- Email: [email protected]
Formation of crystalline titanium dioxide on barrier layer-forming metals in aqueous electrolytes by anodic spark deposition—first mechanistic conceptions
Electrochimica Acta 49 (2004) 3319–3325
Formation of crystalline titanium dioxide on barrier layer-forming metals in aqueous electrolytes by anodic spark deposition—first mechanistic conceptions Susann Meyer∗ , Roger Gorges, Günter Kreisel Institute of Technical Chemistry and Environmental Chemistry, Friedrich Schiller University Jena, Lessingstraße 12, 07743 Jena, Germany Received 20 November 2003; received in revised form 4 March 2004; accepted 5 March 2004 Available online 17 April 2004
Abstract A spark deposition process for the generation of crystalline titanium dioxide layers on barrier layer-forming metals such as Al, Ti, Mg, Zr, etc. was investigated. The process was carried out at high voltages and currents in an aqueous electrolyte. The electrolyte composition is provided and it could be shown that the electrolyte system used has great influence on the properties of the oxide layers. From the titanium balance, it was proven that most of the layer originates from the deposition of electrolyte compounds rather than from conversion of substrate material. Mechanistic conceptions of the layer formation are presented and supported by analytical determination of some reaction intermediates. The titanium dioxide layers generated were characterised regarding their physical and chemical properties. © 2004 Elsevier Ltd. All rights reserved. Keywords: Titanium dioxide; TiO2 ; Spark deposition; Photocatalytic; Crystalline
1. Introduction Titanium dioxide is a universal semiconductor that has found its way into many application areas. Properties such as a high dielectric constant, purity and homogeneity lead to a variety of applications in the semiconductor industry, in photovoltaic devices, for waste water cleaning, for air detoxification and for surface functionalisation [1–4]. Many chemical and technological problems, however, require the fixation of titanium dioxide onto a support. Various methods are known for the fixation of nanoscale titanium dioxide onto a support matrix. However, the immobilisation method has considerable influence on the properties of the obtained TiO2 layer. Possible production processes for these oxide layers include simple dipping processes [5–7], sol–gel processes [8], electrochemical processes [9], hydrolysis and chemical vapour deposition (CVD) [10]. With all these methods at hand, it still remains a challenge to produce crystalline titanium dioxide layers on inexpensive and easily-processable substrates.
∗ Corresponding author. Tel.: +49-3641-948435; fax: +49-3641-948402. E-mail address: [email protected] (S. Meyer).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.03.003
An alternative method for the preparation of titanium dioxide layers are spark deposition processes. A good overview of past and current research in this field is given by Yerokhin et al. [11]. In this paper, we report the generation of crystalline titanium dioxide layers on different substrates via anodic spark deposition [12–14]. In addition to their crystalline structure, the layers also exhibit photocatalytic properties. The anodic spark process, having a rather simple experimental set-up, employs complex electrolyte systems, which are difficult to characterise exactly. So far, electrolyte compositions were mainly found empirically, and the layer formation process of the crystalline titanium dioxide has only been understood insufficiently. The elucidation of this process is the aim of this paper.
2. Experimental 2.1. Electrolyte The preparation of the electrolyte is carried out in two consecutive steps. EDTA-Na2 is dissolved in water and ammonium acetate and ammonia are added. This solution is stirred until it becomes clear. In a second batch, acetylacetone and 2-propanol are mixed, and then tetraethyl
3320
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
Table 1 Composition of the used electrolyte system Compound
Concentration (mol l−1 )
Tetraethyl orthotitanate Acetylacetone 2-Propanol EDTA-Na2 Ammonium acetate Ammonia
0.05 0.5 0.65 0.1 0.013 0.007
orthotitanate is added slowly under constant stirring. Then, the organic mixture is slowly added to the aqueous solution under stirring. Water is added to reach the anticipated volume. A clear yellow solution is obtained. The concentrations of the individual electrolyte compounds are summarised in Table 1 [15]. 2.2. Coating conditions The plasma-chemical process is carried out in a stirred and thermostatted electrolyte bath (5 l) with the substrate connected to the anode of a pulsed dc high-voltage source. The cathode is a stainless steel mesh running around the edges of the electrolyte bath. Typical coating conditions for the production of crystalline titanium dioxide layers are: U = 160 V, I < 10 A, dU/dt = 30 V s−1 , T = 18 ◦ C, f = 1.5 kHz. 2.3. Photocatalytic testing For the photocatalytic testing of the obtained TiO2 layer, a sample (50 mm × 10 mm) was put in a Suprasil cuvette and 3 ml of a 0.25 mmol l−1 aqueous 4-chlorophenol solution were added. The cuvette was then illuminated with a
450 W xenon arc lamp. Samples were taken after 60 and 120 min and the measurement was repeated five times. The concentration of 4-chlorophenol was determined with HPLC (Dionex C18 RP column, flow: 1.5 ml min−1 , eluent: water/methanol (50:50), detection wavelength: 280 nm).
