Synthesis of para-amino benzoic acid-TiO2 hybrid nanostructures of controlled functionality by an aqueous one-step process

Article abstract of DOI:10.1002/ejic.200700971

In situ amino acid surface-modified TiO₂ nanoparticle syntheses were performed by a simple one-pot hydrolysis of heteroleptic titanium alkoxide [Ti(OiPr)₃(O₂CC₆H₄NH ₂)]_m in water with NⁿBu₄Br. This process allowed precise control of the surface grafting rate by varying the amount of precursors and provided highly functionalized nanomaterials. Their compositions and microstructures were determined by C, H and N elemental analyses, TGA-MS, ¹³C CP-MAS NMR, XRD, TEM, BET, Raleigh diffusion, FTIR, Raman, XPS and UV/Vis experiments. The results indicated that (i) the aggregation rate increased with an increase in the loading of the organic substrate and (ii) the amino acid is chemisorbed as a carboxylate group onto the TiO₂ nanoparticles, which leads to a strong interaction between the amino acid and the TiO₂ nanoparticle and good stability of these hybrids. Applications of low-aggregated nanomaterials were demonstrated as efficient protection additive against UVA + UVB radiations. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200700971

FULL PAPER
DOI: 10.1002/ejic.200700971
Synthesis of para-Amino Benzoic Acid–TiO2 Hybrid Nanostructures of
Controlled Functionality by an Aqueous One-Step Process
Raed Rahal,[a] Stéphane Daniele,*[a] Liliane G. Hubert-Pfalzgraf,[a][†]
Véronique Guyot-Ferréol,[b] and Jean-François Tranchant[b]
Keywords: Molecular precursor / Sol–gel / Nanoparticles / Hybrids / Titanium
In situ amino acid surface-modified TiO2 nanoparticle syn- gation rate increased with an increase in the loading of the
theses were performed by a simple one-pot hydrolysis of het-
eroleptic titanium alkoxide [Ti(OiPr)3(O2CC6H4NH2)]m in
water with NnBu4Br. This process allowed precise control of
the surface grafting rate by varying the amount of precursors
and provided highly functionalized nanomaterials. Their
compositions and microstructures were determined by C, H
and N elemental analyses, TGA-MS, 13C CP-MAS NMR,
XRD, TEM, BET, Raleigh diffusion, FTIR, Raman, XPS and
UV/Vis experiments. The results indicated that (i) the aggre-
organic substrate and (ii) the amino acid is chemisorbed as a
carboxylate group onto the TiO2 nanoparticles, which leads
to a strong interaction between the amino acid and the TiO2
nanoparticle and good stability of these hybrids. Applications
of low-aggregated nanomaterials were demonstrated as ef-
ficient protection additive against UVA + UVB radiations.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
and organic compounds (e.g., carboxylic acids,[4] phos-
phonic acids[5] or amino-based[6]) or by (ii) modification of
molecular precursors such as alkoxides by polymers (e.g.,
polyimide, polyether sulfone, etc.)[2f,2h,2i,7] and by β-dike-
tonates[8] and carboxylic,[9] phosphonic[10] and sulfonic ac-
ids.[11] The conventional post-functionalization method im-
plied the immersion of the nanoparticles or the films into
an organic or aqueous solution containing the organic moi-
eties. However, difficulties in getting well-dispersed nanoob-
jects and/or in filling efficiently the pores of nanosized ag-
gregates are encountered, which can limit the control of the
surface coverage. In addition, the pH of the aqueous solu-
tion hardly affected the adsorption, whereas heat treatment
(e.g., of TiO2 Ͼ 500 °C) noticeably lowered the adsorp-
tion.[12] In contrast, the synthesis of organic–inorganic hy-
brids from modified precursors by sol–gel processing usu-
ally led to amorphous as-prepared materials and required
acidic conditions to favour the ligand exchange for reorga-
nization to generate the crystallization of the inorganic
part. Sanchez et al. reported a one-step sol–gel method to
obtain surface-protected nanocrystalline zirconium, tita-
nium and tin dioxide particles in the presence of acetyl-
acetone and para-toluenesulfonic acid.[8] A nonaqueous, in
situ surface functionalization process, which requires the
addition of TiCl4 to a mixture of benzyl alcohol and dopa-
mine or 4-tert-butylcatechol, was also described by Nieder-
berger et al. to tailor the surface and the solubility proper-
ties of the hybrid titania nanoparticles.[13] However, under
acidic conditions variation of the amount of surface-ad-
sorbed species was possible only by changing the structure
