- Email: [email protected]
micro-porous layers
Electrochimica Acta 55 (2010) 5786–5792
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Investigation on hydrophilicity of micro-arc oxidized TiO2 nano/micro-porous layers M.R. Bayati a,∗ , Roya Molaei a , Amir Kajbafvala b,∗∗ , Saeid Zanganeh c , H.R. Zargar d , K. Janghorban e a
School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box 16845-161, Tehran, Iran Department of Materials Science and Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695-7907, USA School of Engineering, University of Connecticut, 261 Glenbrook Rd., Storrs, CT, USA d Department of Materials Engineering, University of British Columbia, Vancouver, Canada e Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran b c
a r t i c l e
i n f o
Article history: Received 18 February 2010 Received in revised form 3 May 2010 Accepted 4 May 2010 Available online 11 May 2010 Keywords: Titanium oxide Micro-arc oxidation Porous materials Hydrophilic
a b s t r a c t Titania porous layers with a rough surface were synthesized via micro-arc oxidation (MAO) and the effect of the applied voltage and electrolyte concentration on surface structure, and chemical composition of the layers was studied. Morphological and topographical investigations, performed by SEM and AFM, revealed that pore size and surface roughness of the layers increased with the applied voltage and the electrolyte concentration. Based on the XRD and XPS results, the grown layers consisted of anatase and rutile phases with varying fractions depending on growth conditions. It was found that anatase/rutile relative content reached its maximum value at medium applied voltages or electrolyte concentrations. Finally, hydrophilicity of the grown layers was determined using a water contact angle apparatus, and a correlation between measured contact angles and MAO-parameters was suggested. It was observed that the layers synthesized under the applied voltage of 400 V in the electrolytes with a concentration of 10 g l−1 exhibited the highest hydrophilicity. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Titanium dioxide (TiO2 ) has been widely studied due to its great variety of applications by many researchers. Photocatalysis [1,2], solar cells [3,4], gas sensors [5], lithium batteries [6], antibacterial activity [7,8], and self-cleaning and antifogging surfaces [9–11] are some of its applications. Compared with its powder form, TiO2 films can be easily separated, recovered, and efficiently recycled [12]. In 1997, Wang et al. [13] reported that UV-irradiation of titania surfaces produces a super-hydrophilic surface. So far, hydrophilic titanium dioxide layers have attracted many theoretical and practical applications such as self-cleaning and antifogging mirrors. However, the hydrophilicity of the TiO2 layers needs to be further enhanced for practical applications [14]. One way to improve its hydrophilicity is by fabricating porous layers. Since photo-chemical reactions mainly occur on the surface, the surface properties of TiO2 such as surface area, defects, surface acidity, surface functional groups, particle size, and crystalline phase will greatly affect photo-chemical activity and related mechanisms [12,15]. Yu and Wark studied the effect of specific surface area on photocat-
∗ Corresponding author. Tel.: +98 21 77920727. ∗∗ Corresponding author. Tel.: +1 919 515 7217. E-mail addresses: [email protected] (M.R. Bayati), [email protected] (A. Kajbafvala). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.05.021
alytic performance and hydrophilicity of titania and demonstrated the more the specific surface area the better the photo-induced hydrophilicity [16,17]. Titanium dioxide with varying morphologies has been synthesized by different methods including sol–gel [18,19], chemical vapor deposition [20,21], physical vapor deposition [22,23], hydrothermal process [24,25], electrochemical methods [26,27], liquid phase deposition [28], and spray pyrolysis [29,30]. TiO2 can also be obtained via micro-arc oxidation (MAO) process [31,32]. MAO is an electrochemical technique for formation of anodic films by spark/arc micro-discharges which move rapidly on the vicinity of the anode surface [33–36]. It is characterized by high productivity, economic efficiency, ecological friendliness, high hardness, good wear resistance, and excellent bonding strength with the substrate [37–39]. This process is carried out at voltages higher than the breakdown voltage of the gas layer enshrouding the anode. Since the substrate is connected to positive pole of the rectifier as anode, the gas layer consists of oxygen. When the dielectric gas layer completely covers the anode surface, electrical resistance of the electrochemical circuit surges and the process continues providing that the applied voltage defeats the breakdown voltage of the gas layer. Applying such voltages leads to formation of electrical discharges via which electrical current could pass the gas layer. MAO process is characterized by these electrical sparks [34,40] which are responsible for formation of the structural pores [41,42]. Due to strong electrical field (106 –108 V m−1 ) between anode and
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Fig. 1. SEM top-view of the TiO2 layers grown in the electrolytes with a concentration of 5 g l−1 under applied voltages of (a) 350 V, (b) 400 V, (c) 450 V, (d) 500 V, and (e) 550 V.
