Solid-state reduction of silica nanoparticles via oxygen abstraction from SiO₄ units by polyolefins under mechanical stressing

Abstract

Metal oxides with an oxidation number lower than the highest often exhibit attractive functional properties. However, conventional chemical or thermal reduction of the stable oxides is often laborious and cannot be stopped at an appropriate level of reduction. Therefore, we here try to explore non-conventional reduction processes in a solid-state without external heating. Unique features of reduction processes of SiO₂ toward suboxides, SiO_x (1 ≤ x < 2), were made possible by milling fumed silica nanoparticles with polyolefins (POL), i.e., polypropylene (PP) or polyethylene (PE) and a fluorine-containing one, polyvinylidene difluoride (PVDF). We mainly examined the electronic and coordination states of Si by Si2p XPS spectra and ²⁹Si MAS NMR, respectively. They significantly differ from a similar commercial product obtained via a thermal route. Judging from the chemical shift of ²⁹Si MAS NMR as a criterion of the degree of reduction of SiO₂, the function of POL as a reductant is in the order PP ≈ PE > PVDF. Since the present solid-state reaction is free from the formation of unstable gaseous SiO as an intermediate, the products are free from the Si component in a Q⁰ state close to that of metallic Si. From these results we conclude that the present silicon suboxides obtained by co-milling silica with POL are closer to those defined as a random bonding model of SiO, than a random mixture model, the former being unachievable by a thermal process. The main mechanism of the present solid-state reduction is the oxygen abstraction from the SiO₄ units by the polarized POL, with its simultaneous oxidative decomposition up to the state of carbon. The reaction process is simple and scalable so that it may offer a new affordable fabrication method of silicon suboxides.

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Introduction

Silicon suboxides, SiO_x (1 ≤ x < 2), including nominal SiO, gather increasing interests in view of their diverse application potentials, e.g. coating materials for anti-reflection,^1–3 photoluminescence^4,5 or anode materials for Li ion batteries.^6–9 SiO_x is usually prepared via a thermal reduction of SiO₂,^10,11 although oxidation of Si metal is also explored as a minor option.^12,13 Associated research works have been predominantly motivated by semiconductor technology.^14,15

Reduction of silica is already engineered for semiconductor, industrial or solar grade Si.^16,17 Therefore, it seems possible to obtain SiO_x with varying x as an appropriate intermediate stage. However, to stop the thermal reduction process at a desired stage is not so easy. It is generally recognized that gaseous SiO is formed by heating to temperatures up to 2000 K, followed by its condensation. Cooling of SiO is accompanied by its disproportionation to Si and SiO₂.^18–20 An example of a typical fabrication method via a thermal route is heating a mixture of Si and SiO₂ at 1400 °C to obtain gaseous SiO and subsequently condensing at 600 °C.²⁰ The kinetic process of the disproportionation is dominated by many parameters among others the rate of quenching, including the distance from the heat source and the substrate or the collecting vessel, and hence, difficult to bring under precision control.

Therefore, the stoichiometry, crystallography and microstructure of the nominal SiO are not uniquely defined and remain controversial.¹¹ There have been some doubts regarding the phase purity of nominal, commercial SiO.²¹ Hohl et al.²⁰ elaborated the structure determination of amorphous SiO in detail and concluded that it is a mixture of nanoclusters of Si dispersed in the matrix comprising SiO₂ or slightly reduced silica. Their conclusions were confirmed by different researchers as well.^22,23

We therefore try in this study to reduce SiO₂ via an alternative route without passing through the gaseous SiO. We thought it could be done in a solid-state process by milling SiO₂ nanoparticles with solid polyolefin (POL) species. The idea is based on our recent findings, that the co-milling of a mixture of metal oxide nanoparticles and hydrocarbon-containing polymers results in the oxidative decomposition of polymers with simultaneous reduction of the oxides and anion exchange, when the POL contains anionic species other than oxygen.^24,25 Hitherto, we preferentially examined TiO₂ and SnO₂ with fluorine-containing hydrocarbons without external or subsequent heating.

