Preparation of electrochromic thin films by

Journal of Physics and Chemistry of Solids 74 (2013) 1433–1438

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Preparation of electrochromic thin films by transformation of manganese(II) carbonate Sasho Stojkovikj a, Metodija Najdoski a,n, Violeta Koleva b, Sani Demiri a a Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Sts. Cyril and Methodius University, POB 162, Arhimedova 5, 1000 Skopje, Republic of Macedonia b Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Republic of Bulgaria

art ic l e i nf o

a b s t r a c t

Article history: Received 31 December 2011 Received in revised form 27 March 2013 Accepted 2 May 2013 Available online 13 May 2013

A new chemical bath method for deposition of manganese(II) carbonate thin film on electroconductive FTO glass substrates is designed. The homogeneous thin films with thickness in the range of 70 to 500 nm are deposited at about 98 1C from aqueous solution containing urea and MnCl2. The chemical process is based on a low temperature hydrolysis of the manganese complexes with urea. Three types of films are under consideration: as-deposited, annealed and electrochemically transformed thin films. The structure of the films is studied by XRD, IR and Raman spectroscopy. Electrochemical and optical properties are examined in eight different electrolytes (neutral and alkaline) and the best results are achieved in two component aqueous solution of 0.1 M KNO3 and 0.01 M KOH. It is established that the as-deposited MnCO3 film undergoes electrochemically transformation into birnessite-type manganese (IV) oxide films, which exhibit electrochromic color changes (from bright brown to pale yellow and vice versa) with 30% difference in the transmittance of the colored and bleached state at 400 nm. & 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Thin films B. Chemical synthesis C. Raman spectroscopy C. Infrared spectroscopy D. Electrochemical properties

1. Introduction One of the most important properties of the materials used in various fields of high technology is electrochromism. A material is electrochromic if it has the capability to maintain reversible and persistent change in the optical properties (color change) when an electrical potential is applied to it. The reversible change in the color is induced by the change in the oxidation state of the metal ions which is associated with relevant insertion/extraction of ions from the electrolyte into/from the material. WO3 is one of the most extensively studied electrochromic material [1]. In comparison to this material, the electrochromism of manganese based compounds is less explored. Among them the most promising compound is MnO2, which is an anodically coloring electrochromic oxide. The Mn4+ state has an optical absorbance due to the d–d transition in the visible light range, giving a brownish color and the reduction to Mn3+ state changes the color to pale yellow [2]. The electrochromic properties of manganese oxide thin films with different structures (γ-Mn2O3/Mn3O4 and layered Mn7O13∙ 5H2O) prepared by electrodeposition at different potentials have been reported by Chigane et. al. [3]. Long et al. [4] have found that sol−gel derived thin films of Na-birnessite show relatively high electrochromic efficiency. The influence of the number of layers of MnO2 nanosheets prepared by layer-by layer assembly on the electrochromic properties has been examined by Sakai et al. [2]. n

Corresponding author. Tel.: +389 78 570 700; fax: +389 2 3226 865. E-mail address: [email protected] (M. Najdoski).

0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.05.001

It is established that appropriate change in the optical density can be achieved by controlling the number of deposition cycles. This result can be used for tailoring the optical properties. Generally, manganese based thin films are prepared using various methods such as: dip coating [5], electrodeposition [6], potentiostatic electrolyses [2], chemical vapor deposition [7], thermal decomposition [8], atomic layer deposition [9], aqueous precipitation (chemical bath deposition) [10] etc. Each of the coating methods has various advantages and disadvantages, though all of them are capable to produce the desired material. Among them the chemical bath deposition (CBD) is low-cost, very simple and large-area applicative technique that requires only aqueous solution of the chemical precursors. The reaction mechanism usually engages two stages: nucleation and particle growth, based on formation of the solid phase from the solution. In addition, CBD allows fabrication of thin films at low temperature. Most recently we have developed a chemical method for deposition of K-birnessite-type manganese oxide thin film with electrochromic properties [11]. The method is based on the successive immersion of the substrates in aqueous solutions of MnCl2 and KMnO4 and it allows the preparation of films with thickness from 50 nm to 250 nm for about 2–5 min. Both as-prepared and annealed films demonstrate good electrochromic characteristics with a difference in the transmittance of 40% between the reduced and oxidized state in aqueous KNO3 electrolyte [11]. In the search for new manganese electrochromic thin films we have examined films based on manganese(II) carbonate. Manganese(II) carbonate by itself has been widely applied in solid oxide

