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Home Search Collections Journals About Contact us My IOPscience Preparation and characterization of titanium dioxide nanotube array supported hydrated ruthenium oxide catalysts This content has been downloaded from IOPscience Please scroll down to see the full text 2012 Adv Nat Sci: Nanosci Nanotechnol 015008 (http://iopscience.iop.org/2043-6262/3/1/015008) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 80.82.77.83 This content was downloaded on 11/04/2017 at 10:26 Please note that terms and conditions apply You may also be interested in: Structure and dye-sensitized solar cell application of TiO2 nanotube arrays fabricated by the anodic oxidation method Seon-Yeong Ok, Kwon-Koo Cho, Ki-Won Kim et al The large diameter and fast growth of self-organized TiO2 nanotube arrays achieved viaelectrochemical anodization H Yin, H Liu and W Z Shen Fast-rate formation of TiO2 nanotube arrays in an organic bath and their applications 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dioxide nanotube array supported hydrated ruthenium oxide catalysts Thi Phuong Ly Giang1 , Thi Nhu Mai Tran2 and Xuan Tuan Le3 University Paris-Sud, UMR-CNRS 8612, Laboratory of Proteins and Nanotechnologies in Separation Sciences, 92296, Faculté de Pharmacie, Châtenay-Malabry, France Faculty of Chemistry, Hanoi University of Science, Vietnam National University in Hanoi, 19 Le Thanh Tong Street, Hanoi, Vietnam MiQro Innovation Collaborative Centre (C2MI), 45, boul de l’Aéroport, Bromont (Québec), Canada E-mail: xuan.tuan.le@ulb.ac.be and maitrannhu@gmail.com Received 27 July 2011 Accepted for publication 26 September 2011 Published March 2012 Online at stacks.iop.org/ANSN/3/015008 Abstract This work aimed at preparing and characterizing TiO2 nanotube supported hydrated ruthenium oxide catalysts First of all, we succeeded in preparing TiO2 nanotube arrays by electrochemical anodization of titanium metal at 20 V for h in a 1M H3 PO4 + 0.5 wt% HF solution as evidenced from scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) results The hydrated ruthenium oxide was then deposited onto TiO2 nanotubes by consecutive exchange of protons by Ru3+ ions, followed by formation of hydrated oxide during the alkali treatment Further XPS measurements showed that the modified samples contain not only hydrated ruthenium oxide but also hydrated ruthenium species Ru(III)-OH Keywords: TiO2 nanotube, anodization, hydrated ruthenium, catalytic oxidation Classification numbers: 2.03, 4.00, 5.06 a catalytic reaction The semiconducting properties of such new materials may result in strong electronic interaction between the support and a catalyst, which could improve catalytic performance in redox reactions [15] As a result, studies on supporting ruthenium-based compounds on TiO2 nanotubes are of potential interest [15–17] In this sense, this work focuses on preparation of titanium dioxide nanotube array supported hydrated ruthenium oxide catalysts As a cost-effective and high rate method, the use of electrochemistry to prepare TiO2 nanotube arrays is described in the first section of the present paper Part of the work is then devoted to the loading of hydrated ruthenium oxides onto the electrochemically anodized TiO2 nanotubes It is remarkable to note that as a promising new catalyst for selective oxidation of many alcohols in aqueous media, hydrated ruthenium oxides have frequently been used in wastewater treatments [15] However, the role of Introduction Incorporating metal-based species onto titanium dioxide surface is one of the well-known methods to improve catalytic activity of the resulting modified TiO2 materials [1–6] Among various metals such as copper, nickel, tin, gold, platinum, palladium the TiO2 supported Ru catalysts have been proven to play an indispensable role with respect to wastewater treatment and energy storage applications [7–10] During the past ten years, TiO2 nanotubes have been widely investigated due to their practical applications in areas such as biomaterials, solar cell, rechargeable lithium batteries, gas sensor, and catalysts in particular [1, 10–14] Indeed, the large cation exchange capacity of TiO2 nanotubes allows a high