Megawatt Ultraviolet Laser Photolysis of Dichloroethenes for Gas

Sep 15, 2010 - E-mail: [email protected] (J.P.), [email protected] (A.O.)., † ... FTIR, Raman, X-ray photoelectron and Auger spectroscopy, and...
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J. Phys. Chem. C 2010, 114, 16153–16159

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Megawatt Ultraviolet Laser Photolysis of Dichloroethenes for Gas-Phase Deposition of Nanosized Chlorinated Soot Josef Pola,*,† Anna Galı´kova´,† Snejana Bakardjieva,‡ Jan Sˇubrt,‡ Zdeneˇk Bastl,§ Vladimı´r Vorlı´cˇek,| Miroslav Marysˇko,| and Akihiko Ouchi*,⊥ Laboratory of Laser Chemistry, Institute of Chemical Process Fundamentals, ASCR, 16502 Prague, Czech Republic, Institute of Inorganic Chemistry, ASCR, 25068 Rˇezˇ, Czech Republic, J. HeyroVsky´ Institute of Physical Chemistry, ASCR, 18223 Prague, Czech Republic, Institute of Physics, ASCR, 18221 Prague, Czech Republic, and National Institute of AdVanced Industrial Science and Technology, AIST Tsukuba, Ibaraki 305-8565 ReceiVed: March 23, 2010; ReVised Manuscript ReceiVed: September 2, 2010

Highly intense ArF and KrF laser radiation-induced photolysis of gaseous dichloroethenes allows chemical vapor deposition of ultrafine amorphous Cl-substituted hydrogenated carbon soot, which was characterized by FTIR, Raman, X-ray photoelectron and Auger spectroscopy, and electron microscopy, and diagnosed for magnetic and thermal properties. The deposited submicroscopic material was shown to contain C(sp3)-H, C-Cl, C-O, and CdO bonds and upon heating to 700 °C to evolve H2, HCl, Cl2, and C/H fragments and transform to graphite-like carbon. The deposited soot is diamagnetic, has a large surface area, and has a potential for synthesis of soot modified at the C-Cl bonds by other substituents. 1. Introduction IR and UV laser-induced chemical vapor deposition of carbonaceous materials from various hydrocarbons has become a well-established field of research. These low-pressure processes occur strictly in the gas phase, obviate hot-wall effect in carbonization taking place in conventional pyrolysis, and can lead to specific forms of carbon materials that are difficult to obtain in surface-assisted hot-wall decomposition of hydrocarbons. The IR laser-induced (thermal) processes take place as a pulsed TEA CO2 laser induced infrared multiple-photon decomposition or a continuous-wave-CO2 laser-induced decomposition in flame, and they allow formation of graphitic hydrogenated films,1 amorphous carbon and spherical diamond particles,2 graphitic carbon nanopowders,3 shell-shaped carbon particles,4 carbon clusters and soot particles,5 and highly aromatic hydrogenated or turbostratic concentric nanoparticles,6 to name just a few. The UV laser-induced photolytic decompositions lead to hydrogenated carbon films,7 graphite-like and highly C(sp3)-based films,8,9 or H-rich and unsaturated C films.10 We have recently reported a new approach to chemical vapor deposition of nanosized carbon materials that incorporate poly(oxocarbosilane)11,12 or CN-bonds containing poly(oxocarbosilane)-containing crystalline nanoregions of rare chaoite13 and found that the materials composition, structure, and properties (surface area, magnetism) were remarkably affected by laser source. The procedure consists of high-fluence excimer laser irradiation of gaseous hydrocarbons (butadiyne,11 toluene12) or pyridine13 in silica window-furnished reactor, which affords decomposition of hydrocarbon adjacent to the silica window, etching of silica, and deposition of the nanosized composites. * Corresponding author. Tel.: +420 2 20390308 (J.P.), +81-29-8614550 (A.O.). Fax: +420 2 20920661 (J.P.), +81-29-861-4421 (A.O.). E-mail: [email protected] (J.P.), [email protected] (A.O.). † Institute of Chemical Process Fundamentals, ASCR. ‡ Institute of Inorganic Chemistry, ASCR. § J. Heyrovsky´ Institute of Physical Chemistry, ASCR. | Institute of Physics, ASCR. ⊥ National Institute of Advanced Industrial Science and Technology.

