Surface Science 592 (2005) 124–140

www.elsevier.com/locate/susc

Surface interactions between Y2O3 nanocrystalsand organic molecules—an experimental

and quantum-chemical study

Henrik Pedersen a,*, Fredrik Soderlind a, RodrigoM. Petoral Jr. b, Kajsa Uvdal b,Per-Olov Kall a, Lars Ojamae a

a IFM Chemistry, Linkoping University, SE-581 83 Linkoping, Swedenb IFM Applied Physics, Linkoping University, SE-581 83 Linkoping, Sweden

Received 15 December 2004; accepted for publication 5 July 2005Available online 15 August 2005

Abstract

The surface interactions between Y2O3 nanocrystals and the organic molecules formic acid, diethylene glycol (DEG),and tetramethoxy silane (TMOS), have been studied experimentally and by quantum chemical calculations with theintent to elucidate the chemisorption characteristics such as adsorbate vibrational spectra and adsorption structures.Nanocrystal synthesis was performed by a colloidal method based on polyols and by a rapid combustion method.The products were experimentally characterized by X-ray powder diffraction (XRD), transmission electron microscopy(TEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS).

In the quantum chemical calculations, the B3LYP hybrid density functional ab initio method was used to study thechemisorption of formic acid, DEG and TMOS at the surface of Y12O18 clusters. From a comparison of calculated andexperimental vibrational spectra, the binding mode for formic acid on Y2O3 was inferred to be of bridge or bidentatetype. The XPS and FT-IR experiments showed that DEG is chemisorbed on the particle surface. The experimental IRspectra of DEG chemisorbed on Y2O3 were consistent with an adsorption mode where the hydroxyl groups are depro-tonated according to the quantum-chemical computations. The adsorption energy is of the order of 370 kJ mol�1 forformic acid, 550 kJ mol�1 for DEG, and 60 kJ mol�1 for TMOS, according to the quantum chemical calculations.� 2005 Elsevier B.V. All rights reserved.

Keywords: Ab initio quantum chemical methods and calculations; X-ray photoelectron spectroscopy; Chemisorption; Yttrium;Alcohols; Carboxylic acid; Silane

0039-6028/$ - see front matter � 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.susc.2005.07.027

* Corresponding author. Tel.: +46 13 281719; fax: +46 13 281399.E-mail address: henke@ifm.liu.se (H. Pedersen).URL: http://www.ifm.liu.se/inorgchem/nanochem.html (H. Pedersen).

H. Pedersen et al. / Surface Science 592 (2005) 124–140 125

1. Introduction

Over the last few years there has been a rapidlygrowing interest in nanocrystals of magnetic metaloxides for use as contrast agents in magnetic reso-nance imaging (MRI). Among the materials ofinterest are, in particular, oxides of iron and gad-olinium. Gadolinium (III) is strongly paramag-netic due to its seven unpaired f-shell electronsand gadolinium containing chelates. For example,Gd-DTPA (diethylene triamine pentaacetic acid),as well as super-paramagnetic iron oxide nanopar-ticles, are routinely used in MRI examinations ofpatients. The role of the contrast agent in MRIis to reduce the relaxation times T1 and/or T2 of1H in the human body [1]. To be used as contrastagents in vivo, the nanocrystals have to be coated,‘‘functionalised’’, by molecules rendering thembio-compatible and water soluble while enhancingtheir tissue-specific targeting properties. It is thusimportant to understand how attached molecules,e.g., carboxylic acids, bind to a particle�s surfaceand to assess the stability and geometry of theresulting bonds.Details of the molecule–nanocrystal interac-

tions, such as adsorption energies and coordinationgeometries, not readily obtained experimentally,can be extracted from first-principles calculations[2–6]. Theoretical IR spectra, density of states(DOS), and band-gap energies [7] of the nanoparti-cles can be calculated as well. However, an obstaclein quantum chemical calculations for gadolinium-containing compounds is the large number ofunpaired electrons of Gd3+ (electron configuration[Xe]4f7) which enhances the computational burden.One way to circumvent this obstacle is to choose asuitable reference system that has only a few or nounpaired electrons, but is chemically very similar tothe studied one. In terms of chemistry, Y2O3 is asuitable reference system for Gd2O3. Y

3+ has nounpaired electrons (electron configuration [Kr]),while cubic Gd2O3 and Y2O3 are isostructural(space group Ia�3, No. 206), with 80 atoms in theunit cell [8,9]. The RE2O3 (RE = Y or Gd) struc-ture can be described as consisting of approxi-mately close-packed oxygen atoms with Gd or Yin 2/3 of the octahedral holes. The cell parameter(a-axis) is 10.809 A for Gd2O3 [8] and 10.604 A

for Y2O3 [9], due to a small difference in the ionicradii of Gd3+ (108 pm) and Y3+ (104 pm). Thestandard electrode potential values (E�) for thehalf-cell reaction M3+ + 3 e� !M are quite simi-lar for the two metals (�2.37 V and �2.29 V forM = Y and Gd, respectively) [10]. Furthermore,nanocrystalline materials of both oxides can besynthesized by the same synthesis routes. Nano-crystalline Y2O3 is in itself an important system,and unusual photoluminescence properties of cubicand monoclinic Y2O3:Eu

3+ have recently beenreported [11–15]. In a previous work, the chemi-sorption of various organic acids, e.g., oleic acidand citric acid, onto Gd2O3 nanocrystals was stud-ied [16]. It was shown that the carboxylic group ofthe organic acids coordinates to the surface in abidentate fashion, i.e., with both carboxylate oxy-gens binding to one metal atom, or in a bridgingfashion, where the oxygens bind to two nearbymetals [16]. The size distribution and MR proper-ties of Gd2O3 nanocrystals prepared by chemicalmethods will be discussed in a forthcoming article.In this work, we report the adsorption of formicacid, diethylene glycol, tetraethylene glycol, andtetramethoxy silane onto nanocrystals of cubicY2O3, prepared by the polyol [11,17,18] and com-bustion [12] methods. The chemisorption of themolecules onto the oxide particle surfaces was stud-ied experimentally by IR and XPS, and theoreti-cally by molecular quantum chemical hybridDFT-ab initio calculations.

