ZnSe quantum dots modified with a Ni(cyclam) catalyst for efficient visible-light driven CO2 reduction in water

Article abstract of DOI:10.1039/c7sc04429a

A precious metal and Cd-free photocatalyst system for efficient CO₂ reduction in water is reported. The hybrid assembly consists of ligand-free ZnSe quantum dots (QDs) as a visible-light photosensitiser combined with a phosphonic acid-functiona

Full text of DOI:10.1039/c7sc04429a

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ZnSe quantum dots modified with a Ni(cyclam) catalyst
for efficient visible-light driven CO2 reduction in water
Received 00th January 20xx,
Accepted 00th January 20xx
Moritz F. Kuehnel,a†§ Constantin D. Sahm,a† Gaia Neri,b Jonathan R. Lee,b Katherine L. Orchard,a
Alexander J. Cowanb* and Erwin Reisnera*
DOI: 10.1039/x0xx00000x
A precious metal and Cd-free photocatalyst system for efficient CO2 reduction in water is reported. The hybrid assembly
consists of ligand-free ZnSe quantum dots (QDs) as a visible-light photosensitiser combined with a phosphonic acid-
functionalised Ni(cyclam) catalyst, NiCycP. This precious metal-free photocatalyst system shows a high activity for aqueous
CO2 reduction to CO (Ni-based TONCO >120), whereas an anchor-free catalyst, Ni(cyclam)Cl2, produced three times less CO.
Additional ZnSe surface modification with 2-(dimethylamino)ethanethiol (MEDA) partially suppresses H2 generation and
enhances the CO production allowing for a high Ni-based TONCO of >280 and more than 33 % selectivity for CO2 reduction
over H2 evolution, after 20 h visible light irradition (λ >400 nm, AM 1.5G, 1 sun). The external quantum efficiency of
3.4±0.3 % at 400 nm is comparable to state-of-the-art precious metal photocatalysts. Transient absorption spectroscopy
showed that band-gap excitation of ZnSe QDs is followed by rapid hole scavenging and very fast electron trapping in ZnSe.
The trapped electrons transfer to NiCycP on the ps timescale, explaining the high performance for photocatalytic CO2
reduction. With this work we introduce ZnSe QDs as an inexpensive and efficient visible light-absorber for solar fuels
generation.
www.rsc.org/
catalysts on a semiconductor surface using suitable anchoring
groups5 enables efficient charge transfer from the semi-
conductor to the catalyst.6
Introduction
Artificial photosynthesis allows for the storage of solar energy
through the conversion of water and carbon dioxide into
chemical fuels and represents
a promising strategy to
overcome the global dependence on fossil energy sources.1
Economic viability and scalability of this approach, however,
greatly benefit from the development of environmentally
benign photocatalysts consisting of inexpensive and abundant
materials that operate in water. Hybrid photocatalysts have
the potential to achieve this aim, because they combine the
selectivity of molecular electrocatalysts with the photophysics
of nanoparticulate photosensitisers.2 Ultra-fast electron-
transfer kinetics often observed with quantum dots (QDs)3 are
favourable for the overall efficiency, because productive
electron transfer from the photosensitiser to the catalyst
competes with charge recombination.4 Immobilising molecular
Figure 1. Schematic representation of the photocatalyst system developed in this work:
Ligand-free ZnSe QDs (ZnSe-BF4) combined with a molecular electrocatalyst, NiCycP, for
aqueous CO2 reduction to CO. H2 evolution can be partially suppressed by a surface
inhibitor (MEDA). AA: Ascorbic acid.
A
growing number of earth-abundant molecular
electrocatalysts capable of selective CO2 reduction in water,
both in homogeneous phase7 and immobilised on electrodes8
have recently been reported. However, photocatalytic CO2
reduction is rarely achieved in aqueous solution without using
precious metals.9 The potentials required to drive CO2
reduction at these catalysts are often very negative, resulting
in competing proton reduction due to faster kinetics, and thus
an overall low selectivity. Moreover, the necessary driving
force is typically supplied by expensive light-absorbers such as
a.Christian Doppler Laboratory for Sustainable SynGas Chemistry, Department of
Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
e-mail: reisner@ch.cam.ac.uk
http://www-reisner.ch.cam.ac.uk
b.Stephenson Institute for Renewable Energy, Department of Chemistry, The
University of Liverpool, Crown Street, Liverpool L69 7ZD, UK
e-mail: A.J.Cowan@liverpool.ac.uk
† These authors contributed equally.
§ Present address: Department of Chemistry, Swansea University, College of
Science, Singleton Park, Swansea SA2 8PP, UK
https://moritz-kuehnel.com
Electronic Supplementary Information (ESI) available: Experimental details,
additional QD characterisation, control experiments, time-resolved spectroscopy.
See DOI: 10.1039/x0xx00000x. Additional data related to this publication are
available at the University of Cambridge data repository.
