Improved degradation of methyl orange dye using bio-co-catalyst Se nanoparticles impregnated ZnS photocatalyst under UV irradiation

Highlights

• Se nanoparticles was biosynthesized using Se tolerant Bacillus sp.

• Improved photocatalytic activity of Se co-catalyzed ZnS under UV irradiation.

• Se-ZnS nanocomposite completely photodegraded methyl orange to CO₂.

• 1 wt% Se impregnated ZnS catalyst exhibited the highest photoactivity.

Abstract

The present work outlines the importance of biosynthesized selenium nanoparticles (SeNPs) as co-catalyst for the enhanced photocatalytic activity of as-prepared ZnS photocatalyst for degradation of methyl orange (MO) dye under UV light irradiation. SeNPs of different wt% were impregnated onto ZnS by calcination at 200 °C to make Se-ZnS nanocomposites (Se-ZnSNCs) and characterized by UV–Vis spectroscopy, DLS, XRD, SEM-EDX, TEM and BET surface area analysis. The specific surface area notably increased from 54 m² g⁻¹ (ZnS) to 75 m² g⁻¹ with different wt% of Se loading. The band edge absorption at 330 nm (3.77 eV) of bare ZnS significantly red-shifted to 357 nm (3.30 eV), after 1 wt% Se loading (3.30 eV). 1 wt% Se-ZnSNCs exhibited the highest degradation efficiency (95%) of MO as compared to bare ZnS (55%) under 160 min UV light irradiation. The reaction followed pseudo first-order kinetics with an apparent rate constant k = 8.42 × 10⁻³ min⁻¹ where CO₂ evolution gradually increased depending on the extent of MO degradation by Se-ZnSNCs under different time period of UV light irradiation.

Introduction

With the increasing emphasis on green chemistry processes in the field of nanotechnology, the new approaches for synthesis of nanomaterials have become essential. Although the physical and chemical approaches are being used successfully for their synthesis, the techniques are often tedious, hazardous and environmentally challenging [1]. Therefore, there is a growing concern to develop eco-friendly and sustainable methods for the synthesis of nanomaterials. The use of biological means for synthesis of nanoparticles is gaining greater attention as it is a ‘green’ approach that utilizes the living cells [2], [3], [4], [5]. The biological approaches assist in eliminating intense processing conditions by allowing the synthesis at physiological pH, temperature, and at relatively low cost and less time. Prokaryotic bacteria have received the most attention in this area due to ease of handling and genetic manipulation [6].

This study was aimed at producing and using Se nanoparticles as co-catalysts keeping in view their unique physical and chemical properties and large inherent applications. Elemental selenium (Se) has narrow band gap of 1.7–2 eV with relatively low melting point (∼217 °C), high refractive index and high reactivity making it suitable catalyst for carrying out organic hydration and oxidation reactions [7], [8], [9]. Se nanoparticles (SeNPs) are reported to exhibit potential to photocatalytically degrade methylene blue and congo red under UV light irradiation [10], [11], [12]. The biosynthesis of these SeNPs has been reported in various bacterial species [13]. The synthesis of SeNPs is induced through reduction of selenate and selenite oxyanions into non-toxic elemental selenium (Se⁰) which is insoluble in water [14].

Among the various photocatalysts being explored till date, ZnS is an important II–IV direct semiconductor material with a wide band gap i.e., 3.5–3.8 eV and high excition binding energy i.e. 40 meV [15]. This makes it a suitable material to be used as heterogeneous photocatalyst for the degradation of organic dye pollutants. However, due to limitations in its efficacy [16] it is imperative to improve the photocatalytic efficiency of ZnS nanoparticles.

Nanomaterial co-catalysts are known to play an important role in improving the photocatalyst activity. In general, when co-catalyst (metal/semiconductor) nanoparticles come in contact with a charged semiconductor nanoparticle, they undergo Fermi-level/energy-band equilibration. Electrons can easily be trapped by the noble metals (Pt, Au, Pd etc.) due to their low Fermi level. However, the noble metals are costly and scarce, which largely inhibit their practical applications. The development of the highly efficient and low-cost co-catalysts is needed. The heterojunction from integrating two or more semiconductors enhances the capture of light, provided active sites and promotes the separation of photogenerated e⁻/h⁺ by energy-band equilibration [17]. This helps in storing and shuttling of photogenerated electrons from one semiconductor to another, acting as a sink for photoinduced charge carriers and hence promoting charge separation process (Scheme 1) [18], [19]. Therefore, the coupling of semiconductor photocatalyst with semi-conductor co-catalyst provides a unique pathway for maximizing the efficacy of photocatalytic reaction system.

Approaches attempted to enhance ZnS based photocatalytic activity includes loading ZnS with different metal ions such as Mn, Ni, Cu [20], Fe [21] by chemical precipitation and electrochemically coupling ZnS with narrow band gap semiconductor like Se [22]. These methods are related with various problems like complex reaction conditions, use of expensive metal salt solutions and harmful chemical agents (hydrazine, surfactants) [23], [24], [25], [26], [27]. As an effective alternative, biologically formed nanoparticles are expected to offer good co-catalytic activity as their surfaces are not affected due to chemical reactions. SeNPs are expected to act as narrow-band semiconductors and improve the activity of ZnS photocatalyst. Thus, the aim of the present work was to synthesize the Se-ZnS nanocomposites (Se-ZnSNCs) using biogenic Se nanoparticles (SeNPs) as co-catalyst and determine their photocatalytic activity for photodegradation of organic pollutant viz., methyl orange (MO). MO is used in this study, as it is an azo dye, water-soluble and highly stable which is extensively used in many industries like textiles, foodstuffs, leather and paper. The release of MO and its products in the environment cause serious pollution problems. Hence, its removal from waste water is important.