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Microporous and Mesoporous Materials 91 (2006) 1–6

Titania aerogel prepared by low temperature supercritical drying

V. Stengl *, S. Bakardjieva, J. Subrt, L. Szatmary

Institute of Inorganic Chemistry, Academy of Czech Science (AS CR), 250 68 Rez, Czech Republic

Received 31 March 2005; received in revised form 28 September 2005; accepted 31 October 2005Available online 27 December 2005

Abstract

Mesoporous titania aerogel with high surfaces area (1086 m2 g�1) were prepared by two steps: heterogeneous precipitation of aqueoussolution containing TiOSO4 and ammonia, followed by continuous supercritical extraction with CO2 after replacement of the water withn-propanol/benzene mixture. The total pore volume of as obtained aerogel is 3.44 cm�3 g�1 and the main pore diameter is 20–80 nm. Thestructure evolution of titania aerogel during heating was studied by DTA and high temperature XRD analyses. At 450 �C titania aerogeltransforms to polycrystalline anatase. The morphology and microstructure characteristics of both titania aerogel and anatase sampleswere obtained by SEM, HRTEM, BET and BJH methods. Anatase reveals high photocatalytic activity during the degradation of 4-chlo-rophenol in aqueous suspension.� 2005 Elsevier Inc. All rights reserved.

Keywords: Aerogels; Titanium oxide; Supercritical drying; Carbon dioxide

1. Introduction

Nanophase materials, with their gram sizes or phasedimensions in the nanometer size regime, are now beingproduced by a wide variety of synthesis and processingmethods. The interest in these new ultrafine-grained mate-rials results primarily from the special nature of theirvarious physical, chemical, and mechanical propertiesand the possibilities of controlling these properties duringthe synthesis and subsequent processing procedures. Sinceit is now becoming increasingly apparent that their proper-ties can be engineered effectively during synthesis andprocessing, and that they can also be produced in quantity,nanophase materials should have considerable potential fortechnological development in a variety of applications.

Among these materials, one of the very important,is nanocrystalline anatase (TiO2), widely used now forphotocatalytic air and water purification and many otherpurposes based on photocatalytic oxidation and decompo-

1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2005.10.046

* Corresponding author. Tel.: +420 2 6617 3534; fax: +420 2 2094 0157.E-mail address: stengl@iic.cas.cz (V. Stengl).

sition of organic pollutants [1–4]. The material can be alsoused for solar energy storage and conversion [5,6] organicsyntheses [7], etc. Titanium dioxide is one of the most pop-ular and promising materials for these purposes, because ofits stability, commercial availability and ecological safety.According to the literature [8–10], the photocatalytic activ-ity of suspended TiO2 in solution strongly depends on thephysical properties of TiO2 (e.g., crystal structure, surfacearea, surface hydroxyls, and particle size). In this respect,attempts have been made to prepare TiO2 particles withhigh surface area, suitable porosity and a distinct shape(films, spheres, rods, etc.) in order to suit this material tothe demands of its application [11,12]. To achieve goodphotocatalytic activity, the material should contain aslow as possible amount of amorphous material [13]. Suffi-cient amount of surface OH� groups is also needed inorder to stabilize the active ion–hole pairs in the form ofsurface OH� and O�2 radicals on the surface of the TiO2

photocatalyst [14]. Current attention has been focused onthe preparation of nanometer TiO2 particles using super-critical fluid drying. According to Matson et al. [15,16]supercritical fluid state is a state of material that its temper-ature and pressure are higher than its critical temperature

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and critical pressure. In supercritical fluid drying of nano-meter particle preparation, solvent can be removed withoutsurface tension effect and the coagulation of particles, the-oretically can be avoided. Recently, Hu et al. [17] preparednanometer titanium oxide with n-butanol supercriticaldrying. The technique mainly consisted of sol preparation,replacement of water in precipitate using n-butanol andsupercritical drying. Anatase and rutile mixture with aparticle size of 10–20 nm was obtained using the prepara-tion technique. The present state of supercritical dryingca be divided into the high-temperature and low tempera-ture methods, usually based on alcoholic solvents andCO2, respectively [18].

