A two-dimensional bismuth coordination polymer with tartaric acid: Synthesis,characterization and thermal decomposition to Bi 2 O 3 nanoparticles

2.1 Materials and instrumentation

All chemicals were used without further purification and purchased from commercial sources. IR spectra were recorded on the Shimadzu spectrometer 470 (KBr pellets, 4000-400 cm^–1). Elemental analyses were performed using the Costech ECS 4010 CHNS analyzer. X-ray powder diffraction (XRD) measurements were performed using the Bruker, Advance D8 with Cu Kα (λ= 1.5406 Å) incident radiation. The size distribution and morphology of the thermally decomposed sample was analyzed by scanning electron microscopy (SEM, Tescan Vega3SB – EasyProbe, Czech Republic) and transmission electron microscopy (TEM, Philips CM120)

2.2 X-ray crystallographic analysis

The data collection was carried out on a SuperNova diffractometer of Rigaku Oxford Diffraction, using mirror-collimated MoKα(λ= 0.71073 Å)radiation of a microfocus X-ray tube, and CCD detector Atlas S2. The data reduction and cell refinement were performed using CrysAlis PRO [29]. The structures was solved by SHELXT [30] and Superflip [31] and refined by the full-matrix least-squares methods based on F²using Jana2006 [32]. Due to the presence of the quite heavy atom Bi, refinement of anisotropic displacement parameters of light atoms was unreliable and did not improve R values. Therefore, we only refined anisotropic displacement of bismuth.

Hydrogen atoms bonded to carbon were attached geometrically. Hydrogen atoms of OH group could be found (with difficulties cause but very noisy difference Fourier maps due to heavy atom Fourier maps due to the heavy atom) in the difference Fourier map and refined using a bond length restraint 0.84Å and C-O-H angle restraint 109.47°. Determination of the lattice water hydrogen atoms was not possible.

The structure visualizations were done with ORTEP-III [31] and MERCURY (Version 3.5.1) [33]. The crystal parameters, data collection and refinement results for 1 summarized in Table 1 (more details in Supplementary material).

2.3 Synthesis of the complex

Preparation of { 4H₂O}_∞ (1): L-tartaric acid (0.75 g, 5 mmol) and Bi(NO₃)₃·4H₂O (2.42 g, 5 mmol) were dissolved in distilled water (30 mL) and the mixture solution was stirred for 1 h at room temperature. Bi(NO₃)₃·4H₂O (2.42 g, 5 mmol) was added into the above-mentioned solution. Reaction mixture was stirred at 25 °C for 6 h. The precipitate was filtered off and the clear solution was kept at 4 °C until the colorless crystals of 1 suitable for X-ray diffraction were obtained.

IR (KBr) (v ˜_, cm^–1): 3530-3330 (b), 1577 (s), 1453 (m), 1405 (s) 1375 (s), 1330(s), 1310 (s), 1296 (s), 1152 (m), 1119 (m), 947 (m), 674 (m), 599 (m), 492 (w).Anal. Calc. for C₄H₁₄BiNO₁₄: C, 9.43, H, 2.75, N, 2.75%. Found: C, 9.34, H, 2.68, N, 2.70%.

2.4 Preparation of Bi₂O₃ Nano structure

For thermal decomposition of complex and preparation of nanostructure of Bi₂O₃, 1 was mixed with polyethylene glycol 2000 in a 2 : 1 weight ratio and calcinated at 500 °C for 3 h. The obtained pale yellow precipitate was washed with water/ethanol (1 : 1 ratio v/v) and dried at 60 °C. IR spectroscopy was used to confirm that complex 1 has been decomposed completely.

3.1 Crystal structure

The asymmetric unit of 1 (Fig. 1) contains one bismuth, one tartrate ligand, one nitrate ligand and five lattice water molecules. Complicated network of bonds is generated by relatively high symmetry. The structure of 1 consists of a bismuth(III) ion coordinated by two tartrate ligands bound in O,O’-bidentate fashion (O1, O7 and O2, O9), two tartrate ligand bound in O-monodentate fashion (O11 and O6),one lattice water molecule, one nitrate ion bound in O,O’-bidentate fashion and four lattice water molecules of (Fig. 2). The bismuth(III) ion in 1 is nine-coordinated forming a distorted tricappedtrigonal prism. The water molecule O14, one oxygen atom from nitrate ion O4, and carboxylate O6 atomsare placed on one base of the trigonal prism, tartrate O2, O11 and O7 atoms are placed on the opposite base, and O1, O9 (from tartrate ligands) and O13 (from nitrate ion) atoms are placed at the three additional caps of the prism (Fig. 3, Table 2). Tartrate is a multidentate ligand, bridging four symmetry related bismuth(III) ions. It is bound in O,O’-bidentate fashion via O2 and O9 atoms to one bismuth(III) ion (forming five-membered ring from carboxylate and hydroxyl groups through (H)O–C–C–O-), and in O,O’-bidentate fashion via O1 and O7 atoms to the adjacent bismuth(III) ion. Moreover, O11 and O6 atoms act as bridging atoms joining the neighboring bismuth(III) ions. All these bonds give rise to an infinite 2D coordination network extended in the (0 0 1) plane (Fig. 4). The Bi–O bond distances are in the range of 2.387(7)–2.637(7) Å. All distances are in accordance with the values known from the literature [11–15, 26].

There is some hydrogen bond of the O–H...O type between crystalline water molecule and complex (Fig. 5, Table 3).

3.2 IR spectra

The IR spectra of the complex show two sets of vibrations due to the water molecules and tartrate ligands. Bands at 1590–1119 cm^–1 areassigned to the L-tartrate ligand while broad strong bands at the region 3500–3300 cm^–1 are assigned to O–H...O hydrogen bonds between water molecules [34, 35]. The coordinated nitrate ion show strongly split bands at 1453–1405 and 1296–1152 cm^–1. The magnitude of the peak splitting (157 and 253 cm^–1) corresponds with the bidentate coordination mode of the nitrate ion [36, 37].

3.3 Characterization of Bi₂O₃ nanoparticles

The conversion of the complex to Bi₂O₃ is accompanied by disappearance of the IR bands characteristic for the L-tartrate ligand at 1590–1119 cm^–1 and broad strong bands at the region 3500–3300 cm^–1 and bands at 1453–1405 and 1296–1152 cm^–1 for nitrate ion. The remaining absorption bands at 580 and 505 cm^–1 can be assigned to Bi–O vibrations of Bi-O bonds in BiO₆ octahedral units [38].

The PXRD profile (Fig. 6) confirms that the product of the thermal decomposition of 1 is monoclinic α-Bi₂O₃. The d values of the powder profile match with the reported values (JCPDS Card Pattern: 27-0053). The sharp diffraction peaks indicate that the prepared powder is well-crystalline. The crystallite size is calculated from the Debye-Scherer formula D = kλ/β cosθ, where D is the crystallite size (nm), k is an empirical constant (0.9, assuming that the particles are spherical), λ is the wavelength of the X-ray radiation, β is the full width at half maximum (obtained after correction for the instrumental broadening) and θ is the diffraction angle. The average crystallite size obtained from PXRD data is ∼65 nm.

The morphology of the sample was examined by a scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM and TEM images of the nanosized Bi₂O₃ powder is shown in Figs. 7 and 8.