Low-temperature solution route to macroscopic amounts of hydrogen terminated silicon nanoparticles

Article abstract of DOI:10.1021/ja064177q

A new solution route for preparing gram-scale, hydrogen terminated silicon nanoparticles is presented. Dimethoxyethane and diocytl ether have been used to prepare silicon nanoparticles via a solution reaction between sodium silicide and ammonium bromide. The reaction products are isolated as a clear yellow-orange solution and a dark black powder. Both the solution and the powder have been characterized. Copyright

Full text of DOI:10.1021/ja064177q

Published on Web 08/05/2006
Low-Temperature Solution Route to Macroscopic Amounts of Hydrogen
Terminated Silicon Nanoparticles
Doinita Neiner, Hsiang Wei Chiu, and Susan M. Kauzlarich*
Department of Chemistry, One Shields AVe, UniVersity of California, DaVis, California 95616
Received June 13, 2006; E-mail: smkauzlarich@ucdavis.edu
The production of hydrogen terminated silicon (Si) nanoparticles
is currently an area of intense research. The primary methods of
producing large amounts of hydrogen terminated Si is either from
1
the mechanical fracturing of porous Si or the gas-phase decom-
position of silane or organosilane gas.2 In both cases, further
-4
etching is necessary using nitric and hydrofluoric acid to produce
5
a stable, well-passivated hydrogen surface. Once the surface is
hydrogen passivated, further functionalization via hydrosilation is
5
possible. This chemistry has been well-developed in the case of
porous Si and opens the door for biological and technological
applications, such as for quantum dot probes, optical and electronic
applications, hydrogen storage, and solar energy.
Reports of ambient solution reduction have invigorated research
6-9
to improve colloidal synthesis of Si nanoparticles. These reported
syntheses provide new solution routes to free-standing Si nano-
particles that can be further functionalized. In addition, further
doping and shape control of Si has been documented.9 However,
the yields of nanoparticles from these methods are still very low,
and physical methods, such as mechanical fracturing of porous Si
and gas-phase decomposition of silanes, still provide significantly
larger amounts of nanoparticles for further applications. A simple
chemical synthesis of hydrogen-capped Si nanoparticles would
provide a major advance in the application of functionalized silicon
nanoparticles.
,10
Figure 1. TEM and X-ray powder patterns for the powders obtained from
the reactions in (a) DME and (b) DOE. The powder diffraction of (a) can
be fully indexed as diamond-structured silicon.
temperatures less than 300 °C: the production of sodium bromide
as a reaction byproduct and the generation of ammonia and
hydrogen gases. The solvent provides a medium that allows for
better mixing of the reagents and prevents the particles from
agglomerating. In addition, the solvent allows for continuous access
to the surface of NaSi, thus facilitating the reaction. In both of the
Metal silicides have been useful starting materials for the
production of silicon nanoparticles. Si nanoparticles have been
1
1,12
prepared by a metathesis route in solution using SiCl
4
,
by
oxidation in solution using bromine,13 and by sonication. Metal
silicides can produce amorphous Si with embedded nanocrystallites
of Si by the solid-state reaction of metal silicides with metal and
14
investigated solvents, for a 1:1 molar ratio, the reaction product is
15
a mixture of Si nanoparticles with Na
8
Si46 clathrate phase. The
1
5
ammonium halides. However, size and shape control, along with
further functionalization, is not possible via a solid-state reaction.
Herein, we report a new solution route to the production of
macroscopic amounts of either amorphous or crystalline hydrogen
terminated silicon nanoparticles. This method provides a simple
route that can be further optimized to control size and possibly
shape and allows for additional surface functionalization with
organic moieties.
final product consists of a dry black hydrophobic powder (0.2 g)
and a yellow solution (DME) or a brown-orange solution (DOE).
The solutions can be dried into powders (0.016 g) and dispersed in
chloroform for further characterization.
The X-ray powder patterns along with the TEM images for the
solid product obtained from DME and DOE are presented in Figure
1. The TEM images show well-dispersed nanoparticles. The size
distribution for the nanoparticles obtained in DME is 4.91 ( 1.23
nm and for the DOE is 3.85 ( 1.03 nm (based on 1000
nanoparticles). The TEM images for the solution products in DME
and DOE are similar to those presented for the solids. Energy-
dispersive X-ray (EDX) spectroscopy of the samples shows that
they contain silicon, and there is no evidence for bromine or sodium.
The X-ray powder diffraction pattern for the powder obtained from
DME clearly shows peaks consistent with diamond structure,
crystalline silicon. The crystallite size based on the diffraction line
width is 8 nm. Similar X-ray powder patterns of less than 10 nm
particles have been obtained by laser-induced heating of silanes,
All manipulations for the synthesis of Si nanoparticles were
performed in a N -filled glovebox or Ar inert atmosphere Schlenk
2
line. The silicon nanoparticles were prepared by a solution reaction
using sodium silicide (0.5 g) and ammonium bromide (0.55 g) in
dimethoxyethane (DME, 80 °C) and dioctyl ether (DOE, 260 °C)
using standard Schlenk techniques. Sodium silicide was prepared
by direct reaction of stoichiometric amounts of the elements in sealed
niobium tubes, sealed under vacuum in fused silica tubes, at 650
°
C for 3 days.16 The chemical reaction is provided in (1), where
Si/H is used to indicate hydrogen terminated silicon nanoparticles.
4
followed by HF etching. Calculated scattering with particle size
NaSi + NH Br f NaBr + NH + Si/H + H (1)
There are two possible driving forces for this reaction to occur at
4
3
2
suggests that the diffraction pattern changes to that of diamond
structure Si at about 1.7 nm diameter and that once diameters of 6
11016
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J. AM. CHEM. SOC. 2006, 128, 11016-11017
10.1021/ja064177q CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
5
Swihart to give rise to luminescence. The yellow and yellow/orange
solution can be dried into a powder and redissolved in chloroform.
The yellow solution shows a photoluminescence emission maximum
at about 420 nm.
In summary, we have developed a simple solution route to
hydrogen terminated silicon nanoparticles. Macroscopic amounts
of powder can be obtained of either crystalline or amorphous
nanoparticles, depending upon solvent and reaction conditions. The
hydrogen termination of these nanoparticles provides an ideal
surface for further functionalization and application of silicon
nanoparticles.
Acknowledgment. The authors thank the DOE’s Centre of
Excellence for Chemical Hydrogen Storage for funding and
Xiaoming Zhang for assistance.
Supporting Information Available: TEM images, size distribu-
tions, EDX spectra, and FTIR data before and after wash of the
nanoparticles. Photoluminescence spectra (excitation and emission).
This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 2. FTIR spectra for Si nanoparticles obtained from (a) DME and
b) DOE, after washing with acidified water.
(
nm are reached, there is very little change in form factors or line
width.17 Further experimental results are necessary to test this
prediction. The X-ray powder diffraction for the product obtained
from DOE shows two broad diffraction peaks. These peaks, at about
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The black powder does not have detectable photoluminescence
(by eye), but it can be etched in a similar manner as reported by
JA064177Q
J. AM. CHEM. SOC.
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