Fabrication of pure and Ag-doped TiO2 nanorods and study of the lattice strain and the activation energy of the crystalline phases

Journal of the Korean Physical Society, Vol. 62, No. 3, February 2013, pp. 459∼468

Fabrication of Pure and Ag-doped TiO2 Nanorods and Study of the LatticeStrain and the Activation Energy of the Crystalline Phases

Mehran Riazian,∗ Shima Daliri Rad and Reza Ramezani Azinabadi

Department of Engineering, Tonekabon Branch, Islamic Azad University, Tonekabon, Iran

(Received 12 November 2012)

TiO2 nanorods can be used as dye-sensitized solar cells and as various sensors and photocata-lysts. These nanorods are synthesized by using a thermal corrosion process in a NaOH solutionat 200 ◦C with TiO2 powder as a source material. In the present work, the synthesis of TiO2

nanorods in anatase, rutile and Ti8O15 phases and the synthesis of TiO2 nanorods by using thesol-gel method and alkaline corrosion to incorporate silver and silver-oxide dopants are reported.The morphologies and the crystalline structures of the TiO2 nanorods are characterized using fieldemission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), tunneling elec-tron microscopy (TEM) and X-ray diffraction (XRD) techniques. The obtained results show anaggregation structure at high calcining temperatures with spherical particles and with Ti-O-Ti,Ti-O and Ag-O bonds. The effects of the chemical composition and the calcining temperature onthe surface topography, lattice strain and phase crystallization are studied. The activation energy(E) of nanoparticle formation in a pure state during thermal treatment is calculated.

PACS numbers: 61.10.-i; 61.46.+w; 61.50.-f; 61.16.Ch; 61.16.BgKeywords: Lattice strain, Nanorods, TiO2, Silver and silver-oxide dopant, Sol-gel methodDOI: 10.3938/jkps.62.459

I. INTRODUCTION

TiO2 is an important material that is used in manyindustrial applications related to photo-splitting of wa-ter [1], photocatalysis [2], photovoltaic devices [3], etc.It is known to have three natural polymorphs, i.e., ru-tile, anatase, and brookite. Only anatase is generally ac-cepted to have significant photocatalytic activity. Thephotocatalytic performance of this compound dependson the characteristic of the TiO2crystallites, such as thesize and the surface area. Therefore, modifications of itsphysical and chemical properties are of interest for re-searchers [4–9]. One possible way to modify the proper-ties of TiO2crystallites is by adding a second semicon-ductor into the TiO2matrix. Silver particles have an-tibacterial effect and can improve the purification role ofTiO2.

Many studies addressing both TiO2 doping with differ-ent metals and the synthesis and characterization of one-dimensional (1D) nanostructures (nanowires, nanotubes,nanorods) have received considerable attention due tounique properties and novel applications of those nanos-tructures [10–13]. However, more investigations are stillneeded to study the synthesis processes and the dopanteffects on the photocatalytic semiconductor surface in or-der to determining the synergism of the micro- and the

∗E-mail: [email protected]

nanoscale hierarchical surface structures, the orientationof the crystal planes and the surface photosensitivity.

Many methods have been successfully developed forthe fabrication of 1D nanostructures, including vapor-so-lid, vapor-liquid-solid and solution-liquid-solid template-based synthetic approaches and laser ablation [14–19].However, almost all of these methods use either catalystmaterials or physical templates, which unavoidably bringsome contamination to the products. Therefore, one hasto explore a new approach to synthesize 1D nanomateri-als without using preformed templates or catalysts. Thesol-gel process is employed quite often for the synthesisof nanosize catalytic materials. The incorporation of anactive metal in the sol during the gelation stage allowsthe metal to have a direct interaction with the support,so the material possesses special catalytic properties.

