NANO LETTERS
Synthesis of Anatase TiO2 Nanocrystals with Exposed {001} Facets
2009 Vol. 9, No. 6 2455-2459
Yunqian Dai,†,‡ Claire M. Cobley,† Jie Zeng,† Yueming Sun,‡ and Younan Xia*,† Department of Biomedical Engineering, Washington UniVersity, St. Louis, Missouri 63130, and School of Chemistry and Chemical Engineering, Southeast UniVersity, Nanjing, Jiangsu 211189, People’s Republic of China Received April 13, 2009
ABSTRACT This paper reports a facile synthesis of anatase TiO2 nanocrystals with exposed, chemically active {001} facets. The nanocrystals were prepared by digesting electrospun nanofibers consisting of amorphous TiO2 and poly(vinyl pyrrolidone) with an aqueous acetic acid solution (pH ) 1.6), followed by hydrothermal treatment at 150 °C for 20 h. The as-obtained nanocrystals exhibited a truncated tetragonal bipyramidal shape with 9.6% of the surface being enclosed by {001} facets. The use of electrospinning is critical to the success of this synthesis as it allows for the generation of very small particles of amorphous TiO2 to facilitate hydrothermal crystallization, an Ostwald ripening process. The morphology of the nanocrystals had a strong dependence on the pH value of the solution used for hydrothermal treatment. Low pH values tended to eliminate the {001} facets by forming sharp corners while high pH values favored the formation of a rodlike morphology through an oriented attachment mechanism. When acetic acid was replaced by inorganic acids, the TiO2 nanocrystals further aggregated into larger structures with various morphologies.
Nanomaterials have attracted much attention due to their different and often superior properties when compared to the bulk phase. For example, the unique optical and electronic properties of nanostructured titania (TiO2) have already led to breakthroughs in applications such as photocatalysis, photovoltaics, sensing, and photonic crystals.1 Most of these applications require control of both size and shape (or facets exposed on the surface) for the nanostructures. For anatase TiO2 crystals, they are usually dominated by {101} facets, which are thermodynamically stable due to a low surface energy (0.44 J/m2). Although {001} facets with a higher energy (0.90 J/m2) are more interesting and important for higher reactivity,2 they usually diminish rapidly during a crystal growth process.3 Therefore, single crystals of anatase TiO2 with exposed {001} facets are rarely observed. Recently, an important breakthrough in preparation of anatase TiO2 crystals with exposed {001} facets was achieved by Lu and co-workers.4 They reported the synthesis of anatase TiO2 microcrystals with 64% {001} facets on the surface by reversing the relative stability of {101} and {001} facets through the use of hydrofluoric acid as a shape controlling agent. Soon after, Xie and co-workers used a similar strategy to synthesize anatase TiO2 nanosheets with 89% exposed {001} facets and excellent photocatalytic efficiency.5 However, using hydrofluoric acid to decrease the * To whom correspondence should be addressed. E-mail: xia@ biomed.wustl.edu. † Washington University. ‡ Southeast University. 10.1021/nl901181n CCC: $40.75 Published on Web 05/13/2009
2009 American Chemical Society
surface energy of {001} facets is undesirable for mass production due to the high toxicity in both liquid and vapor forms. Furthermore, the crystal surface tended to be highly passivated by fluoride ions, which must be removed by heat treatment before these crystals can be used in a catalytic application. During this process, the {001} facets may have lost much of the reactivity as a result of surface reconstruction. For example, under ultrahigh vacuum (UHV) conditions, the anatase (001) surface reconstructs to form a microfaceted surface composed of (103) and (1j03) facets.6 In addition to these solution-phase methods, Liu and coworkers have reported the fabrication of oriented anatase TiO2 films by directly growing on surface-modified glass substrates.7 The films typically showed a tetragonal bipyramidal morphology with a tunable degree of truncation at the corners. The sizes of the anatase TiO2 crystals were approximately 90-150 nm, which are still too big for a number of applications that require high specific surface areas. It is still a challenge to synthesize nanometer-sized anatase TiO2 single crystals with clean and exposed {001} facets. In this report, we employed an electrospinning process to manipulate the hydrolysis of a sol-gel precursor and thus obtain well-defined anatase TiO2 nanocrystals (Figure 1A,B). In the first step, we prepared nonwoven mats of composite nanofibers consisting of amorphous TiO2 and poly(vinyl pyrrolidone) (PVP) using a method previously developed in our group (Supporting Information, Figure S1).8 Typically, an ethanol solution containing titanium tetraisopropoxide
Figure 1. (A) TEM image of a typical sample of anatase TiO2 nanocrystals prepared by hydrothermal treatment at 150 °C for 20 h. The solution was obtained by digesting the as-spun TiO2/PVP composite nanofibers with aqueous acetic acid with pH ) 1.6. (B) TEM image of the same sample at a higher magnification with the inset showing a schematic drawing of a truncated tetragonal bipyramid. The edge lengths labeled A and B, respectively, can be used to define the degree of truncation. (C) High-resolution TEM image taken from part of an individual nanocrystal; the inset shows the corresponding fast-Fourier transform (FFT) pattern. (D) XRD pattern taken from the same sample as in (A), together with the expected diffraction peaks for anatase TiO2.
(Ti(OiPr)4), PVP (Mw ≈ 1.3 × 106), and acetic acid was electrospun onto a grounded aluminum foil electrode. The diameters of the fibers varied in the range of 150 to 400 nm.9 As soon as the electrospinning process began, Ti(OiPr)4 hydrolyzed by reacting with the moisture in air to form very small particles of amorphous TiO2 in a PVP matrix. Because of a small diameter for the nanofibers and the presence of the PVP matrix, the amorphous TiO2 nanoparticles are expected to be very small and uniform in size. When the nonwoven mats of fibers were peeled off from aluminum foil and immersed in an acetic acid solution with a pH value of 1.6, the acid solution dissolved the PVP matrix and digested the fibers into discrete particles and eventually formed a transparent and colorless solution. Because of a high molecular weight for the PVP, the solution was heated in air at 80 °C for 18 h with magnetic stirring to ensure homogeneity. Finally this solution was loaded into an autoclave and hydrothermally treated at 150 °C for 20 h to generate anatase TiO2 nanocrystals through a crystallization (or Ostwald ripening) process. Figure 1A,B shows TEM images of the resultant nanocrystals, characterized by a truncated tetragonal bipyramidal morphology and enclosed by eight equivalent {101} facets and two equivalent {001} facets. The inset in Figure 1B shows a model of the truncated tetragonal bipyramidal shape exhibited by the as-synthesized TiO2 nanocrystals, which is similar to the equilibrium shape of anatase crystals predicted by Wulff construction from 2456
