tio2-aerogels morphology-synthesis correlation

TIO2-AEROGELS MORPHOLOGY-SYNTHESIS CORRELATION

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TIO2-AEROGELS MORPHOLOGY-SYNTHESIS

CORRELATION

Virginia Danciua, Veronica Coşoveanua, I. Mariana, Anca Peterb, P.Margineanc, E. Indreac a “Babeş-Bolyai”University, Faculty of Chemistry and Chemical Engineering,

Electrochemical Research Laboratory, Arany Janos Str., 11, Cluj-Napoca, RO-3400,

ROMANIA

b North University of Baia Mare, Faculty of Sciences, Department of Chemistry,

Victoriei Str., 76, Baia Mare, RO-4800, ROMANIA

C National Institute for Research and Development of Isotopic and Molecular

Technologies, Donath Str., 71-103, Cluj-Napoca, RO-3400, ROMANIA

SUMMARY

Aerogels are highly porous materials that are produced via sol-gel processing and supercritical drying. Titanium dioxide aerogels were prepared using a variety of sol composition from the system Ti(OC3H7)4 / H2O/ EtOH / HNO3. The morphology of titanium dioxide aerogel was characterized by krypton adsorption, X-ray diffraction, scanning and transmission electron microscopies. The surface area, pore volume and pore size distribution of TiO2 aerogels depend on the gel preparation, drying and annealing. The photoassisted oxidation of salycilic acid on the TiO2 aerogels was compared with commercial (P-25)TiO2 powder. The aerogels showed a much higher photocatalytic activity. Keywords: aerogel, TiO2, sol-gel method, supercritical point drying, photocalytic activity

INTRODUCTION

Aerogels are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. Then, the obtained wet gels are dried by supercritical method. As a result, the dry samples known as aerogels, keep the very unusual porous texture which they had in the wet stage. In general, the aerogels have very low bulk densities and large specific surface areas. Owing to these interesting physical properties, aerogels are potentially important precursors for the manufacturing of catalysts, adsorbents, sensors and thermal or sound insulation materials.

TiO2 aerogels combine the properties of the aerogels with the photocatalytical

V. D A N C I U , V. C O Ş O V E A N U , I . M A R I A N , A . P E T E R , P. M A R G I N E A N , E . I N D R E A

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properties of the semiconductor oxides. The ability of TiO2 to photocatalytically decompose organic pollutants is understood to originate with the band structure of TiO2. Band-gap of TiO2 is constant of about 3,2eV and shows no particle size dependence (in the range 10D <Rpart < 134 D). The electron-hole pair created by absorption of UV radiation will catalyze the decomposition of organic pollutants. The electron and hole will either recombine immediately or they will migrate to the surface and take part in a chemical process. On the other hand the lifetime of charges-carrier and surface-recombination velocities determine the response of a photoactive system to a varying light intensity. The quantum yield of this system is limited by the rate at which electrons can be transferred to the electrolyte and this is a function of the particle size as well as the presence of shallow surface states on the particles which can trap electrons.

The goal of the present work is to realize some aerogel morphology (surface area, porosity) by controlling the various synthetic parameters (reactant ratios, washing, annealing) to optimize the aerogel’s photocatalytic ability

MATERIALS AND METHODS

MATERIALS Preparation of TiO2 gels. TiO2 gels were prepared by acid-catalyzed sol-gel method.

The wet gels were prepared by mixing Ti(IV) isopropoxide (>98%, Merck-Schuchardt or 97%, Aldrich) with anhydrous ethanol (analytical grade, Aldrich), H2O ( and HNO3 (analytical grade, 65%, Primexchim) , at room temperature with stirring. A series of gels, with a constant molar ratio between titanium isopropoxide (TIP) and the acid, and different water and alcohol contents (EtOH), were prepared. The compositions of the gels prepared are shown in Table 1. The sols were poured into closed plastic boxes and maintained at room temperature until gelled. The gels were allowed to age in the mother liquor for at least 40 days. Before drying, the gels were successively washed with excess of fresh alcohol, at least four times per day and were kept in alcohol for a week to several months.

