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Photocatalytic behavior of different titanium dioxide layers
Thin Solid Films 515 (2007) 6309 – 6313 www.elsevier.com/locate/tsf
Photocatalytic behavior of different titanium dioxide layers M.F. Brunella a,⁎, M.V. Diamanti a , M.P. Pedeferri a , F. Di Fonzo b , C.S. Casari b , A. Li Bassi b b
a Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy NEMAS — Center for NanoEngineered MAterials and Surfaces, CNISM and Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Via Ponzio 34/3, 20133 Milano, Italy
Available online 20 December 2006
Abstract Many recently developed applications are related to the photocatalytic behavior of semiconductive oxides. Among the different oxides, titanium dioxide (TiO2) is one of the most interesting due to its high photocatalytic efficiency towards a great number of reactions and to its hydrophilic properties. Aim of this work is the evaluation and comparison of the photocatalytic properties of different crystalline titanium dioxide films, directly grown on titanium substrates by surface anodization (eventually followed by thermal annealing) and by Pulsed Laser Deposition (PLD) on titanium and silicon substrates, followed by thermal annealing. The structure and morphology of the layers were characterized by Scanning Electron Microscopy and X-Ray Diffraction and photocatalytic tests on stearic acid mineralization were performed. Results showed that the PLD layers possess a higher photocatalytic efficiency than anodized titanium. This can be attributed to the microstructured/microporous morphology of the related surfaces. Instead, PLD TiO2 layers with a relatively high content of the rutile phase have a reduced photocatalytic efficiency with respect to mainly anatase containing layers. © 2006 Elsevier B.V. All rights reserved. Keywords: Titanium oxide; Photocatalysis; Anodic oxidation; Pulsed laser deposition PLD
1. Introduction The need for the development of simple, rapid and economical purification processes has opened up new research fields, particularly in the attempt to find an antipollution treatment able to deal with polluting agents present in the atmosphere or in wastewaters in low concentrations. Photocatalysis is thus gaining an increasing interest, as it represents an efficient and useful method for the degradation of both organic and inorganic substances [1,2]. Many semiconductors have the right qualities to be employed as photocatalysts; titanium dioxide (TiO2) is certainly the most studied and utilized, thanks to its outstanding efficiency. TiO2 photocatalytic activity strictly depends on its crystal structure: in fact, both the anatase and the rutile allotropic forms show photocatalytic efficiency, while the amorphous oxide does not. Even though both oxides allow an efficient oxidation of
⁎ Corresponding author. Tel.: +39 2 23993157; fax: +39 2 23993180. E-mail address: [email protected] (M.F. Brunella). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.194
most organic molecules, the anatase phase presents a higher photoactivity, thanks to its ready and efficient transfer of the electrons necessary for the reduction reactions, needed in most pollutants degradation processes. Nevertheless, due to their high band gap values (rutile 3 eV and anatase 3.2 eV), for both phases light radiation absorption and the subsequent photoactivity are established only under UV irradiation (λ b 400 nm). Therefore, attention is now focusing on the shift of light absorption towards lower photon energies, in order to allow the photoactivity of the semiconductor under common daylight exposure. A fundamental role is played by the effective surface area of the photocatalytic substrate on which pollutants must adsorb before being degraded: this leads to another research topic, i.e. the obtaining of nanostructured TiO2 layers or TiO2 crystallites or powders with low dimensionality. Anodizing techniques [3] and traditional deposition techniques, either chemical vapor deposition (CVD) or physical vapor deposition (PVD), such as pulsed laser deposition (PLD), allow the growth of TiO2 thin films, either consisting of bare amorphous oxide or composed by anatase and/or rutile
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Table 1 Different treatment conditions and samples labels Sample identification
Substrate material
Growth process
Treatment conditions
PLD1
Silicon
PLD2
Silicon
PLD3
Titanium
PLD4
Titanium
A1
Titanium
PLD + thermal treatment PLD + thermal treatment PLD + thermal treatment PLD + thermal treatment Anodizing
A2
Titanium
Anodizing
A3 A4
Titanium Titanium
Powder
Titanium
– Pretreatment + anodizing + thermal treatment –
30 min; 40 Pa dry air; 4 J/cm2 30 min; 100 Pa dry air; 4 J/cm2 30 min; 40 Pa dry air; 4 J/cm2 30 min; 100 Pa dry air; 4 J/cm2 1 M H2SO4; 1 min; 90 V;400 A/m2 0.5 M H2SO4;2 min; 120 V;400 A/m2 – H3PO4; 130 V [13]
TiO2 anatase powder deposition
