Various forms of platinum deposited on carbon nanotubes A . D ...

Volume 75

International Scientific Journal

Issue 2

published monthly by the

October 2015

World Academy of Materials

Pages 53-62

and Manufacturing Engineering

Various forms of platinum deposited on carbon nanotubes A.D. Dobrzańska-Danikiewicz a *, W. Wolany a, D. Łukowiec a, D. Cichocki a, M. Burda b a

Faculty of Mechanical Engineering, Silesian University of Technology, Konarskiego 18A, 44-100 Gliwice, Poland b Department of Materials Science & Metallurgy, University of Cambridge UK, 27 Charles Babbage Rd, CB3 0FS Cambridge, United Kingdom * Corresponding e-mail address: [email protected]

ABSTRACT

Purpose: The main purpose of the article is to present interesting forms of platinum at a nanometric scale. There are multiple fabrication methods of nanoparticles, nanowires and other forms of platinum, and the methods proposed in the article are simple and effective. They employ carbon nanotubes in the form of a so-called forest, manufactured by CVD methods and nanotubes dispergated (in a water or ethylene glycol solution) as templates for deposition of Pt nanoforms. Design/methodology/approach: Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were applied for showing the structure and morphology of platinum nanoforms deposited on carbon nanotubes, and Energy Dispersive Spectroscopy (EDS) was used for confirming the chemical composition of the analysed structures. Findings: The microscope examinations carried out with scanning electron microscopy have shown that platinum may crystallise by assuming the form of, notably, nanoparticles, nanowires and nanocubes. The structure of carbon nanotubes covered with nanoparticles of Pt at a nanoscale could have been observed by applying high-resolution transmission electron microscopy. Practical implications: Carbon nanotubes decorated with Pt nanoparticles and platinum at a nanometric scale are used as, in particular, an active layer of chemical and biochemical sensors. In addition, excellent catalytic properties of platinum are used in various industrial processes, including chemical, automotive and petroleum industry. Originality/value: Chloroplatinic acid H2PtCl6 is an input substance for producing various forms of platinum. Platinum exhibits unique physiochemical properties at a nanoscale, different than its properties at a macro scale. It was confirmed that the selected fabrication method of platinum nanoforms is effective and simple. Keywords: Nanomaterials; Platinum; Nanoforms; Carbon nanotubes Reference to this paper should be given in the following way: A.D. Dobrzańska-Danikiewicz, W. Wolany, D. Łukowiec, D. Cichocki, M. Burda, Various forms of platinum deposited on carbon nanotubes, Archives of Materials Science and Engineering 75/2 (2015) 53-62. MATERIALS

© Copyright by International OCSCO World Press. All rights reserved. 2015

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A.D. Dobrzańska-Danikiewicz, W. Wolany, D. Łukowiec, D. Cichocki, M. Burda

1. Introduction Introduction Nanomaterials, also referred to as materials with a nanometric structure, are characterised by the fact that their structural elements in at least one direction have a dimension of not more than 100 nanometres. The nanostructural layers are referred to as layers deposited or produced on a given substrate with the predefined thickness of below 1 µm [1-7]. The following groups of nanostructured materials (NSMs) can be distinguished [1,2,6]: (i) zero-dimensional (0D), also called points, e.g. quantum dots, nanoparticles; (ii) one-dimensional (1D), e.g. nanowires, nanorods, nanotubes; (iii) two-dimensional (2D), e.g. nanolayers, nanoplates, nanostructured films; (iv) three-dimension groups (3D) consisting of 0D, 1D and/or 2D objects. Nanoparticles are usually defined as material particles dimensioned between 1 and 100 nm and exhibiting properties differing largely from the properties of the same bulk material consisting of larger structural elements [4,8]. Nanoparticles can either have a crystalline or amorphous structure [5]. The following materials can be distinguished among other nanomaterials having, similar to nanoparticles, a small aspect ratio, defined as a ratio of length to diameter: nanopyramids [9], nanocubes [10,11], nanobeads [12], nanodiamonds [13]. Research circles are also very interested in diverse one-dimension nanomaterials (1D). There is consensus that whiskers and nanorods are shorter than fibers and nanowires, but definitions of such nanomaterials have been modified over the last several years [14-16]. Initially, all one-dimensional nanostructured materials with the diameter varying between several nanometres to several hundred microns were referred to as whiskers and nanofibers, while currently the term fibre rather refers to carbon or polymer materials [14,17-19]. One-dimensional nanostructured metallic materials of the similar type with the diameter of between one atom to several hundred nms are referred to as nanowires in the current literature [16,20-22], contrary to nanorods where an aspect ratio, expressed with the dimension product, is between 3 to 5 [16,20-23]. Onedimension nanostructured materials also encompass cylindrical carbon nanotubes (CNTs) [24-28] and other inorganic nanotubes [29,30]. Various nanoparticles fabrication methods exist, including [7,10,31,32]: (i) vapour condensation consisting of evaporation of metal in the liquid state and fast condensation of deposit in the form of clusters with nanometric dimensions; (ii) grinding, being a disintegration process, and nanoparticles’ sizes are mainly

