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X‑ray Induced Formation of Metal Nanoparticles from Interpolyelectrolyte Complexes with Copper and Silver Ions: The Radiation-Chemical Contrast Vladimir I. Feldman,*,†,‡ Alexey A. Zezin,†,‡ Sergey S. Abramchuk,† and Elena A. Zezina† †

Department of Chemistry, Lomonosov Moscow State University, Moscow 119991 Russia Institute of Synthetic Polymer Materials, Russian Academy of Sciences, ul. Profsoyuznaya 70, Moscow, 117393 Russia

‡

ABSTRACT: Tuning the size and spatial distribution of metal nanoparticles is a key issue for obtaining metal−polymer nanocomposites with desirable properties. It was shown that the action of white X-ray radiation (bremsstrahlung, Emax ∼ 30 keV) on swollen films of interpolyelectrolyte complexes with copper and silver ions in an aqueousalcoholic environment resulted in the formation of a specific type of metal nanostructures characterized by transmission electron microscopy (TEM). The structure pattern was explained by autocatalytic enhancement of the rate of chemical transformations in the vicinity of preformed metal nanoparticles. The effect was attributed to increasing absorption of X-ray photons by compact metal nanoparticles leading to local sensitization described as the radiation-chemical contrast. The nanostructure pattern exhibited a remarkable evolution with increasing irradiation dose. In particular, at a certain stage, the big primary nanoparticles induce formation of “clouds” of ultrasmall secondary particles due to local distribution of the secondary electrons. The scale of the radiation-induced events is determined by the photon energy and electronic properties of absorbers. In the case of copper ions, major interaction occurs with K-electrons, whereas the interactions with both K- and L-electrons are significant for silver ions, which results in bimodal distribution of nanoparticle size. formation of metal nanoparticles.16 The above-mentioned studies made use of monochromatic X-ray radiation. On the other hand, recently we have shown that white X-ray radiation (bremsstrahlung) can be used for effective reduction of metal ions and preparation of nanocomposites from interpolyelectrolyte films in the swollen state.17−19 In this work we focus on the specific features of the formation and growth of nanoparticles in the films polyacrilic acid (PAA)−polyethyleneimine (PEI)−metal ions irradiated with white X-rays (maximum energy of ca. 30 keV), which are determined by basic mechanisms of interaction of X-ray radiation with matter. The key point is strong dependence of the absorption cross-section on atomic number Z and photon energy. In particular, the metal ions and atoms with relatively large Z values are extremely strong absorbers of photons with E = 10−30 keV. As revealed by the results obtained in this study, this leads to specific size and spatial distribution of nanoparticules and may induce extraordinary changes of local structure of polymer matrix in the vicinity of metal nanoparticles, which act as “concentrators” of the absorbed energy. This specific effect can be described as “radiation-chemical contrast”, and it opens novel opportunities for effective control

1. INTRODUCTION Unique electrophysical properties and high catalytic activity of polymer materials containing metal nanoparticles have stimulated continuous interest to synthesis and investigation of metal−polymer nanocomposites with variable structure and distribution of nanoparticles.1,2 Among different approaches to fabrication of such materials, the radiation-chemical reduction of metal ions was found to be a promising tool due to the feasibility of controlling the generation of reducing agents and reduction rate. The mechanism of the radiation-induced reduction of metal ions and formation of metal nanoparticles was studied in detail,3−9 and potential applications were outlined.10−12 It is worth noting that the cited works were carried out with γ-irradiation or fast electrons. A similar-type approach was used in our laboratory for a single-stage synthesis of nanocomposites containing copper and nickel nanoparticles from interpolyelectrolyte films in an aqueous-alcohol environment.13 Meanwhile, to our knowledge, the applications of X-ray radiation for reduction of metal ions are still rather limited. Muller et al. have introduced this method for surface treatment of a Langmuir monolayer leading to the formation of nanostructured films,14 and later it was extended to the preparation of nanoparticles in bulk aqueous solutions.15 Also, the irradiation with soft X-rays emitted by the XPS spectrometer source was used for surface reduction of noble metal ions embedded in a polymer matrix resulting in the © 2013 American Chemical Society

Received: September 12, 2012 Revised: February 19, 2013 Published: February 27, 2013 7286

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of formation and assembling of metal nanoparticles in polymer matrices.

