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CuO nanostructures: Synthesis, characterization,

growth mechanisms, fundamental properties,

and applications

Qiaobao Zhang a, Kaili Zhang a,, Daguo Xu a, Guangcheng Yang b, Hui Huang b,Fude Nie b, Chenmin Liu c, Shihe Yang d

a Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kongb Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, Chinac Nano and Advanced Materials Institute, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kongd Department of Chemistry, William Mong Institute of Nano Science and Technology, Hong Kong University of Science and

Technology, Clear Water Bay, Kowloon, Hong Kong

a r t i c l e i n f o

Article history:

Received 21 May 2013Receivedin revised form 27 September 2013Accepted 29 September 2013Available online 5 October 2013

a b s t r a c t

Nanoscale metal oxide materials have been attracting much atten-tion because of their unique size- and dimensionality-dependentphysical and chemical properties as well as promising applicationsas key components in micro/nanoscale devices. Cupric oxide (CuO)nanostructures are of particular interest because of their interest-ing properties and promising applications in batteries, supercapac-itors, solar cells, gas sensors, bio sensors, nanofluid, catalysis,photodetectors, energetic materials, field emissions, superhydro-phobic surfaces, and removal of arsenic and organic pollutantsfrom waste water. This article presents a comprehensive reviewof recent synthetic methods along with associated synthesis mech-anisms, characterization, fundamental properties, and promisingapplications of CuO nanostructures. The review begins with adescription of the most common synthetic strategies, characteriza-tion, and associated synthesis mechanisms of CuO nanostructures.Then, it introduces the fundamental properties of CuO nanostruc-tures, and the potential of these nanostructures as building blocksfor future micro/nanoscale devices is discussed. Recent develop-ments in the applications of various CuO nanostructures are alsoreviewed. Finally, several perspectives in terms of future researchon CuO nanostructures are highlighted.

2013 Elsevier Ltd. All rights reserved.

0079-6425/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.pmatsci.2013.09.003

Corresponding author.

E-mail addresses:[email protected],[email protected](K. Zhang).

Progress in Materials Science 60 (2014) 208337

Contents lists available at ScienceDirect

Progress in Materials Science

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2092. Synthesis of CuO nanostructures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

2.1. Solution-based methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

2.1.1. Hydrothermal synthetic method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2122.1.2. Solution-based chemical precipitation methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

2.2. Solid-state thermal conversion of precursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2272.3. Electrochemical method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2312.4. Thermal oxidation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2322.5. Other synthetic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

3. Growth mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2363.1. Oriented attachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2363.2. Ostwald ripening process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2423.3. Scroll of Cu(OH)2nanosheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2443.4. Stress and grain-boundary diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2453.5. Stress-induced cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

4. Fundamental properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2514.1. Crystal structures and phase transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2514.2. Electronic band structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2534.3. Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2594.4. Electrical conductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2624.5. Photoelectrochemical (PEC) properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2664.6. Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2705.1. Application in LIBs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2705.2. Application in supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2745.3. Application in sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

5.3.1. Application in gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

5.3.2. Application in enzyme-free glucose electrochemical sensors . . . . . . . . . . . . . . . . . . . . 2885.4. Application in solar cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2905.5. Application in photodetectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2925.6. Application in catalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2985.7. Application in photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3015.8. Application in enhancement of thermal conductivity of nanofluid . . . . . . . . . . . . . . . . . . . . . . . 3075.9. Application in nEMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3085.10. Application in field emission displays (FEDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3115.11. Application in superhydrophobic surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3125.12. Application in removal of arsenic (As) and organic pollutants from waste water . . . . . . . . . 3165.13. Toxicity of CuO nanoparticles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318

6. Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322

1. Introduction

Nanostructured transition metal oxides (MOs), a particular class of nanomaterials, are the indisput-able prerequisite for the development of various novel functional and smart materials. These transi-tion MO nanocrystals have been attracting much attention not only for fundamental scientificresearch, but also for various practical applications because of their unique physical and chemical

properties [128]. These physical and chemical properties are strongly dependent on the sizes, shapes,compositions, and structures of the nanocrystals. Interesting phenomena such as remarkable increasein surface-to-volume ratio, significant change in surface energy, and quantum confinement effectsoccur when transition MOs are reduced to nanoscale dimension [7,20,21]. These phenomena result

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in a variety of new physical and chemical properties that are not feasible for materials with bulkdimensionality. Therefore, the manipulation of well-controlled synthesis and fabrication of nanostruc-tured transition MOs with different sizes, shapes, chemical compositions, and structures is crucial inthe advancement in nanoscience and nanotechnology. Consequently, various nanostructured transi-tion MOs have been synthesized by diverse chemical, physicochemical, and physical strategies[17,9,1417,20,21,25,28]. Compared with their micro or bulk counterparts, nanostructured transitionMOs 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 theoreticalstudies and practical applications in micro/nanodevices.

Cupric oxide (CuO) has been a hot topic among the studies on transition MOs because of its inter-esting properties as a p-type semiconductor with a narrow band gap (1.2 eV in bulk) and as the basisof several high-temperature superconductors and giant magneto resistance materials [25,2935]. CuOnanostructures with large surface areas and potential size effects possess superior physical andchemical properties that remarkably differ from those of their micro or bulk counterparts. These nano-structures 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 environmentalfriendliness[36]. They are also promising materials for the fabrication of solar cells because of theirhigh solar absorbance, low thermal emittance, relatively good electrical properties, and high carrierconcentration[37]. Furthermore, CuO nanostructures are extensively used in various other applica-tions, including gas sensors [38], bio-sensors [39], nanofluid [40], photodetectors [41], energetic mate-rials (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 nano-scale CuO can be used to prepare various organicinorganic nanocomposites with high thermal con-ductivity, high electrical conductivity, high mechanical strength, high-temperature durability, and soon [32,33,49,50]. Moreover, the nanoscale CuO is an effective catalyst for CO and NO oxidation as wellasin the oxidation of volatile organic chemicals such as methanol[5153]. In addition, some reports

have demonstrated the excellent activities of nanoscale CuO as catalyst in the CN coupling andCS cross-coupling of thiols with iodobenzene reactions[51,54,55]. The superhydrophobic propertiesof CuO nanostructures render these materials as promising candidates in Lotus effect self-cleaningcoatings (anti-biofouling), surface protection, textiles, water movement, microfluidics, and oilwaterseparation [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 interest-ing applications of CuO nanostructures are being explored.

Cuprous oxide (Cu2O), another important copper (Cu)-based oxide, is also one of the first knownp-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 andcovalent nature with cubic structure (space group, O4h pn3m) that exhibits various excitonic levels.By contrast, CuO has an iron-dark color with a more complex monoclinic tenorite crystallographicstructure (space group, C2/c) and displays promising antiferromagnetic ordering [58,59]. Cu2O isexpected 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 bandgap (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 lowercarrier mobility[61].

Although these two Cu-based oxides have contrasting properties, both oxides are of considerableinterest 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, nontoxicnature, and low-cost [30,31,62,63]. In recent years, the size- and morphology-controlled synthesisand application of Cu2O and CuO have been intensively investigated[25,2831]. However, CuO ismore stable than Cu2O because Cu(II) ions are much more stable in ambience, which makes it moreimportant in practical applications. Furthermore, the synthesis, properties, and applications of various

210 Q. Zhang et al. / Progress in Materials Science 60 (2014) 208337

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Cu2O nanostructures have been extensively reviewed[28,31,6466]. Therefore, the recent advance-ment 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], andSnO2 [17], CuO nanostructures have more interesting magnetic and superhydrophobic properties.Additionally, these nanostructures demonstrate unique applications in heterogeneous catalysis inthe complete conversion of hydrocarbons into carbon dioxide, enhancement of thermal conductivityof nanofluid, nanoenergetic materials (nEMs), and superhydrophobic surfaces. CuO nanostructures asanode 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 CuOnanostructures is expected to increase the application of these nanomaterials as anode materialsfor LIBs. MOs, including SnO2, ZnO, TiO2along with their various sub-stoichiometric forms[38], arewidely considered for gas sensor applications. Thus, the study of CuO for gas sensors is expectedto increase rapidly because of the easy synthesis of high-quality and single-crystalline CuOnanostructures.

