Surface stoichiometry manipulation enhances solar hydrogen evolution of CdSe quantum dots

Article abstract of DOI:10.1039/c8ta00385h

Surface stoichiometry is a sensitive parameter affecting the decay dynamics of photogenerated hole-electron pairs of QDs. However, the effect of this manipulation on artificial photocatalytic H₂ evolution is unclear. Here, we report that surface stoichiometry manipulation is a facile and feasible approach for enhancing H₂ photogeneration of QDs. In the absence of an external cocatalyst, a decrease in the surface Se ratio of CdSe QDs from ～16.7% to ～4.9% gives a more than 10-fold increase in solar H₂ evolution. Taking Ni(ii) as an external cocatalyst, CdSe QDs with a surface Se ratio of ～4.9% can produce ～1600 ± 151 μmol H₂ gas during 27 h of visible-light irradiation, giving a total turnover number of (1.24 ± 0.12) × 10⁵ on CdSe QDs and an apparent quantum yield of 10.1%, which is about 8 times that of CdSe QDs with a surface Se ratio of ～16.7% under the same conditions. Mechanistic insights obtained by a combination of steady-state and time-resolved spectroscopic techniques indicate that surface stoichiometry exerts a significant influence on the exciton kinetics of CdSe QDs: a higher ratio of surface Se would increase the possibility of exciton recombination through hole trapping, thus depressing the performance of solar H₂ evolution.

Full text of DOI:10.1039/c8ta00385h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Surface stoichiometry manipulation enhances solar hydrogen
evolution of CdSe quantum dots
Mao-Yong Huang,a,b Xu-Bing Li,*,a,b Yu-Ji Gao,a,b Jian Li,a,b Hao-Lin Wu,a,b Li-Ping Zhang,a,b Chen-Ho
Tung,a,b and Li-Zhu Wu*,a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Surface stoichiometry is a sensitive parameter to affect the decay dynamics of photogenerated hole-electron
pairs of QDs. However, this manipulation is unclear towards artificial photocatalytic H2 evolution. Here, we
www.rsc.org/
report that surface stoichiometry manipulation is
a facile and feasible approach to enhance H2
photogeneration of QDs. In the absence of external cocatalyst, decreasing surface Se ratio of CdSe QDs from
~16.7% to ~4.9% gives a more than 10-time increase of solar H2 evolution. Taking Ni(II) as an external
cocatalyst, CdSe QDs with surface Se ratio of ~4.9% can produce ~1600 ± 151 mmol H2 gas during a 27 h
visible-light irradiation, giving a total turnover number of (1.24 ± 0.12) × 105 on CdSe QDs and an apparent
quantum yield of 10.1%, which is about 8-fold to that of CdSe QDs with a surface Se ratio of ~16.7% under
the same conditions. Mechanistic insights obtained by a combination of steady-state and time-resolved
spectroscopic techniques indicate surface stoichiometry exerts significant influence on the exciton kinetics of
CdSe QDs: higher ratio of surface Se would increase the possibility of exciton recombination through hole
trapping, and thus depressing the performance of solar H2 evolution.
parameter to affect the decay dynamics of photoinduced hole-
electron pairs of CdSe QDs.28, 29 In this regard, one may wonder
Introduction
Due to the intrinsic advantages in visible-light absorption,
exciton generation and charge separation, colloidal
semiconductor quantum dots (QDs) have outperformed their
bulk counterparts as well as organic dyes and metal complexes
in photosynthetic hydrogen (H2) evolution.1-12 Coupling with
cocatalysts,13-14 forming hybrid composites,15-18 and
constructing core-shell19,20 or dot-in-rod nanostructures,21
QDs have been used for favored H2 evolution with turnover
number (TON) high up to hundreds of thousands.22-25 Despite
the promises, all of these studies consider QDs as a whole, and
the surface stoichiometry of QDs that influences the relaxation
process of excitons kinetics is unclear towards artificial
photocatalytic H2 evolution.
whether the performance of solar H2 evolution of QD-based
artificial photosystems would be very much changed if the
photogenerated holes are most captured by sacrificial electron
donors and/or the electrons are utilized for proton reduction.
