In-situ synthesis of mixed phase electrospun TiO2 nanofibers: a novel visible light photocatalyst
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TiO2 nanofibers were synthesized by electrospinning method using mixed titanum isopropoxide and polyvinylpyrrolidone precursors in ethanol followed by calcination at high temperature. The obtained TiO2 e-spun nanofibers were characterized by FESEM, TEM, XRD, XPS and BET surface area analytical techniques. XRD result shows that the nanofibers contain both anatase and rutile mixed phases. FESEM and TEM study indicates the development of precise fine tiny nanoparticles with diameter in the range of 10–20 nm, which are oriented along one dimensional direction to form the fibrous structure with diameter in the range of 300–500 nm. The mixed phase TiO2 nanofibers were used as photocatalysts for degradation of Rhodamine-B and Methyl orange under solar light irradiation and also under visible light irradiation. More than 99% degradation was achieved within 90 min of irradiation time for both the dyes. Furthermore, it is observed that the TiO2 photocatalyst is efficiently degrading (98%) the tetracycline hydrochloride solution within 70 min of contact time. The excellent visible light photocatalytic activity may be attributed to the combine factors of hetero-conjunction at anatase–rutile interface and immediate charge transfer due to 1D structure, which inhibit the electron–hole recombination.
KeywordsElectrospinning Anatase Rutile Nanofibers Photocatalysts
The textile industries utilize about 10,000 dyes and pigments. Wastewaters from printing and dyeing units of the industries are highly hazardous to aquatic living and human beings because they cause serious damage to the surrounding environment [1, 2, 3]. Thus, it is very essential to remove it from water. Many technologies have been proposed for the treatment of dyeing effluents, including biological, adsorption, membrane and chemical oxidation. However, these technologies still have some problems such as secondary pollution or insufficient treatment. Removal of toxic dyes by photocatalysis with semiconductors is widely investigated as a useful technique not only for the efficient degradation of contaminants in waste water, but also for utilization of abundant solar energy without the need for additional chemical regents [4, 5].
Among the photoactive semiconductors, nanostructured TiO2 has received a great attentation as a photocatalyst due to its suitable band structure, high chemical and thermal stability, non-environmental effect and low cost [6, 7, 8]. However, there are several limitations like low absorbance in visible reason, high recombination rate of photo generated electron–hole pairs which restricts its practical application as an efficient photocatalyst . Therefore, there is a need for the development of visible light responsive photocatalyst. Recently many efforts have been taken to improve the visible light active photocatalytic properties of TiO2 such as by doping with metals [10, 11, 12, 13, 14] and nonmetals [15, 16, 17, 18], deposition of noble metals [19, 20, 21], surface fluorination  and controlling crystal facets , by creating heterojunction by coupling with other semiconductor materials having suitable band gap [24, 25, 26, 27, 28]. Design of nano structures having different morphologies like nanoflower , nanofibers , nanorod , coupling with carbanious materials like grapheme , carbon nanotube , and development of porous nanostructures  have also been proposed to increase visible light absorbance and to reduce the electron–hole pair recombination rate of TiO2.
Apart from this, development of 1D TiO2 nanomaterials has received great attention throughout the field. It is because 1D TiO2 nanostructures cannot only improve light absorption but also can reduce electron–hole pair recombination rate. The reasons behind the extraordinary properties of 1D nanostructure over nanoparticles are: faster electron diffusion than in nanoparticles; presence of additional energetic barrier to recombination due to the formation of a space-charge region; 1D nanofibers ensure the rapid collection of carriers generated by the reducing surface [34, 35, 36].
It is well known that TiO2 exists in three structural forms namely anatase (3.2 eV), rutile (3.0 eV) and brookite (3.4 eV) . Among these forms inspite of larger band gap anatase exhibits better photocatalytic activity than rutile because anatase has unique ability to separate photo generated electron hole pairs more than rutile. It has been reported that creating a heterojunction with anatase and rutile phase exhibits higher photo catalytic activity than individual pure anatase and rutile structures. Unlike the normal heterojunction, due to proper alignment of energy levels and similarity of crystal lattice, formation of heterojunction between two TiO2 based materials can prevent formation of charge transfer barrier at the heterojunction interface . Thus developing 1D TiO2 nanomaterials in which there exist a heterojunction between anatase and rutile can generate highly efficient visible light responsive photocatalyst.
