Laser-Induced Conversion of Silica into Nanosized Carbon

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J. Phys. Chem. C 2007, 111, 16818-16826

Laser-Induced Conversion of Silica into Nanosized Carbon-Polyoxocarbosilane Composites Josef Pola,*,† Snejana Bakardjieva,‡ Miroslav Marysˇko,§ Vladimı´r Vorlı´cˇ ek,§ Jan Sˇ ubrt,‡ Zdene´ k Bastl,⊥ Anna Galı´kova´ ,† and Akihiko Ouchi*,| Laboratory of Laser Chemistry, Institute of Chemical Process Fundamentals, ASCR, 16502 Prague, Czech Republic, Institute of Inorganic Chemistry, ASCR, 25068 Rˇ ezˇ, Czech Republic, Institute of Physics, ASCR, 18223 Prague 8, Czech Republic, J. HeyroVsky´ Institute of Physical Chemistry, ASCR, 18223 Prague 8, Czech Republic, and National Institute of AdVanced Industrial Science and Technology, AIST, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: June 1, 2007; In Final Form: August 27, 2007

ArF and KrF laser-induced decomposition of gaseous toluene adjacent to silica leads to silica etching and chemical vapor deposition of fluffy, amorphous, and nanosized carbon-polyoxocarbosilane composites that were analyzed by Fourier tranform infrared, Raman, X-ray photoelectron, Auger and electron paramagnetic resonance spectra, and electron microscopy and were diagnosed for magnetic and thermal properties. It is shown that the composite composition, structure, and properties are remarkably affected by the laser source. The diamagnetic, high-surface area and Si/O-rich composite produced with ArF laser radiation is a blend of nanosized bodies of polyoxocarbosilane and carbon, merged into agglomerates and spheres. The very rare ferromagnetic, lower-surface area and Si/O-poor composite produced with KrF laser radiation is composed of less than 10 nm sized carbonaceous bodies. The formation of polyoxocarbosilane is judged to be a sequence of reactions of carbonaceous fragments with silica (carbothermal reduction) and with intermediary silicon monoxide. The reported laser-backside gas-phase etching has potential for fabrication of novel nanocomposite materials.

1. Introduction Carbon and silica are important entities in chemistry, physics, and material science and their composites become of growing interest due to their high electric conductivity, intercalating capability, high-thermal stability, and corrosion resistance as well as promising applications as low-cost solar absorbers1,2 and novel catalytic systems.3 Interesting classes of these composites are silica-rich/carbon-silica nanocomposites and diamondlikesilica nanocomposite coatings. The former were prepared by carbonization inside silica pores,1 using sol-gel synthesis of silica-carbon precursor composites,2,4 intercalation and hydrolysis of alkoxysilane in graphite oxide5,6 and similar mechanochemical approach,7 addition of carbon precursor to silica sol solution,8 and monomer polymerization in the course of solgel process.9 Other approaches are thermal decomposition of silicone grease10 and carbonization of phenylene/silica hybrid.11 The latter consist in networks of amorphous carbon and silica and were obtained as films from silicon-organic precursors in a plasmatron12 and by ion-beam assisted deposition of Si/SiO2 in ethyne.13 They represent thermally stable systems,14 wherein the two networks stabilize each other by weak chemical forces. The different form of coexistence of the three elements is manifested in inorganic silicon oxycarbide (e.g., see refs 1518) and in H-poor polyoxocarbosilanes (e.g., see refs 19,20). * Corresponding authors. (J.P.) Tel.: +420-2-20390308. Fax: +4202-20920661. E-mail: [email protected]. (A.O.) Tel: +81-29-861-4550. Fax: +81-29-861-4421. E-mail: [email protected]. † Institute of Chemical Process Fundamentals, ASCR. ‡ Institute of Inorganic Chemistry, ASCR. § Institute of Physics, ASCR. ⊥ J. Heyrovsky ´ Institute of Physical Chemistry, ASCR. | National Institute of Advanced Industrial Science and Technology, AIST.

Figure 1. Schematic of batch reactor (top view).

The former amorphousmaterial of the general formulas SiOxC4-x has a silica network structure in which O atoms are substituted by carbons. It shows high-thermal, chemical, and mechanical stability and finds many applications as, for example, catalyst support, lubricants, ceramic matrix composite and sealants, and coatings for high-temperature materials. The latter material is best described as silicon oxycarbide possessing low contents of C and Si elements bonded to H. Various Si/C/O (and Si/C/ O/H) phases and nanophases were produced by different procedures like pyrolysis of polysiloxanes,17,18,21 polysiloxanes gels,15,16,22-24 pyrolytic laser-organosilicon aerosol interaction,25 by plasma19,26 and by IR laser thermolysis,27,28 and UV laser photolysis20,29,30 of gaseous disiloxanes. Composites of silicon oxycarbide and carbon have not been particularly examined so far, although they are accessible by

10.1021/jp074243q CCC: $37.00 © 2007 American Chemical Society Published on Web 10/18/2007

Conversion of Silica to Carbon-Polyoxocarbosilane

J. Phys. Chem. C, Vol. 111, No. 45, 2007 16819

Figure 2. Depletion of toluene irradiated with ArF (0) and KrF (]) laser pulses as dependent on irradiation time.

