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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 36933−36940
In Situ Formation of Micropore-Rich Titanium Dioxide from Metal− Organic Framework Templates Linzhi Zhai,†,‡,§ Yuhong Qian,†,§ Yuxiang Wang,† Youdong Cheng,† Jinqiao Dong,† Shing Bo Peh,† and Dan Zhao*,† †
ACS Appl. Mater. Interfaces 2018.10:36933-36940. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 11/01/18. For personal use only.
Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore ‡ School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China S Supporting Information *
ABSTRACT: Phase and porosity control in titanium dioxide (TiO2) is essential for the optimization of its photocatalytic activity. However, concurrent control over these two parameters remains challenging. Here, a novel metal−organic framework templating strategy is demonstrated for the preparation of highly microporous anatase TiO2. In situ encapsulation of Ti precursor in ZIF-8 cavities, followed by hydrolysis and etching, produces anatase TiO2 with a high Brunauer−Emmett−Teller surface area of 335 m2·g−1 and a micropore surface area ratio of 48%. Photocatalytic hydrogen generation catalyzed by the porous TiO2 can reach a rate of 2459 μmol·g−1· h−1. The measured photocatalytic activity is found to be positively correlated to the surface area, highlighting the importance of porosity control in heterogeneous photocatalysts. KEYWORDS: microporous titanium dioxide, in situ incorporation, metal−organic frameworks, templating synthesis, photocatalytic water splitting
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INTRODUCTION Hydrogen can be a promising alternative to fossil fuels if its mass production can be achieved using sustainable energy sources. As a result, hydrogen production from photocatalytic water splitting has attracted wide attention because of its sustainable feature.1 Among the available photocatalysts, TiO2based ones have raised great interest due to their low cost, favorable band gap (3.2 eV), good photostability, and environmentally friendly properties.2 Porous TiO2 materials have exhibited superior photocatalytic activity owing to their high surface areas and prosperous pore structures that can facilitate the exposure of active sites during the water splitting.3−6 Since the late 1990s, the templating approach has been successfully extended to the fabrication of porous materials.7 The templates can be classified into soft templates (e.g., surfactants,8 biopolymers,9 and microemulsions10) and hard templates (e.g., mesoporous silica11−14 and carbon15). Stucky and co-workers have prepared porous anatase TiO2 using poly(alkylene oxide) block copolymer EO20PO70EO20 (P123).16 The product possesses a pore size of 6.5 nm and a Brunauer−Emmett−Teller (BET) surface area of 205 m2·g−1. Zhao’s group has obtained three-dimensional (3D) mesoporous anatase TiO2 using P123 as a template, possessing large and uniform pores (pore size ∼ 4.5 nm) with a specific surface area of 145 m2·g−1.17 Mesoporous rutile (pore size ∼ 5.92 nm) and anatase-phase (pore size ∼ 5.33 nm) TiO2 were synthesized via a soft template-assisted hydrothermal proce© 2018 American Chemical Society
dure by employing cetyltrimethyl ammonium bromide and P123, respectively.18 However, porous TiO2 synthesized by soft templates usually possesses mesopores with pore sizes larger than 2 nm, and it is relatively challenging to obtain micropores (pore size less than 2 nm) using this approach. Besides soft templates, hard templates can also be used to prepare porous materials.19−24 Recently, metal−organic frameworks (MOFs) have grown as flourishing hard templates in preparing porous metal oxides including TiO2.25−33 For example, using MIL-101(Fe) as the template, Lin’s group has fabricated a Fe2O3@TiO2 core−shell composite with a BET surface area of 11.6 m2·g−1 for photocatalytic water splitting.34 Rutile and anatase TiO2 prepared from MIL-125 and NH2MIL-125 (Ti) have been reported as well.35,36 Xu and Zhao’s groups have synthesized porous TiO2/C composites with specific surface areas of 125 and 329 m2·g−1, respectively, by calcining Ti-MIL-125 precursor.37,38 Chen and Qian have reported porous anatase TiO2 with a BET surface area of 220 m2·g−1 and a pore diameter of about 1.9 nm through the calcination of MIL-125 (Ti).39 Wei and coauthors have prepared the porous anatase TiO2 from MIL-125 (Ti) precursor, with a specific surface area of 147 m2·g−1 and a mean pore size volume of 10 nm.40 Mallouk’s group has Received: July 16, 2018 Accepted: October 8, 2018 Published: October 8, 2018 36933