This process involves the laser backside etching in the gas phase through reduction of silica by carbonaceous fragments and can be important for deposition of novel nanosized composites of carbon and other materials. It was of our further interest to investigate UV laser-induced formation of carbon materials from volatile chlorinated alkenes. These compounds are considered to belong among hazardous materials, and their carbonization to cokes14 as well as mechanistic studies on their thermal decomposition15 have attracted only limited attention. The dichloroethenes are well-known to be UV photolyzed via a number of pathways (cleavage of HCl, Cl, and H216-23) and to allow formation of C/H/Cl fragments whose recombination in the gas phase may result in deposition of carbon nanoparticles with Cl-C bonds. Such products of UV laser-induced carbonization of chloroethenes have not been yet studied, and we report on ArF and KrF laser photolysis of gaseous cis-, trans-, and gem-dichloroethenes and elucidate the structure of the gas-phase deposited products. 2. Experimental Section Laser irradiation experiments were carried out on gaseous cis-, trans-, and gem-dichloroethene (150 Torr) in argon (total pressure 760 Torr) admitted to a reactor described previously.12 Briefly, the reactor (140 mL in volume) had two orthogonally positioned tubes: one furnished with UV-grade synthetic silica and the other with KBr windows. The reactor possessed two side arms: one fitted with a rubber septum and the other connecting to a standard vacuum manifold. The dichloroethene samples were irradiated with an LPX 210i excimer (ArF and KrF) laser operating with a repetition frequency of 10 Hz. The laser pulses at 193 nm (280 mJ) and 248 nm (400 mJ) were, respectively, focused to an incident area of 0.50 and 0.36 cm2. (The respective incident fluences 560 and 1100 mJ/cm2 correspond to MW (megawatt) outputs and represent critical threshold, as the described process (sooting) does not occur at lower values.) The progress of dichlorethene photolysis was monitored directly in the reactor by FTIR (Fourier transform infrared)

10.1021/jp102600k  2010 American Chemical Society Published on Web 09/15/2010

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spectroscopy (a Shimadzu FTIR IR Prestige-21 spectrometer) using diagnostic absorption bands at 580 cm-1 (cis-dichloroethene), 1660 cm-1 (trans-dichloroethene), and 1140 cm-1 (gemdichloroethene). Aliquots of the irradiated reactor content were sampled by a gastight syringe (Dynatech Precision Sampling) and analyzed by gas-chromatography-mass spectroscopy (a Shimadzu QP 5050 mass spectrometer, 60 m capillary column Neutrabond-1, programmed temperature 30-200 °C). The decomposition products were identified through their FTIR diagnostic bands (HCl, 2880 cm-1, ethyne, 730 cm-1) and through mass spectra using the NIST library. The samples of the deposited ultrafine soot were analyzed by X-ray photoelectron (XPS), Auger, FTIR, and Raman spectroscopies, and electron microscopy and were also diagnosed for magnetic, physical absorption, and thermal properties and examined for reactivity toward gaseous ammonia. The C 1s, N 1s, Cl 2p, and O 1s photoelectron and C KLL Auger electron spectra of the deposit were measured by using an ESCA 310 (Scienta) electron spectrometer with a base pressure better than 10-9 Torr and 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. The Raman spectra were recorded on a Renishaw (a Ramascope model 1000) Raman microscope with a CCD detector using the exciting beam of an Ar-ion laser (514.5 nm, 50 mW). The beam was attenuated to obtain incident energy densities lower than 5 × 10-3 W/cm2. The FTIR spectra were obtained on thin layers of the deposited soot accommodated between KBr plates using a Nicolet Impact 400 spectrometer. The SEM (scanning electron microscopy) analyses were conducted using a Philips XL 30 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 Torr) was used for quantitative determination of C, O, N, and Cl elements. The transmission electron micrographs were obtained using a JEOL JEM 3010 microscope (LaB6 cathode) operating at 300 kV and equipped with EDS detector (INCA/Oxford) and CCD Gatan (Digital Micrograph software). The samples were prepared by grinding and subsequently dispersing the powder in ethanol and applying a drop of very dilute suspension on Ni grid. The suspension was dried by slow evaporation at room temperature. The thermogravimetric analysis of the solid deposit (sample weight 0.5 mg) was carried out by heating the sample to 700 °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) enabling multiple (16 channel) ion monitoring as a time-dependent plot. The reactivity of the solid deposit obtained from cisdichloroethene (10 mg) toward gaseous ammonia (50 Torr) was examined by heating the deposit at 100 °C for 2 h in a Pyrex tube (250 mL) connected to a vacuum line. The magnetic measurements were performed in the temperature region 5-350 K using a SQUID (Superconducting Quantum Interference Device) magnetometer MPMS-5S. cis-1,2-Dichloroethene and 1,1-dichloroethene (Tokyo Chemical Industry, stabilized with MEHQ, purity better than 99%) and trans-1,2-dichloroethene (Wako Pure Chemical Industries, purity 95%) were distilled prior to use.