(https://doi.org/10.17863/CAM.17941)
2+
Ir(ppy)3, Ru(bpy)3 or GaP.10 Entirely precious metal-free
photocatalyst systems are scarce and only a few Fe11 and Co12
complexes have been reported to efficiently reduce CO2 when
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combined with inexpensive photosensitisers, but activity was than the onset potential for electrocatalytic CO2 reduction at
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14b
only observed in organic solvents. Aqueous CO2 reduction Ni(cyclam)Cl2 of approximately –1.0 V DvOs.I: 1N0H.10E39a/tC7pSCH054.4529A
using nickel terpyridine catalysts on CdS QDs has been (E0’CO2/CO = –0.43 V vs. NHE). While the energetics suggest that
reported with an efficiency of 0.28 % (EQECO) and a high CO electron transfer from the ZnSe CB to Ni(cyclam)2+ is
selectivity of 90 %.9a, 9b However, this system was limited by thermodynamically feasible, the electron transfer kinetics are
the durability of the catalyst, which started to degrade after crucial to efficiently drive CO2 reduction at Ni(cyclam)2+.
8 h of illumination. An Fe porphyrin-based system driven by an Electron transfer from the photosensitiser to the co-catalyst
organic dye achieved a TONCO of 120 and a selectivity of 95 %, must be faster than charge recombination, and CO2 reduction
but was suffering from dye instability.9d
additionally competes with H2 evolution.
CO is one of the most versatile CO2 reduction products with We have previously demonstrated that capping ligands,
numerous synthetic applications in industry including the commonly employed to control QD growth and stability, can
synthesis of liquid fuels.13 Ni(cyclam)2+ is a non-precious significantly impact their photocatalytic activity.24 Ligand
electrocatalyst with high stability and activity for selective removal exposes vacant surface sites25 that are needed for the
7e
conversion of CO2 to CO.7d,
Photocatalytic reduction of controlled
immobilisation
of
anchor-functionalised
aqueous CO2 has been achieved by combining Ni(cyclam)2+ and molecules.9a The ligand-free QDs used here enable us to study
its derivatives with various photosensitisers.14 Nonetheless, the effect of first adding defined amounts of a CO2 reduction
these previous systems showed either good activity of up to 38 catalyst, followed by blocking the remaining particle surface
TONCO with a poor selectivity of <10 %14a or vice versa with capping ligands to suppress competing H2 evolution.
(TONCO 2, CO selectivity 94 %)14e despite using expensive ZnSe QDs were prepared using a modified literature procedure
photosensitisers such as Ru(bpy)32+. Moreover, the observed based on heating zinc stearate and selenium in octadecene to
selectivity for CO2 reduction over H2 evolution was often much 300 °C.26 A growth period of 130 min yielded individual, near-
lower than what is achieved in electrocatalytic CO2 reduction spherical stearate-capped ZnSe nanocrystals (ZnSe-St). An
at Ni(cyclam)2+ and the activity and longevity was far below average diameter of 4.55±0.62 nm was determined from
that
of
photocatalysts
based
on
precious-metal transmission electron microscopy (TEM, Figure S1A-B) in good
agreement with a diameter of 5.40±0.93 nm determined from
electrocatalysts.15
Here, we present a novel hybrid photocatalyst system entirely powder X-ray diffraction (XRD, Figure S1C) using the Scherrer
free of precious metals (Figure 1). The combination of equation.27 Ligand-free, charge stabilised ZnSe QDs (ZnSe-BF4)
inexpensive ZnSe QDs as a visible-light photosensitiser with a were obtained from ZnSe-St by reactive ligand stripping using
functionalised Ni(cyclam) electrocatalyst enables efficient CO2 Me3OBF4.28 ZnSe QDs show little change in the particle size,
reduction in aqueous solution with activities and longevities on morphology and structure upon ligand stripping; the mean
par with precious metal-based systems by exploiting partial particle size was 4.50±0.53 nm (from TEM; 5.02±0.41 from
surface capping to enhance activity and selectivity. We use XRD, Figure 2A-C). The UV-vis spectrum features good visible-
transient absorption spectroscopy to demonstrate that light response with a first excitonic absorption maximum at
electrons are transferred from the photosensitiser to the 417 nm (Figure 2D). ATR-IR spectroscopy of dried ZnSe-BF4
immobilised catalyst on the ps-time scale, enabling fast particles shows the expected signatures of surface-adsorbed
catalyst turnover.
Results and Discussion
Quantum Dot Synthesis
In line with ongoing efforts to replace scarce and expensive
photosensitisers with inexpensive and benign materials,16 we
first sought to identify a non-precious semiconductor material
with a suitable conduction band (CB) edge to allow electron
transfer to Ni(cyclam)2+. ZnSe is a stable semiconductor with a
direct band gap of 2.7 eV17 that enables absorption of near-UV
and visible light unlike ZnS which requires excitation by UV
light.18 Additionally, electron mobility in ZnSe (610 cm2/Vs)19 is
significantly higher than in its sulfide analogues ZnS
(200 cm2/Vs) and CdS (385 cm2/Vs).20 Despite these favourable
properties, ZnSe has received little attention for solar fuels
generation. ZnSe-based photocathodes were recently reported
for H2 evolution,21 but unlike their cadmium analogues CdS22
Figure 2. Characterisation of ligand-free ZnSe QDs (ZnSe-BF4): (A) Transmission electron
micrographs with (B) corresponding particle size distribution. (C) Powder X-ray
diffractogram overlaid with cubic zinc blende ZnSe reference (PDF 01-071-5978); (D)
UV-vis spectrum compared to ZnSe-St.
