Sol-Gel Synthesis of Gd₂O₃:Nd³⁺ Nanopowders and the Study of Their Luminescent Properties

Abstract

The synthesis of gadolinium oxide nanopowders doped with neodymium (Gd₂O₃:Nd³⁺) 30 nm in size was carried out using the citrate sol-gel method that included the simultaneous stabilization of nanoparticles using citric acid and polyvinylpyrrolidone (PVP). This study proposes and assesses a sol-gel synthesis process that involves the use of two different organic stabilizers with different thermal stabilities. Citric acid and polyvinylpyrrolidone were used as organic modifying components, playing a double role in the synthesis process, that is, acting as stabilizers of forming nanoparticles in colloidal solutions and serving as fuel additives in the process of heat treatment of materials. The structural and optical properties of Gd₂O₃:Nd³⁺ were investigated using photoluminescence, FTIR spectroscopy, DTA/TG, scanning electron microscope (SEM), and XRD analysis.

1. Introduction

Nanocrystalline materials based on gadolinium oxide (Gd₂O₃) are characterized by high luminescent properties and thermal and chemical stability and are promising for various optical, environmental, and medical applications [1].

Liquid methods are often used to synthesize Gd₂O₃ nanocrystals: sol-gel process [2]; polymer-salt method [3]; precipitation [4]; hydrothermal method [5]; spray pyrolysis [6], etc. The synthesis process has a substantial influence on the crystal structure and luminescent characteristics of gadolinium oxide nanoparticles. Wet chemical processes are straightforward, efficient, and often used in the production of oxide ceramic materials [7]. Using wet chemical techniques enables a substantial reduction in the temperature required for the synthesis of oxide nanoparticles, consequently accelerating the technical process. Gd₂O₃ nanocrystalline powders are synthesized using wet chemical techniques, such as sol-gel or modified Pechini method, and have a stable cubic crystal structure, which is the preferred crystalline form of Gd₂O₃ [8].

The sol-gel process, which is widely known and used to obtain various materials, provides their high homogeneity and relatively low synthesis temperatures [9, 10, 11, 12]. Thus, in [13], the formation of Gd₂O₃ crystals was observed when gels were heat-treated to a temperature of only 400°C, which is significantly lower than the temperatures of technological processes traditionally used in the production of oxide optical materials.

The citrate sol-gel method based on the introduction of citric acid into the initial solutions followed by their heating and formation of homogeneous gels is used to create luminescent nanomaterials based on gadolinium oxide [14].

Citric acid and polyvinylpyrrolidone (PVP) play a dual role in the synthesis process, acting as stabilizers of the formed nanoparticles in colloidal solutions and acting as fuel additive in the process of heat treatment of materials [15, 16].

Citric acid forms chelate compounds with metal ions in solutions and is used to form oxide nanophosphors [15, 17]. During sol-gel synthesis, citric acid molecules undergo evolution at the stage of heating the initial solution and gel formation and completely decompose at T > 175°C [18], that is, temperatures significantly lower than the decomposition temperatures of metal nitrates [19]. For this reason, citric acid is an effective but relatively low-temperature stabilizer of oxide nanoparticles.

PVP is a soluble organic polymer used to stabilize various nanoparticles [20, 21, 22]. Thermal decomposition and oxidation of PVP occur at temperatures of 300–550°C, close to the decomposition temperatures of metal nitrates and the formation of oxide nanoparticles. The presence of PVP in the initial solutions has a significant effect on the size and properties of the formed nanoparticles [8, 22]. Compared to citric acid, PVP is a stabilizer that acts at higher temperatures and has a direct role in the formation of oxide nanoparticles.

The aim of the present work is to develop a low-temperature citrate sol-gel synthesis of Gd₂O₃:Nd³⁺ nanopowders with simultaneous application of two organic stabilizers (presented in the table), to study the evolution of nanoparticle structure during their formation and to investigate the luminescent properties of the obtained nanopowders.

2. Materials and methods

Aqueous solutions of gadolinium (Gd) and neodymium (Nd) nitrates, citric acid, and PVP were used as starting materials. Aqueous solution of the components was obtained by dissolving powdered reagents in distilled water under vigorous stirring. The mixture of specified volumes of component solution was performed at room temperature. The chemical composition of the obtained liquid mixtures is given in Table 1. The mixed solutions were heat-treated at 70°C with stirring.

Sample number	Concentration, wt.%
	Solution					Powder
	Water	Gd(NO3)3	Nd(NO3)3	Citric acid	PVP	Gd2O3	Nd
1	87.74	4.15	0.05	2.80	5.26	99	1

Table 1.

Chemical composition of materials.

The processes of material evolution during sol-gel synthesis in the present work were studied via infrared (IR) spectroscopy, differential-thermal and thermogravimetric (DTA-TG), scanning electron microscope (SEM), and XRD analysis.

IR spectra were captured using FTIR spectrometer Bruker ALPHA; DTA-TG analysis was performed using STA 449F1 Jupiter (Netzsch). The MIRA3 TESCAN scanning electron microscope was used to investigate the morphology of the acquired oxide particles. XRD analysis was carried out using diffractometer Rigaku Ultima IV. The average crystallite size was estimated using the Scherrer equation.

