Elsevier

Applied Surface Science

Volume 377, 30 July 2016, Pages 121-133
Applied Surface Science

Low temperature synthesis of N-doped TiO2 with rice-like morphology through peroxo assisted hydrothermal route: Materials characterization and photocatalytic properties

https://doi.org/10.1016/j.apsusc.2016.03.137Get rights and content

Highlights

  • The N:TiO2 nanorice were prepared using facile peroxo-assisted hydrothermal method at low temperature.

  • The N:TiO2 exhibited rice-like morphology.

  • The nitrogen doping favoured UV/visible light photocatalytic activity.

  • The RhB and Atrazine were chosen as model pollutants.

Abstract

Nanorice-shaped N:TiO2 photocatalysts have been prepared by the peroxo assisted hydrothermal method using stabilized titanium complex as a precursor and urea as a N source. The N:TiO2 nanorices were characterised by XRD, FE-SEM, HRTEM, XPS, UV–vis spectroscopy, Raman spectroscopy and measurements of photocatalytic degradation of organic molecules (atrazine and RhB dye) under the UV and visible-light irradiation. XRD analyses showed that pristine TiO2 crystallizes into anatase polymorph and that the N-doping process at 5% introduced a degree of disorder on the TiO2 crystalline structure. XPS study revealed the successful incorporation of the nitrogen atoms at the interstitial sites of the TiO2 crystal lattice. Microscopy studies revealed that the particle size was in the range 50–80 nm for the pristine TiO2. The photocatalysts were assembled in the form of nanorices with a high surface area (102 m2 g−1). The successful incorporation of nitrogen atoms into the TiO2 crystal lattice is expected to be responsible for enhanced photocatalytic activity of the as-prepared samples for the degradation of pollutants (RhB and atrazine) under UV and visible light irradiation. The rate of radical dotOH radicals formation under visible-light irradiation was examined and found to be correlated with the photocatalytic activity per unit surface area. The N:TiO2 particles with nanorice morphology was efficient photocatalysts for decomposition of organic dyes under UV and visible-light exposure while pristine TiO2 photocatalyst did not show any significant photocatalytic activity when stimulated by visible-light. The 3% doped N:TiO2 sample exhibited the highest photocatalytic activity among all synthesized photocatalysts.

Introduction

Nanostructured semiconductors have been reported as excellent materials for photocatalytic applications. Their photocatalytic efficiency has been improved by assorted and adaptable ways. Reduction of particle size, for instance, results in the quadratic growth of the specific surface area, and hence the photocatalytic efficiency is enhanced due to the availability of many active sites at the photocatalyst surface capable of interacting with pollutant species. Whereas, this effect is sometimes not obvious because of quantum confinement effects which occur owing to a dimensional equivalence between particle size and electron mean free path, and thereby facilitating the recombination of photogenerated electron-hole pairs [1], [2], [3], [4]. These drawbacks have been addressed by addressing special emphasis on hierarchical or complex nanostructures that display enhanced photocatalytic activity [1], [2], [3], [5]. Researchers have shown much interest in constructing a variety of dimensions, morphologies and architectures for functional photocatalyst materials, as these features are considered to be key parameters in the control of their electronic and optical properties. Extensive studies have been made for such complex nanostructures, particularly to those who made up of oxides and sulfides [6], [7], [8].

In recent years, titanium dioxide (TiO2) nanoparticles have played a fundamental role in many chemical processes involving light exposure, such as photocatalyst for remediation of pollutants, environmental compatibility, hydrogen production, and in photo-electrochemical cells for solar energy conversion[9], [10], [11]. This has been mainly attributed to the unique application-worthy properties of TiO2, along with its processing and occurrence in different crystalline phases [1], [12], [13]. TiO2 is a cheap and nontoxic material; however, it absorbs only UV radiation as its band-gap (3.2 eV for the anatase polymorph) is rather large and can capture only approximately 3% of the total solar radiation[14], [15]. Hence, pristine TiO2 nanoparticles cannot be utilized effectively for solar excitation or conversion [16], [17].

Several efforts have been made to improve the photo-response efficiency of TiO2 nanoparticles [18]. Especially, electronic structural modifications of TiO2 by formation of Ti3+ sites by introducing oxygen vacancies or doping the oxide with transition metals and nonmetal impurities has been shown to enhance dramatically the absorption features of TiO2 towards red shifting (the visible region of 400–800 nm) [19], [20], [21] Various transition metals have been used [22] as a dopant for TiO2, but the poor capacity of these dopants in acting as a center for recombination of photo-generated electrons-holes pairs, the use of expensive synthesis protocols, and also thermal instability of doped TiO2 [22], [23] reduce the overall effectiveness of this photocatalyst. Recently, the use of anionic non-metal species such as C [24], S [25], B [26], F [27], and N [28], [29], [30], [31], [32] as dopants has attracted much scientific interest, in particular, nitrogen has been found to improve the photocatalytic efficiency of TiO2 under visible light. Theoretical calculations have revealed that nitrogen is the most effective dopant for TiO2 among other anionic species as predicted by the formation of localized N 2p states above the valence band of TiO2 and/or narrowing of the band gap [29], [33], and also because of its comparable atomic size with oxygen, small ionization energy, metastable center formation, and stability [7], [34], [35], [36]. More recently, the rising number of publications on N:TiO2 materials have discussed the main challenges for the preparation of powders and films of TiO2 doped with nitrogen that could be activated by visible-light. It has been suggested that the N-doping process modifies the optical properties of TiO2 in the visible spectral region, and ultimately prompt the catalytic activity of TiO2 for degradation of pollutant in the various photo-oxidation reaction under visible-light exposure [37], [38], [39], [40].