and CdSe,23 ZnSe QDs have not been used in photocatalytic
reduction of protons or CO2. At pH 5.5, the ZnSe CB is located
at approximately –1.4 V vs. NHE,21a i. e. 400 mV more negative
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Figure 4. Enhanced CO2 photoreduction with ZnSe-BF4/NiCycP in the presence of
MEDA: (A) Changes in product selectivity with different MEDA concentrations (4 h
irradiation). (B) Long-term activity of ZnSe-BF4/NiCycP/MEDA (25 µM MEDA).
Conditions: AM 1.5G, λ >400 nm, 100 mW cm–2, 0.5 µM ZnSe-BF4, 10 µM NiCycP, 0.1 M
AA, pH 5.5, CO2, 25 °C. (C-D) Change of the UV-visible spectrum of aqueous ZnSe-BF4
over 90 min in the absence (C) or presence (D) of added MEDA (0.5 µM ZnSe-BF4, 0.1 M
AA, pH 5.5, 25 °C, with or without 25 µM MEDA, no irradiation).
Figure 3. Photocatalytic reduction of aqueous CO2 in the presence of ZnSe-BF4/NiCycP:
(A-B) Effect of varying co-catalyst loadings on product distribution and selectivity (4 h
irradiation). (C) Performance depending on the employed co-catalyst (4 h irradiation).
(D) Long-term activity of ZnSe-BF4/NiCycP (sample purged with CO2 after 20 h and 0.5
µM ZnSe-BF4, 10 µM NiCycP or nothing added). Conditions: AM 1.5G, λ >400 nm, 100
mW cm–2, 0.5 µM ZnSe-BF4, 10 µM NiCycP, 0.1 M AA, pH 5.5, CO2, 25 °C (unless
otherwise stated).
NiCycP to ZnSe-BF4 promotes the generation of CO at the
expense of H2 evolution (Table S2). This observation suggests
that H2 evolution at the particle surface and CO2 reduction at
the co-catalyst directly compete for CB electrons, thus proving
electron transfer to NiCycP to be kinetically feasible. The
importance of fast electron transfer from ZnSe to the co-
catalyst was further demonstrated by employing the freely
diffusing co-catalyst Ni(cyclam)Cl2 instead of anchored
NiCycP.The CO2 reduction activity was over three times lower
(Figure 3C, Table S2), even though Ni(cyclam)Cl2 shows a
higher level of activity as a homogenous electrocatalyst and
has a similar onset potential for CO2 reduction.14b We have
previously reported that co-immobilisation of NiCycP and a Ru-
dye on a solid support accelerates electron transfer from the
dye to the catalyst compared to a diffusional homogeneous
system.14b, 29
Under optimised conditions (10 µM NiCycP), ZnSe-BF4/NiCycP
achieved a Ni-based TONCO of up to 121±6 and 8.0±0.9 % CO
selectivity after 20 h. Higher catalyst loadings (150 µM NiCycP,
Figure 3B) led to a higher CO selectivity of up to 21.5±1.1 %,
but resulted in a lower TONCO (Table S2). The photocatalyst
system is still active after 20 h irradiation, but the rate of CO
production is lowered while H2 generation remains largely
unchanged. Adding fresh ZnSe-BF4 had little effect on the CO
production, whereas adding fresh NiCycP restored the initial
activity and selectivity (Figure 3D, Figure S5), indicating that
ZnSe-BF4 remains intact while the molecular co-catalyst
undergoes deactivation over time, presumably due to CO
poisoning.30 The stability of the particles was further
corroborated by UV-vis spectra collected after irradiation
(Figure S6). Increased scattering implies some particle
agglomeration, however the absorption onset of the ZnSe-BF4
QDs remains unchanged compared to a sample that was
stirred in the dark or the stock solution. TEM images of the
–
BF4 (1010 cm–1) and DMF (1089, 1375 and 1649 cm–1),
additional bands at 730 and 1537 cm–1 suggest a small amount
of residual stearate on the particle surface (Figure S2). X-ray
photoelectron spectroscopy (XPS) shows a 0.8 eV shift of the
O1s, Zn2p and Se3d binding energies to more positive for ZnSe-
BF4 particles compared to ZnSe-St, suggesting partial oxidation
of the QD surface upon ligand stripping (Figure S3).
Photocatalytic CO2 Reduction
A hybrid photocatalyst system was subsequently assembled,
using ZnSe-BF4 as the light absorber to drive aqueous CO2
reduction at the molecular co-catalyst NiCycP,14b
a
functionalised derivative of Ni(cyclam)Cl2 bearing a phosphonic
acid anchoring group (Figure 1). Adding NiCycP to an aqueous
suspension of ZnSe-BF4 resulted in attachment of the co-
catalyst to the particle surface. ATR-IR spectra of ZnSe-BF4
isolated and washed after NiCycP immobilisation showed the
appearance of additional bands at 1048, 965 and 580 cm–1
corresponding to attached NiCycP (Figure S4). Ion-coupled
plasma optical emission spectroscopy (ICP-OES) confirmed that
7.8±0.5 % of the added NiCycP was immobilised
(corresponding to 2 catalyst molecules per QD), whereas the
non-phosphonated analogue Ni(cyclam)Cl2 showed a lower
attachment of only 2.9±0.2 % (Table S1).
