Al 도핑된 BiFeO3 분말의 국부 구조에 대한 싱크로트론 X선 흡수 분광법 연구

여기서 k _최대 그리고 k _분 변환된 k의 최대값과 최소값입니다. 공백, 각각 χ (R)은 해닝 윈도우 함수, ω (카 )는 가우스 창 함수이고 k ⁿ 가중 계수(n =0, 1, 2, 3). 카 ₂ 그리고 k ₃ BFA_{x 값} O는 각각 2와 10이고 n의 값은 2. Fe K의 푸리에 변환 -BFA_{x의 가장자리 EXAFS} 그림 4a와 같이 O 샘플이 수행됩니다. ~ 1.503 Å에 중심을 둔 비대칭 피크(첫 번째 점선으로 둘러싸여 있음)는 산소 음이온의 산란으로 인해 Fe-O 결합으로 식별됩니다. ~ 3.527 Å 부근의 두 번째 강한 피크(두 번째 점선으로 둘러싸여 있음)는 Fe-Fe/Al 결합에 해당하며, 이는 가장 가까운 이웃 Fe/Al 원자로부터 산소 음이온의 산란으로 설명될 수 있습니다. 조개. 첫 번째 및 두 번째 배위 껍질에 대한 거리는 그림 4의 표 3에 요약되어 있습니다. 피크 위치에서 도핑되지 않은 BFO 샘플과 비교하여 Fe-O 및 Fe-Fe/Al 결합이 더 작은 쪽으로 약간 이동하는 경향이 있습니다. R x가 증가하는 값 . 이것은 Al 이온에 의한 도핑이 중심 Fe 원자의 가장 가까운 이웃 국부 구조에 영향을 미칠 뿐만 아니라 Fe 원자의 다음으로 가장 가까운 배위 껍질에도 영향을 미친다는 것을 나타냅니다. 반면, Fe-O 결합이 더 작은 R로 이동 값은 Al 이온의 반경이 Fe 이온의 반경보다 작기 때문에 발생할 수 있습니다. 이는 XRD 데이터와 일치합니다. Fe-Fe/Al의 작은 R로의 이동 값은 또한 평균 Fe-Fe/Al 결합 길이를 나타냅니다(여기서 두 Fe ³⁺ 이온은 인접한 산소 팔면체의 중심에 있음)이 점차 짧아지고 결합 각도가 Al 도핑된 샘플에서 수정됩니다. 그러나, 도핑된 샘플의 경우 Fe-O 분포의 피크 강도가 약간 증가하는 반면 Fe-Fe/Al 분포의 피크 강도는 거의 변하지 않습니다. 이것은 Fe-O의 철 이웃 구조가 Al 도핑에 의해 수정되었음을 나타냅니다. Al 도핑된 샘플에서 더 짧은 Fe-Fe/Al 결합은 XRD의 주요 피크(101)가 더 높은 2θ로 이동하는 이유를 설명할 수 있습니다. 각도(그림 1b 참조). 이것은 Fe를 Al로 대체하면 산소 팔면체에 영향을 줄 수 있으며, 이는 인접한 두 Fe 원자 사이의 배위 거리를 더욱 감소시킬 수 있음을 나타냅니다. 그림 4b는 Fe K를 보여줍니다. -BFA_{x의 가장자리 EXAFS} k에 처리된 샘플 O ³ × χ (카 ) oscillation with a k space of 0–10 Å ⁻¹ . As can be seen, all the k ³ × χ (카 ) spectra show similar patterns at the smaller k values but different at larger k values with some various noise (surrounded by the dash line). Al-doped samples show a broader k ² × χ (카 ) spectrum than those of undoped BFO in the k space of 8.2–9.3 Å ⁻¹ , implying an enhanced short-range structural disorder in BFA_x O samples. The noises are observed in a k space of 10–10.4 Å ⁻¹ . These changes indicated that the local structure of center atoms has changed due to B-site Al doping, similar to what was reported by Li et al. [51].

아 Fourier transforms of Fe K -edge k ³ -weighted EXAFS data, for the BFA_x O (0 ≤ x ≤ 0.1). ㄴ EXAFS χ (카 ) × k ³ spectra of BFA_x O (0 ≤ x ≤ 0.1). The spectra were aligned along the Y -axis for better comparison

The peak positions, intensities, and shapes of the line in the Bi L ₃ -edge XANES spectrum are well known to depend on the local electronic structure of the Bi atoms, which could provide information on the Bi valence. The full spectra of Bi L ₃ -edge XANES of undoped BFO and BFA_x O samples are also shown in Fig. 5a, while Fig. 5b shows the Bi L ₃ -edge XANES spectrum in the energy region of 13,400–13,480 eV. The analysis of these spectra helps to investigate the local electronic structure of Bi ions in the doped system. From Fig. 5b, it can be seen that the shape of all spectra are the same to each other and there is almost no change of absorption edge in the whole series. The absorption edge energies for our study are found to be 13,429.1 eV, 13,429.4 eV, 13,429.3 eV, 13,429.3 eV, and 13,429.8 eV for x =0, 0.025, 0.05, and 0.1 and the reference compound Bi₂ O₃ , respectively (see Table 4 in Fig. 5). The absorption edges slightly shift toward higher energies with increasing Al concentration. The absorption edge in the Bi L ₃ -edge of the BFA_x O samples matches well with that of the reference compound Bi₂ O₃ , which indicates that the valence state of Bi ions in all the samples is in + 3 valence state. However, there are two post-edge peaks found in all samples and marked as B1 and B2, respectively. These two post-edge peaks are caused by the electric-forbidden transition from 2p _3/2 level to the 6d ones. Compared with undoped BFO, the intensity of peak B2 can be clearly seen to increase for the doped samples (see Fig. 5c), which means the transition from 2p _3/2 to 6d state increases, so does the energy of 6d state. Except these, there is no other significant change in the whole spectrum.

