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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. vol.59 no.2 Concepción jul. 2014

http://dx.doi.org/10.4067/S0717-97072014000200008 

 

SOLID STATE SYNTHESIS OF MICRO AND NANOSTRUCTURED METAL OXIDES USING ORGANOMETALLIC-POLYMERS PRECURSORS

 

CARLOS DÍAZ*a, MARÍA LUISA VALENZUELA*b, GABINO CARRIEDOc AND NICOLAS YUTRONICa

aDepartamento de Química, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Ñuñoa, Casilla 653, Santiago, Chile.
b
Universidad Autónoma de Chile, Dirección de investigación y Postgrado, Carlos Antúnez 1920, Santiago, Chile.
c
Departamento de Química Orgánica e Inorgánica. Facultad de Química. Universidad de Oviedo. C/Julián Clavería S/N. Oviedo 33071. España.
e-mail: cdiaz@chile.cl and maria.valenzuela@uautonoma.cl


ABSTRACT

The organometallic derivatives of poly(styrene-co-4vinylpyridine), PS-co-4-PVP, of the general formula: {[CH2CH(C6HJ)]0.1[CH2CH(C5H4N•MLn)]0.9}n; MLn = W(CO)5 , (1), CpRu(PPh3)2 (2), CpFe(dppe) (3), Cp2TiCl (4) and CH3-C5H4-Mn(CO)2 (5) were prepared from the respective organometallic and the ' co-polymer {[CH2CH(C6H5)]0.1[CH2CH(C5H4N]0.9}n. The solid state pyrolysis of these derivatives under air and at 800 °C give rise to micro and nanostructured powder metal oxides WO3, RuO2, TiO2 Mn2O3 and the iron (III) phosphate (FePO4) in the case of the iron precursor. The pyrolytic products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray scattering (EDX) and infrared spectroscopy (IR). From the TG (thermal gravimetric) and DSC (differential scanning calorimetry) data a possible mechanism for the formation of the solid nanostructured materials is discussed.

The method appears to be a reliable and general way to obtain nanostructured metal oxide in solid-state which could be potentially and easily incorporated in solid-state electronics, catalysts and sensor devices.

Keywords: Micro and nanostructured materials, metal oxide, solid-state, nanoparticles.


 

INTRODUCTION

In recent years intensive research attention has been increase draw in an effort to synthesize micro- and nanomateriales for their fundamental size, morphology-dependent magnetic properties and many important technological applications which are derived from their low dimensionality combined with the quantum confinement effect.1-3 Metal oxides, in particular, have attracted great interest for their applications as anode materials for lithium batteries,4, 5 catalysis, 5-7 sensors,8 solar cells,9 solid-state transistors 10 and metal ion removal.11 Although several solution methods to prepare nanostructured metal oxides have been reported 12-16 few solid-state route have appeared.17 The ability to rationally prepare metallic and metal oxide nanoparticles stems from the exploring methods for alternative nanoscale metal deposition in solid-state nanoelectronics and nanotechnology 18-21 and the benefit of being able to deposit both metals and dielectric or semiconducting oxides, both from the same base route. Issues including limitations on good mechanical and thermal stability of nanoscale metals have been found to be related to certain deposition methods for these metals.

Poly(styrene-co-4vinylpyridine) (see figure 1) is an interesting copolymer due to the vinylpyridine block which binds metal ions and the styrene groups which then form shells, leading to stable macromolecular complexes.22-26

 
Figure 1: Formula of the precursor [CH2CH(C6H5)]01[CH2CH(C5H4N•M Ln)]0.9}n.

Although the PS-co-4-PVP has been used in solution as template/stabilizer of metals and other solutions, 27-29 no reported experimental data are available concerning its use as solid-state template/stabilizer of nanoparticles. In this work we report the first example of Poly(styrene-co-4vinylpyridine as solid-state template for the formation of metallic oxides micro and nanoparticles.

