Single-crystalline tungsten oxide quantum dots for fast pseudocapacitor and electrochromic applications

Article abstract of DOI:10.1002/adma.201400447

Tungsten oxide quantum dots (QDs) with an average size down to 1.6 nm are developed, serving as a new class of promising electrode materials, which yield efficient and fast electron/ion transport in the charge/discharge process. With a visually impressive display, the QDs present coloration/bleaching times within 1 s, which is much superior to inorganic analogues and even competitive to organic-related materials.

Full text of DOI:10.1002/adma.201400447

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Single-Crystalline Tungsten Oxide Quantum Dots for Fast
Pseudocapacitor and Electrochromic Applications
Shan Cong, Yuyu Tian, Qingwen Li, Zhigang Zhao,* and Fengxia Geng*
The research and development of devices regarding energy
conversion, storage, and saving including fuel cells, lithium
batteries, supercapacitors, and smart windows have attracted
a great deal of interest due to the rapid economic expansion,
increase in world population, and ever-increasing human reli-
ance on energy-based appliances. A fast electrochemistry with
efficient electron and ion transport sits at the heart, the success
of which would not only create high rate capability for batteries
and capacitors but also bring fast response for electrochromic
devices and therefore achieve many exciting performances for
energy-related devices.[1] To this end, considerable attempts
have been made to innovate upon the design of the energy-
related electrochemical devices from many aspects such as
electrolyte, current collector, and electrode material,[1a,2] among
which reforming the electrode material was particularly actively
explored through surface modification, tailoring pore structure,
doping and making hybrid composites, etc..[3] Nanoparticulate
materials are usually desired, aiming at improving electrolyte
accessibility and shortening ion diffusion length, which is ben-
eficial to accelerate electrochemical process to a certain extent.[4]
Therefore, downsizing electrode material to extremely small,
for example, zero-dimensional quantum dots (QDs) may rep-
resent a promising but challenging strategy. QDs materials
possess several striking advantageous features desirable for
this purpose: 1) the small size would greatly shorten the diffu-
sion paths of intercalation ions in the solid phase, which would
help to realize fast charge/mass transfer, 2) the large surface-to-
volume ratio of QDs would facilitate intimate contacts between
electrode materials and electrolyte/current collector, thereby
providing fast charge-transport and electron-transfer kinetics,
and 3) the significantly high proportion of surface atoms makes
QD electrode more active for electrochemical reactions, and
thus significantly improving reaction rates. However, although
QDs have been documented to be utilized in several state-of-art
applications including photocatalysis, solar cells, light-emitting
devices and bio-imaging sensors,[5] their utilization as electrode
materials in electrochemical reactions remains a formidable
challenge, which may be related with the poor conductivity of
most available QD materials due to the detrimental insulation
of organic ligands capped on QDs and the interparticle resist-
ance raising with the increasing grain boundaries. Thus, it is
highly desired to develop a strategy to overcome the deficiency
and effectively utilize QDs as electrochemical electrode mate-
rials, which would be of great significance both to basic under-
standing of electrochemical processes and advanced design of
electrochemical devices.
Meanwhile, tungsten oxide, WO3−x, a typical electrode mate-
rial with charge storage/delivery property engendered by the
valence change between W6+ and W5+, has been actively pur-
sued because of its superior electrochemical performances
in both electrochromic devices[6] and supercapacitors,[7] with
attractive applications ranging from smart window to energy
storage media. The structure of dehydrated tungsten oxide typi-
cally consists of stacking of infinite corner-sharing WO6 octa-
hedra layers, leading to the formation of tunnels between the
octahedras, which provide sites for ion intercalation and trans-
port. However, limited by the sluggish electrochemical kinetics,
most currently available tungsten oxide materials suffer from
poor rate capability as supercapacitors and slow response times
for electrochromic uses, which closely relates to the time-con-
suming electrochemical double injection of positive ions (H+,
Li+, Na+) and electrons into the host lattice of tungsten oxide
electrodes.[8] The slow Faradaic electrochemical process has
become a bottleneck in achieving desirable performance for
tungsten oxide-based capacitive and electrochromic devices.
