Stabilization of mid-sized silicon nanoparticles by functionalization with acrylic acid
© Bywalez et al; licensee Springer. 2012
Received: 10 August 2011
Accepted: 16 January 2012
Published: 16 January 2012
We present an enhanced method to form stable dispersions of medium-sized silicon nanoparticles for solar cell applications by thermally induced grafting of acrylic acid to the nanoparticle surface. In order to confirm their covalent attachment on the silicon nanoparticles and to assess the quality of the functionalization, X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier spectroscopy measurements were carried out. The stability of the dispersion was elucidated by dynamic light scattering and Zeta-potential measurements, showing no sign of degradation for months.
Silicon nanoparticles received considerable attention in recent years, especially since the discovery of quantum-confined luminescence in silicon. Besides optoelectronic devices [1, 2], silicon nanoparticles are envisioned for a much broader range of applications, especially if they can be processed by printing techniques. Future generations of lithium-ion batteries might rely on printable silicon nanoparticles as a part of the electrode setup, boosting the battery's capacity [3, 4]. Furthermore, their potential in photovoltaics is shifting into the focus of interest. For example, silicon nanoparticles were used as a top layer on commercial polycrystalline solar cells boosting their power performance by 60% in the blue/UV range and also as a principal component of a heterojunction solar cell in combination with P3HT [5, 6]. The reported top efficiencies of 1.15% are promising although the specimens need to be stored under inert conditions.
One of the basic requirements for the industrial applicability of silicon nanoparticles is the availability of printable dispersions, and in cases of electronics applications, a suitable protection against oxidation. The most common approach is to functionalize the particles with various organic substances like alkenes [7–9], amines , and phospholipids . Although it has been shown that this leads to fairly stable dispersions of small nanoparticles with sizes below 5 nm, the situation gets more complicated when dealing with particles of larger sizes. Veinot et al. showed a strong size dependence of hydrosilylation efficiency for silicon nanoparticles. Particles with 5 to 7 nm in diameter required significant longer reaction times than the particles with 2 to 3 nm in diameter and still showed worse functionalization efficiencies . These effects are attributed to changes in reaction chemistry. Together with the observation that smaller nanoparticles require a lower degree of surface grafted molecules  to form stable dispersions and the fact that the decreasing surface curvature of large nanoparticles reduces the specific surface coverage , it is obvious that functionalization of mid-sized silicon nanoparticles is challenging. While the functionalization with alkenes, as also established in our group, yields stable dispersions from small nanoparticles with sizes below 5 nm , the same reaction routes do not lead to stable dispersions with larger particles.
A surface coverage with acrylic acid molecules was used to render our particles hydrophilic and provide stable dispersions even for particles exceeding a few nanometers in diameter. Li et al. and He et al. [15, 16] used UV-grafted polyacrylic acid to render small nanoparticles water soluble. Sato et al.  also used a similar approach on silicon nanoparticles of < 2 nm in diameter to provide a termination using acrylic acid. Nevertheless, there is no sound evidence for the covalent attachment of acrylic acid via a Si-C bond, and polymerization cannot be excluded. The rather high oxidation levels, although only small particles were used, indicate low (insufficient) functionalization efficiencies. Not only surface oxidation, but also ligands with a long chain length as well hamper the applicability of silicon nanoparticles in electric or electroluminescent devices because they prevent an efficient charge transport compared to short ones . Therefore, short functionalization chain lengths are desired as they ensure better charge transport compared to their larger counterparts .
The approach used in this work reduces the thickness of the surface coating compared to commonly used n-alkenes and polymers. We present a fast functionalization route for medium-sized particles of a few 10 nm in diameter, with sound dispersion properties as well as very low oxygen content. Furthermore, clear evidence for the underlying binding mechanism is provided.
Hydrofluoric acid (40%), methanol, acrylic acid, and isopropanol were purchased from VWR International, Darmstadt, Germany and used as received.
Synthesis and functionalization
Particle diameters were calculated from Brunauer, Emmett, and Teller [BET] specific surface measurement with Quantachrome Nova 2200 (Quantachrome Instruments, Boynton Beach, FL, USA) and TEM with a FEI Tecnai F20 ST microscope (FEI Co., Hillsboro, OR, USA). Surface functionalization was confirmed via diffuse reflectance infrared Fourier transform spectroscopy [DRIFTS] utilizing a Bruker IFS66v/S spectrometer (Bruker Optik GmbH, Ettlingen, Germany), and XPS was done with a SPECS Phoibos 100 spectrometer (SPECS GmbH, Berlin, Germany). Dispersion quality and stability were probed via dynamic light scattering [DLS] and Zeta-potential measurements, both performed with a Malvern Nano ZS instrument (Malvern Instruments, Worcestershire, United Kingdom).
Results and discussion
In order to help distinguish the surface termination from polyacrylic acid, the respective Fourier transform infrared [FTIR] spectra are shown as well [see Additional file 3]. The most striking difference here is the missing OH-vibration around 3,574 cm-1.
The Si-CH2 scissoring vibration at 1,450 cm-1 is regularly used as an indicator for the covalent attachment of the functionalization agent onto the molecules [10, 17, 23]. This choice is problematic because the strong C-CHx vibration signal appears in the same frequency range and overlaps with the Si-C signal [21, 24]. A more reasonable selection is the Si-CH2 stretching vibration at 1,259 cm-1, cf. Figure 2. However, this vibration is weak and hardly detectable, as could be seen in the work of Rosso-Vasic et al. .
Highly stable dispersions of silicon nanoparticles stabilized by acrylic acid were formed. Evidence for dispersion stability was provided by DLS and Zeta-potential measurements. FTIR and XPS measurements were used to assess the functionalization quality and elucidated the binding mechanism between acrylic acid and silicon nanoparticles. TEM images provided further insights into the nature of the surface termination. Future experiments will focus on the electrical properties of functionalized particles as well as printed layers.
We thank Anna Elsukova and Zi-An Li for providing the TEM pictures and Alice Sandmann (all University of Duisburg-Essen) for her support with the DLS and Zeta-potential measurements. Financial support by the Deutsche Forschungsgemeinschaft through the Research Training Group GRK 1240 and by the European Union and the Ministry for Innovation, Science and Research of North Rhine-Westphalia in the framework of the ERDF program is gratefully acknowledged.
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