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Synthesis of octagonal microdisks assembled from anatase TiO2 nanosheets with exposed {001} facets
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 15033–15043 www.elsevier.com/locate/ceramint
Synthesis of octagonal microdisks assembled from anatase TiO2 nanosheets with exposed {001} facets XiaoYi Hun College of Materials, Xiamen University, Xiamen 361005, China Received 21 April 2014; received in revised form 23 June 2014; accepted 24 June 2014 Available online 30 June 2014
Abstract Octagonal TiO2 microdisks constructed orderly from anatase nanosheet building blocks (NSBBs) with exposed {001} facets were synthesized using a facile liquid phase precipitation method combined with subsequent heat treatment. The in-situ generated BF4 and F adsorbed onto {001} facets of NH4TiOF3 (as precursor of TiO2) decreased the surface energy instead of extremely poisonous and corrosive HF. Polymer surfactant is likely to further stabilize the {001} facets and it may induce NH4TiOF3 nanocrystals as a linkage to align high-orderly by lateral expansion for the formation of NH4TiOF3 mesocrystals. Sintering temperature for the heat treatment of NH4TiOF3 microdisks has a considerable effect on TiO2 microdisks for the extent of exposure of the {001} surface. Symmetrically growths on [100] and [110] directions parallel to the {001} surface are favored which lead to the well-defined octagonal flat-shape. TiO2 microdisks show an excellent adsorption capacity in dark and enhanced reactive activity under irradiation of UV-light for the degradation of methylene blue, owing to the high-ordered organization of nanosheets and large exposure of high-energy facets. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Electron microscopy; B. Surfaces; D. TiO2; Photocatalysis
1. Introduction Titanium dioxide (TiO2), as one of the most advanced semiconductors, has drawn a great deal of scientific and technological attentions for its application in photocatalysis, solar cells, photonic crystals, and sensors in recent years [1–5]. Among these applications, photocatalysis has been studied most widely due to their potential in relieving the environmental contamination caused by chemical compounds and H2 evolution by water splitting. Generally, the activity of photocatalysts dependents not only on the crystal phase and size but also on the surface states [6,7]. Both theoretical calculations and experimental results reveal that {001} facets of anatase exhibit higher photoactivity than {101} facets [8–10] in some cases. However, the surface free energies in anatase crystals are γ{110} (1.09 J m 2) 4 γ{001} (0.90 J m 2) 4 γ{100} (0.53 J m 2)4 γ{101} (0.44 J m 2) [7,11]. The n
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thermodynamically stable {101} facets make up more than 94% of the crystal surface (according to the Wulff construction) during the process of naturally crystallization [11]. In keeping with the energetics, it turns out to be an arduous challenge to synthesize TiO2 with largely exposed {001} facets. A significant breakthrough in the long-term desirable preparation of well-shaped crystals with exposed {001} facets was achieved by Yang and co-workers [10] in 2008. They proposed a theoretical prediction that the fluorine-terminated effect can reverse the relative stability of {101} and {001} facets and then successfully synthesized micro-sized anatase TiO2 single crystal with 47% {001} surface by using HF as a capping agent and TiF4 as precursor. With the same morphology controlling agent, Xie's group [12] obtained TiO2 nanosheets with the highest percentage of the {001} surface up to 89% from tetrabutyl titanate as the Ti resource. During the following years, HF (or in the form of ammonium salt) remained to be the most common choice for fluorine-mediated formation of active-faceted titania nano, micro-sized [13–16]
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crystals (or hierarchical structures [17,18]). In order to minimize the employ of the extremely poisonous and corrosive HF, some researchers focused on developing less dangerous synthesis systems. For instance, the assistance of 2-propanol [13,19] or EDTA [20] could stabilize the F-adsorbed (001) facets and BF4 [21] might be an alternative fluorine provider. Nevertheless, far more efforts should be made for greener synthesis of TiO2 materials with dominant {001} facets. Moreover, the above attempts were mostly carried through long-drawn hydrothermal treatment at relatively high temperatures (120–210 1C) or assisted by microwave heating. It requires expensive equipments and makes it more difficult for large-scale productions. Besides, nanostructured architectures self-assembled from nanoscale building blocks have been as well a research focus for their potential in practical applications for aspects of environment and energy [22–25]. The superstructures, constructed from primary nanoparticles with high-energy surface, will be the new tendency in upcoming researches, as they are easy to separate and recover during repeated use [23]. It has been expected for high-order organization of nanosheets with the dominate {001} surface into micro-sized assemblies [26], so that the recovery problem of such surface-mediated photocatalysts could be solved after photocatalytic reaction. Herein, we propose a mild and controllable liquid phase precipitation for fabricating NH4TiOF3 and subsequent heat treatment for topochemical transformation [16,27,28] from NH4TiOF3 to anatase TiO2. High-order octagonal TiO2 microdisks in crystallographic orientation can be obtained by selfassembly of anatase nanosheets with largely exposed {001} surface without the addition of HF/NH4F. In this synthesis route, (NH4)2TiF6 is used as Ti source for its controllable hydrolysis and containing for fluoride ions. Polyvinyl pyrrolidone (PVP) acts as both a capping agent to co-adsorb on the {001} surface together with in-situ generated BF4 and F and a linker to connect NH4TiOF3 nanocrystals. The obtained nanostructured TiO2 microdisks perform an excellent adsorption capacity in dark and enhanced photoactivity under UV irradiation.
2. Experimental section 2.1. Preparation of NH4TiOF3 For a typical synthesis, the details would be as follows: 7 mL PVP aqueous solution (15 mg mL−1) was added to 69 mL ethanol ( Z 99.7%), labeled as solution A, then ultrasonic treated for 15 min. A freshly prepared aqueous solution (12 mL) containing 0.002 M (NH4)2TiF6 and 0.006 M H3BO3, labeled as solution B, was added to solution A. After ultrasonic treated for less than 1 min, the mixture was stored at 80 1C in a water bath for 2 4 hours. The obtained particles were separated by centrifugation and washed twice with ethanol and thoroughly with DI water.
