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Materials Chemistry and Physics 106 (2007) 102–108

Synthesis of structured titanium dioxide from carbonaceous templatesPreparation of supported nanoscaled titania particles

Philippe Rodriguez a, Laurence Reinert a,∗, Marc Comet b,Julien Kighelman c, Herve Fuzellier a

a Laboratoire de Chimie Moleculaire et Environnement, ESIGEC, Savoie Technolac, Universite de Savoie,73376 Le Bourget du Lac Cedex, France

b Institut franco-allemand de recherches de Saint-Louis, BP 70034,68301, Saint Louis Cedex, Francec Laboratoire des Materiaux de Construction, Ecole Polytechnique, Federale de Lausanne,

Station postale no. 12, 1015 Lausanne, Switzerland

Received 30 April 2007; accepted 15 May 2007

bstract

Titanium dioxide assemblages of various shapes (plaques, tubes) were synthesized by adsorption of a TiCl solution in n-hexane on various

4

arbonaceous substrates. A following hydrolysis led to the conversion of the chlorides into the oxide and the decomposition of the substrates.anoscaled TiO2 particles were deposited on the whole surface of carbon fibers by a CVD process of respectively POCl3 and TiCl4, followed by

ontrolled heating treatments. Synthesized samples were characterized by X-ray diffraction, scanning electron microscopy and EDS microanalysis. 2007 Elsevier B.V. All rights reserved.

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eywords: Oxides; Chemical synthesis; Chemical vapor deposition; Electron m

. Introduction

Titanium dioxide is one of the most investigated oxide mate-ials owing to its technological importance. Originally, TiO2 wasidely used as white pigment for paints, drugs and cosmetics.ince the 1970s and the pioneering article of Fujishima [1] itas many important applications in areas such as, for exam-le, water purification [2], photocatalysis [3] or gas sensing [4].itania particles and films were synthesized by different meth-ds: sol–gel process [5–7], hydrothermal method [8], sputteringechnology [9], ultrasonic spray pyrolysis [10] and flame oxida-ion process [11]. Plenet et al. showed that the crystalline phasef TiO2 layer changes with the thickness of the deposited film7].

Porous TiO2 assemblages of various shapes were preparedsing different materials as templates. Tubules and fibrils were

btained using porous alumina membranes [12], a “coral-like”iO2 network was produced using a polymer gel templatingrocedure [13], a honeycomb structure was prepared using latex

∗ Corresponding author. Tel.: +33 4 79 75 81 22; fax: +33 4 79 75 86 74.E-mail address: laurence.reinert@univ-savoie.fr (L. Reinert).

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pheres as templates [14] and recently the synthesis of TiO2orous films by polyethylene glycol templating was reported15].

The use of carbonaceous materials as templates for the syn-hesis of titanium dioxide was also reported. TiO2 nanoparticlesere prepared by sol–gel precipitation on activated carbon [16],

ubes were synthesized using carbon nanotubes as supports17], and TiO2 microcoils were prepared from carbon micro-oil templates [18]. Wakayama et al. reported the synthesisf nanoporous TiO2 using activated carbon fibers as templateith supercritical fluids solvents by using a nanoscaled cast-

ng process [19]. As far as we know, the use of exfoliatedraphite as template for the synthesis of titanium dioxide wasot reported yet. Several works describing the use of celluloseor the synthesis of titanium dioxide were reported in the litera-ure: cellulose acetate filter membranes [20] or natural cellulosicubstances [21] were used as templates to produce porous TiO2eplicas.

When suspended particles are used for water purification,

heir resulting highly active surfaces are advantageous. How-ver, technical problems may arise from undesirable particlegglomeration and a post treatment for catalyst recycling andor the obtaining of a clean, powder-free water is needed. Tita-

istry

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ium oxide photocatalysts anchored or embedded onto largeurface areas supports constitute alternative materials that canvoid such recovering problems [22].

