Electrodeposited Prussian Blue in mesoporous TiO2 as electrochromic hybrid material

Microporous and Mesoporous Materials 164 (2012) 67–70

Contents lists available at SciVerse ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Electrodeposited Prussian Blue in mesoporous TiO2 as electrochromic hybrid material Britta Seelandt a, Michael Wark b,⇑ a b

Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, Callinstr. 3a, D-30167 Hannover, Germany Laboratory of Industrial Chemistry, Ruhr-University Bochum, Universitaetsstr. 150, D-44801 Bochum, Germany

a r t i c l e

i n f o

Article history: Available online 17 June 2012 Keywords: Electrochromism Prussian Blue Mesoporous titania Electrodeposition

a b s t r a c t In the ordered mesopores of thin films of semiconductive titanium dioxide, synthesized by a sol–gel approach employing a block-co-polymer as structure-directing agent, and dip-coating, Prussian Blue is electrochemically deposited. Such hybrid films were for the first time prepared and tested for their electrochromic properties. By applying only low current densities in a voltammetric cycle large differences in the optical transmission are recorded. Due to short diffusion lengths for the charge carriers in the hybrid films the electrochromic switching is fast, reversible, highly repeatable and reaches high coloration efficiencies. Thus, this material is a promising new candidate for use in smart windows. Ó 2012 Elsevier Inc. All rights reserved.

been demonstrated for example in combination with viologene dendrimers [10].

1. Introduction Due to global warming the efficient use of energy has become an important topic for our society. In nowadays office building the consumption of electrical energy is high due to simultaneous use of electric lighting and lowered sunblinds to protect against blinding. In order to reduce electric energy consumption and costs so-called smart windows attract increasing attention [1,2]. In these windows thin layers of electrochromic materials are encapsulated which change their color if a small potential is applied [3,4]. In recent years a fast improvement of electrochromic materials took place whereas especially Prussian Blue (PB) rendered high attraction [5,6] and is meanwhile commercially used in some systems [7]. The electrochromic properties of PB are well documented [8]. The blue color disappears if electrons are incorporated due to reduction of FeIII zu FeII according to Eq. (1):

KFeIII ½FeII ðCNÞ6  þ Kþ þ e $ K2 FeII ½FeII ðCNÞ6 

ð1Þ

The most important advantage is the low absorption in the oxidized state which ensures the high transmission difference between colorless and colored state necessary for effective use. In particular, high effectiveness is expected if the PB is highly dispersed. In this paper we use TiO2 thin films possessing an ordered array of mesopores to support PB. As single material titanium dioxide (TiO2) can only be electrochromically switched between colorless and blue–grey, exhibiting low coloration efficiency [9]. However, as host material it can promote the switching efficiency as had ⇑ Corresponding author. E-mail address: [email protected] (M. Wark). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.06.015

2. Experimental 2.1. Sample preparation The mesoporous TiO2 thin films were synthesized on conducting FTO (fluorine-doped tin dioxide, about 10 X/sq resistivity) glass substrates by sol–gel technique using tetraethylorthotitanate (TEOT) and the triblock copolymer Pluronic P123 ((PEO)20(PPO)70(PEO)20). Four grams, i.e. 0.746 mmol, of P123 were dissolved in 48 g ethanol in an ultrasonic bath. In a second solution 16.6 g (72.8 mmol) TEOT were mixed with 10.6 mL conc. HCl. Both solutions were then joined at 281–283 K and subsequently warmed up to room temperature and stirred for 10 min [11]. The films were deposited from the obtained sol by dip-coating [12] utilizing the EISA (evaporation-induced self-assembly) process [13] at 293 K and 15% humidity with a withdrawal rate of 70 mm/min. The geometric surface area of the films was 4.0  2.0 cm2. Finally the FTO glasses with the TiO2 layer were heated in air to 673 K with a heating rate of 1 K/min and calcined at that temperature for 4 h. The PB is introduced into the mesopores by electrochemical deposition [14] from a solution of HCl, K3[Fe(CN)6] and FeCl36H2O. A typical setup consisting of three electrodes was used, in which the mesoporous TiO2 film on the FTO glass was employed as working electrode, a Pt wire is the counter electrode and a Ag/AgCl electrode, connected via a salt bridge containing 0.1 M KCl acts as reference electrode. During the deposition the applied voltage

