Article pubs.acs.org/IECR
Photocatalytic Degradation of Naphthalene by Electrospun Mesoporous Carbon-Doped Anatase TiO2 Nanofiber Mats Kunal Mondal, Souryadeep Bhattacharyya, and Ashutosh Sharma* Department of Chemical Engineering, Indian Institute of Technology, Kanpur, 208016, India ABSTRACT: We have fabricated partially aligned free-standing mesoporous pure anatase TiO2 nanofiber mats (TiO2-NF) for photocatalysis by electrospinning on a rotating drum collector using a blend of titanium isopropoxide (Ti(OiPr)4), with a carrier polymer, polyvinylpyrrolidone (PVP) in acetic acid and ethanol. Calcination removes PVP and generates mesoporous TiO2-NF with fiber diameters in the range of 25−75 nm by optimizing the electrospinning parameters such as electric field strength, polymer concentration, and flow rate of solution, etc. The band gap energy of TiO2 nanofibers from the UV−vis absorption spectra is found to increase with an increase in the calcination temperature, thus allowing band gap engineering for different applications. The surface morphology, phase composition, crystallinity, surface area, and porosity of the TiO2-NF are also investigated. We demonstrate the efficient and reusable photocatalytic action of the partially aligned pure electrospun TiO2-NF and residual carbon containing TiO2-NF mats with an average fiber diameter ∼40 nm in the photocatalytic degradation of a polycyclic aromatic hydrocarbon (PAH) dye, naphthalene. Small carbon residue (2.54%) containing TiO2-NF is found to be about twice as efficient as the pristine TiO2-NF in photodegradation of a PAH dye, but the effectiveness declined at higher carbon content.
1. INTRODUCTION Naphthalene (C10H8) the simplest polycyclic aromatic hydrocarbon (PAH), and a white crystalline solid, is a common hazardous waste product in a variety of industries including the coal and petroleum sector.1 The higher water solubility of naphthalene in comparison to other PAHs makes it a major pollutant in the effluent streams of these industries.2 In addition, it is carcinogenic and has well-documented ill effects on human and animal health.3 Conventional wastewater purification methods have little effect on removing naphthalene as it is not biodegradable.1 Although naphthalene removal from wastewater can be accomplished using techniques like biofiltration, membrane bioreactors, ozonolysis, electron beam irradiation, pulse radiolysis, and electrolytic aeration, etc., photocatalysis shows significant advantages as compared to the other methods.4 While photocatalysis is much cheaper than ozonolysis or radiolysis, the removal rate is also much faster than that of bioreactors.4 A wide bandgap semiconductor, titania, exists in three different phases: anatase, rutile, and brookite. The most stable is rutile and anatase is the most photoactive. Anatase titania is also found to have high photocatalytic detoxification and is extremely effective in the degradation of organic pollutants.5,6 Its low cost and nontoxicity also makes it ideal for use in photocatalytic applications;7 hence, TiO2 photocatalysts are usually preferred in the anatase form. Photocatalysts like TiO2 and ZnO become deactivated in aqueous solutions as a layer of H4O4Ti, Zn(OH)2 forms on their surfaces.6 Aqueous solutions of TiO2, on the other hand, exhibit high stability and good catalytic performance when compared to the other nanocrystalline semiconductor particles like CdS, CdSe, and PbS, etc.8 The morphology and structures of nanoparticles are important parameters in determining the extent of photocatalytic activity. Spherical TiO2 nanoparticles show higher photocatalytic efficiency with particle size reduction due to © XXXX American Chemical Society
increase in surface-to-volume ratio compared to titania films.4,6,9,10 However, the separation of these nanoparticles from solution is tedious. Nanoparticles often also have a strong tendency to agglomerate, decreasing the photocatalytic efficiency.11,12 Therefore, a mesh of titania nanofibers with high aspect ratio and a diameter in tens of nanometers should have better application potential than the other forms of titania such as powder, film, etc. Electrospinning or electrohydrodyanmic atomization has been widely recognized and used as a simple and inexpensive fabrication method to produce such high aspect ratio nanofibers.13−16 Typically, polymer solutions or melts had been used in nanofiber fabrication. The injection of such a solution from a nozzle several micrometers in diameter, under the action of an externally applied strong electric field (several kV/cm) leads to charge buildup on its surface and induces jet formation. The electrospun jets undergo splaying and whipping motion due to instabilities, along with solvent evaporation and stretching of the polymer jet, finally producing a mesh of high aspect ratio nanofibers. Fiber morphology can be varied using various parameters such as the solution properties (viscosity, concentration, surface tension, etc.), process parameters (flow rate, applied electric potential, distance of collector from tip) and the ambient properties (temperature, humidity, etc.). Electrospun ceramic nanofibers like those of titania are usually fabricated by spinning a solution containing precursors to the ceramic followed by calcination at high temperature. Special Issue: Ganapati D. Yadav Festschrift Received: June 28, 2014 Revised: August 26, 2014 Accepted: August 27, 2014
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Figure 1. Schematic representation of the electrospinning setup used for the synthesis of aligned TiO2-NF.