2.2. Preparation of TiO2 Anatase TiO2 was prepared by post-heat-treatment of NH4TiOF3 precursors. Typical heat process was carried through in a Muffle furnace at 500 1C for 2 h with a ramping rate of 5 1C min 1. The converted productions were taken out for further characterization after nature cooling to room temperature in the Muffle furnace. 2.3. Characterization X-ray diffraction studies of powder samples were investigated by X'pert X-ray diffractometer (XRD, X'pert PRO, Panalytical, Netherlands) with CuKα1 radiation (λ= 1.54056 Å) at 40 kV and 30 mA. The morphology of asprepared and sintered particles was observed using scanning electron microscopy (FE-SEM, LEO1530). The micrographs and SAED patterns of samples were performed by transmission electron microscopy (HRTEM, JEM2100). 2.4. Photocatalytic activity measurement Photocatalytic activity of TiO2 powders was evaluated in terms of degradation of a methylene blue (MB) dye solution under UV-light irradiation. For the photodegradation measurements, two 8 W UV lamps were used as UV-light (λ=365 nm) source. 50 mg TiO2 powders were dispersed in 50 mL MB aqueous solution (10 mg L 1). The suspension (pH=3) was stirred in the dark for 1 h to reach adsorption equilibrium for MB and then irradiated with UV-light under widely stirring for 2 h. The concentration of the residual MB after every 30 minutes was determined from the absorption at the wavelength of 665 nm using a UV-visible spectrophotometer (UV-723PC). 3. Results and discussion 3.1. Deposition behavior of NH4TiOF3 3.1.1. Effect of ethanol The fabrication of NH4TiOF3 is realized by a simple and mild liquid phase precipitation method with (NH4)2TiF6 as the titanium source. H3BO3 is used as an F-scavenger to remove the generated F ions, pushing the hydrolysis reaction of Ti (IV) ions forward. The involved mechanisms can be explained by following equations: [TiF6]2 þ nH2O⇋[TiF6 n(OH)n]2 þ nHF
(1)
[Ti(OH)6]2 þ 2H þ ⇋TiO2↓þ 4H2O
(2)
H3BO3 þ 4HF⇋H þ þ BF4 þ 3H2O
(3)
The gradually substitution of combined F ions in fluorotitanium complex ions by hydroxyl groups makes precipitation of solid phase go temperately. In our synthesis strategy, a pair of experiments with different composition of solvent was set to discuss the effect of ethanol on the hydrolysis. The solution systems in both experiments were as follows: a freshly
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prepared aqueous solution (12 mL) containing 0.002 M (NH4)2TiF6 and 0.006 M H3BO3 mixed with 69 mL H2O or 69 mL ethanol respectively, incubated at 80 1C in a water bath for 2–4 h. The XRD peaks (as shown in Fig. 1a) of powder samples prepared with or without ethanol can be respectively indexed to crystalline NH4TiOF3 and anatase TiO2. It indicates that the composition of solvent in reaction mixture affects the crystalline phase of precipitates by influencing the degree of hydrolysis. The completely hydrolyzed production of (NH4)2TiF6 in pure H2O would be TiO2 (consistent with Eqs. (1)–(3)), and it would be the intermediate (NH4TiOF3) while ethanol was employed to inhibit hydrolysis which may be described in following equations [29]: [TiF6]2 þ 3H2O⇋[TiF3(OH)3]2 þ 3HF
(4)
[TiF3(OH)3]2 þ H þ þ NH4þ ⇋NH4TiOF3↓þ 2H2O
(5)
Fluoro-titanium octahedron in mixed solvent including ethanol and water surrounds by both –OH and OH . The electrically neutral –OH is likely to be chemical inert to replace fluoride. As a result, titanium and the unsubstituted fluoride precipitated in the form of ammonium oxofuorotitanates. The corresponding SEM images (Fig. 1b and c) of as-synthesized NH4TiOF3 and TiO2 display chopsticks-like and rod-like particles respectively, suggesting that the c-axis preferred crystalline growth dominated in the absence of polymeric surfactant. Growth orientation along the [001] direction is a common rule in equilibrium crystal growth of TiO2, particularly in a naturally-occurred one. Consequently, it is reasonable to believe ethanol is obligatory for stable incompletely hydrolyzed production (NH4TiOF3), and it barely impacts the morphology of products.
Fig. 1. (a) The XRD patterns of powder samples from (NH4)2TiF6 and H3BO3 mixed reaction solution (A) with or (B) without ethanol. (NH4TiOF3 from PDF#54-0239; TiO2 from PDF#21-1272). SEM images of corresponding powder samples (b) with or (c) without ethanol.
3.1.2. Effect of PVP To investigate the effect of PVP on the synthesis, a series of experiments was conducted with different concentrations of original PVP aqueous solution under the same procedures described in the experimental section. The XRD patterns in Fig. 2a demonstrate that samples prepared from different content of PVP (0, 5, 10, 15, 20 mg mL 1 original PVP aqueous solution) are assigned to NH4TiOF3. It reveals that PVP has no influence on the formation of NH4TiOF3 phase in a wide range of concentrations. However, the diffraction intensities of these XRD peaks are inconsistent. It suggests that PVP affects the crystallinity as well as crystal orientation of the generated particles. For the powder sample prepared with 15 mg mL 1 PVP aqueous solution (denoted as s-P-15), the diffraction intensities of all peaks are higher than those of other samples, and that 001 peak is particularly enhanced compared with other directions, implying a good crystallinity and preferred growth on the {001} facets. The SEM images of these NH4TiOF3 samples are shown in Fig. 2b–f. In the absence of PVP, as-prepared NH4TiOF3 sample are chopsticks-like with length of 0.8–1 μm (Fig. 2b) as aforementioned. Aggregations of coupled chopsticks into rodlike particles with similar size are observed for s-P-5 (Fig. 2c). From the SEM image (Fig. 2d) of s-P-10, a long shape with
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Fig. 2. (a) The XRD patterns of powder samples from (NH4)2TiF6 and H3BO3 mixed reaction solution with different contents of PVP (0, 5, 10, 15, 20 mg mL 1 original PVP aqueous solution). SEM images of corresponding powder samples with (b–f) 0, 5, 10, 15, 20 mg mL 1 original PVP aqueous solution.
microstructures much like spool gears can be found under closely observation, and some irregular particles and broken sheets are also exist. NH4TiOF3 particles of s-P-15 (Fig. 2e) grow into well faceted and dispersed octagonal microdisks with a size of 4–6 μm and mean thickness of about 1 μm. However, as the content of PVP further increasing, particles of s-P-20 (Fig. 2f) are almost identical with s-P-10, except for the existence of more irregular aggregations and sheets. These changes of products from SEM images correspond to the above XRD characterization. Morphologies of the as-prepared NH4TiOF3 can be adjusted by the content of PVP as the capping agent. Well faceted and dispersed octagonal NH4TiOF3 microdisks can be successfully synthesized by the facile liquid phase precipitation method with 15 mg mL 1 PVP aqueous solution. Any increase and decrease of PVP
would both inhibit the growth of the {001} facets of crystalline NH4TiOF3 and weaken the crystallinity of products. 3.2. Topochemical conversion from NH4TiOF3 to TiO2 particles 3.2.1. Morphology and microstructure of NH4TiOF3 and TiO2 microdisks As-synthesized NH4TiOF3 samples were transformed into anatase TiO2 samples after heat treatment. That is verified by means of X-ray diffraction, the results of which have not put on here. Morphologies of NH4TiOF3 microdisks and anatase TiO2 converted by heat treatment at 500 1C are analyzed in detail from several SEM images as shown respectively in Fig. 3a–d. The outline of as-prepared NH4TiOF3 particles
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Fig. 3. SEM images of (a) as-prepared NH4TiOF3 and (c) TiO2 microdisks. Low and high-magnification top-view SEM images of representative (b) NH4TiOF3 and (d) TiO2 particle. Insets in (a) and (c) are the side-view SEM images.
