One of the most promising photocatalysts for the degradation of organic pollutants is TiO2 that has attracted significant attention. This material is biologically and chemically inert, mechanically robust, nontoxic, cheap, environmentally friendly and therefore a perfect candidate for wide scale applications with high efficiency [1-4]. Furthermore, TiO2 nanoparticles can be excited by photons to produce electron-hole pairs for photocatalytic activation, if the photons energy provides enough energy for the TiO2 band gap (3.0 for rutile and 3.2 eV for anatase). The anatase TiO2 has higher photocatalytic activity than rutile TiO2 [4-6]. They require near ultraviolet irradiation (λIn this work, nitrogen-doped nanocrystalline TiO2 was successfully synthesized through the sol-gel method. The photocatalytic activity of TiO2 nanoparticles were greatly enhanced by decreasing the N/TiO2 band gap. Organic compounds were used as nitrogen sources such as triethylamine, N,N,N’,N’–tetramethylethane–1,2-diamine, ethyldiamine, 1,2-phenylenediamine, propanolamine and propylenediamine. At the same time, the catalytic activity of the N/TiO2 catalyst is compared with the pure anatase TiO2. The effects of various nitrogen sources on the photocatalytic properties of products were investigated. Then, the photocatalytic activity of the N/TiO2 catalyst was tested for the removal of methyl orange dye under UV and visible light irradiation. Through the comparison of the photocatalytic activities of the N/TiO2 with different nitrogen sources, the best source was optimized.
MATERIALS AND METHODS
The tetraisopropyl orthotitanate (TTIP), triethylamine, N,N,N’,N’–tetramethylethane-1,2-diamine, ethyldiamine, 1,2-phenylenediamine, propanolamine, propylenediamine, acetic acid, ethanol and polyvinylpyrrolidone (PVP) were purchased from Merck and used without any further purification. Deionized water was prepared by a pure water system (Smart-2-Pure, TKA Co., Germany). Methyl orange (MO, M.W. = 695.58 g mol−1) dye was provided by Alvan Co., Iran.
Synthesis of N-doped TiO2 nanoparticles
Part 1. The pure anatase phase of TiO2 and the N-doped TiO2 were prepared by sol–gel method. The products were synthesized using one of the following compounds as N source: triethylamine, N,N,N’,N’–tetramethylethane,1,2-diamine, ethyldiamine, 1,2-phenylenediamine, propanolamine or propylenediamine. Also, TTIP was used as TiO2 source and acetic acid was applied as catalyst. The preparation process was as follow: TTIP, ethanol and acetic acid with mole ration of 1:2:2 were mixed together and the mixed solution was stirred for 3 h. The pH of the mixture was adjusted to about 3.0 using acetic acid to prevent the formation of TiO2 in this step. After that, the nitrogen source, deionized water and PVP with molar ratio of 2:10:1 were mixed together and added slowly to the first mixed solution. The resultant solution was kept under continuous stirring for a further 2 h for perform hydrolysis reaction, then dispersed under irradiation with a high intensity ultrasonic of 20 kHz in a sonication cell for 15 min, outcome is the formation of a transparent solution of TiO2 sol. The prepared light-yellow solution was kept for 24 h in the dark for nucleation process. After this period, the gel was dried at 100 °C and subsequently the catalyst was crushed into fine powder and calcined in a muffle furnace at 500 °C for 2.0 h. The nanosized N-doped TiO2 powders were obtained after adequate rubbing.
Part 2. As in Part 1, but here for the nitrogen source, various certain amount of triethylamine (the molar ratio of N:TiO2 is 0.1, 0.5, 1.0, 1.5, 2.0 and 2.5) were used.
