The size and shape of nanocrystals (nanospheres, nanorods, nanotubes, and nanowires) are crucial parameters for controlling nanocrystal properties such as electrical transport, optical, and magnetic properties. (Hu al., 1999, Kazes et al., 2002) In particular, one-dimensional structures are the fundamental units for anisotropic shape control, presenting general shape and assembly control strategies for more complex structures. The most exciting and rapidly expanding field would be carbon nanotubes from graphite by the rolling of graphen sheets.(Ebbesen & Ajayan, 1992, Iijima, 1991, Iijima & Ichihashi, 1993, Rao et al., 2004) The MX2 layers of metal dichalcogenides (M = Mo, W and X = S, Se) are analogous to the single graphene sheets, being able to roll into curved structures. Considerable progress has been made in the synthesis of the fullerene and nanotube structures of MX2.(Parilla et al., 1999, Rao & Nath, 2003, Tenne, 1995) Nanorods (or nanoneedles) and nanowires have also been extensively explored because the functional nanomaterials with a restricted dimension offer the opportunities for investigating the influence of shape and dimensionality on the physical properties.(El-sayed, 2001, Xia et al., 2003) For instance, ZnO nanowire sensitized by narrow band-gap materials have been studied for use in photovoltaic device applications due to their facile synthesis and excellent optical properties.(Birkmire & Eser, 1997, Law et al., 2005, Levy-Clement et al., 2005, Leschkies et al., 2007) The selective synthesis of such structures can be achieved by a morphology control in the crystallization process including nucleation and growth. Various synthetic methods have been developed for many important materials.
The interest in studying lanthanide oxides and hydroxides stems from their potential applications including dielectric materials for multilayered capacitors, luminescent lamps and displays, solid-laser devices, optoelectronic data storages, waveguides, and heterogeneous catalysts.(Cutif et al., 2004) Recently, lanthanide-doped oxide nanoparticles are of special interests as potential materials for an important new class of nanophosphors. For instance, when the oxide nanophosphors are applied for a fluorescent labeling, there are several advantages such as sharp emission spectra, long life times, and high resistance against photobleaching in comparison with conventional organic fluorophores and quantum dots.(Beaurepaire et al., 2004, Louis et al., 2005) Many lanthanide-doped oxides generate visible light in fluorescent lamps and emissive displays. Excitation of photoluminescent phosphors takes place using ultraviolet (UV) photons generated by a discharge in fluorescent lamp or plasma display panel (PDP). Cathodoluminescence, thermoluminescence, and electroluminescence of phosphors are also important for cathode-ray tube (CRT), field emission display (FED), radiation detector, and electroluminescence display (ELD) applications.(Yen et al., 2007) An improved performance of displays and lamps requires high quality of phosphors for sufficient brightness and long-term stability. In practice, a densely packed layer of small size particles can improve aging problems. On the other hand, when the particle is smaller than a critical value, the luminescence efficiency decreases because of increased light reabsorption and the luminescence quenching by the surface layer. Thus, high-concentration of surface defects and microstrains could greatly reduce the total luminescent intensity of nanosized luminescent materials. The concentration quenching ranges of activators are also different from those of corresponding bulk materials.(Duan et al., 2000, Zhang et al., 2003) Therefore, the morphology and size, the stoichiometry and composition, and the surface characteristics must be controlled in order to achieve the desirable objectives of improved nanophosphors.
