Dual Functional Ta-Doped Electrospun TiO2 Nanofibers with

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Dual Functional Ta-Doped Electrospun TiO2 Nanofibers with Enhanced Photocatalysis and SERS Detection for Organic Compounds Narendra Singh,† Jai Prakash,†,‡ Mrinmoy Misra,† Ashutosh Sharma,†,§ and Raju Kumar Gupta*,†,§ Department of Chemical Engineering and Center for Nanosciences and §Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India ‡ Department of Physics, University of the Free State, Bloemfontein 9300, South Africa ACS Appl. Mater. Interfaces 2017.9:28495-28507. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/29/19. For personal use only.

†

S Supporting Information *

ABSTRACT: There is a growing interest in multifunctional nanomaterials for the detection as well as degradation of organic contaminants in the water. In this work, we report on the development of dual functional TiO2 nanofibers (TNF) with different tantalum (Ta) doping (1−10 mol %) by a simple electrospinning technique. As-prepared TNF show mesoporous dominant structure, which are favorable for photocatalytic activity due to the presence of catalytic spots. Ta doping decreases the crystalline size within TiO2 matrix because of the incorporation of Ta5+ ions and restricts the phase transformation from anatase to rutile. Ta doping slightly enhances the visible light absorption because of the Ti3+ defects sites created upon Ta5+ doping. The effect of Ta doping within TiO2 matrix was systematically studied for the degradation of methylene blue (MB) dye under ultraviolet (UV) and solar light irradiation. The 5% Ta-doped TNF were found to be optimal and showed 5.1 and 2.2 times higher photocatalytic activity as compared to TNF under UV and solar light irradiation, respectively. The effect of Ta doping for the detection of MB molecules was also studied by surface enhanced Raman scattering (SERS). It was observed that 5% Ta-doped TNF exhibit higher photocatalytic activity and enhanced SERS signals of adsorbed MB molecules as compared to the TNF. The enhanced photocatalytic and SERS activities can be explained as combined effects of enhanced visible light absorption, lower crystalline size, and slightly higher surface area. The observed results show that Ta doping induces new energy levels below the conduction band of TiO2 because of Ti3+ defects, which inhibit the photogenerated charge recombination acting as electron traps and promote charge transfer mechanism acting as an intermediate state for TiO2 to MB molecule electron transfer, and are mainly responsible for the enhanced photocatalytic and SERS activities, respectively. KEYWORDS: electrospinning, nanofibers, doping, degradation, photocatalysis, SERS

1. INTRODUCTION Among metal oxide semiconductors, titanium dioxide (TiO2) has received greater attention because of its fascinating properties such as low chemical toxicity, resistance to chemical breakdown, higher stability, and low cost.1−3 It shows multifunctional applications in various fields such as photocatalysis, sensing, solar cell, and ultratrace detection of organic pollutants/biomolecules using surface enhanced Raman scattering (SERS).4−7 Because of the modern industrialization and frequent water pollution problems, there is a growing demand to produce costeffective techniques and multifunctional nanomaterials for the detection as well as degradation of organic contaminants in the wastewater. SERS is an ultrasensitive technique, which gives molecular level information on surface adsorbed molecules. It has the ability to detect even a single molecule and has shown great promises in recent years to detect organic pollutants such as dye molecules.5,8 Recent literature shows that TiO2 and TiO2 © 2017 American Chemical Society

based nanomaterials have gained special attention for such environmental issues because of their great potential to be applied as photocatalyst as well as SERS substrates.6,9 TiO2 is an active nanomaterial mainly in ultraviolet (UV) light due to its wide band gap (energy band gap for rutile -3.0 eV and for anatase −3.2 eV).10 To maximize the effective utilization of TiO2 as a photocatalyst and in other fields, its optical property needs to be extended in visible light range. Effective strategies for reducing the energy band gap of TiO2 remain to be of high interest by the scientific community for its variable applications. There are several approaches to improve the visible light absorption such as metal nanoparticles deposition over TiO2, doping with anions/transition metals, sensitization by quantum dots, and formation of hybrid Received: May 28, 2017 Accepted: August 4, 2017 Published: August 4, 2017 28495

