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Photocatalytic and electronic properties of TiO2 powders elaborated by sol–gel route and supercritical drying
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Solar Energy Materials & Solar Cells 83 (2004) 421–433
Photocatalytic and electronic properties of TiO2 powders elaborated by sol–gel route and supercritical drying b Souhir Boujdaya, Frank Wunsch . , Patrick Portesa, Jean-Fran@ois Bocqueta, Christophe Colbeau-Justina,* 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 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 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 TiO2 electrodes was described in 1972 by Fujishima and Honda [1]. This activity was at first considered harmful as it was *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
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the cause of the degradation of polymeric matrix in which the pigment was incorporated. 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]. The main application of photocatalysis is environmental and deals of photodegradation of organic pollutants in water and air [6–10]. Thus, the general subject of the studies on 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 initiatives are realized in Japan where some companies are selling TiO2 covered tiles which are killing bacteria and sterilizing its surface, sets of tunnel lights that avoid the accumulation of carbon and oil layers, filters to catch cigarette smoke, antifog mirrors, and glasses [13]. However, in spite of this economical importance of titania, its elaboration for photocatalysis is still poorly understood. The main factors that seem to govern the photocatalytic activities of TiO2 powders are their structural and their 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 are semiconductors 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 not recombine, they can migrate to the surface where electrons are trapped by titanium and holes by the superficial OH groups. Trapped holes form OH radicals and trapped electrons react with O2 and H2O to form HO2 radicals [15]. These free radicals are very oxidant entities and are causing the degradation of organic compounds 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 parameters such as crystalline quality, particle size and also the superficial OH groups on chargecarriers. These preliminary results suggest that an anatase type TiO2 of high crystalline quality, with nanosized particles (10 to 100 nm) for a highly reactive surface, generates a lot of charge-carriers with high lifetimes. This titania should then be the best photocatalyst.
Fig. 1. Photocatalytical activity of TiO2.
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We have thus undertaken the present research to deepen these results, and specially to correlate the structural, textural, and electronic properties of TiO2 powders to their photocatalytic activities. We investigate various TiO2 powders synthesized by sol–gel method followed by drying in supercritical CO2. The properties of these powders were also compared to those of other powders elaborated 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-aqueous medium. The sols were prepared by adding an aqueous solution of acid to a solution of titanium isopropoxide Ti(O–iC3H7)4 in anhydrous alcohol at room temperature under continuous stirring. Two series of gels were prepared (Table 1): C-referenced series: For this series, the sols were prepared by adding a 12 M solution of hydrochloric acid (Prolabo, 37%) to a solution of titanium isopropoxide Ti(O–iC3H7)4 (Acros, 98%) and isopropanol (Prolabo, 99.7%). The molecular ratio of Ti/alcohol/H2O/acid is 1/11/2/0.08. N-referenced series: For these samples, we used a 2 M solution of nitric acid (Prolabo, 68%) and we replaced isopropanol by ethanol (Prolabo, 99.9%). The ratio of Ti/alcohol/H2O/acid is 1/18/3/0.08. This procedure was already used by Dagan et al. [17] with Ti/alcohol ratio of 20. We set this ratio to 18 to decrease the gelling time. In each case, the mixture was kept under stirring for 10 min. The gel is formed upon 10 min 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 a highly pure Ti(O–iC3H7)4 (Alfa Aesar, 99.999%).
Table 1 Experimental conditions of gel synthesis and drying Reference
CA
CX
Gel synthesis conditions (mol per mol of Ti(O–iC3H7)4) Alcohol 11 11 isopropanol isopropanol Water 2 2 Acid 0.08 0.08 hydrochloric hydrochloric Drying conditions Pressure 100 bar atmospheric Temperature 50 C 70 C Gas CO2 Air
NA
NX
18 ethanol 3 0.08 nitric
18 ethanol 3 0.08 nitric
100 bar 50 C CO2
atmospheric 70 C Air
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Fig. 2. Supercritical drying setup.
