Electrochromic Behavior of Manganese Oxides/Silver Thin Films from Electrochemical Deposition ^†

Abstract

MnO_x thin films with silver additives were electrochemically deposited on an Indium Tin Oxide (ITO) substrate with silver acetate and potassium permanganate aqueous solution. The addition of Ag enhanced electrochromic behavior during cyclic voltammetry (CV). The morphology of the thin films was examined by using scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). The chemical states of Mn and Ag ions on the surfaces of the thin films were examined using X-ray photoelectron spectroscopy (XPS). Spherical Ag₂O and Ag nanoparticles were homogeneously dispersed on the thin films. The electrochemistry of the thin films was examined by cyclic voltammetry in a conventional three-electrode system and an electrochemically tested system. The electrochromic behavior of the films was demonstrated through the cyclic voltammetry (CV) process in the KNO₃ electrolyte. The electrochromic behavior of the thin films depended on the redox reactions associated with the reaction between Ag and Ag₂O coupled with Mn⁴⁺ ions and Mn³⁺ ions in the KNO₃ electrolyte.

1. Introduction

The electrochemistry of Manganese oxide (MnO_x) electrodes has been extensively studied, starting from their application for active materials in catalysis, batteries, and supercapacitors [1,2,3,4]. Typically, MnO_x electrodes are prepared by galvanostatic electrolysis, and chemical vapor deposition, thermal decomposition, and electron beam evaporation have been proposed [5,6,7]. In our previous study, we applied the electrochemical deposition method to prepare MnO_x/Ag thin films [8]. According to the XPS analysis, thin films composed of Ag₂O and α-Mn₂O₃ nanoparticles had a lower Ag content, while those composed of Ag₂O₂ and β-MnO₂ had a higher Ag content. Ag nanoparticles in the thin film acted as a reduction agent and varied the valence of Mn from 4⁺ to 3⁺. The capacitance of the thin film increased with an increase in the ratio of Ag/Mn up to 8%. The impedance of the cell also increased with the Ag/Mn ratio. In subsequent research, we found that adding silver caused it to change its color in a cyclic voltammetry test due to its electrochromic behavior.

Electrochromic devices have attracted great attention recently due to the growing demand for smart electronics. Color electrochromic cells are operated in the visible light region and display their remaining capacity through instant color changes. Many materials have been explored with regard to this property, and MnO_x is one of the promising candidates. The electrochromic properties of MnO_x with various crystalline phases have been examined in aqueous [9,10,11,12,13] and nonaqueous [14,15,16] electrolytes. The electrochromic properties of materials depend on their crystalline structure and morphology. The electrochromic property of MnO₂-based materials is obtained through instantaneous extraction or insertion of electrons and electrolyte cations into or from the matrix. However, MnO is extensively used in supercapacitors owing to its high energy density, but it has a poor electrochromic performance.

In this study, nanocomposite thin films were obtained by electrochemical deposition on Indium Tin Oxide (ITO) glass substrate in KMnO₄ and Ag(C₂H₅COO) aqueous solution. The electrochromic behavior of the thin film was demonstrated by cyclic voltammetry (CV) in the KNO₃ electrolyte.

2. Experiments

The raw materials used in the synthesis process-potassium permanganate (KMnO₄, ACS reagent and purity > 99%, Sigma Aldrich, St. Louis, MO, USA), silveracetate (AgC₂H₃O₂, purity > 99%, Sigma Aldrich) and Potassium nitrate (KNO₃, purity > 99%, Kingfex, Taichung, Taiwan). The nanocomposite thin films were obtained by electrodeposition with a potentiostatic method [8]. The thin films were examined by cyclic voltammetry with a three-electrode system using the electrochemically tested system (Electrochemical station 5000, Jiehan, Hsinchu, Taiwan). A platinum line and a standard calomel reference electrode (SCE) were used as the reference and the counter electrode, respectively. The cyclic voltammetry (CV) experiment was operated from 0 to 1.0 V in 0.1 M KNO₃ electrolyte at a scan rate of 20 mVs⁻¹. A field emission scanning electron microscope (FESEM, Hitachi S-4700) was used to investigate the morphology of the thin films. The state of the thin films was analyzed with transmission electron microscopy (TEM). The optical absorption spectra were recorded with a UV–visible 560 spectrophotometer (Jasco). The states of the Mn and Ag ions were examined by X-ray photoelectron spectroscopy (XPS, VG ESCA Scientific Theta Probe) with Al Kα (1486.6 eV), and the X-ray spot size was 15 μm.

3. Results and Discussion

Figure 1 shows a CV curve of the MnO_x electrode without the Ag additive in the KNO₃ electrolyte. The CV curve exhibits a quasi-rectangular shape with redox reactions at 0.2 and −0.23 V. The color of the film changed from brownish to dark brown following the first oxidation reaction at 0.2 V. As the voltage decreased, the film changed from dark brown to a brownish color under the reduction reaction at −0.23 V. The color change in the thin film is ascribed to the redox between Mn(3+) and Mn(4+) in the KNO₃ electrolyte [17,18].

