hydrothermally synthesized wo3 nanowire arrays with highly improved

Hydrothermally synthesized WO 3 nanowire arrays with highly improvedelectrochromic performance† Jun Zhang, Jiang-ping Tu,* Xin-hui Xia, Xiu-li Wang and Chang-dong Gu Received 14th Decemb

Hydrothermally synthesized WO 3 nanowire arrays with highly improved

electrochromic performance†

Jun Zhang, Jiang-ping Tu,* Xin-hui Xia, Xiu-li Wang and Chang-dong Gu

Received 14th December 2010, Accepted 11th February 2011

DOI: 10.1039/c0jm04361c

A hexagonal WO3nanowire array film is obtained using a template-free hydrothermal method by

adding ammonium sulfate as a capping agent The WO3nanowires grown vertically on a FTO-coated

glass substrate are woven together at the surface of the film, forming well-aligned arrays at the bottom

part and a porous surface morphology Transmission electron microscopy (TEM) and X-ray

diffraction (XRD) reveal that each nanowire is a hexagonal single crystal and their long axes are

oriented toward the [0001] direction Due to the highly porous surface, good contact with the

conductive substrate and large tunnels of the hexagonal-structured WO3, a fast switching speed of 7.6

and 4.2 s for coloration and bleaching, respectively, and a high coloration efficiency of 102.8 cm2C
1

are achieved for the WO3nanowire array film

Tungsten trioxide (WO3) attracts extensive attention because of

its distinctive physical and chemical properties, making it

suit-able for applications in electrochromic (EC) devices,1–4

photo-catalysis5,6and gas sensing.7,8EC devices are able to change their

optical properties (coloring/bleaching) reversibly by alternating

the polarity of the applied small voltage Compared to other

transition metal oxides, WO3 is the most widely studied EC

material due to its multiple oxidation states, high coloration

efficiency and good cyclic stability

The interest in EC WO3has increased over the last decade due

to its application in solar light control and energy saving in

modern buildings, which always have a large area made up of

windows Generally, WO3 film is fabricated on a transparent

conductive substrate to form a working electrode in an EC

device The approaches to fabricating WO3 films including

sputtering,9,10pulsed laser deposition,11,12sol–gel,13–16

electrode-position,17anodic oxidation,18,19thermal evaporation,20chemical

vapor deposition (CVD),21hot-wire CVD22and electrophoresis

deposition (EPD).23However, each of these methods has one or

more characteristic drawbacks, including being highly energetic,

being vacuum dependent, or requiring exotic and often

dangerous reagents The hydrothermal method is a facile,

dominant tool for the synthesis of crystalline oxides with a high

surface area and unique morphology The significant advantages

of this less complicated method are controllable size, growth at low temperature and cost-effectiveness.24

The hydrothermal method has been used to synthesize one-dimensional (1D) WO3 nanowires or nanorods with Li2SO4, (NH4)2SO4, or NaCl as the capping agent.25–29Their

EC properties have been investigated in coatings fabricated from 1D WO3 nanowires through drop-assembly25,26 or elec-trophoretic deposition (EPD).30Although EC performance was enhanced for some aspects of these WO3 nanowire films, two drawbacks affected the response (coloring/bleaching) speed and the coloration efficiency: (a) the poor contact between the WO3nanowires (especially the upper parts of the films) and the transparent conductive substrate; (b) the active surface area of the nanostructures was not sufficiently utilized due to the compact stacking Growing the nanowires vertically onto the substrate to form the WO3arrays (so each nanowire has good contact with both the substrate and the electrolyte) would probably be a promising approach to resolve this problem

It has been reported that WO3 nanowire arrays can be deposited on metal surfaces, such as Mo and W, using a thermal evaporation method or CVD,31–34while preparing WO3 nano-wire arrays through a template-free solvent path still remains

a challenge In our previous work,35thick films of WO3nanowire arrays were successfully fabricated on an alumina plate and tungsten foil, and they showed fast reversible wettability change between superhydrophilicity and superhydrophobicity Very recently, photoelectrochemical properties of WO3 nanowire/ nanoflake arrays have been investigated by Grimes et al.36In this work, we make an attempt to grow WO3thin films directly on FTO-coated glass by a hydrothermal method Sulfate-assisted, hydrothermally synthesized, porous WO3nanowire array film is

State Key Laboratory of Silicon Materials and Department of Materials

Science and Engineering, Zhejiang University, Hangzhou, 310027, China.

