Fabrication of Visible Light Sensitive Electrospun TiO 2 Nano ﬁ bers Using Squaric Acid for Photocatalytic Application

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Introduction
Most of the photocatalysts available today show a response mainly to UV light.Titanium dioxide (TiO 2 ) is a well-known photocatalyst with a large band gap of 3.2 eV [1].TiO 2 is widely used in organic pollutant degradation applications because of its low cost, availability, and environmental friendliness.However, due to its large band gap, TiO 2 can absorb radiations only between those small regions [2].Inducing visible light sensitivity in such UV-active photocatalysts will improve their absorption efficiency, as the solar spectrum comprises 43% visible light.Various attempts are made earlier to improve the visible light photoactivity of TiO 2 .
One such attempt was to dope nitrogen in the oxygen site of the TiO 2 to reduce the band gap [3].The experiments revealed that the visible light responsiveness may not be due to the reduction of the TiO 2 semiconductor bandgap, but might be due to the introduction of impurity energy levels.Eventually, it increased the recombination of the photoinduced charge carriers, reducing the overall photocatalytic efficiency.This led to the search for similar other dopants to promote less energetic excitations of electrons from mid-gap dopant levels to the conduction band of TiO 2 , without recombination losses.It includes doping TiO 2 with sulphur [4], red phosphorus [5], copper/iodine [6], carbon [7], and transition metals as well as loading TiO 2 with noble metals [8].Another new approach involves the formation of an interfacial charge transfer (ICT) complex by modifying the surface of TiO 2 using organic molecules, mainly derivatives of benzene.The hydroxyl groups present in the organic molecules react with the hydroxyl groups present on the surface of the TiO 2 reacts via condensation reaction forming a covalent Ti-O-C linkage.This surface modification will aid the transfer of charges into the conduction band of TiO 2 , by forming a mid-gap energy level.The alignment of energy levels of the organic-inorganic hybrid leads to a red shift, as the ground state of the ICT complex lies within the band gap of the TiO 2 [9].There are also attempts to use nonbenzene aromatic ligands such as rhodizonic acid to from the ICT complex, which showed significant improvement in the photocatalytic efficiency of the TiO 2 [10].
In this study, a nonbenzenoid aromatic ligand, squaric acid (SqA) was used to modify the optical properties of TiO 2 nanofibers (NFs).TiO 2 NFs were prepared using electrospinning and then the NFs were modified by treating with SqA [11].The SqA-modified TiO 2 NFs were characterized using various techniques and their photocatalytic efficiency was determined by analyzing the degradation of tetracycline.

Experimental Section
One milliliter of tetra butyl titanate (TBT, reagent grade, 97%, Sigma) was mixed with 5 mL of ethanol (analytical grade 99.9%, Sigma) and 2 mL of acetic acid (reagent grade 99%, Sigma) and stirred for 3 hr.Then, 0.78 g of polyvinylpyrrolidone (PVP, Mw 1300000, Aladdin) and 2 mL of N, N-dimethyl formamide (DMF, for molecular biology ≥ 99%, Sigma) were added to the solution and the mixture was stirred for 12 hr.The prepared solution was electrospun using a dispensing needle of Gauge 23 as a nozzle with an injection flow rate of 1 mL/hr and a voltage of 15.6 kV.The obtained fibers were annealed at 600°C for 3 hr to remove the organic residues and produce high-purity TiO 2 NFs.The surface of the TiO 2 NFs was modified by dispersing 0.1 g of TiO 2 NFs in 30 mL of water containing 20 mg of SqA for 24 hr.Treated TiO 2 NFs were washed three times using distilled water and separated by centrifugation.The collected TiO 2 NFs were dried in a vacuum oven.
The morphology and topography of the modified TiO 2 NFs were analyzed using high-resolution transmission electron microscopy (HR-TEM).The crystallinity and phase of the fibers were analyzed using an X-ray diffractometer (XRD).Significant X-ray photoelectron spectroscopy structural changes and the electron distribution of modified TiO 2 NFs were investigated using a Raman spectrometer and electron paramagnetic resonance (EPR) spectrometer.EPR spectra of the solid samples were recorded at room temperature using X-band microwave frequency of 9.5 GHz with modulation frequency of 100 kHz, modulation amplitude of 10 G, and constant microwave power of 1.2 mW.Optical properties were studied by diffused reflectance spectrum using UV-Visible spectrometer.X-ray photoelectron spectroscopy (XPS) was recorded using one color Al Kα radiation (225 W, 15 mA, 15 kV) with a spot size of 100 mm.The photocatalytic efficiencies of TiO 2 NFs and SqA-modified TiO 2 NFs were analyzed by comparing their ability to degrade tetracycline organic dye under 300 W Xenon lamp.The photocatalyst was dispersed in 100 mL aqueous solution of tetracycline at the concentration of 20 mg/L.Next, the dispersion was stirred magnetically in the dark until the adsorption-desorption equilibrium is achieved, by measuring the change in concentration of tetracycline at an interval of 20 min.Then, the dispersion was irradiated using the Xenon lamp and the photocatalytic activity was measured by measuring the degradation of tetracycline at an interval of 20 min, using UV-Visible spectrometer (by measuring the adsorption peak of tetracycline at 357 nm).

