Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 218–224

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journal homepage: www.elsevier .com/locate /saa

Preparation, structural and morphological studies of Ni doped titaniananoparticles

http://dx.doi.org/10.1016/j.saa.2014.02.1161386-1425/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 9443283241.E-mail address: rajamannanb@gmail.com (B. Rajamannan).

B. Rajamannan a,⇑, S. Mugundan b, G. Viruthagiri b, N. Shanmugam b, R. Gobi b, P. Praveen b

a Department of Engineering Physics (FEAT), Annamalai University, Annamalainagar 608002, Indiab Department of Physics, Annamalai University, Annamalainagar 608002, India

h i g h l i g h t s

� No change in anatase phase of bareand Ni doped TiO2 was predicted evenafter calcinations at 500 �C.� No alkoxy groups were identified in

FTIR spectra.� The peaks which were existing in bare

TiO2, were blue shifted as result ofdoping.� Particles are almost in spherical

shape with uniform size distribution.� TEM image that the particle size is 8–

16 nm, which are in good agreementwith the XRD results.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 October 2013Received in revised form 4 February 2014Accepted 19 February 2014Available online 12 March 2014

Keywords:TiO2 nanoparticlesCrystalline sizeFT-IROptical propertiesSpherical uniform structureSHG Efficiency.

a b s t r a c t

TiO2 nanoparticles doped with different weight percentages (4%, 8%, 12% and 16%) of nickel contents wereprepared by a modified sol–gel method using Titanium tetra iso propoxide and nickel nitrate as precur-sors and 2-propanol as a solvent. X-ray diffraction studies show that the as prepared and annealed prod-ucts show anatase structure with average particle sizes running between of 8 and 16 nm. FTIR resultsdemonstrate the presence of strong chemical bonding at the interface of TiO2 nanoparticles. The opticalproperties of bare and doped samples were carried out using UV-DRS and photoluminescence measure-ments. The surface morphology and the element constitution of the nickel doped TiO2 nanoparticles werestudied by scanning electron microscope attached with energy dispersive X-ray spectrometer arrange-ment. The non linear optical properties of the products were confirmed by Kurtz second harmonicgeneration (SHG) test and the output power generated by the nanoparticle was compared with that ofpotassium di hydrogen phosphate (KDP).

� 2014 Elsevier B.V. All rights reserved.

Introduction

Titanium dioxide (TiO2) has been extensively studied in the pastfew years owing to its technological applications. It has been wellrecognized that among the semiconductors employed inphotocatalysis, TiO2 is the most effective photocatalyst due to its

high performance, photostability (i.e., resistance to photo corro-sion), low cost, nontoxicity and availability. It is one of the mostpromising photocatalysts for environmental cleanup, photo gener-ation of hydrogen from water, and solar energy utilization [1–3].

TiO2 crystallizes in three different structures: brookite, anatase,and rutile, among them, the anatase phase has the highest photo-catalytic activity compared with the rutile and brookite phases[4,5]. However, the rutile structure is the most stable form at hightemperature, and anatase and brookite both can be converted to

B. Rajamannan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 218–224 219

rutile upon heating [6]. These three phases can be commonly de-scribed as constituted by arrangements of the same buildingblock-Ti–O6 octahedron in which Ti atom is surrounded by six oxy-gen atoms situated at the corners of a distorted octahedron. Inspiteof the similarities in building blocks of Ti–O6 octahedra for thesepolymorphs, the electronic structures are significantly different.

Titanium dioxide (TiO2) offers a great attention due to itsunique physical and chemical properties such as wide energy gap(3.2 eV), high electrochemical stability, refractive index, anddielectric constant [7,8].

Titanium dioxide (TiO2) is a well known semiconductor photo-catalytic material that has attracted much attention due to itsbiological and chemical inertness, strong photo oxidizing power,cost effectiveness, and long term stability against photo and chem-ical corrosion [9]. Among the semiconductors, titanium dioxide isthe most active photocatalyst and commonly used in organic com-pounds degradation. However, the widespread technological use ofTiO2 is impaired by its wide band gap (3.2 eV for crystallineanatase phase), the fast charge-carrier recombination, and thelow interfacial charge-transfer rates [10].