The formed gels were dried under supercritical conditions to form aerogels. Experimental drying conditions are displayed in Table 1. The alcohol contained in the gel is replaced with liquid CO2 and then, the system is brought above the critical point in temperature and pressure (>35 C and 70 bar). The supercritical drying setup is displayed on the Fig. 2. A second ‘‘classical’’ procedure of drying in an oven at 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 at 450 C, 500 C, and 550 C. Treatment was 6 h ramping to final temperature and 10 h stage at this temperature. Beneath 450 C, thermal gravimetric analyses show that organic residues are not totally removed, the color of the powder is then gray. Above 550 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 to xerogel or aerogel. The heating temperature of each sample is indicated at the end of each reference. The samples prepared starting from highly pure Ti(O–iC3H7)4 have the 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 1729 diffractometer using Cu (Ka) radiation. Crystallite sizes of anatase powders were estimated 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 meaning of Scanning Electron Microscopy (LEO 440S, Leica, Cambridge). BET: Adsorption–desorption measurements of N2 using COULTER SA 3100 have been realized to determine surface area by multipoint Brunauer–Emmett–Teller method (BET) [19] and pore size distribution by Barrett–Joyner–Halenda method (BJH) [20].
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2.3. Electronic properties characterization: TRMC The charge-carrier lifetimes in TiO2 after UV illumination has been determined by microwave 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 the microwave power reflected by a sample, DPðtÞ; induced by its laser-pulsed illumination. The relative difference DPðtÞ=P can be correlated, for small perturbations of conductivity, to the difference of conductivity DsðtÞ considering the Eq. (1) [21]: X DPðtÞ ¼ ADsðtÞ ¼ Ae Dni ðtÞmi ; P i
ð1Þ
where Dni ðtÞ 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 microwave frequency 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 valence band. In the specific case of TiO2, the TRMC signal can be attributed to electrons because 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. Imax reflects the number of the excess charge-carriers created by the UV pulse. It must be noted that this information is weighted by the mobility of the charge-carriers and the influence of charge-carrier decay processes during the excitation. As better chargecarrier transport properties lead to a higher mobility and less decay during the excitation, a higher value of Imax corresponds to better charge-carrier transport properties. The parameters t1=2 ; 2t1=2 and 3t1=2 are the half-times of signal corresponding 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 the signal decay is not purely exponential, thus, the general decay shape is characterized by several half-time lives linked to charge-carrier lifetimes. As recombination phenomena occur mainly between 0 and 40 ns after the pulse [16], the ratio of the intensity of the signal 40 ns after the beginning of the pulse by Imax notifies on the speed of the recombination processes: A high I40 ns =Imax indicates a low speed recombination. TRMC measurements were carried out in the Hahn–Meitner–Institut, Berlin. The incident microwaves are generated by a Gunn diode of the Ka band (28–38 GHz). The experiments were performed at 31.4 GHz. Pulsed light source is a Nd:YAG laser providing an IR radiation at l ¼ 1064 nm. Full-width at half-maximum (FWHM) of one pulse is 10 ns; repetition frequency of the pulses is 10 Hz. UV light (355 nm) is obtained by tripling the IR radiation. The light energy density received by the sample is 1.3 mJ cm2.
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2.4. Photocatalytic properties The photocatalytic activities of the synthesized TiO2 powders were tested in the reaction of photodegradation of phenol in water. Photocatalytic tests were carried out in the static mode in a batch reactor. 400 mg of photocatalyst were added to 400 ml of a 50 mg l1 phenol solution (Aldrich, 99%). The suspension was kept under vigorous stirring with oxygen bubbling. The mixture was then illuminated by a mercury lamp (Philips HPK 125 W, 8.3 106 Einstein s1). This lamp was placed in a cooling jacket dipped in the reaction mixture to set the temperature constant (25 C). The reaction progress was followed by systematic sampling (10 min interval). The taken samples were filtered through 0.22 mm pore size Millipore filter and the concentration 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 as reference. The scan range was from 900 to 190 nm with a 1.0 nm interval, and the averaging 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 amorphous and exhibit a high specific surface area. The structural parameter of the prepared TiO2 powders after thermal treatment are reported in Table 2. The 10 compounds are 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).
