Probing Elusive Intermediates by Synchrotron VUV Mass Spectrometry—The Formation of Aluminium Containing Intermediates in the Gas Phase

Solid layers of Al₂O₃ are widely used in industry as functional surface coatings because of its high electrical resistivity and its hardness.^1,2 An important method to deposit such layers is chemical vapour deposition, where a metal-containing precursor is evaporated and finally forms a film in a surface reaction. Often, gas-phase reactions are the initial decomposition step, but gaseous intermediates can lead to unwanted film morphology or a depletion of the precursor by side reactions, leading to a reduction in growth rate. Consequently, the analysis of the early stages of growth is important and requires fast and sensitive analytical techniques with sufficiently low detection limits for elusive gas-phase species. Because of limitations in experimental techniques, it was not possible so far, to detect most of the postulated intermediate species and their temperature-dependent kinetics often remained unknown. We have overcome some of these challenges and demonstrated, that by using a microreactor coupled to a very mild ionization source, aided by numerical simulation, we are capable to detect and characterize elusive species, especially metal-containing intermediates with short lifetimes below 100 μs.³<br/>Here, we present insights into the vacuum pyrolysis and reduction of aluminium tris(acetylacetonate), Al(C₅H₇O₂)₃. In brief, the precursor is sublimed, subsequently transported by argon or a mixture of argon and hydrogen carrier gas, and expanded through a pinhole into a resistively heated 1 mm inner diameter SiC-microreactor of 10 mm length. Species leaving the reactor are probed, ionized by tuneable vacuum ultraviolet (VUV) synchrotron radiation, and characterized by imaging photoelectron photoion coincidence spectroscopy (i²PEPICO) and mass spectrometry at the Swiss Light Source. We recorded photoionization efficiency curves (PIE) and threshold photoelectron spectra (TPES) at photon energies of 7.5-11.5 eV, which give us direct evidence for the characterization of reactive intermediates and products.<br/>In the experiments, 49 hydrocarbons, oxygenated and aluminium-containing species were detected and characterized unambiguously in the gas-phase. Supplemented by the temperature-dependent photoionization mass spectra recorded at temperatures of 413-923 K, this data provides insights into the underlying decomposition mechanisms. First, at lower temperatures we probed and assigned a substituted pentalene ring species (C₁₀H₁₂O₂) most likely formed from Al(OH)₂(C₅H₇O₂). Second, and most importantly, we detected and characterized aluminium bis(diketo)acetylacetonate-H, Al(C₅H₇O₂)C₅H₆O₂ at <i>m/z</i> 224 as major initial decomposition product in the gas-phase at temperatures above 600 K. Additionally, some hydrocarbons and oxygenated species were detected and assigned as decomposition species for the first time, i.e. C₁₄H₁₈, C₁₂H₁₀O₂, and C₉H₆O₂ and their formation mechanisms will be discussed. The influence of H₂ addition on the decomposition mechanism will be adressed and Arrhenius parameters will be presented on the gas-phase decomposition kinetics of the pyrolysis and reduction of Al(C₅H₇O₂)₃. Together with the simultaneous numerical simulation of the flow field in a microreactor, the application of the synchrotron radiation coupled to the i²PEPICO experimental apparatus enables us to unravel the decomposition mechanisms and kinetics of metal-organic precursors in the gas-phase.<br/><br/>References<br/>1. J.P.B. Silva, K.C. Sekhar, H. Pan, J.L. MacManus-Driscoll and M. Pereira, <i>ACS Energy Lett.</i>, 2021, <b>6</b>(6), 2208.<br/>2. S. Wu, Y. Zhao, W. Li, W. Liu, Y. Wu and F. Liu, <i>Coatings</i>, 2021, <b>11</b>(1), 79.<br/>3. S. Grimm, S.-J. Baik, P. Hemberger, A. Bodi, A.M. Kempf, T. Kasper and B. Atakan, <i>Phys Chem Chem Phys</i>, 2021, <b>23</b>(28), 15059.