Supercritical point drying of TiO2 gels. For supercritical point drying the samples were immersed in anhydrous ethanol (analytical grade, Chimopar) and dried using liquid CO2 (99,99%, Linde Gaz Romania).

Adsorption and photodegradation of salicylic acid on TiO2 aerogels. In this study salicylic acid (analytical grade, 99,9%, Fluka), was used as a prototype substance to study photocatalytical activity of TiO2 aerogels. Stock solution of 10mM salicylic acid in 1.1mM KCl (ionic conductivity of 670µS) at pH 3.7 (adjusted by HCl) was prepared. In each experiment, 10 ml of the salicylic acid solution was added to TiO2 aerogel (0.25 g) and the cell was kept for 1h in the dark before the degradation experiment


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Table I. Reactant compositions (molar ratio) used for preparation of TiO2 gels and gel features

Gel EtOH/TIP

[mol/mol] H2O/TIP

[mol/mol] Gelation time

[min] Gel features

A 18.66 3.5 7 Transparent, strong

B 19 3.325 6 Transparent, strong

C 20 3 8 Transparent, strong

D 20 3.25 5 Transparent, strong

E 20 3.5 3 Transparent, strong

F 20 3.75 2 Cloudy, strong

G 21 3.675 4 Light opaque, strong

H 21.53 3.5 10 Transparent, strong

K 22 3.85 2 Transparent, strong

L 23.33 3.5 45 Transparent, strong

METHODS

Supercritical point drying of TiO2 gels was performed in a SAMDRI-PVT 3D

(Tousimis) critical point dryer. In this drying method, the alcohol contained in the gel was replaced with liquid CO2. The system was brought to conditions above the CO2 critical point (T>35°C, p>1200 psi) for a slow removal of the CO2.

To remove residues of organic substances, the aerogels were annealed at temperatures of 400-600°C for 1-3 hours, using a CARBOLITE type ELF 11/6 chamber furnace. Surface area of TiO2 aerogels was determinated by Brunauer-Emmett-Teller measurements. Kr adsorption isotherms were obtained using a all glasse installation, at a partial pressure of 0.05-0.35. The samples were preheated for 2h at 600C before measurement. After their degasification in vacuum, the samples were freezed at temperature of liquid nitrogen.

Structure of TiO2 aerogels was obtained by Warren-Averbach method using a standard DRON-3M powder diffractometer, working at 45 kV and 30 mA, and equipped with scintillation counter with single channel pulse height discriminator counting circuitry. The Cu Kα ( λCuK = 1.54178 Å) radiation, Ni filtered, was collimated with Soller slits. The data of the X-ray diffraction patterns were collected in a step-scanning mode with 0025.02 =∆ θ steps and then transferred to a PC for processing.


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Photodegradation and adsorption profiles were obtained from the concentration of salicylic acid, measured on a Jasco V- 530 UV/VIS spectrophotometer (from the decrease in the peak at 295nm). The detection limit for salicylic acid was 3 µM. Adsorbtion of salicylic acid on TiO2 was studied under the same experimental conditions (concentration, pH, ionic strenght, temperature) as the photodegradation studies. These experiments were performed in the dark to avoid photooxidation. Photodecomposition experiments were performed in a Teflon cell with a quartz window for illumination. The cell was irradiated by a 200 W Hg-Xe lamp. Irradiation intensity between 300-400 nm, was measured with an Eppley PSP radiometer and cutoff filters. Oxygen used in the reaction was atmospheric oxygen dissolved in the solution. Photodegradation was quenched by filtration of the samples.

