DOI: 10.1021/cg901351s
Self-Organization of Anatase TiO2 Nanoparticles to Regular Shape Clusters
2010, Vol. 10 1721–1724
Yahya Alivov*,†,‡ and Sabee Molloi§,† †
Department of Radiological Sciences, University of California, Irvine, Medical Sciences I, B-140, Irvine, California 92697, and ‡Nano Tech Center, Texas Tech University, P.O. Box 43102, Lubbock, Texas 79409-3102 Received October 29, 2009; Revised Manuscript Received February 7, 2010
ABSTRACT: Self-organization of titanium dioxide (TiO2) nanoparticles to regular shape clusters (regular triangle, hexagon, pyramid, and rhomb) has been observed. The accumulation of TiO2 nanoparticles to clusters occurs during transformation of TiO2 nanotubes to nanoparticles, which is achieved by annealing of electrochemically grown TiO2 nanotubes in fluorine ambient. Size of clusters varied in range 0.5-30 μm, which is determined by the time elapsed from the start of the nanotubenanoparticle transformation. Systematic studies of cluster size and density have been done on preparation conditions that revealed, in particular, that accumulation of nanoparticles to clusters occurs when nanoparticle size is less than 100 nm. The origin of this interesting phenomenon is discussed.
Introduction Titanium dioxide (TiO2) nanostructures is a very promising oxide semiconductor material for a number of applications, especially, for dye-sensitized solar cells1 and hydrogen generating devices.2 The large crystal surface area of nanostructures, nanoparticles (NP) and nanotubes (NT), make them an excellent choice for such applications; surface area is of significant importance. One of the simplest methods for TiO2 nanotube growth is electrochemical oxidation of titanium sheets in fluorine-based electrolytes.3,4 Despite the large number of studies that were conducted, many properties of such nanotubes are still to be discovered. Recently, we reported on transformation of TiO2 NTs electrochemically grown in ethylene glycol and NH4F into nanoparticles (NPs) when annealed on glass slide with close contact with it or in a sealed glass container.5,6 The results were explained by a catalytic reaction of fluorine residues in NTs with TiO2.5 The formed NPs have a truncated bipyramidal shape with high fraction of reactive (001) surface area. The size of NPs depends on fluorine concentration and ranges within 20-500 nm. This transformation happens in several steps before they completely convert to nanoparticles: first, they contract in size, reducing in height and increasing in wall thickness, and later break down and merge to truncated shape pyramids or bipyramids.6 This new method for TiO2 nanoparticle fabrication is very simple and controllable method that makes it much more preferential to other existing methods for TiO2 nanoparticle fabrication. It is very important to understand the properties of TiO2 nanoparticles to better govern their fabrication, so further studies are needed to shed more light on the NT-NP transformation mechanism. In this study, further efforts have been done to reveal interesting features of such NT-NP transformations. In particular, depending of the nanoparticle size, NPs accumulate to clusters, and these clusters may acquire regular geometric shapes, such as regular triangles, hexagons, rhombs, pyramids, etc. In this paper, we
report on this phenomenon and discuss its origins. We want to emphasize the difference of this paper from the previous paper on transformation mechanism.6 While in earlier paper,6 the transformation process between two phases, nanotubes and nanoparticles, was studied, the present work examined the behavior of the nanoparticles after transformation and proved their tendency to accumulate to regular shape clusters. Experimental Section
*To whom correspondence should be addressed. E-mail: yalivov@ uci.edu (Y.A.). § E-mail:
[email protected].
