Formation and Evolution of the High-Surface-Energy Facets of

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Formation and Evolution of the High-Surface-Energy Facets of Anatase TiO2 Baohua Zhang,‡ Fan Wei,‡ Qian Wu,† Lingyu Piao,*,† Min Liu,*,† and Zhong Jin*,‡ †

CAS Key Laboratory for Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology, 11 Zhongguancun North 1st Alley, Beijing 100190, China ‡ Supercomputing Center, Computer Network Information Center, Chinese Academy of Sciences, 4 Zhongguancun Nansijie, Beijing 100190, China S Supporting Information *

ABSTRACT: The anatase TiO2 single crystal exposed highsurface-energy facets have attracted much attention. However, the evolution mechanism and process of the high surface energy facets are still not clear. Here, based on the analysis of experimental results and theoretical calculation results, the evolution process and balanced coexistence for high-surfaceenergy TiO2 facets, such as {001} and {110} facets, was well explicated. Thus, this work will help for better understanding and controlling of the morphology of metal oxide crystals with different facets.

1. INTRODUCTION Anatase TiO2, as one of the most important transition metal oxides, has attracted considerable attention because of its wide applications in catalysis, photovoltaic cells, self-cleaning devices, sensors, Li-ion battery materials, optical emission, water splitting, paints, paper, and cosmetics.1−9 These applications originate from its unique physical and chemical properties, which depend not only on the crystal phase and particle size but also on the exposed facet.10−14 The surface energies of its clean {110}, {001}, and {101} facets are 1.09, 0.90, and 0.44 J· m−2, respectively. However, for anatase TiO2, both theoretical and experimental studies have revealed that minority {001} facets are much more oxidative reactive than {101} facets.12−15 As a result, synthesized anatase nanocrystals are usually dominated by energetically stable {101} facets. The production of anatase crystals with exposed {001} facets is important and challenging. For this reason, the interest in controlling the synthesis of TiO2 single crystals with {001} facets has been increased.15−31 Yang and coworkers made an important breakthrough in the fabrication of anatase single-crystals with 47% of the highly reactive {001} facets by using hydrofluoric acid as a capping agent.15 Soon after, they obtained anatase TiO2 nanosheets with 64% of the {001} facets by using 2propanol as a synergistic capping agent and reaction medium together with hydrofluoric acid.16 Subsequently, anatase TiO2 nanosheets with 89% exposed {001} facets were synthesized by Han and coworkers using a similar strategy.21 Interestingly, {110} facets with the highest surface energy in anatase TiO2 can also be exposed by controlling the experimental conditions. For example, members in our group © XXXX American Chemical Society

prepared {001} and {110} facet-exposed TiO2 single crystals for the first time by using metal titanium powders as raw materials, hydrofluoric acid as a capping agent, and hydrogen peroxide as a synergistic capping agent and buffer medium.22 Later, Pan and coworkers reported TiO2 single crystals with exposed {110} facets by using titanyl sulfate as raw materials and hydrofluoric acid as a capping agent.32 Although many researchers have conducted controllable synthesis of TiO2 single crystals with high-surface-energy facets and studied the influences of the experimental parameters, it is still unclear what influences the morphology and size of exposed surface facets of TiO2 single crystals. Here we calculate the surface energies and differentiate adsorption energies of {110}, {001}, and {101} facets with different F adsorption ratio, 25, 50, 75, and 100%, by using firstprinciple quantum chemical calculations. Through the theoretical calculation, it was found that adsorbed fluorine ions could reduce the surface energies of facets and the exposed ratio of facets depended on the differentiate adsorption energies. On the basis of these theoretical calculation results, we prepared and characterized the highest percentage, 11%, of {110} facetexposed anatase TiO2 single crystals by controlling the experimental conditions.33 This will contribute greatly to controllable synthesis of TiO2 crystals with exposed highsurface-energy facets, and the study method in the present work is also very helpful for other functional crystals. Received: January 5, 2015 Revised: February 26, 2015