RESULTS AND DISCUSSION

Characterization of TiO2 aerogels. Stable gels were obtained by using by using

water/TIP molar ratio of 2-4. It was observed that longer gelation time are needed when a smaller H2O/TIP ratio was used. Increasing water concentration determined a faster hydrolysis process. It is known that the balance between the rate of hydrolysis, condensation and surface ionization determines the structure and homogeneity of the gel [1].

BET surface area (SBET) of these materials is shown in Table II. The largest surface area was obtained for H2O/TIP = 3.325 and [EtOH]/[TIP] = 20. The heat treated aerogels have 4-5 times smaller surface area. This fact is explained by the change of structure from amorphous to crystalline, due to the heat treatment – determineted by X-ray diffraction.

The average crystallite size was determinated from X-ray diffraction spectrum. To estimate the crystallite size, the full width at half maximum (FWHM) of the largest peak was used through the Scherrer formula:

)cos(/ θλ Bkt =

where, t is the average dimension of the crystallite, k is a shape factor constant (assumed to be 1 for spherical particles), B is the FWHM, λ is the wavelength and θ is the incident angle of the X-rays.

Figure 1 shows the X-ray diffraction patterns resulting by the analysis of the heat-treated aerogels and suggests the formation of a anatase-like crystalline network structure with a = b = 0.37(8) nm and c = 0.95(7) nm lattice parameters, quite close to the ASTM reported values. X-ray diffraction patterns of the non heat-treated aerogels are typically for amorphous systems.


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TABLE II BET AREA, EFFECTIVE CRYSTALLITE MEAN

SIZE (DEFF) AND THE ROOT MEAN SQUARE OF THE

MICROSTRAIN SIZE (<(Ε2)>1/2) FOR SEVERAL TIO2

aerogel samples heat and non heat treated

Tipul de aerogel

SBET[m2/g] Deff(nm) <e2>1/2101x103

A A5000 C

484 132

- 7.51

- 3.981

B B5000 C

759 145

- 7.38

- 3.637

C C5000 C

596 127

- 6.96

- 5.338

D D5000 C

706 130

- 7.35

- 4.561

E E5000 C

555 139

- 7.91

- 3.408

F F5000 C

651 121

- 8.27

- 2.873

H H5000 C

685 125

- 7.65

- 3.581

K K5000 C

730 142

- 9.10

- 2.715

Pulbere Degussa

54 >20 -

20 30 40 50 60 70 801000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

600oC

500oC

400oC

(2 2 5)

(2 2 0)

(1 1 6)

(2 0 4)

(2 1 1)

(1 0 5)

(2 0 0) (0 0 4)

(1 0 1)

inte

nsity

[a

.u.]

diffraction angle 2θ [ degrees ]

Figure 1. X-ray diffraction patterns for the heat-treated K-type TiO2 aerogels


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The single (101) anatase X-ray diffraction line was analyzed in order to determine the microstructural parameters of the deposed films. The X-ray diffraction line broadening is caused by the small size of the crystallites, the lattice strains and lattice faults, and the experimental diffraction geometry [2]. The microstructural informations were obtained by a single X-ray profile Fourier analysis, with a method developed by Aldea and Indrea by a XRLINE computer program [3].

The microstructural information obtained by the single X-ray profile Fourier analysis of the heat-treated TiO2 aerogels were : the Deff(nm) effective crystallite mean size, the <ε2>1/2hkl root mean square (rms) of the microstrains averaged along [hkl] direction, and the α staking fault probability [2,4]. Tables II summarizes the microstructural parameters of the TiO2 samples. The morphology and crystallinity were controlled by adjusting the water contents of reactants and by heating the dry aerogels at the specified conditions. Average crystal size, grows with the temperature of thermal treatment and depends by H2O:TIP molar ratio (see table III).

Table III. Microstructural parameters of the TiO2 heat-treated K type TiO2 aerogels.

