Elsevier

Catalysis Today

Volume 350, 15 June 2020, Pages 171-183
Catalysis Today

Efficiency enhancement of photocatalytic degradation of tetracycline using reduced graphene oxide coordinated titania nanoplatelet

https://doi.org/10.1016/j.cattod.2019.06.038Get rights and content

Highlights

  • Structured TiO2-RGO-TiO2 catalyst was produced using a cost sol-gel method.

  • Graphene scaffolding provided improved efficiency for photodegradation of TCH.

  • Generation of surface defeats resulted in hole creation for photodegradation of TCH.

  • Another dominant route for photodegradation of TCH was via reactive oxygen species.

Abstract

The compound tetracycline hydrochloride (TCH) is an antibiotic used in the veterinary and medical application, however, it's over usage and accidental release into riverine and estuaries raising up serious threats to the ecology, environment and public health. This study focused on developing a low-cost catalyst to photodegrade TCH. Different formulations of TiO2 photocatalysts scaffolded with reduced graphene (denoted as TiO2-RGO-TiO2) were produced using a cost-effective sol-gel method. The mesoporous structured TiO2-RGO-TiO2 was tuned using citric acid as a complexing agent, Pluronic P-123 as a grafting agent and graphene oxide (GO) as structural “core” to guide the TiO2 growth. Our findings documented that the TiO2-RGO-TiO2 catalysts displayed high degradation efficiency towards TCH. This efficiency was determined to be improved by 22.8% under ultraviolet light and 32.80% under simulated solar energy within 2 h exposure respectively, relative to catalysis by TiO2 alone. It was found that 6 wt percent GO resulted in the highest degradation percentage (˜ 94%) due to the synergistic effects of inhibition of the electron-hole (e and h+) recombination and improvement of effective reactive sites. The holes were found to the major factor in facilitating the photocatalytic degradation of TCH (75.4%), while the reactive oxygen species (ROS) accounted for the remaining 24.63%. Another factor to contribute to the superior performance of the TiO2-RGO-TiO2 compared to TiO2 alone lies in the decrease of the band gap energy of TiO2 from 3.20 to 2.87 eV upon introduction of reduced GO.

Graphical abstract

Degradation efficiency of tetracycline was improved using reduced graphene oxide coordinated titania photocatalyst under different irradiations. Mechanistic study indicated a biphasic model was followed with the holes as dominant factor to directly oxidize tetracycline.

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Introduction

The tetracycline hydrochloride (TCH, C22H25ClN2O8) antibiotics have been widely used to provide first-line therapy for different diseases, such as Rocky Mountain spotted fever, Lyme disease, and series infection in mammals [1]. The TCH antibiotic is used due to its wide spectrum of action, low lethal dose (LD50) and persistence for antibacterial action [2]. Its cost-effectiveness allows the wide application in human, veterinary and medicine. Riverine runoff in the transportation of contaminants from terrestrial sources to the open oceans has led to a toxic, biotic, microbial resistance, acute and chronic effects [3,4]. The chronic dose of TCH residues in water resulted in permanent discolor of teeth from the prenatal period through childhood and adulthood, generation of microvascular fatty tissue in the liver, photosensitivity skin to ultraviolet (UV) light, drug-induced lupus, and hepatitis [5]. It was observed that above 70% of the deposited TCH was excreted into the water table and soil, which caused the enrichment of antibiotic-resistant pathogens and bacterial drug resistance [6]. It was found that 193 tonnes runoff of antibiotics from pearl river delta and pearl river estuary, South China in 2013 was correlated with the emergence of antibiotic-resistance microbes, such as Escherichia coli [7]. In addition, the TCH antibiotics are difficult to be adsorbed by animal metabolism due to its polyketone molecular structure [8]. Further serious threats to the ecological environment and human healthcare are raising up due to the extensive use in human and veterinary medicine and require a simple and effective approach for removal [9,10].

