Cocrystal of {Ti4} and {Ti6} Clusters with Enhanced Photochemical

Feb 10, 2017 - Research reported in this publication was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Gran...
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Cocrystal of {Ti4} and {Ti6} Clusters with Enhanced Photochemical Properties Jian-Fang Wang,†,‡ Wei-Hui Fang,‡ Dong-Sheng Li,† Lei Zhang,*,‡ and Jian Zhang‡ †

College of Materials and Chemical Engineering, Research Institute of Materials, China Three Gorges University, Yichang 443002, P. R. China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

cocrystallization in PTC-57 endow it with better solar-driven photocatalytic activity than the individual PTC-55 and PTC-56 clusters. The crystals of PTC-55−PTC-57 were made by a one-pot solvothermal reaction (Scheme 1), which has proven to be a

ABSTRACT: Herein we report the construction of a supramolecular cocrystal {Ti4 + Ti6} through the combination of isolated {Ti4} and {Ti6} clusters. Moreover, the combined {Ti4 + Ti6} complex presents better solar-driven photocatalytic dye degradation activity than the individual {Ti4} and {Ti6} compounds.

Scheme 1. Illustration of the Synthesis and Assembly of PTC55−PTC-57

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he intense pursuit and extraordinary developments of cluster chemistry are attributed to the size-dependent physical properties of quantum-confined systems.1−4 The interclusters are connected by weak interactions to form stable supramolecular networks in the solid state.5,6 These combined clusters usually belong to homogeneous units with the same structure and character. If heterogeneous clusters could be introduced into one complex, not only fascinating structures but also significant properties would be generated. Recently, the synthesis and characterization of crystalline polyoxotitanate clusters (PTCs) have attracted considerable attention for their precise structural information and potential applications in photoelectronic and photocatalytic chemistry.7−15 So far, a variety of structurally known crystalline titanium oxo clusters have been reported, whose nuclearities are already developed to 34 Ti atoms in Ti34O50(OiPr)36,11 42 Ti atoms in a fullerene-like polyoxotitanium cage,16 and 52 Ti atoms in the largest Ti52 cluster to date.17 However, studies on the cocrystallization of titanium oxo clusters with different nuclearities and core structures still remain very rare.9 With these kinds of special structures, it would be very interesting to investigate how the diverse components finally affect the properties of the complexes. Herein, we report a crystalline intercluster complex, [CuBr 2 ] 2 [Ti 4 O(O 3 P-Phen) 2 (OiPr) 9 ] 2 [Ti 6 O 4 (O 3 P-Phen)2(OiPr)10(L)2] (PTC-57; H2O3P-Phen = phenylphosphinic acid; L = 2-chloroisonicotinic acid), which not only contains two different titanium oxo clusters but also incorporates a doped copper halide anion. Interestingly, we have also obtained the crystalline phases of two subclusters, [CuBr2][Ti4O(O3PP h e n ) 2 ( O iP r ) 9 ] (P T C- 5 5 ) a n d [ T i 6 O 4 ( O 3 P - P h e n)2(OiPr)10(L)2] (PTC-56), allowing the implementation of comparable structure and property studies. Solid-state UV−vis absorption studies on these three compounds revealed that metal doping could efficiently increase the absorption of solar energy. Moreover, the synergic effects of metal doping and cluster © XXXX American Chemical Society

facile synthetic approach for crystalline PTCs. Tetraisopropoxytitanium, isopropyl alcohol, and phenylphosphinic acid were selected as the titanium source, solvent, and bridging ligand, respectively. When a copper bromide salt was added to the reaction system, an ion-separated PTC-55 was synthesized; when 2-chloroisonicotinic acid was put in as the organic auxiliary ligand, neutral PTC-56 was isolated; when both of them were introduced, cluster-combined PTC-57 was obtained. The prepared compounds were fully characterized by energydispersive X-ray spectroscopy (EDS), Fourier transform infrared spectrometry, and thermal gravimetric analysis (TGA). Their phase purity was identified by powder X-ray diffraction (PXRD) studies. Their detailed structural information was acquired Received: November 30, 2016

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DOI: 10.1021/acs.inorgchem.6b02913 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

ABB-layered model (Ti6Ti4Ti4) is observed along the b or c axis (Figure S16). Solid-state UV−vis absorption spectra of the complexes were obtained at room temperature. As shown in Figure 2a, they all

through single-crystal X-ray diffraction analysis. The valence states of the Ti and Cu ions are confirmed to be 4+ and 1+ by bond-valence-sum (BVS) calculations (Table S2). A prominent feature of PTC-57 is cocrystallization of the individual PTC-55 and PTC-56 clusters. In the previously reported Ti10 structures (Figure 1a), Ti ions are connected

Figure 2. (a) Normalized solid-state absorption spectra of PTC-55− PTC-57. (b) Sunlight-driven photocatalytic degradation profiles for MB by P25 and PTC-55−PTC-57.

