GENG Lin FANG Wei-Hui② ZHANG Lei ZHANG Jian

a (College of Chemistry and Materials Science, Fujian Normal Uniνersity, Fuzhou350007, China)

b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou350002, China)

ABSTRACT In this text, we present researches on the synthesis of polyoxo-titanium clusters (PTCs) by the use of gadolinium pivalate precursor.During the synthesis process, we discover that solvents play an important role.When acetonitrile was used as solvent, Ti8Gd(μ2-O)(μ3-O)5(Sal)5(OPiv)2(μ2-OiPr)3(μ-OiPr)8 (PTC-82) (H2Sal = salicylic acid, HOPiv = pivalic acid) was obtained.When N,N-dimethylformamide (DMF) was used as solvent under the same reaction conditions, Ti4Gd2(μ3-O)2(Sal)4(OPiv)4(μ2-OiPr)2(μ-OiPr)4(DMF)2 (PTC-83) was synthesized.The solid-state UV absorption spectra revealed that the dopant of lanthanide and the incorporation of salicylic acid ligands have successfully narrowed the bandgap of PTCs.In addition, they exhibited rapid and reproducible photo-responses.

Keywords: lanthanide pivalate, dopant, polyoxo-titanium clusters, bandgap, photocurrent;

1 INTRODUCTION

So far, there is a particular interest in the applications of titania in the photocatalytic[1,2].However, the common polymorphs of titania possess band gaps of 3.2 eV[3].In addition to non-metal doping of titania, metal doping has proved to be another efficient way to narrow the bandgap of titania[4-8].Among the metal doping, most researches focus on the transition metal doping[9-12].Lanthanide (Ln) doped titania nanomaterials with appropriate bandgap energies can act as effective light harvesting antennae[8].However, there is a competition between the high oxophilicity Ti and Ln ions.In addition, Ln ions possess flexible coordination number and geometry.So, there are still great challenges in making Ln ions doped in titania.

Crystalline Ln doped polyoxo-titanium clusters (PTCs) could be interesting precursors for homogeneous mixed-oxide materials[13,14].Moreover, the Ln doped PTCs with precise structural information offer an opportunity to understand the relationship between the structures and properties.Nevertheless, examples of Ln doped PTCs remain rare.Till now, a group of famous chemists have contributed in this area.In 2014, Schubert and co-workers studied the ion-size- dependent formation of mixed titanium-Ln oxo clusters[15].Wright’s group reported a series of single Ln ions doped PTCs, including Ti6Ln, Ti8Er and Ti28Ln cages[16-18].Dai et al systematically studied the fluorescent of Ti11Ln and Ti10Ln2clusters[19,20].Long’s group devoted to the research of high-nuclearity Ln-titanium oxo clusters, like Ti10Ln8and wheel like Ti8Eu24[21,22].

According to literatures, different Ln salts, like rare earth alkoxide[20], ErX3(X = Cl, ClO4, NO3, OAc)[14,15,17-19,21,24-28], europium(III) 2,4-pentanedionate hydrate[22]has been applied in the preparation of Ln doped PTCs.As is known, Ln salts are much more expensive than Ln oxide.In this text, by facile refluxing gadolinium oxide in pivalic acid (HOPiv), we prepared a kind of crystalline gadolinium pivalate precursor of Gd(OPiv)3(H2O)3·2HOPiv.Successfully, by using this precursor in the synthesis of gadolinium doped PTCs, we isolated two compounds, formulated as Ti8Gd(μ2- O)(μ3-O)5(Sal)5(OPiv)2(μ2-OiPr)3(μ-OiPr)8(PTC-82) (H2Sal = salicylic acid) and Ti4Gd2(μ3-O)2(Sal)4(OPiv)4(μ2-OiPr)2(μ- OiPr)4(DMF)2(PTC-83).From the synthesis aspect, these two compounds were isolated under the same reaction conditions with different solvents.Structurally, PTC-82 exhibits overall distorted trigonal prism geometry plus a pair of Ti ions, where Gd ion locates in the trigonal prism center.While PTC-83 consists of two Ti2Gd subunits linked by four salicylic ligands.The UV-visible diffuse absorption measurement revealed that these two compounds display semiconductor feature.In addition, their photocurrent response properties were also studied.

