Journal of Sol-Gel Science and Technology 21, 109–113, 2001 c 2001 Kluwer Academic Publishers. Manufactured in The Netherlands.
Synthesis and Characterization of Photocatalytic Porous Fe3+ /TiO2 Layers on Glass NATALIE SMIRNOVA, ANNA EREMENKO∗ AND OLGA RUSINA Institute of Surface Chemistry, Ukrainian Academy of Sciences, 17 Gen. Naumov Str. 03164 Kiev, Ukraine
[email protected]
WERNER HOPP Fraunhofer Institut f¨ur Silicatforschung, Neuner Platz 2, 97082 W¨urzburg, Germany LUBOMIR SPANHEL Fakult¨at f¨ur Chemie und Pharmazie, Universit¨at W¨urzburg, R¨ontgenring 11, 97070 W¨urzburg, Germany Received June 2, 2000; Accepted October 18, 2000
Abstract. Wet chemical synthesis and preliminary photocatalytical characteristics of titania and Fe(III)-containing TiO2 layers are presented. A highly stable coating colloids could be prepared under base- as well as acid-catalyzed condensation conditions. Structural properties of the as-prepared wet gels and sintered films were investigated using SEM, TEM, XRD as well as optical absorption spectroscopy, DTA-TG analysis and photomineralisation studies. X-ray amorphous wet titania gel layers start to crystallize at 500◦ C forming the characteristic anatase phase. In the presence of iron ions (Fe/Ti = 1), nanocrystalline FeTiO3 ilmenite phase forms. Both TiO2 and Fe-containing TiO2 films demonstrate a photocatalytic activity in the process of the photomineralization of dichloroacetic acid. Keywords: Fe3+ /TiO2 films, nanocrystalline layers, solar energy materials, photocatalysis
1.
Introduction
Sol-gel processing of metal oxide glasses and ceramics have been increasingly employed to produce nanomaterials for innovative applications, especially in the field of composites, porous monoliths and coatings [1–3]. Especially titanium oxide films on glass belong to the class of the most currently investigated functional materials with a large application potential in photonics [4, 5], solar cell technology [6] and photocatalytic environment cleaning [7]. In the field of photocatalysis, much work was devoted to optimize the performance of the TiO2 photocatalysts in order to enhance the efficiency ∗ To
whom all correspondence should be addressed.
of light harvesting in the visible region of the solar spectrum. For example, covalently linked Co2+ tetrasulfophtalocyanine antennas [8], entrapped Fe3+ colour centers [9], photostable CdS sensitizers [10] or more recently Pt4+ -halide molecules [11] were used to functionalize titania crystallites in dispersions and colloids. Of special interest are the cost effective Fe3+ /TiO2 -dispersions thoroughly investigated with respect to their use in solar water detoxification processes [12]. From these studies it is known that iron supports the photocatalytic effect of titania in the visible range. However, a few obstacles need to be overcome before a commercialization of this photocatalyst becomes possible. On the one hand, one needs to avoid the release of iron out of the photoirradiated TiO2 interior as well as to block the phase separation at large
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iron contents (appearance of non-active hematite- and pseudobrookite phases). On the other hand, a suitable form of the photocatalyst will be required, e.g., transparent thick and highly porous layers on large area glass supports. This paper deals with the synthesis, structural and optical characterization of concentrated stable TiO2 and Fe3+ /TiO2 -sols and films. In addition, results from photocatalytic studies are presented where a photomineralisation of dichloroacetic acid (DCA) on supported TiO2 - and Fe3+ /TiO2 -membranes is demonstrated. 2. 2.1.
Experimental Synthesis of TiO2 - and Fe3+ /TiO2 Films
Ti(i-C3 H7 O)4 , Ti(C4 H9 O)4 , butoxyethanol (BuEtOH), diethanolamine (DEA), tetramethyl-ammonium hydroxide (TMAOH) and ethanol were used as received. Figure 1 depicts the general flow scheme of the synthesis of the titania photocatalysts. The titania sol preparation starts with a dissolution of titanium alkoxide (ABCR) in a half of prescribed amount of isopropyl alcohol. Then, the solution is mixed with an alkaline or acidic H2 O in the remainder of i-C3 H7 OH under snowbath temperature and magnetic stirring conditions. The Fe3+ /TiO2 coating solutions were obtained by dissolving a high purity FeCl3 salt in a EtOH/Ti(i-C3 H7 O)4 — mixture which was further used to coat pre-cleaned glass substrates under ambient laboratory conditions.