Different materials were used as supports for TiO2: zeo-ites [23], glass or glass fibers [24], different polymers [25],ilica [26] or ZrO2 particles [27]. Various carbonaceous mate-ials such as carbon nanotubes [28], carbon microspheres [29],arbon black [30], exfoliated graphite [31] and activated carbon32,33] were also used as supports for TiO2. The preparationf thin titania films on activated carbon fibers by moleculardsorption–deposition was reported by several authors [34,35].he modified carbon surface showed good adsorptive andatalytical properties. Oxidation-resistant TiO2-coated carbonbers were successfully prepared by a sol–gel method [36], car-on fiber electrodes coated with titanium oxide were used forlectrochemical applications [37]. Yamashita et al. were the firsto propose photocatalysis applications: titanium oxide supportedn activated carbon fibers exhibited higher photocatalytic reac-ivities towards water purification than TiO2 powdered catalystsrepared by conventional methods [38].

This study introduces the synthesis and characterization ofitanium oxide obtained by replica of various carbonaceous

aterials. Titania nanoscaled particles supported on carbonbers were also obtained by using a chemical vapor depositionethod. To the best of our knowledge, no study was reported

n the synthesis of TiO2 using exfoliated graphite as templatend on the synthesis of titanium dioxide particles anchored onarbon fibers by chemical vapor deposition.

. Experimental

The crystallinity of the samples was characterized by X-ray diffractionXRD). Patterns were recorded on an Inel XRG 3000 diffractometer using Cu�1 radiation (λ = 154 pm).

The morphology of the samples was observed by scanning electronicroscopy (SEM), using a Cambridge Stereoscan 440 microscope coupled

o a Kevex Energy Dispersive Spectrometer (EDS).Surface area of the samples (SBET) was measured using a Pyrex apparatus

onceived in our laboratory. Before the analysis, all the samples were degassedt a temperature of 250 ◦C for 2 h under vacuum. Surface areas were calculatedccording to the BET equation after successive nitrogen expansion–adsorptionycles.

. Results and discussion

.1. TiO2 prepared from carbonaceous materials: the

emplate process

In general, the template method is technically simple and con-ists of two principal steps: (i) deposition of a precursor by liquid

as

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able 1yntheses conditions and characterization of obtained TiO2

ubstrate [TiCl4] (mol L−1) Substrate elimination

T (◦C) t (h

xfoliated graphite 0.9 650 48arbon fibers 0.09 600 4ellulose fibers 0.09 500 15

and Physics 106 (2007) 102–108 103

r gaseous process on a templating material; (ii) elimination ofhe substrate by calcination. The obtained replica keeps the ini-ial template morphology. The heating rate during calcinationffects the surface area, a rapid temperature ramp is favorableo obtain a high-surface area and thermally stable material [19].

Three different carbonaceous substrates were used as sup-ort:

an exfoliated graphite (Papyex®, Carbone Industrie,SBET ∼ 16 m2 g−1)carbon fibers (Mitsubishi, SBET ∼ 8 m2 g−1)cellulose fibers (SBET ∼ 8 m2 g−1)

Substrates were immersed in titanium tetrachloride solutionsTiCl4, Prolabo, 99.5%) of various concentrations (Table 1) in-hexane (Riedel de Haen, >99.9%). This solvent was chosenecause of its low density (d = 0.655) and hydrophobic char-cteristics. These properties allow a good wettability of thearbon substrates. Moreover, the low boiling point of n-hexanebp = 69 ◦C) makes its elimination easy after adsorption of theitanium precursor. The conversion of the precursor into thexide and the decomposition of the substrate were achieved by aubsequent heating treatment at temperatures ranging from 500o 650 ◦C, depending on the nature of the substrate (Table 1).omplete elimination of the carbon substrate was very long in

he case of exfoliated graphite, i.e. 2 days, due to the morphol-gy of the substrate. Indeed, this latter is composed of a stackf micron sized carbon sheets.