Table 2 Structural and textural properties of TiO2 powders Reference
CA450 CA500 CA550 CAP550 CX550 NA450 NA500 NA550 NAP550 NX550
Structure
Texture
Crystallite size (nm)
SBET(m2 g1)
Porous volume (ml g1)
10 16 23 — 32 13 17 21 — 38
112 106 82 71 46 98 81 63 60 3
0.546 0.541 0.526 0.319 0.128 0.418 0.373 0.262 0.251 —
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Fig. 3. SEM pictures of CA550 (a, c) and CX550 (b, d).
After heating at 450 C, all the samples have an anatase structure-type. The crystallite size increases with calcination temperature while specific surface area decreases. However, there are no big differences between the two kinds of gels (C and N) even though the C crystallites are slightly smaller. Nevertheless, the procedure of drying influences considerably the crystallite size: aerogel crystallites are distinctly smaller than xerogel ones. This effect is also observed on specific surface areas which are much higher for aerogel than for xerogel. Moreover, the nature of the acid and alcohol used to prepare the gels modifies the specific surface area: 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 on Table 3.
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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 References
Imax (mV)
I40 ns =Imax (%)
t1=2 (ns)
2t1=2 (ns)
3t1=2 (ns)
CA450 CA500 CA550 CAP550 CX550 NA450 NA500 NA550 NAP550 NX550
10.6 15.0 19.0 13.6 21.3 10.0 11.8 17.8 15.2 16.7
43 44 50 54 71 50 51 56 53 48
20 20 30 40 250 35 35 60 30 25
250 250 750 400 >104 1500 1000 3000 750 180
5000 4000 >104 2000 — >104 >104 >104 4000 1200
For the two series of powders, thermal treatment enhances the number of excess charge-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 for the 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 after the pulse are lower in number and in lifetimes than the corresponding samples in the two 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 created excess charge-carriers on the xerogel are larger than in aerogel and their lifetimes are higher. Whereas for the N-samples, the supercritical drying enlarges the number of the created excess charge-carriers and increases their lifetimes.
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3.3. Photocatalytic properties The percentage of the photodegraded phenol as a function of reaction time is shown 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 enhances the photocatalytic activities, the best percentage of phenol degradation are obtained when TiO2 is heated to 550 C. There are no big differences between the activities of the two kinds of gels when heated at 450 C, 500 C, and 550 C. Moreover, the use of highly pure titanium precursor does not influence significantly the photocatalytic behavior of TiO2 powders. Finally, the drying procedure of the gels seems to be the major 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 and catalysis) separately. The first part (photo part) concerns phenomena linked to light– material interaction which includes photons absorption, charge-carrier creation and dynamics, and also surface trapping. The second part (catalysis part), concerns phenomena linked to surface radicals formation and surface reactivity, i.e. the interaction between H2O, O2, organic pollutant and the oxide surface.
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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Table 4 Percentage of photodegraded phenol on TiO2 powders after 75 min Reference
CA450 CA500 CA550 CAP550 CX550 NA450 NA500 NA550 NAP550 NX550
% of degraded 64.5 phenol after 75 min
81.6
94.9
92.7
42.6
67.0
78.1
92.2
94.0
8.7
For the photo part, the most effective structural parameter on photocatalysis is the crystalline quality [16]. Actually, as shown on Fig. 1, the oxidant radicals, which are the active species in photocatalysis, are formed when the charge-carriers created by absorbed UV-photons are trapped in the surface. Thus, recombination and bulk trapping phenomena that decrease charge-carrier lifetimes and prevent their arrival to the surface penalize the formation of the oxidant radical. Yet, recombination and bulk trapping are promoted by defects, doping elements, and impurities or amorphous domains. Consequently, to enhance the charge-carriers lifetime, the crystalline quality should be as high as possible. Thus, for titania, the TRMC measurements can be considered an indicator of the level of crystalline quality. High values of Imax and slow decay indicate an important amount of charge-carriers created 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 specific surface area induces higher number of accessible active sites and consequently better reactivity. In the following sections, we checked the influence of some synthesis parameters on these two structural parameters and observed the resulting influence on photocatalysis. 4.1. Influence of the thermal treatment The dried gels were calcinated at 450 C, 500 C and 550 C to examine the influence of the thermal treatment. This range was chosen because at lower temperatures we cannot avoid the presence of organic residues, and at higher temperatures, we make a start on the crystallite transformation from anatase to rutile 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, CA and NA. The specific surface areas corresponding to these two series are equal to 112 and 98 m2 g1. Indeed, when these samples are heated at a higher temperature, their particle diameter increases and their specific surface areas decrease as shown in Table 2. Consequently, the thermal treatment go slightly against the catalysis part in the photocatalysis mechanism. Meanwhile, TRMC measurements indicate the enhancement of the electronic properties when the samples are heated at higher temperatures. Indeed, for the two series of gels, the highest amount of created charge-carriers with the longest lifetimes
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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 improvement of 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 influential than the catalysis one. Yet, we must notice that the change consequent to thermal treatment is less perceptible on the textural properties than to electronic ones, which may explain the enhancement of photocatalytic activities by thermal treatment.
