Novel Nanocrystal Heterostructures: Crystallographic-OrientedGrowth of SnO2 Nanorods onto r-Fe2O3 Nanohexahedron

Mutong Niu, Yao Cheng, Yuansheng Wang,* Lifeng Cui, Feng Bao, and Lihua Zhou

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,Chinese Academy of Sciences, and Graduate School of Chinese Academy of Sciences,Fuzhou, Fujian 350002, China

ReceiVed October 24, 2007; ReVised Manuscript ReceiVed January 11, 2008

ABSTRACT: The novel SnO2/R-Fe2O3 semiconductor nanocrystal heterostructures (SNCHs) were fabricated by virtue of thecrystallographic-oriented growth of the SnO2 nanorods onto the R-Fe2O3 nanohexahedrons via a reverse microemulsion route. Thevariation of the exposure surfaces of the R-Fe2O3 precursor crystals was found to result in the change of the preferential crystallographicorientation of the SnO2 nanorods. The investigations on the structural characteristics and the morphological evolution of the SnO2/R-Fe2O3 SNCHs suggested that the heteronucleation and homonucleation of SnO2 occurred simultaneously at the early stage of thereaction, and the SnO2/R-Fe2O3 SNCHs were then well-formed by a mechanism of Ostwald ripening of the heteronucleated SnO2

particles. The “lattice dislocations” generated at the SnO2/R-Fe2O3 interfaces took the role to relieve the interfacial strain and thusfacilitated the formation of the definite interfaces.

1. Introduction

Semiconductor nanocrystal heterostructures (SNCHs) consist-ing of two or more chemically distinct components haveattracted increasing attention recently for their great potentialitiesto revolutionize nanomaterial research by providing a meansto define multiple functionalities within a single nanostructure.1

In recent years, some one- and two-dimensional SNCHs, suchas nanowires with either radially2 or axially3 modulatedcompositions, core–shell4 nanoparticles, and some dot-rod5 orrod-rod6 heterostructures, etc., have been successfully fabricated.

However, studies on the control of the growth mode of thesecondary phases on the precursor crystals for the SNCHs arestill lacking for the time being.7 The significance of this subjectis due to the knowledge that the properties of the single-component nanocrystals are very sensitive to their shape, sizeand assembling organization.8 Therefore, with elaborate alteringthe growth mode of the secondary phases, the complex fun-ctionalities of the SNCHs are expected to be tailored or enriched.

Both Fe2O3 and SnO2 crystals, the important wide band gapsemiconductors, have been intensively investigated as gas sen-sors,9 electrode materials,10,11 catalysts,12,13 and so on. Recently,Yan’s group prepared a kind of SnO2/R-Fe2O3 SNCHs,7 withSnO2 rods about 100 nm in diameter and several micrometerssin length grew perpendicularly into dense arrays on the surfacesof the R-Fe2O3 nanotubes. The SnO2 was found to offer the{101} planes to form interfaces with the {110} planes of theR-Fe2O3 nanotubes resulting from the lower lattice mismatchat the interface. In this work, we employed the R-Fe2O3

nanohexahedrons13 as precursors, which might provide thedistinct exposure surfaces of {012} planes to form the interfaceswith SnO2, and tried to explore how the nucleation and growthmodes of SnO2 would be affected. Interestingly, we found thatalthough the identical {101} planes of the SnO2 rods are chosento form the interfaces with the R-Fe2O3 precursors, they grewslantwise to the interfaces forming a morphologically andstructurally novel SnO2/R-Fe2O3 SNCHs. Observations on thestructural and morphological evolutions of the reaction product

indicated that the SNCHs were formed via the Ostwald ripeningof the heteronucleated SnO2 particles. These results not onlyenriched the SNCH species, but also displayed the possibilityto elaborately design and control the SNCH structure via alteringthe crystallographic growth mode of the secondary phase onthe precursor crystals.

2. Experimental Section

2.1. Synthesis. The R-Fe2O3 nanohexahedrons were prepared by asolvothermal method reported in our previous work.13 0.01 g of theR-Fe2O3 nanohexahedrons were dispersed into a reverse microemulsionsolution containing n-octane (10 mL), n-pentanol (3.0 mL), sodiumdodecyl sulfate (SDS) (1.44 g), and Sn(OH)6

2- (VSnCl4,0.5mol/L:VNaOH,5.0mol/L ) 1:2, 2.0 mL). After ultrasonication for 10 min, themixture was transferred into a 25 mL Teflon-lined autoclave and heatedat 220 °C for 6 h. When cooled to the room temperature, the resultingprecipitates were collected by centrifugation, washed several times withthe deionized water and absolute ethanol, and finally dried in a vacuumat 40 °C for 4 h. SnCl4 ·5H2O and n-octane were purchased fromSinopharm Chemical Reagent Co. Ltd., China; SDS and n-pentanolfrom Tianjin Fuchen Chemical Reagent Plant, China. All Reagents wereof analytical grade and used as received.

