Monodisperse Samarium and Cerium Orthovanadate …

 

 

Monodisperse Samarium and Cerium Orthovanadate Nanocrystals and
Metal Oxidation States on the Nanocrystal Surface

Thanh-Dinh Nguyen, Cao-Thang Dinh, and Trong-On Do*

Department of Chemical Engineering, Laval University, Quebec G1K 7P4, Canada

Received April 19, 2009. Revised Manuscript Received May 31, 2009

A new solvothermal method has been developed for the synthesis of monodisperse SmVO4 and CeVO4 nanocrystals
with controlled size and shape. The obtained materials were characterized by X-ray photoelectron spectroscopy (XPS),
X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED)
techniques. The results reveal that uniform nanocrystals and pure tetragonal phase of SmVO4 and CeVO4 can be
achieved. To investigate the oxidation states of the metals on the mixed oxide nanocrystal surface, the XPS technique
was employed. The results exhibit that only one oxidation state of samarium, cerium, and vanadium for each metal (e.g.,
Sm3þ, Ce3þ, V5þ) was surprisingly well stable on the particle surface at the nanoscale, even after calcination, while
the existence of two oxidation states of these metals is observed (e.g., Sm3þ/Sm2þ, Ce4þ/Ce3þ, V5þ/V4þ) in the
corresponding single metal oxide nanocrystals.

1. Introduction
Rare earth orthovanadate (Re-V-O) crystals have potential
applications as catalysts and support owing to their high surface
area, thermal stability, and oxygen storage/release capacity
(OSC).1-3 These materials have been used as oxygen ion con-
ductors in solid oxide fuel cells, as three way catalysts (TWCs),
and as high-activity catalysts in oxidative dehydrogenation of
propane to propene and selective oxidation of hydrogen sulfide to
elemental sulfur at low reaction temperatures.4-7

In recent years, the synthesis of these nanosized Re-V-O
crystal systems with controlled size and shape has attracted much
interest due their extensive potential applications in different
fields, for example, optics, electrical, magnetic, and catalytic
fields.8,9 Their unique properties are due to not only their size-
dependent properties but also the combination of the properties
of individual components and of stoichiometry and symmetry in
the mixed oxide crystals which are not available in single metal
oxide nanocrystals (NCs).10,11 Furthermore, the metal oxidation
states in the mixed oxide NCs as well as the crystal structure are
also considered to be responsible for their catalytic performance12
and potential application.13,14

*To whom correspondence should be addressed. E-mail: Trong-On.

Do@gch.ulaval.ca.

1185–1189.

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2005, 105, 1025–1102.

In order to better understand the metal oxidation states on the
mixed oxide surface at the nanoscale, we have developed a novel,
simple, and reproducible method for the synthesis of monodis-
perse SmVO4 and CeVO4 nanocrystals. In this work, we have
focused on the synthesis of nanocubes and nanospheres. How-
ever, different shapes and sizes of these monodisperse mixed oxide
NCs can be also obtained using this same approach. Although
several methods have been reported,15-18 however, to the best
of our knowledge, up to date, few studies have been focused on
the synthesis of uniform SmVO4 and CeVO4 nanospheres.
Furthermore, our method is fundamentally different from the
previous methods for the synthesis of these types of materials in
aqueous19,20 and organic21,22
solutions. The monodisperse
SmVO4 and CeVO4 NCs obtained by our approach are char-
acterized by X-ray diffraction (XRD), transmission electron
microscopy (TEM), and selected area electron diffraction
(SAED). The oxidation states of the metals in the lattice are
examined by X-ray photoelectron spectroscopy (XPS) both
before and after calcination, compared to those of the corre-
sponding single metal oxide NCs (e.g., Sm2O3, CeO2, V2O5).

2. Experimental Section

Starting Materials. All chemicals were used as received
without further purification. Samarium nitrate hexahydrate (Sm
(NO3)3 3 6H2O, 99.9%), cerium nitrate hexahydrate (Ce(NO3)3 3
6H2O, 99.9%), vanadium pentaoxide (V2O5, 99.6%), tetraocty-
lammonium bromide ([CH3(CH2)7]4NBr or TOABr, g98%),
potassium oleate (C17H34COOK or KOA, 40 wt % paste in
water), and oleylamine (C18H35NH2 or OM, tech. grade, 70%)

(15) Mahapatra, S.; Ramanan, A. J. Alloys Compd. 2005, 395, 149–153.
(16) Liu, J.; Li, Y. Mater. Chem. 2007, 17, 1797–1803.
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(18) Deng, H.; Liu, C.; Yang, S.; Xiao, S.; Zhou, Z. K.; Wang, Q. Q. Cryst.

