Novel Gd(OH)₃, GdOOH and Gd₂O₃ Nanorods: Microwave-Assisted Hydrothermal Synthesis and Optical Properties

Abstract

We present a study of the controlled synthesis and optical properties of single-crystals Gd(OH)₃, GdOOH and Gd₂O₃ nanorods. In this work, Gd(OH)₃ nanorods were synthesized by a simple and fast microwave-assisted hydrothermal method. This process combined with the thermal decomposition oxidation of Gd(OH)₃ nanorods as precursors enabled the preparation of single-crystalline GdOOH and Gd₂O₃ structures with well-defined morphology at low temperatures. The crystal structure dependence on the optical properties was investigated. We observed a green shift effect on the photoluminescence (PL) emission spectra from Gd(OH)₃ to Gd₂O₃ nanorods, which can be attributed to different types of surface defects, as well as intrinsic properties that contribute significantly to the modified PL behavior.

Keywords
Gd(OH)₃; GdOOH; Gd₂O₃; nanorods

1. Introduction

Nowadays, rare earth hydroxide/oxide nanomaterials have received special attention because of their excellent physical and chemical properties, which provide a wide variety of applications in the nanotechnology field^1-6. Among various types of rare earth hydroxide/oxide materials, gadolinium hydroxide (Gd(OH)₃), gadolinium oxyhydroxide (GdOOH) and gadolinium oxide (Gd₂O₃) exhibit important functional properties that depend on the structure. Gadolinium oxyhydroxide is a stable phase obtained by the thermal dehydration of gadolinium hydroxide and has a simple layered structure. So, these materials have been investigated as promising candidates in the field of high throughput luminescent devices, catalysis and other functional devices based on their excellent electronic, optical and physicochemical responses arising from 4f electrons^5-7.

Despite the recent improvements, few studies have been reported concerning the synthesis control of new rare earth hydroxide/oxide nanomaterials. Therefore, more studies leading to the development of new synthetic strategies that allow a superior control in the processing of such materials are needed and this remains a highly challenging field.

Some methods are reported in the literature to obtain well defined Gd(OH)₃, GdOOH and Gd₂O₃ nanostructures such as electrochemical route, microwave, sol-gel synthesis, combustion, solid state reaction, and polyol method^3-8. However, most of these methods require long processing times due to slow reaction kinetics. Therefore, the microwave-assisted hydrothermal (MAH) method has shown numerous advantages such as the use of an environmentally friendly solvent (water) and low processing temperatures (≤200 °C). This method provides a simple and fast approach for the large-scale production of new emergent complex functional materials. In addition, it also enables the shape-controlled synthesis of micro- and/or nanomaterials at a lower cost, for their industrial exploitation with unprecedented capabilities in the new technologies^9-14.

Herein, we report a facile and simple, low-temperature synthesis of single-crystalline Gd(OH)₃, GdOOH and Gd₂O₃ nanorods obtained by the MAH method combined with thermal decomposition oxidation process. These microcrystals were structurally characterized by different techniques and the optical properties of the crystal structures obtained were investigated using ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy and photoluminescence (PL) measurements at room temperature. Moreover, this paper discuss the electronic effects in order to establishes a close correlation between the structure and properties for the single-crystalline Gd(OH)₃, GdOOH and Gd₂O₃ nanorods.

2. Experimental

The Gd(OH)₃ nanorods were prepared using a MAH method. Initially, a solution of Gd³⁺ was prepared by dissolving gadolinium nitrate (99.9%, Aldrich) in distilled water to obtain a concentration of 0.1 mol L^-1. Under magnetic stirring, an ammonium hydroxide solution was added dropwise to complete the precipitation (final pH = 10), resulting in a white gel. In the second stage, the white gel is dispersed in water and the aqueous precursor solution was transferred into a Teflon autoclave, which was properly sealed and placed inside a domestic MAH system (2.45 GHz, maximum power of 800 W). MAH processing was performed at 130ºC under a constant pressure of approximately 3 bar for 20 min. After the hydrothermal treatment, the dispersed powder was centrifuged and washed with distilled water until the residual ions in the solution were eliminated. The resulting product was dried at room temperature for 24 h.

GdOOH and Gd₂O₃ nanocrystals were obtained by a thermal decomposition oxidation process using the Gd(OH)₃ nanocrystals prepared by the MAH synthesis as precursors. In both cases, the heating rate was fixed at 3ºC min^-1 using a conventional furnace (ambient air), obtaining a white powder. The Gd(OH)₃ was calcined at either 380 or 600ºC for 60 min, depending on the type of nanocrystals envisaged (GdOOH or Gd₂O₃, respectively).

