Effects of Magnesium and Zirconium Dopants on Characteristics of

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Effects of Magnesium and Zirconium Dopants on Characteristics of Titanium(IV) Oxide Fibers Prepared by Combined SolGel and Electrospinning Techniques Jerawut Kaewsaenee,† Pinpan Visal-athaphand,† Pitt Supaphol,‡ and Varong Pavarajarn*,§ †

Department of Physics, Faculty of Science, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand The Petroleum and Petrochemical College and The Center for Petroleum, Petrochemical and Advanced Materials, and § Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand ‡

ABSTRACT: The solgel and electrospinning techniques were combined to produce poly(vinylpyrrolidone) (PVP)/titanium(IV) oxide composite fibers from solutions containing PVP and titanium tetraisopropoxide. Either magnesium nitrate hexahydrate or zirconyl(IV) nitrate hydrate was added to the solutions as a source of magnesium or zirconium dopant, respectively. Upon calcination of the as-spun fibers, metal-doped titania fibers were obtained. The presence of either magnesium or zirconium dopant affected both the physical and chemical properties of the as-synthesized titania fibers and even retarded the formation of the rutile phase. Various characterization techniques were employed to confirm the increased content of Ti3þ defects and oxygen vacancies within the titania structures resulting from the presence of the metal dopants. These defects were found to trap photoexcited charges (i.e., electrons and holes), thus retarding the recombination of electronhole pairs. This led to an enhancement of the photocatalytic activity of the titania structures.

1. INTRODUCTION Titanium(IV) oxide or titania (TiO2) is a truly versatile material. It has been proposed for use as a major component in devices for various applications, such as catalysts,1 sensors,2 solar cells,3 and other types of electrodes.4 Titania exists in three natural polymorphic structures: anatase, rutile, and brookite. Titania can be synthesized in various shapes, such as nanoparticles,5 nanowires,6 nanotubes,7 and nanofibers8 by different preparation methods such as solgel,9 hydrothermal,10 solvothermal,11 and thermal plasma12 approaches. Titania can also be fabricated into nanofibers by a combination of electrospinning and solgel techniques.13 In a typical process, a mixture of a titania precursor and a polymeric template, where the latter provides the fiber-forming ability of the resulting mixture, is ejected as a jet from a small nozzle under the influence of a high electric field. The jet solidifies during its flight to a target and is collected as an unwoven fiber mat on the target. The fibers are subsequently calcined to remove the polymeric template and to transform amorphous titania into crystalline entities. Currently, titania nanofibers are of great interest for utilization in special applications, such as sensors,14 dye-sensitized solar cells,15 thermophotovoltaics,16 dental and bone implants,17 and photocatalysts.18 However, a major drawback is the rapid recombination of electronhole pairs that exist on the surface of the titania fibers, yielding a low photocatalytic efficiency. Procedures that have been used to overcome this drawback include, for example, the deposition of noble metals onto the titania fibers,19 the mixing of metal oxides with titania,20 and the doping of metal ions into the titania lattice.21 For the doping of a secondary metal, zirconium has been reported to significantly enhance the stability of the photoactive anatase phase,22 whereas magnesium has been reported to be easily incorporated into the lattice of titania.23 r 2011 American Chemical Society

The main objective of the present work was to investigate the effects of magnesium and zirconium as secondary metal dopants on various properties of titania nanofibers, with particular interest being focused on the defects formed within the lattice of titania in response to the presence of the dopant. The photocatalytic activities of the fibers were also investigated, based on the photo-oxidation reaction of methylene blue (MB) as a model compound.

