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Applied Surface Science 257 (2010) 132–137

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

icrowave-assistant synthesis of ordered CuO micro-structures on Cu substrate

uLin Mina,b,c, Tao Wanga,b,∗, YouCun Chena,b

School of Chemistry and Chemical Engineering, Anqing Normal College, Anqing 246011, PR ChinaKey Laboratory of Functional Coordination Compounds, Anhui, PR ChinaDepartment of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China

r t i c l e i n f o

rticle history:eceived 19 January 2010

a b s t r a c t

A mild template-free mixed solution medium with the assistant of microwave method was successfullyestablished to synthesize well-aligned CuO nanostructures. By varying process parameters such as the

eceived in revised form 26 May 2010ccepted 18 June 2010vailable online 25 June 2010

eywords:anostructure

volume ratio of the mixed solution, different kinds of architectural structures can be controllably synthe-sized in large quantities. On the basis of the results, it is found that the polarity of the solution plays thekey role in controlling the growth of the CuO crystal. The as-prepared CuO products were characterizedusing diverse techniques including X-ray diffraction, scanning electron microscopy, and transmissionelectron microscopy. The gas sensor property investigation shows that the sensitivities to ethanol of the

uctur

anoscalexidation

as-prepared CuO nanostr

. Introduction

Recently, how to synthesize nanostructures with controllablerientation, density and crystalline morphology without templateas attracted more and more attention. Their unique propertiesould be harnessed for the design and fabrication of nanosen-ors, switches, nanolasers, and transistors [1]. Also, many possiblepplications of 1D nanomaterials require the formation of well-ligned arrays to accentuate the anisotropy and satisfy the criteriaf device design. As we know, CuO, a well-known photovoltaicaterial, is a p-type semiconductor having a narrow band gap

.2 eV. It could be widely used in many fields magnetic storageedia, solar energy transformation, electronics, sensors, cataly-

is and batteries, resonance imaging and so forth [2–5]. Differentinds of CuO nanostructures could be prepared by thermal meth-ds [6,7], electrochemical methods [8,9], sol–gel methods [10,11],olid-state reactions [12], chemical reduction and decompositionoute [13,14], sonochemical methods [15,16], and photochemicalethods [17–20].On the other hand, the microwave-assisted synthesis [21,22],

hich is generally quite fast, simple and efficient in energy, has

een developed and is widely used in various fields such as molec-lar sieve preparation, radiopharmaceuticals, the preparation of

norganic complexes and oxide, organic reactions, plasma chem-stry, analytical chemistry and catalysis. Generally speaking, the

∗ Corresponding author at: School of Chemistry and Chemical Engineering, Anqingormal University, Anqing 246011, PR China. Tel.: +86 556 5500090;

ax: +86 556 5500090.E-mail address: ahaqmylin@yahoo.com.cn (T. Wang).

169-4332/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.apsusc.2010.06.049

es are higher than those of normal CuO nanoparticles.Published by Elsevier B.V.

power, heating frequency, and on/off irradiation cycles are themain heating parameters of a microwave oven, and each of themmay have a great effect on the structure and properties of theproducts. To the best of our knowledge, although such a heatingmethod has been a focus of research, most of previous reportswere limited to fixed working conditions of the microwave oven[23–26], and there is no comprehensive report addressing theeffects of these heating parameters on the synthesis of nanoma-terials [27–30]. Furthermore, there are few reports on synthesizingthe CuO nanoplates without template at mild condition by the useof microwave method.

Here, we report a simple microwave method to fabricate well-aligned CuO nanostructures on a copper surface without anytemplate and surfactant.

2. Experiment

Well-aligned CuO nanostructures were grown directly on cop-per foil substrate. In a typical synthesis process, fresh copper foilsubstrates were inserted in 10 mL of acetone and absolute ethanolat room temperature, by supersonic cleaning method for 10 min,and rinsed with deionized water. The experimental procedure wasdesigned as follows: a fixed amount of mixed solution and 1 × 1 cm2

copper were added to a 25 mL microwave-sealed vessel and then

put into the microwave reflux system. The mixture was heated to150 ◦C by microwave under magnetic stirring and maintained atthis temperature for 10 s. Then the foil was collected from solution,rinsed with deionized water and dried in air at 60 ◦C. The mixedsolution could be deionized water, absolute ethanol, ethylenedi-amine or diethanolamine.