The aim of our research was to obtain nanosized TiO2 inpure anatase form with high surface area. We try todevelop relatively cheap n-propanol/benzene supercriticalfluid drying technique—the water of the primary precipi-tate obtained from aqueous solution containing TiOSO4

and ammonia was replaced with n-propanol/benzene mix-ture followed by continuous supercritical extraction withCO2.

2. Experimental

2.1. Preparation of aerogel of titanium oxide

A wet gel of titanium(IV) hydroxide was prepared bymixing aqueous solutions of ammonia (25%) and 0.2 Msolution of TiOSO4 under stirring at temperature 0 �C.The wet gel (denoted as SAMPLE1) was separated byfiltration, washed few time with distilled water by decanta-tion, and loaded into the flask of an apparatus for solventreplacement by distillation, as described in [17]. The flaskwas filled with a 1:2 mixture of propanol and benzene.The distillate, composed of a mixture of water and organicphase, was collected in a separating funnel, the organicmaterial was recycled in to flask and the process was con-tinued until all water had been removed from the gel.After solvent replacement, the gel was filtered of andloaded in to flask with acetone on 24 h. The wet gel fromacetone was converted to an aerogel (denoted as SAM-PLE2) by low temperature supercritical drying with CO2

in supercritical extraction system (model CPD7501 Quo-rum Technologies) at pressure 80 bars, drying times 24 hand temperature �12 �C, flow rate 10 mL min�1 at tem-perature 33 �C.

2.2. Heat treatment of aerogel of titanium oxide

On the basis of high temperature XRD results (Fig. 1)the aerogel of titanium oxide obtained from the low tem-perature supercritical drying was heated under dynamicvacuum (80 mbar) at temperature 450 and 550 �C for 2 hby using furnace controlled by the PID controller in thestainless steel tube. The temperature increase rate was1 �C/min. After the heat treatment, the sample was allowedto cool in room temperature.

2.3. Characterization methods

Specific surface area of the samples was determinedfrom nitrogen adsorption–desorption isotherms at liquidnitrogen temperature using a Coulter SA 3100 instrumentwith outgas 15 min at 120 �C. Specific surface area was cal-culated by the BET [19] method, while the pore size distri-bution (pore diameter and pore volume) was determined bythe BJH [20] method.

Transmission electron micrographs were obtained usinga JEOL JEM 3010 microscope (LaB6 cathode), operatingat accelerating voltage of 300 kV and equipped withCCD GATAN MULTISCAN (model 794) and EDS spec-trometer (INCA x-stream module OXFORD). The sam-ples for electron microscopy were prepared by grinding inan agate mortar and subsequent dispersing the powder inethanol. The suspension was treated in ultrasonic bathfor about 10 min. A drop of very dilute suspension wasapplied on carbon-coated grid.

SEM studies were performed using a Philips XL30 CPmicroscope equipped with EDX, Robinson, SE and BSEdetectors. The sample was placed on an adhesive C sliceand coated with Au–Pd alloy 10 nm thick layer.

X-ray powder diffraction patterns were obtained on aSiemens D5005 instrument using Cu Ka radiation (40 kV,30 mA) and diffracted beam monochromator. Qualitativeanalysis was performed with the Eva Application and theXpert HighScore using the JCPDS PDF-2 database. Crys-tallite size of the heated samples was calculated from theScherrer [21] equation using the X-ray diffraction peak at2H = 25, 29�.

DTA–TG–MS measurements were carried out by usinga simultaneous Netzsch Instrument STA 409 coupled toQuadrupol Mass Spectrometer Balzers QMS-420 underdynamic conditions in the air (flow rate 75 mL min�1).The samples were heated at the rate of 10 K min�1.

