Article pubs.acs.org/Langmuir
Cladding Layer on Well-Defined Double-Wall TiO2 Nanotubes Chaorui Xue, Tetsu Yonezawa,* Mai Thanh Nguyen, and Xu Lu Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan S Supporting Information *
ABSTRACT: Highly ordered double-wall TiO2 nanotube arrays were obtained by a two-step anodization method in a fluoride-containing glycerol based electrolyte. The low water and fluoride content and high viscosity of the electrolyte support a partly undissolved fluoride-rich layer, and its hydrolyzed products remain on the tube walls. The double-wall structure and a cladding layer originating from the fluoride-rich layer were clearly observed after annealing. The morphology and crystal structure of the cladding layer were investigated. The study of the cladding layer gives a fundamental insight into the wall structure design of the anodic TiO2 nanotubes in the glycerol-based electrolyte. process, reacting with oxidized Ti4+ and building up a fluoriderich layer (e.g., TiF4) between the metal−oxide interface of the anode Ti.19 Accompanying the outward growth (with respect to the anode surface) of the oxide cells, fluoride-rich layers are taken in with and exist at the boundary of the growing cells, which will then suffer chemical dissolution/etching to give the final individual tubes in self-aligned nanotube arrays.18 As a result, controlling the formation and dissolution of the fluoriderich layer and the growth of the oxide shell become important factors in modifying the intertube structure, tube wall morphology, long-range organized structure, and stability of the nanotube arrays.11−20 For example, high fluoride concentration accelerates the dissolution of the fluoride-rich layer and thins the tube wall by chemical etching of TiO2, which can eventually stop the growth of the nanotube array.21 The anodic TiO2 nanotube arrays can be obtained using aqueous or organic electrolytes such as ethylene glycol, glycerol, DMSO (dimethyl sulfoxide), or their solutions with water. While fluoride containing aqueous based electrolytes often gave nanotube arrays with side-wall inhomogeneity and short length, fluoride containing organic based electrolytes offered smoother nanotubes without tube wall inhomogeneity and with a great improvement in the nanotube aspect ratios, nanotube array thickness (micrometer scale), and ordering.22−26 Under fine control of anodizing conditions, well-defined single- and more often double-wall nanotube arrays were obtained in ethylene glycol based electrolytes. Additionally, wall thickness, tube diameters, tube wall morphology (smooth, rough, ripple, bamboo) were controlled, and multilayered nanotube arrays were fabricated, thanks to massive research conducted on ethylene glycol based electrolytes.24,25 Because double-wall nanotube arrays provide higher specific surface areas, which
1. INTRODUCTION TiO2 has become a major subject of tremendous research, owning to its high stability, nontoxicity, and important applications for photocatalysis, dye-sensitized solar cells, removal of pollutants, and sensing.1−3 In particular, onedimensional TiO2 nanotubes and their higher ordered structures, named nanotube arrays or thin films, are anticipated to be promising in these applications, due to their unique nanostructure, electronic properties, high surface area, and long-range ordering.3−5 Even though various synthesis methods have been studied to obtain TiO2 nanotubes and/or nanotube arrays,6,7 as simple anodic oxidation of metal Ti foil in the presence of fluoride ions resulted in oxide nanotube arrays,8 it has become a popular approach to fabricate self-aligned TiO2 nanostructures.5−10 Over the past ten years, a huge amount of attention has been given to this approach to elucidate the formation of self-organized structures, specifically the role of related synthesis factors and their applications.5,10 Important parameters were highlighted, including the nature of the electrolyte (aqueous, organics, or ionic liquid), content of fluoride ion, pH, applied potential, temperature, anodizing time, and annealing, to name a few;11−17 however, a detailed and complete picture has not been achieved. The formation of self-ordered anodic TiO2 nanopore/ nanotube arrays using fluoride-containing electrolytes was emphasized with the crucial role of the fluoride ion to form the water-soluble complex TiF62−, which helped maintain a certain degree of anodic oxide dissolution in the steady state equilibrium between metal oxide formation/growth and dissolution, leading to the continuous growth and selfalignment of TiO2 pore/nanotube arrays.3,18 However, without fluoride ions, oxide dissolution is prohibited, and a compact anodic TiO2 layer is formed. It was also demonstrated that, due to the higher mobility of the fluoride anion compared to that of the oxygen anion, fluoride ions were accumulating at the metal−oxide interface of the anode Ti during the anodization © 2015 American Chemical Society
Received: December 1, 2014 Revised: January 5, 2015 Published: January 10, 2015 1575