2.2. Structural Characterization. To investigate the microstructureof the products, we recorded the powder X-ray diffraction (XRD)patterns of the as-synthesized samples by a RIGAKU-DMAX2500X-ray diffractometer using Cu KR radiation (λ ) 0.154 nm) at ascanning rate of 5°/min for 2θ ranging from 5 to 85°, and thetransmission electron microscopy (TEM) and high-resolution transmis-sion electron microscopy (HRTEM) observations were carried out bya JEM-2010 transmission electron microscope working at 200 kV. Ina brief procedure for TEM specimen preparation, powder samples weredispersed in ethanol by ultrasonic agitation, and a small volume of thesuspension was dropped on the carbon-enhanced copper grids and driedin air.

3. Results and Discussion

3.1. Structure Characteristics of the SnO2/r-Fe2O3 SN-CHs. The XRD pattern of the as-synthesized sample is presentedin Figure 1a, which identifies the reaction product as a mixtureof the hexagonal R-Fe2O3 (with lattice constants of a ) b )0.504 nm, c ) 1.375 nm, PDF 860550) and the tetragonal SnO2

(with lattice constants of a ) b ) 0.475 nm, c ) 0.319 nm,PDF 770450). Compared to those of SnO2, the peaks of R-Fe2O3

* Corresponding author. Tel: 86-591-8370-5402. Fax: 86-591-8370-5402.E-mail: yswang@fjirsm.ac.cn.

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10.1021/cg701048d CCC: $40.75 2008 American Chemical SocietyPublished on Web 04/16/2008

are quite weak, similar to the result reported by Yan,7 probablydue to the coverage of SnO2 nanorods on the surfaces of R-Fe2O3

nanohexahedrons to form the SnO2/R-Fe2O3 SNCHs, which isrevealed by TEM observation shown in Figure 1b. The inset ofFigure 1b offers more morphological details of the SnO2/R-Fe2O3 SNCHs that several SnO2 nanorods, with 8-15 nm indiameter and 30-50 nm in length, slantwise stand on the edgesand corners of the R-Fe2O3 nanohexahedron sized about 30–50nm, ascribing to the higher surface energy of these sites thanthe others.

The HRTEM image of the SnO2/R-Fe2O3 SNCHs, shown inFigure 2, clearly demonstrates the structural characteristics ofthe SNCHs. It had been demonstrated in our previous paper13

that the R-Fe2O3 nanohexahedron is enclosed by six identicalplanes indexed as {101} in the trigonal system (herein, in orderto simplify the analysis, it is transformed to {012} in thehexagonal system), with an interplanar distance of 0.368 nm.On the HRTEM image, the {101}, {11j2}, and {110} planes ofthe SnO2 crystals are also marked according to the fast Fouriertransform (FFT) analysis. The {101} planes are parallel to the

SnO2/R-Fe2O3 interface, indicating that SnO2 nanorods providethe {101} planes to form the interfaces with the {012} planesof the R-Fe2O3 precursor. However, the preferential growthdirection of the SnO2 nanorods is along the normal of the {11j2}planes, with a degree of about 65° to the interface as indicatedin Figure 2, which is different from Yan’s result,7 where theSnO2 nanorods grew along the normal of the {101} planes andwere perpendicular to the interface. It is noticed that in the XRDpattern of the as-prepared SnO2/Fe2O3 SNCHs sample, shownin Figure 1a, the relative intensity of the {112} peak is muchstronger than that in the standard SnO2 XRD pattern from thePDF card 770450, which also implies the preferential growthof SnO2 nanorods along the normal of {112}, consistent withthe TEM observations. The distinct exposure surfaces of theR-Fe2O3 precursors in the two experiments are believed to bethe origin of this difference. Detailed observation to the inter-face of SnO2/R-Fe2O3 SNCHs reveals that most of {110} latticefringes of SnO2 consecutively join with the {012} ones ofR-Fe2O3, forming a definite interface (i.e., no alloyed interme-dium is formed to connect the different phases14) with “latticedislocations” to relieve the interfacial strain. The interplanarmismatch between {101}SnO2 and {012}R-Fe2O3 is about 9.9%;therefore, every 12 lattice planes in SnO2 joining with 11 latticeplanes in R-Fe2O3 form a repeating unit with one “dislocation”to modify the lattice strain, as illustrated by the white lines andthe right inset in Figure 2. It is worth noticing that theobservations on the interfacial structure of other SnO2/R-Fe2O3

SNCHs in the sample gave similar results.

