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Fabrication of superhydrophobic vanadium pentoxide nanowires surface by chemical modification
Applied Surface Science 258 (2012) 7455–7459
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of superhydrophobic vanadium pentoxide nanowires surface by chemical modification Karuppanan Senthil a,b,∗ , Guenjae Kwak a , Kijung Yong a,∗∗ a Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea b School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 March 2012 Accepted 9 April 2012 Available online 18 April 2012 Keywords: Oxides Nanowire Morphology Surface energy Superhydrophobicity
a b s t r a c t Vanadium pentoxide (V2 O5 ) nanowires have been synthesized on Au-coated Si substrates by a physical vapor deposition process. The synthesized nanowires are randomly oriented with a diameter around 40–200 nm and length of several micrometers. The crystalline structure of the nanowires analyzed by using X-ray diffraction and Raman spectroscopy corresponds to single crystalline orthorhombic V2 O5 phase with [0 0 1] growth orientation. The transmission electron microscopy and energy-dispersive X-ray analysis suggests a possible vapor–solid (VS) growth mechanism for the V2 O5 nanowires. A selfassembled monolayer (SAM) of octadecyltrichlorosilane (OTS) was deposited on the V2 O5 nanowires to obtain superhydrophobic V2 O5 nanowire surfaces with water contact angle (CA) of 157.5◦ . The superhydrophobic behavior is attributed to the high surface roughness provided by the nanowire surface and low surface energy due to SAM layer deposition. The impact dynamics of water droplets impinging on the superhydrophobic surface is also investigated. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The wettability of a solid surface is an important property because controlling the surface wettability is one of the most important aspects for a wide range of biological, chemical and electronic applications [1–6]. The wettability of a surface can be expressed directly from the contact angle (CA) of a water droplet on the surface. Surfaces with very high contact angles (CA larger than 150◦ ) are called as superhydrophobic surfaces and with low contact angles (CA lower than 5◦ ) are called as super-hydrophilic surfaces. Recently superhydrophobic surfaces have attracted more attention because of their anti-sticking, anti-contamination and self-cleaning properties [7–10]. These properties are desirable for many industrial and biological applications such as antibiofouling paints for boats, anti-sticking of snow for antennas and windows, self-cleaning windshields for automobiles, metal refining, stain resistant textiles and antisoiling architectural coatings
∗ Corresponding author at: Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea; School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea. Tel.: +82 31 2907401. ∗∗ Corresponding author. E-mail addresses: [email protected] (K. Senthil), [email protected] (K. Yong). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.060
[11–14]. Recently, the fabrication of superhydrophobic surfaces on one-dimensional nanostructures has attracted much due to their inherent surface roughness and unique properties [15–19]. It is well-accepted that the wettability of nanostructured surfaces is dominated by the two key factors: (i) surface roughness and (ii) surface energy. The superhydrophobic surfaces are normally characterized by their low surface energy. Changing the surface chemistry effectively modifies the surface energy: a low surface energy gives rise to reduced wettability and thus larger CAs. Nanostructures with low surface energies reduce the water droplet contact area and prevent the penetration of water into the interstices between the nanostructures, leading to a large contact angle and a small hysteresis (CAH). Artificial superhydrophobic surfaces have been mostly fabricated on nanostructures by utilizing a combination of surface roughness and surface modification by chemical coating [15–21]. Of the various transition-metal oxide materials, vanadium oxides are especially interesting because of their wide range of applications [22,23]. Among the various vanadium oxide phases, vanadium pentoxide (V2 O5 ) is one of the most extensively studied of the more stable phases of vanadium oxides because of their outstanding electrochromic, photochromic, catalytic and chemical sensitive properties. Nanostructured V2 O5 has potential application in the fields of chemical sensors, catalysis, actuators, lithiumion batteries, electrochemistry, field emission displays and spintronic devices [24–26]. Nanostructures of various transition-metal
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oxides (ZnO, WO3 and TiO2 ) are well-known to exhibit controllable wettability [18,19,27–30]. According to our knowledge, there are not many reports available about the wettability of V2 O5 nanostructures. Thus the incorporation of water-repellent properties into V2 O5 nanostructures will greatly extend their applications to many other important fields. In the previously reported wettability work of V2 O5 nanostructures [31], the superhydrophobic nanostructured V2 O5 films were fabricated by simply drop-casting an ethanolic suspension of sol–gel synthesized V2 O5 particles onto Si wafers. The intrinsic superhydrophobic behavior of the fabricated V2 O5 films was attributed to the presence of densely packed alkyl chains introduced by the hexadecylamine (used as an organic templating agent during synthesis) and the nanoporous network structures provided by the rose-like V2 O5 films. To our knowledge, there are no previously reported works available on the fabrication of superhydrophobic V2 O5 nanostructures by chemical modification using a self-assembled monolayer (SAM). In the present work, we report for the first time, the fabrication of superhydrophobic V2 O5 nanowire surfaces by chemical modification. We first synthesized V2 O5 nanowires on Au-coated Si substrates by a simple physical vapor deposition process to enhance the surface roughness. Then we have demonstrated that the combination of the surface roughness of the as-grown V2 O5 nanowires with chemical modification by the deposition of an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) leads to the formation of superhydrophobic surfaces. 