WO2020075194A1

WO2020075194A1 - Heteroatom-induced ferromagnetism in antiferromagnetic hematite

Info

Publication number: WO2020075194A1
Application number: PCT/IN2019/050751
Authority: WO
Inventors: Sundara Ramaprabhu; Dipsikha GANGULY; Anamika GHOSH
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2018-10-12
Filing date: 2019-10-09
Publication date: 2020-04-16

Abstract

The present invention relates to a single step method for synthesizing ferromagnetic hematite iron oxide(α- Fe₂O₃) by combustion or pyrolysis. The combustion synthesis of α- Fe₂O₃ includes heating a composite admixture including at least one iron precursor and at least one heteroatom precursor in a predetermined weight ratio to a predetermined temperature range under an inert gas atmosphere.Further, the admixture is subsequently exposed to atmospheric air. The pyrolytic synthesis of α- Fe₂O₃ includes heating a composite admixture including at least one iron precursor and at least one heteroatom precursor in a predetermined weight ratio to a predetermined temperature range under an air atmosphere. The method includes inducing ferromagnetism by inclusion of heteroatoms probably in the sites of crystal defect of α- Fe₂O₃. The method results in a large yield of magnetic α-Fe₂O₃ having high magnetization. The synthesized α-Fe₂O₃ may be used in nanobiosensors, batteries or giant magnetoresistance devices.

Description

HETEROATOM-INDUCED FERROMAGNETISM IN ANTIFERROMAGNETIC

HEMATITE

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority to Indian provisional patent application No. 201841038764 dated October 12, 2018.

FIELD OF THE INVENTION

[0002] The disclosure relates generally to heteroatom induced ferromagnetism and in particular to ferromagnetic hematite and a controlled method of preparation thereof.

DESCRIPTION OF THE RELATED ART

[0003] Transition metals have emerged as most researched material as it possesses various fascinating properties like variable valences and photoluminescence. Iron is the most well-known magnetic transition metal having three metal oxides: Fe₃0₄, FeO and Fe₂0₃. Among different oxides, Fe₃0₄ is highly magnetic in nature whereas a-Fe₂0₃ is antiferromagnetic a- Fe₂0₃ possess various interesting properties like wide band gap, absorption in visible region, fluorescence, corrosion resistant, biocompatibility, low cost which makes it suitable candidate for technological applications. However, due to its antiferromagnetic nature magnetic field assisted potential applications are limited. Hence enhancing the magnetic properties of antiferromagnetic a- Fe₂C>₃ can foster vast applications in the field of magnetism.

[0004] Few approaches are attempted for the synthesis of ferromagnetic a- Fe₂C>₃ which mainly includes solution based synthesis and except a few which involves tedious methods e.g. template assisted solution combustion synthesis, multistep combustion process, all other synthesis methods have resulted magnetization of 1-4 emu/g.

[0005] A method for preparing a magnetic iron oxide-graphene composite is disclosed in US20180206366A1.PCT application W02010120964A2 discloses nanoscale materials and a method of preparation thereof. An iron oxide-carbon composite particle powder and a method for producing the iron oxide-carbon composite particle powder are disclosed in PCT application WO2017082338A1. Among various synthesis procedures, solution combustion method is found to be most common method, but one of the major disadvantages of this method is that it is a time consuming method and involves a lot of intermediate steps as well as chemicals. The present invention discloses a ferromagnetic hematite iron oxide composite and a method of preparation thereof that overcomes some of the drawbacks of the existing methods.

SUMMARY OF THE INVENTION

[0006] In various embodiments provided herein, is a method of combustion synthesis of ferromagnetic hematite iron oxide composite. The method includes heating a composite admixture including at least one iron precursor and at least one heteroatom precursor in a predetermined weight ratio to a temperature between 300 °C to 600 °C under an inert gas atmosphere. Further, the method includes subsequently exposing the admixture to atmospheric air to synthesize ferromagnetic hematite iron oxide.