(Fig. 3a) has been retained intact after sintering, as presented in general view of TiO2 particles (Fig. 3c). For clarity, a typical microdisk is picked out before (inset of Fig. 3b) or after (inset of Fig. 3d) sintering. These octagonal microdisks are welldefined and highly symmetric with eight nearly equal sides around 2 μm in size. The specific directions and planes of the octagonal shape in crystallography will be discussed below in the TEM analysis section. High-magnification top-view (Fig. 3b) or side-view (inset of Fig. 3a) SEM image of a certain NH4TiOF3 microdisk show a relatively smooth surface and unabridged edges, demonstrating the as-prepared precursor has a compacting structure. For TiO2 microdisk, the top-view (Fig. 3d) image displays discrete and linked nanosheets of dimensions between tens and hundred of nanometers. And the side-view image (inset of Fig. 3c) also confirms the existence of nanoparticles, we speculate, which are sides of nanosheets in the whole large disk with a uniform thickness of about 20 nm. So it is almost credible that TiO2 microdisks consist of much smaller subunits as building blocks. The lattice mismatch between NH4TiOF3 and anatase TiO2 in the z-axis causes loosened packing of subunits [29]; a visible porous structure has obtained as a consequence. Further characterization for the microstructure of NH4TiOF3 microdisks is shown in Fig. 4. The majority of NH4TiOF3 microdisks present the smooth surface and unabridged edges (Fig. 3a and b), while microdisks with a low crystalline quality are also observed (typically in Fig. 4a and b). A distinguishable layered feature is found at the margin of some microdisk, and usually accompany with more evident defects. So the substance in dotted rectangular region from down right corner of Fig. 4b might be a vertical disk. Layered structure of NH4TiOF3 can be obtained from liquid chemical system in the
presence of 2-propanol [16,19]. As elaborated in Refs. [16,29,30], crystal structure of NH4TiOF3 is characterized by inserted ammonium between layers composed by Ti-centered octahedra. It probably causes the layered morphology. In relatively integrated particles, there are bits of interior pores (Fig. 4c). Selected area electron diffraction (SAED) patterns (Fig. 4d and g) from defective or integrated disks display “single-crystal-like” diffraction which can be indexed into [001] zone of NH4TiOF3; that is, {001} facets are successfully exposed. The growth process of such crystals perform not ionby-ion, but via the assembly of clusters. That comes up to a concept emerged within a decade, defined the term “mesocrystal” by Colfen and Antonietti [31], which has attracted intensive interest in crystal design, synthesis and their promising properties [32–34]. It requires crystallographic uniformity of nanocrystals for mesocrystals, therefore their electron diffraction behaves as same as that from single crystals. Fusion of nanocrystals occurred in mesocrystals obscures the difference between the mesocrystal and single crystal (as shown in Fig. 4b and f, respectively) [28]. The representative HR-TEM images demonstrate poorly crystallized NH4TiOF3 particles (Fig. 4e) with mesopores (2–5 nm in size) and well crystallized NH4TiOF3 particles (Fig. 4h). TEM image of a typical TiO2 microdisk (Fig. 5a) displays a loosened structure and the “single-crystal-like” diffraction spots as shown in SAED pattern from the center of particle (Fig. 5b) verified that it is TiO2 mesocrystal. The SAED pattern can be indexed into diffraction spots of the [001] zone [10]. SAED pattern of subunits/subunit from the right margin of particle (down right corner of Fig. 5c) shows identical diffraction spots with that from the central region. The crystallographic coincidence of the diffraction patterns
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Fig. 4. (a) SEM, (b, c and f) TEM, (e and h) HR-TEM images and (d and g) SAED pattern of representative NH4TiOF3 particles. Insets in (a) and (b) are highmagnification images of NH4TiOF3 particles from corresponding dotted rectangular regions.
suggests that the whole architecture is a crystallographically ordered assembly of NSBBs. The high-magnification TEM image of a nanosheet (Fig. 5e) shows a lattice spacing of 0.189 nm which is identical to the (020) and (200) atomic planes of anatase TiO2. The interfacial angle of 901 conforms to the angle between two planes. A tetragonal atomic arrangement on the (001) plane of anatase TiO2 is shown in the corresponding fast-Fourier-transform-filtered TEM image (Fig. 5f) with high clarity. SAED pattern from a vertical microdisk (Fig. 5d) is not so bright because of the thickness but distinguishable with the marked angle of 68.31 which is identical to the theoretical value for the angle between {101} and {001} facets of anatase. That is, the microdisks are mesocrystal in nature but of anatase TiO2. The top and bottom facets of disks are {001} facets and nanosheets as the building blocks are dominated with {001} facets. The eight sides of
regular octagon are perpendicular to the [100] and [110] directions for each orthonormal four sides respectively. Detailed TEM images of TiO2 nanosheets and their assembly are performed as shown in Fig. 6a and b. From the highmagnification TEM image, primary nanosheets with a size about 25 nm exist in isolation of approximate square with round corner or linked into much larger sheets with serrated or smooth edges. The above two type of links (along the [110] or [100] direction) lead to different sides of octagon as labeled in dotted rectangular region I and II. Fig. 6c gives the schematic of possible attachments of nanosheets. 3.2.2. Formation process of octagonal TiO2 microdisks After the discussion of deposition behavior of NH4TiOF3 and the analysis of microstructure of NH4TiOF3 and TiO2 microdisks, we have a hypothesis here about the formation
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Fig. 5. (a) Low-magnification TEM image and (b) SAED pattern of the central region of TiO2 microdisks. TEM images and SAED pattern from (c) the right margin and (d) side of particle. (e) High-magnification TEM image of an individual nanosheet from dotted rectangular region in (c) and (f) the corresponding fast-Fouriertransform-filtered TEM image.
process of octagonal TiO2 microdisk. The illustration is shown in Fig. 7. From the supersaturated (NH4)2TiF6 solution, NH4TiOF3 nuclei form gradually under the controlled hydrolysis of [TiF6]2 with mediation of ethanol; then the interactions between these NH4TiOF3 nuclei drive the attachment of ambient nucleus and create primary clusters which stepwise grow up to nanocrystals [29], as depicted in the “Growth” (the contents inside double quotes corresponding to illustrations in Fig. 7, similarly hereinafter in this section) process. This speculation is based on the mesopores (Fig. 4c and e) which
produced during “Oriented attachment” process in consequence of various shape and size of NH4TiOF3 nanocrystals [28,29], although the boundary of nanocrystals in the final mesocrystal (Fig. 4f) tends to be unclear because of the fusion process (“Fusion into SC”) [28]. The layered feature of assembly and distortion of oriented attachment (“Lamellar loosen”; “Distortion”) strengthen such speculation. Macromolecules of polymer in the solution may act as a matrix inducing the crystallographically oriented assembly [29]. The resulted NH4TiOF3 mesocrystals are exposed with {001} facets whose surface energy is lowered notablely by coverage of the in-situ
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Fig. 6. (a) High and (b) low-magnification TEM images of nanosheets for TiO2 microdisks. (c) Illustration of possible attachments of nanosheets as shown in dotted rectangular region I and II of (b).