E-mail: tujp@zju.edu.cn; Fax: +86 571 7952573; Tel: +86 571 87952856

† Electronic supplementary information (ESI) available: Figs S1 and S2

and Table S1 See DOI: 10.1039/c0jm04361c

Materials Chemistry

Cite this: J Mater Chem., 2011, 21, 5492

expected to have a high surface area and an enhanced EC

performance

2.1 Chemical materials

All solvents and chemicals were of analytical grade and were used

without further purification FTO-coated glass was purchased

from Nippon Sheet Glass (Japan) Sodium tungstate,

ammo-nium sulfate and hydrochloric acid (35%) were obtained from

Sinopharm Chemical Reagent Co., Ltd (China) Lithium

perchlorate (anhydrous) and propylene carbonate (PC) were

purchased from Aladdin Chemistry Co., Ltd (China) All

aqueous solutions were freshly prepared with de-ionized water

2.2 Preparation of WO3films

A WO3seed layer was prepared on FTO-coated glass through

a sol–gel method Firstly, FTO-coated glass (15 mm 20 mm in

size) was washed with acetone, then ethanol, and finally

de-ionized water in an ultrasonic bath for 10 min per wash The

WO3sol was prepared according to a literature method,37then

the sol was cast onto the FTO-coated glass through spin-coating

technology, followed by annealing at 400C for 30 min to form

a seed layer (see ESI†, Fig S1)

WO3 nanowire arrays were synthesized by a sulfate-assisted

hydrothermal method Briefly, 3.29 g sodium tungstate powder

was dissolved in 76 ml de-ionized water, and a 3 M HCl aqueous

solution was used to adjust the pH value to 2.0 Afterwards,

ammonium sulfate (2.64 g) was added to the reaction precursor

to control the morphology of the WO3product After stirring for

1 h, the clear solution obtained was transferred into a

Teflon-lined stainless autoclave The FTO-coated glass with the

WO3seed layer was placed vertically in the autoclave, and then

the autoclave was sealed and heated at 180 C for 4 h The

obtained film with a thickness of 1.5mm was washed and dried in

a vacuum oven at 60C for 12 h For comparison, a WO3film

with equal thickness was prepared by the same process without

the addition of ammonium sulfate

2.3 Characterizations

X-ray diffraction (XRD) measurements were conducted with

a PANalytical X’Pert PRO diffractometer, Cu-Ka radiation (l ¼

1.54056 A) The 2q range was 10–80 with a step of 0.02and

a scanning speed of 2.4min
1 The morphology of the films was

characterized by a field emission scanning electron microscope

(FESEM, Hitachi S-4800) and a high resolution transmission

electron microscope (HRTEM, Philips CM200 UT, operated at

160 kV) The Brunauer–Emmett–Teller (BET) surface area of the

WO3nanowire array film was studied using nitrogen adsorption

at 77 K using an Autosorb-1-C analyzer (Quantachrome)

Electrochemical measurements were carried out on an

electro-chemical workstation (CHI660C, Shanghai Chenhua

Instru-ments, Inc.) using a conventional three-electrode test cell The

working electrode was the WO3film on FTO glass An Ag/AgCl

electrode and Pt foil were used as reference and counter

elec-trodes, respectively Cyclic voltammetry (CV), electrochemical

impedance spectroscopy (EIS) and chronoamperometry (CA)

tests were performed in a propylene carbonate (PC) solution of

1 M LiClO4 CV measurements were carried out at a scanning rate of 50 mV s
1between
1.0–1.0 V at room temperature EIS tests were carried out with a superimposed 5 mV sinusoidal voltage in the frequency range 100 kHz to 0.01 Hz The EIS results obtained experimentally were analyzed using a non-linear least squares fitting program EQUIVCRT CA tests were con-ducted under a square-wave voltage of2.0 V with a pulse width

of 100 s The optical properties were recorded using a UV-vis spectrometer (Shimadzu UV-240)

