Growth, Properties and Application of Copper Oxide, Nickel Oxide-hydroxide Nanostructures

CHAPTER 2

Growth, Properties, andApplications of Copper Oxideand Nickel Oxide/HydroxideNanostructures

Ahmad Umar, Mohammad Vaseem, Yoon-Bong Hahn

School of Semiconductor and Chemical Engineering, Centre for Future EnergyMaterials and Devices, Chonbuk National University, Jeonju 561-756, South Korea

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Growth and Properties of Copper Oxide Nanostructures . . . . 3

2.1. One-Dimensional Nanostructures of Copper Oxide . . . 32.2. Complex Nanostructures of Copper Oxide . . . . . . . . . 92.3. Spherical, Urchin, and Flower-Shaped

Nanostructures of Copper Oxide . . . . . . . . . . . . . . . . 133. Applications of Copper Oxide Nanostructures . . . . . . . . . . 23

3.1. Photocatalytic Properties of CopperOxide Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2. Field-Emission Properties ofCopper Oxide Nanostructures . . . . . . . . . . . . . . . . . . 27

3.3. Sensor Applications of CopperOxide Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . 29

4. Growth and Properties of Nickel Oxide andHydroxide Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . 30

5. Concluding Remarks and Future Directions . . . . . . . . . . . . 37References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

1. INTRODUCTIONThe science of nanotechnology is based on using the smallest units of matter todesign/process new materials/devicesatom by atomin order to obtain superior per-formance on the basis of atomic-scale architecture. Thus, fabrication of nanomaterials

ISBN: 1-58883-170-1Copyright 2010 by American Scientic PublishersAll rights of reproduction in any form reserved.

1

Metal Oxide Nanostructures and Their ApplicationsEdited by Ahmad Umar and Yoon-Bong Hahn

Volume 2: Pages 139

2 Growth, Properties, and Applications of Copper Oxide and NiO/Hydroxide Nanostructures

with well-dened structures and precisely controlled sizes is crucial to the develop-ment of nanotechnology. The investigation of nanostructures can provide unprecedentedunderstanding of materials and devices; nanostructures exhibit novel and signicantlyimproved physical/chemical/biological properties, phenomena, and processes comparedto their bulk counterparts [1, 2]. Previous work has been done with various materials tosystematically investigate the experimental conditions for fabricating nanostructures suit-able for desired applications. Synthesis of inorganic nanostructures controlled in termsof size and shape has been strongly motivated by the desired practical applications thatdepend on the size and shape of such structures [172]. Therefore, over the last fewdecades, signicant effort has been made to synthesize inorganic nanostructures withthe desired physical/chemical properties for possible applications in the fabrication ofefcient nanodevices [428]. Metal oxide semiconducting nanostructures are one of themost versatile classes of semiconducting materials due to their diverse properties andfunctionalities. Metal oxide nanostructures exhibit unique properties that can be used ina variety of applications for the fabrication of novel and efcient nanodevices. Therefore,rapid research developments have been made in the eld of metal oxide nanostructuresin terms of their growth and applications. The study of metal oxides has attracted agreat deal of interest due to the importance of their size- and shape-dependent propertiesin electronic/optoelectronic applications. Hence, investigations have attempted to con-trol the morphology of metal oxide nanomaterials. Among the variety of nanostructuredmorphologies, the so-called 1-D nanostructured materials (e.g., nanotubes, nanorods,nanowires, nanobelts, nanoribbons) have been intensively studied due to their substantialimportance and potential applications. However, for novel technologies based on nano-scale machines/devices, not only 1-D nanomaterials are needed but also other complexnanostructures are desirable. Nanomaterials with higher degrees of engineering and morecomplex architectures, such as 2-D and 3-D nanostructures, have potential applicationsin light emission/detection, eld-emission applications, biomedical devices, selective gassensors, electrode materials in batteries, and so forth. Numerous 2-D or 3-D nanostruc-tures of various materials have already been synthesized and reported in the literature;these complex nanostructures exhibit excellent physical properties and hence provideopportunities for scientists to use these nanostructures for the fabrication of highly ef-cient nanodevices. Therefore, the development of rational, general strategies for fabri-cation of multidimensional, interconnected, patterned assemblies of nanoscale buildingblocks will be signicant accomplishments. These developments are key to the success ofbottom-up approaches toward future nanodevices able to exploit these building blocks.Among different semiconducting metal oxides, copper oxide (CuO) has been stud-

ied as a unique and attractive mono-oxide material for both fundamental investigationsand practical applications. CuO is an important p-type/positive-doped, transition-metaloxide semiconductor, having a narrow bandgap (bandgap energy Eg = 12 eV). CuOsemiconductors exhibit a versatile range of applications such as fabrication of electrical,optical, and photovoltaic devices; selective gas sensing devices; heterogeneous catalysts;magnetic storage media; eld-emission devices (e.g., eld-emission gun); solar cells; andLi-ion electrode materials. CuO also possesses complex magnetic phases and forms thebasis for several high-TC superconductors (i.e., superconductors for which the Curie tem-perature TC is high) and materials with high magnetoresistance. Moreover, CuO can beused to prepare a variety of organicinorganic nanostructured composites with uniquecharacteristics, which include high thermal and electrical conductivity, high mechanicalstrength, and high-temperature durability. Due to their versatile properties and wideapplications, various CuO nanostructures have already been synthesized using a varietyof fabrication techniques; these techniques have included hydrothermal methods, solgeltechniques, gas-phase oxidation, and microemulsion. Solution processes present an easy,low-energy, low-temperature, and cost-effective approach to obtain CuO products withgood yields. Therefore, up to now, a variety of procedures for the preparation of CuOnanostructures by solution methods have been reported in literature; such proceduresyield nanorods, nanowires, nanotubes, nanosheets, and nanoribbons. Other than theseso-called 1-D structures, some complex nanostructures of CuO have also been reported

Growth, Properties, and Applications of Copper Oxide and NiO/Hydroxide Nanostructures 3

in the literature. In addition to these, nanostructures of nickel hydroxide (Ni(OH)2)one of the most important transition-metal hydroxideshave received increasing atten-tion due to their exotic properties and extensive applications. Notably, applications ofNi(OH)2 include use as an active material in positive electrodes and use in Ni-based,alkaline rechargeable batteries with the characteristics of high power density, excellentcyclability, high specic energy, and low toxicity. As for nickel oxide (NiO), it formsp-type, wide-bandgap material, which can be used as a transparent, semiconductinglayer. As an important material, NiO can be used in various applications, such as mag-netic materials, catalysts, electrochromic lms, fuel-cell electrodes, selective gas sensors,active optical bers, and battery electrodes. It has been observed that the performanceof Ni-based, alkaline rechargeable batteries and other devices depends on the struc-tural/morphological features of Ni(OH)2 and NiO; hence, considerable work has beendone to prepare and investigate nanocrystalline Ni(OH)2 and NiO structures.In this chapter, we assemble information about the growth, properties, and applica-

tions of various kinds of copper oxide, nickel oxide, and nickel hydroxide nanostructuresgrown using fabrication techniques reported in the literature to date.

2. GROWTH AND PROPERTIES OF COPPER OXIDENANOSTRUCTURES

Among the various semiconducting metal oxides, copper oxide (CuO) has been stud-ied as a unique mono-oxide material that is attractive for both fundamental investiga-tions and practical applications. Due to its versatile properties, CuO is highly usefulfor fabrication of various highly efcient devices. Nanostructures of CuO have beenused in various applications such as high-TC superconductors, heterogeneous catalysts,selective gas sensors, Li-ion electrode materials, eld-emission devices, magnetic stor-age media, and solar energy transformation [128]. Due to the excellent properties andversatile applications of CuO, a wide range of CuO nanostructures have been syn-thesized using various fabrication techniques. The CuO nanostructures reported in theliterature include nanowires, nanorods, nanobelts, nanotubes, nanoplatelets, nanorib-bons, nanobers, nanoparticles, nanosheets, ower-shaped structures, and sea urchin-likenanostructures. Here, we present growth procedures and properties of many CuO nanos-tructures fabricated via a variety of techniques.

2.1. One-Dimensional Nanostructures of Copper Oxide

The so-called 1-D nanostructures (i.e., nanorods, nanowires, nanotubes) have gener-ated great interest due to the prospects of these materials. For example, the study oftransport processes in 1-D systems has provided insight into the transport processes oflow-dimensional systems. These 1-D nanostructures have versatile applications such asfabrication of nanoelectronics, where these nanostructures can be used as single-electrontransistors. Hitherto, a variety of 1-D CuO nanostructures have been reported in theliterature. In this section, we review growth procedures and properties of 1-D CuO nano-structures fabricated using various methodologies.Hsieh and colleagues demonstrated the synthesis and structural characterizations of

well-ordered CuO nanobers [29]. These well-ordered CuO nanobers were synthesizedvia a self-catalytic growth process using a polycarbonate membrane as a template andCu nuclei sites (Cu(111)). The Cu nuclei sites were uniformly deposited on a Cu sub-strate via high-voltage input (electric eld = 15 V cm1) in a copper sulfate solution. Twosizes of Cu nuclei were obtained depending upon the pore diameter of the polycarbonatemembranes used as template. Upon heat treatment in an oxygen (O2) atmosphere, theelectrodeposited Cu nuclei were transformed into CuO nanober arrays. X-ray diffraction(XRD) and transmission electron microscopy (TEM) analyses conrmed the nanocrys-talline nature of the as-grown CuO nanobers. In the synthesis of CuO nanobers, Hsiehand colleagues observed that the self-catalytic reaction involved proceeds through thethermal oxidation of Cu nuclei on the (111) plane [29]. By this process, well-orderedCuO nanobers with different diameters were formed; interestingly, the diameter of an


individual nanober approximated the size to its Cu nucleus. This indicates that thesize of the Cu nucleus determines the diameter of the as-grown nanober. The possiblegrowth mechanism for the formation of CuO nanobers was also demonstrated by Hsiehand colleagues; they conrmed that the nanobers were grown via a self-catalytic growthprocess using Cu nuclei as basic units [29].Cao and colleagues reported the synthesis of Cu, Cu2O, and CuO nanotubes/nanorods