2 EXPERIMENTAL

2.1 Materials and methods

All the reagents and solvents were purchased commercially and were not further purified when used.Ti(OiPr)4and Gd2O3were purchased from Energy Chemical, while salicylic acid was bought from Macklin Chemical Reagent Shanghai.N,N-Dimethylformamide (DMF), pivalic acid, n-hexane, toluene and CH3CN were acquired from Sinopharm Chemical Reagent Beijing.

We collected the Fourier transform infrared spectroscopy (FTIR) data (KBr pellets) on a PerkinElmer Spectrum 100 FT-IR Spectrometer.Thermogravimetric analyses (TGA) were performed on a Mettler Toledo TGA/SDTA 851e analyzer in N2with a heating rate of 10 ℃·min-1from 25 to 800 ℃.Powder X-ray diffraction (PXRD) data analyses were collected on a Rigaku Mini Flex II diffractometer using CuKαradiation (λ= 1.54056 Å) in the 2θrange of 5～50oat a scan rate of 5o·min−1.The UV diffuse reflection data were recorded at room temperature using a powder sample with BaSO4as a standard (100% reflectance) on a PerkinElmer Lamda-950 UV spectrophotometer and scanned at 200～800 nm.The compositions of PTC-82 and PTC-83were studied with an Oxford X-max energy dispersive spectrometer (EDS) equipped on the JEOL JSM6700F.

2.2 Syntheses of complex PTC-82-PTC-83

2.2.1 Synthesis of the Gd precursor

Gd(OPiv)3(H2O)3·2HOPiv was synthesized by refluxing Gd2O3(3.62 g, 10 mmol) and excess pivalic acid (30.6 g, 300 mmol) at 160 ℃ for 5 h to form a clear solution.Followed by cooling the solution to room temperature and white precipitate came out.50 mL toluene and 50 mL n-hexane was added to dissolve the excess pivalic acid and filtered in vacuum.Single crystals of Gd(OPiv)3(H2O)3·2HOPiv were grown in the mixed solution of toluene and n-hexane.

2.2.2 Synthesis of PTC-82

A mixture of salicylic acid (150 mg, 1.09 mmol), Gd(OPiv)3(H2O)3·2HOPiv (100 mg, 0.14 mmol), tetraiso- propoxytitanium (0.92 mL, 3.0 mmol) and CH3CN (6 mL) was sealed in a 25 mL vial, then transferred to a preheated oven at 80 ℃ for 4 days.After cooling to room temperature, yellow crystals of PTC-82 were obtained (Yield: about 38% based on Gd).Elemental analysis calcd.(%) for C78H115GdO36Ti8(MW 2169.14): C, 43.19; H, 5.34.Found (%): C, 42.50; H, 5.02.The experimental Ti:Gd molar ratio of 7.8:1 was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

2.2.3 Synthesis of PTC-83

PTC-83 was synthesized by a procedure similar to that of PTC-82 except that the CH3CN was replaced by N,N- dimethylformamide.Instead of partially dissolved, the solution here was a clear yellow liquid.After cooling to room temperature, yellow crystals of PTC-83 were obtained (Yield: about 71% based on Gd).Elemental analysis calcd.(%) for C72H108Gd2N2O30Ti4(MW 1987.70): C, 43.51; H, 5.48; N, 1.41.Found (%): C, 43.02; H, 5.53; N, 1.76.The experimental Ti:Gd molar ratio of 2.3:1 was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

2.3 Photocurrent measurement

Photocurrent measurement electrodes of compounds can be prepared by solution coating method.The new prepared samples (5 mg) and nafion (10 μL) were dissolved in 0.5 mL isopropyl alcohol, then sonicated and dripped the 40 μL solution evenly on the precleaned FTO glass (1.5 × 4.0 cm2, 50 Ω per square cm) to obtain the coating film.The photocurrent test was carried out on the IM6/Zennium electrochemistry workstation using a three-electrode setup, in which the sample coated FTO glass (0.25 cm) as the working electrode, a Pt plate as the counter electrode and an Ag/AgCl electrode as the reference electrode.The Na2SO4(0.2 mol·L-1) solution was used as the electrolyte and a 300 W high pressure xenon lamp as a full-wavelength light source.