(items in parentheses stand for the radiation-chemical yields in species per 100 eV of absorbed energy, 1 molecule/100 eV ≈ 10−7 mol/J). The hydrated electrons and H-atoms usually act as reducing agents for parent metal cations, while the OH radicals may act as oxidizers for metal atoms and ions in intermediate oxidation states. In order to increase the efficiency of reduction, it is practicable to use the scavengers of OH radicals (e.g., aliphatic alcohols or formate ions20). In particular, in the present case, ethanol completely converts the OH radicals to the CH3·CHOH radicals with reducing properties. Basic scheme of the processes leading to the formation of metal clusters and nanoparticles is given elsewhere.6,20 In general, the formation of metal nanostructures in interpolyelectrolyte films irradiated in aqueous−organic media is determined by the diffusion of reducing species and the transport of metal ions throw ligand sites of the polymer matrix.19 Different mechanisms of cluster formation may lead to different structure pattern. In particular, in the case of Cu2+, the nanoparticles are localized selectively at the subsurface of X-ray irradiated films,18,19 while the reduction of Ag+ results in the occurrence of regular zones for the formation and growth of metal nanostructures in the polymer matrix.19 3.1. Complexes PAA−PEI−Cu2+. The micrographs shown in Figure 1 demonstrate the development of nanostructure depending on the irradiation time for the swollen film PAA− PEI−Cu2+. Figure 1a shows the initial formation of ultrafine nanoparticles in the subsurface film layer with a thickness of 15−20 nm after a 45-min exposition to X-rays . Increasing irradiation time to 90 min leads to increase in the number of nanoparticles localized selectively in the layer with the thickness of 30−40 nm and efficient growth of these nanoparticles. At more prolonged irradiation (180 min, Figure 1c) the depth of the layer filled by nanoparticles increases to ca. 100 nm. The microdiffractograms (Figure 2) clearly demonstrate the metal nature of nanoparticles revealing reflections corresponding to the interplanar distances in the nearly undistorted copper lattice (2.08, 1.81, 1.28, and 1.09 Å). Regarding the localization of nanoparticles in the vicinity of film surface, first of all, it cannot be explained by the small penetration depth of the X-ray radiation used in this study. Indeed, in the case of X-rays with E = 20 keV, the length corresponding to one-half attenuation in aqueous and organic media is definitely above 7 mm,21 which is incomparable with the film thickness (0.3 mm). Therefore, such pattern of nanostructure in the polymer films may be explained basically by two reasons: (1) peculiarities of metal ions reduction and cluster generation in irradiated heterogeneous system and (2) specific features of the mechanism of X-ray interaction with matter. As will be shown below, both reasons are crucial. As to reason 1, in fact, the diffusion of reducing agents generated by irradiation from aqueous−organic media surrounding the films provides favorable conditions for the formation of clusters in the subsurface layer. It is worth noting that the formation of copper clusters is a slow process due to specific features of the two-stage reduction of copper(II) ions.20 The growth of clusters is essentially determined by the transport of copper ions through the polymer matrix, that is, formation of metal nanoparticles near the film surface should be accompanied by exhausting the bulk film. The effect of formation of nanoparticles selectively in the subsurface film layer was first observed in our recent study of the X-ray radiolysis of interpolyelectrolyte film with relatively