2. Methods

2.1. Synthesis methods

In all the syntheses, in the following denoted byS1–S4, chemicals of at least synthesis grade (�99%)were used. Polyol synthesis (S1) of cubic Y2O3

nanocrystals was performed using diethylene glycol(C4H10O3, DEG) as the solvent. Y(NO3)3 Æ 6H2O,deionised water and NaOH were added to 10 mlof DEG so as to yield total concentrations of�0.2 M Y(NO3)3, �2 M H2O and �0.2 M NaOH.The mixture was magnetically stirred and heated to140 �C. When the reactants had dissolved com-pletely, the temperature was raised and held

126 H. Pedersen et al. / Surface Science 592 (2005) 124–140

constant at 180 �C for 4 h, yielding a dark yellowcolloid [11].Surface modification with formic acid (S2) was

performed by dissolving formic acid in DEG(1 mmol in 5 ml of DEG). The mixture was thenadded to hot yttrium containing DEG colloid,causing precipitation. The precipitate was centri-fuged and washed several times with methanoland dried in air.Combustion synthesis (S3) of Y2O3 nanocrys-

tals was performed by mixing equal volumes ofwater solutions of Y(NO3)3 and glycine (each100 mM) in an E-flask, magnetically stir for afew minutes and then heated on a hot plate. Whenthe water had evaporated, a dried, brownish pre-cipitate was left. After further heating, a rapidreaction takes place, forming a white, fine powder[12].Surface modification with tetramethoxy silane

(S4) was performed by placing the powder ob-tained in (S3) in a test tube and adding deionisedwater. The powder slurry was first treated by ultra-sonification, heated under agitation in a hot waterbath, and then centrifuged. The procedure was re-peated several times in order to clean the powderand adsorb water to it before drying a few hoursin an evacuated desiccator. The dried powderwas stirred with 300 mM tetramethoxy silane intoluene: �2 mg powder to 5 ml of solution, at�50 �C, over night. The powder was washedtwice with toluene and methanol and treated byultrasonification after each wash. The presumedreaction between TMOS and the yttria particlesis [19]:

(Y2O3)surf (–OH)x + Si(OCH3)4! (Y2O3)surf (–O–)x

SiðOCH3Þ4�x þ xCH3OH ð1Þwith 1 6 x 6 3.

2.2. Experimental characterization methods

The crystallinity of the samples was examined byX-ray powder diffraction (XRD) using a Philipspowder diffractometer PW 1820 with Cu Ka radia-tion (k = 1.5418 A). The size and shape of thenanocrystals were examined by transmission elec-tron microscopy (TEM) with a Philips CM20instrument, operated at 200 kV. Before being

examined by TEM, the samples were dispersed inmethanol and a drop of the liquid was placed ona copper grid with an amorphous, holey carbonfilm. The molecular coating of the obtained prod-ucts was analyzed by infrared spectroscopy (FT-IR), with a Perkin Elmer FT-IR Spectrum 1000spectrometer using pressed KBr tablets. For ele-mental surface analysis, X-ray photoelectron spec-troscopy (XPS) was used. XPS analysis wasperformed in a VG instrument with a CLAM2analyzer and a twin Mg/Al anode. The base pres-sure in the analysis chamber was approximately5 · 10�10 mbar. The measurements were carriedout with unmonochromated Mg Ka photons(1253.6 eV). The resolution was determined fromthe full width at half the maximum (FWHM) ofthe Au (4f7/2) line, which was 1.3 eV with a pass en-ergy of 50 eV. The power of the X-ray source waskept constant at 300 W. The binding energy scaleof the spectra was aligned through the C (1s) peakat 284.6 eV. Measurements were made using pho-toelectron take-off angles (TOA) of 30� and 80�with respect to the surface normal of the sample.The VGX900 data analysis software was used tocalculate the elemental composition from the peakareas and to analyze the peak positions. Curve fit-ting was done using the program XPSPEAK [20].For XPS sample preparation, the nanocrystalswere dissolved in methanol and an ample amountof the solution is nitrogen-gas-cast-dried forminga multilayer on either a Si(100) wafer or a cleangold surface.

2.3. Computational methods

The quantum chemical calculations were per-formed using the B3LYP [21,22] functional in theGaussian03 program [23]. The basis set used foryttrium was a split valence basis set in combina-tion with an effective core potential for inner shellelectrons by Hay and Wadt [24], where all primi-tive Gaussian basis functions with exponents lessthan 0.06 had been removed. The split valence ba-sis set by Bouteiller et al. [25] was used for oxygen,carbon and silicon, and the 6-31G basis set wasused for hydrogen [23]. Geometry-optimized struc-tures and vibrational spectra were computed forthe molecular clusters consisting of an organic

