Nano Superstructures for

Research Article pubs.acs.org/journal/ascecg

Facile Ultrasound Route To Prepare Micro/Nano Superstructures for Multifunctional Applications H. J. Amith Yadav,† B. Eraiah,*,† H. Nagabhushana,*,‡ G. P. Darshan,§ B. Daruka Prasad,∥ S. C. Sharma,⊥ H. B. Premkumar,# K. S. Anantharaju,∇ and G. R. Vijayakumar⊗ †

Department of Physics, Bangalore University, Bangalore 560056, India Prof. C.N.R. Rao Centre for Advanced Materials, Tumkur University, Tumkur 572103, India § Department of Physics, Acharya Institute of Graduate Studies, Bangalore 560107, India ∥ Department of Physics, BMS Institute of Technology and Management, VTU Affiliated, Bangalore 560064, India ⊥ Professor and Advisor, Department of Mechanical Engineering, Jain University, Jain Group of Institutions, Kanakapura, Ramanagara District− 562112, India # Department of Physics, Dayananda Sagar Academy of Technology and Management, Bangalore 560082, India ∇ Department of Chemistry, Dayananda Sagar College of Engineering, Bangalore 560078, India ⊗ Department of Chemistry, University College of Science, Tumkur University, Tumkur 572103, India ‡

ABSTRACT: Dy3+ doped zirconium dioxide (ZrO2) nanophosphors were prepared by cetyltrimethylammonium bromide (CTAB) assisted ultrasound method. The powder X-ray diffraction profiles showed pure cubic phase. Morphology changes were observed as there is a change in sonication time, CTAB concentration, pH and sonication power. The energy band gaps were varied from 4.13 to 4.53 eV. PL emission spectra exhibits sharp peaks at ∼483, 584 and 674 nm were ascribed to the transitions of 4F9/2→6H15/2, 4F9/2→6H13/2 and 4F9/2→6H11/2, respectively. The spectroscopic properties of the samples were evaluated by Judd−Ofelt theory. Photometric characterization of prepared samples shows white emission and suitable for lightemitting diodes. The optimized ZrO2:Dy3+ (3 mol %) nanopowders (NPs) was utilized to reveal latent fingerprint on various surfaces. The photocatalytic behavior of ZrO2:Dy3+ NPs was extensively studied by degrading hazardous methylene blue dye. Overall, results confirmed that the method of preparation was significant to achieve white light emitting diodes, UV-lasers, photodegradation and forensic applications. KEYWORDS: Latent fingerprints, Photoluminescence, Photocatalyst, Solid state lighting, Impedance spectroscopy, Judd−Ofelt analysis

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INTRODUCTION Rare earth ions (RE) doped nanopowders (NPs) exhibit high color rendering index, energy efficient and stable towards radiations. Hence, they have uses in optoelectronics, catalysis and forensic fields.1,2 The properties of these materials were mainly dependent on the synthesis routes where structural, morphological and optical properties can be tuned.3 Due to the tunable hypersensitive transitions of Dy3+ in the yellow and blue regions, Dy3+ doped materials showed the possibility of obtaining the pure white light emission from single material.4,5 Hence, these materials were promising materials for white light generation and possibility to use in various optoelectronic applications.6 ZrO2 NPs are useful in various applications viz., optoelectronics, fuel-cell technology, gas sensing, coating to protect the optical components etc. which is due to its high melting point, wide band gap, low optical loss and transparent to EM waves of visible and near-infrared region.2,7−10 ZrO2 exhibits the stable © 2017 American Chemical Society

monoclinic phase at room temperate (RT) and transforms into tetragonal at 1443 K and to cubic at 2643 K. At high temperature, tetragonal and cubic phases were unstable in bulk forms. Gu et al. investigated the influence of dopant (Dy3+) and calcination temperature on ZrO2 NPs which exhibited enhanced PL property for optimum dopant concentration of 3 mol %.10 Effect of Dy3+ of 2 mol % in ZrO2 shows prominent PL intensity at 480 nm was reported by Torres et al.11 Presently modified ultrasound assisted sonochemical method was used to prepare various micro/nanostructured ZrO2:Dy3+ NPs. In this method of preparation, there is a production of intense heat and high pressure due to the collapse of cavitation bubbles within few nanoseconds which favors the formation of NPs with interesting surface modifications. Received: July 24, 2016 Revised: January 2, 2017 Published: January 7, 2017 2061

DOI: 10.1021/acssuschemeng.6b01693 ACS Sustainable Chem. Eng. 2017, 5, 2061−2074

Research Article

ACS Sustainable Chemistry & Engineering Fingerprints (FPs) were one of the most important and commonly used forms of physical evidence in forensic investigations.12−14 Usually, latent FPs which were not having clear visualization at offense sights need an improvement for imaging and recognition. Powdering method (PM) was used where networked metals of lead, gold and silver were used for the detection of FPs. Because of toxic chemicals used, this method requires appropriate ventilation and safety equipment’s. Hence as an alternative, the powder based luminescent materials can be used for the effective detection of FPs. In this technique, the surface modification of the NPs has a large influence on the strong bonding efficiency to the FP. In this paper, various concentrations of ZrO2:Dy3+ NPs were prepared by versatile and energy efficient ultrasound route using cetyltrimethylammonium bromide (CTAB) as a surfactant. Dy3+ ions substitution changes the photoluminescence (PL) properties, which opens the possibility to use of these compounds in optoelectronic and forensic uses. In addition, diverse self-assembled superstructures (SS) were obtained by tuning the experimental influential factors. The probable formation mechanisms to obtain such structures were also proposed. For the first time, we reported on the various hierarchical SS of ZrO2:Dy3+ NPs.