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dependent on the material and grinding time; (iii) chemical synthesis being the most popular technique where nanoparticles are formed in a liquid environment due to reduction reactions of appropriately selected chemical compounds, e.g. precursors of metals. Nanowires and nanorods are usually manufactured using the following techniques (Fig. 1) [15,16,20-23,30,33-36]: (i) chemical synthesis with the chemical growth process in a gas or liquid environment; (ii) lithography; (iii) selfassembly with templates; (iv) Chemical and Physical Vapour Deposition; (v) sol-gel method.

Fig. 1. Exemplary methods for fabrication of zero and/or the one-dimensional nanomaterials The chemical synthesis method with different reagents was applied in order to obtain various nanostructured forms of platinum, including: nanocubes, nanowires, sea urchins, nanoparticles and nanoaphids, which are presented in the following chapters of the article. Platinum is a ductile metal crystallising in the A1 lattice, with the density of 21.45 g/cm3 and a melting point of 1772°C and boiling point of 3800°C. Platinum has widespread practical applications due to its unique physiochemical properties, very good catalytic properties, good electrical conductivity. Platinum, also this occurring at a nanometric scale in diverse forms, is used in particular in the chemical, petroleum, automotive, electrical and jewellery sector and in medicine and dentistry. Chemical and biochemical substance sensors are an attractive area of such materials’ potential applications, especially Pt nanoparticles deposited on the surface of carbon nanotubes. It happens

Archives of Materials Science and Engineering

Various forms of platinum deposited on carbon nanotubes

so because it is possible to measure resistance of such nanomaterials in a time unit in the presence of specific chemical substances in a liquid and gas environment. The sensitivity threshold, selectivity and operating speed are heightened due to deposition of Pt nanoparticles onto the surface of CNTs as compared to sensors produced with undeposited CNTs. The nanocomposites consisting of carbon nanotubes modified with Pt are appropriate for production of electrodes, they also may be used in a catalysis process of different chemical reactions and in fuel cells [33-41].

2. Materials and methodology

2. Materials and methodology

Various forms of platinum in a nanometric form have been fabricated in the course of research works, according to the three different variants. Multiwalled Carbon Nanotubes (MWCNTs) were used each time in the manufacturing process as a carrier on which a growth process of various platinum nanoforms took place. CNTs were employed for the experiments, which were fabricated in the forest form by Catalytic Chemical Vapour Deposition (CCVD) with an EasyTube® 2000 device being the equipment of the Department of Nanotubes and Nanomaterials of the Scientific and Didactic Laboratory of Nanotechnology and Materials Technologies at the Institute of Engineering Materials and Biomaterials, the Silesian University of Technology. The Authors’ earlier presentations provide thorough descriptions of the carbon nanotubes forest fabrication method in the CCVD process together with a description of the structure of the fabricated MWCNTs [28,42,43]. The carbon nanotubes produced on a silicon substrate in the form of a forest, i.e. simple cylindrically rolled graphene layers densely laid next to each other, were employed twice during the research works undertaken. A fragment of a nanotubes forest produced on a silicon plate was used directly in variant 1 for further experiments, while in variant 2, after separating nanotubes mechanically from the substrate, they were functionalised, filtered and dried, and then dispergated in ethylene glycol. Non-functionalised MWCNTs were used for implementing variant 3 of the Pt nanoforms production process, Chemical Vapour Deposition (CVD) was used at a Cambridge University laboratory [44], which were dispergated in deionised water in advance. The detailed conditions of the platinum manufacturing

