Journal of Chemistry
Volume 2019 (2019), Article ID 5749702, 13 pages
https://doi.org/10.1155/2019/5749702

Research Article

Sonochemical Synthesis and Properties of YVO₄:Eu³⁺ Nanocrystals for Luminescent Security Ink Applications

Chinh Dung Trinh,1,2 Phuong Thi Pham Hau,¹ Thi My Dung Dang,¹ and Chien Mau Dang¹

¹Institute for Nanotechnology, Vietnam National University, Ho Chi Minh City, Vietnam
²The University of Science, Vietnam National University, Ho Chi Minh City, Vietnam

Correspondence should be addressed to Chien Mau Dang; dmchien@vnuhcm.edu.vn

Received 30 January 2019; Revised 1 April 2019; Accepted 18 April 2019; Published 10 July 2019

Guest Editor: Van Duong Dao

Copyright © 2019 Chinh Dung Trinh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Solutions and redispersible powders of nanocrystalline, europium-doped YVO₄, are prepared via a wet chemical method using the ultrasonic processor (sonochemical) and microwave and thermal stirring. From X-ray diffraction (XRD) results, YVO₄:Eu³⁺ nanoparticles synthesized using sonochemical method have better crystallinity than those prepared using thermal stirring and microwave methods exhibiting the tetragonal structure known for bulk material. From field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results, it is found that the size of nanoparticles is around 25 nm and increasing after annealing at 900°C. From UV-Vis result, there is a peak at 270 nm corresponding to the absorption of groups. The photoluminescence (PL) results clearly show the strongest red emission peak at the wavelength around 618 nm. The highest luminescent intensity is obtained for the sample prepared by the sonochemical method at pH = 12 and annealing temperature at 900°C for 4 h. The average lifetimes of the Eu³⁺ ions in the samples annealed at 300, 600, and 900°C for 1 h at 618 nm emission under 275 nm excitation are 0.36, 0.62, and 0.64 ms, whereas sample annealed at 900°C for 4 h has lifetime of 0.70 ms. The security ink, containing synthesized YVO₄:Eu³⁺ nanoparticles, is dispersed in glycerol and other necessary solvents. The experimental security labels are printed by inkjet using the electrohydrodynamic printing technique. The resulting lines represented to the security labels are analyzed by the 3D microscope equipment and UV 20 W mercury lamp with a wavelength of ∼254 nm. The seamless line of the printed security label has the value of the width at ∼230 μm, thickness at ∼0.68 μm, and distance between two adjacent lines at 800 μm. This result is compatible for producing security labels in small size (millimeter) in order to increase security property.

1. Introduction

The study of rare earth doped luminescence materials has been largely motivated by the prospect of original specific applications such as electroluminescent techniques, biological labels, and integrated optics [1–6].

Moreover, rare earth nanoparticles are interesting due to their marked improvement in lumen output, color rendering index, energy efficiency, and greater radiation stability [6–11].

YVO₄:Eu³⁺ has large application in color television cathode ray tube displays [12] and high-pressure mercury lamps [13] as a red phosphor. The photoluminescence quantum yield of the europium emission is as high as 70% in YVO₄ with the excitation by UV light [14]. It is also applied in biology [15–17] and specially in producing security ink. The vanadate group (V⁵⁺–O²⁻ charge transfer band) in YVO₄:Eu³⁺ phosphor is excited by ultraviolet radiation, and this provides efficient energy transfer to Eu³⁺ [18].

YVO₄:Eu³⁺ phosphor can be prepared by different methods, for example, high-temperature solid state method [19], combination method [20], microwave rapid heating method [21–23], sol-gel method [24], and hydrothermal reaction method [25]. Recent studies have shown that there is a tremendous potential in nanoscale rare earth doped luminescence materials in abovementioned fields.