2008, 130, 2032–2040.

Growth Des. 2008, 8, 4432–4439.

Chem.;Eur. J. 2009, 15, 1233–1240.

(19) Qian, L.; Zhu, J.; Chen, Z.; Gui, Y.; Gong, Q.; Yuan, Y.; Zai, J.; Qian, X.

(20) Luo, F.; Jia, C. J.; Song, W.; You, L. P.; Yan, C. H. Cryst. Growth Des.

(13) Burda, C. B.; Chen, X. B.; Narayanan, R.; Sayed, M. A. E. Chem. Rev.

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2005, 5, 137–142.

2009, 31, 1032–1037.

11142DOI:10.1021/la901387qLangmuir2009,25(18),11142–11148PublishedonWeb07/02/2009pubs.acs.org/Langmuir©2009AmericanChemicalSocietyDownloaded by UNIV LAVAL on September 8, 2009 | http://pubs.acs.org Publication Date (Web): July 2, 2009 | doi: 10.1021/la901387q Scheme 1. Schematic Illustration for the Formation of the Re(OA)3 and VO4(TOA)3 Complexes (i) and the ReVO4 Nanocrystals (ii)

were purchased from Sigma-Aldrich. Sodium hydroxide (NaOH,
97%) and all solvents used such as toluene and ethanol were of
analytical grade and purchased from Reagent ACS.

Synthesis of Rare Earth Orthovanadate (ReVO4, Re =
Sm and Ce) Nanocrystals. Preparation of Re(OA)3 Com-
plexes. To prepare rare earth oleate complexes, an organic
solution was produced from adding 40 mL of toluene into the
ethanol solution (10 mL) containing potassium oleate (KOA,
6.4 g or 0.008 mol). The organic solution was mixed to 30 mL of a
Re(NO3)3 3 6H2O aqueous solution (0.067 mol/L) with a Sm3þ/
OA- molar ratio of 1:3 and then transferred to a flask. The two-
phase mixtures were heated to 70 (cid:1)C for 60 min with vigorous
stirring, and the organic solution became light yellow after being
reacted, indicating the occurrence of the coordinated reaction
of Re3þ and OA- for the complex formation. Subsequently,
the upper deep orange supernatant toluene solution (40 mL)
containing Re(OA)3 complexes (0.059 mol/L) was isolated.

Preparation of VO4(TOA)3 Complexes. A total of 0.24 g
(0.0013 mol) of commercial bulk V2O5 powders was dissolved
in 20 mL of a 0.12 M NaOH aqueous solution and vigorously
stirred at room temperature for 30 min. V2O5 powders were
completely dissolved in the dilute NaOH solution to give a
homogeneous light orange Na3VO4 aqueous solution (∼0.133
mol/L). Then, 40 mL of toluene solution (0.040 mol/L) con-
taining a cationic phase-transfer reagent ([CH3(CH2)7]4NBr
or TOABr, 0.87 g) was added to the above solution, and the
3-/TOAþ molar ratio was close to 1:3. The two-phase
VO4
mixture was vigorously stirred at room temperature. After 30
3- anions in the aqueous phase were completely
min, VO4
extracted into the toluene phase, and a light orange toluene
solution was observed. Subsequently, the upper light orange
supernatant toluene solution (40 mL) containing VO4(TOA)3
complexes (0.059 mol/L) was isolated.

Synthesis of ReVO4 Nanocrystals. Typically, the two above
toluene solution containing Re(OA)3 complexes and VO4(TOA)3
complexes (0.059 mol/L) were mixed together with a Re/V molar
ratio of 1:1, and 0.011 mol (5 mL) of oleylamine (OM) was added
under stirring. The reaction solution was transferred into a 120
mL Teflon-lined stainless steel autoclave and was heated at 180 (cid:1)C
for 16 h in an oven. After the reaction, the autoclave was cooled in
tap water, and the as-obtained products were easily precipitated
by excess ethanol and redispersed preferably in nonpolar solvents
(e.g., toluene, hexane, etc.). The precipitation-redispersion pro-
cess was repeated several times to purify the producing ReVO4
nanocrystals. Different reaction temperatures were tested to
control the sizes and the shapes of the ReVO4 nanocrystals. The
obtained samples for all of the measurements shown in this
communication are directly from synthesis without any size-
selection process.

