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Sol–gel synthesis of nanosized titanium oxide in a porous coordination polymer
Microporous and Mesoporous Materials 195 (2014) 31–35
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Sol–gel synthesis of nanosized titanium oxide in a porous coordination polymer Cho Rong Kim a, Takashi Uemura a,b,⇑, Susumu Kitagawa a,c,⇑ a
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan c Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan b
a r t i c l e
i n f o
Article history: Received 16 December 2013 Received in revised form 8 April 2014 Accepted 9 April 2014 Available online 18 April 2014 Keywords: Titanium oxide Porous coordination polymer Nanocomposite Photoresponsive material
a b s t r a c t Nanosized titanium oxide (TiO2) was synthesized in the channels of a porous coordination polymer (PCP) [La3+(1,3,5-benzenetrisbenzoate)]n by the sol–gel reaction of titanium tetraisopropoxide. XRD, IR, UV–vis, and gas sorption measurements demonstrated that nanosized TiO2 was formed in channels of the PCP. In this system, the resultant PCPTiO2 composite showed enhancement of the adsorption of water. Furthermore, the sorption behavior of the composite could be changed by UV irradiation. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction The design and synthesis of PCPs with unique structural topologies and electronic functions using the self-assembly approach of metal ions and organic ligands have been studied. Because of their regular porous structures with large surface areas, permanent porosity, and surface functionality, PCPs have emerged as an important new class of nanoporous materials with potential for many applications in storage, separation, catalysis, and chemical sensing [1–8]. The fabrication of metal and metal oxide particles accommodated in PCPs has made rapid progress because the doping of only a small amount of the materials can afford new functions to PCPs [9–14]. For example, we have previously reported the synthesis of nanosized silica in the channels of HKUST-1 ([Cu3(btc)2]n; btc = benzene-1,3,5-tricarboxylate) which showed higher sorption amounts for a hydrophilic molecule compared to that of only the host [15–16]. Furthermore, magnetic property of a zeolitic
⇑ Corresponding authors. Address: Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan. Tel.: +81 753 2733; fax: +81 753 2732 (T. Uemura). Address: Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan (S. Kitagawa). E-mail addresses: [email protected] (T. Uemura), kitagawa@icems. kyoto-u.ac.jp (S. Kitagawa). http://dx.doi.org/10.1016/j.micromeso.2014.04.020 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.
imidazolate framework (ZIF) was exhibited by the incorporation of Fe3O4 particles, which allowed recovery of the PCP from a solvent suspension by applying a magnetic field [17]. For the enhancement of adsorption property for hydrogen molecules, utilization of nanocomposite consisting of Pd in PCP pores has been studied [18,19]. This methodology is useful enough to alter the properties of PCPs without changing the host structures, such as the pore size, shape, and surface. Titanium oxide (TiO2) is a very useful functional material due to its wide range of applications in the fields of photocatalysis, optical materials, dye-sensitized solar cells, lithium-ion batteries, and superhydrophilic materials [20–22]. These functions of TiO2 are derived from the change of the energy transfer of TiO2 by UV irradiation [23–25]. Recently, the use of nanoporous matrices as host media for the formation of nanosized TiO2 has become considerable interest for specific nanosize properties and host–guest synergistic functions [26–28]. In this work, we successfully performed the synthesis of TiO2 within one-dimensional nanochannels of [La3+(1,3,5-benzenetrisbenzoate)]n (1; pore size = 10 10 Å2) by sol–gel polycondensation of titanium(IV) tetraisopropoxide (TTIP). The resulting material was characterized by XRD, IR, UV–vis, and SEM-EDX measurements. Alteration of the adsorption behaviors of the obtained composite by irradiation of UV light was investigated. We expect that this work will contribute to the development of PCPs for use as photosensitive materials.
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C.R. Kim et al. / Microporous and Mesoporous Materials 195 (2014) 31–35
2. Experimental 2.1. Materials All reagents and chemicals used were obtained from commercial sources, unless otherwise noted. The host PCP (1) was prepared using the previously reported method [29]. 2.2. Sol–gel synthesis of accommodating TiO2 within 1 The dried host compound 1 (53 mg) was prepared by evacuation (<0.1 kPa) at 120 °C for 3 h in a Pyrex tube. Subsequently, 31 mg of titanium(IV) tetraisopropoxide (TTIP) was added to 1 suspended in 3 mL of dehydrated 2-propanol, and the resulting mixture was stirred for 10 min under nitrogen atmosphere to incorporate TTIP into the nanochannels of 1. The solvent was then evaporated to give 1TTIP adduct (1TTIP). 1TTIP adduct was heated at 250 °C for 24 h under exposure to water vapor to facilitate the sol–gel reaction of TTIP. Further condensation was achieved by incubating the material under vacuum at 120 °C to remove byproducts such as 2-propanol and water in the condensation process, which gave PCP-TiO2 composite (1TiO2, 60 mg).
