dr

er

K

Heat capacity

Copper oxide

monnd Zteri(03ts frna

inherehas bations [46]

These hydration layers are essential to the stability of the particles,and if they are removed the particles coarsen and form macro-scopic particulates [18]. As the behaviour of water conned tonanoscale domains is known to be distinctly different from thatof bulk water [1926], it is of paramount importance that a com-prehensive understanding of the thermodynamic properties ofthese hydration layers be achieved, especially as the presence of

surface of several, Co3O4, PdO andse investiical comp

of the particles has a profound effect on the heat capathe particle hydration layers, although the polymorphismparticles does not, at least in the case of TiO2. Furthermore, thedegree of water coverage of the particles has a complex inuenceon the thermodynamic properties of the water conned on thesurface of SnO2 particles. Herein we discuss the VDOS for hy-drated CuO, ZnO and CeO2 nanoparticles, and show how the heatcapacity and vibrational entropy for the hydration layers ad-sorbed on the surfaces of these particles are inuenced by thenature of the underlying metal oxide lattice.

Corresponding author.

Chemical Physics xxx (2013) xxxxxx

Contents lists available at

al

lseE-mail address: nross@vt.edu (N.L. Ross).and phase stability of nanoscale particles relative to their bulkcounterparts [1417].

An additional factor that further complicates the chemistry ofnanoparticles is the presence of water conned to their surface.

modynamic properties of water conned on themetal oxide nanoparticle materials such as CoOisostructural TiO2 and SnO2 [2933]. From thewe have been able to conclude that the chem0301-0104/$ - see front matter 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.chemphys.2013.06.019

Please cite this article in press as: E.C. Spencer et al., Chem. Phys. (2013), http://dx.doi.org/10.1016/j.chemphys.2013.06.019gationsositioncity ofof theantibacterial agents [8], fuel additives [9], pharmacology [10,11],and environmental remediation [12,13], to name but a few. Whatis clear from the wealth of evidence available is that it cannot beassumed that the macrosopic properties of nanomaterials, whichmake them so amenable to practical applications, are equivalentto those of the corresponding bulk materials. This is a consequenceof the potentially dramatic differences in physicochemical proper-ties such as magnetism, electronic properties, surface structure,

ative to transition metals and the absence of selection rules per-mit the use of INS experiments to probe the dynamics of wateradsorbed on the surface of the particles without signicant inter-ference from the underlying metal oxide lattice. No other tech-nique allows for such accurate determination of thethermodynamic properties of the hydration layers situated onthe surface of nanoscale oxide particles. We have successfullydemonstrated the utility of this method for evaluating the ther-1. Introduction

Metal oxide nanoparticles are anwhich we live, and their versatilitywidespread development and utiliscatalysis [1,2], sensors [3], solar cellnt part of the world ineen exploited for theirin areas as diverse as, enzyme mimetics [7],

this surface water may signicantly alter the surface chemistryof the CuO, ZnO and CeO2 nanoparticles [27,28].

Inelastic neutron scattering (INS) techniques are uniquely suit-able for determining the heat capacity and vibrational entropy ofwater conned on the surface of metal oxide nanoparticles. Thelarge incoherent neutron scattering cross-section of hydrogen rel-Cerium oxideZinc oxide

hydration layers. 2013 Elsevier B.V. All rights reserved.Inelastic neutron scattering studies of hynanoparticles

Elinor C. Spencer a, Nancy L. Ross a,, Stewart F. ParkaDept. of Geosciences, Virginia Tech, Blacksburg, VA 24061, USAb ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UcDept. of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA

a r t i c l e i n f o

Article history:Available online xxxx

Keywords:NeutronSpectroscopy

a b s t r a c t

In this contribution we dethe surface of CeO2, CuO ature, inelastic neutron scatisochoric heat capacitieshydration layers. The resulposition of the metal oxide

Chemic

journal homepage: www.eated CuO, ZnO and CeO2

b, Rebecca E. Olsen c, Brian F. Woodeld c

strate how the vibrational density of states (VDOS) for water conned tonO nanoparticles can be determined from high-resolution, low-tempera-ng (INS) spectra. These VDOS have been employed in the calculation of the00 K) and room temperature vibrational entropies of the nanoparticleom this analysis clearly demonstrate that the structure and chemical com-noparticles has a notable effect on the thermodynamic properties of their

