Solar Energy Materials & Solar Cells 83 (2004) 421–433 Photocatalytic and electronic properties of TiO 2 powders elaborated by sol–gel route and supercritical drying Souhir Boujday a , Frank W . unsch b , Patrick Portes a , Jean-Fran@ois Bocquet a , Christophe Colbeau-Justin a, * a Laboratoire d’Ing ! enierie des Mat ! eriaux et des Hautes Pressions, CNRS-UPR 1311, Universit ! e Paris 13, 99 avenue J. B. Cl ! ement, F-93430 Villetaneuse, France b Hahn-Meitner-Institut, Abteilung Solare Energetik, Glienicker Strasse 100, D-14109 Berlin, Germany Received 16 June 2003; received in revised form 12 January 2004; accepted 2 February 2004 Abstract Nanosized TiO 2 powders for photocatalytic degradation of organic pollutants are prepared by sol–gel method. The –Ti–O–Ti– network is synthesized by hydrolysis and controlled condensation of titanium isopropoxide Ti(O–iC 3 H 7 ) 4 . The resulting alcogels are dried under air to form xerogels or under supercritical CO 2 to form aerogels. In both cases, drying is followed by thermal treatment under air and gives crystallized white and dry powders. These powders are characterized by XRD, SEM, and specific surface area measurements. Their electronic properties are determined through Time Resolved Microwave Conductivity. Their photocatalytic activities are tested in the photodegradation of phenol in water. Results establish a strong correlation between synthesis, structure, charge-carrier lifetimes and photocatalytic activity. The influence of the supercritical drying on the final properties of materials is also evidenced. r 2004 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; Sol–gel method; Supercritical drying; Titanium dioxide; Wastewater treatment 1. Introduction The photocatalytic splitting of water over TiO 2 electrodes was described in 1972 by Fujishima and Honda [1]. This activity was at first considered harmful as it was ARTICLE IN PRESS *Corresponding author. E-mail address: [email protected] (C. Colbeau-Justin). 0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2004.02.035
Transcript
Solar Energy Materials & Solar Cells 83 (2004) 421–433
Photocatalytic and electronic properties of TiO2powders elaborated by sol–gel route and
supercritical drying
Souhir Boujdaya, Frank W .unschb, Patrick Portesa,Jean-Fran@ois Bocqueta, Christophe Colbeau-Justina,*
aLaboratoire d’Ing!enierie des Mat!eriaux et des Hautes Pressions, CNRS-UPR 1311, Universit!e Paris 13,
99 avenue J. B. Cl!ement, F-93430 Villetaneuse, FrancebHahn-Meitner-Institut, Abteilung Solare Energetik, Glienicker Strasse 100, D-14109 Berlin, Germany
Received 16 June 2003; received in revised form 12 January 2004; accepted 2 February 2004
Abstract
Nanosized TiO2 powders for photocatalytic degradation of organic pollutants are prepared
by sol–gel method. The –Ti–O–Ti– network is synthesized by hydrolysis and controlled
condensation of titanium isopropoxide Ti(O–iC3H7)4. The resulting alcogels are dried under
air to form xerogels or under supercritical CO2 to form aerogels. In both cases, drying is
followed by thermal treatment under air and gives crystallized white and dry powders.
These powders are characterized by XRD, SEM, and specific surface area measurements.
Their electronic properties are determined through Time Resolved Microwave Conductivity.
Their photocatalytic activities are tested in the photodegradation of phenol in water.