As one of lanthanide oxides and hydroxides, Gd2O3 nanopaticle is a promising host matrix for multiphoton and up-conversion excitation.(Guo et al., 2004, Hirai & Orikoshi, 2004, Zhou et al., 2003) The gadolinium oxide doped with Eu3+ (Gd2O3:Eu) exhibits paramagnetic behavior (S = 7/2) as well as strong UV and cathode-ray excited luminescences, which are useful in biological fluorescent label, contrast agent, and display applications.(Blasse & Grabmaier, 1994; Goldys et al., 2006; Nichkova et al., 2005) Very recently, the magnetic resonance relaxation property of the colloidal solution of layered gadolinium hydroxide exhibited the sufficient positive contrast effect for magnetic resonance imaging (MRI).(Lee et al., 2009) In addition, Gd2O3:Eu is a very efficient X-ray and thermoluminescent phosphor.(Rossner & Grabmaier, 1991, Rossner, 1993) Diverse preparation methods have been developed to reduce the reaction temperature and achieve a small particle size of high quality Gd2O3:Eu phosphors.(Erdei et al., 1995, Ravichandran et al., 1997, Shea et al., 1996,Yan et al., 1987)
In an attempt to produce various inorganic materials in the form of isotropic or anisotropic nanostructures, the solvothermal reaction has widely been adopted due to its simplicity, high efficiency, and low cost. The hydrothermal synthetic routes to the nanostructures of lanthanide hydroxides are well introduced in the literatures. The hydrothermal treatments for colloidal precipitates of lanthanide hydroxides resulted in diverse nanostructures such as nanospheres, nanorods, nanowires, nanoplates, nanotubes, and nanobelts.(Wang et al., 2003, , Wang & Li 2002, Wang & Li 2003) It is noted that the hydrothermal reaction is generally sensitive to the temperature, pH, or aging conditions. Thus, when we apply the hydrothermal technique to synthesize Gd(OH)3:Eu as a precursor for Gd2O3:Eu nanophosphor, a strong pH dependence is observed in particle shape. In this chapter, we introduce a selectively controlled low-temperature hydrothermal synthesis of Gd(OH)3:Eu phosphor at different pHs and subsequent dehydration into Gd2O3:Eu in the form of nanorods with different aspect ratios, nanowires, nanospheres, and nanotubes.(Bae et al., 2009, Lee et al., 2009) Because the concentration quenching behavior has not yet been well investigated for the phosphor nanowires, a better understanding is required for the parameters affecting the radiative or nonradiative relaxation behaviors in nanowire structures. Therefore, the growth behavior and photoluminescence property of high quality nanowires of Gd2O3:Eu phosphor are also described here. Gd2-x Eu x O3 solid solution is examined to get an insight for the relationship between the quenching concentration of activator (Eu3+) and the optimum photoluminescence characteristics of nanowires with high aspect ratio.
2. Hydrothermal synthesis of Gd2O3:Eu phosphor
2.1. pH dependent hydrothermal synthesis
Several shapes (i.e. nanospheres, nanorods, nanotubes, and nanowires) of Gd(OH)3:Eu nanoparticles are synthesized at different pHs by using the hydrothermal method. Thus, the shape selective synthesis of Gd(OH)3:Eu from nanorods with considerably different aspect ratios to nanowires, nanospheres, and nanotubes can be successively achieved with increasing the pH of initial solution for hydrothermal reaction from about 6 to 14. In a typical synthesis, the stoichiometric amounts of Gd2O3 and Eu2O3 are dissolved in HNO3 solution. After clear solution is formed by uniform stirring, aqueous KOH solution is added until the pH of solution is adjusted to be in the range of 6 14 for the formation of colloidal hydroxide precipitates. For the hydrothermal growth of Gd(OH)3:Eu particles, the resulting colloidal mixture is put into a Teflon-lined stainless steel autoclave at room temperature. The autoclave is then sealed and maintained at 120 – 180 C for several hours. In general, the solution is continuously stirred during the hydrothermal treatment. But the reaction can be performed without stirring for aging. After the reaction is completed, the solid product Gd(OH)3:Eu is collected by filtration, washed with distilled water, and dried. Subsequent dehydration of Gd(OH)3:Eu by heating at 500 C for several hours yields Gd2O3:Eu oxide. To maintain the morphology of Gd(OH)3:Eu, the heating rate is controlled at slower than 3 C /min. In order to compare the crystallization, morphology change, and luminescence behaviors at different temperatures, the additional heat treatments for all Gd2O3:Eu powders are successively carried out at 700 and 800 C for several hours.