In this work, initial TiO2 nanotubes were grown from titanium (Ti) sheets with 0.25 mm thickness and 99.97% purity by electrochemical oxidation in electrolyte, prepared using NH4F (98%) and ethylene glycol (99.8%). Electrochemical anodization was carried out in DC voltage range 30-60 V with NH4F concentration varied in a range of 0.1-2 wt %. Water (H2O, 10%) was added to the electrolyte to increase the growth rate.7,8 The obtained highly ordered TiO2 nanotube arrays were annealed in the sealed glass container in temperature range 500-800 °C. The gas in the container was air at atmospheric pressure. The annealing was performed immediately after the sample growth so that the samples contained residual electrolyte. Alternatively, previously grown samples were soaked in NH4F aqueous solution before annealing. NH4F concentration in the electrolyte and in the solution was changed in the range 0.1-2% to control the size of transformed NPs. More details on sample preparation, annealing, and other treatments can be found elsewhere.5,6 Typical scanning electron microscope (SEM) images of the initial samples are shown in Figure 1, where top view (a), and side and bottom views (b) of TiO2 NT arrays can be seen. A well-defined and highly aligned tubular structure of arrays is confirmed from this figure. NT arrays grew on both surfaces of the Ti sheet without noticeable differences in NT thickness, diameter, wall thickness, and surface morphology. As-grown nanotubes were of gray color, which turned yellow after thermal annealing on a glass slide or in a container at 300 °C and above for 30 min with ramping rate 16 °C/min. The yellow color resulted from nanoparticle nature of the transformed layer. The SEM image of the converted NP layer is shown in Figure 2. Nanoparticles have truncated bipyramidal or pyramidal shapes, as can be seen in the insert to Figure 2. The thickness of NP layer was 13-17 μm after full transformation of original 56-60 μm thickness NT films. This contraction is explained by a collapse of the original hollow NTs with amorphous structure, and densification, and crystallization into NPs. It should be emphasized that the NT-NP transformation occurs only when samples are annealed in the presence of fluorine and starts at temperatures as low as 300 °C. No such a transformation is observed
r 2010 American Chemical Society
Published on Web 02/18/2010
pubs.acs.org/crystal
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Figure 3. GAXRD spectra of NP side (labeled NP) and the NT side (labeled NT) of the TiO2 nanostructure sample. T refers to the diffraction peak of Ti foil.
Figure 1. Representative SEM images of TiO2 nanotube arrays grown in ethylene glycol þ NH4F electrolyte by electrochemical anodization: (a) top view, inset corresponds to magnification of the same area; (b) 3D image showing side view and bottom view of NT arrays.
Figure 2. SEM images of TiO2 nanoparticle layer formed after transformation of NT film; inset corresponds to magnification of the same area showing truncated pyramidal shape of nanoparticles. when TiO2 nanotubes are annealed in nonflourine ambient as was demonstrated in previous reports,9,10 in which the morphology of TiO2 NTs remained unchanged until annealing temperatures as high as 800 °C. Studies of crystalline properties, performed by glancing angle X-ray diffraction (GAXRD), revealed anatase phase for both nanotubes and nanoparticles, as can be seen from GAXRD pattern in Figure 3. The GAXRD patterns in this figure were taken from NT and NP sides of the same sample, which was annealed at 500 °C for 30 min. As can be seen from the GAXRD pattern for both NTs and NPs consisted of 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° in Figure 3 refers to the Ti sheet peak. Higher intensities of diffraction peaks of NPs indicate higher crystal quality compared to NTs. To study transformation mechanism of NTs to NPs, a series of experiments were performed by annealing of grown NT samples in fluorine ambient at 500 °C for 1-5 min. The aim of this procedure was to monitor initial stages of NT-NP transformation and subsequent evolution of nanoparticles. Selected
Figure 4. Representative SEM images of regular triangle (a), hexagon (b), pyramid (c), and rhomb (d) shape nanoparticle clusters. Drawings next to the images emphasize regular shapes of the clusters. samples with clusters were annealed at 600 o C, 700 o C, and 800 °C to check the effect of annealing temperature on the cluster shape.
Results and Discussion It was discovered that nanoparticles accumulate to clusters and the clusters get impressively high order regular shapes. Figure 4 shows several SEM images of triangle (a), hexagon (b), rhomb (c), and pyramid (d) shape clusters on the top of nanotube arrays. The triangles (a) have equilateral sides with 60° angle with each other, while the hexagon (b) clusters have equal sides with 120° angle between two neighboring sides. The angle of the rhomb changed slightly from cluster to cluster, with most typical value of ∼115°, as shown in Figure 4c. More examples of regular shape clusters are provided in the
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Figure 6. Top view of partially transformed NTs layer with ∼300 nm size nanoparticles, uniformly spread on the NT film surface. This image demonstrates that large size (>100 nm) NPs do not accumulate to clusters. Figure 5. SEM images of small size clusters corresponding to initial stage of nanoparticle cluster formation.
