Elsevier

Journal of Aerosol Science

Volume 128, February 2019, Pages 50-61
Journal of Aerosol Science

Aerosol synthesis of germanium nanoparticles supported by external seeding: Theoretical and experimental analysis

https://doi.org/10.1016/j.jaerosci.2018.11.013Get rights and content

Highlights

  • Novel concept for combining a hot wall reactor with a hot wire generator as an external seeding unit.

  • Influence of heterogeneous nucleation is studied theoretically and experimentally.

  • Heterogeneous nucleation is essential for synthesis of narrowly distributed germanium particles.

  • The predicted particle distributions agree well with the experimental results.

Abstract

In this work we present a detailed study of the effect of heterogeneous nucleation on the formation of germanium nanoparticles (Ge NPs) produced from monogermane (GeH4) in a hot wall reactor gas phase synthesis. As external seeding unit we use a hot wire generator (HWG) which produces a stable concentration of aerosol particles. The particle concentration and size of the seeds is easily controllable via the applied voltage and used metal. We demonstrate the importance of seed particles for the production of narrowly distributed Ge NPs with geometrical standard deviations (GSD) < 1.1 in our setup. Thereby, molybdenum and tungsten as wire metal show the best seeding results. The size of the final Ge NPs can be precisely controlled in the range of 20–60 nm. The expansion of Ge lattice seen in XRD and tiny amount of Mo probed by high-resolution STEM-EDXS analysis indicate that the Mo seed particles are most likely dissolved and incorporated in the Ge lattice during growth. Furthermore, we demonstrate an approach to investigate the effect of heterogeneous nucleation on the particle formation by population balance modeling. The results are in good agreement with experimental data. The seed concentration is the key parameter and has a major influence on the nucleation rate.

Introduction

Semiconducting nanoparticles (NPs) out of silicon (Si) or germanium (Ge) have attracted attention in a wide field of applications over the last decades. The NPs are used as building blocks for the fabrication of functional devices e.g. thin film transistors (Meric et al., 2015), solar cells (Fan et al., 2010, Wang et al., 2013), lithium ion batteries (Duveau et al., 2015, Li et al., 2017), sensors (Chu et al., 2016), and thermoelectric generators (Basu et al., 2014). The properties of the functional devices can be tailored by particle properties like size, shape or composition. Contrary to Si, Ge has a 10–50 times larger absorption coefficient throughout the visible spectrum (Dash & Newman, 1955) and a 104 times higher electronic conductivity (Wang, Bao, Lou, Liang, & Zhou, 2016). Therefore, Ge is a promising material for NP based device fabrication. However, a well-controlled synthesis is essential to provide building blocks with defined size, shape and composition for thin-film formation.

The synthesis of Ge nanoparticles is achieved in the liquid phase by a variety of methods like colloidal synthesis in molten salts (Liu, Giordano, & Antonietti, 2012) or thermal reduction of GeO2 with magnesium (Wang et al., 2016). However, for a continuous and scalable production route with controllable product properties aerosol processes are more suited (Kruis, Fissan, & Peled, 1998). Synthesis strategies are well demonstrated in literature for non-thermal plasma (Ahadi et al., 2016, Holman and Kortshagen, 2011), laser ablation (Amoruso et al., 2004, Erogbogbo et al., 2011), and hot wall reactor processes (Onischuk et al., 2000, Wiggers et al., 2001).

In previous works we have shown the high versatility and scalability of hot wall reactors. Quite narrow particle size distribution of Si NPs can be obtained with tailor-made properties by low concentration and pressure to overcome broad distribution predicted by the self-preserving theory for coagulation (Dekkers and Friedlander, 2002, Körmer et al., 2012). Besides pure Si NPs also the formation of Si/Ge hybrid structures and alloys were demonstrated (Mehringer et al., 2014, Mehringer et al., 2015). It was shown that for the synthesis of well-defined Ge NPs the low concentration and pressure are not sufficient. Additional seed particles have to be added to suppress homogenous nucleation. Otherwise, very high nucleation rates occur which lead to high particle number concentrations. Due to the high number concentration agglomeration and sintering becomes relevant. Similar observations were made by Okuyama, Ushio, Kousaka, Flagan, and Seinfeld (1990) and Alam and Flagan (1986) who evaluated experimentally and theoretically the role of seed particles in the formation of TiO2 and Si. For Ge NP formation a detailed study and comparison of simulated and experimental data is not present. Therefore, in this work we introduce a hot wire generator (HWG) directly connected to the first stage as flexible and small external seeding unit. The HWG allows a stable and controllable production of ultrafine metal particles in high concentrations (Khan et al., 2014, Peineke et al., 2006) suitable for providing seed or catalyst particles (Nasibulin, Moisala, Brown, Jiang, & Kauppinen, 2005). The amount and size of the seed particles can be easily adjusted in the HWG which allows a detailed investigation of the effect of seed particles on particle formation. The experimental results are supported theoretically by a population balance model including global reaction kinetics, homogenous and heterogeneous nucleation and particle growth to describe the evolution of the particle size distribution.

Section snippets

Theoretical approach for heterogeneous nucleation

The model for the particle synthesis of Ge NPs with seed particles applied in this work is based on the work of Körmer, Schmid, & Peukert (2010), who successfully implemented and validated an one-dimensional model of Artelt, Schmid, and Peukert (2003); Artelt, Schmid, and Peukert (2005) for the particle synthesis of Si in an aerosol reactor. Therefore, only the most important mechanisms and equations are briefly described in the next sections. Agglomeration and sintering are not considered in

Reactor setup

The reactor setup consists of two consecutive hot wall reactors (HWR I and HWR II) and is described in detail elsewhere (Mehringer et al., 2014). In this study, only the HWR I is used for particle synthesis. The standard operation pressure is 25 mbar. The metallic seed particles are produced by a hot wire generator by condensation from a supersaturated vapor (see Fig. 1). The vapor is generated by passing an electric current through a metallic wire, which is quenched by an inert gas to reach

Characterization of the seeding metal

In order to use the hot wire generator as an external seeding unit, a constant emission of seed particles with defined aerosol properties must be ensured during synthesis. One of the most critical properties is the particle concentration emitted into the hot wall reactor. The amount of particles depends on the material which evaporates over time and the resulting supersaturation around the wire (Peineke et al., 2006). Therefore, an essential parameter for the usability as seeding material is

Conclusion

We demonstrated the relevance and applicability of a hot wire generator as flexible seeding unit for synthesis of narrowly size distributed Ge NPs. The small particles from the HWG prevent the formation of small strongly agglomerated Ge NPs by suppressing the homogenous nucleation. We systematically investigated the influence of different process parameters on the seed particle production by the hot wire generator and the resulting Ge NPs morphology. To ensure exclusive heterogeneous nucleation

Acknowledgements

The authors acknowledge the funding of the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence “Engineering of Advanced Materials” at the Friedrich-Alexander-Universität Erlangen-Nürnberg

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