polycondensation of titanium alkoxide leading to the formation of a titanium oxide [30]. TTIP was incorporated into the channel by immersion of 1 in a 2-propanol solution of TTIP, followed by evaporation of 2-propanol (1TTIP). 1TTIP adduct was left under H2O atmosphere for hydrolysis of TTIP in the pore and was heated at 250 °C to initiate the polycondensation. Further condensation was achieved by evacuating the resulting material at 120 °C to provide a composite (1TiO2). We checked the XRD of 1TiO2 and found that the pattern of 1TiO2 was almost the same as that of 1, indicating that the framework was retained after the sol–gel reaction (Fig. 1). In the UV–vis spectroscopy results, an increase in the absorption at wavelength around 350–400 nm can be assigned to the intrinsic absorption of TiO2, representing the existence of TiO2 particles in the composite (Fig. 2) [31]. Quantitative analysis of 1TiO2 showed that the number of Ti atoms per unit cell of 1 was 0.48 as determined by the SEM-EDX result. The energy dispersive X-ray analysis for elemental mapping measurements (EDX) revealed that TiO2 particles were homogeneously dispersed in the channels of 1 (Fig. S1). Because the elemental map for 1TiO2 showed almost the same ratio of Ti atoms in the unit of 1 at the different accelerating
2.3. Isolation of nanosized TiO2 from the host framework To isolate TiO2 particles inside 1, the composite (60 mg) was stirred overnight in a 0.05 M aqueous solution (40 mL) of sodium ethylenediaminetetraacetate (Na-EDTA) for the complete dissolution of 1. The isolated TiO2 (8 mg) was washed with water (5 mL 3) and dried under a reduced pressure at room temperature. 2.4. Photoirradiation of 1TiO2 An activated sample of 1TiO2 in a cylindrical Pyrex glass tube was irradiated with a 500 W ultrahigh pressure mercury lamp (385–425 nm) for 12 h equipped with glass filters at 298 K. 2.5. Measurements X-ray powder diffraction (XRPD) data were collected using a Rigaku RINT 2000 Ultima diffractometer employing CuK radiation. SEM-energy-dispersive X-ray (EDX) measurements were conducted by using a JEOL JED-2300 detector in a JEOL JSM-5600 at an accelerating voltage of 15 kV and 30 kV. The thermogravimetric (TG) analysis was carried out from room temperature to 500 °C using a Rigaku Instrument Thermo plus TG 8120 in a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ULVAC-PHI Model 5500 spectrometer with 15 kV, 400 W Mg Ka emission as the X-ray source. The charging effect was corrected by adjusting the binding energy of the C 1s peak to be 284.6 eV. Nitrogen adsorption measurements at 77 K and solvent vapor adsorption measurements at 298 K were carried out using a Belsorp-Max and Belsorp-Aqua volumetric adsorption instruments, respectively. The samples were activated under high vacuum (<10 2 Pa) at 120 °C for 6 h before the sorption measurements.
Fig. 1. XRPD of 1, 1TiO2, TiO2 isolated from 1, and bulk synthesized TiO2.
3. Results and discussion 3.1. Synthesis of nanosized TiO2 in the nanochannels of PCP We carried out the synthesis of nanosized TiO2 in the onedimensional nanochannels of 1 using a sol–gel polycondensation of TTIP. The sol–gel process is based on hydrolysis and
Fig. 2. UV–vis spectra of 1, 1TiO2, and TiO2 isolated from 1.
