Chapter 5 Results and Discussion on NED and PS Removal Process
5.2 Fabrication of Cylindrical Nickel Inverse Opals
Electrodeposition was carried out on CCCs to form the cylindrical inverse opals. Figure 5.13 presents the plot of current vs. time. Similar to the planar case, the electroplating process can be divided into two parts, as presented in the inset. The primary regime left to the dash line included a peak in current, followed by the regime right to the dash line that revealed an increasing current density. Consistent with earlier planar case, the values for current density were first increased and ascended to the highest point within 20 sec. It suggested that a rapid reduction of nickel ions occurring at the CF-electrolyte interface. We observed a larger current jump for the CCC-660 over that of CCC-460 because its defective colloidal structure allowed better electrolyte percolation.
Figure 5.13 Time dependence of current density during the electrodeposition at pH of 5.2 and 1 V on CCC-460 (☆) and CCC-660 (■). The inset sketches illustrate the stages of deposits inside the PS template at each point indicated by arrows, and the plot shows the transition points that were determined by variation of current density at initial stage. Inset axes are identical with the principal plot.
After the current density reached the valley, the nickel deposits just filled the void around the semi-spheres, as shown in sketch (2). This suggested that a semi-egg-shell structure deposited on the CF can be achieved by controlling the deposition time. After that, the replenishment of Ni ions from the bulk electrolyte was funneled by the colloidal template, as shown in sketch (3). Therefore, the NED rate increased with various slopes, and the effective path for electroplating was extended with diameter of deposits, especially in CCC-660 case.
Figure 5.14 shows the CIO-660 with semi- and multi-layer deposits. The inset in Fig.
5.14(a) provides the semi-layer of CIO. The semi-layer was bent and its morphology was strongly affected by the CF. When the thickness of CIO was increased, its surface was becoming flatter and more uniform, as shown in Fig. 5.14(b). As shown in the inset, the CIO was in a disorder structure, but the inter-pores were still observed. This indicated that the PS microspheres were stacked together during the NED process. Figure 5.15 provides the EDX analysis of the as-prepared CIOs. The carbon and nickel peaks were from the substrate and coating, while the oxygen peak was from the oxide layer produced by heat treatment.
(a) (b)
Figure 5.15 EDX analysis of the as-prepared CIOs.
Figure 5.16 provides the SEM images in cross-sectional and side views for the CIOs. As shown, the cylindrical inverse structures replicated the arrangement of primary CCCs. As shown in Fig. 5.16(a), the arrangement of 660 nm microspheres near the substrate was in a close-packed structure with a slightly bent angle. In addition, the ordered structure could be found at about 12~15 layers from the substrate.
In principle, when the circumference of any layer is at integral times of microsphere diameter, the arrangement for those microspheres in that particular layer is expected to stay ordered close-packed. However, in practical case, the stacking in a specific layer is also affected by the layers above and below. As a result, the possibility of close-packing in any layer is greatly reduced. Hence, occurrence of close-packed structure can only be observed when the 2πR (R is the radius) is much larger than the r (radius of microspheres). Because of this, the pores of the CIO-660, shown in Fig. 5.16(b) were still in disordered state.
On the contrary, the PS microspheres of 460 nm were in good order and formed a close-packed array near the substrate, as shown in Fig. 5.16(c). The ordered structure could be obtained after only 3~4 layers. In addition, more hexagonal pores appeared as the thickness for the CIO-460 was increased. Ordered domains could also be obtained on the
Figure 5.16 SEM images in the (a) cross-sectional and (b) side views for CIO-660, as well as (c) cross-sectional and (d) side views for CIO-460. The fabrication time was fixed at 10 min.
The scale bars are 5 μm.
5.2.2 Deposition Rate of Cylindrical NED
Figure 5.17 demonstrates the OM images for the CIO-660 under different NED time. In Figure 5.17(a), the carbon fiber was covered with a semi-layer nickel inverse opal. Like the NIOs, some color segments were observed on the surface as a result of the concave morphology. The inset in Fig. 5.17(a) exhibits the fringes of a carbon fiber, and these fringes were also observed on the CIO surface shown in Fig. 5.17(a). When the fabrication
(a) (b)
(c) (d)
CIO was increased. In this way, the incident light was reflected easier than a thinner CIO.
