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Highly photoactive and stable TiO2 coatings on sintered glass
Applied Catalysis A: General 277 (2004) 183–189 www.elsevier.com/locate/apcata
Highly photoactive and stable TiO2 coatings on sintered glass M.C. Hidalgo*, S. Sakthivel, D. Bahnemann Institut fu¨r Technische Chemie, University of Hannover, Callinstrasse 3, D-30167 Hannover, Germany Received 19 July 2004; received in revised form 9 September 2004; accepted 9 September 2004 Available online 19 Ocotober 2004
Abstract TiO2 coatings on sintered borosilicate glass (100–160 mm pore size) were prepared employing a synthetic route based on the hydrolysis of titanium oxysulfate (TiOSO4). The morphology of the coatings was studied by scanning electron microscopy (SEM), showing TiO2 layers with a smooth and homogeneous structure covering the entire surface of the particles forming the sintered glass. The photocatalytic activity of the TiO2 coatings was analysed employing the photooxidation of glucose in water as test reaction. The influence of the preparation conditions of the coatings on their photoactivities was studied. It was found that the starting concentration of precursor and the ageing time were two parameters of significant relevance for the final properties of the catalysts. With specific preparation parameters, photonic efficiencies up to 4% for the total mineralisation of glucose in water could be reached. The effect of the reaction pH during the photocatalytic tests on the efficiency of these coatings was also studied, with the glucose degradation being more favourable at high values of pH. Stability and lifetime of the coatings were tested following the evolution of their photocatalytic activity for more than 4000 h of continuous operation. No appreciable degradation of the coatings could be observed during this time. # 2004 Elsevier B.V. All rights reserved. Keywords: TiO2; Titanium oxysulfate; Sintered glass; Coatings; Photocatalysis; Glucose
1. Introduction In the last 20 years, heterogeneous photocatalysis has become one of the most studied processes as convenient route for water and air purification [1–3]. The mineralisation of last traces of organic contaminants in water together with the removal of microorganisms make photocatalysis into a technique suitable not only for the treatment of wastewater but as an alternative to conventional drinking water treatment as well as for the production of ultrapure water for pharmaceutical applications [4–7]. Titanium dioxide is the most commonly used semiconducting photocatalyst since it is highly photoactive, photostable, biologically and chemically inert, nontoxic and relatively inexpensive. Regarding economical and practical reasons, it has become clear that for many applications the most useful form of the * Corresponding author. Tel.: +49 511 762 2773; fax: +49 511 762 2774. E-mail address: [email protected] (M.C. Hidalgo). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.09.011
photocatalyst is that of a film or coating on a convenient support [8,9]. The supporting material plays a major role in the stability and the final photocatalytic properties of the supported catalysts as well as for the design of the most appropriate reactor. Several materials have already been studied as supports for titanium dioxide or other photocatalysts, such as glass, quartz, glass fibers, wool, steel, etc. [10] and references therein. This paper describes the preparation of TiO2 coatings supported on sintered borosilicate glass, with nominal pore size between 100 and 160 mm. This material presents some advantageous features as a support in photocatalysis, since it should be feasible to form strong surface bonds with the catalyst without negatively affecting its photoefficiency, and it exhibits a high transparency to ultraviolet light down to a wavelength of 300 nm. Additionally, the sintered glass with this porosity allows the flow of fluid through its structure without appreciable pressure drop and is flexible from the point of view of the reactor design.
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It is well known that structural and morphological properties of catalysts are greatly dependent on the procedure followed for their preparation [11]. This is fully applicable to the synthesis of TiO2, where different experimental conditions and parameters during the preparation can lead to very different final properties of this material, including its photocatalytic activity [12–15]. In the present work, the TiO2 coatings were prepared by a synthetic route based on the hydrolysis of TiOSO4, and the influence of the main preparation parameters on the final properties of the coatings was studied. However, a stable fixation and long lifetime of the coatings will for practical applications be as important as a high photocatalytic activity and therefore the stability of the coatings were tested in repeated successive experiments.
2. Experimental 2.1. Preparation of the coatings The TiO2 coatings were prepared on sintered borosilicate glass (100–160 mm nominal pore size) by a dip-coating method, using square sintered glass plates (90 mm 90 mm) with a thickness of 4 mm. The dipping solutions were prepared by mixing TiOSO4 (Alfa Aesar, titanium(IV) oxide sulfate dihydrate) with distilled water at a concentration of 12.3 wt.% (4 wt.% as TiO2). The mixtures were stirred at room temperature until a clear solution was obtained, and then heated up to 85 8C. Then, a 5% NaOH solution was slowly added until pH 5.5 was reached. The formed gels were kept under stirring at this temperature for 6 h. After this ageing time, the gels were filtered, repetitively washed with water, and finally resuspended in distilled water, using twice the volume of the original amount of water. Then, the final solutions were sonicated for 6 h. After sonication, the dispersions are ready for the preparation of the coatings and can be stored at 5 8C for long time without affecting their properties and without any apparent sedimentation. For the preparation of the coatings, the sintered glass plates were dipped in the dispersions for 15 s and pulled up at a controlled speed of 1 mm/s. After dipping, the plates were dried at 110 8C for 2 h and calcined at 500 8C for 1 h, following heating rates of 5 8C/min. Other dipping solutions were also prepared changing different experimental parameters, such as the initial concentration of the precursor or the ageing time, and the influence of these variables on the photocatalytic activity and stability of the coatings was studied. Every solution with specific experimental parameters and their corresponding coated plates were prepared at least twice to ensure reproducibility in stability and photoactivity.
