Mechanism of Optical Reflectance Enhancement on Ag Electrodeposition-based Electrochromic Cells by Introduction of Citric Acid

Abstract

A citric acid was added to the electrolyte as an capping agent to effectively enhance the reflectance of the Ag deposition-based electrochromic (EC) cells. The electrochemical properties of EC electrolytes containing different concentrations of citric acid were compared and the surface morphology of Ag deposited in these electrolytes were also analyzed. The redox potential of Ag⁺/Ag was not changed in the presence of citric acid, but the rate of consumption of Ag⁺ in the deposition was significantly affected by the citric acid especially in case of high concentration. As adding citric acid, the increasing of reflectance of Ag deposits in total wavelength region is increased, typical at the [Silver nitrate] : [Citric acid] is 1 : 2, the reflectance in the wavelength range (400–580 nm) is increased more, it is leading to specular mirror appearance. The mechanism for enhancement of the reflectance was discussed in terms of surface morphology of the Ag deposits and deposition behavior of the Ag nanoparticle.

1. Introduction

The Ag deposition-based electrochromic (EC) cell are novel intelligent devices that operate using the principle of the electrochemical redox reaction of silver. These EC cell possess novel optical properties and advantages such as low energy consumption, which make them highly promising for applications in electronic paper, smart windows and automobiles. Compared to other electrochromic devices, the charm of Ag deposition-based EC cell lies in their unique optical reflection characteristics, allowing them to switch reversibly between mirror and transparent states.¹ Previously, the basic working principles of Ag deposition-based cell and the influence of the composition of electrolyte on the morphology and optical properties of the deposited silver are also researched.² In general, Ag deposition-based EC cells are divided into reflective and multicolor types according to different functions. The former has broad application prospects in automotive rear-view mirrors and reflective type heat insulation devices, while the latter is great significance for the development of multi-color smart windows and e-papers.^3,4 Functional integration is very important for the practical application of Ag deposition-based EC cells. For example, smart windows that seamlessly integrate heat insulation with multicolor function have great potential for future applications in the automotive industry and smart homes. However, achieving seamless integration of high reflectivity and multi coloration is currently challenging, mainly due to the low Ag⁺ concentration (10 mM) (M = mol L⁻¹) in EC electrolytes needed for achieving good multi-coloration by the Ag deposition-based EC cells. Therefore, it is important to explore the method of obtaining high reflectance Ag deposits in the EC electrolyte with low Ag⁺ concentration. It is now common practice to optimize the reflectance of Ag films in these EC cell by replacing the electrode material or by adding extra supporting salts to electrolyte. However, these methods have a fatal disadvantage, although the bright mirror in the electrolyte with low Ag⁺ concentration is formed, the silver deposition rate of the EC cell is significantly slower, and the required time for mirror state accomplished is up to 30 min, which considerably limits the practical application of silver deposition-based EC cell.⁵

The objective of this study is to directly modify the surface morphology of the deposited Ag by introducing environmentally friendly citric acid into the EC electrolyte as a capping agent. Capping agent is often used in the chemical synthesis of Ag nanoparticles.^6–8 In particular, citric acid is often used to regulate the morphology of Ag nanoparticles.⁹ However, there are few research on the electrochemical behavior of citric acid added EC electrolyte. The interface interactions of the citric acid studied with the Ag deposit and the influence on the kinetic characteristics of Ag deposition are very important in order to gain insight into the functions and mechanisms of the actions of citric acid in the electrodeposition.

In this study, the effects of citric acid concentration on the reduction and kinetic characteristics of Ag deposition were studied by cyclic voltammetry (CV). The relationship between the content of citric acid and the morphology of Ag deposits was also studied by chronoamperometric (CA) method. The results of further reflectance measurement were also consistent with the surface morphology analysis. The Scharifker-Hills (S-H) model was used to analyze the effect of the presence of citric acid on the Ag nucleation. The adsorption behavior of citric acid on specific crystal plane of Ag deposits was proved by XRD. Cottrell equation is introduced to analyze the dynamic change of concentration gradient in deposition and a hypothesis is proposed to explain the effect of concentration gradient on morphology of silver deposit. Combined with the above results, we get insightful views into the function mechanisms of citric acid on the electrochemical behaviors of Ag deposits.

