Dysprosium Doped Indium Oxide

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J. Phys. Chem. C 2008, 112, 6781-6785

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Indium Oxide and Europium/Dysprosium Doped Indium Oxide Nanoparticles: Sonochemical Synthesis, Characterization, and Photoluminescence Studies Dimple P. Dutta,*,† V. Sudarsan,† P. Srinivasu,‡ A. Vinu,‡ and A. K. Tyagi*,† Chemistry DiVision, Bhabha Atomic Research Centre, Mumbai - 400 085, India, and Nano-Ionics Materials Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305 0044, Japan ReceiVed: January 21, 2008; In Final Form: February 20, 2008

Indium oxide and europium/dysprosium doped indium oxide nanoparticles were prepared using a sonochemical technique where indium ethoxide was used as a precursor. The obtained indium oxide nanoparticles were characterized using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) techniques. The band gaps of the particles were estimated using diffuse reflectance spectroscopy. With change in size of particles from 8 to 14 nm, the band gap varied between 4.11 and 3.79 eV. The photoluminescence (PL) spectra of In2O3 nanoparticles showed peaks in visible region characteristic of shallow traps present within the nanoparticles. Weak luminescence was observed in europiumdoped indium oxide nanoparticles,

Introduction Transparent conducting oxides popularly called TCOs have been extensively studied by many researchers for many years because of their diverse practical applications.1 Indium oxide (In2O3), which is an important TCO material, is an n-type semiconductor with a wide band gap (Eg = 3.70 eV). It has high electrical conductivity and optical transparency in the visible range2 and hence is useful as material for low-emissivity windows, solar cells, flat panel displays, and so forth. Nanostructures of In2O3 find applications not only in UV lasers and detectors but also as gas sensors for ozone and nitrogen dioxide.3 Consequently, In2O3 nanoparticles have been prepared by a variety of methods, including thermal hydrolysis,4 thermal decomposition,5 sol-gel technique,6 microemulsion,7 reactive magnetron sputtering,8 pulsed laser ablation,9 spray pyrolysis,10 mechanochemical processing,11 and laser photolysis.12 However, the particles formed have a high degree of agglomeration and a wide size distribution. The particles prepared by pulsed laser ablation10 showed luminescence around 328 nm, whereas the In2O3 nanoparticles dispersed within pores of mesoporous silica13 exhibited a broad luminescence band between 430 and 520 nm when excited at 250 nm. In the latter case, the intensity of the band was found to decrease with an increase in annealing temperature. Generally, rare earth ions like Eu3+, Er3+, and so forth are incorporated in semiconductors to improve the luminescence efficiency by energy transfer process. The optical properties of rare earth ions trapped in host lattices have received much attention in terms of fundamental and technological importance.14 These matrices effectively reduce the quenching of surface rare earth emission by shielding the rare earth ions present on the surface of the nanoparticles from the external ligands. However, there are very few reports on synthesis and optical studies of rare earth doped In2O3 films,15,16 and to the * Email: [email protected], [email protected]. Tel.: +91 22 25592308, Fax: +91 22 25505331. † Bhabha Atomic Research Centre. ‡ National Institute for Materials Science.