Fig. 2. SEM top-view of the TiO2 layers grown in the electrolytes with a concentration of 10 g l−1 under applied voltages of: (a) 300 V, (b) 350 V, (c) 400 V, (d) 450 V, (e) 500 V, and (f) 550 V.
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Fig. 3. SEM top-view of the TiO2 layers grown in the electrolytes with a concentration of 20 g l−1 under applied voltages of: (a) 250 V, (b) 300 V, (c) 350 V, (d) 400 V, (e) 450 V, (f) 500 V, and (g) 550 V.
cathode, electrolyte anions are drawn into the structural pores where they can attend electrochemical reactions. Structural pores are formed by electron avalanches taking places on the vicinity of the anode. Characteristics of electrolyte have a great influence on the film formation kinetics. Phosphates, sulfates, silicates and borates are four conventional kinds of electrolytes employed in previous researches, and the formed TiO2 films usually contain the element of the electrolytes (P, S, Si, B, etc.) [34,36,37,40]. In this research, results of growth, characterization, and especially hydrophilicity performance of the TiO2 layers fabricated via MAO process were discussed. A correlation between MAOparameters and hydrophilicity of the layers was proposed. To the best of our knowledge, this is the first study on the hydrophilicity of the MAO-grown titania layers. 2. Experimental A DC-rectifier with a maximum output of 600 V/30 A was employed as current source. 3 cm × 3 cm × 0.5 mm commercially pure grade 2 titanium pieces, surrounded by an ASTM 316 stainless steel cylindrical container (cathode), was also used as substrate (anode). Three sodium phosphate (Na3 PO4 ·12H2 O, Merck) solutions with different concentrations were used as electrolyte. The electrolyte temperature was fixed at 70 ± 3 ◦ C employing a water circulating system. Prior to MAO treatment, substrates underwent a cleaning process including mechanical polishing followed by washing in distilled water. Afterward, the titanium plates were chemically etched in diluted HF solution (HF:H2 O = 1:20 vol.%) at room temperature for 30 s, and then washed in distilled water again. In the last stage of cleaning procedure, the substrates were ultrasonically cleaned in acetone for 15 min and finally washed by distilled water. After surface cleaning, MAO treatment was accomplished under different voltages in a range of 250–550 V with +50 V intervals. Meanwhile, the deposition time for each run was considered as 3 min. Surface morphology and topography of the layers were examined by scanning electron microscope (TESCAN, Vega II) and atomic force microscope (Veeco auto probe) in contact mode with a 10 nm
radius silicon tip. X-ray diffraction (Philips, PW3710) and X-ray photoelectron spectroscopy (VG Microtech, Twin anode, XR3E2 Xray source, using AlK␣ = 1486.6 eV) techniques were used to study phase structure and chemical composition of the synthesized layers. The photo-induced hydrophilicity of the layers was evaluated by screening photos and measuring the contact angle of DI-water droplets. The grown layers were first UV-irradiated by a 25 W lamp ( = 365 nm) for 1 h; then, the hydrophilicity photos were obtained by a water contact angle apparatus. The hydrophilicity studies were performed in the atmosphere. 3. Results and discussion SEM morphologies of the layers synthesized at different applied voltages for 3 min in electrolytes with concentrations of 5, 10, and 20 g l−1 are shown in Figs. 1–3. No pore was observed in the structure of the layers grown at applied voltages less than 350 V in the electrolytes with a concentration of 5 g l−1 , but the samples which were fabricated under higher applied voltages were porous, and the pore size increased with the applied voltage. It should be mentioned that no electrical sparking occurred at the applied voltages less than 350 V, and increasing the applied voltage resulted in generation of stronger and long-living sparks. It is to emphasize that applying higher voltages caused electrical sparks with higher energy due to higher electrical current passing through the electrochemical cell. Stronger electric avalanches resulted in the formation of wider pores. A similar behavior was also observed for the layers synthesized under various voltages in electrolytes with concentrations of 10 and 20 g l−1 for 3 min. It was noted that the voltage at which structural pores began to appear decreased with the electrolyte concentration. As the electrolyte electrical resistance, and, hence, total resistance of the electrochemical cell decreased with increasing the electrolyte concentration, the voltage which was applied on the anode surface increased and reached the breakdown voltage of the surface gas layer more suddenly. As a consequence, the electrical sparks appeared at lower applied voltages. Furthermore, it was observed that the pore size increased with the electrolyte concentration. Any decrease in circuit resistance resulted in increased
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Fig. 4. AFM surface topography of the TiO2 layers grown in the electrolytes with a concentration of 10 g l−1 under applied voltages of (a) 300 V, (b) 350 V, (c) 400 V, (d) 450 V, (e) 500 V, and (f) 550 V.
