Abstract
The chemical synthesis of nitrogen-doped titanium dioxide (N-TiO2) is explored in an attempt to understand the mechanisms of doping. Urea is used as precursor in a sol gel synthesis of N-TiO2. Chemical and structural changes during thermal treatment of the precursors were followed by several techniques. The effect of doping on band gap, morphology, and microstructure was also determined. The byproducts produced during firing correspond to those obtained during urea thermal decomposition. Polynitrogenated colored compounds produced at temperatures below 400°C may act as sensitizer. Incorporation of N in the TiO2 structure is possible at higher temperatures. Degradation experiments of salicylic acid under UVA and visible light ( nm) in the presence of TiO2 or N-TiO2 indicate that doping decreases the activity under UVA light, while stable byproducts are produced under visible light.
1. Introduction
The development of novel materials capable of solar-driven chemical transformation or electricity production is one of the more important challenges for now and the following years. Oxide semiconductors are an interesting family of semiconductors that can use solar light to catalyze several chemical processes and/or produce electricity. In this sense, TiO2 is a noble material due to its chemical stability, photo-corrosion resistance, and low toxicity [1, 2]. However, TiO2 can absorb only the relatively small part of the solar spectrum that includes the nm range (around 5% of the solar light). In an attempt to improve light absorption in the visible range, TiO2 was modified by incorporation of transition metals, noble metals and recently by nonmetallic elements [3–7].
During recent years, N-TiO2 became one of the more studied nonmetal-doped systems. The presence of a nontoxic dopant and the reported activity are the principal reasons why N-TiO2 is chosen as a promising photocatalyst to be used under solar light illumination.
The methods to prepare N-TiO2 can be summarized as follows:(1)sputtering and implantation techniques [8–10]: these techniques are adequate for the preparation of films;(2)calcinations of TiO2 or Ti(OH)4 under N-containing atmospheres generated by nitrogen compounds like ammonia [11–13]: these techniques are principally used to prepare powders;(3) sol-gel process [5, 14–16] which can be used to prepare powders or films.
The preparation method has an important role in determining the final properties of the products, because different ways to incorporate N lead to systems with dopant located in different positions of the TiO2 structure and with different activities [15, 17, 18].
N-TiO2 can be synthesized by chemical method if an appropriate N containing precursor is selected. This methodology is attractive because there is no expensive equipment involved, which helps to reduce costs for the massive synthesis of the product. Coprecipitation of TiO2 with urea followed by thermal treatment is one of the most popular procedures to prepare N-TiO2 [14, 19, 20]. However, the mechanism of doping is still not well understood and is a challenge for chemists to determine how TiO2 is doped by N under those conditions. Besides, the synthesis is sometimes difficult to reproduce; so a better understanding of the process is necessary in order to develop reliable synthesis.
In this paper, we present the results of studies designed to understand the mechanisms involved in the synthesis and in the determination of the photocatalytic activity of the materials under black and white light illumination.
2. Materials and Methods
2.1. N-TiO2 Synthesis and Characterization
The synthesis procedure is based on what was reported by Ohno [19]. Three solutions containing Ti-isopropoxide, Ti(C3H7O)4 (Aldrich), urea (Anedra PA), or water dissolved in absolute ethanol (Merck, PA) were prepared: solution (a) Ti(C3H7O)4 0.40 mol dm−3, solution (b) urea 0.80 mol dm−3, and solution (c) water 0.40 mol dm−3. Solution (a) was incorporated into solution (b) under stirring and mixed during 15 minutes. After the stirring period, solution (c) was slowly incorporated to the mixture under stirring. A white slurry was obtained, which was stirred for another 60 minutes. Finally, the slurry was evaporated under vacuum at 40°C until a white precursor powder, with a 4 : 1 N : Ti molar ratio, was obtained. In order to determine the effect of N doping on microstructure, precursor powders with N : Ti ratios 2 : 1 and 8 : 1 were prepared following the same protocol, but using solutions (b) with 0.40 and 1.6 mol dm−3 urea, respectively. The product was air-dried and fired under air at 175, 250, 375, 412, 450, or 500°C, during 15 min with a 10°C/min ramp. The precursor powders used in photocatalysis experiments and those with N : Ti ratios 2 : 1 and 8 : 1 were fired at 250°C for 3 h, heated to 500°C with a 10°C/min ramp, and immediately cooled down in the oven. Finally, they were washed with water to remove possible soluble impurities and dried at 60°C. As control, pure TiO2 was synthesized in a similar way, but without the incorporation of urea.