It is noted that the precipitates showing a plate-type morphology are obtained after hydrothermal treatment at pH < 7, such products show an X-ray powder diffraction (XRD) pattern quite different from that of Gd(OH)3. Recently, it was reported that Gd2(OH)5(NO3) nH2O of layered rare-earth hydroxide structure is obtained in the range of pH = 6 ~ 7.(Lee & Byeon, 2009) Because large capacity and high affinity for the ion-exchange reaction, this compound can be used as host materials for a wide range of active molecules. Fig. 1 shows typical XRD patterns of the as-synthesized Gd2(OH)5(NO3) nH2O:Eu at pH ~ 7, Gd(OH)3:Eu from hydrothermal process at pH ~ 8 (nanorods; see section 3.1) and ~ 11 (nanowires; see section 3.1), and Gd2O3:Eu obtained after heat treatment at 500 C. A series of strong (00l) reflections observed in the XRD pattern of Gd2(OH)5(NO3) nH2O:Eu (Fig. 1a) is characteristic of a layered phase and corresponding to an interlayer separation of ~ 8.5 Å. In contrast, all the reflections for Gd(OH)3:Eu and Gd2O3:Eu can be indexed to the hexagonal (space group P63/m; Figs. 1 b and 1c) and the cubic (space group Ia3; Fig.1d) phases respectively.(JCPDS card 83-2037; Buijs et al., 1987) While Gd(OH)3:Eu obtained at pH ~ 8 is well crystallized, the increase of solution pH to ~ 11 and ~ 13 results in a relatively weak and broad intensity patterns, indicating a poorer crystallinity. The full-widths at half-maximum (FWHM) of the (110), (101), (201) reflections of Gd(OH)3:Eu obtained at pH ~ 8 are smaller than one half in comparison with those obtained at pH ~ 11. Furthermore, it is of particular interest that the relative intensities of (110) and (101) diffractions for Gd(OH)3:Eu are significantly different depending on the solution pH for hydrothermal synthesis. The intensity ratios between (110) and (101) reflections of the hydroxide nanorod is around 0.5 (Fig. 1b). Similar XRD patterns are observed for Gd(OH)3:Eu oxides which are prepared at different pHs other than ~ 11 and the (110)/(101) intensity ratios are typically in the range of 0.5 ~ 1.0. These observations are consistent with the previously reported results for
Gd(OH)3.(Wang & Li, 2002; Wang et al., 2003; Louis et al., 2003) In contrast, such intensity ratio dramatically increased up to ~ 8 in the XRD pattern of the hydroxide prepared at pH ~ 11 (Fig. 1c). This would suggest that the (110) orientation is strongly preferred for Gd(OH)3:Eu prepared at around this pH range. The comparison of the XRD patterns and morphology changes of Gd(OH)3:Eu (see section 3.1) indicates that the growth direction of Gd(OH)3:Eu from nanorods to nanowires is parallel to the (110) planes. Some more details are discribed in the next section.
2.2. Synthesis of high quality nanowires of Gd2O3:Eu at constant pH
The careful adjustment of pH for the hydrothermal reaction is required to induce well developed nanowire structure with sufficiently high aspect ratio. In contrast to the synthesis of lanthanide phosphate nanowires at pH = 1 – 2 under the hydrothermal condition,(Fang et al., 2003) the optimal pH range for the growth of Gd(OH)3:Eu nanowires is close to around 11. Nanowires of Gd1-x/2 Eu x/2 (OH)3 solid solution (x = 0.08, 0.12, 0.16, 0.20, and 0.24) are accordingly synthesized at pH ~ 11 by the procedure similar to section 2.1. The high quality bulk powder of Gd2O3:Eu phosphor can be prepared according to the citrate route.(for example, see Byeon et al., 2002)
It is well known that, besides pH, the concentration of solution can also strongly affect the transport behavior of constituting ions and the growth behavior of particles in the solvothermal synthesis. Thus, the highly anisotropic growth to the more uniform nanowires with several tens of micrometers in length is achieved by maintaining the higher concentration of the precursors dissolved in the initial HNO3 solution for hydrothermal synthesis. The temperature is another factor influencing the growth of Gd(OH)3:Eu nanowires. For instance, no nanowire is induced below 120 C. The nanorods with relatively high aspect ratios are obtained at 120 – 160 C under the same pH condition. The growth of nanowires longer than 10 micrometers can be achieved by raising the reaction temperature above 160 C. Fig. 2 compares the XRD pattern of Gd(OH)3:Eu bulk powder with those of the as-synthesized hexagonal Gd(OH)3:Eu nanowires from hydrothermal process at pH ~ 11 and the cubic Gd2O3:Eu nanowires obtained after heat treatment at 500 C. Similarly to the case in section 2.1, the intensity ratio between (110) and (101) reflections for the hydrothermally synthesized Gd(OH)3:Eu nanowires are significantly different from those of the bulk powder; such relative intensity ratio of the bulk hydroxide is close to 1.0 (Fig. 2a), which is significantly smaller than that (~8) of the hydroxide nanowire (Fig. 2b). Single crystalline nanowires of hydroxides and oxides have been extensively studied.(Sharma & Sunkara, 2002, Tang et al., 2005; Singh et al., 2007) Their formations are related to the fact that the growth rate along one crystallographic direction is significantly faster than along the other directions. It has been reported that the growth direction of Gd(OH)3 to nanorods is parallel to the (110) planes of the hexagonal unit cell.(Du & van Tendeloo, 2005) High resolution transmission electron microscope (HRTEM) images of Gd(OH)3:Eu nanorod and nanowire are compared in Fig. 3. The fine fringes demonstrate that the as-synthesized Gd(OH)3:Eu nanorods and nanowires are single crystalline. The spacing between fringes along both the rod and wire axes is about 0.32 nm which is close to the interplanar spacing of the (110) plane. This agreement confirms that the exceptionally high (110)/(101) intensity ratio of nanowires (Fig. 2b) is attributed to the preferential growth of nanowire parallel to the (110) planes. Although Gd(OH)3 nanorods with relatively high aspect ratios have been described as nanowires in the literatures, true nanowires would show highly increased (110)/(101) intensity ratios in their XRD patterns.
