Electrocatalytic oxidation and reduction of H₂O₂ on vertically aligned Co₃O₄ nanowalls electrode: Toward H₂O₂ detection

Abstract

Single crystal and vertically aligned cobalt oxide (Co₃O₄) nanowalls were synthesized by directly heating Co foil on a hot-plate under ambient conditions. The vertically aligned Co₃O₄ nanowalls grown on the plate show excellent mechanical property and were facilely attached to the surface of a glassy carbon (GC) electrode using conductive silver paint. The prepared Co₃O₄ nanowalls electrode was then applied to study the electrocatalytic oxidation and reduction of hydrogen peroxide (H₂O₂) in 0.01 M pH 7.4 phosphate buffer medium. Upon the addition of H₂O₂, the Co₃O₄ nanowalls electrode exhibits significant oxidation and reduction of H₂O₂ starting around +0.25 V (vs. Ag/AgCl), while no obvious redox activity is observed at a bare GC electrode over most of the potential range. The superior electrocatalytic response to H₂O₂ is mainly attributed to the large surface area, minimized diffusion resistance, high surface energy, and enhanced electron transfer of the as-synthesized Co₃O₄ nanowalls. The same Co₃O₄ nanowalls electrode was also applied for the amperometric detection of H₂O₂ and showed a fast response and high sensitivity at applied potentials of +0.8 V and −0.2 V (vs. Ag/AgCl), respectively. The results also demonstrate that Co₃O₄ nanowalls have great potential in sensor and biosensor applications.

Introduction

Due to its strong oxidizing property, hydrogen peroxide (H₂O₂) is widely used in many fields. For example, H₂O₂ is useful for the synthesis of various organic compounds, food production, pulp and paper bleaching, sterilization, and clinical applications [1], [2]. H₂O₂ is also used as an oxidant for many liquid-based fuel cells [3], [4], [5], [6], [7], [8], [9], and is readily present in a variety of commercial products such as cosmetic and pharmaceutical products [10], [11]. Further, H₂O₂ has emerged as an important by-product of enzymatic reactions in the field of biosensing [12], [13], [14], [15], [16], [17], [18], [19]. Thus, the detection and quantification of H₂O₂ remains a significant endeavor in a variety of fields.

Many analytical methods have been developed for the detection and quantification of H₂O₂ [11]. Among them, titration [20], spectrophotometric [21], [22], [23], fluorometric [24], [25], [26], [27], [28], chemiluminescent [29], [30], [31], and chromatographic [32], [33], [34] techniques are well known. However, electrochemical methods have emerged as preferable, owing to their relatively low cost, efficiency, high sensitivity, and ease of operation [35], [36], [37]. Different materials, such as noble metals, macrocycle complex of transition metals, carbon nanotubes, and enzymes have been used to modify electrodes for the reduction/oxidation as well as the detection of H₂O₂ [1], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. In recent years, new efforts have been emphasized on the use of novel metal oxides nanomaterials to modify electrode surfaces for enhanced oxidation/reduction and sensitive detection of H₂O₂ because metal oxide nanomaterials are easy to be synthesized, have very high surface to volume ratio, and show great potential to enhance electrocatalytic activity and promote electron-transfer reactions at a lower overpotential [1], [11], [47], [48].

Recently, several metal oxide nanomaterials have been deposited on the surface of electrodes and investigated for the reduction/oxidation and detection of H₂O₂. Cobalt oxide (Co₃O₄) [11], [47] and copper oxide (CuO) nanoparticles [48] have been proved to perform well as H₂O₂ reduction catalysts in strong basic solution, as both Co₃O₄ and CuO are useful for transferring electrons between H₂O₂ and an electrode while enabling regeneration after electron exchanges with H₂O₂. However, the nanoparticles cannot stand alone and require, for example, a matrix to entrap them for electrode attachment. A major disadvantage of entrapment of metal oxide nanoparticles is the additional diffusion resistance offered by the entrapment material. Additionally, the strong basic solution (0.1–3 M NaOH) used in these studies greatly limits their application. The direct electrodeposition of cobalt oxide nanoparticles on the surface of a glassy carbon (GC) electrode has also been used to yield both particles with an average size of 100 nm and large agglomerated particles (∼200–600 nm) [1]. The modified rotating electrode (2000 rpm) was then applied to the sensitive detection of H₂O₂ based on the oxidation current at +0.75 V (vs. Ag/AgCl). Even though the performance of the system is impressive, the needs for a rotating electrode and for more work to produce uniform particles with smaller size may limit its further application. Therefore, there remains a need for simpler processes to fabricate novel metal oxide nanomaterials with superior catalytic property for fast, sensitive, reliable, and stable detection of H₂O₂. Preferably, the novel metal oxide nanomaterials are free-standing, have excellent mechanical and electrocatalytic properties, and can be easily manipulated and attached to an electrode surface without the help of an entrapment matrix.

In this communication, we reported the synthesis of single crystal and vertically aligned cobalt oxide nanowalls, the fabrication of Co₃O₄ nanowalls electrode, and its application in the electrocatalytic oxidation and reduction of H₂O₂ in 0.01 M pH 7.4 phosphate buffer. The same electrode was also applied for the sensitive amperometric detection of H₂O₂ based on the oxidation and reduction currents at applied potentials of +0.8 V and −0.2 V (vs. Ag/AgCl), respectively. The as-prepared Co₃O₄ nanowalls show excellent mechanical and electrocatalytic properties, and have great potential for applications in electrochemical detection.