Summary of Chapter 3

Characterizations of the proposed planar DBD APPJ system are summarized as follows:

(1) Direct visualization and OES spectra aided to our understanding the changes in

plume color.

(2) OES measurements showed that abundant metastble nitrogen was generated in the post-discharge jet region, which was important in later applications. However, addition of oxygen species reduced the amount of metastable nitrogen tremendously because of reduced plasma density due to electro-negativity of the oxygen.

(3) Gas temperature measurements showed that gas temperatures were low enough in the post-discharge jet region for most of the applications studied in the thesis.

(4) Measurements showed that addition of oxygen enhanced the amount of ozone, which were important in bacteria inactivation.

Chapter 4

Hydrophilic Modification of the PP Film Using Nitrogen-Based DBD APPJ

In this chapter, hydrophilic modification of the PP film using the developed planar nitrogen-based DBD APP system has been described in detail. Details of the experimental setup and characteristics of the discharge have been shown in Chapter 2 and Chapter 3, respectively, and are skipped here for brevity. Several diagnostic techniques, including contact angle measurements, aging effect, AFM analysis and XS analysis, were employed to study the effect of plasma jet treatment on the hydrophilic property and their corresponding results are described in the following in turn. Some important findings have been summarized at the end of this Chapter.

4.1 Contact Angle Measurements

4.1.1 Stationary Conditions

Figure 4-1 shows the distribution of the measured contact angles of the PP film as functions of the downstream distance (z=2-20 mm) and the ratio of O₂/N₂ (%) (0.004%- 1.6%) immediately after the plasma treatment. Note that the measurement of the contact angle of the untreated sample was 103°. The results were obtained by averaging 3-5 measurements over a region of 10 mm by 10 mm of PP film with ±3°. The results indicated that the favorable operating conditions (contact angles < 30°) included: 1) z=6-9 mm and O2/N2 <0.008%; 2) z=2-10 mm and 0.04%<O2/N2 <0.3%; and 3) z=0-6 mm and 0.6%<O₂/N₂ <1.0%. The above observations could be explained by the

distributions of ozone and UV emission in the near jet region. It was interesting to learn that the lowest contact angle (23°) occurred at the ratio of O2/N2 near 0.06% at z=2 mm in Region 2, which was very close to the exit of the DBD. This could be attributed to the strong UV emission (200-300 nm) near this condition (see Figure 3-9), where the ozone could efficiently absorb and convert into an oxygen radical [Iwasaki et al., 2007], and then easily react with the surface, as will be demonstrated later. In addition, UV emissions in the range of 200-300 nm also played an important role in breaking the chemical bonds of C-H (4.2 eV) and C-C (3.8 eV) since, for example, the photon energy of 236.6 nm was 5.74 eV. In Region 1, the ozone concentration had a maximal value at z=6-9 mm (as shown in Figure 3-14) due to the entrainment of oxygen from the ambient, where the UV emission was still appreciable in order for the ozone to convert efficiently into an oxygen radical. In Region 2, both ozone concentration and UV emission were appreciable, which led to the production of a significant amount of oxygen radicals. In addition, as mentioned earlier, strong UV emission also played an important role when the PP film was non-stationary, as will be discussed next. In Region 3, the ozone concentration was very high (40-80 ppm) (see Figure 3-14), although UV emission became relatively low in the downstream region after z=10 mm (see Figure 3-9).

4.1.2 Non-Stationary Conditions

The measured contact angles for the non-stationary films have been summarized in Tables 4-1 and 3-3 at z=2 mm. Table 4-1 lists the contact angles for various O₂/N₂ ratios (0-1.6%) at a non-stationary speed of v=0.5 cm/s, while Table 4-2 summarizes the data at various non-stationary speeds for PP film (0.5-8 cm/s) for four typical O₂/N₂ ratios in which the contact angles were in the range of 26-39°, as shown in Table 4-1. Only the cases of z=2 mm for the non-stationary films have been presented here, since we found

