One-pot combustion synthesis of nickel oxide and hematite: From simple coordination compounds to high purity metal oxide nanoparticles

This work is the first report of a very simple and fast one-pot synthesis of
 nickel oxide (NiO) and hematite (?-Fe2O3) nanoparticles by thermal
 decomposition of transition metal aqua complexes with camphor sulfonate
 anions. Obtained nanopowders were characterized by X-ray powder diffraction,
 Fourier transform IR analysis, scanning electron microscopy, and
 Energy-dispersive X-ray spectroscopy. X-ray powder diffraction confirmed the
 formation of high purity NiO and ?-Fe2O3 crystal phases. In the case of
 ?-Fe2O3, about five times larger average crystallite size was obtained.
 Fourier transform IR spectra of synthesized materials showed characteristic
 peaks for NiO and ?-Fe2O3 nanostructures. To visualize the morphology and
 the chemical composition of the final products Scanning electron microscopy
 and Energy-dispersive X-ray spectroscopy were performed. The
 thermogravimetric analysis was done for a better understanding of the
 general thermal behavior of precursor compounds. This easy-to-perform and
 fast preparation method opens a broad range of obtained materials? usage,
 particularly due to its economic viability


Introduction
There is a continuous scientific demand for cheap, simple, environmentally sound and economically sustainable routes to develop, design and produce advanced functional nanomaterials. The modern technologies particularly require faster development of metal oxide materials at the nanometric level, striving for cheaper precursors and new synthesis methods of nanostructures with uniform physico-chemical properties [1][2][3][4].
This type of materials can exhibit various size-, shape-, and morphology-dependent properties by a slight change in the preparation procedure [12,[20][21][22][23][24]. Sintering appeared to be one of the best processes for obtaining oxide materials and improving their physical and chemical properties [25][26][27][28][29][30]. Although many synthetic approaches were developed [31][32][33][34][35][36][37][38][39], emphasis should be pointed out to thermal decomposition of suitable precursors for the preparation of functional oxide nanomaterials with tailored properties at mass scale [40]. Coordination compounds, such as metal carbonyls, metal acetylacetonates, and metal carboxylates, have been profoundly investigated as precursors to fabricate nanostructured metal oxides with desired morphology by thermal decomposition [40][41][42][43][44][45][46][47]. Organic solvents are widely used in thermal decomposition methods, to lower reaction temperatures and provide uniform and narrow size distribution of nanoparticles [40,48]. However, extensive use of such solvents is environmentally inexcusable. Stable, large single crystals of isomorphous hexaaquametal(II) D-camphor-10-sulfonate might be potentially used as optical filters and optical materials [49]. Despite their applications in optics, such compounds can be recognized as precursors for the synthesis of oxide nanomaterials by thermal decomposition, due to their high purity and easy, fast, and cheap production. It is expected that the presence of organic anion in these compounds can enable the avoidance of additional organic solvents in one-pot combustion syntheses, opening the way to an eco-friendly preparation method for metal oxides. To the best of our knowledge, hexaaquametal(II) D-camphor-10-sulfonates have not been investigated in this manner.
Having all this in a mind, in this paper, an alternative approach for producing nanosized nickel oxide (NiO) and hematite (α-Fe 2 O 3 ) powders at low cost by thermal decomposition of appropriate camphor sulfonate precursors is presented.
D-Camphor-10-sulfonic acid monohydrate (35.0 g, 0.15 mol) was dissolved in deionized water (60 mL). Iron chips (10.0 g, 0.18 mol) were added in D-camphor-10-sulfonic acid solution. Than the reaction mixture was refluxed for 48 h and filtered off. The solution has been cool down in a refrigerator until the orange crystals of hexaaquairon(II) D-camphor-10-sulfonate were obtained (yield 68 %, 31.95 g).
NiO and α-Fe 2 O 3 were synthesized by thermal decomposition of the solid camphor sulfonate precursors (5 g), in an electrical furnace with a heating rate of 10 °C/min at T = 550 °C for 3 hours, followed by pulverization in an agate mortar. X-ray powder diffraction (XRPD) patterns for the final products of thermal degradation were collected using a Rigaku SmartLab automated powder X-ray diffractometer with Cu Kα1,2 (λ = 1.54059 Å) radiation (U = 40 kV, I = 30 mA) equipped with D/teX Ultra 250 stripped 1D detector in the XRF reduction mode. The diffraction angle range was 20-120° 2θ with a step of 0.01° at a scan speed of 2°/min. The structural and microstructural investigation of the final products after thermal decomposition was conducted by the Rietveld method.
The IR spectra were recorded on a Nicolet 6700 FT-IR instrument (Thermo Scientific), in the ranges of 4000-400 and 700-240 cm -1 using the ATR technique with a Smart Orbit accessory (diamond crystal).
Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) analyses of the final products were performed with a JEOL JSM-6390LV scanning electron microscope. EDS analyses were conducted in the area of 1 × 10 4 μm 2 per sample. A

