Water adsorption, dissociation and oxidation on SrTiO₃ and ferroelectric surfaces revealed by ambient pressure X-ray photoelectron spectroscopy

Abstract

Unveiling surface adsorbates under atmospheric conditions and in surface water redox reactions on TiO2 terminated surfaces and ferroelectric oxides, as studied by AP-XPS.

Surface charged species and electrochemistry of ferroelectric thin films

“…They also suggest that polarization orientation strongly influences the interaction of water molecules with the ferroelectric surface. [33][34][35][36] Near-ambient pressure X-ray photoelectron spectroscopy (AP-XPS), allowing the contributions of H 2 O and OH − groups to be considered separately, 15,17,37 has corroborated these observations, demonstrating an asymmetric adsorption of the charged species as a function of polarization orientation. 35,36 More recently, the combination of SPM with secondary ion mass spectrometry 38 has suggested that the evolution of chemical states during polarization switching, beside changes in surface chemistry, 39 can also include redistribution of bulk cations.…”

“…AP-XPS has the required chemical sensitivity to detect water molecules on the ferroelectric surface, whose adsorption and dissociation are well known to depend on the polarization 16,[33][34][35][36] as well as on surface chemical reactivity, 5,14,16,54 especially for perovskite oxides with TiO 2 terminated surfaces. 15 The total adsorption and the dissociation of water molecules into charged species (-OH groups, protons and peroxide species) is very different as a function of the as-grown sample polarization state. Fig.…”

“…2 shows the O 1s spectrum, which can be decomposed into a complex distribution of peaks, assigned to different adsorbate species according to previously reported XPS measurements, as described in the ESI. † 15,36 Spectra of the O 1s region at 1 mbar and 700 eV are shown in Fig. 2 for three different surfaces: up polarized as-grown ( Fig.…”

“…Quantitative analysis of the fittings can be performed using a multilayer electron attenuation model, 15,36 and plotted as a function of RH (Fig. 2b, c and d).…”

“…[1][2][3][4][5] In this respect, adsorbed water from the environment has long been recognized as playing a crucial role in electrostatic screening under atmospheric conditions due to its polar nature and ionic conductivity, and the associated screening dynamics have been studied as a function of humidity 6,7 and temperature, [8][9][10] leading to the development of a full thermodynamic theory of the screening mechanisms. 11 However, few studies and ab initio modeling have considered the capacity of water molecules to dissociate and participate in redox chemical reactions at ferroelectric surfaces, [12][13][14][15][16] which would be expected to provide a large additional screening contribution, 17,18 in particular on highly reactive perovskite oxides. 19,20 Moreover, surface chemistry is not only important for the stabilization of ferroelectric surfaces, 21 but in fact polarization switching kinetics are intrinsically coupled to surface electrochemical phenomena.…”