Visible light catalysis-assisted assembly of Nih-QD hollow nanospheres in situ via hydrogen bubbles

Article abstract of DOI:10.1021/ja5047236

Hollow spheres are one of the most promising micro-/nanostructures because of their unique performance in diverse applications. Templates, surfactants, and structure-directing agents are often used to control the sizes and morphologies of hollow spheres. In this Article, we describe a simple method based on visible light catalysis for preparing hollow nanospheres from CdE (E = Te, Se, and S) quantum dots (QDs) and nickel (Ni²⁺) salts in aqueous media. In contrast to the well-developed traditional approaches, the hollow nanospheres of QDs are formed in situ by the photogeneration of hydrogen (H₂) gas bubbles at room temperature. Each component, that is, the QDs, metal ions, ascorbic acid (H₂A), and visible light, is essential for the formation of hollow nanospheres. The quality of the hollow nanospheres depends on the pH, metal ions, and wavelength and intensity of visible light used. Of the various metal ions investigated, including Cu⁺, Cu²⁺, Fe²⁺, Fe³⁺, Ni²⁺, Mn²⁺, RuCl ₅^2-, Ag⁺, and PtCl₄^2-, Ni²⁺ ions showed the best ability to generate H₂ and hollow-structured nanospheres under visible light irradiation. The average diameter and shell thickness of the nanospheres ranged from 10 to 20 nm and from 3 to 6 nm, respectively, which are values rarely reported in the literature. Studies using high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), inductively coupled plasma-mass spectroscopy (ICP-AES), and steady-state and time-resolved spectroscopy revealed the chemical nature of the hollow nanospheres. Additionally, the hollow-structured nanospheres exhibit excellent photocatalytic activity and stability for the generation of H₂ with a rate constant of 21 μmol h^-1 mg^-1 and a turnover number (TON) of 137 500 or 30 250 for CdTe QDs or nickel, respectively, under visible light irradiation for 42 h.

Full text of DOI:10.1021/ja5047236

Article
pubs.acs.org/JACS
Visible Light Catalysis-Assisted Assembly of Ni ‑QD Hollow
h
Nanospheres in Situ via Hydrogen Bubbles
Zhi-Jun Li, Xiang-Bing Fan, Xu-Bing Li, Jia-Xin Li, Chen Ye, Jiu-Ju Wang, Shan Yu, Cheng-Bo Li,
Yu-Ji Gao, Qing-Yuan Meng, Chen-Ho Tung, and Li-Zhu Wu*
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry &
University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
*
S Supporting Information
ABSTRACT: Hollow spheres are one of the most promising micro-/nanostructures because of their unique performance in
diverse applications. Templates, surfactants, and structure-directing agents are often used to control the sizes and morphologies
of hollow spheres. In this Article, we describe a simple method based on visible light catalysis for preparing hollow nanospheres
2+
from CdE (E = Te, Se, and S) quantum dots (QDs) and nickel (Ni ) salts in aqueous media. In contrast to the well-developed
traditional approaches, the hollow nanospheres of QDs are formed in situ by the photogeneration of hydrogen (H ) gas bubbles
2
at room temperature. Each component, that is, the QDs, metal ions, ascorbic acid (H A), and visible light, is essential for the
2
formation of hollow nanospheres. The quality of the hollow nanospheres depends on the pH, metal ions, and wavelength and
+
2+
2+
3+
2+
2+
2−
+
intensity of visible light used. Of the various metal ions investigated, including Cu , Cu , Fe , Fe , Ni , Mn , RuCl , Ag ,
5
and PtCl42 , Ni ions showed the best ability to generate H and hollow-structured nanospheres under visible light irradiation.
−
2+
2
The average diameter and shell thickness of the nanospheres ranged from 10 to 20 nm and from 3 to 6 nm, respectively, which
are values rarely reported in the literature. Studies using high-resolution transmission electron microscopy (HRTEM), X-ray
photoelectron spectroscopy (XPS), X-ray diffraction (XRD), inductively coupled plasma-mass spectroscopy (ICP-AES), and
steady-state and time-resolved spectroscopy revealed the chemical nature of the hollow nanospheres. Additionally, the hollow-
structured nanospheres exhibit excellent photocatalytic activity and stability for the generation of H with a rate constant of 21
2
μmol h− mg and a turnover number (TON) of 137 500 or 30 250 for CdTe QDs or nickel, respectively, under visible light
1
−1
irradiation for 42 h.
7
−9
and inorganic hollow spheres,10−12 is promising for
INTRODUCTION
vesicles,
■
light-harvesting and charge transfer in artificial photosynthetic
systems and can be accessible through versatile fabrication
techniques. In general, organic micelles and vesicles are
constructed from surfactants or block copolymers that possess
intrinsic differences in the chemical potentials of the hydro-
phobic/hydrophilic regions of the surfactant or polymer
In natural photosynthetic systems, light-absorbing chlorophylls
are well organized in a specific geometry to form light-
harvesting assemblies, and the primary light absorption and
subsequent transfer of electronic excitation energy and
electrons within these assemblies toward the reaction center
are extremely important for the conversion of sunlight into
13
1
−3
blocks. Conversely, inorganic hollow spheres are often
prepared using various sacrificial templates, including poly-
styrene spheres, silica spheres, resin spheres, vesicles, liquid
chemical energy.
To duplicate the architecture and
functionality of natural photosynthetic systems, scientists have
been devoting considerable effort to developing artificial
systems and exploring the energy and electron transfer
functions of natural solar energy conversion systems. An
abundance of accumulated evidence has demonstrated that the6
organization of elegant architectures, such as organic micelles,
13−16
droplets, and microemulsion droplets.
The most com-
4
,5
Received: January 4, 2014
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja5047236 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
monly used method is a multistep process, in which the
removal of the core template or two immiscible liquid phases
using calcination or dissolution is technically complicated.
irradiation cycle. The current study clearly not only offers an
attractive method for assembling advanced hollow QD
architectures similar to those found in natural photosynthetic
systems, but also provides robust artificial photocatalysts to
realize the practical production of solar fuels.