Nanosized TiO2 has been fabricated using sol–gel, sputtering,combustion flame, and thermal plasma [11,12]. Although the sol–gel method is considered as a suitable method to synthesizeultra-fine particles, this method needs a large quantity of solution,longer processing time and heat treatment for crystallization sinceamorphous TiO2 has a very little photocatalytic activity. In thiswork, we report on the sol–gel synthesis and characterization ofTiO2 nanomaterials doped with nickel in different atomic weightpercentages (4%, 8%, 12% and 16%) capable to have enhanced opti-cal properties. Since, Titanium tetra iso-propoxide is non toxic andecofriendly, in the present in experiment it is used as a titaniumsource. Also, the structural, functional and morphological analysesof the nanopowders were performed. The NLO activity of the TiO2

nanostructures was investigated for the first time.

Sample preparation

Synthesis of bare and Ni doped TiO2 nanoparticles

To synthesize Ni doped TiO2, 90 ml of 2-propanol (C3H8O) andnickel nitrate [Ni(NO3)2] in 10 ml aqueous solution with differentconcentrations (4%, 8%, 12% and 16%) were mixed drop by drop.The mixture was stirred magnetically at room temperature untila homogenous solution was obtained. Then 10 ml of Titanium tetraiso-propoxide was added drop by drop to the above mixture. Theentire was continuously stirred for 5 h using a magnetic stirrer.After stirring the solution was filtered by using whatman filter pa-per and washed several times using deionized water. The precipi-tate formed was dried at 80 �C for 5 h to evaporate organicresidues. Then the dry gel was calcined at 500 �C and 800 �C for2 h to obtain desired anatase and rutile TiO2 nanoparticles. Thenthe calcined powders were grained in an agate mortar to avoidagglomeration.

The bare TiO2 nanoparticles were synthesized by following thesame procedure without doping material.

Fig. 1. X-ray diffraction pattern for bare TiO2 and Ni doped TiO2 nanoparticle.

Characterizations

The crystalline phase and particle size of pure and Ni–TiO2

nanoparticles were analyzed by X-ray diffraction (XRD) measure-ment which was carried out at room temperature by usingXPERT-PRO diffractometer system (scan step of 0.05� (2h),counting time of 10.16 s per data point) equipped with a Cu tubefor generating Cu Ka radiation (k = 1.5406 Å) as an incident beamin the 2-theta mode over the range of 20–80�, operated at 40 kV

and 30 mA. The functional groups were idenified by AVATAR 330Fourier-transform infrared spectrometer in which the IR spectrawere recorded by diluting the mixed powders in KBr and in thewavelength between 4000 and 400 cm�1. The band-gap energyand the particle size were measured at wavelength in the rangeof 200–2500 nm by UV–Vis–NIR spectrophotometer (Varian/Carry5000) equipped with an integrating sphere and the baseline cor-rection was performed using a calibrated reference sample of pow-dered barium sulphate (BaSO4). The photoluminescence (PL)spectra of the samples were recorded with a Spectroflurometer(Jobin Yvon, FLUOROLOG – FL3-11). The surface morphology ofpure and Ni–TiO2 was observed by a scanning electron microscope(SEM: Hitachi, S-3400 N). The dispersion of titanium, oxygen andnickel in the products was characterized by energy dispersiveX-ray elemental analysis (EDX: Thermo Super Dry II) equipped tothe scanning electron microscope. Transmission electronmicroscope (TEM) images were taken using a technai t20 operatedat voltage of 200 kV. The NLO property of the materials wasconfirmed by the Kurtz powder second harmonic generation(SHG) test. The materials were illuminated using Spectra physicsQuanta Ray DHS2. The SHG radiations of 532 nm green light werecollected by a photomultiplier tube (PMT-Hamasu R2059) afterbeing monochromated (monochromator – Czerny-Turner) to col-lect only the 532 nm radiation. A Q-switched Nd–YAG laser whoseoutput was filtered through 1064 nm narrow pass filter was usedfor this purpose. The input power of the laser beam was measuredto be 4.5 mJ/pulse.

