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A Large Titanium Oxo Cluster Featuring a Well-Defined Structural Unit of Rutile Published as part of a Crystal Growth and Design virtual special issue on Crystalline Functional Materials in Honor of Professor Xin-Tao Wu
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Zi-Feng Hong,# Su-Hui Xu,# Zhi-Hao Yan, Dong-Fei Lu, Xiang-Jian Kong,* La-Sheng Long,* and Lan-Sun Zheng Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surface and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China S Supporting Information *
ABSTRACT: Titanium oxo clusters (TOCs) are the well-defined molecular modes for TiO2 materials and provide the opportunity to clarify the relationships between the structures and properties of TiO2. Here, we report a large titanium oxo cluster {Ti14} featuring a well-defined structural unit of rutile by a solventthermal reaction of Ti(OiPr)4, acetic acid, and 1,10-phenanthroline. Crystal structural analysis showed that the 14 Ti4+ ions are connected by 19 bridging oxo ligands, forming a double-decked hexagonal prism structure passivated with acetate and chelate 1,10-phenanthroline ligands. The {Ti14} cluster displays a high photocatalytic H2 production activity because of the conjugated chromophore ligands.
cluster {Ti14} consists of 14 Ti4+, 11 μ2-O2−, 8 μ3-O2−, 8 phen ligands, 12 CH3COO− anions, 2 disordered OiPr−, 4 lattice acetate ligands, and 2 lattice HOiPr (Figure 1). Each Ti4+ is sixcoordinated, exhibiting a distorted octahedral coordination geometry (Ti−N bond lengths of 2.250−2.294 Å and Ti−O bond lengths of 1.784−2.489 Å). Bond valence sum calculations and bond lengths distributions suggested that every titanium atom is tetravalent.30 As shown in Figure 2b, the {Ti14O19} core can be considered as a decked dimer, and each planar is a unit of hexagonal {Ti7} core. In the {Ti7} core, seven Ti atoms were bonded each other with four bridging μ3O and two bridging μ2-O, forming the near-planar building unit. In the center of each {Ti7} unit, there is a six-coordinated Ti4+ ion, which connects six other Ti atoms distributed evenly at the apexes of the hexagon by four μ3-O atoms. In the side of the core unit, four phen ligands are attached on the surface of core. Two {Ti7} cores are connected by seven μ2-O atoms to form the {Ti14} cluster core. As depicted in Figure 2a, there are four π−π stacking interactions in {Ti14} with a distance of 3.576(1) Å and 3.664(2) Å, respectively. Interestingly, the hexagonal {Ti7} core in the {Ti14} is very similar to that of rutile. However, the up and bottom {Ti7} cores in the {Ti14} are deflected 63.19° (Figure 2c,d), while those in the rutile are translated 3.25 Å (Figure 2e,f). The bond lengths of Ti−O in rutile are in the range of 1.95 and
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ince the water electrochemical photolysis of semiconducting n-type TiO2 under visible light was reported, titanium dioxide (TiO2) has received extensive attention driven largely by their useful applications in photocatalysis, solar cells, and environmental pollution removal.1−7 However, surfaced precise structural information on TiO2 particles has not been generally recognized, although the semiconductor surfaces are important components of photocatalytic reactions.8−10 Titanium oxo clusters (TOCs) with well-defined structures are regarded as the ideal molecular modes for TiO 2 materials.11,12 Because the TOCs are often protected by ligands, they provide the opportunity to clarify the relationships between the structures and properties of TiO2,13−15 as well as the surface structural information. Although a great many TOCs have been reported,16−29 none of them exhibit the similar structural unit of pure-phase brookite, rutile, or anatase TiO2. Herein, we report crystal structure of a large titanium oxo cluster of [Ti14 (μ3-O)8(μ2-O)11 (phen) 8(Ac)12 (O iPr) 2]· 2HOiPr·4Ac− (phen = 1,10-phenanthroline, OiPr− = isopropyl alcohol, Ac− = acetate) (namely, {Ti14}), which is prepared through the hydrolysis of Ti(OiPr)4, acetic acid, and 1,10phenanthroline. Interestingly, the core of the cluster consists of two {Ti7} units featuring the structural unit of rutile. Investigation of the photocatalytic H2 evolution of {Ti14} reveals that its photocatalytic activity is about 4 times higher than that of P25. Single-crystal diffraction shows {Ti14} is crystallized in space group C2/c of the monoclinic crystal system. The core of © XXXX American Chemical Society
Received: June 13, 2018 Revised: July 18, 2018 Published: July 26, 2018 A
DOI: 10.1021/acs.cgd.8b00904 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Communication