2. EXPERIMENTAL DETAILS 2.1. Fabrication of Titania Nanofibers. The fabrication of titania nanofibers by electrospinning starts with the preparation of the spinning solution. A spinning solution containing poly(vinylpyrrolidone) (PVP, Mw ≈ 1 300 000 Da; Aldrich, Milwaukee, WI), titanium tetraisopropoxide (TTIP; Aldrich, Milwaukee, WI), acetic acid (Fluka, Steinheim, Germany), and a source of metal dopant [either magnesium nitrate hexahydrate or zirconyl(IV) nitrate hydrate (both from Aldrich, Milwaukee, WI)] was prepared by a method previously developed in our group.21 In the first step, 1.5 g of TTIP was mixed with 3 mL of acetic acid and 3 mL of absolute ethanol (Fluka, Steinheim, Germany). The solution was allowed to stand for 10 min before being added into 7.5 mL of a 10 wt % (based on the total mass of the final mixture) PVP solution in ethanol. The mixture was then stirred for 10 min to give a solution hereafter referred to as the neat spinning solution. To dope magnesium or zirconium into the titania fibers, Received: December 19, 2010 Accepted: May 22, 2011 Revised: May 16, 2011 Published: May 22, 2011 8042

dx.doi.org/10.1021/ie102527p | Ind. Eng. Chem. Res. 2011, 50, 8042–8049

Industrial & Engineering Chemistry Research magnesium nitrate hexahydrate or zirconyl(IV) nitrate hydrate was added to the neat spinning solution, and the resulting mixtures were stirred for an additional 1 h. The high solubility of the nitrates in ethanol ensured total dissolution into the neat spinning solution. These mixtures are hereafter referred to as the metal-doped spinning solutions. In the electrospinning process, each of the freshly prepared spinning solutions was loaded into a plastic syringe equipped with a blunt-ended 20-gauge stainless steel hypodermic needle. The emitting electrode from a Gamma High Voltage Research RC5 30 power supply, capable of generating dc voltages up to 30 kV, was attached to the needle. The grounding electrode from the same power supply was attached to a piece of aluminum foil used as the collector plate and placed approximately 7 cm below the tip of the needle. Upon application of a high voltage of 16 kV across the needle and the collector plate, a fluid jet was ejected from the nozzle. As the jet accelerated toward the collector plate, the solvent evaporated, depositing ultrathin fibers on the collector. The fibers were left exposed to ambient moisture (ca. 75% relative humidity) for approximately 24 h before being subjected to calcination at 500 °C for 3 h to remove organic residues, including the PVP, and to induce crystallization of the amorphous titania. The morphology and size of the as-prepared titania fibers were observed with a JEOL JSM 5800 scanning electron microscope and a JEOL JEM 2100 transmission electron microscope, the latter equipped with a selected-area electron diffraction (SAED) attachment. The crystalline phase of the products was studied with a Siemens D5000 X-ray diffractometer using Cu KR radiation. The anatase/rutile fractions of the products were calculated according to the method proposed by Jung and Park.24 The infrared spectra of the fibers were recorded on a Nicolet Impact 400 Fourier-transform infrared (FTIR) spectrometer. The specific surface areas of the products were calculated based on nitrogen uptake values following the BrunauerEmmettTeller (BET) equation (Micromeritics ChemiSorb 2750). Elements on the surfaces of the titania fibers, as well as their chemical states, were analyzed with a Kratos Analytical AMICUS X-ray photoelectron spectrometer, operated using Mg KR X-rays. Electronspin resonance spectroscopy (ESR) was conducted using a JEOL JES-RE2X ESR spectrometer, operating at 77 K and using a microwave frequency of 9.1555 GHz and a magnetic field range of 303353 mT. The ESR instrument was calibrated with Mn2þ/MgO. Finally, the photoluminescence (PL) emission spectra of the fibers were recorded using a Perkin-Elmer LS 55 fluorescence spectrometer, equipped with an emission and excitation slit width of 7 nm and a xenon lamp as a source for excitation light with a wavelength of 300 nm. 2.2. Photocatalytic Experiments. The photooxidation of methylene blue (MB, C16H18N3S) was used to assess the photocatalytic activity of the titania fibers. In each run, 40 mg of the as-synthesized fibers was placed into 200 mL of an aqueous MB solution (10 ppm) in a borosilicate glass reactor. The mixture was stirred magnetically and kept in total darkness for 1 h prior to ultraviolet (UV) light illumination in order to eliminate the effects from the sorption of MB onto or within the materials. Six low-pressure mercury lamps (15 W, Philips TLD15W/05, UV-A; wavelength ≈ 365 nm), placed around the reactor at a distance of 10 cm, were used to provide the UV radiation required to initiate the reaction. The light intensity at the center of the reactor was measured by a radiometer (IL1700 Research Radiometer) and found to be 7.43 W 3 cm2. The

