Contact Angle Relaxation and Long-Lasting Hydrophilicity of

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Contact Angle Relaxation and Long-lasting Hydrophilicity of Sputtered Anatase TiO Thin Films by Novel Quantitative XPS Analysis 2

Min-Kyo Lee, and Young Chun Park Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03258 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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Manuscript

Contact Angle Relaxation and Long-lasting Hydrophilicity of Sputtered Anatase TiO2 Thin Films by Novel Quantitative XPS Analysis

Min-Kyo Lee, Young-Chun Park* School of Computer Science and Electrical Engineering, Handong Global University, Pohang 37554, Republic of Korea

*Corresponding

Author.

E-mail address: [email protected] (Y. C. Park) Tel.: +82 54 260 1933; Fax: +82 54 260 1976

ACS Paragon Plus Environment

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Abstract The contact angle relaxation of TiO2 surfaces is an important problem that must be understood, particularly for long-lasting hydrophilicity under dark conditions. The relaxation of sputtered anatase TiO2 thin films over a long time (~22 days) in an atmospheric environment was observed using quantitative XPS analysis. A new peak was identified as H2O within a donoracceptor complex at ~2.57 eV above the lattice oxygen peak. This donor−acceptor complex turns out to be a key factor for long lasting hydrophilicity, and our model is presented. Adventitious carbon contamination was not the main cause of the contact angle relaxation. Instead, samples with lower amounts of donor−acceptor complexes (IDAC/Ibulk ≤ ~5%) underwent contact angle relaxation over time, and samples with a high density of donor−acceptor complexes (IDAC/Ibulk ≥ ~10%) showed good hydrophilicity (contact angle ≤ 20°) over 22 days. Larger amounts of basic Ti−OH relative to acidic OHbridge (ITi−OH/Ibridge ≥ 1) resulted in greater amounts of donor−acceptor complexes (IDAC/Ibulk ≥ ~10%). Thus, basic Ti−OH groups interact with H2O by forming a strong electrostatic donor−acceptor complex, leading to long-lasting hydrophilicity. Indeed, TiO2 was transformed to show long lasting hydrophilicity by high-density oxygen plasma treatment by forming sufficient Ti−OH groups and H2O molecules in the donor−acceptor complexes. Contact angle relaxation is closely related to the interactions between water molecules and the TiO2 surface in the dark. It is suggested that the relaxation depends on the number of electrostatic donor−acceptor complexes. This study provides new insight by linking theoretical studies with the experimental contact angle at the TiO2 surface in an ambient environment and is the first study that provides the presented relaxation mechanism.


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1. Introduction Titanium dioxide (TiO2) is a representative photocatalytic oxide showing strong hydrophilicity. Numerous researchers have studied the relationship between UV light and the TiO2 surface to understand the mechanism of photoinduced hydrophilicity. They mostly have focused on the contact angle right after UV exposure. However, the mechanism of contact angle relaxation in the dark has not been well understood yet. Some researchers argued that the relaxation is caused by carbon contamination.1,2 Another proposed mechanism is that hydrophilicity of TiO2 is due to new hydroxyl groups (Ti−OH) induced by UV light, and that the surface can return to being hydrophobic by losing the photoinduced hydroxyl groups in the dark.3 In this study, the long-term hydrophilicity mechanisms for contact angle relaxations in the dark are studied using XPS and then a model is proposed. The contact angle relaxation is closely related to the interactions between water molecules and the TiO2 surface. Water can adsorb molecularly and dissociatively on a TiO2 surface. In dissociative adsorption, the H2O molecule dissociates into an OH and a H atom.4−16,23 The hydroxyl group fills an oxygen vacancy site (Lewis basic site) or binds with a Ti atom (Lewis acidic site) at the surface, resulting in a Brønsted acidic OHbridge (pKa ≈ 2.9) and a Brønsted basic Ti−OH (pKa ≈ 12.7), respectively.6,12 The H atom also forms an acidic bridging OH (OHbridge) by binding to another bridging O2C atom. In molecular adsorption, two different types of interactions can occur between a H2O molecule and active surface sites: a donor−acceptor complex can form4,8,10,17−22 and H-bonding can occur.5,17−19 A donor−acceptor complex is a complex with an electrostatic interaction between H2O∙∙∙OHTi−OH (hydrogen donor−acceptor), OHbridge∙∙∙H2O (hydrogen donor−acceptor), H2O∙∙∙Obridge (hydrogen donor−acceptor), or H2O∙∙∙Ti (electron donor−acceptor). Lastly, H-bonding is a well-known interaction among neighboring Obulk atoms or H2O molecules.