2. EXPERIMENTAL SECTION The films of stoichiometric interpolyelectrolyte complexes were obtained by evaporation of the solvent from a 50% formic acid aqueous solution containing 1 mol/L of PAA and PEI. The following reagents were used to prepare the films of interpolyelectrolyte copper complexes: analytical-grade ethanol, copper sulfate, and silver nitrate; poly(acrylic acid) (Mw = 80 000) manufactured by the Institute of Polymer Chemistry and Technology (Dzerzhinsk, Russia); and polyethyleneimine (Mw = 60 000, Serva). The dried films were cross-linked by annealing for 10 min at 433−438 K for improvement of their mechanical characteristics.17 The resulting films were washed with distilled water to neutral pH. The samples of triple polymer−metal complexes (TPMC) with 25 wt % of Cu2+ and 16 wt % of Ag+ were obtained by sorption of copper ions from the aqueous solution of copper sulfate or silver nitrate. The swollen films of PAA−PEI−Cu2+ and PAA−PEI−Ag+ complexes were irradiated in aqueous−ethanol mixture (10% ethanol) by X-rays with a 5-BKhV-6W tube with a tungsten anode (applied voltage was 30−33 kV, anode current ca. 70 mA). In order to avoid oxidation of the nanoparticles by air oxygen dissolved in the aqueous-alcohol medium, the solution was bubbled with argon of the special purity grade. The dose rate of ca. 20 Gy/s was determined using a ferrosulfate dosimeter irradiated in the same geometry as the test sample. The radiation-chemical yield of Fe3+ ions was taken as G(Fe3+) = 14.5 ions per 100 eV of absorbed energy for X-rays with an effective energy of 20 keV. The actual dose rates for the systems containing metal ions were calculated taking into account the mass absorption coefficients (see below). The shape and size of nanoparticles were characterized with a Leo-912 AB OMEGA transmission electron microscope (TEM) at a resolution of 0.3 nm. 3. RESULTS AND DISCUSSION The PAA and PEI macromolecules yield homogeneous interpolyelectrolyte complexes17 due to cooperative ionic interaction between the units of polyanions and polycations. The functional groups of interpolyelectrolyte complexes may serve as effective ligands for binding of metal ions inside the polymer matrix. The complexes PAA−PEI−metal ions used in this work form samples insoluble in aqueous−organic media with high dialysis permeability for low-molecular-weight polar compounds and ions.17 Due to these properties, they can be used as reaction media for chemical or radiation-chemical reduction of metal ions and obtaining and stabilization of nanoparticles.13,17−19 The irradiation leads to the formation of reactive species both in polymer film and in outer environment. As revealed by previous analysis,17 the reactions of reducing radicals formed from aqueous alcoholic medium play a dominating role in the radiation-induced reduction of metal ions in the swollen interpolyelectrolyte films. The formation of principal products of water radiolysis occurring in a microsecond time scale may be described by the following wellknown scheme:6,20 H 2O ⇝ e− aq(2.8), OH(2.8), H(0.6), H 2(0.45), H 2O2 (0.75)

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Figure 2. Microdiffractogram of the PAA−PEI−Cu2+ complex sample irradiated with X-rays for 180 min.

and small particles (1−3 nm). Further increase in the irradiation time to 180 min (Figure 1c) leads to the growth of primary nanoparticles up to 30−50 nm and an increase of the number of small particles. Simultaneously, the number of small nanoparticles increases progressively. Thus, it appears that the existence of large nanoparticles promotes the formation of ultrasmall particles. This effect is characteristic of X-ray radiolysis and it is not observed in the same system subjected to γ-irradiation, even at high doses. Its meaning will be discussed below. An illustrative result of local enhancement of the radiationchemical effects is presented in Figure 3, which shows strong

Figure 3. TEM micrographs of the cutoff of a PAA−PEI−Cu2+ film irradiated with X-rays for 180 min. Scale bar = 500 nm. Figure 1. TEM micrographs of the cutoff of a PAA−PEI−Cu2+ sample irradiated with X-rays for (a) 45, (b) 90, and (c) 180 min. Scale bar = 50 nm.

