Preparation and optical properties of one dimensional nano hydroxides and oxides

The Yttrium hydroxide nanotubes and nanorods as well as the ZnO nanorods and nanowires were prepared by different methods.. The growth process of the nanotubes may be performed through

dimensional nano hydroxides and oxides

Lamthi Kieu Giang1, Nguyen Vu1, Dinh Xuan Loc1, Man Hoai Nam1,

Gyu- Chul Yi2, Tran Kim Anh1 and Le Quoc Minh1, 3

1 Institute of Materials Science, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay Distr., Hanoi, Vietnam

Email: anhtk@ims.vast.ac.vn

2

Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790- 784, Korea

3

College of Technology, Vietnam National University, Hanoi, Vietnam

Abstract The Yttrium hydroxide nanotubes and nanorods as well as the ZnO nanorods and

nanowires were prepared by different methods Y(OH)3 nanotubes and nanorods were synthesized by polyol – mediation via the metastable precursor YCl3, PEG and were studied by FE SEM and XRD The influence of the polymer molecular weights of PEG and the temperature were studied in detail These results show that the first stage is nanorods, then it is followed by the formation of nanotubes The outer diameter of the tubes is about

150 nm to 600 nm and interior hollow is in the range between 100 nm to 300 nm The length of the rods and the tubes is up to several micrometers We have found that the synthesis temperature, reaction rate and molecular weight of PEG induce the change in shape of obtained nanotubes from a cylindrical to a hexagonal appearance The growth process of the nanotubes may be performed through three stages: first complexing the PEG polymer with Y(NO3)3, then hydrolysis for formating Y(OH)3 nanorods and finally diffusion-controlled developing of Y(OH)3 nanotubes The ZnO nanorods were prepared by Chemical Vapor Deposition (CVD) and Metal Organic Chemical Vapor Deposition

prepared The nanorods have the diameters of about 50 nm and the length of about 1.12 micrometers The influence of the temperature is investigated The luminescent spectra (excited by Cd-He laser at 325 nm) of ZnO nanorods at 10 K- 300 K was measured and investigated The sharp peak is assigned to an exciton bound to a neutral donor

3

MOCVD, CVD

1 Introduction

Since carbon nanotubes have been discovered in 1991 till now, 1-dimensional (1-D) nanostructural materials have gained much of interests in material science Rather than other low-dimensional nanomaterials like 0-dimensional quantum dots, which have been applied in the manufaction of nano-scale devices (quantum lasers, transistors, sensors, probes etc.), or quantum wells, which have applications mainly in semiconductor physics, 1-D nanostructural materials are

Keywords: nanotubes, nanorods from Y(OH) , ZnO, PEG-polymer mediated synthesis,

(MOCVD) method, respectively Aligned growth of ZnO nanorods has been successfully

accompanied with many peculiar and interesting applications in physics and devices manufacture Due to their intrinsic anisotropic structure, 1-D nano-structures have been of interest in research and development of applications in manufacturing of functional materials, in optoelectronics and electronics, electromechanics, etc [1-5]

There are many methods which have been developed to fabricate 1-D nano-structures like template-directed methods, growth of 1-D nanonano-structures from different phases and/or solutions, self-assembling methods, etc [6, 7] However, many of these methods are costly either in price or in time In this paper, we present the fabrication of 1-D nanostructures of Y(OH)3 –by using a polyol-mediated method We have used the length of CH2 links and hydrogen binding ability of PEG plolymer and dynamic conditions to control the formation process

of 1-D nanostructures of Y(OH)3 The Y(OH)3 – nanotubes could be converted to

Y2O3 by using a special treatment at higher temperature

The oxide lattice has proved to be and excellent host material for some of the most powerful laser built The Y2O3 is characterized by low phonon frequencies which makes non-radiative relaxation of the excited states inefficient ZnO nanorods also provide new opportunities for fundamental studies and applications Especially in the last 10 years, major interests are focused on 1-dimensional nanostructures with hollow interior due to their peculiar and fascinating physical and chemical properties Those are quite anomalous and interesting and show new phenomena in the field of material science The low-dimensional structures such

as nanowires, nanorods, nanotubes or nanobelts have been fabricated and studied

by many research groups ZnO nanorods have enhanced luminescence properties compared to other 1-dimensional nanostructures, and opened novel applications such as making new luminescence devices, new generation of displays and monitors, manufacturing of functional materials, new kinds of biosensors, emission labels in biology etc In this paper we present two nanostructures of current interest The first part deals with 1-D nanostructures of Y(OH)3 The second part is about the preparation and the optical properties of ZnO nanorods and ZnO nanowires, which were prepared by Chemical Vapor Deposition (CVD)

and Metal Organic Chemical Vapor Deposition (MOCVD)