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process in the nanometric form for the three undertaken variants are presented in Table 1. Carbon nanotubes in all the variants were used as templates in the process of manufacturing various nanoforms of platinum by way of chemical synthesis. The manufacturing process in variant 1 and 3 was performed at room temperature in a water solution without a stabilising agent. H2PtCl6 as a Pt precursor and formic acid HCOOH as a reducer were used in order to carry out a chemical reduction process and to produce platinum atoms which next, in a self-assembly process, have created its various nanoforms. The chemical synthesis applied is characterised by an easy and effective manufacturing process, as a result of which this method enjoys considerable interest of research environments globally. For example, by applying carbon nanotubes doped with nitride (10.4% N) as templates in the chemical synthesis process, ultra-thin platinum nanowires with the diameter of about 2.5 nm and length of up to 100 nm were produced [40]. Another reducer, i.e. NaBH4, was used in variant 2 [41], which allowed to fabricate an MWCNT-Pt-type nanocomposite through deposition - on the surface of carbon nanotubes - of single platinum nanocrystals with the size of several nm, with the carbon nanotubes functionalised in prior in a mixture of HNO3/H2SO4 acids at a rate of 1:3. The process was assisted mechanically with ultrasounds for the purpose of functionalisation or using a magnetic stirrer to ensure appropriate decoration process. The platinum-decorated nanotubes were filtered and rinsed with deionised water several times at the final stage. The Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) techniques were used to examine the platinum nanoforms achieved in the experiments. SEM images were made using a scanning electron microscope SEM Supra 35 by Carl Zeiss equipped with an X radiation spectrometer of energy dispersion EDS by EDAX. TEM and STEM images were taken using a transmission electron microscope STEM TITAN 80-300 by FEI fitted with an electron gun with FEG field emission, a condenser spherical aberration corrector, STEM scanning system, bright and dark field detectors, HAADF (High Angle Annular Dark Field), and EFTEM energy image filter and an EDS spectrometer. The preparations for the studies for TEM were prepared by collecting materials from particular samples, placing a droplet of mixtures on HRTEM nets and drying the preparations at free air at room temperature.

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Table 1. The conditions of the platinum manufacturing process in the nanometric form for the three undertaken variants Criterion VARIANT 1 VARIANT 2 VARIANT 3 MWCNTs CCVD method at Silesian CCVD method at Silesian CVD method at Cambridge fabrication method University of Technology University of Technology University Form of MWCNTs Forest of carbon nanotubes Carbon nanotubes Non-functionalised carbon used in the functionalised in prior and nanotubes, dispergated in experiment dispergated in glycol ethylene deionised water Metal precursor H2PtCl6 H2PtCl6 H2PtCl6 Reducer HCOOH NaBH4 HCOOH Other substances H2O Ethylene glycol, acetone H2O Decoration process 72 h 8h 72 h time Process temperature Room temperature 140°C Room temperature Mechanical None Ultrasounds, Ultrasounds assistance magnetic stirrer Manufacturing Chemical reduction, selfChemical reduction Chemical reduction, selfmethod assembly, spontaneous assembly, spontaneous growth growth Additional activities Washing with deionised Filtration Filtration water of silicon substrate with Pt deposited on nanotube forest Carpet consisting of different Single Pt nanoparticles with Nanoaphids which are platinum Manufacturing process outcome Pt nanoforms, including: the size of 3-4 nm arranged nanoparticles deposited in uniformly on the surface of clusters on non-functionalised nanocubes and nanowires forming sea urchins carbon nanotubes carbon nanotubes