Electrospinning is an effective, straightforward, and convenient method to synthesize continuous semiconductor metal oxide nanofiber photocatalysts with high photocatalytic activity and favorable recycling characteristics due to their one dimensional (1D) nanostructural property [39, 40]. Metal oxide nanofibers are usually prepared by electrospinning a precursor metal salt solution with the help of a proper polymer, followed by calcination to decompose the polymer completely and turn metal salt into metal oxide .
In this work, we have reported an efficient method for the fabrication of TiO2 nanofibers by calcination of electrospun PVP/Ti(OiPr)4 composite nanofibers at 500 °C for 2 h. The photocatalytic ability of prepared TiO2 nanofibers was investigated by degradation of organic dyes, Rhodamine-B and Methyl orange under solar light irradiation and the photo-degradation process was detected using a simple UV–Vis spectroscopy method.
2 Experimental techniques
Polyvinylpyrrolidone (PVP) (Mw ca. 1,300,000), titanumisopropoxide (TIP), acetic acid, ethanol, Rhodamine-B and Methyl orange dyes were commercially available and used as received.
2.2.1 Synthesis of electrospun TiO2 nanofibers
In a typical synthesis procedure 0.5 g of ethanol, 0.5 g of acetic acid and 0.5 g of Titanumisopropoxide (TIP) were mixed and stirred for 10 min to form Solution-A. 0.24 g of PVP dissolved in 1.2 g of ethanol to form Solution-B. Then solution A was added to solution B with constant stirring and the stirring was continued for 2 h after stirring to get uniform composite solution. Then the prepared solution was delivered to a stainless needle with inner diameter: 0.5 mm) at a constant flow rate of 1.0 mL/h by a plastic syringe pump. As a high voltage of 13 kV was applied, the precursor solution jet accelerated towards the cathode which was placed 10 cm from the needle tip, leading to the formation of nanofiber arrays onto the aluminum foil substrate accompanied by solvent evaporation to form PVP/TIP nanofiber. The as-spun nanofibers were then calcined at 500 °C in air for 2 h to form ultrafine TiO2 nanofiber.
2.2.2 Nanofiber characterization
The morphologies of the as-fabricated nanofibers were observed by FESEM (Nova Nano SEM 450) and transmission electron microscopy (TEM; JEM-2100 HRTEM, Make-JEOL, Japan). The crystal structure of the nanofibers was identified by XRD pattern recorded on a Rigaku Ultima-IV X-ray diffractometer with Cu Kα radiation (λ = 1.54156 Å) at a scan rate of 5°/min in the range of 20°–80°. X-ray photoelectron spectroscopy (XPS) was determined using a VG Scientific ESCA LAB Mk-II Spectrometer with Al Kα radiation (1486.6 eV) at a takeoff angle at 45°. The UV–Visible absorbance spectra of the nanofibers were analyzed with Shimadzu spectrometer (model 2450) with BaSO4 coated integration sphere within the range 200–800 nm. Specific surface area and pore size distribution (PSD) of the nanofibers were determined from nitrogen adsorption/desorption isotherms obtained at the temperature of liquid nitrogen in an automated physisorption instrument (Autosorb-iQ, Quantachrome Instruments). Prior to the analysis, the samples were outgassed under vacuum at 150 °C for 1.5 h.
2.2.3 Photocatalysis experiments
The photodegradation efficiency of all synthesized TiO2 photocatalysts was tested towards decontamination of Rhodamine-B (RhB) and Methyl orange (MO) under solar light radiation. Initially stock solutions of 1 g/L were prepared by dissolving 1 g of RhB/MO in 1000 mL of double distilled water. In a typical experiment, 10 mg of catalyst was added to 100 mL of 20 mg L−1 RhB/MO solution in a 250 mL of beaker. Before irradiation, the suspension was magnetically stirred in dark for 1 h to ensure the establishment of the adsorption/desorption equilibrium of the dye onto the surface of photocatalysts. Afterwards the dye solution along with nanofiber was exposed to sunlight with continuous stirring. All the experiments were performed during the month of December and January (sunny days), from 11:00 AM to 12:30 PM, when the average solar intensity was 0.25 kW m−2 with minimum fluctuation. During photoreaction 5 mL of the suspension was collected at 15 min time intermission and centrifuged to eradicate the particles. After that the RhB/MO concentration in the solution was analyzed by UV–VIS spectrometer (Shimadzu spectrometer, model-2450) at its maximum adsorption wavelength (λmax) of 554 nm for RhB and 463 nm for MO. This degradation method was continued till complete degradation of dye from the aqueous media.