In order to compare the efficiency of these catalysts, coatings on sintered glass were also prepared by a traditional sol–gel route, using titanium tetraisopropoxide (TTIP, Aldrich 99.999%) as titanium precursor and following a preparation procedure described in the literature [16,17]. 2.2. Characterisation Scanning electron microscopy (SEM) was performed using a Zeiss apparatus, model DSM 982, equipped with Xray energy-dispersive analyses (EDX). Prior the measurements, the samples were cut in pieces of ca. 1 cm2 and coated with a platinum/palladium layer employing a standard sputtering technique. Micrographs of both, the surface and the broken-inner part of the sintered glass pieces were taken. UV–vis spectra of the coatings were measured employing a Varian spectrometer model Cary 100, equipped with an integrating sphere, and using Ba2SO4 as reference. All spectra were recorded in diffuse reflectance mode. 2.3. Photocatalytic activities The photocatalytic activity of the TiO2 coatings was tested for the reaction of photooxidation of glucose (D-+glucose, Merck, anhydrous for biochemistry) in water, using a starting concentration of 3 mg/L total organic carbon (TOC) in all cases. The measurements of activity were performed in a 200 mL square reactor made of polyoxylmethylen (POM), where the coated plate is fitted and sealed inside dividing the reactor in two compartments, and which is provided with a UV-transparent PMMA window (80 mm 80 mm) for front illumination and two openings for inlet and outlet of the fluid, respectively. For the tests, a total volume of 2 L glucose solution (at natural water pH) was recirculated through the plates with a flow rate of ca. 40 L/h. The first 20 min were run in the dark to allow the equilibration of the system. Illumination was provided by five parallel Philips UV-lamps (20 W). The incident photon flux on the plates was determined by ferrioxalate actinometry [18] to be 3.0 1017 photons s1, and checked periodically. Measurements with an UV meter (Dr. Ho¨ nle UVA/B meter) where used for detailed corrections. Glucose degradation profiles with illumination time were followed by on-line measurements of total organic carbon (TOC) contents, using a TOC analyser (Shimadzu TOC5000A). Blank experiments were performed in absence of any illumination as well as with uncoated sintered glass plates, without observable change in the initial concentration of glucose in both cases. Photonic efficiencies (j%) of the different coatings for glucose degradation were calculated using the following
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equation [19]: jð%Þ ¼
d½C=dt 100 P0
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3. Results and discussion (1)
where dC/dt is the initial mineralisation rate (in terms of mol organic C/s), zero-order kinetics assumed, and P0 is the incident photonic flux (einstein s1). 2.4. Lifetime tests of the coatings Stability and lifetime of the coatings were tested following the evolution of their photocatalytic activity for more than 4000 h of continuous operation. These tests were performed in a different reactor, consisting in a cylinder made of PMMA with two openings for inlet and outlet, into which a coated sintered glass tube (225 mm long, 28 mm i.d., 100–160 mm nominal pore size) was fitted and sealed. A Philips CLEO compact lamp (15 W) longitudinally placed inside the reactor provided inner illumination. Glucose solution was continuously re-circulated through the reactor at 40 L/h under illumination. The photonic efficiency of the coating was estimated periodically by introducing freshly prepared 3 mg/L glucose solution (pH 6) at a total volume of 2 L. The incident photon flux in this reactor was also determined by ferrioxalate actinometry [18], being 1.6 1018 photon s1. The lamp was exchanged periodically and its intensity was checked before every calculation of photonic efficiency bymeasurements employing the UV meter (Dr. Ho¨ nle UVA/ B meter) and correlated with the corresponding photon flux.
3.1. Morphology of the coatings The morphology of the coatings was studied by scanning electron microscopy (SEM), evaluating the coating on the surface as well as in the inner part of the sintered glass plates. In Fig. 1 selected SEM images are shown for an uncoated (Fig. 1A and B) and for a coated plate (inner part) (Fig. 1C and D), respectively. The micrographs for the uncoated sintered glass, Fig. 1A and B, show a structure formed by glass particles from around 200 to 400 mm in size sintered together. Fig. 1C and D shows the coated sample, where it can be observed that the form and structure of the coating is considerably smooth and homogeneous all over the surface of the particles forming the sintered glass, presenting fissures only at some joints between sintered particles. On the other hand, the small particles that can be seen on the surface of the coating are not TiO2, as confirmed by EDX analyses, and consequently they can be also observed on the surface of uncoated samples, as it is evident from Fig. 1B. These particles originate probably from the process of cutting the plates during the preparation of the samples for SEM. Fig. 1C and D shows the coating in the inner part of the sintered glass plate, however, similar morphology for the coatings could be observed on the surface of the plates (pictures not shown). 3.2. Photocatalytic properties The photonic efficiency (j%) of the coatings prepared on sintered glass from the hydrolysis of titanium sulfate, as
Fig. 1. Scanning electron microscopy images for uncoated borosilicate sintered glass (A and B) and TiO2 coating on borosilicate sintered glass (C and D).
