There are two parts divided in this section. The first part is about the results from the synthesis of the sol-gel method followed by as-prepared acid/base two-step catalyst silica thin films and formation of hydrophobic silica thin films by surface modification while the second part is the characterization for the modified silica thin films. The first part included the effect of heat treatment time on refractive index and thickness of as-prepared silica thin films and surface modification by different silylation temperatures as formation of self-assembled films for hydrophobic properties. The second part is discussion about the results from water contact angles, Tadmor's equation, surface energy from Young's equation, surface roughness from AFM, structural properties from FTIR spectroscopy, optical transmission and optical band gap from UV-VIS spectrophotometer, surface morphologies and percentage of composition in silica thin film and modified silica thin films from FESEM and EDX.
4.2 Silica sols by sol gel method
The silica sols that have been prepared by sol gel method showed a colorless and transparent liquid which is compatible with Brinker and Scherer [64]. The prepared silica sol is said to be in stable state where no sediment were formed [65, 66]. It also has medium viscosity where it can soak all over the substrates' surfaces. A few important observations have been made from this preparation as stated below:
Ethanol should be put first into the beaker before other chemicals. The stabilization of sols is easier to get rather than put the precursor or catalyst first.
All chemical reagents used must follow the calculated amount.
The addition of water must be done at the last stage. It was found that if water was put early, the hydrolysis will take place faster and the solution still not stable yet.
The quantity of water used must be controlled wisely. The stabilization of the sols really dependent on the quantity of water.
All the apparatus and equipment must be dry from excess tap water that used to clean them. This is because tap water will damage the sols.
The fifth observation is compatible with recommendation from Qunyin Xu and Anderson [67] which stated that all the equipment that been washed using water should be dried carefully.
4.2.1 Formation of Silica Thin Films
The example of silica thin films that were spin-coated at two-step timers which are 600 rpm for 10 s followed by 1800 rpm for another 10 s are shown in Figure 4.1. Spin speed timer is very important factor as different set of time will give different thickness and percentage element composition on the films. A systematic spin-deposition will give good films quality and suitable thickness films.
Before the silica thin films can be coated, the aging time of the sols must first be considered. In this study, the silica sols were used at aging time before t = 0.7tgel which approximately around 15 to 21 days from sols preparation. This is because the viscosity of the sols change correlated with time. That is why the spin speed was set at two timers which are suitable with the sols viscosity. After the films were deposited, the films were heated in order to remove excess solvents. The temperature and time during heat treatment are very important as to determine the homogeneity, thickness and refractive index of the prepared films.
Figure 4.1 The example image of the spin-coated samples.
4.2.2 Choose of Heat Treatment Time
The heat treatment time is one of the important parameter need to be consider in deposition of acid/base two-step catalyzed silica thin films using spin coating technique. The heat treatment time were varied from 0 to 3 hours while temperature of heat treatment was maintained at 150°C. During this heat treatment, the silica thin films shrink as trapped water and solvent within the structure is removed. The refractive index, n as a function of thickness, nm is shown in Figure 4.2.
For silica thin film prepared without heat treatment, it shows that refractive index is 1.3507 and with the lowest thickness of 95.23 nm. For silica thin films prepared at 1 hour and 2 hours of heat treatment, it shows that the refractive index is slightly decreased about 0.0020 from 1.3825 to 1.3805 but thickness is increased from 112.40 nm to 134.88 nm. The most interesting part is that at 3 hours of heat treatment, the silica film shows the lowest refractive index about 1.324 but higher thicknesses at 164.75 nm.
Since the refractive index and thickness value is different for those same acid/base two-step catalyzed silica films, it clearly shows that there is a difference measure of the bending ray of light when passing through each the silica films. In order to obtain the optimum results, there are four different spot were taken into measurement for each samples and mean value were calculated.
thicknssRI
Figure 4.2 Refractive index, n as a function of thickness, nm for heat treatment from 0 to 3 hours
From the Figure 4.2, it is found that time of heat treatment influenced the refractive index. The thickness of the silica films were increased as the time of heat treatment increased. Although heat treatment time at 3 hours gives highest value of thickness but the refractive index is still very low. Thus, from this result, the suitable time for heat treatment lies between 1 hour and 2 hours. In this study, the silica films formation were kept at 2 hours at 150°C.