DOI: 10.1021/acsami.7b07571 ACS Appl. Mater. Interfaces 2017, 9, 28495−28507

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ACS Applied Materials & Interfaces composites, etc.10,11 These treatments modify the energy band gap of TiO2 by introducing some defects or subenergy levels between conduction and valence bands, thus facilitating separation of photogenerated charge carriers. Furthermore, the optoelectronic properties are enhanced providing different aspect for multifunctional applications. Metal nanoparticles deposition over TiO2 reduces excitons recombination significantly by creating Schottky barrier acting as an electron trap and improves the photocatalytic activity of the TiO2.12,13 Moreover, metal nanoparticles deposited over TiO2 forming nanocomposites also extend its functionality for the ultratrace detection of organic molecules serving as an active SERS substrate.6,14 Quantum dots sensitization over metal oxides (TiO2, ZnO, etc.) enhances their optical absorption and charge separation at the interface (e.g., TiO2/CdS creates excitons upon solar illumination and photogenerated electrons from CdS transferred to TiO2 and simultaneously quantum dots show tunable optical property with size).15 Anion doping (e.g., N, S, C, etc.) decreases its band gap by some extent because of creation of new discrete energy level above the valence band.11,16−19 Similarly, doping of transition metals (e.g., Cr, W, Ni and Fe, etc.) into TiO2 creates new energy level between conduction and valence bands, which enhances visible light absorption.20−23 Moreover, several dopants such as Mn, Co, Zn etc. in TiO2 have shown enhanced photocatalytic properties due to the improved optical and charge separation ability of the doped TiO2 through the production of surface defects. Additionally, these doped TiO2 materials show the enhanced SERS functionality toward the organic molecules. 24−26 Similarly, Ni and Zn codoping in TiO2 has shown to create abundant surface states and metal doping levels band gap, which facilitates the charge transfer from TiO2 to the probe molecule and enhances the SERS sensitivity.27 Other dopants such as Nb, Ta, etc., have been extensively used in improving the properties of TiO2 for various applications ranging from photocatalysis to photovoltaic devices. However, Ta doping has advantage over Nb doping owing to its higher equilibrium solubility within TiO2 network because of lower energy requirement.28 The Ta ions are soluble in TiO2 matrix due to ionic radius of Ta5+ ions (0.064 nm) which is slightly larger than the Ti4+ ions (0.061 nm).29 There are several studies which report on enhanced photocatalytic properties of Ta-doped TiO2 nanomaterials using different techniques. For example: Wang et al. prepared three-dimensional macroporous TiO2/Ta2O5 mixed oxide structures which showed that photocatalytic activity was increased by 2.6 times than TiO2 under UV light.30 Znad et al. prepared Ta/TiO2 mixed oxide by impregnation method which exhibited decreased band gap and enhanced photocatalytic activity under solar light.31 Bawaked et al. prepared undoped and Tadoped TiO2 thin films by aerosol assisted chemical vapor deposition method and found that doped TiO2 thin films showed good transport properties and hence better photocatalytic activity.32 Similarly, there are few studies demonstrating the application of TiO2 based nanomaterials in SERS. For example: three-dimensional TiO2 nanorods decorated with Ag nanoparticles33 and Ag embedded TiO2 nanotubes34 exhibited enhanced SERS properties. Recently, Ta/N codoped TiO2 nanorods have been reported for their excellent photocatalytic activities.35 However, there are only a few studies showing multifunctional applications based on one-dimensional TiO2 nanostructures. For instance: recently, Kumar et al. have shown

multifunctional Ag-TiO2 nanorods with enhanced photocatalytic degradation and SERS detection of dye molecules.9 However, these composite materials involves multistep synthesis procedure including synthesis of noble metal nanoparticles, their characterizations and then assembly with TiO2, whereas TiO2 itself does not play any role in SERS activity.6,9 On the contrary, as discussed above, doped TiO2 nanostructures have shown their capability to be implemented in both the field of photocatalysis as well as SERS for the ultradetection of the molecules and have easy synthesis process. To enhance the performance of TiO2, one-dimensional structures such as nanotubes, nanowires, and nanofibers have been synthesized to provide unidirectional paths for photogenerated electrons and to minimize its recombination process.36,37 One-dimensional nanostructures can be prepared by a hydrothermal technique, electrochemistry, electrospinning, etc.38−41 Among these techniques, electrospinning is a simple and preferred route for the synthesis of one-dimensional nanofibers at room temperature with high aspect ratio, controlled porosity, and high chemical reactivity. Furthermore, the electrospinning technique provides more flexibility in synthesizing different composites and compounds.42 It is thus interesting to fabricate and study Ta-doped onedimensional TiO2 nanostructures and their multifunctional applications in various directions. This is the first study exploring the dual functional application of Ta-doped TiO2 nanofibers in the field of photocatalysis and SERS for the photodegradation and ultradetection of dye molecules, respectively. In this work, one-dimensional Ta-doped TiO2 nanofibers have been synthesized via electrospinning technique. We have explored the effects of Ta doping on energy band gap and visible light absorption for the Ta-doped TiO2 electrospun nanofibers. Furthermore, the photocatalytic activity of doped and undoped TiO2 nanofibers has been studied to degrade MB (model pollutant) under the irradiation of UV and solar light to evaluate the effect of doping. In addition, these nanofibers have also been explored to be used as SERS substrate for the detection of MB dye molecules. Finally, mechanisms have been proposed for the observed photocatalytic and SERS enhancements.

2. EXPERIMENTAL DETAILS 2.1. Materials Required. Titanium tetraisopropoxide (TTIP, 97%) and tantalum ethoxide were purchased from Sigma-Aldrich Pvt. Ltd. Polyvinylpyrrolidone (PVP, MW= 1,300,000 g mol−1) and MB were purchased from Alfa Aesar. Deionized water (DI) was used for preparation of MB solution. Ethanol (emsure 98%) and acetic acid (HPLC grade) were purchased from Merck Chemicals. All chemicals were used without further purification. 2.2. Synthesis of Undoped and Ta-Doped TNF. To prepare electrospinnable solution, we dissolved 0.45 g of PVP in 7.5 mL ethanol at 55 °C under continuous magnetic stirring for 20−30 min. In another reagent bottle, 1.5 g of TTIP was dissolved in 6 mL of mixed acetic acid and ethanol (volume ratio = 1:1) under continuous stirring at room temperature for 20−30 min. Ethanolic solution of PVP was poured into TTIP solution at room temperature and stirred continuously for another 10 h to get homogeneous solution for electrospinning. For Ta-doped TNF, Ta precursor (tantalum ethoxide) was mixed with TiO2 precursor in the proper molar ratio for different Ta doping. Ta-doped TNF with different molar ratios of 1, 2, 5, and 10% Ta are named as 1% Ta TNF, 2% Ta TNF, 5% Ta TNF, and 10% Ta TNF, respectively. The prepared electrospinnable solution was loaded into a plastic syringe of 10 mL volume. The voltage applied at the needle was 13 kV and rotary drum collector was grounded. The solution was pushed through a syringe pump at a rate 28496


