Ultralight Magnetic Nanofibrous GdPO4 Aerogel.

Anisotropic aerogels are promising bulk materials with a porous 3D structure, best known for their large surface area, low density, and extremely low thermal conductivity. Herein, we report the synthesis and some properties of ultralight magnetic nanofibrous GdPO4 aerogels. Our proposed GdPO4 aerogel synthesis route is eco-friendly and does not require any harsh precursors or conditions. The most common route for magnetic aerogel preparation is the introduction of magnetic nanoparticles into the structure during the synthesis procedure. However, the nanofibrous GdPO4 aerogel reported in this work is magnetic by itself already and no additives are required. The hydrogel used for nanofibrous GdPO4 aerogel preparation was synthesized via a hydrothermal route. The hydrogel was freeze-dried and heat-treated to induce a phase transformation from the nonmagnetic trigonal to magnetic monoclinic phase. Density of the obtained magnetic nanofibrous monoclinic GdPO4 aerogel is only ca. 8 mg/cm3.

Introduction

An interest in synthesis and development of various rare-earth (RE)-based materials is consistently growing because of successful application of such materials in various high-tech and biomedical fields.[1−6] RE orthophosphates possess many beneficial features, such as high thermal (up to 2300 °C) and chemical stability, low solubility, and desirable optical properties [such as high refractive index (YPO4 = 1.76, LaPO4 = 1.85, and GdPO4 = 1.97)].[7−9] Additionally, RE-based compounds are often referred as promising hosts for nuclear waste management.[10,11]

Aerogels are materials with a set of outstanding properties, such as extremely low density, large surface area, and low thermal conductivity. Despite a wide variety of materials that can be processed to very porous and low-density aerogel-like substances, there is still plenty room for specifically tailored and multifunctional systems. Aerogels can be prepared from a variety of different organic and inorganic substances, recent studies even report aerogels prepared from silver and gold.[12−14] Aerogel-based materials are considered for application in numerous fields ranging from tissue engineering and biosensing to catalysis and aerospace applications.[15−21] Introducing porosity in anisotropically structured magnetic materials would lead to the perfect supports for magnetic separations in biotechnology, environmental fields, or for magnetic field-assisted chemical reactions.[22−24] Typically, anisotropic aerogels are made from anisotropic particles (rods, fibers, wires, etc.), which are intertwined in all directions chaotically. The result of such chaotic interweaving of magnetic fibers is fully compensated overall magnetic momentum; therefore, magnetic properties of such an anisotropic aerogel would be significantly quenched.[25] Thus, obtaining a magnetic anisotropic aerogel as a homogeneous system with magnetic properties still remains an enormous challenge. Most of the recent papers report magnetic aerogels as composite materials, where magnetic properties are introduced by incorporating magnetic nanoparticles into an originally nonmagnetic aerogel matrix.[26−29]

RE elements, in turn, can be used to bring a whole new class of aerogels. RE elements would enable us to obtain homogeneous aerogel platforms with both magnetic and optical properties. Such materials could also be used in designing novel composite-based aerogels as templates. For example, Gd3+-containing inorganic materials usually are magnetic because of the strong paramagnetic nature of Gd3+ ions.

Until now, there is only one scientific paper published on RE orthophosphate aerogels by Yorov et al., where the CePO4 aerogel was successfully obtained and characterized.[30] However, the CePO4 aerogel synthesis procedure, reported in this work, needed various organic solvents, was very time-consuming (more than 10 days), and required sophisticated supercritical drying in CO2. Thus, the more environmentally friendly and time-effective synthesis route for RE phosphate aerogel preparation still needs to be developed. Additionally, to the best of our knowledge, there is no information describing synthesis or preparation of homogeneous anisotropically structured magnetic RE aerogels; therefore, such materials can be referred to as innovation.

The synthetic approach reported in this work can be applied to obtain various composite aerogels by simply incorporating different nanomaterials into the hydrogel, which further is used to produce aerogels. In such a way, gadolinium phosphate-based aerogels can be easily doped with other RE ions in order to tailor or enhance the desired properties.

In this work, we present the unique way for homogeneous fibrous magnetic GdPO4 aerogel preparation. The method is simple, scalable, eco-friendly, and does not require any harsh precursors or conditions. The main characteristics of the synthesized nanofibrous GdPO4 aerogel are also evaluated.