Supporting document, Figures S1 and S2. These images were taken from different samples, or from different areas of a sample. To emphasize the nanoparticle nature of clusters, we show a cluster and its magnified surface areas in Figure S3 (Supporting document). Although some clusters slightly deviated from regular shape geometry, as was the case with hexagon (a) and rectangular (b) clusters in Figure S4, Supporting Information, the greater part of clusters had a regular shape, and it can be concluded that there was a strong tendency to such regularity. Among different geometries, pyramid clusters dominated and varied in shape, such as truncated pyramids (Figures 1c and S2a,e,f in the Supporting Information), pyramids capped with inverted pyramid void (Figure S2b,c, Supporting Information), and Egyptian pyramids (Figure S2d, Supporting Information). Self-organization of nanoparticles to arrays and clusters in solid phase was observed before (see refs 11-14); however, to our knowledge, this is the first time when such self-organization of nanoparticles occurs in regular shape clusters. Recently, Prabhakaran et al.11 reported on formation of triangular shape GaN nanoparticle clusters of uniform height. Self-organization of TiO2 nanoparticles in thin films was observed in ref 14. Size of clusters varied in range 0.5-30 μm, which is determined by the time elapsed from the start of the NT-NP transformation, and the height of initial NT film, that is, the amount of TiO2 material supply available for NP formation. The smallest size of nanoparticle clusters was 0.5-1.5 μm; Figure 5 shows two of such small size clusters on the surface of NT array. These small clusters correspond to initial stages of cluster formation when accumulation just began. Studies of NT-NP transformation performed as a function of time showed that as formation of nanoparticles continues, the size of clusters increases, and when the amount of NPs produced is large enough, clusters start merging and form a continuous nanoparticle layer without signs of clusters, similar to the one shown in Figure 2. Different thickness NT films were used in annealing to find out the minimum thickness of NT films that allows for clusters to merge and shape. In our
experiments, only when the thickness of original NT films was greater than 15 μm, the cluster merging occurred. If the thickness of original NT films is not big enough, the resulting clusters, after full transformation of NTs to NPs, may not merge with each other because of insufficient NP supplies. The height of the clusters, as was estimated from SEM analysis, depended on the time elapsed from the beginning of TiO2 NTNP transformation, and varied from several hundred nanometers to several micrometers. Attempts were made to figure out the internal structure of the clusters by scratching the surface of samples with clusters. The performed tests indicated that clusters are continuous spatially. However, this aspect of clusters needs to be further verified by additional studies. Selected samples with clusters were also annealed at 600, 700, and 800 °C to check the effect of the annealing temperature on the clusters shape. The experiments revealed no significant changes in the clusters shape with annealing at higher temperatures. While the size of NPs could be controlled in the range of 20-500 nm during NT-NP transformation (by varying fluorine concentration),5 no clusters were observed with nanoparticle size of more than ∼100 nm. If NP size after NT transformation was, say, 300 nm, the resulting nanoparticles would spread uniformly on the NT film surface, similar to the one shown in Figure 6. One of the explanations for this critical size of NPs could be the existence of attractive forces between nanoparticles. Since the size of NPs determines their weight through the relationship m ≈ d3F, where m, d, and F are mass, size, and density of NPs, respectively, too big NPs have too large weight so that the limited value of attractive forces is not enough to move these heavy NPs. The density of clusters (a number of clusters per unit surface area) was also analyzed. It was found that the density changed from sample to sample, depending on annealing conditions, and varied in range of 103-104 cm-2. Figure 7 shows a representative SEM image of a sample with clusters distributed on top of NT film. The cluster density in this sample was 6.4 103 cm-2. The observed phenomenon, accumulation of TiO2 nanoparticles to regular shape clusters, raises two questions. First, why do nanoparticles accumulate to clusters? And, second, why do clusters form in such regular shape configurations? Currently, we do not have a clear understanding of the cluster formation
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transformation. The density of clusters changed from sample to sample, depending on annealing conditions, and varied in range of 103-104 cm-2. Supporting Information Available: Additional examples of regular shape clusters, SEM images showing nanoparticle nature of the clusters. This material is available free of charge via the Internet at http://pubs.acs.org. Note Added after ASAP Publication. This article posted ASAP on February 18, 2010. Figure 7 has been revised. The correct version posted on February 23, 2010.