Photocatalytic activity characterization. The mineraliza-tion of 4-chlorophenol (4-CP) in an aerated aqueous slur-ries was measured and the kinetics of the reaction wasassessed by fitting. Seventy milligrams of the investigatedphotocatalyst sample were quantitatively transferred, using63 mL of distilled water, into the inner tube of the photo-reactor and kept in dark for about 1 h. Then, 7 mL of0.001 M solution of 4-chlorophenol (4-CP) were added thatcompleted the total volume up to 70 mL. The prepared sus-pension was then magnetically stirred for about 1 h in darkto achieve the adsorption equilibrium. The solutionamount of 1 mL was taken as a blank probe before irradi-ation and, after filtration of the catalyst submitted to theHPLC analysis. After that, the inner tube was put intothe photoreactor and irradiation started. At appropriateirradiation times, 1 mL probes of the irradiated suspensionwere taken and analyzed by HPLC in the same way as theblank probe. To complete a set of HPLC measurements fora sample, the amount of 0.0001 M 4-CP solution in waterwas analyzed under the same conditions. By this way weestimated the amount of 4-CP initially adsorbed on TiO2

V. Stengl et al. / Microporous and Mesoporous Materials 91 (2006) 1–6 3

particles, which was removed together with the powder inthe filtration process. After finishing the HPLC analysisof each sample the integrated areas or heights of the peakswere plotted as a function of the irradiation time.

Quartz water-jacketed laboratory photoreactor, mag-netically stirred and continuously irradiated with one‘‘black light’’ lamp (k = 365 nm, I0 = 5.3 · 10�5 Ein-stein dm�3 s�1) was used. A self-constructed photoreactorconsists of two coaxial quartz tubes placed in the middleof a steel cylinder with an aluminum foil covering its innerwall. Inner quartz tube (diameter 24 mm, length 300 mm)was filled with the investigated suspension (70 mL) andmagnetically stirred. Cooling water was circulatingbetween the inner and the outer quartz tube to keep con-stant temperature of 20 �C. The HPLC measurements werecarried out by using a Merck device with L-6200 IntelligentPump, L-3000 Photo Diode Array Detector and D-2500Chromato-Integrator. Mobile phase methanol/water (2:3;v/v) and a Merck column LiChroCART 125-4 filled withLiChrosphere 100 RP-18 (5 lm) were used, injection loopwas 20 lL, flow rate 1 mL min�1 and detection wavelength280 nm were applied. The solid sample necessarily togetherwith the adsorbed portions of the dissolved molecular spe-cies was removed before HPLC, using a filtration by aMillipore syringe adapter (diameter 13 mm) with filter408 (porosity 0.45 lm). Detailed experimental procedurescan be found elsewhere [22].

3. Results and discussion

3.1. Thermal analysis of the titanium gel

Fig. 1 show a typical thermal analysis of the titanium gelSAMPLE1 used in the synthesis. DTA curve shows, thatwater started evolves at low temperature (endothermicpeak start at temperature 155 �C) and achieves maximum

Fig. 1. DTA curves of the SAMPLE1.

at 250 �C, which correspond fracturing of –OH groups.Carbon dioxide was detected in the gas phase at tempera-ture �150� and peaked at 331 �C. The exothermic maxi-mum at temperature 430 �C correspond with transition toanatase in high temperature XRD. The limit of the calcina-tions temperature to anatase used in the synthesis was450 �C. At these conditions we obtained the final solidsproduct consisting of anatase nanoparticles without anytraces of precursors.

3.2. Adsorptions isotherms and specific surface area

Surface area, pore volume and pore size distribution ofthe materials were obtained from N2 adsorption–desorp-tion isotherms (BET and BJH methods) at temperatureof liquid nitrogen on Coulter SA3100 instrument. Priorto measurements the Ti-aerogels, directly after CO2 extrac-tion, were degassed at temperature 150 �C for 30 min. Atypical nitrogen adsorption–desorption isotherms obtainedfor all samples is shown in Fig. 2. The isotherms are closedhysteresis loops type A and have classification for cylindri-cal pores open at both ends [23]. The specific surface areaand total pore volume of samples are shown in Table 1.Pore size distribution shift to mesoporous using low tem-perature supercritical drying (see Table 2).

