Supersized TiO2 Mesocrystals Prepared by a Successive Topotactic

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Super-sized TiO2 mesocrystals prepared by a successive topotactic transformation reaction and with a high photocatalytic activity Bingyu Lei, Yanna Guo, Huan Xie, Jin Chen, Xiaofan Li, Yaqiao Wu, and Lei Zhou Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00432 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Crystal Growth & Design

Super-sized TiO2 mesocrystals prepared by a successive topotactic transformation reaction and with a high photocatalytic activity Bingyu Lei,a‡ Yanna Guo,a‡ Huan Xie,a Jin Chen,a Xiaofan Li,a Yaqiao Wu,a and Lei Zhou*ab. Department of Biomedical Engineering, School of Life Science and Technology, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China b Advanced Biomaterials and Tissue Engineering Centre, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China. E-mail: [email protected] ‡ These authors contributed equally KEYWORDS anatase, nanostructure, photocatalysis a

ABSTRACT: TiO2 has been widely used in the fields of energy, environment and biology. Most of the researches are focusing on the nano-sized TiO2 materials because of their high specific area, especially in the field of wastewater treatment. However, there are also a few drawbacks for nano-sized materials such as low crystallization degree, troublesome recycling and possible biosafety concern. All these may hinder the large-scale application of TiO2 as photocatalysts. In this work, a new successive topotactic transformation (from (NH4)2TiOF4 to NH4TiOF3 and then to TiO2) method was developed to prepare anatase TiO2 mesocrystals with a length of ~150 μm and a shape of long hexagonal prism. The method belongs to the top-down strategy. It is easy to control the overall shape and size of the final product. The anatase TiO2 mesocrystals have super sizes and are composed of crystallographically ordered assembly of TiO2 nanocrystals with exposed {001} facets. The photocatalytic activity of our materials is close to that of P25 when used in suspension system and superior when immobilized onto glass substrates by a simple pressure-aided method.

As one of the most widely used materials, TiO2 has drawn a lot of attention because of its promising applications in the fields of energy and environment, especially working as photocatalysts for the degradation of various pollutants.1-4 Most of the current researches are focusing on the TiO2 nanoparticles because of their high specific surface area, which facilitates the interaction between them and surrounding pollutants and therefore increases their performance. However, for practical applications, there are a few drawbacks that TiO2 nanoparticles may have. Firstly, both specific surface area and crystallization degree are highly related (usually positively correlated) with the photocatalytic activity.5 To improve the photocatalytic activity of TiO2, we’d better increase specific surface area and crystallization degree simultaneously.6 Unfortunately, for ordinary nanoparticle materials, these two vital properties are usually in tension. That is, increasing specific surface area typically reduces crystallization and vice versa. Secondly, for the slurry system that are widely applied in water treatment, if TiO2 nanoparticles are used, there are high aggregation tendency and difficulty of separation and recovery.7 These lead to a decrease of photocatalytic efficiency, as well as an increase in overall cost.8 Thirdly, most biosafety conclusions about TiO2 materials are based on large-sized TiO2 particles, some researches have suggested that nanoparticles may exhibit potential risks to environment and human body, because of some adverse effects such as

oxidative stress, lipid peroxidation, genotoxicity, lung diseases, inflammation and so on.9-11 Mesocrystals are a new class of hierarchically structured materials.12-15 Their elemental building blocks are nanosized and organized in crystallographically order ways crossing multiple levels from microscale to macroscale. The geometric complexity leads to many advantages not only inherited from the nanosized building blocks but also gained from synergistic effects.16,17 Many researches have suggested that TiO2 mesocrystals may solve the problems mentioned above.18-20 Their nanometer-sized building blocks can provide high surface area, high surface-to-bulk ratio, and therefore a lot of surface reaction sites that can interact with pollutants. The crystallographically oriented interaction among the building blocks leads to high crystallization degree without largely losing the surface area. Their overall micrometersized structure provides desirable mechanical properties, such as robustness, fast species transportation, easy recycling, facile regeneration and higher biosafety. Topotactic transformation reactions have been proved to be feasible routes to prepare TiO2 mesocrystals,21-23 during which materials with the similar crystallographic structure to that of TiO2 are used as precursors. After the transformation reaction, the overall exterior dimensions and shapes of precursors are retained because of the crystallographic similarity. They belong to the top-down strategy, which are helpful for us to get unusual but

