IR LASER-INDUCED CARBOTHERMAL REDUCTION OF TITANIUM MONOXIDE: CARBON- PHASE SHIELD TO NANOSIZED TiO OXIDATION Věra JANDOVÁ a, Zdeněk BASTL b, Jan ŠUBRT c, Josef POLA a a Institute of Chemical Process Fundamentals, ASCR, 16502 Prague b J. Heyrovský Institute of Physical Chemistry, ASCR, 18223, Prague c Institute of Inorganic Chemistry, ASCR, 25068 Řež Abstract Pulsed IR laser-irradiation of titanium monoxide (TiO) leads to ablation and when carried out in gaseous benzene (1-5 Torr) to simultaneous dielectric breakdown of benzene into low molecular carbonaceous species. Both processes allow carbothermal reduction of ablated TiO particles with agglomerating carbonaceous species and deposition of carbon-coated TiO nanoparticles remarkably stable towards oxidation in air. The reported results suggest potential for protection of reactive gas-phase produced nanoparticles by carbon phase. Keywords: titanium monoxide nanoparticles; laser-induced carbothermal reduction; carbon-protection in air. 1. Introduction Carbothermal reduction of titanium oxides for synthesis of titanium oxycarbides represents a current field of interest in ceramics and hard materials research. The past studies were mainly concerned with synthesis and properties of bulk Ti carbides (final products of carbothermal reduction) under thermodynamically equilibrium conditions and were restricted to the use of elemental carbon as reductive reagent. We now report on pulsed IR laser-induced carbothermal reduction of Ti monoxide to intermediary titanium oxycarbides by decomposing gaseous benzene in dielectric breakdown adjacent to TiO surface. The short IR laser pulses bring about hydrocarbon decomposition to reactive C/H species that react with ablated/reduced TiO to yield nanosized TiO x and TiO x C y structures stabilized towards oxidation in air by excess of simultaneously produced and deposited nanostructured carbon phase. This work is related to our previous studies on IR laser carbothermal reduction of silicon oxides and the first synthesis of nanoscopic silicon oxycarbides [1-4] 2. Experimental IR-laser irradiation experiments were conducted in a Pyrex reactor (70 ml in volume) in the presence of benzene (1 and 3 Torr) by using by a pulsed TEA CO 2 laser (model 1300 M, Plovdiv University) operating with a frequency of 1 Hz on the P(20) line of the 00 0 1-10 0 0 transition (944.19 cm -1 ) and a pulse energy of 1.8 J. This radiation was focused with a NaCl lens (f. l. 15 cm) on the titanium monoxide pellet (Aldrich) positioned in the centre of the reactor above copper substrate. The reactor was described elsewhere [1] and it was a tube fitted at each end with KBr windows and having a valve connecting to vacuum manifold and pressure transducer (Fig. 1).
Fig. 1. A reactor for IR laser carbothermal reduction of titanium monoxide. (1, Pyrex vessel; 2, valve to vacuum; 3,NaCl window; 4, laser pulse; 5, lens; 6, TiO pellet; 7, Cu substrate; 8, visible luminescence. The progress of benzene decomposition and volatile decomposition products were analyzed directly in the reactor by FTIR spectrometry (an FTIR Nicolet Impact spectrometer, resolution 4 cm -1 ). The deposited films were analyzed with Raman spectroscopy (a Nicolet Almega XR Raman spectrometer, excitation wavelength 473 nm and power 10 mw), X-ray Ti 2p and C 1s photoelectron spectroscopy (an ESCA 310 Scienta electron spectrometer using Al-Kα radiation (1486.6 ev) for electron excitation) and by scanning electron microscopy (a Philips XL30 CP scanning electron microscope) and transmission electron microscopy (a PHILIPS EM 201 microscope) on samples dispersed in ethanol and applied on a Cu grid. 3. Results and discussion The highly focused pulsed IR laser irradiation induces TiO ablation and when carried out in gaseous benzene, a non-resonant interaction leading to dielectric breakdown (a visible spark) and decomposition of benzene. This decomposition leads to formation of transient species (observation of optical emission from neutral and ionic C and atomic H as well as molecular Swan C 2 bands, [5]) and of gaseous hydrocarbons (ethene, ethyne, butadiyne, phenylethyne) together with CO (and less CO 2 ) which prove the occurrence of carbothermal reduction of TiO (Fig. 2). The black films concomitantly deposited on Cu substrate (and a majority of inner reactor surface) were analyzed by a number of techniques and revealed by EDX-SEM analyses to contain carbon together with almost equal amounts of Ti and O.
