Deciphering the mechanism of water interaction with nanostructural copper oxide: going beyond superwetting

Abstract

The interaction of water molecules with copper oxide nanowires (nW-CuO) was comprehensively studied. The obtained results prove that nW-CuO is a perfect material for water purification via photothermal conversion in solar interfacial evaporation. Measurements of the water contact angle (WCA) close to zero degrees lead to a conclusion about the superwetting state. However, in such a case, the questions appear: do we measure the neat surface properties? To what extent does the water film affect the surface properties? These problems were addressed by synthesizing nW-CuO(X), where X is the desorption temperature, and assessing the effect of H₂O pre-adsorption through air exposure using nW-CuO(25). The sample was characterized on a molecular level by Raman and far-infrared spectroscopy and XRD, proving the synthesis of the typical CuO material. Some meaningful differences in surface energy values between nW-CuO(200) and nW-CuO(25) were detected based on immersion enthalpy. These differences were proved to originate from a different concentration of surface Langmuir-type highly active adsorptive centers determined by the H₂O adsorption isotherm. The adsorption enthalpy measurement also reveals dissociative H₂O adsorption in the low-coverage region on nW-CuO(200). For both samples, the entropic factor plays a dominant role during adsorption under high humidity conditions. As a consequence of mobile adsorption, H₂O molecules retain partial freedom of movement, which reduces entropy loss and heat of evaporation to ca. −7 kJ mol⁻¹. Such a low value of the desorption enthalpy promotes efficient evaporation from the material surface with a rate of 3.02 kg m⁻² h⁻¹ under minimal temperature increase.