3. Results and discussion With the presented electrolyte system and under the given coating conditions, ceramic oxide layers can be formed by anodic spark deposition on barrier layer-forming metals, such as Al, Ti, Mg, Zr, etc. The generation of crystalline titanium dioxide layers by anodic spark deposition becomes only possible using the described electrolytes. In addition, these layers exhibit photocatalytic properties that are produced in a single process step, and mainly result from the composition of the electrolyte. The layers show good photocatalytic properties for the degradation of the model substance 4-chlorophenol (Fig. 1). Reproducible results were found for repeated measurements. Fig. 2 shows a scanning electron micrograph (SEM) of the produced coral-like porous TiO2 layers. The layer thickness can reach up to 100 m, which is thick, compared to other preparation methods. The X-ray diffractogram of a typical TiO2 layer is depicted in Fig. 3. The layer consists of approximately 30% anatase and 70% rutile. Thermogravimetric investigation showed that the layer contains less than 5% organic compounds. After thermal treatment of the layers at 400 ◦ C for 1 h, no organic compounds were found in the layer and the adhesion of the layer to the substrate improved significantly. The band gap of the semiconductor was determined from reflection measurements to be 3.44 eV. Further characteristics
Fig. 1. Photocatalytic degradation of 0.25 mM 4-chlorophenol solution at the investigated TiO2 layers. Samples were taken after 60 and 120 min.
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
3321
Fig. 2. SEM micrograph of the cross-section of a TiO2 /Ti/glass-sample.
of our TiO2 layers, such as grain size or layer composition, are documented in detail elsewhere [16]. It was observed that the used electrolyte, i.e. the composition and concentration of the individual components, largely influenced the deposition process. The properties of the obtained layers are thus directly related to the electrolyte composition. The electrolyte described and the proposed deposition mechanism were investigated for the first time and are not comparable to other electrochemical coating methods described in literature [17–19]. We started our investigation by characterising the electrolyte after it was freshly prepared. The spectroscopic investigation of the electrolyte and the isolation of single compounds from the electrolyte revealed that during the preparation titanium alkoxide (TiIV ) forms a complex with acetylacetone according to Scheme 1. The titanyl acetylacetonate formed1 is very stable in aqueous solutions at pH values around 6.5. No other reactions could be detected. However, it cannot be excluded that titanyl acetylacetonate forms oligomers in aqueous solutions, although mass spectroscopy could not support this hypothesis. After using the electrolyte in the coating process, further changes could be observed that indicated more complex chemical reactions. For example, the used electrolyte showed a hypsochromic shift in the UV-Vis spectrum. The absorption maximum was shifted by 10 nm and a second absorption maximum at 210 nm was observed. Both changes serve as indicators for possible chemical reactions during the layer-formation process (cf. Fig. 4). The second absorp-
1
One litre of the electrolyte was extracted with 250 ml chloroform and then the solvent was removed. 1 H NMR (200 MHz, CDCl3 ), δ (ppm): 2.0 (s, 3H), 2.2 (s, 1H), 3.6 (s, 2H), 5.5 (s, 1H); 13 C NMR (50 MHz, CDCl3 ), δ (ppm): 24.8, 30.8, 100.4, 191.2; MS: m/z = 262 (M+ ).
tion maximum in the used electrolyte could be identified to origin from in situ generated 3,5-diacetyl-2,6-dimethyl-1,4dihdropyridine.2 In order to investigate whether the crystalline titanium dioxide layer was generated through conversion of the substrate or by deposition of electrolyte compounds, a titanium balance was carried out. For this purpose, 1000 cm2 of titanium were coated and the concentration of titanium in 1 l of the electrolyte was determined before and after the coating. It was found that the electrolyte was depleted by 24% titanium. Parallel gravimetric analysis of the coated titanium substrates supported these findings and showed that the mass difference of titanium in the electrolyte was found as titanium dioxide layer on the substrate. These experimental results let us conclude that the layer formation proceeded by the deposition of compounds from the electrolyte solution rather than by electrochemical conversion of the titanium substrates. Considering that the titanium, which forms the oxide layer, is almost entirely depleted from the electrolyte, the layer must be formed from a TiO2 precipitation. In general, metal oxide precipitations result from inorganic polycondensations and are caused by the hydrolysis of metal ions in solution. Such hydrolyses, condensations and complexing reactions are phenomena that are influenced by a variety of factors (pH value, charge, electronegativity, ligands, etc.) and considered difficult to describe theoretically. This is already true for pure aqueous solutions. The conditions present in the electrolyte during the coating process (high voltages
2 During the coating process, a yellow precipitate was formed. This precipitate was filtered, dried and recrystallised from 2-propanol. 1 H NMR (200 MHz, DMSO), δ (ppm): 2.08 (s, 6H), 2.6 (d, 6H), 3.3 (d, 2H), 8.24 (s, NH); 13 C NMR (50 MHz, DMSO), δ (ppm): 18.7, 26.3, 30.1, 107.7, 196.7; MS: m/z = 193 (M+ ); λmax = 208, 252, 278, 409 nm.
