Synthesis of Germanium Dioxide Nanoparticles in Benzyl Alcohols

Home , Germanium dioxide, Germanium monoxide

DOI 10.1515/mgmc-2013-0038 Main Group Met. Chem. 2013; 36(5-6): 209–214

Philipp Kitschke, Steffen Schulze, Michael Hietschold and Michael Mehring* Synthesis of germanium dioxide nanoparticles in benzyl alcohols – a comparison

Abstract: The surfactant-free synthesis of β-phase ger- variation in crystal structure, in morphology and in size manium dioxide nanoparticles in ortho-methoxy benzyl from below 100 nm up to several hundreds of nanome- alcohol and benzyl alcohol has been reported. Characteri- tres. For example, size control was previously achieved by sation of the hexagonal β-GeO2 crystals, which involves hydrolysis of GeCl4 or Ge(OEt)4 in micelles and/or reverse powder X-ray diffraction, nitrogen adsorption measure- micelle systems (Wu et al., 2006; Chiu and Huang, 2009; ments (Brunauer-Emmett-Teller method), dynamic light Zou et al., 2011). A major drawback of these synthesis routes scattering measurements, IR spectroscopy, transmission is the need for several different reactants often including electron microscopy and energy-dispersive X-ray analysis special surfactants and precise adjustment of the synthe- has been presented. Synthesis of β-GeO2 under ambient sis conditions. For instance, Chiu and Huang reported the conditions in benzyl alcohol results in nanoparticles with synthesis of hexagonal GeO2 particles varying in size from diameters below 20 nm, whereas the synthesis under inert around 100 to 300 nm depending on the reaction time by conditions in benzyl alcohol at reflux (205°C) gives larger the use of a mixture of six reactants [precursor: Ge(OEt)4; nanoparticles. In ortho-methoxy benzyl alcohol, agglom- oil phase: cyclohexane; surfactant: 4-octylphenol polyeth- erates with particle sizes above 100 nm are observed oxylate; co-surfactant: n-hexanol; HCl; and water] (Chiu under inert atmosphere conditions at room temperature. and Huang, 2009). However, simple surfactant-free nonaqueous synthesis routes for a great number of nanosized Keywords: benzyl alcohol; germanium dioxide; nanopar- metal oxides were reported by Niederberger et al. using ticles; surfactant-free synthesis. 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; *Corresponding author: Michael Mehring, Technische Universität Chemnitz, Fakultät für Naturwissenschaften, Institut für Chemie, Pinna and Niederberger, 2008). Additionally, the synthesis Professur Koordinationschemie, D-09107 Chemnitz, Germany, of metal oxide nanoparticles by surfactant-free non-hydro- e-mail: [email protected] lytic sol-gel routes starting from the corresponding metal Philipp Kitschke: Technische Universität Chemnitz, Fakultät chlorides were reported by Mutin and his coworkers (Abou- für Naturwissenschaften, Institut für Chemie, Professur laich et al., 2009, 2010, 2011). Nevertheless, reports on the Koordinationschemie, D-09107 Chemnitz, Germany synthesis of GeO nanoparticles by a simple surfactant-free Steffen Schulze and Michael Hietschold: Technische Universität 2 Chemnitz, Fakultät für Naturwissenschaften, Institut für Physik, synthesis route starting from GeCl4 are still lacking. We Professur Analytik an Festkörperoberflächen, D-09107 Chemnitz, are currently studying the process of twin polymerisation, Germany 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- Introduction step synthesis (Grund et al., 2007; Spange et al., 2009; Löschner et al., 2012). Additionally, this method also gives In the last few years, a significant progress has been access to metal oxide nanoparticles (Mehner et al., 2008; reported on the synthesis of GeO2 materials (Adachi et al., Böttger-Hiller et al., 2009). 2005; Zou et al., 2005; Armelao et al., 2012; Chen et al., However, our attempts to synthesise the correspond- 2012; Seng et al., 2013; Zhang et al., 2013). Especially, small ing germanium-containing compounds, which are struc- particles show interesting features such as photolumines- turally related to 2,2′-spirobi[4H-1,3,2-benzodioxasiline], cence under visible light irradiation, high refractive index starting from germanium alkoxides and GeCl4 failed so as compared to that of silica glasses and optical transpar- far. Benzyl alcohol derivatives that were tested as reagents ency in between 280 and 5000 nm (Kim et al., 2007; Chiu gave GeO2 and partial polymerisation of the benzyl alcohol and Huang, 2009; Liu et al., 2010). It is of scientific as derivatives after addtion of the latter germanium contain- well as commercial interest to prepare GeO2 particles with ing reagents. In addition, the formation of the respective