However, only few reports have described the synthesis strategies adopted for CuO nanostructuresalong 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 introduc-tion of the recent progresses of various CuO nanostructures has been published. This article will beginwith a systemic discussion on the synthesis of different CuO nanostructures. For each synthetic meth-od, 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 nano-structures will be addressed. The fundamental properties of CuO nanostructures will also be intro-duced. 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 termsof future research on CuO nanostructures will be highlighted. This review aims to provide a criticaldiscussion of the synthesis of CuO nanostructures. The potential of CuO nanostructures as functionalcomponents 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 re-ported in the literature to date and summarize the various synthetic strategies. Promising selections

Fig. 1. Schematic diagram of a typical hydrothermal synthesis for CuO nanostructures.


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and interesting applications are presented, and finally some perspectives on the future research anddevelopment of CuO nanostructures are provided.

2. Synthesis of CuO nanostructures

The development of synthetic methods has been widely accepted as an area of fundamental impor-tance to the understanding and application of nanoscale materials. It allows scientists to modulate dif-ferent parameters such as morphology, particle size, size distributions, and composition. Numerousmethods have been recently developed to synthesize various CuO nanostructures with diverse mor-phologies, sizes, and dimensions using various chemical and physical strategies. In this review, wepresent the most common synthetic strategies and associated mechanisms for tuning the morphology,size, and structure of the CuO nanostructures along with the studies of the effects of these parameterson the chemical and physical properties of the synthesized nanostructures.

2.1. Solution-based methods

Solution-based synthetic methods are very common and effective ways to prepare various MOnanostructures with good control of shape, composition, and reproducibility. They usually have rela-tively low reaction temperature and are flexible and suitable for large-scale production. Moreover, thesynthesis parameters can be rationally tailored throughout the entire process, which is beneficial formore precise control of compositions, sizes, and dimensions of the resulting materials[12,20,22,30,6972]. Among a variety of solution-based synthetic methods, hydrothermal and chem-ical precipitation techniques have been widely used to synthesize CuO nanostructures.

2.1.1. Hydrothermal synthetic method

Hydrothermal synthetic method, in which the reactions are conducted in water in a pressurized

sealed container and reaction temperature over the critical point of a solution, has been widely usedto generate different nanomaterials because its reaction system is simple/green, with convenient post-treatment [1,12,69]. Furthermore, the hydrothermal method exhibits the following advantages: (i)numerous inorganic salts can be well dissolved in water, allowing a very flexible adjustment of thesource of the metal ions depending on the requirements; (ii) water is low-cost, non-toxic, and envi-ronment friendly; (iii) small coordinating molecules can be easily applied to modulate the growthof the final nanocrystals; and (iv) the strong polarity of water may be favorable to the oriented growthof nanocrystals[69]. A schematic diagram of a typical hydrothermal synthesis for CuO nanostructuresis shown inFig. 1.

Fig. 2. Temperature contour diagram of a T-type micro mixer[75]. Copyright 2011 Elsevier.


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2.1.1.1. Synthesis of CuO nanoparticles. The hydrothermal synthesis of CuO nanoparticles is generallybased on a two-step process. First, cupric hydroxide [Cu(OH)2] particles are formed by the reactionof a cupric salt precursor with a basic solution, such as sodium hydroxide (NaOH) or ammoniumhydroxide. The Cu(OH)2particles are then thermally dehydrated in an autoclave at fixed temperaturesto obtain the final CuO nanoparticles. With the hydrothermal techniques, experimental parameterssuch as cupric concentration, pH, growth time, or growth temperature determine the final dimension,size, and quality of the CuO nanoparticles.

Neupane et al. [73] synthesized flake-like CuO nanoparticles by simply controlling the precipitationreaction temperature between the copper nitrate trihydrate [Cu(NO3)23H2O] and NaOH under ahydrothermal process. The results showed as-prepared samples with sizes of 37 nm, regularflake-like morphology, and uniform size distribution. A series of controlled experiments confirmedthat the temperature and the partial pressure inside the autoclave changed the morphology and phaseof CuO nanoparticles during the hydrothermal process. To study the effect of temperature on the mor-phology and phase control, Neupane et al. fixed the reaction time to 2 h by only adjusting the temper-ature in the reaction system. When the reaction temperature was fixed at 100 C, impure phases ofCu(OH)2and Cu2O appeared. Increasing the reaction temperature to 300 C resulted in the formation

of pure metallic Cu. The uniformly dispersed pure CuO nanoparticles were obtained at an optimumtemperature of 200 C. These results indicated that the temperature and the partial pressure insidethe autoclave are essential in controlling the morphology and phase of CuO nanoparticles duringthe hydrothermal process.

Chakraborty et al.[74]synthesized CuO nanoparticles by the hydrothermal route using two differ-ent organometallic and inorganic precursors of copper acetylacetonate [Cu(C5H7)2:Cu(AA)2] andCu(NO3)23H2O. The resulting flower-like CuO nanoparticles are both in single phase. However, thesynthesized nanoparticles from the two different precursors had different the vibrational properties.The appearance of the Raman peak at 218 cm1 was observed only in the CuO nanostructures synthe-sized using Cu(AA)2as the precursor, which was attributed to a certain concentration of the precursorresulting in particular defect states which induced the structural changes in the synthesized product.

Sue et al. [75] useda T-type micro mixer (Fig. 2) at 673 K and 30 MPato synthesize the CuO, nickel oxide(NiO), and iron oxide (Fe2O3) nanoparticles by the hydrothermal method. The effects of the variousexperimental and sample parameters such as residence time, metal species on conversion, crystalstructure, particles size, and mass loss were investigated. The time variation of conversion, averageparticle size, and coefficient of variation of the particles showed diverse behavior depending on the me-tal species, that is, differences in MO solubilities where Fe2O3< NiO < CuO. For CuO nanoparticles,

Fig. 3. Transmission electron microscope (TEM) images and SAED patterns of CuO nanorods prepared at (a) room temperatureand (b) 100 C[79]. Copyright 2004 American Chemical Society.


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nucleation did not occur at 0.002 s, but it remarkably proceeded to 0.157 s with increasing conversion.The average particle size of CuO was found to increase quickly from 23.7 nm to 28.7 nm during theearly reaction stage (60.257 s), and then it gradually increased to 34.3 nm by increasing the residencetime to 2 s.

In addition, Outokesh et al.[76]reported the hydrothermal synthesis of CuO nanoparticles undernear-critical and supercritical conditions. During synthesis, three targets of the sample parameters,namely, yield of the reaction, size of the nanoparticles, and purity of the products, were optimizedby the authors through a series of controlled experiments. Results show that the optimization ofthe three parameters was obtained under the following reaction conditions: T= 500 C, time = 2 h,[Cu(NO3)2]=0.1moldm

3, and at pH 3. The appropriate mechanisms of the formation of CuOnanoparticles were proposed by the authors as follows. First, nanoparticles are suggested to bein the liquid phase similar to Cu(OH)2. Second, in the presence of HNO3 under relatively high

temperature, some of the initially formed Cu(OH)2or CuO are transformed to Cu2(OH)3NO3. Finally,Cu(OH)2and Cu2(OH)3NO3are decomposed to CuO.

2.1.1.2. Synthesis of CuO 1D nanostructures. Hydrothermal method can also synthesize CuO 1D nano-structures including CuO nanorods, nanotubes, and nanoneedles. Cao et al.[77] reported the synthesisof Cu, Cu2O, and CuO nanotubes as well as nanorods. In a typical process, a 0.8 mM CuCl2and 3 MNaOH were dissolved to prepare CuOH24 solution. Then, a surfactant cetyltrimethyl ammonium bro-mide (CTAB) was added to the solution under vigorous stirring at 50 C for 30 min to ensure completedissolution of CTAB. After adding 0.8 mM glucose, the solution was transferred into a stainless steelautoclave and kept at room temperature for 1 h. Finally, after the CuO nanotubes were collected, they

were washed by ethanol and water, centrifuged, and dried. The concentration of CuOH24 was justadjusted to 15 mM to synthesize the CuO nanorods. Cheng[78]recently synthesized CuO nanorodsin a large scale using the same method and proved that the concentration of surfactant CTAB criticallyinfluences the morphology of CuO nanorods.

Gao et al. [79] reported that the temperature in the hydrothermal treatment during synthesisremarkably influences the crystalline structures and morphology of CuO nanorods. Similarly, Cu(OH)2was prepared from NaOH and CuCl2 and dispersed in NaOH solution. Then, the mixture was trans-ferred into a Polytetrafluoroethylene (PTFE) container in an autoclave and kept at room temperatureat 48 h. The temperature was then increased to 100 C for another 48 h. After washing and drying, thefinal products were collected. The fine CuO nanorods prepared at room temperature have higher as-pect ratio and smaller diameters compared with the bulk CuO nanorods prepared at 100 C(Fig. 3).

