Quantum confinement of lead titanate nanocrystals by wet chemical method

Accepted Manuscript Quantum confinement of lead titanate nanocrystals by wet chemical method K. Kaviyarasu, E. Manikandan, Z.Y. Nuru, M. Maaza PII:

S0925-8388(15)30496-5

DOI:

10.1016/j.jallcom.2015.07.099

Reference:

JALCOM 34790

To appear in:

Journal of Alloys and Compounds

Received Date: 28 May 2015 Revised Date:

7 July 2015

Accepted Date: 12 July 2015

Please cite this article as: K. Kaviyarasu, E. Manikandan, Z.Y. Nuru, M. Maaza, Quantum confinement of lead titanate nanocrystals by wet chemical method, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.07.099. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Quantum confinement of lead titanate nanocrystals by wet chemical method K. Kaviyarasu1,2*, E. Manikandan2,3, Z. Y. Nuru1,2, M. Maaza1,2

UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology Laboratories,

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College of Graduate Studies, University of South Africa (UNISA), Muckleneuk Ridge, P O Box 392, Pretoria, South Africa. 2

Nanosciences African network (NANOAFNET), Materials Research Department (MSD),

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iThemba LABS-National Research Foundation (NRF), 1 Old Faure Road, 7129, P O Box 722, Somerset West, Western Cape Province, South Africa.

Central Research Laboratory, Sree Balaji Medical College & Hospital, Bharath University,

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Chrompet, Chennai, Tamil Nadu, India.

ABSTRACT

Lead Titinate (PbTiO3) is a category of the practical semiconductor metal oxides, which is

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widely applied in various scientific and industrial fields because of its catalytic, optical, and electrical properties. PbTiO3 nanocrystalline materials have attracted a wide attention due to their unique properties. PbTiO3 nanocrystals were investigated by X-ray diffraction (XRD) to identify the PbTiO3 nanocrystals were composed a tetragonal structure. The diameter of a single sphere

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was around 20 nm and the diameter reached up to 3 µm. The chemical composition of the samples and the valence states of elements were determined by X-ray photoelectron

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spectroscopy (XPS) in detail.

Keywords: Lead titinate; Nanoparticles; Composite materials; Colloidal processing; XPS; XRD.

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Corresponding authors:- [email protected] ; [email protected], [email protected] (K. Kaviyarasu), Tel:- +27 630441709 *

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1. Introduction Recently, one-dimensional (1D) nanostructures have stimulated much attention because of their fascinating applications for well-defined interconnects and building blocks for nanodevices. Nanocrystalline materials have attracted a wide attention due to their unique

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properties and immense potential application for opto-device fabrication of one-dimensional (1D) nanostructures, such as nanotubes, nanobelts, nanowires and nanorods because of the distinct geometries, and novel physical and chemical properties of different from those of bulk counterparts [1-3]. Due to the size confinement in the radial direction materials of these

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nanostructures are promising candidates for realizing nanoscale electronic [4, 5] optical and magnetic [6-10] devices. Ferroelectric oxides represents a particular interesting class of

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materials, which exhibits spontaneous polarization that can be reoriented by external electric field, and possess a broad range of properties, such as remnant polarization and high dielectric permittivity as well as piezoelectricity and pyroelectricity [11, 12]. Previous studies on thin film and nanocrystalline samples have indicated that their physical properties are critically dependent on their dimensions [13]. Thus, it is of great significant to investigate the fabrication and physical properties of ferroelectric of 1D nanostructure. So far, long ferroelectric nanowires with

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well-defined structures and a diameter of 5 nm to 10 nm have been fabricated by using different methods [14, 15]. For example, Urban and co-workers have fabricated well-isolated nanowires of BaTiO3 and SrTiO3 with diameters ranging from 10 to 50 nm and lengths reaching up to 10 µm by solution-phase decomposition of bimetallic alkoxide precursors in the presence of

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coordinating ligands [16]. Deng et al have fabricated single crystalline PbTiO3 nanorods (NRs) with diameters of 50 nm to 80 nm by solid-state reaction [17]. Hydrothermal technique is a

