Control of Exposed Facet and Morphology of Anatase Crystals through

Dec 19, 2013 - Control of Exposed Facet and Morphology of Anatase Crystals through TiOxFy Precursor Synthesis and Impact of the Facet on Crystal Phase...
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Control of Exposed Facet and Morphology of Anatase Crystals through TiOxFy Precursor Synthesis and Impact of the Facet on Crystal Phase Transition Yubao Zhao,†,∥ Yifan Zhang,†,∥ Hongwei Liu,§ Hongwei Ji,† Wanhong Ma,† Chuncheng Chen,*,† Huaiyong Zhu,‡ and Jincai Zhao† †

Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Discipline of Chemistry, Queensland University of Technology, Brisbane, Qld 4001, Australia § School of Materials Science and Engineering, Guangxi University, Nanning 530004, P. R. China S Supporting Information *

ABSTRACT: Anatase crystals with 30% and 80% of {001} facet exposures are prepared by decomposition of two fluorinecontained precursors (TiOxFy). The two precursors were obtained via solvothermal process by controlling the F− and Ti4+ concentrations and had a flower-like and hexagonal pencillike structure, respectively. The two anatase products by calcination of the corresponding precursors at 700 °C have unique morphologies: sheet-like single crystals with square pores and oriented assembly of truncated-octahedron anatase crystals, respectively. Both of them are inherited from the morphologies of the precursor particles. We reveal for the first time that the phase transition temperature of the crystal with larger {001} facet percentage is lower, and the in situ SEM experiments indicate that numerous defect sites first form on {001} facet during the phase transformation of anatase to rutile, while the {101} facet on the same crystal remains smooth. The formation of defect sites is attributed to the preferential (relative to {101} facet) diffusion of bulk vacancy defect toward the {001} facet and agglomeration on this facet during the annealing, which is an important process to start the anatase to rutile phase transformation. KEYWORDS: shape controlled synthesis, TiO2, photocatalyst, anatase to rutile phase transition

1. INTRODUCTION

As an emerging technique, synthesis of anatase via decomposition of precursors was recently reported to produce some anatase crystals with novel morphologies. By titanate wire precursors, anatase nanowires have been successfully synthesized, which has provoked several substantial researches on the structure−activity relationships.16−18 Because the precursors usually have their intrinsic crystal structures and morphologies that are different from anatase and the shape of anatase crystals would be sensitively affected by the morphology of precursors, the precursor-based anatase crystal fabrication affords the opportunity for building crystals with unique morphologies and facets exposure that cannot be realized by structure directing reagent. In a previous study, we solvothermally prepared a series of pure anatase crystals with controllable facet exposure by using fluoride anion as the structure directing reagent, which enable us to have a detailed investigation on the facet-dependent

Anatase (tetragonal, space group I41/amd) is one of the four polymorphs of titanium dioxide. It has been widely utilized in photocatalytic degradation of environmental pollutants, water splitting, and the synthesis of organic chemicals,1−3 due to its various advantages, such as abundance, nontoxicity, and high activity. The surface structure of anatase, which depends substantially on the exposed crystal facets, is decisive to its photocatalytic performance.4,5 To obtain anatase with new properties and to investigate the surface structure−photocatalytic activity relationships, controlled fabrication of crystals with different exposed facet is desirable and necessary, which however is still a great challenge so far.6−11 To control the exposed crystals facet, the traditional methods employ the structure directing reagents to adjust the surface energy and promote the growth of certain crystal facet. For example, in hydrothermal synthesis of titania, fluoride ion and diethylenetriamine could induce the formation of anatase {001} facet, which is distinct from the ordinary {101} facet in the photocatalytic performances.10,12−15 © 2013 American Chemical Society

Received: September 12, 2013 Revised: December 13, 2013 Published: December 19, 2013 1014

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dioxygen activation during the photocatalytic reaction.13 In this study, we report that, by controlling the F − and Ti 4+ concentrations in ethanol solutions that undergo solvothermal treatment, one can incorporate F− into the products and produce fluorine-contained precursors with different morphologies. These precursors can be converted to anatase crystals with unique morphologies and very different crystal facet exposures by calcination: one product is the anatase single crystal sheets with square pores through the crystals; the other is the directional assembly of truncated-octahedron anatase crystals. Also importantly, the controllable synthesis of anatase crystals with different facet exposure enables us to examine the effect of crystal facet on essential properties of anatase, such as the phase transition temperature of anatase to rutile. We find that the crystal with larger {101} facet percentage exhibits higher phase transition temperature. An investigation by in situ SEM clearly confirms that numerous defect sites first formed on {001} facet during the phase transformation of anatase to rutile.