best of our knowledge, none on europium or dysprosium doped In2O3 nanoparticles. Keeping this in mind, we have synthesized nearly monodispersed In2O3 nanoparticles and also Eu3+ and Dy3+ doped In2O3 nanoparticles with a narrow size distribution using a sonochemical technique. The nanoparticles have been characterized thoroughly, and their optical properties have been studied. Results of this work are reported herein. Experimental Details All the reactions were carried out at room temperature under ambient conditions. High-purity indium chloride (Trans Metal), cetyltrimethyl ammonium bromide (CTAB) (Aldrich), europium acetate (Aldrich), and dysprosium acetate (Aldrich) were obtained from commercial sources. The solvents (triethylamine, ethyl alcohol, and ammonium hydroxide) used were all of AR grade. Preparation of Indium Ethoxide In(OEt)3. Vacuumdistilled InCl3 (2.97 g, 13.43 mmol) was added to excess absolute ethanol, followed by addition of distilled triethylamine (4.08 g, 40.28 mmol). An exothermic reaction occurred, and after the addition was complete, the temperature of the reaction flask cooled down. A dirty white precipitate was obtained, which was separated by ordinary filtration and dried thoroughly under vacuum. Elemental analysis of the precipitate corresponded to that of In(OEt)3. Preparation of Indium Oxide Nanoparticles. In(OEt)3 (0.62 g, 2.49 mmol) was put in a standard 100 mL 2-necked flask, and a solution of CTAB (0.36 g, 1 mmol) in 20 mL of ethanol was added to it, followed by 60 mL of double-distilled water. The pH of the solution was adjusted to 10 by adding NH4OH. The system was then irradiated with a high-intensity (100 W/cm2) ultrasonic radiation operating at 20 kHz, under air at room temperature for 1.5 h. This was done by the direct immersion of a titanium horn (13 mm diameter) to a depth of 6 cm in the solution. During the reaction, one of the two necks of the flask was linked to the ultrasonic probe and sealed, and the other one was connected with a condensation tube. The reaction cell was not thermostated and the final temperature

10.1021/jp800576y CCC: $40.75 © 2008 American Chemical Society Published on Web 04/03/2008

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Figure 1. Powder XRD of In(OH)3 obtained after (a) 0.5, (b) 1, and (c) 1.5 h sonochemical treatment of the reactants.

was 75 °C. After sonication, the precipitate was washed with water and centrifuged. The grayish powder obtained was heated in a furnace under air at different temperatures for different time periods. Powder XRD of residue obtained confirmed the formation of cubic indium oxide nanoparticles. Preparation of Dysprosium Doped Indium Oxide Nanoparticles. In(OEt)3 (0.59 g, 2.49 mmol) was put in a standard 100 mL 2-necked flask with dysprosium acetate (0.04 g, 0.12 mmol) and a solution of CTAB (0.36 g, 1 mmol) in 20 mL of ethanol was added to it followed by 60 mL of double-distilled water. The rest of the method is similar to that reported for indium hydroxide nanoparticles. The particles obtained were subjected to thermal treatment, and the residues obtained were stored in clean plastic vials. Similarly, europium doped indium oxide nanoparticles were also synthesized. X-ray diffraction (XRD) measurements were carried out on a Philips Instrument, operating with Cu KR radiation (λ ) 1.5417 Å) and employing a scan rate of 0.02°/ s in the scattering angular range (2θ) 20-70°. The lattice parameters were calculated from the least-squares fitting of the diffraction peaks using the POWDERX program. The average crystallite size was calculated from the diffraction line width based on the Scherrer relation: d ) 0.9λ/B cos θ, where λ is the wavelength of X-rays and B is the half-maximum line width. The scanning electron microscopic (SEM) examination was done on a Vega-Tescan instrument. EDX (energy dispersive spectroscopy) analyses were carried out using an Inca Energy 250 instrument coupled to Vega MV2300t/40 scanning electron microscope. Conventional TEM was recorded on JEOL 2000FX. The particulates obtained were dispersed in methanol solution and then deposited on the carbon-coated copper grids for SEM/ EDX/TEM studies. Diffuse reflectance UV-vis spectra were recorded on a JASCO spectrometer using an integrating sphere accessory. All luminescence measurements were carried out at room temperature with a resolution of 3 nm, using a Hitachi Instrument (F4010) having a 150 W Xe lamp as the excitation source. Powder samples were mixed with methanol, spread over a quartz plate, dried at 150 °C, and mounted inside the sample chamber. Results and Discussion The sonication products were characterized by XRD diffraction measurements. Figure 1 illustrates the diffraction patterns of the “room temperature” sonication products for different time

Figure 2. TEM micrograph of In(OH)3 particles obtained after (a) 0.5 and (b) 1 h sonochemical treatment.