C.R. Kim et al. / Microporous and Mesoporous Materials 195 (2014) 31–35
voltage, TiO2 particles were not gathered near the surface of 1, but were dispersed in the channels of 1. As shown in Fig. 3, nitrogen adsorption on 1TiO2 demonstrates a certain decrease in the adsorption amount for 1TiO2 compared to 1 alone can be attributed to the partial filling of the pores in 1 with TiO2 particles. However, this nitrogen adsorption result shows that the micropores in 1TiO2 are available for further adsorption of guest molecules. The pore size distribution of 1 was almost the same as that of 1TiO2 as determined by the non-localized density functional theory (NLDFT) method (Fig. 3b). Furthermore, the adsorption of nitrogen on 1TiO2 started at a similar pressure to that of 1 (Fig. 3a inset). Thus, these results revealed that the pores of 1TiO2 were not covered with a thin layer of TiO2, but were partially occupied with TiO2 particles in the pores. Therefore, in this system, the sol–gel polycondensation successfully occurred only inside the channels, without destruction of the framework and deposition of TiO2 particles outside the pores. In the IR spectrum of 1TiO2, a broad band at 3400 cm 1 corresponded to the characteristics of the hydroxyl group of 1TiO2 (Fig. 4) [32]. In addition, a shoulder peak around 1645 cm 1 was observed which was derived from Ti OH stretching. In the low energy interval of the intensive broad band below 800 cm 1 was due to the Ti O Ti moiety of TiO2. For further understanding of the formation of TiO2, TiO2 was isolated from 1TiO2 by stirring in 0.05 M aqueous EDTA solution. As shown in Fig. 1, amorphous TiO2 was formed in the channels of 1. In contrast, anatase crystals were formed when we carried out the bulk sol–gel reaction in the similar condition. Therefore, TiO2 particles were formed in the inner space of 1, not on the surface of 1. Formation of TiO2 was also confirmed by observing the characteristic peaks for TiO2 in UV–vis and IR spectroscopy (Figs. 2 and 4). Hydroxyl groups on the surface of TiO2 have an influence on adsorption behavior of 1TiO2. As shown in Fig. 3, the nitrogen adsorption amount of 1TiO2 was decreased owing to the occupation of TiO2 in the pores of 1. However, 1TiO2 exhibited a distinctive adsorption behavior for water (Fig. 5). In this system, open metal sites in the host framework would preferentially interact with water molecules prior to the surface OH groups, because TiO2 particles were partially dispersed inside the pore (Fig. S1). Thus, 1 and 1TiO2 showed almost the same adsorption of water below the P/P0 = 0.3 (Fig. 5 inset). However, in the higher pressure region, we observed that 1TiO2 showed a higher uptake of water than 1 alone. This remarkable difference in adsorption
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Fig. 4. IR spectra of 1, 1TiO2, isolated TiO2 from 1, and 1TiO2 irradiated with UV.
Fig. 5. Water adsorption isotherms for 1, 1TiO2, and 1TiO2 irradiated with UV at 298 K. (inset) Isotherms plotted at low pressure region of P/P0 from 0.2 to 0.4.
Fig. 3. (a) Nitrogen adsorption isotherms at 77 K for 1 (black), 1TiO2 (red), and 1TiO2 (blue) irradiated with UV. (inset) Isotherms plotted against a logarithmic relative pressure. (b) Pore size distribution of 1, 1TiO2, and 1TiO2 irradiated with UV using non-localized density functional theory, determined from the N2 adsorption profile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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demonstrated the interaction of surface OH groups of TiO2 with water. In addition, based on the result of IR measurement, the increase of water adsorption of 1TiO2 could be derived from the hydroxyl groups on the surface of the accommodated TiO2, interacting with water molecules effectively. Such enhancement of water adsorption was also observed in the previous work with regard to the modification of the PCP pores with silica [16].
3.2. Alternative photoresponsive properties of 1TiO2 with UV irradiation To examine the photoresponsive property of the composite, 1TiO2 was irradiated with UV for 12 h at room temperature. Then, we carried out nitrogen and H2O adsorption measurements at 77 K and 298 K, respectively. The nitrogen adsorption amount of 1TiO2 was increased when the composite was irradiated with UV as shown Fig. 3a. Compared with the isotherm obtained from 1TiO2, the pore volume was changed from 0.44 to 0.53 cm3/g after UV irradiation. In contrast, H2O adsorption amount of 1TiO2 was decreased by UV irradiation (Fig. 5). IR measurement of 1TiO2 irradiated with UV could explain the reason for the change of the H2O adsorption behavior. According to the IR spectrum of 1TiO2 irradiated with UV, the peak intensity of the OH groups at 3400 cm 1 was reduced (Fig. 4). Therefore, decreasing the amount of H2O adsorption results from the loss of the hydroxyl group in 1TiO2 by UV irradiation. The change of H2O adsorption behavior of composite irradiated with UV has relevance to the chemical state of accommodated TiO2. X-ray photoelectron spectroscopy (XPS) analysis was used to determine the state of TiO2 nanoparticles formed in 1. The XPS data for Ti 2p region showed that the Ti atoms in 1TiO2 were mainly associated with the Ti4+ state while Ti3+ existing in the composite remarkably increased after UV irradiation (Fig. 6) [33]. UV irradiation causes an electron transfer from the lattice oxygen (O2 ) to the titanium ion (Ti4+) to form a charge transfer excited state, providing photoinduced Ti3+ O species [34,35]. In general, water molecules kinetically coordinate to the Ti3+ O moiety to stabilize TiO2 surface, resulting in an increase in the amounts of absorbed H2O [36]. However, uptake of H2O for the composite was decreased after irradiation of UV owing to the reduction of OH groups on the TiO2 surface.