All EDS analyses of the recovered immersed substrates revealhe presence of Ti, O and Cl on their surfaces. These elementsere attributed to the presence of a titanium oxichloride layerhich results from the reaction of TiCl4 with the adsorbed watern the substrate surface. After calcination at higher temper-tures, the absence of the chloride peak on all EDS spectraonfirms the complete conversion of the oxichloride into tita-ium dioxide.

The morphology of the synthesized oxide is very similar tohe morphology of the substrate used as support. Using exfo-iated graphite, TiO2 stacked sheets were obtained (Fig. 1a)hereas twisted hollowed fibers were formed with carbon or cel-

ulose fibers (Fig. 1b and c). A diameter contraction and a smallncrease of the surface area of the TiO2 fibers were observeduring substrate elimination. The surface area variation is prob-

bly due to the presence of heteroatoms on the surface of theubstrate, which act as activating sites.

During heating treatments at higher temperatures∼1000 ◦C), no collapse of the microscopic structure was

Anatase/rutile (from XRD) SBET (m2 g−1)

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bserved, which confirms the validity of this synthesis methodo obtain oxide replicas of a carbonaceous substrate. Theseeplicas present interesting mechanical properties which coulde taken into account in catalysis, when the recovering of aulverulent catalyst is sometimes difficult.

X-ray diffraction patterns of all synthesized materials areharacteristic of crystallized samples. Pure anatase was onlybtained using cellulose fibers as substrate (Table 1). As calci-

ation temperatures had to be higher for the elimination of bothther substrates, a mixture of rutile and anatase was obtained.or the TiO2 replica of cellulose, the presence of rutile was

ig. 1. scanning electron micrographs of TiO2 obtained using exfoliatedraphite (a), carbon fibers (b), and cellulose fibers (c) as template.

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nly detected by XRD after calcinations at 800 ◦C. This result isnteresting as the classical anatase/rutile transition temperatureor pure TiO2 ranges between 450 and 600 ◦C [39]. After calci-ation at 1000 ◦C, traces of anatase remain present. The surfacerea of the resulting oxide is very low (i.e. <1 m2 g−1), whichndicates a collapse of the micro structure.

The interest of cellulose compared to the other carbonaceousubstrates is the presence of the hydroxyl groups on the surfacef the fibers, which favor the interactions with TiCl4. More-ver, the supramolecular structure of this material allows a goodenetration of the reagent within the depth of the fibers. Forther carbonaceous substrates, only the surface of the substrates impregnated.

However, all fibrous material obtained remained friable. Inrder to take advantage of the good mechanical properties ofhe carbon supports, carbon fibers-TiO2 composites were syn-hesized, using a CVD technique. Carbon fibers were chosen asupport as they can easily be activated. Moreover, they are easiero handle than exfoliated graphite.

.2. Synthesis of nanoscaled TiO2 particles supported onarbon fibers by CVD

The reactor for CVD syntheses was composed of a refrac-ory tube inserted into a tubular furnace. At the end of theeactor, calcium chloride and silica gel flasks avoided rehumid-fying. A protection and a water flask completed the systemn order to neutralize reaction gas by-products. The materialsed as support was a rough carbon felt (Mitsubishi, fibersiameter ∼10 �m, SBET = 8 m2 g−1) which was cut at the reac-or diameter (∅ = 45 mm, thickness = 10 mm, weight = 1.5–2 g).eagents were introduced into the reactor in an alumina cruciblehich was placed in a temperature zone closed to their boil-

ng temperature. Titanium tetrachloride (TiCl4, Prolabo 99.5%,p = 136 ◦C) was used as titanium oxide precursor and phospho-

◦

ous oxichloride (POCl3, Acros 99.9%, bp = 107 C) was useds coating reagent. Argon carried reagents into the reactor (rateow: 250 mL min−1).

Two synthesis routes for supported TiO2 were developed:

ig. 2. scanning electron micrograph of a non-adherent TiO2 film on a carbonber (TiCl4, Ar, 350 ◦C, 1 h; dry air, 400 ◦C, 3 h).