periods of sonolysis. The reactant mixture was sonolyzed for 0.5, 1, and 1.5 h, and for each case, the diffraction pattern has been recorded as shown in Figure 1a-c, respectively. It is clear that the structureless features detected for the 0.5 h sonolysis (Figure 1a) are due either to the amorphous nature or to the very small particle size of the product. We can see some broad diffraction peaks at 2θ ) 22.2°, 35.5°, 45.4°, and 56.4°, which are close to the reported data for [In(OH)3] (JC-PDS card 160161). The broad nature of the diffraction peaks imply that nanocrystalline [In(OH)3] was formed. The products obtained after 1 and 1.5 h of sonolysis (Figure 1b,c) show diffraction peaks that clearly match that of [In(OH)3]. A previous report on the synthesis of In(OH)3 nanopowder prepared via the sonication of an aqueous solution of InCl3 at room temperature had some unidentified peaks in the powder XRD pattern.17 Using the Scherrer equation, the sizes of the [In(OH)3] nanoparticles were found to be 36 and 45 nm for the 1 and 1.5 h sonolyzed products, respectively. It can be clearly seen that the duration of sonolysis affects the crystallinity and size of the nanoparticles obtained. It is interesting to note that the particle size and the crystallinity of the nanoparticles significantly increases with an increase in the time of sonolysis. TEM micrographs of the products obtained after 0.5 and 1 h sonolysis of reactants is shown in Figure 2a,b, respectively. The product obtained after 0.5 h of sonolysis shows clusters of slightly elongated particles. The observation of the [200] peak

In2O3 Nanoparticles

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Figure 4. TEM micrograph and SAED pattern of In2O3 particles obtained after 1 h of furnace heating at 350 °C.

Figure 3. Powder XRD pattern of In2O3 particles obtained after furnace heating of In(OH)3 at (a) 750 °C for 1 h, (b) 350 °C for 3 h, and (c) 350 °C for 1h.

in the XRD pattern of the same product is also related to the observation of elongated structures. When the sonication time is increased to 1 h, nanocubes are obtained with dimensions on the order of 30-35 nm. This is in excellent agreement to the size obtained from powder XRD pattern recorded for the same sample. It is reported that the preferential adsorption of molecules and ions in the solution to different crystal faces directs the growth of nanoparticles into various shapes by controlling the growth rates along different crystal axes.18 In our case, the addition of CTAB probably leads to the formation of nanocubes, since it has been reported that they induce the sphere-rod transition of the micelles in aqueous solution.19 The TG curve for the [In(OH)3] nanoparticles exhibited single step decomposition in the temperature range 210-290 °C and the weight loss was around 16.3%. This matches well with the weight loss of 16.28% expected in the case of formation of In2O3 from [In(OH)3]. 4

2In(OH)3 98 In2O3 + 3H2O The [In(OH)3] nanocubes were heated in a furnace under air at 350 °C for 1 h. The powder diffraction pattern matches well with that reported for cubic In2O3 (JC-PDS 06-0416). The effect of calcination temperature and duration of heating on nanocubes of [In(OH)3] was studied. Powder XRD of residues obtained from heating [In(OH)3] at 350 °C for 1 and 3 h and at 750 °C for 3 h are shown in Figure 3. It can be seen that, on increasing the temperature of calcination, peaks get narrower and sharper, indicating an increase in the size and the crystalline nature of the sample. The structure and morphology of the above nanoparticles have been further analyzed by TEM. Figure 4 shows the bright-field TEM image of the 1 h calcined In2O3 sample along with its SAED. The nanoparticles have a mean diameter of 8.5 nm along with a standard deviation of 2.2 nm. The nanocubes of [In(OH)3] on heating yielded nearly spherical In2O3 nanoparticles, and there was a decrease in size from 30 to 35 nm to ∼8.5 nm. The SAED pattern confirmed the nanocrystalline nature of the particles. The indexing of the concentric rings corresponds to cubic In2O3 crystal structure. The optical diffuse reflectance spectroscopy measurement has been carried out on the In2O3 nanoparticles obtained from

