English

• 1. INTRODUCTION

  Nanoscale titanium oxide has been broadly used in solving energy crisis due to its abundance, low-cost, little toxicity, nice photostability, and high photocatalytic efficiency^{[1, 2]}. However, it is difficult to determine the reaction mechanisms while utilized as photocatalysis. Hence, crystalline polyoxo titanium clusters (PTCs) with accurate structures have appealed to researchers, and a great number of crystalline PTCs have been synthesized and characterized recently^[3-9]. One of the most important studies is bandgap engineering^{[10, 11]}. And one method to reduce the bandgap of titanium oxo clusters and enhance their visible light adsorption is the organic ligand modification^[12-15].

  In the family of PTCs, organophosphate-stabilized {Ti₆P₂} cluster with six labile coordination sites has been used as a platform for ligands substitution and metal incorporation^[16-18]. For example, in 2014, Schubert et al. reported a series of stable Ti₆O₄(OⁱPr)₁₀(O₃PR)₂(OAc)₂ (OAc = acetate) cluster with different phosphonate ligands^[19]. Subsequently, our group used carboxylates, phosphonates and sulfonates ligands to replace the active coordination sites of {Ti₆P₂} cluster, and demonstrated that high electron-withdrawing organic species can reduce the bandgaps of these complexes^[20]. However, there are still no studies on the inorganic acid-modified {Ti₆P₂} cluster. As is known to us all, inorganic acids like nitric or sulfuric acid are cheap, stable, and widely available. Thus, it is meaningful to study how inorganic acids occur in such {Ti₆P₂} structure.

  As a continuation of our effort, we researched constructing {Ti₆P₂} cluster-based PTCs using nitric or sulfuric acid. Successfully, two complexes, namely, Ti₆O₄(OⁱPr)₁₀(O₃P-Phen)₂(NO₃)₂ (PTC-251) and Ti₆O₄(OⁱPr)₁₀(O₃P-Phen)₂(HSO₄)₂ (PTC-252) (H₂O₃P-Phen = phenylphosphinic acid), were synthesized and structurally characterized. As expected, the introduced inorganic acids indeed replace the organic carboxylates sites (like acetate, benzoic acid, etc.), and locate on the upper and bottom surfaces of {Ti₆P₂} cluster. Moreover, the bandgap properties and photocatalytic water-splitting hydrogen-evolution activities of these two complexes are also investigated.

  2. EXPERIMENTAL

  2.1 Materials and measurements

  All chemicals except distilled water were purchased commercially and used without further purification. Distilled water was obtained by our laboratory. Ti(OⁱPr)₄ was purchased from Aladdin, and phenylphosphonic acid was purchased from Energy Chemical. Isopropyl alcohol, nitric acid and sulfuric acid were purchased from Sinopharm Chemical Reagent Beijing. Powder X-ray diffraction (PXRD) data were obtained by placing target crystals onto the flat sample holders using a MiniFlex2 X-ray diffractometer with CuKα radiation (λ = 0.1542 nm) in the 2θ range from 5° to 50° with a scanning rate of 5 °/min. The Fourier transform infrared (FTIR) spectroscopic data (KBr pellets) were obtained on a PerkinElmer Spectrum 100 FTIR Spectrometer. The diffuse reflectance ultraviolet (UV) data were collected on powder samples with BaSO₄ as standard (100% reflectance) with a PerkinElmer Lamda-950 UV spectrophotometer at room temperature. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA 851^e analyzer at a heating rate of 10 ℃/min from 25 to 600 ℃ under a nitrogen atmosphere.

  2.2 Synthesis

  2.2.1   Ti₆O₄(OⁱPr)₁₀(O₃P-Phen)₂(NO₃)₂ (PTC-251)

  Phenylphosphonic acid (0.9941 g, 7.0 mmol) and 4 drops of nitric acid were mixed in 5.5 mL anhydrous isopropanol. And then Ti(OⁱPr)₄ (0.92 mL, 3.0 mmol) was added rapidly into the mixture. The resultant solution was sealed and heated at 80 ℃ for 3 days. Colorless block crystals of PTC-251 were obtained after cooling to room temperature (Yield: 53% based on Ti(OiPr)₄). Important IR data (KBr, cm^-1): 2970(m), 2932(w), 2864(w), 1627(w), 1555(s), 1474(w), 1438(w), 1366(w), 1275(s), 1034(m), 989(s), 928(m), 754(m), 684(m), 609(w), 551(w).

