Radiation chemistry of nanocolloids and clusters
J. Belloni and M. Mostafavi
Laboratoire de Physico-Chimie des Rayonnements, UMR 8610 UPS-CNRS, Université Paris-Sud, 91405 Orsay, France
For more than two decades, extensive research work has been devoted to the unique properties of clusters. They are made of a small number (or nuclearity) of atoms or molecules only, and therefore constitute a new state of matter, or mesoscopic phase, between the atom or molecule and the crystal. New methods have been developed in physics and chemistry for their synthesis, their direct observation, the study of their properties, and of their crucial role in number of processes, such as phase transition, catalysis, surface phenomena, imaging. Owing to its specific approach, radiation chemistry offered first the opportunity to reveal the existence of nuclearity-dependent properties of clusters and has then proven to be a powerful method to study the mechanisms of cluster formation and reactivity in solution.
One century ago, X rays discovered by W. Roentgen, then radiation of radioactive elements discovered by H. Becquerel, were detected through their effects on the material they traversed, specially reduction of silver ions of the photographic plates and ion pair formation in irradiated air. Both effects were therefore used from the origin of the ionizing radiation study to indirectly detect their presence and calibrate their intensity, even if the molecular processes of cluster formation in the photographic
Figure 1. Radiation-induced Agn and (AgAu)n clusters under various conditions. From top : Ag73+ with n = 4, stabilized by PA (partial reduction at 4 kGy h-1), alloyed (AgAu)n,PA with n ≈ 500 (at 8 MGy h-1), alloyed,PVA with n ≈ 105 (at 35 kGy h-1),4 and Agn with n ≈ 108 (partial reduction at 3 kGy h-1 and then developed by EDTA) (see text).
latent image or the effects of radiation were at that time unexplained. Progressively, the complexity of the specific absorption of high-energy radiation by matter, including the non-homogeneous spatial distribution of initial ions and radicals, was better understood, at least in aqueous solutions.
In particular, various metal ions were used widely as radical scavengers and redox indicators in the reduction or oxidation processes induced indirectly by the short-lived primary radiolytic species, allowing their identification and the calibration of their formation yield.,, Metal ions such as those of gold or silver irradiated in solutions by or pulse radiolysis underwent reduction to the zero-valence metal, to form colloids and then precipitates.7,
The radiolytic method of reduction allowed, owing to the accurate knowledge of the dose used, a control of the progressive extent of the reduction and an instantaneous distribution of the reducing agent formed throughout the solution. However, quite often, puzzling data were reported when the zero-valent metal was formed, such as an induction time for precipitation, radiolytic yields sensitive to the initial presence or absence of added particles, and only weakly reproducible., Moreover, oxidation of silver atoms by molecular oxygen was observed, though the process was thermodynamically improbable for a noble metal like silver.10 The explanation of some oxidation observed of newly formed zero-valent silver (latent image regression) in nuclear photographic plates used for long in particle track detection was also a difficult question.12
It was observed, in 1973, that the metal that was expected to arise from the reduction of Cu+, was not found when these ions were used as electron scavengers in the radiolysis of liquid ammonia, despite the fast reduction of metal ions by solvated electrons. Instead, molecular hydrogen was evolved. These results were explained by assigning to the "quasi-atomic state" of the nascent metal specific thermodynamical properties distinct from those of the bulk metal that is stable under the same conditions.14 This concept implied that, as soon as formed, atoms and small clusters of a metal, even a noble metal, may exhibit much stronger reducing properties than the bulk metal, and may be spontaneously corroded by the solvent with simultaneous hydrogen evolution. It also implied that for a given metal the thermodynamics depended on the particle nuclearity (number of atoms reduced per particle), and allowed that to provide rationalized interpretation of other previous data, for example that on the spontaneous oxidation of nascent radiolytic silver by oxygen in water, or its higher stability when produced at the surface of added particles.10,14 Soon, experiments on the photoionization of silver atoms in solution demonstrated that their ionization potential was much lower than that of the bulk metal. Also, it was shown that the redox potential of isolated silver atoms in water must be lowered relative to that of the silver electrode E°(Ag+/Agmet) = 0.79 VNHE, by the sublimation energy of the metal equal to 2.6 V and E°(Ag+/Ag0) = - 1.8 VNHE. In early eighties, an increasing number of experimental works emphasized, for metal or semiconductor particles prepared by various ways in the gaseous and condensed phases, the nuclearity-dependent properties of clusters of atoms or molecules, theoretically predicted earlier by Kubo.
Nuclearity-dependent properties of semiconductor particles may be studied by radiolysis too. For example, a soluble anion-cation couple is transformed by electron scavenging into a pair of low solubility product.