With this in mind, we have used surface stoichiometry
manipulation to monitor solar H2 evolution of QDs under both
photocatalytic and photoelectrochemical conditions. Our results
suggest that when the size and crystalline phase of CdSe QDs are
kept unchanged, mere reduction of the atomic ratio of surface Se
within a certain extent can greatly enhance the performance of
solar H2 evolution, and a maximal rate of H2 photoproduction at
~4.9% surface Se is achieved, giving a TON of (1.24 ± 0.12) × 105 and
an apparent quantum yield (AQY) of ~10.1% under the optimal
conditions, which is about 8-fold to that of CdSe QDs with ~16.7%
surface Se. Mechanistic investigation indicates that the lower ratio
of surface Se can restrain the process of hole trapping, which in
turn suppresses the possibility of non-radiative recombination of
photogenerated hole-electron pairs and significantly enhances solar
H2 production.
In fact, the photoluminescent quantum yield and
photobrightening of QDs, taking CdSe QDs as an example, are
greatly influenced by surface stoichiometry.24 In contrast to their
bulk counterparts, Sfeir et al. had reported that higher ratio of
surface cadmium is able to impact the blinking behavior and
subsequently control the photoluminescent lifetime.25 Peng and co-
workers revealed that surface stoichiometry is
a sensitive
Experimental section
Chemicals
3-Mercaptopropionic acid (MPA, ≥ 99%), thioglycollic acid
(TGA,
≥
99%), selenium powder (∼200 mesh), NiO
50 nm), CdCl2·2.5H2O (≥ 99%), Na2SO4 (≥ 99%)
nanoparticles (
∼
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and Na2SO3 (≥ 99%) were purchased from Sigma-Aldrich. The of CdCl2 to Na2SeSO3 and the time of CdSe growth, as
ultrapure water with 18.2 MΩ cm (Mettler Toledo, FE20, demonstrated in following equations.
China) was used for all experiments. Hydrochloric acid (HCl,
Cd2+ + MPA
Cd(MPA)2+ + 2OH- + SeSO3
®
Cd(MPA)2+
(1)
(2)
(3)
36-38%), sodium hydroxide (NaOH, ≥ 96%) and isopropanol
(IPA, ≥98%) were purchased from Beijing Chemicals. All
chemicals were analytical grade without further purification
unless otherwise noted.
2-
2-
® CdSe(MPA) + H2O + SO4
nCdSe(MPA) ® (CdSe)a(Cdn-aSebMPAn-a) + aMPA
where a is the number of core CdSe and (n-a) and b are the
numbers of surface Cd and Se atom, respectively.
Characterizations
Photocatalytic H2 evolution
UV-vis absorption spectrum was recorded with a Shimadzu
Photocatalytic H2 evolution experiments were performed in 5
mL CdSe QDs solution of volume ratio IPA 50%-water 50% in
the absence and presence of NiCl2·6H2O at room temperature
1601-PC
spectrophotometer.
Photoluminescent
(PL)
measurements were performed using
a
Hitachi 4500
fluorescence spectrophotometer. PL decay was performed by
using a time-resolved fluorescence spectrometer (FLS920,
Ediburgh Instruments). High-resolution transmission electron
microscopy (TEM) was performed by JEM 2100F (operated at
an accelerating voltage of 200 kV). TEM samples were
prepared by dropping QDs solution on the carbon-coated
copper grid and drying in the ambient temperature. X-ray
powder diffraction (XRD) spectra was taken on a Bruker D8
Focus under Cu-Kα radiation at (λ = 1.54056 Å). X-ray
Photoelectron Spectroscopy (XPS) measurements were
performed on an ESCALAB 250 spectrophotometer with Al-Kα
radiation. All pH measurements were made with a Model pHS-
3C meter (Mettler Toledo FE20, China). The generated
photoproduct of H2 was characterized by GC analysis (14B
Shimadzu) using N2 as the carrier gas with a molecular sieve
column (5 Å; 30 m × 0.53 mm) and a thermal conductivity
detector. The elemental composition of CdSe was determined
by inductively coupled plasma atomic emission spectrometer
(ICP-AES, Varian 710-ES, USA). Photoelectrochemical
measurement was carried out by using Zennium
electrochemical workstation (Germany, Zahner Company) in a
three-electrode setup with the working electrode of the
sample film electrode, counter electrode of platinum disk, and
reference electrode of Ag/AgCl (3.0 M KCl). The light source
was a 100 W Xe-lamp with a UV cut-off filter unless otherwise
stated. Prior to measurement, the area of the electrode was
fixed by insulating cement and the exposed area was ~0.64
cm2.
