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2009, 113, 12954–12957 Published on Web 07/01/2009
A Method for Fabrication of Pyramid-Shaped TiO2 Nanoparticles with a High {001} Facet Percentage Yahya Alivov* and Z. Y. Fan† Nano Tech Center and Department of Electrical and Computer Engineering, Texas Tech UniVersity, P.O. Box 43102, Lubbock, Texas 79409-3102 ReceiVed: June 2, 2009; ReVised Manuscript ReceiVed: June 19, 2009
A new method is demonstrated for fabricating TiO2 nanoparticles. Transformation of TiO2 nanotubes to truncated pyramid-like-shaped nanoparticles with a high percentage of photocatalytically active {001} facets has been observed in titanium dioxide (TiO2) ordered nanotube arrays after thermal annealing in ambient fluorine. It was found that the nanotube-nanoparticle transformation resulted from catalytic reaction of fluorine ions (F-) from the electrolyte residues in long nanotubes grown by anodization in ethylene glycol + NH4F electrolyte. This transformation occurs at annealing conditions when evaporation of electrolyte residues in nanotubes is slow enough to remain until 500 °C, when this transition starts. The size of the formed TiO2 nanoparticles depends on the fluorine concentration and can be controlled from 20 to 500 nm. The crystal properties of nanoparticle layers are superior compared to those of nanotube arrays. This work demonstrates a simple method for producing high-quality anatase TiO2 nanoparticles and nanoparticle-based electrodes with a high ratio of reactive {001} facets. Nanostructured titanium dioxide (TiO2) has attracted great attention recently1-5 due to its unique properties that make them of considerable scientific interest and practical importance. TiO2, including both nanoparticles (NPs) and nanotubes (NTs), has been used for dye-sensitized solar cells (DSSC), water splitting for hydrogen generation, photocatalysis for purification of air and water, and bio- and chemical sensors.4,6,7 To facilitate vectorial electron transport for efficient photoconversion, the organized nanotube structure has its advantages. Two maskfree techniques, hydrothermal synthesis by NaOH treatment of TiO2 nanoparticles with subsequent acid washing3,8 and electrochemical anodic oxidation of titanium metal foil in fluorinated electrolyte,4,9,10 have been widely reported for the synthesis of TiO2-based nanotubes. It has been explained11 that in the hydrothermal synthesis by treatment with NaOH, some Ti-O bonds are broken, leading to the formation of lamellar sodiumcontaining titanate fragments, which would undergo Na+ exchange with H+ in the post-treatment acid washing. The ion exchange results in the variation of surface charge and peelingoff of the individual layers of titanate (nanosheets). The scrolling of the sheets forms individual nanotubes. In the anodization method, first discovered in 1991 by Zwilling et al.,12 organized nanotube arrays formed under the balance of TiO2 formation and chemical dissolution in the electrolyte during electrochemical oxidation of Ti sheets.4 It should be pointed out that the hydrothermally formed product is a random mixture of individual nanotubes, while the electrochemically formed product is a regular nanotube array organized on the bottom Ti metal foil. This electrochemical anodization of titanium is a simple and cost-effective method for growth of highly ordered TiO2 * To whom correspondence should be addressed. E-mail: Yahya.Alivov@ ttu.edu. † E-mail:
[email protected].
10.1021/jp905174x CCC: $40.75
NTs, and uniform titania nanotube arrays of various pore sizes, lengths, and wall thicknesses can be easily grown by tailoring electrochemical conditions.4 For applications such as solar cells, which require a fast transport of charge carriers, the electrochemically synthesized regular nanotube array clearly has its advantage. There have been reports11,13 on the nanostructure transformation of TiO2 between nanosheets, nanotubes, nanorods, and nanoparticles during hydrothermal synthesis, based on the pH variation of chemical solution, but there is no report on the nanostructure transformation for electrochemical anodizationsynthesized regular nanotube arrays. In this paper, we report for the first time on the transformation of TiO2 NTs grown in ethylene glycol + NH4F by electrochemical oxidation into nanoparticles at certain annealing conditions. We found that TiO2 NTs transform to the nanoparticle phase when annealing is performed at a high-temperature ramping rate with the opening end of NTs sealed by close contact with a supporting plate. This is explained by the catalytic reaction of fluorine residues in NTs with TiO2. The formed NPs have a truncated bipyramid shape14 with a high portion of reactive (001) surface area.15-17 The size of the NPs depends on the fluorine concentration and can be controlled within 20-500 nm. The crystal quality of the formed anatase NPs is superior to that of anatase NTs. Titanium (Ti) sheets with 0.25 mm thickness and 99.97% purity were used for electrochemical oxidation in electrolyte prepared using NH4F (98%) and ethylene glycol (99.8%). The metal sheets were first cleaned in acetone and ethanol, followed by rinsing in deionized water and drying in a nitrogen stream. Electrochemical anodization was carried out in a DC voltage range of 30-60 V with the NH4F concentration varied in a range of 0.1-1 wt %. A 10% water (H2O) solvent was added to the 2009 American Chemical Society
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Figure 2. Pictures of the NT side (top) and the NP sides (bottom) of the annealed on glass samples. Size of the images: 0.5 in. × 1 in.