Results and Discussion
HR-TEM TiO 2 NFs and SqA surface-modified TiO 2 NFs are shown in Figures 1(a) and 1(b), respectively.Both images have similar shapes and morphology, which implies that the surface modification does not induce any morphological changes in the TiO 2 NFs.The image also reveals the presence of nanometer-sized grains of TiO 2 in both samples.The insert in the figures indicates the lattice spacing of the TiO 2 NFs and modified TiO 2 NFs, both have a lattice spacing corresponding to the (101) anatase phase of TiO 2 .The occurrence of impurities or any other crystalline phases were not found in both samples.
XRD pattern of the TiO 2 NFs and modified TiO 2 NFs is shown in Figure 2(a).Both the samples have similar diffraction peaks and the peaks at 25.0°, 37.6°, 47.7°, 53.7°, 54.7°, and 62.4°can be, respectively, indexed to (101), (004), ( 200), (105), (211), and (204) crystal planes.These patterns were indexed to the tetragonal anatase phase of TiO 2 (JCPDS: 21-1272) [12].These XRD results agree with the transmission electron microscopy (TEM) images that there were no impurities and any other crystalline phases.The Raman spectra of the TiO 2 NFs and modified TiO 2 NFs are compared in Figure 2(b).Both the samples showed resonance peaks at 197, 395, 514, and 634 cm −1 , which correspond to the Ti−O bands in the anatase phase of TiO 2 [13].However, the strongest peak at 142 cm −1 for the pristine TiO 2 NFs had shifted to 2 Journal of Nanomaterials 145 cm −1 after the surface modification using SqA.This peak shift and broadening shall be due to the presence of Ti 3+ species and the induction of more oxygen vacancies [14].In addition, a slight increase in the intensity of the strongest peak and peak broadening of the modified TiO 2 NFs when compared with the TiO 2 NFs shall be attributed to the increase in the number of oxygen vacancies in the lattice of modified TiO 2 NFs, due to the introduction of nonbenzenoid aromatic ligand by SqA [15].XPS spectrum of the samples corresponding to O 1s and Ti 2p is shown in Figure 2(c).The strong peak at the 528.4 eV of the TiO 2 NFs corresponds to the lattice oxygen of Ti-O, whereas the strong peak (529.9 eV) of modified TiO 2 NFs had a positive shift of ∼1.27 eV.Han et al. [16] reported a shift of O 1s peaks of the H 2 O 2-modified TiO 2 toward a lower energy side, which they had attributed to the presence of more number of O atoms than Ti atoms in the lattice.In this work, the peak had shifted to a higher energy side, which can be attributed to the lesser number of O atoms than Ti atoms in the lattice (presence of oxygen vacancies).A small new peak found at 531.2 eV of modified TiO 2 NFs can be due to the presence of surface hydroxyl groups (Ti−OH) [17,18].Ti 2p spectra of both TiO 2 NFs and modified TiO 2 NFs showed two similar peaks at 458.1 and 463.7 eV, respectively, which can be assigned to the Ti 4+ oxidation state.However, no additional peaks for Ti 3+ species were observed in the modified TiO 2 NFs, which is in contrast to the Raman spectroscopy.This suggests the formation of a shell-like structure on the surface of the modified TiO 2 NFs, which was dominated by the Ti 4+ species.Whereas, Ti 3+ species were present in the inner core of the modified TiO 2 NFs.The thickness of the shell-like structure should have been more than (5 nm)  Journal of Nanomaterials the detection limit of XPS [15].To confirm the formation of such core/shell structure, the samples were analyzed using EPR spectroscopy.
EPR spectra of the TiO 2 NFs and modified TiO 2 NFs are shown in Figure 2(d).TiO 2 NFs do not show any paramagnetic signal, whereas modified TiO 2 NFs showed strong g values at ∼1.94, which confirms the presence of paramagnetic Ti 3+ species [19,20].In general, the occurrence of Ti 3+ will be followed by the formation of oxygen vacancies.However, no signals were found at 2.005, which suggests that there were no Ti 3+ species at the surface [17,20].Hence, it is inferred in connection with the XPS and Raman spectra data that the Ti 3+ species and corresponding oxygen vacancies exist in the bulk rather than on the surface of modified TiO 2 NFs.Thus, in the modified TiO 2 NFs, oxygen vacancies exist in the core with TiO 2 as a shell.This also backs the formation of an ICT complex, which in turn will enhance the absorption of lower energy visible light spectrum in modified TiO 2 NFs.