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Figure 1. Schematic diagram which shows the synthesis of Dy3+ doped ZrO2 NPs.

EXPERIMENTAL SECTION

ZrO2:Dy3+ (1−11 mol %) were prepared by using a surfactant assisted ultrasound method. The stoichiometric quantity of zirconyl nitrate and CTAB were mixed properly through magnetic stirrer till a clear solution was obtained. Further, a suitable quantity of dysprosium nitrate was added dropwise to the above solution until the stoichiometry conditions were met. The pH value of the solution was adjusted to desired values (5−11) using 1 M NaOH and 1 M HCl. Then the above solution was stirred using an ultrasound sonicator maintained at a frequency of ∼20 kHz, temperature of 333 K and the power of ∼300 W. Finally, a white precipitate formed was filtered and washed by using deionized water and ethanol to remove the unwanted traces of ions. The procedure was repeated for different sonication time (1−6 h). The final powder was dried at 333 K for 3 h in a vacuum oven and used for further studies. Figure 1 shows the schematic diagram for the preparation of ZrO2:Dy3+ NPs. Structural properties were obtained by using powder X-ray diffraction (PXRD) data (Shimadzu 7000, Cu Kα radiation). Scanning electron microscopy (SEM) images were obtained by SEM instrument (Hitachi, TM-3000). Transmission electron microscopy (TEM) images were obtained by TEM instrument (Hitachi H-8100 with an accelerating voltage up to 200 kV, LaB6 filament) equipped along with EDS (Kevex sigma TM Quasar, USA). Diffused reflectance spectra (DRS) were obtained by PerkinElmer (35λ) spectrometer. The photoluminescence (PL) measurement was recorded on a spectroflourimeter (Jobin Yvon Fluorolog-3 equipped with a 450 W xenon lamp). Nikon D3100 digital camera equipped with an AF-S Nikon 50 mm f/1.8G ED lens and a 254 nm UV light was used for the latent FPs. Raman spectra of the prepared samples were acquired by BRUKER RFS 27: Standalone FT-Raman Spectrometer equipped with an Nd:YAG 1064 nm in the spectral range of 4000−50 cm−1. The chemical composition was characterized by X-ray photoelectron spectroscopy (Phoibos 150, specs and dual Al/Mg anode X-ray source was used). Cyclic voltammograms (CH instruments 600E model) was used to study the electrochemical impedance spectroscopy of the sample to measure the impedance response of the prepared samples.

Figure 2. SEM images of ZrO2:Dy3+ (3 mol %) obtained under sonication time of (a) 1 h, (b) 2 h, (c) 3 h, (d) 4 h, (e) 5 h and (f) 6 h without CTAB.

indicates that the sample exhibits various shapes and sizes of flower shaped particles as shown in Figure 2a. The uniform cone shaped structure was obtained after 2 h of sonication irradiation. Further increase in sonication time up to 3 h, flower shaped morphology was observed and it was continued even for the reaction time of 4−6 h. Hence, the time factor during ultrasound treatment greatly influenced the morphologies of the prepared materials. The reaction activity usually happens between the cavitation bubble and the surrounding solution; also, there is a possibility of reaction inside the collapsing bubbles based on the nature of the powder. In the present case, due to crystalline nature of ZrO2 sonochemical reaction took place in the interfacial region only. The surface interactions were closely associated with the structural properties of ZrO2 and surfactants. Figure 3 shows the effect of different quantities of CTAB (5−25 mL) on the morphology with 3 h of sonication. Surfactants with an optimum quantity can influence the growth condition for nano/micro SS and were utilized as efficient structure directing agents agents to prepare NPs with preferred structures.

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RESULTS AND DISCUSSION Figure 2 shows the SEM images of the ZrO2:Dy3+ (3 mol %) NPs in the absence of surfactant CTAB at various sonication times (1−6 h). The SEM micrograph of ZrO2 obtained at 1 h 2062


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ACS Sustainable Chemistry & Engineering

stabilizing effect of CTAB molecules would be favorable for coalesce of ZrO2 crystalline units. However, with further increase of CTAB quantities to 20 and 25 mL, uniform lotus type flower morphology was observed. Hence, the amount of surfactant plays a major role to achieve various morphologies of ZrO2 in the sonochemical route. Based on the SEM studies, the pictorial diagrams for development of SS in ZrO2:Dy3+ (3 mol %) compounds (with and without CTAB) was as shown in Figure 4. By changing the pH value of the solution, growth and shape of the SS could prominently alter. The various morphologies of ZrO2:Dy3+ (3 mol %) material were observed for different pH values (Figure 3). In the growth stage, to reduce the surface energy, many of the tiny assemblies undergo growth to form various structures. The unique formation mechanism was very complex and needs further investigation. Figure 5 shows the SEM images of ZrO2:Dy3+ (3 mol %) NPs

