Article pubs.acs.org/crystal
Chloride Flux Growth of La2TiO5 Crystals and Nontopotactic SolidState Transformation to LaTiO2N Crystals by Nitridation Using NH3 Kenta Kawashima,† Mirabbos Hojamberdiev,† Hajime Wagata,† Kunio Yubuta,‡ Shuji Oishi,† and Katsuya Teshima*,†,§ †
Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ‡ Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan § Center for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ABSTRACT: Oxynitride perovskites and related phases have received considerable attention due to their potential application for visible-light-responsive photocatalyst and nontoxic inorganic pigments. The changes in bonding and structure by a partial replacement of O2− by N3− give rise to interesting dielectric behavior. Here, we report on the fabrication of highly crystalline La2TiO5 crystals by chloride flux growth method and their subsequent nitridation to form the LaTiO2N crystals using NH3 gas. The flux-grown La2TiO5 crystals had a columnar structure grown in the ⟨001⟩ direction. Using the NaCl flux, larger columnar La2TiO5 crystals were grown compared to those grown using the KCl flux. With increasing solute concentration, the aspect ratio of columnar La2TiO5 crystals decreased significantly. The columnar La2TiO5 crystals with smooth surface were readily converted by nitridation at 950 °C for 45 h followed by acid treatment into the LaTiO2N crystals with a highly porous structure that formed from the strong segregation of nanocrystals, leading to the largest specific surface area (16.5−18.4 m2·g−1). For the La2TiO5 crystals grown using the chloride fluxes, the wavelength of the absorption edges was approximately 320 nm (Eg = 3.87 eV), whereas the absorption edges exhibited by the LaTiO2N crystals obtained by nitridation were approximately 600 nm (Eg = 2.06 eV). Particularly, the LaTiO2N crystals prepared in this study by nitriding the precursor La2TiO5 crystals did not show a noticeable absorption in the near-infrared region above 600 nm, which is generally attributable to some reduced Ti3+ species and nitrogen deficiency, even after a long nitridation process. The fabricated LaTiO2N crystals with low defect density will be advantageous for various applications that specially require higher specific surface area.
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INTRODUCTION Lanthanum titanates (e.g., LaTiO3, La2Ti2O7, La2TiO5, and LaTiO2N) in the perovskite structure have been the subject of numerous recent studies because of their variety of attractive characteristics, including metal−insulator transition, semiconducting, dielectric, luminescence, magnetic, ionic conductivity, and photocatalytic properties. The LaTiO3 phase has an orthorhombic GdFeO3 perovskite structure (space group Pbnm).1 It is a Mott insulator with a G-type antiferromagnetic ordered ground state and a Néel temperature of approximately 135 K. LaTiO 3 is a semiconductor and becomes an antiferromagnetic insulator upon cooling through the Néel temperature. When synthesized with a moderate oxygen content, LaTiO3‑x is metallic at high temperature, with a metal−insulator transition occurring at reduced temperature.2 Doped and undoped LaTiO3 has been demonstrated to have promising applications, including resistive switching memory, conducting electrode for ferroelectrics, dielectric-base transistors, and sensors.3 La2Ti2O7 is a monoclinic compound with space group P21 formed of alternating perovskite-like blocks of TiO6 octahedron © 2014 American Chemical Society
slabs. La2Ti2O7 is characterized by its ferroelectric property with very high Curie temperature (TC = 1500 °C),4 spontaneous polarization (PS = 5 μC cm−2), and permittivity (ε = 42−62).5 The high refractive indices and an appropriate level of nonlinear optical coefficient of La2Ti2O7 make it suitable for application as nonlinear optical devices.6 In La2Ti2O7, the substitution of La and Ti sites by other elements promises its potential use as photoluminescence or laser-active materials.7 The photocatalytic property of La2Ti2O7 with a wide bandgap energy of about 3.8 eV is also investigated for overall water splitting8 and degradation of volatile organic compounds.9 As another member of the perovskite lanthanum titanate family, which we mainly focus on in this study, the La2TiO5 phase exhibits orthorhombic structure (space group Pnam), with lattice constants of a = 11.004 Å, b = 11.392 Å, c = 3.944 Å, and α = β = γ = 90°, which is stable up to melting at Received: September 18, 2014 Revised: October 27, 2014 Published: November 10, 2014 333
DOI: 10.1021/cg501397x Cryst. Growth Des. 2015, 15, 333−339
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Crystal Growth & Design temperature above 1700 °C.10 The crystal structure of La2TiO5, shown in Figure 1a, consists of edge-sharing octahedra in which
formation to LaTiO2N crystals by nitridation using NH3. Here, we also discuss structural characterization and chloride flux growth manner of the La2TiO5 crystals and transformability to LaTiO2N crystals under an NH3 flow at high temperature.
