Green synthesis of single phase hausmannite Mn3O4 nanoparticles via Aspalathus linearis natural extract

Nowadays, green synthesis of nanoparticles using plant precursors has been extensively studied. However, less attention has been given to Mn3O4. This contribution validates the synthesis of single-phase Hausmannite Mn3O4 nanoparticles by a green approach without using any standard acid/base compounds, surfactants, and organic/inorganic dissolving agents. The chemical chelation of the Mn precursor was performed via bioactive compounds of the Aspalathus Linearis’ extract, an African indigenous plant. Annealing at 400 °C for ~ 1 h was required to crystallize the small amorphous nanoparticles with an initial bimodal size distribution peaking at ϕ1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left\langle {\phi_{1} } \right\rangle$$\end{document} ~ 4.21 nm and ϕ2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left\langle {\phi_{2} } \right\rangle$$\end{document} ~ 8.51 nm respectively. Such annealing lead to increase in the diameter of the nanoparticles from 17 to 28 nm.The morphological, structural, vibrational, surface, and photoluminescence properties of the single-phase Hausmannite nanoparticles were comprehensively investigated by High Resolution Transmission Electron Microscopy(HRTEM),Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), Raman and X-rays Photoelectron Spectroscopy (XPS), spectroscopy as well as room temperature photoluminescence. Structural and morphological investigations revealed the formation of quasi-spherical nanoparticles having a single phase Hausmannite Mn3O4 crystal structure. XPS results also validated the XRD results about the formation of Hausmannite Mn3O4 nanoparticles. Raman investigations allowed a crystal-clear distinction between the Mn3O4 nature of the nanoparticles from the potential γ -Mn2O3 phase as both phases belong to the same space group and both assume tetragonally-distorted cubic lattices of nearly similar dimensions. The optical studies of the single phase Hausmannite crystalline nanoparticles exhibited a broad photoluminescence in the spectral range of 300–700 nm, which is ideal for emission devices.


Introduction
The Manganese oxide system exhibits a rich range of stoichiometric and crystallographic phases including β-MnO 2 , γ-MnO 2 , α-Mn 2 O 3 , γ-Mn 2 O 3 , α-Mn 3 O 4 , andMn 5 O 8 where the Manganese atoms are obtained in different oxidation states. While burned in the air, such MnOx compounds undergo various electronic and/or crystallographic phase transformations. In the heat ranging from 500-600 °C, MnO 2 is converted to Mn 2 O 3 and to Mn 3 O 4 above 890 °C [1]. Depending on the environment, Mn may expect low or high oxidation states. The capacity to change within such oxidation states together with defects allows the well-established oxygen storage capacity of Mn oxides [2]. Manganese oxides in general and MnO, MnO 2 , and Mn 3 O 4 especially are attractive systems with potential applications in microwave absorption materials, sensors, supercapacitors, anode materials, water splitting and antimicrobial therapeutics [3][4][5]. Mn 3 O 4 could potentially be employed as precursors for the synthesis of LiMn 2 O 4 which is used for battery manufacturing. Also, Mn 3 O 4 is known to be an salts, for instance, the calcination and the thermal decomposition methods [7,8]. These methods require expensive manganese precursors and are also energy waste intensive. The second category is the oxidation of intermediate manganese hydroxide (Mn(OH) 2 ), such as solvothermal, refluxing, hydrothermal, and the self-assembly methods [9][10][11][12].Yet cost effective, these processes are relatively time consuming in general. This is because the Mn 3 O 4 nanoparticles are achieved through a low dynamic gas-solid reaction within O 2 and Mn(OH) 2 . As summarized in Table 1, yet very effective, the bulk of physical and chemical routes to synthesize Mn 3 O 4 nanoparticles seem not be green processes in regard of the rules of green chemistry. There are many physical and chemical methods for synthesis of nanoscale materials, but the ones that use green chemistry and eco-friendly techniques are the most suitable. The synthesis of nanoparticles using green technologies is highly needed for the rapid translation of nanostructures to real-world applications [5,15].This is because the process of formation of the nanoparticles does not require any toxic stabilizing, reducing, and oxidizing agents and can be done under ambient temperature and pressures. Three  [24]. Yet economically of interest, compared to the traditional methods, which use toxic organic solvents or precursors, these processes are relatively energyconsuming in general. Herein, single phase Hausmannite Mn 3 O 4 nanoparticles are synthesized via Aspalathus linearis by a green approach. The process of formation of nano-scaled Mn 3 O 4 particles does not require any standard acid/base compounds, neither surfactants nor organic/inorganic dissolving agents. As one could conclude later, it allows the synthesis of the ultrafine nanoparticles with an initial size distribution of 4.21-8.51 nm at the lowest temperature. The morphological, structural, vibrational, surface, and photoluminescence properties of the nano-scaled particles investigated by HRTEM, EDS, XRD, Raman, XPS, and PL are presented.

