Microwave-Assisted Synthesis of Carbon-Based (N, Fe)-Codoped TiO2 for the Photocatalytic Degradation of Formaldehyde
© Tian et al. 2015
Received: 2 August 2015
Accepted: 28 August 2015
Published: 16 September 2015
A microwave-assisted sol–gel method was used to synthesize (N, Fe)-codoped activated carbon (AC)/TiO2 photocatalyst for enhanced optical absorption in the visible light region. The prepared samples were characterized via X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller analysis, ultraviolet–visible light spectroscopy, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy. The results showed no significant difference in the surface area of AC/TiO2 (approximately 500 m2/g) after doping. TiO2 was uniformly distributed on the surface of AC, which exhibited coexisting anatase and rutile structures with a mean crystallite diameter of approximately 20 nm. N and Fe monodoping on AC/TiO2 reduced the energy band gap of TiO2 to 2.81 and 2.79 eV, respectively, which mainly attributed to the impurity energy formed in the energy gap of TiO2. In (N, Fe)-codoped AC/TiO2, N and Fe are incorporated into the TiO2 framework and narrow the band gap of TiO2 to 2.58 eV, thereby causing a large redshift. Codoping of N and Fe enhanced the production of hydroxyl radicals (⋅OH) and improved the photocatalytic activity of the resultant AC/TiO2 compared with those of undoped and N- or Fe-monodoped AC/TiO2. N-Fe-AC/TiO2 degraded 93 % of the formaldehyde under Xe-lamp irradiation. Moreover, the photocatalyst was easily recyclable. In summary, a novel and efficient method to mineralize low concentrations of HCHO in wastewater was discovered.
KeywordsPhotocatalytic degradation Microwave irradiation Active carbon Codoped AC/TiO2 Formaldehyde
Formaldehyde (HCHO) is a volatile organic compound that irritates the respiratory, cardiovascular, and nerve tissues of humans . Thus, HCHO removal is essential for improving environmental quality. HCHO photodegradation in the presence of a titanium dioxide (TiO2) photocatalyst completely degrades HCHO into CO2 and H2O . Unfortunately, the photocatalytic activity of TiO2 is limited by its low adsorption property and large band gap (3.2 eV) [3, 4].
Significant efforts have been made to overcome these two drawbacks. Preparation of TiO2 photocatalysts loaded with porous materials and characterization of their photocatalytic performance have drawn great research attention . Combination of the distinctive properties of mesoporous carbon materials and TiO2 evidently improves optical absorption and has been adopted in the removal of organic contaminants . Pastravanu et al.  reported that 92 % conversion of methyl orange is achieved after 170 min of ultraviolet (UV) irradiation with AC/TiO2 composite, whereas only 42 % conversion is achieved with pure TiO2. However, in our previous studies , the degradation efficiency of AC/TiO2 (1 g) was only 36 % under UV irradiation for 420 min at a low concentration of HCHO (30 mg/L). Therefore, it is important to improve the photocatalytic performance of TiO2 by improving its internal structure. Metal or nonmetal doping of TiO2 could extend its optical absorption range into the visible light region and modify the generation rate of the electron–hole pairs [9–12]. TiO2 doped with transition metals has recently been prepared, and studies on these materials have shown that the energy band gap of TiO2 decreases with decreasing recombination rate of photogenerated electron–hole pairs [13–15]. Iron is considered one of the most appropriate transition metals for TiO2 doping because the atomic radius of Fe3+ is close to that of Ti4+, thus, the titanium positions in the TiO2 lattice can be easily replaced by iron cations [16–18], remarkably improving photocatalytic efficiency. The results of Safari et al.  showed significant improvements in the photodegradation of Reactive Orange 16 by Fe-doped TiO2 (nearly 93 %) compared with that by pure TiO2 (approximately 71 %) under UV irradiation. Doping TiO2 with nonmetallic anions, such as N, S, and C, has been proposed as a promising method for extending photoresponses from the UV to the visible light regions [20, 21]. Among them, N-doped TiO2 has been demonstrated to be the most effective in narrowing the band gap and increasing photocatalytic activity in the visible light region . Several papers have reported the effects of Fe and N modification of TiO2 in enhancing photocatalytic activity [23, 24]. The photocatalytic activities of these powders are approximately two to four times higher than that of pure anatase TiO2 under visible light irradiation. In addition, using microwave irradiation to synthesize TiO2 nanoparticles is a recent innovation . Compared with conventional methods, microwave irradiation presents several advantages in terms of cleanliness, short reaction times, and energy economy . Because very few reports on the microwave synthesis of (N, Fe)-codoped TiO2 photocatalysts coated on AC (N-Fe-AC/TiO2) are available, the objective of the present work is to develop a rapid method to prepare N-Fe-AC/TiO2 and investigate its photocatalytic effect on HCHO under visible light irradiation.