However, the SAED images show that the fine CuO nanorods are polycrystalline, in contrast to themonocrystalline bulk CuO nanorods.

Shrestha et al.[80]used the following chemical combinations to synthesize CuO nanorods by thehydrothermal method: (1) Cu(NO3)2, lactic acid, and NaOH; (2) CuSO4, sodium lactate, and NaOH; and(3) Cu(NO3)2and NaOH. The chemical reagents were mixed and stirred, and then transferred into an

Fig. 4. Scanning electron microscope (SEM) images of CuO nanorods (a), (b), and (c) synthesized with chemical combinations(1), (2), and (3), respectively[80]. Copyright 2010 American Chemical Society.


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autoclave for 24 h at 140 C. The as-synthesized CuO nanorods have similar morphologies (Fig. 4).However, unlike the other two cases, the CuO nanorods synthesized from combination (1) assembledin a spherical structure without separation of individual nanorods (Fig. 4a) The CuO nanorodsachieved from combination (2) separated, but still tended to aggregate. Interestingly, some of theCuO nanorods obtained from combination (3) show rectangular cross-sections (Fig. 4c). Althoughthe chemical combinations were different, the shapes of the CuO nanorods did not remarkably change.

However, when the concentration of NaOH, aging period, and temperature were altered, the resultingproducts varied from plate-like structure to octahedral structures instead of nanorods.Dar et al.[81]reported a hydrothermal method of synthesizing CuO nanoneedles by the reaction

between Cu(NO3)23H2O and NaOH under continuous stirring followed by heating from 120 C to180 C for 20 h to 60 h. The CuO nanoneedles had very sharp tips and large-diameter bottoms. In

Fig. 5. (a) Schematic representation of the synthesis of CuO using alcohols, EG, and nonionic polymeric surfactants [83].Copyright 2011 Elsevier. (b) The schematic growth of CuO nanostructures with diverse morphologies [47].Copyright 2012Elsevier.

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addition, the nanoneedles were grown by surfactant-free approach. Yang et al. [82] recentlyintroduced microwave-assisted hydrothermal method for the synthesis of CuO nanorods. Similarly,CuSO45H2O, polyethylene glycol 400 (PEG-400), and urea were dissolved and heated to 80 C in anultrasonic bath. NaOH was added to the solution and aged for 5 min under the same condition. Then,

the mixture was transferred to an autoclave, sealed, and treated in the microwave digestion system at120 C for just 20 min to obtain the final CuO nanorods. The size of the as-synthesized CuO nanorodswas smaller than those from normal hydrothermal method. During the microwave-assistedhydrothermal stage, CuO nanorod formation just required 20 min.

Fig. 6. Typical SEM images of (a) butterfly-like[90](Copyright 2011 Indian Academy of Sciences), (b) gear wheel bundles[105](Copyright 2007 American Association of Nanoscience and Technology), (c) flower-like assemblies [32] (Copyright 2007American Chemical Society), (d) dendrite-like [101](Copyright 2007 Elsevier), (e) nanobat-like[108](Copyright 2009 Elsevier),(f) layered hexagonal discs [112] (Copyright 2011 Institute of Physics), (g) honeycomb-like[32](Copyright 2007 AmericanChemical Society), (h) self-assembled leaf-like[99](Copyright 2010 Springer), (i) hierarchical peachstone-like[109](Copyright2010 Elsevier), (j) shrimp-like[47](Copyright 2006 American Chemical Society), (k) sheaf-like[111](Copyright 2009 Elsevier),and (l) urchin-like microspheres[104](Copyright 2009 Elsevier).



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Table 1

Summary of CuO nanostructures obtained hydrothermally and the various synthesis conditions [PEG, sodium dodecyl sulfate

(SDS), CTAB, EG, sodium dodecylbenzenesulfonate (SDBS), dodecylsulfate (DS), hexamethylenediamine (HMDA), poly-sodium

4-styrenesulfonate (PSS)].

Morphology Size (nm) Starting materials Additives Temperature

(C)

Duration

(h)Flower-like, boat-

like, plate-like,and ellipsoid-like[85]

Various Cu(NO3)23H2O PEG 100180 0.52NH3H2O

Honeycomb-like[32]

90100 nm in diameter, tens ofmicrometers long

Cu foil Na2WO4,Na2MoO4,SDS

160 24

Nanoplatelets[9698] 400 nm to 2 mm in width and 1 mm toseveral micrometers long

CuxS 150, 200, 230 1.5NaOH

1 lm in width Cu foil H2O2 150 12120300 nm in width and 480700 nmlong

Cu(DS)2 120 12NaOH

Leaf-like[99] 0.51.5 lm in length, 80110 nm long Cu(OH)2 Urea 150 612Momordica-like

[100]Less than 100 nm long and 3050 nm inwidth

CuSO45H2O 180 15NH3H2O

Dendrite-like[101] About 100 nm in width and severalmicrometers in length

CuSO45H2O EG 200 20NaOH

Branch-like andflake-like[102]

50200 nm width and 300400 nmlength for flake-like nanostructure;hundreds of nanometers long anddiameters of 20100 nm for branch-likenanostructures

CuSO45H2O Sodiumcitrate

160 12

Spherical-like[103] 500 nm in diameter Cu(CH3COO)2 Urea 121 0.67Hierarchical hollow

microspheres [87]3.5lm in diameter Cu(CH3COO)2H2O 120 24

Urchin-like[104] 3 lm in diameter CuCl22H2O EG 100 12

Gear wheel andclew like[105]

10 lm in diameter for gear wheelnanostructure

Cu(NO3)23H2O PEG 150180 10, 21

Flower-like[91,106]

2.53 lm long Cu threads K2Cr2O7,H2SO4

140 12

6 lm in diameter CuCl22H2O CTAB 150 12Hierarchical

flower-like[107]510 lm in diameter CuSO45H2O H2O2 120 6

NaOHNanobat-like[108] 70 nm in diameter, 170 nm long Cu(NO3)23H2O Urea 100150 615Hierarchical

butterfly-like[90]6 lm long and 24 lm in width CuCl22H2O SDBS 80150 115

Nanobundles [89] 2030 nm in diameter and 350500 nmlong

CuCl22H2O SDBS 130 1824

Peachstone-like

[109]

4 lm long and 3 lm in width CuCl22H2O [Omim]TA 100 24

Hollow micro/nanostructures[110]

1.53 lm in diameter CuSO4 Tyrosine 130 4NH3H2O

Shuttle-like[88] 200300 nm in width and severalmicrometers long

Cu(CH3COO)2H2O CTAB 120 12

Sheaf-like[111] 2 lm long Cu(NO3)23H2O Urea 120 8Layered hexagonal

discs[112]34lm long and 1.52.0 lm in width Cu(NO3)23H2O HMDA 130 310

NH3H2OHierarchical

dandelion-likemicrospheres [113]

36lm in size Cu(NO3)23H2O EG 130 16NH3H2ONaOH

Twinned-hemisphere-like

[114]

22.5 lm in size Cu(CH3COO)2H2O PSS 180 2NaF

Hierarchicaldendrite-like[115]

69lm in size Cu(NO3)23H2O Urea 160 12

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2.1.1.3. Synthesis of CuO 2D/3D nanostructures. The simplicity of the hydrothermal method facilitateditsperformance at low temperatures and in a large scale. Additionally, it can be used for the production ofcomplex nanostructured CuO with diverse morphologies and sizes. In most cases, the hydrothermalsynthesis of CuO nanostructures starts with the formation of intermediate compound Cu(OH)2precip-itationor other intermediate phases,whicharepreparedfrom cupric salt/cupricacids/Cu foil precursorsin alkaline media. Then, thesolutionis kept at an elevated temperature in an autoclave for a certainper-iod, allowing the decomposition of Cu(OH)2 or other intermediate phases into the final product of CuO.Therefore, the manipulation of well-controlled synthesis of CuO nanostructures with various shapes ispossible by choosing different solutions andby adjusting the concentrations of precursors. To obtain di-verse morphologies and dimensions of CuO nanostructures, several surfactant and structure-directingagents are normally added into the precursor solution. Systematic studies of the experimental param-eters reveal that the Cu source, reaction temperature, reaction time, and surfactant along with the pHvalue of the precursor solution influence the morphology, growth, size, and dimensions of the resultingCuO nanostructures (Fig. 5)[25,47,83,84].

