Heterometallic Lanthanide–Titanium Oxo Clusters: A New Family of

Communication pubs.acs.org/IC

Heterometallic Lanthanide−Titanium Oxo Clusters: A New Family of Water Oxidation Catalysts Dong-Fei Lu,† Xiang-Jian Kong,*,† Tong-Bu Lu,*,‡ 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 ‡ Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *

Furthermore, although transition-metal-doped TOCs exhibit an enhanced photocatalytic water-splitting performance,11 the investigations of photoinduced water splitting based on the lanthanide-doped TOCs have not been reported. Herein, we report the syntheses, crystal structures, and photoelectrocatalytic activities of three lanthanide−titanium oxo clusters (LTOCs), formulated as [Ln 8 Ti 1 0 (μ 3 O)14(tbba)34(Ac)2(H2O)4(THF)2]·2Htbba [Ln = Eu (1), Sm (2), and Gd (3); Htbba = 4-tert-butylbenzoic acid; Ac− = acetate]. Clusters 1−3 were synthesized through the reaction of Htbba, Ln(Ac)3·xH2O, and Ti(OiPr)4 in acetonitrile and tetrahydrofuran (THF). Single-crystal X-ray diffraction analysis revealed that the metal cores of clusters 1−3 contain 8 Ln3+ and 10 Ti4+ ions organized in a linear geometry. Notably, with the exception of Ti28Ln, Ti10Ln8 clusters are currently the largest LTOCs.10f Because clusters 1−3 are isomorphous, we describe cluster 1 to illustrate the structural features of all three clusters. As shown in Figure 1, the cluster core of 1 consists of 10 Ti4+, 8 Eu3+, 14 μ3O2−, 34 tbba− ligands, 2 CH3COO− anions, and 4 water and 2 THF molecules. Three Eu3+ ions are connected by a pair of tbba− ligands in the tridentate mode (μ3:η1:η2) to form a triangular Eu3 cluster. Each edge of the triangular Eu3 cluster links one Ti4+ ion through one μ3-O2−, generating a planar hexanuclear [Eu3Ti3(μ3O)3]15+ unit. The [Eu3Ti3(μ3-O)3]15+ unit is further connected to

ABSTRACT: We report the synthesis and photoelectrochemical activity of three lanthanide−titanium oxo clusters (LTOCs), formulated as [Ln 8 Ti 1 0 (μ 3 O)14(tbba)34(Ac)2(H2O)4(THF)2]·2Htbba [Ln = Eu (1), Sm (2), and Gd (3); Htbba = 4-tert-butylbenzoic acid; Ac− = acetate]. These stable compounds are efficient catalysts of photoelectrochemical water oxidation with high turnover numbers (7581.0 for 1, 5172.4 for 2, and 5413.0 for 3) and high turnover frequencies (2527.0 for 1, 1724.1 for 2, and 1804.0 for 3). The differences in the photoelectrochemical activity among these three compounds may be related to the differences in their band gaps. This work shows that the heterometallic LTOCs provide a tunable platform for the design of highly effective water oxidation catalysts.

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hotoinduced water splitting is regarded as one of the most promising energy sources for hydrogen preparation.1 In chemical terms, photoinduced water splitting involves two halfreactions, i.e., the oxidation of water to oxygen and the reduction of protons to hydrogen. Because the former half-reaction (2H2O → 4H+ + 4e− + O2) requires a very high redox potential, which often leads to decomposition or deactivation of water oxidation catalysts (WOCs),2 this half-reaction is known as a bottleneck in water splitting.3 Therefore, the preparation of well-defined, robust, and long-lived WOCs is key for photoinduced water splitting. To this end, over the past few decades, intensive research efforts have focused on the synthesis of WOCs; as a result, various types of WOCs have been prepared, including Cdots-C3N4,4 p-type semiconducting materials,5 n-type metal oxides,6 and metal−organic frameworks (MOFs).1c Titanium oxo clusters (TOCs), as the earliest known model WOCs of TiO2,7 offer an opportunity to understand the relationship between the structure and chemical reactivity.8 Because the band gap of the TOCs is often close to or even larger than that of anatase, various transition-metal ions are introduced into the TOCs to reduce their band gaps.9 Recent studies have demonstrated that, in addition to enhancing the photocatalytic activity, lanthanide-doped TiO2 can also increase the stability of the most photoactive anatase phase.7b However, lanthanidedoped TOCs remain rare. To the best of our knowledge, only several lanthanide-doped TOCs have been reported, and the largest number of lanthanide ions doped in TOCs is 2.10 © XXXX American Chemical Society

Figure 1. Ball-and-stick view of the molecular structure of 1. Hydrogen atoms are omitted for clarity. Received: December 15, 2016