The present work focuses on the synthesis of N-Fe-AC/TiO2 prepared using microwave irradiation and its structural characterization. The characteristics of the photocatalysts have been analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), Fourier transform infrared (FTIR), Brunauer–Emmett–Teller analysis (BET), ultraviolet and visible spectroscopy (UV–vis), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL). Studies on the use of catalysts for photodegradation of HCHO in aqueous solution under visible light irradiation are in progress.
AC was prepared based on the precious study . A mixture of solid KOH (10 g) and dried coal at a ratio of 1:1 was placed in a quartzose tube in a microwave reactor and activated under vacuum atmosphere at 693 W for 10 min. The obtained AC samples were pretreated by adding into HNO3 solution with 24 h. The mixture was filtered using distilled water until they became neutral. The pretreated AC was then dried and stored until use.
The TiO2 gel/sol was obtained by conventional sol–gel method. All reagents were of analytical grade and used without further purification. In typical synthesis process, 30 mL of tetrabutyl orthotitanate (TBOT) was dissolved in anhydrous alcohol (EtOH) in proportion of 1:1 (volume ratio). This solution was thoroughly stirred for 40 min and named solution A. Solution B was prepared by mixing 14 mL of glacial acetic acid and 7 mL of distilled water in 35 mL of absolute alcohol. Solution B was added to solution A dropwise and continuously stirred for 1 h. Then, it was obtained pale yellow clear TiO2 sol. Pretreated AC (10 g) was added into TiO2 sol (100 g). The mixture was placed in an oven at 100 °C for 24 h. After solidification, AC/TiO2 was prepared under microwave irradiation at 700 W for 15 min. To prepare Fe-doped AC/TiO2, Fe(NO)3⋅9H2O was mixed with solution B, while for N-doped AC/TiO2, urea was dissolved in solution B. The dosage of Fe iron was 0.008, 0.01, and 0.012 g, and the resulted samples were noted as 0.008 Fe-AC/TiO2, 0.01 Fe-AC/TiO2, and 0.012 Fe-AC/TiO2, respectively. The dosage of N was 0.2, 0.4, and 0.6 g, and the resulted samples were noted as 0.2 N-AC/TiO2, 0.4 N-AC/TiO2, and 0.6 N-AC/TiO2, respectively. Optimum concentrations of N and Fe were obtained by maximizing the photocatalytic activity for the monodoped (Fe or N) AC/TiO2. These optimized concentrations were used for synthesizing (N, Fe)-codoped AC/TiO2.
The crystal structures of the prepared samples were measured through XRD on a Rigaku D/Max-2500/PC powder diffractometer. Each sample powder was scanned using Cu-Kα radiation with an operating voltage of 40 kV and an operating current of 200 mA. The surface micromorphologies of photocatalysts were characterized through SEM (S4800, Hitachi LTD) at an accelerating voltage of 15 kV. TEM was performed on a Tecnai G2 F20 microscope at 100 kV. FTIR spectra were recorded with a Bruker Vertex FTIR spectrometer, resolution of 2 cm−1, in the range of 4000–400 cm−1 by KBr pellet technique. The UV–vis DRS were obtained with a powder UV–vis spectrophotometer (U-4100, Hitachi LTD). Specific surface area (SBET, m2 · g−1) was calculated using the BET equation, and total pore volume (V t , m3 · g−1) was evaluated by converting the adsorption amount at P/P 0 = 0.95 to the volume of liquid adsorbate. XPS analysis of samples was conducted using a PHI5700 ESCA system equipped with a Mg Kα X-ray source (1253.6 eV) under a vacuum pressure <10−6 Pa. The formation rate of ⋅OH at photo-illuminated sample/water interface was detected by the PL technique using terephthalic acid (TA) as a probe molecule. PL spectroscopy of synthesized products was taken at room temperature on a Hitachi F2500 spectrofluorometer using a Xe lamp with an excitation wavelength of 325 nm.