TiO2 aerogel sample Deff (nm) <ε2>1/2101 x 103 α

K400 4.9 3.432 0.043

K500 9.1 0.819 0.028

K600 14.8 0.609 0.016

Actually, the nanocrystallites are not monodisperses; it exists a distribution of sizes. Through scanning and transmission electron microscopies, the structure of the TiO2 aerogel was determinated to have two length scales, one for crystalline nanoparticles and other for mesoaggregates constituted by nanoparticles of anatase closely packed (Fig.2) [5].


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Figure 2. Schematic representation of TiO2 aerogel

Adsorbtion of Salycilic Acid on TiO2. To be able to monitor the dissapearance of salicylic from the solution, either by adsorbtion or by photodecomposition, it studied the adsorbtion profiles on the TiO2 samples before and after annealing at 5000C for 2 hours. Figure 4 shows that adsorbtion on the aerogel is about an order of magnitude higher than on the commercial material. This correlates with the larger surface area (see tab.II).

0 5 10 15 20 25 30-50

0

50

100

150

200

250

300

350

400

450

Adso

rbtia

[µM

/g]

Timp [ore]

Degussa C Ctr B Btr

Figure 3. Adsorbtion profile of salicylic acid adsorbed on annealed ( Btr, Ctr) and nonannealed (B, C) TiO2 aerogels as compared to a commercial TiO2 powder (Degussa)

Photodegradation of salicylic acid on TiO2 aerogel. Adsorption of salycilic acid was indicated by a bright yellow color on the titania surface (no color in the solution). This color stems from formation of a complex on the TiO2 surface that enhances electron charge transfer to the solution [6]. Under illumination, the yellow color changed gradually to dark brown, indicating oxidation of the organic material on the TiO2 surface. When oxidation of salicylic acid was completed the white color of TiO2 surface was restored.

Figure 4 shows photodegradation profiles for the aerogels (B and C) before and after annealing as compared to the commercial (Degussa) powder. It can be observed that the greater area surface aerogels have a greater photocatalytic activity, the nonannealed samples having a much higher activity.


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The numbers in the ordinate represent the solution concentration of salicylic acid, after illumination for the specific time interval and equilibration.

Direct photodegradation of salicylic acid in the absence of TiO2 did not take place under these experimental conditions. This was checked by illumination of a solution of salicylic acid under the same conditions used for the photocatalytic experiments. No decrease in the concentration was observed even after 5h.

0 400 800 1200 1600

0

100

200

300

400

500

Conc

entra

tia [µ

M]

Timp [min]

Degussa B Btr C Ctr

Figure 4. Photodegradation profiles of salicylic acid on annealed and nonannealed C– and D– TiO2 aerogels and on TiO2-powder (Degussa)

CONCLUSION The synthesis of efficient, nonsupported, higly porous TiO2 aerogels was

reported.These materials have a large surface area and very good photocatalytical activity. This, taken together with the bandgap and band edge position, makes the TiO2 aerogels suitable for photocatalytical degradation of organic pollutants. The sol-gel preparation method and critical point drying conditions are advantageous as the desired morphology of such devices can be tailored by modification of the preparation conditions. ACKNOWLEDGMENT

This work was supported by the Research and Education Ministry, through its MATNANTECH Program – under Contract C 101(204). REFERENCES 1. Loftus K.D., Sastry K.V.S., Hunt A.J., Advanced Materials Proceedings SME: Littleton, CO, 1990,

Vol 90, Chapter 26 2. van Bercum J.G.M., Vermeulen A.C., Delhez R., de Keijser T.H. and Mittemeijer E.M., J.Appl.

Phys., 27 (1994) 345-353 3. Aldea N. and Indrea E., Comput. Phys. Commun., 60 (1990) 155-159 4. Indrea E. and Barbu A., Appl. Surf. Sci., 106 (1996) 498-501 5. Zhu Z., Tomkiewicz M, Mat. Res. Soc. Symp. Proc., 346 (1994) 751-756 6. Moser J., Punchihewa S., Infelta P.P., Gratzel M., Langmuir 7 (1991) 3012


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tio2-aerogels morphology-synthesis correlation

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