Shape-controlled synthesis of one-dimensional α-MnO₂ nanocrystals for organic detection and pollutant degradation

Highlights

• One-dimensional α-MnO₂ nanocrystals with different shapes were synthesized.

• α-MnO₂ shows shape related activities for detection of H₂O₂ and l-ascorbic acid.

• α-MnO₂ shows shape related activities in activation of peroxymonosulfate.

• Different activities are due to different surface areas and surface charge densities.

Abstract

Shape control is an important technique for improving the quality and activity of nanomaterials. Two types of one-dimensional manganese dioxide (MnO₂) nanocrystals with different shapes were synthesized by facile hydrothermal methods as the catalyst materials for both sensor fabrication and heterogeneous catalytic reactions. The nanomaterials present an α-crystalline phase (α-MnO₂) in either nanotube or nanowire shapes. The α-MnO₂ nanocrystals were found to have a favorable electrochemical property that can be used to fabricate sensors for rapid detection of hydrogen peroxide and l-ascorbic acid. The α-MnO₂ also functioned well as a catalyst for the oxidation of phenol and chlorophenol by peroxymonosulfate and hydrogen peroxide in an aqueous solution at room temperature. Comparison between the two differently shaped α-MnO₂ catalysts indicated that nanowires performed better than nanotubes in both electrocatalytic detection and catalytic phenol degradation. Compared to α-MnO₂ nanotubes, nanowires have a much greater surface area and lower negative surface charge density, which are probably the main reasons for their higher catalytic activities.

Introduction

Manganese dioxide (MnO₂) is one of a group of attractive inorganic materials that can catalyze electrochemical reactions for various environmental applications [1], [2]. For example, with its transition to different oxidation states, MnO₂ has been found to be sensitive for the detection of chemical oxidants and reductants, such as hydrogen peroxide (H₂O₂) and l-ascorbic acid (AA), in water [3], [4]. Nano-sized MnO₂ particles are also promising catalysts for catalytic degradation of organic pollutants. Ye and coworkers synthesized α-MnO₂ with different shapes for the catalytic oxidation of naproxen [5]. Zhu and coworkers prepared α-, β-, γ- and δ-MnO₂ using a hydrothermal method and evaluated their activities for carbon monoxide oxidation [6]. MnO₂ materials also perform well in Fenton-like reactions, such as β-MnO₂ nanorods for methylene blue oxidation by H₂O₂ [7]. Recently, Wang and coworkers tested various forms of manganese oxides for the catalytic activation of peroxymonosulfate (PMS) and found α-MnO₂ to be the best form for chemical oxidation by PMS [8].

H₂O₂ has been widely applied in industrial processes and domestic uses as a universal oxidant, and it is also a very important intermediate in environmental and biological reactions [9]. AA is known for its antioxidant properties in food and drink and its importance in several human metabolic processes involving oxidations and reductions [10]. Rapid detection of H₂O₂ and AA is therefore of great importance and has attracted research attention for quite some time. It is desirable to develop sensor materials for electrodes that can be readily used to detect H₂O₂, AA and similar chemicals in water.

Water pollution is one of serious environmental problems. Toxic organic pollutants from industrial sources often cause long-term water pollution and threaten ecological systems [11], [12]. It is difficult to remove toxic organics from wastewater discharge by conventional biological wastewater treatment methods [13], [14]. Adsorption can be effective for chemical separation [15]; however, it cannot degrade organic pollutants and may result in secondary pollution during the processing and disposal of the adsorbents [16], [17]. Catalytic oxidation has been promoted as a “green” oxidation technology for wastewater treatment with its advantages of detoxification and organic degradation without generating secondary pollutants [8]. Several oxidants, such as H₂O₂, O₂, O₃ or S₂O₈²⁻, may be used for oxidation processes, for which effective catalysts are needed [18], [19], [20], [21]. Recently, PMS was proposed as a more effective oxidant for the decomposition of organics [22]. PMS can generate sulfate radicals that non-selectively degrade organics to harmless products such as CO₂, H₂O and inorganic ions [23]. Cobalt ions were first used as a homogenous catalyst for the activation of PMS and the generation of sulfate radicals [22]. Nano-sized heterogeneous catalysts were then developed for more effective activation of PMS, including metal-based materials, such as those involving Co [24] and Ru [25], and graphene-based metal-free catalysts [26].

Shape-controlled synthesis of nanomaterials is an important technique for improving their activity in catalysis reactions. Many materials, including noble metals and metal oxides, have shown shape-related activities. However, the shape effect of α-MnO₂ on sensor fabrication and catalytic activity has not been reported. In this study, we synthesized both α-MnO₂ nanotubes and nanowires using different facile hydrothermal methods. The sensors fabricated by α-MnO₂ nanocrystals were characterized for their electrochemical properties in the detection of H₂O₂ and AA in water. The morphology-dependent activity of α-MnO₂ was also investigated for the catalytic degradation of phenol and chlorophenol by PMS at room temperature. The MnO₂ nanowires were found to be more active than the nanotubes in both electrochemical detection and catalytic oxidation reactions.