As ions transport in the lattice is diffusion-controlled, down-
sizing all the three dimensions to an extremely small size and
making QDs stands a great chance to accelerate the Faradaic
process. However, the creation of QDs for tungsten oxide ana-
logues without any template is rather difficult, and attempts
on structure-tailoring usually lead to the generation of two-
dimensional sheet or one-dimensional wire morphologies due
to the unequal growth rate in the plane and normal to the plane
direction.[9]
Here we report developing of tungsten oxide QDs with ali-
phatic amines anchoring on the surface to diminish the aniso-
tropic growth habit and a subsequent exchange of amine ligand
with conductive molecule to endow the QDs with excellent
conductivities, with the premise of which a very fast electro-
chemistry was ultimately realized, schematically illustrated in
Figure 1. The obtained material manifested excellent electro-
chemical and electrochromic behaviors, surprisingly robust to
be charged and discharged without significant distortion in CV
profile shape, well keeping the fine structures even at a scan
rate up to 500 mV s−1, and fast response with both coloration
and bleaching within 1 s.
Dr. S. Cong, Y. Y. Tian, Prof. Q. W. Li, Prof. Z. G. Zhao
Suzhou Institute of Nanotech and Nanobionics
Chinese Academy of Sciences
398 Ruoshui Road, Suzhou Industry Park
Suzhou 215123, China
E-mail: zgzhao2011@sinano.ac.cn
Prof. F. X. Geng
College of Chemistry
Chemical Engineering and Materials Science
Soochow University
Suzhou 215123, China
E-mail: gengfx@suda.edu.cn
DOI: 10.1002/adma.201400447
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Figure 1. Scheme of our strategy for realizing fast electrochemical process through extreme downsizing to QDs region with the assistance of conduc-
tive ligand exchange. (a) Efficient ion diffusion and charge transfer in QDs material in comparison with blocking in bulk materials. (b) The replace-
ment of pristine aliphatic amine ligand formed during QDs growth (left) with conductive pyridine (right), through which reduced diffusion path and
improved conductivity are achieved. Each QD is represented by layer structure of tungsten oxide viewing along the tunnel direction. The tunnels provide
convenience for ion intercalations.
The synthesis of tungsten oxide QDs was achieved through
a two-step procedure, reacting tungsten chloride (WCl6)
and phenol (C6H5OH) to yield tungsten aryloxide precursor
(W(OC6H5)6) followed by thermal heating in octylamine at
180 °C. To avoid possible agglomerations and attain good dis-
persions in characterizations, the as-prepared samples were
redispersed in hexane after collecting by centrifugation. As
shown in the inset of Figure 2a, the dispersion exhibited a
uniform pale-blue color, typical of tungsten oxide of nonstoi-
chiometric composition. The phase purity and crystal structure
of the as-prepared product was examined by X-ray diffraction
studies (Supporting Information), which showed reflections
Figure 2. (a) Low-magnification and (b) high-resolution TEM images of as-prepared tungsten oxide materials confirming the successful achievement
of monodispersed spherical QDs with average sizes of 1.6 nm. Inset in (a), an optical photo of the dispersion in hexane showing pale-blue color, which
suggests the nonstoichiometric nature of the material. Inset in (b), SAED pattern composed of individual reflected spots supporting the single crystal-
line nature of the QDs. (c) XPS and (d) FT-IR spectra for the tungsten oxide QDs, suggesting the wrapping of octylamine ligand over the surface of QDs.
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closely matching (110), (200), and (102) of a tetragonal (P4/nmm)
phase of nonstoichiometric WO3−x with lattice parameters of
a ( =b) = 5.31 Å, c = 3.93 Å. The obvious broadening of reflections
implies ultrasmall crystalline sizes for the particles, which were
roughly estimated with the Scherrer equation, giving a value
as small as 1.8 nm.[10] Figure 2a is a typical low-magnification
transmission electron microscope (TEM) image for the product,
showing monodispersed crystals with nearly spherical shapes
possessing an average diameter of 1.6 nm, in good consistency
with XRD estimations. Notably, the crystal sizes are even below
the exciton Bohr radius for tungsten oxide materials, which was
reported to be 3 nm,[11] suggesting the possible occurrence of
strong quantum confinement effect. The corresponding high-
resolution TEM image in Figure 2b manifests clear fringes in
each individual nanocrystal, suggesting the single-crystalline
nature of the QDs. The distances between the adjacent lattice
fringes was measured to be 2.6 Å, corresponding to the lattice
spacing of (200) or (020) planes. The high crystallinity was sup-
ported by the selected area electron diffraction (SAED) pattern
(inset in Figure 2b), which shows a strong diffraction ring com-
posed of several individual spots indexed as (200) or (020).