Green synthesis of manganese oxide nanoparticles via Aspalathus Linearis
Aspalathus Linearis, known as Rooibos, is a plant native of Southern Africa which has great importance in the synthesis of nanoparticles. Its leave extract contains phenolic compounds with a higher anti-oxidant potential and, as a result, a significant metal ion reduction capacity, making green synthesis of nanoparticle possible. Additionally, the high protein, lipid, and amino acid content aids in the stabilization of nanoparticles growth while preventing agglomeration [25][26][27]. For the synthesis of Manganese oxide nanoparticles, 8 g powders of dried A. linearis leaves from its primary geographical region, i.e. the Cederberg region-South Africa, were cleaned and added to 300 ml of de-ionized water at Room Temperature for 48 h. The filtrated solution was mixed with 3 g of MnCl 2 .4H 2 O (Sigma-Aldrich, purity 99.99%) as a demonstration of the idea of green synthesis of Hausmannite Mn 3 O 4 nanoparticles at low temperatures and by an entirely green process.
As swirling the solution, the precursor was seen to dissolve entirely in the solution, provoking a color turn from brown to greyish-brown. Following such a phase, a gray colored precipitate (Presumably MnO and/or Mn (OH)2 based mixture) was observed throughout 48 h. The precipitates were collected and dried at ~ 100 °C to remove any extra water content. Figure 1 depicts a schematic diagram of a potential mechanism for A. linearis extract-induced nanoparticle formation. In the formation of the nanoparticles, the plant extract acts as a reducing agent as well as a capping or binding agent. However, the interpretation and final chemical reactions taking place are expected to be challenging.The nanoparticles were annealed in air at 400 °C for 1 h to induce their crystallization, as described in Sect. 3.

Characterization techniques
Various characterizations were conducted to study various properties of the synthesized nanoparticles. For morphology and electron microscopy investigations the HRTEM, a Jeol JEM 4000EX electron microscopy unit with a resolution limit of about 0.12 nm, equipped with a Gatan digital camera, was utilized. The EDS spectrum was collected with an Oxford instruments X-Max solid-state Silicon drift detector operating at 20 keV. The structural properties of the annealed nanoparticles were characterized by using XRD Model Bruker AXS D8 Advance using radiation of Cu (K α Having wavelength of 1.5406 Å) and a Jobin-Yvon-SPEX integrated Raman spectroscopy with excitation wavelength of 632.8 nm. For the XPS, a VG Scientific LAB MK-II spectrometer with an Mg-Ka X-ray source (1253.6 eV) was used. The photoluminescence properties were analyzed at 250 nm using Horiba Jobin Yvon Fluorolog III modular spectrofluorometer.