current which passed the cell, and the power of the electrical avalanches. As a consequence, larger pores were formed in thicker electrolytes. According to the SEM images presented in Fig. 1, it can be deduced that the layers grown under medium applied voltages had the highest pore density, smallest pore size, and, consequently, the highest surface area.
Fig. 4 shows the AFM surface topography of the layer synthesized with different applying voltages in electrolytes with a concentration of 10 g l−1 in a scale of 10 m × 10 m. Other AFM images are not presented here. The results depict a rough surface which is usual for MAO-grown layers. Using statistical analysis, it was found that the average surface roughness of the layers (ASR)
Fig. 5. XRD patterns of TiO2 layers grown under different conditions: (a) 550 V, (b) 500 V, (c) 450 V, (d) 400 V, (e) 350 V, (f) 300 V, and (g) 250 V.
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Fig. 6. Anatase mass fraction as a function of applied voltage for different electrolyte concentrations.
increased with the applied voltage because applying higher voltages increased the electrical current passing the electrochemical cell. Higher electrical current generated more heat in the oxide layer resulting in sequential local melting and solidifying of the growing layers in the surrounding electrolyte which made the layers porous. XRD patterns, depicted in Fig. 5, demonstrate that the synthesized layers consisted of anatase and the rutile phases. Since the anatase form of the TiO2 is known as the only photoactive phase among its other forms, the mass fraction of anatase in the synthesized layers determines the sample hydrophilicity performance. Anatase mass fraction of the layers was calculated using the formula WA = (1 + 1.265IR /IA )−1 [43] where IR and IA are the normalized XRD peak intensities of rutile and anatase phases, respectively. The results are shown in Fig. 6. The results revealed that the anatase mass fraction reached a maximum value at a certain applied voltage (Vm ) and then decreased at very high voltages. Applying higher voltages warms the anode up more due to higher electrical current passing the electrochemical circuit and more electrical sparks taking place on the vicinity of the anode [34]. Because of this extra heat, the anatase which is a meta-stable phase transforms to rutile stable phase at higher temperatures. To further confirm the stoichiometry of the synthesized TiO2 layers grown in the electrolytes with a concentration of 10 g l−1 under different applied voltages, XPS technique was employed. All of the binding energies were referenced to the C(1 s) peak at 285.0 eV. Fig. 7 depicts the Ti(2p3/2 ) core level binding energy at 458.7 eV. Hence, existence of titanium in the form of Ti+4 state is confirmed. The O(1 s) peak, shown in Fig. 8, can be resolved into various components using original software with Gaussian rule. All O(1 s) peaks are wide and asymmetric demonstrating that there are different kinds of O-binding states in the layers. The peak A, located at 530.1 eV, is assigned to the crystal lattice oxygen (Ti–O), while the peak B, located at 531.6 eV, represents the oxygen in hydroxyl groups (O–H) formed by adsorbed O2 /OH on the surface. Oxide free surfaces contacting with the atmosphere are always hydrated, i.e. contain water molecules and hydroxyl groups. There are two types of OH-groups on the surface: single M–OH and double OH–M–OH. It was observed that the proportion of this peak increased with applied voltage. The peaks C at the binding energy of 532.7 eV reveal the existence of O− species on the surface. Finally, the peak D represents oxygen in water molecules. Since the layers are porous and are grown in aqueous solutions, water may trap inside the pores. The proportions of the surface area under the peaks OA to that of the peaks Ti(2p3/2 ) were calculated and approximately obtained as 2.09, 1.98, and 1.83 for the applied voltage of 200, 350, and 550 V, respectively. The reason for decreasing the O/Ti value with increasing the voltage is that higher applied voltages warms the anode up.