{Ti14} loses lattice molecules in the temperature range of 30− 90 °C and starts to decompose at 210 °C by losing the organic ligands (Figure S5). The photoelectrochemical activity of {Ti14} was investigated in comparison with P25 (anatase/rutile = 3:1), Nafion, and ligand at the same experimental conditions. The modified electrode used for the photoelectrochemical research was prepared by dropping HOiPr/H2O/Nafion solution containing {Ti14}, ligand or Nafion onto an ITO glass slide. Cyclic voltammetry (CV) experiments were studied in NaAc/HAc buffer solution (0.5 M, pH = 5), under the radiation of highpressured mercury lamp. Interestingly, the oxidation current density of the {Ti14}-modified electrode was 66.0 μA/cm2, which was about three times larger than that of P25 (21.2 μA/ cm2) and ligand (19.2 μA/cm2) (Figure 3a), demonstrating that {Ti14} has a greater sensitivity for light. To get further insight into the photoactivity of {Ti14}, the charge-separation efficiencies were studied by the transient short-circuits photocurrent under an external +0.6 V bias. As shown in Figure 3b, the photocurrent density was increased dramatically with illumination of high-pressured mercury lamp and further tended to a steady value. With closing the light source, the photocurrent density dramatically dropped to nearly zero, reflecting that {Ti14} shows a high charge separation efficiency. Recycling experiments of the shortcircuit photocurrents suggests a high cyclic stability of {Ti14}. Under the identical experimental conditions, the stable photocurrent density of {Ti14} was 1.017 μA/cm2, which is much higher than P25 (0.553 μA/cm2) and ligand (0.221 μA/ cm2). The result suggested that {Ti14} exhibited higher photoelectrochemical activity than P25 and ligand. In addition, on the basis of photocurrent densities and a previously reported method31 (eq S1 and Figure 3b), the transient decay time of {Ti14}, P25, and ligand were calculated to be 6.2, 4.3, and 3.8 s, respectively, indicating a slow charge recombination process for {Ti14}.32 To confirm the stability of {Ti14} after photoelectrochemical experiments, IR spectra were collected. As shown in Figure S3, the IR patterns of {Ti14} matched reasonably well before and after experiments. Meanwhile, inductively coupled plasma optical emission spectrometry (ICP-OES) analysis shows that less than 0.1 ppm Ti4+ leaches to the solution after reaction, which further confirmed the stability of {Ti14} during the experiments. In consideration of the higher photoactivity and the chemically stability of {Ti14}, photocatalytic H2 evolution experiments were studied. {Ti14} (20 mg) was suspended in deionized water (20 mL) containing 10% CH3OH as a sacrificial agent, and the reaction mixture was irradiated with a 300 W xenon lamp. The gas products were analyzed periodically through a gas chromatograph with a thermal conductivity detector (TCD) to quantify the amount of H2 produced. As displayed in Figure 3c, during 8 h reaction, the hydrogen production increased linearly, and with no decrease of hydrogen production rate. The hydrogen production was 37.8 μmol in 8 h, and the H2-generation rate is 236.3 μmol g−1 h−1, which is about 4 times higher than that of P25. The values of TON and TOF of {Ti14} were 6.80 and 0.85 h−1 (Table S3). As shown in Figure 3d, recycling experiments show that the {Ti14} exhibited high photocatalytic activity with similar hydrogen production rate during recycling processes. The PXRD pattern and IR spectrum were collected after recycling experiment. PXRD peaks and IR spectrum were basically matched well before and after experiments (Figure S4 and
Figure 1. Crystal structure of {Ti14}. Green: Ti. Cambridge blue: N. Red: O. Gray: C. H atoms were omitted for clarity.
Figure 2. (a) The π−π stacking interactions in {Ti14}. (b) Ball-andstick structure of the {Ti14O19} core in {Ti14} cluster. (c) The polyhedral drawing of {Ti14O19} core and (d) {Ti7O6} unit in {Ti14} cluster. (e) The polyhedral drawing of {Ti14} unit in rutile and (f) polyhedral drawing of {Ti7} unit in rutile.
1.99 Å for the {Ti14}. Notably, the {Ti14} represents the first example of TOCs containing the rutile-like hexagonal {Ti7} unit. The thermogravimetric analysis (TGA) indicates that B
DOI: 10.1021/acs.cgd.8b00904 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 3. (a) Cyclic voltammograms of {Ti14}, P25, ligand, Nafion. (b) Transient short-circuits photocurrent of {Ti14}, P25, ligand, Nafion. (c) The amount of photocatalytic hydrogen evolution of {Ti14} and P25 at different times. (d) Recycling water-splitting H2 evolution tests of {Ti14}.
