Europium-doped Y2O3 having mainly two different crystal structures; cubic and monoclinic, is one of the materials widely used as red emitting phosphors in industries[1]. Since crystal structure and morphology of Y2O3:Eu affects its optical properties leading different applications, a number of studies on the synthesis methods and controlling structure and morphology have been carried out[2]. In this study, the techniques applied to generate the desired structure and morphology were reviewed. Based on the wide literature review, the nature of Y2O3:Eu crystal structure and morphology can be understood, and a new method can be developed which can be used in the synthesis of desirable Y2O3:Eu structure.
2. INTRODUCTION
Y2O3:Eu is an element which emits red light in ultraviolet irradiation[3]. Such a fluorescent property of Y2O3:Eu makes it possible to be variously applied in many industries or research areas. These areas include lamps and plasma display panels[4], emission display[5], high density television, and medical imaging[6-7].
Many methods to synthesize Y2O3:Eu has been studied. These methods can be combustion synthesis [8], chemical deposition [9], laser ablation [10], spray pyrolysis in hot-wall or flame reactors[11]. Flame aerosol synthesis is one of the methods that uses the flame as a reactor. Flame aerosol synthesis can be sort of flame spray pyrolysis(FSP) and flame assisted spray pyrolysis(FASP)[2, 12]. This classification is related to the solvent of the precursor solution and fuel. FSP uses the solvent of the precursor solution as a fuel[2]. FASP uses water as the solvent of the precursor solution and a gas as a fuel[13]. Flame aerosol synthesis is the one-step method to generate Y2O3:Eu[3], that is why we review some articles about flame aerosol synthesis prepared to synthesize Y2O3:Eu in this paper.
According to the previous studies, it has been found that Y2O3:Eu has multiple crystal structures[11]. Among the crystal structures, two main crystal structures are cubic and monoclinic[3]. Cubic and monoclinic crystal structures of Y2O3:Eu have different photoluminescence[14]. The cubic structure Y2O3:Eu has higher fluorescence intensity than the monoclinic structure[11]. This makes the cubic structure Y2O3:Eu be a commercial product[12].
There are many factors that affect the crystal structure and morphology of Y2O3:Eu. One of the factors is the temperature. At a high flame temperature, spherical, densely packed Y2O3:Eu is produced. In contrast, hollow and porous particles of Y2O3:Eu are produced at a low flame temperature[2]. Moreover, the monoclinic structure of Y2O3:Eu is mostly synthesized at a high temperature while the cubic structure of Y2O3:Eu is only synthesized at a low temperature[3].
This morphologic and structural difference on temperature is one of the problem we need to solve to get a desirable property of Y2O3:Eu. To develop the previous studies, we need to look into articles about the relationship among temperature, morphology, and structure. Also, we need to find other factors that would affect the structure and morphology of Y2O3:Eu. As a result, the purpose of this review is to go through some articles to synthesize Y2O3:Eu, compare the methods and factors that affect the structure and morphology of Y2O3:Eu, then present new possibilities to control the structure and morphology of Y2O3:Eu. Hereafter, the detailed contents are described.
3. Methods
3.1 Y2O3:Eu Phosphor Particles Characteristics Prepared By Flame Spray Pyrolysis[11]
3.1.1 Experimental
The shape and morphology Y2O3:Eu particles are important issues when preparing these particles due to the influence of these parameters on photoluminescence of these particles. For this reason new preparation technique has been developed to obtain best photoluminescence characteristics. In this technique in addition to the spray pyrolysis, a flame is applied to the droplets that were prepared by ultrasonic spray generator.
By applying this technique high temperatures can be achieved and high temperature allows obtaining more dense structures [11]. In conventional spray pyrolysis technique hollowness and high porosity is the main problems. High porosity and hollowness cause decrease in luminescence and structural stability of particles which is a problem for commercial applications [11]. This problem arises from the low temperature process of droplets which is the reason of defected morphology. To produce high temperature flame, fuel gas such as hydrogen and propane can be used. This provides temperature above 20000 C. In a schematic diagram of flame spray pyrolysis is shown [11].
Fig : Simple diagram of flame spray pyrolysis system [11].