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described in Section 2.1, was measured to be j = 2.5% for the photocatalytic mineralisation of glucose in water. The values of photonic efficiencies showed high reproducibility when calculated for different plates coated employing the same conditions as well as for different measurements of the same plate. Following the same preparation procedure, other coatings were prepared but from dipping solutions containing different initial amounts of titanium oxysulfate (TiOSO4), covering the range between 3.8 and 30.6 wt.% (1.2–10 wt.% in terms of TiO2 content). A strong influence of the concentration of the precursor on the activity of the coatings was found, as shown in Fig. 2. A significant increase of the photoactivity of the coatings is observed as the initial concentration of TiOSO4 is increased, starting from a photonic efficiency of not over 1% when using a starting concentration of 3.8 wt.% TiOSO4 and rising to a photonic efficiency of 4% when the initial concentration of TiOSO4 increases to 30.6 wt.%. As also seen from Fig. 2, the photonic efficiency for the coating prepared by the alcoxide route (TTIP) is lower than that for any coating prepared by the hydrolysis of TiOSO4. Even if both preparation methods cannot be compared directly, this effect cannot be due to a different initial concentration of precursor (since for the alcoxide method this would be around 3.5 wt.% based upon TiO2) but rather to the preparation route itself. The amounts of TiO2 deposited on the sintered glass plates were determined by weighing the plates before and after the coating. Employing these values, and assuming that the coatings are homogeneously distributed over all the sintered glass (as observed by SEM (Fig. 1C and D)), the thickness of the TiO2 layers was estimated from these weight differences. For these calculations the density of TiO2 anatase was taken as 3.84 g/cm3 and the surface area of the sintered glass as 0.03 m2/g (value provided by the supplier of the sintered glass). These results are presented in Table 1.
Fig. 2. Influence of the initial concentration of titanium oxysulfate present in the dipping solutions on the photonic efficiencies (j%) of the final coatings on sintered glass. In brackets: initial concentrations of the dipping solutions in terms of the content of TiO2. TTIP: value for the coating prepared by the alcoxide route. Photonic efficiencies calculated for the total photocatalytic mineralisation of glucose in water.
Table 1 Calculated mean thickness and photonic efficiency of the coatings prepared from the indicated initial concentrations Concentration Concentration Mass of the Mass of Mean j(%) TiOSO4 as TiO2 uncoated TiO2 on the thickness plate (g) (nm) (wt.%) (wt.%) plate (g) 3.8 6.4 12.3 18.3 30.6
(0.18 M) 1.2 (0.28 M) 2 (0.58M) 4 (0.93 M) 6 (1.82 M) 10
n.d. 39.442 44.967 40.493 40.285
n.d. 0.101 0.219 0.263 0.603
n.d. 22 42 57 130
1.3 1.5 2.5 3.0 4.0
Photonic efficiencies calculated for the total photocatalytic mineralisation of glucose in water. n.d.: not determined.
The theoretical thicknesses of the coatings start at 22 nm and end at 130 nm depending on the initial concentration of TiOSO4. As it can be observed in Fig. 3, there is a linear increase in the thickness of the layers as well as a linear increase in the photocatalytic activity with the increase of the initial concentration of precursor. This is suggesting that the high photonic efficiencies reached for these coatings are due to the capacity of the sintered glass to incorporate by this preparation method high amounts of TiO2 as thick and stable layers. Thus, the higher activities of the coatings prepared from highly concentrated initial solutions could be explained by the higher amount of TiO2 in the coatings, resulting in a more efficient utilisation of the incident photon flux. The ageing time was also found to have an effect on the activity of the coatings, even if this was not as significant as that of the concentration of the precursor. To study this effect, different coatings were prepared using ageing times between 2 and 8 h during the preparation of the dipping solutions (initial concentration 12.3 wt.% TiOSO4). As Fig. 4 shows, preparations with ageing times longer than 6 h resulted in the highest values of efficiencies for the coatings under these experimental conditions. During the period of ageing, processes of polycondensation and syneresis as well as of dissolution and reprecipitation in the gel network are produced, driven by differences in solubility between surfaces with different radii of curvature [20]. This causes the growth of necks between particles, increasing the strength and stiffness of the gel structure that later will form the coating. Stronger and more stable structures will resist better the process of shrinkage during the steps of drying and calcination, keeping the original pore structure to a higher degree. Both, the more stable as well as the more defined pore structure of the TiO2 layer could be responsible for the observed improvement in the photocatalytic efficiency of coatings prepared with extended ageing times. During the preparation procedure, the pH reached in the hydrolysis is also known to have an important influence on the final properties of the catalysts. In the work described here a pH 5.5 has been chosen based on Ref. [21], where this value of pH was found to be the optimum for the
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Fig. 3. Calculated mean thickness (circles, right y-axis) and photonic efficiency (squares, left y-axis) of the coatings vs. the initial concentration of titanium oxysulfate used in their preparations. Photonic efficiencies calculated for the total photocatalytic mineralisation of glucose in water.