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Figure 1. (a) XRD patterns and (b) magnified XRD patterns in the region between 24 and 27° of TNF and Ta-doped TNF. of 20 μL min−1. The Rotary drum collector was placed at 10 cm from the tip of needle. The drum was wrapped with aluminum foil to collect nanofibers mat. The nanofibers mat collected on aluminum foil was placed in the oven at 60 °C to peel off from aluminum foil. The collected freestanding mat was placed in a furnace at 500 °C for 2 h under air to remove polymer and solvent. The sample was then allowed to cool naturally inside the furnace to room temperature for further use. 2.3. Photocatalytic Experiments. Prior to the photocatalysis experiments, adsorption−desorption experiment was carried out for 2 h in dark with constant stirring. Twenty micromolar and 40 μM MB solutions were prepared for the photocatalytic process under UV and solar light, respectively. Photocatalytic study was carried out in a petridish by dispersing 10 mg of TNF photocatalyst (both separately doped and undoped) in 10 mL of 20 μM MB dye and then the solution was exposed to UV lamp (12 W, 365 nm). To study the visible light photocatalytic activity, we performed the photodegradation study under simulated light source (1 Sun, OAI TriSOL). For this experiment, 10 mg of each photocatalyst was dispersed in 50 mL (40 μM) of MB solution and exposed to solar light. Reusability test of 5% Ta TNF was carried out for 5 cycles under illumination of UV and solar light following the same experimental conditions as above. To monitor the time-dependent photocatalytic degradation process, we collected samples (MB) at regular time intervals and recorded the absorbance of MB dye using an Agilent Technologies Cary 7000 UV−vis−NIR absorption spectrophotometer. 2.4. SERS Experiments. For the SERS experiment, first of all, an aqueous solution of 20 μM of MB was prepared, then 5 mg of TNF and Ta-doped TNF were dispersed in 5 mL of 20 μM MB aqueous solution separately. The solution was stirred for 2 h. An aliquot of the solution was then dropped on a glass slide and kept overnight for drying at room temperature. These glass slides were used as a sample for SERS measurements. The Raman spectra were taken at many places and average of Raman spectra was used for the analysis. 2.5. Characterization Techniques. The surface morphology of TNF and Ta-doped TNF were investigated through field emission scanning electron microscope (FESEM, Quanta 200, Zeiss) and transmission electron microscope (TEM, Tecnai 20G2). Element composition of nanofibers was determined by energy dispersive X-ray spectroscopy (EDX linked to FESEM, Oxford Instrument). Crystal structure of prepared TNF and Ta-doped TNF was investigated by Xray Diffraction (XRD, X’Pert Pro, PANanalytical) using Cu Kα as Xray source (λ = 1.5406 Å). Raman spectra of all samples were collected by Raman spectrometer (WiTec, using laser light of λ = 532 nm). The surface area and pore size of synthesized sample were investigated using Brunauer−Emmett−Teller instrument (BET, Quantachrome Instruments). Thermal behavior of as-collected electrospun TiO2 mats was determined through thermo gravimetric analysis (TGA) using TA Instruments. Electronic structure of TNF and Ta-doped TNF were investigated by X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe II, FEI Inc.) and XPS spectra was analyzed using the XPSPEAK 4.1 software. UV−vis absorption spectra of all the samples

were taken using UV−vis−NIR spectrophotometer (Agilent technologies, Cary 7000). Fourier transform infrared spectra (FT-IR, PerkinElmer) were recorded using KBr pellet method. Electron paramagnetic resonance (EPR) spectra were collected using BrukerEMX spectrophotometer. Surface potential of undoped and 5% Ta TNF were investigated using Kelvin probe force microscopy (KPFM, Asylum MFP-3D AFM).

3. RESULTS AND DISCUSSION 3.1. Characterization of TNF and Ta-Doped TNF. Figure 1a shows XRD patterns of TNF and Ta-doped TNF in the range of 20−80°. XRD pattern of TNF is well matched with anatase phase of TiO2 and consistent with standard card (JCPDS, No. 01−084−1285). It also shows small content of rutile phase (JCPDS, No. 01−077−0440). The main characteristic peak of anatase TiO2 is observed at 25.30° that corresponds to (101) plane. The XRD patterns do not contain any peak corresponding to carbon or polymer which confirms that the polymer and solvent were removed completely during heat treatment. XRD patterns of 1, 2, and 5% Ta-doped TNF are matched with anatase phase of TiO2 and no other peaks corresponding to Ta or Ta2O5 are observed. It indicates the good distribution of doped Ta ions within the TiO2 matrix. However, XRD pattern of 10% Ta TNF shows small peak at 22.94° that may be due to the segregation of Ta2O5 (JCPDS No. 01−073−0005). Ta doping restricts the phase transformation from anatase to rutile in Ta-doped TNF even at higher temperature as reported previously.43 Interestingly, it can be observed that a peak corresponding to anatase (101) plane is slightly shifting to lower 2θ with an increase in Ta doping concentration (Figure 1b). The peak corresponding to plane (101) also gets broadened with an increase in doping, which is a signature of decreasing crystalline size. This peak is shifted from 25.30 to 25.21° for TNF to 10% Ta TNF which is attributed to the incorporation of Ta5+ at the expense of Ti4+ into anatase phase of TiO2 matrix. The crystalline size of anatase (101) plane for TNF and Tadoped TNF samples was calculated using Debye−Scherrer formula, as given below44 D=

Kλ β cos θ

(1)

where K is 0.89 taken as a shape factor, λ is the wavelength of X-ray used, β is full width at half-maximum for (101) plane of anatase phase of TiO2, and θ is the diffraction angle. The calculated crystalline sizes of (101) plane for TNF, 1% Ta TNF, 2% Ta TNF, 5% Ta TNF, and 10% Ta TNF are found to be 28497


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Figure 2. (a) Raman spectra and (b) magnified Raman pattern in the region of 300−450 cm−1 of TNF and Ta-doped TNF.

Figure 3. (a) UV−vis diffuse reflectance spectra (DRS) of TNF and Ta-doped TNF; Plot of transformed Kubelka−Munk function vs energy for (b) TNF and (c) 5% Ta TNF.