The phenomenon of different values of thickness and refractive index can be explained from the reaction process shown in Figure 4.3. From Figure 4.3 (a), by adding basic catalyst, the growth model of silica sol is biased towards spherically expanding particles and the pore volume of the formed film is quite large, and the porosity is high giving the film a very low refractive index. On the other hand, with an acidic catalyst, the growth of silica sol tends to form linear chains as shown in Figure 4.3 (b). Thus for using acid/base two-step catalyzed shown in Figure 4.3 (c), the silica gel with the linear chains is likely incorporated into spherical particles structure so that porosity is decreases [68].
Thus it can be concluded that for silica films prepared without heat treatment and 3 hours of heat treatment contains more spherical particles from basic catalyst compared to silica films that went through 1 and 2 hours of heat treatment. This is also because during spin deposited, the acid/base silica sols were spin off and at the end the tabulation of acid/base particles are at the edge of the films.
(b)
(c)
(a)
Figure 4.3 Schematic diagram of reaction process of (a) base-catalyzed silica sol, (b) acid-catalyzed silica sol and (c) acid/base-cayalyzed silica sol [68].
4.3 Formation of Hydrophobic Silica Thin Films
4.3.1 Silylation Temperatures
The prepared acid/base two-step catalyzed silica films were went through silylation process that leads to formation of self assembled films thus create hydrophobic surfaces. The silylation agent, time and temperature are important factors to create hydrophobic surfaces. TMCS was chosen as silylating agent and time of reaction was maintained at 30 minutes for each samples. The temperatures were varied from 40 to 100 °C as the boiling point for TMCS is low and it is easily evaporate. That is why in this experiment, simple chemical evaporation was carried out and rate of evaporation/reaction were determined prior to hydrophobic surfaces.
Figure 4.4 shows the rate of evaporation/reaction, (°C/min) as a function of silylation temperatures, °C and the data is tabulated in Table 4.1. From the data, it shows that when temperature is increased the rate of evaporation/reaction is also increased. There are slightly increasing from 40 to 80 °C but from 80 to 100 °C there is drastically increasing rate of reaction.
Table 4.1 Silylation temperatures and Rate of evaporation/Reaction
Silylation Temperature (°C)
Rate of Evaporation/Reaction
40
0.087
60
0.100
80
0.120
100
0.170
D:\rateofEvaporation.JPG
Figure 4.4 The rate of evaporation/reaction, (°C/min) as a function of silylation temperature, °C
The relationship between rate of evaporation/reaction and water contact angle due to silylation temperatures will be discussed further in next section.
4.4 Characterization of Modified Silica Thin Films
4.4.1 Water Contact Angle
Hydrophobic properties synonymy with water contact angle and it is very important as to determine which the optimum silylation temperature is. Figure 4.5 shows the water contact angles and their silylation temperatures of the modified silica thin films.
From the Figure 4.5, it is clearly shows that as-prepared silica thin film has the lowest value of water contact angle compared to other modified silica thin films with silylation process. When temperature of silylating is increase to 40 °C, it shows great increment from the as-prepared silica thin film. Although it is not over 90° for make it hydrophobic, but it shows an improvement in our self assembled film. The silylation temperature was increased again up to 60 °C and 80 °C and it gives the water contact angle value over than 90°. On the other hand, when the silylation temperature was increased to 100 °C, the water contact angle value decreased back to below 90 °. Figure 4.6 shows the simplified relationship pattern between water contact angle, (deg.) and silylation temperatures, (°C).
G:\CONTACT ANGLE\service\sample A\Sample A\sample 2.jpg
G:\CONTACT ANGLE\service\sample b\Sample B\sample 2.jpg
CA : 57.6°
(as-prepared)
(b) CA : 85.0°
G:\CONTACT ANGLE\service\sample C\Sample C\sample 2.jpg
(c) CA : 96.5°
G:\CONTACT ANGLE\service\sample D\Sample D\sample 2.jpg
(e) CA : 74.3°
(d) CA : 97.6°G:\CONTACT ANGLE\service\sample E\Sample E\sample 2.jpg
Figure 4.5 Water Contact Angle for (a) as-prepared and modified silica thin films at silylation temperature (b) 40 °C (c) 60 °C (d) 80 °C (e) 100 °C
(as-prepared)
Figure 4.6 Water contact angle (deg.) vs silylation temperature (°C).
