Original research articleStrain influence on the structural properties of nitrogen and fluorine codoped TiO2
Introduction
It is well known that TiO2 is responsive to ultraviolet (UV) light [1]. To efficiently extend the optical response of TiO2 from ultraviolet to visible light region, different routes have been explored [[2], [3], [4]]. In recent years, anion doping [2,5,6] with p-block elements has been widely investigated. A bunch of studies concentrated on mono-doping with these elements and inferred that the dopants significantly altered the physicochemical properties of materials [7]. Asahi et.al [8]. carried out doping of TiO2 with nitrogen and proposed that the band gap narrowing in TiO2 was due to nitrogen occupying substitutional sites in the TiO2 lattice. Again, Li et.al [9] carried out co-doping of TiO2 with nitrogen and fluorine and found that due to synergistic effect of two elements co-doping, the materials showed a higher visible light absorption. However, the peculiar optical absorption by mono-doped and co-doped systems is still a matter of discussion [10,11]. Even the dynamics of charge carriers proposed by different researchers are in strict contradiction with each other [12]. This gives a subtle hint that there are other factors affecting the fate of photogenerated charge carriers. It is expected that lattice strain could be one such factor. It is a measure of the distribution of lattice constants arising from crystal imperfections [13]. Strain may be uniform or non-uniform but in either cases it affects the width and intensity of the Bragg peaks [13]. Thus, there exists a possibility that it could modify the structural properties of materials as well. In spite of this; the strain factor control on the structural properties of doped materials is poorly studied and seldom reported to the best of our knowledge.
In the present study, N-doped (NT) and N, F codoped TiO2 (NFT) having different loadings of precursor were prepared using hydrothermal synthesis. Inorganic precursors; liquid ammonia (NH3-H2O) and ammonium fluoride (NH4F) were used as dopant sources in the two cases so as to avoid hydrocarbon remnants in the lattice. The strain due to lattice deformation associated with each of the samples was estimated following Williamson-Hall (W-H) methodology [14]. It was observed that lattice distortion caused an increase in crystallinity. For higher doping amounts, strain decreased and a blue shift of the absorption edge was noticed.
Section snippets
Materials and method
A facile hydrothermal method was adopted to prepare nitrogen doped TiO2 [NT]. 5 mL tetrabutyl titanate [Sigma Aldrich] was hydrolyzed in 50 mL deionized water under vigorous magnetic stirring. To this, a certain amount of ammonia solution (25 %) [Merck] was added. The pH of the mixture was found to be 11. The solution was stirred for 6 h which resulted in a yellowish brown color. The sol was then filtered and dried at 120 °C for 24 h in an oven. For doping TiO2 with nitrogen and fluorine
Effect of doping on the crystallite size and lattice strain
XRD (X’Pert PRO PANalytical X-ray diffractmeter) was employed to evaluate peak broadening for the determination of crystallite size and lattice strain originating due to dislocation [15]. The weak Bragg peaks in N doped TiO2 (NT) showed anatase phase (JCPDS 21-1272) [Fig. 1] with a wider half width indicating destruction of crystallinity. No peaks related to nitrogen or its compounds were detected. This could be because of nitrogen occupying the interstitial or substitutional sites in the TiO2
Conclusions
In summary, N and F codoped TiO2 with tunable particle sizes and enhanced visible light sensitivity were successfully synthesized by a facile hydrothermal route using inorganic economic precursors. The insertion of fluorine was held responsible for visible light response by narrowing the band gap. XRD revealed that all the samples have the simplex anatase crystal structure. The most striking fact deduced from the study was the impact of strain on crystallinity and morphology of the samples.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors acknowledge the DST FIST (SR/FST/PSI-175/2012) and DST-PURSE programme for instrumental help. Authors are also grateful to ECR project (ECR/2017/000707) from DST, India and Ramanujan Fellowship grant (SB/S2/RJN-006/2016) for financial support.
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