WO2013035124A1 - Method to prepare supported nanomaterials based on iron(iii) oxide by the cvd technique and synthesis method of fe(hfa)2tmeda - Google Patents

Method to prepare supported nanomaterials based on iron(iii) oxide by the cvd technique and synthesis method of fe(hfa)2tmeda Download PDF

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WO2013035124A1
WO2013035124A1 PCT/IT2012/000276 IT2012000276W WO2013035124A1 WO 2013035124 A1 WO2013035124 A1 WO 2013035124A1 IT 2012000276 W IT2012000276 W IT 2012000276W WO 2013035124 A1 WO2013035124 A1 WO 2013035124A1
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hfa
substrate
tmeda
precursor
flow
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PCT/IT2012/000276
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French (fr)
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Alberto GASPAROTTO
Giorgio Carraro
Davide BARRECA
Chiara MACCATO
Eugenio TONDELLO
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Università Degli Studi Di Padova
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
    • C23C16/30—Deposition of compounds, mixtures or solid solutions, e.g. borides, carbides, nitrides
    • C23C16/40—Oxides
    • C23C16/406—Oxides of iron group metals
    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07F—ACYCLIC, CARBOCYCLIC OR HETEROCYCLIC COMPOUNDS CONTAINING ELEMENTS OTHER THAN CARBON, HYDROGEN, HALOGEN, OXYGEN, NITROGEN, SULFUR, SELENIUM OR TELLURIUM
    • C07F15/00—Compounds containing elements of Groups 8, 9, 10 or 18 of the Periodic Table
    • C07F15/02—Iron compounds
    • C07F15/025—Iron compounds without a metal-carbon linkage

Definitions

  • the object of the present invention is a method to prepare supported iron(III) oxide-based nanomaterials by CVD and a synthesis method for Fe(hfa) 2 TMEDA.
  • the nanomaterials, and in particular the nanostructured films, obtained thanks to method according to the present invention are suitable for use in solid state gas sensors, in systems for photocatalytic applications and in energy generation/storage devices, thanks to their morphology, tuneable as a function of processing conditions, to their phase purity and to the visible light absorption.
  • Systems based on iron oxides with the a-Fe 2 0 3 (hematite), P-Fe 2 0 3 (bixbyite), 8-Fe 2 0 3 (orthorhombic phase), and y-Fe 2 0 3 (maghemite) structure can be used in various technological applications, ranging from electro-optic devices to data storage, from gas sensors to electrodes for lithium-ion batteries, up to photocatalytic pollutant degradation and H 2 evolution from water activated by solar radiation.
  • Fe 2 0 3 -based materials both as powders and supported films, have been obtained by means of a large variety of processes, including hydro- and solvothermal methods, sol-gel, spray pyrolysis, electrochemical routes, evaporation, sputtering and epitaxial growths.
  • CVD Chemical Vapor Deposition
  • CVD techniques present lower restrictions regarding the shape and dimensions of the substrate to be covered (conformal step coverage), and allow the deposition of a wide range of thin films and nanostructures on various types of substrates.
  • the main aim of the present invention is the suppression of drawbacks of the current above sited technique, providing a simple method for the preparation of supported iron(III) oxide-based nanomaterials.
  • a further aim of the present invention is the development of a method for the preparation of supported iron(III) oxide-based nanomaterials by CVD, capable of controlling the morphology of the obtained iron(III) oxides.
  • a further aim of the present invention is to provide a method for the preparation of supported iron(III) oxide-based nanomaterials by means of CVD, enabling the selective deposition of c - Fe 2 0 3 and P-Fe 2 0 3 phases.
  • a further aim of the present invention is to provide a method for the preparation of supported iron(III) oxide-based nanomaterials by means of CVD capable of yielding high purity iron(III) oxides.
  • Figure 1 shows a representative sketch of the cold-wall CVD equipment used to implement the preparation route to obtain supported iron(III) oxide-based nanomaterials according to the present invention
  • Figure 2 shows a scheme of the synthetic process according to the invention for the preparation of Fe(hfa) 2 TMEDA and for its subsequent use according to the invention in CVD process for the development of Fe 2 0 3 -based materials;
  • Figures 5 a-b display plane- view and cross-sectional FE-SEM images, respectively, for a representative P-Fe 2 0 3 sample deposited on Si(100) at 400°C;
  • Figures 5 c-d show plane-view and cross-sectional FE-SEM images, respectively, for a representative cc-Fe 2 0 3 sample deposited on Si(100) at 500°C;
  • Figure 6 shows a representative SMS profile for a P-Fe 2 0 3 sample deposited on Si0 2 ;
  • Figure 7 display transmittance IR spectra for two P-Fe 2 0 3 and cc-Fe 2 0 3 samples deposited on Si(100) substrates at temperatures of 400°C and 500°C, respectively, and
  • Figure 8 shows the visible optical absorbance spectrum for a P-Fe 2 0 3 sample deposited on Si0 2 at 400°C.
  • the present invention describes a method to prepare supported iron(III) oxide-based nanomaterials by means of the Chemical Vapor Deposition (CVD) technique.
  • CVD Chemical Vapor Deposition
  • the nanosystem realm includes materials possessing at least one of the aggregate dimensions in the range 1-100 nm, so that the major solid portion is formed by interfacial regions and possesses a very high surface-to-volume ratio.
  • This feature along with charge carrier quantum confinement effects within such low-sized structures, is responsible for unique properties, rendering these systems considerably different from the conventional bulk materials. This is particularly true as regards gas sensing, photoactivated hydrogen production and energy storage, ideal application fields for the materials developed according to the present invention.
  • supported nanomaterials defines nanostructures grown on a substrate, for instance in the form of nanostructured films.
  • the Fe(hfa) 2 TMEDA precursor has the following structural formula (II):
  • This complex can be easily handled even in air, with great benefits in terms of simplified operation steps. Furthermore, it possesses improved mass transport properties and a suitable reactivity, resulting in significant advantages in terms of growth rates of iron oxide nanostructures.
  • TGA ThermoGravimetric analysis
  • DTA Differential Thermal Analysis
  • the Fe(hfa) 2 TMEDA precursor used in the method according to the present invention allows to advantageously overcome these drawbacks thanks to its monomelic nature and to the complete saturation of the metal center coordination sphere by hfa and TMEDA ligands.
  • This phenomenon enabling to prevent the formation of oligomers, provides the target compound not only with the above discussed improved thermal features, but also with a remarkable stability towards parasitic hydrolysis processes promoted by humid atmospheres. Consequently, this compound can be easily handled on open benches under ordinary conditions without any particular caution, a highly appealing feature from an applicative point of view.
  • the synthesis and structure of Fe(hfa) 2 TMEDA have been already reported in literature (for example by N.A. Bayley, D.E. Fenton, M.S.
