Investigation of the Hydrophobic Nature of Metal Oxide Surfaces

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Investigation of the Hydrophobic Nature of Metal Oxide Surfaces Created by Atomic Layer Deposition Jongyoon Bae, Izabela A. Samek, Peter C. Stair, and Randall Q. Snurr Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00577 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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Investigation of the Hydrophobic Nature of Metal Oxide Surfaces Created by Atomic Layer Deposition Jongyoon Bae†║, Izabela A. Samek†, Peter C. Stair‡*, Randall Q. Snurr†*

†Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States

║Current Address: School of Engineering, Brown University, 184 Hope Street, Providence, Rhode Island, 02912, USA ‡Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States

ABSTRACT: Surface hydrophobicity can be exploited in the design of catalyst materials to improve their activity and selectivity. One versatile method for modifying the hydrophobicity of the environment surrounding an active site is atomic layer deposition (ALD). In this work, Al2O3, TiO2, and SiO2 deposited by ALD as well as CeO2 deposited

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Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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by electron beam evaporation – all on α-Al2O3 wafers – are investigated to determine their intrinsic hydrophobicity and any changes upon exposure to the atmosphere. The properties of the metal oxide thin films are compared to those of single crystal α-Al2O3, αSiO2, and Y:ZrO2. Contact angle measurements with water combined with XPS studies are applied to determine the hydrophobicity and elemental content of the metal oxides. Both the single crystal and thin film metal oxides are found to be intrinsically hydrophilic following a rapid thermal processing procedure. Upon exposure to air, the investigated metal oxide surfaces become increasingly hydrophobic, correlated to adsorption of carbonaceous species. Metal oxide thin films deposited by ALD exhibit the same hydrophobicity behavior as their single-crystal equivalents.

INTRODUCTION Metal oxides are widely utilized in fields such as electronics, optics, and catalysis because of their unique and desirable properties including mechanical, thermal, and chemical stability.1-4 While bulk metal oxides are often used, application as thin films is often preferred, because thin films can modify the properties of the platform material or take


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advantage of platform stability, high surface area, crystallinity, etc.5 Atomic layer deposition (ALD) has become an important tool for depositing metal oxides owing to the precise film thickness control that can be achieved.3,

6-9

ALD is a vapor-deposition

technique that uses a sequence of two or more self-limiting reactions in a series of cycles to form the film. A metal precursor reacts with surface functional groups in the first halfcycle. Upon depletion of the active sites, the reaction ceases and another reagent is introduced to regenerate the active sites in the second half-cycle. The self-terminating characteristics of the process enable precise thickness control at the angstrom level.10-11 Hydrophobicity is an important surface property for several applications, and it can provide insights into how a surface interacts with molecules. Hydrophobicity is especially important for certain applications of ALD-grown materials such as protective coatings on microelectromechanical systems (MEMS)12 and gas diffusion barrier layers on polymer substrates,13-14 as water adsorption deteriorates the performance of MEMS and can damage polymer structures. A number of studies previously reported in the literature suggest that metal oxides deposited by ALD are hydrophobic. However, their hydrophobicity may not be intrinsic to the metal oxides but a result of the presence of 3 ACS Paragon Plus Environment

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adsorbed hydrocarbons on the surface.15-17 It is well-known that metal oxides are generally hydrophilic due to the presence of metal cations, oxygen anions, and hydroxyl groups on the surface. Therefore, a variety of modifications to alter the hydrophobicity of metal oxides have been investigated. In the study by Halfpenny and co-workers, the surface of a silica was dehydroxylated by annealing and laser irradiation to condense out water molecules from vicinal hydroxyl groups and promote a M-O-M structure.18 However, as shown by Lamb et al., quartz surfaces remain hydrophilic even at the lowest achievable hydroxyl group density.19 In another study, self-assembled monolayers with hydrophobic alkyl tails were attached to metal oxide surfaces.12 The length of the attached alkyl tails was found to have an effect on the hydrophobicity of the surface. Longer tails aligned without creating large gaps, preventing water molecules from having contact with the oxide substrates and creating a more hydrophobic surface.20 Another (often unintentional) modification involves the spontaneous adsorption of hydrocarbons from the atmosphere.21-25 Among the surfaces studied after exposure to the atmosphere, CeO2 was found to have a transition from hydrophilic to hydrophobic, while SiO2 remained hydrophilic even after prolonged exposure to the atmosphere.22, 24-25 In the present study, 4 ACS Paragon Plus Environment