Table 1.
Figure 1. Schematic flow diagram for the synthesis of titania sols and films.
Dip-coating speed was 6.9 cm/min, in individual cases 13 cm/min. The wet gel films were finally sintered in air between 450◦ C and 500◦ C for 5 min. All compositions investigated are listed in Table 1. 2.2.
Structural and Optical Characterizations
DTA/TG measurements were made with a Setaram TAG 24 analyser. The heating rate was 10 K/min. X-ray
Synthesis conditions and properties of planar titania photocatalysts.
N
Ti(OR)4 Mol
Catalyst Mol
Solvent Mol
H2 O Mol
BuEtOH or MCS Mol
TiO2 % in precursor
Structure crystal size
1
1
DEA, 1
16
1–4
4
12, 15 , 20, 24
Anatase after 500◦ C d = 140A
1
16
2
4
Film thickness 200 nm
MCS
20
650 nm
2
1
2
16
1–4
4
15, 20
Peeled
3
1
TMAOH
EtOH
2
BuEtOH
8
Precipit
0.1–1
10
HAc
20
4
1
1 1–4
1–4
8–14
Anatase after 500◦ C
Peeled
10
Anatase after 500◦ C
150–290 nm
1–3 5
6
Ti(BuO)4
HNO3
EtOH
1
0.03
16
Ti(i-pro)4
FeCl3
EtOH
5%Ti
1
1
20
5%Fe
490 nm
Photocatalytic Porous Fe3+ /TiO2 Layers
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erage titania film thickness of about 500 nm) were used for the investigations. 3. 3.1.
Figure 2. Laboratory scale photoreactor for the examination of titania films in the process of photomineralization of chlorinated hydrocarbons.
diffraction measurements on sintered TiO2 coatings were performed with a Cu-Kα1 STOE/STADIP system using Bragg-Brentano geometry. Crystallite sizes, morphology and film thickness of titania samples were determined using a 120 kV Philips CM12 TEM/STEM microscope. Optical absorption spectra of the sintered TiO2 - and Fe3+ /TiO2 -films were taken with a Hitachi U 3000 spectrophotometer.
2.3.
Results and Discussion Thermal and Structural Analysis of Titania Sols and Films
The XRD data of the all coating sols investigated revealed an amorphous structure prior to air sintering. Above 500◦ C, the titania films undergo crystallization yielding porous Anatase phase. Figure 3 shows the characteristic XRD pattern of the air sintered titania films on glass. The mean crystallite size has been determined to be around 12 nm (using Debye-Scherrer formula). In the presence of 1–5 at.% Fe3+ , there were no significant changes in the XRD patterns detected whereas on film samples containing larger amounts of Fe3+ (Ti/Fe = 1), FeTiO3 ilmenite phases were found with its own distinctive X-ray diffraction pattern with d = 2.75; 2.54; 1.73. TG analysis data collected on sample N1 (see Table 1) indicated a weight loss at 120◦ C (8%), 380◦ C (14%) and 580◦ C (20%) attributed to water removal (at 120◦ C) and pyrolysis of unhydrolysed organics and evaporation of combustion gas in the films. No explicit exothermic peak associated with anatase formation was recognised from the DTA data. TEM investigations of air sintered titania films and the selected area diffraction (SAD) also gave evidence of the anatase phase. The films exhibited a broad variety of different aggregate sizes (ranging between 30 nm
Photocatalytic Investigations
Figure 2 shows the scheme of a laboratory scale photoreactor used to test the photocatalytical activity of titania films which is similar to that employed by Bahnemann et al. [12] for the investigations of colloidal dispersions. As a model substance for the photocatalytic decompositions, dichloroacetic acid was employed. The rate of photomineralization was followed by pH measurements through the titration of evolved protons with NaOH solution at 17◦ C [12]. Reference reacting solution contained of 0.2525 g KNO3 , 0.151 g DCA, 1 ml 1 N NaOH all diluted with water up to 250 ml. A 200 W Hg-Xe lamp was used as the photoexcitation source. The reactor was held at constant temperature of 17◦ C during the photoirradiations and purged with app. 10 L oxygen per hour, respectively. Typically, three single dip coated glass slides (with av-
Figure 3. Characteristic XRD-pattern of air-sintered titania anatase photocatalysts.