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2. EXPERIMENTS AND THEORETICAL CALCULATION 2.1. Synthesis of TiO2 Crystal with Exposed {001} Facets. In a typical synthesis, 0.01 g of Ti powder was dissolved in aqueous solutions of HF (0.1 mL, 40 wt %) and 27 mL of H2O (18.0 MΩ cm). Then, 3 mL of H2O2 (30%) was transferred to the previously described solution. The mixture was transferred to a Teflon-lined autoclave. Then, the mixture was kept at 150−180 °C for 0.25−10 h for different crystals with different {001} facets ratio, respectively. After the solution was cooled to room temperature, the products were collected by centrifugation and washed with deionized water several times until the pH of the solution was neutral. The samples were then dried at 80 °C for 10 h and calcined at 500 °C for 2 h in air, respectively. To obtain the TiO2 single crystals with different percentage of {001} facets, we adjusted the amount of HF from 0.1 to 0.15 mL. When the amount of HF exceeded 0.15 mL or reaction temperature exceeded 180 °C, the TiO2 crystals with selective erosion {001} facets were obtained. We have controllably synthesized the TiO2 crystal-like Chinese classic window lattice when reaction time was extended (from 1 to 10 h). 2.2. Synthesis of TiO2 Crystal with Exposed {110} and {001} Facets. In a typical synthesis, hydrofluoric acid (40 wt %, 0.06 mL) was placed into a Teflon-lined autoclave containing 0.01 g Ti powder and 27 mL of deionized water, and the mixture was stirred vigorously for 10 min. After the addition of 1.5 mL of H2O2 (30 wt %) to the solution, the mixture was maintained at 150 °C from 0.25 to 2.5 h to obtain the TiO2 crystal with different percentage {110} and {001} facets. The products were collected by centrifugation, washed with deionized water several times until the pH of the solution was neutral, and then dried at 80 °C for 10 h and calcined at 600 °C for 2 h in a static air atmosphere in a furnace to remove the surface fluorine. 2.3. Characterizations. The morphologies of the samples were determined by scanning electron microscopy (SEM) on a Hitachi S-4800 system. 2.4. Methodology. The present computational work will determine the adsorption modes, surface free energies, and adsorption energies of fluorine on {001}, {110}, and {101} surface of anatase TiO2 with different F adsorption ratio: 25, 50, 75, and 100%. All surfaces are modeled with (2 × 2) periodic slab models for the calculation and a symmetric adsorption on both sides of the slabs. All atoms are relaxed without any constraint. All calculations have been carried out using the density functional theory (DFT) within the generalized-gradient approximation (GGA) with the exchange-correlation functional of Perdew−Burke−Ernzerhof (PBE).34,35 This has been implemented in the Vienna Ab Initio Simulation Package (VASP), which spans reciprocal space with a plane-wave basis and uses the projector-augmented wave (PAW) method.36 For the plane-wave basis set, a cutoff of Ecut = 450 eV has been used. We have used an 11 × 11 × 11 Monkhorst−Pack k-point mesh for bulk anatase, 5 × 5 × 1 for (2 × 2) slabs and 3 × 3 × 3 for dimers of F for the energy calculations. During the relaxations, all structures have been relaxed to an energy convergence of 10−4 eV and forced convergence of 0.05 eV/Å. In the case of slabs, the vacuum space is larger than 15 Å, and for the dimers of F, a cubic unit cell with a = b = c = 16 Å has been employed.

3. RESULTS AND DISCUSSION The TiO2 single crystals obtained under different conditions were investigated by XRD and Raman, and all were confirmed

Figure 1. Clean and different number of F− terminated atomic structural models of {001}, {110}, and {101} surfaces. Ti, O, and F atoms are labeled a−c, respectively.

to be the anatase phase. Figure S1 in the Supporting Information (SI) shows the XRD pattern of the typical TiO2 single crystals with {001} and {110} facets. The synthesized TiO2 was well-crystallized anatase, and the main diffraction peaks in the spectrum were identical with the standard cards (JCPDS 1-562). The Raman spectrum also shows that the TiO2 products were the pure anatase phase. No surfaces defects can be revealed from the Raman spectrum. The photoreactivities of above typical TiO2 crystals were estimated by degradation of MB under UV light irradiation. The photoreactivities results are shown in Figure S3 in the SI. The higher the percentage of {001} facets, the higher the photoreactivity. From Figure S4 in the SI, the specific activity of anatase TiO2 crystal with exposed {110} and {001} facets was approximately two times that P-25 and was significantly higher than that for only {001} facets. The high-energy facets of the TiO2 anatase single crystals have an important influence on their photocatalytic reactivities. On the basis of the analysis of experimental results and theoretical calculation results, we analyzed the evolution process and balanced coexistence for high-surface-energy TiO2 facets, such as {001} and {110} facets. In this work, the calculated cohesive energy of TiO2 in bulk anatase is −21.94 eV, which is very close to Yang’s result (−21.60 eV).15 The surface energies of clean {001}, {110}, and {101} surfaces are 0.91, 1.03, and 0.37 J/m2, respectively. The results are close to Yang’s work (0.93 J/m2 for {001} surface and 0.39 J/m2 for {101} surface) and Diebold’s result (1.09 J/ m2 for {110} surface).11,15 Therefore, the methods and models we used are reliable, and the accuracy of the following calculations can be ensured. The models of (2 × 2) slab have been build for these three surfaces with different ratio of fluorine on the slab to examine surface free energies and adsorption energies of fluorine on {001}, {110}, and {101} surfaces of anatase TiO2 with different adsorption ratio. For example, for {001} surfaces, 25% B