Y. Min et al. / Applied Surface Science 257 (2010) 132–137 133

Table 1Experimental conditions for typical samples and their morphologies.

Sample Solution Morphology

1 Water + 1 mL H2O2 Nanoplate2 Absolute ethanol Thick layered nanorock

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3 Water:absolute ethanol (V:V = 1:1) Thin layered nanorock4 Diethanolamine Nanotower5 Ethylenediamine Nanoflower

The morphology of sample is investigated on a JEOLJSM-6700Fcanning electron microscope (SEM). The oriented growth direc-ion and crystallinity can also be analyzed by X-ray diffraction

easurement (Rigaku X-ray diffractometer with Cu Ka radiation)nd high-resolution transmission electron microscope (HRTEM,EOL 2010, 200 kV). The microwave system was a CED discover

icrowave synthesis (CEM Corporation, USA).Gas sensors were fabricated using thin films which were pre-

ared by depositing the CuO powder scratched from Cu substrate.nd the samples were dispersed in ethylene glycol and ultrason-

cated into slurry before fabricating the sensors. No conductiveinder was added. The slurry was coated on aceramic tube on whichpair of Au electrodes was previously printed, and the tube was

hen subsequently calcinated at 450 ◦C for 2 h in air. Finally, therepared sensors were fixed into the gas sensor apparatus.

. Result and discussion

The morphologies of CuO under different conditions have beenisted in Table 1. When the solution is changed from water tothylenediamine, the morphology of the CuO would vary drasti-ally, as shown in Table 1. The solvent effect on the sample woulde investigated in the following text.

Fig. 1 shows the XRD data of CuO samples on the Cu substrate.he pattern for the products after microwave reaction at 150 ◦C cane indexed as monoclinic phase copper oxide with cell constants of

= 4.689 ´̊A, b = 3.426 ´̊A and c = 5.132 ´̊A (JCPDF card NO:801917) andubic phase face-centered lattice copper metal from the substrate

ith cell constants of a = 3.615 ´̊A (JCPDF card NO:040836), com-ared with the XRD pattern presented by Yang et al. [31] The XRD

ates of the other samples from different conditions are similar tohose shown in Fig. 1.

Apparently, the CuO nanoplates still keep the aligned structuren the Cu substrate. Fig. 2b shows HRTEM image of a single CuOanoplate. The CuO crystallites have a preferential orientation in

Fig. 2. (a) The representative TEM image of a CuO nanoplate sample scratc

Fig. 1. XRD pattern of CuO nanoplates on the Cu substrate.

the nanoplates; that is, the [1 1 0] is along the longest dimensionof the nanoplates. Crystallographic analyses with both XRD andHRTEM indicate that axis direction of the nanoplates is along [1 1 0].

Fig. 2a shows the TEM picture of CuO nanoplates scratchedfrom the Cu substrate, which reveals that the ends of the obtainedCuO nanoplates are very plane. The conclusion that the nanoplatesgrowth axis direction is along [1 1 0] is also supported by the HRTEMobservation (Fig. 2b), in which the clearly observed lattice fringesreveal that the CuO nanoplates are highly crystalline and the pref-erential growth occurred along the [1 1 0] direction.

Fig. 3a clearly shows the morphology of the final product. TheCu foil is covered with uniform and dense CuO nanoplates. Alter-natively, Fig. 3b shows the morphology on the edge region of thesample with the Cu substrate (corresponding to the sample shownin Fig. 3a) and the nanoplates evidently grow on the substratedespite the poor alignment due to the edge region. The local magni-fication of FESEM images exhibits that the products are composedof vertically standing nanoplates (Fig. 3c and d) and each of themhas a clear rectangle on their top section (Fig. 3d). These nanoplatesare 4 �m in width, up to 7 �m in length, and about 1.5 �m in thick-ness.

