Shape-selective synthesis of gold nanoparticles and their catalytic activity towards reduction of p-nitroaniline

Highlights

• Rapid synthesis of AuNPs in 6 min under microwave irradiation using NaOH, PVP and DMF.

• Shape of the AuNPs, tuned among truncated cube, cube, tetrahedral and octahedral.

• Preferential adsorption of Na ion on the surface of Au leads to the formation of different shapes.

• AuNPs exhibited excellent catalytic activity towards reduction of p-nitroaniline.

Abstract

Microwave assisted shape-selective synthesis of gold nanoparticles has been achieved in a short reaction time of 6 min. Shape of the gold nanoparticles has been found to be tuneable among truncated cube, cube, tetrahedral and octahedral by changing the concentration of aqueous NaOH into the reaction mixture containing HAuCl₄, N,N-dimethylformamide (DMF) and polyvinylpyrrolidone (PVP) under microwave irradiation. The preferential adsorption of sodium ions on gold surface during crystal growth controls the shape of the nanoparticles. The chemical state, crystallographic structure, elemental composition and morphology of the as-prepared gold nanoparticles were analyzed using respective XPS, powder XRD, EDS and FEG-SEM analytical techniques. Catalytic performance and recyclability of the as-prepared gold nanoparticles were examined towards the reduction of p-nitroaniline into p-phenylenediamine using sodium borohydride and continuous monitoring the reaction progress using UV-Visible spectrophotometer under ambient conditions.

Introduction

Shape-selective synthesis of noble metal nanoparticles has gained considerable attention of researchers because of their shape-dependent properties and applications [[1], [2], [3], [4], [5]]. Noble metal nanoparticles crystallized in face centered cubic (fcc) structure have very different surface atom densities in their various crystal facets such as {111}, {100} and {110} hence exhibit different chemical reactivities [[6], [7], [8], [9]]. Among the metal nanoparticles of various shapes, those enclosed by high-index facets often display different or enhanced catalytic properties in comparison to those enclosed of low-index facets [[10], [11], [12], [13], [14]]. Among the noble metals, gold nanoparticles (AuNPs) have been explored extensively due to a combination of properties including molecular sensing [[15], [16]], surface plasmon resonance [17], catalytic applications and tailorable surface enhancing spectroscopic effects [[18], [19], [20]]. In recent years, synthesis of anisotropic AuNPs has been found to be fascinating hence, various effort have been made in synthesis of anisotropic AuNPs [[21], [22], [23], [24], [25], [26], [27], [28], [29]].

One of the most effective method for shape-selective synthesis of AuNPs involves seed mediated growth of nanoparticles under the influence of shape-directing additives [[3], [30], [31], [32]]. Seed mediated synthetic approach generally involves two steps first, the fast reduction of gold precursor into fine seed particles using strong reducing agent and the second step facilitates growth of the seed particles into various shape using a mild reducing agent and shape directing additive [33]. Another widely used method having precise control over AuNPs shape involves the use of silver (Ag) ions taking the benefit of Ag under potential deposition (UPD) on the gold (Au) surfaces, which hinder newly generated Au atoms from being deposited over the Au surfaces covered with Ag [[34], [35]]. In this regard, Personick et al. have demonstrated the use of Ag ions in the synthesis of AuNPs having predominant growth of various facets [[36], [37], [38]]. M.F. Mohamad et al. have reported the microwave assisted polyol method for the synthesis of AuNPs and described the effect of sodium chloride concentration on the shape of the gold nanoparticles. However their results indicate poor control over the size and shape of the obtained AuNPs [39].

In recent years, DMF has been used as an reducing agent for Au${}^{3+}$, Pt${}^{2+}$ and Pd${}^{2+}$ ions into the corresponding metal under appropriate conditions [[40], [41]]. In this regard, Yamamoto et al. [[42], [43]] and Yao et al. [44] have carried out the synthesis of AuNPs using DMF as an reducing as well as a capping agent. AuNPs With various shapes such as, trisoctahedron, nanostars [45] and polyhedron [46] have been prepared using DMF and PVP. Herein, we report a rapid, microwave assisted process for shape-selective synthesis of gold nanoparticles. AuNPs in various shapes such as truncated cube, cube, tetrahedrons and octahedrons are obtained by introducing a trace amount of aqueous NaOH into the synthesis mixture containing HAuCl₄, DMF and PVP under microwave irradiation in 6 min. Catalytic performance and recyclability of the as-prepared gold nanoparticles were examined towards the reduction of p-nitroaniline into p-phenylenediamine using sodium borohydride and continuous monitoring the reaction progress using UV-Visible spectrophotometer under ambient conditions.

Microwave energy enhances the nanoparticle formation rate by providing the uniform volumetric heating throughout the reaction medium [[47], [48], [49]]. The dielectric loss constant of a particular solvent determines its ability to gain heat during microwave irradiation hence, selection of a solvent is crucial in microwave assisted synthesis. The dielectric loss constant value of DMF is 36.7; hence, DMF has been widely utilized as a solvent for synthesis of nanoparticles under microwave irradiations.