Unique sharp photoluminescence of size-controlled sonochemically synthesized zirconia nanoparticles

Ultrasonics Sonochemistry 23 (2015) 174–184

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Ultrasonics Sonochemistry

journal homepage: www.elsevier .com/ locate/ul tson

Unique sharp photoluminescence of size-controlled sonochemicallysynthesized zirconia nanoparticles

http://dx.doi.org/10.1016/j.ultsonch.2014.10.0041350-4177/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 44 22358689.E-mail address: [email protected] (D. Manoharan).

Divinah Manoharan ⇑, Aswaghosh Loganathan, Vishista Kurapati, Victor Jaya NesamonyDepartment of Physics, Anna University, Chennai 600 025, Tamil Nadu, India

a r t i c l e i n f o

Article history:Received 20 June 2014Received in revised form 3 October 2014Accepted 4 October 2014Available online 12 October 2014

Keywords:ZirconiaNanophosphorsPhotoluminescenceEnergy band gapKubelka–Munk plotRietveld refinement

a b s t r a c t

The present study explores the features of tetragonally stabilized polycrystalline zirconia nanophosphorsprepared by a sonochemistry based synthesis from zirconium oxalate precursor complex. The sonochem-ically prepared pristine zirconia, 3 mol%, 5 mol% and 8 mol% yttrium doped zirconia nanophosphors werecharacterized using thermo-gravimetric analysis (TGA), X-ray diffraction (XRD), Raman spectroscopy,field emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy(EDS), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and photolumi-nescence spectroscopy (PL). The reaction mechanism of formation of zirconia nanophosphors is discussedin detail. The probable sonochemical formation mechanism is being proposed. Stabilization of tetragonalphase of pristine zirconia even at room temperature was effectively established by controlling the parti-cle size using ultrasonic waves. Improved phase purity and good surface morphology of the nanophos-phors is being achieved via sonochemical route. FE-SEM micrographs reveal that the nanoparticleshave uniform spherical shape and size. The narrow particle size distribution (�15–25 nm) of the zirconiananoparticles was found from FE-SEM statistical analysis and further confirmed by TEM. Zirconia nano-phosphors exhibit a wide energy band gap and which was found to vary with yttrium dopant concentra-tion. The highlight of the present study is the synthesis of novel nanocrystalline ZrO2 and Y-ZrO2

phosphor which simultaneously emits extremely sharp as well as intense UV, violet and cyan light onexciting the host atom. The yttrium ion dopant further enhances the photoluminescence property of zir-conia. These nanocrystalline phosphors are likely to have remarkable optical applications as light emit-ting UV-LEDs, UV lasers and multi color displays.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Pure ZrO2 exists in three polymorphic forms namely mono-clinic, tetragonal and cubic. The monoclinic form is thermodynam-ically stable at room temperature, however transforms reversiblyto the tetragonal structure above 1170 �C and above 2370 �C, a sta-ble cubic structure is formed [1]. Yet, by incorporation of somealiovalent species, the metastable cubic and tetragonal phase canbe well stabilized at room temperature. High temperature poly-morphs (tetragonal and cubic) have to be stabilized at room tem-perature either by adding stabilizers such as Y2O3, MgO and CaOor by reducing the crystallite size to nanometer scale so as toenable effective application in various fields [2]. Among the dopedalloys of ZrO2, yttria-stabilized zirconia (YSZ) plays an importantrole owing to its wide applications and exceptional properties suchas high mechanical strength, good chemical stability, high level of

oxygen-ion conductivity, corrosion resistance, low thermal con-ductivity, and interesting luminescent functions [3,4].

Nanophosphors are basically those nanomaterials which exhibitthe phenomenon of luminescence, i.e. the nanomaterials have theability to absorb the high energy (short wavelength) radiation suchas ultraviolet light and down-convert them into lower energy(longer wavelength) radiation. Nanoscale phosphors are expectedto be more advantageous and beneficial than micron-sized phos-phors, because the nanomaterials often exhibit important differ-ences in the physical and chemical properties compared to thebulk. These unique properties are related to the large number ofsurface atoms. Obviously, the quantum effects due to the high sur-face to volume ratio may enhance the luminescence characteristics[5]. This has currently invoked advanced research in new inorganicnano-phosphor materials for new lighting devices. Inorganic oxidenanophosphors have gained much interest in recent years and areconsidered as the most potential candidate material for wideapplication in optical communication fields, fluorescent lamps,solid-state lasers, lighting and displays. Owing to the importance

D. Manoharan et al. / Ultrasonics Sonochemistry 23 (2015) 174–184 175

in optical applications, the study on luminescence property in YSZhas gained much interest in the past decade [6–9]. Materials sci-ence research has provoked identification of new materials withshorter wavelength light emitting property for application inshort-wavelength lasers which play important roles in laser print-ing and information storage. This is because of the fact that shorterthe wavelength of the laser, the higher the information storagedensity of the compact disc [10].