Fig. 7. XPS Ti(2p3/2 ) core level binding energy of the layers grown in the electrolytes with a concentration of 10 g l−1 under different voltages.
It has been suggested [44] that titania begins to lose its oxygen at higher temperatures. Moreover, applying such voltages increases the growth rate resulting in formation of surface defects namely oxygen vacancies. It was observed that the amount of –OH-groups, present on the surface of the films, increased with the applied voltage. This phenomenon asserts the formation of more surface oxygen vacancies under higher applied voltage, because hydroxyl groups occupy the sites corresponding to such vacancies. Results of the hydrophilicity tests are presented in Fig. 9 where the measured contact angles are plotted as a function of the applied voltage for different electrolytes concentrations. It was deduced that water contact angle decreased with the applied voltages and reached its minimum value at medium applied voltages, and, then, increased at higher voltages. Two reasons were proposed for such a behavior. First, anatase/rutile relative content, affecting the hydrophilicity of the layers, reached its maximum value at medium voltages, as elucidated in Fig. 6. The mechanism of photo-induced hydrophilicity of TiO2 layers has been previously studied [45–47]. It was suggested that preferential adsorption of water molecules on the photo-generated surface defective sites led to the formation
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Fig. 8. XPS O(1 s) core level binding energy of the layers grown in the electrolytes with a concentration of 10 g l−1 under different voltages.
of highly hydrophilic TiO2 films [16,48]. Photo-generated electrons and holes could either recombine or move to the surface to react with species adsorbed on the surface. Some of the electrons react with lattice metal ions Ti4+ to form Ti3+ defective sites [49]. The formation processes of defective sites on TiO2 surface was expressed [16] as follows: TiO2 + h → e− + h+
(1)
O2− + 2 h+ → 1/2O2 + oxygen vacancy
(2)
Ti4+ + e− → Ti3+
(3)
In air, the surface trapped electrons tend to react immediately with O2 adsorbed on the surface to form O2 − or O2 2− ions. Meanwhile, water molecules may coordinate into the oxygen vacancy sites, which lead to dissociative adsorption of the water molecules on the surface [50,51]. This process gives rise to the increase of hydroxyl content on the illuminated TiO2 surface. Based on this mechanism, electrons and holes are two prerequisites of hydrophilicity reactions. Since, they are mostly generated by anatase phase, the layers which have the highest anatase/rutile ratios exhibit the maximum hydrophilicity. Secondly, as explained before, the layers grown under medium applied voltages had higher surface areas where the hydrophilicity reactions take place. The more the surface area the higher the hydrophilicity. Although the layers grown under the applied voltage of 250 V had the highest anatase relative content (see Fig. 6), they did not reveal a high hydrophilicity due to their low effective surface area. Results show that the MAO-synthesized titania layers exhibit a high hydrophilicity due to their rough surface. Wenzel has proposed a theoretical model to describe the contact angle () of a rough surface [52].
