Visible light-induced degradation of antibiotic ciprofloxacin over Fe–N–TiO₂ mesoporous photocatalyst with anatase/rutile/brookite nanocrystal mixture

Highlights

• Controlled iron and nitrogen contents are crucial for the photocatalytic properties of the doped TiO₂.

• The Fe–N–TiO₂ photocatalysts exhibit anatase, rutile and brookite phase mixtures with mesoporous nanorice morphologies.

• Nitrogen incorporated into the modified TiO₂ lattice inhibits the phase transformation from anatase to rutile and brookite.

• Nitrogen dopant suppresses the role of iron as a recombination center for the photogenerated electrons and holes.

• The synergetic effect of different crystal phases plays an essential role in the photocatalytic degradation of ciprofloxacin.

Abstract

Titanium dioxide photocatalysts co-modified with iron and nitrogen (Fe–N–TiO₂) were produced via hydrothermal method using iron(III) nitrate nonahydrate and urea as iron and nitrogen sources, respectively. The effects of different dopant concentrations were investigated. The modified TiO₂ catalysts were characterized for phase composition, surface morphology, specific surface area, degree of doping, charge states and bandgap energy combining various techniques. The results showed that controlled iron and nitrogen concentrations significantly altered the physicochemical properties of the catalysts. The photocatalysts displayed the anatase/rutile/brookite crystal phase mixture. The rutile and brookite phase contents increased with increasing iron content. On the other hand, increasing nitrogen content inhibited the formation of rutile and brookite phases and the catalysts displayed predominantly the anatase phase. High iron and low nitrogen contents led to the highest BET surface areas. The surface morphology changed from nanorice to spherical shape with increasing iron content. The bandgap energy of all Fe–N–TiO₂ samples was in the range 2.7–3.1 eV, being lower than that of undoped TiO₂ and pure anatase phase. Nitrogen was incorporated into the TiO₂ lattice on interstitial positions (Ti–O–N). The iron, substituting some Ti⁴⁺ in the lattice was presented in Fe²⁺ and Fe³⁺ oxidation state, as confirmed by XANES measurements. The photocatalytic degradation of antibiotic ciprofloxacin was performed under visible light using a LED illumination source and nearly 70 % of the antibiotic was removed in 6 h by using the most active sample (2.5 %N–1.5 %Fe). As verified by photoluminescence results, the iron and nitrogen dopants synergistically enhanced the charge separation, since they promoted the formation of the different TiO₂ phases.

Introduction

Over the past several decades, the worldwide growth of industrialization has resulted in the generation of an increasing quantity of wastewater containing a high level of organic and inorganic pollutants. The pharmaceutical industries are one of the major contributors that produced toxic and hazardous effluents [1]. More importantly, the rapid growth in population has massively influenced the production of pharmaceutical and personal healthcare products due to heavy usage, leading to extensive spread in the environment, especially in the water [2,3]. Majority of these substances enter the environment via discharge into the surface water by the excretion of humans and animals, effluent from hospitals, wastewater treatment plants and cosmetic manufacturing facilities [4]. Antibiotics have been widely used to remove a bacterial infection in humans and animals for several decades and usually detected in the environment [5,6]. The existence of these compounds and their transformation in the environment can cause serious effects in the ecosystem, such as disruption of ecological balance, a threat to water quality as well as the emergence of antimicrobial resistance, even at low concentration [6,7]. Unfortunately, most of these compounds cannot remove by conventional physical and biological methods [5,6]. Therefore, effective treatment methods are required for removing the pharmaceutical wastewater. Advanced oxidation processes (AOPs) are another class of treatment technology that can decompose a wide range of organic pollutant including pharmaceutical wastewater. Some of the well-known AOPs are ultrasonication, Fenton-like, ozonation, photolysis and photocatalysis. Currently, many researchers reported that photocatalysis using titanium dioxide (TiO₂) can be employed for utilizing pharmaceutical wastewater treatment.