Results and discussion

Powder X-ray diffraction study (XRD)

Fig. 1 shows X-ray powder diffraction patterns of pure and Nidoped TiO2 nanoparticles calcined at 500 �C. All observed peakscould be corresponding to the anatase phase tetragonal structureof TiO2 (JCPDS card No. 21-1272). There is no indication of rutileand brookite phase formation on 500 �C of sintering. The peaks at2h = 25.5� (101) and 48� (200) were in anatase phase of TiO2

[13]. However, few peaks related to nickel were identified at2h = 33.10� and 36.88� indicate the presence of nickel [14]. The Dif-fraction Plane (101) shows a sharp and higher intensity for un-doped and Ni doped samples due to nature TiO2 nanoparticles.From the obtained peak the average nanocrystalline sizes wasmeasured according to Debye Scherrer formula (Eq. 1) and werepresented in Table 1.

D ¼ KkbCosh

Å ð1Þ

Fig. 2. FTIR spectra pattern for bare TiO2 and Ni doped TiO2 nanoparticle.

Fig. 3. UV-DRS spectra for bare TiO2 and Ni doped TiO2 nanoparticle-directtransition.

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where D is the crystallite size; K is the shape factor; k is the0.154 nm; b is the full width at half maximum and h is reflectionangle.

The mean crystallite size of bare TiO2 is 15.31 nm, whereas ondifferent levels (4%, 8%, 12% and 16%) of Ni doping the estimatedsizes are 8.77, 16.04, 16.82 and 11.77 nm respectively.

When compared to bare TiO2 all the Ni doped products show in-creased size as a result of improved crystallinity (except 4% and16%). The reduction of particle size from 15.31 to 8.77 nm on4 wt.% of Ni doping reveals its optimum level.

Fourier Transform Infrared Spectroscopy (FTIR)

Fig. 2 shows the FTIR spectra of the obtained bare and Ni dopedTiO2 nanoparticles after calcined at 500 �C for 5 h. The broad bandaround 3400 cm�1 is attributed to O–H stretching, and the peaknear 1600 cm�1 is due to O–H bending which are related to phys-ically absorbed moisture. The IR band observed from 400 to900 cm�1 corresponds to the Ti–O stretching vibrations [15,16].The band at 1634 cm�1 was attributed to stretching vibrations ofOH. The peaks in between 2924 and 2843 cm�1 were assigned toC–H stretching vibrations of alkane groups. The alkane and carbox-ylic groups come from Titanium tetra isopropoxide and 2-Propanol(precursor material), when we used in the synthesis process. Inaddition, a broad absorption band between 500 and 1000 cm�1 isascribed to the vibration absorption of the Ti–O–Ti linkages inTiO2 nanoparticles [17]. However, the vibrational bands between1300 and 4000 cm�1 are mainly assigned to the chemisorbed andphysisorbed H2O and CO2 molecules on the surface of the com-pound. In addition, the band centered at 1371 cm�1 is assignedto bending vibrations of C–H bond in the species linking–Ti–O–Ti– structural network [18,19]. When metal ions are dopedto the surface of TiO2, the absorption band significantly transformsand simultaneously new absorption band appears.

Ultra violet-diffuse reflectance spectra (UV-DRS)

Fig. 3 shows UV-DRS spectra of bare and Ni doped TiO2 nano-particles obtained after calcined at 500 �C for 5 h. The TiO2 materialshowed an intense absorption in the UV region and the absorptionedge of titania can be easily discerned. Compared with bulk TiO2 ashift of the reflectance spectra of the synthesized TiO2 nanocrystalstowards the lower wavelength region were observed. This result isidentical with the reported one [20].

The band gap energy (Eg) of pure TiO2 was obtained from thewavelength value corresponding to the intersection point of thevertical and horizontal part of the spectrum, using (Eq. (2))

Eg ¼hck

eV; Eg ¼1240

keV ð2Þ

where Eg is the band gap energy (eV); h is the Planck’s constant(6.626 � 10�34 Js); c is the light velocity (3 � 108 m/s) and k is thewavelength (nm).

