Oriented organization of shape-controlled nanocrystalline TiO2

Wenjun Dong, Guangsheng Pang, Zhan Shi, Yaohua Xu, Haiying Jin,Rui Shi, Jingjing Ma, Shouhua Feng*

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130023, China

Received 6 February 2003; received in revised form 13 October 2003; accepted 25 October 2003

Abstract

The self-assemble propensity of the controlled shape and size distribution titania nanoparticles makes the

particles oriented organization to ordered structures without substrate by a hydrolysis-hydrothermal route. The

procedure offers the possibility of a generalized approach to the production of patterned organization of single

and complex oxide nanoparticles with size and morphology tunable.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; A. Oxides; D. Photoluminescence

1. Introduction

The oriented organization of nanocrystals has shown considerable challenges for constructingfunctional ordered structures with size-dependent properties in applications to the microelectric,magnetic and optical devices [1–5]. Current studies on the oriented organization of nanoparticlesemphasize on finding effective and simple methods in specific functional systems. To date, a method forcontrolling the rate of evaporation of the solvent was reported in which the TiO2 nanoparticles film wasorganized on the surface of silicon wafer or glass [6]. A reverse micelle route for controlledorganization of BaCrO4 was proposed by Mann et al. [7], and more recently, a sol–gel process for theself-organization of BaTiO3 nanoparticles was developed by O’Brien et al. [8]. A number of reports onthe prepared nanoparticles to build up a network for interactions were also reported for years [9]. Dueto the easy precipitation and difficulty in the shape control for the TiO2 nanoparticles, the orientedorganization of TiO2 nanoparticles without substrates is still an unsolved problem. Our motivation is

Materials Research Bulletin 39 (2004) 433–438

* Corresponding author. Tel.: þ86-431-849952; fax: þ86-431-5671974.

E-mail address: shfeng@mail.jlu.edu.cn (S. Feng).

0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2003.10.012

searching for an effective technique to construct the oriented organization of nanoparticles, which isbased on creating an interfacial interaction by chemical modification on the surfaces of nanoparticles.Chemically, the surfactants may modify to some degree the surfaces of nanoparticles, which providesthe protection of further crystal growth and precipitation as well as the possible interaction such asthe interdigitation. The interfacial activated nanoparticles may be linked in some ways to produce theordered structure. The force of the surfactant molecules attached to specific crystal faces of thenanoparticle is expected [6]. Here, we report a hydrolysis-hydrothermal preparation method associatedwith the use of the surfactant to create the oriented organization of the shape-controlled TiO2

nanoparticles without substrate.

2. Experimental

The preparation of a TiO2 ordered structure was based on the combination of the hydrolysis andcrystallization of titanium n-butoxide (Ti(OR)4) (Beijing Reagent Factory, China) in the presence ofprimary amines (Aldrich), and followed by the hydrothermal crystallization at 180 8C. The molar ratiosof hexadecylamine (C16H33NH2) to Ti(OR)4 were varied from 1/4 to 2. In a typical procedure, 2.41 gC16H33NH2 and 1.70 ml titanium n-butoxide were dissolved in 10 ml absolute ethanol to form asolution, to which 10 ml distilled water was added slowly at 0 8C with vigorous stirring. A whiteviscous precipitate was formed immediately and the precipitate was aged for 24 h at room temperature.The result precipitate was filtered and placed in a Teflon-lined vessel with addition of 10 ml distilledwater. The hydrothermal treatment was carried out at 180 8C for 5 days and then the white powderproduct was obtained.

The morphology and size of TiO2 nanoparticles were observed on a Hitachi-8100IV TransitionElectronic Microscopy (TEM). X-ray powder diffraction (XRD) patterns were obtained using aSiemens D5005 X-ray diffractometer with Cu Ka (l ¼ 1:5418 A) at room temperature. The particlesize was calculated from the X-ray line broadening, using the Debye-Scherrer equation. The thermalgravimetric analysis (TGA) was carried out on a Perkin-Elmer DTA 1700 differential thermal analyzerin air with a heating rate of 10 8C/min. The photoluminescence measurements were performed ona Perkin-Elmer LS 55 luminescence spectrometer at room temperature under the excitation light of240–330 nm.

