Cryochemistry of Metal Nanoparticles

Cryochemistry of metal nanoparticles

Gleb B. Sergeev Laboratory of Low Temperature Chemistry, Chemistry Department, Moscow State University, 119899, Moscow, Russia (Tel.: +7(095)939 5442; Fax: +7(095)939 0283; E-mail: [email protected])

Received 20 March 2003; accepted in revised form 23 May 2003

Key words: metal nanoparticles, low temperature, solid state, encapsulation, sensor materials, spectroscopy, explosive reactions

Abstract

The interaction of metal atoms, clusters and nanoparticles with different organic and inorganic substances were studied at low temperature (10–40 K). Combination of matrix isolation technique and preparative cryochemistry was applied for the investigation of activity and selectivity of metal particles of different size. Encapsulation of metal nanoparticles in polymers was studied. The metal–polymer ﬁlms thus obtained exhibited satisfactory sensitivity to ammonia.

Introduction isolation technique and preparative cryochemistry. The interaction of metal particles with different organic The last decade of the 20th century was marked by the and inorganic substances will be presented in the first increased attention of scientists in the fields of physics, part of this article. In the second part we will consider chemistry, material science, etc., devoted to nanoparti- the encapsulation of metal nanoparticles in polymer cles, their synthesis, properties and different reactions. films and thus obtaining the nanosystems, exhibited The reason for this lies in the fact that particles of sensor activity. nanometer size exhibit peculiar properties. At present time the most interesting subject is the connection of chemical properties of metallic particles Reactions in low temperature with their size. Small metal particles with sizes in range co-condensate films from 1 to 10 nm exhibit high and sometimes unusual chemical reactivity and show a strong variation in their General remarks activity depending on cluster size. These are size effects in nanochemistry. The most successful are the study The method of low temperature chemistry is based of such effects in gas phase reactions and chemisorp- on the condensation of reagent vapors on the cooled tion. Simultaneous application of jet methods, pulsed surface in special cryostats under conditions, which laser characterization and different mass-spectroscopic exclude interaction in the gas phase. The scheme techniques allow to define the activity of metal par- in Figure 1 illustrates the fundamental possibility of ticles with different number of atoms (Binns, 2001; using low temperature in order to obtain metal clus- Knickelbein, 1999). ters, mono- and polynuclear metal complexes and Low and superlow temperature may be also used ligand-stabilized nanoparticles. for distinguishing the activity of metal atoms and Chemical interactions in low temperature co- nanoparticles (Sergeev, 2001; Sergeev & Shabatina, condensates begin with metal atoms. During the 2002). This method is based on combination of matrix condensation and annealing of the samples the two 530

Figure 1. Scheme of chemical processes taking place in metal/ligand co-condensates at low temperatures. Metal and ligand interactions start from atoms and proceed as a number of parallel and sequential reactions lead to aggregation and metal nanoparticles’ formation and formation of organometallic compounds and complexes of different nuclearity. competing processes take place: the aggregation of The size of metal particles, which can be formed by metal species and their stabilization in the matrix. Addi- such technique and their reactivity are determined by tion of ligand into the systems may cause the formation the combination of different experimental conditions. of metal particles of different size and their stabilization The main experimentally controllable factors are the or reaction with ligand molecules. The aggregation of substrate temperature, metal/ligand ratio, reagent con- metals atoms and interaction with ligands occur prac- densation rate and the rate of sample annealing. It was tically without activation barrier. The high reactivity shown that the lower is the temperature of the substrate of small metal species is the main difficulty in estab- surface, the less are the interactions, controlled by the lishing the relation between the size of the particles diffusion and the more the possible is the formation and their chemical activity. There are also problems of high energetic and reactive species at low tem- in producing and isolating the compounds with def- peratures. Low temperature co-condensates belong to inite compositions. The reactions shown in Figure 1 non-equilibrium dynamic systems, which possess the are complex multifactor processes taking place under internal accumulated energy. The metal/ligand ratio highly non-equilibrium conditions and for a wide or strongly affects the size of metal particles, obtained narrow distribution of the reactive species on their via low temperature co-condensation. Increasing this chemical activity. This fact is reflected usually in the ratio usually leads to raising of the part of clus- kinetics of low temperature processes. ters and the more aggregated metal particles. The The general scheme of our cryochemical synthe- component condensation rate has a complex effect sis and reactions with metals, encapsulated in organic, on the properties of low temperature co-condensate inorganic or inert polymer matrices are presented in film. The lifetime of such highly active species, as Figure 2. The first step is the co-condensation of metal metal atoms, their dimers or trimers, during the co- and ligand vapors on surface at low or very low tem- condensation on cold surface is inversely proportional perature. Thus we produce solid co-condensate film. It to the condensation rate and depends on the nature is possible to observe the stabilization or reactions of of relaxation and diffusion processes in the system. metal species in such films using IR, UV-vis and ESR Intensity of particle beam determines the number spectroscopy and electrical measurements. During the of collisions of atoms and molecules with surface annealing to room temperature the dispersion of metal and each other. Together with chemical nature of nanoparticles or metal-containing polymer films are reagents all mentioned factors determine the path- formed. At last we produce nanosized metal-containing way of processes leading, or not, to a reaction. Pro- organosols or solid films and use these materials cesses, which occur during the real condensation, are to study various chemical transformations, catalytic more complicated than the given scheme (Sergeev, activity, gas sensor properties. 2003). 531