Figure 5. SEM images of ZrO2:Dy3+ (3 mol %) NPs with different sonication time in the presence of 20 mL of CTAB surfactant. Figure 3. SEM image of ZrO2:Dy3+ (3 mol %) NPs with various pH values (left) and different concentration of CTAB (right) obtained after subjecting to 1 h of ultrasonic irradiation.

with different sonication time varied from 1 to 6 h in the presence of 20 mL of CTAB surfactant. Numerous factors required to achieve flower-like structures: to name few, Vander Waals forces, electrostatic, dipolar fields, etc. As compared to other synthesis routes, the optimized sonication factors can reduce the crystalline sizes, uniformly distributed particles and reduced synthesis time. Therefore, SS for different sonication power (20, 22 and 24 kHz) were achieved. Initially, the particles start opening to form flower-like structure observed when the frequency of 20 kHz was applied, (Figure 6a).When

Without the use of surfactant, no specific morphology was observed (Figure 2). But after the addition of surfactant, at lower quantity (5 mL), a flower-like growth with various shaped ZrO2:Dy3+ (3 mol %) nanostructures was observed. When the quantity of CTAB was increased to 10 and 15 mL, individual flowers attached side by side to form bigger sized particles (Figure 3). This may be due to the fact that the weaker

Figure 4. Schematic diagram showing the formation of SS in ZrO2:Dy3+ (3 mol %) with and without CTAB. 2063


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This confirms that the sonication route requires less time to synthesize nanostructures and also helps to achieve the formation of better crystallization. The reaction scheme and plausible mechanism for the formation of ZrO2:Dy3+ SS is provided in Figure 8. In the

Figure 6. SEM images of ZrO2:Dy3+ (3 mol %) NP with different sonication power: (a) 20 kHz, (b) 22 kHz, (c) 24 kHz. (d) Magnified image of panel c.

the power was increased to 22 and 24 kHz, flower-like particles were formed (Figure 6b,c). In the absence of ultrasonication, it was evident that the product yield traces of byproducts and their particles shows broader size distributions. When mechanical stirring was applied in the place of ultrasound irradiation along with CTAB, the branched structures with small agglomerates (irregular) with micrometer sized particles were observed, shown in Figure 7. All the samples showed wellcrystallized nanoflowers for all the quantities. SEM micrographs, reveals the flower-like structure after stirring over 3 h (Figure 7). This is because of extended time to complete the precipitation as compared to the 1 h period of sonication route.

Figure 8. Reaction steps for the formation of ZrO2 SS in the presence of CTAB.

process of formation of ZrO 2:Dy 3+, reaction mixtures containing nitrates of Zr and Dy sources and CTAB were subjected to ultrasonication which in the beginning produces free radicals of H+ and OH−.15 The free radicals of H2O reacted with nitrates and gave hydroxides of Zr and Dy with the elimination of HNO3. Further hydroxides undergo dehydration at high temperature under sonication to afford micro/nano grains with a white precipitate that is highly insoluble in water, stable and chemically inert. But in the presence of CTAB, the morphology and structural changes occur as represented by a schematic diagram that shows the plausible mechanism of ZrO2:Dy3+ (Figure 8). CTAB acts as a cationic surfactant that helps the reorientation of the crystalline particles, and micelle formation occurred. This micelle rearrangement and crystallization process occurred during sonication to yield SS (Figure 9). Herein, sonication time has an important role to obtain the regularly packed particles compared to without CTAB. TEM, HRTEM images and SAED patterns of ZrO2: Dy3+(3 and 9 mol %) with 1 h sonication time are shown in Figure 9. The TEM images shows thin sheet-like nanostructures of ZrO2:Dy3+ material. The HRTEM image shows the clear

Figure 7. SEM images of ZrO2:Dy3+ (3 mol %) NPs obtained in the presence of 20 mL CTAB and subjected the precursors to mechanical stirring for various time. 2064


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Figure 9. TEM and HRTEM images of ZrO2:Dy3+ Nps (a, b) for 3 mol % and (d, e) 9 mol % of Dy3+. (c−h) Magnified images of HRTEM and (i) EDAX spectrum.

atomic planes with an average interplanar spacing of ∼0.32 nm and ∼0.28 nm, respectively for the 3 and 9 mol % of ZrO2:Dy3+. The SAED patterns were well matched with the (hkl) values corresponding to the prominent peaks of the PXRD profiles, which is discussed in the subsequent next section. The PXRD patterns of ZrO2:Dy3+ (1−11 mol %) materials are shown in Figure 10a. The observed PXRD profiles

Figure 11. (a) Rietveld refinement fitted curve and (b) packing diagram of cubic ZrO2:Dy3+ SS.