3. Results and and discussion discussion

The material manufactured in the technological conditions characterised in Table 1, as variant 1, was subject to observations using a scanning electron microscope. The observations made have permitted to conclude that the surface formed by a forest of carbon nanotubes is a place where platinum agglomerates are attached. It was confirmed in particular that, by way of self-assembly, a specific platinum carpet was created by itself on the surface being a forest of MWCNTs, containing the clusters of platinum creating a less or more uniform layer (Fig. 2). The topography of the so created platinum carpet is therefore complex and inhomogeneous. The microscope observations performed for the magnification of 20 KX enable to confirm the existence of oval Pt agglomerates with their size of usually not more than 1 µm, which is clearly noticeable in a microscope image in Fig. 3. Observations with even higher magnification of 100-300 KX allow to state that the carpet created consists of smaller or larger clusters of Pt nanocubes and nanowires. The side length of the

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nanocubes (Fig. 4) is less than 100 nm, and nanowires (Fig. 5) are similar in appearance to thin hairs with the length of less than 100 nm, which grow in different directions and create clusters, referred to in the literature [46,47] as sea urchins, due to the their shape similar to such underwater animals.

Fig. 2. Platinum precipitates forming a carpet covering a forest of nanotubes; SEM image



Fig. 3. Oval platinum agglomerates covering a forest of carbon nanotubes; SEM image

Fig. 4. Pt nanocubes; SEM image

Fig. 5. Pt nanowires forming sea urchins; SEM image By using the recipe determined in Chapter 2 of the article, single platinum nanoparticles were deposited onto the surface of carbon nanotubes as variant 2 of the

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manufacturing process. A series of experiments was carried out in particular using preparations with the 5, 10 and 20% mass fraction of platinum [41]. MWCNT-5% Pt, MWCNT10% Pt, MWCNT-20% Pt type nanocomposites were manufactured this way by combining permanently the carbon-metal material. The experiments made with highresolution electron microscopes have revealed that a nanocomposite with 5% mass fraction volume of platinum nanoparticles exhibits the best structure for the nanocomposites produced during own works [41]. The material obtained is characterised by highly dispersed Pt particles on the surface of carbon nanotubes and the lack of tendency to agglomerate nanoparticles, pointing out that a deposition process had been performed correctly (Fig. 6). The uniform arrangement of platinum nanoparticles on the surface of carbon nanotubes was shown by using an HAADF detector in the investigations in the STEM mode and such nanotubes are visible as clear light precipitates (Fig. 7). It was also ascertained and observed within the whole volume of the studied nanocomposites that the size and spherical shape of the fabricated Pt nanoparticles with the diameter of 3-4 nm was uniform (Fig. 8). Platinum nanoparticles have a crystalline structure with crystallographic planes clearly visible during STEM observations using an HAADF detector (Fig. 9). The images of nanostructured forms of Pt made in a transmission electron microscope, created according to the recipe as variant 3 of the manufacturing process (Table 1), are shown in Figs 10-13. In such case, the selfassembling Pt atoms create nanoforms called nanoaphids by the authors of the article. The observations made indicate that platinum nanoparticles exhibit a tendency to agglomerate in small clusters which are dispersed across the surface of carbon nanotubes. An HAADF detector was used during imaging in the STEM mode (Figs 10 and 11) in order to obtain more exact information about the arrangement of platinum nanoparticles on the surface of MWCNTs. Considering the large differences in the value of atomic numbers of platinum (Z = 78) and carbon (Z = 6), Pt nanoparticles are visible in the dark field as light precipitates with an oval, slightly elongated shape, located in small clusters on the surface of carbon nanotubes. Changes in the number of Pt nanoparticles on the surface of nanotubes were noticed in few samples (Fig. 12) while carrying out observations in the bright field in the STEM mode, starting with their clear agglomerates (1), through a high degree of arrangement of small agglomerates (2), ending with occasional nanoparticle (3). The presence of platinum nanoparticles as fine agglomerates similar to aphids in a stalk of a plant are observed in the majority of cases, which is clearly visible in the dark field (Figs 10-11), and in the light field (Fig. 13).