  • the present invention also describes an alternative synthetic route to Fe(hfa) 2 TMEDA.
  • the method advantageously involves an extraction stage d) of Fe(hfa) 2 TMEDA from the solution into chloroform and a subsequent recovery e) of Fe(hfa) 2 TMEDA by solvent evaporation under reduced pressure and mild heating.
  • the synthesis method according to the present invention is appreciably simpler, since the synthesis is performed in a single solution, using water instead of toluene as a solvent, with a significantly lower environmental impact.
  • the compound was subjected to elemental analysis: found, C, 32.86 %; H, 3.24 %; N, 4.72 %; calculated, C, 32.79 %; H, 3.09 %; N, 4.78 %.
  • the obtained compound was analyzed by Electron Impact (EI) mass spectrometry (70 eV).
  • the preparation method of iron(III) oxide-based nanomatenals by CVD technique involves the decomposition of Fe(hfa) 2 TMEDA vapors in presence of gaseous 0 2 on a pre-heated deposition surface.
  • the precursor decomposition, as well as the concomitant reaction with 0 2 enables the formation of solid iron(III) oxides on the above heated surface, yielding Fe 2 0 3 nanostructures, for instance nanostructured films.
  • the deposition surface should be preferably heated in a temperature range between 300°C and
  • the variation of the deposition surface heating temperature between 480°C and 520°C, with a preferable value of 500°C, advantageously enables the selective formation of a-Fe 2 0 3 (hematite) on the deposition surface.
  • the deposition surface is preferably contacted with a gas flow containing both 0 2 and Fe(hfa) 2 TMEDA vapors.
  • This gas flux enables the removal of carbonaceous residuals (derived by the precursor decomposition) from the deposition surface.
  • This strategy allows to obtain substrate cleaning from these residuals, resulting hence in the formation of high purity iron(III) oxide nanostructures (in particular nanostructured films).
  • the above mentioned gas flow may advantageously include even water vapor. It has been verified that the presence of water vapor enables to enhance the purity of the obtained iron(III) oxide nanostructures.
  • the above gas flux preferably contacts the deposition surface with a laminar flow.
  • the absence or the minimization of turbulences close to the deposition surface enables its uniform coverage by iron oxide structures.
  • the method to prepare supported nanomaterials based on iron(III) oxides by CVD involves the following operating stages:
  • the substrate heating temperature is preferably controlled in such a way to selectively obtain P-Fe 2 0 3 or a-Fe 2 0 3 formation.
  • controlling the substrate heating temperature in the 380-420°C interval, and maintaining it preferably at 400°C enables to achieve the selective formation of P-Fe 2 0 3 on the substrate.
  • controlling the substrate heating temperature in the 480-520°C interval, and maintaining it preferably at 500°C enables to achieve the selective formation of a-Fe 2 0 3 on the substrate.
  • the transport gas flow profitably crosses the chamber, contacting at least one substrate.
  • carbon-containing residuals produced by the decomposition of Fe(hfa) 2 TMEDA precursor are removed from the substrate itself.
  • the transport gas flow crosses the chamber with laminar motion, at least in the proximity of the substrate.
  • the transport gas flow can be formed by pure 0 2 or can even comprise water vapor.
  • water partial pressure ranges between 10 and 50%.
  • the transport gas flow within the reaction chamber can have an 0 2 flow rate ranging between 10 and 400 seem (standard cubic centimeters per minute).
  • the Fe(II) precursor is advantageously vaporized by heating, as also discussed below.
  • the Fe(II) precursor is vaporized by maintaining an amount of solid precursor at a temperature between 50°C and 90°C, and preferably of 60°C, to avoid precursor thermal decomposition.
  • the method according to the invention to prepare supported nanomaterials based on iron(III) oxide by CVD can be utilized on a variety of substrates.
  • the substrate can be formed by single-crystal silicon, silica, or, in general, by glassy and dielectric materials, in particular polycrystalline alumina and homologous systems.
  • the synthesis of Fe 2 0 3 -based nanomaterials with controlled phase composition on highly rough polycrystalline alumina supports enables the development of resistive gas sensors, taking advantage of the unique substrate morphology to enhance the surface area and nano- organization of the target material.
  • the substrate can be formed by transparent conducting materials, such as, for instance, tin- doped indium oxide, or by non-transparent conducting materials, such as metallic titanium. Thanks to the versatility of the method according to the invention by the CVD technique, depositions can be extended to metallic substrates, in particular titanium, for instance for the development of anodes for last-generation lithium batteries.
  • the substrate can be formed by zinc oxide, cobalt oxide, copper oxide or their mixtures.
  • the reactor consists of a cold-wall pyrex cylindrical pipe 1, equipped with a resistive heater 2.
  • the latter is fabricated by a cylindric aluminum plate, mounted on a ceramic support 2a, in which a heating resistance 2b and a temperature sensor 2c are inserted. Terminals 2d of both components, resistance 2b and sensor 2c, are directly connected to external power source and transduction units respectively (not reported in the Figure), through a metallic flange 2e located on the lower heater side.
  • a chamber end 3 is connected, through a metal connector 4, to a pyrex vaporizer 5 maintained at a temperature of 60°C by means of an oil bath and containing Fe(hfa) 2 TMEDA.
  • the Fe(hfa) 2 TMEDA precursor is preferably, but not necessarily, synthesized by using the method according to the invention.
  • a pyrex reservoir 6 containing liquid water, heated at 40°C by means of external tapes throughout the process duration, can be connected to the chamber 1 through the metal connector 4.
  • the produced water vapor is subsequently transported into the reaction chamber under a pure oxygen flow.
  • the opposite side of the chamber 1 with respect to the one with the connector 4 presents another connector 7 for the outcoming gas flow.
  • the instrument is equipped with two flow-meters and two pressure gauges, resistive and capacitive, located respectively before and after the reaction chamber 1 (not reported in Figure 1).
  • substrate 8 is composed by:
  • the Fe(hfa) 2 TMEDA compound obtained according to the above described synthesis method, is vaporized in vaporizer 5 and subsequently transported by means of a pure 0 2 flow into the reaction chamber 1.
  • the connector 4 is heated at 120°C by means of external tapes.
  • the carrier gas is flowed out of the reaction chamber 1 through the connector 7, whereas at least a part of the precursor has been decomposed on the substrate 8, resulting in an homogeneous and crack- free deposit with color ranging from gray to reddish/brown.
  • Fe(hfa) 2 TMEDA free from Fe-C bonds, containing both the Fe and O elements desired in the final product, Fe 2 0 3 , directly bonded in the same molecule, possessing an appreciable volatility already at moderate temperatures (60°C), it is possible to obtain homogeneous Fe 2 0 3 nanomaterials.
  • the latter are deposited, for instance, on Si(100) or on Si0 2 substrates, by using a cold-wall CVD apparatus and an oxygen atmospheres, at relatively moderate temperatures ( ⁇ 550°C).