under visible-light irradiation (λ = 410 nm). The mixed solution
was adjusted to pH 5.0 ± 0.2 by using NaOH or HCl aqueous
solution and degassed by bubbling argon for 30 min. Then, 0.6
mL high purity CH4 was injected as the internal standard for
quantitative GC analysis. The sample was irradiated under a 3
W LED lamp (λ = 410 nm) with water-cooling equipment to
keep at room temperature. The generated H2 gas was
quantified by GC analysis. After a certain time of irradiation,
0.6 mL of the mixed gas was extracted from the sample tube
and injected into GC immediately. The response factor for
H2/CH4 was about 3.82 under the experimental condition.
Photoelectrochemical measurements
T
he working electrode of CdSe QDs was prepared as following:
the stock solution (30 mL) of CdSe QDs was concentrated to a
volume of 3 mL, centrifuged and discarded the supernate after
adding adaptable mount of hydrochloric acid to remove the
passivate ligands. Then, the ligands free CdSe QDs were
redispersed into aqueous solution of thioglycollic acid (TGA) at
pH 11. Lastly, a mesoporous NiO/FTO electrode was immersed
into the above mentioned solution for two hours for the
binding of CdSe QDs on the surface of NiO electrode.
Combining with the counter electrode of Pt and the reference
electrode of Ag/AgCl (3.0 M KCl), the amperometric I-t curves
were recorded under the irradiation of a Xe-lamp with a UV
cut-off filter (~100 mW cm-2) for 30 s light-on-off cycles.
Results and discussion
Synthesis of MPA-CdSe QDs
Water-soluble CdSe QDs with varied surface stoichiometry were
prepared according to the reported method with slight
modification.30 The size, morphology and structure of the obtained
CdSe QDs are characterized by UV-vis absorption spectrum, X-ray
diffraction (XRD), and high-resolution transmission electron
microscope (TEM), respectively. As shown in Figure 1a, the first
excitonic peaks of CdSe QDs with surface Se varied from ∼16.7% to
∼2.1% were around 440 nm, indicating the similar size distribution
of the as-synthesized CdSe QDs. CdSe QDs with different surface
compositions exhibited three representative diffractions at about
12.7°, 21.0°, and 24.8°, belonging to the lattice planes of (1 1 1), (2 2
0) and (3 1 1) of CdSe QDs, respectively (Figure 1b). Evidently, the
structure of obtained CdSe QDs were zinc blende (International
Centre for Diffraction Data, JCPDS Card No. 19-0191).33 Due to no
apparent peak shift, the structure of CdSe skeleton framework was
3-Mercaptopropionic acid (MPA) stabilized CdSe QDs were
prepared according to previous reports with necessary
modifications.30 Briefly, CdCl2·2.5H2O (92 mg) and MPA (52 μL)
were dissolved with 380 mL ultra-pure water in a three-neck
round-bottom flask. Then, the mixture solution was adjusted
to pH 11 with 10 M NaOH and was de-aerated with high-purity
Ar bubbling for 30 min. And then, 20 mL newly prepared
Na2SeSO3 solution (5 × 10-3 M) was injected and further de-
aerated with high-purity Ar bubbling for another 30 min.
Finally, the colourless solution was heated and refluxed for the
growth of CdSe QDs. Aliquots of the reaction solution were
taken out at regular intervals for UV-vis absorption and PL
emission characterizations. CdSe QDs with different surface
stoichiometry were prepared by controlling the precursor ratio
2 | J. Name., 2012, 00, 1-3
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retained with decreasing surface Se from ∼16.7% to ∼2.1%. The precursor, molar ratio of Cd/Se in the as-synthesized CdSe QDs
slightly narrower peak width at half height (PWH) of lattice plane (1 varied in a similar trend. By quantifying the surface atomic
1 1) of CdSe QDs suggested the better degree of crystallinity of composition, the percentage of Cd increased from 37.0% to
CdSe QDs with lower surface Se. Dynamic light scattering (DLS) was 47.0%, while surface Se decreased from 16.7% to 2.1%. The
then used to provide the size distribution of CdSe QDs in aqueous surface S derived from surface ligands of thiols increased from
solution. As shown in Figure S1 and Table S1, CdSe QDs with 46.3% to 50.9%, which was in line with the variation trend of
∼
∼
∼
∼
∼
∼
different stoichiometry gave very uniformed hydrodynamic size surface Cd. Hence, we could claim that surface stoichiometry of
distribution around 2.6 nm, which was larger than the real particle CdSe QDs had been successfully modified, while maintaining
size due to the capping of surface ligands.31,32 High-resolution TEM original size distribution and structure.