The radiation-induced method, in the - or pulse regime, provides a particularly powerful means to understand the exotic phenomena which occur any time a new phase of oligomeric particles is formed in the bulk of a homogeneous mother phase,,,,, phenomena which are thus rather frequent in physics and chemistry.
2.1. Metal cation reduction
The atoms are produced in deaerated solution by radiation-induced reduction of the metal ion precursors. The species solvated electrons eaq-, and H. atoms arising from the radiolysis of water are strong reducing agents (E°(H2O/eaq-) = - 2.87 VNHE and E°(H+/H.) = - 2.3 VNHE). They easily reduce metal ions up to the zero-valent state :
M+ + eaq- M0 (1)
M+ + H. M0+ H+ (2)
where M+ is the symbol of monovalent metal ions possibly complexed by a ligand. Similarly, multivalent ions are reduced by multistep reactions, also including disproportionation of intermediate valencies. Such reduction reactions have been observed directly by pulse radiolysis for a lot of metal ions. Most of their rate constants are known and the reactions are often diffusion controlled. In contrast, OH. radicals, which are also formed in water radiolysis,25 are able to oxidize the ions or the atoms into a higher oxidation state and thus to counterbalance the previous reductions (1) and (2). For that reason, the solution is generally added with a scavenger of OH. radicals. Among various possible molecules, the preferred choice is for solutes whose oxidation by OH. yields radicals which are unable to oxidize the metal ions but in contrast exhibit themselves strong reducing power such as the radicals of secondary alcohols or of formate anion.
(CH3)2CHOH + OH. (CH3)2C.OH + H2O (3)
HCOO- + OH. COO.- + H2O (4)
H. radicals (E°(H./H2) = + 2.3 VNHE ) oxidize these molecules as well :
(CH3)2CHOH + H. (CH3)2C.OH + H2 (5)
HCOO- + H. COO.- + H2 (6)
The radicals (CH3)2C.OH and COO.- are almost as powerful reducing agents as H. atoms: E°((CH3)2COH/(CH3)2C.OH) = - 1.8 VNHE at pH 7 and E°(CO2/COO.-) = - 1.9 VNHE, respectively. In some cases they can reduce directly metal ions into lower valencies or into atoms for monovalent cations:
M+ + (CH3)2C.OH M0 + (CH3)2CO + H+ (7)
M+ + COO.- M0 + CO2 (8)
In other cases, the reduction proceeds via complexation of the ion with the radical :
M+ + (CH3)2C.OH (M(CH3)2C.OH)+ (9)
(M(CH3)2C.OH)+ + M+ M2 + + (CH3)2CO + H+ (10)
Then the charged dimer is formed.
2.2. Metal atom coalescence
The atoms are formed with a homogeneous distribution throughout the solution. The binding energy between two metal atoms is stronger than the atom-solvent or atom-ligand bond energy. Therefore the atoms dimerize when encountering or associate with excess ions. Then, by a cascade of coalescence process these species progressively coalesce into larger clusters:
M0 + M0 M2 (11)
M0 + M+ M2+ (12)
M n + M+ Mn+1+ (13)
Mm+xx+ + Mn+yy+ Mp+zz+ (14)
Mn+1+ + eaq- Mn+1 (15)
where m , n, and p represent the nuclearities, i.e. the number of reduced atoms they contain, x, y and z the number of associated ions. The radicals (CH3)2C.OH, or COO.- also reduce Mn+1+ as eaq- in reaction (15). The radius of Mn increases as n1/3 (Figure 1). The fast reactions (12) and (13) of ion association with atoms or clusters play an important role in the cluster growth mechanism. Firstly, the homolog charge of clusters slow down their coalescence (reaction (14)). Secondly, the subsequent reduction of the ions fixed on the clusters (reactions (13 and 15)) favors their growth rather than the generation of new isolated atoms (reactions (1, 7, 8)). The competition between reduction of free ions (1, 7, 8) and of absorbed ions (15) is controlled by the rate of reducing radical formation. Coalescence reactions (11 or 14) obey second order kinetics. Therefore, the cluster formation by direct reduction (1, 7, 8) followed by coalescence (11) is predominant at high irradiation dose rate.
