ARTICLE
Journal of
Department of Chemistry, Northeast Normal University, Changchun 130024, P. R. China. E-mail:
[email protected].
Materials Chemistry
Guanggang Gao, Lin Xu,* Wenju Wang, Wenjia An and Yunfeng Qiu
www.rsc.org/materials
Electrochromic ultra-thin films based on cerium polyoxometalate
Received 7th January 2004, Accepted 19th April 2004 First published as an Advance Article on the web 26th May 2004
Ultra-thin multilayer films consisting of polyoxotungsceriumate cluster K17[CeIII(P2W17O61)2]?30H2O [Ce(P2W17)2] and polyallylamine hydrochloride were fabricated on quartz and ITO substrates by a layer-bylayer self-assembly method. The multilayer films were characterized by UV-vis spectra, cyclic voltammetry (CV) and chronoamperometric (CA) measurement. Compared with other polyoxometalates, the electrochromic film exhibits suitable response times, low operation potentials, high optical contrast and thermal stability, which provide valuable information for exploring the possibility of application to polyoxometalate-based electrochromic materials.
DOI: 10.1039/b400021h
Introduction
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In the past few years, polyoxometalates have attracted increasing attention worldwide due to their intriguing structures and diverse properties,1,2 such as catalytic activity for chemical transformation,3 molecular conductivity,4 magnetism,5 and luminescence as well as photochromism and electrochromism.6 Obviously, polyoxometalates are extremely versatile inorganic building blocks for the construction of functional materials in particular, and may be a good candidate for electrochromic materials. Although a number of electrochromic materials, such as transition metal oxides,7–12 Prussian blue,13 viologens,14 conduction polymers,15 lanthanide complexes,16 transition metal complexes17 and metal phthalocyanines18 have widely been studied, these materials have had little success because of the lifetime requirements, their cost-intensive production process, and their high price. For instance, WO3 has to be applied on the glass surface in a cost-intensive sputtering process under high vacuum. Furthermore, transition metal oxide films exhibit only slow response times for obtaining full contrast.19 While these materials should be modified in order to improve performance, the further exploration of polyoxometalate-based electrochromic materials still remains a challenge. In 1978, Tell and co-workers investigated the electrochromic properties of H3PW12O40?29H2O and H3PMo12O40?29H2O,20,21 but these electrochromic cells have a disadvantage in that they bleach relatively slowly when heavily colored. The cell’s life is also limited by an increase in cell impedance over time.22 Twenty years later, Moriguchi and Fendler began to explore the use of electrostatic assembly techniques to form electrochromic films containing the [W10O32]42 polyanion and poly(diallyldimethylammonium) polycation. However, the slow response time and low optical contrast limited their future application.23 More recently, Kurth and co-workers have reported the preparation of multilayer films containing the polyoxometalate cluster of [Eu–(H2O)P5W30O110]122 by a layer-by-layer self-assembly method. Such a multiplayer film material of [Eu–(H2O)P5W30O110]122 displayed strong electrochromism,24 suggesting that more polyoxometalates with various structures should be investigated to search for applicable electrochromic materials. Moreover, in comparison with a large number of researches on other electrochromic compounds, the study of electrochromism for polyoxometalates has been reported rarely. It will be of practical importance to J. Mater. Chem., 2004, 14, 2024–2029
find some polyoxometalate-based electrochromic materials that have less power consumption, a suitable response time and a long use life. Therefore, in this paper, we successfully identify a polyoxometalate of K17[Ce(P2W17O61)2]?30H2O [abbreviated as Ce(P2W17)2] for use as an inorganic electrochromic matrix and incorporate it into the ultra-thin film material. The reason for selecting this polyoxometalate is that it can display light-brown as a modified colour before being reduced and can be bleached automatically from blue to lightbrown without any power consumption, which is more suitable for application in electrochromic devices. On the basis of the fabrication of multilayer films by the layer-by-layer method, the electrochromism of the film was studied in detail.
Experimental Materials Polyoxotungsceriumate, K17[Ce(P2W17O61)2]?30H2O [Ce(P2W17)2], whose structure is shown in Fig. 1, was prepared according to the literature method and identified by UV-vis absorption spectra and cyclic voltammetry.25 Poly(styrenesulfonate) (PSS) (MW 70 000), Poly(allylamine hydrochloride) (PAH) (MW 70 000) and 3-aminopropyltrimethoxysilane were purchased
Fig. 1 Structure of [Ce(P2W17)2], 3-aminopropyltrimethoxysilane (1), poly(allylamine hydrochloride) (2), and poly(styrenesulfonate) (3).