Fig. 7. Illustration of formation process of assembly. “SC” is short for single crystal.
generated BF4 and F . As another capping agent, PVP trend to adsorb on the {001} facets for the high polarity, thus further stabilizing the surface [35]. The fact that undercoordinated Ti atoms and their influence on the Ti–O–Ti bond angles makes adsorbates interact selectively with specific surfaces of NH4TiOF3 [36]. Polymer surfactants preferentially adsorb on the {001} surface with relatively large Ti–O–Ti bond angles and for the high polarity. The F [10] and BF4 groups [19,21]
as rich fluorine carriers bond with fivefold-coordinated Ti atoms on {001} facets. The growth of the [001] direction is inhibited and the highly reactive surface can be largely preserved. The residual molecules of PVP may play a role of linking the nanocrystals preferentially on the ab planes with highly orientation. The self-assembly process go in the way of lateral expansion in eight directions at much the same growth rate. As a result, octagonal NH4TiOF3 microdisks deposit from
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Fig. 8. XRD patterns of TiO2 particles converted by sintering at different temperatures and the calculated ratio of the 004 diffraction intensity to the 101 diffraction intensity.
the reaction liquid phase. It can be topochemically converted to TiO2 by subsequent heat treatment because of the similarity of Ti atoms arrangement between NH4TiOF3 and TiO2 on {001} facets. The square-shaped nanosheets (Fig. 6a) dominated with {001} facets (see in Fig. 5e) serve as NSBBs in the obtained TiO2 mesocrystals (“Sintering”). Meanwhile, voids will appear since the dehydration and the shrinkage of unit cells of TiO2 compared to those of NH4TiOF3 in the z-axis causes contraction of the primary nanoparticles. As the external thickness barely alters, visible voids formed by combination of much smaller voids appear among the nanoparticles. Thus octagonal TiO2 microdisks assembled from nanosheets with exposed {001} facets are obtained. 3.2.3. Effect of sintering temperature on conversion of TiO2 particles Interestingly, sintering temperature for the heat treatment of NH4TiOF3 microdisks has a considerable effect on TiO2 particles. XRD patterns (Fig. 8) of samples (sintered at 500, 600 and 700 1C) indicate all of them are pure anatase TiO2. As the sintering temperature increases from 500 to 700 1C, XRD peaks get sharper and the relative diffraction intensity of 004 peak remarkably increase, while the ratio of the 004 diffraction intensity to the 101 diffraction intensity for randomly oriented anatase powder samples is averagely 0.2 as recorded in JCPDS data (PDF#21-1272). The significantly enhanced 004 peak suggests the larger exposure of {001} facets or the possibility of self-induction of TiO2 flat parallel to the sample holder. Fusion of crystallines and stability of crystal planes are extra sensitive to sintering temperature. So we suppose that the TiO2 nanosheets perhaps have a greater extent of lateral fusion at a higher temperature. In a similar research, Zhou et al. [27] have already pointed out appropriate temperature range and highlighted the advantage of rapid heating. It is different with this work, suggesting that effects of heating parameters on topochemically conversion change with precursor (NH4TiOF3)
Fig. 9. (a) Adsorption capacity and photocatalytic activity of TiO2 samples prepared with different content of PVP and after sintering at 500 1C and (b) adsorption capacity and photocatalytic activity of P25 TiO2 and TiO2 microdisks after heat treatment at different temperatures.
precipitated from certain reaction solution. Further investigation on the structure of these particles will be our next work. 3.3. Photocatalytic activity Photocatalytic activity of TiO2 powders was evaluated in terms of degradation of methylene blue (MB) dye solution under UV-light irradiation. The adsorption behavior of MB molecules in dark on different TiO2 samples and degradation efficiency of TiO2 samples under UV-light irradiation are integrated into account. Fig. 9a shows the adsorption capacity and photocatalytic activity of TiO2 samples prepared with different contents of PVP and after sintering at 500 1C. TiO2 microdisks (i. e. heat-treated s-P-15, donated as s-P-15-HT500) exhibit much higher adsorbability (24% of MB molecules) than those of other TiO2 samples. The degradation efficiency of TiO2 microdisks is also enhanced. Although the sizes of s-P-0-HT500 and s-P-5-HT500 are smaller than that of TiO2 microdisks, their activities are extremely low. With the
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existence of some broken sheet-like particles, S-P-10-HT500 and s-P-20-HT500 show minor adsorption capacity. It has been already confirmed that {001} facets are more favored for dissociative adsorption of reactant molecules than {101} facets [37]. The chemical dissociation of adsorbed water molecules is activated on {001} facets [38]. These surface interactions are benefit to transfer the photogenerated carrier and provide more active sites for degradation of pollutants. In addition, superstructure of assembly with uniform subunits and crystallographic order is believed to be promising for the transfer of reactants and degradation products. Fig. 9b shows the adsorption capacity and photocatalytic activity of TiO2 microdisks after heat treatment at different temperatures. Commercial P25 TiO2 is used as a benchmark sample. The degradation efficiency of TiO2 microdisks slightly increases with the rise of sintered temperature. The adsorption capacity in dark of TiO2 microdisks sintered at 700 oC is extremely high because of the larger exposure of {001} facets. The complete adsorption of MB molecules can be accomplished in a relatively transient process at the same time of photocatalysis, drastically promoting catalytic activity to a high efficiency even better than P25 TiO2. We perform the comparison with P25 in order to offer a referable evaluation of our samples. However, it is unfair to ignore the enormous difference between them in size, crystal phase and related properties while draw the conclusion that these TiO2 microdisks are less active than P25. Micro-sized TiO2 organizations are easy to be separated from the suspension by sedimentation. The residual adsorbed MB molecules on separated TiO2 microdisks can be degraded within less than 12 min under the irradiation of UV light. Photocatalytic activity has been enhanced through facet-controlled synthesis of anatase TiO2, and the excellent adsorption capacity of TiO2 microdisks organizations may be applied in prospective applications such as catalysis, dye-sensitized solar cells or can be substrate materials in related areas. 4. Conclusion As a summary, octagonal TiO2 microdisks assembled from NSBBs with exposed {001} facets have been successfully synthesized. Adjusting morphology of NH4TiOF3 is realized by a PVP-assisted liquid phase precipitation method from (NH4)2TiF6. Transformation from NH4TiOF3 to anatase TiO2 is accomplished by solid topochemical conversion. The in-situ generated BF4 and F are proved to be milder alternatives as the capping agents, instead of adscititious highly poisonous and corrosive HF. PVP provides dual functions: one is stabilizing the high-reactive facets and the other is linking the nanocrystals to alignment in three-dimensions. The formation process of TiO2 microdisks is discussed in detail. TiO2 microdisks show both an excellent adsorption capacity in dark and enhanced photocatalytic activity owing to the superstructures of assembly of nanosheets and exposed {001} facets. TiO2 microdisks sintered at 700 1C exhibits a comparable activity to that of P25. The facet-controlled synthesis of anatase TiO2 with high adsorption capacity and easy recycling
of products may have promising features for other chemical synthesis and applications.