METAL (M) ORGANIC OR INORGANIC In evaporation source of COMPOUND low temperature In constant temperature bath or vacuum set-up evaporation source

EVAPORATION AND CO-DEPOSITION OF VAPOURS ONTO THE SURFACE AT REACTIONS OF METAL LOW TEMPERATURE ATOMS AND CLUSTERS

SOLID CO- ESR-SPECTROSCOPY IR-SPECTROSCOPY CONDENSATE FILM

ELECTROPHYSICAL SLOW HEATING UP TO UV-Vis-SPECTROSCOPY PROPERTIES ROOM TEMPERATURE

TEM - ANALYSIS DISPERSION OF METAL NANOPARTICLES IN LIQUID ORGANIC SOLVENT (MONOMER) OR SOLID METAL-CONTAINING POLYMER FILM SLOW POLYMERIZATION OF MONOMER UNDER ARGON EVAPORATION OF UNREACTED MONOMER

VISCOUS METAL COLLOID THIN METAL – CONTAINING SOLUTION POLYMER FILM metal content up to a few percent depending on experimental conditions; the same particle size

STORAGE AT ROOM TEMPERATURE

NANOSIZE METAL- CHEMICAL REACTIVITY CONTAINING MATERIAL CATALYTIC ACTIVITY (ORGANOSOL OR SOLID METAL-POLYMER FILM)

GAS SENSOR PROPERTIES

Figure 2. Cryochemical synthesis of metal nanoparticles and their reactions with various ligands.

Reactions of magnesium species In the case of carbon tetrachloride we have the competition of chemical reactions and formation of dif- In organic chemistry of poly-halogen compounds it is ferent chemical intermediates and products: Grignard known that at ambient temperature carbon tetrachlo- reagent (CCl3MgCl), tri-chloromethyl radical (CCl3), ride does not react with bulk magnesium. The situation dichlorocarbene (CCl2). The reaction mechanism was changes dramatically in Mg-CCl4 co-condensates on studied in detail (Zagorsky and Sergeev,1990). cold surface at temperature, close to the temperature Magnesium clusters of different nuclearity can be of liquid nitrogen (77 K). In ﬁlm co-condensates we stabilized in low temperature matrices upon depo- have reactions, which are presented by the following sition at 77 K. The process of cluster formation is scheme: simply controlled by changing the reagent ratio. Mag- nesium atoms initiate radical reaction resulting in a number of recombination products. Magnesium atoms H2O CCl3MgCl CHCl3 aggregate in the systems with Mg : RX ratio in the range 1 : 100 and yield the Grignard reagent. The par- CCl C Cl Mg + CCl4 3 2 6 ticle size and the C–Hal bond energy are important. Recently, we have studied the interaction of magne- C Cl C Cl 2 2 2 4 sium species with halogen butanes and compared the 532 yields of octane (product of recombination) with C–Hal after clusters and possibly the reactivity changes in the bond energy (Ivashko, 2001). The results are corre- row Mg2 ≥ Mg3 > Mg4 ≥ Mg. lated with the C–Hal bond energy and our scheme of IR-spectroscopic study of this system have shown reaction. that the only stable products of the reaction are C2Cl4 New data were obtained for reactions of magnesium and C2Cl6. The formation of Grignard reagent was not particles with poly-halogenmethanes CCl4, CHCl3 and detected. We suppose that the reaction proceeds over CFCl3 in temperature range 12–70 K (Mikhalev et al., the sequential abstraction of chlorine atoms accompa- 2002). Details of experimental setup may be found in nied by metal cluster decomposition. The intermediate the work of Soloviev et al. (1997). The system Mg-CCl4 product is probably the complex between magnesium was studied more thoroughly. UV-visspectra of magne- chloride and dichlorocarbene. Further transformation sium particles in argon are shown in Figure 3. The mag- of this complex lead to formation of stable products. nesium particles were identiﬁed using the data from This scheme of the reaction agrees with theoretical work (Knight & Ebener, 1976). Atoms, dimers, trimers calculations (Granovsky et al., 2001). and tetramers are all stable at 12 K and may be found in matrix immediately after the condensation. Annealing of the condensate, deposited at 12 K leads to simultane- The comparison of reactions of ous linear decrease with increasing the temperature of magnesium and samarium all absorption peaks of magnesium atoms and clusters in argon. The spectrum totally disappear at 35 K. We have studied the interactions of magnesium and