Fig. 6. XPS analysis in Ti 2p region of 1TiO2 before (black) and after (red) UV irradiation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
According to the previous reports, doping of transition metal ions with TiO2 offers a way to trap charge carriers and extend the lifetime of one or both of the charge carriers [37,38]. Lanthanide ions are known to form complexes with a Lewis base by the interaction of the f-orbitals of lanthanides. Likewise, La3+ ion of 1 might play an important role as a Lewis acid, interacting with an oxygen ion on the TiO2 surface. As a result, a decrease of OH groups on the surface of TiO2 led to the reduction of the water adsorption by UV irradiation. Therefore, this host–guest interaction may influence modifications of the pores of PCPs which could result in the alteration of the sorption behavior owing to the interaction between the host PCP and the accommodated TiO2 particles. 4. Conclusion TiO2 nanoparticles deposited in the channels of La(1,3,5-tribenzenecarboxylate) were prepared by sol–gel polycondensation without damaging the host structure and depositing particles outside the pores. Although small amounts of TiO2 were introduced into the PCP pores, the water adsorption property of 1TiO2 was drastically enhanced, because of the surface hydroxyl groups on the incorporated TiO2. Furthermore, the adsorption behavior of the composite was changed by UV irradiation owing to the reduction of the hydroxyl groups. Therefore, modification of 1 with TiO2 particles could allow for the change of adsorption behaviors by UV irradiation. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Innovative Area ‘‘New Polymeric Materials Based on ElementBlocks (No. 2401)’’ from the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan. We thank Prof. Y. Chujo of Kyoto University for access to the SEM-EDX apparatus. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2014. 04.020. References [1] O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998) 474–484. [2] B. Moulton, M.J. Zaworotko, Chem. Rev. 101 (2001) 1629–1658. [3] G. Férey, Chem. Soc. Rev. 37 (2008) 191–214. [4] S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem. Int. Ed. 43 (2004) 2334–2375. [5] A. Corma, H. García, F.X. Llabrés i Xamena, Chem. Rev. 110 (2010) 4606–4655. [6] A.P. Côté, G.K.H. Shimizu, Coord. Chem. Rev. 245 (2003) 49–64. [7] D. Bradshaw, J. Claridge, E. Cussen, T. Prior, M. Rosseinsky, Acc. Chem. Res. 38 (2005) 273–282. [8] D. Maspoch, D. Ruiz-Molina, J. Veciana, Chem. Soc. Rev. 36 (2007) 770–818. [9] M. Meilikhov, K. Yusenko, D. Esken, S. Turner, G. Van Tendeloo, R.A. Fischer, Eur. J. Inorg. Chem. (2010) 3701–3714. [10] F. Schröder, R.A. Fischer, Top. Curr. Chem. (2010) 77–113. [11] H.-L. Jiang, Q. Xu, Chem. Commun. 47 (2011) 3351–3370. [12] J. Juan-Alcañiz, J. Gascon, F. Kapteijn, J. Mater. Chem. 22 (2012) 10102–10118. [13] A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 41 (2012) 5262–5284. [14] H.R. Moon, D.-W. Lim, M.P. Suh, Chem. Soc. Rev. 42 (2013) 1807–1824. [15] T. Uemura, D. Hiramatsu, K. Yoshida, S. Isoda, S. Kitagawa, J. Am. Chem. Soc. 130 (2008) 9216–9217. [16] T. Uemura, Y. Kadowaki, C.R. Kim, T. Fukushima, D. Hiramatsu, S. Kitagawa, Chem. Mater. 23 (2011) 1736–1741. [17] G. Lu, S. Li, Z. Guo, O.K. Farha, B.G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han, X. Liu, J.S. DuChene, H. Zhang, Q. Zhang, X. Chen, J. Ma, S.C.J. Loo, W.D. Wei, Y. Yang, J.T. Hupp, F. Huo, Nat. Chem. 4 (2012) 310–316. [18] C. Zlotea, R. Campesi, F. Cuevas, E. Leroy, P. Dibandjo, C. Volkringer, T. Loiseau, G. Férey, M. Latroche, J. Am. Chem. Soc. 132 (2010) 2991–2997. [19] M. Sabo, A. Henschel, H. Fröde, E. Klemm, S. Kaskel, J. Mater. Chem. 17 (2007) 3827–3832. [20] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [21] K. Hashimoto, H. Irie, A. Fujishima, Jpn. J. Appl. Phys. 44 (2005) 8269–8285. [22] A. Fujishima, X. Zhang, D. Tryk, Surf. Sci. Rep. 63 (2008) 515–582.