of the precursor (increasing the complexant/metal ratio)
Introduction
Hybrid oxide (TiO2) nanostructured materials, which
combine the properties of inorganic and organic constitu-
ents, have a large range of uses in the fields of composite
materials, separation, catalysis, electrical and optical devices
or in the biomedical area.[1] The control of the aforemen-
tioned properties of the resulting nanomaterials depends
strongly on the control of parameters such as size, shape,
crystallinity, phase composition of the oxide nanoparticles,
type of bonding and interface between the organic and the
inorganic phases, the nature and the amount of the organic
component and finally the degree of dispersion of the
nanoobjects. Whereas elaborations of nanostructured TiO2
hybrid particles of controlled size, morphology and phase
are well documented in the literature, a synthesis method
that allows good control over the compositions and surface
chemistries still remains a challenge.[2]
The attachment of organic moieties onto transition-
metal oxide nanoparticles has been accomplished by either
(i) post-functionalization processing with the use of func-
tionalized polymers (e.g., polyaniline, polythiophene, etc.)[3]
[a] Université Claude Bernard-Lyon1, Institut de Recherches sur
la Catalyse et l’Environnement de LYON-UMR 5256,
69626 Villeurbanne cedex, France
Fax: +33-472-44-53-99
E-mail: stephane.daniele@ircelyon.univ-lyon1.fr
[b] LVMH Recherche, Parfums & Cosmétiques, Département In-
novation Matériaux et Technologies,
185 avenue de Verdun, 45804 Saint Jean de Braye, France
[†] Deceased.
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
980
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Eur. J. Inorg. Chem. 2008, 980–987

p-Amino Benzoic Acid–TiO2 Hybrid Nanostructures of Controlled Functionality
and the size of the nanoparticles (thus increasing the sur-
face-to-volume ratio).
Recently, we reported the aqueous synthesis of nanocrys-
talline particles of titania at low temperature (100 °C) by a
sol–gel process using titanium alkoxide and bromide am-
monium salts.[14] We then grafted iron phthalocyanine com-
plexes onto the in situ generated titanium dioxide nanopar-
ticles to prepare catalysts for an oxidation reaction.[11a]
Such experimental conditions allowed high loading of the
catalytic complex on the surface of the particles, along with
excellent activity in the oxidation of various substituted
phenols that compared well with the parent homogeneous
system. These results led us to investigate “ecosynthesis”
and controlled functionality processing of nanohybrid tita-
nia materials starting from modified alkoxides.[15]
Scheme 1. Syntheses of crystalline 1 and 2.
Hydrolysis of 1 was performed without any organic sol-
vent; the neat powder was added to deionized boiling water
(100 mL) containing the NnBu4Br salt (0.1 equiv.). The sus-
pension was heated under reflux for 2 h and then centri-
fuged. The as-prepared precipitate was washed with deion-
ized water and ethanol and then dried at 70 °C for ca. 12 h
to yield a yellow powder (denoted nTiO2/PABA-I to -VI).
Our interests lay in commercial sunscreen products that
are creams containing opaque inorganic oxides, especially
crystalline zinc oxide and/or titanium dioxide nanoparticles,
and organic reagents in order to adjust the sun protection
factor (SPF).[16]Among the more widely used organic selec-
tive agents are amino acids such as p-amino benzoic acid
(PABA), which absorbs UV-B radiation (290–320 nm) while
letting UV-A radiation (320–400 nm) pass through to pro-
duce a tan. However, the less energetic UV-A zone is not
quite innocuous, as it adds to the ability of UV-B rays to
inflict damage, and repeated exposure to UV radiation over
a longer period produces skin cancer, especially among fair-
haired, light-skinned people. Hence, for the new generation
of sunscreens, research is mainly focussing on developing a
broad spectrum of active ingredients that provide consistent
protection across all wavelengths, particularly in the UV-A
range. Herein, we report the aqueous in situ synthesis and
the characterization of inorganic–organic hybrid nanomat-
erials formed from crystalline TiO2 nanoparticles and para-
aminobenzoic acid. The effects of the amount of PABA an-
chored onto the TiO2 nanoparticles were also addressed in
terms of the interaction and optical properties of the nano-
particles.
