Article pubs.acs.org/Langmuir
Hydrophilicity Reinforced Adhesion of Anodic Alumina Oxide Template Films to Conducting Substrates for Facile Fabrication of Highly Ordered Nanorod Arrays Chuanju Wang,†,‡ Guiqiang Wang,†,‡ Rui Yang,† Xiangyu Sun,†,‡ Hui Ma,†,‡ and Shuqing Sun*,† †
Shenzhen Key Laboratory for Minimal Invasive Medical Technologies, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China ‡ Department of Physics, Tsinghua University, Beijing 100084, China S Supporting Information *
ABSTRACT: Arrays of ordered nanorods are of special interest in many fields. However, it remains challenging to obtain such arrays on conducting substrates in a facile manner. In this article, we report the fabrication of highly ordered and vertically standing nanorod arrays of both metals and semiconductors on Au films and indium tin oxide glass substrates without an additional layering. In this approach, following the simple hydrophilic treatment of an anodic aluminum oxide (AAO) membrane and conducting substrates, the AAO membrane was transferred onto the modified substrates with excellent adhesion. Subsequently, nanorod arrays of various materials were electrodeposited on the conducting substrates directly. This method avoids any expensive and tedious lithographic and ion milling process, which provides a simple yet robust route to the fabrication of arrays of 1D materials with high aspect ratio on conducting substrates, which shall pave the way for many practical applications in a range of fields.
1. INTRODUCTION The fabrication of arrays of one-dimensional nanostructures with high density and high aspect ratio has attracted much attention owing to their potential applications in metamaterials,1 supercapacitors,2 and optoelectronic devices.3,4 In the past few years, much effort has been made to generate highly ordered arrays of 1D materials. Among those various synthetic methods, template synthesis is a very promising one. Owing to the structure-oriented property of templates, the size, arrangement, and structure of the desired material can be controlled by adjusting the property of the template. As an easily achievable material, anodic aluminum oxide (AAO) has been extensively used as a template because the aspect ratio, diameter, and length of the core in AAO can easily be controlled by voltage, temperature, electrolytes and anodization time. Different methods have been applied to the fabrication of nanostructure arrays using AAO, such as atomic layer deposition5 and electrodeposition.6−8 Alternating current (Ac) electrodeposition is often used to fabricate the nanorod arrays in AAO templates because it is easy to operate.7,9 The key aspect of the method is that during the second anodization, the voltage is gradually decreased to a certain point to thin the barrier layer for the subsequent electrodeposition of materials. One advantage of the Ac electrodeposition is that without removing the barrier layer, the nanorod arrays can be electrodeposited using Al substrates as the conducting substrate. However, the formation of bifurcating structures at the base of the template during the barrier-thinning step is inevitable. Irregularity of the barrier © XXXX American Chemical Society
layer will result in different electrical resistances in each pore of the AAO template, which can lead to a poor filling rate and nonuniform nanorod arrays. Sputtering an electrically conductive layer (300−500 nm) on one side of the through-hole AAO template as a working electrode has also been successfully adopted by some groups.10,11 In this procedure, the thickness of the AAO template should be at least 20 μm, so that the sputtering and the subsequent operations can be performed,12 whereas for most applications, the thickness of the template needs only to be around a few hundred nanometers.2−4 Another drawback of using this method is that the thick conductive layer can heavily influence the optical properties of the obtained arrays. A method that involves the anodization of a vacuumdeposited Al film onto a supporting substrate has also been introduced.3,4,6,12,13 In this approach, the deposited Al film barely reaches enough thickness, which is essential in the first anodization to form ordered masks.3,13 To overcome the problem caused by the limited thickness of the deposited Al film, prepatterning techniques, such as interference lithography, were also used to form dimple structures on the Al film to replace the function of the first anodization.14 A redundant layer, such as W, is needed to enhance the adhesion between the Al film and the substrate and to avoid the cracking of the anodized film.3,8 In this case, the influence of this new layer on Received: November 4, 2016 Revised: December 22, 2016 Published: December 23, 2016 A
DOI: 10.1021/acs.langmuir.6b03999 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 1. Schematic diagram depicting the synthesis of highly ordered nanorod arrays fabricated directly on the conducting substrates. The final SEM image shows the top view of CdSe nanorod arrays on the ITO glass, and scale bar in the SEM images is 200 nm. least 1 h to eliminate the organic residue. A 2 nm Cr followed by a 50 nm Au film was magnetron-sputtered onto the SiO2 substrate. The SiO2 substrate coated with the Au film was dipped into 50 mM 3MPT for 12 h at 30 °C. Then, the SiO2 substrate was dipped into 0.2 M HCl for 1 h at 30 °C to form hydroxyl groups on its surface. The ITO glass was ultrasonically cleaned in acetone, alcohol, and deionized water in succession for 10 min each. Finally, the ITO glass was irradiated in an ultraviolet (UV) cleaning apparatus of 30 W for 1 h. 2.3. Transfer of the AAO Template onto Conducting Substrates. With the aid of a home-made funnel, the AAO template was transferred to acetone to thoroughly dissolve the nail polish, and then the template was floated on a mixture of deionized water and acetone. After the solution was drawn off using a syringe, the template was finally placed on the gold film or ITO glass followed by transferring the samples to vacuum oven at 150 °C for 30 min. 2.4. Electrodeposition. Before electrodeposition, the nail polish was evenly applied on the conducting substrate that was not covered by the AAO template. Electrodeposition was carried out in a threeelectrode cell at room temperature; the deposition potential is relative to the Ag/AgCl reference electrode. 