3.1 Structure and morphology Fig 1 shows the SEM images of the WO3films prepared from

a tungstate acid sol using a simple hydrothermal method with and without the addition of ammonium sulfate The plan-view SEM images of the films prepared in the presence of ammonium sulfate show a macroporous surface morphology where nano-wires are interwoven together (Figs 1a and b) From the sectional view and back view images of the films, it is found that the well-aligned WO3nanowires grow vertically on the substrate (Figs 1c and d) The nanowires first grow in a parallel manner from the seed layer, and then as the nanowires are lengthened, they change their direction gradually It is reasonable to form such

a morphology under the hydrothermal conditions of high

Fig 1 SEM images of WO 3 films prepared in the presence of ammonium sulfate: (a) a plan view of the front, (b) a magnified view, (c) a cross-sectional view, (d) a view of the back; without ammonium sulfate: (e)

a plan view, and (f) a cross-sectional view.

temperature and high pressure The WO3 film grown in the

solution without ammonium sulfate is composed of bulk

micro-bricks (Figs 1e and f) and has a similar surface morphology to

that reported by Jiao et al.37 The XRD patterns of the

FTO-coated glass, the WO3micro-brick film and the WO3nanowire

array film are shown in Fig 2 From the XRD patterns, it is

confirmed that the nanowire array film is a pure phase of

hexagonal WO3 corresponding to JCPDS No 85-2459 A

strongly preferential growth direction along the c-axis [0001] can

be found The synthesis in the absence of ammonium sulfate

yielded an orthorhombic phase of WO3hydrate with the XRD

pattern corresponding to JCPDS No 87-1203 It is found that

the WO3nanowires have a BET surface area of 116.5 m2g
1(see

Fig S2†) Fig 3 shows the TEM images of the WO3nanowires,

which are scraped from the substrate and ultrasound-treated in

alcohol The nanowires have a typical length of 1.5 mm with

diameters of 20–40 nm The selected area electron diffraction

(SAED) pattern and the HRTEM image show that each

nano-wire is a single hexagonal crystal that preferentially grows along

the [0001] direction, which is in accordance with the results of the

XRD characterization

The precipitation of WO3from a tungstate ion solution using

concentrated acid is a well-known synthetic route described as

follows:38,39

WO42
+ 2H++ nH2O# H2WO4$nH2O (1)

H2WO4$nH2O# WO3+ (n + 1)H2O (2) First, tungstate acid hydrate is synthesized from sodium

tungstate in acidic solution at room temperature to form a WO3

sol Then WO3 nucleates to form primary particles from the

precursor under 180C hydrothermal conditions In the presence

of an appropriate amount of ammonium sulfate in the solution,

WO3primary particles aggregate along the [0001] direction of the

hex-WO3unit cell via self-assembly, because sulfate ions

prefer-entially adsorb on the faces parallel to the c-axis of the WO3

nanocrystal and thus 1D single crystal nanowires are formed.27

Although WO3nanorods can be synthesized with NaCl as the

capping agent,25,26it is likely that only the sulfate ions lead to the

formation of 1D hex-WO3 nanostructures with a high aspect

ratio.27,28On the other hand, ammonium ions play a crucial role

in the formation of the hex-WO3structure.28,40Fig 4 summarizes the formation process of WO3nanowires and micro-bricks under different hydrothermal conditions

3.2 Electrochemical and electrochromic properties Fig 5 shows the cyclic voltammograms (CVs) of the WO3films prepared with (a) and without ammonium sulfate (b), i.e micro-brick film and nanowire array film, respectively As can be seen from the CVs, the current density of the micro-brick film first increases then becomes stable, indicating an activation process

Fig 2 XRD patterns of FTO and WO 3 films prepared with or without

ammonium sulfate.