Furthermore, it is suggested that the luminescence propertiesare dependent on the size, crystallinity and morphology of thenanoparticles which in turn is dependent on the synthesis proce-dure and reaction conditions [11–13]. Numerous methods havebeen used to produce zirconia nanoparticles such as ball milling[14], precipitation method [15], sol–gel processing [16], hydrother-mal treatments [17], microemulsion techniques [18], chemicalvapor synthesis [19], inert gas condensation [20], combustion syn-thesis [21] and microwave approach [22]. Hitherto limitedresearch has been reported on the synthesis of ZrO2 by sonochem-ical method [23,24]. In the present study, an oxalate precipitationtechnique coupled with ultrasonic irradiation as a physical aid hasbeen employed to prepare the zirconia and yttrium stabilized zir-conia nanophosphors which has not been reported elsewhere.

One of the most widely used solution techniques for synthesisof nanostructured materials is the co-precipitation method. Themajor negative aspect of the process is the inability to controlthe size of the precipitating particles and their subsequent aggre-gation. To overcome this problem, a secondary aid such as surfac-tant assisting is required during synthesis process. However,removal of surfactant from the material is complicated and oftenpossesses residual impurities which in turn may deteriorate thematerials properties. Alternatively, the better choice to controlthe grain growth during precipitation is physical agitation throughultrasonication process. If the precipitation technique could be tai-lored with ultrasonication such that the de-agglomeration of thesynthesized nanomaterial could be accomplished then that hybridmethod would be able to produce fine grained crystalline nanom-aterials. Sonochemical synthesis under the irradiation of ultra-sound in solution is thus an effective method that can be used toprepare uniform sized ultrafine nanostructures. In this paper, wereport the structural, vibrational, electronic and optical propertiesof the sonochemically prepared zirconia and yttrium stabilized zir-conia nanophosphors.

2. Materials and methods

2.1. Starting materials

All the chemicals and reagents used were analytical grade andused as such without any further purification. Zirconium (IV) oxy-chloride octahydrate (Himedia), Yttrium (III) nitrate hexahydrate(Himedia), Oxalic acid dihydrate (Merck), 32% extrapure ammoniasolution (Merck), Ethanol (Merck) and double distilled water wereused for the synthesis.

Fig. 1. Schematic representation of the experimental set-up.

2.2. Synthesis of pristine zirconia and yttria stabilized zirconiananophosphors

50 ml of 0.5 M precursor solution in the ratio Zr:Y::1 � x:x(where x = 0, 0.03, 0.05 & 0.08) was made by dissolving the precur-sor salts in double distilled water by stirring. 0.5 M oxalic acid wasadded drop-wise to the above mixed solution under vigorous stir-ring until the solution turns pale white gel. Addition of oxalic acidand stirring was continued till the gel gets dissolved and forms awhite solution. The molar ratio of precursor solution to oxalic acidwas 1:1. Precipitation of the precursor solution by drop-wise addi-

tion of 10 M liquid ammonia was done to maintain the pH of thesolution at 8.

The schematic representation of the sonochemical experimen-tal set-up is shown in Fig. 1. High intensity ultrasound (20 KHz,40 W/cm2) was applied to the zirconium hydroxide precipitatefor 30 min while maintaining the solution at room temperatureby immersing it in a cold water bath. The Titanium horn was keptwell immersed into precursor solution taken in a round bottomflask in order to facilitate uniform acoustic wave propagation inthe reaction mixture. Ultrasound waves were applied to the pri-mary precursor complex solution to intensify the formation of zir-conia nuclei and subsequent control over the nuclei growth. Theobtained precipitate was aged for 12 h at room temperature andthen the supernatant water was decanted. The obtained precipitatewas again subjected to ultrasonication for another 15 min by main-taining the temperature at room temperature by using a cold waterbath. The precipitate was centrifuged and washed with water andethanol several times. Complete removal of chloride ion wasassured by the silver nitrate test conducted with the supernatantwater. Then the precipitate was dried in hot air oven at 100 �C.The dried precipitates were calcined at 800 �C for 2 h to form nano-crystalline zirconia particles. For convenience, the samples purezirconia, 3, 5 and 8 mol% yttria stabilized zirconia prepared bysonochemical method are named as ZrO2, 3YSZ, 5YSZ and 8YSZ.For the purpose of comparison, zirconia was also prepared by con-ventional precipitation method by stirring for 12 h without theultrasonication process which was named as ZrO2 (P).

