ARTICLE pubs.acs.org/Langmuir
Role of Substrate Wettability in the “Bubble Deposition Method” Applied to the CeVO4 Nanowire Films Claire Costa-Coquelard,† Pascale Jegou,‡ and Jean-Jacques Benattar*,† † ‡
Service de Physique de L'Etat Condense, DSM/IRAMIS/SPEC, CEA, 91191 Gif sur Yvette Cedex, France Laboratoire de Chimie des Surfaces et Interfaces, DSM/IRAMIS/SPCSI, CEA, 91191 Gif sur Yvette Cedex, France
bS Supporting Information ABSTRACT:
Homogeneous two-dimensional structures of CeVO4 nanowires (NWs) deposited on silicon substrates are obtained by means of the bubble deposition method (BDM). Surface wettability (i.e., surface energy) and film ripening (i.e., film thickness) are two major parameters in nanoparticle confinement and deposition. As the presence of surfactant could be detrimental to applications, a washing treatment is developed without CeVO4 chemical changes or NW film modifications. Careful investigations of the film topography are carried out by atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) is used to check the chemical composition of the film at different stages. Finally, samples made by BDM are compared to those made by dip-coating method, demonstrating the higher efficiency of the BDM in providing large areas of well-organized and dense CeVO4 monolayer.
1. INTRODUCTION Controlling the organization of nano-objects is of great scientific interest for functional materials fabrication and has led to the development of various ways to direct nanoparticle assembly. Langmuir-Blodgett (LB), Langmuir-Schaeffer (LS), spin-coating, and dipping methods are widely used to prepare controlled two-dimensional assemblies.1-4 The LangmuirBlodgett method is up to now the most commonly used technique to obtain large area assemblies of nanoparticles. This technique has some specific requirements such as a sophisticated experimental device and hydrophobicity of the nanoparticles.5,6 In the present work, we apply the emerging “bubble deposition method” (BDM) to hydrophilic CeVO4 nanowire systems. The BDM is a powerful way to control the deposition and organization of various nano-object films. Its efficiency has already been proven on different hydrophobic substrates showing its usefulness for the transfer of pure surfactant films.7-9 On this basis, we recently achieved the formation of densely packed single layers made of various nano-objects such as proteins,10,11 gold12 and silica nanoparticles,13 and single-walled carbon nanotubes.14 Self-assemblies of CeVO4 nanowires (NWs) present attractive optical, catalytic, and electrical properties. They could be used as counter electrodes in “electrochromic devices”15-18 or as lowtemperature catalysts for the oxidative dehydrogenation of propane.19 Deng and co-workers have demonstrated the upconverted avalanche luminescence from these oxides and thereby their applications as luminescent materials.20,21 The case of CeVO4 r 2011 American Chemical Society
NWs, is original since it involves, for the first time, hydrophilic nanoparticle organization by means of BDM. NWs are deposited onto silicon substrates with different contact angles under controlled experimental deposition conditions. Indeed, in the present work, we specifically investigate the influence of surface wettability and ripening time (i.e., film thickness before deposition) on the film transfer quality. After studying the deposition stage, we have developed a simple washing procedure to enable elimination of the surfactant necessary for bubble formation. Finally, in order to demonstrate the efficiency of the BDM for CeVO4 NW organization, we present a comparison between films made by the dip coating method (limited to a simple adsorption process22) to those made by the BDM.
2. EXPERIMENTAL SECTION 2.1. Materials. All standard research grade chemicals and solvents were purchased from Sigma Aldrich and used as received. All solutions were prepared with ultrapure water (18.2 MΩ, Milli-Q system). CeVO4 nanowires (about 50-200 nm long and 5 nm thick) have been selectively synthesized by hydrothermal process.21 The nanowires were dissolved in an aqueous solution of sodium dodecylbenzene sulfonate (SDBS) surfactant at 5 CMC (critical micellar concentration). Received: November 22, 2010 Revised: February 15, 2011 Published: March 17, 2011 4397
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Table 1. Contact Angle Data for Etched Silicon Substrates
Figure 1. BDM (a) with adhesion of the bubble onto the substrate is compared to the dip-coating method (b).
silicon substrate
θ°
S-1
90
S-2
35
S-3
20
using a monochromatic Al KR source. The vacuum in the analysis chamber during the measurements was below 5 10-9 Torr. The spectrometer was calibrated by assuming the binding energy (BE) of the Au 4f7/2 line at 84 eV with respect to the Fermi level. All spectra were recorded under the following conditions: pass energy 20 eV, 0.05 eV step-1, 2s step-1; these parameters correspond to a resolution of 0.4 eV. After a Shirley-type background subtraction, the raw spectra were fitted using a nonlinear least-squares fitting program adopting GaussianLorentzian peak shapes for all peaks. The atomic compositions were evaluated using Scofield sensitivity factors.