The UV-Visible diffused reflectance spectrum of the samples is shown in Figure 3.The pristine TiO 2 NFs exhibit a sharp absorption at 400 nm [21] without any significant absorption in the visible and near infrared region.While the modified TiO 2 NFs, which were brown in color when compared with the white color of pristine TiO 2 NFs, displayed a broad absorption in the 400-700 nm region.A significant red shift of 300 nm (1.3 eV) was observed for the modified TiO 2 NFs, as the absorption onset for modified TiO 2 NFs was at 700 nm when compared with that of the pristine TiO 2 NFs, which was at 400 nm.This broadening of the optical absorption can be attributed to the induction of oxygen deficiencies in the subband excitation near the conduction band leading to the formation of the ICT region.Formation of the ICT complex significantly improved the optical absorption of the modified TiO 2 NFs.The hydroxyl groups on the SqA molecule interact with the surface of TiO 2 NFs through coordination bonding.This process led to the formation of a strongly bonded monolayer of SqA molecules on the surface of the TiO 2 NFs, altering the surface properties of the TiO 2 .This SqA acts as a sensitizer, injecting electrons into the conduction band of the TiO 2 semiconductor through the ICT complex [22,23].The actual mechanism involves the formation of hydroxyl radicals ( • OH) on the surface of SqAtreated TiO 2 NFs under light irradiation.Upon absorbing a photon, the ICT complex undergoes excitation.The energy from the absorbed photon promotes an electron from the highest occupied molecular orbital of the SqA to the lowest unoccupied molecular orbital of TiO 2 , leading to the formation of electron-hole pairs.This excitation process is facilitated by the strong electronic coupling between the SqA and TiO 2 .These electron-hole pairs then react with water and oxygen adsorbed on the TiO 2 surface to form hydroxyl radicals (•OH).The hydroxyl radicals are highly reactive and attack the tetracycline molecules, leading to their degradation into smaller, less harmful compounds.
The photocatalytic activity of the samples against tetracycline is shown in Figure 4. Spectroscopic measurements of the concentration of tetracycline in dark conditions revealed that the sorption-desorption equilibrium between the photocatalyst and the tetracycline was achieved between 40 and 60 min.Therefore, all the dispersions of photocatalyst and tetracycline were stirred in the dark conditions for 60 min, before exposing them to the Xenon illumination.Reduction in the concentration of the tetracycline was observed for both samples.However, modified TiO 2 NFs exhibited a drastic degradation efficiency against tetracycline when compared with pristine TiO 2 NFs.SqA creates carboxylic acid groups on the TiO 2 surface.This modification not only enhances the separation of electron-hole pairs but also reduces recombination and increases the efficiency of photocatalytic degradation.In addition, the carboxylic acid groups can also act as active sites for adsorption of tetracycline molecules, leading to their degradation by the generated reactive oxygen species.
The rate of degradation for blank, TiO 2 NFs, and modified TiO 2 NFs at dark and illuminated conditions is shown in Figure 4(c).No reduction in the concentration was observed in the blank solution without any photocatalyst, both in dark and illuminated conditions.In dark conditions, TiO 2 NFs and modified TiO 2 NFs removed 4% and 16% of the tetracycline, respectively, in 60 min by absorption.Whereas under illuminated conditions, the TiO 2 NFs and modified TiO 2 NFs exhibited degradation efficiencies of 60% and 85%, respectively, in 80 min.Modified TiO 2 NFs have also showed an increased rate of degradation of 0.21 mg/L/min, when compared with the 0.13 mg/L/min of unmodified TiO 2 NFs.This increase in the photocatalytic efficiency of modified TiO 2 NFs can be attributed to the absorption of more photons in the visible region by the modified TiO 2 , due to the presence of an ICT layer as shown in Figure 4(d).This ICT complex was a result of the condensation reaction between the hydroxyl groups in SqA and the surface hydroxyl groups in the TiO 2 NFs, forming the Ti-O-C linkage [10].