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regular shapes of TiO2 particles that are not able to be obtained by conventional crystal growth methods. The most investigated precursor is NH4TiOF3. There are two advantages of using NH4TiOF3 as precursor. Firstly, TiO2 mesocrystals tend to be obtained with exposed highly active {001} facets. In addition, it is easy to get N (or F)doped TiO2 mesocrystals, which are supposed to have visible photocatalytic activity.24 However, the growth habit of NH4TiOF3 limits the variety of the particle shape and possible applications. In this work, a novel successive topotactic transformation method is proposed to prepare TiO2 mesocrystals based on the crystallographic similarities among (NH4)2TiOF4, NH4TiOF3 and anatase TiO2 (Figure S1), which includes two simple topotactic transformation processes. One is from (NH4)2TiOF4 to NH4TiOF3. The other is from NH4TiOF3 to TiO2. The obtained TiO2 particles are ultra-large, mesocrystalline, rod-like and with largely exposed {001} facets. The combination of the large size, hierarchical structure and exposed {001} facets endues the obtained TiO2 particles with better handling facility without large sacrifice of high photocatalytic ability in comparison with commercially available P25. When immobilized on glass surface using a simple pressureaided method, our materials even show a better performance. The starting materials of the successive topotactic transformation process are super-sized rod-like (NH4)2TiOF4 particles and prepared through the reaction between (NH4)2TiF6 and NH4OH. Experimental details are given in SI file. Briefly, (NH4)2TiF6 solution was mixed with dilute NH4OH solution and kept at 25 oC for 30 min to 16 hours. The XRD patterns of the precipitates collected from the reaction solution after different reaction times are shown in Figure 1a and Figure S2. The results indicate after 16 h reaction the main component of the precipitates is (NH4)2TiOF4 (PDF No. 49-0161) without evident impurities. Before 16 h, the proportion of impurities decreases with the reaction time increasing. Morphology details of the 16 h sample are observed by SEM and TEM. The low-resolution SEM image (Figure 1b) indicates that these (NH4)2TiOF4 particles have a rod-like shape. The length is ~ 150 μm. The cross-sectional width is ~ 15 μm. The higher resolution image (Figure 1c) indicates that these (NH4)2TiOF4 rods are hexagonal prisms, which is also confirmed by the TEM image (Figure 1d). The SAED pattern (Figure 1d, inset) shows a “single-crystal-like” spot pattern. The diffraction spots correspond to the [001] zone axis of (NH4)2TiOF4, indicating that six rectangular facets correspond to (001) facet and its equivalent facets. It has to be pointed out that the rod-like particles are tending to lie on the substrate with the {001} facets facing up or parallel to the substrate, which makes {001} facets more easily to be observed in SEM and TEM and leads to a much stronger intensity of the (002) peak in comparison with other peaks (Figure 1a). The successive topotactic transformation is triggered by a quick sintering process of the rod-like (NH4)2TiOF4 particles. The XRD pattern (Figure 2a) indicates that after sintering at 800 oC for 12 min the rod-like (NH4)2TiOF4 particles can be completely converted to anatase TiO2 (PDF

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Figure 1. The rod-like (NH4)2TiOF4 precursors. a) XRD pattern, b) SEM image; inset, 3D model of a (NH4)2TiOF4 particle. c) HRSEM image. d) TEM image; inset, the corresponding SAED pattern.

No. 86-1156).25 No evident impurity is detected. The XPS (Figure S3) and STEM-mapping (Figure S4) results are consistent with the results of XRD. The particles are pure TiO2 without evident impurities such as N and F. The SEM image (Figure 2b) suggests both the shape and size of the TiO2 particles are similar to the (NH4)2TiOF4 precursors, which is a typical feature of topotactic transformation. The HR-SEM image (Figure 2c) shows that the rod-like TiO2 particles are built of TiO2 nanoparticles with a size of 50200 nm and a shape of decahedron, which is typical for TiO2 particles with dominantly exposed {001} facets.26 The TEM image (Figure 2d) confirms this hierarchical structure and suggests that these nanoparticles are crystallographically aligned at a similar orientation. The marked hexagon is a combination of two symmetrical trapezoids, suggesting that these nanoparticles are viewed along [100]/ [010] direction of decahedral TiO2.20 The inset of Figure 2c gives the schematic illustration of the decahedral building block. The SAED pattern (Figure 2d, inset) shows a “single-crystal-like” diffraction with minor distortions coming from small mismatches between boundaries of the nanoparticles, which is typical for mesocrystals. The arrangement of the spots indicates that this particle is viewed along [010] direction of anatase TiO2, consistent with SEM and TEM results. The corresponding high-resolution image is given in Figure S5. The (101) and (004) atomic planes with lattice spacing of 0.35 nm and 0.24 nm respectively are clearly shown. The (101) and (004) facets form the interfacial angle of 69o, identical to the theoretical value. Based on XRD, SEM and TEM analyses, it is unambiguous that the sintering products are anatase mesocrystals with a shape of hexagonal prism and constructed of nanoparticles with highly exposed {001} facets. In order to figure out what happens during the sintering process, TG tests were performed. Results are shown in Figure 3a. The TG curve can be roughly divided into four stages. The first stage is from room temperature to 100.28