H. Pedersen et al. / Surface Science 592 (2005) 124–140 127

adsorbate and an yttrium oxide cluster as de-scribed in Section 3.4 below. Adsorption energieswere calculated as the energy difference betweenthe sum of energies for the starting species, i.e.,the cluster and the molecule (each geometry opti-mized), and the total energy for the system whenthe molecule was adsorbed to the surface.These basis sets have previously been success-

fully utilized in studies of molecular adsorptionon e.g., MgO [26], ZnO [27–29] and TiO2 [29–31]. The B3LYP method is well established formolecular studies [2] and has been shown to bea suitable choice for studies of metal oxides[32]. For example, in a recent study of phos-phonic and formic acid adsorption at TiO2 [31],the B3LYP functional combined with the samebasis set type as in the present study gave resultsin line with previous DFT calculations of formicacid adsorption [33], and furthermore managedto reproduce the experimental trend in bindingenergy for phosphonic and formic acid. (In Ref.[31] the results obtained using full versus partialgeometry optimizations are also discussed.) Inthe present study, some additional tests of themethod were performed. First, we investigatedwhether the B3LYP functional and the presentbasis set could reproduce the Y2O3 crystal struc-ture. Using the Crystal03 program [34,35], whichis capable of handling periodic systems such ascrystals using the hybrid ab initio-DFT func-tional together with Gaussian-type basis sets,the crystal structural parameters were geometry-optimized to find the energy-minimum structure.The resulting cell axis was 10.66 A, in excellentagreement with the experimental value 10.60 A[9]. Also the interatomic distances agreed favour-ably, e.g., the Y–O nearest neighbour distanceswere 2.26, 2.29 and 2.33 A from the calculationsversus 2.23, 2.29 and 2.32 A from the experi-ments.For comparison, a few calculations using the

larger standard CEP-31G basis set [36–38] wereperformed (the number of basis functions for Yis 34 as compared to 13 for the basis set ordinarilyused in the present study). The use of the CEP-31G basis set turned out to be impractical, partlybecause of the larger size but mostly due to theslow SCF convergence typically encountered using

that basis set. Hence, only single-point calculationsfor the geometries obtained for the ordinary basisset were performed. The adsorption energies forformic acid on an yttrium oxide cluster were calcu-lated with the CEP-31G basis set for one particularbridge-type adsorption mode (where two O[formicacid]–Y bonds are formed involving two differentY ions, see below) and for a bidentate mode(where two O[formic acid]–Y bonds are formedto the same Y ion). 158 and 249 kJ mol�1, respec-tively, were obtained. If the ordinary basis set wasused, the corresponding adsorption energies were170 and 257 kJ mol�1. The basis set used in thepresent study thus overestimates the magnitudesof the adsorption energies by some 10 kJ mol�1

compared to the CEP-31 G basis set, but the influ-ence on the relative difference between the differentadsorption modes is smaller (91 kJ mol�1 for CEP-31G compared to 87 kJ mol�1 using the ordinarybasis set).

3. Results and discussion

3.1. XRD and TEM

The powder XRD patterns of the sample ob-tained by the polyol method using DEG as the sol-vent and subsequently treated with formic acid(S2) is shown in Fig. 1a. The pattern exhibitsone broad peak centred at 2h � 30�, reasonablyidentifiable as the 222 reflection of cubic Y2O3.By heating the S2 sample in a furnace at 300 �Cfor 24 h in ambient atmosphere, the reflection be-comes more salient and also the 440 reflectioncan be seen as a weak intensity at �48.3�(Fig. 1b). The powder XRD pattern of the sampleprepared by the combustion method (S3) is shownin Fig. 1c and is very similar to that in Fig. 1b. Ifone compares the diffraction patterns in Fig. 1a–cwith that obtained from a commercial Y2O3 pow-der (Alfa 99.99%) used as reference (Fig. 1d), it isclear that the peak-broadening is considerable forthe above materials resulting in overlap of the 222and 400 intensities. The peak-broadening can beattributed to the small crystallite size, or to thepresence of poorly crystallized product(s). Bazziet al. [11] reported the synthesis of sub-5-nm

15 25 35 45 552θ (°)

Int.

(a. u

.)

d

c

b

a

Al

(622)(440)

(400)

(222)

Fig. 1. Powder diffraction pattern of (a) formic acid cappedY2O3 (S2), (b) S2 after heated to 300 �C for 24 h, (c) Y2O3

nanocrystals prepared by the combustion method (S3), and (d)a commercial Y2O3 powder. The sharp peak at 2h � 45� seen inseveral patterns is due to the aluminium sample holder.

Fig. 2. HRTEM image of Y2O3 nanocrystals from combustionmethod (S3). The micrograph shows crystalline nanoparticles,seen in different crystallographic directions.

128 H. Pedersen et al. / Surface Science 592 (2005) 124–140

RE2O3 crystals (RE = Y or Gd) via the DEGroute, claiming highly crystalline materials wereobtained despite the low synthesis temperature(180 �C). Although no TEM investigation wasundertaken of the S1 sample in this study, XPSdata indicated it to be crystalline, as discussed be-low. The HRTEM image of S3 (Fig. 2), shows thatthe particles obtained in the combustion synthesisare crystalline with 5 nm or smaller size. As seen inthe micrograph, the nanocrystals tend to aggre-gate, perhaps as a result of their lack of coating.

3.2. FT-IR

A comparison of the IR spectrum of pure formicacid with that of formic acid-capped nanocrystals(S2) in Fig. 3a and b, respectively, reveals the split-ting of the m(C@O) stretch peak at 1746 cm�1 inpure formic acid, into an asymmetric, mas(COO

�),and a symmetric, ms(COO

�), stretch band at,

respectively, 1592 cm�1 and 1366 cm�1, indicativeof the formation of a chemical bond between theY2O3 surface and the carboxylic group. InFig. 3d and e the IR spectra of oleic acid cappedY2O3- and Gd2O3 nanocrystals are shown. Thetwo spectra are identical, suggesting that theadsorption geometry of oleic acid is the samefor both systems. Compared to pure oleic acid(Fig. 3c), the m(C@O) stretch peak at �1711 cm�1

is split into an asymmetric, mas(COO�), and a sym-

metric, ms(COO�), stretch band at, respectively,

1563 cm�1 and 1447 cm�1, explainable by the for-mation of a chemical bond between the RE2O3 sur-face and the carboxylic group.The IR spectrum of the nanocrystals obtained

from the combustion method (S3) (Fig. 4c) differsfrom that of the reference Y2O3 powder (Fig. 4d)by the presence of a broad peak at 3453 cm�1,and a split one around 1450 cm�1. These peakswere also observed for nanocrystalline Gd2O3 pre-pared by the combustion method and ascribed towater, nitrate and un-reacted glycine [14]. Thespectrum of the sample treated with TMOS (S4)(Fig. 4b) differs from that of S3 mainly by thebroad band at 1050 cm�1, attributable to the Si–O and O–CH3 stretches of the molecule. The

a

b

e

d

c

Tra

nsm

itta

nce

(a. u

.)