XRD measurements were performed on a Shimadzu XRD 6100 with CuK_α radiation (40 kV, 30 mA, λ = 0.15418 nm). The diffraction pattern was measured between 5º and 75º with a step size of 2º s^-1. The size and morphologies were characterized by FE-SEM (FEG-VP Zeiss Supra 35) and high-resolution transmission electron microscopy (HRTEM, Jeol, JEM-2100). The thermogravimetric (TG) analyses were carried out with a Netzsch-409 STA simultaneous thermogravimetric-differential thermal analysis (TG-DTA) apparatus with a heating rate of 10ºC min^-1 under flowing air. DR spectra were produced using a Varian Cary 5G(USA) spectrophotometer in diffuse reflectance mode. PL spectra were collected with a thermal Jarrel-Ash Mono Spec Monochromator and a Hamamatsu R446 photomultiplier. A krypton ion laser (Coherent Innova) with an excitation wavelength of 350.7 nm was used, with the laser nominal output kept at 200 mW. All measurements were performed at room temperature.

3. Results and discussion

Figure 1(a) presents the XRD patterns of the as-prepared Gd(OH)₃ powder and Gd(OH)₃ powder after the MAH treatment. For the sample processed with the MAH method, all the diffraction peaks can be perfectly indexed to the hexagonal structure of the Gd(OH)₃ powders, in agreement with the respective Joint Committee on Powder Diffraction Standard (JCPDS) cards nº 083-2037. No impurities were detected, which in turn reveals the important role of the MAH method in the processing and characteristics of the obtained Gd(OH)₃ powders. Moreover, this method enabled the synthesis of highly crystalline products featuring a long-range-ordered single-phase (Figure 1(a)).

Figure 1
XRD patterns of (a) as-prepared Gd(OH)₃ with and without the use of MAH treatment (b) TG/DTA curves of the Gd(OH)₃ prepared by the MAH method (c) XRD patterns of the Gd(OH)₃, GdOOH and Gd₂O₃ nanocrystals.

Figure 1(b) shows the TG/DTA curve for the Gd(OH)₃ sample prepared by the MAH method at 130ºC for 20 min. Our measurements reveal the presence of two major stages with rapid weight losses at about 312 and 437ºC (8.9 and 4.4%, respectively), suggesting the existence of an intermediate phase during the thermal conversion process of Gd(OH)₃ to Gd₂O₃. Interestingly, the total weight loss is in agreement with the theoretical value found for the complete dehydration of Gd(OH)₃ to produce Gd₂O₃ (13.3%), as previously reported^15,16. The thermal dehydration process of the Gd(OH)₃ and subsequent conversion to Gd₂O₃ observed in the Figure 1(b) is supposed to occur in two steps, as summarized by equations (1) and (2).

(1) $\mathrm{Gd}{\left(\mathrm{OH}\right)}_3\stackrel{}{\to }\mathrm{GdOOH}+H_{2}O$

(2) $2\mathrm{GdOOH}\stackrel{}{\to }{\mathrm{Gd}}_2{O}_3+H_{2}O$

In this study, a two-step approach was used to synthesize the GdOOH and Gd₂O₃ powders at optimal conditions. Based on these results, we chose two different temperatures (380 and 600ºC) to produce GdOOH and Gd₂O₃ powders, respectively, using a thermal decomposition process of the Gd(OH)₃ powder obtained previously by the MAH method.

As shown in Figure 1(c), all the diffraction peaks can be perfectly indexed to the hexagonal structure of Gd(OH)₃ (JCPDS 083-2037), monoclinic structure of GdOOH (JCPDS 075-3267) and cubic structure of Gd₂O₃ (JCPDS 072-6362), respectively^16-19. However, the peak widths for Gd₂O₃ are relatively broader than those observed in the XRD patterns of GdOOH and Gd(OH)₃, suggesting a smaller crystallite size. The crystallite sizes were estimated using the Scherrer equation¹⁷, obtaining 34.9, 19.9 and 15.7 nm for Gd(OH)₃, GdOOH and Gd₂O₃, respectively. Therefore, the average crystallite size decreases as a function of the dehydration process. This behavior is explained by the dehydration process during the phase change, as well as the synthesis method employed, which facilitates the preparation of materials with small particle sizes.