[0007] In some embodiments, the heteroatom precursor is selected from nitrogen, sulfur, boron or carbon precursor. In one embodiment, the carbon precursor is selected from paracetamol, urea, melamine, graphitic carbon nitride, carbon nanotubes, graphene oxide, graphene, or carbon nanofiber. In some embodiments, the sulfur precursor is selected from sulfur powder or hydrogen sulfide. In one embodiment, the boron precursor is selected from boric oxide, sodium borohydride or boric acid. Further, the nitrogen precursor is selected from sodium nitrate or ammonia.

[0008] In some embodiments, the iron precursor is selected from iron nitrate, iron chloride or iron sulphate. In one embodiment, the heat treatment is carried out for 2h to 3h. In various embodiments, the predetermined weight ratio of heteroatom precursor to iron precursor is in the range of 0.5: 1 to 1.5: 1. In some embodiments, the heating causes incorporation of the heteroatom into hematite iron oxide crystal structure to induce ferromagnetism.

[0009] In various embodiments provided herein, is a pyrolytic synthesis method of ferromagnetic hematite iron oxide composite. The method includes heating a composite admixture including at least one iron precursor and at least one heteroatom precursor in a predetermined weight ratio to a temperature between 400 °C to 750 °C under an air atmosphere to form ferromagnetic hematite iron oxide.

[0010] In some embodiments, the heteroatom precursor is selected from nitrogen, sulfur, boron or carbon precursors. In one embodiment, the carbon precursor is selected from paracetamol, urea, melamine, graphitic carbon nitride, carbon nanotubes, graphene oxide, graphene, or carbon nanofiber.

[0011] In some embodiments, the sulfur precursor is selected from sulfur powder or hydrogen sulfide. The boron precursor is selected from boric oxide, sodium borohydride or boric acid. The nitrogen precursor is selected from sodium nitrate or ammonia. In one embodiment, the iron precursor is selected from iron nitrate, iron chloride or iron sulphate. In some embodiments, the heat treatment is carried out for 2h to 3h. In one embodiment, the predetermined weight ratio of heteroatom precursor to iron precursor is in the range of 2: 1 to 1:2.

[0012] In various embodiments provided herein, is a method of pyrolytic synthesis of ferromagnetic hematite iron oxide from antiferromagnetic hematite iron oxide. The method includes heating a composite mixture comprising the antiferromagnetic hematite iron oxide with at least one heteroatom precursor in a predetermined weight ratio to a temperature between 300°C to 320°C in an air atmosphere to synthesize ferromagnetic hematite iron oxide. [0013] In some embodiments, the heteroatom precursor is selected from nitrogen, sulfur, boron or carbon precursors. In one embodiment, the carbon precursor is selected from paracetamol, urea, melamine, graphitic carbon nitride, carbon nanotubes, graphene oxide, graphene, or carbon nanofiber. In various embodiments, the sulfur precursor is selected from sulfur powder or hydrogen sulfide. The boron precursor is selected from boric oxide, sodium borohydride or boric acid. The nitrogen precursor is selected from sodium nitrate or ammonia.

[0014] In one embodiment, the heat treatment is carried out for 2h to 3h. In some embodiments, the predetermined weight ratio of heteroatom precursor to antiferromagnetic hematite iron oxide is in the range of 2: 1 to 1:2.

[0015] In various embodiments provided herein, is a ferromagnetic hematite iron oxide having rhombohedral crystal structure (space group R-3c). The crystal structure is characterized by oxygen and iron site defects. In some embodiments, an oxygen occupancy at l8e site in a unit cell of the structure ranges from 1 to 0.499. Further, the iron occupancy at l2c site in the unit cell ranges from 1.005 to 0.978. In some embodiments, an increase in the oxygen site defects is configured to cause a magnetization of 7 emu/g or greater. In some embodiments, an increase in the oxygen site defects is configured to cause a magnetization of 7-17 emu/g. In various embodiments, the oxygen vacancy distribution is obtained by doping with a heteroatom selected from carbon, nitrogen, sulphur or boron.