crystallites embedded in an amorphous matrix; in the former case, the amorphous layer can be partially converted into crystalline by means of a thermal annealing treatment. Moreover, both anodizing and deposition methods can be used to grow a micro- or nanostructured oxide structure, thus leading to the possibility of controlling the layer properties (e.g. energy gap) and of enhancing its specific surface area [4–11]. Aim of this work is the evaluation of the photocatalytic efficiency of TiO2 films obtained by different processes. In photocatalytic tests, stearic acid was the model organic material undergoing TiO2-assisted mineralization under UV irradiation, with the following chemical reaction:
phosphoric acid, according to Ref. [14], at 130 V, followed by thermal treatment. PLD of titanium oxide thin films was accomplished by ablating a pure Ti target with laser pulses from a KrF excimer laser (wavelength 248 nm, duration 10–15 ns, energy density ∼ 4 J/cm2) in different dry air background pressures (40 Pa and 100 Pa). Laser ablation in the presence of a dry air background pressure results in cluster formation in the ablation plume and thus in the production of titanium oxide nanostructured layers with a degree of porosity at the nano- and microscale, which increases the specific surface [15,16]. PLD thin films were grown on both titanium and silicon substrates. The use of the former ensures a better comparison with anodized samples, in which the oxide is grown on titanium, too. It would have been interesting, in principle, to study the photoactivity of PLD titanium oxide layers grown on glass substrates, since the production of photocatalytic windows is a potentially appealing commercial application; unfortunately, FT-IR tests require an opaque substrate. The thermal treatment consisted in heating at a temperature of 400 °C in air, maintained for 2 h. Previous work has shown that annealing of PLD films deposited in different pressure conditions (and thus with a different nanostructure) leads to the growth of a
hvzEbg TiO
CH3 ðCH2 Þ16 CO2 H þ 26O2 Y 18CO2 þ 18H2 O gap
Its decomposition can be revealed directly by FTIR spectroscopy, by monitoring the signals of the asymmetric C–H stretching mode (CH3 group) and of the asymmetric and symmetric stretching modes (CH2 group), at 2958, 2923, and 2853 cm− 1 respectively [12]. The absorbance values versus UV irradiation time are representative of the decomposition kinetics. 2. Experimental The tested TiO2 growth processes consisted of two different anodizing methodologies, followed or not by thermal treatment, and PLD in a dry air background atmosphere at two different pressures (40 Pa and 100 Pa), followed by thermal treatment. The anodizing treatments were carried out on titanium platelets about 10 mm wide, 15 mm long and 0.5 mm thick. The first anodizing treatment was performed in sulphuric acid solution (0.5–1 M) at two different potentials (90 V and 120 V) with 400 A/m2 current density: the process parameters were chosen in order to maximize the anatase phase content [13]. The second type of anodic oxidation treatment was performed in
Fig. 1. SEM micrographs of sample PLD1: a) cross section; b) surface.
M.F. Brunella et al. / Thin Solid Films 515 (2007) 6309–6313
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3. Results The sample anodized in phosphoric acid is characterized by a green surface; the sample anodized in sulfuric acid at the lower voltage is dark pink colored, while the other samples show a grey surface. Colors are due to interference phenomena that establish at the metal–oxide interface: this indicates that the two colored surfaces are covered by homogeneous oxide films, a few hundred nanometers thick. The surfaces of these samples are rather smooth, exception made for the roughness due to the pretreatment of sample A4 [14] and the microroughness related to the presence of small craters on sample A2 (see Fig. 2b).
Fig. 2. SEM micrographs of: a) sample PLD2; b) sample A2.
titanium oxide crystalline structure with different rutile/anatase compositions [16]. Two more samples were used for comparison: one bare titanium substrate and a titanium substrate covered by TiO2 anatase powder (Aldrich). Deposition, treatment conditions and sample labels are shown in Table 1. X-ray thin film diffraction (Philips PW3020, Cu Kα radiation) was used to characterize the oxide allotropic phases; in some cases Raman spectroscopy was used, too. All samples were observed by Scanning Electron Microscopy (Cambridge Stereoscan 360) and analyzed by energy dispersive spectrometry (Oxford INCA 200). Photocatalysis tests were carried out by irradiating samples with a UV lamp (Xe 300 W HAMAMATSU Super Quiet). Light was focused by a convergent lens; a quartz cell filled with water was used to absorb the IR component of radiation in order to avoid stearic acid sublimation. The radiation intensity was 0.2 mW/cm2. Standardized tests were performed by covering the sample surface with a controlled amount of stearic acid diluted in hexane. The mineralization degree was determined at several irradiation times by FTIR (Nicolet 510P) measurements. Optical microscope observations were performed to evaluate the distribution of stearic acid crystals and their time-dependent degradation.
