Bi0.9Ho0.1FeO3/TiO2 Composite Thin Films: Synthesis and Study of Optical, Electrical and Magnetic Properties

A visible light active Bi0.9Ho0.1FeO3 nanoparticles/TiO2 composite thin films with different mol.% of Bi0.9Ho0.1FeO3 were successfully prepared via non-aqueous sol-gel method. The incorporation of 5, 10 and 20 mol.% Bi0.9Ho0.1FeO3 nanoparticles in the precursor solution of TiO2 brings modifications in the functional properties of the composite thin films. XPS analysis indicates that interdiffusion of Fe3+, Ho3+, Bi3+/Ti4+ ions through the interfaces between Bi0.9Ho0.1FeO3 nanoparticles and TiO2 matrix reduces the concentration of Ti3+ ions. X-ray diffraction analysis affirms that TiO2 and Bi0.9Ho0.1FeO3 retain anatase and orthorhombic phase respectively in composite films. The composite thin film containing 20 mol.% Bi0.9Ho0.1FeO3 nanoparticles exhibits the most prominent absorption phenomenon in visible region and has significantly reduced indirect band gap of 2.46 eV compared to that of pure TiO2 (3.4 eV). Hall effect measurements confirm that the resistivity of composite film increases by ∼2.33 orders of magnitude and its carrier concentration decreases by 1.8 orders of magnitude at 5 mol.% Bi0.9Ho0.1FeO3 nanoparticles addition compared to those of pure TiO2 film. Moreover, the pure film exhibits diamagnetism, whereas the composite films have both large ferromagnetic and small diamagnetic components. The findings in this research justify that the composite film can be a potential candidate for making improved photocatalyst, resistors and spintronic devices.


XPS analysis
Figure S1. XPS spectra of (a) Fe 2p and (b) O 1s core levels of the BHFO nanoparticles Figure S1 depicts the core level spectra of Fe 2p and O 1s of BHFO nanoparticles. The XPS spectra clearly indicates that peak intensity of Fe is relatively lower than that of O' s peak intensity. That is why Fe 2p peaks near 720 eV were not easily detectable by XPS in T 80 B 20 for its low intensity.

XRD analysis
Generally, pure BiFeO3 (BFO) crystallizes in a rhombohedrally distorted perovskite structure, which belongs to the space group R3c. The perovskite structure of BFO transforms from rhombohedral (R3c) to tetragonal (P4 mm) or orthorhombic (Pnma) or cubic (Pm-3m) phase depending on the nature of the A site and B site doping. 1 2,3 As a result, diversified structural reports are available for Bi0.9Ho0.1FeO3 (BHFO) in which the neighboring peaks such as (012), (110)/(104), (006)/(202), (116)/(112) and (018/214) are merged together. 4,5 In current study, it can be appreciated by observing the XRD patterns in Fig. S2 that substantial structural modification has occurred in the Ho substituted BFO nanoparticles. Magnified portions of the XRD patterns (see Fig. S3) for the ranges of 2=21.5°-23° and 2=30°-34° there is partial merging of (110)/(104) peaks, which immediately indicate a possible structural transition in the synthesized BHFO nanoparticles. This possible merging due to ion doping has been reported in previous studies as an indication of rhombohedral to orthorhombic or tetragonal phase transition. 6 XRD patterns collected for all the sample have been fitted by the Rietveld method using FULLPROF. 7 Rietveld refinement was carried out for rhombohedral, tetragonal, triclinic, monoclinic and orthorhombic structure. The Goodness of Fit (GoF) of our sample with orthorhombic structure (space group -Pnma) and with a second phase Bi2Fe4O9 (space group-Pbam) has been found to be ~1.44 indicating the reliability of the refinement. Indeed, rare earth metal doped BFO crystallizes with orthorhombic structure. 4,5 . Figure

EDX studies
EDS analysis and elemental mapping were performed to gain insight on spatial distribution of elements, as shown in Fig. S4. Although the EDS spectrum and elemental mappings in Fig S4. b-g are exhibiting good homogeneous distribution of the elements (Ti (green), Bi (blue), Ho (pink), Fe (yellow), and O(red)), this homogeneous distribution could be attributed to large interaction volume produced during the EDS operation. An incident electron beam of 15 kV utilized during this operation was expected to penetrate a depth of 1 to 2 m into the film in all directions. The percentage of these elements has increased in T 80 B 20 film as the mol.% of BHFO has also increased.

Optical studies
The positions of conduction and valence band edge of semiconductors were calculated using electronegativity which is the geometric mean of the electronegativity of the constituent atoms: Where  , q and N are the electronegativity of the constituent atom, the number of spices, and the total number of atoms in the compound respectively. Herein the electronegativity of an atom is the arithmetic mean of the atomic electron affinity (EA) and the first ionization potential (FIP):

=5.86
Similarly the electronegativity TiO2 is found to be 5.81. Figure S5. Diamagnetic susceptibility of films with different mol. % of BHFO nanoparticles