Porous lanthanide oxides via a precursor method: Morphology control through competitive interaction of lanthanide cations with oxalate anions and amino acids

Article abstract of DOI:10.1039/b916167h

Porous lanthanide oxides were fabricated by a precursor-thermolysis method. The precursors were synthesized by a hydrothermal reaction with lanthanide (La, Ce, Pr and Nd) salts, sodium oxalate and asparagine (or glutamine). Under hydrothermal conditions asparagine and glutamine exhibited greatly different complexation abilities with lanthanide cations. The competitive interactions of lanthanide cations with oxalate anions and asparagine (or glutamine) gave rise to the formation of precursors with different structures and morphologies. ESI-MS detection further confirmed the different complexation abilities of asparagine or glutamine with lanthanide cations at the molecular level. Variation of oxalate anion concentration or the pH value of the reaction solution could tune the morphology of the products. After calcination, porous lanthanide oxides were obtained with the morphologies of their corresponding precursors. Our work suggests that the complexation ability of organic molecules with metal cations could be a crucial factor for morphological control of the precursors. Moreover, considering the diversity of organic additives and metal salts, other metal oxides with complex composition and morphology could be fabricated via this organic molecule-modified precursor method.

Full text of DOI:10.1039/b916167h

View Article Online / Journal Homepage / Table of Contents for this issue
This article is published as part of the Dalton Transactions themed issue entitled:
Synthetic Solid State Chemistry
Guest Editor Duncan Gregory
University of Glasgow, UK
Published in issue 26, 2010 of Dalton Transactions
Image reproduced with the permission of Svilen Bobev
Articles in the issue include:
PERSPECTIVES:
Syntheses and characterization of some solid-state actinide (Th, U, Np) compounds
Daniel E. Bugaris and James A. Ibers,
Dalton Trans., 2010, DOI: 10.1039/b927026d
Hybrid materials through linkage of chalcogenide tetrahedral clusters
Paz Vaqueiro, Dalton Trans., 2010, DOI: 10.1039/c000130a
COMMUNICATIONS:
Increasing the dimensionality of hybrid vanadium oxyfluorides using ionothermal synthesis
Farida Himeur, Phoebe K. Allan, Simon J. Teat, Richard J. Goff, Russell E. Morris and Philip
Lightfoot, Dalton Trans., 2010, DOI: 10.1039/c000318b
One-step synthesis of high-purity fluorous-capped inorganic nanoparticles
Rakesh Voggu, Ajmala Shireen and C. N. R. Rao,
Dalton Trans., 2010, DOI: 10.1039/b927355g
Visit the Dalton Transactions website for more cutting-edge inorganic and solid-state research
www.rsc.org/dalton

PAPER
www.rsc.org/dalton | Dalton Transactions
Porous lanthanide oxides via a precursor method: Morphology control
through competitive interaction of lanthanide cations with oxalate anions and
amino acids†
Zhu-Rui Shen, Jin-Gui Wang, Ping-Chuan Sun, Da-Tong Ding and Tie-Hong Chen*
Received 5th August 2009, Accepted 9th November 2009
First published as an Advance Article on the web 11th January 2010
DOI: 10.1039/b916167h
Porous lanthanide oxides were fabricated by a precursor-thermolysis method. The precursors were
synthesized by a hydrothermal reaction with lanthanide (La, Ce, Pr and Nd) salts, sodium oxalate and
asparagine (or glutamine). Under hydrothermal conditions asparagine and glutamine exhibited greatly
different complexation abilities with lanthanide cations. The competitive interactions of lanthanide
cations with oxalate anions and asparagine (or glutamine) gave rise to the formation of precursors with
different structures and morphologies. ESI-MS detection further confirmed the different complexation
abilities of asparagine or glutamine with lanthanide cations at the molecular level. Variation of oxalate
anion concentration or the pH value of the reaction solution could tune the morphology of the
products. After calcination, porous lanthanide oxides were obtained with the morphologies of their
corresponding precursors. Our work suggests that the complexation ability of organic molecules with
metal cations could be a crucial factor for morphological control of the precursors. Moreover,
considering the diversity of organic additives and metal salts, other metal oxides with complex
composition and morphology could be fabricated via this organic molecule-modified precursor method.
Introduction
in which the precursor of a molecular compound contains all
27–29
the necessary elements of the final product.
Nanocrystalline
In recent years, the fabrication of porous metal oxides with pore
size ranging from several to hundreds of nanometres has been
extensively studied.
is the template-based method, and soft templates (e.g. block
copolymers, surfactants and emulsions)
lanthanide oxides have been fabricated by a cellulose-precursor
30
method. Inspired by these results, recently, our group has
1
–10
One common route to produce porosity
developed an amino-acid-modified oxalate precursor method to
31
prepare hierarchically-structured mesoporous ceria. Complex
11–15
and hard templates
morphologies of the cerium oxalate precursors were fabricated
under the control of amino acids as crystal-growth modifiers. After
decomposition of the precursors by calcination, the morphologies
16–19
(
e.g. mesoporous silica, carbon and polymer beads)
have been
used. Another method named the ‘metal salt precursor method’
was developed as early as the 1980s, and it was successfully
used to fabricate nanocrystalline metal oxides by simple thermal
decomposition of metal salt precursors. The resultant oxides
generally showed aggregation of fine crystallites, giving rise to
mesopores (or macropores) as well as relatively large surface
2
0–26
areas.
size of 10–20 nm by thermal decomposition of Zn
Sankaranarayanan and co-workers fabricated ultrafine amor-
phous and crystalline Dy Fe 12 using citrate Dy Fe
(cit)25·36H
as the precursor. Cu-Zn based manganites with specific surface
For example, Lou e¨ r et al. prepared ZnO with a crystallite
20
3
(OH)
4
(NO ) .
3
2
3
5
O
3
5
2
O
21
2
-1
area from ca. 30 to 50 m g were obtained after calcination of their
carbonate precursors. Besides, metal oxides nano-composites
have also been prepared by a “single-source precursor” method
2
5
Institute of New Catalytic Materials Science, Key Laboratory of Energy-
Material Chemistry (Tianjin) and Engineering Research Center of Energy
Storage & Conversion (MOE), College of Chemistry, Nankai University,
Tianjin 300071, PR China. E-mail: chenth@nankai.edu.cn; Fax: +86-22-
Fig. 1 XRD patterns of Ce-based products of (a) the initial products
2
3507975
without hydrothermal treatment, (b) without additive, (c) with 6 mmol glu-
†
Electronic supplementary information (ESI) available: Optical photos,
3+
TEM and SEM images, N
DOI: 10.1039/b916167h
2
sorption isotherms and XRD patterns. See
tamine, (d) with 6 mmol asparagine, at reactant molar ratio (Ce :oxalate)
◦
of 2:1.5 mmol after reaction at 160 C for 48 h, respectively.
6
112 | Dalton Trans., 2010, 39, 6112–6123
This journal is © The Royal Society of Chemistry 2010