17
The light-assisted method has recently become a new
strategy for fabricating unique structured nanomaterials. For
18
example, Kotov et al. reported a light-controlled self-assembly
method for preparing twisted ribbons using CdTe nano-
RESULTS AND DISCUSSION
19
■
particles. Suib et al. developed a self-templating method that
Ni -CdTe Hollow Nanosphere Preparation and Char-
uses UV-light to synthesize γ-manganese oxide (γ-MnO2),
h
acterization. The Ni -CdTe hollow nanospheres were
cobalt oxyhydroxide (CoOOH), and cerium oxide (CeO )
h
2
20
synthesized in an aqueous solution of 3-mercaptopropionic
hierarchical nanoarchitectures. Fan et al. employed densely
packed optically active porphyrins to prepare well-defined
hollow platinum (Pt) nanostructures. Here, we present a simple
approach for creating hollow-structured nanospheres of
quantum dots (QDs) using visible light catalysis.
acid (MPA)-stabilized CdTe QDs, NiCl ·6H O, and H A at
2
2
2
room temperature. Because MPA-capped CdTe QDs (Figures
S1,S2, see details in the Supporting Information) are extremely
sensitive to the pH of the solution, we kept the initial pH value
36
of the solution approximately to the pK of H A at 4.65.
Semiconductor QDs have attracted widespread attention
because their nanoscale physical properties are quite different
a
2
During irradiation (λ > 400 nm), the solution became colorless
with the formation of precipitates. However, no change was
observed when the same solution was kept in the dark.
Evidently, the precipitates resulted from visible light irradiation,
and the photochemical reaction occurred on the surface of the
MPA-CdTe QDs rather than in the solution.
21,22
from those of bulk materials.
Because of their light
absorption and large extinction coefficients over a broad
spectral range, rich surface binding properties, and high surface-
to-volume ratios, QDs are ideal for light-harvesting and charge
23−34
separation applications.
QDs are expected to be a versatile
Examination of the precipitates by HRTEM revealed that
uniform hollow nanospheres were formed at a large scale after
irradiation (Figure 1a−c). Unlike the MPA-CdTe QDs (Figure
1d), the average size of the hollow structure ranged from 10 to
20 nm, and its shell thickness ranged from 3 to 6 nm. However,
the typical lattice plane distance of 3.52 Å (Figure 1c) is
type of building block for creating hollow structures such as
those found in natural light-harvesting antenna complexes.
However, few examples of these QD assemblies have been
reported, and the salient features associated with their
assembly are unclear.
1
2
In this Article, we demonstrate that hollow-structured
nanospheres of QDs can be easily synthesized in a one-pot,
visible light-catalyzed reaction under ambient conditions. In
contrast to well-developed methods that use polymer latex,
3
7,38
identical to that of CdTe QDs
(Figure 1d). The well-
defined diffraction spots exhibited three obvious diffraction
1
3,15,16
silica, and emulsion/microemulsion as templates,
the
hollow nanospheres are formed in situ via H gas bubbles.
2
Upon visible light irradiation of an aqueous solution that
2
+
contains QDs, ascorbic acid (H A), and nickel (Ni ) salts at
2
pH 4.65, the QDs are capable of assembling around the gas−
liquid interface to form hollow-structured nanospheres. The
2
+
absence of any of the components, that is, QDs, Ni ions, H A,
2
or visible light, resulted in no appreciable Ni -QDs assembly.
h
Furthermore, the quality of the hollow nanospheres depended
on the wavelength and intensity of the visible light used.
+
2+
2+
3+
2+
Various metal ions, such as Cu , Cu , Fe , Fe , Mn ,
RuCl52 , Ag , and PtCl , exhibited different behaviors in the
−
+
2−
4
formation of hollow-structured nanospheres and in the
photocatalytic generation of H . Of the metal ions studied,
2
2+
Ni ions possessed the best ability to generate H and hollow-
2
structured nanospheres under visible light irradiation. The
average diameter and shell thickness of the nanospheres ranged
from 10−20 nm and 3−6 nm, respectively. These types of
small-sized hollow nanospheres are rarely observed in the
3
5
literature. Notably, the nanospheres exhibited excellent
photocatalytic activity and stability for H generation. Here,
2
the QDs serve as antenna chromophores to harvest light, and
2+
earth-abundant Ni -centered complexes formed in situ with
2
−
2−
dangling bonds (Te , S ) on the surface of the QDs serve as
reaction centers to capture the photoexcited electrons of the
QDs for H2 generation. Consequently, Ni -CdTe hollow
h
nanospheres, for example, enable the photogeneration of H2
from an aqueous H A solution with a rate constant of 21 μmol
2
−1
−1
h
mg and a TON of 75 000 or 16 500 for CdTe QDs or
Figure 1. HRTEM images of Ni -CdTe hollow nanospheres (a−c)
h
nickel (referring to 0.16 wt % nickel, ICP-AES), respectively,
under visible light irradiation for 21 h. No obvious variation was
(
scale bars: a, 100 nm; b, 20 nm; c, 10 nm), and their SAED pattern
(e) with three diffraction rings; HRTEM image of the MPA-CdTe
observed in the rate of H formation in one continuous
QDs (d), and their SAED pattern (f) with three diffraction rings.
2
B
dx.doi.org/10.1021/ja5047236 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
rings corresponding to the (111), (220), and (311) lattice
planes, respectively (Figure 1e), which is consistent with typical
CdTe QDs (Figure 1f). All of the results suggested that the
crystalline structure of the shell of the nanospheres is composed
of CdTe QDs. The as-obtained hollow nanospheres were
generated in situ via the assembly of small CdTe QDs rather
1
4,16
than through the Ostwald ripening process.
The in situ assembly of the hollow-structured nanospheres
was further characterized using XRD. The average size of the
corresponding lattice plane (111) was estimated to be 3.4 nm
39
using the Debye−Scherrer formula, and this value is similar to
that of the MPA-CdTe QDs determined using HRTEM
(
Supporting Information Figure S2). Although the shapes and
widths of the three peaks shown in Figure 2 negligibly changed
Figure 3. S 2p XPS spectra recorded from (a) Ni -CdTe hollow
h
nanospheres and from (b) MPA-CdTe QDs; the vertical lines are
guides for the positions of S 2p1/2 and S 2p3/2 from MPA-Cd and CdS,
respectively.
weak, a Ni(II) 2p3/2 4peak at ca. 854.8 eV and a Ni(II) 2p
1/2
5,46
peak at ca. 871.4 eV
were clearly observed. Furthermore,
the amount of Ni was determined to be 0.16 wt % using the
ICP-AES method (Supporting Information Table S1).