3. EXPERIMENTAL RESULTS 2.2. Substrate Preparation. Silicon substrates (n-type Si(111)
wafers (1-20 Ω 3 cm)) were provided by Neyco SA. The first stage of a well-known etching method is to degrease the substrates using solvents during 5 min (ultrapure water, ethanol, and acetone). They were then rinsed with Mili-Q water and cleaned in a mixture of concentrated hydrochloric and nitric acid (volume ratio 3:1) for 1 h. After a new vigorous rinsing with Milli-Q water, the substrates were rendered hydrophobic by placing them alternately in piranha mixture and 40% NH4F solution (sample denoted S-1); the details can be found elsewhere.8,9 Before being treated with the NH4F solutions, the substrates were hydrophilic (sample denoted S-2). Another treatment was performed by an UV-ozone exposure during 10 min (sample denoted S-3). All samples were immediately dried under N2 gas flow and sealed to prevent surface pollution. 2.3. Deposition Methods. In the present work, we compare the two simplest methods, BDM and dip-coating. The BDM is a recent and simple experimental method to deposit nano-objects confined in a bubble onto solid substrates at large scale (Figure 1a). Concisely, the bubble is formed inside a closed cell using a very small solution quantity. Bubbles with different sizes can be formed. In this paper, the bubble diameter is typically 1 cm corresponding to a deposited surface area of ∼0.8 cm2. After the drainage, the substrate is approached until touching the bubble, which sticks to the surface, forming a large circular area. Then the bubble bursts, and after the evaporation of remaining water, the substrate is removed from the cell to be characterized. The details of the experimental apparatus can be found elsewhere.7 In the dip coating method,23,24 the substrates are immersed in the CeVO4 solution (with or without SDBS) and are vertically kept in the solution for 2 min before being withdrawn (Figure 1b). 2.4. Contact Angle Determination. Surface wettability is characterized by contact angle measurements performed at room temperature with an Apolo instrument controlled by SCA20 Image software. Each contact angle represents an average value of three separated drops on different sample areas. 2.5. Atomic Force Microscopy (AFM). Topographical imaging of CeVO4 nanowire films was performed in tapping mode at room temperature with a 5500LS atomic force microscope (Agilent Technologies) using Si tips. Images were systematically taken from different areas of the same deposition. The imaging treatment was carried out with the PicoImage software. The height distributions were determined by counting more than 300 NWs from at least six cross sections on each AFM image. The height distribution plots were then fitted using a Gaussian model with Origin 6.1 graphing software. 2.6. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed on a KRATOS Axis Ultra DLD spectrometer,
3.1. Chemical Treatment of the Silicon Surfaces. Three different substrates were prepared to study the influence of surface wettability on the deposition stage. Silicon wafers have been chemically treated to obtain hydrophobic and hydrophilic surfaces.25 The hydrophilic surface of silicon wafers do not require any heavy additional and time-consuming chemical treatment such as those required for the hydrophobic ones. In addition, the deposition on hydrophilic substrates enlarges the variety of substrates that can be used with the BDM and thus, the number of applications. Contact angle measurements were performed on each surface and are summarized in Table 1. Sample S-1 is prepared by NH4F etching treatment and presents a high contact angle characteristic of a hydrophobic state. This property is due to the presence of Si-H groups that are the predominant surface chemical bonds.26 Cleaning procedures with a strong oxidizing acid mixture (S-2) or by UV-ozone exposure (S-3) are widely used to obtain a hydrated SiO2 surface displaying a low contact angle (i.e., hydrophilic properties).26,27 3.2. Film Deposition and Topographic Investigation. 3.2.1. Influence of Film Ripening. Experiments have been performed at different film thicknesses to determine the ripening time influence on the final NW density. It is necessary to emphasize that the term “ripening” corresponds to the time required for the drainage before deposition. Consequently, a given film thickness is related to a well-defined “ripening time”. Typically, a freshly formed soap film is composed of two external air/water interfaces covered by surfactant. As the water drains out of the bubble, the film gets thinner (i.e., there is a decrease of the distance between the surfactant monolayers) and finally becomes black when its thickness is much smaller than the wavelength of the visible light.28-33 The thickness of the water layer ranges from a few angstroms to a few nanometers. Common black film (CBF) possesses a rather large water core, whereas Newton black film (NBF) is much thinner and contains only bonded water molecules.8,34 In this work, the BDM has been only applied to CBFs. The drainage induces a variation in the film thickness producing different colors on the bubble corresponding to the Newton color scale.10,30,35 The experiments have been performed on type S-2 substrates with three different film thicknesses (controlled by the ripening time) of ∼200, 80, and 40 nm corresponding to green, yellow, and gray bubble stages, respectively. 4398
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Table 2. Surface Coverage and Height Distribution from AFM Data According to the Film Thickness before Depositiona film thickness
200 nm
80 nm
40 nm
bubble color
green
yellow
gray
surface coveragea
100%
100%
37%
a
height distribution
a
monolayer
40%
60%
89%
bilayer
47%
34%
11%
trilayer
13%
6%
In average.