in the presence of a structure-directing surfactant, cetyl trimethylammonium bromide(CTAB) by varying reaction conditions and using Cu(OH)24 as an inorganic precursor[30]. The authors claimed that by using this method, not only the products but also themorphologies could be controlled. The Cu and Cu2O nanostructures were obtained byreducing Cu(OH)24 with hydrazine hydrate (NH2NH2 H2O) and glucose at room tem-perature (i.e., 300 K), respectively. Facile hydrothermal treatment of Cu(OH)24 results theformation of CuO nanostructures; results of structural observations conrmed that theCuO nanostructures grown were single-crystal structures. In this synthesis, the investi-gators observed that at a lower concentration of the Cu(OH)24 precursor tubular struc-tures were formed, while at a higher concentration rod-like structures were formed.Regarding the growth process of the CuO nanostructures grown (i.e., nanorods, nano-tubes), the authors speculated that, due to electrostatic interaction, the inorganic pre-cursor Cu(OH)24 and the cationic surfactant CTAB could form different conformationalsurfactantinorganic composites under different reaction conditions; the different com-posites could then serve as templates. Thus, by manipulating surfactantinorganic aggre-gates in solution, various kinds of CuO nanostructures can be obtained [30].In another report, Xu and colleagues presented the synthesis of CuO nanowires grown

on Cu foil by thermal oxidation of the Cu foils at 400C and 500C under various gaseousenvironments (e.g., air, N2, O2, with/without water vapor) [31]. These authors observedthat nanowires were formed exclusively from monoclinic CuO crystals under a gaseousatmosphere with a sufciently high oxygen partial pressure; no nanowires were found insamples oxidized in N2, with/without water vapor. Moreover, uniform nanowires wereformed at high density in wet air; nanowires were formed in a only small amounts indry air. The CuO nanowires formed in pure oxygen (O2) had the highest density. Finally,the authors concluded that the high oxygen partial pressure enhanced both the nucleationprobability and the growth rate of the nanowires, while the effect of water vapor wasmainly to assist nucleation. Results of detailed structural observations conrmed that thenanowires grown possessed monoclinic structures. A vaporsolid mechanism was pro-posed by the authors for the growth of these CuO nanowires presented in this study [31].In work by Mei and colleagues, CuO nanowire arrays on Si-based (i.e., SiO2) nano-

scale islands were fabricated via nanochannels of Si-based, porous anodic alumina (PAA)as templates at room temperature under pulse voltage in a conventional solution forelectrodeposition of Cu [32]. Detailed structural characterizations by XRD and X-ray pho-toelectron spectrometry (XPS) revealed that the main component of the nanowires wasCu2O and that the nanowires had a preferential growth direction along the (111) planesthat were connected with the nanoscale SiO2 islands (as conrmed by TEM results).Regarding the growth of the nanowires obtained, it was proposed that the formation ofCu2O was due to the alkalinity of the solution of anodized material [32].By using single-crystal Cu2(OH)2CO3 nanoribbons as precursors, various kinds of 1-D

CuO nanostructures/morphologies (e.g., nanoribbons, scroll-like structures, rod-likestructures, arrays of CuO nanoparticles) have been prepared via heat treatment by Zhuand colleagues [33]. These authors observed that the morphologies of CuO nanostructuresare strongly dependant on heat-treatment conditions. For instance, at relatively low-heattreatment and low heating rates, the morphological features of the precursor can be pre-served; arrays of CuO nanoparticles can be obtained at high heating rates; CuO rod-likestructures can be prepared with increased heat treatment. Moreover, the authors suggestedthat the formation of scroll-like structures of CuO in a transition stage of Cu2(OH)2CO3decomposition may be understood according to the position/arrangement of Cu atoms inthe crystal structure of a Cu2(OH)2CO3 cell. Detailed structural characterization indicatedthat the CuO nanostructures obtained were monoclinic, nanocrystalline structures [33].


Xu and colleagues reported the synthesis and detailed structural characterization ofCuO nanowires grown onto Cu foils heated in wet air at controlled temperatures [34].Two different morphologies of nanowires (i.e., straight and curved) were obtained fromtwo temperature zones. Regarding the growth of the nanowires obtained, the authorsclaimed that growth behavior of nanowires can be understood in terms of the kinetics(that is, a short circuit of diffusion of atoms/ions during the reaction), the strength ofthe nanowires, and the thickness ratio of the oxide scale/coating versus the metal foil.Thus, increasing the oxidation temperature may produce a reduction in the density ofthe growing nanostructures and an increase in their diameter and strength, causing themto adopt the straight morphology of nanowires. Moreover, deformation of the thin oxidescale/coating under further thermal stress may contribute to the formation of curvednanowires [34].Large-scale synthesis of CuO nanowires has been achieved by thermal evaporation

of Cu foils in ambient oxygen (O2) at 300C900C by Huang and colleagues [35].These authors observed that the evaporation conditions can affect the formation of thenanowires grown by this method. Thus, in the lower end of the temperature range exam-ined, nanowires with small diameters were formed. The amount of nanowires obtainedalso varied with the evaporation temperature. When the evaporation temperature was ashigh as 800C, only a small amount of CuO nanobers formed; however, when the evap-oration temperature was 400C750C, large amounts of CuO nanowires were obtained.Additionally, the authors observed that with increases in evaporation time, the lengthof nanowires obtained increases; however, the growth rate decreases. Results of exten-sive studies of the as-grown CuO nanowires conrmed the nanocrystalline nature for thenanostructures grown by this large-scale method. A vaporsolid growth mechanism wasproposed for the growth of nanowires; the authors claimed that by heating the Cu foilin an oxygen (O2) atmosphere, CuO lms were easily obtained on the substrate. Thus,when Cu foil was heated in an oxygen (O2) atmosphere, the foil was oxidized to formCu2O in a rst step, and CuO was formed in a second step (i.e., oxidation of Cu2O) [35].Additionally, growth of CuO nanowires depended on supersaturation.Lu and colleagues demonstrated the high-yield, low-cost, controlled synthesis of

Cu(OH)2 nanowires and nanoribbons in solution-phase reactions by simply drippingKOH and ammonia (NH3) solutions into an aqueous solution of CuSO4 at ambienttemperature [36]. Well-dened CuO nanostructures (e.g., nanoplatelets, nanoleaets,nanowires) were then produced by thermal dehydration of the as-prepared Cu(OH)2nanostructures in solution or in the solid state. These investigators observed that theconversion from Cu(OH)2 to CuO in solution occurred mainly through a reconstructivetransformation involving a dissolution process followed by CuO crystallization. The ther-mal dehydration of 1-D Cu(OH)2 nanostructures in the solid state normally resulted inmorphology-reserved, 1-D CuO nanostructures [36]. Results of detailed structural studiesconrmed that the CuO nanostructures obtained were monoclinic, nanocrystalline struc-tures (see Fig. 1 for studies on nanowires). Regarding the conversion of Cu(OH)2 nanos-tructures into CuO nanostructures, the authors stated that, when Cu(OH)2 nanowires(obtained by the KOHNH3 route) were heated in the original solution immediately afterthe addition of the two alkaline solutions, a complete transformation to CuO occurred attemperatures as low as 50C [36]. The authors also suggested that, during the solution-phase synthesis of CuO nanostructures, the composition and morphology of the nalproduct are largely dependent on the synthesis conditions (e.g., basicity, solvent); further,the solid precursors (e.g., Cu(OH)2) may exist only momentarily if at all.Du and Van Tendeloo synthesized CuO nanowires and nanobelts using copper nitrate

(Cu(NO3)2), NH3, and NaOH in a Teon-lined stainless autoclave at 130C for 10 h [37].The products were characterized by various techniques such as XRD, HRTEM, and elec-tron energy-loss spectrometry (EELS). By XRD, the nanostructures obtained were pureCuO with a monoclinic structure having the lattice constants of a = 4685 , b = 3425 ,c = 5 , = 99549; these constants were very similar to the reported data (JCPDS: 45-0937). Regarding the growth process of CuO nanostructures from Cu(OH)2 nanowires,the authors suggested that during the heating process the Cu(OH)2 nanowires lost H2O


(A)

(C)

(B)

(D)

Figure 1. Results of SEM (A), TEM (B), ED (C), and XRD (D) studies of CuO nanowires obtained by thermaldehydration of as-prepared Cu(OH)2 nanowires in the solid state at 120C for 2 h. Scale bars: 500 nm (A);200 nm (B). Reprinted with permission from [36], C. Lu et al., J. Phys. Chem. B 108, 17825 (2004). 2004, American Chemical Society. ED = electron diffraction, SEM = scanning electron microscopy, TEM =transmission electron microscopy, XRD = X-ray diffraction.

and transformed into CuO while the basic morphology of Cu(OH)2 nanowires remained[37]. According to Cudennec and Lecerf, the transformation from Cu(OH)2 to CuO canbe understood by the simple chemical reaction [38] described in reaction (I).

CuOH2s CuOs +H2Og (I)It was suggested that the loss of water was performed by an oxolation mechanism,

which would involve a dehydration process and the formation of OCuO bridges [38].These bridges would be formed after the loss of water and would be followed by con-traction of the structure along the [010] direction. Simultaneously, shifts of CuO4 groupsor Cu atoms along the [001] direction would take place to promote evolution towardcrystallized CuO; this would lead to the formation of CuO nanowires.Regarding the growth of CuO nanobelts, Du and Van Tendeloo suggested that the

growth mechanism was different from that of CuO nanowires and was due to a process ofthermal dehydration and then recrystallization as the Cu(OH)2 nanobelts [37]. In a typicalprocess, divalent copper ions (Cu2+) were rst dissolved in the form of complex anionsCu(OH)24 , which result in the square planar surrounding. These anions can be consideredthe precursors for the formation of CuO. Here, a condensation phenomenon combineswith a loss of two hydroxyl ions and one water molecule leading to the formation ofchains of square planar CuO4 groups and then to the formation of CuO in solid form.This transformation can be described according to reactions (II) and (III).

