Ag₂SO₄ decorated with fluorescent Ag_n nanoclusters

Abstract

Here we report on the production of an Ag₂SO₄/Ag₂O mixed-grain powder during the anodization of Ag foil in a HF-H₂SO₄ electrolyte. We propose that there are three competing reactions during the anodization process: (i) the production of Ag₂O at the Ag foil anode surface from the presence of water in the electrolyte, (ii) the dissolution of the Ag₂O in the presence of HF releasing Ag⁺ ions, (iii) the precipitation of Ag⁺ and SO₄²⁻ ions, as Ag₂SO₄ on the Ag foil anode surface. This co-precipitation/dissolution process ultimately results in a mixed-grain powder. We then show that the Ag₂O embedded within the mixed-grain is photo-decomposed to produce highly fluorescent silver nanoclusters (Ag_n) which decorate the Ag₂SO₄ crystals. The Ag₂SO₄ salt offers a stable matrix for the photo-decomposed Ag_n nanoclusters to emit their strong fluorescence.

Introduction

The strong fluorescence of silver nanoclusters (Ag_n) has been widely studied [1], [2]. A limiting factor is the stability of these nanostructured systems due to the small number of atoms that make up the clusters. The challenge therefore is to design a matrix which stabilizes the Ag_n nanoclusters and maximizes the fluorescent effect. Previously reported matrices include glass [2], dendrimer encapsulation [3], thiol-capping [4], peptide-capping [5], zeolites [6] and argon droplets [7]. In this report, we will demonstrate that synthesized Ag₂SO₄ powders can effectively be used to stabilize Ag_n nanoclusters so that the fluorescence is preserved.

The investigation of silver salts has, in the past, focussed on their application in superconductive materials [8], [9]. Recent work has shown that the preparation of a sulfate of divalent silver, AgSO₄, is neither a mixed valence Ag(I)/Ag(III) compound nor a peroxodisulfate of Ag(I) [10], [11]. However, a more widely available silver salt is non-fluorescent Ag₂SO₄ which has been used in a broad range of applications, including uses as a synthetic reagent, catalyst, photographic agent and, more recently, as an anti-microbial and anti-fungal agent [12], [13].

An attractive option for the fabrication of Ag₂SO₄ is electrochemical synthesis which facilitates fast, easy, cost-effect operation and allows the tuning of key growth parameters such as current density, potential, temperature and solution concentrations [14]. For example, cathode reduction was employed to fill deep silicon pores with metal via electrodeposition [15], to fabricate semiconductive single nanowires or crystalline thin films via electrocrystallization [16] whilst anodization to etch porous structures by breaking through the double-charged layer [17] and to synthesize Ag₂O nanowires or hollow platelets by oxidation, etc. [14].

In this work we have anodized silver foil in HF-H₂SO₄ solution to synthesize an Ag₂SO₄ amorphous powder with particles sizes in the range of 1–3 μm. Interestingly, we observed that the as-prepared salt exhibited a strong fluorescence. We propose that the Ag₂SO₄ amorphous powder has been decorated with Ag₂O sub-structures, which are easily photo-decomposed to Ag_n silver nanoclusters [1], [2]. The unexpected fluorescence thus arises from the intra-band transitions of the free electrons within the Ag_n silver nanoclusters and/or the energy transfer from Ag_n silver nanocluster surface plasmon to the silver salt/oxide matrix. This novel material has potential uses as fluorescent markers in anti-microbial and anti-fungal processes, chemical and biological systems [12], [13].