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Powder Technology 1

Chemical strategy for tuning the surface microstructures of particles

Chenglin Yan, Longjiang Zou, Jiasheng Xu, Junshu Wu, Fei Liu, Chao Luo, 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, China

Available online 22 November 2007

Abstract

Particles with tailored surface microstructures exhibit unique structure-dependent phenomena and subsequent utilization of them for thepractical applications are of significant interest. In this work, we have developed some potential chemical strategies to tune the surfacemicrostructures of functional materials, such as hollow ZnO microspheres, semiconductor films or arrays, LiNbO3 spheres, and cubic phase Cu2Oparticles. We describe their surface microstructure-guiding processes and illustrate the detailed key factors controlling their growth by examiningvarious reaction parameters. The proposed surface structure-guiding mechanisms are presented and the important pioneering studies on therational design and fabrication of particles with tunable surface microstructures are discussed. Current results demonstrate that our suggestedchemical strategies for tuning surface microstructures of particles can be used as a versatile and effective route to the controllable synthesis ofother inorganic functional materials.© 2007 Elsevier B.V. All rights reserved.

Keywords: Chemical strategy; Particles; Surface microstructure

1. Introduction

Surface microstructure control of particle growth provides ameans of tailoring the interfacial arrangement of atoms and hasa vital role in enhancing the desired reactivity or stability of amaterial [1]. A challenge in materials engineering is thecontrolled assembly of purposefully designed molecules orensembles of molecules into micro- and nanostructures to pro-vide an increasingly precise control at molecular levels over thesurface microstructure of functional materials [2–4]. This isparticularly interesting since precise control of such factorsallows one not only to observe unique properties of the particlesbut also to tune their chemical and physical properties as desired[5,6]. New chemical strategies for tuning the surface micro-structure of particles are of fundamental importance in theadvancement of science and technology [7–11]. Much efforthas been invested in developing new chemical strategy for thesynthesis of particles with a controlled surface microstructuresand size [12,13]. However, no efficient synthetic routes areavailable for the synthesis of a wide-range of function materials.

⁎ Corresponding author. Tel./fax: 86 411 88993623.E-mail address: dfxue@chem.dlut.edu.cn (D. Xue).

0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2007.11.016

Recently, solution-based methods have served as powerfultools for the controllable synthesis of surface microstructures ofparticles due to their various advantages such as a large-scaleproduction process at low temperature, a high quality withoutimpurity pollution, the wide selection of surface microstructureand size control [14–18]. The chemical synthesis of particlesinevitably involves the process of precipitation of a solid phasefrom solution [19–21]. A good understanding of the processand parameters controlling the precipitation helps to improvethe engineering of the growth of particles to the desired surfacemicrostructures [22–26]. In the case of particle formation, fornucleation to occur, the solution must be supersaturated eitherby directly dissolving the solute at higher temperature and thencooling to low temperature or by adding the necessary reactantsto produce a supersaturated solution during the reaction [27]. Toobtain more advanced-structured particles and to have theability to control the surface microstructures of particles asdesired, the unraveling and systematic understanding of surfacemicrostructure control through effect chemical strategies arenecessary.

Oxides have been an attractive topic in the field of functionalmaterials. ZnO semiconductor has attracted considerableattention due to its many attractive properties, such as the

