Water Adsorption on Hydrophilic and Hydrophobic Surfaces of Silicon

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Water Adsorption on Hydrophilic and Hydrophobic Surfaces of Silicon Lei Chen, Xin He, Hongshen Liu, Linmao Qian, and Seong H. Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01821 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 6, 2018

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Water Adsorption on Hydrophilic and Hydrophobic Surfaces of Silicon

Lei Chen,† Xin He,‡ Hongshen Liu,‡ Linmao Qian,†,* Seong H. Kim†,‡,* †

Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China

‡

Department of Chemical Engineering and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA *Corresponding author. E-mail: [email protected], [email protected]

ABSTRACT: The isotherm thickness and hydrogen-bonding interactions of water layers adsorbed on hydrophilic and hydrophobic surfaces were quantified and compared. The hydrophilic and hydrophobic surfaces were modeled with an OH-terminated native oxide layer on silicon and a HF-etched silicon terminated with hydrogen, respectively. The silicon substrate allows the use of attenuated total reflection infrared (ATR-IR) spectroscopy for quantitative measurement of adsorbed water without interferences from the gas phase water. On the hydrophilic Si-OH surface, the average thickness of the strongly-hydrogen-bonded water layer increases up to ~2 molecular layers as relative humidity (RH) increases, beyond which the weakly-hydrogen-bonded structure is dominant. On the hydrophobic Si-H surface, the adsorbed water layer consists predominantly of the weakly-hydrogen-bonded structure and its average thickness remains less than a monolayer even at RH = 90%. The differences in the thickness and structure of adsorbed water layers on hydrophilic versus hydrophobic surfaces found from ATR-IR measurements provide critical insights needed for better 1

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understanding of various physical processes affected by water adsorption in ambient conditions.

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INTRODUCTION Most inorganic materials exposed to ambient air adsorb water and such adsorbed water layers can play a crucial role in biology,1,2 material science,3,4 and tribology.5-7 The unique properties of water in the liquid state stem from its large dipole moment, high polarizability and hydrogen bonding capability.8,9 In the case of water adsorbed on solids, additional interactions with the solid surface make its property or structure deviate from the liquid water.10,11 Understanding the structure of interfacial water is of fundamental interest in many physical processes such as separation of hydrophilic surfaces or macromolecules in water,12 fabrication of uniform films in atomic layer deposition,13 or control of surface wear and interfacial friction of solid materials.14-16 The isotherm thickness and structure of adsorbed water films can vary substantially depending on the surface chemistry and the relative humidity (RH).17 On hydrophilic silicon oxide surfaces, the adsorbed water layer can form strongly-hydrogen-bonded or ordered structures, which was often denoted as solid-like water, ice-like water, or quasi-ice.18,19 The formation of ordered structures is in agreement with density functional theory (DFT) and molecular dynamics (MD) simulations.20-22 With the increase of RH (or partial pressure of water vapor), more disordered or liquid-like structure grows in the adsorbed water layer. The double-layer structure was also reported for mica and metal surfaces.23-25 At hydrophobic surfaces, MD simulations showed that water molecules have a diffusive behavior due to the lack of strong interactions with the surface.26 For example, simulations predicted that water molecules on graphite do not have any hydrogen bond interactions with the surface.27 This paper reports spectroscopic evidence on differences in structural configuration of 3

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water molecules adsorbed on hydrophilic and hydrophobic surfaces of silicon. The thickness and hydrogen-bonding interactions of water adsorbed on hydrophilic native oxide surface (terminated with Si-OH, water contact angle < 5o) and hydrophobic HF-etched surface (terminated with SiH, water contact angle ≈ 83o) were measured as a function of RH using attenuated total reflectance infrared (ATR-IR) spectroscopy. The silicon substrate allows the use of ATR-IR spectroscopy for measurement of adsorbed water without interferences from the gas phase water. Also, silicon is a technically important material in various scientific and engineering applications.28-32 One advantage of ATR-IR over other techniques such as polarization-modulation reflection-absorption infrared spectroscopy (PM-RAIRS) is that it allows quantitative determination of the adsorbed water layer thickness.33 In the past, silane-based self-assembled monolayers (SAMs) were frequently used for modification of silicon surfaces; however, the use of SAMs complicates spectral analysis due to ingress of water to subsurface sites.17,34,35 For this reason, the Si-OH surface of the native oxide on Si and the Si-H surface obtained right after the HF etching were used to model hydrophilic and hydrophobic surface chemistry, respectively. The ATR-IR analysis of these surfaces quantitatively shows how the isotherm thickness and hydrogen-bonding interactions in the adsorbed water layer vary with the surface chemistry of silicon-based materials.