3. Results and discussion

Highly oriented organization of TiO2 nanoparticles were obtained through hydrolysis and thehydrothermal method. The molar ratio of C16H33NH2 to Ti(OR)4 was 2. Morphology was observed byTEM on the samples collected directly from the as-made white powder product after dried in air andwashed with absolute ethanol on the TEM grids. Fig. 1a shows a TEM image of the chain-like orderedarrays of rod-shaped TiO2 nanoparticles along their short axes. The ordered structure consists of rod-shaped particles with uniform length of 8 � 2 nm and width of 3 � 0:5 nm. The tiled lattice made therod-shaped nanoparticles align in one direction along the short axes. TGA indicated that the sample ofthe ordered structure contained ca. 30 wt.% of the surfactant. The even-separation suggests the organicamine remains bound to the surface of the particles. One possibility is that the network spontaneously

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self-assembled in the autoclave from interactions between the hydrophobic tails of the moleculesabsorbed onto the faces of developing TiO2 nanoparticles, and the interdigitated arrangement ofC16H33NH2 between the adjacent nanoparticles must be made, which was similar to the process thatproposed by Li et al. [7]. The amine molecules, acting as the oriented attachment, were considered tocouple the nanoparticles by self-assembly. Their strong van der Waals interactions in the way ofinterdigitation gave rise to the formation of the ordered structure [6,7]. Another ordered structure wasalso observed during hydrothermal treatment in a Teflon-lined vessel with an addition of 5 ml distilledwater and 5 ml glycol at 180 8C for 5 days, Fig. 1b. The TiO2 nanorods form a hexagonal, close-packedarray. TGA indicated that the sample of the ordered structure contained ca. 40 wt.% of the surfactant,and the white-space observed around the nanorods (Fig. 1b) is probably occupied by organic amine,which is similar to the mechanism proposed by Chemseddine and Moritz [6]. The selected area electrondiffraction pattern of the aggregates (Fig. 1b inset) could be indexed to the cell of anatase-type of TiO2.

The role of the hydrothermal crystallization in the process of the oriented organization structure isessential. XRD of the initial hydrolysate showed amorphous (Fig. 2a), the well-formed material showeda series of Bragg reflections that could be indexed as the cell of anatase TiO2 (Fig. 2b). This shows thatthe crystallization of the initial amorphous hydrolysate TiO2 is essential. The particle sizes estimatedfrom the broadening of the XRD are the same as the TEM for all samples. TEM studies showed the

Fig. 1. TEM images of TiO2 nanocrystals. (a) The chain-like ordered arrays of the rod-shaped TiO2 nanoparticles and the inset

of the enlarged image in the edge of the arrays and (b) the self-assembly of the ordered hexagonal close-packed arrays and the

inset of the electron diffraction pattern of anatase.

Fig. 2. Powder XRD patterns of (a) the amorphous hydrolysate of the initial reaction mixture and (b) the white powder

product of TiO2 nanoparticles.

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TiO2 particles at their final stage in the ordered structure, which implies that TiO2 growth, associatedwith the self-organization of TiO2 nanoparticles, occurs in the hydrothermal crystallization process.

The morphologies of TiO2 nanoparticles depended upon the preparative conditions. The particlemorphologies can be tuned simply by changing the molar ratio of C16H33NH2 to Ti(OR)4. When themolar ratio of C16H33NH2 to Ti(OR)4 was in accord, uniform TiO2 nanoparticles of 10 nm with inter-particle space of ca. 1 nm were separated (Fig. 3a). All nanoparticles appeared in cubic forms. A furtherincrease in the surfactant content to 1.5 led to the formation of the rhombus and rod-like TiO2

nanoparticles (Fig. 3b). When the ratio was 2, almost exclusively rod-like nanoparticles gave orderedchain-like arrays (Fig. 1). When the ratio was 1:3, ca. 20 nm nanoparticles were organized into meso-spheres with diameter nearly 200 nm [10] (Fig. 3c).