Figure 5. FTIR spectra of ArF laser-obtained (a) and KrF laserobtained (c) deposits (in KBr pellets) and of these deposits heated to 600 °C (b,d, respectively).

Figure 3. Figure 3. GC/MS trace of volatile products in ArF laserinduced process. (Peak designation: 1, CO; 2, ethyne; 3, C3H4; 4, 1-buten-3-yne; 5, 1,3-butadiyne; 6, 3-penten-1-yne; 7, C5H4; 8, benzene; 9, hex-3-en-1,5-diyne; 10, C6H2; 11, toluene; 12, ethylbenzene; 13, ethynylbenzene.

Figure 4. SEM image of ultrafine deposit obtained by using KrF (a) and ArF (b) laser.

pyrolysis of phenylsiloxanes,18 polyphenylsilsesquioxanes,17 or sol-gel organosilicon polymers15,24 and represent a useful intermediate material to bulky silicon carbide. These materials are known only as bulk glassy blends of both constituents. Until now, no nanoscopic forms have been intentionally prepared and examined. In this paper, we introduce a new, one-step approach to fabrication of nanosized carbon/polyoxocarbosilane composites by employing laser-induced toluene decomposition near silica and show that structure and physical properties of chemicalvapor-deposited nanostructured carbon-polyoxocarbosilane composites are strongly affected by laser radiation source. 2. Experimental Section Laser irradiation experiments were carried out on gaseous toluene (16 Torr) in helium (total pressure 760 Torr) admitted to a reactor (140 mL in volume) having two orthogonally positioned tubes, one furnished with UV grade synthetic silica and the other with KBr windows. The reactor had two side-

arms, one fitted with a rubber septum and the other connecting to a standard vacuum manifold (Figure 1). Toluene samples were irradiated with an LPX 210i excimer laser (ArF and KrF radiation) operating with a repetition frequency 10 Hz. The laser pulses (full width at half maximum (fwhm) typically 23 ns, 230 mJ at 193 nm and 600 mJ at 248 nm) were focused to an incident area of 0.5 × 0.2 cm (fluences 2.3 and 6 J/cm2) and corresponded to 4.6 and 12 MW outputs. These fluences represent a critical threshold, as the described process does not take place at lower values. The progress of toluene decomposition was monitored directly in the reactor by Fourier transform infrared (FTIR) spectrometry (Shimadzu FTIR Prestige-21 spectrometer) using diagnostic absorption bands of toluene at 1610 cm-1. Volatile products were examined directly in the reactor by FTIR spectrometry. Aliquots of the irradiated reactor content were sampled by a gastight syringe (Dynatech Precision Sampling) and analyzed by gas chromatography-mass spectroscopy (GC/MS) (Shimadzu QP 5050 mass spectrometer (60 m capillary column Neutrabond-1, programmed temperature 30-200 °C)). The decomposition products were identified through their FTIR spectral diagnostic bands (C2H2, 731 cm-1; CH4, 1305 and 3016 cm-1; CO, 2175 and 2130 cm-1) and through mass spectra using the NIST library. The deposited ultrafine material was analyzed by X-ray photoelectron (XPS), Auger, FTIR, electron paramagnetic resonance (EPR) and Raman spectroscopy, electron microscopy, thermogravimetry, and for magnetic and thermal properties. The X-ray C 1s, and Si 2p photoelectron and C KLL Auger electron spectra of the deposit were measured using an ESCA 310 (Scienta) electron spectrometer with a base pressure better than 10-7 Pa and using Al KR radiation (1486.6 eV) for electron excitation. The surface composition of the deposited film was determined by correcting the spectral intensities for subshell photoionization cross sections.31 The Raman spectra were recorded on a Renishaw (Ramascope model 1000) Raman microscope with a charge-coupled device (CCD) using the exciting beam of an Ar ion laser (514.5 nm) 50 µW. The beam was attenuated to obtain incident energy densities in the range of 102-103 W/cm2. The FTIR spectra were obtained on powders in KBr pellets using a Nicolet Impact 400 spectrometer.