In recent years, the inkjet technique has been in research to make spare parts such as electric circuits and biosensor [26–35]. The advantage of this technique is having fewer steps in preparation and ability to print in many different bases such as conductive base, unconductive base, solid base, and flexible base. This inkjet technique requires research in the printing process and inkjet ink. In 2012, Meruga et al. investigated security ink from the rare earth nanoparticles ß-NaYF₄-doped Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ to print security QR code on paper and PET by Optomec direct-write aerosol jetting [36]. According to the work of Gupta et al. in 2010, they evaluated the security ink made of rare earth nanoparticle Y₂O₃ doped Eu³⁺ (Y₂O₃:Eu³⁺), and the samples in this research were printed by screen printing techniques [37]. The purpose of our security inkjet technique is making products with high security. With inkjet technology and this security ink, we study printing the labels with the demand of high security on money, visa, certificate, and military products. The high security characteristic of the product is determined by two factors—the first is small size and delicacy of the printed label (related to the printing technique) and second is the strong emissivity of printed label under UV (related to optical emission of YVO₄:Eu³⁺ nanoparticles in the ink). The emissivity of YVO₄:Eu³⁺ nanoparticles is affected by many factors such as particle size and crystallinity, doping level of Eu³⁺ ions into YVO₄, etc. All these parameters are affected by the production method applied for nanoparticles [38–42].

In this study, we use the wet chemical method to synthesize YVO₄:Eu³⁺. Our purpose is to apply YVO₄:Eu³⁺ as phosphor in security ink. Hence, performing research in producing the YVO₄:Eu³⁺ nanoparticles with strong luminescent intensity is crucial along with the research in inkjet process. Regarding the references on synthesis of YVO₄:Eu³⁺ nanoparticles by wet-chemical method [12–21, 38–42], we use three different routes to synthesize YVO₄:Eu³⁺ nanoparticles, which are thermal stirring, microwave methods, and ultrasonic methods, in order to compare the luminescent level of the nanoparticles. According to the published results, the best doping level of Eu³⁺ in YVO₄ for luminescence properties is 5 mol% [38, 43–55]. We use YVO₄:Eu³⁺ nanoparticles to produce ink for PS JET 300 V inkjet printer. This printer is operated by the electrohydrodynamic (EHD) inkjet technique. The advantage of EHD inkjet technique is the ability to print labels in small size (in micrometers) on various material substrates.

2. Materials and Methods

2.1. Materials

Y(NO₃)₃·6H₂O (99.8%, Aldrich), Eu(NO₃)₃·5H₂O (99.9%, Aldrich), and Na₃VO₄ (99.98%, Aldrich) are used as starting materials. NaOH (99%, Merck) is used to control pH.

C₃H₈O₃ (99.5%, Merck), C₂H₆O (99.5%), C₄H₈O₂ (99.8%, Merck), and C₂H₆O₂ (99.5%, Merck) are solvents and binding agents used in the synthesis of security ink.

2.2. Synthesis of YVO₄:Eu³⁺ Nanoparticles and Security Ink

YVO₄:Eu³⁺ nanoparticles are synthesized by the wet chemical method. Dissolving 0.88 g of Y(NO₃)₃·6H₂O (2.3 mmol) and 0.05 g of Eu(NO₃)₃·5H₂O (0.12 mmol) into 15 ml DI water (Eu³⁺ doping molar concentration is 5%) in 15 min leads to the formation of solution A, whereas 0.44 g of Na₃VO₄ (2.4 mmol) in 15 ml DI water is solution B. Mixing solution A with solution B dropwise leads to white precipitation in the mixture. The pH value of mixture is adjusted to 12 by using 5 M of NaOH.

This mixture is heat-treated by three different ways for comparison, which are thermal stirring at ∼150°C in 1 h, microwave in 15 min by Mars 6 (CEM Corporation) [21–23], and sonochemical by ultrasonic liquid processors VCX 750 (Sonics and Materials) with electrical frequency 20 kHz in 15 min. Synthesized YVO₄:Eu³⁺nanoparticles are dried at 60°C and annealed at 300, 600, and 900°C in an oven (Carbolite Gero, Max. temp. up to 1100°C) for 1 h. The formation of YVO₄:Eu³⁺ nanoparticles follows equation (1):

The security ink using YVO₄:Eu³⁺ nanoparticles is synthesized by mixing YVO₄:Eu³⁺ nanoparticles in such solvents as glycerin, ethanol, ethyl acetate, and ethylene glycol at appropriate ratios.