(OiPr)4), which is a sol−gel precursor to titania, by a process slightly modified32 from the one described by Xia et al.13,33 Briefly, 0.45 g of PVP was added to 7.5 mL of ethanol and kept stirring for a few minutes. Separately, 1.5 g of titanium isopropoxide was mixed with a 6 mL 1:1 by volume solution of acetic acid and ethanol, stirred, and mixed with the first solution. Then the solution (∼15 mL) was stirred for about 1 h followed by its immediate loading into a 2 mL plastic syringe having a stainless steel needle (26 gauges). The needle was connected to a high-voltage power supply (Gamma High Voltage Inc., High Bridge, NJ) that could generate DC voltages up to 20 kV. The solution feed rate was controlled using a syringe pump (Harvard Apparatus, Holliston, MA). The carrier polymer concentration indicated above was adjusted to provide the correct viscoelasticity needed for the formation of bead-free continuous fibers. The electrospun fibers were collected for several hours (∼5 h) to make a thick mat on a rotating drum electrode which was wrapped with aluminium foil and placed ∼5 cm away from the tip of the needle horizontally. The thick nanofibrous membrane could be detached from the drum collector in the form of a free-standing mat. The PVP/Ti(OiPr)4 composite nanofiber mats were then left overnight in air before calcination at 300, 500, and 700 °C for 3 h (2 °C/min ramp rate) to selectively remove the polymer. TiO2-NF containing different residual carbon loadings were prepared by controlling the calcination time. Figure 1 shows a schematic of the experimental setup used. 2.3. Characterization. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Raman spectroscopy, and UV−vis spectroscopy were used to characterize the synthesized titania nanofibers. The size and morphology of the nanofibers under different synthesis conditions were characterized by scanning electron microscopy (Quanta 200, Zeiss, Germany). The internal morphology and the phase composition of the fibers were characterized using transmission electron microscopy (TEM) (Tecnai G2, USA). The elemental composition of the nanofibers was determined through energy dispersive X-ray spectroscopy, EDX (Oxford instruments) which was attached to the scanning electron microscope. The X-ray diffraction (XRD) measurements were conducted (X’Pert Pro, PANanalytical, Netherlands) on an X-ray system with Cu Kα emission to get the phase information on the synthesized titania nanofibers. Raman spectral data were recorded (WiTec CRM 200 micro-Raman spectrometer, Germany) using a laser light of 532 nm wavelength. UV−visible absorbance spectra of the nanofibers were determined using a Varian Cary 50 Bio UV−
In the past decades, there have been many reports on the photocatalytic degradation of PAHs in aqueous solutions (Theurich et al.,1 Sigman et al.,17 Lin et al.18). David et al.,19 Reyes et al.20,21 and Jang et al.,22 have also reported on the adsorption of PAHs on alumina, SiO2, and other metaloxide surfaces. Recently, the nanostructured TiO2 photocatalystassisted degradation of various organics has been studied (Gupta et al.,23 Haarstrick et al.,24 He et al.,25 Kesselman et al.,26 Liu et al.27). However, there are only a few reports on the photocatalytic degradation of PAHs by TiO2 nanoparticles (Das et al.28 Pramauro et al.29) and on the thin film of suspended TiO2 in an aqueous system (Pal et al.30). Recently, Singh et al.31 showed that electrospun ZnO nanofibers are effective in the UV-assisted photocatalytic degradation of naphthalene and anthracene pollutants. To the best of our knowledge, there is no study on the photocatalytic naphthalene degradation with the electrospun mesoporous anatase TiO2 in the nanofiber form. In this work, we have investigated the photocatalytic property of electrospun, free-standing, pristine and carbondoped anatase TiO2-NF mat toward the degradation of a PAH dye, naphthalene, which is a common organic water pollutant. Our synthesis procedure for the titania nanofibers utilizes a mixture of Ti(OiPr)4 and high molecular weight PVP before calcinating them to obtain the ceramic nanofibers.28 Variations of fiber morphology under different electrospinning and calcination conditions are performed to get optimized freestanding, partially aligned, mesoporous anatase TiO2 nanofiber mats. The fibers have been characterized using X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and UV−vis absorption spectra. The photocatalytic activity and reusability of the pure and carbon-doped titania nanofibers are then tested in the degradation of naphthalene and compared with previous reports of degradation by the use of titania films and nanoparticles.