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we explored the hydrophobicity of clean metal oxide surfaces, as well as those exposed to atmospheric hydrocarbons, to mimic practical conditions of many ALD metal oxide applications. In addition to the application of hydrophobic metal oxide surfaces as coatings, a number of studies have exploited hydrophobicity as a means to enhance catalyst activity and selectivity.26-30 For example, Corma et al. achieved selective transport of non-polar reactants in cyclohexene epoxidation catalysis. In this study, a hydrophobic Ti-MCM-41 catalyst was synthesized by introducing organosilanes directly into the titanium-zeolite gel synthesis. As a result, hydrophilic silanol groups were substituted with hydrophobic organosilane groups. By creating a hydrophobic environment around the active sites, the authors were able to prevent the over-oxidation of epoxide products and the formation of diols. Additionally, the absence of diols limited catalyst deactivation caused by diol adsorption on hydrophilic surfaces.27 Similarly, metal oxide layers with varying wetting properties could, conceivably, be deposited by ALD to create catalysts selective to hydrophobic reactants, as shown in Scheme 1. The hydrophobic nanochannel formed by ALD should favor the interaction of 5 ACS Paragon Plus Environment

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the Ti active site with the hydrophobic cyclohexene molecule over the hydrophilic epoxide. An investigation of the hydrophobic nature of metal oxide films deposited by ALD will therefore aid in the design of hydrophobicity-selective catalysts. ALD has also been applied to modify the environment around nanoparticles by depositing metal oxide layers to create size-selective catalysts.31-32

Scheme 1. Schematic of selective cyclohexene epoxidation with hydrophobic nanostructures deposited by ALD. In this work, we deposited Al2O3, SiO2, and TiO2 by ALD and CeO2 by electron beam evaporation onto α-Al2O3 wafers to study both their intrinsic hydrophobicity and how it changes upon exposure to air. SiO2 and CeO2 were specifically chosen because it has been shown that upon air exposure the hydrophobicity of the former does not change significantly while that of the latter is highly dependent on hydrocarbon adsorption.22, 24-25


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Additionally, we compared the hydrophobicity of several single crystal metal oxides, αAl2O3, α-SiO2, and Y:ZrO2, to that of the metal oxides deposited by ALD. We measured water contact angles to quantify hydrophobicity and employed X-ray photoelectron spectroscopy (XPS) to determine the amount of carbon adsorbed on the surface. The hydrophobicity of the CeO2 surface was of particular interest due to conflicting results in the literature. Azimi et al. reported that ceria is intrinsically hydrophobic,33 which contradicted earlier studies where the hydrophobicity of ceria was attributed to hydrocarbon adsorption.25, 34 In the study by Azimi et al., the hydrophobicity of lanthanide oxide pellets was attributed to the low extent of hydrogen bonding due to shielding of the

4f electron shell by the 5s2p6 core shell.33 A follow-up study by Preston et al. indicated that ceria is intrinsically hydrophilic.35 However, another study by Khan et al. reported that freshly sputtered ceria is hydrophilic due to the excess of oxygen present on the surface, but it becomes hydrophobic when it is relaxed under an ultra-high vacuum environment with a stoichiometric O/Ce ratio.36 Our results in the present study aim to provide additional data to clarify the intrinsic hydrophobicity or hydrophilicity of CeO2.


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EXPERIMENTAL METHODS Table 1 shows a summary of the materials included in this study. Single-crystal metal oxides of α-Al2O3 (0001) (99.99%), α-SiO2 (0001) (99.99%), and Y:ZrO2 (100) (99.99%, 8 mol% yttrium) were purchased from MTI Corporation as square wafers with dimensions of 10 mm × 10 mm × 0.5 mm. Table 1. Description of the investigated materials. Chemical

Preparation

Crystal

Symbol

Method

Facet

α-Al2O3

Single Crystal

(0001)

(MTI Corporation) α-SiO2

Single Crystal

(0001)

(MTI Corporation) Y:ZrO2

Single Crystal

(100)