3. Morphological aspects of Gd2O3:Eu nanophosphor
3.1. pH dependent shape evolution of Gd(OH)3:Eu and Gd2O3:Eu
The selective synthesis of diverse nanomaterials is challenging for understanding physical properties derived from well difined shape dimensionality. For example, the anisotropic growth of lanthanide orthophosphates (LnPO4 where Ln = lanthanides) nanostructure can be enhanced by kinetically controlled hydrothermal processes using carefully selected chelating ligands.(Yan et al., 2005) Among many parameters affecting the solvothermal synthesis, the adjustment of pH was found to play a key role in selectively controlling the morphology of Gd(OH)3:Eu nanostructures. In particular, KOH would influence on the nucleation and anisotropic growth of particles. For instance, the layered gadolinium hydroxynitrate Gd2(OH)5(NO3) nH2O:Eu is obtained at pH = 6 – 7. The field emission scanning electron microscopy (FE-SEM) image (Fig. 4a) of Gd2(OH)5(NO3) nH2O:Eu obtained at pH ~ 7 shows that they comprise a plate-like microcrystalline powders. Because of very weak intensity and insufficient number of non-(00l) reflections observed in XRD patterns of Gd2(OH)5(NO3) nH2O:Eu (Fig. 1a), the arrangement mode within the ab plane is measured by the selected area electron diffraction (SAED). As shown in Fig. 4b, the clear spots observed in the reciprocal lattice, which are rectangularly arranged, confirm an order within the ab plane of crystals (a ~ 7.3 Å, b ~ 12.9 Å).
On the contrary, at pH > 8, Gd(OH)3:Eu particles are produced in the form of nanorods with variable aspect ratios, nanowires, or the mixture of nanosheets, nanotubes, and nanorods, depending on the solution pH. Fig. 5 shows FE-SEM images of Gd(OH)3:Eu synthesized at pH ~ 8 and Gd2O3:Eu obtained by subsequent heat treatment at 500 and 800 C. It can be seen in Fig. 5a that the as-synthesized Gd(OH)3:Eu are composed of highly uniform nanorods. The aspect ratio of nanorods is tunable by controlling the experimental conditions of pH, temperature, and concentration. After dehydrating this hydroxide to Gd2O3:Eu at high temperatures up to 800 C, the nanorod shape is completely retained as shown in Figs. 5b and 5c. It has already been reported that the nanorod shape of Gd(OH)3:Eu is maintained after the dehydration into Gd2O3:Eu.(Chang et al., 2005) Their average diameter and length are around 200 nm and 1 – 1.5 μm, respectively. The morphology of Gd(OH)3:Eu synthesized at pH ~ 9 is shown in Fig. 6a. This powder also consists of uniform rod-like particles. Compared with those obtained at pH ~ 8, the diameter of Gd(OH)3:Eu nanorods is smaller but the length is longer when they are prepared at pH ~ 9.0. The nanorod structure is not collapsed after thermal dehydration (Figs. 6 b and 6c). Their average diameter and length are around 150 nm and 1.5 – 2 μm, respectively.
