Full Length ArticleCarbon-based heterostructure from multi-photo-active nanobuilding blocks SrTiO3@NiFe2O4@Fe0@Ni0@CNTs with derived nanoreaction metallic clusters for enhanced solar light-driven photodegradation of harmful antibiotics
Graphical abstract
The present study focuses on the environmentally relevant solar light-driven photodegradation of tetracycline using a rational designed penta-component inorganic bulk SrTiO3@NiFe2O4@Fe0@Ni0@CNTs heterostructure for a better understanding of the fundamental aspects of the involved photo-initiated processes. The presence of Fe0 and Ni0 in the multi-component based heterostructure have direct implications on the enhanced photodegradation activity.
Introduction
Emerging pollutants in aquatic matrices, such as those produced by the pharmaceutical industry [1], urgently require advanced degradation processes, because, at the moment, the wastewater treatment plants are not completely capable to remove these molecules, releasing them back into the environment [2], [3]. Particularly, the contamination of water with drugs, such as tetracycline (TC) antibiotics, is of major concern due to their extensive use in medical treatments. Since they have been the most widely used antimicrobial drugs for more than 80 years, they still constitute the most important group of antibiotics currently in use in human and animal medical treatments [4], [5]. TCs being the second largest produced antibiotics are relatively often reported as present, sometimes at elevated levels, in various environmental matrices including surface, underground, or wastewater [6], [7]. Besides the direct impact on the biota, their presence in such environmental compartments is more often reported as being responsible for the increasing antibiotics resistance, one of the biggest threats to global health, food security, and development today [8], which has led to decreased tetracycline usage for certain conditions. Therefore, the removal of TCs from the wastewater has become a critical challenge for sustainable techniques and materials development to facilitate the total degradation up to the mineralization of these contaminants [9], [10]. The recent advances in nanotechnology suggest that several problems involving emerging pollutants in wastewater could greatly benefit by using metal oxide (MO) nanostructures [11], semiconducting metal oxides (SCMO) [12], nanocatalysts [13], nanostructured catalytic membranes [14], metal–organic frameworks (MOFs) [5], and nanoparticles beds enhanced filtration [15]. In this regard, novel photocatalysts were developed aiming at the removal of emerging contaminants from wastewaters, such as polymeric materials [16], clay minerals [17], activated carbons [18], multiwalled carbon nanotubes [19], and magnetic nanoparticles [20], [21]. For efficient removal and mineralization of large drug molecules, the photocatalysts should have a well-developed mesoporous structure (pore size: 2–50 nm), a proper band gap for visible-light-driven processes, more active sites, extended electron-hole carrier lifetimes, photochemical stability, and reusability [22], [23], [24]. Moreover, the photocatalysts should be sustainable materials with high abundance. NiFe2O4, a well-known material with small band gap of ∼1.5 eV [25], stability and good magnetic properties [26] is becoming increasingly interesting for photocatalytic applications, alone [27], [28] or in various combinations, such as NiFe2O4@TiO2 core@shell [29], SrTiO3/NiFe2O4 [30], [31], NiFe2O4/Bi2O3 [32], γ-Fe2O3@NiFe2O4 [33], NiFe2O4/MWCNT [34], [35], [36]. Although depending on the method of synthesis and morphology, in some cases it has been reported with a very good photocatalytic activity, NiFe2O4 presents fast recombination of e- / h+ pairs [29]. SrTiO3 is a cubic perovskite with an indirect band gap of 3.2 eV [37], which is usually used for dye degradation [38], water splitting [24], CO2 reduction [39] only under UV light irradiation [40]. Tuning the capability of SrTiO3 in photocatalytic applications is feasible by altering the chemical composition and/or physical appearance [24], [41]. To overcome three of the most pressing problems of these two materials, namely the fast recombination process for NiFe2O4, low efficiency under visible-light irradiation, and difficult recycling of SrTiO3 after use, recently, they were combined by various techniques and tested in photocatalytic processes. Jing et al. [30] reported the synthesis of SrTiO3/NiFe2O4 porous nanotubes (PNTs) and SrTiO3/NiFe2O4 particle-in-tubes (PITs) via single-spinneret electrospinning and showed remarkable photocatalytic efficiency in the degradation of rhodamine B. Yongmei Xia et al. [26] synthesized the multifunctional SrTiO3/NiFe2O4 by coupling SrTiO3 with NiFe2O4 through a two-step route for the quenching of the e- and h+ recombination and increase the photocatalytic efficiency for degradation of RhB dye under simulated solar light irradiation [26]. However, even with these advances, at this time, few materials allow both photo-oxidation and photo-reduction under visible light, because an efficient photocatalyst should have a large surface area, superior sensitivity