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direct wide band gap (3.37 eV), large exciton binding energy(60 meV at room temperature), good piezoelectric character-istics, chemical stability and biocompatibility, which has manyremarkable applications in electronics, photoelectronics, andsensors [19]. Cu2O is another important semiconductor withmany potential applications, such as solar energy conversion,magnetic storage devices, and catalysis [11]. As an importantferroelectric materials, LiNbO3 has also been drawn a great dealof interest due to its excellent pyroelectrical, piezoelectrical,electro-optical and photorefractive properties [14]. However, allthese applications demand the production of powders or filmswith the highest possible purity, well-defined particle morphol-ogy, and the uniform size. It is expected that specific crystalmorphologies enhance the performance and allow a fine-tuningof various properties. Meeting the increasing demands of thesepractical applications, however, will require facile and eco-nomic processes for the growth of the functional materials. Inthe present work, we will discuss several novel chemical strate-gies based on the solution method aiming at chemicalmodification of surface microstructure of particles. Specifically,A new type of complex ZnO hollow structures constructed bypolyhedral particles has been prepared via an oriented aggre-gation process, in which ZnO precursors (ZnCO3) are synthe-sized through the reaction between Zn(CH3COO)2 andNH4HCO3 in aqueous solution. Similar morphology of hollowZnO microspheres can be effectively obtained by the pyrolysismethod. The hollow ZnO microsphere exhibits a uniquegeometrical shape, as its surface is composed of uniform andmonodisperse small polyhedral particles. ZnO films with differ-ent surface microstructures (such as nanorods, flower-likeparticles, petals) are also synthesized from the aqueous solutionon the zinc substrate. In addition, LiNbO3 particles with tunablesurface microstructures have been obtained via a quasireverseemulsion route. Ethylenediaminetetraacetic acid tetrasodiumsalt dihydrate (EDTA) reduction route has been demonstratedfor the surface microstructure controllable growth of Cu2Oparticles.

2. Experimental section

All reagents in the current experiments were of analyticalgrade (purchased from Shanghai Chemical Industrial Co.) andwere used as-received without further purification. The hollowZnO, LiNbO3 microspheres, Cu2O crystals were synthesizedthrough the hydrothermal reaction, solvothermal method, andEDTA reduction route, respectively. In the first route, the reac-tion solution was prepared by mixing the appropriate amount ofZn(CH3COO)2·2H2O and NH4HCO3 in 30 mL solution waterunder stirring, in the second route the solution was prepared bymixing niobium acid and LiOH, and ethylene glycol, while inthe last route, reaction solution was obtained by dissolvingequimolar quantities of EDTA and Cu(NO3)2 in deionized wateror stoichiometric ratios of EDTA/Cu(II). The above reactionsolutions were then respectively transferred into Teflon-linedstainless steel autoclaves. The autoclave was maintained at adesigned temperature and then cooled down to room tempera-ture. The powders were collected, filtered off, and washed with

deionized water and absolute ethanol several times, respectively.Finally, the samples were dried in air at 60 °C. The as-preparedsamples were characterized by an X-ray diffractometer (XRD)on a Rigaku-DMax 2400 diffractometer equipped with thegraphite monochromatized CuKα radiation flux at a scanningrate of θ of 0.02°s−1 in the 2θ range 5–80°. Scanning electronmicroscopy (SEM) images were taken with a JEOL-5600LVscanning electron microscopy, using an accelerating voltage of20 kV. UV/vis diffuse reflectance spectra were obtained using aUV–vis–NIR spectrophotometer (JASCO, V-550).

3. Results and discussion

3.1. Polyhedral construction of hollow ZnO microspheres andtheir tunable surface microstructures

Particles with hollow structures show a lower density, highersurface area, and distinct optical property, which thus makethem attractive from both a scientific and a technologicalviewpoint [28]. Applications for such particles with hollowstructure are diverse, including capsule agents for drug delivery,catalysis, coatings, composite materials, and protecting sensi-tive agents [18,29]. Previous investigations have demonstratedthat catalyst- or high temperature-based methods have shownthe possibility of successfully controlling the hollow ZnOstructures. These methods, however, often suffer from the dis-advantage of introducing metal catalysts and requiring hightemperature, which could make the synthesis procedures morecomplex and introduce catalyst impurities to influence theproperties of particles.

