- Email: [email protected]
Efficient photocatalysis on hierarchically structured TiO2 nanotubes with mesoporous TiO2 filling
Electrochemistry Communications 42 (2014) 21–25
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Efficient photocatalysis on hierarchically structured TiO2 nanotubes with mesoporous TiO2 filling Vesna Müller 1, Patrik Schmuki ⁎ Department of Material Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstraße 7, D-91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 7 January 2014 Received in revised form 22 January 2014 Accepted 23 January 2014 Available online 8 February 2014 Keywords: TiO2 nanotubes Mesoporous TiO2 Photocatalytic degradation
a b s t r a c t In the present manuscript we demonstrate an efficient procedure for the preparation of a hierarchically structured 4.5 μm long TiO2 nanotube layer filled with Pluronic-templated mesoporous TiO2. The photocatalytic activity of this hierarchically structured TiO2 was compared to plain nanotubes and to classic particle-decorated TiO2 nanotubes through UV-light induced decomposition of a dye, acid orange 7. The hierarchically structured TiO2 nanotubes clearly show higher constant decomposition rate in comparison to the pure TiO2 nanotubes or mesoporous layers. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Since 1972 when Fujishima and Honda [1] reported on light-induced water splitting on TiO2 surface, the TiO2 started to be widely explored in photocatalytic reactions [2,3]. The key for efficient photocatalytic activity of the TiO2 comes from its semiconducting nature with suitable band-edge positions that allow formation of highly oxidative species from aqueous media [4,5] that for example are used to degrade organic waste or pollutants. The mechanism of photocatalytic activity is mainly ascribed to photogeneration of electron–hole pairs under illumination of TiO2 with UV light, where the holes are strong oxidizing agents. The holes in essence can undergo two reactions: 1. oxidize organic compound directly or, 2. react with water and form hydroxyl radical (•OH) which acts as secondary oxidizing agent for organic compounds [6,7]. Among other factors, in order to obtain a high photocatalytic conversion rate it is crucial to have a high surface area of the catalyst [8]. In this light, many efforts have been made in order to prepare TiO2 with a high surface area. Many attempts were based on using commercially available TiO2 nanoparticles (such as Degussa P25) which can be either dispersed in the suspension or deposited on the substrate forming a thin film [4,9]. Suspensions of nanoparticles have a slight drawback, as they require a filtration step for the removal of the catalyst. In comparison to compacted particle films, mesoporous films [10–13], nanowires
⁎ Corresponding author. Tel.: +49 9131852 7575; fax: +49 9131 852 7582. E-mail address: [email protected] (P. Schmuki). 1 Present address: Chair for Process Systems Engineering, Wissenschaftszentrum Weihenstephan für Ernährung Landnutzung und Umwelt, Technische Universität München, Gregor-Mendel-Straße 4, 85354 Freising. 1388-2481/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2014.01.018
[14], or particularly nanotubular films [9,15,16] were reported to have higher efficiencies [6,17–20]. Although TiO2 in the form of mesoporous films has the highest surface area, the preparation of thick films is tedious, i.e. it requires several multicoating procedures with annealing steps in between, and a maximum practical limit exists with TiO2 films at 2.7 μm [21]. Present work demonstrates a fast and efficient route for the preparation of high surface area, thick films of TiO2 by using TiO2 nanotubes with wide opening as a host material for incorporation of mesoporous TiO2. Such hybrid material with a high surface area offers a large interface highly needed in photocatalytic reactions.