Following the same method, Ni -CdSe and Ni -CdS hollow-
h
h
structured nanospheres were successfully synthesized using
MPA-CdSe QDs and MPA-CdS QDs, respectively (Supporting
Information Figure S4). The average size (10−20 nm) and
shell thickness (3−6 nm) of the obtained hollow structures are
difficult to realize using external templates. With unique
structural characteristics, the hollow-structured nanospheres
of the QDs could provide a rich platform for exploring new
functional materials.
Figure 2. XRD patterns of (a) MPA-CdTe QDs and (b) Ni -CdTe
hollow nanospheres.
h
as a result of the aggregation of the CdTe QDs, their positions
moved slightly toward higher angles as compared to those of
MPA-CdTe QDs (bulk cubic (zinc blende) CdTe, Interna-
tional Centre for Diffraction Data, JCPDS card no. 65-1046).
The higher shift of the diffraction pattern of the hollow
nanospheres toward that of CdS (bulk cubic (zinc blende) CdS,
JCPDS card no. 10-0545) may be a result of the partial
decomposition of MPA during illumination leading to the
incorporation of CdS on the surface of the CdTe nanocryst-
Mechanistic Insight into the Formation of Hollow-
Structured Nanospheres. To determine the factors that
control the formation of hollow-structured nanospheres, a set
of parallel experiments were carefully conducted. It was
determined that MPA-QDs, NiCl ·6H O, H A, and visible
2
2
2
light are all essential to formation of hollow nanospheres. The
absence of any of these components resulted in no appreciable
Ni -QDs assembly, including that of Ni -CdTe (Supporting
h
h
Information Figures S5,S6). Furthermore, commercially avail-
able CdTe powders were investigated for comparison, but no
hollow nanospheres were obtained under identical conditions.
This finding confirms that the unique characteristics of MPA-
CdTe QDs are considerably different than those of bulk
materials.
4
0
als, which is consistent with the increment of sulfur species in
the CdTe QDs after irradiation (confirmed by ICP-AES, see
Supporting Information Table S1).
To determine the exact chemical composition and electronic
structure, the XPS spectrum was carefully investigated. Note
that the electron binding energy depends not only on the
element it comes from but also on the chemical state of the
The formation of the Ni -CdTe hollow nanospheres was
h
strongly dependent on the pH of the solution (Figure 4). When
the pH was <4, only aggregates of CdTe QDs were observed.
41,42
element in the given nanoparticles (Figure 3).
The peak at
approximately 162 eV arises from S 2p with a doublet structure,
corresponding to S 2p and S 2p3/2, which are separated by
When the pH was >4, Ni -CdTe hollow nanospheres formed
h
with variable quality. The highest quality Ni -CdTe hollow
1
/2
h
4
3
1
.3 eV. Before irradiation, the XPS spectrum of the MPA-
CdTe QDs could be well fit with one doublet for S 2p1/2 at
62.95 eV and S 2p3/2 at 161.65 eV, which suggests the
existence of chemical bonds between the thiols of MPA and the
nanospheres were formed at pH 4.65 (Figure 4).
Because no additive or template was used in any part of the
process, we examined the photoproducts of the reaction system
in addition to the hollow-structured nanospheres. Notably, H2
was detected as the only product in the reaction mixture. In this
situation, it can be assumed that the hollow nanospheres are a
result of the assemblies around the gas−liquid interface of the
1
44
cadmium ions on the surface of the CdTe QDs. After
irradiation, the S 2p doublet increased significantly and shifted
to a lower binding energy. A new doublet peak at 162.25 eV for
S 2p1/2 and at 161.23 eV for S 2p3/2 implied that irradiation
14,15
H gas bubbles generated in situ in water.
2
This assumption
40
leads to the decomposition of MPA to CdS. Thus, the
photodecomposition process resulted in changes in the
chemical properties of the CdTe QDs. Additionally, the Ni
signals from the hollow-structured nanospheres were detected
using XPS (Supporting Information Figure S3). Despite being
was well corroborated by the fact that a higher H2
photogeneration efficiency led to higher quality hollow
nanospheres. A sharp maximal rate for H evolution was
2
observed at pH 4.65 (Supporting Information Figure S7), while
significant amounts of H were also obtained at lower or higher
2
C
dx.doi.org/10.1021/ja5047236 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
H bubbles in situ is essential for assembling QDs into hollow-
2
structured nanospheres.
In addition to the facile morphology control offered by the
optimal pH and metal ions of the reaction system, the quality of
hollow-structured nanospheres also depends on the wavelength
and intensity of visible light used (Figure 6). When monitoring
a specific wavelength of incident visible light from an LED
−2
(
410, 455, 530, 590, and 630 nm; 80 mW cm ), we observed
that the only light wavelengths that QDs absorb are 410, 455,
and 530 nm, which provide good to excellent performance for
the generation of H gas bubbles and thus lead to the formation
2
of high-quality hollow nanospheres of QDs (Figure 6).
−2
Moreover, the light intensity (410 nm LED, 160 mW cm ,
−2
−2
8
0 mW cm , and 40 mW cm ) for a given reaction system
also significantly influenced the quality of the hollow
nanospheres, which is related to the same trend of H2
evolution activity (Supporting Information Figure S8).
In an effort to understand the entire process, we further
2
+
studied the interaction of the necessary components: Ni ions,
H A, and MPA-CdTe QDs. For the solutions at pH 11 and pH
2
Figure 4. HRTEM images of hollow Ni -CdTe nanospheres obtained
at different pH values (scale bar: 10 nm).
h
4
.65, the MPA-CdTe QDs exhibited zeta potentials of −40.6 ±
1
.6 and −13.8 ± 0.7 mV, respectively. The negative zeta
potential values along with the FT-IR spectral study
(
Supporting Information Figure S9) identified the negatively
pH values. Furthermore, the same trend was observed in the
charged surface, which depends on the degree of deprotonation
of the carboxyl groups and on the surface dangling bond (Te ,
S ) of QDs (see details in the Supporting Information). When
NiCl ·6H O was added to the aqueous solution of MPA-CdTe
formation of Ni -CdTe hollow-structured nanospheres (Figure
h
2
−
4
).