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Sol–gel synthesis of nanosized titanium oxide in a porous coordination polymer Cho Rong Kim a, Takashi Uemura a,b,⇑, Susumu Kitagawa a,c,⇑ a
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan c Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan b
a r t i c l e
i n f o
Article history: Received 16 December 2013 Received in revised form 8 April 2014 Accepted 9 April 2014 Available online 18 April 2014 Keywords: Titanium oxide Porous coordination polymer Nanocomposite Photoresponsive material
a b s t r a c t Nanosized titanium oxide (TiO2) was synthesized in the channels of a porous coordination polymer (PCP) [La3+(1,3,5-benzenetrisbenzoate)]n by the sol–gel reaction of titanium tetraisopropoxide. XRD, IR, UV–vis, and gas sorption measurements demonstrated that nanosized TiO2 was formed in channels of the PCP. In this system, the resultant PCPTiO2 composite showed enhancement of the adsorption of water. Furthermore, the sorption behavior of the composite could be changed by UV irradiation. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction The design and synthesis of PCPs with unique structural topologies and electronic functions using the self-assembly approach of metal ions and organic ligands have been studied. Because of their regular porous structures with large surface areas, permanent porosity, and surface functionality, PCPs have emerged as an important new class of nanoporous materials with potential for many applications in storage, separation, catalysis, and chemical sensing [1–8]. The fabrication of metal and metal oxide particles accommodated in PCPs has made rapid progress because the doping of only a small amount of the materials can afford new functions to PCPs [9–14]. For example, we have previously reported the synthesis of nanosized silica in the channels of HKUST-1 ([Cu3(btc)2]n; btc = benzene-1,3,5-tricarboxylate) which showed higher sorption amounts for a hydrophilic molecule compared to that of only the host [15–16]. Furthermore, magnetic property of a zeolitic
⇑ Corresponding authors. Address: Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan. Tel.: +81 753 2733; fax: +81 753 2732 (T. Uemura). Address: Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan (S. Kitagawa). E-mail addresses: [email protected] (T. Uemura), kitagawa@icems. kyoto-u.ac.jp (S. Kitagawa). http://dx.doi.org/10.1016/j.micromeso.2014.04.020 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.
imidazolate framework (ZIF) was exhibited by the incorporation of Fe3O4 particles, which allowed recovery of the PCP from a solvent suspension by applying a magnetic field [17]. For the enhancement of adsorption property for hydrogen molecules, utilization of nanocomposite consisting of Pd in PCP pores has been studied [18,19]. This methodology is useful enough to alter the properties of PCPs without changing the host structures, such as the pore size, shape, and surface. Titanium oxide (TiO2) is a very useful functional material due to its wide range of applications in the fields of photocatalysis, optical materials, dye-sensitized solar cells, lithium-ion batteries, and superhydrophilic materials [20–22]. These functions of TiO2 are derived from the change of the energy transfer of TiO2 by UV irradiation [23–25]. Recently, the use of nanoporous matrices as host media for the formation of nanosized TiO2 has become considerable interest for specific nanosize properties and host–guest synergistic functions [26–28]. In this work, we successfully performed the synthesis of TiO2 within one-dimensional nanochannels of [La3+(1,3,5-benzenetrisbenzoate)]n (1; pore size = 10 10 Å2) by sol–gel polycondensation of titanium(IV) tetraisopropoxide (TTIP). The resulting material was characterized by XRD, IR, UV–vis, and SEM-EDX measurements. Alteration of the adsorption behaviors of the obtained composite by irradiation of UV light was investigated. We expect that this work will contribute to the development of PCPs for use as photosensitive materials.
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C.R. Kim et al. / Microporous and Mesoporous Materials 195 (2014) 31–35
2. Experimental 2.1. Materials All reagents and chemicals used were obtained from commercial sources, unless otherwise noted. The host PCP (1) was prepared using the previously reported method [29]. 2.2. Sol–gel synthesis of accommodating TiO2 within 1 The dried host compound 1 (53 mg) was prepared by evacuation (<0.1 kPa) at 120 °C for 3 h in a Pyrex tube. Subsequently, 31 mg of titanium(IV) tetraisopropoxide (TTIP) was added to 1 suspended in 3 mL of dehydrated 2-propanol, and the resulting mixture was stirred for 10 min under nitrogen atmosphere to incorporate TTIP into the nanochannels of 1. The solvent was then evaporated to give 1TTIP adduct (1TTIP). 1TTIP adduct was heated at 250 °C for 24 h under exposure to water vapor to facilitate the sol–gel reaction of TTIP. Further condensation was achieved by incubating the material under vacuum at 120 °C to remove byproducts such as 2-propanol and water in the condensation process, which gave PCP-TiO2 composite (1TiO2, 60 mg).
