Chapter 5. 97 influence the wetting has not been explored systematically. .... Chapter 5. 104. 5.2.2 Results and Discussion. Nano crystalline ZrO2 thin films are ...
Chapter - 5 Dispersed liquid-solid systems Introduction The presence of surfactants in the liquid is an important feature that controls the wetting behavior of a liquid over a substrate. Surfactants are those materials on whose addition the surface tension of a given liquid reduces. In this chapter a systematic study of the effect of surfactant on the particle dispersion is presented. As a part of this thesis work ZrO2 and surfactant assisted ZrO2 thin films by sol-gel method are deposited on amorphous fused silica substrates to explore the wetting behavior in terms of surface energy and morphology of the substrate. Liquid crystals are coated over quartz substrates to observe that wetting behavior in terms of liquid crystalline phases.
5.1 Surfactant System Surfactants are used for variety of applications such as emulsion, wetting and dispersion. Wetting of solids by surfactant is important for many applications such as detergents, printings, coatings. The surfactant is essential on those cases where the surface energy of solid is lower than liquid and as a result liquid is not able to spread over the substrate (Zdziennicka, 2010). The addition of surfactant to liquid reduces the surface tension of liquid and able to wet the surface (Stoebe et al., 1996, 1997). The surfactant has the advantage that it can reduce the solid-vapor and solid-liquid interfacial tension and promote wetting. Thus it is interesting to study influence of various surfactants on the wettability behavior surfaces with different surface energies (Dutschk et al., 2003). Influence of surfactant on contact angle is studied by many groups but how substrate surface energy
Chapter 5
influence the wetting has not been explored systematically. Since surfactant molecule assemble spontaneously to form aggregate near its CMC, therefore it is very important to study the physico-chemical properties such as surface tension, contact angle of surfactant aggregates on that concentration.
5.1.1 Results and Discussion The wetting behavior of aqueous solutions of cetyl trimethyl ammonium bromide (CTAB), Sodium dodecyl sulphate (SDS) is determined on the basis of contact angle measurement by sessile drop method. CTAB and SDS aqueous solution are prepared from deionized water.
The substrates used to investigate the spreading behavior are highly
hydrophobic Teflon, moderately hydrophobic ITO, and hydrophilic Si to explore the influence of substrate surface energy on spreading behavior. The measured contact angle indicates that the wettability of surfactant solutions depends upon the surfactant concentration and type of substrate used. The concentration of surfactant is chosen both well below and closed to critical micelle concentration (CMC) to explore its wetting properties when micelle forms. The measured values of contact angle of aqueous solutions of CTAB and SDS are shown in Fig. 5.1 and Fig. 5.2 respectively. The surface tension of surfactant solution are measured by the pendant drop method and shown in Fig. 5.3 and Fig. 5.4 respectively. It is observed that surface tension as well as contact angles for all the substrates decreases approximately linearly with increasing surfactant concentration till the CMC is reached. The surface tension and contact angle remain unaltered once CMC is reached.
97
Contact angle
o
Chapter 5
130 125 120 115 110 105 100 95 90 85 80 75 70 65 60 55
Teflon ITO Si
0.0
0.2
0.4
0.6
0.8
1.0
Concentration of CTAB
Figure 5.1: Variation of contact angle with concentration of CTAB in water over Teflon, ITO and Si. 130
Teflon ITO Si
120
Contact angle
o
110 100 90 80 70 60 50 40 30 0
2
4
6
8
10
Concentration of SDS
Figure 5.2: Variation of contact angle with concentration of SDS in water over Teflon, ITO and Si. 98
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It is also observed that higher the surface energy of the substrates lower the contact angle. For highly hydrophobic surfaces like Teflon the contact angles of solutions do not change much. The spreading is relatively low for highly hydrophobic surface like Teflon having surface energy 23 mJ/m2. The spreading is significant for moderately hydrophobic (ITO) and hydrophilic surface (Si) having surface energies 37.27mJ/m2 and 41.46 mJ/m2 respectively. At low surfactant concentration change in contact angle is significant. It can be concluded that initially adding of surfactant decreases the surface tension of water and attains a stable value near its CMC. At that particular concentration contact angle remains constant irrespective of type of surfactant and substrates. The results indicate that the wettability of aqueous surfactant depends upon concentration of surfactant. The relation between contact angle and concentration of surfactant suggest that the contact angle will be constant near its CMC and after that further addition of surfactant contact angle remain altered. So when CMC is reached it is the lowest value of contact angle for a particular substrate. However the spreading is less in case of hydrophobic low energy surfaces like Teflon. The spreading behavior is good for intermediate surface energy material in contrast with low surface energy material. When surfactant are added to water they decreases the surface tension of liquidvapor and liquid-solid interface and as a consequence aqueous solution of surfactant is able to spread over hydrophobic substrate. The presence of surfactant influences the interfacial surface tension and enhances water wettability. Contact angle results suggest that complete spreading over different substrate is not possible even though the surface tension of the liquid is less than the surface energy of the solid.
