Design of molecular precursors for nanomaterials

1 Introduction

Understanding and manipulating nanosized particles is an active research area. The family of nanomaterials and their synthesis is a challenge to material scientists due to their fascinating size dependent properties which have distinct properties from their corresponding bulk counterparts as well as from the atomic or molecular precursors from which they are derived.

Modern technologies in nanoage require new materials to improve our life span and style. Metal alkoxides are considered to be best precursor due to the soft reaction conditions with their better compositional uniformity in the final oxides. Metal alkoxides have attracted much attention due to their solubility in a wide variety of organic solvents and their ability to form heterometallic oxides species (Mehrotra, 1992; Brinker et al., 1984; Trevor and Sakka, 2004; Bradley et al., 1978; Turova et al., 2002; Bradley et al., 2001). Alkoxides that display the highest nuclearity are often non-volatile and non-soluble when compared to other alkoxides with low nuclearity. The M–O–C bond polarity and the polymerization mainly govern their solubility, volatility and thermal stability for the design of functional nanomaterials (Bradley et al., 1978; Turova et al., 2002; Bradley et al., 2001).

To protect the green house effect for our modern lifestyle, it is necessary to protect our self with the help of nanodevices such as sensors (electronic, biological) and nanodrug delivery systems in cost-effective techniques. Scientists are trying their best to improve the synthetic techniques in an environmental friendly approach. The role of precursor plays an important role for the synthesis of nanomaterials with controlled particle size and its distribution with a favorable surface morphology. The favorable particle chemistry can be achieved with the help of cost-effective soft chemistry approach (Klabunde, 2000; Sapra and Sarma, 2004; Jolivet, 2000).

In this study the synthesis of heterometallic precursors was carried out to design nanopowder with favorable surface chemistries via wet chemical routes. As we know the chemical reactivity of metal alkoxides towards nucleophilic reactions depends largely on the electrophilic character of the metal ion and its ability to increase the coordination number. The coordination unsaturation is the main driving force behind the reactivity of metal alkoxides. A metal oxide has emerged as promising engineering materials due to their ultra pure nature to evaluate the functional properties for new potential applications in sensor devices.

2 Experimental

All manipulations were performed under dry argon using Schlenk line techniques. Solvents and alcohols were purified by standard methods and stored over molecular sieves. FT-IR spectra were recorded from Nujol mulls on a Perkin–Elmer Paragon 500 FT-IR spectrometer. UV–visible spectra were recorded on a unicam UV2-100 spectrometer. The C and H analyses were carried out by using an EA 1110 (CE instrument) elemental analyzer. Ti, Sn, Sb and Ce were determined quantitatively as metal oxide (Vogel, 1989). Isopropoxy content was estimated by the oxidotitration method using standard chromic acid (Mehrotra, 1954). ¹H NMR spectra were recorded in CDCl₃ or C₆D₆ on a Bruker DRX–300 MHz spectrometer. Mass studies were recorded for fragmentation on a JMS-DX 303 spectrometer. Powder X-ray diffraction measurements were performed at room temperature on D8-Discover (with GADDS) Bruker operating with Cu Kα₁ radiation. The powder morphology of the compounds before and after heat treatment was investigated using a scanning electron microscope (SEM) Philips XL30S FEG XL V.5.50. Thermal analysis was performed on a TA Instruments SDT960 in a nitrogen atmosphere. The compounds were heated in nitrogen at a rate of 5 °C/min. The experiments were performed in Quartz crucibles, which also served as the reference.

3 Results and discussion

The synthesis of bimetallic alkoxides was confirmed with the help of quantitative yield of the product and as well as quantitative yield of KCl. The formation of these alkoxides was further confirmed with the help of microanalysis of C and H, IR and ¹H NMR studies. When freshly prepared they are extremely susceptible to moisture to form the hydrated oxides. All bimetallic alkoxides were soluble in mother liquor and thermally labile but could not be distilled or sublimed at 200 °C/10⁻³ torr. Elemental analyses were performed under ambient conditions; a partial hydrolysis is unavoidable and invariably associated with the handling procedure. The analytical data of alkoxides and their corresponding oxides as shown in Table 3 support the formation of the product.

The metal alkoxides and their corresponding metal oxides were characterized with the help of FT-IR by observing the main characteristic absorption peaks as shown in Table 4. The absence of ν_C–O bands (terminal and bridging) in corresponding metal oxides clearly supports the formation of oxides from their corresponding bimetallic alkoxides. The significant down field shift was observed in metal oxides for the absorption band of ν_M–O (asymm), ν_Ce–O (asymm), ν_M–O (symm) due to M–O stretching bond.

A UV absorption edge was found to be very sensitive with a change in band gap and the coordination around the central metal ion. The absorptions of UV–visible spectra of the alkoxides can be attributed to the CT band of unshared electron pairs on the oxygen atoms to the empty Ce³⁺ metal orbital. The sharpness of the absorption band shows the presence of 4f electrons of Ce³⁺. Ce³⁺ was found to be suitable for the studying changes within the coordination number and oxygen donor properties of the ligands (Westin et al., 1998; Yatsimirskii and Davidenko, 1979). The CT bands of all three metal alkoxides was found to be due to the presence of the Ce³⁺ ion. The fine structural peak in isopropanol is virtually unchanged due to a weak interaction with the solvent. In solution these alkoxides does not show any ageing effect for many days in dry atmosphere. UV spectra are shown in Fig. 1.