2. Experimental

2.1 Materials

All chemicals were commercially available and were used as received. Silver nitrate (AgNO₃, Wako Pure Chemical Co., Inc., Japan) was purchased as the Ag⁺ source. The lithium bromide (LiBr, Kanto Chemical Co., Inc., Japan) was used as the supporting electrolytes, and anhydrous citric acid (Kanto Chemical Co., Inc., Japan) was used as the capping agent (The structure of citric acid is shown in Fig. 1a). The dimethyl sulfoxide (DMSO, Sigma Aldrich, USA) was received as the organic solvent. The indium tin oxide-coated electrode (ITO, <10 Ω/□, Cyoshin Electronics Co., Ltd., China) was used after being adequately washed.

Figure 1.

(a) The chemical structure of citric acid and (b) Schematic diagram of three-electrode type EC cell.

2.2 Preparation of the EC electrolyte

AgNO₃ is dissolved as an EC material in DMSO in concentration of 10 mM (mM (= m mol L⁻¹)). In addition, LiBr were dissolved in DMSO with 5 times the concentration of AgNO₃ (50 mM) in an argon atmosphere in a glove box. The concentration of citric acid is 0–2 times higher than that of AgNO₃ (0, 5, 10, 20 mM). All of the electrolytes are bubbled with nitrogen gas for 30 minutes before used.

2.3 Measurement of the electrochemical and optical properties of EC electrolytes

In this study, a standard three-electrode type electrochemical cell was used for electrochemical measurements (the schematic diagram is shown in Fig. 1b). The cell is consisted by transparent ITO-coated electrode was used as the working electrode, platinum wire (Pt wire) as the counter electrode, and an Ag/Ag⁺ reference electrode. To fix the distance between the working and counter electrodes, the electrode system was combined with a optical glass colorimetric cell (10 × 10 × 45 mm³), and the EC electrolytes were poured into EC cells. The distance between the working and the counter electrode were fixed at 1 cm, and the effective contact area of electrode between the ITO electrode and electrolyte was fixed at 10 × 40 mm². The reflectance spectra of Ag deposits were recorded in situ using an Ocean Optics USB 2000 detection system. A high reflectance specular standard (STAN-SSH Ocean Insight Company) was used to correct the standard values for maximum reflectance measurements and the standard for minimum reflectance measurement was calibrated using the low reflectivity standard (STAN-SSL). Cyclic voltammogram (CV) and Ag deposition were performed using an ALS model 660A potentiostat equipped with a computer controller and the scan rate was fixed at 50 mV/s. Reflectance measurements require a working electrode to be deposited on a silver deposits under different conditions. The obtained electrodes were rinsed with methanol and deionized water and dried with nitrogen gas flow. The Ag deposits were individually placed under a reflectance sensor for measurement. The surface morphologies of the Ag deposits were observed using field-emission scanning electron microscopy (FE-SEM, JSM-6700F, JEOL, Japan). The crystalline characteristics of the samples were further examined in detail by powder X-ray diffraction measurements (D8 Advance, Bruker, U.S.).

3. Results and Discussion

3.1 Electrochemical measurement

In order to investigate the effect of citric acid concentration on the electrochemical redox properties of Ag, we conducted a series of CV measurement experiments. Four sets of CV measurements were performed in three-electrode type EC cells with various concentrations of citric acid ([10 mM Silver nitrate] : [Citric acid] = 1 : 0; 1 : 0.5; 1 : 1; 1 : 2). The potential range for the CV experiments were −2.5 V to 0.5 V vs. Ag/Ag⁺. Figure 2 shows that reduction current of Ag⁺ is started to appear when the potential is scanned in the negative direction to −1.35 V vs. Ag/Ag⁺. Another obvious fact is the addition of citric acid has no obvious promoting effect on the redox reaction of Ag. In particular, the reduction potential of Ag⁺ was almost not affected by the presence of citric acid. On the contrary, the reduction current is attenuated when the [Silver nitrate] : [Citric acid] is 1 : 2, which mean that the reduction rate of Ag is affected by citric acid.

Figure 2.

Cyclic voltammograms of three-electrode type EC cell containing electrolyte [Silver nitrate] = 10 mM with various [citric acid] = 0–20 mM.