However, almost in the early steps, the redox potential of the clusters, which decreases with the nuclearity, is quite negative. Therefore, the growth process undergoes another competition with a spontaneous corrosion by the solvent which may even prevent the formation of clusters, as mostly in the case of non noble metals. Monomeric atoms and oligomers of these elements are so fragile to reverse oxidation by the solvent and radiolytic protons that H2 is evolved and the zerovalent metal is not formed:14
M0 + H+ M+ + H. (16)
H. + H. H2 (17)
For that reason, it is preferable in these systems to scavenge the protons by adding a base to the solution and to favor the coalescence by a reduction faster than the oxidation which obeys first order kinetics (§ 4.2)
2.3. Semiconductor cluster formation
Nanoparticles of semiconductor compounds (MA)n may be formed from scavenging of radiolytic species produced by irradiation. The cationic part M+ is for example provided by a soluble salt, while the anionic part A- is generated by cleavage after electron attachment to a soluble electrophile substitute RA as a precursor. M+ and A- are selected for their very low solubility product :
RA + eaq- R. + A-(18)
M+ + A- MA (19)
MA + MA (MA)2(20)
(MA)m + (MA)n (MA)m + n(21)
Other semiconductor monomers are formed from A- provided by a soluble salt and M+ resulting from the radiolytic reduction (for instance by eaq-) of a higher valency metal ion :
M2+ + eaq- M+(22)
Similarly, reaction (22) is followed by the formation reactions (19 - 21) of the (MA)n cluster. Adsorption of precursor ions M+ (or A-) on clusters confer them identical charges which slow down the coalescence due to electrostatic repulsion. But it favors their growth by further reaction with A- (or M+) (ripening) :
(MA)n + M+ (MA)n M+(23)
(MA)n M+ + A- (MA)n+1(24)
Nucleation by reactions (19, 20) is in competition with growth by (23, 24) and is favored by fast A- generation, thus at high dose rate. Multivalent anionic Ay- and cationic Mx+ precursors react also by successive ion fixation on the growing cluster according to the stoichiometry and yield eventually (MyAx)n clusters.
2.4. Cluster stabilization
Metal atoms or semiconductor monomers formed by irradiation or any other method tend to coalesce into oligomers which themselves progressively grow into larger clusters and eventually into precipitates, as found in early radiolytic experiments. However, for studying stable clusters or for applications, the coalescence must be limited, by adding a polymeric molecule acting as a cluster stabilizer. Functional groups with high affinity for the metal ensure the anchoring of the molecule at the cluster surface while the polymeric chain protects the cluster from coalescing with the next one and thus inhibits at an early stage further coalescence through electrostatic repulsion or steric hindrance. Some of these polymeric systems are at the same time the stabilizer and the reducing agent used to chemically synthesize the metal clusters. When metal or semiconductor clusters are to be prepared by irradiation, the stabilizing polymers must be selected instead for their unability to directly reduce the ions before irradiation. Poly(vinyl alcohol) (PVA), sodium dodecyl sulfate (SDS),29, sodium poly(vinylsulfate) (PVS), poly(acrylamide) (PAM), or poly(N-methylacrylamide) (PNPAM),32 carbo-wax, poly(ethyleneimine) (PEI),, polyphosphate (PP), gelatin,, do not reduce ions and fulfill the conditions for the stabilization. Some functional groups such as alcohol are OH. scavengers and may contribute to the reduction under irradiation. The final size of metal clusters stabilized by these polymers lies in the nanometer range. Sodium polyacrylate (PA) is a much stronger stabilizer which allows the formation of long-lived metal oligomers (Figure 1).3,
Some ligands (e.g. CN-, or EDTA5) are able by themselves to stabilize small sized particles (Figure 1). The coalescence of atoms into clusters may also be restricted by generating the atoms inside confined volumes of microorganized systems or in porous materials. The ionic precursors are included prior to penetration of radiation. The surface of solid supports adsorbing metal ions is a strong limit to the diffusion of the nascent atoms formed by irradiation at room temperature, so that quite small clusters can survive. The stabilization of radiation-induced clusters at the smallest sizes by a polymer or a support is the way to benefit the specific properties appearing for the lowest nuclearities.
3. METAL CLUSTER NUCLEATION AND GROWTH
3.1. Cluster nucleation
As the early species produced after reduction such as atoms, dimers and oligomers are short-lived, time-resolved observations of the reactions of these transients are carried out by the pulse radiolysis method, coupled with optical absorption or conductivity.25 Generally, kinetics are studied in the absence of oxygen or stabilizer unless their specific interaction has to be known. The earliest10,11,16 and most complete data38,,, on the nucleation mechanism were obtained on silver clusters. Indeed, silver may be considered as a model system owing to the one-step reduction of the monovalent silver ions, hydrated or complexed with various ligands, and to the intense absorption bands of the transient oligomers and final clusters. As for other metal oligomers, the specific feature of the spectral properties is to be nuclearity-dependent. The wavelengths of the absorption band maxima of the atom Ag0 and of the
Figure 2. Optical properties of hydra-ted silver clusters.
Top : Transient absorption spectra of silver oligomers.22
Bottom : Calibrated absorption spectra of Ag0, Ag2+ and Ag32+ in water obtained by pulse radiolysis.
charged dimer Ag2+ (Figure 2), and the rate constants of their formation (reactions 1 and 12, respectively) in aqueous solutions are given in Table 1. Ag32+ is formed by reaction of Ag2+ with an additional Ag+ cation (Figure 2). Atoms and charged dimers of other metals are formed by homolog reactions (Table 1).
