- Morphology analysis of Co3O4/TiO2 secondary heterostructures
Four metal oxides (Co2O4, Fe2O3, Fe3O4, and CuO) can be grown as secondary structures on the surface of TiO2 nanofibers, confirming the feasibility of our combined electrospinning technique and hydrothermal method. We first characterized the morphology analysis of the Co3O4/TiO2 secondary heterostructure. Figure 1(a) shows the scanning electron microscope picture of TiO2 nanofibers. It can be seen from Figure 1(a) that the surface of TiO2 nanofibers is smooth without any secondary structure, and the diameter is 200~500nm
between. Then, after the TiO2 nanofibers were placed in a reactor containing Co(NO3) 2•6H2O, the surface of the nanofibers changed significantly after hydrothermal reaction. It can be seen from the SEM images of the composites [Fig. 1(b) and Fig. 1(c)] that some flake-like secondary structures have grown on the surface. Then, the composites were calcined at high temperature, and interestingly, compared with the Co3O4/TiO2 secondary heterostructure before calcination [Fig. 2(a) and Fig. 1(b)], calcination resulted in the growth of TiO2 nanofibers surface Many holes are generated on the Co3O4 nanosheets, and these holes increase the specific surface area of the material, which is beneficial to enhance the performance of the electrode material for lithium-ion batteries. Next, we characterized the morphology of the Co3O4/TiO2 secondary heterostructure by transmission electron microscopy. It can be clearly seen from Fig. 1(d) that the porous nanosheets are uniformly grown on the surface of TiO2 nanofibers. From the lattice spacing measured by high-resolution transmission electron microscopy [Fig. 1(e) and Fig. 1(f)], we can judge that the nanosheet-like material is Co3O4. SEM images of Co3O4/TiO2 secondary heterostructures with different resolutions before calcination are shown in Fig. 2.
- Structural Analysis of Co3O4/TiO2 Secondary Heterostructure
It can be confirmed from the X-ray diffraction (XRD) spectrum of Fig. 3(a) that all diffraction peaks can be indexed as anatase TiO2 (JCPDS No. 21-1272) and face-centered cubic.
Co3O4 [spacegroup: Fd3m(227), JCPDS No. 43-1003]. In the Raman spectrum [Fig. 3(b)], 4 peaks located at 143.6 (Eg) cm-1, 399.1 (B1g) cm-1, 519.3 (A1g) cm-1 and 639.7 (Eg) cm-1 are assigned to the typical Raman peaks of anatase TiO2. The reacted samples, in addition to the typical Raman peaks of anatase TiO2, appeared at 193.6 (B1g) cm-1, 476.9 (Eg) cm-1, 517.1 (F2g) cm-1, 615.1 (F2g) cm-1 1. A typical Co3O4 Raman peak of 684.3 (A1g) cm-1. Therefore, we can confirm the formation of Co3O4/TiO2 secondary heterostructures by XRD and Raman spectra.
Next, we used XPS to characterize the purity, composition, and fine structure of the samples. The typical C 1s, N 1s, O 1s and Co 2p peaks present in Fig. 4 can confirm that the sample contains carbon, nitrogen, oxygen and cobalt elements, and the carbon and nitrogen are derived from the pyrolysis of urea. A closer look at Figure 4 reveals that the presence of titanium was not detected. This may be because XPS is a surface detection technique and cannot detect the elements inside. Such results can confirm in disguise that the secondary structure of Co3O4 completely covers the surface of TiO2 nanofibers , so that the existence of internal elements cannot be detected when XPS tests are performed on them.
Based on the above characterization methods, it has been confirmed that Co3O4/TiO2 secondary heterostructures can be successfully prepared by combining electrospinning technology and hydrothermal method.
- Morphology and structural analysis of metal oxide/TiO2 secondary heterostructures
To demonstrate the generality of this method, we extend this synthetic strategy combining electrospinning technique and hydrothermal method to prepare other metal oxide/TiO2 secondary heterostructures. Under the condition of keeping other conditions unchanged, we replaced Co(NO3)2•6H2O and urea in the hydrothermal reactor with FeC13 as precursors. After reacting at 90℃ for 6h, Fe2O3/ TiO2 secondary heterostructure. Its scanning and transmission electron microscope pictures are shown in Figure 5. Apparently, the surface of the nanofibers has changed significantly, and some nanorods have grown on its surface. The transmission electron microscope can clearly see that the tiny nanorods grow uniformly on the surface of TiO2 nanofibers. Then, by measuring the lattice spacing of high-resolution transmission electron microscopy, we can determine that these tiny nanorods are Fe2O3.