CuOH2S + 2OHaq CuOH24 aq (II)

CuOs + 2OHaq +H2O (III)


Therefore, it is predicted that the presence of water facilitates the morphological trans-formation from nanowire to nanobelt.Yu and colleagues reported the growth of rod-shaped CuO nanostructures and inves-

tigated the polarized micro-Raman scattering for an individual CuO nanorod [39]. TheCuO nanorods were synthesized by a one-step annealing process using commerciallyavailable Cu plates as starting material. For the synthesis, a Cu plate was annealed at400C for 24 h in air. After the desired reaction time, a black ash-like top layer formedon the Cu plate; this layer contained CuO nanorods. The authors investigated individualCuO nanorods with different aspect ratios using polarized micro-Raman scattering. Anobvious anisotropy in the intensity of the Raman modes was observed when the electriceld vector of the incident laser beam was parallel/perpendicular to the long axis ofa nanorod. The mechanism responsible for the polarized Raman spectra observed wasattributed to the polarization effect produced by the large length:diameter ratio of theCuO nanorods and the large contrast in the dielectric properties of these nanorods versusthe surrounding environment [39].CuO nanorods of various crystalline structures/morphologies were synthesized in an

NaOH solution by a simple, efcient method by Gao and colleagues [40]. These investi-gators observed that the crystalline structures/morphologies of the products were highlydependent upon the temperature of the hydrothermal treatment in the synthesis proce-dure. Results of structural characterization by XRD conrmed the monoclinic, nanocrys-talline state of the as-grown CuO nanostructures. When the as-grown CuO nanomaterialswere used as anode materials for Li-ion batteries, ne polycrystalline nanorods exhibitedhigh electrochemical capacity (766 mAhg1) as compared to single-crystal bulk nanorods(416 mAhg1); this was due to the large surface area and numerous structural defects ofthe polycrystalline nanorods. However, the capacity retention of polycrystalline nanorodswas not as good as that of single-crystal nanorods due to the Li-driven, irreversiblemorphological/crystalline changes during the electrochemical reaction cycles to whichanode materials are exposed [40].Chang and Zeng reported several wet chemical methods for synthesis of 1-D CuO

nanostructures in which waterethanol solutions were used as solvents at 77C82Cand 1 atm [41]. Various 1-D CuO nanostructures (e.g., nanorods, nanowires, nanorib-bons, nanoplatelets, nanosheets) were obtained by such methods. It was observed thatat low reaction temperatures and normal atmospheric pressure, monodisperse, single-crystal CuO nanorods (with a selected breadth of 515 nm) could be prepared by simplychanging the starting Cu ion concentration. For nanorods, preferential growth is alongthe [010] direction. By using a two-step, continuous process in which seeding and lengthgrowth could be controlled under a pseudo-steady-state operation, these investigatorsdemonstrated the synthesis of rigid or exible nanorods/nanoribbons with lengths upto 1 m. The morphologies of the pristine nanorods obtained were further modiedby an aging treatment. These authors also reported the self-assembly of small nanorodsinto 2-D netted structures. These netted structures were formed after prolonged heat-ing, and the nanorods were attached to each other using their {001}, {100}, and {110}crystal planes. Nanoplatelets or nanosheets of CuO were also synthesized using a highconcentration of NaOH; in this procedure, growth along the [100] direction becomespronounced. Results of detailed structural characterization using XRD and selected areaelectron diffraction (SAED) conrmed that the CuO nanostructures grown were mono-clinic, crystal structures [41].Yao and colleagues demonstrated the synthesis/characterization of uniform, monodis-

perse CuO nanorods using a process in which size could be controlled [42]. This pro-cess involved spontaneous aggregation/crystallization of CuO nanoparticles generatedfrom a solidliquid, arc-discharge process under ambient conditions in the absence ofsurfactants/additives. These investigators found that newly formed Cu nanoclusters gen-erated by this method rapidly oxidized into CuO nanoparticles, and these CuO nanopar-ticles spontaneously aggregated/self-organized and then crystallized into uniform CuOnanorods via a prolonged aging time under ambient conditions. Therefore, by choosinga suitable reducing agent to prevent oxidation of the Cu nanoclusters, selective synthesis


of CuO, Cu2O, and Cu nanostructures was achieved. Results from XRD conrmed thatthe CuO nanorods obtained were monoclinic, crystalline structures [42].The growth and properties of CuO nanowires grown by thermal oxidation of Cu

sheets in an oxygen (O2) atmosphere were investigated by Kaur and colleagues [43].This procedure was carried out in a resistively heated furnace at various tempera-tures (400C800C) and various times under a ow of oxygen (O2). These investigatorsobserved that branched CuO nanowires were obtained by long oxidation times. Thelength of the nanowires obtained increased with annealing time; at 22 h, nanowiresof >20 m length were obtained. Results of detailed structural studies conrmed thatthe nanowires and their branches had grown along specic crystallographic directions;that is, nanowires grew along the [010] direction and the branches grew along the [210]direction. The bandgap of as-grown nanowires was determined using UV-visible absorp-tion spectrometry; the bandgap of the as-grown CuO nanowires was larger than that ofbulk material [43].Wang and colleagues reported the synthesis and sensing applications of hydrother-

mally grown CuO nanorods [44]. The growth of CuO nanorods was performed in aTeon-lined stainless steel autoclave (at 120C150C for 12 h) using the cationic sur-factant CTAB, copper acetate, and NaOH as source materials. The authors reported thatduring the reaction, the formation of CuO occurred according to the following reaction:Cu2+ + 2OH CuO+H2O. The XRD pattern of the as-prepared product was indexedto the tenorite structure of CuO with lattice constants of a = 496 , b = 343 , andc = 513 (JCPDS: 41-0254). Results of detailed structural characterization using high-resolution TEM (HRTEM) conrmed that the nanorods synthesized were grown parallelto the [001] direction [44].A solution-phase route for the synthesis of single-crystal CuO nanoribbons was devel-

oped by Wang and colleagues [45]. The nanoribbons grown had widths and thicknessesof 1080 nm and 520 nm, respectively; their lengths ranged from several hundred nano-meters to several micrometers. In addition to nanoribbons, CuO nanorings (diameter =100300 nm) were fabricated by the same procedure [45]. These CuO nanostructures werefabricated by the reaction of CuCl2 and NaOH in the presence of sodium dodecylben-zenesulfonate (NaDBS). The as-prepared products were characterized by powder XRD,TEM, and HRTEM; the CuO nanoribbons and nanorings both had monoclinic, single-crystal structures. The nanorings were closed but not by the simple superposition of thetwo ends of nanoribbons. On the basis of TEM observations, the formation of nanorib-bons and nanorings was interpreted to be a multistage process in which initial nanoakeswould split into nanoribbons due to Brownian motion of the surfactant molecules; thennanoribbons, which would have polar surfaces, would coil into nanorings to reduce elec-trostatic energy. Moreover, it was proposed that single-crystal nanorings were formed viaa spontaneous, self-coiling process during the growth of polar nanoribbons that wouldhave an alternating stack of Cu2+ and O2 ions along the [002] axis [45].Wen and colleagues synthesized aligned, CuO nanoribbon arrays (that were approxi-

mately perpendicular to the Cu substrate surface) by a solution-treatment process with asubsequent heat-treatment process [46]. For the synthesis of CuO nanoribbons, the initialCu(OH)2 nanoribbons were fabricated by a simple, coordinated self-assembly methodin which Cu2+ ions were oxidized from the surface of Cu foil in an alkaline solution.Heat treatment in a horizontal, quartz-tube furnace was used to remove water from theCu(OH)2 nanoribbons to yield CuO nanoribbons. The CuO nanoribbons synthesized were50 nm in width and several nanometers in thickness; nanoribbon length was controlledby varying reaction temperature and time [46]. The CuO nanoribbons obtained were char-acterized using scanning electron microscopy (SEM), TEM, HRTEM, and XRD (Fig. 2). Bylow-magnication TEM, the average diameter of the nanoribbons obtained was 50 nm(Fig. 2(A)). The SAED pattern from an ensemble of nanoribbons (Fig. 2(A), inset) wastypical of monoclinic CuO with three diffraction rings of (110), (111), and (111). The cor-responding XRD pattern again conrmed the monoclinic nature of the CuO nanoribbonsobtained (Fig. 2(B)). SEM images (as in Fig. 2(C)) showed that the CuO nanoribbons


(A)

(C)

(D)

(B)

Figure 2. TEM (A), XRD (B), SEM (C), and HRTEM (D) analyses of CuO nanoribbons obtained from Cu(OH)2nanoribbons by heat treatment. Inset of panel A represents the SAED pattern of an ensemble of CuO nanorib-bons. Inset of panel D represents the SAED pattern of single CuO nanoribbons. Reprinted with permissionfrom [46], X. Wen et al., Langmuir 19, 5898 (2003). 2003, American Chemical Society. HRTEM = high-resolution transmission electron microscopy, SAED= selected area electron diffraction, SEM= scanning electronmicroscopy, TEM = transmission electron microscopy, XRD = X-ray diffraction.

obtained after heat treatment were still aligned along the surface of the Cu foil sub-strate where the initial Cu(OH)2 nanoribbons had been fabricated by oxidation. HRTEMimages (as in Fig. 2(D)) revealed that the CuO nanoribbons obtained were crystals, butthe crystallinity was not perfect; fringes were often wavy or discontinuous. Nevertheless,the CuO crystallites had a preferential orientation in the nanoribbons; that is, the [110]direction is along the longest dimension of the nanoribbon. This agreed with the SAEDpattern (Fig. 2(D), inset). Crystallographic analysis by both XRD and SAED indicatedthat the nanoribbon axis was along the [110] direction. It is plausible that the (110) and(002) planes of CuO crystallites (with dhkl values of 2.75 and 2.52 , respectively) wereconverted from the (100) and (002) planes of Cu(OH)2 crystallites (with dhkl values of2.95 and 2.63 , respectively) by release of H2O [46].