Figure 1. Representative SEM images of TiO2 nanotube arrays grown in ethylene glycol + NH4F electrolyte by electrochemical anodization; (a) and inset, top views at different magnifications; (b) and insert, side views at different magnifications.
electrolyte to increase the growth rate. The use of ethylene glycol with the addition of water as a solvent in the electrolyte was found to dramatically increase the TiO2 nanotube growth rate.18,19 A two-electrode cell was used with a platinum meshed plate as the counter electrode, separated from the titanium anode with a distance of 2 cm. The obtained highly ordered TiO2 nanotube arrays were annealed in air in the temperature range of 300-800 °C. The morphology and geometry (diameter, wall thickness, and height) of the TiO2 nanotubes were studied using scanning electron microscope (SEM). Figure 1 represents typical SEM images of the as-grown samples (a) with inset showing the top views and (b) with the inset showing cross sections at different magnifications. A well-defined tubular structure can be observed from this figure. The average diameter of the NTs depended on the anodization voltage, being 80 and 160 nm for 30 and 60 V, respectively. The NT wall thickness depended on the acid concentration and ranged over 10-30 nm. NT arrays grew on both surfaces of the Ti sheet without noticeable differences in the NT film thickness, diameter, wall thickness, and surface morphology. Both sides of the as-grown nanotubes have a gray color, as shown in Figure 2a. After rapid annealing on a glass plate at 500 °C for 30 min with a ramping rate of 16 °C/min, the bottom side facing the glass transformed to yellow color, as shown in Figure 2b, while the color of the top side remained the same (although the amorphous TiO2 was converted into the anatase phase). SEM analysis of the yellow color side revealed a nanoparticle pattern, as shown in Figure 3. In this figure (a-c) correspond to top views at different magnifications. Thus, the nanotube array in contact with the glass plate was
Figure 3. (a-c) SEM images of TiO2 nanoparticle layers at different magnifications. Most of the nanoparticles have a shape of a truncated pyramid and bipyramid, as shown in (c).
transformed into nanoparticles during annealing, while the nanotube array on the other side of the sample without covering retained its nanotube array morphology. Hereafter, we will refer the sample side with nanoparticles as the NP side, while the side with NT arrays will be referred to as the NT side. The NP
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Figure 4. GAXRD spectra of the NP side (red) and the NT side (blue) of the TiO2 nanostructure sample.
size of the sample shown in the Figure 3 (grown in 0.5% NH4F) was in the range of 300-400 nm. The formed NP layers are mechanically stable and do not change in morphology and thickness even after treatment in ultrasound. This implies that the particles have catenated with each other, ensuring, in particular, good electrical contacts. Most of the nanoparticles have truncated bipyramid or pyramid shape, as shown in Figure 3c. The cross section of the NP side is shown in Supporting Information Figure 1S. The thickness of the NP side was found to be 13-17 µm compared to the 56-60 µm thickness of the original NT films. Such a reduction of the NT film thickness by 4-5 times can be explained by a collapse of the original hollow NTs with amorphous structure and densification and crystallization into NPs. Crystalline and optical properties of the samples were studied by a glancing angle X-ray diffractometer (GAXRD) and photoluminescence (PL) methods, which revealed a great enhancement of these properties for the NP side of the sample. Figure 4 presents the XRD spectra of the NP and NT sides of the samples. Both sides have anatase (101), (103), (004), (112), and (200) diffraction peaks at 25.3, 36.95, 37.75, 38.45, and 47.95°, respectively. A diffraction peak at 44.55° refers to the Ti sheet peak. However, the major anatase peak (101) intensity of the NP side is 2.5 times greater than that of NT side, and the full width at half-maximum (fwhm) of the NP side of the (101) peak is smaller compared to that of the NT side, being 0.31 and 0.38°, respectively. These data indicate that the crystal quality of the NP side is superior to that of the NT side. The anatase-rutile transformation temperature for the NP side was higher by 100 °C compared to that for the NT side, being 750 and 650 °C, respectively. A series of experiments was performed using different annealing conditions to understand the mechanism of NT-NP transformation. It was found that no NPs can be formed when the annealing temperature is at or below ∼300 °C, and NTs started transforming to NPs at 400 °C with a partial NP pattern on NT arrays after a limited annealing time (images are shown in Figure S2 in the Supporting Information). Rapid NT-NP transition occurs when annealed at 500 °C or above. The temperature ramping rate was found to be critical for NT-NP transition. When the temperature ramping rate was as low as 1 °C/min, no NP pattern was observed. Only at ramping rates as high as 16 °C/min or above did NT-NP transformation occur. At ramping rates around 10 °C/min, a partial NP phase was observed. These results indicate that a high-temperature ramping rate is necessary for NP formation. In further experiments, samples were annealed in a way that both sides were exposed to flowing air without blockage. No NP pattern was observed on any side of the sample after annealing. In contrast, both sides of the sample had a well-defined NP pattern when samples were