2 Experiment

All used chemicals like NaOH, PEG with different molecular weight, Y(NO3)3, without further purification Deionized water was used throughout The micro autoclave instrument (MMM Med center Einrichtungen GmbH) allows the control

of all heating processes by a programme in time and temperature; the minimum increment/decrement is 1°C

The synthesis of Yttrium hydroxide nanorods and nanotubes were based on the preparation of colloidal hydroxide precipitates at room temperature and the

to 50h

Yttrium Nitrate and PEG (Mw = 4000) are mixed up and added by water with a desired concentration The obtained mixture then is stirred continuously for 1h at subsequent hydrothermal treatment at 120 -200°C and reaction time from 16 etc were of analytical grade, with high purity (>99.9%) and used as received

room temperature Alkali hydroxide of 10% solution is used for pH control in order to reach a suitable pH value in the range of 8 to 14 The stirring is carried out for an additional half hour and than the mixture is filled into a Teflon container and put into the autoclave The temperature of autoclave is maintained at the different temperatures from 120 to 200°C The reaction time was lasted from

16 h to 50 h The reaction product is cooled to room temperature Then cleaning process of the obtained product is done by ultrasonic vibration, centrifugal separation and washing in de-ionized water The product is dried at 60°C in air The morphology of the obtained product has been investigated by a FESEM (Hitachi at VAST) and a TEM (JEM- 1010 at National Institute of Hygiene and Epidemiology) instrument The phase structures were revealed by X-ray diffraction measurements carried out on XRD systems Siemens D5000, D5005

3 Results and Discussion

1 Construction of different one–dimensional nanoformations of yttrium hydroxide and yttrium oxide

XRD patterns have shown that most of the obtained products under these experimental conditions have hexagonal [P63/m] structures and the lattice constants got from calculation are a = 6.2793 and c = 3.5472 These values are highly compatible with standard ones (a = 6.268 and c = 3.547, 24-1422) Figure 1 shows the reflection patterns of Y(OH)3 at 190°C

Fig 1 XRD patterns of Y(OH)3 nanorods and nanotubes at 190°C

The morphology of Y(OH)3 nanotubes has been investigated by FESEM and

the result is shown in Figure 2

In this synthesis one is able to control the morphology and dimensions of 1-D nanostructures by controlling dynamic condition of the process When using small weight molecular-dimmer, diethylenglycol (DEG), the heating process can be performed only at 180°C to obtain spherical nanoparticles of Y2O3 with 5nm in size However, with larger MW of polymer chains (MW=4000) and changing the pressure of the solution, the obtained products are 1-D nanostructures such as rods (at 170–180°C) and tubes (at 190–200°C) with diameters of 50–600 nm and

Fig 2 FESEM images of Y(OH)3 nanorods and nanotubes which are prepared at the different temperatures (a:180°C; b:190°C)

lengths of about tens µm Ends of these nanotubes are opened and have hexagonal shape with inner diameters in the range of (150–300 nm) and outer ones in the range of (400-600 nm) Especially there is a change in the shape of the nanotubes from cylindrical cross-section to hexagonal one when temperature steps from 190°C to 200°C XRD patterns show that obtained 1-D nanostructures of Y(OH)3 are in hexagonal phase [P63/m] with lattice constants a = 0.6248 nm and c = 0.3525 nm This is satisfied with the original values

2 Formation mechanism of the Y(OH) 3 nanotubes

The tubular shape of the obtained products were found by HRTEM and TEM investigations The growth and formation processes of Y(OH)3 nanotubes have been studied by investigation of the samples at different conditions (120-200°C, 24-30h) and from the result, it can be concluded that the growth process of 1-D nanostructures may be performed as follows: the first stage is formation of metastable complexes between Y3+ and PEG in which, OH- substitutes NO3- to form Y(OH)3 When the concentration of ions was high enough, they would aggregate to form small clusters or so-called growing nuclei through a homo-geneous nucleation Because of their antistrophic hexagonal structure, there was

a tendency to grow into Y(OH)3 nanorods In order to form perfect crystals, generally, it is needed to have reversible routes between nuclei at the surface and those in liquid phase This condition allows the nuclei easily occupied suitable locations for crystallization in large scales, and controlling of adding rate of nuclei

is essential to obtain crystals with homogeneous phase structure and shape When the crystal has grown, dynamic diffusion will occur from the surface to the growing regions and become under saturation the central part of each nanorod So when the consumption of Y(OH)3 reaches the end, it is naturally that nanotubes were formed as a result of dynamic diffusion This growth process can be considered as a diffusion-controlled process There are two directions of diffusion: radial diffusion and parallel diffusion [2] The change from cylindrical shape to hexagonal shape occurs at a certain temperature can be explained as follows: at that temperature, the rate of parallel diffusion was at critical value

3 ZnO nanorods morphology and photoluminescence

One-dimensional semiconductor nanorods and nanowires have attracted increasing interest due to their novel physical properties and diversity for potential electronic and photonic device applications [8-10] ZnO nanorods were grown on

