Fabrication of Ag nanoparticles modified TiO₂–CNT heterostructures for enhanced visible light photocatalytic degradation of organic pollutants and bacteria

Abstract

TiO₂ synthesized using polymeric template consisting of polyethylene glycol and polyvinyl alcohol (Tev) and loaded with different wt% of Ag (2%, 6%) was exploited to create covalent bonds with carboxylate functionalized SWCNT/MWCNT moieties. The synthesized Ag free Tev–SWCNTs as well as Ag containing Tev–SWCNTs/MWCNTs have been characterized by UV–visible diffuse reflectance, powder XRD, HRTEM, and selected area electron diffraction (SAED), photoluminescence, Raman, FTIR and N₂ sorptiometry. The materials containing Ag displayed high photocatalytic activity towards degradation of rhodamine B dye under visible irradiation (λ_max > 450 nm). Specifically TevAg₆–SWCNT has shown the best performance (0.3 g/l catalyst, 20 ppm RhB conc. and 80 min reaction time) due to the synergistic effects derived from TiO₂/Ag°/SWCNT heteroarchitectures. The antibacterial activity of synthesized photocatalysts; under visible light irradiations, towards Escherichia coli and Staphylococcus aureus was tested by performing bacterial DNA and agar well diffusion method. The results revealed that TevAg₆–SWCNT was able to effectively kill both gram-positive and gram-negative bacteria. Although TevAg₆–SWCNT indicated higher E_g values (1.9 eV) than TevAg₂–MWCNT (1.75 cV) and they both exposed not only Ag° nanoparticles but also Ag₂O, the former sample confirmed more lethal action against bacterial growth as well as superior photodegradation activity. This was due to delaying the recombination of electrons and holes, increasing the S_BET value as well as decreasing the spherical nanoparticles of Ag° to 3 nm diameter. The mechanisms of the dye degradation and destruction of bacterial cell membranes indicate the efficacy of OH and O₂⁻ as reactive radical intermediates in both processes.

Introduction

Carbon nanotubes (CNTs) have attracted considerable attention because of their excellent electrical, mechanical, magnetic properties, high surface areas and high chemical stability [1], [2]. On the other hand, TiO₂ is used widely as a photocatalyst for solving environmental problems. In particular, it has been used to remove different chemical pollutants from water as well as from air [3], [4], [5]. However, the low photocatalytic efficiency has restricted the application of TiO₂, in practical water treatment [6], [7], [8], to be active only under ultraviolet irradiation. Composite materials containing CNTs and TiO₂ have attracted considerable attention owing to their unique properties. They are used widely in waste water treatment to remove a range of pollutants because of their large surface area, high adsorption capacity as well as shifting the photocatalytic reactions to take place under visible light irradiation [9], [10]. Accordingly, when TiO₂ is coated on nanocarbon materials, the binding to pollutants will increase and the electron–hole recombination at the TiO₂–CNT interfaces is hindered by electron trapping and thus yields superior photocatalytic activities compared with individual analogue [11], [12], [13]. The morphology and structure of CNTs enable them to serve as specific templates for preparing metal nanoparticle–CNTs nanohybrids [14]. The combination of these two materials (i.e., CNTs and nanoparticle) is specifically useful when integrating the properties of the two components of the hybrid materials for use in catalysis, energy storage and nanotechnology [15]. Previous studies were carried out to find a low-cost, stable and effective nanocatalytic material as a substitute for noble metal catalysts. For instance Ag supported by CNTs showed high electrocatalytic activity towards hydrazine oxidation [16]. As reported by Mohamed and Al-Sharif [17], the photocatalytic performance of Ag/TiO₂ for 4-nitrophenol reduction to 4-aminophenol, which was accomplished in less than 2 min reaction time, was also investigated and attributed to Ag°/Ag⁺ species formed at the interface of TiO₂. On the other hand, the higher activity of Ag°-modified mesoporous TiO₂ for herbicide degradation was predominant due to the presence of small Ag particles and to the interface formed between Ag and TiO₂ [18]. However, to date, little attention has been paid to the antibacterial properties of TiO₂–CNT composites.

In present days, resistance to commercially available antimicrobial agents by pathogenic microorganisms has been increasing at an alarming rate and has become a serious problem [19]. The inorganic agent silver has been employed as the most antimicrobial agent since ancient times to fight infection pathogens [20], [21]. Accordingly, there are many in-vitro studies on antibacterial and antifungal properties of silver nanoparticles [22]. In daily life, human beings are often infected by microorganisms like bacteria, molds, viruses, etc. To impart sterility (e.g., hospital trays, contaminated water) and avoid infection (e.g., wound dressing), the use of antimicrobial agents is important. Research has been intensively carried out in antibacterial materials containing various natural and inorganic substrates [23], [24], [25], [26] over the last few years. The mode of action of silver nanoparticles (Ag-NPs) on bacteria is probably the same as the silver ion that possesses higher toxicity comparatively [27]. It is believed that nanoscale Ag possesses superior reactivity than the bulk metallic counterpart because of high large surface area and varieties of engineered nanoscale Ag-modified nano-architectures [22], [23], [24], [25]. In addition, Ag-NPs can bind to the DNA inside the bacterial cells, preventing its replication [28], or interact with the bacterial ribosome preventing translation [29], so Ag-NPs have attracted considerable attention as antimicrobial active materials [30]. Several techniques were used to manufacture Ag-NPs including chemical reduction of silver ions in aqueous solutions, with or without stabilizing agents, thermal decomposition in organic solvents and chemical and photo reduction in reverse micelles [31], [32]. Most of these techniques are extremely expensive and capital intensive, they also involve the use of toxic, hazardous chemicals that may pose potential environmental and biological risks as well as is inefficient in materials and energy use [33]. In this study, the photocatalytic degradation of Ag/TiO₂–CNT towards rhodamine B (RhB) dye and the inactivation of bacteria; including Escherchia coli (E. coli) and Staphylococcus aureas, will be investigated together with the effect of nanoparticles on bacterial DNA. The effect of either Ag₂O and/or Ag° will be examined together with inspecting the reactive intermediates might be emitted while irradiating the photocatalysts and their lethal effects on the dye degradation and on the bacterial growth. Structural variations, optical and morphological properties were examined for the fabricated CNT/TiO₂ and Ag–CNT/TiO₂ composites. X-ray diffraction (XRD), transmission electron microscopy (SEM), energy dispersive X-ray (EDX), N₂ sorptiometry, FTIR, Raman, photoluminescence and UV–vis diffuse reflectance spectroscopy were used to characterize these synthesized photocatalysts.