Results establish a strong correlation between synthesis, structure, charge-carrier lifetimes
and photocatalytic activity. The influence of the supercritical drying on the final properties of
The photocatalytic splitting of water over TiO2 electrodes was described in 1972by Fujishima and Honda [1]. This activity was at first considered harmful as it was
0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2004.02.035
the cause of the degradation of polymeric matrix in which the pigment wasincorporated. Thus, the first studies on TiO2 activity were to obtain its inhibition [2].Ever since, growing attention has been paid to photocatalytic activities [3–5]. Themain application of photocatalysis is environmental and deals of photodegradationof organic pollutants in water and air [6–10]. Thus, the general subject of the studieson photocatalysis is the characterization of the activity (kinetic parameters,degradation mechanisms) of commercial powders over various organic solutes(benzoic acid, salicylic acid, phenol, chlorophenol, acetone, etc.).More than water and air depollution applications, photocatalysis is widely used in
the self-cleaning of TiO2 covered surface [11,12]. In this field, significant initiativesare realized in Japan where some companies are selling TiO2 covered tiles which arekilling bacteria and sterilizing its surface, sets of tunnel lights that avoid theaccumulation of carbon and oil layers, filters to catch cigarette smoke, antifogmirrors, and glasses [13]. However, in spite of this economical importance of titania,its elaboration for photocatalysis is still poorly understood. The main factors thatseem to govern the photocatalytic activities of TiO2 powders are their structural andtheir electronic properties [14].Actually, TiO2 crystallizes mainly in two phases: anatase and rutile. Anatase is a
metastable form that converts to rutile at high temperatures. Both phases aresemiconductors with a band gap equal 3.23 eV for anatase and 3.1 eV for rutile.Under UV light illumination, the absorption of a photon with a higher energy
than the band gap creates an electron–hole pair (Fig. 1). If the charge-carriers do notrecombine, they can migrate to the surface where electrons are trapped by titaniumand holes by the superficial OH groups. Trapped holes form OH� radicals andtrapped electrons react with O2 and H2O to form HO2
� radicals [15]. These freeradicals are very oxidant entities and are causing the degradation of organiccompounds such as phenol.In a previous paper [16], we investigated the charge-carrier dynamics in TiO2
under UV illumination. We stressed the importance of several structural parameterssuch as crystalline quality, particle size and also the superficial OH groups on charge-carriers. These preliminary results suggest that an anatase type TiO2 of highcrystalline quality, with nanosized particles (10 to 100 nm) for a highly reactivesurface, generates a lot of charge-carriers with high lifetimes. This titania should thenbe the best photocatalyst.
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Fig. 1. Photocatalytical activity of TiO2.
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We have thus undertaken the present research to deepen these results, andspecially to correlate the structural, textural, and electronic properties of TiO2powders to their photocatalytic activities. We investigate various TiO2 powderssynthesized by sol–gel method followed by drying in supercritical CO2. Theproperties of these powders were also compared to those of other powderselaborated by classical sol–gel method.
2. Experimental
2.1. Material and procedure
TiO2 gels were prepared by acid-catalyzed sol–gel method in a non-aqueousmedium. The sols were prepared by adding an aqueous solution of acid to a solutionof titanium isopropoxide Ti(O–iC3H7)4 in anhydrous alcohol at room temperatureunder continuous stirring. Two series of gels were prepared (Table 1):
C-referenced series: For this series, the sols were prepared by adding a 12Msolution of hydrochloric acid (Prolabo, 37%) to a solution of titanium isopropoxideTi(O–iC3H7)4 (Acros, 98%) and isopropanol (Prolabo, 99.7%). The molecular ratioof Ti/alcohol/H2O/acid is 1/11/2/0.08.
N-referenced series: For these samples, we used a 2M solution of nitric acid(Prolabo, 68%) and we replaced isopropanol by ethanol (Prolabo, 99.9%). The ratioof Ti/alcohol/H2O/acid is 1/18/3/0.08. This procedure was already used by Daganet al. [17] with Ti/alcohol ratio of 20. We set this ratio to 18 to decrease the gellingtime.In each case, the mixture was kept under stirring for 10min. The gel is formed
upon 10min for the N-referenced series and upon 48 h for the C-referenced one.In order to observe any potential influence of the purity of titanium precursor on
the final properties, we followed the same procedure described previously using ahighly pure Ti(O–iC3H7)4 (Alfa Aesar, 99.999%).
