Electrochromism of mixed phosphotungstic acid - titanium oxide xerogeI thin solid films Elektrokromizem tankih filmov fosfovolframove kisline v kserogelu titanovega oksida U. Lavrenčič - Štangar, B. Orel, National Institute of Chemistry, Hajdrihova 19, 61115 Ljubljana Thin solid films composed of phosphotungstic acid (PWA) incorporated in titanium oxide xerogel have been made via sol - gel route by dip coating technique. Electrochromism of as - deposited films has been studied with the help of cyclic voltammetric (C V) measurements in HC104 electrolyte. In situ UV - VIS spectroelectrochemical measurements have been applied for quantification of optical changes of the films during colouring / bleaching process. With the help of ex situ Near Grazing Incidence Angle (NGIA) reflection spectroscopy it has been demonstrated that electrochromism is related to the intensity change of the W-0-W vibrations indicating redistribution of charges in Keggin 's ions. Key words: electrochromism, electrochromic and ion conductive material, in situ spectroelectrochemistry, infrared spectra Po sol-gel metodi in z dip coating tehniko nanašanja tankih plasti smo pripravili stabilne in optično kvalitetne mešane PWA/Ti tanke filme kserogelov na različnih substratih. Tako pripravljeni filmi so homogeni in amorfni. Elektrokromizem P WA/Ti smo preučevali s pomočjo cikličnovoltametričnih in kronokulometričnih meritev. Stabilnost filmov v HC104 elektrolitu je dobra (>200 ciklov), interkaliran naboj doseže vrednosti do 35 mC/cm2. In situ UV-VIS spektroelektrokemijske meritve smo uporabili za ovrednotenje optičnih sprememb med obarvanim in razbarvanim (prepustnim) stanjem. Prepustnost se spremeni za -40% pri filmu z debelino 400 nm, kar pomeni, da je vsebnost vgrajene PWA v mreži Ti-kserogela dovolj velika, da opazimo elektrokromni efekt mešanega sistema. Ex situ NGIA FT-IR spektroskopske meritve na obarvanih in razbarvanih vzorcih so pokazale, da se interkalacija H ionov iz elektrolita (proces obarvanja) odraža na intenzitetnih spremembah W-0-W valenčnih nihanj Kegginovega aniona zaradi prerazporeditve naboja. Ključne besede: elektrokromizem, elektrokromni in ionsko prevodni material, in situ spektroelektrokemija, infrardeči spektri 1 Introduction Electrochromism is described as reversible change of colour of the material under the application of electric current or voltage. Schematically, phenomenon could be visualised as double injection ofelectrons and ions into the structure of the material. For practical utilisation of the electrochromic effect five layers of materials each of them performing specific function in device are required; trans- parent electronicconductors1-5, electrochromic material2, fast ionic conductor3 and ionic storage material'1. For attaining transmissive electrochromic device ali of the layers have to be optically transparent giving rise to undistorted view through the device. From practical and technological reasons it is vvise to reduce number of layers by using material exhibiting combined properties required for functioning of the device, e. g. electronic conductor - ion storage material or electrochromic material - ion conductor. Phosphotungstic acid (H3PW]2O40*xH2O - noted PWA) consisting of P04 tetrahedra vvhich are surrounded by four W3013 sets has been proposed as a candidate which could perform simultaneously as electrochromic material and ion conductor1 Teli2 and Mohapatra3 exploited electrochromism of PWA by making diffusely reflecting electrochromic device where P W A was used in a powder form. Electrochromic properties of PWA diluted in aqueous solution have been demonstrated by Maheswari and Habib". The correspon-ding colouring efficiency was found to be 16.8 cm2/C vvhich is stili lower compared to the colouring efficiency of W03 (51 cm2/C). The main drawback which prevents that PWA could be used in transmissive electrocliromic devices stems from the fact that beside having