Single-phase α-Cr2O3 nanoparticles’ green synthesis using Callistemon viminalis’ red flower extract

ABSTRACT This contribution reports for the first time on the synthesis and the main physical properties of single-phase pure α-Cr2O3 nanoparticles synthesized by a facile, rapid and eco-friendly process using Callistemon viminalis flower's extract as an effective oxidizing/reducing agent. These crystalline nanoparticles exhibit a cubic-like platelet shape with sharp edges with an average particle diagonal size of ∼92.2 nm. The room temperature physical properties of these pure highly crystalline Eskolaite α-Cr2O3 nanoparticles were carried out using complementary techniques such as high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), XRD, FTIR-ATR, Raman and XPS. GRAPHICAL ABSTRACT


Introduction
The trivalent oxide, Cr 2 O 3 , is recognized as a stable oxide in the Cr-O binary system. Other well-known oxides in this system are CrO 3 and CrO 2 (1). Although many other formulae have been reported as higher oxides in the past few decades, the accepted ones are Cr 3 O 8 , Cr 5 O 12 , Cr 2 O 5 and Cr 6 O 13 (2,3). Except for the purely hexavalent chromium in CrO 3 and tetravalent Cr in CrO 2 , the valencies of chromium in these higher oxides are combinations of Cr 3+ and Cr 6+ . Of these higher oxides, α-Cr 2 O 3 is the most stable and is the oxide of choice. Known to have a wide band gap (E g ∼ 3.4 eV), it crystallizes in a corundum-type structure and is antiferromagnetic with a Neel temperature of T N -307 K. Depending on its growth conditions, α-Cr 2 O 3 can exhibit n-type or p-type semiconductor behaviour. With one of the highest hardness values for metal oxides in the bulk form, 29 GPa as compared to 12 GPa for zirconia, it is useful for applications that require high wear resistance. Its optical band gap makes it the green colourant of choice in the pigment industry for ceramics, coatings, paints and printing. Because of its high chemical stability, α-Cr 2 O 3based catalysts are also of importance for the chemical industry in the preparation of several important commodity chemicals and have also been investigated for their potential as Li cathodes (4)(5)(6)(7)(8)(9)(10). As a result of its high refractive index, it is also finds use in cermets designed for low and medium temperature-selective solar absorbers (11)(12)(13)(14)(15)(16).
Several physical and chemical process methods have therefore been used to prepare α-Cr 2 O 3 particularly in its nano-scaled form among which are laser-induced deposition, hydrothermal synthesis, thermal decomposition, microwave-plasma assisted growth, sonochemical synthesis, sol-gel synthesis, hydrothermal synthesis and combustion synthesis (17)(18)(19)(20). In the case of nanoscaled α-Cr 2 O 3 for functional and colour pigment applications, two mass-scale methods of synthesis have been used which consists of solid state and hydrothermal processes. With a host of these methods, the use of environmentally harmful chemicals is generally needed. To mitigate the problem of adding to the environment toxic and hazardous wastes generated by these processes, greener methods of synthesis are being researched. One such promising method for green synthesis of metal oxide nanoparticles is that of the bio-reduction/oxidation of metal precursor salts using plant-based extracts. This biosynthesis method makes possible rapid synthesis of stabilized metal/ metal oxide nanoparticles in a manner that is easy and cost-effective (21)(22)(23). While significant work has been reported on the use of plant-based extracts to prepare various metal/metal oxide nanoparticles, not much is reported on the use of these extracts for the green synthesis of Cr 2 O 3 nanoparticles.
Through this contribution, we report for the first time, the use of the dye extract from the red Callistemon viminalis as an effective agent for facile and rapid biosynthesis of pure Eskolaite α-Cr 2 O 3 cubic-like nanoparticles. The fact that no inorganic/organic solvents or surfactants are used in this method of synthesis makes the process eco-friendly.

