Structural characterization of cuboidal α-Fe2O3 nanoparticles synthesized by a facile approach

α-Fe2O3 nanoparticles were synthesized using Tabebuia aurea leaf extract by a facile approach. The signature peaks for Fe and O in the EDX spectrum verified the formation of Fe2O3 nanoparticles. Cuboidal-shaped nanoparticles were observed in the FE-SEM image. In the XRD pattern, it was observed that the peaks belong to α-Fe2O3 nanoparticles. These particles were pure and crystalline with an average particle size of 25.69 nm. The signals at 538 and 494 cm−1 in the FTIR image confirmed the formation of hematite nanoparticles. BET analysis showed a comparatively greater surface area (31.03 m2/g) than the commercial α-Fe2O3 nanoparticles, and the pores were mesoporous. XPS analysis confirmed the existence of α-Fe2O3 by showing the specific oxidation states for iron and oxygen at 710.34 and 529.67 eV, respectively. The saturation magnetization value of 13.97 emu/g confirmed the superparamagnetic nature. The TGA, which determined the thermal stability of the nanoparticles, reported a total weight loss of 12.75%. Hence, the highly crystalline, pure, mesoporous, superparamagnetic α-Fe2O3 nanoparticles with high surface area synthesized using T. aurea leaf extract can be potentially applied in diverse fields.


Introduction
Nanotechnology is the field of technology that deals with the study, application, and manufacture of materials on a nanoscopic scale, often between 1 and 100 nm. The key characteristics of these particles are their large surface to volume ratio that is responsible for their extensive usage in medicinal, optical, biotechnological, microbiological, electronics, environmental, and other branches of engineering and materials science (Dihom et al. 2022).
IONPs are synthesized using several methods, among which physical and chemical methods are the most common (Gottimukkala et al. 2017). The synthesis of IONPs requires certain strong chemicals that are necessarily reducing agents, which can lead to a severe impact on the environment. Hence, it is very essential to look for a reliable, efficient, and ecological method for the synthesis of nanoparticles (Al-Buriahi et al. 2022). The method adopted should be economically reasonable, sustainable, and cheap. Also, the availability of raw materials and natural resources are the factors that have to be given extreme importance while choosing a method (Saif et al. 2016).
Recently, researchers are developing methods that are effective, reliable, and pose minimal or zero impact on the environment. Various organisms such as algae, yeast, plants, and parts of plants act as the source to produce stable and well-functioning nanoparticles (Kamaruzaman 2022). However, biomass-based synthesis is expensive and consumes more time (Mukherjee et al. 2006).
Unlike the biomass-based process, the green synthesis based on plant extract is rapid, inexpensive, and simple (Hernandez 2022). Specific metabolites of plants such as proteins, phenolic compounds, alkaloids, polyphenols, terpenoids, acids, and sugars play an essential part in reducing metallic ions and yielding nanomaterials (Makarov et al. 2014). These metabolites also stabilize nanoparticles by capping them. The increase in the reports on the production of nanomaterials based on plant extracts indicates that this approach is particularly prevalent .
Tabebuia aurea is an ornamental tree belonging to Tabebuia genus which is common in tropical and subtropical areas. The Caribbean trumpet tree, tree of gold, and silver trumpet tree are common names for T. aurea. They are deciduous trees that have bright yellow flowers and grow to 8 m tall in the dry season. Silver nanoparticles were synthesized using T. aurea leaf extract (Prathibha and Priya 2017). No report on the synthesis of IONPs using T. aurea has been found in the literature. Therefore, the key focus of the current investigation is to synthesize α-Fe 2 O 3 nanoparticles using T. aurea leaf extract and characterize them using various techniques.

Materials
Analytical-grade chemicals were utilized for all the experimentations and procured from Merck, India. Iron (II) sulfate heptahydrate (FeSO 4 .7H 2 O) was used as a precursor and sodium hydroxide was used to maintain the pH of the system. T. aurea plant leaves were collected from Manipal Campus, India.

T.aurea leaf extract (TALE) preparation
The collected T. aurea leaves were thoroughly rinsed with tap water and further with distilled water. They were air dried, made into small pieces, and kept in a plastic cover before any further use. 30 g leaves were added to 1 L distilled water and heated for 1 h at 80 °C. The dark brown filtrate was cooled, named TALE, and stored in a fridge.