Addition of 10% of CCl4 changes the annealing samarium particles with different organic and inorganic behavior of the system. The data, presented in inset of compounds. These metals have been chosen as a model Figure 3 clearly shows that dimers and trimers are con- because they both form organometallic reagents at sumed ﬁrst. Magnesium tetramers are a bit more stable room temperature with similar properties. It was inter- and it is interesting to consider the stability of atomic esting to compare the properties of small particles and magnesium adsorption. The adsorption of atoms was we studied the reactions of Mg and Sm atoms and nearly constant up to 27 K, while the absorption of all clusters at temperature 10–40 K. clusters decreases. Only when remained less than 20% Metal reactions with light hydrocarbon molecules of the initial cluster concentration the atomic peak also are of signiﬁcant interest in investigations of starts to decrease. One can see that atoms start to react metal–ligand bonding and reaction mechanisms.

1,0 Mg 35240 12K /I T 0,8 I Mg Mg 3 4 Mg4 Mg 2 Mgx Mgx Mg 0,6 32680 Mg2 10 15 20 25 30 35 37620 Mg Temperature,K 2 34260 Mg Mg Absorbance 0,4 3 2 29000 40460 27220 0,2

0,0 40000 35000 30000 25000 Energy,cm-1

Figure 3. Temperature dependence of UV-vis spectra of Mg/CCl4/Ar = 1/100/1000 condensate. Shown are traces in the range (from top to bottom 12–34 K). Inset shows the variation of normalized integral intensity of several bands. 533

Reaction of magnesium with CH3Cl and CH3Br led The interactions of magnesium and samarium with to Grignard reagent formation. Quantum chemistry cal- ethylene in low temperature co-condensates were stud- culation show that only one Mg atom incorporates ied by IR- and UV-vis spectroscopy. The spectroscopic into carbon–halogen bond (Soloviev et al., 1997). For data and calculations at MP2-6-31G (2p, 2d) level of the system Sm–CH3X, (X = Cl, Br) the formation of Mg–C2H4 system show that the interaction of Mg with methane is observed. C2H4 is realized in Van-der-Waals complex formation. The reactions of Mg and Sm particles with CO2, The structure of the complex (Figure 4b) corresponds to C2H4 and mixtures of CO2 and C2H4 have also been 5-member cycle with two ethylene molecules. Binding studied. In the system Mg–CO2 the higher activ- energy is E = 20, 6 kcal/mol. ity of magnesium clusters in comparison to atoms For the system Sm–C2H4 we suppose the formation was observed. The product of the reaction is a com- of complexes Sm(C2H4) and Sm(C2H4)2 with metal plex (Soloviev et al., 1999). The complex have been atom co-ordinated by ethylene π-system. The com- characterized by the IR- and UV-vis spectroscopic plex of samarium with ethylene dimer probably has a techniques and ab initio quantum chemistry. The opti- sandwich-like structure. mized structure parameters for Mg–CO2 complex have The results, obtained for the reactions of samarium been presented in the paper (Soloviev et al., 1999). and magnesium species are summarized in Table 1. Atomic charges are shown in Figure 4a. The values The analysis of the data presented in table allow to of the charges allow to consider this compound as an conclude that samarium particles possess higher redun- + − ion–radical pair Mg CO2 . dant energy and different reactivity than magnesium For the system Sm–CO2 the higher activity of particles in the same reactions. One important general Sm clusters in comparison to atoms was found. peculiarity in the behavior is that the metal clusters are Figure 5 presents the temperature dependence of rela- more active than atoms. tive intensity of several atomic and cluster transitions Another peculiarity in the behavior of magnesium in Sm/CO2/Ar co-condensate. The rate of consumption and samarium particles was observed in their reac- of atoms and particles is clearly different. Samarium is tions with mixtures of two ligands, CO2 and C2H4.It more active than magnesium in reaction with carbon was found that under comparable conditions samar- dioxide. Just after the co-condensation it formed not ium react only with CO2 and do not react with + − only Sm CO2 complexes, but also secondary products. ethylene. Magnesium forms complex with both lig- The IR- and UV-vis spectra together with the scheme ands, which probably corresponds to 5-member cyclic of the reaction were presented in the work (Sergeev structure, and it is responsible for the formation of et al., 2000). chemical bonds between ligands. The results obtained

