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Cite This: Inorg. Chem. 2019, 58, 7989−7996
Conversion of a 2D Lepidocrocite-Type Layered Titanate into Its 1D Nanowire Form with Enhancement of Cation Exchange and Photocatalytic Performance Mohamed Esmat,†,‡,§ Ahmed A. Farghali,§ Samaa. I. El-Dek,§ Mohamed H. Khedr,§ Yusuke Yamauchi,∥ Yoshio Bando,†,⊥,# Naoki Fukata,†,‡ and Yusuke Ide*,†
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International Center for Materials Nanoarchitechtonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan § Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University (BSU), Beni-Suef 62511, Egypt ∥ Australian Institute for Bioengineering and Nanotechnology (AIBN) and School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia ⊥ Australian Institute for Innovative Materials, University of Wollongong, Squires Way, North Wollongong, NSW 2500, Australia # Institute of Molecular Plus, Tianjin University. No. 11 Building, No. 92 Weijin Road, Nankai District, Tianjin 300072, P. R. China S Supporting Information *
ABSTRACT: Layered titanates with one-dimensional (1D) shapes have been an important class of nanomaterials due to their combination of 1D and 2D fascinating properties. Among many layered titanates, lepidocrocite-type layered titanates have significant advantages such as superior intercalation and exfoliation properties, while the synthesis of the 1D-shape forms is still challenging. Here, we report on a facile one-pot hydrothermal conversion of a lepidocrocite-type layered titanate into the corresponding nanowire-shape form. The reaction mechanism involves the decomposition of the starting layered titanate into 1D small segments which assemble into the nanowire. This new nanowire shows properties resulting from the combination of 1D and 2D nanostructural features, excellent cation exchange ability, and high photoinduced charge separation and photocatalytic efficiency. As a demonstration, we evaluate the nanowire as a sequestrating material capable of collecting toxic cations, like Cd2+, from water and photoreducing them (immobilizing them tightly). We find that the nanowire shows an efficient and ultrafast photoimmobilization activity, whereas the starting layered titanate and a benchmark TiO2 photocatalyst (P25) show no activity under the identical conditions. The photoimmobilization rate (within 1 min) is considerably faster than the cation exchange rates reported for state-of-the-art cation exchangers (with no photoimmobilization ability). The nanowire used for photoimmobilization reactions is easily recovered from water by decantation, showing the possible practical use for safe disposal of toxic cations in the environment.
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INTRODUCTION In the past few decades, one-dimensional (1D) TiO2 nanostructured materials have attracted the attention of scientists due to their fascinating physical and chemical properties. TiO2 nanowires (NWs) are an especially important class of 1D nanostructured materials because they offer unique charge transport (separation) properties.1,2 TiO2 NWs, nowadays, find many applications that deal with environmental and energy issues, such as photocatalysts, solar cells, and supercapacitors.3−5 Besides 1D TiO2 nanostructures, two-dimensional (2D) layered titanates and their exfoliated nanosheets have been extensively investigated for many environmental and energy applications.6−8 Compared with other layered titanates, © 2019 American Chemical Society
lepidocrocite-type layered titanates, with a chemical formula of AxTi2−yMyO4 (A, interlayer cation; M, metal or vacancy), have significant advantages, such as rich intercalation chemistry, excellent exfoliation ability, and composition-tunable properties.9−11 The synthesis of lepidocrocite-type layered titanates with a shape of NWs is promising for further enhancement of the properties and exploitation of new applications. However, only a few studies have been reported for the synthesis of lepidocrocitetype layered titanate NWs, and the conventional synthetic methods are relatively complicated without using lepidocrociteReceived: March 12, 2019 Published: May 28, 2019 7989
DOI: 10.1021/acs.inorgchem.9b00722 Inorg. Chem. 2019, 58, 7989−7996
Article
Inorganic Chemistry
Figure 1. Schematic illustration for the formation of KTLO NWs via decomposition/recrystallization of KTLO under hydrothermal conditions (upper), and that of anatase TiO2 via dissolution/deposition of KTLO is also shown (lower). Color coding: red = O, purple = K, green = Ti.
Figure 2. SEM images of (a) KTLO and (b) KTLO NWs. (c) XRD patterns and (d) Raman spectra of KTLO and KTLO NWs. (e) HRTEM and ED pattern of KTLO NWs.
type layered titanates as starting materials.12,13 Additionally, no studies have been reported on the enhanced charge separation efficiency of lepidocrocite-type layered titanate NWs, which is crucial for the applications such as photocatalysis and photovoltaics. Here, we report on a facile synthesis of NWs of a lepidocrocite-type layered titanate, KxTi2−x/3Lix/3O4 (named KTLO), one of the most studied layered titanates. We show that the starting 2D KTLO is converted into the corresponding 1D NWs via a specially designed hydrothermal reaction (Figure 1). We demonstrate that the new NWs exhibit properties originating from a combination of 1D and 2D structural features, excellent cation exchange ability, and high photoinduced charge separation/photocatalytic efficiency.
