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Materials Research

Print version ISSN 1516-1439
On-line version ISSN 1980-5373

Mat. Res. vol.19 no.6 São Carlos Nov./Dec. 2016  Epub Oct 10, 2016

http://dx.doi.org/10.1590/1980-5373-mr-2016-0288 

Articles

Titanium Dioxide Nanoparticles: Synthesis, X-Ray Line Analysis and Chemical Composition Study

Hossein Mahmoudi Chenaria  * 

Christoph Seibelb  c 

Dirk Hauschildb  c 

Friedrich Reinertb  c 

Hossein Abdollahiand 

aDepartment of Physics, Faculty of Science, University of Guilan, Namjoo Ave, Po Box 41335-1914, Rasht, Iran

bExperimental Physics VII and Röntgen Research Center for Complex Materials - RCCM, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany

cKarlsruhe Institute of Technology - KIT, Gemeinschaftslabor für Nanoanalytik, D-76021 Karlsruhe, Germany

dNanotechnology Research Center of Urmia University, Urmia, Iran

ABSTRACT

TiO2 nanoparticleshave been synthesized by the sol-gel method using titanium alkoxide and isopropanolas a precursor. The structural properties and chemical composition of the TiO2 nanoparticles were studied usingX-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy.The X-ray powder diffraction pattern confirms that the particles are mainly composed of the anatase phase with the preferential orientation along [101] direction.The physical parameters such as strain, stress and energy density were investigated from the Williamson- Hall (W-H) plot assuming a uniform deformation model (UDM), and uniform deformation energy density model (UDEDM). The W-H analysis shows an anisotropic nature of the strain in nanopowders. The scanning electron microscopy image shows clear TiO2 nanoparticles with particle sizes varying from 60 to 80nm. The results of mean particle size of TiO2 nanoparticles show an inter correlation with the W-H analysis and SEM results. Our X-ray photoelectron spectroscopy spectra show that nearly a complete amount of titanium has reacted to TiO2.

Keywords TiO2; Nanoparticles; X-ray analysis; SEM; XPS

1. Introduction

Nanometer-scale materials have recently attracted considerable scientific attention because of their beneficial high surface to volume ratio and therefore unique chemical, electronic, and physical properties. In particular TiO2 nanoparticles are in the focus of research and thus many reports on electrical, optical, and structural properties of TiO2 nanoparticles can be found1-4. B. Sathyaseelanet al.5 investigated structural, optical and morphological properties of post-growth calcined TiO2 nanopowder. The size dependent reflective properties of TiO2 nanoparticles synthesizedusing arc discharge method were studied by F. Fang et al.6 Nano-crystalline TiO2 is a promising candidate for a wide range of applications such as photocatalysis, solar cells, dielectric materials, and photoconductors7-10.Modification of TiO2 with metal and nonmetal elements has received much attention and Doped TiO2 nanoparticles exhibit novel properties and according to impurity type, dopants improve the physical and optoelctronic paroperties of TiO2 nanoparticles. For instance,K. Kaviyarasu et al.11 reported the fabrication, optical and microscopic studies of magnesium doped TiO2 NCs .optical, structural, and electronic properties of carbon-modified titanium dioxide nanoparticles synthesized by ultrasonic nebulizer spray pyrolysis have been investigated by R. Taziwa et al.12 S. Ivanov et al.13 studied one-step synthesis of TiO2 nanoparticles based on the interaction between thiourea and metatitanic acid and reported the photocatalytic activity of the doped SC-TiO2 powders. TiO2 nanoparticles in both powder and film form can be synthesized using various methods such as chemical vapor deposition14, chemical spray pyrolysis15, sol-gel technique16, hydrothermal treatment17, and arc discharge method18. In this paper, the sol-gel technique has been successfully employed to synthesize TiO2 particles on the nanometer scale.

Most of the research reports on the structural properties of nanoparticles dealt with the determination of structure type, physical and different microstructural parameters. X-ray diffraction line broadening studies give more useful information about the physical parameters such as crystallite size, dislocation density and strain.There are many analytical methods to evaluate the microstructure properties of materials such as the Scherrer's equation19, Williamson -Hall analysis20, the integral breadth method21 and size strain plot method22. This study highlightsthe microstructure analysis and chemical composition of TiO2 nanoparticles. Whereas only a few reports on TiO2 nano-crystals perform X-ray photoelectron spectroscopy (XPS)23-26.

In this work, TiO2 nanocrystals were prepared by a simple sol-gel method. A structural characterization and chemical composition study was performed by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. We give more information on strain-stress and the energy density of crystal by using the Williamson- Hall procedure. We also include a quantitative analysis of chemical composition of a very successful synthesized nanoparticle by surface sensitive XPS known as electron spectroscopy for chemical analysis (ESCA). The measurements suggest that nearly a complete amount of titanium has reacted to TiO2.

