The diffractogram of SBT, SBTF0.05, SBTF0.1, SBTF0.15 and SBTF0.2 compounds were shown in Fig. 1. All diffractograms indexed to SBT standard data (Joint Committee on Powder Diffraction Standard (JCPDS) No. 43–0973) have an orthorhombic crystal system with a space group of A21am. There is no additional peaks were identified on the diffractogram that indicated the absence of an impurity phase in all samples.
The crystallography data of all samples were obtained from the refinement of samples diffractograms that were processed by the Rietica software using the Le Bail method. The refinement process used SBT standard data (JCPDS No. 43–0973), with a space group of A21am. The refinement plot is depicted in Fig. 2, and the results of refinement are summarized in Table 1. The obtained residual profile (Rp) and residual weighted profile (Rwp) values are below 10. These data indicate that the diffractogram of the sample had good agreement with SBT standard data [30]. The trends of lattice parameters and cell volume are depicted in Fig. 3, and it can be seen that the lattice parameter values of a, b, c, and cell volume slightly decrease with increasing Fe concentration. It relates to the substitution of Ti4+ (ionic radii (r) = 0.068 nm) by Fe3+ (r = 0.064 nm), causing the shortening of B-O bond. The structural distortion can be determined from the orthorhombic distortion (b/a) depicted in Fig. 4. Trends show that the structural distortion becomes more orthorhombic with increasing Fe doping concentration which indicates the Fe's occupancy in the pseudo perovskite layer. It phenomenon give advantage for photocatalyst material because the increased structural distortion enhance the photocatalytic activity [31].
Table 1
Parameters | | Sample |
| SBT | SBTF0.05 | SBTF0.1 | SBTF0.15 | SBTF0.2 |
Crystal system | | Orthorhombic | Orthorhombic | Orthorhombic | Orthorhombic | Orthorhombic |
Space group | | A21am | A21am | A21am | A21am | A21am |
Z | | 4 | 4 | 4 | 4 | 4 |
a (Å) | | 5.4297(9) | 5.4273(4) | 5.4253(3) | 5.4224(6) | 5.4213(3) |
b (Å) | | 5.4383(7) | 5.4369(9) | 5.4355(5) | 5.4337(4) | 5.4329(4) |
c (Å) | | 40.9296(6) | 40.8896(9) | 40.8602(8) | 40.8551(2) | 40.8421(1) |
V (Å3) | | 1208.2830(8) | 1206.5904(5) | 1204.9549(6) | 1203.7663(6) | 1202.9528(6) |
b/a | | 1.00158 | 1.00177 | 1.00188 | 1.00208 | 1.00214 |
Rp (%) | | 8.30 | 8.50 | 8.66 | 9.17 | 8.95 |
Rwp (%) | | 6.85 | 6.45 | 7.32 | 8.68 | 8.10 |
GoF (X2) | | 1.142 | 1.043 | 1.217 | 1.320 | 1.408 |
$$D=\frac{K \lambda }{\beta Cos \theta }$$
1
The crystallite size of all the samples is shown in Fig. 5, which was calculated using the Debye Scherrer equation (Eq. 1) on the most intensive diffraction peak data of 2𝜃 (o) = 30.4 [33]. D is the crystal size (nm), λ is the wavelength of X-ray radiation with Cu k-α (1.5406 nm), K is the Scherer constant (0.9), β is the integration of the reflection peak area (FWHM, radians), and θ is diffraction angle at the highest intensity [32, 33]. The trend shows that increasing Fe concentration resulted in the decrease in crystalline size as result the shortening of the B-O bond caused by Fe dopant (ionic radii of Fe < Ti).
The Raman spectra of Fe doped SBT are shown in Fig. 6, and the obtained Raman vibration peaks have a similar pattern to that previously reported by Prasetyo et al [34]. Based on group theory, SBT with space group symmetry A21am have 141 Raman-active vibration modes ΓRaman = 36A1 + 35A2 + 34B1 + 36B2 [34, 36]. As shown in Fig. 6, the number of the identified peak are ten peaks that are less than predicted, which caused (a) peak overlapping and/or (b) very weak Raman vibration mode intensity [35, 36]. The Raman vibration modes of the Aurivillius compound can be classified into two groups: (a) the vibration peak below 200 cm− 1, which is related to the vibration modes of Bi2+ motion, and the relative motion of the bismuth layer to the pseudo-perovskite layer (RL mode), and (b) the vibration modes above 200 cm− 1 which is corresponding to the internal BO6 motion vibration [34]. The group of Raman vibration modes of Fe doped SBT are: (a) P1 (58 cm− 1) related to RL mode, (b) P2 (99 cm− 1), P3 (130 cm− 1), P4 (153 cm− 1) related to translational modes of cation-A/Sr, (c) P5 (268 cm− 1) related to BO6 bending and tilting, (d) P6 (341 cm− 1) related to BO6 tilting (combination bending and stretching BO6, (e) P7 (473 cm− 1) related to BO6 torsional mode, (f) P8 (559 cm− 1), P9 (724 cm− 1) related to BO6 stretching mode, and (g) P10 (864 cm− 1) related to symmetric BO6 stretching [34, 36]. The inset of Fig. 6 showed the slightly shifted vibration mode of P9 (BO6 stretching mode) that correlated to the local structural changes in TiO6 due to the replacement of Ti4+ by Fe3+.
SEM image of Fe doped SBT is shown in Fig. 7 and tt can be seen that all samples have a typical anisotropic plate-like morphology of the Aurivillius material [27]. All samples have non-uniform particle sizes, and there is no found agglomerated particles. It indicates that Fe dopant's addition did not affect the morphology as well as particle size of SBT.
Table 2
The band gap energy of Fe doped SBT.
Sample | Band gap energy | Wavelength |
(eV) | (nm) |
SBT | 3.00 | 414 |
SBTF0.05 | 2.34 | 530 |
SBTF0.1 | 2.01 | 617 |
SBTF0.15 | 1.87 | 664 |
SBTF0.2 | 1.76 | 705 |
The UV-Vis DRS spectra and the Tauc plot of all samples are shown in Fig. 8. The band gap energy of samples is summarized in Table 2, and obtained that increasing concentration of Fe dopant caused the decreasing of band gap energy. The existence of Fe dopant was also revealed to red-shifted absorption to the wider visible light range (530–705 nm). The decrease in band gap energy was due to the formation of a new conduction band from Fe-3d metal dopant, which replaces some sites of Ti-3d metal in the Aurivillius host structure as results the electronic transition mechanism changes (Bi-6s + O-2p (VB) to Ti-3d (CB) orbitals changes to Bi-6s + O-2p (VB) to Fe-3d (CB)) [15, 17].