2. Experimental 2.1. Synthesis and characterization of V2 O5 nanowires Vanadium pentoxide nanowires (V2 O5 NWs) were synthesized in a hot wall horizontal tube furnace using a physical vapor deposition method. The V2 O5 NWs have been fabricated on Si(1 0 0) substrates which were ultrasonically cleaned using acetone and ethanol. The cleaned Si substrates were coated with a thin gold film of thickness 10 nm, to catalyze the growth of the V2 O5 NWs. The V2 O5 powder source was placed at the center of the furnace in a stainless boat and heated above its melting point of 690 ◦ C. The Au-coated Si substrates were placed at the down-stream end (about 17 cm from the source) of the furnace. The tube furnace was evacuated to a pressure of about 1.0 × 10−3 torr and a flow of argon/oxygen (1%) gas was introduced in to the furnace. In a typical experiment, the temperature of the source was increased to about 875 ◦ C at a rate of 17 ◦ C/min to evaporate the V2 O5 powder in the stainless boat. The evaporated source vapor was transported by the Ar/O2 gas (100 sccm) to the location of the substrates in the down-stream end of the furnace. The temperature range in the location of the substrates was found to be 475–500◦ C. The nanowire growth was carried out for about 2 h and the furnace was allowed to cool naturally to room temperature. The structural characteristics of the nanowires were analyzed by X-ray diffraction (XRD; Rigaku D-Max1400) and Raman spectroscopy (SENTERRA dispersive Raman microscope, 532 nm laser wavelength) measurements. The surface morphology and composition of the synthesized V2 O5 NWs were investigated by using field-emission scanning electron microscopy (FE-SEM; JEOL JSM 330F), high-resolution transmission electron microscopy (HR-TEM; JEOL 2100F) and energy-dispersive X-ray spectroscopy (EDX) measurements.
monolayer (SAM) to fabricate superhydrophobic V2 O5 NW surfaces. The OTS layer was deposited onto the V2 O5 NW surface by immersing the substrates in 3 mmol toluene solutions of OTS for 3 h at 4 ◦ C. The resulting surfaces were rinsed with toluene to remove the excess OTS and dried under a gentle stream of nitrogen. The water contact angles were measured using 5 L droplets of deionized water using a contact angle system (Kruss. Model DSA-10) under ambient atmospheric conditions. The dynamic behavior of the impinging water droplets on the fabricated superhydrophobic surfaces was observed using a high-speed charge coupled camera (Fastcam, Model Ultima 512) operated at 2000 frames/s. 3. Results and discussion The structural characteristics and purity of the synthesized vanadium oxide products were analyzed from the X-ray diffraction measurements. Fig. 1 shows the XRD pattern obtained for the synthesized nanostructures. All the diffraction peaks correspond well to an orthorhombic V2 O5 phase (JCPDS card: 89-0162) with ˚ b = 3.571 A˚ and c = 4.383 A. ˚ Any the lattice parameters, a = 11.544 A, characteristic peaks due to impurity phases were not observed in the XRD pattern. The most intense peak corresponding to (0 0 1) plane indicates [0 0 1] as the preferential growth direction of the synthesized nanostructures. The surface morphology of the synthesized V2 O5 NWs was analyzed using SEM measurements. Fig. 2 shows the SEM images of the V2 O5 NWs synthesized on Si substrates. The SEM images clearly showed straight, curved, randomly oriented and free standing nanowires. The nanowires were typically about 40–200 nm thick and several m in length, with the presence of a negligible amount of particles with a different morphology. The crystalline phase of the synthesized nanowires was also identified using Raman spectroscopy measurements. The Raman spectrum acquired for the synthesized nanostructures is shown in Fig. 3. Peaks characteristics of orthorhombic V2 O5 were clearly observed in the Raman spectrum. The Raman peaks identified at the wavenumbers 143, 196, 283, 301, 403, 480, 525, 699 and 993 cm−1 can be assigned to the Raman characteristics of orthorhombic V2 O5 phase. The obtained Raman spectrum was very similar with the spectrum obtained for the previously reported V2 O5 nanowire samples [24,32]. A predominant low-wavenumber peak at 143 cm−1 corresponds to the skeleton bent vibration (B3g mode), while the peaks at 196 and 283 cm−1 can be attributed to the bending vibrations of OC V OB bond (Ag and B2g ) modes. The peaks at 301, 403, 525 cm−1 are assigned to the bending vibration of V OC (Ag mode), the bending vibration of V OB V bond (Ag mode) and the stretching vibration of V OB V bond (Ag mode), respectively. The peaks at 699 and 993 cm−1 can be assigned to the
2.2. Fabrication of superhydrophobic V2 O5 NWs/Si surfaces After the growth of V2 O5 NWs on Si substrates, the substrates were rinsed with deionized water and dried in a stream of nitrogen. We used octadecyltrichlorosilane (OTS) as the self-assembled
Fig. 1. X-ray diffraction pattern of the synthesized V2 O5 nanowires on the Si substrate corresponding to orthorhombic V2 O5 (JCPDS No: 89-0612).