surface energy calculations.10 The edge lengths labeled A and B can be used to define the degree of truncation as the ratio of {001} facets to total surface area can be derived from the ratio of B/A.4 For this sample, the percentage of {001} facets on the surface was estimated to be approximately 9.6%. Because of the high specific surface areas of nanometer-sized samples, the total area of exposed {001} facets of our nanocrystals may be comparable to that of micrometer-sized TiO2 crystals or sheets reported in previous studies (despite their higher percentages of {001} facets). Figure 1C shows high-resolution TEM analysis of a truncated tetragonal bipyramidal TiO2 nanocrystal with clear crystalline lattice fringes. The fringe spacing of 3.5 Å corresponds to the {101} planes, while the fringe spacing of 4.7 Å corresponds to the {002} planes, indicating that the top/ bottom surface exposed by truncation is bound by a {001} facet. The angle labeled in the corresponding fast-Fourier transform (FFT) image inset is 68.3°, which is identical to the theoretical value for the angle between the {101} and {001} facets in an anatase crystal.4,7 Figure 1D shows X-ray diffraction (XRD) pattern of the resulting sample, in which the diffraction peaks match those of anatase TiO2 (JCPDS No. 21-1272). The broadening of the diffraction peaks can be attributed to the relatively small sizes of the TiO2 nanocrystals.11 To verify the function of the electrospinning process in producing such single crystals, a solution identical to the one for electrospinning was directly injected into an acetic acid solution of pH 1.6 and then subjected to similar hydrothermal treatment. As clearly shown in Supporting Information, Figure S2B, irregular shapes with a broad range of edge length A dominated this sample and less than half of the nanoparticles exhibited a truncated tetragonal bipyramidal shape. The difference in morphology can probably be ascribed to the different hydrolysis behaviors of Ti(OiPr)4 under dissimilar conditions. During the electrospinning process, Ti(OiPr)4 hydrolyzed with trace moisture in air as the solution was gradually ejected from the spinneret. Because of the small diameters of the as-spun nanofibers, the moisture in air could facilely diffuse into the interior to ensure complete hydrolysis for Ti(OiPr)4 at a more or less constant rate. It typically took 5 h for the Ti(OiPr)4 in asspun nanofibers to completely hydrolyze in air.8 This relative slow hydrolysis rate favored the formation of nonbranched Ti-O-Ti chains for better packing in the three-dimensional space.12 The PVP matrix could also prevent the small particles of amorphous TiO2 from aggregating into larger ones. In comparison, when the same solution was injected into the acidic medium without undergoing electrospinning, the Ti(OiPr)4 quickly hydrolyzed in the presence of water from the solution phase. Because hydrolysis and condensation proceeded simultaneously and very quickly, white precipitates (i.e., large particles of amorphous TiO2) appeared as a result of uncontrolled branching of the Ti-O-Ti network.13 Both the large size and branched structure make it harder to convert these amorphous TiO2 into anatase nanocrystals during hydrothermal treatment. We also tried to cast films from the same solution. In this case, it was Nano Lett., Vol. 9, No. 6, 2009
Figure 3. TEM images of the TiO2 nanocrystals obtained by hydrothermal treatment for 20 h with the pH value of the acetic acid solution being adjusted to (A) 0.23 (showing sharp tetragonal bipyramids); (B) 0.80 (showing a mixture of sharp and truncated tetragonal bipyramids); and (C) 3.0 (showing a rodlike morphology). (D) HRTEM image of the same sample in (C) with the inset showing an FFT pattern.
Figure 2. TEM images of TiO2 nanocrystals obtained by hydrothermally treating for (A) 4 h, (B) 8 h, (C) 13 h, and (D) 16 h while the pH value of the aqueous acetic acid solution was adjusted to 1.6. (E) The dependence of average edge length A and aspect ratio B/A on the treatment time. The edge length A (black bars) was proportional to the treatment time while the aspect ratio B/A (white bars) remained almost unchanged. (F) XRD patterns of samples obtained at different times of hydrothermal treatment.