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Figure 2. Surface free energies (a) and differentiate adsorption energies (b) for {001}, {110}, and {101} facets.

Figure 3. SEM images of TiO2 single crystals prepared with different reaction time: (a) 25, (b) 45, (c) 65, (d) 80, (e) 100, and (f) 150 min.

Figure 6. FE-SEM images of TiO2 single crystals obtained at different reaction temperature: (a) 150, (b) 180, and (c) 200 °C.

Figure 4. SEM images of anatase TiO2 crystals prepared with different amount of HF: (a) 0.10, (b) 0.12, and (c) 0.15 mL.

Figure 5. FE-SEM images of TiO2 single crystals obtained at different HF concentration: (a) 0.15 and (b,c) 0.2 mL.

adsorption ratio indicated that two fluorines symmetrically have been adsorbed on both sides of the (2 × 2) slab. The optimization geometry models with different number of fluorines are shown in Figure 1. Figure 1 shows the clean and different number of F−terminated atomic structural models of {001}, {110}, and {101} surfaces. Because of the strong attraction of F−Ti and repulsion of F−O, the F-terminated relaxed slab models are

Figure 7. FE-SEM images of TiO2 single crystals obtained at different reaction times: (a) 1, (b) 3, (c) 5, (d) 7, and (f) 10 h, respectively. Red dotted line represents the reversed corner of the etched quasi-square.

distorted relative to the clean ones that bridged titanium on surface moves outward and bridged oxygen inward, and the C

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Coexistence of the {110} facets and {001} facets in equilibrium (about the growth of {110} facets) has been observed. At the beginning of reaction, because of higher hydrofluoric acid concentration, the dynamic factor is dominant in this stage. The surface energy of the clean {001} facets is not the biggest, but the adsorption energy undergoes the largest decrease from clean surface to the one with 25% of fluorine adsorption. So, in the initial stage, adsorption occurs primarily on {001} facets until fluorine adsorption reaches ∼30% (Figure 2). After that, the adsorption ability of {001} facets is weaker than that of {110} facets because the differentiate adsorption energy of {110} facets is greater than that of {001} facets. So {110} facets are emerged and grown obviously due to their largest differentiate adsorption energy, which is in accord with the experimental phenomena shown in Figure 3a−c. The {110} facets begin to appear and grow after {001} facets with a certain stage of growth. At the same time, the crystal size has also become larger. This is consistent with the Wullf construction law and experimental result that the distance between {110} facets and crystal center is the shortest. According to Figure 2a, the surface energy of {001} facets with 25% fluorine adsorption is larger than that of {110} facets with 75 and 100% fluorine adsorption. Because of the requirements of crystal stabilization, the energy of {001} facets will decrease with the aid of outer environment such as high reaction temperature. {001} facets continue to be adsorbed by fluorine, and the adsorption proportion reached ∼45. At this time, the surface energy of {001} facets reduced to 0.228 J/m2, and this value is equal to and will be less than that of {110} facets with complete adsorption. Therefore, the growth of {110} facets ceased after {001} facets obtained 45% absorption. This can be verified by the experimental result shown in Figure 3d. The distance between {001} facets and the crystal center is shorter than that of {110} facets. If the adsorption percentage of F− on {110} facets reached 75−100%, while it reached 25−45% on {001} facets (the surface free energy and differentiate adsorption energy are close between {001} and {110} facets), then the growths of {110} and {001} facets are competitive and also synchronous. When {001} facets get 45% F− adsorption, the surface energy decreases to 0.228 J/m2, which is equal to that of saturation adsorption on the {110} facets. As the reaction proceeds, when the adsorption density of F− on {001} facets is 50%, the surface energy of {001} facets decreases to 0.18 J/m2. This is lower