3322
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
Fig. 3. X-ray diffractogram of a TiO2 layer generated by anodic spark deposition.
Ti
O
O
O
O
+ 2
O
O
O - 2 C2H5OH
+
-
O
Ti
O O
-
H2O
O O
- 2 C2H5OH
O
+
Ti
O -
O O
O
Scheme 1. Complex formation of TiIV during the preparation of the electrolyte system. The intermediate titanium(IV)-diethoxybisacetylacetonate could also be isolated and identified. The organic part of the electrolyte was prepared according to the procedure described in Section 2 and stirred for 3 h. Then the reaction products were isolated by cold distillation. 1 H NMR (200 MHz, CDCl3 ), δ (ppm): 1.1 (s, 3H), 1.9 (d, 3H), 4.4 (d, 2H), 5.4 (s, 1H); 13 C NMR (50 MHz, CDCl ), δ (ppm): 18.3, 25.8, 72.2, 102.8, 191.1; MS: m/z = 307 (M+ ). 3
Fig. 4. UV-Vis spectra of the investigated electrolyte system. Freshly prepared electrolyte (dashed line) and after the coating of 1000 cm2 titanium substrates (solid line).
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
3323
Fig. 5. Part 1: schematic voltage vs. time (dashed line) and current vs. time (solid line) plots of titanium dioxide coating process with the described electrolyte system. The process can be subdivided into two phases (A and B). Further explanations in the text. Part 2: plot of real experimental results.
and currents) even add to the complexity of these processes and make it difficult to investigate them thoroughly. Fig. 5 depicts the time-dependence of the voltage and the current during the coating process. Understanding this diagram contributes to understanding the processes that take place at the anode and the cathode. In the first process, phase A (cf. Fig. 5), the natural surface oxide layer of titanium is reinforced. The formation of the so-called barrier layer (thickness 0.06–0.8 m) represents an anodic oxidation in the classical sense. The resulting amorphous oxide layer is formed already at lower voltages solely by conversion of the substrate material. The insulation character of this layer causes a significant current decrease in phase A during the first process seconds denoted by (a). When no voltage is applied, the electrolyte has a pH value of 6.5 and titanyl acetylacetonate is present in a hydrolysisimpeding TiIV transport modification [20]. After applying a potential, the pH values of the anode and cathode change due to the electrolysis of water according to Scheme 2. The formation of a pH gradient could be shown by pH measurements in the vicinity of the anode and cathode (Fig. 6). As a consequence of the emerging pH gradient, a number of transfer processes and parallel chemical reactions are initiated. Anode: 2 H 2O
O2 + 4 H + + 4e −
Cathode: 2 H 2 O + 2e −
2OH − + H 2
Scheme 2. Anode and cathode reactions during the first process seconds.
The increase in pH value at the cathode leads to hydrolysis of titanyl acetylacetone. It is known that besides the coordination sphere, the hydrolysis of metal cations in aqueous solutions is mainly dependent on the formal charge of the complex and the pH value of the solution. Scheme 3 gives a simple and formally stoichiometric description of this process. Depending on the pH value, differently coordinated titanium complexes exist in the aqueous system. Common ligands include hydroxo (OH− ), aquo (H2 O) and oxo (O2− ) groups. Since the exact complex constitutions could not be determined, the transport modification of the TiIV complex is described in the following by the general formula [Ti(L)n ]trans . In phase B voltage increases and current reincreases (b) until the pre-set hold voltage (c) is reached. Small current discharges on the surface could be observed. Those discharges are viewed as barrier plasmas [21] and can only occur because of the dielectric properties of the barrier layer. In the barrier plasma, ultra-short-lived micro-bubbles are formed, which represent reaction chambers with high temperatures and pressures. The formation of these micro-bubbles is considered crucial to the crystalline layer formation. Phase B starts with the formation of the crystalline titanium dioxide layer and is characterised by a constant voltage, which must be maintained at 145 V or above in order to ensure the dielectric breakdown that yields the desired plasma discharges. Phenomenologically, the plasma discharge process is associated with the emission of sparks and sounds. Meanwhile, the cathodically formed [Ti(L)n ]trans m− anion is transported by the electric field to the anode (substrate) due to its negative charge. During the transport, the coordination of the ligands on the titanium changes multiple times induced by the pH gradient between cathode and anode. At the anode (substrate) conversion,
3324
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
Fig. 6. pH value at the anode and cathode over the coating time.