The silver deposits were prepared by electrodeposition with a constant potential of −2.5 V vs. Ag/Ag⁺ and charge amount of 150 mC for the above-mentioned electrolytes. The reflectance spectra of the silver deposits are shown in Fig. 3. It can be observed that after addition of citric acid, the increasing of reflectance of Ag deposits in total wavelength region increased. Especially at the [Silver nitrate] : [Citric acid] is 1 : 2, the reflectance of the corresponding silver deposits in the wavelength range (400–580 nm) is increased in certain level. In addition, the silver deposits prepared in the electrolyte without citric acid appeared darker to the naked eye, with porous and rough surface morphology, the average size of silver nanoparticles is also relatively large as shown in the FE-SEM images in Fig. 4a. Contrarily, the addition of citric acid makes the average size of the silver nanoparticles became smaller and more uniform, the dense cohesion between the particles also makes the resulting silver deposits with smoother surface morphology and the corresponding appearance is shinier (Figs. 4b–4d).

Figure 3.

Reflection spectra of Ag deposits in three-electrode type EC cell after an application of constant potential (−2.5 V, 150 mC) at various concentrations of citric acid; 0–20 mM.

Figure 4.

FE-SEM images of Ag deposits in three-electrode type EC cell after an application of constant potential (−2.5 V, 150 mC) at various concentrations of citric acid; 0–20 mM.

In order to explain the effect of citric acid on the morphology of silver deposits, we combined the deposition kinetics and concentration gradient theory for analysis. The formation of silver deposits is mainly divided into initial nucleation and subsequent growth stages. The nucleation as the first step in the formation of Ag deposits has a direct impact on the surface morphology of the subsequent growth of these Ag nucleus stage. A three-dimensional infinite the diffusion mass-transfer electrodeposition model that was established by Scharifker and Hills, called S-H model.^10,11 This model categorizes the growth behavior of metals deposits during electrochemical deposition into two main types; instantaneous and progressive nucleation. Progressive nucleation involves continuous and simultaneous nucleation and the subsequent growth of the metal nuclei during electrodeposition. And on the other hand, the instantaneous nucleation process refers to the formation of metal nucleus at only one instance and their eventual growth; The progressive and instantaneous nucleation process can be described as follows:^12–14

Progressive nucleation:

  

\begin{equation} (i/i_{\textit{max}})^{2} = 1.2254(t_{m}/t)\{1 - \exp[-2.3367(t/t_{m})^{2}]\}^{2} \end{equation} (1)

Instantaneous nucleation:

  

\begin{equation} (i/i_{\textit{max}})^{2} = 1.9542(t_{m}/t)\{1 - \exp[-1.2564(t/t_{m})]\}^{2} \end{equation} (2)

where i_max and t_m are the maximum currents in the electrodeposition and the corresponding time for the instantaneous and progressive nucleation, respectively.

Instantaneous nucleation occurs when all nuclei form simultaneously at the beginning of the deposition process, and new nuclei do not form during the growth phase. Because of this, all nuclei tend to be more uniform in size. On the other hand, the progressive nucleation occurs when new nuclei continue to form over time during the electrodeposition process. Instantaneous nucleation tends to produce more uniform and dense deposits, while progressive nucleation can lead to a wider distribution of particle sizes and potentially more porous structures.¹⁵

The Ag deposition process in case of the 1 : 0 and 1 : 2 of [Silver nitrate] : [Citric acid] were selected for fitting analysis by using the S-H model. The fitting results are shown in Fig. 5. The nucleation mode in electrolytes without citric acid closely followed a progressive nucleation. However, the Ag deposition in the electrolyte containing citric acid follows an instantaneous nucleation process. In terms of nucleation process, the addition of citric acid contributes to the formation of uniform and dense Ag deposits during subsequent nuclei growth.

Figure 5.

Scharifker-Hill’s (S-H) models correspond to the electrodeposition in EC electrolyte with and without citric acid.