Figure 5.17(d) shows the magnified picture of CIO-660 of 10 min, and the pattern of inverse opal was clearly observed. It was noted that the bright spots represented the position of holes, while the protrusion-like grain boundaries appeared as dark lines.
Figure 5.17 Optical microscopic images of the CIO-660s after NED process for (a) 30 sec, (b) 1 min, and (c) 10 min. Figure (d) is the magnified image of (b). The inset in (a) provides the image of a pure carbon fiber. The scale bars are 20 μm in (a) to (c), and 10 μm in (d).
(c) (d)
(a) (b)
Figure 5.18 demonstrates the time dependence of thickness for single CIO during the NED process. The deposition rate for the CIO-660 and CIO-460 were 0.4904 μm-min-1 and 0.2508 μm-min-1, respectively. Apparently, the NED occurred faster on the CIO-660 case, which was consistent with the current density versus time plot of NED. This also suggested that difference in the deposition rate became obvious as the thickness of EPD layer in NED process was increased.
Figure 5.18 Thickness versus NED time for the CIO-660 (■) and CIO- 460 (☆) prepared at an applied voltage of 1 V.
Figure 5.19 presents the SEM image of CIO for different NED time. The depositing layers were around the carbon fibers, and the tubular structures were filled with periodic holes. The carbon fiber in the middle could be removed by thermal oxidation, as shown in Fig. 5.19(b) and Fig. 5.19(c). Figure 5.20 exhibits the SEM images of many CIOs. These
Figure 5.19 SEM images for the CIOs made of NED for different deposition time; (a) CIO-660 for 10 min, (b) CIO-660 for 30 min, (c) CIO-460 for 10 min, and (d) CIO-460 for 30 min. All scale bars are 10 μm.
Figure 5.20 SEM images of a bundle (3000 CFs) in (a) CIO-660 and (b) CIO-460 made of NED for 20 min. Insets show the magnified view for the CIOs. The scale bars are 20 μm in the figures and 2 μm in the insets.
(c) (d)
(a) (b)
(a) (b)
5.3 PS Removal Process 5.3.1 Chemical Dissolution
Figure 5.21 exhibits the OM image for the as-prepared CIO. In appearance, the Ni coating was covered with a pale film, which was attributed to the non-filled CCCs. To remove the PS template, ethyl acetate was used to dissolve the polystyrene at room temperature and 50℃, respectively.
Figure 5.21 Optical image for the as-deposited CIO without removing the PS microspheres.
The scale bar is 100 μm.
Figure 5.22 presents the SEM images for NIO-460 after immersion in ethyl acetate for 2 days at room temperature. As clearly seen in the top-view, the PS microspheres were partially dissolved, becoming a gel state. In Fig. 5.22(b), only the layers near the surface were PS-free, while those near the bottom were still filled by PS. Because the dissolution rate of PS was relatively slow at room temperature, the viscosity of dissolved particles remained high. Therefore, the etching temperature was further raised to 50℃, as shown in
dissolved PS, the PS microspheres at the bottom part were also dissolved, as shown in Fig.
5.22(d).
Figure 5.22 SEM images in (a) top and (b) cross-sectional view for the NIO460 after PS removal at room temperature, as well as (c) top and (d) cross-sectional view at 50℃. The immersion time was 2 days. The scale bars are 2 μm.
5.3.2 Thermal Oxidation
Figure 5.23 demonstrates the TGA profiles for the PS, as-deposited nickel, and CFs. In the TGA curve for the PS, event of notable weight loss occurred at 250 ℃, indicating volatile COx was formed from the oxidation of PS. On the other hand, the TGA response for the as-deposited Ni displayed a negligible weight increase until 500 ℃, at which point a rapid weight gain was detected. The sharp rise in the sample weight inferred considerable nickel oxidation above 500 ℃. The oxidation for CFs took place as the temperature was above 400
(c) (d)
(a) (b)
℃, but obvious weight loss occurred at 520 ℃. Combining these three curves, the heating temperature was set above 250 ℃ to remove the PS. Hence, heat treatments in 250, 350, and 450 ℃ for 2 days were carried out on NIO-460 samples to remove the colloidal template.