2.2. Complex Nanostructures of Copper Oxide

In addition to 1-D CuO nanostructures, a variety of complex nanostructures of CuO havebeen reported in the literature. In this section, we summarize the growth proceduresand properties of complex CuO nanostructures synthesized using various methodologiesreported in the literature.Complex dendrite-like CuO nanostructures, consisting of a rod-like main stem and

rod-like subbranches, have been synthesized via a simple, ethylene glycol (EG)assistedhydrothermal method by Zhang and colleagues [47]. Detailed structural characterization


by XRD and SAED indicated that the dendrite-like CuO nanostructures were in a mon-oclinic state and an individual branch had a single-crystal nature. The authors observedthat the structures/morphologies of the products strongly depend on growth conditionssuch as pH and temperature. The function of EG in the hydrothermal synthesis of CuOnanostructures was investigated; EG not only promoted growth of branches but also hadreducing ability within a certain range of alkaline conditions. These investigators pro-posed that in the EG-assisted hydrothermal synthesis of CuO nanostructures, pH hadthree functions: providing a source of OH ions, facilitating the reducing ability of EG,and stimulating branched structural growth. The quantity of OH ions, the reducingability of EG, and the branched growth of CuO nanostructures all increased as the pHincreased. When the pH of the solution was


were formed. However, under high rates of hydrolysis (achieved by increasing the con-centration of the precipitation agent (i.e., urea) or by conducting the reaction at hightemperatures (i.e., 120C)), only Pre-CuO nanoparticles with featureless morphologywere formed. In one set of experiments, spherical Pre-CuO architectures were formedand converted to a porous structure (CuOx) after removal of octylamine via calcination.Compared to 1-D and 2-D aggregates, the porous architecture obtained was highly ther-mostable and did not collapse even after calcination at 500C [50].Chen and colleagues demonstrated the growth and properties of shuttle-like CuO

nanostructures synthesized using the process known as pulsed-laser-induced liquidsolidinterfacial reaction (PLIIR) [51]. In a typical reaction process, deionized water was usedas the reactive liquid, and the solid target was Cu bulk material (purity = 997%). TheCu target was rst xed on the bottom of a reaction chamber. Subsequently, deion-ized water was poured slowly into the chamber until the target was covered by 5 mm.Finally, the pulsed laser was focused on the target surface, and during the laser abla-tion, the target and reaction uid were maintained at room temperature while the targetwas rotated at a slow speed (i.e., 10 rotations min1). After the pulsed laser interactedwith the target for 45 min, the PLIIR chamber was heated to 120C for 30 min. Thepowders synthesized were collected from the reaction uid and analyzed in terms oftheir structural and optical properties. SEM, XRD, and TEM (in conjunction with anenergy-dispersive X-ray spectrometer) were used to determine the morphology, structure,and composition of the as-prepared products. The XRD pattern of the as-prepared CuOnanostructures conrmed the monoclinic crystal nature of the nanostructures grown bythis procedure. HRTEM images indicated that the interplanar spacing of 0.25 nm and0.27 nm corresponded to the {110} and {200} crystallographic planes of CuO nanostruc-tures, respectively. An anomalous peak (max = 258 nm) was observed in the UV absorp-tion spectrum of the shuttle-like CuO nanoparticles; the authors proposed from theirexperimental analyses that the anomalous UV absorption peak was induced by the shapeof the CuO nanoparticles. Moreover, these investigators employed theoretical calculationsbased on discrete dipole approximation to pursue the physical origin of the anomalousUV absorption peak. Thus, both experimental and theoretical evidence were used toshow that the shape of nanoparticles has a great inuence on the optical properties ofnanomaterials [51].Xiao and colleagues demonstrated the controlled synthesis of CuO nanostructures with

various morphologies [52]. The 1-D, 2-D, and 3-D CuO nanostructures obtained weresynthesized in the presence of sodium citrate by a hydrothermal process under con-trolled the reaction conditions. These investigators observed that, when the molar ratio ofsodium citrate to CuSO4 (represented as the SC:Cu2+ ratio) was 1.0, 3-D branch-likeCuO nanostructures with lengths of hundreds of nanometers and diameters of 20100 nmwere formed. UV-visible studies were performed to assess the optical properties of the as-prepared CuO nanostructures. By UV absorption measurements, the CuO nanostructuresobtained displayed a blueshift in the bandgap relative to bulk CuO material. Moreover,the optical bandgap energy (Eg) of CuO nanostructures obtained could be tuned throughmorphological control of the structures. UV absorption measurements revealed that theestimated Eg of the 1-D rod-like, 2-D ake-like, and 3-D branch-like CuO nanostructureswas 2.36 eV, 1.60 eV, and 1.40 eV, respectively; all of these values were larger than thereported values (Eg = 12 eV) of bulk CuO materials [52].Wang and colleagues demonstrated the synthesis and characterization of CuO

nanowhiskers by a one-step, solid-state reaction in the presence of a nonionic surfactant,polyethylene glycol (PEG) 400 [53]. For the growth of CuO nanowhiskers, CuCl2 2H2O,NaOH, and PEG were used as source materials. In detailed structural observations, XRDdata conrmed that the nanowhiskers obtained were monoclinic, nanocrystalline struc-tures with a space group of C62h. TEM observations conrmed that the nanowhiskers


grown were shaped as rods with smooth, clean surfaces; also, the nanowhiskers had uni-form diameters of 8 nm and lengths > 100 nm. By HRTEM, the interplanar spacing ofthe nanowhiskers grown was 0.248 nm, which corresponds to the {002} crystal planesof CuO nanostructures [53].Zhang and colleagues reported the large-scale growth of CuO nanostructured lms

including nanotube arrays, ordered nanotube arrays with a special nanoplate wall struc-ture, and nanoower lms [54]. These investigators fabricated such novel nanostructuresby manipulating the conditions of the chemical reactions. The CuO nanotube arrays weresuccessfully synthesized by heating CuO nanotubes under a nitrogen (N2) atmosphere(Cu(OH)2 nanotubes were initially prepared under N2 and used as a precursor for thegrowth of CuO nanotubes). These investigators observed that CuO nanotubes (even afterheating) retain morphology similar to that of initially prepared Cu(OH)2 nanotube pre-cursor. In addition, ordered CuO nanotube arrays with a special nanoplate wall structurewere obtained by heating Cu(OH)2 nanotubes in an autoclave with benzene (C6H6) as thesolvent. The CuO nanotube arrays formed still kept the initial nanotube structure, butexhibited features different from other CuO nanotubes. Two complex nanostructuresCuO nanotube arrays (prepared under a nitrogen (N2) atmosphere) and ordered CuOnanotube arrays with a special nanoplate wall structure (prepared in benzene)were dis-tinguished by two aspects. (1) The latter CuO tubes had average diameters in the rangeof 300500 nm, which were thicker than those of the CuO tubes formed under a nitro-gen (N2) atmosphere. (2) The special nanoplate wall structure (formed in benzene) hada maize-like morphology, which was made up of well-aligned nanoplates (with uniformthicknesses of 15 nm) stacked along the direction of the tube axis; the walls of the CuOnanotubes formed under a nitrogen (N2) atmosphere were smooth. Also, some of thenanotube tips grew closed under the solvothermal condition. Films of CuO nanoowerswere prepared by oxidizing Cu foil hydrothermally in an autoclave. By various analyticmethods, all the CuO nanostructures reported were conrmed to be well crystallizedwith monoclinic structure [54].Liu and colleagues reported the large-scale synthesis and the characterization of single-

crystal CuO nanoplatelets grown via a hydrothermal process (120C for 12 h) using cupricdodecylsulfate (Cu(DS)2) and NaOH as source materials [55]. In this synthetic process,the Cu(DS)2 functions both as structure-directing agent and source material for the pro-duction of Cu2+ ions. The CuO nanoplatelets synthesized are monoclinic, single-crystalstructures (lattice constant a= 0468 nm) as conrmed by XRD results. Detailed structuralanalyses by SEM and TEM revealed that the CuO nanoplatelets obtained were of thick-ness 50 nm, width 120300 nm, and length 480700 nm. The width-to-thickness ratioswere 26. XPS and other analytic methods indicated that the CuO nanoplatelets grownwere highly pure. In addition, these investigators conducted several time-dependantreactions to suggest the possible growth mechanism involved. In UV-visible absorptionspectral analyses, the as-prepared CuO nanoplatelets exhibited an absorption peak at287 nm. From absorption edge analyses, the optical bandgap of the as-grown CuOnanoplatelets was calculated at 2.60 eV, much larger than the reported value for bulkCuO materials (Eg = 185 eV) [55].Zhao and colleagues demonstrated the synthesis and characterization of a variety of

CuO nanostructures including radially arrayed nanowhiskers, cubic nanoparticles, andspherical nanoparticles [56]. In a typical reaction process, an aqueous solution of copperacetate and ethylene glycol (EG) were place into a microwave reux system under ambi-ent conditions for 15 min at a power of 365 W. Various concentrations of EG in aqueoussolutions were used to control the morphologies of the CuO nanostructures obtainedduring the microwave heating/irradiation. Reaction temperature, irradiation time, andcopper acetate concentration were also found to be critical factors affecting the morphol-ogy of the products. Detailed structural observations conrmed that all the productssynthesized were monoclinic, nanocrystalline structures [56].Interestingly, 2-D hierarchical CuO nanosheets were synthesized by Zheng and Liu via

a facile, solution-phase method at near-neutral pH (7.5) and near room temperature (i.e.,30C) without the use of any additives or templates [57]. From low-magnication SEM


images, it was clearly seen that the as-prepared CuO products were grown on a largescale. It was also observed that most of the nanosheets were ellipsoidal with averagehorizontal-axis and longitudinal-axis sizes of 400 nm and 200 nm, respectively. Resultsof high-resolution SEM conrmed the 2-D sheet-like conguration and indicated that thethickness of the nanosheets obtained was 25 nm. Moreover, the 2-D sheet-like structureswere actually composed of small CuO nanocrystals; hence, these structures were termedhierarchical 2-D nanosheet structures. The XRD pattern conrmed that the as-preparedhierarchical 2-D CuO nanosheets were monoclinic, nanocrystalline CuO structures withthe lattice constants a0 = 4684 , b0 = 3425 , c0 = 5129 . The optical properties ofthe as-grown hierarchical 2-D CuO nanosheets were observed by UV-visible studies, andEg was estimated at 1.94 eV [57].Well-aligned arrays of CuO nanoplatelets synthesized by a hydrothermal route with-