Chemical composition of the as-prepared samples was fur-
ther examined. The presence of peaks belonging to W = O bond
in Fourier Transform Infrared (FT-IR) and Raman spectra (Sup-
porting Information) confirmed the nonstoichiometric nature
of the as-synthesized QDs. The quantitative information on the
relative number of W atoms in valence states different from +6
can be obtained from deconvolution analysis of W4f core-level
spectrum (inset in Figure 2c), which was well fitted into two
spin-orbit doublets. The main peaks at 35.4 and 37.6 eV corre-
spond toW4f7/2 and W 4f5/2 lines of the W atoms in a +6 formal
oxidation state, and the second doublet with a lower binding
energy at 34.6 and 36.2 eV are originated from W4f5/2 and
W4f7/2 of the atoms in an oxidation state of +5. The relative per-
centage of W6+ and W5+ in the as-synthesized QDs was deter-
mined to be 87.5 and 12.5%, respectively. The O/W atomic ratio
is therefore ∼2.94. Additionally, the wide X-ray photoelectron
spectrum (XPS) in Figure 2c identified signals from C, N along
with W, O, and the FT-IR spectrum in Figure 2d displayed char-
acteristic IR peaks derived from organic C−H, N−H, C−C, C−N
bonds besides those from inorganic O−W−O bonds, implying
the coexistence of tungsten oxide structures and organic amine
moieties. Particularly, a broad peak centered at ∼2028 cm−1 was
discerned, which arises from a combination of the bending
peak in the ultraviolet window. For a sample behaving like a
bulk indirect semiconductor, an estimate of the band gap can
be obtained from the intercept of a function plotting (αhv)0.5 vs
hv, where α represents the absorption coefficient and hv is the
photon energy, as shown in inset of Figure 3a. A clear region
of linearity was observed and the band gap was estimated to be
Eg ≈ 4.19 eV. In sharp contrast, the absorption for bulk mate-
rial is broad and much red shifted, the band gap of which was
determined to be 2.55 eV, agreeing well with the value in lit-
erature, which is 2.6 eV (Supporting Information).[6,13] Mean-
while, the photoluminescence (PL) spectrum in Figure 3b
featured a strong and sharp (fwhm = ∼34 nm) UV emission
at 299 nm (4.15 eV) under the excitation wavelength of
λex = 220 nm, which should be relevant to the band-to-band
radiative recombination of photoexcited electrons and holes.
With varying excitation wavelengths, no obvious shift in the
emission peak was observed (Supporting Information). It
should be mentioned that emission of bulk tungsten oxide
materials is rarely observed, and even when compared with
previously reported tungsten oxide nanostructures such as
nanorods with a diameter of 4 nm, the emission of QDs dis-
cussed herein also shifts to lower numbers by nearly 50 nm.[14]
To determine the band electronic structure, the total densities
of states of XPS valence band spectra for bulk tungsten oxide
and as-synthesized QDs were measured (Figure 3c), which
shows that the valence band edge for QDs shifts to the higher
binding-energy side by 0.57 eV. The valence-band (VB) max-
imum for bulk tungsten oxide materials was documented to be
3.1 eV relative to a normal hydrogen electrode,[13,15] and accord-
ingly the VB for QDs can be determined to be 3.67 eV. The
conduction-band (CB) minimum was subsequently obtained
by subtracting the band gap from VB maximum, yielding −0.51
and 0.5 eV for QDs and bulk materials, respectively. Therefore
a simplified energy diagram comparison between bulk tung-
sten oxide and QDs is constructed and provided in Figure 3d.
The upward shit of CB minimum, downward shift of VB max-
imum, and the resultant widened band gap unambiguously
reveal a strong quantum confinement effect due to the ultra-
small dimensions of the QDs material and provide possibilities
for unusual behaviors.