In this work we descried a useful and general solid-state methods to prepare metal oxides micro and nanoparticles from the pyrolysis of the mac-romolecular organometallic-complexes {[CH2CH(C6H5)]0.1[CH2CH(C5H4N•MLn)]0.9}n; MLn = W(CO)5, CpRu(PPh3)2, CpFe(dppe), Cp2TiCl and CH3-C5H4-Mn(CO)2.

The poly(styrene-co-4vinylpyridine) co-polymer can therefore act as a solid-state template which after combustion goes away as volatile products. Although isolated method to prepare nanostructured metal oxides have been reported 17 no general solid-state way have appeared. The here reported method constitute a easy and general way toward micro and nanostructured metal oxide.

In the last year, organometallic derivatives of oligo and polyphosphazenes have also shown to be useful solid-state precursors of M°, MxOy and MxPyOz nanostructured materials.30-39 Solid-state pyrolysis of organometallic derivatives at 800 °C affords metallic nanostructures.

However, due to the presence of phosphorus in the polymeric chain, the nanostructured materials usually involve phosphates and/or pyrophosphates metallic phases. Therefore, if we want to obtain pure metal oxides or metal nanoparticles, a polymer not containing phosphorus within the polymeric chain could be desirable. It is expected that phosphorus-less organic polymers have the potential to be good solid-state template of metallic and organometallic-macromolecular complexes during their pyrolysis. The aim of this work is to prepare pure metal oxides nanoparticles from the solid-sate pyrolysis of the macromolecular organometallic-complexes {[CH2CH(C6H5)]0.1[CH2CH(C5H 4N•MLn)]0.9}n; MLn = W(CO)5, CpRu(PPh3)2, CpFe(dppe), Cp2TiCl and CH3-C5H4-Mn(CO)2. It is thus anticipated that in the precursors containing phosphorus atoms (as auxiliary ligands or as counterions), metal phosphates or metal pyrophosphates nanoparticles could eventually also be obtained.

The poly(styrene-co-4vinylpyridine) co-polymer can therefore acts as a solid-state template which after combustion goes away as volatile products.

EXPERIMENTAL

Physical-Chemical Measurements

All reactions were carried out under dinitrogen using standard Schlenk techniques. IR spectra were recorded on an FT-IR Perkin-Elmer Spectrum BX spectrophotometer. Solvents were dried and purified using standard procedures. The polymer {[CH2CH(C6H5)]0.1[CH2CH(C5H4N]0.9}n W(CO)6, CpRu(PPh3)2Cl, Cp2TiCl2 and CH3-C5H4-Mn(CO)3 were purchased from Sigma-Aldrich. CpFe(dppe)I was prepared according a previous reported method. 36 Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were performed on a Mettler TA 4000 instrument and Mettler DSC 300 differential scanning calorimeter, respectively. The polymer mixtures samples were heated at a rate of 10 °C/min from ambient temperature to 800° C under a constant flow of nitrogen. X-ray diffraction (XRD) was carried out at room temperature on a Siemens D-5000 diffractometer with θ-2θ geometry. The XRD data was collected using Cu-Kα radiation (40 kV and 30 mA). SEM photographs were taken with a Philips EM 300 microscope. EDAX (energy dispersive X-ray analysis) microanalysis was performed on a NORAN Instrument micro-probe attached to a JEOL 5410 scanning electron microscope. TEM analysis was conducted on a JEOLSX100 transmission microscope. The finely powered samples were dispersed in n-hexane and dropped on a conventional holey carbon copper grid and dried under a lamp.

Preparation of the organometallic precursors

Preparation of [CH2CH(C6H5)]0.1[CH2CH(C5H4N•W(CO)4]0.9 (1):

To a solution of W(CO)5 MeOH generated photochemically (from W(CO)6 0,65 g, 1.86 mmol ) in MeOH (75 ml) 0.178 g, 9.17 mmol of the polymer was added and the mixture stirred for 2.5 h. The solution was evaporated under reduced pressure and the solid washed with n-hexane and diethylether. The resulting yellow solid was dried under vacuum at room temperature.Yield 67%. Anal. Calc for C9.62H7.1N0.9O2.52 W0.63 ,C, H, N Calc: C 37.9 %; H 2.33 % ; N 5.10%. Found C 38.55 % , H 3.32 % , N 4.19% . IR (KBr pellet,cm-1) 3089m, 2927m, v(CO) 2071vw, 1974w, 1926vs, (py coordinated) 1637s, 1602s, 1557w, 1503m, 1452w, 1418m, 1221w, 1097,vw 1068vw, 976s, 959s, 893s, 812vs.