determines largely the {001} facet exposure of the crystals.11,13 However, when the concentration of Ti4+ and HF was increased to ca. 0.7 M and 2.5 wt %, respectively, and the solvothermal reactions were carried out in ethanol solvent, fluorine-contained precursors were formed. At HF concentration of 2.57 wt %, a flower-like structure assembled by sheets with tens of nanometers thickness is obtained (precursor A, Figure 1a−c). XRD characterization demonstrates that

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The fluorine-contained TiO xF y precursors with different morphologies were prepared by solvothermal method. Then 9.80 mL of titanium n-butoxide (Alfa Aesar) was dissolved in 29.44 mL of ethanol (99.9%). Given amount of HF aqueous solutions (40% v/v, J&K Scientific) were slowly dropped in the ethanol solution under vigorous stirring (HF is highly corrosive and must be operated in well ventilated fume hood with caution). The mixture was then transferred into autoclaves with a volume of 45 mL. The autoclaves were rotating in the oven at 180 °C for 12 h. By controlling the amount of HF, we obtained two fluorine-contained precursors with difference crystalline phases and morphologies: 2.75 mL of HF aqueous solution was used for synthesis of precursor A, and 2.93 mL of HF solution was used for the preparation of precursor B. The fluorine-contained precursors were washed with sodium hydroxide solution and water until no fluoride ions were left in the filtrate (measured by ion chromatograph), and then dried for use. These two precursors were calcined at 700 °C for 3 h in the tube furnace with air flow to transform into target anatase crystals (represented by A700 and B700, respectively). The truncated octahedron crystals with {001} facet percentage of 32% and 63% in this experiment was obtained by the method described in our previous work (ref 13), which is adopted from the reports by Yang et al.9,11 2.2. Characterization. The morphology of the samples was determined by scanning electron microscope (SEM) on a Hitachi S4300 with an accelerating voltage of 15 kV and high resolution transmission electron microscope (HRTEM) on a Tecnai F30 operating at 200 kV. In situ high temperature X-ray diffraction (XRD) patterns of the catalysts were obtained on a PANalytical X’Pert PRO MPD diffractometer with Cu radiation (Cu Kα = 0.15418 nm). The samples were heated in Anton Paar HTK 16 High-Temperature Chamber, and the temperature of the chamber was controlled by Anton Paar TCU 2000 Temperature Control Unit. The temperature program: temperature ramping rate was 20 °C/min; when the temperature reached the preset value, there were 5 min for temperature stabilization before XRD scanning. The scan step was 0.0167° and the scan speed was 8°/min. The contents of rutile in the samples were calculated from WR = AR/(0.884AA + AR), in which AA represents the integrated intensity of anatase (101) diffraction peak and AR represents the integrated intensity of rutile (110) diffraction peak.

Figure 1. SEM images of precursor A (a−c) and precursor B (d−f) with different magnifications. SEM image f shows the cross-section of the hexagonal column.

precursor A consists of anatase and face-centered cubic (fcc) TiOF 2 (Figure S1). When HF concentration in the solvothermal system was slightly increased just to 2.74 wt %, a distinctly different product with a hexagonal pencil-like shape is obtained (precursor B, Figure 1d−f). As shown in Figure S2, this precursor exhibits sharp diffraction peaks indicating its high crystallinity. TEM characterizations demonstrate that the hexagonal pencil-like precursor is hexagonal close packed (hcp) TiOF2 (a = 0.3532 nm, c = 1.3232 nm) (See details in Figure S3). As indicated by the XRD patterns in Figure S4, after calcination at 700 °C for 3 h in the air flow, both precursors A and B will transform to pure anatase phase (denoted by A700 and B700, respectively). Sample A700 from the flower-like precursor A possesses an interesting sheet-like morphology structure with square pores (Figure 2a,b). As shown in Figure 2c, the lattice distance of 0.37 nm corresponds to the spacing of (010) planes; and the selected area electron diffraction presents a typical pattern of anatase with crystal zone axis of [001], which further implies that the sheets (petals) in precursor A are composed of anatase and that TiOF2 phases assembling along [100] and [010] directions are due to the similarity of the lattice constant of the two phases (tetragonal anatase a = b = 0.3785 nm; cubic TiOF2 a = b = 0.3798 nm). During

3. RESULTS AND DISCUSSION As reported in the literature, in the mixed solution of isopropanol/H2O (v/v = 1) containing about 2.3 mM Ti4+ and 0.15 wt % HF, solvothermal process directly produced pure anatase crystals, and the concentration of hydrogen fluoride 1015

