Insight into the precursor nanofibers on the flexibility of La₂O₃-ZrO₂ nanofibrous membranes

2.1 Materials

Citric acid and zirconium acetate solution were obtained from Sigma-Aldrich (Seoul, Korea). N,N-dimethylformamide (DMF) was purchased from Samchun Pure Chemical (Daejeon, Korea). PVP (M_w=1,300,000) and lanthanum nitrate hexahydrate were purchased from Aladdin (Shanghai, China). All the chemicals were used as received without further purification. DMF was used as the solvent.

2.2 Fabrication of nanofibrous membranes

The precursor solution was prepared by dissolving different amounts of zirconium acetate solution, citric acid, and lanthanum nitrate hexahydrate in DMF and stirring for 2 h. Then, PVP powder (10 wt%) was added and stirred for 12 h under ambient conditions. The conductivity of precursor solutions were 2.81 (Zr/La: 0 mol%), 3.82 (Zr/La: 70 mol%), and 4.02 m cm^-1 (Zr/La: 100 mol%) at 25°C, respectively. After the precursor solution was transferred into a plastic syringe for electrospinning with a flow rate of 1 ml h^-1, a high voltage of 20 kV from was applied from a power source (HV30, NanoNc). The nanofibers were collected on a rotating roller (80 rpm), and the needle-to-roller distance was 15 cm. The relative humidity and ambient temperature were 42±3% and 25±2°C, respectively. Thus, the precursor nanofibrous membranes with various Zr contents were obtained. The precursor membranes were named as LZPNM-x [x is the molar proportion (mol%) of Zr/La: 0, 70, and 100, respectively]. Then, the precursor nanofibrous membranes were calcined at 650°C for 4 h under air atmosphere. Various La₂O₃ nanofibrous membranes doped with different contents of Zr were fabricated, and the La₂O₃-ZrO₂ membranes were named as LZNM-y [y is the molar proportion (mol%) of Zr/La: 0, 70, and 100, respectively].

2.3 Characterization

The morphology and microstructure of the LZPNM and LZNM were analyzed using a FE-SEM (S-4700, Hitachi, Japan). The crystal structure of the precursor membranes was investigated by powder XRD analysis (X’ Pert-PRO, PANalytical, Netherlands) with Cu Kα radiation (λ=0.15405 nm). The surface functional groups of the precursor membranes were measured using an FTIR spectrometer (Varian 1000 FTIR Scimitar Series, PIKE Technologies, USA). The tensile mechanical properties were investigated using a tensile tester (XQ-1C, Instrument Research Center of Donghua-Lipu, China). The thermal properties of the precursor membranes were studied using a TGA (Pyris 1, PerkinElmer, USA).

3.1 FE-SEM

The morphologies of the LZPNM and LZNM were first investigated using FE-SEM. Figure 2A–C show the representative FE-SEM images of LZPNM-0, LZPNM-70, and LZPNM-100, respectively. The diameters of the nanofibers increased from 283±36 nm to 535±94 nm with the increase in the Zr contents. The corresponding FE-SEM images with a higher magnification (Figure 2A–C) show that these randomly oriented nanofibers had a smooth surface and uniform diameters. The diameters of the nanofibers were controlled by the precursor concentration. Figure 2D–F show the FE-SEM images of LZNM-0, LZNM-70, and LZNM-100, respectively. The diameters of the nanofibers increased from 174±29 nm to 286±40 nm. The composition of the nanofibers was controlled by the Zr/La ratio of the precursor. However, as shown in Figure 2D, LZNM-0 consisted of short and fragile nanofibers. The high-magnification FE-SEM images show that the nanofibers had a rough surface (Figure 2D inset). Compared to LZNM-0, LZNM-70 and LZNM-100 showed nanofibers with a smooth surface and robust flexibility during the bending (Figure 2E and F, inset). After the sintering at 650°C, nanofibers with a smooth surface, continuous structure, and almost uniform diameter were obtained. Several process parameters affected the morphology of the electrospun nanofibers, including the operating conditions and intrinsic properties of the precursor solution. The flexibility of LZNM-70 and LZNM-100 depends on the morphology of the nanofibers. To obtain flexible nanofibers with the desired morphology, it is important to optimize the experimental conditions such as the properties of the precursor solution, electrospinning parameters, and calcination conditions.