Characterization of Hybrid Nanoparticles
The compositions and structures of the hybrid nanopar-
ticles were characterized by nitrogen elemental analysis,
thermogravimetric analysis (TGA), 13C CP-MAS NMR
spectroscopy, dynamic light scattering, X-ray powder dif-
fraction (XRD), transmission electron microscopy (TEM),
BET, Raman and UV/Vis spectroscopy. The organic load-
ing of each hybrid material was determined by ICP experi-
ments on nitrogen atoms and by thermogravimetric analy-
ses.
Interestingly, as shown in Figure 1, the organic content
could be easily controlled by hydrolyzing various amount of
1 (from 1 to about 6 g) and was demonstrated to be highly
reproducible (average standard deviation of nitrogen per-
centage = Ϯ0.01%). The use of higher quantities of precur-
sor (i.e., 6.77 g) resulted in ammonium contamination in
the final nTiO2/PABA-VI material, as confirmed by solid-
state NMR spectroscopic data, and indicated a threshold
for the amount of organic grafting. Other parameters such
as the number of equivalents of NR4Br (from 0.1 to
1 equiv.), the time of reflux (exceeding 2 h) and the volume
of washing solutions (H2O and ethanol, from 25 to 75 mL)
did not show any significant effect on the final organic con-
tent. It is noteworthy that the same procedure applied to
the glycine [Ti(OEt)3(O2CCH2NH2)]2 complex afforded
nanocrystalline TiO2 with a trace amount of amino acid.
Results and Discussion
Chemical Design and Nanoparticle Syntheses
The equimolar reaction between Ti(OiPr)4 and para-ami-
nobenzoic acid or benzoic acid in toluene at room tempera-
ture afforded [Ti(OiPr)3(O2CC6H4NH2)]m (1) and
[Ti6O(OiPr)16 (O2CC6H5)6]m (2), respectively (Scheme 1).
The difference in the formulation of 1 and 2 (presence of
oxido ligands) is attributed to the immediate precipitation
of 1 from the medium; thus, the hydrolysis reaction is avo-
ided as a result of an esterification reaction between the
acid and the alcohol generated by the substitution reaction.
Previous similar species such as [Ti(OEt)3(O2CCH2-
NH2)]2,[17] [Ti6(µ3-O)6(OCH2C(CH3)3)6(O2CR)6] (R = H,[18]
C6H5[19]) and [Ti6(µ3-O)6(OEt)6(O2CC6H4OC6H5)6][20] were
structurally identified.
Figure 1. Nitrogen percentage versus weight of modified precursor.
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This result showed that the aromatic moiety was necessary ford 0.4 g of nanohybrids], we treated TiO2 (0.4 g) with
to strongly attach the carboxylic group onto the surface PABA (0.68 g, 1 equiv.) in water (100 mL) at 100 °C for 2 h.
during this process.
The TGA results, according to the weight losses between
Thermogravimetric analyses, carried out in air up to 175 and 500 °C, showed lower organic contents and large
900 °C, exhibited weight loss within the temperature range data discrepancies between three crops of the same sample
of 20–500 °C. Samples nTiO2/PABA-I to -V showed only of the as-prepared (6.8Ϯ1.3%) or of the Hombikat UV 100
two weight losses (Figure 2). TGA coupled to mass spec- (7.3Ϯ1.1%) titania samples, whereas P25-degussa TiO2 was
trometry (TGA-MS) experiments confirmed that the first slightly functionalized (around 1%). The same procedure
step (20–175 °C) corresponds to the loss of residual solvent was applied to the as-prepared TiO2 samples in order to
(H2O, ethanol) and the further steps (175–500 °C) result synthesize nTiO2/PABA-V; a weight loss of 8.4Ϯ1.6% was
from the release and combustion of the organic groups obtained, which is in contrast to 20.8Ϯ0.4% obtained with
(–OH, –O2CC6H4NH2). Weight losses between 175 and our process. This comparative study showed that in a classi-
500 °C of our nTiO2/PABA-I to -V samples ranged from cal adsorption experiment the nature of the surface, the de-
about 7 to 21% of organics (TG average weight losses: 6.9, gree of dispersion and/or the specific surface area can dras-
10.3, 11.7, 14.8, 20.8%), and each sample exhibited highly tically influence the ability to modify the surface of the
reproducible data as a very interesting feature (TG average nanoparticles in a large range and with high precision. In
standard deviation = Ϯ0.3%).
