Article pubs.acs.org/JPCC
N‑Doped TiO2 Nanofibers Deposited by Electrospinning Daniela Di Camillo,† Fabrizio Ruggieri,† S. Santucci,‡ and Luca Lozzi*,‡ †
Department of Physical and Chemical Sciences and ‡Department of Physical and Chemical Sciences and CNISM, University of L'Aquila, 67100 L'Aquila, Italy ABSTRACT: N-doped TiO2 nanofibers were deposited by electrospinning onto silicon and then annealed in air. They have been analyzed by secondary electron microscopy, X-ray diffraction, and X-ray photoemission. After the thermal processes, the fibers, having a diameter between 300 and 400 nm, contain a small amount of nitrogen and still show the TiO2 anatase crystalline structure. Different solutions and annealing processes were used to vary the nitrogen concentration and the crystalline phase. These nanofibers were successfully tested as photocatalytic devices for methylene blue degradation under visible light irradiation.
prepared by annealing TiO2-based NFs in ammonia15 and decorating them with metal atoms, showing excellent results for the production of hydrogen using UV photons.16 Very recently, Bi-doped TiO2 NFs have been deposited by electrospinning, showing very interesting photocatalytic properties using photons with wavelengths higher than 420 nm.17 In this paper, we present our results on the deposition of Ndoped TiO2 NFs prepared by electrospinning. The N-doping was performed by introducing a nitrogen precursor in the solution used for the NF preparation. These NFs were characterized by X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and scanning electron microscopy (SEM). Finally, their photocatalytic properties were tested, monitoring the photodegradation of methylene blue (MB) under visible light.
1. INTRODUCTION Titanium dioxide (TiO2), thanks to its interesting properties of nontoxicity, low cost, and high chemical stability, has been extensively investigated for several applications in which, following light absorption, the generated charges can be usefully applied, such as for photovoltaic systems1 and for photocatalytic devices.2 However, because of the wide intrinsic energy gap of TiO2 (between 3.0 and 3.2 eV, depending on the crystalline structure), only a small fraction of the solar spectrum can be used to promote the electron−hole pair formation.3 In photocatalytic devices, to increase the application of TiO2based systems where UV is not easily available, as, for example, to treat wastewater in remote regions where the electrical infrastructure is lacking, one of the most important goals of the recent research is to be able to prepare photocatalysts that can be activated by absorbing visible light.4,5 To reduce the energy gap and, therefore, increase the fraction of the solar spectrum that can be absorbed, different approaches have been used, mainly by doping TiO2 with metals6 or anions.3,4,7 Depending on the doping system, a narrowing of the energy gap or new states in the energy forbidden region can be observed. In fact, metal doping or codoping (two metals at the same time) reduces the band gap,8 while doping with nonmetal elements generally induces new states in the band gap.4 One of the most used doping elements is nitrogen. Density functional theory (DFT) calculations and optical data indicate that the N doping, as both substitutional and interstitial impurities, introduces new states in the band gap close to the TiO2 valence band, reducing the photon energy required for the creation of electron−hole pairs.9,10 TiO2 doped with nitrogen atoms can be prepared in different ways, as powders,11 as thin films by sol−gel7 or reactive sputtering,12 or as powders deposited onto carbon nanofibers (NFs).13 Electrospinning is a very cheap, versatile, and easy technique to deposit metal oxide as NFs.14 N-doped TiO2 NFs have been © 2012 American Chemical Society