polycondensation of titanium alkoxide leading to the formation of a titanium oxide [30]. TTIP was incorporated into the channel by immersion of 1 in a 2-propanol solution of TTIP, followed by evaporation of 2-propanol (1TTIP). 1TTIP adduct was left under H2O atmosphere for hydrolysis of TTIP in the pore and was heated at 250 °C to initiate the polycondensation. Further condensation was achieved by evacuating the resulting material at 120 °C to provide a composite (1TiO2). We checked the XRD of 1TiO2 and found that the pattern of 1TiO2 was almost the same as that of 1, indicating that the framework was retained after the sol–gel reaction (Fig. 1). In the UV–vis spectroscopy results, an increase in the absorption at wavelength around 350–400 nm can be assigned to the intrinsic absorption of TiO2, representing the existence of TiO2 particles in the composite (Fig. 2) [31]. Quantitative analysis of 1TiO2 showed that the number of Ti atoms per unit cell of 1 was 0.48 as determined by the SEM-EDX result. The energy dispersive X-ray analysis for elemental mapping measurements (EDX) revealed that TiO2 particles were homogeneously dispersed in the channels of 1 (Fig. S1). Because the elemental map for 1TiO2 showed almost the same ratio of Ti atoms in the unit of 1 at the different accelerating
2.3. Isolation of nanosized TiO2 from the host framework To isolate TiO2 particles inside 1, the composite (60 mg) was stirred overnight in a 0.05 M aqueous solution (40 mL) of sodium ethylenediaminetetraacetate (Na-EDTA) for the complete dissolution of 1. The isolated TiO2 (8 mg) was washed with water (5 mL 3) and dried under a reduced pressure at room temperature. 2.4. Photoirradiation of 1TiO2 An activated sample of 1TiO2 in a cylindrical Pyrex glass tube was irradiated with a 500 W ultrahigh pressure mercury lamp (385–425 nm) for 12 h equipped with glass filters at 298 K. 2.5. Measurements X-ray powder diffraction (XRPD) data were collected using a Rigaku RINT 2000 Ultima diffractometer employing CuK radiation. SEM-energy-dispersive X-ray (EDX) measurements were conducted by using a JEOL JED-2300 detector in a JEOL JSM-5600 at an accelerating voltage of 15 kV and 30 kV. The thermogravimetric (TG) analysis was carried out from room temperature to 500 °C using a Rigaku Instrument Thermo plus TG 8120 in a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ULVAC-PHI Model 5500 spectrometer with 15 kV, 400 W Mg Ka emission as the X-ray source. The charging effect was corrected by adjusting the binding energy of the C 1s peak to be 284.6 eV. Nitrogen adsorption measurements at 77 K and solvent vapor adsorption measurements at 298 K were carried out using a Belsorp-Max and Belsorp-Aqua volumetric adsorption instruments, respectively. The samples were activated under high vacuum (<10 2 Pa) at 120 °C for 6 h before the sorption measurements.
Fig. 1. XRPD of 1, 1TiO2, TiO2 isolated from 1, and bulk synthesized TiO2.
3. Results and discussion 3.1. Synthesis of nanosized TiO2 in the nanochannels of PCP We carried out the synthesis of nanosized TiO2 in the onedimensional nanochannels of 1 using a sol–gel polycondensation of TTIP. The sol–gel process is based on hydrolysis and
Fig. 2. UV–vis spectra of 1, 1TiO2, and TiO2 isolated from 1.
