Nanostructured titanium dioxide (TiO2) is widely used as a photocatalyst for the degradation of environmental pollutants in water and air. The physicochemical properties of TiO2, such as thermal and chemical stability, relatively high photocatalytic activity, low toxicity and low cost make TiO2 the most interesting photocatalyst for environmental remediation . The anatase phase has been applied as an excellent photocatalyst in purification . This process is accomplished by activation of photocatalyst using ultraviolet or visible light to produce primarily hydroxyl and superoxide radicals which are the active sites on TiO2 surfaces for oxidizing organic compounds and antibacterial to water vapor and carbon dioxide . TiO2 with a wide band gap about 3.2 eV needs to UV irradiation for degradation of contaminants which constitute 4% of the sunlight . Many research groups have studied the synthesis of particular TiO2 that can effectively be activated by visible light, the major portion of solar light. Reports demonstrate that doping of TiO2 with various transition metal and nonmetal can be shifted the optical absorption edge of TiO2 from UV range to visible range [5-7]. Recently, intensive attempts have been directed to improve the photocatalytic treatment of TiO2 under visible light using metal ions (such as Fe, Co, Ag, Ni) [8-10] and nonmetal elements (e.g. C, N, F, S) [11,12]. Nitrogen can be inserted into TiO2 crystal lattice. Nitrogen is more attractive than all other anionic elements due to its closer atomic size to oxygen, small ionization energy, metastable center formation and stability [13,14]. Some investigations have shown that slightly depositing of noble metals on TiO2 surface can effectively capture the photo-induced electrons or holes, omit the recombination of electron–hole pairs and also extend the light response of TiO2 in the visible light region [15-17]. It is also known that loading of silver nanoparticles can boost the photocatalytic activity of TiO2. The enhancement is ascribed to the fact that the loading of silver makes the formation of Schottky barriers at each Ag@TiO2 contact regions, thus promoting charge separation and preventing the recombination of electron–hole pairs and leaving holes in the valence band of TiO2 [18, 19].
In this work, firstly nanocrystalline nitrogen-doped TiO2 (N/TiO2) was successfully synthesized through the sol-gel method, subsequently N/TiO2 has been coated with silver metal through the photochemical method. The photocatalytic activity of N/TiO2 nanoparticles was plenty enhanced by decreasing the Ag@N/TiO2 band gap. Then, the photocatalytic activity of the Ag@N/TiO2 catalyst was tested for the removal of methyl orange dye under UV and visible light irradiation.
Materials and methods
All the chemicals were purchased from Merck and were used without any further purification. Tetraisopropyl orthotitanate (TTIP, C12H28O4Ti, MW=284.25 g/mole, d=0.96 g/mL) was used as TiO2 source, Triethylamine (N(CH2CH3)3 MW=101.19 g/mole, d=0.7255 g/mL) was used as Nitrogen precursor and Silver nitrate (AgNO3 Mw=169.87 g/mole, d=4.35 g/mL) was used as Silver precursor. Deionized water was obtained from ultra-pure water system (type smart-2-pure TKA, Germany). Methyl Orange (MO, M.W. = 695.58 g mole−1) dye was provided by Alvan Co., Iran.
XRD patterns have been recorded from a diffractometer of Philips Company with X’pert pro filtered by Cu Kα radiation (λ = 1.54 Å). The diffractgrams have been recorded in the 2θ range of 10-80º. The morphology and size of nanoparticles have been characterized using scanning electron microscope (SEM) (Philips XL-30ESM) equipped with an energy dispersive X-ray (EDX). The diffuse reflectance UV-Vis spectra (DRS) of the samples have been recorded by an Ava Spec-2048TEC spectrometer. FT-IR spectra of the samples have been recorded on a Nicolet Magna IR 550 spectrometer. The extent of MO degradation was monitored using UV-Vis spectrophotometer (Perkin Elmer Lambda2S).
The mole ratio of N/TiO2 in preparation of Ag@N/TiO2 was 2:1 for N/TiO2. The N/TiO2 prepared by sol–gel method. The products were synthesized using TTIP as TiO2 source and triethylamine as N sources. The preparation process was as follow: TTIP, ethanol and acetic acid with the mole ratio of 1:2:2 were mixed together and the mixed solution was stirred for 3 h. After that, the nitrogen source, deionized water and PVP with the molar ratio of 2:10:1 were mixed together then added to the first mixed solution. The resultant solution was kept under continuous stirring for 2 h and then under irradiation with a high intensity ultrasonic of 20 kHz in a sonication cell for 15 min, result is the formation of a transparent solution of TiO2 sol. The prepared 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 deformed into fine powder and calcined in a muffle furnace at 500 °C for 2 h. This precursor has been dispersed in deionized water by ultrasonic irradiation. Then, Required amount of Ag salt (for covering of N/TiO2) have been added to solution and stirred for 30 min and for prevent of oxidation of metals the solutions have been deoxygenated and irradiated for 18 h by UV light. The result solution has been dried in oven on 120 ˚C.