2. EXPERIMENTAL SECTION 2.1. Materials Used. Absolute ethanol and glacial acetic acid (99.8%) were purchased from Merck and Fischer Scientific, respectively. Titanium isopropoxide (97%), polyvinylpyrrolidone (PVP) (Mw ≈ 1 300 000) and naphthalene was obtained from Sigma-Aldrich, USA. Deionized water from Millipore water systems (resistivity 18.2 MΩ.cm) was used in all experiments. All chemicals were of analytical grade and were used as received with no further purification. 2.2. Synthesis of TiO2 Nanofibers. The TiO2-NF were electrospun from a solution of titanium isopropoxide (TiB
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vis spectrophotometer. The total surface area and pore size distribution were analyzed using a Brunauer−Emmett−Teller (BET) method and Autosorb1 software (Quantachrome Instruments, USA). The TiO2-NF was sonicated thoroughly in ethanol to form a homogeneous solution to record the absorbance data. 2.4. Photocatalytic Experiment. A 2.5 mg sample of naphthalene powder was carefully weighed and dispersed after thorough stirring and sonication in 100 mL of deionized water to form a 25 ppm naphthalene solution. Then, the photocatalyst, that is, the free-standing titania nanofibers mat, was immersed into 50 mL of this solution, and both were exposed to a UV lamp (power = 12 W, intensity = 2.05 mW/cm2, and wavelength = 254 nm) to promote the degradation reaction. The UV source was kept at 5 cm distance from the naphthalene solution. The strong odor of naphthalene in the solution slowly decreased indicating the advancement of the reaction which was monitored by a UV−vis spectrophotometer at a wavelength of 275 nm by using Varian Cary 50 Bio UV−vis spectrophotometer. The effects of the catalyst loading on the photocatalytic activity of the TiO2-NF were assessed for three loadings, 1, 5, and 15 mg. The reaction was carried out in a clean glass Petri dish, and 2.0 mL of the solution was taken out from the stock solution by a cuvette for recording the absorbance data at different time gaps which was added back to the previous stock solution. All the photocatalytic reactions were carried out in a controlled atmosphere at the room temperature of 23 ° C, 32% humidity, and at a pH value of 7.
Figure 2. FE-SEM micrograph of the PVP/(Ti(OiPr)4) fibers electrospun at a constant flow rate of 40 μL/min with different PVP concentration. (a) Beaded, nonuniform polymer fibers form at 1 wt %, (b) more uniform diameter and bead-free nanofibers form at 6 wt %, and (c) thick fibers form at 12 wt %.
3. RESULTS AND DISCUSSION 3.1. Characterization of TiO2 Nanofibers. Electrospinning of TiO2-NF is based on a sol−gel precursor to TiO2, titanium isopropoxide (Ti(OiPr)4). This precursor undergoes rapid hydrolysis in the presence of moisture and forms TiO2 gel in the electrospun nanofibers. The hydrolysis reaction of (Ti(OiPr)4) was controlled by the addition of acetic acid, which can also help to manipulate the surface morphology of the TiO2 nanofibers such as porosity.33 The as-spun nanofiber mats were left in air overnight to permit the complete hydrolysis of Ti(OiPr)4. Finally, the carrier polymer PVP was removed upon calcination leaving a mesoporous TiO2 continuous nanofiber mat with some residual carbon content. The role of sacrificial polymer PVP in the electrspun solution was to increase the viscosity and therefore to control its viscoelastic behavior.13 The diameter of the electrospun nanofiber could be varied, which is a determining factor in the reaction rate of photocatalysis. Obviously, a lower fiber diameter leads to higher specific surface area therefore resulting in a larger number of reaction sites enhancing catalytic efficiency. In our experiments, we kept the electric field strength and ambient properties such as the temperature and moisture level fixed while varying process parameters such as solution flow rate and polymer concentration to optimize the fiber diameter. The distance of the rotating drum from the needle tip was kept constant at 5 cm. Figures 2 and 3 show FE-SEM micrographes of the synthesized electrospun PVP/Ti(OiPr)4 composite nanofibers with near optimal (Figure 2b; Figure 3a) and suboptimal morphologies (Figures 2a,c; Figure 3b) under different conditions of electrospinning. The optimization of fiber diameter with polymer solution concentration and flow rate are shown in Figure 4. The figure shows formation of defect-
Figure 3. FE-SEM images of 6 wt % PVP/(Ti(OiPr)4) solution electrospun at different flow rate: (a) 2 μL/min and (b) 100 μL/min.