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A detailed understanding of the mechanism is necessary, specifically focusing on which of the proposed interactions have more influence on the contact angle. Atomic-level studies of both the structural and dynamic behavior of the water−TiO2 interface have been conducted over the last few decades. Dynamics in the first few water molecule layers is critical to determine the adsorption of liquid water.4,22 Electrostatic interactions between H2O molecules and TiO2 at the surface may provide a stronger complex than H-bonding in bulk liquid due to the strong charge polarity.10,20−25 Huang et al.25 reported that the surface hydroxyl groups (OHbridge and Ti−OH) affect the dipole orientation of water molecules by the electrostatic H-bond complex in the first layer of the water molecules, bringing the second layer of water closer to the surface. Further, the Ti−OH group has been reported as a key factor to determine surface reactivity and drive hydrophilicity.15,25,26 However, these theoretical results have not been clearly correlated with experimental results that represent the actual relationship between contact angle and the TiO2 surface. Here, an experimental study on contact angle relaxation and long-lasting hydrophilicity of anatase TiO2 thin films is presented using contact angle measurement and XPS quantitative analysis. Unlike some researchers who use single crystal TiO2 and mainly focus on H2O directly adsorbed on Ti or O atoms, polycrystalline anatase TiO2 thin films were deposited by DC magnetron sputtering, which can be used for practical industry applications. We discuss whether adventitious carbon contamination is the cause of relaxation or not. In the desorption experiment using XPS, the new O 1s peak is discovered as donor−acceptor complex (D.A.C.) and verified by temperature desorption experiment. The results are compared with the conventional XPS peakfitting analysis. Over 22 days, water-related adsorbates and contact angles are observed in order to understand their dynamics as a function of time. After 22 days, it is confirmed that


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donor−acceptor complex (D.A.C.) induced mainly by Ti−OH is the key factor to determine the contact angle relaxation and long-lasting hydrophilicity. It is further discussed for practical application that TiO2 can be transformed to show long lasting hydrophilicity by high-density oxygen plasma treatment. The focus of this study is on their fundamental mechanisms by investigating donor−acceptor complex at water−TiO2 interface. This study provides new physical/chemical insights into the fundamentals of contact angle relaxation against the previous arguments.

2. Experimental Methods 2.1. Preparation of TiO2 Thin Films TiO2 thin films were deposited using a DC magnetron sputtering system with a 3-inch diameter Ti (99.995%) target disc. The DC power was fixed at 600 W, and the distance between the target and substrate was about 80 mm. All samples were reactively sputtered onto glass substrates (20 × 20 mm2) using pure Ar and O2 gases (≥99.99%). Before the deposition, substrates were cleaned in an ultrasonic cleaner for 5 minutes with acetone and then isopropyl alcohol, followed by rinsing with de-ionized water for 10 minutes. In our previous study, the contact angle varied among samples deposited with various deposition parameters.27 In-situ pure anatase crystals tended to form at higher total pressure (≥ 2 Pa) and a mixture phase of anatase and rutile were formed at 1 Pa of total pressure. Interestingly, TiO2 thin film tended to grow as A(211) oriented polycrystalline with the higher oxygen partial pressure (≥~5.67, PO2/PAr) in sputtering deposition. It was further observed that the higher the XRD intensity ratio of A(211)/A(101), the lower the contact angle measured 24 hours after UV exposure. In the following, four representative deposition conditions were selected because the sputtered TiO2 films show different A(211)/A(101)


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ratios and are expected to show different contact angle relaxation over time. The detailed deposition parameters are shown in Table 1. Table 1. Deposition Parameters and Characteristics of TiO2 Thin Films No.

PTa [Pa]

tsb [min]

PO2/PArc

Phased

Rae [nm]

df [nm]

A(211)g

#1

1

23

2.3

A+R

4.827

318

14.8%

#2

2

16.6

1.22

A

5.146

429

33.6%

#3

2

25

5.67

A

4.002

327

43%

#4

2

40

5.67

A

5.356

610

152%

aP T

: Total pressure of the chamber. bts : Sputtering time. cPO2/PAr : The ratio of oxygen partial pressure to argon partial pressure. d‘A’ and ‘R’ in phase represent anatase and rutile, respectively. eR : Surface roughness. fd : Film thickness. gA(211) : XRD intensity ratio of A(211) to A(101). a

2.2. Contact Angle Experiments and Characterization The contact angle relaxation of various sputtered TiO2 thin films was observed. Four samples with different conditions were sputtered as one set in the same day, and each set (four sets in total) was deposited at a different point in time over a long period. All samples were irradiated with UV light (UV−A, 33 W/cm2) in the air for 10 minutes to initialize the contact angle. They were then kept dry at ~15% humidity using dehumidifier crystals in an airtight container. Two specimens of each sample were used to analyze the surface. One (12 × 12 mm2) was for the contact angle measurement using a contact angle analyzer (SmartDrop Lab, FemtoBioMED inc.) with 1 µL droplets (deionized pure water, 99.9%). The other (6 × 6 mm2) was for quantitative analysis using X-ray photoelectron spectroscopy (ESCALAB™ 250Xi XPS, Thermo Scientific) with focused, monochromatized Al Kα (hυ = 1486.6 eV) radiation. Charge compensation was required with an electron flood gun for the analysis of insulating TiO2 samples. The spectra (Ti 2p, O 1s, and C 1s) were acquired with 10 scans, a take-off angle of 90°, and a pass energy of 50.0 eV in UHV (~10-8 Torr). The hydrocarbon contaminant was taken to be 284.5 eV as the reference peak