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TABLE 1: FTIR Spectra of Powders Laser-Deposited from Toluene absorbancea/wavelength (cm-1) KrF laser-obtained deposit

KrF laser-obtained deposit heated to 600 °C

ArF laser-obtained deposit

ArF laser-obtained deposit heated to 600 °C

0.04/470 0.34/793 1.0/1080 0.16/1260

0.22/468 0.47/793 1.0/1093 0.26/1260

0.22/470 0.32/794 1.0/1093 0.12/1260

ν(CdC)

0.12/468 0.14/780 1.0/1080 0.04/1260 0.17/1390 0.24/1450 0.48/1620

0.25/1620

0.05/1430 0.24/1625 0.28/1680

ν(CdO) ν(C-H) ν(C-H) ν(O-H)

0.28/1733 0.30/2920 0.50/2962 0.70/3480

0.10/1733 0.03/2920 0.02/2962 0.50/3400

vibration δ(OSiO) δ(H-CdC), ν(Si-C) ν(SiOSi) δ(HnC-Si) δ(H-CdCH-)

a

0.06/2919 0.05/2963 0.24/3480

0.03/2920 0.02/2962 0.10/3480

Normalized to absorbance of ν(SiOSi) band.

The scanning electron microscopy (SEM) analyses were carried out by using a Philips XL30 CP scanning electron microscope equipped with energy-dispersive analyzer (EDAX DX-4) of X-ray radiation. A PV 9760/77 detector in low-vacuum mode (0.5 mbar) was used for quantitative determination of Si, C, and O elements. The given stoiochiometries reflect an average of three different analyses and the deviation in atomic % for all elements lower than 4% of the value. The transmission electron micrographs were obtained using a JEOL JEM 3010 microscope (LaB6 cathode) operating at 300 kV and equipped with an energy dispersive system detector (INCA/Oxford) and CCD Gatan (Digital Micrograph software). The samples were prepared by grinding and subsequently dispersing the powder in ethanol and then applying a drop of the very dilute suspension on Ni grid. The suspensions were dried by slow evaporation at ambient temperature. The thermogravimetric analysis of the solid deposit (sample weight 0.5 mg) was conducted by heating the sample up to 600 °C at the rate of 4 °C/min, using Cahn D-200 recording microbalances in a stream of argon. The composition of outgoing gases was analyzed by a quadrupole mass spectrometer (VG GAS ANALYSIS LTD, Middlewich, England) enabling multiple (16 channel) ion monitoring as a time-dependent plot. The EPR spectra of the deposits were measured at room temperature in vacuum and in air by a cw EPR spectrometer working in X band (9.3 GHz and 1 mW) with 100 kHz magnetic field modulation (0.115 mT). The spectrometer was equipped with a digital frequency counter and an NMR magnetometer used for g-factor calculation. Quantitative estimation of spin concentration is based on Tempol and Mn2+ (internal standard) The magnetic measurements were performed in the temperature region 5-350 K using a SQUID magnetometer MPMS5S (Quantum Design). Toluene (Wako, purity 99.7%) was distilled prior to use. 3. Results and Discussion 3.1. Toluene Decomposition. The ArF and KrF laser irradiation of gaseous toluene with pulses focused several mm behind the entrance silica window leads to visible luminescence, depletion of toluene, and efficient formation of dark fog (very tiny soot) that flows in the reactor and slowly deposits to the reactor surface as ultrafine powder. During the irradiation, depletion of the silica window at the side exposed to toluene vapors takes place from ca. 0.2 cm2 area through which laser pulses penetrate to the reactor and is a base of ca. 1 cm long luminescence zone. Although being etched, the silica window remains clean, allowing quite long irradiations at both 193 and

248 nm (Figure 2). Considerably high progress in toluene decomposition achieved at 248 nm allowed the collection of ca. 8 mg of black ultrafine particles in one run with 248 nm irradiation and 2.4 mg of grayish ultrafine particles in one run with 193 nm irradiation. The MW laser irradiations are consistent with multiple photon dissociations and ionization of toluene possibly leading to plasma formation. The UV spectrum of toluene32 shows incomparably more intense absorption at 193 nm than at 248 nm and the observed more efficient depletion by 248 nm photons (Figure 2) is in line with more efficient absorption of these photons in hot toluene, primarily leading to benzyl radical.33 The final volatile products are carbon monoxide, methane (observed only by FTIR spectra), and hydrocarbons poor in H, which are the same in both irradiations. Typical GC/MS trace is illustrated for the sake of brevity only for the ArF laserinduced process (Figure 3), which yields higher amounts of CO. The highly unsaturated hydrocarbons [ethyne, propadiene, propyne, 1-buten-3-yne, 1,3-butadiyne, 3-penten-1-yne, C5H4 (pentadiynes, 1,2-pentadiene-3-yne, or 1,2,3,4-pentatetraene (m/ z, intensity in %), 64, 100; 63, 64; 62, 32; 61, 28; 38, 20), benzene, hex-3-en-1,5-diyne (m/z, intensity in %), 76, 93; 75, 10; 74, 19; 73, 10; 64, 35; 65, 36; 62, 12; 61, 13; 50, 100; 38, 23; 37, 25; 28, 60), 1,3,5-hexatriyne (m/z, intensity in %, 74, 100; 73, 30; 37, 26), ethylbenzene, and ethynylbenzene] are formed only in minor amounts (less than few percent of depleted toluene, which confirms that the decomposed toluene is mainly consumed for the formation of the dark soot). The absence of methane and ethane (formed in toluene lamp photolysis34) and the presence of highly unsaturated compounds are in keeping with important dehydrogenation steps and formation of H2. We assume that cleavage of primary benzyl and phenyl radicals gives rise to ethyne and ethynyl radical and is ensued by isomerization of ethyne to ethylidene,35 recombination, Habstraction and attack onto the π-electron density of the intermediates,36 dissociation of ethynyl radical into C2 + H,37 and by agglomeration of C2 and highly unsaturated species into the observed soot. The presence of ethynylbenzene suggests this molecule as a source of cyclic carbonaceous structures. The observed silica depletion (etching) is ascribed to interaction of hot carbonaceous products with silica surface; the formation of CO is in keeping with carbothermal reduction of silica. Independent high-fluence irradiations of the reactor filled with only atmospheric pressure of He do not lead to any silica spallation. We note that similar silica etching has not been yet observed in the gas phase but was reported in the liquid phase (and referred to as laser backside wet etching, e.g., see refs