Then we discuss Ag nanoparticle growth stage that is also affecting surface morphology of the Ag deposits. Some studies propose a hypothesis to elucidate the subsequent growth of the Ag nucleus involved several steps.¹² First, as the first step in the formation of Ag deposits, Ag nucleus are formed on the ITO electrode shown in Fig. 6a. As the Ag nucleus gradually grows into a Ag deposits during electrodeposition, a diffusion layer formed around it due to the consumption of Ag⁺. The initial diffusion layer was three-dimensional (3D hemispherical shape), and these diffusion layers could be viewed as separate from each other without contact, as shown in Fig. 6b. As the deposition is proceeded, the Ag deposits gradually grow larger, and the surrounding 3D diffusion layer expanded continuously as shown in Fig. 6c, finally merging to form an overlapping state of these diffusion region similar to a one-dimensional layer (1D diffusion layer), as shown in Fig. 6d. Our previous studies demonstrated the effect of the transition rate of diffusion process from 3D to 1D on the morphology of Ag deposits.¹⁶ Simultaneously, when the deposition in the 1D diffusion stage continued for a long time, the growth of Ag deposits became anisotropic structure with vertical orientation owing to the strong concentration gradient, which is unfavorable for the formation of a Ag film with a smooth surface morphology.¹⁷ In order to elucidate the effect of citric acid on the change of the diffusion layer during silver deposition, we tested various electrolytes containing different concentrations of citric acid. The resulting changes in the diffusion layer were analyzed to determine the influence of citric acid on the deposition process. The transformation of diffusion layer from 3D to 1D is analyzed by using Cottrell equation, the Cottrell equation is described as follows,

  

\begin{equation} I(t) = zFAcD^{1/2}\pi^{-1/2}t^{-1/2} \end{equation} (3)

where, z is number of electron transfer/Ag⁺, F is faraday constant [96,500 C mol⁻¹], A is electrode area [cm⁻²], c is concentration [mol cm⁻³], and the D is the diffusion coefficient [cm² s⁻¹]. Therefore, in the 1D diffusion regime, the current should be proportional to t^−1/2. The plot of i vs. t^−1/2 is referred to as the Cottrell plot shown in Fig. 7.

Figure 6.

Schematic illustrations during electrodeposition of Ag controlled by diffusion layer changed. (a) The formation of the Ag nucleus, (b) The formation of 3D diffusion layer (c) The transition of diffusion layer from 3D to 1D and (d) The formation of 1D diffusion layer.

Figure 7.

Curves of Cottrell equation of electrolyte containing 0–20 mM citric acid during an application of constant potential (−2.5 V).

The Cottrell plots were calculated from the chronoamperograms (CA) datas. Times (T_{3D to 1D}) corresponding to the transition to the 1D diffusion regime were determined from the each Cottrell plot and the amount of reaction charge (Q_{3D to 1D}) accumulated before the transition behavior from 3D type the 1D diffusion stage was calculated by integrating the current over reaction time. Both the time (T_{3D to 1D}) and the charge (Q_{3D to 1D}) from 3D to 1D diffusion stage in Ag deposition varied with the addition of citric acid, and gradually increased with the concentration of citric acid, as shown in Table 1. The amount of reaction charge accumulated before the 1D diffusion control stage increased with increasing of the citric acid concentration in the EC electrolyte, such that it was higher when using the citric acid (20 mM) (98.8 mC) than the case of citric acid-free electrolyte (4.64 mC). In the above result, the silver deposits prepared by applying −2.5 V vs. Ag/Ag⁺ using the electrolyte containing citric acid is easily formed a smoother surface morphology. Thus, in this case, growth in the direction parallel to the ITO electrode is also promoted, and amount of charge in Ag deposition reaching to the 1D diffusion stage should be larger. On the other hand, the silver deposits with porous and rough surface morphology are mostly formed in the electrolyte without citric acid; thus, in this case, the amount of charge for Ag deposition before reaching the 1D diffusion stage should be smaller since a 1D diffusion layer is formed quickly by coupling between 3D diffusion layers. The accumulated reaction amount of charge Q prior to reach to the 1D diffusion control stage (shown in Table 1) also support this hypothesis.

Table 1. Critical time T_{3D to 1D} (s) and critical charge Q_{3D to 1D} (mC) in EC electrolyte containing citric acid (5–20 mM) during transition from 3D to 1D diffusion stage.