  • the obtained nanomaterials possess a high purity and are selectively formed by the ⁇ - ⁇ 2 0 3 or a-Fe 2 0 3 phases as a function of the sole heating temperature, 400°C or 500°C respectively.
  • An independent oxygen flow saturated by water vapor, with the same rate of the previous one, is separately introduced through reservoir 6 into the chamber itself. In the latter, maintained at a total pressure of 3 mbar and at a temperature of 300-550°C, a Si0 2 or p-type Si(100) substrate 8 with the above described characteristics had been previously located.
  • the distance between the precursor contained in vaporizer 5 and the substrate 8 was of 20 cm.
  • the process duration was one hour.
  • the obtained specimens were characterized in detail in their structure, composition and morphology by GIXRD, FE-SEM and IR analyses.
  • the invention enables to obtain various advantages, some of which have already been described.
  • the method for the preparation of supported nanomaterials based on iron(III) oxide according to the present invention is practically simple to be operated.
  • the air stability and favorable thermal properties of the Fe(hfa) 2 TMEDA precursor enable its easy manipulation.
  • the method according to the present invention enables to control the morphology of the resulting iron(III) oxides. It is also possible to selectively obtain a-Fe 2 0 3 or p-Fe 2 0 3 phases.
  • the method according to the present invention enables also to obtain iron(III) oxide nanostructures endowed with high purity and enhanced surface area, amenable for applications in gas sensing, catalysis/photocatalysis and for energy storage in last-generation lithium batteries.
  • the preparation of Fe 2 0 3 -based materials by the CVD method of the present invention offers various operating advantages with respect to other preparation techniques.
  • the method according to the invention enables to avoid the use of solvents, with considerable advantages in terms of final material purity, process costs and environmental impact.
  • the method enables to obtain, directly during the synthesis, supported materials ready for use, without the need of post-treatment processing to anchor the material to the substrates (such as those requested by powdered systems). Beside reducing production times and costs, this feature improves the adhesion between the substrate and the deposit, enhancing the mechanical stability of the latter and its service-life for various technological applications.
  • the developed invention thus meets the initial goals.

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Abstract

The invention concerns a method to prepare supported nanomaterials based on iron(III) oxide by the CVD technique, characterized by the fact that an iron(II) precursor, having general formula Fe(hfa)2TMEDA, where hfa indicates 1,1,1,5,5,5 - hexafluoro - 2,4 - pentanedionate and TMEDA indicates Ν,Ν,Ν',Ν' - tetramethylethylenediamine, is used.

Description

Description of the invention with the title:
Method to prepare supported nanomaterials based on iron(III) oxide by the CVD technique and synthesis method of Fe(hfa)2TMEDA filed on University of Padua
Inventors: Gasparotto Alberto, Carraro Giorgio, Barreca Davide, Maccato Chiara,
Tondello Eugenio
DESCRIPTION
Application Field
The object of the present invention is a method to prepare supported iron(III) oxide-based nanomaterials by CVD and a synthesis method for Fe(hfa)2TMEDA.
The nanomaterials, and in particular the nanostructured films, obtained thanks to method according to the present invention, are suitable for use in solid state gas sensors, in systems for photocatalytic applications and in energy generation/storage devices, thanks to their morphology, tuneable as a function of processing conditions, to their phase purity and to the visible light absorption.
Technological State of the Art
Systems based on iron oxides with the a-Fe203 (hematite), P-Fe203 (bixbyite), 8-Fe203 (orthorhombic phase), and y-Fe203 (maghemite) structure, can be used in various technological applications, ranging from electro-optic devices to data storage, from gas sensors to electrodes for lithium-ion batteries, up to photocatalytic pollutant degradation and H2 evolution from water activated by solar radiation.
In this context, one of the main critical issues is the availability of iron oxide materials with high purity and controlled phase composition and morphology, key features for the obtainment of chemico-physical properties and functional performances optimized in view of the required technological applications. Up to date, Fe203-based materials, both as powders and supported films, have been obtained by means of a large variety of processes, including hydro- and solvothermal methods, sol-gel, spray pyrolysis, electrochemical routes, evaporation, sputtering and epitaxial growths.
In this context, chemical vapor deposition routes, known as CVD (Chemical Vapor Deposition), possess a strategic potential for the preparation of Fe203 films since they allow an improved control of stoichiometry, phase composition and impurity concentration with respect to other production technologies.
In addition, CVD techniques present lower restrictions regarding the shape and dimensions of the substrate to be covered (conformal step coverage), and allow the deposition of a wide range of thin films and nanostructures on various types of substrates.
Nevertheless, the full control of a CVD process depends on the availability of suitable molecular compounds as precursors for the desired material and on the careful modulation of the synthetic conditions for the obtainment of the final product.
CVD syntheses of Fe203 thin films and nanomaterials have been up to date carried out starting from various iron compounds, such as halogenated salts, Fe(CO)5, ferrocene and its derivatives, alkoxides and Fe(II)/Fe(III) β-diketonates, as reported, for instance, in US patents US2008038482, US2005164011 and US3695908.
Nevertheless, these compounds present various drawbacks due to one or more of the following issues:
• High toxicity and pyrophoricity, resulting in difficult precursor handling under ordinary conditions;
• Narrow temperature window between vaporization and decomposition, resulting in a difficult process control;
• Presence of parasitic reactions in the gas phase and/or uncontrolled precursor decomposition, resulting in the formation of powders and contamination of the deposit material, with parallel degradation of its functional properties;
• Low vapour pressure, determining too low growth rates for technological process exploitation;
• Necessity of high deposition temperatures in order to obtain the desired crystalline phase, with considerable economical drawbacks and technical difficulties for the instrumental apparatus operation and maintenance.
In the framework of CVD processes aimed at the synthesis of Fe203, the majority of scientific papers in the specialized literature describes the obtainment of the a phase (hematite), and its utilization for the photocatalytic hydrogen production from water and for the fabrication of solid state gas sensors. The technological potential of this phase can be traced back not only to its low cost, but also to the non-toxicity and low environmental impact of iron oxide, joined with its chemical stability and interesting optical properties (band gap « 2.0-2.2 eV).
In spite of these advantages, it is worth noticing that some a-Fe203 electrical properties (short charge carrier diffusion length, position of the conduction band edge) lower its functional performances in the above fields.
On this basis, the selective obtainment of the less studied p-Fe203 phase can be extremely promising in order to overcome the intrinsic hematite drawbacks.