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Figure 1. Schematic illustration of the two-step anodization of double-wall TiO2 nanotube arrays. glycerol (C3H8O3, 97.0% purity), NH4F, and acetone were purchased from Wako Pure Chemical, Japan. Double-wall TiO2 nanotube arrays were prepared by a two-step anodization method. Prior to anodization, the Ti foil was cut into rectangular pieces (10 × 15 mm2), ultrasonicated in acetone and water, and dried under N2 stream. Anodization was carried out in a two-electrode cell with Pt as the counter electrode and the Ti foil as the working electrode, separated by a distance of 30 mm, as in our previous report.32 The ramp rate from open-circuit to final voltage was 0.1 V s−1. A schematic illustration of the two-step anodization process is shown in Figure 1. Briefly, Ti foil was first anodized at 60 V for 1 h. Electrolyte was prepared by mixing 56 g of ethylene glycol, 0.9 g of H2O (1.6 wt %) and 0.18 g of NH4F (3 wt %). After the first anodization, the as-prepared nanotube layer was removed by means of ultrasonication in purified water. Finally, a Ti foil with a mirror-like surface was obtained and used for the second anodization at 60 V for 1.5 h at room temperature in 60 g of glycerol-based electrolyte containing 3 wt % H2O and 0.5 wt % NH4F. Resulting samples were washed with purified water several times and then dried under ambient air. Thermal annealing of samples was carried out in the air using an electric muffle furnace (FT-101FM, Fultech, Japan) at different temperatures (300, 500, and 600 °C) for 2 h with a heating and cooling rate of 2 °C min−1. 2.2. Sample Characterization. The surface morphologies were characterized using a field emission scanning electron microscopy (JEOL JSM-6701F) with an acceleration voltage of 15 kV. Films were scraped with a steel blade for the observation of their side and bottom views. The scraped films were examined using a transmission electron microscope (JEOL JEM-2010, 200 kV). X-ray photoelectron spectroscopic (XPS) measurements were carried out on anodic TiO2 oxide nanotube array on a Ti substrate using a JPS-9200 (JEOL) equipped with a monochromatic MgKα source, operating at 100 W under ultrahigh vacuum (∼1.0 × 10−7 Pa). XPS depth profiles were obtained using Ar+ ions bombardment. X-ray diffraction patterns were collected on an X’pert Philips PMD diffractometer with a Panalytical X’celerator detector, using CuKα radiation at a scan rate (in 2θ) of 0.02° s−1. A UV−vis spectrophotometer (V-550DS, JASCO, Japan) with an ISV-469 integrating sphere attachment was used to record the UV−vis diffuse reflectance spectra (DRS). IR-RAS (Infrared Reflection-Absorption Spectroscopy) spectra were measured with a JASCO FT−IR 4200 with a reflection apparatus (RAS PR041-H) in the wavenumber range of 3600−600 cm−1, with a resolution of 4.0 cm−1.
positively contribute to the enhanced photocatalytic performance, the tube wall design of anodic TiO2 nanotubes (doubleor even multiple-wall nanotube arrays) is not only of structural engineering interest, but also of practical importance.26,27 However, different from the widely used ethylene glycol solution, studies on self-aligned anodic TiO2 nanotube arrays prepared using glycerol-based electrolytes are often reported for single-wall structures22−29 while the others did not clearly mention and/or characterize the nanotube structure in terms of single- or double-wall.13,17 It is probable, due to the fact that even at low fluoride concentrations in glycerol-based electrolytes, other often used synthesis conditions such as long anodization time (tens of hour) or a large water content (more than 5%) or even mild applied potentials (normally 20 V) may prefer to form single-wall structures. Recently, by anodizing in a glycerol-based solution, we prepared TiO2 nanotube arrays with double-wall structure, which revealed that the glycerol can be an alternative to the ethylene glycol for the tube wall design of anodic TiO2 nanotubes.30,31 In the case of glycerol-based solution, for the higher viscosity of glycerol than that of ethylene glycol, the diffusion and transport of anions and dissolution products could vary and influence the nanotube growth and tube wall geometry.29 Therefore, it can be expected that the fluoride-rich layer surrounding nanotubes may be chemically etched differently from that happen in water or ethylene glycol. Additionally, regarding to the fluoride-rich layers in double-wall TiO2 nanotube arrays, there is lack of deep investigation. So far, no direct evidence of fluoride-rich layers on the walls of double-wall TiO2 nanotubes prepared in glycerol-based electrolytes and their transformation upon annealing has yet been reported. In this paper, specific anodizing conditions (low water content, relatively high anodizing potential, and short time) in glycerol-based electrolytes allowed a fluoride-rich layer and its hydrolyzed products to survive as the cladding on double-wall TiO2 nanotube arrays. This remaining layer on the assynthesized anodic oxide nanotube array was annealed and clearly observed as a cladding layer on the tube wall, which suggests an important role for the fluoride-rich layer in the wall structure design of anodic TiO2 nanotubes. The well-defined inner wall of the nanotubes was obtained by annealing.
