Solvothermal Synthesis, Crystal Structure and Photocurrent Property of a Ti6-Cor

GUO Yu-Hu YU You-Zhu NIU Yong-Sheng WANG Zhen SHI Wei-Yun WU Xin-Li

Solvothermal Synthesis, Crystal Structure and Photocurrent Property of a Ti6-Core-based Titanium Oxo Cluster①

GUO Yu-HuaaYU You-Zhua②NIU Yong-ShengaWANG ZhenaSHI Wei-YunaWU Xian-Lib

a(455000)b(450001)

A Ti6-core-based titanium oxo cluster (TOC) [Ti6(2-O)2(3-O)2(Bpo)2(PhCOO)8(OPr)4] assembled by 2,2΄-biphenol (H2Bpo) and benzoic acid has been synthesized and characterized by IR, elemental analyses, thermogravimetric analysis and X-ray diffraction technique.Single-crystal X-ray diffraction analysis revealed that the complex crystallizes in monoclinic system, space group21/.The structure contains two Ti3(3-O) units featuring a flat mode as building blocks.Solid-state UV/Vis absorption spectrum reveals that complex 1showsa wide range from 240 to 650 nm.Moreover, the optical band gap of 1 is estimated to be 2.35 eV.Additionally, the Ti6-TOC exhibits good photocurrent response.

titanium oxo cluster, crystal structure, band gap, photocurrent response;

1 INTRODUCTION

The semiconductor materials-based photocatalysts have attracted much attention due to their applications in the fields of energy and environment[1-4].Titanium dioxide (TiO2) is one of the most promising photocatalysts because of its high efficiency, biocompatibility and non-toxic nature[5-8].However, only UV-light (< 5% of the solar spectrum) can be absorbed by pristine TiO2due to its large band gap value (.3.20 eV), which greatly inhibits its application for photocatalysis[9-11].Sensitizing TiO2by chromophore ligands may efficiently reduce its band gap value and enlarged its light absorption to Vis-light region[12, 13].Despite advantages of the sensitized TiO2materials, it is still a long-term challenge to illustrate the sensitized mechanisms and structure-property relationships because of their inhomogeneous and imprecise characteris- tics[14, 15].

Titanium oxo clusters (TOCs) are the ideal structure and reaction models for TiO2which have drawn wide attention in the very recent years[16-18].Though a large number of TOCs and metal-doped TOCs have been reported, the investigations and studies on the Ti-based complex are still less than other transitional metal-based complex.On the other hand, the chromophore ligands-based TOCs with narrow band gap values are also insufficiently among the reported TOCs, especially for band gap engineering study of TiO2and TOCs[16, 19].As is well known, TOCs with narrow band gap values can always be obtained by using the phenolic ligands[20].For example, phenol and catechol are usually used as chromophore ligands to construct TOCs featuring narrow band gap values[21, 22].While as for H2Bpo, it is not easy to predict the structures of TOCs because of the rotatability of the bond axis between benzene rings.Taking account of the above, it is appealing to construct TOCs using H2Bpo as chromophore ligands.In this work, the solvothermal reactions of H2Bpo and benzoic acidwithTi(OPr)4in mixed organic solvent at 80 °C gave rise to a new Ti6-core-based TOCs, namely [Ti6(2-O)2(3-O)2(Bpo)2(Ba)8(OPr)4] (1).Addi- tionally, the band gap and photocurrent response property of 1 were also investigated.

2 EXPERIMENTAL

2.1 Materials and general methods

All starting reagents of AR grade were commercially purchased and used without further purification.The crystal data were collected on a Bruker Apex II CCD diffractometer.IR spectrum was determined on a Tensor 27 OPUS (Bruker) FT-IR spectrometer with KBr pellets in the range of 4000～400 cm–1.Analyses of C and H were recorded on a Perkin- Elmer 240 Elemental analyzer.The powder X-ray diffraction patterns (PXRD) were performed on a Rigaku D/Max-2500 diffractometer at 40 kV and 100 mA, using a graphite- monochromator and a Cu-target tube under ambient con- ditions.Thermogravimetric analysis (TGA) experiments were recorded on a NETZSCH STA 449F3 thermal analyzer at a heating rate of 10 °C/min from 40 to 800 °C under N2.The solid UV/Vis spectra were measured on a UV25500 UV-VIS-NIR Spectrophotometer (Shimadzu Corp.).