Table 1. Estimated Crystallite Size, Strain and Energy Gap (Eg) values of ZrO2:Dy3+ (1−11 mol %) SS Dy3+ conc. (mol %)

Crystallite size (nm) [D-S approach]

Crystallite size (nm) [W-H approach]

Strain (×10−4)

Eg (eV)

1 3 5 7 9 11

36 34 28 32 33 34

42 41 39 36 46 44

0.45 0.46 0.44 0.45 0.46 0.48

4.13 4.17 4.25 4.29 4.36 4.53

were estimated by the Hall−Williamson fitting method.19 A slope fitted between β cos θ/λ and sin θ/λ along y-axis and xaxis respectively gives strain as depicted in Figure 10b. The crystalline size and strain were calculated, and the results are listed in Table 1. Further the lattice factors and atomic coordinates were provided with statistical validation through Rietveld refinement using Fullproof software with the PseudoVoigt function.20 The refinement result reveals that ZrO2:Dy3+ (3 mol %) sample crystallized in cubic phase and space group of Fm3̅m (225). Figure 11a shows the best fit curve with the difference and the Bragg’s positions of the sample evaluated with the raw data of PXRD after subtracting the background corrections. The refined structural factors obtained were tabulated in Table 2. The GoF was found to be 0.93, which confirms that the prepared sample was in the above-mentioned phase. The packing diagram obtained using Diamond Software by providing the necessary structural outputs obtained from Rietveld analysis is as shown in Figure 11b.

Figure 10. (a) PXRD patterns and (b) W−H plots of ZrO2:Dy3+ (1− 11 mol %) NPs.

were fitted better with the standard JCPDS card No. 49-1642. The spectra of these samples exhibit pure cubic phase without any additional phases.16 The formation of cubical phase was due to the oxygen vacancies. The tolerable percentage change (Dr) between the radii of doped ion Dy3+ (0.947 Å) and Zr4+ (0.89 Å) ions was estimated and found to be 6.40%, which confirms the proper replacement of Zr4+ ions by Dy3+ ions as explained elsewhere.17 The calculated crystallite size D as per18 values of ZrO2:Dy3+ (1−11 mol %) are listed in Table 1. The crystallite size and strain present in the prepared compounds 2065


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ACS Sustainable Chemistry & Engineering Table 2. Rietveld Refinement Factors of Undoped and ZrO2:Dy3+ (1−11 mol %) NPs Compounds

ZrO2:Dy3+ 1 mol %

ZrO2:Dy3+ 3 mol %

ZrO2:Dy3+ 5 mol %

ZrO2:Dy3+ 9 mol %

ZrO2:Dy3+ 11 mol %

Crystal system Space group Hall symbol Lattice factors (Å) a=b=c Unit cell volume (Å3) Atomic coordinates

Cubic Fm3m ̅ (225) -F 4 2 3

Cubic Fm3m ̅ (225) -F 4 2 3

Cubic Fm3m ̅ (225) -F 4 2 3

Cubic Fm3m ̅ (225) -F 4 2 3

Cubic Fm3m ̅ (225) -F 4 2 3

5.1085 133.31

5.1129 133.66

5.1187 134.12

5.132 135.17

5.112 133.56

0.0000 0.0000 0.0000 0.9687

0.0000 0.0000 0.0000 0.9036

0.0000 0.0000 0.0000 0.9026

0.0000 0.0000 0.0000 0.0348

0.0000 0.0000 0.0000 0.0500

0.0000 0.0000 0.0000 0.0700

0.2500 0.2500 0.2500 1.6494 4.90 6.56 7.17 0.91 2.65 2.83 5.94

0.2500 0.2500 0.2500 1.6963 6.51 7.26 7.73 0.93 4.94 3.69 6.06

0.2500 0.2500 0.2500 1.7982 2.70 3.44 4.37 0.87 0.87 0.57 6.26

Zr4+ X Y Z Occupancy

0.0000 0.0000 0.0000 0.9997

0.0000 0.0000 0.0000 0.9066 Dy3+

X Y Z Occupancy

0.0000 0.0000 0.0000 0.9997

0.0000 0.0000 0.0000 0.0100

X Y Z Occupancy RP RWP RExp χ2 RBragg RF X-ray density (g/cm3)

0.2500 0.2500 0.2500 1.9887 2.81 5.26 5.74 0.96 1.20 1.02 5.96

0.2500 0.2500 0.2500 1.6964 3.83 8.15 8.27 0.98 2.43 1.63 6.04

O2−

recorded in the range of 200−800 cm−1 is shown in Figure 13. The intense Raman peaks were recorded at 277 cm−1 (Eg), 325

The DRS of ZrO2:Dy3+ (1−11 mol %) material is shown in Figure 12a. The spectra exhibits sharp peaks at ∼1073, 1227

Figure 12. (a) Diffuse reflectance spectra and (b) energy band gaps of ZrO2:Dy3+ (1−11 mol %) SS.