The absorption edge of nano TiO2 was noted at 345.49 nm, cor-responding to a band gap of 3.5891 eV, which is usually ascribed tocharge-transfer from the valence band (mainly formed by 2p

Table 1Crystallite sizes for the bare and Ni doped TiO2 nanoparticle.

Samples Crystallite size (nm)

Bare TiO2 15.314% Ni doped TiO2 8.778% Ni doped TiO2 16.0412% Ni doped TiO2 16.8216% Ni doped TiO2 11.77

orbitals of the oxide anions) to the conduction band (mainlyformed by 3dt2g orbitals of the Ti4+ cations) [21]. However, on dop-ing the absorption edge was blue shifted. Therefore, the increase ofincorporating Ni content, the band-gap energy of the TiO2 nanopar-ticles increased systematically.

At the same time on a higher level of doping (8–12%) in additionto UV band visible bands are also predicted. These bands are posi-tioned at 571,600 and 585 nm respectively for 8%, 12% and 16% ofNi doping.

The obtained values of Eg are 3.58, 3.65, 3.61, 3.83 and 4.0 eV forthe TiO2 and TiO2 doped with 4%, 8%, 12% and 16% nickel as shownin Table 2 respectively, suggesting the change the band gap energyof TiO2 on Ni doping.

Photoluminiscence (PL)

The room temperature PL spectra were recorded for bare and 8%of nickel doped TiO2 with an excitation wavelength of 325 nm(Fig. 4). From the PL spectra, it is predicted that the bare TiO2

exhibits three emission bands, one at UV region (377 nm) and

Table 2Bandgap energy for bare and Ni doped TiO2 nanoparticle.

Samples Bandgap (eV)

Bare TiO2 3.584% Ni doped TiO2 3.658% Ni doped TiO2 3.6112% Ni doped TiO2 3.8316% Ni doped TiO2 4.0

Fig. 4. PL spectra for bare TiO2 and Ni doped TiO2 nanoparticle.

B. Rajamannan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 218–224 221

the other two at visible regions (431 and 587 nm). A band at377 nm is attributed to the first vibronic fluorescence band. Theobvious PL peak at about 431 nm may be due to band edge freeexcitons and this surface emission is attributed to indirect transi-tion X1a to C1b and linked to exciton recombination in shallowtrapped surface state [22,23].

The PL spectrum of doped TiO2 is dominated by the broad bandemission in the range of 580–620 nm. The PL intensity was

Fig. 5. SEM micrograph of TiO2 nanoparticles (a a

drastically decreased with the incorporation of Ni in the TiO2 nano-particles. The broad band emission from the PL spectrum at roomtemperature indicates that the TiO2 and Ni–TiO2 nanoparticle havegood luminescence quality [24]. The intensity of the PL peaks is anindicator for the electrons/hole recombination rate, low intensityindicates low recombination and vice versa. As shown in the figure,incorporation of Ni–TiO2 nanoparticle leads to decrease the inten-sity which preliminarily claims good photoactivity for theNi-doped TiO2 nanoparticles compared with the undoped ones.

On doping these bands are shifted to lower wavelength regionswith reduced intensity. However, the broadness of the bands hasbeen enhanced on doping; the broadness may be open up anenhanced luminescent property of the doped product.

Scanning electron microscope (SEM with EDX)

Fig. 5(a and b) shows the scanning electron microscope (SEM)image of bare TiO2 nanoparticles. As shown in figure most of theparticles are almost in spherical shape with uniform size distribu-tion and they consists of either some single particles or cluster ofparticles. But, on (8 wt.%) Ni doping multi sized particles are pre-dicted with non uniform distribution (Fig. 6(a and b)). This resultis in accordance with the fact that Ni doping can alter the TiO2 par-ticles growth [25].

The energy dispersive X-ray (EDX) spectrum is used to analyzethe Ni doped TiO2 nanoparticles, and the results are shown inFigs. 5(c) and 6(c). Peaks of Ti, O and Ni are obviously found inthe spectra, confirming the formation of Ni doped TiO2 nanoparti-cles [26].