The objective of silylation agent (TMCS) is to increase the number of -Si-(CH3)3 groups that attached to silica surface that will increase in water contact angle. From the Figure 4.5, it can be described that in the modified silica thin films, the number of -Si-(CH3)3 groups is the least at silylation temperature 100 °C, followed by silylation temperature at 40 °C. The difference in value of water contact angle is because of difference in physical properties of silica films thus gives different reaction in kinetics of the silylation agents. So it can be concluded that if silylating temperature is high enough (100°C), it will changes the kinetics reaction of TMCS with silica film surface and consequently lower the water contact angle by the excess TMCS. From the Figure 4.4 that shows the rate of evaporation/reaction (°C/min) vs temperature (°C) earlier, it is also prove that the highest rate of reaction is at 100 °C. At this time, it is actually turns back to Si - OH functional group. As for the lower silylation temperature at 40°C, low water contact angle is because the boiling point of TMCS is 57 °C, so it is below rate of evaporation/reaction thus give water contact angle below 90°.
Venkateswara Rao et al. [69] stated that if further increase in the TMCS percentage above 5%, the contact angle remains constant due to almost completion of the surface modification of the silica surface. This can be seen for the silylation temperature at 60 and 80 °C where their water contact angles are slightly changed. Hence it can be concluded that by increasing silylation temperature up to 80 °C but below 100 °C, it will give hydrophobic films.
Meanwhile, according to Tadmor [35], an accurate relation between the contact angle of the water droplet and its diameter can be obtained by knowing a volume on a flat horizontal surface sample, by means of geometrical consideration. From the equation 2.10 and 2.11 mentioned earlier in Chapter 2, the values from Tadmor's equations were calculated in Table 4.2 and plotted in Figure 4.7. From the figure, it is possible to see that the relation between drop diameter and contact angle has a hyperbolic trend and it is compatible with Tadmor's [35] result.
The drop shape of water droplet on modified silica thin films surface analysis is one of the conventional methods to evaluate wettability and surface energy. Hence further discussion about surface energy will be presented in the next section.
Table 4.2 Drop diameter (nm) with its contact angle (deg.)
Contact Angle (deg.)
Drop Diameter (nm)
57.6
30.0
74.3
25.7
85.0
23.5
96.5
21.4
97.6
21.2
tadmorEq
Figure 4.7 Relationship between drop diameter and water contact angle.
4.4.2 Relationship between Water Contact Angle and Surface Energy
Table 4.3 shows a distribution of the Young's equation for each of the water contact angles obtained from equation 2.12. It defines the balances of forces caused by a water droplet on a dry surface. If the surface is hydrophobic enough then the contact angle of a drop of water will be larger. Meanwhile if the surface is hydrophilic it is indicated by smaller contact angles and higher surface energy.
Table 4.3 Surface energy and its water contact angle at silylation temperatures 40 to 100°C
Water Contact Angle (deg.)/ θc
Surface Energy (J/m2)
57.6
38. 61
85.0
06.28
96.5
-8.10
97.6
-9.56
74.3
19.54
From the table, it shows that higher water contact angles gives lower value of surface energy. This estimation theory on surface energy or surface tension really dependence on the materials and liquid used as droplet for measuring water contact angle. Figure 4.8 shows the simplified relationship between water contact angle and surface energy. It is clearly confirmed the theory about lower surface energy gives higher water contact angle which compatible with Nakajima et. al. [70].
C:\Users\User\Desktop\SurfaceEnergy.JPG
Figure 4.8 Relationship between water contact angle and surface energy.