to the visible region, appropriate band energetics, and agile carrier transport to inhibit recombination processes [42], [43]. For this purpose, carbon nanotubes (CNTs) with large surface area, special morphology, high chemical stability, and exceptional electrical properties have become interesting conductive material supports for the immobilization of various photocatalytic semiconductors [44], [45], [46]. However, the literature reported very few heterostructures based on CNTs for tetracycline photodegradation, although the benefits of using this material, such as large electron-storage capacity and as well as the ease with which it accepts photon-excited electrons, acting as an electron pool [34], [35], [47], [48], are well known. Inspired by the vide-supra ideas, in the present work, a pentanary hybrid nanostructured material of SrTiO3@NiFe2O4@Fe0@Ni0@CNTs with fibrous structure network was synthesized by a facile sequential synthetic strategy. As a novelty, the heterostructure based on CNTs was prepared by chemical vapor deposition with some essential changes in pre-deposition catalyst nanoparticles. Our inspiration came from the idea that NiFe2O4 from the SrTiO3@NiFe2O4 starting composite could function as a catalyst for the growth of the CNTs matrix. Moreover, taking into account the extremely strong reducing environment (continuous flow of acetylene) in the CVD installation and the high temperature of 830 °C, we assumed that Fe(III) and Ni(II) in the nickel ferrite will be partialy reduced to metallic state on the CNTs surface. Our results showed that, as-synthesized SrTiO3@NiFe2O4@Fe0@Ni0@CNTs fibrous structure network can eficientlly achieve a high-to-total degradation of tetracycline antibiotic under visible light irradiation in less than two hours. The photodegradation mechanism and reusability of SrTiO3@NiFe2O4@Fe0@Ni0@CNTs were examined. Finally, corroborating all the experimental evidences monitored by applied complementary characterization techniques, such as XRD, FESEM, HR-TEM, BET, XPS, and soft XAS in both total electron yield (TEY) and fluorescence yield (TFY), we proposed the breakthrough development of a high-operative photocatalyst with penta-component inorganic bulk heterojunction SrTiO3@NiFe2O4@Fe0@Ni0@CNTs with improved charge trapping characteristics at particle–particle interfaces for enhanced solar light-driven degradation of tetracycline antibiotic. Specifically, the development and usage of the new concept of photocatalytic composite, their multiscale characterization, interfacial charge/energy transfer investigation, and exploitation of their optoelectronic properties, will be discussed in detail. Moreover, it will be shown that the heterostructure allows a controlled integration of complementary components, each having structural and/or compositional features with dimensions at the nanoscale to exhibit synergistic effects that combine multiple functionalities in one structure.
Section snippets
Multistep synthesis method of pentanary hybrid nanostructured material of SrTiO3@NiFe2O4@Fe0@Ni0@CNTs fibrous structure network
The protocol synthesis of the SrTiO3@NiFe2O4 composite was extensively described in a previous study [49]. The main stages of the synthesis consist of a two-step protocol, as follows: the SrTiO3 powder obtained by solid-state method at 1100 °C was mixed with nickel nitrate, iron nitrate, and glycine with a molar ratio of 1:2:1 in distilled water. The gel, obtained after 30 min of energetic magnetic stirring at 80 °C, was heated on a sand bath with an increasing step of 50 °C up to 350 °C, where
Structural and morphological characterization
The powder XRD technique was used to investigate the crystal structure and phase formation for all the samples of the present study. The XRD profile of pristine CNTs (Fig. 1 a), showed the characteristic (0 0 2) and (1 0 0) peaks of the graphitic reflection. The morphology of the CNTs was first characterized by field emission scanning electron microscopy (FE-SEM) and shown in Fig. 1 b–e. The fibrous morphology was obtained by using the well-known ferric chloride-mediated chemical vapor deposition
Conclusions
In this study, a novel high-operative photocatalyst with penta-component inorganic bulk heterojunction SrTiO3@NiFe2O4@Fe0@Ni0@CNTs was successfully prepared via a slightly modified chemical vapor deposition technique. FE-SEM and HR-TEM images showed that the starting composite SrTiO3@NiFe2O4 provided physical anchor points for the CNTs matrix growth, NiFe2O4 acting as a catalyst for the graphitic matrix growth. Due to the extremely strong reducing environment in the CVD installation and the
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.
Acknowledgments
This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI – UEFISCDI, project number PN-III-P1-1.1-TE- TE-2019-2037, within PNCDI III and by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI – UEFISCDI, project number PN-III-P2-2.1-PED-2019-4215, within PNCDI III.; D.G acknowledges the financial support of the grant of the Ministry of Research, Innovation and Digitization, CNCS - UEFISCDI, project number
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