2−
To examine the general applicability of the approach studied
in this work, we used other metal ions available in our
laboratory to clarify the formation of hollow-structured
2
2
QDs, the band edge emission was dramatically quenched
Figure 7a) with a shortened lifetime (Supporting Information
(
nanospheres and the photocatalytic generation of H . As
2
+
2+
2+
Figure S10), which suggested that the interaction of the MPA-
shown in Figure 5, various metal ions, such as Cu , Cu , Fe ,
Fe , Mn , RuCl , Ag , and PtCl , with different valence
CdTe QDs and Ni2+ ions was in the excited state. The linear
3
+
2+
2−
+
2−
5
4
47
2+
states and reduction potentials (Supporting Information Table
S2) exhibit different behaviors. With the irradiation of an
aqueous solution containing MPA-CdTe QDs, metal ions, and
plot of the Langmuir law indicated that Ni ions were
binding to the surface dangling bond (Te , S ) of the
QDs
Ni -centered complex in situ with Te and S ligands,
described by Talapin et al.
Because the conduction band energy level (Ecb) of the MPA-
2−
2−
3
0,39,47,48
(Supporting Information Figure S11) to form a
29,46
2
+
2+
3+
2+
2−
2−
H A at pH 4.65, only the Ni , Fe , and Fe ions that evolve
as
2
4
8,49
H are noticeably capable of forming the hollow-structured
2
2+
nanospheres (Figures 1 and 5). Of the metal ions studied, Ni
25
ions were found to possess the best ability to generate H and
CdTe QDs (3.4 nm) (−1.6 V vs NHE) is more negative than
2
2+ 0
hollow nanospheres under visible light irradiation. For the
the redox potential of the Ni /Ni couples (−0.25 V vs
50
metal ions that gave rise to small amounts or no H evolution
NHE), the free energy change of the electron transfer from
2
2
+
+
2+
2−
+
2+
from the system, that is, Mn , Cu , Cu , RuCl , Ag , and
the excited MPA-CdTe QDs to the Ni ions is thermodynami-
cally favorable. The photoinduced electron transfer process was
directly investigated using a nanosecond transient absorption
5
PtCl42 , only aggregates of CdTe QDs were observed (Figure
−
5). All of these results demonstrate that the photogeneration of
Figure 5. Visible light catalysis-assisted assembly of hollow nanospheres and H generation using other metal ions: (a) Time course of H evolution
2
2
−7
−2
−4
+
2+
2+
3+
2+
2+
from MPA-CdTe QDs (2.4 × 10 M), H A (2.84 × 10 M), and metal ions (2.1 × 10 M), Cu , Cu , Fe , Fe , Ni (as a reference), Mn ,
2
RuCl52 , Ag , and PtCl4 suspended in 5 mL of an aqueous solution (pH = 4.65) under monochromated light from a 410 nm LED; HRTEM
−
+
2−
+
2+
2+
3+
2+
2−
images of the precipitates obtained from the reaction system with various metal ions, (b) Cu , (c) Cu , (d,e) Fe , (f,g) Fe , (h) Mn , (i) RuCl
,
5
+
2−
(
j) Ag , and (k) PtCl4 .
D
dx.doi.org/10.1021/ja5047236 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 6. Visible light wavelength dependence of H evolution and Ni -CdTe hollow nanosphere formation: (a) Time course of H evolution, NiCl ·
2
h
2
2
−4
−7
−2
6
H O (2.1 × 10 M), MPA-CdTe QDs (2.4 × 10 M), and H A (2.8 × 10 M) suspended in 5 mL of an aqueous solution (pH = 4.65) under
2
2
monochromated light from an LED at 410, 455, 530, 590, and 630 nm; HRTEM images of the hollow nanospheres obtained at specific light
irradiation wavelengths for 21 h, (b,c) 410 nm, (d,e) 455 nm, (f,g) 530 nm, (h,i) 590 nm, and (j,k) 630 nm.
−
7
−5
Figure 7. Emission spectra of the MPA-CdTe QDs (2.4 × 10 M) with a concentration of (a) NiCl ·6H O (0 to 7.0 × 10 M), (c) H A (0 to 3.6
2
2
2
−3
×
10 M), and (e) H A (0 to 0.378 M) at pH = 4.65 in water; (b) nanosecond recovery kinetics of bleaching for MPA-CdTe QDs in the absence
and presence of Ni ions monitored at 556 nm and normalized to the maximal signal. The fits were deconvolved with a Gaussian function with a 4
ns fwhm representing the instrument response; emission decays of the MPA-CdTe QDs (2.4 × 10 M) with various concentrations of (d) H A (0
to 3.6 × 10 M), and (f) H A (0 to 0.378 M) at pH = 4.65 in water (excitation wavelength: 406 nm).
2
2+
−7
2
−3
2
study. After being laser-pulsed at 410 nm, the MPA-CdTe QDs
displayed a kinetic decay of the characteristic absorption (556
nm) with a lifetime of 46.2 ns. When Ni ions were introduced
into the solution, the lifetime of the absorptions decreased to
to adjust the solution pH to 4.65 and then determined the
5
1,57
interaction between the CdTe QDs and H A.
As shown in
2
2
+
Figure 7e, introducing H A to an aqueous solution of CdTe
2
QDs significantly quenched the band edge emission. In the
2
4
6.8 ns, which is shorter than that of the CdTe QDs alone (τ =
absence and presence of H A, the lifetime of the emission
2
6.2 ns) (Figure 7b). The faster recovery of exciton bleaching
decreased by a factor of ∼2 from 81.8 to 40.1 ns (Figure 7f),
which was attributed to the photogenerated holes trapped by
the H A. Neither the Ni -CdTe hollow nanospheres nor H
2
+
in the presence of Ni ions suggested an effective electron
transfer from the excited MPA-CdTe QDs to the Ni active
sites, a crucial step in the photogeneration of H gas (see
2+
2
h
2
5
1
were obtained when H A was replaced by HCl (Supporting
2
2
details in the Supporting Information) and the subsequent in
situ assembly process.