The electrochemical measurement electrode of complex 1 was prepared by a solution coating method.Generally, the grinded crystals (5 mg) were ultrasonically dispersed in 100 uL ethanol and 10 uL Nafion solutions (5 wt%, Sigma-Aldrich).After 10 min, 40 uL suspensions were dropped on the cleaned ITO glass (1 cm2area) by pipette.After evaporation under ambient atmosphere for 2 hours the coating film was obtained and used as the working electrode.Photocurrent measurements and electrochemical impedance spectroscopy were conducted using CHI 760E electroche- mical workstation in a three-electrode system, using a Pt plate as the auxiliary electrode and an Ag/AgCl electrode as the reference electrode.All the tests were performed at the same bias potential of +0.4 V and an aqueous solution of Na2SO4(0.2 mol·L–1) was used as the electrolyte.A 300 W high-pressure xenon lamp was used as a full-wavelength light source located 20 cm away from the ITO electrode, and the on-off cycling irradiation intervals are 10 s.

2.2 Synthesis of [Ti6(m2-O)2(m3-O)2(Bpo)2(PhCOO)8(OiPr)4] (1)

H2Bpo (186 mg, 1 mmol), benzoic acid (611 mg, 5 mmol) and Ti(OPr)4(0.5 mL, 1.5 mmol) were added with stirring to isopropanol (1 mL) and acetonitrile (5 mL).After 5 min, the resulting mixture was sealed in a Teflon-lined stainless vessel (15 mL) and heated at 80 °C for 36 h under autogenous pressure.The vessel was then cooled by air cooling to room temperature spontaneously.Orange-red and block single crystals were obtained by filtration, washed with acetonitrile, and dried in air.Yield: 0.255 g (53%, based on Ti(OPr)4).Elemental analysis: calcd.(found) for C92H84O28Ti6(%): C, 57.41(57.29); H, 4.40(4.35).IR (KBr, cm-1): 3062(w), 2971(w), 2927(w), 2888(w), 1595(s), 1550(s), 1490(m), 1419(s), 1269(s), 1126(s), 1020(s), 717(s), 678(m), 642(s), 605(s) and 482(s).

2.3 Crystallographic measurements and structure determination

The suitable single crystal of complex 1 used for X-ray diffraction experiment was obtained directly from the above experiment.Crystallographic data of 1 were collected at room temperature on a Bruker APEX II diffractometer by using an-2scan mode.The X-ray source is a graphite-monochro- matized Moradiation (= 0.71073 Å).The structure was solved by direct methods using the SHELXS-97 programs[23]and refined by full-matrix least-squares on2using the SHELXL-2014 program package[24].The non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were added theoretically, riding on the concerned atoms and refined with fixed thermal factors.Crystal data for 1: monoclinic system, space group21/with13.8085(6),= 16.5884(9),20.5623(14) Å,= 99.502(5)°,4645.4(5) Å3,2, C92H84O28Ti6,M = 1924.99,D = 1.376 g/cm3,(000) = 1984,= 1.076, the final= 0.0796,= 0.1482 (= 1/[2(F2) + (0.0720)2+ 4.06], where= (F2+ 2F2)/3), (Δ)max= 0.744 and (Δ)min= –0.521 e/Å3.The selected bond lengths and bond angles and the hydrogen bond information for 1 are given in Table 1.

Table 1.Selected Bond Lengths (Å) and Bond Angles (º) for 1

Symmetry code:#1–+ 1, –+ 1, –+ 1

3 RESULTS AND DISCUSSION

3.1 Description of the crystal structure

Complex 1 crystallizes in monoclinic system, space group21/.The crystal structure of complex 1 is revealed in Fig.1a and b, and the stacking structure is shown in Fig.2.As shown in Fig.1a, complex 1 consists of six Ti4+ions, two2-O ions, two3-O ions, two Bpo anions, eight benzoates and four isopropoxide groups.All Ti4+ions show octahedral TiO6coordination environments.Two edge-sharing Ti4+ions are linked by one3-O and one2-O atom which can generate a Ti2subunit, then the Ti2subunits and another Ti4+ion linked by one3-O can generate a Ti3O subunit featuring a nearly flat mode by vertex-sharing mode.Finally, the two Ti3O subunits are connected by two2-O atoms via vertex-sharing mode to form a Ti6core structure (Fig.1b).In complex 1, the substitued ratio of PhCOO:Ti is 4:3 which is a higher degree than most of the reported Ti6cores-based TOCs.In addition, the average bond lengths of Ti1–O, Ti(2)–O and Ti(3)–O in 1 are 1.969, 1.965 and 1.963 Å respectively, which are consistent with the literature[11, 16, 21].Notably, the Bpo anions as CLs ligand in complex 1 make its crystals orange red.