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depends on the facet. In the present study, the two anatase samples, A700 with 80% of {001} facet and B700 with only 30% of {001} facet, were investigated using in situ high temperature XRD. As shown in Figure 3, for sample A700, the transition from anatase phase to rutile phase begins at 850 °C. The contents of

Figure 2. (a,b) SEM and TEM images of sample A700; (c) HRTEM image of Sample A700, the inset is the SAED pattern obtained with the electron beam perpendicular to the predominant exposed crystal facet of sample A700; (d−f), SEM images of B700.

calcination, the TiOF2 is transformed to anatase and merges with the original anatase phase. This process would result in the loss of the TiOF2 phase in cubic shape, which would reasonably leave the square holes in the sheet. These characterizations confirm that the predominant exposed surface of the sheet-like crystal is {001} facet, which accounts for more than 80% of the exposed surface. Furthermore, the whole micrometer-sized sheet is a single anatase crystal, rather than assembly of nano crystals. Evidently, the sheet-like shape of A700 is inherited from the flower-like morphology of precursor A. During the calcination, the sheet structure of the petals in precursor A is retained. This result suggests that the precursor-based method is a promising way to creating mesoporous single crystals in addition to template based routes.19 The square pores in the anatase single crystal sheets have never been realized by the method employing a template or structure directing reagent to control the crystal shapes. Calcination of the pencil-shaped precursor B produces an aggregation of oriented anatase crystals (Figure 2d−f). These in situ formed crystals have the typical anatase morphology of truncated-octahedron. An important feature is that these crystals assemble orderly along [010] direction, which has not been realized by wet chemistry method yet. The percentage of {001} facet of the crystals is ca. 30%. It is found that the aggregates of oriented anatase crystals have a similar morphology to that of the pencil-shaped precursor particles. Furthermore, there are some similarities in crystal structures between anatase and hexagonal TiOF2. The atomic configuration of the close-packed (0001) plane of hcp-TiOF2 is similar to that of the closed-packed (110) plane of anatase. By elimination of F atoms in hexagonal TiOF2 structure, this precursor could easily convert into anatase without large strain or structural modification. Similar situation of crystal transformation also happens in cubic TiOF2 (precursor A to sample A700). These results demonstrate that the morphology of the precursor particles could be retained during the transformation in calcinations. Hence, it is possible to control the morphology of product anatase crystals through the precursor synthesis. The obtained anatase crystals with different percentage exposed {001} facet provide us opportunities to study the effect of crystal facet on the physiochemical properties of TiO2. For instance, phase transition of anatase to rutile is a process attracting scientific and technologic attention.20 It is of fundamental interest to know whether the phase transition

Figure 3. XRD patterns of sample A700 (a) and B700 (b) under different temperatures. (c) Content of rutile in the samples under different calcination temperatures measured by in situ high temperature XRD. The rutile contents of the samples were calculated by using WR = AR/(0.884AA + AR), in which AA represents the integrated intensity of anatase (101) diffraction peak and AR is the integrated intensity of rutile (110) diffraction peak. The same temperature rising and data acquisition program was used for the measurements of all the samples.

rutile and anatase were estimated by integrating the intensities of rutile (110) peak and anatase (101) peak.21 Sample A700 contains 1.9 wt % of rutile at 850 °C and is transformed thoroughly to rutile phase at 1050 °C. While for sample B700 the transition of anatase to rutile starts at 950 °C, there is still 2.5 wt % anatase phase left at 1100 °C. The phase transition temperature of sample A700 is ca. 100 °C lower than that of sample B700. The observation suggests that it is easier to initiate phase transition to rutile at the anatase {001} facet than at {101} facet. In order to avoid the possible effect of crystal size on the phase transition temperature, the truncated-octahedron anatase crystals with different {001} facet percentages but similar 1016