4
1,42
of the precursors remained and ceria nanoparticles were obtained
to form mesoporous structures. Up to now, the metal salts
precursor method has been widely used to prepare porous metal
were obtained after calcination.
These results inspired us
to further study how the complexation ability of the amino
acids influenced their morphology control effect on metal salt
precursors. In this work we studied the competitive interaction of
oxalate anions and amino acids (asparagine or glutamine) with
lanthanide cations and lanthanide-based compound precursors
with different structures and morphologies were obtained by
tuning the competition effect of oxalate anions and amino acids.
32–40
oxides assembled by nanoparticles.
MnO and NiO were fabricated by decomposition of their oxalate
precursors porous TiO spheres were produced by in situ
For instance, mesoporous
2
32,33
2
34
hydrolysis of titanium glycolate precursors, and mesoporous
MgO has been obtained through decomposition of magnesium
35
carbonate hydrate. Dimension-manipulated ceria nanostructures
36
were obtained from a cerium octylate precursor.
Very recently, we found asparagine had very strong com-
plexation ability with lanthanide cations under hydrothermal
conditions. Using asparagine as a ligand, lanthanide-based com-
pound precursors with the morphologies of macroporous foams,
microspheres and hollow spheres were fabricated successively, and
porous lanthanide oxides with the morphology of their precursors
Experimental
Glutamine (99%), asparagine (99%) and lanthanide salts
(AR, including La(NO ) ·6H O, Ce(NO ) ·6H O, CeCl ·7H O,
3
3
2
3
3
2
3
2
Pr(NO ) ·6H O, Nd(NO ) ·6H O) were commercial products and
3
3
2
3
3
2
used as-received.
3+
Fig. 2 SEM images of (a, b) cerium oxalate obtained at a reactant molar ratio (Ce :oxalate) of 2:1.5, (c, d) cerium oxlate obtained at a reactant molar
3
+
3+
ratio (glutamine:Ce :oxalate) of 6:2:1.5, (e, f) Ce-organic complexation compound foam obtained at reactant molar ratio (asparagine:Ce :oxalate) of
6
◦
:2:1.5, after hydrothermal reaction at 160 C for 48 h.
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6112–6123 | 6113

3+
◦
Fig. 3 SEM images of cerium oxalate products obtained at a reactant molar ratio (glutamine:Ce :oxalate) of 6:2:1.5 after reaction at 160 C for (a, b)
h, (c, d) 5 h, (e, f) 12 h. The quantity of glutamine was 6 mmol in the synthesis.
0
Fig. 5 XRD patterns of products obtained at reactant molar ratio
3+
◦
Fig. 4 XRD patterns of Ce-based products obtained at reactant molar
ratio (glutamine:Ce :oxalate) of 6:2:1.5) after reaction at 160 C for (a)
(asparagine:Ce :oxalate) of 6:2:1.5 after reaction at 160 C for (a) 0 h,
(b) 1 h, (c) 2 h, (d) 6 h, (e) 24 h. The quantity of asparagine was 6 mmol in
the synthesis.
3+
◦
0
h, (b) 5 h, (c) 12 h. The quantity of glutamine was 6 mmol in the synthesis.
6
114 | Dalton Trans., 2010, 39, 6112–6123
This journal is © The Royal Society of Chemistry 2010

