Advanced Powder Technology 21 (2010) 197–202

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Advanced Powder Technology

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Rapid Communication

Structure and growth mechanism of CuO plates obtainedby microwave-hydrothermal without surfactants

A.P. Moura a, L.S. Cavalcante b,*, J.C. Sczancoski a, D.G. Stroppa c, E.C. Paris a, A.J. Ramirez c,J.A. Varela b, E. Longo b

a LIEC-DQ, Universidade Federal de São Carlos, P.O. Box 676, 13565-905, São Carlos, SP, Brazilb LIEC-IQ, Universidade Estadual Paulista, P.O. Box 355, 14801-907, Araraquara, SP, Brazilc Laboratório Nacional de Luz Síncrotron, P.O. Box 6192, 13083-970 Campinas, SP, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 September 2009Received in revised form 2 November 2009Accepted 5 November 2009

Keywords:CuOPlatesRietveld refinementMicroscopyCrystal growth

0921-8831/$ - see front matter � 2009 The Society ofdoi:10.1016/j.apt.2009.11.007

* Corresponding author. Tel.: +55 16 3361 5215,fax: +55 16 3361 8214.

E-mail address: laeciosc@bol.com.br (L.S. Cavalcan

CuO plates were obtained by microwave-hydrothermal processing at 130 �C for 30 min without any sur-factant. X-ray diffraction, Rietveld refinement and selected area electron diffraction showed that the CuOplates present a monoclinic structure without secondary phases. The nitrogen adsorption isotherm mea-surements revealed a specific surface area of approximately 30 m2/g. Field-emission gun scanning elec-tron microscopy and transmission electron microscope micrographs indicated that the growth process ofthese plates was through Ostwald ripening and aggregation of plates surface by Van der Waals forcesalong to the two [100] and [010] directions.� 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

In recent years, copper oxide (CuO) has received special attentionfrom many technological fields and scientific areas because of theirwide potential for industrial applications. For example, this oxide isconsidered an efficient heterogeneous catalyst for the conversionof hydrocarbons into carbon dioxide and water [1–3]. CuO nanopar-ticles have been employed as additives in oil lubricant since theirgood tribological properties, including high friction coefficient andlow wear [4]. The narrow band gap of this material (1.2 eV) favorsits use in photoconductive and photothermal applications [5–7].CuO nanoparticles dispersed in semiconductor tin oxide (SnO2) im-prove the sensitivity of SnO2 sensors for H2S gas detection [8]. More-over, this oxide also is an important material for the fabrication oflithium ion battery [9,10]. Due to these applications, morphology-controlled synthesis and well-defined shapes for the formation ofCuO are technologically interesting due to the chemical and physicalproperties of this material depend on its composition, structure,phase, shape, size, and size distribution [11–13]. Thus, different mor-phologies have been obtained for the CuO with and without the useof surfactants or templates, such as nanowires [14,15], nanorods[16], nanoribbons [17,18], nanowhiskers [19], nanoplatelets [20–22], feather-like [23], urchin-like [24,25], and plates-like [26].

Powder Technology Japan. Publish

mobile: +55 16 9176 49 43;

te).

Currently, CuO has been formed by different preparation methods,mainly including: self-catalytic mechanism [27], simple solution[28,29], simple hydrolysis [30], hydrothermal reduction [31], hydro-thermal [32–36], solvothermal [37] microwave heating using ionic liq-uids [38] and microwave irradiation [39]. In particular, microwave-hydrothermal (MH) is an interesting synthesis method due to reactionkinetics increase of reaction by one to two orders of magnitude, forma-tion of materials with different morphologies on micro or nanoscale,rapid heating, low synthesis temperatures, reduced processing times,low costs with electric energy and environmentally friendly [40–42].

Therefore, in this communications, we report on the synthesisof CuO plates by MH at 130 �C for 30 min without any surfactantor template. These structures were analyzed by X-ray diffraction(XRD), Rietveld refinement, nitrogen adsorption isotherms mea-surements, field-emission gun scanning electron microscopy(FEG-SEM), transmission electron microscope (TEM), high resolu-tion transmission electron microscope (HRTEM) and selected areaelectron diffraction (SAED). Finally, a model was proposed to ex-plain the growth mechanism of these plates.