C.R. Kim et al. / Microporous and Mesoporous Materials 195 (2014) 31–35
voltage, TiO2 particles were not gathered near the surface of 1, but were dispersed in the channels of 1. As shown in Fig. 3, nitrogen adsorption on 1TiO2 demonstrates a certain decrease in the adsorption amount for 1TiO2 compared to 1 alone can be attributed to the partial filling of the pores in 1 with TiO2 particles. However, this nitrogen adsorption result shows that the micropores in 1TiO2 are available for further adsorption of guest molecules. The pore size distribution of 1 was almost the same as that of 1TiO2 as determined by the non-localized density functional theory (NLDFT) method (Fig. 3b). Furthermore, the adsorption of nitrogen on 1TiO2 started at a similar pressure to that of 1 (Fig. 3a inset). Thus, these results revealed that the pores of 1TiO2 were not covered with a thin layer of TiO2, but were partially occupied with TiO2 particles in the pores. Therefore, in this system, the sol–gel polycondensation successfully occurred only inside the channels, without destruction of the framework and deposition of TiO2 particles outside the pores. In the IR spectrum of 1TiO2, a broad band at 3400 cm 1 corresponded to the characteristics of the hydroxyl group of 1TiO2 (Fig. 4) [32]. In addition, a shoulder peak around 1645 cm 1 was observed which was derived from Ti OH stretching. In the low energy interval of the intensive broad band below 800 cm 1 was due to the Ti O Ti moiety of TiO2. For further understanding of the formation of TiO2, TiO2 was isolated from 1TiO2 by stirring in 0.05 M aqueous EDTA solution. As shown in Fig. 1, amorphous TiO2 was formed in the channels of 1. In contrast, anatase crystals were formed when we carried out the bulk sol–gel reaction in the similar condition. Therefore, TiO2 particles were formed in the inner space of 1, not on the surface of 1. Formation of TiO2 was also confirmed by observing the characteristic peaks for TiO2 in UV–vis and IR spectroscopy (Figs. 2 and 4). Hydroxyl groups on the surface of TiO2 have an influence on adsorption behavior of 1TiO2. As shown in Fig. 3, the nitrogen adsorption amount of 1TiO2 was decreased owing to the occupation of TiO2 in the pores of 1. However, 1TiO2 exhibited a distinctive adsorption behavior for water (Fig. 5). In this system, open metal sites in the host framework would preferentially interact with water molecules prior to the surface OH groups, because TiO2 particles were partially dispersed inside the pore (Fig. S1). Thus, 1 and 1TiO2 showed almost the same adsorption of water below the P/P0 = 0.3 (Fig. 5 inset). However, in the higher pressure region, we observed that 1TiO2 showed a higher uptake of water than 1 alone. This remarkable difference in adsorption
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Fig. 4. IR spectra of 1, 1TiO2, isolated TiO2 from 1, and 1TiO2 irradiated with UV.
Fig. 5. Water adsorption isotherms for 1, 1TiO2, and 1TiO2 irradiated with UV at 298 K. (inset) Isotherms plotted at low pressure region of P/P0 from 0.2 to 0.4.
Fig. 3. (a) Nitrogen adsorption isotherms at 77 K for 1 (black), 1TiO2 (red), and 1TiO2 (blue) irradiated with UV. (inset) Isotherms plotted against a logarithmic relative pressure. (b) Pore size distribution of 1, 1TiO2, and 1TiO2 irradiated with UV using non-localized density functional theory, determined from the N2 adsorption profile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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C.R. Kim et al. / Microporous and Mesoporous Materials 195 (2014) 31–35
demonstrated the interaction of surface OH groups of TiO2 with water. In addition, based on the result of IR measurement, the increase of water adsorption of 1TiO2 could be derived from the hydroxyl groups on the surface of the accommodated TiO2, interacting with water molecules effectively. Such enhancement of water adsorption was also observed in the previous work with regard to the modification of the PCP pores with silica [16].
3.2. Alternative photoresponsive properties of 1TiO2 with UV irradiation To examine the photoresponsive property of the composite, 1TiO2 was irradiated with UV for 12 h at room temperature. Then, we carried out nitrogen and H2O adsorption measurements at 77 K and 298 K, respectively. The nitrogen adsorption amount of 1TiO2 was increased when the composite was irradiated with UV as shown Fig. 3a. Compared with the isotherm obtained from 1TiO2, the pore volume was changed from 0.44 to 0.53 cm3/g after UV irradiation. In contrast, H2O adsorption amount of 1TiO2 was decreased by UV irradiation (Fig. 5). IR measurement of 1TiO2 irradiated with UV could explain the reason for the change of the H2O adsorption behavior. According to the IR spectrum of 1TiO2 irradiated with UV, the peak intensity of the OH groups at 3400 cm 1 was reduced (Fig. 4). Therefore, decreasing the amount of H2O adsorption results from the loss of the hydroxyl group in 1TiO2 by UV irradiation. The change of H2O adsorption behavior of composite irradiated with UV has relevance to the chemical state of accommodated TiO2. X-ray photoelectron spectroscopy (XPS) analysis was used to determine the state of TiO2 nanoparticles formed in 1. The XPS data for Ti 2p region showed that the Ti atoms in 1TiO2 were mainly associated with the Ti4+ state while Ti3+ existing in the composite remarkably increased after UV irradiation (Fig. 6) [33]. UV irradiation causes an electron transfer from the lattice oxygen (O2 ) to the titanium ion (Ti4+) to form a charge transfer excited state, providing photoinduced Ti3+ O species [34,35]. In general, water molecules kinetically coordinate to the Ti3+ O moiety to stabilize TiO2 surface, resulting in an increase in the amounts of absorbed H2O [36]. However, uptake of H2O for the composite was decreased after irradiation of UV owing to the reduction of OH groups on the TiO2 surface.