Figure 4. FESEM micrographs of (a) TNF, (b) 1% Ta TNF, (c) 2% Ta TNF, (d) 5% Ta TNF, and (e) 10% Ta TNF; TEM micrographs of (f) TNF and (g) 5% Ta TNF; (h) digital image of TNF and 5% Ta TNF.

to Eg(1), B1g(1), (A1g+B1g(2)) and Eg(2) modes of anatase TiO2, respectively.45 A very small peak at 196 cm−1 is also observed corresponds to Eg(2) mode. Raman spectra of all Ta-doped TNF are also harmonized with Raman active modes of anatase TiO2. Furthermore, Raman spectra do not show any other detectable peak corresponding to any other form of Ti, TiO2, or Ta compounds. Thus, Raman spectra of Ta-doped TNF confirm the presence of anatase phase, which is in good agreement with XRD results. It is also found that with an increase in Ta doping concentration, Raman spectra of doped TNF show a peak shift of B1g(1) (397 cm−1) mode to a lower

14.66, 10.98, 10.63, 10.51, and 10.19 nm, respectively, using eq 1. To acquire more information about TNF and Ta-doped TNF, Raman spectra of TNF and Ta-doped TNF (different doping %) were recorded at room temperature upon excitation with 532 nm laser beam. It is already specified that anatase TiO2 contains six Raman active characteristics peaks [Eg(1), Eg(2), B1g (1), (A1g + B1g(2)) and Eg(2)] corresponding to different modes as shown in the Figure 2a. Figure 2a shows Raman spectra of TNF and Ta-doped TNF. Raman spectrum of TNF shows major peaks at 144, 397, 517, and 640 cm−1 correspond 28498


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Figure 5. XPS profiles of (a) Ti2p spectra of TNF; (b) Ti2p spectra and (c) Ta4f spectra of 5% Ta TNF.

TEM micrographs of undoped and 5% Ta-doped solid nanofibers. TEM analysis also shows that all TNF are solid and porous in nature, which is in good agreement with FESEM results. TEM analysis reveals that Ta doping does not affect the morphology of the TNF. Also, digital images of TNF and 5% Ta TNF does not show any significant difference in color (Figure 4h). As discussed in absorbance spectra analysis, the absorbance of Ta-doped TNF is increased and band gap is decreased with an increase in Ta doping concentration up to 5% Ta TNF. Therefore, the XPS of TNF and 5% Ta TNF was performed to study the electronic structure and chemical changes due to Ta doping as shown in Figure 5. The high resolution XPS spectrum of Ti for TNF sample shows a 2p3/2 characteristic peak corresponding to Ti4+ oxidation state at a binding energy of 458.6 eV (Figure 5a).49,50 Moreover, the other peak at 464.3 eV (2p1/2) corresponds to Ti4+ oxidation state with separation of 5.7 eV. High-resolution XPS spectra of Ti and Ta for 5% Ta TNF are shown in Figure 5b, c. It can be observed that the Ti has a doublet characteristic peak at 458.6 and 464.3 eV corresponding to 2p3/2 and 2p1/2 of Ti4+ state as shown in the Figure 5b. A detectable peak can also be observed at lower binding energy than the peak appeared at 458.6 eV. This additional peak at 457.1 eV appears after the Ta doping, which refers to the Ti3+ oxidation state due to incorporation of Ta5+ ions within the crystal structure. High-resolution XPS spectrum of Ta shows two 4f7/2 and 4f5/2 peaks corresponding to the Ta5+ oxidation state, appearing at 26.0 and 27.9 eV, respectively (as shown in Figure 5c) with a separation of 1.9 eV.51 The XPS study reveals that Ta doping creates Ti3+ oxidation states within the TiO2 matrix in doped TNF, which serve as defect sites and are responsible for the observed changes in optical properties. Furthermore, generation of Ti3+ states is also confirmed by EPR analysis as shown in Figure S4. It shows a room temperature EPR signal that lacks the signature of diamagnetic TiO2. A characteristic signal for paramagnetic Ti3+ was observed at average g = 1.97, which clearly suggests its paramagnetic nature and that is from the Ti3+. We have explored the Fermi level shift of undoped and 5% Ta TNF. We conducted Kelvin probe force microscopy (KPFM) of the undoped and 5% Ta TNF. KPFM is a technique which measures the distributed contact potential difference (CPD) between the tip and sample and provides surface potential using eq 3

wavenumber (Figure 2b). It confirms that Ti ions are substituted by Ta ions because the B1g(1) peak at 397 cm−1 is a signature of the Ti−O stretching mode.46 To investigate the optical absorption behavior of TNF and Ta-doped TNF, we carried out the UV−vis absorption study in the range of 200−800 nm as shown in Figure 3a. Ta doping slightly alter the light absorption properties as observed in Figure 3a. The absorption curves indicate that with an increase in Ta doping concentration, absorption increases up to 5% Ta TNF and then decreases. Figure 3b, c and Figure S1 show transformed Kubelka−Munk function vs energy of light, indicating the energy band gap alteration with Ta doping. The energy band gap of TNF and Ta-doped TNF was estimated using the following eq 247 (F(R )hν) ∝ (hν − Eg )2