K. Senthil et al. / Applied Surface Science 258 (2012) 7455–7459
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Fig. 2. FE–SEM images of the randomly oriented V2 O5 nanowires on Si substrate.
Fig. 3. Raman spectrum acquired from the V2 O5 nanowires on Si substrate.
stretching vibration of V OC bond (B2g mode) and the stretching of vanadium atoms connected to oxygen atoms through double bonds (Ag mode), respectively. The morphology and crystalline quality of the synthesized V2 O5 nanowires was further investigated by transmission electron microscopy (TEM) measurements. Fig. 4(a) shows the TEM
Fig. 4. (a) TEM image of the V2 O5 nanowires, (b) HR-TEM image of a V2 O5 nanowire, (c) selected area electron diffraction (SAED) pattern obtained from the V2 O5 nanowire, (d) EDX spectrum from the V2 O5 nanowire showing the absence of Au signal.
image of V2 O5 nanowires, which indicates that the synthesized nanowires have smooth surface morphology. The high-resolution TEM (HR-TEM) image of the synthesized V2 O5 nanowire and its corresponding selected area electron diffraction (SAED) pattern are shown in Fig. 4(b) and (c), respectively. The clear lattice images and the sharp diffraction spots in the SAED pattern demonstrate that the synthesized nanowires are single crystalline. The lattice spacing (0.215 nm) measured from the HR-TEM image (Fig. 4(b)) is consistent with that of the (0 0 2) plane of orthorhombic V2 O5 , indicating the nanowire growth direction of [0 0 1]. The EDX spectrum (Fig. 4(d)) acquired from an individual nanowire shows strong signals corresponding to vanadium (V) and oxygen (O). The atomic ratio of V and O is approximately equal to 2:5, indicating the stoichiometry of the V2 O5 nanowires. The signals corresponding to copper (Cu) and carbon (C) are from the copper supporting TEM grid. The EDX spectrum (acquired on the tip of the nanowire) did not show any signals from the gold (Au) catalyst used for the growth of V2 O5 nanowires. The TEM image of the nanowires also did not show any Au particles on the nanowire surfaces. Based on the TEM and EDX observations, we can conclude that vapor–solid (VS) growth process may be the possible growth mechanism for the growth of V2 O5 nanowires. The Au catalyst particles merely provide a low energy interface to collect the precursor materials and acts as a nucleation layer for reducing the nucleation barrier of V2 O5 . When the V2 O5 source powder is evaporated, the V2 O5 source will be continuously generated and the VS phase transition will occur as soon as the vapor reached the Au-coated Si substrate. Superhydrophobic surfaces were obtained by depositing OTS SAM layer onto hydrophilic V2 O5 NWs/Si surfaces. Fig. 5 shows the schematic diagram showing the modification of a hydrophilic V2 O5 NWs/Si surface into an OTS modified superhydrophobic V2 O5 NWs/Si surface. The V2 O5 material on smooth crystal surfaces is usually a hydrophilic material. The water contact angle (CA) for the as-grown V2 O5 NWs/Si surface was almost 0◦ (<5◦ ) as shown in Fig. 5. The water droplet instantaneously soaked into the as-grown nanowire surface with a CA of <5◦ within 0.5 s, which confirms the
Fig. 5. Schematic illustration showing the modification of a hydrophilic V2 O5 NWs/Si surface into an OTS modified superhydrophobic V2 O5 NWs/Si surface.
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Fig. 6. Photograph of the water droplet impinging on the superhydrophobic V2 O5 nanowire surface.