1.0 (a) O -0.91

0.8 Mg +0.87 C+0.95 Sm 501 nm.

O -0.91 0.6 (b) Mg 0.4 2.09(0.78) 2.09(0.78) H H Relative int. C C Smx 600 nm. H H 0.2 1.55(1.05) 1.55(1.05)

C Sm2 541 nm. 1.56(0.99) C H H 0.0 H H 15 20 25 30 35 T, (K) Figure 4. Calculated structures for (a) MgCO2 complex including atomic charges (4-member ring) (b) MgC2H4 complex with bond Figure 5. Temperature dependence of relative intensity of Sm length (Å) and bond orders given in parentheses (5-member ring). atomic and cluster absorptions. 534

Table 1. Reactions of magnesium and samarium species in matrices at 10–40 K

Metal CO2-ligand C2H4/C2D4-ligand CH3X(X= Cl, Br)-ligand species

+ −. Mg Mg CO2 by matrix Cycle Mg(C2H4)2 by CH3MgX by irradiation annealing matrix annealing (λ ≈ 280 nm) + −. Mg2–4 Mg CO2 by CH3MgX by irradiation co-condensation (λ>300 nm) + −. Mgx Mg CO2 by matrix annealing +. −. + Sm Sm CO2 , CO, SmCO3 Complexes Sm(C2H4)/C2D4)SmCH3Xby by matrix annealing and Sm(C2H4)2/C2D4)2 co-condensation +. −. Sm2 Sm CO2 , CO, SmCO3 By co-condensation

