Chapter 2 Literature Review
2.3 Electrodeposition
Electrodeposition is the most common method for surface modification to alter the material property. The process is carried out with two- or three-electrodes arrangement under an applied voltage. Moreover, the overpotential to drive the electroplating can be estimated via the Nearnst equation, and the amount of deposits can be calculated by Faraday’s law [42]. In electrodeposition, variables like pH value, ion concentration, temperature, and additives are usually adjusted and studied to achieve bottom-up fabrications. Likewise, to fill the void in colloidal crystals, the electrodeposition has been carried out in potentiostatic [23] or galvanostatic [43] modes. Au [44], Pt [45], Ag [44], Pd [45], Co [45], and Ni [17, 23, 46-50]
inverse opals were recently fabricated through a potentiostatic electrodeposition. Because of the layer structure of colloidal crystals, the current response revealed an oscillation with deposition time, as shown in Fig. 2.10.
Figure 2.10 Time dependence of current in fabrication of inverse opals by electrodeposition.
The numbered arrows indicate the valleys of current. Inset shows the times when the valleys appeared (●) and the intervals between each valley (△) [50].
Chapter 3
Experimental
This chapter introduces a facile process to prepare high quality colloidal crystals on planar and rod-like substrates through electrophoretic depositons (EPD). In addition, nickel inverse-opals were fabricated via nickel electrodeposition (NED) in the colloidal crystals.
As shown in Figure 3.1, the EPD process can fabricate ordered arrays of PS microspheres in a desirable three-dimensional pattern, while the NED can successfully convert the periodic structure into an inverse one after dissolving the PS templates. Lastly, the as-prepared samples were characterized by various instrument to determine their electrical and optical properties. Figure 3.2 provides the flow chart for the sample preparation steps involved.
Figure 3.1 Schematic diagrams for fabrication of (a) planar and (b) cylindrical inverse opals.
Figure 3.2 Flow chart for the experimental steps involved in this research work.
Cleaning of Substrates Synthesis of Polystyrene
EPD Process for Colloidal Crystals
PS Template Removal
Preparation of Electrolyte
NED Fabrication for Inverse Opals
Chemical Dissolution Thermal Oxidation
Measurement & Analysis Preparation of Suspension
3.1 Materials
3.1.1 Synthesis of Polystyrene Microspheres
PS microspheres with diameter of 460 and 660 nm were synthesized via an emulsifier-free emulsion polymerization process. Styrene was served as the monomer after removing inhibitors and K2S2O8 was used as the initiator. A minute amount (0.5 g/L) of sodium bicarbonate was added in preparation of 460 nm PS. The polymerizations took place at 75℃
under a nitrogen atmosphere for 18 hrs. Afterward, the colloidal suspension was diluted with deionized (DI) water and filtrated through a filter paper (ADVANTEC 5C 110 mm) with a mechanical pump in order to remove remaining monomers. Mono-disperse PS microspheres were later collected from the filter and suspended in DI water. After evaporation under 60 ℃ for 3 to 4 days, we were able to obtained dry PS powders.
3.1.2 Preparation of Suspensions
1g of 660 nm PS microspheres and 0.25 g of 460 nm ones were mixed respectively with 100 mL of 99.5 wt% ethanol to form stable suspensions, followed by ultrasonication and vigorous stirring for 5 hrs to reach a stable state. To adjust the properties of suspensions, PS microsphere concentration, pH value, and microsphere size were systematically controlled to stabilize those microspheres. In these suspensions, values for zeta-potential and electrophoretic mobility were -65.19 mV and -1.31×10-4 cm2V-1s-1 for the PS 460 suspension, and -61.17 mV and -1.23×10-4 cm2V-1s-1 for the 660 nm one. These values were recorded by a zeta-potential instrument (Malvern Zetasizer Nano Zs).
3.2 EPD for Colloidal Crystals 3.2.1 Pretreatment on Planar Substrates
ITO on glass (30×35 cm2) purchased from Uni-Onward company with a sheet resistance of 7 ohm and 0.7 mm in thickness was used as a planar substrate in EPD process. Pieces of the ITO-glass (15×15 and 10×5 mm2) were pre-cleaned with DI water and degreased in acetone under ultrasonication for 1 hr, followed by oven drying at 50 ℃. Then, they were treated with ethanol for 10 min at room temperature prior to the EPD process.