2.5. Characterization. The morphologies of the nanorod arrays were characterized by field-emission scanning electron microscopy (FE-SEM, Zeiss Supra 55) and transmission electron microscopy (TEM) (Tecnai F30). The X-ray diffraction (XRD) analysis of these samples was measured using Cu Kα radiation (Bruker D8 Advance, λ = 1.5418 Å). Absorption curves of CdSe were measured using a Cary 5000 spectrometer (Agilent).
the optical properties and photovoltaic behavior of the substrate, however, should be assessed.3,8,13 By roughly putting the AAO templates onto different substrates, nanodot arrays can be obtained through vapor deposition.15−17 However, the shadowing effect of AAO templates impeded us to obtain more important high-aspectratio nanorod arrays through physical vapor deposition.18 Limited by the adhesion between the AAO templates and the substrates, electrodeposition cannot be implemented in those experiments because vigorous electrochemical reactions will delaminate the template from conducting substrates. Until now, despite the enormous efforts, a facile and satisfactory method, to fabricate highly ordered nanorod arrays in a large area, has not yet been proposed. In this article, we developed a fairly simple method to integrate highly ordered and vertically standing nanorod arrays onto the indium tin oxide (ITO) glass and metal (gold) films. To achieve this, the hydrophilic treatment of both conducting substrates and AAO templates was carried out to improve the adhesion between the two materials. These hydrophilic modifications resulted in strong adhesion between the AAO templates and the substrates, thus enabling the use as templates for the fabrication of nanorod arrays through electrodeposition.
2. EXPERIMENTAL METHODS
3. RESULTS AND DISCUSSION Figure 1 shows the schematic of our experimental procedure. A highly ordered AAO template was obtained using a two-step anodization method. The AAO templates fabricated using a two-step anodization method display well-arranged nanopore arrays with an areal density of approximately 1 × 1010 per cm2. The nail polish was evenly applied onto the AAO template (Figure 1a). This step is essential to obtain an ultrathin AAO template because during the detachment of the Al substrate, the AAO would inevitably bend in saturated SnCl4. Under the protection of nail polish, the AAO template maintained its structural integrity even if it was extruded vigorously.20 The nail polish filled in the pores of the AAO template can also prevent the diameter of the AAO template from expanding during the removal of the barrier layer in 5 wt % phosphoric acid (Figure 1b). The protective nail polish was dissolved in acetone at room temperature for 10 min. To improve the adhesive force
2.1. AAO Template Fabrication. The AAO template was fabricated using a two-step anodization method.19 High-purity aluminum (40 × 12 × 0.5 mm3) was thoroughly degreased in acetone and 1 M sodium hydroxide for 10 min each. Then, electrochemical polishing was performed in a 4:1 mixture of ethanol/perchloric acid at 20 V and 10 °C to smooth the surface of the aluminum sheet. The first anodization of 10 h at 40 V and 10 °C produced a sacrificial layer of approximately 100 μm. After that, the irregular AAO layer was removed in a mixture of 6 wt % phosphoric acid and 1.8 wt % chromic acid at 65 °C for 7 h. The thickness of the AAO template can be controlled by the time of the second anodization, whereas the pore size can be controlled by the corrosion time in phosphoric acid. Then, the Al substrate was detached in saturated SnCl4. The through-hole AAO template was fabricated after the barrier layer was removed in 5 wt % phosphoric acid at 30 °C for 40 min. Finally, the AAO template was immersed in hydrogen peroxide (30 wt %) for 2 h at room temperature. 2.2. Modification of Substrates. Before vacuum sputtering, the SiO2 substrate was dipped into H2SO4/H2O2 (1:1, v/v) solution for at B
DOI: 10.1021/acs.langmuir.6b03999 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir between AAO templates and the conducting substrates, hydrophilic treatment was carried out to improve the wetting of the two materials. For the AAO template, it was immersed in hydrogen peroxide (30 wt %) for 2 h at room temperature to improve its hydrophilic property (Figure 1b).21 Hydrophilic treatment of the Au film and ITO glass was performed using different methods to decrease the contact angle of the conducting substrates. After the AAO template was placed on the conducting substrate with good adhesion (Figure 1c), electrodeposition was performed using different electrolytes to obtain metal or semiconductor nanorod arrays (Figure 1d). Following the removal of the template in sodium hydroxide (Figure 1e), freeze-drying was used to remove the remaining water to obtain vertically standing nanorod arrays.22 SEM images of AAO templates etched for different lengths of time in 5 wt % phosphoric acid at 20 °C are shown in Figure 2a. The AAO templates fabricated using a two-step anodization
Figure 3. Water contact angle of the unmodified Au film (78°) and Au film modified with 3MPT (98°). After being hydrolyzed in acid media (5°) (top). Water contact angle of the ITO glass before (80°) and after (10°) hydroxide treatment (bottom).