and 351 nm were due to 6H5/2→6F9/2, 6H5/2→6F7/2 and 6 H15/2→6P7/2.21 The Kubelka−Munk theory was employed to estimate optical energy gap (Eg) from DRS for the prepared compounds. Figure 12b shows the tangents drawn to the plots of [F(R∞) hν]1/2 versus hν.22 The estimated values of Eg were calculated and tabulated in Table 1. Changes in Eg were due to deviated ordered structures in the host matrix and changes in the energy level distributions.23 In addition, synthesis routes and conditions can change the type of structural defects which leads to changes in the Eg. Raman spectroscopy is a nondestructive tool for the characterization of defects and to obtain the structural information directly related to the vibrational energies of the molecules. The RT Raman spectra of Dy3+ doped ZrO2 NPs

Figure 13. Raman spectra of the ZrO2:Dy3+ (1−11 mol %) NPs.

cm−1 (B1g), 469 cm−1 (Eg), 617 cm−1 (A1g) and 655 cm−1 (Eg). The observed peak positions matches well with the reported Raman spectra for cubic ZrO2.24,25 Further, these results are in correlation with the PXRD results as shown in Figure 10a. To know the elementary oxidation states of the ZrO2:Dy3+ (3 mol %) NPs, the XPS study is represented as shown in 2066


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Figure 14. XPS spectra of ZrO2:Dy

3+

(3 mol %) NPs. (a) Overall scan range; (b) Zr 3d; (c) O 1s (d) Dy 4d and (e) C 1s peaks.

oxygen produced by hydroxyl ions of the surface, respectively. These values were very close to those reported in the literature.28 Figure 14d shows that the binding energy of Dy 4d at about 161.0 eV, which is similar to the reported date in the literature.29 Figure 15 (inset) shows the excitation spectrum of ZrO2:Dy3+ (3 mol %) NPs monitored at emission wavelength of 484 nm. The spectrum exhibits peaks centered at ∼351, 366, 386 and 435 nm due to 6H15/2→6P17/2, 6H15/2→6P15/2, 6 H15/2→4I13/2 and 6H15/2→4G11/2 transitions of Dy3+ ions, respectively.30,31 The PL emission spectra of Dy3+ doped ZrO2 excited at 351 nm at RT shown in Figure 15 exhibits sharp peaks at ∼483, 584 and 674 nm attributed to the transitions of 4 F9/2→6H15/2, 4F9/2→6H13/2 and 4F9/2→6H11/2 respectively.32 The peaks at ∼483 nm and ∼584 nm were attributed to electric and magnetic dipole transitions of the Dy3+ ions, respectively. The schematic representation for the possible energy

Figure 14a−e. The full range spectra shown in Figure 14a specifies that all the peaks on the curve were validated to Dy, Zr, O and C elements. The presence of C at 291.0 eV comes from the carbon adhesive tape used for the XPS study (Figure 14e). Figure 14b−d displays the high resolution spectra that corresponds to Zr, O and Dy elements. The Zr-3d spectrum (Figure 14b) consists of 3d5/2 at 181.3 eV and 3d3/2 at 184.5 eV electronic states. Deconvolution of the spectra produces peaks attributed to the existence of two kinds of Zr4+ species at low and high binding energies pertaining to the different chemical atmosphere. The characteristic spectrum of Zr 3d with Zr at +4 oxidation state confirms the formation of ZrO2.26,27 Further, the investigation for the presence of oxygen vacancies is shown in Figure 14c for O 1s state, which is unequal in nature, representing the occurrence of multicomponent oxygen ions on the surface. The deconvoluted curve showed two separate peaks located at 536.5 and 532.1 eV can be indexed to O2− and 2067


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Figure 15. PL emission spectra of ZrO2:Dy3+ (1−11 mol %) NPs at λexc = 351 nm with 6 h sonication time and 22 kHz power. (Inset: excitation spectrum of ZrO2:Dy3+ (3 mol %) NPs at λemi = 484 nm.).

transitions of ZrO2:Dy3+ NPs is shown in Figure 16. The consequence of increased concentration of Dy3+ on PL

Figure 17. (a) Effect of concentration of Dy3+ on 484 nm peak emission, variation of asymmetric ratio and (b) variation of log(x) and log(I/x) in ZrO2:Dy3+ (1−11 mol %) NPs.

categorize the structural background and symmetry of Dy3+ ions in the host matrix. Intensity factors were evaluated by utilizing the PL emission spectra of the prepared materials. The estimated radiative emission changes (A0‑J) associated with forced electric dipole according to the procedure explained elsewhere.38 The estimated values of Ω2 and Ω4 are listed in Table 3. The factor Ω2 was dependent on the covalence of Dy3+ ions with synchronizing ligands and the site symmetry of the local environment of Dy3+ ion. If the asymmetry around Dy3+ was higher, the value of Ω2 will be high. The Ω2 factor value attributes to the covalency and mechanical deviations in the vicinity of Dy3+ ions, whereas Ω4 factor is a long-range effect dependent on the dielectric and viscosity of the host matrix. Smaller values of Ω4 obtained for ZrO2:Dy3+ NPs shows appreciable rigidity of the crystalline host matrix. Ω6 is the intensity factor that could not be calculated in the present case because the 4F9/2→6H11/2 transition was very weak. Radiative lifetime, branching ratios and asymmetric ratio were also determined according to the method as reported earlier.39 The measured branching ratios were found to be 0.99, which was greater than 0.50. As a result, the present phosphor was good source for laser due to its high emission rates. The CIE chromaticity coordinates of the present phosphor were estimated and were very close to the NTSC standard values (Table 4 and Figure 18a). The CCT factor was relating to its color rendering reference light when heated up to a specific temperature, in kelvin (K). To find quality of white light CCT values were calculated by using the McCamy empirical formula.40−42 Table 4 shows the CCT values estimated were