measurement of CdSe QDs with different surface compositions
confirmed monodispersed QDs with a diameter around 2.0 nm
(Figure 1c, outlined by white circles). TEM results manifested that
CdSe QDs with lower surface Se ratio had better crystallinity, well
catered with XRD results and reported results.29
Figure 2. XPS spectra of CdSe QDs with surface Se of ~4.9%. (a) Full XPS
spectrum. (b) Cd 3d. (c) Se 3d. (d) S 2p. The black dotted lines
represented raw XPS data, blue dashed lines are Gaussian curve fits at
energies corresponding to core atoms, and red dashed lines were
Gaussian curve fits at energies corresponding to surface atoms.
Next, the influence of surface stoichiometry on solar H2 evolution
of CdSe QDs was examined. To our surprise, CdSe QDs with
different surface compositions exhibited distinct performances of
solar H2 production, taking 410 nm LEDs as light source and
isopropanol (IPA) as sacrificial reagent, see details in experimental
Figure 1. (a) UV-vis spectra of CdSe QDs with different surface Se% (b) and section. Initially, the photocatalytic H2 evolution activity of CdSe
the corresponding powder XRD patterns. (c) High-resolution TEM images of QDs was significantly enhanced along with decreasing the atomic
CdSe QDs with different surface stoichiometry: (i) 16.7% surface Se; (ii) 13.5% ratio of surface Se from
∼
16.7% to
mol) during 5.0 h irradiation, more than 10 times
higher than that of CdSe counterparts with 16.7% surface Se
(Figure 3a). However, further decreasing the ratio of surface Se to
2.1%, the rate of photocatalytic H2 evolution dramatically declined.
∼4.9%, and reached a maximum
surface Se; (iii) 9.1% surface Se; (iv) 4.9% surface Se; (v) 3.4% surface Se; and of
(vi) 2.1% surface Se. Scale bar represents 2.0 nm.
∼1.88 mL (∼84 m
∼
Inductively coupled plasma-atomic emission spectrometry (ICP-
∼
AES) and X-ray photoelectron spectroscopy (XPS) were carried out The decreased H2 evolution was probably due to the blocked active
to provide surface composition details of CdSe QDs. As shown in site Cd for proton reduction by the increased amount of surface
Table S2, the elemental composition of CdSe QDs had been ligands (MPA), which inhibited the penetration of substrate
successfully controlled by gradually increasing the molar ratio of molecules across QD/water interface.41,42 This assumption was well
cadmium (Cd) to selenium (Se) precursor. High-resolution XPS manifested by the facts, 1) H2 photogeneration of CdSe QDs with
spectra of Cd 3d, Se 3d and S 2p of obtained CdSe QDs are shown in lowest surface Se ratio could be significantly enhanced by removing
Figure 2. Clearly, Cd 3d had two distinct peaks with a binding energy surface MPA ligands, and 2) the photocatalytic activity of CdSe QDs
of 405.1 eV and 411.9 eV (Figure 2b), respectively. The binding capped with other organic ligands (i.e. EDTA or TGA) showed much
energies of Se 3d 5/2 and 3/2 were 53.5 eV and 55.1 eV, and the lower rate of H2 evolution (Figure S2).
A series of control
unique binding energies of S 2p 3/2 and 1/2 were 161.6 eV and experiments demonstrated that the system could not evolve H2
162.2 eV, respectively (Figure 2c and 2d).33,34 The atomic surface under dark, sacrificial reagent and QDs were necessary for efficient
compositions of CdSe QDs were shown in Table 1, see detailed raw H2 evolution (Figure S3).