contrast, our in situ process removed such drawbacks, and
it appeared to be an efficient functionalization-controlled
method for the synthesis of hybrid TiO2 nanoparticles that
could be used to adjust the organic/inorganic balance and
thus the combined properties of the hybrid.
The XRD patterns of each nTiO2/PABA sample were
similar and contained only anatase as the crystalline phase
(see Supporting Information). The average particle size, as
estimated from line-broadening according to the Scherrer
equation, was about 5 nm for all samples. The morphology
of the particles was examined by transmission electron mi-
croscopy (TEM) and dense, homogeneous, spheroidal
nanocrystallites of about 5–6 nm aggregated into submic-
ron-sized secondary particles were revealed (Figure 3A).
HRTEM images revealed the fringes from the continuous
lattice structure of a typical crystallite, which illustrates the
Figure 2. Thermogravimetric analyses of nTiO2/PABA samples.
Correlations between nitrogen ICP data and TGA weight high crystallinity of TiO2 even at 100 °C (Figure 3B). In
losses match only nanomaterials having a mean raw for- addition, no amorphous layer was observed on the surface
mula of TiOx(OH)y(O2CC6H4NH2)(solvent)w (solvent = of the nanoparticles and all diffraction peaks could be as-
H2O and/or ethanol), which indicates that hydroxy groups signed (Figure 3B,C). The results showed that the size, mor-
(–OH) are present on the surface (see Supporting Infor- phology, nature of the phase and crystallinity of the TiO2
mation). Under our experimental conditions, the ratio be- crystallite were the same for all the hybrid nanomaterials,
tween
surface
bound
O2CC6H4NH2
and
free and hence, the variation of the organic content was not due
HO2CC6H4NH2 varied strongly (from 0.018 to 0.135 for to the variation of the surface-to-volume ratio. One effect
nTiO2/PABA-I and -V, respectively), which indicated that of the higher amount of organic surface species was the
the reactions were not governed by a classical adsorption higher aggregation state of the nanocrystalline TiO2 par-
mechanism so that the carboxylate groups remained appar- ticles.
ently bonded to the titanium atom during the process. The
Figure 4 presents characteristic nitrogen adsorption–de-
solid-state 13C CP-MAS NMR spectra of nTiO2/PABA-I sorption isotherms for desorbed samples nTiO2/PABA-I
and -V exhibited a complex pattern with different chemical to -V (100 °C/6 h). The specific surface areas of the samples
shifts for the COO carbon atom (174 ppm) and the aro- decreased in the order nTiO2/PABA-I, -II, -III, -IV, -V (261,
matic carbon atoms [151 (C-NH2), 131, 118 and 113 ppm] 247, 232, 208, 149 m2 g–1, respectively) as a consequence of
(see Supporting Information).
the continuous shortening of the interparticulate distances
To compare with existing or standard methods, we car- (TiO2 obtained by the same procedure: 243 m2 g–1). The to-
ried out post-functionalization reactions between para- tal pore volume was 0.34, 0.28, 0.28, 0.23 and 0.22 cm3 g–1
amino benzoic acid and different nanoparticulate titania for samples nTiO2/PABA-I, -II, -III, -IV and -V, respec-
samples as-prepared from Ti(OiPr)4 (243 m2 g–1) and com- tively (determined by the BJH method). The isotherm, a
mercial P25-Degussa (50 m2 g–1) and Hombikat UV 100 type IV with a capillary hysteresis loop of type-H2 (accord-
ones (250 m2 g–1). For instance, under the same conditions ing to IUPAC terminology), attested that nTiO2/PABA-I
as those used in our process to obtain nTiO2/PABA-II with was a well-characterized mesoporous system with intercon-
10.3Ϯ0.2% loss of organics [in a typical experiment: 1.7 g nected particles and that it was different from TiO2 (less
of Ti(OiPr)3(PABA) and 100 mL of water were used to af- rigid texture). This self-assembly process gave an average
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p-Amino Benzoic Acid–TiO2 Hybrid Nanostructures of Controlled Functionality
nol suspensions, respectively, and we will mainly focus our
further discussion on these three representative nanohybrid
samples.
