Preparation of undecanoic acid-terminated Si particles from rice husks

Rice is the third most-produced crop worldwide, following wheat and corn. Rice husk is the outer covering of a rice grain, ¹⁾ and 1.2 × 10⁸ tons of rice husk is disposed of globally every year. Most rice husks are incinerated in air or in an incineration facility, which causes air pollution and CO₂ emission. Additionally, the incineration of rice husk deposits large amounts of rice husk ash because of the high concentration (20% in dry weight) of SiO₂ particles in rice husks. ²⁾

There have been several studies on and attempts to recycle rice husks. For example, attempts have been made to recycle rice husks into biomasses, fertilizers, livestock beddings, and so on. ³⁾ However, these products from rice are examples of low value-added products, which hinder the commercial recycling of rice.

Recently, Si-based inorganic materials, such as SiC and SiN, were formed from SiO₂ powder extracted from rice husks. ^2,4–7) From this organic material, Si fine powder is attractive because of its interesting room temperature luminescence. ^2,8–22) Si is not typically considered a suitable material for use in light-emitting devices as it is an indirect transition semiconductor. In the 1990s, the observation of room temperature photoluminescence from Si nanomaterials was reported, which led to several studies of the optical properties of Si nanomaterials for their application in light-emitting devices. ^23–33)

Additionally, luminescent Si nanomaterials have an expected application as fluorescent bio labels since they are nontoxic materials. ^21,22) Thus, the preparation of luminescent Si colloid from rice husks could lead to high value-added recycling. In our previous study, Si nanocrystals were successfully prepared via the Mg reduction of SiO₂ powder extracted from rice husks, and room temperature luminescence was observed in the Si nanocrystals. In this study, the surface of luminescent Si nanoparticles was terminated by hydrophilic organic molecules of undecanoic acid to produce a Si colloid. The optical properties and hydrophilicity of the Si colloid obtained from rice husks were also characterized.

Rice husks weighing 3 g are put into an HCl solution to remove metal elements, such as Na and K. The rice husks are then rinsed with ethanol and incinerated in air at 700 °C for 3 h to extract SiO₂ powder. Next, 100 mg of Mg powder is mixed with an equal weight of the extracted SiO₂ powder. The powder mixture is heated in N₂ gas for 2 h at 700 °C to reduce the SiO₂ content. After the reduction process, the powder is soaked in ethanol and HCl solution for 24 h to remove the MgO formed during the reduction. The rice husk Si powder is then soaked in undecanoic acid solution and heated at 100 °C to terminate the Si surface with undecanoic acid.

Fourier transform infrared (FTIR) spectroscopy (FT-IR-4300, Jasco) was performed at room temperature to evaluate each sample preparation step used to form the rice husks into Si powder and alter the surface termination. Room temperature PL spectra of the Si nanoparticles were observed using the 405 nm line of a GaN laser (TC35, NEOARK) and a spectrometer (SP 2150, Princeton Instruments) equipped with an electrically cooled charge-coupled device (CCD; PIXIS 256, Princeton Instruments). PL decay spectra were observed using an image intensifier CCD (i-CCD; PI-MAX4, Princeton Instruments) with a 7 ns pulse excited by the third harmonic of a Nd:YAG laser (λ = 355 nm; Ultra 50, Quantel) as the excitation source.

The structure and morphology of the Si powder were assessed using a transmission electron microscope (TEM; HITACHI, HF-2000) with an accelerating voltage of 200 kV. The dispersibility of the surface-terminated Si powder created from rice husks was evaluated via the observation of time-transient light transmittance using a wavelength of 1000 nm.

Figure 1 shows the FTIR spectra of samples at each stage in the process used to produce Si powder from rice husks. In the FTIR spectra of just the rice husks and the rice husks after HCl treatment, sharp absorption peaks were observed at approximately 1100 and 2800 cm⁻¹, which correspond to C–O and C–H bonds, respectively [Figs. 1(a) and 1(b)]. These absorption peaks were due to the organic elements such as cellulose in the rice husks. After the incineration process at 700 °C, the sharp absorption peaks at approximately 1100 and 2800 cm⁻¹ disappeared, indicating that the organic matter in the rice husks was burned out. Relatively broader peaks were observed at approximately 1100 cm⁻¹, corresponding to Si–O bonds, as shown in Fig. 1(c). Thus, the white powder that formed on the specimen after incineration is SiO₂ powder originally contained in the rice husks. Figure 1(d) shows FTIR spectra of samples after the Mg reduction process. The absorption peaks at approximately 1100 cm⁻¹ dispersed after the reduction process, indicating that the SiO₂ powder was successfully reduced and Si powder was formed.

Zoom In Zoom Out Reset image size

Figure 2(a) shows the changes in the FTIR spectra of the Si nanoparticles created from rice husks after their HCl treatment to remove MgO. Absorption peaks at approximately 2100 cm⁻¹, corresponding to Si–H bonds, were observed in all the absorption spectra. This bonding is most likely due to the surface termination of Si nanoparticles by H⁺ ions produced during the HCl treatment. Absorption peaks at approximately 1100 cm⁻¹ due to Si–O bonds were also observed in the sample immediately after HCl treatment. The Si–O bond was formed by surface oxidation of rice husk Si nanoparticles during HCl treatments. The intensity of the absorption peak increased as time lapsed, up to 5 days, indicating that the natural oxidation of the rice husk Si nanoparticles progressed in ambient air. Figure 2(b) shows the FTIR spectra of rice husk Si nanoparticles after the hydrosilylation process. Absorption peaks at approximately 1450 cm⁻¹ and corresponding to Si–C bonds were observed, indicating that the surface of the Si nanoparticles was successfully terminated by undecanoic acid. Absorption peaks due to Si–O and Si–H bonds were also observed. These results indicate that the most of surface of the rice husk Si nanoparticles were terminated by undecanoic acid. Conversely, the intensity of absorption peaks corresponding to Si–O bonds did not change during 5 days of aging in distilled water, indicating that the surface oxidation of the Si nanoparticles was suppressed by the undecanoic acid termination.

