The experimental procedure is to add the carbon nanotubes into the deposition solution which was in order to explore whether its compound electroplating behavior is feasible or not. In addition, it was expected to seek for the correct concentration of carbon nanotubes that required to be added. Furthermore, uniform carbon nanotubes which can not only reduced the quantity of using carbon nanotubes, but also can make it to be evenly spread on the deposition solution; as a result, it can be equally deposited on the surface of Ti Substrate then. The experimental procedure is shown in Fig 3-1, which will be explained in the following sections.
3-1 Pre-treatment of Substrate
The substrate used in this study is the commercial 99.99% titanium (Ti) with size of 1×1 cm2. The reason we choose Ti as the substrate is that, both Ti and ruthenium oxide have the same crystal structure. Before deposition, Ti substrate was first cleaned thoroughly by acetone and followed by coarse chemical etching of 5 wt%
hydrofluoric acid (HF) for 5 minutes to make the larger intensive voids. And the substrate was then etched by 50wt% HCl for 15 minutes at 90℃ to produce smaller voids on the surface of surface.
These processes will produce many intensive voids, which not only increase the mechanical locking between the coating layer and the substrate, but also deposits more coating material. Figure3-2 (a) illustrates the microstructure of Ti substrate after chemical etching by 5 wt% HF for 5 minutes. It shows many micro-voids were produced through this process. Figure3-2 (b) shows the etched surface after 50 wt%
HCl for 15 minutes followed the first 5% HF etching process.
After the etching process, the Ti substrate was placed in the distilled water-alcohol (99.9%, Merck)-acetone mixture which can avoid Ti substrate been oxidized.
3-2 Deposition Solution
To make the deposition solution, distilled water was first mixed with alcohol, and then added the surface-active agent (Triton-X100) into the distilled water-alcohol mixture. The solution was evenly dissolved by ultrasonic vibration and the purpose of adding surface-active agent was that it will make the surface of substrate to be moistened which allows the coating process to be easier [103, 104]. The advantage of adding surface-active agent was not only getting surface moister, but also had the large effect of added carbon nanotubes.
Carbon nanotube not only has the characteristics of high surface area but also is a good conductor. It can be expected that the conductivity of solution should be increased when carbon nanotube was added. Therefore, the deposition condition should be adjusted accordingly comparing that without adding carbon nanotube.
Deposited the carbon nanotubes on the titanium substrate successfully, and the current density should been adjusted necessarily.
It was found that carbon nanotube will agglomerate in the solution. To avoid agglomeration of carbon nanotube, carbon nanotube has been dispersed by high-power ultrasonic vibration. Adding surfactants into the deposition solution is also efficient to disperse the carbon nanotube.
In the process of dispersing carbon nanotubes, the cationic surface-active agent and the non-ionic surface-active agent could be chosen. Because of few OH- ions exist on the carbon nanotubes surface, the carbon nanotubes surface will be attached a surface-active agent layer with positively charge by the cationic surface-active agent
working effect. There will produce a Van Der Waals force between each carbon nanotube that disperses the carbon nanotube evenly in the solution.
The non-ionic surface-active agent, as the cationic surface-active agent that has the effect of dispersed carbon nanotubes evenly. However, the non-ionic surface-active agent can not only supply the cationic to combine with the OH- from the carbon nanotube surface closely, but also make the carbon nanotubes to be dispersed without agglomeration. The non-ionic surface-active agent can form a moist film layer on the carbon nanotubes surface, those had the positive effect of hydrous ruthenium complex deposited [5, 67, 78, 80]. Triton-X100 used in this study is a kind of non-ionic surface-active agent that served a function of carbon nanotubes dispersion.
Hydrous ruthenium chloride (RuCl3‧xH2O, Alfa, 99.9%) 0.02M was added into the above solution which the required amount of carbon nanotubes (NTP, Multi wall Carbon nanotubes-1020) was also added. And then add potassium chloride (KCl) 0.3M into the deposition solution [152]. Table 3-1 shows the specification of carbon nanotube used in this study. There are two kinds of sample prepared in this study; one is the sample which carbon nanotube was adding without ultrasonic dispersion, another was the sample which carbon nanotube after ultrasonic dispersion was added.
That high-power ultrasonic machine is called “the cell disintegrator”, which is Autotune Series High Intensity Ultrasonic Processor, 750 Watt Model. In pH adjustment, HCl and KOH aqueous solutions had been used in this study. And then, KCl was used into the solution which can not only increase the conductivity of the solution, but also can reduce the resistance of solution. It is important to mention that the deposition solution should be kept away of light to prevent ruthenium ions from occurring the self-reduction action, and stirred the solution evenly to be prepared.
3-3 Cathodic Deposition
Before deposition, the substrate should be dried. And the substrate was weighted by the precision electronic microbalance (Sartorios, MC-5). The configuration of deposition for the solution is shown in Fig 3-4 which platinum plate was used as the counter electrode. The specimens were prepared using a fixed current at various deposition periods.
During deposition process, magnet stirring should be used. It was found that the stirring speed is important to control the quality of coating [56-64, 153, 156, and 216].
