Electrochemical generation and property modification of supported metal and alloy nanoparticles

  • Terryn, Herman (Administrative Promotor)
  • Van Haesendonck, C. (Co-Promotor)
  • Bals, Sara (Co-Promotor)

Project Details

Description

Aim of the project The aim of this project is threefold. First, we aim at the generation of supported metal and alloy particles on different substrates by surface mediated chemical or electrochemical deposition. Second, we concentrate on the modification of these particles by changing their surface properties. In view of the complexity surrounding the investigation of the structure and electronic properties of systems with nanoscale dimensions, a variety of complementary experimental methods will be used. The unification of those techniques into a methodologically homogeneous approach is the third goal of the project. Approach 1. Nanoparticle synthesis 1.1 Surface mediated chemical deposition The nanoparticle preparation will be carried out from solutions containing metal salts and a suitable reductor. The particles will be sprayed in such a way that a multitude of separate drops is delivered to a given substrate. The crystallization process will be initialized at activation centers present on the substrate surface. Typical for this approach is that the crystal growth stops upon the exhaustion of the reagents available in each single drop of the solution. As a result, the size of the precipitated clusters can be controlled through a variety of parameters, including size of the drop, number of the activation centers, solution concentration, and temperature. The approach is rather universal with respect to the substrates that can be used, as it does not depend on the substrate conductivity. A major limitation is related to the fact that it is only applicable for deposition of single metals with relatively noble electrochemical potentials. The method is currently applied at VUB for deposition of Ag on Cu. The experimental data show that the surface mediated chemical deposition delivers well defined and strongly attached spherical particles in a dimension range of 10-50 nm. In the framework of the project, this approach will be used to form noble metal particles such as Pd and Cu. The dimensions and distribution of the particles will be characterized in detail for different values of the tunable parameters that have been mentioned above. 1.2 Electrocrystallisation Considering the fact that the chemical deposition is only applicable for a limited number of metals, electrocrystallization of nanostructures remains one of the most useful alternatives for extending the range of accessible material compositions. Despite the intensive exploration of galvanic deposition, many problems related to the initial steps of the growth process remain, however, unclear. There remains a lack of detailed information about the dimensions, shape, structure, and composition of critical size nuclei. The question about the properties of the critical size nuclei is very important, as it defines the smallest nanoparticles that are stable at given external conditions. While the significance of this knowledge is well appreciated, it is not easily achievable in view of the necessity to use a combination of electrochemical control and analysis methods with high spatial resolution. One part of the project will therefore concentrate on the use of high resolution analysis techniques, some of them being performed in-situ. Our purpose is to study the dependence of the nucleation process on the electrochemical overpotential, electrolyte composition, temperature and convection for systems where mass transfer is important. One innovative point of the project is related to the use of ionic liquids for the electrochemical synthesis of metallic and alloy clusters. Ionic liquids are newly created materials that mimic the electrochemical properties of melts, but at room temperature. They have extremely wide electrochemical windows and were already used for the electrocrystallisation of Al and Ti [5]. The necessity to introduce the ionic liquids is dictated by two facts. First, it is well known that the electrochemical reduction in aqueous solutions is only possible for metals with reasonably positive electroreduction potentials, which is usually the case for the metals located on the right hand side of the Periodic Table. Second, some very recent data indicate that ionic liquids provide a much more suitable environment when compared to water for the controlled crystallization of nanoparticles. The reason is that the metal ions have other transport properties in the ionic liquids than in the aqueous ones, and the electrocrystallization process can be slowed down in a better way. It has, e.g., been shown that in ionic liquids a step-like growth of Pd particles from 2 nm to 3.5 nm with step size of 0.5 nm can be achieved [6]. It is expected that with this approach we will have an improved control of particle size and distribution when compared to the previous method. 