DOI: 10.1021/acsami.8b11920 ACS Appl. Mater. Interfaces 2018, 10, 36933−36940
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic illustration of the synthesis of microporous TiO2 derived from ZIF-8 template. FESEM images of (b) ZIF-8, (c) Ti(OiPr)4@ZIF-8, (d) TiO2@ZIF-8, and (e) TiO2; TEM images of (f) ZIF-8, (g) Ti(OiPr)4@ZIF-8, (h) TiO2@ZIF-8, and (i) TiO2; the inset shows HRTEM images of TiO2 nanoparticles. hydroxide were purchased from Merck & Co., Inc. All chemicals are of analytical grade and used without further purification. Synthesis of TiO2@ZIF-8 Composite. The synthesis process includes three stages: (1) in situ Ti-precursor encapsulation into ZIF8 during the formation of ZIF-8, (2) hydrolysis of Ti-precursor inside ZIF-8, and (3) removal of ZIF-8 template. First, 2-MeIm and Zn(Ac)2·2H2O were ground into fine powder and dried at 120 °C. The Ti(OiPr)4-embedded ZIF-8 composite was obtained through one-pot in situ solvothermal reaction. In a typical experiment, 2-MeIm (2.1 g), Zn(Ac)2 (2.2 g), and Ti(OiPr)4 (9 mL) were added into an autoclave and heated at 200 °C for 24 h. After reaction, the powder was centrifuged and washed with anhydrous ethanol several times to remove the excess Ti(OiPr)4 from the exterior surface of the ZIF-8. The resultant powder was then dried overnight to yield the Ti(OiPr)4@ZIF-8 composite. Subsequently, the Ti(OiPr)4@ZIF-8 powder was suspended in an ethanol/water mixture solution containing various modification agents and heated at 150 °C for 24 h. The resultant solid powder was centrifuged. TiO2@ZIF-8 was obtained after washing several times with water to remove the excess ethanol. Preparation of Microporous TiO2. TiO2@ZIF-8 powder was treated with HCl aqueous solution (0.1 M) to etch away the ZIF-8 template. Afterward, the powder was washed with water several times and dried under vacuum for 24 h. Photocatalytic Water Splitting. The photocatalytic water splitting reactions were carried out in a Pyrex irradiation reaction vessel connected to a glass enclosed gas system (Beijing Zhongjiao Jinyuan Technology Co., Ltd, CEL-SPH2N-D). In a typical experiment, TiO2 powder (50 mg) was added into aqueous solution (100 mL) containing methanol (20 mL) as a sacrificial agent. Platinum (1 wt %) was loaded onto the surface of TiO2 powder as a cocatalyst using an in situ photodeposition method with H2PtCl6 solution. A 300 W Xe lamp (CEL-HXF 300) was used as the light source. The wavelength of the light was controlled through an appropriate VisREF cutoff filter (350−780 nm). The temperature of the suspensions was maintained at 6 °C. H2 produced from water splitting was automatically analyzed by a gas chromatograph (TDX-01 packed column) equipped with a thermal conductivity detector. The experiment duration for each run was at least 6 h. The activity and
reported the synthesis of a brookite-phase titania with a micro/ mesoporous binary structure using HKUST-1 as the template by a two-step method (i.e., HKUST-1 preparation and liquidphase impregnation).41 The resultant TiO2 exhibits a higher BET surface area of 270 m2·g−1 and micropores with a diameter of about 6 Å. However, the impregnation of Ti precursor is subsequent to the synthesis of HKUST-1 template, which leaves a possibility that impregnating the Ti precursor simultaneously during the template (MOF) synthesis may simplify the preparation, increase the loading content of Ti precursor, and enhance the surface area of the obtained porous TiO2. Herein, we propose a new approach for the fabrication of MOF templates by the in situ encapsulation of Ti precursors for the preparation of micropore-rich TiO2 with high BET surface areas and decent photocatalytic activity. The judicious selection of encapsulating MOF templates and titanium precursors enables the rapid one-pot fabrication of Ti(OiPr)4@ZIF-8 composites. Subsequently, by applying optimized hydrolysis and post-synthetic etching processes, anatase TiO2 with high BET surface areas and substantial microporosity can be developed. The photocatalytic performance of the resultant porous TiO2 in catalyzing water splitting reaction displays a positive correlation between surface area and hydrogen evolution rate, underling the role of porosity in heterogeneous photocatalysis.