The applicative potential of both a-Fe203 and P-Fe203 phases can anyway be improved thanks to the obtainment of high active area materials with controlled nanoscale organization (for instance, nanotubes, nanowires, dendritic structures). Such systems offer in fact several degrees of freedom for the obtainment of materials with tailored properties, as requested by current technologies. In the framework of CVD technologies devoted to the preparation of Fe203 films, the main technical problems representing up to date an open challenge must be traced back both to the disadvantages of the actually used precursors (as already discussed above) and to the non- optimal features of the final material due to some process drawbacks, such as:
i) High toxicity and/or air reactivity of some precursors, making difficult their use and manipulation prior to deposition, as well as complicating the CVD equipment maintenance;
ii) The reduced volatility and/or uncontrolled decomposition during the process, resulting in moderate growth rates (and, as a consequence, too long deposition times for industrial applications) and materials with non-optimal features for technological applications (for instance, for the low purity or for the gas-phase formation of powders than can be incorporated in the film, altering its electrical, mechanical and optical properties,...);
iii) The difficulty in obtaining materials selectively containing a single crystalline phase (phase purity) and with a morphology tuneable as a function of the target application (for instance, dense systems, porous matrices, nanotubes/nanowire arrays,...).
Invention Presentation
As a consequence, the main aim of the present invention is the suppression of drawbacks of the current above sited technique, providing a simple method for the preparation of supported iron(III) oxide-based nanomaterials.
A further aim of the present invention is the development of a method for the preparation of supported iron(III) oxide-based nanomaterials by CVD, capable of controlling the morphology of the obtained iron(III) oxides.
A further aim of the present invention is to provide a method for the preparation of supported iron(III) oxide-based nanomaterials by means of CVD, enabling the selective deposition of c - Fe203 and P-Fe203 phases.
A further aim of the present invention is to provide a method for the preparation of supported iron(III) oxide-based nanomaterials by means of CVD capable of yielding high purity iron(III) oxides.
Short Figure Description
The technical features of the invention, according to the above aims, are clearly evidenced by the content of the claims reported below, and its advantages will be further highlighted in the following detailed description, with reference to the enclosed figures, that represent one or more of its preferred development forms, provided only as non-restrictive examples and, in which:
• Figure 1 shows a representative sketch of the cold-wall CVD equipment used to implement the preparation route to obtain supported iron(III) oxide-based nanomaterials according to the present invention;
• Figure 2 shows a scheme of the synthetic process according to the invention for the preparation of Fe(hfa)2TMEDA and for its subsequent use according to the invention in CVD process for the development of Fe203-based materials;
• Figure 3 shows the results of thermogravimetric TGA and DTA analyses performed on a Fe(hfa)2TMEDA sample, obtained according to the synthetic process of the invention;
• Figure 4 shows GIXRD patterns (incidence angle = 1°) of two p-Fe203 and a-Fe203 samples deposited on Si(100) substrates at 400°C and 500°C, respectively;
• Figures 5 a-b display plane- view and cross-sectional FE-SEM images, respectively, for a representative P-Fe203 sample deposited on Si(100) at 400°C;
• Figures 5 c-d show plane-view and cross-sectional FE-SEM images, respectively, for a representative cc-Fe203 sample deposited on Si(100) at 500°C;
• Figure 6 shows a representative SMS profile for a P-Fe203 sample deposited on Si02;
• Figure 7 display transmittance IR spectra for two P-Fe203 and cc-Fe203 samples deposited on Si(100) substrates at temperatures of 400°C and 500°C, respectively, and
• Figure 8 shows the visible optical absorbance spectrum for a P-Fe203 sample deposited on Si02 at 400°C.
Detailed Description
The present invention describes a method to prepare supported iron(III) oxide-based nanomaterials by means of the Chemical Vapor Deposition (CVD) technique.
In general, the nanosystem realm includes materials possessing at least one of the aggregate dimensions in the range 1-100 nm, so that the major solid portion is formed by interfacial regions and possesses a very high surface-to-volume ratio. This feature, along with charge carrier quantum confinement effects within such low-sized structures, is responsible for unique properties, rendering these systems considerably different from the conventional bulk materials. This is particularly true as regards gas sensing, photoactivated hydrogen production and energy storage, ideal application fields for the materials developed according to the present invention.
The term "supported nanomaterials" defines nanostructures grown on a substrate, for instance in the form of nanostructured films.
According to a general implementation form of the present invention, the preparation route of supported iron(III) oxide-based nanomaterials by the CVD technique involves the use of a Fe(II) precursor, with general formula Fe(hfa)2TMEDA (hfa = 1,1,1,5,5,5 - hexafluoro - 2,4 - pentanedionate, TMEDA = Ν,Ν,Ν',Ν' - tetramethylethylenediamine). The Fe(hfa)2TMEDA precursor has the following structural formula (II):
Figure imgf000008_0001
The Fe(hfa)2TMEDA is a solid deep-purple powder at room temperature (melting point = 82 °C).
The complete saturation of the iron (II) coordination sphere by two fiuorinated β-diketonate ligands and a bidentate diamine one provides to the Fe(hfa)2TMEDA precursor very favorable thermal and chemical properties, making it a preferred choice for CVD applications with respect to all iron compounds adopted up to date.
This complex can be easily handled even in air, with great benefits in terms of simplified operation steps. Furthermore, it possesses improved mass transport properties and a suitable reactivity, resulting in significant advantages in terms of growth rates of iron oxide nanostructures.
The precursor thermal properties have been investigated by means of ThermoGravimetric analysis (TGA) and Differential Thermal Analysis (DTA). These analyses (carried out with a heating rate = KTCxmin"1) have provided analogous results both in air and nitrogen atmospheres (see Figure 3). As can be observed from Figure 3 graphs, the precursor is stable up to 110°C, whereas at higher temperatures a considerable weight loss, associated to a single- step powder vaporization, occurs. The quantitative sublimation of the target adduct free from parasitic side decompositions is confirmed by the residual weight close to zero for temperatures higher than 190°C. This feature appears extremely favorable for use of Fe(hfa)2TMEDA in CVD processes as an alternative to commonly adopted iron precursors, such as acetylacetonate and alkoxide derivatives, showing residual weights from 20 to 70 %, along with multi-stage decompositions significantly decreasing the mass supply during the process, as well as the reproducibility and control of the latter.
The Fe(hfa)2TMEDA precursor used in the method according to the present invention allows to advantageously overcome these drawbacks thanks to its monomelic nature and to the complete saturation of the metal center coordination sphere by hfa and TMEDA ligands. This phenomenon, enabling to prevent the formation of oligomers, provides the target compound not only with the above discussed improved thermal features, but also with a remarkable stability towards parasitic hydrolysis processes promoted by humid atmospheres. Consequently, this compound can be easily handled on open benches under ordinary conditions without any particular caution, a highly appealing feature from an applicative point of view. The synthesis and structure of Fe(hfa)2TMEDA have been already reported in literature (for example by N.A. Bayley, D.E. Fenton, M.S. Leal Gonzalez, Inorg. Chim. Acta, 1984, 88, 125; M. Dickman, Acta Crystallogr. Sect. C-Cryst. Struct. Commun., 1998, IUC9800048, or in the USA patent US4180386). Up to date, the application of this compound was as antiknock additive in fuels. It has never been used before now to obtain Fe203-based materials via CVD routes.