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70
Surface Tension (mN/m)
65 60 55 50 45 40 35 30 0.0
0.2
0.4
0.6
0.8
1.0
Concentration of CTAB in water(mM)
Figure 5.3: Variation of surface tension with concentration of CTAB in water. 70
Surface Tension (mN/m)
65 60 55 50 45 40 35 30 0
2
4
6
8
10
Concentration of SDS in water (mM)
Figure 5.4: Variation of surface tension with concentration of SDS in water.
5.1.2 Summary Normal wetting of liquid over a substrate is hindered by the micelle formation. Contact angle of aqueous solutions of CTAB and SDS vary below CMC, but remain constant for concentrations higher than CMC. 100
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5.2 Zirconium Dioxide (ZrO2) Nano-crystalline ZrO2 (Tyagi et al., 2006) has drawn considerable attention due to its physical, chemical and optical properties (Kumari et al., 2009). Zirconia films have a high refractive index and a large optical band gap (Yu et al., 2005; Suhail et al., 1994) and high transparency in the visible and near infrared region. Due to these properties, ZrO2 is used in ceramics, gas sensors, catalysts and opto-electronics (Xu et al., 2003). Sol-gel method has been commonly used to prepare metal oxides and their composites (Nishide et al., 2000; Liang et al., 2007; Lu et al., 2010; Hemissi et al., 2007; Kumar et al., 2011; Zhang et al., 1999) and it has been used widely for the formation of ZrO2 films (Perdomo et al., 1999; Parralejo et al., 2010). Further sol-gel processes provide a convenient way for forming a nanostructure layer (Park et al., 2009). The controlled synthesis of nanostructural metal oxides has received interest due to their potential applications in nanodevices (Lu et al., 2006). Cost effective surfactant mediated sol-gel method is efficient in controlling microstructure and particle size by the formation of aggregate like micelles (Blin et al., 2001). Complex surfactant structure provides a cage like effect which limits particle agglomeration and proved to be a versatile soft template by self-assembling to form different conformations in nanomaterial synthesis. Annealing temperature is also an important factor for changing the phases of sol-gel derived zirconia powder (Santos et al., 2008). Recently, there have been reports on the preparation of ZrO2 powder with various morphologies such as nanorods, nanowires and nanotubes (Pavasupree et al., 2005; Tsai et al., 2008). Although there are reports of surfactant assisted ZrO2 by sol-gel however the how effect of surfactant concentration is able to control the microstructure and consequently 101
Chapter 5
affecting the surface energy of ZrO2 film is not studied in great detail. The main objective of this work is to show how surfactant dispersion in water controls the particle growth and adversely affect the corresponding surface energy.