The various morphologies of the CuO nanostructures achieved using the hydrothermal technique isshown in Fig. 6. The obtained CuO nanostructures exhibit diverse optical, electrical, and catalytic prop-

erties[8587].The hydrothermally synthesized CuO nanostructures fabricated with and without additives exhibit

evidently different morphologies and properties. However, the exact mechanism for the formation ofthese architectures by the addition of inorganic or organic materials has not been fully understood. Interms of organic additives, several reports have suggested that certain ions in the additives are ad-sorbed on the CuO surface, which alters the growth mechanism[78,8890]. Other studies have dem-onstrated that additives can act as a template for the formation of CuO nanostructures with differentmorphologies [83,91]. The addition of basic media such as NaOH, urea, NH3H2O, and (CH2)6N4 leads tothe formation of intermediate compound Cu(OH)2that is transformed into CuO under heat treatmentby the oriented attachment growth mechanism as detailed in Section3. The final morphology of CuOwas determined by the internal crystallographic structure of Cu(OH)2. Cudennec et al. [92]showed

that the transformation of Cu(OH)2into CuO is a reconstructive transformation involving a dissolutionreaction followed by the precipitation of CuO according to the following scheme: CuOH22s2OHaq ! CuOH

24aq $ CuOs 2OH

aq H2O. Moreover, the application of microwave power

during the hydrothermal process to synthesize CuO nanostructure has emerged as an important topicin the scientific community because of its low energy consumption, rapid heating process, and fastkinetics of crystallization[93]. The use of hydrothermal microwave to synthesize CuO nanostructureleads to small diameter at short annealing time with high yield compared with the conventionalhydrothermal process[94,95].

In summary, various CuO nanostructures, including CuO nanoparticles, 1D CuO nanowires/rods/tubes, 2D, and several complex 3D CuO nanostructures, have been hydrothermally synthesized byadding various reactants to the cupric salt/cupric acids/Cu foil precursor solution in the presence of

some capping agents. Moreover, morphologies of the synthesized CuO nanostructures can be con-trolled by selecting certain types of structure-directing and dispersing modifying agents. A brief sum-mary of the obtained CuO nanostructures that are hydrothermally synthesized with or withoutdifferent additives is shown inTable 1.

The possible growth scheme for the formation of rectangular-shaped nanobat-like CuO nanostruc-tures through a typical hydrothermal synthetic process is shown in Fig. 7[108]. The reagents areCu(NO3)23H2O and urea, where urea acts as pH buffer to control the supply of OH

ions. Duringthe initial stage, water reacts with urea to form ammonia and CO2. In addition, ammonia further reactswith water to produce ammonium and OH ions. The reaction slowly generates solid building unitsinto solution (b). The concentration of solid building units in the solution increases continuouslyand nucleation starts (c) after a crucial super-saturation level is reached. The crystallographic struc-

tures of the nuclei and seeds greatly affect the final shapes of the nanocrystals (d). The variation inthe CuO nuclei is caused by the increasing pH of the reaction solution (c). The small CuO crystalincreases and forms a bigger crystal with increasing reaction time. O and Cu2+ are stacked alternatelyalong with the specific directions to form alternating planes, which causes the anisotropic growth ofthe CuO crystal. The different growth rates are due to the crystallographic faces, where the growth rate


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is sequenced as [01 0]length> [100]breadth> [001]height. The slowest growing (010) planes dominate thecrystal into the typical rectangular-shaped structure (e). The formed nanobat-like structure thenforms a circular fashion via self-assembly to build hollow microspheres. This phenomenon was causedby the formation of several unbalanced charge centers as a result of the dissimilarities in the surfacecharges. These charge centers attract the nanobats in the solution through rotating adjacent structure(e) to share identical orientation. Finally, sphere-like CuO microsphere assemblies are formed withincreasing reaction time.

2.1.2. Solution-based chemical precipitation methods

Chemical precipitation synthesis is similar to the hydrothermal method with a reaction also occur-ring in the solution, but the chemical reactions can be conducted in an open container with a relativelylow reaction temperature (normally below 100 C). This process can be simply defined as the chemicalreaction between the precursors to produce monomers that subsequently aggregate into final result-ing materials [69]. A schematic drawing of the solution-based chemical precipitation to producenanoscale CuO is shown inFig. 8.

2.1.2.1. Synthesis of CuO nanoparticles. Cupric salt (normally nitrate or sulfate) and alkaline compounds(normally NaOH) are often used in the synthesis of CuO nanoparticles. A typical process of chemicalprecipitation is as follows [116]: Cu(NO3)2 solution (300 mL 0.02 M) is prepared by dissolvingCu(NO3)23H2O in deionized water. The solution is placed into a round-bottom flask equipped with

a refluxing device. The Cu(NO3)2solution is kept at an appropriate temperature (6100 C) with vigor-ous stirring. Then, 0.5 g solid NaOH (platelet) is rapidly added into the solution, resulting in the pro-duction of a large amount of blue or black precipitates and the crystallization temperature ismaintained for 10 min. Next, the precipitates are heated at 100 C for another 10 min. After completereactions, the resulting products are centrifuged, washed with water and ethanol for several times,

Fig. 7. Schematic illustration of the growth scheme for the formation of rectangular-shaped nanobat-like CuO nanostructuresby the hydrothermal synthesis method[108]. Copyright 2009 Elsevier.

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and then dried in air at room temperature. Zhu et al. [116]synthesized CuO nanoparticles by thisroute. The average diameter of the as-prepared particles is 10 nm.

The proportion of reactive compounds and reactive temperature markedly affects the size and mor-phology of the synthesized CuO nanoparticles. Using a similar method and three sets of recipes, Zhouet al.[52]obtained three types of CuO products with distinct sizes and morphologies: nanoparticles,nanobelts, and nanoplatelets (Fig. 9). The recipes were (a) Cu(NO3)23H2O + NaOH, (b) Cu(OAc)2H2O+ NaOH, and (c) Cu(NO3)23H2O + N a2CO3.

The synthesized CuO nanoparticles using ordinary chemical precipitation methods usually have acommon problem, that is, the achieved nanoparticles are apt to agglomerate. To solve this limitation,chemical precipitation methods have been extensively investigated to improve the separation of thenanoparticles through the application of some external energy such as ultrasonic or high pressure. Thesonochemical method is used to apply ultrasonic cavitation during the synthesis procedure. Kumaret al.[48]synthesized well-separated 26 nm CuO nanoparticles by irradiating the reactive solutionwith a high-intensity ultrasonic horn under 1.5 atm of argon at room temperature for 3 h. However,this method requires expensive apparatus and excessive organic solvent as well as severe reactionconditions. In addition to the sonochemical synthesis of dispersed CuO nanoparticles, Zhu et al.[117]developed a simple quick-precipitation procedure to prepare highly dispersed CuO nanoparti-cles with the size of about 6 nm in aqueous solution. The authors noted that the tendency of CuOnanoparticles to aggregate during preparation may have been caused by the low nucleation andgrowth rates of CuO particles at mild reaction condition. A large amount of well-dispersed CuO nano-particles were obtained by the rapid addition of the NaOH solid to the mixture of an aqueous solutionof Cu(CH3COO)2 and a glacial acetic acid at 100 C. This result indicates that higher temperatures cause

Fig. 8. Schematic of a typical chemical precipitation synthetic process for CuO nanostructures.

Fig. 9. TEM images of CuO (a) nanoparticles, (b) nanobelts, and (c) nanoplatelets [52]. Copyright 2008 Institute of Physics.




higher reaction rates, resulting in large amounts of nuclei to form in a short period and the inhibitionof the aggregation of crystals. Consequently, well-dispersed CuO nanoparticles with small sizes wereachieved at relatively high reaction temperature. However, Mahapatra et al.[118]introduced a wetchemical method to synthesize ultrafine dispersed CuO nanoparticles. The authors claimed that thesize of as-prepared CuO nanoparticles can be controlled by simply changing the concentration ofthe basic Cu precursor. In addition, hot-solution decomposition and microemulsion process, whichhave been widely applied to synthesize well-dispersed quantum dots, may be applied in the prepara-tion of dispersed CuO nanomaterials with various shapes and sizes. Other synthetic technologies suchas by introducing sonic waves or microwaves in the synthetic process can also be tried to obtain well-dispersed CuO nanoparticles in the future[119].