The data from AFM measurements are summarized in Table 2; they exhibit differences in their film organization. For the thicker films (200 and 80 nm), the surfaces are entirely covered by the deposited film. Nevertheless there are some differences concerning the film organizations (Figure SI-1 and SI-2, Supporting Information). For the green stage deposition, a resulting bilayer film predominates with some trilayer forms. For the intermediate film thickness (yellow stage), the largest part of the film is a single layer. Thus, this experiment suggests that the NWs become more confined by decreasing the film thickness before deposition, resulting in a more single layer structure. Finally, for the thinnest film (gray stage), drainage was in an advanced state, resulting in a large decrease of the surface coverage. In this case, the film is inhomogeneous with mainly monolayers in the external part of the deposition but with no NWs in the rest of the deposited area (Figure SI-3, Supporting Information). Therefore, during the film transfer onto a substrate, the film thickness exerts a large influence on the NW density: thicker films (≈200 nm) favor multilayer formation while thinner films (≈40 nm) involving a long ripening time lead to a low surface coverage. An intermediate thickness (80 nm) appears to be the best compromise between the NW density and homogeneity and was chosen for the subsequent experiments. 3.2.2. Influence of Substrate Wettability on the Film Transfer Efficiency Hydrophobic Substrates (θ = 90°). As described previously,8 hydrophobic substrates are necessary to interact with a pure surfactant bubble through the hydrophobic-hydrophobic interactions via the alkyl chains pointing outward from the surfactant film. Therefore, first deposition experiments were carried out on hydrophobic surfaces of S-1 type (θ = 90°) with a 80 nm thick film. In Figure 2, the AFM topographic image exhibits an inhomogeneous deposition containing NW aggregates. Some isolated SDBS surfactant patches can be observed on the rest of the substrate. These results clearly indicate a dewetting phenomenon. Here, the NWs are partially covered by surfactants but preserve their hydrophilic character. Thus, the hydrophilic balance of the NW-surfactant system (see further in the text) favors aggregation phenomena. Moderately Hydrophilic Substrates (θ = 35°). As described in section 3.2.1 (Figure SI-2, Supporting Information), the NW film deposited on a hydrophilic substrate (type S-2) is largely monolayered at the center of the deposition. A presence of multilayers can be observed in the rim surrounding the deposited area. Contrary to the hydrophobic substrates, there is no dewetting phenomenon on a lower contact angle surface.
Figure 2. AFM topography image of CeVO4 NW-SDBS film deposited during the yellow bubble stage onto the S-1 silicon substrate.
Figure 3. AFM topography image of CeVO4 NW-SDBS film deposited during the yellow bubble stage onto the S-3 silicon substrate.