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Figure 1. GC/MS trace of volatile products in ArF laser photolysis of cis-1,2-dichloroethene (a) and gem-dichloroethene (b) and in KrF laser photolysis of cis-1,2-dichloroethene (c). (Peak designation: 1, Ar; 2, HCl; 3, chloroethyne; 4, 1,1-dichloroethene; 5, trans-1,2-dichloroethene; 6, cis-1,2-dichloroethene; 7, trichloroethene; 8, trichloromethane; 9, 1,1,2,2-tetrachloroethane; 10, 1,1,1,3,-tetrachloropropene; 11, 1,2,3,3tetrachloro-1-propene, 12, chlorethene; 13, buta-1,3-diyne; 14, 3,4dichloro-1,3-butadiene; 15, 3,3,3-trichloro-1-propene; 16, dichloromethane; 17, 3,3-dichloropropyne; 18, 1,1,1,3-tetrachloropropane.)

3. Results and Discussion 3.1. UV Laser Decomposition of Dichloroethenes. The ArF (193 nm) and KrF (248 nm) laser irradiation of gaseous cis-1,2dichloroethene, trans-1,2-dichloroethene, and gem-dichloroethene (all 150 Torr) in Ar (total pressure 760 Torr) leads to visible luminescence, depletion of the dichloroethene, formation of HCl and a number of volatile products, and a black fog that flows in the reactor and slowly descends to its surface as ultrafine black soot. All three dichloroethenes have a strong absorption band centered near 193 nm, but they practically do not absorb at 248 nm.16 This suggests that the visible luminescence (spark) observed in both ArF and KrF laser irradiations is possibly due to multiple photon absorption and dissociation at 193 nm and dielectric breakdown at 248 nm. The main volatile products of the irradiations of all the dichloroethenes at 193 and 248 nm are HCl, ethyne, chloroethyne, and Cl2C2H2 isomers. These are accompanied by very minor products of which only some differ for each edduct. The same type major products together with minor products are, for the sake of brevity, illustrated only for ArF laser photolysis of cis1,2-dichloroethene (Figure 1a) and gem-dichloroethene (Figure 1b) and for KrF laser photolysis of cis-1,2-dichloroethene (Figure 1c). The products divide into one-carbon molecules (dichloromethane and trichloromethane), two-carbon molecules (isomeric dichloroethenes, chloroethyne, chloroethene, trichloroethene, tetrachloroethane), and three- and four-carbon molecules (trichloropropene, tetrachloropropenes, dichloropropyne, and tetrachloropropane; butadiyne and dichlorobutadiene). All of them reveal that the MW laser irradiation at 193 and 248 nm of dichloroethenes is a complex process involving isomerization of cis-1,2-dichloroethene to trans-1,2-dichloroethene, HCl elimi-

Laser Photolytic Deposition of Chlorinated Soot

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Figure 2. SEM images of soot obtained upon (a) KrF laser irradiation of cis-dichloroethene and (b) ArF laser irradiation of gem-dichloroethene.