super-hydrophilic behavior of the as-grown V2 O5 NWs/Si surface. The hydrophilic nature of the V2 O5 NWs/Si surface can be explained on the basis of the Wenzel model [33]. In Wenzel’s model, the air trapped in the rough surface can be piled out by water and the substrate can be easily wetted. When the surface is intrinsically hydrophilic, the solid–liquid interaction is favored and the water CA will be decreased with the increase of surface roughness. The low contact angle favored water movement into the rough surface structures by a 3D capillary effect [28]. The observed intrinsic hydrophilic behavior can be attributed to the formation of the hydroxyl groups ( OH) on the V2 O5 NW surface when exposed to humidity in ambient. The hydroxyl group is referred to as a hydrophilic group because it forms hydrogen bonds with water and enhances the hydrophilicity. With the deposition of OTS layer on the V2 O5 NWs/Si surface, the surface energy will be changed. The water droplets on the OTS-modified V2 O5 NWs/Si surface were nearly spherical with a static CA of 157.5◦ (Fig. 5). This superhydrophobic behavior can be explained by the Cassie model. Cassie proposed a equation that describes water CA at a surface composed of solid and air. According to the Cassie model [34], water CAs are directly influenced by the surface fraction of solid (f1 ) versus air pockets (f2 ), where the sum of these two parameters is 1: cos ∗ = f1 cos − f2 The decreased surface energy produced by the OTS coating indicated a low solid surface fraction (f1 ) and a high air surface fraction (f2 ), yielding a high water CA. In this Cassie state, the rough surface is not wetted and can trap air in the interstices between the nanostructures. As a result, the adhesion between the water droplet and the surface is weak and can be even neglected. The capillary effects and the air pockets supported the droplet and prevent the droplet from penetrating the nanostructure [28]. These OTS-coated V2 O5 NWs/Si surfaces showed stable superhydrophobic behavior, in which there were no apparent changes in CA even after 1 month under ambient atmospheric conditions (data not shown). The superhydrophobic behavior of the OTS-coated V2 O5 NWs/Si surfaces can be explained on the basis of the lowering of the surface energy due to chemical modification of the V2 O5 NWs/Si surface. The OTS [CH3 (CH2 )17 SiCl3 ] is one of the widely used SAM molecules used to lower the surface energy in various applications. OTS is an amphiphilic molecule consisting of a long-chain alkyl group (C18 H37 ) and a polar head group (SiCl3 ), which forms SAMs on various oxide substrates. The formation of the monolayer of OTS on the V2 O5 NWs/Si surface involves a first step synthesis of the chloride bonds in silicon. The hydrolysis, which is necessary step in the formation of the cross-linked chain, happens by mass transfer of water vapor from the ambient. All the chloride bonds of silicon undergo hydrolysis causing with the elimination of HCl and formation of Si OH bonds, as shown in the reaction scheme [35].
formation of Si O Si cross linkages, with the elimination of water, resulting in a permanently grafted monolayer of carbon chains. The formation of the cross-linked network of OTS molecules over the V2 O5 NWs/Si surface reduces the surface free energy which thereby increases the hydrophobicity of the surface. Thus the high roughness provided by the nanowire surface and the low surface energy due to OTS deposition, contributes to the formation of superhydrophobic surface. We studied the impact dynamics of droplets impinging on the OTS-coated V2 O5 NWs/Si surfaces to understand how the superhydrophobic coating influenced the dynamic shape of the water droplets. Fig. 6 shows the sequential video images of water droplet with diameter of 1 mm and velocity of 0.2 m/s impinging on the OTS-coated nanowire surface. It was observed that the droplet clearly bounced off the OTS-coated V2 O5 NWs/Si surface without penetrating the nanostructure. The air pockets and capillary forces supported the droplet throughout the impact event. Finally, the droplet rested on the surface and maintained a high CA without undergoing a transition to a wetted state, suggesting the formation of a solid–air–liquid interface. A video file showing a water droplet impinging on the OTS-coated V2 O5 NWs/Si surface is given in the supplementary information. It could be clearly seen that the water droplet bounced from the OTS-coated surface without penetrating into the nanowires. There was almost no resistance to the water droplet to bounce from the prepared superhydrophobic surface.
4. Conclusions Vanadium pentoxide (V2 O5 ) nanowires have been synthesized on Au-coated Si substrates using a horizontal tube furnace by a physical vapor deposition process. The synthesized nanowires are characterized by using SEM, XRD, Raman, TEM and EDX measurements. The nanowires are randomly oriented with 40–200 nm in diameter and several m in length. The XRD and Raman measurements indicate that the synthesized nanowires are single crystalline corresponding to orthorhombic V2 O5 phase oriented along the [0 0 1] growth direction. The absence of any catalyst particles on the tip of the nanowires indicates that vapor–solid (VS) mechanism might be the possible growth mechanism. We fabricated superhydrophobic V2 O5 nanowire surfaces by chemical modification using octadecyltrichlorosilane (OTS) as the SAM layer to lower the surface energy. The water contact angle (CA) increased from about 0◦ (<5◦ ) to 157.5◦ after the deposition of OTS layer on the nanowire surface. This superhydrophobic behavior has been explained on the basis of high roughness induced by the nanowires and lowering of surface energy due to chemical modification by OTS SAM layer. The capability of controlling water droplet dynamics on the superhydrophobic V2 O5 NWs/Si extends the possibility of using V2 O5 nanowire surfaces to many industrial and biological applications.
Acknowledgments
In the next step, the OTS moieties undergo polymerization by formation of covalent bonds between adjacent units leading to the
This work was supported by grants from the National Research Foundation (NRF2010-0009545, NRF2010-0015975), and by the Korean Research Foundation Grants funded by the Korean Government (MOEHRD) (KRF-2008-005-J00501).