Monolayers of 3MPT on the Au film can effectively protect the underlying metal film from corrosion in solutions, which contains halogen ions, such as chloride (Cl−).26 The terminal methoxyl groups of 3MPT were subsequently converted to hydroxyl groups after hydroxylation in an acid medium (0.2 M HCl), and as a result, the contact angle of the modified Au film decreased to 5°. Transparent conducting substrates are of special interest in many areas. For example, in optoelectronic technology, ITO glass is widely used as the substrate in hybrid solar cells.27,28 Integrating conducting materials onto the ITO glass is of wide interest because ITO features low resistivity but allows transmission of the majority of incoming light. For the ITO glass, hydrophilic modification was performed under irradiation in an UV cleaning apparatus.29,30 This apparatus could emit UV wavelengths of 254 and 185 nm, which can thoroughly clean the surface of the ITO glass. The 185 nm UV ray can be absorbed by O2 to generate O3, whereas the wavelength of 254 nm can disassociate O3 into O2 and active oxygen (O). Large quantities of O produced during this continuous process can thoroughly oxidize the organic residue on the ITO glass to volatile products, such as H2O, N2, and CO2. Meanwhile, a variety of hydrophilic groups, such as −OH and −COOH groups, were introduced under UV irradiation,31 and the contact angle of the UV-irradiated ITO glass decreased from 78 to 10°. The adhesive force between the AAO template and the conducting substrates will be greatly enhanced with the decreased contact angle between the two solid substrates. To qualitatively analyze the relationship between the wetting of two different solid substrates and their interfacial force, a simplified model of two circular substrates with a drop of water on each of them is adopted. Influence of gravity is neglected for simplification. As shown in Figure 4a, surface tensions at three different interfaces can be connected by Young’s equation32−35
Figure 2. Plot shows pore diameter as a function of pore-widening time. SEM images of AAO templates etched for different lengths of time are shown in (a). Contact angle of AAO templates of different states (b).
method display well-arranged nanopore arrays with an areal density of approximately 1 × 1010 per cm2. The pore diameter is calculated from an average of 150 pores with the help of ImageJ software. The etch rate was 0.4 nm per minute, which was consistent with previous experiments.8,23 Figure 2b shows the corresponding contact angle of AAO templates in Figure 2a. The contact angle of AAO templates gradually increase from 80 to 110° with the widening of diameter from 30 to 75 nm, which can be implied using a modified Laplace model.24 The wetting of AAO templates can be greatly improved after modified with hydrogen peroxide. The contact angle of AAO templates with different diameters can all be reduced to less than 10°; the rightmost image in Figure 2b shows the contact angle of AAO with diameters of 75 nm after hydrophilic treatment. Hydrophilic treatment was carried out to improve the wetting of the conducting substrates (Figure 3). For Au film, it was dipped into the ethanol solution of (3-mercaptopropyl) trimethoxysilane (3MPT), which bears two active functional groups. The thiol group of 3MPT can form stable covalent bonds with different metals, such as gold, silver, and copper, whereas the terminal groups of 3MPT (methoxyl groups) can form a hydrophobic layer,25,26 with the contact angle of the modified metallic surface increasing from 78 to 98°.
γSG = γSL + γLG cos θ
0
