Abstract

A novel N–H–TiO2 photocatalyst was prepared via hydrothermal synthesis followed by a thermal treatment in NH3 and H2 atmospheres. The results showed that benzene with the initial concentration of 150 ppm (75 ppm) could be thoroughly removed by N–H–TiO2 under visible light irradiation. The inflfluences of doping and hydrogenation on properties of TiO2 samples were investigated. It is suggested that the disordered surface layer introduced by hydrogenation contributes to the excellent photocatalytic activity of N–H–TiO2. The synergistic effect of N doping and surface oxygen vacancies was also confifirmed by ultraviolet–visible diffuse reflflectance spectra and photoluminescence. Moreover, the diverse role of oxygen vacancies in different positions of TiO2 has been focused on.

1. Introduction

Benzene, as one of the confirmed carcinogenic substances, is difficult to degrade due to its conjugated π-bond [1]. Hence, there is absolutely a need to develop an effective approach to remove it. It is widely accepted that TiO2 has many advantages, such as abundance, chemical stability and high activity under ultraviolet (UV) irradiation, providing an efficient way to solve the environment problems [2]. However, the wide band gap and rapid recombination of photogenerated electrons and holes hinder visible light absorption and photocatalytic activity of TiO2 photocatalyst. Hence various approaches have been developed to engineer band structure and introduce defects, which will improve optical absorption and photocatalytic performance of TiO2. Among the efforts to modify TiO2, nitrogen doping and hydrogenation have been extensively investigated [3,4]. Unfortunately, the effifi- ciency of current TiO2 photocatalyst in decomposing pollutants under visible light remains low so far [5]. Furthermore methods for preparing highly active TiO2 usually required complicated procedure or rigorous conditions, which definitely became the obstacles for practical application [6].

Here, we present a highly efficient N–H–TiO2 photocatalyst prepared by a simple hydrothermal method. Surprisingly N–H– TiO2 exhibites amazing photocatalytic activity in quickly and thoroughly degrading high concentration benzene under visible light irradiation, which is timesaving, economical and efficient with simple preparation technology for large-scale application in removing indoor pollutants.

2. Experiments

TiO2 was synthesized by a hydrothermal method. Typically, 2 mL tetra-n-butyl titanate was added dropwise to 75 mL distilled water under vigorous magnetic stirring and then the hydrolysate transferred into a 100 mL Teflflon-lined autoclave to react at 160 1C for 12 h. Afterwards, the obtained precipitate was separated by a centrifuge and washed several times, and then dried in an oven at 80 1C overnight to acquire TiO2 nanoparticles. TiO2 was further modifified by annealing at 600 1C in H2 for 2 h as well as in NH3 and H2 for 4 h, denoted as H–TiO2 and N–H–TiO2 respectively.

The structures and morphologies of samples were observed by X-ray diffraction (XRD, X'Pert PRO diffractometer with Cu Kα radiation) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F30 fifield-emission TEM). Ultraviolet– visible diffuse reflflectance spectra (UV–vis DRS) were obtained on a Shimadzu U-3010 spectrometer using BaSO4 as a reference. Photoluminescence (PL) emission spectra were acquired under excitation at 325 nm using an Edinburgh Instruments PLSP920 spectrometer.

Photocatalytic activities of the samples were tested by degrading benzene under visible light irradiation for 3 h. A 300 W Xe-arc lamp (CEL-HXF300) with a UV-cutoff filter (λo400 nm) was used as the light source. Generally, 100 mg TiO2 was dispersed on the loading plate. Afterwards, 150 ppm (75 ppm) benzene was injected into the reaction chamber of a gas chromatograph (GC) to measure the concentration of benzene and CO2. Then the samples H–TiO2, N–H–TiO2 and N–H–TiO2-s (N–H–TiO2 stored over 6 months) were tested successively. The degradation effifi- ciency is calculated by the functions C/C0 and C/ C0, where C0 is initial concentration of benzene or CO2, and C is concentration during the photodegradation process. A blank control test without photocatalyst was conducted for reference.

3. Results and discussion

The visible light photocatalytic activity of TiO2, H–TiO2, N–H–TiO2 and N–H–TiO2-s samples was evaluated with a blank control test, as shown in Fig. 1. The result shows that N–H–TiO2 and N–H–TiO2-s exhibit the best photocatalytic activity, which indicates the unique performance and good stability of N–H–TiO2. The conversion of benzene over N–H–TiO2 under visible light irradiation is as high as 100% within 3 h. Meanwhile, the performance of H–TiO2 is better than that of TiO2 while the blank test gives almost no activity when decomposing benzene. Obviously, the fundamentally enhanced performance of N–H–TiO2 can be attributed to the synergistic effect of N doping and hydrogenation.

Fig. 2 displays the crystalline structures of all the samples characterized by XRD. The diffraction peaks of all the samples can be indexed to anatase (JCPDS File no. 21-1272). Moreover, H–TiO2 and N–H–TiO2 have increased crystallinity after annealing. The average particle sizes of TiO2, H–TiO2 and N–H–TiO2 are, respectively estimated to be 19.46, 36.72 and 33.55 nm by the Scherrer formula, consistent with TEM images presented in Fig. 3. Furthermore, surface amorphous structure introduced by hydrogenation can be clearly observed in H–TiO2 depicted in the range 25–351 of XRD and the blurry edge of HRTEM image (e). This result is similar to those of previous investigations for TiO2 nanoparticles annealed in reducing atmospheres, which show that the disordered phase separates fully crystalline inner part from disordered (amorphous) surface of a nanoparticle [7,8].

Fig. 4 shows the optical absorption properties examined by UV– vis DRS. The onset of optical absorption edge of N–H–TiO2 has extended to 550 nm compared with TiO2, which indicates the typical redshift of nitrogen doped TiO2 [9]. This can be assigned to the signifificant cause for enhanced visible light absorption and band gap narrowing of N–H–TiO2. Besides, a new absorption band up to infrared region emerging in H–TiO2 means that band structure of TiO2 has been modified by hydrogenation, which has been reported by previous studies [10,11].