damage of the film surface with a regular etched layer of the depth of 700−800 nm at high doses of irradiation (180 min). This phenomenon actually reveals the scale of the radiationinduced processes initiated by X-rays in the system and study. 3.2. Complexes PAA−PEI−Ag+. In the case of complexes with silver ions, the nanostructure pattern was found to be somewhat different. The basic difference results from the fact that the radiation-induced reduction of silver ions is a singlestage process, and the formation of silver clusters is fast.6 For

low initial content of copper ions (4 wt %).18 Meanwhile, the present study using the sample with high content of copper ions reveals specific evolution of size distribution of nanoparticles, which can be explained only by the peculiarities of Xray absorption. This is particularly visible for prolonged irradiation. Indeed, irradiation for 90 min (Figure 1b) results in the appearance of both relatively large particles (15−20 nm) 7288

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Figure 4. TEM micrographs of the cutoff of a PAA−PEI−Ag+ sample irradiated with X-rays for (a) 15, (b) 30, (c) 60, and (d) 90 min. Scale bar = 100 nm.

this reason, the formation of silver nanoparticles occurs not only in the subsurface layer, but also in the bulk of the films. The TEM images of the samples irradiated to different doses (Figure 4) demonstrate development of the nanostructure pattern in different zones of films. The intense formation of nanoparticles occurs in the surface layer of films due to favorable conditions for reduction of silver ions near in the interface region, similar to the case of reduction of copper ions. Increase of the irradiation time from 15 to 30 min leads to growth of nanoparticles from 2−6 to 20−40 nm (Figure 4a,b). Irradiation for 60 min results in further increase of nanoparticle size up to 60 nm and higher filling of subsurface layer with nanoparticles (Figure 4c). However, in the case of longer irradiation (90 min), the TEM image does not demonstrate nanoparticles in the subsurface layer. In this case, the damaged layer is just removed from the surface, which looks like a sharp cutoff (Figure 4d). Again, the metal nature of nanoparticles is confirmed by the microdiffraction data (Figure 5), which demonstrate the reflections corresponding to the interplanar distances of the crystal lattice of metal silver (2.36, 2.04, 1.45, 1.23, and 1.17 Å). Meanwhile, the most intriguing events occur inside the polymer matrix (Figure 6). As the irradiation dose increases, one can see rapid growth of primary metal nanoparticles and the formation of new (secondary) small nanoparticles. The growth of primary nanoparticles up to ca. 30 nm occurs at the irradiation time up to 30 min. Increasing irradiation time from

Figure 5. Microdiffractogram of the PAA−PEI−Ag+ complex sample irradiated with X-rays for 90 min.

30 to 90 min leads to both growth of existing nanoparticles and formation of new small nanoparticles. It is worth noting that the total mass of metal silver increases, which implies progressive reduction of silver ions in the bulk of swollen film. This process occurs due to migration of metal ions through ligand sites of polymer film. The ion transport should be rapid enough to provide compensation for local exhausting 7289

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and small nanoparticles surrounding large primary nanoparticles with sizes 30−60 nm. Thus, similar to the case of copper, the formation of primary silver nanoparticles results in dramatic local sensitization of the radiation-induced processes and, therefore, provokes intensive reduction of silver ions in the environment. High rate of generation of silver clusters leads to formation of ultrasmall particles, which act as precursors for further assembling of larger particles at later stages of irradiation in specific “microcontainers”. However, it is worth noting that the scale of local processes induced by the presence of nanoparticles in the case of silver is only 30−50 nm, that is, much smaller than that in the case of copper. 3.3. The Radiation-Chemical Contrast: The Effect of Absorber Nature and Photon Energy. The results obtained in this work clearly reveal unique pattern of development of metal nanostructures upon X-ray induced reduction of copper and silver ions in the swollen films of interpolyelectrolyte complexes. This pattern is specific only for X-ray radiolysis, and it depends strongly on the nature of metal ion. An important point in further consideration is concerned with the mode of local energy deposition determined by the specific features of interaction of X-rays with condensed matter. In the energy range used in this work, the principal mechanism is photoelectric effect. In this case, the atomic absorption crosssection τp above the energy of the K-edge (EK) can be approximated as τp ∼ Z 4 /Ep3

(I)