photoactivity of TiO2 powders prepared employing a similar synthetic method. On the other hand, other parameters in the preparation were found to be important not for the activity of the coatings but for their stability. Preliminary experiments showed that the repetitive washing of the gel strongly affected the stability of the final coating. When this step was omitted, the fixation of the TiO2 layer to the glass was found to be very poor and unstable, probably due to high amounts of SO42 and Na+ ions incorporated in the gels. 3.3. Absorption properties The absorptive properties of the coatings were studied by diffuse reflectance UV–vis spectroscopy (DRS). This technique is especially useful in photocatalytic applications,
since it can yield information about the absorption capacities and the band gap values of the semiconductors. Fig. 5 shows the diffuse reflectance spectra for the coatings prepared from solutions of different starting concentrations. The spectrum of the uncoated sintered glass (Fig. 5a) shows that this material is transparent to UV irradiation of wavelengths exceeding 300 nm, important prerequisite for a support of photocatalysts. Regarding the coated samples, no significant differences can be observed for the spectra of the coatings prepared at higher starting concentrations (Fig. 5d–f), showing in any case absorption of light with wavelengths below 375 nm. However, the coatings prepared from lower starting concentrations (Fig. 5b and c) exhibit a notable shift of the absorption edge to lower wavelengths, which can be explained by a higher band gap energy, probably due to the smaller nano-sized TiO2 particles present in these samples. The lower range of light absorption would also explain the lower photoactivity found for these two coatings, as shown in Fig. 2. No important differences could be observed in the spectra for the coatings prepared with different ageing times, being in each case similar to the spectrum shown in Fig. 5d (results not shown). 3.4. Lifetime of the coatings
Fig. 4. Influence of the ageing time on the photonic efficiencies (j, %) of the final coatings on sintered glass. Value for the coating prepared by the alcoxide route (TTIP) is also shown. Photonic efficiencies calculated for the total photocatalytic mineralisation of glucose in water.
In order to study the stability and lifetime of the coatings on sintered glass, the photocatalytic efficiency of a coating (12.3 wt.% TiOSO4 initial concentration, 6 h ageing) was followed during 4000 h of continuous operation under the conditions stated in Section 2.4. Fig. 6 shows the evolution of the photonic efficiency for the glucose oxidation employing this coating, which was calculated periodically every 500–1000 h during this non-
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Fig. 5. Diffuse reflectance spectra for the uncoated sintered glass (a), and for the coatings prepared with different initial concentrations of TiOSO4: 3.8 wt.% (b); 6.4 wt.% (c); 12.3 wt.% (d); 18.3 wt.% (e); 30.6 wt.% (f, non-continuous line).
The influence of the reaction pH on the degradation of glucose during the photocatalytic tests for our coatings on sintered glass was studied in the pH range between 2 and 9.
The pH of the glucose solutions used for these tests was adjusted to the desired values by addition of small amounts of HNO3 or NH3. In each experiment the reaction pH was monitored on-line and no notable changes of the initially adjusted pH could be observed during the irradiation time. Fig. 7 shows the photonic efficiencies for the mineralisation of glucose at different pH values employing the same coating (prepared from 12.3 wt.% TiOSO4 initial concentration, 6 h ageing). It is evident that the pH used during the tests has a strong influence on the efficiency of the coating for the degradation of glucose, improving considerably with increasing reaction pH. It is well known that the reaction pH is an important parameter in heterogeneous photocatalysis, since it has a strong influence on the surface charge of the catalysts, and therefore it also affects their properties of absorption
Fig. 6. Percentage of the original activity of the TiO2 coating over 4000 h of continuous operation in the photocatalytic degradation of glucose in water. Flow rate = 40 L/h, photon flux = 1.6 1018 photons s1.
Fig. 7. Influence of the reaction pH on the photonic efficiency of the total photocatalytic mineralisation of glucose in water.
stop operation. It can be noticed that the coating efficiency did not change considerably at any moment, exhibiting after 4000 h of operation still 100% of its initial activity (assuming as 100% activity the value of photonic efficiency shown during the first 12 h of operation). Thus, it can be assumed that even using a relatively high flow rate (40 L/h) for more than 4000 h, there was no notable degradation of the coating caused by abrasion due to the flow of the substrate solution. 3.5. Influence of the substrate pH on the photocatalytic tests
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concerning the compounds that will be degraded in the process [22,23]. Consequently, the influence of the pH has especial importance for the degradation of polar molecules, such as glucose, the model compound chosen in this work for the photocatalytic tests. Some studies have been performed about the influence of pH on the photocatalytic degradation of many compounds, i.e. carboxylic acids were found to be more efficiently degraded at low pH values and chloroform at high pH values, while 4-chlorophenol was found not significantly pH-dependent [24]. Nevertheless, as far as we know, the pH influence on the photocatalytic degradation of glucose has never been studied. The negatively charged TiO2 surface at high pH values should accommodate more glucose, probably helped by a specific orientation and stronger adsorption of the molecules at these pH values. This local higher concentration of glucose on the TiO2 surface at high pH would improve the efficiency of the process of photooxidation of this compound.
4. Conclusions Photoactive TiO2 coatings were prepared by dip-coating, following a clean and simple synthetic route based on the hydrolysis of TiOSO4. Sintered glass plates, the support chosen for the coatings, proved to be very good supporting materials for titania, allowing the preparation of thick and stable coatings of relatively high photonic efficiency. However, a strong influence of the preparation conditions on the final activity of the coatings was found, with the starting concentration of precursor and the ageing time being two parameters possessing significant effects. Employing specific preparation parameters, the photonic efficiency for these coatings reached values up to 4% for the total mineralisation of glucose in water. The coatings produced on sintered glass by the method described in this paper appeared mechanically very robust and were not degraded even after repeated successive applications (4000 h of continuous operation did not result in any appreciable decrease in the photocatalytic activity). Nevertheless, an important influence of the reaction pH on the efficiency of these coatings was found during the photocatalytic tests, with the glucose degradation being more favourable at higher pH values.