Characterization of the Chemical Interactions in TiO2
Hybrid Nanoparticles
To shed light on the chemical interactions between
PABA and the TiO2 nanoparticles, FTIR, Raman and X-
ray photoelectron spectroscopy (XPS) were carried out.
Interestingly, all of the hybrid nanoparticles were found to
be stable (no total leaching of the organic part, as con-
firmed by FTIR experiments) between ca. pH 2 (HNO3)
and 10 (NaOH) for at least 12 months. FTIR analysis of
each of the as-prepared nTiO2/PABA precipitates showed
absorption peaks for the ν(OH), ν(CO2) and ν(TiO) vi-
brations (see Supporting Information). The broad feature
between 3700 and 2500 cm–1 was due to the O–H stretching
vibrations of the hydroxy groups and adsorbed water and
ethanol molecules, whereas the band at 1620 cm–1 was as-
cribed to the bending vibrations of the adsorbed molecules.
It is worth noting that the C=O stretching band at
1665 cm–1 for the pure para-aminobenzoic acid is absent in
the spectra. New sharp bands at 1606, 1534 and at 1486,
Figure 3. (A) TEM, (B) HRTEM and (C) SAED images of nTiO2/
PABA-I.
pore diameter with a large distribution of about 4–7 nm.
Samples with a higher organic content (from nTiO2/PABA-
II to -V) possessed an unusual type-H4 loop hysteresis at
p/p0 between 0.4 and 0.5. Such hysteresis loops have already
been observed in samples having aggregates of plate-like
particles, which leads to a slit-shape structure,[21] and in
mesoporous aluminosilicate MCM-41 with void defects.[22]
These N2-sorption isotherm patterns reflect an extensively
interconnected porous network and a so-called “pore-
blocking effect”. This is due to the opening of many inter-
granular pores at the same pressure, and this happens only
for pore sizes lower than 4 nm. As a common feature, the
aggregation state increased with an increase in the surface
organic amount, which could be attributed to the increase
in the bond interactions between the para-aminobenzoic
groups of close particles (confirmed by TEM experiments).
In addition, particle size measurements, carried out by dy-
namic light scattering, displayed average aggregates sizes of
160, 240 and 2200 nm for nTiO2/PABA-I, -II and -V etha-
–
1406 cm–1 were attributed to asymmetric νas(CO2 ) and
–
symmetric νs(CO2 ) stretches, respectively, as a result of the
lower hydrolytic susceptibility of the carboxylate ligands
relative to that of the alkoxides. This reveals that para-ami-
nobenzoic acid is chemisorbed as a carboxylate onto the
surface of TiO2, and the value ∆ν = [νas(CO2) – νs(CO2)] of
around 120–130 cm–1 suggests a bidentate bridging/chelat-
ing coordination. This value agrees well with that found by
Finnie et al., who concluded that benzoic acid attached to
TiO2 films through bridging or bidentate coordination.[23]
The low wavenumber region exhibited a strong vibration
between 880 and 500 cm–1, which can be assigned to ν(Ti–
O–Ti) of the titanium oxide framework. Raman experi-
ments on various nTiO2/PABA samples displayed charac-
teristic anatase bands at 169, 400, 520 and 644 cm–1 besides
several bands of weak intensity corresponding to the PABA
moiety (see Supporting Information). The shift in the in-
tense Raman band at greater wavenumbers (169 cm–1) rela-
tive to that of TiO2 powder at 143 cm–1 was due to the as-
prepared nanosized TiO2.[24] Raman spectroscopy was less
informative than FTIR and, in particular, new bands corre-
sponding to the Ti–O–C(PABA) linkage could not be ob-
served under our experimental conditions.
