Elsevier

Progress in Materials Science

Volume 60, March 2014, Pages 208-337
Progress in Materials Science

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

https://doi.org/10.1016/j.pmatsci.2013.09.003Get rights and content

Abstract

Nanoscale metal oxide materials have been attracting much attention because of their unique size- and dimensionality-dependent physical and chemical properties as well as promising applications as key components in micro/nanoscale devices. Cupric oxide (CuO) nanostructures are of particular interest because of their interesting properties and promising applications in batteries, supercapacitors, solar cells, gas sensors, bio sensors, nanofluid, catalysis, photodetectors, energetic materials, field emissions, superhydrophobic surfaces, and removal of arsenic and organic pollutants from waste water. This article presents a comprehensive review of recent synthetic methods along with associated synthesis mechanisms, characterization, fundamental properties, and promising applications of CuO nanostructures. The review begins with a description of the most common synthetic strategies, characterization, and associated synthesis mechanisms of CuO nanostructures. Then, it introduces the fundamental properties of CuO nanostructures, and the potential of these nanostructures as building blocks for future micro/nanoscale devices is discussed. Recent developments in the applications of various CuO nanostructures are also reviewed. Finally, several perspectives in terms of future research on CuO nanostructures are highlighted.

Introduction

Nanostructured transition metal oxides (MOs), a particular class of nanomaterials, are the indisputable prerequisite for the development of various novel functional and smart materials. These transition MO nanocrystals have been attracting much attention not only for fundamental scientific research, but also for various practical applications because of their unique physical and chemical properties [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. These physical and chemical properties are strongly dependent on the sizes, shapes, compositions, and structures of the nanocrystals. Interesting phenomena such as remarkable increase in surface-to-volume ratio, significant change in surface energy, and quantum confinement effects occur when transition MOs are reduced to nanoscale dimension [7], [20], [21]. These phenomena result in a variety of new physical and chemical properties that are not feasible for materials with bulk dimensionality. Therefore, the manipulation of well-controlled synthesis and fabrication of nanostructured transition MOs with different sizes, shapes, chemical compositions, and structures is crucial in the advancement in nanoscience and nanotechnology. Consequently, various nanostructured transition MOs have been synthesized by diverse chemical, physicochemical, and physical strategies [1], [2], [3], [4], [5], [6], [7], [9], [14], [15], [16], [17], [20], [21], [25], [28]. Compared with their micro or bulk counterparts, nanostructured transition MOs exhibit unique structural characteristics and size confinement effects as well as novel properties. These properties contribute to the potential of transition MOs as candidates for both theoretical studies and practical applications in micro/nanodevices.

Cupric oxide (CuO) has been a hot topic among the studies on transition MOs because of its interesting properties as a p-type semiconductor with a narrow band gap (1.2 eV in bulk) and as the basis of several high-temperature superconductors and giant magneto resistance materials [25], [29], [30], [31], [32], [33], [34], [35]. CuO nanostructures with large surface areas and potential size effects possess superior physical and chemical properties that remarkably differ from those of their micro or bulk counterparts. These nanostructures have been extensively investigated because of their promising applications in various fields. CuO nanostructures are also considered aselectrode materials for the next-generation rechargeable lithium-ion batteries (LIBs) because of their high theoretical capacity, safety, and environmental friendliness [36]. They are also promising materials for the fabrication of solar cells because of their high solar absorbance, low thermal emittance, relatively good electrical properties, and high carrier concentration [37]. Furthermore, CuO nanostructures are extensively used in various other applications, including gas sensors [38], bio-sensors [39], nanofluid [40], photodetectors [41], energetic materials (EMs) [42], field emissions [43], supercapacitors [44], removal of inorganic pollutants [45], [46], photocatalysis [47], and magnetic storage media [48]. Recent studies have demonstrated that nanoscale CuO can be used to prepare various organic–inorganic nanocomposites with high thermal conductivity, high electrical conductivity, high mechanical strength, high-temperature durability, and so on [32], [33], [49], [50]. Moreover, the nanoscale CuO is an effective catalyst for CO and NO oxidation as well asin the oxidation of volatile organic chemicals such as methanol [51], [52], [53]. In addition, some reports have demonstrated the excellent activities of nanoscale CuO as catalyst in the C–N coupling and C–S cross-coupling of thiols with iodobenzene reactions [51], [54], [55]. The superhydrophobic properties of CuO nanostructures render these materials as promising candidates in Lotus effect self-cleaning coatings (anti-biofouling), surface protection, textiles, water movement, microfluidics, and oil–water separation [56]. Thus, nanoscale CuO with different shapes and dimensions, such as zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanotubes, 1D nanowires/rods, two-dimensional (2D) nanoplates, 2D nanolayers, and several complex three-dimensional (3D) nanoflowers, spherical-like, and urchin-like nanostructures have been synthesized using numerous methodologies. More interesting applications of CuO nanostructures are being explored.

Cuprous oxide (Cu2O), another important copper (Cu)-based oxide, is also one of the first known p-type semiconductor materials [57]. However, Cu2O and CuO have striking contrasting colors, crystal structures, and physical properties [58]. Cu2O is a reddish p-type semiconductor of both ionic and covalent nature with cubic structure (space group, Oh4=pn3m) that exhibits various excitonic levels. By contrast, CuO has an iron-dark color with a more complex monoclinic tenorite crystallographic structure (space group, C2/c) and displays promising antiferromagnetic ordering [58], [59]. Cu2O is expected to have an essentially full Cu 3d shell with a direct forbidden band gap of 2.17 eV in bulk, which can only absorb light up to the visible region. CuO has an open 3d shell with a direct band gap (1.2 eV in bulk) of charge-transfer type, which can absorb light up to the near infraredregion [59], [60]. Recent reports have demonstrated that CuO has higher conductivity than Cu2O but with lower carrier mobility [61].