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Table 1
Experimental conditions of gel synthesis and drying
Reference CA CX NA NX
Gel synthesis conditions (mol per mol of Ti(O–iC3H7)4)
Alcohol 11 11 18 18
isopropanol isopropanol ethanol ethanol
Water 2 2 3 3
Acid 0.08 0.08 0.08 0.08
hydrochloric hydrochloric nitric nitric
Drying conditions
Pressure 100 bar atmospheric 100 bar atmospheric
Temperature 50�C 70�C 50�C 70�C
Gas CO2 Air CO2 Air
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The formed gels were dried under supercritical conditions to form aerogels.Experimental drying conditions are displayed in Table 1. The alcohol contained inthe gel is replaced with liquid CO2 and then, the system is brought above the criticalpoint in temperature and pressure (>35�C and 70 bar). The supercritical dryingsetup is displayed on the Fig. 2. A second ‘‘classical’’ procedure of drying in an ovenat 70�C under air and atmospheric pressure was used. The resulting powders,xerogels, were compared to aerogels to observe the influence of drying.After drying, samples were heated to obtain anatase crystallized phases and also to
remove any residue of organic substances. Thermal treatment was carried out at450�C, 500�C, and 550�C. Treatment was 6 h ramping to final temperature and 10 hstage at this temperature. Beneath 450�C, thermal gravimetric analyses show thatorganic residues are not totally removed, the color of the powder is then gray. Above550�C, anatase phase begins to turn into rutile.Samples are referenced using two letters. The first is C or N whether hydrochloric
or nitric acid is used. The second, X or A, refers to the kind of drying, leading toxerogel or aerogel. The heating temperature of each sample is indicated at the end ofeach reference. The samples prepared starting from highly pure Ti(O–iC3H7)4 havethe same reference added by P indicating the purity.
2.2. Structural and textural characterization
XRD: X-Ray Diffraction measurements were performed with a Philips PW 1729diffractometer using Cu (Ka) radiation. Crystallite sizes of anatase powders wereestimated from Scherrer equation [18] using the X-ray diffraction peak at y=12.7�.
SEM: Morphology and size particles of powders were determined by the meaningof Scanning Electron Microscopy (LEO 440S, Leica, Cambridge).
BET: Adsorption–desorption measurements of N2 using COULTER SA 3100have been realized to determine surface area by multipoint Brunauer–Emmett–Tellermethod (BET) [19] and pore size distribution by Barrett–Joyner–Halenda method(BJH) [20].
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Fig. 2. Supercritical drying setup.
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2.3. Electronic properties characterization: TRMC
The charge-carrier lifetimes in TiO2 after UV illumination has been determined bymicrowave absorption experiments using Time Resolved Microwave Conductivity
method (TRMC) [16,21,22].The principle of TRMC and the experimental setup were widely described in a
previous paper [16]. This technique is based on the measurement of the change of themicrowave power reflected by a sample, DPðtÞ; induced by its laser-pulsedillumination. The relative difference DPðtÞ=P can be correlated, for smallperturbations of conductivity, to the difference of conductivity DsðtÞ consideringthe Eq. (1) [21]:
DPðtÞP
¼ ADsðtÞ ¼ AeX
i
DniðtÞmi; ð1Þ
where DniðtÞ is the number of excess charge carriers i at time t and mi their mobility.The sensitivity factor A is independent on time, but depends on the microwavefrequency and on the conductivity of the sample.Considering that the trapped species have a small mobility which can be neglected,
Dni is reduced to mobile electrons in the conduction band and holes in the valenceband. In the specific case of TiO2, the TRMC signal can be attributed to electronsbecause their mobility is much larger than that of the hole [23].The main data provided by TRMC are given by Imax; t1=2; 2t1=2; 3t1=2;
and I40 ns=Imax: The parameter Imax is the TRMC signal maximum value. Imaxreflects the number of the excess charge-carriers created by the UV pulse. It must benoted that this information is weighted by the mobility of the charge-carriers and theinfluence of charge-carrier decay processes during the excitation. As better charge-carrier transport properties lead to a higher mobility and less decay during theexcitation, a higher value of Imax corresponds to better charge-carrier transportproperties. The parameters t1=2; 2t1=2 and 3t1=2 are the half-times of signalcorresponding to the time necessary to reduce the intensity of the signal, respectively,to Imax=2; Imax=4; and Imax=8: These three parameters are important because thesignal decay is not purely exponential, thus, the general decay shape is characterizedby several half-time lives linked to charge-carrier lifetimes. As recombinationphenomena occur mainly between 0 and 40 ns after the pulse [16], the ratio of theintensity of the signal 40 ns after the beginning of the pulse by Imax notifies on thespeed of the recombination processes: A high I40 ns=Imax indicates a low speedrecombination.TRMC measurements were carried out in the Hahn–Meitner–Institut, Berlin. The
incident microwaves are generated by a Gunn diode of the Ka band (28–38GHz).The experiments were performed at 31.4GHz. Pulsed light source is a Nd:YAG laserproviding an IR radiation at l ¼ 1064 nm. Full-width at half-maximum (FWHM) ofone pulse is 10 ns; repetition frequency of the pulses is 10Hz. UV light (355 nm) isobtained by tripling the IR radiation. The light energy density received by the sampleis 1.3mJ cm�2.