In our work, we report the fabrication of ZnO hollow struc-ture with spherically polyhedron constructed surface micro-structure through a simple solution-phase route. Specifically,hollow ZnCO3 microspheres are prepared through the reactionbetween Zn(CH3COO)2 and NH4HCO3 in the aqueous solution.The calcination of hollow ZnCO3microspheres results in hollowZnO microspheres showing similar morphological features asthe precursor. The growth of the hollow ZnO structure isbelieved to be an oriented aggregation process. The generalmorphology of as-prepared ZnO samples is shown in Fig. 1 (A),which exhibits interesting hollow microspheres constructed bysmall polyhedral particles. It is clearly shown from Fig. 1 (B) thatthe microsphere is hollow inside and many polyhedral particlesare self-assembled into the hollow microspheres. In particular,the surface microstructures of ZnO precursor (ZnCO3) micro-spheres can be tuned by simply altering the reaction temperature.SEM results shown in Fig. 2 indicate that these microsphereshave a decreasing number of polyhedral particles on the surfacewith decreasing the temperature. A smooth surface can be finallyobtained at around 45 °C, which is shown in Fig. 2 (D). Duringthe reaction system, the small ZnCO3 particles are thermo-dynamically unstable from the viewpoint of growth kinetics. Thethermodynamic equilibrium condition requires minimization ofthe interfacial energy. These small particles, driven by theminimization of interfacial energy, have a tendency to aggregateat the microsphere surface, which finally leads to microsphereswith different surface microstructures.

Fig. 1. SEM images of polyhedron constructed hollow ZnOmicrospheres obtained by calcinations of ZnCO3 microspheres at 450 °C: (A) detailed views on the surfacemicrostructure of ZnO microspheres; (B) two broken shells of hollow microspheres.

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3.2. Surface microstructure controllable growth of ZnO on thezinc substrate

Despite significant progresses in semiconductor ZnO synth-esis and many promising applications, developing a simple, one-step, environmentally benign, and mild solution method for thesynthesis of ZnO nanomaterials for the desired size, surfacemicrostructures would be nice toward understanding fundamen-tal properties. In the case of the growth of ZnO on the zinc

Fig. 2. SEM images of ZnO precursors (ZnCO3) prepared at different r

substrate, the usage of Zn(OH)42− solution and zinc foil are two

crucial conditions to the formation of well-aligned ZnO nano-arrays. The Zn(OH)4

2− solution is initially formed through astarting reagent reaction between Zn2+ and excessive KOH.Subsequently, ZnO nanoarrays can preferentially grow on zincsubstratewhen zinc foil is immersed into Zn(OH)4

2− solution at anelevated temperature without any templates or additives. It can beseen from Fig. 3 (A) that well-aligned ZnO nanorod arrays withthe strongest orientation preferentially growing along c axis are

eaction temperatures: (A) 80 °C; (B) 70 °C; (C) 55 °C; (D) 45 °C.

Fig. 3. SEM images of ZnO particles with different surface microstructure grown on the zinc substrate: (A) ZnO nanorods grown on the zinc foil substrate with a Zn2+

concentration of 0.12mol/L; (B) ZnO nanorods grown on the zinc foil substrate with a Zn2+ concentration of 0.06mol/L; (C) XRD pattern of ZnO nanorod arrays grownon the zinc foil substrate with a Zn2+ concentration of 0.12 mol/L; (D) ZnO nanoflakes grown on zinc foil in ethanol/water solvothermal condition at 105 °C for 12 h.

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achieved with Zn2+ concentration of 0.2 mol/L. When Zn2+

concentration is decreased to 0.06 mol/L, only random ZnOnanorods can be obtained, as shown in Fig. 3 (B). Since Zn2+

concentration was below 0.06 mol/L, there was small quantity of[Zn(OH)4]

2−, resulting in the deficiency of [Zn(OH)4]2− growth

units and thus ZnO nanorods obtained at this Zn2+ concentrationare only random ZnO nanorod arrays. A typical dissolutioncrystallization mechanism is responsible for the growth of well-aligned ZnO nanorods. The zinc foil surfaces are firstly oxidizedby naturally dissolved oxygen from air as the ZnO nuclei.Subsequently, the constituent atoms enter into ZnO nuclei in theform of Zn(OH)4