Information Figures S5,S6), indicating that the hole transfer
from the CdTe QDs to H A is also important. Here, H A (a
weak dibasic acid in aqueous solution ) serves not only as a
2
2
58
On the other hand, the addition of H A to the MPA-CdTe
2
QD solution led to a remarkable decrease in the emission
intensity and lifetime (Figure 7c,d), which is a result of the
protonation or detachment of the MPA from the QDs, causing
proton source for H production but also as a sacrificial electron
donor for regenerating QDs after the photoinduced electron
transfer.
2
5
2,53
aggregation of soluble QDs.
spectra (Figure 7c) but not in the absorption spectra
Supporting Information Figure S12) could be explained by
The red-shift in the emission
On the basis of the above results and analyses, a potential
mechanism for the formation of Ni -CdTe hollow-structured
h
(
nanospheres can be proposed. First, the negative surface
2
−
2−
the fact that the energy transfers from the aggregated smaller
MPA-CdTe QDs (larger band gap due to the quantum
dangling bonds (Te , S ) on the surface of the CdTe QDs
2
+
interact with positively charged Ni ions to form a Ni -QD
photocatalyst in situ,
h
53−56
30,48
confinement effect) to the larger ones (smaller band gap).
which has a tendency to aggregate in
To avoid the interference of further aggregation and energy
transfer of the suspended QDs, we used hydrochloride (HCl)
the presence of H A. Upon visible light irradiation of the Ni -
QDs, electron transfer from the excited QDs to the Ni -
2
h
2
+
E
dx.doi.org/10.1021/ja5047236 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
2
+
centered complex occurs. The reduced Ni -centered complex
further reacts with protons to generate H . On the other hand,
2
the formed hole that remains in the Ni -QDs after the electron
h
transfer is regenerated by the sacrificial electron donor of H A.
2
Consequently, the catalytic cycle for the evolution of H is
2
achieved, the detailed mechanism of which is presented in the
Supporting Information (Scheme S1). Notably, the H bubbles
2
generated in situ under visible light irradiation may act as the
center of the Ni -QDs assembly. Driven by the minimization of
h
1
4,59−61
interfacial energy
and the photodecomposition of
h
3
0,40
MPA,
the aggregated Ni -QDs are organized around the
gas−liquid interface of H bubbles to form hollow-structured
2
nanospheres of Ni -CdTe to scale. On the basis of the size and
h
shell thickness of the nanospheres, the number of QD units in
one hollow nanosphere was calculated to range from 95 to 286
(
see details in the Supporting Information). In contrast to the
59−66
traditional gas bubble template methods
physical processes or thermochemical reactions, the
superiority of the current approach is the ability to prepare
that use specific
67
6
6
hollow nanospheres using H bubbles generated in situ by
2
visible light catalysis. The shell thickness of 3−6 nm, which
corresponds to the monolayer to bilayer of the CdTe QDs (the
diameter is 3.4 nm), and the dimensions of <20 nm are rarely
Figure 8. Photosynthetic H2 production from Ni -CdTe hollow
h
−2
13−16,35
nanospheres in 10 mL of a 5.68 × 10 M (100 mg/10 mL) H2A
aqueous solution (pH = 4.65) under visible light irradiation (λ > 400
nm). Error bars represent the mean ± s.d. of three independent
experiments.
realized in the preparation of hollow spheres.
Photocatalytic Activity of Ni -QD Hollow Nano-
h
spheres. The appealing architecture and function of the
hollow-structured nanospheres are similar to that observed in
the photosynthetic reaction center for storing sunlight as
chemical energy in the form of H . When the Ni -CdTe hollow
2
h
nanospheres and in the photocatalytic generation of H . Of the
2
nanospheres (referring to 0.16 wt % nickel, ICP-AES) were
2+
metal ions studied, Ni ions possessed the best ability to
−
2
dissolved in an aqueous H A solution (5.68 × 10 M, 10 mL,
2
generate H and hollow-structured nanospheres. Under visible
2
pH 4.65), for example, the rate of H production (∼21 μmol
2
light irradiation, the hollow nanospheres can be successfully
−1
−1
h
mg , 180 μmol of H ) was nearly linear even after 21 h of
2
created via H gas bubbles generated in situ, rather than using
2
irradiation by visible light, and the TON approached 75 000 or
6 500 for MPA-CdTe QDs or nickel, respectively (see TON
external templates. The average diameter and shell thickness of
the nanospheres range from 10−20 nm and 3−6 nm,
respectively. These types of small-sized hollow spheres are
1
determination in the Supporting Information). One continuous
irradiation cycle was conducted to examine the reproducibility
of each photocatalyst bed (Figure 8). No apparent variation in
the rate of H2 formation was observed throughout the
35
rarely observed in the literature. Although QDs have been
demonstrated to be excellent building blocks for assembling
ordered structures, the challenge still remains to achieve
hierarchical architectures using bubble templates, especially
bubbles generated in situ through chemical reactions. The
strategy presented here is general and can be extended to other
cadmium chalcogenides (CdE; E = Se, S) and metal salts under
ambient conditions. By taking advantage of visible light
catalysis, this novel method offers an attractive one-pot route
for the fabrication of advanced functionalized materials for
artificial photosynthetic devices. The mechanistic information
will help us design various hollow nanospheres capable of light-
harvesting, charge separation, and catalysis, which occur in
natural photosynthesis systems.
irradiation process. The amount of H was evaluated as an
2
average value from two cycles. More than 330 μmol of H2
(
TON of 137 500 or 30 250 for CdTe QDs or nickel,
respectively) was produced during the course of 42 h of
irradiation. The internal quantum efficiency (IQE, see details in
the Supporting Information) is a function of the visible light
wavelength used (410 nm, 455 nm, 530 nm, 590 nm, 630 nm;
−2
8
0 mW cm ) (Supporting Information Figure S13), and the
highest IQE was determined to be 1.8% at 455 nm. These
results greatly motivate us to construct hollow-structured
nanospheres as effective and robust artificial photosynthetic
systems for H generation.