2. Experimental section TiCl4 (purum ≥ 98%), Pluronic P123, Pluronic F127, ethanol (anhydrous), ethylene glycol (anhydrous, 99.8%), ammonium fluoride (puriss, p.a. ≥ 98%), acid orange 7 were purchased from Aldrich and were used without further purification. Ti foil (99.6%) of 0.1 mm in thickness was purchased from Advent and used for anodic growth of TiO2 nanotubes. For the preparation of TiO2 nanotubes the Ti foil was pretreated by ultrasonication in acetone, isopropanol and methanol for 5 min in each solvent, afterwards rinsed with water and dried in a nitrogen stream. The TiO2 nanotubes were grown by anodization of the Ti foil in 87 vol.% ethylene glycol-13 vol.% water-0.08 M NH4F electrolyte at 120 V for 2 h. The electrochemical anodization was performed with a two electrode arrangement with a platinum foil as a counter electrode and Ti foil as a working electrode using a high voltage potentiostat. The as-prepared nanotubes are amorphous and were used for filling with titania sol–gel solution. The filling was performed by dipping the
22
V. Müller, P. Schmuki / Electrochemistry Communications 42 (2014) 21–25
foil with TiO2 nanotubes into sol–gel solution and keeping it immersed for 15 min. After this the foils were slowly taken out, dried in the air and transferred into furnace for annealing. The annealing was carried out in the air at 450 °C for 3 h. The titania sol–gel solution, which was used for filling of the nanotubes, was prepared by adding TiCl4 (1.4 ml) to ethanol (10 ml) under continuous stirring. For the preparation of the sol–gel solution “meso1”, the Pluronic P123 (0.4 g) was added into the above described titania sol–gel solution. In the case of the sol–gel solution “meso-2” the Pluronic F127 (0.4 g) was added to the titania sol–gel solution. Both solutions were vigorously stirred for 3 h prior to use. For reference also a classic TiCl4 treatment that decorates the tubes with nanoparticles [22] was investigated. For this, a 0.2 M aqueous solution of TiCl4 was prepared under vigorous stirring in ice-cooled conditions. Previously annealed TiO2 nanotubes were treated with 0.2 M TiCl4 solution in a closed vessel at 70 °C for 30 min. Afterwards, the TiCl4 treated nanotubes were washed subsequently with distilled water and ethanol, and dried in a nitrogen stream. The TiCl4 treated nanotubes were additionally annealed at 450 °C for 30 min. The morphology of the samples was investigated using a high resolution field emission scanning electron microscope (FE-SEM) Hitachi S4800 operated at 10 kV. The crystalline structure of the samples was investigated using an X-ray diffractometer X'pert Philips MPD PW 3040 instrument with Cu Kα radiation. The photocatalytic experiments were carried out by periodically measuring photodegradation of the dye acid orange 7 during the UV illumination of the TiO2 samples which were immersed in the dye
solution. For this, a 2.5 μM solution of acid orange 7 was prepared. The TiO2 samples were immersed in acid orange 7 solution which was placed in a quartz cuvette. The solution in the cuvette was stirred at 300 rpm during the experiment. Prior to the UV illumination the solution was kept for 1.5 h in the dark. Afterwards the samples were irradiated with a laser with a wavelength of 325 nm and intensity of 60 mW cm− 2 (He–Cd, Kimmon, Japan). The absorbance of the dye solution was periodically measured at λ = 486 nm by using a UV–vis spectrometer (Libra S2, Biochrome) in order to determine the decomposition rate of the AO7 dye.
3. Results and discussion Fig. 1 shows the morphology of pure mesoporous films (meso-1 and meso-2) and pure TiO2 nanotubes. It can be seen that the meso-1 film has a more compact structure in comparison to the meso-2 film which has larger pores (Fig. 1a, b). This behavior was expected since the template which was used for the preparation of meso-1 films has lower molecular weight and therefore forms smaller micelles which subsequently during annealing procedure form porous structure [23]. Both films are compact and crack-free. The thicknesses of the films were estimated from the SEM measurements (Fig. 1c, d) and were found to be 350 nm for meso-1 and 450 nm for meso-2, respectively. For preparation of thicker mesoporous films, a time consuming repeated deposition and annealing procedure is needed. Moreover, in 2008 Procházka et al. [21] showed that such multicoating procedure leads to the deterioration of the bottom mesoporous layers of the film.
Fig. 1. SEM images of mesoporous TiO2 films and TiO2 nanotubes. Images (a) and (b) present top views of meso-1 and meso-2 films, (c) and (d) present the cross sections of the mesoporous films, (e) and (f) present top view and cross section SEM images of pure TiO2 nanotubes.