As an application, we also report on the use of these NWs to collect toxic cations from water and photocatalytically reduce them (immobilize them tightly). Due to the large ion exchange capacity and chemical and thermal stabilities, layered inorganic solids, composed of inorganic nanosheets and interlayer exchangable ions, have been extensively studied as ion exchangers for purposes such as removal of toxic elements from water.14−16 Generally, layered inorganic solids readily release collected ions via back exchange reactions, which limits their practical applications. In contrast, several layered inorganic solids, like synthetic micas, undergo a structure collapse during the ion exchange, resulting in tight immobilization of collected ions in the interlayer space (so-called irreversible ion exchange).17,18 As a result, the complete removal of target ions from water and the safe disposal of collected hazardous ions 7990
DOI: 10.1021/acs.inorgchem.9b00722 Inorg. Chem. 2019, 58, 7989−7996
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Inorganic Chemistry
Figure 3. SEM images of hydrothermal products of KTLO obtained by using (a) two, (b) four, and (c) six times the amount of TPAOH. (d) XRD patterns of KTLO and hydrothermal products of KTLO obtained by using two, four, and six times the amount of TPAOH.
the product, and a little effect of this impurity on the performance will be discussed later). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (ED) revealed the single-crystal nature and layered structure of KTLO NWs with an interplanar distance of 0.79 nm (Figure 2e), consistent with the XRD data. The composition of KTLO NWs was investigated because KTLO has the general composition of KxTi2−x/3Lix/3O4 (the range of x explored was 0.6−0.8) and the properties, like hydration/swelling ability, depend on the composition (layer charge density).11 From elemental analysis (Table S1 in the Supporting Information), the composition of KTLO and KTLO NWs was calculated as K0.80Ti1.73Li0.27O4 and K0.63H0.21Ti1.72Li0.28O4, respectively. Their layer charge density was almost identical ([Ti1.73Li0.27O4]0.80− vs [Ti1.72Li0.28O4]0.84−), while KTLO NWs had a smaller amount of the interlayer K+ (H+ should exist to compensate the negative charge of the layers). As a result, KTLO NWs can accommodate a larger amount of interlayer H2O (or H3O+) molecules (as shown in Figure S1) and thus have a slightly larger basal spacing than KTLO (Figure 2c). We discussed the formation mechanism of KTLO NWs by changing hydrothermal conditions. Figure 3a−c shows the SEM images of the hydrothermal products obtained by using larger amounts of TPAOH at 170 °C for 1 week. With the increasing amount of TPAOH, the amount of NWs decreased while the amount of spindle-shaped particles increased. As shown in Figure 3d, diffraction peaks due to KTLO were weakened and those due to TiO2 anatase were strengthened with the increasing amount of TPAOH. Considering that TPAOH should promote the decomposition/dissolution of starting layered titanates (while NH4F acts as mineralizer to recrystallize the decomposed/dissolved species) according to our previous experiments,21−25 we can consider a possible mechanism of the formation of KTLO NWs from KTLO (Figure 1). KTLO decomposes into small segments, 1D chains of edge- and cornershared TiO6 octahedra; these segments assemble into 1D KTLO NWs under lower TPAOH concentration, while KTLO
to avoid leaching from the ion exchangers can be achieved. However, diffusion of ions into layered materials, generally, is not fast;11 thus, it is clearly necessary to develop advanced layered material-based ion exchangers exhibiting ultrafast tight immobilization of target ions. In this study, we show that KTLO NWs show efficient and ultrafast photoimmobilization for Cd2+ in water, which cannot be delivered by even state-of-the-art cation exchangers and a benchmark TiO2.