4000 3000 2000 1500Wavenumber (cm-1)

1000 400

Fig. 3. FT-IR spectra of (a) formic acid, (b) Y2O3 capped withformic acid (S2), (c) oleic acid, (d) Y2O3 nanocrystals cappedwith oleic acid, and (e) Gd2O3 nanocrystals capped with oleicacid.

g

ab

f

e

d

c

Tra

nsm

itta

nce

(a. u

.)

4000 3000 2000 1500

Wavenumber (cm-1)

1000 400

Fig. 4. FT-IR spectra of (a) TMOS, (b)Y2O3 from combustionsynthesis treated with TMOS (S4), (c) Y2O3 from combustionsynthesis (S3), (d) a commercial Y2O3 powder, (e) DEG, (f)Y2O3 nanocrystals prepared in DEG (S1), and (g) Gd2O3

nanocrystals prepared by the same synthesis route as describedin S1.

H. Pedersen et al. / Surface Science 592 (2005) 124–140 129

Si–O stretch is found at 1110 cm�1 in the spectrumof pure TMOS, Fig. 4a. The only Y–O vibrationsdetected in the reference powder in the measuredinterval appear at 562 and 464 cm�1 (Fig. 4d).A comparison of the IR spectrum of pure dieth-

ylene glycol with that of the Y2O3 nanocrystalsprepared in the polyol (S1) (Fig. 4e and f) revealsseveral similarities and differences. For example,the C–H stretch band at �2900 cm�1, the O–Hstretch at �3400 cm�1 and the broad C–O andC–C stretch bands around 1100 cm�1 are foundin both spectra. However, the spectra differstrongly in the 1200–1700 cm�1 region where var-ious C–H vibration modes in S1 are modified bymolecule–surface interactions. Comparing the IRspectra of Y2O3 and Gd2O3 nanocrystals prepared

by the same DEG synthesis route (Fig. 4f and g),one finds that the peaks are slightly better resolvedfor Y2O3, the two spectra are identical.

3.3. XPS

Three of the samples, the Y2O3 particles pre-pared from DEG (S1), those obtained from thecombustion synthesis (S3), and those reacted withTMOS (S4), were investigated by XPS. The Y

130 H. Pedersen et al. / Surface Science 592 (2005) 124–140

(3d), O (1s) and C (1s) XPS spectra of S1 nanocrys-tals are shown in Fig. 5a–c, respectively. In the Y(3d) spectrum of S1 spin-coated onto a SiOx sub-strate (Fig. 5a), the spin–orbit, split doublet ofthe Y (3d) level is well defined. Fig. 5a (i) showsthe Y (3d) narrow scan measured in the bulk mode(TOA = 30�), while Fig. 5a (ii) shows it in surfacemode (TOA = 80�). The Y (3d) level consists of aspin orbit split doublet, with the Y (3d5/2) and Y(3d7/2) peaks at 157.7 and 159.7 eV, respectively,which is clearly seen at TOA = 30�. The line shape

150155160165170

Inte

nsity

(a.u

.)

Binding Energy (eV)

Y (3d)

(i)

(ii)

a b

290295

Inte

nsity

(a.u

.)

Binding Energ

C (1s)

c

Fig. 5. XPS spectra of DEG capped Y2O3 nanocrystals (S1): (a) thespectrum. See text for explanations.

and peak positions are in good agreement with re-cently published data for a cubic Y2O3 thin film ona sulfonated, self-assembled monolayer producedby enhanced hydrolysis of yttrium nitrate solutionin the presence of urea at 80 �C [39]. When measur-ing in the surface-sensitive mode (TOA = 80�), asmall contribution of another form of yttrium isobserved, with binding energy 160.2 eV (Y (3d5/2)).The fitted binding energy confirmed a form ofyttrium oxide, i.e., YOx [40]. Thus, the XPS dataindicate that S1 mostly consists of crystalline cubic

525530535540

Inte

nsity

(a.u

.)

Binding Energy (eV)

O (1s)

280285y (eV)

Y (3d) spectrum, (b) the O (1s) spectrum, and (c) the C (1s)

H. Pedersen et al. / Surface Science 592 (2005) 124–140 131

Y2O3, but with small amounts of other yttriumoxides.The O (1s) spectrum of S1 (Fig. 5b) has the

main binding-energy peak at 531.4 eV, which ismeasured in the bulk mode. The peak consists ofcontributions from the substrate (SiOx) and nano-crystals (Y2O3 and YOx). A broadening on thehigh binding energy side of the peak includes con-tributions from the DEG and possible side prod-ucts in the synthesis. The same binding-energypeak and line shape of the peak are observed whenmeasurement is done in the surface-sensitive mode.The C (1s) spectrum measured in the bulk mode

is shown in Fig. 5c. The position of the main peakis at 284.6 eV and a smaller one at 288.6 eV. Themain peak consists of aliphatic carbons presentin the DEG. A fitted peak at 286.2 eV represents

Fig. 6. The geometry-optimized structure using B3LYP computationscrystal plane parallel to the plane of the page (orthogonal to the main SO ions marked (a), a side view of the aforementioned cluster (b), a geocluster formed by doubling the cluster in the direction of the main sy

the hydroxyl carbons, present as the terminatingcarbons in DEG. The fitted peak at 288.6 eV is as-signed to possible carbonyl containing side prod-uct(s) in the synthesis.Synthesis products from S3 and S4 were also

investigated. The spectra measured for S3 indi-cates a more complicated composition. Again,the line shape and binding energy position of Y(3d) spectrum (figure not shown) are similar tothose observed for S1, but several different formsof yttrium oxide (YOx) can be deduced, based onthe binding energy position of the main peak, themeasured energy of which is about 160 eV. ForS4, the presence of silicon from chemisorbedTMOS was confirmed, using a gold substrate inthe XPS measurements. The relative intensity ratioof yttrium to silicon (Y/Si), which reflects the

of the Y12O18 cluster cut from the bulk structure, with the (111)

6 symmetry axis) and with the chemically non-equivalent Y andmetry-optimized cluster of S2 symmetry (c), and a larger Y24O36

mmetry axis (d). Interatomic distances in Angstrom.