References
Figure 7. SEM image of large area of sample with clusters showing distribution of clusters on the NT film surface.
mechanism. To answer these questions, direct measurements of the magnetic and electric properties of TiO2 nanoparticles are necessary. Currently we are involved in these measurements, and plan to report on the findings in our subsequent papers, while this study examines possible mechanisms for nanoparticle accumulation. The fact that nanoparticles accumulate into clusters implies the existence of attractive forces between nanoparticles. It was often reported that oxygendeficient oxides TiO2, HfO2, In2O3, and ZnO have defectrelated ferromagnetism15-20 due to unpaired electrons of cations or due to lattice distortion.20 Therefore, it is possible that TiO2 nanoparticles are magnetized because of oxygen deficiency, which is common for as-grown TiO2.21 However, only direct measurements of the TiO2 nanoparticle properties can shed more light into this phenomenon. Conclusion In conclusion, self-organization of TiO2 nanoparticles to regular shape clusters (regular triangle, hexagon, pyramid, and rhomb) has been observed during the transformation of TiO2 nanotubes to nanoparticles. This transformation was achieved by annealing of electrochemically grown TiO2 nanotubes in fluorine ambient. Systematic studies were performed to reveal conditions at which cluster formation occurs and their parameters. In particular, it has been found that accumulation of nanoparticles to clusters occurs when nanoparticle size is less than 100 nm. Size of clusters varied in range 0.5-30 μm, depending on the time elapsed from the start of the NT-NP
(1) Gr€atzel, M. Nature 2001, 414, 338. (2) Fujishima, A.; Honda, M. Nature 1972, 238, 37. (3) Zwilling, V.; Aucouturier, M.; Darque-Ceretti, E. Electrochim. Acta 1991, 45, 921. (4) Gong, D.; Grimes, C. A.; Varghese, O. K.; Hu, W.; Singh, R. S.; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331. (5) Alivov, Y; Fan, Z. J. Phys. Chem. C 2009, 113, 12954–12957. (6) Alivov, Y.; Fan, Z. Y. TiO2 Nanotechnology 2009, 20, 405610. (7) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E; Varghese, O. K.; Mor, G. K. J. Phys. Chem. B 2006, 110, 16179. (8) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 065707. (9) Alivov, Y.; Pandikunta, M.; Nikishin, S.; Fan, Z. Y. Nanotechnology 2009, 20, 225602. (10) Varghese, O. K.; Gong, D.; Paulose, M.; Grimes, C. A.; Dickey, E. C. J. Mater. Res. 2003, 18, 156. (11) Prabhakaran, K.; Schwenzer, B.; DenBaars, S. P.; Mishra, U. K. Appl. Surf. Sci. 2007, 253, 4773. (12) Verdes, C.; Chantrell, R. W.; Satoh, A.; Harrell, J. W.; Nikles, D. J. Magn. Magn. Mater. 2006, 304, 27. (13) Temmyo, J.; Kuramochi, E.; Kamada, H.; Tamamura, T. J. Cryst. Growth 1998, 195, 516. (14) Burnside, S. D.; Shklover, V.; Barbe, C.; Comte, P.; Arendse, F.; Brooks, K.; Gratzel, M. Chem. Mater. 1998, 10, 2419. (15) Hong, N. H.; Barla, A.; Sakai, J.; Huong, N. Q. Phys. stat. sol. (c) 2007, 4, 4461–4466. (16) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Nat. Mater. 2005, 4, 173–179. (17) Khalid, M.; Ziese, M.; Setzer, A.; Esquinazi, P.; Lorenz, M.; Hochmuth, H.; Grundmann, M.; Spemann, D.; Butz, T.; Brauer, G.; Anwand, W.; Fischer, G.; Adeagbo, W. A.; Hergert, W.; Ernst, A. Phys. Rev. B 2009, 80, 035331. (18) Yoon, S. D.; Chen, Y.; Yang, A.; Goodrich, T. L.; Zuo, X.; Arena, D. A.; Ziemer, K.; Vittoria, C.; Harris, V. G. J. Phys.: Condens. Matter 2006, 18, L355–L361. (19) Coey, J. M. D.; Venkatesan, M.; Stamenov, P.; Fitzgerald, C. B.; Dorneles, L. S. Phys. Rev. B 2005, 72, 024450. (20) Kim, D.; Hong, J.; Park, Y. R.; Kim, K. J. J. Phys.: Condens. Matter 2009, 21, 195405. (21) Nowotny, M. K.; Bak, T.; Nowotny, J.; Sorrel, C. C. Phys. Stat. Solidi (b) 2005, 242, R88.