The only reported method up to date for the synthesis of Fe(hfa)2TMEDA [Bayley 1984, ibidem] involves a multi-step process characterized by the following working stages:
i) reaction of iron(II) sulphate with Hhfa/NaOH to obtain Fe(hfa)2;
ii) Fe(hfa)2 recovery on silica gels at reduced pressures;
iii) subsequent reaction between Fe(hfa)2 and TMEDA in toluene, to obtain Fe(hfa)2TMEDA; iv) recovery of the desired product by filtration.
The present invention also describes an alternative synthetic route to Fe(hfa)2TMEDA.
In detail, the synthesis procedure according to the present invention involves the following steps (as sketched in Figure 2):
a) reaction of iron(II) chloride (FeCl2) with Hhfa (molecular formula CF3COCH2COCF3) in an acqueous NaOH solution, yielding Fe(hfa)2, with the following structural formula
(I):
Figure imgf000010_0001
b) addition of TMEDA (molecular formula (CH3)2NCH2CH2N(CH3)2);
c) reaction between Fe(hfa)2 and TMEDA with consequent formation of Fe(hfa)2TMEDA. The method advantageously involves an extraction stage d) of Fe(hfa)2TMEDA from the solution into chloroform and a subsequent recovery e) of Fe(hfa)2TMEDA by solvent evaporation under reduced pressure and mild heating.
The synthesis method according to the present invention is appreciably simpler, since the synthesis is performed in a single solution, using water instead of toluene as a solvent, with a significantly lower environmental impact.
An example of Fe(hfa)2TMEDA preparation according to the invention method is reported below.
To an aqueous solution of NaOH (0.93 g, 23.5 mmol, in 10 mL of deionised H20), maintained under vigorous stirring, 3.3 ml of Hhfa (d = 1.47 gxmL"1, 23.3 mmol) were added dropwise. A solution of FeCl2 (1.49 g, 11.73 mmol, in 50 ml of deionized H20) was dropped into the previous one, resulting in a color change from dark yellow to maroon. After reacting for 1 h under stirring, 1.8 mL of TMEDA (d = 0.77 gxmL"1; 12.59 mmol) were slowly added to the obtained solution, that turned to a darker purple color. After stirring for 2.5 h, the complex was extracted from the aqueous phase into chloroform, and the organic solution was concentrated by mild heating. Finally, solvent evaporation under reduced pressure (30 °C, 10"3 mbar) yielded a deep purple solid (melting point = 82°C; process yield = 70 %).
The compound was subjected to elemental analysis: found, C, 32.86 %; H, 3.24 %; N, 4.72 %; calculated, C, 32.79 %; H, 3.09 %; N, 4.78 %.
The obtained product was analyzed by Fourier Transform JR Spectroscopy (FT-IR). The results are reported below: 1630 cm"1 (s), C=0 stretching modes of hfa ligands; 1380-1550 cm" 1 (s), C=C stretching and P(C-H) modes; 1344 cm"1 (m), C-CF3 and C-C stretching modes; 1257 cm"1 (s), 1188 cm"1 (s) and 1144 cm"1 (s): combination of p(C-H), v(C-CF3) and v(C-F) modes; 1099 cm"1 (m), C-C stretching in hfa ligands; 930-1050 cm"1 (m), combination of C- C/C-N stretching and CH3/C¾ deformation modes of TMEDA. For the assignment of IR bands on the basis of homologous compounds, the following papers have been taken as references: G. Bandoli, D. Barreca, A. Gasparotto, R. Seraglia, E. Tondello, A. Devi, R.A. Fischer, M. Winter, E. Fois, A. Gamba, G. Tabacchi, Phys. Chem. Chem. Phys., 2009, 11, 5998; G. Bandoli, D. Barreca, A. Gasparotto, C. Maccato, R. Seraglia, E. Tondello, A. Devi, R.A. Fischer, M. Winter, Inorg. Chem., 2009, 48, 82.
The obtained compound was subjected to Ή-NMR spectroscopic analysis (400.13 MHz, CDC13, 298 K, TMS). The obtained data are reported below: δ = 121.1 and 47.8 (6H each, TMEDA CH3- protons), 80.2 and 47.7 (2H each, TMEDA CH2- protons) and 5.9 (2H, hfa CH- protons). The obtained compound was analyzed by Electron Impact (EI) mass spectrometry (70 eV). The obtained results are reported below: m/z (%) = 586.4 (11), [Fe(hfa)2TMEDA]+*; 567.3 (3), [Fe(hfa)2TMEDA - F]+; 470.2 (57), [Fe(hfa)2]+'; 420.1 (25), [Fe(hfa)2 - CF2]+; 401.1 (54), [Fe(hfa)2 - CF3]+; 379.3 (36), [Fe(hfa)TMEDA]+; 351.1 (5), [Fe(hfa)2 - CF2 - CF3]+"; 282.1 (7), [Fe(hfa)2 - CF2 - 2CF3]+; 263.1 (13), [Fe(hfa)]+; 232.0 (10), [Fe(hfa)2 - 2CF2 - 2CF3]+; 213.0 (85), [Fe(hfa) - CF2f; 116.2 (19), TMEDA+"; 72.1 (12), (CH3)2N(CH2)2 +; 69.0 (93), CF3 +; 58.1 (100), (CH3)2NCH2 +; 42.1 (29), N(CH2)2 +. This fragmentation pathways is in line with data reported for rare-earth adducts containing the same β-diketonate ligand, see R. Lo Nigra, R.G. Toro, M.E. Fragala, P. Rossi, P. Dapporto, G. Malandrino, Inorg. Chim. Acta, 2009, 362, 4623.
According to a preferred implementation form, the preparation method of iron(III) oxide-based nanomatenals by CVD technique involves the decomposition of Fe(hfa)2TMEDA vapors in presence of gaseous 02 on a pre-heated deposition surface. The precursor decomposition, as well as the concomitant reaction with 02, enables the formation of solid iron(III) oxides on the above heated surface, yielding Fe203 nanostructures, for instance nanostructured films.
The deposition surface should be preferably heated in a temperature range between 300°C and
550°C.
It has been verified that a suitable control of the deposition surface heating temperature allows a direct control on the phase composition and morphology of the grown iron(III) oxides.
The variation of the deposition surface heating temperature between 380°C and 420°C, with a preferable value of 400°C, advantageously enables the selective formation of P-Fe203 (bixbyite) on the deposition surface.