In addition, the wetting ability of the hydrophobic silica films can also be discussed here. The wetting ability of liquid to maintain contact with silica films is affected by intermolecular interactions between them. The degree of wetting is determined by a force balance between adhesive and cohesive force. A low contact angle which is below 90 °C of silica thin films and modified silica thin films with silylation temperatures at 40 and 100 °C indicates that wetting of the surface is at advantageous and the liquid drop spread over a large area of the surface. On the other hand, for water contact angle greater than 90 °C which are silylating temperatures at 60 and 80 °C indicates that the liquid drop minimized their contact with the surface.
The wetting ability of the silica films is also dependent on the topology and surface morphology of the films. This will be discussed in the next section. The structural studies of the as-prepared and modified silica thin films will be discussed later.
4.4.3 Atomic Force Microscopy
Typical three-dimensional image from AFM of as-prepared silica thin film and modified silica thin films with silylation temperatures between 40 - 100 °C are shown in Figure 4.9. The images were recorded at 1 x 1 µm2 planar in contact mode. The root-mean-square (RMS) roughness value of the films can be observed from the micro and nano-scale features.
C:\Users\User\AppData\Local\Temp\Rar$DI84.024\C 1U 3D.JPGC:\Users\User\AppData\Local\Temp\Rar$DI34.320\B 1U 3D.JPGC:\Users\User\AppData\Local\Temp\Rar$DI88.096\Aa 1U 3D.JPG
(b)
(a)
(c). C:\Users\User\AppData\Local\Temp\Rar$DI56.136\E 10U 3D.JPG
(e)
(d)C:\Users\User\AppData\Local\Temp\Rar$DI39.728\D 1U 3D.JPG
Figure 4.9 AFM images of (a) as-prepared silica thin film and modified silica thin films with silylation temperatures at : (b) 40 °C (c) 60 °C (d) 80 °C and (e) 100 °C.
The RMS roughness of prepared silica thin films and modified silica thin films are shown in Table 4.4. The surface of the prepared silica thin film and modified silica thin film at 100 °C silylation temperature consist of dispersed island while the modified silica thin films with silylation temperatures of 40, 60 and 80 °C shows the formation of steeply crater like.
Silylation Temperature (°C)
Water Contact Angle
(deg.)
Surface Roughness (nm)
0 (as-prepared)
57.6
111.67
40
85.0
140.20
60
96.5
140.67
80
97.6
142.22
100
74.3
113.04
Table 4.4 Surface roughness of the silica thin films
In order to determine the effect of silylation temperatures on surface roughness, data from Table 4.4 is plotted in Figure 4.10. From the result, it shows that the silylation temperatures between 40 to 80 °C have higher value of surface roughness. Meanwhile, the surface roughness for silylation temperature 100 °C has almost similar value with the as-prepared silica thin film. The surface roughness also influenced the value of water contact angle. The Cassie-Baxter's model suggested that the surface traps air in the hollow spaces of the rough surface, so that the droplet essentially rests on a layer of air [10]. This explains to the silylation temperatures at 60 and 80 °C where their surface roughness are large and water contact angle greater than 90°. For smaller roughness, the profile of the surface and the water contact angle is given by Wenzel's law as described earlier in the literature. Hence, surface modification with various silylation temperature does change the surface roughness along with the water contact angle.
D:\surfaceRoughness.JPG
Figure 4.10 Correlation between silylation temperature and surface roughness.
4.4.4 Structural studies
The infrared spectra of the as-prepared silica thin film and modified silica thin films at silylation temperatures of 40, 60, 80 and 100 °C were recorded and shows in Figure 4.11. Because of silica sols contained with TEOS as precursor, ethanol as solvent, water, and acid/base catalysts followed by surface modification using TMCS, it gives quite a complex infrared spectrum.
Figure 4.11 (a) shows the structure of the as-prepared silica thin film while Figure 4.11 (b) to (e) shows the changes in infrared spectra of the modified silica thin films at different silylation temperature. The absorption band tends to have a broad peak which indicates the amorphous state of the thin films. The absorption peaks were observed in the range of 600 - 4000 cm-1. The most important stretching band located at about 1000 cm-1 where it can determine the stoichiometry of the modified silica thin films. The Si-O-Si bands existed at 1030 cm-1 which confirms the formation network structure inside the silica thin films [69]. The absorption bands actually become broadened and shifted to lower wavenumbers. During the silylation temperature at 80°C, the absorption band at 1030 cm-1 show sudden decreased in its intensity. This might due to the increasing CH3 units when react with TMCS which contribute to high water contact angle.