B. Seelandt, M. Wark / Microporous and Mesoporous Materials 164 (2012) 67–70

2.2. Characterization Scanning electron microscopy (SEM) micrographs were obtained from a Jeol JSM-6700F field-emission instrument with the secondary electron detector at a low acceleration voltage of 5 kV. Transmission electron microscopy (TEM) was performed on a Jeol JEM-2100F field-emission instrument at 200 kV with an ultra-high resolution pole piece and a spherical aberration constant of CS = 0.5 mm that provides a point resolution better than 0.19 nm. Moreover, the microscope was equipped with a Gatan GIF 2001 energy filter with a 1k-CCD (charge-coupled device) camera. For spatially resolved EDXS the microscope was equipped with an ultrathin window detector (Oxford Instruments INCA 200 TEM). Specimens for the TEM investigation were prepared by epoxy agglutination of two film pieces. Subsequently, these samples were first cut into 1  1  2 mm3 pieces, ground, and polished on polymer-embedded diamond lapping films to approximately 0.01  1  2 mm3. Finally, Ar+ ion sputtering was employed at 3 kV under an incident angle of 4° until electron transparency was achieved. The film thickness was determined by use of a Dektak 6 M stylus (Veeco) surface profile measuring system in combination with TEM measurements. Texture properties of the mesoporous films were determined from Kr adsorption isotherms carried out at 77 K on a Micromeritics ASAP 2010 apparatus. Prior to the adsorption experiments, the samples were outgassed at 423 K overnight. The pore size of a mesoporous film was calculated from a correlation between pore size and point of inflection of the desorption branch of the corresponding adsorption/desorption isotherm. For estimation of texture parameters of the films from Kr isotherms, the molecular crosssectional area of krypton of 0.21 nm2 and the molar volume of solid krypton were used according to the software of the adsorption apparatus producer. The UV–vis spectra of the transparent thin films were recorded over a range of 200–700 nm with a Varian Cary 4000 Scan UV–vis system in transmission. For the electrochromic measurements a 3electrode setup was realized in a cuvette with a K+ or Li+ ions containing electrolyte. With this either cyclovoltamograms over a range of 0.2 to 0.6 V vs. Ag wire (scan rate: 50 mV/s) and simultaneously the transmission at a constant wavelength of 700 nm were recorded or complete UV–vis spectra were taken at different applied potentials. Coloration efficiencies g were determined from cyclovoltamogramms taken between 0.6 and 0.2 V vs. Ag wire and simultaneously recorded transmission at the fixed wavelength of 700 nm the charge density Q and the change in optical density DOD, i.e. the ratio of the transmission in the bleached state Tb and the colored state Tc (Eq. 2).

g¼

DODðkÞ Tb with DOD ¼ log : Q Tc

0.008 0.007 2

0.006

3

and the current are controlled by a potentiostat (Autolab PGSTAT12). The electrolyte consisted of 15 mL 0.05 M HCl, 30 mL 0.05 M K3[Fe(CN)6] and 30 mL 0.05 M FeCl36H2O in ultrapure water (18.2 MX cm). The deposition was performed galvanostatic with a current density of 40 lA/cm2 for 240 s.

Kr adsorption [cm /cm ]

68

0.005 0.004 0.003 0.002 0.001 0.000 0.0

0.2

0.4

0.6

0.8

1.0

p/p0 Fig. 1. Scanning electron microscopy (SEM) image and Kr adsorption of a mesoporous TiO2 film.