He modified Young’s equation as follows: cos() =
r(SV − SL ) = r cos(˛) LV
(4)
where r is defined as the surface ratio, SV , SL , and LV are the interfacial free energy per unit area of solid–vapor, solid–liquid, and liquid–vapor interfaces, respectively, and ˛ is the contact angle of a smooth area. This equation indicates that the surface roughness enhances the hydrophilicity of a hydrophilic surface ( < 90◦ ) and the hydrophobicity of a hydrophobic one ( > 90◦ ) because r is always greater than 1. 4. Conclusions Micro-arc oxidation was used to grow titania layers with a porous and rough surface which are suitable for hydrophilicity applications. The layers consisted of anatase and rutile phases whose fraction was observed to vary with the applied voltage and electrolyte concentration. It was found that the samples fabricated under medium applied voltages had highest anatase/rutile relative content. In addition, pore size as well as surface roughness of the layers increased with voltage and electrolyte concentration. It was also found that the layers prepared under the applied voltage of 400 V in the electrolytes with a concentration of 10 g l−1 from sodium three phosphate salt exhibited maximum hydrophilicity. Acknowledgement The authors would like to thank the personnel in the advanced ceramic synthesis laboratory of Iran University of Science and Technology for their technical assistance. References
Fig. 9. Water contact angle as a function of the applied voltage for different electrolyte concentrations.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Investigation on hydrophilicity of micro-arc oxidized TiO2 nano/micro-porous layers M.R. Bayati a,∗ , Roya Molaei a , Amir Kajbafvala b,∗∗ , Saeid Zanganeh c , H.R. Zargar d , K. Janghorban e a
School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box 16845-161, Tehran, Iran Department of Materials Science and Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695-7907, USA School of Engineering, University of Connecticut, 261 Glenbrook Rd., Storrs, CT, USA d Department of Materials Engineering, University of British Columbia, Vancouver, Canada e Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran b c
a r t i c l e
i n f o
Article history: Received 18 February 2010 Received in revised form 3 May 2010 Accepted 4 May 2010 Available online 11 May 2010 Keywords: Titanium oxide Micro-arc oxidation Porous materials Hydrophilic
a b s t r a c t Titania porous layers with a rough surface were synthesized via micro-arc oxidation (MAO) and the effect of the applied voltage and electrolyte concentration on surface structure, and chemical composition of the layers was studied. Morphological and topographical investigations, performed by SEM and AFM, revealed that pore size and surface roughness of the layers increased with the applied voltage and the electrolyte concentration. Based on the XRD and XPS results, the grown layers consisted of anatase and rutile phases with varying fractions depending on growth conditions. It was found that anatase/rutile relative content reached its maximum value at medium applied voltages or electrolyte concentrations. Finally, hydrophilicity of the grown layers was determined using a water contact angle apparatus, and a correlation between measured contact angles and MAO-parameters was suggested. It was observed that the layers synthesized under the applied voltage of 400 V in the electrolytes with a concentration of 10 g l−1 exhibited the highest hydrophilicity. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Titanium dioxide (TiO2 ) has been widely studied due to its great variety of applications by many researchers. Photocatalysis [1,2], solar cells [3,4], gas sensors [5], lithium batteries [6], antibacterial activity [7,8], and self-cleaning and antifogging surfaces [9–11] are some of its applications. Compared with its powder form, TiO2 films can be easily separated, recovered, and efficiently recycled [12]. In 1997, Wang et al. [13] reported that UV-irradiation of titania surfaces produces a super-hydrophilic surface. So far, hydrophilic titanium dioxide layers have attracted many theoretical and practical applications such as self-cleaning and antifogging mirrors. However, the hydrophilicity of the TiO2 layers needs to be further enhanced for practical applications [14]. One way to improve its hydrophilicity is by fabricating porous layers. Since photo-chemical reactions mainly occur on the surface, the surface properties of TiO2 such as surface area, defects, surface acidity, surface functional groups, particle size, and crystalline phase will greatly affect photo-chemical activity and related mechanisms [12,15]. Yu and Wark studied the effect of specific surface area on photocat-
∗ Corresponding author. Tel.: +98 21 77920727. ∗∗ Corresponding author. Tel.: +1 919 515 7217. E-mail addresses: [email protected] (M.R. Bayati), [email protected] (A. Kajbafvala). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.05.021
alytic performance and hydrophilicity of titania and demonstrated the more the specific surface area the better the photo-induced hydrophilicity [16,17]. Titanium dioxide with varying morphologies has been synthesized by different methods including sol–gel [18,19], chemical vapor deposition [20,21], physical vapor deposition [22,23], hydrothermal process [24,25], electrochemical methods [26,27], liquid phase deposition [28], and spray pyrolysis [29,30]. TiO2 can also be obtained via micro-arc oxidation (MAO) process [31,32]. MAO is an electrochemical technique for formation of anodic films by spark/arc micro-discharges which move rapidly on the vicinity of the anode surface [33–36]. It is characterized by high productivity, economic efficiency, ecological friendliness, high hardness, good wear resistance, and excellent bonding strength with the substrate [37–39]. This process is carried out at voltages higher than the breakdown voltage of the gas layer enshrouding the anode. Since the substrate is connected to positive pole of the rectifier as anode, the gas layer consists of oxygen. When the dielectric gas layer completely covers the anode surface, electrical resistance of the electrochemical circuit surges and the process continues providing that the applied voltage defeats the breakdown voltage of the gas layer. Applying such voltages leads to formation of electrical discharges via which electrical current could pass the gas layer. MAO process is characterized by these electrical sparks [34,40] which are responsible for formation of the structural pores [41,42]. Due to strong electrical field (106 –108 V m−1 ) between anode and
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Fig. 1. SEM top-view of the TiO2 layers grown in the electrolytes with a concentration of 5 g l−1 under applied voltages of (a) 350 V, (b) 400 V, (c) 450 V, (d) 500 V, and (e) 550 V.
