Introduction

The preparation of nanocomposites with well-controllable morphology and properties nowadays is a really important task [1, 2]. Many different methods exist for synthesizing various composites, one of them is electrospinning [3,4,5,6], by which technique uniform nanofibers can be produced with high specific surface area. Although it has a simple setup, the reproducibility is exceptional and the properties (e.g., diameter, specific surface area, composition) of the product can be controlled precisely. During the process, high voltage is applied to a polymer solution or melt, which can also contain the precursor salt of metal oxides. Due to the electric field, charged jets erupt from the surface of the liquid, which stretch to ultrathin continuous fibers and dry mid-flight before reaching the grounded collector. The use of electrospinning allows the production of nanomaterials with diverse properties; thus, there is a wide variety of applications for them. There are examples when the electrospun nanofibers were applied in air filtration [7], photocatalysis [8, 9], gas sensing [10, 11], thermal energy storage [12], optical sensors [13], or for encapsulating biomaterials [14], etc.

Numerous metal oxide nanofibers (e.g., ZnO, TiO2, MnO2, WO3, V2O5, NiO, SnO2, Fe2O3) [15] or composites (e.g., NiO/ZnO [16], SnO2/TiO2 [17], carbon/MnO2 [18]) containing different metal oxides can be easily prepared by electrospinning. Out of these oxides, TiO2 nanofibers are the most widely researched. They can be used in gas sensing [19], photocatalysis [20], solar cells [21], electrochemical biosensing [22], etc. For the preparation of TiO2, many precursors are available, e.g., titanium tetrabutoxide, titanium chloride, titanium tetraisopropoxide, etc. These all have different properties (solubility, thermal stability, etc.), and consequently, choosing the precursor is based on what purpose the nanomaterial is prepared for. In many of the applications, the thermal stability of the sample is of great importance [23], and because of this, it has to be studied thoroughly.

In this work, polyvinylpyrrolidone/titanium tetraisopropoxide (PVP/TTIP) nanofibers were prepared from their alcoholic solution by electrospinning. Later from the PVP/TTIP composite, anatase and rutile TiO2 nanofibers were prepared by annealing at different temperatures (550 °C and 900 °C, respectively) and in different atmospheres (oxidative and inert). The nanocomposite prepared by electrospinning and the different TiO2 nanofibers were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy.

To prepare oxide nanofibers from the electrospun fibers and also to be able to control the crystallinity and composition of the fibers, it was important to study the thermal properties of the PVP/TTIP fibers thoroughly. The thermal properties of the composite were investigated in air and nitrogen atmospheres by simultaneous thermogravimetry/differential thermal analysis (TG/DTA). The evolved gases were analyzed by on-line coupled mass spectrometry (TG/DTA-MS) [24,25,26], which was previously not done in the literature when these precursors were used for the preparation of TiO2 nanofibers [27].

Experimental

Electrospinning was done using a homemade setup. For the preparation of the PVP/TTIP composite fibers, 1 g of titanium tetraisopropoxide (TTIP, Ti(OiPr)4 by Sigma-Aldrich) was dissolved in the mixture of 1 cm3 of acetic acid (AcOH) and 1 cm3 of ethanol (EtOH). The solution was stirred for 15 min at room temperature. After this, 0.5 g of polyvinylpyrrolidone [PVP, (C6H9NO)n, K-90 by Merck] was separately dissolved in 3 cm3 of ethanol. The two solutions were then mixed together and stirred for 1 h at room temperature before the electrospinning [28]. The electrospinning was done at 25 kV voltage, the characteristic distance was approximately 25 cm, and a feeding rate of 4 cm3 h−1 was used. The fibers were collected on an Al foil screen covered by a polyethylene sheet.

After the preparation of the composite nanofibers, the thermal properties were investigated. The measurements were carried out in an SDT 2960 Simultaneous DTA/TGA (TA Instruments Inc.) thermal analyzer. The samples were heated up to 900 °C using a heating rate of 10 °C min−1 in air and inert (nitrogen) atmospheres as well. Evolved gases were measured by a ThermoStar GSD 200 (Balzers Instruments) quadrupole mass spectrometer (MS) in multiple ion detection (MID) mode. The selected m/z ions could be measured in 64 channels simultaneously. A heated (200 °C) 100% methyl deactivated fused silica capillary provided the on-line coupling.

Based on the TG results, the PVP/TTIP fibers were later annealed at different temperatures in order to produce both anatase and rutile TiO2 nanofibers. The temperatures were chosen based on the known phase transformations of TiO2. Except for extremely small particles, at atmospheric pressure, the anatase-to-rutile transition is around 600 °C [29]. Thus, for preparing anatase TiO2 fibers, the composite was heated up to 550 °C by which temperature the polymer completely burned out. Rutile TiO2 nanofibers were achieved by annealing at 900 °C. The annealing was carried out in both air and nitrogen. In all cases, the heating rate was 10 °C min−1.

The morphology of all samples was studied by scanning electron microscopy (SEM) in a JEOL JSM-5500LV scanning electron microscope. The measurements were done at 20 kV voltage. Before the measurement, the nanofibers were coated with a thin Au/Pd layer in a sputter coater.

The XRD patterns were recorded by a PANalytical X’pert Pro MPD X-ray diffractometer using Cu Kα irradiation.

Raman spectra were measured by a LabRam system (Horiba Jobin–Yvon, Lyon, France) coupled with an external 532 nm Nd-YAG laser source (Sacher Lasertechnik, Marburg, Germany).

Results and discussion

Based on the SEM pictures, the as-prepared PVP/TTIP fibers (Fig. 1a) were fairly uniform and had a diameter of about 800–900 nm. No bead formation could be observed.