[0016] The ferromagnetic hematite iron oxide nanoparticles produced by combustion synthesis or pyrolysis are configured to have a size ranging from 20-60 nm.

[0017] This and other aspects are disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0019] FIG. 1 illustrates ferromagnetic hematite iron oxide crystal structure.

[0020] FIG. 2 depicts results of TGA analysis of Fe2C>₃ samples.

[0021] FIG. 3A depicts XRD pattern of magnetic a- FeiCVC by combustion.

[0022] FIG. 3B depicts XRD pattern of magnetic a- FeiCF -gCN-400 by solid-state pyrolysis using graphitic carbon nitride.

[0023] FIG. 3C depicts XRD pattern of magnetic a- FciCF /C from antiferromagnetic a- FciCf NP.

[0024] FIG. 4 depicts results of Raman spectra of magnetic a- FeiCF /C from antiferromagnetic a- FciCf NP.

[0025] FIG. 5A depicts TEM image of magnetic a- FeiC C by combustion.

[0026] FIG. 5B depicts TEM image of magnetic a- FeiCE/C by solid-state pyrolysis using graphitic carbon nitride.

[0027] FIG. 5C depicts TEM image of magnetic a- FeiCE/C by solid-state pyrolysis using antiferromagnetic a- FeiCE NP.

[0028] FIG. 6A depicts magnetic hysteresis curves of ferromagnetic a- FeiCE/C synthesized by combustion.

[0029] FIG. 6B depicts magnetic hysteresis curves of a- Fe₂0₃-gCN-400 synthesized by solid state pyrolysis using graphitic carbon nitride.

[0030] FIG. 6C depicts magnetic hysteresis curves of ferromagnetic a- FeiCE/C synthesized from antiferromagnetic a- FciCf NPa- Fe₂0₃-gCN-400 by solid state pyrolysis.

[0031] FIG. 7A illustrates the simulated crystal structure of pure a-Fe20₃. [0032] FIG. 7B illustrates the simulated crystal structure of synthesized a-FeiCF/ C.

[0033] FIG. 8A illustrates the simulated crystal structure of a-FciCf synthesized using combustion method in the absence of carbon precursor.

[0034] FIG. 8B illustrates the simulated crystal structure of a-FeiC C synthesized using combustion method in the presence of carbon precursor.

[0035] FIG. 9A illustrates the simulated crystal structure of a-Fe203/C synthesized with C:Fe(carbon to iron) in the ratio of 0.5: 1 .

[0036] FIG. 9B illustrates the simulated crystal structure of a-FeiC C synthesized with C:Fe in the ratio of 1 : 1 .

[0037] FIG. 9C illustrates the simulated crystal structure of a-FeiC C synthesized with C:Fe in the ratio of 1.5: 1 .

[0038] FIG. 10 illustrates the magnetization variation with change in C:Fe ratio.

DETAILED DESCRIPTION

[0039] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0040] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0041] The invention in its various embodiments proposes a simple, facile single step and low-cost method of producing magnetic hematite iron oxide (a- FeiCE). The method includes incorporating a heteroatom with tunable magnetic properties. Further, ferromagnetic hematite iron oxide synthesized using said method is disclosed. The method results in synthesis of a large yield of magnetic a-FciCf with high magnetization.

[0042] In various embodiments, a method of combustion synthesis of ferromagnetic a- FciCf includes grinding and blending at least one iron precursor and at least one heteroatom precursor in a predetermined weight ratio to form an admixture. In some embodiments, the admixture may be obtained by grinding at least one iron precursor and at least one heteroatom precursor in a mortar pestle for a predetermined time. In one embodiment, the predetermined time is 15 to 30 minutes.

[0043] In various embodiments, the combustion synthesis method further includes heating the composite admixture under an inert gas atmosphere and subsequently exposing the admixture to atmospheric air. In some embodiments, the method includes heating the admixture to a temperature between about 300 °C to about 600 °C under an inert gas atmosphere and causing the incorporation of the heteroatom precursor into hematite iron oxide crystal structure thereby inducing ferromagnetism. In one embodiment, the combustion synthesis method includes the formation of ferromagnetic a-Fe203with incorporation of carbon due to the decomposition of iron nitrate at 550°C in argon atmosphere and sudden exposure in air at 300°C in presence of carbon precursor. In some embodiments, the combustion synthesis method includes heating the admixture for 2 to 3 hours. In one embodiment, the ferromagnetic a-FeiCf nanoparticles synthesized using combustion method exhibits a magnetization of 17 emu/g or greater.