Figure 8. (a) Bright-field TEM image of hollow single crystal recorded along the [001] axis. (b) SAED pattern of the hollow single crystal. (c) HRTEM image recorded from the hollow single crystal with [001] orientation. (d) Fast-Fourier-transform-filtered TEM image recorded from the dotted rectangular area in panel c.

distortion degree becomes larger as the F-adsorption ratio grows. This is consistent for {001}, {110}, and {101} surfaces. The surface free energies and differentiate adsorption energies for all the models are calculated, and the results are shown in Figure 2. The surface free energy, which is a sign of thermodynamically relative stability of facets, gradually decreases with the increase in adsorption ratio because of bonding energy DF−F < DF−Ti (Figure 2a). It indicates that 0 0 stability gradually increased with more F−Ti coordination. So the existence of F stabilizes the surfaces. Figure 2b is differentiating adsorption energy, which is a dynamic factor that indicates the adsorption ability of F on the surfaces relative to its previous adsorption state. The higher the differentiate adsorption energy, the easier the adsorption of F on the surface and vice versa. From Figure 2, we can find that the change of surface free energy does not quite agree with the change of differentiate adsorption energy for different surfaces. As we all know, the rate of surface growth is a balance between dynamic and thermodynamics effects. On the basis of these theoretical calculation results and our previous experimental results, we can explain some experimental phenomenon.

Figure 9. Speculated growth mechanism of {001} and {110} for anatase TiO2. D

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compressed flat and center perforated morphology in the experiment, which is in Figures 5 and 6. There are two conditions to cause this erosion phenomenon: one is the enough fluorine (TiO2 single crystals obtained at 0.2 mL HF concentration shown in Figure 5b,c are more etched than that at 0.15 mL of HF concentration shown in Figure 5a) and the other is outside energy such as high temperature or pressure. (TiO2 single crystals obtained at 180 and 200 °C reaction temperature shown in Figure 6b,c are more etched than that at 150 °C shown in Figure 6a.) We have controllably synthesized the TiO2 crystal-like Chinese classic window lattice according to the negative surface energy of {001} facets (Figure 7). With the increase in the reaction time, the etching process occurred on these single crystals. The obtained hollow structure clearly showed that its shape is a symmetrical quasi-square and its corners were reversed with their regular direction in a normal square. The TEM image and its corresponding selected-area electron diffraction (SAED) pattern confirmed that the obtained products have systematical hollow structure and exhibit single-crystal characteristics, which can be seen in Figure 8a,b. As shown in Figure 9, on the basis of the above results, we proposed the growth mechanism of {001} and {110} in the different stages of fluorine adsorption, including the following stages (Figure 9): (i) {001} facets are grown first in the beginning stage due to their high adsorption energy. (ii) {110} facets are beginning to emerge with the increase in fluorine concentration, and the growth rates of {001} facets are greatly slowed down. (iii) {001} facet growth is obtained obviously again, and the growth process competition is formed with {110} facets when fluorine adsorption ratio on {110} facets reaches 75−100%. (iv) {110} facet growth is stopped and {001} facet growth is faster when the fluorine adsorption ratio on {001} facets is >45%. (v) {001} facets have shown visible disintegration when fluorine adsorption ratio is >75%, whose surface energy is below zero. So, the corrosion phenomenon of {001} facets has been observed.