During hydrothermal synthesis, it is revealed that the higher the pH of starting solution is, the smaller diameter of Gd(OH)3:Eu nanorods is induced. In contrast, the average length of the nanorods is increased so that the aspect ratio of Gd(OH)3:Eu nanorods becomes larger with the increase of solution pH at the same concentration and reaction temperature. Fig. 7 shows typical images of the as-synthesized Gd(OH)3:Eu at pH ~ 10 and its dehydrated oxide
Gd2O3:Eu. Both compounds exhibit the nanorod structures with average diameter of about 100 nm and length of 3 – 4 μm. The maximal aspect ratios of Gd(OH)3:Eu and Gd2O3:Eu particles are achieved in the higher pH range. The typical FE-SEM and TEM images of Gd(OH)3:Eu synthesized at pH ~ 11 are shown in Figs. 8 a and 8b, respectively. As can be seen in these figures, the as-synthesized Gd(OH)3:Eu is composed of uniform nanowires whose lengths are close to several tens of micrometers. The images shown in Fig. 8b with higher magnification suggests that the diameters of nanowires are in the range of 20 – 30 nm. Figs. 8c and 8d display the FE-SEM of Gd2O3:Eu nanowires converted by thermal treatment of Gd(OH)3:Eu nanowires at 500 C and 800 C, respectively. The temperature and activator (Eu3+) concentration dependence of Gd2O3:Eu nanowires are additionally described in next section (3.2.). Further addition of KOH solution to adjust the pH to higher than 11 significantly reduces the aspect ratio of Gd(OH)3:Eu particles to produce essentially
nanorods. The XRD pattern of this hydroxide is quite similar to Fig. 1b, the (110)/(101) intensity ratio being close to 0.7. It was proposed that the high OH- ion concentration is preferable for the high aspect ratio but greatly reduce the ionic motion for the one- dimensional growth.(Wang & Li, 2002) This would imply that an optimal pH condition is required for the growth of true nanowires with high aspect ratio. Fig. 9a shows a FE-SEM image of the as-synthesized Gd(OH)3:Eu powder by hydrothermal reaction at pH ~ 12. Although the particle shapes are uniform and wire-like, the aspect ratio is significantly decreased in comparison with that of true nanowires (Fig. 8) prepared at pH ~ 11. The morphology images of Gd2O3:Eu also exhibit the nanorod shapes (Figs. 9b and 9c), indicating the maintenance of morphology. The diameter and length of rigid nanorods are in the range of 20 – 30 nm and 2 – 3 μm, respectively.
Compared with those prepared at lower pH ranges, the morphology of Gd(OH)3:Eu obtained at pH ~ 13 strongly depends on the reaction temperature; the formation of nanowires is not induced but instead both nanorod and nanotube structures seems in competition in this pH range. The formation of nanorods with low aspect ratio is preferred at higher than 160 C while the nanotubes are mainly obtained at lower than 140 C. When the hydrothermal reaction is carried out with the initial solution of pH ~ 13, the Gd(OH)3:Eu crystallites synthesized at 180 C display the uniform morphology of nanorods with 20 – 30 nm in diameter and 200 – 300 nm in length (Fig. 10a). In contrast to the observations in other nanorods prepared at lower pHs, the morphology of Gd(OH)3:Eu nanorods obtained at this pH range was not retained after the thermal transformation into Gd2O3:Eu. Figs. 10b and 10c are the FE-SEM and TEM images of Gd2O3:Eu obtained from heat treatment of Gd(OH)3:Eu at 500 C. Relatively regular Gd2O3:Eu particles are all quasi-spherical and the average particle size is close to 30 – 60 nm. Observation of the fine fringes supports a formation of regular crystalline lattice. On the other hand, the mixture of nanorods, nanotubes, and nanosheets is produced when Gd(OH)3:Eu is synthesized at 120 C. Fig. 10d shows that the nanotubes have outer diameters less than 30 nm and lengths of 150 – 200 nm. The nanorods have the aspect ratio smaller than that prepared at 180 C. Considering that the nanosheets are also observed, the formation of sheet-structure is likely followed by the formation of nanotubes at a lower temperature, which in turn grows to more stable nanorods at a higher temperature. The optimal condition to synthesize uniform nanotubes of Gd(OH)3:Eu is strongly dependent on the concentration of KOH and temperature. Similarly to the nanorods prepared at 180 C, no shape is retained in the mixture of nanosheets, nanotubes, and nanorods obtained at 120 C but the spherical nanoparticles with size of 10 – 40 nm are mainly obtained after dehydration into Gd2O3:Eu (Fig. 10e). The observed fine fringes are associated with the regular crystalline lattice (Fig. 10f). The spacings between fringes, 0.32, 0.27, and 0.20 nm correspond to the interplanar spacing of the (220), (400), and (440) plane of cubic cell, respectively. The origin of collapse of this nanorod shape is not straightforward. If we consider that the nanosheet, nanorod, and nanotube morphologies are in competition, such collapse would be attributed to a metastable nanorod structure. Many different strategies for the synthesis and characterization of inorganic nanotubes have been reported.(Rao & Nath, 2003) In the majority of the cases, the nanotube structures are induced by a rolling of single sheets from the layered lattices. Some oxide nanotubes have been synthesized by employing a hydrothermal technique. For instance, the hydrothermal synthesis of single-crystalline α-Fe2O3 nanotubes has been accomplished.(Jia et al., 2005)
When the direct synthesis of oxide nanotubes is difficult, a precursor prepared by hydrothermal reaction can be used under the appropriate conditions. Highly crystalline TiO2 nanotubes were synthesized by hydrogen peroxide treatment of low crystalline TiO2 nanotubes prepared by hydrothermal methods.(Khan et al., 2006) In particular, CeO2 nanotubes have been prepared by the controlled annealing of the as-synthesized Ce(OH)3 nanotubes from hydrothermal synthesis.(Tang et al., 2005) A similar result was expected for a thermal dehydration of Gd(OH)3:Eu nanotubes to Gd2O3:Eu nanotubes. Unfortunately, Gd(OH)3:Eu nanotubes are collapsed to yield spherical nanoparticles, no Gd2O3:Eu nanotubes being obtained after dehydration at 500 C. The nanotubes of cubic Gd2O3:Eu, which is not a lamella structure, will require numerous defects and twin orientation relationships, which is energetically unfavorable, during rearrangement for the structural transformation. The difference in strain and curvature between the outer and inner surfaces of nanotubes will induce a different contraction tensions around defects and twins. This torsion would consequently lead to a collapse of tube structure. The hydrothermal reaction at pH ~ 14 results in Gd(OH)3:Eu nanorods with low aspect ratios. This hydroxide is not well crystallized and the rod shape is not completely retained after thermal dehydration into Gd2O3:Eu. Instead, the mixture of nanospheres andnanorods is obtained.