4.2. Influence of the nature of the gel The two series of studied gels, C- and N-referenced, differ in the parameters of synthesis: precursor concentration, hydrolysis rate, complexing agent, pH and solvent. We compared the properties of the final powders in order to check the influence of theses parameters. Moreover, to discuss the influence of the precursor purity, 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 textural properties, despite the lower specific surface areas for the N-samples. As for the electronic properties, NA-samples show slower recombination phenomena associated to longer charge-carrier lifetimes than CA-sample. Nevertheless, the photocatalytic behavior of these two series is quite similar. This result may be explained 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 photodegradation of phenol solutions, benzoquinone and hydroquinone, by UV–Visible spectroscopy (Fig. 6), indicates that the photocatalysis mechanism is the same for both powders [25]. Therefore, the nature of the gel after supercritical drying and thermal treatment, even though influencing the structural and electronic properties of the samples, has no perceptible influence on their photocatalytic properties. 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. The properties 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 chargecarrier lifetimes. This sample is also slightly less active than CA550. NAP550 has the same surface as NA550, and quite the same number of excess charge-carriers created with shorter charge-carrier lifetimes; this last point has no great influence on the activity which is slightly higher than NA550. Thus, the purity of the precursor clearly does 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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Fig. 6. Photocatalysis measurements on TiO2 powders. UV-visible spectra of phenol solutions after various irradiation times on CA550 (a) and NA550 (b).
the same influence on the two kinds of gel. These differences will be discussed in the following section. 4.3. Influence of the drying procedure Comparatively for the other synthesis parameters, the drying procedure seems to be the most influential on the final properties of TiO2 powders. Both, structural and photocatalytic 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. In the particular case of the N gels, a supercritical drying enhances the specific surface area 23 times. The catalysis aspect is therefore, clearly promoted by supercritical drying. Nevertheless, TRMC results vary in an opposite manner. The xerogels generate upon UV absorption the creation of an important amount of charge-carriers with high lifetimes. In the case of the C-samples, the charge-carriers created on the xerogel are in a higher number and have longer lifetimes than those observed on the corresponding aerogel. The supercritical drying is therefore slightly harmful to the photo aspect. This weak decrease of the electronic properties does not go against the improvement of photocatalytic properties by supercritical drying. At the end of the reaction, as shown on Table 4, the xerogels degraded two and ten times less phenol than the corresponding aerogels, respectively, for the C and N samples.
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5. Conclusion The present work shows that the structural, electronic, and photocatalytic properties of sol–gel made of titanium dioxide can be arranged by controlling the elementary steps of their preparation. The particular steps of drying and thermal treatment of the photocatalysts are crucial on their final properties. We stressed the improvement provided by supercritical drying on the specific surface area of titanium dioxide. This parameter, which is highly significant in an interfacial reaction, such as photocatalysis, promotes the activity of titanium dioxide and can increase 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 the possibility of modifying the surface state of titanium dioxide to improve its electronic properties while keeping constant its specific surface area. This way will be more deeply studied and discussed in a near-future publication.