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 15033–15043 www.elsevier.com/locate/ceramint
Synthesis of octagonal microdisks assembled from anatase TiO2 nanosheets with exposed {001} facets XiaoYi Hun College of Materials, Xiamen University, Xiamen 361005, China Received 21 April 2014; received in revised form 23 June 2014; accepted 24 June 2014 Available online 30 June 2014
Abstract Octagonal TiO2 microdisks constructed orderly from anatase nanosheet building blocks (NSBBs) with exposed {001} facets were synthesized using a facile liquid phase precipitation method combined with subsequent heat treatment. The in-situ generated BF4 and F adsorbed onto {001} facets of NH4TiOF3 (as precursor of TiO2) decreased the surface energy instead of extremely poisonous and corrosive HF. Polymer surfactant is likely to further stabilize the {001} facets and it may induce NH4TiOF3 nanocrystals as a linkage to align high-orderly by lateral expansion for the formation of NH4TiOF3 mesocrystals. Sintering temperature for the heat treatment of NH4TiOF3 microdisks has a considerable effect on TiO2 microdisks for the extent of exposure of the {001} surface. Symmetrically growths on [100] and [110] directions parallel to the {001} surface are favored which lead to the well-defined octagonal flat-shape. TiO2 microdisks show an excellent adsorption capacity in dark and enhanced reactive activity under irradiation of UV-light for the degradation of methylene blue, owing to the high-ordered organization of nanosheets and large exposure of high-energy facets. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Electron microscopy; B. Surfaces; D. TiO2; Photocatalysis
1. Introduction Titanium dioxide (TiO2), as one of the most advanced semiconductors, has drawn a great deal of scientific and technological attentions for its application in photocatalysis, solar cells, photonic crystals, and sensors in recent years [1–5]. Among these applications, photocatalysis has been studied most widely due to their potential in relieving the environmental contamination caused by chemical compounds and H2 evolution by water splitting. Generally, the activity of photocatalysts dependents not only on the crystal phase and size but also on the surface states [6,7]. Both theoretical calculations and experimental results reveal that {001} facets of anatase exhibit higher photoactivity than {101} facets [8–10] in some cases. However, the surface free energies in anatase crystals are γ{110} (1.09 J m 2) 4 γ{001} (0.90 J m 2) 4 γ{100} (0.53 J m 2)4 γ{101} (0.44 J m 2) [7,11]. The n
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thermodynamically stable {101} facets make up more than 94% of the crystal surface (according to the Wulff construction) during the process of naturally crystallization [11]. In keeping with the energetics, it turns out to be an arduous challenge to synthesize TiO2 with largely exposed {001} facets. A significant breakthrough in the long-term desirable preparation of well-shaped crystals with exposed {001} facets was achieved by Yang and co-workers [10] in 2008. They proposed a theoretical prediction that the fluorine-terminated effect can reverse the relative stability of {101} and {001} facets and then successfully synthesized micro-sized anatase TiO2 single crystal with 47% {001} surface by using HF as a capping agent and TiF4 as precursor. With the same morphology controlling agent, Xie's group [12] obtained TiO2 nanosheets with the highest percentage of the {001} surface up to 89% from tetrabutyl titanate as the Ti resource. During the following years, HF (or in the form of ammonium salt) remained to be the most common choice for fluorine-mediated formation of active-faceted titania nano, micro-sized [13–16]
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crystals (or hierarchical structures [17,18]). In order to minimize the employ of the extremely poisonous and corrosive HF, some researchers focused on developing less dangerous synthesis systems. For instance, the assistance of 2-propanol [13,19] or EDTA [20] could stabilize the F-adsorbed (001) facets and BF4 [21] might be an alternative fluorine provider. Nevertheless, far more efforts should be made for greener synthesis of TiO2 materials with dominant {001} facets. Moreover, the above attempts were mostly carried through long-drawn hydrothermal treatment at relatively high temperatures (120–210 1C) or assisted by microwave heating. It requires expensive equipments and makes it more difficult for large-scale productions. Besides, nanostructured architectures self-assembled from nanoscale building blocks have been as well a research focus for their potential in practical applications for aspects of environment and energy [22–25]. The superstructures, constructed from primary nanoparticles with high-energy surface, will be the new tendency in upcoming researches, as they are easy to separate and recover during repeated use [23]. It has been expected for high-order organization of nanosheets with the dominate {001} surface into micro-sized assemblies [26], so that the recovery problem of such surface-mediated photocatalysts could be solved after photocatalytic reaction. Herein, we propose a mild and controllable liquid phase precipitation for fabricating NH4TiOF3 and subsequent heat treatment for topochemical transformation [16,27,28] from NH4TiOF3 to anatase TiO2. High-order octagonal TiO2 microdisks in crystallographic orientation can be obtained by selfassembly of anatase nanosheets with largely exposed {001} surface without the addition of HF/NH4F. In this synthesis route, (NH4)2TiF6 is used as Ti source for its controllable hydrolysis and containing for fluoride ions. Polyvinyl pyrrolidone (PVP) acts as both a capping agent to co-adsorb on the {001} surface together with in-situ generated BF4 and F and a linker to connect NH4TiOF3 nanocrystals. The obtained nanostructured TiO2 microdisks perform an excellent adsorption capacity in dark and enhanced photoactivity under UV irradiation.
2. Experimental section 2.1. Preparation of NH4TiOF3 For a typical synthesis, the details would be as follows: 7 mL PVP aqueous solution (15 mg mL−1) was added to 69 mL ethanol ( Z 99.7%), labeled as solution A, then ultrasonic treated for 15 min. A freshly prepared aqueous solution (12 mL) containing 0.002 M (NH4)2TiF6 and 0.006 M H3BO3, labeled as solution B, was added to solution A. After ultrasonic treated for less than 1 min, the mixture was stored at 80 1C in a water bath for 2 4 hours. The obtained particles were separated by centrifugation and washed twice with ethanol and thoroughly with DI water.