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position. Then, peak fitting was performed with proper parameter conditions including a Shirley background, Lorentzian-Gaussian ratio ( #3). Based on XRD spectra reported in our previous work,2 Figure 1 indicates that pure anatase TiO2 has better hydrophilicity than the samples having a rutile phase (#1) under dark conditions. The number of active sites (oxygen vacancies or Ti4⁺ atoms) affecting the surface energy and charge polarity depends on the surface structure (crystallinity). Thus, the crystallinity has an impact on the contact angle relaxation process for relaxing samples. However, some samples in Figure 1a show long-lasting hydrophilicity (contact angle < 20° for 22 days), and it appears that the crystallinity is not the only factor. Thus, some other factors determine the hydrophilicity over time, considering that #2, #3, and #4 conditions all showed long-lasting hydrophilicity.


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Figure 2. Change of [C]/[Ti] ratio over time (Figure 1b). The effect of carbon contamination on the relaxation process was first investigated by quantitative analysis using XPS. XPS C 1s spectra were measured every week during the contact angle experiments (Figure 1b). Figures S1 and 2 show raw data of the XPS C 1s spectra and the relationship between adventitious carbon and contact angle, respectively. Peak fitting was


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performed with three different peaks: C−C (284.5 ± 0.03 eV), C−O (285.8 ± 0.03 eV), O−C=O (288.4 ± 0.02 eV). As shown in Figure 2, the C−C peak which is the majority of total adventitious carbon exhibits different trends among the samples. The other peaks are a small portion of the total carbon and show insignificant changes over time. Therefore, there is no clear relationship between the contact angle and adventitious carbon (also shown in Figure S3). All samples have the same probability of carbon contamination under an ambient environment, but they show different contact angle relaxations in Figure 1b. Even for #3 sample, the contact angle remains low even though the amount of adventitious carbon seems to increase. These results show that carbon contamination is not the main cause of the contact angle relaxation. In addition, #1−4 samples have similar surface roughness (Ra) in Table 1 and Figure S2, but their contact angles show different relaxation profiles over time (Figure 1). Therefore, the contact angle relaxation on anatase TiO2 thin films is not only due to physical factors (crystallinity, carbon contamination, or surface roughness) but also other physical/chemical reactions on the TiO2 surface. Now we focus on the water-related adsorbates on TiO2 surface and observe the relationship between the adsorbates and contact angles over time.

3.2. Relationship between Surface Hydroxyl Groups and Contact Angle Relaxation As mentioned above, water molecules can be adsorbed as four different adsorbates on a TiO2 surface: OHbridge, Ti−OH, H2O in a donor−acceptor complex, and H-bonded H2O molecules. Since desorption experiment is an effective method to analyze water-related surface adsorbates, the reported knowledge was applied to in-situ temperature programmed XPS analysis. However, in previous XPS studies, only three water-related peaks of the O 1s signal have been reported. They are OHbridge, Ti−OH, and H-bonded H2O peaks, and their peak positions vary in the literature.


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For instance, the OHbridge peak position has been reported at 0.9−1.6 eV above the lattice oxygen peak.8,10,12−16,29−31 The reported Ti−OH peak position range also varies 2.0−2.7 eV above the lattice oxygen peak.8,10,12−16,29,30 These OH peaks show very wide fitting position ranges. Although all XPS instruments have their own observational errors, it seems that there should be another waterrelated peak in the range of 0.9−2.7 eV above the lattice oxygen peak. The H-bonded H2O peak identified at 3.2−3.5 eV above the lattice oxygen peak shows a relatively stable position range.8,10,29,30 For accurate analysis of the relationship between hydroxyl groups and contact angle relaxation, it is necessary to have better curve fitting of water-related O 1s peaks at the TiO2 surface, so a new peak fitting with four peaks is proposed. Two representative samples (#2 and #3) showing different contact angle relaxations (Figure 1b) were used to observe a clear difference, and their O 1s signals were in-situ measured at different temperatures.