The optimization of experimental parameters of sonochemicalreaction was done by varying the ultrasound power, irradiationtime and reaction temperature. The best experimental parameters(Table 1) from the above optimization were chosen for the sono-chemical reaction as 40 W/cm2 power, 30 min irradiation timeand room temperature. The synthesis at various stirring speedhad no observed distinguished effect as it enables only homoge-neous mixing of the reaction mixture; however, the stirring timeaffects the yield. Therefore, the stirring time was chosen to be 12 h.

2.3. Characterization of the prepared nanophosphors

The thermal analysis of the as-prepared zirconia in air atmo-sphere was done using SDT Q600 V8.0 Instrument in the tempera-ture range 29–980 �C. FE-SEM images were recorded using JEOL6500 instrument. TEM images were recorded using Tecnai 20 G2(FEI). The identification of phases in the synthesized zirconia pow-der sample was carried out by X-ray powder diffraction studies atroom temperature on a PANanalytical X’pert PRO X-ray diffractom-eter using Cu Ka radiation (1.54 Å), and comparing the interplanardistances and intensity values with those of the corresponding

Table 1Optimization of reaction parameters for sonochemical synthesis method.

Power (W/cm2) Irradiation time (min) Average crystallite size (nm) and dominant phase present at different temperatures

Room temperature 40 �C 50 �C 60 �C

6 10 42 (m, t) 45 (m, t) 49 (m, t) 57 (m)20 39 (m, t) 42 (m, t) 45 (m, t) 52 (m)30 37 (m, t) 40 (m, t) 42 (m, t) 46 (m, t)45 36 (m, t) 40 (m, t) 42 (m, t) 45 (m, t)60 37 (m, t) 40 (m, t) 42 (m, t) 46 (m, t)

20 10 34 (m, t) 38 (m, t) 41 (m, t) 44 (m, t)20 30 (m, t) 33 (m, t) 37 (m, t) 43 (m, t)30 28 (m, t) 32 (m, t) 35 (m, t) 43 (m, t)45 28 (m, t) 33 (m, t) 36 (m, t) 43 (m, t)60 27 (m, t) 32 (m, t) 36 (m, t) 44 (m, t)

30 10 27 (m, t) 33 (m, t) 35 (m, t) 42 (m, t)20 23 (t) 29 (m, t) 34 (m, t) 39 (m, t)30 20 (t) 26 (t) 33 (m, t) 37 (m, t)45 20 (t) 26 (t) 34 (m, t) 38 (m, t)60 20 (t) 27 (m, t) 35 (m, t) 38 (m, t)

40⁄ 10 27 (t) 32 (m, t) 34 (m, t) 40 (m, t)20 19 (t) 26 (t) 33 (m, t) 40 (m, t)

30⁄ 14 (t)⁄ 21 (t) 29 (t) 37 (m, t)45 14 (t) 22 (t) 29 (t) 37 (m, t)60 14 (t) 22 (t) 29 (t) 39 (m, t)

50 10 27 (m, t) 33 (m, t) 36 (m, t) 43 (m, t)20 19 (t) 27 (t) 35 (m, t) 40 (m, t)30 14 (t) 22 (t) 28 (m, t) 36 (m, t)45 14 (t) 22 (t) 29 (m, t) 38 (m, t)60 16 (t) 22 (t) 28 (m, t) 38 (m, t)

100 10 25 (t) 31 (m, t) 33 (m, t) 36 (m, t)20 24 (t, c) 26 (c, t) 29 (c, t) 26 (c, t)30 22 (t, c) 25 (c, t) 28 (c, t) 29 (c, t)45 31 (m, t) 38 (m, t) 46 (m, t) 54 (m)60 39 (m, t) 43 (m, t) 52 (m, t) 54 (m)

m, t and c denote the monoclinic, tetragonal and cubic phase of zirconia.The corresponding values are made bold for easy indentification.

* denotes the optimized reaction parameters chosen for the present work.

Fig. 2. Reaction flowchart for the sonochemical synthesis.

176 D. Manoharan et al. / Ultrasonics Sonochemistry 23 (2015) 174–184

Fig. 3. Thermogram of the sonochemically as-prepared zirconia nanophosphor.