out the assistance of any kind of template were reported by Zou and colleagues [58].In a typical reaction process, cuprous chloride (CuCl) was dissolved in a concentratedNH3 solution, and the resultant solution was added to deionized water with continuousstirring. The mixture obtained was then transferred to a 60-mL Teon-lined stainless steelautoclave, which was then sealed and maintained at 140C for 40 h. After terminating thereaction, the autoclave was cooled to room-temperature, and the products synthesizedwere scratched, centrifuged, washed with distilled water and ethanol, and dried at 40Cfor 2 h in air. The CuO nanostructures synthesized were characterized in detail in termsof their structural and optical properties using various analytic tools. XRD peaks for theas-prepared CuO nanostructures could be clearly indexed to the monoclinic structure of(space group C2/c). Moreover, compared with the standard diffraction peaks for crys-talline CuO structures, no other peaks were observed (i.e., peaks belonging to impuritiessuch as Cu(OH)2 or Cu2O) conrming the high purity of as-prepared CuO nanostruc-tures. Field-emission SEM (FESEM) observations provided detailed information aboutthe morphologies of the as-grown CuO nanostructures. It was observed that the CuOnanocrystals self-organized into wall-like structures and that most of the nanocrystalswith uniform morphology were upright, outward, densely packed, and well aligned.From FESEM observations, it was also found that these nanocrystals possessed platelet-like structures (i.e., nanoplatelets). It was observed that nanoplatelets were 5080 nmin thickness, 150250 nm in width, and 0.81.5 m in length. Most of the as-preparednanoplatelets had four clear edges, resembling prisms; however, the nanoplatelets did notobtain uniform tops. In addition, a few patch-like nanocrystals were also found amongthe products. Electron microscopic analysis showed that the nanoplatelets grew alongthe [010] direction. The Ostwald ripening mechanism was used to describe the growthof CuO nanoplatelets. The optical/electrochemical properties of the as-prepared prod-ucts were also investigated. The arrays of CuO nanoplatelets exhibited a blueshift in theUV-visible spectra, a slow capacity fading rate, and a relatively high coulombic efciencyin the chargedischarge process.

2.3. Spherical, Urchin, and Flower-Shaped Nanostructures of Copper Oxide

There are a few reports in the literature that present the synthesis and characterization ofspherical and sea urchin-shaped CuO nanostructures. In this section, we summarize thegrowth procedures and properties of such CuO nanostructures synthesized using variousmethodologies.Keyson and colleagues demonstrated the synthesis and characterization of urchin-

shaped CuO nanostructures [59]. The synthesis was carried out by a simple, novel,hydrothermal microwave method using PEG, CuCO3 Cu(OH)2, and NH4OH at 120C in1 h. Various techniques were used for detailed structural characterization of the urchin-shaped CuO nanostructures obtained. All diffraction peaks of the structures synthesizedcould be indexed to CuO monoclinic structures. The lattice parameters were calculatedby least-squares renement using the UnitCell-97 program (Department of Earth Sci-ences, University of Cambridge); the estimated parameters were a = 4692 , b = 3428 ,and c = 5136 with volume cell of 81.52 3. These values were consistent with those


reported in the literature (JCPDS: 45-0937). The FESEM images revealed that the urchin-shaped particles grown were uniform spheres with diameters of 0.71.9 m. The specicsurface area of the CuO nanostructured microspheres was 170.5 m2 g1. In the pro-posed growth mechanism of urchin-shaped CuO nanostructures, PEG acted as a templateof sorts for the formation of the nanostructures obtained. During the rst stage of thegrowth process, the aqueous solution of Cu2+ plus PEG and NH4OH gave rise to the rstnucleation seeds, which acted as initial nuclei for particle growth. When the particlesreached a critical dimension, PEG absorbed the small particles by exposed OH bonds;these PEG-absorbed particles then acted as templates for the formation of CuO nanos-tructures. In the nal stage of the growth process, urchin-shaped CuO nanostructureswere formed.Vaseem and colleagues also reported sea urchin-like and sheet-like CuO nanostruc-

tures synthesized in large quantities via simple, solution processes [60]. Urchin-like CuOnanostructures were obtained from Cu powder present in a strong alkali solution of cop-per nitrate [Cu(NO3)2]; in this process, small CuO nanosheets were nucleated and grownon the outer surfaces of Cu powder particles. Sheet-like structures, some arranged inower-shaped morphologies, were also found in the same reaction solution. Figure 3shows typical morphologies of the urchin-like CuO nanostructures synthesized. Metal-lic Cu powder was used to synthesize urchin-like CuO nanostructures that were madeof thin CuO nanosheets. Aggregates of almost-spherical Cu particles with diameters of12 m were observed in samples of the Cu powder (Figs. 3(A and B)). These Cu par-ticles were oxidized and converted into CuO in the presence of a strong alkali solutionof Cu(NO3)2. Upon examination of the general morphology of the as-prepared urchin-like CuO nanostructures, it was revealed that these structures were created by the nearrangement of thin CuO nanosheets. The authors proposed that the Cu particles were

(A) (B)

(C) (D)

Figure 3. Typical low-magnication (A) and high-magnication (B) FESEM images of Cu powder prior toprocessing and low-resolution (C) and high-resolution (D) FESEM images of as-grown, sea urchin-like CuOnanostructures grown on the Cu powder by a simple, solution process. Reprinted with permission from [60],M. Vaseem et al., Mater. Lett. 62, 1659 (2008). 2008, Elsevier B. V., Amsterdam, The Netherlands. FESEM =eld-emission scanning electron microscopy.


oxidized and converted into CuO particles in the presence of the strong alkali solutionof Cu(NO3)2; CuO nanosheets were then arranged on these CuO particles as the reac-tion proceeded for longer time. Interestingly, these investigators found that the sizes ofthe Cu particles were smaller than the urchin-like CuO nanostructures formed; thus,they believed that some converted CuO particles adhered to each other and formedthe larger, urchin-like CuO nanostructures. The typical diameter of urchin-like morpho-logies was 24 m (Fig. 3(C)), while some smaller, urchin-like morphologies were alsoseen in micrographs (Fig. 3(D)). In addition to urchin-like morphologies, 2-D sheet-likestructures were also observed in samples of the reactant solution (Fig. 4). Figure 4(A)presents a low-magnication image of the as-grown nanosheets and reveals that thesheets were grown in large quantity. Interestingly, it is also seen that some nanosheetswere arranged in such a fashion that they made ower-like morphologies with an averagediameter of 25 m (Fig. 4(B)) while some nanosheets were arranged in irregular man-ner (Fig. 2(C)). As revealed by high-resolution FESEM, the thicknesses of the as-grownnanosheets were in the range of 4060 nm (Fig. 4(D)); the as-grown nanosheets were24 m wide. TEM images (Figs. 5(A and B)) of the as-grown urchin-like CuO nano-structures are fully consistent in terms of morphology and dimensionality with FESEMimages (Figs. 3(C and D)). The XRD pattern of the as-prepared products agreed well withthose of nanocrystalline CuO structures, and the peaks could be indexed to a monoclinic,crystalline CuO structure (JCPDS: 48-1548) (Fig. 5(C)). Moreover, in the XRD pattern,two dominant peaks (located at 2 values of 35.6 and 38.8 and indexed as (111)(002)and (111)(200) planes, respectively) were characteristic for pure, monoclinic CuO crys-tallites. Several bands appeared in the Fourier transform infrared (FTIR) spectrum of anas-grown sample (Fig. 5(D)). The presence of weak absorption at 3334 cm1 is due to thestretching vibration of adsorbed water and surface hydroxyl groups. Several other bandsin the FTIR spectrum (at 598, 525, and 430 cm1) are characteristic of monoclinic CuOstructures and conrm the monoclinic structure of the as-grown nanostructures [60].

(A) (B)

(C) (D)

Figure 4. Typical low-magnication (A, B) and high-magnication (C, D) FESEM images of ower-like nanos-tructures composed of thin nanosheets and irregularly arranged sheet-like nanostructures grown in the reactantsolution. Reprinted with permission from [60], M. Vaseem et al., Mater. Lett. 62, 1659 (2008). 2008, ElsevierB. V., Amsterdam, The Netherlands. FESEM = eld-emission scanning electron microscopy.


(A)

(C) (D)

(B)

Figure 5. Typical TEM images (A, B), XRD pattern (C), and FTIR spectrum (D) of the as-grown, sea urchin-likeCuO nanostructures grown by a simple, solution process. Reprinted with permission from [60], M. Vaseemet al., Mater. Lett. 62, 1659 (2008). 2008, Elsevier B. V., Amsterdam, The Netherlands. FTIR = Fourier transforminfrared, TEM = transmission electron microscopy, XRD = X-ray diffraction.