Figure 1. XRD pattern of the as-made SmVO4 nanospheres.

Characterization. The particle sizes and morphologies of the
synthesized samarium and cerium orthovanadate nanocrystals
were determined at 120 kV by using a JEOL JEM 1230 transmis-
sion electron microscope. Samples were prepared by placing
a drop of a dilute toluene dispersion of nanocrystals onto a 200
mesh carbon coated copper grid and immediately evaporated at
ambient temperature. Particle size distribution diagrams were
obtained from statistical treatment of representative TEM images
of each sample by measuring the diameter of typically not less
than 300 particles using the software ImageJ.23 Average particle
diameters were determined from these size distribution dia-
grams. The crystalline phases of products were characterized on
a Bruker SMART APEXII X-ray diffractometer operated at
1200 W power (40 kV, 30 mA) to generate Cu KR radiation
(λ=1.5418 A˚ ). The XPS spectra were taken on a photoelectron
spectrometer (KRATOS AXIS-ULTRA) with a monochromatic
X-ray source of Al KR. The operating conditions for recording
Sm 3d, Ce 3d, V 2p, O 1s, C 1s, and N 1s high-resolution spectra
were 1486.6 eV and 225 W, pass energy of 160 eV with an
operating pressure of 10-9 Torr, acquisition time of 5.75 min,
and a pressed mixed oxide pellet under an Arþ bombardment. The
peaks were deconvoluted by means of standard CasaXPS soft-
ware (v.2.3.13; product of CasaXPS Software Ltd.) in order to
resolve the separate constituents after background subtraction.
The UV-vis spectra of the as-synthesized ReVO4 nanocrystals
were recorded on a Hitachi U-3010 spectrometer, and pure MgO
was used as a blank. The thermal analyses of the as-synthesized

(23) Igathinathane, C.; Pordesimo, L. O.; Batchelor, W. D. Food Res. Int. 2009,
42, 76–84.Public domain software to be downloaded from http://rsb.info.nih.gov/
ij/http://rsb.info.nih.gov/ij/.

DOI:10.1021/la901387q11143Langmuir2009,25(18),11142–11148Nguyenetal.ArticleDownloaded by UNIV LAVAL on September 8, 2009 | http://pubs.acs.org Publication Date (Web): July 2, 2009 | doi: 10.1021/la901387q Figure 2. TEM images of 15 nm sized SmVO4 nanocrystals synthesized at different temperatures for 16 h: (a) nanocubes at 150 (cid:1)C and (c)
corresponding particle size distribution; (b) nanospheres at 180 (cid:1)C (inset, SAED) and (d) corresponding particle size distribution.

synthesis of these materials is reported in the 2Experimental
Section. Metal oxide nanocrystals of Sm2O3, CeO2, and V2O5
were also synthesized following the methods reported previously
by our group.24-26

Figures 1 and 3 show the X-ray diffraction (XRD) patterns of
the as-made SmVO4 and CeVO4 nanocrystal samples, respec-
tively. All the XRD peaks are well indexed to tetragonal zircon-
type structure with the following lattice contents: SmVO4: a=b=
7.265 A˚ , c=6.389 A˚ (Fm3hm, JCPDS card no. 17-0876);15 CeVO4:
a=b=7.265 A˚ , c=6.389 A˚ (I1/amd, JCPDS card no. 12-0757).17
The intense broadening of the peaks is caused by the small particle
sizes. The average crystalline size of these as-made SmVO4 and
CeVO4 NC samples calculated from the broadening of the (200)
peak using the Scherrer formula are 17.5 and 18.5 nm, respec-
tively. Furthermore, for the corresponding as-made single metal
oxide NCs, the XRD patterns exhibit cubic phase (JCPDS card
no. 78-0429) for Sm2O3,24 cubic phase (JCPDS card no. 78-0429)
for CeO2,26 and orthorhombic phase (JCPDS card no. 78-0429)
for V2O5 (Supporting Information S-Figure 1).25 Their XRD
peaks did not match at all those of the mixed oxide SmVO4 and
CeVO4 NCs. Otherwise, no XRD peaks of the SmVO4 and
CeVO4 NC samples which correspond to single metal oxide
NCs of Sm2O3, CeO2, and V2O5 were detected, indicating that
the pure tetragonal zircon-type structure of SmVO4 and CeVO4
NCs can be achieved by this method. It can be also concluded that
the formation of the pure tetragonal ReVO4 phase results in the

(24) Nguyen, T. D.; Mrabet, D.; Do, T. O. J. Phys. Chem. C 2008, 112, 15226–

15235.