2. EXPERIMENTAL SECTION The NFs were deposited, following the method reported in ref 18, by electrospinning using titanium tetrabutoxide (Ti(OBu)4) as the Ti source, ethylenediamine (C2H8N2) as the N source, polyethylene oxide (PEO, Mv = 300.000) as the polymer for the NF formation, and acetonitrile (CH3CN) as the solvent. The solution was prepared in the following way (the reported values refer to the weight percentages for the preparation of a solution with N/Ti = 1 atomic ratio): a mixture composed of PEO (5%), acetonitrile, glacial acetic acid (CH3CO2H, 12%, added to stabilize the solution and to control the hydrolysis reaction of the sol−gel precursor), titanium tetrabutoxide (32%), and ethylenediamine (2.8%) was stirred and ultrasonically agitated for 8 h to form a homogeneous solution. The concentrations of the Ti and N source compounds were varied to prepare Received: March 15, 2012 Revised: August 9, 2012 Published: August 11, 2012 18427
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(b) magnification, respectively. The mean fiber diameter is between 300 and 400 nm. Increasing the magnification, the nanostructured nature of the fibers is clearly observed, while it misses before the thermal process because of the presence of the polymer and the amorphous nature of the as-deposited NFs.18 The NF morphology does not change varying the N/Ti atomic ratio concentration. The chemical composition of the NFs was investigated by XPS. In Figure 2, the Ti 2p (a) and N 1s (b) XPS spectra of the
solutions with different N/Ti atomic ratios (from 1 to 3). For the deposition of undoped NFs (pure TiO2 NFs), ethylendiamine was not used. The substrate (a wafer of silicon) was placed at about 10−12 cm below the tip of the needle, and the NFs were deposited by applying an electrostatic voltage of about 10 kV between the needle tip and the substrate. Subsequently, the samples were annealed in an oven (in air ambient) for 1 h at 300, 400, or 500 °C (named in the following as 300, 400, and 500 °C/1 h samples, respectively) with a heating rate of 3 °C/min, to remove PEO and achieve the crystallization of titania. The morphology of the electrospun NFs was examined by SEM. The crystallographic and chemical structures of the NFs were characterized with grazing incidence XRD and XPS. The photocatalytic properties were evaluated looking at the degradation of MB (1 mg/103 cm3 in water) under visible light (halogen lamp, 50 W; sample-to-lamp distance, 5 cm). The variation, as a function of the exposure time, of the MB absorption peak intensity (at 664 nm wavelength) was measured. During the illumination, the solution temperature was monitored, and it never exceeded 35 °C. The photon flux of the halogen source was measured by means of a calibrated radiometer.
3. RESULTS AND DISCUSSION In Figure 1, the SEM images of the sample prepared using N/ Ti = 1 atomic ratio solution after the annealing at 500 °C for 1 h are reported. The images were acquired at low (a) and high
Figure 2. Ti 2p (a) and N 1s (b) XPS spectra of N-doped TiO2 NFs annealed at different temperatures.
N/Ti = 1 samples are reported. The Ti 2p spectra refer to the N-doped TiO2 sample before the thermal process (as deposited, dotted line) and after the annealing process at 400 °C/1 h in air (dashed line). Moreover, the spectrum for the undoped TiO2 sample after the annealing process at 400 °C/1 h in air (continuous line) is also reported for comparison. Both of the annealed spectra show the 2p3/2 and 2p1/2 peaks at about 458.8 and 464.5 eV, respectively. These values are in good agreement with published data for titanium dioxide films (Ti4+ chemical state).19 The spectra of the annealed samples are equal, indicating that there is no difference in the oxidation state of the titanium atoms in the doped and undoped samples after the annealing. Instead, the as-deposited N-doped TiO2 sample presents a light tail toward low binding energies (it is quite clear for the 2p3/2 peak). This could be ascribed to a small concentration of TiO2−x chemical state (as Ti3+).20
Figure 1. SEM images of NFs after the annealing at 500 °C for 1 h at low (a) and high (b) magnifications. 18428
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In Figure 4, the XRD spectra recorded on the samples (N/Ti = 1) annealed in air at different temperatures (300, 400, and