3.3. X-ray diffraction (XRD)

Fig. 3 shows the X-ray diffraction patterns of both, anaerogel SAMPLE2 prepared by low temperature supercrit-ical drying and SAMPLE3 prepared by thermal treatmentof titania aerogel at 450 �C. They are all similar and exhibit

Fig. 2. Nitrogen adsorption–desorption isotherm of the SAMPLE2.

Table 1Specific surface areas, SBET and total pore volume, Vp, of Ti-aerogels

Sample Temperature of heating (�C) SBET (m2 g�1) Vp (cc g�1) Phase identified by XRD L25,29(2H) (nm)

SAMPLE1 – 785.2 0.78 Amorphous –SAMPLE2 – 1085.2 3.44 Amorphous –SAMPLE3 450 364.3 2.23 Anatase 4.2SAMPLE4 550 253.1 1.74 Anatase 4.4

Table 2Desorption BJH pore size distribution of prepared samples

SAMPLE1 SAMPLE2 SAMPLE3 SAMPLE4

Porediameter(nm)

Porevolume(cc/g)

% Pore diameter(nm)

Pore volume(cc/g)

(%) Pore diameter(nm)

Pore volume(cc/g)

(%) Pore diameter(nm)

Pore volume(cc/g)

(%)

Under 6 0.29 55.03 Under 6 0.15 4.42 Under 6 0.02 0.98 Under 6 0.05 3.186–8 0.09 17.96 6–8 0.09 2.71 6–8 0.07 3.00 6–8 0.03 1.998–10 0.04 7.42 8–10 0.06 2.00 8–10 0.08 3.63 8–10 0.02 1.4210–12 0.02 5.28 10–12 0.09 2.75 10–12 0.15 6.21 10–12 0.03 1.9012–16 0.01 3.56 12–16 0.15 4.57 12–16 0.18 7.56 12–16 0.05 2.9316–20 0.01 2.34 16–20 0.25 7.31 16–20 0.19 7.70 16–20 0.08 4.4420–80 0.03 6.11 20–80 2.23 63.95 20–80 1.39 56.31 20–80 0.93 49.44Over 80 0.01 2.32 Over 80 0.42 12.29 Over 80 0.36 14.61 Over 80 0.65 34.72

Fig. 3. XRD patterns of the specimens SAMPLE2 and SAMPLE3.

Fig. 4. High temperature XRD of the SAMPLE2.

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characteristic peaks for TiO2 (anatase modification, PDF21-1272).

The XRD spectrum of the Ti-aerogel (SAMPLE2) syn-thesized by low temperature supercritical drying with CO2

indicate a diffuse pattern with relatively broad peaks andweak intensity and confirms X-ray amorphous sample.Energy-dispersive X-ray microanalysis was performed onthe Ti-aerogel due to acquire semiquantitative elementalinformation. EDX analysis confirmed the empirical com-position of 58.47 wt% Ti and 41.53 wt% O. This value werelooked to be in accordance with the theoretical composi-tions of TiO2. The EDX information was believed to becorrect to about ±3%.

The XRD spectrum labeled SAMPLE3 belongs to theheated Ti-aerogel at 450 �C. Calcination in oxygen at450 �C using a heating rate of 1 �C min�1 was an effective

heat treatment procedure for obtaining pure polycrystallineanatase TiO2 (ICDD PDF 21-1272).

High temperature XRD diffractogram of SAMPLE2 isin Fig. 4. The transition to anatase modification is at tem-perature 420 �C, and transition anatase–rutile at tempera-ture 920 �C. The temperature of transition to anatasecorrespond with the results of DTA–TG (see Fig. 1).