loading of an active catalyst with even distribution and high dispersion The open mesoporous morphology of the nanotubes, absence of micropores, and high specific surface area should facilitate transport of reagents during 2043-6262/12/015008+05$33.00 © 2012 Vietnam Academy of Science & Technology Content from this work may be used under the terms of the Creative Commons Attribution-NonCommercial ShareAlike 3.0 licence Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI Adv Nat Sci.: Nanosci Nanotechnol (2012) 015008 T P L Giang et al dissolved in 23 ml of water with addition of ml of 0.5 M HCl Then TiO2 nanotube array samples were immersed in this solution for 60 at 25 ◦ C After washing with a large amount of DI water, the samples were put in a beaker containing 1.0 M NaOH (Sigma Aldrich) After h, the samples were taken out of the solution, rinsed by DI water, and then dried at 80 ◦ C under vacuum condition for h 2.3 Microscopy study The SEM images were recorded by a Hitachi S4800 equipped with a field emission gun (FEG-SEM) 2.4 XPS Figure Two-electrode electrochemical cell for preparation of TiO2 nanotube arrays XPS measurements were carried out with a Theta 300 (Thermo Scientific Instrument) equipped with a microfocusing monochromator x-ray source The data were collected at room temperature, and the operating pressure in the analysis chamber was always below 10−9 Torr The core level spectra were referenced to the pollution C s binding energy at 284.9 eV Data treatment and peak-fitting procedures were performed using Avantage software ruthenium supported catalysts towards methanol oxidation in fuel applications is still an open topic for discussion [18, 19] While some authors refer to the active ruthenium compound mainly as metallic Ru0 in a bimetallic alloy, early research revealed that hydrated ruthenium oxide as a part of bimetallic Pt–Ru systems is the most active catalyst for methanol oxidation [18] It is thus interesting to investigate the oxidation state of the Ru components deposited on our TiO2 nanotube arrays In this work, the formation of Ru-based species on the surface of TiO2 nanotubes will finally be discussed as clearly as possible on the basis of x-ray photoelectron spectroscopy (XPS) results Results and discussions 3.1 Electrochemical preparation of T i O nanotube arrays When a potential of 20 V is applied through the two-electrode configuration described in the experimental section, first of all, TiO2 is electrochemically formed (Ti + H2 O → TiO2 + 4e− ) Dissolution of titania then takes place thanks to the presence of fluoride ions in the solution and leads to the formation of soluble hexafluorotitanium complexes (TiO2 + 6F− + 4H+ → TiF2− + 2H2 O) With the help of electrical field, TiO2 nanotubes finally formed as a function of the time [20, 21] Figure presents the SEM images of the resulting layers at different scales The zoom-out SEM image (figure 2B) clearly shows the self-organized nanotubes as expected It is also observed that the nanotube diameter is of approximately 100 nm (figure 2C) Such an obtained result is in a good agreement with the work of Bauer et al [22], where the TiO2 nanotube diameter was reported to be linearly dependent on the applied voltages and a diameter of about 100 nm was obtained with an applied potential of 20 V On the other hand, XPS measurements allow us to confirm that the self-organized nanotubes are titanium dioxide As can be seen in figure the survey spectrum of the sample is dominated by signals of titanium and oxygen as expected Besides Ti and O peaks, we equally observe the presence of unavoidable contaminated carbon peak This peak will be further discussed in the next section of this work It is important to point out that the Ti 2p core level spectrum (figure 4(a)) shows the typical characteristics of titanium in TiO2 with the 2p3/2 and 2p1/2 peaks centred at 458.8 and 464.3 eV, respectively [23] XPS analysis also revealed that fluoride ions are strongly absorbed on TiO2 surface, indicating the migration of F− ions is driven by the electrical field (figure 4(b)) In fact, under the influence of the electrical field, fluoride ions can even penetrate into the bottom of the nanotube as reported Experimental 