In situ vibrational spectroscopy analysis of adsorbed phosphate species on gold single crystal electrodes

The adsorption of phosphate species on Au(100) electrode at different pHs is studied by using in situ Fourier Transform Infrared Spectroscopy. The results are compared with those published previously for Au(lll) and also for polycrystalline gold. H2PO4- ions present two spectral features at 1000 and 1120cm(-1), consistent with a C-2v symmetry, as expected. At pH 12.6 for the adsorption of PO(4)(3-)on Au(100) an abrupt change in the band intensity is observed which is consistent with the lifting of the surface reconstruction. This change in the intensity coincides with a current peak observed in the cyclic voltammogram and has been related to the surface lifting of the reconstruction. The PO(4)(3-)and H2PO4- ions when adsorbed on Au(100) show the same characteristic bands compared to Au(lll). The bands of the adsorbed species are located at ca. 1120 cm(-1) and 1050 and 1085 cm(-1) for HPO42- and PO43- respectively. At Au(100) surfaces the three-fold coordination for both PO43- and HPO42- is very unlikely due to the mismatch between the oxygen-oxygen distances and the atomic distances at the (100) surface. For PO43-, where the bands for the solution and adsorbate phosphate species do not overlap, it is possible to identify a consistent difference in the band half-width, when compared with PO(4)(3-)adsorbed on Au(lll). This difference in band width is tentatively assigned to a different surface morphology of Au(100) after the lifting of the reconstruction. (C) 1998 Elsevier Science B.V.