Parametric Study of Zinc Oxide Nanomaterials Grown from Aluminum Substrates

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1 Parametric Study of Zinc Oxide Nanomaterials Grown from Aluminum Substrates Aaron Anesgart Chloe Cho Saransh Chopra Michael Higgins Saira Reyes New Jersey Governor s School of Engineering and Technology 2016 Abstract Control over the micro and nanoscale morphology of three dimensional molded parts is an important challenge in the field of materials science engineering. Costeffectiveness, time-effectiveness, reproducibility, and consistent results are desirable factors in the nanofabrication process. However, nanoimprinting does not allow for the creation of reentrant features unless the master mold is destroyed, which makes the process costly and inefficient. Zinc oxide (ZnO) nanostructures can be grown onto various surfaces, such as aluminum, through a seeded growth solution. The aluminum and ZnO nanostructures become a master mold which can be imprinted onto a target polymer. During this process, the zinc oxide becomes a sacrificial material that allows the master material, or the mold, to be removed and reused. In this research, hot water etching and base etching were used to modify aluminum substrates. These pretreatments provide a degree of control over the resulting morphology of ZnO nanostructures. The aluminum/zno substrates were then imprinted into the polymer Kraton D1102. Afterwards, the surface properties, such as hydrophilicity, were analyzed by measuring the contact angles of water droplets on the surface of the material. The results demonstrate how the ZnO nanostructures and the properties of the final material were affected by the various pretreatments. The pretreatments of aluminum substrates were shown to alter the growth of ZnO nanostructures and the physical properties of the materials in which these structures were imprinted. Specifically, ZnO nanostructures in the hot water and base etched samples grew to lengths of about 2 to 3 micrometers. In contrast, the ZnO nanostructures on the untreated aluminum substrate only grew to lengths of about 1 micrometer. 1. Introduction The unique properties of surface micro and nanostructures have many diverse and promising biomedical and industrial applications. For instance, the electrical conductivity, chemical and thermal stability, high electron mobility, and easy fabrication 1

2 of zinc oxide (ZnO) nanostructures make them highly sought in the development of inverted organic solar cells (IOSCs) [1]. The ability of these structures to produce and strengthen desired surface features, such as tunable wettability, anti-icing, and selfcleaning capabilities, is incredibly valuable to the production of optoelectronics such as high-performance LEDs and ultraviolet photodetectors [2], as well as to the future of materials science engineering. One process by which the properties of a material can be controlled to better fit its applications is nanofabrication. This process allows for the modification of a substance s chemical and physical properties, such as wettability and optical features. This research examines several methods of controlling the micro and nanoscale morphology of ZnO nanostructures grown on aluminum substrate by combining nanoimprint lithography and self-assembly processes. 2. Background 2.1 Nano-feature Manufacturing Nanostructures are fabricated through a variety of conventional methods, including three dimensional writing, nanoimprint lithography, and self-assembly [3]. Three dimensional writing and printing allow an extensive control of nanofeatures; however, the fabrication process is slow and only offers precise resolution down to 70 nanometers. Nanoimprinting and interference lithography are faster methods but provide only moderate control over the nanofabrication process as the master material, or the mold, is unable to be altered once the master and imprint are formed. In addition, conventional nanoimprinting requires deforming the target polymer using high temperatures and removing the mold after the polymer cools down, preventing the creation of reentrant features as the polymer grips onto the master and is unable to detach [4]. Self-assembly allows the creation of high resolution surface features from 0.1 nm to 10 nm, but the randomness of self-assembly causes the process to be relatively uncontrollable. Combining the methods of self-assembly and nanoimprinting allows reentrant features to be imprinted at a faster rate while maintaining the high resolution achieved through self-assembly. Although selfassembly is an uncontrolled process, combining it with surface pretreatments and growth parameters can be used to retain a degree of control on nanofeatures. 2.2 ZnO Nanostructures ZnO nanostructures possess many intriguing properties that make them highly attractive for biomedical, electrical, and optical applications [5]. They have highly varied morphologies, which can be easily affected by the different parameters affecting their growth [6]. These parameters include substrate roughness, growth time, and growth temperature. As a result, ZnO nanostructures can take a number of forms, including nanorods, nanosheets, nanobelts, and nanohelixes, because ZnO structures can grow in three different directions at different rates [7]. ZnO nanostructures can be grown using spontaneous growth or a seeded growth solution. When a seeded growth solution is used, the nature of the growth depends on both the type of growth as well as seed properties such as size and distribution. ZnO nanostructures also tend to be very fragile, which can affect the nanoimprinting process. 2.3 Properties of Aluminum Aluminum is a metal widely used for machining purposes such as drilling, bending, and milling. The cost-effectiveness, 2