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DOI: 10.1021/acs.inorgchem.6b03072 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry one dinuclear [Ti-(μ3-O)-Ti]2+ unit by two μ3-O2−, producing the [Eu3Ti5(μ3-O)6]17+ core of the asymmetric unit of 1 (Figure 2a). The metal−oxo core [Eu8Ti10(μ3-O)14]36+ of 1 can be

the oxidation current density for 1−3 under irradiation is significantly larger than that for P25, 4-tert-butylbenzoate, and ITO/Nafion. The oxidation current density is 0. 327 mA·cm−2 for 1, 0.233 mA·cm−2 for 2, and 0.255 mA·cm−2 for 3 at +1.597 V (vs NHE), which is obviously larger than the values of 0.0756, 0.0570, and 0.0635 mA·cm−2, obtained for the ITO/Nafion, 4tert-butylbenzoate, and P25. This result is further verified by electrochemical impedance spectroscopy (EIS). As shown in Figure 3b, 1−3 exhibit smaller radii than P25, 4-tertbutylbenzoate, and ITO/Nafion in a 0.2 M NaOAc/HOAc buffer solution without irradiation, indicating that 1−3 exhibit lower charge-transfer resistance. To study the charge-separation efficiency, the transient shortcircuit photocurrents of 1−3, P25, 4-tert-butylbenzoate, and ITO/Nafion with the on−off cycle’s illumination under an external bias of +0.6 V (vs NHE) have been carried out. As shown in Figure 3c, without illumination, the transient dark current density was nearly zero. Upon illumination, the photocurrent density dramatically increased and reached a steady state of 0.744 μA·cm−2 for 1. When the light was switched off, the photocurrent density rapidly decayed to nearly zero, suggesting a high chargeseparation efficiency for 1 under irradiation. Investigation of the transient short-circuit photocurrents of 2 and 3 revealed that they exhibited behavior similar to that of 1, with steady-state photocurrent densities of 0.605 and 0.691 μA·cm−2 for 2 and 3, respectively. On the basis of the photocurrent densities of 1−3, the transient decay time (tad) was obtained via a previously reported method.14 The transient time constant (τ), defined as the decay time when ln tD = −1, was 6.5 s for 1, 5.3 s for 2, and 5.7 s for 3, indicating the presence of a slow charge recombination process in 1−3.15 Because clusters 1−3 exhibit relatively larger oxidation current densities and long transient decay times, the photoelectrochemical properties of 1−3 were examined using a three-electrode photoelectrochemical cell with 1−3 as photoanodes, platinum foil as the cathode, an Ag/AgCl quasi-reference electrode, and a 0.2 M NaOAc/HOAc buffer solution as the electrolyte under an argon atmosphere.1c Electrolysis was held at 1.4 V (vs Ag/AgCl) for 3 h, and 16.29, 11.0, and 14.29 C of charge had been passed for 1−3, as shown in Figure 3d. The measured stable photocurrent densities were 1.43, 0.876, and 1.02 mA·cm−2 for 1−3, respectively. For comparison, the current densities were only 0.289 and 0.391 mA·cm−2 for ITO/Nafion and P25, respectively, under the same conditions, demonstrating that the current densities in 1−3 arise from the intrinsic properties of these systems. The amounts of O2 and H2 detected by gas chromatography, the ratio of H2 to O2, and the turnover number (TON) and turnover frequency (TOF) values for O2 after 3 h of irradiation are summarized in Tables S3 and S4. The ratio of H2 to O2 was 2.11:1 for 1, 2.06:1 for 2, and 2.26:1 for 3. Deviation of the ratio of H2 to O2 from 2:1 is attributed to the fact that some of O2 could not be detected by headspace sampling because of its solubility in the electrolyte.1c,16 This deduction is consistent with the faraday efficiencies of 1−3 (91.4% for 1, 73.1% for 2, and 79.2% for 3). Additionally, the TON and TOF values for O2 were 7581.0 and 2527.0 h−1 for 1, 5172.4 and 1724.1 h−1 for 2, and 5413.0 and 1804.0 h−1 for 3. The TON values, especially that for 1, are substantially higher than those for some of the robust molecular electrocatalytic WOCs based on earth-abundant metals,1d,17 demonstrating for the first time that LTOCs are promising WOCs.

Figure 2. (a) Ball-and-stick structure of the [Eu3Ti5(μ3-O)6]17+ unit. (b) Metal−oxo core [Eu8Ti10(μ3-O)14]36+. (c) Coordination sphere of metal ions in 1.

viewed as a pair of [Eu3Ti5(μ3-O)6]17+ units connected by a [Eu2(μ3-O)2]2+ unit, as shown in Figure 2b. The core is stabilized by 34 tbba− and 2 acetate(Ac−) ligands, and the metal coordination sphere is completed with 2 THF and 4 water molecules. Each of the Ti4+ ions is coordinated by six oxygen atoms and displays a distorted octahedral geometry with Ti−O distances of 1.731(2)−1.951(2) Å. The bond distance distribution and calculated bond valence sum values indicate that all of the titanium atoms are tetravalent.12 The Eu−O distances are 2.282(5)−2.598(1) Å, similar to the reported europium clusters.13 Cyclic voltammetry (CV) of 1−3, P25, 4-tert-butylbenzoate, and indium−tin oxide (ITO)/Nafion in a 0.2 M NaOAc/HOAc buffer solution (pH 6.0) under irradiation by a mercury lamp (500 W) was studied, and the control experiments without irradiation were investigated (Figure S1). As shown in Figure 3a,