3.2.2 Fabrication of Planar Colloidal Crystals
Two electrodes were arranged vertically at a distance of 10 mm for the EPD process, as shown in Fig. 3.3. They were stainless steel plate (counter electrode) and ITO-glass piece (working electrode). A voltage of 10 V was applied for 10~60 min to reach desirable colloid thickness. Upon finished, the samples were carefully removed from the suspension, followed by a controlled drying at 20 ℃ for 30 min. This is to avoid unnecessary evaporation that might alter the colloidal crystal structure.
Figure 3.3 A schematic diagram of experimental setting for the planar EPD process.
3.2.3 Preparation of Circular Electrodes
bunch of CFs (TORAYCA-T300). Then, they were pre-cleaned with DI water and degreased in acetone under ultrasonication. Subsequently, a segment of copper tape with silver colloidal gel was employed as a conductive cladding to fix the CF. Afterward, the as-prepared sample was further heated in oven at 50 ℃ to achieve complete dehydration.
Next, the sample was cut into length of 1.5 cm to proceed EPD.
3.2.4 Fabrication of Cylindrical Colloidal Crystals
The EPD was carried out with two electrodes arranged vertically in a co-axial configuration.
An as-prepared CF sample was immersed into the suspension at 1 cm depth. A stainless steel (A304) in circular shape was used as the counter electrode, as presented in Fig. 3.4.
Figure 3.4 A schematic diagram of experimental setup for the cylindrical EPD process.
A voltage of 10 V was applied to drive the EPD, and the distance between the counter and working electrodes was 2.3 cm. The assembly process lasted 0-60 min. When finished, samples of CCC were removed and kept at 20 ℃ for 10~20 min to evaporate ethanol slowly.
3.3 NED for Inverse Opals 3.3.1 Preparation of Electrolyte
The electrolyte for the nickel electrodeposition included NiSO4•6H2O (130 gL-1), NiCl2•6H2O (30 gL-1), H3BO3 (18 gL-1), and H2O2 (3 mLL-1). A minute amount of H2SO4 and NaOH were added to adjust the pH value of the electrolyte. In addition, the solution was stirred for 1 day before electroplating, and raised to 45℃ prior to the NED process.
3.3.2 Fabrication of Inverse Opals
To fabricate Ni inverse opals, planar and CF substrates underwent similar EPD process in different PS suspensions with various processing variables. Before the electrodeposition, samples were immersed in the plating electrolyte with moderate stirring for 1 hr, and a hotplate coupled with a thermal couple was employed to control the temperature of the system.
Next, the electrodeposition was carried out on EPD-derived colloidal crystals under an applied voltage of 1 V at 45 ℃ in electrolyte of pH 5.2. Upon finished, the samples were cleaned with DI water and dehydrated in oven at 50 ℃ for 1 hr. Table 3.1 provides relevant processing variables in the fabrication process. Current output was recorded with a Keithley 2400 during the NED process.
Table 3.1 EPD parameters of samples for sequent NED process.
Particle Size PS-460 PS-660
Concentration 2.5 gL-1 10 gL-1
Shape Planar Cylindrical Planar Cylindrical
Substrate Size 5×5 mm2 7×10-2 π-mm2 15×15 mm2 7×10-2 π-mm2 Electric Field/
Applied Voltage 15 V/cm 10 V 10 V/cm 10 V
EPD Time 7.5 min 30 sec 30 min 45 sec
3.4 Removal of Polystyrene Template 3.4.1 Chemical Dissolution
PS microspheres after the NED process were carefully etched away by immersing the samples in an ethyl acetate (95 wt%) solution. The color for the sample would immediately change from white to purple/red as the solution percolated throughout the structure, making a larger contrast in the refractive index. After 2 days, the remaining Ni skeleton was cleaned with ethanol several times until the odor of ethyl acetate was removed. At this stage, we successfully made the Ni inverse opals.
3.4.2 Thermal Oxidation
PS microspheres were directly removed by a thermal oxidation treatment at temperatures of 250 ℃, 350 ℃, and 450 ℃ for 2 days. It is noted that the Tg (glass transition temperature) and Tm (melting point) of PS are 95 ℃ and 240 ℃, respectively. Upon finished, the samples were kept in oven for furnace cooling in order to avoid undesirable thermal distortion of the inverse structure.