The evolution of the powder precursor during firing was followed by thermal gravimetric analysis (TGA; Shimadzu TG-50) and differential thermal analysis (DTA; Shimadzu DTA-50) under air atmosphere. The nature of the byproducts obtained at the different firing temperatures was determined by Fourier transformed infrared spectroscopy (FTIR, Thermo Nicolet 8700). The crystalline phases predominant at each temperature were determined by X-ray diffraction (XRD; Siemens D-5000) using the Cu Kα wavelength. The bandgap of the different samples was determined by reflectance spectroscopy (Shimadzu UV-3101PC), using BaSO4 as reference. Surface area and porosity of the powders were measured by N2 sorptometry, using the BET formalism (Micromeritics ASAP 2020 V3.00 H). The morphology of the particles of powders was determined by Field Emission Scanning Electron Microscopy (FEGSEM Zeiss LEO 982 GEMINI).
2.2. Photocatalytic Activity
The photocatalytic activity of the different photocatalysts was determined through the degradation of salicylic acid (Fluka, PA, USA) as model contaminant. Salicylic acid is a nice target because the presence of aromatic and phenol functionality, with negligible toxicity at the used concentrations M. A homemade photoreactor was used for all the experiments. A borosilicate glass cylindrical container (200 mL) was surrounded by four 12 W light tubes, symmetrically placed at 15 cm from the axis of the cylinder. The photocatalyst was incorporated to the salicylic acid solution and suspended with the application of ultrasound for 10 minutes. The final concentration of photocatalyst was 1.0 g/L. The suspension was magnetically stirred, and O2 was bubbled during all the experiment. The system was stirred in the dark during 30 minutes to reach adsorption equilibrium. The lamps were warmed up during 15 minutes. Once the reactor was under illumination, 5 mL samples were taken at regular periods. The samples were filtered through a 0.45 μm pore diameter polycarbonate membrane (Sartorius) and stored in glass containers at −20°C until analysis was performed.
The experiments run under UVA light were carried out using 12 W black light tubes, while for the experiments with visible light 12 W, white tubes were used. In the latter case, a 4 mm thick Plexiglas cylinder was placed surrounding the borosilicate glass reactor to eliminate all the emissions with wavelength lower than 400 nm. The intensity of light inside the photoreactor was determined with a radiometer at 360 nm, using a combination of two filters: WG 335 and UG 11. The radiance was calculated through: where P is luminic potency; A is photodiode area; E is photon energy (hν/360 nm); h is Planck’s constant; f is correction factor due to filter attenuation (f = 13.43).
The concentration of the remaining salicylic acid in the irradiated samples was determined by High-Performance Liquid Chromatography (HPLC), Shimadzu, equipped with an Econosphere C-18, mm, column, and UV-Vis detector. The solvent carrier was a 25 : 75 methanol:acetic acid (2% in water), with a flow rate of 1.4 mL/min. Salicylic acid was detected at 298 nm. To detect the presence of byproducts, standards of dihydroxybenzoic acid and hydroquinone were analyzed by HPLC with UV detection.