Fig. 2. SEM top-view of the TiO2 layers grown in the electrolytes with a concentration of 10 g l−1 under applied voltages of: (a) 300 V, (b) 350 V, (c) 400 V, (d) 450 V, (e) 500 V, and (f) 550 V.
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Fig. 3. SEM top-view of the TiO2 layers grown in the electrolytes with a concentration of 20 g l−1 under applied voltages of: (a) 250 V, (b) 300 V, (c) 350 V, (d) 400 V, (e) 450 V, (f) 500 V, and (g) 550 V.
cathode, electrolyte anions are drawn into the structural pores where they can attend electrochemical reactions. Structural pores are formed by electron avalanches taking places on the vicinity of the anode. Characteristics of electrolyte have a great influence on the film formation kinetics. Phosphates, sulfates, silicates and borates are four conventional kinds of electrolytes employed in previous researches, and the formed TiO2 films usually contain the element of the electrolytes (P, S, Si, B, etc.) [34,36,37,40]. In this research, results of growth, characterization, and especially hydrophilicity performance of the TiO2 layers fabricated via MAO process were discussed. A correlation between MAOparameters and hydrophilicity of the layers was proposed. To the best of our knowledge, this is the first study on the hydrophilicity of the MAO-grown titania layers. 2. Experimental A DC-rectifier with a maximum output of 600 V/30 A was employed as current source. 3 cm × 3 cm × 0.5 mm commercially pure grade 2 titanium pieces, surrounded by an ASTM 316 stainless steel cylindrical container (cathode), was also used as substrate (anode). Three sodium phosphate (Na3 PO4 ·12H2 O, Merck) solutions with different concentrations were used as electrolyte. The electrolyte temperature was fixed at 70 ± 3 ◦ C employing a water circulating system. Prior to MAO treatment, substrates underwent a cleaning process including mechanical polishing followed by washing in distilled water. Afterward, the titanium plates were chemically etched in diluted HF solution (HF:H2 O = 1:20 vol.%) at room temperature for 30 s, and then washed in distilled water again. In the last stage of cleaning procedure, the substrates were ultrasonically cleaned in acetone for 15 min and finally washed by distilled water. After surface cleaning, MAO treatment was accomplished under different voltages in a range of 250–550 V with +50 V intervals. Meanwhile, the deposition time for each run was considered as 3 min. Surface morphology and topography of the layers were examined by scanning electron microscope (TESCAN, Vega II) and atomic force microscope (Veeco auto probe) in contact mode with a 10 nm
radius silicon tip. X-ray diffraction (Philips, PW3710) and X-ray photoelectron spectroscopy (VG Microtech, Twin anode, XR3E2 Xray source, using AlK␣ = 1486.6 eV) techniques were used to study phase structure and chemical composition of the synthesized layers. The photo-induced hydrophilicity of the layers was evaluated by screening photos and measuring the contact angle of DI-water droplets. The grown layers were first UV-irradiated by a 25 W lamp ( = 365 nm) for 1 h; then, the hydrophilicity photos were obtained by a water contact angle apparatus. The hydrophilicity studies were performed in the atmosphere. 3. Results and discussion SEM morphologies of the layers synthesized at different applied voltages for 3 min in electrolytes with concentrations of 5, 10, and 20 g l−1 are shown in Figs. 1–3. No pore was observed in the structure of the layers grown at applied voltages less than 350 V in the electrolytes with a concentration of 5 g l−1 , but the samples which were fabricated under higher applied voltages were porous, and the pore size increased with the applied voltage. It should be mentioned that no electrical sparking occurred at the applied voltages less than 350 V, and increasing the applied voltage resulted in generation of stronger and long-living sparks. It is to emphasize that applying higher voltages caused electrical sparks with higher energy due to higher electrical current passing through the electrochemical cell. Stronger electric avalanches resulted in the formation of wider pores. A similar behavior was also observed for the layers synthesized under various voltages in electrolytes with concentrations of 10 and 20 g l−1 for 3 min. It was noted that the voltage at which structural pores began to appear decreased with the electrolyte concentration. As the electrolyte electrical resistance, and, hence, total resistance of the electrochemical cell decreased with increasing the electrolyte concentration, the voltage which was applied on the anode surface increased and reached the breakdown voltage of the surface gas layer more suddenly. As a consequence, the electrical sparks appeared at lower applied voltages. Furthermore, it was observed that the pore size increased with the electrolyte concentration. Any decrease in circuit resistance resulted in increased
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Fig. 4. AFM surface topography of the TiO2 layers grown in the electrolytes with a concentration of 10 g l−1 under applied voltages of (a) 300 V, (b) 350 V, (c) 400 V, (d) 450 V, (e) 500 V, and (f) 550 V.