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Acknowledgements S. Sakthivel thanks the DAAD for financial support to perform his research work in Germany.
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Highly photoactive and stable TiO2 coatings on sintered glass M.C. Hidalgo*, S. Sakthivel, D. Bahnemann Institut fu¨r Technische Chemie, University of Hannover, Callinstrasse 3, D-30167 Hannover, Germany Received 19 July 2004; received in revised form 9 September 2004; accepted 9 September 2004 Available online 19 Ocotober 2004
Abstract TiO2 coatings on sintered borosilicate glass (100–160 mm pore size) were prepared employing a synthetic route based on the hydrolysis of titanium oxysulfate (TiOSO4). The morphology of the coatings was studied by scanning electron microscopy (SEM), showing TiO2 layers with a smooth and homogeneous structure covering the entire surface of the particles forming the sintered glass. The photocatalytic activity of the TiO2 coatings was analysed employing the photooxidation of glucose in water as test reaction. The influence of the preparation conditions of the coatings on their photoactivities was studied. It was found that the starting concentration of precursor and the ageing time were two parameters of significant relevance for the final properties of the catalysts. With specific preparation parameters, photonic efficiencies up to 4% for the total mineralisation of glucose in water could be reached. The effect of the reaction pH during the photocatalytic tests on the efficiency of these coatings was also studied, with the glucose degradation being more favourable at high values of pH. Stability and lifetime of the coatings were tested following the evolution of their photocatalytic activity for more than 4000 h of continuous operation. No appreciable degradation of the coatings could be observed during this time. # 2004 Elsevier B.V. All rights reserved. Keywords: TiO2; Titanium oxysulfate; Sintered glass; Coatings; Photocatalysis; Glucose
1. Introduction In the last 20 years, heterogeneous photocatalysis has become one of the most studied processes as convenient route for water and air purification [1–3]. The mineralisation of last traces of organic contaminants in water together with the removal of microorganisms make photocatalysis into a technique suitable not only for the treatment of wastewater but as an alternative to conventional drinking water treatment as well as for the production of ultrapure water for pharmaceutical applications [4–7]. Titanium dioxide is the most commonly used semiconducting photocatalyst since it is highly photoactive, photostable, biologically and chemically inert, nontoxic and relatively inexpensive. Regarding economical and practical reasons, it has become clear that for many applications the most useful form of the * Corresponding author. Tel.: +49 511 762 2773; fax: +49 511 762 2774. E-mail address: [email protected] (M.C. Hidalgo). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.09.011
photocatalyst is that of a film or coating on a convenient support [8,9]. The supporting material plays a major role in the stability and the final photocatalytic properties of the supported catalysts as well as for the design of the most appropriate reactor. Several materials have already been studied as supports for titanium dioxide or other photocatalysts, such as glass, quartz, glass fibers, wool, steel, etc. [10] and references therein. This paper describes the preparation of TiO2 coatings supported on sintered borosilicate glass, with nominal pore size between 100 and 160 mm. This material presents some advantageous features as a support in photocatalysis, since it should be feasible to form strong surface bonds with the catalyst without negatively affecting its photoefficiency, and it exhibits a high transparency to ultraviolet light down to a wavelength of 300 nm. Additionally, the sintered glass with this porosity allows the flow of fluid through its structure without appreciable pressure drop and is flexible from the point of view of the reactor design.
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It is well known that structural and morphological properties of catalysts are greatly dependent on the procedure followed for their preparation [11]. This is fully applicable to the synthesis of TiO2, where different experimental conditions and parameters during the preparation can lead to very different final properties of this material, including its photocatalytic activity [12–15]. In the present work, the TiO2 coatings were prepared by a synthetic route based on the hydrolysis of TiOSO4, and the influence of the main preparation parameters on the final properties of the coatings was studied. However, a stable fixation and long lifetime of the coatings will for practical applications be as important as a high photocatalytic activity and therefore the stability of the coatings were tested in repeated successive experiments.
2. Experimental 2.1. Preparation of the coatings The TiO2 coatings were prepared on sintered borosilicate glass (100–160 mm nominal pore size) by a dip-coating method, using square sintered glass plates (90 mm 90 mm) with a thickness of 4 mm. The dipping solutions were prepared by mixing TiOSO4 (Alfa Aesar, titanium(IV) oxide sulfate dihydrate) with distilled water at a concentration of 12.3 wt.% (4 wt.% as TiO2). The mixtures were stirred at room temperature until a clear solution was obtained, and then heated up to 85 8C. Then, a 5% NaOH solution was slowly added until pH 5.5 was reached. The formed gels were kept under stirring at this temperature for 6 h. After this ageing time, the gels were filtered, repetitively washed with water, and finally resuspended in distilled water, using twice the volume of the original amount of water. Then, the final solutions were sonicated for 6 h. After sonication, the dispersions are ready for the preparation of the coatings and can be stored at 5 8C for long time without affecting their properties and without any apparent sedimentation. For the preparation of the coatings, the sintered glass plates were dipped in the dispersions for 15 s and pulled up at a controlled speed of 1 mm/s. After dipping, the plates were dried at 110 8C for 2 h and calcined at 500 8C for 1 h, following heating rates of 5 8C/min. Other dipping solutions were also prepared changing different experimental parameters, such as the initial concentration of the precursor or the ageing time, and the influence of these variables on the photocatalytic activity and stability of the coatings was studied. Every solution with specific experimental parameters and their corresponding coated plates were prepared at least twice to ensure reproducibility in stability and photoactivity.
