Figure 1(a) showed the XRD (5°/min) patterns of the as-prepared LZ1. No characteristic reflections of impurities were detected, demonstrated a face-centered cubic pure pyrochlore type structure (JCPDS No. 01-071-2363). Notably, the peaks marked with black peach for the LZ1 were characteristic superlattice diffraction peaks of pyrochlore structure [25].
The Raman spectra of the LZ1 were presented in Fig. 1(b). According to symmetry theory, the Raman spectra are very sensitive to the vibrations of the O cation. The pyrochlore structure usually consists of six Raman active vibration modes as follows:
For an ideal pyrochlore structure, the six vibration modes are associated with an O cation, including one mode (F2g) related to the O cation at the 8a site and five modes (A1g + Eg+3F2g) assigned to the O cation at the 48ƒ site, where Eg is derived from the O–RE–O bending vibrations [26, 27]. The LZ1 presented four typical vibration modes in the range of 200–800 cm− 1 without other vibration modes, indicating a pyrochlore structure.
The simulated model of an ideal crystal structure diagram of pyrochlore was depicted in Fig. 1(c). An ideal pyrochlore structure belongs to the space group Fdm (Z = 8), where La3+ distributed randomly and equally at the 16c site, and the Zr4+ equally and homogeneously occupy the 16d site. The O occupies the 8b and 48f sites, while the O vacancy occupies the 8a site [28].
The FT-IR spectra of LZ1 in the wave range of 400 cm− 1 to 4000 cm− 1 were presented in Fig. 1(d). The peaks centered approximately at 1187 cm− 1 and 1108 cm− 1 are ascribed to the stretching vibration of Zr atoms against O atoms in Zr-O-Zr bond [29]. While the peaks at the vicinity of 540 cm− 1 and 3690 cm− 1 are correspond to the vibration of the chemical bond Zr-O stretching modes. A relatively weak band centered at about 436 cm− 1 associated with the vibration of O-La-O bending modes [30]. Also, the appearances of the band centered at approximately 1630 cm− 1 are evidence of the existence of water molecules attached to the powders.
Figure 2(a, b) depicted the SEM and TEM images of the calcined LZ1. Clearly, the samples have a good microstructural integrity with a typical “pearl chain” microstructure, which presented a 3-D porous network skeleton with numerous nanoscale particles and nanopores. The nanoscale particles (approximately 25–35 nm) were uniformly distribute in the whole field of vision with an average particle size. The nanopores presented open and close pore structures with an average size of approximately 40–50 nm. HRTEM was performed to further verify the information of the LZ1 with pyrochlore structure (Fig. 2(c)). The observed interplanar spacing of 0.182 nm and 0.173 nm were corresponded to the (222) and (440) planes, respectively. The SAED pattern of LZ1 were shown in Fig. 2(d). The Miller indices (hkl) of all the crystal planes of LZ1 obtained from SAED pattern are in good agreement with XRD reference patterns. The LZ1 were further examined through EDS mapping to investigate the element distribution homogeneity on the nanoscale, as shown in Fig. 2(e), all the elements contained were uniformly distributed without any noticeable elemental segregation or clustering.
Figure 3 showed the microstructures and the corresponding XRD patterns of the La2Zr2O7 powders calcined at different temperature. It can be seen from Fig. 3(a) that the enlarged particles and decreased size of pores. Nevertheless, the 3-D porous network skeleton of the microstructure still present. As for sample LZ3 (Fig. 3(b)), we can clearly see that the particles were grow bigger in the whole field of view and many of them seemed closely combined. The pores still exist though most of them disappeared and the interconnected assembly forms a relatively dense structure. The corresponding XRD patterns of three samples calcined at different temperatures were shown in Fig. 3(c). Obviously, the three samples presented a single-phase pyrochlore structure without other phases. However, because of the lattice constant increase, the diffraction peak positions slightly shifted toward a lower diffraction angle as the calcined temperatures increase [31].
Figure 4 showed the N2 adsorption-desorption isotherms and corresponding BJH pore size distribution analysis of the samples. It appeared a typical type IV adsorption isotherm with an H3-type hysteresis loop in accordance with mesoporous structure [32]. The BET specific surface areas were calculated to be 325.17, 221.43 and 62.57 m2/g for samples LZ1, LZ2 and LZ3, respectively. The pore size distribution curve (inset images) depicted that the samples exhibited mesopore and macropore size regions with a small average pore width and a high pore volume. The BET specific surface area of the samples decreased because the grains further grow. Simultaneous, the pore size of the samples decreased as the calcination temperature increases.
Figure 5(a) depicted the photograph of the as-prepared LZ1 bulk reconstructed under a uniaxial pressure of 50 MPa, which shows an intact overall structure. The surface morphology with low magnification which demonstrated a uniform, regular and dense morphology, as shown in Fig. 5(b). The magnified surface morphology (Fig. 5(c)) showed a 3-D porous network skeleton with no structural destroyed.
The room temperature thermal conductivity of the samples were shown in Fig. 6(a). Clearly, the room-temperature thermal conductivity were 0.07 W/(m·K), 0.09 W/(m·K) and 0.13 W/(m·K) for the sample LZ1, LZ2 and LZ3 bulk, respectively. The lower room temperature thermal conductivity can attributed to the unique 3-D interconnected porous structures with rich porosity, and the ceramics grains were not continuous that can block the heat conducting pathways [33]. We can conclude that the high porosity can greatly reduce the effective thermal conductivity of the porous La2Zr2O7 bulk. What’s more, combining with the results of Fig. 4, the specific surface area (SSA) is another factor for decreasing thermal conductivity of porous La2Zr2O7 bulk. For a given density, the smaller the size of the pores, the higher the SSA, and the better will be the insulation performance.