The variation of the deposition surface heating temperature between 480°C and 520°C, with a preferable value of 500°C, advantageously enables the selective formation of a-Fe203 (hematite) on the deposition surface. The deposition surface, as will be also discussed below, is preferably contacted with a gas flow containing both 02 and Fe(hfa)2TMEDA vapors. This gas flux enables the removal of carbonaceous residuals (derived by the precursor decomposition) from the deposition surface. This strategy allows to obtain substrate cleaning from these residuals, resulting hence in the formation of high purity iron(III) oxide nanostructures (in particular nanostructured films). The above mentioned gas flow may advantageously include even water vapor. It has been verified that the presence of water vapor enables to enhance the purity of the obtained iron(III) oxide nanostructures.
The above gas flux preferably contacts the deposition surface with a laminar flow. The absence or the minimization of turbulences close to the deposition surface enables its uniform coverage by iron oxide structures.
According to a preferred implementation form of the invention, the method to prepare supported nanomaterials based on iron(III) oxides by CVD involves the following operating stages:
a) setting up a cold-wall chamber, within which at least one substrate, defining the Fe203 deposition surface, is located;
b) vaporizing the Fe(hfa)2TMEDA precursor under a flowing transport gas comprising 02; c) introducing precursor vapors into the reaction chamber;
d) heating the substrate at temperature between 300°C and 550°C;
e) decomposing the vapors of the Fe(II) precursor on the substrate by the combined action of heat provided to the substrate itself and of 02 contained in the transport gas, leading to the formation of solid Fe203 on the substrate itself.
As already observed, the substrate heating temperature is preferably controlled in such a way to selectively obtain P-Fe203 or a-Fe203 formation.
In particular, controlling the substrate heating temperature in the 380-420°C interval, and maintaining it preferably at 400°C, enables to achieve the selective formation of P-Fe203 on the substrate. Conversely, controlling the substrate heating temperature in the 480-520°C interval, and maintaining it preferably at 500°C, enables to achieve the selective formation of a-Fe203 on the substrate.
The transport gas flow profitably crosses the chamber, contacting at least one substrate. In this way, as already observed, carbon-containing residuals produced by the decomposition of Fe(hfa)2TMEDA precursor are removed from the substrate itself.
Preferably, the transport gas flow crosses the chamber with laminar motion, at least in the proximity of the substrate.
The transport gas flow can be formed by pure 02 or can even comprise water vapor.
In the case of a transport gas flow comprising water vapor, water partial pressure ranges between 10 and 50%.
In particular, the transport gas flow within the reaction chamber can have an 02 flow rate ranging between 10 and 400 seem (standard cubic centimeters per minute).
The Fe(II) precursor is advantageously vaporized by heating, as also discussed below.
In particular, the Fe(II) precursor is vaporized by maintaining an amount of solid precursor at a temperature between 50°C and 90°C, and preferably of 60°C, to avoid precursor thermal decomposition.
The method according to the invention to prepare supported nanomaterials based on iron(III) oxide by CVD can be utilized on a variety of substrates.
In particular, the substrate can be formed by single-crystal silicon, silica, or, in general, by glassy and dielectric materials, in particular polycrystalline alumina and homologous systems. The synthesis of Fe203-based nanomaterials with controlled phase composition on highly rough polycrystalline alumina supports enables the development of resistive gas sensors, taking advantage of the unique substrate morphology to enhance the surface area and nano- organization of the target material.
The substrate can be formed by transparent conducting materials, such as, for instance, tin- doped indium oxide, or by non-transparent conducting materials, such as metallic titanium. Thanks to the versatility of the method according to the invention by the CVD technique, depositions can be extended to metallic substrates, in particular titanium, for instance for the development of anodes for last-generation lithium batteries.
The substrate can be formed by zinc oxide, cobalt oxide, copper oxide or their mixtures.
The method according to the invention to prepare supported nanomaterials based on iron(III) oxide by CVD will now be described in detail in a preferred implementation form, reported as a non-restrictive example, with reference to the enclosed figures.
In detail, the preparation method according to the invention has been implemented on a CVD apparatus schematically represented in Figure 1. The reactor consists of a cold-wall pyrex cylindrical pipe 1, equipped with a resistive heater 2. The latter is fabricated by a cylindric aluminum plate, mounted on a ceramic support 2a, in which a heating resistance 2b and a temperature sensor 2c are inserted. Terminals 2d of both components, resistance 2b and sensor 2c, are directly connected to external power source and transduction units respectively (not reported in the Figure), through a metallic flange 2e located on the lower heater side. A chamber end 3 is connected, through a metal connector 4, to a pyrex vaporizer 5 maintained at a temperature of 60°C by means of an oil bath and containing Fe(hfa)2TMEDA. The Fe(hfa)2TMEDA precursor is preferably, but not necessarily, synthesized by using the method according to the invention.
Pure oxygen flows through vaporizer 5. A pyrex reservoir 6 containing liquid water, heated at 40°C by means of external tapes throughout the process duration, can be connected to the chamber 1 through the metal connector 4. The produced water vapor is subsequently transported into the reaction chamber under a pure oxygen flow. The opposite side of the chamber 1 with respect to the one with the connector 4 presents another connector 7 for the outcoming gas flow. The instrument is equipped with two flow-meters and two pressure gauges, resistive and capacitive, located respectively before and after the reaction chamber 1 (not reported in Figure 1).
On the resistive heater 2, a solid substrate 8 for deposition of the target Fe203-based materials is mounted. According to the present example, substrate 8 is composed by:
- p-type Si(100) (MEMC®, Merano, Italy; size = 1 cm 1 cm x 0.1 cm), or
- Si02 (Herasil silica, Heraeus®), with the same size of the previous one, preliminarily subjected to a suitable cleaning procedure for the removal of undesired contaminants.
The Fe(hfa)2TMEDA compound, obtained according to the above described synthesis method, is vaporized in vaporizer 5 and subsequently transported by means of a pure 02 flow into the reaction chamber 1. An additional 02 flux, possibly saturated by water vapor after passing through reservoir 6 (water partial pressure = 0.3 mbar for a total pressure of 3.0 mbar), is separately introduced into the same chamber 1. In order to prevent precursor condensation phenomena between the vaporizer 5 and the reaction chamber 1, the connector 4 is heated at 120°C by means of external tapes.
The substrate 8 mounted on the aluminum sample holder on the ceramic support 2a, heated at temperatures between 300 and 550°C, is located in the reaction chamber 1, maintained at a total pressure ranging between 1 and 1000 mbar. The distance between the precursor contained in vaporizer 5 and the substrate 8, for instance of 20 cm, is sufficient to ensure a laminar flow of the gas mixture over the deposition and growth surface.
The carrier gas is flowed out of the reaction chamber 1 through the connector 7, whereas at least a part of the precursor has been decomposed on the substrate 8, resulting in an homogeneous and crack- free deposit with color ranging from gray to reddish/brown.