210 P. Kitschke et al.: Synthesis of GeO2 nanoparticles in benzyl alcohols

O O n SiO Si 2 n O O 2 n

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). 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

Figure 1 X-ray powder diffraction pattern of the as-prepared β-GeO2 Germanium dioxide was synthesised starting from GeCl4 particles. The red, blue and black curves depict the X-ray powder in benzyl alcohol by the following different synthetic diffraction patterns of the GeO2 particles of samples A, B and C. The violet bars display the standard diffraction pattern of hexagonal protocols: (i) GeCl4 was stirred in ortho-methoxy benzyl β alcohol under inert conditions at room temperature for -GeO2 (ICDD no. C00-036-1463).

22 h (sample A); (ii) GeCl4 was dissolved and stirred in benzyl alcohol under inert conditions at reflux for 24 h the particles are larger than 100 nm, which is significantly

(sample B); (iii) GeCl4 was added to benzyl alcohol and larger than the size of the primary particles as determined stirred under ambient conditions at 40°C for 24 h (sample from the PXRD data and they vary in size. This is ascribed

C). All these methods gave β-GeO2, which was isolated to agglomeration of the crystalline nanoparticles and the by centrifugation followed by washing with tetrahydro- presence of some amorphous material. A lower extent of furan (THF) and ethanol, respectively. The GeO2 particles agglomeration and smaller particles are obtained by syn- were characterised by powder X-ray diffraction (PXRD), thesising GeO2 in benzyl alcohol as demonstrated by the nitrogen physisorption [Brunauer-Emmett-Teller (BET) TEM images of samples B and C. A noteworthy observa- method], dynamic light scattering (DLS) measurements, tion is that GeO2 particles isolated after their synthesis infrared (IR) spectroscopy, transmission electron micros- in benzyl alcohol under inert conditions exhibit sizes in copy (TEM) and energy-dispersive X-ray (EDX) analysis. between 30 nm and several tens of nanometres (sample The PXRD patterns of these particles are given in Figure 1. B), whereas the synthesis under ambient conditions

The PXRD data confirm the formation of crystalline gives GeO2 particles with diameters mainly below 20 nm

β-GeO2 particles for all samples. Determination of the par- (sample C). The deviations in primary particle sizes as ticle sizes by applying the Scherrer equation based on the obtained from the PXRD data result from agglomeration (101) reflection gave particle sizes of (43 ± 4) (sample A), as observed for sample A, but to a lower extent. The EDX

(27 ± 2) (sample B) and (9 ± 1) nm (sample C) for the primary spectra support the formation of GeO2 without any chlo- particles. Nitrogen adsorption measurements gave BET rine impurities. Absence of the latter was also verified surface areas of 9, 10 and 146 m2/g for samples A, B and C, by the lack of any precipitation after addition of a 0.2 m respectively. The corresponding TEM images of the β-GeO2 AgNO3 solution to the filtrate of an aqueous suspension particles (samples A–C) are shown in Figure 2. of the particles that was heated at reflux for 1 h. However,

The TEM image of the β-GeO2, which was synthesised small amounts of carbon and hydrogen were detected by in ortho-methoxy benzyl alcohol (sample A), shows that CH analysis for all samples in the range from 0.46% to

P. Kitschke et al.: Synthesis of GeO2 nanoparticles in benzyl alcohols 211

Figure 2 TEM images for samples A–C.