Characterization of photocatalysts
X-ray diffraction (XRD) patterns were recorded by a Philips-X’PertPro, X-ray diffractometer using Ni-filtered Cu Kα radiation at scan range of 10<2θ
Photocatalytic decomposition of methyl orange
Photocatalytic activity of un-doped and N/TiO2 nanoparticles was evaluated by the decomposition of MO solution in water under UV and visible light irradiation. In each experiment, the recyclable photocatalyst (0.1 g) was added into 100 mL of methyl orange aqueous solution (pH =2-3) with 11 mg/L concentration. The decomposition of MO was performed in a glass vessel with a diameter of 10 cm. A Xenon lamp (500 W) was hanged perpendicularly to the glass vessel as light source. A quartz filter was placed between lamp and vessel that allow to pass the wavelength
RESULTS AND DISCUSSION
Fig. 1 shows the FT-IR spectra of pure TiO2 and N/TiO2 powders obtained from various nitrogen sources. The FT-IR spectra of the N/TiO2 catalyst show a strong peak at 3000–3700 cm−1 and narrow band at 1628 cm−1 that assign to the O-H stretching and H-O-H bending vibrations of adsorbed water molecules. The peaks observed in 1384, 1163 and 1019 cm-1 are typical of N–O stretching and O–N–O bending vibrations, respectively [43, 44]. Additionally, the peak at 514 cm-1 for N/TiO2 resulted from Ti–O–Ti bending vibrations, being red-shifted compared with the peak for TiO2 at 539 cm-1 and the peak at 653–550 cm-1 is ascribed to the Ti–O stretching vibration [36, 45].
XRD patterns for pure TiO2 and N/TiO2 nanoparticles obtained from various nitrogen sources, are shown in Fig. 2. In all the XRD patterns, TiO2 anatase diffraction lines could be seen and no other crystal phase could be detected. Characteristic peaks of anatase (2θ = 25.2, 37.76, 48.02, 54.05, 55.03, 62.80, 68.85, 70.19, and 75.07) can be associated with (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes respectively, were observed. This indicates that the produced doped TiO2 and undoped TiO2 are in anatase phase (JCPDS no. 36-1451). It could also be seen from the XRD patterns that N/TiO2 has the broader peaks compared to pure TiO2. This means smaller crystallite size, according to Scherrer equation, and thus increase the photocatalytic activity of N/TiO2 . Generally, crystallite growth in TiO2 is considerably affected by the dopant .
The photocatalytic degradation of MO under UV and visible lights were measured for pure and N/TiO2 samples. Fig. 3 clearly shows photodegradation of MO by N/TiO2 versus time under UV and visible irradiation. According to Fig. 3, the results show that the type of used nitrogen source is effective in photocatalytic activity of synthesized N/TiO2 in the degradation of MO dye. All N/TiO2 samples show better catalytic activity compared to pure TiO2, except one that was used from 1,2-phenylenediamine as nitrogen source (sample 2). The degradation percent for N/TiO2 samples (except sample 2) against 240 minutes visible irradiation or 30 minutes UV light irradiation were 57 to 76 and 72 to 87 percent. The photocatalytic activity of N doped TiO2 samples was best when used triethylamine as source of nitrogen. In the next stage, the results of different ratio of triethylamine were studied.
Fig. 4 and Fig. 5 show the FT-IR spectra and XRD patterns of pure TiO2 and N/TiO2 powders from different mole ratio of trimethylamine, respectively. The average crystallite size of pure TiO2 and N/TiO2 were calculated using the Scherrer equation. Pure TiO2 had a particle size of 16.3 nm; doped TiO2, about 13.2 nm. N/TiO2 powders showed smaller size than pure TiO2 prepared at the same calcinations temperature. In general, crystallite growth in TiO2 is considerably affected by the dopant. This smaller crystallite size enhanced the photocatalytic activity of N/TiO2.
The surface morphology of the N-doped TiO2 nanoparticles have been investigated by SEM, respectively, shown in Fig. 6. It is observed that the N/TiO2:2.0 is composed of large quantity of relatively uniform particles with diameter from 20 to 30 nm, which indicates sample could have good dispersion in solution. As it can be seen in Fig. 6, the larger particles are obtained by the aggregation of smaller particles.