The photocatalytic performance of this hybrid assembly was
studied in CO2-saturated water under UV-filtered simulated
solar light irradiation (AM 1.5G, λ >400 nm, 100 mW cm–2)
using ascorbic acid (AA, 0.1 M, pH 5.5) as sacrificial electron
donor. In the absence of
a
co-catalyst, ZnSe-BF4
photocatalytically reduces protons to H2, indicating that
electron transfer to aqueous protons can, indeed, compete
with charge recombination. Figure 3A-B shows that adding
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particles after photocatalysis confirmed the formation of production of liquid fuels such as meVtihewanAroticlle Oannlinde
aggregates which retained a nanocrystalline morphology hydrocarbons.13b In addition, improvedDOCIO: 10p.1r0o3d9/uCc7tSioCn044w29aAs
(Figure S7). Only 13CO was generated when photocatalysis was observed, enabling NiCycP to reach an unprecedented TONCO
performed under 13CO2, confirming CO2 as the sole carbon of 283±23 after 20 h (Figure 4B, Table S4), presumably because
source (Figure S8). Control experiments in the absence of more excited charges are available for the molecular catalyst.
NiCycP yielded only traces of CO; no CO was formed without ICP-OES confirmed that at the optimum MEDA concentration
ZnSe or in the dark (Table S3). Experiments in the absence of (25 µM), the QDs are only partially covered (23 equiv. MEDA
AA gave only negligible amounts of CO and H2, suggesting that adsorbed per QD with approx. 100 surface Zn atoms per QD).
particle self-oxidation is not a major process as previously seen At higher MEDA concentrations where full coverage is
for CdSe.31
expected, CO production was inhibited, suggesting that the
NiCycP catalyst is displaced from the QD surface. UV-vis
spectroscopy showed that MEDA, in addition to enhancing the
Enhanced Selectivity with Partial Ligand Capping
Having demonstrated how co-catalyst immobilisation can product selectivity, improves ZnSe-BF4 particle stability in
accelerate the desired electron transfer kinetics, we sought to solution (Figure 4C-D). In the absence of MEDA, a decrease in
further enhance the product selectivity by decelerating H2 absorbance and strong scattering is observed over time,
evolution from the particle surface. Our previous work has whereas MEDA-containing samples remain unchanged over
shown that capping ligands can inhibit photocatalytic H2 the course of 90 min. Zeta potential measurements confirmed
evolution at CdS quantum dots.24 Here, we exploit this effect that adding MEDA results in an increase in positive particle
for the first time to control the product selectivity in CO2 charge (Table S5), presumably due to protonation of the
reduction. We studied the influence of adding 2- tertiary amine at the employed conditions (pH 5.5). This
(dimethylamino)ethanethiol (MEDA) on the photocatalytic positive charge can stabilise the colloidal suspension by
activity of ZnSe-BF4/NiCycP at
a
catalyst loading coulombic repulsion. The presence of MEDA did not have a
(10 µM NiCycP) where the CO selectivity was low (8.0±0.9 %). significant impact on surface oxidation of the ZnSe-BF4
CO2 photoreduction was performed in the presence of 25, 50 particles as confirmed by XPS measurements (Figure S3). We
or 100 µM MEDA to partially passivate the particle surface and expect suppressed H2 evolution to result from MEDA blocking
thereby lower H2 evolution activity. Figure 4A confirms that Zn sites on the particle surface.
adding MEDA suppresses proton reduction at ZnSe-BF4/NiCycP For ZnSe-BF4/NiCycP/MEDA, the average external quantum
already at low concentrations, while CO evolution was even efficiency for CO production (EQECO) under 400 nm mono-
enhanced at low MEDA concentrations and only suppressed at chromatic light was 3.4±0.3 % (1 mW cm–2, average taken over
higher loadings. At MEDA concentrations >150 µM, overall 6 h; Table S6); H2 evolution showed an EQEH2 of 4.2±0.2 %. This
photocatalytic activity was negligible, demonstrating that fully performance considerably exceeds previous results on
ligand-capped QDs are inactive. At low concentrations (25 photocatalytic CO2 reduction at Ni(cyclam)2+ and derivatives
µM), MEDA therefore increases the selectivity for CO2 (Table 1).14 The highest photocatalytic activity has been
reduction from 8.0±0.9 % to 33.8±1.7 % by selectively previously reported for Ni(cyclam)2+ embedded in a Cu-azurin
suppressing the competing H2 evolution. The generated syngas protein scaffold, achieving a TONCO of 38 and <10 % selectivity
had an ideal H2:CO ratio of 2:1 as required for the industrial
Table 1. Literature comparison of different catalysts for visible-light driven reduction of CO2 to CO in aqueous solution.