Figure 4. Variation of as-spun fiber diameter on PVP concentration and solution flow rate.
free sub-150 nm fibers electrospun at 40 μL/min flow rate, under 6 wt % PVP concentration with a constant application of 3 kV/cm electric field strength. Lowering polymer concentration yields smaller diameter fibers, but these have increasingly nonuniform diameter and a beaded appearance as seen in Figure 2a at 1 wt % concentration of PVP. For the low concentration, low viscoelasticity PVP solution, the liquid jet breaks readily and produces an electrospray of droplets/ beads or heavily beaded fibers of nonuniform diameter. An increase in the polymer concentration and viscoelasticity was required to fabricate continuous bead-free fibers. A much higher concentration forms thicker fibers, for example, the results shown in Figure 2c at 12 wt % concentration. We have C
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also observed that the application of higher electric fields did not lead to further substantial reduction in fiber diameter. Also, a higher voltage offers more probability of forming bead-like structures and nonuniform diameters. On increasing the solution flow rate from 2 to 100 μL/min under an applied electric high voltage of 15 kV, the fiber diameter increased from ∼100 to 400 nm (Figure 3). This increase in the fiber diameter on increasing flow rate is expected theoretically and in accord with previous observations in electrospinning.34 The production rate of nanofibers obviously is higher at a higher flow rate, which is desirable. Hence, all subsequent characterizations and degradation studies were carried out with fibers produced by 6 wt % polymer concentration and electrospun with 40 μL/min solution flow rate. Figure 5 shows the morphology and SAED pattern (Figure 5d) of calcined fibers made of TiO2. The latter confirms the
Figure 6. EDX spectrum of titania nanofibers calcined at 500 °C. The Si signature is because the TiO2-NF was dispersed on a Si wafer during sample preparation.
Table 1. EDX Data Confirming the Elemental Composition of TiO2-NF Calcined at 500 ° C element
wt %
wt % S.D.
atomic %
carbon oxygen titanium
6.451 62.761 30.788
3.443 8.168 7.933
10.525 76.878 12.587
percentage ∼6% in the TiO2-NF calcined at 500 °C. The presence of the small amount of carbon may be due to incomplete removal of the carrying polymer (PVP) through calcination which can be removed by heating PVP/(Ti(OiPr)4 fibers at a relatively higher calcination temperature. However, it is known that a higher calcination temperature leads to decrease in porosity35 and also encourages the formation of TiO2 in the rutile form, which is photocatalytically less active than TiO2 in the anatase phase. As shown previously, the fibers produced by this method are flexible.34 Although their tensile strength decreases with calcination, the Young’s modulus is improved.36 The flexibility of the TiO2-NF mats offers prospect for new applications of these materials. As discussed, a 6 wt % polymer solution electrospun at 40 μL/min flow rate yields 120−150 nm diameter composite fibers, which upon calcination finally produce 40−60 nm diameter TiO2 nanofibers. It is interesting to note that the fiber diameter strongly decreases after calcination, and around 60−70% shrinkage in the fiber diameter was observed. The calcination temperature was optimized at 500 °C for all the synthesized samples to ensure the pure anatase phase of the TiO2 photocatalyst. For analyzing the BET surface area, the sample was subjected to degassing in vacuum at 200 °C for 8 h, and then N2 adsorption−desorption isotherms were recorded. The pore-size distribution was analyzed using NLDFT (nonlocal density functional theory) from the desorption branch of the isotherm. Figure 7 shows the BET isotherm along with the pore size distribution in the inset. The TEM micrograph in Figure 5c also depicts the porous morphology of the TiO2 nanofibers. The total pore volume was calculated from the adsorbed N2 gas at relative pressure P/Po of 0.99665. The total BET surface area was found to be 29.39 m2/g and the total pore volume was 0.0504 cc/g with mesopore, micropore, and macropore volumes of 0.026, 0.007, and 0.019 cc/g, respectively, with the average pore diameter of 15 nm. Thus, the results indicate
Figure 5. FE-SEM micrographs of (a) TiO2 nanofiber mat produced at 500 °C calcination temperature; (b) higher magnification view of the single fiber; (c) TEM micrograph of TiO2-NF; and (d) the SAED pattern showing the crystal planes of anatase phase of TiO2.