Smx demonstrate that no unambiguous relationship exists Intensity between the activity and selectivity of metal nanoparti- 2000 cles (Sergeev, 2001). The frontier task is to study how 1500 1000 to control the activity and selectivity of metal species A B 500 C depending on their size from atoms to clusters and 0 polynuclear complexes and solvated nanoparticles. 109 -500 Ag -1000 107 -1500 Ag Explosive reactions at low temperature -2000 -2500 The high activity of small metal particles in some 2800 3000 3200 3400 3600 cases causes the possibility of explosive reactions in H, G low temperature co-condensates. Such reactions have Figure 6. Typical ESR-spectrum of Ag-5CB co-condensate sam- been observed in growing co-condensate films for the ple at 80 K. magnesium–1,2-dichloroethane systems (Sergeev & Efremov, 1996). organic materials for obtaining and stabilization of The explosion takes place at a certain critical thick- metal nanoparticles. ness Lcr, the reaction yield after the explosion is close to 100%. Critical thickness depends on the reagent Nanometal–liquid crystal systems ratio and substrate temperature. The probability of The possibilities of stabilization of silver nanoparticles explosions decreases with the increase of surface tem- using polar liquid crystals like cyanobiphenyls were perature and lowering the reagent condensation rate. studied in temperature range 80–300 K (Shabatina Our investigations have demonstrated that all men- et al., 2001; 2002). Most of the experiments were done with 4-pentyl-4-cyanobiphenyl (5CB) and 4-octyl- tioned factors determine the size of metal particles and the redundant energy of the system. The results were 4 -cyanobiphenyl (8CB). Spectroscopic data obtained analyzed by the model, which involves the mechan- in combination with the results of quantum chemistry ical stresses formation during film growth and posi- calculations of the model cyanophenyl system show the tive feedback, generating a wave explosive chemical existence of sandwich-like Ag(CB)2 complexes with reaction (Sergeev & Shabatina, 2002). cyanobiphenyl ligand molecules by the ‘head-to-tail’ principle. The electron spin resonance spectrum of Ag–5CB Organometallic nanostructures co-condensate showed two doublet lines at high and low fields due to silver atoms (J = 1/2) with two iso- The synthesis of organometallic hybrid nanomateri- topes (Ag107: g = 1.9996, a = 558.5 G and Ag109: als is of great interest from fundamental and applied g = 1.9994, a = 641 G) included in π-complex struc- viewpoints. We used low temperature and different ture (Figure 6). The obtained value of the hyperfine 535 structure constant in comparison with the data for silver polymerization is initiated by silver atoms. This caused atoms stabilized in the inert matrices (Michlik et al., the smaller particle sizes for bimetallic system. 1996) allowed us to estimate the electron spin density Optical spectroscopy in the UV and visible on silver as ρM = 0.87. This value is characteristic for ranges in combination with dynamic light scatter- metal atom π-complexes and shows the electron den- ing technique were used to study the properties sity donation from silver atom to π ∗-orbital of ligand. of polymer-stabilized silver nanoparticles (Sergeev The central wide anisotropic singlet line (C in et al., 2001). Co-condensation of vapors of silver Figure 6) with g-factor close to 2.0025 could be and 2-methylaminoethylmethacrylate (DMAEMA) in referred to non-valent silver small clusters (Michalik vacuum was carried out on the walls of a glass vessel, et al., 1996). The size estimation of silver clusters, cooled by liquid nitrogen. According to the electron stabilized in the cyanobiphenyl matrix at 90 K with microscopic data, heating to room temperature resulted the ratio Ag : 5CB = 1 : 10 gives the average value of in the formation of both silver particles of 5–12 nm several (1–2) nanometers. in diameter, stabilized by surface polymer layer and By heating the Ag–5CB co-condensate film sample their aggregates. Dynamic light scattering studies have from 90 up to 200 K the relative integral intensity of shown that the size distribution of these particles is doublet lines decreases and intensity of the central com- bimodal. Such a distribution seems to suggest that ponent increases. The process reveals thermal decom- both isolated silver particles and their aggregates are position of the complex and simultaneous growth of present simultaneously in the system. The properties the silver nanoparticles. The same effect can be caused of cryochemically synthesized silver nanoparticles in by irradiation of the sample at 90 K by UV-light with water, acetone and toluene are found to be stabilized

λmax = 340–360 nm. by macro-molecular poly-DMAEMA. In Table 2 the data for the mean radii of light scattering particles are presented. The values for radii of isolated parti-

Nanometal–monomer and polymer systems cles (R1) and for radii of aggregated particles (R2) were calculated by the Stokes–Einstein equation. The For stabilization of nanosized metal particles we used sizes, presented in the table are larger than the values, acrylic acid, methyl acrylate (MA) and dimethyl- found by electron microscopy. The reason for this is the aminoethylacrylate (Sergeev et al., 1998; 2001). value, determined from light scattering experiments, is Low temperature co-deposition of vapors of metal the radius of metal core and polymer adsorption layer, and MA and subsequent heating to room temperature which depends on the nature of the solvent. One can produce stable organosols of silver with metal particles see from Table 2 that in aqueous systems radius R1 is of size smaller than 15 nm and also bimetallic silver– more that twice greater than the particle size in ace- lead nanoparticles with the sizes less than 5 nm. The tone and toluene. In our opinion, the difference is due particle sizes were determined by transmission electron to the fact that in water systems polymer molecules microscopy (TEM). are bounded to the surface of silver nanoparticles and The system, containing two metals is closer to form a more bulky layer than in acetone and toluene. Pb–MA than to Ag–MA system. In our opinion such Thus, we can conclude that light scattering technique behavior can be connected with non-additive changes can give interesting information about the processes of of nanoparticle properties on transition from binary nanoparticles self-assembly and effect of the solvent to ternary system. The inhibition by lead of MA on such processes.