difficult to achieve uniform hydrolysis for the sample because the quick hydrolysis at the solution-air interface tended to block the precursor inside the film from being hydrolyzed. In addition, it was not easy to remove the dried film from the substrate (more difficult than peeling off the nonwoven mat of electrospun nanofibers). The evolution of nanocrystals as a function of treatment time was analyzed by TEM and X-ray diffraction and the results are shown in Figure 2. After 4 h, nanocrystals with a truncated tetragonal bipyramidal structure were observed (Figure 2A), although the shape was not as defined as for other further reacted samples (Figure 2B-D). The average edge length of A was proportional to the duration of the hydrothermal treatment and can be seen in Figure 2E (black bars) (4 h, 12.6 nm; 8 h, 14.8 nm; 13 h, 16.9 nm; 16 h, 19.3 nm; and 20 h, 21.8 nm). As shown in Figure 2E (white bars), the average ratio of B/A stayed almost constant, indicating that the nanocrystals kept their degree of truncation during the hydrothermal crystallization process. The distributions of edge length A at different treatment times are shown in Nano Lett., Vol. 9, No. 6, 2009
Supporting Information, Figure S3. The increase of edge length A is consistent with the sharpening of (101) diffraction peak as depicted in Figure 2F. The XRD patterns also indicate the gradual increase of crystallinity with time. This observation is in agreement with an Ostwald ripening process, in which the molecules on the surface of amorphous TiO2 particles tended to diffuse through solution to grow on anatase TiO2 seeds. As a result, anatase TiO2 tended to grow over time while amorphous particles tended to shrink in number and eventually disappear. The sizes of TiO2 nanocrystals could also be tailored by adjusting the temperature used for the hydrothermal treatment. At a lower temperature of 120 °C (as compared to 150 °C), nanocrystals with an average side length A of 17.6 nm (Supporting Information, Figure S4) were observed indicating growth at a lower temperature resulted in nanocrystals of a smaller size, similar to previous reports.12 According to studies of the growth kinetics of TiO2 nanoparticles in aqueous solutions, the rate constant for coarsening decreases at lower temperatures due to the temperature dependence for both the viscosity and TiO2 solubility.12 Figure 3 shows the morphologies of the TiO2 nanocrystals when acetic acid solutions with different pH values were used. When the pH value was 0.23, the TiO2 nanocrystals exhibited a sharpened tetragonal bipyramidal shape bounded by eight triangular surfaces without exposed {001} facets, as shown in Figure 3A. The percentage of exposed {001} facets is estimated to be below 1.0%. At a slightly higher pH value of 0.80, both truncated tetragonal bipyramidal shapes and tetragonal bipyramidal shapes were observed (Figure 3B) with approximately 3.8% exposed {001} facets. 2457
When the pH value was increased to 1.6, the TiO2 nanocrystals showed primarily a truncated tetragonal bipyramidal shape with approximate 9.6% of the surface being enclosed by {001} facets (Figure 1A,B). It is well known that the relative surface energies of crystal facets of metals and metal oxides can be varied by selective adsorption of surfactant molecules.14 In our synthesis, PVP in the original electrospinning solution likely acts as a surfactant and shape controlling agent. It has been suggested in previous studies that PVP may adsorb preferentially onto {101} anatase facets, thereby modifying the shape of TiO2 nanocrystals.7 This binding specificity may be caused by variations in surface atom densities among different facets.15 PVP is known to bind to polar molecules exceptionally well due to its polarity. Therefore, in more acidic solutions PVP would have easily adsorbed onto the positively charged TiO2 surfaces. At lower pH values, PVP molecules were preferentially adsorbed onto the {101} surfaces, so the growth of {001} facets proceeded more quickly and the TiO2 nanocrystals exhibited a {101}-plane-dominated tetragonal bipyramidal shape.7 At higher pH values, the tendency for PVP to adsorb onto {101} surfaces would be weaker as a result of less positively charged TiO2 surface, so the {001} facets were not eliminated through preferential growth. When the pH value was further increased to 3.0, two to four nanocrystals aggregated together thereby forming a rodlike morphology. These rodlike nanoparticles seem to form through an oriented attachment mechanism (Figure 3C).16 Oriented attachment can occur in the early stage of crystal growth, leading to the fusing of several particles into a single crystal by sharing a common crystallographic orientation despite the presence of strong surface-bound ligand.17 Figure 3D shows high-resolution TEM image of the rodlike TiO2 nanocrystal, where the fringe spacing of 3.5 Å corresponds to {101} planes. Another set of lattice fringes (4.7 