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

Abstract

Hydrophobic surfaces that are robust can have widespread applications in drop-wise condensation, anti-corrosion, and anti-icing. Recently, it was shown that the class of ceramics comprising the lanthanide series rare-earth oxides (REOs) is intrinsically hydrophobic. The unique electronic structure of the rare-earth metal atom inhibits hydrogen bonding with interfacial water molecules resulting in a hydrophobic hydration structure where the surface oxygen atoms are the only hydrogen bonding sites. Hence, the presence of excess surface oxygen can lead to increased hydrogen bonding and thereby reduce hydrophobicity of REOs. Herein, we demonstrate how surface stoichiometry and surface relaxations can impact wetting properties of REOs. Using X-ray Photoelectron Spectroscopy and wetting measurements, we show that freshly sputtered ceria is hydrophilic due to excess surface oxygen (shown to have an O/Ce ratio of ∼3 and a water contact angle of ∼15°), which when relaxed in a clean, ultra-high vacuum environment isolated from airborne contaminants reaches close to stoichiometric O/Ce ratio (∼2.2) and becomes hydrophobic (contact angle of ∼104°). Further, we show that airborne hydrocarbon contaminants do not exclusively impact the wetting properties of REOs, and that relaxed REOs are intrinsically hydrophobic. This study provides insight into the role of surface relaxation on the wettability ofmore » REOs.« less

Authors:
;  [1];  [2];  [3]
  1. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)
  2. Department of Chemical Engineering and Applied Chemistry and Materials Science and Engineering, University of Toronto, Toronto, Ontario M5S 3E5 (Canada)
  3. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)
Publication Date:
OSTI Identifier:
22412594
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; Journal Volume: 106; Journal Issue: 6; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ATOMS; CERAMICS; CERIUM OXIDES; CORROSION; ELECTRONIC STRUCTURE; HYDRATION; HYDROCARBONS; HYDROGEN; ICE; INTERFACES; MOLECULES; OXYGEN; RELAXATION; SPUTTERING; STOICHIOMETRY; SURFACES; WETTABILITY; X-RAY PHOTOELECTRON SPECTROSCOPY