(MTI Corporation) Al2O3

ALD

SiO2

ALD

TiO2

ALD

CeO2

E-Beam Evaporation


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Al2O3, SiO2, and TiO2 were deposited onto α-Al2O3 wafers by ALD in a stainless-steel viscous flow reactor. The resulting surfaces will be referred to as simply Al2O3, SiO2, and TiO2. The metal oxide films were grown by sequential dosing of metal oxide precursor A (Table 2), purging of A with ultra-high purity (UHP) nitrogen (Airgas, 99.999%), dosing of 18.2 MΩ∙cm Millipore water at 25°C as the oxidant (precursor B) and purging of water by UHP N2. Therefore, the total time for one ALD cycle is composed of dose time of A, purge time of A, dose time of B, and purge time of B, which is denoted below as t1-t2-t3-t4. UHP nitrogen at a flow rate of 360 sccm was used as the carrier gas. ALD reactor pressures ranged from 1 to 2 Torr throughout the deposition. Table 2 shows the detailed conditions for the metal oxides deposited by ALD. The values for expected growth per cycle are based on information reported in the literature.6, 37-40 Table 2. ALD precursors and growth conditions for Al2O3, SiO2, and TiO2. Metal

Expecte

Oxide

d

Precursor Metal Oxid

Metal Oxide

e

Precursor

t1-t2-t3-t4 (s)

Numbe Growth

Reservoir

Reactor

r

of Rate

Temperatu

Temperatu

Growth (Å/cycle

re (°C)

re (°C)

Cycles

)


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Al2O3 Trimethylaluminum (TMA,

60-60-60-60

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25

200

10

1.1-1.26

300

25

50

40

0.737-38

30-30-30-30

65

100

60

0.139-40

Sigma-

Aldrich, 97%) 120-240-300SiO2

Tetraethoxysilane (TEOS,

Strem,

98%) Titaniumisopropoxi TiO2

de (TTIP,

Sigma-

Aldrich, 97%) CeO2 thin films were deposited on α-Al2O3 (0001) wafers with an Auto 500 electron beam evaporator (Edwards). The base pressure of the growth chamber was approximately 5 ×10-6 Torr prior to evaporation. An electron gun filament current of 80 mA was applied during the deposition. CeO2 pieces (Kurt J. Lesker 99.9%) and a graphite crucible liner (Kurt J. Lesker) were used. The substrate wafers were placed directly above the CeO2 pieces and packed closely to achieve uniform growth. We attempted to grow CeO2 films by ALD, but the resulting materials were not stable to the rapid thermal processor cleaning treatment described below.


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The hydrophobicity of the metal oxide surfaces was quantified by the measurement of static water contact angles using an FTA125 goniometer (First Ten Angstrom). 18.2 MΩ∙cm Millipore water was the probe liquid. Approximately 4 µL was dispensed by a microliter syringe (Gilmont) and deposited on the test surface with a gentle contact. There was a 30 s delay between drop suspension and image capture to allow for stabilization of the water droplet. Following the delay, the contact angle was measured using FTA software by spherical fitting of the captured image. After each measurement, the entire sample surface was rinsed with DI water and dried under nitrogen flow. Contact angles were measured immediately after (ca. 5-10 min of transportation) carbon removal treatments as well as after varying exposure times to room atmosphere (24 °C with 1430 RH%). An AFM Dimension Icon (Bruker) was used to image surface morphology and measure surface roughness. For all seven sample surfaces, probe areas of 2 µm by 2 µm were analyzed at a scan rate of 1.2 Hz in tapping mode, and the measurements were repeated three times at three different regions on the sample. The AFM images for a given sample were visually compared to check for any noticeable inconsistency. All samples showed 11 ACS Paragon Plus Environment

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consistent AFM images, and among the three images, the one with the highest AFM image quality, capturing the most features, was selected for the roughness calculation. The root-mean-squared roughness was calculated based on the height sensor data. We explored three different methods of surface cleaning since the surfaces exposed to the atmosphere can adsorb water and carbon-containing contaminants.22, 24, 40 In the first method, all seven samples were treated with an oxygen plasma using a PC-2000 Plasma Cleaner (South Bay Technology). The vacuum chamber pressure was ~0.05 Torr prior to dosing. Oxygen gas (Airgas, 99.8%) was introduced until the pressure was increased to ~0.1 Torr. The RF power was set at 50 W, and samples were treated for 10 minutes. This method effectively removed adsorbed carbon. However, it introduced fluorine contamination as discussed in the Supporting Information (SI). In the second method, the surfaces of α-Al2O3, SiO2, TiO2, and CeO2 were sequentially rinsed with acetone (EMD Millipore, 99.5%), methanol (Sigma-Aldrich, 99.93%), and DI water. Each surface was rinsed for approximately 10 s with acetone, and then 10 s with methanol, followed by a 30 s rinse with DI water. This procedure was performed after