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Fig. 6. Pt nanoparticles deposited on MWCNTs; TEM image [45]

Fig. 8. Pt nanoparticles deposited on MWCNTs; HRTEM image [41]

Fig. 7. Pt nanoparticles being light precipitates; STEM image made with HAADF detector [41]

Fig. 9. Pt nanocrystal with clearly visible crystalline planes, STEM image made with HAADF detector [41,45]

Qualitative analyses of chemical composition from the microarea of the carbon-metal preparations were performed for all three variants of manufacturing process with Energy

Dispersive Spectroscopy (EDS) by EDAX. The elements forming the given material were each time identified through an EDS analysis based on a radiation energy

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value recorded by the instrument expressed with [keV], which is characteristic for each element. Spectral qualitative analyses of chemical composition made for all three variants of manufacturing process show that the composition of the investigated preparations contain Pt and C, which is associated to the presence of carbon nanotubes. Spectroscopic research results made for

variant 2 and variant 3 of the manufacturing process show the presence of oxygen and cooper. The presence of Cu is explained by a copper mesh used at the stage of preparation, onto which the studied materials were deposited. One from registered EDS spectra made for variant 2 of nanocomposite manufacturing process is presented in Fig. 14.

Fig. 10. MWCNTs as stalks covered with Pt nanoaphids; STEM-HAADF image

Fig. 12. Different number of Pt nanoparticles on carbon nanotubes; STEM-BF image

Fig. 11. Pt nanoaphids deposited on MWCNTs; STEMHAADF image

Fig. 13. Pt nanoaphids deposited on MWCNTs; STEM-BF image

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fine aphids situated on a stalk of a plant, which, due to their similarity to the world of nature, have been called nanoaphids. The experiments undertaken allow to conclude that, depending on the fabrication method and on the form of the applied carbon nanotubes, the reducer type and other chemical compounds used in the manufacturing process, the time and temperature of the process and the additional activities performed, nanostructured forms of platinum can be fabricated with very interesting and diversified morphology.

Acknowledgements Acknowledgements

Fig. 14. Results of EDS analysis from microarea of nanomaterial manufactured by variant 2 of the manufacturing process

4. Conclusions Conclusions The microscope examinations carried out with scanning and transmission electron microscopy confirmed that diverse nanostructured forms of platinum were fabricated, obtained by carrying out a manufacturing process in line with three different variants. The authors have developed diverse nanostructured forms of platinum using a surface of a forest of multiwalled carbon nanotubes manufactured by CCVD and walls of functionalised and non-functionalised MWCNTs as the nucleus places of Pt. Variant 1 of the manufacturing process has allowed to produce a carpet, on the surface of a carbon nanotube forest, composed of oval Pt agglomerates. It can be pointed out by using higher magnification of 100 KX that such agglomerates include the clusters of nanocubes and sea urchins made up of platinum nanowires. The nanowires produced are less than 100 nm long, are homogenous, and the shape of their agglomerates is similar to carbon nanohorns. By acting according to variant 2 of the manufacturing process, single platinum nanoparticles can be deposited onto the surface of pre-functionalised carbon nanotubes, and the best effect is achieved for 5% mass fraction of Pt. Single Pt nanoparticles with the size of 3-4 nm are deposited uniformly across the entire surface of MWCNTs in such case. Variant 3 of the manufacturing process enables to deposit fine Pt agglomerates dispergated in prior in deionised water onto the surface of non-functionalised carbon nanotubes, with such agglomerates being similar to

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The works have been implemented within the framework of the project entitled “Determining the importance of the effect of the one-dimensional nanostructural materials on the structure and properties of newly developed functional nanocomposite and nanoporous materials”, funded by the Polish National Science Centre in the framework of the “OPUS” competitions. The project was awarded a subsidy under the decision DEC2012/07/B/ST8/04070.

Additional Additionalinformation information Selected issues related to this paper are planned to be presented at the 22nd Winter International Scientific Conference on Achievements in Mechanical and Materials Engineering Winter-AMME’2015 in the framework of the Bidisciplinary Occasional Scientific Session BOSS'2015 celebrating the 10th anniversary of the foundation of the Association of Computational Materials Science and Surface Engineering and the World Academy of Materials and Manufacturing Engineering and of the foundation of the Worldwide Journal of Achievements in Materials and Manufacturing Engineering.

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