This journal is ß The Royal Society of Chemistry 2004
from Aldrich and used without further treatment. The water used in all experiments was deionized to a resistivity of 18 MV.
absorbance at the maximum absorption wavelength as a function of time during potential steps from 2900 to 0 mV.
Preparation of the {[Ce(P2W17)2]/poly(allylamine hydrochloride)}n film
Results and discussion
Quartz substrates and ITO-coated glass were used for the film fabrication of self-assembly. The surface of the quartz (1.0 6 2.0 cm) and ITO-coated glass (1.0 6 2.0 cm) was cleaned in an H2O/H2O2/NH4OH (1:1:1) bath for 30 min, then the [Ce(P2W17)2]/poly(allylamine hydrochloride) multilayer films were prepared as follows. First, a [3-aminopropyltrimethoxysilane/poly(styrenesulfonate)/poly(allylamine hydrochloride)] precursor film was deposited onto a cleaned substrate by immersing the substrate alternately in 3-aminopropyltrimethoxysilane, poly(styrenesulfonate) (containing 1 M NaCl; pH ~ 4.0), and poly(allylamine hydrochloride) (containing 1 M NaCl; pH ~ 4.0) solutions for 12 h, 20 min, and 20 min, respectively, followed by rinsing with deionized water and drying in nitrogen after each immersion. Then the substrate-supported precursor film was alternately dipped into the [Ce(P2W17)2] (1023 M in HOAc–NaOAc buffer pH ~ 4.0– 4.2) and poly(allylamine hydrochloride) solutions for 20 min, rinsed with deionized water and dried in a nitrogen stream after each dipping. This process can be repeated until the desired number of bilayers of [Ce(P2W17)2]/poly(allylamine hydrochloride) is obtained. All adsorption procedures were performed at room temperature. The multilayer architectures thus obtained here are expressed as {[Ce(P2W17)2]/poly(allylamine hydrochloride)}n and the schematics of the self-assembly of an {[Ce(P2W17)2]/poly(allylamine hydrochloride)}2 film are illustrated in Fig. 2.
After the substrate is modified with a precursor [3-aminopropyltrimethoxysilane/poly(styrenesulfonate)/poly(allylamine hydrochloride)], the layer-by-layer self-assembly of the anionic [Ce(P2W17O61)2]172 and the cationic poly(allylamine hydrochloride) onto the positive surface of the film basically depends upon the electrostatic attraction between the oppositely charged species. The key to a regular multilayer buildup is the reversal of the surface charge in each adsorbed layer. The precursor film is essential for achieving a more homogeneous, positive charge distribution on the surface of the substrate and for the subsequent reproducible deposition.26 It was also necessary to rinse the films with deionized water and then dry with nitrogen after each adsorption step. With the fabrication procedure mentioned above, highly reproducible films with controlled thickness can be obtained. UV-vis spectroscopy was used in the present work to monitor the layer-by-layer assembling process of {[Ce(P2W17)2]/ poly(allylamine hydrochloride)}n films owing to its facility in evaluating the growth process of the multilayers. Fig. 3(b) shows the UV-vis absorption spectra of {[Ce(P2W17O61)2]/ poly(allylamine hydrochloride)}n multilayers (with n ~ 1–8) assembled on a precursor [3-aminopropyltrimethoxysilane/ poly(styrenesulfonate)/poly(allylamine hydrochloride)] film on a quartz substrate (on both sides). The inset in Fig. 3(b) displays the plots of the absorbance values for quartzsupported {[Ce(P2W17)2]/poly(allylamine hydrochloride)}n
Physical measurements UV-vis absorption spectra of quartz- and ITO-supported films were recorded on a 756CRT UV-visible spectrophotometer. The cyclic voltammetric and chronoamperometry measurements were carried out in 0.1 M HOAc–NaOAc buffer solution (pH ~ 4.0) at ambient temperature (25 uC). The ITO electrode coated with the self-assembled films were placed in solution with a platinum coil as counter electrode and Ag/AgCl (3 mol L21 KCl) as reference electrode. Spectroelectrochemical measurements of {[Ce(P2W17)2]/poly(allylamine hydrochloride)}n were performed by in situ UV-vis spectroscopy to monitor the
Fig. 2 Schematics of the self-assembly of an {[Ce(P2W17)2]/poly(allylaminehydrochloride)}2 film.