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S. Stojkovikj et al. / Journal of Physics and Chemistry of Solids 74 (2013) 1433–1438

fuel cell (SOFC), inorganic–organic hybrid composite materials, pigments and is often used as solid precursor to synthesize the respective metal oxides [12]. However, to our knowledge, there are no reports in literature of electrochromic thin films based on manganese(II) carbonate. In the present paper we propose a chemical bath deposition method for preparation of manganese (II) carbonate thin films. The method is very simple and consists of heating a solution containing manganese(II) chloride and urea. Three types of films originating from the initial manganese(II) carbonate films have been considered: as-deposited, annealed and electrochemically transformed thin film. The structure, electrochemical behavior and electrochromic properties of the three types of manganese thin films have been studied and compared.

recorded in a computer file. The film thickness is measured using Alpha Step D-100 profilometer with parameters: Stylus force – 8 mg, Length: 8 mm, Speed: 0.07 mm/s, Range 10 mm. Optical measurements are conducted using Varian Cary 50 Scan UV–visible Spectrophotometer. The transmittance is recorded in the wavelength range from 400 to 900 nm at potentials+2.5 and −2.5 V. The in situ optical analysis is performed in 0.1 M LiClO4(aq) and in two component electrolyte. The IR spectra are measured by a Perkin-Elmer System 2000 infrared interferometer, using KBr disks in the 4000–400 cm−1 region. The Raman spectra are recorded by Horiba JobinYvon LabRam 300 Infinity using He:Ne laser excitation at 632.8 nm (f 50, resolution 4 cm−1). In order to avoid the sample photo transformation the Raman spectra are recorded at a low excitation power of 3–4 mW.

2. Experimental 3. Results and discussion 2.1. Preparation of the FTO glass substrates

2.2. Chemical deposition of MnCO3 thin films The chemical bath solution is prepared at room temperature by dissolving 0.5 g manganese(II) chloride tetrahydrate (MnCl2d4H2O) and 2 g urea (H2NCONH2) in 50 cm3 deionized water in a 100 cm3 beaker. The reagents used are of analytical grade purity. Thus prepared solution has pH ¼6. The glass substrates were immersed in the chemical bath in such a way that their nonconductive side faces the wall of the beaker. During the deposition the solution was continuously stirred by electromagnetic stirrer and the beaker was covered with Petri dish (in order to avoid water vaporization). Then, the chemical bath was heated up to 98 1C. When the solution became opaque the substrates are removed from the solution and the conductive side of substrates was quickly wiped with wet cotton. After the deposition, the substrates were washed with large amount of deionized water and then dried at room temperature. The thickness of the thin films was controlled by variation on the deposition time. Thin films were deposited on both sides of each substrate. The material deposited on the nonconductive sides of the substrates was carefully removed using cotton and aqueous solution of 3 M hydrochloric acid. Then, they were washed with deionized water and dried again at room temperature. The thin as-deposited films are transparent colorless and the thick films are opaque white. 2.3. Characterization of the thin films The structure of the thin films is examined by X-ray powder diffraction, using Bruker D8 Advance X-ray diffractometer (Sol-X detector) with CuKα radiation. The electrochemical properties are characterized by cyclic voltammetry measurements. Cyclic voltammograms are recorded using a microAUTOLAB II equipment (Eco-Chemie, Utrecht, Netherlands). A KCl saturated Ag/AgCl electrode is used as a reference and a platinum wire as an auxiliary electrode. The samples were studied in different electrolytes: 1 M LiClO4 in propylene carbonate (PC) solution; 0.1 M LiClO4(aq); 0.1 M KNO3(aq), aqueous solution of KOH with concentrations 1, 0.1, 0.05 and 0.01 M; a two component aqueous solution containing 0.1 M KNO3 and 0.01 M KOH. Hereafter, the latter electrolyte will be simply denoted as two component electrolyte. The data are