2.3. Security Printing

The test patterns are printed onto glass substrate by a commercial printer (PS JET 300 V). This printer is operated by the electrohydrodynamic (EHD) inkjet technique.

2.4. Characterization

The prepared YVO₄:Eu³⁺ nanoparticle samples are studied by UV-Vis absorption spectroscopy by using a double-beam spectrophotometer in the wavelength range from 200 to 900 nm. Particle size is determined by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM). Samples for TEM measurements are suspended in ethanol and ultrasonically dispersed. The suspension drops are placed on a copper grid coated with carbon. The crystallite structure of YVO₄:Eu³⁺ is analyzed by X-ray diffraction spectroscopy. The emission spectra are recorded at room temperature using a Hitachi F-4500 spectrophotometer. The decay of luminescence is measured by a Horiba Deltaflex™ with 275 nm SpectraLED excitation source.

The security ink viscosity is analyzed by an m-VROC™ VISCOMETER. The samples after printed by inkjet printer are synthesized by a 3D microscope (Sensofar Metrology) and UV 20 W mercury lamps (Germicidal lamp, Sankyo Denki Co.) having a wavelength of around 254 nm.

3. Results and Discussion

3.1. Effect of Different Synthesis Methods on the Formation of YVO₄:Eu³⁺ Nanoparticles

The wet chemical method is applied in all experiments; however, there is difference in the heating method during synthesis process as mentioned above. Three synthesized samples correspond to three different methods with the same chemical components and ratios. The doping concentration of Eu³⁺ is 5 mol% in YVO₄ host (Y0.95Eu0.05VO₄), which had been optimized previously by Georgescu et al. [43], Kumar et al. [44], and He et al. [45]. Each heating method has different effect to YVO₄ particles crystallinity and doped ability of ion Eu³⁺ to host matrix. The crystallization and doped ability have crucial effect to luminescent intensity of YVO₄:Eu³⁺nanoparticles. The XRD patterns recorded for the YVO₄:Eu³⁺ samples are shown in Figure 1. All diffraction peaks were successfully attributed to known tetragonal phase of YVO₄ (JCPDS, No. 17-0341) [41, 42, 46].

Figure 1: The XRD pattern of YVO₄:Eu³⁺ nanoparticles synthesized with a different method.

Among three preparation methods, the highest peak intensity is observed for the sample fabricated by the sonochemical treatment, that reveals the best crystallinity level of YVO₄:Eu³⁺ nanoparticles in this sample. According to theory and references of sound waves, the constitution of ultrasound region happens with sound wave frequency above 20 kHz. The ultrasound region can be divided into two parts: one where the cavitation phenomenon takes place (20–100 kHz), called power ultrasound, and the other where no cavitation occurs (5–10 MHz), used for diagnostics. Sonochemical effects depend on the cavitation phenomenon. According to the “hot spot” theory, each cavitation bubble behaves like a microreactor, which, in aqueous systems, at an ultrasonic frequency of 20 kHz each cavitation bubble collapse acts as a localized “hotspot” generating temperatures of about 4000 K and pressures in excess of 1000 atmospheres [47–51].

A series of radicals, such as and , are formed, by the irradiation process of ultrasound to water, at the gas-phase interface of the cavitation bubbles, and the responsibility for the enhanced reactivity to a lesser extent, in the bulk solution [51].

These factors have positive effect to the crystalline nanoparticle formation and incorporation of Eu³⁺ ions into YVO₄ lattice. These factors have effect to the luminescent intensity of YVO₄:Eu³⁺ nanoparticles.

The TEM micrographs and size distribution diagrams of YVO₄:Eu³⁺ nanoparticles prepared by sonochemical and thermal stirring methods are shown in Figure 2. The monodispersion state of YVO₄:Eu³⁺ nanoparticles is evident. In Figure 2(a), more even size distribution is observed with maximum at ∼25 nm. The wider and nonhomogeneous size distribution is seen in Figure 2(b). The TEM results agreed with the XRD results shown in Figure 1.

