Growth of sillenite Bi12FeO20 single crystals: structural, thermal, optical, photocatalytic features and first principle calculations

Ideal sillenite type Bi12FeO20 (BFO) micron sized single crystals have been successfully grown via inexpensive hydrothermal method. The refined single crystal X-ray diffraction data reveals cubic Bi12FeO20 structure with single crystal parameters. Occurrence of rare Fe4+ state is identified via X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The lattice parameter (a) and corresponding molar volume (Vm) of Bi12FeO20 have been measured in the temperature range of 30–700 °C by the X-ray diffraction method. The thermal expansion coefficient (α) 3.93 × 10–5 K−1 was calculated from the measured values of the parameters. Electronic structure and density of states are investigated by first principle calculations. Photoelectrochemical measurements on single crystals with bandgap of 2 eV reveal significant photo response. The photoactivity of as grown crystals were further investigated by degrading organic effluents such as Methylene blue (MB) and Congo red (CR) under natural sunlight. BFO showed photodegradation efficiency about 74.23% and 32.10% for degrading MB and CR respectively. Interesting morphology and microstructure of pointed spearhead like BFO crystals provide a new insight in designing and synthesizing multifunctional single crystals.

www.nature.com/scientificreports/ solar spectra making them photoactive with potential applications in water splitting and dye degradation under visible light 12,13 . Not much research has been carried out on related ideal sillenite type Bi 12 FeO 20 compound. In this communication, to the best of our knowledge, we report for the first time a spearhead like ideal sillenite structured Bi 12 FeO 20 single crystals . Non-ideal Fe based Bi-rich sillenite materials, such as crystallites of Bi 12 Fe 0.63 O 18.945 and Bi 25 FeO 40 single crystals, have been investigated before 14,15 . An ideal Sillenite structure of polycrystalline Bi 12 FeO 20 , synthesized at elevated temperatures by typical solid state reaction method, has also been reported by Elkhoun et al 16 . First report on low temperature synthesis of ideal structured Bi 12 FeO 20 single crystals with rare Fe 4+ state via inexpensive hydrothermal process is revealed here, along with systematic characterization and application related studies.

Experimental
Crystal growth. High pure Bi(NO 3 ) 3 .5H 2 O and Fe(NO 3 ) 3 .9H 2 O as starting precursors and 0.2 M concentration each were dissolved in 50 ml of deionized water. Few drops of HNO 3 were added to get clear transparent solution. After vigorous stirring, 50 g of KOH was added and the temperature of solution was cooled down to room temperature before transferring it into a 100 mL Teflon-lined autoclave, upto 70% of its maximum capacity. Crystallization takes place at 200 °C for 72 h. Post this, the autoclave was cooled and depressurized, product was washed with distilled water, sonicated and then the sample was harvested as fine, reddish brown crystals.
