1. Synthesis of V2O5 nanomaterials with different morphologies
Figure 1 shows the synthesis process of V2O5 nanomaterials with different morphologies. First, non-woven films were prepared by electrospinning. The vanadium acetylacetonate/polyvinylpyrrolidone/N,N-dimethylformamide spinning mixed liquid is prepared by electrospinning technology to prepare the vanadium acetylacetonate/polyvinylpyrrolidone non-woven nanofiber film. Secondly, the specific calcination temperature is adjusted during the calcination process. When the calcination temperature is 400°C, porous V2O5 nanotubes can be obtained, when the calcination temperature rises to 500°C, secondary V2O5 nanofibers are obtained, and when the temperature is further increased to 600°C, single crystal V2O5 nanobelts are obtained.
2. Crystal structure analysis of V2O5 nanomaterials with different morphologies
Figure 2 shows the X-ray diffraction (XRD) spectra of the prepared V2O5 nanomaterials with different morphologies, and we give the crystal plane parameters of typical crystal plane peaks. It can be seen from Figure 2 that the synthesized sample has the structure of orthorhombic vanadium pentoxide, Pmmn (no.59); a=1.1516nm, b=0.3565nm, c=0.4372nm; JCPDS card no. 41-1426, and there are no other impurity peaks in the XRD patterns of the samples obtained at the three calcination temperatures, indicating that the crystallinity of the samples is single. By observing and comparing the XRD patterns of the nanomaterials calcined at different temperatures, we can see the change history of the XRD patterns of the samples during the temperature increase. The intensity of the diffraction peaks increases with increasing temperature. By observing the intensity of the diffraction peaks of the nanomaterials calcined at 600℃, especially the (001) plane, we can preliminarily judge that the nanomaterials obtained by calcination at this temperature are single crystals.
3. Morphology analysis of V2O5 nanomaterials with different morphologies
Figure 3 shows the SEM images of V2O5 nanomaterials with different morphologies. It can be seen from Figure 3(a) that the uncalcined nanofibers have a smooth surface with a diameter of about 560 nm. The calcined samples no longer have a smooth surface, and even the fibrous structure has changed. First, the material calcined at 400 °C has a tubular structure with an obvious internal hollow, and its surface also has many holes. And the fiber diameter became significantly smaller, only 340 nm [Figure 3(b)]. As the calcination temperature was increased to 500 °C, the porous tubular structure disappeared and a fibrous structure composed of many smaller nanoparticles appeared instead [Figure 3(c)]. When the calcination temperature was increased to 600 °C, the smaller particles further grew to form a ribbon-like structure [Figure 3(d)]. It is obvious that the controllability of the morphology of V2O5 nanomaterials can be achieved by adjusting the calcination temperature.
To further study the morphologies and structures of porous V2O5 nanotubes and single-crystalline V2O5 nanoribbons, we tested TEM of both types of materials. It can be seen from Figure 4(a) that the sample exhibits an obvious tubular structure, which is consistent with the results observed by scanning electron microscopy. High-resolution transmission electron microscopy not only confirmed the existence of the hollow tubular structure, but also confirmed that the tubular material is composed of many small particles with many voids between them, resulting in a porous hollow tubular material [Figure 4(b)]. Transmission electron microscopy also confirmed the formation of an obvious band-like structure, which is also consistent with the results observed by scanning electron microscopy [Figure 4(c)]. Moreover, by analyzing its high-resolution transmission electron microscope, it can be confirmed that the lattice spacing of 0.58 nm is the lattice spacing of the V2O5 (200) plane [Figure 4(d)]. It can be confirmed by Fourier transform that the V2O5 nanomaterials calcined at 600 °C have a single crystal structure grown along the (010) plane.
4. Thermogravimetric analysis of V2O5 nanomaterials with different morphologies
Thermogravimetric analysis was used to observe the relationship between the weight of vanadium acetylacetonate/polyvinylpyrrolidone nanofibers as a function of calcination temperature (Figure 5). Apparently, the weight loss of the electrospun fiber membrane was caused by the decomposition of vanadium acetylacetonate and polyvinylpyrrolidone throughout the temperature increase. Combined with the changes of the DSC curve, it can be seen that the weight loss process mainly includes the following three stages.