5.2.1 Experimental Details
All chemicals are AR grade and used as received. 3.137ml of zirconium isopropoxide diluted in 6ml isopropanol is used as the source of zirconium dioxide. After dissolving of the zirconium alkoxide in isopropanol, 6ml glacial acetic acid (CH3COOH) is added to a solution homogenized while being subject to ultrasound vibrations. A homogeneous mixture is obtained after 20 min at which point 3ml of water is added to complete the hydrolysis. A clear and transparent sonosol is obtained. Surfactant-assisted films are prepared by the same method with the difference that instead of water, 1mM (CTAB) dissolved in water is used as cationic surfactant and on the other case 1mM (SDS) dissolved in water is used as anionic surfactant. ZrO2 and surfactant assisted ZrO2 thin films are deposited on fused silica substrate using spin coater revolving at a speed of 2500 rpm for 20s. After deposition of one layer the substrate are kept at 150oC in a hot air oven for 10 min to remove the solvent in the film and the film is allowed to cool to room temperature. Then next layer is then deposited and the procedure repeated for four layers. The samples are annealed at a temp of 400oC, 600oC and 800oC respectively for 2 hours in a preheated furnace. After annealing, the samples are allowed to cool up to room temperature. The schematic representation of the preparation is shown in Fig. 5.5
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Chapter 5 Zirconium isopropoxide Zr(OC3H7)4
Isopropanol (C3H7OH)
Ultrasound agitation
Glacial acetic acid (CH3COOH)
1mM SDS dissolved in water
Ultrasound agitation
Spin Coating
1mM CTAB dissolved in water
Water
Spin Coating
4 Cycles
Drying at 150oC
Annealed at 400oC, 600oC and 800oC
SDS assisted ZrO2 films
Ultrasound agitation
Spin Coating 4 Cycles
4 Cycles
Drying at 150oC
Annealed at 400oC, 600oC and 800oC
ZrO2 films
Drying at 150oC
Annealed at 400oC, 600oC and 800oC
CTAB assisted ZrO2 films
Figure 5.5: Schematic representation of experimental procedure
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5.2.2 Results and Discussion Nano crystalline ZrO2 thin films are prepared by sol-gel route and spin coating the sol solutions on fused silica followed by annealing at 400oC, 600oC and 800oC for 2 hours each. Transmission spectra for ZrO2 thin film are shown in Fig. 5.6 and the transmission spectra for CTAB/SDS assisted ZrO2 thin films are shown in Fig. 5.7 and Fig. 5.8 respectively.
100
80 % Transmittance
o
ZrO2 thin film annealed at 400 c o
ZrO2 thin film annealed at 600 c
60
o
ZrO2 thin film annealed at 800 c 40
20
0 500
1000
1500
2000
2500
3000
Wavelength(nm)
Figure 5.6: Optical transmission spectra of ZrO2 thin films annealed at different temperature.
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100
% Transmittance
80
60
o
CTAB assisted ZrO2 thin film annealed at 400 c o CTAB assisted ZrO2 thin film annealed at 600 c o CTAB assisted ZrO2 thin film annealed at 800 c
40
20
0 500
1000
1500
2000
2500
3000
Wavelength(nm)
Figure 5.7: Optical transmission spectra of CTAB assisted ZrO2 thin films and annealed at different temperature.
100
% Transmittance
80 o
SDS assisted Zro2 thin film annealed at 400 c
60
o
SDS assisted Zro2 thin film annealed at 600 c o SDS assisted Zro2 thin film annealed at 800 c
40
20
0 500
1000
1500
2000
2500
3000
Wavelength(nm)
Figure 5.8: Optical transmission spectra of SDS assisted ZrO2 thin films annealed at different temperature. 105
Chapter 5
High transmission values are obtained for all the films. The high transmission indicates that a homogeneous film has been formed. The ZrO2 thin films are found to show clear interference fringes in the transmission spectra. The interference pattern in the transmission spectra of ZrO2 on fused silica is due to the refractive index contrast between the two materials. The refractive index of ZrO2 film is in between 1.99-2.01 where as in the CTAB/SDS assisted ZrO2 film 1.99-2.05 and 1.99-2.05 respectively. The refractive index of the film is calculated by envelope method proposed by Swanepoel (1983). The band gap is determined, in the range of 5.80-5.88eV for ZrO2 film synthesis with and without surfactant and annealed at different temperature and approximately equal to 5.74 reported elsewhere (Kumar et al., 2011). The XRD profiles of ZrO2 thin films prepared without surfactants are shown in Fig. 5.9 and with surfactant are shown in Fig. 5.10 and Fig. 5.11 respectively. ZrO2 thin films are seen to be polycrystalline with tetragonal phase up to 600oC and an additional monoclinic phase for surfactant-assisted films annealed at 800oC. This is in good agreement with JCPDS data for tetragonal (JCPDS 88-1007) and monoclinic (JCPDS 86-1451) phases of ZrO2. XRD result reveals that surfactant favors the formation of monoclinic phase of ZrO2 at 800oC. Presence of surfactant reduces the surface tension of sol which decreases the energy needed to form the new phase and as a result high temperature monoclinic phase at low annealing temperature is observed but which is not observed in case of ZrO2 film without surfactant. SDS-ZrO2 films show a tetragonal phase at 4000C annealing unlike CTAB-ZrO2 and ZrO2 annealed at the same temperature, which exhibit an amorphous phase. It is evident from the XRD that film prepared in SDS template the crystallisation temperature decreases. Higher 106
Chapter 5
crystalline phase at lower annealing temperature is observed. Intensity of peaks increases with increase in annealing temperature. With increasing annealing temperature, the crystallinity of the samples improves and the crystallite size is also increased resulting in a decrease in defect levels.