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EXPERIMENTAL SECTION
Growth of La2TiO5 Crystals. Reagent-grade La2O3, TiO2, KCl, and NaCl (Wako Pure Chemical Industries, Ltd.) were used for the flux growth of La2TiO5 crystals. A stoichiometric mixture of La2O3 and TiO2 was used as solute, and KCl and NaCl were used as flux. The solute concentration was varied from 1 to 50 mol %, and the total mass of a solute−flux mixture was approximately 10 g for each run. After mixing, each solute−flux mixture was placed in a platinum crucible with a capacity of 30 cm3, and a platinum lid was loosely closed. The mixture-containing platinum crucible was placed in an electric furnace, heated to 1100 °C at a heating rate of 45 °C·h−1, and held at this temperature for 10 h. Subsequently, the platinum crucible was cooled to 500 °C at a cooling rate of 150 °C·h−1 using a cooling program and then allowed to cool naturally to room temperature. The flux-grown La2TiO5 crystals were separated from the remaining flux by washing the obtained crystal product with hot water and dried at 100 °C for 12 h. Fabrication of LaTiO2N Crystals. To produce the LaTiO2N crystals, 1.0 g of the flux-grown La2TiO5 crystals was wrapped with quartz wool, placed in a vertical tubular furnace with a quartz rod, and heated at 950 °C for 45 h with a heating rate of 600 °C·h−1 under an NH3 flow (200 mL·min−1) and then cooled naturally to room temperature. Afterward, the product was collected and washed with dilute HCl (0.1 M) and deionized water several times to remove unreacted La2O3 and dried at 100 °C for 12 h. Characterization. The crystal phases formed were identified using X-ray diffraction (XRD, MiniflexII, Rigaku) with Cu Kα radiation (λ = 0.154 nm). The X-ray diffractometer was operated at 30 kV and 20 mA in the 2θ range from 10° to 70°. The crystal morphology and size of the obtained products were examined by field-emission-type scanning electron microscopy (SEM, JSM-7600F, JEOL) at an acceleration voltage of 15 kV. The crystallographic characteristics of the obtained crystals were analyzed by high-resolution transmission electron microscopy (HR-TEM, EM-002B, TOPCON) operated at 200 kV. The specific surface area (SBET) was obtained by using the Brunauer, Emmett, and Teller (BET) method from N2 adsorption−desorption isotherm at 77 K (BELSORP-mini, BEL Japan, Inc.) on the crystal product degassed at 100 °C for 5 h in vacuum. The ultraviolet−visible (UV−Vis) diffuse reflectance spectra of the obtained crystals were recorded on a JASCO V-630 spectrophotometer. The optical band gap energy was estimated from the UV−Vis diffuse reflectance spectrum.