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EXPERIMENTAL SECTION
Materials. 2-Methylimidazole (2-MeIm, 99%), chloroplatinic acid, and titanium dioxide P25 were purchased from Sigma-Aldrich. Zinc acetate dihydrate [Zn(Ac)2·2H2O, 98.0−101.0%] and titanium(IV) isopropoxide [Ti(OiPr)4, 97%] were obtained from Alfa Aesar. Ethanol was purchased from VWR. Titanium oxides and sulfuric acid were purchased from Sinopharm Chemical Reagent Co., Ltd and Fisher Scientific, respectively. Sodium sulfate and ammonium 36934
DOI: 10.1021/acsami.8b11920 ACS Appl. Mater. Interfaces 2018, 10, 36933−36940
Research Article
ACS Applied Materials & Interfaces
Figure 2. PXRD patterns of (a) simulated ZIF-8, synthesized ZIF-8, Ti(OiPr)4@ZIF-8, and TiO2@ZIF-8; (b) TiO2 obtained under different hydrolysis conditions; (c) XPS spectra of ZIF-8, Ti(OiPr)4@ZIF-8, TiO2@ZIF-8, and TiO2; (d) Ti 2p XPS spectrum of Ti(OiPr)4@ZIF-8 and TiO2@ZIF-8; (e) N2 sorption isotherms and (f) pore size distributions of ZIF-8, Ti(OiPr)4@ZIF-8, and TiO2@ZIF-8. recyclability of the photocatalysts were evaluated for at least three cycles. Characterization. N2 sorption isotherms were measured using a Micromeritics ASAP 2020 surface area and pore size analyzer. Before each measurement, the sample was degassed under high vacuum ( T2 > T3 > T6 > T5 > T1 (Table 1). With the exception of T1, the measured hydrogen production rate of TiO2 samples is found to be positively correlated to the surface area. T4 exhibits the highest photocatalytic activity for H2 production, which may mainly depend on its outstanding surface area that is in agreement with the previous results.13 Moreover, the band gap of T4 was lower than that of synthesized TiO2 samples (Figure 4d, Table S7). Hydrogen evolution activities of T5 and T6 are 3.92 and 4.03 times higher than that of T1, respectively, indicating that Smicro/St proportion and particle size may also play important roles.
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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/acsami.8b11920. XPS, PXRD, gas sorption isotherm, TEM image, TiO2 porosity, and photocatalytic activity (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].. ORCID
Dan Zhao: 0000-0002-4427-2150
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Author Contributions §
CONCLUSIONS In summary, we herein report the preparation of microporerich TiO2 obtained from TiO2@ZIF-8 composite prepared in situ. Our results confirm the possibility of in situ trapping the Ti precursors inside the pores of ZIF-8 during the formation of MOF template. The in situ-MOF-templated strategy can obviously enhance the surface areas and micropore portions of resultant porous TiO2. The highest surface area of TiO2 prepared in this study can reach 335 m2·g−1, with a micropore portion of 48%. The corresponding photocatalytic hydrogen production rate is as high as 2459 μmol·g−1·h−1, and photocatalytic hydrogen production activity mainly depends on the surface area. This work opens a new strategy in using MOFs as hard templates to synthesize micropore-rich metal oxides with high surface areas and larger micropore portions.
L.Z. and Y.Q. contributed equally to this work.
Author Contributions
The manuscript was written through 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 work is supported by National University of Singapore (CENGas R-261-508-001-646), Ministry of Education Singapore (MOE AcRF Tier 1 R-279-000-472-112, R-279000-540-114), and Agency for Science, Technology, and Research (PSF 1521200078, IRG A1783c0015). 36938
DOI: 10.1021/acsami.8b11920 ACS Appl. Mater. Interfaces 2018, 10, 36933−36940
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ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.8b11920 ACS Appl. Mater. Interfaces 2018, 10, 36933−36940