In the present work, first, a TiO2 nanorod is synthe-sized by using a hydrolysis procedure in simple wet chem-ical approach with titanium tetra isopropoxide at 300,600, and 900 ◦C. Shortly after that, it is doped into thesilver and the silver-oxide matrices. The morphologiesand the crystalline structures of the TiO2 nanorods arecharacterized using (FE-SEM), (AFM), (TEM), (XRD),and (FTIR). The obtained results indicated the nanorodproperties depend on the preparation procedures and thecalcining temperature.

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-460- Journal of the Korean Physical Society, Vol. 62, No. 3, February 2013

Table 1. Composition of starting solutions and experimental conditions for TiO2 nanorod preparation.

Sol-gel Method Step Precursor Molar Ratio(MR) Stirring Time(h) pH

TTIPTTIP/EtOH/H2O

= 1:0.022/9.5 × 10−4

Alkoxide 1 AgNO3 TTIP/ AgNO3 = 282.05 24 5

routeFe(NO3)3(9H2O)

TTIP/Fe(NO3)3(9H2O)

= 666.6

2Dropwise Precipitation

0.5 8Precipitation with Ammonia

Fig. 1. Schematic flowchart illustrating the steps in thesynthesis pathway of TiO2 nanorods.

II. EXPERIMENT

The composition of the starting solution and the ex-perimental conditions used for the TiO2 nanorods arelisted in Table 1. Figure 1 illustrates the preparation pro-cedures. The precursors, AgNO3 (Merck≥99%), titaniumtetra isopropoxide (TTIP) (Ti(OPri)4, Merck≥98%),0.1 N nitric acid (Merck ≥65%), ethanol (Merck≥97%) anddistilled water are used without further purification.

The starting point for the synthesis of a targeted sys-tem is a solution prepared by mixing the precursors: Indetail, according to the molar ratio in Table 1. Precur-sors are chosen (TTIP, deionized water, ethanol). AgNO3

is dissolved in 20 cc of deionized water and stirred atroom temperature (RT) for 48 h to get a solution ofpH = 5. Then, for precipitation, ammonia solution isadded dropwise so that pH = 8 and is then centrifugedat 1500 rpm to gather the precipitate. The precipitateis heated at 50 ◦C for 24 h. For the formation of a rodshape, the powders are immersed in 10 N NaOH solu-

tion in a teflon balloon. Once more, the powders aregathered with a centrifuge and are purified with 0.1 Nnitric acid and distilled water to eliminate the Na ions.Finally, the powders are dried at 50 ◦C for 48 h in airand calcined at three different temperatures (300, 600,and 900 ◦C). The effects of the variation in the calciningtemperature are studied with as-prepared, 300, 600, and900 ◦C samples. Doped samples calcined at temperatureshigher than 600 ◦C tended to melt.

III. CHARACTERIZATION OF THE TIO2

NANORODS

XRD patterns are measured on a GBC-MMA 007(2000) X-ray diffractometer. The diffractograms arerecorded with kα(Cu), (1.54056 A, 0.02◦ step size, and10◦/min speed) radiation over a 2θ range of 10 – 80◦.Transmission electron microscopy (TEM, CM10 Philips)is used to investigate the structure and the morphol-ogy of the nanorods. Field-emission electron microscopy(FE-SEM, S-4160 Hitachi) is routinely used to investi-gate the morphology of the nanoparticles. AFM (EasyScan 2 Flex (Switzerland)), with a silicon tip is used.The measurements are made at 20 ◦C and 45% relativehumidity. FTIR measurements are performed on a 1730Infrared Fourier Transform Spectrometer (Perkin-Elmer)with potassium bromide as the background.

IV. RESULTS AND DISCUSSION

The crystallographic phases of the composite ceramicare investigated by using the XRD technique, and theresults are shown in Figs. 2 and 3. The characteristicsof the XRD peaks are summarized in Tables 2 and 3.As shown in these figures, different crystalline phases areformed at different calcining temperatures. The powdersare obtained from gels after 2 h drying. The powders arecalcined at 300, 600, and 900 ◦C with a 10

◦Cmingradient.