V. Müller, P. Schmuki / Electrochemistry Communications 42 (2014) 21–25
In order to eliminate the multicoating procedure for the preparation of mesoporous films with micrometer thickness, TiO2 nanotubes were used as a scaffold. For this purpose, TiO2 nanotubes which possess a wide opening of 270 nm and the length of approximately 4.5 μm were chosen as a host matrix for the preparation of such hierarchically structured material (Fig. 1e, f). The hierarchically structured TiO2 nanotubes and the reference samples (pure TiO2 nanotubes and pure mesoporous films) were all annealed through the same annealing program at 450 °C in the air. Fig. 2a–f shows SEM images of the hierarchically structured TiO2 samples. The top view of both types of hierarchically structured TiO2 (Fig. 2a, b, c, d) nanotubes shows complete coverage and filling of the nanotubes with mesoporous material with a presence of some cracks which are a result of the annealing step. The cross section (Fig. 2e) and cross cut (Fig. 2f) of the nanotubes indicate a presence of the mesoporous material down to the bottom of the nanotubes. In
23
order to explain the hierarchical structure more clearly, the mesoporous material is marked with white arrows, in the Fig. 2e and f while, the walls of the nanotubes are marked with black arrows. XRD measurements (Fig. 2g) show the samples to be completely crystallized to anatase. The additional lines which can be visible in regions 35–40 2θ° and around 55 2θ° correspond to the Ti foil substrate. These peaks are marked additionally with gray lines in the graph. Photocatalytic activity of the pure mesoporous TiO2 films, pure TiO2 nanotubes and hierarchically structured TiO2 nanotubes was followed through degradation of AO7 as a function of irradiation time with UV light (Fig. 3a, b). The data are plotted according to the classic pseudofirst rate approach ln(C / C0) = k · τ, where k is the rate constant and τ is time. From the linear shape of the data the k values can be extracted (Fig. 3c). From the obtained k values it can be seen that both hierarchically structured TiO2 nanotubes possess higher decomposition rate than
Fig. 2. (a–f) SEM images of hierarchically structured TiO2 nanotubes and (g) XRD patterns of the pure mesoporous TiO2 films (meso-1 and meso-2), pure TiO2 nanotubes and hierarchically structured TiO2 nanotubes. (a–d) show top view SEM images of hierarchically structured nanotubes. (c) cross section and (d) cross cut of the hierarchically structured nanotubes, respectively. The gray lines in the XRD graph present diffraction lines of the Ti foil substrate.
24
V. Müller, P. Schmuki / Electrochemistry Communications 42 (2014) 21–25
Fig. 3. Photocatalytic degradation of AO7 (C0 = 2.5 × 10−5 M in aqueous solution) under UV light (λ = 325 nm). a: photocatalytic degradation performed with pure TiO2 nanotubes, meso-1 film, meso-2 film, and hierarchically structured nanotubes. b: photocatalytic degradation of TiCl4 decorated nanotubes (TiCl4 decorated NT), once annealed (1-Ann. Pure NT) and double annealed pure TiO2 nanotubes (2-Ann. Pure NT), double annealed hierarchically structured nanotubes (2-Ann. hierarchically structured NT-1 and 2-Ann. hierarchically structured NT-2). c: k values of the samples obtained from the graphs a and b. d: SEM top view image of TiCl4 decorated nanotubes. With black arrows are marked walls of the nanotubes whereas, with white arrows is marked mesoporous filling.
pure TiO2 nanotubes, mesoporous films, or classic TiCl4-treated nanotube layers. This behavior can be correlated with the fact that hierarchically structured TiO2 nanotubes-1 have much higher contact area with the electrolyte and consequently with the dye than pure TiO2 nanotubes. This is in line with the hierarchically structured TiO2 nanotubes which contain smaller pore sizes that consequently have a larger surface area and therefore a higher decomposition rate. As a possible way to increase the surface area of the nanotubes, in 2009 Roy et al. showed that nanotubes decorated with TiO2 nanoparticles have an increased efficiency in dye sensitized solar cell applications [22]. In order to compare our results with such system we accordingly prepared TiO2 nanotubes and decorated them with TiO2 nanoparticles, Fig. 3d. This type of preparation procedure requires two annealing steps, this in contrast to the present approach where typically only one annealing step is required. We have also additionally performed a second annealing step