The heat conductivity mechanisms within aerogels were systematically sketched out in Fig. 6(b). Heat transfer in monolithic non-evacuated aerogels involves three components, since the convection heat transfer can be neglected in porous materials where void spaces are smaller than 4 mm. Thus, the total thermal conductivity can be described as follows [34].
k T = ks + kg + kr (2)
where kT is the total thermal conductivity and ks, kg, and kr refer, respectively, to the solid, gaseous and radiative transfer components of the first.
The ks relates to the solid fraction of aerogels which is influenced by the extent of crosslinking and network connectivity. Heat conduction in the solid network occurs through the atomic lattice, due to the excitation of vibrational energy levels of interatomic bonds or even by free electron transport under thermal gradient [35]. When the heat conducted in the gaseous state, the collision phenomenon happens and the faster ones transfer part of their kinetic energy to the slower molecules. But the movements are of minor relevance in the monolithic native aerogels because the average pore dimensions are typically below 70 nm, and the mean free path of air molecules is about 66 nm [36]. The kr is based on electromagnetic waves, correlated to the photon's mean free path. Therefore, this component is highly temperature-dependent, with a high factor-scaling exponent. According to Fomitchev [37], the radiative thermal conductivity can be computed as follows:
K r = 16n2σT3ε−1 (3)
where n is the index of refraction, σ is the Stefan-Boltzmann constant, T is the temperature (K) and ε (ε = ρbe) is the spectral attenuation coefficient.
In conclusion, the extremely low thermal conductivity of the as-prepared porous La2Zr2O7 bulks were result from the combination effect of the three heat conduction modes.
The pore structure and morphology of the particles also have a significant effect on the mechanical properties of porous La2Zr2O7 ceramic. Figure 7 depicted that the compressive strength values were 5.22, 8.34 and 11.95 MPa for sample LZ1, LZ2 and LZ3 bulk, respectively. Even though the compressive strength values here were lower compared with reported work [38], the as-prepared porous La2Zr2O7 ceramic in this work had much higher porosity. According to Griffith theory, the critical crack size has a vital effect on the mechanical strength and the cracks are mainly result from the pores. Since the stress concentrates on the surrounding region of pores, and the pores can decrease the load area and weaken the load capability [39]. The porosity decreased with the increasing of sintering temperature and the interconnected adjacent particles formed a strong neck, which enhancing the compressive strength.
The LZ1 bulk samples were utilized as photocatalyst for the photo-degradation of RhB dye. As depicted in Fig. 8(a), different absorption spectrums of RhB solution taken out at different times were characterized by UV-light irradiation. At the initial stage, the spectrum showed the maximum absorption peaks of RhB at 553 nm. The absorption peak intensity decreased with prolonging irradiation time, demonstrating the gradual degradation of the organic dye. The inset showed the photograph of RhB solution at beginning and after 60 min of UV-light exposure. The color of the solution was almost faded away, manifesting the degradation of the dye molecules. The characterization of degradation efficiencies of the three photocatalysts were presented in Fig. 8(b). It is the plot of degradation C/C0 as a function of irradiation time, where C is the absorbance of RhB solution measured every 10 min in the process of photodegradation and C0 is the original RhB solution. It was clear that the degradation efficiencies of the LZ1 photocatalyst was ideal and the dye is decomposed 73% after UV-light treated for 60 min, demonstrating a better photocatalytic degradation performance than its counterparts. Figure 8(c) depicted the re-use performances of LZ1 photocatalyst. Though the photocatalytic degradation under UV-light irradiation has a little changed, the yield was still 60% after repeating 4 times. The enhanced photocatalytic performance of LZ1 photocatalyst can ascribed to its unique 3-D porous structure and high specific surface area, which provided more electrons-holes (h+-e−) that facilitating the enhancement for photocatalytic properties.
The La2Zr2O7 photocatalyst has received significant attention due to the absorption of oxygen (O2) and water molecules (H2O) on the porous structures surface of the photocatalyst by the effect of UV light irradiation [40]. In order to explore the photocatalytic mechanism, the band energy levels of La2Zr2O7 samples were depicted in Fig. 8(d). When exposed to UV light, the La2Zr2O7 absorbed photon of energy greater than the energy band gap (Eg) electrons (e−) and excited from valence band (VB) into the vacant conduction band (CB). Based on the reports [41, 42], a possible mechanistic pathway of the La2Zr2O7 catalyst is shown as follows:
La2Zr2O7 + hv → La2Zr2O7 (h+) + La2Zr2O7 (e−)
La2Zr2O7 (e−) + O2 → O2· −
O2· − + H2O → H2O2 + OH·
La2Zr2O7 (h+) + H2O → OH·
OH· + Dye → CO2 + H2O
The electrons (e−) at CB of La2Zr2O7 were scavenged by dissolved O2 and produce superoxide radicals (O2·−), and the produced O2·− then reacts with H2O to form hydroxyl radicals (OH·). Also, the holes (h+) in VB of La2Zr2O7 can react with H2O and generate OH·. The produced OH· can decompose the RhB organic dye molecule into the smaller organic molecules [43], H2O, CO2, and mineral acids.