Figure 3. (a) CV plots. (b) EIS Nyquist plots. (c) Transient short-circuit current responses to on−off cycles of illumination with 0.6 V (vs NHE) bias. (d) Controlled potential electrolysis data performance of 1−3, P25, 4-tert-butylbenzoate, and ITO/Nafion in a 0.2 M NaOAc/HOAc buffer under irradiation. B

DOI: 10.1021/acs.inorgchem.6b03072 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

band-gap theory.20 Because of their isomorphous characterization, the difference of the photocatalytic activities of 1−3 can be rationally attributed to the difference of Ln3+ ions, which lead to the differences in their band-gap values. In summary, three 18-metal-ion LTOCs Ln8Ti10(Ln = Eu, Sm, and Gd) were synthesized and characterized. The 8 Ln3+ and 10 Ti4+ ions were connected by μ3-O2− ligands into a linear structure. All three LTOCs are chemically stable and exhibit efficient photoelectrochemical water oxidation. Given that photoelectrochemical water oxidation has not previously been performed using LTOCs, the present work will open a new route for the preparation of non-noble-metal-based WOCs.

As shown in Figure 4a, cluster 1 exhibits a high activity with an oxygen production rate of 509.8 mmol·g−1, about 6 times higher

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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.6b03072. Synthesis and characterization details (PDF) CIF file for 1 (CIF) CIF file for 2 (CIF) CIF file for 3 (CIF)

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Figure 4. (a) Photoelectrocatalytic amount of O2 of 1−3 and P25. (b) Recycling performance for 1. (c) PXRD curves of 1 after a 3 h experiment, as-synthesized and simulated. (d) Diffuse-reflectance spectrum of 1 (black), 2 (red), and 3 (blue).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-J.K.). *E-mail: [email protected] (T.-B.L.). *E-mail: [email protected] (L.-S.L.).

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than that of P25 (75.7 mmol·g ) after a 3 h experiment. The oxygen production rates of 2 and 3 are 377.9 and 386.8 mmol· g−1, respectively. Recycling experiments show that 1 was used three times without a noticeable change in the activity. To further confirm that the catalytic efficiency of 1−3 arises from the intrinsic properties of the clusters, we collected the powder X-ray diffraction (PXRD) pattern and IR spectrum of 1 and analyzed the Ti4+ and Ln3+ contents in solution after a 3 h experiment. As shown in Figures 4c and S5, the PXRD pattern of 1 after 3 h of reaction was similar to those of the as-synthesized pristine catalyst. Meanwhile, the Ti4+ and Eu3+ contents in solution after 3 h of reaction, as detected by inductively coupled plasma optical emission spectrometry, indicated that less than 0.05% of the cluster was dissolved in the solution. Thus, the clusters were chemically stable during the photoelectrochemical water splitting, and the catalytic efficiency is an intrinsic property of the clusters. That 1−3 exhibited relatively higher catalytic activity for photoelectrochemical water splitting may be attributed to the following. First, the introduction of Ln3+ ions may be lead to a lower overpotential for the oxygen evolution reaction; this behavior had been reported by doping various transition metals into TiO2.18a Second, theoretical calculations predicted that oxidative cleavage of the first O−H bond is the rate-limiting step for the water oxidation reaction on the TiO2 surface.18b The coordination of water molecules and the hydrogen bonding between coordinated water molecules and adjacent oxygen atoms may be favorable for the oxidative cleavage of the first O− H bond.18b Finally, we investigated the band gap of the clusters by collecting UV/vis diffuse-reflectance spectra of 1−3. As shown in Figure 4d, the band gaps for 1−3 were 2.90, 2.95, and 2.96 eV, respectively. These values are substantially smaller than 3.53 eV for the Ti28Ln cage10g and 3.2 eV for anatase.19 The significant band-gap red shifts lead to large absorption in the UV/ vis region, which is favorable for the catalytic activity, although some opposite effect has been reported due to the shortfall of the

ORCID

Xiang-Jian Kong: 0000-0003-0676-6923 La-Sheng Long: 0000-0002-0398-4709 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Project (Grant 2014CB845601) from the Ministry of Science and Technology of China and by the National Natural Science Foundation of China (Grants 21422106, 21371144, 21431005, and 21390391), the Fok Ying Tong Education Foundation (Grant 151013), and the Fundamental Research Funds for the Central Universities (Grant 20720150158). We thank K.-N. Chen, D. Q. Wang, J. W. Wang, O. Y. Ting, and S. H. Wang for experimental help and discussions.

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DOI: 10.1021/acs.inorgchem.6b03072 Inorg. Chem. XXXX, XXX, XXX−XXX