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doi:10.1016/j.jc

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Journal of Crystal Growth 310 (2008) 1708–1712

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Surface fabrication of oxides via solution chemistry

Chenglin Yan, Congting Sun, Yong Shi, Dongfeng Xue�

State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering,

Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, PR China

Available online 22 November 2007

Abstract

A template- and catalyst-free strategy has been successfully designed to prepare MgO and hydrated magnesium carbonate hydroxide

(Mg5(CO3)4(OH)2 � 4H2O) nanosheets with different patterns (such as chrysalides- and rose-like morphology) on the substrate surface.

Experimental results reveal that the temperature and substrate allow us to tune the morphology of patterns. Mg5(CO3)4(OH)2 � 4H2O

thermodynamically prefers to grow into the sheet-like crystal at the current solution growth environment, which has been successfully

explained by using the chemical bonding theory. The predicted morphology can accord well with the current experimental results. The

obtained MgO and its precursor Mg5(CO3)4(OH)2 � 4H2O with novel patterns might find enhanced applications in catalysis, refractory

materials, plastics, fire retardants, and functional nanodevices.

r 2007 Elsevier B.V. All rights reserved.

PACS: 81.10.Dn; 75.47.Pq; 68.65.+g

Keywords: A1. Nanostructures; A2. Growth from solutions; B1.Oxides

1. Introduction

Low-dimensional nanostructures, such as nanotubes[1,2], nanorods [3], nanowires [4] and nanosheets [5], haveattracted intensive attention because of their novel physicalproperties and potential applications. Recently, research isexpanding into the synthesis of nanostructures with three-dimensional (3D) ordered structures [6,7]. This is particu-larly interesting since the properties of the materials aredependent on their aggregation state, and thus the proper-ties are in general different from the isolated crystals. Thestrategy of using organic templates and/or catalysts hasbeen widely applied to controlling the morphology ofinorganic materials with 3D structures [8–10]. It is obviousthat the introduction of templates or catalysts into thesynthetic route undoubtedly leads to more syntheticprocedures and causes impurities in the final products.Thus, the development of template- and catalyst-freemethod is still highly desired for the formation of 3Dstructures in the current material synthesis and device

e front matter r 2007 Elsevier B.V. All rights reserved.

rysgro.2007.11.092

ing author.

ess: dfxue@chem.dlut.edu.cn (D. Xue).

fabrication. Herein we report a solution-based approachfor the synthesis of MgO and hydrated magnesiumcarbonate hydroxide (Mg5(CO3)4(OH)2 � 4H2O) na-nosheets with 3D structures on the different substratesurfaces without any template or catalyst. Furthermore,the nanosheet morphology and growth habit ofMg5(CO3)4(OH)2 � 4H2O crystals were quantitatively cal-culated and compared with the experimental observations.Both the calculation and experimental studies indicate thatMg5(CO3)4(OH)2 � 4H2O thermodynamically prefers togrow into the sheet-like morphology crystal.The MgO patterns have been used as buffer layers for

superconducting due to their thermodynamic stability andwide-bandgap insulator characteristics and ferroelectricmaterials [11–13]. Also, their use as secondary emissionmaterials for plasma display panels has recently attractedgreat interest [14]. Mg5(CO3)4(OH)2 � 4H2O is widelyused in rubber, plastics, and fire retardants.Mg5(CO3)4(OH)2 � 4H2O may also be used as a precursorto prepare MgO [15–17]. Accordingly, designingMgO and Mg5(CO3)4(OH)2 � 4H2O with novel patternsare of great importance for basic fundamental research aswell as of relevance for various fields. In this work,

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Fig. 1. XRD patterns of: (A) rose-like Mg5(CO3)4(OH)2 � 4H2O and (B)

rose-like MgO obtained by calcination of Mg5(CO3)4(OH)2 � 4H2O

precursor.

C. Yan et al. / Journal of Crystal Growth 310 (2008) 1708–1712 1709

well-defined patterns with different morphologies ofMg5(CO3)4(OH)2 � 4H2O are prepared by a heterogeneousnucleation and subsequent growth. MgO with the similarmorphology can be obtained by calcination ofMg5(CO3)4(OH)2 � 4H2O precursor. This soft-solution ap-proach provides a general route to the synthesis of MgOpatterns with unique morphologies. Furthermore, this newstrategy may be applicable for making other interestingstructures on the other substrates.