Morphology and electron microscopy investigations
Morphology of the un-annealed sample, i.e. the isolated precipitates dried at ~ 100 °C was analysed with the TEM. Figure 2a indicates that the dried precipitate consists of dispersed quasi-spherical nanoparticles. Following a digitization analysis from multiple TEM images, the nanoparticles' size was found to obey a bi-modal distribution with average diameters peaking at ⟨ 1 ⟩ ~ 4.21 nm and ⟨ 2 ⟩ ~ 8.51 nm respectively (Fig. 2b). Of those views in different areas of the sample, it was found that the initial dried precipitate consists of amorphous nanoparticles (Fig. 2c). The support for such a conclusion is justified by the high grade of the chemical Mn precursor (99.99% Sigma Aldrich) and the fact that all required precautions during the synthesis were considered. As no other elements were detected in the EDS spectrum, this suggests the chemical formation of an Mn x O y or a slightly hydrated phase such as MnO or Mn(OH) 2 respectively, but certainly no MnCl 2 as there is no trace of chlorine.

Structural and crystallographic analysis
Following the thermogravimetry and differential calorimetry preliminary investigations, it was found that a heat treatment above 371 °C is required to crystallize the initially amorphous Mn x O y . Hence, the initial precipitates were dried at ~ 100 °C and thereafter annealed at about 400 °C for 1 h. The duration of 1 h was the optimum time extent of annealing to avoid significant sintering. In terms of crystallographic structure, Hausmannite Mn 3 O 4 exist into two forms: low-temperature tetragonal and hightemperature cubic structure by a transition happening at around 1170 •C [28]. Likewise, it is established that the cubic Mn 3 O 4 structure stabilizes toward ambient temperature in films produced by MOCVD on single-crystal (100) MgO substrate [29]. Figure 4 exhibits an XRD pattern of the annealed nanoparticles (400 °C, 1 h) in air. The pattern of the annealed powder is indexed as pure Hausmannite  Table 2. Further, data in Table 2 reveals that the quotient (d hkl Exp -d hkl Bulk )/d hkl Bulk ranges between 5.0-0.3% each h,k,l indices. This variation suggests the existence of compressive strain in the annealed powder [31]. In fact, more accurate analysis of the JCPDS database shows that the pattern assignable to the tetragonal Hausmannite Mn 3 O 4 can also be assigned to γ-Mn 2 O 3 if one considers the bar-errors on the lattice parameters. If so, the XRD can not easily distinguish between Hausmannite Mn 3 O 4 and γ-Mn 2 O 3 because both materials belong to the same space group and both assume tetragonallydistorted cubic lattices of nearly similar dimensions. Consequentially, it is necessary to carry out at least Raman studies as the vibrational characteristics of the Hausmannite Mn 3 O 4 and γ-Mn 2 O 3 are indeed different.