Figure 5.23 TGA profiles for the CFs, PS microspheres, and Ni coating.
Figure 5.24 provides the SEM images for the NIO-460 after heat treatment at different temperatures. As shown in Fig. 5.25(a), a heat treatment at 250 ℃ for 2 days could effectively evaporate the PS template without any alteration on the structure. Some PS residues were still trapped at the bottom of NIOs. However, they could be removed as the heating time was prolonged. As the temperature was raised to 350 ℃, the structure of NIO maintained moderate disorder in pores, as shown in Fig. 5.24(b). Meanwhile, the surface roughness for the NIOs was increased by the oxidation, and particles of nickel oxide were formed on the surface, as shown in the inset. Also, lattice distortion was observed on the
Figure 5.24 SEM images for the NIO-460 after thermal oxidation at (a) 250, (b) 350, and (c) 450 ℃ for 2 days. Insets show the cross-sectional view for the samples. The scale bars are 1 μm in the figures and 500 nm in the insets.
Figure 5.25 provides the XRD pattern for the NIO-460 after heat treatments. As mentioned before, the as-deposited NIO-460 revealed only one peak in (111) plane. Its intensity was largely increased by the heat treatment at 250 ℃ for 2 days. It suggested that the crystallinity of NIO-460 was improved considerably without formation of other preferred orientations. Moreover, the presence of (200) plane in the NIO was observed by heat treatment at 350 ℃. These peaks were converted to nickel oxide peaks at 450 ℃. This suggested that the oxidation was rapidly taking place above that temperature. In sum, with a heat treatment at 250 ℃, the PS templates in the NIO could be removed entirely, and the crystallinity of the samples was improved simultaneously.
(a) (b)
(c)
Figure 5.25 X-ray diffraction patterns for (a) the NIO-460 after heat treatment at various temperatures, (b) Ni from JCPDS 04-0850, and (c) NiO from JCPDS 47-1049.
Chapter 6
Measurements and Analyses
6.1 Electrical Measurements
6.1.1 Sheet Resistance of Planar Inverse Opals
Before taking the measurement, the surface of NIOs was deposited with gold electrodes by e-beam evaporation, as shown in Fig. 6.1. These gold electrodes were 300 nm in thickness, and the contact area between gold and NIOs was 0.0314 mm2. The inset sketch provides a cross-sectional profile for the samples. The I-V curves were recorded with a 2-point arrangement, which was in the middle of the nearest two gold electrodes. The diameter for the measured electrodes was 200 μm, and the distance between them was 500 μm.
Figure 6.1 Optical microscopic image of gold electrodes on the NIO for electrical measurements. The sketch illustrates the setting in measurement.
To obtain the sheet resistance, the measured value was calculated with following equation;
𝝆𝒔 = 𝝆
measured electrodes, Rito is the resistance of ITO substrate, and R is the resistance derived from the linear I-V curve.
Figure 6.2 displays the sheet resistance of NIOs in different layers. Also provided are the sheet resistances for ITO and Ni film. As shown, the values for NIOs were much lower than that of pure ITO substrate. It is because the Ni was relatively conductive. The values for NIO-660 were close to that of Ni film. In contrast, the values for NIO-460 were considerably larger than both Ni film and NIO-660. We surmised that the smaller porous structure of NIO-460 provided a longer path not only on surface but in its structure for electron transport.
Figure 6.2 Plot of sheet resistance with respect to the layer number for NIO-660 (■) and NIO-460 (☆). The values for the as-deposited nickel film (○) and ITO substrate (◆) are shown on the left for comparison purpose.
curves were recorded with a two-probe measurement, and the distance between electrodes was controlledbetween 0.3 to 0.7 mm.
Figure 6.3 Optical image of silver electrodes on the CIO for electrical measurement. The sketch illustrates the setting in measurement.