that the contact angles became very large after this distance (e.g. >70° at z=4 mm with O₂/N₂ ratio of 0.06%) because of a low residence time and very low UV emission at further downstream locations. At the distance of z=2 mm, we found that the minimal contact angle occurred at O₂/N₂ ratios in the range of 0.06-0.2%. However, as the non-stationary speed increased to v=1.0 cm/s, only at the condition of an O2/N2 ratio of 0.06% was the contact angle low enough (33°), while all other contact angles were greater than 45°. This was attributed to the very strong UV emission that occurred when the ratio of O₂/N₂ was between 0.02% and 0.2% near the jet exit (as shown in Figure 3-9). This led to a direct breaking of the surface chemical bonds, such as C-H (4.2 eV) and C-C (3.8 eV), without the help of oxygen radicals. Unlike the stationary film, the residence time on the surface for the conversion of ozone into an oxygen radical (very short life-time) by absorbing UV emissions (200-300 nm) became increasingly insufficient with the increasing non-stationary speed of the PP film. Thus, only the direct bond breaking by the UV emissions played an important role in the surface modification of non-stationary PP film. In general, the contact angle increased with the increasing non-stationary speed of the PP film. As the speed reached v=8 cm/s, surface modification of the PP film using a nitrogen-based DBD jet became effectively useless under the current operating conditions.

4.2 Aging Effect

Figure 4-2 presents the typical aging test within 24 h (at room temperature) of the improved hydrophilic property (contact angle) of stationary and non-stationary PP films after plasma treatment. The results showed that for all the cases as presented in Figure 4-2, the contact angles were all less than 30° right after plasma treatment. However, the contact angles for the non-stationary PP films rose very quickly to ~80° (v=0.5 cm/s)

and ~90° (v=1.0 cm/s) after 2 h of exposure to air and became saturated until the end of the test (24 h), while the contact angles for the stationary PP films increased 5-15° after the first 2-4 h and then became saturated at ~40-50° until the end of the test. This showed that the improved hydrophilic property, formed because of the bond breaking by the UV emission (non-stationary PP film), deteriorated quickly; however, the one caused by both the UV emission and oxygen radical (stationary PP film) could be sustained much longer. This will be further explained by the XPS analysis.

4.3 AFM Analysis

Table 4-3 summarizes the typical roughness data of stationary PP film, as measured by AFM, after treatment using the pure nitrogen discharge jet. Note that the RMS roughness data were taken from a 3µm by 3µm section. The untreated RMS roughness was 40.4 nm, but it decreased to 17.8 nm at z=6 mm and gradually increased at further downstream locations (28.6 nm at z=20 mm). This trend coincided with that of the measured contact angles, in which a minimal value occurred in the range of z=6-8 mm. This meant that the less the level of roughness, the smaller the contact angle. The DBD treatments thus produced various effects, such as the removal of contaminants, oligomers and amorphous layers existing on the surface, allowing for chemical activation of the material [Borcia et al., 2006]; however, this will require further investigation in order to understand the underlying physics.

4.4 XPS Analysis

Figure 4-3 shows the XPS scan spectra of untreated and treated stationary PP (N₂+0.06% O₂) films. After plasma treatment, the O1s peak increased dramatically, while the C1s peak decreased in a milder fashion. This meant that an appreciable

amount of oxygen radical had been incorporated into the surface compound, while carbon had been removed efficiently.

4.5 Discussion

Table 4-4 summarizes the corresponding quantitative atomic compositions and the fractions of peak area of untreated and treated samples, as calculated from the C1s core level spectra on the surface of the PP film for the various typical test conditions. The results showed that after plasma treatment, tremendous increases in the O/C ratio at z=2 and 6 mm were found. In the stationary cases, at z=2 and 6 mm, we found a large increase in the O/C ratio. A possible explanation could be that at z=2 mm, the ozone absorbed the abundant NO-γ UV emission (200-300 nm) and produced abundant highly chemically active oxygen radicals by photo dissociation [O₃+hυ→O P(³ )+O₂] which further reacted with the PP film. The N2 1^st positive lines [N²(B³

∏

_g)→N²(A³

∑

_u⁺)^]

