Effect of Flow Rate on Characterizations of TiO 2 Nano fibers using Electro spinning Method

TiO 2 nanofibers with anatase structure were synthesized by an electrospinning method. X-ray diffraction measurements showed that TiO 2 nanofibers were polycrystalline with anatase phase. The effects of flow rate parameter on TiO 2 nanofibers were examined using atomic force microscopy, field emission scanning electron microscopy. The energy gap was estimated and optical behavior was studied using UV-Vis spectroscopy, and Photoluminescence spectroscopy. It was noted that the average diameter of these nanofibers increases from (82-320) nm with increasing the flow rate from (1-10 ml/h) respectively. The length of the nanofibers reached to several microns.


INTRODUCTION
itanium dioxide or titania (TiO 2 ) is one of the most common materials for a variety of applications such as catalytic devices, sensors, solar cells, and other optoelectronic devices [1,2].Titania is a wide bandgap semiconductor with many interesting properties, such as transparency to visible light, high refractive index and low absorption coefficient.Other than these properties, it has been known to be an excellent catalyst in the field of photocatalytic decomposition of organic materials.Therefore, titania has been employed for organic pollutant treatment in the environmental applications [3].Titania is known to have three natural polymorphs, i.e. rutile, anatase, and brookite.One-dimensional nanostructures such as nanorods, nanowires, nanofibers and nanotubes of various oxide materials have been the subject of considerable attention in recent years.The enormous amount of attention results from fundamental scientific interest in these materials as well as from their potential applications in various types of functional devices [4][5][6].Numerous methods have been developed for the fabrication of one-dimensional TiO 2 nanostructures, such as self-assembling, template growth, strong alkali treatment, thermal evaporation and electrospinning.
Electrospinning has been widely used to synthesize nanofibers of oxide materials [16,17].In the typical process of electrospinning, an electrical potential is applied between a syringe needle and a grounded target.When the electrostatic force exceeds the surface tension of the droplet formed at the tip of a syringe needle, charged fluid jet is ejected which is subsequently stretched to form a continuous ultrathin fiber and finally nanofibers are deposited on a target plate [18].During its flight to a collective target, the ejected, charged jet dries out, leaving ultra thin fibers on the target.The non-woven mat has a high surface area with relatively small pore size [1,2,19].
The following parameters and processing variables affect the electrospinning process: (i) system parameters such as molecular weight, molecular weight distribution and architecture (branched, linear, etc.) of the polymer, and polymer solution properties (viscosity, conductivity, dielectric constant, and surface tension, charge carried by the spinning jet) and (ii) process parameters such as electric potential, flow rate and concentration, distance between the capillary and collection screen.(iii)ambient parameters such as temperature, humidity and air velocity in the chamber .For instance, the polymer solution must have a concentration high enough to cause polymer entanglements yet not so high that the viscosity prevents polymer motion induced by the electric field.The solution must also have a surface tension low enough, a charge density high enough, and a viscosity high enough to prevent the jet from collapsing into droplets before the solvent has evaporated [20].
The aims of the present work are to prepare TiO 2 nanofibers by electrospinning technique using polyvinylpyrrolidone (PVP) and Ti tetraisopropoxide as precursors, and the effect of the flow rate as one of important processing parameters was then investigated on the crystal structure, morphology, and microscopy of electrospun TiO 2 nanofibers.TiO 2 nanofibers were characterized by XRD, AFM, SEM, UV-Vis, and PL spectroscopy.
In a typical procedure, 1 g of titanium tetraisopropoxide (TIP) was mixed with 2 ml of acetic acid and 2 ml of ethanol.The solution was rested for 10 min before being added into 5 ml of PVP solution in ethanol.The concentration of the PVP solution was kept 7 wt% and the resulting mixture was constantly stirred for 10 min.The

Figure (1) Schematic view of electrospinning technique.
The x-ray diffraction (XRD) measurements, which were used to characterize the crystalline phase of the TiO 2 fiber, were carried out on an x-ray diffractometer (type miniflex II Rigaku, Japan) using (CuKα) radiation.Atomic force microscopy (AFM) micrographs were taken with a Digital Instruments, Inc. Nanoscope III and Dimension 3100).The samples were observed under a (Hitachi S-4160 Japan) field emission scanning electron microscope (FESEM) after being gold-coated.
The optical absorption spectra of the TiO 2 film were recorded using (Optima Sp-3000 plus UV-Vis-NIR, Split-beam Optics, Dual detectors) spectrophotometer equipped with a xenon lamp and a wavelength range of (300-900 nm).The photoluminescence (PL) spectra were recorded at room temperature (25 o C) using the (Perkin Elmer Spectrophotometer Luminescence LS 55) by exciting the samples at 260 nm.