2− fundamental growth units produced by thestrong chemical bonds. ZnO nanorod arrays can be effectivelygrown in this solution system. The orientation of ZnO nanorodarrays synthesized with Zn2+ concentration of 0.2 mol/L areinvestigated by XRD diffraction. Fig. 3 (C) shows XRD patternof the as-prepared ZnO nanorods. The highly enhanced (002)peak can be clearly seen as a result of the vertical orientation ofnanorods. The indexed diffraction peaks can be ascribed to thepure hexagonal phase of wurtzite-type ZnO (space group:P63mc) with lattice constants a=3.249 Å and c=5.206 Å,which are consistent with the reported data (JCPDS, 36-1451).Some weak peaks (unindexed peaks) are originated from those ofthe zinc substrate. It is worth noting that, comparing to thestandard values of hexagonal phase ZnO, the relative intensity ofthe peaks corresponding to (002) plane is significantly enhanced

in the obtained XRD pattern, which suggests that ZnO pref-erentially grows along the [001] direction.

When ethanol is present in the reaction system, ZnO nano-flake arrays can be obtained, which is shown in Fig. 3 (D).However, when citrate is induced into a reaction system for thegrowth of ZnO arrays, tower-like ZnO structures can befabricated, which are stacked by a series of nanoplates with adiameter of about 1 μm and 100 nm in thickness, as shown inFig. 4 (A). Citrate plays a role as crystal growth director in thesesystems [30], it was reported having the ability to selectivelyadsorb on the (001) surface of ZnO crystal and suppress thegrowth rate of [001] direction. The formation of tower-like ZnOmight be based on the assembly of nanoplate which can beobtained directly without adding citrate, some unassembledplates at the bottom of ZnO tower can confirm this point. Whencitrate is added, it directs the nanoplates to stack together,forming tower-like structure. Interestingly, some tubular struc-tures were also observed, as shown in Fig. 4 (B), which may beformed due to helical stacking to ZnO nanoplates.

3.3. Surface microstructure controllable growth of LiNbO3

particles with structure-dependent optical properties

The quasireverse emulsion route to LiNbO3 particles withtunable surface microstructures have been successfully demon-strated in the H2O/ethylene glycol reaction system. SEM image

Fig. 4. SEM images of ZnO nanostructures obtained on the zinc foil substrate in the presence of 0.4 M sodium chloride, 0.05 M zinc sulfate and 0.05 M sodium citrateat 120 °C for 12 h: (A) tower-like ZnO structures; (B) ZnO tubular structures.

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shown in Fig. 5 (A) demonstrates that monodisperse LiNbO3

nanoparticles are synthesized after a reaction time of 3 days. It isinteresting that flower-like structures with a diameter of about1.0 μm can be obtained when increasing the reaction time up to4 days, which is shown in Fig. 5 (B). TEM image of the inset of

Fig. 5. SEM and TEM of samples: (A) nanoparticles synthesized for 3 days; (B) flow5 days; (D) UV/vis diffuse reflectance spectra of the as-obtained LiNbO3 particles:

Fig. 5 (B) indicated that the flower-like particles are composed ofmany petals. Fig. 5 (C) shows SEM image of hollow micro-spheres prepared at the proper volume ratio of H2O/ethyleneglycol for 5 days, from which monodisperse micrometer spherescan be clearly seen. TEM images shown in the inset of Fig. 5 (C)

er-like particles synthesized for 4 days; (C) hollow microspheres synthesized for(a) nanoparticles, (b) hollow microspheres, and (c) flower-like particles.

Fig. 6. SEM images of novel Cu2O microcrystal morphologies, which display 8-pod branching growth along the b111N direction, based on 8 corners of the cube.

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clearly show the bright center, obviously confirming that thehollow nature interior of microsphere is hollow. At the beginningof the reaction, LiNbO3 nanoparticles are obtained. Subse-quently, the oriented outgrowth step of LiNbO3 nuclei leads tothe formation of the flower-like particles for eliminating thehigher surface-energy faces [14,15]. With increasing reactiontime, LiNbO3 crystallites located in the inner cores begin todissolve, while new depositions grow on the edge, which leadsto the formation of hollow microspheres.