2.1.2.2. Synthesis of CuO 1D nanostructures. Wang et al.[120]reported a simple wet-chemical methodfor preparing CuO nanorods. In a typical process, a non-ionic surfactant, polyethylene glycol (PEG; Mw20,000), and CuCl22H2O are dissolved in water. After stirring for 15 min, NaOH is added into the solu-tion to generate Cu(OH)2precipitate. The Cu(OH)2is placed into a steam trace for 30 min and is trans-formed into CuO precipitate, followed by washing, filtering, and drying. The as-synthesized CuO

nanorods have monoclinic structure up to 400 nm long with a diameter ranging from 5 nm to15 nm. Lu et al.[121]synthesized CuO nanowires by dehydration of the precursor of Cu(OH)2nano-wires. First, KOH solution was added dropwise into CuSO4solution under rigorous stirring, followedby the dropwise addition of ammonia solution. Then, the Cu(OH)2 nanostructures were heated at120 C for 2 h and at 180 C for another 3 h. The resulting CuO nanowires have structures similar tothose of the precursor Cu(OH)2nanowires. Ethiraj and Kang [122] introduced an organic molecule thi-oglycerol (TG) as a stabilizer in the synthesis of CuO nanowires. TG was first added in a copper acetatesolution under stirring. Then, the NaOH solution was added dropwise into the mixture, followed byimmediate addition of water under continuous stirring for a few minutes. The precipitate was col-lected by centrifuging, washing and overnight drying. By comparing the samples with and withoutthe use of TG, the authors found that a small amount of TG led to well-dispersed CuO nanowires,

otherwise the CuO nanowires would assemble into a flower-like structure. Zhang et al. [44]reportedthe preparation of CuO nanobelts by adding ammonia to a Cu(NO3)2solution directly under constantstirring for 15 min. After the formation of a blue precipitate, the mixture was then sealed and heatedat 60 C for 4 h. The product was collected by repeated washing and centrifugation. The widths andlengths of the as-obtained CuO nanobelts were 510 nm and 13 lm, respectively. Moreover, thethickness of the CuO nanobelts was estimated to be 25 nm. Interestingly, Yangs group [123]synthe-sized vertically aligned CuO nanorod arrays on a Cu substrate. In a typical synthesis, a Cu foil reactedwith NaOH under the presence of some oxidants or surfactants. Then the foil was removed, washed,and dried. The CuO nanorod arrays were found to uniformly cover the surface of the Cu foil.

Remarkably, by using PEG 200 as the capping agent, ultralong CuO nanowire bundles with lengthsranging from tens to hundreds of micrometers as shown inFig. 10a were selectively synthesized on a

large scale at room temperature by a facile solution-phase method by Li et al. [124]. Transmissionelectron microscope (TEM) characterizations (Fig. 10b) demonstrated that the obtained CuO nanowirebundles are polycrystalline. Moreover, a series of controlled experiments performed by the authorsrevealed that the presence of PEG 200 and the concentration of OH affected the morphology andphase control of CuO nanostructures during the reaction process. No CuO nanowire bundles can beobtained, but CuO nanoleaves (NLs), without PEG 200 (Fig. 10c). These NLs were single crystals andgrew along the [111] crystal plane with diameters ranging from 200 nm to 500 nm (Fig. 10d). TheCuO nanowire bundles could only be synthesized when the molar ratio of OH/Cu2+ was higher thanfour, indicating that PEG200 and the concentration of OH are essential in the formation of CuO nano-wire bundles.

Wang et al.[125]synthesized large-scale ultralong CuO nanowires with an average diameter of

8 nm and lengths of up to several tens of micrometers by a facile room temperature solution-phasechemical route without any capping agent, as depicted in Fig. 11. Cu(OH)2 nanowires were first formedand subsequently served as template to direct the formation of CuO nanowires. Compared with thecommercial CuO powders, the obtained ultralong CuO nanowires exhibit enhanced photocatalyticactivity for RhB degradation and the potential for applications in LIBs and catalysis.

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2.1.2.3. Synthesis of CuO 2D/3D Nanostructures. The solution-based chemical precipitation method canalso be used to synthesize more complex 2D/3D CuO nanostructures. A brief summary of the obtainedCuO nanostructures prepared by the solution-based chemical precipitation method with differentadditives are listed in Table 2. Some examples of CuO nanostructures with various morphologiesachieved by this technique are shown inFigs. 13 and 14. In the whole synthetic process, two keypoints, namely, nucleation and growth, dominate the formation of different CuO nanoarchitectures.An appropriate precursor, a rational condition for the reaction system together with additional ligandsto adjust the surface energies will significantly influence the nucleation and growth of the CuO

Fig. 10. (a) SEM image of the CuO nanowire bundles. (b) TEM image and the corresponding SAED pattern taken in a zone rich innanowires. (c) SEM image and TEM image (inset). (d) High-resolution (HRTEM) image of one CuO NL and the corresponding

SAED pattern, indicating growing along the

111direction[124].Copyright 2010 Elsevier.

Fig. 11. (a) TEM image of CuO nanowires. (b) A histogram of the CuO nanowires, revealing the diameter distribution[125].Copyright 2012 Royal Society of Chemistry.

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nanostructures. Thus, to achieve the controlled synthesis of CuO nanoarchitectures by solution-basedchemical precipitation, various experimental parameters including the type of precursors, reactiontemperature, reactant concentration, and surfactant can be manipulated to control the morphologyand structure of the obtained CuO nanocrystals. Moreover, recent reports have determined that thepH value together with concentration of OH ions can significantly affect the nucleation and growth

behavior (for example, the number of nuclei and concentrations of growth units) of the CuO crystals,resulting in the formation of the diverse morphologies of CuO nanostructures as shown in Fig. 12. Asimilar phenomenon is also observed in controlling the hydrothermal synthesis of CuOnanostructures.

In addition, the various additives used in the preparation process have distinct functions. Severalstudies have suggested that ionic surfactants, such as SDBS and CTAB, can be absorbed onto the sur-faces of CuO nanocrystals[130,131]. This effect can reduce the interfacial tension between the crys-tallizing phase and the surrounding solutions, which strongly affects the growth rate andorientation of the crystals, resulting in the formation of CuO nanostructures with diverse morpholo-gies. Non-ionic surfactants, polyvinylpyrrolidone (PVP) and PEG were thought to serve as templatesfor the formation of CuO nanostructures. However, the mechanism responsible for the growth of

CuO nanostructures by solution-based chemical precipitation method remains unclear. Someresearchers have used oriented attachment as the possible statistical growth mechanism as detailedin Section3[128,138].

The schematic growth mechanism for the formation of flower-shaped CuO nanostructures througha typical solution-based chemical precipitation process is shown in Fig. 15[33]. When Cu nitrate is

Table 2

Summary of CuO nanostructures obtained by solution-based chemical precipitation methods and their synthesis conditions (PEG,

CTAB, SDBS, DS, HMDA, and PVP).

Morphology Starting Materials Additives Temperature(C)

Duration(h)

Nanowires, rectangles, and seed-, belt- and sheet-like[132]

Cu(NO3)23H2O 80 0.5NaOH

Urchin-like[51] Cu(NO3)23H2O Urea 100 6Flower-shaped[33] Cu(NO3)23H2O HMTA 100 3

NaOHBelt-, bamboo leaf- and shuttle-like[130] CuCl22H2O PEG, CTAB 75 0.5

NaOHLeaflet-like nanosheet[133] CuSO45H2O 25 2

KOH, NH3H2OFlower-like hierarchical assemblies[134] Cu(NO3)23H2O 25, 100 1272

NaOHCarambola-like[135] Cu(NO3)23H2O HMTA 95 3

NaOH

Shuttle- and flower-like[136] Cu(NO3)23H2O HMTA, CTAB 100 3NaOH

Nanoribbons and nanorings[137] CuCl22H2O SDBS 120 0.5NaOH

NLs[138] CuCl22H2O 35 40NH3H2O

Hierarchical nanochains[139] CuCl22H2O PVP,(NH4)2SO4

100 12NaOH

Hierarchical nanosheets[140] Cu(CH3COO)2H2O 30 72NH3H2O

Oval nanosheets and nanoellipsoid[128] Cu(CH3COO)2H2O 65 24NH3H2O

Flower-like microspheres[141] Cu power, NaOH (NH4)2S2O8 25 20Lenticular-like, pseudo-like and elliptical[84] Cu(CH3COO),

NaOH

25 24

Spindle-like and plate-like[142] Cu(NO3)2, NaOH 80 0.5Flower-like hierarchical nanostructures[143] Cu(NO3)23H2O 80 4

NH3H2O

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mixed only with HMTA (a), a turbid solution containing the building units (b) is obtained. After theturbid solution is refluxed at 100 C for >1 h, Cu(OH)2 nuclei are formed from the origination ofOH ions by the hydrolyzation of HMTA, and then nuclear aggregation occurs (c). With increasingreaction time, the Cu(OH)2 is converted into small CuO crystals. Moreover, the CuO crystals are ar-ranged with time and finally form petal-like structures (d). Interestingly, if the Cu(NO3)2is mixed withboth HMTA and NaOH, the solution immediately changes to blue color because of the instant forma-tion of Cu(OH)2nuclei (e) caused by the fast formation of OH

ions from NaOH. The Cu(OH)2nuclei aretransformed into CuO through a simple chemical reaction of Cu(OH)

2? CuO+H

2O. With increasing

reaction time, the initially formed CuO nuclei are assembled, forming individual petals (f) and finallyflower-like morphologies (g).

