Borazine adsorption and decomposition at Pt(111) and Ru(001) surfaces

Abstract

The adsorption and decomposition of borazine (B 3N 3H 6) was examined over the substrate temperature range 140–1200 K on Pt(111) and Ru(0011) surfaces. The surface reactivity was examined by conventional ultra high vacuum (UHV) methods that included Auger electron spectroscopy (AES), thermal desorption mass spectroscopy (TDMS), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), low energy electron diffraction (LEED), and low energy ion surface scattering (LEISS). For room temperature adsorption, borazine adsorbed irreversibly with effective BN coverages of 0.36 monolayers (ML) on Pt(111) and 0.15 ML on Ru(001) (determined from attenuation of substrate AES signals). The drastic difference in room temperature borazine saturation coverage between the two substrates is attributed to a significant degree of dissociation on Pt(111) compared to the Ru surface. For room temperature adsorption, the TDMS data indicated only hydrogen (H 2) desorption from both surfaces over a very broad temperature range (e.g., 350–900 K). Borazine exposures of > 5.0 × 10 −5 Torr. s at either surface at 1000 K produced ordered close-packed hexagonal BN overlayers with saturation BN surface coverages of 1.22 ML on Pt(111) and 1.16 ML on Ru(001). LEED from the BN M ( M=Pt(111) or Ru(001)) surfaces prepared in this manner gave hexagonal spot splittings of approximately 10th and 12th order, respectively. This is consistent with the formation of a coincidence lattice h-BN overlayer on both substrates. Producing thicker BN overlayerrs by thermal exposures in the high vacuum regime was not possible. The evidence for h-BN overlayers is derived from EELS and LEED data from the annealed BN M surfaces. Multilayer adsorption/desorption of borazine was observed to occur at ∼ 150 K for initial substrate adsorption temperatures of 140 K.