Titanium dioxide (TiO₂), an inexpensive and nontoxic material, has been recognized for several decades as one of the materials which can decompose a large number of organic and inorganic pollutants in air and water [5,8,9]. However, the use of TiO₂ demonstrates some drawbacks, for example, the wide bandgap energy, that limit their absorption under visible light region and poor charge carrier separation both of which hinder their practical applications. Accordingly, several attempts were made to overcome the existing limitation and enhance the photoactivity, especially under the visible light domain. Based on the previous research, impurity ions doping is one of the typical approaches to extend the photoactivity of TiO₂ towards longer wavelengths (visible light). Some dopants such as transition metal, noble metal, rare earth metal and nonmetal doping have been employed to tune the electronic structure and improved the photocatalytic activity of the TiO₂ [[10], [11], [12], [13]]. Among these, nonmetal ion doping with nitrogen into TiO₂ lattice is widely used as a visible light active photocatalyst for environmental remediation applications. Additionally, nitrogen dopant can enhance the purity of the anatase crystalline phase in TiO₂ and also delay the phase transition, as published in previous studies [[14], [15], [16]]. At the same time, there are some problems of N–TiO₂ photocatalyst which may also be an important issue of concern. For instance, it is somewhat difficult to obtain the TiO₂ doping with high nitrogen concentration. On the one hand, the stability of the N–TiO₂ is reported to become worst after photocatalytic reaction of a pollutant under UV and visible light illumination due to a loss of nitrogen content at the surface of TiO₂ [[16], [17], [18], [19]]. Moreover, the single doping of nitrogen into the TiO₂ lattice usually results in the formation of oxygen vacancies in bulk. This defect can lead to the strongly localized N 2p state above the valence band. These isolated empty states act as traps for photogenerated electrons leading to loss of photocatalytic activity [20,21].

In order to improve the photocatalytic performance of N–TiO₂, co-doping with transition metals is greatly explored by many researchers [[22], [23], [24]]. The reports demonstrated that co-doping with nonmetal and metal not only can extend the optical absorption spectra of TiO₂ into the visible light region but also serve as efficient electron traps, thus inhibiting the recombination rate of the photogenerated charge carriers [[25], [26], [27]]. Among transition metals, iron is deemed to be a worthy co-dopant to enhance the photocatalytic activity of N–TiO₂. Moreover, iron is inexpensive and abundant for practical uses. In addition, the ionic radius of iron in the Fe³⁺ form (0.64 Å) is close to Ti⁴⁺ (0.68 Å) that can easily be incorporated into the TiO₂ lattice [28,29]. Besides, the co-doping of iron and nitrogen with a suitable concentration can inhibit the recombination of electron and hole, and also displays a greater photocatalytic efficiency than single N–TiO₂ or Fe–TiO₂ under visible light illumination [20,22,25].

Apart from extending the optical absorption spectra of TiO₂ photocatalyst into the visible light range by co-doping with nonmetal and metal, there are other factors that can affect the photocatalytic activity of TiO₂ as well. It is well known that the photoactivity of TiO₂ catalyst can be influenced by many factors, for example, surface area, particle size, surface morphology, crystal structure, porosity as well as the chemical reaction condition including temperature, pH and light intensity [16,[30], [31], [32]]. Furthermore, tuning the phase compositions and the crystal structure are the most promising strategies for photocatalytic activity enhancement. Many researchers have discussed the effect of crystalline phase on the photoactivity of TiO₂ catalyst [[32], [33], [34]]. However, most of these reports have focused on the anatase, rutile or their mixtures [34,35]. In contrast, the brookite phase has not received much attention because it is rather difficult to synthesize. Currently, there are few studies focused on the phase mixing (anatase, rutile and brookite) on the characteristics and visible light-driven photoactivity of TiO₂. For instance, Wang et al. [36] reported that the anatase-brookite-rutile mixed-phase TiO₂ prepared by hydrothermal method presents slower recombination rate and higher photocatalytic activity than its pure phase content. Additionally, Cao et al. [31] reported that the TiO₂ composed with 72 % brookite and 28 % rutile displays the highest photoactivity in the degradation of phenol. Therefore, we were also greatly interested in the study of this phase mixing effect, which was not reported in our previous researches [15,16,30].

In this paper, the Fe–N–TiO₂ mesoporous materials with anatase/rutile/brookite nanocrystal phase mixture were prepared by hydrothermal method. The reactions were carried out under low temperature and time, as reported [30,37], with various iron and nitrogen dopants concentrations using iron(III) nitrate nonahydrate and urea as the dopant sources, respectively. This work focused on optimizing the dopants level and investigating the influence on the structural, morphological, textural and optical properties, surface chemical composition, oxidation state as well as photocatalytic activity of the modified TiO₂ sample. The photocatalytic performance of Fe–N–TiO₂ with the three-mixed phase catalysts was examined through the decomposition of ciprofloxacin antibiotic under visible light using LED irradiation source. For comparison, the undoped TiO₂ anatase sample synthesized via the same hydrothermal condition was tested as well.