2. Experimental Methods

2.1. Preparation

The preparation of TiO2 nanoparticles was performed by the sol-gel method as follows:

The high reactivity of alkoxides in the presence of water or solutions containing isopropanol causes a formation of three-dimensional oxide networks, producing metal hydroxides (i) or hydrated oxides (ii). Hence, the general chemical reactions are given by the following equations:

TiOR4+4H2OTiOH4+4ROHhydrolysis, (I)
TiOH4TiO2×H2O+2xH2Ocondensation, (II)

Where R is ethyl, i-propyl, n-butyl, etc.27. The molar ratio of water to titanium strongly affects the stability, shape, size, and morphology of the produced alkoxide-sol. Also the size distribution of nanoparticles is dependent on the pH of solution28. Due to the high reactivity of titanium alkoxide (Ti{OCH(CH3)2}4), 10 ml of this precursor was diluted with 40 ml of isopropanol (C3H7OH) at room temperature in a dry atmosphere with about 8 %relative humidity. The mixture was then added dropwise into a solution that consist of deionized water and isopropanol in a 1:1 ratio. For adjustment of the pH valuehydrochloric acid and ammonium hydroxide were added, respectively. With this addition, the acidity-alkalinity of the gel was stabilized to a pH value of 3. Subsequently, the solution was vigorously stirred for 30 minutes and a yellowish gel was formed. Afterwards, the prepared materials were washed with ethanol and the obtained gel was then dried at 120°C for two hours. A Scientific furnace(NaberthermLVD 73/23/EC) was used for calcinations of the synthesized materials at 450°C for four hours29. The resulting product was TiO2 nanopowder. We will now begin to discuss the results of the structure analysis and the chemical composition measurements.

2.2. Characterization techniques

The bulk sensitive X-ray diffraction (XRD) patterns were taken with Philips X'Pertdiffractometerat room temperature using monochromatic Cu Kα (hν=8042.55 eV) excitation. Measurements were taken under beam acceleration conditions of 40 kV/35 mA. Whereas the surface sensitive X-ray photoelectron spectroscopy (XPS) measurements were performed under ultra-high vacuum (UHV) condition, in a system exhibiting a base pressure of better than 2×10-10 mbar. In order to study the chemical state of titanium and oxygen in the nanoparticles we used a standard non-monochromatized Mg Kα (hν = 1253.6 eV) X-ray source and a VG Clam 4 electron spectrometer. The spectra were corrected for X-ray satellites and secondary electron background (Shirley30) prior to analysis. Scanning electron microscopy (SEM) images of the same samples were recorded with a LED-1430VP microscope using an electron beam energy of 15 keV and a beam current of 2.62 A.

3. Results and Discussion

3.1. X-ray analysis

X-ray profile analysis is apowerfultoolin extracting the microstructure information of nanocrystallinesamples. Figure 1 shows the XRD pattern of TiO2 nanoparticles, in the 2θ range of 10-70°.The diffraction peaks corresponding to the (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4) and (1 0 6) crystal planes with the lattice constants a = 3.755 Å and c = 9.5114Å confirms the anatase phases of the TiO2 nanoparticles according to the JCPDS file 21-1272 31. X-ray diffraction profiles are usually influenced by crystallite size and lattice strain. According to the W-H method the individual contribution due to the size and strain can be expressed as17

Bhkl=BS+βD (1)
BhklCosθ=kλ/D+4εsinθ (2)

Where ε is the strain and D is the average crystallite size of a x-ray peak. Βhkl is the peak width at half-maximum intensity and Βs, βD are the peak broadeningduetothe crystallite size and lattice strain,respectively.In the Eq. 2 the strain was assumed to be uniform in all crystallographic direction implying a uniform deformation model ('UDM'). Figure 2 (a) shows the UDM analysis.The effective crystallite size can be estimated from the extrapolation of Βhkl cosθ versus 4sinθ and the slop of the fitted line represents the strain.Inthe uniform deformation energy density model (UDEDM)has been replaced by ε=σ/E in equation (2); where σ is the stress of crystal and E is the modulus of elasticity in the direction perpendicular to the set of Bragg reflection. The elastic moduli E, for the TiO2 anatase-tetragonal assumed to be ≈174 GPa32. In the modified form of W-H equation called (UDEDM) model, the strain energy density u is considered and the modulus of elasticity is no longer independent. The energy density u can be determined from u=(ε2E)/2 using Hooke's law. Then equation (2) can be modified again according to the energy density as

BhklCosθ=kλ/D+4sinθ×2u/Ehkl1/2 (3)

In this model Βhkl cosθ were plotted against 4 sinθ / (Ehkl/2)1/2. The anisotropic energy density (u) from the slope of fitted line and the crystallite size calculated from the y-intercept; see Figure 2(b). The results obtained from the UDM and UDEDM models are collected in Table 1. As it is evidentfromTable1, the mean crystallite sizes obtained from the W-H models are more or less similar implying that strain in different form has very small contribution on the mean crystallite size.