for the hierarchically structured nanotubes and compared their behavior with the nanoparticle-decorate nanotubes. From the plot of the degradation of the AO7 with the time for these samples (Fig. 3b), the nanoparticle-decorated nanotubes show higher decomposition rate than our pure one-step annealed nanotubes. However, the second annealing step applied on pure nanotubes leads to an increase in their photocatalytic activity but the k value is still lower than for the hierarchically structured nanotubes. The photodegradation rate of the AO7, with double annealed hierarchically structured nanotubes, shows in contrast to the pure TiO2 nanotubes a decrease in the decomposition rate. This behavior results from the double annealing procedure applied on mesoporous systems which leads to pore shrinkage and consequently to a decrease in the surface area; this finding is in good agreement with the work by Procházka et al. [21]. In summary, the results presented highlight the advantage of using mesoporous TiO2 filling in TiO2 nanotubes in comparison to
pure mesoporous films or classic TiCl4-treatment of TiO2 nanotubes for the photocatalytic degradation of AO7. Acknowledgment The authors acknowledge Ulrike Martens-Jahns for XRD measurements and Anja Friedrich for SEM images. DFG is acknowledged for financial support. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] F. Kiriakidou, D.I. Kondarides, X.E. Verykios, The effect of operational parameters and TiO2-doping on the photocatalytic degradation of azo-dyes, Catal. Today 54 (1999) 119–130. [3] F. Zhang, J. Zhao, T. Shen, H. Hidaka, E. Pelizzetti, N. Serpone, TiO2-assisted photodegradation of dye pollutants II. Adsorption and degradation kinetics of eosin in TiO2 dispersions under visible light irradiation, Appl. Catal. B Environ. 15 (1998) 147–156. [4] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. [5] U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (2003) 53–229. [6] N.K. Shrestha, M. Yang, Y.-C. Nah, I. Paramasivam, P. Schmuki, Self-organized TiO2 nanotubes: visible light activation by Ni oxide nanoparticle decoration, Electrochem. Commun. 12 (2010) 254–257. [7] J.M. Macak, M. Zlamal, J. Krysa, P. Schmuki, Self-organized TiO2 nanotube layers as highly efficient photocatalysts, Small 3 (2007) 300–304. [8] K. Lee, D. Kim, P. Roy, I. Paramasivam, B.I. Birajdar, E. Spiecker, P. Schmuki, Anodic formation of thick anatase TiO2 mesosponge layers for high-efficiency photocatalysis, J. Am. Chem. Soc. 132 (2010) 1478–1479. [9] M. Zlamal, J.M. Macak, P. Schmuki, J. Krýsa, Electrochemically assisted photocatalysis on self-organized TiO2 nanotubes, Electrochem. Commun. 9 (2007) 2822–2826. [10] L.V.-E. Juan, C. Ya-Dong, C.W.W. Kevin, Y. Yusuke, Recent progress in mesoporous titania materials: adjusting morphology for innovative applications, Sci. Technol. Adv. Mater. 13 (2012) 013003.
V. Müller, P. Schmuki / Electrochemistry Communications 42 (2014) 21–25 [11] Y. Bian, X. Wang, Z. Zeng, Z. Hu, Preparation of ordered mesoporous TiO2 thin film and its application in methanol catalytic combustion, Surf. Interface Anal. 45 (2013) 1317–1322. [12] J.B. Joo, M. Dahl, N. Li, F. Zaera, Y. Yin, Tailored synthesis of mesoporous TiO2 hollow nanostructures for catalytic applications, Energy Environ. Sci. 6 (2013) 2082–2092. [13] L. Kuang, Y. Zhao, L. Liu, Photodegradation of Orange II by mesoporous TiO2, J. Environ. Monit. 13 (2011) 2496–2501. [14] G.-R. Xu, J.-N. Wang, C.-J. Li, Template directed preparation of TiO2 nanomaterials with tunable morphologies and their photocatalytic activity research, Appl. Surf. Sci. 279 (2013) 103–108. [15] P. Roy, S. Berger, P. Schmuki, TiO2 nanotubes: synthesis and applications, Angew. Chem. Int. Ed. 50 (2011) 2904–2939. [16] D. Kowalski, D. Kim, P. Schmuki, TiO2 nanotubes, nanochannels and mesosponge: self-organized formation and applications, Nano Today 8 (2013) 235–264. [17] J. Papp, H.S. Shen, R. Kershaw, K. Dwight, A. Wold, Titanium(IV) oxide photocatalysts with palladium, Chem. Mater. 5 (1993) 284–288.
25