The photoluminescence spectra of obtained powders in the wavelength range of 250–800 nm were investigated using a Perkin–Elmer LS 50B luminescence spectrometer.

The study of emission properties of materials was carried out on an experimental setup including a laser source on a YAG:Nd crystal (wavelength λ = 532 nm) generating pulses with duration τ = 10 ns and energy E = 30 mJ. Acton-300 monochromator (Acton Research) and InGaAs-photodetector ID-44 (Acton Research) were used to record emission spectra. During the research process, Gd₂O₃:Nd³⁺ powders were tightly fixed in the space between two plane-parallel polished quartz glass plates, with the thickness of the nanopowder layer between the plates being 150 μm. Laser radiation was focused into a spot with a diameter of about 130 μm on the surface of the plates.

3. Results and discussion

3.1 FTIR spectra

Figure 1 displays the infrared absorption spectra of the wet gel 1. The wide and strong absorption band within the 3300–3500 cm⁻¹ range is linked to the stretching vibrations of O–H groups. The strong absorption band at 1631 cm⁻¹ in the wet gel is attributed to the presence of citrate anions and PVP molecules, which cause vibrations in the carbonyl group C– –O. Prior to the heat treatment, the gel contains nitrate anions, which is why the absorption band at 1360 cm⁻¹ is attributed to NO₃ anions. Additionally, it is important to mention that the wet gel has a weak peak at 537 cm⁻¹, which could be attributable to the vibrations of the Gd-O bond.

Figure 1.

Absorption spectrum of composite gel in the IR spectral region.

Previous investigations have shown comparable results in the production of Y₂O₃:Eu materials with a similar structure via the citrate sol-gel method [23]. The spectra of cubic Y₂O₃ crystals exhibited an absorption band at a wave number of 560 cm⁻¹, corresponding to the vibrations of the Y-O bond. This absorption band was identified after drying the gels at 100°C, and its intensity increased with increasing temperatures throughout the heat treatment of the materials [23].

3.2 DTA/TG analysis

Heating of the gel during the heat treatment process leads to the decomposition of citric acid, metal salts, and PVP, as well as the formation and growth of oxide crystals. Figure 2 displays the results of the DTA-TG analysis, which reveals the various processes that take place during the heat treatment of the gel. Figure 2a shows that, during the heating of gel, there is a stepwise decrease in the sample mass accompanied by several exothermic effects (Figure 2b).

Figure 2.

Results of the thermal gravimetric (a) and differential thermal (b) analyses of the processes occurring during heat treatment of the gel.

The removal of residual water from the material determines the loss of sample mass and small heat absorption at the initial stages of heat treatment (20–150°C). Citric acid decomposes at a temperature of 175°C, causing a noticeable decrease in the sample mass and the release of heat.

The most significant variations in sample mass changes and exothermic effects are observed in a wide temperature range of 340–850°C. At temperatures of 340–550°C, decomposition of metal nitrates and PVP occurs, accompanied by the significant release of heat and gaseous products.

3.3 XRD analysis

Figure 1 displays the X-ray diffraction (XRD) patterns of Gd₂O₃:Nd³⁺ nanocrystalline powders. The powder only comprises peaks that are indicative of the cubic Gd₂O₃structure. Both the amorphous and parasitic crystalline phases, as well as the halo, were absent in the XRD patterns.

Referring to the results presented in Ref. [8], the addition of PVP in the initial solution significantly accelerates the formation of the crystalline phase (Figure 3).

Figure 3.

XRD patterns of the Gd₂O₃:Nd³⁺ nanocrystalline powders.

The average size of the Gd₂O₃ nanocrystals was determined using the Sherrer equation [24]: d = kλ/(β cos θ), where k is the Sherrer constant (k = 0.9), λ is the x-ray wavelength (λ = 0.15406 nm) for Cu Kα, β is the full width at half maximum of the corresponding XRD peak, and θ is the Bragg angle. The average crystallite size of Gd₂O₃ was determined to be around 30 nm. According to the findings shown in Ref. [8], the addition of PVP has slight impact on the overall decrease in the average size of the nanocrystals.

3.4 SEM analysis

Figure 4 displays SEM images of Gd₂O₃:Nd³⁺ nanocrystalline powder at resolution 20 μm (Figure 4a) and 5 μm (Figure 4b). The powder is composed of highly aggregated nano-sized Gd₂O₃∶Nd³⁺ crystals of around 30 nm in diameter. Based on the scanning electron microscope (SEM) image, the average diameter of Gd₂O₃ nanocrystals is consistent with the value obtained by calculating it using the Scherrer equation.

Figure 4.

SEM images of Gd₂O₃:Nd³⁺ nanocrystalline powder at resolution 20 μm (a) and 5 μm (b).

The higher magnification image (Figure 4b) reveals that the powder is more widely dispersed and comprises small particles, including thin micro-plates. The inclusion of PVP in the sol-gel process results in a material with a highly dispersive form.