3.2. Structure and Morphology Evolution of the Reac-tion Products. In order to track the formation process of theSnO2/R-Fe2O3 SNCHs, time-dependent experiments were carriedout. Figure 3a-3d are TEM micrographs of the products atdifferent reaction stages, which present the structural andmorphological evolution from R-Fe2O3 nanohexahedrons toSnO2/R-Fe2O3 SNCHs. When reacted for 0.5 h, only bareR-Fe2O3 nanohexahedrons with smooth surfaces were observed,as shown in Figure 3a. After reaction for 1.5 h, many SnO2

particles sized 5 nm appeared in the sample as the result of thedehydration of Sn(OH)6

2-, some of them heteronucleated onthe surfaces of the R-Fe2O3 nanohexahedrons, while othershomonucleated in the solution. HRTEM images e and f in inFigure 3 exhibit, respectively, the detailed structures of theheteronucleated and homonucleated SnO2 particles. The simul-taneous occurrence of hetero and homonucleation of SnO2 inthe solution is surprising since some authors held the opinion

Figure 1. (a) XRD pattern of the SnO2/R-Fe2O3 SNCHs, the arrows indicate the peaks from the R-Fe2O3 nanohexahedrons, whereas the indexedones are from SnO2 nanorods; the inset shows a standard SnO2 XRD pattern from PDF card 770450; (b) TEM micrograph of the SnO2/R-Fe2O3

SNCHs, the inset shows the magnified SNCHs.

Figure 2. HRTEM image showing the crystallographic-orientatedgrowth of SnO2 nanorods on an R-Fe2O3 nanohexahedron forming theSNCHs. The bottom inset is the FFT pattern of the squared area in aSnO2 rod, and the right inset is the magnified image showing a“dislocation” at the SnO2/R-Fe2O3 interface.

1728 Crystal Growth & Design, Vol. 8, No. 5, 2008 Niu et al.

that the homonucleation must be prevented, otherwise the systemwill result in the single-component crystals instead of SNCHs.15,16

When the reaction was further prolonged to 3 and 5 h, theheteronucleated SnO2 particles grew into nanorods and formedthe SnO2/R-Fe2O3 SNCHs shown in images c and d in Figure3, whereas the number of the homonucleated SnO2 particlesdramatically decreased and eventually vanished when thereaction time reached 6 h (see Figure 1b). It is known that thegrowth of nanocrystals in solution follows two primary mech-anisms: one is Ostwald ripening and the other oriented attach-ment.17 The SnO2 nanorods at different reaction stages all exhibitsmooth surfaces without “bottleneck”, which is the typicalcharacter of the particle–particle bonding;18 furthermore, theirlattices are perfect without any dislocations that should beinvolved in the process of oriented attachment among particlesbecause of small misorientation or atomic nonflatness at thebonding interface.19 Therefore, the growth mechanism for theSnO2 nanorods in this experiment is believed to be Ostwaldripening of the heteronucleated SnO2 particles at the expenseof the homonucleated ones.

Under similar reaction conditions but without the directingof R-Fe2O3 precursors, the growth of tiny SnO2 nanoparticlesinto nanorods had been found to follow the oriented attachment

mechanism.20 The Ostwald ripening growth of SnO2 in ourexperiment is thought to be attributable to the strong interactionat the SnO2/R-Fe2O3 interfaces where the {110} planes of theheteronucleated SnO2 particles join consecutively with the {012}ones of the R-Fe2O3 nanohexahedrons, which stabilizes theheteronucleated SnO2 nanoparticles. Therefore, the Ostwaldripening of the heteronucleated SnO2 particles at the expenseof the homonucleated ones with relatively higher chemicalpotential is favorable. The reason why the SnO2 particles“coarse” as nanorods onto the R-Fe2O3 hexahedrons is asfollows: the (110) plane is the plane with lowest surface energyon SnO2; therefore, when the (110) plane was induced becauseof the iron oxide hexahedrons, a thermodynamically favorablegrowing process of the SnO2 nanorods may be expected.

4. Conclusions

The morphologically and structurally novel SnO2/R-Fe2O3

SNCHs, of several SnO2 nanorods slantwise standing on thesurfaces of the R-Fe2O3 nanohexahedrons, were fabricated viaa facile reverse microemulsion route. Study on the formationprocess of the SNCHs indicates that the hetero- and homo-nucleation of SnO2 occurred simultaneously at the early stageof the reaction, and the SnO2/R-Fe2O3 SNCHs were formedthrough Ostwald ripening of the heteronucleated SnO2 particlesat the expense of the homonucleated ones.

Acknowledgment. This work was supported by the NationalNatural Science Foundation of China (50672098), the MajorScience & Technology Project of Fujian Province (2005HZ01-1, 2007HZ0002-2), and the projects of the Chinese Academyof Sciences (KJCX2-YW-M05) and FJIRSM (SZD07004).

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CG701048D

Figure 3. TEM micrographs of the products at the different reactionstages: (a) reacted for 0.5 h; (b) reacted for 1.5 h; (c) reacted for 3 h;(d) reacted for 5 h; (e) enlarged HRTEM image from the circled areain b showing the detailed configuration of the SnO2/R-Fe2O3 SNCHs;(f) enlarged HRTEM image of the homonucleated SnO2 nanopaticlesafter reaction for 1.5 h.

Growth of SnO2 Nanorods onto R-Fe2O3 Nanhexahedron Crystal Growth & Design, Vol. 8, No. 5, 2008 1729