Gold-decorated TiO₂ nanofibrous hybrid for improved solar-driven photocatalytic pollutant degradation

Highlights

• •Facile fabrication of gold nanoparticles decorated TiO₂ nanofibrous hybrid photocatalyst.

• •Highly efficient solar-driven photocatalytic pollutant degradation.

• •Dual functions of gold nanoparticles for effective charge separation and plasmon enhanced solar light photocatalysis.

• •Successful demonstration of effective solar-driven photocatalytic degradation of Rhodamine B (RhB) and methyl blue (MB).

Abstract

TiO₂-based nanomaterials are among the most promising photocatalysts for degrading organic dye pollutants. In this work, Au–TiO₂ nanofibers were fabricated by the electrospinning technique, followed by calcination in air at 500 °C. Morphological and structural analyses revealed that the composite consists of TiO₂ nanofibers with embedded Au nanoparticles that are extensively distributed throughout the porous fibrous structure of TiO₂. The photocatalytic performance of these Au-embedded TiO₂ nanofibers was evaluated in the photodegradation of Rhodamine B and methylene blue under solar simulator irradiation. Compared with pristine TiO₂ nanofibers, the Au-embedded TiO₂ nanofibers displayed far better photocatalytic degradation efficiency. The plasmon resonance absorption of Au nanoparticles in the visible spectral region and the effective charge separation at the heterojunction of the Au–TiO₂ hybrid are the key factors that have led to the considerable enhancement of the photocatalytic activity. The results of this study clearly demonstrate the potential of Au–TiO₂ electrospun nanofibers as solar-light-responsive photocatalysts for the effective removal of dye contaminants from aquatic environments.

Introduction

With the onset of the 21st century, the globalization of industries has led to an unprecedented increase in the manufacturing of both consumer goods and industrial products. This massive scale of production comes with the unavoidable drawback of a proportionally large amount of industrial waste. Of the many concerns brought about by these waste products, wastewater pollution caused by chemical factories and manufacturing plants is one of the most immediate ones. A large group of pollutants that is responsible for wastewater contamination consists of organic dyes, which are widely used in the manufacturing of textiles, cosmetics, paper, and leather, and can consequently be found in the effluent of these industries (He et al., 2018; Berradi et al., 2019). It has been estimated that 15% of the dyes used in these manufacturing processes are released into different bodies of water. Because these compounds are non-biodegradable, they can remain in water systems for extended amounts of time, adversely affecting the aquatic ecosystems as well as humans who rely on these bodies of water for sustenance. Moreover, due to biomagnification, larger concentrations of these dyes are found in the wildlife commonly consumed by humans (Lellis et al., 2019).

To tackle the presence of hazardous organic dyes in our water systems, various physical and chemical treatment processes have been developed. One such process is the use of photocatalysts to degrade these organic dyes. In photocatalysis, a light-harvesting material is used to promote the decomposition of organic pollutants into less harmful intermediates in the presence of light. Metal oxide semiconductors such as TiO₂, ZnO, and SnO₂ have been the main focus of photocatalyst-based wastewater treatment as they have been shown to exhibit notable photocatalytic efficiency (Hoffmann et al., 1995; Chan et al., 2011; Al-Hamdi et al., 2017; Thuong et al., 2019). A typical photocatalytic process begins with the absorption of photons with energy greater than the band gap of the semiconductor. Electrons in the valence band are promoted to the conduction band and leave behind a hole, forming an exciton pair. Exciton pairs can either migrate to the surface of the material or recombine (Ma et al., 2014). The photogenerated electrons and holes that successfully reach the surface can generate free radicals that interact with organic dye pollutants in redox reactions and break them down into more environmentally favorable intermediates (Ajmal et al., 2014). The photocatalytic activity for dye degradation is thus largely determined by the ability of the semiconductor to strongly absorb light and create electron–hole pairs that can drive the dye decomposition reactions.