Fig. 3 (a) UV spectrum of 1025 mol L21 [Ce(P2W17)2] solution. (b) UV spectra of {[Ce(P2W17)2]/poly(allylamine hydrochloride)}n film with n ~ 0–8 for on the precursor film-modified quartz substrate (on both sides); from bottom to top, n ~ 0, 1, 2, 3, 4, 5, 6, 7 and 8; the inset shows plots of the absorbance values at 205 and 290 nm. J. Mater. Chem., 2004, 14, 2024–2029
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(n ~ 1–8) multilayer films at 205 and 290 nm as a function of the number of {[Ce(P2W17)2]/poly(allylamine hydrochloride)} bilayers. As shown in Fig. 3(b), these films all exhibit the characteristic absorption of the [Ce(P2W17)2]172 polyanions in the UV region with characteristic bands at 205 and 290 nm owing to the oxygen A tungsten charge transfer transitions,27 which confirms the incorporation of [Ce(P2W17)2] into the composite films. However, there is a slight red shift of about 10 nm for the shoulder band at 290 nm, when compared to that of the solution absorption spectrum of [Ce(P2W17)2] [see Fig. 3(a)]. This may be related to the strong electrostatic interaction between the [Ce(P2W17)2] and poly(allylamine hydrochloride). The feature band at 225 nm for the precursor film is due to the benzene chromophores in PSS, while PAH does not absorb above 200 nm.28 Cyclic voltammetry (CV) of an aqueous 1 mM [Ce(P2W17)2] solution (in 0.1 M HOAc–NaOAc buffer at pH ~ 4.0), using a bare ITO-coated glass electrode (immersion area 1.0 6 1.0 cm), consists of four anodic peaks of A1 ~ 20.4214, A2 ~ 20.5366, A3 ~ 20.7343 and A4 ~ 20.8742 V, and four cathodic peaks of C1 ~ 20.5317, C2 ~ 20.6617, C3 ~ 20.8493 and C4 ~ 20.9619 V (see Fig. 4, dotted line). The C1/A1 and C2/A2 pairs of peaks correspond to the 1e2/1H1 redox process, while the other two paris are undetermined according to the literature25,29 due to the possible occurrence of irreversible or quasi-reversible processes. In the following electrochromic investigation, however, the {[Ce(P2W17)2]/poly(allylamine hydrochloride)}20-modified ITO electrode is still stable without any structure change, even if the potential is applied to 21.0 V, which indicates that a little irreversible or quasi-reversible processes do not affect the electrochromism of the multilayer films. In addition, the peak values are somewhat different to those reported in the literature25,29 because those reports were under different experimental conditions with the use of a different electrolyte. Cyclic voltammograms of the {[Ce(P2W17)2]/poly(allylamine hydrochloride)}20-modified ITO electrode (immersion area of 1.0 6 1.0 cm) in HOAc–NaOAc buffer solution with pH ~ 4.0, using a platinum coil as the counter electrode and Ag/ AgCl/KCl (3 mol L21) as reference electrode, displays three cathodic peaks 20.4273 (c1), 20.6632 (c2) and 20.9645 V (c3) and three anodic peaks 20.3663 (a1), 20.5080 (a2) and 20.7813 V (a3) (Fig. 4, solid line). The peak potential values of a1, a2, a3 and their counterparts of c1, c2, c3 have little difference compared to the peaks of 1023M [Ce(P2W17)2] in solution. Both the first and second pairs of peaks, a1/c1, a2/c2,
Fig. 4 Cyclic voltammograms of 1 mM [Ce(P2W17)2] in solution (dotted line, ITO-coated glass as working electrode) and a {[Ce(P2W17)2]/poly(allylamine hydrochloride)}20-modified ITO electrode in HOAc–NaOAc buffer solution (solid line) [platinum coil as the counter electrode and Ag/AgCl/KCl (3 mol L21) as reference electrode with a scan rate ~ 25 mV s21]. 2026
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Fig. 5 Visible spectra of oxidized (dotted line) and reduced (solid line) [Ce(P2W17)2] in soluton {reduced [Ce(P2W17)2] was obtained at a bare ITO electrode versus Ag/AgCl at 20.9 V}.