3 Results and discussion
3.1 Characterization and properties of the prepared nanofibers
3.1.1 XRD analysis
3.1.2 XPS and FTIR analysis
More information on the elemental composition and chemical state of the calcined TiO2 nanofibers is provided by XPS. The fully scanned spectra (Fig. 2a) show that the TiO2 nanofibers contain Ti, O and C elements. The C element may be ascribed to an adventitious carbon-based contaminant. The high resolution XPS spectra of Ti 2p and O 1s are displayed in Fig. 2b, c. The Ti 2p spectrum of the TiO2 nanofibers in Fig. 2b contains peaks at 459.4 and 464.9 eV, which correspond to Ti 2p3/2 and Ti 2p1/2, respectively. The peak separation between Ti 2p3/2 and Ti 2p1/2 peaks is 5.5 eV, suggesting the existence of the Ti4+ oxidation state . In the O 1s region (Fig. 2c), binding energy values at 530.5 and 531.3 eV are observed. The binding energy value at 530.5 eV is corresponding to the characteristic peak of Ti–O–Ti. The other peak located at 531.6 is assigned to Ti–OH, which is due to surface adsorbed hydroxyl groups. The FTIR spectra of the TiO2 nanofiber in the frequency range of 400–4000 cm−1 is shown in Fig. 2d. The existence of a peak corresponding to the stretching vibration of O–H and bending vibrations of adsorbed water molecules around 3200–3800 cm−1, 2350 cm−1 and 1600 cm−1, respectively. The broad intense band in the range of 450–700 cm−1 is due to the bending vibration of Ti–O bonds .
3.1.3 FESEM and EDS analysis
3.1.4 TEM analysis
3.1.5 Surface area and porosity measurement
3.2 Photocatalytic degradation of RhB and MO dye solutions under solar light irradiation
Percentage of degradation and pseudo-first-order kinetic parameters
Furthermore, we have carried out the visible light photocatalytic degradation of non-colored tetracycline hydrochloride (20 mg/L) compound. It is observed that the TiO2 photocatalyst is efficiently degrading (98%) the tetracycline hydrochloride solution within 70 min of contact time (supporting information, Fig. S2). The obtained result suggests that the prepared TiO2 nanofibers are efficient visible light active photocatalyst and can be used for photocatalytic degradation of toxic organic compounds.
3.3 Photocatalytic mechanism
In order to inspect photogenerated electron transfer pathways, we have carried out the photoluminescence (PL) emission spectra of the prepared TiO2 nanofibers by exciting at wavelengths of 350 nm. The PL spectrum is associated to the transfer behavior of the photoinduced electrons and holes so that it can reflect the separation and recombination rate of photoinduced charge carriers. The result obtained from photo luminesce (PL) experiments is presented in the supporting information (Fig. S2). Very low PL intensity in case of mixed TiO2 nanofibers is due to the efficient separation of photo induced electron–holes charge carriers.
PVP/TIP composite nanofibers were fabricated via electrospinning technology using a solution of Titanumisopropoxide (TIP) and Polyvinylpyrrolidone (PVP) precursors in ethanol. Then the PVP/TIP composite nanofibers were calcined at 500 °C to form mixed phase mesoporous TiO2 nanofibers. XRD analysis confirmed the major presence of the anatase phase accompanied by a rather significant fraction of the rutile phase. From FESEM and TEM analysis, it is observed that very fine tiny nanoparticles with diameter in the range of 10–20 nm are oriented along one dimensional direction to form the fibrous structure with diameter in the range of 300–500 nm. The prepared dual phase TiO2 nanofibers exhibit superb photocatalytic activity for the degradation of toxic dyes Rhodamine B and methyl orange under solar light irradiation. The excellent visible light photocatalytic activity may be attributed to the combine factors of heteroconjuction at anatase–rutile interface and immediate charge transfer due to 1D structure, which inhibit the electron–hole recombination.
Dr. Dhal is thankful to NIT Rourkela, and MHRD, Govt. of India for providing research facility and infrastructure to carry out this works in the form of an Institute fellowship (SRF).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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