Fig. 3. X-ray diffraction patterns of: a) sample PLD1; b) sample PLD2; c) sample A4; d) sample A2.
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The PLD oxide layers are thicker, about ten microns, and nanostructured. Samples PLD1 and PLD3 show a columnar growth, with typical “cauliflower” appearance and poor adherence to the silicon platelet, which improves on the titanium surface (Fig. 1). PLD2 and PLD4 surfaces show no arranged growth, thus having a “fluffy” appearance, and better adherence (Fig. 2a). X-ray diffraction patterns show the presence of the anatase peaks in all the samples, with different main peak (at 25.5°) intensity. Low intensity, broader peaks were detected in the PLD films, while narrower peaks of higher intensity were detected in the anodized samples (Fig. 3). The non-treated titanium platelet showed neither anatase nor rutile phase presence. Rutile phase was detected in the PLD oxide layer grown in 100 Pa dry air (PLD2), where it is prevalent with respect to the anatase phase, as revealed by the higher intensity of the rutile main peak. Raman analysis (see e.g. Ref. [17]) of peaks at 143, 399, 514, 639 cm− 1 (corresponding to anatase phase) and 235, 447, 612 cm− 1 (rutile phase) confirms that the main crystalline phase obtained by PLD in 40 Pa dry air followed by thermal treatment is anatase, while mainly rutile phase is obtained at 100 Pa deposition pressure followed by thermal treatment [16]. Fig. 4 shows the IR spectra obtained at different irradiation times from sample A4. In order to compare the photocatalytic efficiency of the different TiO2 films, the peak of the asymmetric stretching mode (CH2 group) at 2923 cm− 1 was considered. The percentage of stearic acid degradation was calculated as the 100 complement of the ratio between the area of the peak at different irradiation times and its area before irradiation: Fig. 5 shows stearic acid degradation in time for the different layers. PLD deposition in 40 Pa dry air followed by thermal treatment (PLD1 and PLD3) showed the most efficient photocatalytic behavior, as 90% stearic acid degradation was achieved in less than one day (and in particular 100% degradation was reached in 8 h for the film deposited on titanium). This result is even better than that achieved with pure anatase powder (70% stearic acid degradation is reached in about 24–27 h). PLD in 100 Pa dry air followed by thermal treatment (PLD2 and PLD4) showed a behavior quite similar to the powder. Samples PLD1 and PLD2 (i.e. films deposited on Si) did not reach complete degradation of stearic acid. Observation by optical microscope showed that the remaining stearic acid
Fig. 4. IR transmittance curves as a function of irradiation time for sample A4.
Fig. 5. Residual percentage of stearic acid as a function of time for all tested samples.
crystals were located at points were the silicon surface was not covered by the deposit. Photodegradation of stearic acid on anodized samples was slower: 50% of stearic acid degradation was obtained after one day exposure on the A2 sample and after two days on samples A1 and A4. At the end of the test, sample A2 reached 100% degradation, while A1 only reached 85% degradation and sample A4 was not able to reach the complete acid degradation even after two weeks of irradiation (and lost its photocatalytic activity after 60% stearic acid mineralization). The bare titanium surface did not exhibit significant photocatalytic behavior. 4. Discussion Results demonstrated that all depositions or treatments promoting anatase growth led to an increased photocatalityc efficiency of the substrates. In particular, the photocatalytic behavior of the dioxide layers appears to be mostly influenced by anatase content, crystalline structure and layer morphology. By comparing all the tested samples, it emerges that films deposited by PLD followed by thermal treatment have the best photocatalytic behavior. The irradiation time necessary to obtain the degradation of about 70% of the stearic acid is less than one day for the PLD growth in 40 Pa dry air and less than two days for the PLD growth in 100 Pa dry air. Sample A2 showed the best behavior among the anodized titanium samples, but after one day exposure only 50% of the stearic acid was mineralized. Other anodizing treatments (A1 and A4) led to the same result (i.e., 50% degradation) with a delay of one day, and moreover sample A4 reached 60% mineralization only after 2 weeks. The