In particular, a significantly lower threshold voltage and an increased electrochromic contrast can be achieved compared with unloaded neat TiO2nanotube layers.. We show a facile route to
Trang 1Decoration of TiO 2 nanotube layers with WO 3 nanocrystals
for high-electrochromic activity
A Benoit1, I Paramasivam, Y.-C Nah, P Roy, P Schmuki*
Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany
a r t i c l e i n f o
Article history:
Received 14 January 2009
Accepted 20 January 2009
Available online 24 January 2009
Keywords:
TiO 2 nanotubes
WO 3 nanoparticle
Electrochromism
a b s t r a c t
We report a simple approach to decorate ordered TiO2nanotube (TiNT) layers with tungsten trioxide nanocrystallites by the controlled hydrolysis of a WCl6precursor These WO3nanocrystallites, when formed, are amorphous, but can be annealed to a monoclinic crystal structure The WO3crystallites on the TiO2nanotube skeleton are electrochemically active, and hence ion insertion reactions are possible
As a result, the decorated nanotube layers show remarkable enhancement of the electrochromic proper-ties In particular, a significantly lower threshold voltage and an increased electrochromic contrast can be achieved compared with unloaded (neat) TiO2nanotube layers
Ó 2009 Elsevier B.V All rights reserved
1 Introduction
Over the last years the anodic formation of ordered TiO2
nano-tube (TiNT) layers has created significant scientific interest (see
Refs.[1,2]for an overview) The application of these nanotube
lay-ers has been explored for example in photocatalysis [3],
photo-chromism[4], or biomedicine[5] Due to the tubular nature and
large surface area, they were used as hosts for insertion of H+or
Li+ions[6,7], and considerably high-electrochromic contrast can
be obtained due to the nanotubular architecture This
electrochro-mic effect is based on the fact that when electrons and ions are
in-jected under cathodic polarization, the electronic structure of TiO2
is modified It was reported[8,9]that the reduced form of Ti3+by
electron injection leads to local electronic states 0.7 eV under the
conduction band, which results in an absorption in the visible
range TiO2based devices are mostly built with nanoparticulate
systems to shorten the solid-state diffusion path and time[10]
Re-cently, we reported that this solid-state diffusion step can be
dras-tically accelerated by widening of the host TiO2lattice by doping
with Nb2O5[11]which also allows for the insertion of Na+ions
However, other transition metal oxides, in particular WO3, are
typically more efficient electrochromic materials than TiO2, i.e
typical electrochromic criteria such as coloration efficiency and
threshold voltage are reported to be better[12–14] Recently our
group investigated WO3nanoporous structures that show a
drasti-cally enhanced electrochromic contrast, and a faster switching
re-sponse than the compact anodic WO3layers[15] However, a key drawback of pure W is that up to now no highly defined tubular structures could be grown[15,16], and thus the full electrochromic potential of WO3 based nanotubular systems could not be exploited We showed that one strategy to overcome this problem
is anodizing Ti–W alloys[16] By suitable substrate alloying, highly defined mixed oxide TiO2–WO3nanotubes with strongly enhanced electrochromic properties could be grown
In this work, we explore another approach to combine the out-standing WO3 electrochromic properties with the defined mor-phology of TiO2 nanotubes We show a facile route to decorate the TiO2 nanotubes with WO3nanocrystallites and demonstrate that these decorated tubes have significantly enhanced electro-chromic characteristics
2 Experimental TiO2nanotube layers were grown by anodic oxidation of tita-nium foils with 99.6% purity (from Goodfellow, England) of 0.1 mm thickness Prior to the experiments the titanium foils were sonicated in acetone, isopropanol and methanol (for 3 min each) followed by rinsing with deionized water and then dried in a nitro-gen stream Anodization was carried out using a high-voltage potentiostat Jaissle IMP 88 using an electrolytic mixture of glycerol (1, 2, 3-propanetriol) and water (60:40 vol%) + 0.27 M NH4F[17]at
30 V for 3 h Ti samples were pressed against an O-ring in an elec-trochemical cell where 1 cm2was open to the electrolyte The
set-up[18]consisted of a three electrode configuration with a Pt gauze
as counter electrode and a Haber-Luggin capillary with Ag/AgCl as reference electrode The anodization process forms nanotube lay-ers with a tube length of 1.4lm and a diameter of 100 nm
1388-2481/$ - see front matter Ó 2009 Elsevier B.V All rights reserved.