Figure 5. Diffuse reflectance UV-visible spectra of In2O3 particles obtained after furnace heating of precursor at (a) 750 °C for 1 h, (b) 350 °C for 3 h, and (c) 350 °C for 1 h.

furnace heating of indium hydroxide precursor at 350 °C for 1 and 3 h and at 750 °C for 1 h. An estimate of the optical band gap (Eg) can be obtained using the following equation

a(ν) ) A(pν - Eg)m/2 where p ) h/2π, pν ) photon energy, a is the absorption coefficient, and m is dependent on the nature of the transition. For a direct transition, m is equal to 1 or 3, while for an indirect allowed transition, m varies from 4 or 6. Since A is proportional to F(R), the Kubelka-Munk function, then the energy intercept of a plot of (F(R) × pν)2 and (F(R) × pν)1/2 versus pν yields the Eg,dir for a direct-allowed transition and Eg,ind for an indirectallowed transition, respectively, when the linear regions are extrapolated to the zero ordinate.20 For In2O3, however, we have opted to use the equation for a direct semiconductor in accordance with common practice. Using this method, from the spectrum (Figure 5) we calculated the band gap of In2O3 to be 4.11 eV for the 1 h and 3.998 eV for the 3 h furnace-heated (350 °C) samples, respectively. For the sample heated at 850 °C for 1 h, the band gap was found to be 3.79 eV. The value of the band gap energy of nano In2O3 is slightly higher than that of bulk In2O3 particles reported in literature (3.7 eV) and is probably due to the size quantization effect, which results from the smaller size of the In2O3 nanoparticles. Emission spectrum of In2O3 nanoparticles obtained after heating at 350 °C for 1 h is shown in Figure 6a. On exciting at 235 nm, the pattern is characterized by a strong, broad asymmetric peak centered around 460 nm (blue) and two relatively less intense peaks centered around 548 nm (yellow) and 618 nm (orange). Similar results have been recently reported

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Figure 6. Emission spectrum after 235 nm excitation (a) and excitation spectrum for 460 nm emission (b) for In2O3 nanoparticles obtained after 1 h annealing of precursor at 350 °C.

for indium oxide (In2O3) nanocrystals prepared by a one-pot solvothermal route using indium oleate as the precursor.21 However, in their case, the yellow emission had the highest intensity. On the basis of the previous luminescence studies on In2O3 nanoparticles,5,9,21 the emission peaks in the visible region have been attributed the shallow traps/defects present with the nanoparticles. The emission peak maximum remained unaffected even after varying the excitation wavelength from 300 to 235 nm. The excitation spectrum corresponding to 460 nm emission is shown in Figure 6b. The pattern consists of a shoulder peak characteristic of semiconductor absorption around 300 nm, and this value agrees well with the band gap of In2O3 particles (∼4.1 eV), obtained after heating at 350 °C for 1 h. For the samples annealed at higher temperatures (850 °C), no change in peak positions in photoluminescence spectra was observed though there was a slight increase in the intensity. Figures 7 a and 8a show the XRD pattern for the In2O3 and In2O3:Eu (5 at %) and In2O3 and In2O3:Dy (5 at %) nanoparticles. With Eu3+/Dy3+ incorporation, the lattice undergoes distortion as revealed by the broadening of diffraction peaks corresponding to that of undoped In2O3. The lattice parameter for In2O3:Eu was found to be a ) 10.17(1) Å as compared to a ) 10.098(3) Å for undoped In2O3, while that of 5% Dy doped In2O3 was 10.123(3) Å. This indicates that Eu3+/Dy3+ has gone into the In2O3 lattice, as the ionic radius of Eu3+/Dy3+ is slightly larger than that of In3+. The photoluminescence spectrum of the In2O3:Eu particles (Figure 7b), obtained after excitation at 235 nm, showed weak emission bands corresponding to 5D0 f 7F0 (∼575 nm), 5D0 f 7F1 (587 and 601 nm), and 5D0 f 7F2 (612 nm) transitions of Eu3+. The emission peak position showed no change on varying the excitation wavelength from 280 to 235 nm. Excitation spectrum corresponding to the 5D0 f 7F0 transition of Eu3+ is shown in Figure 7c. The spectrum consists of a broad peak centered at 265 nm, which is attributed to the charge-transfer transition between the Eu3+and O2- ions.