3. Results
Figures 1(a), 1(b) and, 1(c) show TGA and DTA plots of urea, TiO2 powder, and coprecipitated TiO2-urea powder, respectively. As shown in Figure 1(a), decomposition of pure urea starts after melting, characterized by a sharp endothermic peak at 133°C, with a steep loss of mass that involves at least two processes, as indicated by the endothermic peaks at 191 and 230°C. There is a small plateau in the 260–300°C range, followed by another steep loss of mass that ends at 360°C and corresponds with one endothermic process centered at 330°C. Finally, there is another endothermic process centered at 395°C, associated with a smooth loss of mass in the range 330–450°C. In the case of pure TiO2, synthesized as explained previously, the thermal behavior is much simpler. There is a steep loss of mass in the range 33–300°C that corresponds with a broad endothermic process that likely contains, at least, two superimposed processes centered at 56 and 110°C. The coprecipitated mix of urea and TiO2 displays similar behavior to pure TiO2. As shown in Figure 1(c), mass is lost from the beginning of the heating, with a steep loss at 115°C. The slope of the curve decreases as the temperature rises. DTA indicates that decomposition starts immediately after urea melting (133°C); probably the different processes observed in Figure 1(a) are superimposed in the peak centered at 150°C in Figure 1(c). It should be considered that the mass of urea is lower than in the experiment shown in Figure 1(a), because in the case of Figure 1(c), there is a mixture of urea and TiO2. Consequently, some processes that involve small losses of mass may be hidden.
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Figure 2 shows FTIR spectra of raw coprecipitated TiO2-urea and fired at different temperatures. As can be seen in the figure, the raw mixture displays the typical features of urea corresponding to N-H stretching (3450 and 3300 cm−1), C=O (1600 cm−1), and N-C (1340–1250 cm−1) [21]. After firing at different temperatures, new features can be characterized and others disappear. The more important changes take place in two ranges: 175–375°C and 375–500°C. In the first range, the typical peaks of urea are replaced by others typical of biuret (see also Figure 7), (1405 cm−1, 1330 cm−1, 1075 cm−1, and 1025 cm−1), cyanuric acid (1158 cm−1), ammelide, ammeline, and melanine with typical bands in the 1800–1300 cm−1 range among others (see [22] and references cited therein). There are also bands at 2048, 2195, and 2340 cm−1 that correspond with cyanides and cyanates; these bands increase as the other decrease with the temperature. In the range 412–500°C, only the bands typical of cyanates and cyanides remain in the spectra in the range 3500–1000 cm−1, and the bands corresponding to O-Ti-O stretching, in the range 400–600 cm−1, increase with the temperature.
Figure 3 shows X-Ray diffractograms of TiO2-urea coprecipitated samples fired at different temperatures. The raw sample displays features that correspond to crystalline urea. As the temperature increases, the peaks become less intense, until in the range 250–300°C the samples are noncrystalline. At 375°C, the sample is slightly crystalline, while at higher temperatures, it becomes crystalline. Anatase is the only crystalline phase present in the range 375–500°C. It is noteworthy that when the solid starts to crystallize, polynitrogenated cyclic compounds, such as ammelide, ammeline, and melanine are present on the TiO2 particles, as determined by FTIR (see Figure 2).
Figure 4 shows SEM images of pure TiO2 and N-TiO2 samples prepared from different N : Ti ratios. In all the cases, the particles that form the aggregates are nanometric, with an average size of approximately 10 nm. As the urea/Ti ratio increases, the morphology of the systems notably changes. The particles seem bigger and the porosity decreases. The N2 sorption experiments displayed in Table 1 agree with the SEM images. The surface area and the pore volume decrease as the urea/Ti ratio in the synthesis increases.
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Figure 5 shows the reflectance spectra and the band gap of TiO2, N-TiO2, and a commercial sample (Finnit by Kemira). The remission function was calculated from the reflectance data through the Kubelka-Munk function: where F(R) is the remission function and R is the measured reflectance.
The band-gap energy can be obtained by extrapolation to zero of a plot of (F(R) × E)1/2 versus E, where E is the energy of the incident light in eV. The band gap of the commercial sample corresponds with anatase (3.26 eV). The TiO2 synthesized in the laboratory has a band gap shifted to lower energies, compared with pure anatase. N-TiO2 displays a shoulder at 2.5–2.7 eV, which can be associated with the deep yellow color displayed by the N-doped samples.