Figure 16. Estimated energy transitions of Dy3+ doped ZrO2NPs.

intensity is shown in Figure 17a. It was observed that the PL intensity was increased with increase of Dy3+ concentration up to 3 mol % and later diminished. The decrease in PL intensity was due to self-concentration quenching phenomena attributed to energy transfer among dopant ions.33 The separation between Dy3+ ions was less at higher activator concentration, which results in nonradioactive energy transfer between dopant ions. The critical distance (Rc) was estimated and found to be ∼5.347 Å, which was greater than 5 Å and leads to the multipole−multipole interaction in the ZrO2 host.34 To identify the type of multipolar interaction, the Dexter and Schulman theory was used. From Figure 17b, the curve of log I/x vs log x clearly shows that the slope is ∼−1.415. Hence, the value of Q was found to be ∼6.0353, which was close to the theoretical value of 6. So, the electric dipole−dipole interaction was the main reason for the concentration quenching in ZrO2:Dy3+ NPs. The asymmetry ratio (A21) was determined as explained elsewhere, which confirms that the intensity ratios of various transition of PL profile influences the luminescent property of a material.35 To study the site symmetry and luminescence behavior of ZrO2:Dy3+ (1−11 mol %) NPs Judd−Ofelt (J-O) factors were evaluated.36,37 The three J-O factors, Ωλ (λ = 2, 4 and 6) can 2068


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Table 4. Photometric Features of ZrO2:Dy3+ (1−11 mol %) SS Dy3+ concentration (mol %)

X

Y

CP (%)

CCT (K)

1 3 5 7 9 11

0.3102 0.3152 0.2907 0.2952 0.2964 0.2894

0.3252 0.3244 0.3178 0.3111 0.3145 0.3149

77 88 95 90 92 96

6671 6395 8015 7841 7654 7468

Table 5. CIE Values of Doped ZrO2:Dy3+ SS under Different Excitation Wavelength CIE values (x, y)

Excitation wavelength (nm)

ZrO2:Dy3+ (0.5 mol %) ZrO2:Dy3+ (1 mol %)

(0.363, 0.407)

350

(0.364, 0.414)

286

(0.359, 0.405) (0.353,0.395) (0.347, 0.388)

3.

ZrO2:Dy3+ (3 mol %) ZrO2:Dy3+ (5 mol %) ZrO2:Dy3+ (10 mol %) ZrO2:Dy3+ (15 mol %) ZrO2:Dy3+, Li (0 wt %) ZrO2:Dy3+, Li (0.5 wt %) ZrO2:Dy3+, Li (1.0 wt %) ZrO2:Dy3+, Li (3.0 wt %) ZrO2:Dy3+, Li (5.0 wt %) ZrO2:Dy3+, Li (10 wt %) ZrO2:Dy3+ (7 mol %)

(0.308, 0.310)

4.

ZrO2:Dy3+ (7 mol %), Gd3+ (1.8 mol %) ZrO2:Dy3+ (5 mol %)

5.

ZrO2:Dy3+ (5 mol %)

SL no. 1.

2.

Figure 18. (a) CIE and (b) CCT diagrams of ZrO2:Dy3+ (1−11 mol %) NPs.

well acceptable ranges and quite useful in home appliances (Figure 18b). The list CIE values for doped ZrO2 NPs obtained under different excitation wavelengths are tabulated in Table 5. The detection of LFPs by using luminescent material was a physical phenomenon and depends on the sticking nature of phosphor particles to the fingerprint residue.47 The general mechanism for the revelation of LFPs by using ZrO2:Dy3+ (3 mol %) NP is illustrated in Figure 19. The LFPs were collected from the donor by washing hands thoroughly with soap and were pressed gently on various forensic related materials at RT. The dry ZrO2:Dy3+ (3 mol %) NPs were spread carefully to the area where LFPs were taken, and then images were captured using a digital camera. ZrO2:Dy3+ (3 mol %) NPs was used for LFPs recognition technique because it exhibited unique property of UV radiation-dependent PL emission and enhanced adhesive property. The noninfiltrating surfaces namely glass, plastic and marble were considered to demonstrate the background interference of the fingerprints stained by

Sample

References L. A. DiazTorres et al.43 A. BaezRodriguez et al.44

(0.330, 0.376) (0.360, 0.400) (0.354, 0.392) (0.356, 0.385) (0.355, 0.384) (0.340, 0.370) (0.347, 0.360) (0.363, 0.362)