Table 2. Properties of silver sols sterically stabilized by poly-DMAEMA in water, acetone and toluene

Dispersion Mean Mean radius, R/nm Isolated Aggregated medium diffusion particles, R/nm particles, coefﬁcient, R/nm D/108 cm2 s−1

Water 1.23 ± 0.06 179.2 ± 9.4 53.3 ± 14.4 323.5 ± 79.3 Acetone 8.22 ± 0.09 80.8 ± 0.9 18.5 ± 2.1 111.9 ± 11.5 Toluene 3.06 ± 0.15 125.6 ± 6.4 19.8 ± 5.9 183.9 ± 22.0 536

Polymer films with nanosized metal particles and metallic nature of the particles was shown by X-ray studies (Sergeev et al., 1995). Low temperature co-deposition of vapors of different The poly-p-xylylene is a typical insulator. The addi- metals, such as Ag, Cu, Mg, Cd, Zn, Pb, Sm, Sn, tion of lead up to 10 wt% does not change the charac- Mn with the monomer p-xylylene, which is reactive teristics of the materials. The electrical conductivity of at 100–120 K, has been used for preparation of nanometal–polymer films was studied during the formation sized metal particles, encapsulated in a polymer matrix of the samples and they annealing in temperature range (Sergeev et al., 1995; 1999; Sergeev & Petrukhina 80–300 K (Zagorskii et al., 1999). 1996). Lead-containing poly-p-xylylene material was Polymerization of the film system and formation of found to be gas sensitive (Bochenkov et al., 2002; a polymer network at low temperature prevents in this Sergeev et al., 1997). Thin films, containing lead case the high aggregation of small metal species. Metal nanoparticles were prepared by vacuum deposition vapors are obtained by resistive heating of the bulk technique at low temperature. Electrical resistance metal at 500–1200◦C depending on the volatility. The measurements of poly-p-xylylene films with lead reactive monomer p-xylylene was prepared by pyroly- particles in the presence of ammonia and water in sis of di-p-xylylene at 600–700◦C in the same vacuum gas phase at room temperature have shown that the cryostat. After co-deposition of monomer and metal resistance varies reversibly by a factor 104–106. The vapors on the surface, cooled by liquid nitrogen the co-operative effect of ammonia and water vapors on co-condensate was slowly heated during 3–30 min up film resistance has been found. Lead–poly-p-xylylene to ambient temperature. The scheme of the synthesis films, prepared by sequential technique were charac- of poly-p-xylylene with nanosized metal is presented terized by atomic force microscopy (Bochenkov et al., (Sergeev et al., 1995). 2002). The films, prepared by sequential method also The poly-p-xylylene films, containing metal possess sensitivity to humidity or humid ammonia. nanoparticles, can be extracted from vacuum cryo- The co-operative effect of water and ammonia vapors stat for further investigations. The metal-containing on film resistance is much higher than for individual poly-p-xylylene films were examined by TEM. It was components. found, for example, that globular lead particles are uniformly distributed within poly-p-xylylene at room temperature (Figure 7). The size of these particles was Conclusions found to be 3–8 nm and independent of lead content in range 0.1–6.5 wt%. The metal content was determined Low and superlow temperatures allow researchers to by means of X-ray fluorescent analysis. The crystalline study the highly active metal species: atoms, small clusters and their nanosized aggregates. Small metal species can be formed on different cooled surfaces and they can migrate on the surface by heating in more large-sized particles. Such processes can be accompa- nied by unusual chemical reactions. The interactions between two different substances on cold surface or included in thin condensed films usually are initiated by the reactive molecules, contacting via the formation of their unstable complexes. The formation of hybrid metal–organic supramolecular structures metal clusters and nanosized metal particles can take place during the low temperature co-condensation and during further thermal treatment of the samples. The most interesting in cryochemistry of metal nanoparticles are the size effects of various sorts. As such effects we consider the effect of metal species size (the number of atoms in structure) on their reactivity in different processes. Figure 7. Particles size distribution in poly-p-xylylene films with The existence of such effects leads to reconsidera- 6.5 wt% Pb. tion of many widely used thermodynamic and kinetic 537 concepts. At low temperature diffusion processes, of silver nanoparticles stabilized by acrylic polymers. Mol. mechanical stability and reactivity in thin condensed Cryst. Liq. Cryst. 356, 121–129. films are highly dependent on the sample thickness. Sergeev G., V. Zagorsky, M. Petrukhina, S. Zaviyalov, Thus, low temperature open also new opportunities for E. Grigoriev & L. Trakhtenberg, 1997. Preliminary study of the interaction of metal nanoparticle-containing poly-p-xylylene studies of physical–chemical properties of nanoparti- films with ammonia. Anal. Comm. 34, 113–114. cles and size effects in thin films. Sergeev G.B., 1998. 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