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Figure 1. FTIR spectra of (a) precalcined undoped fibers compared with (b) neat, (c) 0.7 mol % Mg-doped, and (d) 0.7 mol % Zr-doped titania fibers after calcination at 500 °C for 3 h.

Figure 2. XRD patterns of (a) undoped titania fibers and titania fibers doped with (bd) magnesium at (b) 0.1, (c) 0.3, and (d) 0.7 mol % and (eg) zirconium at (e) 0.1, (f) 0.3, and (g) 0.7 mol %. All samples were calcined at 500 °C for 3 h.

temperature of the solution was constantly monitored and controlled at 35 ( 1 °C. The concentration of MB in the reactor was periodically monitored by measuring the absorbance of the solution at 665 nm using a Shimadzu UV-2550 UVvisible (UVvis) scanning spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Prepared Fibers. Electrospinning of each spinning solution resulted in the ejection of the solution as a fine stream (i.e., the jet) that accelerated toward the collector plate and deposited on the plate as ultrathin fibers in the form of an unwoven fabric. Upon exposure to the ambient moisture, hydrolysis and subsequent condensation of TTIP occurred within 8043

dx.doi.org/10.1021/ie102527p |Ind. Eng. Chem. Res. 2011, 50, 8042–8049

Industrial & Engineering Chemistry Research

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Table 1. Physical Properties of Fibers Obtained after Calcination at 500 °C for 3 h average fiber diameter (nm) fraction of anatase phase (%)

anatase crystallite size (nm)

precalcined

calcined

BET surface area (m2/g)

undoped

65.5

21.9

121

86

50.2

0.1% Mg-doped

70.2

20.9

134

87

51.4

0.3% Mg-doped

74.8

17.7

145

96

51.3

0.7% Mg-doped

84.1

15.6

168

125

55.7

0.1% Zr-doped

84.3

17.2

134

88

51.6

0.3% Zr-doped

84.7

17.0

141

95

53.0

0.7% Zr-doped

88.5

15.3

152

105

54.4

sample

the fibers, resulting in the formation of PVP/titania composite fibers. Subsequent calcination of the fibers removed the residual solvent, PVP matrix, and other organic residues. The removal of PVP from the fibers was confirmed by FTIR spectroscopy, as shown in Figure 1. The absorption bands in the range of 10002000 cm1, corresponding to both ethanol25 and PVP,26 disappeared completely following calcination. At the same time, a broad band associated with the vibration modes of TiOTi bonding within the range of 400800 cm1 appeared instead.25 According to the X-ray diffraction (XRD) results shown in Figure 2, the crystallographic structures of the as-calcined fibers were mainly anatase with only small amounts of rutile present. The anatase phase fractions in the as-calcined fibers increased significantly upon addition of either magnesium nitrate or zirconyl(IV) nitrate to the spinning solution (see Table 1). No XRD patterns associated with either oxide or titanate of magnesium or zirconium were observed in any of the samples. Nevertheless, slight shifts in the positions of the XRD peaks were observed, with the effect of zirconium doping being less pronounced than that of magnesium doping. The position of the (101) peak of anatase shifted from 25.325° to 25.307° and 25.320° after doping with 0.1 mol % of magnesium and zirconium, respectively. As the doping content was increased to 0.7 mol %, the peak shifted further to 25.317° for zirconium doping, whereas the shift for magnesium doping remained similar to that for 0.1% doping. This indicates that the magnesium and zirconium were incorporated well within the lattice structure of titania without the formation of discrete oxide clusters within the fibers. The crystallite sizes of the anatase phase, calculated from the XRD line broadening using the Scherrer equation, are also included in Table 1. Clearly, the crystallite size decreased in the presence of and with increasing contents of the metal dopants. More importantly, the observed decrease in the anatase crystallite size correlated with the observed increase in the anatase phase fraction within the samples. This agrees well with an earlier report that the presence of a metal dopant within the lattice structure of anatase inhibits the growth of nanocrystals, thereby decreasing the probability of phase transformation from anatase to rutile.23 The morphology of the fibers, both before and after calcination, was investigated by scanning electron microscopy (SEM). As shown by the representative SEM images in Figure 3, all of the fibers after calcination were circular in cross section, with diameters and surface areas in the ranges of 80120 nm and 5054 m2 3 g1, respectively (see Table 1). The fiber diameters were measured using JEOL SemAfore 4.0 image-analytical software. The values reported in Table 1 are average values calculated