microstructural characterization of the samples shows that PLD deposits are thick, with a porous micro- and nanostructured surface, while anodizing produces thin films with different thickness (the oxide layer in sample A2 is thicker than in sample A4, as evidenced by the loss of interference color). Furthermore, on the surface of sample A2 it is possible to notice the presence of craters, due to the establishment of anodic spark deposition conditions, thus enhancing the surface roughness. As for the phase compositions, XRD showed a higher anatase content in anodized samples. Therefore, the better photocatalytic behavior of PLD samples seems to result from their micro/nanoporous morphology (and thus larger effective surface), and maybe also from the smaller grain size of
M.F. Brunella et al. / Thin Solid Films 515 (2007) 6309–6313
anatase domains formed in the amorphous oxide matrix, as evidenced by broader XRD anatase peaks in PLD samples. Some differences can be evidenced between the two types of PLD deposition, probably due to the presence of anatase phase only in the 40 Pa dry air deposition with respect to the 100 Pa dry air deposited oxide, which mainly contains rutile [16]. No significant differences could be observed between the PLD films deposited on different substrates, i.e. titanium and silicon. Nevertheless, it is worth noticing that oxide layers obtained by PLD do not exhibit a good adhesion to the substrate, especially on silicon, and are less reproducible (and their preparation more expensive) than those obtained by anodizing techniques. 5. Conclusions TiO2 layers obtained by Pulsed Laser Deposition followed by thermal treatment showed higher photocatalytic efficiency in the degradation of stearic acid than anodized titanium. The simple evaluation of the percentage of anatase phase content in the oxide layer is not sufficient to determine the photocatalytic properties of the surface, since a fundamental role is played by surface morphology, oxide thickness and anatase crystallite size. The microporous structure of PLD deposits shows a photocatalytic activity similar to or better than that encountered in testing a pure anatase powder. Anodized titanium samples show a lower efficiency with respect to PLD samples, but can be attractive since the oxidation treatment is more reproducible and cheaper than with other techniques. References [1] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33. [2] M.A. Fox, in: Schiavello (Ed.), Photocatalytic Oxidation of Organic Substrates, Photocatalysis and Environment: Trends and Applications, Kluwer, 1988, p. 445.
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[3] M.P. Pedeferri, C.E. Bottani, D. Cattaneo, A. Li Bassi, B. Del Curto, M.F. Brunella, in: NSTI Nanotech (Ed.), Surfaces and Films, Technical Proceedings of the 2005 NSTI Nanotechnology Conference and Trade Show, Anaheim May 8–12, 2005, p. 332. [4] G.P. Wirtz, S.D. Brown, W.M. Kriven, Mater. Manuf. Process. 6 (1991) 87. [5] J.P. Schreckenbach, G. Marx, F. Schlottig, M. Textor, N.D. Spencer, J. Mater. Sci., Mater. Med. 10 (1999) 453. [6] K.H. Dittrich, W. Krysmann, P. Kurze, H.G. Schneider, Cryst. Res. Technol. 19 (1984) 93. [7] W. Krysmann, P. Kurze, K.H. Dittrich, H.G. Schneider, Cryst. Res. Technol. 19 (1984) 973. [8] T.B. Van, S.D. Brown, G.P. Wirtz, Am. Ceram. Soc. Bull. 56 (1977) 563. [9] J.M. Macak, K. Sirotna, P. Schmuki, Electrochim. Acta 50 (2005) 3679. [10] A. Ghicov, H. Tsuchiya, J.M. Macak, P. Schmuki, Electrochem. Commun. 7 (2005) 505. [11] H. Tsuchiya, J.M. Macak, L. Taveira, E. Balaur, A. Ghicov, K. Sirotna, P. Schmuki, Electrochem. Commun. 7 (2005) 576. [12] N.P. Mellott, C. Durucan, C.G. Pantano, M. Guglielmi, Thin Solid Films 502 (2006) 112. [13] M.V. Diamanti, M.P. Pedeferri, Corrosion Science 49 (2007) 939. [14] P. Pedeferri, EP Patent N° 1 199 385 A2, 19 october 2000. [15] D.H. Lowndes, C.M. Rouleau, T. Thundat, G. Duscher, E.A. Kenik, S.J. Pennycook, Appl. Surf. Sci. 355 (1998) 127. [16] A. Li Bassi, C.S. Casari, F. Di Fonzo, A. Bailini, M. Fusi, D. Dellasega, V. Russo, A. Baserga, D. Cattaneo, C.E. Bottani, in: Cengiz S. Ozkan, Federico Rosei, Gregory P. Lopinski, Zhong L. Wang (Eds.), Mater. Res. Soc. Symp. Proc., Boston, U.S.A. November 27- December 1, 2005, 901E - Ra24–04. [17] A. Li Bassi, D. Cattaneo, V. Russo, C.E. Bottani, E. Barborini, T. Mazza, P. Piseri, P. Milani, F.O. Ernst, K. Wegner, S.E. Pratsinis, J. Appl. Phys. 98 (2005) 074305.