3 durability, and machinable standards of aluminum make it ideal for macroscale use [8]. In addition, aluminum is a preferred material for production and utilities due to its strength, lightness, conductivity, and recyclability. Aluminum can also be used as a substrate for ZnO nanostructure growth [9]. Different methods of etching may be used as a pretreatment to affect the properties of aluminum prior to the growth process. Base etching and water etching, for example, are performed to create surface roughness. This roughness can potentially affect nanostructure growth. The availability of aluminum makes aluminum-zno templates cost-effective for sacrificial methods of nanoimprinting. In sacrificial nanoimprint lithography, only the ZnO layer is sacrificed, but the substrate can be reused for multiple imprints. In another nanoimprinting method, both the ZnO layer and the aluminum substrate must be sacrificed. ZnO nanostructures, however, are much easier and faster to regrow than conventional nanoimprinting molds. By using aluminum as a substrate, instead of more expensive alternatives such as silicon, the sacrificial methods of nanoimprinting become applicable in the industrial environment. material. In a sacrificial method of nanoimprinting, ZnO nanostructures can be grown onto small aluminum substrates. The resulting aluminum-zno complex becomes the master mold that can be imprinted onto a target polymer. After the imprinting process, both the aluminum substrates and ZnO can be etched off the master. SNIL involves a removal of the aluminum substrate which replaces chemical etching. ZnO nanostructures can be grown onto larger blocks of aluminum and imprinted onto larger disks of a target polymer. The aluminum block is separated after the imprinting process and can be reused for other imprints. Then, ZnO, which is amphoteric, is rinsed off the imprinted polymer using a dilute acetic acid (Figure 1). 2.4 Sacrificial Nanoimprint Lithography Nanoimprinting does not allow for the creation of reentrant features without destroying the original mold, which makes the process slow and costly. Through sacrificial nanoimprint lithography (SNIL), reentrant features are created while allowing the substrate material to be reused after the separation process [10]. The use of a sacrificial film replaces the conventional demolding process after imprinting, as the film can be etched or rinsed off the master ZnO is a component of the sacrificial film in both methods. In comparison with conventional nanoimprint lithography, ZnO can be regrown at a faster speed and in a more cost-effective manner. Similarly, although the chemical etching technique destroys the original aluminum substrate, the process of recreating the aluminum-zno complex is both cost-effective and time-effective. The reagents are very affordable, and SNIL can also be performed without the need of a 3

4 controlled manufacturing environment, making the process ideal for macroscale industrial use. 2.5 Polymers Polymers exemplify a wide range of physical and chemical properties depending on their monomer, shape, and size. Additionally, the presence of cross-links, or bonds between two polymer chains, prevents a polymer from being pulled apart without breaking the bonds. Thus, materials with crosslinks generally are unformable whereas materials without crosslinks are able to be molded. For example, polydimethylsiloxane (PDMS) is a translucent, flexible polymer that is easy to mold. The lack of crosslinks causes PDMS to be super elastic. Because it is also biocompatible and cost-effective, PDMS has many potential applications and is used widely in research. Block copolymers are comprised of more than one monomer and are formed by chemically bonding two distinct monomers. These two materials are connected by an inter-material dividing surface (IMDS), but they repel one another and attempt to separate. Depending on the relative sizes and components of the two materials, these block copolymers form various shapes, such as spheres and cylinders. Kraton D1102 is a type of copolymer can easily be molded into different shapes. 2.6 Wetting Control Wetting control, a common but extremely important surface function of materials and complex organisms, explains the hydrophilic and hydrophobic nature of surfaces [11]. The hydrophobicity or hydrophilicity of a surface depends on the surface-air, surface-liquid, and air-liquid forces acting on a system. In order to quantify the wetting control features materials, the surface-air, surface-liquid, and liquid-air boundaries can be examined through image data analysis, from which contact angles can then be calculated. Contact angles are defined as the degree measure between the surface-liquid boundary and liquid-air boundary of a surface when a liquid is introduced to a surface. They provide keen insight into the degree of hydrophobicity of a particular surface. Contact angles that measure less than 90 indicate that a surface is hydrophilic, and contact angles that measure less than 30 indicate that a surface is superhydrophilic. Contact angles that measure greater than 90 indicate a surface that is hydrophobic, and contact angles that measure greater than 150 indicate that a surface is superhydrophobic. The manipulation of this surface property has many potential industrial applications. For instance, superhydrophobic surfaces can be used in self-cleaning products and machines, such as solar panels, mirrors, and lenses. Additionally, the ability of superhydrophobic surfaces to retain air when immersed in water leads to a number of potential applications, such as the drag reduction of underwater vessels [12]. 3. Methods/Experimental Design 3.1 Substrate Preparation To prepare aluminum foil substrates for the initial pretreatments, small sections of commercial aluminum foil were cut and placed on glass slides. The aluminum foil was then affixed to the glass slide using Kapton tape on both sides of the aluminum foil substrate, and the corners of the glass slide were trimmed so that the slide would fit in the petri dish. Afterwards, the aluminum foil substrates were flattened using a roller. 4

