Synthesis of α-Fe2O3 rhombus nanoplates for photocatalytic investigation of cationic and anionic dyes and antibacterial aspect

In the present investigation, Hematite rhombus nanoplates (α-Fe2O3-RNPs) were synthesized from Ferric chloride salts by hydrothermal method. The α-Fe2O3-RNPs were characterized by X-ray powder diffraction (XRD), Scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy (EDX), Fourier transformation infrared (FTIR), X-ray photon spectroscopy (XPS), Brunauer–Emmett–Teller (BET) specific surface area, UV-visible spectroscopy, Photoluminescence (PL) and Electrochemical impedance spectroscopy (EIS) techniques. The photocatalytic response was confirmed via the degrading of two different pollutant dyes such as cationic methylene blue (MB) and anionic methylene orange (MO). The degradation was performed under diverse optimizing specifications of process variables. The response time, catalyst amount, dye initial concentration, and recyclability showed versatile effects during dye degradation. The α-Fe2O3-RNPs hold potent antibacterial activity against P. acne and Enterococcus faecium (E. faecium). Based on the results, the hydrothermal method is feasible and facile for the synthesis of α-Fe2O3-RNPs for photocatalytic and antibacterial activity.


Introduction
In the last few decades, with the development taking place in every part of life, one of the most critical environmental challenges is water quality. The eminent population compactness and the degree of industrial enterprise have activated the hydrosphere to embody impure with the inorganic and organic substances at an accelerative order. To some extent, this problem is solved by efficient development in the field of nanotechnology. There is an immense involution of researchers in the discipline of nanotechnology imputable to tunable attributes of the immiscible materials at the nanoscale level [1][2][3][4][5][6].
Recently, nanostructured metal or metallic oxide semiconducting materials own specific optic, electric, biosensing and photocatalyst attributes compared to their bulk form [7,8]. Particularly, Iron oxide nanostructures with their unique properties have attracted many investigators because of their facile manufacturing ways, low-priced, atoxic behaviour, magnetic, electronic and optic appliances [9,10]. Amongst diverse forms of iron oxides, hematite (α-Fe 2 O 3 ), maghemite (γ -Fe 2 O 3 ) and wustite (FeO), the hematite is an n-type semiconductor (Eg = 2.1 eV) photocatalyst (A photocatalyst is a material that stimulated by light photon and whose presence raises the value of a chemical response but itself persists unvaried at the final stage of the chemical reaction) has acquired intense interest owing to bio-safe, inexpensive, abundance existence, the most familiar sort of iron oxide, high index to absorb a photon of visible light (absorbs light up to 600 nm), ineradicable in most aqueous solutions and environment-friendly nature for eloquent solar energy (adequate to 40% of the solar-spectrum) conversion [11][12][13]. The structure of α-Fe 2 O 3 is a hexagonal crystal system in which the Fe atom is surrounded by six O atoms. However, as it is reported its photocatalytic response is limited due to its short diffusion length (2-4 nm), short excited-state lifetime (1 ps) and α-Fe 2 O 3 also has crystal size-dependent photocatalytic activity [14][15][16][17]. The physical and chemical properties of these materials depend largely on the shape, size and morphology of the nanostructure which ultimately affects the surface to volume ratio and generates quantum confinement effects [18][19][20].
Recently, two-dimensional nanostructures have got so much attention compared to three multidimensional nanostructures of the same materials owed to substantial anisotropy which stimulates specific attributes [21][22][23]. The surface of two-dimensional nanostructures incorporates a multitude of atoms ordered in a single crystal. The presence of an eminent part of atoms on the surface establishes two-dimensional nanostructures as superior catalysts [24,25]. There are many reported methods regarding the preparation of α-Fe 2 O 3 catalysts including sol-gel, co-precipitation, hydrolysis, solvothermal and hydrothermal methods [26][27][28][29][30]. One of these methods hydrothermal method is efficient, facile, and a protocol that gives high productivity, furthermore morphology could be controlled by controlling temperature, surfactant and reaction time [31,32].
Gastrointestinal problems are the major health issues in the global village due to the massive application of antibiotics, development in the medicinal industry and contaminated foods [33]. About 400 species have been identified in the gastrointestinal tract or on the skin, including Staphylococcus aureus, which produces a toxin and causes infections [34]. Due to this, nanoparticles get the attention of researchers as an antimicrobial agent to stunt the growth of bacteria. The efficacy of nanoparticles as antimicrobial agents depends upon the species of microbial, size, shape, and nature of nanoparticles [35]. In this study, we prepared the α-Fe 2 O 3 -RNPs to check the antibacterial activity of bacterial species via the agar diffusion method.
In this study, we synthesized α-Fe 2 O 3 powder by hydrothermal method and then characterized for their morphological, optical, and structural correspondences by applying versatile characterization techniques, such that XRD, FE-SEM, EDX, FTIR, XPS, UV-visible, BET surface area, PL and EIS techniques. The photocatalytic response of as prepared α-Fe 2 O 3 -RNPs was investigated against two different anionic and cationic dyes. It was observed that the photocatalytic attributes and efficiencies are toned through the surface structures of the photocatalysts letting in expanse, porousness, and particle size. PL spectra of as produced α-Fe 2 O 3 -RNPs catalyst revealed, owing to local d-band transition nature Fe 2 O 3 doesn't exhibit photoluminescent expelling in bulk form but a significant emission peak in the spectrum green region owed to quantum confinement effects. The photoluminescent emissions were caused by the surface defects which may arise from the deep level impurities and owed to improve Fe-O bonding separation. This excellent PL behaviour endows it with potential applications in the optical field. Articles reporting all this information about hematite α-Fe 2 O 3 -RNPs are very rare.