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(b)

Figure 2: TEM micrographs and the size distribution diagram of YVO₄:Eu³⁺ nanoparticles synthesized by sonochemical (a) and thermal stirring (b).

There are three major steps in the excitation-emission process of YVO4:Eu³⁺ under UV radiation. Firstly, UV radiation is absorbed by groups. However, UV radiation can be directly absorbed by Eu³⁺ ions, and this is dependent on the wavelength. Secondly, the migration of activation energy through vanadate sublattice leads to the transferring of the excited energy to Eu³⁺ ions, and the last step is the production of strong red emission by the de-excitation process of excited Eu³⁺ ions [15, 41, 52, 56]. A proposed energy transfer mechanism demonstrating the above process is shown in Figure 3.

Figure 3: Energy levels and transitions scheme of Eu³⁺. Vertical arrows: absorption and emission transitions.

In Figure 4 (left inset), the UV-V is spectra of YVO₄:Eu³⁺ nanoparticles prepared by sonochemical and wavelength and thermal stirring methods are shown. The redispersed dry powder of YVO₄:Eu³⁺ in the same amount of deionized water was stirred 2-3 min, resulting in the transparent colloid representing the absorption spectrum. There is a peak at around 270 nm in these three samples that proved the existence of absorption in the groups [39, 41, 52, 56]. According to references, it is explained as the attribution to the charge transfer from oxygen ligands to the central vanadium atom in group [15, 40, 41]. The UV-Vis spectra in Figure 4 (left inset) are crucial regarding to the luminescence mechanism of YVO₄:Eu³⁺ nanoparticles, and it proves that there is an energy absorption and transfer from to Eu³⁺. The sample of the sonochemical method exhibits the strongest absorption while the one of the thermal stirring methods exhibits the weakest absorption.

Figure 4: Photoluminescence spectra of YVO₄:Eu³⁺ nanoparticles synthesized with a different method. Right inset shows UV-V is spectra of YVO₄:Eu³⁺ nanoparticles synthesized with a different method. Left inset is image of YVO₄:Eu³⁺ powder synthesized with sonochemical method under UV lamb 254 nm.

In Figure 4, the photoluminescence emission spectra of YVO₄:Eu³⁺ nanoparticles prepared by three synthesis methods are shown. The samples were excited at ∼275 nm. The sharp lines in the range from 550 to 750 nm correspond to the transitions from the excited ⁵D₀  ⟶  ⁷FJ of Eu³⁺ ions (Figure 3) [15, 41, 57–59]. There is no obvious vanadate group emission band indicating the transfer of absorption energy of the vanadate groups to Eu³⁺ ions. There is assignation of strongest red emission peak at 618 nm to the ⁵D₀  ⟶  ⁷F₂ transition, emission peak at 590 nm to ⁵D₀  ⟶  ⁷F₁ of Eu³⁺ ions, and strong emission peak at 692 nm to the ⁵D₀  ⟶  ⁷F₄ transition [15, 40, 41]. Eu³⁺ ions occupy asymmetry inversion center instead of Y³⁺ in the strongest emission from ⁵D₀  ⟶  ⁷F₂ transition [15, 41]. The sample produced by the sonochemical method has the strongest intensity at 618 nm, as compared to that of other samples. Regarding this result, the characteristics of the ultrasonic processor, high local temperature and pressure, are attributed to the forming of YVO₄:Eu³⁺ nanoparticles with high crystalline structure and better doping of Eu³⁺ ions to YVO₄ lattice. We also assumed that sound waves with characteristic of mechanical waves affect the doping of Eu³⁺ ions to the YVO₄ lattice. And, it leads to the highest intensity of emission peak [51]. Figure 4 (right inset) is the image of YVO₄:Eu³⁺ nanoparticles powder, synthesized by sonochemical method, under excitation at 254 nm by the UV lamp. The powder turns red under the UV lamp corresponding to dominant emission at 618 nm.

