3.1 Characterization of LDHs
Figure 2 showed the XRD patterns of NiAlCe-LDHs prepared under different conditions. The patterns at 11.50°, 23.41°, 34.54°, 39.11°, 60.28° and 62.11° were corresponded to (003), (006), (009), (015), (110) and (113), respectively, which indicated the existence of a multilayer hydrotalcite structure [21-23]. The diffraction peaks appearing at 2θ=28.51°, 47.52° were corresponded to the hydrotalcite characteristic peaks of (111) and (220), indicating that a metal oxide containing Ni and Ce was produced [24]. The XRD diffraction peaks of NiAlCe-LDHs prepared under co-solvent systems were relatively stronger and sharper, which means that the hydrotalcites contained more metal oxides of Ni and Ce. NiAl2O4 can be determined by XRD patterns with the corresponding peak at about 18.98°, and CeO2 could be measured at about 56.24°. The peak at about 46.76° was inferred to be cerium carbide or cerium oxide, while the peak at about 31.78° represented alumina. The presence of NiAl2O4, cerium carbide and cerium oxide would affect the structure of the catalyst and might increase the activity of the photocatalyst. Compared with XRD pattern of NiAlCe-LDHs sample prepared using water as solvent, the diffraction peaks of NiAlCe-EG and NiAlCe-PEG samples prepared under co-solvent system were higher and sharper, which indicated the samples prepared under co-solvent system had stronger crystallinity [25].
Table 1 showed the average particle size and lattice parameters of NiAlCe-LDHs samples. The estimated error of the average particle size generally corresponded to ±0.5 nm. NiAlCe-10%EG(h) and NiAlCe-15%PEG(h) had smaller average particle size than those samples prepared by co-precipitation method. The basal spacing of each diffraction peak was calculated by the Bragg equation (2) as follows:
2dsinθ=λ (2)
Where, d was the basal spacing, θ was the angle between the incident X-ray and the corresponding crystal plane, and λ was the wavelength of the X-ray. d003 of all samples were around 0.79 nm, which was similar to the reported results of the (003) basal spacing of carbonate hydrotalcite [26]. d110 reflected the laminates density of the atomic arrangement. The larger the atomic arrangement laminates density was, the smaller the lattice parameter a was, as well as the smaller d110 was [27]. It could be seen from Table 1 that NiAlCe-10%EG(h) and NiAlCe-15%PEG(h) had larger laminates density of the atomic arrangement than other samples which was beneficial to its light utilization efficiency.
Table 1 Average particle size and lattice parameters of NiAlCe-LDHs samples
Sample
|
Average particle size
/(nm)
|
d003
/(nm)
|
d110
/(nm)
|
a
/(nm)
|
NiAlCe-10%EG(c)
|
31
|
0.7936
|
0.1525
|
0.9541
|
NiAlCe-10%EG(h)
|
19
|
0.7964
|
0.1522
|
0.9483
|
NiAlCe-15%PEG(c)
|
26
|
0.7894
|
0.1532
|
0.9498
|
NiAlCe-15%PEG(h)
|
25
|
0.7908
|
0.1524
|
0.9499
|
NiAlCe-LDHs
|
7
|
0.7950
|
0.1530
|
1.0577
|
Figure 3 was the XPS spectrums of Ce3d of NiAlCe-EG(c) and NiAlCe-EG(h), respectively. In Figure 3(a), the XPS spectrum of Ce3d of NiAlCe-EG(c) contained the following four peaks: V (879.68), V1 (883.5), U (898.43), U1 (916), where the labeled peaks were associated with Ce 3d3/2 and Ce 3d5/2, respectively [28]. The peaks labeled V (879.68), U (898.43) were assigned to Ce3+, while the peaks labeled V1 (883.5), U1 (916), were assigned to Ce4+. Ce3+ and Ce4+ could coexist near the XPS binding energy region of Ce3d, which confirmed the coexistence of Ce3+ and Ce4+ in NiAlCe-EG(c) sample [29]. The ratio of Ce3+/Ce4+ of NiAlCe-EG(c) sample was calculated to be 45% from the area of the XPS peaks. In Figure 3(b), the XPS spectrum of Ce3d of NiAlCe-EG(h) contained the following five peaks: V (880.13), V1 (885.03), U (899.45), U1 (903.31), U2 (916.27), where the peaks correspond to Ce 3d3/2 and Ce 3d5/2, respectively. The peaks labeled V (880.13), U (899.45), U1 (903.31) were assigned to Ce3+, while the peaks labeled V1 (885.03), U2 (916.27) were assigned to Ce4+. The ratio of Ce3+/Ce4+ of NiAlCe-EG(h) sample was calculated to be 55%.