Mechanochemical reduction of a complex oxide like magnesium ferrite was worked out in terms of milling media, whereby the stainless-steel media have supposedly played a role of reductant.^26,27 A redox reaction between dissimilar oxides, e.g. between α-Fe₂O₃ and SnO, also takes place mechanochemically.²⁸ These mechanochemical reduction processes were between oxides or oxides and metals. What we are going to discuss in this work, in contrast, deals with those between metal oxides and hydrocarbon species serving as reductants.

Our explicit objectives in this study are to elucidate the process of reducing SiO₂ to SiO_x via a solid-state route by co-milling with a POL and to compare the products with commercial SiO as an example of the thermal reduction product. Emphasis is laid on the difference in the electronic and coordination states of Si between the products of mechanochemical and thermal processes by using XPS and ²⁹Si MAS NMR as main analytical tools.

Experimental

Sample preparation

Amorphous pyrogenic (fumed) silica (Aerosil 200, Evonik) without any surface modification was used as a source of SiO₂. Following 3 POL species were chosen as partners of co-milling: polypropylene, PP (IUPAC name, poly(1-methylethylene)), (Seishin Enterprise, PPW-5); polyethylene, PE (IUPAC name, poly(methylene)), (Seishin Enterprise, SK-PE-20L) and a fluorine containing POL, polyvinylidene difluoride, PVDF (IUPAC name poly(1,1-difluoroethylene)), (Aldrich). All the POLs were used as supplied, without pre-treatment. A nominal SiO, supplied by Osaka Titanium (SiO powder, purity 99.95%), fabricated via a thermal route, was used for comparison.

Co-milling was carried out by a planetary mill (Fritsch Pulverisette 6) at 300 rpm for up to 3 h. A vial of 80 mL and 15 mm and 5 mm balls, 6 pieces each, were used for milling. All these parts were made of yttria stabilized zirconia, YSZ. The staring mixture comprised silica and POL, with the weight ratio 9 : 1, and the total mass 2.0 g. We will denote those co-milled products henceforth as OXM.

Characterization

The long-range ordering of the products was examined by X-ray diffractometry (XRD, Rigaku MultiFlex). Vibrational spectroscopy i.e. Fourier transform infrared spectroscopy (FT-IR, JASCO FT/IR 6200, KBr disk in N2) and Raman scattering spectroscopy (Jasco, NRS-3100 spectrometer with an incident laser beam of 532.0 nm), were performed to characterize the short-range atomic interaction.

Chemical states of each atomic species were monitored mainly by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI5000 Versa Prove). Si2p and O1s signals were obtained by using a monochromated AlKα beam (1486.6 eV, 25 W), with a spot diameter 100 μm. Calibration of the binding energy was based on the C1s signals on the sample of commercial SiO.

The coordination states of Si were examined by magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR, VARIAN INOVA-400 plus, PDMS) for ²⁹Si nuclear species. Details of NMR measurement are as follows; offset: 0 ppm, sweep: 500 ppm, scans: 256, acquisition time: 17.2 ms, resolution: 58.2 Hz, observed angle 90 deg: observed width 90: 13 μs × 90, amp. pulse: 100%, and relaxation delay: 120 s. The chemical shift was expressed with a reference of polydimethylsiloxane (PDMS), −34.44 ppm.

Color change was observed by diffuse reflectance spectroscopy (UV-vis DRS, Shimadzu, UV-3150). Morphology and fine structures were observed by a field emission scanning electron microscope (FE-SEM: JEOL, JSM-7600F).

Results and discussion

Apparent reduction of silica by co-milling with POL

To examine whether and to what extent silica was reduced by OXM, we first observed O1s XPS spectrum. The results are summarized in Fig. 1. From the spectrum of the starting SiO₂, we observe a single peak at around 533 eV, which was established as that of pure silica.²⁹ Nominal SiO exhibits also a single peak at around 532.2 eV. Lower binding energy of O1s is a clear indication of reduction of silicon oxides.³⁰

Fig. 1 XPS spectra (O1s) of SiO₂, SiO and the mixtures with SiO₂:POL after milling for 3 h.