2. Structural and electronic properties of the chemically or electrochemically generated nanoparticles In the project we will study chemically or electrochemically deposited nanoparticles of Pd and V and of some of their alloys. Ultra-thin layers and clusters of Pd containing materials have practical applications, e.g., in catalysis, fuel cells, magnetic storage materials, and electronics. Moreover, both Pd and V are of large interest for hydrogen storage. The nanoparticles also provide nice possibilities to explore the influenc of, e.g., a reduced dimensionality on structural ordering, expansion or compression, difference between surface and bulk, compositional segregation, and chemical interactions. The Pd and V metal and alloy nanoparticles will be deposited on different types of substrates. Flat metallic samples, glassy carbon, and transmission electron microscopy (TEM) grids will be used for freely growing nuclai. The advantage of the latter type of substrate is that the samples can be directly used for TEM, without the need for dedicated TEM sample preparation that requires polishing and/or (focused) ion milling. Patterned electrocrystallization will be performed in porous anodic aluminum oxide templates [7,8]. Relying on a special anodizing treatment procedure, which has already been used in the past at VUB, highly ordered templates with pore dimensions ranging from 5 to 50 nm can be formed. The columnar aluminum oxide films have at the bottom thin aluminum barrier films, which allow tunneling of electrons so that reduction of cations can take place locally. This method is commonly used to grow nanowires inside the pores by several other groups, including the group of Luc Piraux at UCL. Here, we, however, focus on the formation of nanopartciles in the pores. In order to investigate such samples by TEM, patterned aluminum oxide surfaces will be made electron transparent by Ar ion milling, prior to deposition of the particles. 3. Modification of the particles 3.1. Modification of the bulk properties: Alloying with H: Pd-H and V-H particles The interest in the Pd-H and V-H alloys is inspired by the fact that both elements are able to take up large amounts of hydrogen. The difference between Pd and V is that Pd only forms solid solutions with hydrogen, while V forms solid solutions as well as two hydride compounds. The study of the two V hydrides is intriguing because of their distinctive properties. One of them is the only known metal-hydrogen compound that decomposes at room temperature, while the second is more stable. In view of their specific properties, Pd-H and V-H modified particles are therefore suitable systems to investigate the influence of a low dimensionality on the structural and electronic properties of metal-hydrogen alloys and metal hydrides. We note that, while a significant number of investigations considered bulk Pd-H and V-H systems, only a very limited number of articles reported such studies on Pd nanoparticles. No investigation on nanosized V-H has been reported so far. Since the particles are formed on a conductive substrate, hydrogen uptake can be simply carried out by cathodic polarization in a low pH aqueous solution. Alloying of Pd with Co or Ni Once we have a good control over the formation of Pd nanoparticles, we will modify the bulk composition by alloying with Co or Ni. The closeness of Pd in the electrochemical activity row to Co and Ni makes the electrodeposition of their alloys with controlled concentration feasible. 3.2 Surface (core shell type) modification of the particles Two ways of changing the surface of the nanoparticles, i.e., oxidation and adsorption will be considered. The modification of the oxidation/reduction state of the metal will be carried out in solutions by electrochemical polarization to a passive state or by a gas oxidation. VUB has studied the oxidation behavior of metal surfaces extensively in the past. It is expected that this way we will be able to create and control the oxide shell around the metallic particles. The influence of multiple oxidation-reduction cycles on the oxide shell will be studied as well. For the project we will focus on Cu and Co nanoparticles. We will rely on the magnetic properties to evaluate the successful modification of the particles. In particular, we will focus on measuring the magnetic hysteresis loops of oxidized Co particles with SQUID magnetometry at the KULeuven. It is well known that the direct contact between the ferromagnetic Co and the antiferromagnetic CoO coating gives rise to exchange bias that results in a shift of the hysteresis loop with respect to zero field and a broadening of the hysteresis loop [9]. The strength of the exchange bias effect can be directly related to the the quality of the CoO layer (homogeneous thickness, absence of crystalline defects, etc). In view of the limited sensitivity of the SQUID magnetometry we will investigate substrates that are densely covered with particles having a size of 20 nm and larger. Nevertheless, for this particle size we should be able to observe possible finite-size effects that are otherwhise difficult to access with lithographic patterning techniques [10]. Production of core-shell structures by adsorption of organic molecules will be the second topic of this part of the project. Interaction of various organic compounds with macroscopic and nanosize metal and oxide surfaces is one of the major research directions at VUB [11-13]. The recent work focuses more and more on making a transition from deposition by self-assembly to a potential