Simple fabrication of N-doped mesoporous TiO2 nanorods with the enhanced visible light photocatalytic activity
© Zhou et al.; licensee Springer. 2014
Received: 15 October 2013
Accepted: 6 January 2014
Published: 16 January 2014
N-doped mesoporous TiO2 nanorods were fabricated by a modified and facile sol–gel approach without any templates. Ammonium nitrate was used as a raw source of N dopants, which could produce a lot of gasses such as N2, NO2, and H2O in the process of heating samples. These gasses were proved to be vitally important to form the special mesoporous structure. The samples were characterized by the powder X-ray diffraction, X-ray photoelectron spectrometer, nitrogen adsorption isotherms, scanning electron microscopy, transmission electron microscopy, and UV-visible absorption spectra. The average length and the cross section diameter of the as-prepared samples were ca. 1.5 μm and ca. 80 nm, respectively. The photocatalytic activity was evaluated by photodegradation of methylene blue (MB) in aqueous solution. The N-doped mesoporous TiO2 nanorods showed an excellent photocatalytic activity, which may be attributed to the enlarged surface area (106.4 m2 g-1) and the narrowed band gap (2.05 eV). Besides, the rod-like photocatalyst was found to be easy to recycle.
Since the exciting discovery of the synthesis of TiO2 - xN x film with an enhanced visible light absorption, N-doped TiO2 nanoparticles have been widely studied in the fields of degrading recalcitrant organic contaminants under visible light in recent years[2, 3]. However, practical applications of N-doped TiO2 nanoparticles are greatly limited due to their low recycle rate. To solve this problem, N-doped TiO2 with different morphologies such as nanowires, nanotubes, hollow spheres, and nanorods were prepared[7, 8]. It is well known that N-doped TiO2 nanorods can be fabricated by chemically nitriding TiO2 nanorods. However, with this route, the nitridation is limited in the surface of the nanorods at a very low level, and thin nitridation layer can be easily removed during the photocatalytic reaction. Besides, the rod-like structure leads to the formation of small surface areas in many cases due to the accumulation of the nanoparticles.
In this work, N-doped TiO2 nanorods with mesoporous structure were fabricated by a modified and facile sol–gel approach without any templates. The photocatalytic activity was evaluated by photodegradation of methylene blue (MB) in aqueous solution. The reasons why the N-doped mesoporous TiO2 nanorods showed an excellent photocatalytic activity and photochemical stability had been investigated.
In the experiments, deionized water was used. All of the chemicals were analytical grade. TiO2 used for comparison was Degussa P25 (Frankfurt, Germany), whose surface area and particle size were reported as 50 m2 g-1 and 21 nm, respectively.
Preparation of N-doped mesoporous TiO2 nanorods
Typically, 5 mL of tetrabutyl titanate (TBOT), 30 mL of ethanol, and certain ammonium nitrate were mixed together in the reaction flask of the rotary evaporator, and ten agate granules with a diameter of about 1 cm were added into the system for better stirring. The rotary evaporator was turned on and the system was maintained at 25°C. In the mean time, an air blower connected with a round bottom flask containing some deionized water was turned on to transport air at a rate of 40 L min-1. A small amount of water vapor was carried into the reaction flask with air to react with the TBOT. The TBOT solution was hydrolyzed slowly to form a cream color emulsion. Reaction stopped after 3 h and then the emulsion was distillated at 50°C for 15 min under vacuum. Finally, the samples were annealed at different temperatures for 2 h to obtain the N-doped mesoporous TiO2 nanorods, designated as NMTNR-x-y, where x represents the theoretical molar ratio of N (%) and y represents the calcination temperature (°C).
Characterization of the samples
The crystalline phase identification and structural analysis were carried out by X-ray diffraction (XRD) instrument with Cu Kα radiation. A Japan ULVAC-PHI PHI 5000 VersaProbe X-ray photoelectron spectrometer (XPS; Kanagawa, Japan) was applied to analyze the elemental composition and state of the samples. The microstructures were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). N2 adsorption-desorption isotherms were measured at 77 K on a Micromeritics Tristar 3020 system (Norcross, GA, USA). The UV-visible (UV–vis) absorbance spectra of the samples were characterized using a Japan Shimadzu UV240 UV–vis spectrophotometer (Kyoto, Japan).
The photocatalytic activity of the samples was estimated by MB degradation performed in a 500-mL cylindrical glass photocatalytic reactor, and a 500-W xenon lamp was selected as the visible light source. Between the xenon lamp and reactor, a cut filter was inserted to eliminate ultraviolet light. In a typical experiment, 0.08 g of photocatalyst was dispersed into 250 mL of MB solution (10 mg L-1). The actual effect of photocatalytic activity by chemical reaction was studied by maintaining the solutions in the dark for 1 h before irradiation. The MB solution (5 mL) was taken out every 5 min and analyzed using UV–vis spectrophotometer. The degradation of MB can be calculated via the formula η = (1 – A i /A0) × 100%, where A0 is the absorbance of the original MB solution before irradiation and Ai is the absorbance of MB solution measured every 5 min. The photodegradation of MB follows pseudo-first-order kinetics. Its kinetics can be expressed as ln(C0/C) = kt, where k (per minute) is the degradation rate constant.
The stability of photocatalyst was evaluated by the degradation of MB with reused photocatalyst, and 250 mL of new MB solution (10 mg L-1) was added into the reactor each time.
Structural properties of the different samples
Crystal size A/Ra(nm)
Accurate N contentb(at.%)
Based on the data in Table 1, the excellent photocatalytic performance of N-doped mesoporous TiO2 nanorods might be explained by the following factors. Firstly, N doping could extend the spectral response to visible light and greatly improve the utilization of visible light[1, 20]. Secondly, it is known that mesoporosity can improve surface adsorption capacity of the reactants due to the increased surface area[21, 22]. It is obvious that with the increase of N proportion, the photocatalytic efficiency was improved. This may be resulting from the narrowed band gap and the enlarged surface area of N-doped mesoporous TiO2 nanorods. In addition, the calcination temperature also plays an important role in the catalytic efficiency. On the one hand, with the increase of the temperature, the grain size and band gap increased and the specific surface area decreased, which are responsible for the depress of photocatalytic activity. On the other hand, under lower temperature, TiO2 had a lower crystallinity, which results in the lower photocatalytic activity.
In summary, the N-doped mesoporous TiO2 nanorods had been successfully fabricated by a template-free modified sol–gel approach. Ammonium nitrate was used to form the mesoporous structure and provided the source of N dopants. The average length and the cross section diameter of the as-prepared samples were ca. 1.5 μm and ca. 80 nm, respectively. The BJH adsorption average pore diameters were in the range of 5 to 10 nm. The mesoporous TiO2 nanorods doped with 6% theoretical molar ratio of N and annealed at 500°C showed the best photocatalytic performance. The photodegradation rate constant of this sample is 0.092 min-1, which is 7.6 times higher than that of P25. Furthermore, the rod-like photocatalyst can be easily separated and recycled, which could enhance the stability of the photocatalyst. The results provide useful insights for designing highly active photocatalyst.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF- 2013-R1A1A2009154), the fund from a key project for Industry-Academia-Research in Jiangsu Province (BY2013030-04), and the fund from Colleges and Universities in Jiangsu Province Plans to Graduate Research and Innovation (CXLX13-812).
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