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CHEMICAL PROBLEMS 2022 no. 4 (20) ISSN 2221-8688
289
UDC: 544.228
COMPARATIVE ANALYSIS OF ELECTROCHROMIC PROPERTIES OF CuWO4'WO3,
Bi2WO6^WO3 AND WO3 THIN FILMS
V.O. Smilyk, S.S. Fomanyuk*, I.A. Rusetskiy, M.O. Danilov, G.Ya. Kolbasov
V.I. Vernadskii Institute of General and Inorganic Chemistry of the Ukrainian NAS, Palladina prospect 32-34, Kyiv 03142, Ukraine; * e-mail: [email protected]
Received 31.06.2022 Accepted 03.09.2022
Abstract: A comparative analysis of electrochromic properties of composites CuWO4'WO3, Bi2WO6'WO3 and WO3 films obtained by electrochemical and chemical methods was carried out. The study into the kinetics of light transmission and spectral characteristics of electrochromic coloration revealed some differences in electrochromic processes. It found that in the WO3, Bi2WO6'WO3, CuWO4'WO3 series, lithium intercalation in the film is slowed down, which is due to diffusion limitations in the process of coloring of the Bi and Cu oxides. Spectral characteristics of light transmission Bi2WO^WO3 and CuWO4'WO3 also differ from WO3 in that the contribution to light absorption is also made by Bi and Cu oxides, which are partially reduced by lithium in the process of their coloring. It is shown that the metal tungstates can be effective electrochromic materials with an additional absorption band in the visible region. Keywords: electrochromism, metal tungstates, electrochromic composites. DOI: 10.32737/2221-8688-2022-3-289-296
Introduction
There is not enough published studies about electrochromic properties of metal tungstates what delays their use in real devices as smart windows, multicolor bistable displays, solar heat regulation, optical telecommunications and applications in aerospace and military camouflage [1-5]. In this article, thin films of CuWO4*WO3 and Bi2WO6*WO3 were selected as objects of research into the electrochromic properties of metal tungstates. The tungstates of these metals can have similar spectral characteristics of electrochromic coloration to tungsten oxide with
some differences typical for these materials. For the synthesis of films, preference was given to electrodeposition methods with interference control of film thickness. According to the literature analysis, the method of electrochemical deposition provides films with a high degree of hydration [6, 7] and allows controlling the thickness [8] and surface morphology [9] of films, while the choice of the optimal thickness in the range of 150 - 200 nm [10, 11] allows, in turn, expecting the most optimal parameters of electrochromic efficiency and coloration rate.
Experimental part
CuWO4*WO3 films were obtained in two stages by Cu2O electrochemical deposition at the cathode current 1 mA/cm2 from solution of the composition (CuSO4- 0.05 mol/l, citric acid and 2 mol/l KOH, pH = 10) and followed by anodized Cu2O in a solution of 1 mol/l K2WO4 at a voltage of 3.5 V for 0.5 hours. The
peroxide electrolyte Na2WO4 -0.1 mol/l, H2O2 -0.2 mol/l and H2SO4 (pH =1.1) was used to obtain WO3 film. The deposition was carried out in the galvanostatic mode with a cathodic current of 1.5 mA/cm2. Bismuth tungstate was taken off by electrochemical precipitation from an electrolyte containing Bi2O3= 45 g/l,
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CHEMICAL PROBLEMS 2022 no. 4 (20)
Na2WO4 = 100 g/l, 35% H2O2 - 50 ml/l, adjusted to pH = 1 with nitric acid (55 ml) at a cathodic current of 1 mA/cm2. Bismuth tungstate was also obtained by ion layering using solutions of Bi2O3 = 45 g/l, pH = 1 (nitric acid 55 ml) and Na2WO4 = 100 g/l. All used chemical reagents were analytical grade purchased from Sigma-Aldrich. The XRD study of films was performed using a DRON 4 diffractometer. The optical properties of the films were studied using a Perkin Elmer UV -Vis Lambda 35 spectrophotometer.