For electrospray ionization mass spectrometry (ESI-MS),
Table 1 The fractional ions detected by ESI-MS after different times of
3+ 3+
◦
the Nd –asparagine and Nd –glutamine reaction systems at 120 C
6
4
mmol asparagine was dissolved in 24 mL deionized water at
◦
5
C, and then 0.5 mmol Nd(NO
3
)
3
·6H
2
O was added under
3+
3+
Reaction
time
Nd –asparagine reaction
system
Nd –glutamine reaction
stirring. After 10 min, the solution was transferred to a 30 mL
system
Teflon-lined autoclave, sealed and hydrothermally treated at
Fractional ions detected
Fractional ions detected
◦
120 C for a certain time. After hydrothermal treatment the
+
+
0
[Nd(C
4
H
H
7
O
O
3
3
N
N
2
2
)
)
2
] (m/z
[Nd(C
432.02)
[Nd(C
5
H
9
O
O
3
N
N
2
)
)
2
] (m/z
solutions were diluted with methanol and subjected to ESI-MS
analysis.
For the synthesis of lanthanide oxalate nanofibers and their
4
00.99)
+
+
[
Nd(C
533.05)
[Nd (C
74.52)
Nd (C
878.04)
[Nd (C
64.72)
[Nd (C
4
7
3
+H] (m/z
5
H
9
3
2
3
+H]
(m/z 578.10)
2
+
+
3
6
9
1
0 min
0 min
0 min
20 h
3
4
H
7
7
7
7
O
O
O
O
3
3
3
3
N
N
N
N
2
2
2
2
)
7
]
(m/z
[Nd
1013.12)
[Nd (C
2
(C
5
H
9
O
3
N
2
2
)
5
7
] (m/z
oxides, 6 mmol glutamine was dissolved in 12 mL deionized
6
◦
water at 45 C and then 2 mmol lanthanide salt (La(NO
3
)
3
·6H
2
O,
2+
2+
[
4
4
H
H
H
)
)
)
9
+H] (m/z
3
5
H
9
O
3
N
)
]
(m/z
CeCl
3
·7H
2
O, etc.) was added under stirring. Next the solution was
solution (1.5 mmol) and immediately
723.59)
—
3+
mixed with 12 mL Na
2
C
2
O
4
5
4
12
] (m/z
7
a suspension was obtained. After being stirred for 30 min the
suspension was transferred to a 30 mL Teflon-lined autoclave,
2+
+
3
4
6
(C
4
H
6
O
4
N)]
[Nd (C
m/z 726.16)
[Nd(C
5 9 3 2 4
H O N ) +2H]
(
◦
2+
sealed and hydrothermally treated at 160 C for 48 h. The product
(m/z 675.01)
5
H
9
O
3
N
2
)
6
+5H]
(
m/z 509.66)
powders were collected and were washed with deionized water
and ethanol thoroughly. Ceria nanofibers were obtained after
+
2+
[
4
Nd
72.02)
2
(C
4
H
7
O
3
N
2
)
5
] (m/z
[Nd
2
(C
5
H
9
O
3
N
2
)
7
+3H]
(m/z 653.13)
[Nd (C
(m/z 726.16)
◦
calcination of the precursors at 360 C for 1 h, while other
+
No fractional ions detected
(solid particles appeared)
—
5 9 3 2 4
H O N ) +2H]
lanthanide oxide nanofibers were obtained after calcination of
2+
◦
[Nd(C H O N ) +5H]
5
9
3
2
6
the precursors at 720 C for 4 or 5 h.
(
m/z 509.66)
[Nd (C
m/z 653.13)
2+
—
2
5
H
9
O
3
N
2
)
7
+3H]
(
No fractional ions detected
Monolithic foam product
No fractional ions
detected
No solid product
obtained, dark-brown
solution
obtained, light-brown solution
For the synthesis of 3D macroporous lanthanide compound
foams and their oxides, the procedure was same as the above
paragraph, except that 6 mmol asparagine was used instead of
glutamine. The quantity of Na
2
C
2
4
O was tuned as 0, 1.5 and
3
mmol, respectively. Dilute NaOH or HCl solution was used
to tune the pH value of the solution if necessary. As-synthesized
samples were usually collected as monoliths and were washed
with deionized water and ethanol thoroughly. Ceria foams were
◦
obtained after calcination of precursors at 360 C for 1 h, while
other lanthanide oxide foams were obtained after calcination of
◦
precursors at 720 C for 2 or 5 h.
XRD patterns were recorded with a Rigaku D/max-2500
diffractometer. SEM and TEM images were measured on Shi-
madzu SS-550 and Philips Tecnai F20 instruments, respectively.
N adsorption isotherms were measured on a BELSORP mini II
2
analyzer. Elemental analysis data was obtained with an Elementar
Vario-EL instrument. Electrospray ionization mass spectra were
obtained on an IonSpec 7.0 T FTICR-MS (ESI source) instru-
ment.
Results and discussion
Cerium oxalate based precursors in the presence of glutamine and
asparagine, respectively
In the synthesis process, upon the addition of sodium oxalate to
Fig. 6 SEM images of Ce-based products obtained at reactant molar
3
+
3+
◦
the solution of Ce , a white precipitate was formed immediately,
which was identified as Ce (C O by its XRD pattern
ratio (asparagine:Ce :oxalate) of 6:2:1.5 after reaction at 160 C for (a)
h, (b, c) 1 h, (d, e) 2 h, (f) 6 h, (g) 12 h, (h) 24 h. The arrows in (f, g)
O
4
)
3
·10H
2
0
2
2
indicate some hollow spheres. The quantity of asparagine was 6 mmol in
the synthesis.
(Fig. 1a). However, during the subsequent hydrothermal treat-
ment, different products were obtained depending on the presence
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6112–6123 | 6115