Preparation of TA-Fe 2 O 3 nanoparticles
TALE and freshly prepared FeSO 4 .7H 2 O were combined in 1:1 volume at room temperature. The mixture completely changed to a dark hue indicating the formation of nanoparticles. After increasing the pH to 13 using 2N NaOH, the contents were incubated at 80 °C for 40 min (Vinayagam et al. 2020). The formation of black color colloidal suspension confirmed the synthesis of TA-Fe 2 O 3 NPs, which were extracted subsequently from the solution using a powerful neodymium magnet (Fig. 1). The obtained contents were dried at 100° C for 1d and kept in a vial.

Characterization
The prepared TA-Fe 2 O 3 NPs were examined by measuring the absorbance of plant extract and the nanoparticle colloidal suspension by employing a UV-Vis spectrophotometer (SHIMADZU-UV1700). Field emission scanning electron microscope (FE-SEM) (Carl ZEISS, Sigma, Oxford Instruments) was employed to analyze the topological properties of TA-Fe 2 O 3 NPs. The porous nature and specific surface area were determined by Brunauer-Emmett and Teller apparatus (Smart Instruments, Mumbai). The X-ray diffraction (Rigaku Miniflex 600) pattern examined the crystallinity. X-ray photoelectron microscopy (PHI 5000 VersaProbe III, Physical cryotonics) analyzed the oxidation state of the components. The vibrating sample magnetometer (VSM 8600 series, Lakeshore cryotonics, USA) analyzed the magnetic property of TA-Fe 2 O 3 NPs. For studying chemical bonding in TA-Fe 2 O 3 NPs, Fourier transform infrared spectroscopy (FTIR-SHIMADZU-8400S) was employed. The thermal stability of TA-Fe 2 O 3 NPs was analyzed using thermogravimetric analysis (TGA55 (0550-0543) equipment) at a temperature range 33-800 °C in an N 2 surrounding at 10 °C per min heating rate.

Zero point of charge (ZPC)
The pH needed to provide zero net charge on the surface of the nanoparticles is referred to as zero point of charge (ZPC). The surface charge on the nanoparticles is neutral at ZPC (Qin et al. 2015). The nanoparticle surface is negatively charged at pH > ZPC and vice versa at pH < ZPC. It also describes the pollutant removal potential at different pHs. Anionic pollutants are efficiently removed in acid conditions (pH < ZPC) and cationic pollutants in basic conditions (Tang et al. 2015). As reported in an earlier study, the pH drift method was used to determine ZPC with few modifications (Vinayagam et al. 2020). For the measurement of ZPC, 20 mg of synthesized nanoparticles was taken in 50 mL of 5 mM NaCl with pH ranging from 2 to 12 and incubated at room temperature. The final pH was noted after 48 h of equilibration. Then, ZPC was obtained at the intersection point of the straight line of the initial and final pH.

UV-Vis spectra analysis
The nanoparticle formation was established by the black color development after mixing the precursor and the TALE. This black-colored colloidal suspension was subjected to UV spectral studies after multiple dilutions. For both TALE and TA-Fe 2 O 3 NPs, a broad absorption band was observed with no specific peaks between 300 and 800 nm, confirming the formation of IONPs as shown in Fig. 2. A similar kind of absorption band was obtained for α-Fe 2 O 3 nanoparticles synthesized from various plant sources such as root extract of Arisaema amurense (Narayanan and Han 2016), Spondias dulcis (Vinayagam et al. 2020), Pheonix dactylifera leaf extract (Laouini 2019), and henna leaf extract (Abid and Kadhim 2020). The band gap for the TA-Fe 2 O 3 NPs was calculated as 2.67 eV by Tauc plot (Fig. 2 inset). The value matches with the reported value of 2.6 eV for the synthesis of Fe 2 O 3 by the chemical method (Suman et al. 2020).  1 3 average size of 43 nm was determined by Image J software. Many researchers have documented the cuboidal-shaped IONPs using fruit extracts of Hyphaene thebaica (Mohamed et al. 2020) and Tridax leaf extract (Yadav and Fulekar 2018). The elemental nature of nanoparticles was studied using EDX (Fig. 3b). The strong peak of iron and oxygen confirmed the formation of IONPs. The other peaks such as Na, Ca, and Si mainly originated because of the glass specimen used for the analysis (Dash et al. 2019). The carbon peak may be due to the polyphenols in the extract.