Transmission electron microscope (TEM with SAED)

Fig. 7(a and b) shows the transmission electron microscope(TEM) micrographs of bare TiO2 nanoparticles. As shown in figure,most of the particles are get agglomerated and showing a crosslinked chain structure with sizes in the range of 15.31 nm. TheSAED pattern of bare TiO2 nanoparticles reveals the formation of

nd b) and corresponding to EDX diagram (c).

Fig. 6. SEM micrograph of 8% Ni doped TiO2 nanoparticles (a and b) and corresponding to EDX diagram (c).

Fig. 7. TEM image of TiO2 nanoparticles (a and b) and corresponding to SAED pattern (c).

222 B. Rajamannan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 218–224

Fig. 8. TEM image of 8% Ni doped TiO2 nanoparticles (a and b) and corresponding to SAED pattern (c).

Table 3Comparison of SHG efficiency of bare and 8% Ni doped TiO2 nanoparticles.

Samples Input (mJ/pulse)

Output(mV)

KDP(mV)

SHGefficiency

Bare TiO2 4.5 8.0 10.2 0.78Ni doped TiO2 2.6 5.6 17.6 0.31

B. Rajamannan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 218–224 223

well separated concentrate rings indicating its higher crystallinenature (Fig. 7c).

Further, on Ni (8 wt.%) doping well separated individual parti-cles are noted in the TEM images (Fig. 8(a and b)). However, thesizes of the particles are in the range of 8–16 nm. The SAED patternof Ni doped TiO2 exhibiting well isolated concentric rings, reveal-ing the improved crystalline nature (Fig. 8(c)).

Nonlinear optical studies

The oven dried powders of pure and 8% Ni doped TiO2 were keptinto muffle furnace and annealed at 800 �C for 4 h to obtain rutilephase. In order to confirm the NLO property, the specimens wassubjected to a Kurtz powder test [27] using a Q-switched, modelocked Nd: YAG laser of 1064 nm and a pulse width of 8 ns. The in-put laser beam was directed on the synthesized powders to getmaximum powder SHG then the emitted light passed through anIR filter was measured by means of a photomultiplier tube andoscilloscope assembly. The SHG efficiency of the materials wasevaluated by taking the crystalline powder of KDP as the referencematerial. The SHG behavior was confirmed by the output of intense

green light (532 nm) emission from both the materials (bare & Nidoped TiO2) and KDP. Thus the efficiency results show an outputof 8.0 mV for pure TiO2, when compared to 17.6 mV of KDP withsame input laser power of 2.6 m J/pulse. The relative SHG conver-sion efficiency of bare TiO2 is found o be about 0.78 times that ofKDP. However, the SHG efficiency of Ni doped TiO2 is 0.31 timesless than that of KDP respectively. The SHG/NLO measurement re-sults of bare and doped materials are presented in Table 3.

Conclusion

The bare and Ni (4%, 8%, 12% and 16%) doped TiO2 nanoparticleswere successfully synthesized by sol–gel method at room temper-ature and then annealed at 500 �C and 800 �C for getting anataseand rutile phases, respectively. The XRD analyses reveal that theprepared products were attributed to the tetragonal system andanatase phase was found to be present in all the samples. Whencompared to bare TiO2, the presences of hydroxyl ions were con-firmed by FTIR analysis in doped TiO2. However, no alkoxyl groupsare present in the samples. The optical cut-off wavelengths of Nidoped TiO2 were blue shifted towards the lower wavelength side,resulting larger band gap energies compared to pure TiO2. PL Spec-tra observed a broad and sharp range of emission spectra. Thepeaks which were existing in bare TiO2 were blue shifted as a re-sult of doping. The morphological analysis of 8% Ni–TiO2 nanopar-ticles indicates that the uniform distribution of spherical shapedparticles compared to pure TiO2. As can be seen from the TEM im-age of 8 wt.% of Ni doped TiO2, the particles are having sizes in therange of 8–16 nm, which are in good agreement with the crystallite

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size obtained from XRD results. When compared with doped prod-uct bare TiO2 shows more than two fold NLO efficiency.

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