data in Table S3. With increasing the original ratio of Cd to Se
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To facilitate proton reduction, nickel salts (NiCl2), which had been the Ni2+ concentration from 2.1 × 10-5 mol L-1 to 2.1 × 10-4 mol L-1
reported to couple with QDs in situ to form hybrid (Figure 3c). However, further increasing the amount of cocatalyst to
photocatalysts,35,36 were introduced to evaluate the influence of 8.4 × 10-4 mol L-1 led to decline of H2 evolution rates. As shown in
surface composition on the performance of H2 photogeneration. As Figure 3d, increasing the concentration of CdSe QDs from 0.1 × 10-5
presented in Figure 3b, introduction of Ni2+ salts to work as mol L-1 to 1.5 × 10-5 mol L-1, the activity of CdSe QDs with
∼4.9%
cocatalyst gave similar trend of solar H2 production with that in the surface Se and Ni2+ (2.1 × 10-4 mol L-1) was dramatically improved
absence of nickel species (vide ante; Figure 3a). Importantly, CdSe for solar H2 evolution. Further increasing CdSe QDs to 5.9 × 10-5 mol
QDs with
photocatalytic H2 evolution. During 5.0 h light irradiation, a total solar H2 production was CdSe QDs (
volume of 13.7 mL (~610
mol) H2 gas was evolved for CdSe QDs mol L-1 and Ni2+ of 2.1 × 10-4 mol L-1. Under the optimal condition,
with 4.9% surface Se, about 8-fold to 16.7% counterparts. 5.0 mL solution could produce 36 ± 3.4 mL (1600 ± 150 mol) H2
∼4.9% surface Se gave the highest activity of
L
-1 showed negligible variation. As a result, the optimal condition of
∼
4.9% surface Se) of 1.5 × 10-5
∼
m
∼
∼
∼
m
Introduction of other cocatalysts, such as Pt nanoparticles, Co2+ during 27 h visible-light irradiation, giving a TON of (1.24 ± 0.12) ×
salts, and Co(dmgH)2pyCl complex, could also enhance solar H2 105 based on CdSe QDs (Figure 4a). By using 410 nm LEDs as
evolution of QDs with similar tendency (Figure S4). To exclude the monochromatic light source, the apparent quantum yield (AQY) was
varied solar H2 evolution caused by different loading amount of determined to be
∼10.1%, which is about 8-fold to that of CdSe QDs
cocatalyst, the weight content of nickel species incorporated into with
QDs was investigated by XPS after irradiation. The almost same
loading amount of nickel species on QDs (Figure 5a and Table S4)
indicated that the varied solar H2 evolution was hardly determined
by the varied proton reduction sites.
∼16.7% surface Se ratio under the same conditions.
Figure 4. (a) Solar H2 evolution of CdSe QDs with 4.9% surface Se
under visible light (λ = 410 nm). (b) The photocurrent density of CdSe
QDs photocathodes with different surface Se ratio under on-off visible
light (λ > 400 nm). Error bars represent mean ± s.d. of at least three
independent experiments.
The performance of photoelectrochemical (PEC) H2 evolution of
CdSe QDs could also be enhanced by surface stoichiometry
manipulation. When similar amounts of CdSe QDs were loaded on
the surface of NiO/FTO electrode to construct a photocathode,
which was confirmed by diffuse transmission spectrum (Figure S5),
the photocurrent was significantly improved from -48 μA cm-2 to -
Figure 3. Solar H2 evolution under visible light (410 nm). (a) and (b) CdSe 100 μA cm-2 only with decreasing the ratio of surface Se from
QDs in the absence and presence of NiCl2·6H2O (2.1 × 10-4 mol L-1).
Investigating the influence of NiCl2·6H2O (c) and CdSe QDs (surface Se ~4.9%) ratio of surface Se to
concentration (d) on solar H2 evolution at pH 5.0 ± 0.2. Error bars represent cm-2. Clearly, simply manipulating the surface stoichiometry could
∼16.7% to
∼
4.9% (Figure 4b and Figure S6). Further decreasing the
∼2.1% led to decline of photocurrent to -55 μA
mean ± s.d. of at least three independent experiments.
improve the PEC H2 evolution performance of CdSe QDs based
photocathodes. More importantly, the variation tendency was
To investigate the optimal H2 evolution conditions of CdSe QDs highly consistent with that of solar H2 production in solution,
with 4.9% surface Se, we examined the influence of concentrations further confirming the vital effect of surface stoichiometry
of cocatalyst (Ni2+) and QDs. The rate of H2 evolution increased with manipulation on H2 production of QD-based photosystems.
Table 1. Surface atomic compositions of CdSe QDs from ICP-AES and XPS.