While the octylamine wrapping outside of QDs is critical
to keep the desirable dispersibility of QDs, such long-chain
aliphatic molecules often impose a disastrous effect on the
interparticle charge transport because of their electrical insu-
lation.[16] Therefore, before employing the WO3−x QDs as elec-
trode materials, the pristine octylamine ligand on QDs surface
was exchanged with short-chained and conductive pyridine.
This process can improve the conductivity of electrode material
accompanied by changing surface to hydrophilic, which would
greatly facilitate the infiltration of aqueous electrolytes into the
electrode materials (schematic illustration in Figure 1b). The
successful exchange of octylamine ligand with pyridine was
evidenced by FT-IR characterizations with the appearance of a
peak characteristic of C−N−C bonds for pyridine molecules at
1445 cm−1 and the simultaneous disappearance of signals for
octylamine formerly located at 2028 and 2920 cm−1 (Figure 4a).
The QDs have excellent structural integrity and no significant
morphological changes were found during the exchange pro-
cess, as evidenced by the findings of monodispersed QDs of
+
vibration and torsional oscillation of −NH3 group when it
interacts with the apical oxygen of the W−O framework and
+
forms R-NH3 …−O −W bonds.[12] Considering the WO3−x QDs
are produced in the solution of octylamine, probably the QDs
are wrapped by a layer of octylamine molecules with amine
end pointing inward and interacting with dangling bonds on
the surface of QDs as illustrated in scheme Figure 1b. This also
explains the good dispersibility in hexane.
The exciton Bohr radius of tungsten oxide materials was esti-
mated to be 3 nm,[11] and consequently the QDs with average
crystalline sizes down to 1.6 nm are in the strong-quantum-
confinement regime and would display clear features associated
with size quantization effects such as a blue shift in absorption
and near band edge emission. Figure 3a shows the UV-Vis
optical absorption data, which detected a strong absorption
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Figure 3. (a) UV-Vis absorbance spectrum of the QDs solution held in a quartz cell. Inset: plot of (αhv)0.5 against hv to achieve the bandgap of 4.19 eV
for QDs, greatly blueshifted compared to 2.6 eV for bulk materials. (b) PL emission spectrum under excitation wavelength of 220 nm showing an
obvious and sharp UV emission peak at 299 nm. Even when compared with other nanostructured materials such as nanorods or nanofilms, the emis-
sion is shifted to high-energy regime. (c) Total densities of states of XPS valence band showing the higher-energy-shift of VB maximum for QDs by
0.57 eV. (d) A comparison of modulated CB and VB energy levels of as-synthesized QDs with bulk tungsten oxide materials. The numbers with asterisk
are from previous reports.[13,15]
comparable sizes in TEM (Figure 4b) and AFM (Supporting
Information) observations. It is noted, however, after the
exchange treatment the particle size distribution is slightly
broadened with increased percentage of particles with slightly
bigger sizes, which can be explained by some aggregations of
nanoparticles due to the relatively thinner shell of pyridine
ligand and the resultant decreased interpar-
remove possible unbonded organics. Electrode of bulk mate-
rials was made similarly with the same mass loading. A com-
parison view of the CV profiles at an example scan rate of
80 mV s−1 is shown in Figure 5a. For bulk tungsten oxide, one
pair of weak and broad redox peaks is observed at the potentials
of +0.13 and −0.12 V, which derives from the redox reactions
ticle distances. Electrochemical impedance
spectroscopy characterizations confirmed
that the QDs after pyridine exchange pos-
sess both fast ion diffusion and low charge
transfer resistance, which may provide
advantages for excellent electrochemical per-
formances (Supporting Information).
The behavior of the QDs after pyridine-
ligand exchange as electrode materials was
first examined by electrochemical studies of
cyclic voltammograms (CV) and galvanostatic
charge/discharge measurements and com-
pared with that of bulk tungsten oxide mate-
rials. The QDs samples after ligand exchange
were coated onto FTO glasses as working
Figure 4. (a) FT-IR spectrum supporting the successful ligand exchange with the occurrence
of peak corresponding to C-N-C of pyridine and disappearance of signals for octylamine. (b) a
electrode, followed by sintering at 140 °C to typical TEM image showing that ligand exchange did not bring obvious morphology changes.
