Growth of arrays of oriented epitaxial platinum nanoparticles with controlled size and shape by natural colloidal lithography
© Komanicky et al.; licensee Springer. 2014
Received: 26 May 2014
Accepted: 27 June 2014
Published: 5 July 2014
We developed a method for production of arrays of platinum nanocrystals of controlled size and shape using templates from ordered silica bead monolayers. Silica beads with nominal sizes of 150 and 450 nm were self-assembled into monolayers over strontium titanate single crystal substrates. The monolayers were used as shadow masks for platinum metal deposition on the substrate using the three-step evaporation technique. Produced arrays of epitaxial platinum islands were transformed into nanocrystals by annealing in a quartz tube in nitrogen flow. The shape of particles is determined by the substrate crystallography, while the size of the particles and their spacing are controlled by the size of the silica beads in the monolayer mask. As a proof of concept, arrays of platinum nanocrystals of cubooctahedral shape were prepared on (100) strontium titanate substrates. The nanocrystal arrays were characterized by atomic force microscopy, scanning electron microscopy, and synchrotron X-ray diffraction techniques.
Supported transition metal nanoparticles are widely used as catalysts and electrocatalysts in many industrial applications. Carbon-based electrically conducting supports are frequently used in the low-temperature proton exchange membrane fuel cells, while the refractory metal-oxide supports are used in moderate- and high-temperature applications such as automotive catalytic converters. Platinum is one of the most commonly used catalysts. Studies with single crystals  showed that catalyst activity can be influenced by the atomic arrangement of the catalyst surface as well as the presence of the defect sites. In the case of nanoparticulate catalysts, the shape can be an important governing factor in overall catalyst activity  because the nanoparticle shape is dictated by the crystallography of facets with the lowest surface energy. Each facet can have different specific catalytic activities. Particle-substrate interface crystallography and interfacial energy are an additional shape-controlling factor of supported catalysts . The ability to fabricate well-defined model systems on various substrates where one can systematically vary the size, shape, and spacing between nanoparticles is of high fundamental  and practical importance . Nanofabricated supported model catalyst systems can be probed with traditional scanning probe imaging techniques and synchrotron X-ray surface characterization tools. In the past, top-down nanofabrication techniques such as electron beam lithography (EBL) have been successfully used to produce platinum catalyst arrays [2, 6, 7]. Expensive instrumentation and multistep pattern transfer procedures make production of these systems challenging and costly. Additionally, EBL is a rather slow serial technique, and patterning of several square millimeters of the substrate area with densely packed arrays of dots can take many hours. For the practical applications, e.g., fuel cells, the total catalyst area has to be in the order of hundreds of square meters. There is clearly a motivation to produce well-defined catalyst samples supported on various substrates using cheaper and faster techniques. Natural lithography  alone or in combination with other techniques has been successfully used to produce metallic nanostructures and nanoparticle crystallites of random shape  and orientation . The purpose of this report is to present a simple two-step process based on mask templates of a self-assembled silica colloidal sphere monolayer suitable for production of epitaxially oriented platinum nanoparticle arrays with precisely controlled shape. Shape and orientation of the particles are controlled via substrate crystallography, and particle size and spatial distribution are controlled via size of colloidal silica spheres used in monolayer template. We demonstrated only preparation of one type of particle shape, but it is possible to make different particle shapes if substrates with other crystallographic orientations are used [2, 7]. Since the nanoparticles are supported on the annealable and electrically conducting Nb-doped strontium titanate (STO) substrates, the samples can be used both in electrocatalysis and gas phase catalysis.
Preparation of monodispersed colloidal silica spheres
Silica nanospheres were synthesized following the Stöber-Fink-Bohn method  starting from tetraethyl orthosilicate (TEOS 98%, Sigma-Aldrich, St. Louis, MO, USA), deionized water, ammonia (25%, Merck, Whitehouse Station, NJ, USA), and absolute ethanol (99.9%, Riedel-de Haën, Seelze, Germany) as precursor alkoxide, hydrolyzing agent, catalyst, and solvent, respectively. Two mother solutions were prepared: one containing ammonia-water and another one containing TEOS-ethanol. First, we add the ammonia-water solution to a solution of TEOS-ethanol kept at 50°C ± 1°C, in one step. Then, the solution was mixed and put back into the controlled water bath (50°C ± 1°C), for 1 h (no mixing). After 60 min, the resulting spheres were separated from the liquid phase with centrifugation and then ultrasonically dispersed in deionized water. The procedure was repeated three times. Then, the particles were dried in an oven at 50°C. Note that using this method, the final particle size critically depends on the reagent concentrations, molar ratio, and reaction temperature, so that difficulties are usually encountered in obtaining both a good control of the sphere size in a wide dimensional range and monodispersity with size distribution as narrow as possible. In this paper, we applied conditions for the synthesis of silica particles with well-defined particle size as described in . We synthesized samples with nominal particle sizes of 150 and 450 nm.