  2.2.2   Ti₆O₄(OⁱPr)₁₀(O₃P-Phen)₂(HSO₄)₂ (PTC-252)

  PTC-252 was prepared by the same procedure as that for PTC-251, except the nitric acid was replaced by sulfuric acid. Colorless blocks crystals of PTC-252 were obtained (60% yield based on Ti(OⁱPr)₄). Important IR data (KBr, cm^-1): 2973(m), 2930(w), 2866(w), 2311(w), 1625(m), 1445(s), 1363(m), 1327(w), 1202(m), 1140(w), 1038(w), 993(vs), 939(w), 756(m), 693(s), 621(w), 548(m), 430(w).

  2.3 X-ray crystallography

  The intensity crystallography data of PTC-251 and PTC-252 were collected on a Supernova and Xcalibur single crystal diffractometer with graphite-monochromatized CuKα radiation (λ = 1.54178 Å) and MoKα radiation (λ = 0.71073 Å), respectively. All absorption corrections were applied by using SADABS^[21]. The structures were solved by direct methods and refined by full-matrix least-squares on F² with SHELXTL-2014 and OLEX-2 programs^[22]. All atoms except hydrogen atoms were refined anisotropically while hydrogen atoms were generated geometrically. Crystal data for PTC-251: C₄₂N₂O₂₆P₂Ti₆H₈₀ (M_r = 1378.42 g/mol): orthorhombic, space group Pbca (No. 61), a = 16.9346(3), b = 17.4591(4), c = 22.3238(5) Å, V = 6600.3(2) ų, Z = 4, T = 293(2) K, μ(CuKα) = 6.984 mm^-1, D_c = 1.387 g/cm³, 16721 reflections measured (7.92°≤2θ≤148.58°), 6624 unique (R_int = 0.0321, R_sigma = 0.0326) which were used in all calculations. The final R = 0.0685 (I > 2σ(I)) and wR = 0.2257 (all data). Crystal Data for PTC-252: C₄₂H₈₂O₂₈P₂S₂Ti₆ (M_r = 1448.53 g/mol): triclinic, space group P$ \overline 1 $, a = 11.4280(10), b = 12.6293(18), c = 13.3326(14) Å, α = 118.223(13)°, β = 92.481(8)°, γ = 107.367(10)°, V = 1578.8(4) ų, Z = 1, T = 293(2) K, μ(MoKα) = 0.918 mm^-1, D_c = 1.524 g/cm³, 11324 reflections measured (4.85°≤2θ≤56.46°), 6400 unique (R_int = 0.0293, R_sigma = 0.0538) which were used in all calculations. The final R = 0.0455 (I > 2σ(I)) and wR = 0.1198 (all data).

  3. RESULTS AND DISCUSSION

  3.1 Crystal structures of PTC-251 and PTC-252

  The organic modifications on {Ti₆P₂} cluster have been reported in 2014 and 2016^{[19, 20, 23]}. Herein, it's the first time to decorate the {Ti₆P₂} cluster with different inorganic acids. The crystal belongs to monoclinic system with space group Pbca. {Ti₃(μ₃-O)} subunit is a common and low-nuclearity moiety existing in the hydrolysis of titanium^[24-26]. This fragment usually acts as a second building block generating larger aggregation^{[19, 27-33]}. In PTC-251, such {Ti₃(μ₃-O)} triangle block was equatorially bridged by isopropoxide groups (Fig. 1). Five isopropoxide ligands alternately adopted bridging bidentate and monodentate coordination mode. Axially, it was then linked by a nitrate to form Ti₃(μ₃-O)(μ₂-OⁱPr)₂(OⁱPr)₃(NO₃)⁴⁺ ({Ti₃}). Two parallel {Ti₃} are connected through oxo bridges and phenylphosphinic ligands to form a {Ti₆P₂} cluster (Fig. 2). Ti1 and Ti2 ions in the {Ti₆P₂} core are six-coordinated whilst Ti3 is only five-coordinated.

  Figure 1

  

  Figure 1.  Perspective view of the {Ti₃(μ₃-O)} subunit in PTC-251. Color code: O red; C grey; Ti green; N blue

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  Figure 2

  

  Figure 2.  Molecular structure of PTC-251. Color code: O red; C grey; Ti green; N blue

  DownLoad: Full-Size Img PowerPoint

  The cluster core of PTC-252 is isostructural to that of PTC-251. However, the attachment of the "outer" Ti coordination environment is different. Sulfate took tridentate coordination mode capping on the {Ti₃(μ₃-O)} subunit (Fig. 3). As a result, all of the Ti ions in PTC-252 are in octahedral coordination geometry (Fig. 4). The Ti–O bond lengths between nitrate and sulfate were longer than the other Ti–O bond in the structure (Tables S1 and S2). These clusters packed differently largely attributed to the steric hindrance from the two outer faces (Fig. S1).