anatase phase as discussed later. The nanofibers are partially aligned (shown in Figure 5a) and form a 3-D fibrous mat. It also could be seen (Figure 5b,c) that TiO2 nanoparticles stacked densely along the fiber direction. Figure 5b shows a single TiO2-NF which is very thin and has a diameter of ∼23 nm. The calcinations process substantially changed the surface morphologies of the electrospun nanofibers which are presented in Figure 5b,c. After calcination the TiO2 fibers become very thin and show a smooth surface with varying fiber diameters ranging from 25 to 75 nm, which are calculated by analyzing several FE-SEM and TEM micrographs. The elemental composition of the electrospun nanofibers was determined using energy dispersive X-ray spectroscopy or EDX. The EDX spectrum in Figure 6 shows the presence of titanium and oxygen, as well as some residual carbon as discussed later. The unassigned high peak in the figure is due to the silicon wafer used in sample preparation. The quantitative EDX results in Table 1 show a residual carbon weight D
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NF, the as-spun (Ti(OiPr)4)/PVP fibers were calcined at 500 °C. For a final confirmation of the phase of the titania nanofibers, X-ray diffraction (XRD) analysis was performed. Figure 9 shows the XRD characterization of the electrospun
Figure 7. N2 adsorption/desorption isotherm and pore size distribution plot (inset) of the TiO2-NF.
that the calcinated TiO2-NF are mesopore dominated accounting for 51.2% of the total pore volume (microporosity and macroporosity account for 15.24% and 33.48% of the total pore volume, respectively). Figure 8 shows the Raman spectroscopy results of the synthesized titania nanofibers. The Raman spectrum indicates
Figure 9. XRD plot shows the pure anatase phase of TiO2-NF calcined at 500 °C. Inset shows rutile and anatase mixed phase of TiO2-NF calcined at 700 °C.
nanofibers. The observed XRD peaks at 25.43° corresponding to (101) plane represent the anatase phase of titanium dioxide. While the other peaks at 2θ position of 75.46°, 36.39°, 48.06°, 53.93°, and 68.82° corresponding to (215), (103), (200), (105), and (116) plane reveal the anatase phase of titania. It was also observed that the calcinations of the electrospun fibers up to 700 °C leads to the transformation of anatase to the rutile phase, which is seen in the inset of Figure 9 at 2θ = 27.45° for the (110) diffraction plane of rutile TiO2 in addition to the other anatase peaks. The XRD data for the anatase and rutile TiO2 were matched with Pearson’s Crystal Data (PCD) from reference number 1420258 and 1906244, respectively. The selected area electron diffraction (SAED) pattern in Figure 5d also confirms the above XRD results. An average crystallite size of TiO2-NF (Lc) can be calculated from the XRD diffraction pattern using the Debye−Scherrer equation: Lc = 0.90λ/β × cos(θ) where β is equal to the full peak width at half height (FWHM) corresponding to the X-ray diffraction angle (θ) and 0.90 is the Scherrer’s constant.37 The crystallite sizes calculated using the Debye-Scherrer equation for the different samples which were calcined at 300, 500, and 700 °C were 23.6 nm, 42.40 nm, and 53.33 nm, respectively. Therefore, the average crystallite size of TiO2 is increased with the calcination temperature, which is also supported by a literature report.35 The Raman and the XRD results thus show that the synthesized titania nanofibers calcined at 500 °C are of the pure anatase phase, which is more photoactive and hence desirable for photocatalytic applications. 3.2. Optical Band Gap Calculation for the TiO2 Nanofibers. To get an idea about the opical band gap of the mesoporous TiO2-NF present in the free-standing mat, the bandgap energy was determined by means of the optical absorption spectrum recorded by a UV spectrophotometer compatible for solid sample analysis. Figure 10 shows the plot of (αhυ)1/2 versus photon energy, which is linear for a broad
Figure 8. Raman spectra of TiO2-NF (calcined at 300 °C, 500 °C) taken at room temperature. Inset figure shows the rutile TiO2 peaks obtained after calcination at 700 °C.