To obtain the resistivity of CIOs, the measured value was calculated with following equation;
𝝆 = 𝑹 × 𝑳
𝑨 𝑨 = 𝝅 × 𝑫 𝟐
𝟐
[eq. 6.2]
where ρ is the resistivity of the composite, R is the resistance derived from the linear I-V curve, L is the distance between two measured electrodes, A is the cross-sectional area of the CIO, and D is the diameter of the CIO. As shown in Fig. 6.4, the resistivity of CIO was lower than that of pure CF. This indicated that the conductivity of the CF was improved by the Ni overcoats. Because the packing density of Ni for the CIO-460 and CIO-660 are almost equal, their resistivity both revealed at the same order as expected.
Figure 6.4 A plot of resistivity with respect to layer number for CIO-660s (■) and CIO- 460s (☆).
6.2 Optical Analyses
6.2.1 Reflection Spectra of Colloidal Crystals
Figure 6.5 presents the hybrid band structure of FCC performed by BandSOLVE Method with a commercial software (RSOFT). The index difference used in the simulation was 0.59 (PS:1.59; Air:1). In calculation, the forbidden band extended on Γ－L was at a region of 0.58~0.62. The corresponding photonic band gap (PBG) for the PCC-460 and PCC-660 were at the wavelength of 1049.26~1121.62 and 1505.45~1609.28 nm, respectively. To verify the calculation, near-IR reflectance spectra were obtained with an incidence light normal to the PCCs. Their spectra are provided in Fig. 6.6. The as-prepared PCC-460 and PCC-660 displayed peaks at 1088.87 and 1627.49 nm, which were close to the expected values from simulation. The slight discrepancy between the calculated values and the measured ones was resulted from the deviation in size of the PS microspheres, which was mentioned in chapter 4.
Figure 6.5 Simulated hybrid band structure of FCC with an index difference of 0.59.
Figure 6.6 Reflectance spectra recorded from (a) PCC-460 and (b) PCC-660.
In addition, the reflectance at the PBG revealed a relatively high intensity about 60%.
This level of reflectance justified the quality of PCCs made by the EPD process. The exact position of PBG can also be approached by the size of latex spheres via following equation [9]:
𝝀𝒄 = 𝟐𝒏𝒆𝒇𝒇𝒅 𝒏𝒆𝒇𝒇𝟐 = 𝒏𝒍𝒂𝒕𝒆𝒙𝟐𝒇 + 𝒏𝒂𝒊𝒓𝟐(𝟏 − 𝒇) [eq. 6.3]
where λc is the wavelength where the PBG located, neff is the effective refractive index composed of latex and air, and d is the lattice spacing of (111) plane. In the equation [6.3], f is the filling factor of 0.74 for a close packed structure, and nlatex/nair is the refractive index in each material. Following the equation, the positions of stop bands were expected to be 1104 and 1573 nm for PCC-460 and PCC-660, respectively. Notably, the measured values were also consistent with the ones from the equations above.
(a) (b)
Figure 6.7 presents the reflectance spectra of CCCs prepared in different EPD time.
Apparently, peak changes were observed in Fig. 6.7(a) of CCC-460 since the amount of effective (111) plane was increased with thickness. In contrast, a negligible difference was present for CCC-660, as provided in Fig. 6.7(b). It suggested that the structure of colloidal crystals was influenced by both the line defects and curved surface. Because the signal reflected from the CCCs was rather limited (in order to be collected by the detector), the peaks on these spectra only displayed a low reflectance of 5~20%.
Figure 6.7 Reflectance spectra recorded from (a) CCC-460 and (b) CCC-660 for different EPD time.
A slight red shift from 1113.66 to 1135.61 nm was observed among the peaks from CCC-460. Theoretically, the filling factor (f) of air could be increased in a bent structure, making the peak closer to shorter wavelengths. Therefore, the position of peaks would move to longer wavelengths as the diameter of CCC was increased, until the distortion was fully compensated. It suggested that the bent structure in layer by layer formation might provide a continuous PBG. On the contrary, the peaks for CCC-660 remained unchanged with the increase of EPD time. This indicated that the stacking of PS microspheres was not sufficiently thick to compensate the bending in the structure within 10 min. Compared with these two cases, we can derive that the layers near the surface offered a distributed PBG
(a) (b)
nature by its distortion, while layers far from the substrate contributed to the intensity of PBG just like a typical ordered structure.