(2)

where Eg is the band gap of the material, ν is the frequency of light, F(R) is the Kubelka−Munk function, and h is the planks constant. The band gap of TNF, 1% Ta TNF, 2% Ta TNF, 5% Ta TNF, and 10% Ta TNF are found to be 3.11, 3.01, 2.99, 2.96, and 3.00 eV, respectively, as shown in Figure 3b, c and Figure S1. An increase in Ta doping concentration results in a decrease in the energy band gap slightly up to 5% Ta TNF along with visible light absorption. The possible reason for the observed decrease in the energy band gap may be due to the presence of Ti3+ states within TiO2 matrix upon Ta5+ doping, as evidenced by XPS (discussed later), which can provide d−d transitions.32,48 The absorbance study reveals that 5% Ta doping is the optimum doping in TNF. Figure 4a−e shows typical FESEM micrographs of TNF and Ta-doped TNF. It has been observed that all doped and undoped samples show nanofiber morphology, which are continuous in nature. The diameter distribution and lowmagnification images of all the samples are shown in Figure S2. The diameters of all the samples are distributed in the range of 20−100 nm, whereas average diameters of different doped and undoped matrix are in the range of 40−60 nm. All nanofibers are of several micrometers in length as observed by magnified FESEM images shown in Figure 4a−e and Figure S2. To investigate the doping concentration of the Ta in the nanofibers matrix, we performed an EDX study, the results of which are presented in Figure S3. As the Ta doping concentration is increased in primary electrospun solution, the atomic concentration of the Ta within the TiO2 matrix also increased, as confirmed by EDX (Figure S3). The surface of nanofibers both doped and undoped TNF is rough in nature, which suggests the porous nature of nanofibers. To further investigate the morphology in detail, we studied TNF and 5% Ta TNF samples using TEM. Figure 4f, g shows

VCPD =

φtip − φsample e

(3)

Here, e is the electronic charge and ϕtip and ϕsample are the work functions of the tip and the sample, respectively; the 5% Ta 28499


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Figure 6. (a) N2 adsorption−desorption isotherm and (b) pore size distribution of TNF; (c) N2 adsorption−desorption isotherm and (d) pore size distribution of 5% Ta TNF.

TNF sample showed a CPD value of around −40 mV in comparison to undoped TNF, which showed a CPD value of ∼90 mV. The ϕtip has a value of 5.0 eV, and thus the Fermi level of the sample can be calculated using eq 3. The calculated value for undoped TNF sample is 4.9 eV, whereas for the 5% Ta TNF sample, it is ∼5.1 eV. Thus, this study reveals that Ta doping shifts the CB and Fermi level downward, which is in agreement with the UV−vis results. The N2 adsorption−desorption isotherms and pore size distributions of TNF and 5% Ta TNF have been shown in Figure 6. From N2 adsorption−desorption isotherms (Figures 6a, c) of TNF and 5% Ta TNF, it can be observed that N2 adsorption−desorption isotherms of both samples are a typical type IV isotherm (according to IUPAC classification) with a hysteresis loop (capillary condensation).52 Figure 6b, d show pore size distributions of TNF and 5% Ta TNF as calculated by Barrett−Joyner−Halenda (BJH) method. The BET surface area of TNF and 5% Ta TNF has been estimated as 36.2 and 46.1 m2 g−1, respectively. The enhanced surface area of 5% Ta TNF is due to the lower crystalline size as estimated by XRD results. The total pore volume of TNF and 5% Ta-doped TNF are 0.110 and 0.163 cm3 g−1, respectively. TNF contains both mesopores and macropores with mesopores in dominance (90% of the total pore volume). On the other hand, 5% Ta TNF contains all types of pores (micropores, mesopores, and macropores) with mesopores in dominance (about 80% of the total pore volume). FT-IR spectra of TNF and Ta-doped TNF (2 and 10%) are shown in Figure S5. FTIR spectra of TNF, 2% Ta TNF, and 10% Ta TNF show two absorption bands at 3420 and 1635 cm−1 due to O−H stretching and O−H bending modes of

adsorbed water, respectively.53 All samples show absorption band at 2346 cm−1 due to the presence of atmospheric CO2 during measurement.53,54 TNF exhibits broad absorption bands in the range of 400−800 cm−1 due to the presence of vibrational modes of anatase TiO2. Two absorption bands at 670 and 470 cm−1 are assigned to the Ti−O and Ti−O−Ti vibrational modes of TiO2.53,55 Ta2O5 shows two absorption bands at 540 and 670 cm−1, and these vibrational modes are due to the presence of Ta−O and Ta−O−Ta.56 For 2% Ta TNF and 10% Ta TNF, the absorption band corresponding to 670 cm−1 becomes stronger as compared to absorption band at 470 cm−1 because of the overlapping of Ti−O and Ta−O vibrational modes. Figure S6 shows the TGA graph of TNF and 5% Ta TNF. The complete removal of solvent and PVP can be observed during calcination of spun nanofibers. TNF and 5% Ta TNF follow the same trend due to the removal of solvent and polymer. It is found that the weight loss occurred during the temperature increment from room temperature to 200 °C because of desorption of water and removal of solvent. A further rise in temperature degrads the polymer to volatile compound and it removes completely around 480 °C. An additional rise in temperature does not cause any weight loss, which confirms the formation of TNF and Ta-doped TNF. The estimated weight loss during the 100−500 °C for TNF and 5% Ta TNF (same volume of solvent and the same quantity of PVP) is around 47.2 and 45.8%, respectively. The weight loss is less for doped TNF as compared to undoped one due to the incorporation of Ta ions in the TiO2 crystal lattice. The TGA analysis supports XRD as well as EDX results, which show the absence of carbon in calcined nanofibers. 28500


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Figure 7. Photocatalytic activity of TNF and Ta-doped TNF photocatalyst for degradation of MB (a) under UV light (photocatalyst loading = 1 mg/ mL of 20 μM MB), (b) under solar light (photocatalyst loading = 0.2 mg/mL of 40 μM MB); (c, d) Photocatalytic reaction kinetics of MB degradation under UV and solar light, respectively; inset of (b) shows digital image of MB solution before and after degradation in the presence of 5% Ta TNF.