the presence of TiO2 crystalline nanofibers and showed four strong peaks at 141, 394, 513, and 635 cm−1. These peaks are matched to the anatase phase of titania37,38 calcined at 300 °C to 500 °C and another two strong Raman peaks at 429 and 608 cm−1 in the 700 °C calcined sample corresponding to the Eg and A1g vibration for the rutile phase which is in agreement with the literature.38 The spectrum of the 300 °C calcined sample contains two extra peaks along with the anatase TiO2 peaks, at 1352 and 1595 cm−1 corresponding to the D and G bands for carbon, respectively, which are to be defined as the main signature of amorphous carbon materials.39 This has further revealed that a significant amount of carbon was present in this sample due to incomplete removal of carbon. There was no characteristic Raman peak (in the inset of Figure 8) for carbon at a calcination temperature of 700 °C implying that the higher calcination temperature is needed for a complete removal of carbon from (Ti(OiPr)4)/PVP derived electrospun nanofibers. As we focused on the pure anantase form of TiO2E
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Figure 10. Tauc plot for calculating band gap energy of TiO2-NF. The inset shows that band gap increases with increase in the calcination temperature.
Figure 11. Pathways for the electron−hole pair generation in TiO2NF.
Migration of the charge carriers to the surface leads to reactions with the adsorbed chemicals resulting in their oxidation by the holes or reduction by the electrons. Usually • one or more intermediates like H2O2, O•− 2 , OH, or O2 play a significant part in the reaction mechanism (schematic shown in Figure 12).
range of photon energies (hυ) indicating the indirect band type of transitions. The extrapolations of these straight line plots on the hυ axis give the energy band gap of the TiO2 nanofibers. A shift in the absorption edge among the three samples prepared at three different calcination temperatures is also observed. The band gap energies are found to be 3.02, 3.21, and 3.39 eV (from Tauc plot40 in Figure 10) for TiO2-NF calcined at 300, 500, and 700 °C, respectively. The band gap for the TiO2NF calcined at 500 °C is found to be an indirect band gap with energy ∼3.21 eV which further confirms the pure anatase phase composition of the synthesized titania. The inset plot shows that the band gap energy of TiO2 increases as the calcination temperature increases. As calcination temperature increases, a blue shift from 411 nm to 386 nm and 366 nm is observed (respectively, for 300, 500, and 700 °C). This shift of the absorbance spectra toward shorter wavelengths can be elucidated as a consequence of the quantum size effect initiated by the reduction of TiO2-NF particle size.40,41 It is reported that when a wide band gap semiconductor such as TiO2 is combined with conductive materials, the band gap reduces.42 In our case, lower calcination temperature left greater carbon residue in the TiO2-NF. This residual carbon creates new energy states between the top of the valence band and the bottom of the conduction band of the semiconductor, which decreases the band gap energy of the TiO2-NF.43 Additionally, greater shrinkage in fiber diameter at higher temperature may lead to the widening of the band gap in the nanodomian.40,44 Also, the higher calcination temperature produces higher surface stress of the nanofibers.41 Therefore, another possible reason may be the surface stress related with the surface energy of the TiO2-NF for the shift in the band gap.41 For nanoparticles and nanofibers, higher stress is likely to be caused due to surface energy, which further noticeably increases with the lowering of the size of nanofiber/particle.41 3.3. Photocatalytic Degradation of Naphthalene. Typically a semiconductor gets activated upon its irradiation with energy in the form of photons of light which is equal or slightly greater than its band gap. If excitation of an electron from the conduction band to the valence band occurs, this leads to an electron−hole pair generation. There are three pathways for the produced electron−hole pair, as illustrated in Figure 11, namely: (i) bulk recombination, (ii) surface recombination with energy dissipation, (iii) consumption by the species adsorbed on the photocatalyst surface or in contact with the photocatalyst surface.
Figure 12. Schematic illustration of the photoinduced charge transformation on TiO2 nanofibers.