Next absorption bands are observed at 1260 cm-1 respectively. This absorption bands are attributed to Si-C stretching vibrations [68]. The strong absorption band of Si-C vibration is observed when the silylation temperature reaches 80°C. This absorption bands confirms that the surface of silica thin films have been modified from hydrophilic to hydrophobic. The absorption bands around 1400 cm-1 indicates to symmetric deformation vibration of C-H bonds, represents methyl group attached to silicon atom [22]. The last strong and broad absorption bands are observed at 3400 cm-1 which corresponding to Si-OH vibrations.
The structural comparison between as-prepared and modified silica thin film shows that the as-prepared silica thin film and modified silica thin films, both exhibits the Si-O-Si bonding. The intensity of the prepared silica thin film however is greater than the modified silica thin film. Because of heat treatment, the Si-O-Si band at 821 cm-1 also appeared [51]. The new absorption band is also observed at 1260 cm-1 for modified silica thin film respectively, due to surface modification by TMCS.
4000
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
Wavenumbers, cm-1
Si-O-Si
C-H
Si-C
Si-OH
(e)
(d)
(c)
(b)
Intensity a.u
(a)
Figure 4.11 Infrared spectra of the (a) as-prepared and modified silica thin films at silylation temperature of (b) 40 (c) 60 (d) 80 and (e) 100 °C.
4.4.5 Optical Transmission
Silica thin films are well known for its high transparency. Figure 4.12 shows the percentage of transmission in wavelength range between 200 and 800 cm-1 for as-prepared silica thin film and modified silica thin films using TMCS at silylation temperatures 40 °C, 60 °C, 80 °C and 100 °C. From the figure, it is clearly shows that the as-prepared silica thin film transmitted more light compared to the modified silica thin film. This is due to the fact that when the silica films react with TMCS, the content of Si-CH3 groups attached to the already formed silica clusters from the TEOS precursor is increased. This will decrease in the cross-linking between the clusters leading to particles of non-uniform sizes [21].
Transmission
Figure 4.12 Optical transmittance spectra of the as-prepared silica thin film and modified silica thin films at various silylation temperatures.
The absorption coefficient of as-prepared silica thin film and modified silica thin films with various silylation temperatures have been calculated from 3.80 - 4.50 eV. The absorption coefficient as the function of photon energy at different silylation temperatures are shown in Figure 4.13. All graphs shows almost similar trend where the absorption coefficient is increase with increasing photon energy around 4.10 until 4.40 eV. The absorption coefficient for silylation temperature at 40°C and 80°C exhibit high value of absorption coefficient followed by 60°C and 100°C. It was found that the absorption coefficient is in the range of 0.7-1.7x106 cm-1.alphahv
Figure 4.13 Absorption coefficients, α vs photon energy, hv for as-prepared silica thin film and modified silica thin films at various silylation temperatures.
4.4.6 Optical Band Gap (Eopt)
The graph of ln α vs ln hv is plotted in Figure 4.14 to determine the type of transition of the samples. The gradient, m is calculated base on linear equation and represent as r in the equation as stated below:
αhv = A(hv - Eopt)r (4.1)
m=2.16
m=3.96
m=4.23
m=1.84
m=2.70
m=1.96
m=3.25
m=1.64
m=4.25
m=1.65
m=4.086
Figure 4.14 Graph ln α vs ln hv for as-prepared silica thin film and modified silica thin films at various silylation temperatures.
The calculated gradient values varies within 1.64 - 3.25 which are between allowable indirect transition (r = 2) and forbidden indirect transition (r = 3). However allowable indirect transition (r = 2) shows great dominant points compared to forbidden indirect transition (r = 3). The optical band (Eopt) of the prepared silica thin film and modified silica thin films can be obtained by projecting the vertical linear until it crosses the x-axis in graphs of (αhv)1/2 versus photon energy. The graph of allowable indirect transition is shown in Figure 4.15.