5 3.2 Pretreatments Hot Water Etching For each hot water etching sample, an aluminum foil substrate was placed in a petri dish and submerged in 20 ml of distilled water. Samples were placed in an oven at 90 C for 30, 60, 90, and 120 minutes, for a total of four hot water etched aluminum foil substrate samples. respective petri dishes, rinsed in distilled water, and dried after the one hour of growth (Figure 2). 3.4 Nanoimprinting Kraton D1102 pellets were individually heated to 120 C and flattened using heated platens between glass slides coated with PDMS on the Kraton facing side, Base Etching Three aluminum foil substrates were placed in three different petri dishes and submerged in 20 ml of a 0.25 M potassium hydroxide (KOH) solution. The substrates were submerged in the KOH solution for 10, 20, and 30 minutes respectively, for a total of three base etched samples. After removal from the KOH solution, each sample was submerged in 20 ml of distilled water to rinse off any excess base solution. 3.3 ZnO Growth A ZnO growth solution was prepared for each aluminum sample. First, 20 ml of distilled water and 37 mg of zinc acetate dihydrate were mixed in a 50 ml centrifugal tube for optimal solution formation. Once the zinc acetate dihydrate was fully dissolved, 50 mg of hexamethylenetetramine were added to the solution and mixed until it was dissolved. Then, the ZnO growth solution was poured into a petri dish. The hot water etched, base etched, and untreated aluminum substrates were placed upside down over O-rings in the prepared petri dishes containing the ZnO growth solution. Each petri dish was placed inside an oven at 90 C for one hour. The substrates were removed from their Figure 2: Untreated aluminum foil substrate, Post Growth creating Kraton disks. The use of PDMS prevented the Kraton from sticking to the glass slide. These disks were then allowed to cool before separation from the PDMS slides. For the aluminum foil substrate imprints, each substrate was placed on top of a Kraton disk, growth side down, and placed between PDMS coated glass slides, with the PDMS sides facing towards the Kraton disk and aluminum substrate. This combination was pressed until the top of the heated platen made contact with the glass slide coated with PDMS. The platens were then reheated to 120 C. Once the platens reached 120 C, the two slides of PDMS-coated glass, a Kraton disk, and the aluminum-zno substrate were compressed for approximately two minutes until the unit cooled and was separated. The aluminum substrate was carefully removed from the imprinted Kraton, and the imprint was rinsed in a 0.1 M acetic acid bath (Figure 3). 5

6 Figure 3: Prepressed Kraton disk (left) and imprinted disk (right) 3.5 Scanning Electron Microscopy Scanning Electron Microscopy (SEM) of the aluminum substrates can provide important insight into the morphology of ZnO nanostructures. The images obtained from SEM can qualitatively demonstrate how structures are affected by different substrate pretreatments. 3.6 Contact Angles In order to determine the contact angles of imprinted Kraton D1102, 10 µl water droplets was introduced to the surface of each material and detailed gray-scale images were taken from the flat surface plane (Figure 4). Images were then analyzed for high pixel contrast using Python Imaging Library (PIL). Each pixel within a confined region was compared to its neighboring pixels in all four directions: up, down, left, and right. The regions analyzed in each image consisted of the pixels that made up the water-surface boundary ( flat line ) and the water-air boundary closest to the material ( tangent line ) from both the left and right side of the water droplet. The program measured the brightness of these pixels on a scale of (0 being pure black and 255 being pure white), and printed all coordinates at which the difference in brightness was above a specific threshold (typically between 20 and 40, depending on the amount of points returned). As a general rule, enough points should be returned to make a scatter plot in Microsoft Excel with a distinguishable best fit line, but excess data should be avoided as it can introduce outliers and skew results. It should additionally be noted that Figure 4: Camera side-view of imprinted Kraton disk with a 10 µl water droplet. 6