3.5 Instrument
3.5.1 Morphological Observation Scanning Electron Microscope (SEM)
A SEM (JEOL-LSM-6700) was employed to observe relevant morphologies for colloidal crystals and inverse opals. In SEM sample preparation, the specimens were mounted on a copper holder, while sputtering of Pt was carried out at 20 mA for 90 sec (for insulating samples) or 40 sec (for conductive samples). Afterward, the SEM specimens were maintained under a pressure of 9.63×10-5 Pa, and observed with an acceleration voltage of 15 kV and an emission current of 10 μA.
Optical Microscope (OM)
With light incidence directly normal to the sample surface, the OM (Olympus CX41) was used to observe the light diffracting from cylindrical colloidal crystals. The images were captured by a coupled digital CCD camera (CY-100A).
3.5.2 Characterizations
Energy Dispersive X-ray (EDX)
EDX attached on a SEM was used to perform the semi-quantatative composition report and elemental mapping for the inverse opals.
High Resolution X-ray Diffractometer (HRXRD)
A HRXRD (Bede D1) was used to determine the crystallinity and relevant composition for the nickel inverse opals before and after thermal treatments.
Four-Point Measurement (4pp)
Keithaley 4200 to obtain the conductivity for the planar nickel inverse opals. For cylindrical nickel inverse opals, samples were fixed on a non-conductive glass with two silver bumps serving as the probe contact points, and a forward bias of 0~400 mV was applied during the measurement.
UV-Vis-IR Spectrum
Optical response for the colloidal crystals and their inverse opals were acquired through a Fourier Transform Infrared (FTIR) microscope (Hyperion 2000, Bruker) in a nitrogen atmosphere. During the measurement, the samples were illuminated with a selective light source (CaF2), and the reflectance was obtained by normalizing the measured signals detected from the samples to that from a flat aluminum mirror.
Chapter 4
Results and Discussion on EPD Process
This chapter provides the results and discussion on fabrications of colloidal crystals by the EPD in various processing parameters. Variables under studies were suspension concentration, applied electric field, and deposition time. In general, the deposit weight PS microspheres in suspension, S is the area of substrate, ε and ξ are the dielectric constant and zeta potential of PS microspheres, η is the viscosity of solvent in suspension, V is the applied voltage, L is the distance between working and counter electrodes, and t is the deposition time. In equation [4.2], there are three more variables in cylindrical EPD system;
l and a are the length and radius of the working electrode, and b is the radius for the coaxial
counter electrode (b > a). In this work, suspensions made of 460 and 660 nm PS microspheres (PS-460 and PS-660) were prepared in different Cs and examined to achieve a smooth EPD coating. By driving the EPD process with suitable applied voltage (electric field), the planar and cylindrical colloidal crystals (PCC-460/660 and CCC-460/660) were successfully fabricated. The following sections provide detailed discussion on individual variables and their effects on the quality of colloidal crystals.
4.1 Suspensions and Substrates
The as-synthesized PS microspheres are demonstrated in Fig. 4.1. The larger ones exhibited a diameter of 660 nm with a standard deviation of 16.4 nm while the smaller ones revealed a diameter of 460 nm with a standard deviation of 10.1 nm. Assembly of PS microspheres could be easily achieved simply by solvent evaporation. As shown, we obtained reasonable uniformity in size distribution and this attribute was critical for our EPD process. The PS microspheres were further mixed with ethanol to prepare the suspensions, PS-460 and PS-660.
Figure 4.1 SEM images of PS microspheres with diameter of (a) 460 nm and (b) 660 nm.
The scale bars are 2 μm.
Figure 4.2 presents the plots of electrophoretic mobility in PS-460 and PS-660. Their concentrations were 2.5 g/L and 10 g/L, respectively. The mobility for the PS microspheres in each suspension appeared in a single peak with corresponding zeta-potential was above 60 mV. Both results suggested that our suspensions were in excellent stability and the size of PS microspheres was reasonably mono-dispersive.
(a) (b)
Figure 4.2 Electrophoretic mobility of suspension (a) PS-460 and (b) PS-660.
4.1.1 Microsphere Concentration in Suspensions
While the PS microspheres were driven to deposit on the substrate from bulk suspension, it took some time to assemble them into a desirable close-packed arrangement. It is necessary for the first layer to be close-packed before the second layer can be deposited. Therefore, a reduced deposition rate was considered critical in fabricating high-quality colloidal crystals.
After rearranging those variables in equation 4.1, we could derive that the Cs is proportional to the deposition rate (Y/t). Thus, the Cs plays significant influence over the quality of the colloidal crystals.