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Figure 3. XPS O 1s spectra of two TiO2 samples at different temperatures (293, 333, 453, and 573 K). #2 and #3 represent the typical TiO2 samples having few hydroxyl groups and a large number of hydroxyl groups, respectively. Heating rate β was ~5 K/min. Figure 3 shows the XPS O 1s spectra of two selected TiO2 samples at different temperatures (293, 333, 453, and 573 K). In this experiment, water-related surface adsorbates desorbed with increasing temperature. However, some adsorbates remained on the #3 surface at 573 K. Some of them further increased even though the temperature increased. As mentioned above, the peak fitting of these surface adsorbates was carried out with four peaks unlike the literature. First, two peaks at lower binding energies were assigned as dissociative adsorbed OH peaks, which are OHbridge (blue line) and Ti−OH (green line). They were at 1.2 ± 0.1 eV and 2.0 ± 0.03 eV above the lattice peak, respectively. The other two peaks at higher binding energies were assigned as molecular adsorbed H2O peaks, which are H2O in donor−accepter complexes (red line)


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and H-bonded H2O (orange line). They were fitted at 2.57 ± 0.03 eV and 3.25 ± 0.05 eV above the lattice peak, respectively. Detailed information related to peak-fitting is shown in Table 2. Table 2. XPS Quantitative Analysis and Peak Position of O 1s Peaks IOH/Ibulka [%]

Peak

Erelb [eV]

293 K

333 K

453 K

573 K

293 K

333 K

453 K

573 K

1

OHbridge Dissoc.c

#2

15

10.7

8.28

8.11

1.2

1.1

1.2

1.2

#3

11.4

10

11.9

14.5

1.2

1.2

1.2

1.2

2

Ti−OH Dissoc.c

#2

7.84

6.9

5.94

3.04

2.0

2.0

2.0

2.0

#3

15.7

18.4

22.2

22.4

2.01

1.98

1.98

1.97

3

D.A.C.d

#2

5.54

3.8

0.88

0.5

2.57

2.57

2.57

2.57

#3

22.1

19

11.4

6.67

2.55

2.55

2.57

2.57

4

H-bonded H2O

#2

1.32

#3

7.69

3.25 5.69

3.25

aI

3.25

: O 1s signal (area) ratio of hydroxyl groups to the oxide bulk. bErel : the difference in binding energy between a sub-peak and the bulk-peak. cDissoc. denotes dissociative adsorbates. dD.A.C. stands for donor−acceptor complex. OH/Ibulk


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Figure 4. Change in O 1s signal ratio IOH/Ibulk with temperature. Figure 4 shows a graph of the quantitative analysis of XPS O 1s signals (Figure 3) with increasing temperature. In case #2 shown in Figure 4a, the H-bonded H2O peak disappears when the temperature reaches 333 K. Donor−acceptor complexes also decrease in the temperature range of 293–453 K and are very slowly desorbed after 453 K because the desorption energy of low H2O coverage is higher than that of high H2O coverage.5 Thus, Figure 4a indicates that H-bonded H2O molecules are easier to remove than donor−acceptor complexes. This result is consistent with a previous report that the desorption activation energy of the complex is higher than that of Hbonding among water molecules.18 According to the literature,3,18,32,33 H2O molecular desorption at a defective TiO2 surface occurs in the range of 300−500 K. Gun’ko et al.18 observed two peaks (Tmax = ~340 and 400 K) due to the difference in the energy of formation and sizes of the H-bonded clusters of water molecules. Assuming that the two peaks (Tmax = ~340 and 400 K) reflect the


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desorption of weakly H-bonded H2O and desorption of strongly H-bonded H2O (i.e., H2O in a donor−acceptor complex), respectively, this result is in complete agreement with our data. Dissociative adsorbed hydroxyl groups (OHbridge and Ti−OH) in Figure 4a are also desorbed between 300 and 573 K, where two types of desorption take place. One is associative desorption, which is reported to occur between 320 and 700 K (Tmax = ~550 K).3,17,18,32,33 Neighboring hydroxyl groups that are H-bonded together (vicinal) can be desorbed by forming H2O molecules at the surface (Hbridge + OHTi−OH → H2O and dense OH islands → H2O).17,18 The other type is dissociative desorption (Ti−OH → OH− and Ti−O(H) −Ti → OH−), which is reported to occur between 450 and 800 K (Tmax = ~650 K).18,32,33 This desorption is relatively difficult because the energy for bond cleavage of surface hydroxyl groups is very high (Ebridge≠ ≈ 790 kJ/mol and ETi−OH≠ ≈ 1,019 kJ/mol).17 Thus, desorption of hydroxyl groups does not actively occur after 453 K in Figure 4a. Compared to Figure 4a, desorption also takes place in Figure 4b. However, the amount of the surface hydroxyl groups in #3 increases with increasing temperature. The intensity of the OHbridge in Figure 4b decreases until 333 K and then smoothly increases with temperature. The curve of OHbridge in Figure 4b may then be resolved into two curves: the associative/dissociative desorption curve and the formation of surface hydroxyl groups. The formation of surface hydroxyl groups is dominant when there are a sufficient number of donor−acceptor complexes at the surface. Otherwise, the desorption process is dominant. It is assumed that H2O desorption and dissociative adsorption take place at the same time. When H2O molecules within the complex are desorbed rapidly with temperature increase, some of them form surface hydroxyl groups (OHbridge and Ti−OH) at the unfilled active sites (Ti atom, bridging O atom, or vacant site) if they overcome the activation barrier of dissociative adsorption, in this case with higher temperature. High temperature