Photocatalyst system
Light absorber Catalyst
TONCO
[molCO / molcat
selectivity[a]
[%]
EQECO
[%]
Ref.
]
2+
2+
2+
2+
2+
14a
14e
14d
14g
14f
14b
14c
9c
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
Ni(cyclam)@Cu-azurin
38
2.2
2.1
0.1
n.r.
4.8
5.2
35
130
190
27,000
2,100
20
<10
94
87
13
80
19
71
<20
81
98
n.r.
n.r.
0.14
n.r.[b]
0.06
n.r.
n.r.
n.r
13
n.r.
4+
[Ni(cyclam)]2
Ni(cyclam)2+
Ni(cyclam)2+
Ni(cyclam)2+
NiCycP
RuP/ZrO2
Ru-cyclam-Ni dyad
Ru(bpy)32+/TiO2
Co(cyclam)3+
Ru(dmb)2-Re(CO)3Cl2+ dyad
15c
15f
9b
[Ru(dtb)(bpy)2]2+
CdS
Re(dtb)(CO)3Cl
CODH enzyme
CODH enzyme
n/r
n/r
>90
95
0.01
n.r.
0.28
n.r.
32
RuP/TiO2
CdS
purpurin
2+
9a
Ni(terpyS)2
9d
Fe-p-TMA
NiCycP
120
283±23
ZnSe-BF4/MEDA
34±2
3.4±0.3
this work
[a] selectivity = 100 % × nCO / (nCO + nH2); [b] n.r. = not reported.
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Figure 5. TA spectroscopy of the ZnSe-BF4/NiCycP/MEDA photocatalyst under different conditions: Band gap excitation of ZnSe-BF4/MEDA (A) in the absence of AA produces a
ground state bleach and a positive feature (hole), (B) in the presence of AA produces a long-lived red-shifted bleach (trapped electrons), and (C) in the presence of NiCycP and AA
accelerates recovery of the trap state bleach (Conditions: 0.5 µM aqueous ZnSe-BF4, pH 6.5, with or without 0.1 M AA, with or without 25 µM MEDA, with or without 10 µM NiCycP
under Ar at room temp.; 400 nm excitation, 10 nJ; 450-900 nm probe). (D) Kinetics of the observed trapped electrons derived from exponential fitting († amplitude-weighted
average lifetime, see Figure S9-S11 and Table S7 for details).
2+
with a Ru(bpy)3 sensitiser.14a 94 % selectivity and a TONCO of elucidate the electron transfer kinetics in ZnSe-BF4/NiCycP to
2.2 have been reported for a dinuclear Ni(cyclam)-derivative unravel the mechanism behind its high photocatalytic
2+ 14e
sensitised by Ru(bpy)3
for
.
An EQECO of 0.14 % was reported performance (Figure 5). The TA spectrum of ZnSe-BF4/MEDA in
2+
a
Ni(cyclam)2+/Ru(bpy)3 combination in
a
biphasic water was measured following 400 nm excitation (Figure 5A,
H2O/scCO2 mixture.14d An analogous Co(cyclam)3+ catalyst see Figures S9-S11 for detailed description). The spectra show
immobilised on Ru(bpy)32+-sensitised TiO2 was reported to a ground state bleach at 425 nm, which decays very rapidly
achieve a TONCO of 35 and <20 % selectivity. The best synthetic (τ1 = 0.9±0.2 ps, τ2 = 35±15 ps; see Figure S10, Table S7 for full
non-precious photocatalyst system in water, a Fe porphyrin fitting parameters). We also observe photoinduced
sensitised by a homogeneous organic dye achieved a TONCO of absorptions at 470 nm and ca. 590 nm within 2 ps of
120 and 95 % selectivity after 94 h irradiation, however the excitation. The absorptions at 470 nm and 590 nm have a
organic dye degraded over time with more dye consumed than significantly longer lifetime (470 nm: τ1 = 25±4 ps, τ2 = 409±106
CO produced (TONsensitiser = 0.6),9d whereas in our system the ps; 590 nm: τ1 = 21±6 ps, τ2 = 475±72 ps) than the ground state
ZnSe QDs are stable and achieve a TONQD >5,000 mol CO mol–1 bleach. The bleach feature in such systems typically arises due
QD. A CdS/Ni(terpyridine) hybrid achieved a TONCO of 20 and to state-filling by excited electrons in conduction band states,34
an EQECO of 0.28 %.9a TONs in water comparable to the present while the broad positive signal has been ascribed to trapped
work have only been reported for precious metal-based charge carriers.35 The lifetimes of the bleach and photoinduced
catalysts. A Ru-Re vesicle system achieved a TONCO of 190 and absorption do not match, indicating that different relaxation
showed an excellent 98 % selectivity in aqueous solution at pH pathways are available. This observation is in line with past
6.5 – 7.1 (no EQE reported).15f A homogeneous Ru-Re dyad studies which show the presence of a range of mid-gap states
achieved a TONCO of 130, 72 % selectivity and a remarkable with similar ZnSe materials36 and our expectation of additional
EQECO of 13 % in pH 9.8 aqueous solution.15c In mixed surface states that are likely to be present as a consequence of
organic/aqueous solution, a ReP catalyst immobilised on dye- ligand stripping. In the presence of AA, the positive features at
sensitised TiO2 showed a TONCO of 86 with tuneable selectivity 470 and 590 nm are removed (Figure 5B). This supports their
up to 78 % selectivity.33 Considerably higher TONs have been assignment to trapped charge carriers, specifically holes, and is
achieved using a carbon monoxide dehydrogenase (CODH) consistent with a reductive quenching mechanism. We now
32
enzyme immobilised on Ru-sensitised TiO2 or on CdS observe a broad bleach centred at 550 nm. Previous TA studies
nanorods,9b but these systems showed low quantum yields of ZnSe report the occupation of defect states below the CB
and the enzymes are extremely fragile as well as expensive edge by photoelectrons,35a, 37 with the state filling leading to
and difficult to isolate and purify. Higher performances have an increase in transmission at energies below the band gap.