Fig. 2. FTIR spectrum of benzene (5 Torr) and decomposition products upon laser irradiation (250 pulses) of TiO surface. Designation: 1, butadiyne; 2, ethyne; 3, phenylethyne; 4, ethene; 5, carbon monoxide. Typical Raman spectra of the deposit show G and D bands positioned at 1540 and 1360 cm -1 (Fig. 3) which respectively reflect bond stretches of all pairs of sp 2 atoms in rings and chains, and breathing modes of rings, and are assignable to graphitic a-c:h carbon. Fig. 3. Typical Raman spectrum of deposit obtained by laser ablation of TiO in presence of 1-5 Torr of benzene. The SEM images show remarkably different morphology for the deposit obtained by ablation of TiO in vacuum and in the presence of benzene (Fig. 4). The former consists of irregular several µm-sized agglomerates, whereas the latter reflects smaller one up to several µm-sized and almost uniformly distributed particles that have spongy structures.
12. - 14. 10. 2010, Olomouc, Czech Republic, EU Fig. 4. SEM images of laser-ablated deposit from TiO in vacuum (a) and from TiO in presence of 1 Torr of benzene (b,c). The TEM analysis and electron diffraction of these particles is seen on Fig. 5 and are consistent with amorphous several tens nm-sized bodies. These features have been also found in several other gas-phase obtained sediments obtained with IR laser pulsed irradiation of metal (Co, Ni, Ga) targets in feasibly carbonizing gaseous hydrocarbons (benzene, ethyne) [5,6]. Fig. 5. TEM image and diffraction of laser ablated deposit from TiO in presence of 1 Torr of benzene. + The XPS Ti 2p spectra of the deposit are compared to those of the commercial and Ar sputtered TiO samples (Fig. 6). These spectra respectively indicate the prevalence of the Ti 2+ Ti, Ti 3+ 4+ and Ti 4+ state and almost equal populations of states. Further analyses indicate that the samples obtained by ion sputtering and subsequently 4+ exposed to air (Fig. 6b) recover the spectra of Ti (Fig. 6a), which indicates fast atmospheric oxidation of the topmost layers. Remarkably, such oxidation is almost hindered in samples obtained by the laser ablation in the presence of 1 and 3 Torr of benzene (Fig. 6c), which confirms that nano-sized TiOx particles are efficiently enveloped and protected by the carbonaceous phase.
Fig. 6. Spectra of Ti 2p photoelectrons of TiO (commercial sample), the sample after Ar + ions sputtering (b) and of TiO sample obtained by laser ablation in presence of 1 Torr of benzene (c). 4. Conclusions Pulsed IR laser-irradiation of titanium monoxide (TiO) carried out in gaseous benzene (1-5 Torr) allows carbothermal reduction and coating of ablated TiO x nanoparticles with carbon layer, the process of expected importance in synthesis of reactive nanoparticles produced in the gas phase. Acknowledgement: The support of Ministry of Education. Youth and Sports of the Czech Republic (grant no. LC523) and the Czech Science Foundation (GAAVCR grant no. 400720619) are gratefully acknowledged. The authors thank RNDr. J. Kupčík for TEM analysis. References [1] POLA, J., OUCHI, A., BAKARDJIEVA, S., VORLÍČEK, V., MARYŠKO, M., ŠUBRT, J., BASTL, Z. Nanodomains of Crystalline Chaoite and Silica in Amorphous C/Si/O/N Phase. J. Phys. Chem. C 112 (2008) 13281-13286.
[2] POLA, J., BAKARDJIEVA, S., MARYŠKO, M., VORLÍČEK, V., ŠUBRT, J., BASTL, Z. GALÍKOVÁ, A., OUCHI, A., Laser-Induced Conversion of Silica into Nanosized Carbon- Polyoxocarbosilane Composites. J. Phys. Chem. C 111 (2007) 16818-16826. [3] POKORNÁ, D., URBANOVÁ, M., ŠUBRT, J., BASTL, Z., POLA, J. IR Laser-Induced Carbothermal Reduction of Silicon Monoxide. J. Anal. Appl. Pyrol. 83 (2008) 180-184. [4] URBANOVÁ, M., POKORNÁ, D., BAKARDJIEVA, S., ŠUBRT, J., BASTL, Z., POLA, J. IR Laser-Induced Carbothermal Reduction of Silica. Eur. J. Inorg. Chem. (2008) 4111-4116. [5] SANTOS, M., DÍAZ, L., CAMACHO, J.J., URBANOVÁ, M., POKORNÁ, D., ŠUBRT, J., BAKARDJIEVA, S., BASTL, Z., POLA, J. IR Laser-induced Metal Ablation and Dielectric Breakdown in Benzene. Infrared Phys. Technol. 53 (2010) 23-28. [6] POLA, J., URBANOVÁ, M., POKORNÁ, D., ŠUBRT, J., BAKARDJIEVA, S., BEZDIČKA, P., BASTL, Z. IR Laser-induced Formation of Amorphous Co-C Films with Crystalline Co, Co 2 C and Co 3 C Nanograins in a Graphitic Shell. J. Photochem. Photobiol. A: Chem. 210 (2010) 153-161.