Applications of Silver Nanowires on Transparent Conducting Film and Electrode of Electrochemical Capacitor

Silver nanowire has potential applications on transparent conducting film and electrode of electrochemical capacitor due to its excellent conductivity. Transparent conducting film (G-film) was prepared by coating silver nanowires on glass substrate using Meyer rod method, which exhibited better performance than carbon nanotube and graphene. The conductivity of G-film can be improved by increasing sintering temperature. Electrode of electrochemical capacitor (I-film) was fabricated through the same method with G-film on indium tin oxide (ITO). CV curves of I-film under different scanning rates had obvious redox peaks, which indicated that I-film exhibited excellent electrochemical pseudocapacitance performance and good reversibility during charge/discharge process. In addition, the specific capacitance of I-film was measured by galvanostatic charge/discharge experiments, indicating that I-film exhibits high special capacitance and excellent electrochemical stability.


Introduction
In recent years, noble metal nanomaterials, especially silver nanomaterial become the focus of research because of their unique physical and chemical properties, which has been widely used in catalysis [1], optical, electrical [2,3], and antibacterial [4] areas.Among these various silver nanostructures, nanowire has attracted intense forces due to its high dc conductivity and optical transmittance.As optoelectronic devices become smaller and lighter, there is an increasing need for efficient transparent electrodes.The most common material of transparent electrodes is indium tin oxide (ITO); however, ITO cannot keep pace with the development of optoelectronic devices because of its high cost, brittleness, and critical preparation process.Although people have tried to use other materials to fabricate transparent electrodes, such as carbon nanotubes (CNTs) [5][6][7][8], graphene [9][10][11], and conducting polymer [12][13][14], the problem that how to achieve ratio of transmittance to sheet resistance (Rs) as high as ITO still cannot be solved.Therefore, many groups put efforts on metallic nanowires, particularly silver nanowires.Leem et al. [15] have pioneered silver nanowires as the electrode in solar cells, and the transmittance of it was 89.3% with low Rs of 10 Ω/sq.Since then, silver nanowire films have been fabricated by rod-coating technique [16] and spay-coating method [17].Therefore, silver nanowire can be used as a replacement of ITO in the future.In order to further decrease the Rs of silver nanowire film, Bergin et al. [18] studied the effects of the length and diameter of silver nanowires on their properties.Longer nanowires can result in lower Rs due to fewer connections between nanowires.Therefore, the preparation of ultralong nanowires is an urgent issue.Apart from increasing the length of nanowire to improve its properties, Hu et al. applied mechanical pressing method to reduce the resistance of junctions, which can make connection of silver nanowires closer leading to the increase of the conductivity [19].They also found that coating gold on the film is an efficient way, which can make the surface of silver nanowire smooth leading to the decrease of the junction resistance.Zhu et al. [20] used plasma treatment to remove the polymer coated on the surface of silver nanowire and welded the junctions, improving the performance of silver nanowire film.However, the large contact resistance of internanowires is still a limitation of the development of silver nanowire films in optoelectronic and electronic devices.
In addition, silver nanowire can also be used as electrodes of electrochemical capacitor.Transparent capacitors have potential application on energy storage [21][22][23].Sorel et al. [24] prepared transparent capacitor by spray-coating silver nanowires on polymer films, which exhibited capacitor properties with 1.1 uF/cm 2 .However, compared with other electrodes of capacitor, the specific capacitance was much lower.Pan et al. [25] found that nanostructured AgO electrode showed excellent electrochemical properties, and silver nanowires can be oxidized to Ag 2 O forming Ag/Ag 2 O core-shell nanostructures during the electrochemical process [26]; therefore, silver nanowire is a promising candidate of electrochemical capacitor.
In this paper, we prepared long silver nanowires by a simple method reported in our previous work.Based on this, transparent conducting film (G-film) and electrode of electrochemical capacitor (I-film) were fabricated by coating silver nanowires on glass or ITO, respectively, and their characteristics were investigated.The relationship between transmittance and Rs of G-film was discussed.The conductivity of G-film was improved by increasing sintering temperature.By cyclic voltammetry and galvanostatic charge/discharge experiments, the capacitor properties of I-film were studied, indicating that silver nanowire has high and stable electrochemical capacitance which can be used as material of electrode of electrochemical pseudocapacitance.
The morphologies and Energy Dispersive Spectrometer (EDS) of silver nanowires were measured by scanning electron microscope (SEM) (SIRION, USA).The Rs of silver nanowire film was measured by four-probe technique with a Keithley 2701 source meter.UV-vis spectra were recorded by a fiber-optic spectrometer (PG2000, Ideaoptics Technology Ltd., Shanghai, China).Electrochemical capacitance property of silver nanowire electrode is investigated through cyclic voltammetry (CV) and galvanostatic charge/discharge measurements using an electrochemical workstation (CHI 760D, CH Instruments Co., Ltd.).