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initial oxygen plasma treatment and subsequent exposure to air. This method was found to be ineffective in removing adsorbed carbon species as discussed in the SI. In the third method, carbonaceous deposits were removed thermally/oxidatively using an AS-Micro (Annealsys) Rapid Thermal Processor (RTP) unit. This procedure was performed on samples previously treated with oxygen plasma and subject to solvent rinsing. The heating chamber pressure was ~0.04 Torr with a 100 sccm flow of ultra-pure carrier oxygen (Airgas, 99.996%). The chamber was purged with ultra-pure carrier nitrogen (Airgas, 99.9993%) following the annealing process. The samples were initially heated with a ramp rate of 5°C/s to 800°C, held at 800°C for 5 min and then cooled down to 25°C. Subsequently, the samples were heated with a ramp rate of 20°C/s to 1000°C, held at 1000°C for 30 s and then cooled down to 25°C. Only results from the RTP cleaning are shown in the main manuscript. An ESCALAB 250Xi (Thermo Scientific) X-ray photoelectron spectrometer was used to identify and quantify surface chemical species. Excitation by a monochromatic Al-Kα Xray source with a beam size of 500 µm was used with the sample surface normal to the detector. Additional XPS spectra were taken at variable sample tilt angles for oxygen 13 ACS Paragon Plus Environment

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plasma treated α-Al2O3. Charge correction was performed by adjusting the binding energy of the C 1s peak to 285 eV. Peaks were fitted with a modified Shirley background, and atomic percentages were calculated using Scofield sensitivity factors41 modified with an instrument-specific transmission function and instrument factor. C 1s peaks were deconvoluted with three peaks assigned to C-C, C-O, and O=C-O in order to quantify the total amount of adsorbed adventitious carbon.40 The C-O and O=C-O peaks were located at 1-1.8 eV and 4.1-5 eV higher binding energy than the C-C peak, respectively. An example of the fitted C 1s peak is shown in Figure 1.


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Figure 1. C 1s spectrum for adsorbed carbon on a TiO2 surface. Black line: experimental data; red lines: Lorentzian/Gaussian fitted peaks; blue dashed line: sum of the fitted peaks. A detailed description of the experimental procedures is provided in the SI.

RESULTS AND DISCUSSION Characterization of Metal Oxide Deposition by XPS The chemical species present on the surface of the metal oxide films deposited by ALD and e-beam evaporation were identified and quantified by XPS. The Al 2p signal for the Al2O3 sample included contributions from both the α-Al2O3 substrate and the Al2O3 film grown by ALD and hence the Al:O ratio of the ALD film itself could not be determined by XPS. Figure 2 shows the Si 2p, Ti 2p, and Ce 3d spectra. The splitting and binding energies, and the corresponding expected values reported in the literature are summarized in Table 3. These results confirm the deposition of SiO2, TiO2, and CeO2 films.42-46 The Ce 3d5/2 binding energy was 1 eV higher than the reported values. However, the peak shape indicates the presence of oxidized CeO2, rather than reduced


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Ce2O3.47 The Si peak showed a broad feature at the lower binding energy region, which was attributed to the α-Al2O3 substrate as shown in Figure 2. The corrected Si spectrum in Figure 2b was obtained by subtracting the α-Al2O3 substrate signal from the SiO2 spectrum.