Journal of Nanomaterials
Tetracycline was used as a typical persistent organic pollutant and its mineralization was determined by measuring total organic carbon (TOC).Modified TiO 2 NFs (200 mg) were used to degrade tetracycline under illumination and the TOC was measured at a regular interval of 30 min and shown in Figure 4(e).A 47% decrease in the TOC was observed, which exhibits the efficiency of the modified TiO 2 NFs to mineralize the tetracycline into H 2 O and CO 2 .
Furthermore, recycling efficiency of the modified TiO 2 NFs was analyzed by separation, washing, and reusing the photocatalyst in multiple cycles.The results are shown in Figure 4(f ).Ninety-one percent efficiency was observed in the first cycle and it further stayed consistent at 84% over three cycles.

Conclusions
The modification of electrospun TiO 2 NFs using SqA has significantly improved the absorption efficiency of the NFs in the visible spectrum by the formation of the ICT complex.It should also be noted that the modification does not create any changes in the morphology or crystallinity of the TiO 2 NFs.The photocatalytic efficiency of the modified TiO 2 NFs against the tetracycline was significantly higher than that of the pristine TiO 2 NFs, which can be attributed to the formation of the ICT complex by condensation of hydroxyl groups.This proves and opens up the possibility of using organic ligands to enhance the visible light activity of the higher band gap photocatalysts.

FIGURE 1 :
FIGURE 1: TEM images of (a) TiO 2 NFs and (b) modified TiO 2 NFs.The inserts in images are HR-TEM images of the selected areas.

FIGURE 4 :
FIGURE 4: Photocatalytic degradation of tetracycline by (a) pristine TiO 2 NFs, (b) modified TiO 2 NFs, (c) rate of photocatalytic degradation in dark and illuminated conditions, (d) schematic illustration of the energy alignment in modified TiO 2 NFs, (e) rate of degradation of total organic carbon by modified TiO 2 NFs, and (f ) recycle efficiency of the modified TiO 2 NFs.