7 computer pixels run on a reversed y-axis. As a result, all calculated slopes must be multiplied by -1. It is critical that this step is performed before determining a contact angle. Using the slope of a flat best-fit line and a tangent best fit line, the angle at which they intersect can be calculated using the formula: (1) hydrophobic, the calculated angle must be subtracted from 180 to achieve a measure for its obtuse supplement. Additionally, the formula is invalid when the slope of a tangent is determined to be undefined. In this case, the only necessary function is as follows: (2) where θ is the angle and m is the slope of the flat surface. (3) where θ is the contact angle, and m 1 and m 2 are the linear slopes of the lines graphed. The stated formula always finds the acute angle between two intersecting lines; therefore, if the sample appears to be where θ is the angle and m is the slope of the flat surface. Contact angles calculated on both the left and right side of a water droplet were averaged to Figure 5: SEM images of ZnO growth samples at 5000X magnification. A) Untreated aluminum B) Base etching 10 minutes C) Base etching 20 minutes D) Base etching 30 minutes 7

8 Figure 6: SEM images at 5000X magnification A) Hot Water Etching 30 minutes B) Hot water Etching 60 minutes C) Hot Water Etching 90 minutes D) Hot Water Etching 120 minutes represent the contact angle of the material. 4. Results and Discussion 4.1 SEM Imaging The unaltered aluminum substrates were shown to contain small, sheet-like ZnO nanostructures after growth. The individual structures grew to lengths of approximately 1 micrometer. he hot water etched aluminum substrates also exhibited small, sheet-like ZnO nanostructures. However, the individual structures grew to lengths of about 2 to 3 micrometers. The structures appeared to be separate from one another, but they grew at such a high density that there was negligible space between them. Significant differences were observed in the nanostructures of the hot water etched samples. Generally, as the hot water etching time on the substrate increased, the amount of ZnO nanostructures also increased. From 60 to 120 minutes of hot water pretreatment, the data followed the general trend. This trend, however, was not observed when the pretreatment time increased from 30 minutes to 60 minutes. In this instance, the size of the ZnO nanostructures increased while the density decreased. The results indicate that perhaps an optimal hot water etching time can be achieved at a time between 60 and 90 minutes, in which the size of ZnO nanostructures reaches a maximum. This 8

9 could possibly be due to a switch in the way that the hot water etching starts to affect the aluminum substrate within this time period. On the other hand, the base etched samples yielded much larger ZnO structures. The individual structures of the based etched samples grew to about 3 micrometers in length. While the ZnO structures were also densely packed and sheet-like, they were more structurally connected throughout. The T time of the base etching pretreatment also impacted the size and spatial distribution density of the ZnO nanostructures. As the base etching time on the aluminum substrates increased from 10 to 30 minutes, the ZnO nanostructures increased in size. At the same time, the spatial density of nanostructures decreased. In addition, the sheet-like shape of the nanostructures became more defined as base etching time increased. Therefore, the results indicate that base etching has a fairly consistent effect on the morphology of ZnO nanostructures (Figures 5 and 6). 4.2 Imprinting Data The images of 10 µl water droplets on imprinted Kraton disks had varying degrees of hydrophilicity, supporting the hypothesis that the diverse pretreatments used on the aluminum substrates affect the physical properties of imprinted materials differently (Figure 7). Unimprinted Kraton has an average contact angle of 87.03, indicating that Kraton disks are slightly hydrophilic in their unmodified form. When imprinted with an unmodified aluminum substrate with ZnO growth, the polymer disk becomes more hydrophilic, approaching an average contact angle of The Kraton disks imprinted with the post-growth hot water etched samples were slightly more hydrophobic than nonimprinted Kraton disks. These Kraton samples were also hydrophobic overall, as the contact angles for the imprinted disks imprinted were all greater than or equal to 90. Additionally, the imprinted Kraton disks became more hydrophobic as the aluminum substrates they were imprinted on were etched for longer periods of time. However, this did not happen at a very fast rate; the difference in contact angle between Kraton imprinted with unmodified aluminum and Kraton imprinted with 120 minute hot water etched aluminum was only 5. The results found that hot water etching slightly increased the contact angle of the Kraton, however the results were within the standard error bounds rendering the results statistically insignificant. Further testing should be carried out to determine the extent to which the surface properties of Kraton can be affected by hot water etched aluminum substrates. The Kraton disks imprinted with base etched aluminum substrates displayed widely disparate physical properties. Kraton disks imprinted with 10 minute base etched aluminum substrates had low contact angles when compared to Kraton disks imprinted with unmodified aluminum substrates, indicating that these surfaces were highly hydrophilic. In contrast, Kraton disks imprinted with 20 minute and 30 minute base etched aluminum substrates displayed hydrophobic properties, as the contact angles averaged over 110 and 100, respectively. Therefore, base etched aluminum substrates create a peak in hydrophobicity of the Kraton disks. Further experimentation should be carried out to determine a more specific relationship between these two variables. It can be concluded though, that base etching of aluminum substrates can modify the physical properties of imprinted materials to a great extent. It is important to note that the Kraton 9