nation to chloroethyne, and cleavage and addition (or rearrangement) of H and Cl in 1,2-dichloroethenes leading to 1,1dichloroethene. Other plausible reactions are a three-center elimination of HCl and isomerization of chloroethyne, both leading to chlorovinylidene, which is capable of addition to dichloroethenes and chloroethyne to yield three-membered cyclic compounds that further rearrange to unsaturated threemembered linear compounds. The four-carbon molecules may form through recombination of C-centered radicals and ensuing HCl, Cl2, and H2 eliminations. We thus assume that the earlier observed H and Cl formation and Cl2, HCl, and H2 elimination,17,19,21,23 along with the formation and rearrangement of three-membered cyclic compounds, are ensued by further C-H and C-Cl bond homolyses, rearrangements, molecular (Cl2, HCl, and H2) elimination, and carbene additions, allowing formation of high-molecular C/H/Cl unsaturates. The observed three- and four-carbon compounds serve as examples of intermediate products arising from addition of carbenes and/or addition/recombination of C-centered radicals, while the observed dichloromethane and trichloromethane illustrate the possibility of cleavage of bonds between carbon atoms in intermediary products. The black ultrafine soot is accumulated in amounts of 35-45 mg from five photolytic runs, each carried out with 8 min irradiation and driving photolysis to about 50%. 3.2. Properties of Deposited Soot. Scanning Electron Microscopy. Morphology and composition of the soot do not depend on the structure of dichloroethene: scanning electron microscopy confirms fluffy submicrometer structures merged to larger spongy agglomerates (illustrated in Figure 2), and SEM-energy dispersive X-ray (EDX)-derived stoichiometry C1.00Cl0.08-0.10 reveals chlorocarbon structure with C:Cl ratio ∼10-12 and a tiny (ca. 5 at. %) incorporation of oxygen. The substantial decrease in Cl content as compared to the initial dichloroethene and the low incorporation of O element are in keeping with the efficient cleavage of the C-Cl bonds and with reactivity of the soot toward atmospheric oxygen. Physical Absorption Measurements. The determined Brunauer-Emmett-Teller (BET) surface area 70-200 m2/g and average pore diameters 6-7 nm of the deposited soot indicate that both parameters are comparable to those of carbon nanocomposites prepared by MW laser photolysis of toluene and pyridine.12,13 FTIR Spectra. The FTIR spectra of the deposited soot typically consist of absorption bands at 3400, 2970-2840, 1730-1740, 1450-1460 cm-1 and a multitude of bands at 1260-740 cm-1 that are, in the given order, assignable24 to C-H, CdO, and C-O-C stretches and C-H deformation and skeletal vibrations (Figure 3a). The presence of the ν(C-H)