Synthesis procedure
All the chemicals and reagents were of analytic grade and applied with no additional refinement. For the typical synthesis, Ferric chloride (FeCl 3 ) 0.70 g was dissolved in 30 ml water and stirred vigorously for 1 h, then 3% wt of 2, 2-bipyridine and CTAB was added drop wisely into the solution. The pH of the solution was adjusted at 4; the resultant solution was shifted to a 100 mL Teflon-lined stainless-steel autoclave and put into a muffle furnace at 200°C. The reaction lasted for 22 h and then allowed the autoclave to cool near room temperature naturally. The colour of the solution was changed during this hydrothermal treatment from yellow to red. The reddish precipitates were collected and Water-washed complied by ethanol washing (thrice) to assure the remotion of unreacted ions then calcined at 600°C for 2 h. The colour of the reddish precipitates powder changed into deep brown powder after calcined treatment and the obtained product was taken for further physiochemical characterizations.

Characterization
To explore the Hematite catalyst various characterization techniques were applied. For crystallization and phase, XRD (Bruker D8-Advance X-ray diffractometer) was used. For shape, dimensions and structure study, SEM (Hitachi-S5500) was used. For chemical composition, Energy Dispersive X-ray Spectroscopy (EDX) was used. For elemental analysis and bond location and surface analysis, XPS (Thermo Fischer Scientific) was used. For the attached functional group's analysis, FTIR (Nicolet Magna-550 spectrometer) was used. For the surface area analysis, BET (Micro metrics ASAP 2010) was used. For the charges transportation within the α-Fe 2 O 3 -RNPs catalyst, PL (Hitachi F-4500) and EIS (Chi-760D) were used.

Photocatalytic activity
The photocatalytic activity (PCA) was measured under visible radiation using a Xe-lamp. For the photolysis and photocatalysis of the aqueous solution of MB and MO dyes, a photochemical setup equipped with a 300W Xe-lamp (Perkin Elmer-35) was used. A magnetized scaremonger was exploited for stimulating the sample to attain a consistent flow of visible light throughout the solution. The dyes solutions of varying concentrations were set up in ultrapure water and the catalyst was imparted at circumstantial ratios. To dissolve the catalyst, the sonication process was executed for 10 min. To attain the adsorption-desorption equilibrium, unenlightened stirring was accomplished for 30 min. A sample of unenlightened stirring was picked up and examined to get the spectra. The visible light irradiated samples were collected at different times and filtered out through a micron syringe filter. Then the persisting absorption of dyes was checked through spectrophotometric analysis. While the experiments were carried out in a glass beaker with a capacity of 250 millilitres. A precise quantity of catalyst was mixed with 50 millilitres of dyes solutions. The pH of the dye's solutions was attuned using the acidbased solution. The percentage of dyes degradation was evaluated by applying the relation as presented in equation (1).
The analytical analysis of the kinetics of MO and MB dyestuffs debasement is measured by applying the pseudo-first-order model as given below [36,37].
where C e is the initial dyestuff concentration at equilibrium, and Ct represents the concentration subsequently at the time "t" Whereas, k is the pseudo-first-order rate constant. Pseudo-first-order kinetic model is typically worked out for the system where the quantity of the initial pollutant is low [38][39][40].