Fig. 4. (a, b) FE-SEM image and the corresponding EDAX spectrum of ZrO2 prepared by s8YSZ prepared by sonochemical method, (e, f) FE-SEM image and the corresponding ED


standard peaks using JCPDS files. Rietveld powder structure refine-ment method by using PANanalytical X’Pert High Score Plus soft-ware was utilized to determine the microstructural parametersof the synthesized powders. The starting micro-structural parame-ters of the phases were taken from the database of the Rietveldsoftware. All the peaks in the XRD pattern have been indexed totheir respective (hkl) planes. The crystallite size of the zirconiananophosphors have been evaluated from X-ray powder diffrac-tion data using Scherrer formula [25] given in Eq. (1)

D ¼ kk=ðb cos hÞ ð1Þ

where D is the crystallite size; k = 0.89, a correction factor toaccount for particle shape; b is the full width at half maximum(FWHM) of the most intense diffraction plane; k is the wavelengthof Cu target = 1.54 Å; and h is the Bragg angle.

The volume fraction of tetragonal and monoclinic phase wasdetermined by the relative intensity relationship of X-ray diffrac-tion [26] using the Eq. (2a) and (2b),

onochemical method, (c, d) FE-SEM image and the corresponding EDAX spectrum ofAX spectrum of ZrO2 prepared by conventional precipitation method.

Fig. 5. (a) Particle size distribution of sonochemically synthesized ZrO2 and (b)particle size distribution of sonochemically synthesized 8YSZ.

Fig. 6. TEM image of sonochemically synthesized zirconia nanophosphors and theirrespective histogram of particle size distribution (inset).


%mXRD ¼ ½Imð111Þ þ Imð�111Þ�=½Imð111Þ þ Imð�111Þ þ Itð101Þ�ð2aÞ

%tXRD ¼ 100� ð%mXRDÞ ð2bÞ

where the subscripts m and t refer to the monoclinic and tetragonalphases and I refers to the X-ray diffraction intensity of the corre-sponding peak.

The visible Raman spectroscopy was done in Jobin Yvon LabRaman HR 800 with laser source of wavelength 632.8 nm in therange 100–800 cm�1. The volume fraction of tetragonal and mono-clinic phase [27] was determined by using the following equations,

%mRaman ¼ ½Imð180Þ þ Imð192Þ�=½Itð148Þ þ Imð180Þþ Imð192Þ� ð3aÞ

%tRaman ¼ 100� ð%mRamanÞ ð3bÞ

where the subscripts m and t refer to the monoclinic and tetragonalphases and I refers to the area under the respective Raman modes.

The energy band gap was determined from UV Diffuse reflec-tance data by transforming it into a function of reflectance as pro-posed by Kubelka–Munk. KM plot is plotted with the nth power ofproduct of function of reflectance F(R) and photonic energy (Eg = -ht) against the photonic energy. Since zirconia is considered as adirect band gap semiconductor, the value of n is taken as 2 forallowed transitions [28]. The energy band gap is found out byextrapolating the linear portion of the graph to the X-axis. Photo-luminescence spectrum was recorded with Jobin YvonSpectrofluorometer.

3. Results and discussion

3.1. Reaction mechanism for the formation of zirconia nanophosphors

The reaction flowchart shown in Fig. 2 clearly describes the stepby step process involved in the sonochemical synthesis. Accordingto the starting materials of the chemical reaction, a zirconiumhydroxy-oxalate complex is suggested to be formed according tothe reaction in Eq. (4a). This zirconium hydroxy-oxalate complexis insoluble however dissolves in excess of oxalic acid facilitatingthe formation of zirconium oxalate.

ZrOCl2 � 8H2Oþ ðCOOHÞ2 � 2H2Oþ nH2O

! ZrðOHÞ2ðCOOÞ2 � nH2Oþ 2HClþ nH2O ð4aÞ

Hydrolysis of zirconium oxalate occurs on increase of pH byaddition of ammonia solution which neutralizes the HCl in solu-tion. Zirconium hydroxide (Zr(OH)4) is immediately precipitatedin basic condition (pH = 8) according to the Eq. (4b).

ZrðOH2Þ2þ þ 2OH� ! ZrðOHÞ4 ð4bÞ

Moreover, ultrasonic wave irradiation aids in hydrolysis by pro-moting the presence of OH� free radicals by decomposition ofwater molecules [23] as given in the Eq. (4c).

H2O! Hþ þ OH� ð4cÞ

Table 2Comparison of size and phase of zirconia nanoparticles.

Synthesis route Reaction parameters Calcinationtemperature

Mean crystallitesize (nm)

Particle size(nm)

Phasespresent

Dominantphase

Impurity

Emulsion assisted direct precipitation[2]

Precursor: ZrOCl2

room temperaturereagent concentration:water:surfactant::10:1(molar ratio)

800 �C �13–23 13–29 m + t – NaCl

Direct precipitation [23] Precursor: 0.3 MZrO(NO3)2

temperature: 30 ± 2 �Creagant concentration:0.3 M

800 �C �48 – m + t + c m –

Ultrasound assisted precipitation [23] ’’ 800 �C �39 – m + t + c m –Oxalate precipitation [30] Precursor: ZrOCl2

room temperaturereagent concentration:1 M

600 �C – 15–40 m + t t –

⁄Conventional precipitation method[present work]

Precursor: ZrOCl2

room temperaturereagent concentration:0.5 M

800 �C �57 �400–1000 m + t m –

⁄Ultrasound assisted oxalateprecipitation [present work]

’’ 800 �C �14–19 �18–22 m + t t –

m, t and c denote the monoclinic, tetragonal and cubic phase of zirconia.The corresponding values are made bold for easy indentification.