strong aciditv, it is extremely soluble in water and some organic solvents. Thin films of PWA are unstable in a humid environment and tarmsh quickly by absorbing water molecules. Recently, Tatsumisago and Minami5 succeeded to prepare thin solid films from phos-phomolybdic acid (PMoA) mbeded into the Si02 xerogel. Films containing 42 wt. % PMoA exhibit conductivity 3x10° S/cm at 90 % of relative humidity, but their electrochromic properties have not been investigated yet. We followed the same approach (5) but PWA was included in Ti02 xerogel. The mam objective of our vvork was to prepare stable films of mixed PWA/Ti and to demonstrate their electrochromic properties. Beside this, structural features of films and the corresponding gels and xerogels have been elucidated. Finally, near grazing mci-dence angle FT-IR spectroscopy has been used in order to shovv in which way the FT-IR spectra of films change upon the inserted charge. 2 Experimental Preparation of Sols, Gels and Coatings PWA/Ti sols were prepared by dilutmg 1 g of phosphotungstic acid PWA (Fluka) m 20 ml of 2-methoxy-ethanol (Fluka). Titanium isopropoxide (Aldrich), which served as a precursor leading to a host material for PWA, was added to this solution. At the first stage the precipitation of titanium was observed vvhich turned over to a clear solution when the molar ratio PWA/Ti(OPri)4 = 0.03 was achieved. In order to obtain maximum concentration of electrochromic aetive species (PWA) in the Ti02 gel matrix the smallest amount of Ti(OPr')4 (~4ml) assurmg a clear and relatively stable solution was used. As prepared sol should be kept dry smee it tends to hydrolyse rather easilv m a humid environment. The reason of usmg 2-methoxy-ethanol as a solvent is in slovving down this reaction due to steric elfect of a bigger -OCH CH OCH3 group, whereas in some more common solvents, e.g. ethanol, propanol, isopropanol, precipitation cannot be overcome. Viscous yellow gels vvere prepared by removing volatiles on a rotary evaporator at an ambient temperature. For obtaining pure Ti02 sols, gels and films an analogous procedure was applied to compare properties of both sys-tems. Coatings vvere made on thedip coating unit with pulling speeds up to 5 cm/min. The supporting conductive ITO glassplates(ps~ 100 Q/D)vverepreviouslyultrasonically cleaned. Substrates vvere repeatedly dipped up to 3 times in order to inerease the film thickness. Since larger number of dips is not efficient, more viscous sols have to be used to achieve thicker films. Instrumental Thickness measurements vvere performed on a Surface Profiler Alfa Step 200 having maximal resolution of 5 nm /100 nm. Cyclic voltammetric and chronocoulometric experi-ments were done vvith an EG&G PAR model 273 compu-ter controlled potentiostat - galvanostat, driven by a model 270 Electrochemical Analysis software. A Metrohm type voltammetric celi (50 ml) and a three - electrode system vvere employed. A vvorking (test) electrode consisted of a roughly 2 cm2 ITO /( PWA/Ti) covered glass positioned so that exactly 1 cm2 (one side) of it vvas in a physical contact vvith an electrolyte solution. A Pt rod served as a counter electrode and Ag/AgCl/0.2 M KC1 as a reference electrode. Single scan or multi scan cycling and chronocoulometric measurements vvere performed vvithin and at potentials +0.4 V and -1.2V, respectively. In situ UV-VIS spectroelectrochemical measurements vvere performed on HP 8451 A diode array spectropho-tometer vvith EG&G PAR model 264 A polarographic analyser. Samples vvith dimensions ~lxl cm2 vvere accommodated mto spectroelectrochemical celi equipped vvith Pt counter and Ag/AgCl reference electrode and IIC104 electrolyte. In situ chronocoulometric measurements vvere done at cathodic potential, i.e. -1.2 V vs. Ag/AgCl within the spectral range 350 - 800 nm as a function of t ime. FT-IR spectra vvere recorded on a FTS-80 Digilab Spectrometer at resolution 8 cm"' (256 