Article pubs.acs.org/Langmuir
High Surface Water Interaction in Superhydrophobic Nanostructured Silicon Surfaces: Convergence between Nanoscopic and Macroscopic Scale Phenomena Á lvaro Muñoz-Noval,*,†,§ Mercedes Hernando Pérez,‡ Vicente Torres Costa,† Raúl J. Martín Palma,† Pedro J. de Pablo,‡ and Miguel Manso Silván*,† †
Departamento de Física Aplicada, and ‡Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, Madrid, Spain § Instituto de Cerámica y Vidrio, Consejo Superior de Investigaciones Científicas, Madrid, Spain ABSTRACT: In the present work, we investigate wetting phenomena on freshly prepared nanostructured porous silicon (nPS) with tunable properties. Surface roughness and porosity of nPS can be tailored by controlling fabrication current density in the range 40−120 mA/cm2. The length scale of the characteristic surface structures that compose nPS allows the application of thermodynamic wettability approaches. The high interaction energy between water and surface is determined by measuring water contact angle (WCA) hysteresis, which reveals Wenzel wetting regime. Moreover, the morphological analysis of the surfaces by atomic force microscopy allows predicting WCA from a semiempiric model adapted to this material.
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sen θCB = Φ(cos θ Y − 1) + 1
INTRODUCTION Water contact angle (WCA) measurements have become a routinely well established characterization technique in surface science.1,2 However, the application of the wetting formalisms to particular surfaces requires special attention. Recent developments in multiscale and dynamic systems have demonstrated that the most accepted models for wetting on surfaces with controlled roughness cannot be applied in general to every surface.3,4 In the classical wetting theory of rough surfaces (the Wenzel model5) a liquid is considered to penetrate into the grooves of the surface. In a sessile drop configuration, the real area of contact at the liquid−solid interface is higher than the flat projection of the droplet contact area to the surface. The liquid phase under the Wenzel model should present a contact angle θW following the relationship expressed below in terms of Young’s equilibrium contact angle θY (contact angle measured for a flat surface with identical surface chemistry):
where Φ is the ratio between the real contact surface and the projected flat surface (i.e., Φ < 1). For systems with topographical features of sizes in the order of the scale of the solid−liquid interaction area, Wenzel and Cassie−Baxter models do not describe the solid−liquid interaction8 and a dynamic approach is required.9 More recent studies have proved the univocal dependence of the contact angle with interactions along the 3-phase contact line, instead of an interaction through the whole solid-droplet contact surface.10 The Wenzel and Cassie−Baxter approaches are considered a particular case to this model. When the surface is homogeneous and isotropic and the surface roughness elements are significantly smaller than the probing droplets, both models are useful to describe the wetting behavior. WCA hysteresis (HWCA) measurements are a clue point in the determination of the characteristic wetting of a surface. In the nondynamic approach, the HWCA gives the range of possible WCA values between two equilibrium states.11 Moreover, HWCA gives an estimation of the interaction energy between liquid and solid phase in the system, and can be used to elucidate the wetting regime that takes place in a surface.12,13 In this approach, systems with a high HWCA value (higher than the flat surface) correspond to a Wenzel wetting regime. On the
cos θ W = rW cos θ Y (1) where rW is the so-called Wenzel’s roughness parameter, which expresses the ratio between real and apparent solid−liquid contact area. In the Cassie−Baxter model6,7 the liquid is considered not to penetrate inside the surface grooves being locally repelled. The solid−liquid contact area is thus lower than the flat projection of the contact surface. In this case the observed Cassie−Baxter contact angle θCB and Young’s contact angle follow the relation: © 2011 American Chemical Society
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Received: October 21, 2011 Revised: December 9, 2011 Published: December 11, 2011 1909
dx.doi.org/10.1021/la2041289 | Langmuir 2012, 28, 1909−1913
Langmuir
Article
Figure 1. FESEM images of a nPS surface, obtained by electrochemical anodization at 100 mA/cm2, for 100 s in a HF solution 1:1:4 (HF:water:ethanol): (a) 50 000× and (b) 100 000×. amplitude. The images were processed and analyzed for determination of root-mean-square roughness (σ) by using WSxM software. Water contact angle (WCA) measurements were carried out in a KSW 100. For the determination of static WCA (θst), measurement in three to five droplets in four substrates for each condition were averaged. Droplet volumes of 5 μL were used for all the static measurements. For determining advancing contact angle (θAv) and deriving HWCA the syringe was placed over the surface and the water supplied at 0.25 μL/s during 20 s. Meanwhile, 1 image/s was acquired. The inverse process was applied also for determining receding contact angles (θRe). No less than four droplets in four different samples for each condition were measured to determine these values. An image analysis software (CAM2008 and SFECAM2008) was used to determine Static, Advancing and Receding contact angles for each experiment. Once determined, the HWCA values were calculated from
opposite case, low HWCA values correspond to Cassie−Baxter or more complex interaction regimes.1−3,5 Nanostructured porous silicon14 (nPS) is a well-known material with numerous properties and applications (i.e., optoelectronics15 and biomedicine16,17). Its main characteristics are semiconductor behavior, high surface area, high reactivity, photoluminescence, and biocompatibility with several tissues.18,19 These properties can be tailored by tuning the nPS porosity to control the optical response of the material.15 Moreover, it allows tuning the surface topography, which determines wetting as a key parameter in the design of biomedical scaffolds20−24 and in the final distribution of elements in-filtered into nPS.25 In the present work we study the relationship between the surface nanotopography and the wettability of nPS with water. Such comprehensive study is particularly attractive for the optimization of the interactions of nPS with biomedical fluids, in which nPS is increasingly applied.