from 50 individual fiber segments, randomly selected from several locations in the samples. On comparison with fibers reported in a previous work,21 the average diameter of the neat titania fibers in this work was lower because of the lower PVP content in the spinning solution and the greater electrical potential applied during the electrospinning process. Moreover, the addition of either the magnesium or zirconium precursor affected the size of the fibers. Increasing the amount of dopant increased the diameters of the fibers, an effect that is likely due to the increase in the viscosity of the spinning solutions.27 Visual observation of the solutions revealed that they became gel-like when the doping content exceeded 0.7 mol %. Furthermore, transmission electron microscopy (TEM) revealed the polycrystalline nature of the as-calcined fibers. Clearly, each individual fiber segment showed evidence of nanosized entities, distributed throughout its mass. These nanosized entities were shown to be single nanocrystals, as confirmed by the interference patterns observed in high-resolution TEM images (see Figure 4) and by the fact that the sizes of the nanocrystals, as suggested by TEM, were consistent with those calculated by the XRD line-broadening method. ESR analysis was conducted on all of the as-calcined fibers, and the results are shown in Figure 5. The detected ESR signals were located at the g values of 1.978, 1.999, and 2.018. The spectra were similar to those reported by Joung et al.,28 in which the signals at the g values of 1.978 and 1.999 were thought to correspond to Ti3þ and that at 2.018 to oxygen radicals (O•). The results also indicate that doping of either magnesium or zirconium into titania significantly increased the fractions of both Ti3þ and oxygen radicals, as evidenced by the observed increases in the intensities of the ESR signals in comparison to those of the undoped fibers (see Figure 6). Interestingly, zirconium doping resulted in a greater increase in the ESR signal for the doped fibers than magnesium doping at the same loading. Photoluminescence (PL) was employed to investigate the charge-carrier diffusivity within the mass of the as-synthesized titania fibers. PL emissions of semiconductor materials are the result of the recombination of photoinduced electrons and holes.29,30 Figure 7 compares the PL spectra of all of the samples in the range of 370 to 520 nm. Two main peaks were observed at 423 and 485 nm, neither of which correspond to the direct electron transition from the conduction band to the valence band. Instead, it has been reported that these PL signals are attributable to the surface states resulting from oxygen vacancies and defects on the surface of titania particles.29 Although the peak positions observed in the PL spectra of the metal-doped titania were in agreement with those of the undoped spectra, the PL intensity was quite sensitive to the metal doping. The 8044



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Figure 4. TEM micrographs and corresponding SAED patterns of pieces of (a) undoped, (b) 0.7 mol % Mg-doped, and (c) 0.7 mol % Zr-doped titania fiber after calcination at 500 °C for 3 h. Figure 3. SEM micrographs of (a) undoped, (b) 0.7 mol % Mg-doped, and (c) 0.7 mol % Zr-doped titania fibers after calcination at 500 °C for 3 h.

