Synthesis of germanium dioxide nanoparticles in benzyl alcohols – a comparison

Introduction

In the last few years, a significant progress has been reported on the synthesis of GeO₂ materials (Adachi et al., 2005; Zou et al., 2005; Armelao et al., 2012; Chen et al., 2012; Seng et al., 2013; Zhang et al., 2013). Especially, small particles show interesting features such as photoluminescence under visible light irradiation, high refractive index as compared to that of silica glasses and optical transparency in between 280 and 5000 nm (Kim et al., 2007; Chiu and Huang, 2009; Liu et al., 2010). It is of scientific as well as commercial interest to prepare GeO₂ particles with variation in crystal structure, in morphology and in size from below 100 nm up to several hundreds of nanometres. For example, size control was previously achieved by hydrolysis of GeCl₄ or Ge(OEt)₄ in micelles and/or reverse micelle systems (Wu et al., 2006; Chiu and Huang, 2009; Zou et al., 2011). A major drawback of these synthesis routes is the need for several different reactants often including special surfactants and precise adjustment of the synthesis conditions. For instance, Chiu and Huang reported the synthesis of hexagonal GeO₂ particles varying in size from around 100 to 300 nm depending on the reaction time by the use of a mixture of six reactants [precursor: Ge(OEt)₄; oil phase: cyclohexane; surfactant: 4-octylphenol polyethoxylate; co-surfactant: n-hexanol; HCl; and water] (Chiu and Huang, 2009). However, simple surfactant-free non-aqueous synthesis routes for a great number of nanosized metal oxides were reported by Niederberger et al. using one single metal containing precursor and simple organic solvents such as benzyl alcohol, n-butyl ether or ethanol (Niederberger et al., 2002; Pinna et al., 2004; Ba et al., 2005; Pinna and Niederberger, 2008). Additionally, the synthesis of metal oxide nanoparticles by surfactant-free non-hydrolytic sol-gel routes starting from the corresponding metal chlorides were reported by Mutin and his coworkers (Aboulaich et al., 2009, 2010, 2011). Nevertheless, reports on the synthesis of GeO₂ nanoparticles by a simple surfactant-free synthesis route starting from GeCl₄ are still lacking. We are currently studying the process of twin polymerisation, which has been reported recently for silicon-containing compounds such as 2,2′-spirobi[4H-1,3,2-benzodioxasiline] (see Scheme 1). This is a convenient method to form nanostructured organic-inorganic hybrid materials by a one-step synthesis (Grund et al., 2007; Spange et al., 2009; Löschner et al., 2012). Additionally, this method also gives access to metal oxide nanoparticles (Mehner et al., 2008; Böttger-Hiller et al., 2009).

Scheme 1

Twin polymerisation of 2,2′-spirobi[4H-1,3,2-benzodioxasiline] resulting in nanostructured, interpenetrating SiO₂/phenolic resin networks (Spange et al., 2009).

However, our attempts to synthesise the corresponding germanium-containing compounds, which are structurally related to 2,2′-spirobi[4H-1,3,2-benzodioxasiline], starting from germanium alkoxides and GeCl₄ failed so far. Benzyl alcohol derivatives that were tested as reagents gave GeO₂ and partial polymerisation of the benzyl alcohol derivatives after addtion of the latter germanium containing reagents. In addition, the formation of the respective dibenzyl ether derivative as a by-product was observed. These observations prompted us to study the reactivity of benzyl alcohols in the presence of GeCl₄ in more detail. We present here the preparation of β-GeO₂ particles in ortho-methoxy benzyl alcohol under inert conditions as well as in benzyl alcohol under inert and ambient conditions starting from GeCl₄. Variation of the mean particle size from tenths up to hundreds of nanometres depends on both, the benzyl alcohol used and the synthesis conditions.

Results and discussion

Germanium dioxide was synthesised starting from GeCl₄ in benzyl alcohol by the following different synthetic protocols: (i) GeCl₄ was stirred in ortho-methoxy benzyl alcohol under inert conditions at room temperature for 22 h (sample A); (ii) GeCl₄ was dissolved and stirred in benzyl alcohol under inert conditions at reflux for 24 h (sample B); (iii) GeCl₄ was added to benzyl alcohol and stirred under ambient conditions at 40°C for 24 h (sample C). All these methods gave β-GeO₂, which was isolated by centrifugation followed by washing with tetrahydrofuran (THF) and ethanol, respectively. The GeO₂ particles were characterised by powder X-ray diffraction (PXRD), nitrogen physisorption [Brunauer-Emmett-Teller (BET) method], dynamic light scattering (DLS) measurements, infrared (IR) spectroscopy, transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) analysis. The PXRD patterns of these particles are given in Figure 1.