At this system Y and Eu nitrates salts are solved in distilled water and 1M concentration of solution is prepared. Prepared solution is supplied to the flame nozzle by using oxygen as carrier gas. At the tip of the nozzle fuel gas and oxygen is combined and desired temperatures can be obtained easily by varying the fuel gas and oxygen amount. In the reactor droplets evaporate, decompose and melt then desired structure is obtained. At this method the time of residence in rector is less than in the spray pyrolysis method. However the main factor in determining the product morphology and shape is not the residence in rector but the temperature reached during evaporation and decomposition [11]. This is an important result observed in the studies of preparation of Y2O3:Eu with flame spray pyrolysis. Although in conventional method (spray pyrolysis method) the residence time is longer, the temperature obtained in reactor with an electric furnace is below 15000 C [11]. After the travel of evaporated and decomposed particles in reactor they reached to bag filter which is used to collect particles produced in reactor [11]. Also a pump is used to sustain flow of droplets to the bag filter from droplet generator through the flame nozzle and reactor. This system provides necessary control the flame temperature. However the process may not be enough to obtain cubic Y2O3:Eu structure due to the high temperature and fast cooling during the reaction in reactor. Thus a second step of post heat treatment is required. The cubic phase of Y2O3:Eu has higher photoluminescence intensity than the monoclinic phase of Y2O3:Eu [11].
3.1.2 Results
According to the studies Y2O3 particle shape is highly related to the structure of Yttrium oxide. Yttrium oxide shows cubic and monoclinic crystal structure. Cubic structure is stable until approximately 17000 C above this temperature Yttrium oxide has monoclinic stable crystal structure. Cubic yttrium oxide has higher photoluminescence intensity however the particles obtained with conventional methods shows hollow and agglomerated which reduces photoluminescence intensity of cubic crystal Yttrium oxide. Flame spray pyrolysis solves the hollowness and agglomeration problem. However, because of the fast cooling during reaction, high temperature monoclinic structure remains at low temperature. Monoclinic structure can be transformed to the cubic structure by applying a post treatment. The important point is to preserve the desired spherical shape and better morphology of monoclinic particles at high temperatures where the phase transformation to the cubic structure occurs [11]. When temperature of treatment furnace increased to the 12000 C, phase transformation from monoclinic phase to the cubic phase occurs however the shape of the particles cannot be preserved and highly agglomerated and irregular shapes obtained. To avoid this morphological shape a stepwise temperature increase can be applied. These two heat treatment approaches is shown in . As shown in figure in second method temperature is increased slowly to keep shape and morphology of monoclinic structure and at 10000 C monoclinic to cubic phase transformation occurred. The SEM images particles of without and with post treatment is shown in and respectively. According to these SEM images the particle shapes was preserved with the second method of post-treatment. Also any agglomeration is avoided.
When the SEM images analyzed it can be seen that actually two particle sizes are distributed in two different ranges and these ranges can be defined as nanometer size and submicron size ranges [11]. Nanometer size particles are usually formed at the surface of the sub micron particles. These particles are inevitably formed due to the closeness of the evaporation and melting points of Yttrium oxide. However these nanometer size particles can be dissolved into the large particles after post treatment. This is another reason of applying a post-treatment to the as-prepared particles.
In this process flame temperature is the key point to get desired particle size and morphology. If the estimated adiabatic flame temperature is reduced to a lower temperature, particles have cubic structure and their morphology and shape is similar to the Y2O3:Eu particles obtained from other conventional methods. This is showed at . On the other hand when high flame temperature is applied undesired nanometer size particles are formed in high concentration and after post treatment these nanometer size particles can survive and cause inappropriate shape and morphology [11].
XRD results of particles before and after post treatment shows that before the post treatment crystal structure of particles are not completely cubic or monoclinic but the composition of these structures and crystallinity is low. However after post treatment completely cubic structure is obtained and crystallinity is high. Also this structure is same with the particle structure obtained by conventional spray pyrolysis [11].
In shows the photoluminescence spectra of particles prepared by conventional and flame spray pyrolysis. It can be seen that particles prepared by flame spray pyrolysis have higher PL intensity than the PL intensity of particles prepared by conventional spray pyrolysis method. Former is approximately 120% of the latter. This is essentially due to the dense, spherical structure and better morphology of the particles obtained with the flame spray pyrolysis method. Thus it is obvious applying high temperature flame to the conventional spray analysis and applying a careful post-treatment improves the PL performance of Y2O3:Eu which is critical for commercial applications [11].