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promising way for fabricating ferroelectric nanomaterials because it can realize a low processing temperature of 200 °C or less, and can obtain products with high purity [18]. For example, Xu et al have fabricated single-crystalline tetragonal perovskite NRs and nanowires (NWs) using hydrothermal process assisted by polymers [19]. As a ferroelectric material, PbTiO3 exhibits a perovskite structure and a high Curie temperature (Tc= 763 K) compared to other ferroelectric materials, such as BaTiO3, SrTiO3 etc., which makes it useful over a wide temperature range. It has many potential applications in electronic and microelectronic devices, belonging to the most important ferroelectric and piezoelectric families [20-22]. For this material, many early studies were mainly concerned with thin films and particles [23]. Although high quality nanosphere 2

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were thought to be quite difficult to obtain. Therefore, there is considerable interest for studying such materials not only for future applications but also from a fundamental point of view.

2. Experimental

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2.1 Chemicals & Typical synthetic process of lead titinate nanocrystal

All chemical reagents (analytical grade) were used as received without further purification. Lead titinate (PbTiO3) were used as the starting materials, and Di-methyl formaldehyde (DMF)

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was served as a capping agent. In 1 mol % of PbTiO3 was dissolved in 100 ml deionized water and 1 mol % of DMF was dissolved in 50 ml ethanol solution. After ultrasonication for an hour the suspension was transferred into 150 ml Teflon lined stainless steel autoclave. The autoclave

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was maintained at a temperature of 390 °C for 12 hrs. After cooling to room temperature (RT) naturally, the black yellowish product was washed with distilled water for several times and then dried under vacuum at 110 °C for 36 hrs. Then the black yellowish dispersion was rinsed three times with deionized water by centrifugation. Finally nearly 1g of H2SO4 was added into the suspension and stirred for 5 hrs. The as-obtained samples were filtered and washed 3 times with

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distilled water and ethanol and then dried in vacuum at 90 °C for 7 hrs. 2.3 Sample characterization

The X-ray powder diffraction (XRD) experiments were measured on a RigaKu D/maxRB diffractometer with Ni-filtered graphite monochromatized Cukα radiation (λ = 1.54056 Å)

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under 40 kV, 30 mA and scanning between 10° to 90° (2θ). The XPS spectrum was recorded on a ESCALAB 250 photoelectron spectrometer (Thermo-VG Scientific, USA) with Al Kα

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(1486.6 eV) as the X-ray source. High-resolution Transmission electron microscopy (HRTEM) measurements were made on a HITACHI H-8100 electron microscopy (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. The sample for HRTEM characterization was prepared by placing a drop of colloidal solution on carbon-coated copper grid and dried at room temperature. The elemental composition was determined using the selected area electron diffraction (SAED) (IH-300X) analysis was performed at several points in the HRTEM system respectively.

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3. Results and discussion 3.1 X-ray Powder Diffraction Fig.1. shows the X-ray diffraction (XRD) pattern of PbTiO3 nanocrystals sample by

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wet – chemical synthesized at 390 °C for 12 hrs. Most of the diffraction peaks in this pattern can be assigned to a tetragonal phase with the lattice parameter, a = 3.904 Å, corresponding well with the reported data (JCPDS card file no. 01-074-2495). The strong and sharp reflection peaks

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suggest that the as-prepared products are well crystallized [24, 25].

3.2 XPS analysis

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Fig. 2. shows the XPS wide scan spectra of PbTiO3 nanocrystals in the binding energy ranging up to 600 eV. It can be seen that the nanocrystals contains Pb, Ti and O elements and no other elements are detected expect for carbon, the atomic ratio of Pb:Ti:O is respectively. In two peaks at 325 eV and 545 eV for O1s were identified; each component peak in the spectrum was fitted with Lorentzian function respectively [26]. The major peak of lower binding energy was assigned to lattice oxygen (Ti-O-) in the PbTiO3 nanocrystal, while the smaller peak of higher

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binding energy was assigned to the hydroxyl group of oxygen, which is very common in samples with high surface energy [27]. This may be due to the variation of the lattice constants a & c (a=0.391 Å, c = 0.415 Å, which are calculated from XRD pattern), together with the shrinking of lattice for PbTiO3 nanocrystal, which means that the oxide anions form octahedral of TiO6

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enclosing the titanium ions and the Ti-O bonds in PbTiO3 lattice become much more stronger. This result reveals that the lattice shrinkage plays a significant role in the spin-orbit splitting of

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Ti2p state.