3.2. Photocatalytic Activity of TNF and Ta-Doped TNF. Photocatalytic tests were performed under UV and simulated solar light irradiation, separately. The controlled experiments were performed in dark conditions in the presence of photocatalysts as shown in Figure S7. Analysis shows that MB molecules were adsorbed onto the surface of TNF and doped TNF photocatalysts due to the presence of surface area availability as well as porosity existing on the photocatalyst material. MB adsorption over TNF, 1% Ta TNF, 2% Ta TNF, 5% Ta TNF, and 10% Ta TNF was 18.6, 16.3, 18.9, 21.8, and 28.9%, respectively, after 2 h under continuous stirring (Figure S7). The MB molecules show its characteristic absorption peak at 664 nm, which diminishes gradually upon illumination in the presence of photocatalyst material (Figure S8). The blue color of the dye also disappears with time and becomes colorless at the end, as shown in the Figure 7b inset. During photocatalysis process, MB molecules are initially adsorbed on the photocatalyst surface, further degraded into intermediates via opening of central aromatic ring, followed by degradation of intermediates into CO2, SO42−, NH4+, and NO3−.57 When MB solution is exposed to UV light irradiation in the presence of photocatalysts, it is observed that the doping improves the photocatalytic activity in comparison to undoped one. The concentration ratio (C/Co) vs time graph for all the photocatalysts was obtained using a time-dependent study of MB degradation under UV light as shown in Figure 7a. According to the Langmuir−Hinshelwood model with approximation, the kinetic rate constant with different photocatalyst was determined by following eq 458 ⎛C ⎞ ln⎜ o ⎟ = K app t ⎝C⎠

were calculated using ln(Co/C) vs time graph, as shown in Figure 7c and tabulated in Table 1. Table 1. Photocatalytic Rate Constant of Different Photocatalyst under UV Light photocatalyst (1 mg of photocatalyst/mL of 20 μM MB)

rate constant (min‑1) (under UV light)

TNF 1% Ta TNF 2% Ta TNF 5% Ta TNF 10% Ta TNF

0.00991 0.01308 0.02993 0.05095 0.02105

Interestingly, the photocatalytic activity of 5% Ta TNF sample is found to be increased by 5.1 times than the TNF. 5% Ta TNF show the best photocatalytic activity as compared to undoped and other doped TNF as summarized in Table 1. The other Ta-doped TNF samples (less than 5% Ta doping) also show better photocatalytic activity as compared to the undoped sample. The photocatalytic activity is enhanced by 1.3 and 2.9 times for 1% Ta TNF and 2% Ta TNF, respectively, as compared to TNF. When the Ta doping concentration is increased to 10%, the photocatalytic activity of the 10% Ta TNF sample is decreased slightly in comparison to the 5% Ta TNF sample. However, it shows 2.1 times higher photocatalytic activity than the undoped TNF. The decrement of photocatalytic activity at higher Ta doping concentration (10% Ta TNF) may be due to the segregation of Ta2O5 as observed in XRD pattern (Figure 1a). To explore the effectiveness of TNF and Ta-doped TNF on dye degradation, we also performed the photocatalytic degradation studies at low photocatalyst material loading (10 mg of photocatalyst/50 mL of 40 μM MB) under simulated solar light irradiation. The concentration ratio (C/Co) vs time was obtained using time-dependent MB degradation results

(4)

where Co is the initial concentration of MB, C is the concentration of MB with time, Kapp is the apparent kinetic rate constant and t is exposure time. The kinetic rate constants 28501


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be due to the lost catalyst during recovery by centrifugation and reduced catalytic surface area.59,60 Figures 8c-d show FESEM images of 5% Ta TNF after 5 cycles of the reusability test under UV and solar light. It is observed that there is no significant change in morphology of the nanofibers even after 5 repetitive usages. Figure 8e shows the XRD of 5% Ta TNF after reusability test which does not show any significant change in its crystal structure even after 5 cycles of reuse. FT-IR and Raman spectra of the 5% Ta TNF sample after 5 cycles of reuse have also been shown in Figure S9. FT-IR spectrum shows a few characteristics peaks (1316, 1395, 1496 cm−1) of MB (Figure S9a).61 Also, Raman spectra show the characteristics peaks of adsorbed MB molecules (Figure S9b) (discussed in section 3.3), which reduces the available catalytic sites for other incoming molecules. It can be concluded that MB molecules adsorb on the surface of photocatalyst and reduce the catalytic surface area for the other molecules, which decreases the photocatalytic activity with an increase in the number of cycles. 3.3. SERS Detection of MB Molecules. SERS detection of MB molecules in 20 μM aqueous solution was performed using TNF and 5% Ta TNF as 5% Ta doping is the optimum doping in TNF (as revealed by UV−vis absorption study) and also shows the best photocatalytic activity as discussed above. All Raman and SERS spectra were recorded using 532 nm laser, which rules out any contribution of resonant Raman Effect of MB because the absorbance peak of MB falls above 550 nm with characteristic absorbance peak at 664 nm as shown in Figure S8. Figure 9a shows the normal Raman spectra of the solid MB in powder form and MB aqueous solution along with the chemical structure of the MB. It is clear that the Raman signals of MB in aqueous solution are not so prominent because of the low concentration of MB molecules. The Raman spectrum of solid MB shows several Raman signals with

(Figure 7b). The kinetic rate constants for different photocatalysts were calculated using ln(Co/C) vs time graph (Figure 7d). The kinetic rate constants of different photocatalysts follow the order: 5% Ta TNF > 2% Ta TNF > 10% Ta TNF > 1% Ta TNF > TNF, as shown in Table 2. The photocatalytic degradation of MB is found to be enhanced by 2.2 times for 5% Ta TNF sample in comparison to the undoped one. Table 2. Photocatalytic Activity of Different Photocatalyst under Solar Light photocatalyst (1 mg of photocatalyst/5 mL of 40 μM MB)

rate constant (min−1) (under solar light)