* Corresponding author Tel.: +49 9131 852 7575; fax: +49 9131 852 7582.
E-mail address: schmuki@ww.uni-erlangen.de (P Schmuki).
1
On leave from: Université de Nantes, Nantes Atlantique Universités,
Poly-tech’Nantes, Materials Science Department, Rue Christian Pauc, BP 50609, F-44306
Nantes Cedex 3, France.
Contents lists available atScienceDirect Electrochemistry Communications
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / e l e c o m
Trang 2[17] The TiO2nanotube layers were then annealed in air using a
thermal treatment in a Rapid Thermal Annealer – Jipelec JetFirst,
at 450 °C for 1 h with heating and cooling rate of 30 °C min 1to
form an anatase structure[17]
For WO3 nanocrystallite deposition, WCl6 (Aldrich 99.9%) in
powder form was dissolved in ethanol (>99.9% Purity,
Sigma–Al-drich) to obtain 0.1 M of stock solution and then further diluted
to obtain a 0.001 M solution In this solution tungsten is very
sen-sitive to moisture (hydrolysis) and oxygen present in atmosphere
Therefore, a fresh solution was used for each new experiment In
order to preserve the solutions and minimize contact with air, vials
with septa and syringe are used for storage handling and dilution
of the solutions
For tube decoration, annealed TiNT layers are placed in a beaker
containing 10 mL of the fresh 0.001 M WCl6solution The color of
the solution is at this moment light green to yellow (if the solution
is fresh) Five milliliters of a mixed solution of water and ethanol
(50:50) is added to the stirred 0.001 M WCl6solution using a
syr-inge and the color turns to dark blue Finally, the closed beaker is
placed in water bath The temperature is raised slowly from
20 °C to 70 °C and afterwards it is kept for 1 h at 70 °C Now the
color turns from dark to light blue The sample is slightly rinsed
in ethanol and dried in a nitrogen stream The WO3 decorated
tubes are then re-annealed at 450 °C, 1 h in the rapid thermal
annealer
A Scanning Electron Microscopy, HITACHI SEM FE S4800, was
used to acquire micrographs of the tube morphology The chemical
composition of the deposited materiel was analyzed using X-ray
photoelectron spectroscopy (PHI 5600 XPS Spectrometer) with Al
Ka radiation at an incident angle of 45° to the surface normal
XRD measurements were performed using a Philips X’Pert PRO
dif-fractometer with monochromatic Cu Karadiation To characterize
the electrochemical and electrochromic behaviour of the TiO2
nanostructures with and without WO3decoration, a conventional
three electrode system was used Samples were pressed against
an O-ring with a Cu plate in an electrochemical cell A platinum
plate and a Haber-Luggin capillary with Ag/AgCl (1 M KCl) were
used as a counter and a reference electrode, respectively The
elec-trolyte was 0.1 M HClO4 The wall of the cell opposite to the sample
surface consists of a quartz glass window to allow optical
measure-ments during electrochemical cycling Reflectance measuremeasure-ments
were carried out using USB 2000 Fiber Ocean Optics Spectrometer
The cyclic voltammograms and chronoamperometric
measure-ments were performed using an Autolab PGSTAT30 Potentiostat/
Galvanostat Optical images were recorded using a CCD camera
3 Results and discussion
Fig 1shows the top view of the TiO2nanotube layers used in
this work before (Fig 1a) and after the decoration with WO3
nano-crystallites (Fig 1b–d) From the images after the WCl6treatment it
can be seen that some of the tubes are covered with a hazy very
thin layer (Fig 1b) but most of the surface area shows open and
ni-cely decorated tubes (Fig 1c and d) where individual WO3
nano-particles have a diameter of 5 nm XRD investigations were
carried out with the decorated tubes before and after additional
annealing at 450 °C Before annealing the XRD spectra only reveal
TiO2 anatase peaks, after annealing clearly peaks of monoclinic
WO3could be detected (most characteristic at h = 23°) This
indi-cates that the as deposited WO3 crystallites are amorphous and
only the annealing treatment converts them to the crystalline
material
In order to investigate the chemical composition and oxidation
state of WO3on the decorated and annealed nanotubes, XPS
mea-Fig 1 SEM images showing top views of TiO 2 nanotubes (TiNT): (a) as grown by anodization in a mixture of glycerol and water (60:40 vol%) + 0.2 M NH 4 F at 30 V for
3 h, (b–d) after decoration with WO 3 (WO 3 /TiNT).