With a further decrease in the surfactant content, an increased nanoparticle size and irregularity ofmorphology, the TiO2 nanoparticles gave random patterns (Fig. 3d).

Using different primary amines in the reaction has proven to play a substantial role in theorganization of TiO2 nanoparticles. TiO2 nanocrystals of different patterns have been obtained byadjusting the amine (CnH2nþ1NH2) from C16H33NH2 to Me4NOH. The TEM images show that there isno appreciable difference in the crystal structure, size and shape of TiO2 nanoparticles as a function ofvarious amines, but the ordered structure appeared only when the amine chain length is n ¼ 10–20. Theprimary amine molecules acting as the surfactant in the experiment played an important role in reactingwith the surface of the nanoparticles and coupling the individual TiO2 nanoparticles. It could beproposed that the TiO2 nanoparticles were spontaneously self-assembled under hydrothermalconditions by the interactions of the hydrophobic tails of the amine molecules bound onto thesurfaces of the TiO2 nanocrystallites. The preferential aggregation to form a certain pattern was due to astrong thermodynamic driving force for oriented attachment to reduce overall energy by removingsurface energy associated with unsaturated bonds and freely transferred TiO2 nanoparticles. While theamine chain length n ¼ 10, the short hydrophobic tails could not join the TiO2 nanoparticles into theordered structure.

The effects of crystallization duration and temperature on the morphology and size of TiO2

nanoparticles were examined by fixing the ratio of C16H33NH2 to Ti(OR)4, for the ratio of C16H33NH2

to Ti(OR)4 was 2, as the crystallization time increased from 5 to 15 days, the length of nanorod

Fig. 3. TEM images of the systems with molar ratios of C16H33NH2 to Ti(OR)4 at (a) 1.0, (b) 1.5, (c) 1/3 and (d) 1/4.

436 W. Dong et al. / Materials Research Bulletin 39 (2004) 433–438

increased slightly, but further prolonged hydrothermal reaction time had no evident influence on thesize of the products. When the molar ratio of C16H33NH2 to Ti(OR)4 was in agreement, the reactiontemperature increased from 180 to 240 8C, the diameter of the nanoparticles (cubic) increased from 10to 20 nm accordingly. Thus, crystallization time and temperature have some influence on the size of theTiO2 nanoparticles, but the ordered structures exist at the lower limits of preparation time andtemperature. It was confirmed that the primary amine molecules as an oriented attachment coupled thenanoparticles to the TiO2 ordered structure.

The oriented-organized TiO2 nanoparticles exhibit photoluminescences at 390, 409, 423, 461 and486 nm with an excitation of 265 nm light at room temperature. The luminescence spectrum of TiO2

nanorod ordered structure is shown in Fig. 4. The 390, 409 and 423 nm emissions are the characteristicemissions of anatase titania [11]. The fluorescence bands at 460 and 485 nm were attributed to therecombination of excitons and/or shallowly trapped electron/hole pairs, which caused the band edgeluminescence. Hexadecylamine became the trap center of the exciton binding energy significantly, andinduced the absorption potential, which was similar to the literature [11,12]. Compared with the widefluorescence band near 500 nm at 77 K for bulk TiO2 observed by Deb [12], and those of the ultrafineTiO2 particles, TiO2 nanosheet Crystallites and dye-sensitized TiO2 particles [12–16], the oriented-organized TiO2 nanoparticles gave novel photoluminescences.

4. Conclusion

In summary, we have obtained the patterned anatase TiO2 nanoparticles without substrate by thehydrolysis-hydrothermal combination route in the presence of the surfactant. The size and morphologyof the nanoparticles varied with the preparative conditions. The surfactant coupled the nanoparticles bystrong van der Waals interaction in the way of interdigitation. Our route offers a ready and generalizedproduction of the patterned organization of the single and complex oxide nanoparticles with size andmorphology tunable.

Acknowledgements

This study was supported by the National Nature Science Foundation of China.

Fig. 4. Photoluminescence spectrum of the oriented-organized TiO2 nanoparticles.

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