TNF 1% Ta TNF 2% Ta TNF 5% Ta TNF 10% Ta TNF

0.00502 0.00569 0.00983 0.01121 0.00761

Figure 8 shows the reusability test of the 5% Ta TNF toward the degradation of MB under UV and solar light over 5 cycles. All experiments were performed under the same conditions as mentioned earlier. After each cycle, the photocatalyst was removed by centrifugation and again dispersed in the same volume of dye for the next cycle. In Figure 8a, It can be observed that there is not much loss in the photocatalytic activity in the first 2 cycles under UV light (reaction time = 1 h). However, a considerable loss is observed in subsequent third cycle, while fourth and fifth cycle do not show a significant loss in photocatalytic activity. While a considerable loss in photocatalytic efficiency can be observed in second, third, fourth and fifth cycle under solar light (reaction time = 4 h) (Figure 8b). This observed loss in photocatalytic activity may

Figure 8. Reusability test of 5% Ta TNF for the degradation of MB (a) under UV light (reaction time = 1 h), (b) under solar light (reaction time = 4 h); FESEM image of 5% Ta TNF after 5 cycles of reuse, (c) under UV light, (d) under solar light; (e) XRD pattern of 5% Ta TNF before and after 5 cycles of reuse. 28502


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Table 3. Normal and SERS Raman Shifts and Their Assignments for MB Molecules62−65 normal (cm−1)

SERS (cm−1)

Raman peak assignments

1035 1070 1153 1301 1396

1040 1084 1157 1295 1390 1436 1467 1528 1634

in plane C−H bending symmetric C−N stretching in plane C−H bending C−C ring stretching symmetric C−N stretching asymmetric C−N stretching C−C ring stretching asymmetric C−C stretching C−C (and C−N−C) ring stretching

1469 1622

TNF. SERS enhancement is generally described as either chemical enhancement or electromagnetic enhancement,6,8,66 which are responsible for the shift in Raman peak position and change in intensity as observed in the present case of MB adsorbed on TNF and 5% Ta TNF. The electromagnetic SERS enhancement takes place as a result of coupling of surface plasmon resonance (SPR) of noble metals with excitation wavelength. Therefore, the possibility of electromagnetic enhancement is ruled out in case of TiO2, however, in such metal oxide semiconductors, the surface plasmon resonant frequency falls in IR region, whereas the excitation source used is 532 nm.67 The chemical SERS enhancement mainly takes place because of the charge transfer (CT) between the adsorbed molecule and semiconductor surfaces.67 In the present case, a shift in Raman peak positions, intensity improvement, and splitting of peak (1469 cm−1) therefore result because of chemisorption of the MB molecules on semiconductor-based TNF and 5% Ta TNF. These observations are also consistent with other results for MB adsorbed on TiO2 surfaces and are explained on the basis of chemical SERS enhancement due to the CT mechanism from TiO2 to MB molecules;62,63 the detailed mechanism is given in the next section. 3.4. Mechanism of Enhanced Photocatalytic Performance and SERS Detection. The photocatalytic property as well as the SERS activity of TiO2-based nanomaterials are determined by several material properties such as energy band gap, specific surface area, crystallinity, and their surface structure. These properties can be modified and improved for the particular applications by many ways. In the present work, we have reported the improvement in such properties of TNF by Ta doping and observed their enhanced photocatalytic degradation and SERS performance toward MB molecules. As observed in XRD analysis, the crystalline size of (101) plane is decreased with the Ta doping, thus helping in photocatalytic activity. The BET surface area of the 5% Ta-doped TNF sample is slightly higher than that of the undoped one, thus offering more MB molecules to adsorb on the surface and hence enhancing photocatalytic and SERS activity. Therefore, it can be predicted that the combined effect of these enhanced properties of the materials (Ta-doped TNF) is responsible for the enhancement of photocatalytic and SERS activity. As the Ta5+ ions have a similar ionic radius to Ti4+ ions, they can easily replace a few Ti4+, which causes increased electron concentration within the TNF matrix along with the replacement of a few Ti4+ with Ti3+ sites. The created Ti3+ defects introduce new energy levels below the conduction band, which enhance the visible light absorption.68 These electrons can also moderately fill the surface traps, thereby shifting the Fermi level to more

Figure 9. (a) Normal Raman spectra of solid MB powder and MB aqueous solution, and (b) SERS spectra of MB molecules adsorbed on TNF and 5% Ta TNF.

characteristic peaks at 1153, 1396, and 1622 cm−1 assigned to symmetric in plane C−H bending, symmetric C−N stretching (and in plane C−H ring deformation) and C−C (and C−N− C) ring stretching mode, respectively.62−65 SERS activity of TNF and 5% Ta TNF was studied by Raman spectra of MB (aqueous solution) adsorbed on these TNF. Raman spectra below 1000 cm−1 have not been shown to avoid any overlapping with TiO2 Raman modes. Figure 9b shows the SERS spectra of MB molecules adsorbed on the TNF and 5% Ta TNF, where each peak corresponds to a characteristic SERS signals of the adsorbed MB molecules. The SERS and adsorption of MB molecules can be understood by shift in the Raman peak position and enhancement in the Raman intensity. Moreover, it can be observed from the SERS spectra (Figure 9b) that there is a shift in Raman peak positions as well as change in their intensities as compared to the normal Raman spectra of solid MB (Figure 9a). As can be compared in Figure 9, that the main characteristic Raman peaks at 1396 and 1622 cm−1 in normal Raman spectra are shifted to 1390 and 1634 cm−1 in SERS spectra. This indicates SERS enhancement of MB molecules adsorbed on TNF and 5% Ta TNF. The major Raman shifts in normal Raman and SERS spectra with proper assignments of the different Raman modes of MB molecule have been given in Table 3. Interestingly, very strong and high intensity characteristic Raman peaks (1040, 1153, 1390, and 1634 cm−1) of adsorbed MB molecules on 5% Ta TNF can observed in addition to some new Raman peaks (1335, 1436, and 1528 cm−1). It demonstrates the enhanced SERS activity of 5% Ta TNF as compared to TNF. These results indicate that Ta doping plays an important role in the SERS enhancement of MB adsorbed of 28503