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helps the H2O molecule to dissociate more readily into the surface hydroxyl groups. The amount of hydroxyl groups can increase with temperature if the dissociation rate of H2O is higher than the desorption rate due to sufficient H2O molecules at the surface. Therefore, it is suggested that the formation of surface hydroxyl groups (OHbridge and Ti−OH) is more favorable than that of donor−acceptor complexes at higher temperature. Another possible reaction is proton diffusion, which can take place from dense clusters to unfilled/isolated active sites with increasing temperature. Proton transfer further occurs among H2O molecules, the surface hydroxyl groups, and active sites even at lower temperatures as well as at higher temperatures.33,34 So, an increase in temperature causes rehydration of active sites by promoting H2O or dense clusters to adsorb dissociatively on the surface due to these mechanisms. Primet et al.38 also suggested a similar mechanism, dehydroxylation, which first removes the hydrogen-bonded OH groups (i.e. associative desorption). Then, some H2O molecules dissociate into unfilled active sites with proton or hydroxyl migration with increasing temperature. At higher temperature, some isolated OH groups can be removed by reaction with migrating protons or hydroxyl groups. This proposed mechanism supports our desorption results. Therefore, an increase in the amount of surface hydroxyl groups can be explained.


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Figure 5. Change in O 1s signal ratio IOH/Ibulk fitted with three typical peaks. Typical previous XPS analyses used three peaks for fitting the data. In fact, our XPS O 1s signals can also be fitted with three typical peaks. For our data, three peaks (OHbridge, Ti−OH, and H2O) can be assigned at 1.3 ± 0.2 eV, 2.35 ± 0.2 eV, and 3.3 ± 0.1 eV above the lattice oxygen peak, respectively. Quantitative analysis of #2 with three peaks (Figure 5) seems to be acceptable. The O 1s signal ratio of all water related adsorbates (OHbridge, Ti−OH, and H2O) to the lattice oxygen decreased with temperature. However, if the same analysis is performed, the desorption results of #3 show problems. ITi−OH/Ibulk rapidly decreased from 39.74% (293 K) to 22.19% (573 K) with a large binding energy shift (+2.35 → +2.18 eV). The molecular adsorbed water IH2O/Ibulk also decreased from 5.29% (293 K) to 0% (453 K). In contrast, Ibridge/Ibulk increased from 11.06% (293 K) to 27.1% (573 K). Thus, it seems that the OHbridge and Ti−OH starts to desorb at 293–333 K if the amount of Ti−OH is not enough at the surface. If there are a sufficient number of Ti−OH


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groups, they help to increase OHbridge by filling increased vacant sites or bridging O atoms due to diffusion or rehydration of H2O molecules. This result indicates that Ti−OH desorbs dissociatively, and then the rehydration occurs at the bridging oxygen sites (bridging O atoms and vacant sites). Further, the formation of OHbridge seems to be more favorable than that of Ti−OH (Ed,Ti-OH < Ed,bridge). However, this is contradictory to known studies.17 It is more reasonable to assume that OHbridge and Ti-OH have similar desorption activation energies because of bond strengths and desorption mechanisms. As mentioned above, the dissociative desorption of hydroxyl groups rarely occurs under 573 K due to the strong bond strength. Instead, associative desorption actively takes place in this region (≤573 K). Thus, the amount of Ti−OH should decrease with that of OHbridge at lower temperature, not by itself. Even if OHbridge is known to be thermally stable,37 there is no clear evidence showing that the formation of OHbridge is more favorable than that of Ti−OH at 293−573 K. Therefore, the three-peak analysis is contradictory to the known adsorption and desorption facts of hydroxyl groups, and our four-peak analysis is a more reasonable analysis.

Figure 6. Change in contact angle with temperature increase. Shifting our focus toward the relationship between contact angle and surface water-related adsorbates, Figure 6 shows the contact angle change of TiO2 thin films with increasing temperature. The samples were irradiated by strong UV light (UV−A, 33 W/cm2) 24 hours before the


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experiment. So, the initial contact angle of each sample (293 K) is quite low. As shown in the figure, the contact angle further increases with temperature because the water-related adsorbates are desorbed. As shown, #2 sample with a few water-related adsorbates shows a rapid increase in the contact angle at temperatures from 293 to 573 K. In contrast, #3 sample with sufficient adsorbates shows a slow increase in its contact angle at temperatures from 293 to 333 K, and then the angle increases rapidly after 333 K. Note that #2 and #3 show contact angles of 38° and 15° at 333 K, respectively, and show higher contact angles above 453 K, where TiO2 samples predominantly have surface hydroxyl groups. This observation implies that the number of donoracceptor complexes affects the contact angle difference at 333 K, and the surface hydroxyl groups (known as the key factors related to hydrophilicity) are not directly related (also shown in Figures 8 and 9). No matter how many surface hydroxyl groups are present, the contact angle is high when the number of the complexes is low on the surface. Moreover, above 453 K, the contact angles continue to increase with a similar tendency. Thus, the surface hydroxyl groups interact weakly with H2O molecules as heating chemically stabilizes the surface. As a result, Figure 6 illustrates that the only donor−acceptor complexes are directly related to the contact angle.


