In situ H₂O₂ generation for tuning reactivity of V₄O₇ nanoflakes and V₂O₅ nanorods for oxidase enzyme mimic activity and removal of organic pollutants

Highlights

• Hydrothermal Synthesis and characterization of V₄O₇ and V₂O₅ Nanorod.

• Oxygen vacancies tuning photophysical, catalytic, and photocatalytic activity of V₄O₇ and V₂O₅.

• Oxidase-like enzyme activity of V₄O₇ enhanced due to V⁺³/V⁺⁴ equilibrium.

• Oxygen vacancies in V₄O₇ is a dominant factor to generate hydrogen peroxide.

Abstract

Surface oxidation state mediated oxygen vacancies of metal oxide enhance oxidase-enzyme mimic, catalytic, and photocatalytic activity of the catalyst. Herein, we prepared the mixed valence V₄O₇ (V⁺³/V⁺⁴), which was thermally (650 °C) oxidized to produce V₂O₅ (V⁺⁵) nanorods. For the first time, we demonstrate that vanadium oxides (V₄O₇ and V₂O₅) exhibit catechol oxidase-like activity using the in situ production of H₂O₂. In addition, both metal oxides exhibit photocatalytic activity for organic pollutants removal in absence and enhanced in presence of H₂O₂. Catalytic and photocatalytic activities were modulated by the change in surface oxidation sate and oxygen vacancies tuned through the facile charge immobilization on V₄O₇ (V⁺³/V⁺⁴) to the less active V₂O₅ (V⁺⁵). The V₄O₇ shows enzyme activity higher than the corresponding V₂O₅ nanorods. On the other hand, the mixed valance V₄O₇ (V⁺³/V⁺⁴) was superior photocatalyst for remediation of methylene blue as it completely removes the dye in less than 15 min in presence of visible light.

Introduction

Artificial enzymes have always been the interest of researchers to mimic the natural enzymes [1]. In addition to the low cost, high stability and long-term storage; artificial enzymes are easy to mass-produce. In particular, nanomaterials have attracted a considerable interest as an enzyme mimic materials not only for their high surface to volume ratio, which is advantageous for bioconjugation for further applications [2]. The potential activity of nanomaterials as peroxidases was intensively investigated. For example, Fe₃O₄ magnetic nanoparticles show a peroxidase mimic activity, which is used for H₂O₂ detection [3]. Peroxidase mimic nanomaterial, Fe-doped and N-doped carbon nanostructure, was developed by Zhang and coworkers [4] and further used for H₂O₂ detection. A modified glassy carbon electrode with Fe₃O₄/reduced graphene oxide nanocomposite was used for electrochemical sensing of H₂O₂ [5]. The bimetallic Au_xPt_y nanomaterial were prepared and investigated for their peroxidase mimic activity and subsequently used in biothiols detection [6]. On the other hand, reports related to the ability of nanomaterials as oxidase-like enzyme activity is comparatively scarce. For example, cerium oxide nanoparticles (nanoceria) [7], [8], [9], selenium nanoparticles [10], MnFe₂O₄ [11]_, CoFe₂O₄ [12]_, Au@Pt [13], Pt nanoparticles [14] exhibit oxidase-like enzyme activity.

Metal oxides (MOs) semiconductors have been recently used in different applications [15], [16], [17]. In particular, MOs are widely used as photocatalytic agents due to the efficient and economic catalytic materials [18], [19]. This is due to the special features for the MOs such as high surface to volume ratio and efficient surface charge separation upon light exposure. Several techniques have been used to increase the charge separations in MOs such as doping and impregnation with metal ions [20], [21], [22]. The oxides of vanadium (V_xO_y) have been intensively investigated due to the widely use in different applications. The oxidation state of vanadium varies in its oxides from V³⁺, V⁴⁺ and V⁵⁺ for V₂O₃, VO₂, and V₂O₅ with a change in color from green, blue and yellow, respectively. While mixed valance oxides such as V₄O₇ and V₃O₆ have been reported [23], [24]. For examples, V₂O₅ nanoparticles prepared by sol gel method were investigated for their high rate lithium batteries applications [25]. In another study, V₂O₅ nanoparticles prepared by hydrothermal method were used for photocatalytic activity with methyl orange dye (MO) under UV irradiation [26]. RuO₂ quantum dots@V₂O₅ were prepared and showed a larger electrochemical performance than the pure V₂O₅ cathode [27]. Furthermore, several studies have reported fabrication of different vanadium oxidation nanomaterials with different oxidation states. For example, synthesis of ultra-long VO₂ (A) nanobelts were performed by hydrothermal method using V₂O₅ sol as precursor. Then, polyethylene glycol was used both as reducing and surfactant agent [28]. Gao and coworkers [29] uses the solvothermal process for the synthesis of VO₂ (B) by the reduction of V₂O₅ in ethylene glycol (EG). Both the phase and morphology of the resulting nanomaterials were controlled by the heating rate. On the other hand, Du and coworkers [23] reported the first preparation of nanocross V₄O₇ using solvothermal method in ethylene glycol-water mixed media, which show a potential applications in lithium ion batteries due to the exceptional cyclic stability [23]. Both EG and PEG were used to as template agents in order to control VO morphology [28], [29], [30]. Oxygen vacancies in V₂O₅ were generated by annealing under N₂ atmosphere and high temperature, which enhanced the reversible capacity and lithium storage properties [31]. In addition, oxygen vacancies in VO₂ were a dominant factor to enhance the CO₂ sensing [32].

In addition to the numerous applications of VO, several reports investigated the peroxidase activity of such nanomaterials [24], [33], [34], [35]. However, to the best of our knowledge, no reports were related to the use of VO as oxidase-like enzymes. In addition, effect of the surface oxidation state of vanadium on the reactivity is of huge interest. Herein, we report the hydrothermal preparation and characterizations of V⁺⁵ (V₂O₅) and mixed valance V⁺³/V⁺⁴ (V₄O₇) nanowires. The samples were systematically investigated by analytical techniques such as XRD, XPS, FTIR, BET, EPR, TEM and SEM microscopes. The potential activity of V₂O₅ nanosheets and V₄O₇ nanowires oxidase-like enzymes were explored in 10 mM sodium acetate buffer (pH 7.2). In addition, the photocatalytic activity of V₂O₅ and V₄O₇ was tested for removal of organic pollutants.