3.4. Electron microscopy (SEM and HRTEM)

Fig. 5 shows the SEM micrographs of Ti-aerogel (SAM-PLE2) obtained by continuous supercritical extraction with

Fig. 5. SEM image of the SAMPLE2.

V. Stengl et al. / Microporous and Mesoporous Materials 91 (2006) 1–6 5

CO2 after replacement of the water with n-propanol/ben-zene mixture. It can be seen that titania aerogel consistsof large micron-scale agglomerates. This agglomerates arenot accurate spheres in shape and not very uniform in size.We suppose that the morphology of a SAMPLE2 dependon the weak coagulation among particles, which may beresulted from the influence of H2O and solvent mixture(n-propanol:benzene = 1:2).

Fig. 6. HRTEM image

Fig. 6(a) and (b) are the morphologies of the TiO2, ana-tase modification (ICDD PDF 21-1272), obtained by heat-ing of titania aerogel (SAMPLE2) at 450 �C. The sphericalparticles consist of primary 4–5 nm anatase nanocrystalscan be observed. The result indicates that there existed nomarked the sintering between particles at this temperature.Fig. 6(c) and (d) reveal polycrystalline anatase with highquality of nanocrystallites without fraction of amorphoussubstance. The fine fringe spacing is 0.37 nm correspondingto the (103) plane of anatase. This fact seems to be the mainreason for the highest catalytic activity of SAMPLE3 dur-ing the degradation of 4-chlorophenol in H2O solution.

Selected area diffraction (SAD) patterns (inserted inFig. 6) include a series of continuous Debye–Scherrer ringsthat correspond to (1) 0.36, (2) 0.24, (3) 0.19 nm in thed-spacing, denoting anatase crystal lattice planes withMiler indices (010), (004), (200), respectively. The ringpatterns confirmed the polycrystalline anatase specimen(tetragonal, space group I4/amd) consisting of many nano-crystals (4–5 nm) with a random orientation.

3.5. Photocatalytic activity

It followed from Fig. 7 that the prepared specimenSAMPLE3 and SAMPLE4 show good photocatalytic

of the SAMPLE3.

Fig. 7. Photocatalytic activity of the SAMPLE3, SAMPLE4 and DegussaP25.

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properties for the decomposition of 4-chlorophenol in anaqueous slurry under UV radiation. The time dependencesof the 4-CP decomposition were fitted by using Eq. (1) forthe first-order kinetics reaction [24]. It is obvious fromFig. 7 that the first-order kinetics curves (plotted as lines)fitted to all experimental points.

d½CP�dt¼ kða0 � ½CP�Þ ð1Þ

where [CP] is concentration of 4-CP, a0 is initial concentra-tion of 4-CP and k is rate constant.

The rate constant k of the SAMPLE3 heated at temper-ature 450 �C is 0.0291 and SAMPLE4 heated at tempera-ture 550 �C is 0.135. It is evident that the photoactivityof SAMPLE3 is higher to the standard photocatalyst P25Degussa (rate constant 0.0203). Lower photoactivity ofSAMPLE4 depend on decreasing of surface area (see Table1).

4. Conclusion

Amorphous TiO2 Æ nH2O (SAMPLE1, surfacearea = 785 m2 g�1) were prepared reaction of titanium sul-phate with aqueous solution of ammonium hydroxide attemperature 0 �C. The wet gel was converted to titaniaaerogel (SAMPLE2, surface area = 1085 m2 g�1) bymethod low temperature supercritical drying with CO2.The titania aerogel was converted to anatase (SAMPLE3,surface area = 364 m2 g�1, SAMPLE4, surface area =

253 m2 g�1), by heating at temperature 450 and 550 �C.The conversion of modification was determined by hightemperature XRD. The anatase SAMPLE3 prepare byheating of titanium aerogel at temperature 450 �C havehigher photocatalytic activity at decomposition 4-chloro-phenol then Degussa P25.

Acknowledgement

This work was supported by the Ministry of Education,Youth and Sports of the Czech Republic (Project No.1M4531477201).

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