Results and Discussion

The evolution of this research started by preparation of GdPO4 nanoparticles with different morphologies via a hydrothermal synthesis route[31] at relatively high pH values and different Gd3+/PO43– ratios in the solution. Then, we observed that long and interwoven GdPO4 nanofibers also can be obtained if pH of the synthesis solution is lowered to 7 and the Gd3+/PO43– ratio is fixed to 1 (Section S1 in the Supporting Information). Reducing the pH to 2 yields urchin-like microparticles, as shown in Figure S1a, whereas the increase in pH to 10 yields much shorter nanofibers (see Figure S1b); therefore, they cannot interweave sufficiently enough to form a hydrogel. The stirring should also be avoided because it breaks the delicate nanofibers (see Figure S1c). The synthesis at pH = 7 yielded a nearly transparent hydrogel with the final volume of 10 mL. We have also diluted this hydrogel to 20 and 30 mL before freeze-drying in order to check whether there is any influence to the aerogel formation. The best results were obtained for the 10 mL sample, where no anisotropic pore structures were detected. However, the samples with higher volume possessed the large anisotropic pores formed by ice crystals. This is supported by low-resolution scanning electron microscopy (SEM) images, as given in Figure S2. Moreover, if the samples were diluted to 30 mL, the aerogel structure collapsed during the freeze-drying procedure. The 10 mL volume hydrogel was washed and freeze-dried resulting in the “as-prepared” aerogel. Subsequently, this aerogel was annealed at 915 °C for 12 h in air in order to induce phase transformation from trigonal to monoclinic. The schematic representation of the synthesis process conducted to obtain GdPO4 nanowires, hydrogel, and aerogel is provided in Figure .

Figure 1

Preparation scheme for the as-prepared aerogel (trigonal phase) and annealed GdPO4 aerogel (monoclinic phase).

Both, the as-prepared and annealed aerogels were investigated by powder X-ray diffraction (PXRD) to evaluate the phase composition and purity. The obtained XRD patterns for the as-prepared and annealed gadolinium phosphate aerogels are given in Figure a,b, respectively. XRD analysis reveals that the as-prepared GdPO4 aerogel consists of the pure rhabdophane-type phase with the composition GdPO4·nH2O, which has trigonal lattice symmetry with the space group P3121 (#152) (Figure a). In the rhabdophane structure, Gd3+ ions are 8-coordinated with oxygen ions. The resulting polyhedron, in turn, shares edges with four other GdO8 polyhedrons and two PO4 tetrahedra and vertexes with four PO4 tetrahedrons. The visual representation of this building block is given in Figure S3. The lattice parameters of the as-prepared GdPO4 aerogels were also calculated by applying the Le Bail method. The calculations yielded lattice parameters a = 0.6935 nm and c = 0.6349 nm, which are slightly larger than those reported in the reference structure GdPO4·H2O, PDF ICDD 00-039-0232 (a = 0.69055 nm, c = 0.63257 nm).

Figure 2

XRD patterns of the as-prepared (a) aerogel (trigonal phase) and annealed (b) GdPO4 aerogel (monoclinic phase).

One-dimensional growth of the rhabdophane structure is well-known.[32] Anisotropic particles (nanorods) of various RE orthophosphates are the result of such an anisotropic growth and are reported in numerous papers and is characteristic structural feature of such compounds.[32−34] The crystalline structure of the trigonal rhabdophane phase contains zeolitic channels (pointed out by the black dashed circle in Figure a), which accommodate crystalline water, yielding GdPO4·nH2O stoichiometry. Water molecules, accommodated in zeolitic channels, go along the crystalline structure of anisotropic particles (in our case—nanofibers) or to be more exact, along the c-axis of the GdPO4·nH2O structure.[31] This crystalline water is responsible for stabilization of the trigonal phase. Therefore, loss of this crystalline water results in phase transformation to monoclinic.[35]

The anisotropic nature of lanthanide orthophosphates can be related to their crystal structure. From the thermodynamic point of view, the activation energy of growth on the ab-plane is lower in comparison with other crystal planes (ac and bc), implying that the ab-plane is the preferable plane for growth.[8] Similarly, Wang et al.[32] explain that different chemical potentials on different crystalline planes are responsible for the anisotropic growth rhabdophane phase crystals. According to the Thompson–Gibbs theory, chemical potential is directly proportional to the number of dangling bonds on the particular crystal plane.[36] Because the ab-plane (perpendicular to the c-axis) has the largest number of dangling bonds, this plane is preferable for further growth.