(25) Nguyen, T. D.; Do, T. O. Langmuir 2009, 25, 5322–5332.
(26) Nguyen, T. D.; Do, T. O. J. Phys. Chem. C 2009, 113, 11204–11214.

Figure 3. XRD pattern of the as-made CeVO4 nanospheres.
ReVO4 nanocrystals (∼5 mg) were carried out at a heating rate of
10 (cid:1)C/min under an air flux up to 650 (cid:1)C using a Perkin-Elmer
TGA thermogravimetric analyzer. Fourier transform infrared
(FTIR) absorption spectra were measured with a FTS 45 infrared
spectrophotometer with the KBr pellet technique.

3. Results and Discussion

The synthesis of ReVO4 nanocrystals consists of two steps:
(i) the preparation of Re(OA)3 complexes (Re=Sm and Ce) and
VO4(TOA)3 complexes from inexpensive precursors; Re(NO3)3
and V2O5 powders, respectively; (ii) the formation of ReVO4
nanocrystals (e.g., SmVO4 and CeVO4 NCs) from the presynthe-
sized Re(OA)3 and VO4(TOA)3 complexes. The formation of
ReVO4 NCs is schematically illustrated in Scheme 1, and detailed

11144DOI:10.1021/la901387qLangmuir2009,25(18),11142–11148ArticleNguyenetal.Downloaded by UNIV LAVAL on September 8, 2009 | http://pubs.acs.org Publication Date (Web): July 2, 2009 | doi: 10.1021/la901387q Figure 4. TEM images of 15 nm sized CeVO4 nanocrystals synthesized at different temperatures for 16 h: (a) round-shaped nanocrystals at
150 (cid:1)C and (c) corresponding particle size distribution; (b) nanospheres at 180 (cid:1)C (inset, SAED) and (d) corresponding particle size
distribution.
complete reaction of the 1:1 ratio of Re3þ cation and VO4
monomers in the bulk solution during the synthesis.

3- anion

The shape of SmVO4 and CeVO4 NCs could be controlled
by the synthesis temperature. TEM images, SEAD patterns,
and particle size distribution (PSD) diagrams of the samples
synthesized solvothermally at 150 and 180 (cid:1)C for 16 h are
shown in Figures 2 and 4. Nearly cubic-shaped SmVO4 and
round-shaped CeVO4 nanocrystals with an average diameter
of 15 nm were found at 150 (cid:1)C. When the synthesis tempera-
ture increased to 180 (cid:1)C for 16 h, both uniform SmVO4 and
CeVO4 nanospheres were observed; however, the diameter is
unchanged. A slight change in color of the toluene containing
nanoparticles from light yellow to brown was observed, when
the reaction temperature was increased from 150 to 180 (cid:1)C,
indicating a change in shape of the nanoparticles (Supporting
Information S-Figure 3). The inset SAED patterns taken from
single particles are also indexed to a tetragonal SmVO4 and
CeVO4 single crystal with strong ring patterns from the (200),
(112), (301), (312), and (420) planes which are consistent with
the XRD results. PSD diagrams of these samples were
obtained from the statistical treatment of particle diameters
from representative TEM images,23 as shown in Figures 2c, d
and 4c, d. It is found that the PSD becomes narrower,
indicating more unifrom nanoparticles, when the reaction
temperature is increased from 150 to 180 (cid:1)C. The transforma-
tion of both cubic-shaped SmVO4 and round-shaped CeVO4
NCs into uniform nanospheres while preserving the particle