10 Figure 7: Contact angle analysis of imprinted Kraton samples disks, as well as the aluminum substrates that were used for the imprinting process, were susceptible to contamination throughout the experiment, which could have affected both the growth of the ZnO nanostructures on the aluminum substrates and the final hydrophobicity of the Kraton surfaces. 5. Conclusions Surface pretreatments on the aluminum substrates were shown to have altered the physical properties of the aluminum template, thus affecting the size and shape of the ZnO nanostructures grown, as shown through the varying morphology of ZnO nanostructures in the SEM images. Additionally, the durations of the pretreatments were shown to allow further control over ZnO nanostructure growth. ZnO nanostructures grown on aluminum substrates were shown to have effectively changed the physical properties of imprinted Kraton disks, as shown through the contact angle measurements made. However, these results need to be verified, and the correlations that we established must be further investigated in order to create more accurate depictions of the relationships between our variables. As a result of these findings, it seems that ZnO nanostructures grown from aluminum substrates affected by various pretreatments can in fact be an effective method for manipulation of the surface properties of polymers. Therefore, sacrificial nanoimprint lithography has the potential to pave the way for easier manipulation of the surface properties of materials both on the nanoscale and, in the future, the macroscale. Acknowledgments The authors of this paper would like to thank Professor Jonathan P. Singer for 10

11 providing the opportunity to work in his lab and to conduct this research. The authors would also like to extend their gratitude towards laboratory assistants Imrhankhan Shajahan, Lin Lei, and Tim Ma for all their help in this research. Additionally, the authors are thankful for the assistance of the Residential Teaching Assistants, especially Sandra Pelka, the RTA assigned to this project, and the entire staff at the New Jersey Governor s School of Engineering and Technology. The authors would also like to thank Dean Jean Patrick Antoine and Dean Ilene Rosen, Rutgers University, and the NJ Governor s School of Engineering and Technology for the many opportunities that they have provided throughout the Governor s School program. Finally, the authors would like to thank the sponsors of NJ GSET, including Rutgers School of Engineering, Rutgers State University of New Jersey, Lockheed Martin, South Jersey Industries, and printrbot. References [1] X. Fan, G. Fang, S. Guo, N. Liu, H. Gao, P. Qin, S. Li, H. Long, Q. Zheng and X. Zhao, "Controllable synthesis of flake-like Al-doped ZnO nanostructures and its application in inverted organic solar cells", Nanoscale Res Lett, vol. 6, no. 1, p. 546, [2] A. Djurišić, X. Chen, Y. Leung and A. Man Ching Ng, "ZnO nanostructures: growth, properties and applications", Journal of Materials Chemistry, vol. 22, no. 14, p. 6526, [3] J. Lee, J. Singer and E. Thomas, "Micro-/Nanostructured Mechanical Metamaterials", Adv. Mater., vol. 24, no. 36, pp , [4] C. Huang and K. Ekinci, "Fabrication of freely suspended nanostructures by nanoimprint lithography", Appl. Phys. Lett., vol. 88, no. 9, p , [5] J. Gomez and O. Tigli, "Zinc oxide nanostructures: from growth to application", J Mater Sci, vol. 48, no. 2, pp , [6] Z. Wang, "Zinc oxide nanostructures: growth, properties and applications", Journal of Physics: Condensed Matter, vol. 16, no. 25, pp. R829- R858, [7] J. M. Hancock, Formation and Analysis of Zinc Oxide Nanoparticles and Zinc Oxide Hexagonal Prisms and Optical Analysis of Cadmium Selenide Nanoparticles, Ph.D. dissertation, Dept. Chemistry and Biochemistry, Brigham Young Univ., Provo, UT, [8] "Aluminum metal property & uses: production, transformation & recycling", Constellium.com, [Online]. Available: um-company/aluminium-propertiesand-uses. [Accessed: 16- Jul- 2016]. [9] Y. W. Koh, M. Lin, C. K. Tan, Y. L. Foo, and K. P. Loh, Self-Assembly and Selected Area Growth of Zinc Oxide Nanorods on Any Surface Promoted by an Aluminum Precoat, The Journal of Physical Chemistry B J. Phys. Chem. B, vol. 108, no. 31, pp ,

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