P. Rodriguez et al. / Materials Chemistry and Physics 106 (2007) 102–108 105

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Fig. 3. scanning electron micrographs and corresponding EDS analyses of

(I) Carbon fibers were first heated at 350 ◦C in 35 minunder argon. Respectively about 0.2 mL of phospho-rous oxichloride (Ar, 45 min) and then 0.5 mL oftitanium tetrachloride (Ar, 1 h) were introduced intothe reactor. Finally samples were dried under air(1 L min−1, 400 ◦C, 3 h) in order to convert chlorides intooxide.

II) Carbon fibers were activated at 350 ◦C during 7 h by an

air flow (1 L min−1) which first bubbled into water. Then0.5 mL of titanium tetrachloride were introduced into thereactor (Ar, 1 h). The conversion was achieved under dryair (1 L min−1, 400 ◦C, 3 h).

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Cl3 covered fiber (a, b) and supported TiO2 nanoscaled particles (c, d, e).

When TiCl4 was directly deposited on untreated carbon fibersy CVD (Ar, 350 ◦C for 1 h), an oxide layer of about 200 nmhickness and composed of agglomerated TiO2 particles wasormed after conversion (dry air, 1 L min−1, 400 ◦C for 3 h).owever, as observed by scanning electron microscopy (Fig. 2),

his layer was not well adherent to the support. It thus appearedssential to use a compound which would play the role of binderetween carbon fibers and titanium oxide particles. In order to

acilitate the covering of the fibers by TiCl4 and the furtherermination and growth of the TiO2 particles on the fibers sur-ace, fibers were first covered by a POCl3 layer. Indeed, manyhosphorous reactants (e.g. phosphorous acid) are usually used

106 P. Rodriguez et al. / Materials Chemistry and Physics 106 (2007) 102–108

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ig. 4. XRD pattern of a covered felt obtained at 400 ◦C (a) and after calcinationt 700 ◦C (dry air, 2 h) (b).

uring the activation process of carbon fibers. Moreover, POCl3,hich presents the advantage of being liquid at room tempera-

ure, was already used as oxidation inhibitor for carbon–carbonomposites [40].

.3. Synthesis route (I)

After deposition of POCl3 (Ar, 350 ◦C, 45 min), the sur-ace of the carbon fibers appears very rough (Fig. 3a), whichs characteristic of an activated surface. The presence of theovering phosphorous oxichloride film was confirmed by EDSnalyses (Fig. 3b). The scanning electron micrographs of theonverted samples (air, 400 ◦C, 3 h) submitted to both consecu-ive POCl3 and TiCl4 treatments show that the fibers are coveredy nanoscaled titanium dioxide particles spread all over their sur-ace (Fig. 3c–e). The average particles diameter ranges between00 and 200 nm.

However, scattered micron sized aggregates are sometimesresent. Nevertheless, both sides of the felt are equivalent.

As the distribution of the particles on the surface of the fiberss homogeneous, it can be assumed that POCl3 generates acti-ated sites. These sites probably facilitate the covering of thebers by TiCl4 and thus the local germination and growth of theiO2 particles. The measured surface area of the TiO2 coveredbers (150 m2 g−1) is higher than the surface area of raw car-on fibers, i.e. 8 m2 g−1, confirming of the activation of carbonubstrate during the different steps of the synthesis.

The X-ray diffraction pattern of the covered felt obtained at00 ◦C (Fig. 4a) is characteristic of a very low crystallized mate-ial. The two observed peaks were attributed to the main peaksf anatase (2Θ = 25.3◦) and titanium pyrophosphate (TiP2O7,Θ = 27.7◦). After complete elimination of the amorphous car-on substrate by calcination under air at 700 ◦C for 2 h, theecovered white powder is well crystallized (Fig. 4b). The result-ng material is composed of a mixture of mainly the anatase formf TiO2 and also titanium pyrophosphate. This latter compoundrobably results from the reaction between titanium tetrachlo-

ide or titanium oxide with the phosphorous oxichloride layer.his oxichloride was probably already present in the composite

reated at 400 ◦C as small intensity peaks attributed to P and Clere observed on its EDS spectrum.