Fig.1.Ball-stick (a) and polyhedral (b) views for the structure of 1.The H atoms have been omitted for clarity

Fig.2.Stacking structure of complex 1 (view along axis).All H atoms are omitted for clarity

3.2 IR spectrum

The absorption peak at 3062 cm–1should be assigned to the stretching vibrations of the C–H bonds of benzenes.The peaks at 2971, 2927 and 2888 cm–1can be ascribed to the stretching vibration of the saturated C–H bonds, whereas peaks at 1595, 1550 and 1490 cm–1could be assigned to the vibration of benzene rings.Notably, the characteristic sharp vibrations of the Ti-OR bands are in the region of 400～750 nm, whereas the peaks at 1126 and 1020 cm–1can be ascribed to the vibration of TiO-C bonds[25].

Fig.3.Simulated and experimental PXRD patterns for complex 1

3.3 Powder X-ray diffraction (PXRD) and thermal stability

The pure phase of complex 1 was confirmed by the powder X-ray diffraction performed at room temperature.As shown in Fig.4, the RXPD experimental pattern was in good agreement with the computer-simulated pattern, which confirmed the pure phase of 1.The different intensities of reflections between the experimental and simulated patterns are mostly because of the powder size and variation in preferred orientation of the powder samples during the collection of experimental PXRD data.

Fig.4.Simulated and experimental PXRD patterns for complex 1

The thermogravimetric analysis (TGA) of complex 1 was also performed.As shown in Fig.5, complex 1 shows thermal stability under 200 °C.It can be observed that the obvious weight loss step is in the range of 250～500 °C with the weight loss of about 64.07%, which can be ascribed to the release of organic ligands.Then, the last residue remains about 28.24% at 800 °C.

Fig.5.TGA curve of complex 1

3.4 Diffuse-reflectance UV-Vis spectra and photoluminescence properties

Solid-state UV/Vis absorption spectrum of complex 1 was measured at room temperature using BaSO4as a standard reference (Fig.6a).Complex 1 showsa wide absorption range from= 240 to 650 nm.The absorption range from 240 to 400 nm could be ascribed to the aromatic ligands and the Ti oxo cores, whereas the absorption range from 400 to 650 nm should be mainly affected by the chromophore ligands in the structure.To be noted, the wide absorption range of complex 1 from UV to Vis-light region may greatly improve its photo-absorption efficiency for photocatalysis application.According to the Kubelka-Munk function method[18, 26], the optical band gap of complex 1 is estimated to be 2.35 eV from the solid-state UV/Vis reflection spectrum (Fig.6b), which is a larger decrease than that of TiO2.

Fig.6.Solid-state UV/Vis absorption (a) and band gap (b) of complex 1

As shown in Fig.7a, the electrochemical impedance spectroscopy (EIS) of complex 1 has been carried out to evaluate its charge transfer resistance.The small semicircle of complex 1 in the high frequency range indicates its small charge transfer resistance.The charge-separation efficiency of semiconductor material is also very important for their photocatalytic reaction.The transient short-circuit photo- current test of complex 1 has been carried out.As shown in Fig.7b, complex 1 showed clear photocurrent response property.Obviously, the photocurrent was quickly generated with the light on, and the photocurrents rapidly decayed with the light off.The photocurrent density of complex 1 is about 0.062A·cm−2, which is nearly correspondent with 0.08A·cm−2of the same type of complex[16].

Fig.7.Nyquist plot (a) and photocurrent response under repetitive irradiation (b) of complex 1

4 CONCLUSION

In conclusion, by the application of H2Bpo and benzoic acid, we successfully synthesized a Ti6-core-based TOC in a simple and general approach.The title complex shows a wide light-absorption range of 240 to 650 nm and a narrow band gap value of 2.35 eV.The charge transfer resistance and photocurrent response were also investigated.This work may not only enrich the family of TOCs but also provide important information for sensitized method of TiO2.

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28 June 2020;

11 August 2020 (CCDC 2011093)

①This work was financially supported by the National Natural Science Foundation of China (No.21702005), the Foundation of the Henan Joint International Research Laboratory of Nanocomposite Sensing Materialsand the Foundation of Anyang Institute of Technology (YPY2019003)

.Yu You-Zhu, born in 1978, professor, majoring in coordination chemistry.E-mail: 119yyz@163.com

10.14102/j.cnki.0254–5861.2011–2921