Next, we go on to demonstrate the generalizability of the method. Under the condition of keeping other conditions unchanged, we replaced the aqueous solution of Co(NO3)2•6H2O and urea in the hydrothermal reactor with FeCl3•6H2O, polyethylene glycol and sodium acetate solution, and reacted at 200℃ After 16 h, Fe3O4/TiO2 secondary heterostructures can be obtained without calcination. The scanning and transmission electron microscope pictures are shown in Fig. 6(a)~(c), the surface of the nanofibers has changed obviously, and a layer of nanoparticles has grown on the surface. And these nanoparticles wrap the TiO2 nanofibers, forming a dense core-shell structure. Transmission electron microscopy can clearly see the existence of the core-shell structure formed by the uniform growth of fine nanoparticles on the surface of TiO2 nanofibers. Keeping other conditions unchanged, when we replaced Co(NO3)2•6H2O with CuCl2•2H2O in the hydrothermal reactor, after 12h reaction at 180℃, calcined at 500℃ for 2h in air, CuO can be obtained /TiO2 secondary heterostructure. Its scanning and transmission electron microscope images are shown in Fig. 6(d)~(f), the surface of the nanofibers has also undergone significant changes, and some larger nanospheres have grown on the surface. However, these nanospheres do not completely wrap the TiO2 nanofibers, forming a composite structure in which the nanofibers are connected in series with the nanospheres. Transmission electron microscopy can also clearly see that large-sized nanospheres are connected in series on the surface of TiO2 nanofibers.
In order to fully confirm the crystal structure of the secondary materials grown on the surface of TiO2 nanofibers, we also carried out XRD characterization of these secondary nanomaterials (Fig. 7). Figure 7(a) is the XRD pattern of the prepared Fe2O3/TiO2 secondary heterostructure, and we give the crystal plane parameters of typical crystal plane peaks. As can be seen from Fig. 7(a), in addition to the diffraction peaks of typical anatase TiO2 (JCPDS No. 21-1272), the diffraction angles 2θ are 24.15°, 33.3°, 35.8°, 40.6°, 49.4° , 54.1°, 57.4°, 62.3° and 63.9° belong to (012), (104), (110), (113), (024), (116), (122), (214) and (300) crystals, respectively surface diffraction peaks. The diffraction peaks of this sample correspond to rhombohedral hematite (JCPDS No. 33-0664). Figure 7(b) is the XRD pattern of the prepared Fe3O4/TiO2 secondary heterostructure, and we give the crystal plane parameters of typical crystal plane peaks. As can be seen in Fig. 7(b), all diffraction peaks can be indexed as anatase TiO2 (JCPDS No.21-1272) and Fe3O4 (JCPDSfile No.19-0629). While Fig. 7(c) is the XRD pattern of the prepared CuO/TiO2 secondary heterostructure, we give the crystal plane parameters of typical crystal plane peaks. As can be seen in Fig. 7(c), all the diffraction peaks can be indexed as anatase TiO2 (JCPDS No. 21-1272) and CuO (JCPDS No. 48-1548). It can be seen that this synthetic strategy combining electrospinning technology and hydrothermal method is not only simple and feasible, but also has great universality and can be used to prepare various metal oxide/TiO2 secondary heterostructures.
- Formation mechanism of Co3O4/TiO2 secondary heterostructure
Based on the above analysis, various metal oxide/TiO2 secondary heterostructures can be prepared by combining electrospinning technology and hydrothermal method. Importantly, there is no need for any surface modification and chemical grafting of TiO2 nanofibers during the whole preparation process. In order to deeply study the formation mechanism and evolution process of secondary heterostructures, we describe the formation process of such structures in detail by changing the time of the hydrothermal reaction.
Figure 8 shows the SEM images of the composites generated by TiO2 nanofibers after different reaction times (1h, 1.5h, 2h, 4h, 6h and 9h) in the reactor. Obviously, the surface of TiO2 nanofibers has no obvious change after 1 h of hydrothermal reaction [Fig. 8(a)]. However, after continuing the reaction for 1.5 h, fine nanowires started to nucleate and grow on the surface of TiO2 nanofibers [Fig. 8(b)]. When the reaction time reached 2 h, some nanosheet-like materials began to form on the surface of TiO2 nanofibers [Fig. 8(c)]. When the reaction time exceeds 4 h [Fig. 8(d)], the continuous growth leads to the disappearance of fine nanowires and the appearance of nanosheet-like structures instead. Continuing to increase the reaction time to 6h and 9h [Fig. 8(e) and Fig. 8(f)], the structure of the composite material hardly changed, indicating that the secondary structure did not continue to change after further prolonging the time, so there was no further change in the formation of the composite material. Impact.
Based on the above experiments, we propose the formation mechanism of the secondary heterostructure [shown in Fig. 8(g)]. First, we tested the zeta potential of TiO2 nanofibers, and the test result was 29.4 mV, which indicated that the surface of TiO2 nanofibers was negatively charged. Then, the positively charged metal cations are added to the aqueous solution containing the negatively charged titanium dioxide fiber membrane, and driven by the mutual attraction of the positive and negative charges, the metal cations are adsorbed to the surface of the fiber membrane. Then, the metal cations adsorbed on the surface of TiO2 nanofibers reacted with urea to form a cobalt-containing precursor.
Apparently, the TiO2 fiber membrane was used as a substrate throughout the nucleation process, inducing the growth of its surface secondary structure. At this stage, the generation of uniform and fine nuclei is very important because it is the key to decide the growth of Co3O4 nanosheets on the surface of TiO2 nanofibers [22~23]. With the prolongation of the reaction time, the nucleated particles grow further, thus forming nanosheet-like structures on the surface. Finally, a Co3O4/TiO2 secondary heterostructure with many holes on the nanosheets was produced by high temperature calcination.
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