Hydrophobicity of rare-earth oxide ceramics

Abstract

Hydrophobic materials that are robust to harsh environments are needed in a broad range of applications. Although durable materials such as metals and ceramics, which are generally hydrophilic, can be rendered hydrophobic by polymeric modifiers, these deteriorate in harsh environments. Here we show that a class of ceramics comprising the entire lanthanide oxide series, ranging from ceria to lutecia, is intrinsically hydrophobic. We attribute their hydrophobicity to their unique electronic structure, which inhibits hydrogen bonding with interfacial water molecules. We also show with surface-energy measurements that polar interactions are minimized at these surfaces and with Fourier transform infrared/grazing-angle attenuated total reflection that interfacial water molecules are oriented in the hydrophobic hydration structure. Moreover, we demonstrate that these ceramic materials promote dropwise condensation, repel impinging water droplets, and sustain hydrophobicity even after exposure to harsh environments. Rare-earth oxide ceramics should find widespread applicability as robust hydrophobic surfaces.

Superhydrophobic drag reduction in laminar flows: a critical review

“…In particular, biofouling on the surface is a serious issue for underwater vehicles as it incurs additional frictional drag and considerable maintenance effort. Nevertheless, we expect that these issues can be overcome eventually with a rapid development of low-cost micro-and nanofabrication techniques (Ahn and Guo 2009), the development of regenerative SHPo surfaces , damage-tolerant SHPo surfaces (Lu et al 2015), rare-oxide-based hydrophobic coatings (Azimi et al 2013), and the functioning of SHPo surfaces as anti-biofouling surfaces (Genzer and Efimenko 2006). Lastly, oil-infused nanostructured surfaces (Wong et al 2011), which have attracted much interest for their liquid repellency as well as their stability under a high liquid pressure, have been hailed as an alternative to the SHPo surface.…”

Raman spectra of R2O3 (R—rare earth) sesquioxides with C-type bixbyite crystal structure: A comparative study

“…They are suitable to be used as electroluminescence 1 and cathodoluminescence 2 sources, catalysts for chemical organic reactions, 3,4 high-k gate dielectrics, 5 optical parts of high power lasers, 6 oxygen ion conducting electrolyte in solid oxide fuel cells, 7 and materials with strongly hydrophobic surface. 8 Depending on the type of the rare earth, five different crystal structures are known. 9 Two of them, the so-called X-phase and H-phase, are stable only above 2000 K. 10 The other three, the hexagonal A-phase, the monoclinic B-phase, and the cubic C-phase, can be observed at ambient conditions.…”

Role of surface oxygen-to-metal ratio on the wettability of rare-earth oxides

“…[42][43][44][45][46][47] REOs are capable of sustaining their intrinsic hydrophobicity after exposure to extreme processing conditions such as high temperature, abrasive wear, and steam. 42 The underlying phenomenon of hydrophobicity in REOs arises from the unique electronic structure of the rare-earth metal atoms where the unfilled 4f orbitals are shielded from interactions with the environment by a full octet of electrons in the 5s 2 p 6 outer shell. 42 This shielding effect minimizes the hydrogen bonding with interfacial water molecules and renders REOs hydrophobic 42 as shown in Figure 1.…”

“…42 The underlying phenomenon of hydrophobicity in REOs arises from the unique electronic structure of the rare-earth metal atoms where the unfilled 4f orbitals are shielded from interactions with the environment by a full octet of electrons in the 5s 2 p 6 outer shell. 42 This shielding effect minimizes the hydrogen bonding with interfacial water molecules and renders REOs hydrophobic 42 as shown in Figure 1. In contrast, in the case of a common ceramic like alumina, the lack of such a shielding effect allows the metal atom to hydrogen bond with interfacial water molecules.…”