3.2. The growth behavior of Gd2O3:Eu nanowire
Oxide nanowires can be synthesized by diverse methods. Single crystalline cubic spinel LiMn2O4 nanowires were prepared by using Na0.44MnO2 nanowires as a self-template.(Hosono et al., 2009) ZnSnO3 nanowires were prepared by using D-fructose as a molecule template.(Fang et al., 2009) Non-catalytic resistive heating of pure metal wires or foils at ambient conditions was developed to grow the nanowires of metal oxides.(Rackauskas et al., 2009) A vapor–solid route was employed for the catalyzed synthesis of vertically aligned V2O5 nanowire arrays with tunable lengths and substrate coverage.(Velazquez & Banerjee, 2009) The hydrothermal synthesis also provides an effective route to fabricate the oxide nanowires. Single-crystalline tetragonal perovskite-type PZT oxide nanowires has been synthesized by applying a polymer-assisted hydrothermal method.(Xu et al., 2005) Uniform single-crystalline KNbO3 oxide nanowires have also been obtained by employing the hydrothermal route.(Magrez et al., 2006) However, a direct hydrothermal synthesis of Gd2O3:Eu oxide nanowires is difficult because of high stability of the hydroxide form at high pH conditions. Instead, Gd2O3:Eu nanowires can be obtained via dehydration of hydrothermally synthesized hydroxide forms as described in section 3.1. Fig. 11a shows again the FE-SEM image of Gd2O3:Eu oxide of several tens of micrometers in length. Fig. 11b with higher magnification suggests that the diameters of nanowires are in the range of 10 ~ 30 nm. It is generally expected that the highly anisotropic shapes of nanoparticles would collapse when they transform into a different structure of phase by heat treatment.(Liang & Li, 2003) In this respect, it is of interest that the nanowire shape of Gd(OH)3:Eu are not broken after the transformation into Gd2O3:Eu structure by heat treatment. The structural transformation from hexagonal Gd(OH)3 to cubic Gd2O3 proceeds through the formation of intermediate monoclinic GdOOH. If we consider that the single-crystalline character of Gd(OH)3 nanorods is retained in GdOOH,(Chang et al., 2006) a sequential transformation from hexagonal to monoclinic and finally to cubic structure, rather than an abrupt transformation, could be responsible for the maintenance of wire-shapes in Gd2O3:Eu. The hydrothermal method for the synthesis of Gd(OH)3:Eu nanowires consequently provides a selective route to the nanowires of red-emitting Gd2O3:Eu oxide nanowires. Despite the maintenance of anisotropic particle shape, however, the structural transformation of nanowires from hexagonal to cubic symmetry will require numerous defects associated with the presence of microstrains. HRTEM image of Gd2O3:Eu nanowires shown in Fig. 11c supports this feature. Besides the fine fringes spaced by about 0.27 nm, which is close to the interplanar spacing of the (400) plane, the other weak fringes and wave-like images likely attributed to the stacking faults are observed as indicated by dotted line in the circle of this figure. The concentration of Eu3+ also influences on the formation of Gd1-x/2Eux/2(OH)3 and consequently Gd2-xEuxO3 nanowires. SEM images of nanowires are
compared as a function of Eu3+ concentration(x) and heating temperature in Fig. 12. As shown in these images, when the concentration of doped Eu3+ increases from 0.08 up to ~ 0.20, the morphology of Gd2O3:Eu nanowire tends to be improved. Further increase of doping concentration results in an abrupt decrease in aspect ratio and then nanorods are obtained. The nanowire morphology of Gd2O3:Eu is maintained even after heat treatment up to 700 and 800 C. Partial collapse of nanowires to the irregular nanorods is induced at higher than 900 C.