Photocatalytic decomposition of methyl orange
Photocatalytic activity of N/TiO2 and Ag@N/TiO2 nanoparticles was evaluated by the decomposition of MO solution under UV and visible light irradiation. The degradation reaction was performed in a quartz photocatalytic reactor. The photocatalytic degradation was carried out with 100 mL aqueous MO solution (10 mg L-1) containing 100 mg of catalyst nanoparticles. This mixture was aerated for 30 min to reach adsorption equivalency. Then, the mixture was placed inside the photoreactor in which the vessel was 40 cm away from the UV and 25 cm away from visible lamps. The quartz vessel and light source were placed inside a black box equipped with a fan to prevent UV leakage. The experiments were accomplished at room temperature and the pH of MO solution was adjusted to 2-3 . Aliquots of the mixture were taken at periodic pauses during the irradiation and after centrifugation they were analyzed with the UV–Vis spectrometer.
Results and discussion
Fig. 1 shows the X-ray diffraction patterns of the Ag@N/TiO2 with different weight percentages of Ag. In these patterns have not been observed any peaks of Ag and all of diffraction peaks have been closed to TiO2 nanocrystals which shows that Ag ions uniformly have been dispersed among the anatase crystallites. The nanocrystalline anatase structure was conﬁrmed by (1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 1 1) diffraction peaks . Characteristic peaks of anatase were observed at 2θ values 25.620, 38.10, 48.330, 54.200, 55.330, 62.030, 69.10, 70.590, and 75.410 that illustrate the structure of supplied doped-TiO2 was anatase (JCPDS no. 36-1451). The crystallite size measurements were done using the Scherrer equation: Dc=0.9λ/βcosθ where β is the width at half maximum intensity of the observed diffraction peak, and λ is the X-ray wavelength (Cu Kα radiation, 0.154 nm) . The estimated crystallite size is about 37 nm.
The FT-IR spectra of 1.5 weight percentage (Wt%) of Ag covered N/TiO2 (1.5%Ag@N/TiO2) have been illustrated in Fig. 2. The broad intense band below 1200 cm-1 is due to Ti-O-Ti vibrations. Two absorption peaks in this spectrum have been corresponded to stretching vibrations of the O-H and bending vibrations of the adsorbed water molecules around 3350-3450 cm-1 and 1620-1635 cm-1, respectively. In synthesis procedure for preparation of TiO2 a lot of deionized water has been applied to enhance the nucleophilic attack to Ti precursors and caused to a super-fast condensation and preparing of TiO2. On the other hand, presence of remained alkoxy groups could be decreased the rate of formation of less dense anatase phase .
SEM and EDS analysis
Fig. 3 shows SEM image of 1.5% Wt Ag coated on N/TiO2 catalyst. It is clear that 1.5% Wt Ag coated on N/TiO2 is composed of big particles which are aggregation of small and relatively uniform particles with diameter from 40 to 60 nm.
The EDS data of 1.5% Wt Ag coated on N/TiO2 sample (Fig. 4) 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, oxygen and silver are clearly distinct at 0.3, 0.6 and 4 keV, respectively. These results confirm the existence of Ti, O, N, and Ag atoms in the catalyst structure.
UV-Vis Diffuse Reflectance Spectroscopy (DRS)
The electronic bands of the different titania samples were studied and corresponding spectra are provided in Fig. 5. The absorption spectrum of TiO2 consists of a single broad intense absorption around 400 nm due to the charge-transfer from the valence band to the conduction band . As shown in Fig. 6, covering of N/TiO2 by different amount of Ag causes the red shift of the absorption peaks to higher wavelength (visible region. The band gap of Ag@N/TiO2 has changed from 3.24 eV in N/TiO2 to 1.9 eV for 1.5%Ag@N/TiO2 sample that shown in Fig. 6.
Degradation of MO under UV and visible irradiation has been followed by UV-Vis spectroscopy and the results have been represented in Fig. 7 and Fig. 8. The degradation of methyl orange under visible and UV light by N/TiO2 nanoparticles shows 74% degradation after 240 min and 86.3% after 30 min, respectively . Fig. 7 and Fig. 8 has been shown the photocatalytic behavior of Ag covered to N/TiO2 with different percentage of Ag. After covering of N/TiO2 with metal the recombination of electron and hole-pair has been reduced and the photocatalytic behavior has meliorated. The photocatalytic behavior in covering all percentage have been increased.but, the best photocatalytic activity has been observed for 1.5% of Ag that shows 89% degradation after 240 min of visible light irradiation and 99% after 30 min of UV light irradiation.
N/TiO2 and Ag@N/TiO2 nanoparticles were prepared by the sol-gel and photochemical method, respectively. From all of the samples only anatase phase was confirmed from the XRD results. From the XRD, SEM-EDX, UV-Vis and FT-IR results, it was confirmed that the incorporation of Ag in N/TiO2 decreases the grain size, shifts the absorption peaks to higher wavelengths (red shift) and lowers the surface area due to agglomeration of the particles. The photocatalytic degradation of MO under UV and visible irradiation revealed higher activity in the presence of the Ag coated on N/TiO2 than the N/TiO2. Among the Ag coated on N/TiO2 samples, the 1.5% Ag@N/TiO2 catalyst exhibited the highest photocatalytic activity, while under visible and UV irradiation.
Conflict of interest
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.
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