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Figure 5. (a) CV profiles at a scan rate of 80 mv s−1 with QDs material showing two pronounced redox pairs but bulk material having no clear features.
CV profiles for (b) bulk and (c) QDs materials at various scan rates of 5, 10, 20, 50, 80, 150, 300, and 500 mV/s. Fine features for QDs are well main-
tained even at a high scan rate of 500 mV/s while those for bulk material are lost at increasing scan rates. (d) Galvanostatic charge/discharge profiles
for QDs and bulk materials under different currents, 0.2, 0.5, 1, 2, 4, 6, and 8 mA, in the potential range of −0.5 to 0.2 V. With increase in applied
current, the triangle shape for QDs electrode in both charge and discharge process is well maintained and only limited capacity sacrifice was found,
supporting the fluent ion transport in QDs materials.
between W6+ and W5+ accompanied by H+ insertion/extraction
WO3−x + e− + H+ ⇔ HWO3−x. Such nearly featureless CV pro-
files have been usually observed with various tungsten oxide
analogues of both stoichiometric and nonstoichiometric com-
positions as well as other oxides, especially at high scan rates,
which results from the difficult and long distance ion diffusion
in the oxide lattice.[17] In contrast, the profile for QDs possesses
a richer structure, which features two well-resolved pairs of
redox peaks with the anodic voltage peaks lying at −0.02 and
−0.18 V and the cathodic ones at −0.14 and −0.30 V. Excluding
the possibilities that the signals arise from existing organics or
FTO substrate, they can be both ascribed to WO3−x QDs, sug-
gesting that the ion insertion probably occurs in two distinct
stages, WO3−x + ne− + nH+ ⇔ HnWO3−x and HnWO3−x + (1−n)
e− + (1−n)H+ ⇔ HWO3−x (0 < n < 1). A similar split of redox
peaks was observed in a nanostructured V2O5, reflecting the
phase transformations from α-V2O5 to ε-V2O5 and from ε-V2O5
to δ-V2O5, which, however, did not occur to bulk materials
except scanning at very low rate of 1 mV s−1.[18] For tungsten
oxide materials, the visibility of such fine structures is believed
to be a great profit from the dimensional downsizing of elec-
trode materials. Furthermore, the potential difference between
the oxidation and reduction potential (EO-ER) is an important
measure of the reversibility of the redox reaction, with 0.12 V
for WO3−x QDs but 0.25 V for bulk counterpart, suggesting the
improved reversibility of QDs materials.[19]
The charge storage/delivery capability was further examined
by recording CV profiles at various sweep rates ranging from 2
to 500 mV/s (Figure 5b and c). At increasing scan rate, although
both bulk and QDs materials exhibit current increase together
with shift of oxidation peaks to a more positive position and
reduction peaks to a more negative position due to an increase
in the internal diffusion resistance within the pseudocapactive
material, QDs material is robust enough to be charged and
discharged without significant distortion in profile shape, well
keeping the fine structures even at a scan rate up to 500 mV s−1,
in distinctive difference from the loss of reaction information at
high scan rates for bulk counterpart, corroborating a low internal
resistance and fast charge transfer in the QD materials. Figure 5d
depicts the charge/discharge behaviors at various currents, with
the downward lines corresponding to charging and upward
ones for discharging. In obvious contrast from the unsymmetric
curve shape for bulk tungsten oxide, the WO3−x QDs manifest
typical pseudocapacitive triangular shapes over a wide range of
applied currents, from 0.2 up to 8 mA, indicative of the revers-
ible occurring of efficient and fast ion transport for QDs mate-
rials. Voltage change during current switching is known to be
proportional to the resistance of electrode materials. Much
smaller voltage drop is observed with QDs at the beginning of
the discharge compared with those of bulk WO3 recorded at
the same current, which is attributed to the fluent ion transport
through the conducting surface. With an increase in applied
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Figure 6. (a) Transmittance spectra taken for electrodes of tungsten oxide QDs on deposited FTO in the colored (−0.8 V) and bleached (+0.2 V)
states. The material has high transparency at bleached state, over 90% of transmittance, and large modulation window, >80%, which are very desirable
for electrochromic devices. Inset: optical images of the electrode at bleached and colored states, appearing almost transparent and dark blue color,
respectively. (b) Plot of optical density variation as a function of charge density monitored at wavelength of 633 nm, which gives coloration efficiency of
154 cm2 C−1. (c) Transmittance switching for bulk (green) and QDs (black) tungsten materials showing the significant fast response for QDs martials.