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Crystal Growth & Design

Figure 2. The sample prepared by sintering the precursor (NH4)2TiOF4 at 800 oC for 12 min. a) XRD pattern. b) SEM image; inset, 3D model of a rod-like anatase particle. c) HR-SEM image; inset, 3D model of an anatase nanoparticle. d) TEM image; inset, the corresponding SAED pattern. oC,

the mass loss is about 1.11%, which is attributed to the evaporation of the physically adsorbed water and other volatile impurities. The second stage is from 100.28 oC to 298.38 oC. The mass loss is about 20.97%, which is consistent with the mass loss of NH4F from (NH4)2TiOF4 to NH4TiOF3. The third stage is from 298.38 oC to 370.68 oC, in which the mass loss is about 23.56% to initial sample and approximately consistent with the mass loss of the transformation from NH4TiOF3 to TiOF2.21 In the fourth stage, from 370.68 oC to 498.58 oC, there is 10.75% mass loss to yield final product TiO2. It is worth noting that, from the end of the second stage to the last ending, the mass loss is about 34.31% to the initial sample and 44% to the product of the second stage, which almost matches the mass loss behavior and amount of NH4TiOF3 under heating conditions and strongly proves that NH4TiOF3 will occur as a metastable intermediate during the whole heating process of (NH4)2TiOF4 in this work. In addition to TG test, a semi in-situ XRD analysis test (Figure 3b, S4) was also carried out to investigate the process of the transformation process. The as-prepared (NH4)2TiOF4 was heated and collected when the temperature of the furnace reached a pre-set value (i.e. 200, 250, 300 and 350 oC). The XRD patterns (Figure 3b) suggest that almost pure NH4TiOF3 was obtained when the temperature is 250 oC. In addition, similar examination of different sintering time was also done, in which the furnace was preheated to 800 oC, and (NH4)2TiOF4 was sintered in the preheated furnace for 5 s, 40 s, 1 min and 2 min respectively. The XRD patterns are shown in Figure S6a. It is clear that with the sintering time increasing, the content of NH4TiOF3 increases and then declines. For the 40 s sample, there is almost pure NH4TiOF3. In order to get more details about the intermediate, the 250 oC sample and 40 s sample were used for further characterization. The SEM images (Figure 3c, S6b) show that NH4TiOF3 powders are hexagonal rod with a length of around 150 μm and a cross-sectional width of around 15

Figure 3. a) TG curve of the rod-like (NH4)2TiOF4. b) XRD patterns of the samples prepared by sintering the precursors to different temperature. ((NH4)2TiOF4: JCPDS no. 49-0161; NH4TiOF3: JCPDS no.54-0239; TiO2: JCPDS no. 86-1156.) c) and d) SEM images of the sample prepared by sintering the precursor (NH4)2TiOF4 to 250 oC.