of asparagine or glutamine, respectively. Without the addition of
amino acid, the XRD pattern (Fig. 1b) shows the product was a
crystalline phase. To our knowledge it could not be ascribed to any
known structure, and this product was just denoted as “oxalate A”,
which should be a more stable cerium oxalate phase and its growth
cations, the electrostatic interaction between lanthanide cations
and oxalate anions is obviously stronger, and after the hy-
drothermal treatment only lanthanide oxalate crystals were
obtained.
However, in the presence of asparagine, the final product was a
white monolith (Fig. S1 of the ESI†) floating in the reaction so-
lution. The XRD pattern of the product was featureless (Fig. 1d),
indicating its amorphous nature. As shown in Fig. 2e and f, the
monolith displayed a 3D macroporous foam-like morphology,
with the cellular size ranging from 20 to 50 mm. Elemental analysis
showed that it had a high organic species content (5.1 wt% of nitro-
gen), and the foam-like monolith was a kind of cerium compound.
During the reaction process, after hydrothermal treatment for 1 h,
was accompanied with the dissolution of Ce
2
(C
2
O
4
)
3
·10H
2
O. The
morphology of the product was dumbbell-like bundles of densely
packed microrods (Fig. 2a and b). When glutamine was added, the
crystal structure was also determined to be “oxalate A” (Fig. 1c)
and the morphology of the product was dumbbell-like particles
assembled as nanorods (Fig. 2c and d). The products synthesized
with or without glutamine exhibited the same crystal structure,
which implies that glutamine was not involved in the crystal
structure of “oxalate A”, and it acted only as the crystal-growth
the initial microcrystals of Ce
merge together and some microspheres formed (Fig. 6a, b and c).
The corresponding XRD pattern displayed lower crystalline peaks
2
(C
2
O
4
)
3
·10H
2
O (Fig. 5a) began to
43
modifier, which could control the nucleation and epitaxial growth
of the cerium oxalate crystals.
The time dependent syntheses of Ce-precursors were performed
under 160 C in the presence of glutamine. The initial precipitates
produced without hydrothermal treatment were microcrystals
with a size of ~1 mm (Fig. 3a and b), and the XRD patterns
which were assigned to Ce
that the microspheres formed at the expense of Ce
2
(C
2
O
4
)
3
·10H
2
O (Fig. 5b). This indicated
(C
·10H
◦
2
2
O
4
)
3
2
O
crystals, and the asparagine in the solution competed with oxalate
anions to bind with lanthanide cations to form the amorphous
compound. After 2 h of reaction, the monolithic blocks from the
aggregation of microspheres became the main product (Fig. 6d
and e), meanwhile several weak crystalline peaks corresponding
(
5
(
Fig. 4a) showed that they were pure Ce
h of reaction, the dumbbell-like nanorod aggregation appeared
Fig. 3c and d), and its XRD pattern (Fig. 4b) indicated
the structure of “oxalate A”. With the reaction proceeding to
2 h, the assembly of nanorods became the majority of the
2
(C
2
O
4
)
3
·10H
2
O. After
to residual Ce
2
(C
2
O
4
)
3
·10H
2
O crystals could be found in the XRD
1
pattern (Fig. 5c). After 6 h and 12 h, many holes began to emerge
on the block and some spheres not totally fused into the block
became hollow simultaneously (Fig. 6f and g). This implies that the
solid spheres totally merged into the block would undergo the same
final products (Fig. 3e and f), meanwhile its corresponding
XRD pattern showed they were pure “oxalate A” (Fig. 4c).
Although glutamine is able to form complexes with lanthanide
3+
Fig. 7 SEM images of Ce-based products obtained at different reactant molar ratios (asparagine:Ce :oxalate) of (a) 6:2:0, (b) 6:2:075, (c) 6:2:1.5, (d)
◦
6
:2:3, after reaction at 160 C for 48 h.
6
116 | Dalton Trans., 2010, 39, 6112–6123
This journal is © The Royal Society of Chemistry 2010

hollowing process, gradually developing into the 3D macroporous
foam-like structure. When the reaction proceeded to 24 h, the 3D
macroporous foam was obtained (Fig. 6h) and its XRD pattern
finally became featureless (Fig. 5e). From the above results, it is
found that the hollowing process was similar to the previously
reaction and determined the composition and morphology of
the precursors during the succeeding reaction stages, as discussed
later.
Complexes formation by asparagine/glutamine and lanthanide
cations as monitored by ESI-MS
41
reported one based on an Ostwald ripening mechanism. This is
to say that during the hydrothermal reaction, asparagine grabbed
the lanthanide cations from the starting Ce
via complexation interaction to form the compound
foam.
Compared with the case of glutamine addition, it is proposed
that under hydrothermal conditions the complexation interaction
between asparagine and lanthanide cations is stronger than the
electrostatic interaction between oxalate anions and lanthanide
cations, and the latter is stronger than that between glutamine
and lanthanide cations, as discussed above. Although glutamine
2
(C
2
O
4
)
3
·10H
2
O
From the time-dependent experiments described above, it was
found that asparagine and glutamine induced different morpholo-
gies for cerium oxalate precursors. Moreover, similar phenomena
a
3
+
3+
were also found when other lanthanide cations (e.g. La , Pr ,
3
+
Nd ) were used instead of cerium ions, and we ascribed these
phenomena to different complexation ability of asparagine and
glutamine with lanthanide cations under hydrothermal conditions.
Asparagine binds to lanthanide cations more strongly than glu-
tamine under these conditions, and was able to grab cations from
the crystalline lanthanide oxalates and finally form an amorphous
compound. The cation–ligand complexes and the monomer of
the coordination polymer could be effectively examined by ESI-
and asparagine differ only by a CH group (Scheme S1 of the
2
ESI†), they exhibited different morphology control processes
due to their difference in complexation ability with lanthanide
cations. The competitive complexation of amino acids and oxalate
anions occurred from the initial stage of the hydrothermal
4
4-48
MS.
Our testing system was simple, and contained only
3
+
asparagine (or glutamine), Nd and H
2
O (please note that no
3+
Fig. 8 SEM images of Ce-based precursors obtained at reactant molar ratios (asparagine:Ce :oxalate) of 6:2:1.5 and with the pH value of 3.0 (a), 7.0
◦
(
b, c) and 9.0 (d, e, f) after reaction at 160 C for 48 h.
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6112–6123 | 6117