3-4 Analytical Method
The analysis of coating can be divided into two categories. One is the coating characteristic and structure investigation [154]; and the other is the electrochemical characteristic [158 and 217]. In coating characteristic and structure investigation, SEM, TEM, XPS, XRD, ESCA, and TGA were utilized to define the characteristics of the micro-coating.
For the analysis of electrochemical characteristics, it includes the measurements of Capacity, Cycle Life, Polarization Curves, Bode Curves, Capacitor Defined, and Electrochemical Impedance.
3-4-1 The Characteristic of Coating by Various Instruments A. SEM
Scanning electron microscope is usually used to observe the surface structure for objects. This instrument can be used as the preliminary observations of the coating. In
addition to the surface morphology investigation, analyze the chemical composition of micro-area can be analyzed by using EDAX which is attached on SEM.
B. TEM
Transmission Electron Microscope has very high resolution, which is one of the very effective tools for the research of materials. According to the electronically and physically generated signal, the major analyses of transmission electron microscopy information detection were divided into three types: (1) imaging of catch Transmitted Electron or Elastic Scattering Electron; (2) make the Diffraction Pattern (DP), to micro-organizations and the study of crystal structure; (3) with X-ray spectroscopy analyzer (EDAX) or Electron Energy Loss Spectroscope (EELS) for chemical composition analysis. Due to its super resolution, micro/nano structure of coating can be studied.
C. XRD
XRD is utilized the of X-ray diffraction phenomenon to analyze the material’s crystal, crystalline structure, lattice parameters, crystalline defects, different content structure, and internal stress.
XRD can analyze various materials, which included metals, ceramics, semiconductors, various thin-film materials, single crystalline materials, and epitaxial wafers.
D. XPS、ESCA
XPS is generally utilized to detect the electronic binding energy of elements.
Therefore it can be used to determine the elements in unknown materials according to the electronic binding energy measured [72, 112, 212, and 213].
E. TGA
Thermo-gravimetric Analyzer (TGA) is the equipment in used of measuring the weight of material changes under particular temperature. The process is to place the sample in a programmable furnace which the temperature and the environment can be controlled. When the temperature reached to the evaporation temperature, pyrolysis temperature, oxidation temperature of material, the sample will be evaporation, pyrolysis, and oxidation which will cause the material to lose its weight. Recorded the weight change of samples with the temperature or time which can be determined the properties of decomposition temperature, thermal stability, composition ratio, purity, moisture content, inoxidizability of material, and etc. This method is mainly to measure the thermal stability and components of material [108].
In this study, we had used the characteristics of TGA to determine the crystal water percentage of coatings [109].
3-4-2 Analysis Electrochemical Characteristic
Potentiostat/Galvanostat (AUTOLAB, PGSAT12) is the important apparatus for electrochemical experiments and measurement. The metals corrosion potential, corrosion current, polarization potential, curve of electrochemical reaction and electrochemical impedance testing can be measured by using Potentiostat/Galvanostat.
voltage. The result will be recorded by computer and energy can be obtained by integrating the curve area which was done during the experiment. To integrate with the curve areas that can obtain the amount of energy. Testing device, as shown in Figure 3-5, is a typical three-electrode electrochemical analysis. Reference Electrode (RE) is the calomel electrode Hg/Hg2Cl2 with saturated KCl (Saturated Calomel Electrode, SCE). Counter Electrode (CE) is the platinum plate, and the Working Electrode (WE) is titanium with coating. 0.5M sulfuric acid solution was used as electrolyte, which the hydrogen ion concentration was 1M.
A. Calculating Capacitance
For capacitance, it was calculated by cyclic voltammetry with fixed current.
Theoretically, if we did a CV scan to an electrode with constant capacitance, the curve will be a closed rectangular curve (as shown in Figure 3-6). From the definition of theorize capacitance equation [1], the capacitor is the power which can stored in each proceeded voltage.
V
C= Q……….……… [1]
Where C is the electrical capacity, Q is the charges and V is the applied voltage.
However, continuous surface redox reaction had occurred in this study. Therefore, the ruthenium compound on the electrode surface was reacted during the scanning process, which an irregular closed inverse S-shaped curve was produced, as shown in Figure 3-7. Due to the oxidation-reduction reaction and other external factor, the characteristic of capacitor are unstable. To measure the capacitance in the real situation, Eq.1 is differentiated vs. time. Therefore Eq.1 can be rewritten as Eq.2.
υ i dVdtdt dQ
C = = …………..………[2]
Where i is the density of electric current and υ is the screen rate of electric current.
The potential scan range was 0V-1V, and scan rate was 25 mV/sec. The steps were as follows:
(A) Samples were dried at low-temperature to avoid the microstructure change of coating. 60℃ was the temperature that we used to dry the sample. After deposition process, the samples were weighted by electronic microbalance.
(B) Calculating the capacitance (F) by CV scanning. Which capacitance [C] is defined as units of voltage [V] that contained in the charge, which is the capacitor for farad, and charge [Q] is Coulomb, voltage for [V]
∵
VC=Q and Q=I×Δt ⇒Q=Δt×
∫
Idi∴
VIdi C= Δt×
∫
B. Cycle Lifes
The cycle life is also important for supercapacitor. In order to test the cycle life, the electrode was going through charge/discharge cycles for 105 times.