Annealing the as-prepared GdPO4 aerogel results in phase transformation from trigonal (rhabdophane) to monoclinic (monazite) with a space group P21/n (#14). The XRD pattern shows that the pure GdPO4 phase is obtained (see Figure b). In the monoclinic GdPO4 structure, Gd3+ ions are nine-coordinated forming GdO9 polyhedrons. Each GdO9 polyhedron shares edges with six other GdO9 polyhedrons and two PO4 tetrahedra and vertexes with five PO4 tetrahedrons.[37] The visual representation of this building block is given in Figure S4. Calculated lattice parameters for the annealed aerogel are as follows: a = 0.6334 nm, b = 0.6854 nm, c = 0.6664 nm, β = 104.1°, and are very similar to the ones provided in the ICCD database (PDF ICDD-00-032-0386 card) (a = 0.63342 nm, b = 0.68451 nm, c = 0.66525 nm, and β = 104.0°).

It is also necessary to mention that the monoclinic phase is more preferable as it possesses no crystalline water within its structure. The high content of crystalline water within the trigonal rhabdophane structure is known to quench luminescence emission significantly.[38,39] Therefore, such a monoclinic phase aerogel is much more suitable to be doped with RE ions to introduce luminescent properties. After annealing of the as-prepared aerogel at several temperatures, we found out that 915 °C is the highest temperature that the aerogel can withstand. Annealing at higher temperature resulted in collapse of the porous aerogel structure. Even samples annealed at 915 °C have already shown some signs of nanofiber melting, as demonstrated in transmission electron microscopy (TEM) images given in the Supporting Information Figure S5.

Furthermore, SEM and high-resolution TEM were used for morphological characterization of the as-prepared and annealed GdPO4 aerogel samples. The obtained images are given in Figure . Several studies reported that during the annealing process at 900 °C temperature, when phase transformation occurs, anisotropic nanoparticles of trigonal LnPO4 tend to deform and sinter badly.[38,40,41] This, however, is not true in our case, where virtually identical morphological features of both the as-prepared and annealed aerogel samples were observed. This can be evaluated from Figure . Both GdPO4 aerogels are formed from high-aspect-ratio fibers; therefore, they can be regarded as anisotropic porous structures. High porosity of the structure ensures effective accessibility of the GdPO4 aerogel surface throughout the entire volume. SEM images also revealed that GdPO4 nanofibers within an aerogel structure were randomly woven together, forming a three-dimensional (3D) nanofibrous network. It is necessary to emphasize again that a nanofibrous GdPO4 aerogel withstands the annealing process and phase transformation, while maintaining the original 3D structure undeformed and undamaged.

Figure 3

SEM and TEM images of the GdPO4 aerogel: as-prepared trigonal (a,c,e) and annealed monoclinic (b,d,f).

The TEM images taken confirm the anisotropic nature of the nanofibrous structure of both the as-prepared and annealed GdPO4 aerogels (see Figures c–f and S5). It is evident that fibers wider in diameter consist of axially agglomerated single crystalline nanofibers with a diameter ranging from 5 to 15 nm. TEM analysis of annealed samples has also confirmed that the annealing process causes significant changes in the morphology of nanofiber junctions—they become noticeably more agglomerated. Figure c demonstrates that prior to sintering, individual nanofibers were mostly separate throughout the junctions and contact areas. However, TEM images of the annealed aerogel (Figure d) show that nanofibers in contact places were more agglomerated and sintered, while in the as-prepared aerogel nanofibers were less affected by agglomeration. This is also supported by numerous TEM images given in Figure S6.

We have also evaluated the surface area and weight of the obtained GdPO4 aerogels because these parameters are often referred to as the main characteristics defining aerogels. The as-prepared GdPO4 aerogel had a density of around 10 mg/cm3 and a specific surface area of around 29 m2/g. On the contrary, the density of the annealed aerogel decreased even more, that is, to around 8 mg/cm3, and the determined specific surface area was around 35 m2/g. It is evident that the specific surface area of annealed GdPO4 aerogels increases. Such an increase in the surface area is a result of crystalline water removal during the calcination step when the trigonal rhabdophane phase transforms to the monoclinic monazite phase. Because the total volume of the annealed aerogel virtually does not change, the weight loss results in the increased surface area.