Fig : Post treatment methods applied to get monoclinic to cubic transformation[11].
Fig : SEM image of particles without
post treatment[11].
Fig : SEM image of particles with
post-treatment[11].
Fig : SEM image of particles
at low flame temperature[11].
Fig : SEM image of particles at high
flame temperature[11].
Fig : XRD results of particles before
and after post-treatment[11].
Fig : PL spectra of particles prepared
by Flame Spray Pyrolysis and
Conventional Spray Pyrolysis
Methods[11].
3.2 One step flame spray pyrolysis (FSP) for Y2O3:Eu3+ nanoparticles[3].
3.2.1 Experimental
Many different techniques have been studied for nano-sized europium-doped yttria, such as combustion synthesis, chemical deposition, laser ablation, spray pyrolysis in hot-wall or flame reactors. From these studies, it was proposed that the flame temperature play a very important role determining the crystal structure of the Y2O3:Eu3+. At this part, Y2O3:Eu3+ nanoparticles were produced by controlling the temperature of the flame and the residence time of the particles in the flame. Therefore, the particles which were synthesized in low enthalpy flame have monoclinic crystalline structure. On the other hand, the particles which were synthesized in high enthalpy flame have cubic crystalline structure. This phenomenon was investigated by transmission electron microscopy (TEM) and photoluminescence (PL) emission.
Europium-doped yttria is one of the most commonly used substances for red emitting phosphors, fluorescent lamps and plasma display panel[15]. New ideas about using high resolution display have increased interesting about small Y2O3:Eu3+ particles. A newly created technology that phosphors are formed by methane-oxygen diffusion flames is developed[3].
Y2O3:Eu3+ particles are formed while combustible precursor solution is dispersed through a nozzle which forms a fine spray with ignition[16]. FSP setting in the production rate, flame temperature, flame residence time of particles in flame controls the particle size and shape depending on the conditions. Typically, FSP is versatile and simple process of synthesis of nano-particles for ceramics, electronics, catalysis and optical materials[3].
Precursor mixture of constant 5~8 ml/min supply rate was fed into a nozzle using syringe pump was supplied. Precursor solution at the end of the nozzle has been distributed to form a spray in the rate of 3∼5 L/min of oxygen with the pressure drop of 1.5 bar. Oxygen (2.4 L/min) and methane (1.13 L/min) which were in a ring around the nozzle exit were supplied for the spray to be ignited[3]. Through the combustion, the Y2O3:Eu3+ particles which were formed on glass microfibre filter and collected by using vacuum pump.
Materials were prepared as YEu-x/y, where x is the precursor feed rate of ml/min and y is the oxygen dispersion gas flow rate of L/min. All YEu-x/y contain about 5 wt% of europium content which is standard for Y2O3:Eu3+ phosphors.
※ Samples : YEu-5/5, YEu-6/3, YEu-7/3, YEu-8/3
3.2.2. Result
The approximate size of 11 nm spherical particles (a) were formed by a relatively low enthalpy content and short flame. On the other hand, the size of 15∼25 nm rhombohedrally shaped particles (b) were formed by hotter and longer flame. In addition, YEu-5/5 are pure monoclinic while YEu-8/3 are mostly cubic[3].
Fig 9. TEM images of flame-made nanoparticles
(a) Low enthalpy flame (YEu-5/5) (b) High enthalpy flame (YEu-8/3) [3]
Depending on the condition of specific combustion enthalpy and visual flame height, different size of Y2O3:Eu3+ particles were made. The dotted line is the best fit between the centers of the circles. At the fig 2, the flame height (from the nozzle to the end of the visual flame) and the specific combustion enthalpy (feed rate [KJ/min] divided by the dispersion gas flow rate [ldisp/min] is showed. The specific combustion enthalpy of YEu-8/3 was about 89.1 (KJ/ldisp). On the other hand, the specific combustion enthalpy of YEu-5/5 was about 33.4(KJ/ldisp). From this test, it was observed that smaller flame at lower specific combustion enthalpy made monoclinic crystalline structure and longer at higher specific combustion enthalpy made cubic crystalline structure[3].