3
+
◦
Fig. 9 SEM images of (a, b) ceria nanofibers obtained at reactant molar ratio (glutamine:Ce :oxalate) of = 6:2:1.5 after reaction at 160 C for 48 h,
◦
and was followed by calcination at 360 C for 1 h. The glutamine was 6 mmol in the synthesis. (c, d) TEM images, the inset in (d) displays the interplanar
spacing of the (111) facet.
3
+
2+
sodium oxalate was added). Here we chose Nd as the model
lanthanide cation because of its specific isotope distribution in
and [Nd
3
(C
4
H
7
O
3
N
2
)
6
(C
4
H
6
O
4
N)] (m/z 675.01 with its isotope
peaks) were detected. This implied that multi-nuclear complexes
were further connected and the framework of compounds began to
emerge. Notably, parts of the asparagine ligands were transformed
into aspartic acids, providing direct evidence for the structural
transformation of asparagine to aspartic acid (C
was proposed by EA, XPS and NMR analyses in our previous
work. Furthermore, after 90 min of hydrothermal treatment,
no fractional ions containing asparagine or aspartic acid ligands
could be detected, and it is quite reasonable to speculate that
much larger molecular complexes beyond ESI-MS detection were
◦
ESI-MS detection. A lower temperature of 120 C was selected for
a slower reaction process. The initial solution before hydrothermal
treatment and the solution after hydrothermal reaction for 30, 60
and 90 min, respectively, were measured by ESI-MS and the results
are listed in Table 1.
It was found that in the initial solution asparagine
N ) ]) coordinated with Nd , and ESI-MS peaks
at m/z 400.99 (with its isotope peaks) and 533.05 (with its
4
H
7
4
O N), which
41
+
-
3+
(
[H (C
4
H
7
O
3
2
isotope peaks) could be assigned to the mononuclear complexes
+
+
◦
[
Nd(C
4
H
7
O
3
N
2
)
2
] and [Nd(C
4
H
7
O
3
N
2
)
3
+ H ]. After hydrother-
obtained. Finally after hydrothermal treatment at 120 C for
mal treatment for 30 min, the initial mononuclear complexes
began to connect and multi-nuclear complexes were formed,
120 h, a solid Nd–organic foam with aligned macropores was
41
produced. At this stage, for the remaining solution, there were
no fractional ions detected by ESI-MS. This could be ascribed
to the formation of compound monoliths which consumed most
of the molecular complexes in the solution. Although there were
molecular complexes left in the solution, their thermal stability
was much weaker than that of the rigid coordination framework.
So after hydrothermal treatment as long as 120 h, those complexes
in solution would be decomposed (the reaction solution became
light brown) and thus no fractional ions could be detected by
ESI-MS.
2
+
such as [Nd
3
(C
4
2
2
H
7
3
O
3
N
2
)
7
]
(m/z 674.52 with its isotope peaks),
+
[
[
Nd
Nd
4
(C
(C
4
H
H
7
O
O
3
N
N
)
)
9
]
(m/z 878.04 with its isotope peaks) and
] (m/z 764.72 with its isotope peaks). When
3
+
5
4
7
3
12
asparagine reacted with lanthanide cations, the multi-nuclear
complexes with four or five metal ions might be crucial for
the formation of larger frameworks of compound precursors.
When the hydrothermal reaction proceeded for 60 min, the
four- and five-nuclear complexes were absent while the fractional
+
ions [Nd
2
(C
4
H
7
O
3
N
2
)
5
]
(m/z 472.02 with its isotope peaks)
6
118 | Dalton Trans., 2010, 39, 6112–6123
This journal is © The Royal Society of Chemistry 2010

+
-
In the case of glutamine ([H (C
tion the mononuclear complexes [Nd(C
with its isotope peaks) and [Nd(C
5
H
9
O
3
N
2
) ]), in the initial solu-
+
5
H
9
O
3
N
2
)
2
] (m/z 432.02
+
5
H
9
O
3
N
2
) + H ] (m/z 578.10
3
with its isotope peaks) were found, which indicated that glutamine
could also bind to lanthanide cations by complexation interaction.
After hydrothermal reaction for 30 min, the mononuclear complex
also began to become connected, however, the multi-nuclear com-
+
plexes only contained two or three metal ions: [Nd
2
(C
(C
5
H
9
O
3
N
2
)
5
]
2
+
(
(
m/z 1013.12 with its isotope peaks) and [Nd
3
5
H
9
O
3
N
2
)
7
]
m/z 723.59 with its isotope peaks), and this could be ascribed
to larger spatial resistance when lanthanide cations coordinated
with glutamine, whose molecular size is larger than that of
asparagine. Moreover, it is speculated that the complexes formed
based on these multi-nuclear complexes might not be large enough
to form the amorphous compound particles. After 60 min of
+
hydrothermal treatment, fractional ions [Nd(C
5
H
9
O
3
N
2
) + 2H]
4
2
+
+
(
(
(
m/z 726.16 with its isotope peaks), [Nd(C
5
H
9
O
3
N
2
)
6
+ 5H]
+ 3H]
2
m/z 509.66 with its isotope peaks) and [Nd (C
2
5
H
9
O
3
N
2 7
)
m/z 653.13 with its isotope peaks) were found in the reaction
solution. However, when the hydrothermal treatment proceeded
to 90 min, the fractional ions detected remained the same as the
results after hydrothermal treatment of 60 min, indicating that the
molecular complexes containing only two or three metal ions could
not further connect to form the larger compound framework.
◦
Finally after hydrothermal treatment at 120 C for 120 h, no solid
products could be obtained when using glutamine as ligand, and
Fig. 10 Nitrogen adsorption–desorption isotherms of (a) CeO
nanofibers and (b) CeO foam.
2
2
3
+
◦
Fig. 11 SEM images of (a, b) CeO
was followed by calcination at 360 C for 1 h. (c, d) TEM images of CeO
picture in (d) displaying the interplanar spacing of the (111) facet.
2
foam obtained at reactant molar ratio (asparagine:Ce :oxalate = 6:2:1.5) after reaction at 160 C for 48 h, and
2
◦
foam, the inset picture in (c) is its corresponding SAED pattern, and the inset
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6112–6123 | 6119