Fig. 7. Geometry-optimized structures of formic acid at the Y12O18 cluster using B3LYP computations. Crystal bulk-like cluster withoptimized adsorbate positions and the different modes: (a) bridge, (b) bidentate, (c) monodentate. S2 symmetry-like cluster from fulloptimizations of all coordinates for the different adsorption modes: (d) bridge, (e) bidentate, and (f) monodentate. Distances inAngstrom, the angular values refer to the O–C–O angle.

132 H. Pedersen et al. / Surface Science 592 (2005) 124–140

H. Pedersen et al. / Surface Science 592 (2005) 124–140 133

amount of TMOS chemisorbed on the nanocrys-tal, is measured to be 0.5 suggesting that relativelylarge amounts of TMOS are adsorbed.

3.4. Computational results

3.4.1. Yttrium oxide nanoparticle computations

The quantum-chemical B3LYP calculationswere performed mainly for an yttrium oxide clus-ter consisting of 30 atoms, Y12O18 (see Fig. 6aand b) as a substrate for the adsorption. Thecluster was cut from the bulk crystal structurein such a way that the (111) crystal plane (per-pendicular to the S6 axis) became the dominatingsurface [9]. The surface is stoichiometric, so thatno net charge is associated with the upper andlower atomic layers of the cluster. Furthermore,in an attempt to find a cluster that is as stableas possible, the cluster was chosen in such away that it possessed no net dipole moment andso that the ions had as high coordination num-bers as possible (for a cluster with the same coor-dination as in the bulk structure). The latter wasachieved by avoiding single-coordinated O ionsand Y ions with coordination number less thanthree. The so-obtained Y12O18 cluster in Fig. 6ahas two and three chemically non-equivalent Yand O atoms, respectively. This cluster containsboth ions with unsaturated bonds, typical of sur-face atoms, as well as oxygen ions with full coor-dination, similar to bulk atoms (the Y ion has atmost the coordination number five in the cluster,compared to six in the crystal). The lattice energyof the optimized cluster is 14400 kJ mol�1 perY2O3 unit. The atoms in the cluster are generallyshifted relative to their crystallographic positions,

Table 1Adsorption energies and binding modes for formic acid at the Y12O1

Binding mode Adsorption energy (kJ mol�

Unrelaxed Y12O18 cluster

Bridge 233Bidentate 257Monodentate 258

The terms ‘‘unrelaxed’’ and ‘‘relaxed’’ refer to whether the coordingeometry optimization of the adsorbate plus Y12O18 cluster supermoFig. 6a and b. The adsorption energies at two different relaxed clustersand Fig. 7d–f, and the second set within parentheses for a more amo

e.g., the interatomic distances between the Y–Oatoms 1–2, 1–3, 1–4, 1–5 and 1–6 in Fig. 6aand b are 2.49, 2.21, 2.34, 2.14 and 2.14 A inthe cluster, whereas in the B3LYP-optimized bulkstructure the corresponding distances are 2.29,2.26, 2.33, 2.33 and 2.29 A. The overall size ofthe cluster is 8.9 A as obtained from the distancebetween the O nuclei farthest apart, or 1.2 nm asobtained from the distance between the most dis-tant parts of the ionic radius surfaces at the bothoxygens. Due to its limited size the cluster maystrictly speaking not be considered as fullycrystalline.Clusters that were lower in energy than the ori-

ginal bulk-like cluster were also found when thecluster was distorted to lower symmetry. Thosemore amorphous-like structures represent variouslocal energy minima possible for the given clustersize. One local-minimum structure of S2 symmetry(Fig. 6c) was found to be among the most stablestructures encountered (527 kJ mol�1 lower inenergy than the bulk-like cluster) and was usedextensively in the adsorption studies. This clusteris 1.1 nm across or 1.4 nm if the ionic radii are in-cluded. In some comparative computations oneadditional cluster was used (not shown) thatlacked symmetry, had a more spherical shape,and, without adsorbates, was slightly more stablethan the S2-type cluster (541 kJ mol

�1 below thebulk-like cluster).A larger cluster was also investigated: Y24O36.

The geometry-optimized structure is shown inFig. 6d. It is formed by doubling the size of thebulk-like cluster in Fig. 6a and b in the directionof the main symmetry axis. Due to the size of thecluster, it could only be used in a limited number

8 cluster from B3LYP computations

1)

Relaxed Y12O18 cluster

369 (308)335 (297)269 (291)

ates of the Y12O18 cluster were allowed to vary or not in thelecule. The unrelaxed cluster is the crystal bulk-like clusters inare shown, the first set for the S2 symmetry-like cluster in Fig. 6crphous-like cluster (not shown).

134 H. Pedersen et al. / Surface Science 592 (2005) 124–140

of adsorption studies, for the purpose of com-parison.