Figure 4.3 presents the SEM images of EPD layers fabricated in PS-460 with Cs of 10 g/L and 2.5 g/L under 10 V/cm. As clearly shown in Fig. 4.3(a), the PS microspheres were in random arrangement with approximate domain size in 2~3 μm. In addition, a number of voids was observed among particles. When we reduced the PS microsphere concentration to 2.5 g/L, the domain of colloidal crystals became larger, as exhibited in Fig 4.3(b). These behaviors suggested that the EPD fabrication with an excess Cs could not deliver high-quality colloidal crystals. Moreover, increase in the Cs enhanced the volume fraction of PS microspheres, which raised op the viscosity of suspension and zeta potential. Thus, a larger Cs not only rendered a faster deposition rate but also affected the Brownian motion among
(a) (b)
Figure 4.3 SEM images of EPD layers prepared in PS-460 with various microsphere concentrations of (a) 10 g/L and (b) 2.5 g/L. The electrical field was fixed at 10 V/cm for 3 min. The scale bars are 3 μm.
Identical experiments were also performed in PS-660 suspensions, with their results presented in Fig. 4.4. Similarly, the suspension of 10 g/L demonstrated a better quality in colloidal crystals over 25 g/L one. Howerer, this great difference in the optimized PS microsphere concentration between PS-460 and PS-660 suspension was resulted from their distinct volume fraction.
Figure 4.4 SEM images of EPD layer made in PS-660 with microsphere concentrations of (a) 25 g/L and (b) 10 g/L. The electrical field was fixed at 10 V/cm for 10 min. The scale bars
(a) (b)
(a) (b)
the values for electrophoretic mobility in PS-460 (2.5 g/L) and PS-660 (10 g/L) were almost the same, as shown in Fig. 4.2. This suggested that the stability of suspension was more relevant to the volume fraction but not mass fraction. Therefore, in following experiments we chose the 2.5 g/L of PS-460 and 10 g/L of PS-660 to fabricate colloidal crystals.
4.1.2 Shape of Substrates
Figure 4.5 exhibits the SEM images of EPD layers on planar ITO glass and carbon fiber prepared under 10 V in 0.25 g/L PS-460. The EPD layer on the ITO glass revealed a (111) plane in FCC lattice. In contrast, a defective structure was observed in the EPD layer on CF.
We surmised that linear defects and curved surface of the CF might convert the state of arrangement from close-packed structure into a non-close-packed one. In the insets of Fig.
4.5, the thickness of ITO layer was 200 nm, and the diameter of CF was about 7 μm. In cylindrical case, the diameter size of the CF (7 μm) was only 10~15 times larger than that of PS microspheres (460 nm and 660 nm). Namely, EPD process was carried out on this curved surface (CF) with PS microspheres assembly, leading to a disordered EPD layer.
Figure 4.5 SEM images of EPD layer on (a) ITO glass and (b) carbon fiber. The insets provide the SEM images of each substrate. The scale bars are 5 μm in the figure and 2 μm in the inset.
(a) (b)
4.2 Fabrication of Planar Colloidal Crystals (PCCs) 4.2.1 Electric Field in EPD process
Figure 4.6 provides the thickness of PCC-660 made of various electric fields for 15 min on ITO glasses. The deposit thickness was linearly increased when the electric field was larger than 10 V/cm. However, a reduced deposition rate was present at 5 V/cm, which suggested that there was a transition regime for EPD rate between 5 V/cm and 10 V/cm.
Figure 4.6 Thickness versus electric field for the colloidal crystals made of PS-660. The EPD fabrication time was 15 min.
Figure 4.7 displays the SEM images for the colloidal crystals from 5 V/cm, 10 V/cm, and 20 V/cm, respectively. The PS microspheres were in close-pack structure at 5 V/cm, and became disordered as the applied electric field was increased. This suggested a larger driving force would engender a defective assembly. It is also known that a large electric
Figure 4.7 SEM images of PCC-660s fabricated in PS-660 under (a) 5 V/cm, (b) 10 V/cm, and (c) 20 V/cm for 15 min. The scale bars are 2 μm.