Synthesis and characterization of CuO nanostructured microspheres using a novel,solid-stabilized emulsion method was reported by He [61]. For the growth of CuOmicrospheres, Cu(NO3)2, dimethyl oxalate, 1-hexanol, and acetone were used as sourcematerials; the synthesis was performed at 65C for 5 h. The synthesized structures werecharacterized in detail using various analytic tools such as SEM, TEM, XRD, size anal-ysis, and measurements based on BET theory (so-called from the originators, Brunauer,Emmett, and Teller). The average diameter of the CuO microspheres obtained was2.8 m. From the SEM structural characterization, the investigator observed that thesurfaces of the CuO microspheres were made of pin-like nanostructures (with a pindiameter of 95 nm and a pin length of at least 600 nm); from these ndings, the authorcoined the term nanostructured microspheres. The XRD analysis indicated that the CuOnanostructured microspheres were monoclinic lattices. The BET measurements showedthat the specic surface area of the CuO nanostructured microspheres was 56.8 m2 g1.All peaks in the XRD pattern obtained for the products synthesized could be indexedto monoclinic CuO lattices (JCPDS: No. 5-661), and no peaks from other structures orimpurities were observed. Thus, XRD results conrmed that the CuO nanostructuredmicrospheres synthesized were high purity. The author proposed that the CuO nanos-tructured microspheres obtained were formed via interfacial precipitation with dropletsof a solid-stabilized emulsion serving as templates [61].A simple method for the synthesis of hierarchical CuO microspheres was reported by

Song and colleagues [62]. The synthesis was done under microwave irradiation usingCuCl2 2H2O, N ,N ,N ,N -tetramethylethylenediamine (TMEDA), and NaOH as sourcematerials., A microwave oven (at 100% output power for 2 h) was used in this synthesis.The XRD pattern conrmed that the products obtained were monoclinic CuO structures


with lattice constants of a = 46837 , b = 34226 , c = 51288 , and = 9954. Noother XRD peaks were detected for the CuO structures synthesized, indicating that theCuO products obtained were free of impurities. From SEM and TEM observations, theCuO microspheres synthesized had an average diameter of 1.52.5 nm. The authors con-cluded that the microspheres were formed via self-assembly of nanoakes (width of250 nm, length of up to 1 m); each nanoake was formed via self-assembly of dozensof smaller nanoplates (width 25 nm). These investigators found that, as the concentra-tion of TMEDA was increased and the concentration of NaOH was decreased, the mor-phology of the CuO products changed from simple aggregates of nanoakes to sphericalassemblies (with the external shape of the hierarchical CuO microspheres resembling achestnut bur) [62].Dandelion-shaped CuO nanostructures were synthesized and characterized by Liu and

Zeng [4] and have been reported in the literature by others. In a typical reaction process,an ethanolic solution of Cu(NO3)2 was mixed with the NH3, NaOH, and NaNO3 solu-tions, transferred to a Teon-lined stainless steel autoclave, and heated at 100C180Cfor 224 h. XRD patterns conrmed the nanocrystalline nature and monoclinic symme-try of the as-prepared CuO dandelion-like nanostructures. From detailed experiments,the authors observed that the CuO nanostructures synthesized seemed to be built fromsmall crystalline strips, which were, in turn, build from even smaller 1-D nanoribbons.The crystalline strips were aligned perpendicular to the spherical surface (i.e., pointingtoward a common center). Thus, the structures seemed like dandelions that were coreless(i.e., with a hollow cavity); the thickness of the shell wall was about one-third to one-quarter of the sphere diameter. Regarding the formation of the spherical structures, theauthors suggested that the geometric shapes of the reactants (i.e., building blocks) playeda key role, since no surfactants/emulsions had been used. Further, a simple array ofrhombic CuO crystal strips could easily generate curvature, and the lateral engagementof these building units could naturally lead to a shell-like structure [4].The synthesis and characterization of hollow CuO microspheres grown by a simple,

hydrothermal route, in which neither sophisticated techniques nor catalysts/surfactantswere required, was reported by Zhang and colleagues [63]. The synthesis was performedusing copper acetate as a Cu source and hexamethylenetetramine ((CH2)6N4, HMTA)as a complexing reagent. The authors observed that, under hydrothermal treatment,the copper acetate (as precursor/reactant) and (CH2)6N4 (as indirect template) coopera-tively controlled the synthesis of hollow CuO microspheres. Regarding the formation ofsuch structures, the authors predicted that hollow microspheres could be formed withthe assistance of a soft template of gas bubbles of NH3 produced by decomposition of(CH2)6N4. Detailed structural observations were performed using various analytic tech-niques. Results from XRD conrmed that the CuO microspheres grown were mono-clinic, CuO nanocrystalline structures. As no peaks from other structures or impuritieswere observed; hence, the CuO structures formed were high purity. FESEM observa-tions conrmed that the products grown were spherical microstructures with diametersof 11.5 m. The authors also observed that the hollow CuO microspheres were formedfrom smaller CuO nanoparticles [63].Zhang and colleagues demonstrated aggregation-based formation of nanostructured

CuO particles achieved through a simple oxidation of Cu metal in formamide and sub-sequent hydrolysis of the resulting Cuformamide complexes in aqueous solution [64].The XRD pattern conrmed the monoclinic, crystalline nature of the CuO nanostruc-tures obtained. By TEM, the structures appeared similar to ellipsoids, having the averagelengths and widths of 130 nm and 45 nm, respectively; hence, the products werecalled nanoellipsoidal particles. By high-resolution TEM, the nanostructures synthesizedappeared to be accumulations of several thousand small (67 nm) CuO nanoparticles;the small primary nanoparticles appeared to be connected to one another to form thelarger, secondary ellipsoidal architecture with recognizable boundaries/voids betweenthe component subunits. The anisotropic growth rates of the ellipsoidal particles wereordered as b > a > c or [010] > [100] > [001]. This analysis included different particleaggregation potentials; aggregation/attachment occurred preferentially between the (010)


lattice planes of the nanoparticles. Importantly, the anisotropic growth of the nanostruc-tures obtained [64] was different from the classical crystal growth of CuO nanowiresalong the [111] direction [65, 66]. Additionally, the authors observed that the selectiveadsorption of formamide molecules on various crystallographic planes could play animportant role in the anisotropic growth of ellipsoidal nanoarchitectures [64].There are several reports in the literature on the growth and properties of ower-

shaped CuO nanostructures [4, 6769]. In this section, we assemble information on theower-shaped CuO nanostructures reported to date.Flower-shaped CuO nanostructures were synthesized and characterized by Li and col-

leagues [67]. In a typical reaction process, equimolar solutions of Cu(NO32 and HMTAwere transferred to a vial and heated at 90C120C for 3 h. The substrate (i.e., Si, indiumtin oxide [ITO]coated glass) was placed at the bottom of the vial. Reactions were per-formed over a range of pH; to adjust the pH of the reaction solution, nitric acid (HNO3)or NaOH was added during the reaction. The investigators studied the inuence ofreaction parameters on the morphologies of the CuO nanostructures produced. Reactantconcentration, pH, synthesis temperature, and presence of seed layers on the substratesignicantly affected the morphologies obtained. Additionally, adhesion of the products(i.e., CuO nanostructures) onto the substrate was also affected by reaction parameters.In the absence of a seed layer or at low concentrations of reactants, spherical assembliesof CuO nanoplatelets were obtained. The shape of the nanoplatelets and the density oftheir packing were strongly affected by pH, synthesis temperature, and concentrationsof reactants. Synthesis from solutions with high concentrations enabled fabrication ofnanostructured lms on the substrate; the morphology and adherence of such lms tothe substrate was dependent on pH and synthesis temperature. Results of detailed struc-tural analyses conrmed the monoclinic, crystalline nature of the CuO nanostructuressynthesized [67].Zhu and colleagues demonstrated the synthesis and characterization of ower-like

CuO nanostructures by hydrolyzing copper acetate in an aqueous solution in the absenceof any surfactants/additives [68]. All diffraction peaks in the XRD pattern of the as-prepared CuO nanostructures were well consistent with monoclinic, crystalline CuOstructures. No peaks due to for other structures or impurities were detected in the XRDpatterns; these results conrm that the CuO products synthesized were pure. By SEMand TEM analyses, the products synthesized were owery spheres with diameters of0.40.8 m. Moreover, the ower-shaped structures were composed of many intercon-nected, needle-like crystallites with diameters of 1015 nm [68]. Regarding the sphericalappearance of the products synthesized, it was proposed that the geometric shape ofthe reactants (i.e., building blocks) played a key role, since no surfactants/emulsionswere used [4, 68]. A simple array of needle-shaped CuO crystallites could easily gener-ate curvature, and the lateral engagement of these building units could naturally lead tospherical structures [4, 68].A simple, solution route to prepare ower-shaped CuO nanostructures was developed

by Yang and colleagues [69]. For the growth of ower-shaped nanostructures, Cu(NO3)2and NaOH were used as starting materials. By controlling the molar ratio of NaOH toCu(NO3)2, reaction temperature, and concentration of the starting NaOH solution, CuOnanoribbons or nanorods and their assemblies into hierarchical structures were synthe-sized. For example, 1-D CuO nanorods/nanoribbons were prepared with a molar ratioof NaOH to Cu(NO3)2 of 40; when the molar ratio of NaOH to Cu(NO3)2 was increasedto 200, the products were spherical or ower-like hierarchical assemblies, which werebuilt up from 1-D CuO nanorods/nanoribbons. In addition, ethanol greatly inuencedthe morphologies of CuO nanostructures and enhanced the dehydration rate of Cu(OH)2by reducing the surface tension of the reaction solution [69].Vaseem and colleagues also demonstrated the growth and characterization of ower-

shaped CuO nanostructures prepared by a simple, solution process [69]. These inves-tigators heated Cu(NO3)2, NaOH, and HMTA at 100C for 3 h without the use of anycomplex apparatus or reagents [70]. Results of morphological investigations by FESEMrevealed that the ower-shaped nanostructures were monodisperse and high density.


FESEM images also revealed that the owers were built up from many triangular petals.The diameters of the petals varied from the bases to the tips; that is, petals exhibitedsharpened tips with wider bases. These petals were connected to each other throughtheir wider bases and were rooted in one center to form the ower-like morphol-ogy. The typical length of one petal was 600800 nm; the diameter at the base andtip were 150 nm (range 50 nm) and 50 nm (range 20 nm), respectively. The fullarray of a single ower-shaped structure was 23 m. Interestingly, it appeared thatthese nanostructures were formed by the accumulation of many layers, with each layercontaining several petals. In addition, the sizes of the petals differed from the upperportion to the lower portion of the nanostructures; this arrangement created beautifulower-like structures (Figs. 6(A and B)). Detailed structural characterization by TEM andHRTEM conrmed the monoclinic, crystalline nature of the as-prepared ower-shapedCuO nanostructures (Figs. 6(C and D)). The researchers found that the ower-shapedmorphologies obtained were strongly dependent on several parameters; the concentra-tion of HMTA, presence/absence of NaOH or HMTA, and reaction time were the mostinuential parameters. The authors conducted extensive studies on these topics [70].To optimize reaction time for creation of CuO ower-like morphologies, Vaseem and

colleagues performed various time-dependant experiments with reaction times in theranging of 0.517 h [70]. Figure 7 presents FESEM images of the time-dependent mor-phology of CuO nanostructures. During the early stage of the reaction (0.5 h), shuttle-likemorphologies were formed (Fig. 7(A)). But with increasing reaction time (12 h), theseshuttle-like morphologies began to adhere/assemble as somewhat ower-like morpho-logies (Fig. 7(B)). The authors observed that 3 h was the optimal reaction time to obtainmonodisperse, ower-like morphologies (Fig. 7(C)). Further increases in reaction timecaused the ower-shaped structures to blur. As the best ower-like structures formed

(A) (B)

(C) (D)

Figure 6. Low-magnication (A) and high-magnication (B) FESEM images; low-magnication (C) and high-magnication (D) TEM images of the ower-shaped CuO nanostructures grown by a solution process over3 h. Reprinted with permission from [70], M. Vaseem et al., J. Phys. Chem. C. 112, 5729 (2008). 2008,American Chemical Society. FESEM = eld-emission scanning electron microscopy, TEM = transmission elec-tron microscopy.