correspond to two 1e2/1H1 redox processes. However, in comparison with the third and fourth paris of peaks of the [Ce(P2W17)2] solution, we could only observe one pair of broad peaks for [Ce(P2W17)2] multilayer films; this may result from the possible interaction between [Ce(P2W17)2] and polyallylamine hydrochloride. Additionally, the protonation plays an important role in the charge compensation of the electrochromic self-assembly films.23 When protons are difficult to incorporate into the reduction process in a self-assembled film, the neighboring two cathodic peaks will be combined into one. Similarly, deprotonation of reduced species also depends on a more positive potential, which may make the adjacent two anodic peaks one. The CV curve of [Ce(P2W17)2] in solution and the multilayer film demonstrates that the electrochemical properties of [Ce(P2W17)2] are fully maintained in the multilayer film. The colour change, being observed during the cyclic voltammetry of [Ce(P2W17)2] in 0.1 M HOAc–NaOAc buffer solution, results from the optical absorption of an intervalence charge transfer band (WV–O–WVI or WVI–O–WV) as produced by an applied potential, so that it appears in an electrochromic feature. As can be seen from Fig. 5, the oxidized form of [Ce(P2W17)2] is light brown in the visible region, with an absorption band at approximately 500 nm (Fig. 5, dotted line), while the reduced [Ce(P2W17)2] exhibits a broad
Fig. 6 {[Ce(P2W17)2]/poly(allylamine hydrochloride)}20 film-modified ITO glass electrode during different invariable potentials from 20.4 to 21.0 V (solid lines from bottom to top are 20.4, 20.5, 20.6, 20.7, 20.8, 20.9 and 21.0 V, respectively), and {[Ce(P2W17)2]/poly(allylamine hydrochloride)}100 film-modified ITO glass electrode at 20.9 V (dash-dot line).
adsorption band at about 600 nm (Fig. 5, solid line). Such a visual optical constrast of the {[Ce(P2W17)2]/poly(allylamine hydrochloride)}20 film can also be seen during the potential scanning between 0 and 21.0V, indicating that the film is electrochromic. The visible spectrum (Fig. 6, solid lines) of the {[Ce(P2W17)2]/poly(allylamine hydrochloride)}20 film-modified ITO glass electrode (immersion area 1.0 6 1.5 cm) was recorded by using different potentials from 20.4 to 21.0V. The maxmum absorbance increases to 0.21 at 21.0 V for the n ~ 100 film and the absorbance reaches 0.75 at 21.0 V (see Fig. 6, dash-dot line), which is sufficient for practical devices. The response times of the film were investigated by the double-potential experiment as well as the absorbance measurement. The coloration (tc) and bleaching (tb) times are 4.0 and 5.5 s, respectively, and the maximum absorbance at about 585 nm increases by 0.18 unit. At the same time, the Fig. 8 Repetitive response time for {[Ce(P2W17)2]/poly(allylamine hydrochloride)}100 by potential turning on at 20.9 V and turning off.
electrochromic reversibility of the {[Ce(P2W17)2]/poly(allylamine hydrochloride)}20 film was evaluated by performing repetitive double-potential steps from 2900 to 0 mV [Fig. 7(a)]. Both the response times for coloration and bleaching and the absorbance of the electrochromic film do not change noticeably even after 200 cycles, except for a small decrease of the maximum absorbance (5%) [Fig. 7(b)]. This demonstrates a stable electrochromic property of the self-assembled films. Fig. 8 shows the repetitive response time for {[Ce(P2W17)2]/ poly(allylamine hydrochloride)}100 by potential turning on at 20.9 V and turning off, in which the coloration and bleaching times become 108 and 350 s, respectively. The longer response
Fig. 7 (a) Potential current and absorbance at 585 nm of the {[Ce(P2W17)2]/poly(allylamine hydrochloride)}20 film-modified ITO during subsequent double-potential steps (2900 to 0 mV). (b) Absorbance changes at 585 nm in a {[Ce(P2W17)2]/poly(allylamine hydrochloride)}20 film during repeated potential steps (the numbers indicate the number of steps after which the spectra were taken).
Fig. 9 Potential current and absorbance at 650 nm of {[Ce(PW11)2]/ poly(allylamine hydrochloride)]20 film-modified ITO during subsequent double-potential steps [2900 (turning on) to 0 mV (turning off)]. J. Mater. Chem., 2004, 14, 2024–2029
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period of this investigation. To examine the thermal stability of these films, UV-vis spectra were measured after heating the film at different temperatures for 30 min. The UV-vis absorption spectra for the quartz-coated {[Ce(P2W17)2]/poly(allylamine hydrochloride)}8 film after heating are almost the same as that without heating, and the two characteristic bands of [Ce(P2W17)2] at 205 and 290 nm still remain. In particular, when the heating temperature is raised to 150 uC, the absorbances of the two characteristic bands still remain to be the same as those without heating, suggesting that such a film possesses good thermal stability.
