Solvent effects on arc discharge fabrication of durable silver nanopowder and its application as a recyclable catalyst for elimination of toxic p-nitrophenol

Highlights

• Long-lasting, low cost silver nanopowder (AgNP) is produced by arc discharge.

• The recyclable AgNP is used as a heterogeneous catalyst for purification of water.

• Highly toxic p-nitrophenol is efficiently converted to p-aminophenol by AgNP.

• Hydrogen is produced in the process as a clean source of energy from polluted water.

Abstract

Various types of nanosilver have been synthesized by a variety of methods, among them “arc discharge” stands with numerous advantages and calls for a much higher attention. Here, an unprecedented durable powder form of silver nanopowder (AgNP) is produced by DC arc discharge through a simple, convenient, and economical route, at a current of 5–10 A/cm². Effects of media are probed against the quality of the resulting nanoparticles. Among eleven media explored, glycerin/water (10% w/w) is found to afford the highest yield, purity, and relatively smaller size of AgNP with the desired morphology. To our joy, the as-prepared-AgNP proves efficient in catalytic green reduction of p-nitrophenol (PNP) which is one of the most potent water pollutants. Our reduction of PNP by NaBH₄ in the presence of AgNP in water is monitored by UV–Vis spectrophotometry. A provisional mechanism is proposed for this reaction which appears fast and completes in two minutes. The AgNP is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Hence, arc discharge fabrication of durable AgNP is maximized by adapting glycerin/water as the medium of choice; and the resulting AgNP catalyzes toxic PNP conversion into p-aminophenol.

Introduction

Nanotechnology is developing new materials and examining their properties by altering the particle size, morphology and distribution [1]. Nanoparticles have potential applications especially in fabrication of electronic devices, sensors, nonlinear optical materials, ultrafast data communication, optical data storage, photochemical catalysis, biomedicine, etc. [2], [3], [4]. Synthesis of metal nanoparticles has fascinated scientists because of their catalytic, optical, electronic, antimicrobial, and magnetic properties [5], [6], [7], [8], [9]. Transition metal nanoparticles are prepared and used extensively since they display unique physicochemical properties due to their size-induced quantum effects (large surface to volume ratio) and thus differ significantly from their bulk counterparts [10]. Between nanometals, Ag nanoparticles have attracted considerable attention as a result of its significant applications in fundamental sciences and nanotechnology [9]. Its characteristics strongly depend on the size, morphology, surrounding media, and aggregation state [11], [12], [13], [14], [15], [16].

Normally, physical and chemical methods have been widely used for preparation of metal nanoparticles. The physical preparation includes sonochemical, photolytic, and radiolytic reductions, along with laser ablation, microwave, and ultrasound metal evaporation–condensation [17], [18], [19], [20]. Pulsed wire evaporation (PWE) technique, a physical process, allows high production rate and particle size control with high efficiency, has been introduced to produce metal nanopowders, namely, Ni [21], Fe [22], [23], Al [24], [25], Cu [26], [27], Ag [28], [29], [30] and so forth, or nano-powder alloys, such as brass [31], Pt–Ni [32], Cu–Ni [33], and Cu–Ni–P [34]. In this technique, a high power pulsed dc current passing through a thin metal wire leads to the wire explosion, and the large amount of heat energy causes the wire to melt, followed by subsequent evaporation and formation of plasma. The plasma formed during the process expands and cools when it interacts with a coolant such as an inert gas or liquid, and then nanoparticles are formed through the nucleation process [21], [24], [35], [36].

The chemical methods incorporate reducing the metal ion precursors with alcohol, citrate, borohydride, polyol, N₂H₄, Na₃(C₆H₅O₇)₅, etc. [37], [38], [39], [40], [41], [42].

In order to control the size of Ag nanoparticles and prevent its aggregation, it is often dispersed in a liquid or solid medium or embedded in a polymer matrix [10], [16]. Actually, shrinking of Ag into nanosilver with large surface area makes it a highly effective catalyst for heterogeneous reactions [10], [43]. Yet its performance depends on the selection of the catalyst support/matrices.

On the other hand, p-nitrophenol (PNP) is one of the most important intractable contaminants that can arise in industrial and agricultural wastewaters [10]. It is recorded in “the U.S. Environmental Protection Agency List of Priority Pollutants” [16], [44], [45]. To eliminate 1, the wastewater is often treated with a catalyst which reduces it into p-aminophenol (AmP) [10]. Most classic methods for reduction of such nitro compounds are costly, messy, and environmentally hazardous. They often use metallic reagents in the presence of an acid [6]. Therefore, safer and more economical synthetic routes are necessary. One method is employing NaBH₄ in water as the hydride source. But reduction of nitro groups with NaBH₄ in the absence of any catalyst is highly tedious. Many different metals including gold have been used to carry out this task [46], [47], [48], [49]. Silver metal has not yet been employed, but its nanocomposites are used as heterogeneous catalysts for reduction of PNP [10], [16], [41], [43], [48], [50]. These catalysts while are rather inexpensive and often reusable, carry the burden of lower catalytic activity than the homogeneous ones [10]. Hence, effort towards green preparation of heterogeneous, durable, economical nanocatalysts with enhanced activity is of great current interest.

Here we probe the effects of media on size, morphology, purity, durability, and yield of silver nanopowder (AgNP) fabricated by arc discharge; and examine its green application as a recyclable catalyst for easy and economical reduction of PNP.