As mentioned in Section 2.1, the solution-based synthesis of CuO nanostructures is commonly per-formed in solutions containing Cu(II) salts, and the obtained final products are normally free-standing.In addition, this simple method can also be used to synthesize CuO nanostructures directly on the sur-face of Cu substrates. Generally, Cu substrates are washed in an HCl solution for a few minutes andsubsequently rinsed with deionized water and absolute ethanol to remove the surface impuritiesalong with oxide layers. Then, the treated Cu substrates are immersed into alkaline solutions withor without additives of oxidative reagents (e.g., (NH4)2S2O8 or K2S2O8) at certain temperature for afixed period[129,144147]. The reaction system without additives of oxidative reagents commonlyrequires a longer time to yield the final CuO nanostructures on the Cu surface[146,147].

In 2003, Yang et al.[129]synthesized sheet- and whisker-like CuO nanostructures on a Cu sub-strate surface by a simple liquidsolid reaction under alkaline and oxidative conditions at room tem-perature. They demonstrated an evolution of the film structures as a function of the solutiontreatment time and the concentration of NaOH aqueous solution from the fibers of Cu(OH)2 onto

Fig. 12. Schematic illustration of the growth of CuO nanostructures under different pH conditions and the corresponding SEMimages[127]. Copyright 2010 Elsevier.




the scrolls of Cu(OH)2onto the sheets or whiskers of CuO. The formation of Cu(OH)2and CuO nano-structures on Cu surfaces involved inorganic polymerization (polycondensation) reactions. The ob-tained CuO nanostructures are phase-pure single crystallites. This method for the preparation of Cucompound films with ultrafine structures is advantageous because of its simplicity, high yield, andmild reaction conditions. It can also produce single crystal nanomaterials in array form, which offersan attractive and convenient path to the large-scale engineering of ordered inorganic nanostructureson metal electrodes.

Large-area flower-like 3D CuO nanostructures were also successfully synthesized on a Cu surfaceby a template-free solution route under alkaline and oxidative conditions at 70 C[145]. The forma-tion of flower-like nanostructures was found to strongly depend on the concentration of oxidantK2S2O8. Liu et al.[146]reported the fabrication of CuO hierarchical nanostructures on Cu substratesby the oxidation of Cu in alkaline conditions at 60 C without the addition of oxidative reagents. They

Fig. 13. Typical SEM images of (a) urchin-like nanostructure[51](Copyright 2009 American Chemical Society), (b) hierarchicalnanochains[139](Copyright 2011 Elsevier), (c) Nanosheets[128](Copyright 2006 American Chemical Society), (d) flower-like[32](Copyright 2008 American Chemical Society), (e) flower-like assembles [134](Copyright 2007 Elsevier), (f) nanoellipsoid[128] (Copyright 2006 American Chemical Society), (g) spindle-like [142](Copyright 2012 Royal Society of Chemistry), (h)microplates [84] (Copyright 2012 Royal Society of Chemistry), (i) flower-like microspheres [141], (Copyright 2012 Royal Societyof Chemistry), (j) lenticular-like[84](Copyright 2012 Royal Society of Chemistry), (k) Pseudo-like[84](Copyright 2012 RoyalSociety of Chemistry), and (l) elliptical-like nanostructure[84](Copyright 2012 Royal Society of Chemistry).

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demonstrated that CuO flower-like structures composed of hierarchical 2D nanosheets and sphericalarchitectures constructed by ultrathin nanowalls could be selectively generated by controlling thealkaline reactant. When NaOH was used, 3D CuO flower-like structures on a Cu foil was obtained

Fig. 14. Typical TEM images of (a) sheet-like[136](Copyright 2009 Springer), (b) flower-like[136](Copyright 2009 Springer),(c) NLs[138](Copyright 2007 American Chemical Society), (d) spindle-like[142](Copyright 2012 Royal Society of Chemistry),(e) lenticular-like[84](Copyright 2012 Royal Society of Chemistry), and (f) elliptical-like nanostructures[84](Copyright 2012Royal Society of Chemistry).

Fig. 15. Schematic growth mechanism for the formation of flower-shaped CuO nanostructures [33].Copyright 2008 AmericanChemical Society.




(Fig. 16a). Whereas NaOH was replaced by NH3H2O, CuO with 3D spherical architectures was formedon the Cu foil as illustrated inFig. 16b. In addition, by reducing the concentration of the alkaline solu-tion, well-defined 2D nanosheets and nanowall arrays can be fabricated on a large scale on the Cu foilaccordingly. A new hierarchical CuO microcabbage architecture[147]consisting of densely packednanoplates and nanoribbons was also directly fabricated on Cu foils via a similar synthetic processat room temperature for 6d. In addition, CuO nanostructures such as nanoneedles, nanoflowers, and

stacking of flake-like structures on Cu foils can also be obtained by Cu oxidation under alkaline con-dition by a simple wet chemical route[144].

2.2. Solid-state thermal conversion of precursors

Various CuO nanostructures can be generated through the thermal conversion of the precursors,and the morphological features of the precursors can be well-preserved in the final products giventhat heat treatment is appropriately performed. Cu(OH)2 and basic Cu salts have become favorableprecursor candidates to exploit the interesting morphologies of CuO because of their unique andwell-known layered structure [148,149]. The process usually starts with the synthesis of the Cuprecursors via the reaction of cupric salt (normally nitrate or chloride) with alkaline compounds

(normally NaOH). The obtained cupric precursors are then centrifuged and washed with distilledwater and absolute ethanol. Finally, these cupric precursors are calcined in solid state to obtain thefinal CuO nanostructures. Moreover, the corresponding Cu(OH)2particles are formed and precipitatedin H2O by adding a basic solution (usually NaOH) to the obtained cupric precursors solution. Theresulting nitrate or chloride salts are then washed away, and the corresponding Cu(OH)2 particlesare thermally dehydrated after filtration and washed to obtain the final CuO nanostructures.

This method is similar to the solution-based chemical precipitation method, but the thermal dehy-dration of cupric precursors, such as Cu2(OH)2CO3, Cu2Cl(OH)3, CuC2O4, and Cu(OH)2is in solid statewith relatively higher treatment temperature. Additionally, the morphological features of the cupricprecursors can be well-preserved in the final CuO nanostructures. The mechanism of shape-reservedtransformation from Cu(OH)2 nanowires to CuO nanowires as illustrated inFig. 17clearly demon-

strates that the morphology can be properly reserved during the transformation from Cu(OH) 2 toCuO because of the topotactic transformation in the dehydration process[151].

In addition, a schematic diagram of the metamorphosis of CuO as given by Dey et al. [152]illus-trates the formation process (Fig. 18). This diagram presents a simple depiction of the transformationthat occurs at the atomic level for the first time.

Fig. 16. SEM images of (a) 3D CuO flower-like structures on Cu foil and (b) 3D spherical CuO architectures on Cu foil [146].Copyright Royal Society of Chemistry 2006.

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However, synthesis parameters such as alkaline content (pH) of the solutions during hydrolysisplay a key role in the formation of various morphologies and dimensions of the Cu(OH)2nanostruc-tures, leading to a series of fascinatingly shaped nanostructures of final CuO by subsequent heattreatment.

Dey et al.[152]reported that different morphologies of CuO nanostructures have been synthesizedby a simple chemical route, in which the Cu(OH)2 nanostructures are first synthesized, and theprecipitate is subsequently annealed at 130 C. A variety of shapes such as seed-like, ellipsoidal, rods,and leaves of CuO can be obtained by simply varying the pH value during synthesis (Fig. 19), indicatingthat alkaline content (pH) of the solutions are essential in the formation of various CuOnanostructures.

Fig. 17. (a) Crystal structure of Cu(OH)2. (b) When the temperature rises, long CuO and HO bonds break. The structure ofCu(OH)2changes to a layered Cu(OH)4structure. (c) Projection along thea-axis for (b), (d), and (e). The dehydration process intheb, cplane of Cu(OH)2. (f) Crystal structure of CuO[151]. Copyright 2012 Royal Society of Chemistry.