Surface microstructure is the main factor that determines thechemical and physical properties of nanocrystals, which mayserve as bases for the development of new fields. From Fig. 5 (D),it is observed that UVabsorption edge position of the as-obtainedLiNbO3 samples can be modified by controlling their surfacemicrostructure; therefore, their optical properties can also betuned in this way. LiNbO3 nanoparticles exhibit an absorptionedge at 320 nm, while the absorption edge of hollow micro-spheres is shown to be 340 nm. The absorption onsets of theflower-like particles have a red-shift relative to that of hollowmicrospheres. The absorption edge of LiNbO3 particles in UV/

Fig. 7. SEM images of Cu2O microcrystal morphologies, which display 6-pod bran

vis diffuse reflectance spectra can be thus tuned by the currentsurface microstructure control strategies.

3.4. Face-specified growth of cubic Cu2O particles by a novelreduction route

In our work, a novel reduction route has been demonstratedto the surface microstructure controllable growth of Cu2Oparticles by using EDTA as both chelating reagent and reduc-tant. For the cubic crystallographic Cu2O, both the {111} and{100} faces can be easily maintained in the final appearance. Itis reported that the formation of various surface microstructuresof a cubic nanocrystal depends on the ratio of the growth ratealong [100] to that along [111]. Cu2O particles with differentexposed surfaces were obtained by the hydrothermal treatmentof Cu(II)–EDTA2− solution in an alkaline environment. Subse-quent experiments suggest that the morphology of the productshas a strong dependence on the reaction conditions. When themolar ratio of EDTA/Cu(II)=1, Cu2O crystal branching growthtakes place along all orientations of [111], which results in the

ching growth along the b100N direction, based on 6 corners of the octahedron.

Fig. 8. SEM images of Cu2Omicrocrystal morphologies, which display 12-pod branching growth along the b110N direction, based on 12 corners of the cuboctahedron.

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formation of 8-pod branching growth as shown in Fig. 6 (A).The edges of a Cu2O hopper crystal extend outwards from itscore with a tiny cubic center leaving step-like faces, eight smallcubes are developed respectively from the octa-pods, giving riseto a set of crystal aggregates (Fig. 6B and C). When the molarratio of EDTA/Cu(II) is fixed at 1.5 with different other reactionparameters. Cu2O crystals with a cuboctahedral branchinggrowth pattern as observed from Fig. 7 (A), crystal branchinggrowth taking place along 12 [110] directions, as shown in Fig.7 (B) and (C). In the case of the growth of Cu2O crystal, {100}surface is an unstable. The intrinsic surface energy of {100}faces of cubic Cu2O, containing Cu or O only, is higher than thatof the {111} faces, which contain Cu+ cation and O2− anion.EDTA selectively stabilizes the {100} faces since it interactsstrongly with the charged {100} faces rather than the uncharged{111} faces [11]. Therefore, the cubic shape of Cu2O can beobtained in a lower amount of EDTA. When the amount ofEDTA is increased, the effect of selective adsorption of EDTAmolecules on the different crystallographic planes is signifi-cantly eliminated, and cuboctahedral Cu2O are effectivelyobtained. By manipulating synthetic conditions (such as pHvalue and temperature) that are directly responsible for thesystematic change in the degree of branching, various elegantCu2O architectures can be observed, which is shown in Fig. 8(A). From Fig. 8 (B) and (C), it can be seen that the six legs ofthese architectures grow from the six corners of the octahedralcrystal.

4. Conclusions

The present results indicate that the recent development ofchemical synthesis of particles with controllable surfacemicrostructures may be helpful for designing novel applicationswith desired performances. New synthetic strategies are alwaysthe bases for the development of materials science while newand novel properties discovered in particles with various surfacemicrostructures systems push the whole field forward. With thefurther development of synthetic strategies, particles withprecisely controlled surface microstructures would be obtained

in a much easier way. So a general understanding on thechemical strategy for the controllable synthesis of particles withtuning surface microstructures seems important for scientistsinvolved in this field.

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