3.5 Luminescence in UV spectral range

The luminescent properties of the synthesized powders in the UV spectral range are mainly determined by electronic transitions occurring inside the crystalline matrix of Gd₂O₃. These transitions are comprehensively explained in [25]. The UV luminescence excitation spectra (Figure 5a) exhibit bands with peak wavelengths of 238, 257, 268, and 282 nm. The investigation reported in [26] examined comparable luminescence excitation spectra in Gd₂O₃:Eu³⁺ nanophosphors produced by thermochemical and hydrothermal synthesis methods.

Figure 5.

Luminescence excitation spectrum (wavelength of the luminescence 309 nm) of Gd₂O₃:Nd³⁺ powder (a); photoluminescence spectrum (wavelength of the luminescence excitation 238 nm) of Gd₂O₃:Nd³⁺ powder (b).

The strongest band with a maximum of λ_max = 238 nm is linked to the ⁶P_7/2 → ⁸S_7/2 transition of Gd³⁺ ions inside the crystalline matrix of Gd₂O₃, according to [25]. The band with a maximum of λ_max = 268 nm corresponds to the electronic transition ⁸S_7/2 → ⁶I_7/2–17/2 of Gd³⁺ ions in the crystalline matrix of Gd₂O₃ [27]. The charge transfer band with a maximum λ_max = 257 nm between oxygen ions and Eu³⁺ ions is given in [27]. Upon excitation with a wavelength of λexc = 238 nm, the synthesized powders exhibit luminescence in the ultraviolet region of the spectrum. This luminescence is caused by electronic transitions occurring inside the crystalline matrix of Gd₂O₃, as seen in Figure 5b.

The experimental findings obtained in this study demonstrate that the use of two stabilizers in the sol-gel process has no impact on the ratio of band intensities exhibited in the luminescence excitation spectra and practically has no effect on the shape of the photoluminescence spectrum in the UV region of the spectrum.

3.6 Luminescence in IR spectral range

The intense luminescence observed in the synthesized powder in the near infrared spectral region is determined by electronic transitions of the Nd³⁺ ion-activator. The photoluminescence spectrum of the synthesized Gd₂O₃:Nd³⁺ powder is shown in Figure 6. The three groups of luminescence bands observed in the spectrum, located at wavelengths of 940, 1064, and 1360 nm, are associated with the electronic transitions of Nd³⁺ions ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2, respectively. The most intense luminescence peak of neodymium ions with a maximum at λ_max = 1064 nm corresponds to the electronic transition ⁴F_3/2 → ⁴I_11/2.

Figure 6.

Photoluminescence spectrum (λex = 980 nm) of Gd₂O₃:Nd³⁺ powder in the NIR region.

Analysis of the results from experiments is shown in Figure 6. The results presented in Ref. [28] indicate that Gd₂O₃:Nd³⁺ materials synthesized by the polymer-salt process have photoluminescence spectra that are comparable to powders produced by other liquid methods. The reason for this phenomenon may be attributed to the fact that the luminescent properties, which are determined by the f-f electronic transitions of Nd³⁺, are not significantly impacted by changes in the structure of their immediate surroundings.

4. Conclusion

Neodymium-doped gadolinium oxide nanopowders, with an average crystal size of around 30 nm, were synthesized using the citrate sol-gel method, using two organic stabilizers simultaneously. The evolution of nanoparticle structure during their formation has been studied, and the luminescent properties of the obtained nanopowders have been investigated. Citric acid and polyvinylpyrrolidone were used as modifying organic stabilizers, which acted as stabilizers of the formed Gd₂O₃:Nd³⁺ nanoparticles in colloidal solutions, and also played the role of “combustible” additives in the process of heat treatment of materials. The data of infrared spectroscopy, differential-thermal, and thermogravimetric analyses showed that the formation of Gd₂O₃ nanoparticles starts at the raw gel stage, and the evolution process develops during drying and heat treatment of materials. The XRD data analysis reveals that the powder only consists of peaks that are typical of the cubic Gd₂O₃ structure. The SEM investigation demonstrates that the inclusion of polyvinylpyrrolidone has a substantial impact on the structure of the synthesized powder. It effectively separates the individual nanoparticles within the produced powders, resulting in a distinct morphology. The obtained Gd₂O₃:Nd³⁺ nanopowders showed intense photoluminescence in the ultraviolet and near-infrared spectral regions. The study shows that PVP addition has no influence on luminescence spectra of powders synthesized. The results obtained can be used in the development of technology for the production of bulk ceramics for disc laser elements, in the creation of luminescent nanopowders for nanothermometry in medicine, as well as in the development of technology for the synthesis of various composite luminophores.

Additional information

“This chapter is an English translation of a previously published article by the same authors: Moussaoui A, Bulyga DV, Kuzmenko NK, Ignat’ev AI, Evstropiev SK, Nikonorov NV. Sol-gel synthesis of Gd₂O₃:Nd³⁺ nanopowders and the study of their luminescent properties. Scientific and Technical Journal of Information Technologies, Mechanics and Optics. 2021;21(2):198–205 (in Russian). doi: 10.17586/2226-1494-2021-21-2-198-205.”