  Figure 3

  

  Figure 3.  Perspective view of the {Ti₃(μ₃-O)} subunit in PTC-252. Color code: O red; C grey; Ti green; N blue

  DownLoad: Full-Size Img PowerPoint

  Figure 4

  

  Figure 4.  Molecular structure of PTC-252. Color code: O red; C grey; Ti green; S orange

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  3.2 Characterization

  The experimental PXRD patterns of PTC-251 and PTC-252 are well-matched with their simulated PXRD patterns (Fig. S2), evidencing that the experimental samples are in good phase purity. The different reflection intensity between experimental and simulated is attributed to the variation of the powder sample in the preferred orientation. The TGA of PTC-251 and PTC-252 was analyzed in a dry air atmosphere from 25 to 600 ℃ (Fig. S3). The TGA of PTC-251 exhibits overall one-step weight loss while PTC-252 undergoes two stages of weight loss. The UV absorption spectra of PTC-251 and PTC-252 present bandgaps of 3.39 and 3.34 eV, respectively (Fig. S4).

  The IR spectra of PTC-251 and PTC-252 have been recorded in the range of 4000~400 cm^–1 from solid samples palletized with KBr, which are presented in Fig. S5. In the high wavenumber region (ν > 1000 cm⁻¹), the weak absorption bands at 3010~2910 cm⁻¹ are observed, which can be ascribed to the stretching vibration modes of C–H bonds in OⁱPr groups. The characteristic skeletal vibrations of benzene rings are observed at 1640~1430 cm⁻¹. Besides, the peaks appearing at 1430~1260 cm⁻¹ and 1110~1040 cm⁻¹ can be respectively assigned to the characteristic bending vibrations of δ_C–H and stretching vibration of ν_C–O. In the low wavenumber region (ν < 1000 cm⁻¹), the absorptions in the region ca. 800~642 cm⁻¹ can be attributed to the C–H in-plane or out-of-plane bends, ring breathing, and ring deformation absorptions of benzene rings. What's more, the characteristic vibration of inorganic acid can be also observed at 861~827 cm⁻¹ for NO₃^– in PTC-251, 620~550 cm⁻¹ for HSO₄^2– in PTC-252. Thus, the result of IR spectra coincides with that from the X-ray single-crystal structural analysis.

  3.3. Photocatalytic properties

  To evaluate the photocatalytic performances of PTC-251 and PTC-252, photocatalytic hydrogen production studies were carried out under UV-light. Although acetate decorating {Ti₆P₂} cluster didn't show any hydrogen evolution activities^[20], PTC-251 and PTC-252 with efficient photocatalytic abilities in water-splitting hydrogen-evolution reactions can be observed. The nitrate decorating PTC-251 gives the hydrogen production of 14.5 μmol·g^-1·h^-1, while the sulfate decorating PTC-252 presents the higher hydrogen production of 110.95 μmol·g^-1·h^-1. The photocatalytic performance of PTC-252 was comparable to some PTCs and metal organic frameworks^{[34, 35]}. The rate trends of these two compounds are shown in Fig. 5. Especially to PTC-252, the steady H₂ evolution rate indicates that the {Ti₆P₂} cluster decorated by sulfate is quite stable. These results also confirm that the photocatalytic H₂ evolution activity of PTCs can be influenced by modified ligands.

  Figure 5

  

  Figure 5.  Comparison of H₂ evolution behaviors under UV-vis light-driven with PTC-251 and PTC-252

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  4. CONCLUSION

  In summary, we have successfully utilized {Ti₆P₂} clusters as a platform and constructed two inorganic acids decorating PTC-251 and PTC-252. The coordination modes of capped inorganic acid on {Ti₆P₂} clusters are inconsistent, which leads to totally different supramolecular packing. Moreover, the photocatalytic H₂ evolution activities of these two PTCs are also different. The H₂ evolution rate of PTC-252 can reach up to 110.95 μmol·g^-1·h^-1 while that of PTC-251 is only 14.5 μmol·g^-1·h^-1. Our results not only enrich structures of organophosphate-stabilized PTCs but also provide an interesting model for better understanding the structure-property relationships of Ti–O materials.

  
  
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• 

  Figure 1  Perspective view of the {Ti₃(μ₃-O)} subunit in PTC-251. Color code: O red; C grey; Ti green; N blue

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  Figure 2  Molecular structure of PTC-251. Color code: O red; C grey; Ti green; N blue

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  Figure 3  Perspective view of the {Ti₃(μ₃-O)} subunit in PTC-252. Color code: O red; C grey; Ti green; N blue

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  Figure 4  Molecular structure of PTC-252. Color code: O red; C grey; Ti green; S orange

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  Figure 5  Comparison of H₂ evolution behaviors under UV-vis light-driven with PTC-251 and PTC-252

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•