BET analysis
The specific surface area of the synthesized TA-Fe 2 O 3 NPs was determined by BET analysis to be 31.03 m 2 /g and pore volume was 0.0339 cc/g. The pore diameter was 4.37 nm, which substantiated the mesoscopic nature, and the particle size was 39 nm. The particle size determined from the BET analysis is consistent with that from XRD and SEM analysis. The synthesized TA-Fe 2 O 3 NPs showed better results than other studies. Moreover, the specific surface area of commercially available α-Fe 2 O 3 is 6 m 2 /g (Ahmmad et al. 2013). This indicates that the specific surface area of the synthesized TA-Fe 2 O 3 NPs is five times larger than the commercial one. Besides, the specific surface area observed herein is higher than that reported for other α-Fe 2 O 3 nanoparticles as shown in Table 1. The specific surface area of the nanomaterials is one of the important properties which help in determining the effectiveness of nanoparticles in pollutant removal by adsorption or photocatalytic degradation.

FTIR analysis
Many sharp bands were observed at (cm −1 ) 3734, 3277, 2357, 1545, 1207, 762 and 494 in the FTIR spectrum (Fig. 5). The signal observed at 2357 cm −1 may be assigned to stretching of the alkyne C≡C or C-H because of aliphatic  (Luo et al. 2016). The broadband found in the range from 3000 to 3500 cm −1 in addition to a small signal at 3734 cm −1 was correlated to the vibrations of the OH group in polyphenol TALE. A sharp band at 1545 cm −1 was associated with the C=O stretching, suggesting that the amide region was mainly due to enzymes and proteins that were accountable for reducing metallic ions (Madubuonu et al. 2019).
The weak bands at 1207 cm −1 are designated for C-CO-C stretching of carboxyl compounds (Pan et al. 2019). The 762 cm −1 signals may be allocated to the C-H bending of aromatic components (Yi et al. 2019). The respective frequencies of the low wavenumbers (< 700 cm −1 ) originate from the Fe-O vibrations of the iron oxide bond. The bands at 538 and 494 cm −1 corroborated the presence of hematite (Kumar et al. 2014). This signal can also be traced to oxygen in α-Fe 2 O 3 (Iacovita et al. 2015). Figure 6 inset shows the C signal at 284.79 eV, which affirmed the presence of the alkyl or aliphatic C in the biomolecules of TALE. Further, the peaks at 710.34 eV and 724.04 eV are signature signals of Fe 2p 1/2 and Fe 2p 3/2 , respectively. The peaks at 710.34 eV are related directly to Fe 3+ octahedral species. This confirmed the existence of Fe 3+ ions of α-Fe 2 O 3 (Shen et al. 2014 ). The core levels of Fe 2p and O 1 s were + 3 and −2, respectively, for the oxidation states of iron and oxygen. Hence, it may be concluded that the nanoparticles formed are pure α-Fe 2 O 3 and devoid of other impurities like zero-valent iron and another form of iron oxide.

VSM analysis
VSM analysis provides the magnetic behavior of the synthesized nanoparticles by determining the magnetic parameters such as magnetization, coercivity, and retentivity. Figure 7  provides measurements of magnetic hysteresis performed for TA-Fe 2 O 3 NPs at room temperature in an applied magnetic field from −15 kOe to + 15 kOe. The magnetization studies showed that magnetic particle saturation occurs in the higher field due to the nanoscale size of TA-Fe 2 O 3 NPs, which was obtained as 43 nm from SEM analysis. Also, an increase in the magnetic force favored particle magnetization (Sebastian et al. 2018). The analysis showed that magnetic saturation was 13.97 emu/g, 23 times higher than the commercial α-Fe 2 O 3 nanoparticles (0.6 emu/g) (Tang et al. 1989). This high magnetization is attributed to the surface disordered spin which is more likely to be oriented in the direction of the external magnetic field that is applied. Hence, it is hypothesized that the nanoparticle core-shell structure causes the elevated magnetic moment of the particles (Karade et al. 2019). TA-Fe 2 O 3 NPs were attracted to the strong magnet as seen in the inset of Fig. 7, demonstrating its superparamagnetic property. Moreover, this research shows a higher magnetic saturation value than other published green synthesis literature for α-Fe 2 O 3 . The magnetic saturation of α-Fe 2 O 3 synthesized from the root extract of Arisaema amurense was 1.25 emu/g (particle size: 24 nm) (Narayanan and Han 2016), from Salvadora persica aqueous extract 1.8 emu/g (particle size: 50-20 nm) (Miri et al. 2020), and from Gardenia resinifera plant extract 8.5 emu/g (particle size: 5 nm) (Karade et al. 2019).