scans) equipped vvith NGIA (Barnes) celi usmg p-polariser for obtaining LO spectra of thin films deposited on aluminised glass substrate. The incidence angle vvas 80° 3 Results and discussion Electrochemical Investigations Cyclic voltammetric recordings vvere performed m order to obtain electrochemical fingerprints of the studied xerogel films asa funetion of different conditions and parameters. The potentia 1 in each measurement vvas swept from anodic to cathodic rcgion and then reversed to the initial value. A multi-svveep mode vvas applied to check voltammetric stability of a film (fig. 1). (PWA/Ti )/ HC104 system revealed a relative inerease m peak current densitv during the mitial stabilisation process, vvhereas only a small change vvas observed betvveen voltammograms after fur-ther50.and200.cycles (fig. 1). Atcathodicreductionblue coloration is observed (-1.0--1.2 V) and bleaching occurs at anodic oxidation (-0.2 - -0.1 V) In spite ofthe absence of the cathodic peak ali the CV measurements vvere performed in cathodic potentials up to -1.2 V because fiirther voltage increase produced irreversible film chang- es. Electrochemical stability of pure Ti xerogel coating pre-pared under the same conditions as mixed PWA/Ti films vvas poor since destroying of the film was achieved already after 10 cycles in HC104water solutionelectrolyte. This is surprising because it is known that Ti xerogel film derived from Ti(OBu°)4 gave stable response and exhibited electrochromic properties6. The dependence of the CV response on the film thick-ness is depicted in fig. 2 Thicker PWA/Ti films exhibited larger peak current densities but a relative decrease in reversibility. It means that a greater anodic potential has to be applied to achieve a complete bleaching (from -0.45 V for 80 nm to -0.15 V for 430 nm tluck film). -0.4 -0.8 -0.3 -0.2 < -0.1 E ^ 0 0.1 0.2 0.3 200---------------------- a 50 3 0.4 0.2 0 -0.2 -0.4 -0.6 E / V vs. Ag/AgCI Figure 1. Multi-cyclic voltammograms of a PWAATi film (thickness = 135 nm) in 0.001 M HC104; number of cycles are indicated, potential scan rate: 20 mV/s. Slika 1. Ciklični voltamogrami PWA/Ti filma debeline 135 nm v 0.001 M HC104 (1., 3., 50. in 200. cikel); hitrost preleta potencila: 20 mV/s. Chronocoulometric measurements were performed at con-stant potentials, determined from the sweep voltammetric measurements (-1.1 V and 0.4 V). The total charge (Q) loaded or unloaded into or out of a film is recorded as a fiinction of time (t) (fig. 3a) The coulombic efficiency (ratio of anodic tocathodiccharge) is smaller witha thicker film and when more concentrated electrolyte is used vvhile the total charge density is larger. In this čase, total charge density exceeds 30 mC/cm2 in cathodic direction taking intoaccount limiting time (30 s) and 20 mC/cm2 in anodic direction (corresponding cyclic voltammogram is shown in fig. 3b). In situ UV-VIS spectroelectrochemical measurements vvere done to prove the electrochromism of PWA/Ti films since they don't exliibit a long time open circuit memory and therefore the ex situ measurements may be difficult to perform. The process of cathodic coloration and anodic bleaching expressed with absorbance changes of the film as a fiinction of applied potential (spectral range 3 5 0 - 800 nm) vvas recorded (not shown here). The potential vvas swept from 0.4 to -1.2 V and then reversed to the mitial -0.4 < E 0.4 0.8 0.4 0 -0.4 -0.8 E / V vs. Ag/AgCI -1.2 -0.8 -1 Figure 2. Cyclic voltammograms of PWA /Ti films at different thickness: (1) 80 nm, (2) 135 nm, (3)255 nm, (4)430 nm; electrolyte and scan rate: as in fig. 1. Slika 2. Ciklični voltamogrami PWA/Ti filmov različnih debelin: (1) 80 nm, (2) 135 nm, (3) 255 nm, (4) 430 nm. elektrolit in hitrost preleta potenciala: kot pri si. 1. value at the scan rate 20 m V/s. From the absorbance changes in cathodic and anodic scan a relative change in transmit-tance was calculated yielded about 0.3 for