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2.4. Photocatalytic properties
The photocatalytic activities of the synthesized TiO2 powders were tested in thereaction of photodegradation of phenol in water. Photocatalytic tests were carriedout in the static mode in a batch reactor. 400mg of photocatalyst were added to400ml of a 50mg l�1 phenol solution (Aldrich, 99%). The suspension was kept undervigorous stirring with oxygen bubbling. The mixture was then illuminated by amercury lamp (Philips HPK 125W, 8.3� 10�6 Einstein s�1). This lamp was placed ina cooling jacket dipped in the reaction mixture to set the temperature constant(25�C). The reaction progress was followed by systematic sampling (10min interval).The taken samples were filtered through 0.22 mm pore size Millipore filter and theconcentration of phenol measured by UV-Visible spectroscopy.The UV-Visible spectra of the filtered reaction mixture were recorded in the
transmission mode on a Cary UV 300—Varian spectrometer using distilled water asreference. The scan range was from 900 to 190 nm with a 1.0 nm interval, and theaveraging time at each point was 0.1 s.
3. Results
3.1. Structural and textural characterization
After drying and before any thermal treatment, all the powders are amorphousand exhibit a high specific surface area. The structural parameter of the preparedTiO2 powders after thermal treatment are reported in Table 2. The 10 compoundsare referenced, as described previously, according to the synthesis method (C or N),procedure of drying (A or X), precursor purity (P), and thermal treatment (450, 500,and 550).
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Table 2
Structural and textural properties of TiO2 powders
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After heating at 450�C, all the samples have an anatase structure-type. Thecrystallite size increases with calcination temperature while specific surface areadecreases. However, there are no big differences between the two kinds of gels(C and N) even though the C crystallites are slightly smaller. Nevertheless, theprocedure of drying influences considerably the crystallite size: aerogel crystallitesare distinctly smaller than xerogel ones. This effect is also observed on specificsurface areas which are much higher for aerogel than for xerogel. Moreover, thenature of the acid and alcohol used to prepare the gels modifies the specific surfacearea: the C samples exhibits higher surfaces than the N ones.The SEM pictures of CA550 and CX550 at two scales (1 mm and 200 nm) are
depicted on Fig. 3. The pictures of all the aerogels are quite similar to that of CA550.The second xerogel, NX550 exhibits also the same pictures than CX550.For both aerogels and xerogels, we observe 10 mm clusters. However, a zoom on
the aerogels shows that the clusters are constituted of 20 nm spherical particles,whereas, for the xerogels, the clusters are densely built-up.
3.2. Electronic properties characterization: TRMC
Fig. 4 shows TRMC experiments (0 to 200 ns) on TiO2 powders obtained from C(a) and N (b) gels. The values Imax; I40 ns=Imax; t1=2; 2t1=2; and 3t1=2 are reported onTable 3.
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Fig. 3. SEM pictures of CA550 (a, c) and CX550 (b, d).
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For the two series of powders, thermal treatment enhances the number of excesscharge-carriers and slows down their recombination.On the C-samples, the pulse generates the creation of more excess charge-carriers
than on the N-samples, yet on the former, charge-carrier lifetimes are higher than forthe C-samples.The use of highly pure titanium precursor influences lightly the electronic
properties of the powders. For the P-samples, the created excess charge-carriers afterthe pulse are lower in number and in lifetimes than the corresponding samples in thetwo series of gel.The influence of the supercritical drying on the electronic properties is more
perceptible and differs for the two kinds of gel. Thus, for the C-samples, the createdexcess charge-carriers on the xerogel are larger than in aerogel and their lifetimes arehigher. Whereas for the N-samples, the supercritical drying enlarges the number ofthe created excess charge-carriers and increases their lifetimes.
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Fig. 4. TRMC experiments on TiO2 powders obtained from C (a) and N (b) powders.