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Photocatalytic and electronic properties of TiO2 powders elaborated by sol–gel route and supercritical drying b Souhir Boujdaya, Frank Wunsch . , Patrick Portesa, Jean-Fran@ois Bocqueta, Christophe Colbeau-Justina,* 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 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 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 TiO2 electrodes was described in 1972 by Fujishima and Honda [1]. This activity was at first considered harmful as it was *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
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the cause of the degradation of polymeric matrix in which the pigment was incorporated. 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]. The main application of photocatalysis is environmental and deals of photodegradation of organic pollutants in water and air [6–10]. Thus, the general subject of the studies on 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 initiatives are realized in Japan where some companies are selling TiO2 covered tiles which are killing bacteria and sterilizing its surface, sets of tunnel lights that avoid the accumulation of carbon and oil layers, filters to catch cigarette smoke, antifog mirrors, and glasses [13]. However, in spite of this economical importance of titania, its elaboration for photocatalysis is still poorly understood. The main factors that seem to govern the photocatalytic activities of TiO2 powders are their structural and their 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 are semiconductors 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 not recombine, they can migrate to the surface where electrons are trapped by titanium and holes by the superficial OH groups. Trapped holes form OH radicals and trapped electrons react with O2 and H2O to form HO2 radicals [15]. These free radicals are very oxidant entities and are causing the degradation of organic compounds 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 parameters such as crystalline quality, particle size and also the superficial OH groups on chargecarriers. These preliminary results suggest that an anatase type TiO2 of high crystalline quality, with nanosized particles (10 to 100 nm) for a highly reactive surface, generates a lot of charge-carriers with high lifetimes. This titania should then be the best photocatalyst.
Fig. 1. Photocatalytical activity of TiO2.
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We have thus undertaken the present research to deepen these results, and specially to correlate the structural, textural, and electronic properties of TiO2 powders to their photocatalytic activities. We investigate various TiO2 powders synthesized by sol–gel method followed by drying in supercritical CO2. The properties of these powders were also compared to those of other powders elaborated 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-aqueous medium. The sols were prepared by adding an aqueous solution of acid to a solution of titanium isopropoxide Ti(O–iC3H7)4 in anhydrous alcohol at room temperature under continuous stirring. Two series of gels were prepared (Table 1): C-referenced series: For this series, the sols were prepared by adding a 12 M solution of hydrochloric acid (Prolabo, 37%) to a solution of titanium isopropoxide Ti(O–iC3H7)4 (Acros, 98%) and isopropanol (Prolabo, 99.7%). The molecular ratio of Ti/alcohol/H2O/acid is 1/11/2/0.08. N-referenced series: For these samples, we used a 2 M solution of nitric acid (Prolabo, 68%) and we replaced isopropanol by ethanol (Prolabo, 99.9%). The ratio of Ti/alcohol/H2O/acid is 1/18/3/0.08. This procedure was already used by Dagan et al. [17] with Ti/alcohol ratio of 20. We set this ratio to 18 to decrease the gelling time. In each case, the mixture was kept under stirring for 10 min. The gel is formed upon 10 min 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 a highly pure Ti(O–iC3H7)4 (Alfa Aesar, 99.999%).
Table 1 Experimental conditions of gel synthesis and drying Reference
CA
CX
Gel synthesis conditions (mol per mol of Ti(O–iC3H7)4) Alcohol 11 11 isopropanol isopropanol Water 2 2 Acid 0.08 0.08 hydrochloric hydrochloric Drying conditions Pressure 100 bar atmospheric Temperature 50 C 70 C Gas CO2 Air
NA
NX
18 ethanol 3 0.08 nitric
18 ethanol 3 0.08 nitric
100 bar 50 C CO2
atmospheric 70 C Air
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Fig. 2. Supercritical drying setup.