Zoom In Zoom Out Reset image size

Figure 3(a) shows a bright-field TEM image of the fine Si powder formed via the Mg reduction process of amorphous SiO₂ nanoparticles extracted from rice husks. Aggregates composed of several Si nanoparticles were observed. Lattice fringes corresponding to the Si (111) plane were also observed in several nanoparticles, indicating the crystallinity of the nanoparticles. Figure 3(b) shows the corresponding selected area transmission electron diffraction of the rice husk Si fine powder. A Debye–Scherrer ring pattern containing several spots was observed. Effects of the electron irradiation such as morphology changes of Si nanoparticles and TED pattern were observed during the TEM observation. Additionally, in our previous study, halo pattern of TED was observed from the SiO₂ nanoparticles form rice husks. ²²⁾ These results indicate that most of the Si particles formed through the Mg reduction process of amorphous SiO₂ from rice husks have a crystalline structure.

Zoom In Zoom Out Reset image size

Figure 4(a) shows the room temperature PL spectra of the Si fine powder synthesized from rice husks via the Mg reduction process. Broad spectra ranging from 600 to 1000 nm were observed. The PL intensity of the Si fine powder decreased as times lapsed, for 5 days, in ambient air. The decreasing PL intensity was accompanied by PL peak shifts to shorter wavelengths. The instability of the PL properties was probably due to the formation of nonradiative recombination centers and decreasing size of the Si nanoparticles during natural oxidation in ambient air. For the case of rice husk Si nanoparticles after surface termination by undecanoic acid, the PL properties were stable and the peak wavelength did not change after 5 days of aging in distilled water. This result indicates that the surface termination by undecanoic acid suppressed surface oxidation and therefore improved PL stability of the Si nanoparticles. During the 5 days of aging in distilled water, the PL intensity of the rice husk Si nanoparticles slightly increased. This increasing PL may be due to the precipitation of larger-sized Si nanoparticles that do not contribute luminescence.

Zoom In Zoom Out Reset image size

To discuss details of the optical properties, such as the origin of the luminescence, the PL decay dynamics of the rice husk Si nanoparticles were observed and are shown in Fig. 5. Microsecond-order PL decay curves were observed and were wavelength dependent.

Zoom In Zoom Out Reset image size

It is well known that the PL decay curves of luminescent materials, such as Si nanomaterials, can be fit with the stretched exponential function

$$\begin{eqnarray*}&&{I}={{I}}_{0}\exp \left\{-{\left({t}/\tau \right)}^{\beta }\right\},\end{eqnarray*}$$

where I₀ is the PL intensity at t = 0, τ is the lifetime, and β is a fitting distribution parameter representing the curvature of the decay. ^34–38) Figure 5(b) presents the τ of the rice husk Si nanoparticles evaluated by fitting the stretched exponential function of the PL decay curves. As the wavelength of the decay curves increased, the PL lifetime increased. These results indicate that the origin of the luminescence is the quantum size effect, which is the recombination of a hole–electron pair confined in the rice husk Si nanoparticles. Lifetimes estimated by the fitting were 26–60 μs in the wavelength ranges from 650 to 850 nm. This value is approximately one order of magnitude slower than 2–10 μs that previously reported. ²²⁾ Thus, surface termination by undecanoic acid likely suppressed the formation of nonradiative recombination centers on the rice husk Si nanoparticles.

Figure 6 shows the results of time-transient light transmission measurements of the Si nanoparticles dispersed in distilled water with a wavelength of 1000 nm. In the case of rice husk Si nanoparticles before surface termination, the transmittance of the light was near 0% at the first stage (time = 0 min) of the measurements, which was due to the light scattering of the Si powder dispersed in distilled water. The transmittance of light drastically increased as time elapsed due to the precipitation of the rice husk Si fine powder. The transmittance of light finally reached 80% after 60 min.

Zoom In Zoom Out Reset image size

Conversely, when the Si nanoparticles terminated with unacademic acid were dispersed in distilled water, light transmittance slightly increased as time elapsed, reaching only approximately 20% after 60 min. This result indicates that surface termination with undecanoic acid improved the hydrophilicity (dispersibility in distilled water) of the rice husk Si nanoparticles and, consequently, the transmittance of light was drastically increased as time elapsed.

Si nanoparticles were produced from rice husks, and their surface was terminated by undecanoic acid to prepare a luminescent Si colloid. Almost the entire surface of the Si nanoparticles was terminated by undecanoic acid with partial termination by oxygen via surface oxidation during the hydrosilylation process. After surface termination with undecanoic acid, the PL properties of the rice husk Si nanoparticles were stable and degradation of PL intensity was not observed. In the PL decay curves of the Si nanoparticles fit by the stretch exponential function, a lifetime of 20–60 μs that was wavelength dependent was observed, indicating that the luminescence formed by Si nanoparticles was due to the quantum size effect. Improvement of the dispersibility of the rice husk Si nanoparticles after surface termination with undecanoic acid was confirmed via light transmittance measurements. Preparation of luminescent Si colloid from rice husks has a potential for high value-added recycling of rice husks.