Fig 10. Diagram of the employed FSP conditions[3]
The particle diameter ranged from 11 nm (YEu-5/5) to 23 nm (YEu-8/3) depending on the flame conditions. In addition, the fraction of monoclinic/cubic ranged from 100 % (YEu-5/5) to 16 % (YEu-8/3).
The emission spectrum with a 0.25 nm size spectral resolution was recorded at room temperature. 100mg of each sample was loaded into a cylindrical powder holder and exposed to UV photons supplied by a flash xenon lamp. The emission spectrum in the range of 570∼640 nm were measured at a scan speed of 167 nm/min.
Fig 11. Photoluminescence emission pattern[3]
PL emission spectrum characteristic of cubic phase strongly differs from monoclinic one. Short and cold flame (YEu-5/5) produced pure monoclinic particles while hot and long flame (YEu-8/3) produced mostly cubic[3].
Cubic Y2O3: Eu3 + nanoparticles were directly prepared by FSP of appropriate organometallic precursor without post-processing. The crystal size and composition from monoclinic to cubic can be controlled by selecting FSP-process parameters which determind the high temperature particle residence time. The monoclinic structure particles which were made by YEu-5/5 samples changed to cubic structure particles by increasing the enthalpy density of the process (YEu-8/3 samples). Higher feed rates of precursor which were combined with lower dispersion gas flow rates led to elongated flames. This means that the YEu-55 flame is shorter than the YEu-8/3. As a result, nano-particles which were produced in longer residence time at high temperature form the cubic structure.
3.3 Crystal structure and particle size of Eu3+:Y2O3 synthesized by flame spray pyrolysis[2]
3.3.1. Experimental
Even though the material particles were produced by same process, each particle can have different crystal structure depending on the synthesized particle size. The small particles (smaller than 50 nm) had monoclinic structures on the other hand the lager particles (larger than 50 nm) had cubic structures. What kind of factors made this result? Can we control the factors to obtain specific materials which have distinct crystal structure? In this part, doping with the lanthanide(Eu) ion and flame spray pyrolysis are introduced as an useful diagnostic method for determining the crystal structure. The reason why the doping of lanthanide was used is that Y2O3 is one of the best hosts for lanthanide ions because its ionic radius and crystal structure are very similar to lanthanide oxide[16]
Processing the gas for nanostructure materials has an advantage compared to the liquid phased chemistry. For example, this process makes it possible to raise the high productive rates. Therefore, the high pure and wide range of materials can be produced under this method. This process is not only environmentally friendly but also efficient method. There are some important characteristics which include the grain size distribution, composition and morphology. FSP makes it possible to predict the crystal structure with some materials, such as yttrium oxide. From this method, the nanoparticles were separated by size groups and then characterized by TEM and XRD.
The atomizer is consisted of a nebulizer and co-flow jacket. The nebulizer is furnishing one inside nozzle which is made of capillary tube and the external jacket. The inner nozzle which has approximately 1 mm diameter is located inside the outer jacket and flushes at the end of the nozzle. The outer jacket and inner nozzle are connected with a narrow annular gap[2].
Fig 12. Schematic description of the burner used for synthesis of the nanoparticles[2]
Through the inner nozzle of the nebulizer, ethanol solution which include 2.5 mm Eu(NO3)3 and 50 mm Y(NO3)3 is supplied by using a syringe pump at 40 ml/h. The air is flowed into the annular gap at high speed, 2 L/min, to atomize the solution which is including precursor materials. The flame temperature in the flame was measured with a coated thermocouple to be about 2100℃. The H2 diffusion flame was formed by the combustible ethanol droplets containing the europium and yttrium precursors.
Fig 13. Size distribution of the spray droplets injected in the flame[2]
The reaction happens in order to form Eu3+:Y2O3 particles inside the flame. The productive ratio of this synthetic is about 400∼500 mg/h[2].
3.3.2. Results
The methanol was used as solvent for synthesized particles and the suspension was put into an ultrasonic bath for 30 min to break the particles which were formed during collection process. By using TEM, it was observed that the particles(Eu3+:Y2O3) had a wide size distribution from 5 to 300 nm. (A) showed smaller particles and (B) showed larger particles[2].
Fig 14. Size distributions of the two nanoparticle fractions, separated by
centrifugation . (A) small particles (B) large particles[2]
The smaller particle was figured as faceted. However, the larger particle was figured as spherical shape[2].