2.4 X-ray crystallography

Crystallographic data of crystals of Gd(OPiv)3(H2O)3· 2HOPiv and PTC-83 were collected on a Mercury single crystal diffractometer equipped with graphite-monochromatic MoKαradiation (λ= 0.71073 Å).While crystallographic data of crystal PTC-82 were collected on a Supernova single crystal diffractometer equipped with graphite-monochromatic CuKαradiation (λ= 1.54178 Å) at room temperature.Their crystallographic data are summarized in Table 1.

Table 1.Crystal Data and Structure Refinement for Gd Precursor, PTC-82 and PTC-83

3 RESULTS AND DISCUSSION

3.1 Synthesis

Compared with the Dy pivalate complexes synthesized from Dy(NO3)3[29], the Gd pivalate precursor was obtained by refluxing Gd2O3at 160 °C.Such method is similar to the established synthesis of dimeric Gd pivalate complex[30].The crystals of PTC-82 and PTC-83 were respectively synthesized by solvothermal reactions of Gd precursors with Ti(OiPr)4and salicylic acid in acetonitrile and DMF solvent.However, further exploration attempts in other solvents were failed for the poor crystallinity.The Gd:Ti molar ratio was further proved by ICP elemental analysis.The purity of samples was confirmed by PXRD patterns (Fig.S1-S2).The IR spectra of PTC-82 and PTC-83 presented similar shapes (Fig.S3-S4).The absorption bands at about 1460～1610 cm-1indicate the skeleton of benzene rings.Isopropoxy groups were assigned by theνC-H(between 2865 and 2975 cm−1) andνO-C(about 1010 cm−1) vibrations.The absorption bands around 600～750 cm-1were attributed to the Ti-O vibrations.EDS analyses for PTC-82 and PTC-83 have confirmed the presence of Gd and Ti metals in these compounds (Fig.S5-S6).To study the thermal stability of PTC-82 and PTC-83, the thermogravimetric analysis (TGA) was performed with samples under a N2atmosphere (Fig.S7-S8).For PTC-82, the TGA curve showed two stages.The weight loss of isopropoxy groups and pivalic acid were from 25 to 320 °C .Then the weight loss of the salicylic acid was at the temperature range of 400～520 °C .For PTC-83, the TGA curve shows three stages, the gradual weight loss of DMF molecules is below 200 °C , and the removal of isopropoxy groups and pivalic acid is from 220 to 330 °C .Then the weight loss of salicylic acid is at the temperature range of 385～550 °C .

3.2 Crystal structures

3.2.1 Structure of Ti8Gd(μ2-O)(μ3-O)5(Sal)5- (OPiv)2(μ2-OiPr)3(OiPr)8(PTC-82)

Single-crystal X-ray diffraction analysis reveals that PTC-82 crystallizes in the orthorhombic system, space groupP212121.The asymmetric unit of compound PTC-82 consists of eight crystallographically independent Ti4+ions, one Gd3+ion, five deprotonated salicylic acid ligands, two pivalic acid molecules, oneμ2-O, fiveμ3-O oxo bridges and eleven isopropyl groups (Fig.1a).In the structure, all of the five Sal ligands present one type of coordination mode:μ3-η1:η2:η1.Pivalic acid molecule exhibits a bridging bidentate coordination mode.Among the eleven isopropyl groups, three are bridging bidentate and the other eight are terminal molecules.

One prominent structure feature of compound PTC-82 is its overall distorted trigonal prism geometry plus a pair of Ti ions.It is worth noting that the connection of the two Ti ions is approximately parallel to the arris of the trigonal prism.Interestingly, five Sal ligands cap on the base, while the two pivalic ligands ride on the arris of the trigonal prism (Fig.1b).The Gd ion lies in the center of the trigonal prism, which is coordinated by five carboxylate oxygen atoms from five Sal ligands and fourμ3-O oxo bridges.Ti ions within the trigonal prism adopt an octahedral geometry, while the other two present five-coordinated tetragonal pyramid geometries.Eight Ti ions and the Gd ion are connected by inorganic oxo bridges and organic ligands to give a {Ti8Gd} cluster.

Fig.1.(a) Ball-stick view of the molecular structuree of compound PTC-82.(b) Perspective view showing the core geometry of PTC-82.H atoms are omitted for clarity

As shown in Fig.2a, {Ti8Gd} clusters adopt the same orientation along the [010] direction, but take different orientations along with the [001] direction.Such layers are packed to form a 3D structure.Moreover, square channels with 12.98 × 12.98 Å2dimensions can be observed along thecdirection (Fig.2b).Significant intermolecular hydrogen bonding interactions (C-H· O) exist within the structure.