(Z is an atomic number, and E is a photon energy). Since τp is approximately proportional to Z4, the mass absorption coefficient (μ/ρ) for most elements increases roughly as Z3. As shown in Table 1, this approximation works relatively well 21

Table 1. Mass Absorption Coefficients of X-ray Photons for Different Elements in the Energy Range of 10−30 keV22 μ/ρ, g/cm2 Ep, keV

Ag

Cu

C

N

O

30 20 15 10

16.6 16.9 38 115

9.3 28 58 148

0.066 0.22 0.56 2.08

0.10 0.39 0.97 3.5

0.17 0.62 1.55 5.6

(within 20%) for copper and lighter elements with EK < 10 keV (some deviation from I is observed for copper in the vicinity of the K-edge, i.e., at Ep = 10 keV, which is not crucial for further semiquantitative consideration). On the other hand, in the case of silver, EK is high enough (≈ 25.5 keV) and, for this reason, mass absorption coefficients are lower than those for copper at Ep ≤ 20 keV. The consequences of this difference will be considered in detail below. Anyway, one may note that, due to huge difference in the absorption coefficients, the heavy atoms (copper or silver) act as “chromophores” for X-rays. From the point of view of radiation chemistry, this effect should result in two basic consequences. First, the total absorbed dose and, hence, the total reduction rate should increase with increasing the concentration of heavy ions, as was actually demonstrated for copper ion reduction in our previous work.23 Second, this should naturally lead to a nonuniform dose rate distribution at the microscopic scale in the heterogeneous system under study, which is the focus of our present consideration. Indeed, the concentration of metal ions is nearly

Figure 6. TEM micrographs of the cutoff of a PAA−PEI−Ag+ sample irradiated with X-rays for (a) 30, (b) 60, and (c) 90 min (inner part). Scale bar = 50 nm.

of silver ions in the vicinity of growing clusters and nanoparticles. The dramatic difference in the mobility of metal ions and large clusters explains both growth of large nanoparticles and the formation of new small nanoparticles. In other words, clusters and nanoparticles “pump out” the silver ions from the film bulk, and it works up to rather high degree of ion reduction. The micrographs (Figure 6) clearly demonstrate the formation of contrast “clouds” of secondary silver clusters 7290

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zero in the outer environment and sharply increases in the interpolyelectrolyte complex films. Considering the films containing copper ions, one should keep in mind that EK ≈ 9 keV for copper.22 Above this threshold, photon interaction mainly occurs with K-electrons, and the kinetic energy of photoelectrons Ee is given by Ee ≈ Ep − E K

It is clear that a major part of absorption is due to low-energy photons. Indeed, using eq V, we can estimate the contribution of photons with Ep < E′ (below the maximum in Figure 7) to the total absorption E0 ln N (E′) = N (E 0 ) E0 ln

(II)

Each photoelectron typically produces several hundreds of ionizations in the local environment, so these secondary photoelectrons actually determine the energy deposition and the absorbed dose rate profile in the sample. Accurate numeric calculation of the dose rate distribution in our experimental system is a very difficult task because of a large number of uncertain factors (the contribution of Auger electrons, build-up factors, cross sections for interactions of low-energy electrons, etc.). Meanwhile, at this stage we can restrict our consideration to a qualitative picture. The X-ray source yields a continuous spectrum of photons with the spectral distribution function Y (Ep) = C(E0 − Ep)Ep2

(III)

Figure 7. Intensity distribution of white X-radiation (bremsstrahlung) with Emax = 30 keV. The positions corresponding to K- and L- edges of Cu and Ag are marked with arrows.

absorption cross-section drops sharply, when the photon energy becomes below the binding energy for K-electrons since the interaction with L-electrons is less effective. Thus, in first approximation, we can consider only the interactions with photons at Ep ≥ EK. In this case, combining eqs I and III, one can obtain the spectral distribution of absorbed photons under irradiation with a continuous X-ray spectrum (IV)

where EK ≤ Ep ≤ E0 and B is constant for a given absorber. The total number of absorbed photons is expressed as N (E 0 ) =