[18] C.M. Wang, A. Heller, H. Gerischer, Palladium catalysis of O2 reduction by electrons accumulated on TiO2 particles during photoassisted oxidation of organic compounds, J. Am. Chem. Soc. 114 (1992) 5230–5234. [19] I. Paramasivam, J.M. Macak, P. Schmuki, Photocatalytic activity of TiO2 nanotube layers loaded with Ag and Au nanoparticles, Electrochem. Commun. 10 (2008) 71–75. [20] I. Paramasivam, Y.-C. Nah, C. Das, N.K. Shrestha, P. Schmuki, WO3/TiO2 nanotubes with strongly enhanced photocatalytic activity, Chem. Eur. J. 16 (2010) 8993–8997. [21] J. Procházka, L. Kavan, V. Shklover, M.T. Zukalová, O. Frank, M. Kalbáč, A.T. Zukal, H. Pelouchová, P. Janda, K. Mocek, M. Klementová, D. Carbone, Multilayer films from templated TiO2 and structural changes during their thermal treatment, Chem. Mater. 20 (2008) 2985–2993. [22] P. Roy, D. Kim, I. Paramasivam, P. Schmuki, Improved efficiency of TiO2 nanotubes in dye sensitized solar cells by decoration with TiO2 nanoparticles, Electrochem. Commun. 11 (2009) 1001–1004. [23] C. Sanchez, C. Boissière, D. Grosso, C. Laberty, L. Nicole, Design, synthesis, and properties of inorganic and hybrid thin films having periodically organized nanoporosity, Chem. Mater. 20 (2008) 682–737.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Efficient photocatalysis on hierarchically structured TiO2 nanotubes with mesoporous TiO2 filling Vesna Müller 1, Patrik Schmuki ⁎ Department of Material Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstraße 7, D-91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 7 January 2014 Received in revised form 22 January 2014 Accepted 23 January 2014 Available online 8 February 2014 Keywords: TiO2 nanotubes Mesoporous TiO2 Photocatalytic degradation
a b s t r a c t In the present manuscript we demonstrate an efficient procedure for the preparation of a hierarchically structured 4.5 μm long TiO2 nanotube layer filled with Pluronic-templated mesoporous TiO2. The photocatalytic activity of this hierarchically structured TiO2 was compared to plain nanotubes and to classic particle-decorated TiO2 nanotubes through UV-light induced decomposition of a dye, acid orange 7. The hierarchically structured TiO2 nanotubes clearly show higher constant decomposition rate in comparison to the pure TiO2 nanotubes or mesoporous layers. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Since 1972 when Fujishima and Honda [1] reported on light-induced water splitting on TiO2 surface, the TiO2 started to be widely explored in photocatalytic reactions [2,3]. The key for efficient photocatalytic activity of the TiO2 comes from its semiconducting nature with suitable band-edge positions that allow formation of highly oxidative species from aqueous media [4,5] that for example are used to degrade organic waste or pollutants. The mechanism of photocatalytic activity is mainly ascribed to photogeneration of electron–hole pairs under illumination of TiO2 with UV light, where the holes are strong oxidizing agents. The holes in essence can undergo two reactions: 1. oxidize organic compound directly or, 2. react with water and form hydroxyl radical (•OH) which acts as secondary oxidizing agent for organic compounds [6,7]. Among other factors, in order to obtain a high photocatalytic conversion rate it is crucial to have a high surface area of the catalyst [8]. In this light, many efforts have been made in order to prepare TiO2 with a high surface area. Many attempts were based on using commercially available TiO2 nanoparticles (such as Degussa P25) which can be either dispersed in the suspension or deposited on the substrate forming a thin film [4,9]. Suspensions of nanoparticles have a slight drawback, as they require a filtration step for the removal of the catalyst. In comparison to compacted particle films, mesoporous films [10–13], nanowires
⁎ Corresponding author. Tel.: +49 9131852 7575; fax: +49 9131 852 7582. E-mail address: [email protected] (P. Schmuki). 1 Present address: Chair for Process Systems Engineering, Wissenschaftszentrum Weihenstephan für Ernährung Landnutzung und Umwelt, Technische Universität München, Gregor-Mendel-Straße 4, 85354 Freising. 1388-2481/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2014.01.018
[14], or particularly nanotubular films [9,15,16] were reported to have higher efficiencies [6,17–20]. Although TiO2 in the form of mesoporous films has the highest surface area, the preparation of thick films is tedious, i.e. it requires several multicoating procedures with annealing steps in between, and a maximum practical limit exists with TiO2 films at 2.7 μm [21]. Present work demonstrates a fast and efficient route for the preparation of high surface area, thick films of TiO2 by using TiO2 nanotubes with wide opening as a host material for incorporation of mesoporous TiO2. Such hybrid material with a high surface area offers a large interface highly needed in photocatalytic reactions.