than that of {110} facets so that the growth of {110} facets is decreased (even stop) dramatically and the {001} facets obviously increased. It means that the theoretical growth limit exists for {110} facets. This is in accord with the experimental results that the percentage of {110} facets in anatase is at most 11% and no longer expands again no matter what action is taken. The percentage cannot be increased by increasing the number of F ions. With lower F− concentration, when the reaction time is long enough, {001} facets will be formed directly and {110} facets will not be formed because the adsorption process of F ions is so fast. It can be shown from Figure 3e,f that {110} facets are nearly not observed. The results from our previous work indicate that the photoreactivity of TiO2 with exposed {001} and {110} facets synchronously is higher than that of TiO2 with exposed {001} facets or {110} facets.33 So, how one controls the morphology of TiO2 single crystals to obtain coexistence of {110} and {001} facets is significant to obtain better photoreactivity. The exposed ratio of high surface energy facets is mainly dependent on the number of adsorbed fluorine ions, such as {001} facets. In our previous reports, we clearly showed that the percentage of {001} facets was increased with the number of fluorine ions.26,27 Here our calculation results showed that after fluorine ions were adsorbed on the surface of TiO2 sufficiently the surface energy of {001} facets could be lower than that of {101} facets because of the higher differentiate adsorption energy of {001}. As a result, {001} facets could be exposed on the surface of TiO2. According to Wullf construction law, the distance from crystal facets to the crystal center should be proportional to the surface energy of the facets. With sufficient adsorption of fluorine ions, the surface energy of {001} facets will be reduced constantly. Therefore, the distance between {001} facets and the crystal center decreased continually. The thickness of the obtained single crystals decreased steeply and the {001} facets became the dominated facets on the single crystals, which can be seen in Figure 4. The etching phenomenon is observed on the {001} facets. According to Figure 2a,b, the surface free energy of {101} facets is lower than that of {001} and {110} at the same F adsorption percentage. The differentiate adsorption energy for {101} facets is the lowest among the three facets, which indicates that the obvious growth of {101} facets cannot be obtained due to the slowest adsorption speed. When the adsorption ratio of F− on {001} facets reached up to 75%, the surface free energy of {001} facets could be reduced to below zero. As we all know, the surface with negative surface energy cannot maintain stable morphology and shows soaring disintegration.11 So, the selective etching phenomenon on the {001} facets appears.30 This is very valuable for controlling synthesis of the hollow nanostructures, which have attracted considerable attention as a unique class of functional materials with well-defined cavities. The shell materials with chemical functionality can be used in catalysis, drug delivery, gas sensors, and energy storage systems owing to their intriguing structural features, such as large surface area, low density, a kinetically favorable open structure, and surface permeability. Meanwhile, because of lower differentiate adsorption energy, adsorption ratio of F− on {101} facets is less than that on {001} facets, and the surface free energy of {101} facets is still a positive value. This is why we only observed the corrosion phenomenon of {001} facets. The final crystal is shown in

4. CONCLUSIONS We figured out that the impact of the differentiate adsorption energy on the crystal growth could not be neglected. Surface growth is a balance process between differentiate adsorption energy and surface energy. These points have been confirmed by our experimental results. In this work, we explain why the percentage of obtained {110} facets of anatase TiO2 can be at most 11% and why only {001} facets have shown corrosion during the crystal growth by first-principle quantum chemical calculations, Wullf’s law, and experimental results. The exposed ratio of high-surface-energy facets is dependent on the number of adsorbed fluorine ions. The growth process of two highenergy-surface facets of anatase TiO2 is competitive and balanced coexistence. This is very important for better understanding and controlling the different morphology of metal oxide crystals at a different adsorption period. Finally, the {001} facets of anatase TiO2 are shown to be self-etching when the fluorine adsorption ratio is >75%, whose surface energy is below zero. This is very valuable for controlling the synthesis of the hollow nanostructures. E

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ASSOCIATED CONTENT

S Supporting Information *

Typical XRD pattern of the TiO2 single crystals with {001} and {110} facets. Typical Raman spectrum of the TiO2 single crystals with {001} and {110} facets. Photoreactivity of anatase TiO2 crystals with different percentage of {001} facets under UV light radiation. Normalized reaction rate constant per unit surface area with different photocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*L.P.: E-mail: [email protected]. Fax: (+86) 10-82545653. Tel: (+86) 10-82545653. *M.L.: E-mail: [email protected]. Fax: (+86) 10-82545653. Tel: (+86) 10-82545653. *Z.J.: E-mail: [email protected]. Fax: (+86) 10-58812131. Tel: (+86) 10-58812131. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support form the Ministry of Science and Technology of China (2011FY130104), the National Science and Technology Pillar Program (2011BAK15B05), and the National Basic Research Program of China (973 Program, 2011CB932802). We are grateful to the support from CAS Key Laboratory for Standardization and Measurement for Nanotechnology.

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DOI: 10.1021/acs.jpcc.5b00087 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b00087 J. Phys. Chem. C XXXX, XXX, XXX−XXX