* denotes the present work and the corresponding values are made bold for easy identification.

Fig. 7. (a) XRD pattern of sonochemically synthesized zirconia nanophosphors and (b) Gaussian fit of the high intensity peak (101) in the XRD.


When the zirconium hydroxide precipitate is irradiated withultrasonic wave, hot spots are formed which induces the formationof hydrous zirconia nanoparticles in the solution. This was con-firmed by suspending hydrous zirconia nanoparticles in nitric acidand on further filtering, washing and drying, zirconia nanoparticlesare obtained. In contrast, the zirconium hydroxide precipitate dis-solves in nitric acid and the precipitate soon disappears to form atransparent solution [29]. However, the as-prepared zirconia nano-particles are amorphous as inferred from the XRD analysis (notshown). The final prepared sol contains the amorphous hydrouszirconia solid nanoparticles along with the reaction by-products.The chloride ions can be effectively removed by washing and theoxalate can be decomposed by heat treatment.

In this regard, the thermogram can help to elucidate the decom-position process. The TG/DTG curve of as-prepared zirconia nano-phosphor is presented in Fig. 3. The decomposition occurs inthree steps: (i) dehydration (ii) oxalate decomposition and (iii)hydrous zirconia decomposition. The first weight loss of around10.66% up to 200 �C corresponds to the partial evaporation of

water and alcohol residues. The second stage of weight loss ofabout 20.49% in the temperature range 200–250 �C is due to thedecomposition of oxalate. The third weight loss of 27.71% in thetemperature range 250–350 �C corresponds to the removal of car-bonaceous residues. The final weight loss of 14.19% in the temper-ature range 500–700 �C corresponds to the decomposition ofhydrous zirconia followed by the removal of crystal water. More-over, the derivative thermogram resolves these changes moreclearly. The final zirconia obtained was about 25.91% at 980 �C. Itis to be noted that the horizontal nature comes in effect from700 �C in the TG/DTG curve which indicates that the reaction offormation of zirconia nanophosphor is complete. Then after, thereis no phase change observed till 980 �C.

3.2. Structural and vibrational characterization

The FE-SEM images in Fig. 4(a) and (c) reveals the surface mor-phology of the zirconia nanophosphors prepared by sonochemicaltechnique. The FESEM image and the corresponding EDAX

Fig. 8. XRD pattern of zirconia prepared by conventional precipitation methodwithout ultrasonication.

Table 3Summary of calculated parameters of zirconia nanophosphors.

Samplename

Meancrystallitesize DXRD

(nm)

r.m.sstrain < e2 >� 10�3

Dislocationdensity � 1015 lines/m2

Energyband gapEg (eV)

ZrO2 14 4.03 4.63 4.853YSZ 18 3.24 2.99 4.805YSZ 18 3.22 2.99 4.768YSZ 19 3.04 2.73 4.73

Table 4Microstructure parameters determined from Rietveld refinement for zirconiananocrystals.

Sample name a = b (Å) c (Å) a = b = c d(101) (Å) V (�106 pm3)

ZrO2 3.596 (2) 5.180 (3) 90� 2.96099 66.995073YSZ 3.599 (7) 5.184 (2) 90� 2.96053 67.148245YSZ 3.600 (9) 5.188 (2) 90� 2.95103 67.246808YSZ 3.6049 (7) 5.181 (1) 90� 2.94544 67.32614

Table 5Summary of Reliability factors of Rietveld refinement for zirconia nanocrystals.