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 spectra of the as-prepared GeO particles; samples IR spectroscopy reveals a significantly higher extent of OH Figure 3 2 A (red), (blue) and (black). groups at the surface of the particles for sample C, which B C is in accordance with the reverse behaviour of samples A and B as determined by the DLS measurements. It is note- conditions at room temperature. The NMR spectroscopic worthy that the IR spectra of the samples A–C exhibit the analysis of the reaction mixture confirmed the presence absorption band maxima (517, 552, 586, 728, 753, 882, 890, of different soluble ortho-methoxy benzyl alcohol deriva- 931 and 961 cm-1) which were recently reported for the hex- tives, which most likely are intermediate species in the agonal β-modification of GeO2 (Figure 3). The characteris- formation process of an ortho-methoxy benzyl alcohol -1 tic three absorption band maxima at 517, 552 and 586 cm resin. It is to be noted that the as-prepared β-GeO2 (sample are indicative of the hexagonal structure. The absorption -1 band maximum at 1642 cm indicates the presence of Table 1 Analytical data for the as-synthesised GeO2 particles water at the surface of the particles (Atuchin et al., 2009). (samples A–C). The IR spectra reveal the presence of minor amounts of Sample A Sample B Sample C adsorbed organic material, which is in accordance with a the low carbon content (C < 1.6%) as determined by the CH Primary particle size 43 ± 4 nm 27 ± 2 nm 9 ± 1 nm BET surfaceb 9 g/m2 10 g/m2 146 g/m2 analysis. Hydrodynamic radius: In Table 1, a summary of the primary particle sizes In THF 535 nm 482 nm 179 nm as determined by PXRD and the results of the nitrogen In toluene 360 nm 198 nm 536 nm adsorption measurements (BET method), DLS measure- CH analysis (C/H%) 1.06/0.24% 0.46/0.13% 1.58/0.33% ments and CH analysis of all samples are given. aDetermined by the Scherrer equation (K = 1) using the data of the b The formation of GeO2 particles in ortho-methoxy (101) reflection of the X-ray powder diffraction pattern; Single benzyl alcohol (sample A) was observed under inert point BET analysis p/p0 = 0.150.

212 P. Kitschke et al.: Synthesis of GeO2 nanoparticles in benzyl alcohols

1 2 1 2 A) exhibits a pale pink colour, which is typical for benzyl R 3M-Cl+ R OH R 3M-OH + R -Cl 1 2 1 2 alcohol resins obtained by cationically induced polymeri- R 3M-Cl+ R OH R 3M-OR + H-Cl 1 1 1 1 sation. Thus we propose that the formation of GeO2 parti- R 3M-Cl + R 3M-OH R 3M-O-MR 3 + H-Cl 1 2 2 1 2 2 cles in ortho-methoxy benzyl alcohol can be explained by R 3M-Cl + R -OR R 3M-OR + R -Cl a Lewis acid, here GeCl , initiated polymerisation (oligom- 1 1 2 1 1 2 4 R 3M-Cl + R 3M-OR R 3M-O-MR 3 + R -Cl erisation) of the ortho-methoxy benzyl alcohol, which in situ generates water (Scheme 2). Consecutively, the water Scheme 3 Summary of the proposed reactions of metal(IV) halides in the non-hydrolytic sol-gel process as suggested by Debecker and reacts with GeCl4 to give GeO2 particles. Mutin (2012); R1 = Cl, OH and/or benzyl alcoholate; R2 = benzyl group. 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 chloride (4.6 mol% in the mixture), some dibenzyl ether (1.4 of the GeO2 particles consisted of 29.8 mol% benzyl alcohol, mol% in the mixture) and germanium alkoxide species. The

12.6 mol% benzyl chloride, 38.7 mol% dibenzyl ether and reaction of GeCl4 with dibenzyl ether to give benzyl chloride 18.9 mol% water, which was determined by NMR spec- (see Scheme 3) was also observed in a separate experiment. troscopic analysis. It is noteworthy that the formation of Here, GeCl4 was added to pure dibenzyl ether in the same benzyl chloride was quantitative with regard to the amount ratio as compared to the synthesis of sample B. Formation of GeCl4 added. The quantitative formation of benzyl chlo- of benzyl chloride was proven by NMR spectroscopic analy- ride indicates formation of GeO2 particles by an alkyl chlo- sis of the mixture after stirring at 206°C for 24 h, whereas ride elimination reaction mechanism in accordance with this was not observed at room temperature. These observa- the results reviewed by Debecker and Mutin (2012) for the tions support the hypothesis of an alkyl chloride reaction synthesis of a variety of metal oxides (Scheme 3). mechanism for GeO2 in benzyl alcohol. By contrast, the for-

However, the presence of dibenzyl ether (in a yield of mation of GeO2 in benzyl alcohol under ambient conditions 29% with regard to the amount of benzyl alcohol used for (sample C) was already observed by stirring the mixture at the synthesis) and water in the resulting reaction mixture 40°C for 24 h (yield was 9%). Further stirring of the mixture indicates condensation reactions to take place as well. at 40°C at ambient conditions gave an additional portion