(A) (B) (C)

(D) (E) (F)

(G) (H) (I)

Figure 7. FESEM images of time-dependant assembly of ower-shaped CuO nanostructures grown via a solu-tion process using 0.1 M Cu(NO3)2 and 0.05 M HMTA at 100C (pH = 6.0). (A) 0.5 h; (B) 1 h; (C) 3 h; (D) 5 h;(E) 7 h; (F) 9 h; (G) 11 h; (H) 14 h; and (I) 17 h reaction times. Reprinted with permission from [70], M. Vaseemet al., J. Phys. Chem. C 112, 5729 (2008). 2008, American Chemical Society. FESEM = eld-emission scanningelectron microscopy, HMTA = hexamethylenetetramine.

in only 3 h, the authors suggested that, with prolonged reaction time, petals precipi-tated over already-formed owers and lled gaps between two adjacent petals (Fig. 7(D),5 h reaction time). By further extending the reaction time (717 h), the authors foundthat precipitation at interstices of two petals was increased, and only blurred ower-likemorphologies were obtained (Fig. 7(EI)) [70].Figure 8 presents FESEM images of ower-shaped CuO nanostructures as a function of

HMTA concentration. During these experiments, Vaseem and colleagues varied HMTAconcentration in the range of 0.0250.2 M, but held constant other variables, such astemperature (100C), pH (6.0), time (3 h), and Cu(NO3)2 concentration (0.1 M) [70]. Ingeneral, as the concentration of HMTA increased, blurring/lling (i.e., lling of the spacesbetween two adjacent petals) in the ower-shaped nanostructures was observed. Theoptimal HMTA concentration for the monodisperse ower-shaped morphologies was0.05 M. In these experiments, an aqueous solution of 0.1 M Cu(NO3)2 was mixed withvarious concentrations of HMTA (0.0250.2 M). Importantly, in the course of mixingCu(NO3)2 and HMTA, no immediate precipitation was apparent, but the clear light-bluesolution of Cu(NO3)2 turned turbid with the addition of HMTA. Moreover, to maintainthe pH 6.0 for all reactions, a few drops of 1 M NaOH were included in the solution,and interestingly, a blue-colored precipitate of Cu(OH)2 appeared immediate. From thisobservation, the authors suggested that Cu(NO3)2 initially reacts with NaOH to forms theblue precipitate of Cu(OH)2 via a simple chemical reaction (expressed in reaction (IV)).

CuNO32 3H2O+ 2NaOH CuOH2 + 2NaNO3 + 3H2O (IV)

The authors further suggested that the formation of Cu(OH)2 was important for growthof the CuO crystallites, which initially served as building blocks for the formation of


(A) (B)

(E) (F)

(C) (D)

Figure 8. FESEM images of HMTA concentration-dependant morphologies of ower-shaped CuO nanostruc-tures while keeping other reaction parameters constant (0.1 M Cu(NO3)2, 100C, 3 h, pH 6.0). (A) 0.025 M,(B) 0.05 M, (C) 0.075 M, (D) 0.1 M, (E) 0.15 M, and (F) 0.2 M HTMA. Reprinted with permission from [70],M. Vaseem et al., J. Phys. Chem. C 112, 5729 (2008). 2008, American Chemical Society. FESEM = eld-emissionscanning electron microscopy, HMTA = hexamethylenetetramine.

the nal products; hence, at appropriate heating, Cu(OH)2 led the formation of CuOcrystallites according to reaction (V).

CuOH2 CuO+H2O (V)

At the early stages of the synthesis reaction, only a few drops of NaOH were added;hence, there would not be enough OH ions to produce sufcient Cu(OH)2. There-fore, Vaseem and colleagues proposed that, during the reaction, HMTA provided twothings: rst, HTMA was hydrolyzed and OH ions were produced by reactions (VI) and(VII) [70]. (It had previously been reported that, at elevated temperature, HMTA couldhydrolyze in the distilled water and slowly generate OH ions [71].)

CH26N4 + 6H2O 6HCHO+ 4NH3 (VI)NH3 +H2O NH4+ +OH (VII)


Hence, from the reactions (VI) and (VII), an increase in Cu(OH)2 could be stimu-lated by HMTA-generated hydroxyl ions; this would be a second step for the generationof Cu(OH)2 with reaction (IV) being the rst step. Therefore, a continuous supply ofCu2+ ions and OH ions from the Cu precursor and HMTA, respectively, could leadto the continuous generation of Cu(OH)2, which is nally converted into CuO crystal-lites and forms the ower-shaped CuO nanostructures [70]. Additionally, it had pre-viously been observed that the concentration of OH ions can signicantly affect thenucleation and growth behaviors (e.g., number of nuclei, concentration of growth units)of CuO crystals [48]. Therefore, to obtain a specic nanostructure, there should be anoptimal concentration of OH ions in the solution. In this synthesis, the concentrationof HMTA was varied while keeping other reaction parameters constant, and from theexperimental results, Vaseem and colleagues found that the concentration of HMTA wasimportant for the growth of the best quality, monodisperse ower-shaped nanostruc-tures [70]. The concentration of HMTA for the best results from these experiments wasdetermined to be 0.05 M (Fig. 8(B)). At a lower concentration of HMTA (0.025 M), theower-shaped structures obtained were not well developed, and it seemed as though theywere in initial stages of growth (Fig. 8(A)). Moreover, it had previously been reportedthat higher OH ion concentrations could create diffusion layers on certain surfaces ofCuO nanostructures; these layers could produce additional growth anisotropy, allow-ing only energetically favorable crystallographic planes to grow [41]. Vaseem and col-leagues also observed this phenomena by increasing the concentration of HMTA; thequantity of the OH ions was increased, and overgrowth over the previously formedower-shaped structures was observed [70]. By increasing the concentration of HMTAabove 0.05 M, overgrowth increased and lled the vacant spaces between adjacent petals(Fig. 8(CE)); nally, blunt shaped owers were obtained at highest HMTA concentrationused (Fig. 8(F)). From these results, one can conclude that the HMTA concentration was

(A) (B)

(C) (D)

Figure 9. Low-magnication (A) and high-magnication (B) FESEM images of the CuO structures grown in thepresence of NaOH alone (i.e., without HMTA) using 0.1 M Cu(NO3)2 at 100C for 3 h. Low-magnication (C)and high-magnication (D) FESEM images of the CuO structures grown in presence of HMTA alone (i.e.,without NaOH) by using 0.1 M Cu(NO3)2 and 0.05 M HMTA at 100C for 3 h. Reprinted with permission from[70], M. Vaseem et al., J. Phys. Chem. C 112, 5729 (2008). 2008, American Chemical Society. FESEM = eld-emission scanning electron microscopy, HMTA = hexamethylenetetramine.


one of the most important/prominent factors affecting the ower-shaped morphology ofCuO nanostructures.Second (and in addition to providing the OH ions to the solution), HMTA acted as

an additive that controlled the shape of the nanostructures [70, 72]. To clarify the roleof HMTA, Vaseem and colleagues conducted additional experiments in the absence orpresence of HMTA (keeping constant other reaction parameters such as temperature,pH, and time) [70]. Figures 9(A) and (B) presents low- and high-magnication images ofCuO structures prepared without HMTA (but with NaOH); the reaction was maintainedat pH 6. It can be seen from these images that blunt-shaped nanopetals were formedwithout HMTA; some petals were connected to each other as if in the preliminary stagesof forming ower-like morphologies. On the other hand, in the experiments done in thepresence of HMTA (but without NaOH), the nanopetals formed were well dened withsharp, clean tips; ower-shaped nanostructures were also obtained (Figs. 9(C and D)).Hence, from these observations, one can conclude that, in this case, HMTA was acting asan effective shape-directing agent and helped control the shape of the petals and nallythe ower-shaped structures [70].

3. APPLICATIONS OF COPPER OXIDE NANOSTRUCTURES

3.1. Photocatalytic Properties of Copper Oxide Nanostructures

Vaseem and colleagues reported the growth, photocatalytic, and X-ray absorption nearedge structure (XANES) properties of ower-shaped CuO nanostructures apparentlyassembled from triangular leaves with sharp tips and wide bases [73]. Detailed struc-tural observations conrmed the monoclinic, nanocrystalline structure of the as-preparednanostructures. In Figure 10, it is clearly shown that ower-shaped structures were grownin very large quantity (Fig. 10(A)). A clear view of the single ower-shaped structure

(A) (B)

(C)

Figure 10. Low-magnication (A), high-magnication (B), and very high-resolution (C) FESEM images ofower-shaped CuO nanostructures grown by a simple, solution process. Reprinted with permission from [73],M. Vaseem et al., Catal. Commun. 10, 11 (2008). 2008, Elsevier B. V., Amsterdam, The Netherlands. FESEM =eld-emission scanning electron microscopy.


is shown in Figure 10(B); this ower consists of many triangular petals. The diametersof the petals are varied from the base to the tips; that is, petals have wide/bases andthin/sharp tips. The wide bases of the petals are connected to each other, rooted in onecenter, and (nally) constructed into beautiful, ower-like morphologies. Each petal of theower-shaped structures appears to be created by accumulation of a large number (per-haps thousands) of CuO nanoparticles; this is shown in Figure 10(C), a high-resolutionFESEM image. A low-resolution TEM image of the ower-shaped CuO structure is con-sistency with FESEM observations (Fig. 11(A)). Here, the straight/parallel lattice fringesof the structures reveal that the petals are single-crystal in nature. Moreover, the spacingbetween two neighboring fringes are 0.27 nm, which corresponds to the distance of the(110) plane of monoclinic CuO (Fig. 11(B)). All the observed peaks in the XRD patterncan be ascribed as monoclinic CuO structures and yield data close to the reported datafor such structures (JCPDS: 05-0661; a = 4684 , b = 3425 , c = 5129 , = 9947;space group C2/c). Moreover, the major peaks located at 2 values of 35.6 and 38.8

that indexed as (111)(002) and (111)(200) planes, respectively, are characteristic of pure,monoclinic, crystallite CuO structures (Fig. 11(C)). Figure 11(D) shows the FTIR spec-trum of the as-prepared ower-shaped CuO nanostructures. The sample exhibited weakabsorption bands at 3365 and 1634 cm1 and strong absorptions bands at 432, 526, and596 cm1. These weak absorption bands could be attributed to the stretching and bendingvibrations of absorbed water and surface hydroxyls, respectively [74]. Moreover, thesestrong absorption bands are characteristic of monoclinic CuO structures [75]. Therefore,the FTIR spectral data conrmed that the products synthesized were pure CuO withmonoclinic structures.