Sample name Space group Rexp (%) Rp (%) Rwp (%) v2

ZrO2 P42/nmc 12.43 10.62 13.58 1.193YSZ P42/nmc 12.56 10.78 14.41 1.325YSZ P42/nmc 12.21 10.99 15.18 1.558YSZ P42/nmc 12.49 9.72 12.50 1.00


spectrum of zirconia prepared by conventional precipitation with-out ultrasonication are shown in Fig. 4(e) and (f) respectively. Inconventional precipitation method without ultrasonication pro-cess, the particles are larger in size (�0.4–1 lm) and irregular inshape. The surface morphology of zirconia nanophosphors pre-pared by ultrasound assisted precipitation is well improved com-pared to those prepared by conventional precipitation withoutultrasonication. It is noteworthy that this well controlled nanopar-ticle size with good morphology has been achieved with ultrasonicwave as an physical aid to the reaction and without the use of anysurfactant in the synthesis. The EDAX spectrum shown in Fig. 4(b)

and (d) clearly confirms that the sonochemical route producedhighly pure zirconia nanophosphors. The atomic ratio of Zr:Yobtained from the EDAX analysis agrees well with the initial com-position taken for synthesis of the nanomaterial. Statistical analy-sis was carried out to confirm the narrow particle size distributionby selecting particles via random sampling technique. A total of100 particles were taken under consideration and their diameterwere measured for analyzing the particle size distribution. TheGaussian fit of the particle size distribution as shown in Fig. 5 con-firms the narrow size distribution. The particle size was estimatedto be in the range �18–22 nm from the FESEM analysis. It is evi-dent from TEM image shown in Fig. 6 that there is no agglomera-tion and formation of secondary particles. Moreover, thehistogram shown as inset in Fig. 6 confirms the narrow size distri-bution of particles. Ultrasonic irradiation resulted in cavitationphenomenon which accounts for the reduced particle size of thezirconia nanophosphors. An assessment between the particle sizedistribution and phase of the zirconia nanoparticles prepared bysonochemical method and chemical precipitation in the presentwork and those reported in the literature (Table 2) entails that son-ochemistry can be effectively utilized as a suitable and convenientmethod to achieve nanocrystalline zirconia.

The XRD pattern of zirconia nanophosphors prepared by sono-chemical method shown in Fig. 7(a) confirms the presence oftetragonal phase predominantly in pure zirconia with only twoidentifiable poor intensity peaks (�111) and (111) correspondingto the monoclinic phase. Whereas, zirconia prepared by conven-tional precipitation technique without ultrasonication process ispredominately monoclinic with larger crystallite size �57 nm(Fig. 8). According to the literature [1], only the stable monoclinicphase exists at room temperature. Stabilization of high tempera-ture tetragonal phase requires stabilizing agents. However, in thiscase pure zirconia without any stabilizing agent is predominatelyin tetragonal form at room temperature. This is due to reducedgrain size [2], which can be appreciated as a consequence of acous-tic wave controlled reaction. The size effect is attributed to thelower surface free energy of the tetragonal form as compared tothat of the monoclinic phase, which becomes more prominent fornanometer sized particles. In order to fully stabilize zirconia to thetetragonal phase Yttria has been added at different molar concen-trations. The percentage of monoclinic phase in these nanomateri-als reduces due to addition of stabilizer yttria. This is evident fromthe reduction of peak intensity corresponding to monoclinic phase.The change in crystal phase is marked by peak shift (as shown inFig. 7(b)) towards larger diffraction angles (2h) along with narrow-ing in diffraction peak implying that the stress is released whenyttrium content increases from 0 to 8 mol%. This indicates thatY3+ ions have well entered into crystal lattice of zirconia and theymainly substituted into the Zr sites. The crystallite size, strain anddislocation density are given in Table 3. The micro-structuralparameters calculated via Rietveld refinement method are summa-rized in Table 4. This analysis method almost fits the peaks in theexperimental pattern (i.e., the observed X-Ray powder diffractionpattern of the sample) with the respective patterns of the simu-lated pattern. The reliability factors are summarized in Table 5.

In the Raman spectra shown in Fig. 9(a), six prominent peakscorresponding to the six Raman-active modes of tetragonal phaseare observed for pure ZrO2 and yttria stabilized ZrO2 [31,32]. Thepeaks can be directly related to the 3Eg, 2B1g, and lA1g symmetriespredicted by Anastassakis et al. [33] and reported by Ishigame andSakurai [34] for the tetragonal phase. The Raman spectra are foundto be very similar except for the observed peak broadening and aslight shift in the Eg(2) peak to lower frequencies with yttrium dop-ant concentration (Fig. 9(b)). This may be due to the presence ofdefects such as oxygen vacancies created by large ionic radii Y3+

incorporation into zirconia lattice. Moreover, the two bands at

Fig. 9. (a) Raman spectra of sonochemically synthesized zirconia nanophosphors and (b) Gaussian fit of the high intensity Eg(2) peak in the Raman spectra.

Fig. 10. Phase content of zirconia nanophosphors.