The purpose is synthesizing YVO₄:Eu³⁺ nanoparticles with strong luminescence. There are many factors which affect the light emitting of YVO₄:Eu³⁺ nanoparticles (related to the light emitting mechanism) such as the doping level of Eu³⁺ ions, particle crystalline, and nanoparticle size. Among these factors, the nanoparticle crystallinity and the incorporation of Eu³⁺ ion into the Y³⁺ positions in the lattice of YVO₄:Eu³⁺ nanoparticles play important roles in the increasing of luminescence. As to the above result, the YVO₄:Eu³⁺ nanoparticles with highest luminescent intensity are synthesized by the sonochemical method, and so, this method will be used for the future research.

YVO₄:Eu³⁺ nanoparticles are synthesized by the wet chemical method with De-ion water as solvent, in which the pH parameter affects the forming of particles as well as the crystalline growth of nanoparticles. Hence, the pH of aqueous vanadate solution is an important parameter in the synthesis [22, 23, 43, 60]. The samples are synthesized with the pH value variation (adjusted by 5 M·NaOH) in the solution as follows: 8, 10, 12, and 14 (with sonochemical method and the ratio of chemical as mentioned above). The change in the luminescence intensity of YVO₄:Eu³⁺ nanoparticles is shown in Figure 5.

Figure 5: Photoluminescence spectra of samples with different pH values. Inset shows pH value dependence on ⁵D₀ ⟶ ⁷F₂ intensity.

Figure 5 shows the main emission is at 618 nm due to ⁵D₀  ⟶  ⁷F₂ transition with the strongest emissivity. The samples are excited at ∼275 nm. At 618 nm, the luminescence intensity of sample with pH = 12 is strongest, the second is pH = 14, and the weakest is pH = 8. This can be explained that the reaction does not occur when pH > 12 and only the precipitation of very small particles of Y(OH)₃ takes place instead of YVO₄. When the pH is smaller than 12, the color of the solution changes slowly from light to dark yellow. This may be the attribution to the forming of polyvanadate species [23, 43]. Hence, pH = 12 is optimal for emissivity of YVO₄:Eu³⁺nanoparticles.

3.2. Effect of Annealing Temperature on the Characteristics of YVO₄:Eu³⁺ Nanoparticles Synthesized by Sonochemical Method

The annealing process is carried out after nanoparticles formation, and it plays an important role in increasing the crystallinity and decreasing structure distortion of rare earth nanocrystals [38, 40, 45]. YVO₄:Eu³⁺nanoparticles are synthesized by the sonochemical method and then centrifuged at 9000 rpm. The final powder is dried at 60°C in 6 h and then annealed at 300, 600, and 900°C in 1 h.

Figure 6 shows the XRD patterns, as recorded for the YVO₄:Eu³⁺ nanoparticles prepared at 300, 600, and 900°C. Only the tetragonal phase of YVO₄ (JCPDS, No. 17-0341) is observed in all samples. The sample annealed at 900°C for 1 h has better crystallinity in comparison to that of the samples annealed at 300 and 600°C for 1 h. This result shows that annealing at 900°C is preferable for the sample synthesis.

Figure 6: XRD samples of YVO₄:Eu³⁺ nanoparticles synthesized at different annealing temperatures.

Figure 7 shows the FE-SEM patterns of the samples annealed at 300, 600, and 900°C for 1 h and the energy dispersive X-ray (EDX) spectra of YVO₄:Eu³⁺ nanoparticles annealed at 900°C for 1 h. Using ImageJ software for calculation, the correlative average particle sizes are found to be 47 nm, 78 nm, and 170 nm. Thus, when the annealing temperature increases, the particle size also increases. The increasing of YVO₄:Eu³⁺ nanoparticles size along with increasing annealing temperature has been observed by Georgescu et al. [38] and Li et al. [61]. This phenomenon can be explained by the nephelauxetic effect which is related to the metal-ligand bond covalency. Therefore, the covalency of Eu³⁺–O²⁻ bonds in YVO₄:Eu³⁺ increases as the temperature increases in thermal treatment. There is a relation between the increase of covalency with nanoparticle size and the expansion of the cell parameters of YVO₄:Eu³⁺ nanoparticles in rapport with the bulk material.