Naphthalene degradation occurs by the following mechanism: TiO2 + hυ → TiO2 (e−CB + h+ VB)
(i)
h+ VB + H 2O → •OH + H+
(ii)
e−CB +O2 → O•− 2
(iii)
+ • O•− 2 + H → OOH
(iv)
Under these conditions, TiO2-NF absorbs ultaviolet radiation ≥3.21 eV (band gap energy) and produces electron−hole pairs (eq i). These generated holes from the valence band (h+VB) of the TiO2 semiconductor nanofibers are successively captured by OH− ions or by the water molecules and generate H+ and • OH free radicals (eq ii), whereas the conduction band electrons (e−CB) are trapped by O2 molecules to generate O•− 2 (superoxide radical) species and interact with H+ to create • OOH radicals (eqs iii, iv) (Singh et al.31). The •OH, •OOH, and electron−hole pairs have strong oxidation activity. These species can oxidize and even entirely mineralize almost all organic compounds. The •OH and/or •OOH possibly play a foremost role in the photocatalytic oxidation process in the suspensions. However, on the surface of the photocatalyst the F
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naphthalene dye concentration became almost zero within 13 min at a higher catalyst loading. It was observed that with 1 mg, 5 mg, and 15 mg of TiO2 nanofiber loadings (Figure 15), the
electron−hole pairs might be reacting with the organic substrate. If these holes and electrons recombine slowly the photocatalyst will be more effective in the dye degradation. Formation of •OH, •OOH reactive species assists to diminish the electron−hole recombination rate which provides a longer time for the TiO2 catalyst to interact with the organic dyes. We did not use any separate aeration setup for degradation of the naphthalene. If O2 is injected, it acts as electron acceptor and delays the electron−hole recombination thus strengthening the degradation reaction. Also, time to complete degradation of the dyes raises with the reduced loading of TiO2-NF photocatalyst. The naphthalene concentration was monitored by probing its absorbance at 275 nm with a UV−vis spectrophotometer. Two control experiments were done. There is a change in the dye concentration (∼10%) when the dye was kept in the dark with TiO2-NF for ∼1 h (Figure 13) owing to adsorption. Further,
Figure 15. Progress of reaction for different loadings of TiO2-NF in 25 ppm initial naphthalene solution.
degradation times for 50 mL of 25 ppm aqueous naphthalene solution were 43, 22, and 13 min, respectively. The rate constant for naphthalene degradation by TiO2-NF photocatalyst was determined by pseudo-first-order kinetics. The rate expression is determined by −dC/dt = kC equation, where k is the apparent rate constant influenced by the concentration of the dye. After integrating this equation, we can obtain the equation −ln(C/C0) = kt where C is the concentration of naphthalene at time t and C0 is the initial concentration of naphthalene. The plot −ln(C/C0) versus t is linear, and the slope gives the values of the rate constant k. The maximum absorption wavelength (λmax) was 275 nm. The measurements were stopped when the concentration of the dye solution becomes small. The absorbance at λmax = 275 for the naphthalene solution was recorded after a certain interval of times of reaction with constant UV light exposure to produce a plot of (C/C0) against irradiation time. The rate of the photocatalytic reactions from Figure 15 was calculated and found to be 0.1059, 0.1655, and 0.2612 min−1 for the TiO2-NF photocatalyst loading of 1, 5, and 15 mg, respectively. These high rate constants indicate the efficacy of the anatase mesoporous TiO2-NF mat in photocatalytic degradation of naphthalene dye. We also investigated the effect of alignment in the nanofiber mats by a control experiment in which the performance of an unaligned TiO2-NF mat was compared. There was no significant improvement in the photcatalytic activity (data not shown). The rate constant of the TiO2 nanofiber-assisted naphthalene degradation reaction is found to be very high as compared to the other form of TiO2.30 One main reason for this higher photocatalytic activity of titania nanofibers is the high surface to volume ratio of electrospun TiO2-NF mats due to its nanosized diameters. This high specific surface area provides a large number of reaction sites which increases the catalytic efficiency. The multiscale porosity in the TiO2-NF provides further advantage on the photocatalysis. The high mesoporosity (∼62%) provides higher surface area for the photoreaction to
Figure 13. UV−vis absorption spectra showing a 25 ppm naphthalene solution under UV without the photocatalyst, and with the photocatalyst under dark condition.