Figure 7. Si2p core level spectra of KrF laser-obtained (a) and ArF laser-obtained (b) deposits.

Figure 6. Raman spectra of KrF laser-obtained deposit (a), ArF laserobtained deposit (b), and deconvoluted KrF laser-obtained deposit (c).

38-40) and for the adsorbed toluene layer(s) (and referred to as laser etching at the surface-adsorbed layer41). In the latter process,41 etching is claimed to occur at the interface of the adsorbed film due to a high-toluene pressure. In contrast, under our conditions a real gas-phase surface interaction is dominant as a result of the low-toluene pressure and therefore a deeper penetration of the laser beam into the gaseous toluene, manifesting itself as the sizable visible zone in the gas phase. 3.2. Properties of Deposited Materials. Scanning Electron Microscopy. Both black and grayish ultrafine powders are difficult to handle as they flow in air and keep an electrostatic charge. The SEM images of the powders show fluffy and spongy morphology (Figure 4) and the SEM-energy dispersive X-ray (EDX)-derived stoichiometries, Si1.00O1.30C1.10 (ArF laser irradiation) and Si1.00O2.10C20.0 (KrF laser irradiation), prove more efficient etching of silica, that is, more incorporation of Si and O elements in the deposited soot when using the 193 nm radiation more absorbed in toluene. FTIR Spectra. The IR spectra of both deposits (Figure 5) show similar pattern of absorption bands. The spectra assigned42 in Table 1 are dominated by a broad ν(SiOSi) band at 1000-1200 cm-1 and involve bands due to δ(OSiO) and ν(Si-C), δ(C-H), ν(C-H), ν(CdC) and ν(O-H) vibrations. They indicate the absence of SiO2, whose diagnostic bands are located43 at 510 cm-1 (δ(OSiO)) and 1118 cm-1 (νasym(SiOSi)). Although the 1118 cm-1 band may be overlapped by a broad v(SiOSi) band at 1000-1200 cm-1, the 510 cm-1 band is missing. The presence of the CdO band is indicative of some oxidation at carbon. The relatively strong ν(O-H) band is associated with Si-OH...O bridges and indicates Si-OH bonds. Relative absorbances of the CdC, C-H, and Si-C moieties (Table 1) show that the C-rich KrF laser-obtained deposit contains more CdC and C-H bonds and that the Si/O-rich ArF laser-obtained deposit has more Si-C bonds. The presence of Si-CHn moieties indicates that silica was transformed into oxocarbosilane structures. Raman Spectra. The Raman spectra of both deposits (Figure 6) do not show bands44 of high purity fused silica at 495 and 1200 cm-1. The spectrum of the KrF laser-obtained deposit shows the presence of broad bands at 1352 cm-1 (D-band) and 1594 cm-1 (G-band) of unsaturated sp2 carbon with intensity ratio ID/IG ) 0.68. The G-band reflects bond stretches of all

Figure 8. C KLL X-ray-excited derivative Auger spectra of natural diamond (a), graphite (b), glassy carbon (c), ArF laser-obtained deposit (d), and KrF laser-obtained deposit (e) .