Intensity (arb units)
T
T
T
T
o
800 C T
T
T
T
o
600 C
o
400 C
20
30
40
50
60
70
80
2 (degree)
Figure 5.9: X-ray diffraction pattern of ZrO2 thin films annealed at 400oC, 600oC and 800oC respectively.
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Chapter 5
T M M
Intensity (arb units)
T
T
o
T
800 C
T
600 C
T T
o
o
400 C
20
30
40
50
60
70
80
2 (degree)
Figure 5.10: X-ray diffraction pattern of CTAB assisted ZrO2 thin films annealed at 400oC, 600oC and 800oC respectively.
T M M
Intensity (arb units)
T
T
T
o
800 C
T
T
T
T o
600 C T T
20
30
T
40
50
T
60
o
400 C 70
80
2 (degree)
Figure 5.11: X-ray diffraction pattern of SDS assisted ZrO2 thin films annealed at 400oC, 600oC and 800oC respectively. 108
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SEM images and energy dispersive X-ray spectra of ZrO2 and surfactant assisted ZrO2 film and annealed at 400oC, 600oC and 800oC are shown in Fig. 5.12, Fig. 5.13 and Fig. 5.14 respectively. The FE-SEM images reveal that the films are very smooth and the grains are very small. Energy dispersive X-ray spectroscopy (EDX) measurements and quantitative elemental analysis made on films show peak due to Zr, O and Si in all the samples. The Si peak in the EDS spectrum appears from the substrate. There is no evidence of surfactant either in the XRD pattern or in the elemental analysis by annealing the film from 400 oC to 800oC.
Figure 5.12: SEM images and energy dispersive X-ray spectra of ZrO2 thin films annealed at a) 400oC b) 600oC and 800oC. 109
Chapter 5
Figure 5.13: SEM images and energy dispersive X-ray spectra of CTAB assisted ZrO2 thin films annealed at a) 400oC b) 600oC and 800oC.