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Figure 7. Change in O 1s signal ratio IOH/Ibulk over time. XPS spectra were obtained every week (Figure 1). ‘Dissoc.’ denotes dissociative adsorbates, which refers to the sum of two hydroxyl group peaks; i.e., IOH/Ibulk (Dissoc.) = IOH/Ibulk (OHbridge) + IOH/Ibulk (Ti−OH). ‘CA’ stands for the contact angle of each film.


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Figure 8. Relationship between contact angle and IOH/Ibulk of four samples (Figure 1) for 22 days: (a) OHbridge, (b) Ti−OH, (c) Dissoc., (d) donor-acceptor complex. The arrows indicate the direction in which data points move over time. ‘Dissoc.’ denotes dissociative adsorbates, which refers to the sum of two dissociative hydroxyl group peaks.


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As shown in Figure 1b, TiO2 surfaces were analyzed every week for 22 days based on the four-peak-fitting of XPS. Figures S1 and 7 show the raw data of the XPS O1s signal and the quantitative analysis of the change in the O 1s signal ratio IOH/Ibulk over time, respectively. The relationship between contact angle and each water-related peak is shown in Figure 8. ‘Dissoc.’ in Figures 7 and 8 indicates the total number of dissociated hydroxyl groups (Ibridge/Ibulk + ITi-OH/Ibulk). According to the figures, all water-related peaks change over time. Interestingly, the surface hydroxyl groups increase over time because the ambient environment (e.g., moisture) influences the water-related adsorbates. It was assumed that some water molecules in the air slowly dissociate into hydroxyl groups and fill the unfilled/isolated active sites over time. Kettler et al.10 reported the experimental result that oxygen vacancies are filled with hydroxyl groups by water dissociation even in high vacuum and at very low humidity. Boehm6 also supports our result with the idea that metal oxides are hydroxylated under normal conditions, i.e., at room temperature and when moisture in the air exists. Thus, the unfilled/isolated vacant sites may be hydrated by the hydroxyl groups in the ambient environment (293 K, humidity ~15%) for a long time. Moreover, donor−acceptor complexes of all the samples (except #3) increase in Figure 7. This observation implies that the surface hydroxyl groups induce an increase in the donor−acceptor complexes. Indeed, another specimen sputtered with #2 deposition condition showed an increase in the hydroxyl groups and donor−acceptor complexes for 8 months, which shows long-lasting hydrophilicity (Figure 1). As shown in Figure 8c, the total amount of the hydroxyl groups further increases with the contact angle. It may appear that the hydroxyl groups cause the contact angle relaxation, but it is suggested that the effect of the hydroxyl groups on the contact angle is getting weaker, and then they lose control over the contact angle (as discussed in Figures 9c and 11c below).


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Figure 8d shows an interesting relationship between donor−acceptor complex and the contact angle. Samples of #1, #2, and #4 with low IDAC/Ibulk (≤5%) show a contact angle increase. However, the #3 sample with high IDAC/Ibulk (≥18%) shows long-lasting hydrophilicity. This indicates that TiO2 with a high-density donor−acceptor complex has no or much less contact angle relaxation and maintains hydrophilicity for a long time. This fact supports our suggestion mentioned above that the hydrophilicity depends on the amount of donor−acceptor complex.


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Figure 9. Relationship between contact angle and IOH/Ibulk of all samples after 22 days (saturated) from UV irradiation: (a) OHbridge, (b) Ti−OH, (c) Dissoc. (d) donor−acceptor complex. After 22 days, all the contact angles were considered to be saturated contact angles.


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Several sputtered samples in four different conditions were analyzed 22 days after the UV exposure to observe the relationship between saturated contact angle and water-related adsorbates, as shown in Figure 9. According to Figures 9a−c, the contact angle is not directly related to the surface hydroxyl groups. This observation is also shown in Figures 7–8 and contradicts the previous argument that the reciprocal of contact angle is proportional to the number of hydroxyl groups. The argument is true only right after UV exposure (discussed in Figure 11 below), but not until after a while. Note that donor−acceptor complex in Figure 9d shows the same relationship observed in Figure 8d. It seems that there is a threshold (~5%, IDAC/Ibulk) for hydrophilicity and the upper limit of the contact angle (~20°) is present above the threshold. This indicates that the samples having a large amount of the complexes above the threshold (~5%, IDAC/Ibulk) can maintain hydrophilicity, otherwise the contact angle can relax over time. Furthermore, IDAC/Ibulk vs. contact angle (Figure 9d) can be a useful indicator to determine whether TiO2 has the ability to maintain the hydrophilicity. Hosseinpour et al.26 reported that water molecules at the topmost layer are strong H-bond donors interacting with Ti−OH group. The interaction is stronger than H2O∙∙∙H2O interactions in the bulk, driving the superhydrophilicity. This previous work supports our argument that the complex induces persistent hydrophilicity. It is suggested that the contact angle is not directly related to the amount of hydroxyl groups but related more to the electrostatic interaction caused by the hydroxyl groups. The hydroxyl groups are background adsorbates, leading to stronger interactions than H-bonding in the bulk liquid.