From the optical transmission and optical band gap, we can actually forecast the behavior of the material with the electromagnetic radiation such as light. In this work, the optical bands obtained from Figure 4.15 are in range from 3.60 eV to 3.90 eV. The optical band gap from pure thin film silica gel prepared by Battisha et al. [71] sintering at 1200°C is ~4.3 eV. The slightly difference in the value of the optical band gap might due to the difference temperature in making the thin films.
Apart from that, because of these prepared and modified silica thin films are amorphous, the values of the optical band gap is lower than the crystalline silica thin films or bulk silicone dioxide. The complex infrared spectra that shows in Figure 4.11 revealed that there is an internal stress in the silica films which can lowering the value of the optical band gap. In addition, when the surface of the films was modified with chemical evaporation, the surface energy is lowering thus lowering the optical band gap. In this study, all the silica thin films were prepared at the same parameters thus assuming that all of them have same thicknesses which about 134.88 nm and refractive index about 1.3805. Pure silicon dioxide exhibits refractive index about 1.46. Because of difference in the refractive index itself, the values of optical gap is also varies.
Figure 4.15 The plot of (αhv)1/2 versus photon energy, hv of the silica thin film and modified silica thin films at various silylation temperatures.
The values of optical band gap according to the silylation temperatures and water contact angles are summarize in Table 4.5 and optical band gap, Eopt as a function of silylation temperature, (°C) is plotted in Figure 4.16.
.
Table 4.5 Values of optical band gap obtained experimentally of silica thin film and modified silica thin films at different silylation temperatures with their water contact angles.
Silylation Temperature (°C)
Water Contact Angle (deg.)
Allowable Indirect Transition, Eopt
0 (as-prepared)
57.6
3.70
40
85.0
3.90
60
96.5
3.80
80
97.6
3.90
100
74.3
3.60
The exact pattern of optical band gap with silylation temperature is not properly shows here. However it can be seen that, silylation temperature does influenced the optical band gap of our silica thin films. According to Benno and Joachim [58], the amorphous silicon, the band gap is blurred because there exist many quantum mechanical energy levels outside the actual conduction and valence bands. This is proved from the infrared studies earlier which showed that the silica thin films structures prepared from sol-gel process have complex structures. Moreover, the difference in optical gap is due to the difference intensity of Si-O-Si showed in Figure 4.11.
EG_temperature.JPG
Figure 4.16 The optical gap, Eopt as a function of silylation temperature (°C).
4.4.7 Urbach Energy (Eu)
The Urbach energy can be deduced from a plot of ln α versus photon energy, hv as shown in Figure 4.17. It is important in the low photon energy range where tail state absorption is dominant. The absorption coefficient at the photon energy below the optical gap (tail absorption) depends exponentially on the photon energy as shown in equation 4.2 where Eu is the Urbach energy.
.
α(hv) ~ exp (hv/Eu) (4.2)
The straight line slope in this region is proportional to 1/Eu and the calculated Eu is shown in Table 4.6. It is found that the Urbach energy, Eu is higher at as-prepared and modified silica thin films at 100 °C. Meanwhile, the Eu is decreased with increasing silylation temperature from 40 °C to 80 °C.
D:\MSc\Transmission\UrbachEnergy.JPG
Figure 4.17 The plot of ln α vs hv for as-prepared and modified silica thin films at various silylation temperatures.
Table 4.6 The Urbach energy for as-prepared and modified silica thin films at various silylation temperatures.
Silylation Temperature (°C)
Urbach energy, Eu (eV)
0 (as-prepared)
1.355
40
1.024
60
0.843
80
0.839
100
1.092
According to Zammit et. al. [72] the Urbach energy in fully amorphized samples using ion-bombardment experiments gives a value between 170 meV to 240 meV while Lorenzi et. al. [73] gives the Urbach energy for fluorine-modified sol-gel silica is between 45 and 55 meV which is higher than crystalline quartz and lower than in commercial synthetic commercial silica. However, there is a relevant effect which gives different values in the Urbach energy of amorphous materials. Pan et. al. [74] mentioned that the variation in the properties or values of band tail states due to the topological fluctuations and the overall structure of the network. This finding is compatible with the structural result shows in Figure 4.11 earlier which shows that the as-prepared and modified silica thin films exhibit complex structures. The structure is said to be 'distorted' and consequences of structural relaxation [74].