been achieved with precious-metal-based catalysts for CO2 Therefore, we assign the broad bleach in Figure 5B to trapped
5b
photoreduction to formic acid.5a,
Anchoring dinuclear Ru photoelectrons.
complexes on Ag-loaded TaON allowed for a TON up to 750 Figure 6 shows the first 10 ps following excitation of ZnSe-
and 85 % selectivity in water (0.47 % EQE).15b Replacing TaON BF4/MEDA in the presence of AA. Here we observe loss of the
with graphitic carbon nitride showed enhanced performance initially generated photoinduced absorptions and formation of
(TON >2,000, up to 98 % selectivity, EQE 0.2 %) in water,15a and the trapped photoelectron signal within ca. 1 ps of excitation,
a remarkable TON >33,000 when organic solvents were used indicating that both hole scavenging and electron trapping can
instead.15d
occur on the ultrafast timescales. Hole scavenging rates on the
order of 1012 s–1 and greater have been previously reported for
II-VI nanocrystals elsewhere38 and recent studies have also
observed sub-ps trapping in ZnSe, potentially of hot
electrons37a making accurate measurement of the kinetics of
Ultrafast Transient Absorption (TA) Spectroscopy
TA spectroscopy can offer important insights into the factors
controlling photocatalyst activity through the analysis of
charge carrier dynamics. Here, we use TA spectroscopy to
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these processes beyond the resolution of our spectrometer further demonstrate for the first time that the selectivity for
View Article Online
(400 fs).
CO2 reduction could be increased by suDpOreI:s1s0in.10g3H9/2C7eSvCo0l4u4t2io9nA
In the presence of AA and NiCycP (Figure 5C), recovery of the and enhancing CO generation through blockage of available
bleach centred at 550 nm assigned to trapped electrons is QD surface sites by a capping ligand. The optimised ZnSe-
clearly accelerated, suggesting electron transfer from the trap BF4/NiCycP/MEDA photocatalyst achieved
a performance
to the molecular catalyst (Figure 5D), and further confirming comparable to the best precious metal-based photocatalysts
the assignment of the negative feature to trapped with a TONCO >280 and 3.4 % EQECO, producing CO and H2 in a
photoelectrons. Data shown in Figure 5 was collected under Ar 1:2 ratio, i.e. solar syngas.
and the electron transfer will correspond to the initial
reduction of NiIICycP to NiICycP. CO2 reduction requires the
accumulation of an additional electron on the catalyst making
it important that the initially reduced state is sufficiently long-
lived. In the ps-TA experiments carried out, we are unable to
directly probe NiCycP, a complex that only absorbs very weakly
in the visible region,14b however Figure 5 indicates that under
the conditions employed in the catalysis and TA studies, AA
efficiently scavenges ZnSe photoholes which would minimise
back electron transfer from NiICycP to ZnSe. Fitting of the
bleach recovery is complicated and requires a minimum of a
triexponential function with a small residual signal (17 % for
QD/AA, 10 % for QD/AA/NiCycP) remaining at the longest
timescale we can study (3 ns, Figure S11, Table S7). In similar
systems where complex multi-exponential decay kinetics have
Figure 6. TA spectroscopy of ZnSe-BF4/MEDA (0.5 µM ZnSe-BF4, 25 µM MEDA) in
been observed in the study of charge transfer from QDs, the
amplitude-weighted averaged lifetime has been reported,39
which can provide a simple measure of the average lifetime. In
the presence of NiCycP, a large decrease in the amplitude-
weighted average lifetime from 595 to 186 ps is observed,
giving an approximate electron transfer lifetime to the
catalyst.39 Electron transfer between ZnSe-BF4 and NiCycP is
thus much faster than in previous work using NiCycP and a Ru
dye14b (transfer lifetime of 130 μs) or Ni(cyclamCO2H)2+ on TiO2
(t50% = 1.2 ms).40 Faster charge transfer times have been
reported for other systems, e. g. from CdSe QDs to an
immobilised Re complex (2.3 ps),6a from N-Ta2O5 to an
anchored Ru complex (12 ps)6b and from CuInS2 QDs to an
adsorbed Fe porphyrin (<200 fs).41 The rapid electron transfer
coupled to the observed ultrafast formation (approximately 1
ps) of longer-lived trapped electrons gives an explanation for
the high performance of the presented system.
aqueous solution in the presence of AA (0.1 M, pH 6.5, Ar, room temperature), showing
the first 10 ps following excitation (400 nm).