K. Senthil et al. / Applied Surface Science 258 (2012) 7455–7459
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of superhydrophobic vanadium pentoxide nanowires surface by chemical modification Karuppanan Senthil a,b,∗ , Guenjae Kwak a , Kijung Yong a,∗∗ a Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea b School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 March 2012 Accepted 9 April 2012 Available online 18 April 2012 Keywords: Oxides Nanowire Morphology Surface energy Superhydrophobicity
a b s t r a c t Vanadium pentoxide (V2 O5 ) nanowires have been synthesized on Au-coated Si substrates by a physical vapor deposition process. The synthesized nanowires are randomly oriented with a diameter around 40–200 nm and length of several micrometers. The crystalline structure of the nanowires analyzed by using X-ray diffraction and Raman spectroscopy corresponds to single crystalline orthorhombic V2 O5 phase with [0 0 1] growth orientation. The transmission electron microscopy and energy-dispersive X-ray analysis suggests a possible vapor–solid (VS) growth mechanism for the V2 O5 nanowires. A selfassembled monolayer (SAM) of octadecyltrichlorosilane (OTS) was deposited on the V2 O5 nanowires to obtain superhydrophobic V2 O5 nanowire surfaces with water contact angle (CA) of 157.5◦ . The superhydrophobic behavior is attributed to the high surface roughness provided by the nanowire surface and low surface energy due to SAM layer deposition. The impact dynamics of water droplets impinging on the superhydrophobic surface is also investigated. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The wettability of a solid surface is an important property because controlling the surface wettability is one of the most important aspects for a wide range of biological, chemical and electronic applications [1–6]. The wettability of a surface can be expressed directly from the contact angle (CA) of a water droplet on the surface. Surfaces with very high contact angles (CA larger than 150◦ ) are called as superhydrophobic surfaces and with low contact angles (CA lower than 5◦ ) are called as super-hydrophilic surfaces. Recently superhydrophobic surfaces have attracted more attention because of their anti-sticking, anti-contamination and self-cleaning properties [7–10]. These properties are desirable for many industrial and biological applications such as antibiofouling paints for boats, anti-sticking of snow for antennas and windows, self-cleaning windshields for automobiles, metal refining, stain resistant textiles and antisoiling architectural coatings
∗ Corresponding author at: Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea; School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea. Tel.: +82 31 2907401. ∗∗ Corresponding author. E-mail addresses: [email protected] (K. Senthil), [email protected] (K. Yong). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.060
[11–14]. Recently, the fabrication of superhydrophobic surfaces on one-dimensional nanostructures has attracted much due to their inherent surface roughness and unique properties [15–19]. It is well-accepted that the wettability of nanostructured surfaces is dominated by the two key factors: (i) surface roughness and (ii) surface energy. The superhydrophobic surfaces are normally characterized by their low surface energy. Changing the surface chemistry effectively modifies the surface energy: a low surface energy gives rise to reduced wettability and thus larger CAs. Nanostructures with low surface energies reduce the water droplet contact area and prevent the penetration of water into the interstices between the nanostructures, leading to a large contact angle and a small hysteresis (CAH). Artificial superhydrophobic surfaces have been mostly fabricated on nanostructures by utilizing a combination of surface roughness and surface modification by chemical coating [15–21]. Of the various transition-metal oxide materials, vanadium oxides are especially interesting because of their wide range of applications [22,23]. Among the various vanadium oxide phases, vanadium pentoxide (V2 O5 ) is one of the most extensively studied of the more stable phases of vanadium oxides because of their outstanding electrochromic, photochromic, catalytic and chemical sensitive properties. Nanostructured V2 O5 has potential application in the fields of chemical sensors, catalysis, actuators, lithiumion batteries, electrochemistry, field emission displays and spintronic devices [24–26]. Nanostructures of various transition-metal
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oxides (ZnO, WO3 and TiO2 ) are well-known to exhibit controllable wettability [18,19,27–30]. According to our knowledge, there are not many reports available about the wettability of V2 O5 nanostructures. Thus the incorporation of water-repellent properties into V2 O5 nanostructures will greatly extend their applications to many other important fields. In the previously reported wettability work of V2 O5 nanostructures [31], the superhydrophobic nanostructured V2 O5 films were fabricated by simply drop-casting an ethanolic suspension of sol–gel synthesized V2 O5 particles onto Si wafers. The intrinsic superhydrophobic behavior of the fabricated V2 O5 films was attributed to the presence of densely packed alkyl chains introduced by the hexadecylamine (used as an organic templating agent during synthesis) and the nanoporous network structures provided by the rose-like V2 O5 films. To our knowledge, there are no previously reported works available on the fabrication of superhydrophobic V2 O5 nanostructures by chemical modification using a self-assembled monolayer (SAM). In the present work, we report for the first time, the fabrication of superhydrophobic V2 O5 nanowire surfaces by chemical modification. We first synthesized V2 O5 nanowires on Au-coated Si substrates by a simple physical vapor deposition process to enhance the surface roughness. Then we have demonstrated that the combination of the surface roughness of the as-grown V2 O5 nanowires with chemical modification by the deposition of an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) leads to the formation of superhydrophobic surfaces. 