The XPS spectrum of the C1s core level of the free para-
amino benzoic acid displayed four peaks at 284.6, 285.8,
288.6 and 290.7 eV, which are ascribed to the carbon atoms
in the C=C, C–NH2 and COOH bonds and to a shake-up
line, respectively (Figure 5a). Interestingly, this latter peak
due to πǞπ* transition of the aromatic ring disappeared in
the spectra of all nanohybrid nTiO2/PABA samples, which
confirms the absence of free acid on the TiO2 surface, even
for highly functionalized type V nanoparticles (Figure 5b).
We observed the same phenomenon in the XPS spectrum
Figure 4. Nitrogen adsorption–desorption isotherms of nTiO2/
PABA samples (sample III was omitted for clarity).
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S. Daniele et al.
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of the C1s core level of the sodium para-aminobenzoate
salt. Such data are very scarce in the literature, and this is,
to the best of our knowledge, the first example in which
C1s shake-up lines are used to investigate the manner in
which the carboxylate molecules interact with the TiO2 sur-
face. In addition, the O1s XPS spectrum obtained from the
nTiO2/PABA samples showed one single peak at 531.3 eV
(besides the TiO2 peak at 529.9 eV) instead of two non-
equivalent peaks at 531.8 and 532.2 eV for the free para-
aminobenzoic acid (Figure 5a). These results also confirm
that the carboxylate group is bound symmetrically through
its two oxygen atoms onto the TiO2 surface.
Figure 6. UV/Vis absorbance spectra of (a) p-amino benzoic acid,
(b) TiO2 (Degussa P25) and (c) nTiO2/PABA-I.
diffuse reflectance spectra of Ti-MCM-41 materials was
broadened and shifted to higher wavelengths as a result of
the inserted NH3 molecules and the coordination change
of the TiIV sites from tetrahedral to octahedral.[27a] In our
samples containing a higher amount of amino acid, the ra-
tio between the six- and four-fold titanium sites increased,
which again confirmed the strong surface chemisorption of
PABA molecules (Figure 7).
Figure 7. UV/Vis absorbance spectra of the nTiO2/PABA samples.
Figure 5. XPS C1s and O1s core level spectra of (a) p-amino ben-
zoic acid and (b) nTiO2/PABA-V.
UV/Vis absorbance spectra of TiO2 (Degussa P25), para-
amino benzoic acid and nTiO2/PABA-I are given in Fig-
ure 6. The same spectra were observed for their ethanol sus-
pensions. Measurements on the sample show that the
Applications of TiO2 Hybrid Nanoparticles: Solar
Protection
Preliminary results of solar protection evaluation were
bandgap absorption onset of the nTiO2/PABA nanocrystals obtained with 300-µm thick films by using the same mass of
is redshifted from 380 to 650 nm relative to TiO2 with an hybrid nanoparticles. The nTiO2/PABA-I sample displayed
efficient and interesting protection in the UV-A domain uniform and grain-free films with low intensity of trans-
(Figure 6). The large change in the PABA absorption spec- mitted light. Such a nanomaterial appeared efficient to
trum in the presence of TiO2 colloids implied strong bind- block out all the UV radiation at sea level, and it also
ing of the PABA onto the TiO2 surface.[25] Such a material, seemed promising for cosmetic applications (lowest amount
which is based on dye-sensitized nanostructured semi- of organic UV filter).[28] The aggregation effect, that is, for
conductors (i.e., ZnO, SnO2 and TiO2), was intensively in- the PABA/TiO2-V sample, led to inhomogeneous films and
vestigated as a promising photoelectrochemical solar cell to an increase in the intensity of transmitted light in the
for low-cost solar energy conversion.[4a,26]
UV-A radiation zone (Figure 8). These results demon-
The spectra of nTiO2/PABA samples can be deconvol- strated that precise control of the organic loading was an
uted into two separate peaks. The lower wavelength peak important parameter for adjusting the performance of
(220–245 nm) can usually be assigned to the tetrahedral co- nanohybrid titania for UVA solar protection applications.