2
EXPERIMENTAL SECTION
Preparation of Sodium Hydrogen Telluride. The reported
■
CONCLUSION
In summary, we have developed a new approach to create
hollow nanoarchitectures of Ni -CdTe QDs, where water-
soluble MPA-CdTe QDs, Ni ions, and H A serve as the
■
68
method for preparing sodium hydrogen telluride (NaHTe) was used
with some modifications. Briefly, 80 mg of sodium borohydride was
transferred to a small flask. Next, 1 mL of ultrapure water was added.
After 127.5 mg of tellurium powder was added to the flask, the
reaction system was cooled using ice. A small outlet connected to the
flask was kept open to discharge the pressure from the hydrogen
generated during the reaction. Within 3−5 h, the black tellurium
powder generally disappeared, and a white sodium tetraborate
precipitate appeared on the bottom of the flask. The resulting
NaHTe in the supernatant was separated to prepare the CdTe
particles.
h
2
+
2
subcomponents. The absence of any of the components, that is,
QDs, Ni2+ ions, H A, or visible light, resulted in no appreciable
2
amount of the Ni -QD hollow-structured nanospheres. The
h
quality of the hollow nanospheres depends on the wavelength
and intensity of the visible light used. Various metal ions, such
+
2+
2+
3+
2+
2−
+
2−
as Cu , Cu , Fe , Fe , Mn , RuCl , Ag , and PtCl4
,
5
exhibited different behaviors for the formation of the hollow
F
dx.doi.org/10.1021/ja5047236 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Synthesis of Water-Soluble MPA-CdTe QDs. An aqueous
known amounts of H and CH and determined before and after a
series of measurements.
2
4
colloidal MPA-CdTe QDs solution was prepared using the reaction
2
+
68
between Cd and NaHTe solutions according to the literature. The
2+
Cd precursor solutions were prepared by mixing solutions of CdCl2·
ASSOCIATED CONTENT
Supporting Information
■
2
1
1
.5H O and stabilizer (MPA) followed by a pH adjustment to 11 with
2
*
S
M NaOH. The typical molar ratio of Cd:MPA:Te was maintained at
:1.2:0.2 in our experiment. This solution was placed in a three-necked
Additional experimental details, Tables S1,S2, and Figures S1−
S13. This material is available free of charge via the Internet at
http://pubs.acs.org.
flask, which was fitted and deaerated with N bubbling, for 30 min.
2
The resulting solution was then heated to 99−100 °C and refluxed for
different reaction times to control the sizes of the MPA-CdTe QDs.
Aliquots of the reaction solution were collected at regular intervals for
characterization via UV−vis absorption and emission.
AUTHOR INFORMATION
Corresponding Author
lzwu@mail.ipc.ac.cn
■
General Procedure for the Preparation of Ni -CdTe Hollow
h
−7
Nanospheres. An aqueous colloidal MPA-CdTe solution (2.4 × 10
M, 0.4 mg/10 mL), H A (2.84 × 10 M, 50 mg/10 mL), and NiCl2·
H O (2.1 × 10 M, 0.5 mg/10 mL) were added to a Schlenk tube.
−2
Notes
2
−
4
6
The authors declare no competing financial interest.
2
The total volume of each sample was 10 mL. Prior to irradiation, the
pH value of the solution was adjusted using an aqueous 1.0 M NaOH
or 1.0 M HCl solution and verified using a pH meter. The sample was
degassed by bubbling nitrogen for 30 min and then irradiated under a
high-pressure Hanovia mercury lamp (light intensity of 100 mW cm
at the Schlenk tube surface) with a light wavelength longer than 400
nm.
ACKNOWLEDGMENTS
We are grateful for the financial support from the Ministry of
Science and Technology of China (2013CB834505,
014CB239402, and 2013CB834804), the National Natural
Science Foundation of China (91027041, 21090343, 21390404,
and 51373193), and the Chinese Academy of Sciences.
■
−2
2
For the conditioning experiments on metal ions and light
wavelength and intensity, an aqueous colloidal MPA-CdTe solution
−
7
−2
(
2.4 × 10 M, 0.2 mg/5 mL), H A (2.84 × 10 M, 25 mg/5 mL),
REFERENCES
2
+
2+
2+
3+
2+
2+
2−
+
■
and metal ions (Cu , Cu , Fe , Fe , Ni , Mn , RuCl , Ag , and
5
(
1) Balzani, V.; Credi, A.; Venturi, M. ChemSusChem 2008, 1, 26.
2) Lubitz, W.; Reijerse, E. J.; Messinger, J. Energy Environ. Sci. 2008,
PtCl42 (2.1 × 10 M)) were added to a Schlenk tube. The total
volume of each sample was 5 mL. Prior to irradiation, the pH value of
the solution was adjusted to 4.65 using an aqueous 1.0 M NaOH or
−
−4
(
1
(
, 15.
3) Magnuson, A.; Anderlund, M.; Johansson, O.; Lindblad, P.;
Lomoth, R.; Polivka, T.; Ott, S.; Stensjo, K.; Styring, S.; Sundstrom, V.;
Hammarstrom, L. Acc. Chem. Res. 2009, 42, 1899.
4) Frischmann, P. D.; Mahata, K.; Wurthner, F. Chem. Soc. Rev.
013, 42, 1847.
1
.0 M HCl solution and verified using a pH meter. The sample was
̈
̈
degassed by bubbling nitrogen for 30 min and then irradiated under
̈
−2
monochromated light from an LED (80 mW cm ) at 410, 455, 530,
90, or 630 nm or under varied light intensity (monochromated light
(
2
(
(
5
−2
−2
−2
from a 410 nm LED, 160 mW cm , 80 mW cm , and 40 mW cm ).
The suspension of the QDs tends to form precipitates because the
protonation of MPA (the thiol group) results in a partial dissociation
of the ligand from the QD surface under acidic conditions combined
with irradiation (λ > 400 nm). To overcome this problem, the H2
evolution experiment was performed under rapidly rigorous stirring to
5) Nocera, D. G. Acc. Chem. Res. 2012, 45, 767.
6) Wang, H.-Y.; Wang, W.-G.; Si, G.; Wang, F.; Tung, C.-H.; Wu,
L.-Z. Langmuir 2010, 26, 9766.
