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Aggregation Behavior of Ligand-Protected Au Clusters on Sputtered ALD TiO Hassan S. Al Qahtani, Gregory Francis Metha, Rick B Walsh, Vladimir B. Golovko, Gunther G Andersson, and Tomonobu Nakayama J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11590 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017
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The Journal of Physical Chemistry
Aggregation Behavior of Ligand-Protected Au9 Clusters on Sputtered ALD TiO2
Hassan S. Al Qahtani,± Gregory F. Metha,+ Rick B. Walsh,‡ Vladimir B. Golovko,ɤ Gunther G. Andersson,±* and Tomonobu Nakayama §*
±
Flinders Centre for NanoScale Science and Technology, Flinders University, Adelaide SA 5001, Australia + Department of Chemistry, The University of Adelaide, Adelaide, SA 5005, Australia ɤ The MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Christchurch 8140, New Zealand. § National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Department of Applied Mathematics, Research School of Physics and Engineering, The Australian National University, Canberra
ACT 0200 Australia
Corresponding Authors Email: Tomonobu Nakayama:
[email protected] Gunther Andersson:
[email protected] Abstract
[Au9(PPh3)8)](NO3)3 (Au9) clusters were deposited onto sputtered ALD titania surfaces. Atomic force microscopy (AFM) was used to determine the height and distributions of the Au9 clusters over the titania surface fabricated using atomic layer deposition (ALD). Synchrotron X-ray photoelectron spectroscopy (XPS) was used to derive information about the degree of agglomeration of the Au9 clusters due to the annealing process. Both AFM and XPS show that the Au9 clusters deposited on ALD titania are partially
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agglomerated after annealing. Deposition of the [Au9(PPh3)8)](NO3)3 clusters on sputtered ALD titania is compared with deposition of the same cluster on titania nanosheets of previous work.
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The Journal of Physical Chemistry
I.
INTRODUCTION
Gold nanoparticles (Au NPs) are of interest for modifying surface in order to tailor surface properties to specific functionalities by adsorbing nanoparticles onto the surface.1-9 Nanoparticles modify surfaces due to their specific chemical composition and crystal structure. Gold nanoclusters (Au NCs) are smaller in size than Au NPs and are used to modify surface properties as well. In contrast to nanoparticles it is not the crystal structure but the geometric fluxionality and the size dependence of the electronic structure which is important for the surface modification by clusters.10-27 Au NCs form specific atomic structures depending on the number of atoms forming the NCs, while Au NPs typically adopt a facecentered cubic (fcc) structure.18,
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Gold NCs, like molecules, have discrete electronic energy levels,
while NPs have a continuous electronic energy band structure similar to bulk Au.29-30 For this reason, the number of atoms forming a Au NC strongly influences the properties of NC modified surfaces.31-35 One example is the catalytic properties of surfaces modified with Au NCs, which depend on the size, morphology and the electronic structure of the Au NCs.36-49 The catalytic properties of cluster-modified surfaces change with the size of Au NCs and the size of the NCs needs to be preserved to retain specific surface properties.50 Agglomeration of NCs leads to an increase in NC size, eventually forming NPs, and should be avoided. Thus it is important to understand the conditions which lead to agglomeration of the NCs deposited on surfaces. There is no accepted definition allowing to distinguish between NCs and NPs. The transition between NCs and NPs is not sharp, depends on the number of atoms and the elements forming them and could roughly be drawn at about 100 atoms. Au NCs can be prepared and deposited onto surfaces from the gas phase under UHV conditions37, 51-53 or from the liquid phase using chemical synthesis processes.29,
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Compared with the gas phase
method, chemical methods have the benefit that they can be easily scaled-up. The series of chemically synthesized, atomically-precise Au NCs, formulated as Aun(PPh3)m (where n and m represent the respective number of Au atoms and triphenylphosphine ligands) deposited on titania surfaces have been 3
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studied by us previously.55-56 The size of the cluster core of this phosphine-coordinated Au NC family is well defined
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and has been shown to exhibit clearly distinguishable properties, depending on the
cluster size.55-58, 61, 64 The XPS peak position of Au 4f electron is influenced by the size of the clusters through the final state effect and has been used to estimate the size of metal clusters.29, 51, 55-56, 65-66 When depositing NCs onto surfaces it is important to understand whether or not clusters agglomerate during the deposition process, which deposition and activation conditions lead to agglomeration and how the electronic properties and the size of the clusters are changing during deposition. The characterization of the electronic and geometrical properties of Au NCs deposited on surfaces requires a combination of different surface science techniques, each of them providing specific information that is complementary to other techniques. In this work, a combination of microscopic and spectroscopic techniques has been applied to investigate [Au9 (PPh3)8](NO3)3 clusters (hereafter abbreviated as Au9) deposited on titania surface fabricated using atomic layer deposition (ALD) in order to determine the agglomeration of Au9 clusters after deposition. Atomic force microscopy (AFM) allows us to image and determine the cluster size distribution of the Au NCs and their distribution over the titania surface. X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical state and electronic structure of the Au NCs deposited onto titania. Samples have been investigated before and after heat treatment to 200 ºC. The aim of the present work is to estimate the degree of agglomeration of the cluster after application of heat treatment needed to remove the protecting phosphine ligands. The height distribution of Au particles determined using AFM is correlated to XPS results. The latter allows estimating the Au cluster size through the final state effect. II. EXPERIMENTAL [Au9(PPh3)8](NO3)3 clusters were prepared and purified using a procedure reported elsewhere.56,
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Boron doped Si wafers with a resistivity of