Figure 1

X-ray powder diffraction pattern of the as-prepared β-GeO₂ particles. The red, blue and black curves depict the X-ray powder diffraction patterns of the GeO₂ particles of samples A, B and C. The violet bars display the standard diffraction pattern of hexagonal β-GeO₂ (ICDD no. C00-036-1463).

The PXRD data confirm the formation of crystalline β-GeO₂ particles for all samples. Determination of the particle sizes by applying the Scherrer equation based on the (101) reflection gave particle sizes of (43±4) (sample A), (27±2) (sample B) and (9±1) nm (sample C) for the primary particles. Nitrogen adsorption measurements gave BET surface areas of 9, 10 and 146 m²/g for samples A, B and C, respectively. The corresponding TEM images of the β-GeO₂ particles (samples A–C) are shown in Figure 2.

Figure 2

TEM images for samples A–C.

The TEM image of the β-GeO₂, which was synthesised in ortho-methoxy benzyl alcohol (sample A), shows that the particles are larger than 100 nm, which is significantly larger than the size of the primary particles as determined from the PXRD data and they vary in size. This is ascribed to agglomeration of the crystalline nanoparticles and the presence of some amorphous material. A lower extent of agglomeration and smaller particles are obtained by synthesising GeO₂ in benzyl alcohol as demonstrated by the TEM images of samples B and C. A noteworthy observation is that GeO₂ particles isolated after their synthesis in benzyl alcohol under inert conditions exhibit sizes in between 30 nm and several tens of nanometres (sample B), whereas the synthesis under ambient conditions gives GeO₂ particles with diameters mainly below 20 nm (sample C). The deviations in primary particle sizes as obtained from the PXRD data result from agglomeration as observed for sample A, but to a lower extent. The EDX spectra support the formation of GeO₂ without any chlorine impurities. Absence of the latter was also verified by the lack of any precipitation after addition of a 0.2 m AgNO₃ solution to the filtrate of an aqueous suspension of the particles that was heated at reflux for 1 h. However, small amounts of carbon and hydrogen were detected by CH analysis for all samples in the range from 0.46% to 1.58% and from 0.13% to 0.33% for carbon and hydrogen, respectively, which might be explained by a small amount of residual organic material attached to the surface of the particles. The hydrodynamic radii as determined by DLS measurements are 535, 482 and 179 nm in THF and 360, 198 and 536 nm in toluene for samples A, B and C, respectively. These hydrodynamic radii are in agreement with the agglomeration of the particles as observed in the TEM images of the samples A–C. However, the differences in the hydrodynamic radii of each sample in different solvents indicate specific agglomeration behaviour of the particles with respect to the polarities of the employed solvents. Samples A and B exhibit less agglomeration in toluene, whereas sample C shows a larger hydrodynamic radius in toluene as compared to its value determined in THF. The IR spectroscopy reveals a significantly higher extent of OH groups at the surface of the particles for sample C, which is in accordance with the reverse behaviour of samples A and B as determined by the DLS measurements. It is noteworthy that the IR spectra of the samples A–C exhibit the absorption band maxima (517, 552, 586, 728, 753, 882, 890, 931 and 961 cm^-1) which were recently reported for the hexagonal β-modification of GeO₂ (Figure 3). The characteristic three absorption band maxima at 517, 552 and 586 cm^-1 are indicative of the hexagonal structure. The absorption band maximum at 1642 cm^-1 indicates the presence of water at the surface of the particles (Atuchin et al., 2009). The IR spectra reveal the presence of minor amounts of adsorbed organic material, which is in accordance with the low carbon content (C<1.6%) as determined by the CH analysis.

Figure 3

IR spectra of the as-prepared GeO₂ particles; samples A (red), B (blue) and C (black).

In Table 1, a summary of the primary particle sizes as determined by PXRD and the results of the nitrogen adsorption measurements (BET method), DLS measurements and CH analysis of all samples are given.

The formation of GeO₂ particles in ortho-methoxy benzyl alcohol (sample A) was observed under inert conditions at room temperature. The NMR spectroscopic analysis of the reaction mixture confirmed the presence of different soluble ortho-methoxy benzyl alcohol derivatives, which most likely are intermediate species in the formation process of an ortho-methoxy benzyl alcohol resin. It is to be noted that the as-prepared β-GeO₂ (sample A) exhibits a pale pink colour, which is typical for benzyl alcohol resins obtained by cationically induced polymerisation. Thus we propose that the formation of GeO₂ particles in ortho-methoxy benzyl alcohol can be explained by a Lewis acid, here GeCl₄, initiated polymerisation (oligomerisation) of the ortho-methoxy benzyl alcohol, which in situ generates water (Scheme 2). Consecutively, the water reacts with GeCl₄ to give GeO₂ particles.