Figure 1. Schematic illustrations of crystal structures of (a) La2TiO5 and (b) LaTiO2N.
the La cation is coordinated with seven oxygen atoms, while Ti is five-coordinated with oxygen in an off-center square pyramidal configuration that shares two edges and a corner with neighboring La-site polyhedra.11 The La2TiO5 phase also shows dielectric behavior, semiconducting nature, and photocatalytic property. LaTiO2N is an orthorhombic compound with space group Imma, as shown in Figure 1b, and an n-type semiconductor with a perovskite structure and a band gap of 2.1 eV. LaTiO 2 N is demonstrated to have potential applications, such as nontoxic pigment and visible-light-driven water splitting for the production of H2 and O2 in the presence of sacrificial reagents due to its suitable band positions and relatively cheap elements. LaTiO2N was conventionally synthesized by nitridation of corresponding La2Ti2O7 precursor under an NH3 flow at high temperature.12 Although the La2TiO5 crystals have been mainly fabricated by solid-state reaction11,13 and their pressure-induced structural transformation and formation enthalpy and heat capacity were studied,13,14 the chloride flux growth of the La2TiO5 crystals and their subsequent nitridability to form the LaTiO2N crystals have not been reported yet. As known, low crystallinity and defects on crystal structure have an unfavorable impact on photocatalytic activity because of the recombination of electrons and holes in the bulk. Therefore, it is desirable to produce idiomorphic crystals with well-developed faces, enhancing the photocatalytic activity. In this work, we demonstrate the chloride flux growth of the La2TiO5 crystals with low defect density and nontopotactic solid-state trans-
Figure 2. XRD patterns and SEM images of the La2TiO5 crystals grown using KCl flux with different solute concentrations. 334
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Figure 3. XRD patterns and SEM images of the La2TiO5 crystals grown using NaCl flux with different solute concentrations.
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some crystals was reduced from ca. 0.9 to ca. 0.8 μm. Further increase in the solute concentration up to 50 mol % resulted in the formation of columnar crystals with shorter lengths (ca. 0.7 μm) and some agglomerated vague small crystals of La2TiO5. With increasing solute concentration, the aspect ratio of columnar La2TiO5 crystals decreased in the following order: 3.81 for 1 mol % > 3.32 for 5 mol % > 2.97 for 20 mol % > 2.25 for 50 mol %. On the contrary, the La2TiO5 crystals grown using the NaCl flux were significantly affected by the solute concentration, as shown in Figure 3. Particularly, with increasing solute concentration, the aspect ratio of columnar La2TiO5 crystals decreased considerably in the following order: 7.92 for 1 mol % > 5.14 for 5 mol % > 2.93 for 20 mol % > 1.80 for 50 mol %. It is assumed that solute with lower concentration was easily soluble in the chloride flux, and thus the La2TiO5 crystals could grow gradually through a process involving the formation of nuclei and crystallization during the cooling of the high-temperature solution. Hence, the columnar well-developed La2TiO5 crystals grown with lower solute concentration have larger sizes compared to those grown with higher solute concentration because of the presence of a great number of nuclei, hindering the full growth of crystals even as a high-temperature solution becomes critically supersaturated.15 It is also believed that the solute with higher concentration might not have completely dissolved in the chloride flux. The main difference between the La2TiO5 crystals grown using the KCl and NaCl fluxes is that the La2TiO5 crystals grown using the NaCl flux have much larger crystal sizes compared to those grown using the KCl flux. It is conjectured that the solute was more soluble in the NaCl flux at 1100 °C than in the KCl flux, allowing easy transport and attachment of the atoms from the supersaturated hightemperature solution into the adjacent growing crystal that becomes much longer at undercooling. The crystal growth manner seems to have originated from different energy of interfaces between liquid phase of NaCl or KCl and the La2TiO5 crystal surface, depending on the flux used and crystallographic orientation of the facets. However, more systematic investigation of interfacial energy is needed to fully explain the mechanism of La2TiO5 crystal growth in the presence of different chloride fluxes. Next, in order to assess the transformability of highly crystalline well-developed La2TiO5 crystals into the LaTiO2N crystals, the La2TiO5 crystals grown using the KCl and NaCl fluxes with solute concentration of 20 mol % were subject to