Preparation of {[CH2CH(C6H5)]0.1[CH2CH(C5H4N•CpRu(PPh3)2){PF6}]0.9}n (2):

To a solution of CpRu(PPh3)2Cl (0,55 g, 0.758 mmol ) in CH2Cl2 (75 ml) 0.073 g, 0.69 mmol of the polymer, were added in presence of NH4PF6 (0.18 g) and the mixture stirred for 3 h. The solution was evaporated under vaccum and the solution extracted with dichloromethane and concentrated to 30 ml. Addition of a mixture of diethylether/n-hexane gives yellow-brown solid. Yield 12 %.

IR (KBr pellet, cm-1) 3045m, 2932m, (py coordinated) 1616s, 1475w, 1451m, 1432w, 1160w, 1120 m, 1097,vw 1088vw, 9945s, 836 vs, 721vs , 694s, 558m, 535m, 518.

Preparation of {[CH2CH(C6H5)]0.1[CH2CH(C5H4N•CpFe(dppe)(PF6)]0.9}n (3):

To a solution of CpFe(dppe)I (0,60 g, 0.929 mmol ) in CH2Cl2 (80 ml) 0.082 g, 0.77 mmol of the polymer were added, in presence of TlPF6 (0.32 g.) and the mixture stirred for 18h. The solution was filtered through Celite and the resulting red solution was concentrated to 30 ml. Addition of a mixture of diethylether/n-hexane gives a yellow-brown solid. Yield 15 %.

IR (KBr pellet, cm-1) 3054m, 2924m, (py coordinated) 1608s, 1455w, 1436m, 1181m, 1169w, 1120 m, 1097,vw 1088vw, 999.5s, 844 vs, 743vs , 691s, 557m, 524m.

Preparation of {[CH2CH(C6H5)]0.1[CH2CH(C5H4N•Cp2TiCl)(PF6)]0.9}n (4):

To a solution of Cp2TiCl2 (0,58 g, 2.33 mmol ) in CH2Cl2 (80 ml 0.233 g, 3.36 mmol of the polymer, in presence of NH4PF6 (0.57 g) were added and the mixture stirred for 3h. The solution was evaporated under vaccum and the red solution extracted with dichloromethane and concentrated to 30 ml. Addition of a mixture of diethylether/n-hexane gives a red solid. Yield 23 %.

IR (KBr pellet, cm-1) 2962m, (py coordinated) 1637m, 1508w, 1448m, 1252s, 1049vs, 847vs, 558m, 535m, 400m.

Preparation of {[CH2CH(C6H5)]0.1[CH2CH(C5H4N•CH3-C5H4-Mn(CO)2]0.9}n (5):

To a solution of CH3-C5H4-Mn(CO)2THF generated photochemically (from CH3-C5H4-Mn(CO)3; 0,45 g, 1.74 mmol) in THF (75 ml) 0.164 g, 1.55 mmol of the polymer were added and the mixture stirred for 2.5 h. The orange-red solution was filtered of and the solution evaporated under reduced pressure. The resulting dark-red solid was dissolved in CH2Cl2 and filtered through Celite. The solvent was evaporated from filtrate and the solid washed with n-hexane. The solid was somewhat unstable in solution and not re-dissoluble.

The solid was washed with n-hexane and diethylether. The resulting yellow solid was dried under vacuum at room temperature. Yield 32 %.

IR (KBr pellet, cm-1) 3029m, 2924m, v(CO) 1947w, 1855s, (py coordinated) 1600s, 1419s, 1221w, 1120w, 1068w, 1003w, 823vs, 761s, 700 w, 561m.