The decrease in the O1s binding energy from that of intact silica was also observed by OXM. In the case of PP and PE as POL, peak position is close to that of commercial SiO, but the binding energy is slightly larger than that of SiO. In the case of PVDF, the redshift of the O1s peak is much larger, due to coexisting fluorine, which will be discussed later. From those O1s XPS spectrum, reduction of SiO₂ by OXM is evident.

Electronic states of Si

Change in the electronic states of silicon was examined by Si2p XPS spectra. As shown in Fig. 2, the Si2p binding energy of SiO₂ was at around 103.7 eV, matching well with the literature.^31,32 For the OXM products, the Si2p peaks consistently decreased from that of SiO₂. The decrease in the Si2p binding energy indicates the increase in the electron density around Si, and hence, reduction of silicon from its nominal state of Si⁴⁺ to lower oxidation states. The decrease in the binding energy is more substantial with PVDF. We think the larger shift by PVDF for the same reason with O1s, although the difference in the binding energy between PVDF and other POLs of Si2p is not so large as those of O1s.

Fig. 2 XPS spectra (Si2p) of SiO₂, SiO and the mixtures with SiO₂:POL after milling for 3 h.

The most remarkable difference in the spectra between the samples via OXM and the commercial SiO is the bimodality of the spectrum of the latter. The peak at around 98.4 eV is very close to those ascribed to the Si2p_3/2 of metallic silicon.^33,34 The other peak, at around 102.7 eV, corresponds to a slightly reduced state of silicon from that of SiO₂.³¹ This quasi two-phase state of nominal SiO will be discussed below in detail.

Change in the coordination states of Si

The profiles of ²⁹Si MAS NMR spectra are shown in Fig. 3. The spectrum of the starting, intact fumed silica exhibited a simple spectrum peak at around −108 ppm, ascribed to Q⁴ from Si(O₄).^20,35 Commercial SiO, on the other hand, exhibits two distinct peaks, i.e. one close to Q⁴ of silica and the other, peaked at around −65 ppm ascribed to Q⁰.^20,35 The OXM products exhibited broad peaks ranging between −120 ppm and −80 ppm. We tried to deconvolute these peaks to examine the change in the coordination states of silica, in terms of the relative intensities of Qⁿ (1 ≤ n ≤ 4), representing the states of Si(O_n). The results are summarized in Table 1.

Fig. 3 ²⁹Si MAS NMR spectra of SiO₂, SiO and the mixtures with SiO₂:POL after milling for 3 h.

Table 1 Relative intensities (chemical shift) of ²⁹Si MAS NMR signals with different coordination states

Sample/coordination state	Q^1	Q^2	Q^3	Q^4
Intact SiO2	0	0	25.29 (98 ppm)	74.71 (108 ppm)
Commercial SiO	0	0	16.74 (100 ppm)	83.26 (110 ppm)
Co-milled with PP	0	8.16 (90 ppm)	35.49 (98 ppm)	56.35 (108 ppm)
Co-milled with PE	0	9.48 (90 ppm)	34.49 (98 ppm)	56.03 (108 ppm)
Co-milled with PVDF	0	0	29.74 (100 ppm)	70.25 (110 ppm)

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Intact silica already contains ca. 25% of Q³ state. This is not surprising because of its amorphous and nanocrystalline feature with significant amount of broken Q⁴ networks associated with the dangling bonds, particularly in the near surface region. In the commercial SiO, the percentage of Q⁴ state is slightly higher. Note that the portion of Q⁰ state in the result of SiO was excluded from Table 1. The OXM products exhibit significant amounts of Q³ and Q² states at the cost of the Q⁴ contribution. This is a clear indication of loss of oxygen atoms from Q⁴ units. Among 3 POL species, PP and PE showed similar proportions of Qⁿ states. For the OXM treatments with PVDF, the portion of Q³ is smaller than other two POLs, and Q² states was not detected.

