Hierarchically porous nanostructures through phosphonate-metal alkoxide condensation and growth using functionalized dendrimeric building blocks

Article abstract of DOI:10.1039/c1cc12310f

Controlled titanium alkoxide mineralization in the presence of phosphonated, dendrimeric nano-building blocks provides a new family of hierarchically porous dendrimer-bridged titanium dioxide materials. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc12310f

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 8626–8628
www.rsc.org/chemcomm
COMMUNICATION
Hierarchically porous nanostructures through phosphonate–metal
alkoxide condensation and growth using functionalized dendrimeric
building blocksw
Younes Brahmi,a Nadia Katir,ab Aurelien Hameau,ab Abdellatif Essoumhi,a
El Mokhtar Essassi,ac Anne-Marie Caminade,b Mosto Bousmina,ac Jean-Pierre Majoral*ab
and Abdelkrim El Kadib*a
Received 20th April 2011, Accepted 3rd June 2011
DOI: 10.1039/c1cc12310f
Controlled titanium alkoxide mineralization in the presence of
phosphonated, dendrimeric nano-building blocks provides a new
family of hierarchically porous dendrimer-bridged titanium
dioxide materials.
Here, we present an alternative strategy for making porous
organic–inorganic multifunctional hybrid materials, in a one-pot
synthesis by condensation between a branched dendrimer and
an inorganic alkoxide monomer. This route takes advantage
of the well-controlled topology of the rationally designed
phosphorus dendrimers6 in terms of their organisation on
the nanoscale, their nanoporous cavities, their compacted
hydrodynamic volume in solution, their controlled dispersion
and the large density of functional groups at the peripheral
branches.7 With this aim, the external surface of the dendrimers
has been functionalized with phosphonic acid salts that form
a strong covalent bond with transition metals (P–O–M:
M = Ti, Zr, Al)8 as demonstrated during the post-grafting
of hybrid material.9,10 Herein, the targeted dendrimers were
designed to cross-link metal alkoxides and to template mineral
growth from the processable sol–gel monomers.
The strategic development of rationally designed, multi-
functional, architectural materials lies at the forefront of the
field of material synthesis.1 Of particular interest is that the
nano-scale association of soft and hard matter in the same
building blocks by the so-called self-assembly process2 provides
access to exciting nanohybrids with synergetic inorganic–
organic properties such as thermal and structural stability
(from the inorganic matrix) and specific functional group
reactivity (from the organic fragment).
There are various synthetic pathways to generate hybrid
materials, which span from conventional sol–gel polymerisation
to more sophisticated coating and self-assembly techniques,
intercalation, nano-replication and casting, pillaring and
exfoliation.3 The crucial challenge in the synthesis of these nano-
structured materials concerns the necessary use of a sacrificial
template in order to generate ordered porous networks.4 Such
nanostructures are used in a variety of applications as, for
example, adsorbents, sensors and indicators, drug delivery
systems, or as heterogeneous catalysts. Unfortunately, the
difficulties associated with the costly synthesis of the lyotropic
templates and environmental concerns associated with their
removal overwhelm the real benefits of these materials. There
is therefore a real need for sustainable and less energy-intensive
synthetic protocols to produce multifunctional hybrid materials
featuring hierarchical substructures.5
Three generations of phosphonated dendrimers (G2, G3 and
G4) have been utilized in this study (Fig. 1 and S1, ESIw).
Titanium tetraisopropoxide, Ti(OiPr)4, has been selected as
the inorganic source, ethanol as the solvent and water as the
catalyst.
In a typical procedure for the material synthesis, a
phosphonated dendrimer was solubilised in an ethanol/water
(5/2 v/v) solution. After 15 minutes of stirring, a titanium
alkoxide precursor in a given molar ratio ([dendrimer] : [Ti] =
1 : 5, 1 : 10 or 1 : 20) was added to the transparent dendritic
solution at room temperature. Upon addition, the solution
became cloudy through polymerisation of the titanium
precursor and the resulting solution was heated at 60 1C for
10 hours. After filtration and extensive washing of the
precipitate with ethanol, the collected solid was dried at
60 1C for 2 hours. Hereafter, the materials are denoted M–Gx–R
(where x refers to the dendrimer generation (2, 3 or 4) and R is
the dendrimer : Ti ratio used (1 : 5, 1 : 10 or 1 : 20)).
a (INANOTECH) Institute of Nanomaterials and Nanotechnology –
MAScIR (Moroccan Foundation for Advanced Science,
´
Innovation and Research), ENSET, Avenue de l’Armee Royale,
Madinat El Irfane, 10100 Rabat, Morocco.
E-mail: a.elkadib@mascir.com
b LCC (Laboratoire de Chimie de Coordination),
205 route de Narbonne, 31077 Toulouse cedex, France.