Surface (%)
CdSe
Cd/Se1
Cd/Se
Cd/S
S/Se
Cds/Cdi
Ses/Sei
Ss/Si
Cd
S
Se
a
b
c
d
e
f
2.01
3.46
4.31
6.56
7.29
8.57
2.95
4.60
5.65
9.07
9.45
10.2
0.83
0.90
0.90
1.00
0.89
0.74
3.55
5.14
6.25
9.05
10.6
13.8
0.72
0.67
0.42
0.86
1.23
1.50
0.96
0.77
0.83
0.71
0.77
0.63
0.83
0.91
0.47
0.80
1.31
0.95
37.0
46.3
16.7
38.9
42.8
44.8
46.1
47.0
47.6
48.1
49.3
50.5
50.9
13.5
9.1
4.9
3.4
2.1
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1Cd/Se ratio was measured by ICP-AES. Cd-S-Se ratio was obtained from XPS results. Surface-to-inner ratios of Cd, S, and Se, Cds/Cdi, Ss/Si and Ses/Sei, were
calculated from the integrated peak area fitted by Cd, S and Se signals. Cd%, S% and Se% represented the surface percentage of CdSe QDs.
QDs with a higher surface Se stoichiometry (> 4.9%), which provides
an external pathway of non-radiative annihilation of exciton pairs.
This claim was further confirmed by time-resolved spectroscopic
measurements. The time-resolved luminescent decays of CdSe QDs
with different surface Se ratio were measured. As shown in Figure
6c, the shorter lifetime channel decreased with higher ratio of
surface Se, see details in Table 2. According to reported results,28
the short lifetime channel were induced by excess Se sites on the
surface, which would reduce the quantum yield of radiative decay
Figure 5. (a) Band gap (eV) variation of CdSe QDs with different surface
of excitons by hole trapping at Se sites. On the other hand, the rate
of electron transfer from QDs to cocatalysts almost kept unchanged
(Figure S7 and Table 2), further indicating that the process of
interfacial electron transfer was not the main factor in determining
the performance of solar H2 evolution during the process of surface
stoichiometry variation here.
Se and cocatalyst loading amount of on CdSe QDs with different
surface stoichiometry. (b) The relative variations of surface Cd and Se
ratios versus the change of surface stoichiometry.
To get insights into the surface stoichiometry effect, the band
gap of CdSe QDs with varied surface stoichiometry, surface Se from
16.7% to 2.1%, was determined (Figure 5a and Table S5). The
similar bandgap alignment indicated that similar light absorption of
QDs and driving forces of redox reactions for proton reduction and
sacrificial reagent oxidation. As the ratio of surface Se was
systematically tuned from ∼16.7% to ∼2.1%, however, the change
of surface Se was much more significant than that of Cd (Figure 5b).
It has been reported that the majority of hole traps originate from
surface Se sp3-hybridized dangling bond orbitals.38-40 Conversely,
the conduction band levels are primarily made up from the metal
Cd s orbitals, and the empty surface metal ion s orbitals are
therefore intrinsic electron traps. From the performance of solar H2
evolution of CdSe QDs with varied surface stoichiometry, it seemed
that the kinetics of photogenerated electrons were not the key, but
the kinetics of photogenerated holes were dominant.
In order to confirm our speculation, steady-state luminescent
spectrum and time-resolved luminescent decay were performed.
Distinct quenching of band-gap emission and growth of trap-state
emission were clearly observed from CdSe QDs with higher surface
Se ratio (Figure 6a). As shown in Figure 6b, the contribution of trap-
state emission increased from 0.2% to nearly 80% with increasing
Figure 6. (a) The PL spectra of CdSe QDs with different surface composition
(400 nm excitation). (b) The ratio of ITrap/(IBand + ITrap) of CdSe QDs with
different surface compositions. (c) Time-resolved PL decay of CdSe QDs with
different surface Se ratio (405 nm laser excitation). (d) The PL quenching
degree of CdSe QDs with different surface stoichiometry by adding the same
the ratio of surface from ∼2.1% to ∼16.7%. It is reported that the
band-edge emission could be quenched by anion-rich QDs due to
the formation of mid-gap surface states.38,39,43 That is to say, hole
traps, originated from surface Se, become more apparent for CdSe
amount of IPA. Error bars represent mean
independent experiments.
± s.d. of at least three
Table 2. Kinetics of photogenerated excitons of CdSe QDs with different surface Se ratios.