2. Experimental section TiCl4 (purum ≥ 98%), Pluronic P123, Pluronic F127, ethanol (anhydrous), ethylene glycol (anhydrous, 99.8%), ammonium fluoride (puriss, p.a. ≥ 98%), acid orange 7 were purchased from Aldrich and were used without further purification. Ti foil (99.6%) of 0.1 mm in thickness was purchased from Advent and used for anodic growth of TiO2 nanotubes. For the preparation of TiO2 nanotubes the Ti foil was pretreated by ultrasonication in acetone, isopropanol and methanol for 5 min in each solvent, afterwards rinsed with water and dried in a nitrogen stream. The TiO2 nanotubes were grown by anodization of the Ti foil in 87 vol.% ethylene glycol-13 vol.% water-0.08 M NH4F electrolyte at 120 V for 2 h. The electrochemical anodization was performed with a two electrode arrangement with a platinum foil as a counter electrode and Ti foil as a working electrode using a high voltage potentiostat. The as-prepared nanotubes are amorphous and were used for filling with titania sol–gel solution. The filling was performed by dipping the
22
V. Müller, P. Schmuki / Electrochemistry Communications 42 (2014) 21–25
foil with TiO2 nanotubes into sol–gel solution and keeping it immersed for 15 min. After this the foils were slowly taken out, dried in the air and transferred into furnace for annealing. The annealing was carried out in the air at 450 °C for 3 h. The titania sol–gel solution, which was used for filling of the nanotubes, was prepared by adding TiCl4 (1.4 ml) to ethanol (10 ml) under continuous stirring. For the preparation of the sol–gel solution “meso1”, the Pluronic P123 (0.4 g) was added into the above described titania sol–gel solution. In the case of the sol–gel solution “meso-2” the Pluronic F127 (0.4 g) was added to the titania sol–gel solution. Both solutions were vigorously stirred for 3 h prior to use. For reference also a classic TiCl4 treatment that decorates the tubes with nanoparticles [22] was investigated. For this, a 0.2 M aqueous solution of TiCl4 was prepared under vigorous stirring in ice-cooled conditions. Previously annealed TiO2 nanotubes were treated with 0.2 M TiCl4 solution in a closed vessel at 70 °C for 30 min. Afterwards, the TiCl4 treated nanotubes were washed subsequently with distilled water and ethanol, and dried in a nitrogen stream. The TiCl4 treated nanotubes were additionally annealed at 450 °C for 30 min. The morphology of the samples was investigated using a high resolution field emission scanning electron microscope (FE-SEM) Hitachi S4800 operated at 10 kV. The crystalline structure of the samples was investigated using an X-ray diffractometer X'pert Philips MPD PW 3040 instrument with Cu Kα radiation. The photocatalytic experiments were carried out by periodically measuring photodegradation of the dye acid orange 7 during the UV illumination of the TiO2 samples which were immersed in the dye
solution. For this, a 2.5 μM solution of acid orange 7 was prepared. The TiO2 samples were immersed in acid orange 7 solution which was placed in a quartz cuvette. The solution in the cuvette was stirred at 300 rpm during the experiment. Prior to the UV illumination the solution was kept for 1.5 h in the dark. Afterwards the samples were irradiated with a laser with a wavelength of 325 nm and intensity of 60 mW cm− 2 (He–Cd, Kimmon, Japan). The absorbance of the dye solution was periodically measured at λ = 486 nm by using a UV–vis spectrometer (Libra S2, Biochrome) in order to determine the decomposition rate of the AO7 dye.
3. Results and discussion Fig. 1 shows the morphology of pure mesoporous films (meso-1 and meso-2) and pure TiO2 nanotubes. It can be seen that the meso-1 film has a more compact structure in comparison to the meso-2 film which has larger pores (Fig. 1a, b). This behavior was expected since the template which was used for the preparation of meso-1 films has lower molecular weight and therefore forms smaller micelles which subsequently during annealing procedure form porous structure [23]. Both films are compact and crack-free. The thicknesses of the films were estimated from the SEM measurements (Fig. 1c, d) and were found to be 350 nm for meso-1 and 450 nm for meso-2, respectively. For preparation of thicker mesoporous films, a time consuming repeated deposition and annealing procedure is needed. Moreover, in 2008 Procházka et al. [21] showed that such multicoating procedure leads to the deterioration of the bottom mesoporous layers of the film.
Fig. 1. SEM images of mesoporous TiO2 films and TiO2 nanotubes. Images (a) and (b) present top views of meso-1 and meso-2 films, (c) and (d) present the cross sections of the mesoporous films, (e) and (f) present top view and cross section SEM images of pure TiO2 nanotubes.