Under these conditions, it is urgent to develop an efficient and cost-effective treatment to remove antibiotics from the aquatic environment to minimize acute and chronic effects on aquatic animals [11,12]. In reality, it difficult to treat the TCH residues by the traditional methods, such as physical adsorption and biological degradation due to the difficulties faced on the post-treatment of sorbents and the antibacterial nature of tetracyclines [13,14]. Titania-(TiO2)-based photocatalytic oxidation processes are considered suitable for the degradation of TCH antibiotics with high efficiency, rapid rate, simple operation, and low cost [15]. However, much of photocatalytic processes using TiO2-based photocatalysts, are effective under ultraviolet (UV) irradiation (λ < 380 nm) due to its high band gap energy (˜ 3.2 eV) [16]. The high energy and harmfulness of the UV light collectively hindered the practical application of TiO2 under solar light or indoor usage [17]. In addition, a post-filtration to remove TiO2 catalysts from the matrix also limited its application during continuous use [18]

A preferred approach is the use of modified titania (TiO2) which has a number of advantages, such as low cost, engineering control, tunability, which is difficult with other approaches coupled with the generation of reactive by-products [19]. In the case of titania, degradation of TCH antibiotic is accomplished in-tandem with activation with UV irradiation and simulated solar energy (SSE), resulting in high efficiency at concentrations as high as 20 parts-per-million (ppm) [20]. By engineering photocatalytic properties under visible light conditions, system costs can be lowered, whilst the loading of transition metal oxide can be increased to degrade and remove TCH based antibiotics from water or soil. Tuning the structure and then the property of TiO2 may be achieved through different methods, such as coordination with reduced graphene oxides (RGO). We demonstrated the maximum energy transfer occurs between TiO2 and carbon-based conductive materials due to the closeness in size of hopping energy within RGO (˜ 2.80 eV) [21]. and the decreased band gap of RGO modified TiO2 (˜ 2.87 eV, current study). The efficiency of light absorption can be also affected by absorption depth, tunable pores, the propensity of trap states generated at the interfacial zone [22]. This phenomenon, in turn, would prevent electron-hole pair (e-h+) recombination and enable Fermi level pinning [23]. The combination of modified semiconductor TiO2 with GO will affect the energy barrier between the metal oxide and maximum photocatalytic reactive sites [24].

The reasons of graphene incorporation to tune photocatalyst lie in the intrinsic mechano-optical properties such as high electrical conductivity (106 s⋅ cm−1), thermal conductivity (5000 W⋅ m−1⋅k−1), fracture strength (124 GPa), specific surface area (2630 m2⋅ g−1), charge mobility (105 cm2⋅V−1⋅s−1), and low density (1 g⋅ cm-3) [25]. The graphene is densely packed in a honeycomb crystal lattice with a planar geometry and sp2 hybridization, which will further facilitate electron transfer [26]. It was reported that the corrugations and local electrical properties [27] have been presented for strain-induced local conductance modulations for bigger ripples (2–3 nm in height). The presence of TiO2 can alter the local electrical and optical properties of graphene through the so-called “ripple-engineering” [21,28]. Intrinsically, the graphene is composed of corrugations, however, other defects also exist in three-dimensional space, which includes topological defects (pentagons, heptagons, and /or their combination), vacancies, adatoms, edges and /or cracks, and adsorbed impurities [29]. These defects which lower electrical conductance can be mitigated and controlled by coordinating Ti-complex with the functional groups on the surface of GO. A series of experiments have demonstrated that defects in a graphene oxide could be induced locally by electron or UV irradiation. Graphene oxide (upon reduction) was selected to serve as ideal catalyst support to improve the photocatalytic reactivity towards antibiotic degradation [30,31].

However, the susceptibility of the catalyst support to oxidative environments, limited capacitance, and easy aggregation hindered applications of the pure graphene sheets, despite the intrinsic advantages [32,33]. It was found that RGO composites, such as RGO and TiO2 materials, are more practical for diversified applications, such as water treatment, alternative energy, and sensors, due to lower material usage [34,35]. Recently, researchers focused on controllable functionalization of graphene to provide a platform to produce nanocomposites as photocatalysts [36,37]. Our aim was to use the RGO as support to construct spherical TiO2 photocatalysts to degrade antibiotic residues in water [30,38].