①The weight loss in the 20~200℃ stage can be attributed to the volatilization of solvents such as water and N,N-dimethylformamide.
②In the DSC curve at 200~300℃, there is an obvious exothermic peak at 243℃, which is caused by the decomposition of vanadium acetylacetonate and polyvinylpyrrolidone.
③300~650℃ stage There is a sharp differential peak and a large mass loss at this stage, which is mainly because the residue of polymer decomposition is further oxidized, so an obvious exothermic peak appears at 408℃, which is consistent with many The results reported in the literature are consistent. It can be clearly seen from Figure 5 that the weight loss starts from 243 °C, and the differential curve after 400 °C becomes relatively flat, that is, the weight loss is basically completed, so V2O5 can be generated at 400 °C. In view of this, we choose 400 °C, 500 °C and The calcined products at 600 °C were used to study their corresponding electrochemical properties.
5. Formation mechanism of V2O5 nanomaterials with different morphologies
In order to illustrate the formation mechanism of V2O5 nanomaterials with different morphologies in detail, the morphologies of nanomaterials obtained by a series of calcinations at different temperatures were characterized, and the possible formation mechanisms were proposed according to the obtained morphological evolution process. It can be seen from Figure 6(a) that when the calcination temperature is 350 °C, the surface of the nanofibers is very smooth without any holes and secondary structures, and the average diameter is about 410 nm. According to the thermogravimetric spectrum (Figure 5), the fiber film of the non-woven fabric has already begun to be thermally decomposed at this temperature, so the diameter has decreased, but the fiber structure has not been destroyed due to incomplete decomposition. As the temperature was slowly increased to 400 °C, porous V2O5 nanotubes were formed, which shrunk by 50 nm in diameter [Figure 6(b)]. This is due to the decomposition of vanadium acetylacetonate and polyvinylpyrrolidone and the formation of V2O5. In fact, in this temperature range, since the sample is in an air atmosphere, the organics in the composite are oxidized to carbon dioxide and volatilize rapidly. With the outward volatilization of carbon dioxide, V2O5 is forced to accumulate on the surface of the fiber, so a hollow tubular structure is formed. At the same time, due to the oxidation of vanadium acetylacetonate, the formed V2O5 nanoparticles began to aggregate, and finally holes appeared on the tube wall. When the temperature was increased to 450 °C, the formed V2O5 nanoparticles grew further, which resulted in the enlargement of the pores formed on the tube wall. Despite this, the porous nanotube morphology was still maintained at this temperature [Figure 6(c). )]. But when the temperature was increased to 500 °C, the porous nanotube-like structure disappeared with the further growth of V2O5 nanoparticles, and a secondary nanofibrous structure composed of V2O5 nanoparticles appeared instead [Figure 6(d)]. Continuing to increase the temperature, when the temperature exceeded 550 °C, the further grown V2O5 nanoparticles destroyed the formed nanofibrous structure, and individual nanoribbons appeared instead. At this temperature, the nanoribbons are smaller in size and even partially connected [Figure 6(e)]. Until the temperature was raised to 600 °C, the material with V2O5 ribbon structure of larger size was formed [Figure 6(f)].
Based on the above results, the evolution process of V2O5 nanostructures with different morphologies can be divided into three stages [Figure 6(g)]: when the temperature is lower than 400 °C, this temperature is not enough to make the vanadium acetylacetonate and polyvinylpyrrolidone completely decomposed, so the obtained fibers still maintain a smooth surface fiber morphology; when the temperature is between 400 and 500 °C, the complete decomposition of polyvinylpyrrolidone and the formation of V2O5 lead to the appearance of a porous tubular structure; when the temperature is higher than 500 °C At °C, the growth of V2O5 nanoparticles leads to the appearance of nanoribbon structures. Therefore, one-dimensional V2O5 nanomaterials with different morphologies can be realized by simply adjusting the calcination temperature of the nonwoven fiber films.