Formation rate constants (mol l-1 s-1) and optical absorption maxima of metal atoms, hydrated or complexed, and of the corresponding charged dimers in water.Metal ions / kM++ eaq- / (nm) of M0 / kM0+ M+ / (nm) of M2+
Ag+ / 3.6 x 1010 / 36016, 47 / 8 x 109 / 290, 31516, 47
Tl+ / 3 x 1010 / 450, 260, / 1.4 x 109 / 700, 420, 24548,49
In+ / - / 50060 / 1.5 x 109 / 310, 46060
Ag(CN)2- / 5 x 109 / 450, 50056 / 2 x 1010 / 350, 410, 49056
Ag(EDTA)3- / 1.7 x 109 / 400, 45056 / 1.6 x 109 / 310, 340, 400, 47556
Ag(NH3)2+ / 3.2 x 1010 / 350,11 38557 / - / 315, 34058
Au(CN)2- / 1.1 x 1010 / 420,59 / - / -
AgI,PA / 3.6 x 109 / 36051 / 8.9 x 109 / 310, 450
AgI,gelatin / 1.1 x 1010 / 36038 / 1.1 x 1010 / 290, 315, 308, 32538
Cu(Cl)32- / 2.7 x 1010 / 38060 / 4.9 x 107 / 36060
The band maxima of metal species are different in the gaseous and condensed phases. The absorption bands of Ag0 and Ag2+ are highly dependent on the environment. They are red-shifted with the decreasing polarity of the solvent as in EDA and liquid NH3, where they appear at a longer wavelength than in water. Moreover, the maximum in NH3 is red-shifted with increasing the
Figure 3. Absorption spectra of silver atom Ag0 (top) and charged dimer Ag2+ (bottom) complexed by CN-,56 EDTA,56 and NH357,58 in solution. The spectra of uncomplexed Ag0 and Ag2+ are shown for comparison.47
temperature. Electron spin echo modulation analysis of Ag0 in ice or methanol glasses has concluded to a charge transfer character to solvent (CTTS) of the absorption band. As shown in Table 1 and Figure 3, the interaction of ligands CN-, NH3, or EDTA with the atom or the dimer has also a strong influence on the absorption spectra.
The transient product of the reduction of complexed silver ions is not the isolated atom but a complexed silver atom, Ag0(CN)22-, Ag°(NH3)2, or Ag0(EDTA), 56, respectively, as well as for Au°(CN)22-  or CuCl33-. Though less complete, the results on the reduction of other monovalent metals cations into atoms,26 such as Tl0, In0, Au0(CN)22-, Cu0Cl33-, are comparable with silver (Table 1).
The multistep reduction mechanism of multivalent cations are also partially known from pulse radiolysis studies.26 For example, the reduction of AuIIICl4- into AuIICl3- and the disproportionation of AuII into AuI and AuIII have been directly observed and the rate constants determined. However, the last step of reduction of AuI complexed by Cl- into Au0 is not observed by pulse radiolysis because the eaq- scavenging by the precursors AuIII is more efficient. Moreover, the disproportionation of AuI or of other monovalent cations is thermodynamically hindered by the quite negative value of E°(MI/M0) (§ 4).
3.2. Cluster growth
After reactions (1-12), the reaction of Ag2+ with Ag+ yielding Ag32+ (max = 315 and 260 nm)47 or its dimerization into Ag42+ (max = 265 nm) and the multi-step coalescence of oligomers result in clusters of increasing nuclearity
Figure 4. Growth kinetics of silver clusters observed through their absorbance at 400 nm in the presence of (a) cyanide or (b) sulfate.65
Agn (reaction 14). The absorption spectrum is shifted to the surface plasmon band at 380-400 nm (§ 5). It is known indeed according to Mie theory and its extension that the interaction of light with the electrons of small metal particles results in an absorption band whose shape and intensity depend on the complex dielectric constant of the metal, the cluster size and the environment. During the coalescence, the total amount of silver atoms formed by the pulse is constant, but they are aggregated into clusters of increasing nuclearity with a decreasing concentration. Thus the absorbance increase observed at 400 nm (Figure 4) is assigned to the increase with the nuclearity of the extinction coefficient per silver atom. It was shown from the kinetics analysis that the plasmon band is totally developed with the constant value per
atom = 1.5 x 104 l mol-1cm-1 beyond n = 13. Then, the coalescence into larger clusters is still continuing, though the spectrum is unchanged. Note that the coalescence rate constant depends on the ligand as shown in Figure 4.  At the same initial concentration of atoms, the plateau is reached at almost 103 longer time for Agn(CN-) than Agn(SO4 2-).