Antibacterial activity
The agar diffusion method was employed to determine the antimicrobial activity of α-    [41].

Results and discussion
To further confirm the purity, surface state and composition of α-Fe 2 O 3 XPS analysis was performed. Figure  4(a) shows survey scan XPS spectra of the α-Fe 2 O 3 -RNPs. The detailed XPS elemental analysis confirms the presence of iron, carbon, and oxygen as deliciated in Figure 4(b-d). Figure 4(b) indicates the high-resolution spectrum of C1s from 280 to 292 eV, where the energy bonds of C at 284.70 and 288.20 eV endure to C-C and C-H. Figure 4(c) shows the O1s core level spectrum where the dominant oxide peak at around 529.9 eV. Figure 4(d) shows the photoelectron peaks of Fe at 711 and 725 eV is the characteristic doublets of Fe 2p 3/2 and 2p 1/2 . The core levels suggest that the valency of iron and oxygen are + 3 and − 2, respectively. These results also confirmed our XRD, and EDX results as well as are in good correspondence with the literature esteems of α-Fe 2 O 3 [42].

Photocatalytic evaluation
The photocatalytic activity of α-Fe 2 O 3 -RNPs was investigated by the decomposition of methylene blue (MB)  [18,35]. The photocatalytic efficiency of α-Fe 2 O 3 -RNPs reaches up to 92 and 98% for MO and MB dyes, respectively in 3 h (Figure 5a). The offend difference in the productivity is attributed to the dye's structural characters and photodynamic derived bandgap of α-Fe 2 O 3 -RNPs. The effect of catalyst dose was also examined, and it was observed that a 5-7 mg amount of α-Fe 2 O 3 -RNPs gives optimum results (Figure  5b), at a higher amount the excess generation of OHr adicals starts recombination within the photocatalyst instead of at the surface that leads to a reduction in the efficiency. On the contrary case at a low dose of α-Fe 2 O 3 -RNPs the generation of OH¯radicals is insufficient to obtain the optimum yield, as a result, there is a low photocatalytic response. Dye concentration influence was also examined by taking different dilutions of 10, 20 and 30 mg/L of solutions (Figure 5c, d). An increment in the dye concentration reduces the photocatalytic activity because the number of dye molecules is increased while the OH¯radicals are the same secondly the light penetration power also reduces due to high scattering. For stability and reusability of α-Fe 2 O 3 -RNPs photocatalyst, the photoactivity was repeated thrice and the obtained error bar graphs are inserted in Figure 5(e, f). From these outcomes, it is observed that α-Fe 2 O 3 -RNPs photocatalyst comprises good stability and recyclability which is very important for practical purposes.
The optical energy band gap is a fundamental characteristic that adds to the efficiency of the semiconductor photocatalyst, as is well known. The energy band locations of α-Fe 2 O 3 are determined using the Kubelka-Munk function corresponding to (αhν)2 = A(hν -E g ) n/2 and the UV-visible spectra of Fe 2 O 3 . Where A is a constant value, Eg is the energy of the bandgap, h is Planck's constant, and n is the frequency of light. In Fe 2 O 3 , the energy band "Eg" value is evaluated to be 2.2 eV, which is consistent with prior literature [43][44][45].
The photocatalytic decaying chemical mechanism is interconnected with the adsorption-degradationreleasing process, as the adsorbent of pollutants on the surface of photocatalysts is involved in the photoinduced response. The BET surface area measurements   were also carried out to assure the surface area of the α-Fe 2 O 3 -RNPs in the relative pressure at (P/P o ) ranges from 0.05-0.25 as demonstrated in Figure 6. The specific surface area (S BET ) of α-Fe 2 O 3 -RNPs is 7.907 m 2 g −1 .
The expanse is sufficient and causes raised dye surface assimilation over the α-Fe 2 O 3 -RNPs surface. The adsorption curve represents a type IV curve revealing the creation of nanopores within the nanostructure and characters of H 3 hysteresis. The mean volume and diameter of the pore were calculated as 0.0554 cm 3 g −1 and 1.685 nm, respectively. The presence of nanopores enables fast mass transport, more light-harvesting and helps to trap more and more dye molecules across the α-Fe 2 O 3 -RNPs catalyst surface. Hence, these aggregated light reflectance and surface assimilation over the α-Fe 2 O 3 -RNPs surface collectively enrich the photocatalytic activity and results in fast dye degradation [42,[46][47][48][49][50]. The rate of electron-hole pair formation and recombination reaction affects photocatalytic activity, as is well known. The photoluminescence (PL) spectra reveal information on charge generation, migration, and reintegration in photocatalysts. According to the literature, the lower peak intensity of the resulting PL spectra indicates a successful charge transfer within the semiconductor. The low PL intensity suggests a low charge carrier recurrence rate. As a result, the low recombination concentration allows for greater dye photocatalyst degradation. Figure 7(a) shows the PL spectra of as produced α-Fe 2 O 3 -RNPs catalyst, Owed to the local d-band transition nature it does not exhibit photoluminescent expelling in bulk form but a significant emission peak in the spectrum green region when reached in quantum confinement nanoscale, resulting in a small bandgap and increased photocatalytic activity of α-Fe 2 O 3 -RNPs. These photoluminescence emissions were maybe either emitted by Superficial defects which might originate by the deep level trap constituted ascribable Fe vacancy or possibly referable to alteration in the oxygen atomic co-ordinates in α-Fe 2 O 3 -RNPs, besides improving Fe-O bonding interval, resulting in enhancement of the magnetic coupling of the neighbouring Fe 3+ [45,51].
Electrochemical impedance spectroscopy (EIS) is a valuable tool for analysing the conduction, and transport of electrons and materials. Figure 7(b) shows a modified graph of α-Fe 2 O 3 -RNPs-GCE. The α-Fe 2 O 3 -RNPs spectrum collective with a semicircle at high frequencies and low frequencies is a straight line. The results show that Fe 2 O 3 -RNPs-GCE processes wellcontrolled electron transfer at high frequencies and a well-ordered distribution at low frequencies. The Nyquist frequency range of Fe 2 O 3 -RNPs-GCE is incredibly high compared to the bare GCE which indicates that the modified Fe 2 O 3 -RNPs-GCE has higher electrochemical performance and electron transmission is higher than that of bare GCE. The result shows that the modified Fe 2 O 3 -RNPs-GCE lowers the electron transport impedance and progressive transformation into an electron transfer process, suggestive of a reduced recombination rate [52][53][54]. Table 1 represents the antibacterial activity of α-Fe 2 O 3 -RNPs is screened against P. acne and E. faecium. The α-Fe 2 O 3 -RNPs nanoparticles presented the antibacterial activity towards the examined human pathogenic gram-negative strain P. acne at different concentrations of 25 μL (12 ± 0.985 mm), 50 μL(11.5 ± 0.985 mm) and 100 μL (12 ± 0.985 mm). Antibacterial activity of α-Fe 2 O 3 -RNPs was recorded as follows 25 μL (17 ± 0.985 mm), 50 μL(13.25 ± 0.985 mm) and 100 μL (13.5± 0.985 mm). The exact antimicrobial mechanism of nanoparticles fluctuates from species to species, particles size and shape. For possible mechanism, as α-Fe 2 O 3 is    [55]. Palanivel Sathishkumar et al., 2016 examined the anti-acne activity of silver nanoparticles AgNPs against P.acnes [56]. Jing Huang et al., 2021 proved that prepared chitosan-α-Fe 2 O 3 /GdF exhibit strong antibacterial activity against Escherichia coli, Pseudomonas aeruginosa, Bacilus subtilis, and Staphylococcus aereus [57]. However, the exact role of α-Fe 2 O 3 -RNPs on the antibacterial action is yet to be understood clearly. But the possible mechanism of antibacterial activity of α-Fe 2 O 3 -RNPs was shown in Figure 8. Thus α-Fe 2 O 3 -RNPs can act as an alternative antimicrobial agent to prevailing antibiotics.

Conclusion
In conclusion, we used a hydrothermal technique to successfully manufacture Fe 2 O 3 with Hematite rhombus nanoplates (α-Fe 2 O 3 -RNPs) shape. For physiochemical analysis, XRD, SEM, FTIR, XPS, BET, PL, and Electrochemical Impedance spectroscopy were used extensively. The photocatalytic response of α-Fe 2 O 3 -RNPs was tested against Mo and MB dyes for practical applications. Because of the low impedance, α-Fe 2 O 3 -RNPs had strong photolytic degradation activity, resulting in improved electron-hole separation. As a result, the α-Fe 2 O 3 -RNPs are a very efficient photocatalyst that aids and guides future photocatalyst development to address dye contamination and industrial water pollution. Finally, the antibacterial and anti-acne ability of the α-Fe 2 O 3 -RNPs against P. acne and E. faecium bacteria is also determined and discussed.