Fig. 11. Probable sonochemical format


approximately 165–190 cm�1 weakens considerably with increas-ing concentration of yttria signifying the decreasing trend ofmonoclinic phase percentage in the nanomaterials marked by pro-nounced tetragonal phase stabilization. The observed peak broad-ening of Eg(2) peak (Fig. 9(b)) with increase in Y3+ dopant ion is aconsequence of the increase of tetragonal structure characteristicswhich in turn marks the depletion of monoclinic phase therebyindicating stabilization process. Fig. 10 clearly illustrates the stabil-ization of tetragonal phase with decrease in monoclinic phase vol-ume with increasing Y3+ dopant ion.

3.3. Probable sonochemical formation mechanism of nanophosphors

The sonochemical formation of nanoparticles with good mor-phology is still ambiguous. By hypothetical approach, the probablemechanism of formation of nanophosphors with narrow size dis-tribution has been proposed. Fig. 11 shows the probable sono-chemical formation mechanism of nanophosphors. The Zr4+ metalions in the solution chelates to the organic ligand to form

ion mechanism of nanophosphors.

Fig. 12. (a) Diffuse reflectance spectra of zirconia nanophosphors and (b) Kubelka–Munk plot for determining the energy band gap of zirconia nanophosphors.


zirconium oxalate complex, which encloses each metal ion at thecenter core of the polymeric complex network. At high pH, ahydrolyzed polymeric gel is formed by attachment of –OH ligandsto the metal ion present in the complex network. Upon applicationof power ultrasound, nuclei formation is initiated rapidly duringprecipitation. Moreover, in general, a supersaturated solutionstarts forming nuclei out of a highly concentrated solution consist-ing of metal ions. During the sonochemical precipitation process,ultrasonic waves consisting of compression and rarefaction cyclesduring their propagation through the media produce cavitationbubbles in the media. After several acoustic cycles, the cavitationbubbles collapse violently and adiabatically generating extremelyhigh temperatures and pressures [35]. It is believed that suchextreme temperatures and pressures within a small reactor caninduce precipitative formation of nuclei.

Although, it is complicated to disentangle the growth fromnucleation, both are considered as two distinct but chronologicalstages during the formation of nanoparticles. Ultrasonication pro-cess increases the mass-transfer to the nuclei surface. This leadsto smaller particle size with higher uniformity. The small nucleimay bump with other small nuclei due to the turbulence producedin the solution by acoustic waves leading to self assembly, coales-cence and consequent growth. The larger particles formed by coa-lescence, by chance, may get fragmented by the violentinstantaneous collapse of acoustic bubbles formed on the surfaceof the larger particles or around their surrounding environment.This produces high speed liquid microjets that drives apart thefragmented particles. These microjets push liquid at high pressurebetween the particles and separate them from each other therebyestablishing growth control. This hypothesis may hold good for thereason behind the achieved uniform size nanoparticles with goodmorphology.

3.4. Optical and electronic properties

Fig. 12(a) shows the diffuse reflectance spectra of the zirconiananophosphors with an extended view shown as inset for betterclarity as all the spectrum are almost similar. The Kubelka–Munkplot for determining band gap energy is shown is Fig. 12(b). Thezirconia nanophosphors exhibit a wide band gap and the bandgap energy decreases with increasing Yttria content due to forma-tion of mid-band states upon incorporation of dopant ion. More-over, the band gap decreases with increasing crystallite size(Table 3) thereby revealing quantum confinement phenomena.

In an ideal ZrO2, only Zr–O bonds exist, which will not show anyluminescence. The transformation of phase and the presence ofstructural defects in different phases will introduce electronicstates into the band gap, resulting in luminescence in these mate-rials [36]. Three kinds of luminescence have been described toexplain ZrO2 luminescence [37,38]; (a) from an impurity/dopant(b) intrinsic self trapped excitons (c) from intrinsic defects – (Fand F+ centers, Zr3+). The sonochemical process leads to the exis-tence of large amounts of surface defects (oxygen vacancy or inter-stitial oxygen, Zr vacancy or Zr interstitial) in ZrO2 due to theirlarge surface area of nano-sized particles [39]. Moreover, largeionic radii Y3+ dopant ion incorporation into zirconia lattice createsoxygen vacancies in ZrO2 crystal. According to Kröger’s notation,the formation of oxygen vacancies can be described by the follow-ing Eqs. (5a–5d).

ZrO2 $ ZrO2�n þn2

O2ðgÞ ð5aÞ

O�o $12

O2ðgÞ þ Vo ð5bÞ

Vo $ V�o þ e� ð5cÞ

V�o $ V��o þ e� ð5dÞ

where Vo, V�o, V��o , Oo⁄ and e� represents a neutral oxygen vacancy, a

singly ionized oxygen vacancy, a doubly ionized oxygen vacancy, anoxide ion in a regular lattice site and an electron in conduction bandrespectively.

