Article pubs.acs.org/Langmuir
Anatase Nanoparticle Surface Reactivity in NaCl Media: A CD−MUSIC Model Interpretation of Combined Experimental and Density Functional Theory Studies Moira K. Ridley,*,† Michael L. Machesky,‡ and James D. Kubicki§ †
Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, United States Illinois State Water Survey, University of Illinois, Champaign, Illinois 61820, United States § Department of Geosciences and The Earth & Environmental Systems Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡
ABSTRACT: The effect of particle size on the primary charging behavior of a suite of monodisperse nanometer diameter (4, 20, and 40 nm) anatase samples has been quantitatively examined with macroscopic experimental studies. The experimental results were evaluated using surface complexation modeling, which explicitly incorporated corresponding molecular-scale information from density functional theory (DFT) simulation studies. Potentiometric titrations were completed in NaCl media, at five ionic strengths (from 0.005 to 0.3 m), and over a wide pH range (3−11), at a temperature of 25 °C. From the experimental results, the pH of zero net proton charge (pHznpc) for the 4 and 20 nm diameter samples was 6.42, whereas the pHznpc was 6.22 for the 40 nm sample. The slopes of the net proton charge curves increased with an increase in particle size. Multisite surface complexation and charge distribution (CD) models, with a Basic Stern layer description of the electric double layer, were developed to describe all experimental data. Fits to the experimental data included an inner-sphere Na-bidentate species, an outer-sphere Na-monodentate species, and outer-sphere Cl-monodentate species. DFT simulations found the Na-bidentate species to be the most stable species on the (101) anatase surface (the predominant crystal face). The CD value for the Nabidentate species was calculated using a bond valence interpretation of the DFT-optimized geometry. The Stern layer capacitance value varied systematically with particle size. The collective experimental and modeling studies show that subtle differences exist in the interface reactivity of nanometer diameter anatase samples. These results should help to further elucidate an understanding of the solid−aqueous solution interface reactivity of nanosized particles.
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INTRODUCTION The interface between a mineral and aqueous solution is a structural transitional domain where many important chemical and geochemical reactions occur. The chemical reactivity within the interface domain is driven by the undercoordination of surface oxygen atoms and cations, which are reactive toward water. The reactivity toward water results in the protonation and deprotonation of surface functional groups, giving rise to a pH-dependent surface charge at the mineral surface. The development of surface charge is balanced by the interaction of electrolyte counterions and the specific adsorption of ions. As a result, the interfacial region, known as the electric double layer (EDL), has molecular-level structure dependent on the mineral surface structure, the coordination geometry of specifically adsorbed ions, surface hydration, and ordering of water molecules at the mineral surface.1 For macroscopic particles, the complex mineral−water transition region has been systematically studied via experimental measurements and computational studies.2,3 Additionally, ion adsorption reactions have been studied using macroscopic experiments and X-ray techniques.1 Surface complexation models (SCM), specifically © XXXX American Chemical Society
the multisite (MUSIC) charge distribution (CD) models, have been used successfully to integrate molecular-level computational results with macroscopic experimental data.4,5By incorporating molecule-level information into SCMs, the inherent ambiguity in rationalizing macroscopic experimental ion adsorption data has been reduced.6,7 This has led to a better understanding of the physical and chemical properties at mineral−solution interfaces. Advances in the description and characterization of the surface properties of macroscopic particles have not yet been extended as fully to particles at the nanoscale.8,9 The surface reactivity of nanosized particles is of particular importance and interest, as the unique physical and chemical properties of nanoparticles are likely the result of their large surface area. As the size of nanoparticles decreases, a greater proportion of the atoms present exist at the surface, increasing the ratio of undercoordinated surface oxygen atoms and cations. A greater Received: April 2, 2013 Revised: June 6, 2013
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dx.doi.org/10.1021/la4011955 | Langmuir XXXX, XXX, XXX−XXX
Langmuir
Article
Table 1. Summary and Physical Characteristics of the Three Anatase Powders supplier
particle size (nm)
BET surface area (m2/g)
approximate mass of anatase per titration (g)
total weight loss at 35−300 °C (%)a
Ishihara Techno Corp. ST-01 ST-21 Altair Nanomaterials Inc.
4 20 40
300.3 ± 0.41 66.43 ± 0.34 43.98 ± 0.26
0.04 0.22 0.32
7.6 2.4 1.75
a
Thermogravimetric analysis.
allows for the evaluation of possible curvature effects on the EDL. We also present the results of density functional theory (DFT) calculations, performed to identify possible adsorption geometries of Na+ and Cl− ions on the (101) anatase surface. The first step in interpreting the experimental data is to estimate protonation affinity constants for the various surface functional groups on specific crystal faces. The MUSIC model is the most fully developed surface protonation model that explicitly incorporates site specific information for relevant crystal faces and, therefore, is used to develop a surface protonation description for each anatase sample. The resulting surface protonation schemes are consistent with the experimentally determined pHznpc values. A CD model is used to provide a thorough description of the interaction of Na+ and Cl− ions with the anatase surface. Information from the DFT calculations is used explicitly to constrain the CD model fits of the NaCl primary charging titration curves. That is, the innersphere Na species suggested by the DFT simulations is considered when fitting the titration data. Moreover, the CD value for the inner-sphere Na species is calculated on the basis of the coordination geometry obtained from the DFT simulations. Specifically, we combine the experimental and DFT simulation results within a MUSIC and CD modeling framework, which allows us to quantitatively evaluate and better understand the effects of particle size on the surface reactivity of nanometer-sized metal oxide particles.
ratio of unsatisfied surface bonds relative to the bulk oxygen− metal bonds, an increase in surface defects, and an increase in particle edges can change the pH-dependent surface charging properties of mineral surfaces.10,11 It may therefore be expected that the pH-dependent surface reactivity of nanoparticles will vary as a function of particle size. Any changes in the surface charging behavior of nanoparticles will have a direct impact on the binding of electrolyte ions and specific adsorption of ions. Changes in surface charging and ion adsorption behavior, including that of nanoparticles, are measurable at the macroscopic scale.12 Experimental information examining the surface−aqueous interface reactivity of nanoparticles is, however, limited.13,14 Several studies have investigated the charging and adsorption behavior of metal oxide particles