RESULTS AND DISCUSSION The well-developed columnar crystals of La2TiO5 were grown using the KCl and NaCl fluxes. First, to study the effect of solute concentration on the phase formation and crystal shape and size of La2TiO5, the solute (La2O3 + TiO2) concentration was changed from 1 to 50 mol %. Figures 2 and 3 show XRD patterns of the crystal products grown with different solute concentrations using the KCl and NaCl fluxes at 1100 °C for 10 h and the International Centre for Diffraction Data Powder Diffraction File (ICDD PDF) data for La2TiO5 (54-0179) with space group Pnam. The XRD patterns of the crystal products grown using the KCl flux with the solute concentration ranging from 1 to 20 mol % can be well-indexed as orthorhombic La2TiO5 without any indicative diffraction lines of impurity phases, implying that all the solute was consumed for the formation of the La2TiO5 crystals. No diffraction peak shift in the XRD patterns due to a possible incorporation of Na+ and K+ was observed. However, with further increase in the solute concentration up to 50 mol %, the secondary phase La2Ti2O7 (ICDD PDF 81-1066) started to form along with La2TiO5, indicating that single-phased highly crystalline La2TiO5 crystals could be obtained with the solute concentration of < 20 mol % using the KCl flux. In contrast, the XRD patterns of the crystal products grown using the NaCl flux confirm that phase-pure highly crystalline La2TiO5 crystals could only be obtained with the solute concentrations of 5 and 20 mol %. With the solute concentration of 1 mol %, the La2TiO5 crystals were grown along with minor phase LaOCl (ICDD PDF 71−4813), whereas the crystal products grown with the solute concentration of 50 mol % using the NaCl flux could be identified as La2TiO5, La2Ti2O7, and La4Ti3O12 (ICDD PDF 71-6698). As shown in the XRD patterns of the crystal products synthesized using the KCl and NaCl fluxes, the relative peak intensity of the (201) plane is slightly increased with increasing the solute concentration from 1 to 50 mol %, implying that the La2TiO5 crystals have a preferential growth in the facet of (201) with higher solute concentration. Figures 2 and 3 show the typical SEM images of the crystal products grown with different solute concentrations using the KCl and NaCl fluxes at 1100 °C for 10 h. In Figure 2, the SEM images clearly show that columnar La2TiO5 crystals were grown using the KCl flux with the solute concentration of 1 mol %. With the solute concentrations of 5 and 20 mol %, the crystal morphology almost remained unchanged, whereas the size of 335
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Figure 4. XRD patterns (a) and SEM images of the La2TiO5 crystals (b) grown using KCl flux, LaTiO2N crystals obtained by nitridation of the La2TiO5 crystals (c), and LaTiO2N crystals after acid treatment (d).
Figure 5. XRD patterns (a) and SEM images of the La2TiO5 crystals (b) grown using NaCl flux, LaTiO2N crystals obtained by nitridation of the La2TiO5 crystals (c), and LaTiO2N crystals after acid treatment (d).
nitridation at 950 °C for 45 h under an NH3 flow (200 mL· min−1). Figures 4a and 5a show XRD patterns of the La2TiO5 crystals grown using the KCl and NaCl fluxes with the solute concentration of 20 mol % before and after nitridation and after acid treatment. It is obvious that the transformation of the La2TiO5 crystals into the LaTiO2N (ICDD PDF 48-1230) crystals with orthorhombic structure (space group: Imma) by nitridation was successful; however, some diffraction lines assignable to the La2O3 phase (ICDD PDF 05-0602) and some unknown phases were still present. The byproducts were formed during the high-temperature transformation process, as expressed by the following reaction:
Figures 4 and 5 show SEM images of the La2TiO5 crystals grown using the KCl and NaCl fluxes with the solute concentration of 20 mol % before and after nitridation and after acid treatment. As shown in Figures 4b and 5b, the La2TiO5 crystals have dense columnar structures with smooth surface. Figures 4c and 5c show that the crystal products after nitridation still possess columnar structures with size ca. 0.7− 3.0 μm similar to that of the crystal size of the samples synthesized with the solute concentration of 20 mol % before nitridation. This indicates that although the transformation of the La2TiO5 crystals into the LaTiO2N crystals occurred, the original shape of the La2TiO5 crystals was preserved unaffected during a high-temperature nitridation process. However, the crystal products after nitridation have an interesting highly porous structure formed from the strong segregation of nanocrystals. The average size of discriminable units within the porous aggregates synthesized from the La2TiO5 crystals grown using the KCl flux is approximately 71 nm, whereas the average size of discriminable units within the porous aggregates prepared from the La2TiO5 crystals grown using the NaCl flux is approximately 79 nm. These kinds of porous structures of LaTiO2N were formed possibly by the transformation from a
2La 2TiO5 + N2 + 3H 2 → 2LaTiO2 N + La 2O3 + 3H 2O
Further, to remove byproducts, the crystal products obtained after nitridation were treated with dilute HCl La 2O3 + 6HCl → 2LaCl3 + 3H 2O
and washed with deionized water several times. As expected, the XRD patterns of the crystal products after nitridation followed by acid treatment evidence that the final products contain only LaTiO2N crystals. 336
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Figure 6. TEM and lattice images and SAED patterns of the La2TiO5 crystals grown using (a−c) KCl and (d−f) NaCl fluxes.