아 Bi L ₃ -edge XANES spectra of BFA_x O (0 ≤ x ≤ 0.1) and reference Bi₂ O₃ . ㄴ XANES spectrum in the range of 13,400–13,480 eV. ㄷ Enlarged view of the peak D and peak E

Fourier transform of Bi L ₃ -edge EXAFS radial distribution functions is performed, as shown in Fig. 6a. A high-intensity peak located at around 1.618 Å corresponds to the nearest Bi-O coordination shell (surrounded by the dash line), which is a result from scattering from the nearest-neighbor atomic shell of Bi, i.e., oxygen anions. However, the position of Bi-O bond shifts toward larger R values for the doped samples (see Table 5 in Fig. 6). This indicates that the substitution of Al for Fe could affect the nearest-neighbor local structure of the central Bi atom. It also indicates the extension of Bi-O bond length. The peak intensity of the Bi-O distribution exhibited a small increase with increasing Al content, which suggests that the iron-neighboring structure of Bi-O has changed. Figure 6b shows the Bi L ₃ -edge k ³ × χ (카 ) EXAFS spectra with a k space of 0–14 Å ⁻¹ . From Fig. 6b, it can be seen that all the spectra shape shows similar patterns except some error noises. The error noises are observed in the k space of 12–14 Å ⁻¹ (surrounded by the dash line). This result may imply that the k ³ × χ (카 ) EXAFS function of the center Bi atoms has changed with Al doping. This also suggests that the B-site Al substitution influences short-range structural disordering.

아 Fourier transforms of Bi L ₃ -edge k ³ -weighted EXAFS data, for the BFA_x O (0 ≤ x ≤ 0.1). ㄴ EXAFS χ (카 ) × k ³ spectra of BFA_x O (0 ≤ x ≤ 0.1). The spectra were aligned along the Y -축

The optical properties of the samples are studied by using RT UV-Vis, which is used to characterize the optical properties of the materials. The UV-Vis absorption spectra of undoped BFO and BFA_x O samples in the wavelength of 300–800 nm are shown in Fig. 7a. As a result, the UV-Vis spectra of the undoped BFO and BFA_x O samples show two absorption edges (marked by dashed arrows). One is a band around 650 nm, which is due to the metal-to-metal transition. The other is a band around 760 nm, which is caused by crystal field transition [52]. In addition, the strong absorption band is observed at about 490 nm (marked by dashed arrow), which is attributed to the electronic transition from O 2p to Fe 3d state in the BFO. These strong bands indicate that the BFO prepared by hydrothermal method could be a promising visible-light photocatalytic material. BFO is of direct transition with a value of n as 2. The absorption edge of the doped samples shifted from 659 to 619 nm, suggesting that the BFA_x O powders absorb visible light in the wavelength range of 600–659 nm (see Fig. 7a). A similar blue-shift phenomenon was observed earlier in other element-doped BFO [53,54,55]. This blue shift in the absorption spectra of Al-doped samples in comparison with the undoped BFO shows that doping Al causes a change in the local structure for BFO. From Fig. 7a, one can see that the absorption spectra of the Al-doped samples exhibit a sharp increase around 490 nm and it suggests that all samples can absorb remarkable amounts of visible light. For the sample with x =0.025, the absorption spectrum shows a sudden increase. It means that it has a wider absorption range than the other samples in this range of visible light. The optical band gap of the samples has been calculated by Tauc’s formula, as follows:

아 UV-Vis absorption spectrum of BFA_x O (0 ≤ x ≤ 0.1). ㄴ Plots of (ahν ) ² vs. photon energy. ㄷ 이 g values as a function of Al concentration

여기서 a is the absorption coefficient, A is the parameter, h is the Planck’s constant, m is the frequency of the incident photon, E g is the optical band gap, and n (for direct n =2, for indirect n =0.5) is a constant associated with different types of electronic transitions, as shown in Fig. 7b. 계산된 E g values are found to be 1.833 eV, 1.888 eV, 1.866 eV, and 1.905 eV for x =0, x =0.025, x =0.05, and x =0.1 samples, respectively. It is easy to see that the band gap increases with the substitution ratio, as shown in Fig. 7c. The increase in the band gap is attributed to the doping effect. E g value for the undoped BFO is about 1.833 eV, which is lower than the previous reports [56, 57].