Although these two Cu-based oxides have contrasting properties, both oxides are of considerable interest in photovoltaics, gas sensors, CO oxidation catalysts, various heterogeneous catalysts, and LIBs, because of their low band-gap energy, high optical absorption, high catalytic activity, nontoxic nature, and low-cost [30], [31], [62], [63]. In recent years, the size- and morphology-controlled synthesis and application of Cu2O and CuO have been intensively investigated [25], [28], [29], [30], [31]. However, CuO is more stable than Cu2O because Cu(II) ions are much more stable in ambience, which makes it more important in practical applications. Furthermore, the synthesis, properties, and applications of various Cu2O nanostructures have been extensively reviewed [28], [31], [64], [65], [66]. Therefore, the recent advancement in Cu2O will not be covered in this article to avoid overlapping reviews.

Additionally, compared with other MO nanostructures, such as TiO2 [7], [9], ZnO [14], WO3 [21], and SnO2 [17], CuO nanostructures have more interesting magnetic and superhydrophobic properties. Additionally, these nanostructures demonstrate unique applications in heterogeneous catalysis in the complete conversion of hydrocarbons into carbon dioxide, enhancement of thermal conductivity of nanofluid, nanoenergetic materials (nEMs), and superhydrophobic surfaces. CuO nanostructures as anode materials for LIBs have not been paid as much attention as SnO2 [17], [67] and TiO2 [67], [68]. However, the simplicity of preparation, scalability, non-toxicity, abundance, and low-cost of CuO nanostructures is expected to increase the application of these nanomaterials as anode materials for LIBs. MOs, including SnO2, ZnO, TiO2 along with their various sub-stoichiometric forms [38], are widely considered for gas sensor applications. Thus, the study of CuO for gas sensors is expected to increase rapidly because of the easy synthesis of high-quality and single-crystalline CuO nanostructures.

However, only few reports have described the synthesis strategies adopted for CuO nanostructures along with the introduction of their related applications [25], [29], [31]. Furthermore, most of these review papers only focused on the 1D CuO nanostructures [25], [30], [31]. No review for the systematic introduction of the recent progresses of various CuO nanostructures has been published. This article will begin with a systemic discussion on the synthesis of different CuO nanostructures. For each synthetic method, critical comments will be provided based on our knowledge and related research experience. Next, the associated synthesis mechanisms for controlling the size, morphology, and structure of CuO nanostructures will be addressed. The fundamental properties of CuO nanostructures will also be introduced. The promising applications of 0D CuO nanoparticles, 1D CuO nanotubes, 1D nanowires/rods, 2D CuO nanostructures, and several complex 3D CuO nanostructures along with perspectives in terms of future research on CuO nanostructures will be highlighted. This review aims to provide a critical discussion of the synthesis of CuO nanostructures. The potential of CuO nanostructures as functional components for fabrication of micro/nanodevices are also evaluated and highlighted. In particular, we focus on the fundamental properties and various nanostructured forms of CuO that have been reported in the literature to date and summarize the various synthetic strategies. Promising selections and interesting applications are presented, and finally some perspectives on the future research and development of CuO nanostructures are provided.

Section snippets

Synthesis of CuO nanostructures

The development of synthetic methods has been widely accepted as an area of fundamental importance to the understanding and application of nanoscale materials. It allows scientists to modulate different parameters such as morphology, particle size, size distributions, and composition. Numerous methods have been recently developed to synthesize various CuO nanostructures with diverse morphologies, sizes, and dimensions using various chemical and physical strategies. In this review, we present

Growth mechanisms

The development of nanotechnology has resulted in the fabrication of CuO nanostructures with various morphologies and sizes using different synthetic methods. However, the growth mechanisms responsible for the formation of CuO nanostructures with various morphologies during syntheses are still not fully understood, and extensive studies have been conducted to determine the growth mechanisms of different CuO nanostructures. In this section, we briefly review the most important mechanisms that

Fundamental properties

Table 3 lists the key physical properties of bulk CuO. However, the reduction of CuO dimensions to the nanoscale or even smaller scale results insignificant deviation of some of its physical properties from its bulk counterpart because of the “quantum-size effects.” Therefore, a thorough understanding of the fundamental properties of CuO nanostructures is crucial to their synthesis and applications and a key to the rational design of CuO nanostructure-based functional devices. In this section,

Applications

This section we focus on the recent developments in the different CuO nanostructures as building blocks for applications in a wide range of fields. These fields include LIBs, supercapacitors, sensors, solar cells, photodetectors, catalysis, nanofluid, nanoenergetic materials (nEMs), field emissions, superhydrophobic surfaces, and removal of arsenic and organic pollutants from waste water. The toxicity of CuO nanoparticles is also briefly addressed.

Conclusion and outlook

In summary, CuO nanostructures have been widely studied and are receiving increasing interest because of their interesting properties and promising applications in various areas. In this study, we present a comprehensive review of the state-of-the-art research activities of different CuO nanostructures. We focus on the main synthetic strategies along with associated formation mechanisms, their interesting fundamental properties, and promising applications. Investigation of the synthetic

Acknowledgments

This work was supported by Hong Kong Research Grants Council (Project No. CityU 125412) and NSAF (Grant No. U1330132).

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