layered perovskite-type oxide into the simple perovskite-type oxynitride accompanied by a lattice condensation process due to the partial replacement of O2− with N3− in the La2TiO5 crystals during the long nitridation process and lattice shrinkage.16−18 In contrast, the LaTiO2N particles previously obtained by nitridation of La2Ti2O7 precursor in a flux did not have a porous structure that was reasoned due to the presence of the flux evoking dissolution and recrystallization steps and facilitating atom mobility during the nitridation process.17 Note that a detailed analysis of pore sizes, pore shapes, pore types, lattice defects, and oxidation states of LaTiO2N obtained by nitridation of the La2TiO5 crystals is left open for future investigation. The crystallographic characteristics of the La2TiO5 crystals were evaluated by TEM. Figure 6 shows bright-field TEM and lattice images and corresponding selected-area electrondiffraction (SAED) patterns of the La2TiO5 crystals grown using KCl and NaCl fluxes. The magnified TEM images of the La2TiO5 crystals grown using KCl and NaCl fluxes, shown in Figure 6a,d, confirm that the columnar crystals have singlecrystalline nature without any intergrowth and with flat surfaces despite a slight difference in length between the crystals grown using KCl and NaCl fluxes. The TEM results reveal that using lower solute concentration and chloride flux resulted in the growth of La2TiO5 crystals in the ⟨001⟩ direction possibly due to attractive interaction between the lanthanum and titanium species and the growth surface in supersaturated solution at high temperature, leading to a columnar structure with the reduced facets and high aspect ratio. As the SAED patterns (Figure 6c,f) obtained with the incident beam along the [110] direction were highly ordered and no defects were found in the lattice images (Figure 6b,e), the La2TiO5 crystals grown using KCl and NaCl fluxes have good crystalline quality. Indexing of the SAED patterns reveals that the diffraction spots correspond to the {110} and {200} faces, indicating that the La2TiO5 crystals were grown along the ⟨001⟩ direction. From the TEM results, we conclude that high-quality La2TiO5 crystals were successfully grown at 1100 °C by cooling the chloride fluxes. Figure 7 shows bright-field TEM and lattice images and corresponding SAED patterns of the LaTiO2N crystals obtained by nitridation of the La2TiO5 crystals grown using KCl and NaCl fluxes followed by acid treatment. As shown in
Figure 7. TEM and lattice images and SAED patterns of the LaTiO2N crystals obtained by nitridation of the La2TiO5 crystals grown using (a, b) KCl and (c, d) NaCl fluxes followed by acid treatment.