(d) Enlarged in situ transmittance variation curve between the colored and bleached states for QDs. The switching time for QDs is within 1 s, whereas
for bulk tungsten oxide even 50 s could not give complete coloring or bleaching.
current, the decrease in capacity for bulk material is much more
severe than that of QDs, especially in cathodic capacity (down-
ward direction), indicative of improved rate capability or better
behaviors under high current charge/discharge conditions for
QDs (estimated specific capacitance in Supporting Information).
Electrochromism is one most visualized phenomenon that
reflects the on-going electrochemical process. The electro-
chromic optical modulation range for QDs was initially exam-
ined, and Figure 6a shows the obtained UV-Vis transmission
spectra in its colored and bleached states at different poten-
tials with the corresponding optical images in the inset. At
bleached state under applied voltage of +0.2 V, the electrode
is transparent with optical transmittance of ∼94% at a wave-
length of 633 nm, while it is in a deep blue color with trans-
mittance at the same monitored wavelength decreased to 9.0%
at the colored state under the voltage of −0.8 V. Therefore, the
QDs material possesses both a high transparency over 90%
under bleached state and an extremely large modulation range
of visible light, up to 80%, which is very desirable in the elec-
trochromic devices. Coloration efficiency (CE) is a very impor-
tant parameter for electrochromic materials, which is defined
as the change in optical density (OD) per unit of inserted
charge, i.e., CE = ΔOD/ΔQ = log(Tb/Tc)/ΔQ, where ΔQ is the
inserted charge that promotes the ΔOD change in the optical
absorbance, Tb and Tc refer to the bleached and colored trans-
mittance at a certain wavelength, respectively. The relative
chronoamperometric transient for ΔQ is provided in Sup-
porting Information. A high CE gives a large optical modula-
tion at small charge insertion or extraction, which can desir-
ably induce long-term cycling stability.[8] Figure 6b plots the
relation of OD with current density and the CE can be calcu-
lated from the slope of the linear region of this curve, yielding
154 cm2 C−1 for tungsten oxide QDs. The value is much larger
than that for bulk tungsten oxide and reported tungsten oxide
nanomaterials, either amorphous or crystalline, which typi-
cally ranges from 10 to 108 cm2 C−1,[8,20] suggesting the much
improvement in charge utilization by the size quantization
and conductive ligand wrapping. As a direct gauge parameter
for electrochemical process speed, the switching time between
coloration and bleach in electrochromism was tested. Figure 6c
depicts the reversible switching between colored and bleached
states monitored by transmittance changes at a wavelength of
633 nm with time over a range of 500 s. The switching time is
defined as the spanning period required for 90% of the change
between the steady bleached and colored states. The electro-
chromic response times of QDs is significantly faster compared
with bulk counterpart. For bulk tungsten oxide material, even
given 50 s, the plateau of coloration or bleaching could not be
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and hexane were from Sinopharm Chemicals. All reagents were used
as received without further purifications. The substrate for making
electrode was commercial SnO2: F-coated glass provided by NSG, the
sheet resistance for which was 15 Ω square−1.
achieved. In contrast, a zoon-in view on one cycle in Figure 6d
shows that the response time for both coloration and bleaching
are surprisingly within 1 s. The stability with extended tests of
50 cycles is included in the Supporting Information. To the best
of our knowledge, the new tungsten oxide QDs material herein
is among the best in documented inorganic materials and even
competitive with organic compounds in electrochromic perfor-
mances. For reference, a table listing the performances of some
typical materials is provided in the Supporting Information.