Conclusions Fig. 10 {[Ce(PW11)2]/poly(allylamine hydrochloride)]20 film-modified ITO glass electrode during different invariable potentials (from bottom to top are 20.4, 20.5, 20.6, 20.7, 20.8, 20.9 and 21.0 V, respectively).
time indicates that, with the increasing bilayers of the film, the redox process becomes slow. To make a further comparison with the relative electrochromic polyoxometalate, we also investigated the electrochromic properties of K11[Ce(PW11O39)2]?17H2O [Ce(PW11)2], which is another kind of lacunary Keggin structure polyoxometalate prepared according to the literature25 method. Similarly, we prepared the {[Ce(PW11)2/poly(allylamine hydrochloride)}20 film using the same procedure as that of the [Ce(P2W17)2] films. The double-potential experiment as well as absorbance measurement are also used here to evaluate the electrochromic property for the {[Ce(PW11)2]/poly(allylamine hydrochloride)}20 film. From Fig. 9 and Fig. 10, it can be seen that both the response time and the maximum absorbance at the operation potential 21.0 V of the {[Ce(PW11)2]/ poly(allylamine hydrochloride)}20 film are not as good as [Ce(P2W17)2]. In addition, Table 1 illustrates the detailed comparison of the applied potential, response time and maximum absorbance for [Ce(P2W17)2]-, [Ce(PW11)2]-, [Eu–(H2O)P5W30O110]122-,24 and [W10O32]42-based electrochromic films.23 Both the [Ce(P2W17)2]-, and [Eu–(H2O)P5W30O110]122 -based films show the short response time, but the latter film needs a high potential (21.8 V) to gain a suitable absorbance. Further, the absorbance value (0.12) of the latter film is smaller than the [Ce(P2W17)2]-based film (0.18), showing that it is possible for [Ce(P2W17)2] to become a promising electrochromic material in view of the suitable response time, lower operation potential and high optical contrast. Furthermore, the different electrochromic properties of [Ce(P2W17)2] and [Ce(PW11)2] give valuable information that the polyoxometalate-containing lacunary Wells–Dawson type ligand has a higher electrochromic activity than the related compound with a ligand of lacunary Keggin structure, which provides valuable information for selecting and designing more suitable polyoxometalates as the future electrochromic materials. As described above, the electrochromic multilayer films are robust and stable under ambient conditions, because the UVvis spectra of these films have hardly changed during the whole Table 1 Comparison of the applied potential, response times and maximum absorbance for multilayer films containing different polyoxometalates
A new polyoxometalate-based thin film of {[Ce(P2W17)2]/ poly(allylamine hydrochloride)}n film has been successfully fabricated by a layer-by-layer self-assembly method. From the experimental results, the film material consisting of both polyelectrolyte and [Ce(P2W17)2] shows electrochromism with reversibility, suitable response time, high optical contrast and thermal stability, which suggest that this polyoxometalate cluster may become a promising candidate for use in electrochromic materials. This work not only identifies the useful electrochromism of the polyoxometalate [Ce(P2W17)2] but also proves the preferable electrochromic properties of [Ce(P2W17)2] to [Ce(PW11)2] and other polyoxometalates, providing valuable information for selecting the most suitable polyoxometlates as electrochromic materials.
Acknowledgements The authors are thankful for the financial supports from the National Natural Science Foundation of China (Grant No. 20371010), the State Key Laboratory for Structural Chemistry of Unstable and Stable Species in Peking University (No. 03-12), and the Natural Science Foundation of Jilin Technology Office of China (No. 20030512-1).
References 1 2 3 4 5 6 7 8 9 10 11
Polyoxometalate
Potential step/V
tc/s
tb/s
Amax
[Ce(P2W17)2]172 a [Ce(PW11)2]112 a [Eu–(H2O)W30O110]122 a [W10O32]42 b
20.9 < 0 20.9 < 0 21.8 < 20.4 20.4 < 0
4.0c 20c 4.2d 50d
5.5c 21c 4.4d 180d
0.18 0.08 0.12 0.035
12
Contained 20 bilayers (ref. 24). b Contained 10 bilayers (ref. 23). c Values obtained from 90% absorbance changes. d Values obtained from 95% absorbance changes.
13 14
a
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