EXPERIMENTAL DETAILS A double-side-polished p-doped Si(100) wafer (~725 µm thick) was cut into the ATR crystal shape (50 mm × 10 mm, with 45o bevel-cut and polish in both ends for IR entrance and exit). The total reflection of IR beams from the probe surface was ~35 times. One crystal 4

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was prepared by cleaning with the RCA-1 solution (aqueous solution with H2O2 and NH3, 80oC), rinsing with DI water and drying with nitrogen. After that, the crystal was exposed to UV/O3 for 30 min, which produced an organic-contaminant-free native oxide surface.36 This sample is named as “Si-OH”. Another ATR crystal was treated with wet etching in hydrofluoric acid (~40 wt.% HF in aqueous solution) for about 5 min, followed by rinsing with DI water and drying with nitrogen. This crystal is denoted as “Si-H”. X-ray photoelectron spectroscopy (XPS) analysis confirmed that the oxygen content at the surface is below the detection limit (see Figure S1 in Supporting Information, SI). The water contact angles were measured to be 98% of the peak area measured before the 9% RH exposure (see Figure S2 in Supporting Information). This result implies that the oxidation of the Si-H surface due to exposure to water vapor at room temperature is negligible over the duration of the measurement time (typically, ~5 hours for a full series from 9% RH to 90% RH measurements) and most SiH groups remain intact. For comparison, the ATR-IR spectrum of liquid water is also shown in Figure 1d. Note that the spectral feature of the bulk liquid is not sensitive to the surface chemistry since the penetration depth of evanescent wave into liquid water is several orders of magnitude larger than the thickness of the adsorbed water layer.42 The difference in hydrogen-bonding interactions of water molecules adsorbed on Si-OH and Si-H surfaces are manifested as variances in peak shape of the O-H stretch mode in the range from 3000 cm-1 to 3800 cm-1 (Figures 1a and 1b).43,44 As the strength of hydrogen bonding interaction increases, the O-H stretching peak position shows a larger red-shift.45 Although the exact peak shape is a complicated function of hydrogen bond dynamics,46 the O-H stretch mode of water in this region can be fitted with two main components - (I) the strongly hydrogen-bonded configuration at 3200~3275 cm-1 and (II) the weakly hydrogen-bonded configuration at 3400~3450 cm-1.37,46,47 The IR spectrum of ice water measured at 100o) even at near-saturation RH.33 In the case of the Si-H surface, the silicon substrate with high polarizability may reduce the water contact angle to 83o, compared to the CH3-terminated SAM surface. However, this may not be sufficient to hold a full monolayer of water on the Si-H surface (Figure 3b). Considering the lack of hydrogen bonding interactions between the water molecule and the Si-H surface, the physisorbed water molecules are likely to form clusters via hydrogen bonding interactions with each other.59 Such clusters may form around defect sites at the Si-H surface (such as a trace amount of Si-OH groups). Since the concentration of oxidized Si sites is below the detection limit of XPS (Figure S1) and the average thickness of the adsorbed water molecules from ATR-IR is nearly half a monolayer (Figure 3b), the water clusters near defect sites must be three-dimensional (3D) islands, instead of two-dimensional patches covering a large area of the surface. The structural configuration of the adsorbed water layers implicated by the ATR-IR data is schematically illustrated in Figure 4. On the hydrophilic Si-OH surface, the water 12

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molecules in the ordered layer are under strong influences of the surface hydroxyl groups, and the disordered layer exists at the air-side (Figure 4a). This configuration is congruent with the structures previously proposed from other spectroscopic observations19,49,60 and MD simulations.61,62 The ordered water is likely to be a fully connected network at the monolayer coverage, based on MD simulations predicting the formation of 2D ice-like water layer.63,64 In the case of the hydrophobic Si-H surface, the adsorbed water layer is very little and they are likely to exist in clusters.65 Those clusters may be formed near defect sites which may have some polarity or contain OH groups. Based on the spectral shape of the OH stretch mode, the structure of these clusters appears to be highly disordered or dynamically changing, regardless of RH conditions of the environment (Figure 2).66 The total averaged thickness is less than the full monolayer coverage even at 90% RH.67 This is due to the lack of hydrogen bonding interactions with the Si-H surface.

(a) (b)

Disordered structure

Water cluster Disordered structure

Ordered structure

Hydrophilic Si-OH surface

Hydrophobic Si-H surface

Figure 4. Schematic illustrating the structural of adsorbed water layer on (a) hydrophilic Si-OH and (b) hydrophobic Si-H surfaces. The dotted yellow lines represent hydrogen bonds. Note that illustration is not to scale. In (a), the water layer adsorbed on the hydrophilic Si-OH surface grows up to full coverage at RH < 10% with highly-ordered structures. Inset in (b) shows the schematic of clusters of water molecules on the hydrophobic Si-H surface.