110
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Figure 5.14: SEM images and energy dispersive X-ray spectra of SDS assisted ZrO2 thin films annealed at a) 400oC b) 600oC and 800oC. SEM image reveals the formation of nano crystalline spherical zirconia particles. In water surfactant form spontaneously defined aggregates such as spherical micelles and as a result the morphology of the particles becomes spherical. For SDS assisted film also getting spherical particles are observed although the concentration of surfactant is below is critical micelle concentration (CMC) because presence of zirconium reduces the CMC of SDS in water in considerable extent. For ionic surfactant presence of metal complex decreases the 111
Chapter 5
CMC by screening the electrostatic repulsion between head groups and a result CMC of SDS decreases. Surfactant reduces the interfacial tension between the surfaces. On raising the temperature, particle size increases because of agglomeration. During the agglomeration the overall surface energy reduces. The AFM images of ZrO2 and surfactant assisted ZrO2 film is shown in Fig. 5.15. By using AFM image analysis, the rms roughness of the film are calculated and reveals that films are very smooth with rms roughness of the order of 0.3-3 nm for all the films. The AFM images show that the ZrO2 film grown by surfactant template is homogeneous and dense. The surfaces of the films are smooth with fine grains. CTAB and SDS at lower concentration forms spherical micelle with water and as result particle size becomes nearly spherical. The change in particle size adversely affected the surface energy of the film. In the crystallization process, surfactant molecules serve as a growth controller as well as an agglomeration inhibitor. Contact angle, adhesive energy, surface energy using Geometric mean method (Chapter 2) and crystallite size of ZrO2 and surfactant assisted ZrO2 thin films are presented in Table-5.1. It has been observed that wetting behavior is improved by using cationic surfactant such as CTAB, which in turn reduces the grain size that will result in increased surface energy. The smallest crystallite size from the XRD is obtained for CTAB assisted film. The probable reason for it may be the critical micelle concentration (CMC) of CTAB is lower than SDS. From crystallite size data it is seen that surfactant concentration above its CMC is responsible for low crystallite size.
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Figure 5.15: AFM images with and without surfactant assisted ZrO2 films deposited on fused silica and annealed at different temperature. Left panel is for films annealed at 400oC whereas middle and right panel for films annealed at 600oC and 800oC respectively.
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Table. 5.1. Contact angle, adhesive energy and surface energy of ZrO2 thin films deposited under different conditions.
Samples
Contact Adhesive Geometric mean method angle energy Polar Dispersive Total (water) (mJ/m2) (mJ/m2) (mJ/m2) (mJ/m2) ZrO2 400oC 94.52o 67.06 1.3 29.59 30.89 o o ZrO2 600 C 94.18 67.49 1.31 30 31.31 o o ZrO2 800 C 93.37 68.52 1.31 31.25 32.55 CTAB assisted ZrO2 at 400oC 67.33o 100.86 10.95 32.93 43.89 o o CTAB assisted ZrO2 at 600 C 73.43 93.56 8.14 31.97 40.12 CTAB assisted ZrO2 at 800oC 86.53o 77.21 4 27.11 31.12 o o SDS assisted ZrO2 at 400 C 90.46 72.22 2.25 29.57 31.82 o o SDS assisted ZrO2 at 600 C 91.35 71.08 1.95 29.96 31.92 SDS assisted ZrO2 at 800oC 90.98o 71.55 1.8 31.45 33.25
Crystallite size (nm) Amorphous 15.23 20.61 Amorphous 11.47 22.11 15.9 18.2 24.22
The polar and dispersive component of surface energy of the ZrO2 film, and hence its total surface energy, remains almost constant as the annealing temperature is increased. In CTAB-ZrO2 thin film the two components of surface energy gradually decrease and resulting in a decrease in surface energy with an increase in the annealing temperature. The CTABZrO2 films show increase in hydrophobicity with increasing annealing temperature. At low temperature, CTAB binds the ZrO2 by micelle formation and these results in high surface energy. The increase in surface energy is attributed to the increase in surface to volume ratio of nano particles. With increasing annealing temperature, surface energy decreases because particle size increases since at higher temperature surfactant molecules do not bind the ZrO2 as efficiently leading to agglomeration of grains and ultimately leading to the surface energy approaching to that of ZrO2 film without surfactant assistance. For SDS-ZrO2 thin film, the two components of surface energy remain constant and consequently the surface energy also remains same. The adhesive energy of ZrO2 films and SDS-ZrO2 films is almost the same 114
Chapter 5
while the CTAB-ZrO2 film shows a higher adhesive energy. It is concluded that the addition of CTAB at concentrations above CMC yields lower grain size resulting in higher adhesive energy and a hydrophobic to hydrophilic transition. Schematic representation of surface energy of ZrO2, CTAB/SDS assisted ZrO2 thin film with annealing temperature is shown in Fig. 5.16
CTAB assisted ZrO2
Surface Energy(mJ/m2)
40
o
Annealed at 400 C o Annealed at 600 C o Annealed at 800 C
SDS assisted ZrO2
ZrO2 30
20
10
0 1
2
3
Figure 5.16: Variation of surface energy of ZrO2 and surfactant assisted ZrO2 film with annealing temperature.