Figure 2 shows the representations of the unit cells for the hexagonal Gd(OH)₃ structure with a P63/m space group, the monoclinic GdOOH structure with a P121/m1 space group, and the cubic Gd₂O₃ structure with an Ia-3 space group^18-20. VESTA program was used for the construction and visualization of these three models²¹. In this study, the most important difference between the crystalline phases is the coordination of the gadolinium ions in the ideal crystal structure. The hexagonal Gd(OH)₃ structure has a nine-coordination complex with [Gd(OH)₉] clusters, the monoclinic GdOOH structure has a seven-coordination complex with [GdO(OH)₇] clusters, and the cubic Gd₂O₃ structure has a six-coordination complex with [GdO₆] clusters. It is well known that lanthanide ions have a wide variety of coordination environments, and in general tend to form complex clusters with high coordination numbers, nine-coordination being particularly important in the structure of f-block elements²².

Figure 2
Schematic representation of the unit cells corresponding to the hexagonal, monoclinic and cubic structure of the materials.

FE-SEM analysis was performed in order to determine the particle morphology in detail. Figures 3(a-f) present FE-SEM images of the Gd(OH)₃, GdOOH and Gd₂O₃ nanorods. We can see that the as-prepared Gd(OH)₃ sample consists of particles with undefined shape (Figure 3(a)). On the other hand, the Gd(OH)₃ precursor treated by the MAH method exhibits a controlled and well-defined morphology, where nanorods prevail (Figure 3(b)). Similarly, GdOOH and Gd₂O₃ also exhibit a nanorods-like morphology, even after subsequent thermal treatment (Figures 3(b-f)).

Figure 3
FE-SEM images of (a) as-prepared Gd(OH)₃ without the MAH treatment (b) Gd(OH)₃ (c-d) GdOOH and (e-f) Gd₂O₃ nanorods.

It can be seen that Gd(OH)₃ nanorods (Figure 3(b)) have a narrow size distribution with an average length of about 0.9 µm and diameter of about 12.4 nm, respectively. Figures 3 (c-d) exhibit the same morphology for the intermediate GdOOH compound formed with the thermal treatment at 380 ºC. There was no morphological change after the calcination process, indicating a stability of the nanorods obtained. However, there was a reduction in the size of the nanorods (average length of about 0.64 µm and diameter of about 14.3 nm), confirming the crystallite size results presented earlier. Moreover, similar results were found for the Gd₂O₃ nanorods (Figures 3(e-f)) formed after the thermal treatment at 600 ºC, in which a reduction of the nanorods size (average length of about 0.60 µm and diameter of about 13.2 nm) was also observed.

Then, we can verify that the synthetic strategy employed in this study allows the preparation of materials with high purity and well-defined morphologies at low temperatures and short synthesis times. Successful conditions for this synthesis using the MAH method are attributed to the rapid and effective interaction between the electromagnetic radiation and the permanent dipole moment of water molecules^23,24. The permanent dipoles of water are induced in solution and can produce a rapid heating of the system because they interact directly with microwaves. This kind of interaction is directly related to the water capacity for electromagnetic radiation absorption and efficiency in converting electromagnetic radiation to thermal energy^25-28. Therefore, the reaction time and temperature necessary for obtaining of such materials in this work is significantly lower than those described in the literature^29-32 providing a significant improvement on crystallinity and morphology.

In this work, the Kubelka-Munk method was employed to calculate the E_g values. The direct E_g values were found to be 5.56, 5.12 and 5.04 eV for the Gd(OH)₃, GdOOH and Gd₂O₃ nanorods, respectively (Figures 4 (a)). These values agree with those reported in the literature as compared to bulk band gap values^33,34. Some small variations are attributed to the fact that the E_g values depend on the method of preparation and the experimental conditions like the type of surfactant, temperature and processing time. These key factors can favor or inhibit the formation of structural defects, which can control the degree of structural order-disorder of the material and consequently, the number of intermediate energy levels within the band gap³⁵.

Figure 4
(a) Diffuse reflectance spectra of nanorods and (b) PL at room temperature.

However, it has been revealed that the E_g values are strongly dependent on the crystalline structure, and are very sensitive to the effects of the structural order-disorder, as well as the conditions of synthesis used in the preparation of these complex functional materials. The crystal growth process reported in this study might lead to both surface and bulk modifications due to structural changes, which are directly linked to the appearance of new intermediary energy levels within the forbidden band gap³⁶. This has a major impact on the physical and chemical properties of the nanorods, namely over its PL emission behavior (Figure 4(d)).