Figure 7a,c, although the LaTiO2N crystals seemed to have a mesocrystal-like structure, compared to the precursor La2TiO5 crystals, the diffraction spots in the SAED patterns (Figure 7b,d, insets) confirming the single crystalline characteristics of the mesocrystals did not reveal any evidence. In fact, a set of distinct concentric rings in the SAED patterns confirm polycrystallinity of porous columnar LaTiO2N structures. Therefore, it can be deduced that although the LaTiO2N crystals preserved the original shape of La2TiO5 crystals, single crystals were transformed into polycrystalline structures under nitridation. The crystal products obtained after acid treatment have porous structures with higher porosity compared to those obtained after nitridation. This is thought to have resulted from the dissolution of La2O3 from the structure of crystal products 337
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Figure 8. UV−vis diffuse reflectance spectra of the La2TiO5 crystals grown using (a) KCl and (b) NaCl fluxes, LaTiO2N crystals obtained by nitridation of the La2TiO5 crystals, and LaTiO2N crystals after acid treatment.
the absorption onsets of the LaTiO2N crystals obtained after acid treatment tended to become higher and shifted slightly toward longer wavelengths. That was likely to be because of the presence of La2O3 after nitridation, which was removed by acid treatment. It is noteworthy to mention here that the LaTiO2N crystals obtained by nitriding the La2TiO5 precursor synthesized in this study did not show a noticeable absorption in the near-infrared region above 600 nm that is generally attributable to some reduced Ti3+ species and nitrogen deficiency,21,22 even after long nitridation process of 45 h, compared to the LaTiO2N crystals conventionally synthesized by nitriding the La2Ti2O7 precursor. It can be said that the La2TiO5 crystals synthesized by chloride flux growth method are a suitable oxide precursor to obtain the LaTiO2N crystals with low defect density for better performance.
treated with dilute HCl. Moreover, acid treatment was expected to additionally remove the surface defects formed during the nitridation because the nitridation of oxide precursor particles takes place from the surface to the bulk. Thus, the photocatalytic activity of the LaTiO2N crystals must be higher by eliminating such surface defects which act as a trap for photoexcited carriers.12 The specific surface areas (SBET) of the crystal products obtained were also evaluated by using the BET method from the N2 adsorption isotherms. The SBET values of the La2TiO5 crystals grown using the KCl flux with the solute concentration of 20 mol % before and after nitridation and after acid treatment are < 1.6 m2·g−1, 9.4 m2·g−1, and 16.5 m2·g−1, respectively, whereas the La2TiO5 crystals grown using the NaCl flux before and after nitridation and after acid treatment are < 2.0 m2·g−1, 12.1 m2·g−1, and 18.4 m2·g−1, respectively. A difference between the specific surface areas of the two series of the samples resulted from the difference in the features of porous networks formed. Probably, lattice strain accumulation upon structural transformation in smaller crystals of La2TiO5 was slightly lower, resulting in a less porous structure. Similar results were previously reported by Maegli et al.17 for the LaTiO2N particles converted from La2Ti2O7 precursor by nitriding without flux. Nevertheless, compared to the crystal structures of SrTaO2N previously obtained by our group19 by nitriding flux-grown Sr2Ta2O7 crystals, the crystal structures of LaTiO2N obtained by nitriding flux-grown La2TiO5 crystals seem to be more highly porous stemming from the long nitridation process and acid treatment, and will be advantageous for various applications that specially require higher specific surface area. Figure 8 shows UV−Vis diffuse reflectance spectra of the La2TiO5 crystals grown using KCl and NaCl fluxes, LaTiO2N crystals obtained by nitridation of the La2TiO5 crystals, and LaTiO2N crystals after acid treatment. As for the La2TiO5 crystals grown using the KCl and NaCl fluxes, the wavelength of the absorption edges is approximately 320 nm (Eg = 3.87 eV). In contrast, the absorption edges exhibited by the crystal products obtained after nitridation are approximately 600 nm (Eg = 2.06 eV). Pure LaTiO2N crystals obtained after acid treatment exhibited light absorption onsets at approximately 600 nm, which corresponded to the band gap energy of approximately 2.06 eV, which is consistent with the previously reported data.20 The absorption edge shift was caused due to transforming the top of valence-band shift from O 2p orbitals in La2TiO5 to N 2p orbitals in LaTiO2N. Compared to the absorption onsets of crystal products obtained after nitridation,