7) Zhu, L.; Khairutdinov, R. F.; Cape, J. L.; Hurst, J. K. J. Am. Chem.
Soc. 2005, 128, 825.
8) Sasaki, R.; Nako, Y.; Murata, S. Tetrahedron 2009, 65, 7364.
(
(
maintain the QDs in suspension. The generated photoproduct of H2
was characterized by GC analysis (Techcomp 7890-II) using nitrogen
(9) Watanabe, K.; Moriya, K.; Kouyama, T.; Onoda, A.; Minatani, T.;
Takizawa, S.-y.; Murata, S. J. Photochem. Photobiol., A 2011, 221, 113.
as the carrier gas with a molecular sieve column (5 Å) and a thermal
conductivity detector. After 21 h of irradiation, we isolated the QD
precipitates from the solution using centrifugation, and each
component was further dispersed using ultrasonication. The samples
then were prepared for HRTEM observation by dropping dispersed
suspended precipitates onto ultrathin carbon-coated copper grids with
the excess solvent evaporated.
(
10) Sun, J.; Zhang, J.; Zhang, M.; Antonietti, M.; Fu, X.; Wang, X.
Nat. Commun. 2012, 3, 1139.
11) Wang, D.; Hisatomi, T.; Takata, T.; Pan, C.; Katayama, M.;
Kubota, J.; Domen, K. Angew. Chem., Int. Ed. 2013, 52, 11252.
12) Li, Z.-J.; Li, X.-B.; Wang, J.-J.; Yu, S.; Li, C.-B.; Tung, C.-H.; Wu,
L.-Z. Energy Environ. Sci. 2013, 6, 465.
13) Zeng, H. C. J. Mater. Chem. 2011, 21, 7511.
(
(
(
General Procedure for Photocatalytic H2 Generation. The
(14) Lou, X. W.; Archer, L. A.; Yang, Z. Adv. Mater. 2008, 20, 3987.
(15) Liu, J.; Liu, F.; Gao, K.; Wu, J.; Xue, D. J. Mater. Chem. 2009, 19,
6073.
Ni -CdTe hollow nanospheres, which were formed from a system
h
−4
containing NiCl ·6H O (2.1 × 10 M, 0.5 mg/10 mL), the MPA-
CdTe QDs (2.4 × 10 M, 0.4 mg/10 mL), and H A (2.84 × 10 M,
2
2
−
7
−2
2
(16) Hu, J.; Chen, M.; Fang, X.; Wu, L. Chem. Soc. Rev. 2011, 40,
5
0 mg/10 mL) in water at pH 4.65 under visible light irradiation (21
5
(
472.
17) Lai, X.; Halpert, J. E.; Wang, D. Energy Environ. Sci. 2012, 5,
h, λ > 400 nm), were obtained by centrifugation and immediately
−2
added to 10 mL of a 5.68 × 10 M (100 mg/10 mL) H A aqueous
5604.
2
solution (pH = 4.65). CH (500 μL in concentration effect, control
(18) Srivastava, S.; Santos, A.; Critchley, K.; Kim, K.-S.; Podsiadlo, P.;
Sun, K.; Lee, J.; Xu, C.; Lilly, G. D.; Glotzer, S. C.; Kotov, N. A. Science
2010, 327, 1355.
4
experiments, and optimized concentration experiments to ensure that
sufficient CH was extracted from the samples) was injected as the
4
internal standard for quantitative GC analysis. The sample was then
irradiated under a high-pressure Hanovia mercury lamp (light intensity
(19) King’ondu, C. K.; Iyer, A.; Njagi, E. C.; Opembe, N.; Genuino,
H.; Huang, H.; Ristau, R. A.; Suib, S. L. J. Am. Chem. Soc. 2011, 133,
4186.
(20) Bai, F.; Sun, Z.; Wu, H.; Haddad, R. E.; Xiao, X.; Fan, H. Nano
Lett. 2011, 11, 3759.
(21) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737.
(22) Kamat, P. V.; Tvrdy, K.; Baker, D. R.; Radich, J. G. Chem. Rev.
2010, 110, 6664.
−2
of 100 mW cm at the Schlenk tube surface) with a light wavelength
longer than 400 nm. The generated photoproduct of H2 was
characterized through GC analysis (Techcomp 7890-II) using nitrogen
as the carrier gas with a molecular sieve column (5 Å) and a thermal
conductivity detector. Five hundred microliters of mixed gas was
extracted from the sample tube and immediately injected into the GC.
The response factor for H /CH was approximately 4.86 under the
(23) Greene, B. L.; Joseph, C. A.; Maroney, M. J.; Dyer, R. B. J. Am.
2
4
experimental conditions, which was established by calibration with
Chem. Soc. 2012, 134, 11108.
G
dx.doi.org/10.1021/ja5047236 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(
24) Brown, K. A.; Dayal, S.; Ai, X.; Rumbles, G.; King, P. W. J. Am.
Chem. Soc. 2010, 132, 9672.
25) Wang, F.; Wang, W.-G.; Wang, X.-J.; Wang, H.-Y.; Tung, C.-H.;
Wu, L.-Z. Angew. Chem., Int. Ed. 2011, 50, 3193.
26) Nann, T.; Ibrahim, S. K.; Woi, P. M.; Xu, S.; Ziegler, J.; Pickett,
C. J. Angew. Chem., Int. Ed. 2010, 49, 1574.
27) Huang, J.; Mulfort, K. L.; Du, P.; Chen, L. X. J. Am. Chem. Soc.
012, 134, 16472.
28) Han, Z.; Qiu, F.; Eisenberg, R.; Holland, P. L.; Krauss, T. D.
Science 2012, 338, 1321.
29) Yang, D.; Wang, F.; Yan, J.; Gao, Y.; Li, H. J. Nanopart. Res.
013, 15, 1.
30) Li, Z.-J.; Wang, J.-J.; Li, X.-B.; Fan, X.-B.; Meng, Q.-Y.; Feng, K.;
Chen, B.; Tung, C.-H.; Wu, L.-Z. Adv. Mater. 2013, 25, 6613.