Citation Formats

Khan, Sami, Varanasi, Kripa K., E-mail: varanasi@mit.edu, Azimi, Gisele, and Yildiz, Bilge. Role of surface oxygen-to-metal ratio on the wettability of rare-earth oxides. United States: N. p., 2015. Web. doi:10.1063/1.4907756.
Khan, Sami, Varanasi, Kripa K., E-mail: varanasi@mit.edu, Azimi, Gisele, & Yildiz, Bilge. Role of surface oxygen-to-metal ratio on the wettability of rare-earth oxides. United States. doi:10.1063/1.4907756.
Khan, Sami, Varanasi, Kripa K., E-mail: varanasi@mit.edu, Azimi, Gisele, and Yildiz, Bilge. Mon . "Role of surface oxygen-to-metal ratio on the wettability of rare-earth oxides". United States. doi:10.1063/1.4907756.
@article{osti_22412594,
title = {Role of surface oxygen-to-metal ratio on the wettability of rare-earth oxides},
author = {Khan, Sami and Varanasi, Kripa K., E-mail: varanasi@mit.edu and Azimi, Gisele and Yildiz, Bilge},
abstractNote = {Hydrophobic surfaces that are robust can have widespread applications in drop-wise condensation, anti-corrosion, and anti-icing. Recently, it was shown that the class of ceramics comprising the lanthanide series rare-earth oxides (REOs) is intrinsically hydrophobic. The unique electronic structure of the rare-earth metal atom inhibits hydrogen bonding with interfacial water molecules resulting in a hydrophobic hydration structure where the surface oxygen atoms are the only hydrogen bonding sites. Hence, the presence of excess surface oxygen can lead to increased hydrogen bonding and thereby reduce hydrophobicity of REOs. Herein, we demonstrate how surface stoichiometry and surface relaxations can impact wetting properties of REOs. Using X-ray Photoelectron Spectroscopy and wetting measurements, we show that freshly sputtered ceria is hydrophilic due to excess surface oxygen (shown to have an O/Ce ratio of ∼3 and a water contact angle of ∼15°), which when relaxed in a clean, ultra-high vacuum environment isolated from airborne contaminants reaches close to stoichiometric O/Ce ratio (∼2.2) and becomes hydrophobic (contact angle of ∼104°). Further, we show that airborne hydrocarbon contaminants do not exclusively impact the wetting properties of REOs, and that relaxed REOs are intrinsically hydrophobic. This study provides insight into the role of surface relaxation on the wettability of REOs.},
doi = {10.1063/1.4907756},
journal = {Applied Physics Letters},
number = 6,
volume = 106,
place = {United States},
year = {Mon Feb 09 00:00:00 EST 2015},
month = {Mon Feb 09 00:00:00 EST 2015}
}
  • Determination of oxygen in Y/sub 2/O/sub 3/, La/sub 2/O/sub 3/, CeO/sub 2/, Nd /sub 2/O/sub 3/, Pr/sub 2/O/sub 3/ and Gd/sub 2/O/sub 3/) and in YF/sub 3/ and GdF/sub 3/ was ach ieved by vacuum melting method using a platinum bath, graphite case, and argon chamber. The sensitivity of the method was 0.01%. (R.V.J.)
  • An understanding of the interaction between NO and metal-oxide surfaces is of interest for the development of catalysts reducing the nitrogen oxide emissions. Adsorbed surface oxygens are formed on CaO, SrO, and BaO during exposure to N{sub 2}O and their presence is shown to affect the room-temperature NO adsorption. Information about the adsorbed intermediates is contained in the desorption products and in the desorption temperatures during the subsequent heating ramp in Ar. The presence of adsorbed oxygen species increases the total amount of adsorbed NO for CaO and BaO substrates, whereas for SrO the adsorbed intermediate is stabilized. Two NOmore » desorption peaks are found for CaO and SrO, one at low and one at high temperature. The former is assigned to adsorbed NO, whereas the latter is assigned to adsorbed -NO{sub 2} and/or -NO{sub 3} species. NO adsorption as -NO{sub 2} and/or -NO{sub 3} species finds evidence in the corresponding O{sub 2} desorption. Only one NO desorption peak is found for BaO. This NO desorption peak disappears in the absence of preadsorbed surface oxygens. O{sub 2} desorption is observed, even in the absence of any preadsorbed surface oxygens, for CaO and SrO substrates. This suggests NO bond dissociation upon NO adsorption. The effect of the promotion of CaO by Pt has also been investigated. The respective desorption profiles are similar to those for the unpromoted CaO with preadsorbed surface oxygens, although the amounts are significantly increased.« less
  • We have improved the application of mixed rare-earth oxides (REOs) as hot gas desulfurization adsorbents by impregnating them on stable high surface area supports and by the inclusion of certain transition metal oxides. We report comparative desulfurization experiments at high temperature (900 K) using a synthetic biomass gasifier effluent containing 0.1 vol % H 2S, along with H 2, CO 2, and water. More complex REO sorbents outperform the simpler CeO 2/La 2O 3 mixtures, in some cases significantly. Supporting REOs on Al 2O 3 (~20 wt % REO) or ZrO 2 actually increased the sulfur capacities found after severalmore » cycles on a total weight basis. Another major increase in sulfur capacity took place when MnO x or FeO x is incorporated. Apparently most of the Mn or Fe is dispersed on or near the surface of the mixed REOs because the capacities with REOs greatly exceeded those of Al 2O 3-supported MnO x or FeO x alone at these conditions. In contrast, incorporating Cu has little effect on sulfur adsorption capacities. Both the REO and transition metal/REO adsorbents could be regenerated completely using air for at least five repetitive cycles.« less