2.2. Preparation of TiO2 Anatase TiO2 was prepared by post-heat-treatment of NH4TiOF3 precursors. Typical heat process was carried through in a Muffle furnace at 500 1C for 2 h with a ramping rate of 5 1C min 1. The converted productions were taken out for further characterization after nature cooling to room temperature in the Muffle furnace. 2.3. Characterization X-ray diffraction studies of powder samples were investigated by X'pert X-ray diffractometer (XRD, X'pert PRO, Panalytical, Netherlands) with CuKα1 radiation (λ= 1.54056 Å) at 40 kV and 30 mA. The morphology of asprepared and sintered particles was observed using scanning electron microscopy (FE-SEM, LEO1530). The micrographs and SAED patterns of samples were performed by transmission electron microscopy (HRTEM, JEM2100). 2.4. Photocatalytic activity measurement Photocatalytic activity of TiO2 powders was evaluated in terms of degradation of a methylene blue (MB) dye solution under UV-light irradiation. For the photodegradation measurements, two 8 W UV lamps were used as UV-light (λ=365 nm) source. 50 mg TiO2 powders were dispersed in 50 mL MB aqueous solution (10 mg L 1). The suspension (pH=3) was stirred in the dark for 1 h to reach adsorption equilibrium for MB and then irradiated with UV-light under widely stirring for 2 h. The concentration of the residual MB after every 30 minutes was determined from the absorption at the wavelength of 665 nm using a UV-visible spectrophotometer (UV-723PC). 3. Results and discussion 3.1. Deposition behavior of NH4TiOF3 3.1.1. Effect of ethanol The fabrication of NH4TiOF3 is realized by a simple and mild liquid phase precipitation method with (NH4)2TiF6 as the titanium source. H3BO3 is used as an F-scavenger to remove the generated F ions, pushing the hydrolysis reaction of Ti (IV) ions forward. The involved mechanisms can be explained by following equations: [TiF6]2 þ nH2O⇋[TiF6 n(OH)n]2 þ nHF
(1)
[Ti(OH)6]2 þ 2H þ ⇋TiO2↓þ 4H2O
(2)
H3BO3 þ 4HF⇋H þ þ BF4 þ 3H2O
(3)
The gradually substitution of combined F ions in fluorotitanium complex ions by hydroxyl groups makes precipitation of solid phase go temperately. In our synthesis strategy, a pair of experiments with different composition of solvent was set to discuss the effect of ethanol on the hydrolysis. The solution systems in both experiments were as follows: a freshly
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prepared aqueous solution (12 mL) containing 0.002 M (NH4)2TiF6 and 0.006 M H3BO3 mixed with 69 mL H2O or 69 mL ethanol respectively, incubated at 80 1C in a water bath for 2–4 h. The XRD peaks (as shown in Fig. 1a) of powder samples prepared with or without ethanol can be respectively indexed to crystalline NH4TiOF3 and anatase TiO2. It indicates that the composition of solvent in reaction mixture affects the crystalline phase of precipitates by influencing the degree of hydrolysis. The completely hydrolyzed production of (NH4)2TiF6 in pure H2O would be TiO2 (consistent with Eqs. (1)–(3)), and it would be the intermediate (NH4TiOF3) while ethanol was employed to inhibit hydrolysis which may be described in following equations [29]: [TiF6]2 þ 3H2O⇋[TiF3(OH)3]2 þ 3HF
(4)
[TiF3(OH)3]2 þ H þ þ NH4þ ⇋NH4TiOF3↓þ 2H2O
(5)
Fluoro-titanium octahedron in mixed solvent including ethanol and water surrounds by both –OH and OH . The electrically neutral –OH is likely to be chemical inert to replace fluoride. As a result, titanium and the unsubstituted fluoride precipitated in the form of ammonium oxofuorotitanates. The corresponding SEM images (Fig. 1b and c) of as-synthesized NH4TiOF3 and TiO2 display chopsticks-like and rod-like particles respectively, suggesting that the c-axis preferred crystalline growth dominated in the absence of polymeric surfactant. Growth orientation along the [001] direction is a common rule in equilibrium crystal growth of TiO2, particularly in a naturally-occurred one. Consequently, it is reasonable to believe ethanol is obligatory for stable incompletely hydrolyzed production (NH4TiOF3), and it barely impacts the morphology of products.
Fig. 1. (a) The XRD patterns of powder samples from (NH4)2TiF6 and H3BO3 mixed reaction solution (A) with or (B) without ethanol. (NH4TiOF3 from PDF#54-0239; TiO2 from PDF#21-1272). SEM images of corresponding powder samples (b) with or (c) without ethanol.
3.1.2. Effect of PVP To investigate the effect of PVP on the synthesis, a series of experiments was conducted with different concentrations of original PVP aqueous solution under the same procedures described in the experimental section. The XRD patterns in Fig. 2a demonstrate that samples prepared from different content of PVP (0, 5, 10, 15, 20 mg mL 1 original PVP aqueous solution) are assigned to NH4TiOF3. It reveals that PVP has no influence on the formation of NH4TiOF3 phase in a wide range of concentrations. However, the diffraction intensities of these XRD peaks are inconsistent. It suggests that PVP affects the crystallinity as well as crystal orientation of the generated particles. For the powder sample prepared with 15 mg mL 1 PVP aqueous solution (denoted as s-P-15), the diffraction intensities of all peaks are higher than those of other samples, and that 001 peak is particularly enhanced compared with other directions, implying a good crystallinity and preferred growth on the {001} facets. The SEM images of these NH4TiOF3 samples are shown in Fig. 2b–f. In the absence of PVP, as-prepared NH4TiOF3 sample are chopsticks-like with length of 0.8–1 μm (Fig. 2b) as aforementioned. Aggregations of coupled chopsticks into rodlike particles with similar size are observed for s-P-5 (Fig. 2c). From the SEM image (Fig. 2d) of s-P-10, a long shape with
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Fig. 2. (a) The XRD patterns of powder samples from (NH4)2TiF6 and H3BO3 mixed reaction solution with different contents of PVP (0, 5, 10, 15, 20 mg mL 1 original PVP aqueous solution). SEM images of corresponding powder samples with (b–f) 0, 5, 10, 15, 20 mg mL 1 original PVP aqueous solution.
microstructures much like spool gears can be found under closely observation, and some irregular particles and broken sheets are also exist. NH4TiOF3 particles of s-P-15 (Fig. 2e) grow into well faceted and dispersed octagonal microdisks with a size of 4–6 μm and mean thickness of about 1 μm. However, as the content of PVP further increasing, particles of s-P-20 (Fig. 2f) are almost identical with s-P-10, except for the existence of more irregular aggregations and sheets. These changes of products from SEM images correspond to the above XRD characterization. Morphologies of the as-prepared NH4TiOF3 can be adjusted by the content of PVP as the capping agent. Well faceted and dispersed octagonal NH4TiOF3 microdisks can be successfully synthesized by the facile liquid phase precipitation method with 15 mg mL 1 PVP aqueous solution. Any increase and decrease of PVP
would both inhibit the growth of the {001} facets of crystalline NH4TiOF3 and weaken the crystallinity of products. 3.2. Topochemical conversion from NH4TiOF3 to TiO2 particles 3.2.1. Morphology and microstructure of NH4TiOF3 and TiO2 microdisks As-synthesized NH4TiOF3 samples were transformed into anatase TiO2 samples after heat treatment. That is verified by means of X-ray diffraction, the results of which have not put on here. Morphologies of NH4TiOF3 microdisks and anatase TiO2 converted by heat treatment at 500 1C are analyzed in detail from several SEM images as shown respectively in Fig. 3a–d. The outline of as-prepared NH4TiOF3 particles
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Fig. 3. SEM images of (a) as-prepared NH4TiOF3 and (c) TiO2 microdisks. Low and high-magnification top-view SEM images of representative (b) NH4TiOF3 and (d) TiO2 particle. Insets in (a) and (c) are the side-view SEM images.