Pyrolysis

The pyrolysis experiments were carried out, as previously reported,30-39 by pouring a weighed portion (0.05-0.15 g) of the respective precursor on aluminum oxide boats placed in a box furnace, heated from 25 to 300 °C and then to 800 °C, and annealed for 2 h. The pyrolytic products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray scattering (EDX) and infrared spectroscopy (IR).

RESULTADOS AND DISCUSSION

Macromolecular complexes 1-5 are insoluble solids. Elemental analysis only gave good values for compound 1, indicating 100% coordination of the fragment in the polymer chain. For the other compounds were poor analytical results indicating incomplete combustion and / or a lower degree of coordination that 100%.

Polyphosphazenes containing the organometallic fragments : W(CO)5, CpRu(PPh3)2, CpFe(dppe) , Cp2TiCl and CH3-C5H4-Mn(CO)2 anchored to their polymeric chain have been reported previously 30-37. Coordination of the organometallic fragments to polymeric pyridine chain, can be evidenced by the typical emergence of a new band centered at 1600 cm- 1 characteristic of pyridine coordination 33. On the other hand the presence of the fragments W(CO)5 and CH3-C5H4-Mn(CO)2 in the polymeric chain can be evidenced by the presence of the v(CO) stretching bands of the carbonyls groups of the W(CO)5 and Mn(CO)2 moieties 37. Some IR bands data of the precursors are summarized in Table 1. The compounds are in general insoluble which preclude an additional solution characterization. TG/DSC characterization was made and discussed as follow.

Table 1 Selected Infrared data for the macromolecular precursor 1-5.
 

The residual mass of the precursors 1-5 was investigated by TG analysis. A representative TG curve for precursor 1 is shown in figure 2.

 
Figure 2: TG curve for the precursor {[CH2CH(C6H5)]0.1[CH2CH(C5H4NW(CO)5]0.9}n

The curve exhibits a primary mass loss around 311 °C which can be attributed to the oxidation of the organic matter.30-39 The previous smaller weight loss around 80 °C can be assigned to the loss of residual solvent molecules. A strong weight loss at 526 °C can be assigned to loss of the CO groups from the W(CO)5 moiety, calculated 31.8% and found 29.9 %. In agreement with this, exothermic peaks were observed in the DSC curve at 303 °C, 458 °C and 514 °C, see Electronic supplementary materials S1. The exothermic peak at 458 °C can be attributed to the carbonization of the organic matter.30-39 The final mass residue was 52.7 % which is in approximate agree with the calculated for the formation of WO3, 58.39 %. The small difference can be due to a small amount of carbon arise from the incomplete combustion of the precursor (1). 37 For the other precursors a similar TG/DSC behavior holds (See table 1 of Electronic supplementary materials, S2 which summarizes the data for the other precursors). Pyrolytic residues, in general, are in agreement with the respective formulation of the product, ie. the metal oxides WO3, RuO2, TiO2, Mn2O3 and the iron (III) phosphate FePO4 .

Main products of pyrolysis from the precursors 1-5 were identified by X-ray diffraction. In Table 2 the composition of the products are summarized along with some morphological as well as size characteristics.

Table 2 Summary of the morphology and particle size data for the pyrolytic products from precursors 1-5.

 

A detailed discussion of the pyrolytic materials from each of the precursors is given below.

{[CH2CH(C6H5)]0.1[CH2CH(C5H4NW(CO)5]0.9}n (1)

Pyrolysis of precursor (1) under air and at 800 °C affords pure WO3. The XRD shown in figure 1a, exhibits clearly the pattern of WO3. The main peaks corresponding to (002), (020), (200), (202), (120), (112) and (400) of monoclinic WO3 (ICDD Card Nr01-083-pure phase 09509) are clearly observed as is shown in figure 3a. The less intense peaks -for reason of clarity-were not indicated in the figure, but all can be indexed to WO3 phase. Few pure WO3 phases have been reported. Lu reported a nearly pure monoclinic WO3 from calcinations of H2WO4 at 500 °C 40 with a similar X-ray diffraction to that is shown in figure 3a. Monoclinic WO3 was also obtained by an arc discharge method from W.41

 

Figure 3: XRD (a) SEM image (1 cm = 10 nm) (b), EDS (c) and TEM image (d) of the pyrolytic product from precursor (1).

