decomposition of molecular
photosensitizer, such as ruthenium(II) trisbipyridyl complex, Ru
As a secondary energy carrier, hydrogen is one ideal energy in the
future, and lots of synthetic systems have been developed to get this
renewable energy [1]. Among them, the direct photocatalytic water
splitting into hydrogen is an effective way. To improve the efficiency
for hydrogen production, a challenging work for overcoming this issue
would be to develop efficient catalysts and assembly new photocatalytic
systems with good stability, and high turnover rate [2,3]. To develop
improved catalysts for this application, 3d metals and the related
(bpy)
3
Cl , during irradiation [9]. Considering that CdS has a narrow
2
band gap (with an Eg of 2.4 eV), and the potential of its conduction
band (CB) is more negative than the reduction potential of hydrogen
⁎
Corresponding author.
E-mail address: shzhzhan@scut.edu.cn (S.-Z. Zhan).
https://doi.org/10.1016/j.inoche.2019.01.027
Received 16 November 2018; Received in revised form 20 January 2019; Accepted 22 January 2019
Available online 24 January 2019
1387-7003/ © 2019 Elsevier B.V. All rights reserved.

W.-X. Jiang et al.
Inorganic Chemistry Communications 102 (2019) 5–9
+
proton (H /H
2
), letting it more proper for the H generation [10], CdS
2
is selected as photosensitizers. However, the photocatalytic activity of
CdS itself for water reduction is very low because of high-rate charge
recombination of photogenerated electron [11]. To inhibit this re-
combination, it is necessary to load a cocatalyst on CdS [12,13]. Based
on that the formation of metal hydride intermediate is essential for the
hydrogen evolution reaction [14], much effort has been reported pre-
paring tetra- and pentadentate ligands and assembling coordinatively
unsaturated complexes [15]. Our interests have been focusing on the
design and research of new catalysts [7,14], and a new nickel-based co-
catalyst, Ni-ATSM has been prepared in our lab. Combining CdS NRs
and H
2
A, this nickel complex serves as an efficient co-catalyst for H
2
production.
2
. Results and discussion
Physical measurements and “X-ray Crystallography” for this paper
were showed in “Supplementary Materials”. CdS NRs was prepared by
using the reported method [16] and characterized by Transmission
Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM)
(
Fig. S1). The ligand, diacetyl‑2‑(4‑N‑methyl‑3‑thiosemicarbazo-
ne)‑3‑(4‑N‑amino‑3‑thiosemicarbazone (H
2
ATSM) was prepared based
on the reported method [17].
2.1. General characterization for the nickel complex
The reaction of NiCl and H ATSM afforded a new nickel complex,
2
2
Ni-ATSM (Scheme 1) [18], which is in agreement with the X-ray ana-
lysis in solid state. The structure of the nickel complex was character-
ized by single crystal X-ray diffraction, with the results shown in
Fig. 1a, Table 1 and Table S1. The nickel atom is four-coordinated by
two nitrogen atoms and two sulfur atoms from the ligand (ATSM). The
Ni(1)–N(1) and Ni(1)–N(2) distances are 1.856(6) and 1.843(6) Å, re-
spectively, whilst the bond lengths of Ni(1)–S(1) and Ni(1)-S(2) are
Fig. 1. (a) X-ray structure of the nickel complex, Ni-ATSM. (b) Cyclic voltam-
mograms (CVs) of a 1.50 mM of Ni-ATSM with varying concentrations of acetic
acid. Conditions: Glassy carbon working electrode (1.0 mm diameter), Pt
counter electrode, Ag/AgNO
ternal standard (*).
3
reference electrode, scan rate 100 mV/s. Fc in-
2
.154(2) and 2.142(2) Å, respectively. The result is also consistent with
the ESI-MS analysis. As shown in Fig. S2, the nickel complex exhibited
an ion at a mass-to-charge ratio (m/z) of 317.0148, which is assigned to
Table 1
+
Selected bond lengths (Å) and angles (°) for the nickel complex, Ni-ATSM.
[
Ni-ATSM-H] .
Considering that the nickel complex is in a coordinatively un-
Ni(1)–N(1)
1.856(6)
2.154(2)
84.6(3)
Ni(1)–N(2)
1.843(6)
2.142(2)
101.46(8)
saturated state, leaving empty positions on the nickel center, amenable
Ni(1)–S(1)
Ni(1)-S(2)
+
N(2)-Ni(1)-N(1)
S(2)-Ni(1)-S(1)
to binding to H or H
2
O for hydrogen generation [14], we tried to set
up a photocatalytic system based on the nickel complex. We first in-
vestigated its electrochemical performance, with the result shown in
Fig. 1b. The nickel complex displayed three good reversible redox
Scheme 1. Synthesis of the ligand, H
2
ATSM and the nickel complex, Ni-ATSM.
6

W.-X. Jiang et al.
Inorganic Chemistry Communications 102 (2019) 5–9
waves at −1.89, −1.31 and 0.82 V versus Ag/AgNO
3
, which are as-
NRs, 0.12 M H
2
A, and 0.01 mM Ni-ATSM. According to Fig. 2, H
2
I/0
II/I
III/II
signed to the Ni , Ni
and Ni
couples, respectively. Moreover,
production started immediately upon light irradiation of this system
addition of varied contents of acetic acid from 0.0 to 15.75 mM resulted
in a systematic increase in the voltammetric currents emerging at
and their rates increased sharply and could last for about 80 h with a
TON of 11,500 mol of H per mol of catalyst.
2
−
1.93 and −1.34 V, respectively and an additional positive shift in the
To get apparent quantum yields (AQYs), a photocatalytic system
was irradiated for 10 h under monochromic light with a band-pass filter
(λ = 420 nm + 5 nm). Based on Eq. (1) [21], the AQYs of the photo-
potential from −1.93 to −1.68 V, which is consistent with a catalytic
process [19]. The results indicate that the reductions of Ni(II) to Ni(I) or
Ni(I) to Ni(0) and protonation are responsible for hydrogen generation.
Next, electrochemical properties of this nickel complex were in-
vestigated in aqueous media. As shown in Fig. S3, with decreasing pH
values from 7.0 to 3.5, the strength of the reduction wave of Ni-ATSM
increased, and the onset of the catalytic wave were shifted to higher
potentials, which are consistent with a catalytic process [20]. More-
over, in a pH 4.5 buffer, the nickel complex exhibited a quasi-reversible
redox wave at −0.44 V versus Ag/AgCl (Fig. S3-insert), which is as-
catalytic system for H generation were estimated. As shown in Fig. S7,
2
in the first 1 h, the AQY was ~15.7%, then it increased until a highest
value of 26.7% was reached at 4 h. The average value of AQY was
~21.2% during 10 h irradiation.
AQY (%) = (2·nH2·N
A
·h·c)/(tirr· ·I·A)·100
(1)
n
H2 is the hydrogen generation (mol H
2
), N
A
is the Avogadro constant, h
is the Planck constant, c is speed of light, tirr is the irradiation time, I is
the intensity, A is the irradiated area of the photoreactor, where, I is
II
I
signed to the Ni /Ni couple.
−2
2
5
mW cm , A is 19.63 cm , tirr is 7200 s.
To confirm factors responsible for H
2
generation in this photo-
2
.2. Heterogeneous photocatalytic system based on the nickel complex for
catalytic system, any two of the three components (ascorbic acid, CdS
NRs, or Ni-ATSM) were combined. According to Fig. S8, a mixture of
H
2
generation
Ni-ATSM and CdS NRs only afforded 0.20 μmol H
2
, the integration of
was
Based on that CdS NRs has a flat-band potential (Efb) of −0.58 V
ascorbic acid and CdS NRs gave 4.1 μmol H
2
, and 1.8 μmol H
2
II
I
[
14], and Ni-ATSM exhibits a Ni /Ni couple at −0.44 V versus Ag/
produced when ascorbic acid and Ni-ATSM was combined. Thus, the
combination of the nickel complex, ascorbic acid and CdS NRs is es-
sential for the photocatalytic system.
AgCl, the electron transfer from the excited CdS NRs to the nickel center
is thermodynamically favorable. Such distinctive potential prompted
possible usage of this complex as a co-catalyst for hydrogen generation.
To characterize the catalytic activity of the nickel complex, a photo-
catalytic system was designed by employing Ni-ATSM as a co-catalyst,
2
.3. Investigation for the stability and durability of the photocatalytic
system
H
2
A as an electron donor and CdS NRs as a photosensitizer.
To obtain an optimal photocatalytic system, a series of measure-
A long-time photolysis was carried out to test photo- stability and
ments and analysis were carried out. First, we explored the effect of pH
durability of the nickel complex. According to Fig. S9, 80 h irradiation
led to an increase in pH values from 4.5 to 5.6, which is consistent with
of media on the photocatalytic activity for H
Fig. S4, the best pH for photocatalytic H generation mediated by Ni-
ATSM (0.01 mM) was found at pH 4.5, with a turnover number (TON)
2
evolution. As shown in
−
−
2
accumulation of OH by water reduction, 2H
2
O + 2e → H
+ 2OH .
2
However, this catalytic function could be recovered in 7% when the pH
was adjusted back to the original 4.5. This result indicates that CdS
−
1
of 410 mol of H
Next, effects of amounts of CdS NRs and H
activity for H evolution were investigated. As shown in Fig. S5, to
photocatalytic systems containing 0.10 M H A, 0.010 mM Ni-ATSM and
a varying content of CdS NRs, the TON during 3 h of photolysis in-
2
(mol of cat)
during 3 h of irradiation.
2
A on the photocatalytic
NRs, H
A, the nickel complex or all of them are unstable during pho-
2
2
tocatalysis.
2
To give answers for these equations, several physical or physio-
chemical methods were employed for measurements and analysis. As
shown in Fig. S10, the XRD signs were same as those before 80 h irra-
diation, showing that CdS NRs is stable during photolysis. From Fig.
S11a, both survey spectra were quite similar, with the presence of Cd, S,
O, and C elements. Before photocatalysis, two main peaks were located
at 160.726 eV and 163.093 eV (Fig. S11b), which can be attributed to S
−1
creased with increasing CdS NRs until 3260 mol of H
2
(mol of cat)
−
1
was reached at 0.045 mL . To photocatalytic systems containing
−
1
0
.045 mg·mL
CdS NRs, 0.01 mM Ni-ATSM and varying contents of
ascorbic acid, the TON increased with increasing the concentration of
−1
ascorbic acid until a highest value of 1920 mol of H (mol of cat) was
2
reached at 0.12 M (Fig. S6). These observation and analysis resulted in
an optimal three-component system, containing 0.045 mg·mL−1 CdS
2
4
p in CdS NRs [22], and two obvious Cd 3d peaks were located at
04.756 eV and 411.129 eV (Fig. S11c), which are consistent with the
Cd character in CdS NRs [22]. According to the data shown in Fig. S11b
and c, the position or strength of both S 2 s and Cd 3d peaks remained
almost constant, indicating that CdS NRs was stable as a photosensitizer
during photocatalysis.
As shown in Fig. S11d, before irradiation, two appreciable Ni 2p
peaks were observed at 854.219 eV and 871.503 eV, indicating the
2
+
presence of a Ni
ion. In contrast, the same Ni 2p peaks were found
after irradiation, indicating that the nickel complex is stable over a
period of 80 h photocatalysis. Next, photocurrent response versus time
of Ni-ATSM/CdS NRs and CdS NRs was investigated. According to
Fig. 3, the current can reproducibly increase violently under each ir-
radiation and recover rapidly in the dark, showing that the photo-
current response of Ni-ATSM/CdS NRs was reversible and stable.
Next, electronic spectra were measured to understand the stability
of the photocatalytic system. As shown in Fig. S12, before irradiation,
the three-component system exhibited a main peak at 259 nm, which is
assigned to that of ascorbic acid alone. And the photochemical reduc-
tion catalyzed by Ni-ATSM resulted in no new absorption, but a de-
crease in the strength of peak at 259 nm (Fig. S13), suggesting that the
amount of ascorbic acid decreased after photolysis. However, the
Fig. 2. Hydrogen evolution kinetics obtained upon continuous visible irradia-
−1
tion (λ = 469 nm) of a pH 4.5 buffer solution containing 0.045 mg·mL
NRs, 0.12 M ascorbic acid, and 0.01 mM Ni-ATSM.
CdS
7

W.-X. Jiang et al.
Inorganic Chemistry Communications 102 (2019) 5–9
Scheme 2. Possible mechanism for H production by the photocatalytic system
2
with Ni-ATSM.
band of CdS (−3.8 eV) [23] is smaller than ELUMO (−3.384 eV) of
NiATSM, the photogenerated electrons transfer from CdS to the nickel
complex is energetically favorable.
Moreover, at the excitation wavelength of 450 nm, CdS NRs ex-
hibited two emission waves at 531 and 676 nm (Fig. S16), respectively.
However, with the introduction of the nickel complex, the peak in-
tensity of CdS NRs at 531 nm decreased, showing a lower possibility of
electron–hole pair recombination because of the fast electron transfer
from CdS NRs to Ni-ATSM [24,25]. Therefore, as an electron acceptor
loaded on the CdS NRs surfaces, the nickel complex can trap efficiently
Fig. 3. Current responses versus time of Ni-ATSM/CdS NRs under visible-light
irradiation (λ > 420 nm) at 0.20 V using Ag/AgCl as a reference electrode.
+
photo-generated electrons and promoted their combination with H to
form H .
2
Electrochemical impedance spectroscopy was used to investigate
charge transfer properties and separation efficiency of photogenerated
charge carriers, giving the results shown in Fig. 4. Compared with CdS
NRs, the mixture of CdS NRs and Ni-ATSM showed much smaller arc
radius, representing a faster interfacial charge transfer and higher se-
paration efficiency of photogenerated charge carriers [26–29]. On the
basis of these analysis and literature precedents [14,28–30], we put
forward a possible mechanism for the photocatalytic H production. As
2
outlined in Scheme 2, a reduced nickel(I) species was afforded by the
transfer of the photoexcited electron from the conduction band (CB) of
CdS NRs to the nickel(II) ion of Ni-ATSM. Then the introduction of
+
+
III
hydrogen proton (H ) yielded a Ni -H species. Further addition of
III
hydrogen proton (H ) to the Ni -H species gave dihydrogen, and re-
generated the starting nickel complex.
3
. Conclusion
Fig. 4. Electrochemical impedance spectroscopy Nyquist plots of CdS NRs and
We described a new photocatalytic system based on the nickel(II)
Ni-ATSM/CdS NRs with 0.010 M K
conditions.
3
Fe(CN)
6
/K
4
Fe(CN)
6
electrolyte in dark
complex, Ni-ATSM, a new co-catalyst for hydrogen production. The
studies showed that the photocatalytic activities depend on con-
centrations of CdS NRs, ascorbic acid, the nickel complex and the pH
values of media. CdS NRs and the nickel complex were stable, but as-
corbic acid was decomposed during light irradiation. Our ongoing ef-
forts are focused on modifying the sacrificial electron donor for more
stable photocatalytic system.
adjustment of H A back to the original 0.12 M resumed nearly 34.6%
2
activity of the photocatalytic system under another illumination. Then,
when the solution pH was adjusted back to the original value of 4.5, the
original photocatalytic function can be recovered about 89%. These
results suggest that the decomposition of H A is the primary reason for
2
Acknowledgements
the cease of hydrogen evolution after illumination, although several
components of the photocatalytic system may deactivate during pho-
tolysis.
This work was supported by the National Science Foundation of
China (No. 21271073 and 21875074).
2.4. Investigation for photocatalytic mechanism
Appendix A. Supplementary material
To understand the photocatalytic mechanism, several methods were
CCDC 1843757 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html. Supplementary data to
this article can be found online at doi:10.1016/j.inoche.2019.01.027.
employed. As shown in Fig. S14, the introduction of the nickel complex
into CdS NRs resulted in a red shift of the absorption onset and re-
duction of Eg of CdS NRs, which obviously improves the range and
ability of visible light absorption of CdS NRs.
We also used “the Materials Studio DMol^3” to carried out orbital
calculation for the nickel complex, giving the results shown in Fig. S15.
As shown in Fig. S15, the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) of NiATSM were
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