pairs of sp2 atoms in rings and chains, and the D-band corresponds to breathing modes of rings (e.g., see refs 45,46). The deconvolution of the G-band to two contributions peaked at 1540 and 1600 cm-1 is indicative of two differently hydrogenated skeletons and the ID/IG value is consistent with a-C/H films.47 The Raman spectrum of the ArF laser-obtained deposit lacks both D and G bands and is therefore in line with a low content of the CdC bonds (as inferred from the FTIR spectrum). Both deposits are highly luminescent, which indicates the presence of a polymer-like carbon. X-ray Photoelectron Spectra. The XPS analysis-derived stoichiometry of few superficial layers in the ArF laser-obtained deposit, C1.00O1.62Si1.18, and the KrF laser-obtained deposit, C1.00O0.18Si0.06, indicate some oxidation of topmost silicon in air. The Si (2p) spectra of both samples are identical and can be decomposed in two components located at 100.8 and 102.4 ( 0.2 eV having respective populations 35 and 65% (Figure 7). The less intense component is assigned48 to Si/C and the more intense to Si/C/O arrangement. The presence of Si/C in the Si-rich sample is further supported by the presence of a component at 283 eV in the spectra of C 1s electrons. An estimation of the sp3/sp2 hybridization ratio49,50 is performed by using the energy difference between the most positive maximum and most negative minimum of the C KLL derivative spectra. The comparison of the C KLL spectra of both deposits with those of reference carbon samples is given in Figure 8. The separations obtained for diamond, graphite, and glassy carbon (13.1, 21.9, and 16.8 eV, respectively) agree well with literature,49-51 and those for the ArF laser-obtained deposit (13.9 eV) and KrF laser-obtained deposit (18.6 eV), respectively, correspond to about 10 and 75% of sp2 hybridized C atoms.


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Figure 10. (a) Hysteresis loops measured at temperatures T ) 5, 15, and 300 K. A diamagnetic background χ ) -4.13 × 10-6 emu/(g Oe-1) was subtracted from the original data. (b) Temperature dependence of the coercivity. Figure 9. EPR spectrum of KrF laser-obtained deposit in vacuum and air.

TABLE 2: EPR Spectra of the Deposits deposit

g-factor

ca, spin/g

dYvac/dYairb

KrF laser-obtained deposit ArF laser-obtained deposit heated KrF laser-obtained deposit heated ArF laser-obtained deposit

2.0022 2.0020 2.0027 2.0027

8.75 × 1017 6.82 × 1016 9.47 × 1016 5.35 × 1016

1.79 2.22 5.87 3.11

a Concentration of paramagnetic centers. bdY ) (A , peak-to-peak pp amplitude).

EPR Spectra. The EPR spectra of the deposits (Table 2) show single lines without hyperfine structure and very similar g-factors. The absence of a resolved hyperfine interaction is consistent with the delocalization of unpaired electron in an aromatic π-system52 or with an unpaired electron localized in the orbital of an atom having zero nuclear spin, which is part of a three-dimensional network preventing electron transfer and recombination. These two alternatives cannot be discerned as g-values for graphitic carbon are anisotropic and amount to53 2.0050-2.0026, and those for silicon depend on the number of the neighboring oxygen atoms and range54 between 2.0008 and 2.0056. Both deposits change their EPR spectra when exposed to air (Table 2). This behavior is illustrated in Figure 9. The reversible decrease and broadening of the line in the EPR spectra in air compared to those in vacuum is typical for some charcoals possessing C-centered radicals.55,56 The higher concentration of paramagnetic species and higher response to air observed for the C-rich deposit is in keeping with the presence of an unpaired electron in the delocalized aromatic system. Heating of both samples to 600 °C decreases the signal intensity and increases the response to air; these changes are more pronounced with the C-rich sample, and we assume that the Si/O-rich sample contains both an unpaired electron in the delocalized aromatic system and thermally stable54,57 (SixO3-x)Si· radical centers. Magnetic Properties. The magnetic measurements revealed that the ArF laser-obtained (Si/O-rich) deposit has a very small magnetic moment given by the intrinsic diamagnetism of the carbon, whereas the KrF laser-obtained (C-rich) deposit possesses a relatively strong ferromagnetic moment. The presence of a ferromagnetic state is clearly indicated by the hysteresis loops (Figure 10a) with a finite coercivity (Figure 10b) and the remnant magnetization (mrem). Information on the temperature dependence of the saturation magnetization was obtained by measuring the magnetization m at the field H ) 8 kOe (Figure 11a,b). This measurement was performed during cooling of the sample from T ) 350 K to T ) 5 K. The remnant magnetization

Figure 11. (a) Magnetization measured under the applied magnetic field 8 kOe after the subtraction of the diamagnetic background. (b) Magnetization on a larger scale. The contribution of the classical ferromagnetism is denoted by the solid line. (c) Temperature dependence of the remnant magnetization.

