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Article Cite This: ACS Omega 2018, 3, 8874−8881
http://pubs.acs.org/journal/acsodf
Lepidocrocite-Type Titanate Formation from Isostructural Prestructures under Hydrothermal Reactions: Observation by Synchrotron X‑ray Total Scattering Analyses Satoshi Tominaka,*,†,‡ Hiroki Yamada,§,‡ Satoshi Hiroi,∥,‡ Saori I. Kawaguchi,‡ and Koji Ohara‡
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†
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Research and Utilization Division, Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-gun, Hyogo 679-5198, Japan § Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan ∥ Synchrotron X-ray Station at SPring-8, Research Network and Facility Services Division, National Institute for Materials Science (NIMS), 1-1-1 Koto, Sayo, Hyogo 679-5148, Japan S Supporting Information *
ABSTRACT: The formation of titanium dioxides, such as rutile and anatase, is known to proceed through the formation of a lepidocrocite-type layered structure under hydrothermal conditions, but the nucleation of this intermediate is still not understood well. Here, the nucleation of lepidocrocite-type layered titanates under hydrothermal conditions is observed by tracking the structural changes by in situ time-resolved pair distribution function analyses. We found that titanate clusters or corrugated layered prestructures having 2.8 Å). We used PDFfit2 to simulate the PDF based on the structural model with fixed coordinates, and then we judged the acceptance or rejection of the structure modification on the basis of the R value exported from PDFfit2. Performing these cycles of structure modification and evaluation of the curve fitting is a type of reverse Monte Carlo simulation using a large-scale structure model. This can also be considered as the simulated annealing method, which is often used in XRD analysis because of the use of a structure model of a unit cell.32 Since the surface O atoms coordinate to only two Ti atoms and thus are considered not to be well restrained by the data, we fixed their coordinates. This model can effectively simulate the experimental PDF (P1 space group; a = 6.169(13), b = 7.68(2) Å, c = 100 Å, Rw = 16.1% in the range from 1 to 15 Å), meaning that the titanate is a lepidocrocite-type structure having relaxed atom positions within the restraints. It is apparent that the Ti positions are disordered (Figure 6c). Thus, we consider that this disordered prestructure of the lepidocrocite grows to form larger 2D lepidocrocite structures upon thermal treatment.
rules out the formation of double bonds (Figure S9). The peak at 1.95 Å is assigned to Ti−O nearest neighbors. The PDF was analyzed by curve fitting using an isolated nanosheet model, as shown in Figures 5 and S10, where an
Figure 5. (a) PDF curve fitting result for the titanate in the heated solution measured at RT. (b, c) Lepidocrocite-type layered titanate structure model.
orthorhombic unit cell in the Pmmn space group was used (a = 2.987 (3) Å, b = 3.803 (3) Å, c = 100 Å, Rw = 16.5% in the range from 1 to 20 Å; the large c value was used to simulate the isolated structure; two Ti sites and four O sites). The lattice constants a and b are almost half of those of the crystalline product shown in Figure 1c, where the interlayer TMAH molecules lower the symmetry of the crystal structure. Note that we did not include the interlayer TMAH molecules found in the crystalline products because their contribution was not significant even in the crystalline product although they were considered to exist around the titanate layers. Thus, the model does not simulate the peak at 1.4 Å, which is assigned to C−N distance though it might be a termination ripple of the Fourier transform. Moreover, the model has an empty space along the c axis to simulate in-plane atomic correlation only using PDFfit2, but this model cannot properly simulate the background of the PDF, which reflects averaged number density of atoms. Thus, we simulated background of the PDF separately as shown in Figure S10, subtracted it from the experimental PDF, and then performed the curve fitting using PDFfit2. Looking closely at the PDF of the titanate in the as-prepared solution, we found that the peak positions were slightly different from those in the grown lepidocrocite-layered titanate. First, there are no peaks above 7 Å (Figure 4b), meaning that the structure is tiny or highly corrugated. We simulated the PDFs of these typical TiO2 crystals and layered hydroxide crystals (H2Ti2O3, H2Ti3O7, and lepidocrocite-type TiO2) as shown in Figure S11. It is apparent that the Ti−Ti pairs in corner-shared TiO6 octahedra are richer in the titanate in the as-prepared solution than those in typical TiO2 crystals of anatase, rutile, and brookite. We confirmed that the lepidocrocite-type TiO2 PDF most closely matched the experimental PDF (Figure S11f). The experimental PDF was
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CONCLUSIONS This work has revealed that (i) titanium isopropoxide forms segregated tiny cluster-like or disordered corrugated lepidocrocite-type structures with
