The interaction of water with ZnO nanoparticles has been studied by means of diffuse reflectance infrared spectroscopy (DRIFTS) and ultra-high vacuum FTIR spectroscopy (UHV-FTIRS). Exposing clean ZnO powder to water at 323 K leads to both molecular and dissociative adsorption of H₂O forming a number of hydroxyl species. All the OH bands are clearly identified by the adsorption of D₂O showing the expected isotopic shifts. According to the vibrational and thermal stability data obtained from single crystal surfaces, the OH species observed on ZnO nanoparticles are identified as follows: (1) OH group (3620 cm⁻¹) on the polar O–ZnO(000) surface formed via dissociation of water on oxygen vacancy sites; (2) partial dissociation of water on the mixed-terminated ZnO(100) surface yielding coexistent H₂O (∼3150 and 3687 cm⁻¹) and OH species (3672 cm⁻¹), where the molecularly adsorbed H₂O is further identified by the characteristic scissoring mode at 1617 cm⁻¹; (3) isolated OH species (3639 and 3656 cm⁻¹) formed on the mixed-terminated ZnO(100) surface; (4) interaction of water with defects forming hydroxyl (or O–H⋯O) species (3564 and 3448 cm⁻¹).

The interaction of water with the non-polar ZnO(100) surface has been studied by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). Adsorption of water at room temperature leads to the partial dissociation of water molecules giving rise to a well defined (2 × 1) superstructure. This observation was confirmed by the HREELS data which show the water-induced O–H stretching modes at 396 and 460 meV (3193 and 3709 cm⁻¹) as well as the peak at 456 meV (3677 cm⁻¹) arising from the OH species. The large red shift of the loss at 396 meV indicates unusually strong hydrogen bonding interactions of water to both neighbouring adsorbate molecules and the surface O atoms which are responsible for the partial dissociation of water molecules on the perfect ZnO(100) surface.

A newly designed ultrahigh vacuum (UHV) infrared spectroscopy apparatus dedicated to the spectroscopic characterization of oxides, singles crystals as well as powders, is described. It combines a state-of-the-art vacuum Fourier transform infrared (FTIR) spectrometer (Bruker, VERTEX 80v) with a novel UHV system (PREVAC) consisting of load-lock, distribution, measurement, and magazine chambers. The innovative design allows carrying out both reflection-absorption IR spectroscopy experiments at grazing incidence on well-defined oxide single crystal surfaces and FTIR transmission measurements for powder particles. A further unique feature of the apparatus is the entirely evacuated optical path to avoid background signals from gas phase H2O, CO2, and other species, thus creating the possibility to record high-quality IR data with high sensitivity and stability, an essential prerequisite for monitoring molecular species adsorbed on oxide single-crystal surfaces. The unique performance of this new apparatus with regard to the spectroscopic characterization of adsorbates on oxide single crystals as well as on powder particles is demonstrated by case studies for two different materials, TiO2 and ZnO.