Fig. 18. Schematic representation of CuO formation at the molecular level from a crystallographic perspective. The blue ballsrepresent Cu atoms, and the red balls represent O atoms. (a) FCC lattice of Cu and (b) monoclinic cell of CuO. (c) Two unit cells

joined by a common face, showing the stacking of the 1 11 planes[152]. Copyright 2012 Royal Society of Chemistry.

Fig. 19. Diagram of the formation of various shapes of CuO under different synthesis conditions [152].Copyright 2012 RoyalSociety of Chemistry.

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With the method mentioned above, CuO with different morphologies such as nanoribbons[153], hierarchical sphere-like [154], peanut-shaped superstructures [150], sisal-like [148], fish-bone-like [155], pillow-shaped [156], perpendicularly cross-bedded microstructure [157], NCline-assembled bundle-like [158], butterfly-sheet-like or nanotubes [159], and worm-like [160]were obtained through thermal conversion of their corresponding Cu salts. In addition, otherCuO nanostructures, including CuO nanoribbon arrays, CuO nanotube arrays, nanotube arrays withspecial nanoplate wall structure, and quasi-aligned submicrometer CuO ribbons formed on Cu foils,achieved by heating the corresponding Cu(OH)2 precursors have also been reported [161163].Chen et al. [164]fabricated novel hierarchically mesoporous nanosheet-assembled gear-like pillarCuO arrays directly grown on Cu foil by the vapor-phase corrosion approach and subsequent heattreatment [164]. The hierarchically nanosheet-assembled gear-like pillar arrays (HNGPAs) ofCu(OH)2 were first formed on Cu foil. Hierarchical micro/nanostructure CuO was obtained by thesolid-state thermal transformation of the formed Cu(OH)2 arrays on Cu foil. The obtained CuOnanostructures were found to inherit the intact hierarchical superstructures with retained radialsymmetry and nanosheets subcomponents from Cu(OH)2 HNGPAs without collapse and aggrega-tion (seeFig. 20).

Novel CuO mesoporous nanosheet cluster arrays that are directly grown on a Cu substrate via thesame ammonia vapor-phase corrosion route were also found by Chen et al.[165]. This simple andeffective fabrication strategy shows promising potential for the preparation of other nanoarchitec-tured materials for both high-energy and high-power applications.

The advantages of solid-state thermal conversion of precursors are simple, easy and safe to use,controllable, allows the mass-production of CuO nanostructures with unique superstructures, practi-cal, and promising for various applications ranging from catalytic reaction to sensing.

Fig. 20. (a) Schematic illustration of the fabrication of CuO HMNGPAs via a vapor-phase corrosion route; SEM images ofCu(OH)2HNGPAs and (c) CuO HNGPAs; and (d) schematic of the growth process of Cu(OH) 2 HNGPAs[164]. Copyright 2012American Chemical Society.




2.3. Electrochemical method

Electrochemical method is widely used for the preparation of nanoporous MOs because of its sim-plicity, low-temperature operation process, and viability of commercial production. This method is

also advantageous because the growth orientation, morphology, and size of the resulting productscan be modestly controlled by adjusting the deposition parameters (deposition voltage, current den-sity, temperature, etc.). The setup of an electrochemical deposition facility is illustrated in Fig. 21.

A typical fabrication process for the electrochemical synthesis of CuO nanostructures with differentshapes at room temperature was described by Yuan et al.[166]. In this fabrication, a two-electrodesystem was used with Cu plates as the anode and stainless steel plate as the cathode. The electrolytecontained 1 M NaNO3dissolved in distilled water. Systematic studies of the experimental parameterssuch as electrolyte, composition of electrolyte solution, and current density reveal that the composi-tion of electrolyte solution and the use of current density during synthesis remarkably influence thesize and morphology of the resulting CuO nanostructures. When H2OEtOH mix was used as an elec-trolytic solvent, CuO nanorods with sharp-end morphology and with 2050 nm in diameter and

200300 nm in length were obtained. The products obtained with pure distilled water as an electro-lytic solvent are almost uniform and mono-disperse spindle-like nanoparticles (i.e., nanospindles)with 80100 nm in diameter and 200300 nm in length. However, by simply increasing the currentdensity from 5 mAcm2 to 10 mAcm2 and then to 20 mAcm2, the morphologies of CuO nanostruc-tures tend to vary from nanospindles to nanorods and then to irregular nanoplates. These results sug-gest that electrochemical shape- and size-controlled synthesis of CuO nanostructure could be easilyrealized by controlling the current density or changing the electrolytic solvent.

Toboonsung et al.[167]synthesized CuO nanorods and their bundles on a glass substrate using anelectrochemical dissolution and deposition process. The deposition time, electrode separation, andvoltage were found to play key roles in the formation of CuO nanorods and the ratio of bundles to

Fig. 21. Schematic for typical setup of an electrochemical deposition facility for synthesizing CuO nanostructures.

Fig. 22. TEM images of the leaf-like CuO mesocrystals under (a) low and (b) high magnification [171].Copyright 2012 RoyalSociety of Chemistry.


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nanorods. In addition, Xu et al. [168] synthesized large-scale CuO honeycombs using two-step electro-chemical deposition and subsequent heat-treatment. The Cu(OH)2 precursor was first fabricated bytwo-step electrochemical deposition of a Cu foil in an aqueous solution of KOH. The CuO honeycombswere then synthesized by heating the Cu(OH)2precursor at 160 C for 3 h under nitrogen protection.The applied potential and deposition time were found to be the most important parameters affectingthe honeycomb-like CuO growth. The CuO honeycombs comprised well-oriented nanowires with uni-form average diameters of approximately 100 nm and lengths of tens of micrometers. In addition tothe nanorods and honeycombs, flower-shaped CuO nanostructures can also be achieved electrochem-ically. Lu and Huang [169] recently reported the synthesis of flower-like CuO microspheres from nano-flakes by electrochemical anodic dissolution of pure Cu in an NaOH aqueous solution at roomtemperature. Whisker-shaped CuO nanostructures electrochemically fabricated from a metallic Cuprecursor, followed by annealing at 600 C for 30 min in air, have also been recently reported[170].

Xu et al.[171]reported the synthesis of leaf-like CuO mesocrystals using the electrochemical pro-cess (Fig. 28), in which Cu foils were used as the working and counter electrodes. The electrodes weresubmerged into an aqueous solution of NaNO3. The distance between the two electrodes was main-tained at 25 mm with moderate magnetic stirring being applied throughout the process. The CuO

mesocrystals were electrochemically grown at a constant voltage of 3 V for 200 s at 70 C. The ob-tained precipitates were finally harvested from the solution by centrifugation and dried at 70 C.TEM images (Fig. 22) reveal that the width of the CuO NLs is 50 nm, and the length is estimatedto be approximately several hundred nanometers. Each CuO NL comprised numerous small particles,causing the very rough surfaces of the obtained CuO NLs. When usedas LIB anode materials, the ob-tained leaf-like CuO mesocrystals showed high specific capability and good cycle performance becauseof the novel feature of the CuO mesocrystals.

2.4. Thermal oxidation method

Different methods have been employed to synthesize 1D CuO nanostructures[25,30,31]. The mostcommonly used method is to directly heat Cu substrates in air, during which the reaction between Cu

Fig. 23. SEM images of the CuO nanowires synthesized by directly heating Cu grids in air at 500 C for (ac) 4 h and (d) 2 h[172].Copyright 2002 American Chemical Society.




and oxygen (O2) results in the growth of CuO nanowires[31]. Typically, the process of thermal oxida-tion just involves heat treatment of pure Cu substrates in either ambient air or O 2atmosphere. Themorphology of the grown CuO nanowires depends on the oxidation temperature, growth time, andgas flow rate.

In 2002, Jiang et al.[172]synthesized CuO nanowires by directly heating Cu substrate in air. Typ-ically, the Cu substrate was cleaned using HCl solution to remove the native oxidation layer and rinsedwith distilled water. After blow-drying with N2gas, the Cu substrate was placed in a furnace. Then, thesubstrate was heated at 500 C for 4 h and naturally cooled to room temperature. The CuO nanowireswith large aspect ratio grew on the surface of the Cu grid or wire (Fig. 23). These nanowires had diam-eters ranging from 30 nm to 100 nm and can be up to 15lm long. The authors also reported that CuOnanowires only grow within the temperature range from 400 C to 700 C.