Figures 2 and 3 also show the amorphous structures forthe as-prepared and the 300 ◦C sample due to the short-range ordering of the network [13,17,20].

Fabrication of Pure and Ag-Doped TiO2 Nanorods and Study of the Lattice · · · – Mehran Riazian et al. -461-

Table 2. The 2θ angle, d-space, Miller indexes, grain size of a pure TiO2 nanorod.

Pure state

Crystalline PhaseAs-prepared Calcined at 300 ◦C Calcined at 600 ◦C Calcined at 900 ◦C

2Θd-space size

2Θd-space size

2Θd-space size

2Θd-space size

(A) (nm) (A) (nm) (A) (nm) (A) (nm)

Anatase Tetragonal

a = 3.8040 A 25.33 3.51 11 25.15 3.52 34 25.60 3.46 54 25.20 3.55 60

c = 9.6140 A

Rutile Tetragonal

a = 4.5940 A 27.55 3.23 7 27.68 3.26 18 27.56 3.23 31 27.80 3.20 145

c = 2.9590 A

TI2O3 Hexagonal

a = 5.1490 A 33.2 2.69 4 33.20 2.65 9 33.00 2.71 13 33.35 2.69 24

c = 13.6420 A

Ti3O15 Hexagonal

a = 4.8440 A 29.10 3.06 4 29.00 3.08 7 29.28 3.05 12 29.8 2.99 24

c = 13.2700 A

Table 3. The 2θ angle, d-space, Miller indexes, and grain size of a TiO2 nanorod doped with silver and silver-oxide.

Doped state

Crystalline Phase As-prepared Calcined at 300 ◦C Calcined at 600 ◦C2Θ d-space (A) size (nm) 2Θ d-space (A) size (nm) 2Θ d-space (A) size (nm)

Anatase Tetragonal

a = 3.8040 A 25.21 3.50 9 25.30 3.50 97 25.26 3.53 102

c = 9.6140 A

Rutile Tetragonal

a = 4.5940 A 27.40 3.26 6 27.50 3.22 24 - - -

c = 2.9590 A

Ti Hexagonal

a = 2.9500 A 40.18 2.24 9 40.07 2.25 14 40.23 2.28 19

c = 4.6860 A

Ag Cubic38.04 2.36 23 38.06 2.36 29 38.07 2.36 142

a = 4.1090 A

Ag2O Cubic33.00 2.71 12 33.19 2.70 17 33.11 2.70 33

a = 4.7600 A

Ag2O3 Monoclinic

a = 4.8520 A33.70 2.65 13 33.30 2.68 15 33.80 2.65 44

b = 9.5530 A

c = 3.2550 A

Samples calcined at 600 and 900 ◦C have a high degreeof the crystallinity. The grain size values are calculatedfrom the Scherrer equation:

r =0.9λ

2β cos θ, (1)

where λ = 0.154 nm, β is the full width at half maxi-mum (FWHM), and θ is the reflection angle. Data inTables 2 and 3 show the influence of the calcining tem-perature on the grain size of different phases. The size

of grains increases when the calcining temperature is in-creased. This is due to enlarged chemical bonding andrearranged crystalline structure caused by the higher cal-cining temperature. The rutile phase has not been seenin doped state calcined at 600 ◦C. Precursor chemistry,experimental conditions and the presence of a dopantmaterial influence the nucleation and the growth of thedifferent polymorphs of TiO2 [21,22].

The averages of the crystalline nanopowder sizes in thepure state are calculated based on Scherrer’s formula.


Fig. 2. (Color online) XRD patterns of pure TiO2

nanorods obtained without hydrothermal treatment (as-prepared), calcined at 300 ◦C, calcined at 600 ◦C and calcinedat 900 ◦C.

Fig. 3. (Color online) XRD patterns of TiO2 nanoroddoped with silver and silver-oxide obtained without hy-drothermal treatment (as-prepared), calcined at 300 ◦C andcalcined at 600 ◦C.

