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J. Phys. Chem. C 2010, 114, 20062–20067
Reflection High-Energy Electron Diffraction Characterization of Cobalt Clusters Electrodeposited on a Au(100) Electrode Hengliang Wu,† Mauscheng Zei,*,‡ and Shuehlin Yau*,† Departments of Chemistry and Physics, National Central UniVersity, Taiwan 320 ReceiVed: July 15, 2010; ReVised Manuscript ReceiVed: October 13, 2010
Cobalt was electrodeposited onto a reconstructed Au(100) electrode in 0.25 M Na2SO4 + 0.1 mM CoSO4 under potentiostatic conditions. The Au(100) electrode loaded with the Co deposit was transferred via an argon environment to ultrahigh vacuum and subsequently examined with reflection high-energy electron diffraction (RHEED). The resultant RHEED results were analyzed in detail to unveil the structures of the as-prepared Co deposit, which assumed nanometer-sized clusters on the Au(100) substrate. Aided by coulometric results, profile analysis of the RHEED results indicated that the Co clusters on average were 3 nm in width and 15 atomic layers in height. The packing habits of the Co clusters, determined by the obtained RHEED results, were the face-centered cubic (fcc) phase, rather than the hexagonal closed-packed (hcp) phase. Cobalt clusters grew with their fcc (1j10) plane aligned parallel to the Au(100) substrate and with their [110] axis aligned in the [011] direction of the Au(100) substrate. The preferential growth of the fcc Co phase is driven by the optimal lattice match between the (1j10) plane of Co clusters and the Au(100) substrate network. 1. Introduction When embedded in amorphous alumina, Co clusters less than 2.5 nm in diameter can crystallize in the face-centered cubic (fcc) structure, but change to the hexagonal close-packed (hcp) structure as the Co clusters grow to 3 nm or larger.1 It is also found that a Co cluster forms a β-phase at room temperature under the stress of epitaxy.2 The reasons behind these unusual results found with Co are still unclear. Meanwhile, epitaxially grown Co films have revealed a variety of interesting phenomena in physics, including giant magnetoresistance3 and magnetooptical effects.4,5 The new magnetic properties of these films have attracted a great deal of attention lately. These results exemplify that nanometer-scaled aggregates can arrange differently from their counterparts in the bulk phase. On the other hand, it can be challenging to unveil the atomic arrangements of nanoparticles, particularly for those deposited on a solid support. Sophisticated, structure-sensitive techniques are needed to explore the atomic structures of a miniscule amount of materials, such as Co nanomaterials deposited on well-defined surfaces by physical or electrochemical means. Extended X-ray absorption fine structure used to examine a Co thin film deposited on Au(111) in vacuum reveals hexagonal packing of Co with a lattice constant resembling that of bulk Co.6 Scanning tunneling microscopy (STM) is used to examine cobalt films grown on Au(111) from 0.3 to 3 monolayers (MLs).7,8 Although no apparent structural change is observed at coverages between 4 and 7 MLs, the magnetization axis changes from perpendicular to parallel to the surface.8 The dependence of magnetization on the structure of the Co deposit has not been investigated thoroughly, partly because of limited information on the structure of the Co deposit, particularly when the Co deposit is merely a few atoms in depth. According to * To whom correspondence should be addressed. E-mail:
[email protected] (M.Z.);
[email protected] (S.Y.). † Department of Chemistry. ‡ Department of Physics.
transmission electron microscopy and X-ray diffraction measurements, Co films thicker than 10 MLs on Au(111) pack in the hcp structure.9 Electrodeposition of cobalt on gold, copper, etc. is also examined using structure-sensitive tools, such as X-ray diffraction and scanning probes, which elucidate the packing of the cobalt deposit and the nucleation-and-growth mechanism.10-12 A substrate with a hexagonal atomic structure, such as Au(111), has been most popular. STM imaging provides a direct view of Co deposition, where surface defects such as steps or vacancies serve as the nucleation sites. The Co deposit can wet the Au(111) substrate as deposition continues, as the Co adlayer adopts a 2D incommensurate structure up to a thickness of 4 MLs.11,12 Given the significant (13%) difference in the diameter of Co and Au atoms, deposition of Co on a gold substrate is unlikely to proceed in a layer-by-layer fashion, as revealed by STM.12 The Co film is found to grow on Au(100) in a 3D growth mode, yielding a mostly rough morphology.11 Although STM renders direct visualization of Co deposition, the packing habit of Co, particularly on Au(100), is not known. Meanwhile, the process of cobalt electrodeposition on a gold electrode has been scrutinized by conventional means based on analyzing the current transient due to the reduction of cobalt. This tactic has resulted in the nucleation-and-growth mechanism for metallization and the possibility of coupled hydrogen evolution for cobalt deposition on gold or carbon electrodes.13-16 This study is a detailed examination of the atomic structure of a Co deposit on a Au(100) electrode using reflection highenergy electron diffraction (RHEED), because this technique is particularly capable of probing epitaxial growth of metallic thin films, as illustrated in a number of studies.17,18 It was found that Co clusters amounting to 3 nm in diameter on a Au(100) electrode arranged in the fcc structure, rather than its bulk hcp structure. We contend that the Co deposit tended to arrange in a way to minimize lattice strain existing between the atop Co deposit and the underlying Au(100) substrate.
10.1021/jp1065924 2010 American Chemical Society Published on Web 11/05/2010
RHEED of Co Clusters on a Au(100) Electrode
J. Phys. Chem. C, Vol. 114, No. 47, 2010 20063
Figure 1. RHEED patterns obtained at the [011] (a) and [001] (b) azimuths for Au(100) after argon ion sputtering and annealing at ∼1000 K under UHV conditions, showing the 1/5-fractional-order beams due to the reconstructed Au(100) surface.
2. Experiment RHEED experiments were performed with an apparatus consisting of a UHV chamber (base pressure