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Figure 10. Relationship between donor−acceptor complex and the surface hydroxyl groups for all the samples measured 22 days after UV irradiation: (a) OHbridge, (b) Ti−OH, (c) Dissoc., (d) ITi−OH/Ibridge. In graph (d), the region where ITi−OH/Ibridge is less than 1 is regarded as the OHbridge dominant region, and the other region (ITi−OH/Ibridge ≥ 1) is regarded as the Ti−OH dominant region.


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Figure 10 shows the relationship between donor−acceptor complex and other waterrelated adsorbates. The amount of H2O molecules in the electrostatic complexes increases exponentially with the dissociated hydroxyl groups (Figure 10a−c). This indicates that more surface hydroxyl groups induce more H2O molecules to form electrostatic donor−acceptor complexes by forming electric charges at the surface.6,35,36 This shows that the donor−acceptor complex is due to the surface hydroxyl groups (OHbridge∙∙∙H2O and OHTi−OH∙∙∙H2O). Note that the basic Ti−OH group contributes more to the formation of the donor−acceptor complex compared to acidic OHbridge. As shown in Figure 10d, the TiO2 surface has just few complexes in the acidic OHbridge dominant region (ITi−OH/Ibridge < 1), whereas it has a large amount of electrostatic complexes in the basic Ti−OH dominant region (ITi−OH/Ibridge > 1). This tendency implies that the basic Ti−OH group prefers electrostatically attracting H2O molecules more than acidic OHbridge. Parfitt39 suggested that OHbridge has greater thermal stability and interacts weakly with adsorbed H2O molecules, but Ti−OH strongly interacts. It was also reported that Ti−OH groups have stronger charge polarity than the acidic OHbridge.25,26 Further, the point of zero charge, pHpzc, where the positive and negative charge are balanced (i.e., σnet = 0) is ~5.9 for anatase TiO2.40 TiO2 having sufficient Ti−OH groups shows a more basic surface due to the basicity of the Ti−OH group (pKa ≈ 12.7), which implies that strong negative charge is present when de-ionized water liquid (pH ≈ 7.0) is in contact with the surface. Furthermore, Figure 10d provides a strong evidence that the basic Ti−OH group is reactive, thereby supporting the idea that the polarizable Ti−OH group prefers to interact with H2O by the electrostatic donor−acceptor interaction, which is stronger than H-bonding among water molecules.20,21


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Figure 11. The relationship between contact angle and water-adsorbates within 24 hours after UV exposure: (a) Contact angle relaxation of some samples (IDAC/Ibulk ≤ 10% in Figure 9). Relationship between contact angle and (b) H2O with donor−acceptor (c) total adsorbates (Itotal = Ibridge + ITi−OH + IDAC + IH2O). (d) Relationship between H2O with donor−acceptor and ITi−OH/Ibridge for samples


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within 24 hours of the UV exposure. To analyze the relaxation process, UV light (33 W/cm2) was irradiated on samples showing lower IDAC/Ibulk (≤~10%). Then, XPS spectra were measured after 2, 6, and 24 hours of exposure. Figure 11b shows that the contact angle is inversely proportional to the amount of donor−acceptor complex. Figure 11c also shows that the total amount of adsorbates, known as the sum of hydroxyl group peaks in the previous XPS study, is inversely proportional to the contact angle.3 However, the relationships break over time due to contact angle relaxation for the samples under ~6% of IDAC/Ibulk. Note that the spectra shown in Figure 11d follow the same curve shown in Figure 10d. Therefore, the Ti−OH group provides a major contribution to the formation of the electrostatic complex. ITi−OH/Ibridge can a useful indicator for determining the ability to form a strong donor−acceptor complex and to maintain hydrophilicity over time. Based on all the above results, following mechanism of contact angle relaxation and longlasting hydrophilicity on the TiO2 surface is proposed. Electron-hole pairs in TiO2 are generated by UV exposure, interact with H2O molecules, and disappear due to relaxation in the dark. Note that UV light activates both acidic OHbridge and basic Ti−OH groups by hole transfer to the hydroxyl groups.21 The activated sites with polarity prefer to attract H2O molecules by an electrostatic complex, enabling hydrophilicity. When the number of Ti−OH groups is sufficient, the dense clusters of hydroxyl groups can keep their surface charges and attract H2O molecules within the complex. Then, the upper layers are polar when the samples have a large amount of donor-acceptor complex, causing long-lasting hydrophilicity.21,23-25 The lack of H2O molecules within the complex on the surface means that the hydroxyl groups have weak polarity and do not maintain hydrophilicity. Thereby, TiO2 loses its hydrophilicity and undergoes the contact angle relaxation. Upon UV exposure again, the TiO2 can recover the surface charge polarity by EHP and


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regain its hydrophilicity. Therefore, the formation of a high-density strong electrostatic donor−acceptor complex is essential to make long-lasting hydrophilic TiO2 thin films.