The electrochemical measurements of deposited films were performed using PGSTAT Elins P-8S Potentiostat. Platinum was used as a counter electrode and SnO2 / glass substrate as working electrode. The EC study was carried
out in 1 M LiClO4 in propylene carbonate solutions. The change in the light transmittance (electrochromic color) of the films was measured using a universal setup based on a single-beam diffraction spectrophotometer of the C-302 type, which provided measurements in the wavelength range X from 300 to 1300 nm. The monochromator was controlled using a complex based on a personal computer. The galvanostatic current change was provided with the help of the G6-26 signal generator, which supplies a current control pulse to the potentiostat and electrochromic cell. At the same time, the dynamics of changes in the intensity of the transmitted light through the film were recorded.
Results and discussion
The mechanism of electrochemical formation of Bi2WO6 films is similar to the processes of formation of WO3 as a result of electroreduction of peroxide-complex compounds based on tungstate ions. The interaction of Na2WO4 and H2O2 forms a peroxotungstate complex [(O2)2(O)W-O-W(O)(O2)2]2- [12]. The work [13] shows that the process of electrodeposition of WO3 from acidic solutions containing this complex is carried out in two stages: 1 - electrochemical breaking of the O-O bond in the molecule of the peroxotungstate complex and 2 - the chemical stage of polymerization to tungstic acid that forms tungstate ions (H2WO4=WO42-+2H+) and is present in the near-cathode space. If ions (Bi3+) are added to the deposition solution, along with the formation of H2WO4 the Bi2WO6 will also be co-precipitated. Studies of the structure of the obtained films proved this assumption. From the interpretation of X-rays, fig. 1, it is established that along with Bi2WO6, hydrated phases of WO3 are also observed. Fig. 1 presents the results of X-ray phase analysis of bismuth tungstate obtained by ion layering (a) and electrochemical method (b). X-ray diffraction analysis established that the composition of the materials is mixed and includes, in addition to bismuth orthorhombic tungstate, tungsten trioxide with a hexagonal
structure. The comparison of X-ray phase analysis for chemically and electrochemically obtained Bi2WO6 showed that the films obtained by electrochemical deposition have an amorphous structure, with interspersed crystallites of orthorhombic Bi2WO6 and hexagonal WO3 (Fig. 1 (b)) [14, 15], while the films obtained by ion layering have a polycrystalline structure with broadened peaks (Fig. 1 (a), which indicates the fine-grained nature of the obtained crystallites. This is explained as being due to the fact that during ion layering, crystal points are created for further crystal growth, and during electrochemical deposition, a process similar to polymerization takes place following which an amorphous mixture of mixed phase composition Bi2WO6*WO3 is formed. To obtain thin CuWO4*WO3 films with optimal electrochromic parameters [10, 11], voltammetric studies of Cu2O electrodeposition as a precursor for the formation of CuWO4*WO3 were performed. The choice of Cu2O current electrodeposition is based on voltammetric studies. Fig. 2 shows voltammetry of the Cu2O precipitation process from citrate solution based on CuSO4 and alkali. From the analysis of the curve of Fig. 2 it was found that within the potentials of the reduction wave from Cu2+ to Cu+ [16] the current is 1 mA/cm2. As a result, Cu2O films are formed.
Fig.1. a - XRD pattern of chemically obtained bismuth tungstate, where *- orthorhombic Bi2WO6, A - hexagonal WO3; b - X-ray phase analysis of electrochemically obtained bismuth tungstate.
The resulting Cu2O films were anodized in 1 mol / l K2WO4 solution at 3.5 V for 30 min. The anodization resulted in the dissolution of
Cu2O and the formation of CuWO4 sediment. The process of anodizing Cu2O can be described by reactions [17] as follows:
Cu2O + H2O = 2Cu2+ + 2OH- + 2e
WO42- +Cu2+ = CuWO4
(1) (2)
Upon completion of these reactions, thin films of CuWO4 copper tungstate with WO3 impurities were obtained. In parallel with the CuWO4 formation there is the reaction of water
decomposition at the anode with the release of oxygen and H+ protons that interact with WO42-to form hydrated forms of WO3:
2H2O = 4H+ +O2+4e-
WO42- + 2H+ = WO3 *H2O
(3)
(4)
X-ray phase analysis CuWO4*WO3 on the Fig. 3 showed the presence of monoclinic structure CuWO4 • 2H2O (standard card (PDF 33-0503)) [18,19] and undeciphered tungstate structure, possibly, phase CuWO4 (standard card (JCPDS No 88-0269)) [20] with impurities of
hydrated forms WO3 monoclinic WO3*2H2O (JCPDS Card No.18-1420) [21], orthorhombic WO3'0.33H2O (JCPDS Card No. 35-0270) [22] and orthorhombic structure WO3, (JCPDS card 20-1324) [23].
