Effective photodegradation of dyes using in-situ N-Ti3+ co-doped porous titanate-TiO2 rod-like heterojunctions
Graphical abstract
Introduction
Over the past several decades, photocatalytic oxidation processes have become regarded as effective methods for the decomposition of pollutants in wastewater. A variety of semiconductors have been utilized as the photocatalysts such as TiO2, V2O5, WO3 and ZnO [1]. Particularly, TiO2 has been widely investigated as a promising photocatalyst owing to its advanced properties, including non-toxicity, low cost, superior oxidizability and long-lived photo-induced charge carriers [2]. Among the three common polymorphs of TiO2, namely, anatase, rutile and brookite, anatase exhibits a higher photoreactivity than rutile and brookite do. The poor photocatalytic activity of rutile is mainly attributed to its higher recombination rate of electrons and holes. TiO2 has low photoreactivity under visible light due to its intrinsically wide bandgap energy (∼3.2 eV for anatase and 3.0 eV for rutile). To overcome this issue for feasible application of TiO2 as a photocatalyst under visible light, several studies have focused on doping TiO2 with non-metals such as N, C and F [3], [4], [5]. Moreover, introducing Ti3+ or oxygen vacancies (Ov) into the lattice of TiO2 was also documented. Fabrication of such Ti3+ self-doped TiO2 has been reported to not only increase the electrical conductivity of TiO2 but also accelerate the transfer of charge carriers [6].
To enhance the charge carrier separation of TiO2, it is feasible to couple TiO2 with other semiconductors, while sensitizing them to visible light. Alkali-metal titanates (A2TinO2n + 1, where A is Li, Na or K and n = 3–8), which have an edge-sharing TiO6 octahedral layered structure (n = 3–4) or tunnel structure, have been reported to exhibit advantageous characteristics for photocatalytic performance, particularly when forming a composite with TiO2 [7]. Several studies have focused on the synthesis of titanate-TiO2 hybrids for applications under visible light [7], [8], [9]. However, most synthesis methods require multiple steps and extremely high temperature (∼900 °C). Moreover, the reported hybrids exhibited a high photogenerated charge recombination rate under visible light, which reduced their photocatalytic activity.
Several studies have reported that the physical and chemical properties of TiO2-based materials (hierarchical TiO2-based nanowires [10], rutile TiO2-based hollow nanorods [11], layered TiO2-based nanosheets [12], etc.) were strongly dependent on the operational parameters such as the calcination temperature, the reaction time, solution pH and reactant concentration. Particularly, the calcination temperature has a strong effect on the surface morphology, phase composition, crystallinity and photocatalytic activity of TiO2-based materials. Nevertheless, comprehensive investigation has not yet focused on the effect of calcination temperature on the physical properties (surface morphology, crystallinity, optical property and porosity) and visible-light driven photocatalytic performance of TiO2-titanate heterostructures.
In this work, we provide a one-pot approach to the synthesis of N-Ti3+ co-doped porous titanate-TiO2 rod-like heterostructures with effective electron-hole separation under visible light by using ethylenediaminetetraacetic disodium salt (EDTA-Na2) as a template and titanium alkoxide as a precursor. The influence of the calcination temperature on the physical properties and photocatalytic activity of TiO2-titanate heterostructures was tested via the degradation of methylene blue (MB) in dark and under visible light. Moreover, the MB removal mechanism was also investigated in detail.
Section snippets
Chemicals
EDTA-Na2 (≥99.0%), acetic acid (≥99.7%), titanium (IV) isopropoxide (TTIP, ≥99.99%), isopropanol (ISP, ≥99.7%), tert-butanol (t-BuOH, ≥99.0%), p-benzoquinone (BQ, ≥98%) and MB (≥95.0%) were purchased from Sigma-Aldrich and were used for experiments without further purification. Deionized (DI) water from a Millipore water (18 MΩ cm) purification system was used for all experiments.
Preparation of photocatalysts
First, 3.60 g of EDTA-Na2 was premixed with 100 mL of acetic acid under vigorous stirring at 4 °C for 3 h. A 50.0 mL
Morphology
The surface morphologies of TiO2-400 and TiO2-BTs were observed and are shown in Fig. 1. The top-view SEM image of TiO2-400 (Fig. 1a) revealed the deposition of clusters of nano- to micro-sized particles onto the TiO2 surface, which was tentatively attributed to the agglomeration of ultrafine TiO2 powders into larger particles [13]. Ti and O are the most abundant elements in TiO2-400 (Fig. 1b). After heat treatment with EDTA-Na2, the surface of TiO2-BT-400 (Fig. 1c) clearly shows the presence
Conclusions
We have developed a simple method to synthesize novel N and Ti3+ co-doped mesoporous titanate-TiO2 rod-like heterostructures using EDTA-Na2 as a template. The obtained rod-like heterostructures were thermally less stable at high temperatures (>400 °C). The Na2Ti3O7–anatase rod-like heterostructure underwent structural conversion into the Na2Ti3O7- Na2Ti6O13 heterostructure at calcination temperatures higher than 400 °C. The photocatalytic activity of the rod-like heterostructures under visible
Acknowledgment
This work was financially supported through a project of the environmental technology development for chemical accident countermeasures funded by the Korea Environmental Industry and Technology Institute, South Korea.
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