In order to compare the efficiency of these catalysts, coatings on sintered glass were also prepared by a traditional sol–gel route, using titanium tetraisopropoxide (TTIP, Aldrich 99.999%) as titanium precursor and following a preparation procedure described in the literature [16,17]. 2.2. Characterisation Scanning electron microscopy (SEM) was performed using a Zeiss apparatus, model DSM 982, equipped with Xray energy-dispersive analyses (EDX). Prior the measurements, the samples were cut in pieces of ca. 1 cm2 and coated with a platinum/palladium layer employing a standard sputtering technique. Micrographs of both, the surface and the broken-inner part of the sintered glass pieces were taken. UV–vis spectra of the coatings were measured employing a Varian spectrometer model Cary 100, equipped with an integrating sphere, and using Ba2SO4 as reference. All spectra were recorded in diffuse reflectance mode. 2.3. Photocatalytic activities The photocatalytic activity of the TiO2 coatings was tested for the reaction of photooxidation of glucose (D-+glucose, Merck, anhydrous for biochemistry) in water, using a starting concentration of 3 mg/L total organic carbon (TOC) in all cases. The measurements of activity were performed in a 200 mL square reactor made of polyoxylmethylen (POM), where the coated plate is fitted and sealed inside dividing the reactor in two compartments, and which is provided with a UV-transparent PMMA window (80 mm 80 mm) for front illumination and two openings for inlet and outlet of the fluid, respectively. For the tests, a total volume of 2 L glucose solution (at natural water pH) was recirculated through the plates with a flow rate of ca. 40 L/h. The first 20 min were run in the dark to allow the equilibration of the system. Illumination was provided by five parallel Philips UV-lamps (20 W). The incident photon flux on the plates was determined by ferrioxalate actinometry [18] to be 3.0 1017 photons s1, and checked periodically. Measurements with an UV meter (Dr. Ho¨ nle UVA/B meter) where used for detailed corrections. Glucose degradation profiles with illumination time were followed by on-line measurements of total organic carbon (TOC) contents, using a TOC analyser (Shimadzu TOC5000A). Blank experiments were performed in absence of any illumination as well as with uncoated sintered glass plates, without observable change in the initial concentration of glucose in both cases. Photonic efficiencies (j%) of the different coatings for glucose degradation were calculated using the following
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equation [19]: jð%Þ ¼
d½C=dt 100 P0
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3. Results and discussion (1)
where dC/dt is the initial mineralisation rate (in terms of mol organic C/s), zero-order kinetics assumed, and P0 is the incident photonic flux (einstein s1). 2.4. Lifetime tests of the coatings Stability and lifetime of the coatings were tested following the evolution of their photocatalytic activity for more than 4000 h of continuous operation. These tests were performed in a different reactor, consisting in a cylinder made of PMMA with two openings for inlet and outlet, into which a coated sintered glass tube (225 mm long, 28 mm i.d., 100–160 mm nominal pore size) was fitted and sealed. A Philips CLEO compact lamp (15 W) longitudinally placed inside the reactor provided inner illumination. Glucose solution was continuously re-circulated through the reactor at 40 L/h under illumination. The photonic efficiency of the coating was estimated periodically by introducing freshly prepared 3 mg/L glucose solution (pH 6) at a total volume of 2 L. The incident photon flux in this reactor was also determined by ferrioxalate actinometry [18], being 1.6 1018 photon s1. The lamp was exchanged periodically and its intensity was checked before every calculation of photonic efficiency bymeasurements employing the UV meter (Dr. Ho¨ nle UVA/ B meter) and correlated with the corresponding photon flux.
3.1. Morphology of the coatings The morphology of the coatings was studied by scanning electron microscopy (SEM), evaluating the coating on the surface as well as in the inner part of the sintered glass plates. In Fig. 1 selected SEM images are shown for an uncoated (Fig. 1A and B) and for a coated plate (inner part) (Fig. 1C and D), respectively. The micrographs for the uncoated sintered glass, Fig. 1A and B, show a structure formed by glass particles from around 200 to 400 mm in size sintered together. Fig. 1C and D shows the coated sample, where it can be observed that the form and structure of the coating is considerably smooth and homogeneous all over the surface of the particles forming the sintered glass, presenting fissures only at some joints between sintered particles. On the other hand, the small particles that can be seen on the surface of the coating are not TiO2, as confirmed by EDX analyses, and consequently they can be also observed on the surface of uncoated samples, as it is evident from Fig. 1B. These particles originate probably from the process of cutting the plates during the preparation of the samples for SEM. Fig. 1C and D shows the coating in the inner part of the sintered glass plate, however, similar morphology for the coatings could be observed on the surface of the plates (pictures not shown). 3.2. Photocatalytic properties The photonic efficiency (j%) of the coatings prepared on sintered glass from the hydrolysis of titanium sulfate, as
Fig. 1. Scanning electron microscopy images for uncoated borosilicate sintered glass (A and B) and TiO2 coating on borosilicate sintered glass (C and D).