E-mail: majoral@lcc-toulouse.fr
c Hassan II Academy of Science and Technology, Rabat, Morocco
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc12310f
Fig. 1 Dendrimer G4.
c
8626 Chem. Commun., 2011, 47, 8626–8628
This journal is The Royal Society of Chemistry 2011

View Article Online
The structural properties of these materials were elucidated
by means of CP MAS NMR and DRIFT analysis. 13C CP
MAS NMR clearly shows the characteristic signals of the
organic skeleton at 33, 46, 55 and 72 ppm and those of the
aromatic rings at 122, 130, 139 and 150 ppm, which indicate
the preservation of the dendritic structure during the sol–gel
polymerization (see ESIw, S2). The 31P MAS NMR spectra of
the solid materials exhibit signals characteristic of the parent
dendrimers—in particular, those corresponding to the cyclo-
phosphazene core at 5 ppm, internal phosphorus at 65 ppm
and peripheral phosphonate groups at 18 ppm (see ESIw, S3).
Compared to the native dendrimers, the broadness of the
signals for the phosphonic groups in the hybrid materials
indicates the significantly reduced mobility of the peripheral
fragments, which is consistent with their covalent bonding
to the titania network.9,11 DRIFT analysis revealed signals
characteristic of the dendritic substructures (see ESIw, S4). The
disappearance of the absorption peak at 910 cmÀ1 assigned to
the phosphonate P–OH and the merging new bands at 1080 cmÀ1
(due to the P–OÁ Á ÁTi stretching vibrations) provide evidence
for extensive condensation between the titanium alkoxide and
the phosphonate groups of the dendritic structures to form
bridged P–O–Ti hybrid materials.9–11
the microstructure. At higher magnification, the individual
crystals (B5 nm in width) of anatase TiO2 are clearly observed.
The HRTEM image allows for the resolution of lattice fringes
of the crystals assigned to the {101} planes of the anatase
titania (Fig. 2c and d and S6 in ESIw). The calculated inter-
planar distance is 3.5 A, which is in line with the results
obtained from the X-ray diffraction patterns. The XRD
pattern exhibits peaks that can be assigned to the anatase
phase of the titanium dioxide (ESIw, S7). The size of
the nanocrystals estimated from the half-height width of the
{101} peak using the Scherrer formula corresponds to the
average crystallite size ranging from 4 to 7 nm, in accordance
with the results obtained from TEM analysis. The nanocrystals
are homogeneous in their composition as evidenced by local
mapping of the distribution of titanium dioxide in the hollow
network by EDX analyses (ESIw, S8). In three different zones,
the same Ti/P (or S) ratio (P and S from the dendrimer) was
found, which excludes the formation of core–shell structures
and unambiguously precludes the formation of two phase-
separated heterogeneous zones. Moreover, this highlights
the pivotal role played by the dendrimer substructures as
anchoring points for the nucleation and further mineral
growth of the nanocrystalline titanium dioxide.12
The textural properties of these hybrid materials were
investigated using scanning and transmission electron microscopy
techniques (SEM and TEM, respectively), X-ray diffraction
analysis and nitrogen sorption measurements. As evidenced by
SEM micrographs, the network of the materials is composed
of spherical particles connected to each other to form a
continuous network. The size of the spheres varies slightly
within the series of the materials from 100 to 150 nm (see
Fig. 2a and S5, ESIw). TEM provides useful information
concerning the organization on the nano-scale of these cross-
linked dendritic networks. Interestingly, small particles seem
to be well entangled in the network of the material and no
bulk material or separated phase was observed. Disordered
mesopores (intercrystalline pores) are abundant throughout
The texture of the solid network of these hybrid materials
was further investigated by nitrogen sorption analysis.
Regardless of the generation of the dendrimer and the
[dendrimer] : [Ti(OiPr)4] ratio used, the materials displayed
high porosity with surface areas ranging from 203 to 514 m2 gÀ1
(Table 1). Although the surface area decreased significantly
after increasing the ratio from 1 : 5 to 1 : 20, the mesoporous
network became more pronounced, as evidenced by the
appearance of hysteresis in the adsorption–desorption isotherm
(Fig. 2b and S9 in ESIw).