O
+
Ti O
O O
O
+
3OH-
TiO(OH)3-
+
O
2
O
Scheme 3. Possible reaction of titanyl acetylacetone at the cathode with starting coating potential.
of the [Ti(L)n ]trans m− anion to crystalline titanium dioxide takes place. Anodically formed protons undergo acid–base reactions that yield intermediates such as [Ti(OH)4 ·(OH)2 ]0 . These polycations of the tetravalent element undergo spontaneous hydrolysis based on their electronegativity. The strongly polarising TiIV , however, does not form stable hydroxides. Therefore, dehydration occurs and yields titanium dioxide. The plasma processes, combined with the special hydrothermal conditions at the surface of the anode, create ideal reaction spaces for accelerated dehydration, condensation and crystallisation. Thus, crystalline and strongly adherent precipitations of TiO2 are obtained on the substrate. The reinforced amorphous barrier layer, which has been formed in phase A, serves as condensation core for newly precipitating titanium dioxide on the substrate. The barrier layer is thus, besides its dielectric properties, essential for an accelerated growth of the oxide layer. Phase B of the coating process contributes mostly to the growth of the titanium dioxide layer. Current plateaus at a constant value (d). The layer thickness and properties are a function of the energy input and the coating time. After the first crystalline oxide layer is closed and enough crystallisation cores are present, more crystalline titanium dioxide aggregates on top of the already formed layer. Apparently, the deposition of the particles takes place in a self-organised fashion that results coral-like but reproducible surface struc-
tures. Micro-bubbles most often occur during the first process seconds in the vicinity of the anode. The diminishing of sound emission with progressing coating time confirms the theory that the layer formation is initiated in these bubbles. After initiation, the deposition process proceeds by hydrolysis of titanium species from the electrolyte. Increasing anatase content (the low temperature modification of titanium dioxide) in the layer with increasing layer thickness supports this thesis since the newly generated titanium dioxide experiences less thermal stress after ceasing bubble formation. Furthermore, the porous layer morphology and the irregular layer thickness support the hypothesis of such adsorption and condensation processes. From these findings, also other barrier layer-forming metals should be able to be coated with titanium dioxide from this electrolyte. We found that also zirconium and tantalum could be coated with titanium dioxide using the electrolyte described.
4. Conclusion In summary, for the coating process presented, a deposition of crystalline TiO2 from the electrolyte could be proven. Although not every single process step could be identified, a clear idea of this anodic spark deposition process is provided, and hence may be referred to as “electrochemically
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
induced sol–gel process”. The method thus combines the advantages of electrochemistry and plasmachemistry (onestep preparation and possibility to coat complicated substrate geometries) with the positive characteristics of sol–gel methods (such as the formation of crystalline mesoporous oxides for the generation of photocatalytic titanium dioxide layers). The usual rate-determining factors in sol–gel processes such as the formation of condensation cores, growth and ageing of the oxides proceed in a single process step in an accelerated fashion due to the hydrothermal conditions on the substrate surface. Acknowledgements The authors would like to thank Dr. R. Ohser-Wiedemann and M. Martin of the Technical University Freiberg, Germany, for the X-ray investigations and Techno Coat GmbH Zittau, Germany, for the preparation of titanium PVDcoatings. Financial support from the German Ministry of Education and Research (BMBF) in the framework of NOA—Netzwerk für Innovative Oberflächentechnik und Anlagenbau is gratefully acknowledged. References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (1998) 135.
3325
[3] M. Grätzel, Prog. Photovoltaics Res. Appl. 8 (2000) 171. [4] K. Talagi, T. Makimoto, H. Hiraiwa, T. Negishi, J. Vac. Sci. Technol. A 19 (2001) 2931. [5] J.A. Byrne, B.R. Eggins, N.M.D. Brown, B. McKinney, M. Rouse, Appl. Catal. B 17 (1998) 25. [6] M. Bideau, B. Claudel, C. Dubien, L. Faure, H. Kazonan, J. Photochem. Photobiol. A 91 (1995) 137. [7] G. Chester, M. Anderson, H. Read, J. Photochem. Photobiol. A 71 (1993) 291. [8] S. Lee, M. Kang, S.M. Cho, G.Y. Han, B. Kim, K. Yoon, C. Chung, J. Photochem. Photobiol. A 146 (2001) 121. [9] M.L. Hitchman, R. Spackman, A. Anthony, L. Cesar, J. Chem. Soc., Faraday Trans. 92 (1996) 4049. [10] H.L.M. Chang, H. You, J. Guo, D.J. Lam, Appl. Surf. Sci. 48–49 (1991) 12. [11] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [12] J. Käßbohrer, G. Kreisel, S. Meyer, R. Gorges, Galvanotechnik 94 (2003) 1374. [13] S. Meyer, R. Gorges, G. Kreisel, Galvanotechnik 94 (2003) 1634. [14] J. Schreckenbach, G. Marx, U. Bayer, G. Kreisel, Galvanotechnik 94 (2003) 816. [15] I. Hennig, J. Käßbohrer, G. Kreisel, U. Bayer, U. Weinzierl, Patent DE19841650A1 (2000). [16] S. Meyer, R. Gorges, G. Kreisel, Thin Solid Films 450 (2004) 276. [17] J.P. Schreckenbach, D. Butte, G. Marx, Microchim. Acta 133 (2000) 295. [18] P. Kurze, Metalloberflächen 48 (1994) 104. [19] U. Bayer, Galvanotechnik 90 (1999) 2116. [20] D.A. Antonelli, J.Y. Ying, Angew. Chem. Int. Ed. Engl. 34 (1995) 2014. [21] H. Drost, Plasmachemie, 1st ed., Akademie-Verlag, Berlin, 1978.