(Fig. 3a) has been retained intact after sintering, as presented in general view of TiO2 particles (Fig. 3c). For clarity, a typical microdisk is picked out before (inset of Fig. 3b) or after (inset of Fig. 3d) sintering. These octagonal microdisks are welldefined and highly symmetric with eight nearly equal sides around 2 μm in size. The specific directions and planes of the octagonal shape in crystallography will be discussed below in the TEM analysis section. High-magnification top-view (Fig. 3b) or side-view (inset of Fig. 3a) SEM image of a certain NH4TiOF3 microdisk show a relatively smooth surface and unabridged edges, demonstrating the as-prepared precursor has a compacting structure. For TiO2 microdisk, the top-view (Fig. 3d) image displays discrete and linked nanosheets of dimensions between tens and hundred of nanometers. And the side-view image (inset of Fig. 3c) also confirms the existence of nanoparticles, we speculate, which are sides of nanosheets in the whole large disk with a uniform thickness of about 20 nm. So it is almost credible that TiO2 microdisks consist of much smaller subunits as building blocks. The lattice mismatch between NH4TiOF3 and anatase TiO2 in the z-axis causes loosened packing of subunits [29]; a visible porous structure has obtained as a consequence. Further characterization for the microstructure of NH4TiOF3 microdisks is shown in Fig. 4. The majority of NH4TiOF3 microdisks present the smooth surface and unabridged edges (Fig. 3a and b), while microdisks with a low crystalline quality are also observed (typically in Fig. 4a and b). A distinguishable layered feature is found at the margin of some microdisk, and usually accompany with more evident defects. So the substance in dotted rectangular region from down right corner of Fig. 4b might be a vertical disk. Layered structure of NH4TiOF3 can be obtained from liquid chemical system in the
presence of 2-propanol [16,19]. As elaborated in Refs. [16,29,30], crystal structure of NH4TiOF3 is characterized by inserted ammonium between layers composed by Ti-centered octahedra. It probably causes the layered morphology. In relatively integrated particles, there are bits of interior pores (Fig. 4c). Selected area electron diffraction (SAED) patterns (Fig. 4d and g) from defective or integrated disks display “single-crystal-like” diffraction which can be indexed into [001] zone of NH4TiOF3; that is, {001} facets are successfully exposed. The growth process of such crystals perform not ionby-ion, but via the assembly of clusters. That comes up to a concept emerged within a decade, defined the term “mesocrystal” by Colfen and Antonietti [31], which has attracted intensive interest in crystal design, synthesis and their promising properties [32–34]. It requires crystallographic uniformity of nanocrystals for mesocrystals, therefore their electron diffraction behaves as same as that from single crystals. Fusion of nanocrystals occurred in mesocrystals obscures the difference between the mesocrystal and single crystal (as shown in Fig. 4b and f, respectively) [28]. The representative HR-TEM images demonstrate poorly crystallized NH4TiOF3 particles (Fig. 4e) with mesopores (2–5 nm in size) and well crystallized NH4TiOF3 particles (Fig. 4h). TEM image of a typical TiO2 microdisk (Fig. 5a) displays a loosened structure and the “single-crystal-like” diffraction spots as shown in SAED pattern from the center of particle (Fig. 5b) verified that it is TiO2 mesocrystal. The SAED pattern can be indexed into diffraction spots of the [001] zone [10]. SAED pattern of subunits/subunit from the right margin of particle (down right corner of Fig. 5c) shows identical diffraction spots with that from the central region. The crystallographic coincidence of the diffraction patterns
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Fig. 4. (a) SEM, (b, c and f) TEM, (e and h) HR-TEM images and (d and g) SAED pattern of representative NH4TiOF3 particles. Insets in (a) and (b) are highmagnification images of NH4TiOF3 particles from corresponding dotted rectangular regions.
suggests that the whole architecture is a crystallographically ordered assembly of NSBBs. The high-magnification TEM image of a nanosheet (Fig. 5e) shows a lattice spacing of 0.189 nm which is identical to the (020) and (200) atomic planes of anatase TiO2. The interfacial angle of 901 conforms to the angle between two planes. A tetragonal atomic arrangement on the (001) plane of anatase TiO2 is shown in the corresponding fast-Fourier-transform-filtered TEM image (Fig. 5f) with high clarity. SAED pattern from a vertical microdisk (Fig. 5d) is not so bright because of the thickness but distinguishable with the marked angle of 68.31 which is identical to the theoretical value for the angle between {101} and {001} facets of anatase. That is, the microdisks are mesocrystal in nature but of anatase TiO2. The top and bottom facets of disks are {001} facets and nanosheets as the building blocks are dominated with {001} facets. The eight sides of
regular octagon are perpendicular to the [100] and [110] directions for each orthonormal four sides respectively. Detailed TEM images of TiO2 nanosheets and their assembly are performed as shown in Fig. 6a and b. From the highmagnification TEM image, primary nanosheets with a size about 25 nm exist in isolation of approximate square with round corner or linked into much larger sheets with serrated or smooth edges. The above two type of links (along the [110] or [100] direction) lead to different sides of octagon as labeled in dotted rectangular region I and II. Fig. 6c gives the schematic of possible attachments of nanosheets. 3.2.2. Formation process of octagonal TiO2 microdisks After the discussion of deposition behavior of NH4TiOF3 and the analysis of microstructure of NH4TiOF3 and TiO2 microdisks, we have a hypothesis here about the formation
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Fig. 5. (a) Low-magnification TEM image and (b) SAED pattern of the central region of TiO2 microdisks. TEM images and SAED pattern from (c) the right margin and (d) side of particle. (e) High-magnification TEM image of an individual nanosheet from dotted rectangular region in (c) and (f) the corresponding fast-Fouriertransform-filtered TEM image.
process of octagonal TiO2 microdisk. The illustration is shown in Fig. 7. From the supersaturated (NH4)2TiF6 solution, NH4TiOF3 nuclei form gradually under the controlled hydrolysis of [TiF6]2 with mediation of ethanol; then the interactions between these NH4TiOF3 nuclei drive the attachment of ambient nucleus and create primary clusters which stepwise grow up to nanocrystals [29], as depicted in the “Growth” (the contents inside double quotes corresponding to illustrations in Fig. 7, similarly hereinafter in this section) process. This speculation is based on the mesopores (Fig. 4c and e) which
produced during “Oriented attachment” process in consequence of various shape and size of NH4TiOF3 nanocrystals [28,29], although the boundary of nanocrystals in the final mesocrystal (Fig. 4f) tends to be unclear because of the fusion process (“Fusion into SC”) [28]. The layered feature of assembly and distortion of oriented attachment (“Lamellar loosen”; “Distortion”) strengthen such speculation. Macromolecules of polymer in the solution may act as a matrix inducing the crystallographically oriented assembly [29]. The resulted NH4TiOF3 mesocrystals are exposed with {001} facets whose surface energy is lowered notablely by coverage of the in-situ
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Fig. 6. (a) High and (b) low-magnification TEM images of nanosheets for TiO2 microdisks. (c) Illustration of possible attachments of nanosheets as shown in dotted rectangular region I and II of (b).