The N 1s spectra refer to the N-doped TiO2 sample (N/Ti = 1) as deposited (dotted line), after the annealing process in air at 400 °C/1 h (dashed line) and at 500 °C/1 h (continuous line). The as-deposited sample shows a quite intense peak at about 400 eV, which can be assigned to NO bond or interstitial N atoms.12 The annealing process strongly decreases the N 1s peak intensity. It is still well evident in the 400 °C/1 h sample, while this peak is almost missing in the 500 °C/1 h sample. This last intensity is similar to that observed in undoped TiO2 sample, in which the nitrogen signal is due to the physisorbed molecular nitrogen because of the exposure to air of the sample after the preparation. These data indicate that nitrogen atoms are very weakly bonded in the TiO2 cage and can be very easily lost. The reduction of the N concentration, after the annealing, is also confirmed, determining, by XPS, the atomic concentrations from the area underneath the main XPS peak of the different elements and using the atomic sensitivity factors for the used analyzer.21 For the N/Ti = 1 samples, the N concentration varies from 6 (as deposited) to 2% (400 °C/1 h) and to 1% (500 °C/1 h). This atomic concentration is similar to that observed in other N-doped TiO2 films, which showed good photocatalytic properties.4,12 Increasing the N/Ti concentration in the solution, no variation of the Ti 2p spectrum is observed, while the N1s spectrum changes. In Figure 3, the nitrogen 1s spectra recorded
Figure 4. XRD spectra acquired in grazing incidence geometry (incidence angle = 0.5°) on N-doped TiO2 samples annealed at (a) 300, (b) 400, and (c) 500 °C. The spectrum recorded onto a undoped TiO2 sample is also reported for comparison (d). The vertical bars show the position of the XRD peaks for the TiO2 crystal in the anatase phase.
500 °C) for 1 h are reported. The vertical bar shows the position of the XRD peaks for the TiO2 crystal in the anatase crystalline form.22 For comparison, the diffraction spectrum recorded on a sample composed of undoped TiO2 NFs grown by electrospinning and annealed at 500 °C/1 h in air is also reported (d). This sample shows the typical anatase shape. The spectrum recorded on the N-doped TiO2 annealed at 300 °C (bottom curve) does not show any crystalline phase. Increasing the annealing temperature, all of the XRD spectra indicate that the samples present a crystalline phase (anatase), which is very similar to that observed for undoped TiO2. The only sizable difference is a small increase of the peak width in the N-doped NFs with respect to the undoped ones, which can be attributed to a different size of the nanocrystals. No differences in the XRD spectra between N-doped TiO2 NFs deposited using different N/Ti atomic ratios (1 or 3) are observed. From the width of the main XRD peak (at about 25.3°) and using the Sherrer method, the mean crystallite size has been evaluated.23 This size is between 14 (400 °C/1 h) and 15 nm (500 °C/1 h) for the N-doped NFs (N/Ti = 1). The undoped NFs present a crystallite dimension of about 16 nm (500 °C/1 h). These NFs were tested as photocatalytic nanomaterials for MB degradation under visible light to evaluate the effect of the N doping. In Figure 5, the photocatalytic results obtained using the different samples are reported. The graph shows the variation of the monitored MB absorption peak intensity (C) as a function of the time under the illumination with a halogen lamp (visible). C0 indicates the initial absorption peak intensity. The variation of MB absorption intensity when no NFs (blank curve, stars) or undoped TiO2 NFs were used are also reported for comparison. In the insert, the photon flux hitting the samples is reported. The shown data refer to the results obtained performing five degradation tests on five different samples prepared using the same procedure, and the vertical bars show the error of the experimental data. In all of the samples, the normalized absorption intensity decreases as a
Figure 3. N 1s XPS spectra of N-doped TiO2 NFs deposited with different N/Ti atomic concentrations and annealed at 400 °C/1 h.