Table 3
Electronic properties of TiO2 powders characterized by TRMC
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3.3. Photocatalytic properties
The percentage of the photodegraded phenol as a function of reaction time isshown on Fig. 5 (a) for C-samples and (b) for N-samples.We reported on Table 4 the percentage of photodegraded phenol on TiO2 powders
at the end of the reaction. For the two series of powders, thermal treatment enhancesthe photocatalytic activities, the best percentage of phenol degradation are obtainedwhen TiO2 is heated to 550
�C. There are no big differences between the activities ofthe two kinds of gels when heated at 450�C, 500�C, and 550�C. Moreover, the use ofhighly pure titanium precursor does not influence significantly the photocatalyticbehavior of TiO2 powders. Finally, the drying procedure of the gels seems to be themajor factor. The supercritical gels degrade two and ten times more phenol,respectively, for the C and N-referenced powders, than the corresponding xerogels.
4. Discussion
In order to get a better understanding of the correlation between the structural,textural and electronic properties of TiO2 powders and their photocatalytic activities,we considered the two linked parts of the photocatalysis mechanism (photo andcatalysis) separately. The first part (photo part) concerns phenomena linked to light–material interaction which includes photons absorption, charge-carrier creation anddynamics, and also surface trapping. The second part (catalysis part), concernsphenomena linked to surface radicals formation and surface reactivity, i.e. theinteraction between H2O, O2, organic pollutant and the oxide surface.
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Fig. 5. Evolution of the percentage of photodegraded phenol as a function of reaction time obtained from
C (a) and N (b) powders.
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For the photo part, the most effective structural parameter on photocatalysis is thecrystalline quality [16]. Actually, as shown on Fig. 1, the oxidant radicals, which arethe active species in photocatalysis, are formed when the charge-carriers created byabsorbed UV-photons are trapped in the surface. Thus, recombination and bulktrapping phenomena that decrease charge-carrier lifetimes and prevent their arrivalto the surface penalize the formation of the oxidant radical. Yet, recombination andbulk trapping are promoted by defects, doping elements, and impurities oramorphous domains. Consequently, to enhance the charge-carriers lifetime, thecrystalline quality should be as high as possible. Thus, for titania, the TRMCmeasurements can be considered an indicator of the level of crystalline quality. Highvalues of Imax and slow decay indicate an important amount of charge-carrierscreated with long lifetimes and reveal a high crystalline quality.For the catalysis part, the specific surface area is the most effective structural
parameter. Indeed, photocatalysis is an interfacial reaction. Thus, higher specificsurface area induces higher number of accessible active sites and consequently betterreactivity.In the following sections, we checked the influence of some synthesis parameters
on these two structural parameters and observed the resulting influence onphotocatalysis.
4.1. Influence of the thermal treatment
The dried gels were calcinated at 450�C, 500�C and 550�C to examine theinfluence of the thermal treatment. This range was chosen because at lowertemperatures we cannot avoid the presence of organic residues, and at highertemperatures, we make a start on the crystallite transformation from anatase torutile form, whose photocatalytic activity is lower than anatase [24].When the powders are heated at 450�C, their structure is anatase type and the
diameter of the particles is 10 and 13 nm, respectively, for the two series of gel, CAand NA. The specific surface areas corresponding to these two series are equal to 112and 98m2 g�1. Indeed, when these samples are heated at a higher temperature, theirparticle diameter increases and their specific surface areas decrease as shown in Table2. Consequently, the thermal treatment go slightly against the catalysis part in thephotocatalysis mechanism.Meanwhile, TRMC measurements indicate the enhancement of the electronic
properties when the samples are heated at higher temperatures. Indeed, for the twoseries of gels, the highest amount of created charge-carriers with the longest lifetimes
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Table 4
Percentage of photodegraded phenol on TiO2 powders after 75min
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is observed upon calcination at 550�C. Thus, oppositely to textural properties,electronic properties are promoted by thermal treatment indicating an improvementof the crystalline quality with heating temperature.The results of photocatalytic degradation of phenol show a better activity for
samples dried at higher temperatures. The photo aspect is therefore more influentialthan the catalysis one. Yet, we must notice that the change consequent tothermal treatment is less perceptible on the textural properties than to electronicones, which may explain the enhancement of photocatalytic activities by thermaltreatment.