2. Experimental procedure

All chemicals used in this experiment, such as magne-sium chloride (MgCl2), urea (CO(NH2)2) and hydrochloricacid (HCl) were of analytical grade. The experimentalprocedures were shown as following. In a typical synthesis,the copper grid, MgO single crystal and glass substratewere cleaned in an aqueous 1.0M HCl solution for about20 s, followed by repeated rinsing with distilled water anddried in air at 60 1C. Subsequently, 0.06mol MgCl2 and0.24mol CO(NH2)2 were added into a Teflon-linedautoclave of 40ml capacity. The autoclave was then filledwith water up to 70% of the total volume. Then, thesubstrate (e.g. copper grid, MgO single crystal, and glass)was placed in the bottom of the Teflon-lined autoclave. Theautoclave was sealed into an electric oven and maintainedat 90–130 1C for 4–18 h. After cooling down to roomtemperature naturally, the substrate was washed withdeionized water and absolute ethanol several times toremove any of residual salts or impurity. Finally, thesamples were dried in air at 60 1C for 4 h. The phaseidentification of the sample was determined by X-raypowder diffraction (XRD) patterns employing a scanningrate of 0.021 s�1 using XRD that was performed on aRigaku-DMax 2400 powder X-ray diffractometer with CuKa radiation (k=1.5418 A). The morphology of thesample was measured by scanning electron microscopy(SEM, JSM-5600LV, JEOL).

3. Results and discussion

Fig. 1(A) shows a typical XRD pattern of the as-synthesized rose-like Mg5(CO3)4(OH)2 � 4H2O sample,which is obtained from copper grid substrate surface.All peaks in the XRD pattern can be indexed as themonoclinic structure with lattice constants a ¼ 10.07,b ¼ 8.90, and c ¼ 8.32 A (JCPDS, 70–1177). No peaksof impurities were detected, indicating the high purityof our as-prepared samples. The pyrolysis of rose-likeMg5(CO3)4(OH)2 � 4H2O pattern results in similar mor-phology of MgO sample. The XRD pattern of the obtainedrose-like MgO sample is shown in Fig. 1(B). All strongpeaks could be indexed as the cubic MgO phase with thecell constant a ¼ 4.21 A, which is well in agreement withthe value in the literature (JCPDS, 75–0447).

The different reaction conditions and the corresponding as-prepared sample morphologies are summarized in Table 1.

Sheet-like Mg5(CO3)4(OH)2 � 4H2O patterns synthesized atdifferent temperatures are found to grow on the copper gridsubstrate surface, which is shown in the SEM image of Fig. 2.It can be seen that chrysalides-like Mg5(CO3)4(OH)2 � 4H2Opattern can be obtained at 130 1C (Fig. 2(A)). After carefulobservation from Fig. 2(B), it can be found that thechrysalides-like Mg5(CO3)4(OH)2 � 4H2O are infact built fromindividual nanosheets (as shown in the inset of Fig. 2(B)),which is different from the nest-like Mg5(CO3)4(OH)2 � 4H2Oreported by our previous work [12]. In the presentreaction system, we find that the reaction temperatureis vital in the formation of Mg5(CO3)4(OH)2 � 4H2Onanosheets with different morphologies. By carefullyadjusting the synthesis temperature of 95 1C, the rose-likeMg5(CO3)4(OH)2 � 4H2O pattern can be obtained on thecopper grid substrate surface, as shown in Fig. 2(C).Numerous nanosheets are self-assembled into the rose-like Mg5(CO3)4(OH)2 � 4H2O (Fig. 2(D)). However, whenthe reaction temperature is maintained at 110 1C, somerandom Mg5(CO3)4(OH)2 � 4H2O nanosheets are observedon the copper grid surface (Fig. 3(A)). It is clearlydemonstrated that temperature has significant influenceon the morphology of Mg5(CO3)4(OH)2 � 4H2O samples.The formation of the rose- or chrysalides-likeMg5(CO3)4(OH)2 � 4H2O patterns can be related todifferent growth dynamics mechanism at differentreaction temperatures. In addition, the rose-like MgOpattern can be effectively prepared by calcination ofthe rose-like Mg5(CO3)4(OH)2 � 4H2O at 700 1C. Further-more, it is found that the thermal treatment does notalter the surface morphology significantly. The sizeand morphology are similar to that of their precursors(Fig. 5). Therefore, all Mg5(CO3)4(OH)2 � 4H2O samplescan be converted into the similar morphology of MgOsamples.Experimental results indicate that the basic building

units of the obtained pattern (on the substrate surface) aresheet-like Mg5(CO3)4(OH)2 � 4H2O crystals. The formedsheet-like Mg5(CO3)4(OH)2 � 4H2O structure may be

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Fig. 2. SEM images of Mg5(CO3)4(OH)2 � 4H2O nanosheets with different morphologies grown on the copper grid substrate surface synthesized at

different temperatures: (A and B) chrysalides-like Mg5(CO3)4(OH)2 � 4H2O patterns synthesized at 130 1C and (C and D) rose-like Mg5(CO3)4(OH)2 � 4H2O

patterns synthesized at 95 1C.