Morphology analysis by SEM (see Fig. 3b) evidences a fused-grain material. EDS analysis exhibits the expected presence of W and O, see figure 3c. TEM image indicates an agglomeration of clusters of WO3 nanoparticle with sizes around 200 nm for the smaller species and 600 nm for the larger ones see figure 3d.

{[CH2CH(C6H5)]0.1[CH2CH(C5H4NCpRu(PPh3)2){PF6}]0.9}n (2)

Pyrolysis of this precursor affords a XRD consistent with the presence of the tetragonal RuO2 phase. The main two typical (110) and (101) diffraction peaks corresponding to tetragonal RuO2 were observed,42-44 as is shown in figure 4a. The enhancing of the (101) orientation, respect to the bulk material is consistent with the presence of nanostructured domains.42

 

Figure 4: XRD (a), SEM image (b) and TEM image (c) of pyrolytic product from precursor (2).

Although the macromolecular precursor contains phosphorus from the triphenylphosphine ligand, no metallic phosphates were observed after their pyrolysis.

SEM of the thus obtained RuO2 exhibits a porous morphology as shown in figure 4b. On the other hand, TEM image exhibits a linear arrangement of nearly circular nanoparticles with size in the range 100-60 nm as is shown in figure 4c. Nanostructured RuO2 exhibits interesting properties such as low resistivity, high chemical and thermodynamic stability under electrochemical environment. The most known application of RuO2 is as an electrode in energy storage electrochemical supercapacitors. 42-44

{[CH2CH(C6H5)]0.1[CH2CH(C5H4NCpFe(dppe)(PF6)]0.9}n (3) Pyrolysis of precursor (3) affords nanostructured FePO4 as can be observed from figure 5a, and further confirmed from the characteristics Bragg diffraction peaks, (100), (012), (104), (112) corresponding to hexagonal FePO4.45-49. Minor intensity peaks can be due to traces of unidentified Fe phases. The formation of FePO4 and the absence of pure iron oxides arises from the presence of P in the ligand dppe. 26

 

Figure 5: XRD (a), SEM image (b) EDS (c) and TEM image (d) of pyrolytic product from precursor (3).

SEM images indicate a porous 3-D network as is shown in figure 5b. The EDAX confirmed the presence of Fe, P and O atoms as is shown in figure 5c. The TEM images (Fig. 5d) evidence the presence of agglomerates composed of nanoparticles with various shapes and sizes.

FePO4 is an interesting material due to its use in catalysis, waste water purification systems, ferroelectrics and lithium batteries. 45-48

Most typical preparation methods for nanostructured FePO4 involved coprecipitation 46 by a solvothermal approach using dodecyl sulfate as template 47 and using microwave irradiation to a solution containing (NH4)2Fe(SO4)2•6H2O and H3PO4 in presence of CTAB as stabilizer.49 All of these methods are in solution and no solid-state methods to obtain these types of Fe nanoparticles have been reported.

{[CH2CH(C6H5)]0.1[CH2CH(C5H4NCp2TiCl)(PF6)]0.9}n (4)

For the pyrolytic products of precursor of 4, the (101), (103), (004), (112), (200), (105), (211) diffraction lines, which are characteristic of anatase-TiO2, were observed, see Electronic supporting information S3.50-53 No significant amounts of other TiO2 brookite or rutile 49 phases were found. However, some residuals from another TixOy might be present as previously observed in other TiO2 preparations. 51

The morphology analysis by SEM exhibits a dense shape. The TEM images indicate the presence of only some big agglomerates. Despite several preparation methods of nanostructured TiO2 have been reported 50-53 few solid state routes are known.36 Titania nanocrystals have received great attention in recent years for their extensive applications in conventional catalyst support, optics, cosmetics and solar cells. 50-53 Most of these applications require their direct incorporation into solid-state devices.