∫E

E0 K

− E′ + E K − E0 + E K

(VI)

In our case (E0 = 30 keV, E′ = 20 keV, and EK = 9 keV), one can see that >80% interactions are induced by photons with Ep < 20 keV. Accordingly, taking into account eq II, we can conclude that at least 80% of photoelectrons possess kinetic energy Ee ≤ 11 keV. These electrons with low penetration ability are responsible for local energy deposition. Indeed, the distances corresponding to the 90% absorption of electron energy in water are ca. 2 μ, 700 nm, and 50 nm for electrons with initial kinetic energy Ee = 10, 5, and 1 keV, respectively.25 This means that most of the photoelectrons produced from the copper ions on the film surface deposit their energy within a thin solution layer le (several hundreds of nanometers). Furthermore, the angular distribution of photoelectrons (preferential ejection is normal to the X-ray beam axis, i.e., in the film surface), makes this thickness even smaller. Meanwhile, it is worth noting that this picture corresponds to the initial stage of the process (short irradiation time), when the film is nearly virgin. As mentioned above, due to diffusion reasons, the formation of larger clusters should occur first at the interface between film and solution. In the case under consideration, this will lead to enhancement of the local absorbed dose in this region because of increased local concentration of absorbers (heavy copper atoms included in clusters). In turn, the growth rate in this layer will again increase because of higher local concentration of reducing species produced by radiolysis. This implies an autocatalytic ef fect in the formation of copper clusters and nanoparticles in the surface region, which is specific for X-ray irradiation. Indeed, as stated above, this effect was not observed for nanoparticles produced by γ-irradiation in the same system.13 Also to be mentioned, the nanoparticles obtained under X-ray irradiation are substantially larger that those observed in the γirradiated systems. Thus, the initial dose rate distribution is modified during irradiation, resulting in the appearance of a maximum in the surface region (within ± le from the film surface). This local dose enhancement is illustrated well by the picture observed after prolonged irradiation (Figure 1 b,c). Concentration of deposited energy in the surface layer results in extensive radiation-induced damage of the film edge with the etching depth of ca. 700−800 nm (Figure 3), that is, roughly comparable with the le value estimated above. As mentioned previously, in the case of silver, the situation appears to be different, because the K-electrons have substantially larger binding energy (between 22 and 25.5 keV, with the K-edge energy EK ≈ 25.5 keV). This means that major fraction of photons available from the source (>75% as follows from integration of eq III) may interact only with L-electrons (EL ≈ 3.8 keV), and these interactions can be no more neglected, even though their cross-section is lower. The resulting photoelectrons should have energy up to 18 keV, thus providing more diffuse dose rate distribution. On the other hand, there is still a fraction of photons with Ep > EK with high absorption cross-section. They produce low-energy photoelectrons with Ee < E0 − Ek ≈ 4.5 keV. Using formula VI, as

where E0 is maximum photon energy determined by the applied voltage and C is constant depending on anode material and anode current value.24 It is easy to see that Y(Ep) exhibits a maximum at Ep = E′ = (2/3) E0, that is, in our case (E0 ≈ 30 keV), E′ ≈ 20 keV (Figure 7). It is worth noting that the