3. RESULTS AND DISCUSSION 3.1. Double-Wall TiO2 Nanotube Arrays with FluorideRich Layer on Tube Wall. First, the formation of the doublewall TiO2 nanotube array was confirmed using SEM, TEM, and TEM-EDS analysis, and the results are given in Figure 2. The nanotubes show highly uniform size and well aligned tubular
2. EXPERIMENTAL SECTION 2.1. Preparation of Double-Wall TiO2 Nanotube Arrays. Ti foil of 250 μm in thickness (99.6% purity) was obtained from Nilaco, Japan. Water was purified by an Organo/ELGA Purelabo-II system (resistivity of 18.2 MΩ). Ethylene glycol (C2H6O2, 99.5% purity), 1576
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A second important feature of the obtained double-wall nanotube array is that fluoride was found in both the bottom (point A) and the side wall (point B) for a single nanotube, areas (Figure 2d) with comparable values (Figure 2e). In Figure 3, SEM-EDS measurements were further performed for various
Figure 2. (a) SEM image of a part of the scraped nanotube array laying on the surface of the nanotube array, (b) the zoomed-in SEM image for the side wall and top view areas, (c) the cross-sectional TEM images of the nanotube, (d) the side view TEM image for the bottom area of a few nanotubes, and (e) the EDS spectra collected at spots A and B in (d). δouter and δinner in (c) are the thickness of outer wall (44.63 nm) and the inner wall (63.67 nm), respectively. Figure 3. (a) SEM images of the Ti substrate (after partial removal of the TiO2 nanotube array) and the TiO2 nanotube array. The EDS spectra of three blue spots (b) and three red spots (c) shown in (a).
structure, with outer diameters of 151.48 nm and lengths of 2.48 μm. In Figure 2a, a single nanotube can be captured. Even though the double-wall structure is not clearly seen near the tube mouth (Figure 2a,b), the cross-section and side view TEM images (Figure 2c,d) indicate the hollow and double-wall structure of the nanotubes, where the inner walls (thickness, 63.67 nm) are evidently enclosed in the outer wall (thickness, 44.63 nm) of the nanotube (Figure 2c). The EDS measurements were conducted on a nanotube for point A and B shown in Figure 2d. The EDS results indicate higher carbon contamination in the tube wall than that at the bottom, which is consistent with the results obtained for double-wall nanotube arrays synthesized using ethylene glycol and suggested a somehow similar nature of the inner tube wall.33 The XPS depth profile of Supporting Information (SI) Figure S1 further demonstrates an increasing carbon content with sputtering, of which the higher carbon content of inner wall could be a main reason. It was found that glycerol could be electrochemically decomposed to form glycolic acid, glyceric acid, and glycolaldehyde.34 In highly viscous glycerol-based electrolyte, under the electric field, these species can react with outward Ti4+ at the TiO2/electrolyte interface and/or deposit inside the outer wall and hardly release or transport back to the electrolyte, resulting in a highly carbon contaminated oxide (e.g., titanium carboxylates) as the inner wall of nanotube, which is similar to the formation of the inner wall in an ethylene glycol based electrolyte.35 The loose structure of the inner wall of nanotube can be seen in Figure 2c,d. It can also be expected that the chemical dissolution of both the inner and outer walls took place more significantly at the tube mouth, where the nanotube was formed at the start of the anodizing process and suffered severe top etching for longer immersion in the fluoride-containing electrolyte.35,36 As a result, the inner wall should be thinner toward the tube mouth, and not clearly observed from the top view (Figures 2a,b). The conical architecture of the tube wall was also recorded in other research using glycerol-based electrolytes, but, for densely single-wall nanotubes.27
areas on the metal substrate, where the nanotube was scraped away (B-1, B-2, and B-3), and on the anodic oxide nanotube array (R-1, R-2, and R-3). The EDS results given in Table 1 not Table 1. Summary of the Elemental Atomic Composition from EDS Measurements at Different Positions Shown in Figure 3aa atomic concentration in % position TiO2 nanotube arrays
Ti substrate
a
R-1 R-2 R-3 B-1 B-2 B-3
C
O
F
Ti
4.71 4.64 3.99 0 0 0
55.61 55.89 56.50 13.67 10.96 9.52
13.20 12.83 13.58 4.57 2.99 4.19
26.48 26.64 25.93 81.76 86.50 86.29
CKα, OKα, FKα and TiKα were used to calculate the concentration.