V. Müller, P. Schmuki / Electrochemistry Communications 42 (2014) 21–25
In order to eliminate the multicoating procedure for the preparation of mesoporous films with micrometer thickness, TiO2 nanotubes were used as a scaffold. For this purpose, TiO2 nanotubes which possess a wide opening of 270 nm and the length of approximately 4.5 μm were chosen as a host matrix for the preparation of such hierarchically structured material (Fig. 1e, f). The hierarchically structured TiO2 nanotubes and the reference samples (pure TiO2 nanotubes and pure mesoporous films) were all annealed through the same annealing program at 450 °C in the air. Fig. 2a–f shows SEM images of the hierarchically structured TiO2 samples. The top view of both types of hierarchically structured TiO2 (Fig. 2a, b, c, d) nanotubes shows complete coverage and filling of the nanotubes with mesoporous material with a presence of some cracks which are a result of the annealing step. The cross section (Fig. 2e) and cross cut (Fig. 2f) of the nanotubes indicate a presence of the mesoporous material down to the bottom of the nanotubes. In
23
order to explain the hierarchical structure more clearly, the mesoporous material is marked with white arrows, in the Fig. 2e and f while, the walls of the nanotubes are marked with black arrows. XRD measurements (Fig. 2g) show the samples to be completely crystallized to anatase. The additional lines which can be visible in regions 35–40 2θ° and around 55 2θ° correspond to the Ti foil substrate. These peaks are marked additionally with gray lines in the graph. Photocatalytic activity of the pure mesoporous TiO2 films, pure TiO2 nanotubes and hierarchically structured TiO2 nanotubes was followed through degradation of AO7 as a function of irradiation time with UV light (Fig. 3a, b). The data are plotted according to the classic pseudofirst rate approach ln(C / C0) = k · τ, where k is the rate constant and τ is time. From the linear shape of the data the k values can be extracted (Fig. 3c). From the obtained k values it can be seen that both hierarchically structured TiO2 nanotubes possess higher decomposition rate than
Fig. 2. (a–f) SEM images of hierarchically structured TiO2 nanotubes and (g) XRD patterns of the pure mesoporous TiO2 films (meso-1 and meso-2), pure TiO2 nanotubes and hierarchically structured TiO2 nanotubes. (a–d) show top view SEM images of hierarchically structured nanotubes. (c) cross section and (d) cross cut of the hierarchically structured nanotubes, respectively. The gray lines in the XRD graph present diffraction lines of the Ti foil substrate.
24
V. Müller, P. Schmuki / Electrochemistry Communications 42 (2014) 21–25
Fig. 3. Photocatalytic degradation of AO7 (C0 = 2.5 × 10−5 M in aqueous solution) under UV light (λ = 325 nm). a: photocatalytic degradation performed with pure TiO2 nanotubes, meso-1 film, meso-2 film, and hierarchically structured nanotubes. b: photocatalytic degradation of TiCl4 decorated nanotubes (TiCl4 decorated NT), once annealed (1-Ann. Pure NT) and double annealed pure TiO2 nanotubes (2-Ann. Pure NT), double annealed hierarchically structured nanotubes (2-Ann. hierarchically structured NT-1 and 2-Ann. hierarchically structured NT-2). c: k values of the samples obtained from the graphs a and b. d: SEM top view image of TiCl4 decorated nanotubes. With black arrows are marked walls of the nanotubes whereas, with white arrows is marked mesoporous filling.
pure TiO2 nanotubes, mesoporous films, or classic TiCl4-treated nanotube layers. This behavior can be correlated with the fact that hierarchically structured TiO2 nanotubes-1 have much higher contact area with the electrolyte and consequently with the dye than pure TiO2 nanotubes. This is in line with the hierarchically structured TiO2 nanotubes which contain smaller pore sizes that consequently have a larger surface area and therefore a higher decomposition rate. As a possible way to increase the surface area of the nanotubes, in 2009 Roy et al. showed that nanotubes decorated with TiO2 nanoparticles have an increased efficiency in dye sensitized solar cell applications [22]. In order to compare our results with such system we accordingly prepared TiO2 nanotubes and decorated them with TiO2 nanoparticles, Fig. 3d. This type of preparation procedure requires two annealing steps, this in contrast to the present approach where typically only one annealing step is required. We have also additionally performed a second annealing step for the hierarchically structured nanotubes and compared their behavior with the nanoparticle-decorate nanotubes. From the plot of the degradation of the AO7 with the time for these samples (Fig. 3b), the nanoparticle-decorated nanotubes show higher decomposition rate than our pure one-step annealed nanotubes. However, the second annealing step applied on pure nanotubes leads to an increase in their photocatalytic activity but the k value is still lower than for the hierarchically structured nanotubes. The photodegradation rate of the AO7, with double annealed hierarchically structured nanotubes, shows in contrast to the pure TiO2 nanotubes a decrease in the decomposition rate. This behavior results from the double annealing procedure applied on mesoporous systems which leads to pore shrinkage and consequently to a decrease in the surface area; this finding is in good agreement with the work by Procházka et al. [21]. In summary, the results presented highlight the advantage of using mesoporous TiO2 filling in TiO2 nanotubes in comparison to