262

R. C. MartinezOlmos et al.45

(0.340, 0.370)

355

(0.290, 0.317)

351

Rodriguez et al.46 Present work

ZrO2:Dy3+ (3 mol %) NP (Figure 20a−c). Well-defined minutiae friction ridges can be identified without any background hindrance, and color disruption on the fingerprints are shown in Figures 20d−f. A series of experiments were carried out on pet bottle, mouse and mobile screen, showing clearly evident for well-defined ridge patterns (Figure 20g−i)) without any background interference. Therefore, the optimized compound can be used

Table 3. Judd−Ofelt Analyzed Factors of ZrO2:Dy3+ (1−11 mol %) SS (λex = 351 nm) Judd−Ofelt intensity factors (×10−20 cm2) ZrO2:Dy3+ conc. (mol %)

Ω2

Ω4

Emission peak wavelength λp (nm)

AT (s−1)

τrad (ms)

βR

A21

1 3 5 7 9 11

11.97 30.82 8.21 14.08 36.35 38.84

6.04 9.50 25.14 14.53 43.01 10.64

584.5 583.5 583.7 584.5 583.7 587.3

5.76 14.85 3.95 6.78 17.51 18.71

173.3 67.31 25.26 147.2 57.08 53.42

0.998 0.999 0.997 0.998 0.999 1.01

0.25 0.69 0.19 0.30 0.01 0.83

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Figure 19. Illustration that shows the development of LFPs using ZrO2:Dy3+ (3 mol %) SS dusting method. (i) Impressions of fingerprints on different substrates, (ii) applied onto the substrate to stain the fingerprint, (iii) a UV light was then used to irradiate the fingerprint to emit white light and (iv) consequently revealing the fingerprint with high sensitivity and contrast.

Figure 21. LFPs stained by ZrO2:Dy3+ (3 mol %) NP. Some minutiae ridges of the FPs were identified, and images are shown (c−k).

Table 6. Various Nanomaterials Used for Develop FPs as Reported by Various Authors SL no.

Figure 20. LFPs stained by ZrO2:Dy3+ (3 mol %) SS and imaged on the surface of (a) glass, (b) knife handle, (c) marble, (d) transparent plastic sheet, (e) CD, (f) aluminum foil, (g) pet bottle, (h) mouse and (i) mobile screen after 254 nm light irradiation.

for revealing the LFPs in a simplistic and adaptable way on various surfaces as a labeling agent due to its small crystalline size and intense photoluminescence property. The LFPs stained by ZrO2:Dy3+ (3 mol %) NPs on a glass slide under 254 nm UV light are shown Figure 21. From the figure, it was confirmed that fingerprint ridges clearly indicated minutiae ridges effortlessly. Hence, it was confirmed that preparation of nanophosphor was quite useful in order to enhance LFPs in different porous and nonporous materials. Table 6 shows different hosts used to develop FPs by different authors. It was successfully demonstrated that the developed LFPs exhibit higher efficiency (because procedure involves simple setup and fast and performed within 5 min) and high sensitivity (because no color hindrance and chemical constituents can be observed) compared to the other reported method. In addition, the prepared compounds can be stored for longer time without any loss of luminescence efficiency. Electrochemical impedance spectroscopy (EIS) was carried out at RT using platinum as the counter electrode and silver as the reference electrode. The experiment was carried out in 0.1

Sample

Synthesis technique

Emission color

1

NaYF4:Yb,Er

Solvothermal

Green

2

NaYF4:Yb,Er/Ce

Hydrothermal

Green

3

Sr4Al14O25:Eu2+,Dy3+

Combustion

Blue-green

4

ZnO

Solvothermal

Green

5

CdSe

Hydrothermal

Blue

6

Eu3+:Y2Ti2O7/SiO2

Red

7

YAlO3:Tm3+

Sol−gel method Combustion

8

CdTe

Reflux

Multicolor

9

CdTe-MMT

Multicolor

10

ZrO2:Dy3+

Lowtemperature synthesis Sonochemical

Blue

White

References Meng Wang et al.48 Han-Han Xie et al.49 Vishal Sharma et al.50 Mi Jung Choi et al.51 Yuan Feng Wang et al.52 Saif et al.53 Darshan et al.54 Jianjun Liu et al.55 Feng Gao et al.56 Present work

M NaNO3 test solution for Dy doped ZrO2 NPs with an a.c. bias voltage of 0.005 V in the frequency range from 1 Hz to 0.1 MHz, and the results are shown in Figure 22. The part of semicircle region in the Nyquist plot signifies a high-frequency component and a linear region represents a low-frequency component. The diameters estimated were 76, 105, 94, 85, 58 and 73 Ω of the semicircle links to the interfacial charge transfer resistance (Rct). Depending on the Rct, the split-up efficiency between the electrons and holes can be explained clearly. The diameter of charge transfer resistance follows the order: ZrO2:Dy3+11 mol % < ZrO2:Dy3+9 mol % < ZrO2:Dy3+1 mol % ≪ ZrO2:Dy3+7 mol % < ZrO2:Dy3+5 mol % < ZrO2:Dy3+3 mol %. The smaller diameter of the circle signifies the higher Rct and lower charge recombination rates. This provides noteworthy influence to the boosting of photocatalytic activity. 57,58 Thus, ZrO2:Dy11 mol % having lower diameter is expected to exhibit high photocatalytic activity. This result was also evident from PL studies where the same mol % compound resulted in lower 2070