Preparation of Silver Nanowires.
Silver nanowire was prepared by the method reported in our previous work [27].In each synthesis, l mL EG solution of AgNO 3 (0.9 M) and 0.6 mL EG solution of NaCl (0.01 M) were added into 18.4 mL EG solution of PVP (0.286 M).Then the mixture was refluxed at 185 ∘ C for 20 min.After the above processes, the excess PVP and EG were removed by adding deionized water centrifuging at 14000 rpm for 10 min, 3 times.

Procedure of Silver Films on Glass and ITO.
The glass and ITO substrates were treated by the mixture solution of concentrated sulfuric acid and hydrogen peroxide under ultrasonication for 30 min, which can make them hydrophilic.In this case, uniform film can be obtained.Silver nanowires were coated on glass or ITO substrate with treatment, using Meyer rod, and then heated in 150 ∘ C for 20 min.The film obtained on glass substrate was named G-film.Samples 1 to 5 are G-films fabricated with 2 mM, 1.75 mM, 1.5 mM, 1 mM, and 0.5 mM silver nanowires solution, respectively.The film obtained on the ITO was named I-film.The two kinds of films have different properties because of different substrates.

Results and Discussions
3.1.Morphology of Silver Nanowire Film.As shown in Figure 1, uniform silver nanowire film was prepared using Meyer rod.The length of most silver nanowire exceeds 5 um, which is long enough to be connected into a network.The inset in Figure 1 is silver nanowire colloids.The color of silver colloids is yellowish white, similar to the highly purified silver nanowire colloids obtained after cross-flow filtration [28].Preparation of high yield and long silver nanowires has been studied by many groups; however, these reaction processes are usually complex or difficult to control [29,30].Without fine control of reactant concentrations and growth process, the obtained silver nanowires are always in low yield accompanied by large amounts of by-products such as nanocubes or nanospheres growing from isotropic seeds, which influences the properties of silver nanowire films.

Transparent Conducting Film.
Optical transmittance over a large wavelength range is an important property for transparent and conductive film. is very low.When the concentration decreased from 2 mM to 0.5 mM, the transmittance of samples showed an increasing tendency reaching 31%, 58%, 62%, and 65%, respectively.In addition, it can be seen in Figure 2 that the transmittances of G-films keep stable in the near-infrared regions, which is important for solar cells.However, the transmittance of ITO decreased from 1100 nm described to its plasmon resonance peak at 1300 nm [19].The conductivity of G-films is also affected by the thickness of film.As shown in Figure 2, with the increase of thickness, the Rs of G-film drops.
As mentioned in the introduction, it is a big problem to decrease the junction resistance of silver nanowire film.We found that increasing the sintering temperature is a facile and effective way to improve the conductivity of silver nanowire film.As shown in Table 1, when the sintering temperature was 150 ∘ C, the Rs of sample 4 was 80 Ω/sq.Increasing sintering temperature to 200 ∘ C, the Rs dropped to 45 Ω/sq.Because the PVP coated on surface of silver nanowires was decomposed partially at 200 ∘ C, the surfaces of silver nanowires can connect together leading to higher conductivity [31].In addition, at 200 ∘ C some silver nanowires can be welded together.When the sintering temperature was 250 ∘ C, PVP was almost removed and most of the junctions between silver nanowires were melted resulting in the lower Rs with 15 Ω/sq, which can be seen in Figure 3(a).When the sintering temperature was 300 ∘ C, although some of silver nanowires were broken, the film still was a conductive network with lower Rs (6 Ω/sq) shown in Figure 3(b).However, when thinner sample 5 was sintered at 300 ∘ C, many silver nanowires were broken leading to nonconductive film which can be seen in Figure 3(d).At 400 ∘ C, the silver nanowires of sample 4 were almost broken (in Figure 3(c)).According to (1) [20], we can calculate  dc / op which can evaluate the performance of transparent conducting film, the higher  dc / op means the higher ratio of transmittance to Rs.The  dc / op of sample 4 after treated at 300 ∘ C was 116.5 which is higher than that of carbon nanotube [32,33] and graphene [34].Therefore, G-films have potential application on optoelectronic devices: . (1)