The ATR-IR analysis of water adsorption provides the structure and thickness 13

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information of the physisorbed water layers on the hydrophilic Si-OH and hydrophobic Si-H surfaces in equilibrium with the gas phase water, which could be related to various physical processes occurring such surfaces. For examples, the protein adsorption on biomaterials is a function of surface wettability.68 Hydrophilic surfaces with a water contact angel less than 60° show a resistance to protein adsorption, whereas a hydrophobic surface can readily adsorb proteins in aqueous solution.68 This could be attributed to the hydrogen bond strength of water molecules at or near the surface (Figure 4). At the hydrophilic surface, water molecules are associated into a strongly hydrogen bonded network which may not be displaced easily. In contrast, the weakly-associated disordered water layers near the hydrophobic surface could be easily displaced by protein molecules, changing the interfacial energy. Furthermore, the lack of hydrogen bonding interactions between water molecules and a hydrophobic surface may be the main cause for the formation of a lower density “depleted” interface at the hydrophobic surface in liquid water.69,70 The energetic instability of such interfaces may play a significant role in collapsing or coagulation of hydrophobic particles in aqueous solution.71,72 The strongly hydrogen-bonded water structure would play a pivotal role in interfacial water flow. MD simulations showed that hydrodynamic slip of water can occur even on hydrophilic surfaces depending on the surface density of hydrophilic sites.73 If the OH groups on the Si-OH surface are assumed to be distributed in a hexagonal array pattern (a close-packed array with equal distance among each other), then the areal density of 5/nm2 would correspond to the distance between the nearest OH groups of ~0.48 nm.38 This is ~1.7 time of the size of water molecule.50 Because the silica surface is known to be a non-slip 14

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surface for water, this distance might be too far to allow the slip of water molecules among adjacent OH sites;73 but, it is close enough to induce strongly hydrogen bonded network covering the entire surface, as evidenced by ATR-IR (Figures 1-3). Also, the structure of the adsorbed water layer is found to be important in nanotribology. The adhesion and friction forces measured at nanoscale are found to be strongly dependent on the amount of ordered water layer formed around the annulus of the contact area on the hydrophilic surface, which is a function of RH in the environment. But, they are much smaller and relatively independent of RH on the hydrophobic surface because the adsorbed water layer is so thin (less than a monolayer in average, Figure 3) and its structure does not change (Figure 2).74-76 Another example is water-assisted mechanochemical reactions of silicon materials. Compared to the disordered water layer, the ordered water layer appears to facilitate mechanochemical reactions that lead to hydrolysis of silicon oxide surfaces, which is manifested as surface wear.77,78 A similar mechanism may pertain to the mechanochemical wear of multicomponent silicate glass materials. 79 The mechanochemical wear of hydrophilic silicon increases with the growth of the ordered water layer; but this is somewhat suppressed upon the growth of disordered water layers at high RH.80

Conclusions The structure and isotherm thickness of physisorbed water layers strongly depend on the surface chemistry of the substrate and RH in the surrounding environment. ATR-IR analysis of the adsorbed water in equilibrium with the gas phase provided the spectroscopic evidence for the strongly hydrogen-bonded ordered network of water molecules on the hydrophilic 15

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Si-OH surface and the lack or rarity of such structures on the hydrophobic Si-H surface. As RH increases, the thickness of the ordered water layer on the hydrophilic Si-OH surface reaches a full monolayer at 60%, and increases fast as RH approaches the saturation value. The water layer on the hydrophobic Si-H surface is highly disordered and less than a monolayer (probably forming isolated clusters) even at RH = 90%.

Supporting Information XPS measurements on Si-H and Si-OH surfaces, ATR-IR analysis of the SiH stretch region of the Si-H surface. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was financially supported by the Natural Science Foundation of China (51505391, 51527901)

and

the

Fundamental

Research

Funds

for

the

Central

Universities

(2682016CX026). ATR-IR measurements were carried out with the support from the National Science Foundation of the USA (Grant No. DMR-1609107 and CMMI-1435766).

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80. Alazizi, A.; Barthel, A. J.; Surdyka, N. D.; Luo, J. W.; Kim, S. H. Vapors in the Ambient—A Complication in Tribological Studies or an Engineering Solution of Tribological Problems? Friction 2015, 3, 85-114. 81. Wang, X. D.; Kim, S. H.; Chen, C.; Chen, L.; He, H. T.; Qian, L. M. ACS Appl. Mater. Interfaces 2015, 7(27), 14785-14792.

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