Fig. 1(a) shows the XRD pattern of the as-deposited film. As seen it is well crystalline and the observed reflections match very well with rhodochrosite phase, MnCO3 (JCPDS 83–1763). Some weak reflections from the FTO layer (SnO2, JCPDS 46–1088) are also seen. The chemical deposition of MnCO3 thin films is based on two consecutive chemical processes. The first process consists of formation of Mn2+ adducts with urea at room temperature and it has been already described as follows [13,14] in Eq. (1): Mn2+(aq)+xH2NCONH2(aq)-[Mn(H2NCONH2)x]2+(aq)

(1)

where x ¼4 and/or 6 We suppose that in the second step at temperature above 90 1C a hydrolysis of the manganese adducts occurs (Eq. (2)), which is accompanied with the precipitation and growth of manganese(II) carbonate on the surface of the substrate. [Mn(H2NCONH2)x]2++(x+y+1)H2O-MnCO3 yH2O+(x−1) CO2+2NH4++2(x−1)NH3

(2)

The formation of the rhodochrosite phase is confirmed by the FT-IR spectrum of the as-deposited film recorded after the scraping away of the film (Fig. 2). The IR spectrum is dominated by the characteristic vibrations of the carbonate ions: stretching C-O vibrations at 1072 (ν1) and 1430 cm−1 (ν3); bending OCO vibrations at 864 (ν2) and 725 cm−1 (ν4). The weak bands at 1796 and 2484 cm−1 arise from the combination vibrations like (ν1+ν4) and (2ν2+ν4), respectively.

*

*

*

b *

*

Intensity (a.u.)

Glass substrates coated with electroconductive fluorine doped tin(IV) oxide (FTO) having electric resistance of 10−20 Ω/mm2 and dimensions of 5 cm 2.5 cm 0.2 cm were used for thin films deposition. The substrates were preliminary cleaned with hexane and acetone, and dried at room temperature. Then, they are washed with deionized water and dried again.

*

*

a *

* * 20

* 30

* 40

50

60

* 70

2θ (degree) Fig. 1. XRD patterns of the as-deposited MnCO3 film (a) and annealed at 400 1C film (b); The asterisks denote the peaks from the SnO2 substrate.


1435

5.0

1796

4.0

1072

3.0

725

864

I (A ·10-4)

Transmittance (a.u.)

2484 3430

2.0 1.0 0.0 -1.0

1430 3500

3000

2500

2000

Wavenumber

1500

1000

500

(cm-1)

Fig. 2. IR spectrum of the scraped as-deposited MnCO3 film.

Thickness (nm)

450 350 250 150 50 0

5

10

15

20

25

Time (min) Fig. 3. Thin films thickness vs. deposition time.

The observed CO32− frequencies coincide very well with the reported values for MnCO3 xH2O obtained by urea precipitation method [14] as well as with hydrothermally prepared manganese carbonate [12]. In addition, a weak broad band centered at 3430 cm−1 with a shoulder at around 3530 cm−1 are also visible in the ν(OH) mode region. More probably, they are due to water molecules adsorbed on the surface. By varying the deposition time we have prepared thin films with different thicknesses (Fig. 3). The deposition time was measured from the moment when the opaque state of the chemical bath solution was achieved and it usually was after heating of 25 min. The thinnest 70 nm film was obtained after 3 min of deposition time, while the thickest one with a thickness of about 480 nm was obtained after 23 min deposition time. The electrochemical and optical studies are performed with films having about 200 nm thickness. The electrochemical behavior of the as-deposited thin films is analyzed by cyclic voltammetry in different electrolytes in the range of −2.5 V to+2.5 V at a scan rate of 10 mV/s. The asdeposited film did not show considerable electrochemical activity in 0.1 M LiClO4(aq), 0.1 M KNO3(aq) and 1 M LiClO4 in propylene carbonate as electrolytes. A pronounced electrochemical activity was recorded in alkaline aqueous KOH solutions with concentrations between 1 and 0.01 M. We established, however, that in these solutions the observed redox process is unstable which refer to irreversible redox changes. The best reversibility and reproducibility of the redox processes is achieved with the two component electrolyte (aqueous solution of 0.1 M KNO3 and 0.01 M KOH) and the corresponding cyclic voltammogram is shown in Fig. 4. It is characterized by one