Fig.2.(a) Rectangle grid arrangement of {Ti8Gd} clusters on the bc plane.(b) Diagram of the packing structure of compound PTC-82

3.2.2 Structure of Ti4Gd2(μ3-O)2(Sal)4(OPiv)4(μ2- OiPr)2(μ-OiPr)4(DMF)2(PTC-83)

Compound PTC-83 crystallizes in another orthorhombic space groupPbca.The asymmetric unit of PTC-83 contains one unique Gd3+ion, two Ti4+ions, two Sal ligands, two pivalic acid molecules, three isopropyl groups and one DMF molecule (Fig.3a).As the same as PTC-82, two Sal ligands present the coordination mode ofμ3-η1:η2:η1and pivalic acid molecule exhibits a bridging bidentate coordination mode.Among all three isopropyl groups, one is bridging bidentate and the other two are terminal molecules.The two Ti4+ions are both octahedrally coordinated.Different from nine-coor- dinated Gd3+ion PTC-82, the Gd3+ion herein is eight- coordinated.Its coordination environment is completed by six carboxylate oxygen atoms from two pivalate and four Sal ligands, oneμ3-O oxo bridge, and oxo bridge from DMF.The Gd-O bond lengths fall in the range of 2.330(4)～2.468(4) Å, which consists with the reported related compounds[20].

Fig.3.(a) Molecular structure of PTC-83.(b) {Ti4Gd2} clusters in PTC-83

Two Ti and one Gd ions are connected by aμ3-O to form a Ti2(μ3-O)Gd unit.Intuitively, the structures of PTC-83 can be viewed as two inorganic Ti2(μ3-O)Gd subunits linked by four organic Sal ligands (Fig.3b).Instead of linking four neighbouring paddle wheel clusters, the two Gd ions in PTC-83 are connected to four Ti ions to give a {Ti4Gd2} cluster herein.As shown in Fig.4a, {Ti4Gd2} clusters adopt rhombic grid arrangement on theabplane.Such layers are packed in -ABAB- alternation to form a 3D structure (Fig.4b).

Fig.4.(a) Rhombi grid arrangement of {Ti4Gd2} clusters on the ab plane. (b) Diagram of the packing structure of compound PTC-83

3.3 Spectroscopic results and photoelectrochemistry

The optical properties of PTC-82 and PTC-83 were explored using UV-visible reflectance measurements.The main absorption bands of PTC-82 and PTC-83 located at 400～480 and 380～430 nm respectively in the solid-state UV/Vis absorption spectra (Fig.S9-S10).According to Kubelka-Munk function[31], the optical bandgaps were estimated to be 2.5 and 2.8 eV for PTC-82 and PTC-83 (Fig.5a), which are in accordance with their yellow crystals.

To study the charge-separation efficiency, we measured the photocurrent responses of PTC-82 and PTC-83 (Fig.5b).The photocurrent response properties are measured using a three-electrode cell with cluster coated FTO working electrodes.The xenon lamp (interval 10 seconds) was used for the switching cycle, and a clear photocurrent response was observed with an applied potential of 0.6 V.Upon repetitive irradiation, steady photocurrent responses were observed.Photocurrent densityy of PTC-82 (0.65μA·cm−2) was a little bit higher than that of PTC-83 (0.57 μA·cm−2).The result is comparable to the reported Ti10Ln8[21].

Fig.5.(a) Kubelka-Munk transformation of diffuse reflectance data for PTC-82 and PTC-83.(b) Photocurrent responses of PTC-82 and PTC-83 in a 0.2 M Na2SO4 aqueous solution under repetitive irradiation

4 CONCLUSION

By employing low price gadolinium oxide as the starting material, we developed a Gd precursor for the preparation of Ln doped PTCs.In the course of synthesis, we found that solvent is an important factor.When keeping the other reaction condition parameter but changing the solvent from acetonitrile to DMF, mononuclear Gd doped PTC-82 changes to binuclear Gd doped PTC-83.Further photo-physical measurements indicated PTC-82 exhibited low bandgap and better photo-response than that of PTC-83.This work not only vividly demonstrated the solvent factor in the synthesis of PTCs, but also opened a new route to homogenous Ln doped titania.In view of the presence of pivalic acid in the precursor, there is a great potential of making high nuclearity of Ln doped PTCs materials.