Characterization. For structural analysis, Single crystal X-ray diffraction (SC-XRD, Bruker Kappa ApexII) was performed and the lattice parameters were obtained by refining Single Crystal XRD data by SHELXTL refining software 17,18 . High Resolution transmission electron microscopic (HRTEM) images and selected area electron diffraction patterns (SAED) were recorded on an FEI Tecnai TF-20 operating at 200 kV. X-ray photoelectron spectroscopic (SPECS GmbH, Germany) measurements were conducted to investigate the Fe and Bi oxidation states. X-ray absorption spectroscopy (XAS) measurements were carried out to examine the valence state of Fe in Bi 12 FeO 20 (BFO). The experiments were performed at the Energy-Scanning EXAFS beamline (BL-9) at the Indus-2 Synchrotron Source (2.5 GeV, 200 mA), Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India 19, 20 . Synchrotron based powder X-ray diffraction measurements. Synchrotron based powder X-ray diffraction measurements were carried out on well ground powder samples of Bi 12 FeO 20 single crystals at Extreme Conditions X-ray diffraction (EC-XRD) beamline (BL-11) at Indus-2 synchrotron source, Raja Ramanna Centre for advanced Technology (RRCAT), Indore, India. High temperature measurements were carried out on STOE high temperature attachment 0.65.3 with Eurotherm 2416 controller. Desired wavelength (0.6285 Å) for ADXRD diffraction experiments was selected from white light from the bending magnet using a Si(111) channel cut monochromator. The monochromatic beam is then focused on to the sample with a Kirkpatrick-Baez mirror or K-B mirror. A MAR345 image plate detector (which is an area detector) was used to collect 2-dimensional diffraction data. Sample to detector and the wavelength of the beam were calibrated using NIST standards LaB 6 and CeO 2 . Calibration and conversion/integration of 2D diffraction data to 1D, intensity vs 2θ, was carried out using FIT2D software 21, 22 . Electronic structure calculations. The density functional calculations are carried out under full potential linear augmented plane wave (FP-LAPW), as implemented in Wien2K 23 . The modified Becke-Johnson parameterization is used as an exchange correlation function 24 . The unit cell is divided in muffin tin region (with R mt as radius) and interstitial region (IR). The muffin tin radii for bismuth, iron and oxygen atoms are chosen in such a way that there is no overlap among different atomic elements. The plane wave cut off parameters R mt × K max = 7 and G max = 12 are used for structural, electronic and optical properties of Bi 12 FeO 20 . The maximum value of l (l max ) is considered 10 and cut-off energy is at -7.0 Ry, defining the separation between the core and valence states. The self-consistent calculations are carried out under total energy convergence of∼ 0.001 Ry. A large plane wave cut-off of 150 Ry is used throughout the calculation and initially 125 K-points are considered in Brillouin zone for optimization while 1000 K points are used for computing the other properties like electronic and optical properties of the material.

Results and discussion
Single crystal X-ray diffraction data for Bi 12 FeO 20 (BFO) crystals were measured at 296 K using a Bruker Kappa Apex II with a wavelength of MoKα radiation of 0.7107 Å. The structure was refined using SHELXTL refining software 17,18 . The refined XRD pattern and crystal structure is shown in Fig. 1a. The lattice parameters of BFO are a, b and c = 10.1713(10) Å and α, βand γ = 90° and the cell volume is V = 1052.28(3) Å 3 . Single crystal structure refinement data is presented in Table S1 and the refined data indicates the formation of Bi 12 FeO 20 with body centered cubic crystal system and I23 space group. The optimization of crystal structure is performed and the calculated variation of volume versus energy is shown in Fig. S1. A clear minimum noticed at − 523,513.3 eV, correspond to the minimum energy structure. The computed lattice parameter is a = 10.2707 (19.4154 Bohr) Å, and is in good agreement with experimental refined XRD data. A possible growth mechanism of spearhead like BFO crystals based on microscopic revelations has been illustrated and represented with a schematic in Fig. 3. SEM images recorded at various stages of crystallization of BFO proves that the growth mechanism involves various steps starting from nucleation to single crystal formation.  www.nature.com/scientificreports/ During the hydrothermal growth at applied temperature and autogenerated pressure, supersaturation takes place and initiates nucleation in the precursor solution. An increase in growth time tends to aggregate the nuclei to form clusters. This step is followed by secondary aggregation of clusters initiating oriented growth which is desirable in single crystal. At an optimized growth time and temperature complete growth of crystal takes place. XPS survey spectrum in Fig. 4a, calibrated using carbon binding energy of 284.6 eV, indicates the presence and oxidation states of constituent Bi, Fe and O elements in Bi 12 FeO 20 single crystals. XPS has been extensively used to determine the oxidation state of b-site cation, which is expected to possess tetravalency in an ideal sillenite structured Bi 12 FeO 20 . In Fe-based non-ideal sillenite structure such as Bi 12     Sillenites are also referred to as pyroelectric materials, which can enable them to generate voltages when they experience heat energy. Hence, these materials can be used for thermal sensing devices 36 and determination of thermal properties of such materials can give new insights into their behavior. Thermal expansion of these materials can be analyzed by finding the coefficient of thermal expansion / thermal expansivity (α). Temperature dependent XRD is the one of the accurate methods for determining the molar volume (V m ) of a solid because thermal expansion can be calculated per unit cell (atomic level). In the present work, we have attempted to find the thermal expansivity of sillenite BFO using ADXRD within a temperature range of 30-700 °C shown in Fig. 6.