Fig. 6. XPS analysis in Ti 2p region of 1TiO2 before (black) and after (red) UV irradiation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
According to the previous reports, doping of transition metal ions with TiO2 offers a way to trap charge carriers and extend the lifetime of one or both of the charge carriers [37,38]. Lanthanide ions are known to form complexes with a Lewis base by the interaction of the f-orbitals of lanthanides. Likewise, La3+ ion of 1 might play an important role as a Lewis acid, interacting with an oxygen ion on the TiO2 surface. As a result, a decrease of OH groups on the surface of TiO2 led to the reduction of the water adsorption by UV irradiation. Therefore, this host–guest interaction may influence modifications of the pores of PCPs which could result in the alteration of the sorption behavior owing to the interaction between the host PCP and the accommodated TiO2 particles. 4. Conclusion TiO2 nanoparticles deposited in the channels of La(1,3,5-tribenzenecarboxylate) were prepared by sol–gel polycondensation without damaging the host structure and depositing particles outside the pores. Although small amounts of TiO2 were introduced into the PCP pores, the water adsorption property of 1TiO2 was drastically enhanced, because of the surface hydroxyl groups on the incorporated TiO2. Furthermore, the adsorption behavior of the composite was changed by UV irradiation owing to the reduction of the hydroxyl groups. Therefore, modification of 1 with TiO2 particles could allow for the change of adsorption behaviors by UV irradiation. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Innovative Area ‘‘New Polymeric Materials Based on ElementBlocks (No. 2401)’’ from the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan. We thank Prof. Y. Chujo of Kyoto University for access to the SEM-EDX apparatus. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2014. 04.020. References [1] O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998) 474–484. [2] B. Moulton, M.J. Zaworotko, Chem. Rev. 101 (2001) 1629–1658. [3] G. Férey, Chem. Soc. Rev. 37 (2008) 191–214. [4] S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem. Int. Ed. 43 (2004) 2334–2375. [5] A. Corma, H. García, F.X. Llabrés i Xamena, Chem. Rev. 110 (2010) 4606–4655. [6] A.P. Côté, G.K.H. Shimizu, Coord. Chem. Rev. 245 (2003) 49–64. [7] D. Bradshaw, J. Claridge, E. Cussen, T. Prior, M. Rosseinsky, Acc. Chem. Res. 38 (2005) 273–282. [8] D. Maspoch, D. Ruiz-Molina, J. Veciana, Chem. Soc. Rev. 36 (2007) 770–818. [9] M. Meilikhov, K. Yusenko, D. Esken, S. Turner, G. Van Tendeloo, R.A. Fischer, Eur. J. Inorg. Chem. (2010) 3701–3714. [10] F. Schröder, R.A. Fischer, Top. Curr. Chem. (2010) 77–113. [11] H.-L. Jiang, Q. Xu, Chem. Commun. 47 (2011) 3351–3370. [12] J. Juan-Alcañiz, J. Gascon, F. Kapteijn, J. Mater. Chem. 22 (2012) 10102–10118. [13] A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 41 (2012) 5262–5284. [14] H.R. Moon, D.-W. Lim, M.P. Suh, Chem. Soc. Rev. 42 (2013) 1807–1824. [15] T. Uemura, D. Hiramatsu, K. Yoshida, S. Isoda, S. Kitagawa, J. Am. Chem. Soc. 130 (2008) 9216–9217. [16] T. Uemura, Y. Kadowaki, C.R. Kim, T. Fukushima, D. Hiramatsu, S. Kitagawa, Chem. Mater. 23 (2011) 1736–1741. [17] G. Lu, S. Li, Z. Guo, O.K. Farha, B.G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han, X. Liu, J.S. DuChene, H. Zhang, Q. Zhang, X. Chen, J. Ma, S.C.J. Loo, W.D. Wei, Y. Yang, J.T. Hupp, F. Huo, Nat. Chem. 4 (2012) 310–316. [18] C. Zlotea, R. Campesi, F. Cuevas, E. Leroy, P. Dibandjo, C. Volkringer, T. Loiseau, G. Férey, M. Latroche, J. Am. Chem. Soc. 132 (2010) 2991–2997. [19] M. Sabo, A. Henschel, H. Fröde, E. Klemm, S. Kaskel, J. Mater. Chem. 17 (2007) 3827–3832. [20] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [21] K. Hashimoto, H. Irie, A. Fujishima, Jpn. J. Appl. Phys. 44 (2005) 8269–8285. [22] A. Fujishima, X. Zhang, D. Tryk, Surf. Sci. Rep. 63 (2008) 515–582.
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