{[CH2CH(C6H5)]0.1[CH2CH(C5H4NCH3-C5H4-Mn(CO)2]0.9}n (5)

The pyrolytic product exhibits the typical XRD diffraction peaks of cubic Mn2O3 at 2θ = (211), (222), (321) and (400) [54-56], see figure 6a. Minor intensity peaks can be due to traces of unidentified Mn phases The morphology analysis by SEM exhibits a 3-D grain network as is shown in figure 6b. EDS analysis confirms the presence of Mn and O, see figure 6c.

 

Figure 6: XRD (a), SEM image (b) EDS (c) and TEM image (d) of pyrolytic product from precursor (5).

The TEM images show a diverse arrangement of near circular nanoparticles joined in various shapes and showing a broad range of sizes as is shown in figure 6d.

Mn2O3 constitutes an interesting target due to potential applications as catalyst for carbon monoxide removing, 54 for the preparation of soft magnetic materials 54 and as constituent of electrode materials for rechargeable lithium batteries. 56 The main preparation methods involve the reaction of aqueous solution of MnCO3 with KMnO4,54 decomposition of the manganese coordination polymer [Mn(Pht)(H2O)]n in presence of oleic acid triphenylphosphine as stabilizer and capping 55 and using hydrothermal method starting from MnO2 56 However, no solid-state method to prepare nanoparticles of Mn2O3 have been reported.

Pyrolysis mechanism

The probable formation mechanism of WO3, RuO2, TiO2, Mn2O3 and the iron (III) phosphate FePO4 nanoscale materials described here involves the cross-linking 36 of the PSP-co-4-PVP chains by the organometallic metal centers during the initial annealing step, followed by the carbonization of the organic matter to produce holes where the metal centers begin to coarsen and grow. 57 Carbonization of the organic matter usually occurs in the pyrolysis of metallic and organometallic derivatives of polymers around 350 °C. 57 Additionally, some incomplete degree of carbonization can produce a carbon host 57 where the nanoparticles are subsequently stabilized in solid state. This carbon matrix formed during this solid state synthetic method constitutes the analogue of the stabilization effect exploited in the synthesis of nanoparticles and nanocrystals in solution, which is typically provided by a coordination stabilizer such as TOPO, TOP, alkylamines, alkylthiols and related ligands. 58

CONCLUSIONS

Macromolecular organometallic derivatives of poly(styrene-co-4vinylpyridine), PS-co-4-PVP, of the general formula: {[CH2CH(C6H5)]0.1[CH2CH(C5H4N•MLn)]0.9}n; ML = W(CO)5 (1), CpRu(PPh3)2 (2), CpFe(dppe) (3), Cp2TiCl (4) and CH3-C5H4-Mn(CO)2 (5) are useful precursors of the micro and nanostructured materials WO3, RuO2, TiO2, Mn2O3 and the iron (III) phosphate FePO4 for the case of iron precursor. In general, pure micro and nanostructured oxides can be obtained except when the metallic salt contains phosphorus atoms. In most cases the nanoparticles are somewhat large and in some cases form agglomerates. The smallest particles correspond to RuO2.

The synthesis reported here may constitute a useful and general method to obtain metallic oxides micro and nanoparticles in solid-state. Solid-state methods to produce metallic nanoparticles are necessary to incorporate the particles into solid-state device such as electronic parts, sensors, high temperature catalysts, etc.18,19 Thus, the chemical and mechanical stability of these materials are crucial for the fabrication of nanodimensional optoelectronic circuits and optical memory with ultrahigh recording speed and storage density.21 Experiments to include these micro and nanostructured metal oxide and FePO4 into solid matrix are in course.

ACKNOWLEDGEMENTS

To Project Fondecyt 1120179 for financial support.

 

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