-2.0 -1.1 -0.8 -0.5 -0.2

0.1

0.4

0.7

1.0

1.3

E (V) Fig. 4. Cyclic voltammogram of as-deposited MnCO3 thin film.

oxidation peak at 0.8 V and two reduction peaks at −0.45 and −0.58 V. It is also seen that the area of the oxidation peak is rather larger than that of the two reduction peaks. Previous studies have shown that the electrochemical oxidation of Mn2+ species at potential higher than 0.3 V results in the formation of different types of MnO2 oxides, where the Mn4+ sites are predominant [3,15,16]. For example, birnessite-type manganese oxides have been prepared by potentiostatic anodic electrolysis of manganese(II) ammine complex with pH ¼8 at potentials higher than 0.3 V [3] and of MnSO4 in aqueous K2SO4 at potentials higher than 0.8 V [15]. The formation of Mn4+ at this potential agrees well with the established that it is the thermodynamically stable ionic form at the potentials higher than 0.4 V [17]. Taking into account the literature data the oxidation peak around 0.8 V in the CV curve of the as-deposited film is attributed to the oxidation of Mn2+ mainly to Mn4+, but presence of Mn3+ is also assumed. At around −0.5 V a reduction of Mn4+ sites to Mn3+ takes place, which explains the smaller area of the reduction peaks (smaller number of electrons involved in the process) compared to the oxidation peak (Fig. 4). It is known that the reduction of Mn3+ to Mn2+ with formation of Mn(OH)2 is observed only in concentrated alkaline solutions (for example, 7 M) at potentials below −0.43 V [18]. The optical transmittance spectra of the as-deposited film recorded in the two component electrolyte at 7 2.5 V are shown in Fig. 5a and b. It should be mentioned that the change in the film color at the potential variation is hardly visible. As seen from Fig. 5a and b the transmittances of the oxidized state at +2.5 V (hereafter colored state) and reduced state at − 2.5 V (hereafter bleached state) are very close. The maximum difference in the two states is only 5% at 400 nm, which is insignificant result regarding the electrochromic properties. More probably, just a little part of the MnCO3 phase is electrochemically transformed in active manganese oxide phase which is responsible for the poor result. Looking for improvement in the optical characteristics of the prepared films in the next step we explored the so called “cycled films”. They were prepared by multiple cycling (300 and 700 cycles) in 0.1 M aqueous LiClO4 at voltages of 72.5 V with 5 s time interval of each cycle By the multiple cycling we expected to induce a complete electrochemical transformation of the MnCO3 phase in an electrochromic active state. Thus prepared cycled films were firstly analyzed in 1 M LiClO4 in PC, where electrochemical activity was not observed. The films showed electrochemical reproducibility in the two component electrolyte (Fig. 6). The CV curves of the cycled films show well-defined oxidation and reduction peak at 0.95 and −0.60 V, respectively (Fig. 6). The shapes of the CV curves are similar to that of the as-deposited film, but the redox potentials have slightly higher values (Figs. 4 and 6).


Transmittance (%)

80

a

75 70 65

b

60

c

55 50

- as-deposited (a,b)

d

45

- annealed (c,d)

40 400

500

600

700

800

Wavelength (nm) Fig. 5. In situ optical transmittance spectra of: as-deposited film at+2.5 V (a) and at − 2.5 V (b); annealed film at 400 1C at+2.5 V (c) and at (d) −2.5 V.

10.0 8.0 I (A ·10-4)

6.0 4.0 2.0 0.0 -2.0 -4.0 -1.1 -0.8 -0.5 -0.2

0.1

0.4

0.7

1

1.3

E (V) Fig. 6. Cyclic voltammogram of cycled film.

Transmittance (%)

90 80

a

70 60 50 40 400

b

500

600

700

800

Wavelength (nm) Fig. 7. In situ optical transmittance spectra of cycled film at +2.5 V (a) and at (b) −2.5 V.