SciVerse ScienceDirect

Physics

vier .com/locate /chemphys

2. Experimental methods

2.1. Synthesis

Samples were prepared following a method developed by the

E = energy transfer: E = Ef + Ei, where Ei = incident energy andEf = nal energy. Ef is a constant, and for TOSCA the values for thefront and back detectors are equal to 3.35 meV and 3.32 meV,respectively. Q = magnitude of momentum transfer that was calcu-

lue that ensured the E was appropriately scaled to the spectrum

urfa

2 E.C. Spencer et al. / Chemical Physics xxx (2013) xxxxxxauthors: [34]CeO2 nanoparticles: 20 g of Ce(NO3)32H2O was mixed with 11 g of

NH4HCO3 in amortar and pestle for approximately 1min. Distilled H2O(12ml) was added to facilitate mixing. A precursor was formed thatwas then dried in air at 100 C for at least 24 h, rinsed with 0.5 L of dis-tilledH2O, andcalcined inair at550 C for2 hat a ramprateof26min1.Two separate samples weremade and combined before analysis.

CuO nanoparticles: 53 g of Cu(NO3)22.5H2O was mixed with38 g of NH4HCO3 in a mortar and pestle for approximately 1 min.Distilled H2O (10 ml) was added to facilitate mixing. A precursorwas formed that was rinsed with 0.5 L of distilled H2O, and cal-cined in air at 250 C for 1 h at a ramp rate of 11min1.

ZnO nanoparticles: 29 g of Zn(NO3)26H2O was mixed with 17 gof NaHCO3 in a mortar and pestle for approximately 1 min. Dis-tilled H2O (4 ml) was added to facilitate mixing. A precursor wasformed that was rinsed with 4 L of distilled H2O, and calcined inair at 325 C for 1 h at a ramp rate of 15min1.

2.2. Characterization

The phase purities of the CeO2, CuO, and ZnO samples were con-rmed by powder X-ray diffraction (PXRD) analyses that were per-formed with a PANalytical XPert Pro diffractometer (Cu-Ka1radiation, k = 1.540598 ) operating at 45 kV and 40 mA. Data wereacquired over the 2h range of 1090 (PXRD patterns for all sam-ples are provided in the SI). Average crystallite diameters wereestimated with the Scherrer equation [35]. The water contents ofthe samples were determined by thermogravimetric analyses(TGA) that were performed with a Netzsch STA 409 PC Thermo-gravimetric Analyzer. Specic BET surface areas and pore volumeswere determined from full-range N2 adsorption isotherms ob-tained at 77 K with a Micromeritics TriStar II surface analyzer:0.250.50 g samples were degassed at 473K prior to data collec-tion. Surface areas were calculated by the BrunauerEmmetTeller(BET) method, and pore volumes were calculated by the BarrettJoynerHalenda (BJH) method. Table 1 summarises the character-istics of the CeO2, CuO, and ZnO samples employed in this study.

2.3. INS Data collections and processing

Low temperature (413 K) INS spectra for samples were col-lected on the TOSCA spectrometer at the ISIS Facility (pulsed neu-tron source) at the Rutherford Appleton Laboratory (Oxford, UK)[36]. TOSCA is a time-of-ight (TOF) spectrometer that has excel-lent resolution (DE/E 1.5%) at low energy transfers (

al PhE.C. Spencer et al. / Chemicredistribution of the translational bands to higher energies in theVDOS of CuO and ZnO relative to the corresponding bands in theCeO2 VDOS. Moreover, the rst acoustic peak that is clearly appar-ent at 9 meV in the VDOS of CeO2 is strongly suppressed in theVDOS of CuO and ZnO. This energy redistribution indicates thatwater conned on the surface of CuO and ZnO nanoparticles expe-rience stronger watersurface interactions, and thus exhibit morerestricted motion, than water on CeO2 particles. Furthermore, thetranslational bands in the VDOS of the nanoparticle systems arehighly structured, which is suggestive of strong anisotropic inter-actions between the conned water and particle surfaces.