Fig. 7. Illustration of formation process of assembly. “SC” is short for single crystal.
generated BF4 and F . As another capping agent, PVP trend to adsorb on the {001} facets for the high polarity, thus further stabilizing the surface [35]. The fact that undercoordinated Ti atoms and their influence on the Ti–O–Ti bond angles makes adsorbates interact selectively with specific surfaces of NH4TiOF3 [36]. Polymer surfactants preferentially adsorb on the {001} surface with relatively large Ti–O–Ti bond angles and for the high polarity. The F [10] and BF4 groups [19,21]
as rich fluorine carriers bond with fivefold-coordinated Ti atoms on {001} facets. The growth of the [001] direction is inhibited and the highly reactive surface can be largely preserved. The residual molecules of PVP may play a role of linking the nanocrystals preferentially on the ab planes with highly orientation. The self-assembly process go in the way of lateral expansion in eight directions at much the same growth rate. As a result, octagonal NH4TiOF3 microdisks deposit from
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Fig. 8. XRD patterns of TiO2 particles converted by sintering at different temperatures and the calculated ratio of the 004 diffraction intensity to the 101 diffraction intensity.
the reaction liquid phase. It can be topochemically converted to TiO2 by subsequent heat treatment because of the similarity of Ti atoms arrangement between NH4TiOF3 and TiO2 on {001} facets. The square-shaped nanosheets (Fig. 6a) dominated with {001} facets (see in Fig. 5e) serve as NSBBs in the obtained TiO2 mesocrystals (“Sintering”). Meanwhile, voids will appear since the dehydration and the shrinkage of unit cells of TiO2 compared to those of NH4TiOF3 in the z-axis causes contraction of the primary nanoparticles. As the external thickness barely alters, visible voids formed by combination of much smaller voids appear among the nanoparticles. Thus octagonal TiO2 microdisks assembled from nanosheets with exposed {001} facets are obtained. 3.2.3. Effect of sintering temperature on conversion of TiO2 particles Interestingly, sintering temperature for the heat treatment of NH4TiOF3 microdisks has a considerable effect on TiO2 particles. XRD patterns (Fig. 8) of samples (sintered at 500, 600 and 700 1C) indicate all of them are pure anatase TiO2. As the sintering temperature increases from 500 to 700 1C, XRD peaks get sharper and the relative diffraction intensity of 004 peak remarkably increase, while the ratio of the 004 diffraction intensity to the 101 diffraction intensity for randomly oriented anatase powder samples is averagely 0.2 as recorded in JCPDS data (PDF#21-1272). The significantly enhanced 004 peak suggests the larger exposure of {001} facets or the possibility of self-induction of TiO2 flat parallel to the sample holder. Fusion of crystallines and stability of crystal planes are extra sensitive to sintering temperature. So we suppose that the TiO2 nanosheets perhaps have a greater extent of lateral fusion at a higher temperature. In a similar research, Zhou et al. [27] have already pointed out appropriate temperature range and highlighted the advantage of rapid heating. It is different with this work, suggesting that effects of heating parameters on topochemically conversion change with precursor (NH4TiOF3)
Fig. 9. (a) Adsorption capacity and photocatalytic activity of TiO2 samples prepared with different content of PVP and after sintering at 500 1C and (b) adsorption capacity and photocatalytic activity of P25 TiO2 and TiO2 microdisks after heat treatment at different temperatures.
precipitated from certain reaction solution. Further investigation on the structure of these particles will be our next work. 3.3. Photocatalytic activity Photocatalytic activity of TiO2 powders was evaluated in terms of degradation of methylene blue (MB) dye solution under UV-light irradiation. The adsorption behavior of MB molecules in dark on different TiO2 samples and degradation efficiency of TiO2 samples under UV-light irradiation are integrated into account. Fig. 9a shows the adsorption capacity and photocatalytic activity of TiO2 samples prepared with different contents of PVP and after sintering at 500 1C. TiO2 microdisks (i. e. heat-treated s-P-15, donated as s-P-15-HT500) exhibit much higher adsorbability (24% of MB molecules) than those of other TiO2 samples. The degradation efficiency of TiO2 microdisks is also enhanced. Although the sizes of s-P-0-HT500 and s-P-5-HT500 are smaller than that of TiO2 microdisks, their activities are extremely low. With the
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existence of some broken sheet-like particles, S-P-10-HT500 and s-P-20-HT500 show minor adsorption capacity. It has been already confirmed that {001} facets are more favored for dissociative adsorption of reactant molecules than {101} facets [37]. The chemical dissociation of adsorbed water molecules is activated on {001} facets [38]. These surface interactions are benefit to transfer the photogenerated carrier and provide more active sites for degradation of pollutants. In addition, superstructure of assembly with uniform subunits and crystallographic order is believed to be promising for the transfer of reactants and degradation products. Fig. 9b shows the adsorption capacity and photocatalytic activity of TiO2 microdisks after heat treatment at different temperatures. Commercial P25 TiO2 is used as a benchmark sample. The degradation efficiency of TiO2 microdisks slightly increases with the rise of sintered temperature. The adsorption capacity in dark of TiO2 microdisks sintered at 700 oC is extremely high because of the larger exposure of {001} facets. The complete adsorption of MB molecules can be accomplished in a relatively transient process at the same time of photocatalysis, drastically promoting catalytic activity to a high efficiency even better than P25 TiO2. We perform the comparison with P25 in order to offer a referable evaluation of our samples. However, it is unfair to ignore the enormous difference between them in size, crystal phase and related properties while draw the conclusion that these TiO2 microdisks are less active than P25. Micro-sized TiO2 organizations are easy to be separated from the suspension by sedimentation. The residual adsorbed MB molecules on separated TiO2 microdisks can be degraded within less than 12 min under the irradiation of UV light. Photocatalytic activity has been enhanced through facet-controlled synthesis of anatase TiO2, and the excellent adsorption capacity of TiO2 microdisks organizations may be applied in prospective applications such as catalysis, dye-sensitized solar cells or can be substrate materials in related areas. 4. Conclusion As a summary, octagonal TiO2 microdisks assembled from NSBBs with exposed {001} facets have been successfully synthesized. Adjusting morphology of NH4TiOF3 is realized by a PVP-assisted liquid phase precipitation method from (NH4)2TiF6. Transformation from NH4TiOF3 to anatase TiO2 is accomplished by solid topochemical conversion. The in-situ generated BF4 and F are proved to be milder alternatives as the capping agents, instead of adscititious highly poisonous and corrosive HF. PVP provides dual functions: one is stabilizing the high-reactive facets and the other is linking the nanocrystals to alignment in three-dimensions. The formation process of TiO2 microdisks is discussed in detail. TiO2 microdisks show both an excellent adsorption capacity in dark and enhanced photocatalytic activity owing to the superstructures of assembly of nanosheets and exposed {001} facets. TiO2 microdisks sintered at 700 1C exhibits a comparable activity to that of P25. The facet-controlled synthesis of anatase TiO2 with high adsorption capacity and easy recycling
of products may have promising features for other chemical synthesis and applications.
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