Various synthesis parameters, including oxidation temperature, oxidation time, O2flow rate, anddifferent type of Cu substrates, have been extensively studied to achieve better results from thermaloxidation. Chen et al.[173]annealed a Cu foil in air using various temperatures and growth times.Their results showed that the density and the length of the CuO nanowires increased as the growthtime was prolonged at 400 C. Instead of directly heating in ambient air, Kumar et al. [174]synthe-

sized CuO nanowires by thermal annealing Cu foil in an oxygen atmosphere. The results showed thatthe oxygen flow rate and the annealing temperature both affected the aspect ratio and density of CuOnanowires, but annealing time primarily affected only the aspect ratio. Compared with the direct oxi-dation in air, the presence of oxygen flow during thermal oxidation can reduce the necessary growthtime for CuO nanowires to no more than 1 h. Mema et al.[175]improved the growth density of CuOnanowires by applying bending stresses on the Cu surface. The Cu foil was initially bent at a 10 mmradius, followed by typical cleaning, annealing in 200210 Torr oxygen pressure, and cooling. Theupper surface of the Cu foil was shortened by dL, whereas the bottom surface was elongated by dL,

Fig. 24. A schematic diagram illustrating the generation of stresses at the upper and bottom surfaces of the Cu foil [175].Copyright 2011 Elsevier.

Fig. 25. SEM images of the oxide surface for (a) unbent Cu, (b) upper surface of the bent Cu, and (c) bottom surface of the Cu[175]. Copyright 2011 Elsevier.


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causing the compressive and tensile stresses in the upper and bottom surfaces, respectively (Fig. 24).The results shown inFig. 25indicated that the tensile stress in the bottom surface promoted thegrowth density of the CuO nanowires. However, the length of the CuO nanowires is not significantlydependent on tensile stress.

Yuan and Zhou[176]reported that the growth density and length of the CuO nanowires could beenhanced by increasing the surface roughness of the Cu substrate. The Cu substrates were first sand-blasted with different durations to generate different surface roughness, followed by typical thermaloxidation process under 200 Torr oxygen pressure. Their results proved that surface roughness signif-icantly increases the length and density of the grown CuO nanowires. However, the synthesis of CuOnanowires directly on Cu substrates (foils, plates, or grids) is not suitable for device integration withthe current semiconductor industry. Therefore, novel techniques to directly grow CuO nanowires onother substrates (especially semiconducting silicon) are desired to achieve CuO nanowire-based func-tional devices. Zhang et al.[177]reported large-scale and aligned CuO nanowires synthesized onto asilicon substrate by thermal oxidation of a Cu thin film deposited onto silicon. Comparative results oftwo Cu thin films deposited by thermal evaporation and electroplating show that a uniform and largeamount of CuO nanowires grew on the electroplated thin film only, which provided higher roughness

and larger surface grain size. Given that the CuO nanowires were synthesized onto silicon, a basicmaterial for microelectronics and microsystems, integrating CuO nanowires into silicon-based micro-systems is more convenient to achieve promising functional devices. The introduction of high-purityN2and O2gas during annealing results in more vertically aligned and uniform CuO nanowires[178].

Furthermore, Zhang et al.[179]presented a novel localized thermal oxidation method in ambientair instead of heating the entire Cu foils/films to synthesize the CuO nanowires. The CuO nanowiresonly grew on the surface of the heated area realized by localized joule heating ( Fig. 26). This methodis CMOS-compatible and can potentially integrate CuO nanowires with conventional microelectronics.

Vertically aligned CuO nanowires by thermal oxidation of a Cu thin film deposited onto 30 nmCu/Ti film-coated silicon substrates were fabricated by Cheng and Chen [180]. The length of the

Fig. 26. (a) SEM image of the suspended microheaters, (b) highly magnified SEM image of the tip in (a), (c) highly magnifiedSEM image of CuO nanowires in (b), and (d) optical image of one tip heater being heated [179]. Copyright 2010 Institute ofPhysics.




CuO nanowires was observed to be tuned from several to tens of micrometers by tailoring theoxidation temperature and time. The obtained CuO nanowires were single-crystalline with differentaxial crystallographic orientations, and the average length of CuO nanowires produced at eachtemperature followed a parabolic relationship with the oxidation time (Fig. 27).

Tsai et al. [181] synthesized uniformly aligned single-crystal CuO nanowires using a Co WP cappinglayer as a nanofilter to catalyze CuO nanowires on Cu (100 nm)/TaN/Ta/SiO2/Si blanket substrates bythermal oxidation. Their results suggested that the obtained CuO nanowires can grow at a relativelyhigher growth rate than those reported and become longer and denser with increasing calcinationtime. Park et al.[182]demonstrated that CuO nanowire growth can be achieved by thermal oxidationof Cu metal deposited on CuO (20 nm)/SiO2/Si substrate. The CuO nanowires were grown by a contin-uous supply of both Cu from the Cu films and O2from air. The growth of CuO nanowires by heating Cufilm deposited on glass substrates in the air was realized by Hsueh et al.[183]and Chang and Yang[184](Fig. 28). To avoid cracking of the obtained CuO nanowires/film during oxidation, a 100-nm-thick CuO film was first deposited onto the glass substrate to serve as an adhesion layer ( Fig. 28).Moreover, the average length of the CuO nanowires was determined by the initial Cu film thickness.

In summary, various methodologies have been reported to synthesize nanostructured CuO. The

hydrothermal synthesis, a wet chemical process with a reaction occurring in solution, is well knownfor low temperature, simple equipment, environmentally safe process, and good potential forhigh-quantity production. The solution-based chemical precipitation method that utilizes chemicalsolutions is another promising synthetic route because of its high efficiency, relatively low-cost,and its advantages in adjusting the size and morphology of the CuO nanostructures. The electrochem-ical method for the formation of nanostructured CuO is also of particular interest because of its manymerits over other methods including low-temperature, ease of process, and viability of commercialproduction. Furthermore, with the right selection of pH level and/or potential, the Cu or CuO phasecan be well controlled. Thermal oxidation is a simple, efficient, and low-cost method for synthesizing1D CuO nanostructures, and it is suitable for batch fabrication and mass production. The achieved 1DCuO nanostructures are uniform and vertically aligned with low impurity. In addition, the morphol-

ogy, density, diameter, and length ofthe1D CuO nanostructures can be easily tailored by adjustingthe synthesis parameters.

Fig. 27. Photographs of as-deposited Cu/Ti thin film and electrodeposited Cu film and SEM images of nanowires. Photographs ofthe (a) as-deposited Cu/Ti thin film on a silicon substrate and the electrodeposited Cu film-coated silicon samples (b) before and(c) after thermal oxidation. (d) A typical cross-sectional SEM image of nanowires grown on an oxidized Cu film-coated siliconsubstrate. (e) A typical high magnification SEM image of an individual nanowire [180]. Copyright 2012 Springer.


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2.5. Other synthetic methods

In addition to the methods described in Section 2.4, other synthetic techniques have also been usedto prepare CuO nanostructures. These techniques include sonochemical synthesis [48,50,185189],microwave irradiation synthesis [190196], template-assisted method [197205], solgel [206],microemulsion [207210], electrospinning technique [211213], synthesis combined with somephysical methods (e.g., spray pyrolysis) [214,215], thermal-based chemical vapor deposition[216],andc-irradiation[217]. For example, nanorods, nanoparticles, and submicrospheres from CuO havebeen obtained by sonochemical synthesis. Various CuO nanostructures, which include nanorods, shut-tle-like, flower-like, plate-like, leaf-like, dandelion-like, and hollow structures, together with CuOnanoparticles have been synthesized by microwave irradiation synthesis. CuO nanowires and nanof-ibers have been achieved by template-assisted method and electro-spinning technique.

3. Growth mechanisms

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

3.1. Oriented attachment

Oriented attachment is defined as a special kind of crystal growth in which small crystallites attachto each other through their suitable crystal planes or facets along the same crystallographic directions.

In this sense, the final aggregates can be considered as large single crystals built from the pristine crys-tallite in an irreversible and highly oriented manner[218,219].

Zhang et al.[220]demonstrated an anisotropic aggregation-based crystal growth of a few hundredmonoclinic CuO nanoparticles into uniform ellipsoidal monocrystalline architectures by taking advan-tage of the oriented attachment (Fig. 29). Stepwise orientation and aggregation in three dimensionswere observed at room temperature, caused by the formation of primary CuO nanoparticles in amother solution via the preferential 1D [001] or

cuo nanostructures- synthesis, characterization, growth mechanisms, fundamental properties, and...

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