current which passed the cell, and the power of the electrical avalanches. As a consequence, larger pores were formed in thicker electrolytes. According to the SEM images presented in Fig. 1, it can be deduced that the layers grown under medium applied voltages had the highest pore density, smallest pore size, and, consequently, the highest surface area.
Fig. 4 shows the AFM surface topography of the layer synthesized with different applying voltages in electrolytes with a concentration of 10 g l−1 in a scale of 10 m × 10 m. Other AFM images are not presented here. The results depict a rough surface which is usual for MAO-grown layers. Using statistical analysis, it was found that the average surface roughness of the layers (ASR)
Fig. 5. XRD patterns of TiO2 layers grown under different conditions: (a) 550 V, (b) 500 V, (c) 450 V, (d) 400 V, (e) 350 V, (f) 300 V, and (g) 250 V.
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Fig. 6. Anatase mass fraction as a function of applied voltage for different electrolyte concentrations.
increased with the applied voltage because applying higher voltages increased the electrical current passing the electrochemical cell. Higher electrical current generated more heat in the oxide layer resulting in sequential local melting and solidifying of the growing layers in the surrounding electrolyte which made the layers porous. XRD patterns, depicted in Fig. 5, demonstrate that the synthesized layers consisted of anatase and the rutile phases. Since the anatase form of the TiO2 is known as the only photoactive phase among its other forms, the mass fraction of anatase in the synthesized layers determines the sample hydrophilicity performance. Anatase mass fraction of the layers was calculated using the formula WA = (1 + 1.265IR /IA )−1 [43] where IR and IA are the normalized XRD peak intensities of rutile and anatase phases, respectively. The results are shown in Fig. 6. The results revealed that the anatase mass fraction reached a maximum value at a certain applied voltage (Vm ) and then decreased at very high voltages. Applying higher voltages warms the anode up more due to higher electrical current passing the electrochemical circuit and more electrical sparks taking place on the vicinity of the anode [34]. Because of this extra heat, the anatase which is a meta-stable phase transforms to rutile stable phase at higher temperatures. To further confirm the stoichiometry of the synthesized TiO2 layers grown in the electrolytes with a concentration of 10 g l−1 under different applied voltages, XPS technique was employed. All of the binding energies were referenced to the C(1 s) peak at 285.0 eV. Fig. 7 depicts the Ti(2p3/2 ) core level binding energy at 458.7 eV. Hence, existence of titanium in the form of Ti+4 state is confirmed. The O(1 s) peak, shown in Fig. 8, can be resolved into various components using original software with Gaussian rule. All O(1 s) peaks are wide and asymmetric demonstrating that there are different kinds of O-binding states in the layers. The peak A, located at 530.1 eV, is assigned to the crystal lattice oxygen (Ti–O), while the peak B, located at 531.6 eV, represents the oxygen in hydroxyl groups (O–H) formed by adsorbed O2 /OH on the surface. Oxide free surfaces contacting with the atmosphere are always hydrated, i.e. contain water molecules and hydroxyl groups. There are two types of OH-groups on the surface: single M–OH and double OH–M–OH. It was observed that the proportion of this peak increased with applied voltage. The peaks C at the binding energy of 532.7 eV reveal the existence of O− species on the surface. Finally, the peak D represents oxygen in water molecules. Since the layers are porous and are grown in aqueous solutions, water may trap inside the pores. The proportions of the surface area under the peaks OA to that of the peaks Ti(2p3/2 ) were calculated and approximately obtained as 2.09, 1.98, and 1.83 for the applied voltage of 200, 350, and 550 V, respectively. The reason for decreasing the O/Ti value with increasing the voltage is that higher applied voltages warms the anode up.