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CONCLUSIONS In summary, highly crystalline La2TiO5 crystals were grown using the KCl and NaCl fluxes. The flux-grown La2TiO5 crystals had a columnar structure grown in the ⟨001⟩ direction and dimensions 0.7−3.1 μm. Using the NaCl flux, larger columnar La2TiO5 crystals were grown compared to the KCl flux. With increasing solute concentration, the aspect ratio of columnar La2TiO5 crystals decreased significantly from 3.81 to 2.25 using KCl flux and from 7.92 to 1.80 using NaCl flux. The LaTiO2N crystals with highly porous structure formed from the strong segregation of nanocrystals were prepared by nitridation of the La2TiO5 crystals with dense columnar structures at 950 °C for 45 h using NH3 gas followed by acid treatment. The SBET values of the crystal products obtained using the KCl flux before and after nitridation and after acid treatment were < 1.6 m2·g−1, 9.4 m2·g−1, and 16.5 m2·g−1, respectively, whereas using the KCl flux they were < 2.0 m2·g−1, 12.1 m2·g−1, and 18.4 m2· g−1, respectively. The optical absorption edges of the La2TiO5 and LaTiO2N crystals obtained by nitridation followed by acid treatment were approximately 320 and 600 nm, respectively, and the band gaps were estimated to be located at 3.87 and 2.06 eV, respectively. The LaTiO2N crystals obtained by nitriding the La2TiO5 precursor synthesized in this study did not show a noticeable absorption in the near-infrared region above 600 nm which is generally attributable to some reduced Ti3+ species and nitrogen deficiency, even after long nitridation process. Therefore, the La2TiO5 crystals synthesized by chloride flux growth method are a suitable oxide precursor to obtain the LaTiO2N crystals with low defect density and high surface area that will be advantageous for various applications. 338
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(16) Park, N.-Y.; Kim, Y.-I. Morphology and Band Gap Variations of Oxynitride LaTaON2 Depending on the Ammonolysis Temperature and Precursor. J. Mater. Sci. 2012, 47, 5333−5340. (17) Maegli, A. E.; Pokrant, S.; Hisatomi, T.; Trottmann, M.; Domen, K.; Weidenkaff, A. Enhancement of Photocatalytic Water Oxidation by the Morphological Control of LaTiO2N and Cobalt Oxide Catalysts. J. Phys. Chem. C 2014, 118, 16344−16351. (18) Lu, D.; Hitoki, G.; Katou, E.; Kondo, J. N.; Hara, M.; Domen, K. Porous Single-Crystalline TaON and Ta3N5 Particles. Chem. Mater. 2004, 16, 1603−1605. (19) Mizuno, Y.; Wagata, H.; Yubuta, K.; Zettsu, N.; Oishi, S.; Teshima, K. Flux Growth of Sr2Ta2O7 Crystals and Subsequent Nitridation to Form SrTaO2N Crystals. CrystEngComm 2013, 15, 8133−8138. (20) Zhang, F.; Yamakata, A.; Maeda, K.; Moriya, Y.; Takata, T.; Kubota, J.; Teshima, K.; Oishi, S.; Domen, K. Cobalt-Modified Porous Single-Crystalline LaTiO2N for Highly Efficient Water Oxidation Under Visible Light. J. Am. Chem. Soc. 2012, 134, 8348−8351. (21) Kim, Y.-I.; Woodward, P. M.; Baba-Kishi, K. Z.; Tai, C. W. Characterization of the Structural, Optical, and Dielectric Properties of Oxynitride Perovskites AMO2N (A = Ba, Sr, Ca; M = Ta, Nb). Chem. Mater. 2004, 16, 1267−1276. (22) Siritanaratkul, B.; Maeda, K.; Hisatomi, T.; Domen, K. Synthesis and Photocatalytic Activity of Perovskite Niobium Oxynitrides with Wide Visible-Light Absorption Bands. ChemSusChem 2011, 4, 74−78.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through equal contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was partially supported by the Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem).
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REFERENCES
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DOI: 10.1021/cg501397x Cryst. Growth Des. 2015, 15, 333−339
