Thermal properties of electrospun polyvinylpyrrolidone/titanium tetraisopropoxide composite nanofibers
In this work, polyvinylpyrrolidone/titanium tetraisopropoxide (PVP/TTIP) composite nanofibers were prepared by electrospinning from alcoholic solutions. The morphology and the properties of the fibers were investigated by SEM, Raman spectroscopy, and XRD. The thermal properties and the evolved gases were studied in detail by TG/DTA-MS. The as-spun 800–900-nm-thick PVP/TTIP fibers were then, respectively, annealed in different atmospheres (air and nitrogen) and at different temperatures (550 °C, 900 °C) in order to obtain anatase and rutile TiO2 nanofibers. The investigation of the thermal properties was important before the preparation of the oxide nanofibers, because based on them the crystallinity and the composition of the TiO2 nanofibers could be controlled. Metal oxide nanofibers with a slightly smaller diameter, made up mostly by anatase TiO2, were successfully prepared when the annealing was done at 550 °C, while at 900 °C, rutile TiO2 fibers were obtained. If nitrogen atmosphere was applied, the nanofibers contained some carbon residue as well.
KeywordsTG/DTA-MS Polyvinylpyrrolidone Titanium tetraisopropoxide TiO2 Nanofiber Electrospinning
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 , photocatalysis [8, 9], gas sensing [10, 11], thermal energy storage , optical sensors , or for encapsulating biomaterials , etc.
Numerous metal oxide nanofibers (e.g., ZnO, TiO2, MnO2, WO3, V2O5, NiO, SnO2, Fe2O3)  or composites (e.g., NiO/ZnO , SnO2/TiO2 , carbon/MnO2 ) 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 , photocatalysis , solar cells , electrochemical biosensing , 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 , 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 .
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 . 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 . 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
The thermal analysis of these fibers was done in both oxidative (air) and inert atmospheres (N2).
Fragments of the materials in TG/DTA-MS measurement
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).
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 , because the light absorption could be shifted to the visible range if the sample also contains a small amount of carbon in it.
Open access funding provided by Budapest University of Technology and Economics (BME). I. M. Szilágyi thanks for a János Bolyai Research Fellowship of the Hungarian Academy of Sciences and an ÚNKP-18-4-BME-238 New National Excellence Program of the Ministry of Human Capacities, Hungary. A GINOP-2.2.1-15-2017-00084, an NRDI K 124212, and an NRDI TNN_16 123631 Grant are acknowledged. The research within project No. VEKOP-2.3.2-16-2017-00013 was supported by the European Union and the State of Hungary, co-financed by the European Regional Development Fund. The research reported in this paper was supported by the Higher Education Excellence Program of the Ministry of Human Capacities in the frame of Nanotechnology and Materials Science research area of Budapest University of Technology (BME FIKP-NAT).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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