2. Experimental 2.1. Synthesis and characterization of V2 O5 nanowires Vanadium pentoxide nanowires (V2 O5 NWs) were synthesized in a hot wall horizontal tube furnace using a physical vapor deposition method. The V2 O5 NWs have been fabricated on Si(1 0 0) substrates which were ultrasonically cleaned using acetone and ethanol. The cleaned Si substrates were coated with a thin gold film of thickness 10 nm, to catalyze the growth of the V2 O5 NWs. The V2 O5 powder source was placed at the center of the furnace in a stainless boat and heated above its melting point of 690 ◦ C. The Au-coated Si substrates were placed at the down-stream end (about 17 cm from the source) of the furnace. The tube furnace was evacuated to a pressure of about 1.0 × 10−3 torr and a flow of argon/oxygen (1%) gas was introduced in to the furnace. In a typical experiment, the temperature of the source was increased to about 875 ◦ C at a rate of 17 ◦ C/min to evaporate the V2 O5 powder in the stainless boat. The evaporated source vapor was transported by the Ar/O2 gas (100 sccm) to the location of the substrates in the down-stream end of the furnace. The temperature range in the location of the substrates was found to be 475–500◦ C. The nanowire growth was carried out for about 2 h and the furnace was allowed to cool naturally to room temperature. The structural characteristics of the nanowires were analyzed by X-ray diffraction (XRD; Rigaku D-Max1400) and Raman spectroscopy (SENTERRA dispersive Raman microscope, 532 nm laser wavelength) measurements. The surface morphology and composition of the synthesized V2 O5 NWs were investigated by using field-emission scanning electron microscopy (FE-SEM; JEOL JSM 330F), high-resolution transmission electron microscopy (HR-TEM; JEOL 2100F) and energy-dispersive X-ray spectroscopy (EDX) measurements.
monolayer (SAM) to fabricate superhydrophobic V2 O5 NW surfaces. The OTS layer was deposited onto the V2 O5 NW surface by immersing the substrates in 3 mmol toluene solutions of OTS for 3 h at 4 ◦ C. The resulting surfaces were rinsed with toluene to remove the excess OTS and dried under a gentle stream of nitrogen. The water contact angles were measured using 5 L droplets of deionized water using a contact angle system (Kruss. Model DSA-10) under ambient atmospheric conditions. The dynamic behavior of the impinging water droplets on the fabricated superhydrophobic surfaces was observed using a high-speed charge coupled camera (Fastcam, Model Ultima 512) operated at 2000 frames/s. 3. Results and discussion The structural characteristics and purity of the synthesized vanadium oxide products were analyzed from the X-ray diffraction measurements. Fig. 1 shows the XRD pattern obtained for the synthesized nanostructures. All the diffraction peaks correspond well to an orthorhombic V2 O5 phase (JCPDS card: 89-0162) with ˚ b = 3.571 A˚ and c = 4.383 A. ˚ Any the lattice parameters, a = 11.544 A, characteristic peaks due to impurity phases were not observed in the XRD pattern. The most intense peak corresponding to (0 0 1) plane indicates [0 0 1] as the preferential growth direction of the synthesized nanostructures. The surface morphology of the synthesized V2 O5 NWs was analyzed using SEM measurements. Fig. 2 shows the SEM images of the V2 O5 NWs synthesized on Si substrates. The SEM images clearly showed straight, curved, randomly oriented and free standing nanowires. The nanowires were typically about 40–200 nm thick and several m in length, with the presence of a negligible amount of particles with a different morphology. The crystalline phase of the synthesized nanowires was also identified using Raman spectroscopy measurements. The Raman spectrum acquired for the synthesized nanostructures is shown in Fig. 3. Peaks characteristics of orthorhombic V2 O5 were clearly observed in the Raman spectrum. The Raman peaks identified at the wavenumbers 143, 196, 283, 301, 403, 480, 525, 699 and 993 cm−1 can be assigned to the Raman characteristics of orthorhombic V2 O5 phase. The obtained Raman spectrum was very similar with the spectrum obtained for the previously reported V2 O5 nanowire samples [24,32]. A predominant low-wavenumber peak at 143 cm−1 corresponds to the skeleton bent vibration (B3g mode), while the peaks at 196 and 283 cm−1 can be attributed to the bending vibrations of OC V OB bond (Ag and B2g ) modes. The peaks at 301, 403, 525 cm−1 are assigned to the bending vibration of V OC (Ag mode), the bending vibration of V OB V bond (Ag mode) and the stretching vibration of V OB V bond (Ag mode), respectively. The peaks at 699 and 993 cm−1 can be assigned to the
2.2. Fabrication of superhydrophobic V2 O5 NWs/Si surfaces After the growth of V2 O5 NWs on Si substrates, the substrates were rinsed with deionized water and dried in a stream of nitrogen. We used octadecyltrichlorosilane (OTS) as the self-assembled
Fig. 1. X-ray diffraction pattern of the synthesized V2 O5 nanowires on the Si substrate corresponding to orthorhombic V2 O5 (JCPDS No: 89-0612).