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described in Section 2.1, was measured to be j = 2.5% for the photocatalytic mineralisation of glucose in water. The values of photonic efficiencies showed high reproducibility when calculated for different plates coated employing the same conditions as well as for different measurements of the same plate. Following the same preparation procedure, other coatings were prepared but from dipping solutions containing different initial amounts of titanium oxysulfate (TiOSO4), covering the range between 3.8 and 30.6 wt.% (1.2–10 wt.% in terms of TiO2 content). A strong influence of the concentration of the precursor on the activity of the coatings was found, as shown in Fig. 2. A significant increase of the photoactivity of the coatings is observed as the initial concentration of TiOSO4 is increased, starting from a photonic efficiency of not over 1% when using a starting concentration of 3.8 wt.% TiOSO4 and rising to a photonic efficiency of 4% when the initial concentration of TiOSO4 increases to 30.6 wt.%. As also seen from Fig. 2, the photonic efficiency for the coating prepared by the alcoxide route (TTIP) is lower than that for any coating prepared by the hydrolysis of TiOSO4. Even if both preparation methods cannot be compared directly, this effect cannot be due to a different initial concentration of precursor (since for the alcoxide method this would be around 3.5 wt.% based upon TiO2) but rather to the preparation route itself. The amounts of TiO2 deposited on the sintered glass plates were determined by weighing the plates before and after the coating. Employing these values, and assuming that the coatings are homogeneously distributed over all the sintered glass (as observed by SEM (Fig. 1C and D)), the thickness of the TiO2 layers was estimated from these weight differences. For these calculations the density of TiO2 anatase was taken as 3.84 g/cm3 and the surface area of the sintered glass as 0.03 m2/g (value provided by the supplier of the sintered glass). These results are presented in Table 1.
Fig. 2. Influence of the initial concentration of titanium oxysulfate present in the dipping solutions on the photonic efficiencies (j%) of the final coatings on sintered glass. In brackets: initial concentrations of the dipping solutions in terms of the content of TiO2. TTIP: value for the coating prepared by the alcoxide route. Photonic efficiencies calculated for the total photocatalytic mineralisation of glucose in water.
Table 1 Calculated mean thickness and photonic efficiency of the coatings prepared from the indicated initial concentrations Concentration Concentration Mass of the Mass of Mean j(%) TiOSO4 as TiO2 uncoated TiO2 on the thickness plate (g) (nm) (wt.%) (wt.%) plate (g) 3.8 6.4 12.3 18.3 30.6
(0.18 M) 1.2 (0.28 M) 2 (0.58M) 4 (0.93 M) 6 (1.82 M) 10
n.d. 39.442 44.967 40.493 40.285
n.d. 0.101 0.219 0.263 0.603
n.d. 22 42 57 130
1.3 1.5 2.5 3.0 4.0
Photonic efficiencies calculated for the total photocatalytic mineralisation of glucose in water. n.d.: not determined.
The theoretical thicknesses of the coatings start at 22 nm and end at 130 nm depending on the initial concentration of TiOSO4. As it can be observed in Fig. 3, there is a linear increase in the thickness of the layers as well as a linear increase in the photocatalytic activity with the increase of the initial concentration of precursor. This is suggesting that the high photonic efficiencies reached for these coatings are due to the capacity of the sintered glass to incorporate by this preparation method high amounts of TiO2 as thick and stable layers. Thus, the higher activities of the coatings prepared from highly concentrated initial solutions could be explained by the higher amount of TiO2 in the coatings, resulting in a more efficient utilisation of the incident photon flux. The ageing time was also found to have an effect on the activity of the coatings, even if this was not as significant as that of the concentration of the precursor. To study this effect, different coatings were prepared using ageing times between 2 and 8 h during the preparation of the dipping solutions (initial concentration 12.3 wt.% TiOSO4). As Fig. 4 shows, preparations with ageing times longer than 6 h resulted in the highest values of efficiencies for the coatings under these experimental conditions. During the period of ageing, processes of polycondensation and syneresis as well as of dissolution and reprecipitation in the gel network are produced, driven by differences in solubility between surfaces with different radii of curvature [20]. This causes the growth of necks between particles, increasing the strength and stiffness of the gel structure that later will form the coating. Stronger and more stable structures will resist better the process of shrinkage during the steps of drying and calcination, keeping the original pore structure to a higher degree. Both, the more stable as well as the more defined pore structure of the TiO2 layer could be responsible for the observed improvement in the photocatalytic efficiency of coatings prepared with extended ageing times. During the preparation procedure, the pH reached in the hydrolysis is also known to have an important influence on the final properties of the catalysts. In the work described here a pH 5.5 has been chosen based on Ref. [21], where this value of pH was found to be the optimum for the
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Fig. 3. Calculated mean thickness (circles, right y-axis) and photonic efficiency (squares, left y-axis) of the coatings vs. the initial concentration of titanium oxysulfate used in their preparations. Photonic efficiencies calculated for the total photocatalytic mineralisation of glucose in water.