2. Experimental details

2.1. Synthesis and characterization of CuO plates

CuO plates were obtained by the MH processing without the useof surfactants. The typical experimental procedure is described as

ed by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Fig. 1. XRD pattern of CuO plates processed in microwave-hydrothermal at 130 �Cfor 30 min. The vertical lines indicate the position and relative intensity of ICSD cardNo. 67850.

Fig. 2. Rietveld refinement of CuO plates processed in microwave-hydrothermal at130 �C for 30 min.

Table 2Atomic coordinates used to model the CuO unit cell.

Atom Site x y z

Copper 4c 0.25 0.25 0.0Oxygen 4e 0.0 0.57894 0.25

a ¼ 90; b ¼ 99:4692; c ¼ 90;RBragg ¼ 2:32;Rp ¼ 1:35;Rwp ¼ 1:72 and v2 ¼ 1:96:

198 A.P. Moura et al. / Advanced Powder Technology 21 (2010) 197–202

follows: 1 molar of sodium hydroxide (NaOH) (99.5% purity, Synth)and 2.5 � 10�3 mol of copper chloride dihydrate (CuCl2�2H2O)(99.5% purity, Aldrich) were dissolved in 100 mL of deionizedwater under constant stirring. A blue solution was formed due tothe presence of Cu2+ ions. In the sequence, this solution was trans-ferred into a Teflon autoclave, which was sealed and placed intothe microwave-hydrothermal (2.45 GHz, maximum power of800 W) system. MH synthesis was performed at 130 �C for30 min. The heating rate in this system was fixed at 20 �C/minand the pressure into the autoclave was stabilized at 294 kPa. AfterMH synthesis, the autoclave was naturally cooled to room temper-ature. The resulting solution was washed several times with deion-ized water to neutralize the solution pH ð� 7Þ. Finally, the blackprecipitates were collected and dried in a hot plate at 60 �C for 5 h.

CuO plates were structurally characterized by X-ray powder dif-fraction (XRD) using a Rigaku-DMax/2500PC (Japan) with Cu Karadiation ðk ¼ 1:5406 ÅÞ in the 2h range from 20� to 75� with anangular step of 0.02�/min. The Rietveld routine was performed inthe 2h range from 30� to 110�, using an angular step of 0.02�/minand exposure time of 2 s. The nitrogen adsorption isotherms weremeasured through an ASAP 2000 (USA) equipment. BET (Brunauer,Emmett and Teller) method [43] was employed to estimate thespecific surface area. The morphologic characteristics were ana-lyzed using a FEG-SEM Supra 35-VP Carl Zeiss (Germany) andTEM samples were prepared by dropping diluted solution of CuOnanoplates on copper grids covered with a thin amorphous carbonfilm ð� 5 nmÞ. HRTEM characterization was performed using aJEM-2100 TEM at 200 KeV with a lanthanum hexaboride electron

Table 1Comparative results between lattice parameters and specific surface area values of CuO o

Method Temperature (�C) Time (min) Average a (Å)

HC 180 720 4.683SM 180 1440 4.665QPM 100 – 4.676FZM 950 4320 4.68HM 130 30 4.6856(2)JCPDS card No. 45-0937 4.685

HC = hydrothermal conventional, SM = solvothermal method, QPM = quick-precipitz ¼ this work and Ref. = reference.

gun and equipped with a 1024� 1024 thermoelectrically cooledCCD camera. HRTEM multislice image simulation was performedusing JEMS software [44]. The growth direction of CuO plateswas determined by the SAED technique.

3. Results and discussion

3.1. X-ray diffraction and Rietveld refinement analyses

Fig. 1 shows the XRD patterns of CuO plates obtained by MHprocessing at 130 �C for 30 min.

In this figure, all diffraction peaks of CuO plates can be indexedto the monoclinic structure with space group C2/c. This resultshows that the MH system is able to promote the formation of thisoxide with low synthesis temperature and reduced processingtime.