Fig. 7. XPS Ti(2p3/2 ) core level binding energy of the layers grown in the electrolytes with a concentration of 10 g l−1 under different voltages.
It has been suggested [44] that titania begins to lose its oxygen at higher temperatures. Moreover, applying such voltages increases the growth rate resulting in formation of surface defects namely oxygen vacancies. It was observed that the amount of –OH-groups, present on the surface of the films, increased with the applied voltage. This phenomenon asserts the formation of more surface oxygen vacancies under higher applied voltage, because hydroxyl groups occupy the sites corresponding to such vacancies. Results of the hydrophilicity tests are presented in Fig. 9 where the measured contact angles are plotted as a function of the applied voltage for different electrolytes concentrations. It was deduced that water contact angle decreased with the applied voltages and reached its minimum value at medium applied voltages, and, then, increased at higher voltages. Two reasons were proposed for such a behavior. First, anatase/rutile relative content, affecting the hydrophilicity of the layers, reached its maximum value at medium voltages, as elucidated in Fig. 6. The mechanism of photo-induced hydrophilicity of TiO2 layers has been previously studied [45–47]. It was suggested that preferential adsorption of water molecules on the photo-generated surface defective sites led to the formation
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Fig. 8. XPS O(1 s) core level binding energy of the layers grown in the electrolytes with a concentration of 10 g l−1 under different voltages.
of highly hydrophilic TiO2 films [16,48]. Photo-generated electrons and holes could either recombine or move to the surface to react with species adsorbed on the surface. Some of the electrons react with lattice metal ions Ti4+ to form Ti3+ defective sites [49]. The formation processes of defective sites on TiO2 surface was expressed [16] as follows: TiO2 + h → e− + h+
(1)
O2− + 2 h+ → 1/2O2 + oxygen vacancy
(2)
Ti4+ + e− → Ti3+
(3)
In air, the surface trapped electrons tend to react immediately with O2 adsorbed on the surface to form O2 − or O2 2− ions. Meanwhile, water molecules may coordinate into the oxygen vacancy sites, which lead to dissociative adsorption of the water molecules on the surface [50,51]. This process gives rise to the increase of hydroxyl content on the illuminated TiO2 surface. Based on this mechanism, electrons and holes are two prerequisites of hydrophilicity reactions. Since, they are mostly generated by anatase phase, the layers which have the highest anatase/rutile ratios exhibit the maximum hydrophilicity. Secondly, as explained before, the layers grown under medium applied voltages had higher surface areas where the hydrophilicity reactions take place. The more the surface area the higher the hydrophilicity. Although the layers grown under the applied voltage of 250 V had the highest anatase relative content (see Fig. 6), they did not reveal a high hydrophilicity due to their low effective surface area. Results show that the MAO-synthesized titania layers exhibit a high hydrophilicity due to their rough surface. Wenzel has proposed a theoretical model to describe the contact angle () of a rough surface [52].
He modified Young’s equation as follows: cos() =
r(SV − SL ) = r cos(˛) LV
(4)
where r is defined as the surface ratio, SV , SL , and LV are the interfacial free energy per unit area of solid–vapor, solid–liquid, and liquid–vapor interfaces, respectively, and ˛ is the contact angle of a smooth area. This equation indicates that the surface roughness enhances the hydrophilicity of a hydrophilic surface ( < 90◦ ) and the hydrophobicity of a hydrophobic one ( > 90◦ ) because r is always greater than 1. 4. Conclusions Micro-arc oxidation was used to grow titania layers with a porous and rough surface which are suitable for hydrophilicity applications. The layers consisted of anatase and rutile phases whose fraction was observed to vary with the applied voltage and electrolyte concentration. It was found that the samples fabricated under medium applied voltages had highest anatase/rutile relative content. In addition, pore size as well as surface roughness of the layers increased with voltage and electrolyte concentration. It was also found that the layers prepared under the applied voltage of 400 V in the electrolytes with a concentration of 10 g l−1 from sodium three phosphate salt exhibited maximum hydrophilicity. Acknowledgement The authors would like to thank the personnel in the advanced ceramic synthesis laboratory of Iran University of Science and Technology for their technical assistance. References
Fig. 9. Water contact angle as a function of the applied voltage for different electrolyte concentrations.
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