P(Ep) = B(E0 − Ep)/Ep

E′ EK E0 EK

⎞ ⎛ E P(Ep) dEp = B⎜E0 ln 0 − E0 + E K ⎟ EK ⎠ ⎝ (V) 7291

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significant enhancement of local dose rate and the rate of the radiation-induced reduction of metal ions in the vicinity of nanoparticles, which leads to the formation of new (“daughter”) nanoparticles. This effect is related to local sensitization of environment to X-ray irradiation by pre-existing metal nanoparticles, which was reported in recent studies on biomolecules26 and radiation-induced polymerization.27 An important feature of the effect is that the spatial distribution of radiation-induced events is determined by the photon energy and electronic structure of absorber. Furthermore, in the case of white radiation (bremsstrahlung), interaction of photons with K- or L-electrons may lead to selective response in terms of kinetic energy of secondary electrons. Thus, adjusting the maximum energy of photons (applied voltage) or the composition of the system, one can tune the energy of secondary electrons and, therefore, the scale of the radiation-chemical effects and nanostructure pattern. Finally, we may note that the potential applications of the radiation−chemical contrast are definitely not limited to the fabrication of nanohybride structures from heterogeneous systems. In particular, they may include selective radiation sensitization of nanocomposites, controlled modification of surfaces and interfaces between nanoparticles and organic matrices, lithography, and other kinds of X-ray induced manipulations with organized systems. In general, it may be considered as further implementation and development of the fascinating concept of X-ray induced nanochemistry.27

described above, we can estimate that more than 80% of electrons ejected from K-shells of silver should have Ee < 2 keV. This may be the reason for the very local energy deposition (within several tens of nanometers). The effect of these local processes should be particularly important under prolonged irradiation, when relatively big silver nanoparticles already exist. Due to the mechanism described above, such primary nanoparticles should act as absorbers of X-rays generating low-energy electrons, which produce ionization in the film at a short distance (within several tens of nanometers). As a result, one can see the production of silver clusters and ultrasmall secondary nanoparticles surrounding big primary nanoparticles, which is illustrated by Figure 6. Thus, a specific size distribution of silver nanoparticles in the systems under consideration is related to bimodal energy distribution of secondary electrons resulting from interaction of X-rays with L- and K-electrons, respectively. It is worth noting that the linear energy transfer function (−dE/dx) for electrons with initial energy of several kiloelectron-volts increases with decreasing their energy (see, e.g., ref 21). From general consideration, one might expect maximum density of deposited energy and maximum local concentration of active species (radicals) at a certain distance from primary nanoparticle. On the other hand, the nanoparticle size should be significant because of self-attenuation of the secondary electrons. Thus, an optimal size of nanosensitizers providing maximum emission of the X-ray induced electrons into the matrix should be on the order of several nanometers to several tens of nanometeters, that is, comparable to the path length of low-energy secondary electrons in dense media. Larger particles (R ≫ le) would provide attenuation of secondary electrons, while filling the surface layer with large nanoparticles finally just builds a “radiation shield” for X-rays. Obviously, the formation of new nanoparticles will be affected by other factors, including kinetics and peculiarities of ion and cluster transport. In our view, it is impossible to give a detailed picture for this complex system, so we stay with the estimate of the scale of effects, which appears to reasonably explain the experimental findings. The key point is that only consideration of the electron energy deposition length may explain the scale of observed chemistry, specific for X-ray irradiation, and its dependence on the absorber nature.

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AUTHOR INFORMATION

Corresponding Author

*Mailing address: Department of Chemistry, Lomonosov Moscow State University, Moscow 119991 Russia. Tel.: +7495-9394870; Fax: +7-495-9394870; E-mail: feldman@rad. chem.msu.ru. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by The Russian Foundation for Basic Research (Projects 11-03-91374-CT_a and 12-03-00762-a).

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CONCLUSIONS In summary, we have to emphasize specific meaning and implications of the effect of “radiation-chemical contrast” revealed in this work. First, it is worth mentioning that most previous studies focused on manipulations with the radiationinduced chemical reactions for controlling the growth and distribution of metal nanoparticels in solutions and films, whereas the results obtained in this work clearly show an impact of the mode of energy absorption and transfer (physical stage) for X-ray irradiation. On the other hand, it should be stressed out that the X-ray induced formation of nanostructure in heterogeneous systems is essentially different from simple “physical imaging” (like in a solid-film lithography). Indeed, the structure pattern is determined by both selective absorption of X-ray photons due to effect of heavy atom and peculiarities of diffusion-controlled chemical reactions leading to the growth of nanoparticles preferentially in the interface layer. The key point is an autocatalytic ef fect occurring at a certain stage of irradiation, when compact metal nanoparticles start to act as strong collective absorbers of X-ray photons. This results in

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3090765 | J. Phys. Chem. C 2013, 117, 7286−7293