where C o and C t are the concentrations of HCHO at initial and different irradiation times, respectively.
Powders with different dopant concentrations
Anatase size (nm)
Rutile size (nm)
Ratio of A and R %
Physicochemical properties of undoped, mono-doped and co-doped AC/TiO2 powders
Anatase size (nm)
Rutile size (nm)
Band gap (eV)
Ratio of A and R %
0.4 N-0.01 Fe-AC/TiO2
Several factors may result in the high generation rate of ⋅OH radicals and photocatalytic activity. First, codoped AC/TiO2 contains anatase (57 %) and rutile (43 %) phases, which reduce the recombination of photogenerated electrons and holes and enhance the formation rate of ⋅OH . Electrons from the VB can be excited and moved to the CB of TiO2 by photon absorption. In undoped AC/TiO2, the band gap energy between the VB and CB of TiO2 is 3.86 eV. Fe ion occupancy in the Ti sites of the TiO2 lattice of Fe-AC/TiO2 can be seen in the XPS results shown in Fig. 9. When the catalysts were subjected to solar irradiation, the 3d electrons of Fe3+ were excited into the CB of TiO2, which introduces a new energy level . Electrons can be excited in two stages; the first stage involves electron excitation from lower new states to the CB photon adsorption and the second stage involves excitation from the VB to the lower Fe3+ states. Thus, an interaction among the d electrons of Fe and the TiO2 CB or VB occurs, eventually narrowing the energy gap of TiO2 through the formation of new intermediate energy levels. In N-AC/TiO2, the Ti–N linkage (Fig. 8) is believed to lead to the formation of the N1s peak (Fig. 9), which is due to substitutional N doping in the TiO2 lattice. Doping of N into TiO2 forms a new state on N1s just above the O2p VB, leading to the strong absorption of visible light and enhancing the separation efficiency of photoinduced electrons and holes. Fe3+ and N doping can suppress the recombination rate of electron–hole pairs and improve the photocatalytic activity of the resultant catalysts. Therefore, the cooperation of Fe3+ and N induces the formation of new energy levels close to the CB and VB, respectively, leading to a much narrower band gap and greatly improved photocatalytic activity.
N-Fe-AC/TiO2 photocatalyst was successfully synthesized through an efficient and rapid microwave-assisted sol–gel method. The sphere-like TiO2 showed anatase and rutile phases with a particle size of 20 nm in codoped AC/TiO2, which features a specific surface area of 550 m2/g. N and Fe ions occupied the TiO2 lattice, replacing some Ti4+ and O2−, respectively, and extending the absorption range of the catalyst to the visible light region. The N-Fe-AC/TiO2 photocatalyst exhibited better photocatalytic activity than undoped and Fe/N-monodoped AC/TiO2, which degraded 93 % of HCHO within 120 min under Xe-lamp irradiation. N-Fe codoping may have induced the formation of new states between the VB and CB. Moreover, N-Fe codoping can promote the separation of photogenerated electrons and holes to accelerate the transmission of photocurrent carriers. The produced photocatalyst can be easily recycled, which reveals its enhanced stability. These results suggest that the prepared (N, Fe)-codoped AC/TiO2 exhibits the characteristics of a highly effective photocatalyst under visible light irradiation.
This work was supported financially by funding from the National Natural Science Foundation of China (51262025), International scientific and technological cooperation project of Xinjiang Bingtuan (2013BC002), and Graduate Research Innovation Project in Xinjiang (XJGRI2014053).
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