Materials Letters 63 (2009) 2655–2658

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Materials Letters

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Several shape-controlled TiO2/TiB2 hybrid materials with a combinedgrowth mechanism

Fei Huang a, Zhengyi Fu a,⁎, Aihua Yan b, Weimin Wang a, Hao Wang a, Yucheng Wang a,Jinyong Zhang a, Qinjie Zhang a

a State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR Chinab College of Materials Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China

⁎ Corresponding author. Tel.: +86 27 87662983; fax:E-mail address: zyfu@whut.edu.cn (Z. Fu).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.09.027

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 June 2009Accepted 14 September 2009Available online 20 September 2009

Keywords:Crystal growthMicrostructureHybrid materialsGrowth mechanism

Several kinds of TiO2/TiB2 hybrid materials with different morphologies, including hollow bipyramid struc-ture with truncations, pineapple structure, urchin structure and nanowall structure, have been successfullysynthesized by a facile solvothermal approach in the aqueous solution of ethylenediamine. The effect ofethylene ediamine on the shape change of the final products was investigated in detail. With the increaseof ethylenediamine, anatase TiO2 on the TiB2 core is gradually evolved from nanoparticles, nanorods tonanosheets. Based on attachment theory, Oswald ripening phenomenon, chelating effect and Kirkendalleffect, a possible formation mechanism for such TiO2/TiB2 hybrid materials was proposed to explain themorphology evolution.

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1. Introduction

Anatase titaniumdioxide (A-TiO2) has been extensively investigatedfor its broad application in the field of photocatalysis [1], heterogeneouscatalysis [2], chemical sensor [3], and solar energy conversiondevice [4].In general, the intrinsic properties of A-TiO2 are strongly dependent onthe choice of morphology, microstructure and crystallite size, as well assynthesized condition [5,6]. Therefore, a great deal of researching workhas been concentrated on valence bond theory, thermodynamics orsurface chemistry. However, for some hybrid systems with complexhierarchical structure, these theories limit the general understanding ofthe microstructure and possible growth mechanism. Recently, severalgroups synthesized some A-TiO2 nanostructure based on Ostwaldripening theory, Kirkendall effect or attachment theory [7–9]. However,above expatiation is based on a single theory. For some complex A-TiO2

micro/nanoarchitectures or binary hybrid systems, it is hard tomake anunconvincing explanation using only a single theory.

On the other hand, binary hybrid materials have attracted sub-õstantial attention since such materials combine the properties ofeach unit and provide a great deal of opportunity to explore theirnovel properties or enhanced properties [10,11]. However, currentinvestigation of binary A-TiO2 hybrid materials mainly focuses onA-TiO2/metal oxides ceramic and A-TiO2/organic materials [12,13].So far A-TiO2/nonoxide ceramic hybrid material has been scarcely

investigated. Herein we report a facile solvothermal route to fab-ricate several new classes of TiO2/TiB2 hybrid materials. A com-bined crystal growth mechanism is proposed to explain theformation process of such TiO2/TiB2 hybrid materials.

2. Experimental

In a typical procedure, TiB2 (99.6%, 0.746 g) and HF (30%, 2 g) wereadded into the aqueous solution of ethylenediamine (EDA) with aVH2O/VEDA of (40−x): x, respectively (Here x=0 ml, 2 ml, 4 ml, and8 ml). Then the mixed solution was transferred into an autoclave witha 50 ml teflon-line, followed by sealing, maintaining at 140 °C for 24 hand naturally cooling down to room temperature. After centrifuga-tion, the collected precipitate was washed with deionized water andabsolute ethanol for several times, respectively. The final productswere dried under vacuum at 80 °C for 12 h.

The morphologies and structures of synthesized products wereinvestigated by FESEM (Hitachi S-4800, Japan) and TEM (Tecnai G2 20,Holland). The composition of the products was analyzed by X-raydiffraction (XRD, Rigaku Ultima П, Japan) with Cu Kα irradiation andhigh-resolution transmission electronmicroscopy (HRTEM, JEOL 2100F,Japan).

3. Results and discussion

3.1. Morphology and structure

Fig. 1 presents a set of typical FESEM images of the samplessynthesized at 140 °C for 24 h. It is clearly observed that the EDA plays

Fig. 1. FESEM images of the samples synthesized using different amounts of EDA: a) x=0 ml; b) x=2ml; c) x=4 ml; d) x=8 ml. The insets are the corresponding high-magnification images.

Fig. 2. FESEM and TEM images of the samples treated under strong ultrasonic wave. a) bipyramid structure with truncations; b) pineapple structure; c) urchin structure; andd) nanowall structure.

2656 F. Huang et al. / Materials Letters 63 (2009) 2655–2658

Fig. 3. a) XRD patterns of the samples synthesized at 140 °C for 24 h with differentamounts of EDA; and b) HRTEM image of urchin sample.