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ig. 5. scanning electron micrographs of supported TiO2 particles obtained byynthesis route II (a, b) and corresponding EDS analysis (c).

However, titanium pyrophosphate seems interesting as it ishermally stable and presents catalytic properties. TiP2O7 wasor example used as catalyst for the dehydrogenation of n-butane41]. A TiP2O7 carbon composite showed catalytical propertiesowards the decomposition of 2-propanol [42].

.4. Synthesis route (II)

In order to avoid the formation of titanium pyrophosphate,he carbon felt was first activated at 350 ◦C during 7 h by anir flow (1 L min−1) which bubbled into water. The aim of this

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ig. 6. XRD pattern of a calcined sample obtained by synthesis route (II) (dryir, 700 ◦C, 2 h).

reliminary treatment was the generation of oxygenated activeites, which could be used for the grafting of TiO2. It was shownhat during such surface treatments three processes generallyccur: (i) cleaning of the surface by removal weakly adherentebris, (ii) modification of the surface morphology in order toncrease the “keying” effect and (iii) activation of the surface byormation of chemical groups [43]. This preliminary treatmentas followed by the deposition of titanium tetrachloride on the

arbon fibers (0.5 mL of TiCl4, Ar, 350 ◦C, 1 h). Chlorides werehen converted into oxide by a dry air flow at 400 ◦C for 3 h1 L min−1).

After complete treatment, homogeneous samples werebtained: the carbon fibers appeared to be entirely recoveredy a continuous titanium dioxide film (Fig. 5a and c). This films constituted by a multitude of nanoscaled particles of averageiameter about 100 nm (Fig. 5b).

The activation treatment increases the adsorption capacity ofarbon fibers and develops the superficial water layer. Titaniumetrachloride reacts with this adsorbed water to produce a tita-ium oxichloride layer (TiOCl2) which is converted into oxideuring the dry air treatment. The low intensity K�1 chlorineeak which is present on the EDS spectrum (Fig. 5c) indicates anncomplete conversion of chlorides into oxide. This was resolvedy a longer conversion time (4 h). The surface area of the TiO2overed felt is 250 m2 g−1. This high value is ascribable to thectivation step by water, which increases the surface area byodifying surface morphology.The X-ray diffraction pattern of the covered felt (not pre-

ented) characterizes an amorphous material. After calcinationt 700 ◦C for 2 h and complete elimination of the carbon sub-trate, both rutile and anatase forms are crystallized (Fig. 6). Thenatase/rutile ratio, determined according to the formula giveny Arroyo et al. [44] is about 60/40. This is not surprising as thenatase to rutile transition temperature for oxides synthesizedith mineral precursors is closed to 700 ◦C [45].

. Conclusions

The syntheses methods described in this paper are originals the morphology of TiO2 results from a chemical process thats governed by the surface properties of the carbonaceous mate-ials. A superficial water layer on the carbon fibers allowed

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and Physics 106 (2007) 102–108 107

he obtaining of TiO2 microtubes having a section of a fewicrometers. Other carbonaceous substrates led to the synthesis

f structured titanium dioxide with various shapes. After acti-ation by POCl3, the carbon fibers can be homogeneously andasily coated by submicron sized TiO2 particles.

Synthesized oxides and composites may find promisingpplications as photocatalysts or gas sensors. In addition to theirxcellent mechanical properties, composite materials shouldffer a high adsorption and retention potential towards aqueousollutants or gaseous substances.

These developed processes can be easily adapted to the syn-hesis of others oxides having interesting catalytical or sensingroperties, e.g. Al2O3, ZrO2, CeO2, etc.

cknowledgement

The authors would like to thank Olivier Romeyer for hiselpful assistance in the observations by scanning electronicroscopy.

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