(500-700 nm in Figure 3-8a) show that abundant metastable N2₍A³

∑

_u⁺₎ (energy=6.2 eV) was generated, which could be transported to the substrate by the jet stream and easily break the C-C (3.8 eV) chemical bonds, as has been observed previously [Herron 1999;

Klages et al., 2008]. At downstream z=6 mm, the UV emission became weaker but the ozone became more abundant because of ambient entrainment, and it could still produce appreciable amounts of oxygen radicals. In addition, the amount of long-lived metastable _N2_(A³∑_u⁺₎ was still high enough to reach the PP film and break its chemical bonds. The above two mechanisms resulted in a tremendous O/C increase at both z=2 and 6 mm. However, at the further downstream position z=20 mm, no UV emission was possible and the amount of metastable _N2_(A³∑_u⁺₎ became smaller, making the O/C ratio increase less obvious, as shown in Table 4-4.

For example, the O/C ratio increased from 0.1 to 0.9 for stationary PP film at z=6 mm for the condition of N2+0.06% O2, in which the measured contact angle was 23°.

This again confirmed that the polar functional groups containing oxygen were introduced on the PP film surface. By comparing the corresponding measurements of the contact angles in Table 4-4, we concluded that the larger the O/C ratio, the smaller the contact angle. Figure 4-4 presents a graph by separating the peak of the samples in the C1s level through curve fitting. As shown in Figure 4-4, the polar functional groups, such as C-O (286.8 eV), C=O (287.8 eV) and COO (289.7 eV), were introduced on the PP film surface after an AP nitrogen plasma treatment, although the fraction of COO was very small and less than the experimental uncertainties. Nevertheless, these could be only considered qualitative since they were fitted values.

The results showed that the greater the amount of functional groups C-O and C=O, the better the hydrophilic property of the PP film. In addition, the appearance of the C=O group may have explained the longer measured aging time for stationary film cases as found earlier (see Figure 4-2), given the higher binding energy as compared to that of C-O. As trace oxygen was added into the nitrogen (O₂/N₂=0.06%), more ozone was generated (see Figure 3-14) and more UV (200-300 nm) emitted (see Figure 3-9), which produced more abundant oxygen radicals to react with the PP film. This was confirmed by the increase of the O/C ratio (0.8-0.9) as compared to the pure nitrogen case (0.7) in Table 4-4, resulting in a better hydrophilic property at z=2 mm. At z=6 mm, much lower UV emission caused the role of this mechanism to be less important than the metastable nitrogen, as can be seen from almost the same contact angle (23° vs.

26°).

Based on this observation, methods to efficiently incorporate C=O and even COO functional groups, which have higher binding energies, are sure to become an important issue for longer aging time and should be subject to further investigation.

4.6 Summary of Chapter 4

Major findings of the study of hydrophilic modification of the PP film using the developed planar DBD APPJ system are summarized as follows:

(1) The DBD jet was applied to treat the PP film under stationary and non-stationary conditions with various O2/N2 ratios ranging from 0 to 1.6% at different treating distances in the range of 2-20 mm.

(2) Results show that, for stationary PP films, the surface hydrophilic property improves dramatically from 103° (untreated) to a value less than 30° (treated) for contact angles with a wide range of O2/N2 ratios (< 1%) and treating distances (< 10 mm). For the non-stationary PP films, highly hydrophilic surfaces can only be obtained when the PP film is placed near the jet exit (treating distance of 2-4 mm) with an O₂/N₂ ratio of 0.06-0.2%.

(3) These observations are explained through measured optical emission spectra and ozone concentration data, in which the metastable nitrogen plays a key role in breaking the surface chemical bounds and UV emission (200-300 nm) participates in the process of converting the ozone into oxygen radical. Aging tests show that for stationary PP films the contact angle can still be maintained at

~40° after 24 h, while for the non-stationary test cases it can only be maintained at ~80-90° when the non-stationary speed is near 1 cm/s.

(4) By AFM analysis, we also observed that the less the surface roughness the smaller the contact angle is. Finally, XPS analysis shows that O/C ratio increases dramatically which results from the incorporation of several polar functional groups containing oxygen into the surface of PP films during plasma treatment.