Figure 1 X-raydiffraction pattern of TiO2 nanoparticles 

Figure 2 The W-H analysis of TiO2 nanoparticles assuming (a) UDM, and (b) UDEDM model. 

Table 1 Micro structural parameters of TiO2 nanoparticles. 

UDMmodel UDEDMmodel
D ε D u ε σ (MPa)
(nm) (no unit) (nm) (Kj.m-3) (no unit)
72.2 2×10-5 72.2 5.2054 5.469×10-6 0.952

3.2. SEManalysis

Figure 3 shows SEM image obtained from the titanium oxide nanoparticles. The structure of this particle cluster consisting of agglomerated nanoparticles can be identified as a non-ordered and porous. In Figure 3,two particle sizes were exemplarily determined, exhibiting width of 64.82 nm (Pa 1) and 74.08 nm (Pa2). In general, the width of the nanoparticles varies from 60 to 80nm.

Figure 3 SEM image of theTiO2 nanoparticles. 

3.3. X-ray photoelectron spectroscopy

The powder was prepared ex-situ before transferring to the UHV chamber. Subsequent, X-ray photoelectron spectroscopy was performed and a survey scan of TiO2 nanopowder is presented in Figure 4. To compensate charging effects, we calibrated the C1s peak to 286.0 eV since at this certain binding energy C-N and C-O bonds overlap energetically [25].The spectrum is dominated by the signals of Ti and O. The binding energies of the spin-orbit split Ti2p1/2,3/2 (464.6 eV and 458.9 eV, respectively) signals,as well as the O 1s (530.7 eV) level are corresponding to the titanium dioxide chemical environment (see Table 2).In addition, we find a significant amount of carbon (see C 1s in Figure 4) and nitrogen (see N 1sin Figure 4) in the spectrum. The detected carbon and nitrogen contaminations stem probably from adsorbents at grain boundaries and crystallite surfaces. In general, contaminations are inevitable for samples exposed to air. However,the residual build-in contaminations resulting from the manufacturing process cannot be rule out. Furthermore, we derived the Ti:O ratio at the particle surfaces by using the corresponding cross section26, the inelastic mean free path33, and the transmission function T ~ Ekin-0.6 of our electron spectrometer. Hence, we determined a total Ti:O ratio of 1:(2.2 ± 0.4). We explain the slightly surplus of the oxygen content with adsorbents like COx and hydro carbons. A detailed analysis of the O1s signal reveals a small deviation from its intrinsic lineshape, which indicates several oxygen species. However, it is beyond the aim of this paper to evaluate their individual quantities and to assign them to certain contaminations.The inset of Figure 4 shows the detailed measurement of the Ti 2p region with the spin-orbit split doublet peaks at 463.9 eV and 458.2 eV for Ti 2p1/2 and Ti 2p3/2, respectively. A comparison of the obtained binding energies to the respective literature values for Ti 2p shows a good agreement between the binding energy of the main spectral 2p component and the one of pure TiO2 (see Table 2)34,35. Furthermore, the absence of a signal at lower binding energies of approximately 453.8 eV36 indicates that during the synthesis in the frame of measurement accuracy and probing depth all Ti atoms have fully reacted to titanium dioxide. The inset shows a detailed spectrum of the Ti 2p signal with a subtracted Shirley background and therefore a dashed horizontal line indicates the base line. The grey bar represents the reported Ti 2p3/2 binding energy values for TiO235.

Figure 4 X-ray photoemission spectrum of TiO2 nanoparticles taken at room temperature using a Mg X-ray source. 

Table 2 Energetic positions of the Ti 2p1/2, Ti 2p3/2, and O 1s of the investigated nanoparticle sample and adsorbate. Literature values are added for comparison. 

Element Orbital EB,Exp (eV) EB,Lit (eV)
Titanium 2p1/2 464.6 464.334, 464.4 35
Titanium 2p3/2 458.9 458.834,35
Oxygen 1s 530.7 529.634, 528.4 35

4. Conclusions

In this study, we have successfully prepared titanium dioxide nanoparticles applying sol-gel technique. The performed X-ray diffraction measurement suggests that the precursor materials have reacted to the anatase phase .The evolution of the crystallite size and the microstrain was studied using the X-ray peak broadening analysis by the Williamson-Hall method. The obtained mean crystallite size of TiO2 nanoparticles show an inter correlation with the value obtained from the W-H analysis and SEM results.The results of the surface sensitive X-ray photoelectron spectroscopy measurements indicate that nearly the complete amount of titanium has reacted to TiO2.

5. Acknowledgments

The authors would like to acknowledge the Experimental Physics VII and Röntgen Research Center for Complex Materials (RCCM),UniversitätWürzburg, Am Hubland, D-97074 Würzburg, Germany and University of Guilan Research Council for the support of this work.

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Received: April 08, 2016; Revised: July 25, 2016; Accepted: August 26, 2016

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