3.4.2. Formic acid adsorption computations

The adsorption of formic acid at the differentY12O18 clusters was studied computationally withrespect to adsorption modes and geometries,adsorption energies and vibrational frequencies.First, adsorption at the highly symmetric bulk-likecluster was studied. In these calculations, the coor-dinates of the formic acid molecule were geometry-optimized to find the energy-minimum adsorptiongeometries while the coordinates of the yttriumoxide cluster were kept fixed. (If the yttrium oxidecoordinates of this cluster were not fixed, the per-turbation from the molecular interaction oftentransformed the cluster into, for example, the S2-type cluster. One could speculate that keepingthe yttrium oxide cluster frozen provides a bettermodel for a much larger and thus more crystal-likecluster.)Different coordination modes of the molecule to

the surface were sampled by placing the molecule

1592

150016001700

Wavenumber

Tra

nsm

itta

nce

(a.

u.)

νas(COO-) 1543

1587 1575

Fig. 8. Experimental IR spectrum of formic acid capped Y2O3 nanoB3LYP computations for the bridge (solid), bidentate (dashed) and(middle spectra). At the bottom are marked the corresponding peaks

at different starting positions at the surface withplausible geometries and then performing geo-metry optimizations. The adsorption modes testedwere either (1) a bridging mode involving a depro-tonated molecule where the two oxygens of thecarboxylate group are coordinated to two differentsurface Y ions and the proton has been transferredto a surface O ion, (2) a bidentate mode similar toabove but where the two carboxylate oxygens arebonded to the same Y ion, or (3) a monodentatemode where one formic acid oxygen is coordinatedto a surface Y ion and the formic acid OH grouphydrogen bonds to a surface O ion. In the lastcase, either the proton of the OH group can betransferred to the surface or the molecule can beleft intact. Initially we tried to coordinate the mol-ecule to the surface ions that had a low number ofdangling bonds, i.e., the ‘‘inner’’ ions denoted YI

and OI in Fig. 6a, but during the geometry-optimi-zation process the molecules moved spontaneouslytowards the edge atoms (YII, OII and OIII). Thismay be expected since the most energy may begained by saturating bonds of ions that have many

1457

13001400

(cm–1)

1366

1403

14151350

HCO bend.

1317

νs(COO-) 1330

crystals (S2) (top spectrum) and calculated IR spectrum usingmonodentate (dotted line) at the S2 symmetry Y12O18 clusterfor a low-symmetry Y12O18 cluster.

Fig. 9. Geometry-optimized structures of DEG at the Y12O18

cluster using B3LYP computations: (a) with unrelaxed crystalbulk-like cluster, (b) undissociated hydroxyl groups at the S2symmetry-like cluster, (c) deprotonated hydroxylgroups at theS2 symmetry-like cluster, where ‘‘unrelaxed’’ refers to that thecoordinates of the Y12O18 cluster were not allowed to vary inthe geometry optimization. Distances in Angstrom.

H. Pedersen et al. / Surface Science 592 (2005) 124–140 135

dangling bonds. Only in one case was a stable localminimum involving coordination to the inner ionsfound and that was for a non-dissociated mono-dentate binding mode involving the ions markedby ‘‘1’’ and ‘‘2’’ in Fig. 6a, which possessed a mea-gre adsorption energy of 61 kJ mol�1. Many differ-ent ways of coordinating the formic acid moleculein the bridge, bidentate and monodentate fashionswere investigated by a trial-and-error approach,and the structures of each adsorption mode withthe lowest energy that were found are shown inFig. 7a–c. The corresponding adsorption energiesare listed in Table 1 and are of the order of250 kJ mol�1 with the monodentate mode beingthe most stable. However, the differences in energybetween the different modes are rather small andprobably only a low energy barrier separates themonodetate mode from the bidentate mode. Wenote that in the monodentate mode the protonhas been transferred to the surface O ion.The adsorption energy is also highly dependent

upon the position of the proton at the surface. If,for the bridge mode, the proton is placed at thebottom side of the cluster at the O ion marked‘‘A’’ in Fig. 7a, the adsorption energy will be170 kJ mol�1. If it is placed at the site marked‘‘B’’ the energy will be only 104 kJ mol�1. In thesame way for the bidentate mode, if the protonis placed instead at the ‘‘B’’ site the adsorption en-ergy diminishes and is only 95 kJ mol�1.The adsorption energies of the bridge and

bidentate adsorption modes above were also com-puted for the large Y24O36 cluster (Fig. 6d) as thesubstrate. After geometry optimization of thecoordinates of the formic acid molecule the ob-tained energies were 258 and 291 kJ mol�1 forthe bridge and bidentate, respectively. The adsorp-tion energy magnitudes are thus slightly more than10% larger than for the smaller cluster, which indi-cates that the adsorption energy magnitude mayincrease with the size of the nanoparticle. The dif-ference in energy between the two modes is33 kJ mol�1, to be compared with 24 kJ mol�1

for the smaller cluster.The adsorption conformations of formic acid at

the S2 symmetry-like cluster in Fig. 6c were alsoinvestigated. In this case, all the coordinates ofthe adsorbate plus nanoparticle system were re-

laxed in the geometry optimization. The lowest-energy structures of each of the bridge, bidentate

136 H. Pedersen et al. / Surface Science 592 (2005) 124–140

and monodentate modes are shown if Fig. 7d–f.The adsorption energies are listed in Table 1.The molecules are more strongly adsorbed on thiscluster, probably mainly due to the relaxation ofthe yttrium oxide structure caused by the adsorp-tion. Now the bridge mode is the most stable,followed by the bidentate and then by the monod-entate mode. The adsorption energies at the low-symmetry Y12O18 cluster are in addition listed inTable 1. The bridge mode is also in this case themost stable, but the energy differences betweenthe different modes are less compared to adsorp-tion at the previous cluster.The calculated vibration spectra for formic acid

adsorbed in the bridge, bidentate and mono-dentate modes at the S2 symmetry-like cluster(Fig. 7d–f) are compared with the experimentallyobserved in Fig. 8. The general shape of the exper-imental spectrum in the C@O stretch region is wellreproduced by the bridge and monodentate vibra-tional calculations, with at least three large peaksin the interval 1300–1700 cm�1, whereas themonodentate mode exhibits only one major andone minor peak. The monodentate peak at1587 cm�1 actually agrees very well with the exper-imental peak with maximum at 1592 cm�1, butthis is accidental (the computed vibrational fre-quencies shown in Fig. 8 have not been scaled byany ad hoc factor). The C@O stretch frequencyof the HCOOH molecule in the gas phase is1775 cm�1 [41], whereas the corresponding com-puted frequency for the gas phase molecule usingB3LYP and the current basis set is 1723 cm�1.We would thus expect the calculated spectrum tobe displaced downwards by about 50 cm�1 com-pared to the experimental spectrum, and a shiftof similar magnitude is indeed what is observed.