The greater the amount of functional groups C-O and C=O, the better the

hydrophilic property of the PP film.

(5) In addition, the number of polar functional groups, such as C=O, having higher binding energy can directly influence the aging time of the hydrophilic property of PP film after the plasma treatment.

Chapter 5

Surface Cleaning of the ITO Glass Using Nitrogen-Based DBD APPJ

In this chapter, surface cleaning of ITO glass using the developed planar nitrogen-based DBD APP system has been described in detail. Details of the experimental setup have been shown in Chapter 2 and are skipped here for brevity.

Several diagnostic techniques, including contact angle measurements, aging effect, XPS analysis and AFM analysis were employed to study the effect of plasma jet treatment on the hydrophilic property and their corresponding results are described in the following in turn. Some important findings have been summarized at the end of this Chapter.

5.1 Contact Angle Measurements

5.1.1 Stationary Conditions

Figure 5-1 shows the measured contact angle (CA) of the stationary ITO glass surface after the plasma jet treatment as a function of both the downstream distance and the ratio of O2 to N2 (%). The contact angle before plasma treatment was 84°. The results showed that there existed two distinct regimes with lower CAs in the range of 20-30°. The first one was the regime with an oxygen addition of less than 0.05% and a treating distance in the range of 6-16 mm. The second one was the regime with an oxygen addition larger than 0.06% and a treating distance in the range of 2-10 mm.

Measured CAs in both regimes were less than 30° in general; for some conditions in the first regime they were well below 25°, demonstrating that this post-discharge jet region could be used to effectively improve the hydrophilic property of ITO glass by adding

trace amounts of oxygen.

5.1.2 Non-Stationary Conditions

Figures 5-2a and 5-2b show the measured contact angles (CA) of the ITO glass surface after pure N₂ and 0.04% O₂/N₂ plasma jet treatment for the non-stationary case (1-9 cm/s) as a function of downstream distance, respectively. The measured contact angle generally increased with increased non-stationary speed at the fixed downstream distance from the jet exit, which meant that the hydrophilic property deteriorated because of the reduced effective treatment time. However, the range of smaller contact angles for the 0.04% O2/N2 plasma jet treatment was obviously wider than that of the pure N₂ one; this showed that the addition of a trace amount of oxygen produced a wider operating window. These results are explained below.

5.2 XPS Analysis

Table 5-1 summarizes the measured chemical composition of the ITO glass surface using XPS analysis and the measured CAs after plasma jet treatment. Clearly, the O/C ratio increased dramatically from 1.16 (untreated) to 2.95-2.96 (z=10 mm for pure nitrogen and 0.06% oxygen cases), which was similar to the result in [Yi et al., 2004].

This meant that some carbon atoms were effectively removed by the plasma jet, which could be explained by considering the measured concentrations of O₃, UV emission (e.g., 236.6 nm, NO-γ) and other OES spectra in the jet region, which are described next.

5.3 AFM Analysis

Table 5-2 summarizes the typical roughness data of stationary ITO glass under

various test conditions after surface treatment using the pure N2 and O2/N2 (0.1%) discharge jet with different distances. Note that the RMS roughness data were taken from a 2 mµ by 2 mµ section. All RMS roughness was in the range of 0.53-0.69 nm, signifying that the application of APPJ did not significantly modify the surface morphology. APPJ only acted to remove the attached organic contaminants, which was important for this type of application.

5.4 Discussion

These data have shown that the amount of metastable nitrogen decreased rapidly with the addition of too much oxygen, since oxygen is an electronegative gas which has a very strong electron affinity. However, the quantitative reason for observing the maximal amount of the 1st positive N₂ at O₂/N₂=0.06% is still unknown and deserves further investigation. Note that the metastable N₂ [N₂(A³

∑

_u⁺)] energy state is 6.2 eV above the ground state and its lifetime is ~13 s [Fridman et al., 2004]. It is very reactive towards saturated hydrocarbons and can transfer about 6.2 eV efficiently to these molecules to generate dissociative triplet states that break C-H bonds (4.2 eV) and C-C bonds (3.8 eV) [Herron 1999; Klages et al., 2008]. This effect may have been especially important in improving the hydrophilic property of ITO glass at the far downstream locations where the amount of O₂/N₂ was very small (e.g., 0.05-0.06%), and the amount of ozone was also very small (see Figure 5-1).