The formed gels were dried under supercritical conditions to form aerogels. Experimental drying conditions are displayed in Table 1. The alcohol contained in the gel is replaced with liquid CO2 and then, the system is brought above the critical point in temperature and pressure (>35 C and 70 bar). The supercritical drying setup is displayed on the Fig. 2. A second ‘‘classical’’ procedure of drying in an oven at 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 at 450 C, 500 C, and 550 C. Treatment was 6 h ramping to final temperature and 10 h stage at this temperature. Beneath 450 C, thermal gravimetric analyses show that organic residues are not totally removed, the color of the powder is then gray. Above 550 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 to xerogel or aerogel. The heating temperature of each sample is indicated at the end of each reference. The samples prepared starting from highly pure Ti(O–iC3H7)4 have the 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 1729 diffractometer using Cu (Ka) radiation. Crystallite sizes of anatase powders were estimated 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 meaning of Scanning Electron Microscopy (LEO 440S, Leica, Cambridge). BET: Adsorption–desorption measurements of N2 using COULTER SA 3100 have been realized to determine surface area by multipoint Brunauer–Emmett–Teller method (BET) [19] and pore size distribution by Barrett–Joyner–Halenda method (BJH) [20].
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2.3. Electronic properties characterization: TRMC The charge-carrier lifetimes in TiO2 after UV illumination has been determined by microwave 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 the microwave power reflected by a sample, DPðtÞ; induced by its laser-pulsed illumination. The relative difference DPðtÞ=P can be correlated, for small perturbations of conductivity, to the difference of conductivity DsðtÞ considering the Eq. (1) [21]: X DPðtÞ ¼ ADsðtÞ ¼ Ae Dni ðtÞmi ; P i
ð1Þ
where Dni ðtÞ 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 microwave frequency 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 valence band. In the specific case of TiO2, the TRMC signal can be attributed to electrons because 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. Imax reflects the number of the excess charge-carriers created by the UV pulse. It must be noted that this information is weighted by the mobility of the charge-carriers and the influence of charge-carrier decay processes during the excitation. As better chargecarrier transport properties lead to a higher mobility and less decay during the excitation, a higher value of Imax corresponds to better charge-carrier transport properties. The parameters t1=2 ; 2t1=2 and 3t1=2 are the half-times of signal corresponding 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 the signal decay is not purely exponential, thus, the general decay shape is characterized by several half-time lives linked to charge-carrier lifetimes. As recombination phenomena occur mainly between 0 and 40 ns after the pulse [16], the ratio of the intensity of the signal 40 ns after the beginning of the pulse by Imax notifies on the speed of the recombination processes: A high I40 ns =Imax indicates a low speed recombination. TRMC measurements were carried out in the Hahn–Meitner–Institut, Berlin. The incident microwaves are generated by a Gunn diode of the Ka band (28–38 GHz). The experiments were performed at 31.4 GHz. Pulsed light source is a Nd:YAG laser providing an IR radiation at l ¼ 1064 nm. Full-width at half-maximum (FWHM) of one pulse is 10 ns; repetition frequency of the pulses is 10 Hz. UV light (355 nm) is obtained by tripling the IR radiation. The light energy density received by the sample is 1.3 mJ cm2.
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2.4. Photocatalytic properties The photocatalytic activities of the synthesized TiO2 powders were tested in the reaction of photodegradation of phenol in water. Photocatalytic tests were carried out in the static mode in a batch reactor. 400 mg of photocatalyst were added to 400 ml of a 50 mg l1 phenol solution (Aldrich, 99%). The suspension was kept under vigorous stirring with oxygen bubbling. The mixture was then illuminated by a mercury lamp (Philips HPK 125 W, 8.3 106 Einstein s1). This lamp was placed in a cooling jacket dipped in the reaction mixture to set the temperature constant (25 C). The reaction progress was followed by systematic sampling (10 min interval). The taken samples were filtered through 0.22 mm pore size Millipore filter and the concentration 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 as reference. The scan range was from 900 to 190 nm with a 1.0 nm interval, and the averaging 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 amorphous and exhibit a high specific surface area. The structural parameter of the prepared TiO2 powders after thermal treatment are reported in Table 2. The 10 compounds are 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).