Experimental Section
Materials. All chemicals were obtained from commercial
sources and used as received. L-ascorbic acid (99 %), N,N-
dimethylformamide (DMF, 99.8 %), octadecene (90 % techn.),
selenium powder (99 %), trimethyloxonium tetrafluoroborate
(96 %) and zinc stearate (purum) were purchased from Sigma-
Aldrich. 2-Dimethylaminoethanethiol hydrochloride (MEDA,
95 %) was purchased from Acros Organics, 1-butanol (99 %)
was purchased from Alfa Aesar. All aqueous experimental
solutions were prepared with distilled water and all aqueous
analytical samples were prepared with ultrapure water (DI
water; Milli-Q®, 18.2 MΩ cm). 13CO2 (>99 atom% 13C) was
7e
purchased from Sigma Aldrich. Ni(cyclam)Cl2 and NiCycP14b
were prepared by literature procedures.
ZnSe-St. Stearate capped ZnSe-QDs were prepared by a
modified literature procedure26 as follows: Zinc stearate (1
mmol), Se powder (1 mmol) and 65 mL octadecene were
added to a 250 mL three-necked flask and degassed for 1.5 h
at 50 °C. The reaction was triggered by raising the temperature
to 300 °C under a N2 atmosphere, resulting in an initially
colourless reaction mixture that turned progressively yellow.
To monitor particle growth, aliquots (100 µL) were taken
periodically, diluted with CHCl3 to 1 mL total volume, filtered
with a syringe filter (Merck Millex-GN, 0.20 µm nylon
membrane) and analysed by UV-vis spectroscopy. After 2 h
10 min (counted from the time when the temperature was
raised above 50 °C) the reaction was stopped by removing the
heating and blowing N2 into the flask. The particles were
precipitated using an acetone/methanol mixture (20:80),
Conclusions
In summary, we have established ZnSe QDs as an inexpensive,
Cd-free and stable photosensitiser for artificial photosynthesis.
A hybrid photocatalyst consisting of ZnSe-BF4 QDs and
immobilised NiCycP reduces aqueous CO2 to CO with high
TONs under visible-light irradiation without using precious
metals. Anchor-free Ni(cyclam)2+ shows a substantially lower
affinity for the QD surface, which is consistent with a much
lower photocatalytic activity. A detailed study of the electron
transfer kinetics using TA spectroscopy revealed ultra-fast
trapping of conduction band electrons, followed by fast and
efficient electron transfer from these long-lived trap states to
the immobilised NiCycP on the ps timescale. These rapid
electron transfer dynamics are thus key to the high
performance of the ZnSe-NiCycP photocatalyst system. We
6 | Chem. Sci., 2017, 00, 1-3
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followed by centrifugation (7,000 rpm, 10 min). The residue concentration (0.5 µM QD-BF4, 10 µM catalyst inV2ie6w mArtLicle0O.1nliMne
DOI: 10.1039/C7SC04429A
was washed with methanol (twice) and butanol and re- aq. AA, pH 6.5). Samples were purged with CO2 for 10 min,
dispersed in CHCl3.
ZnSe-BF4. Ligand-free ZnSe QDs were prepared by reactive by centrifugation (6,500 rpm, 30 min). The supernatant was
ligand removal using a modified literature procedure for CdS discarded and the precipitate was dissolved in mL
stirred for 2 h in the dark before the particles were separated
1
stripping.24, 28 A ZnSe-St solution in CHCl3 was dried in vacuo. concentrated HNO3 (trace metal analysis grade), diluted with
Under a N2 atmosphere, the residue was re-dispersed in a water (1:200 for Zn2+, 1:10 for Ni2+) and analysed by ICP-OES.
mixture of anhydrous CHCl3 (3 mL) and anhydrous DMF (0.2 The catalyst loading was calculated from the relative Ni2+ and
mL). Aliquots of stripping agent (Me3OBF4, 1.0
M
in Zn2+ concentrations in the precipitate. To study immobilised
acetonitrile, typically 2-3 mL) were added slowly until the NiCycP on ZnSe-BF4 by ATR-IR spectroscopy, one drop of QD
particles precipitated. The resulting ligand-free particles were stock solution was dried on a fluorine-doped tin oxide (FTO)-
centrifuged (7,000 rpm, 10 min), dried in air for 1 min, and re- coated glass slide in vacuo and the FTO/ZnSe-BF4 was
dispersed in DMF (2-3 mL). The resulting slightly cloudy incubated with water or aqueous NiCycP (1.0 mM) for 2 h,
solution of ZnSe-BF4 in DMF was further purified by carefully washed with water and dried in vacuo before
centrifugation (7,000 rpm, 10 min) to give a black precipitate, a collecting spectra.