The photoluminescence emission spectra were recorded corre-sponding to an excitation wavelength centered at 293 nm andthe photoluminescence excitation spectra were recorded corre-sponding to an emission wavelength 400 nm. The PL spectra ofpure zirconia and 3, 5 and 8 mol% yttria stabilized zirconiarecorded at room temperature are shown in Fig. 13. Upon excita-tion in the optical absorption region, a UV emission centered at361 nm, a sharp intense violet emission centered at 400 nm anda less intense cyan emission at 493 nm was observed. It is foundthat the PL excitation band around 293 nm can be attributed todefect states due to oxygen vacancies that exist at the grain bound-aries in the YSZ samples which is an inherent aspect of nanocrys-tallinity [40]. At these sites, Y3+ ions are distributedheterogeneously and are likely to cause the defect states. Theseresults are consistent with a proposed model by Petrik et al. [41]that oxygen vacancies with one Y3+ ion and one Zr4+ ion as nearestneighbors to the vacancy lead to one PL excitation band at 266 nm(293 nm in the present study), whereas those with two Zr4+ ions asnearest neighbors to the vacancy lead to the excitation band at213 nm.

The principal intrinsic defects in nanocrystalline zirconia areanion vacancies. This has been assigned to the formation of elec-trons (F and F+ centers) [42]. These singly ionized oxygen vacancies

Fig. 13. (a) Photoluminescence excitation spectra of zirconia nanophosphors and(b) photoluminescence emission spectra of zirconia nanophosphors.


lead to UV emission and the emission results from the radiativerecombination of a photo-generated hole with an electron occupy-ing the oxygen vacancy. The PL emission bands in the violet regionin the present ZrO2 might be due to the transitions from the surfacetrap states in the conduction band to lower energy levels close tothe valance band. The low intensity band centered at 493 nm emis-sion may be due to the presence of oxygen vacancies.

The sharp emission observed may be due to the uniformity inparticles size and shape of the sonochemically synthesized highquality nanophosphors. Such unique sharp emission of�27.2 meV (full width at half the maximum) in zirconia have notbeen reported elsewhere and is considered advantageous foradvanced optical applications. The Y3+ ion dopant influences thephotoluminescence property as revealed by intensity enhance-ment in the emission peak. The increase in density of oxygenvacancy due to Y3+ ion dopant as revealed by down shift of Eg peakin Raman spectra may be the reason behind the observed increasein PL intensity with increasing Y3+ ion dopant. Therefore, it can beconcluded that the multiple emission (i.e.) simultaneous emissionof UV, violet and cyan light on exciting the host is the characteristicproperty of the sonochemically prepared nanophosphor.

4. Conclusion

In summary, pristine and yttria stabilized zirconia nanophos-phors have been successfully synthesized by an easy, rapid and

environmentally friendly green sonochemical synthesis withoutthe use of surfactants. As a consequence of well controlled grainsize by acoustic wave, the crystal structure of nano-zirconia wasfound to be predominantly tetragonal at room temperature fromthe XRD analysis, and further confirmed by Raman spectroscopicanalysis. The yttrium ion incorporation into zirconia lattice hasbeen confirmed from the peak shift towards higher diffractionangle in XRD. The stabilization of tetragonal phase of zirconia atroom temperature with increasing molar concentration of yttriumin zirconia was evident from the peak broadening and down shiftof tetragonal Eg peak in Raman spectra. The particle size has beeneffectively controlled by the use of ultrasonic wave as physical aidto the synthesis process. The particle size determined were in therange �15–25 nm. The nanoparticles have uniform spherical shapeand narrow size distribution evidently proving the advantage ofemploying sonochemical synthesis for procuring good well definedmorphology. Upon excitation with UV light in the range centeredabout 293 nm, remarkable multiple sharp emission was observedfrom zirconia nanophosphors at room temperature as a result ofstructural defects in quantum confined nanoparticles establishedvia sonochemical synthetic route by cavitation phenomenon. Theunique sharp PL emission of �27.2 meV observed elucidates thequality of the sonochemically synthesized zirconia nanophosphorsand moreover, incorporation of Y3+ ion enhanced the PL properties.Besides its scientific interests for the mechanism of the electronicexcitation in YSZ, the resulted luminescent zirconia nanoparticlesis also an interesting photo-active material for UV light emitters,violet light emitters and light-emitting diodes. Therefore, zirconiananophosphors are a promising good candidate inorganic materialfor new lighting devices.

Acknowledgement

The financial support rendered by AICTE, New Delhi, Indiathrough National Doctoral Fellowship Scheme is greatlyacknowledged.

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Unique sharp photoluminescence of size-controlled sonochemically synthesized zirconia nanoparticles

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