2.1 Anodization of titanium metal For the electrochemical anodization, a typical two-electrode configuration (figure 1) was employed with platinum foil as the counter electrode and titanium foil as the working electrode Thickness of titanium foils (99.6% purity) was 0.5 mm Effective area of the O-ring on the working electrode in contact with electrolyte solution (as shown in figure 1) was 1.0 cm2 Prior to any electrochemical treatment the foils were sonicated in acetone, isopropanol and methanol successively, followed by rinsing with deionized (DI) water and drying in a nitrogen stream All anodization experiments were realized at room temperature in a M H3 PO4 (Merck) + 0.5 wt% HF (Sigma-Aldrich) solution A potential of 20 V was applied through the system for h After each anodization, the obtained sample was rinsed by DI water and dried in a nitrogen stream The as-anodized TiO2 nanotubes were then recrystallized by heating at 400 ◦ C for 10 h under nitrogen atmosphere The obtained samples were characterized by means of scanning electron microscopy (SEM) and XPS techniques 2.2 Deposition of hydrated ruthenium oxides onto T i O nanotubes The hydrated ruthenium oxide was deposited on TiO2 nanotubes by consecutive exchange of protons by Ru3+ ions, followed by formation of hydrated oxide during the alkali treatment 200 mg of RuCl3 ·3H2 O (Sigma Aldrich) was Adv Nat Sci.: Nanosci Nanotechnol (2012) 015008 T P L Giang et al O 1s Ti 2p O KLL Ti LMM F 1s F KLL C 1s 40 kCPS Ti 3p 1000 800 600 400 200 Binding Energy /eV Figure XPS survey spectrum of anodized TiO2 nanotube arrays a) Ti 2p 2p3/2 kCPS 2p1/2 468 466 464 462 460 458 456 Binding energy /eV Figure SEM images of TiO2 nanotubes formed at 20 V for h in 1M H3 PO4 + 0.5 wt% HF at different scales b) F1s elsewhere [24] Furthermore, it should be kept in mind that fluoride anions are involved in the dissolution process of TiO2 as mentioned above Here, the most important point to be underlined is that simple anodization of titanium metal led to the formation of TiO2 film which consists of individual tubes with a diameter of ≈100 nm as evidenced from the XPS and SEM results 500 CPS 3.2 Titanium dioxide nanotube array supported hydrated ruthenium oxide catalysts 690 688 686 684 682 680 Binding energy /eV Figure shows the XPS survey spectrum of TiO2 nanotube arrays supported Ru As is seen here, the XPS survey spectrum of TiO2 nanotubes modified with Ru-based species looks very similar to that of pristine TiO2 nanotubes We not observe clearly the presence of ruthenium on the spectrum This however can be easily understood by noting the fact that the positions of Ru 3p are found to be very close to those of Ti 2p and also the Ru 3d3/2 peak appears superposed to the C s line In order to bing out the difference between the two samples, we wish next to concentrate on the C s Figure Ti 2p and F s high-resolution spectra of TiO2 nanotube arrays and C s + Ru d high-resolution spectra of the pristine and modified samples Before modification with ruthenium, the C s core level can be fitted by three components located at 284.9, 286.4 and 288.8 eV respectively (figure 6(a)) After modification, a typical behaviour of C s + Ru d mixed spectrum as already reported in many published works [9, 25–27] is Adv Nat Sci.: Nanosci Nanotechnol (2012) 015008 T P L Giang et al one, Ru2 (3d3/2 ) and Ru2 (3d5/2 ), locate at 282.1 and 286.3 eV Note that a separation distance of 4.2 eV between 3d3/2 and 5d5/2 peaks found for both pairs in this work is very close to the expected value of 4.1 eV [26] One can deduce that there are two components of Ru-species on the surface of the modified TiO2 nanotube Nevertheless, discussion on the nature of the two components is quite complicated Mazzieri et al [25] reported that by using RuCl3 as precursor for catalyst preparation, ruthenium oxychloride species characterized by 3d3/2 peak at 280.9 eV are present on the sample surface In our case, it is however worth mentioning that we not observe any significant amount of chloride on the spectrum This allows us to exclude the presence of the chloride compounds (ruthenium oxychloride and ruthenium chloride) in our catalysts Actually, the Ru component standing for a 