Fig. 12 SEM images of bread-like ceria particles assembled as nanoflakes obtained by the thermolysis of the corresponding cerium-based precursors.
the solution turned to brown due to the decomposition of the
glutamine which could not form solid compound particles with
lanthanide cations. As a result, we could not find any amino-acid-
containing fractional ions from the solution after hydrothermal
lanthanide 3D macroporous foams with higher porosity were
obtained (Fig. 7b and c). However, when the concentration of the
oxalate ions was increased further, only crystalline microparticles
were obtained (Fig. 7d, for the XRD pattern see Fig. S2 of the
ESI†). EA analysis showed that the N content in those crystalline
microparticles is 1.48 wt%, which is nearly one third of that
detected in the 3D macroporous foam produced with the addition
of 1.5 mmol oxalates (5.1 wt%). This means that the oxalate anions
could still competitively bind with the lanthanide cation depending
on its concentration.
On the basis of the results mentioned above, it is verified
again that there existed a complexation–dissociation balance
between oxalate anions, lanthanide cations and asparagine, and
this balance determined the growth process of the compound.
When the concentration of oxalates was changed, a new balance
would be formed and thus a new growth process would produce a
final product with a different macroporosity.
◦
treatment at 120 C for 120 h.
Based on the results of ESI-MS, we could conclude that: i)
for asparagine, the final product originated from mononuclear
complexes that grew to form some crucial multi-nuclear complexes
with more than four metal ions, and further connect to form the
solid compound; ii) for glutamine, it could also bind with lan-
thanide cations to form mononuclear complexes initially, however,
only transient multi-nuclear complexes with fewer than three metal
ions could be formed because of larger spatial resistance, and these
multi-nuclear complexes could not connect together to form any
solid product.
Morphology control of lanthanide-based precursors in the presence
of asparagine by tuning the concentration of oxalate anions and pH
values
The ionization balance of asparagine determined the concen-
49
tration of deprotonated asparagine in solution, and it could be
altered easily by tuning the pH value of the reaction solution.
Considering complexes were formed by deprotonated asparagine
and lanthanide cations, according to ESI-MS, the complexation–
dissociation balance between oxalate anions, lanthanide cations
and asparagine could also be changed by altering the pH value of
the reaction solution, thus the morphology of cerium precursors
were tuned simultaneously. When the pH value of the reaction
solution was tuned to 3.0, macroporous foams (Fig. 8a) with
lower porosity and thick walls were produced. When the pH value
reached 7.0, it is interesting that the foams had two kinds of
pore size distribution of 20–40 mm and 4–10 mm (Fig. 8b and
As discussed above, the cerium macroporous precursor was
proven to be an amorphous compound and there were mutual
3
+
complexation interactions between Ce , oxalate anions and
asparagine. Varying the concentration of any species would change
the complexation interactions between them, and then would
influence the morphology of the product. So we performed
synthesis with different concentrations of the oxalates. SEM
observation showed that without the addition of oxalate ions, a
macroporous foam with low porosity and thick walls was produced
(
Fig. 7a). After addition of a certain quantity of oxalate ions,
6
120 | Dalton Trans., 2010, 39, 6112–6123
This journal is © The Royal Society of Chemistry 2010

2 3 6 2 3
Fig. 13 SEM images of porous–macroporous materials of (a, b) La O , (c, d) Pr O11, (e, f) Nd O .
c). However, when the pH value was tuned to 9.0 the macroporous
foam disappeared and crystalline bread-like particles assembled
as rhombic nanoflakes were obtained (Fig. 8d–f). The XRD
patterns showed it had the same crystal phase (Fig. S3†) to the
microparticles produced under high oxalate concentrations, which
was noted as “oxalate B”.
calcination in air, the monolithic shape remained with a yellowish
color as well as a foam-like morphology (Fig. 11a and b). Ceria
nanoparticles were generated, as revealed by the TEM images
(Fig. 11c and d) and the relatively broad peaks in the XRD pattern
(Fig. S4b) of the calcined foam. The 0.32 nm spacing between two
adjacent lattice planes corresponds to the separation of the (111)
lattice planes of the ceria. The selected-area electron diffraction
(
SAED) pattern (inset of Fig. 11c) clearly displays diffraction
Porous lanthanide oxides obtained by thermolysis of
lanthanide-based precursors
rings, indicating the polycrystalline nature of the foam wall. The
BET surface area of the CeO foam was 78 m g , with a pore
volume of 0.084 cm g (Fig. 10b). After calcination, bread-like
ceria particles assembled as nanoflakes were produced (Fig. 12)
with a BET surface area of 35 m g (Fig. S5).
2
-1
2
3
-1
Ceria materials were produced by the thermolysis of cerium-
based precursors. After calcination in air, the morphology of the
precursors obtained under control of glutamine remained stable
2
-1
(
Fig. 9a) and the XRD pattern showed it was pure ceria (Fig. S4a).
This amino-acid-modified precursor method could be easily
From the SEM image (Fig. 9b), it is observed that the dumbbell-
like particles were composed of oriented nanofibers (200–400 nm
in diameter). The TEM image showed the aggregation of ceria
expanded to prepare other lanthanide oxides, such as La
Pr . When using glutamine as the additive, after
decomposition of the corresponding precursors, bi-pyramidal
particles of La nanofibers (Fig. S6a and b), dumb-bell as-
semblies of Pr 11 nanofibers (Fig. S6c and d) and dispersed
2
O
3
,
6
O
11 and Nd
2
O
3
2
-1
nanoparticles (Fig. 9d) with a BET surface area of 99 m g
2
O
3
3
-1
as well as a pore volume of 0.072 cm g (Fig. 10a). For the
Ce-coordination compound monolithic foam as precursor, after
6
O
Nd
2
O
3
nanofibers were prepared (Fig. S6e and f), respectively.
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6112–6123 | 6121