Figure 2. Si 2p spectra from SiO2 deposited on α-Al2O3 (solid line) and pure α-Al2O3 (dashed line) (a). Si 2p spectrum from SiO2 deposited by ALD obtained by subtracting the signal due to the α-Al2O3 substrate from the SiO2 on α-Al2O3 spectrum (b). Ti 2p spectrum from TiO2 on α-Al2O3 (c). Ce 3d spectrum from CeO2 on α-Al2O3 (d). Table 3. Peak binding energies obtained from XPS and expected values reported in the literature. The Δ values refer to binding energy differences between the Ti 2p3/2 and Ti 2p5/2 peaks and between the Ce 3d5/2 and Ce 3d3/2 peaks. Metal

Spectru

Oxide

m

Expected Peak

Peak

FWHM a

Expected Δ(eV)

Δ(eV ) 16

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Binding Energy

Bindin

(eV)

g Energy (eV)

SiO2

Si 2p

103.342, 103.843

102.6

2.1

AlSiOx Si 2p

102.642

TiO2

Ti 2p3/2

458.842, 44

458.8

1.2

5.542, 5.744

5.7

CeO2

Ce 3d5/2

882.645, 882.746

883.7

2.6

15.845, 15.646

15.5

aFWHM

stands for full width at half maximum.

Quantitative elemental analysis results obtained from XPS for samples cleaned by rapid thermal processing are summarized in Table 4. Metal impurities detected by XPS accounted for approximately 2 atomic % of the sample composition. The low Si content indicated in Table 4 for SiO2 can be associated with limited and inhomogeneous deposition of SiO2 by ALD. The expected growth rate cited in Table 2 corresponds to ALD of SiO2 by TEOS and oxygen plasma as well as TEOS and H2O catalyzed by NH3.37-38 It has been previously reported that the use of TEOS and H2O in the ALD process results in a decrease in mass gain after 20 ALD cycles which is consistent with lower than expected deposition of SiO2 in this work.48 The O:Al ratio increased from 1.4 to 1.6 upon


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SiO2 deposition on α-Al2O3. The increase in oxygen is likely caused by the deposition of silanol groups during the ALD process. The XPS signal for the SiO2 sample includes significant contribution from the underlying α-Al2O3 substrate. The intense Al signal could be a result of exposed α-Al2O3 surface due to incomplete SiO2 coverage, the α-Al2O3 substrate underneath the SiO2 thin film or both. This contribution is not observed for TiO2 or CeO2. We attribute the low Al content in the XPS signals for TiO2 and CeO2 to the growth of thicker and more uniform films of the two metal oxides on sapphire. Table 4. Summary of all detected elements in atomic % calculated on the basis of XPS data collected after the RTP treatment.


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Element

Atomic % per Sample α-Al2O3

α-SiO2

Y:ZrO2

Al2O3

SiO2

TiO2

CeO2

Al

37.1

-

-

36.9

34.5

1.5

4.8

Si

-

31.6

-

-

0.4

2.4

-

Y

-

-

6.6

-

-

-

-

Zr

-

-

21.1

-

-

-

-

Ti

-

-

-

-

-

30.4

-

Ce

-

-

-

-

-

-

26.7

O

58.0

64.6

62.0

58.0

60.6

59.0

56.7

C

2.8

3.6

10.3

4.6

3.1

4.7

10.5

F

0.7

-

-

-

-

-

-

Surface Roughness and Morphology Surface roughness was measured for all of the examined samples because it can significantly affect the results of contact angle measurements.49 Atomic Force Microscopy (AFM) images used to determine surface roughness can be found in Figure S6. All of the investigated sample surfaces were very smooth, with RMS roughness values less than 1 nm, which is the same order of magnitude as the thickness of the films deposited by ALD. Specific surface roughness values can be found in Table S2. This implies that none of


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the surfaces have any specific microstructures or patterns and, as a result, that surface roughness should have a negligible effect on the contact angle measurements. The results also indicate that conformal growth of metal oxide films was obtained by ALD. Surface Hydrophobicity Studies Contact angles were measured immediately after RTP treatments, and subsequently at intervals of 24 hours of room atmosphere exposure for 7 days as shown in Figure 3 for the single crystals (a) and thin films (b). The reported contact angles are averages from experiments on two separate surfaces and the error bars are the standard deviations. Systematic errors ranging from 1° to 10° are expected for contact angle measurements as a result of uncertainties associated with the placement of the baseline.50 A significant increase in hydrophobicity was observed for all of the samples in agreement with previous studies.21-25 There was an initial rapid increase followed by a leveling off in the measured contact angles. After 7 days of atmospheric exposure, the CeO2 film showed the greatest increase in hydrophobicity, with the highest contact angle. As shown in Figure 3c, where the single crystal and metal oxide films of Al2O3 and SiO2 deposited by ALD are compared directly, there was no significant difference in either the intrinsic hydrophobicity or that 20 ACS Paragon Plus Environment

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measured upon exposure to air between the two types of surfaces. The comparable hydrophobicity properties of SiO2 and α-SiO2, and their distinctness from α-Al2O3, confirm that a continuous thin film of SiO2 was deposited by ALD. This implies that the Al signal detected by XPS for SiO2 is solely due to the α-Al2O3 support beneath the thin film.