intensities of the overall PL signals increased with increasing dopant content. This should be a direct result of the increase in the defect contents of the titania crystals, such as those detected by ESR analysis, that could bind photoinduced electrons to form free or bound excitons.30 This is also supported by the fact that the zirconium-doped titania fibers, which contained higher defect

contents as indicated by the ESR results, also showed higher PL intensities than their magnesium-doped counterparts. Finally, elements on the surfaces of the as-calcined fibers, as well as their chemical states, were analyzed by X-ray photoelectron spectroscopy (XPS). The representative spectra for Ti 2p and O 1s are shown as insets in Figure 8. The shapes of the XPS spectra obtained from all samples were quite similar. The Ti 2p3/2 signal could be deconvoluted into two peaks: a main peak at 458.8 eV for Ti4þ and a minor peak at 457.5 eV for Ti3þ.31 In 8045



Figure 5. ESR spectra of (a) undoped titania fibers and titania fibers doped with (bd) magnesium at (b) 0.1, (c) 0.3, and (d) 0.7 mol % and (eg) zirconium at (e) 0.1, (f) 0.3, and (g) 0.7 mol %. All samples were calcined at 500 °C for 3 h.

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Figure 7. Integrated intensities of PL signals of titania fibers as functions of ()) magnesium and (9) zirconium doping contents. The inset shows PL spectra of undoped titania fibers (thick solid line) and titania fibers doped with 0.7 mol % magnesium (dashed line) and 0.7% mol zirconium (thin solid line). All samples were calcined at 500 °C for 3 h.

Figure 6. Intensities of ESR signals corresponding to (a) Ti3þ at a g value of 1.978 and (b) oxygen radicals at a g value of 2.018 as functions of ()) magnesium and (9) zirconium doping contents.

contrast, the signal for O 1s was found to comprise three different chemical states, as reported in the literature, namely, oxygen in the lattice of TiO2 (at 530.1 eV), oxygen vacancies (at 531.5 eV), and oxygen in the hydroxyl group (at 532.4 eV).32,33 The fractions of titanium and oxygen in each chemical state in all samples were calculated based on the ratios of the areas of the deconvoluted XPS peaks. The relative abundances of Ti3þ and Ti4þ and of oxygen vacancies (OV) compared to oxygen in the

Figure 8. Ratios of XPS signals of (a) Ti3þ and Ti4þ ions and (b) oxygen vacancies (OV) to oxygen in the TiO2 lattice (OL) as functions of ()) magnesium and (9) zirconium doping contents.

lattice of TiO2 (OL) are reported graphically in Figure 8. Although both the ESR and XPS techniques detected the presence of Ti3þ, the trends for the change in the fraction of Ti3þ with 8046



Figure 9. Apparent rate constant, kapp, for MB photodegradation on titania fibers doped with magnesium ()) or zirconium (9) as a function of doping content. The inset shows the degradation of MB in the presence of 0.7% mol zirconium-doped titania fibers with (0) and without (Δ) UV illumination. The dashed line represents results calculated from the LangmuirHinshelwood kinetics model.

