Materials Research Bulletin 45 (2010) 329–332

Assembly of nanoscale building blocks at solution/solid interfaces

Fei Liu, 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

A R T I C L E I N F O

Article history:

Received 4 June 2009

Received in revised form 15 July 2009

Accepted 17 July 2009

Available online 16 December 2009

Keywords:

A. Nanostructures

A. Oxides

B. Semiconductors

B. Chemical synthesis

B. Crystal growth

A B S T R A C T

A chemical strategy has been purposely designed to hierarchically assemble nanoscale building blocks at

the interface of solution/solid. Typically, a solution containing precursor of one component and a metal

foil as metal source of another component were employed, on the basis of proposed chemical reactions

on expected interfaces. Proper reaction parameters including temperature, pH value etc. were selected to

adapt both chemical reactions in solution and on the metal surface. Consequently, at the interface of

solution and metal foil, two kinds of nanoscale building blocks deposited simultaneously leading to the

current hierarchical assembly. This strategy has been applied to the fabrication of a series of functional

materials, including Nb2O5/TiO2, Nb2O5/LiF and ZnO/Co3O4. The current strategy provides a convenient

one-step route to achieve complex functional structures, which may have potential applications in a

variety of fields such as solar cells, Li-ion batteries, electrochemical supercapacitors, catalysts as well as

chemical, gas, and bio-sensors.

� 2009 Elsevier Ltd. All rights reserved.

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1. Introduction

The generation and fabrication of nanoscopic structures are ofcritical technological importance for future applications in areassuch as nanodevices and nanotechnology. Much effort has beendirected to the bottom-up synthesis of well-defined nanomaterialswith precisely controlled dimensions. Ordering nanoscale buildingblocks such as nanorods, nanotubes and nanoparticles intocomplex structure is a crucial step to harvest the full potentialapplication of nanomaterials, and represents a significant chal-lenge in the field of nanoscale science [1–8]. Heterogeneousnanostructures are particularly expected to have complex devicefunctionalities due to their diverse properties, which can betailored by fine-tuning the morphology, composition, and organi-zation pattern of primary building blocks [9–12]. Various methods,such as vapor–solid growth [13], solution-based reactions [14],and templating techniques [15] have been applied to achieve theassembly of these building blocks. Despite this progress, it stillremains a challenge to fabricate complex morphologies withcontrolled structures by simple, economical and environmentallyfriendly protocols.

Herein, we demonstrate a facile one-step wet chemical route toassemble different nanoscale building blocks at solution/solidinterfaces. All these structures have novel geometrical architec-

* Corresponding author.

E-mail address: dfxue@chem.dlut.edu.cn (D. Xue).

0025-5408/$ – see front matter � 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2009.12.009

ture. The method used here provides an effective approach forcomplex architecture fabrication and is evidently to be a generalroute.

2. Experimental

All products were synthesized under hydrothermal condition,employing a precursor solution of one component and a metal foilas the source materials for another component. As an example, forthe synthesis of Nb2O5/TiO2 structures, a niobium foil(1 cm � 1 cm) was carefully cleaned with absolute alcohol anddeionized water, respectively, in an ultrasonic bath to removesurface impurities, and then purposely fixed at the bottom of a30 mL Teflon-lined stainless steel autoclave (used as Nb source andsubstrate for harvesting samples). 0.1–0.2 g fresh TiO2 (preparedby hydrolysis of TiCl4) was dissolved in hydrofluoric acid solution,the solution was transferred into the autoclave. The autoclave wasthen filled with an appropriate amount of water up to 70% of thetotal volume and kept in an electric oven at 160–180 8C for 18–24 h. After cooling down to room temperature naturally, theproducts on the foil were collected and washed several times withdistilled water to remove impurities. The final samples were driedat 60–80 8C (more than 4 h). Nb2O5/LiF structures were obtainedby replacing the TiO2 precursor solution with LiF suspension liquidand ZnO/Co3O4 structures were fabricated by using Zn foil andmixture solution of NaOH and Co(NO3)2. The synthesized productswere characterized using X-ray powder diffraction (XRD, D/Max2400, Rigaku Corp., equipped with graphite monochromatized

Fig. 1. Illustration of the assembly of nanoscale building blocks at solution/solid interfaces.

F. Liu, D. Xue / Materials Research Bulletin 45 (2010) 329–332330

CuKa radiation), scanning electron microscope (SEM, JSM-5600LV,JEOL, operated at 20 kV).

3. Results and discussion

The one-step assembly of nanoscale building blocks at solution/solid interfaces performs in a spontaneous and self-organizedmanner. Fig. 1 is an illustration of the current formation process. Inthe reaction system, metal foil is employed as cation source of onekind of building blocks, and the other kind of building blocks isprecipitated from the solution. Particles formed in the solutiondeposited at the interface of the solution and metal foil, by thehydrothermal treatment, while metal ions released from the metalfoil condensed and nucleated on the existing particles deposited atthe interface, finally grown into heterogeneous nanostructures.