Table 2Adsorption energies from quantum-chemical B3LYP computations fo

Cluster Adsorption geometry

Unrelaxed bulk-like Undissociated (Fig. 9a)S2 symmetry-like Undissociated (Fig. 9b)S2 symmetry-like Deprotonated (Fig. 9c)

The term ‘‘unrelaxed’’, in the first row, refers to that the coordinatesoptimization. For the second to fourth rows full geometry optimizati

Taking into account this shift, the bridge or mayberather the bidentate binding mode is the mostlikely candidate to explain the observed spectrum.This is in agreement with the computed adsorptionenergies, where the bridge and bidentate adsorp-tions are stronger than the monodentate adsorp-tion for the relaxed cluster (Table 1).In the lower part of Fig. 8, the computed fre-

quencies using the low-symmetry cluster as a sub-strate instead of the S2 symmetry cluster aremarked as vertical lines. Some minor shifts in fre-quencies occur, but the general spectral featuresare the same as for the computations using the pre-vious cluster.In the calculated spectrum (Fig. 8) the

mas(COO�) and ms(COO

�) stretches for the bridgemode appear at 1575 cm�1 and 1330 cm�1, respec-tively. In the measured IR spectrum, the corre-sponding peaks are found at 1592 cm�1 and1366 cm�1, respectively. The wave number differ-ence between the two peaks in the observed IRspectrum is D = 1592–1366 = 226 cm�1. Accord-ing to Nakamoto [42], monodentate binding ofthe carboxylate group should be expected for sucha D value, while Nara et al. [43] claim a bridge-typecoordination. Our computations indicate that for-mic acid binds to the Y2O3 surface in a bridge or abidentate binding mode in agreement with Naraet al. [43]. One can argue that the interpretationof the D values in Refs. [42,43] pertain to variouscoordination compounds of carboxylic acids andnot to one chemisorbed on a metal oxide surface,and therefore should be used with care.

3.4.3. DEG adsorption computations

The adsorption of diethylene glycol on theY12O18 cluster was studied; here we tried to place

r DEG at the Y12O18 cluster

Adsorption energy (kJ mol�1)

255228547

of the Y12O18 cluster were not allowed to vary in the geometryons were performed.

H. Pedersen et al. / Surface Science 592 (2005) 124–140 137

the DEG molecule at the cluster surface in such away that the strain in the molecule was as small aspossible and so that the molecular oxygens werebonded to under-coordinated Y ions. This hadbeen found to be advantageous in the studies offormic acid. The crystal bulk-like cluster wasstudied first, where the geometry of the Y12O18

cluster was held frozen, allowing optimization onlyof the position of the DEG molecule (Fig. 9a).Then the S2 symmetry-like cluster was studied,and here the whole DEG–cluster system was opti-mized. Two different adsorption modes weretested, one mode in which protons had beentransferred from the molecular OH groups tothe surface and one mode in which the moleculedid not dissociate (Fig. 9b). The most stableadsorption mode for DEG was found to be disso-ciated hydroxyl groups and the bond formed be-tween the hydroxyl oxygen atoms of the DEGmolecule and cluster yttrium atoms (Fig. 9c). Allcalculated adsorption energies for DEG are given

C-Oterminal str.C-Oterminal str.

1025

105210771115

1050110011501200Wavenumbe

Tra

nsm

itta

nce

(a.

u.)

1058

1079

C-O-C str.1036

1147 1112

Fig. 10. (a) Calculated IR spectrum using B3LYP computations forhydroxyl groups, dotted line: undissociated hydroxyl groups. (b) ExpDEG (S1).

in Table 2, it was found that the deprotonatedform was more stable than the undissociated formby 319 kJ mol�1.The theoretically calculated IR spectrum of the

DEG molecule adsorbed at the Y12O18 cluster andthe experimentally observed one for the Y2O3

nanocrystals prepared in DEG (S1), are shownin Fig. 10a and b, respectively. All peaks in the cal-culated spectrum (Fig. 10a) emanate from theDEG molecule, where a solid line represents thespectrum from the most stable adsorption geo-metry and the spectrum from the undissociatedadsorption mode is given as a dotted line for com-parison. The comparison between the computedand experimental vibrational spectra thus corrob-orates the conclusion from the consideration ofthe adsorption energies that the DEG moleculesare present in the dissociated form. The C–O(H)and O–CH3 stretch frequencies of 2-methoxyetha-nol, a molecule very similar to DEG, in the gasphase, are 1067 and 1133 cm�1, respectively [44],

b876

922

8509009501000r (cm–1)

a

878 926

C-C str.925

996

C-O str. 897

the DEG molecule at the Y12O18 cluster. Solid line: dissociatederimental IR spectrum of Y2O3 nanoparticles with chemisorbed

Table 3Calculated DEreact for reactions involving tetramethoxy silane from B3LYP computations

Reaction energy (kJ mol�1):Y12O18 + H2O! (Y12O18)–OH,H 288(Y12O18)–OH,H + Si(OCH3)4 ! (Y12O18)–O–Si(OCH3)3,H + CH3OH 62

Fig. 11. Geometry-optimized structure using B3LYP compu-tations of (Y12O18)–O–Si(OCH3)3 distances in Angstrom.