Table 2 Structural and textural properties of TiO2 powders Reference
CA450 CA500 CA550 CAP550 CX550 NA450 NA500 NA550 NAP550 NX550
Structure
Texture
Crystallite size (nm)
SBET(m2 g1)
Porous volume (ml g1)
10 16 23 — 32 13 17 21 — 38
112 106 82 71 46 98 81 63 60 3
0.546 0.541 0.526 0.319 0.128 0.418 0.373 0.262 0.251 —
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Fig. 3. SEM pictures of CA550 (a, c) and CX550 (b, d).
After heating at 450 C, all the samples have an anatase structure-type. The crystallite size increases with calcination temperature while specific surface area decreases. However, there are no big differences between the two kinds of gels (C and N) even though the C crystallites are slightly smaller. Nevertheless, the procedure of drying influences considerably the crystallite size: aerogel crystallites are distinctly smaller than xerogel ones. This effect is also observed on specific surface areas which are much higher for aerogel than for xerogel. Moreover, the nature of the acid and alcohol used to prepare the gels modifies the specific surface area: 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 on Table 3.
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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 References
Imax (mV)
I40 ns =Imax (%)
t1=2 (ns)
2t1=2 (ns)
3t1=2 (ns)
CA450 CA500 CA550 CAP550 CX550 NA450 NA500 NA550 NAP550 NX550
10.6 15.0 19.0 13.6 21.3 10.0 11.8 17.8 15.2 16.7
43 44 50 54 71 50 51 56 53 48
20 20 30 40 250 35 35 60 30 25
250 250 750 400 >104 1500 1000 3000 750 180
5000 4000 >104 2000 — >104 >104 >104 4000 1200
For the two series of powders, thermal treatment enhances the number of excess charge-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 for the 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 after the pulse are lower in number and in lifetimes than the corresponding samples in the two 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 created excess charge-carriers on the xerogel are larger than in aerogel and their lifetimes are higher. Whereas for the N-samples, the supercritical drying enlarges the number of the created excess charge-carriers and increases their lifetimes.
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3.3. Photocatalytic properties The percentage of the photodegraded phenol as a function of reaction time is shown 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 enhances the photocatalytic activities, the best percentage of phenol degradation are obtained when TiO2 is heated to 550 C. There are no big differences between the activities of the two kinds of gels when heated at 450 C, 500 C, and 550 C. Moreover, the use of highly pure titanium precursor does not influence significantly the photocatalytic behavior of TiO2 powders. Finally, the drying procedure of the gels seems to be the major 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 and catalysis) separately. The first part (photo part) concerns phenomena linked to light– material interaction which includes photons absorption, charge-carrier creation and dynamics, and also surface trapping. The second part (catalysis part), concerns phenomena linked to surface radicals formation and surface reactivity, i.e. the interaction between H2O, O2, organic pollutant and the oxide surface.
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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Table 4 Percentage of photodegraded phenol on TiO2 powders after 75 min Reference
CA450 CA500 CA550 CAP550 CX550 NA450 NA500 NA550 NAP550 NX550
% of degraded 64.5 phenol after 75 min
81.6
94.9
92.7
42.6
67.0
78.1
92.2
94.0
8.7
For the photo part, the most effective structural parameter on photocatalysis is the crystalline quality [16]. Actually, as shown on Fig. 1, the oxidant radicals, which are the active species in photocatalysis, are formed when the charge-carriers created by absorbed UV-photons are trapped in the surface. Thus, recombination and bulk trapping phenomena that decrease charge-carrier lifetimes and prevent their arrival to the surface penalize the formation of the oxidant radical. Yet, recombination and bulk trapping are promoted by defects, doping elements, and impurities or amorphous domains. Consequently, to enhance the charge-carriers lifetime, the crystalline quality should be as high as possible. Thus, for titania, the TRMC measurements can be considered an indicator of the level of crystalline quality. High values of Imax and slow decay indicate an important amount of charge-carriers created 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 specific surface area induces higher number of accessible active sites and consequently better reactivity. In the following sections, we checked the influence of some synthesis parameters on these two structural parameters and observed the resulting influence on photocatalysis. 4.1. Influence of the thermal treatment The dried gels were calcinated at 450 C, 500 C and 550 C to examine the influence of the thermal treatment. This range was chosen because at lower temperatures we cannot avoid the presence of organic residues, and at higher temperatures, we make a start on the crystallite transformation from anatase to rutile 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, CA and NA. The specific surface areas corresponding to these two series are equal to 112 and 98 m2 g1. Indeed, when these samples are heated at a higher temperature, their particle diameter increases and their specific surface areas decrease as shown in Table 2. Consequently, the thermal treatment go slightly against the catalysis part in the photocatalysis mechanism. Meanwhile, TRMC measurements indicate the enhancement of the electronic properties when the samples are heated at higher temperatures. Indeed, for the two series of gels, the highest amount of created charge-carriers with the longest lifetimes
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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 improvement of 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 influential than the catalysis one. Yet, we must notice that the change consequent to thermal treatment is less perceptible on the textural properties than to electronic ones, which may explain the enhancement of photocatalytic activities by thermal treatment.