The morphologies of these products were examined by SEM. Atypical low-magnification SEM image (Fig. 2a) indicates that a highyield and good uniformity of products can be achieved in ourcurrent reaction conditions. Fig. 2b shows typical SEM images ofNb2O5/TiO2 heterogeneous structures, these novel structures areregularly assembled by a square plate like TiO2 core and four

Fig. 2. SEM images of Nb2O5/TiO2 heterogeneous structures

bundles of well-aligned secondary nanowires. The nanowire arraysstand perpendicularly to the side surfaces of the TiO2 plate. The 4-fold symmetry feature can be clearly observed from Fig. 2b, eachnanowire bundle is perfectly perpendicular to its neighbouringones within the same core. We propose that the symmetry featureis determined by the geometry of TiO2 plate. Fig. 2c and d showsSEM images of the products obtained at shorter reaction times (5 hand 12 h, respectively). From Fig. 2c, the square plate like TiO2

cores are clear to be seen, and nanowires begin to grow at the edgesof these cores. At a reaction time of 12 h, Nb2O5/TiO2 heteroge-neous structures with short nanowires can be obtained as shown inFig. 2d, a 24 h hydrothermal reaction leading to the final product(Fig. 2a).

To confirm the fact that the core material is TiO2 and thenanowires are composed of Nb2O5, a comparative experiment hasbeen done. By keeping other reaction parameters unchanged whilefixing the Nb foil standing vertically in the autoclave to avoid thedeposition of TiO2 on the foil, pure Nb2O5 nanowires can beobtained on Nb foil. The SEM image of these nanowires has beenshown in Fig. 3a, and the XRD pattern (Fig. 4a) verified thesenanowires are pure pseudo-hexagonal phase Nb2O5 (JCPDS card

at the reaction time of (a, b) 24 h, (c) 5 h and (d) 12 h.

Fig. 3. SEM images of (a) pure Nb2O5 nanowires and (b) pure TiO2 plates.

Fig. 4. XRD patterns of (a) pure Nb2O5 nanowires, (b) pure TiO2 plates, (c) Nb2O5/

TiO2 heterogeneous structures; standard XRD patterns of (d) pseudo-hexagonal

Nb2O5 and (e) anatase TiO2.

F. Liu, D. Xue / Materials Research Bulletin 45 (2010) 329–332 331

no. 28-0317). Products collected at the bottom of autoclave arepure anatase TiO2, as shown in Fig. 4b (JCPDS card no. 21-1272),and SEM image in Fig. 3b indicated they are single crystals with 4-fold symmetry, which is consistent with the final products.According to the XRD pattern of the final products (Fig. 4c), it isapparent that all the peaks can be indexed to the anatase andpseudo-hexagonal phase Nb2O5, therefore, the current results canconfirm that the core is anatase and the nanowires are Nb2O5.

Fig. 5 shows the heterogeneous structures of Nb2O5/LiF andZnO/Co3O4 obtained by the similar method, which also presented a

Fig. 5. SEM images of (a) Nb2O5/LiF and (b)

high organization and good symmetry. From Fig. 5a, we can seethat Nb2O5 nanosheets are perpendicularly grown on cubic LiFsurfaces, and as seen in Fig. 5b, heterogeneous structurescomposed of ZnO nanowires and Co3O4 nanoplates can also befabricated, by the Zn foil and mixture solution of NaOH andCo(NO3)2. These results demonstrated that the present strategy isversatile and may be extended to many other systems.

The formation of heterogeneous structures at the solution/solidinterfaces can be regarded as the nucleation of one component onthe surface of another component. Generally, two kinds ofnucleation are involved in the precipitation process, homogeneousnucleation and heterogeneous nucleation. Homogeneous nucle-ation takes place in the condition that no foreign nucleating aidsare involved [16–23]. Heterogeneous nucleation involves foreignsubstances as nucleating aids on which the nucleation occurs.Heterogeneous nucleation generally occurs with much moredifficulties. The creation of nuclei is an energy consuming process,because it brings new phase, and generate an interface at theboundary of new phase, heterogeneous nucleation can take placeat significantly lower supersaturation than homogenous nucle-ation, because some energy is released by the partial destruction ofexisting interface.

For the fabrication of these heterogeneous structures, heteroge-neous nucleation process is mostly involved, nucleation and particlegrowth are firstly taken place in solution, forming one kind ofbuilding blocks and directly deposited at the interface of substrateand saturated solution. Because the heterogeneous nucleation ismuch easier to take place, the newly forming species are morereadily to nucleate on the surface of existing particles which areprecipitated from solution. This crystallization phenomenon leads to

ZnO/Co3O4 heterogeneous structures.

F. Liu, D. Xue / Materials Research Bulletin 45 (2010) 329–332332

preferentially formation of heterogeneous structures at the solution/solid interfaces.

4. Conclusion

In summary, we have developed a facile one-step solution-basedsynthetic route to hierarchically assemble different nanoscalebuilding blocks at the solution/solid interfaces. To illustrate thevalidation of the current strategy, heterogeneous structuresincluding Nb2O5/TiO2, Nb2O5/LiF and ZnO/Co3O4 were, respectivelyfabricated. The products obtained herein presented a highorganization and good symmetry. The current strategy provides aconvenient chemical route to achieve complex functional structuresin nano- to microscale, which may have potential applications in avariety of fields. This chemical strategy would be extended to othersystems for the fabrication of functional heterogeneous structures.

Acknowledgment

The financial support of the Natural Science Foundation ofChina (Grant No. 50872016) is gratefully acknowledged.

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