These condensation reactions may be catalyzed by GeCl4, of β-GeO2 (yield was 37%). Therefore, we conclude that other in situ generated germanium species or even by GeO2 the formation of GeO2 particles in benzyl alcohol under particles as shown in Scheme 4. ambient conditions results from hydrolysis and that benzyl The eliminated water might additionally give hydroly- alcohol acts as a solvent and a surfactant in the formation sis of germanium-containing species that finally results process of the nanoparticles. 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, Conclusions the quantitative formation of benzyl chloride indicates that Germanium(IV) oxide nanoparticles were synthesised the alkyl chloride reaction mechanism is the predominant in ortho-methoxy benzyl alcohol under inert conditions reaction in benzyl alcohol under inert conditions. Although and in benzyl alcohol under inert and under ambient no formation of GeO was observed under inert conditions 2 conditions, respectively, starting from GeCl without at lower temperature, the NMR spectroscopic analysis of 4 addition of any further reactant. The as-prepared β-GeO a reaction mixture consisting of benzyl alcohol and GeCl 2 4 particles were characterised by PXRD, nitrogen adsorp- in a molar ratio of 42:1 that was stirred under argon atmos- tion measurements (BET method), DLS measurements, phere at 40°C for 2 weeks revealed the formation of benzyl IR spectroscopy, TEM and EDX analysis. The synthesis

OMe OMe in ortho-methoxy benzyl alcohol under inert conditions

OH GeCl CH OH GeR4 / n 4 + n H O 2 2 GeO2 O 2 + H2O n

Scheme 2 Reaction scheme to show the formation of water caused Scheme 4 Reaction scheme showing the formation of dibenzyl by Lewis acid straight composition initiated polymerisation (oligomeri- ether under inert conditions; R = Cl, OH and/or benzyl alcoholate sation) of ortho-methoxy benzyl alcohol under an inert atmosphere. intermediates.

P. Kitschke et al.: Synthesis of GeO2 nanoparticles in benzyl alcohols 213

IQ2 (Quantachrome, Odelzhausen, Germany), which were evaluated gave GeO2 particles with sizes > 100 nm. It is proposed by the BET method using p/p ratio of 0.150. that Lewis acid initiated partial polymerisation of ortho- 0 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 Syntheses conditions. Their formation is assumed to be a result of Reactions using ortho-methoxy benzyl alcohol were performed in an an alkyl chloride elimination reaction mechanism. The inert atmosphere (argon) glovebox. Reactions using benzyl alcohol β formation of -GeO2 starting from benzyl alcohol under were performed in an inert atmosphere (argon) glovebox as well as ambient conditions is a result of slow diffusion of mois- under ambient conditions to allow water diffusion into the reaction mixture. Solvents were purified and dried by applying standard tech- ture into the reaction mixture and gave the smallest GeO2 nanoparticles that exhibit sizes below 20 nm. The results niques. Benzyl alcohol and ortho-methoxy benzyl alcohol were purchased from Merck Schuchardt OHG (Hohenbrunn, Germany) and demonstrate that hydrolysis must be considered as an Alfa Aesar (Ward Hill, MA, USA), respectively. Benzyl alcohol was used important side reaction, even in a so-called non-hydro- without any purification for the synthesis under ambient conditions lytic processes, if water might be formed as a side product (sample C). Ortho-methoxy benzyl alcohol and benzyl alcohol were or the presence of moisture is not fully excluded. dried over molecular sieves for 3 weeks before using them for the syn-

theses under inert conditions. GeCl4 was purchased from ABCR GmbH & Co. KG (Karlsruhe, Germany) and used without further purification. Experimental