Of all the semiconductors that are used as photocatalysts, nanosized TiO₂ is the most extensively studied owing to its low toxicity, chemical and thermal stability, resistance to photocorrosion, cheap cost, and widespread availability (Al-Mamun et al., 2019; Gopinath et al., 2020). Both the anatase and rutile forms of TiO₂ have been shown capable of degrading organic dyes, such as methylene blue, methyl orange, Rhodamine B, indigo carmine, and Eriochrome Black T, under ultraviolet (UV) irradiation (Gautam et al., 2016; Kochkina et al., 2017). However, a significant drawback seen in these studies is the fact that a UV light source must be used to induce the photocatalytic process due to the wide band gap of TiO₂ (E_g = 3.2 eV for anatase, 3.0 eV for rutile) (Khalid et al., 2017). This can severely limit its application for large-scale industrial use. As a response to this concern, researchers have studied the modification of TiO₂ by creating a hybrid with a photosensitizer that allows for absorption of visible light (Djurišić et al., 2014). This would allow for photocatalysis under solar light, which is strongly favorable for industrial and commercial use. Another limitation of TiO₂ is its fast recombination rate, which is detrimental to photocatalysis as less photogenerated charge carriers become available to participate in the dye degradation process (Nam et al., 2019). The pairing of TiO₂ with plasmonic Au nanoparticles is a promising remediation to the disadvantages of pristine TiO₂. Au nanoparticles exhibit a phenomenon called localized surface plasmon resonance (LSPR), which can boost the photocatalytic performance through energy transfer mechanisms that can enhance the charge carrier concentration in TiO₂ (Linic et al., 2011). As the LSPR of Au nanoparticles lies in the visible spectral range, coupling TiO₂ with Au nanoparticles enables absorption of visible photons, which constitute a large portion of the solar spectrum. In addition, Au has been found to inhibit electron–hole recombination by separating photogenerated electron–hole pairs and promoting interfacial charge transfer (Bumajdad et al., 2014).

The synthesis of Au–TiO₂ nanocomposites is typically done in solution. One of the most common protocols is the deposition–precipitation method, where Au nanoparticles are randomly deposited onto the surface of pre-synthesized TiO₂ nanoparticles (Amrollahi et al., 2014; Bumajdad et al., 2014). However, the nanoparticles that are prepared through this approach have a tendency to flocculate in solution, which may negatively affect their photocatalytic activity and limit their reusability. Another technique is to allow TiO₂ to anisotropically grow onto pre-synthesized Au nanostructures in the presence of surfactants. A variety of hybrid configurations, such as Janus, core–shell, and flower-like architectures, have been produced through this strategy (Li and Zeng, 2005; Seh et al., 2011). The foremost challenge faced by solution-phase methods is that they are limited to small-scale production. A commercially viable way of producing Au–TiO₂ hybrid nanostructures is through the polymer-assisted electrospinning process. Electrospinning is an economical and scalable method for fabricating one-dimensional (1D) nanomaterials with inherent porosity. For instance, electrospun TiO₂ with mesoporous 1D fiber-like structures have been successfully prepared through this approach (Li and Xia, 2003; Cossich et al., 2015; Someswararao et al., 2018; Roongraung et al., 2020). The general setup involves loading a syringe with the precursor solution and connecting its conductive needle tip to a high voltage power supply (Soo et al., 2019). The solution is subsequently ejected out of the needle by a syringe pump into a stable jet, which is elongated by the electric current to form a continuous thin fiber (Ligon et al., 2018). Several copies of the resultant fiber can be formed in a short amount of time under the continuous-feeding mode. These nanofibers are then deposited onto the surface of a grounded collector, and are later subjected to calcination in air. To create Au–TiO₂ electrospun nanofibers, pre-synthesized Au nanoparticles are included in the precursor solution prior to the electrospinning process (Duan et al., 2019; Kumar et al., 2020). The resulting hybrid consists of TiO₂ nanofibers that are sparsely decorated with Au nanoparticles.

In this study, Au–TiO₂ electrospun nanofibers have been synthesized through a more facile procedure that does not require the pre-synthesis of Au nanoparticles. Analysis of the morphological structure of the calcined sample revealed that our protocol produced porous polycrystalline TiO₂ nanofibers with embedded Au nanoparticles that are extensively distributed throughout the fibrous structure. The photocatalytic performance of the Au–TiO₂ composite nanofibers was compared against that of pristine TiO₂ nanofibers under solar irradiation for dye pollutant degradation using Rhodamine B (RhB) and methylene blue (MB) dyes. Our results indicate that the embedded Au nanoparticles can effectively enhance the photocatalytic activity of TiO₂.