Figure 1. (a) Representative Ti10 clusters in the literatures. (b) {Ti4 + Ti6} cluster in PTC-57 and its relationship with PTC-55 and PTC-56.

display intense absorption in the ultraviolet region up to ∼400 nm. Compared with the PTC-56 cluster, PTC-55 and PTC-57 crystals present stronger adsorption between 400 and 500 nm due to the Cu+ doping. More surprisingly, they also show weak peaks in the visible range, which should be attributed to the presence of Cu2+ ions. Although both the BVS calculation and the linear two-coordinated environment indicate the 1+ state of Cu ions in PTC-55 and PTC-57, X-ray photoelectron spectroscopy studies indeed show small signals of Cu2+ (Figure S18). The presences of these Cu2+ ions might be due to the percolation of a small amount of Cu2+ that could not be refined. To explore the photocatalytic performances of the combinational PTC-57 and the separated PTC-55 and PTC-56, their activities toward solar-driven methylene blue (MB) photodegradation were investigated. The characteristic absorption of MB at about 665 nm was selected for monitoring of the photocatalytic degradation process (Figures 2b and S17). For comparison, the commercial TiO2 (P25) and blank solution were also investigated under the same conditions. Under sunlight irradiation, the photocatalytic activities of PTC-55−PTC-57 all appear to be higher than those of P25. Notably, the combinational PTC-57 displays a better photocatalytic performance than the isolated PTC-55 or PTC-56. It can be seen that both transition-metal doping and cluster combination benefit the photocatalytic activities. The PXRD patterns of the samples after photocatalytic degradation studies were basically identical with

through inorganic oxo bridges in the belt18 and supertetrahedral7 clusters or linked by organic (aminoethyl)phosphonic acid19 and pyrocatechol ligands.20 To be different from them, PTC-57 is composed of isolated Ti4 and Ti6 clusters. Notably, there is an additional [CuBr2]− counterion in the same lattice to balance the charge of the [Ti4O(O3P-Phen)2(OiPr)9]+ moiety. Usually, the doping metal ions are directly connected to the Ti−O cluster core through coordination bonding (named molecular doping).21 The lattice [CuBr2] species found in PTC-55 and PTC57 is quite unusual and may provide a new mode for Ti−O doping (named lattice doping). Four Ti ions in the Ti4 cluster of PTC-55 are linked by a μ4-O atom to form tetrahedral geometry.19,22−24 Among the four triangular faces of the tetrahedral cluster, two of them are capped by a pair of O3PPhen ligands. The octahedral coordination environment of each Ti ion is completed by peripheral isopropyl alcohol groups. In our recent work, we demonstrated that the labile surface sites in O3P-Phen-stabilized hexatitanium clusters can be used for ligand substitution.25 Herein, the labile surface sites in the Ti6 cluster of PTC-56 are occupied by 2-chloroisonicotinic acid at each end. The external coordination sites of the Ti6 cluster are surrounded by 10 OiPr molecules, two O3P-Phen ligands, and two 2chloroisonicotinic acids, resulting in five- or six-coordinated Ti centers. The two O3P-Phen ligands constitute the two vertexes of the cubic cluster core. In the packing diagram of PTC-57, an B

DOI: 10.1021/acs.inorgchem.6b02913 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry ORCID

those of the original compounds, indicating that these complexes are stable during the photocatalysis experiments. In addition, we measured the photocurrent responses of the prepared materials. The crystal-coated fluorine-doped tin oxide electrodes were examined in a three-electrode cell. Upon on−off cycling irradiation with xenon light (intervals of 10 s), clear photocurrent responses were observed for PTC-55 and PTC-57 (Figure 3). The cathodic photocurrents were quickly generated

Dong-Sheng Li: 0000-0003-1283-6334 Lei Zhang: 0000-0001-7720-4667 Jian Zhang: 0000-0003-3373-9621 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Research reported in this publication was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB20000000) and the NSFC (Grants 21673238, 21473202, and 21501176).

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Figure 3. Photocurrent responses of PTC-55−PTC-57 in a 0.2 M Na2SO4 aqueous solution under repetitive irradiation.

and kept stable without an obvious intensity decrease, indicating their good photoelectric response and high photophysical stability. However, PTC-56 did not show any observable photocurrent under the applied experimental conditions. The IR spectra of the samples after photocurrent studies were similar to those of the original crystals, indicating that the compounds were stable during measurement (Figures S7−S9). By comparing the photocurrent responses of the complexes, we can conclude that copper doping might be the main influential factor for the observed differences. In summary, we successfully established an unprecedented structural model to construct complexes with heterogeneous titanium oxo components. In the presence of the unusual lattice transition-metal doping, isolated {Ti4} and {Ti6} clusters have been combined to form a cocrystal of {Ti4 + Ti6}. Photocatalytic activity and photocurrent response studies confirm that both copper halide doping and cluster combination can improve their physical properties. For the development of titanium oxo cluster chemistry, this work provides a new model for transition-metal doping and also an effective method for the construction of cocrystallizing heterogeneous cluster structures.

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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.6b02913. Experimental details, TGA diagrams, PXRD patterns, and additional figures (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. C

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DOI: 10.1021/acs.inorgchem.6b02913 Inorg. Chem. XXXX, XXX, XXX−XXX