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ACS Applied Materials & Interfaces negative potential. The enhanced visible light absorption and photocatalytic activity is due to Ti3+ sites present in TNF matrix. Increasing doping concentration of Ta increases the Ti3+ defects and leads to the stronger optical absorption and thus higher photocatalytic activity. However, higher doping can create higher defects, which can act as charge carrier recombination sites.31 Moreover, it can be explained as Ta doping induces new energy levels below the conduction band of TiO2 because of Ti3+ defects, which inhibit the photoexcited charge recombination acting as electron traps and promote the CT mechanism acting as an intermediate state for TiO2 to MB molecule electron transfer, and are mainly responsible for the enhanced photocatalytic and SERS activities, respectively, as shown schematically in Figure 10. Because TiO2 has a wide bandgap

nanoparticles because of the synergic contribution of both CT and surface plasmon resonance of Ag nanoparticles. Similarly, the same group reported that energy levels could be created between the CB and VB of the TiO2 nanoparticles by metal ion doping, which facilitated the CT from TiO2 to molecules, resulting in the enhanced SERS signals of the adsorbed molecules.70 In the present case, it can be explained that the SERS spectra of a MB molecule adsorbed on undoped TNF and Ta-doped TNF have been obtained without the resonance condition and by means of chemical SERS enhancement through CT mechanism. In the case of Ta-doped TNF, the energy levels are introduced below the conduction band of TiO2 because of the Ti3+ defects created by Ta doping. Under the laser excitation, the excited electrons can be transferred to these Ti3+ defect energy levels and then transit to the lowest unoccupied molecular orbital (LUMO) level of the chemisorbed MB molecules on the doped TNF surface, thus facilitating the CT process. This CT mechanism provides an additional Ta-doped TNF to molecule CT besides the undoped TNF-to-molecule CT, and is responsible for the enhanced SERS enhancement. The present work is motivation for synthesizing one-dimensional multifunctional novel nanomaterials.

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CONCLUSIONS TNF and Ta-doped TNF were successfully prepared by electrospinning technique. The prepared nanofibers were continuous, solid and uniform throughout the matrix. Undoped and Ta-doped TNF samples were mesoporous in nature, indicating a positive effect for photocatalysis application. Ta doping in TNF slightly decreased its crystalline size because of the incorporation of Ta5+ within the TiO2 matrix, which restricted crystal growth and hence enhanced photocatalytic activity. Ta doping showed enhancement of visible light absorption due to the presence of Ti3+ states created by Ta5+ ions doping; 5% Ta-doped samples showed higher photocatalytic activity among all the photocatalysts because of the combined effect of low crystalline size, higher surface area, and enhanced visible light absorption. The 5% Ta TNF showed 5.1 and 2.2 times higher photocatalytic activity in comparison to TNF under UV and solar light irradiation, respectively. The 5% Ta TNF also showed the enhanced SERS functionality as compared to the TNF in detection of MB molecules. The observed results are explained on the basis of the Ta doping induced new energy levels below the conduction band of TiO2 due to Ti3+ defects. These Ti3+ defects energy levels play an important role in improving the photoexcited charge separation acting as electron traps and promote CT mechanism acting as an intermediate state for TiO2 to MB electron transfer, and are mainly responsible for the enhanced photocatalytic and SERS activities, respectively. The material synthesized here is also likely to be useful in other applications such as water splitting, solar cell, sensors, etc.

Figure 10. Schematic of mechanism of enhanced photocatalytic degradation and SERS performance of Ta-doped TNF.

and excitation with the 532 nm laser would not excite the valence band electrons to conduction band. As in the case of noble metal nanostructures, such excitation is important for resonance condition and for electromagnetic SERS enhancement, which is governed by surface plasmon resonance on noble metal nanostructures. However, in the case of semiconductors (i.e., TiO2), such a resonance condition is not essential for the SERS enhancement, on the other hand, chemical enhancement as a result of charge interaction between the TiO2 surface and the adsorbed molecules is responsible for the SERS activity. For example: Leon et al.69 reported that photoinduced CT processes mainly occurred between the adsorbed molecules and the semiconductor substrates and SERS enhancement can be taken place without the resonance conditions. They performed the SERS experiments at different excitation wavelengths (532 and 632.8 nm) and observed that the SERS signals of N719 dye molecules adsorbed on TiO2 nanoparticles could be obtained without the resonance condition at the excitation wavelength of 632.8 nm. In another experiment by Yang et al.14 SERS effect of TiO2 and a series of silver-deposited TiO2 nanoparticles were studied using Raman spectroscopy with an excitation wavelength of 514.5 nm. It was observed that pure TiO2 nanoparticles showed SERS signals of 4-MBA molecules adsorbed on its surface, which was attributed to the intrinsic TiO2-to-molecule CT mechanism. Furthermore, these SERS signals could be enhanced on Ag-TiO2 nanocomposite nanoparticles as compared to those on pure TiO2

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07571. Kubelka−Munk function vs energy graph for 1, 2, and 10% Ta TNF, low-magnification FESEM micrographs and diameter distribution of doped and undoped TNF, EDX of Ta-doped and undoped TNF, FT-IR, adsorption 28504


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study with different photocatalysts, FT-IR spectrum and Raman spectra of 5% Ta TNF before and after 5 cycles of reuse (under UV and solar light irradiation) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-5122596972. Fax: +915122590104. ORCID

Mrinmoy Misra: 0000-0002-6284-2532 Raju Kumar Gupta: 0000-0002-5537-8057 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.K.G. acknowledges financial assistance from Department of Science and Technology (DST), India, through the INSPIRE Faculty Award (Project IFA-13 ENG-57) and Grant DST/TM/ WTI/2K16/23(G). J.P. acknowledges DST, India, for the prestigious INSPIRE faculty award (INSPIRE/04/2015/ 002452). DST support to the Center for Nanosciences is acknowledged.

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