Figure 7. (a) Powder XRD of 5% Eu doped In2O3 and undoped In2O3; (b) emission spectrum after 235 nm excitation; and (c) excitation spectrum for 575 nm emission for In2O3:Eu nanoparticles.

The absence of In2O3 host band in the excitation spectrum of Eu3+ indicates that there is almost no energy transfer from the In2O3 host to the doped Eu3+. Photoluminescence spectra of the doped In2O3:Dy nanoparticles were also recorded. However, no lanthanide ion emission could be detected. The corresponding strains in the undoped and Dy doped In2O3 are also shown in Figure 8b. It can be clearly seen from the plot that the slopes of the linear fit for In2O3:Dy are much higher than that in undoped In2O3, indicating the presence of high strain in the doped nanoparticles. The absence of luminescence may be possibly due to the highly strained and distorted environment

In2O3 Nanoparticles

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6785 In(OH)3 surfaces and lead to oriented growth of the planes. Finally, calcinations of the In(OH)3 nanocubes yielded In2O3 nanoparticles. To make a comparison, we have also carried out the reaction with vigorous electromagnetic stirring at room temperature and also at 75 °C instead of ultrasound irradiation. However, only bulk In(OH)3 instead of nanocubes precipitated out in both cases, which on furnace heating yielded bulk In2O3 and not nanoparticles. The decrease in dimensions may be related to the surface corrosion and fragmentation of solids in the presence of high-intensity ultrasound irradiation. Thus, sonication facilitates the formation of nanosized In2O3 and also its doped counterpart. It is a faster and cost-effective technique compared to hydrothermal or solvothermal synthesis. Also, the scaling up provision is greater in this method compared to that of reactive magnetron sputtering, pulsed laser ablation, spray pyrolysis, mechanochemical processing, and laser photolysis techniques. Conclusions A convenient method for synthesis of In2O3, In2O3:Eu, and In2O3:Dy nanoparticles using a sonochemical technique has been reported. The particles have been characterized using powder XRD and TEM. Diffuse reflectance spectroscopic studies on In2O3 have confirmed quantum confinement effects expected for nanoparticles. Emission peaks observed from In2O3 nanoparticles in the visible region have been attributed to the presence of shallow defect levels in the annealed samples. Relatively weak Eu3+ emission could be detected in In2O3:Eu nanoparticles possibly due to the highly strained and distorted environment around the lanthanide ions in the In2O3 lattice. In2O3:Dy nanoparticles, however, did not show any luminescence. References and Notes

Figure 8. (a) Powder XRD of 5% Dy doped In2O3 and undoped In2O3 and (b) corresponding strain in the sample.

around the dysprosium ions in the In2O3 lattice. Similar strain was also observed in the In2O3:Eu particles. This might be the reason for observation of low emission intensity from this sample. However, in the case of Dy3+, the problem is manifold due to the fact that Dy3+ ions exhibit poor absorption and emission cross section characteristics compared to Eu3+. Mechanism of Formation of Nanoparticles. Under the irradiation of a high-intensity ultrasonic source, H‚ and OH‚ radicals are formed in the solution.22 At the same time, highintensity ultrasound results in the effects of acoustic cavitation. As a result, a transient temperature of about 5000 K, a pressure of about 1800 atm, and a cooling rate of 1010 K S-1 are produced. This results in the formation of In(OH)3 nanoparticles from the In(OEt)3 precursor. During the formation process, the addition of the surfactant and the time of the sonication directly affect the morphology of In(OH)3. In our case, CTAB plays an important role in the formation process of In(OH)3 structures. When no CTAB was added, only irregular In(OH)3 particles were obtained after sonication for 2 h (not shown here). At the cube formation stage, the addition of CTAB may bond to the

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