The experimental lattice parameters were calculated using Riet-veld refinement. The obtained results were compared with thosereported in the literature by different preparation methods, asshown in Table 1. The specific surface area values obtained in thiswork are also listed in this table.

btained in this work with those reported in the literature by different methods.

Lattice b (Å) Parameter c (Å) Specific surface area Ref.

3.4212 5.124 10.6 [26]3.419 5.124 – [45]3.417 5.122 22.3 [46]3.420 5.129 – [47]3.4251(5) 5.1341(7) 30 ½z�3.426 5.13 – [48]

ation method, FZT = floating zone technique, MH = microwave-hydrothermal,

Fig. 3. Schematic representation of a monoclinic CuO unit cell.

Fig. 4. FEG-SEM micrographs: (a) and (b) several CuO plates with agglomerate and polyCuO plates. TEM micrographs: (d) low magnification TEM micrographs of CuO plates agsingle CuO plate [001] oriented and (f) HRTEM micrographs simulation of the selected athe reader is referred to the web version of this article.)

A.P. Moura et al. / Advanced Powder Technology 21 (2010) 197–202 199

As it can be seen in Table 1, the lattice parameter values of CuOplates are in agreement with the reported in the literature [45–47]and with the respective ‘‘ICSD” (Inorganic Crystal Structure Data-base) card No. 67850 [48]. The differences verified in the surfacearea can be related to several factors, such as employed method,synthesis conditions, morphology, average particle size, surface de-fects, and formation of agglomerate particles.

Fig. 2 shows the Rietveld refinement of CuO plates obtained byMH processing at 130 �C for 30 min.

The Rietveld refinement [49] was performed using GSAS software[50]. The diffraction peak profiles were better adjusted by theThompson–Cox–Hastings pseudo-Voigt (pV-TCH) function and byan asymmetry function as described by Finger et al. [51]. The strainanisotropy broadening was corrected by the phenomenological

disperse nature formed by microwave-hydrothermal at 130 �C for 30 min; (c) somegregate. Inset shows the SAED pattern of CuO plates; (e) HRTEM micrographs of a

rea (red square). (For interpretation of the references to colour in this figure legend,

(a)

(b)

Fig. 5. (a) Average plate length distribution and (b) average plate thicknessdistribution of CuO plates obtained in microwave-hydrothermal processing at130 �C for 30 min.

200 A.P. Moura et al. / Advanced Powder Technology 21 (2010) 197–202

model described by Stephens [52]. The obtained results from theRietveld refinement are listed in Table 2.

3.2. Unit cell representation of CuO

Fig. 3 shows a schematic representation of a monoclinic CuOunit cell in 1� 1� 1 format. Java Structure Viewer (Version1.08lite for Windows) and VRML-View (Version 3.0 for Windows)[53,54] programs were used to model this crystalline structure.The structural parameters and atomic coordinates obtained fromthe Rietveld refinement (Table 2) were used in these programs.As it can be seen in Fig. 3, CuO unit cell presents the copper atomscoordinated to four oxygens in a planar square configuration.

3.3. FEG-SEM and TEM analyses

Fig. 4 shows the FEG-SEM and TEM micrographs of CuO platesobtained by MH processing at 130 �C for 30 min.

