In situ IRAS and STM of adsorbate structures on an Ir(111) electrode in sulfuric acid electrolyte

The adsorption of hydrogen adatoms and bisulfate anions on an Ir(111) electrode surfaces in 0.5 M H2SO4 solution has been investigated in situ with a combination of cyclic voltammetry (CV), infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM) experiments. The IRAS result shows evidence for the adsorption of hydrogen adatoms occupying the on-top sites of the Ir(111) at potentials below 55 mV versus SHE (standard hydrogen electrode) where current spikes are seen in the CV profile. The STM reveals a (1×1) structure corresponding to the unreconstructed Ir(111) lattice at potentials below 55 mV. At potentials positive of the current spikes, two absorption bands at 1255–1320 and 945–956 cm−1 are present in the IR spectra with the former bands showing a higher frequency shift upon deuteration. The IRAS and STM results demonstrate that HSO4− anions rather than SO42− anions predominantly adsorb on the Ir(111) electrode to form the characteristic (√3×√7) structure.