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Figure 3. Contact angle as a function of atmospheric exposure time for single crystals (a) and metal oxide films deposited by ALD and e-beam evaporation (b) cleaned by RTP.


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Direct comparisons of contact angles on single crystal and deposited sample surfaces (c). The lower detection limit of the software used for contact angle analysis was 3°, which is denoted as 0° in the graphs above. To further investigate the reason for the increase in hydrophobicity upon exposure to air, α-Al2O3, SiO2, TiO2, and CeO2 surfaces were examined using both contact angle measurements and XPS. Contact angles were measured immediately after the RTP treatment as well as after 14 hours, 4 days, and 7 days of atmospheric exposure. XPS was performed immediately after each contact angle measurement to identify and quantify surface elements. As shown in Table 4, small amounts of metal impurities, residual carbon and relevant metals were detected on the sample surfaces after the RTP treatment. The only observed change in the XPS results obtained upon atmospheric exposure was an increase in the carbon content. The spectra collected for the thin film samples included contributions from the underlying supports. However, as discussed above, the α-Al2O3 substrates were completely covered by the oxide films, and thus the extent of carbon adsorption is only associated with the nature of the oxides deposited on the surface. Figure 4a shows the contact angle results as a function of the carbon atomic 23 ACS Paragon Plus Environment

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percent. The carbon content increased with air exposure, and small initial changes in the carbon content correlated with significant increases in the measured contact angles. Relatively low contact angles were measured for the CeO2 surface at carbon contents up to 10 atomic %. The low contact angle is attributed to a high fraction of oxidized carbon species present on the surface, as shown in Figure 4b. These species provide polar sites even at high overall carbon content. The relatively high amount of oxidized carbon can be explained by the reactivity of rare earth oxide films towards hydrocarbons. These oxides have been previously shown to partially or fully oxidize adsorbed hydrocarbons, whereas the rare earth surface species undergo reduction.51


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Figure 4. Contact angle as a function of carbon atomic % for surfaces initially cleaned by RTP (a). Quantitative C 1s chemical state analysis immediately after RTP cleaning and after 14 hours, 4 days, and 7 days of atmospheric exposure (b). Two possible modes of carbon adsorption are hypothesized, based on the α-Al2O3 and CeO2 results in Figure 4a. The contact angle for α-Al2O3 remained relatively constant regardless of the 5.2 atomic % increase in carbon content between the two longest


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exposure times. It is highly unlikely that carbon had covered the entire α-Al2O3 surface at this point because the contact angle was relatively low at 50°. A negligible amount of oxidized carbon was found on the α-Al2O3 surface. If the surface had been completely covered with carbon, the measured contact angle should have been comparable to the contact angle on polyethylene at 94°.52 Therefore, it is reasonable to postulate that carbon adsorbs in a patchy island-like manner instead of forming a continuous monolayer on αAl2O3. In this scenario, water molecules have the possibility of interacting with the metal oxide surface, resulting in a relatively low contact angle. Table 5 shows the result of calculations of theoretical oxygen and nitrogen atomic % (details are given in the SI). A fully carbon-covered CeO2 surface has a theoretical oxygen atomic % similar to that of PMMA or Nylon. The contact angle values of PMMA and Nylon are 80° and 70°, respectively.52 The measured contact angle for CeO2, 75°, is within the range of these values and suggests that the ceria surface is completely or almost completely covered with carbon after 7 days of room atmosphere exposure. This is in agreement with work previously published by Külah et al., where a full carbonate monolayer was observed after a week of exposure of rare earth oxide films to air. CeO2 and Gd2O3 films were shown to 26 ACS Paragon Plus Environment

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readily react with adsorbed hydrocarbons until all of the surface rare earth atoms were saturated, at which point the reaction rate decreased.51 Table 5. Theoretical combined oxygen and nitrogen atomic %. Sample