[0044] In various embodiments, a method of pyrolytic synthesis of ferromagnetic a- Fe2C>3 composite includes includes grinding and blending at least one iron precursor and at least one heteroatom precursor in a predetermined weight ratio to form an admixture. In some embodiments, the admixture may be obtained by grinding at least one iron precursor and at least one heteroatom precursor in a mortar pestle for a predetermined time. In one embodiment, the predetermined time is 15 to 30 minutes. In various embodiments, the pyrolytic synthesis method further includes heating the composite admixture to a temperature between about 400 °C to about 750 °C under an air atmosphere. In one embodiment, the ferromagnetic a-Fe2C>3 synthesized using pyrolytic synthesis method exhibits a magnetization of 10 emu/g or greater.

[0045] In various embodiments, a method of pyrolytic synthesis of ferromagnetic a- Fe2C>3 composite from antiferromagnetic a- Fe2C>3 includes heating a composite mixture including the antiferromagnetic a- Fe2C>3 with at least one heteroatom precursor in a predetermined weight ratio to a temperature between 300°C to 320°C under an air atmosphere. In some embodiments, pyrolytic synthesis of ferromagnetic a- Fe2C>3 composite from antiferromagnetic a- Fe2C>3 includes direct heating of antiferromagnetic a- Fe2C>3 with a heteroatom precursor at low temperature in air. In one embodiment, pyrolytic synthesis of ferromagnetic a-Fe2C>3 from antiferromagnetic a-Fe2C>3 may involve antiferromagnetic a-Fe2C>3 nanoparticles as iron source and paracetamol as carbon source. In one embodiment, the ferromagnetic a-Fe2C>3 synthesized using pyrolytic synthesis method exhibits a magnetization of 6 emu/g or greater. In some embodiments, the ferromagnetic a-FciCh nanoparticles have a rhombohedral structure of size ranging from 20-60 nm.

[0046] In some embodiments, the pyrolytic synthesis methods include heating the admixture for 2 to 3 hours. Further, variation of exposing temperature and duration of heating in air may result in controlled variation of magnetization.

[0047] In various embodiments, the heteroatom precursor is selected from nitrogen, sulfur, boron or carbon precursor. In some embodiments, the carbon precursor is selected from paracetamol, urea, melamine, graphitic carbon nitride, carbon nanotubes, graphene oxide, graphene, or carbon nanofiber. In one embodiment, the sulfur precursor is selected from sulfur powder or hydrogen sulfide. In some embodiments, the boron precursor is selected from boric oxide, sodium borohydride or boric acid. In one embodiment, the nitrogen precursor is selected from sodium nitrate or ammonia.

[0048] In various embodiments, the iron precursor includes one or more iron salts. In one embodiment, the iron precursor is selected from iron nitrate, iron chloride or iron sulphate. In some embodiments, the predetermined weight ratio of heteroatom precursor to iron precursor is in the range of 2: 1 to 1:2. In one embodiment, the predetermined weight ratio of heteroatom precursor to iron precursor is in the range of 1: 1 to 1 : 1.5. In another embodiment, the predetermined weight ratio of heteroatom precursor to iron precursor is 1: 1.44.

[0049] In some embodiments, the synthesized a-FciCF is of high purity of at least 60%. In one embodiment, the synthesized a-FciCf has a purity of at least 80%. In another embodiment, the synthesized a-FciCF has a purity of at least 90%. In some embodiments, the methods results in a large yield of magnetic a-FciCf of at least 30-40 % of the total reactants.