Fig. 2. Voltammetry of the process of obtaining Cu2O from a solution (CuSO4- 0.05 mol / l, citric acid 2 mol / l, KOH -to pH = 10). Scan rate 5 mV/s.
Fig. 3. XRD pattern of CuWO4*WO3 sample where ■- hydrated form of monoclinic CuWO4-2H2O, • - undeciphered tungstate structure [20], ▼- monoclinic WO3*2H2O, o- orthorhombic WO3 •0.33H2O, - orthorhombic structure tungsten trioxide
To assess the stability of the obtained bismuth and copper tungstate films as an electrochromic material, they were cycled in the galvanostatic mode with a current from +2.5 to -2.5 mA/cm2 in 1 M LiClO4 in propylene carbonate solutions while measuring light transmission (Fig. 4) . The comparison of the cycling rate of bismuth and copper tungstates with tungsten trioxide showed different rates of activity for the samples. Also, the comparison of the kinetics of color change processes of electrochromic films of copper and bismuth tungstates with tungsten trioxide showed that in the WO3, Bi2WO6*WO3, CuWO4'WO3 series, lithium intercalation in the films slows down. As can be seen from Fig. 4, the change in the
light transmittance over the same period of time in three samples slows down in the series WO3 (curve 1), Bi2WO6*WO3 (curve 2) and CuWO4*WO3 (curve 3), which is due to the mobility of ions in these structures [24]. In the series WO3, Bi2WO6*WO3 and CuWO4'WO3, the highest mobility of charge carriers is observed in amorphous WO3 films of 20 cm2 /V.s [24], the lowest in CuWO4 0.006 cm2 /V.s [25]. Since the effect of an electric field accelerates electrochromic processes only in those materials in which there are no significant obstacles to the intercalation of ions and the injection of electrons into the films, the materials whose composition includes oxides with the lowest mobility of charge carriers will
have the lowest lithium diffusion rate. At the same time, in the films of Bi2WO6*WO3 and CuWO4*WO3 tungstates, a partial reverse reduction of oxide compounds of bismuth and copper to lower oxides is observed in
comparison to tungsten trioxide. The results obtained show that differences in spectral characteristics are observed in composite films, which is expressed by the shift of the absorption band to the region of shorter wavelengths.
Fig. 4. Cyclic dependences of light transmittance for WO3 films (1), Bi2WO6*WO3 (2) CuWO4*WO3 (3) in the galvanostatic mode with a current of 2.5 and -2.5 (mA/cm2) (X = 1000 nm)
Fig. 5. Optical spectra of electrochromic coloration for WO3 (1) Bi2WO6*WO3 (2) CuWO4*WO3 (3) at j=-2.5 mA/cm2 in 1 M LiClO4 in propylene carbonate solution.
Fig. 5 shows the spectral characteristics of light transmittance of colored films of copper bismuth tungstates and tungsten trioxide. From Fig. 5 curve 1 it follows that for tungsten trioxide the absorption maximum is within X=1000 nm. For bismuth tungstate obtained by electrodeposition, the absorption maximum occurs at 650 nm. And for CuWO4 film with lithium intercalation, an even greater shift of the
absorption band to X=400 nm is observed. The comparison of the spectral characteristics of CuWO4'WO3, Bi2WO6*WO3 and WO3 showed that other oxide components besides WO3 contribute to the color of the film. The analysis of literature data found that the transition from Cu2+ to Cu+ in copper oxide compounds is accompanied by the appearance of an absorption band at 400 nm [26] as in Fig. 4,
curve 3. And the partial reversible electrochemical reduction of Bi2O3 during Li+ intercalation leads to the decrease in the transmission (increase in absorption) of light in the 550-650 nm range [27], which is also observed in our case in fig. 5, curve 2. Thus, in addition to the absorption band of tungsten
trioxide, which falls mainly on the IR region (Fig. 5, curve 1), additional absorption bands appear in the visible part of the spectrum. This fact reveals that the use of metal tungstates is promising, provided that the intercalation of lithium into oxide components is sufficiently large.