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Fig. 2. FE–SEM images of the randomly oriented V2 O5 nanowires on Si substrate.
Fig. 3. Raman spectrum acquired from the V2 O5 nanowires on Si substrate.
stretching vibration of V OC bond (B2g mode) and the stretching of vanadium atoms connected to oxygen atoms through double bonds (Ag mode), respectively. The morphology and crystalline quality of the synthesized V2 O5 nanowires was further investigated by transmission electron microscopy (TEM) measurements. Fig. 4(a) shows the TEM
Fig. 4. (a) TEM image of the V2 O5 nanowires, (b) HR-TEM image of a V2 O5 nanowire, (c) selected area electron diffraction (SAED) pattern obtained from the V2 O5 nanowire, (d) EDX spectrum from the V2 O5 nanowire showing the absence of Au signal.
image of V2 O5 nanowires, which indicates that the synthesized nanowires have smooth surface morphology. The high-resolution TEM (HR-TEM) image of the synthesized V2 O5 nanowire and its corresponding selected area electron diffraction (SAED) pattern are shown in Fig. 4(b) and (c), respectively. The clear lattice images and the sharp diffraction spots in the SAED pattern demonstrate that the synthesized nanowires are single crystalline. The lattice spacing (0.215 nm) measured from the HR-TEM image (Fig. 4(b)) is consistent with that of the (0 0 2) plane of orthorhombic V2 O5 , indicating the nanowire growth direction of [0 0 1]. The EDX spectrum (Fig. 4(d)) acquired from an individual nanowire shows strong signals corresponding to vanadium (V) and oxygen (O). The atomic ratio of V and O is approximately equal to 2:5, indicating the stoichiometry of the V2 O5 nanowires. The signals corresponding to copper (Cu) and carbon (C) are from the copper supporting TEM grid. The EDX spectrum (acquired on the tip of the nanowire) did not show any signals from the gold (Au) catalyst used for the growth of V2 O5 nanowires. The TEM image of the nanowires also did not show any Au particles on the nanowire surfaces. Based on the TEM and EDX observations, we can conclude that vapor–solid (VS) growth process may be the possible growth mechanism for the growth of V2 O5 nanowires. The Au catalyst particles merely provide a low energy interface to collect the precursor materials and acts as a nucleation layer for reducing the nucleation barrier of V2 O5 . When the V2 O5 source powder is evaporated, the V2 O5 source will be continuously generated and the VS phase transition will occur as soon as the vapor reached the Au-coated Si substrate. Superhydrophobic surfaces were obtained by depositing OTS SAM layer onto hydrophilic V2 O5 NWs/Si surfaces. Fig. 5 shows the schematic diagram showing the modification of a hydrophilic V2 O5 NWs/Si surface into an OTS modified superhydrophobic V2 O5 NWs/Si surface. The V2 O5 material on smooth crystal surfaces is usually a hydrophilic material. The water contact angle (CA) for the as-grown V2 O5 NWs/Si surface was almost 0◦ (<5◦ ) as shown in Fig. 5. The water droplet instantaneously soaked into the as-grown nanowire surface with a CA of <5◦ within 0.5 s, which confirms the
Fig. 5. Schematic illustration showing the modification of a hydrophilic V2 O5 NWs/Si surface into an OTS modified superhydrophobic V2 O5 NWs/Si surface.
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Fig. 6. Photograph of the water droplet impinging on the superhydrophobic V2 O5 nanowire surface.