[0050] In various embodiments, provided herein is ferromagnetic hematite iron oxide 100 as shown in FIG. 1. The hematite iron oxide 100 is having a rhombohedral crystal structure and belongs to the space group R-3c. The hematite iron oxide 100 has a crystal structure that includes sites of oxygen 101 and iron occupancy 102 that may have defects such as vacancies. In some embodiments, the crystal structure is characterized by oxygen or iron site defects that occupy a fraction of the oxygen sites. In some embodiments, the fractional oxygen occupancy 101 at l8e site in a unit cell of the structure ranges from 1 to 0.499 and the remaining fraction of oxygen sites form vacancies 103. In some embodiments, the iron occupancy 102 at l2c site in the unit cell ranges from 1.005 to 0.978. In some embodiments, an increase in the oxygen site defects is configured to cause a magnetization of 7 emu/g or greater. In some embodiments, an increase in the oxygen site defects is configured to cause a magnetization of 7-17 emu/g. In various embodiments, the oxygen vacancy distribution in the ferromagnetic hematite iron oxide is obtained by doping with a heteroatom selected from carbon, nitrogen, sulphur or boron. In some embodiments the heteroatoms may occupy interstitial sites or substitutional sites within the hematite lattice and cause distortion of the lattice. In various embodiments, the distortion of the hematite a-FciCf lattice is configured to cause the stabilization of oxygen vacancies, resulting in the ferromagnetism of the a- Fe C> . In some embodiments, the degree of heteroatom doping and thereby variation in oxygen vacancy concentration is configured to cause a variation in magnetization of the ferromagnetic a-FciCf.

[0051] In one embodiment, size of ferromagnetic hematite iron oxide nanoparticles produced using the combustion synthesis, pyrolysis processes ranges from 20-60 nm.

[0052] In one embodiment, the oxygen occupancy 101 at 18e site in a unit cell of the structure is 0.499, the iron occupancy 102 at l2c site in a unit cell of the structure is 1.001 and the corresponding magnetization exhibited by ferromagnetic hematite iron oxide nanoparticle is l7emu/g or greater. In another embodiment, the oxygen occupancy 101 at l8e site in a unit cell of the structure is 0.763, the iron occupancy 102 at l2c site in a unit cell of the structure is 1.005 and the corresponding magnetization exhibited by ferromagnetic hematite iron oxide nanoparticle is 7emu/g or greater. In some embodiments, the oxygen occupancy 101 at l8e site in a unit cell of the structure is 0.918, the iron occupancy 102 at l2c site in a unit cell of the structure is 0.978 and the corresponding magnetization exhibited by ferromagnetic hematite iron oxide nanoparticle is 7emu/g.

[0053] In various embodiments, the synthesized a-FeiCh has potential application in nanomagnetic devices, nanobiosensors, batteries, magnetic field controlled ion separation, giant magnetoresistance devices, and magnetic field controlled photocatalytic reactors and biomedical applications.

EXAMPLES

Example 1: Synthesis of ferromagnetic a-Fe203 by combustion method

[0054] The method employs a tubular furnace with a quartz tube placed inside the furnace and sealed with aluminum coupling with the provision for the passage of gases at the inlet and outlet.1 g of paracetamol was mixed with 1.44 g of iron nitrate nonahydrate in mortar pestle for 15-30 minutes. Further, the mixture was transferred to alumina / quartz boat and placed in the middle of the tubular furnace closed with couplings and heated to 550 °C for 2 h under argon gas at a flow rate of 0.16 l/min. At 300 °C argon was stopped and couplings were opened to initiate the combustion and then sample was cooled to room temperature. It was observed that the method yield 380 mg of ferromagnetic a-FciCf as given in Table 1.

Example 2: Synthesis of ferromagnetic a-FeiOs by pyrolysis method with iron salt as a precursor

[0055] 1 g of graphitic carbon nitride was mixed with 1.44 g of iron nitrate nonahydrate in mortar/ pestle for 15-30 minutes. Then the mixture was transferred to alumina / quartz boat and placed in the middle of the tubular furnace and heated to 400°C at l0°C/min for 2 h. After the reaction, it was observed that the method yield 400 mg of the ferromagnetic a-FeiCF as given in Table 1.

