Temperature programmed desorption of polar adsorbates from (10 [formula omitted]0), (0001) and (000 [formula omitted]) ZnO surfaces

Abstract

Dynamic mass spectrometric techniques - making possible the quasi simultaneous monitoring of the identities and individual rates of evolution of various species - have been utilised to develop temperature programmed desorption spectra (TPDS) following adsorption of water, methanol or isopropanol onto selected faces of ZnO single crystals at ca. 100 K. During TPDS from D 2O/ZnO {10 1 0}, (0001) and (000 1 ) systems, across the temperature range 100 to 540 K, molecular D 2O was the sole species desorbing in significant amount. Desorption of D 2O from several distinguishable surface configurations was detected for each crystal face. Detailed study of the relative extents of development of associated TPDS features, as a function of exposures increasing upward from 0.2 Langmuir, demonstrated strong sensitivity of some features to the choice of crystal face. On this basis features with T(max)at 340 and 190 K are respectively assigned to site-specific interactions of_adsorbed D 2O with zinc sites on ZnO (0001) and with oxygen sites on ZnO (000 1 ). Other features appear to originate from D 2O aggregated into clusters or condensed as two and three-dimensional ice-like layers. For the Alcohol/ZnO systems desorption from condensed layers was again distinguished from that originating from species held more strongly at oxygen sites or zinc sites in the first ad-layer. However in marked contrast to the D 2O/ZnO (0001) system, TPD of methanol from that ‘zinc’ face yielded not only a TPDS feature characteristic of the parent alcohol but also TPDS features at 400–500 K for carbon oxides and hydrogen, such as would be consistent with some dissociative chemisorption of CH 3OH and conversion to a “formate-like” surface intermediate. No evidence for the latter was found in TPDS from CH 3OH/ZnO (000 1 ) oxygen faces. Indications from TPDS of (CH 3) 2CHOH/ZnO systems for dissociative chemisorption of isopropanol were again confi ned to the ZnO (0001) zinc face.