4. Photoluminescence spectra of Gd2O3:Eu nanophosphors
4.1. Correlation between photoluminescence and aspect ratio of Gd2O3:Eu nanoparticles
The optical characteristics and performances of nanometer-sized phosphor materials are generally dependent on their crystal structures, size, and morphologies. For instance, a difference of about 10 nm in the charge-transfer band position is observed between Gd2O3:Eu nanorods and microrods despite the same composition.(Chang et al., 2005) The correlation between morphology and photoluminescence (PL) intensity of Gd2O3:Eu phosphor provides an insight for a particle shape of the oxide nanophosphor with high PL efficiency. Indeed, the photoemission spectra of Gd2O3:Eu with different aspect ratios support that a systematic difference in PL intensity is induced as a function of the particle morphology. Fig. 13a compares the PL emission spectra of Gd2O3:Eu phosphors as a function of the pH value at which corresponding hydroxide precursors are synthesized. For comparison, the emission intensity measured from a commercial Y2O3:Eu is also plotted as a reference in the figure. The observed several emission bands are associated with the 5D0 – 7FJ
(J = 1 ~ 5) transitions of the Eu3+ ion, where the most intense emission at 610 nm is assigned to the 5D0 – 7F2 transition.(Brecher et al., 1967) As shown in this figure, Gd2O3:Eu oxide from the hydroxide prepared at pH ~ 8 exhibits strong emission whose intensity is close to 90 % in comparison with that of commercially available Y2O3:Eu. The commercial Y2O3: Eu phosphor is generally sintered at temperature above 1300 C. An adoption of hydrolysis technique using urea for the synthesis of Y2O3: Eu also requires the firing temperature of 1150 – 1400 C to achieve an optimum luminescent property.(Matijevic & Hsu, 1987, Jiang et al., 1998, Jing et al., 1999) It is accordingly notable that the emission intensity of Gd2O3:Eu nanorods obtained at 800 C is comparable with that of commercial Y2O3: Eu. In Fig. 13b, the pH dependent PL intensities of Gd2O3:Eu phosphors are summarized as a function of the dehydration temperature of corresponding hydroxide precursors. The emission intensity of Gd2O3:Eu exhibits a minimum value when the pH for hydrothermal synthesis of the hydroxide increases. The aspect ratio of Gd(OH)3:Eu (or Gd2O3:Eu) is enhanced with increasing the solution pH from ~ 8 to ~ 11 which is the optimal value for the formation of nanowires. In contrast, the PL emission intensity of Gd2O3:Eu is monotonically reduced with increase of the aspect ratio in this pH range. The lowest emission is observed with the nanowires (pH ~ 11) of the highest aspect ratio. The intensity of Gd2O3:Eu nanowires obtained after dehydration at 800 C is close to 70 % in comparison with that of commercial Y2O3:Eu. When pH > 11, the aspect ratio of Gd2O3:Eu nanorods is decreased again and then the PL emission intensity is enhanced.
Comparing the PL behavior as a function of the particle morphology, the emission intensity of Gd2O3:Eu decreases when the aspect ratio becomes larger. One of the origins for such a correlation could be a difference in the surface area of particles. An important source of luminescence quenching in nanosized particles is the surface, where the coordination of the atoms differs from that in the bulk.(Counio et al., 1998) As a result of higher aspect ratios, the enlarged surface area of crystallites would result in an increase of surface defects which can be the nonradiative recombination centers (see section 4.2). Thus, the high concentration of stacking faults and twins in the surface of nanowires can be an origin for a decrease of the emission intensity observed in higher aspect-ratio-particles.