Zero point of charge (ZPC)
ZPC plays a key role in the analysis of α-Fe 2 O 3 nanoparticle surface properties. It defines the electroneutrality of the nanomaterials and their surface at a particular pH (Lasheen et al. 2014). The graph of the initial vs final pH to estimate ZPC is shown in Fig. 8. The intersection point with the straight line is considered as ZPC and the value was 7.1, which was consistent with α-Fe 2 O 3 nanoparticles synthesized by the precipitation process (Dehbi et al. 2020). The ZPC value of the synthesized TA-Fe 2 O 3 NPs revealed that the nanoparticle surface was negatively charged at a solution pH greater than the ZPC and positively charged at pH smaller than the ZPC.

TGA
TGA measures the changes in the mass of nanoparticles as a function of temperature (or time) under a regulated environment to estimate thermal stability. The TGA plot of synthesized IONPs revealed a high rate of weight loss up to the temperature of 385 °C and it stabilized further ( Fig. 9). In the initial phase, a minimal amount of weight loss, i.e., 2.34%, was detected within the temperature ranges of 33-100 °C. A similar amount of weight loss (2.5%) was observed for α-Fe 2 O 3 synthesized using Cyperus rotundus rhizome extract (Basavegowda et al. 2017). This was because of the evaporation of surface-bound water and the volatile content present in TA-Fe 2 O 3 NPs (Akshay et al. 2020). From 100 to 400 °C, a significant weight loss of about 11.22% was observed. This could be attributed to the destruction of the surrounding phyto-compounds of TA-Fe 2 O 3 NPs (Narayanan and Han 2016). Further, from 400 to 800 °C, a gradual decrement (1.5%) was obtained due to adsorbed oxygen species (Basavegowda et al. 2017). At this temperature range, α-Fe 2 O 3 nanoparticles synthesized utilizing Spondias dulcis leaf extract showed a 7% loss (Vinayagam et al. 2020). Finally, it was shown that the residual weight of TA-Fe 2 O 3 NPs at 750 °C was 87.26% of the initial weight.

Plausible mechanism of TA-Fe 2 O 3 NPs synthesis
The plant extracts contain a range of biomolecules which are responsible for the reduction and stabilization of nanoparticles as reported by many researchers. The present study's FTIR findings clearly demonstrate that proteins and polyphenols are present, which are responsible for the synthesis. These compounds reduce Fe 2+ to Fe 0 which is followed by metal core-shell oxidation, wherein Fe 0 is oxidized to Fe 3+ as explained in the literature (Aksu Demirezen et al. 2019).

Conclusion
α-Fe 2 O 3 nanoparticles were effectively prepared using T. aurea leaf extract by a green synthesis approach without the use of any toxic chemicals. The adopted method proved to be easy and inexpensive. The synthesized TA-Fe 2 O 3 NPs were characterized by various techniques to analyze their size, morphology, surface functional group, and magnetic behavior. The synthesized nanoparticles were cuboidal in shape, pure, crystalline, and thermally stable with a mean diameter of 43 nm, and a specific surface area of 31.03 m 2 /g. They were more strongly magnetic than commercially available α-Fe 2 O 3 nanoparticles with magnetic saturation of 13.97 emu/g. The synthesized TA-Fe 2 O 3 NPs can be used in various fields such as environmental remediation, biomedical application, and construction.
Funding Open access funding provided by Manipal Academy of Higher Education, Manipal.
Data availability Data will be made available on request.

Conflict of interest None.
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