Fig. 1
figure 1

SEM images of a PVP/TTIP fibers; b fibers annealed at 550 °C in N2; c fibers annealed at 900 °C in N2; d fibers annealed at 550 °C in air; e fibers annealed at 900 °C in air

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The thermal analysis of these fibers was done in both oxidative (air) and inert atmospheres (N2).

On the TG curve in nitrogen atmosphere (Fig. 2) till 100 °C, there was just a small mass loss (~ 6%), which could be assigned as the loss of physically adsorbed water and a very small amount of solvent that was used during the electrospinning. After this, on the TG curve, only one larger step could be seen between 250 °C and 600 °C, which was endothermic. At 250 °C, based on the MS spectra, just the TTIP started to decompose, which can be proved by the appearance of the methyl (m/z = 15) and the isopropoxide (m/z = 59) fragments (Table 1). Then, from around 280–300 °C, the PVP begun to decompose as well (a PVP fragment at m/z = 84 and the PVP monomer at m/z = 111 appeared on the MS spectra). In nitrogen, the decomposition of the PVP extremely stretched out and the organic compounds could not burn out totally. Caused by this, the final product with a remaining mass of ~ 30% also contained some carbon residue beside TiO2.

Fig. 2
figure 2

Thermal analysis of the PVP/TTIP composite fibers in nitrogen

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In air (Fig. 3), the decomposition started similarly with a mass loss of about 6% of water by 100 °C. The intense decomposition of the sample started at around 250 °C and consisted of two exothermic steps. Taking into account the MS results as well, it was obvious that the degradation again started with the TTIP (appearance of the fragment m/z = 59) (Table 1). In air, the organic material started to burn, and the endothermic heat of decomposition was overcompensated by the exothermic combustion heat, leading to exothermic DTA peaks. At around 350 °C, the decomposition was so rapid that the generated heat led to the excision of the temperature set by the heating program. This was indicated by the retraction of all the TG, DTG, and DTA curves. The degradation of the PVP also started later in air than that of the TTIP, and at first, the polymer chains of the PVP broke up and the side chains were severed, so only the organic backbone of the polymer remained, which was the last to decompose. The whole process was followed by the evolution of CO2 as well. In air, by 350 °C, the TTIP decomposed totally, and by 550 °C, all the organic char residues from the PVP burnt out completely. The remaining mass was around 24%, and the final product only contained TiO2. This was later confirmed by the XRD measurements.

Fig. 3
figure 3

Thermal analysis of the PVP/TTIP composite fibers in air

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Table 1 Fragments of the materials in TG/DTA-MS measurement
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On the SEM pictures after the annealing (Fig. 1b–e), it was visible that in all the cases the fibrous structure was maintained, but the fibers broke into shorter pieces. The diameter slightly decreased after the annealing in N2 (700–800 nm), and the shrinking of the fibers was more significant when air was used as atmosphere (600–700 nm).

On the XRD diffractograms (Fig. 4), it could be seen that the electrospun PVP/TTIP fibers were amorphous. After annealing either in air or in nitrogen at 550 °C, only peaks referring to TiO2 could be found, the nanofibers contained mostly anatase TiO2 (ICDD 01-083-2243). A small peak at 27° appeared as well, indicating the presence of a smaller amount of rutile phase beside anatase. The chosen temperature was close to the phase transition temperature, so the appearance of this peak could be explained by that the formation of the rutile phase has already begun in a small part of the samples. When the annealing temperature was higher (900 °C), the fibers crystallized mostly in rutile form (ICDD 00-021-1276); however, a part of the sample was still anatase, based on the peaks.

Fig. 4
figure 4

XRD results of the PVP/TTIP and annealed fibers

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The Raman spectra (Fig. 5) were in agreement with the XRD results. At 900 °C, the peaks of rutile TiO2 can be seen on the spectra (the peaks at around 243 cm−1, 447 cm−1, and 612 cm−1 can be, respectively, assigned to the second-order Raman scattering, Eg and A1g modes of rutile TiO2 [28, 30]), and also some small peaks of the anatase remained. However, at 550 °C, only the anatase TiO2 peaks were visible. (The peaks at 144 cm−1, 403 cm−1, 520 cm−1, 639 cm−1 are the Eg, B1g, A1g or B2g, and Eg modes of the anatase phase [8, 31], respectively.) The Raman studies also confirmed that in nitrogen as expected from the TG/DTA-MS results some carbon residue remained (D and G bands of carbon appeared [32, 33]), while in air the polymer was burnt completely, and only the peak of crystalline TiO2 appeared.

Fig. 5
figure 5

Raman measurement of the PVP/TTIP and annealed fibers

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Conclusions

Polyvinylpyrrolidone/titanium tetraisopropoxide (PVP/TTIP) nanofibers with a diameter of 800–900 nm were prepared by electrospinning. The thermal properties of these fibers were thoroughly studied by TG/DTA, and the steps of decomposition were explained based on the results and the evolved gas analysis; the latter was previously not measured for these materials, when they were used for electrospinning. To be able to control the crystallinity and composition of the fibers, when later they were annealed to prepare TiO2 nanofibers, studying the thermal properties was also important. For annealing, two different temperatures (550 °C and 900 °C) were chosen and also two different atmospheres were used. At 550 °C anatase and at 900 °C, rutile TiO2 fibers could be prepared. When the annealing was done in nitrogen, besides the oxide, the fibers contained a small amount of carbon as well. Controlling the carbon content could be useful in the future, if the fibers are used as photocatalyst [28], because the light absorption could be shifted to the visible range if the sample also contains a small amount of carbon in it.