Fig 3 TEM images of nanowires (insets of (b) are the SEAD (bottom left) and HRTEM (top right) images).

Fig 4 A schematic illustration of the formation process of the WO 3

nanowires and micro-bricks under different hydrothermal conditions.

Contrastingly, the nanowire array film shows no activation stage,

and its current density decreases slowly from the first cycle to the

1000th cycle This difference could be attributed to an effect of

the morphology, that the nanowire array film with a porous

surface facilitates Li+ion intercalation/deintercalation, similar to

our previous works.41,42The evolution of the current density in

the CVs of both films also indicates that the nanowire array film

degenerated slightly faster than the micro-brick film The porous

surface is responsible for the degeneration of the current density,

because the porous morphology causes it to dissolve into the

solution faster than that of the dense morphology

The electrochromic properties of WO3films were measured after the film electrodes had been subjected to CV testing for

10 cycles in 1 M LiClO4/PC solution Fig 6 shows the UV-vis transmittance spectra of WO3 films in colored and bleached states, which are applied at
2.0 V and 2.0 V (vs Ag/AgCl) for

100 s, respectively The color of the WO3films changes from deep blue (colored state) to transparent (bleached state) reversibly This process is in accordance with intercalation (deintercalation)

of the Li+into (out from) the WO3films:

WO3þ xLiþþ xe
# LixWO3

It is clearly seen that the WO3 nanowire array film shows

a larger transmittance modulation than the micro-brick film The modulation range of the transmittance of the WO3 nanowire array film is high, up to 58% at 633 nm, while the micro-brick film only exhibits 45% at 633 nm Fig 7 shows the digital photo-graphs of the WO3nanowire array film at different stages It can

be seen from the digital photos that the color is uniform and can

be controlled by polarization at different voltages

The switching characteristics of the WO3films are investigated

by chronoamperometry and the corresponding in situ trans-mittance at 633 nm (Fig 8) The chronoamperometry was per-formed on the WO3 films between
2.0 and +2.0 V The coloration and bleaching times are defined as the time required for a 90% change in the full transmittance modulation at 633 nm For the WO3nanowire array film, the coloration time tcis found

to be 7.6 s, and the bleaching time tbis 4.2 s However, for the

WO3micro-brick film, the coloration time tcis 46.1 s, and the bleaching time tb is 38.8 s The switching speed of the WO3 nanowire array film is faster than the micro-brick one Further-more, it is faster than the compact assembled nanorod films prepared by drop-assembly, which need 272 and 364 s for a 90%

Fig 5 Cyclic voltammograms of the WO 3 films prepared with (a) and

without (b) ammonium sulfate.

Fig 6 The UV-vis transmittance spectrum of the WO 3 films in the

colored and bleached states (solid square: nanowire array film; open

circle: micro-brick film).

Fig 7 Digital photographs of the WO 3 nanowire array film at different stages: (a) as-prepared; (b) colored at
1.0 V for 30 s; (c) colored at

2.0 V for 30 s; (d) bleached at 2.0 V for 30 s.

modulation in coloration and bleaching, respectively.25The fast switching speed of the WO3 nanowire array film is due to the large active surface area of a highly porous structure, good contact between the nanowires and the substrate, and large tunnels in the hexagonal WO3

Coloration efficiency (CE), which is defined as the change

in optical density (OD) per unit of charge (Q) inserted into (or extracted from) the EC films, is a characteristic parameter for comparing different EC materials It can be calculated from the following formulas:

where Tb and Tc refer to the transmittance of the film in its bleached and colored states, respectively A high value of CE indicates that the EC film exhibits a large optical modulation with a small charge inserted (or extracted) Fig 9 shows the plots of OD at a wavelength of 633 nm versus the inserted charge density at a potential of
2.0 V Under the biasing of potential, the OD tends to a constant value with an increase in charge density after a short time The CE is extracted as the slope

of the line fits to the linear region of the curve The calculated CE values are 24.5 and 102.8 cm2C
1, for the micro-brick film and nanowire array film, respectively Combining the results of

