Facile one-step deposition of Co₃O₄-MoS₂ nanocomposites using a vacuum kinetic spray process for non-enzymatic H₂O₂ sensing

Abstract

Co₃O₄-MoS₂ nanocomposites (NCs) were deposited on titanium sheets at room temperature using a vacuum kinetic spray process and Co₃O₄ and MoS₂ micro powders. Co₃O₄-MoS₂ NCs/Ti electrodes were used as electrocatalysts for non-enzymatic detection of H₂O₂ in 0.1 M NaOH. X-ray photoelectron spectroscopy revealed an improvement in the synergy between various cobalt-based active sites and MoS₂ species in all hybrid electrodes. Analysis of the amperometric response of H₂O₂ oxidation in 0.1 M NaOH revealed that titanium-modified electrodes with either nanostructured Co₃O₄ or Co₃O₄-MoS₂ NCs exhibited a wide linear detection range of 20 μM to 1mM. A gradual increase in the amount of Co₃O₄ in the modified electrodes led to a reduction in charge-transfer resistance and the evolution of more electro-active sites at the working electrode surface, accompanied by an overall enhancement in detection sensitivity to a peak of 3000 μA·mM⁻¹·cm⁻² at a Co₃O₄ content of 75 wt.%. Besides, the titanium modified electrodes with Co₃O₄-MoS₂ NCs exhibited high selectivity for H₂O₂ oxidation in 0.1 M NaOH.

Introduction

Accurate and real-time evaluation of hydrogen-peroxide concentrations can serve as a useful tool for the diagnosis of various disease stages [1], [2], [3]. Enzymatic biosensors offer efficient and sensitive H₂O₂ detection under certain circumstances [4]. However, high fabrication costs and low recycling reproducibility have restricted the commercialization of enzymatic H₂O₂ detection sensors. Considerable resources have been devoted to developing alternative techniques for non-enzymatic H₂O₂ detection using direct oxidation of H₂O₂ and an electrocatalytic process at the interface between modified electrodes and various metallic or semiconductor nanostructures characterized by a fast response, high stability, and recyclability [5], [6], [7]. However, a metallic non-enzymatic biosensor reportedly displayed high electrocatalytic efficiency toward the H₂O₂ detection [8,9], and high costs continue to restrict the commercial application of metallic-based biosensors. Transition-metal electrocatalysts with high electrocatalytic activity are popular alternatives for metallic electrocatalysts because of their availability and low costs, as well as the chemical stability associated with resistance to corrosion [10], [11], [12]. Co₃O₄-based materials and composites are promising electrocatalysts for non-enzymatic detection of H₂O₂, either by oxidation or reduction with effective sensitivity and a wide linear-detection range [13], [14], [15], [16], [17], [18]. The pure phase of nanostructured Co₃O₄ offers a wide surface area and a high concentration of electroactive sites, but poor electrical conductivity and the tendency of the internal grains to aggregate may result in slow charge-transfer kinetics and reduce electrochemical performance [19]. One of the most effective techniques to improve the charge transfer kinetics of nanostructured Co₃O₄ is to assemble hybrid composites with layered structures such as carbon-based or dichalcogenide materials, including MoS₂. The synergy enhancement between the electroactive species of spinel Co₃O₄ (i.e., Co²⁺ and/or Co³⁺) and such layered structures increases the electroactive species concentration as well as the development of charge-transfer kinetics, which are accompanied by an overall enhancement in electrochemical performance. In a previous study, we fabricated a hybrid electrocatalyst of Co₃O₄-graphene nanosheets with superior detection sensitivity to H₂O₂ reduction. The hybrid electrocatalyst was associated with enhanced charge transfer and an increase in electroactive-site concentration due to the high spin state of spinel Co₃O₄ (i.e. Co²⁺) [13]. Wang et al. [20] prepared nanostructured Co₃O₄/glassy carbon using pyrolysis, with the results exhibiting a relatively wide linear H₂O₂ detection range of 0.4 μM to 2.2 mM in 0.1 M NaOH and relatively low sensitivity of 120.55 μA·mM⁻¹·cm⁻². Dai et al. [16] synthesized Au@C-Co₃O₄ hybrid electrodes using high-temperature pyrolysis at 450°C for 4 h as well as oxidative calcination at 250°C for 2 h. The modified electrocatalyst achieved a sensitivity toward H₂O₂ reduction of 7553 μA·mM⁻¹·cm⁻² within a small linear detection range of 0.05 to 100 μM. Kong et al. [18] reported preparing Co₃O₄-reduced graphene oxide (rGO) nanostructured composite electrocatalysts using a multi-stage hydrothermal process, with the resulting composite exhibiting relatively high H₂O₂ sensitivity of 1140 μA·mM⁻¹·cm⁻². Kogularasu et al. [21] synthesized polyhedral Co₃O₄-rGO composites electrocatalysts with high activity toward the reduction of H₂O₂, a detection sensitivity of 3450 μA·mM⁻¹·cm⁻², and relatively small linear detection range of 0.05 to 400 μM.

In our previous work, we deposited various thin-film semiconductor nanostructures that demonstrated high electrochemical performance for supercapacitor application [22], high electrocatalytic activity toward oxygen and hydrogen evolution reactions in alkaline medium [23,24], and superior non-enzymatic detection of H₂O₂ reduction in alkaline medium [13] using a nanoparticle deposition system (NPDS), which is a vacuum kinetic spray process. The NPDS involved the deposition of nanostructured ceramic-based materials and their alloys at room temperature without any binders or dangerous chemicals [25,26]. In contrast with other deposition techniques, the NPDS takes into consideration the cost and fabrication time of the desired electrocatalyst [27]. The NPDS can also be scaled up for large deposition areas, such as those involved in WO₃ thin films with areas of 1 × 1 m² [28].

Based on a literature survey, nanostructured Co₃O₄-MoS₂ hybrid electrocatalysts have not been previously examined as non-enzymatic sensors for H₂O₂ oxidation. In the present work, we fabricated titanium-modified electrodes with Co₃O₄-MoS₂ nanocomposites (NCs) containing 25, 50, and 75 wt.% Co₃O₄ using a one-step deposition by the NPDS at room temperature without any post-treatment after the deposition process. The results revealed high electrocatalytic activity toward the oxidation of H₂O₂ in an alkaline medium. Compared with the other reported techniques, ours is a simple, one-step process that can rapidly generate composite catalysts with no chemical hazards, and in a manner suitable for scaling up to large-volume production. We also evaluated the electrochemical performance of non-enzymatic detection of H₂O₂ oxidation by Co₃O₄-MoS₂ NC/Ti with different Co₃O₄ contents in an alkaline medium.