The librational bands in the VDOS for the nanoparticle systemsare broadly distributed over the 40120 meV energy range, yet nobands are observable in the 4065 meV region of the ice-Ih VDOS.The additional vibrational modes in the nanoparticle VDOS arepossibly a consequence of the riding modes that arise due to thecorrelated motion of the hydroxyl groups and physisorbed surfacewater with the optical modes of the metal oxide lattice. The CoGvalues for the librational bands can be calculated in a similar man-ner to those for the translational bands (Eq. (4) with a = 50 meV,

Fig. 2. Cv curves for the hydration layers conned to the surface of CeO2, CuO and ZnO nhave been calculated from the VDOS for these materials. Left: Cv data calculated overtemperature region.

Fig. 1. VDOS plots for hydrated CeO2, CuO and ZnO nanoparticles. Also shown is theVDOS for the reference material ice-Ih (taken from Ref. [41]). The VDOS have beensmoothed with a SavitzkyGolay lter so that their features are more clearlydiscernable [42].

Please cite this article in press as: E.C. Spencer et al., Chem. Phys. (2013), httpb = 120 meV) and are as follows: 84.4 meV, CeO2; 85.4 meV, CuO;85.2 meV, ZnO; 92.2 meV, ice-Ih. The librational motions of theconned water are sensitive to the hydrogen bond environmentin which the water species (i.e. molecular (H2O) and dissociated(hydroxyl groups) forms) reside. The shift of the CoGs of the libra-tional bands of the nanoparticle systems to lower energies relativeto that of ice-Ih reveals an overall softening of the hydrogen bondinteractions between the water species conned to the surface ofthe metal oxide particles; this is likely to be a result of thewatersurface interactions and riding modes that are prevalentin these systems.

Interestingly, despite the strong watersurface interactions thatrestrict the low-energy translational motion of the physisorbedwater, the librational bands present in the VDOS of the hydratedCuO and ZnO nanoparticles display broad peaks, which indicatesthat the structure of hydrogen bond network within these hydra-tion layers are not severely altered relative to ice-Ih. Conversely,the absence of structure in the librational band in the VDOS of hy-

anoparticles. Also shown is the Cv curve for the reference material ice-Ih. All curvesthe 0300 K temperature range; right: expansion of the Cv curves in the 0150 K

ysics xxx (2013) xxxxxx 3drated CeO2 nanoparticles suggests that the hydrogen bond net-work within the hydration layers on the surface of these particlesis disrupted leading to a more isotropic hydrogen bonding environ-ment for the water molecules. It should be noted, that this is not anartefact of the water coverage as both the CeO2 and ZnO particleshave a similar number of water molecules per unit area of surface(Table 1). If we assume that the effective surface area for adsorbedwater molecules is 0.14 nm2 [43], then for monolayer coverageapproximately seven molecules of water can simultaneously occu-py 1 nm2 of particle surface. Thus, based on the surface coveragevalues reported in Table 1 there are approximately 5, 31 and 3water layers on the surfaces of the CeO2, CuO and ZnO particles,respectively. Indeed, if differences in water coverage were respon-sible for the variations in the hydrogen bond networks within thehydration layers then it may be expected that the disruption to thenetwork within hydrated CuO sample, with 610 times the watercoverage of the ZnO and CeO2 particles would be less pronouncedas the watersurface interactions are unlikely to have a signicantimpact on the outer layers of the surface water.

The isochoric heat capacity (Cv) of the hydration layers can beextracted from the VDOS by application of the following expres-sion: [44]

Cv Z 10

vxhx=kT2exphx=kTexphx=kT 1b c2

dx 5

://dx.doi.org/10.1016/j.chemphys.2013.06.019

The subtle differences in the heat capacities for these systems

al Phcan be accentuated by assessment of the room temperature vibra-tional entropies (S) of the hydration layers, which can be deter-mined by application of the following equation: [40]

S Z T0

CpTdT 6

For the reference material ice-Ih S = 36.68 JK1mol1, and forthe hydration layers on CeO2, CuO and ZnO the values of S are41.23, 38.53, and 39.72 JK1mol1.

At low temperatures (

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E.C. Spencer et al. / Chemical Physics xxx (2013) xxxxxx 5Please cite this article in press as: E.C. Spencer et al., Chem. Phys. (2013), http://dx.doi.org/10.1016/j.chemphys.2013.06.019

Inelastic neutron scattering studies of hydrated CuO, ZnO and CeO2 nanoparticles1 Introduction2 Experimental methods2.1 Synthesis2.2 Characterization2.3 INS Data collections and processing

3 Results & discussion4 ConclusionsAcknowledgementsAppendix A Supplementary dataReferences