Identification of a Stable B₂H₂ Intermediate in the Decomposition of Zr(BH₄)₄ on the Pd(111) Surface

Tetrakis(tetrahydroborato)zirconium(IV), Zr(BH₄)₄, is a volatile compound that has been widely used as a single-source precursor to grow carbon-free thin films of zirconium diboride by chemical vapor deposition (CVD) on a wide range of substrates that include oxides, semiconductors, and metals. However, the basic surface chemistry of the compound that underlies the initial stages of the CVD process is largely unknown for any substrate. We studied the adsorption and decomposition of Zr(BH₄)₄ on a Pd(111) surface with the experimental techniques of reflection absorption infrared spectroscopy (RAIRS), temperature-programmed reaction spectroscopy (TPRS), and X-ray photoelectron spectroscopy (XPS) and with density functional theory and first-principles molecular dynamics simulations. After exposing Pd(111) at 90 K to Zr(BH₄)₄ (g), a RAIR spectrum was obtained that closely matched that of the pure compound, indicating that it adsorbs without dissociation at 90 K. However, upon heating to 200 K the RAIR spectrum undergoes dramatic changes indicating that a new surface species is formed that retains both terminal B-H bonds and bridging metal-H-B bonds, as indicated by B-H stretches in the ranges of 2563-2540 and 2143-2135 cm^-1, respectively. Hydrogen desorption is first observed at around 178 K and the presence of a stable hydrogen-containing surface intermediate is revealed by additional H₂ desorption peaks at 330 and 426 K. A combination of theoretical methods reveals that BH adsorbed at a 3-fold hollow site on the Pd(111) surface is the most stable species, but once formed two BH molecules can dimerize to form HBBH. A stable configuration for B₂H₂ is achieved through formation of B-H-Zr bridge bonds with a Zr atom located at one or both ends of a B₂H₂ molecule. Calculated vibrational frequencies and intensities provide an excellent match with the experimental RAIR spectra.