Formation of crystalline titanium dioxide on barrier layer-forming metals in aqueous electrolytes by anodic spark deposition—first mechanistic conceptions Susann Meyer∗ , Roger Gorges, Günter Kreisel Institute of Technical Chemistry and Environmental Chemistry, Friedrich Schiller University Jena, Lessingstraße 12, 07743 Jena, Germany Received 20 November 2003; received in revised form 4 March 2004; accepted 5 March 2004 Available online 17 April 2004
Abstract A spark deposition process for the generation of crystalline titanium dioxide layers on barrier layer-forming metals such as Al, Ti, Mg, Zr, etc. was investigated. The process was carried out at high voltages and currents in an aqueous electrolyte. The electrolyte composition is provided and it could be shown that the electrolyte system used has great influence on the properties of the oxide layers. From the titanium balance, it was proven that most of the layer originates from the deposition of electrolyte compounds rather than from conversion of substrate material. Mechanistic conceptions of the layer formation are presented and supported by analytical determination of some reaction intermediates. The titanium dioxide layers generated were characterised regarding their physical and chemical properties. © 2004 Elsevier Ltd. All rights reserved. Keywords: Titanium dioxide; TiO2 ; Spark deposition; Photocatalytic; Crystalline
1. Introduction Titanium dioxide is a universal semiconductor that has found its way into many application areas. Properties such as a high dielectric constant, purity and homogeneity lead to a variety of applications in the semiconductor industry, in photovoltaic devices, for waste water cleaning, for air detoxification and for surface functionalisation [1–4]. Many chemical and technological problems, however, require the fixation of titanium dioxide onto a support. Various methods are known for the fixation of nanoscale titanium dioxide onto a support matrix. However, the immobilisation method has considerable influence on the properties of the obtained TiO2 layer. Possible production processes for these oxide layers include simple dipping processes [5–7], sol–gel processes [8], electrochemical processes [9], hydrolysis and chemical vapour deposition (CVD) [10]. With all these methods at hand, it still remains a challenge to produce crystalline titanium dioxide layers on inexpensive and easily-processable substrates.
∗ Corresponding author. Tel.: +49-3641-948435; fax: +49-3641-948402. E-mail address: [email protected] (S. Meyer).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.03.003
An alternative method for the preparation of titanium dioxide layers are spark deposition processes. A good overview of past and current research in this field is given by Yerokhin et al. [11]. In this paper, we report the generation of crystalline titanium dioxide layers on different substrates via anodic spark deposition [12–14]. In addition to their crystalline structure, the layers also exhibit photocatalytic properties. The anodic spark process, having a rather simple experimental set-up, employs complex electrolyte systems, which are difficult to characterise exactly. So far, electrolyte compositions were mainly found empirically, and the layer formation process of the crystalline titanium dioxide has only been understood insufficiently. The elucidation of this process is the aim of this paper.
2. Experimental 2.1. Electrolyte The preparation of the electrolyte is carried out in two consecutive steps. EDTA-Na2 is dissolved in water and ammonium acetate and ammonia are added. This solution is stirred until it becomes clear. In a second batch, acetylacetone and 2-propanol are mixed, and then tetraethyl
3320
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
Table 1 Composition of the used electrolyte system Compound
Concentration (mol l−1 )
Tetraethyl orthotitanate Acetylacetone 2-Propanol EDTA-Na2 Ammonium acetate Ammonia
0.05 0.5 0.65 0.1 0.013 0.007
orthotitanate is added slowly under constant stirring. Then, the organic mixture is slowly added to the aqueous solution under stirring. Water is added to reach the anticipated volume. A clear yellow solution is obtained. The concentrations of the individual electrolyte compounds are summarised in Table 1 [15]. 2.2. Coating conditions The plasma-chemical process is carried out in a stirred and thermostatted electrolyte bath (5 l) with the substrate connected to the anode of a pulsed dc high-voltage source. The cathode is a stainless steel mesh running around the edges of the electrolyte bath. Typical coating conditions for the production of crystalline titanium dioxide layers are: U = 160 V, I < 10 A, dU/dt = 30 V s−1 , T = 18 ◦ C, f = 1.5 kHz. 2.3. Photocatalytic testing For the photocatalytic testing of the obtained TiO2 layer, a sample (50 mm × 10 mm) was put in a Suprasil cuvette and 3 ml of a 0.25 mmol l−1 aqueous 4-chlorophenol solution were added. The cuvette was then illuminated with a
450 W xenon arc lamp. Samples were taken after 60 and 120 min and the measurement was repeated five times. The concentration of 4-chlorophenol was determined with HPLC (Dionex C18 RP column, flow: 1.5 ml min−1 , eluent: water/methanol (50:50), detection wavelength: 280 nm).