only confirm that the fluoride is left in the metal substrate, but also that it clearly exists in the nanotube array area, which is consistent with the TEM-EDS results (Figure 2e). Therefore, fluoride was found in both the tube wall and the tube bottom, which proved the migration of fluoride through the nanotube bottoms during anodization. In the general proposal for the formation of tubular structures in fluoride-containing electrolytes, the “fluoride-rich” layers surrounding the nanotubes undergo chemical dissolution by making water-soluble complexes with fluoride ions, creating the gaps among the individual nanotubes.12 Therefore, many reports have shown the fluoride-rich layer left in the metal substrate (anode) or at the bottom of nanotube arrays synthesized using ethylene glycol based electrolytes.5,35 Additionally, in our case, the XPS depth profiles through the bottom of the nanotubes (SI Figure S1) were collected to clarify the existence of the fluoride-rich layer and/or its incomplete dissolution of materials covering the tube wall. The fluoride atomic concentration continuously 1577
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titanium (from metal substrate) can be found. Crystallization of anatase TiO2 occurred after annealing at 300 °C and was improved after annealing at 500 °C, with higher intensity and sharper peaks for the anatase phase. After annealing at 600 °C, the anatase main phase and the rutile as the minor phase were observed. As the annealing temperature increased, the better defined double-wall structure of the nanotubes was observed. Figure 4c−e shows images of the bottom of the nanotube arrays after annealing, in which the inner wall appeared to be separated from the outer wall at 300 °C, became clearly distinguished at 500 °C, and condensed at 600 °C of annealing. The cross-sectional TEM images captured for a single nanotube after annealing furthermore emphasize this tendency (insets in Figure 5a,b). These results can be
decreased from the beginning of sputtering and remained constant after 300 seconds of sputtering (50 nm from the bottom of nanotube). This indicates that the fluoride was enriched at the Ti/TiO2 interface, and the fluoride-rich layer was formed and somehow remained on the nanotube. It is clearly distinguished in the morphology change of the tube bottom after immersing it in water for 24 h (SI Figure S3) by the formation of many tiny islands, due to the further hydrolyzation of fluoride-containing materials and their incomplete dissolution products (such as TiF4, Ti(OH)4, or titanium carboxylates) on the outer surface of the nanotube. Even though the atomic distribution of fluoride on a single nanotube was not available in our study, the EDS point analysis and XPS depth profiles strongly indicate the long-range existence of a fluoride-containing layer on the tube wall of TiO2 nanotube arrays. In a high viscous electrolyte, the remaining fluoride-rich layer covering the outer walls of the nanotubes could occur and can be explained as follows: the high anodizing voltage (60 V) causes the rapid growth of an oxide wall in a viscous electrolyte, the low water and fluoride content of the glycerol-based electrolyte contributes to the partial dissolution of the “fluoride-rich” layer and the remaining of chemicals such as Ti(OH)4, pentafluoroaquotitanate, titanium carboxylates, or even glycerol derivatives12,37 on the walls of the nanotubes in a short anodizing time. Probably, this layer with the low electrical conductivity38 on the surface of the nanotube resulted in the observation of the ambiguous inter tube region (not sharp boundary) in the SEM images (side view, Figure 2a,b). 3.2. Effect of Annealing on the Morphology and Crystal Structure of the Tube Wall. Annealing is a conventional method to obtain the crystal structure of the assynthesized amorphous anodic titanium oxide nanotube arrays. In this study, we focus on the impact of annealing temperature on the crystal structures and morphology of the double-wall nanotubes and the fluoride-rich layer surrounding the tube wall. Figure 4 shows crystal structures and XPS wide scan spectra of double-wall TiO2 nanotube arrays before and after annealing at 300, 500, and 600 °C for 2 h, respectively. The as-prepared TiO2 nanotube arrays are amorphous, only diffraction peaks of
Figure 5. Lateral SEM images of as-prepared double-wall TiO2 nanotube arrays after annealing at (a) 300 °C, (b) 500 °C, and (c) 600 °C for 2 h. Insets in (a) and (b) are TEM images of the TiO2 nanotubes after annealing at 300 and 500 °C, respectively.
