Structure and Stability of SnO₂ Nanocrystals and Surface-Bound Water Species

Abstract

The structure of SnO2 nanoparticles (avg. 5 nm) with a few layers of water on the surface has been elucidated by atomic pair distribution function (PDF) methods using in situ neutron total scattering data and molecular dynamics (MD) simulations. Analysis of PDF, neutron prompt gamma, and thermogravimetric data, coupled with MD-generated surface D2O/OD configurations demonstrates that the minimum concentration of OD groups required to prevent rapid growth of nanoparticles during thermal dehydration corresponds to ~0.7 monolayer coverage. Surface hydration layers not only stabilize the SnO2 nanoparticles but also induce particle-size-dependent structural modifications and are likely to promote interfacial reactions through hydrogen bonds between adjacent particles. Upon heating/dehydration under vacuum above 250 °C, nanoparticles start to grow with low activation energies, rapid increase of nanoparticle size, and a reduction in the a lattice dimension. This study underscores the value of neutron diffraction and prompt-gamma analysis, coupled with molecular modeling, in elucidating the influence of surface hydration on the structure and metastable persistence of oxide nanomaterials.

Uncovering the True Atomic Structure of Disordered Materials: The Structure of a Hydrated Amorphous Magnesium Carbonate (MgCO3·3D2O)

“…The level of agreement reached at the end of the process (χ 2 = 0.30, 0.48 in terms of R w as defined in PDFgui 45 ) is relatively large compared with some high quality PDF refinements of disordered crystalline materials (R w ≈ 0.08 in perovskites 47 and R w ≈ 0.05 in chalcogenides. 48 However, the agreement is similar to what is achieved for other amorphous materials and nanoparticles, including gold nanoparticles (R w ≈ 0.26), 49 water molecules on SnO 2 nanoparticles (R w ≈ 0.19), 50 and our previous DFT-PDF iterative methodology investigation on metakaolin (R w ≈ 0.77). 24 To determine the exact medium range changes that occur during the crystalline-to-amorphous transition, the main contributing atom−atom correlations are displayed in Figure 4a and b for the nesquehonite (after the DFT energy minimization, structure B) and hydrated AMC (structure K), respectively.…”

Vibrational Density of States of Strongly H-Bonded Interfacial Water: Insights from Inelastic Neutron Scattering and Theory

“…25 Upon further hydration, a second (L 2 ) and third layer (L 3 ) will form whose structure depends on the degree of dissociation of L 1 . These layers are defined by distinct minima in the axial density profile of oxygen atoms perpendicular to the surface derived from our previous results, 36,37,43 and constitute 1 ML in L 1 , ∼1.7 ML in L 2 , and ∼1.3 ML in L 3 per Sn 2 O 4 (110) surface unit area. Figure 3 shows the INS spectra (measured at 7 K to minimize Debye−Waller effects on the measured spectra) for SnO 2 nanoparticulate samples at two hydration levels, full hydration (full: L 1 + L 2 + L 3 + a little extra water, or 4.4 ML) and driest achievable experimental state before the onset of nanoparticle destabilization and growth (dry: L 1 + 0.25L 2 , or 1.5 ML).…”

“…In the presence of even low water vapor pressures a complete L 1 layer develops where the oxygen atom of the water forms a covalent bond with the undercoordinated Sn v and creates a terminal water (TW). 36,37,43 If one of the protons of TW transfers to the adjacent BO site, terminal and bridging hydroxyls (TH and BH) will form. These L 1 configurations are shown in Figures 1 and 2 43 demonstrated that (i) the L 1 water can only be removed at high temperature (>250°C) under vacuum, (ii) L 2 stabilizes the nanoparticles studied here and in ref 43, and (iii) the presence of L 1 species subtly influences the nanoparticle crystalline structure (further evidence of strong water−surface interactions).…”

“…36,37,43 If one of the protons of TW transfers to the adjacent BO site, terminal and bridging hydroxyls (TH and BH) will form. These L 1 configurations are shown in Figures 1 and 2 43 demonstrated that (i) the L 1 water can only be removed at high temperature (>250°C) under vacuum, (ii) L 2 stabilizes the nanoparticles studied here and in ref 43, and (iii) the presence of L 1 species subtly influences the nanoparticle crystalline structure (further evidence of strong water−surface interactions). The degree of dissociation of L 1 corresponds to a dynamic equilibrium driven by proton transfers between the associated and dissociated configurations, which depends on temperature, hydration level, 24 and pH, if liquid water is present.…”

“…Synthesis of SnO 2 nanoparticles and micrometer-sized (bulk) crystallites and structural/morphological characterization are described in our previous work 43 and references therein. We emphasize here that SnO 2 nanocrystals synthesized through solution-phase growth possess size/shape morphologies similar to those used in our neutron diffraction work 43 with the exception that nanopowders were capped with H 2 O/OH species rather than D 2 O/OD molecules. Hydrated SnO 2 nanoparticles (40−60 Å) with two different surface water contents, referred to as full and dry samples, are prepared according to the descriptions in ref 43.…”