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3.3.
Photocatalytic Activity
In our studies on titania films functionalized with trivalent iron cations, we used model compound dichloroacetic acid (DCA). From earlier studies on photoirradiated TiO2 particles in the presence of O2 and water, it is known that DCA degrades completely according to [14]: + − CHCl2 CO− 2 + O2 → 2CO2 + H + 2Cl
Figure 4. Optical absorption spectra of air sintered, single coated titania and iron-titania films on glass support (film thickness about 0.5 µm).
and 60 nm) composed of app. 13 nm large primary crystallites which coincides with the above XRD data and pores approximately 10 nm. The typical film thickness of the single coated air sintered titania layers was determined in SEM measurements to be about 0.5– 0.7 µm. In the Fe/Ti = 1 system, the primary crystallite size of the FeTiO3 phase was determined to be 8–10 nm. 3.2.
Optical Absorption Spectra
The optical absorption spectra of the titania sols and wet gel films exhibit a step absorption edge around 320 nm which shifts toward 350 nm after air sintering. Figure 4 shows the characteristic optical absorption spectrum of an air sintered transparent titania film exhibiting a high optical absorption below 370 nm (>99%) as well as interference fringes in the visible range. One also recognizes the characteristic optical absorption spectrum of the iron containing red coloured titania film (Ti/Fe = 1) with the shoulder around 550 nm. At low iron contents this optical absorption in the visible range is rather low, attributed to the excitation of 3d-electrons of the Fe(III) into the TiO2 conduction band. At higher iron concentrations, electronic band to band transitions in entirely new ternary FeTiO3 -compounds seems to be responsible for the pronounced visible absorption, comparable with that observed on iron oxides [12, 13]. As it will be shown in the next chapter, the red coloured film samples are active in the process of photomineralization in the visible spectral range.
(1)
The preliminary results of the photocatalytic degradation of DCA are presented in Fig. 5, where the consumption of the sodium hydroxide is plotted against the photoirradiation time for four different photocatalyst film samples. As it can be seen, the illumination of all TiO2 and FeTiO3 films in the presence of DCA and oxygen lead to proton release due to degradation of the organic molecules. The upper limit for the H+ appearance was achieved after 2.5 h of irradiation. For a given experimental conditions, thin titania and Fe/Ti films are more effective photocatalysts, than those after 2 or more dipcoating coverings. The effectiveness of Fe/Ti patterns is lower when cut-off filters (365 nm + 430 nm) were used. The activity may be recovered by treatment of the films in air at 500◦ C after photocatalytic process. Furthermore, iron containing samples (curves 1, 3) are more active than bare thick titania film (curve 4)
Figure 5. DCA-photodegradation curves collected on titania and iron-titania films immersed in oxygenated water under UV/vis-light excitation conditions (λex > 320 nm). (1) Ti/Fe = 1, single coated; (2) Ti/Fe = 1, double coated; (3) TiO2 , single coated; (4) TiO2 , double coated.
Photocatalytic Porous Fe3+ /TiO2 Layers due to higher optical absorption in the visible spectral range. In addition, single coated films, although thinner, are slightly more efficient than double coated samples. A possible explanation could be the development of a dense interface region between the two layers. However, more detailed morphological investigations would be needed to clarify the lower activity of the double coated thicker layers. 4.
Conclusion
Thin titania films of high uniformity composed of anatase nanocrystals as well as iron-functionalized titania films have been synthesized and their photocatalytic activity in UV/vis-range demonstrated. The quantum yield of the dichlotroacetic acid decomposition was rather low (