Electrode of Electrochemical
Capacitor.The cyclic voltammetry is used to evaluate the electrochemical properties of I-film.All these electrochemical measurements are conducted in 1.0 M KOH using a three-electrode system.Figure 4 showed CV curves of I-film electrode at a scan rate from 10 to 100 mV s −1 .The CV curve of I-film exhibits definitely different capacitance properties from electric double layer capacitance which has rectangular CV curve.Distinct redox peak can be seen from Figure 4 in the applied potential from −0.5 to 0.5 V versus Hg/HgO resulting from the redox reaction between Ag and Ag 2 O [35] described as (2).The capacitance of I-film at different scan rates can be estimated by the area of the closed circle.Changes in capacitance at different scan rates result from that at low scan rates; the diffusion of ions throughout the reaction system is unlimited leading to full use of silver nanowire as electrode, while at high scan rates, the capacitance performs double-layer or non-Faradic behavior so that silver is not fully oxidized or reduced resulting in the decrease of the capacitance [36].
Results indicate that I-film shows excellent electrochemical pseudocapacitance performance and good reversibility during charge/discharge process: Usually, silver experiences a reversed redox in an alkaline condition.In the first step, Ag is electrochemically oxidized to Ag 2 O by OH − , leaving a water molecule and two electrons.In a converse direction, a water molecule was separated into H + and OH − , so that Ag 2 O can be reduced to Ag by H + leaving OH − .As a result, silver nanowires were transformed into Ag/Ag 2 O core-shell nanostructures as Figure 5(a) showed.To detect the production of Ag 2 O during the process, the EDS with a large spot size (approximately 5 m) was performed.In Figure 5(b), we can see the percentages of elements.EDS spectrum exhibited that the atom ratio between Ag and O is less than two.The reason is that sources of oxygen are from Ag 2 O and PVP which is covered on the surface of silver nanowires, and the core of silver nanowires is still Ag element.Thus, the experiment result is consistent with theory and demonstrates the form of Ag 2 O/Ag core-shell nanostructures during the charge/discharge process.
There is a linear relationship between the scan rate and the response current according to (3) [37], where  is the discharge current (mA);  is the capacitance; / is the scan rate of the cyclic voltammetry.The enclosed area of the cyclic voltammetry curve can be used to estimate the electrochemical capacitance.The specific capacitance   is calculated using (4), where  is the area of active material (cm 2 ): The galvanostatic charge/discharge experiments are conducted at a potential window from −0.5 to 0.5 V to study the specific capacitance of I-film.Figure 6 shows the galvanostatic charge/discharge curves of I-film at a current density from 0.5 to 6 mA cm −2 .As Table 2 showed, the specific capacitance of I-film increased from 42.2 to 41.76 mF/cm 2 when the current density increased from 0.5 to 3.0 mA/cm 2 , which is only 1% decay.However, the specific capacitance of I-film sharply declined to 27 mF/cm 2 under 6.0 mA/cm 2 .The reason is that larger current density results in shorter time of redox between Ag/Ag 2 O, so that ions have not enough time to diffuse from electrolyte and interphase [26].In addition, the surface of nanowires is covered by PVP, which also have effect on the charge/discharge rate [38].Figure 7 presented that the capacitance retention of I-film at a current density of 6 mA/cm 2 can achieve 94.2% of initial value after 100 cycles.As a result, the I-film electrode has a good stability during continuous cycles.

Conclusions
G-film and I-film have been fabricated by coating silver nanowires on glass and ITO, respectively.The transmittance of G-film increased with the decrease of the thickness of G-film, and the conductivity can be improved by increasing sintering temperature attributed to the remove of PVP and weld of junctions of silver nanowires.Results showed that G-film had higher ratio of transmittance to Rs than that of carbon nanotube and graphene, which is a promising replacement of ITO applied in optoelectronic areas.In addition, the CV curves of I-film under different scanning rates had obvious redox peaks indicating its good performance of electrochemical pseudocapacitance and good reversibility during charge/discharge process.Through galvanostatic charge/discharge experiments, it can be seen that  the specific capacitance of I-film depends on the current density, and I-film exhibits high electrochemical stability.At low current density, the decay of specific capacitance can be ignored while, at high current density, the specific capacitance decayed dramatically because of short time for the diffusion of ions.Therefore, silver nanowires have great potential applications in optoelectronic devices.

Figure 1 :
Figure 1: The SEM image of silver nanowires: the inset is the corresponding solution.

Figure 5 :
Figure 5: (a) SEM image for I-film; (b) EDS for I-film.

Table 1 :
Rs of samples 4 and 5 under different sintering temperatures.

Table 2 :
Specific capacitances of I-film at different current densities.