Figures 4 and 6 show the variations of XRD crystallinesizes for nanoparticles prepared by using thermal treat-ment of the nanopowders at various temperatures. Theseshow that the crystalline size of rutile increased rapidlyfrom about 31 nm at 600 ◦C to 145 nm at 900 ◦C whilethe crystal size of anatase increased relatively slowly.Furthermore the crystalline sizes of Ti2O3 and Ti8O15

increased rapidly from about 13 and 12 nm at 600 ◦C to24 nm at 900 ◦C, respectively. This is directly related tothe crystallization of nanoparticles. The straight lines of

Table 4. The activation energy of nanoparticle formationin the pure and the doped states.

Pure State

Phase Anatase Rutile Ti2O3 Ti8O15

Activation Energy5.78 6.15 5.41 5.81

kJ/mol

Doped State

Phase Anatase Rutile Ti Ag Ag2O Ag2O3

Activation Energy9.87 7.21 2.73 5.74 3.42 3.83

kJ/mol

the lnD against 1/T curves (Figs. 5 and 7) are plottedaccording to the Scott [23] equation given below underthe condition of homogeneous growth of nanocrystallites,which approximately describes the crystal growth duringannealing:

D = C exp(−E/RT ), (2)

where C is a constant, E is the activation energy, R is thegas constant, and T is the absolute temperature. A goodlinear relationship exists. The E values for the pure TiO2

nanorods phases are calculated from the slope of thestraight line as E = 5.78 kJ/mol for anatase, 6.15 kJ/molfor rutile, 5.41 kJ/mol for Ti2O3, and 5.81 kJ/mol forTi8O15.

In the doped state, E = 9.87 kJ/mol for anatase and7.21 kJ/mol for rutile, which are more than the acti-vation energies of the anatase and the rutile phases inthe pure state. The data in Table 4 show the activa-tion energies for nanoparticle formation in the pure andthe doped state during thermal treatment. It shows thatthe calcining temperature has a remarkable effect on thegrowth of nanocrystallites.

The lattice strains of nanocrystallites in pure TiO2

nanorods are determined from the FWHM (full widthhalf maximum) of the diffraction lines observed in 2θrange of 10 – 80◦, according to the Williamson-Hall’sequation [24]:

β cos θ =kλ

L+ 4ε sin θ, (3)

where β is the FWHM observed, ε is the lattice strain,and the shape factor k is assumed to be 0.9 similar toScherrer’s equation. λ (is the wavelength of Kα(Cu) ra-diation). Plots of βcosθ against 4sinθ for different sam-ples are approximately linear. The lattice strain is deter-mined from the slope of this linear relation. Because oflowly-crystallized powder samples, the linearity betweenβcosθ and 4sinθ is not very evident [25]. The plots ofβcosθ against 4sinθ for diverse diffraction lines are il-lustrated in Fig. 8. For low calcining temperatures, theFWHMs are difficult to measured because the peaks are


Fig. 4. (Color online) Effect of thermal treatment temperature on the particle sizes of (a) anatase, (b) rutile, (c) Ti2O3, and(d) Ti8O15 in the pure state (without dopant).

Fig. 5. (Color online) Plot of LnD as a function of calcining temperature for (a) anatase, (b) rutile, (c) Ti2O3, and (d)Ti8O15 in pure state (without dopant).


Fig. 6. (Color online) Effect of thermal treatment temperature on the particle size of (a) anatase, (b) rutile, (c) Ti, (d) Ag,(e) Ag2O, and (f) Ag2O3 in the doped state.

Fig. 7. (Color online) Plot of LnD as a function of calcining temperature for (a) anatase, (b) rutile, (c) Ti, (d) Ag, (e) Ag2O,and (f) Ag2O3 in the doped state.


Fig. 8. (Color online) Relation between βcosθ and 4sinθ(Williamson-Hall plots) with different calcining temperaturesin the pure state.