Analysis of the surface chemical composition by XPS shown in Table 2 indicates the presence of N in all the studied samples. The presence of N in the bare TiO2 sample can be assigned to physisorbed N (400.9 eV). The concentration of N increases with the urea/Ti ratio. The binding energies can be assigned to substitutional N (396-397 eV), interstitial N (400 eV) [18], graphite-like phases: (400.6 eV, N-Csp2), or polycyanogen (399.0–400.5 eV, (-C=N-)x) [23], due to polycyclic nitrogenated compounds. Table 2 also shows that C is present in all the samples.
Photocatalytic activity of TiO2 and N-TiO2 was determined under UVA (360 nm) and visible light ( nm). When black lights were used, the photon flux at 360 nm inside the reactor was photons/(s cm2). In the case of white lamps, the 360 nm emission was removed by the Plexiglas filter; the photon flux at different wavelength was (400 nm), (436 nm), (492 nm), (547 nm), and photons/(s cm2) (579 nm). As the band-gap energy for TiO2 synthesized in the laboratory is 3.03 eV and for N-TiO2 is 2.38 eV, only the emissions with wavelengths shorter than 400 or 547 nm can be, respectively, absorbed by the photocatalysts.
Salicylic acid was used as target contaminant; Figure 6(a) shows the temporal evolution of the concentration when a M solution containing 1.0 g/L of catalyst is illuminated with black or white light. Under UVA illumination, N-TiO2 displayed much lower efficiency than TiO2 synthesized under similar conditions. Under visible light, N-TiO2 was slightly more efficient than TiO2 and, in both cases, the degradation rate decreases with the time. Figure 6(b) shows the concentration of dihydroxybenzoic acid (DHB) relative to the remnant salicylic acid at different illumination times. It is clear that the highest accumulation of DHB is observed with N-TiO2, followed by TiO2, in both cases under visible light. Direct photolysis of salicylic acid by UVA illumination, as shown in Figure 6(a), or visible light illumination (not shown) was negligible during the studied period. Adsorption of salicylic acid on TiO2 and N-TiO2 was 12–25% of the initial concentration.
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4. Discussion
The transformation of the coprecipitated urea-TiO2 system during thermal treatment starts after the melting of urea, as can be seen in Figures 1(c) and 2. The byproducts detected in samples fired at different temperatures are coincident with that reported by Schaber et al. [22]. The decomposition reaction begins with the condensation of urea yielding biuret. Figure 7 compares FTIR spectra of urea, biuret, and a sample fired at 175°C. Clearly, the sample contains both urea and biuret as shown by the characteristic peaks indicated with arrows “u” or “b,” respectively. The heating at higher temperatures produces further condensation leading to more complex polynitrogenated compounds as melamine. Cyanates and cyanides are also present in the fired samples. It can be noticed in Figure 2 that as the temperature increases, the intensity of the features corresponding to polynitrogenated compounds (1300–1800 cm−1) decreases in comparison with those of cyanates and cyanides (2048–2340 cm−1). As the firing temperature gets close to 500°C, all the features disappear and eventually the samples become white. The synthesis of polynitrogenated compounds on the surface of TiO2 can be a consequence of the condensation reaction as postulated by Mitoraj and Kisch [23, 24], which starts with the synthesis of melamine:
Further condensation of melamine leads to melam, melon, and so forth. All these compounds are dyes that withstand temperatures close to 400°C, which turn the N-TiO2 system yellow. The presence of these compounds or their firing byproducts may be responsible of the reduction of surface area in the N-TiO2 samples, as indicated in Table 1 and Figure 4.