4.2. Influence of the nature of the gel The two series of studied gels, C- and N-referenced, differ in the parameters of synthesis: precursor concentration, hydrolysis rate, complexing agent, pH and solvent. We compared the properties of the final powders in order to check the influence of theses parameters. Moreover, to discuss the influence of the precursor purity, 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 textural properties, despite the lower specific surface areas for the N-samples. As for the electronic properties, NA-samples show slower recombination phenomena associated to longer charge-carrier lifetimes than CA-sample. Nevertheless, the photocatalytic behavior of these two series is quite similar. This result may be explained 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 photodegradation of phenol solutions, benzoquinone and hydroquinone, by UV–Visible spectroscopy (Fig. 6), indicates that the photocatalysis mechanism is the same for both powders [25]. Therefore, the nature of the gel after supercritical drying and thermal treatment, even though influencing the structural and electronic properties of the samples, has no perceptible influence on their photocatalytic properties. 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. The properties 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 chargecarrier lifetimes. This sample is also slightly less active than CA550. NAP550 has the same surface as NA550, and quite the same number of excess charge-carriers created with shorter charge-carrier lifetimes; this last point has no great influence on the activity which is slightly higher than NA550. Thus, the purity of the precursor clearly does 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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Fig. 6. Photocatalysis measurements on TiO2 powders. UV-visible spectra of phenol solutions after various irradiation times on CA550 (a) and NA550 (b).
the same influence on the two kinds of gel. These differences will be discussed in the following section. 4.3. Influence of the drying procedure Comparatively for the other synthesis parameters, the drying procedure seems to be the most influential on the final properties of TiO2 powders. Both, structural and photocatalytic 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. In the particular case of the N gels, a supercritical drying enhances the specific surface area 23 times. The catalysis aspect is therefore, clearly promoted by supercritical drying. Nevertheless, TRMC results vary in an opposite manner. The xerogels generate upon UV absorption the creation of an important amount of charge-carriers with high lifetimes. In the case of the C-samples, the charge-carriers created on the xerogel are in a higher number and have longer lifetimes than those observed on the corresponding aerogel. The supercritical drying is therefore slightly harmful to the photo aspect. This weak decrease of the electronic properties does not go against the improvement of photocatalytic properties by supercritical drying. At the end of the reaction, as shown on Table 4, the xerogels degraded two and ten times less phenol than the corresponding aerogels, respectively, for the C and N samples.
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5. Conclusion The present work shows that the structural, electronic, and photocatalytic properties of sol–gel made of titanium dioxide can be arranged by controlling the elementary steps of their preparation. The particular steps of drying and thermal treatment of the photocatalysts are crucial on their final properties. We stressed the improvement provided by supercritical drying on the specific surface area of titanium dioxide. This parameter, which is highly significant in an interfacial reaction, such as photocatalysis, promotes the activity of titanium dioxide and can increase 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 the possibility of modifying the surface state of titanium dioxide to improve its electronic properties while keeping constant its specific surface area. This way will be more deeply studied and discussed in a near-future publication.
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