modes indicates H bonded to sp3-hybridized carbon, and those of the ν(CdO) and ν(OdC-O) modes are in line with oxidation at CdC bonds upon contact to atmosphere. The broad (almost flat) band at 2970-2840 cm-1 reveals similar absorbance of symmetric and asymmetric stretches of C(sp3)-Hn (n ) 1-3)25,26 contributions. This fact and the lack of absorption above 3000 and 3300 cm-1 (regions of ν(C(spx)-H), x ) 1,2) are in accordance with H preferentially occupying saturated carbon centers. Deformation modes and skeletal vibrations of hydrocarbon framework are of low diagnostic value and seem to overlap with weak absorption ν(C-Cl) bands of aromatics at ∼1090 cm-1 and with weak (less likely) (ν(C(sp3)-Cl) modes at 620-720 cm-1. The region between 620-500 cm-1 corresponds to δ(C-H) and δ(C-C) in aromatics. Raman Spectra. The visible Raman spectra of all soot show (Figure 4) broad bands at 1350-1360 cm-1 (D-band) and 1575-1595 cm-1 (G-band) of unsaturated sp2 carbon with intensity ratio ID/IG ) 0.56-0.66. The G-band reflects stretches of all pairs of sp2 atoms in rings and chains, and the D-band relates to breathing modes of rings.27,28 The shoulder of the G-band corresponds to a contribution of amorphous carbon (A-band)29 peaked at 1537 cm-1, which becomes more apparent upon deconvolution of the G-band (Figure 5). The identified D, G, and A bands, the ID/IG values, and the high photoluminescence background reveal that the soot consists of differently hydrogenated skeletons. High photoluminescence inferred from the background, which increases with wavenumber, is compatible30 with a polymer-like constituent and H-concentration of ca. 40%. X-ray Photoelectron Spectra. The XPS analysis-derived stoichiometry of superficial layers of the soot obtained from all irradiation experiments, C1.00Cl0.16-0.22O0.13-0.29, is in agreement with stoichiometry of the soot determined by the SEM-EDX analysis. Somewhat higher content of O and Cl elements in topmost layers indicates reaction of these layers with Cl2 and/or HCl, and oxidation by atmospheric oxygen. The spectra of C 1s electrons are asymmetric toward higher binding energies (Figure 6a) and resemble in this respect the spectra of graphite. However, they lack the satellite line located 6-7 eV apart from the main C 1s line characteristic of graphite. The Auger C KLL derivative spectra, allowing in some cases an estimation of the sp3/sp2 hybridization ratio,31,32 are compared to those of graphite, diamond, and glassy carbon in Figure 6b. These spectra do not have a peak at 245 eV characteristic33 of diamond and glassy carbon. The separation between the most positive and the most negative minimum used to estimate the sp3/sp2 ratio31-33 corresponds neither to sp2 nor to sp3 hybridization but rather to ratio sp3/sp2 ≈ 1 (see Figure 6b). It comes from the stoichiometry determination that about 30-50% of carbon atoms in the near surface layers are likely involved in

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Figure 3. FTIR spectrum of the soot from KrF laser irradiation of cis-dichloroethene before (a) and after heating to 700 °C (b) and treatment with ammonia (c).

Figure 5. Deconvoluted Raman spectrum of soot deposited from ArF laser photolysis of trans-dichloroethene.

Figure 4. Raman spectra of the soot from KrF laser photolysis of cis-dichloroethene (a), ArF laser photolysis of trans-dichloroethene (b), KrF laser photolysis of gem-dichloroethene (c), ArF laser photolysis of cis-dichloroethene (d), and ArF laser photolysis of gem-dichloroethene (e).

C-O and C-Cl bonds. However, the method of hybridization ratio determination on the basis of analysis of derivative Auger spectra was initially proposed for materials containing predominantly carbon, and the hybridization ratio given above should be considered as a rough estimate.

Figure 6. C 1s core level spectra (a) and C KLL X-ray-excited derivative Auger spectra of the soot from KrF laser photolysis of cisdichloroethene (a), ArF laser photolysis of trans-dichloroethene (b), KrF laser photolysis of gem-dichloroethene (c), ArF laser photolysis of cis-dichloroethene (d), and ArF laser photolysis of gem-dichloroethene (e), diamond (f), graphite (g), and glassy carbon (h).

Laser Photolytic Deposition of Chlorinated Soot

Figure 7. Thermogravimetric analysis of the soot from KrF laser photolysis of cis-dichloroethene (a), ArF laser photolysis of transdichloroethene (b), KrF laser photolysis of gem-dichloroethene (c), ArF laser photolysis of cis-dichloroethene (d), and ArF laser photolysis of gem-dichloroethene (e).

Thermal Stability. The soot deposited in all experiments and heated to 700 °C monotonously decreases weight by ca. 40-50% (Figure 7). Major diagnostic single ion traces of gaseous products are observed to have different progress with different samples, and this is illustrated at m/z 2 (H2), 26 (C2H2), 36 (HCl), 37 (C3H), and 70 (Cl2) for soot from KrF laser irradiation of cis-dichloroethene and ArF laser irradiation of gem-dichloroethene (Figure 8). The different evolution of HCl and Cl2 indicates that both processes take place from structurally different carbonaceous frameworks. Properties of Heated Soot. The TGA residues retain spongy morphology of the deposited samples (Figure 9), and their SEMEDX analysis confirms that they do not contain chlorine. Their FTIR spectra (Figure 3b) show the presence of C-H and CdO bonds, and the pattern between 400-1300 cm-1 indicates skeletal rearrangement of the framework. The Raman spectra of the heated samples (Figure 4) lack the A band and possess the D band positioned at wavenumbers of the unheated samples (1350-1360 cm-1) together with the G band shifted to higher wavenumbers (1585-1600 cm-1). The insignificant photoluminescence observed in the spectra is in keeping30 with ca. 20 at. % of H. These features and a small but systematic increase in the ID/IG ratio (0.70-0.91) are consistent with more ordered