the most prominent (1 0 1), (2 0 0) and (2 1 0) orientations of the anatase modification, demonstrating that the pore walls are at least partially crystalline. By electrochemical Li insertion for comparable mesoporous TiO2 films synthesized with Pluronic P 123 and calcined at 673 K a crystallinity of the walls of about 40% was reported [15]. The SEM image in Fig. 1 shows that the synthesized TiO2 thin films possess ordered arrays of wormlike-mesopores. The average diameter of these mesopores is about 8 nm which can be deduced from the onset of the step in the adsorption branch in the isotherm obtained with Kr as test gas at p/p0 of about 0.85 indicating capillary condensation (Fig. 1). The whole isotherm exhibits the typical type IV form with a broad hysteresis loop, which steepness indicates quite uniform pore sizes. The films exhibit roughness factors of about 70 representing the ratio of the total pore surface in relation to the geometric area. As shown in Fig. 2, transmission electron microscopy cross-section images in combination with energy-dispersive X-ray spectroscopy (TEM–EDXS) prove the successful incorporation of PB exclusively into the pores of the TiO2 film. The grey region represents the TiO2 thin film, in which the mesoporous appear as darker structures. There are no indications for a deposition of PB on top of the TiO2 layer as it was found for mesoporous indium tin oxide (ITO) films exhibiting a higher electrical conductivity than the TiO2 films [16]. The role of the conductivity for the PB deposition is also seen, if different areas of the mesoporous TiO2 films are compared regarding the iron or PB content. The atom ratio Fe/Ti detected by EDXS in the region which is closer to the highly conducting FTO glass substrate (lower area 2 in the TEM image in Fig. 2) is with about 0.115 significantly higher than that in the upper area 1 being 0.085. This shows that the primary iron species from the electrolyte diffuse

with EDXS analyzed area 1 ð2Þ

PB in TiO2 film

with EDXS analyzed area 2

3. Results and discussion The thickness of the transparent porous TiO2 layers obtained from the dip-coating and subsequent calcinations was about 250 nm. Wide-angle X-ray diffraction pattern exhibited slightly broadened reflections at 2h = 25.2°, 47.7° and 55.3° representing

FTO Fig. 2. TEM cross-section image and resulting elemental distribution of PB deposited into the pores of a TiO2 film; bright lower area: FTO.

69

B. Seelandt, M. Wark / Microporous and Mesoporous Materials 164 (2012) 67–70

Table 1 Electrochromic characteristics of a PB/TiO2 hybrid film in different electrolytes; applied charge density Q, obtained change in optical density DOD, transmission in the bleached state Tb and the colored state Tc, and coloration efficiency g (calculated for the first cycle).

1.4 1.2

blue PB/TiO2 film colorless PB/TiO2 film

0.8 0.6

Electrolyte

Q/(mC/cm2)

Tb /%

Tc /%

DOD

g/(cm2/C)

KCl (1 M) KCl/HCl (pH 2) LiClO4/PC (0.5 M) LiClO4/PC (1 M)

9.36 8.66 4.87 6.68

67.2 74.4 40.2 63.8

8.6 7.3 10.8 19.3

0.89 1.01 0.57 0.52

95.35 116.54 117.26 76.59

0.4

400

500

600

700

800

wavelength [nm] Fig. 3. Absorption spectra of a PB/TiO2 hybrid film in the blue state at a applied potential of 0.6 V and in the colorless form at an applied potential of -0.2 V.

through the porous film and PB is then first formed in the pores near the highly conductive FTO. The FeIII ions from the hexacyanoferrate are reduced to FeII and the [FeIIIFeII(CN)6] complex is formed, which finally precipitates together with further FeIII ions as the weakly soluble blue FeIII[FeIIIFeII(CN)6] (PB). Since PB itself is more conductive than the mesoporous TiO2 network, more and more PB is then deposited directly on-top of the pre-formed PB in the pores preventing unwanted deposition of PB on the external surface of the films. By varying the PB deposition time in the range of 210–300 s, deposition time of 240 s was found to be optimum in order to avoid any PB deposition on the external surface, but nevertheless ensuring high coloration of the hybrid films. Under applied potential the PB/TiO2 hybrid films change their color without delay in correlation with the potential change and reversibly from colorless to blue and vice versa. At a potential of 0.6 V vs. Ag/AgCl the blue state is established showing a maximum of absorption at about 700 nm (Fig. 3). High absorption is achieved over a wide wavelength region ranging from about 550–800 nm. If the potential is shifted to 0.2 V vs. Ag/AgCl the films bleach completely. In this colorless state the PB/TiO2 films exhibit at 700 nm a transmission of 74%, which is only slightly lower than that of the pure TiO2 film. It has to be stated that in general the mesoporous TiO2 films exhibit a color change if a potential between 0.5 and 2.0 V is applied and also Li+ ions, e.g. from the electrolyte LiClO4 in propylene carbonate (PC), are present. This color change between blue and grey results because the Li+ ions are encapsulated into the TiO2 network under reduction of some TiIV species to Ti3+ ions leading to a composition LixTiO2 with mixed oxidation state of the titanium [15]. In contrast to the color change presented here, however, in that case the blue color of the Ti3+ ions is only present in the absence of any oxygen. The coloration and decoloration experiments with the PB/TiO2 hybrid films, however, were performed in air and oxygen-containing electrolyte. To determine the coloration efficiencies for PB/TiO2 hybrid films in different electrolytes the transmissions at 700 nm in the colorless and the blue state were measured. Table 1 summarizes the obtained values and compares it with the electric charges needed to induce the color change. For 1 M LiClO4/PC a difference in transmission of 44.59% was detected between colored and non-colored state. For maximum coloration of the whole film an electric charge of 6.68 mC/cm2 is needed, resulting in a coloration efficiency of 76.59 cm2/C (Table 1). In order to get an idea how much PB is chemically reduced or oxidized by that charge we roughly calcu-