3.3 High resolution Scanning Electron Microscopy The high-resolution transmission electron microscopy (HRTEM) of PbTiO3 nanocrystals prepared by wet-chemical route. It is obvious from Fig. 1 that the XRD and SAED patterns are densely deposited, and the average size of the grains is about ~ 10 nm. In order the highmagnification HRTEM images in Fig. 3(a-d) and shows that the spherical aggregates have good crystallinity and are composed of nanoparticles with a diameter less than 25 nm consistent with the XPS spectra and the XRD data discussed above. A high-resolution SAED images shows that 4

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the lattice fringes on the crystal face have a spacing of 3.904 Å, corresponding to the (111) face of TiO3.

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3.4 Atomic Force Microscopy In order to locally characterization of the samples, we have performed measurements by means of an atomic force microscope (AFM) as shown in Fig. 4(a, b). The AFM is a useful instrument for topographic and functional mapping of surfaces, and is used in such diverse fields

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as materials science, biology, physics, chemistry and nanotechnology. The typical accuracy is of the order of the nanometer in the X/Y directions, and of the order of an angstrom in the Z direction. Depending on the exact microscope model and design, the column is used to sweep the

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plane of either the sample or the pyramidal silicon tip and to adjust the distance between the two junctions. In order to measure the interaction between the tip and the sample, a laser shines at a quadrant photo sensitive detector by reflecting off the back of the tip cantilever. The free resonance frequencies of such a micro-sized system are in the ultrasonic range when the cantilever has an approximately rectangular shape and a uniform cross-section its flexural and torsional modes can be easily calculated. The cantilever can vibrate also while the piezo-mode

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images the contrast in the topography image is weaker and the displayed features are not so distinct and easy to differentiate. The images were obtained the same diamond-coated cantilever which was used for the lead titinate sample. The frequency of the applied ultrasound and the ac voltage was equal to 320 KHz. The size of the images is also 10µmx10µm despite the fact that

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the highest scale of the topography image is 20 nm. In contrast the corresponding AFM and ultrasonic piezo mode images show the difference in the size of the ferroelectric domain pattern.

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The figure also reveals that the surface gradient of the sphere changes corresponding to the change of the birefringence. The size of the domains varied from place to place in the same sphere.

4. Conclusion

In summary, PbTiO3 nanocrystals from regular particles shaped of spherical-liked nanocrystals in a wet – chemical process. If the crystallization process happens at high enough

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temperatures for sufficiently long times, than all the nanocrystal have a well-defined shape with relatively sharp facets that preferably consist of (111) and (311) planes as lateral surfaces and (222) planes as top surfaces they are single crystalline, epitaxial, uniform in height and free from

distribution of self-assembling into order arrays on a large scale.

Acknowledgements

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any volume defects. This approach offers a great deal of control of the size and the size

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The authors gratefully acknowledge research funding from UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology Laboratories, College of Graduate Studies, University of South

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Africa (UNISA), Muckleneuk Ridge, Pretoria, South Africa, (Research Grant Fellowship of framework Post-Doctoral Fellowship program under contract number Research Fund: 139000). One of the authors (Dr. K. Kaviyarasu) is grateful for the Prof. M. Maaza, Nanosciences African network (NANOAFNET), Materials Research Department (MSD), iThemba LABS-National Research Foundation (NRF), Somerset West, South Africa. Support Program and the Basic Science Research Program through the National Research Foundation of South Africa for his

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constant support, help and encouragement generously.

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Figure and Caption

Fig. 2. XPS wide scan spectra of PbTiO3 nanosphere Fig. 3(a-d). HRTEM images of PbTiO3 nanosphere

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Fig. 4. Atomic Force Microscopy of PbTiO3 nanosphere

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Fig. 1. X-ray powder diffractogram of PbTiO3 nanosphere

© K. Kaviyarasu et al., 2015.

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Research Highlights

Single crystalline NSs of PbTiO3 fabricated by wet chemical method PbTiO3 NSs were uniform and continuous along the long axis

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Tetragonal perovskite structure with the diameter 20 nm and length 3µm XPS spectrum was fitted with Lorentzian function respectively

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The size of the images is also 10µmx10µm