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(b)

(c)

(d)

Figure 7: FE-SEM micrographs of YVO₄:Eu³⁺ nanoparticles at different annealing temperatures and corresponding EDX spectrum of samples annealed at 900°C (in 1 h).

The EDX spectrum in samples of YVO₄:Eu3⁺ nanoparticles affirms the existence of yttrium (Y), oxygen (O), vanadium (V), and europium (Eu) factors, indicating that Eu³⁺ ions are doped into the YVO₄ nanocrystals.

The photoluminescence spectra which appeared under excitation at 275 nm of samples fabricated at different annealing temperatures are shown in Figure 8. Right inset is the UV-V is spectra of YVO₄:Eu³⁺ nanoparticles with different annealing temperatures—left inset is the diagram presenting the dependence of ⁵D₀  ⟶  ⁷F₂ intensity on annealing temperature. The optical emission mechanism is the absorption of ultraviolet light by groups and transfer of energy to Eu³⁺, the energy which is released in the form of fluorescence [61–63]. Hence, the absorption of is displayed in Figure 8 (right inset). There is an absorption in the groups related to the peaks at 270 nm of three samples. In accordance with references, we can explain that the oxygen ligands attributed to the charge transfer to central vanadium atom in group. The result of UV-Vis spectra is important because of the luminescent characteristic of YVO₄:Eu³⁺ nanoparticles resulting in the energy absorption and transfer from to Eu³⁺ [22, 41]. All three samples have the strongest peak at 618 nm resulting from ⁵D₀  ⟶  ⁷F₂ transition. This leads to the indication of the occupying of Eu³⁺ ions to the asymmetry inversion center instead of Y³⁺. Figure 8 (left inset) shows the dependence of luminescence intensity on annealing temperature. With the annealing at 900°C for 1 h, YVO₄:Eu³⁺ nanoparticles have the strongest luminescence intensity, which can be explained that the increase of temperature will decrease the number of top surface defects. However, the annealing temperature in this research remains not over 900°C in order to control the size of nanoparticles. Due to the nanoparticle size increasing, the disadvantage appeared in the preparation of security ink.

Figure 8: Photoluminescence spectra of samples at different annealed temperatures. Inset (right) shows the UV-V is spectra of YVO₄:Eu³⁺ nanoparticles with different annealed temperatures; left inset is the diagram of dependence of ⁵D₀-⁷F₂ intensity on annealing temperature.

Figure 9 shows the photoluminescence spectra of samples annealed at 900°C for 2, 4, and 5 h with excitation at ∼275 nm. The more increasing of Eu³⁺ local symmetry environment causes the increase of the strongest emission from ⁵D₀ ⟶   ⁷F₂ transition over annealing time from 2 h to 4 h. However, the annealing time of 5 h leads to the lower intensity than that at 4 h. The relative intensity ratio of ⁵D₀  ⟶  ⁷F₂ to ⁵D₀  ⟶  ⁷F₁ peaks can show the symmetry rate of local environment of Eu³⁺ ions [23, 61, 64]. To reveal the influence of heat treating temperature and time on the luminescence properties, the relative intensity ratios of ⁵D₀  ⟶  ⁷F₂ and ⁵D₀  ⟶  ⁷F₁ transitions in the samples are calculated, and the results are shown in Table 1.

Figure 9: Photoluminescence spectra of samples annealed at 900°C in different durations. Inset shows diagram of dependence of ⁵D₀-⁷F₂ intensity on the annealing time at 900°C.

Table 1: ⁵D₀ ⟶ ⁷F₂/⁵D₀ ⟶ ⁷F₁ relative emission intensity ratio.