Letters annealed with both sides covered by two glass plates. These experiments showed that “sealing” the opening ends of the NTs is necessary to observe NT-NP transition. Also, there was a minimum NT film thickness to observe the NP transition. It was found that when the NT film thickness was ∼10 µm, no NP pattern was observed. When the film thickness was ∼17 µm, only a partial NP pattern formed. Only NT arrays with film a thickness of 30 µm and above revealed full NT-NP transformation. The above observations of NT-NP transformation could be explained by the reaction of TiO2 NTs with the electrolyte residues in the nanotubes after growth. Although samples were rinsed and dried in a nitrogen stream after growth, residual electrolyte could still exist in the long (∼60 µm) nanotubes. The glass plate served as a semibarrier to block the rapid evaporation of residual electrolyte during the annealing, and the rapid ramping rate and long nanotube guaranteed that some electrolyte remained in the tube at the transformation temperature (>400 °C). Considering that the electrolyte is a mixture of NH4F, H2O, and ethylene glycol, further experiments were conducted to determine the chemical component responsible for the NT-NP transformation. TiO2 NT samples were first annealed at low temperature to completely drive out the electrolyte residuals, and then, they were soaked in ethylene glycol, 0.4% NH4F aqueous solution, or H2O followed by annealing in a semisealed glass container at 500 °C for 30 min. Experiments showed that only samples soaked with NH4F aqueous solution transformed to the NP phase. In all other cases (ethylene glycol and H2O), no change of the initial NT pattern was observed. There was no thickness limitation or temperature ramping rate limitation for annealing in a sealed container, and all samples were converted to NPs. This indicates that the presence of fluorine (F) is necessary for NT-NP transformation. The NP size was found depending on the amount of fluorine in the NTs during annealing in the sealed glass container. It decreased with an increase of F concentration. Specifically, NP sizes from NT samples presoaked in a 0.1, 0.5, 1, and 2% NH4F aqueous solution were, respectively, 220, 110, 35, and 20 nm. (Representative SEM images of NP patterns with different grain sizes are shown in Figure S3 in the Supporting Information.) The NP size was determined by averaging the size of 20 nanoparticles. The deviation was estimated to be 20%. In all cases, truncated bipyramid-shaped nanoparticles were observed regardless of nanoparticle size. The similar trend of NP size dependence on F concentration was observed for samples annealed on glass plates. The NP size in this case ranged over 60-500 nm when the F concentration decreased from 1 to 0.1%. The variation of the F concentration in this case was achieved by using different NH4F concentrations in electrolyte during electrochemical growth. The NP size range in this case was larger than that in the case where annealing was done in a glass container (20-230 nm). Plots showing the dependence of NP size on F concentration for each case are shown in Figure 5. Thus, by varying the F concentration during heat treatment and using different annealing methods, the NP size can be controlled within 20-500 nm. The bipyramid nanoparticles with {001} and {101} facets were observed. A schematic of anatase the TiO2 truncated bipyramid observed in our work is illustrated in the inset to Figure 5. The degree of pyramid truncation is characterized by a ratio of the B and A parameters (B/A).14,17 Previously, it was reported that most anatase TiO2 crystals are strongly dominated by stable {101} facets (by more than 94%). Recently, Yang et al.20 predicted by first-principle calculations and confirmed
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J. Phys. Chem. C, Vol. 113, No. 30, 2009 12957 pyramid shape with large ratio of photocatalytically active (001) surface area. Acknowledgment. This work was supported by Texas Tech University and the U. S. Army CERDEC (Contract No. W15P7T-07-D-P040). Supporting Information Available: Cross-sectional view of the NP layer, top views of a partial NP sample at different magnifications, and representative SEM images of different size NPs formed from NTs using different NH4F concentrations. This material is available free of charge via the Internet at http:// pubs.acs.org.