3. Results and discussion With the presented electrolyte system and under the given coating conditions, ceramic oxide layers can be formed by anodic spark deposition on barrier layer-forming metals, such as Al, Ti, Mg, Zr, etc. The generation of crystalline titanium dioxide layers by anodic spark deposition becomes only possible using the described electrolytes. In addition, these layers exhibit photocatalytic properties that are produced in a single process step, and mainly result from the composition of the electrolyte. The layers show good photocatalytic properties for the degradation of the model substance 4-chlorophenol (Fig. 1). Reproducible results were found for repeated measurements. Fig. 2 shows a scanning electron micrograph (SEM) of the produced coral-like porous TiO2 layers. The layer thickness can reach up to 100 m, which is thick, compared to other preparation methods. The X-ray diffractogram of a typical TiO2 layer is depicted in Fig. 3. The layer consists of approximately 30% anatase and 70% rutile. Thermogravimetric investigation showed that the layer contains less than 5% organic compounds. After thermal treatment of the layers at 400 ◦ C for 1 h, no organic compounds were found in the layer and the adhesion of the layer to the substrate improved significantly. The band gap of the semiconductor was determined from reflection measurements to be 3.44 eV. Further characteristics
Fig. 1. Photocatalytic degradation of 0.25 mM 4-chlorophenol solution at the investigated TiO2 layers. Samples were taken after 60 and 120 min.
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
3321
Fig. 2. SEM micrograph of the cross-section of a TiO2 /Ti/glass-sample.
of our TiO2 layers, such as grain size or layer composition, are documented in detail elsewhere [16]. It was observed that the used electrolyte, i.e. the composition and concentration of the individual components, largely influenced the deposition process. The properties of the obtained layers are thus directly related to the electrolyte composition. The electrolyte described and the proposed deposition mechanism were investigated for the first time and are not comparable to other electrochemical coating methods described in literature [17–19]. We started our investigation by characterising the electrolyte after it was freshly prepared. The spectroscopic investigation of the electrolyte and the isolation of single compounds from the electrolyte revealed that during the preparation titanium alkoxide (TiIV ) forms a complex with acetylacetone according to Scheme 1. The titanyl acetylacetonate formed1 is very stable in aqueous solutions at pH values around 6.5. No other reactions could be detected. However, it cannot be excluded that titanyl acetylacetonate forms oligomers in aqueous solutions, although mass spectroscopy could not support this hypothesis. After using the electrolyte in the coating process, further changes could be observed that indicated more complex chemical reactions. For example, the used electrolyte showed a hypsochromic shift in the UV-Vis spectrum. The absorption maximum was shifted by 10 nm and a second absorption maximum at 210 nm was observed. Both changes serve as indicators for possible chemical reactions during the layer-formation process (cf. Fig. 4). The second absorp-
1
One litre of the electrolyte was extracted with 250 ml chloroform and then the solvent was removed. 1 H NMR (200 MHz, CDCl3 ), δ (ppm): 2.0 (s, 3H), 2.2 (s, 1H), 3.6 (s, 2H), 5.5 (s, 1H); 13 C NMR (50 MHz, CDCl3 ), δ (ppm): 24.8, 30.8, 100.4, 191.2; MS: m/z = 262 (M+ ).
tion maximum in the used electrolyte could be identified to origin from in situ generated 3,5-diacetyl-2,6-dimethyl-1,4dihdropyridine.2 In order to investigate whether the crystalline titanium dioxide layer was generated through conversion of the substrate or by deposition of electrolyte compounds, a titanium balance was carried out. For this purpose, 1000 cm2 of titanium were coated and the concentration of titanium in 1 l of the electrolyte was determined before and after the coating. It was found that the electrolyte was depleted by 24% titanium. Parallel gravimetric analysis of the coated titanium substrates supported these findings and showed that the mass difference of titanium in the electrolyte was found as titanium dioxide layer on the substrate. These experimental results let us conclude that the layer formation proceeded by the deposition of compounds from the electrolyte solution rather than by electrochemical conversion of the titanium substrates. Considering that the titanium, which forms the oxide layer, is almost entirely depleted from the electrolyte, the layer must be formed from a TiO2 precipitation. In general, metal oxide precipitations result from inorganic polycondensations and are caused by the hydrolysis of metal ions in solution. Such hydrolyses, condensations and complexing reactions are phenomena that are influenced by a variety of factors (pH value, charge, electronegativity, ligands, etc.) and considered difficult to describe theoretically. This is already true for pure aqueous solutions. The conditions present in the electrolyte during the coating process (high voltages
2 During the coating process, a yellow precipitate was formed. This precipitate was filtered, dried and recrystallised from 2-propanol. 1 H NMR (200 MHz, DMSO), δ (ppm): 2.08 (s, 6H), 2.6 (d, 6H), 3.3 (d, 2H), 8.24 (s, NH); 13 C NMR (50 MHz, DMSO), δ (ppm): 18.7, 26.3, 30.1, 107.7, 196.7; MS: m/z = 193 (M+ ); λmax = 208, 252, 278, 409 nm.
3322
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
Fig. 3. X-ray diffractogram of a TiO2 layer generated by anodic spark deposition.