As can be seen from the top view (Fig. 3a, c and d), the nanoplates

cover the copper surface uniformly, smoothly, and compactly. Inaddition, the nanoplates are roughly aligned up to the coppersurface. And the surface of the CuO nanoplates is smooth. Further-more, from the side view (Fig. 3b), it is clearly observed that CuO

hed from Cu substrate. (b) HRTEM of selected area from left picture.

134 Y. Min et al. / Applied Surface Science 257 (2010) 132–137

F e subs

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ig. 3. Typical SEM images of sample 1: (a) the products obtained using Cu foil as thubstrate; (c) and (d) SEM images of different magnifications.

anoplates are grown vertically in high density over the entire sur-

ace of the Cu substrate. The average length and width is about–3 �m and 1–2 �m, respectively.

Interestingly, when the solution is changed into absolutethanol, the morphology would be transformed into thick-layered

ig. 4. Typical SEM images of sample 2: (a) the products obtained using Cu foil as theifferent magnifications; typical SEM images of sample 3; (d) the products obtained usinnlarged SEM image of selected area of different magnifications.

strate with H2O2 + water solution; (b) products grown on the edge region of the Cu

nanorock, as seen in Fig. 4a–c. From Fig. 4a, it is clearly seen that

the sample 2 is composed of layered nanostructure. Furthermore,from Fig. 4b and c, it is easily concluded that the spacing of lay-ers of CuO is about 100 nm, and the depth of the each layer isalso about 100 nm. By comparing Fig. 4c to Fig. 3d, it is obvious

substrate in absolute ethanol; (b) and (c) enlarged SEM image of selected area ofg Cu foil as the substrate in absolute ethanol and water mixed solution; (e) and (f)

Y. Min et al. / Applied Surface Science 257 (2010) 132–137 135

F he subi

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ig. 5. Typical SEM images of sample 4: (a) the products obtained using Cu foil as tmages of different magnifications.

hat the surface of sample 2 shows coarser and more hierarchicaltructure. As shown in Fig. 4a, densely aligned nanorock is ver-

ically arranged. However, while the solution was conversed into

ixed solution which is composed of water and absolute ethanol,he morphology has changed into thin-layered nanorock, as seen inig. 4d–f. From Fig. 4f, it is easily observed that the spacing of layersf the sample 3 is thinner than 100 nm, which is about 50 nm. The

ig. 6. Typical SEM images of sample 5: (a)–(c) the products obtained using Cu foil as thehe Cu substrate.

strate; (b) products grown on the edge region of the Cu substrate; (c) and (d) SEM

depth of each layer is also 30 nm. And, the polarity of the solutionplays the key role in deciding the morphology, which is stated as

below.

While making use of diethanolamine as the solution, the mor-phology of the sample 4 has changed drastically and hexagonalmultilayer structures are well-formed. The SEM images of thenanotower are shown in Fig. 5, which show that the hexagonal

substrate with different magnifications; (d) products grown on the edge region of

136 Y. Min et al. / Applied Surface Science 257 (2010) 132–137

of th

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know, the change of resistance in sensor is mainly caused by theadsorption and desorption of gas molecules on the surface of thesensing materials. From Fig. 8, we could conclude that the sample5 could provide more active sites and enable the detecting gases

Fig. 7. XPS date

ultilayer is built layer by layer, and the diameter of the base isbout 2 �m and that of the top is about 100 nm. The multilayersre almost vertical to the Cu substrate. In addition, from Fig. 5c,t is confirmed that the whole tower is composed of many layers.he thickness of each layer is about 150 nm, which is similar to theample 2. From Fig. 5d, microscale sized tower can be seen clearly.n accordance with this observation, such novel structure has noteen reported previously.

When the solution is changed to ethylenediamine, the morphol-gy of the sample 5 will be converted into vertical nanoslice, ashown in Fig. 6. Furthermore, from Fig. 6b and c, it is clearly seenhat many nanoslices are intercrossed to form microflower struc-ures. And the thickness of each slice is less than 60 nm, as seen inig. 6c. From the side view (Fig. 6d), it is easily concluded that eachanoslice has grown vertically on the Cu substrate. The height ofach nanoslice is about 1 �m.