Experimental and results
2.1. Biosynthesis process via C. viminalis aqueous extract C. viminalis is a plant originally found in Australia. Various phytochemical studies have been carried out on extracts made from different part of these tree species which are the leaves, stem barks, fruits, seeds as well as the flowers.
In general, these studies show that the red dye extracts obtained from the red Callistemon flowers are very rich in flavonoids, saponins, steroids, alkaloids and triterpenoids (24). A summary of the isolated active compounds obtained from this red dye extracts can be viewed in Table 1.
For the synthesis, fresh red flowers of C. viminalis were collected from around the Western Cape site of iThemba  LABS, South Africa. In a typical set-up, 6.65 g of the bottle-brush-shaped red flowers were heated in 250 ml of de-ionized water at a temperature between 75°C and 85°C for 2-3 h yielding a red-coloured extract of pH = 3. 75. To the filtered red aqueous extract obtained after cooling to room temperature, was added to 10.12 g of Cr(NO 3 ) 3 · 9H 2 O salt. At room temperature, while swirling, the Cr(III) salt was observed to dissolve completely in the aqueous extract under 3 min, bringing about a change in colour of the solution from red to black. The resultant solution was allowed to settle over a period of 1-2 h after which a black-coloured precipitate was observed. The precipitate was then separated from the aqueous extract, first by decanting then by centrifuging at 3500 rpm two to three times over successive additions of de-ionized waterthis is to wash the precipitate of any residual aqueous extract. The resultant precipitate obtained after decanting the precipitate/H 2 O separation was then dried at 250°C then heated in air at 500°C for 2 h. Figure 1 reports the high-resolution transmission electron microscopy (HRTEM) micrographs and the corresponding small area electron diffraction (SAED) pattern obtained on the green Cr x Oy powder following annealing at 500°C of the initially brownish green precipitate.

Surface morphology and elemental analysis
The HRTEM was carried out using a FEI Tecnai G2 Field Emission Gun HRTEM operating at 200 kV. From this it can be established that generally, yet agglomerated ( Figure 1(a)), the nanoparticles are cubic-like platelets with sharp edges with a non-negligible degree of polydispersity (Figure 1(c)). Particle size distribution analysis carried out using Image J software showed an average particle diagonal size to of ∼92.2 nm. Likewise, and in view of the HRTEM (Figure 1(c) and 1(d)) and the SAED patterns ( Figure 1(b)), the bulk of the nanoparticles exhibit a significant atomic ordering. An enlargement of Figure 1(c), that is, Figure 1(d) shows the highly ordered atomic reticular planes. The atomic double periodicity within the scale shown in green in Figure 1 is of the order of 9.91 Å corresponding to a lattice periodicity of 4.95 Å, similar to the value of the crystal lattice parameters of α-Cr 2 O 3 (〈a bulk 〉 = 〈b bulk 〉 = 4.953 Å). As indicated in the enlarged section (Figure 1(d)) one can distinguish a variety of defects such as atomic plane dislocations and atomic twisting. The HRTEM of the α-Cr 2 O 3 nanoparticles with nearly nine observable aligned atomic layers seems to suggest that the α-Cr 2 O 3 nanoparticles grow by a layer-by-layer process, that is, a Frank Van der Merwe type growth. The preliminary HRTEM investigation was followed by elemental analysis obtained through energy-dispersive X-ray spectroscopy (EDS) carried out using an Oxford instruments X-Max solid state silicon drift detector (20 KeV) coupled to the Tecnai G2 HRTEM. The obtained spectrum (Figure 2) confirmed the presence of Cr and oxygen in the annealed powders. The observed peaks of Cu are attributed to the Cu grids that make up the support on which the samples were placed. The peaks due to carbon are assigned to the carbon coating layer onto the Cu grid.

Crystallographic structure and phase identification
To identify the crystallographic phase of the suspected α-Cr 2 O 3 nanoparticles, room temperature XRD analysis ( Figure 3) was carried out using a Bruker Advanced D8 diffractometer with monochromated Cu Kα radiation of wavelength 1.5406 Å operating at a current of 40 mA and a voltage of 40 kV in the Bragg-Brentano geometry. The crystallite size of the α-Cr 2 O 3 nanoparticles cannot be estimated using the classical Debye Scherrer approximation as they exhibit a net shape anisotropy.