While QDs with the ligand exchanged with pyridine mani-
fest unparalleled electrochemical and electrochromic behaviors,
the performances exhibited by pristine QDs with long-chain
octyalmine as capping ligand are much inferior. Significantly
different from the good behaviors of QDs with pyridine ligand,
such as occurrence of fine features in CV scans, consistent
keeping of the fine structures even at a very high scan rate of
500 mV/s, the CV profiles of pristine QDs were almost feature-
less, and severe distortion was found with increased scan rates,
which can be explained by the larger internal resistance and
slower charge transfer of pristine QDs compared with pyridine-
exchanged QDs. With regard to electrochromic behaviors, the
pristine QDs showed obviously prolonged coloration/bleaching
time (7.4/6.4 s) compared with pyridine-exchanged sample
(both <1 s) under the same conditions (Supporting Information).
Therefore, the superior performance of pyridine-exchanged QDs
can mainly be accounted by two important factors, 1) fast and
efficient diffusion of the electrolyte ions to the active material due
to the conducting and hydrophilic surface, and 2) excellent mass
transport within the lattice because of the size quantization.
In summary, QDs of a nonstoichiometric tungsten oxide,
WO3−x (3−x ≈ 2.94), with an average crystalline size of 1.6 nm
are successfully synthesized via a simple colloid process using
tungsten aryloxide as the precursor. The QDs are stabilized with
a surface layer of octyalmine ligand, the exchange of which to
pyridine brings about excellent hydrophilicity and conductivity,
providing possibilities for superior electrochemical behaviors.
Electrochemical investigations revealed the reversible occur-
rence of efficient and fast ion transport in charging and dis-
charging for QDs electrodes, and consequently an extremely
excellent electrochromic performance was achieved, with col-
oration/bleaching time within 1 s and coloration efficiency up
to 154 cm2 C−1, which is much superior to inorganic analogues
and even competitive to organic related materials. We believe
the unusually good behaviors benefit from the size quantiza-
tion and the deliberately wrapped conductive pyridine ligand
on the surface, which would create short diffusion path length
and greatly facilitate the mass/charge transfer both at the con-
tact interfaces and within the electrode materials during the
electrochemical process, leading to an enhanced mass/energy
utility especially at high current densities. The findings here
are exciting because they demonstrate the zero-dimensional-
downsizing of normal materials greatly facilitates fast electro-
chemical kinetics, and further pave the way for applications of
QDs materials into ultrafast-response electrochemical devices.
Synthesis of Tungsten Aryloxide (W(OC6H5)6) Precursor: Tungsten
aryloxide (W(OC6H5)6) was synthesized through a modified approach
on a previously reported method.[21] In a typical synthesis route,
phenol (15 g) was added to tungsten chloride (3.6 g) under nitrogen
atmosphere in the absence of any solvent, and the system was put to
refluxing under continuous magnetic stir for 6 h. After reactions the
resultant dark red mixture was extracted with 100 mL ether and washed
with ∼5% NaOH solution for several times to ensure complete removal
of excess phenol. The ether was finally removed in vacuum to give a dark
red solid of tungsten aryloxide with yields estimated to be of ca. 35%.
Synthesis of Tungsten Oxide (WO3−x) QDs: Tungsten aryloxide precursor
(0.2 g) was melted in a flask under 180 °C under N2 fl ow, followed by
immediate injection of octylamine (3 mL). After performing the reaction
at this temperature for 5 min, the QDs appearing in pale blue color
were precipitated by addition of ethanol, collected by centrifugation, and
thoroughly washed with ethanol to remove unreacted raw materials. The
surface of QDs was covered by a layer octylamine ligand to keep the
unusual QD morphology, and therefore the product was redispersed in
hexane to achieve monodispersity.
Pyridine-Exchange for the QDs: To exchange the surface octylamine
ligand to pyridine, the QDs were dissolved in pyridine and heated in an
oil bath for 3 h at 70 °C and a subsequent extended exchange reaction
was performed overnight at room temperature while keeping stirring.
After the exchange was completed, hexane was added to the solution
to precipitate the pyridine-exchanged QDs, which was isolated by
centrifugation and redispersed in pyridine.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by National Natural Science Foundation of
China (No. 51372266, 51102274) and 973 project (No. 2011CB932600).
Received: January 28, 2014
Revised: March 20, 2014
Published online:
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