Synthesis of GeO2 in ortho-methoxy benzyl General alcohol – sample A

1 13 1 H and C{ H}-NMR spectra were recorded on a Bruker (Bruker GeCl4 (529.4 mg, 2.47 mmol) was added to ortho-methoxy benzyl Corporation, Billerica, MA, USA) ‘Avance III 500’ spectrometer at alcohol (12.79 mL, 103.7 mmol) and the mixture was stirred under an ambient temperature. CH analysis was carried out with a ‘vario inert atmosphere at room temperature for 22 h. The suspension was MICRO’ from Elementar Analysensysteme GmbH (Hanau, Germany). centrifuged at 3000 rpm for 15 min for separating the pale pink GeO2 The PXRD patterns were collected using a STOE-STAD IP diffrac- particles, which were then purified by centrifugation (at 3000 rpm for tometer from STOE (STOE and Cie GmbH, Darmstadt, Germany) 15 min) of a suspension of the particles in THF (5 mL) and in ethanol (5 mL) two times each, respectively, to quantitatively give pale pink with Cu-Kα radiation (40 kV, 40 mA) and a Ge(111) monochroma- tor. The crystallite size was estimated using the Scherrer equation: GeO2. The CH analysis [%] found for GeO2: C 1.06 and H 0.24. Both K λ PXRD and EDX analyses proved the formation of GeO2. τ= , where τ is the volume weighted crystallite size, K is the βθcos Scherrer constant here taken as 1.0, λ is the X-ray wavelength, θ is the Bragg’s angle (in rad) and β is the full width of the diffraction line at Synthesis of GeO in benzyl alcohol under inert half of the maximum intensity (FWHM, background subtracted). The 2 conditions – sample B FWHM is corrected for instrumental broadening using a LaB6 standard (SRM 660) purchased from NIST (National Institute of Standards and Technology, Gaithersburg, MD, USA). The value of β was cor- GeCl4 (942.0 mg, 4.39 mmol) was added to benzyl alcohol (20.00 mL, 2 β2 192.3 mmol) and the mixture was stirred under an inert atmosphere at rected from ( βmeasured and instrument are the FWHMs of measured and standard profiles): reflux for 24 h. The pale yellow suspension was centrifuged at 3000 rpm

for 10 min to separate the GeO2 particles, which were then suspended 22 2 β=ββmeasured -.instrument and purified by centrifugation (at 3000 rpm for 10 min) in THF (5 mL) and in ethanol (5 mL) two times each, respectively, to quantitatively give

The DLS measurements were performed on a Modell 802 from colourless GeO2. The CH analysis [%] found for GeO2: C 0.46 and H 0.13. ™ Viscotek (Malvern Instruments GmbH, Herrenberg, Germany) using Both PXRD and EDX analyses proved the formation of GeO2. a laser light emitting diode at 830 nm. The THF and toluene suspen- sions of the particles obtained by ultrasonic treatment were given in a quartz glass cuvette for DLS measurements. Infrared spectra were Synthesis of GeO in benzyl alcohol under ambient recorded with a Nicolet IR 200 spectrometer from Thermo Scientific 2 (Thermo Fisher Scientific Inc., Waltham, MA, USA) in a KBr matrix. conditions – sample C Transmission electron micrographs and energy dispersive X-ray spectroscopy experiments were obtained by a 200 kV-high resolution GeCl4 (713.8 mg, 3.33 mmol) was added to benzyl alcohol (14.54 mL, transmission electron microscope [HRTEM, CM 20 FEG, Co. Philips 139.8 mmol) at 40°C. The mixture was stirred at 40°C under ambient con- (FEI Europe, Europe NanoPort, Eindhoven, The Netherlands)] with ditions for 24 h. The colourless suspension was centrifuged at 3000 rpm an imaging energy filter from Gatan (GIF). Specific surface analyses for 10 min to separate the GeO2 particles, which were then suspended were performed with nitrogen adsorption-desorption isotherms at and purified by centrifugation (at 3000 rpm for 10 min) in THF (5 mL) liquid nitrogen temperature (77 K) using a Quantachrome Autosorb and in ethanol (5 mL) two times each, respectively. Additional stirring of

214 P. Kitschke et al.: Synthesis of GeO2 nanoparticles in benzyl alcohols the transparent, colourless, overlaying liquid at 40°C at ambient condi- inorganic nanocomposites through twin polymerisation’ tions for another 18 h gave further fractions of colourless GeO . Yield: 2 and the Fonds der Chemischen Industrie for a fellowship 56% (all fractions); the CH analysis [%] found for GeO : C 1.58 and H 2 (P. Kitschke). 0.33. Both PXRD and EDX analyses proved the formation of GeO2.

Acknowledgements: We gratefully acknowledge finan- cial support by the DFG Forschergruppe 1497 ‘organic and Received July 12, 2013; accepted October 2, 2013

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