explained based on the improvement in the crystallinity of the inner wall and loss of carbon-containing compounds in the inner wall due to annealing. The later one was evident by the decrease of carbon peak intensity in XPS spectra (Figure 4b) with increasing annealing temperature. The thermal decomposition of carbon-contaminated compounds in the inner walls, such as titanium carboxylates, above 400 °C is responsible for the loss of carbon.39 The minor remaining carbon after annealing at higher temperature (of ca. 600 °C) can exist as a carbonate dopant in the TiO2 structure. It is recognized based on XPS analysis of the annealed TiO2 nanotube array at 600 °C, and the significant light absorption of TiO2 from the ultraviolet to the visible region (see SI Figure S4). This result demonstrates the potential of the tube wall design in terms of crystal and electronic structures for the enhanced photoactivity of anodic TiO2 nanotube arrays under visible light. It is noted that the fluoride peak observed in the XPS spectrum for the as-synthesized double wall nanotube arrays at room temperature disappeared after annealing at 300 °C or higher temperature (Figure 4b), because the fluoride was driven out at elevated temperature.37 However, the SEM images of the annealed double-wall nanotube arrays (Figure 5) show that there is still a very thin layer on the surface of the double-wall nanotubes that could not be seen from the bottom-view images in Figure 4. After annealing at 300 °C in the air, the rough
Figure 4. (a) XRD patterns and (b) XPS spectra of the double-wall TiO2 nanotube arrays before and after annealing. (c−e) SEM images of the bottom of nanotube arrays after annealing at 300, 500, and 600 °C for 2 h, respectively. A, R, and T in (a) are the symbols of anatase, the rutile phase of TiO2, and titanium, respectively. 1578
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anodic TiO2 nanotube arrays by maintaining a cladding layer on the wall of the double-wall nanotube and annealing.
surface of the nanotubes was clearly seen, which is thought to be a layer of the oxidation products of the fluoride-rich layer as fluoride was removed. Annealing at 300 °C for 2 h may not sufficiently smoothen this layer by sintering and crystal growth of small crystallites. At higher annealing temperatures of 500 and 600 °C, a smooth and dense layer can be distinguished on the surface of the nanotubes and is marked by blue arrows in Figure 5b,c. This is the direct capture for a layer left from the fluoride-rich layer on the tube walls due to annealing, which is important evidence indicating the prior existence of the fluoride-rich layer surrounding as-synthesized nanotubes. Hereafter, we call the layer originated from fluoride-rich layer by annealing the cladding layer. We investigated the crystal structure of the cladding layer using HR-TEM, and the results are given in Figure 6. The lattice spacings of 0.35 nm, which
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1, illustration of the bottom structure and XPS depth profile of TiO2 nanotubes; Figure S2, high resolution XPS spectra of C1s and O1s taken at the tube bottom of lift-off TiO2 nanotube arrays; Figure S3, SEM image of the bottom of assynthesized TiO2 nanotube arrays, (b) SEM image of the bottom of TiO2 nanotube arrays after immersion in water for 24 hours; Figure S4, high resolution C1s spectrum, IR-RAS spectrum, and UV−Vis absorption spectrum of the TiO2 nanotube arrays after annealing at 600 °C for 2 h; and additional references. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +81-11-706-7110. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is partially supported by Grant-in-Aid for Scientific Research (A) from JSPS, Japan (to T.Y., 24241041). REFERENCES
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Figure 6. (a−c) TEM images of the double-wall TiO2 nanotube arrays after being annealed at 300, 500, and 600 °C for 2 h. (d−f) HRTEM images of the rectangular areas in (a−c), respectively. The inset of (c) is the electron diffraction pattern of TiO2 nanotubes.
correspond to the (101) planes of the anatase phase,38 were found for the cladding layer calcinated at 300, 500, and 600 °C, respectively. The selected area electron diffraction (SAED) pattern collected for the sample annealed at 600 °C (Figure 6c) only shows the diffraction pattern of anatase. This is consistent with the XRD results (Figure 4), in which rutile appeared as the minor phase. Densification of the cladding layer was revealed at elevated temperatures, and the smoothness was improved after annealing at 500 and 600 °C. It is very clear in this study that the thin cladding layer covering the nanotube was densified and crystallized in the anatase structure with improved smoothness after annealing. These results offer a feasible approach to tailor the tube wall structure by maintaining the fluoride-rich layer and annealing.
4. CONCLUSIONS An anodic double-wall TiO2 nanotube array with a cladding layer surrounding the nanotube was prepared in a glycerolbased electrolyte. The existence of the cladding layer on the tube wall is attributed to partial dissolution and precipitation of the undissolved fluoride-rich layer. By increasing the annealing temperature, the cladding layer becomes smoothened with the anatase structure and clearly distinguished on the well-defined double-wall nanotubes. These results demonstrate the potential of glycerol-based electrolytes for the wall structure design of 1579
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DOI: 10.1021/la504670p Langmuir 2015, 31, 1575−1580