Results and discussion
Figure (2) shows the XRD patterns of the as-prepared samples calcined at 500 0 C for 3 hours in air atmosphere.The peaks shown in the XRD patterns correspond to the (101), (004), ( 112), ( 200), ( 105), (211), and (204) planes of TiO 2 tetragonal anatase phase.These patterns can be well indexed to tetragonal anatase (JCPDS no. .No peaks of brookite or rutile phase were detected, which indicate the high purity of the as-prepared samples.The crystallite size can be estimated using the Scherrer formula [21]: ( ) …(1) Where: D is the crystallite size in nanometers, k a constant (=0.9 assuming that the particles are spherical), λ is the wavelength of the X -ray radiation (CuKα =0.1541 nm), β is the FWHM of the strongest peak, and Ɵ is the diffraction angle.The crystallite sizes of the powders were calculated on the diffraction peaks of anatase (101) crystalline plane.According to these calculations, the crystallite sizes of the fibers were (23.52, 24.15, 26.00 nm), for the fibers prepared in (1, 5, 10 ml/h) respectively.These results revealed that the resultant fibers obtained in different flow rates were all in the TiO 2 anatase phase, and with an increase of the flow rates, crystallites size of the samples was getting larger.Also, it can be found that films prepared in high flow rate show relative sharp peaks indicating the coalescence of crystalline anatase phase TiO 2 .The surface morphology and roughness have been indicated in figure (3) using AFM images, the roughness average is (10.30,5.15, 3.97 nm) for the fibers prepared in (1, 5, 10 ml/h) respectively.The roughness average increased with the flow rate decreased, and this has great advantages in many applications of TiO 2 nanofibers such as sensing and photocatalyst applications.Photon energy (hν) The PL excitation spectra were evaluated in figure (6).For all samples, the PL excitation spectra have many peaks: (338,355,365, and 471 nm) which are corresponding to (3.67, 3.50, 3.40, and 2.63 eV) respectively.Hence, this band corresponds to the photo absorption near the band energy (NBE) edge of the conduction band [20].It has been demonstrated that the occurrence of emission peak in the visible region (471 nm) is due to the presence of defect levels below the conduction band.The native defects in wide gap semiconductors are expected to introduce deep levels inside the band gap [24].The broad luminescence spectrum in the visible region is attributed to the electronic transition mediated by the defect levels, such as oxygen vacancies in the band gap [25].The stronger the PL intensity, the larger the content of oxygen vacancies and defects, and this occurred with increases the flow rate.

T The 5 th
International Scientific Conference for Nanotechnology and Advanced Materials and Their Applications ICNAMA 2015 (3-4Nov.2015)https://doi.org/10.30684/etj.2015.1173642412-0758/University of Technology-Iraq, Baghdad, Iraq This is an open access article under the CC BY 4.0 license http://creativecommons.org/licenses/by/4.0 Effect of FlowRate on Characterizations of TiO 9, 2015 Part (B), No. 33 Eng.&Tech.Journal, Vol.Nano fibers using Electro spinning Method 1601 spinning solution was immediately loaded into a plastic syringe with 21-gauge stainless steel needle was used as the nozzle.The emitting electrode from a Gamma High Voltage Research ES30P power supply capable of generating DC voltages up to 25 kV was attached to the needle.The grounding electrode from the same power supply was attached to a plate of aluminium with clean glass substrates fixed on it which was used as the collector plate and was placed 20 cm in front of the tip of the needle.When an applied voltage of 21 kV is subjected across the needle and the collective plate, a fluid jet was ejected from the nozzle.The feed rate of the precursor solution was (1, 5, and 10 ml/h) which can be controlled using a syringe pump (KDS 200).As the jet accelerated towards the collector, the solvent evaporated, leaving only ultra-thin fibers on the collector as shown in figure (1).The obtained fibers were left exposed to moisture for approximately 10 h to allow complete hydrolysis of TIP and consequently subjected to calcination at a high temperature of 500 o C for 3 h with a heating rate of 5 o C/min to remove residual PVP.

Figure ( 4 )
Figure (4) FESEM images of pre-calcined as-spun fibers on left side and TiO 2 nanofibers on right side prepared in different flow rates (a) 1 ml/h (b) 5 ml/h (c) 10 ml/h (d) 10 ml/h in smaller magnification.

Figure ( 5 )
Figure (5) (a) The transmittance vs. wavelength of TiO 2 nanofibers prepared in different flow rates (b) The allowed direct band gap prepared in different flow rate.