In the first regime (nearly pure nitrogen plasma jet), as indicated in Figure 5-1, at the exit of the DBD, the NO-γ UV emission was relatively appreciable (see Figure 6) since the metastable N₂(A³

∑

_u⁺) in the early portion of the plasma jet was still abundant, although the NO concentration would be low under this condition. Note that

the residence time of the N₂ [N₂(A³

∑

_u⁺)] species for the current test condition was 0.1 ms up to z=16 mm, which was much shorter than its lifetime (~13 sec). Although the amount of NO-γ UV emission decreased rapidly in the downstream direction (because of ambient quenching of the NO and the short-lived excited N₂(B³

∏

_g)), the amount

of long-lived metastable N₂(A³

∑

_u⁺) was still high enough to transfer energy effectively to break the C-H and C-C bonds of the organic compounds on the ITO glass surface.

In the second regime, as indicated in Figure 5-1, at the exit of the DBD, the NO-γ UV emission peaked at ~0.05% of oxygen addition and then dropped rapidly due to the quenching of increased oxygen addition (reduced metastable N2 species), as shown in Figure 3-9. In this peak region near the DBD exit, a high intensity of NO-γ UV emission produced better surface cleaning because of more atomic oxygen O produced following the reaction path ( O₃ +hυ →O P(³ )+O₂), and the abundant metastable N2 [N₂(A³

∑

_u⁺)] transferred energy to break bonds such as C-C (3.8 eV) and C-H (4.2 eV). As more oxygen was added (e.g., 1%), the amount of atomic oxygen decreased accordingly (see Figure 3-9) because the NO-γ UV emission was much lower than in the first regime.

However, in the further downstream location after z=10 mm, the NO-γ UV emission was reduced to a very small amount, as atomic oxygen O could not be produced effectively; the long-lived N₂ [N₂(A³

∑

_u⁺)] then played a key role in surface cleaning, although the CA increased rapidly after this point.

5.5 Summary of Chapter 5

Major findings of the study of surface cleaning of ITO glass using the developed planar DBD APPJ system are summarized as follows:

(1) Results showed that there existed two distinct regimes having lower CAs in the range of 20-30°. Possible mechanisms of surface cleaning have been presented;

they took into account the spatial distribution of O₃, NO-γ UV emission and OES spectra.

(2) For ITO cleaning mechanisms, in the near jet downstream location (z<10 mm), both the metastable N2 [N₂(A³

∑

_u⁺)] and ozone photo-induced dissociation played dominant roles in cleaning ITO glass, although their relative importance was unclear and requires further investigation.

(3) In the far jet downstream location (z>10mm), when the ratio of O₂/N₂ was small, only the long-lived metastable N₂ [N₂(A³

∑

_u⁺)] played a major role in cleaning ITO glass. One final note on using nitrogen as the discharge gas is that nitrogen was easily recycled to a high purity (>99%) using a ceramic membrane [Baker, 2002].

Chapter 6

Inactivation of the E. coli and the B. subtilis Using Air-Based DBD APPJ

In this chapter, inactivation of the E. coli and B. subtilis using the developed planar air-based DBD APP system has been described in detail. Details of the experimental setup and the characteristics of the discharge have been shown in Chapter 2 and Chapter 3, respectively, are skipped here for brevity. Direct visualization of the Petri dish with bacteria inside and SEM images before and after plasma jet treatment are described in

In this chapter, inactivation of the E. coli and B. subtilis using the developed planar air-based DBD APP system has been described in detail. Details of the experimental setup and the characteristics of the discharge have been shown in Chapter 2 and Chapter 3, respectively, are skipped here for brevity. Direct visualization of the Petri dish with bacteria inside and SEM images before and after plasma jet treatment are described in

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