Mendeleev Communications Mendeleev Commun., 2008, 18, 8–9 – 8 – © 2008 Mendeleev Communications. All rights reserved. Preparation and electrophysical properties of the cryocondensates of lead nanoparticles and carbon dioxide Elena V. Shmanova, Vladimir E. Bochenkov, Vyacheslav V. Zagorsky and Gleb B. Sergeev* Department of Chemistry, M. V . Lomonosov Moscow State University, 119992 Moscow, Russian Federation. F ax: +7 495 939 5442; e-mail: gbs@kinet.chem.msu.ru DOI: 10.1016/j.mencom.2008.01.003 Carbon dioxide has been found to affect the morphology and mean size of lead nanocrystallites. Metal clusters and nanoparticles display unique size-dependent physicochemical properties that find use for performing unusual chemical reactions and creating novel catalytic and sensor nano- materials. 1,2 Cryochemical methods are successfully used to obtain and modify nanomaterials. 2,3 For example, low-tempera- ture methods were used to obtain ammonia-sensitive poly(para- xylylene) films containing lead nanoparticles. 4 In the absence of poly(para-xylylene), nanostructural lead films also possessed sensitivity. 5 When the specimens were heated from 80 to 300 K, the microstructure underwent changes that started at T » 200 K [1/3T m (Pb), where T m is the melting point of the metal]. The film sensitivity was shown to depend on the structure and specimen preparation conditions. 5 In order to control the microstructure of lead condensates and to increase the porosity and specific surface of nanoparticles, we used cryococondensation with an inert gas followed by sublimation at T » 200 K. The partial pressure of CO 2 , used as an inactive gas, at 139 K is 1 Torr. 6 Nanostructured films were obtained in low-temperature cryo- stats described previously. 2 The specimens were formed by the vacuum deposition of lead and carbon dioxide vapours onto a support with Ni/Cr comb-shaped electrodes for conductivity measurements; the support was attached to a copper cryogenic unit cooled with liquid nitrogen. The specimens were studied by scanning tunnelling microscopy and FTIR spectroscopy; further - more, the electric conductivity of the films was measured as a function of humidity and exposure to 1% ammonia vapours. Metal condensates usually display the most interesting electro- physical properties near the percolation threshold since the specimens are non-conducting below this threshold, whereas at much higher temperatures they become continuous conductive metallic films. The metal deposition was continued until the conductivity started to increase, which occurred at surface resistances of the specimens around 10 9 Ω cm –2 . According to calibration data, the condensation rate of lead at 700 °C was 2.5×10 –9 mol h –1 , whereas that of CO 2 was 5.33×10 –7 mol h –1 . The condensation times of the reagent vapours were 10 and 50 min. It is well known that metals react with carbon dioxide at low temperatures to give various complexes. 7 Cryococondensates with various Pb:CO 2 ratios were studied by IR spectroscopy at 80–300 K. Figure 1 shows the IR spectra of the test samples. The assignment of absorption bands in the spectra was based on published data 7 and an IR spectrum of solid CO 2 at 80 K. The IR spectrum of lead–carbon dioxide cocondensates contains two intense bands at 2364 and 2339 cm –1 , which correspond to the asymmetric vibrations of carbon dioxide, and a weak band at 1399 cm –1 , which corresponds to the symmetric vibrations of CO 2 . Similar absorption bands are also present in the spectrum of solid carbon dioxide (Figure 1, spectrum 2). Thus, IR-spectroscopic data showed no interaction of lead with carbon dioxide at 80–300 K. The relief of film surfaces was studied by scanning tunnelling microscopy (STM). The maximum scanning area was 5×5 µm. An image of the surface area for a film obtained by cryo- cocondensation of lead and carbon dioxide vapours and the results of statistical analysis of particle size distribution, are presented in Figures 2 and 3. One can see that the morphology of cocondensate films depends considerably on the component ratio during the film formation. ‘Gaps’ with a mean depth of 20 nm appear; we believe that they are formed due to the sublimation of carbon dioxide from the cocondensate. A com- parison of our images with micrographs obtained in the absence of carbon dioxide 8 shows that the presence of CO 2 in the co- condensate affects lead particle size and film surface morphology. Statistical analysis of STM profiles showed that the mean size of particles with an excess of carbon dioxide or lead was 33 or 13 nm, respectively. We believe that, in an excess of lead, the majority of particles are formed in the flow before interaction with the cold surface. The fraction of surface atoms, 2800 2400 2000 1600 1200 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 2 1 Transmission (arbitrary units) n/cm –1 Figure 1 IR spectra of cryococondensates at 80 K: (1) lead–carbon dioxide; (2) carbon dioxide. 0 10 20 30 40 50 60 70 d /nm Figure 2 (a) STM micrograph and (b) particle size distribution bar graph of a nanostructured thin film with excess carbon dioxide after annealing to room temperature. The scanning area is 1.6×1.6 µm. The mean particle size is ~33 nm. The Pb:CO 2 ratio is 1:3. (a) (b)