(Figure 11c) was obtained during warming of the sample after zero-field cooling to T ) 5 K and subsequently setting H ) 9 kOe and H ) 0. (Let us remark that for SQUID magnetometer the field H is not exactly zero but satisfies the condition |H| < 2 Oe.) The temperature dependence m(T) shown in Figure 11a can be well approximated by the function

m(T) ) m(0)(1 - aT1.5 - bT2.5 - cT3.5) + d/(T - T1) where m(0) ) 0.075 emu/g, a ) 1.8844 × 10-5 K2/3, b ) -5.5481 × 10-8 K2/5, c ) 1.1402 × 10-10 K2/7, d ) 0.0684 emu/(g K-1) and T1 ) 0.6922 K. The first prevailing term proportional to the saturation magnetization m(0) corresponds to the classical ferromagnetism (the solid line in Figure 11b) with the parameters a, b, and c describing the decrease of the magnetization with increasing temperature. From the values a, b, and c, we estimate the Curie temperature to be about 700 K. The second term represents a Curie-Weiss like contribution. An increase of the magnetization in the low-temperature region was reported for the C60 crystals58 where the origin was proved to be in the presence of the paramagnetic molecular oxygen. In our case, the measurement of the dependences m(T) for different applied fields has shown that the constant d is not strictly proportional to H but remains finite after extrapolating H f 0. A small low temperature increase of mrem can be seen in the temperature dependence of the remnant magnetization (Figure 11c). The same feature seems to be present in the results obtained for the a-C/H materials.59 Such a behavior could be understood, if paramagnetic centers (e.g., the O2 molecules) were exposed to an effective magnetic field produced by the carbon



Figure 12. HRTEM images and electron difraction of KrF laserobtained deposit.

system. This hypothesis about the coupling between the ferromagnetic and a paramagnetic system seems to be supported by an expressive increase of the coercivity when going to the lowest temperatures (Figure 10b). In discussing the observed ferromagnetism, we note that the elemental analysis by atomic absorption spectroscopy shows Fe impurity less than 20 ppm, which could give the magnetization 0.004emu/g60 and that represents roughly 4% of the observed value. We can conclude therefore that the observed ferromagnetic moment is with the highest probability given by the intrinsic ferromagnetism of the deposit. The magnetism of carbon has been receiving much attention and has been ascribed to electronic instabilities (bonding defects).61 Ferromagnetism was found61,62 in nanographite, fullerenes, highly oriented pyrolytic graphite,60 laser ablatively deposited nanostructured carbon foams,63 and also in hydrogen-containing carbonaceous films (CVD-prepared a-C/H films59 and plasma-enhanced CVDprepared diamondlike carbon films64). From the point of view of the comparison with our results, we notice especially the ferromagnetism of the a-C/H films.63 For these materials, a relatively large saturation magnetization (2.56 emu/g) was achieved by optimizing the ratio H/C in the starting materials. It is worth mentioning that the magnetic properties of the produced films are at least in two aspects similar to those obtained on our C-rich material. The same ratio mrem(0)/ mrem(300) ≈ 1.23 suggests the nearness of the Curie temperatures, and a large increase of the remnant magnetization (H ) 50 Oe) at T < 20 K corresponds to the dependence mrem(T) measured for the C-rich deposit. This supports the hypothesis about the interaction between the carbon and a paramagnetic system. Both the materials have however different coercivities, which is reflected by the ratio mrem/m(0). For the a-C/H film, this quantity is about five times larger than in our case. We note that the presence of the ferromagnetism just for the KrF laser-obtained (C-rich) deposit can be in accord with higher densities of the unpaired electron spins measured by the EPR spectroscopy method (Table 2) and with different ratio of sp2 and sp3 carbons in the C-rich and Si/O-rich samples. One of the theoretical models explaining the existence of a ferromagnetic state in carbon compounds is based on the occurrence of a mixture of the sp2- and sp3-hybridized carbon atoms.60 The less different population of sp3 and sp2 centers observed for the KrF laser-obtained (C-rich) deposit is more favorable for the achievement of a ferromagnetic ordering, because the onset of ferromagnetism in this model requires similar population of the sp3 and sp2 centers. Transmission Electron Microscopy. The HRTEM images of both deposits obtained from both laser irradiations (Figures 12 and 13) do not reveal any crystalline features and confirm fully amorphous states. The KrF laser-obtained deposit consists from fluffy aggregates composed of less than 10 nm sized bodies, whereas the ArF laser-obtained deposit has a heterogeneous structure and is composed of ca. 20 nm and larger bodies merged into irregular agglomerates and ca. 30-200 nm sized spheres.

Figure 13. HRTEM images and electron diffraction of ArF laserobtained deposit.

Figure 14. Thermogravimetric analysis of ArF laser-obtained (a) and KrF laser-obtained (b) deposits.