Vibrational properties
Hausmannite belongs to the I4 1 /amd-D 4h 19 space group with Z = 4 [32][33][34][35] for which the group theory predicts the vibrational representation of a primitive cell as Γ D4h = 2A 1g (R) + 2A 1u (IN) + A 2g (IN) + 4A 1u (IR) + 3B 1g (R) + 2 B 1u (IN) + B 2g (R) + 4B 2u (IN) + 4E g (R) + 6E u (IR), where "R", "IR" and "IN" means Raman, IR, and inactive lattice vibrations. Following [32] vibrational spectra of Manganese oxides can be divided into 3 regions at 750-600, 600-450, and 450-200 cm _1 , where stretching, bending and wagging vibrations of Mn x O y units take place respectively. The most discernible modes in Raman are vibrations centered at 660, 370, and 318 cm −1 . As it was demonstrated in several forms of the tetragonal Hausmannite Mn 3 O 4 , i.e. bulk, thin films, nanocrystals, nanorods and nano-powdered forms, these are 3 main active Raman modes [6,[36][37][38][39][40]. Figure 5 reports the Raman spectrum of the nanoparticles (~ 400 °C, 1 h). As one could observe, it constitutes a relatively intense peak at 657.6 cm −1 and two smaller peaks centered at around 308.1 and 365.8 cm −1 . These three Raman bands are consistent with the green synthesis of Mn 3 O 4 nanoparticles using plant precursor by [23]. The appearance of three Raman bands is attributed to the crystalline Hausmannite structure [41,42] of greenly synthesized nanoparticles, without any other oxides phase. The three Raman band are ascribed to the A 1g , E g , and T 2g respectively. The relatively intense Raman peak i.e., the A 1g (657.6 cm −1 ) is the fingerprint of the Hausmannite with the spinel structure characterizing the unique Hausmannite Mn-O stretching vibrational mode of the Mn ions in the tetrahedral coordinating. If the positions of 3 Raman modes are compared to the bulk values of pure tetragonal Hausmannite Mn 3 O 4 , they are red-shifted. The corresponding shift is about 2.4, 4.2 and 9.9 cm −1 for 660, 370, and 318 cm −1 modes respectively. This shift can be assigned rather to a size effect than to an oxygen deficiency as reported by Zuo et al. in nano-scaled Mn 3 O 4 particles synthesized by γ-radiolysis [40]. More precisely, as proposed by Zuo et al. and other researchers in their studies [43][44][45], the observed spectra can be explained by the phonon confinement effect. The phonon confinement model is based upon the fact that while in an infinite crystal, only phonons near the center of the Brillouin Zone i.e. q ~ 0 contribute to the Raman spectrum in view of the momentum conservation between phonons and probing light (Raman peaks are sharp), the phonons are confined in space by crystal boundaries or defects in a finite crystal such as nano-scaled particles (〈ϕ particle < λ Incident ). This results in uncertainty in the phonon momentum, allowing phonons with q#0 to contribute to the Raman spectrum. This uncertainty is larger for smaller particles, and hence induces the red shift as well as the broadening of

Surface properties
To support the XRD and Raman studies, XPS investigations of annealed were conducted at room temperature. Figure 6 displays the XPS spectrum of synthesized nanoparticles (annealed at 400 °C for1 hour). More specifically, it describes the Mn 2p peaks and equivalent binding energies. The 2p 1/2 and 2p 3/2 are collected at 641.3 and 653.2 eV. The observed spin-orbit splitting is ~ 11.9 eV, which is in good comparison with that of Manganese oxides [14,46]. The binding energy values from the Mn 2p 3/2 (641.3 eV) and spin-orbit splitting (11.9 eV) were in good agreement with those related values of Manganese oxides. Thus, the XPS analysis confirms the Hausmannite Mn 3 O 4 phase of the synthesized nanoparticles. Figure 7 depicts the room temperature photoluminescence spectrum of the synthesized single-phase Hausmannite Mn 3 O 4 nanoparticles using 250 nm excitation wavelength within the spectral range of 160-950 nm; under the conditions which are similar to those of [47]. One can distinguish five major emissions centered at 327.6, 386.6, 469.4, 562.6 and 659.1 nm with the following width at half maximum Δ 1/2 : 62.9, 61.9, 87.8, 92.5 and 77.4 nm respectively. Yet with slight spectral shift, this photoluminescence response is very close to that obtained by Toufiq et al. for hydrothermally-grown single-crystalline tetragonal Mn 3 O 4 nanoparticles using similar Mn precursor MnCl 2 .4H 2 O [47]. The UV emission bands located at 327.6 nm and 386.6 nm correspond to the recombination and emission of free excitons through an exciton-exciton collision process near band edges of the well crystallized crystals [48,49]. The most intense blue emission collected at 469.4 nm could be ascribed to the radial recombination of the photo-generated hole with an electron resulting in singly ionized oxygen vacancy-related defects. The yellow emission located at 562.6 nm can be assigned to the d-d transitions involving Mn 3+ ions [50]. The combined emission covering a large spectral range from the UV to NIR (300-700 nm) could be of interest for technological applications in ultraviolet and visible light emission devices.