size by increasing synthesis temperature from 150 to 180 (cid:1)C
can be explained by the Wuff facets theory.27,28 Because no
monomer precursors were added during the synthesis, at the
relatively low reaction temperature (e.g., 150 (cid:1)C), the cubic-
shaped SmVO4 and round-shaped CeVO4 nanocrystals
were formed in the anisotropic growth of high-energy crystal
faces owing to high monomer metal concentration in bulk
solution.26 The differences in cubic and round shapes of the
SmVO4 and CeVO4 nanocrystals, respectively, could be due
to the different nature of their corresponding rare earth
metals. At higher synthesis temperature (e.g., 180 (cid:1)C), the
transformation of both the cubic- and round-shaped nano-
crystals into uniform nanospheres could result in the deple-
tion of remaining metal monomer concentration in bulk
solution to a level
lower than that required for a given
anisotropic shape.26 Consequently, the monomers of the
dissolved crystals at high energy faces could move to the
lower energy faces because of the differences in chemical
potential between different crystal faces. As a result, the
corners and the tips of the cubic-shaped SmVO4 and round-
shaped CeVO4 nanocrystals were “smoothed”, which leads to
the formation of stable nanospheres with a minimum face
energy.

The UV-vis absorption spectra of the as-made SmVO4 and
CeVO4 nanosphere samples are shown in Figure 5. A strong
absorption peak at 265 and 264 nm for the SmVO4 and CeVO4
samples, respectively, is observed, which is attributed to the
charge transfer from the oxygen ligands to the central vanadium

(27) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann: Woburn,

MA, 1997.

(28) Wulff, G.; Zeitschrift, F. Beitr. Krystallogr. Mineral. 1901, 34, 449.

DOI:10.1021/la901387q11145Langmuir2009,25(18),11142–11148Nguyenetal.ArticleDownloaded by UNIV LAVAL on September 8, 2009 | http://pubs.acs.org Publication Date (Web): July 2, 2009 | doi: 10.1021/la901387q Figure 5. UV-vis absorption spectra of the as-made nanosphere
samples: (a) SmVO4 and (b) CeVO4.

Figure 7. FTIR spectra of the OM-capped nanosphere samples:
(a) SmVO4 and (b) CeVO4.
exothermic peaks at 305 and 248 (cid:1)C, respectively, which are
related to the decomposition and combustion of oleylamine
molecules on the particle surfaces upon heating. The exothermic
peaks of both samples occur at the different two temperatures
which can result from the different binding of oleylamine mole-
cules to their corresponding nanocrystal surfaces. The FTIR
spectra (Figure 7) of these OM-capped samples show at 2920-
2870 cm-1 which are assigned to the C-H stretching modes of
alkyl chains in oleylamine.25 The band at 1440 cm-1 is attributed
to the N-H bending and N-C stretching modes of -NH2 groups
in oleylamine capped on nanocrystal surfaces.25 The IR band at
810 cm-1 is attributed to the Re-O-V vibrations of ReVO4.30
Residual water and a hydroxide group are detected with a large
band around 3450 cm-1, corresponding to the O-H stretching
frequency due to the bending vibration of associated water. These
data suggest that oleylamine molecules were bound to the
nanocrystal surface and further confirm that SmVO4 and CeVO4
were formed under these synthetic conditions.

The metal oxidation states on the particle surface of the SmVO4
and CeVO4 NCs and corresponding single metal oxides (e.g.,
Sm2O3, CeO2, V2O5) were examined by XPS. The survey XPS
spectra of these as-made and calcined mixed oxide NC samples in
a wide energy range are presented in S-Figures 3 and 4 in the
Supporting Information; beside carbons and nitrogens, no im-
purities were found. However, the intense C 1s and N 1s peaks of
the as-made samples as compared to those of the calcined samples
can be assigned to the oleylamine ligand capped on the NC
surface. The positions of XPS peaks were corrected using the C 1s
core level taken at 285 eV as a binding energy (BE) reference.
Surprisingly, the Sm 3d, Ce 3d, and V 2p XPS spectra of both the
as-made and calcined SmVO4 and CeVO4 nanospheres exhibit
only one oxidation state for each metal (e.g., Sm3þ, Ce3þ, V5þ)
at the nanoscale and even in ultrahigh vacuum for XPS mea-
surements (see Figures 8 and 9). The Sm 3d5/2,3/2 peaks at
1083.5-1110.8 eV, the Ce 3d5/2,3/2 peaks at 885.7-904.0 eV,
and the V 2p3/2,1/2 peaks at 517.2-525.5 eV attributed to Sm3þ,
Ce3þ, and V5þ, respectively, were observed.24,31 No peaks char-
acteristic of other metal oxidation states of these metals were
detected. However, for the single NC samples of samaria, ceria,
vanadia, the Sm 3d, Ce 3d, and V 2p XPS spectra exhibit two
additional weak peaks at 1095.9-1121.3 eV for Sm2þ 3d5/2,3/2,

(30) Fang, Z. M.; Hong, Q.; Zhou, Z. H.; Dai, S. J.; Weng, W. Z.; Wan, H. L.