clear yellow solution and a cloudy white phase on top. The Transient absorption (TA) spectroscopy. Femtosecond TA
black precipitate and white phase were removed and the clear spectroscopy was carried out using a PHAROS laser (Light
yellow solution was used for characterisation and Conversion, Ltd) operating at 10 kHz coupled to an ORPHEUS
photocatalytic experiments. Thus-prepared ZnSe-BF4 can be optical parametric amplifier (Light Conversion, Ltd) in tandem
handled in air for hours without decomposition but will with a LYRA harmonic generator (Light Conversion, Ltd) to
gradually degrade over several days in air. To prevent produce the desired wavelength for sample excitation. The
degradation, the ZnSe-BF4 solution was degassed by 4 freeze- pump beam intensity was adjusted with a neutral density filter
pump-thaw cycles and stored under N2 in the dark at 4 °C. The so as to achieve approximately equal photon fluxes at different
mean particle size was determined from TEM images and was wavelengths. Typical pulse energies were on the order of 10
found in good agreement with the average diameter nJ. The pump wavelength was tuned to 400 nm. A portion of
determined from applying the Scherrer equation to the XRD the PHAROS output was also split off to pump a sapphire
pattern (see SI for details).27 To calculate the QD concentration crystal to generate a white light continuum for the probe
in the stock solution, the Zn2+ concentration determined by beam, which provided for spectral observation in the region
ICP-OES, was divided by the number of Zn atoms per QD based 450-900 nm. The probe beam was focused to a spot size of
on the mean particle diameter and the bulk density of ZnSe.
~100 µm diameter on the sample and was overlapped
Photocatalytic CO2 reduction. A ZnSe-BF4 stock solution (144.7 completely by the pump beam. Spectra were acquired with a
µM in DMF, 6.90 µL) and a co-catalyst solution (2.0 mM NiCycP HELIOS transient absorption spectrometer (Ultrafast Systems,
or Ni(cyclam)Cl2 in water, typically 10 µL) were added to a LLC). The time resolution of the setup is ca. 400 fs.
Pyrex glass photoreactor (Chromacol 10-SV, Fisher Scientific) Measurements were performed by randomly stepping the
containing a magnetic stirrer bar. The mixture was diluted with optical delay line and averaging for 1 s at each delay time. 3 to
ascorbic acid (AA, 0.1 M in water, pH adjusted to 6.5 with 5 consecutive scans were collected and aggregated to produce
NaOH) to a total solution volume of 2 mL. The photoreactor each spectrum. Sample solutions were prepared in a similar
was then sealed with a rubber septum and purged with CO2 fashion to those for photocatalysis experiments (0.5 µM QD,
(containing 2 % CH4 as internal standard) for 10 min in the 10 µM NiCycP in 0.1 M aq. AA, pH 6.5) unless otherwise stated,
dark; the solution pH decreased from 6.5 to 5.5 after purging purged with Ar and transferred to a 1 or 2 mm path length
due to saturation with CO2. The photoreactor was then placed quartz cuvette under an inert atmosphere.
in a water bath maintained at 25 °C, stirred and irradiated by a
solar light simulator (Newport Oriel, 100 mW cm–2) equipped
with an air mass 1.5 global (AM 1.5G) filter. IR irradiation was
Conflicts of interest
filtered with a water filter (10 cm path length) and UV
irradiation with a 400 nm cut-off filter (UQG). Product
distribution was quantified through periodic headspace gas
analysis (50 μL) by gas chromatography (see SI for details). For
isotopic labelling, photocatalysis experiments were performed
as described above, but using 13CO2 as the headspace gas (see
SI for details). After 15 h, the photoreactor headspace was
transferred to an evacuated gas IR cell (SpecAc, 10 cm path
length, equipped with KBr windows) and a high-resolution gas-
phase transmission spectrum was collected.
The authors declare no competing financial interests.
Acknowledgements
This work was supported by the Christian Doppler Research
Association (Austrian Federal Ministry of Science, Research
and Economy and the National Foundation for Research,
Technology and Development), the OMV Group (to E.R.), the
World Premier International Research Centre Initiative (WPI),
MEXT, Japan (to K.L.O.), the EPSRC (EP/K006851/1 to A. J. C.)
and the Studienstiftung des Deutschen Volkes (to C. D. S.). We
Analysis of catalyst loading on QDs. Samples were prepared
as described above for photocatalysis experiments, but scaled
up by a factor of 13 for accurate determination of the Ni2+
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thank Miss Anja Schlosser for initial help with ZnSe preparation 10. (a) J. L. White, M. F. Baruch, J. E. Pander IIIV,ieYw.AHrticule, OI.nliCne.
and Dr Chris Amey for recording XPS spectra.
Fortmeyer, J. E. Park, T. Zhang, K. LiaDoO, IJ:.10G.u10,3Y9./CY7aSnC,0T44. 2W9A.
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12888-12935; (b) H. Takeda, C. Cometto, O. Ishitani and M.
Robert, ACS Catal., 2017, , 70-88.
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Chemical Science
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DOI: 10.1039/C7SC04429A
Chemical Science
EDGE ARTICLE
A robust precious metal-free photocatalyst system comprised of ligand-free ZnSe quantum dots and a phosphonic acid-
functionalised Ni(cyclam) catalyst achieves efficient reduction of aqueous CO2 to CO.
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