The selected-area electron diffraction (SAED) analyses of the spheres (Si1.00O1.73C0.30) and of the outside regions (Si1.00O1.63C2.04) reveal great differences in composition: the spheres being rich in silicon and oxygen and very poor in carbon represent bodies of the oxocarbosilane with low content of carbon (∼SiO3C), whereas the outside region possessing the same Si/O ratio and more carbon appears as a blend of the oxocarbosilane and elemental carbon. Physical Adsorption Measurements. The Brunauer-EmmettTeller (BET) surface area and average pore diameters of the ArF laser-obtained (376 m2/g, 17.1 nm) and KrF laser-obtained (195 m2/g, 11.5 nm) powder indicate that the Si/O-rich material has a higher surface area than the C-rich material. This is an interesting observation, since Si/C/O/H structures have normally noticeably lower surface area than carbonaceous materials. We note that the surface area of the Si/O-rich material is considerably higher than that of oxycarbide prepared by pyrolysis of siloxanes65 but comparable to those for polyoxocarbosilane powders obtained by MW UV laser photolysis of disiloxanes20 or by pyrolysis at 1000 °C of alkyl-substituted silica aerogels.16 Thermal Stability. The ArF laser-obtained and KrF laserobtained deposits heated to 600 °C decrease their weight by 13 and 22%, respectively (Figure 14). The Si,O-rich material experiences a weight drop mostly at 400 °C, resembling thermal behavior of highly cross-linked polysiloxanes (e.g., see refs 21,66), whereas the C-rich material decreases its weight monotonously. Major diagnostic single-ion traces of the gaseous products at m/z 2 (H2), 16 (CH4), 29 and 32 (CH3OH), and 28 (CO) are observed with both samples but have different progress (Figure 15). The higher and broader curve for H2 evolution for the C-rich sample reflects more significant dehydrogenation from carbonaceous skeletons than from H-C(Si) bonds. The


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Figure 17. HRTEM images of heated ArF laser-obtained deposit.

SCHEME 1: Plausible Steps of Polyoxocarbosilane Formation Figure 15. Diagnostic single-ion traces obtained upon heating of ArF laser-obtained (a) and KrF laser-obtained (b) deposits.

Figure 16. HRTEM images of heated KrF laser-obtained deposit.

earlier evolution of methane from the C-rich sample is in keeping with a more feasible formation of methane through C-C than C-Si homolysis. Carbon monoxide and methanol are practically produced only from the Si/O-rich sample, and their formation taking place within 400-580 °C is delayed (similarly as that of methane) with respect to the maximum depletion at 400 °C. This indicates that the carbothermal reduction of Si-O bonds (and evolution of methanol) partly occurs from a portion of the sample evaporated and condensed below the hot zone and not passing to mass spectrometer. FTIR spectra of the residues (Figure 5) show smaller fraction of O-H, C-H, and CdC (also CdO) bonds, as well as different shapes of the ν(Si-O) bands. These changes are indicative of C-H bonds dehydrogenation, (Si)OH-group condensation, and CdC bond polymerization. The changes in absorbance at 780 and 1260 cm-1 (Table 1) reflect scrambling of Si-C and Si-O bonds earlier18,67,68suggested to occur in the solid phase in the course of siloxane pyrolysis. The heated residues differ from the deposited samples also in HRTEM images. The heated C-rich material (Figure 16) shows three different regions A, B, and C that we respectively ascribe to amorphous (A) and concentric-like69,70 (B) structures of carbon and nanocrystalline bodies of SiO2 embedded in the amorphous phase (C). The interlayed distance of 0.300 nm observed in high-resolution transmission electron microscopy (HRTEM) patterns is related71 to lattice planes of carbon silicate (C-SiO2 PDF 5-0469).

The heated Si/O-rich deposit remains amorphous and retains spherical features (Figure 17). The SAED analyses of the spheres and of the outside regions do not indicate changes in elemental composition. Few HRTEM images confirm very rare development of crystalline regions of SiO2 (interlayed distance of 0.381 nm denoting to orthorhombic SiO2 crystal lattice plane with Miller indexes (141) PDF 1-0378).71 The FTIR spectra and HRTEM images of the heated samples thus imply that the “additional” heat results in the formation of crystalline SiO2 nanodomains, makes the deposits poorer in H, changes Si(OH) units into SiOSi units (2 ≡SiOH f (≡Si)2O + H2O) and modifies SiOxC4-x arrangements through carbothermal reduction and Si-C/Si-O scrambling. The formation of crystalline silica domains can be ascribed to crystallization enhanced by dissolved water molecules72,73 and/or redistribution of SiOxC4-x arrangements. Pure amorphous silica crystallizes in air only at temperatures above 1400-1500 °C, but crystallization of amorphous silica in magnetite/silica nanocomposite was recently observed74 at 600 °C. 3.3. Possible Mechanism of Polyoxocarbosilane Formation. The identification of volatile products, the presence of CO among them and the recognition of the solid deposit as carbonpolyoxocarbosilane nanocomposites enables us to draw Scheme 1 for carbon-polyoxocarbosilane nanocomposites formation. We suggest that toluene decomposition resulting in the formation of unsaturated hydrocarbons and elemental carbon involves transient occurrence of reactive (electronically excited, hot, or ionic) Cn (n ) 2, 3) species (e.g., see refs 75,76), and that these species and their aggregates, together with Hcontaining carbonaceous aggregates, react with silica in a way similar to the initial step of carbothermal reduction (SiO2 + C(s) f SiO (g) + CO (g), ref 77). We note that pyrolytic soot formation from toluene requires high temperatures (>1700 °C, ref 78) above which soot is produced along with highly unsaturated hydrocarbons. Such temperatures (Boltzmann distribution) can be achieved in the irradiated system (under the given pressure) within a few ns. The initially formed SiO can therefore undergo a variety of gas-phase reactions with both elemental carbon and unsaturated hydrocarbons. As a true gasphase compound (but not as an Si(SinO4-n) (n ) 0-4) solidstate compound, e.g., see refs 79-81), SiO (a silylene-like species82) can insert into the C-H bonds and add to unsaturated CdC and CtC bonds to form Si-H and Si-CHx bonds. We note that IR laser ablation of silicon monoxide in hydrogen83 and hydrocarbons84 respectively leads to the formation of Si-H bonds-containing SiOx material and Si-CHx bonds-containing