Catal. Lett. 1999, 61, 39–44.

Figure 6. TGA-DTA curves of the OM-capped nanosphere sam-
ples: (a) SmVO4 and (b) CeVO4.

3- groups in the samarium and cerium
atom inside the VO4
orthovanadate, respectively.29 However, no significant change
in the position of this peak was observed with different shapes of
these samples This could be due to almost the same of the
nanoparticle size (∼15 nm).

The thermogravimetric analysis/differential thermal analysis
(TGA-DTA) curves (Figure 6) of the oleylamine (OM)-capped
SmVO4 and CeVO4 nanospheres exhibit the weight loss of 22
and 21% appearing around 150-500 (cid:1)C accompanied by the

(29) Kalai Selvan, R.; Gedanken, A.; Anilkumar, P.; Manikandan, G.;

(31) Salvi, A. M.; Decker, F.; Varsano, F.; Speranza, G. Surf. Interface Anal.

Karunakaran, C. J. Cluster Sci. 2008, DOI: 10.1007/s10876-008-0229-y.

2001, 31, 255–264.

11146DOI:10.1021/la901387qLangmuir2009,25(18),11142–11148ArticleNguyenetal.Downloaded by UNIV LAVAL on September 8, 2009 | http://pubs.acs.org Publication Date (Web): July 2, 2009 | doi: 10.1021/la901387q Figure 8. (A) Sm 3d XPS spectra of (a) the as-made SmVO4 nanospheres, (b) the calcined SmVO4 nanospheres, and (c) the calcined Sm2O3
nanospheres. (B) Ce 3d XPS spectra of (a) the as-made CeVO4 nanospheres, (b) the calcined CeVO4 nanospheres, and (c) the calcined CeO2
nanospheres.

the multiple oxidation states, whereas, for the mixed metal
oxides, the presence of mixed Sm-O-V bonds in SmVO4 and
Ce-O-V bonds in CeVO4 requires the formation of Sm3þ-
V5þ and Ce3þ-V5þ species, respectively.5,32,33 In fact, the
formation of the ReVO4 phase (Re=Sm and Ce) results in the
interaction between the nuclear spin of the V atom and
unpaired 4f electron of Re. Fermi contact interaction could
be involved with a transfer of the unpaired 4f electron of the
Re ions to the orbital on the V atom,34-36 and hence a slight
chemical shift to a higher binding energy (shifted ∼0.3-
1.3 eV) of Sm 3d, Ce 3d, and V 2p of ReVO4 NCs compared
with that of Sm2O3, CeO2, and V2O5 NCs.5,37 As a result, V
and Re possess a valence of þ5 and þ3, respectively, in the
ReVO4 lattice.

For the CeVO4 NCs, as seen in Figure 10a, the O 1s peak at
530.2 eV is attributed to both O-Re and O-V bonds in the
tetragonal ReVO4 lattice31 which consists of VO4 tetrahedra
which share corners and edges with ReO8 dodecahedra,15,17 and
another shoulder peak at 532.2-533.1 eV is attributed to the
presence of hydroxyl species or adsorbed water on the nanocrystal
surface.24 However, for the SmVO4 NCs (Figure 10b), the XPS
peak at ∼530 eV is broader with a full width at half-maximum
(fwhm=2.0 eV) as compared to the O 1s peak (fwhm=1.0 eV) for
the CeVO4 NCs. The broader peak at ∼530 eV could be
associated with a Sm Auger peak (∼526.8 eV).38,39 The XPS
analysis confirms that the molar ratio of Re/V in these samples is
very close to 1:1, agreeing with the formula of ReVO4. It is well-
known that stoichiometric defects such as oxygen vacancies and

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Division: 1992.

(39) Turner, N. H.; Schreifels, J. A. Anal. Chem. 1996, 68, 309–332.