Hydrophilic Substrates (θ = 20°). Figure 3 is a typical image of the NW film deposited on a more hydrophilic substrate (type S-3) than the S-2 type surface. In this case, the substrate is entirely capped by a dense, homogeneous, and totally single layer film of NWs. In comparison with the previous hydrophilic type S-2 substrate, an increase of the surface wettability promoting hydrophilic-hydrophilic interactions has allowed a better control of the film structure. Schematic drawings of the film evolution at different stages of the BDM are illustrated Figure 4. The freshly formed film is composed of two surfactant monolayers surrounding a large water core containing nanoparticles (Figure 4a). NWs are long and rigid nano-objects combined to small amounts of surfactant. When the film becomes thinner, they are confined at the air/ water interface (Figure 4b). In spite of the surfactant presence, NWs retain a hydrophilic character which enables the NW film transfer onto the substrate via strong hydrophilic-hydrophilic interactions (Figure 4c). Besides, it must be noticed that, during the CeVO4 film transfer at the CBF state (i.e., with a large water core), spontaneous spreading of the film on the hydrophilic surface can occur.36,37 3.3. XPS Study. 3.3.1. Chemical Integrity. Previously, we demonstrated the possibility to form dense NW films on large scales. As all experiments are conducted in air, it is necessary to perform XPS studies on the CeVO4 oxidation state integrity. The results of CeVO4-SDBS films deposited with the BDM are 4399
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Figure 4. Schematic view (not to scale) of a freshly formed film (a), after drainage stage (b), and deposited on the hydrophilic substrate (c).
Table 3. XPS Binding Energies (eV) of CeVO4-SDBS Film and CeVO4 Film {CeVO4-SDBS}
V 2p3/2
V 2p1/2
O 1s
Ce 3d5/2(1)
Ce 3d5/2(2)
Ce 3d3/2(1)
Ce 3d3/2(2)
517.2
524.4
530.1
881.7
885.7
900.1
904.0
ΔΕ(3/2-5/2)(1) = 18.4 eV CeVO4
517.3
524.5
530.2
881.8
885.9
ΔΕ(3/2-5/2)(1) = 18.3 eV
ΔΕ(3/2-5/2)(2) = 18.3 eV 900.1
904.1
ΔΕ(3/2-5/2)(2) = 18.2 eV
Table 4. Ce/V, O/V, and O/Ce Ratios for Experimental CeVO4-SDBS and CeVO4 Filmsa
a
CeVO4-SDBS film
CeVO4 film
theoretical ratio
Ce 3d5/2/V 2p3/2 O 1s/V 2p3/2
1.1 4.2
1.2 4.0
1.0 4.0
O 1s/Ce 3d3/2
4.1
3.9
4.0
Theoretical ratio is taken as reference.
illustrated in Table 3 and compared to those of a CeVO4 film deposited by dip-coating without surfactant (taken as a reference). The binding energies of Ce 3d doublet (Ce 3d5/2 and Ce 3d3/2), O 1s, and V 2p core levels are in agreement with the results obtained by the Salvi18 and Zhu38 groups. The peak positions of the Ce 3d doublet and the binding energy difference (ΔE) between these two components were similar (18.3 eV), indicating an identical valence state. The XPS results also confirm the film composition with Ce(III) and V(V) oxides such as standard CeVO4. No other peak components and then no Ce4þ species, which indicate surface oxidation, have been detected.39 Moreover, no difference in binding energies appears when the CeVO4 film is deposited in the presence of SDBS. This implies that there are no unexpected interactions between the nanowires and the surfactant and, consequently, no change in the CeVO4 oxidation state. From a quantitative point of view, the chemical film composition obtained after peak fitting shows the same overall st!chiometry as in CeVO4 formula (Table 4). It can be noted that the sulfur S 2s (Figure 5) and the sodium Na 1s signals (spectrum not shown) at 232.6 and 1071.9 eV, respectively, are due to the presence of SDBS. These results confirm the presence of NWs deposited onto the substrate respecting the
Figure 5. XPS spectra of S 2s core levels from CeVO4 NW-SDBS film recorded before (black line) and after (red line) EtOH washing.
CeVO4 stoichiometry and without chemical modifications due to the deposition process. 3.3.2. Surfactant Elimination. In the BDM, surfactants are necessary to stabilize the bubble. However, the surfactant could hinder the NW properties. This problem is often evoked in technology issues dealing with carbon nanotubes because the performances are degraded due to the presence of residual surfactants.40-43 Thus, the surfactant removal is a necessary step after the deposition without affecting the NW film organization. The most efficient treatment consists of a simple vigorous rinsing of the sample with ethanol. XPS study shows the disappearance of the sulfur signal. This indicates a near complete elimination of the surfactant (Figure 5). Moreover the characteristic peak areas of V 2p3/2 are similar to the one obtained before removal (Figure SI-4, Supporting Information) and prove that there is no NW desorption during rinsing. No change in the NW film density has been observed by 4400
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Figure 6. AFM topography images and corresponding cross sections for samples obtained by dipping method (a) onto S-2 substrate with a CeVO4 NW film and (b) onto S-2 substrate with a CeVO4 NW-SDBS film.