Photoluminescence properties
As it is highlighted in Sect. 1, green synthesis using Adalodakam leaf extract and NaOH resulted in formation of nanocrystalline Mn 3 O 4 (44-66 nm) (Prasad et al.). The XRD analysis confirmed the formation of Mn 3 O 4 in a tetragonal body-centered lattice system. Asaikkuti et al.
used Ananas comosus (L.) peel extract and ethanol to produce spherical Mn 3 O 4 nanoparticles with average particle size of 40-50 nm for dietary supplementation. The formation of Mn 3 O 4 nanoparticles was further validated by SEM and EDS. However, NaOH solutions can decompose proteins and lipids in living tissues, which consequently cause chemical burns and may induce permanent blindness upon contact with the eye. Ethanol can lead to malnutrition, and can exert a direct toxicological effect due to its interference with hepatic metabolism and immunological functions. [23] utilized Azadirachta Indica leaf extract as a reducing and capping agent for the green synthesis of Mn 3 O 4 nanoparticles having particles size of (18.2-30 nm) annealed at 400 ºC for 2 h. Synthesis of Mn 3 O 4 nanoparticles (15 nm) at annealed at 500 °C for 2 h using Simarouba Glauca leaf extract and ethanol, was reported by Sreekala et al. However, in comparison to the above green protocol which uses harmful organic or inorganic precursors, these protocols are relatively energy-consuming. Obviously, the current green synthesis protocols characterization studies have many advantages over the previous works, such as the synthesis of ultrafine non-agglomerated nanoparticles having a size distribution of 4.21-8.51 nm at lower temperature, lower cost, use of Aspalathus Linearis extract, which possesses anti-oxidant, antiaging, anticancer, antidiabetic and anti-inflammatory properties [51]. In addition, the nano scaled feature of the synthesized particles was demonstrated by TEM, and XRD investigations, and the combined XRD, EDS, Raman, and XPS spectroscopy studies verified the nature and/or single-phase formation of the Mn 3 O 4 nanoparticles.
Taking into the observed physical responses, and as a follow up study of the current nanoscale single phase Hausmannite, it is intended to carry out the following studies: UV response [66], hydrophobicity [63,67], antibacterial [68] as well as their doping with rare earth elements [69,70]. Likewise, the mechanism of the green synthesis will be investigated to identify the bioactive compounds involved in the chelation process [31,52,71].

Conclusions
It was demonstrated that pure and nano-scaled Hausmannite Mn 3 O 4 can be synthesized by a green chemistry approach using A. linearis natural extract. Comparatively to the literature, this process allows the synthesis of the smallest Hausmannite nanoparticles in size and prepared to lowest temperature. While the kinetics of their formation is relatively slow, their initial size distribution is bimodal, peaking at 〈ϕ 1 ~ 4.21 nm and 〈ϕ 2 ~ 8.51 nm respectively. An annealing at 400 °C for 1 h approximately is required to induce their crystallization which provokes a sintering phenomenon as their average diameter reaches 17-28 nm. The broad photoluminescence generated by the single phase Hausmannite crystalline nanoparticles in the 300-700 nm spectral range is ideal for emission devices. It is intended to carry out studies to shed-light on the mechanism of formation of such Hausmannite Mn 3 O 4 nanoparticles and the evolution of their surface coordination. More precisely, it is hoped that the ongoing investigations will allow us to identify the bioactive compounds which react with the Mn precursor as well as the various chemical phases through which the reaction is taking place.
Funding This research output was supported by grants from the National Research Foundation of South Africa (NRF), iThemba LABS, the UNESCO-UNISA Africa Chair in Nanosciences and Nanotechnology, The World Academy of Sciences (TWAS), the Abdus Salam ICTP through the Nanosciences African Network (NANOAFNET) as well as the African Laser Centre (ALC) to whom we are grateful.

Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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