Preparation of monolayers of silica colloidal spheres on the STO substrates
The substrates are commercially available epi-polished (100)-oriented STO single crystals doped with Nb (MTI Corporation, Richmond, CA, USA; 0.7% to 1% Nb doping, resistivity 0.0035 to 0.007 Ω cm). The samples were etched for 4 min in a 3:1 mixture of concentrated nitric and hydrochloric acid, rinsed in deionized water, placed in a quartz tube, and annealed in air at 800°C; 0.2 wt.% of dried monodispersed colloidal silica was suspended in methanol using an ultrasonic bath. In order to deposit the monolayer of silica spheres, standard monodispersed colloidal spheres can be self-assembled into ordered 2D arrays using several approaches [13, 14]. Initially, we used a method based on the transferring monolayer formed on the air-liquid interface by slowly draining colloid solution. This method works well for silica containing substrates such as glass slides. Investigation by atomic force microscopy (AFM) showed that this method used with STO substrates did not yield a continuous monolayer. Therefore, we directly micropipetted a colloidal silica sphere solution on the substrate squares with an area of 5 × 5 mm2. The solution contained enough silica spheres to give a full monolayer of colloidal silica spheres. A small droplet of water (approximately 10 μl) was also placed on top of the colloidal solution on the substrates. The solution on top of STO has been dried under continuous sonication. AFM images of deposited silica layers were acquired with a Bruker AFM model Icon (Bruker, the Netherlands). The silicone cantilevers were purchased from MikroMasch (Wetzlar, Germany) with a force constant of 14 N m−1. All images were acquired using tapping mode under ambient laboratory conditions. An epitaxial platinum film with a thickness of 8 nm was evaporated by e-beam evaporation using a three-step deposition technique . A monolayer of silica beads was removed by sonication in hot concentrated potassium hydroxide aqueous solution. The nanocrystal arrays were characterized by X-ray diffraction (XRD) to confirm the orientation of crystalline platinum islands with respect to the substrate. The diffraction experiments were performed at the Advanced Photon Source (APS) using the four-circle diffractometer with a vertical scattering geometry at beamline 12BM. The incident energy was 11.5 keV, and beam defining slits were set to 1 mm with an under-focused beam. From our experience, intense synchrotron X-ray beam in the presence of oxygen from air causes damage to platinum single crystal surfaces. Most likely, this damage is a result of interaction between reactive free radicals generated from oxygen and platinum metal. We protected delicate nanocrystal arrays from X-ray damage by flowing ultra-high purity nitrogen gas into a polypropylene bag placed over the sample. For the STO (001) substrates, the Pt (004) and four (113) Bragg peaks were found. It is necessary to use a θ-offset of 0.15° to 0.30° for the θ-2θ scans so that the STO Bragg peak does not saturate the scintillation detector and to reduce background around the platinum Bragg peaks (STO and Pt (004) are separated by approximately 0.3° at 11.5 keV). The samples were also characterized by a high-resolution Hitachi Model S4700 scanning electron microscope (Hitachi, Tokyo, Japan) at the Electron Microscopy Center, Argonne National Laboratory.
Results and discussion
Microscopy characterization of silica monolayers and platinum nanoparticle arrays
X-ray characterization of Pt arrays on STO
We have demonstrated a simple method for the preparation of platinum nanoparticle arrays with control of nanoparticle size, spacing, and shape. This method can be used to produce monodisperse platinum catalyst nanoparticles without need for elaborate nanopatterning equipment. Particle size and spacing can be controlled by the size of the silica beads used to form the monolayer template. The silica monolayers deposited at optimized conditions on Nb-doped STO were used as masks for deposition of epitaxial platinum islands. Because of initial epitaxial relation between platinum and STO, and annealing conditions, cubooctahedral platinum nanoparticles form. The platinum nanocrystal arrays were characterized by scanning electron microscopy and synchrotron X-ray scattering indicating that they are single crystalline and oriented. Because the STO substrate is electrochemically inactive in a very wide range of potentials in aqueous electrolytes, platinum nanoparticle arrays can be used as well-defined model electrocatalysts to study technologically important reactions such as oxygen reduction reaction, oxygen and hydrogen evolution reaction, or carbon monoxide oxidation. These reactions are important in operations of fuel cells and electrolyzers where platinum metal is the main constituent of deployed catalysts.
The authors would like to thank to Dr. Sungsik Lee for the help during X-ray experiments at APS. The work at Safarik University was supported by Slovak Grant VEGA No. 1/0782/12, by the grant of the Slovak Research and Development Agency under Contract No. APVV-0132-11, by project CFNT MVEP - the Centre of Excellence of the Slovak Academy of Sciences, and by the ERDF EU Grant under Contract No. ITMS26220120005. The work in Materials Science Division and the use of the Advanced Photon Source and Electron Microscopy Center at Argonne National Laboratory were supported by the US Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
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