Figure 4. (a) IR spectra for {Ti14} of as-synthesized and after photocatalysis. (b) EPR spectra for {Ti14} under light irradiation or in the dark. (c) Emission spectra of {Ti14} and phen ligand. (d) UV−vis spectra of {Ti14} and phen ligand.
corresponds to paramagnetic Ti3+ (Figure 4b).33,34 On the other hand, the emission intensity of {Ti14} is reduced compared with that of ligand, suggesting charge transfer from phen ligands to Ti4+ via ligand-to-metal charge transfer (LMCT) process (Figure 4c).35,36 Moreover, the existence of phen ligands in {Ti14} can lead to a red shift in UV/vis spectra (3.15 eV band gap), which is favorable for the photocatalytic activity (Figure 4d).
Figure 4a), suggesting that the {Ti14} was stable during the photocatalytic experiments. To investigate the possible mechanism, electron paramagnetic resonance (EPR) experiment on {Ti14} was performed upon irradiation in the presence of CH3OH. The freshly prepared crystalline {Ti14} was investigated in the dark, and no EPR resonance was observed. Expectantly, upon 10 min of light irradiation, the colorless {Ti14} changed to grayblue, and the EPR spectrum shows a signal at g = 2.006, which C
DOI: 10.1021/acs.cgd.8b00904 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Communication
In light of the above experimental results, the proposed photocatalytic mechanism is shown in Scheme 1. Under a
Notes
Scheme 1. Schematic Illustration of Photocatalytic H2 Production for {Ti14}
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants Nos. 21673184, 21431005, 21721001, and 21390391) and the Fok Ying Tong Education Foundation (151013). We thank the staff from BL17B beamline of National Center for Protein Sciences Shanghai (NCPSS) at Shanghai Synchrotron Radiation Facility for assistance during data collection.
The authors declare no competing financial interest.
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DEDICATION In honor of Professor Xin-Tao Wu (Fujian Institute, Chinese Academy of Sciences) for his great contributions in the development of inorganic and hybrid functional material and coordination chemistry.
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xenon lamp irradiation, a photogenerated excited electron was transferred from phen ligands to Ti4+, forming the Ti3+ species. The Ti3+ further reacts with H+ to produce H2 and Ti4+. The electron donor CH3OH scavenges the holes in {Ti14} to restore the excited phen ligands to the ground state, achieving a complete photocatalytic cycle. Interestingly, a photochromic phenomenon was observed when the suspension of {Ti14} was irradiated, and the color changed from colorless to gray-blue, verifying the presence of Ti3+.33 In summary, we report a large titanium oxo cluster {Ti14O19} featuring a well-defined structural unit of rutile by solvent-thermal reaction of Ti(OiPr)4, acetic acid, and 1,10phenanthroline. Photocatalytic hydrogen evolution reaction (HER) studies show that {Ti14} is efficient and chemically stable HER catalysts. Because the core structure of the cluster is very similar to structural unit of rutile, the present work is helpful for our understanding of the relationships between the structures and properties of TiO2.
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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.cgd.8b00904. Synthesis and characterization details (PDF) Accession Codes
CCDC 1849117 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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REFERENCES