Trang 3surements were performed.Fig 2b shows an XPS survey spectrum
that indicates that the WO3/TiNT composite material contains Ti,
W, O and some traces of carbon.Fig 2c shows the high resolution
XPS spectra of the W4f peak with W4f7/2and W4f5/2at 35.3 eV and
37.4 eV, respectively Even though the determination of the exact
position of W4f was difficult because of a partial overlap with
Ti3p peak, their positions are in line with the peak positions of pure
WO3[19]
In order to explore ion insertion properties of the samples,
elec-trochemical and optical characterization was performed Fig 3a
displays the cyclic voltamograms (CVs) of TiNT, WO3/TiNT
as-formed and annealed at 450 °C carried out in a 0.1 M HClO4
solu-tion Peaks I and II in the CVs can be ascribed to proton insertion
and extraction into and out from the host lattice [20,15] This
insertion process can either take place into the TiO2or the WO3
and may be described as TiO2+ xe + xH+?TiOOH or WO3+ xe +
xH+?HxWO3, respectively In both cases it is associated with a
change in color of the material Compared with neat TiNT, the
WO3decorated nanotubes show significantly larger current
densi-ties, which reflect the fact that proton insertion/extraction is much
more favorable in the decorated structures than in the neat tubes
Insertion into neat nanotubes occurs under the same experimental
conditions only at potentials negative to 1.0 V[6] For annealed
WO decorated samples, the current densities are smaller than
for the ones where the WO3is present as amorphous material This indicates that the crystalline phase formed after annealing at
450 °C, shows a lower ion uptake efficiency – which is in line with literature reports for pure WO3[16] Furthermore, the onset poten-tial for the cathodic reaction for WO3/TiNT (with amorphous WO3)
is located at 0.3 V while for annealed WO3/TiNT (crystalline WO3)
it is at 0 V This means that insertion can be achieved at signifi-cantly lower applied voltage for amorphous sample It also means that the underneath anatase skeleton of TiO2is sufficiently conduc-tive to allow electrochemical switching of the WO3
Fig 3b shows the current density response with time when a cycling pulse potential is applied between 0.5 V and 1.0 V The integrated current density with time (charge density) is indicative
of the amount of protons incorporated during the reactions When comparing the charge exchanged during cathodic and anodic reac-tions for TiNT and WO3/TiNT (as-formed) – compiled in Table (in-set inFig 3b) – it is clear that the WO3/TiNT show much higher values in charge density Again, after annealing at 450 °C, the charge density is slightly decreased due to the crystallinity of the material
Fig 3c shows the electrochromic effects for TiNT, WO3/TiNT as-formed, and WO3/TiNT annealed (450 °C) during potential switching between 0.5 V and 1.0 V To quantify the electrochro-mic effects, reflectance spectra were acquired Compared with
Fig 2 XRD patterns of annealed TiNT and WO 3 /TiNT annealed at 450 °C (a); XPS survey spectra of as-formed WO 3 /TiNT (b); detail of the W4d peak for WO 3 /TiNT (c).