Fig. 9. Dependence of lattice strain on the calcining tem-perature in the pure state.

weak and broad. As shown in Fig. 8 for sample with aspecific molar ratio of the precursor, the variation of thelattice strain normalized to the calcining temperature isillustrated. Figures 8 and 9 show that the lattice straindecreases with increasing calcining temperature. Withincreasing calcining temperature up to 300 ◦C, the lat-tice strain decreases from 0.8048 to 0.1019, and with afurther increase from 600 to 900 ◦C, the lattice strain in-creases from −0.2783 to −0.0991. This implies that withincreasing calcining temperature, the form of the latticestrain varies from an external to an internal strain.

As can be seen in Figs. 10 and 11 for the dopedstate, with increasing calcining temperature from the as-prepared state to 300 ◦C, the lattice strain increases from−4.5483 to 2.1497, and with a further increase in thecalcining temperature to 600 ◦C, the lattice strain de-

Fig. 10. (Color online) Relation between β cos θ and 4sinθ(Williamson-Hall plots) with different calcining temperaturesin the doped state.

Fig. 11. Dependence of the lattice strain on the calciningtemperature in the doped state.

Fig. 12. TEM image of the nanorod structure of a puresample calcined at 900 ◦C.


Fig. 13. FE-SEM images of pure TiO2 nanorods for differ-ent calcining temperatures, (a) as-prepared, (b) 300 ◦C, (c)600 ◦C, and (d) 900 ◦C.

Fig. 14. FE-SEM images of TiO2 nanorods doped withsilver and silver-oxide for different calcining temperatures:(a) as-prepared, (b) 300 ◦C, and (c) 600 ◦C.

creases to 1.0511. This implies that in the doped state,with increasing calcining temperature, the lattice strainbecomes more than the strain in the pure state. Fig-ure 12 shows TEM image of nanorod structure in thepure sample calcined at 900 ◦C. As shown in this figure,the diameter is 43 nm, and the length is 400 nm. FE-SEM images of TiO2 nanorods are shown in Figs. 13 and14. The as-prepared sample has a spherical cover, butcontains many rods. Nanoparticles seem to be formed ina thin rope shape. In the other images in Fig. 13 the rodsare formed at high temperature, 600 ◦C and 900 ◦C. Fig-ure 14 shows FE-SEM images of TiO2 nanorods dopedwith silver and silver-oxide. Spherically shaped particlesin the as-prepared state form nanorods with increasingcalcining temperature. The dopants affect length and theshape in such a way that they enlarge the length and sep-arate the nanorods.

The surface morphologies of the TiO2 nanorods andthe TiO2 nanorods doped with silver and silver-oxide

Table 5. Roughness parameter of TiO2 nanorods.

Sample Sa (nm) Sq (nm) Sm (pm)

Pure TiO2 nanorods34.979 54.599 45.987

in Fig. 15(a)

Pure TiO2 nanorods37.021 49.139 45.898

in Fig. 15(b)

Doped TiO2 nanorods54.153 64.903 63.48

in Fig. 16

are presented in Figs. 15 and 16. As shown in these fig-ures, the islands have quite compact shapes with lengthsof 2 to 3.5 µm in the pure state and about 10 µm inthe doped state. After the islands to be as an arm-likeshapes, the growth seems to continue in a dendritic or ir-regular pattern while maintaining an arm width of about6 µm. Two issues affect the rod structures: (1) Becauseof diffusion of adatoms into the matrix at higher con-centrations, a larger fraction of the deposited TiO2 ordopants impinges existing islands. These adatoms candiffuse off the first islands and condense at the step edge,thereby thickening the structure. (2) Fractals are formedwhere nanoaggregates shapes have been formed. As thelinear size of the islands become larger, higher percent-ages of the TiO2 or dopants make up the fractal regionsbetween the arms. The TiO2 or the dopants atoms inthese regions tend to fill in the regions and do not con-tribute to further radial growth. The radial growth, onthe other hand, is slowed to the extent that the islandshave not coalesced and can still be identified as individ-ual entities. The dendritic shapes are due to a kineticlimitation existing at room temperature, which can beconcluded from their thermal instability [26], as shownin Figs. 15 and 16.