Although information of Si2p XPS and ²⁹Si MAS NMR are entirely different from each other, spectral bimodality of commercial SiO is common to both spectra. From these observations, we may safely conclude that the usual, commercially available SiO as a thermal reduction product, consists of two distinct components. One is close to SiO₂ with possible partial loss of oxygen and the other to metallic Si, as frequently claimed in the literature.^20,23,36 Atomic level structure of SiO was interpreted by two different models.²⁰ A random mixture, comprising nanoparticles of Si dispersed in a silica matric, and a random bonding, where continuous random network comprising Si_4−xO_x units is postulated.²⁰ From the observations mentioned above, we think the product of OXM is closer to the random bonding model^23,37,38 rather than to the random mixture model.^20,39 It is reported that the former model is stabilized under high pressure,²³ which is actualized during milling as in the present case.

Correlation with vibrational spectroscopy

The FTIR spectra of the co-milled samples and commercial SiO and SiO₂ are shown in Fig. 4. The commercial SiO exhibits a shallow but unique absorption peak at around 590–600 cm⁻¹. This is an absorption band of Si, as reported in the literature, examined in the interests of silicon as a window material of IR spectroscopy.⁴⁰ The largest peak is Si–O–Si stretching band at around 1100 cm⁻¹ (ref. 41) shifted to a lower wave number by OXM. Note that the degree of the red-shift is different, depending on the POL species. The correlation between the decreases in the Si–O–Si vibrational energy and the O1s binding energy is demonstrated in Fig. 5, including the intact SiO₂ and commercial SiO. The observed simple correlation could be interpreted in terms of SiO₂ reduction, accompanied by the increase in the electron density around O²⁻ and consequent retardation of Si–O–Si stretching vibration. Despite the observed clear difference in the reduction mechanisms between thermal and mechanochemical routes, Si–O–Si bending bands at around 450 cm⁻¹ and 800 cm⁻¹ (ref. 41) are less sensitive to the sample preparation history. Note that we excluded the data with PVDF. The reason will be discussed below.

Fig. 4 FT-IR spectra of the mixtures with SiO₂:POL.

Fig. 5 Correlation between FT-IR absorption and O1s XPS peaks.

Comparison of reducing capabilities among 3 POLs

Among three OXM products, the peak shape in the broad range between around 90 ppm and 120 ppm are significantly different from those of SiO. From the literature, we may assign the peak at around −100 ppm to Q³ and −90 ppm to Q².^42–45 We also note that no significant signal appeared in this range from the commercial SiO.

As already discussed by referring Table 1, the states of the OXM products with PP and PE are similar. The percentage of Q⁴ is significantly larger with PVDF than PP and PE, indicating the smaller ability of PVDF for mechanochemical reduction. We observe no significant difference in the peak position from SiO₂ and SiO at around −108 to −110 ppm, unlike the case of XPS spectra, where the difference of the peak positions of both O1s and Si2p was more significant. This again suggest the difference in the states of Si after thermal and mechanochemical treatments. In the actual OXM process, C–H bonds in olefins are close neighbor to the OXM products and hence they are most probably polarized,⁴⁶ favoring the oxygen abstraction. Similar view was extensively discussed in the interests of catalytic oxidation of hydrocarbons,^47,48 although our experimental results were not extended to examine the extent of polarization.

We also point out that the co-milled sample with PVDF was excluded from Fig. 5, as its properties were widely apart from the other samples, as we already noticed from O1s XPS spectra (Fig. 2). This again must be attributed to the coexistence of fluorine. Researchers in the field of semiconductor etching report that fluorine is a powerful disrupter of silica-based compounds.^49–51 While the related process is conventionally practiced in liquid⁵² or vapor⁵³ phases, same principles could be applicable in the solid phase. Decrease in the O1s binding energy is closely correlated with decreasing interaction parameter of cation and anion of many metal oxides,⁵⁴ which, in turn, parallels to the polarizability.⁵⁵ Incorporation of the chemical species with high polarizability like fluorine reduces the interaction parameter and hence O1s binding energy, although more exact discussion on this point is yet to be done.