respect to the contents of the doped metals reported by these techniques were different. This inconsistency might come from the fact that only the outer surfaces of the fibers were analyzed by XPS, whereas the ESR signals were generated from throughout the whole cross section. It is therefore suspected that the chemical distributions for Ti3þ, oxygen vacancies, and metal dopants across the cross-sectional area of the fibers were nonuniform. Nevertheless, the XPS results agreed well with the results from both ESR and PL analyses in that zirconium doping caused more defects in the titania structure than magnesium doping. 3.2. Photocatalytic Activity of the Prepared Fibers. In photocatalysis on titania, electrons (e) and holes (hþ) produced by the excitation of titania are thought to have the ability to reduce and oxidize, respectively, chemical species adsorbed on the surface of titania.34 The separation of these electronhole pairs is considered to be one of the most significant factors for an effective photocatalysis process.35 In the present work, titania in the form of nanofibers was used instead of nanoparticles, although, as demonstrated the TEM micrographs in Figure 4, all of the as-calcined fibers were simply aggregates of nanosized grains of titania. The measured surface areas of the samples were in the range of 5054 m2 3 g1, which is roughly the same as for the commercially available titania nanoparticles (e.g., Deggusa P25 with surface area of 49 m2 3 g1). Nevertheless, titania in nanofibrous form allows for easy handling compared with that in particulate form, because of its extremely high aspect ratio that could reach millimeters in length. With a proper supporting device such as coarse steel mesh, relatively large sheets of nanofibers can be used without any breakage. The photocatalytic degradation of MB was used to investigate the photocatalytic activity of the as-synthesized titania nanofibers. The data were fitted to the LangmuirHinshelwood kinetics model, which can describe the kinetics of heterogeneous photocatalysis on titania well,36 to obtain the apparent rate constants. The results are reported in Figure 9. The inset in Figure 9 shows representative experimental data for changes in concentration of MB with respect to the initial concentration (C/C0) compared with those calculated from the model (represented by a dashed line). It was confirmed that the

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LangmuirHinshelwood model represented the experimental data well. It was also found from a comparison with the results obtained for a control test without UV illumination that the effect of the loss of MB due to adsorption was negligible. The kinetic rates were in the same range as that reported for MB degradation on Deggusa P25.37,38 The degradation of MB was enhanced by the addition of the metal dopants, with zirconium doping showing better activity than magnesium doping, especially at low doping contents. Many factors might contribute to the enhanced photocatalytic activities of the zirconium- and magnesium-doped titania. Because all of samples had similar surface areas and points of zero charge (∼3.5, as measured by the solid addition method39), one of the main factors could be the increase in the contents of Ti3þ and oxygen vacancies within the metal-doped titania, as observed by the ESR and PL analyses. The presence of either Ti3þ or oxygen vacancies has been reported to capture photoexcited charges, leading to the inhibition of electronhole recombination and thus improving the photocatalytic activity of titania.40,41 However, because electron trapping is a much faster process (τe ≈ 30 ps) than hole trapping (τhþ ≈ 250 ns),42 the effect from Ti3þ should dominate over that from the oxygen vacancies. This is supported by the similarity in the trends of the graphs shown in Figure 9 (plots of the apparent rate constant) and Figure 6a (plots of the Ti3þ ESR signals). Although it has been reported that there should be an optimal dopant concentration that results in an optimal Ti3þ content above which the photoreactivity decreases because the defects become a recombination center for electronhole pairs,42,43 the optimal value was not reached in this work due to the limitation in the spinnability of the spinning solutions. Solutions with doping contents greater than about 0.7 mol % became too viscous to be electrospun into fibers.

4. CONCLUSIONS In the present contribution, magnesium- and zirconiumdoped titanium(IV) oxide fibers were successfully prepared by combined solgel and electrospinning techniques. These fibers were obtained after calcination of the as-spun poly(vinylpyrrolidone) (PVP)/titania composite fibers with the addition of magnesium nitrate or zirconyl(IV) nitrate. The products were fibers of circular cross section consisting of nanosized titania single-crystalline grains. The presence of either the magnesium or zirconium dopant significantly increased the concentrations of both Ti3þ defects and oxygen vacancies within the titania. These defects and vacancies function as traps for photoexcited charges and, in doing so, enhance the photocatalytic activity of titania through the inhibition of electronhole pair recombination. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: (662)-218-6890. Fax: (662)-218-6877. E-mail:Varong.P@ eng.chula.ac.th.

’ ACKNOWLEDGMENT The authors acknowledge the Office of the Higher Education Commission, Bangkok, Thailand, for financial support under the Strategic Scholarships for Frontier Research Network for the Thai Doctoral Degree Program. The authors also thank Professor Robert Molloy, Department of Chemistry, Chiang Mai 8047


Industrial & Engineering Chemistry Research University, Chiang Mai, Thailand, for assistance with revision of the manuscript.

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