μm, which are close to their (NH4)2TiOF4 precursors. The HR-SEM images (Figure 3d, S6c) show that the NH4TiOF3 particles are built of a lot of well-arranged building units. There are elongated gaps between micron-sized building units, which can be attributed to the loss of NH4F and oriented lattice contraction from (NH4)2TiOF4 to NH4TiOF3. The TEM image of the 40 s sample(Figure S6d) confirms this hierarchical structure, and the lattice spacing in the HR-TEM image (Figure S6f) is 0.38 nm, which can be indexed to the mutually vertical facets (200) or (020) of NH4TiOF3.22 The SAED pattern (Figure S6e) shows typical mesocrystal diffraction spots (single-crystal like diffraction with minor distortion), further proving that the intermediate of the sintering process is NH4TiOF3 mesocrystal. Based on the above results and crystal structure features of (NH4)2TiOF4, NH4TiOF3 and anatase TiO2 (Figure S1), a successive topotactive transformation process is proposed and represented in Figure 4. The similarity of the Ti atom arrangements between {001} planes of NH4TiOF3 and anatase TiO2 has been found and applied to synthesize TiO2 mesocrystals from NH4TiOF3.22 According to the analysis of Ti atoms’ positions in (NH4)2TiOF4 and NH4TiOF3, similarities between the two structures can also be found. The lattice distance mismatch between the (NH4)2TiOF4 and NH4TiOF3 crystal direction is 0.47%, between (NH4)2TiOF4 and NH4TiOF3 direction is 0.53%. It means that Ti atoms’ positions are similar in the {001} planes of (NH4)2TiOF4 and {100} planes of NH4TiOF3 (Ti atoms’ arrangement pattern of (NH4)2TiOF4 and NH4TiOF3 are shown in Figure 4 and Figure S1). Combining the results from the TGA and XRD, it is very sure there is a topotactic transformation from (NH4)2TiOF4 to NH4TiOF3. Because of the removal of NH4F, shrinkage occurs vertical to the topological facets, i.e. {001} facets of (NH4)2TiOF4. As a result, after the first step of the transformation, the hexagonal rod is no longer its whole

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Figure 4. Schematic illustration of the transformation from rod-like (NH4)2TiOF4 to hierarchical rod-like anatase TiO2.

design, but comprised of many building units. Then each NH4TiOF3 building units can topotactic transform again to more and smaller decahedral TiO2 nanoparticles with {001} facets reserved and shrinkage along the [001] axis. In a word, the rod-like precursor undergoes a two-step (i.e. successive) transformation to form anatase TiO2. For practical applications as photocatalysts, TiO2 are usually used in two forms: suspended slurries in aqueous solutions and immobilized powders or films on inert supports.27-31 As a commercial product, P25 is widely used in photocatalysis, but still has drawbacks in reuse and immobilization due to its small size. Our rod-like TiO2 mesocrystals have extremely large size in comparison with P25, suggesting a better handling facility. The filtration and sedimentation tests (Figure S7 and S8) of the rod-like TiO2 samples confirm that they are much easier to be separated from the photocatalytic slurries than P25. The photocatalytic activity of the rod-like TiO2 mesocrystal was evaluated by photocatalytic decolorization of methylene blue aqueous solution and compared with commercialized P25. Results are shown in Figure 5a. After 80 min of UV irradiation, the decoloring ratio of methylene blue of the 800 oC/ 12 min sample is 77% and is very comparable to that achieved by P25 (83%). Considering the extremely large size (~150 m) in comparison with that of P25 (~25 nm), the photocatalytic activity of the rod-like anatase mesocrystals is extraordinary. Usually for compact TiO2 materials, large size means low specific surface area and relatively low photocatalytic activity.32 The high photocatalytic activity of our mesocrystals mainly comes from the unique hierarchical structure, which is built of TiO2 nanoparticles with exposed {001} facets. This unique structure provides relatively much higher practical specific surface area (6.99 m2 g-1) in comparison with the calculated value based on a compact model (0.08 m2 g-1, see the SI file), largely exposed {001} facets and high crystal degree as well. The three properties are all beneficial to promote the photocatalytic activity.20 In addition to examine the photocatalytic activity of the rod-like TiO2 mesocrystal particles as powdered photocatalysts, we also evaluated their performance when immobilized on glass substrate and compared it with that of P25. A simple pressure-aided method is used to prepare the immobilization films. Briefly, TiO2 powders (i.e. our sample or P25) are evenly spread and partly inserted into glass surface with the help of a certain pressure at the softening temperature of glass (details are given in SI file).

Figure 5. Photocatalytic activity comparisons between P25 and the TiO2 mesocrystals in a) suspension system and b) immobilized system respectively. The TiO2 mesocrystals are prepared by sintering the (NH4)2TiOF4 precursor at 800 oC for 12 min. C is the concentration of methylene blue after the reaction of time t. C0 is the initial concentration of methylene blue after being stirred in the dark for 1 h. c) and d) SEM images of rod-like TiO2 coating on glass surface.