1205

1154

1100115012001250Wavenumbe

Tran

smit

tan

ce (

a. u

.)

1192 CH3 rock.

1077O-CH3 str. 1102

Fig. 12. (a) Calculated IR spectrum for (Y12O18)–O–Si(OCH3)3nanocrystals (S4).

138 H. Pedersen et al. / Surface Science 592 (2005) 124–140

whereas the corresponding frequencies calcu-lated for the molecule in the gas phase usingB3LYP and the current basis set are 1038 and1067 cm�1, respectively. The calculations shift thefrequencies downwards by 30–50 cm�1, and thusthe calculated spectrum for DEG is shifted tolower frequencies. For the dissociated adsorptionmode the spectrum however appears to be shiftedupwards. Although the overall shape of the calcu-lated spectrum appears to agree with that of theexperimental one, the upward shift in the calcu-lated spectrum makes it hard to judge whichadsorption mode dominates in the experiment.Further studies are needed.

3.4.4. TMOS adsorptionThe chemisorption of silane compounds on

oxide particle surfaces is of interest, because it

b

90095010001050r (cm–1)

1050

a

958 O-CH3 str.

O-CH3 str.

1005 Si-O str.

and (b) experimental IR spectrum of TMOS capped Y2O3

H. Pedersen et al. / Surface Science 592 (2005) 124–140 139

makes it possible to attach various types of bio-active molecules on the particles via (MOx)–Oads–Si–R bonds [19]. As described above, IRand XPS indicate that tetramethoxy silane adsorbson the Y2O3 surface. In our computations, we al-lowed a water molecule to adsorb onto the surfaceof the S2 symmetry-like Y12O18 cluster, either as amolecular entity or as dissociated into a protonand a hydroxyl group. In the subsequent geometryoptimization, both starting geometries resulted ina dissociated water molecule adsorbed on the clus-ter with a proton adsorbed on a cluster oxygenatom, and a hydroxyl group adsorbed on anyttrium atom.To investigate the chemisorption of TMOS, the

hydroxyl hydrogen and one of the methoxy groupsof TMOS were removed and the rest of the TMOSmolecule was allowed to bind to the surface with asingle bond (monodentate, as illustrated in thereaction equation (1)).Reaction energies for the adsorption of water

and TMOS on Y12O18 are given in Table 3. Theoptimized structure of (Y12O18)–O–Si(OCH3)3 isshown in Fig. 11.The calculated vibration spectrum for Y12O18–

O–Si(OCH3)3, is compared with that of theTMOS-treated yttria powder (S4) in Fig. 12aand b, respectively. The calculated spectrum isbased on a simplified model with a singleTMOS molecule on Y12O18while the real particlesurface probably is more or less covered withmolecules, yielding a badly resolved spectrum.The only peaks found in the spectrum of Y2O3

with chemisorbed TMOS (Fig. 12b) is a broadpeak at �1050 cm�1, most likely a poorlyresolved multiple of peaks from O–CH3 and O–Si stretches, where the peak at 1154 cm�1 probablycorresponds to O–CH3 stretching, and the weakone at 1205 cm�1 is likely a weak CH3 rockingpeak.

4. Conclusions

Chemisorption of three molecular species,diethylene glycol (DEG), formic acid, and tetra-methoxy silane (TMOS), on cubic Y2O3 nanocrys-tals was studied experimentally and by quantum

chemical hybrid DFT-ab initio calculations, usingan Y12O18 cluster as a model system.Y2O3 nanocrystals prepared by the polyol

method (S1), resulted in adsorption of the DEGmolecules on the particle surface, as evidenced byIR and XPS The quantum chemical calculationsshow that for a single DEG molecule on anY12O18 cluster, the most stable adsorption geome-try is obtained when the protons of the two hydro-xyl groups dissociate and the remaining oxygensbind to two surface Y atoms. The calculated IRspectrum for the system is in good agreement withthat observed. A bridge or bidentate binding modeis suggested to dominate for formic acid adsorbedon yttria nanocrystals (S2). As the IR spectra forDEG/RE2O3, formic acid/RE2O3 and oleic acid/RE2O3 are practically identical for RE = Y andGd, the above conclusions are expected to holdfor both systems.TEM showed that Y2O3 nanocrystals prepared

by the combustion method (S3) were 65 nm andof good crystallinity. The particles tended to aggre-gate, however, most likely because of the lack ofmolecular coating. As inferred from IR and XPS,the particles seem to have a more complicatedchemical composition than those prepared by col-loidal methods containing other forms of YOx inaddition to the cubic phase. Some of the absorptionpeaks observed in IR are, tentatively, attributed tothe presence of nitrogen containing species, such asnitrate. IR and XPS data indicate that TMOS canbe adsorbed on the surface of Y2O3 particles (S4).Calculations show that both monodentate andbridge bonding are possible for TMOS. The calcu-lations favour adsorption via an adsorbed watermolecule, although the process is not fully clearand further investigations are needed.This work shows that it is possible to use first

principles, quantum chemical calculations to studythe surface interactions between Y2O3 nanocrys-tals and organic molecules. And since there is sucha close similarity between the Y2O3 and Gd2O3

nanocrystal systems, the results are expected tohold for both systems and thereby can the surfaceinteractions between Gd2O3 nanocrystals and or-ganic molecules indirectly be studied by these cal-culations. The next step will be to model morethan one molecule on the cluster.

140 H. Pedersen et al. / Surface Science 592 (2005) 124–140

Acknowledgements

The authors would like to thank Dr. Per Pers-son, Dr. Urban Forsberg and David Ivanssonfor the TEM measurements. This work was sup-ported by the Swedish Research Council (VR)and by the Knowledge Foundation (KKS). Grantsfor computer time from SNAC at the SwedishNational Supercomputer Centre (NSC), High Per-formance Computer Centre North (HPC2N) andSwegrid are gratefully acknowledged.

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