PL emission spectra of the Gd(OH)₃, GdOOH and Gd₂O₃ nanorods, excited at 350 nm are shown in Figure 4(b). All the samples exhibit a broad emission band at room temperature, typical of a multiphonon process. As shown in Figure 4(b), the PL emission with a maximum at 508 nm corresponds to the Gd(OH)₃ phase, while the other two peaks with lower intensities centered at 440 and 538 nm are ascribed to the PL emissions of GdOOH and Gd₂O₃, respectively. Essentially, Gd³⁺ has a simpler emission spectrum in comparison to the other trivalent rare-earth ions²⁹. However, we observe a green shift effect on the emission spectrum (Figure 4b) when we compare the conversion of Gd(OH)₃ on Gd₂O₃. These emissions are attributed to different types of surface defects of Schottky and Frenkel types³⁵. According to Wang's proposal³⁷, the green emission band may attribute to the recombination of a delocalized electron close to the conduction band with a single charged state of surface oxygen vacancy. In oxide systems, typically oxygen vacancies are prevalent and the presence of such defects contribute significantly to the modified photoluminescence response³⁷.

However, different density of oxygen vacancies might arouse differences of PL intensities as shown in emission pattern of Figure 4 (b). The PL emission can be a probe for the structural evaluation of materials^9,36,38. Therefore, direct observation of oxygen vacancies is difficult, but spectroscopic studies can provide indirect evidences in support of them. More information about the nature of these defects may be elucidated in a future study, using quantum mechanical calculations and experimental techniques such as electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge spectroscopy (XANES). Zhang and co-authors³⁹ have demonstrated that luminescence results can be employed as a simple probe for oxygen vacancies in TiO₂ nanoparticles. In another work, studies of visible luminescence in TiO₂ nanotubes and EPR spectrum provide strong evidence characteristic of single-electron-trapped oxygen vacancies⁴⁰.

Indeed, it is well known that the crystal structure depends on the experimental conditions such as heat treatment, which may cause changes to its electronic structure, and thus, has an important role in the PL profile⁹. We can observe that the Gd(OH)₃ compound presents the most intense PL emission (Fig 4b). This effect can be attributed to defects that cause structural disorder yielding intermediate levels in the band gap, which is essential for the PL phenomenon. An improved structural organization has been achieved with further thermal treatment during the annealing process. The resulting GdOOH and Gd₂O₃ compounds present lower PL intensities. These results corroborate with those obtained by UV measurements [Figure 4 (a)]. As shown before, the band gap values of the GdOOH and Gd₂O₃ samples were lower than those presented by Gd(OH)₃. Studies on intermediate levels in band gap and structural order-disorder showed a decrease in the optical band gap according to the structural organization⁴¹. Our group has postulated that the PL intensity in inorganic oxides is also associated with the thermal treatment history, structural order-disorder, and electronic levels in the band gap^41,14.

Figure 5 shows the Commission Internationale de L'Eclairage (CIE) diagram for the compounds with respective assignment of their colors. Specifically, the CIE chromaticity coordinates [x-axis, y-axis] are [x = 0.2801, y = 0.4052], [x = 0.2608, y = 0.2527], and [x = 0.3455, y = 0.4277] corresponding to emission color coordinates for the Gd(OH)₃, GdOOH and Gd₂O₃, respectively. These results clearly demonstrate the green shift effect and corroborate the observed PL behavior.

Figure 5
CIE chromaticity diagram for the nanorods.

Moreover, an earlier study has demonstrated the synthesis of rare-earth doped Gd₂O₃ nanoparticles⁴². Multicolor emissions at selected wavelengths could be obtained by altering the doping concentration in the triple-doped sample. Based on the CIE coordinates, it could be inferred that double-doped Gd₂O₃ with the composition Tb = 0.05% and Dy = 1.95 mol%, when excited at 247 nm, emits white light, which is closest to the standard noon daylight. Hence, it can be inferred that by altering the doping content of the rare-earth ions in a suitable host lattice like Gd₂O₃, it is possible to achieve desired emission colors.

4. Conclusions

In summary, the method employed is simple and fast to obtain highly crystalline nanorods-like oxides and with unique morphology once the reaction time necessary for the synthesis of such materials is significantly lower than that used in the most previous works. The results contribute to an insight of the crystal structure and chemical composition and its impact the on the optical properties of the Gd(OH)₃, GdOOH and Gd₂O₃ obtained. So, shape, size, interaction between particles and synthesis technique play an important role for optical behavior of the studied oxides. Therefore, such properties enable the potential application of these complex functional materials as high-quality phosphors and optoelectronics devices.

5. Acknowledgments

The authors acknowledge the support of the Brazilian agencies FAPESP-CDMF: 2013/07296-2, FAPESP 2013/07437-5, CAPES/PROCAD: 182441, CNPq: 554580/2010-1, CNPq: 485518/2013-9 and FAPEG. Special thanks by INTMF and LabMic/UFG.

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Publication Dates

• Publication in this collection
  29 Aug 2016
• Date of issue
  Sep-Oct 2016

History

• Received
  27 Mar 2016
• Reviewed
  28 June 2016
• Accepted
  11 Aug 2016