ordination of titanium in the framework, whereas the The use of such materials in the formulation of cosmetics
higher wavelength peak (Ͼ300 nm) is typical of five- and is in progress to display their efficacy as UVA and UVB
six-coordinate Ti atoms.[27] Gianotti et al. showed that the sunscreen actives.[29]
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p-Amino Benzoic Acid–TiO2 Hybrid Nanostructures of Controlled Functionality
Powder X-ray diffraction data were obtained with a Siemens D
5000 diffractometer by using Cu-Kα radiation at the scan rate of
0.02°2θs–1 with step times of 1 and 16 s for routine or for expanded
region spectra, respectively. XPS experiments were performed with
a KRATOS spectrometer by using monochromated Al-Kα radia-
tion as the excitation source. TEM images were collected with a
JEOL JEM-2010 electron microscope. BET experiments were car-
ried out with an ASAP 2010 system after the samples were de-
sorbed at 100 °C for 6 h. Raman experiments were carried out with
LabRam HR equipment (Jobin Yvon). 1H liquid and 13C CP-MAS
solid-state NMR spectra were recorded with Avance 300 and Av-
ance DSX 400 Bruker spectrometers (5 and 10 KHz), respectively.
Dynamic light scattering experiments were carried out with a
Coulter N4 Plus (20 °C, ethanol, 90°). C, H and N analytical data
were obtained from the Service Central d’Analyses du CNRS (Ver-
naison, France) by ICP.
Figure 8. UV transmission through nanohybrid films.
Conclusions
1: To a solution of Ti(OiPr)4 (2.5 mL, 8.4 mmol) in toluene (25 mL)
was added dropwise a solution of p-NH2C6H4CO2H (1.15 g,
8.4 mmol) in toluene (10 mL) at room temperature. A dark-yellow
precipitate appeared after 10 min, which was filtered and washed
with hexane. The yellow powder was dried under vacuum to quan-
titatively yield 2.97 g of [Ti(OiPr)3(O2CC6H4NH2)]m (1). Com-
pound 1 is poorly soluble in hydrocarbon and soluble in THF and
We demonstrated the efficiency of a one-pot relevant
synthetic route to elaborate hybrid nanostructure amino
acid-crystalline TiO2 nanoparticles of controlled function-
ality, starting from molecular heteroleptic alkoxides of tita-
nium. This method offers the convenience and economical
1
3
advantages of being environmentally friendly (organic-free 2-propanol. H NMR (300 MHz, CDCl3, 20 °C): δ = 7.83 (d, J =
3
8.7 Hz, 2 H, CHar), 6.55 (d, J = 8.7 Hz, 2 H, CHar), 4.82 (sept.,
solvent). The surface complexation rate could be adjusted
by simply varying the amount of precursors (up to about
21% of organics) and was not governed by values observed
at plateau in classical adsorption isotherms. Chemical and
spectroscopic data were consistent with strong chemisorp-
tion of the amino acid molecules onto the TiO2 nanopar-
ticles surface through bidentate chelate or bridging coordi-
nation. Interestingly, the disappearance of the C1s shake-
up line due to the πǞπ* transition of the aromatic ring in
3
3J = 5.9 Hz, 1 H, CH), 4.46 (sept, J = 5.9 Hz, 2 H, CH), 4.06 (s,
3
2 H, NH2), 1.19 (d, J = 5.9 Hz, 18 H, CH3) ppm. FTIR (nujol):
ν = 3493 (w), 3474 (m), 3394 (m), 3343 (m), 3221 [w, ν(N–H)], 1633
˜
(m), 1621 (m), 1602 (m), 1583 (w), 1533 (w), 1495 (m), 1462 (s),
1443 (s), 1423 (s), 1409 (s), 1377 (s), 1360 [m, ν(C=O), ν(C=C)],
1327 (m), 1309 (m); 1178 (m), 1166 (m), 1133 (s), 1115 [s, ν(C–O),
ν(C–N)], 1016 (m), 989 (m), 949 (w), 933 (m), 873 (w), 853 (m),
825 (m), 788 (m), 706 (w); 645 (m), 624 (m), 596 (m), 559 (m), 506
(m), 476 [w, ν(Ti–O)] cm–1. C16H27O5NTi (360.67): calcd. C 53.20,
the XPS spectra of all nanohybrid nTiO2/PABA samples H 7.48, N 3.88; found C 53.02, H 7.42, N 3.86.