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RESULTS AND DISCUSSION It is well-known that bulk TiO2 is converted to 1D nanostructured TiO2 including nanotubes under alkali hydrothermal conditions.2 In contrast, the presently developed hydrothermal reaction for synthesizing KTLO NWs is inspired by interzeolite conversion, which is nowadays recognized as a useful method to strategically design zeolites.19,20 In interzeolite conversion, it is proposed that the starting zeolites decompose into small segments (so-called nanoparts) and the nanoparts recrystallize into other zeolites with the aid of structure-directing agents and mineralizers under hydrothermal conditions. We sought the synthetic conditions to decompose metal oxides, such as TiO2 nanoparticles and layered niobates, to obtain new nanomaterials using hydrothermal reactions similar to those used in interzeolite conversion.21−25 In this study, we successfully developed a new hydrothermal method to convert KTLO into KTLO NWs. KTLO, prepared by a solid-state reaction at 600 °C,26 was composed of plate-like particles with a lateral size of 100−200 nm and a thickness of several tens of nanometers, as shown in the scanning electron microscopy (SEM) image (Figure 2a). We hydrothermally treated KTLO in the presence of tetrapropylammonium hydroxide (TPAOH) and ammonium fluoride (NH4F) at 170 °C for 1 week. As shown in Figure 2b, the product was shaped in the form of a nanowire having a length of several hundreds of nanometers and a diameter of approximately 10 nm. X-ray diffraction (XRD)27 and Raman spectroscopy28,29 (Figure 2c, d) revealed that the nanowire-shaped product was KTLO (only a small amount of TiO2 anatase was contained in 7991
DOI: 10.1021/acs.inorgchem.9b00722 Inorg. Chem. 2019, 58, 7989−7996
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Inorganic Chemistry
Figure 4. (a) XRD patterns of the hydrothermal products of KTLO obtained for different reaction times. Samples labeled as 0 and 7 d are identical to KTLO and KTLO NWs, respectively. (b) SEM image of the hydrothermal product of KTLO obtained for 3 days.
Figure 5. SEM images (upper) and XRD patterns (lower) of anatase and the hydrothermal product.
dissolves into Ti4+ to deposit anatase under higher TPAOH concentrations. Actually, it was reported that KTLO and other lepidocrocite-type layered titanates decomposed into such small 1D segments upon treatment with concentrated HCl at ambient conditions30 or during swelling/exfoliation in water containing organic ammoniums.31 To obtain a deeper insight into the formation mechanism, we hydrothermally treated KTLO under the similar conditions for a shorter period. Figure 4 shows the XRD pattern and SEM image of the hydrothermal product obtained using a standard amount of TPAOH at 170 °C for 3 days, which demonstrates that a large amount of KTLO still remains in this stage. Thus, the decomposition rate of KTLO under the present hydrothermal condition is slower than the dissolution rate (hour scale) of lepidocrocite-type layered titanates under alkali hydrothermal conditions with NaOH.29 We also performed a hydrothermal reaction of anatase TiO2 under identical conditions (Figure 5). The structure and morphology of the starting anatase scarcely changed after the reaction. This means that anatase is stable enough not to decompose or dissolve under the present alkali condition, while KTLO, a metastable phase, is unstable. The particle size and crystallinity of anatase slightly increased after the hydrothermal
reaction. This is probably due to the dissolution and reprecipitation occurred on only the surface of anatase nanoparticles. We investigated the possibility of KTLO NWs capable of photoimmobilizing metal cations in water. First, we investigated the cation exchange property (without light irradiation) of KTLO NWs if it could show better performance than KTLO. We selected Cd2+ (ca. 50 ppm) to be adsorbed because it is one of the most toxic cations existing in environments and is difficult to adsorb by usual cation-exchangable layered inorganic solids and TiO2.16,32 As shown in Figure 6a, KTLO NWs adsorbed Cd2+ more rapidly than KTLO. The adsorption on KTLO NWs was completed within 20 min, which was considerably shorter than that on KTLO (more than 60 min). The maximum adsorption amount was estimated as 0.7 mmol g−1, which was almost identical to the cation exchange capacity (98%), and TiO2 P25 (Nippon Aerosil) at the molar ratio of 2.4:0.8:10.4 according to a previous report.26 Briefly, the starting materials were mixed together using an agate mortar for 2 h, and the mixture was then calcined in air at 600 °C for 20 h. After it was cooled to room temperature, the powder was mixed again and then calcined in air at 600 °C for an additional 20 h. Synthesis of KTLO NWs. For the synthesis of KTLO NWs, 0.78 g of TPAOH (Tokyo Chemical Industry, 40 wt % aqueous solution), 0.014 g of NH4F (Sigma-Aldrich, >99.9%), and 0.2 g of KTLO were added in a Teflon-lined stainless-steel autoclave, and the mixture was kept at 170 °C for 1 week. After the hydrothermal reaction, the product was washed with ethanol and dried at 60 °C. Depending on the KTLO used, the hydrothermal reaction period was prolonged to 2 weeks to synthesize KTLO NWs reproducibly. The hydrothermal reaction of anatase (JRC-TIO-1, supplied by the Catalysis Society of Japan, with a primary particle size of ca. 20 nm) was also performed under the identical conditions. As a control, the hydrothermal reaction of KTLO was performed using different conditions (TPAOH amount, reaction time) at 170 °C. Characterization. XRD patterns were collected using a powder Xray diffractometer (Smart Lab, RIGAKU) with Cu Kα radiation at 40 kV and 30 mA. The morphology was observed with a HITACHI SU8230 microscope, operating at 10.0 kV. HRTEM images and EDX elemental maps were taken with a JEOL JEM-2100F microscope, operated at 200 kV. Micro-Raman scattering measurements (Photon Design Company) were carried out at room temperature using a 100× objective and a 532 nm excitation light source. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was performed on an Agilent 710-ES spectrometer. Thermogravimetric curves were recorded using a Hitachi TG/DTA6200. UV−vis spectra were obtained with a JASCO V-570 spectrophotometer. XPS was performed with a PHI Quantera SXM instrument, operated with Al Kα radiation at 20 kV at 5 mA. The binding energy shift was calibrated using the C1 s level at 285.0 eV. Photoreduction and Cation Exchange of Metal Cations. The sample (15 mg) was added in an aqueous CdCl2 (STREM Chemicals, 99.9%) or NiCl2 (Sigma-Aldrich, 99.9%) solution (20 mL, ca. 50 and 95 ppm for Cd2+ and Ni2+, respectively) in a Pyrex glass tube (34 mL), and the mixture was deaerated by Ar bubbling. Methanol was added as a sacrificial agent. The glass tube was sealed with a rubber septum and irradiated with a solar simulator (San-Ei Electric, λ > 300 nm, 1000 7994
DOI: 10.1021/acs.inorgchem.9b00722 Inorg. Chem. 2019, 58, 7989−7996
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Inorganic Chemistry Wm−2) under stirring. After separation of the mixture, the amount of Cd2+ or Ni2+ in the supernatant was quantified with ICP-AES. The solid was analyzed by XRD and XPS to confirm the deposition of Cd and Ni (typical data is shown in Figure S2). Cation exchange with Cd2+ or Ni2+ was carried out without the photoirradiation. Quantitative Analysis of Anatase in KTLO NWs. The amount of anatase in KTLO NWs was estimated using a calibration curve, which showed anatase/KTLO NWs intensity ratios as a function of the anatase content in binary mixtures of KTLO NWs and anatase. KTLO NWs and anatase (JRC-TIO-1) were mixed at different ratios by an agate mortar. The obtained mixtures were analyzed by XRD, and the (101) plane of anatase/the (020) plane of KTLO NWs intensity ratios were plotted against the anatase weight percent in the mixtures to prepare the calibration curve. Formic Acid Oxidation. The powder sample (15 mg) was dispersed in an aqueous solution (5 mL) containing 5 vol % of formic acid (Wako Pure Chemical Industry, 98%) in a Pyrex glass tube (34 mL) and then aerated by O2 bubbling. The glass tube was sealed with a rubber septum and irradiated by a solar simulator (San-Ei Electric, λ > 300 nm, 1000 W m−2) under stirring. The headspace CO2 was quantified by a Shimadzu GC-2010 plus gas chromatograph equipped with a barrier ionization discharge (BID) detector. H2 Evolution. The powder sample (15 mg) was dispersed in aqueous methanol solution (5 mL, 1/1 in V/V) in a Pyrex glass tube (34 mL), and H2PtCl6·6H2O (Wako Pure Chemical Industry, 99.9%, Pt per sample = 0.5 wt %) was dissolved in the dispersion. The resulting dispersion was deaerated by Ar bubbling. The glass tube was sealed with a rubber septum and irradiated by a solar simulator (San-Ei Electric, λ > 300 nm, 1000 W m−2) under stirring. The headspace H2 was quantified by a Shimadzu GC-2010 plus gas chromatograph equipped with a BID detector.
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provide nano- and micro-fabrication facilities for Australia’s researchers.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00722.
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REFERENCES
(Table S1) Composition of KTLO and KTLO NWs; (Figure S1) thermogravimetric curves of KTLO and KTLO NWs measured in air; (Figure S2) XPS spectra of P25, KTLO, and KTLO NWs after the Cd2+ photoreduction reaction; (Figure S3) Ni deposition rates from water containing methanol on P25, KTLO, and KTLO NWs under light irradiation; and (Figure S4) HAADFSTEM images and the corresponding EDX elemental maps of KTLO NWs (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Samaa. I. El-Dek: 0000-0003-4564-9455 Naoki Fukata: 0000-0002-0986-8485 Yusuke Ide: 0000-0002-6901-6954 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Science and Technology Development Fund (STDF), Egypt, through the Short-Term Fellowship (STDF-STF) grant (25505). This work was performed in part at the Queensland node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to 7995
DOI: 10.1021/acs.inorgchem.9b00722 Inorg. Chem. 2019, 58, 7989−7996
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DOI: 10.1021/acs.inorgchem.9b00722 Inorg. Chem. 2019, 58, 7989−7996
