ARTICLE pubs.acs.org/JPCC
Influence of Particle Size and Water Coverage on the Thermodynamic Properties of Water Confined on the Surface of SnO2 Cassiterite Nanoparticles Elinor C. Spencer,† Nancy L. Ross,*,† Stewart F. Parker,‡ Alexander I. Kolesnikov,§ Brian F. Woodfield,|| Kellie Woodfield,|| Mckay Rytting,|| Juliana Boerio-Goates,|| and Alexandra Navrotksy^ †
Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom § Neutron Scattering Sciences Division, Oak Ridge National Laboratory, PO BOX 2008, Oak Ridge, Tennessee 37831-6473, United States Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, United States ^ Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, California 95616, United States
)
‡
bS Supporting Information ABSTRACT: Inelastic neutron scattering (INS) data for SnO2 nanoparticles of three different sizes and varying hydration levels are presented. Data were recorded on five nanoparticle samples that had the following compositions: 2 nm SnO2 3 0.82 H2O, 6 nm SnO2 3 0.055H2O, 6 nm SnO2 3 0.095H2O, 20 nm SnO2 3 0.072H2O, and 20 nm SnO2 3 0.092H2O. The isochoric heat capacity and vibrational entropy values at 298 K for the water confined on the surface of these nanoparticles were calculated from the vibrational density of states that were extracted from the INS data. This study has shown that the hydration level of the SnO2 nanoparticles influences the thermodynamic properties of the water layers and, most importantly, that there appears to be a critical size limit for SnO2 between 2 and 6 nm below which the particle size also affects these properties and above which it does not. These results have been compared with those for isostructural rutile-TiO2 nanoparticles [TiO2 3 0.22H2O and TiO2 3 0.37H2O], which indicated that water on the surface of TiO2 nanoparticles is more tightly bound and experiences a greater degree of restricted motion with respect to water on the surface of SnO2 nanoparticles. This is believed to be a consequence of the difference in chemical composition, and hence surface properties, of these metal oxide nanoparticles.
’ INTRODUCTION Water is ubiquitous in metal oxide nanoparticle systems, and its presence on the surface of such particles affects their stability at the nanoscale.1 Yet, significant differences in the structure, dynamics, dissolution properties, and liquidsolid phase transition temperature for confined water relative to bulk liquid water have been reported, demonstrating that water confined to nanoscale domains exhibits unique physical and chemical properties.28 Consequently, if one is to obtain a comprehensive understanding of the surface chemistry of nanoparticle systems, it is imperative that the thermodynamic properties of the surface hydration layers be evaluated. As the principle contribution to the heat capacity of the water species on the nanoparticle surfaces is due to the motion of the hydrogen atoms associated with these species, inelastic neutron scattering (INS) techniques are eminently suitable for determining the heat capacity and vibrational entropy of the nanoparticle hydration layers. The large incoherent neutron scattering cross section of hydrogen relative to transition metals and the absence of selection rules permit the use of INS experiments to probe the r 2011 American Chemical Society
dynamics of water adsorbed on the surface of the particle with minimal interference from the underlying metal oxide lattice.9,10 The effectiveness of this technique has been demonstrated by Klug et al. who compared heat capacity data determined by INS and calorimetric techniques for ice-Ih; indeed this comparison allowed for the elucidation of the anharmonic contribution to heat capacity of this form of ice.11 We have previously reported INS data for both anatase and rutile TiO2 nanoparticles and demonstrated that the heat capacity and vibrational entropy of the hydration layers situated on the surface of these particles were insensitive to the polymorphism of the underlying TiO2 lattice.12,13 Furthermore, by comparing data collected on two rutile TiO2 nanoparticle systems with differing levels of hydration, namely, TiO2 3 0.22H2O and TiO2 3 0.37H2O, we were able to conclude that the thermodynamic properties of the hydration layers were not influenced by Received: March 16, 2011 Revised: September 15, 2011 Published: October 07, 2011 21105
dx.doi.org/10.1021/jp202518p | J. Phys. Chem. C 2011, 115, 21105–21112
The Journal of Physical Chemistry C
ARTICLE
Table 1. Particle Size and Water Content Details for Samples 13 sample ID
particle size (nm)
water content (wt %)
surface area (m2/g)
no. water molecules/nm2
pore volume (cm3/g)
chemical formula SnO2 3 0.82H2O SnO2 3 0.055H2O
1
2(1
8.93
1068 ( 12
3
0.77 ( 0.04
2A
6(2
0.65
39.6 ( 0.5
6
0.084 ( 0.005
2B
6(2
1.12
39.6 ( 0.5
10
0.084 ( 0.005
3A
20 ( 3
0.85
18.2 ( 0.2
16
0.064 ( 0.004
3B
20 ( 3
1.09
18.2 ( 0.2
20
0.064 ( 0.004
Figure 1. TEM image of 1.