80

140 70 60

135

50

130

40

125 30

120

20

115

10

0

100

200

300

2

0.0

lated the PB content in a typical mesoporous TiO2 film. The error of this rough estimation might by as high as about 50%, but it shows the orders of magnitude. From Kr adsorption measurements for a film with a roughness factor of 70 an average pore volume of about 0.014 mm3/cm2 results. If the pores in 1 cm2 of geometric surface of such film (its typical geometric area is 7 cm2) would completely be filled with PB at maximum 0.0226 mg could be electrochemically deposited in this pore volume (density of PB: about 1.9 g/cm3). From the Fe/Ti ratio of about 0.1 measured by EDXS a real filling of about 25% can be assumed. Thus about 0.008 lmol PB might be present per cm2 geometric surface in the mesopores of a typical TiO2 film. To colorize or decolorize this amount a charge Q of about 8 mC is needed. The highest coloration efficiency of the hybrid film is obtained in the 0.5 M LiClO4/PC electrolyte. However, it results only from the very low current density needed for switching; the transmission in the colorless state is with about 40% inadmissible high. The necessary current density is much higher in both aqueous electrolytes, 0.1 M KCl and 0.1 M KCl with HCl; but here the achieved changes in optical density or transmittance are much higher achieving DT values of 58.59% and 67.13%, respectively, in comparison to only 29.39% and 44.53%, respectively, in the organic electrolytes. A second important property of electrochromic materials is their stability upon frequently repeated switching. Thus, the changes in the transmittance values of bleached and colored state must be judged. Fig. 4 shows the result of such testing for a PB/TiO2 film in the KCl/HCl electrolyte. The film switches with an extraordinary high stability; within 400 cycles the transmittance in the bleached state remains almost unchanged, for the bleached state it decreases slightly only by 4.54%. In average a DT value of 65% results which is very high compared to standard systems [17] which show typically values of around 56% transmittance change. The high stability can be explained with almost identical current

coloration efficiency [cm /C]

0.2

T [%]

absorption [a.u.]

1.0

400

cycles Fig. 4. Transmittance of bleached (j) and colored (d) states of a PB/TiO2 hybrid film and resulting coloration efficiency (–).

70

B. Seelandt, M. Wark / Microporous and Mesoporous Materials 164 (2012) 67–70

densities of 8.66 and 8.43 mC/cm2 for coloration and decoloration, respectively, being in general a requirement for good reversibility [18]. Due to the low thickness of the PB layer in the pores, the sufficiently good conductivity of the mesoporous TiO2 host and the thus resulting relatively low current densities for switching, the times necessary to obtain full coloration or de-coloration are only 3 and 2 s, respectively. In addition Fig. 4 shows also the development of the coloration efficiency. Starting from already good values of about 115 cm2/C in the first cycles coloration efficiency increases to about 135 cm2/C. This change results probably due to some reorganization in the films, which is a commonly observed phenomenon [19]. The reorganization mainly lowers the current density needed for coloration and de-coloration. We did not systematically study the stability of the films under constant applied potential, but it was repeatedly observed that at a potential of 0.6 V the blue color was stable (less than 3% change in transmission) for at least 1 h. The high stability of the transmittance values and the at the same time high coloration efficiencies, which are comparable to values for compact PB/ITO films [17], render this new kind of PB/ TiO2 hybrid films very prospective for application as anodic part in electrochromic devices. The advantage of the hybrid system is the optimum conductivity of the mesoporous TiO2 network, being high enough to stabilize to PB, but on the other hand not so high to cause deposition of clustered PB aggregates on the external surface of the films. Such clusters are quite instable and would limit the long-term stability of the switching. 4. Conclusions By electrochemical deposition PB can be introduced and stabilized in the mesoporous channels of sol–gel prepared TiO2 thin films. The mesoporous TiO2 films are a transparent material which conductivity is high enough to allow electrodeposition of PB homogeneously in the continuous pore network film and also to enable electrical switching of the PB. Due to the interaction with the TiO2 pore walls in the only about 8 nm wide channels the PB is highly dispersed and can, thus, easily and fast change its oxidation state. This leads with lower amounts of PB employed to coloration efficiencies comparable to those of benchmark materials (e.g.