Thermal expansion coefficient (α) is the function of change in molar volume (V m ) with respect to temperature, as shown below where α is the thermal expansion coefficient, V m is the molar volume and T is the absolute temperature. The value of 'α' can be determined from the temperature dependent values of V m 37 . Experimentally, this can be done by using temperature dependent XRD to measure the unit cell parameters at various temperatures. For cubic crystal structure materials, the molar volume V m is defined as where a is the lattice parameter, N A is the Avogadro's number (6.022 × 10 23 ) and Z is the number of formula units per unit cell for body centered cubic cells (Z = 2). Cell parameters, as for example d-spacing (d) and lattice constant, were calculated by Reitveld refinement for XRD patterns recorded at various temperatures. Refined XRD pattern is shown in the supporting information (Figs. S2-S16).
The (1) www.nature.com/scientificreports/ From the temperature dependent XRD, we found an increase in lattice parameters and shifting of 2θ value towards lower degree with respect to increase in temperature from 30 °C to 700 °C (Fig. 6). Upon refining the XRD pattern, lattice parameters at corresponding temperatures were calculated. The measured 'a' value was substituted in Eq. (2) and then molar volume (V m ) was calculated. The measured 'a' value as the function of temperature is shown in Fig. 7a.
From the fitting of Eq. (3), volumetric changes with respect to temperature was calculated (Fig. 7b) and substituted in Eq. (1). The coefficient of thermal expansion (α) was found to be 3.93 × 10 -5 K −1 . Differential scanning calorimetry (DSC) measurements (Fig. S18) (in order to determine the phase transition states) were performed up to 1000 °C. Major phase transitions were observed at 770 °C and 827 °C. The endothermic peak at 770 °C and 827 °C might be caused by partial and complete decomposition of the material. Transition above 900 °C is due to melting of decomposed materials.
The absorption spectra of BFO along with the corresponding Tauc plot (inset Fig. 8a) reveal an effective optical bandgap of 2 eV, which falls under visible region, satisfying a major requirement for photoactive applications like photocatalysis in degradation of organic dyes and water splitting etc.
Any possibility of emission of photons of characteristic wavelengths from ideal Bi 12 FeO 20 sillenite single crystals can also be probed by Cathodoluminescence (CL) (Fig. 7b) under high-energy electron bombardment. In CL the excitation source can be focused to a probe in an electron microscope providing us with luminescence  www.nature.com/scientificreports/ information having spatial resolution orders of magnitude higher as compared to other techniques. CL on perovskite materials can provide evidence regarding the photoactivity of these materials as it has the potential to resolve emission characteristics in nanoregime 38 . The spatial distribution of emission recorded at 300 K reveals that the entire Bi 12 FeO 20 crystal radiates predominantly in the visible region. CL spectrum in Fig. 8b obtained at 20 kV for as grown Bi 12 FeO 20 single crystals, reveals strong band edge emission at about 600 nm (2.04 eV) which is in good agreement with the measured bandgap of BFO single crystals. Further, we compute the electronic band structure shown in Fig. 9. The electronic band structure clearly suggests that Bi 12 FeO 20 is a wide bandgap indirect semiconductor. The valence band maxima and conduction band minima lie at Γ and H in Brillouin zone with bandgap ~ 3.17 eV, as marked with a red dashed arrow (Fig. 9). The mismatch in experimental and calculated bandgap values is ascribed to superposition. However, it is known that DFT calculations do not reproduce the bandgap values.