Thanks to the particular precursor used, Fe(hfa)2TMEDA, free from Fe-C bonds, containing both the Fe and O elements desired in the final product, Fe203, directly bonded in the same molecule, possessing an appreciable volatility already at moderate temperatures (60°C), it is possible to obtain homogeneous Fe203 nanomaterials. The latter are deposited, for instance, on Si(100) or on Si02 substrates, by using a cold-wall CVD apparatus and an oxygen atmospheres, at relatively moderate temperatures (<550°C).
As will be discussed below, the obtained nanomaterials possess a high purity and are selectively formed by the β-Ρε203 or a-Fe203 phases as a function of the sole heating temperature, 400°C or 500°C respectively.
Such features, together with the controlled morphology, asscociated to high active area and porosity, render the obtained systems attractive candidates for use in sensing of various toxic or flammable gases (e.g. CO, N02, 03, CH4, H2), in photocatalysis (hydrogen production, pollutant degradation) and in devices for energy generation and storage. In such fields, in fact, the combined control of morphology and phase composition on the nano-scale has a direct beneficial impact on the functional performances of the target materials.
The compound Fe(hfa)2TMEDA (obtained and characterized according to the above description) has been inserted in vaporizer 5 (amount = 300 mg for each growth experiment) and heated at 60°C. The obtained vapors have been transported in a pure oxygen flow (rate = 40 seem) into the reaction chamber 1. An independent oxygen flow saturated by water vapor, with the same rate of the previous one, is separately introduced through reservoir 6 into the chamber itself. In the latter, maintained at a total pressure of 3 mbar and at a temperature of 300-550°C, a Si02 or p-type Si(100) substrate 8 with the above described characteristics had been previously located.
In each of the preformed experiments, the distance between the precursor contained in vaporizer 5 and the substrate 8 was of 20 cm. The process duration was one hour.
After repeated experiments in the same conditions, aimed at ascertaining the reproducibility of experimental results, the obtained specimens were characterized in detail in their structure, composition and morphology by GIXRD, FE-SEM and IR analyses.
The results of GIXRD (Glancing Incidence X-ray Diffraction) investigation are reported below. The patterns for two samples deposited at the temperatures of 400 and 500°C under the above indicated conditions are reported in Figure 4. In both cases, the results evidence the obtainment of crystalline systems, though characterized by a different phase composition. In particular, at 400°C peaks located at 2θ = 23.2°, 33.0° and 38.4° were observed and attributed to (211), (222) and (400) reflections of the p-Fe203 phase (Pattern n° 039-0238, JCPDS, 2000). In a different way, an increase of the deposition temperature to 500°C led to the appearance of peaks at 2θ = 24.2°, 33.1° and 35.6°, ascribable to (012), (104) and (110) reflections of the a- Fe203 form (Pattern n°00-033-0664, JCPDS, 2000). Such a variation can be explained taking into account that, since the a phase is the most thermodynamically stable one, its obtainment is favored by an increase of the growth temperature.
In both cases, a comparison of the relative intensities with those reported for the powder spectra enables to exclude the presence of preferred orientations. In addition, the average crystallite dimensions, estimated by means of the Scherrer equation (40 nm), indicate the obtainment of nanostructured materials. Furthermore, the possibility of selectively obtaining one of the two phases, β or a, through the sole variation of 100°C in the deposition temperature, has a direct applicative outcome for the possibility of tailoring the features of the resulting Fe203-based material, which, in turn, are directly dependent on its phase composition, as already discussed.
The results of FE-SEM (Field Emission-Scanning Electron Microscopy) investigation are reported below. The acquisition of different FE-SEM images in various regions of each sample has enabled to verify their morphological homogeneity. The micrographs corresponding to the sample obtained at 400°C (Figures 5 (a)-(b)) show the presence of pyramidal structures (average dimensions = 100-200 nm) arising from the growth of adjacent triangular planes, resulting in the ultimate formation of a deposit with an average thickness of 700 nm and a considerable porosity. The images of the hematite sample, obtained at 500°C, are reported in Figures 5 (c)-(d). The specimen is predominantly formed by globular aggregates (sizes = 200- 300 nm), along with lamellar pyramidal structures (average dimensions = 800 nm), giving rise to a porous system with an overall thickness of about 1 μιη.
The considerable growth rate, observed for both systems, makes the preparation method by CVD described in the present invention of a considerable interest from an industrial point of view. In particular, the morphological features of the above materials pave the way to their applications in sensing, photocatalysis and energy generation/storage, since they combine a high active area with the presence of a remarkable reactivity, positively affecting their functional performances.
The results of SIMS (Secondary Ion Mass Spectrometry) investigation are reported below. The analysis of the in-depth chemical composition by SIMS has not shown a significant dependence on the operating conditions. As an example, Figure 6 reports a representative profile pertaining to an Fe203 sample deposited on Si02. As can be observed, the system is characterized by a remarkable compositional homogeneity, and the parallel trend of iron and oxygen ionic yields suggests their common chemical origin. Notwithstanding the silicon and carbon contamination observed in the outermost material regions, the overall carbon amount was typically lower than 50 ppm, suggesting a clean conversion of the Fe(hfa)2TMEDA precursor into Fe203 under the adopted operating conditions. Such a result highlights the validity of the method reported in the present invention. For all elements, a net and well- defined interface with the substrate was observed.
The results of IR and optical absorption studies are reported below. R spectra for two a-Fe203 and -Fe203 samples in the 350-800 cm"1 interval are reported in Figure 7. In the former case, two intense bands located at 524 and 435 cm"1 were observed and ascribed to lattice vibration of the a-Fe203 phase, even taking into account possible shifts due to the material nanostructure. In the latter case, a broad absorption band for wavenumbers lower than 700 cm"1 was observed, and ascribed to the P-Fe203 phase. The lack of overlaps between the two spectra further corroborates the selective obtainment of the pure a-Fe203 and P-Fe203 phases, controlling the sole growth temperature, in line with the above discussed diffraction patterns reported in Figure 4.
The optical absorption spectrum in the Visible range for a P-Fe203 sample deposited on Si02 is displayed in Figure 8. The obtained trend agrees to a good extent with the few works available in the literature for such phase (compare T. Maruyama, T. Kanagawa, J. Electrochem. Soc, 1996, 143, 1675 and A. Kumar, A. Singhal, Nanotechnol., 2009, 20, 295606). The absorbance increase for λ<700 nm is associated to the typical band-to-band electronic transitions of iron oxide. An estimation of the energy gap from the absorption onset yields a value of Eg = 1.7 eV, in good agreement with previous literature results (see [Maruyamal996, ibidem] and E.-T. Lee, G.-E. Jang, C.K. Kim, D.-H. Yoon, Sens. Actuators, B, 2001, 77, 221). This result is of relevant practical importance for application of the Fe203-based nanomaterials in fields requiring a strong electromagnetic radiation absorption in the Visible range, such as photocatalysis for the degradation of water and air pollutants and hydrogen production, a field in which the use of the materials prepared according to the method of the present invention has been almost unexplored up to date.