Because of surface modification with various silylation temperatures range 40 to 100 °C, the silica thin films are affected by thermal disorder which is produced by thermal vibrations, from those of static disorder (frozen) into the amorphous network. This finding is compatible with Vella et. al. [75] whom mentioned that the temperature dependence of the Urbach energy in low temperatures (4 to 300K) is the inverse of the semilogarithmetic slope of the Urbach exponential tail and it is generally considered as an indicator of the degree of disorder of the system. Figure 4.18 shows the inverse relationship between the silylation temperatures and Urbach energy, Eu.
From the Figure 4.18, it is observed a drastic reduction from as-prepared to modified silica thin film. This is due to structural differences existing between wet (high Si-OH content) and dry (low Si-OH content) in the materials [76]. As shown earlier in Figure 4.10, the intensity of Si-OH is quite high for as-prepared silica thin film. The increment in the Urbach energy value at silylation temperature 100 °C might due to the modification process where excess TMCS actually turns back into the properties of as-prepared silica films, respectively.
D:\MSc\Transmission\UrbachTemperature.JPG
Figure 4.18 The Urbach energy, Eu as a function of silylation temperature.
4.4.8 Surface morphologies and element compositions
The field emission scanning electron microscopy (FESEM) images of prepared silica thin film and modified silica thin film are shown in Figure 4.19 for comparison. Figure 4.19 (a) shows the field emission scanning electron microscopy image of the as-prepared silica thin film. The as-prepared silica thin film is smooth and compact surface consists of colloidal silica particles as reported by Oh et al. [61]. Figure 4.19 (b) on the other hand shows the silica particles having rough and textured surface in contrast with the smooth surface of as-prepared silica thin film. The surface morphology of the films really influenced the values of water contact angle.
E:\Huda\Smpl 200 007.jpg
(a)
E:\Huda\Smple 300 006.jpg
(b)
Figure 4.19 FESEM image of (a) as-prepared silica thin film and (b) modified silica thin film.
Figure 4.20 (a) and (b) shows the EDX spectrum of the as-prepared and modified silica films at silylation temperature 80 °C, while the element compositions are shown in Table 4.7 for comparisons. The mass and atomic percentage of C, O, Na and Si became the interest in the analysis. From the analysis, the mass and atomic percent of C increase when silica thin film was modified using TMCS. This is because the precursor TMCS vaporized and reacted with -OH group resulting in the formation of self-assembled film.
0.00
0.80
1.60
2.40
3.20
4.00
4.80
5.60
6.40
7.20
8.00
keV
0
800
1600
2400
3200
4000
4800
5600
6400
7200
Counts
C
O
Na
Si
(a)
0.00
0.80
1.60
2.40
3.20
4.00
4.80
5.60
6.40
7.20
8.00
keV
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Counts
C
O
Na
Si
(b)
Figure 4.20 The EDX spectrum of (a) as-prepared and (b) modified silica thin films at silylation temperature 80 °C.
Table 4.7 Element compositions in as-prepared and modified silica thin film at 80 °C.
Element
Mass %
Atom %
As-prepared
Modified at 80 °C
As-prepared
Modified at 80 °C
C K
3.61
5.37
6.78
10.01
O K
34.49
30.04
48.55
42.03
Na K
1.44
3.76
1.41
3.66
Si K
51.19
50.96
41.05
40.62
From both results we can see the difference in the surface microstructures and element composition between as-prepared silica thin film and modified silica thin film. The modified silica thin film at silylation temperature of 80 °C was chosen for comparison because it has the highest value of water contact angle. On such a binary miro-nano rough structure, water drops were actually suspended with trapped air between water and the rough surface which compatible to Cassie's model [77].
In addition, the surface of modified silica thin film shows hydrophobicity due to the combined micro-nanostructure and low surface energy. During chemical evaporation using TMCS, the large number of methyl groups on the surface lowers the surface energy. This was prove from the Young's equation describe earlier and also from the EDX spectra which shows increment of CH3 composition in the modified silica thin film.