Crystallization begins in the range 300–375°C, where polynitrogenated compounds are still present in the samples. N may be incorporated into the TiO2 structure as the samples crystallize with the increase of the firing temperature. Buha et al. reported that nitrides can be formed from oxides by thermal treatment in the presence of urea or cyanamide at 800 C [25]. The conversion to titanium nitride is total with 5 nm TiO2 particles, while for 20 nm particles titanium nitride and titanium oxide coexist. The particle size of the systems studied in this work is close to 20 nm; so based on the previous report, the substitution of oxides by nitrides seems to be possible.
The incorporation of N into the TiO2 structure might explain the shift in the band gap determined by diffuse reflectance (Figure 5). Carbon can also act as a dopant, helping in the shift of the band gap to lower energy. It should be noticed that in the TiO2 sample synthesized following a similar procedure as for N-TiO2 the band gap is lower than in pure anatase. Carbon doping can be the reason of the band-gap shift in this case. Figure 5 shows a shoulder in the visible range, which indicates the presence of surface or intraband-gap states in the samples. Mitoraj et al. [26] proposed that nitrogen-carbon-doped TiO2 (N,C-TiO2) displays an energy band-like structure of intraband-gap states, while N-TiO2 has a manifold of discrete levels. Hole (h+) stabilization by charge delocalization is more likely in N,C-TiO2. Our results do not allow to discriminate between doping or sensitization produced by the polynitrogenated dyes, but it is clear that both effects are related with the band-gap shift and the shoulder observed by diffuse reflectance in the visible range.
The photocatalytic activity of TiO2 is notably affected by doping as shown in Figure 6. N-TiO2 under UVA light displays the lowest activity for the degradation of salicylic acid, while TiO2 displays the highest. The presence of surface states may enhance hole electron recombination in N-TiO2 leading to low activity. When visible light was used in the experiments, degradation of salicylic acid was observed with both photocatalysts. The degradation rate notably decreases after 250 minutes and almost stops completely in the case of N-TiO2. Figure 6(b) shows that DHB acid was detected in these experiments and that its concentration, relative to the remnant salicylic concentration, was higher than in other cases. When the system is illuminated with visible light, the polynitrogenated compounds inject electrons into the TiO2 conducting band, generating excited dye radicals on the surface. Górska et al. proposed that in N,C-TiO2, superoxide radicals or direct charge transfer to the adsorbed organic are involved in degradation paths [27]. The oxidation power of these species is lower than that of OH• or h+, leading to less oxidized byproducts which can poisoned the surface of the catalyst.
The degradation of salicylic acid on TiO2 can be a consequence of C doping, which shifts the band gap to 3.03 eV, allowing the absorption of light with 400 nm wavelength. The light source has a moderate emission at 400 nm which can be used to activate the catalyst. However, another effect should be considered. Salicylic acid can be absorbed on the TiO2 surface in the form of different surface complexes [28]. These complexes display an absorption band in the visible range and can act as sensitizers of TiO2 [29], leading to the self-degradation of salicylic acid. Both phenomena may be responsible for the target degradation, and it is not possible to discriminate which is the most important.
5. Conclusions
Byproducts generated during urea thermal decomposition are key in the synthesis of N-TiO2. Some byproducts, such as biuret, might be used as precursors themselves. Sol-gel synthesis with urea as N-precursor leads to the production of crystalline N-TiO2 sensitized with polycyclic nitrogen compounds at firing temperatures lower than 400 C. At firing temperatures higher than 400 C, N doping is related with the presence of simpler nitrogenated species or N incorporation. N-TiO2 photocatalytic activity is higher under vis light than under UVA illumination. Accumulation of byproducts was detected in TiO2 and N-TiO2 systems illuminated with white light. Target compounds chemisorbed on TiO2 surface can be degraded by vis light. Both phenomena may by superimposed in N-TiO2 photocatalysis.
Acknowledgments
The authors acknowledge funding given by University of Buenos Aires (Projects X411 and 20020090100297) and University of San Martín (UNSAM SA08/011). R. Candal is Member of CONICET. CONICET fellowship given to L. Guz is gratefully thanked. They thank Dr. L. Tribe for her help with English.