ARTICLE pubs.acs.org/JPCC
Experimental Methodologies for Assessing the Surface Energy of Highly Hygroscopic Materials: The Case of Nanocrystalline Magnesia Shmuel Hayun,†,§ Tien Tran,† Sergey V. Ushakov,† Andrew M. Thron,‡ Klaus van Benthem,‡ Alexandra Navrotsky,† and Ricardo H. R. Castro*,† †
Peter A. Rock Thermochemistry Laboratory and NEAT ORU, and ‡Department for Chemical Engineering and Materials Science, University of California, Davis, Davis, California 95616, United States ABSTRACT: Measuring the surface energy of highly hygroscopic materials has remained a thorny problem for many years, mainly because obtaining an anhydrous surface state and maintaining this condition during the surface energy assessment has been considered an impractical task. In this work, we developed synthetic and calorimetric approaches that overcome these difficulties and applied them to measure the surface energy of anhydrous nanocrystalline magnesium oxide. Anhydrous MgO with specific surface area of ∼300 m2 g1 was synthesized by laser ablation in a controlled oxygen partial pressure environment. High resolution transmission electron microscopy and X-ray diffraction showed cubic nanoparticles with sizes ranging from 5 to 10 nm (as controlled by the partial pressure) and with the periclase crystal structure. The surface energy of the anhydrous state was assessed using high temperature oxide melt drop solution calorimetry and differential scanning calorimetry; the surface energies were 1.2 ( 0.1 and 1.3 ( 0.1 J m2, respectively. These values are slightly higher than from previously reported experiments and are consistent with a less hydrated surface.
1. INTRODUCTION Accurate surface energy data are essential for calculating and predicting the thermodynamic stability of nanosized structures.1 However, the surfaces of nanomaterials are usually modified by adsorbed H2O (or other gases) that cannot be completely removed without severe coarsening of the particles. This is particularly problematic for highly hygroscopic materials. For instance, it has been shown that temperatures up to 950 °C are needed to remove all traces of water from the surface of MgO nanoparticles,2 a temperature high enough to overcome the activation energies for coarsening. The rapid H2O adsorption on these materials often leads to phase transformations (e.g., MgO, La2O3, and CaO),35 which makes direct energy measurements of anhydrous surfaces experimentally challenging. The surface energy of magnesia (MgO) has been the subject of numerous experimental and theoretical investigations2,610 Surface energy data are of particular interest for this material since magnesia is used extensively as a model material in densification studies1115 and is one of the most promising candidates for solid-based catalysts.16 Although the surface energy of MgO has been previously reported, published experimental values are questionable due to the material’s hygroscopic nature8 and the difficulties associated with indirect measurement methods.9 For instance, Jura and Garland8 assessed the surface tension of MgO but clearly underscored that “the error present in this work, due to an unknown quantity of adsorbed water, can be removed by improved equipment and technique”. More recently, Navrotsky r 2011 American Chemical Society
and collaborators have reported a systematic approach to estimate the contributing effect of adsorbed water to the surface energetics of oxides.1721 The method has been successfully used to decouple the heat effect of water adsorption from the surface energy effect using thermochemical cycles and the combination of two calorimetric techniques. However, the application of this methodology to MgO is hindered by the difficulty of creating an anhydrous surface state in samples with relatively high surface areas. In this work, we present a new method to assess the anhydrous surface energy of highly hygroscopic nanomaterials. We synthesized anhydrous MgO nanoparticles by laser ablation in a water-free environment and, for the first time ever, were able to directly assess the energy of its anhydrous surface. Although the method was applied to study nanocrystalline MgO, there is no reason to believe that its success is restricted to this material.
2. EXPERIMENTAL PROCEDURES 2.1. Synthesis. High surface area MgO nanoparticles were prepared using a gas-phase condensation technique described elsewhere in detail.2227 MgO targets for ablation were first fabricated by pressing MgO powder (Fisher Scientific, 99.5%) at ∼10 MPa. The compacts were then free sintered at 1400 °C for Received: September 9, 2011 Revised: October 26, 2011 Published: October 28, 2011 23929
dx.doi.org/10.1021/jp2087434 | J. Phys. Chem. C 2011, 115, 23929–23935
The Journal of Physical Chemistry C 1 h in air. Prior to vaporization, the synthesis chamber was evacuated below 9 103 Torr using an oil-free scroll pump. The chamber was then filled with oxygen gas to target pressures of 0.15, 1.0, or 10.0 Torr. The targets were ablated using a CW-CO2 laser operating at 40 W. The laser beam was focused on the rotating target (10 rpm), with the vaporization process lasting about 1 h. A steel shim located 50 mm from the target collected the condensing nanoparticles. After evaporationcondensation, the entire chamber was transferred to an argon-filled glovebox where the powders were collected. The water content in the glovebox atmosphere is