Example 3: Synthesis of ferromagnetic a-F_dOs from antiferromagnetic hematite

[0056] 50 mg of a-FciCfi was mixed with 500 mg of paracetamol and grounded for

15 min. Then the mixture was transferred to alumina / quartz boat and placed in the middle of the tubular furnace and heated to 300 °C for 2 h at 10 °C/min in air atmosphere. Analysis using XRD and Raman spectroscopy of the product confirmed the presence of only a-FeiCU It was observed from Raman spectroscopic analysis that there was no signature other than a-FeiCU The method yield l lOmg of ferromagnetic a-FciCF as given in Table 1.

TABLE 1: Yield Based on Precursors

[0057] TGA analysis of FIG. 2 shows the amount of heteroatom and amount of a- Fe₂0₃ present in the samples as shown in Table 2.

TABLE 2: TGA Analysis for Purity of the Sample and Estimation of a-FciCf

[0058] The peak shift observed in XRD pattern as shown in FIG. 3A, FIG. 3B and FIG. 3C of magnetic a- FciCf/C with respect to antiferromagnetic a- FciCF probably implies doping of some C atoms into the a- FciCf crystal structure which gave rise to lattice strain and shift towards lower angle. Details of planes and 2Q values are mentioned in the Table 3. The XRD pattern of magnetic a- FeiCL/C by combustion is shown in FIG.3A. Further, the XRD pattern of magnetic a- FciCF -gCN-400 by solid- state pyrolysis using graphitic carbon nitride is shown in FIG. 3B and the XRD pattern of magnetic a- FciCF /C from antiferromagnetic a- FciCf NP is shown in FIG. 3C.As illustrated in FIG. 3 A and FIG. 3C, all the peak corresponds to hematite. As shown in FIG. 3 B, some extra peaks are there around 27.3 ° and 43° which corresponds to graphitic carbon nitride peak. Only a- FciO^ phase can be found with the rhombohedral structure (space group R-3c). No other iron phase can be seen in XRD. From XRD data, shift of the 2Q value observed which indicates the strain induced in the FciO^ after the incorporation of carbon. Oxygen vacancies has been created with incorporation of carbon in the a- FciO^ crystal structure.

TABLE 3: Comparison of XRD Pattern for Synthesized Samples with a- FciO^ Standard

[0059] Raman spectra for the magnetic a-FdCE/C that was synthesized from antiferromagnetic a-FciCf nanoparticles is as shown in FIG. 4which confirms the presence of only hematite phase and no other phase of magnetite, maghemite or iron present in the sample.

[0060] FIG. 5A, FIG.5B and FIG. 5C represents TEM images of ferromagnetic a- FeiCE/C synthesized by combustion, solid-state pyrolysis using graphitic carbon nitride and using antiferromagnetic a- FciCf NP respectively. The microscopic images clearly confirmed the presence of carbon coating over a-FciCf nanoparticles. [0061] Room temperature M-H curve of ferromagnetic a-FciO C synthesized using combustion, solid-state pyrolysis using graphitic carbon nitride and using antiferromagnetic a- FeiCh NP respectively are shown in FIG. 6A ,FIG. 6B and FIG.6C respectively. For the combustion technique magnetization of -17 emu/g was attained. For solid-state pyrolysis using graphitic carbon nitride and iron salt, magnetization of -10 emu/g was observed. In presence of carbon, antiferromagnetic a-FciCf showed ferromagnetism with saturation magnetization of 6.67 emu/g.

[0062] Example 4: Ferromagnetic Hematite Iron Oxide Crystal Structure

[0063] Rietveld analysis has been done to determine the occupancy of oxygen(anion) and iron(cation) atoms in the crystal sites of hematite iron oxide crystal structure. Further, crystal lattice parameters and defects present in the crystal structure have been determined.