(1) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37−38. (2) Toivola, M.; Halme, J.; Miettunen, K.; Aitola, K.; Lund, P. D. Nanostructured dye solar cells on flexible substrates. Int. J. Energy Res. 2009, 33, 1145−1160. (3) Abdullah, N.; Kamarudin, S. K. Titanium dioxide in fuel cell technology: An overview. J. Power Sources 2015, 278, 109−118. (4) Han, F.; Kambala, V. S. R.; Srinivasan, M.; Rajarathnam, D.; Naidu, R. Tailored titanium dioxide photocatalysts for the degradation of organic dyes in wastewater treatment: a review. Appl. Catal., A 2009, 359, 25−40. (5) Teh, C. M.; Mohamed, A. R. Roles of titanium dioxide and iondoped titanium dioxide on photocatalytic degradation of organic pollutants (phenolic compounds and dyes) in aqueous solutions: A review. J. Alloys Compd. 2011, 509, 1648−1660. (6) Ge, M.; Cao, C.; Huang, J.; Li, S.; Chen, Z.; Zhang, K.-Q.; AlDeyab, S. S.; Lai, Y. A review of one-dimensional TiO2 nanostructured materials for environmental and energy applications. J. Mater. Chem. A 2016, 4, 6772−6801. (7) Bai, Y.; Mora-Sero, I.; De Angelis, F.; Bisquert, J.; Wang, P. Titanium Dioxide Nanomaterials for Photovoltaic Applications. Chem. Rev. 2014, 114, 10095−10130. (8) Benedict, J. B.; Freindorf, R.; Trzop, E.; Cogswell, J.; Coppens, P. Large Polyoxotitanate Clusters: Well-Defined Models for PurePhase TiO2 Structures and Surfaces. J. Am. Chem. Soc. 2010, 132, 13669−13671. (9) Benedict, J. B.; Coppens, P. The Crystalline Nanocluster Phase as a Medium for Structural and Spectroscopic Studies of Light Absorption of Photosensitizer Dyes on Semiconductor Surfaces. J. Am. Chem. Soc. 2010, 132, 2938−2944. (10) Sokolow, J. D.; Trzop, E.; Chen, Y.; Tang, J.; Allen, L. J.; Crabtree, R. H.; Benedict, J. B.; Coppens, P. Binding modes of carboxylate- and acetylacetonate-linked chromophores to homodisperse polyoxotitanate nanoclusters. J. Am. Chem. Soc. 2012, 134, 11695−11700. (11) Rozes, L.; Sanchez, C. Titanium oxo-clusters: precursors for a Lego-like construction of nanostructured hybrid materials. Chem. Soc. Rev. 2011, 40, 1006−1030. (12) Tomita, K.; Petrykin, V.; Kobayashi, M.; Shiro, M.; Yoshimura, M.; Kakihana, M. AWater-Soluble Titanium Complex for the Selective Synthesis of Nanocrystalline Brookite, Rutile, and Anatase by a Hydrothermal Method. Angew. Chem., Int. Ed. 2006, 45, 2378− 2381. (13) Coppens, P.; Chen, Y.; Trzop, E. Crystallography and properties of polyoxotitanate nanoclusters. Chem. Rev. 2014, 114, 9645−9661.
AUTHOR INFORMATION
Corresponding Authors
*(X.-J.K.) E-mail:
[email protected]. *(L.-S.L.) E-mail:
[email protected]. ORCID
Xiang-Jian Kong: 0000-0003-0676-6923 La-Sheng Long: 0000-0002-0398-4709 Author Contributions
# These authors (Z.-F. H. and S.-H. X) contributed equally to this work.
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DOI: 10.1021/acs.cgd.8b00904 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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(14) Fang, W. H.; Zhang, L.; Zhang, J. Synthetic strategies, diverse structures and tuneable properties of polyoxo-titanium clusters. Chem. Soc. Rev. 2018, 47, 404−421. (15) Lv, H. T.; Li, H. M.; Zou, G. D.; Cui, Y.; Huang, Y.; Fan, Y. Titanium-oxo clusters functionalized with catecholate-type ligands: modulating the optical properties through charge-transfer transitions. Dalton Trans. 2018, 47, 8158−8163. (16) Gao, M. Y.; Wang, F.; Gu, Z. G.; Zhang, D. X.; Zhang, L.; Zhang, J. Fullerene-like Polyoxotitanium Cage with High Solution Stability. J. Am. Chem. Soc. 2016, 138, 2556−2559. (17) Fang, W. H.; Zhang, L.; Zhang, J. A 3.6 nm Ti52−Oxo Nanocluster with Precise Atomic Structure. J. Am. Chem. Soc. 2016, 138, 7480−7483. (18) Fang, W. H.; Zhang, L.; Zhang, J. Synthetic investigation, structural analysis and photocatalytic study of a carboxylate− phosphonate bridged Ti18-oxo cluster. Dalton. Trans. 2017, 46, 803−807. (19) Narayanam, N.; Fang, W. H.; Chintakrinda, K.; Zhang, L.; Zhang, J. Deep eutectic-solvothermal synthesis of titanium-oxo clusters protected by p-conjugated chromophores. Chem. Commun. 