31) Wang, J.-J.; Li, Z.-J.; Li, X.-B.; Fan, X.-B.; Meng, Q.-Y.; Yu, S.; Li,
C.-B.; Li, J.-X.; Tung, C.-H.; Wu, L.-Z. ChemSusChem 2014, 7, 1468.
32) Wang, F.; Liang, W.-J.; Jian, J.-X.; Li, C.-B.; Chen, B.; Tung, C.-
H.; Wu, L.-Z. Angew. Chem., Int. Ed. 2013, 52, 8134.
33) Li, C.-B.; Li, Z.-J.; Yu, S.; Wang, G.-X.; Wang, F.; Meng, Q.-Y.;
Chen, B.; Feng, K.; Tung, C.-H.; Wu, L.-Z. Energy Environ. Sci. 2013,
, 2597.
34) Jian, J.-X.; Liu, Q.; Li, Z.-J.; Wang, F.; Li, X.-B.; Li, C.-B.; Liu, B.;
(59) Aquilano, D.; Costa, E.; Genovese, A.; Roberto Massaro, F.;
Pastero, L.; Rubbo, M. J. Cryst. Growth 2003, 247, 516.
(60) Fan, X.; Zhang, Z.; Li, G.; Rowson, N. A. Chem. Eng. Sci. 2004,
59, 2639.
(61) Liu, J.; Xue, D. J. Cryst. Growth 2009, 311, 500.
(62) Peng, Q.; Dong, Y.; Li, Y. Angew. Chem., Int. Ed. 2003, 42, 3027.
(63) Zhang, H.; Zhang, S.; Pan, S.; Li, G.; Hou, J. Nanotechnology
(
(
(
2
(
2
(
2
004, 15, 945.
64) Jiang, C.; Zhang, W.; Zou, G.; Yu, W.; Qian, Y. Nanotechnology
005, 16, 551.
(65) Li, X.; Xiong, Y.; Li, Z.; Xie, Y. Inorg. Chem. 2006, 45, 3493.
(
2
(
(66) Wang, W.; Zhang, P.; Peng, L.; Xie, W.; Zhang, G.; Tu, Y.; Mai,
W. CrystEngComm 2010, 12, 700.
67) Wang, H.; Liang, J.; Fan, H.; Xi, B.; Zhang, M.; Xiong, S.; Zhu,
Y.; Qian, Y. J. Solid State Chem. 2008, 181, 122.
68) Zhang, H.; Zhou, Z.; Yang, B.; Gao, M. J. Phys. Chem. B 2002,
07, 8.
(
(
(
1
(
(
6
(
Meng, Q.-Y.; Chen, B.; Feng, K.; Tung, C.-H.; Wu, L.-Z. Nat.
Commun. 2013, 4, 2695.
(
35) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G.
A.; Alivisatos, A. P. Science 2004, 304, 711.
36) Bhattacharyya, J.; Das, S.; Mukhopadhyay, S. Dalton Trans.
007, 1214.
(
2
(
37) Rogach, A. L. Mater. Sci. Eng., B 2000, 69−70, 435.
(38) Li, L.; Qian, H.; Fang, N.; Ren, J. J. Lumin. 2006, 116, 59.
39) Chen, J. L.; Zheng, A. F.; Gao, Y. C.; He, C. Y.; Wu, G. H.;
(
Chen, Y. C.; Kai, X. M.; Zhu, C. Q. Spectrochim. Acta, Part A 2008, 69,
1
044.
(
(
40) Bao, H.; Gong, Y.; Li, Z.; Gao, M. Chem. Mater. 2004, 16, 3853.
41) Sarma, D. D.; Santra, P. K.; Mukherjee, S.; Nag, A. Chem. Mater.
2
(
013, 25, 1222.
42) Kulkarni, S. K.; Winkler, U.; Deshmukh, N.; Borse, P. H.; Fink,
R.; Umbach, E. Appl. Surf. Sci. 2001, 169−170, 438.
43) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir
993, 9, 3277.
(
1
(
44) Reiss, P.; Bleuse, J.; Pron, A. Nano Lett. 2002, 2, 781.
(
45) Huang, X.; Li, G.; Cao, B.; Wang, M.; Hao, C. J. Phys. Chem. C
2
009, 113, 4381.
46) Zhang, W.; Wang, Y.; Wang, Z.; Zhong, Z.; Xu, R. Chem.
Commun. 2010, 46, 7631.
47) Dong, C.; Qian, H.; Fang, N.; Ren, J. J. Phys. Chem. B 2006, 110,
1069.
48) Nag, A.; Chung, D. S.; Dolzhnikov, D. S.; Dimitrijevic, N. M.;
Chattopadhyay, S.; Shibata, T.; Talapin, D. V. J. Am. Chem. Soc. 2012,
34, 13604.
49) Kovalenko, M. V.; Scheele, M.; Talapin, D. V. Science 2009, 324,
417.
50) Sun, Q.; Ren, Z.; Wang, R.; Wang, N.; Cao, X. J. Mater. Chem.
011, 21, 1925.
(
(
1
(
1
(
1
(
2
(
51) Chuang, C.-H.; Burda, C. J. Phys. Chem. Lett. 2012, 3, 1921.
(
52) Pradhan, N.; Battaglia, D. M.; Liu, Y.; Peng, X. Nano Lett. 2006,
7
, 312.
53) Aldana, J.; Lavelle, N.; Wang, Y.; Peng, X. J. Am. Chem. Soc.
005, 127, 2496.
54) Zhang, Y.; Mi, L.; Wang, P. N.; Ma, J.; Chen, J. Y. J. Lumin.
008, 128, 1948.
55) Susha, A. S.; Javier, A. M.; Parak, W. J.; Rogach, A. L. Colloids
Surf., A 2006, 281, 40.
56) Xu, S.; Wang, C.; Xu, Q.; Zhang, H.; Li, R.; Shao, H.; Lei, W.;
Cui, Y. Chem. Mater. 2010, 22, 5838.
57) Zhu, H.; Song, N.; Lv, H.; Hill, C. L.; Lian, T. J. Am. Chem. Soc.
012, 134, 11701.
58) Creutz, C. Inorg. Chem. 1981, 20, 4449.
(
2
(
2
(
(
(
2
(
H
dx.doi.org/10.1021/ja5047236 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX