Light-driven hydrogen production from aqueous solutions based on a new Dubois-type nickel catalyst

Article abstract of DOI:10.1039/c7cp00247e

In this work, we report a new photocatalytic system that links multifunctional semiconductor nanocrystals with emerging water-soluble molecular catalysts made of earth-abundant elements for H₂ generation [Ni(P₂^RN₂^R′)₂(BF₄)₂]^4-, R = Ph, R′ = [PhSO₃]^- (NiS). This noble metal free hybrid exhibits remarkable catalytic activity with a turnover number of 511 for H₂ production and a photon-to-H₂ conversion efficiency of 12.5%. The mechanistic insight into such high efficiency in this photocatalytic system was examined using a combination of steady-state emission and time-resolved absorption spectroscopy.

Full text of DOI:10.1039/c7cp00247e

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Light-driven hydrogen production from aqueous
solutions based on a new Dubois-type nickel
catalyst†
Cite this:DOI: 10.1039/c7cp00247e
Received 12th January 2017,
Accepted 20th February 2017
Y. Zhou,ab S. Yangb and J. Huang*b
DOI: 10.1039/c7cp00247e
rsc.li/pccp
In this work, we report a new photocatalytic system that links multi- dependent optical properties, SC nanocrystals have high extinction
functional semiconductor nanocrystals with emerging water-soluble coefficients across a broad spectral range. Their surfaces can be
molecular catalysts made of earth-abundant elements for H2 genera- readily modified to control their solubility in preferred solvents and
0
tion [Ni(P2RN2R )2(BF4)2]4ꢀ, R = Ph, R0 = [PhSO3]ꢀ (NiS). This noble metal they are amenable for coupling with specific functional targets in
free hybrid exhibits remarkable catalytic activity with a turnover number HECs.18–20 Molecular catalysts that attract particular attention are
of 511 for H2 production and a photon-to-H2 conversion efficiency those that contain active sites with earth-abundant elements.21,22 To
of 12.5%. The mechanistic insight into such high efficiency in this date, the most active synthetic electrocatalyst for the HER is the Ni
photocatalytic system was examined using a combination of steady- catalyst developed by Dubois and co-workers, i.e., the bis(1,5-R0-
0
state emission and time-resolved absorption spectroscopy.
diphospha-3,7-R00-diazacyclooctane) nickel core, [Ni(P2R N2R00)2]2+,23–26
and thus is an especially ripe target for further development.
Offering the dual advantages of SC nanocrystal PSs and Dubois-
The photocatalytic generation of H2 from water requires the
efficient coupling of light harvesting materials with hydrogen type Ni catalysts, in this work, we developed a new hybrid photo-
evolution catalysts (HECs). Since the initial report of the light-driven catalytic system that integrates CdSe quantum dots (QDs) with
generation of H2,1,2 different strategies for constructing such photo- Dubois-type Ni catalysts functionalized with –SO3ꢀ groups (NiS,
ꢀ
catalytic systems have evolved. However, these systems typically Scheme 1). The –SO3 group was chosen as the functional
suffer from significant issues that impact their viability in actual group because of its exceptional solubility in H2O, which makes
devices. For example, the use of noble metals or metal oxides as NiS water-soluble, thus facilitating light-driven H2 generation
catalysts and semiconductor (SC) materials as photosensitizers fromꢀaqueous solution. Moreover, the synthesis of NiS with the
(PSs) leads to durable catalytic systems that can efficiently –SO3 group is much more accessible and cost effective com-
catalyze H2 evolution; yet, their long-term practicality is hampered pared to its cousin functionalized with –PO3H2.26,27 In the
by the scarcity and high cost of noble metals.3–7 As a second presence of ascorbic acid (AA), we demonstrate that this hybrid
example, homogeneous catalytic systems composed of synthetic, system can efficiently catalyze the HER in aqueous solution
often bio-inspired, molecular catalysts and molecular PSs have under visible light illumination. Mechanistic studies using
tunable structures and can be readily synthesized.8–11 However, emission and time-resolved absorption spectroscopy show that
most of them suffer from photoinstability during prolonged ultrafast charge separation occurs in CdSe QDs through electron
irradiation, in particular, from degradation of the molecular PS.
With a view towards addressing the issues of photocatalyst
efficiency, stability, and cost effectiveness, hybrid catalysts that
integrate SC nanocrystal PSs and bio-inspired molecular catalysts
have emerged as attractive photocatalytic systems for the H2
evolution reaction (HER).12–17 In addition to their unique size
transfer (ET) to NiS and hole transfer (HT) to AA.
a College of Science, China University of Petroleum (East China), Qingdao, 266580,
China
b Department of Chemistry, Marquette University, Milwaukee, Wisconsin, 53201,
USA. E-mail: jier.huang@marquette.edu
† Electronic supplementary information (ESI) available: Description of materials
and methods. See DOI: 10.1039/c7cp00247e
Scheme 1 Summary of synthetic procedures of the NiS complex.
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NiS was synthesized according to previously published on experimental conditions, such as sacrificial donors, proton
protocols28 with some modifications (Scheme 1). Details of the sources, the pH values of the solution, and the concentrations
synthesis and structural characterization are provided in the ESI.† of the PS and the catalyst.26,33 Therefore, a series of photo-
Briefly, phenylphosphine (330 mg, 3.0 mmol) was added dropwise catalytic experiments were carried out by systematically varying
to a suspension of paraformaldehyde (180 mg, 6.0 mmol) in 5 mL these parameters. The initial survey, among AA, lactic acid, and
of ethanol under N2 protection. After refluxing for 12 hours, a TEOA/TEA, showed that the hybrid system with AA exhibited the
solution of sodium salt of sulfanilic acid (586 mg, 3 mmol) in highest activity and was thus identified as the most appropriate
water (2 mL) was added to the mixture. The resulting solution electron donor and proton source (Fig. S4, ESI†). The higher
was left overnight with vigorous stirring, after which a white activity in the presence of AA than TEOA/TEA is likely due to the
precipitate was isolated. The white precipitate was confirmed to higher acidity of the former than the latter, which is consistent
1
be PPh2NC H SO Na by H NMR (Fig. S1, ESI†), 31P NMR (Fig. S2, with the pH dependent study discussed below. Meanwhile, the
6
4
3
2
ESI†), 13C NMR (Fig. S3, ESI†) and ESI-MS. The final product, i.e. better activity with AA than that with lactic acid can be attributed
Na4[Ni(P2PhN2C H SO Na)2(BF4)2] (NiS), was obtained after mixing to the higher oxidation power of AA than lactic acid, which
1 eq. of Ni precursor salt ([Ni(CH3CN)6][BF4]2) with 2 eq. of the facilitates hole transfer from CdSe QDs to AA. Furthermore, the
6
4
3
PPh2NC H SO Na ligand in H2O. The molecular structure of the pH of the AA solution was screened to find the optimum acidity
6
4
3
2
NiS complex was confirmed by 1H NMR, 31P NMR, and elemental of the solution, from which the solution with pH 3.8 was
analysis.
identified as the best condition for the reaction (Fig. S4, ESI†)
Fig. 1a shows the UV-visible absorption spectrum of NiS in in terms of activity and durability. The decreased durability for
H2O (red plot). The strong absorption band located at 447 nm H2 generation at pH below 3.8 is likely due to more severe
can be assigned to the metal-to-ligand charge transfer band from corrosion of CdSe QDs under highly acidic conditions.
nickel to phosphorus, similar to previously reported Dubois Ni
Using these conditions, the HER rates of the hybrid system
catalysts.29 Fig. 1b shows the cyclic voltammogram (CV) of NiS as a function of time were measured under illumination of a
measured in H2O solution. The observed two quasi-reversible 505 nm LED lamp (22 mW). Because no appreciable amount of
reduction waves at E1/2 = ꢀ0.558 V and E1/2 = ꢀ0.817 V vs. SCE H2 was observed by omitting AA, NiS or CdSe QDs in the
can be assigned to NiII/I and NiI/0 redox couples,26 respectively.
photocatalytic system, all of these components are necessary
Water soluble CdSe QDs were obtained after ligand exchange for the HER. Next, the concentrations of CdSe QDs ([CdSe]) and
of oleic acid capped CdSe QDs with 3-mercaptopropionic acid NiS catalysts ([NiS]) were systematically varied to obtain the
(MPA), following a previously published procedure30 (see the optimum turnover number (TON) for the HER. Fig. 2a shows
ESI† for details). The UV-visible absorption spectrum of the the time profile of H2 production measured from solutions with
synthesized CdSe QDs in H2O is also presented in Fig. 1a (black fixed [NiS] = 0.2 mM and variable [CdSe]. While all samples
plot). These QDs have their first exciton band located at 540 nm. show notable H2 generation immediately following illumina-
According to the model of Brus,31,32 the estimated conduction tion, the HER rate varies significantly among these samples. As
and valence band edges for CdSe QDs (540 nm) are ꢀ1.14 V and reflected by the slope of the curves and the overall TON as a
+1.05 V vs. SCE, respectively. Compared to the 1st and 2nd
reduction potentials of the NiS obtained above, the driving forces
for the 1st and 2nd ET processes from excited CdSe QDs to NiS
are ꢀ0.582 eV and ꢀ0.323 eV, respectively, suggesting that ET
processes from CdSe QDs to NiS in the CdSe/NiS hybrid are
thermodynamically feasible.
The photocatalytic performance of the CdSe/NiS hybrid for
the HER was evaluated under the illumination of an LED lamp
in aqueous solution. Previous studies have shown that the
efficiency of light-driven H2 production is largely dependent
Fig. 2 Hydrogen evolution profiles of CdSe/NiS hybrids in H2O, [AA] = 0.1 M,
irradiated at 505 nm (22 mW): (a) effect of varying [CdSe] at [NiS] = 0.2 mM;
the inset shows TON as a function of CdSe concentration; (b) effect of varying
[NiS] at [CdSe] = 13.7 mM; the inset shows TON as a function of NiS
Fig. 1 (a) UV-visible absorption spectra of NiS, CdSe, and the CdSe/NiS
hybrid, and emission spectrum of CdSe in H2O. (b) Cyclic voltammogram
of NiS measured in H2O.
concentration; (c) the estimated photon-to-H2 quantum yields using various
[CdSe] (mM) and [NiS] (mM); and (d) the stability measurements for the HER
under optimized conditions.
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function of [CdSe] (the inset of Fig. 2a), the HER rate initially suggests that the valence state of Ni in the NiS catalyst is believed
increases and then decreases with increasing [CdSe]. Because to remain unchanged (NiII state) after catalysis, as the NiI state
the maximum TON occurs at [CdSe] = 13.7 mM, this QD prefers to have lower coordination. These results together imply
concentration is used in the subsequent [NiS] dependence that the square planar structure of NiS is essential for its catalytic
experiments. As shown in Fig. 2b, the TON of the hybrid system activity for H2 generation.
increases with decreasing [NiS] until it reaches its optimum
In addition to the photocatalytic reaction, mechanistic insight
number 511 at [NiS] = 0.02 mM, after which further decrease of into the HER was obtained using a combination of steady state
[NiS] slows down the HER rate. These results suggest that the and time-resolved optical spectroscopy. Transient absorption (TA)
maximum TON (511) has been achieved under the optimized spectroscopy, a powerful tool for probing exciton dynamics in QDs,
conditions with [CdSe] = 13.7 mM and [NiS] = 0.02 mM. Mean- was first used to examine the charge separation dynamics in CdSe
while, the H2 generation efficiency of the photocatalytic system QDs in the presence of NiS and AA. The sample conditions which
after replacing NiS by Ni(CH3CN)6(BF4)2 is dramatically lower give rise to optimum photon-to-H2 efficiency were used for TA
than that of NiS (Fig. S4, ESI†), which suggests that NiS is the measurements. Fig. 3a shows the femtosecond TA spectra of
active species responsible for such a high TON.
CdSe QDs after 560 nm excitation. The strong negative feature at
While the TON against PSs or catalysts is often used to evaluate 500–550 nm can be assigned to the 1S exciton bleach band,
HER efficiency for a specific photocatalytic system, the direct which has been attributed to the reduced oscillation strength
comparison of efficiencies using TONs among different systems is of the 1S(h)–1S(e) transition due to the filling of the 1S(e) level
difficult, as these numbers are largely dependent on the illumination upon excitation.42,43 The bleach band centered at B460 nm
power density. In order to have a valid comparison with previously and the positive absorption bands at 480 nm and 580 nm can
reported systems, we evaluated the HER efficiency of the CdSe/NiS be attributed to the bleach and the Stark-effect induced red
hybrids in terms of photon-to-H2 quantum yield. The photon-to-H2 shift of the higher absorption bands, respectively, which are
quantum yield was defined as Z (%) = D(H2) ꢁ 2/D(hn) ꢁ 100%, typically caused by the presence of 1S excitons.42 The 1S exciton
where D(H2) is the H2 generation rate in the first 3 hours and bleach recovery of CdSe QDs (Fig. 3b), reflecting the electron
D(hn) is calculated from the power of the incident light (22 mW). lifetime in the conduction band of QDs, shows negligible decay
Fig. 2c presents the photon-to-H2 efficiency for samples with within 100 ps, suggesting that multiple excitons are barely
different combinations of [CdSe] and [NiS] after 3 h irradiation. generated under the current experimental conditions. In con-
Using 0.6 mM of NiS and 13.7 mM of CdSe QDs, we have achieved trast, the 1S exciton bleach recovery in the CdSe/NiS hybrid
a photon-to-H2 efficiency of 12.5%. Although photocatalytic (Fig. 3b and Fig. S6, ESI†) is enhanced compared to that of
systems based on Ni salts are still benchmarks,15 and the TON of CdSe, indicating that the 1S electrons in the conduction band
the current system is lower than several other systems using Dubois of QDs in the CdSe/NiS hybrid are depopulated due to the
Ni catalysts,26,34,35 the photon-to-H2 quantum yield observed in presence of the NiS catalyst.
the current system compares well with systems using synthetic
Ni molecular catalysts in aqueous solution (0.2–15%),26,27,36–39
largely promising the potential to use this system for solar-to-
fuel conversion.
In order to reveal the reasons for the cessation of H2
generation, NiS and CdSe QDs were alternatively added to an
inactive photocatalytic system after 10 h. As shown in Fig. 2d,
B90% of the H2 generation efficiency is recovered after adding
the same amount of NiS, while adding CdSe QDs only recovers
a small portion of catalytic activity. These results suggest that
the degradation of NiS catalysts is the major limiting factor that
leads to the photoinstability of the system though the degrada-
tion of CdSe QDs contributes some. To gain more information
on how NiS catalysts were degraded, we compared the X-ray
absorption near edge structure (XANES) spectra of the catalytic
system collected at the Ni-K edge before and after catalysis. As
shown in Fig. S5 (ESI†), the XANES spectrum before catalysis
shows a prominent pre-edge feature at B8336 eV (1s–3d
Fig. 3 (a) Femtosecond absorption spectra of CdSe QDs in H2O following
transition), while this feature disappeared in the spectrum after
catalysis. These results are consistent with previous literature
data,40,41 which can be attributed to the conversion of the NiS
geometry from the noncentrosymmetric environment (i.e. dis-
torted square planar geometry) to a centrosymmetric environ-
ment (octahedral geometry). The octahedral geometry of Ni,
together with the negligible edge shift in the XANES spectrum,
560 nm excitation. (b) Comparison of 1S exciton bleach recovery probed
at 520 nm for CdSe QDs and CdSe/NiS in the absence and presence of AA.
These kinetics were rescaled by normalizing the amplitude at 5 ns for
better comparison of the early time kinetics in the absence and presence
of NiS. (c) The Stern–Volmer plot for emission quenching of CdSe QDs by
AA. The inset shows the emission spectra of CdSe QDs with different
concentrations of AA. (d) Cartoon of charge separation dynamics in CdSe
QDs through electron transfer to NiS and hole transfer to AA.
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The depopulation dynamics of 1S electrons in the conduction This also explains the enhanced initial emission intensity of CdSe
band of CdSe QDs can arise from ET or energy transfer (ENT) to QDs upon addition of AA, where AA molecules replaced the surface
the NiS complex. The ET process results in the formation of a trap states of QDs.
reduced state of NiS while ENT leads to the formation of an
excited state of NiS, which in principle can be distinguished by
Conclusions
using TA spectroscopy. Unfortunately, due to the extremely small
extinction coefficient of NiS (e457 = 1160 cmꢀ1 Mꢀ1) compared to
In summary, we have developed a new water-soluble hybrid
photocatalytic system that integrates a functionalized Dubois-
type Ni complex with CdSe QDs. In the presence of ascorbic
CdSe QDs (e540 = 85 555 cmꢀ1 Mꢀ1),41 along with the significant
overlap between the absorption spectra of CdSe QDs and NiS, it
is impossible to identify these species using TA spectroscopy.
acid, this system demonstrates a reasonable photon-to-H2
¨
Instead, we calculated the ENT rate according to the Forster
conversion efficiency compared to other noble-metal free
photocatalytic systems based on Ni molecular catalysts. Using
steady-state emission and time-resolved absorption spectro-
scopy, we show that ultrafast charge separation occurs in CdSe
resonant energy transfer (FRET) model,44 which has been widely
used to estimate the ENT rate in various hybrid systems.45–48
According to this model, the estimated ENT time has an upper
limit of 5.5 ms, corresponding to an ENT rate of 1.8 ꢁ 105 sꢀ1
(ESI†). Because the ENT rate is much slower than the 1S electron
QDs through electron transfer to NiS (B1.8 ꢁ 1010
s
ꢀ1) and
hole transfer to AA (7.2 ꢁ 1010 Mꢀ1
s
ꢀ1). The effective spatial
depopulation rate which is B1.8 ꢁ 1010
s
ꢀ1 (t B 56 ps) obtained
separation of electron–hole pairs in CdSe QDs with electrons in
NiS and holes in AA is likely the reason resulting in highly
efficient photocatalytic H2 evolution in this system.
from fitting the kinetic traces in Fig. 2b using a previously
published model,20 we attribute the faster exciton bleach recovery
in the CdSe/NiS hybrid observed by TA to the ET process. The ET
time (56 ps) in the current CdSe/NiS system appears to be fast in
ꢀ
Acknowledgements
terms of bimolecular diffusion interaction. Given that NiS has SO3
groups which can be used as anchoring groups to nanoparticles,49
it is highly possible that NiS may have directly attached to the
surface of the CdSe QDs, resulting in the ultrafast ET process from
CdSe QDs to NiS. In addition, it is notable that the 1S exciton
bleach of CdSe/NiS has not fully recovered within our 5 ns time
window of the TA experiment (Fig. 3b). Similar uncompleted 1S
exciton bleach recovery kinetics in CdSe/molecular adsorbate has
been observed previously43,50 and can be attributed to the presence
of free CdSe QDs in the CdSe/NiS solution.
This work was supported by the Marquette University new
faculty startup fund and Regular Research Grant. Use of the
Advanced Photon Source in the Argonne National Laboratory
was supported by the U.S. Department of Energy, Office of
Science, and Office of Basic Energy Sciences, under Award No.
DE-AC02-06CH11357.
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