The mechanism responsible for the enhanced photoactivity of N-doped TiO2 in the presence of visible light is still in debate, and the role of N dopant on band structure modifications, chemical nature of doping centers and, consequently, on such mechanism [38] has been presented by several hypotheses. Asahi et al. [29] reported that a significant shift of the absorption edge in the visible region occurs because the N2p states contribute to the band-gap narrowing by mixing with the O2p states, due to substitutional doping of nitrogen into the TiO2 lattice [29], [41]. Similarly, Lindgren et al. applied photoelectrochemical measurements to confirm that newly created states of nitrogen were located close to the valence band edge and that there was no change in the conduction band edge due to nitrogen doping process [42]. Synthesis methods adopted to prepare N-doped TiO2 vary from physical methods to more sophisticated chemical synthesis, which undoubtedly lead to different materials in terms of fine structure [34], [42]. Irie et al. [43] suggested that doping of TiO2 powder by N (TiO2xNx) created an isolated narrow band above the valence band, and it was responsible for the light stimulating behavior as well as the quantum yield under UV irradiation decreased with the increase in nitrogen concentration, showed that recombination centers dominated with the doping sites. Ihara et al. [44] reported that N-doped TiO2 having an oxygen deficient stoichiometry formed in the grain boundaries are responsible for the photocatalytic activity under visible-light irradiation and the doped nitrogen sites could act as a stabilization center for the oxygen deficiencies. There are several protocols adopted to achieve appropriate nitrogen doping such as sputtering [29], [45], ion implantation [46], electrospinning [14], [47], chemical treatments of bare oxides [43], and sintering of TiO2 under high temperature in a nitrogen-containing atmosphere [48], sol-gel process [49], or oxidation of titanium nitride [50]. Li et al. [51] utilized electrospinning technique for the synthesis of N:TiO2 nanofibers by annealing TiO2 nanofibers in the presence of ammonia. Gole et al. [37] synthesized nano-colloids by treating a titania sol with triethylamine at room temperature. Additionally, polished TiO2 (110) single-crystal substrate was treated in a flow of ammonia at 870 K to prepare rutile N:TiO2 [52] and in another protocol, microtubes of N:TiO2 were prepared using simple hydrolysis of titanium tetrachloride in ammonia atmosphere without using an external template [53].

In the present investigation, nanorice shaped N:TiO2 photocatalysts were synthesized using a low-temperature via the OA mechanism by hydrothermal treatment. Structural, optical and morphological studies of the as-prepared samples have been performed by UV–vis spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of the as-synthesized pristine and N:TiO2 nanoparticles was evaluated for degradation of atrazine and RhB dyes. Such photo-decomposition processes have enormous importance for society because they could help to produce cheap hydrogen (another green fuel), curtail pollution, and to produce active colloidal sulfur for agriculture.

Section snippets

Synthesis of TiO2 precursor

The synthesis procedure started with the formation of a stabilized Ti peroxo complex in a pre-mixed hydrogen peroxide/ammonia solution according to the method described by our group [54]. The method is modified with the introduction of a chelating ligand (urea) as N source to form coordination attachment with the Ti-peroxo complex. This attachment among the chelating ligand and Ti-peroxo complex facilitate the replacement of oxygen atoms by nitrogen atoms from TiO2 crystal lattice in the

Results and discussion

Field emission scanning electron microscopy (FEG-SEM) was used to characterize the surface morphology of the as-synthesized pristine TiO2 (SAB-1) and N:TiO2 nanostructures (SAB-2, SAB-3, SAB-4, SAB-5, and SAB-6). Prior to FEG-SEM analysis, 1 mg of photocatalyst was placed on a sample holder and coated with platinum (thickness of 5–10 nm) in order to enhance the sample electrical conductivity. Fig. 1(a) shows the FE-SEM images of concomitant appearance and interconnected nanorice grain morphology

Conclusion

We have demonstrated a facile and template-free low-temperature peroxo assisted hydrothermal method to enhance the photocatalytic properties of light-activated TiO2 photocatalysts, by doping nitrogen in the interstitial site of TiO2. The atrazine and RhB were used as model pollutants to attest the photocatalytic activity. The synthesis conditions have been optimized for the formation of such N:TiO2 nanostructures. The effectiveness of N doping process has been confirmed by different techniques.

Acknowledgments

The authors are very much thankful to TWAS-CNPq post-gratuate program and FAPESP for providing financial support to this project, and to the Brazilian Nanotechnology National Laboratory LNNano for providing XPS facilities. The authors also thank Dr. Vagner Romito for the productive discussions about this work.

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