States of POL after milling

After co-milling SiO₂ with POL, we observed darkening of the sample color (ESI, Fig. S1†). The UV-vis diffuse reflectance spectra are shown in Fig. 6. We observed black dots in the co-milled products with PVDF or PP under scanning electron microscope (Fig. S2 and S3†). Since the micrographs alone cannot determine that the darkening is attributed to the formation of carbon particles, we examined these samples by Raman spectroscopy.

Fig. 6 UV-vis DRS of the co-milled products.

As shown in Fig. 7, the Raman spectra exhibit typical G and D peaks. The former is derived from “graphite”, observed at around 1550–1600 cm⁻¹, and represents the in-plane bond-stretching vibration of a pair of sp² carbon atoms in E_2g symmetry. The D peak, derived from “disorder”, at around 1350 cm⁻¹, is ascribed to the breathing vibration in A_1g symmetry.⁵⁶ This indicates that the carbon species, preferentially in a sp² state, were formed⁵⁷ by co-milling. This indicates that the oxidative decomposition of POL was proceeded up to the state of carbonization.

Fig. 7 Raman spectra of the mixtures with SiO₂:POL after milling for 3 h.

Changes in the crystallographic properties

The nominal solid compound, SiO, is understood to be metastable and amorphous,²² with a unique interfacial nanostructure.²⁰ Although there are some reports mentioning the crystalline SiO,¹⁰ the metastable, amorphous states were supported by computer simulation as well.²³ The XRD profiles shown in Fig. 8 exhibit only hallo pattern, except relatively sharp peaks of cubic ZrO₂, indicated by the arrows, from the milling balls and vessel. We note that the intensity of the ZrO₂ diffraction peaks are not identical but depends on POL specie, i.e. PP ≈ PE ≥ PVDF. Incorporation of zirconia into the milled product cannot simply be associated with simple mechanical abrasion, but because of mechanochemical reaction between POL and ZrO₂, as we have been suggested with various combination of POL and other oxides, TiO₂ (ref. 24 and 58) and SnO₂.²⁵ It is to be noted that the sequences of the degree of reduction and the apparent amount of ZrO₂ exhibit qualitative parallelism. This serves as another, independent evidence of the present mechanochemical reduction, to be the result of mechanochemical reaction between silica and POL. A small diffraction peak at around 28 degree two theta, on the curve with PVDF is ascribed to that of PVDF, a part of which is remained as a crystalline state. This is an indication of lesser amount of oxidative decomposition of PVDF as compared to PP and PE, in line with its lesser reducing ability as mentioned above.

Fig. 8 XRD of PP, PE and PVDF co-milled with SiO₂. Arrows denote c-ZrO₂ (JCPDS-27-0997).

Conclusions

The reduction route of SiO₂ toward silicon suboxides, SiO_x (1 ≤ x ≤ 2), by milling fumed silica nanoparticles with polyolefins (POLs), with and without fluorine (OXM process), is distinctly different from conventional thermal route. This was confirmed by Si2p XPS and ²⁹Si MAS NMR by using 3 POL species, i.e. poly(propene) (PP), poly(methylene) (PE) or poly(1,1-difluoroethylene) (PVDF) with the corresponding signals from commercial SiO as a reference of the thermally reduced product. We found a good correlation between O1s binding energy observed by XPS and Si–O–Si stretching vibration energy by FTIR, except PVDF, with which, O1s signal has been influenced via a different factor due to coexisting fluorine. The change in the coordination states detected by ²⁹Si NMR was significantly smaller by using PVDF than PP and PE. Since the present solid-state reduction does not involve the unstable gaseous SiO as an intermediate, the products are free from Q⁰ state close to that of metallic Si. From these results we conclude that the OXM products are closer to those defined as random bonding model of SiO, than a random mixture model, the former a thermal process could not achieve. The main reduction mechanism of OXM is the oxygen abstraction from the SiO₄ units by the polarized POL, with its simultaneous oxidative decomposition up to the state of carbon. The reaction process is simple and scalable so that it may offer a new affordable fabrication method of silicon suboxides.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support is acknowledged from the Adaptable and Seamless Technology Transfer Program (A-STEP) through target driven R&A from JST.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra07271j

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