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16157 graphite-like carbon possessing more cyclic sp2 structures than in the unheated samples. These conclusions are supported by HRTEM images of the deposited and heated soot. As illustrated in Figure 10, both types of samples are amorphous and can be described as merged chain-like structures composed of bodies smaller than 100 nm. The SAED (selected-area electron diffraction) and HRTEM (high-resolution transmission electron microscopy) images of the heated samples reveal regions of parallel lines that represent organized carbon structures assignable34 to rhombohedral carbon/ graphite (PDF 74_232). Magnetic Properties. The magnetic measurements revealed that the samples of both deposited and heated soot have a very small magnetic moment. In addition to a small diamagnetism, the soot samples exhibit low temperature paramagnetic behavior. The paramagnetic susceptibility approximately follows the relation for noninteracting paramagnetic centers χ ) C/T, where C is the Curie constant. The temperature dependence of the magnetization measured between T ) 5 and 60 K for H ) 1000 Oe enables evaluation of the Cg values (0.8 to 2.0 × 10-5 K · emu/g · Oe) related to the unit of the mass. At T ) 5 K, the magnetization curves are predominantly determined by the paramagnetic contribution, which can be quantitatively described by the Brillouin functions. The curvature of these curves corresponding to spin values of S ) 1-1.5 excludes the presence of clusters. At higher temperatures (e.g., T ) 150 K), the paramagnetic contribution is proportional to the magnetic field, and a nonlinear contribution saturated at higher fields corresponds to a ferromagnetic contribution. The values of these FM magnetizations mFM range between 1.3 × 10-3 and 4.9 × 10-3 emu/g. We note that a typical mFM value for impurity is 10-3 emu/g35 and that the small increase above this value was observed only for heated soot deposited by ArF and KrF laser irradiation of gem-dichloroethene. The determined Cg and mFM values thus indicate that the relative concentrations of carbon paramagnetic and “ferromagnetic” atoms with S ) 1 can be estimated to be 10-4 and 10-6, respectively. We remark that

Figure 8. Diagnostic single-ion traces observed upon heating of the deposit from (a) KrF laser irradiation of cis-dichloroethene and (b) ArF laser irradiation of gem-dichloroethene.

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Figure 9. SEM images of ultrafine powder obtained upon heating of soot from (a) KrF laser irradiation of cis-dichloroethene and (b) ArF laser irradiation of gem-dichloroethene.

Figure 10. TEM images and electron diffraction of deposited (a) and heated (b-e) soot obtained by KrF laser irradiation of cis-dichloroethene.

the diamagnetic chlorinated a-C:H soot and heated graphitelike soot resemble the diamagnetic Si/O-carbon composite deposited from toluene by radiation of ArF laser, but differ remarkably for very rare ferromagnetic Si/O-carbon composite deposited from toluene by KrF laser.12 Possible Use and ReactiWity toward Ammonia. We note that the deposited nanoscopic chlorohydrocarbon may have a potential for synthesis of nanoscopic carbonaceous materials with organic functional groups, because it may, in analogy with aromatic compounds,36-38 undergo reactions with various nucleophiles, chlorosilanes, or hydridosilanes and replace -Cl substituent by, for example, -OH, -OR, amino-, and alkoxysilyl- groups. The nucleophilic substitutions of aromatics occur in solution, and those of aryl halides by thiolate ions39 and of hexachlorobenzene by ammonia and amines39,40 are most feasible. We therefore assumed that the examination of reactivity of the solid nanoscopic chlorohydrocarbon toward gaseous ammonia was most appropriate. The ultrafine soot obtained from cis-dichloroethene was heated for 2 h in gaseous ammonia at 100 °C and then kept at 10-4 Torr for 5 h. We found that the SEM-EDX-derived stoichiometry (C1.00Cl0.04N0.06O0.05) and the XPS-derived stoichiometry of superficial layers (C1.00Cl0.15N0.05O0.15) of the