It is hypothesized that the modification of the space charge layer will enhance the efficient energy coupling at the surface (< 10 nm) [39,40] where the RGO support exhibiting both excitations of surface plasmon’s resonance and local surface plasmon absorbance [41,42]. Therefore, decoration of TiO2 with a small amount of GO would allow the light energy to be collected through localized surface plasmon [43] upon absorption of a photon at the plasmon resonance frequency [44]. The e-h+ pairs can be generated along the TiO2 surface, where the electrons are proposed to be transferred by the RGO to catalyze the generation of reactive oxygen species (ROS) [45,46]. This electron transfer was found to inhibit the e-h+ recombination. The holes and ROS collectively potentiate the TCH degradation, in which the holes were found to be the predominant oxidizing species by this study. The degradation efficiency of TiO2-RGO-TiO2 toward antibiotic relative to TiO2 alone photocatalysis increased under UV–vis and SSE by 22.76% and 32.80%, respectively. An important consideration was to maximize the degradation efficiency by controlling the TiO2 band gap energy, particle size and mesoporous structure of the photocatalyst.

The significance of this research lies in developing nanocomposites via feasible green synthesis techniques to improve photocatalytic efficiency to degrade TCH. The motivations are to (1) provide a guideline for fundamental research in nanomaterials used for antibiotic degradation; (2) understand the mechanism of photocatalytic degradation; and (3) integrate advanced instrumentation to aid optimization of the variables to enhance degradation efficacy. The key research data demonstrated that the TiO2 -RGO-TiO2 nanocomposite with mesopores generated high reactive sites, holes, and ROS.

Section snippets

Workflow

The TiO2-RGO-TiO2 nanocomposites were applied as antibiotic degradation agents to photo-catalytically degrade TCH under ultraviolet (UV) and simulated solar energy (SSE) conditions into carbon dioxide, water, and carbon-based residues. The materials were being synthesized using bottom-up sol-gel techniques. The degradation activity was attributed to the synergetic effects between high reactive sites from both RGO and TiO2 components and inhibited recombination of hole and electron pairs.

Materials

Formation of RGO-coordinated TiO2 catalysts

Four key steps to forming mesoporous spherical nanostructured photocatalysts, denoted as TiO2-RGO-TiO2 were implemented. These steps (abbreviated as CGCR) are: 1) Complexation (Scheme 1a & b), 2) Grafting (Scheme 2a & b) Cross-linking (Scheme 3), and 4) Reduction (Scheme 4a & b).

  • 1)

    Complexation: The citric acid (Cit, containing carboxylic group, C6H8O7) was added as a chelating ligand to form a coordinative complex with Ti central element through a “de-butanolization” step, which was found to

X-ray diffraction (XRD) analyses

The X-ray diffraction (XRD) data (Fig. 2) of TiO2 alone and TiO2-RGO-TiO2 indicated the composite exhibited an anatase TiO2 structure which was well indexed with the standard tetragonal phase (JCPDS 89–4921, a =3.777 Å, c =9.501 Å; α = 90°) and the rutile phase was not detected. The crystallite sizes of TiO2 and TiO2-RGO-TiO2 were calculated to be about 15.0 and 10.0 nm, respectively. The Scherer equation was used based on the full width at half maximum corresponding to (101) plane, (0.81 < K <

Conclusions

The TiO2-RGO-TiO2 nanocatalysts with a crystalline structure, controllable mesopore and morphology were produced by a sol-gel method using citric acid as complexation agent and P-123 as grafting element. The functional group on the surface of RGO served as “riveting sites” to allow for tunable growth of TiO2 nanoparticles. Compared to TiO2 alone, these TiO2-RGO-TiO2 nanocatalysts showed high degradation efficiency (> 93%, 2 h exposure) towards TCH under UV and SSE conditions. It was found the

Author contributions

The research was conceived and oversaw by Dr. C. Li and the first draft of this manuscript was written by Dr. J. Liu through contributions from all authors. A master student, Mr. R. Hu conducted the research and collected data with supervision by Drs. C. Li and X. Lu. All data were plotted and analyzed by Drs. C. Li and J. Liu. Dr. S. Bashir provided the tetracycline tautomer calculations, verified the spectroscopic data and typeset the manuscript for syntax and grammar. All authors have given

Declaration of Competing Interest

The authors declare no financial conflict of interest.

Acknowledgment

The National Natural Science Foundation of China (511764039), Chinese Scholarship Council (201608625038) and the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology (SKLAB02015010) are duly acknowledged for their financial support. The Robert Welch Foundation, departmental grant (AC-006) is also acknowledged. The technical support from the Lanzhou University of Technology and Department of Chemistry, Texas A&M University-Kingsville

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