The surface morphologies are characterized by usingthe average thickness of the sheets, the intervals betweenthe sheets, and roughness parameters such a Sa, Sm, andSq shown in Table 5. The parameter Sa is the roughness

average sa =1N

N−1∑1=0

|Z(x1)|. Sm is the mean value sm =

1N

N−1∑1=0

Z(x1). The parameter Sq is the root-mean-square

value sp =

√√√√ 1n

N−1∑1=0

(Z(x1))2. As can be seen in Figs. 15

and 16 and Table 5, the roughnesses of the surface of thedoped TiO2 nanorods are greater than those of the pureTiO2 nanorods.

The FTIR spectra of TiO2 nanorods calcined at dif-ferent temperatures are recorded in the wave numberrange of 400 – 4000 cm−1 (Fig. 17). In the prepared gel,the 3200 cm−1 band has to be attributed to hydroxylgroups from water and ethanol, which are occluded inthe titania pore. The OH bending band of water in thegel is observed at 1650 cm−1, and in the low-energy re-


Fig. 15. (Color online) AFM images of pure as-prepared TiO2 nanorods in (a) the congestion state and (b) the singlenanoaggregates state.

Fig. 16. (Color online) AFM images of as-prepared TiO2nanorods doped with silver and silver-oxide.

gion, the Ti-O bands are found at 1061 cm−1 and below1000 cm−1. The IR spectra show peaks characteristic ofTi-O-Ti (495 – 436 cm−1). When the composite is cal-cined at 900 ◦C, the high-energy stretching band almostfade and the 1650 and 3200 cm−1 bending vibration bandintensities decrease due to vaporization of the liquid. Acomparison between pure nanorods and doped nanorodsindicates that the band at 1453 cm−1 can be to the silverions in the Ag-O vibration bond.

V. CONCLUSION

The homogeneous hydrolysis of metal alkoxide andcorrosion with NaOH provided an excellent technique toprepare TiO2 nanorod materials. Experimental results

indicated that the homogeneous hydrolysis of tetra iso-propy ortho titanate via the sol-gel route is a promisingtechnique for preparing photosensitive material with uni-form nanoparticles. In this study, nanocrystalline TiO2

nanorod particles were successfully synthesized by us-ing a chemical method and a heat treatment process.The phase transformation of titanium dioxide dependson calcining temperatures. The calcining temperaturesand the addition of other oxides such as silver and silver-oxide affected the structural properties, such as the size,strain and activation energy. The average crystallite sizesincreased with increasing calcining temperature.

With increasing calcining temperature in the purestate, the crystallite size were shown to increased. Inaddition, the lattice strain to decrease from 0.8048 to0.1019 with increasing the calcining temperature up to


Fig. 17. (Color online) FTIR spectra of (a) pure TiO2

nanorods and (b) TiO2 nanorods doped with silver and silver-oxide and calcined at different temperatures.

300 ◦C, and then increased from −0.2783 to −0.0991with further increase from 600 to 900 ◦C. In the dopedstate, with increasing calcining temperature from the as-prepared state to 300 ◦C, the lattice strain increased from−4.5483 to 2.1497, and with a further increase in thecalcining temperature to 600 ◦C, the lattice strain de-creased to 1.0511. This implies that in the doped state,with increasing calcining temperature, the lattice strainwas more than the strain in the pure state. The activa-tion energy of pure TiO2 nanorod phases was determinedfrom Scott equation. The FTIR spectrum of the ternarycomposite was presented and showed possible Ti-O, Ti-O-Ti, Ti-OH, and Ag-O bonds.

ACKNOWLEDGMENTS

The author thank Tonekabon Branch, Islamic AzadUniversity, for financial support through a researchproject .

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Documents

Fabrication of pure and Ag-doped TiO2 nanorods and study of the lattice strain and the activation energy of the crystalline phases