the variation in hydration level of the sample, albeit over a narrow range of water content. In this contribution, we present INS spectra for SnO2 cassiterite nanoparticles of three different sizes with varying levels of hydration from which isochoric heat capacity curves have been calculated. This study has allowed for an assessment of the influence of both particle size and hydration level on the thermodynamic properties of the water confined on the nanoparticle surface. To provide a more in-depth appraisal of the effect of sample water content on the heat capacity of the hydration layers, this aspect of the experiment was conducted over a broader range of hydration levels than previously reported for rutile TiO2.13
’ EXPERIMENTAL METHODS Sample Preparation. The 2 nm SnO2 nanoparticles (1) were synthesized by a previously published synthetic method with some modifications.14 Anhydrous SnCl4 was mixed with excess solid NaOH, then sufficient H2O was added to create a slurry, and then the mixture was ground for 10 min with a pestle and mortar. The resulting solid mixture was rinsed thoroughly with water until Cl ions could no longer be detected by the addition of a concentrated solution of Ag+ ions. The final product was confirmed to be phase pure. The 6 nm SnO2 sample (2A) was synthesized by calcining a portion of the 2 nm SnO2 sample at 500 C for 2 h. Nanoparticles of SnO2 20 nm in size SnO2 3 0.072H2O (3A) were purchased from Sigma-Aldrich and used as received. Samples 2B and 3B were obtained by exposing 2A and 3A to the open atmosphere (294 K, ∼70% relative humidity) for 24 and 30 h, respectively. Sample Characterization. The phase purities and particle sizes (and their distribution) of the samples were determined by
SnO2 3 0.095H2O SnO2 3 0.072H2O SnO2 3 0.092H2O
powder X-ray diffraction analyses that were performed at a scanning rate of 0.2 2θ min1 on a Scintag X-ray diffractometer equipped with a Cu Kα monochromated radiation source powered to 15 kV (powder patterns are given in the Supporting Information). The water contents of the samples were determined by thermogravimetric analyses (TGA) that were performed with a Netzsch 409 thermogravimetric analyzer. The samples were placed in corundum crucibles and heated at a rate of 5 K min1 up to 1173 K in a He atomosphere. Brunauer EmmettTeller (BET) specific surface areas of the samples were determined from N2 adsorption at 77 K (Micromeritics TriStar II). The samples were degassed at 473 K for 12 h prior to data collection. The results of these analyses are presented in Table 1. Inductively coupled plasma optical emission spectrometry (ICPOES) measurements were performed on a Perkin-Elmer ICPOES Optima 4300 DV system (samples solutions were prepared by dissolving appropriate amounts of sample in 5% NHO3), and the data from these analyses confirmed that the sodium and chloride contents of the as-synthesized 2 nm SnO2 sample (1) were less than 100 ppm. A specimen of 1 was prepared for transmission electron microscopy (TEM) by dispersing a small amount of the sample powder in water (or ethanol) and grinding it with an agate pestle and mortor. A drop of this suspension was then placed on a cooper grid with 3 nm carbon backing, and the TEM image was acquired with a Tecnai F30 TEM system. A TEM image for sample 1 is shown in Figure 1. This image clearly shows the high degree of particle uniformity for this sample as well as the spherical morphology of the SnO2 particles. INS Experiments. INS data were collected at 11 ( 2 K on the Thermal Original Spectrometer with Cylindrical Analyzers (TOSCA) at the ISIS Facility (RAL, U.K.).15 ISIS is a pulsed neutron spallation source, and TOSCA is thermal neutron time-of-flight (TOF) spectrometer with excellent resolution (ΔE/E ≈ 2%) at low energy transfers (