the visible range. A bigger change was expected with in situ chronocoulometric measurements since the potential was kept constant at -1.2 V for 60 s (fig. 4). A relative transmittance change (decrease) in the spectral range 350 - 800 nm was determined. Transmission vvas reduced for about 40% after the application of cathodic voltage to the 400 nm thick PWA/Ti film. Spectroscopic Investigations In order to detect spectral changes of intercalated samples the cyclic voltammogram was interrupted at the voltage giving peak current, i.e. at U = -1.9 V for the films deposited on alummised glass plates and cycled in 0.01 M HC104. After first removmg of the coloured coatings from the electrolytic celi (~1 min) the NGIA spectrum has been recorded (fig. 5-2). The basic features of the NGIA FT-IR spectrum of uncharged sample (fig. 5-1) have been retained except pronounced intensity decrease of the LO modes in the spectral range 800- 900 cm"1 was noticed. These modes7 are assigned to inter and intra W-0-W vibrations involving corner-sharing and edge-sharing oxygen atoms in W30[3 units, respectively. Moreover, LO mode at 839 cm-1 attrib-uted to the intra y W-0-W TO mode at 816 cm1 was shifted to 859 cm"1 vvhile the inter y W-0-W band at 901 cm1, W-Omodeat968 cm-1 andy P-Oat 1066 cm"1 remained nearly undisplaced. Additional band at 1144 cm"1 also appeared which assignment remains unclear. We could conclude that due to the intercalation of H+ ions charges which are contained m Keggin's ions become redistributed in such a way that corner-sharing and edge-sharing vibrations become more polar in respect to the vibrations having pure stretching character ( y W-0). 0.4 0 -0.4 -0.8 E / V vs. Ag/AgCI Figure 3. a - Cathodic and anodic total charge density (Q) at different PWA/'fi film thickness and electrolvte concentration as a function of time: (1)255 nm, 0.001 M HC104, (2)430 nm, 0.01 M HC104; b - cyclic voltammogram corresponding to chronocoulometric curve a-2, potential scan rate: 20 mV/s. Slika 3. a - Katodna in anodna celotna gostota naboja (Q) za PWA/Ti film kserogela pri dveh debelinah in koncentracijah elektrolita v odvisnosti od časa: (1) 255 nm, 0.001 M HC104, (2) 430 nm, 0.01 M HC104; b - ciklovoltamogram, ki ustreza kronokulometrični meritvi a- 2, hitrost preleta potenciala: 20 mV/s. Figure 4. In situ chronocoulometric measurement of PWA/Ti film (thickness = -400 nm). Slika 4. In situ kronokulometrična meritev PWA/Ti filma debeline -400 nm. 0.4 -'-'-'-'- 1800 1500 1200 900 600 300 vvavenumbers / cm"1 Reversibility of the colouring / bleaching process was verified by leaving the coloured sample m the NGIA celi and measured again after 10 min. Discoloration was accompa-nied by regaining initial intensity of the LO bands characteristic for the uncoloured PWA/Ti film. 4 Conclusions Stable mixed PWA/Ti xerogel films have been made via sol - gel route by dip coating technique. Films exhibit excellent optical quality and are characterised with homogenous and amorphous structure. Optical density changes vvhich vvere observed during colouring and bleaching cycles vvere about 40% (in trans-mission) indicating that the concentration ofthe incorporat- Figure 5. FT-IR NGIA spectra of bleached (1) and coloured (2) PWA/Ti film in the spectral range 1800 - 300 cm"1. Slika 5. FT-IR NGIA spektra razbarvanega (1) in obarvanega (2) PWA/Ti filma v spektralnem območju 1800 - 300 cm"1. ed PWA in Ti xerogel matrix vvas sufficient for obtammg electrochromic effect of the mixed films. Electrochemical stability ofthe (PWA/Ti) / HC104is very good (> 200 cycles) vvith intercalated charge up to 35 mC/cm2. NGIA FT-IR spectroscopic studies performed ex situ on coloured and bleached films revealed that mtercalation of H+ ions is related to the changes of intensity ofthe W-0-W stretching vibrations of Keggm's ions, indicating redistribution of charges. 5 References ' C. G. 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