Conclusions
By using of the combined chemical and electrochemical methods WO3, Bi2WO6*WO3 and CuWO4*WO3 films were synthesized. The study into the light transmission kinetics and spectral characteristics of electrochromic coloration revealed some differences in electrochromic processes in these films. It was found that in the series of WO3, Bi2WO6*WO3, CuWO4*WO3 there is a slowing down of lithium intercalation in films. This is due to diffusion limitations in the process of coloring complex
oxides of Bi, Cu, and W. The spectral characteristics of electrochromic coloration Bi2WO6*WO3 and CuWO4*WO3 also differ from WO3, in that, in addition to tungsten trioxide, the contribution to light absorption is also made by oxides of Bi and Cu, which also partially are reduced by lithium in the process of their coloring. From this, it can be concluded that tungstates of metals can become effective electrochromic materials with an additional absorption band.
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СРАВНИТЕЛЬНЫЙ АНАЛИЗ ЭЛЕКТРОХРОМНЫХ СВОЙСТВ ТОНКИХ ПЛЕНОК CuWO4^WO3, BiiWO6^WO3 И WO3
В.О. Смилык, С.С. Фоманюк*, И.А. Русетский, М.О. Данилов , Г.Я. Колбасов
Институт Общей и Неорганической Химии им. В.И. Вернадского НАН Украины Пр.Палладина, 32-34, Киев 03142, Украина * e-mail: fomanyuk@gmail. com
Аннотация: Пленки Bi2WO6^WO3, CuWO4^WO3 and WO3 были синтезированы комбинированным химическим и электрохимическим методами. Изучение кинетики светопропускания и спектральных характеристик электрохромного эффекта позволило установить некоторые различия электрохромных процессов в этих пленках. Показано, что в ряду WO3, Bi2WO6^WO3, CuWO4^WO3 происходит замедление интеркаляции лития в пленки, что связано с диффузионными ограничениями в процессе окрашивания сложных оксидов Bi, Cu и W. Характеристики электрохромного окрашивания Bi2WO6^WO3 и CuWO4^WO3 также отличаются от WO3 тем, что, помимо триоксида вольфрама, вклад в поглощение света вносят также оксиды Bi и Cu, которые также частично восстанавливаются литием в процессе их окраски. Сделан вывод о том, что вольфраматы металлов могут стать эффективными электрохромными материалами, имеющими дополнительную полосу поглощения в видимой области спектра.
Ключевые слова: электрохромизм, вольфраматы металлов, электрохромные композиты
CuWO4^WO3, Bi2WO6^WO3 V3 WO3 NAZiK T9B9Q9L9RiN ELEKTROXROM XÜSUSiYYaTLaRiNiN MÜQAYiSOLi TOHLiLi
V.O. Smilik, S.S. Fomanyuk*, I.A. Rusetskiy, M.O. Danilov, Q.Ya. Kolbasov
Ukrayna Milli Elmhr Akademiyasi, V.i. Vernadsky ad. Ümumi vd Qeyri-üzvi Kimya institutu * e-mail: _ fomanyuk@gmail. com
Xülasa: Bi2WO6^WO3, CuWO4^WO3 va WO3 nazik tabaqalar kombina edilmi§ kimyavi va elektrokimyavi üsullarla sintez edilmi§dir. i§igin ötürülmasinin kinetikasinin va elektroxrom effektinin spektral xüsusiyyatlarinin öyranilmasi bu plyonkalarda elektroxrom prosesl arda bazi farqlar müayyan etmaya imkan vermi§dir. Göstarilmi§dir ki, WO3, Bi2WO6^WO3, CuWO4^WO3 sirasinda litiumun tabaqalara interkalyasiyasi langiyir, bu da Bi, Cu va W mürakkab oksidlarinin ranglanmasi prosesinda diffuziya mahdudiyyatlari ila alaqalandirilir. Bela naticaya galinib ki, i§iq spektrinin görünan hissasinda alava udma zolagin olmasi sababindan metal volframatlar effektiv elektroxrom materiallar ola bilarlar. A^ar sözlar: elektroxromizm, metal volframatlar, elektroxrom kompozitl ar