on the NF deposited using two different solutions (N/Ti = 1 and 3 atomic concentrations) and after the annealing at 400 °C/1 h in air are reported. For N/Ti = 3 sample (dashed line), the peak is much larger than for the N/Ti = 1 sample (continuous line). In fact, the full width at half-maximum (fwhm) changes from 2.4 to about 5 eV. This broad peak suggests the presence of several components, which, because of the low resolution, are not resolved. These low binding energy peaks could be attributed to substitutional N atoms or NO molecules.12 These components are also present in the N/Ti = 3 sample annealed at 500 °C/1 h, although for this sample the overall N1s peak intensity is lower than for the 400 °C/1 h sample. Therefore, increasing the nitrogen concentration in the solution, different sites in the TiO2 lattice can be occupied. This is also confirmed by the increased N concentration in N/Ti = 3 samples with respect to the N/Ti = 1 ones. In fact, for example, in the 500 °C/1 h samples, the N concentration grows from 1 (N/Ti = 1) to about 3% (N/Ti = 3). 18429
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other doping configurations, as NO, have to be taken into account.9,12
4. CONCLUSIONS In conclusion, we have shown that TiO2 NFs doped with nitrogen can be easily deposited by electrospinning. These NFs present a small amount of nitrogen, which does not influence the anatase crystalline phase of fibers. However, this small nitrogen concentration is able to modify the light absorption properties of the film, forming a surface with good photocatalytic activity even under visible light.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +39 0862 433030. Fax: +39 0862 433033. E-mail: luca.
[email protected].
Figure 5. Degradation rate of MB under irradiation of visible light of samples prepared with different N/Ti ratios and annealed in air at different temperatures. C0 is the initial MB absorption peak intensity, and C is its intensity at a given time. The stars indicate the degradation of the MB when no NFs are present. The MB degradation results observed for undoped TiO2 NFs is also reported. In the insert, the photon flux of the used source is reported.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been supported by EU FP7 project “NATIOMEM”, contract no. 245513.
function of the time, indicating a photocatalytic activity of the NFs. The intensity reduction is faster for the sample with low nitrogen concentration (N/Ti = 1) and annealed at low temperature (400 °C). Instead, the slowest degradation rate is observed for the sample with the highest nitrogen concentration (N/Ti = 3) and annealed at the highest temperature (500 °C). The reduced photocatalytic activity, increasing the annealing temperature, is due to lower nitrogen concentration in the NFs (as shown in the XPS data, Figure 2). Instead, increasing the nitrogen content in the solution used for the NF preparation, the total amount of nitrogen at the NF surface grows as expected, changing, for example, for the samples annealed at 500 °C/1 h from about 1 (for the N/Ti = 1) to about 3% (N/Ti = 3). However, the increased nitrogen concentration does not enhance the photocatalytic activity of the sample surface. This is probably due to the worst NF quality, in terms of adhesion of the NFs at the surface, determining a high percentage of NFs damaged and lost during the photocatalysis tests. Comparing the MB degradation results obtained using Ndoped TiO2 NFs with those observed when undoped TiO2 NFs are used (that is, pure TiO2 NFs), the effect of the doping is clearly evident. The photocatalysis of undoped NFs is very close to the natural MB degradation and is probably due to the small fraction of near-UV photons emitted by the source (gap energy for bulk TiO2 in anatase phase is about 3.2 eV,3 corresponding to about 388 nm wavelength). For all of the samples, the contribution of the MB molecule adsorption on the NFs to the MB concentration reduction also has been evaluated, leaving for 1 h the samples in the MB solution and in the dark. The C/C0 ratio varies from 1 to about 0.95−0.98 for the different samples, indicating that the MB adsorption process on the NFs is negligible. It also must be underlined that, although in all samples the presence in N1s XPS spectra of any substitutional nitrogen (at about 396 eV) has been never observed, the NFs show interesting photocatalytic properties. This is in contrast with the conclusions reported in ref 4, suggesting that, as reported by other authors, the role of this interstitial nitrogen in photocatalytic processes must be better evaluated and that
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