Formation of GaPd 2 and GaPd intermetallic compounds on GaN(0001) M. Grodzicki 1 • P. Mazur 1 • J. Pers 1 • J. Brona 1 • S. Zuber 1 • A. Ciszewski 1 Received: 20 March 2015 / Accepted: 30 June 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract Palladium was deposited gradually under ultrahigh vacuum onto a well-defined surface of (0001)- oriented n-type GaN, at room temperature. Each deposition step was followed by annealing. Physicochemical proper- ties of the Pd adlayers were in situ investigated prior to and after annealing by the X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, low-energy elec- tron diffraction, scanning tunneling microscopy and atomic force microscopy techniques. Annealing resulted in the formation of GaPd 2 and GaPd intermetallic compounds at 550 °C and at 800 °C. Even for thicker layers, the com- pounds were strongly dispersed, forming 3D nanostruc- tures. The substrate uncovered by the compounds revealed Ga-rich GaN(0001)-(1 9 1) surface. Formation of Ga-Pd- N bonds or Pd nitrides was not detected at the surface. The Ga-Pd intermetallic compound surface engineered on the GaN(0001) substrate can be used as the strongly dispersed catalyst or a model catalyst. 1 Introduction The GaPd, Ga Pd 2 and Ga 7 Pd 3 intermetallic compounds have been verified as highly active and stable, selective catalysts in the semi-hydrogenation of acetylene [1–5]. This industrially very important reaction is widely employed in polyethylene production to remove traces of acetylene from the ethylene stream [6]. In current tech- nologies, Ag-Pd and Au-Pd substitution alloys are used as catalysts for this reaction. Catalytic selectivity of these alloys is intrinsically limited. The Ga-Pd intermetallic compounds are promising candidates for their more selective replacements because in the case of the substi- tutional Ag-Pd and Au-Pd alloys Pd atoms are randomly distributed and in the case of Ga-Pd intermetallic com- pounds they are distributed regularly. Constant and repro- ducible spacing between Pd atoms at the surface of the catalyst strongly enhances its selectivity. From the point of view of electronics, intermetallic compounds fall into the class of metals, similar to the industrially applied alloys. Therefore, an activation of di-hydrogen as a reactant can easily be achieved due to a sufficient density of states near the fermi level. Crystal structure of the compounds pro- vides surfaces with all Pd atoms structurally ordered and isolated by Ga atoms, fulfilling the earlier proposed active site isolation concept [5, 7]. The concept assumes forma- tion of isolated catalytic centers, which consist of active atoms (Pd) isolated by less active spacer atoms (Ga) on the surface of the catalyst [5, 7]. It limits the number of adsorption configurations for the reactants and changes the electronic structure of the active species according to the nature and concentration of the spacer atoms [7–9]. The covalent bonding within the compounds leads to a much higher stability against surface segregation even under reaction conditions [5]. So far, well-defined and repro- ducibly obtainable unsupported Ga-Pd intermetallic com- pounds with a rather low specific surface area and ultrathin Ga films on Pd(111) and Pd(110) surfaces have been used for basic research on physicochemical and catalytic prop- erties of these compounds [1–5, 10]. In this paper, we report the results of our studies on possibilities of the formation of Ga-Pd intermetallic compounds in a dispersed form by engineering the surface of the Pd/GaN(0001) adsorption system. & M. Grodzicki milosz.grodzicki@ifd.uni.wroc.pl 1 Institute of Experimental Physics, University of Wroclaw, Plac Maxa Borna 9, 50-204 Wroclaw, Poland 123 Appl. Phys. A DOI 10.1007/s00339-015-9331-9