confirmed the absence of free acid on the TiO2 surface, even
for highly functionalized nanoparticles. Observed trends in
the physicochemical properties of such hybrid nanomateri-
als were the increase in the aggregation state by increasing
the surface organic rate. N2-sorption isotherm patterns of
highly functionalized nanomaterials demonstrated an ex-
tensively interconnected porous network and a so-called
“pore-blocking effect”. Low aggregated nanomaterials have
shown promising properties as candidate for UVA + UVB
sunscreen additives. As perspectives, these results provided
a simple and successful pathway for the fabrication of TiO2
nanohybrids with mono- or polyfunctionalities by choosing
the appropriate organic group to tailor their chemical and/
or physical properties.
2: C6H5CO2H (1.03 g, 8.4 mmol) in toluene (10 mL) was added
dropwise to a solution of Ti(OiPr)4 (2.5 mL, 8.4 mmol) in toluene
(25 mL). After 12 h of stirring at room temperature, the medium
was dried under vacuum. The resulting powder was recrystallized
from a minimum amount of hexane at –25 °C to afford 1.7 g (62%)
of [Ti6O(O2CC6H5)6(OiPr)16]m (2). Compound 2 is highly soluble
in common solvents. 1H NMR (300 MHz, CDCl3, 20 °C): δ = 8.12
(m, 12 H, CHar), 7.45–7.36 (m, 18 H, CHar), 5.25–4.86 (m, 16 H,
3
3
CH), 1.43 (d, J = 6.0 Hz, 24 H, CH3), 1.20 (d, J = 6.0 Hz, 72 H,
CH ) ppm. FTIR (nujol): ν = 1595 (w), 1597 (m), 1556 (s), 1522
˜
3
(m), 1492 (w), 1462 (m), 1448 (m), 1383 [s, ν(C=O), ν(C=C)], 1332
(w), 1323 (w); 1162 (m), 1130 (s), 1069 [w, ν(C–O), ν(C–N)], 1023
(m), 1010 (s), 990 (s), 960 (w), 853 (m), 837 (w), 718 (s), 700 (m),
674 (m); 625 (s), 601 (m), 554 (w), 480 [m, ν(Ti–O)] cm–1.
C90H142O29Ti6 (1972.02): calcd. C 54.73, H 7.20; found C 54.53,
H, 7.22.
Hydrolysis of 1: In a typical synthesis, to a solution of N(nBu)4-
Br heated at reflux in water (100 mL) was added neat [Ti(OiPr)3-
(O2CC6H4NH2)]m [1; Ti/N(nBu)4Br = 10] under vigorous stirring;
an immediate yellow precipitate appeared. The suspension was
heated at reflux for 2 h and then centrifuged to yield a yellow solid.
The as-prepared precipitate (denoted nTiO2/PABA) was washed
with deionized water (2ϫ25 mL) and ethanol (50 mL) and then
dried at 70 °C for ca. 12 h. The aqueous solution was recovered
Experimental Section
Ti(OiPr)4 was distilled under vacuum and manipulated under an
atmosphere of dry argon by using Schlenk tube techniques. para-
Aminobenzoic acid and ammonium salts (NnBu4Br) were used as
purchased. FTIR spectra were recorded as nujol suspensions (for
1) or as KBr pellets (for hydrolyzed powders) with a Perkin–Elmer
Paragon 500 FTIR spectrometer. TGA/TDA data were collected
and used again for further syntheses. FTIR (KBr): ν = 3393 [m,
˜
ν(OH)], 1620 [m, δ(OH)], 1606 (m), 1534 (w), 1486 (m), 1406 [s,
νs(COO), νas(COO), ν(C=C)], 1294 (m), 1182 [m, ν(C–O), ν(C–N)],
with a Setaram 92 system in air with a thermal ramp of 5 °Cmin–1. 880–500 [s, ν(Ti–O)] cm–1.
Eur. J. Inorg. Chem. 2008, 980–987
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
985

S. Daniele et al.
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Published Online: January 29, 2008
Eur. J. Inorg. Chem. 2008, 980–987
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
987