The electrochemically cycled films change notably its color depending on the applied voltage. The color state (at+2.5 V) is bright brown, while the bleached one (at −2.5 V) is pale yellow. The optical transmittance spectra of the cycled films recorded in the two component electrolyte (Fig. 7) reveal better electrochromic characteristics than that of the as-deposited one. The achieved difference in the transmittance of the two states is now 30% at 400 nm wavelength. The prolonged cycling for 700 cycles does not improve the electrochromic properties of the films. Since the manganese oxides are known to exhibit promising electrocromic properties we also examined the product of thermal decomposition of the MnCO3 film. The as-deposited MnCO3 film was heated in air at 400 1C for 6 h. According to literature data [11,19] the formation of MnO2 was expected at this temperature. The XRD pattern of the annealed film shown in Fig. 1b reveals poor

crystalline nature of the obtained material. Besides the strong reflections from the FTO layer, four broad peaks of low intensity are found at 37.231, 40.801, 42.851 and 56.531 (2θ). Among the numerous varieties of the manganese dioxide the observed peaks correspond well to the hexagonal MnO2 known as ε-MnO2, akhtenskite (JCPDS 89-5171). Usually, MnO2 obtained by thermal decomposition of salts is close to β-MnO2 (pyrolusite), whereas chemically and electrochemically prepared MnO2 is close to γMnO2 or ε-MnO2 [18,20]. Both γ- and ε-MnO2 are closely related to the pyrolusite structure, all comprised of a hexagonally close packed lattice of O2− anions with Mn4+ cations filling one-half the octahedral sites in the oxygen lattice [18]. The difference in the above forms is in the arrangement of Mn4+ within the octahedral sites. In β-MnO2 Mn4+ are arranged in an ordered configuration creating 1 1 tunnels between the MnO6 octahedra, whereas in εMnO2 one-half of the octahedral sites are randomly filled with Mn4+. The structure of γ-MnO2 can be regarded as an irregular intergrowth of pyrolusite domains within a ramsdellite matrix (1 2 tunnels) in terms of De Wolff disorder and microtwinning [18]. Recently, it is proposed that although the ε-MnO2 structure interpreted to be present in XRD patterns, it is not a discrete phase [21]. The observation of ε-MnO2 pattern is considered to be a superstructure signature of long range disordered Mn4+ ions within the hexagonal ordered oxygen framework, while γ-MnO2 pattern is a result of the short range ordering of the Mn4+ ions similar to that of ramsdellite [21]. The formation of ε-MnO2 instead of β-MnO2 after MnCO3 film decomposition could be related to the influence of the substrate, SnO2 in our case. Recently, it has been established that both the type and orientation of the substrate strongly influence the structure and orientation of the MnO2 thin films prepared by atomic layer deposition [9]. Thus, α-MnO2 is obtained on NaCl(100) and KBr(100) substrates, β-MnO2 is obtained on α-Al2O3(012), whereas ε-MnO2 grows on α-Al2O3(001). The electrochemical behavior of the annealed film in the studied electrolytes resembles that of the as-deposited film. No significant signs of electrochemical activity are recorded in 0.1 M KNO3(aq), 0.1 M KOH(aq) and 1 M LiClO4 in PC. Reversible redox processes are observed in 0.1 M aqueous LiClO4, but here the oxidation process is much more unstable than the reduction one (CV curves are not shown). Similarly to the previous films the best results regarding the reversibility are achieved using the two component electrolyte (Fig. 8). The CV curve of the annealed film (Fig. 8) is also characterized by an oxidation/reduction pair. The oxidation peak appears at 1.2 V, i.e. at higher value in comparison with the asdeposited and cycled films, while the reduction peak has potential of − 0.45 V which is slightly lower (Fig. 8). The area of the oxidation peak is also many times larger than that of the reduction

11.0 8.0

I (A ·10-4)

1436

5.0 2.0 -1.0 -4.0 -7.0 -0.7

-0.4

-0.1

0.2

0.5

0.8

1.1

1.4

E (V) Fig. 8. Cyclic voltammogram of annealed film at 400 1C.