The porosity observed in native hybrid dendrimer–TiO2
(i.e. without calcination) is quite surprising. When the native,
unmodified dendrimer networks are assessed by nitrogen
adsorption, these solid materials present virtually no porosity—
indicative of a densely packed dendrimer network. It is well
known that such substructures present an open-framework
when solvated because of their excellent swelling capacity;7 the
internal voids of these stiff nanoscale macromolecules being as
the basis of their use as hosts for small guests and for the
encapsulation of biomolecules.13 The entropic effect due to the
presence of a solvent keeps the dendrimer strands separated in
Table 1 Textural properties of dendrimer–titania hybrid materials
Particle
sizea/nm
SBET
/
Vmeso
/
Db,c
/
nm
b
b
Entry
Material
m2 gÀ1
mL gÀ1
1
2
3
4
5
6
7
8
9
M–G2–1 : 5
M–G2–1 : 10
M–G2–1 : 20
M–G3–1 : 5
M–G3–1 : 10
M–G3–1 : 20
M–G4–1 : 5
M–G4–1 : 10
M–G4–1 : 20
5 (5)d
6.4
4.9
—
511
356
386
514
289
291
456
361
203
0.11
0.25
0.2
0.24
0.21
0.23
0.17
0.2
2.98
3.86
3.12
3.5
3.7
3.7
3.3
2.8
3.12
4
4.5
5.8 (5)d
7
Fig. 2 (a) SEM image of the hybrid dendrimer–titania material
(M–G2–1 : 10). (b) Comparison of the nitrogen physisorption
isotherms of M–Gx–1 : 20 (x = 2, 3, 4). (c) TEM image showing the
crystalline anatase planes in the hybrid material (M–G2–1 : 10).
(d) SAED showing the planes of titania anatase.
4.4
0.2
a
b
From XRD. From nitrogen sorption analysis. From the
c
d
adsorption branch using the BJH method. From TEM.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8626–8628 8627

View Article Online
advanced properties. This work is currently in progress in
our laboratories.
The authors are grateful to Vincent Colliere for SEM and
HRTEM analysis. The Hassan II Academy of Science and
Technology is warmly acknowledged for the financial support.
Notes and references
1 C. Sanchez, G. J. A. A. Soler-Illia, F. Ribot, T. Lalot, C. R. Mayer
and V. Cabuil, Chem. Mater., 2001, 13, 3061–3083.
2 G. M. Whitesides and B. Grzybowski, Science, 2002, 295,
2418–2421; I. W. Hamley, Angew. Chem., Int. Ed., 2003, 42,
1692–1712; S. Mann, Nat. Mater., 2009, 8, 781–792.
3 F. Hoffmann, M. Cornelius, J. Morell and M. Froba, Angew.
Chem., Int. Ed., 2006, 45, 3216–3251; Y. Wan and D. Y. Zhao,
Chem. Rev., 2007, 107, 2821–2860.
Fig. 3 Schematic illustration of the material design: (i) coordination
between the titanium alkoxides and phosphonate ligands, leading to
the formation of reactive building blocks; (ii) cross-linking of the
titanium alkoxides and further densification of the titania network;
(iii) fusing of the elemental grains to form a continuous network.
4 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and
J. S. Beck, Nature, 1992, 359, 710–712; D. Zhao, J. Feng, Q. Huo,
N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky,
Science, 1998, 279, 548–552.
5 N. Baccile, J. Reboul, B. Blanc, B. Coq, P. Lacroix-Desmazes,
M. In and C. Gerardin, Angew. Chem., Int. Ed., 2008, 47,
8433–8437; A. El Kadib, K. Molvinger, T. Cacciaguerra,
M. Bousmina and D. Brunel, Microporous Mesoporous Mater.,
2011, 142, 301–307.
6 Review: J. P. Majoral and A. M. Caminade, Chem. Rev., 1999, 99,
845–880; A. M. Caminade and J. P. Majoral, Acc. Chem. Res.,
2004, 37, 341–348.
7 J. Leclaire, Y. Coppel, A. M. Caminade and J. P. Majoral, J. Am.
Chem. Soc., 2004, 126, 2304–2305; J. Jimenez, A. Laguna,
A. M. Molter, J. L. Serrano, J. Barbera and L. Oriol, Chem.–Eur.
J., 2011, 17, 1029–1039; P. Carbone and L. Lue, Macromolecules,
2010, 43, 9191–9197.
8 X. Tang, N. Yao and M. E. Thompson, Supramol. Sci., 1997, 4,
35–42; G. Alberti, U. Costantino, C. Dionigi, S. Murciamascaros
and R. Vivani, Supramol. Chem., 1995, 6, 29–40; A. Clearfield,
Prog. Inorg. Chem., 1998, 47, 371–510.
9 W. Gao, L. Dickinson, C. Grozinger, F. G. Morin and L. Reven,
Langmuir, 1996, 12, 6429–6435; P. H. Mutin, G. Guerrero and
A. Vioux, J. Mater. Chem., 2005, 15, 3761–3768; E. M. Ferrero,
G. Franc, S. Mazeres, C. O. Turrin, C. Boissiere, A. M. Caminade,
J. P. Majoral and C. Sanchez, Chem.–Eur. J., 2008, 14, 7658–7669.
10 X. J. Zhang, T. Y. Ma and Z. Y. Yuan, J. Mater. Chem., 2008, 18,
2003–2010; T.-Y. Ma, X.-Z. Lin and Z. Y. Yuan, Chem.–Eur. J.,
2010, 16, 8487–8494.