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duration of 90 min are shown in Figure 23b. It can be noted that degradation of MB was almost consistent under UV light. Figure 23c shows the first-order kinetic plots for the photocatalytic degradation of MB under UV light irradiation. The reaction rate constants (k) estimated were 15.63 × 10−3, 6.8 × 10−3, 10.8 × 10−3, 12.7 × 10−3, 21.78 × 10−3 and 29.4 × 10−3 min−1. The stability of the photocatalyst was important for its assessment and application.59 Figure 23d shows the reusability and stable activity toward the degradation of MB for more than six cycle runs of the ZrO2:Dy3+ (11 mol %), under UV irridiation. Hence, it can be concluded that the prepared compounds can be accepted as a basically viable material for environmental cleanup. By flashing UV light on the compound, electrons of that material jump to the conduction band, leaving same number of holes in the valence band. Then, the electrons produced react with atmospheric oxygen and produce superoxide radicals (O2−•). If Dy3+ ions were assumed to trap an electron, it gets reduced to Dy2+ (Figure 24). Alternatively, if Dy2+ ions were assumed to behave as the hole trap, they get oxidized to Dy3+ ions. The trapped holes may transfer to the hydroxyl anion and are adsorbed on the surface, forming a hydroxyl radical or it can also be moved to the adsorbed dye molecule to form a dye anion. These trapped electrons may transfer to an oxygen molecule and form a superoxide radical.60,61 The mechanism

Figure 22. Impedance plot for ZrO2:Dy3+ (1−11 mol %) SS.

intensity emission peak compared to other NPs in which higher recombination rates were observed. Figure 23a represents the spectral absorbance changes with ZrO2:Dy3+ (11 mol %) NPs showing improved degradation of ∼97% for methylene blue (MB) dye under UV light. The extracted data for the percentage of decolorization (% D) as a function of time for the hazardous MB dye using ZrO2:Dy3+ (1−11 mol %) NPs as photocatalysts under UV light for the

Figure 23. (a) Spectral absorbance changes with ZrO2:Dy3+ (11 mol %) SS photocatalyst. (b) Plot of % decolorization of MB under UV light irradiation. (c) Plot of ln(C/C0) versus irradiation time for the degradation of MB under UV light irradiation. (d) Reusability of the ZrO2:Dy3+ (11 mol %) NPs photocatalyst for six consecutive recycle runs. 2071



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AUTHOR INFORMATION

Corresponding Authors

*H. Nagabhushana. Tel.: +91-9945954010. E-mail: [email protected]. *B. Eraiah. E-mail: eeraiah@rediffmail.com. ORCID

H. Nagabhushana: 0000-0001-7552-3373 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author Dr. H.N. thanks VGST, Govt. of Karnataka, India [VGST/K-FIST-L1/GRD-489] for the sanctioning research project.

Figure 24. Schematic representation which shows the mechanism for the photocatalytic degradation of MB using ZnO as a photocatalyst.

■

can be clearly explained with the help of the reactions as summarized below:

REFERENCES

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Dy 3 + + e− → Dy 2 + Dy 2 + + h+ → Dy 3 +

Dy 2 + + e− → Dy 3 + ZrO2 + hυ → e− + h+ O2 + e− → O2−•

h+ + H 2O → OH• O2• + dye → Decolorization OH• + dye → Decolorization

The holes present in the valence band also react with water molecules and get converted to hydroxyl radicals (OH•). Due to the production of hydroxyl radicals and superoxide radicals, the electron−hole pair recombination was avoided easily and hence the photocatalytic activity was enhanced. These produced hydroxyl and superoxide radicals that finally react with the dye molecules and undergo decolorization.62

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CONCLUSIONS The ZrO2:Dy3+ (1−11 mol %) NPs were synthesized by an ultrasound sonochemical method with CTAB as surfactant. At the time of preparation, various complex morphologies were obtained by optimizing the influential factors. To achieve good crystallinity, smaller crystalline size and larger surface area of superstructures in a short time, ultrasonic irradiation was used. Thus, the present method of synthesis was simple, direct, costeffective and eco-friendly. The PXRD patterns confirm the pure cubic phase of the samples, which was further supported by Raman studies and XPS analysis. The photometric studies showed that the samples exhibit white emission that is very close to NTSC standards of pure white color. The developed FPs procedure was simple and sensitive and overcomes many of the drawbacks associated with the existing powder dusting latent FPs development. The possibility of the high photocatalytic activity was supported by the impedance studies. ZrO2:Dy3+ (11 mol %) exhibited high photocatalytic activity and thus a better photodegradation agent for hazardous MB. 2072


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