controlled one [14]. In view of the obtained results it appears to be very interesting to apply a potential in order to influence the charging or the field around a particle and therefore also the adsorption parameters of the organic compounds. This is very important, since the current studies at VUB reveal significant differences between the properties of organic layers deposited by self-assembly and by potential controlled surface-molecular interaction. Recently, it has become clear that covering noble metal nanoparticles with an organic layer may strongly affect the metallic state and induce unexpected new properties, in particular magnetism [15]. We will check for such effects, which can be linked to electron transfer via the chemical bonding between a metallic nanoparticle and its organic coating, by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Since it is possible to cover the nanoparticles in situ during the electrochemical growth process with an organic coating, this should largely avoid oxidation and contamination at the interface between particle and coating. This way, we should be able to perform reliable tunneling experiments at the KULeuven on the samples that are prepared at the VUB, and are next transferred to the KULeuven in a vacuum suitcase. Here, it is important to note that the electrochemically formed particles can be covered in the solution with the organic molecules, avoiding the occurrence of contamination at the interface with the metal particle surface. The STM and STS experiments will be performed under ultra-high vacuum conditions and down to liquid helium temperatures [16]. This way, we will be able to obtain direct information on the electronic structure of the interface between a particle and its organic coating. Moreover, relying on the possibility to perform spin-polarized tunneling experiments, we will be able to also probe the magnetic properties at the level of an individual particle. Nanoparticle analysis methods From the analytical point of view, complementary methods providing information about the size, morphology, composition, electronic structure and electrochemical behavior will be used in this project. Both for particle synthesis and particle modification the same analytical methods can be applied. Ex situ analysis will be in the first place based on transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS) and high annular angular dark field scanning TEM (HAADF-STEM). Structural analysis, i.e. determination of the particle size distribution and particle morphology will be done using conventional TEM in imaging mode. Furthermore, high resolution TEM will allow us to study the atomic structure of the particles. These results will be combined with TEM based EELS measurements. The peaks in the EELS spectra contain chemical as well as electronic information on the object being studied. For a coreshell structure, it is possible to obtain structural, chemical and electronic information on the core and shell separately. Using HAADF-STEM, one is able to combine structural and chemical information, since this technique yields a structural image in which the recorded signal is a direct function of the atomic number Z of the columns being imaged. Recent studies have revealed that this type of information can also be interpreted in a quantitative manner [17]. (S)TEM and EELS measurements will be done at UA. Apart from standard characterization of particle shape and dimensions using AFM with ultrasharp tips, the partner at the KULeuven will also rely on the possibilities to use AFM measurements for determining local variations in the mechanical properties such as the elasticity. Moreover, contact-resonance-frequency AFM [18] will be used to quantitatively determine the adhesion between the nanoparticles and the substrate. In situ analysis can be performed using the recent acquired electrochemical STM/AFM equipment present at VUB and by SERS measurements under potential control. The latter approach is a valuable possibility to study interactions between metal surfaces and organic molecules by measuring the changes that appear in the molecular vibrational spectra due to the adsorption of organic molecules at different polarization potentials. This approach is applicable to core-shell systems with Cu, Au, or Ag cores [19], but there is a growing evidence that SERS can be applied to clusters formed by other metals as well. The other part of the in-situ studies will rely on synchrotron based approaches under electrochemical control. These approaches include small-angle Xray ray scattering (SAXS), wide-angle X-ray ray scattering (WAXS), extended X-ray absorption fine-structure spectroscopy (EXAFS), and X-ray near-edge spectroscopy (XANES) available on the Dubble beam line at ESRF, Grenoble. An electrochemical cell suitable for mounting on the SAXS/WAXS and EXAFS/XANES ports of Dubble has already been successfully tested. The setup allows to exchange solutions without opening or dismounting the cell from the beam line holders. This will lead to a meaningful comparison between the bare nanoparticles and the particles modified by alloying, oxidation, or adsorption.
AcronymFWOAL527
StatusFinished
Effective start/end date1/01/0931/12/12

Flemish discipline codes

  • Other engineering and technology