AFM; this suggests that such a treatment leads to a total surfactant removal without any modification of the NW film organization. 3.3.3. Comparison with a Dip Coating Method. The main goal of this study was to demonstrate the efficiency of the BDM to obtain dense monolayer films of NWs. As this method does not require any sophisticated equipment, it was thus essential to compare the BDM results to those provided by another simple deposition method. To this end, we performed experiments by simple immersion of the sample in a NW solution, i.e. dip coating method. It has been shown that nano-objects can be easily deposited by dip-coating.22,44-46 Dipping of hydrophilic type S-2 substrates has been carried out in NW solution (Figure 6a) and in NW-SDBS solution (Figure 6b). In both cases, we obtained dense and homogeneous films. Nevertheless, as shown by an AFM cross section analysis, the dip-coated films are mostly multilayer structured and similar results are observed on hydrophilic type S-3 substrates (Figure SI-5, Supporting Information). These observations are further confirmed by XPS (Figure SI-4, Supporting Information). If the dip-coating allows the NW deposition, the method cannot produce homogeneous single layer film.
4. CONCLUSION We successfully showed that the bubble deposition method is suitable for the formation of large and dense single layers assemblies of hydrophilic CeVO4 nanowires. Particularly, we demonstrated that the most relevant parameters for deposition are film ripening time and substrate wettability. The “ripening time”, corresponding to the optimal film thickness before deposition, is the main factor influencing nano-object density. Indeed, above the optimal thickness, the formation of multilayers is favored, whereas in a thinner surface coverage is compromised. Thus, optimization of the “ripening time” allows having simultaneously a high NW density and a good height homogeneity. The nature of the substrate (from hydrophobic to fully hydrophilic) plays an important role in the film transfer and, consequently, in its structure. Let us point out that the most homogeneous NW monolayers are obtained with the most hydrophilic substrates. Conversely, experiments carried out on
hydrophobic substrates lead to aggregate formation, thus illustrating the importance of the hydrophilic-hydrophobic balance. Moreover, after film transfer, XPS analysis has clearly proven that ethanol rinsing does not damage the sample. Finally, investigations on CeVO4 NWs extend the applicability of the BDM to control of nano-object organization at a large scale.
’ ASSOCIATED CONTENT
bS
Supporting Information. AFM topography images (Figures SI-1-3 and SI-5-6) and XPS data (Figure SI-4) of samples realized by BDM or dip coating method. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT J.-J.B. and C.C.-C. acknowledge the DGA (Direction Generale de l’Armement) for its financial support. They thank S. Yang for providing the CeVO4 nanowires and S. Nakamae for a critical reading of this manuscript. ’ REFERENCES (1) Tao, A. R.; Huang, J.; Yang, P. Acc. Chem. Res. 2008, 41, 1662. (2) Liu, J.; Li, Y. Adv. Mater. 2007, 19, 1118. (3) Wang, W.; Gu, B.; Liang, L.; Hamilton, W. J. Phys. Chem. B 2003, 107, 3400. (4) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (5) Wang, W.; Gu, B. J. Phys. Chem. B 2005, 109, 22175. (6) Blute, I.; Pugh, R. J.; Van de Pas, J.; Callaghan, I. J. Colloid Interface Sci. 2009, 336, 584. (7) Benattar, J.-J.; Nedyalkov, M.; Lee, F. K.; Tsui, O. K. C. Angew. Chem., Int. Ed. 2006, 45, 4186. (8) Andreatta, G.; Wang, Y. J.; Lee, F. K.; Polidori, A.; Tong, P.; Pucci, B.; Benattar, J.-J. Langmuir 2008, 24, 6072. (9) Andreatta, G.; Benattar, J.-J.; Petkova, R.; Wang, J. Y. J.; Tong, P.; Polidori, A.; Pucci, B. Colloids Surf., A 2008, 321, 211. (10) Benattar, J.-J.; Nedyalkov, M.; Prost, J.; Tiss, A.; Verger, R.; Guilbert, C. Phys. Rev. Lett. 1999, 82, 5297. 4401
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