The computed total and partial density of states are plotted in Fig. 10. We find that valence band consists of oxygen (O) s and p orbitals mainly with partial contribution from iron(Fe) s, p and d atomic orbitals, whereas bismuth(Bi) p and oxygen(O) s orbitals contribute mainly to the conduction band. Iron is coordinated as FeO 4 tetrahedra in Bi 12 FeO 20 lattice, causing the crystal field splitting in t 2g (d xy , d xz and d xy ) and e g (d x2-y2 and d xy ) orbitals, Fig. 10. The tetrahedral splitting difference is about 2.27 eV with e g orbitals lying inside the valence band and t 2g orbitals lying within the bandgap, giving rise to the intra band states near 2.13 eV, with very large contribution to the density of states. We also computed the absorption spectra of single crysatals (Fig. S18). The large absorption coefficient ~ 10 4 cm −1 is noticed with a small peak ~ 2.13 eV, superimposed with bandgap absorption. This is attributed to the presence of iron intra band states within the bandgap, as noticed in partial density of states, and in experimentally recorded optical absorption spectra of BFO.
Being a light sensitive compound, sillenite BFO has the potential to act as a photoactive material. A detailed analysis of photoresponse as well as PEC (Photoelectrochemical) measurements can help us to evaluate its photoactive behavior. In the PEC measurement setup, the working electrodes were prepared as follows: BFO crystals were ground to fine powder which was used to form slurry using distilled water. The slurry was coated on FTO and the photoresponse of BFO sample was measured under Xenon lamp source (100 W/cm 2 , AM1.5). Dark and light current measurements at fixed potential (0.5 V) reveal significant photoresponse with proper ON/OFF response observed by chronoamperometry studies Fig. 11a,b. Impedance spectroscopic measurements under dark and light illumination conditions on the sample showed a rapid decrease in impedance under light   www.nature.com/scientificreports/ illumination Fig. 11b. The results imply an efficient and rapid separation of photogenerated charge carriers under light irradiation which leads to photoconductivity as well as a rapid decrease in impedance of the sample. A direct evidence of photocatalytic activity of as synthesized BFO crystals was found from dye degradation studies carried out with fine ground powder of BFO. BFO was tested to degrade of methylene blue (MB) and Congo red (CR) organic dyes under direct sunlight irradiation. The initial dye concentration of MB and CR was kept at C o : 3.5 mg/L and 10 mg/L respectively. The catalyst loaded dye solution was thoroughly blended using ultra sonication under dark condition. The experiment was conducted for different intervals of time under natural sunlight and dyes were found to readily degrade. The sunlight driven degradation of both MB and CR was recorded at regular intervals using UV-visible absorption spectroscopy. After 3.5 h of sunlight exposure MB and CR degraded by 74.23% and 32.10% of their initial concentration respectively. The degradation profile is shown in Fig. S17 along with C/C 0 ratio graphs and photocatalytic reaction kinetics of MB and CR in Fig. 11c,d. The degradation rate of MB by BFO was found ~0.392 h −1 , whereas for CR it was ~0.098 h −1 . Photodegradation kinetics MB and CR in presence of BFO are compared with few reported semiconductor nanoparticles based photocatalyst. BFO showed better performance as compared to some of the existing semiconductor photocatalysts [as tabulated in Table S2] pointing towards the efficacy of BFO as a good photocatalyst material for organic effluent treatment.

Conclusion
Ideal sillenite spearhead type Bi 12 FeO 20 single crystals were grown by hydrothermal method for the first time with an average size of ~ 1 mm. The refined single crystal XRD data along with supporting XPS analysis confirms the crystallinity and presence of Bi 3+ and rare Fe 4+ oxidation states in BFO respectively. The thermal expansion coefficient was calculated from temperature dependent XRD studies. The bandgap energy from the optical absorption data was found to be ~ 2 eV and the electronic structure was also investigated using first principle calculations. The photoactivity of as grown crystals was proved beyond doubt by PEC measurements, revealing significant photoresponse. CL and photodegradation studies of prepared BFO crystals revealed the luminescence and photocatalytic behavior respectively with promising applications.