635

g

560 500

650 575 f 500 635 560 Raman intensity (a.u.)

peak as established for the as-deposited film. From these data analogous electrochemical transformations can be supposed for the films. In any cases, during the oxidation process Mn2+ ions are oxidized predominantly to Mn4+, while in the reduction process the reduction of Mn4+ to Mn3+ takes place. The optical properties of the annealed thin film are illustrated in Fig. 6c and d. Contrary to our expectations the obtained result is not much better than that for the as-deposited film. Here, the maximum difference in the transmittance of the colored and bleached state is less than 10% at 600 nm (Fig. 5c and d). Similar data in respect to the difference in the transmittance (12%) were obtained for films annealed at a lower temperature, 350 1C for 6 h (not presented). We can only speculate on the possible reasons for the poor electrochromic behavior of the annealed films. Our experiments with pure unannealed and annealed FTO substrate showed that the substrate has no effect on the electrochromic properties of the films and, consequently its influence on the annealed films must be ignored. In our opinion one of the reasons is probably related with the fact that MnO2 phase is formed after thermal decomposition of the rhodochrosite phase. It could be supposed that the evolved pyrolytic CO2 gas destroys the structure of the film creating a large number of cracks. By this manner the FTO substrate is exposed to an undesired contact with the electrolyte. At the same time, the interface surface between the substrate and the annealed material decreases, thus increasing the electrical resistance. But this is only a hypothesis without any experimental evidence. It is worth mentioning that there is no report in the literature on the electrochromic properties of hexagonal MnO2 phase. Since the best optical properties are obtained with the cycled film the further studies are focused on the clarifying the chemical nature and structure of the film. To specify the type of manganese oxide phase obtained electrochemically in the two component electrolyte, the oxidized and reduced films are examined by XRD and Raman spectroscopy. Unfortunately, the XRD patterns of both states (not shown) did not display any reflections in addition to that from FTO layer, most probably due to the amorphous character of the materials. The Raman spectra of the oxidized and reduced cycled films (300 and 700 cycles) with different thicknesses are presented in Fig. 9. They are characterized by a low Raman activity, which is a general peculiarity of the manganese dioxides even with welldefined structure [22,23]. This is particularly seen for the thin films with thickness of 200 nm (Fig. 9a, b, c), where only two/three very week bands are hardly visible. To obtain better ratio signal/ noise we have also recorded the spectra of films with higher thickness of ∼480 nm (Fig. 9d and e). It is important, that the positions of the bands in the Raman spectra obtained from different points of given film coincide in the limit of the experimental resolution, which implies a homogeneous phase composition of the films. In some cases a little change in the intensity of the bands was observed. The spectra of the oxidized and reduced phases (Fig. 9) display bands only below 750 cm−1 related to the Mn−O vibrations. The Raman spectra of the oxidized phase (Fig. 9a, c, d) are dominated by two strong bands at 640 and 560 cm−1 and a less intensive band at 500 cm−1. Very week bands at around 720, 380, 280 and 160 cm−1 can be resolved only for the thicker film (Fig. 9d). The spectrum of the reduced phase is not considerably different from that of the oxidized phase, especially for the thinner 200 nm film (Fig. 9b). More clear difference between the two states is seen for the thicker 480 nm film, where the spectrum of the reduced phase is characterized by a relatively strong band at 635 cm−1 and two less intensive bands at 560 and 480 cm−1 (Fig. 9e). Two possibilities for the structure of the oxidized MnO2 film could be supposed: a layered (2D) birnessite-structure or a tunnel

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d

480

640 560 490

720

c

380 280

160

635 560 500

b a 635 560

500

900

750

600

Raman shift

450

300

150

(cm-1)

Fig. 9. Raman spectra of: cycled film for 300 cycles with 200 nm thickness (a, b); cycled film for 700 cycles with 200 nm thickness (c); cycled film for 300 cycles with 480 nm thickness (d, e); K-birnessite film (f, g); Curves a, c, d and f correspond to the oxidized state (at +2.5 V) and curves b, e and g correspond to the reduced state (at −2.5 V).