300 nm thick compact PB films on ITO glass [17]), and even more important to high stability of the switching. It can be expected that optimization of the used electrolyte, e.g. with polymers like PEDOT, will further improve coloration efficiency and stability of switching. Acknowledgments The authors thank Professor Dr. J. Caro and PD Dr. A. Feldhoff (Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover) for fruitful discussion and recording the TEM image, respectively. We thank Dr. J. Rathousky (Heyrovsky Institute Prague, Czech Republic) for recording the Kr adsorption isotherm. Financial support by German Science Foundation (DFG, WA 1116/14) is gratefully acknowledged. References [1] C.M. Lampert, Sol. Energy Mater. Sol. Cells 52 (1998) 207–221. [2] R. Baetens, B.P. Jelle, A. Gustavsen, Sol. Energy Mater. Sol. Cells 94 (2004) 87– 105. [3] C.G. Granqvist, Sol Energy Mater. Sol. Cells 92 (2008) 203–208. [4] M. Vidotti, S.I. Cordoba de Torresi, J. Braz. Chem. Soc. 19 (2008) 1248–1257. [5] A. Abbaspour, M.A. Kamyabi, J. Electroanal. Chem. 584 (2005) 117–123. [6] K.-C. Cheng, F.-R. Chen, J.-J. Kai, Electrochim. Acta 52 (2007) 3330–3335. [7] A. Kraft, M. Rottmann, Sol. Energy Mater. Sol. Cells 93 (2009) 2088–2092. [8] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism: Fundamentals and Applications, VCH, Weinheim, Germany, 1990. [9] S.C. de Oliveira, R.M. Torresi, S.I. Cordoba de Torresi, Quim. Nova 23 (2000) 79– 87. [10] M. Möller, S. Asaftei, D. Corr, M. Ryan, L. Walder, Adv. Mater. 16 (2004) 1558– 1562. [11] M. Wark, J. Tschirch, O. Bartels, D. Bahnemann, J. Rathousky, Microporous Mesoporous Mater. 84 (2005) 247–253. [12] I. Bannat, K. Wessels, T. Oekermann, J. Rathousky, D. Bahnemann, M. Wark, Chem. Mater. 21 (2009) 1645–1653. [13] P.C. Alberius, K.L. Frindell, R.C. Hayward, E.J. Kramer, G.D. Stucky, B.F. Chmelka, Chem. Mater. 14 (2002) 3284. [14] K. Itaya, T. Ataka, S. Toshima, J. Am. Chem. Soc. 104 (1982) 4767–4772. [15] D. Fattakhova-Rohlfing, M. Wark, T. Brezesinski, B.M. Smarsly, J. Rathousky, Adv. Funct. Mater. 17 (2007) 123–132. [16] T. von Graberg, P. Hartmann, A. Rein, S. Gross, B. Seelandt, C. Röger, R. Zieba, A. Traut, M. Wark, J. Janek, B.M. Smarsly, Sci. Technol. Adv. Mater. 12 (2011) 025005. [17] R.J. Mortimer, J.R. Reynolds, J. Mater. Chem. 15 (2005) 2226–2232. [18] A. Kraft, F. Sepp, M. Rottmann, K.-H. Heckner, EU Patent 1 517 293 B1, 2007. [19] C.G. Granqvist, P.C. Lansåker, N.R. Mlyuka, G.A. Niklasson, E. Avendano, Sol. Energy Mater. Sol. Cells 93 (2009) 2032–2039.