Photocatalytic behavior of different titanium dioxide layers M.F. Brunella a,⁎, M.V. Diamanti a , M.P. Pedeferri a , F. Di Fonzo b , C.S. Casari b , A. Li Bassi b b
a Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy NEMAS — Center for NanoEngineered MAterials and Surfaces, CNISM and Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Via Ponzio 34/3, 20133 Milano, Italy
Available online 20 December 2006
Abstract Many recently developed applications are related to the photocatalytic behavior of semiconductive oxides. Among the different oxides, titanium dioxide (TiO2) is one of the most interesting due to its high photocatalytic efficiency towards a great number of reactions and to its hydrophilic properties. Aim of this work is the evaluation and comparison of the photocatalytic properties of different crystalline titanium dioxide films, directly grown on titanium substrates by surface anodization (eventually followed by thermal annealing) and by Pulsed Laser Deposition (PLD) on titanium and silicon substrates, followed by thermal annealing. The structure and morphology of the layers were characterized by Scanning Electron Microscopy and X-Ray Diffraction and photocatalytic tests on stearic acid mineralization were performed. Results showed that the PLD layers possess a higher photocatalytic efficiency than anodized titanium. This can be attributed to the microstructured/microporous morphology of the related surfaces. Instead, PLD TiO2 layers with a relatively high content of the rutile phase have a reduced photocatalytic efficiency with respect to mainly anatase containing layers. © 2006 Elsevier B.V. All rights reserved. Keywords: Titanium oxide; Photocatalysis; Anodic oxidation; Pulsed laser deposition PLD
1. Introduction The need for the development of simple, rapid and economical purification processes has opened up new research fields, particularly in the attempt to find an antipollution treatment able to deal with polluting agents present in the atmosphere or in wastewaters in low concentrations. Photocatalysis is thus gaining an increasing interest, as it represents an efficient and useful method for the degradation of both organic and inorganic substances [1,2]. Many semiconductors have the right qualities to be employed as photocatalysts; titanium dioxide (TiO2) is certainly the most studied and utilized, thanks to its outstanding efficiency. TiO2 photocatalytic activity strictly depends on its crystal structure: in fact, both the anatase and the rutile allotropic forms show photocatalytic efficiency, while the amorphous oxide does not. Even though both oxides allow an efficient oxidation of
⁎ Corresponding author. Tel.: +39 2 23993157; fax: +39 2 23993180. E-mail address: [email protected] (M.F. Brunella). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.194
most organic molecules, the anatase phase presents a higher photoactivity, thanks to its ready and efficient transfer of the electrons necessary for the reduction reactions, needed in most pollutants degradation processes. Nevertheless, due to their high band gap values (rutile 3 eV and anatase 3.2 eV), for both phases light radiation absorption and the subsequent photoactivity are established only under UV irradiation (λ b 400 nm). Therefore, attention is now focusing on the shift of light absorption towards lower photon energies, in order to allow the photoactivity of the semiconductor under common daylight exposure. A fundamental role is played by the effective surface area of the photocatalytic substrate on which pollutants must adsorb before being degraded: this leads to another research topic, i.e. the obtaining of nanostructured TiO2 layers or TiO2 crystallites or powders with low dimensionality. Anodizing techniques [3] and traditional deposition techniques, either chemical vapor deposition (CVD) or physical vapor deposition (PVD), such as pulsed laser deposition (PLD), allow the growth of TiO2 thin films, either consisting of bare amorphous oxide or composed by anatase and/or rutile
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Table 1 Different treatment conditions and samples labels Sample identification
Substrate material
Growth process
Treatment conditions
PLD1
Silicon
PLD2
Silicon
PLD3
Titanium
PLD4
Titanium
A1
Titanium
PLD + thermal treatment PLD + thermal treatment PLD + thermal treatment PLD + thermal treatment Anodizing
A2
Titanium
Anodizing
A3 A4
Titanium Titanium
Powder
Titanium
– Pretreatment + anodizing + thermal treatment –
30 min; 40 Pa dry air; 4 J/cm2 30 min; 100 Pa dry air; 4 J/cm2 30 min; 40 Pa dry air; 4 J/cm2 30 min; 100 Pa dry air; 4 J/cm2 1 M H2SO4; 1 min; 90 V;400 A/m2 0.5 M H2SO4;2 min; 120 V;400 A/m2 – H3PO4; 130 V [13]
TiO2 anatase powder deposition