[AgNO3] : [Citric acid]	T3D to 1D [s]	Q3D to 1D [mC]
1 : 0	0.78	4.64
1 : 0.5	1.87	19.5
1 : 1	9.64	67.3
1 : 2	12.31	98.8

Another interesting fact is that the presence of citric acid affects the crystal orientation of silver deposits. A literature shows that citric acid is preferentially adsorbed on the Ag ⟨111⟩ crystal plane to act as a surface capping agent, so as to precisely control the morphology and size of silver nanoparticles in the chemical synthesis process. The XRD results shows that in the presence of citric acid, the intensity corresponding to the ⟨111⟩ plane of Ag deposits is relatively higher than that without citric acid, because the growth on the ⟨111⟩ plane is decelerated by the protection of the citric acid on ⟨111⟩ plane (Fig. 8). This allows the ⟨111⟩ crystal orientation to be largely preserved during growth. Interestingly, the orientation of ⟨111⟩ crystal plane seems to have a curious relationship with the surface morphology of Ag deposits.¹⁸ In the supplementary experiment, the Ag deposition experiments by using electrolytes without citric acid but containing three different AgNO₃ concentrations are performed (10, 30, 50 mM) like our previous studies.¹⁶ (constant potential: −2.5 V vs. Ag/Ag⁺, 80 mC) and intermittent potential method (−2.5 V vs. Ag/Ag⁺, 80 mC is divided into three parts: 27 + 27 + 26 mC), XRD analysis of the obtained silver deposits shows that the intensity of ⟨111⟩ crystal orientation is positively correlated with the reflectance of the corresponding silver deposits, and the surface morphology of the deposits with high intensity ⟨111⟩ crystal orientation is relatively smoother. In addition, under the same experimental conditions, the signal intensity of ⟨111⟩ crystal plane of Ag deposits prepared by intermittent potential method is also higher than that of Ag films prepared by constant potential method. (Fig. S1.)

Figure 8.

XRD graph of deposited Ag by the electrolyte with and without citric acid.

4. Conclusion

This study focuses on the reflectance enhancement effect of citric acid on silver electrodeposition-based EC cells containing low concentration of Ag⁺ electrolyte. As a result, in the presence of citric acid, the reflectance of Ag electrodeposition-based electrochromic cell is significantly improved. We further investigated the effect of citric acid on this improvement by electrochemical analysis, FE-SEM and XRD techniques. The redox potential of Ag is not changed by the presence of citric acid, but the citric acid slows down the deposition rate of Ag through the capping effect on crystal planes ⟨111⟩. The transition from 3D to 1D diffusion was largely delayed (0.78 s to 12.3 s) with citric acid concentration is increased. The EC cell including citric acid greatly enhanced the formation of smooth film morphology due to horizontal merging during enlarged 3D diffusion stage. This study is great significance for the fabrication of high reflectivity EC devices and the electrochemical synthesis of metal nanoparticles.

Acknowledgments

This work was partially supported by JSPS KAKENHI grant No. 17H06377 and 22H02154, and JST, the establishment of university fellowships towards the creation of science technology innovation, grant Number JPMJFS2107.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.26352883.

• 1) A citric acid was added to the electrolyte as an capping agent to effectively enhance the reflectance of the Ag deposition-based electrochromic (EC) cells. The electrochemical properties of EC electrolytes containing different concentrations of citric acid were compared and the surface morphology of Ag deposited in these electrolytes were also analyzed. The redox potential of Ag⁺/Ag was not changed in the presence of citric acid, but the rate of consumption of Ag⁺ in the deposition was significantly affected by the citric acid especially in case of high concentration. As adding citric acid, the increasing of reflectance of Ag deposits in total wavelength region is increased, typical at the [Silver nitrate] : [Citric acid] is 1 : 2, the reflectance in the wavelength range (400–580 nm) is increased more, it is leading to specular mirror appearance. The mechanism for enhancement of the reflectance was discussed in terms of surface morphology of the Ag deposits and deposition behavior of the Ag nanoparticle.

CRediT Authorship Contribution Statement

Hao Wang: Conceptualization (Lead), Data curation (Lead), Investigation (Lead), Writing – original draft (Lead)

Takanori Sugita: Investigation (Equal), Resources (Supporting)

Kazuki Nakamura: Resources (Equal), Writing – review & editing (Supporting)

Norihisa Kobayashi: Funding acquisition (Lead), Project administration (Equal), Resources (Equal), Writing – review & editing (Equal)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Society for the Promotion of Science: No. 17H06377

Japan Society for the Promotion of Science: 22H02154

JST, the establishment of university fellowships towards the creation of science technology innovation: JPMJFS2107

Footnotes

K. Nakamura and N. Kobayashi: ECSJ Active Members

N. Kobayashi: ECSJ Fellow

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