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Figure 1

UV–vis spectra for obtained complexes.

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¹H NMR spectra give useful structural information of the precursor. Although no suitable crystals for X-ray studies were obtained due to hydrolytic instability. ¹H NMR (CDCl₃ or C₆D₆) spectra were carried out and exhibits two types resonance peaks for isopropyl protons with correct integration ratio, but the peaks are not sharp due to the para magnetic behavior. The significant ¹H peaks are given in Table 5. Absence of ¹H NMR signal and the microanalysis support the formation of metal oxide.

The formation of bimetallic oxides was further confirmed by X-ray energy dispersive spectroscopy (EDS) micro elemental analysis as shown in Table 6. The mass spectra do not give any conclusive evidence for the formation of heterometallic molecules in gas phase probably due to the breakdown of the compounds under the high vacuum used in mass spectrometer. The limited characteristic fragments ion peak of low intensity has been observed in standard electron impact conditions, which corresponds to simple decomposed metal oxides. The important fragmentation ion peaks are given in Table 7.

The thermal behavior of metal oxides was investigated by thermogravimetry (TG) and differential thermal analysis (DTA) as shown in Fig. 2. The thermal decomposition of oxides may be described as a stepwise process. The first weight loss occurs below 200 °C and is attributed to the removal of solvent trapped in oxides. Prominent exothermic peaks around 190 °C can be attributed to the removal of organic residues. Little mass loss was noticed below 600 °C, thus support the formation of an oxide with a definite chemical composition after the removal of impurities. The exothermic peaks located at 800–850 °C supports the formation of a well-organized metal oxide network after the decomposition of inorganic residue (Alves et al., 2008).

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Figure 2

TG data showing the weight loss expressed as a percentage of weight as a function of temperature. The heating rate was 10 °C/min in argon atmosphere. DTA data shows exothermic peaks.

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The purity of bimetallic oxide was ascertained by powder X-ray diffraction and compared with standard diffraction data files (JCPDS). The XRD patterns shows broad diffraction peak due to their poor crystallinity at room temperature. The same diffraction patterns were observed for other metal oxides at room temperature probably due to similar type of molecular structures (Andrianainarivelo et al., 1997). X-ray diffraction peaks as shown in Fig. 3 exhibits sharp patterns on calcinations supporting the degree of crystallinity with metal–oxygen framework with the removal of impurities. The size of the particles was about 5–10 nm as calculated by using the Scherer equation (Cullity, 1967).

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Figure 3

XRD diffraction patterns describe the nature of the compounds at room temperature and after calcinations.

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Luminescence spectra of homogenous metal oxides as shown in Fig. 4 supports fact the excitation between π → π* transition gives a characteristic ⁵D₀ → ⁷F^g emission. This indicates fast intermolecular energy transfer from oxygen to the central metal ion. The broad emission at around 1550 nm extending from 1500 to 1600 nm was observed and is attributed to π → π* transitions. The spectral half width of the emission band from the maximum is about 50 nm and it is due to heterogeneous nature and Stark splitting of the excited and ground states. The broad spectrum helps for the fabrication of potential optical amplifiers depending on the nature of the precursor chemistry. It is concluded that the luminescent properties depend on the metal oxide morphology, particles size, its distribution and the methodology (Riwotzki and Haase, 1998; Riwotzki et al., 1998).

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Figure 4

Photoluminescence profiles for complexes.

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SEM shows dense structure with unfaceted pores surface with the oxides grain to be micrometer-sized, which build up to form many tiny particles with the polydispersed nature without any observable defects in particle size and shape as shown in Fig. 5. Therefore, it is concluded that the nature of the metal oxides strongly depends on the chemical composition of the metal alkoxides as a molecular precursor and the methodology used for particles synthesis.

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Figure 5

SEM shows surface morphologies at room temperature.

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TEM shows (Fig. 6) that the size of particles was about 5–10 nm in heterometallic oxide which agrees well with XRD determinations using the Scherer equation. Moreover the metal oxide nanopowder has slight an agglomeration due to the presence of their surface energy based on Ostwald ripening process. However dispersibility of particles is related to the role of the dispersing agent and synthetic methodology (Hyeon, 2007; Lee et al., 2006). Though there was considerable distribution in size, redispersed nanocrystal obtained from low concentration reactions appeared isolated or only very loosely aggregated depending on TEM sample preparation. Many particles exhibited irregular shapes with high crystalline in nature. On the basis of above studies, the following plausible structures can be suggested for molecular precursor and their corresponding metal oxides as shown in Fig. 7.

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Figure 6

TEM images of the particles at room temperature.

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Figure 7

Structure proposed based on using the imaging systems for the characterizations of the obtained products.

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4 Conclusion

The design of molecular routes for solid metal oxides with functional properties has become an important area due to their technological importance. Physical properties of the final metal oxides can be controlled at the molecular level by using single source molecular precursor at low temperature via soft chemistry approach. The physico-chemical property of molecular precursor plays a key role in controlling the high compositional purity in the final products. Aging has a significant effect on the texture and the structure of the mixed oxide. However, the research is still underway to find new chemical routes without sophisticated procedure to obtain homogenous and ultra fine multicomponent materials without agglomeration for high tech application in green conditions.