11 M. N. Tchoul, S. P. Fillery, H. Koerner, L. F. Drummy,
F. T. Oyerokun, P. A. Mirau, M. F. Durstock and R. A. Vaia,
Chem. Mater., 2010, 22, 1749–1759.
12 G. J. A. A. Soler-Illia, L. Rozes, M. K. Boggiano, C. Sanchez,
C. O. Turrin, A. M. Caminade and J. P. Majoral, Angew. Chem.,
Int. Ed., 2000, 39, 4249–4254; G. Franc, E. Badetti, V. Colliere,
J. P. Majoral, R. M. Sebastian and A. M. Caminade, Nanoscale,
2009, 1, 233–237.
their solvated state. However, upon drying, the dendritic
branches are compliant enough to be drawn together by
capillary forces. By sharp contrast, the results presented herein
suggest that cross-linking of these objects provides a robust
hydrogel network that can better withstand the tension of a
receding solvent meniscus during drying.14 Thus, in solid state,
the materials exhibit significant accessible porosity (Fig. 3).
From a mechanistic viewpoint, it seems that the initial step
for material growth involves chelation of the metal alkoxide by
the phosphonate ligands, which leads to strong connectivity
between the organic and inorganic domains.15 This constitutes
the primary reactive building blocks for material nucleation.
The complexation of titanium alkoxide by phosphonate
ligands decreases the reactivity of the metal and allows for
more controlled condensation.16 The remaining alkoxy ligands
bridged to titanium centers interact by hydrolysis–condensation,
allowing for the continued formation and densification of the
elemental grains. The process of condensation consumes the
excess titanium alkoxide monomers by anchoring them to the
growing structure and mitigates random aggregate formation.
As the process goes on, the grains react with each other leading
to the spherical network observed by SEM.
The thermal stability of these materials has been examined
by thermogravimetric analysis (TGA) (S10 in ESIw). Thanks
to the high stability of phosphorus dendrimers,17 only 20% of
the skeleton degrades with heating. The preservation of the
crystalline anatase (even after thermal treatment at 800 1C
with a slight increase in the crystal size to B9 nm) demon-
strates the remarkable stability of the dendritic framework
(S11 in ESIw). Surprisingly, no transformation to the brookite
or rutile phase, commonly observed at this temperature range,
has been detected.18 These important outcomes are mainly
due to the steric hindrance of the dendritic skeleton, which
hampers the thermodynamically favored phase transformation
of the crystalline structure.
13 D. Astruc, E. Boisselier and C. Ornelas, Chem. Rev., 2010, 110,
1857–1959; R. M. Crooks, M. Zhao, L. Sun, V. Chechik and
L. K. Yeung, Acc. Chem. Res., 2001, 34, 181–190.
14 A. Bouchara, L. Rozes, G. J. A. A. Soler-Illia, C. Sanchez,
C. O. Turrin, A. M. Caminade and J. P. Majoral, J. Sol-Gel Sci.
Technol., 2003, 26, 629–633.
15 J. R. Errington, J. Ridland, K. J. Willett, W. Clegg, R. A. Coxall
and S. L. Heath, J. Organomet. Chem., 1998, 550, 473–476;
M. Mehring, G. Guerrero, F. Dahan, P. H. Mutin and
A. Vioux, Inorg. Chem., 2000, 39, 3325–3332.
In summary, an efficient strategy to draw nonporous packed
dendritic structures into the realm of stable porous hybrid
materials has been validated by cross-linking their periphery
with nanocrystalline titanium dioxide clusters. This strategy
further provides interesting hierarchical materials in which
photoactive crystalline anatase19 is grown in dendritic sub-
structures. This hierarchical porosity, the photoactivity of the
titanium anatase and the intrinsic properties of phosphorus
dendrimers are likely to provide synergetic materials with
16 C. Sanchez, J. Livage, M. Henry and F. Babonneau, J. Non-Cryst.
Solids, 1988, 100, 65–76; A. Leaustic, F. Babonneau and J. Livage,
´
Chem. Mater., 1989, 1, 240–252; A. El Kadib, K. Molvinger,
C. Guimon, F. Quignard and D. Brunel, Chem. Mater., 2008, 20,
2198–2204.
17 C. O. Turrin, V. Maraval, J. Leclaire, E. Dantras, C. Lacabanne,
A. M. Caminade and J. P. Majoral, Tetrahedron, 2003, 59,
3965–3973.
18 X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891–2959.
19 A. Primo, A. Corma and H. Garcia, Phys. Chem. Chem. Phys.,
2011, 13, 886–910.
c
8628 Chem. Commun., 2011, 47, 8626–8628
This journal is The Royal Society of Chemistry 2011