photoactivity of TiO2 powders prepared employing a similar synthetic method. On the other hand, other parameters in the preparation were found to be important not for the activity of the coatings but for their stability. Preliminary experiments showed that the repetitive washing of the gel strongly affected the stability of the final coating. When this step was omitted, the fixation of the TiO2 layer to the glass was found to be very poor and unstable, probably due to high amounts of SO42 and Na+ ions incorporated in the gels. 3.3. Absorption properties The absorptive properties of the coatings were studied by diffuse reflectance UV–vis spectroscopy (DRS). This technique is especially useful in photocatalytic applications,
since it can yield information about the absorption capacities and the band gap values of the semiconductors. Fig. 5 shows the diffuse reflectance spectra for the coatings prepared from solutions of different starting concentrations. The spectrum of the uncoated sintered glass (Fig. 5a) shows that this material is transparent to UV irradiation of wavelengths exceeding 300 nm, important prerequisite for a support of photocatalysts. Regarding the coated samples, no significant differences can be observed for the spectra of the coatings prepared at higher starting concentrations (Fig. 5d–f), showing in any case absorption of light with wavelengths below 375 nm. However, the coatings prepared from lower starting concentrations (Fig. 5b and c) exhibit a notable shift of the absorption edge to lower wavelengths, which can be explained by a higher band gap energy, probably due to the smaller nano-sized TiO2 particles present in these samples. The lower range of light absorption would also explain the lower photoactivity found for these two coatings, as shown in Fig. 2. No important differences could be observed in the spectra for the coatings prepared with different ageing times, being in each case similar to the spectrum shown in Fig. 5d (results not shown). 3.4. Lifetime of the coatings
Fig. 4. Influence of the ageing time on the photonic efficiencies (j, %) of the final coatings on sintered glass. Value for the coating prepared by the alcoxide route (TTIP) is also shown. Photonic efficiencies calculated for the total photocatalytic mineralisation of glucose in water.
In order to study the stability and lifetime of the coatings on sintered glass, the photocatalytic efficiency of a coating (12.3 wt.% TiOSO4 initial concentration, 6 h ageing) was followed during 4000 h of continuous operation under the conditions stated in Section 2.4. Fig. 6 shows the evolution of the photonic efficiency for the glucose oxidation employing this coating, which was calculated periodically every 500–1000 h during this non-
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Fig. 5. Diffuse reflectance spectra for the uncoated sintered glass (a), and for the coatings prepared with different initial concentrations of TiOSO4: 3.8 wt.% (b); 6.4 wt.% (c); 12.3 wt.% (d); 18.3 wt.% (e); 30.6 wt.% (f, non-continuous line).
The influence of the reaction pH on the degradation of glucose during the photocatalytic tests for our coatings on sintered glass was studied in the pH range between 2 and 9.
The pH of the glucose solutions used for these tests was adjusted to the desired values by addition of small amounts of HNO3 or NH3. In each experiment the reaction pH was monitored on-line and no notable changes of the initially adjusted pH could be observed during the irradiation time. Fig. 7 shows the photonic efficiencies for the mineralisation of glucose at different pH values employing the same coating (prepared from 12.3 wt.% TiOSO4 initial concentration, 6 h ageing). It is evident that the pH used during the tests has a strong influence on the efficiency of the coating for the degradation of glucose, improving considerably with increasing reaction pH. It is well known that the reaction pH is an important parameter in heterogeneous photocatalysis, since it has a strong influence on the surface charge of the catalysts, and therefore it also affects their properties of absorption
Fig. 6. Percentage of the original activity of the TiO2 coating over 4000 h of continuous operation in the photocatalytic degradation of glucose in water. Flow rate = 40 L/h, photon flux = 1.6 1018 photons s1.
Fig. 7. Influence of the reaction pH on the photonic efficiency of the total photocatalytic mineralisation of glucose in water.
stop operation. It can be noticed that the coating efficiency did not change considerably at any moment, exhibiting after 4000 h of operation still 100% of its initial activity (assuming as 100% activity the value of photonic efficiency shown during the first 12 h of operation). Thus, it can be assumed that even using a relatively high flow rate (40 L/h) for more than 4000 h, there was no notable degradation of the coating caused by abrasion due to the flow of the substrate solution. 3.5. Influence of the substrate pH on the photocatalytic tests
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concerning the compounds that will be degraded in the process [22,23]. Consequently, the influence of the pH has especial importance for the degradation of polar molecules, such as glucose, the model compound chosen in this work for the photocatalytic tests. Some studies have been performed about the influence of pH on the photocatalytic degradation of many compounds, i.e. carboxylic acids were found to be more efficiently degraded at low pH values and chloroform at high pH values, while 4-chlorophenol was found not significantly pH-dependent [24]. Nevertheless, as far as we know, the pH influence on the photocatalytic degradation of glucose has never been studied. The negatively charged TiO2 surface at high pH values should accommodate more glucose, probably helped by a specific orientation and stronger adsorption of the molecules at these pH values. This local higher concentration of glucose on the TiO2 surface at high pH would improve the efficiency of the process of photooxidation of this compound.
4. Conclusions Photoactive TiO2 coatings were prepared by dip-coating, following a clean and simple synthetic route based on the hydrolysis of TiOSO4. Sintered glass plates, the support chosen for the coatings, proved to be very good supporting materials for titania, allowing the preparation of thick and stable coatings of relatively high photonic efficiency. However, a strong influence of the preparation conditions on the final activity of the coatings was found, with the starting concentration of precursor and the ageing time being two parameters possessing significant effects. Employing specific preparation parameters, the photonic efficiency for these coatings reached values up to 4% for the total mineralisation of glucose in water. The coatings produced on sintered glass by the method described in this paper appeared mechanically very robust and were not degraded even after repeated successive applications (4000 h of continuous operation did not result in any appreciable decrease in the photocatalytic activity). Nevertheless, an important influence of the reaction pH on the efficiency of these coatings was found during the photocatalytic tests, with the glucose degradation being more favourable at higher pH values.
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Acknowledgements S. Sakthivel thanks the DAAD for financial support to perform his research work in Germany.
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