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HWCA = θAv − θRe
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Fourier transformed infra red (FTIR) spectra were acquired in a BRUKER IFS 66v in diffuse reflectance (DR) set up with a Praying Mantis DR attachment (Harrick, 1994). Spectra were obtained in the 550−7500 cm−1 range.
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EXPERIMENTAL SECTION
RESULTS AND DISCUSSION After nPS fabrication, a preliminary morphological characterization was performed by electron microscopy for a set of selected samples. Figure 1a shows a FESEM image of a nPS100 sample presenting a morphology of pores with irregular shape size (between 20 and 70 nm) and arrangement. This pore range was a common feature for all studied anodization conditions although the distribution maximum varied, modifying consequently the surface topography. In Figure 1b, the morphology of the pores can be appreciated in more detail: in general, a crater-like shape is a common feature for the pores in nPS. Fresh nPS surfaces present a high concentration of Si−H groups conferring, in cooperation with topography, a hydrophobic behavior to the surfaces.15,26 This establishes a radical difference from a standard crystalline Si wafer with a native oxide layer.15,27 The chemical composition of the surface of fresh nPS samples is evaluated by FTIR in order to identify the chemical species present. The DR spectrum of a fresh nPS80 (Figure 2) shows an interference pattern in the nPS layer. Such configuration enhances significantly the signal from surface molecular species which are hidden in transmission measurements from highly doped Si substrates. In fact, this interference pattern is superposed to the absorption bands related to vibrational modes of molecular species on its surface. From higher to lower wavenumbers, the first identified band around 2900−3000 cm−1 can be ascribed to the weak presence of CHx
Preparation of Porous Silicon. nPS has been obtained by electrochemical anodization of crystalline p type (boron doped) Si(100) wafers of high conductivity (0.01−0.02 Ωcm). Electrochemical anodization was carried out in HF: ethanol solutions (1:2 from 48% commercial HF, Sigma Aldrich) under light conditions at RT. Anodization conditions for fabrication of nPS were varied in the range between 40 and 120 mA/cm2, keeping 100 s as the anodization time to obtain nPS layers thicker than 1 μm. After anodization, samples were rinsed in absolute ethanol and dried with N2. Samples will be named in this work from nPS40 to nPS120 to refer to the anodization conditions. The removal of the native oxide layer in Si (100) wafer for the determination of the Young’s WCA (θY) was performed by soaking the Si substrates in HF solutions. The effect of the solution concentration was checked. From a base solution 1:1:1 (HF:water:ethanol), etching solutions were prepared at 1:1:2, 1:1:4, 1:1:8, and 1:1:16. The Si substrates were immersed for 5 min in each of these solutions. Characterization. Field emission scanning electron microscopy (FESEM) images were obtained in a XL 30S-FEG (PHILIPS). No metallization was required to observe the samples. All of the atomic force microscopy (AFM) images were taken with a commercial microscope (Nanotec Electronica S.L; Spain) operating in Amplitude modulation AFM in air. RC800PSA-ORC8 (Olympus, Tokyo) cantilevers were used. Cantilever features were a spring constant of 0.76 N/m, resonance frequency at 67 kHz, and tip radio