Table 1

Summary of experimental conditions versus the morphology of the samples

Sample no. Temperature (1C) Substrate Mg5(CO3)4(OH)2 � 4H2O morphology

1 95 Copper grid Rose-like pattern

2 130 Copper grid Chrysalides-like pattern

3 110 Copper grid Random sheet pattern

4 110 MgO crystal Particle-decorated nanosheets

5 110 Glass Particle-decorated nanosheets

C. Yan et al. / Journal of Crystal Growth 310 (2008) 1708–17121710

related to its intrinsic symmetry of the correspondinglattice and the specific solution growth conditions. Thecrystal structure of Mg5(CO3)4(OH)2 � 4H2O is based ontwo frameworks of MgO6 octahedral, Mg2+ ions arearranged parallel to the (1 0 0) plane, which is terminatedwith Mg2+ (Fig. 4(A)). The flat planes (selected planes) ofMg5(CO3)4(OH)2 � 4H2O constructed by the strong chemi-cal bonds are (1 0 0), (0 1 0), (0 0 1), (0 1 1), (1 1 0), (1 0 2),(1 0 2), (1 1 1), (3 1 0), (2 2 1). The relative growth rate ofevery selected plane is calculated using the chemicalbond method reported in our previous work [17].The ideal growth morphology of Mg5(CO3)4(OH)2 � 4H2Ocrystals can be derived from the calculation results,which is shown in Fig. 4(B). Considering the atomarrangement forms of the specific planes, the (1 0 0)face of Mg5(CO3)4(OH)2 � 4H2O crystal contains Mg atomonly, whereas other faces consist of mixed Mg and Oatoms. During synthesis in solution, the Mg2+ termi-nated (1 0 0) face will be covered by CO(NH2)2 molecules

since it interacts with the Mg2+ terminated faces, andcrystal growth along the [1 0 0] direction is suppressedunder these conditions, and Mg5(CO3)4(OH)2 � 4H2Onanosheets can be thus obtained in the presence ofCO(NH2)2 (Fig. 4(C)). A schematic diagram for theformation of Mg5(CO3)4(OH)2 � 4H2O nanosheet is shownin Fig. 5.Selection of different substrates allow us to tune the

morphology of the Mg5(CO3)4(OH)2 � 4H2O patterns.Different substrates are employed to investigate the effectof substrate on the morphology of Mg5(CO3)4(OH)2 �4H2O samples. Fig. 6 shows typical SEM images ofMg5(CO3)4(OH)2 � 4H2O grown on the different substratesurfaces at the reaction temperature of 110 1C. As shown inFig. 6(A), random Mg5(CO3)4(OH)2 � 4H2O nanosheets aregrown on the MgO single-crystal substrate surface.Interestingly, it can be clearly seen that the cubic particle-decorated Mg5(CO3)4(OH)2 � 4H2O nanosheets can beobtained on the MgO single-crystal substrate surface.

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Fig. 3. (A) SEM images of random Mg5(CO3)4(OH)2 � 4H2O nanosheets

grown on the copper grid substrate surface synthesized at 110 1C and (B)

SEM images of rose-like MgO grown on the copper grid substrate surface

obtained by the calcination of rose-like Mg5(CO3)4(OH)2 � 4H2O.

Fig. 4. (A) Atom arrangement forms in Mg2+ terminated (1 0 0) face of

Mg5(CO3)4(OH)2 � 4H2O, (B) ideal morphology and (C) the actual

morphology of Mg5(CO3)4(OH)2 � 4H2O crystals grown from the solution.

C. Yan et al. / Journal of Crystal Growth 310 (2008) 1708–1712 1711

The monodisperse cubic particles are anchored onto thesurface of Mg5(CO3)4(OH)2 � 4H2O nanosheets, whichcould partly result from the effect of the surface structureof MgO single crystal on the particle morphology. Incontrast, there are fewer cubic particles attached on thesurface of the Mg5(CO3)4(OH)2 � 4H2O nanosheets grownon the glass substrate surface (Fig. 6(B)), while thereare not any cubic particles on the surface ofMg5(CO3)4(OH)2 � 4H2O nanosheets when the coppergrid is used as the substrate (Fig. 3(A)). It can be concludedthat the different substrates with different surface struc-tures may influence the morphology of the obtainedMg5(CO3)4(OH)2 � 4H2O patterns.