Sample
⁵D₀ ⟶ ⁷F₂/⁵D₀ ⟶ ⁷F₁

300°C-1 h
8.80

600°C-1 h
5.58

900°C-1 h
5.26

900°C-2 h
6.38

900°C-3 h
5.97

900°C-4 h
5.71

900°C-5 h
5.87

In three samples annealed at 300, 600, and 900°C for 1 h, a tendency to decrease the intensity ratio with increasing heat treating temperature indicates the increase of symmetry of local environment of Eu³⁺ ions heat treatment temperature. The highest relative intensity ratio was obtained in the sample annealed at 300°C and the lowest in the sample at 900°C, which shows that highest Eu³⁺ local symmetry environment is in the sample treated at 900°C. These results can be explained that the low annealing temperature (at 300°C) leads to the slower crystal growth rate, the sample obtains enough energy due to increase in the annealing temperature (at 900°C) and has better crystallinity.

In four samples annealed at 900°C for 2, 3, 4, and 5 h, the relative intensity ratio decreases gradually from 2 h to 4 h sample, but this ratio in 5 h sample is higher than 4 h sample. It shows that the local symmetry of Eu³⁺ environment is the highest in the 4 h sample in comparison with other samples. The reason is that the particles have enough time and energy to have better crystallite growth [38, 40, 65]. However, increasing of the heat-treating time to 5 h leads to structure distortion and decrease of Eu³⁺ local symmetry environment.

In Figure 10, the room-temperature luminescence decay curves of ⁵D₀  ⟶  ⁷F₂ transition of Eu³⁺ are shown for the samples annealed at 300, 600, and 900°C for 1 h (Figures 10(a)–10(c)) and at 900°C for 4 h (Figure 10(d)). The excitation wavelength is fixed at 275 nm. The common factors affecting the decay kinetics behavior are the number of different luminescent centers, defects, energy transfer, and impurities in the host [62]. The raw data recorded for the decay curves of all samples are well-fitted by a double-exponential function (equation (2)).

where I is the luminescence intensity at time t, A is the fitting parameter, and τ is the decay lifetime, respectively. The average lifetimes of the Eu³⁺ ions in samples annealed at 300, 600, and 900°C in 1 h at 618 nm emission under 275 nm excitation are calculated to be 0.364, 0.623, and 0.644 ms. The average lifetime increases with the increase of annealing temperature from 300°C to 900°C. The decay lifetime difference may appear due to the nonradiative transition caused by the surface defects and/or crystallinity rate [42]. The higher the annealing temperature, the lower the surface effect. As shown in Figures 10(c) and 10(d) the average decay lifetime of Eu³⁺ ions in sample annealed at 900°C for 4 h is 0.703 ms, which is longer than that in the sample annealed at 900°C for 1 h. The measured decay time comprises both radiative and nonradiative transmission. The radiative decay component is dependent upon the number of light emitting activator ions in the nanoparticles [44]. We assume that Eu³⁺ ions doped in host lattice are light emitting activators, meaning that the occupancy of Eu³⁺ ions to Y³⁺ sites in the samples annealed at 900°C for 4 h is better than that in the sample treated for 1 h.

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(b)

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(d)

Figure 10: The luminescence decay curves for the ⁵D₀ excited state of Eu³⁺ under 275 nm excitation for YVO₄:Eu³⁺ samples synthesized with different annealing temperatures and durations.

3.3. Test Security Printing

There are very few reports on the use of rare earth ions as luminescent colloids. The typical report of Gupta et al. is the usage of spherical Y₂O₃:Eu³⁺ nanoparticles as security ink for screen printing technique, and Meruga et al. studied the security ink from the rare earth nanoparticles ß-NaYF₄-doped Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ to print security QR code by Optomec direct-write aerosol jetting. Our purpose is printing label at small size, from micrometer to millimeter, with high sharpness in order to increase the security ability for product. Therefore, we use YVO₄:Eu³⁺ to produce ink for the PS JET 300 V inkjet printer. This printer is operated by the electrohydrodynamic (EHD) inkjet technique. The advantage of the EHD inkjet technique is the ability to print labels at small size (in micrometer) on various material substrates.