Si (100) and Al2O3 (0001) substrates using metal organic chemical vapor deposition (MOCVD) The crystal structure and orientation of the ZnO nanorods was investigated by X-ray diffraction (XRD) including a θ-2θ scan The morphology of the samples was analyzed using field emission scanning electron microscopy (FE-SEM) equipped with a 4 axis motorized stage The optical properties were investigated at room temperature and at low temperature

Fig 3 FE-SEM images of ZnO sample prepared by CVD method 800°C 30 min

Figure 3 presents FE-SEM images of ZnO sample prepared by the CVD method (800°C, 30 min.) Size and length of the nanorods depend on temperature and time as well as the method MOCVD or CVD, the distance between Zn nanopower to the substrate, speed of argon flux, Au or Pt deposition on the Si wafer The detail results will be presented in our other paper In Figure 4 one can

see images of FE-SEM ZnO nanorods prepared by MOCVD, sapphire substrate

(left), ZnO nanorods at tempered 500°C for 1 hour (right)

Fig 4 FESEM ZnO nanorods MOCVD on sapphire substrate (left) and ZnO nanorods at

500°C for 1 hour (right)

The photoluminescent (PL) spectra were studied by using a Spectrapro 2300 monochromator as the dispersive unit, a Pixies 256 CCD as the detector, and a Kimmon He-Cd Laser (325 nm) as the excitation source At room temperature a broad peak was observed at 3.312 eV with a low excitation intensity of 10 W/cm2 This peak is attributed to the exciton transition bound to neutral acceptors or donor [11] For the ZnO nanorods sample prepared by MOCVD at 500°C, with a growth time of 60 minutes, the PL spectra at low temperature (10 K) show peaks of 3.3473 eV, 3.3213 eV, 3.30186 eV, 3.2079 eV and 3.1113 eV [12] Temperature dependance was observed in the range of 10 K to 300 K

PL spectra of ZnO nanorods on sapphire substrate grown at 500°C for 1 hour

by MOCVD and of ZnO sample prepared by CVD method at 800°C for 30 minutes were presented in Figure 5

Fig 5 PL spectra of ZnO Nanorods on sapphire substrate grouwn at 500°C for 1 hour by

MOCVD (a) and ZnO sample prepared by CVD method 800°C 30 min (b)

4 Conclusions

1-D nanostructures of Y(OH)3 with different constructions have been fabricated using a high efficient and high productivity synthesis method in polymer solutions

of PEG The obtained nanotubes have single-crystalline structure, open ends and hexagonal cross-sections with the sizes Nanotubes have been obtained through heat treatment processes; the products were in hexagonal phase structure [P63/m] with lattice constants a = 0.6248 nm and c = 0.3525 nm

From the experimental results and current literatures, we propose the growth mechanism to forming these nanostructures is suggested as a three-stage process Firstly, the complex is formed by PEG polymer and Yttrium Nitrate; the second step is formation of Y(OH)3 nanorods and the third one is developing Y(OH)3 nanotubes when the temperature increases The growth mechanism of these nanostructures can be understood and explained as a three-stages process includes complexion of PEG and Yttrium Nitrate, formation of Y(OH)3 nanorods and, developing nanotubes, finally

In this paper, we have briefly presented an effective route to synthesize ZnO nanorods We have obtained ZnO nanorods with diameters in the range from 20 -

50 nm via MOCVD The length is about 540 nm to 1.12 micrometer ZnO

400 450 500 550 600 650

0

1000

2000

3000

lEXC:325 nm

Wavelength (nm)

(a)

400 450 500 550 600 650 0

1000 2000 3000 4000 5000

6000 (b) 567 l EXC :325 nm

Wavelength (nm) 380

nanorods have strong luminescence in blue region ZnO/ZnMgO nanomaterials are also prepared We have successfully prepared ZnO nanomaterials by CVD method and studied FE-SEM images and optical properties

Acknowledgements

We thank the financial support from the Vietnam-NSF on Physics Science, Basic

research programmes CB 19, CB 20 and Key Laboratory of Electronic Materials

and Devices

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Dai Co Viet Road 01, Hai Ba Trung District, Hanoi, Vietnam

E-mail: stacoproft@hn.vnn.vn

Professor Dr Annemarie Pucci

Universit¨at Heidelberg, Fakult¨at Physik und Astronomie, Kirchhoff-Institut f¨ur Physik

Im Neuenheimer Feld 227, 69120 Heidelberg, Germany

E-mail: a.pucci@urz.uni-heidelberg.de

Universit¨at Bonn, Institut f¨ur Physikalische und Theoretische Chemie

E-mail: k.wandelt@uni-bonn.de

ISSN 0930-8989 ISBN 978-3-540-88200-8 e-ISBN 978-3-540-88201-5

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Springer Proceedings in Physics

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→

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Hydroxides and Oxides 87

Nanotubes 95

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sulfidation of Au(111) 113

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