(A)

(C) (D)

(B)

Figure 11. Low-magnication (A) and high-resolution (B) TEM images of ower-shaped CuO nanostructures.The distance between two adjacent fringes is 0.27 nm; these nanostructures exhibit growth along the [010]direction for petals grown in ower-shaped nanostructures. Typical XRD pattern (C) and FTIR spectrum (D)of ower-shaped CuO nanostructures. Reprinted with permission from [73], M. Vaseem et al., Catal. Commun.10, 11 (2008). 2008, Elsevier. FTIR = Fourier transform infrared, TEM = transmission electron microscopy,XRD = X-ray diffraction.


(A) (B)

Figure 12. (A) UV-DRS and (B) plots of 1/2 versus photon energy h for ower-shaped CuO nanostructures.Reprinted with permission from [73], M. Vaseem et al., Catal. Commun. 10, 11 (2008). 2008, Elsevier B. V.,Amsterdam, The Netherlands. DRS = diffuse reectance spectrometry, UV = ultraviolet.

For the determination of bandgap, UV-DRS (diffuse reectance spectrometry) of ower-shaped CuO nanostructures were carried out and are presented in Figure 12 [73]. Fromthe observed UV-DRS graph, the bandgap of the ower-like CuO nanostructures wascalculated according to the Eq. (1) and was estimated to be 1.78 eV.

h = Ah Egm/2 (1)In Eq. (1), is the absorption coefcient, h is Plancks constant, is the frequency ofphotons, A is a proportionality constant, and m = 1 for direct transitions. Interestingly,the authors observed that the bandgap of the ower-like CuO nanostructures was largerthan the reported bandgap of bulk CuO materials (1.4 eV) [76]; this difference may becaused by the quantum size effect in the synthesized nanostructures (i.e., the blueshiftas reported in the literature) [77, 78]. The petals of the synthesized CuO nanostructureshave diameters in the range of 5070 nm; the authors observed that these petals appearedto be formed by the accumulation of several thousand small CuO nanoparticles [73].Because of their sharp tips and because they are formed by the accumulation of sev-eral thousand small nanoparticles, the petals of ower-shaped CuO nanostructures couldexhibit the quantum connement effect [77]. It was also indicated that the ower-likeCuO nanostructure could catalyze the photocatalytic decomposition of an organic pollu-tant by the formation of an excess of either superoxide radicals (O2 ) or hydroxyl radicals(OH) or both at the CuO surface [79]. Therefore, the authors examined the degrada-tion of methylene blue (MB) to conrm the photocatalytic ability of the ower-like CuOnanostructures [73]. For decomposition of MB, 100 mL of 50 M MB and 0.1 g of photo-catalysts were stirred for 30 min in a glass reactor prior to light illumination. A 300 Wxenon lamp (Oriel) was used as a light source, and the light was passed through eitheran infrared water lter or no lter. The ltered light was focused onto the reactor. Sam-ple aliquots were withdrawn by a 1 mL syringe intermittently during the illuminationand then ltered through a 0.45 m poly(tetrauoroethylene) (PTFE) lter (Millipore,Billerica, MA). Using UV-visible spectrophotometry, the degradation of MB was mon-itored (max 665 nm) as a function of irradiation time (Fig. 13). The results showedthat, although the photocatalytic ability of CuO nanostructures was 1.5 times lower thanother photocatalysts (e.g., TiO2, CuOTiO2 composite), the ower-like CuO nanostruc-tures had the same potential for photocatalytic applications as inactive bulk CuO particles[80]. The reason for the relatively low photocatalytic ability of CuO nanostructures maybe due to recycling of Cu1+ ions under light on the CuO surface [79] combined withthe large surface area of ower-like CuO nanostructures. Additionally, these investiga-tors studied the photocatalytic degradation of MB under UV irradiation to gauge theoxidation capability of CuO nanostructures as well as UV irradiation (i.e., no catalyst)(Fig. 14) [73]. Figure 14 (inset) illustrates typical time-dependent UV-visible spectra of


Figure 13. Absorption spectrum of a methylene blue solution in the presence of ower-shape CuO nanostruc-tures. Reprinted with permission from [73], M. Vaseem et al., Catal. Commun. 10, 11 (2008). 2008, ElsevierB. V., Amsterdam, The Netherlands.

MB solution during photocatalytic degradation. The spectra of MB in the visible regionexhibits a main peak with max 665 nm. Even though the rate of decrease was not fast,the absorption peaks of MB gradually decreased during the photocatalytic reaction. MBwas also degraded by UV irradiation without the CuO catalyst. The electronic/geometricstructure around Cu in the as-prepared material was characterized with X-ray absorptionne-structure spectrometry (XAFS). X-ray absorption spectrometry (XAS) measurementswere conducted at beam-line 3C1 of PAL (2.5 GeV; stored current of 130180 mA). Theradiation was monochromatized using a Si(111) double-crystal monochromator, and theincident beam was detuned by 20% using a piezoelectric translator in order to minimizecontamination from higher harmonics, in particular, the 3rd-order reection of Si crys-tals. The energy was calibrated by measuring the XAS spectrum of Cu metal foil andby assigning the rst inection point in the rising portion of the absorption spectra at8979 eV. The obtained data were analyzed using the IFEFFIT suite of software programs(University of Chicago) [81].

(A)

(B)

Figure 14. Photocatalytic decomposition of methylene blue (MB; 0.1 g. MB solution, 100 mL of 50 M MB)under UV light: (A) no catalyst, (B) CuO catalyst. Light source: 300 W xenon lamp (Oriel, Newport Corporation,Irvine, CA) equipped with an infrared liquid lter. A is the absorbance of MB (max = 515 nm) and A0 is theinitial absorbance. Inset indicates a typical spectral change with irradiation time. Reprinted with permissionfrom [73], M. Vaseem et al., Catal. Commun. 10, 11 (2008). 2008, Elsevier B. V., Amsterdam, The Netherlands.


(A)

(a) (d)(c)

(b) (b)

(c)

(a)

(d)

(B)

Figure 15. Cu K-edge (A) and pre-edge (B) XANES spectra of (a) Cu foil, (b) Cu2O, (c) CuO, and (d) as-preparedower-shaped CuO nanostructures. Reprinted with permission from [73], M. Vaseem et al., Catal. Commun. 10,11 (2008). 2008, Elsevier B. V., Amsterdam, The Netherlands. XANES = X-ray absorption near edge structure.

Figure 15 shows the XANES spectra of as-prepared material and copper references (e.g.,Cu foil/metal, Cu2O, CuO). The authors found that the XANES features of as-preparedmaterial were closer to that of CuO, rather than to those of Cu metal and Cu2O [73].The absorption edge of Cu K-edge XANES was assigned to the main 1s 4p transition.Cu(0) and Cu(+1) with a d0 conguration have no hole/vacancy in the 3d orbital, andCu(+2) is in a d9 conguration. Thus, Cu(2+) represents a weak pre-edge peak, meaningthe quadruple allows the 1s 3d transition, and it serves as a characteristic feature forCu(+2) because there is no 3d hole/vacancy in Cu(0) or Cu(+1). As shown in Figure 15,the Cu K-edge XANES spectrum of the as-prepared sample showed a weak pre-edgepeak 89768978 eV comparable that of a Cu(+2)O reference; these data indicated thatthe oxidation state of Cu in the as-prepared material was divalent. The position of theXANES peaks (obtained as the maximum point of the rst derivative function) wereused to determine the oxidation state of CuO; the position shifts toward higher energyas the oxidation state of the material increases. The positions of the XANES peaks for theas-prepared sample and copper references were determined to be 8979.0, 8980.4, 8983.6,and 8983.7 eV for Cu foil, Cu2O, CuO, and the as-prepared sample, respectively. Thesedata provided clear evidence of the divalent oxidation state of copper in the as-preparedsample [8285].

3.2. Field-Emission Properties of Copper Oxide Nanostructures

Field emission, one of the most fascinating properties of nanostructured materials forthe practical application in vacuum microelectronic devices such as eld-emission dis-plays, X-ray sources, and microwave devices, has been studied extensively in the pastfew decades [8688]. During this time, carbon-based materials, especially carbon nano-tubes, were studied as promising materials for eld emitters due to their high mechan-ical stability, good conductivity, low turn-on eld, and large emission currents [8688].Importantly, it appeared that metal oxide nanostructures emitters, as compared to car-bon nanotubes emitters, are more stable in harsh environments and have controllableelectrical properties [88].Regarding eld-emission properties of CuO nanostructures, Hsieh and colleagues

reported the eld-emission properties of various CuO nanostructures grown by a two-step


fabrication process [29]. The rst step was the rapid formation of Cu nuclei by elec-trodeposition; the second step was a self-catalytic growth mechanism on the copperelement/polycarbonate substrate [29]. These investigators produced high-purity CuOnanobers at 1 atm without the use of complicated/sophisticated instruments. A majoradvantage of this process was that the vertically oriented CuO nanobers did not requiredany adhesive technology on the interface between the bers and the substrate/templatebecause of the

Growth, Properties and Application of Copper Oxide, Nickel Oxide-hydroxide Nanostructures

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