Figure 2 shows a CV curve of the MnO_x/Ag electrode and the valence state of the Mn and Ag ions. The CV curve of the MnO_x/Ag electrode is different from that of MnO_x in KNO₃. There are two oxidation reaction peaks located at 0.1 and 0.5 V, while the reduction peak is located at 0.25 V. During the oxidation process, the color of the electrode changed from a brownish color to black after peak I at a voltage of 0.1 V. The color of the electrode remained black after peak II at a voltage of 0.5 V and turned back to a brownish color after peak III at a voltage of −0.25 V.

Figure 3 shows the changes in the absorbance of the thin films from the ultraviolet to visible wavelength at each step in the redox reaction. More light absorption of the thin films after the oxidation reaction was observed (peak I and peak II in the redox loop in Figure 2) at the 400 nm band. This absorption caused the film to turn black.

Figure 4 shows the XPS spectra of Mn 2p_3/2 for the thin film at each redox step. The binding energy of Mn 2p_3/2 was determined to be 640.9 eV for Mn²⁺, 641.9 eV for Mn³⁺, and 642.5 eV for Mn⁴⁺ which coincided with previous research [19]. The binding energy of Mn 2p_3/2 of the sample increased from 642.0 to 642.6 eV as the Ag content increased. The shift in the XPS peak to higher binding energy indicates that the oxidation state of manganese changed from Mn³⁺ to Mn⁴⁺ upon adding Ag. This was confirmed by the peak-fitting analysis of the XPS spectra, which indicated that the films primarily exhibited an oxidation state of Mn⁴⁺, with a minor contribution from Mn³⁺ (Figure 4). The predominant phase of the thin film was amorphous MnO₂ (denoted as a-MnO₂). Therefore, the thin films were composed of Ag₂O nanoparticles of several nanoparticles dispersed homogeneously in the a-MnO₂ thin films.

The XPS spectra of Ag 3d_5/2 for the thin film with silver are shown in Figure 5. The peaks of the spectra were analyzed by peak-fitting, and the results suggested that only a single ionic state of Ag existed. The binding energy of Ag 3d_5/2 in the matrix at 367.37 eV is shown in Figure 5a. This is attributed to the bonding between Ag and oxygen in Ag₂O [19]. In addition, a binding energy of 368.18 eV was shown for Ag 3d_5/2 as the Ag nanoparticles precipitated on the surface of the thin film (Figure 4b), which was consistent with the value of pure Ag [20].

In this study, the predominant phase of the prepared thin film was MnO_x with a small amount of MnO₂. The valence state of the ions in the film was 93.8% for Mn³⁺, 6.2% for Mn⁴⁺, and 97.4% for Ag⁺. The electrochromic process changed the valence state of Mn and Ag during cyclic voltammetry.

At peak I, the oxidation reaction at 0.1 V is as follows.

Mn³⁺→ ¼Mn³⁺ + ¾Mn⁴⁺, and ½Ag⁺→ ½Ag⁰

(1)

At this time, Mn ions are oxidized at a lower electric potential. At the same time, half of the silver ions are reduced to silver.

At peak II, the redox reaction at 0.5 V is as follows.

¼Mn³⁺ → ¼Mn⁴⁺, and ½Ag⁺→+ ½Ag⁰

(2)

Mn ions are completely oxidized at the original oxidation potential. At the same time, silver ions are completely reduced to silver.

At peak III, the redox reaction at −0.25 V is as follows. When the potential reached −0.25 V, a reduction peak appeared.

Mn⁴⁺ → Mn³⁺, and Ag⁰ → Ag⁺

(3)

The above reactions were repeated 100 times in the CV process. The oxidation of silver ions accelerated the oxidation of manganese ions. Figure 6 shows SEM images of the thin films at each step of the redox reactions. The surface of the prepared thin film is smooth, as shown in Figure 6a. A lot of nanoparticles precipitated at the surface of the thin film after the oxidation reaction (peaks I and II in the redox loop in Figure 2) (Figure 6b,c). These nanoparticles are Ag with a size of about 20 nm. These nanoparticles disappeared after the oxidation reaction (peak III in the redox loop in Figure 2) (Figure 6d).

Figure 7 shows the TEM images and the electron diffraction patterns of the thin films and the dark particles before and after the redox reactions. The film is amorphous with the features in the diffraction pattern before oxidation, as shown in Figure 7a,b. Spherical nanoparticles are dispersed in the matrix of the thin film after the oxidation reaction (Figure 7c,d). The size of the nanoparticles (darker particles) in the thin film ranges from a few nanometers to 30 nanometers. The typical selected-area diffraction pattern in Figure 7d exhibits clear diffraction rings with interplanar spacing, which fits with the FCC structure of metallic silver.

4. Conclusions

MnO_x/Ag nanocomposite films were obtained using silver acetate and potassium permanganate aqueous solution and electrodeposition methods. The electrochromic property of the thin films was observed in CV from 0 to 1.0 V with a scan rate of 20 mVs⁻¹ in 0.1 M KNO₃ electrolyte. The electrochromic process was deduced from the changes in the ion valence of Mn and Ag and examined through XPS analysis using CV. Silver ions were reduced to silver metal nanoparticles when Mn was oxidized from +3 to +4. The size of the Ag nanoparticles was about 20 nm, and they disappeared after the oxidation reaction. The oxidation of silver ions accelerated the oxidation of manganese ions.