[0064] Detailed Rietveld analysis of pure a-FciO^ and a-FciO^/ C was done to probe the changes in the crystal lattice parameters of the crystal structure as provided in Table 4. Rietveld analysis may also be used to determine iron and oxygen distribution in the crystal structure as provided in Table 5. Along with the shift in the crystal lattice parameters, occupancy of the oxygen atoms changes from 1 to 0.499 which confirms the presence of oxygen vacancies and change in cationic-anionic distribution. FIG. 7A shows the simulated crystal structure of pure a-FciCF and FIG. 7B illustrates the simulated crystal structure of a-FciCF/ C from the crystallographic information obtained from Rietveld analysis.

TABLE 4: Comparison of Crystal Lattice Parameter of a-FciCF and a-FeiCF/ C

TABLE 5: Comparison of atomic positional parameter of a-FciCf and a-FciCf/ C obtained from Rietveld analysis

[0065] The combustion reaction has been performed in the presence and absence of carbon precursor by maintaining the same environment and hematite iron oxide was synthesized. It was observed that in the absence of carbon, iron precursor changed to Fe₃_ x0₄ to Fe_2-X0₃ and introduces iron vacancies instead of creation of oxygen defects. Simulated atomic positional parameters of hematite iron oxide crystal structure in the presence and absence of carbon are tabulated in Table 6. It was observed that there is change in iron(Fe) occupancy whereas oxygen (O) occupancy remained same as 1. FIG. 8A shows the simulated crystal structure of a-Fe₂0₃ synthesized by combustion method in the absence of carbon and FIG. 8B shows the simulated crystal structure of a-Fe₂0₃/C synthesized through combustion method in the presence of carbon.

TABLE 6: Comparison of atomic positional parameter of a-Fe₂0₃ and a-Fe₂0₃/ C obtained from Rietveld analysis

[0066] The oxygen vacancy with carbon content was analyzed. The amount of carbon(C) precursor content was varied (C:Fe as 0.5: 1 , 1 : 1 and 1.5: 1) and the change in the oxygen occupancy was determined from the Rietveld analysis. It was observed that with increase in the carbon precursor content from 0.5 to 1, oxygen occupancy decreased indicating the availability of more vacancies in the sample. Further increase of the carbon amount decreased the oxygen vacancies. FIG. 9A, FIG. 9B and FIG. 9C shows the crystal structure of a-FeiCVC with varying carbon(C) to iron(Fe) ratio 0.5: 1, 1 : 1 and 1.5: 1 respectively. Table 7 illustrates atomic positional parameter of a-FeiC C with varied carbon to iron ratio. The structural deviations in a-FeiCVC were observed with change in C:Fe ratio which may also be reflected in the magnetization data. It was observed that upon increase of the amount of carbon initially saturation magnetization increases up to a certain value and it is maximum for a-R¾0₃/0 (1 : 1) where the oxygen occupancy is lowest, then further it decreases with increased carbon content as shown in FIG. 10. Hence, carbon plays a pivotal role in tuning the oxygen and iron vacancies in the crystal structure. Magnetization of a-FeiC C increases with inclusion of carbon up to a definite limit, further increase in the carbon content decreases the oxygen vacancies, which in turn results in reduction in saturation magnetization.

TABLE 7: Comparison of atomic positional parameter of a-FeiCV C with varied carbon to iron ratio content obtained from Rietveld analysis

[0067] Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed herein. Various other modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the system and method of the present invention disclosed herein without departing from the spirit and scope of the invention as described here.

Claims

We claim:

1. A method of combustion synthesis of ferromagnetic hematite iron oxide composite, the method comprising:

heating a composite admixture comprising at least one iron precursor and at least one heteroatom precursor in a predetermined weight ratio to a temperature between 300 °C to 600 °C under an inert gas atmosphere and subsequently exposing the admixture to atmospheric air to synthesize ferromagnetic hematite iron oxide.

2. The method as claimed in claim 1, wherein the heteroatom precursor is selected from nitrogen, sulfur, boron or carbon precursor.

3. The method as claimed in claim 2, wherein the carbon precursor is selected from paracetamol, urea, melamine, graphitic carbon nitride, carbon nanotubes, graphene oxide, graphene, or carbon nanofiber.