2017, 53, 8078−8080. (20) Wu, Y.-Y.; Wang, P.; Wang, Y.-H.; Jiang, J.-B.; Bian, G.-Q.; Zhu, Q.-Y.; Dai, J. Metal−phenanthroline fused Ti17 clusters, a single molecular source for sensitized photoconductive films. J. Mater. Chem. A 2013, 1, 9862−9868. (21) Wu, Y. Y.; Lu, X. W.; Qi, M.; Su, H. C.; Zhao, X. W.; Zhu, Q. Y.; Dai, J. Titanium−Oxo Cluster with 9-Anthracenecarboxylate Antennae: A Fluorescent and Photocurrent Transfer Material. Inorg. Chem. 2014, 53, 7233−7240. (22) Ni, L.; Liang, D.; Cai, Y.; Diao, G.; Zhou, Z. A novel hexanuclear titanium(IV)-oxoiminodiacetate cluster with a Ti6O9 core: singlecrystal structure and photocatalytic activities. Dalton. Trans. 2016, 45, 7581−7588. (23) Narayanam, N.; Chintakrinda, K.; Fang, W. H.; Kang, Y.; Zhang, L.; Zhang, J. Azole Functionalized Polyoxo-Titanium Clusters with Sunlight-Driven Dye Degradation Applications: Synthesis, Structure, and Photocatalytic Studies. Inorg. Chem. 2016, 55, 10294−10301. (24) Hong, K.; Chun, H. Nonporous Titanium−Oxo Molecular Clusters That Reversibly and Selectively Adsorb Carbon Dioxide. Inorg. Chem. 2013, 52, 9705−7. (25) Eslava, S.; Reynal, A.; Rocha, V. G.; Barg, S.; Saiz, E. Using graphene oxide as a sacrificial support of polyoxotitanium clusters to replicate its twodimensionality on pure titania photocatalysts. J. Mater. Chem. A 2016, 4, 7200−7206. (26) Zhao, C.; Han, Y. Z.; Dai, S.; Chen, X.; Yan, J.; Zhang, W.; Su, H.; Lin, S.; Tang, Z.; Teo, B. K.; Zheng, N. Microporous Cyclic Titanium-Oxo Clusters with Labile Surface Ligands. Angew. Chem., Int. Ed. 2017, 56, 16252−16256. (27) Zhang, G.; Liu, C.; Long, D. L.; Cronin, L.; Tung, C. H.; Wang, Y. Water-Soluble Pentagonal-Prismatic Titanium-Oxo Clusters. J. Am. Chem. Soc. 2016, 138, 11097−11100. (28) Zhang, G.; Li, W.; Liu, C.; Jia, J.; Tung, C. H.; Wang, Y. Titanium-Oxide Host Clusters with Exchangeable Guests. J. Am. Chem. Soc. 2018, 140, 66−69. (29) Gao, M. Y.; Chen, S.; Hu, L. X.; Zhang, L.; Zhang, J. Synthesis and photocatalytic H2 evolution properties of four titanium-oxoclusters based on a cyclohex-3-ene-1-carboxylate ligand. Dalton Trans. 2017, 46, 10630−10634. (30) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244−247. (31) Tafalla, D.; Salvador, P. Kinetic Approach to the Photocurrent Transients in Water Photoelectrolysis at n-TiO2 Electrodes. J. Electrochem. Soc. 1990, 137, 1810−1815. (32) Sun, Y.; Sun, Z.; Gao, S.; Cheng, H.; Liu, Q.; Piao, J.; Yao, T.; Wu, C.; Hu, S.; Wei, S.; Xie, Y. Fabrication of flexible and freestanding zinc chalcogenide single layers. Nat. Commun. 2012, 3, 1057.
(33) Fu, Y.; Sun, D.; Chen, Y.; Huang, R.; Ding, Z.; Fu, X.; Li, Z. An Amine-Functionalized Titanium Metal-Organic Framework Photocatalyst with Visible-Light-Induced Activity for CO2 Reduction. Angew. Chem., Int. Ed. 2012, 51, 3364−3367. (34) Horiuchi, Y.; Toyao, T.; Saito, M.; Mochizuki, K.; Iwata, M.; Higashimura, H.; Anpo, M.; Matsuoka, M. Visible-Light-Promoted Photocatalytic Hydrogen Production by Using an Amino-Functionalized Ti(IV) Metal-Organic Framework. J. Phys. Chem. C 2012, 116, 20848−20853. (35) (a) Zhang, G.; Kim, G.; Choi, W. Visible Light Driven Photocatalysis Mediated via Ligand-to-Metal Charge Transfer (LMCT): An Alternative Approach to Solar Activation of Titania. Energy Environ. Sci. 2014, 7, 954−966. (36) Sun, D.; Fu, Y.; Liu, W.; Ye, L.; Wang, D.; Yang, L.; Fu, X.; Li, Z. Studies on Photocatalytic CO2 Reduction over NH2-Uio-66(Zr) and Its Derivatives: Towards a Better Understanding of Photocatalysis on Metal−Organic Frameworks. Chem. - Eur. J. 2013, 19, 14279− 14285.
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DOI: 10.1021/acs.cgd.8b00904 Cryst. Growth Des. XXXX, XXX, XXX−XXX