Ti
O
O
O
O
+ 2
O
O
O - 2 C2H5OH
+
-
O
Ti
O O
-
H2O
O O
- 2 C2H5OH
O
+
Ti
O -
O O
O
Scheme 1. Complex formation of TiIV during the preparation of the electrolyte system. The intermediate titanium(IV)-diethoxybisacetylacetonate could also be isolated and identified. The organic part of the electrolyte was prepared according to the procedure described in Section 2 and stirred for 3 h. Then the reaction products were isolated by cold distillation. 1 H NMR (200 MHz, CDCl3 ), δ (ppm): 1.1 (s, 3H), 1.9 (d, 3H), 4.4 (d, 2H), 5.4 (s, 1H); 13 C NMR (50 MHz, CDCl ), δ (ppm): 18.3, 25.8, 72.2, 102.8, 191.1; MS: m/z = 307 (M+ ). 3
Fig. 4. UV-Vis spectra of the investigated electrolyte system. Freshly prepared electrolyte (dashed line) and after the coating of 1000 cm2 titanium substrates (solid line).
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
3323
Fig. 5. Part 1: schematic voltage vs. time (dashed line) and current vs. time (solid line) plots of titanium dioxide coating process with the described electrolyte system. The process can be subdivided into two phases (A and B). Further explanations in the text. Part 2: plot of real experimental results.
and currents) even add to the complexity of these processes and make it difficult to investigate them thoroughly. Fig. 5 depicts the time-dependence of the voltage and the current during the coating process. Understanding this diagram contributes to understanding the processes that take place at the anode and the cathode. In the first process, phase A (cf. Fig. 5), the natural surface oxide layer of titanium is reinforced. The formation of the so-called barrier layer (thickness 0.06–0.8 m) represents an anodic oxidation in the classical sense. The resulting amorphous oxide layer is formed already at lower voltages solely by conversion of the substrate material. The insulation character of this layer causes a significant current decrease in phase A during the first process seconds denoted by (a). When no voltage is applied, the electrolyte has a pH value of 6.5 and titanyl acetylacetonate is present in a hydrolysisimpeding TiIV transport modification [20]. After applying a potential, the pH values of the anode and cathode change due to the electrolysis of water according to Scheme 2. The formation of a pH gradient could be shown by pH measurements in the vicinity of the anode and cathode (Fig. 6). As a consequence of the emerging pH gradient, a number of transfer processes and parallel chemical reactions are initiated. Anode: 2 H 2O
O2 + 4 H + + 4e −
Cathode: 2 H 2 O + 2e −
2OH − + H 2
Scheme 2. Anode and cathode reactions during the first process seconds.
The increase in pH value at the cathode leads to hydrolysis of titanyl acetylacetone. It is known that besides the coordination sphere, the hydrolysis of metal cations in aqueous solutions is mainly dependent on the formal charge of the complex and the pH value of the solution. Scheme 3 gives a simple and formally stoichiometric description of this process. Depending on the pH value, differently coordinated titanium complexes exist in the aqueous system. Common ligands include hydroxo (OH− ), aquo (H2 O) and oxo (O2− ) groups. Since the exact complex constitutions could not be determined, the transport modification of the TiIV complex is described in the following by the general formula [Ti(L)n ]trans . In phase B voltage increases and current reincreases (b) until the pre-set hold voltage (c) is reached. Small current discharges on the surface could be observed. Those discharges are viewed as barrier plasmas [21] and can only occur because of the dielectric properties of the barrier layer. In the barrier plasma, ultra-short-lived micro-bubbles are formed, which represent reaction chambers with high temperatures and pressures. The formation of these micro-bubbles is considered crucial to the crystalline layer formation. Phase B starts with the formation of the crystalline titanium dioxide layer and is characterised by a constant voltage, which must be maintained at 145 V or above in order to ensure the dielectric breakdown that yields the desired plasma discharges. Phenomenologically, the plasma discharge process is associated with the emission of sparks and sounds. Meanwhile, the cathodically formed [Ti(L)n ]trans m− anion is transported by the electric field to the anode (substrate) due to its negative charge. During the transport, the coordination of the ligands on the titanium changes multiple times induced by the pH gradient between cathode and anode. At the anode (substrate) conversion,
3324
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
Fig. 6. pH value at the anode and cathode over the coating time.
O
+
Ti O
O O
O
+
3OH-
TiO(OH)3-
+
O
2
O
Scheme 3. Possible reaction of titanyl acetylacetone at the cathode with starting coating potential.