The thickness of EPD layer as a function of electric field using the 460 nm microspheres is displayed in Fig. 4.8. It was observed that the thickness of PCC-460 was linearly increased with the electric field. Figure 4.9 provides the SEM images for the samples made of 10 V/cm, 15 V/cm, and 25 V/cm, respectively. As expected, a larger electric field rendered a relatively disordered structure. Therefore, to improve the quality of PCCs, the electric field should be moderately reduced. Hence, we selected 15 V/cm in the EPD process using PS-460. Figure 4.10 demonstrates the relationship between current density (j) and deposition time (t) under the selected electric fields in PS-660 and PS-460. Both j-t curves reveal a decreasing current density at extended deposition time. This behavior indicated that the PS microspheres were self-assembling on the ITO glass, producing a mask to shield the
(a) (b)
(c)
Figure 4.8 Thickness versus electric field for the colloidal crystals made of PS-460. The EPD fabrication time was 10 min.
(a) (b)
(c)
Figure 4.10 Evolution of current density vs. time during the EPD process under (a) 10 V/cm using PS-660 microspheres and (b) 15 V/cm using PS-460 microspheres.
4.2.2 Deposition Rate for Planar EPD
Figure 4.11 demonstrates the plot of thickness versus time for the samples prepared in the EPD process.
Figure 4.11 Thickness versus EPD time for the PCC-660 (■) and PCC-460 (☆). The applied electric field was 10 V/cm in PS-660 and 15 V/cm in PS-460.
The estimated deposition rates were 1.064 μm-min-1 and 1.337 μm-min-1 for the PCC-660 and PCC-460, respectively. As clearly shown, the growth of PCCs was slowing down with
(a) (b)
the EPD time. We attributed this phenomenon to the decreasing electric field as the PS microspheres assembled on the substrate. Figure 4.12 and 4.13 exhibit the SEM images for the samples prepared in the EPD process with the optimized parameters. As expected, the microspheres were assembled properly to form colloidal crystals in multiple layers.
(a) (b)
(c)
(e)
(d)
Figure 4.13 SEM images of PCC-460s fabricated under 10 V/cm for (a) 2.5, (b) 5, (c) 7.5, (d) 10, and (e) 15 min, respectively. All scale bars are 5 μm.
(a) (b)
(c) (d)
(e)
4.3 Fabrication of Cylindrical Colloidal Crystals (CCCs) 4.3.1 Applied Voltage in EPD Process
As shown in equation 4.2, the yield amount from the cylindrical EPD process was modified by additional three parameters, a, b, and l, and it was further reduced by a constant about 8.79 (ln(a/b)) from that of planar EPD process. On the other hand, the applied voltage (V) was present in the equation as an independent parameter. Hence, we only controlled the voltage in the cylindrical EPD process.
Figure 4.14 display the SEM images of CCC-460s made from two different voltages in PS-660 for 30 sec. The morphology for the CCCs revealed a defective packed structure, which was resulted from the curved CF surface, as shown in Fig. 4.14(a). The diameter of CCC made of 5 V was about 9 μm, which was equal to 3 EPD layers. At this stage, the color of substrate remained dark, which can be easily observed by naked eyes.
In contrast, as shown in Fig. 4.14(b), the diameter of sample made of 10 V was about 60 μm, which was almost 30 layers. Because the size of CF was in micron range, its dimension was relatively small compared with the planar substrates. Therefore, under identical yield amount, the EPD layer prepared on the CF would be 103-104 times thicker as opposed to that of planar substrate. As a result, the effect of voltage during the EPD became extremely critical in the cylindrical case.
Figure 4.15 provides the diameter of CCCs under different applied voltages in the PS-460 and PS-660. Obviously, a voltage of 5 V was not able to deposit large amount PS microspheres in a short time, but 10 and 20 V were capable of delivering a much enhanced deposition rate for both PS-460 and PS-660 suspensions.
Figure 4.14 The SEM images of CCC-460s made of (a) 5 V, and (b) 10 V. The fabrication time was 30 sec. The scale bars are 5 μm in (a) and 10 μm in (b).
Figure 4.15 Diameter versus voltage for the CCC-460s (☆) and CCC-660s (■). Their fabrication time was 30 sec.
Figure 4.16 presents the SEM images of CCCs made of 10 V and 20 V by PS-660 and PS-460. Among them, the applied voltage of 20 V rendered a poor packing quality in the EPD layer because unnecessary larger EPD rate. In contrast, the 10 V exhibited a relatively
Figure 4.16 presents the SEM images of CCCs made of 10 V and 20 V by PS-660 and PS-460. Among them, the applied voltage of 20 V rendered a poor packing quality in the EPD layer because unnecessary larger EPD rate. In contrast, the 10 V exhibited a relatively