The X ray diffraction (XRD) patterns of the samples prepared at different reaction times are shown in Fig. 1.The XRD peaks at angles of 24.8, 28.8, 41.2, 48.7, 51, 59.6, 65.6, and 67.5o were indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) and (4 2 0) planes of BaF2, respectively (JCPDS card No.85-1341)[25]. Its lattice parameters are a = b = 0.6189nm, c = 0.6200nm, indicating that the synthesized sample belongs to tetragonal BaF2.The diffraction peaks of all the products are very complete and sharp, denoting the high crystallinities of the samples prepared in the hydrothermal method.the reaction time of 6 h, the diffraction peak of the sample 160-TM6 is obviously weak, indicating that the BaF2 can not be fully synthesized and has low crystallinity. The reaction time is 48 h, some unknown diffraction peaks appear in the sample 160- TM48, which means that some impurities are formed due to the re-dissolution of the crystal nucleus during the longer reaction time, but BaF2 microcrystals still occupy the main body of the product. While both the samples 160- TM12 and 160-TM24 showed rather sharp diffraction peaks without other impurities, indicating that the optimum time for synthetic BaF2 was between 12–24 h.
To compare the effect of surfactants on materials, three surfactants were added during the reaction. Figure 2 is the XRD of BaF2 obtained by hydrothermal reaction at 160°C for 24 h with 0.1 g of different surfactants. It can be clearly found from the figure that the crystallinity of the samples S-CTAB and S-PEG is the highest, and the crystallinity of the sample S-EDTA is slightly worse. This may be because EDTA forms a complex with Ba2+ in the system and cannot combine with F- ions in the system to form a nucleus in time [26]. CTAB and PEG played a role in accelerating and speeding up the reaction process, so a product with better crystallinity was obtained. The three kinds of surfactants with and without surfactants basically have no impurity peaks, which are in good agreement with the standard card peaks, indicating that the addition of surfactants can greatly improve the performance of the material, which is an important factor in the preparation of BaF2.
Figure 3 show the XRD pattern of the samples prepared at different pH values. As shown in Fig. 3, when pH = 6, the diffraction peaks of sample (p9-TM24) are much sharper than others, means impurities and when the pH value drops to 3, the diffraction peaks become much weaker. The most noteworthy is when the pH value is controlled to 9, the diffraction peaks become much sharp obviously. But when the pH value rises to 12, some diffraction peaks of impurities were observed obviously. The results indicate that BaF2 materials have the higher crystallinity when the pH value controlled to 9.
Figure 4 shows the SEM of all samples prepared with different surfactants.The typical surfaces and morphologies of the materials prepared with the addition of different surfactants are also measured by SEM. As shown in Fig. 4, Fig. 4(a) shows that the BaF2 micron particles (S-CTAB) obtained with CTAB added are square morphology. Moreover, the shape of its particles is regular and the size is homogeneous with ~ 3.4 µm in length and ~ 2.3µm in width, the water CA value is 151o. The samples S-EDTA, S-PEG are shown in Fig. 4(b) and Fig. 4(c), respectively. Both of the two samples show the clearly square morphology with ~ 4.3µm in length and ~ 2.4µm in width. But as shown in Fig. 4(d), the size of sample 160-TM24 is much smaller than others and its morphology is not as homogeneous as the samples with surfactants added. Above all, the addition of surfactants can improve the morphology of BaF2 particles and this agrees well with the results of Fig. 2.
Meanwhile, the reaction mechanism is also concluded based on the related research results. It is well known that the reaction time, surfactants and pH values of reaction system have a great effect on the formation of BaF2. Figure 5 shows that the reaction process: BaCl2·2H2O and NH4F become ionic state in deionized water, then Ba2+ and F− ions begin to react with stirring. The BaF2 particles get close to each other under the effect of Van der Waals force which leads to a result of gather. When the surfactants are added, the products is self-assembly under the effects of surfactants and then the squares BaF2 are formed with time going. All of the factors have influences on the reaction together and also improve products performance. In our experiments, the reaction temperature at 160 oC for 24 h with the pH = 6 and the addition of CTAB or PEG can lead to the best result in terms of the formation of BaF2.
To investigate the super hydrophobic properties of the synthesized BaF2 materials, a contrast experiment is firstly carried out to compare the influence of modification with TMFS. As shown in Fig. 6, It can be obviously found that the curves of the contact angles of the modified surfaces with TMFS show the same regularity. No matter the surface modified or unmodified, the sample p9-TM24 all show the highest contact angle and the contact angle is between 169o and 150o. It is clear seen that the TMFS modified has enormous effect on the contact angle, and pH = 9 is the best condition to synthesis the BaF2 materials with super- hydrophobic property.
The BaF2 particles are dip-coated on the three substrates (glass, copper and aluminum) to measure the water contact angles. Figure 7 shows the water contact angles of the modified surface on the different substrates, the results show that all the surfaces coated on the three kinds of substrates exhibit an excellent super-hydrophobic property. Especially, the contact angle of BaF2surface coated on copper reaches to 156o, but the glass surface is just 155o and the aluminum is just 147o. The result shows that BaF2 surface coated on copper have excellent super-hydrophobic property.
Furthermore, the effect of placement time on the super-hydrophobic properties of BaF2 was studied, the results are collected under three drops of water with pH value 3, 6 and 9 on the glass as a substrate. As shown in Fig. 8, the water at pH = 9 have the highest contact angle than water at pH = 3, 6 in the first day. In addition, the water at pH = 9 contact angle reaches around 156o after 10 days and its contact angle maintains above 150 o in 40 days, these results shows that the BaF2 materials have an excellent super-hydrophobic properties which can keep stable for a long time.
Thermogravimetric analysis was also carried out to study the stability of the samples. The thermogravimetric analysis of BaF2 crystals was under the protection of N2 atmosphere, heated from 20°C to 900°C, and the heating rate was 5°C /min. The results are shown in Fig. 9. Obviously, at a temperature of 394.5oC, there is only a 3% weight loss [27], which is most likely due to the combination of unreacted PEG and a small amount of water molecules in the sample. In addition, the weight loss of the sample with increasing temperature is not obvious, losing 3.2% at 697.4°C and 4% at 900°C. This strongly proves the excellent stability of BaF2 material.