3d3/2 peak at 281.4 eV can be assigned to ruthenium in RuO2 [26] or in RuO2 xH2 O [28] This peak is slightly higher than our first Ru component (Ru1 (3d3/2 ) found at 281.0 in comparison with contaminated C peak of 284.9 eV) In a separative work published by Bavykin et al [15], it was reported that Ru(III)-hydrated oxide could be obtained on the TiO2 surface through the same preparation process used in the present work On account of those facts, it is believed that the first Ru component appeared at low binding energies (281.0 and 285.2 eV) in our spectrum should be attributed to the hydrated ruthenium oxides With the aim of clarifying the nature of the second component with Ru2 (3d3/2 ) and Ru2 (3d5/2 ) binding energies of 282.1 and 286.3 eV, it is important to note that the peaks are not at all linked to RuCl3 as mentioned above In this case, the peaks can be attributed to the Ru (III) from hydrous Ru (III) – OH incorporated on the lattice of TiO2 nanotubes through the ion exchange reactions between the Ru3+ cations in the solution and protons in the TiO2 nanotube framework Nanotubular ‘titanium dioxide’ is indeed a protonated form of a layered titanic acid The exact crystal structure of the nanotubes is a matter of current dispute; it probably corresponds either to the layered titanate H2 Ti3 O7 which has a monoclinic structure with stepwise layers of three lengths in each step, or to H2 Ti2 O4 (OH)2 in which the unit cell has an orthorhombic symmetry The nanotube walls have a multilayered structure in which protons occupy positions on either side of the wall surface (convex and concave), as well as in the interstitial cavities between the layers of the nanotube walls Therefore, protons and cations from aqueous solutions (H+ , Men+ ) could easily be exchanged for protons in the nanotube wall, according to the following equation [15]: O 1s Ti 2p + Ru 3p F 1s C 1s + Ru 3d kCPS 1000 800 600 400 200 Binding energy /eV Figure XPS survey spectrum of TiO2 nanotube arrays supported Ru CPS /a.u a) C 1s C3 C2 C1 290 288 286 284 282 280 Binding energy /eV b) C1s + Ru 3d C3 CPS /a.u C2 C1 Ru1(3d5/2) Ru1(3d3/2) Ru2(3d5/2) Ru2(3d3/2) 290 288 286 284 282 280 Binding energy /eV xMen+ + H2 Ti3 O7 → Mex H2−x Ti3 O7x(n−1)+ + xH+ Figure (a) Decomposed C s core level spectra of pristine TiO2 nanotubes (b) Decomposed C s and Ru d core level spectra of TiO2 supported Ru nanotubes The obtained XPS data indicate that the resulting Ru/TiO2 nanotube arrays contain both hydrated ruthenium oxide and hydrated ruthenium species Ru(III)-OH depicted in figure 6(b) As expected, in addition to the C peaks which are quasi-identical to those of the pristine sample, the Ru d peaks appear in the spectrum In particular, the Ru d core level spectrum is characterized by pairs of relatively narrow peaks which correspond to the 5/2 and 3/2 spin–orbits (the red and black lines presented in figure 6(b)) The first pair of d peak, Ru1 (3d3/2 ) and Ru1 (3d5/2 ), are found at 281.0 and 285.2 eV, respectively while the second Conclusion A one-step electrochemical method has been used to prepare TiO2 layers that consist of arrays of individual tubes with a diameter of ≈100 nm Thanks to the ion exchange reaction between the proton of the protonated Adv Nat Sci.: Nanosci Nanotechnol (2012) 015008 T P L Giang et al form of a layered titanic acid and Ru3+ cation in the bulk solution, hydrated ruthenium species Ru(III)-OH can be easily incorporated on TiO2 nanotube surface Part of such a ruthenium species was subsequently converted to hydrated ruthenium oxide by simple alkali and thermal treatments Aside from Ru(III)-OH species, XPS allowed us to evidence the presence of hydrated ruthenium oxide on the surface of supported Ru/TiO2 nanotube catalysts As mentioned in the introduction, supported Ru/TiO2 catalysts have been proven efficient in the selective oxidation of several organic alcohols Testing the catalytic activity of the obtained Ru/TiO2 nanotube catalysts is obviously the subject of our further works [11] Song Y-Y, Schmidt-Stein F, Bauer S and Schmuki P 2009 J Am Chem Soc 131 4230 [12] Mor G K, Shankar K, Paulose M, Varghese O K and Grimes C A 2006 Nano 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