Conversion of Silica to Carbon-Polyoxocarbosilane polyxocarbosilanes. This indicates that SiO reacts under our conditions with carbonaceous fragments and not with H2. The effective temperatures being possibly above 1000 °C makes us think that the polyoxocarbosilane (Si/O/C/H) structures, a blend of SiCnO4-n arrangements, are produced by direct reaction of SiO with carbonaceous fragments and by additional Si-C/Si-O scrambling occurring67,68,85 above 600 °C. 4. Conclusions High-fluence excimer laser irradiation of gaseous toluene in silica window-furnished reactor affords decomposition of toluene adjacent to the silica window, etching of silica, and deposition of fluffy nanosized composites of carbon and polyoxocarbosilane, whose composition, structure, and physical properties depend on the laser irradiation parameters. The process is the first approach to nanosized carbon/ polyoxocarbosilane composite materials. The irradiation by the ArF laser results in significant silica etching and deposition of diamagnetic, high surface area and Si/O-rich composite with carbon prevailing in the sp3 state. This material is a blend of nanosized bodies of polyoxocarbosilane and carbon merged into agglomerates and spheres. The irradiation by the KrF laser allows minor silica etching and deposition of very rare ferromagnetic, lower surface area and Si/O-poor carbonaceous composite that possesses C atoms mostly in sp2 state and is composed of agglomerated less than 10 nm sized carbonaceous bodies. The ferromagnetism of this composite represents a very rare example of metal-free carbon materials.86 Both deposited powders possess rather high concentration of unpaired electrons, and their EPR spectra are sensitive to air. The formation of polyoxocarbosilane is judged to take place via carbothermal reduction of silica by carbonaceous fragments and agglomerates yielding silicon monoxide and via ensuing reactions of carbonaceous fragments and agglomerates with silicon monoxide. Heating of the KrF laser-obtained material to 600 °C leads to partial graphitization, more Si-C bonds and formation of crystalline SiO2 nanodomains. The formation of these nanodomains was also observed upon similar heating of the ArF laser-obtained material. Both crystallizations take place at temperatures by several hundreds of degrees Celsius lower than that of bulky amorphous silica and are judged to be enhanced by water molecules produced during the heating process. The reported laser process shows that the laser backside etching in the gas phase can be important for fabrication of novel nanosized composites of carbon and other materials. Acknowledgment. This work was supported by GA ASCR Grant 400720619. The authors thank Dr. P. Stopka for measurement of EPR spectra and Dr. O. Sˇ olcova´ for surface area measurements. References and Notes (1) Mastai, Y.; Polarz, S.; Antonietti, M. AdV. Funct. Mater. 2002, 12, 197. (2) Katumba, G.; Lu, J.; Olumekor, L.; Westin, G.; Wackelgard, E. J. Sol-Gel Sci. Technol. 2005, 36, 33. (3) Kawashima, D.; Aihara, T.; Kobayashi, Y.; Koytani, T.; Tomita, A. Chem. Mater. 2000, 12, 3397. (4) Anderson, M. L.; Stroud, R. M.; Rolison, D. R. Nano Lett. 2002, 2, 235. (5) Wang, Z. M.; Hoshino, K.; Shishibori, K.; Kanoh, H.; Ooi, K. Chem. Mater. 2003, 15, 2926. (6) Wang, Z. M.; Shishibori, K.; Hoshinoo, K.; Kanoh, H.; Hirotsu, T. Carbon 2006, 44, 2779.

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Laser-Induced Conversion of Silica into Nanosized Carbon

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