In our synthetic system, the obtained novel morphologyof MgO and Mg5(CO3)4(OH)2 � 4H2O on different sub-strate surfaces indicates that the nucleation and growth arewell controlled. In the aqueous-phase synthesis, nanocrys-talline patterns are deposited on a substrate in aqueous

media by a heterogeneous nucleation and subsequentgrowth. The resultant pattern is controlled by the thermo-dynamics parameters such as reaction temperature. Boththe substrate and other reaction parameter play importantroles in controlling nucleation, growth and self-assembly ofMg5(CO3)4(OH)2 � 4H2O. The intrinsically anisotropicstructure of Mg5(CO3)4(OH)2 � 4H2O may also play animportant role in the determination of its final morphologyof pattern. During the hydrothermal reaction, the homo-geneous precipitation of OH– ions and CO2, which areproduced by the decomposition of urea, results in the directformation of sheet-like Mg5(CO3)4(OH)2 � 4H2O particles.Subsequently, the Mg5(CO3)4(OH)2 � 4H2O pattern is con-structed by an assembly of oriented sheets on the substratesurface.

4. Conclusions

In summary, a heterogeneous nucleation route for thesynthesis of MgO and its precursor patterns with novelmorphologies is reported for the first time. The reactiontemperature and substrates can be regarded as to moderatethe nanosheets to form different morphologies of patterns.The growth habit of Mg5(CO3)4(OH)2 � 4H2O nanosheets iswell predicted from the chemical bond viewpoint in thecurrent work. Accordingly, both experimental and theore-tical results indicate that Mg5(CO3)4(OH)2 � 4H2O is readilygrown into the nanosheet on the substrate surface with

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Fig. 5. Modeling morphology and growth habit of Mg5(CO3)4(OH)2 � 4H2O crystals in the absence of any external influence (left, ideal morphology),

molecules adsorbed on (1 0 0) faces of Mg5(CO3)4(OH)2 � 4H2O (middle), and the actual Mg5(CO3)4(OH)2 � 4H2O nanosheet formation in the presence of

CO(NH2)2 molecules (right).

Fig. 6. SEM images of particle-decorated Mg5(CO3)4(OH)2 � 4H2O

nanosheets synthesized at 110 1C grown on MgO single-crystal surface

(A), and glass substrate surface (B).

C. Yan et al. / Journal of Crystal Growth 310 (2008) 1708–17121712

different morphologies at the current hydrothermal reac-tion condition. The current results indicate that thechemical bonding theory can be helpful for us to

comprehensively understand the relationship betweencrystallographic characteristics and corresponding crystalmorphologies.

Acknowledgments

The authors gratefully acknowledge the financial sup-port of Program for New Century Excellent Talents inUniversity (NCET-05-0278), the National Natural ScienceFoundation of China (NSFC #20471012), a Foundationfor the Author of National Excellent Doctoral Dissertationof PR China (FANEDD #200322), the Research Fund forthe Doctoral Program of Higher Education (RFDP#20040141004) and the Scientific Research Foundationfor the Returned Overseas Chinese Scholars, State Educa-tion Ministry.

References

[1] C. Yan, D. Xue, Adv. Mater., doi:10.1002/adma.200701752.

[2] C. Yan, D. Xue, Electrochem. Commun. 9 (2007) 1247.

[3] C. Yan, D. Xue, J. Phys. Chem. B 110 (2006) 25850.

[4] C. Yan, D. Xue, L, Zou, J. Alloys. Compds. doi:10.1016/

j.jallcom.2006.11.064.

[5] C. Yan, D. Xue, J. Phys. Chem. B 109 (2005) 12358.

[6] G.M. Whitesides, B. Grzybowski, Science 295 (2002) 2418.

[7] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 8124.

[8] J. Lao, J. Wen, Z. Ren, Nano. Lett. 2 (2002) 1287.

[9] X. Duan, C. Lieber, Adv. Mater. 12 (2000) 298.

[10] R. Fan, Y. Wu, D. Li, M. Yue, A. Majumdar, P. Yang, J. Am. Chem.

Soc. 125 (2003) 5254.

[11] D.K. Fork, F.A. ponce, J.C. Tramontana, T.H. Geballe, Appl. Phys.

Lett. 58 (1991) 2294.

[12] A.N. Basit, H.K. Kim, J. Blachere, Appl. Phys. Lett. 73 (1998)

3941.

[13] K. Nashimoto, D.K. Fork, T.H. Geballe, Appl. Phys. Lett. 60 (1992)

1199.

[14] M. Putkone, L. Johansson, E. Rauhala, L. Niinisto, J. Mater. Chem.

9 (1999) 2449.

[15] B. Toure, J. Cuesta, P. Gaudon, A. Benhassaine, A. Crespy, Polym.

Degrad. Stab. 53 (1996) 371.

[16] C. Yan, D. Xue, L. Zou, Mater. Res. Bull. 41 (2006) 2341.

[17] X. Yan, D. Xu, D. Xue, Acta. Mater. 55 (2007) 5747.