Surface Se (%)
16.7
13.5
9.1
4.9
3.4
2.1
τ1
(
Rel.%)
0.35 ns (13.8)
2.98 × 1011
0.52 ns (13.9)
3.34 × 1011
0.59 ns (7.1)
3.19 × 1011
0.62 ns (6.0)
2.81 × 1011
0.85 ns (5.2)
3.13 × 1011
0.91 ns (4.0)
3.29 × 1011
KET (M-1 s-1)
KET: the rate of electron transfer to cocatalyst, the value is determined by employing Ni2+ as the external cocatalysts with the same concentration of QDs
(surface Se from 16.7% to 2.1%); τ1: the short luminescent lifetime channel of bare CdSe QDs; Rel.%: the relative percentage of τ1 which can be ascribed to
hole trapping.
As sacrificial reagent could effectively quench the emission of indicating a higher percentage of photogenerated holes were
QDs through hole consumption,16,27 the luminescent quenching consumed by sacrificial reagent. Then, the decreased quenching
experiments of CdSe QDs with varied surface stoichiometry by IPA degree of QDs with a surface Se ratio of ∼2.1% suggested the lower
were compared under identical condition. As shown in Figure 6d, efficiency of hole transfer from QDs to IPA in solution, possibly due
the quenching degree of CdSe QDs increased from 0.44 to 0.64 to the higher coverage of organic ligands (MPA) on Cd-rich QDs
when the surface atomic ratio of Se reduced from
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∼16.7% to
∼
4.9%, leading to the inefficient interaction between QDs and sacrificial
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reagent.42,43 These experimental results reveal that merely changing Unlike the state-of-the-art methods, we have explored the
the ratio of surface Se of CdSe QDs in the range of 16.7% to 2.1% influence of surface stoichiometry on solar H2 evolution efficiency
mainly influences the kinetics of photogenerated holes. The trend of QDs system. The experimental results confirmed that surface
of hole transfer from CdSe QDs to IPA well matches with the stoichiometry had a significantly effect on the exciton kinetics,
variation of solar H2 evolution (vide ante; Figure 3a).
especially holes, of CdSe QDs. With an optimal surface Se-to-Cd
ratio, both photocatalytic and PEC H2 evolution of CdSe QDs can be
significantly enhanced. The main reason of enhanced solar H2
evolution is a result of depressed hole trapping at mid-edge traps.
Our findings imply that surface stoichiometry manipulation of
colloidal QDs is an effective and feasible pathway to improve the
efficiency of solar-to-fuel conversion of QD-based artificial
photosystems.
Conflicts of interests
The authors declare no competing financial interest
.
Figure 7. The interfacial band diagram of this photosystem to explain the
mechanism of solar H2 evolution.
ACKNOWLEDGMENT
We are grateful for financial support from the Ministry of
Science and Technology of China (2014CB239402 and
2017YFA0206903), the National Science Foundation of China
(91427303, 21390404, 21603248 and 51373193), the Strategic
Priority Research Program of the Chinese Academy of Science
(XDB17000000), Key Research Program of Frontier Science of
the Chinese Academy of Sciences (QYZDY-SSW-JSCO29), and
the Youth Innovation Promotion Association of Chinese
Academy of Sciences (2018031).
On the basis of the above mentioned observations, the interfacial
band diagram, which explains the mechanism of solar H2 evolution,
can be described in Figure 7. Under visible light irradiation, the
photogenerated electrons in the conduction band of CdSe QDs
transfer to cocatalysts, such as nickel species, for reducing proton
into molecular hydrogen. Simultaneously, holes in the valence band
oxidize sacrificial reagent (IPA) to regenerate ground-state QDs.
With increasing the surface Se ratio of CdSe QDs, the rate of
radiative decay and electron transfer to cocatalyst are nearly
unchanged, but on the contrary, the defects of trapping holes are
dramatically raised, leading to the increase of hole trapping and
non-radiative recombination of electron-hole pairs. The increased
percentage of hole trapping would reduce the process of interfacial
hole transfer from QDs to sacrificial reagent, which in turn greatly
influences the activity of photocatalytic/PEC H2 evolution (Scheme
1). The higher surface Se ratio causes a larger quantity and a wider
distribution of hole trapping defects on QD surface, which acts as
the non-radiative recombination center of photogenerated excitons
and thus decreases the possibility of hole transfer from CdSe QDs to
sacrificial reagent. As a result, the overall efficiency of solar H2
evolution of QD-based photosystems is greatly decreased.
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