FEG-SEM micrographs showed a large quantity of CuO plateswith agglomerate and polydisperse nature (Fig. 4a–c). These struc-tures exhibited a length between 1.25 lm and 5.75 lm and a thick-ness between 11.2 nm and 58.4 nm. Low magnification TEMmicrographs revealed the presence of bright and dark areas onthe plates (Fig. 4d). The bright areas correspond to the individualplates, while the black areas indicate the regions with stacking be-tween plates on different crystallographic directions. Also, it is ob-served that CuO nanoparticles plate-like are aggregate and selectedarea electron diffraction (SAED) of a single particle (Fig. 4d) indi-cates a highly crystalline CuO monoclinic phase [55]. The com-bined analyses of SAED patterns and HRTEM images (Fig. 4e andinset) indicates the CuO plates are mainly oriented on the [001]zone axis (ZA) and have as [100] and [010] preferential growthdirections. Fig. 4f shows a high magnification TEM micrograph ofa CuO plate selected in Fig. 4e (red square). SAED pattern con-firmed the monoclinic structure for the CuO plates, in agreementwith the XRD patterns and Rietveld refinement (Figs. 1 and 2).According to literature [56,57], the Ostwald-ripening mechanismis responsible for the growth of these CuO structures along the[100] and [010] directions. Ostwald ripening mechanism isresponsible by formation of CuO plates, this mechanism occurswhen both small and large particles (plates) have been producedin nonequilibrium solution of autoclave at the initial stage of theMH processing. Then small CuO nanoparticles have been graduallydissolved to generate free atoms in the solution, and these atomshave been spontaneously transferred onto the surfaces of somelarge plates (see support information with TEM micrographs).Next, a net atomic transport from the particles with sizes smallerthan the average value to larger particles, after the particles smal-ler than the average value will shrink or even disappear. These re-sult are in agreement with the reported by Raksa et al. [58]. Theseauthors verified the growth direction along [110] direction is fa-vored for the formation of CuO nanowires.

FEG-SEM and TEM micrographs were also employed to evaluatethe average plate size distribution (length and thickness) of CuOplates through the measures of approximately 100 plates (Fig. 5aand b).

As it was verified in the FEG-SEM micrographs (Fig. 4a–d), theplates-like CuO exhibited a polydisperse particle size distribution(thickness and width). Therefore, the best fit for this system is alognormal function, which is described by the following equation:

y ¼ y0 þAffiffiffiffiffiffiffiffiffiffiffiffiffi

2pwxp e

� ln xxc½ �

2

2w2 ; ð1Þ

where y0 is the first value in y-axis, A is the amplitude, w is thewidth, p is a constant, xc is the center value of the distribution curve

in x-axis. Thus, the distribution is asymmetrical on the logarithmicscale of average particle size. Fig. 5a shows the average particlelength distribution in the range from 1.25 lm to 5.75 lm for theplates-like CuO. In this case, it was noted that approximately 68%particles presented an average length between 2.25 lm to 3.75lm. On the other hand, approximately 65% particles exhibited anaverage thickness in the range from 22.5 nm to 37.5 nm. Possibly,the differences observed between length and thickness is causedby the increase in the effective collision rates between the nano-plates by the microwave radiation, as previously described in thetext. Also, we believe that these morphologic characteristics proba-bly can be related with the low specific surface area value of CuOplates (Table 1).

3.4. Growth mechanism of CuO plates

Fig. 6 shows a schematic representation of the synthesis andformation mechanism of CuO plates by the MH system.

According to the literature [59], the crystallographic structureof CuðOHÞ2 is able to influence in the formation of CuO plates.

Fig. 6. Schematic representation of the synthesis and growth process of CuO plates. (a) Initial stage from the chemical synthesis for the formation of the plates. (b) Bluesolution formed by the presence of CuðOHÞ2. (c) Schematic representation of the microwave-hydrothermal system employed in the synthesis. (d) Increase of the effectivecollision rates between small plates by the microwave radiation, favoring the dehydration process and the rapid formation process of CuO plates. (e) Growth process of CuOplates along the [100] and [010] directions.