long time UV exposure (∼1 h) without TiO2-NF mats also shows some degradation of naphthalene concentration as reflected in its decreased concentration to about 80%. Thus, both the UV irradiation and TiO2-NF together are necessary for the rapid and total degradation of this PAH dye. The degradation analysis of naphthalene solution was made spectrophotometrically by assessing its absorbance at 275 nm using a steady UV source. UV−visible absorption spectra for 50 mL of 25 ppm naphthalene solution at different time intervals with a loading of 15 mg of porous TiO2 fibers is shown in Figure 14. The photocatalytic reaction was quick and the
Figure 14. UV−vis absorption spectra for 25 ppm naphthalene mixed with 15 mg of TiO2-NF at different irradiation time. The inset shows magnified λ max at 275 nm wavelength. G
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Industrial & Engineering Chemistry Research
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act as a dopant in TiO2 decreasing its band gap owing to the surface states close to the valence-band edge of TiO2-NF.46 However, at larger carbon concentration, active photocatalytic material decreases, which together with increased conductivity could possibly explain poorer performance. The photocatalytic degradation performance of this electrospun TiO2-NF mat for naphthalene degradation is also compared with the electrospun ZnO nanofiber mat studied by Singh et al.31 Under the same conditions of catalyst and solution loadings, the rate constants for the pure TiO2-NF, optimized 2.54% carbon-doped TiO2-NF and ZnO electrospun mats are 10.1 × 10−2, 20.29 × 10−2, and 5.086 × 10−2 min−1, respectively. Thus, optimal carbon containing TiO2-NF is about four times more efficient than ZnO nanofibers. The difference in the photocatalytic activities originates from their different physicochemical properties, such as bandgap, electron−hole generation and recombination, crystallinity, light absorption, and photostability. As discussed, the residual carbon impurity in the TiO2-NF mat may help in faster generation and slower recombination of the electrons and hole pairs during photocatalysis.48 The reusability is a vital issue in applications of a photocatalyst. The option of recycling the TiO2-NF catalyst for photodegradation of naphthalene was monitored with 15 mg of the TiO2-NF mat for five different 50 mL solutions of 25 ppm naphthalene under the same reaction conditions. After completion of the first photocatalysis, the TiO2-NF mat was washed and dried for further use. Thus, recovered TiO2-NF photocatalyst mat after photocatalysis could be used again for the degradation of naphthalene repeatedly as its efficacy and fibrous structure were retained. As seen in Figure 16a, not much loss in the photocatalytic activity of the TiO2-NF was found even after several runs. At the end of five cycles of use, the morphology of the used nanofibers was studied under FESEM (Figure 16b) confirming the preservation of the TiO2-NF mat fibrous morphology.
occur while the 34% macroporosity helps in the transport of reactants and products. The thickness of the electrospun mat can also be easily controlled through parameters like the electrospinning time. Additionally, the movement of photogenerated electrons in nanoparticles is believed to be through the mechanism of hopping.45,46 This is not an effective conduit for electron transportation and causes a high electron−hole recombination loss. Also, one-dimensional electrospun nanofibers have less lattice defects, and they are made up of wellaligned elongated crystallites in the fiber direction45 which prove beneficial for the enhanced photocatalytic activity by allowing slow electro-hole recombination during reaction.47 However, electrospun TiO2-NF offer an uninterrupted transport path for the electrons resulting in higher photocatalytic efficiency. We also investigated the effect of carbon content in the TiO2-NF mat toward its photocatalytic activity. TiO2-NF containing different residual carbon loadings (10.91, 6.45, 2.54 wt % carbon residue in TiO2-NF and pure TiO2-NF) were prepared by controlling the calcination time. The calcination temperature was kept constant at 500 °C to retain the anatase phase of TiO2. The calcination was performed for 1, 3, 5, and 7 h, respectively, to control the residual carbon content in the TiO2-NF. The amount of carbon present in the TiO2-NF was estimated by EDX (data not shown). We used 5 mg catalyst loading for each of the above prepared different carbon contained TiO2-NF mats and tested their photocatalytic degradation performance. The rate constants for different residual carbon loaded TiO2 fiber mats with 50 mL (25 ppm) of naphthalene are given in Table 2. TiO2-NF containing Table 2. Rate Constants for Different wt % Residual CarbonLoaded TiO2-NF with 25 ppm Naphthalene carbon content (wt %) in TiO2-NF 10.91 6.45 2.54 pure TiO2-NF
rate constant (min−1) 7.71 16.55 20.29 10.1
× × × ×
10−2 10−2 10−2 10−2
4. CONCLUSIONS Optimized free-standing mesoporous anatase TiO2 nanofiber mats have been fabricated by electrospinning and the effect of various operating parameters on the fiber morphology have been studied. Continuous, partially aligned mesoporous TiO2NF free-standing mats with an average diameter of 40 nm were thus obtained. The band gap energy of TiO2 nanofibers is found to increase with increase in the calcination temperature. Higher calcination temperature (∼700 °C) leads to an anataseto-rutile phase change while heating at low temperature (