super-hydrophilic behavior of the as-grown V2 O5 NWs/Si surface. The hydrophilic nature of the V2 O5 NWs/Si surface can be explained on the basis of the Wenzel model [33]. In Wenzel’s model, the air trapped in the rough surface can be piled out by water and the substrate can be easily wetted. When the surface is intrinsically hydrophilic, the solid–liquid interaction is favored and the water CA will be decreased with the increase of surface roughness. The low contact angle favored water movement into the rough surface structures by a 3D capillary effect [28]. The observed intrinsic hydrophilic behavior can be attributed to the formation of the hydroxyl groups ( OH) on the V2 O5 NW surface when exposed to humidity in ambient. The hydroxyl group is referred to as a hydrophilic group because it forms hydrogen bonds with water and enhances the hydrophilicity. With the deposition of OTS layer on the V2 O5 NWs/Si surface, the surface energy will be changed. The water droplets on the OTS-modified V2 O5 NWs/Si surface were nearly spherical with a static CA of 157.5◦ (Fig. 5). This superhydrophobic behavior can be explained by the Cassie model. Cassie proposed a equation that describes water CA at a surface composed of solid and air. According to the Cassie model [34], water CAs are directly influenced by the surface fraction of solid (f1 ) versus air pockets (f2 ), where the sum of these two parameters is 1: cos ∗ = f1 cos − f2 The decreased surface energy produced by the OTS coating indicated a low solid surface fraction (f1 ) and a high air surface fraction (f2 ), yielding a high water CA. In this Cassie state, the rough surface is not wetted and can trap air in the interstices between the nanostructures. As a result, the adhesion between the water droplet and the surface is weak and can be even neglected. The capillary effects and the air pockets supported the droplet and prevent the droplet from penetrating the nanostructure [28]. These OTS-coated V2 O5 NWs/Si surfaces showed stable superhydrophobic behavior, in which there were no apparent changes in CA even after 1 month under ambient atmospheric conditions (data not shown). The superhydrophobic behavior of the OTS-coated V2 O5 NWs/Si surfaces can be explained on the basis of the lowering of the surface energy due to chemical modification of the V2 O5 NWs/Si surface. The OTS [CH3 (CH2 )17 SiCl3 ] is one of the widely used SAM molecules used to lower the surface energy in various applications. OTS is an amphiphilic molecule consisting of a long-chain alkyl group (C18 H37 ) and a polar head group (SiCl3 ), which forms SAMs on various oxide substrates. The formation of the monolayer of OTS on the V2 O5 NWs/Si surface involves a first step synthesis of the chloride bonds in silicon. The hydrolysis, which is necessary step in the formation of the cross-linked chain, happens by mass transfer of water vapor from the ambient. All the chloride bonds of silicon undergo hydrolysis causing with the elimination of HCl and formation of Si OH bonds, as shown in the reaction scheme [35].
formation of Si O Si cross linkages, with the elimination of water, resulting in a permanently grafted monolayer of carbon chains. The formation of the cross-linked network of OTS molecules over the V2 O5 NWs/Si surface reduces the surface free energy which thereby increases the hydrophobicity of the surface. Thus the high roughness provided by the nanowire surface and the low surface energy due to OTS deposition, contributes to the formation of superhydrophobic surface. We studied the impact dynamics of droplets impinging on the OTS-coated V2 O5 NWs/Si surfaces to understand how the superhydrophobic coating influenced the dynamic shape of the water droplets. Fig. 6 shows the sequential video images of water droplet with diameter of 1 mm and velocity of 0.2 m/s impinging on the OTS-coated nanowire surface. It was observed that the droplet clearly bounced off the OTS-coated V2 O5 NWs/Si surface without penetrating the nanostructure. The air pockets and capillary forces supported the droplet throughout the impact event. Finally, the droplet rested on the surface and maintained a high CA without undergoing a transition to a wetted state, suggesting the formation of a solid–air–liquid interface. A video file showing a water droplet impinging on the OTS-coated V2 O5 NWs/Si surface is given in the supplementary information. It could be clearly seen that the water droplet bounced from the OTS-coated surface without penetrating into the nanowires. There was almost no resistance to the water droplet to bounce from the prepared superhydrophobic surface.
4. Conclusions Vanadium pentoxide (V2 O5 ) nanowires have been synthesized on Au-coated Si substrates using a horizontal tube furnace by a physical vapor deposition process. The synthesized nanowires are characterized by using SEM, XRD, Raman, TEM and EDX measurements. The nanowires are randomly oriented with 40–200 nm in diameter and several m in length. The XRD and Raman measurements indicate that the synthesized nanowires are single crystalline corresponding to orthorhombic V2 O5 phase oriented along the [0 0 1] growth direction. The absence of any catalyst particles on the tip of the nanowires indicates that vapor–solid (VS) mechanism might be the possible growth mechanism. We fabricated superhydrophobic V2 O5 nanowire surfaces by chemical modification using octadecyltrichlorosilane (OTS) as the SAM layer to lower the surface energy. The water contact angle (CA) increased from about 0◦ (<5◦ ) to 157.5◦ after the deposition of OTS layer on the nanowire surface. This superhydrophobic behavior has been explained on the basis of high roughness induced by the nanowires and lowering of surface energy due to chemical modification by OTS SAM layer. The capability of controlling water droplet dynamics on the superhydrophobic V2 O5 NWs/Si extends the possibility of using V2 O5 nanowire surfaces to many industrial and biological applications.
Acknowledgments
In the next step, the OTS moieties undergo polymerization by formation of covalent bonds between adjacent units leading to the
This work was supported by grants from the National Research Foundation (NRF2010-0009545, NRF2010-0015975), and by the Korean Research Foundation Grants funded by the Korean Government (MOEHRD) (KRF-2008-005-J00501).
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