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However, a careful examination on the calcined {001} facet shows a remarkable phenomenon that the flaws are actually in shape of regular truncated-square pyramid in upside-down manner (Figure S6). These flaws are evidently corresponding to the crystal structure of anatase,11,13 which can rule out the possibility that the flaws are derived from the volume contraction during phase transition. In addition, no newformed TiO2 particle is observed in the whole SEM image with the appearance of the holes, which may exclude the loss of titania by Ostwald ripening mechanism for the formation of the flaws on {001} facet. The positron annihilation experiment by Ghosh et al. showed that the phase transformation from anatase to rutile starts from the agglomeration of the small vacancy defects into bigger vacancy clusters.23 Therefore, it is plausible that the appearance of the surface flaws should result from the diffusion of bulk vacancy defects toward the {001} facet and agglomeration on this facet to generate the observed flaws during annealing. The {001} facet possesses substantially more distorted bond angle of Ti−O−Ti and larger number of 5coordination Ti on anatase {001} facet, and the surface energy of anatase {001} facet is 0.9 J/m2, which is much higher than the {101} facet (0.44 J/m2).24 The higher surface energy of anatase {001} facet may contribute to the preferential diffusion of the bulk vacancy defects toward {001} crystal facet. The in situ SEM studies provide further solid support to the facet-dependence of phase transition. However, the fundamental reason for this phenomenon is still to be clarified. It was found that the retardation in the annealing of defects (in argon atmosphere) would increase the phase transition temperature.23 It is therefore reasonable to deduce that the lower phasetransition temperature for the anatase crystals with higher {001} facet percentage should be closely related to the easier release of the defects from {001} facet. After the defects are annealed out, the followed phasetransition process should also be surface-sensitive. The effect of surface defects or impurities on phase transition from anatase to rutile has been well documented.25,26 For example, CuO, CoO, Li2O, and Na2O on anatase surface and H2 reduction of anatase surface were reported to accelerate the phase transition process, while sulfate and phosphate ions on the anatase surface could inhibit the phase transformation process.27 Particularly, many studies suggested that the surface oxygen defects level could sensitively affect the kinetics of anatase to rutile phase transformation.28 For instance, the anatase particles synthesized at higher pH condition possess higher surface oxygen defects and therefore exhibit easier phase transition, comparing to that obtained in lower pH condition.29 La2O3 could occupy the defect sites and restrain it from diffusing on the anatase surface, and retard the phase transformation.30,31 Therefore, the surface defects (flaws) agglomerated on {001} facet may also play an important role in decreasing the phase transition temperature. One of the explanations for this is that the presence of these surface flaws would activate the surface and enhance the formation of the anatase {112} twin boundaries between two activated neighboring anatase nanoparticles or flaws since these twin boundaries are reported to be essential for the phase transition according to the coalescence mechanism.32,33

crystal size were prepared according to the methods reported earlier (Figure 4, insets).11,13 As shown in Figures 4 and S5, the

Figure 4. Variation of rutile contents with temperature for samples a and b with {001} facet percentage of 63% and 32%, respectively. The insets a and b are the corresponding SEM images.

phase transition temperature of anatase with 63% {001} facet (sample a) starts at 1000 °C, while for that with 32% {001} facet (sample b), this temperature increases to 1100 °C. These results are consistent with those observations on samples A700 and B700. All the results confirm that it is easier for the phase transition from anatase to rutile to take place at {001} facet. To further understand the difference between {001} and {101} facet in the phase transition, truncated-octahedron anatase single crystals with 32% {001} facet, which have comparable {001} and {101} facet size, were investigated by SEM with in situ specimen heating. As shown in Figure 5, the smooth surface of {001} facet becomes full of flaws after heating at 980 °C for 1 h, while the {101} facet on the same crystal remains smooth. One possible reason for the formation of flaws is the volume contraction during anatase to rutile phase transition because of the difference in density of rutile and anatase phase (relative density: rutile 4.25; anatase 3.89.22).

4. CONCLUSIONS Anatase crystals with substantially different {001} facet exposure (from 30% to 80%) and novel morphologies (sheetlike single crystals with square pores and directional assembly of truncated-octahedron anatase crystals) can be prepared by

Figure 5. In situ high temperature SEM images of the anatase crystals with {001} facet percentage of 32%. (a,b) SEM images of the sample before calcination; (c,d) SEM images of the sample calcined under 980 °C for 1 h. See the Supporting Information for high resolution images (Figure S6). 1017

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decomposition of two fluorine-contained precursors in distinct morphologies. The product inherits the particle morphology of the precursors. We reveal that the temperature at which the phase transition of anatase to rutile takes place depends on the percentage of {001} facet of the anatase crystals. The facile formation of defect sites on the {001} facet may contribute to the ease in phase transition. This precursor-based anatase fabrication method has opened up a new avenue for constructing novel nanostructures, so as to reveal more indepth structure−physiochemical properties relationships.

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ASSOCIATED CONTENT

S Supporting Information *

TEM characterizations and XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(C.C.) E-mail: [email protected]. Author Contributions

∥ These authors (Yu.Z. and Yi.Z.) contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial supports of this work from 973 Projects (No. 2010CB933503) and NSFC (Nos. 20920102034, 21077110, and 20877076) and CAS.

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