A.P. Moura et al. / Advanced Powder Technology 21 (2010) 197–202 201

Fig. 6a shows the initial stages from the synthesis of CuO plates.Initially, the dissolution of CuCl2�2H2O and NaOH in water at roomtemperature favors the hydrolysis process on these chemical re-agents. Thus, this mechanism results in the formation of CuðOHÞ2and solvation of Cl� and Naþ ions. After stirring for 15 min, the firstprecipitates of CuðOHÞ2 were formed in the solution (Fig. 6b). The(010) plane of this hydroxide is a corrugated sheet structure. Byedge sharing, a sheet of distorted CuðOHÞ6 octahedra is formed.The 2D layers of these sheets parallel to the (010) plane are con-nected through H-bonds [60]. Fig. 6c illustrates a schematic repre-sentation of the MH system employed in the synthesis of CuOplates. This equipment was developed through adaptations per-formed on a microwave oven (model NN-ST357WRPH, Panasonic)[61], including: temperature controller, thermocouple and Teflonautoclave. Fig. 6d shows a schematic representation of the forma-tion process of CuO plates by MH. The microwave radiation wheninteracts with the permanent dipoles of the liquid phase results invibrations on the charged particles [62–64]. The heating promotedby these vibrations leads to a break of interplanar H-bonds andconsequently accelerates the dehydration rate. As consequence,this mechanism contributes to a rapid transformation process of

CuðOHÞ2 (orthorhombic structure) to CuO plates (monoclinic struc-ture). Moreover, the microwave radiation is able to promote an in-crease of the effective collisions between CuO particles andconsequently favor a growth process on the crystallographic planesof high atomic densities and more favorable thermodynamically[65]. Fig. 6e shows an Ostwald-ripening mechanism that is consid-ered to be the main mechanism involved in early processing, afteroccurs the aggregation of plates surface by Van der Waals forces.The Ostwald-ripening mechanism also called dissolution/recrystal-lization takes place during the MH processing. Thus, the thermody-namically-driven spontaneous process occurs because largerparticles are more energetically favored than smaller ones. There-fore, the heating caused on the CuO plates by the microwave radi-ation probably intensified the aggregation of plates surface by Vander Waals forces along of two [100] and [010] preferential growthdirections (see Fig. 4e). In previous works [22,25], our researchgroup reported on the growth mechanisms of flower and urchin-like CuO nanostructures obtained by the MH processing. In thissynthesis, copper carbonate (0.005 mol) and 0.1 g of polyethyleneglycol (PEG) were dissolved in 100 mL of deionized water. After15 min, 5 mL of ammonium hydroxide was added into this solution.

202 A.P. Moura et al. / Advanced Powder Technology 21 (2010) 197–202

In this case, the ammonia ðNH3Þ (strong field ligand) replaced thewater (weak field ligand) bonded to the Cu2þ ions forming the½CuðNH3Þ4�

2þ ion complex (blue color). Thus, when this systemwas submitted to the MH processing performed at 120 �C for 1 h,the microwave radiation accelerated the dehydration reaction ofCuðOHÞ2, resulting in the formation of CuO nanostructures withdifferent shapes (flower and urchin). The differences observed inthese morphologies can be related to the variations in the densityelectronic of the ligands by steric impediment and/or because ofthe orientation crystallographic induced by the PEG (see supportinformation). Finally, we believe that the irregularities andagglomeration of CuO plates were caused by the fast nucleationfrom the first seeds and by the high collision rates between smallplates during the MH processing. These results are in agreementwith the reported in the literature [66–70].

4. Conclusions

CuO plates were obtained by microwave-hydrothermal process-ing at 130 �C for 30 min without any surfactant or template. XRDpatterns, Rietveld refinement and SAED pattern indicated thatthese structures crystallizes in a monoclinic structure with spacegroup C2/c. FEG-SEM and TEM micrographs revealed that theplates present a agglomerate nature and polydisperse plate sizedistribution (length and thickness). The agglomeration and irregu-larities of CuO plates were attributed to the Van der Waals attrac-tion and effect of microwave radiation. TEM and HRTEMmicrographs evidenced that the growth process of the plates oc-curs by Ostwald-ripening mechanism and aggregation of platessurface by Van der Waals forces along to the two [100] and[010] directions. Finally, a growth mechanism was proposed to ex-plain the formation of CuO plates by the MH processing.

Acknowledgements

The authors thank the financial support of the Brazilian re-search financing institutions: FAPESP-Postdoctorate (No. 2009/50303-4), CNPq and CAPES. Special thanks to Prof. Dr. D. Keysonand Dr. D.P. Volanti by the development of the microwave-hydro-thermal system.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.apt.2009.11.007.

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