Fig. 14 TEM images of the pore wall of (a) La
porous–macroporous materials. The inset picture in (a) is the SAED pattern which indicates the single crystalline property of the La
2
O
3
porous–macroporous materials, (b) Pr
6
O
11 porous–macroporous materials, (c, d) Nd
microflakes.
2 3
O
2
O
3
TEM images (Fig. S7) of lanthanide oxide nanofibers showed a
porous structure after calcination. When using asparagines as the
complexation with lanthanide cations between oxalate anions
and asparagine, variations of oxalate anions concentration and
pH values of the reaction solution could tune the morphology
of precursors. After calcination, porous lanthanide oxides were
obtained which retained the morphologies of their corresponding
precursors. Our work suggests that the complexing ability of
organic molecules with metal cations could be a crucial factor for
morphological control of metal salt precursors. Considering the
diversity of organic additives and metal salts, it is expected that
other metal oxides with complex composition and morphology
could be fabricated by this organic molecule-modified precursor
method.
additive, La
2
O
3
, Pr
6
O11 and Nd
2
3
O foams with porous walls were
obtained (Fig. 13). From the SEM and TEM images (Fig. 14), it is
observed that the macroporous walls of all these lanthanide oxides
were composed of nanoflakes and microflakes, and the aggregation
of nanoflakes on the microflakes gave rise to the porous structure
of the lanthanide oxides.
Conclusion
Porous lanthanide oxides were fabricated by thermolysis of pre-
cursors, which were synthesized hydrothermally with lanthanide
(
La, Ce, Pr and Nd) salts and sodium oxalate in the presence
Acknowledgements
of glutamine or asparagine, respectively. Though the chemical
structures of asparagine and glutamine are similar, they exhibited
greatly different complexation abilities with lanthanide cations
under hydrothermal conditions. Compared to the electrostatic
interaction between oxalate anions and lanthanide cations, the
complexation interaction between glutamine and lanthanide
cations was weaker. Using glutamine as additive, nanofibers of
the crystalline oxalate precursor was produced. In contrast, the
complexation ability of the asparagine with lanthanide cations was
much stronger and an amorphous compound precursor with a 3D
macroporous structure was fabricated. Based on the competitive
This work was supported by NSFC (Grant Nos. 20873070
and 20973095), the National Basic Research Program of China
(2009CB623502) and NCET of the Ministry of Education (NCET-
07-0448).
References
1
2
M. E. Davis, Nature, 2002, 417, 813.
D. M. Lyons, K. M. Ryan and M. A. Morris, J. Mater. Chem., 2002,
12, 1207.
6
122 | Dalton Trans., 2010, 39, 6112–6123
This journal is © The Royal Society of Chemistry 2010