Fig15. TEM images (A) small particle (B) large particle[2]
X-ray diffraction was used to study the crystal structure of the nano-particles. From the result of the XRD, the lager particle had some features. Low background and strong peak was observed. On the other hand, the smaller particle had different features. Higher noise and only two distinguished peaks were observed[2]. Among the two peaks, the weak peak is located at 2θ=32.1Ëš. This is related with the monoclinic phase of Y2O3. In addition, the high peak is located at 2θ=29.41Ëš. This is related with the face centered cubic (FCC) structure[2].
Fig 16. XDR spectra (Top) small particle (Bottom) large particle[2]
In order to confirm the result of TEM, the electron diffraction patterns obtained from two different particles were studied. The electron diffraction pattern obtained from small particles had clearly distinguishable three rings. The radii of the rings were 2.95, 1.82 and 1.51 respectively. This pattern matches with monoclinic structure according to the JCPDS database. On the other hand, he electron diffraction pattern obtained from large particles had clearly distinguishable two rings. The radii of the rings were 2.60 and 1.22 respectively. This pattern matches with cubic structure according to the JCPDS database[2].
Fig 17. Electron diffraction patterns (A) small particle (B) large particle[2]
The particle size of Eu3+:Y2O3 is related with the property of the crystalline which is obtained by flame spray pyrolysis (FSP). The particles which were larger than 50 nm were featured as a cubic structure. On the other hand, the particles which were smaller than 50 nm were featured as a more complex structure. It is possible that the size of the particle morphology depends on the internal pressure of the particles[2].
4. DISCUSSION
These studies show that there are many methods to synthesize Y2O3:Eu particles. To expand the previous studies to synthesize Y2O3:Eu, we need to find other factors that can control the phase-pure monoclinic and cubic Y2O3:Eu. In order to control the structure of Y2O3:Eu particles, the temperature should be controlled. That is because the temperature affects Y2O3:Eu particles' morphology.
The first possible method to control the structure and morphology of Y2O3:Eu is controlling the ratio between the N2/O2 oxidizer when the H2 is used as a fuel and O2 as an oxidizer. It is assumed that when the H2 is used as a fuel of flame and O2 as an oxidizer, the flame temperature can be decreased by flowing N2 because N2 gas would attribute as a heat capacity. By controlling this N2/O2 ratio, the temperature can be controlled. If the N2 flow rate is increased, the temperature would be decreased. As the previous studies shows, the spherical, densely packed pure-phase monoclinic Y2O3:Eu can be obtained at high flame temperature[2]. On the other hand, the hollow and porous cubic structure of Y2O3:Eu can be synthesized at a low flame temperature[3]. Accordingly, when the flame temperature is decreased by increasing N2/O2 ratio, the critical ratio of N2/O2 and the flame temperature would be found, and the desirable cubic structure Y2O3:Eu would be generated. This is the possible solution used to synthesize the densely packed and spherical cubic structure Y2O3:Eu particles. These particles would have the better fluorescence property. This better quality Y2O3:Eu would be used more widely in industries and research areas. The second possible method to control the structure and morphology of Y2O3:Eu can be obtained by controlling the doping concentration. In Kang et al. they used the fixed doping concentration of Eu at 6%[11]. At that doping concentration, the mixed-phase of monoclinic and cubic structure has synthesized at a high temperature[11]. According to this result, it can be supposed that doping concentration would be a critical factor that affect structure and morphology of Y2O3:Eu. It can be assumed that the monoclinic structure would be dominant as the doping concentration increases. As a result, in a high Eu doping concentration, the phase-pure monoclinic Y2O3:Eu would be synthesized. On the other hand, in a low doping concentration, the cubic structure Y2O3:Eu would be obtained. That is because the possibility to get the monoclinic structure would be increased as the doping concentration increases. In this way, the doping concentration which can synthesize the desirable cubic structure Y2O3:Eu would be found.
In sum, two possible methods to control the crystal structure and morphology of Y2O3:Eu were suggested. If it is possible to synthesize the desirable cubic structure Y2O3:Eu by controlling the temperature followed by N2/O2 ratio and the doping concentration, this would be a helpful discovery for many research areas and industries.
5. CONCLUSION