Combined

Oxygen

and

Nitrogen Content (Atomic %) Polyethylen

0

e PMMA

28.6

Nylon

25

α-Al2O3

6.8

CeO2

21

The CeO2 surface treated by RTP shows a significant amount of carbon immediately after the cleaning due to either incomplete carbon removal or rapid re-adsorption prior to the contact angle measurements. Additionally, ceria adsorbs the highest amount of carbon and, as a result, is characterized by the most rapid increase in hydrophobicity following atmospheric exposure. Similar increased hydrocarbon adsorption on CeO2 surfaces resulting in higher contact angles in comparison to other oxide films has been previously reported for ceria pellets.35,

53

These results suggest that carbon adsorption on CeO2 27 ACS Paragon Plus Environment

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surfaces is relatively spontaneous and favorable, which can be explained by the presence of oxygen vacancies in ceria and hence an increase in the number of adsorption sites.54 The findings reported in this work are in agreement with previously published studies, which note that CeO2 is initially hydrophilic but quickly becomes hydrophobic upon atmospheric exposure and the resulting carbon adsorption on the surface.25, 34-35, 53, 55-56 There have been a number of studies which assigned an intrinsic hydrophobic nature to ceria and other lanthanide oxides.16-17, 33, 57-60 However, we found that in order to measure the intrinsic wetting properties of a high-energy surface like ceria, it must undergo a proper cleaning procedure prior to the contact angle measurements. The hydrophobic nature of CeO2 has typically been observed after the surface was rinsed with organic solvents or water, which we found is not effective for removing carbon contaminants. The contact angles and carbon contents of sample surfaces before and after cleaning the surface with an organic solvent shown in Figure S5 support the inability of this treatment to reduce the carbon content. CONCLUSIONS


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We investigated the intrinsic hydrophobicity of metal oxide surfaces and how the hydrophobicity changes upon exposure to the atmosphere. We measured water contact angles and XPS elemental concentrations on single crystals of α-Al2O3, α-SiO2, and Y:ZrO2, as well as films of Al2O3, SiO2, and TiO2 deposited by ALD and CeO2 prepared by electron beam evaporation on α-Al2O3 substrates. Prior to contact angle measurements, RMS surface roughness was measured by AFM. All seven surfaces were very smooth with a roughness not exceeding 1 nm. In this regime, the surface roughness effects on contact angle measurements are negligible. Additionally, rapid thermal processing was employed to minimize carbon contamination on the sample surfaces. Although small amounts of metal impurities (≤2 atomic %) were found, RTP efficiently removed adsorbed carbon. Finally, contact angle measurements and XPS were recorded after various lengths of atmospheric exposure time to quantify hydrophobicity and the amount of adsorbed carbon, respectively. Metal oxide surfaces of α-Al2O3, α-SiO2, Y:ZrO2, Al2O3, TiO2, SiO2, and CeO2 were found to be intrinsically hydrophilic, with contact angles below 11° when adventitious carbon was properly removed. An increase in hydrophobicity upon atmospheric exposure was 29 ACS Paragon Plus Environment

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observed for all seven metal oxide surfaces. It can be attributed to increasing carbon readsorption. It was found that single crystal and ALD-deposited thin films of alumina and silica had very similar intrinsic wetting properties and reacted in an equivalent manner to air exposure. CeO2 adsorbed carbon most readily, which resulted in the highest measured contact angles. In conclusion, metal oxide surfaces are intrinsically hydrophilic, whether in the form of ALD films or bulk oxide surfaces, and any hydrophobic characteristics are attributed to adsorption of carbonaceous materials on the surfaces.

ASSOCIATED CONTENT

Supporting Information

The following file is available free of charge.

Sequence of experiments performed on the investigated materials, hydrophobicity of oxygen plasma treated surfaces, effect of organic solvent cleaning procedures on carbon content, morphology of metal oxide surfaces analyzed by AFM, theoretical combined oxygen and nitrogen content calculations, (PDF).


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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]

*E-mail: [email protected]

Funding Sources DOE DE-FG02-03-ER154757 Award, NSF ECCS-1542205, NSF DMR-1121262.

ACKNOWLEDGMENT

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DOE DE-FG02-03-ER154757. This work made use of the EPIC, Keck-II, and/or SPID facility(ies) of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN.


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Table of Contents/Abstract Graphic


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