Å) parallel to the top surface are assigned to {002} planes indicating the particles were connected along the 〈001〉 direction and terminated with exposed {001} facets. The dislocations formed at the interface indicate the surfaces of adjacently oriented particles were not atomically flat.18 Because of the higher pH value, the nanoparticles were less positively charged and the repulsive interactions between adjacent nanoparticles could be overcome to eliminate the high energy pairs of {001} facets, leading to an overall reduction in surface energy. As a result, the final crystals expose a smaller percentage of the high energy {001} facets. It is also known that the final morphology of TiO2 nanoparticles is sensitive to the surface chemistry during their growth process.2 The choice of anion in an acidic medium would change the TiO2 surface energy, resulting in different aggregation structures.19 Figure 4 shows different morphologies of TiO2 products when the acetic acid was replaced by various inorganic acids. When nitric acid was used, flowerlike structures with sizes ranging from 75 to 150 nm were observed (Figure 4A,B). The inset in Figure 4A shows a typical flower consisting of more than ten tiny TiO2 nanocrystals that fused together at one common juncture. It is notable that the individual tiny crystals exhibited shapes 2458
Figure 4. TEM and SEM images of TiO2 nanostructures showing the effect of different inorganic acids on morphology. The samples were synthesized with (A, TEM; B, SEM) nitric acid, showing a flower shape (scale bars in the inset are 20 nm); (C, TEM; D, SEM) sulfuric acid, showing a hollow, spherical morphology (scale bars in the inset are 50 nm); and (E, TEM; F, TEM) hydrochloric acid. As shown in (F), some of the nanocrystals had assembled to micrometer-sized cubic boxes. The scale bars in the inset of (E) and (F) are 20 and 200 nm, respectively.
and sizes similar to those formed in an acetic acid medium. When sulfuric acid was used for the acidic medium, hollow spherical aggregates consisting of tiny TiO2 nanocrystals were observed, similar to what was reported for SO42-.20 Figure 4C,D show the hollow spheres whose diameters varied from 350 to 450 nm. Some of the spheres were fused together to form peanutlike aggregates. In previous work,21 formation of such hollow TiO2 structures was attributed to the movement of materials from the inside of a solid TiO2 particle to the surface through pores in the loose shell. As shown in the insets of Figure 4C,D, our hollow spheres were also characterized by loose shells, suggesting a similar mechanism. When hydrochloric acid was used, a flowerlike morphology was observed (Figure 4E), which was similar to the sample in Figure 4A but with a smaller size. Interestingly, they continued to assemble into secondary aggregates and formed core-shell cubic boxes with an average outer edge length of 750 nm and an average inner edge length of 350 nm (Figure 4F). This wide variety of Nano Lett., Vol. 9, No. 6, 2009
morphologies may reflect the different anionic affinities to titanium in an aqueous solution.22 Further work is needed to fully understand the formation mechanism of these aggregations. In summary, we have demonstrated that well-defined anatase TiO2 nanocrystals with exposed {001} facets could be synthesized in high yields by controlling the hydrolysis rate of the sol-gel precursor and hydrothermal treatment. The pH value of the medium used for hydrothermal treatment played an important role in controlling the morphology of the as-obtained TiO2 nanocrystals. At pH ) 0.23, we observed a sharp tetragonal shape and the disappearance of {001} facets as a result of significant PVP adsorption onto the {101} facets. At pH ) 1.6, the adsorption of PVP was reduced and the final shape became a truncated tetragonal bipyramid. As the pH value was further increased to 3.0, oriented attachment led to a rodlike morphology. When inorganic acids other than acetic acid were added to the media, the products showed different aggregation morphologies consisting of tiny nanocrystals. This work clearly demonstrates that anatase TiO2 nanocrystals could be prepared with chemically active {001} facets to enable a range of catalytic applications. Acknowledgment. This work was supported in part by startup funds from Washington University in St. Louis and a research grant from the NSF (DMR-0804088). As a visiting student from Southeast University, Y.D. was also partially supported by the National Basic Research Program (973 program, 2007CB936300), the Innovation Program for Graduate Students in Jiangsu Province (CX08B-051Z), and the China Scholarship Council. Supporting Information Available: Detailed descriptions of experimental procedures and four additional figures are provided. This material is available free of charge via the Internet at http://pubs.acs.org.
Nano Lett., Vol. 9, No. 6, 2009
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