(1D) structure. The tunnel structure is mainly represented by γ-MnO2 and α-MnO2 with 2 2 tunnels [18]. Both types of structures are reported to be formed during the electrochemical oxidation of Mn2+ species in aqueous solutions, depending on the pH of solution, counter ions and applied voltage [24]. Julien et al. [22,23,25] have devoted series of papers to IR and Raman spectroscopic characterization of various manganese oxides. To identify our materials we compared the shape and positions of the bands in our spectra with the already published ones. It turned out that among the numerous manganese oxides the Raman spectra of our oxidized phase (Fig. 9a, c, d) resemble in the highest degree that for the birnessite-type compounds (especially Na-, Co-, and SG-birnessites) [22]. More precisely, the oxidized phase should correspond to a K-birnessite taking into account the used electrolyte, being a mixture of aqueous KOH and KNO3. However, we have not found in the literature Raman spectrum of K-birnessite. To confirm unequivocally the formation of the birnessite-type phase, we additionally measured the Raman spectra of a K-birnessite film (oxidized and reduced state) prepared more recently by us [11]. The latter film was synthesized by successive immersion of SnO2 substrate in aqueous solutions of MnCl2 and KMnO4 and its layered birnessite-type structure was proved by XRD analysis. The Raman spectra of that K-birnessite film are also presented in Fig. 9f and g and from the comparison the great similarity with presently considered film is evident.

1438


Birnessite-type structure is described by layers of edge-sharing MnO6 octahedra (usually Mn4+/Mn3+ with predominant Mn4+ ions) where there is a vacancy in one over every six octahedral sites [26]. Between the layers, separated by a distance of around 7 Å, mono-or divalent cations more or less hydrated are located. Following the interpretation given by Julien et al.[22], the bands at around 640 and 560 cm−1 are assigned to the stretching vibrations of MnO6 octahedra, while the lower frequency bands are mainly due to the deformation Mn–O–Mn modes. In the reduced state the partial reduction of Mn4+ should result in a decrease in the stretching mode frequencies that is inherent to the presence of Mn3+ ions. Such shifting (with 15 cm−1) is clearly seen in the spectrum of the reduced K-birnessite film prepared from MnCl2 and KMnO4 solutions (Fig. 9f and g). Concerning the thin cycled films the shifting is not evident which is explicable taking into account the very low intensity and broadness of the observed peaks. A slight shifting to lower frequency with 5 cm−1 could be mentioned for the thicker film (Fig. 9d and e). The electrochemical transformation of MnCO3 film into birnessite thin film can be described by following equation assuming that it takes place similarly to γ-MnO2 [27] Mn2++(2+n)H2O+xA+ - AxMnO2 nH2O+4H++(2+xe−)

(3)

+

where A stands for alkaline ions. Once produced the birnessite phase, containing predominantly Mn4+ ions, can participate in reversible redox reactions between Mn4+ and Mn3+ responsible for the electrochromic properties. The reduction process in layered MnO2 have been ascribed to various mechanisms, including insertion of M+ from the electrolyte between the Mn-O layers [2] or both H3O+ and M+ [28] or more complex proton insertion from molecular water and exchange between H+ and M+ with formation of OH groups [16,27,29]. With this regard it is worth noting that the Raman spectra of the reduced birnessites (Fig. 9b, e, g) do not evidence for the presence of OH groups. Also, they are essentially different from the published Raman spectrum of MnOOH [23]. Based on these observations we could suppose that the charge balance in the reduced birnessite films is probably achieved by insertion of ions from the electrolyte into the Mn–O layers rather than by insertion of protons with formation of OH groups. The made assumption, however, needs to be confirmed or disproved by more thorough mechanistic investigations. 4. Conclusion Electrochromic manganese oxide films with different thickness derived from as-deposited MnCO3 films have been prepared and studied by powder XRD, IR and Raman spectroscopy, cyclic voltamommetry and optical spectroscopy. The redox processes are examined in different neutral and alkaline electrolytes such as LiClO4 in propylene carbonate, LiClO4(aq), aqueous solution of KOH

with various concentrations. The best reversibility of the redox processes is found in two component electrolyte containing 0.1 M KNO3 and 0.01 M KOH. Manganese(II) carbonate thin films have been electrochemically cycled and transformed into anodic coloring electrochromic materials having birnessite-type structure. The films preliminary cycled in LiClO4(aq) for 300 and 700 cycles with thickness 200 nm exhibits the best optical properties with 30% difference in the transmittance value of the colored and the bleached state at 400 nm wavelength. This result makes the films good candidates for electrochromic materials.

Acknowledgments The authors acknowledge their gratefulness to Alexander von Humboldt Stiftung for providing the electrochemical equipment without which the present study would not have been possible.

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