Fig. 7a shows the XPS spectra of binding energy of Cu2p and1s electrons, respectively. The Cu2p peak at 933.6 eV corresponds

o the binding energy of Cu 2p3/2, which is consistent with thebserved one in CuO. As shown in Fig. 7b, the broad O1s peak rang-ng from 526 to 536 eV in the XPS spectrum is well-fitted by twoomponents that have different energies. The binding energies haveeen identified as O2− in CuO at 529.5 eV, and oxygen is adsorbednto the surface of CuO nanofibers at 531.6 eV.

Contrasting with the previous template-induced growth, theormation of different CuO nanostructures on Cu substrate in ourxperiments suggests that the different solvents or capping agentith the different polarity could be adsorbed on different crystallanes, which has been similarly reported [32]. Single crystallineuO nanostructures can also be self-assembled from nanoparti-les by orientation growth, in which the polarity of the solutionlays a key role. The growth procedure may be shown as follows:t the beginning of the microwave reaction, the copper surface wasrst oxidized into a large quantity of CuO dots which served ashe nuclei for the crystal growth. CuO crystal could be stemmedrom these dots and continued to grow into different nanostruc-ures in the different solvents. In the microwave condition a typicalissolution–crystallization state was established. The polarity ofolution will greatly influence subsequent CuO crystallization onhe nuclei, although there is natural tendency for CuO anisotropicrowth. Therefore, the growth was supported by a certain driv-ng force, which seemed to originate from the different, which isonfirmed by a series of experiments.

In our experiments, the polarity of the solution is an important

actor to control the CuO growth. According to Peng’s reference [33],he precursor concentration plays a key role for the determinationnd evolution of the shapes of the resulting nanocrystals. Keepinghe other experiment parameters, low polar solution will not leadimplex structure CuO nanoplates (see Fig. 3). On the other hand,

e CuO samples.

high polar solution will easily lead to hierarchical structure, whichcould be identified by sample 5 (see Fig. 5).

Moreover, the microwave method is the important role in decid-ing the morphology of CuO samples. Compared with conventionalmethods, such as sol–gel, hydrothermal method and CVD, themicrowave method could aggregate mass energy in a few seconds.Therefore kinetic process would control the whole CuO crystalgrowth. With the help of different kinds of polar solution, mon-oclinic phase copper oxide could be orientation growth.

As we know, CuO has been widely investigated in applications inchemical detection such as gas sensitivities because of their prop-erties such as a p-type semiconductor with energy gap of 1.2 eV.Fig. 8a andb show the sensitivity of the as-prepared CuO withnanoplate and nanoflower structures to ethanol at 150 ◦C. And thesensitivity of CuO particles about 50–100 nm has also been inves-tigated as shown in Fig. 8c for comparison. As shown in Fig. 8a, it isapparent that the CuO with nanoflower structures exhibits supe-rior gas sensing capabilities toward ethanol than the other samples,especially when the concentration of ethanol is over 300 ppm.Fig. 8b is the plot of the sensitivity of CuO with nanoplate struc-tures to ethanol. And it can also be seen that the sensitivity of thesample 5 is much higher than that of the nanoplate (sample 1) andthe CuO nanoparticles. Meanwhile, there are no major changes tothe reversibility of the all products when repeating 60 times. As we

Fig. 8. Gas sensitivity of CuO sample to ethanol.

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o access more surfaces, which can remarkably enhance the gasensitivity.

. Conclusions

Different kinds of morphologies of CuO nanostructures haveeen successfully synthesized on a Cu surface without templatesy the use of microwave method. The alignment is induced by theutual support during the simultaneous and space-limited growth

f the nanostructures on the Cu surface. And the polarity of the solu-ion plays the key role in controlling the growth of the CuO crystal.ur experimental results demonstrate that this novel and simple

oute can produce high-quality crystalline CuO nanostructures.

cknowledgements

The present work was supported by the special funding sup-ort from Natural Science Foundation of Anhui Province (NO.J2008B172, No. KJ2010ZD07) and the National Science Foundationf China (NSFC) (Grants No. 20771006).

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