4. The method as claimed in claim 2, wherein the sulfur precursor is selected from sulfur powder or hydrogen sulfide;

the boron precursor is selected from boric oxide, sodium borohydride or boric acid; and the nitrogen precursor is selected from sodium nitrate or ammonia.

5. The method as claimed in claim 1, wherein the iron precursor is selected from iron nitrate, iron chloride or iron sulphate.

6. The method as claimed in claim 1, wherein the heat treatment is carried out for 2h to 3h.

7. The method as claimed in claim 1, wherein the predetermined weight ratio of heteroatom precursor to iron precursor is in the range of 0.5 : 1 to 1.5: 1.

8. The method as claimed in claim 1, wherein the heating causes incorporation of the heteroatom into hematite iron oxide crystal structure to induce ferromagnetism.

9. A method of pyrolytic synthesis of ferromagnetic hematite iron oxide composite, the method comprising:

heating a composite admixture comprising at least one iron precursor and at least one heteroatom precursor in a predetermined weight ratio to a temperature between 400 °C to 750 °C under an air atmosphere to form ferromagnetic hematite iron oxide.

10. The method as claimed in claim 9, wherein the heteroatom precursor is selected from nitrogen, sulfur, boron or carbon precursors.

11. The method as claimed in claim 9, wherein the carbon precursor is selected from paracetamol, urea, melamine, graphitic carbon nitride, carbon nanotubes, graphene oxide, graphene, or carbon nanofiber.

12. The method as claimed in claim 9, wherein the sulfur precursor is selected from sulfur powder or hydrogen sulfide;

13. The method as claimed in claim 9, wherein the iron precursor is selected from iron nitrate, iron chloride or iron sulphate.

14. The method as claimed in claim 9, wherein the heat treatment is carried out for

2h to 3h.

15. The method as claimed in claim 9, wherein the predetermined weight ratio of heteroatom precursor to iron precursor is in the range of 2: 1 to 1 :2.

16. A method of pyrolytic synthesis of ferromagnetic hematite iron oxide from antiferromagnetic hematite iron oxide comprising:

heating a composite mixture comprising the antiferromagnetic hematite iron oxide with at least one heteroatom precursor in a predetermined weight ratio to a temperature between 300°C to 320°C in an air atmosphere to synthesize ferromagnetic hematite iron oxide.

17. The method as claimed in claim 16, wherein the heteroatom precursor is selected from nitrogen, sulfur, boron or carbon precursors.

18. The method as claimed in claim 16, wherein the carbon precursor is selected from paracetamol, urea, melamine, graphitic carbon nitride, carbon nanotubes, graphene oxide, graphene, or carbon nanofiber.

19. The method as claimed in claim 16, wherein the sulfur precursor is selected from sulfur powder or hydrogen sulfide;

20. The method as claimed in claim 16, wherein the heat treatment is carried out for 2h to 3h.

21. The method as claimed in claim 16, wherein the predetermined weight ratio of heteroatom precursor to antiferromagnetic hematite iron oxide is in the range of 2: 1 to

1 :2.

22. A ferromagnetic hematite iron oxide having rhombohedral crystal structure (space group R-3c), the crystal structure characterized by oxygen and iron site defects, wherein an oxygen occupancy at 18e site in a unit cell of the structure ranges from 1 to 0.499, the iron occupancy at l2c site in the unit cell ranges from 1.005 to 0.978, wherein an increase in the oxygen site defects is configured to cause a magnetization of 7emu/g or greater.

23. The ferromagnetic hematite iron oxide as claimed in claim 22, wherein the oxygen vacancy distribution is obtained by doping with a heteroatom selected from carbon, nitrogen, sulphur or boron.

24. The ferromagnetic hematite iron oxide as claimed in claim 22, wherein the magnetization obtained is between 7-17 emu/g.

25. Ferromagnetic hematite iron oxide nanoparticles having a rhombohedral crystal structure as claimed in claim 22, of size ranging from 20-60 nm.