2657F. Huang et al. / Materials Letters 63 (2009) 2655–2658

a key role in controlling the morphology of final products. WithoutEDA, only regular bipyramid structures with 700–900 nm in heightare observed (Fig. 1a). With different amounts of EDA (2 ml, 4 ml and8 ml, respectively), the morphologies are gradually evolved from pine-apple structure, urchin structure to nanowall structure (Fig. 1b–d).To further confirm the inner structure, all the samples were treatedunder strong ultrasonic wave. Interestingly, hollow structure presentsin bipyramid sample and pineapple sample after strong ultrasonictreatment (Fig. 2a and b),which is evidently different from the previousreport [6]. However, TEM images show that urchin structure and nano-wall structure are of a core/shell structure (Fig. 2c–d).

Fig. 4. The schematic illustration for the formation mechani

XRDpatterns show that all the samples can be indexed as TiO2/TiB2hybrid materials (Fig. 3a). The main phase for bipyramid structure isA-TiO2 (JCPDS 84-1286) with minor TiB2 (JCPDS 75-1045), while themain phase is TiB2 with little A-TiO2 for pineapple structure, urchinstructure and nanowall structure. The difference could be ascribed tothe special structure. For bipyramid structure, unreacted TiB2 isembedded by A-TiO2 completely. It should be noted that the peaksbecome broader and weaker when the addition of EDA is over 8 ml.The results could result from the size effect of nanowall and thespecies of EDA, respectively. High-magnification TEM image showsthat the distance between the lattice planes parallel to nanorod is0.349 nm, which corresponds to the (101) d-space of A-TiO2, in-dicating that the urchin structure is actually a kind of heterostruc-ture. That is, high-density A-TiO2 nanorods grow on the surface ofTiB2 (Fig. 3b).

3.2. Formation mechanism

According to above experimental results and the reactions fromEqs. (1)–(4) [14,15], here a combined model is proposed as follows(Fig. 4). Without EDA, precipitation of [TiF6−n(OH)n]2− hydrousoxide attaches each other at random, accompanied by the presenceof dipyramid structure because such structure needs minimum totalGibbs free energy [6,9] (to see complementary Fig. 1S). That is, theprocess is accompanied by attachment and Ostwald ripening. Thehollow structure could be attributed to a mechanism analogous toKirkendall effect. During the dehydrolyzation reaction, an inwarddiffusion flow of HF molecules and an outward flow of H2O moleculesare necessary. Due to its slow reaction rate, the total reaction isdetermined by Eq. (1). So HF molecules diffuse slowly inwards. Whilethe rapid reaction rate in Eq. (4) makes formed products of H2Omolecule quickly diffuse outwards. Therefore, the faster outward dif-fusion results in a net outward diffusion and the formation of a hollowstructure.

2TiB2 þ 5O2 þ 16HF ¼ 2H2½TiF6� þ H2O þ HBF4 þ 3H3BO3 ð1Þ

½TiF6�2− þ nH2O↔½TiF6−nðOHÞn�2− þ nHF ð2Þ

½TiF6−nðOHÞn�2− þ ð6−nÞH2O↔½TiðOHÞ6�2− þ ð6−nÞHF ð3Þ

Oxolation: Ti–OH þ Ti–OH ¼ Ti–O–Ti þ H2O ð4Þ

½TiF6�2− þ nH2O þmðenÞ ¼ ½TiF6−nðOHÞnðenÞm�2− þ nHF ð5Þ

(Here en represents EDA).

sm of bipyramid and urchin TiO2/TiB2 hybrid materials.

2658 F. Huang et al. / Materials Letters 63 (2009) 2655–2658

In our case, the reaction is processed in an acidic environment. Thatis, the concentration of [TiF6−n(OH)n]2− ions is determined by the pHvalue. As a buffering solution and chelating reagent, EDA could result inrapid increase of the pH value, which weakens the stability of theattachment [14,15]. Consequently, the disrupted electrostatic balancebetween [TiF6−n(OH)n]2− ions makes it hardly close with each other.Consequently, the small particles loosely pile together and formpineapple structure. Moreover, the increased pH value results in thelow reaction according to Eq. (1) so that the small amount of H2Omolecules during oxolation could be drained in time. Therefore, thedifference of the diffusion between HF and H2O molecules becomesneglected. That is, EDA weakens the difference between inwarddiffusion and outward diffusion. Finally, the hollow structure decreasesor disappears. This could be the reason why urchin structure andnanowall structure are solid.

With further addition of EDA, a possible chelation between EDA and[TiF6−n(OH)n]2− ions becomes dominated though the attachmentbecomes weaker (Eq. (5)). Enough EDA can provide good chelation in aplane due to its dual active amine. Finally, nanorod is easily evolved tonanosheet, which is in well agreement with Barnard's report [5]. Withthe increase of EDA species, the amorphous nature in XRD patternfurther confirms the model.

4. Conclusions

In summary, we have successfully synthesized a series of TiB2/TiO2

hybridmaterials with differentmorphologies via a simple solvothermal

route. Themorphology canbemodifiedby controlling the concentrationof EDA solvent. A possible formation mechanism has been proposedusing attachment theory, chelating effect, Ostwald ripening effect andKirkendall effect to explain such sequential shape changes.

Acknowledgements

The work was supported by financial support from the NationalNature Science Foundation of China (50772081, 50821140308) andthe Ministry of Education of China (PCSIRT0644).

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