For the preparation of security ink, the YVO₄:Eu³⁺ nanoparticles are synthesized by the sonochemical method and then redissolved into organic solvents. On the basis of the above results, YVO₄:Eu³⁺ nanoparticles, synthesized by the sonochemical method and annealed at 900°C for 4 h, are most appropriate to be used as security ink in inkjet printing due to high luminescent intensity at 618 nm and proper size for not clogging the printhead. Formulation of YVO₄:Eu³⁺ nanoparticles ink is shown in Table 2 with the viscosity at ∼350 cP. The printhead is made of metal with diameter at ∼198 μm, DC voltage at 1500 V, frequency at 500 Hz, distance between print head and substrate at around 150 μm, and printing speed at 20 mm/s. We perform printing experiment with representative line security labels at small size length of 5 mm and distance between two lines set at 300 and 800 μm.

The printed line image on glass substrate under daylight and UV lamp 254 nm is shown in Figure 11. The lines printed in Figure 11(a) are with distance set at 800 μm apart and length at 5 mm and Figure 11(b) with length at 5 mm and distance set at 300 μm apart. In this experiment, the printed lines are nearly transparent under daylight and have red color luminescence under UV lamp working at 254 nm. The lines are clear, sharp, seamless, and not overlapped over each other even at small distance, 300 μm.

The sample with distance of 800 μm between two lines set is analyzed by Sensofar Metrology 3D microscope technique. Figure 12 and inset present the height-width diagram of section of line after being printed and the micrograph of the respective line. The line is seamless with the width at ∼230 μm and thickness at ∼0.68 μm. According to this result, there is a possibility of printing small security labels with micrometer to millimeter size on glass substrate in order to enhance security property.

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Figure 11: The image of lines under daylight and UV lamp, labels with distance between two lines at 300 μm (a) and 800 μm (b).

Figure 12: The height-width diagram of section of line after printed. Inset shows the micrograph of the respective line taken by Sensofar Metrology 3D microscope technique.

Table 2: Formulation of YVO₄:Eu³⁺ nanoparticles ink.

Ingredient
YVO₄:Eu³⁺
Glycerin
Ethanol
Ethyl acetate
Ethylene glycol

Percentage (% wt.)
8
62
16
4
10

4. Conclusion

The YVO₄:Eu³⁺ nanoparticles are synthesized by thermal stirring and microwave and sonochemical methods. According to the analyzed result, the crystallization of YVO₄:Eu³⁺ nanoparticles appears in the tetragonal structure and the phosphor has the strongest luminescence at wavelength of 618 nm due to ⁵D₀  ⟶ ⁷F₂ transition. The creation of the high local temperature and pressure by the sonochemical method has positive effect on the formation of crystalline YVO₄:Eu³⁺ nanoparticles and doping of Eu³⁺ ions into Y³⁺ position. As shown by TEM observation, the average size of YVO₄:Eu³⁺ nanoparticles synthesized by the sonochemical method is ∼23 nm.

The value of pH in the synthesized solution has influence on the emission intensity of YVO₄:Eu³⁺nanoparticles, as pH at 12 is the most appropriate value. Concerning annealing at different temperatures, YVO₄:Eu³⁺ nanoparticles show the strongest luminescence intensity when being annealed at 900°C. The optimal luminescence intensity appears in the sample annealed at 900°C for 4 h. The size of YVO₄:Eu³⁺ nanoparticles is proportional to the annealing temperature, and the average size of YVO₄:Eu³⁺ nanoparticles is ∼170 nm at 900°C applied for 1 h. The average lifetimes of the Eu³⁺ ions emission at 618 nm under 275 nm excitation of the samples annealed at 300, 600, and 900°C for 1 h are 0.364, 0.623, and 0.644 ms, whereas the value in the sample annealed at 900°C for 4 h is 0.703 ms.

Analysis of the results shows that the lines printed by inkjet technique is solid and even with the width at ∼230 μm, the thickness at ∼0.68 μm, and the smallest distance of two adjacent lines at 300 μm. The test lines are nearly invisible under daylight, and they are red under UV mercury lamps with wavelength of ∼254 nm.

Data Availability

No data were used to support this study.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors would like to give the appreciation to the Vietnam National University, Ho Chi Minh City (VNU-HCM), no. C2017-32-01/HĐ-KHCN.

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