Another contribution from the curved surface was that the half-height width was much broader than those recorded from the PCC. Because of the bending in the EPD layer, the lattice spacing of colloidal crystals on the (111) plane was slightly changed with a small angle.
Its equilibrium was to produce a distribution of lattice constant. It was notable that the self-assembling of PS microspheres on CF demonstrated a model of scales. Hence, as difference in the size ratio of substrate to microsphere was close to 10~15, the colloidal crystals were bent into a non-closed packed structure. This bending behavior would decrease the intensity of PBG peaks. Nevertheless, the same bending structure could render a broadened PBG peak with a half-height width of 300 nm.
6.2.2 Light-Diffracting Fringes of Cylindrical Colloidal Crystals
Figure 6.8 exhibit the OM images for CCC-460 and CCC-660. Their surface displayed sparkling appearance in various color strips, and the color strips were aligned to each other.
With a fixed light source, these color fringes were attributed to the light diffracting by the colloidal crystals from different angles.
(a) (b)
Previously, V. Rastogi et al. [52] provided a model to demonstrate the reflection of parallel light waves that underwent a constructive or destructive interference, as shown in Fig. 6.9.
The following equation derived from geometry in the figure illustrates the path difference of light waves;
𝒎𝝀 = 𝒅 × [𝒔𝒊𝒏(𝟗𝟎 − 𝜽) + 𝒔𝒊𝒏(𝝓 − 𝜽)] [eq. 6.4]
where m is the integrals for the number of sets of fringes, λ is the wavelength of individual color fringe, d is the lattice spacing of (111) plane, and θ and 𝝓 are two specific angles defined from horizontal plane to the position of incident beams.
Because of the small diameter of the as-prepared CCCs, only the fringes in first order (m = 1) were obtained on the surface. Hence, the wavelength for the diffracting lights could be calculated through the equation by measuring each set of angles. Meanwhile, these fringes were observed under optical microscope, which inferred the incident light was normal to the surface (φ ~ 90°). Therefore, the equation could be further simplified to:
𝝀 = 𝒅 × 𝟐𝒔𝒊𝒏 𝟗𝟎 − 𝜽 = 𝟐𝒅 𝐜𝐨𝐬 𝜽 = 𝟒𝒅𝒔/𝑫 [eq. 6.7]
where s is the distance from the middle line to the fringe, and D is the diameter of the CCCs.
From the modified equation, the wavelength of reflection can be easily calculated by the position of their fringes from OM images, as shown in Fig. 6.10. The inset bars exhibits the corresponding color of each wavelength via the RGB value translated from standard CIE 1931 chromaticity diagram [53]. The colors shown in Fig. 6.10 were partially matched with that obtained in OM observation. The deviation in color matching might be induced by a small tilt of the incident light.
Figure 6.9 A schematic for theoretical calculation of fringe formation.
Figure 6.10 Wavelength of fringes versus the s/D plot of the CCCs for m = 1. The inset bars show the corresponded color created with RGB values on CIE 1931 chromaticity diagram.
NIO-660 with different number of layers. The Ni film provided a large reflectance in Mid-IR region, and maintained its value at 80% before entering the near-IR range. The noises detected at 2.62 and 4.25 μm were generated by the H2O and CO2, respectively. With a semi-layer NIO, the sample displayed a curve similar to that of the Ni film but at a slightly reduced reflectance. It suggested that the concave semi-layer structure engendered additional light scattering. Moreover, the PBG behavior was recorded as the layer number
NIO-660 with different number of layers. The Ni film provided a large reflectance in Mid-IR region, and maintained its value at 80% before entering the near-IR range. The noises detected at 2.62 and 4.25 μm were generated by the H2O and CO2, respectively. With a semi-layer NIO, the sample displayed a curve similar to that of the Ni film but at a slightly reduced reflectance. It suggested that the concave semi-layer structure engendered additional light scattering. Moreover, the PBG behavior was recorded as the layer number