crystallites embedded in an amorphous matrix; in the former case, the amorphous layer can be partially converted into crystalline by means of a thermal annealing treatment. Moreover, both anodizing and deposition methods can be used to grow a micro- or nanostructured oxide structure, thus leading to the possibility of controlling the layer properties (e.g. energy gap) and of enhancing its specific surface area [4–11]. Aim of this work is the evaluation of the photocatalytic efficiency of TiO2 films obtained by different processes. In photocatalytic tests, stearic acid was the model organic material undergoing TiO2-assisted mineralization under UV irradiation, with the following chemical reaction:
phosphoric acid, according to Ref. [14], at 130 V, followed by thermal treatment. PLD of titanium oxide thin films was accomplished by ablating a pure Ti target with laser pulses from a KrF excimer laser (wavelength 248 nm, duration 10–15 ns, energy density ∼ 4 J/cm2) in different dry air background pressures (40 Pa and 100 Pa). Laser ablation in the presence of a dry air background pressure results in cluster formation in the ablation plume and thus in the production of titanium oxide nanostructured layers with a degree of porosity at the nano- and microscale, which increases the specific surface [15,16]. PLD thin films were grown on both titanium and silicon substrates. The use of the former ensures a better comparison with anodized samples, in which the oxide is grown on titanium, too. It would have been interesting, in principle, to study the photoactivity of PLD titanium oxide layers grown on glass substrates, since the production of photocatalytic windows is a potentially appealing commercial application; unfortunately, FT-IR tests require an opaque substrate. The thermal treatment consisted in heating at a temperature of 400 °C in air, maintained for 2 h. Previous work has shown that annealing of PLD films deposited in different pressure conditions (and thus with a different nanostructure) leads to the growth of a
hvzEbg TiO
CH3 ðCH2 Þ16 CO2 H þ 26O2 Y 18CO2 þ 18H2 O gap
Its decomposition can be revealed directly by FTIR spectroscopy, by monitoring the signals of the asymmetric C–H stretching mode (CH3 group) and of the asymmetric and symmetric stretching modes (CH2 group), at 2958, 2923, and 2853 cm− 1 respectively [12]. The absorbance values versus UV irradiation time are representative of the decomposition kinetics. 2. Experimental The tested TiO2 growth processes consisted of two different anodizing methodologies, followed or not by thermal treatment, and PLD in a dry air background atmosphere at two different pressures (40 Pa and 100 Pa), followed by thermal treatment. The anodizing treatments were carried out on titanium platelets about 10 mm wide, 15 mm long and 0.5 mm thick. The first anodizing treatment was performed in sulphuric acid solution (0.5–1 M) at two different potentials (90 V and 120 V) with 400 A/m2 current density: the process parameters were chosen in order to maximize the anatase phase content [13]. The second type of anodic oxidation treatment was performed in
Fig. 1. SEM micrographs of sample PLD1: a) cross section; b) surface.
M.F. Brunella et al. / Thin Solid Films 515 (2007) 6309–6313
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3. Results The sample anodized in phosphoric acid is characterized by a green surface; the sample anodized in sulfuric acid at the lower voltage is dark pink colored, while the other samples show a grey surface. Colors are due to interference phenomena that establish at the metal–oxide interface: this indicates that the two colored surfaces are covered by homogeneous oxide films, a few hundred nanometers thick. The surfaces of these samples are rather smooth, exception made for the roughness due to the pretreatment of sample A4 [14] and the microroughness related to the presence of small craters on sample A2 (see Fig. 2b).
Fig. 2. SEM micrographs of: a) sample PLD2; b) sample A2.
titanium oxide crystalline structure with different rutile/anatase compositions [16]. Two more samples were used for comparison: one bare titanium substrate and a titanium substrate covered by TiO2 anatase powder (Aldrich). Deposition, treatment conditions and sample labels are shown in Table 1. X-ray thin film diffraction (Philips PW3020, Cu Kα radiation) was used to characterize the oxide allotropic phases; in some cases Raman spectroscopy was used, too. All samples were observed by Scanning Electron Microscopy (Cambridge Stereoscan 360) and analyzed by energy dispersive spectrometry (Oxford INCA 200). Photocatalysis tests were carried out by irradiating samples with a UV lamp (Xe 300 W HAMAMATSU Super Quiet). Light was focused by a convergent lens; a quartz cell filled with water was used to absorb the IR component of radiation in order to avoid stearic acid sublimation. The radiation intensity was 0.2 mW/cm2. Standardized tests were performed by covering the sample surface with a controlled amount of stearic acid diluted in hexane. The mineralization degree was determined at several irradiation times by FTIR (Nicolet 510P) measurements. Optical microscope observations were performed to evaluate the distribution of stearic acid crystals and their time-dependent degradation.