4.2. Photoluminescence and quenching concentration of Gd2O3:Eu nanowires
Few researchs have been addressed to the photoluminescence and cathodoluminescence of oxide phosphor nanowires. Tin oxide nanowires grown by vapor-liquid-solid process showed the strong UV emission at low temperatures related to the near-band-edge transition.(Chen et al., 2009) Ga2O3-SnO2 nanowires of heterostructure showed a sharp transition region with emission bands from green to red cathodoluminescences.(Maximenko et al., 2009) In Fig. 14A, the photoluminescence (PL) emission spectra of Gd2O3:Eu nanowires are compared as a function of heating temperature. As shown in this figure, the emission intensity of Gd2O3:Eu nanowires measured at 610 nm is enhanced with increasing the dehydration temperature. When obtained even after dehydration at 800 C, however, the nanowire phosphor exhibits about 60 % of emission intensity in comparison with that of commercial Y2O3:Eu. As pointed out in section 4.1, the high concentration of defects such as stacking faults in the surface of Gd2O3:Eu nanowires (Fig. 11c) would act as the nonradiative recombination centers and accordingly be responsible for the low PL emission intensity. The large surface of nanoparticles provides efficient quenching centers for the deexcitation via traps. The density of phonons of the nanoparticles, which is crucial for the resonant energy transfer, is also much lower than that of normal crystal. All of these effects result in a decreased probability of energy transfer among activator ions. As a consequence, higher activator concentrations have been generally observed for the optimized radiative recombination in nanosized particles than in normal materials.(Duan et al., 2000; Zhang et al., 2003) While an increase of the activator concentration improves the probability of the energy transfers to the activator ions and therefore radiative recombination, the probability
of energy transfer between activator ions also increases to induce a concentration quenching. Because there are no intermediate energy levels between the excited 5D0 state and the 7FJ states of Eu3+ to act as a bridge for cross-relaxation, the concentration quenching effect is essentially ascribed to the possible nonradiative transfer between neighboring Eu3+ ions. (Zhang et al., 1998, Dhanaraj et al., 2001) The quenching concentration of Eu3+ activators for Gd2-x Eu x O3 nanowires is determined by measuring the PL intensity in the range of 0.08 ≤ x ≤ 0.24. The relationship between photoemission intensity of 5D0 – 7F2 at 610 nm and Eu3+ activator concentration (x) for the Gd2-x Eu x O3 nanowires is shown in Fig. 13B. The quenching concentration for nanowires is in the range 0.16 < x< 0.20, which is much higher value in comparison with that (x< 0.08 ~ 0.10) for the bulk Gd2O3:Eu powder. Although the PL intensity of x = 0.24 member is comparable to that of x = 0.20 member, the nanowire shape is no longer observed and instead the nanospheres or nanorods with low aspect ratios is formed after high temperature annealing (Fig. 12).
Gadolinium oxide (Gd2O3) with a cubic structure is used as an efficient host matrix for trivalent rare earth ions for the fabrication of nanocrystalline phosphor materials. Because of the high refractive index, the large band gap (5.4 eV), the high resistivity and the high relative permittivity, gadolinium oxide is a promising material for high-k gate dielectrics, waveguides, and high resolution X-ray image detectors. Gd3+ is also a known contrast agent for magnetic resonance imaging (MRI), and thus the rare-earth doped Gd2O3 labels can be used for dual, fluorescence and MR imaging applications. In this chapter, we described that various single crystalline nanostructures (nanorods, nanowires, nanotubes, nanospheres) of red-emitting Gd2O3:Eu phosphor can be prepared by selective hydrothermal synthesis of Gd(OH)3:Eu at different pHs and subsequent dehydration at high temperatures. The aspect ratios of phosphor particles are tunable by simply adjusting the pH of the initial solution for hydrothermal synthesis of Gd(OH)3:Eu. In particular, the nanowires of Gd(OH)3:Eu can be selectively prepared at pH ~ 11. Highly uniform nanowires of 20 ~ 30 nm in diameter can grow up to several tens of micrometers in length. The shape of nanowire obtained under hydrothermal pressures are retained after the structural transformation from hexagonal Gd(OH)3:Eu to cubic Gd2O3:Eu at high temperatures. Therefore, the selective control of Gd(OH)3:Eu morphology provides a strategy for the selective control of one-dimensional oxide nano-phosphor Gd2O3:Eu. This method for the synthesis of Gd2O3:Eu nanowires is quite simple and facile. No catalyst is required to serve as the energetically favorable site for the absorption of reactants. No template is added to direct the growth of nanowires. The relative emission intensity of Gd2O3:Eu is reduced with increasing the aspect ratio of nanoparticles and the quenching concentration of activators is significantly increased in Gd2O3:Eu nanowires (x = 0.20) compared with that of the bulk powder (x = 0.08 ~ 0.10). Thus, the low luminescence efficiency of Gd2O3:Eu nanowires can be highly improved by doping more Eu3+ into the host lattice. This compensation of luminescence efficiency would be of great benefit to the practical use of Gd2O3:Eu phosphor nanowires.
The authors acknowledge the grant R01-2008-000-10442-0 from the Korea Science and Engineering Foundation.