The invention enables to obtain various advantages, some of which have already been described.
The method for the preparation of supported nanomaterials based on iron(III) oxide according to the present invention is practically simple to be operated. The air stability and favorable thermal properties of the Fe(hfa)2TMEDA precursor enable its easy manipulation. The method according to the present invention enables to control the morphology of the resulting iron(III) oxides. It is also possible to selectively obtain a-Fe203 or p-Fe203 phases. The method according to the present invention enables also to obtain iron(III) oxide nanostructures endowed with high purity and enhanced surface area, amenable for applications in gas sensing, catalysis/photocatalysis and for energy storage in last-generation lithium batteries.
The preparation of Fe203-based materials by the CVD method of the present invention offers various operating advantages with respect to other preparation techniques.
At variance with liquid phase routes such as hydro- and solvothermal ones, sol-gel and electrochemical processes, the method according to the invention enables to avoid the use of solvents, with considerable advantages in terms of final material purity, process costs and environmental impact.
In addition, the method enables to obtain, directly during the synthesis, supported materials ready for use, without the need of post-treatment processing to anchor the material to the substrates (such as those requested by powdered systems). Beside reducing production times and costs, this feature improves the adhesion between the substrate and the deposit, enhancing the mechanical stability of the latter and its service-life for various technological applications. The developed invention thus meets the initial goals.
Of course, the invention can also assume, in its practical implementation, even forms and configurations different from the above illustrated one, without exiting from the present protection field.
In addition, all the details could be substituted by technically equivalent elements and the dimension, shapes and employed materials could be anyone depending on the specific needs.

Claims

1. Method for preparing supported nanomaterials based on Fe(III) oxide using the CVD technique, characterised by the fact of using an Fe(II) precursor, having the general formula Fe(hfa)2TMEDA, where hfa indicates 1,1,1,5,5,5 - hexafluoro - 2,4 - pentanedionate and TMEDA indicates Ν,Ν,Ν',Ν' - tetramethylethylenediamine.
2. Method according to claim 1, wherein the precursor Fe(hfa)2TMEDA in vapour form is decomposed through the action of gaseous 02 on a heated deposition surface and wherein the precursor, decomposing and reacting with 02, forms oxides of Fe(III) in the solid state on said heated surface generating nanostructures of Fe203.
3. Method according to claim 1 or 2, wherein the deposition surface is heated to a temperature of 300°C to 550°C.
4. Method according to claim 3, wherein the temperature which the deposition surface is heated to is controlled within the range of 380°C to 420°C, and is preferably equal to 400°C, determining the selective formation of -Fe203 on the deposition surface.
5. Method according to claim 3, wherein the temperature which the deposition surface is heated to is controlled within the range of 480°C to 520°C, and is preferably equal to 500°C, determining the selective formation of a-Fe203 on the deposition surface.
6. Method according to one or more of the previous claims, wherein said heated deposition surface is struck by a flow of gas comprising 02 and vapours of Fe(hfa)2TMEDA.
7. Method according to claim 6, wherein said flow of gas comprises aqueous vapour.
8. Method according to claim 6 or 7, wherein said flow of gas strikes said surface with laminar motion.
9. Method according to one or more of the previous claims, comprising the following operating steps: a) preparing a cold wall chamber inside which at least one substrate is placed which defines the deposition surface of Fe203; b) vaporising the precursor of Fe(II) Fe(hfa)2TMEDA in a flow of carrier gas comprising O2; c) making the vapours of the precursor flow inside the chamber; d) heating the substrate to a temperature between 300°C and 550°C; e) decomposing the vapours of the Fe(II) precursor on the substrate by a combined action of the heat supplied to the substrate itself and of the 02 present in the carrier gas, leading to the formation of solid state Fe203 on the substrate itself, the heating temperature of the substrate being controlled so as to selectively obtain the formation of P-Fe203 or of a-Fe203.
10. Method according to claim 9, wherein the temperature which the substrate is heated to is controlled within the range of 380°C to 420°C, and is preferably equal to 400°C, determining the selective formation of P-Fe203 on the substrate.
11. Method according to claim 9, wherein the temperature which the substrate is heated to is controlled within the range of 480°C to 520°C, and is preferably equal to 500°C, determining the selective formation of a-Fe203 on the substrate.
12. Method according to one or more of the claims from 9 to 11, wherein said flow of carrier gas traverses said chamber striking at least one substrate.
13. Method according to claim 12, wherein the flow of gas removes the carbon residues deriving from the decomposition of the precursor Fe(hfa)2TMEDA from the substrate.
14. Method according to claim 12 or 13, wherein the flow of carrier gas traverses the chamber with laminar motion at least in correspondence with the substrate.
15. Method according to one or more of the claims from 9 to 14, wherein the flow of carrier gas consists of pure 02.
16. Method according to one or more of the claims from 9 to 14, wherein said flow of carrier gas comprises 02 and aqueous vapour.
17. Method according to claim 16, wherein the partial pressure of the water is 10 to 50%.
18. Method according to one or more of the previous claims, wherein the precursor of Fe(II) is vaporised by providing heat.
19. Method according to claim 18, wherein the precursor of Fe(II) is vaporised keeping a quantity of solid precursor at a temperature of 50 ° to 90°C, and preferably equal to 60°C, so as not to chemically decompose the precursor.
20. Method according to one or more of the previous claims, wherein said substrate is composed of monocrystalline silicon, silica or glassy materials.
21. Method according to one or more of the claims from 1 to 19, wherein said substrate is composed of dielectric materials, and preferably of polycrystalline alumina.
22. Method according to one or more of the claims from 1 to 19, wherein said substrate is composed of transparent conductor materials, in particular tin doped indium oxide, or by non-transparent conductor materials, in particular titanium.
23. Method according to one or more of the claims from 1 to 19, wherein said substrate is composed of zinc, cobalt or copper oxides or mixtures of the same.
24. Method of synthesis of Fe(hfa)2TMEDA, where hfa indicates 1,1,1,5,5,5 - hexafluoro - 2,4 - pentanedionate and TMEDA indicates N, N, N', N' - tetramethylethylenediamine, said method comprising the following steps: a) reaction of ferrous chloride FeCl2 with Hhfa, having the formula
CF3COCH2COCF3, in aqueous NaOH solution, with formation of Fe(hfa)2, having the formula (I):
Figure imgf000025_0001
(I)
b) addition of TMEDA diamine, having the formula (CH3)2NCH2CH2N(CH3)2, to the reaction environment; c) reaction of Fe(hfa)2 with TMEDA with formation of Fe(hfa)2TMEDA having the formula (II):
Figure imgf000025_0002
(Π)
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