of the [Ti(L)n ]trans m− anion to crystalline titanium dioxide takes place. Anodically formed protons undergo acid–base reactions that yield intermediates such as [Ti(OH)4 ·(OH)2 ]0 . These polycations of the tetravalent element undergo spontaneous hydrolysis based on their electronegativity. The strongly polarising TiIV , however, does not form stable hydroxides. Therefore, dehydration occurs and yields titanium dioxide. The plasma processes, combined with the special hydrothermal conditions at the surface of the anode, create ideal reaction spaces for accelerated dehydration, condensation and crystallisation. Thus, crystalline and strongly adherent precipitations of TiO2 are obtained on the substrate. The reinforced amorphous barrier layer, which has been formed in phase A, serves as condensation core for newly precipitating titanium dioxide on the substrate. The barrier layer is thus, besides its dielectric properties, essential for an accelerated growth of the oxide layer. Phase B of the coating process contributes mostly to the growth of the titanium dioxide layer. Current plateaus at a constant value (d). The layer thickness and properties are a function of the energy input and the coating time. After the first crystalline oxide layer is closed and enough crystallisation cores are present, more crystalline titanium dioxide aggregates on top of the already formed layer. Apparently, the deposition of the particles takes place in a self-organised fashion that results coral-like but reproducible surface struc-
tures. Micro-bubbles most often occur during the first process seconds in the vicinity of the anode. The diminishing of sound emission with progressing coating time confirms the theory that the layer formation is initiated in these bubbles. After initiation, the deposition process proceeds by hydrolysis of titanium species from the electrolyte. Increasing anatase content (the low temperature modification of titanium dioxide) in the layer with increasing layer thickness supports this thesis since the newly generated titanium dioxide experiences less thermal stress after ceasing bubble formation. Furthermore, the porous layer morphology and the irregular layer thickness support the hypothesis of such adsorption and condensation processes. From these findings, also other barrier layer-forming metals should be able to be coated with titanium dioxide from this electrolyte. We found that also zirconium and tantalum could be coated with titanium dioxide using the electrolyte described.
4. Conclusion In summary, for the coating process presented, a deposition of crystalline TiO2 from the electrolyte could be proven. Although not every single process step could be identified, a clear idea of this anodic spark deposition process is provided, and hence may be referred to as “electrochemically
S. Meyer et al. / Electrochimica Acta 49 (2004) 3319–3325
induced sol–gel process”. The method thus combines the advantages of electrochemistry and plasmachemistry (onestep preparation and possibility to coat complicated substrate geometries) with the positive characteristics of sol–gel methods (such as the formation of crystalline mesoporous oxides for the generation of photocatalytic titanium dioxide layers). The usual rate-determining factors in sol–gel processes such as the formation of condensation cores, growth and ageing of the oxides proceed in a single process step in an accelerated fashion due to the hydrothermal conditions on the substrate surface. Acknowledgements The authors would like to thank Dr. R. Ohser-Wiedemann and M. Martin of the Technical University Freiberg, Germany, for the X-ray investigations and Techno Coat GmbH Zittau, Germany, for the preparation of titanium PVDcoatings. Financial support from the German Ministry of Education and Research (BMBF) in the framework of NOA—Netzwerk für Innovative Oberflächentechnik und Anlagenbau is gratefully acknowledged. References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Adv. Mater. 10 (1998) 135.
3325
[3] M. Grätzel, Prog. Photovoltaics Res. Appl. 8 (2000) 171. [4] K. Talagi, T. Makimoto, H. Hiraiwa, T. Negishi, J. Vac. Sci. Technol. A 19 (2001) 2931. [5] J.A. Byrne, B.R. Eggins, N.M.D. Brown, B. McKinney, M. Rouse, Appl. Catal. B 17 (1998) 25. [6] M. Bideau, B. Claudel, C. Dubien, L. Faure, H. Kazonan, J. Photochem. Photobiol. A 91 (1995) 137. [7] G. Chester, M. Anderson, H. Read, J. Photochem. Photobiol. A 71 (1993) 291. [8] S. Lee, M. Kang, S.M. Cho, G.Y. Han, B. Kim, K. Yoon, C. Chung, J. Photochem. Photobiol. A 146 (2001) 121. [9] M.L. Hitchman, R. Spackman, A. Anthony, L. Cesar, J. Chem. Soc., Faraday Trans. 92 (1996) 4049. [10] H.L.M. Chang, H. You, J. Guo, D.J. Lam, Appl. Surf. Sci. 48–49 (1991) 12. [11] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73. [12] J. Käßbohrer, G. Kreisel, S. Meyer, R. Gorges, Galvanotechnik 94 (2003) 1374. [13] S. Meyer, R. Gorges, G. Kreisel, Galvanotechnik 94 (2003) 1634. [14] J. Schreckenbach, G. Marx, U. Bayer, G. Kreisel, Galvanotechnik 94 (2003) 816. [15] I. Hennig, J. Käßbohrer, G. Kreisel, U. Bayer, U. Weinzierl, Patent DE19841650A1 (2000). [16] S. Meyer, R. Gorges, G. Kreisel, Thin Solid Films 450 (2004) 276. [17] J.P. Schreckenbach, D. Butte, G. Marx, Microchim. Acta 133 (2000) 295. [18] P. Kurze, Metalloberflächen 48 (1994) 104. [19] U. Bayer, Galvanotechnik 90 (1999) 2116. [20] D.A. Antonelli, J.Y. Ying, Angew. Chem. Int. Ed. Engl. 34 (1995) 2014. [21] H. Drost, Plasmachemie, 1st ed., Akademie-Verlag, Berlin, 1978.