Figure 9. V 2p XPS spectra of the spherical NC samples: (a) as-
made SmVO4 NCs, (b) calcined SmVO4 NCs, and (c) calcined
V2O5 NCs.

at 879.5-914.0 eV for Ce3þ 3d5/2,3/2, and at 516.2-524.1 eV for
V4þ 2p5/2,3/2, as compared to those of the mixed oxide SmVO4 and
CeVO4 nanosphere samples. It means that, in these single metal
oxide NC samples, these metals exist in two oxidation states, and
the fit peaks revealed Sm3þ (60%) and Sm2þ (40%) for samar-
ium,24 Ce4þ (54%) and Ce3þ (46%) for cerium,26 V5þ (65%)
and V4þ (35%) for vanadium.25

The XPS results revealed that, for each metal, only one
oxidation state (e.g., Sm3þ, Ce3þ, V5þ) exhibits in the SmVO4
and CeVO4 NCs, and meanwhile, two oxidation states (Sm3þ/
Sm2þ, Ce4þ/Ce3þ, V5þ/V4þ) in the single Sm2O3, CeO2, V2O5
NCs were found. This difference is due to the fact that, for each
single metal oxide, the single Sm-O-Sm bonds in Sm2O3, the
Ce-O-Ce bonds in CeO2, and the V-O-V bonds in V2O5 allow

DOI:10.1021/la901387q11147Langmuir2009,25(18),11142–11148Nguyenetal.ArticleDownloaded by UNIV LAVAL on September 8, 2009 | http://pubs.acs.org Publication Date (Web): July 2, 2009 | doi: 10.1021/la901387q Figure 10. O 1s XPS spectra of the as-made spherical mixed oxide samples: (a) CeVO4 NCs and (b) SmVO4 NCs.

their mobility on the metal oxide surface at reaction temperature
are of great importance for redox reactions.35,40,41 Further work
to determine the amount of oxygen released of both lattice oxygen
and adsorbed oxygen species from these mixed oxide NCs using
the TPD-O2 technique as well as the nature of oxygen species
involved in catalytic oxidation is in progress in our laboratory.

4. Conclusions

We have developed a simple and reproducible method for the
synthesis of uniform SmVO4 and CeVO4 nanocrystals from
the solvolysis of a mixture of Re(OA)3 (Re = Sm and Ce) and
VO4(TOA)3 complexes in toluene in the presence of oleylamine
ligand. By increasing the reaction temperature from 150 to
180 (cid:1)C, SmVO4 and CeVO4 NCs can converted from cubic-
shaped SmVO4 and nearly round-shaped CeVO4 to uniform

(40) Do, T. O.; Nguyen, S. V.; Kaliaguine, S. Phys. Chem. Chem. Phys. 2003, 5,

2724–2729.

(41) Levasseur, B.; Kaliaguine, S. J. Solid State Chem. 2008, 181, 2953–2963.

nanospheres. The XPS results revealed that only one oxidation
state of samarium, cerium, and vanadium for each metal (e.g.,
Sm3þ, Ce3þ, V5þ) exists in the mixed oxide spherical NCs, SmVO4
and CeVO4, while two oxidation states for each metal (Sm3þ/
Sm2þ, Ce4þ/Ce3þ, V5þ/V4þ) exist in the corresponding single
metal oxide spherical NCs, Sm2O3, CeO2, and V2O5. Our syn-
thetic approach could also be useful for the synthesis of other
uniform rare earth orthovanadate, molybdate, and tungstate
nanocrystals as well as doped NCs and multicomponent NCs.

Acknowledgment. This work was supported by the Natural
Sciences and Engineering Research Council of Canada (NSERC)
through a strategic grant.

Supporting Information Available: XRD patterns, TEM
images, corresponding SAED patterns, and survery XPS
spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.

11148DOI:10.1021/la901387qLangmuir2009,25(18),11142–11148ArticleNguyenetal.Downloaded by UNIV LAVAL on September 8, 2009 | http://pubs.acs.org Publication Date (Web): July 2, 2009 | doi: 10.1021/la901387q Monodisperse Samarium and Cerium Orthovanadate Nanocrystals and

Metal Oxidation States on the Nanocrystal Surface

Thanh-Dinh Nguyen, Cao-Thang Dinh, and Trong-On Do*

Department of Chemical Engineering, Laval University, Quebec G1K 7P4 Canada

To whom correspondence should be addressed. E-mail: Trong-On.Do@gch.ulaval.ca

SUBMITTED TO Langmuir – April 2009

Supporting Information

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S-Figure 1. XRD patterns of the calcined single metal oxide spherical NCs: (a) Sm2O3, (b)

CeO2, (c) V2O5.

S-1