pure mesoporous films or classic TiCl4-treatment of TiO2 nanotubes for the photocatalytic degradation of AO7. Acknowledgment The authors acknowledge Ulrike Martens-Jahns for XRD measurements and Anja Friedrich for SEM images. DFG is acknowledged for financial support. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] F. Kiriakidou, D.I. Kondarides, X.E. Verykios, The effect of operational parameters and TiO2-doping on the photocatalytic degradation of azo-dyes, Catal. Today 54 (1999) 119–130. [3] F. Zhang, J. Zhao, T. Shen, H. Hidaka, E. Pelizzetti, N. Serpone, TiO2-assisted photodegradation of dye pollutants II. Adsorption and degradation kinetics of eosin in TiO2 dispersions under visible light irradiation, Appl. Catal. B Environ. 15 (1998) 147–156. [4] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. [5] U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (2003) 53–229. [6] N.K. Shrestha, M. Yang, Y.-C. Nah, I. Paramasivam, P. Schmuki, Self-organized TiO2 nanotubes: visible light activation by Ni oxide nanoparticle decoration, Electrochem. Commun. 12 (2010) 254–257. [7] J.M. Macak, M. Zlamal, J. Krysa, P. Schmuki, Self-organized TiO2 nanotube layers as highly efficient photocatalysts, Small 3 (2007) 300–304. [8] K. Lee, D. Kim, P. Roy, I. Paramasivam, B.I. Birajdar, E. Spiecker, P. Schmuki, Anodic formation of thick anatase TiO2 mesosponge layers for high-efficiency photocatalysis, J. Am. Chem. Soc. 132 (2010) 1478–1479. [9] M. Zlamal, J.M. Macak, P. Schmuki, J. Krýsa, Electrochemically assisted photocatalysis on self-organized TiO2 nanotubes, Electrochem. Commun. 9 (2007) 2822–2826. [10] L.V.-E. Juan, C. Ya-Dong, C.W.W. Kevin, Y. Yusuke, Recent progress in mesoporous titania materials: adjusting morphology for innovative applications, Sci. Technol. Adv. Mater. 13 (2012) 013003.
V. Müller, P. Schmuki / Electrochemistry Communications 42 (2014) 21–25 [11] Y. Bian, X. Wang, Z. Zeng, Z. Hu, Preparation of ordered mesoporous TiO2 thin film and its application in methanol catalytic combustion, Surf. Interface Anal. 45 (2013) 1317–1322. [12] J.B. Joo, M. Dahl, N. Li, F. Zaera, Y. Yin, Tailored synthesis of mesoporous TiO2 hollow nanostructures for catalytic applications, Energy Environ. Sci. 6 (2013) 2082–2092. [13] L. Kuang, Y. Zhao, L. Liu, Photodegradation of Orange II by mesoporous TiO2, J. Environ. Monit. 13 (2011) 2496–2501. [14] G.-R. Xu, J.-N. Wang, C.-J. Li, Template directed preparation of TiO2 nanomaterials with tunable morphologies and their photocatalytic activity research, Appl. Surf. Sci. 279 (2013) 103–108. [15] P. Roy, S. Berger, P. Schmuki, TiO2 nanotubes: synthesis and applications, Angew. Chem. Int. Ed. 50 (2011) 2904–2939. [16] D. Kowalski, D. Kim, P. Schmuki, TiO2 nanotubes, nanochannels and mesosponge: self-organized formation and applications, Nano Today 8 (2013) 235–264. [17] J. Papp, H.S. Shen, R. Kershaw, K. Dwight, A. Wold, Titanium(IV) oxide photocatalysts with palladium, Chem. Mater. 5 (1993) 284–288.
25
[18] C.M. Wang, A. Heller, H. Gerischer, Palladium catalysis of O2 reduction by electrons accumulated on TiO2 particles during photoassisted oxidation of organic compounds, J. Am. Chem. Soc. 114 (1992) 5230–5234. [19] I. Paramasivam, J.M. Macak, P. Schmuki, Photocatalytic activity of TiO2 nanotube layers loaded with Ag and Au nanoparticles, Electrochem. Commun. 10 (2008) 71–75. [20] I. Paramasivam, Y.-C. Nah, C. Das, N.K. Shrestha, P. Schmuki, WO3/TiO2 nanotubes with strongly enhanced photocatalytic activity, Chem. Eur. J. 16 (2010) 8993–8997. [21] J. Procházka, L. Kavan, V. Shklover, M.T. Zukalová, O. Frank, M. Kalbáč, A.T. Zukal, H. Pelouchová, P. Janda, K. Mocek, M. Klementová, D. Carbone, Multilayer films from templated TiO2 and structural changes during their thermal treatment, Chem. Mater. 20 (2008) 2985–2993. [22] P. Roy, D. Kim, I. Paramasivam, P. Schmuki, Improved efficiency of TiO2 nanotubes in dye sensitized solar cells by decoration with TiO2 nanoparticles, Electrochem. Commun. 11 (2009) 1001–1004. [23] C. Sanchez, C. Boissière, D. Grosso, C. Laberty, L. Nicole, Design, synthesis, and properties of inorganic and hybrid thin films having periodically organized nanoporosity, Chem. Mater. 20 (2008) 682–737.