Results and Discussion
Thermal decomposition of hexaaquametal(II) D-camphor-10-sulfonates was performed at 550 °C in electrical furnace, without the addition of any organic compound. The obtained powders were characterized by XRPD, IR spectroscopy, SEM and EDS analyses. In the aim to better understand the degradation processes of precursors, TG and DTG analyses are performed.
XRPD was performed to identify the composition and crystalline phase of the final products. The obtained XRPD results were analyzed by the Rietveld method to gain deeper insight into the structural and microstructural parameters, by the fundamental parameters approach [51], as implemented in PDXL2 Rigaku software. Fig. 1 shows the XRPD patterns of the face-centered cubic phase of NiO (ICDD PDF 47-1049) and the rhombohedral structure of α-Fe 2 O 3 (ICDD PDF 33-0664), respectively. No other diffraction peaks corresponding to impurities were observed. The X-ray diffraction patterns show broad peaks indicating the ultrafine nature and small crystallite size of the particles. The values of average crystallite size and microstrain obtained by the Rietveld methods are listed in Table I. An average crystallite size in NiO was 10.8(1) nm, while approximately 5 times larger average crystallite size was observed for α-Fe 2 O 3 . This result can be explained by a higher rate of crystal growth in the rhombohedral crystal structure of α-Fe 2 O 3 than in the face-centered cubic crystal lattice of NiO. It is a consequence of lower energy (i.e. lower temperature) necessary for the growth of α-Fe 2 O 3 phase than in the case of crystal growth of NiO phase. However, the preparation of single-phase nanocrystalline metal oxides by thermal decomposition of solid camphor sulfonate precursors at relatively low temperatures without using additional organic compounds should not be neglected. The vibration modes are strongly influenced by the crystallite sizes. Thereby, the IR spectrum is a fingerprint of nanocrystalline structure. The IR spectroscopy results confirmed the formation of nanocrystalline NiO and α-Fe 2 O 3 . The IR spectrum of NiO shows a characteristic peak at 457 cm −1 , Fig. 2, assigned to Ni−O stretching vibration as was reported earlier by other researchers [34,52]. The IR spectrum of α-Fe 2 O 3 shows two characteristic bands at 441 and 521 cm −1 of the Fe-O bond, resulting from the tetrahedral and octahedral sites of hematite, respectively [53,54]. A broad peak approximately 3300 cm −1 (~3400 cm −1 in the IR spectrum of hematite) arises due to the presence of water, Fig. 2. A band observed about 1650 cm −1 in the spectra of both investigated samples can be attributed to the bending vibration of water.

Tab. I
IR spectra of camphor sulfonate precursors contain a strong band at 3416 cm −1 belonging to coordinated water molecules. Furthermore, two very strong bands at 1168 and 1045 cm −1 , corresponding to -SO 3 group of camphor sulfonate anion, are present. The bands assigned to the vibrations of the aliphatic alkyl groups at about 2950 cm −1 , as well as the band of the carbonyl group at 1734 cm −1 originate from camphor sulfonate moiety [49].
In summary, in the samples obtained by thermal decomposition of appropriate camphor sulfonate precursors, bands characteristic for camphor sulfonate anion are not detected in IR spectra, Fig. 2. Bearing in mind that the nanocrystalline form of metal oxides strongly affects material microstructure, SEM and EDS analyses were performed to visualize the morphology and the chemical composition of the final products obtained by thermal decomposition of appropriate camphor sulfonate precursors. From the obtained SEM results, Fig. 3, it was clear that the obtained powders were highly agglomerated, as a consequence of their nanometric sizes. NiO sample is consisted of spherically shaped agglomerates, while in α-Fe 2 O 3 powder two forms of aggregates, spherical and plate-like, are observed. The EDS spectrum of NiO powder shows only Ni and O, indicating that material with high degree of purity was obtained, Fig. 4

Conclusion
The one-pot combustion syntheses of NiO and α-Fe 2 O 3 powders at a nanoscale level were conducted by thermal decomposition of appropriate hexaaquametal(II) D-camphor-10sulfonate precursors since the chemical composition of these compounds allows the avoidance of additional organic solvents. Furthermore, here described a new method for preparation of NiO and α-Fe 2 O 3 was fully optimized at relatively low temperatures in a short time and using inexpensive, carefully chosen, precursors. The obtained metal oxide nanopowders are of high purity. Their microstructure indicates that such materials can be highly exploited. Therefore, the further step of our investigations will encompass the study on their electrochemical, sensing, and/or catalytic performances.