4.2. Influence of the nature of the gel
The two series of studied gels, C- and N-referenced, differ in the parameters ofsynthesis: precursor concentration, hydrolysis rate, complexing agent, pH andsolvent. We compared the properties of the final powders in order to check theinfluence of theses parameters. Moreover, to discuss the influence of the precursorpurity, equivalent gels using highly pure titanium precursor were studied.After drying and thermal treatment, the obtained crystallized powders from the
CA- and NA- referenced samples have comparable structural and texturalproperties, despite the lower specific surface areas for the N-samples. As for theelectronic properties, NA-samples show slower recombination phenomena asso-ciated to longer charge-carrier lifetimes than CA-sample. Nevertheless, thephotocatalytic behavior of these two series is quite similar. This result may beexplained by balancing the electronic properties (which are better for the N-samples)with the specific surface (which is lower for NA-samples than CA-samples).Moreover, the following of the intermediate species formed during photodegrada-tion of phenol solutions, benzoquinone and hydroquinone, by UV–Visiblespectroscopy (Fig. 6), indicates that the photocatalysis mechanism is the samefor both powders [25]. Therefore, the nature of the gel after supercritical dryingand thermal treatment, even though influencing the structural and electronicproperties of the samples, has no perceptible influence on their photocatalyticproperties.The best photoactivities were obtained with the two kinds of gel, dried with
supercritical CO2 and thermally treated at 550�C. To observe the influence of the
precursor purity, equivalent gels with highly pure precursor were also studied. Theproperties of the P-samples are quite close to those of the corresponding samples.CAP550 exhibit a weakly smaller surface than CA550 and less and shorter charge-carrier lifetimes. This sample is also slightly less active than CA550. NAP550 has thesame surface as NA550, and quite the same number of excess charge-carriers createdwith shorter charge-carrier lifetimes; this last point has no great influence on theactivity which is slightly higher than NA550. Thus, the purity of the precursor clearlydoes not promote the photocatalytic activity of the powders.Important differences of structural, electronic and photocatalytic properties are
observed on CX500 and NX550 samples. It shows that the classical drying has not
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the same influence on the two kinds of gel. These differences will be discussed in thefollowing section.
4.3. Influence of the drying procedure
Comparatively for the other synthesis parameters, the drying procedure seems tobe the most influential on the final properties of TiO2 powders. Both, structural andphotocatalytic properties are dramatically modified by the drying method.Thus, the crystallite diameter of xerogels is perceptibly higher than that of aerogels
and the corresponding specific surface areas, as shown in Table 2, are very low. Inthe particular case of the N gels, a supercritical drying enhances the specific surfacearea 23 times. The catalysis aspect is therefore, clearly promoted by supercriticaldrying.Nevertheless, TRMC results vary in an opposite manner. The xerogels generate
upon UV absorption the creation of an important amount of charge-carriers withhigh lifetimes. In the case of the C-samples, the charge-carriers created on the xerogelare in a higher number and have longer lifetimes than those observed on thecorresponding aerogel. The supercritical drying is therefore slightly harmful to thephoto aspect.This weak decrease of the electronic properties does not go against the
improvement of photocatalytic properties by supercritical drying. At the end ofthe reaction, as shown on Table 4, the xerogels degraded two and ten times lessphenol than the corresponding aerogels, respectively, for the C and N samples.
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Fig. 6. Photocatalysis measurements on TiO2 powders. UV-visible spectra of phenol solutions after
various irradiation times on CA550 (a) and NA550 (b).
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5. Conclusion
The present work shows that the structural, electronic, and photocatalyticproperties of sol–gel made of titanium dioxide can be arranged by controlling theelementary steps of their preparation. The particular steps of drying and thermaltreatment of the photocatalysts are crucial on their final properties. We stressed theimprovement provided by supercritical drying on the specific surface area oftitanium dioxide. This parameter, which is highly significant in an interfacialreaction, such as photocatalysis, promotes the activity of titanium dioxide and canincrease it up to ten times.We have also shown that the photocatalytic activities of the studied aerogels are
closely correlated to their electronic and textural properties. This opens up thepossibility of modifying the surface state of titanium dioxide to improve its electronicproperties while keeping constant its specific surface area. This way will be moredeeply studied and discussed in a near-future publication.
References
[1] A. Fujishima, K. Honda, Nature 37 (1972) 238–241.