Fig 8 Chronoamperometry curves and the corresponding in situ

transmittance at 633 nm for the nanowire array film (a, b) and

micro-brick film (c, d). Fig 9 The variation of the in situ optical density (OD) vs the charge

density for the WO 3 nanowire array film (a) and micro-brick film (b) The

OD was measured at 633 nm at a potential of
2.0 V.

chronoamperometry and in situ transmittance vs time shown in

Figs 8a and b, it can be concluded that, for the WO3nanowire

array film, a major optical modulation is completed in a short

time after voltage switching However, for the micro-brick film it

takes a longer time and more charge to accomplish the saturation

of OD Compared to the hydrothermally-prepared micro-brick

film in this work (24.5 cm2C
1) and a previously reported work

(38.2 cm2C
1),37the CE value of the WO3nanowire array film is

much higher The obtained CE value is also higher than that for

the compact films assembled by nanowires or nanorods,26while it

is comparable with the thin film derived from nanorods prepared

by a solvothermal method (89–141 cm2C
1).43

To further understand the electrochemical behavior of the

as-prepared film electrodes, EIS measurements were conducted by

applying an AC voltage of 5 mV in a frequency range of 10 mHz

to 10 kHz at their bleached state (about 0.33 V vs Ag/AgCl) As

shown in Fig 10, the plots of the nanowire array and micro-brick

films show two semicircles in the high frequency and medium

frequency ranges, respectively Fig 11a presents the equivalent

circuit for the nanowire array and micro-brick films to simulate

the experimental EIS plots,44–46and the fitting curves are drawn

in Figs 10a and b, correspondingly Re designates the solution

resistance; Rsl(i) and Csl(i) (i¼ 1, 2) denote the migration of

lithium ions and the capacity of the layer, respectively Rctand

Cdlrepresent the charge-transfer resistance and a double-layer capacitance ZWis the Warberg impedance The detailed kinetic steps involved in Li+intercalation into WO3films are illustrated schematically in Figs 11b and c These parameters can be calculated using ZView software (see Table S1†) It is found that the nanowire array film shows much lower Rsland ZWthan the micro-brick one, indicating that the porous and well-aligned structure is more favorable for charge transfer and Li+ ion diffusion than the compact structure, resulting in higher reac-tivity and reaction kinetics

WO3 thin films were prepared by a hydrothermal method on FTO-coated glass Nanowire arrays of WO3 were synthesized successfully by adding an appropriate amount of ammonium sulfate as the capping agent into the hydrothermal solution Comparing with the micro-brick structured WO3film, the highly porous and well-aligned WO3 nanowire array film exhibited

a much better electrochromic performance The WO3nanowire array film showed a transmittance modulation of up to 58% at

633 nm and the coloration efficiency was calculated to be 102.8 cm2C
1, while the WO3micro-brick film only gave values

of 45% and 24.5 cm2 C
1 at 633 nm, respectively The WO3 nanowire array film also showed faster coloration and bleaching

Fig 10 EIS plots of the nanowire array film (a) and micro-brick film (b).

Insets are an enlargement of the high frequency range.

Fig 11 The equivalent circuit used for fitting the experimental impedance data (a); schematic presentations of the kinetic steps involved in Li +

intercalation into the WO 3 nanowire array film (b) and micro-brick film (c).

speed during voltage switching The CV and EIS measurements

revealed that higher reactivity and reaction kinetics were

obtained for the nanowire arrays The highly improved

electro-chromic performance of the WO3nanowire array film enables it

to be a promising prospect for application in electrochromic

devices In addition, the WO3 nanowire arrays will also have

applications in field-emission devices, gas sensors and

photo-catalysis

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