Figure 5. Dependence of NP size on fluorine concentration during annealing of NT samples in a sealed glass container and on glass plate. Inset: A sketch of truncated bipyramid-shaped TiO2 NPs showing (001) and (101) facets; A and B are length parameters, as illustrated in the figure.
experimentally that termination with F atoms yields the lowest value of surface energy and makes (001) surfaces more stable compared to (101) surfaces. This was explained by a balancing of O-O/O-F repulsions and Ti-O/Ti-F attractions, which stabilizes Ti and O atoms on the surface.20 The B/A parameters of the pyramids in our NPs changed in the range of 0.55-0.82, although not in predictable way. However, the B/A values in our samples are much higher than the predicted maximum value of 0.3714 when no surface energy modification atoms are present. It is known that the (001) surface is more photocatalytically active than the (101) surface.15,21,22 However, what forces drive the NTs in ambient fluorine to contract and further to break down to form nanoparticles? It is well-known that the equilibrium shape of a crystal is dictated by the minimization of surface energy. It is likely that F termination of the TiO2 NT surface is not energetically favorable at high temperatures, which results in NT contraction, then in breaking, and then in forming nanoparticles. However, whether this is a true mechanism or not requires further investigations. In conclusion, nanotube-nanoparticle transition was observed after annealing of NT films in fluorine ambient. The NP size can be controlled in the range 20-500 nm by changing fluorine concentration at annealing. The formed NPs have enhanced crystal properties compared to NT arrays and have a truncated
References and Notes (1) Gong, D.; Grimes, C. A.; Varghese, O. K.; Hu, W.; Singh, R. S.; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331. (2) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550. (3) Yao, B. D.; Chan, Y. F.; Zhang, X. Y.; Zhang, W. F.; Yang, Z. Y.; Wang, N. Appl. Phys. Lett. 2003, 82, 281. (4) Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A. Sol. Energy Mater. Sol. Cells 2006, 90, 2011. (5) Macak, J. M.; Tsuchiya, H.; Ghicov, A.; Yasuda, K.; Hahn, R.; Bauer, S.; Schmuki, P. Curr. Opin. Solid State Mater. Sci. 2007, 11, 3. (6) Gra¨tzel, M. Nature 2001, 414, 338. (7) Fujishima, A.; Honda, M. Nature 1972, 238, 37. (8) Khan, M. A.; Jung, H.-T.; Yang, O.-B. J. Phys. Chem. B 2006, 110, 6626. (9) Macak, J. M.; Schmuki, P. Electrochim. Acta 2006, 52, 1258. (10) Alivov, Y.; Pandikunta, M.; Nikishin, S.; Fan, Z. Y. Nanotechnology 2009, 20, 225602. (11) Tsai, C.-C.; Teng, H. Chem. Mater. 2006, 18, 367. (12) Zwilling, V.; Aucouturier, M.; Darque-Ceretti, E. Electrochim. Acta 1991, 45, 921. (13) Nian, J. N.; Teng, H. J. Phys. Chem. B 2006, 110, 4193. (14) Barnard, A. S.; Curtiss, L. A. Nano Lett. 2005, 5, 1261. (15) Gong, X. Q.; Selloni, A. J. Phys. Chem. B 2005, 109, 19560. (16) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Grat¨zel, M. Phys. ReV. Lett. 1998, 81, 2954. (17) Lazzeri, M.; Vittadini, A.; Selloni, A. Phys. ReV. B 2001, 63, 155409. (18) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Mor, G. K. J. Phys. Chem. B 2006, 110, 16179. (19) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 065707. (20) Yang, H.-G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Nature 2008, 453, 638. (21) Gong, X. Q.; Selloni, A. J. Phys. Chem. B 2005, 109, 19560. (22) Zaban, A.; Aruna, S. T.; Tirosh, S.; Gregg, B. A.; Mastai, Y. J. Phys. Chem. B 2000, 104, 4130.
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