Figure 4 was the XPS spectrums of Ce3d of NiAlCe-PEG(c) and NiAlCe-PEG(h), respectively. In Figure 4(a), the XPS spectrum of Ce 3d of NiAlCe-PEG(c) contained the following two peaks: V (878.52), U (898.43), corresponding to Ce 3d3/2 and Ce 3d5/2, respectively. The peak labeled V (878.52) was assigned to Ce3+, while the peak labeled U (898.43) was assigned to Ce4+, and the ratio of Ce3+/Ce4+ of NiAlCe-PEG(c) sample was calculated to be 38%. In Figure 4(b), the XPS spectrum of Ce 3d of NiAlCe-PEG(h) contained the following three peaks: V (878.49), U (897.24), U1 (916), corresponding to Ce3d3/2 and Ce3d5/2, respectively. The peaks labeled V (878.49), U (897.24) belonged to Ce3+, and the peak labeled U1 (916) assigned to Ce4+. The Ce3+/Ce4+ of NiAlCe-PEG(h) sample was calculated to be 43%.
Compared with NiAlCe-LDHs, the ratio of Ce3+/Ce4+ of which was calculated to be 24%, NiAlCe-EG samples and NiAlCe-PEG samples had larger Ce3+/Ce4+ values. NiAlCe-EG(h) and NiAlCe-PEG(h) samples had larger Ce3+/Ce4+ values than that of NiAlCe-EG(c) and NiAlCe-PEG(c) samples. Ce3+ ions acted as hole traps, thereby delaying the recombination of photogenerated electrons and holes [30]. Therefore, higher Ce3+/Ce4+ value had better photocatalytic performance.
The UV-Vis spectra of NiAlCe-LDHs prepared under different conditions was shown in Figure 5. All NiAlCe-LDHs samples showed absorption bands around 280 nm and 630 nm, indicating that the samples exhibited absorption in the visible and the ultraviolet region. According to the relationship between absorbance (A) and incident photo energy (hv), as well as the linear relationship between (Ahv)1/2 and (hv), the band gap (Eg) could be calculated [31]. The results were shown in Table 2. It was known that the narrow band gap favored to strong photo adsorption of the photocatalyst to generate electron-hole pairs, which could promote the photocatalytic reactions. Compared with the samples prepared by the co-precipitation method, thoses prepared by the hydrothermal method had smaller band gaps, which indicated that NiAlCe-10%EG(h) had better photocatalytic effect.
Table 2 The band gap of NiAlCe-LDHs samples
Samples
|
Eg(eV)
|
NiAlCe-10%EG(c)
|
1.610
|
NiAlCe-10%EG(h)
|
1.298
|
NiAlCe-15%PEG(c)
|
2.370
|
NiAlCe-15%PEG(h)
|
2.222
|
NiAlCe-LDHs
|
1.717
|
The FT-IR of NiAlCe-LDHs prepared under different conditions was shown in Figure 6. The absorption peak at 3200-3700 cm−1 represented the hydroxyl group between the LDHs sample laminated [32, 33]. The absorption peaks around 638 cm−1 and 1384 cm−1 corresponded to the bending vibration and expansion of CO32- in LDHs [34, 35]. The addition of EG and PEG affected the synthesis of LDHs by affecting the hydrogen bonding in the LDHs synthesis system. The addition of both solvents increased the content of hydroxyl groups in the system. This caused the release temperature of the crystal water in the LDHs sample to increase, thereby affecting the dehydration of the hydroxyl groups on the base layer, and the temperature of the CO32- releasing CO2 between the layers. The crystallization water with high hydration degree could be easily formed during the reaction process, thereby affecting the accumulation of the ordered structure of the hydrotalcite and the grain growth. But when EG was added too much, the role of hydration-hydrogen bonds was stronger. At this time, it was not conducive to the adjustment of the structure inside the hydrotalcite sample, reducing the regularity of LDHs.
3.2 Photocatalytic activity
Figure 7 showed the photocatalytic activity of NiAlCe-LDHs prepared under different conditions. As could be seen in Figure 7(a), the photocatalytic activity of NiAlCe-LDHs with 10% volume fraction of EG was significantly better than those with other volume fractions. The degradation rate of methyl orange could reach up to 100% in 42 min with NiAlCe-10%EG(c). NiAlCe-10%EG(h) sample showed better photocatalytic performance than NiAlCe-10%EG(c), which could degrade methyl orange in 36 min. As shown in Figure 7(b), the photocatalytic activity of NiAlCe-LDHs with 15% volume fraction of PEG was significantly better than those with other volume fractions. The degradation rate of methyl orange could reach up to 100% in 48 min with NiAlCe-15%PEG(c). Also, NiAlCe-15%PEG(h) sample showed better photocatalytic performance than NiAlCe-15%PEG(c), which could degrade methyl orange in 36 min.