3
4
5
6
7
8
9
Y. S. Hu, Y. G. Guo, W. Sigle, S. Hore, P. Balaya and J. Maier, Nat.
Mater., 2006, 5, 713.
A. S. Aric o` , P. Bruce, B. Scrosati, J. M. Tarascon and W. Van Schalkwijk,
Nat. Mater., 2005, 4, 366.
Z. Tebby, O. Babot, T. Toupance, D. H. Park, G. Campet and M. H.
Delville, Chem. Mater., 2008, 20, 7260.
C. Cannas, A. Ardu, A. Musinu, D. Peddis and G. Piccaluga, Chem.
Mater., 2008, 20, 6364.
R. Zhu, C. Y. Jiang, B. Liu and S. Ramakrishna, Adv. Mater., 2009, 21,
26 E. Traversa, P. Nunziante, M. Sakamoto, Y. Sadaoka, M. C. Carotta
and G. Martinelli, J. Mater. Res., 1998, 13, 1335.
27 M. Veith, J. Sol-Gel Sci. Technol., 2008, 46, 291.
28 M. Veith, J. Chem. Soc., Dalton Trans., 2002, 2405.
29 M. Veith, M. Haas and V. Huch, Chem. Mater., 2005, 17, 95.
30 E. V. Tsipis, V. V. Kharton, I. A. Bashmakov, E. N. Naumovich and
J. R. Frade, J. Solid State Electrochem., 2004, 8, 674.
31 G. J. Zhang, Z. R. Shen, M. Liu, C. H. Guo, P. C. Sun, Z. Y. Yuan,
B. H. Li, D. T. Ding and T. H. Chen, J. Phys. Chem. B, 2006, 110,
25782.
32 C. C. Yu, L. X. Zhang, J. L. Shi, J. J. Zhao, J. H. Gao and D. S. Yan,
Adv. Funct. Mater., 2008, 18, 1544.
33 M. Liu, G. J. Zhang, Z. R. Shen, P. C. Sun, D. T. Ding and T. H. Chen,
Solid State Sci., 2009, 11, 118.
9
94.
T. Sun, S. Donthu, M. Sprung, K. D’Aquila, Z. Jiang, A. Srivastava, J.
Wang and V. Dravid, Thin Solid Films, 2009, 57, 1095.
S. Li, Z. W. Li, Y. Y. Tay, J. Armellin and W. Gao, Cryst. Growth Des.,
2
008, 8, 1623.
1
1
0 T. Z. Ren, Z. Y. Yuan and B. L. Su, Chem. Phys. Lett., 2004, 388, 46.
1 D. M. Antonelli and J. Y. Ying, Angew. Chem., Int. Ed. Engl., 1996, 35,
34 S. L. Zhong, J. S. Hu, L. J. Wan and W. G. Song, Chem. Commun.,
2008, 1184.
4
26.
35 C. L. Gao, W. L. Zhang, H. B. Li, L. M. Lang and Z. Xu, Cryst. Growth
1
1
2 D. M. Antonelli and J. Y. Ying, Chem. Mater., 1996, 8, 874.
3 P. D. Yang, D. Y. Zhao, D. I. Margolese, B. F. Chmelka and G. D.
Stucky, Chem. Mater., 1999, 11, 2813.
Des., 2008, 8, 3785.
36 H. P. Zhou, Y. W. Zhang, R. Si, L. S. Zhang, W. G. Song and C. H.
Yan, J. Phys. Chem. C, 2008, 112, 20366.
1
1
4 B. H. Jones and T. P. Lodge, J. Am. Chem. Soc., 2009, 131, 1676.
5 J. Sarkar, V. T. John, J. B. He, C. Brooks, D. Gandhi, A. Nunes, G.
Ramanath and A. Bose, Chem. Mater., 2008, 20, 5301.
6 T. Katou, B. Lee, D. L. Lu, J. N. Kondo, M. Hara and K. Domen,
Angew. Chem., Int. Ed., 2003, 42, 2382.
37 H. D. Yu, D. S. Wang and M. Y. Han, J. Am. Chem. Soc., 2007, 129,
2333.
38 M. Y. Li, W. S. Dong, C. L. Liu, Z. T. Liu and F. Q. Lin, J. Cryst.
1
1
Growth, 2008, 310, 4628.
39 Z. A. Zang, H. B. Yao, Y. X. Zhou, W. T. Yao and S. H. Yu, Chem.
Mater., 2008, 20, 4749.
7 B. Z. Tian, X. Y. Liu, L. A. Solovyov, Z. Liu, H. F. Yang, Z. D. Zhang,
S. H. Xie, F. Q. Zhang, B. Tu, C. Z. Yu, O. Terasaki and D. Y. Zhao,
J. Am. Chem. Soc., 2004, 126, 865.
40 J. Y. Liu, Z. Guo, F. L. Meng, T. Luo, M. Q. Li and J. H. Liu,
Nanotechnology, 2009, 20, 125501.
41 Z. R. Shen, G. J. Zhang, H. J. Zhou, P. C. Sun, B. H. Li, D. T. Ding
and T. H. Chen, Adv. Mater., 2008, 20, 984.
42 Z. R. Shen, J. G. Wang, P. C. Sun, D. T. Ding and T. H. Chen, Chem.
Commun., 2009, 1742.
43 H. Tong, W. T. Ma, L. L. Wang, P. Wan, J. M. Hu and L. X. Cao,
Biomaterials, 2004, 25, 3923.
44 P. Cheng and D. K. Bohme, J. Phys. Chem. B, 2007, 111, 11075.
45 M. Oh and C. A. Mirkin, Nature, 2005, 438, 651.
46 G. A. Santillan and C. J. Carrano, Cryst. Growth Des., 2009, 9, 1590.
47 T. J. Burchell, D. J. Eisler, M. C. Jennings and R. J. Puddephatt, Chem.
Commun., 2003, 2228.
48 J. A. Rood, W. C. Boggess, B. C. Noll and K. W. Henderson, J. Am.
Chem. Soc., 2007, 129, 13675.
1
1
2
2
2
2
8 B. T. Holland, C. F. Blanford, T. Do and A. Stein, Chem. Mater., 1999,
1
1, 795.
9 X. Y. Lai, X. T. Li, W. C. Geng, J. C. Tu, J. X. Li and S. L. Qiu, Angew.
Chem., Int. Ed., 2007, 46, 738.
0 D. Lou e¨ r, J. P. Auffredic, J. I. Langford, D. Ciosmak and J. C. Niepce,
J. Appl. Crystallogr., 1983, 16, 183.
1 V. K. Sankaranarayanan and N. S. Gajbhiye, Thermochim. Acta, 1989,
1
53, 337.
2 V. K. Sankaranarayanan and N. S. Gajbhiye, J. Am. Ceram. Soc., 1990,
3, 1301.
7
3 J. I. Langford, A. Boultif, J. P. Auffredic and D. Louer, J. Appl.
Crystallogr., 1993, 26, 22.
2
2
4 N. Guillou, J. P. Auffredic and D. Louer, Powder Diffr., 1995, 10, 236.
5 G. Fierro, S. Morpurgo, M. Lo Jacono, M. Inversi and I. Pettiti, Appl.
Catal., A, 1998, 166, 407.
49 S. F. Shao, G. J. Zhang, H. J. Zhou, P. C. Sun, Z. Y. Yuan, B. H. Li,
D. T. Ding and T. H. Chen, Solid State Sci., 2007, 9, 725.
This journal is © The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6112–6123 | 6123