Fig. 3. X-ray diffraction patterns of: a) sample PLD1; b) sample PLD2; c) sample A4; d) sample A2.
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The PLD oxide layers are thicker, about ten microns, and nanostructured. Samples PLD1 and PLD3 show a columnar growth, with typical “cauliflower” appearance and poor adherence to the silicon platelet, which improves on the titanium surface (Fig. 1). PLD2 and PLD4 surfaces show no arranged growth, thus having a “fluffy” appearance, and better adherence (Fig. 2a). X-ray diffraction patterns show the presence of the anatase peaks in all the samples, with different main peak (at 25.5°) intensity. Low intensity, broader peaks were detected in the PLD films, while narrower peaks of higher intensity were detected in the anodized samples (Fig. 3). The non-treated titanium platelet showed neither anatase nor rutile phase presence. Rutile phase was detected in the PLD oxide layer grown in 100 Pa dry air (PLD2), where it is prevalent with respect to the anatase phase, as revealed by the higher intensity of the rutile main peak. Raman analysis (see e.g. Ref. [17]) of peaks at 143, 399, 514, 639 cm− 1 (corresponding to anatase phase) and 235, 447, 612 cm− 1 (rutile phase) confirms that the main crystalline phase obtained by PLD in 40 Pa dry air followed by thermal treatment is anatase, while mainly rutile phase is obtained at 100 Pa deposition pressure followed by thermal treatment [16]. Fig. 4 shows the IR spectra obtained at different irradiation times from sample A4. In order to compare the photocatalytic efficiency of the different TiO2 films, the peak of the asymmetric stretching mode (CH2 group) at 2923 cm− 1 was considered. The percentage of stearic acid degradation was calculated as the 100 complement of the ratio between the area of the peak at different irradiation times and its area before irradiation: Fig. 5 shows stearic acid degradation in time for the different layers. PLD deposition in 40 Pa dry air followed by thermal treatment (PLD1 and PLD3) showed the most efficient photocatalytic behavior, as 90% stearic acid degradation was achieved in less than one day (and in particular 100% degradation was reached in 8 h for the film deposited on titanium). This result is even better than that achieved with pure anatase powder (70% stearic acid degradation is reached in about 24–27 h). PLD in 100 Pa dry air followed by thermal treatment (PLD2 and PLD4) showed a behavior quite similar to the powder. Samples PLD1 and PLD2 (i.e. films deposited on Si) did not reach complete degradation of stearic acid. Observation by optical microscope showed that the remaining stearic acid
Fig. 4. IR transmittance curves as a function of irradiation time for sample A4.
Fig. 5. Residual percentage of stearic acid as a function of time for all tested samples.
crystals were located at points were the silicon surface was not covered by the deposit. Photodegradation of stearic acid on anodized samples was slower: 50% of stearic acid degradation was obtained after one day exposure on the A2 sample and after two days on samples A1 and A4. At the end of the test, sample A2 reached 100% degradation, while A1 only reached 85% degradation and sample A4 was not able to reach the complete acid degradation even after two weeks of irradiation (and lost its photocatalytic activity after 60% stearic acid mineralization). The bare titanium surface did not exhibit significant photocatalytic behavior. 4. Discussion Results demonstrated that all depositions or treatments promoting anatase growth led to an increased photocatalityc efficiency of the substrates. In particular, the photocatalytic behavior of the dioxide layers appears to be mostly influenced by anatase content, crystalline structure and layer morphology. By comparing all the tested samples, it emerges that films deposited by PLD followed by thermal treatment have the best photocatalytic behavior. The irradiation time necessary to obtain the degradation of about 70% of the stearic acid is less than one day for the PLD growth in 40 Pa dry air and less than two days for the PLD growth in 100 Pa dry air. Sample A2 showed the best behavior among the anodized titanium samples, but after one day exposure only 50% of the stearic acid was mineralized. Other anodizing treatments (A1 and A4) led to the same result (i.e., 50% degradation) with a delay of one day, and moreover sample A4 reached 60% mineralization only after 2 