Scheme 2

Reaction scheme to show the formation of water caused by Lewis acid straight composition initiated polymerisation (oligomerisation) of ortho-methoxy benzyl alcohol under an inert atmosphere.

The formation of GeO₂ particles in benzyl alcohol under inert conditions (sample B) was observed by stirring the mixture under reflux. The resulting mixture after removal of the GeO₂ particles consisted of 29.8 mol% benzyl alcohol, 12.6 mol% benzyl chloride, 38.7 mol% dibenzyl ether and 18.9 mol% water, which was determined by NMR spectroscopic analysis. It is noteworthy that the formation of benzyl chloride was quantitative with regard to the amount of GeCl₄ added. The quantitative formation of benzyl chloride indicates formation of GeO₂ particles by an alkyl chloride elimination reaction mechanism in accordance with the results reviewed by Debecker and Mutin (2012) for the synthesis of a variety of metal oxides (Scheme 3).

Scheme 3

Summary of the proposed reactions of metal(IV) halides in the non-hydrolytic sol-gel process as suggested by Debecker and Mutin (2012); R¹=Cl, OH and/or benzyl alcoholate; R²=benzyl group.

However, the presence of dibenzyl ether (in a yield of 29% with regard to the amount of benzyl alcohol used for the synthesis) and water in the resulting reaction mixture indicates condensation reactions to take place as well. These condensation reactions may be catalyzed by GeCl₄, other in situ generated germanium species or even by GeO₂ particles as shown in Scheme 4.

Scheme 4

Reaction scheme showing the formation of dibenzyl ether under inert conditions; R=Cl, OH and/or benzyl alcoholate intermediates.

The eliminated water might additionally give hydrolysis of germanium-containing species that finally results in GeO₂. It is to be noted that the formation of water and consecutively the formation of GeO₂ may also be explained by condensation of germanoles generated by the alkyl chloride elimination reaction (see Scheme 3). However, the quantitative formation of benzyl chloride indicates that the alkyl chloride reaction mechanism is the predominant reaction in benzyl alcohol under inert conditions. Although no formation of GeO₂ was observed under inert conditions at lower temperature, the NMR spectroscopic analysis of a reaction mixture consisting of benzyl alcohol and GeCl₄ in a molar ratio of 42:1 that was stirred under argon atmosphere at 40°C for 2 weeks revealed the formation of benzyl chloride (4.6 mol% in the mixture), some dibenzyl ether (1.4 mol% in the mixture) and germanium alkoxide species. The reaction of GeCl₄ with dibenzyl ether to give benzyl chloride (see Scheme 3) was also observed in a separate experiment. Here, GeCl₄ was added to pure dibenzyl ether in the same ratio as compared to the synthesis of sample B. Formation of benzyl chloride was proven by NMR spectroscopic analysis of the mixture after stirring at 206°C for 24 h, whereas this was not observed at room temperature. These observations support the hypothesis of an alkyl chloride reaction mechanism for GeO₂ in benzyl alcohol. By contrast, the formation of GeO₂ in benzyl alcohol under ambient conditions (sample C) was already observed by stirring the mixture at 40°C for 24 h (yield was 9%). Further stirring of the mixture at 40°C at ambient conditions gave an additional portion of β-GeO₂ (yield was 37%). Therefore, we conclude that the formation of GeO₂ particles in benzyl alcohol under ambient conditions results from hydrolysis and that benzyl alcohol acts as a solvent and a surfactant in the formation process of the nanoparticles.

Conclusions

Germanium(IV) oxide nanoparticles were synthesised in ortho-methoxy benzyl alcohol under inert conditions and in benzyl alcohol under inert and under ambient conditions, respectively, starting from GeCl₄ without addition of any further reactant. The as-prepared β-GeO₂ particles were characterised by PXRD, nitrogen adsorption measurements (BET method), DLS measurements, IR spectroscopy, TEM and EDX analysis. The synthesis in ortho-methoxy benzyl alcohol under inert conditions gave GeO₂ particles with sizes >100 nm. It is proposed that Lewis acid initiated partial polymerisation of ortho-methoxy benzyl alcohol gave water, which results in the formation of β-GeO₂ upon hydrolysis. Nanoparticles below 100 nm in size were obtained in benzyl alcohol under inert conditions. Their formation is assumed to be a result of an alkyl chloride elimination reaction mechanism. The formation of β-GeO₂ starting from benzyl alcohol under ambient conditions is a result of slow diffusion of moisture into the reaction mixture and gave the smallest GeO₂ nanoparticles that exhibit sizes below 20 nm. The results demonstrate that hydrolysis must be considered as an important side reaction, even in a so-called non-hydrolytic processes, if water might be formed as a side product or the presence of moisture is not fully excluded.

Experimental