C. Polarization Method
The potential produced from reaction that can be obtained by using the polarization curve; in addition, the polarization curve can be described by using Butler-Volmer
model equation [3].
I=ioA[e-αfη-e(1-α)fη]...[3]
Where, I is the current amount, io is the exchange current, A is the role of area, α is the charge transfer coefficient, f = F / (RT), F is Faraday constant, R is gas constant, T is absolute temperature, η for the reaction potential.
In the potential, the working area was important. In the experiments, the potential can be used to investigate the difference of sample with or without adding the carbon nanotube, as shown in the Figure 3-8. From the potential measurement, the role of carbon nanotube can be understood during the reaction process.
D. Distinguish Capacitance
As mentioned before, there are two categories of capacitor, one is double-layer capacitor, and the other is pseudo-capacitor. In order to distinguish these two capacitors in this study, the electronic conversion brought from the pseudo-capacitive effect that has adopted to use in this study. Although the effect of double-layer will also be produced, it can be distinguished from the calculation.
There were many layers that near the electrode by electronic distribution. If the closest layer to electrode as the based layer, the boundary separated other layers were named the Helmholtz or Stern layer. The Helmholtz layer which is closest to the electrode surface was named the IHP (Inner Helmholtz Plane) that is the one of the main part of the double-layer structure. And the second closest layer from the IHP to the electrode is called OHP (Outer Helmholtz Plane). Other layers that are outside the OHP layer were all referred to name as the Diffuse Layer. If the current density between IHP and electrode is σi, the current density of IHP to the Diffuse Layer is σd,
and the total current density of solution is σS, the following equation can be obtained.
σS=σi+σd=-σM……….…………..[4]
Where σM is the current density of electrode surface.
From the literature [201], the electric double layer capacitance was determined by the slope of potential decline. Thus, when (dη/dt) t=0, Faraday current through the interface between electrode and electrolyte is:
J(η) =j0exp[(αηF)/(RT)]……………[5]
Where j0 is exchange current density, α is transmitted coefficient.
When the exchange current time is zero, the potential will be declined relatively, and equation will be:
-C(dη/dt)=j(η)………...………[6]
Combined with above function, to get the
-C(dη/dt)=j0exp[(αηF)/(RT)]………[7]
Where C is the capacitance of the electrolyte and the interface of electrode.
If we combine with the former equation letting C is the fixed function, C as the fixed function, the equation can be modified as:
Where τ is the time of released voltage, when t is very larger than τ.
) 0
(_
) 0 (
=
= −
dt t
d t
C jη ………..………[9]
In our experiment, it is easier to measure the producing capacitance from pseudo-capacitor. And it is difficult to distinguish that from double-layer. To measure the capacitance from the double-layer, the principle mentioned above was used. First, we charge the electrode for a period of time which is enough to fill up the electric. In the moment of zero-voltage, a small and residual instantly current can be detected, that was the current charged from double layer, as shown in Figure3-9.
From the experimental results, the total electrode current value is Itotal = Ifrard + Idouble-layer. If Ifrard is 0, then all current value Itotal will be Idouble-layer. Therefore it should be able to know the Cdouble-layer=Idouble-layer • △t/△V (voltage change). The capacitance that measured from pseudo-capacitor is equated to the capacitor which the double-layer capacitor was deducted from the total capacitance.
【Table 3-1】 The specification of carbon nanotube additive in this study
Purity Diameter Length Amorphous carbon
Ash (catalyst residue)
Special surface area
Thermal conductivity
>95% <10 nm 5-15or1-2μm < 3% < 0.2% 40-300 m2/ ~ 2000W/m·k Form MCL of ITRI
【Figure 3-1】 The experimental processes
【Figure 3-2】 The microstructures of Ti substrate after chemical etching. (a) the after 5 wt% HF, (b) the detailed etching after 50 wt% HCl at 90℃.
【Figure 3-3】 The nanostructure of carbon nanotube additive in this study.
(a) (b)
【Figure 3-4】 The schematic diagram of cathodic deposition.
【Figure 3-5】 The schematic diagram of CV scanning.
0 0.4 0.8 1.2 1.6 2
-0.8 -0.4 0 0.4 0.8
(E/V V.S SCE)
(i/A)
【Figure 3-6】 Theoretical CV curve
0 0.2 0.4 0.6 0.8 1
-0.01 -0.005 0 0.005 0.01 0.015
(E/V V.S SCE) (i/A)
【Figure 3-7】 Real CV curve
-0.8 -0.6 -0.4 -0.2 0 E (V)
-0.003 -0.0025 -0.002 -0.0015 -0.001 -0.0005 0
I (A)
Polarization Original Add 0.1% CNT
【Figure 3-8】 Polarization curve.
0 0.4 0.8 1.2 1.6 2 E (V)
0 .994 0 .996 0 .998 1
I (A)
【Figure 3-9】 The curve of distinction of double layer capacitor.