Shape control mechanism of cuprous oxide nanoparticles in aqueous colloidal solutions

Abstract

Cuprous oxide (Cu₂O) nanoparticles with various particle shapes were synthesized via colloidal chemistry approach. Using ascorbic acid as a reducing agent and polyvinylpyrrolidone (PVP) as a surfactant, cuprous oxide monocrystalline nanoparticles with various sizes have been successfully synthesized by reaction of bivalent copper in aqueous solution. The samples were characterized by XRD, SEM and UVs spectroscopy. The results indicate the shape of as-prepared cuprous oxide nanoparticles has close relationship with thermodynamic conditions, kinetic conditions and stirring rate. Self-assembly mechanism of cubic cuprous oxide monocrystalline nanoparticles has also been investigated.

Introduction

Cuprous oxide and cupric oxide have been investigated for decades due to their unique semiconductor and optical properties [1]. As a P-type semiconductor material, the theoretical direct band gap of cuprous oxide is about 2.2 eV [2]. In aspect of photoluminescence, cuprous oxide has a very long excited lifetime (about 10 μs) [3], [4]. Solid-state cuprous oxide also shows a light-emitting coherence character [5]. Cuprous oxide has potential applications in solar cells [6], nano-magnetic devices [7], chemical industry [8], biosensors [9] and so on. It is also reported that cuprous oxide microspheres have been used as cathode material of lithium battery and photocatalyst in the visible light which led to photochemical decomposition of H₂O and generation of O₂ and H₂ [10].

Controlled fabrication of semiconductor nanomaterial is of special interest in the field of nanoscience and nanotechnology [11], because a structure–function relationship is the underlying motive for discovering novel nanoscale structures. The preparation of different forms of cuprous oxide particles are mainly prepared by liquid chemistry method.

In the past few years, numerous Cu₂O nanostructures, including nanosheets [12], nanocubes [13], octahedra [14], spherical particles [15], nanoboxes [16], and nanowires [17], [18] have been synthesized. The geometry shape control and detailed crystal structure analysis of cuprous oxides have been performed on these Cu₂O nanocrystals. However, the growth mechanism, which is important for the controlled synthesis of Cu₂O nanocrystals, still needs a detailed investigation.

Among the above methods for the preparation of Cu₂O nanoparticles, flammable or corrosive reducing agent such as titanium tetrachloride [12] and hydrazine [15], [17] was used. Other wet-chemical synthesis methods adopt toxic organic reactant such as ethylene glycol [16]. Ethylene glycol is easy oxidized in air to glycolic acid which is corrosive, in turn, oxidized to oxalic acid, which is toxic. Such toxic organic reactant was not suitable in industrial production because of toxicity and relatively high material cost. Additional assistant reducing agent such as sodium tartrate was required in certain method [14]. High temperature (over 100 degrees) heating step (oil bath) was required in some synthesis process [16]. On the other hand, significant energy was demanded for high temperature reaction environment [16] and electrodeposition [18] in case of large-scale synthesis and there are many factors of insecurity in procedure. Templates, cetyl trimethyl ammonium bromide (CTAB) [17] and anodic aluminum oxide (AAO) [18] have been successfully employed to prepare Cu₂O nanocrystals with specific morphologies, such as nano-whiskers and nanowires. A simple and green synthetic route for nanosized Cu₂O nanostructures with single-crystalline is sorely needed. The single-crystalline Cu₂O with varied nanostructures are produced by a one-pot, solution-phase and template free method.

In this paper, we adopt a colloidal chemistry approach for the syntheses of monodispersed Cu₂O nanocubes, truncated nanocubes, cuboctahedra, nanosphere, and octahedra by adjusting experimental conditions, detecting the influence of experimental conditions on the morphology evolution of Cu₂O nanocrystals. Based on our observation and analysis, the growth mechanism of Cu₂O nanocrystals is elucidated.