Photocatalytic activities of the rod-like TiO2 mesocrystal coating (named in ROD-Coating) and P25 coating are shown in Figure 5b. The rod-like TiO2 coating performs much better than the P25 coating. In order to better understand the result, XRD and SEM technologies were used to investigate the different coatings. The XRD pattern is shown in Figure S9a, suggesting that the preparation process of the coating doesn’t change the phase of the rodlike TiO2 particles. They are still anatase. The SEM image (Figure 5c) indicates the rod-like particles are firmly “grown” into the glass and partially exposed to the outside. Close inspection (Figure 5d) at the junction of the TiO2 particles and the glass substrate suggests that the TiO2 rods are not just loosely attached on the substrate but has their roots into the substrate. The corresponding HR-SEM image (Figure S9b) indicates that the preparation process doesn’t change the hierarchical structure of the rod-like TiO2 particles. They are still mesocrystals and composed of TiO2 nanocrystals, which are beneficial for reactants to transfer from the exposed outside part to the inside part during the photocatalytic reactions. It has to be pointed out that the super-large sizes of the rod-like TiO2 particles are very important for their better photocatalytic performance. Because of the super-large sizes, the rod-like TiO2 particles can spread on the substrates evenly without much aggregation (Figure 5c). It also makes the rod-like particles to be compounded with the substrate firmly without much sacrificing the photocatalytic performance of the nanocrystals inside. In comparison, the sizes of P25 particles are nanoscale, these nanoparticles tend to aggregate on the substrates (Figure S10) when processed by the same method, which may largely reduce the exposed surface area of the P25 particle and undermines their photocatalytic performance.

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Crystal Growth & Design In summary, we have developed a successive topotactic transformation method to prepare rod-like TiO2 materials with a hierarchical structure. As a top-down strategy, this method gives opportunity to obtain materials with a designed size and shape. Furthermore, the successive topotactic transformation broadens the selection range of precursors. For instance, the obtained TiO2 materials transformed from (NH4)2TiOF4 have a whole new morphology (hexagonal rod shape) rather than the previous cubic or sheet, as well as ultra-large size. Taking advantages of the new hierarchical structure, rod-like anatase mesocrystal performs very well in both suspension and immobilized systems. It is very important for some practical applications, in which anatase mesocrystals can show both high photocatalytic efficiency and good ability in separation, recovery and reuse.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Additional schematic illustration. Detailed characterizations of precursor (NH4)2TiOF4, intermediate NH4TiOF3 and TiO2. Details of the filtration and sedimentation tests. Photocatalytic activities of P25 and Rod-like TiO2 mesocrystals. XRD pattern and HR-SEM image of the rod-like TiO2 coating on glass substrate. SEM images of P25 coating on glass substrate.

AUTHOR INFORMATION Corresponding Author * [email protected].

Author Contributions B. Y. Lei led the experimental work including preparing samples (both powder and coating) and carrying out XRD, SEM, TEM, TG, BET and photocatalytic activity analyses; Y. N. Guo contributed equally in sample preparing and data collection of the powder samples. J. Chen participated in sample preparation for TEM and TG analyses; H. Xie, X. F. Li and Y. Q. Wu assisted with analysis and discussion of XRD and BET; L. Zhou originated and supervised the project. All authors have given approval to the final version of the manuscript. ‡ These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Key Research and Development Program of China (No. 2017YFC1104402), the National Natural Science Foundation of China (No. 51172084) and the Fundamental Research Funds for the Central Universities (No. 2016YXMS259). The authors also thank the Analytical and Testing Center of Huazhong University of Science and Technology for providing SEM, XRD and TEM measurements.

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Crystal Growth & Design For Table of Contents Use Only Super-sized TiO2 mesocrystals prepared by a successive topotactic transformation reaction and with a high photocatalytic activity Bingyu Lei, Yanna Guo, Huan Xie, Jin Chen, Xiaofan Li, Yaqiao Wu, and Lei Zhou*

A successive topotactic transformation method is developed to prepare anatase TiO2 mesocrystals from (NH4)2TiOF4 crystals. The TiO2 mesocrystals have ultra-large sizes, rod shapes, hierarchical structures and good photocatalytic activity. The process of the transformation is also examined in detail.

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