3.3. Plasma Treatment for High Density Donor−Acceptor Complex at TiO2 Surface Oxygen plasma is a useful source to produce the water-related adsorbates at a TiO2 surface. Surface hydroxyl groups generated by the plasma treatment can induce hydrophilicity. Generally, the induced hydrophilicity can disappear over time and change to hydrophobicity. However, if high density oxygen plasma treatment is performed for enough time to produce sufficient amounts of donor−acceptor complexes, the treated TiO2 thin film can maintain long-lasting hydrophilicity. To test the hypothesis, oxygen plasma was treated onto the TiO2 surface deposited with #2.


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Figure 12. XPS O 1s spectra of TiO2 thin film (#2) with plasma treatment: (a) un-treated, (b) O2 plasma treatment for 2 minutes, (c) O2 plasma treatment for 3.5 minutes.


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Figure 13. (a) Change in the number of water-related adsorbates due to O2 plasma treatment. (b) Relationship between donor−acceptor complex and ITi−OH/Ibridge for the untreated and treated samples. Figures 12 and 13 show the results of oxygen plasma treatment of TiO2 thin films that did not show long-lasting hydrophilicity before plasma treatment. According to the data, the TiO2 sample having low water-related adsorbates has enough adsorbates due to the oxygen plasma treatment. The amount of Ti−OH gradually increases over plasma treatment time, whereas the amount of OHbridge gets saturated in the treatment. This may be because the amount of OHbridge is limited by the number of vacant sites at the thin film surface. Note that the amount of donor−acceptor complex increases sharply with Ti−OH as the plasma treatment time increases. Figure 13b also shows the same tendency in Figures 10d and 11d. The observations imply that H2O molecules within the complex are mainly attracted by basic Ti−OH groups supplied by the oxygen plasma. Indeed, the treated samples showed hydrophilicity (contact angle ≤ 20° for ~22 days, not shown here). Therefore, the high-density plasma treatment can be a useful method to


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control the surface hydroxyl groups and donor−acceptor complex. Further, this result is in line with our model and demonstrates that typical TiO2 can be converted to show long lasting hydrophilicity.

4. Conclusion The contact angle relaxation of sputtered TiO2 thin films was investigated using quantitative XPS analysis. The adventitious carbon was not the cause of the contact angle relaxation. Using in-situ temperature-programmed XPS, a new peak was identified, specifically H2O in a donor−acceptor complex at ~2.57 eV above the lattice oxygen peak. This complex is a key factor in determining the contact angle relaxation and long-lasting hydrophilicity. This peak is evidence of strong electrostatic interactions between H2O and the surface hydroxyl groups (OHbridge and Ti−OH), which is stronger than H-bonding between water molecules. Samples having fewer donor−acceptor complexes experience contact angle relaxation. In contrast, the samples having more than a critical number of donor−acceptor complexes show long-lasting hydrophilicity. The surface hydroxyl groups (the number of which is known to be closely linked to the contact angle) are not directly related to the contact angle after a certain period. Note that the basic Ti−OH group strongly induces electrostatic interactions with H2O molecules. Based on the results, it is suggested that the contact angle relaxation is related to the electrostatics due to surface charge. UV light activates the surface hydroxyl groups, and the activated polarizable basic Ti−OH groups strongly interact with H2O molecules by forming the electrostatic complexes. The islands of high-density H2O in the complex can maintain their polarity and overcome the relaxation. It was further confirmed that the long-lasting hydrophilicity can be achieved using a high-density oxygen plasma treatment.


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In summary, the principles of contact angle relaxation and hydrophilicity of TiO2 surfaces in an ambient environment were experimentally studied. A new and better XPS analysis is presented, and a new peak was identified as H2O with a donor-acceptor complex. The complex density is an important factor in understanding long-term hydrophilicity, and a model has been presented. To our knowledge, this is the first model encompassing initial hydrophilicity due to UV exposure and long-term hydrophilicity. This model is consistent with reported facts and supports the phenomena related to contact angle relaxation in the ambient environment.

5. Supporting Information XPS raw data of C 1s & O 1s of four samples over 22 days; AFM images and surface roughness profiles of #1 to #4; The relationship between the contact angle and [C]/[Ti] of all samples in Figure 1a after 22 days.

6. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2017R1D1A3B03030005).

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Contact Angle Relaxation and Long-Lasting Hydrophilicity of

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