Chemical bonding and vibration spectroscopy
To re-confirm that the prepared powder is Eskolaite α-Cr 2 O 3 and to detect any additional surface/interface bonded compounds, attenuated total reflection-FT-IR spectroscopy was carried out on the heat-treated nanopowder at room temperature. The ATR-FT-IR typical spectrum as shown in Figure 4 could be split into two regions, the first lying between the 400 and 800 cm −1 and the second between 2100 and 2700 cm   attributed to E u modes while that centred at 418 cm −1 can be attributed to the IR active A 2u vibration mode. The relatively strong broad band observed at 3440 cm −1 can be attributed to O-H stretching modes of what could be waters of hydration. The weak broad band at 1600 cm −1 may well be attributed to the presence of waters of adsorbed moisture at the surface of the α-Cr 2 O 3 powders. As a complimentary study to the ATR-FTIR investigation above, Raman spectroscopy was carried out. The Raman spectrum observed for the α-Cr 2 O 3 nanopowder ( Figure 5) was obtained at room temperature using a 514.5 nm excitation line of an Ar + laser source in the spectral range of 300-700 nm. Bulk α-Cr 2 O 3 is known to have a corundum structure that belongs to the D 6 3d group. The site symmetry for the Cr atoms is C 3 , while the O atoms are on sites having C 2 symmetry. The corresponding optical modes in the crystal are 2A1g, 2A1u, 3A2g, 2A2U, 5Eg and 4Eu vibrations with only two A1g and five Eg vibrations which are Raman active. From the Raman spectrum, one can distinguish one intense peak centred at about 575.4 cm −1 with a shoulder at ∼543.3 cm −1 that can be ascribed to A1g modes. Relatively less-intense broad peaks at 356. 9

Chemical valence states by X-ray photoelectron spectroscopy
To further confirm the Eskolaite nature of the green nanopowders, X-ray photoelectron spectroscopy was carried out. The initial calibration of the instrument was conducted with a binding energy (BE) of 284.5 eV for a C 1s electron. As shown in Figure 6, high-resolution energy scans on the sample give two main peaks at BE 576.9 and 586.8 eV correspond, respectively, to Cr 2p3/ 2 and Cr 2p1/2 valence states of α-Cr 2 O 3 (27). The splitting between these two states is about 10 eV. The O1s peak at a B.E of 530.6 eV can be ascribed to O atoms in the α-Cr 2 O 3 nanoparticles and is in agreement with data reported by Jin et al. (27). The O1s peaks can be deconvoluted into three peaks centred at 529.5, 531.1 and 532.8 eV as reported by Maetaki and Kishi (28). These deconvoluted peaks can respectively be assigned to lattice oxygen (O 2-) surface adsorbed oxygen (O − ) and an O hydroxyl from O1s, and respectively, account for ∼29.1%, 41.8% and 29.1% of the total oxygen content. The observed XPS profile strongly suggests the absence of impurities within the synthesized nanoparticles of Eskolaite α-Cr 2 O 3 .
In view of the potential applications of the synthesized pure Eskolaite α-Cr 2 O 3 , it was shown that they do exhibit superior supercapacitance response in a composite form with graphene (12). Likewise, they display a tunable photo-induced magnetism as demonstrated recently (14,16) and hence the possibility to engineer novel photo-tunable magnetic devices. From the biomedical aspect, the ongoing antimicrobial studies of such nano-scaled α-Cr 2 O 3 indicate a significant efficiency against several bacteria such as E. coli (K12), S. aureas (MRSA), A. aureas (25923), P. vulgarias (ATCC). More precisely, using disc diffusion methods α-Cr 2 O 3 nanoparticles showed a large inhibition in all bacterial species and this were compared with the standard positive control (antibiotic). In addition, emulsions of the α-Cr 2 O 3 nanoparticles are in process for green ink and paints applications.

Conclusion
Green synthesis of pure Eskolaite α-Cr 2 O 3 nanoparticles via the bio-modification, at room temperature, of a precursor Cr(NO) 3 · 9H 2 O salt was carried out using aqueous extracts from red flowers of C. viminalis. The resultant brownish green precipitate when annealed in air, for 2 h, at 500°C, gave rise to Cr 2 O 3 stabilized in the base-centred monoclinic phase. Besides HRTEM investigations, XRD, ATR/FT-IR, X-Ray photoelectron spectroscopy as well as Raman spectroscopy were used to confirm the formation of Eskolaite α-Cr 2 O 3 nanoparticles. Future work is required to identify the specific bioactive components in the aqueous extract of the red flowers of C. viminalis that are responsible for the bio-transformation of the Cr(NO) 3 · 9H 2 O salt.

Disclosure statement
No potential conflict of interest was reported by the authors.