The EDS data of N/TiO2:2.0 sample (Fig. 7) shows a peak around 0.4 and 0.5 keV and another intense peak appears at 4.5 and 4.9 keV for Ti . The peaks due to nitrogen and oxygen are clearly distinct at 0.3 and 0.6 keV, respectively. These results confirm that Ti, O, N exist in the catalyst structure.
Fig. 8(a) shows the DRS spectra of the pure TiO2 and N/TiO2 samples. The pure TiO2 shows absorption only in the UV region. The optical absorption of the N/TiO2 samples was extended to the visible region. It is noted that the band gap was expanded from 380 to 480 nm upon N doping. Noticeable shifts of the absorbance shoulder from a wavelength below 400 nm to the visible light region were observed for the N/TiO2. The main absorption edges of the N/TiO2 change significantly compared to that of the un-doped sample. It is likely that nitrogen doping creates a new N 2p state slightly above the valence band top consists of O 2p state, and this pushes up the valence band top and leads to visible light response as a consequence .
In addition, the bandgap of the titania was determined from the Eq. (1) 
where A is a constant, hν is the photon energy, Eg is the optical energy gap of the material and γ is characteristic of the optical transition process, which is equal to 2.0 for an indirect allowed optical transition of an amorphous semiconductor. The bandgap of N/TiO2 has changed from 3.38 eV (pure TiO2) to 3.26 eV for N/TiO2:2.0 sample is shown in Fig. 8(b). The first bandgap reflects the effect of N-doping on the main band edges of the oxide. The second gap, which is narrower than the original value, suggests that nitrogen doping contributed to the red shift of the bandgap . Accordingly, it can be presumed that the N/TiO2 sample may exhibit high photocatalytic activity under visible irradiation.
The photocatalytic degradation of MO under UV and visible lights were measured for pure and N doped TiO2 samples (triethylamine as source of nitrogen). The results in Fig. 9 clearly show that, under the irradiation of UV and visible, the photocatalytic performance of anatase TiO2 is greatly improved with the doping of N. The photocatalytic activity of N doped TiO2 sample that triethylamine was source of nitrogen shows comparable activity to that of pure TiO2 and the samples under UV light with increase the nitrogen concentration in the degradation of MO dye was more, resulting in a degradation of N/TiO2:2.0 sample was 87% and selected as the optimum sample. Also, the samples under visible light with increase the nitrogen concentration in the degradation of MO dye was more, finally in a degradation of N/TiO2:2.0 sample was 76.4% and was selected as the optimum sample.
The higher photocatalytic activity of N/TiO2 than pure TiO2 under UV and visible irradiation may be due to the substitution of nitrogen for oxygen atoms in the crystal structure of TiO2 that improves the visible light sensitivity by introducing a mid-gap (N 2p) level, which formed slightly above the top of the (O 2p) valence band .
In summary, in this work N/titania nano-photocatalyst with a spherical shape was prepared using sources of nitrogen such as N,N,N’,N’-tetramethylethane-1,2-diamine, ethyldiamine, 1,2-phenylenediamine, propanolamine, propylenediamine (the molar ratio of 2:1). After that, using triethylamine (with difference molar ratios) as the optimize nitrogen source, by the sol–gel method. The doping mechanism was explained by XRD, FT-IR, EDS, SEM, and UV–visible absorption analyses. The photocatalytic performance of pure TiO2 was greatly improved by the N-doping. The nitrogen doping had predominant effects on the improvement of the photocatalytic activity: On the other hand, it could limit the band gap of titania and extends its absorption to the visible light region, furthermore, it could increase the separation efficiency of the photoinduced electron and hole. The prepared N/TiO2:2.0 shows a great potential as a catalyst for photocatalytic applications.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.