Figure 2:

FE-SEM images of precursor membranes: (A) LZPNM-0, (B) LZPNM-70, (C) LZPNM-100, and inorganic membranes: (D) LZNM-0, (E) LZNM-70, and (F) LZNM-100.

The insets are the digital photograph and magnified FE-SEM images of the corresponding membranes.

3.2 XRD

The crystalline structure of LZPNM was investigated by XRD. The XRD patterns of LZPNM-0, LZPNM-70, and LZPNM-100 from 20° to 80° are shown in Figure 3. All the samples demonstrated a broad diffraction peak at 2θ≈20°, and sharp crystalline peaks were not observed. This clearly indicates that all the LZPNM materials belong to a completely amorphous state.

Figure 3:

XRD patterns of LZPNM-0, LZPNM-70 and LZPNM-100.

3.3 FTIR

Figure 4 shows the FTIR spectra of LZPNM-0, LZPNM-70, and LZPNM-100 in the wavenumber range 500–4000 cm^-1. The broad peaks at ~3450 cm^-1 can be assigned to the symmetric stretching vibration of the OH groups of PVP. The peaks at ~1445 cm^-1 can be attributed to the CH₂ groups of PVP (30). The LZPNM shows characteristic stretching peaks corresponding to the C=O group of PVP (31) at 1642 cm^-1, 1645 cm^-1, and 1644 cm^-1. The peaks at ~1294 cm^-1 can be assigned to the absorption of the C-N group of PVP (32). Figure 4 shows no significant change in the position of the peak, and no new peak appeared.

Figure 4:

FTIR shows peaks of LZPNM-0, LZPNM-70 and LZPNM-100.

3.4 Tensile strength

Figure 5 shows the stress-strain curves of the LZPNM. All the stress curves of the samples first showed a significant increase under a stress load. However, LZPNM-0 exhibited an increase until breaking without obvious slipping, because a small fiber diameter has a large packing density. Figure 5 clearly shows the slipping of LZPNM-70 and LZPNM-100. The fractured membranes are marked with red circles (Figure 6). LZPNM-70 and LZPNM-100 with a fiber diameter of 447±61 nm and 535±94 nm, respectively, showed an ultimate stress of 1.18 MPa and 1.73 MPa, respectively, indicating a significant improvement compared to LZPNM-0 (0.77 MPa). The results indicate that LZPNM-70 and LZPNM-100 had a good tensile strength. This can be attributed to the inhibition of fiber breaking by the doped Zr. It is important to obtain flexible inorganic membranes.

Figure 5:

Stress-strain curves for LZPNM-0, LZPNM-70 and LZPNM-100.

The inset is the dependence of its stress and the Zr contents in the LZPNM.

Figure 6:

Digital photograph of rupture process by stress-strain test for (A) LZPNM-0, (B) LZPNM-70 and (C) LZPNM-100.

3.5 TGA

To investigate the thermal decomposition of the precursor membranes, LZPNM was analyzed using a TGA from ambient temperature to 800°C under a heating rate of 10°C min^-1 (Figure 7). The calcination process can be divided into four stages from 100°C to 800°C. The first stage occurred from room temperature to 140°C, corresponding to the removal of the residual solvent (DMF) and absorbed water. In the second stage from 140°C to 330°C, the nitrate and side-chain of PVP decomposed, resulting in a large weight loss. In the third stage from 330°C to 500°C, the weight loss can be attributed to the cleavage of the C-C bonds of the PVP. Above 500°C, the TGA curves slightly decreased. This indicates that the water, organic compounds, and nitrate in the composite fibers were removed completely, and pure inorganic oxide nanofibers were obtained.

Figure 7:

TGA curves of LZPNM-0, LZPNM-70 and LZPNM-100.

In the design of flexible metal-oxide nanofiber membranes, it is important to determine the change in the structure and properties of the metal-oxide nanofibers with the composition. Hence, flexible LZNM nanofibers were fabricated by careful preparation of the precursors, including the type of metal salt, proportions of metal element, and concentration of polymer. Moreover, the calcination temperature should be optimized. Particularly, after the precursor nanofibers were calcined, the inorganic nanofibers almost automatically shrinked because the polymer template was removed. The nanofibers are usually more brittle because of their thinner section and the internal stress generated by the shrinkage (Figure 2D) (33). Therefore, to prevent the shrinkage of fibers and maintain the morphology of fibers (Figure 2E and F), Zr was incorporated into the fibers during the electrospinning.