5.2.3 Summary ZrO2 thin films are deposited on fused silica substrates with and without surfactants using the sol-gel method and are annealed at different temperatures. Crystallite size determined from the XRD pattern reveals that particle size reduces when the molar concentration of surfactant is above its CMC. On the other hand if the molar concentration of surfactant is below its CMC crystallite size increases and crystalline phase is obtained even 115
Chapter 5
at lower annealing temperature. The variation of contact angle of CTAB assisted film is due to change in crystallite size of the film. For SDS assisted film although there is little variation of particle size after the film is annealed at different temperatures but contact angle and surface energy remain approximately the same. It is seen that contact angle measurements can throw light on the grain size of the surface layer of nano-crystalline film. Concentration below critical micellar concentration led to the formation of larger particles. These results show that the addition of surfactant has a significant effect on controlling the morphology and particle size of Zirconium dioxide thin film.
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5.3 Interaction of liquid with Liquid crystals Surface phenomena have attracted a lot of interest, particularly the interface between liquid crystals and solid substrates (Chung et al., 1999). However it is necessary to study surface properties of LC and adhesion properties to be used in MEMS devices.
5.3.1 Results and Discussion Liquid
crystal
(LC)
6O(OH).2
[N-(4-hexyloxy-2-hydroxybenzylidene)-4/-
ethylaniline (Fig. 5.17)] is dissolved in ether and coated over quartz substrate by gel casting method in which few drops of solution is poured over substrate and allowed to dry. The optical phase sequences of the compound is shown in Fig. 5.18. Polarising optical microscope texture of the compound is shown in Fig.5.19
C6H13O N
C2H5
OH
Figure 5.17: Chemical Structure of N-(4-hexyloxy-2-hydroxybenzylidene)-4/ethylaniline
55.3 83.4 Crystal Nematic Isotropic
Figure 5.18: Optical phase sequence of N-(4-hexyloxy-2-hydroxybenzylidene) 4/ethylaniline
117
Chapter 5
Figure 5.19: POM texture of N-(4-hexyloxy-2-hydroxybenzylidene)-4/-ethylaniline.
Contact angle of water is measured over coated substrate at different temperatures. At room temperature substrate is hydrophobic but at a temperature ~60oc hydrophobic substrate becomes hydrophilic. Thermally induced transition from anisotropic to liquid crystalline phase is responsible for such a dramatic change in wetting properties. Initially the hydroxyl groups are hidden and hence no interaction between LC molecules and H2O. As the temperature increases the aliphatic chain melt and hence thermal fluctuation allow the molecules to start separating. At this temperature the H2O molecules move through the LC molecules and interact with hydroxyl moiety through H-bonding to promote hydrophilic character. Strong temperature induced variations of contact angle is observed. Further this effect is also observed for polar viscous liquids like glycerol. The graphical representation of contact angle of water and glycerol over LC coated quartz substrate is shown in Fig.5.20 and Fig.5.21 respectively.
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Figure 5.20: Variation of contact angle of water over LC coated quartz substrate at different temperature.
Figure 5.21: Variation of contact angle of glycerol over LC coated quartz substrate at different temperature. 119
Chapter 5
5.3.2 Summary Wettability studies reveals that near the transition temperature (Anisotropic-Nematic) hydrophobic surface becomes hydrophilic.
5.4 Conclusions The wetting behavior of a liquid over a substrate is dependent on both physicochemical properties of liquid as well as substrate. Various physical parameter such surface energy, particle size, morphology of film affects its wetting properties. For liquid crystal coated substrate a temperature induced wetting is observed and it is attributed to the transition of anisotropic to nematic phase. In the present study surfactant induced wetting is observed and it can be concluded that droplet spreading of aqueous solution of surfactant not only as a function of surfactant concentration but also surface energy of the substrate. Interestingly once the micelle forms even though the surface tension is less than the surface energy of substrate like ITO and Si but droplet is not able to spread completely.
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