Trang 4TiNT, the decorated WO3/TiNT shows a strong effect as apparent
from the reflectance difference (DR) At a wavelength of 600 nm,
for neat TiO2nanotube structures only a 3% change could be
ob-tained whereas for the WO3loaded systems 45% for the as-formed,
and 21% for the annealed structure can be achieved It is interesting
to note that the bleached state of annealed sample does not
abso-lutely recover to the original state after the first potential pulsing
These findings indicate that a higher crystallinity not only affects
the insertion amount but also the electrochemical reversibility
The response time for the as-formed WO3/TiNT is 3.6 s and 2.8 s
for the coloration and the bleaching, respectively, while for
an-nealed WO3/TiNT the values are 11.4 s and 10.1 s for coloration
and bleaching, respectively This again is in accord with literature
that proton movement is faster in amorphous than in crystalline
WO3[16]
It may be noteworthy that the switching threshold voltage of
the WO3is in the range of 0.3 VAg/AgCl This is very close to the
flatband potential for the underneath TiO2 (anatase) nanotubes
[21] This means that the threshold voltage for WO3to a certain
ex-tent may be dominated by the switching of the underneath
(n-type) material from depletion to accumulation conditions; in other
words, electron supply (conductivity) over the TiO2nanotube
skel-eton may determine the switching threshold voltage However, the
results inFig 3a show that a significant onset of the
electrochem-ical reaction in the WO occur even at a potentials of 0V
which shows that the nanotubes at this voltage are not entirely
in a current blocking state; i.e are still sufficiently conducting to allow switching of the WO3crystallites
In summary, this work demonstrates how TiO2nanotubes can
be decorated with WO3 nanocrystallites The decoration signifi-cantly enhances the contrast and insertion capacity of a TiO2 nano-tube based electrochromic system Decoration of the nanonano-tubes with WO3may also have a significant impact on other TiO2 nano-tube applications
Acknowledgements The authors would like to greatly acknowledge DFG for financial support We extend our sincere thanks to Helga Hildebrand and Ullrike Marten-Jahns for XPS and XRD measurements and also
to Hans Rollig and Martin Kolacyak for their valuable technical help
References
[1] J.M Macak, H Tsuchiya, A Ghicov, K Yasuda, R Hahn, S Bauer, P Schmuki, Curr Opin Solid State Mater Sci 11 (2007) 3.
[2] A Ghicov, P Schmuki, Chem Commun., submitted for publication [3] I Paramasivam, J.M Macak, P Schmuki, Electrochem Commun 10 (2008) 71 [4] I Paramasivam, J.M Macak, A Ghicov, P Schmuki, Chem Phys Lett 445 (2007) 233.
Fig 3 Electrochromic measurements of as-formed WO 3 on TiNT, monoclinic WO 3 on TiNT and TiNT (reference): (a) Cyclic voltammograms of the samples performed between 0.5 V and 1 V with a scan rate of 50 mV in 0.1 M HClO 4 electrolyte; (b) current–density time curves acquired by chronoamperometry measurement applied between 0.5 V and 1 V with 30 s duration; (c) optical images of the electrochromic effect of the different samples; (d) in situ reflectance measurements of the samples obtained during chronoamperometry measurement of Fig 3 b.
Trang 5[5] H Tsuchiya, J.M Macak, L Muller, J Kunze, F Muller, P Greil, S Virtanen, P.
Schmuki, J Biomed Mater Res Part A, doi:10.1002/jbm.a30677
[6] A Ghicov, H Tsuchiya, R Hahn, J.M Macak, A.G Munoz, P Schmuki,
Electrochem Commun 8 (2006) 528.
[7] R Hahn, A Ghicov, H Tsuchiya, J.M Macak, A.G Munoz, P Schmuki, Phys Stat.
Sol (a) 204 (2007) 1281.
[8] D.C Cronemeyer, Phys Rev 87 (1952) 876.
[9] D.C Cronemeyer, Phys Rev 113 (1939) 1222.
[10] A Hagfeldt, N Vlachopoulos, M Graetzel, J Electrochem Soc 141 (1994)
L82.
[11] A Ghicov, M Yamamoto, P Schmuki, Angew Chem., Int Ed 47 (2008) 1.
[12] K Deb, Philos Mag 27 (1973) 801.
[13] M.S Monk, R.J Mortimer, D.R Rosseinsky, Electrochromism: Fundamentals
and Applications, VCH, Weinheim, 1995.
[14] G Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995.
[15] Y.-C Nah, A Ghicov, D Kim, P Schmuki, Electrochem Commun 10 (2008) 1777.
[16] Y.-C Nah et al., J Am Chem Soc 130 (2008) 16154.
[17] J.M Macak, H Tsuchiya, S Aldabergerova, P Schmuki, Angew Chem., Int Ed.
44 (2005) 7463.
[18] L.V Taveira, J.M Macak, H Tsuchiya, L.F.P Dick, P Schmuki, J Electrohcem Soc 152 (2005) B405.
[19] P Cheng, C Deng, X Dai, D Liu, J Xu, J Photochem Photobiol A – Chem 195 (2008) 144.
[20] J.M Macak, B.G Gong, M Hueppe, P Schmuki, Adv Mater 19 (2007) 3027 [21] L Taveira, A Sagües, J.M Macak, P Schmuki, J Electrochem Soc 155 (6) (2008) C293.