Figure 11. N 1s core level spectrum of the soot treated with ammonia.

ammonia-treated soot are consistent with partial substitution of Cl atoms. Furthermore, the XP N 1s spectrum (Figure 11) shows the dominant contribution at 399.2 eV (88% of total N signal), which is assignable41 to the C-N bond in the R-NH2 group. The FTIR spectrum of the ammonia-treated soot (Figure 3c) consists of absorption bands at 3140 and 3040 cm-1 and an intense band at 1410 cm-1, which are stronger than the band of

Laser Photolytic Deposition of Chlorinated Soot the C-H stretching mode in the initially deposited samples (Figure 3a) and which fall in the region of the stretching and deformation frequencies of -NH2 groups. Salient Features of Soot. The employed complementary analyses thus show that the deposited carbon soot is a rare ultrafine material composed of fluffy submicrometer structures containing carbon, hydrogen, and chlorine, and having a high surface area. The atomic C/Cl ratio is ∼10-12, and H atoms are attached to C(sp3) centers. The identified D, G, and A bands, the ID/IG values, and the high photoluminescence background are in accordance with coexistence of different hydrogenated skeletons and a polymerlike constituent possessing H-concentration of ca. 40%. The material is reactive toward atmosphere and incorporates tiny amounts of oxygen. It also reacts with gaseous ammonia and forms C-NH2 bonds. This reaction is an example of nucleophilic substitution of the C-Cl bond. Soot decreases its weight upon heating to 700 °C, evolves H2, C2H2, HCl, C3H, and Cl2, and undergoes skeletal rearrangement of the framework into more ordered graphite-like carbon (containing rhombohedral graphite). The deposited soot and the heated graphite-like soot are diamagnetic and differ from the very rare ferromagnetic Si/O-carbon composite deposited from toluene by KrF laser.12 We point out that similar chlorinated a-C:H material has not been yet prepared, because the only chlorinated carbon was, to the best of our knowledge, grown on hot reactor surface in highpressure pyrolysis of ethylene dichloride and consisted of partly chlorinated 5-40 µm-sized columnar carbon.14 We also stress that the possibility of nucleophilic substitution at the C-Cl bonds by nucleophiles opens the door for synthesis of nanoparticles with different functional groups and to nanoparticles that are of interest in synthetic chemistry and coupling reactions for construction of novel nanobodies. 4. Conclusions MW UV laser-induced decomposition of gaseous cis-, trans-, and gem-dichloroethenes in Ar leads to formation of HCl, ethyne, chloroethyne, and Cl2C2H2 isomers along with a number of minor C1-C4 chlorohydrocarbons. The same major and somewhat different minor products are formed in ArF laser-induced multiple-photon photolysis and KrF laser-induced dielectric breakdown of dichloroethenes together with ultrafine chlorinated soot. These products suggest that the laser irradiation induces a complex process involving initial cleavage of C-Cl and H-C bonds and ensuing cleavages, additions, and rearrangement steps yielding high-molecular C/H/Cl unsaturates. The deposited soot is revealed as a nanoscopic Cl-substituted hydrogenated carbon having C/Cl ratio ∼10-12 and some graphitic features. The soot has more Cl and O atoms contained in topmost layers, which is presumably due to reactions with Cl2 and HCl and superficial oxidation in air. Thermal treatment of the soot results in evolution of HCl, Cl2, H2, and carbon-containing fragments and leads to more graphitization. Both deposited and thermally treated soot have very low magnetic moments. The relatively high BET surface area and possibility of substitution at the C-Cl by ammonia suggest that the soot will be useful for synthesis of large-surface area carbon powders bearing polar functional groups and can be also used for coupling with other nanobodies. Acknowledgment. This work was supported by GAASCR grant no. 400720619.

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