Because the magnetic properties are the most interesting feature of the reported GdPO4 aerogels, the magnetization measurements were performed in order to analyze magnetic behavior of the annealed GdPO4 aerogel. Figure shows magnetization versus applied field at T = 300 and 80 K of the sample containing randomly oriented fiber-like GdPO4 nanocrystals. We point out the absence of any hysteresis either at 300 or 80 K temperature. The observed nonlinear (S-shaped) magnetization curves have been fitted well using a standard Langevin function[42] that is usually used to describe magnetization of superparamagnetic (SPM) systemswhere N is the number of magnetic nanoparticles in the sample, m is the averaged magnetic moment of a particle, kB is the Boltzmann’s constant, and L(x) is the Langevin function: L(x) = cot h(x) – 1/x. The magnetic moment per nanoparticle of about 762 and 1190 Bohr magnetons at 300 and 80 K, respectively, was estimated in this work and corresponds to the best fitting of the magnetization curves with the Langevin function (see red solid curves in Figure ). The observed nonlinear M–H relation of the Langevin type and absence of hysteresis certify that the rod-like GdPO4 nanofibers may be considered as a system of SPM particles. A similar nonlinear M–H relation and absence of hysteresis both at RT and low temperatures have been reported earlier for the crystalline GdPO4 material exhibiting the monoclinic crystalline structure and prepared in various forms, namely, polycrystals,[43] star-like nanocrystals,[44] and Tb- and Eu-doped nanotubes.[45,46] It is worth noting, however, that the room-temperature magnetization value, indicated in this work, is by factor 2–5 lower compared to those reported earlier. The observed magnetic properties of the nanotubes is a result of competing antiferromagnetic and ferromagnetic interaction of the paramagnetic Gd3+ ions with unpaired inner 4f electrons in the outer orbital in the monoclinic unit cell.[43] The reduced magnetization value at room temperature may probably be understood taking into account importance of the surface effects on magnetization of the nanocrystalline material and significant shape anisotropy of the rod-like nanocrystals.

Figure 4

Mass magnetization vs magnetic field of the GdPO4 nanotubes. Solid (red) lines show fitting of the experimental data by the Langevin function.

Conclusions

In this work, we reported the synthesis and properties of the GdPO4 aerogels composed of nanofiber-like crystals of a monoclinic phase, demonstrating chemical and thermal stability similar to that of a bulk material. Ultralight magnetic fibrous-structured GdPO4 aerogels were prepared by a simple and eco-friendly aqueous hydrothermal synthesis method. The aerogel was produced by freeze-drying the hydrogel of GdPO4 nanofibers, followed by a calcination step at 915 °C for 12 h. Various measurements were conducted to characterize the crystalline phase, structure, morphology, surface area, density, and magnetic properties. After the calcination step, the density of the aerogel decreased from ∼10 to ∼8 mg/cm3. Morphological investigation revealed that both the as-prepared and annealed aerogels are highly anisotropic and possess a highly porous structure formed from the 3D network of overlapping GdPO4 nanowires. The applied calcination step induced phase transformation from trigonal rhabdophane to monoclinic monazite.

One can expect that magnetic GdPO4 aerogels (a promising platform for composite aerogels) are a very interesting class of novel materials because they offer magnetic properties, low density, and large surface area. These properties make GdPO4 aerogels promising candidates in applications such as catalysts, nuclear waste management, electronic devices, neutron-absorbing materials, and so on.

We believe that fibrous, open-pore 3D-structured magnetic materials offer a flexible way for anisotropic texturing under a magnetic field. Magnetic RE orthophosphate-based aerogels offer a unique way for structuring because there is no need to introduce magnetic nanoparticles, leading to a homogeneous system. Finally, we enabled a relatively simple and fast route to obtain RE-based aerogels, which are yet emerging as a new class of materials.

Methods

GdPO4 Nanofiber Preparation

GdPO4 nanofibers were prepared using an adopted procedure described in our previous publication with some minor adjustments.[31] The whole procedure in detail is provided in the Supporting Information Section S1. For the instrumental setup used for aerogel characterization, please see the Supporting Information Section S2.

GdPO4 Aerogel Preparation

About 10 mL of the washed hydrogel was frozen in a freezer for 6 h at −24 °C. The frozen GdPO4 hydrogel (containing GdPO4 nanofibers) was then freeze-dried. As a result, a highly porous aerogel, composed of the 3D network of GdPO4 nanofibers (with a diameter ranging from 5 to 15 nm), was obtained. This aerogel, hereafter, will be referred to as the “as-prepared”. It should also be noted that the total volume of the as-prepared aerogel was virtually the same as the frozen GdPO4 hydrogel, that is, ca. 10 mL. Finally, the as-prepared GdPO4 aerogel was annealed at 915 °C for 12 h in order to induce phase transformation from trigonal to monoclinic.