weeks. The microstructural characterization of the samples shows that PLD deposits are thick, with a porous micro- and nanostructured surface, while anodizing produces thin films with different thickness (the oxide layer in sample A2 is thicker than in sample A4, as evidenced by the loss of interference color). Furthermore, on the surface of sample A2 it is possible to notice the presence of craters, due to the establishment of anodic spark deposition conditions, thus enhancing the surface roughness. As for the phase compositions, XRD showed a higher anatase content in anodized samples. Therefore, the better photocatalytic behavior of PLD samples seems to result from their micro/nanoporous morphology (and thus larger effective surface), and maybe also from the smaller grain size of
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anatase domains formed in the amorphous oxide matrix, as evidenced by broader XRD anatase peaks in PLD samples. Some differences can be evidenced between the two types of PLD deposition, probably due to the presence of anatase phase only in the 40 Pa dry air deposition with respect to the 100 Pa dry air deposited oxide, which mainly contains rutile [16]. No significant differences could be observed between the PLD films deposited on different substrates, i.e. titanium and silicon. Nevertheless, it is worth noticing that oxide layers obtained by PLD do not exhibit a good adhesion to the substrate, especially on silicon, and are less reproducible (and their preparation more expensive) than those obtained by anodizing techniques. 5. Conclusions TiO2 layers obtained by Pulsed Laser Deposition followed by thermal treatment showed higher photocatalytic efficiency in the degradation of stearic acid than anodized titanium. The simple evaluation of the percentage of anatase phase content in the oxide layer is not sufficient to determine the photocatalytic properties of the surface, since a fundamental role is played by surface morphology, oxide thickness and anatase crystallite size. The microporous structure of PLD deposits shows a photocatalytic activity similar to or better than that encountered in testing a pure anatase powder. Anodized titanium samples show a lower efficiency with respect to PLD samples, but can be attractive since the oxidation treatment is more reproducible and cheaper than with other techniques. References [1] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33. [2] M.A. Fox, in: Schiavello (Ed.), Photocatalytic Oxidation of Organic Substrates, Photocatalysis and Environment: Trends and Applications, Kluwer, 1988, p. 445.
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[3] M.P. Pedeferri, C.E. Bottani, D. Cattaneo, A. Li Bassi, B. Del Curto, M.F. Brunella, in: NSTI Nanotech (Ed.), Surfaces and Films, Technical Proceedings of the 2005 NSTI Nanotechnology Conference and Trade Show, Anaheim May 8–12, 2005, p. 332. [4] G.P. Wirtz, S.D. Brown, W.M. Kriven, Mater. Manuf. Process. 6 (1991) 87. [5] J.P. Schreckenbach, G. Marx, F. Schlottig, M. Textor, N.D. Spencer, J. Mater. Sci., Mater. Med. 10 (1999) 453. [6] K.H. Dittrich, W. Krysmann, P. Kurze, H.G. Schneider, Cryst. Res. Technol. 19 (1984) 93. [7] W. Krysmann, P. Kurze, K.H. Dittrich, H.G. Schneider, Cryst. Res. Technol. 19 (1984) 973. [8] T.B. Van, S.D. Brown, G.P. Wirtz, Am. Ceram. Soc. Bull. 56 (1977) 563. [9] J.M. Macak, K. Sirotna, P. Schmuki, Electrochim. Acta 50 (2005) 3679. [10] A. Ghicov, H. Tsuchiya, J.M. Macak, P. Schmuki, Electrochem. Commun. 7 (2005) 505. [11] H. Tsuchiya, J.M. Macak, L. Taveira, E. Balaur, A. Ghicov, K. Sirotna, P. Schmuki, Electrochem. Commun. 7 (2005) 576. [12] N.P. Mellott, C. Durucan, C.G. Pantano, M. Guglielmi, Thin Solid Films 502 (2006) 112. [13] M.V. Diamanti, M.P. Pedeferri, Corrosion Science 49 (2007) 939. [14] P. Pedeferri, EP Patent N° 1 199 385 A2, 19 october 2000. [15] D.H. Lowndes, C.M. Rouleau, T. Thundat, G. Duscher, E.A. Kenik, S.J. Pennycook, Appl. Surf. Sci. 355 (1998) 127. [16] A. Li Bassi, C.S. Casari, F. Di Fonzo, A. Bailini, M. Fusi, D. Dellasega, V. Russo, A. Baserga, D. Cattaneo, C.E. Bottani, in: Cengiz S. Ozkan, Federico Rosei, Gregory P. Lopinski, Zhong L. Wang (Eds.), Mater. Res. Soc. Symp. Proc., Boston, U.S.A. November 27- December 1, 2005, 901E - Ra24–04. [17] A. Li Bassi, D. Cattaneo, V. Russo, C.E. Bottani, E. Barborini, T. Mazza, P. Piseri, P. Milani, F.O. Ernst, K. Wegner, S.E. Pratsinis, J. Appl. Phys. 98 (2005) 074305.