Oxidation Of Cu Clusters

The study of chemical reactions in the case of particles prepared by inert gas aggregation is discussed for Cu in the case of oxidation. Two oxides are discussed, cuprite (Cu2O) and tenorite (CuO), although the latter has not been observed directly. Only one example of a transition between cuprite and tenorite will be shown. Transitional states such as Cu4O, Cu8O, Cu64O [48, 49], and paramelaconite (Cu3O4) [50] are observed, and complex structures like suboxides are also present. They will not be discussed in the present work. The oxidation can be performed in different ways:

(a) Additional insertion of O2 at controlled partial pressure during the evaporation process to argon.

Figure 26. HRTEM image (a) of a Ag cylinder deposited on NaCl (100). (b) Detail. (c) PS. Reprinted with permission from [47], N. Nepi-jko et al., Chem. Phys. Chem. 3, 140 (2000). © 2000, Wiley-VCH.

(b) Exposure of deposited clusters to oxygen (in the present examples to air at ambient pressure and temperature) for several hours.

In general, it can be concluded that with the addition of very small amounts of oxygen on the order of less than 10-3 mbar to the argon at about 1 mbar partial pressure (cf. case (a)), the structure is modified even for smaller particles less than 5 nm in diameter. No icosahedra or deca-hedra were observed after this treatment. Only cuboctahe-dral particles with the Cu lattice parameters were detected. The reactivity of these clusters is also decreased. Even after exposure to air at ambient temperature for more than 100 h, splitting the reflections in the PS could very often prove the existence of Cu and oxide. One example is given below. It is assumed that small amounts of oxygen are built into the lattice of Cu, leaving the lattice constants like those of Cu. Therefore, this cannot be deduced by HRTEM. Increasing the amount of oxygen during the evaporation period leads to the creation of both Cu and oxide in the same particle or to the total oxidation of the particle, which can be directly observed from the PS by splitting the reflections or by observing only the cuprite reflections, respectively.

In the other case (i.e., case (b), preparation without oxygen but exposure to air) this process causes a total oxidation after 100 h at room temperature. Transitional states showing the appearance of Cu and Cu2O reflections can also be observed after less than 100 h of air exposure. Some examples are given in the following.

Figure 27 shows a transition between Cu and Cu2O. The preparation was performed without oxygen but after air exposure for 26 h. Two pairs of reflections are observed which represent the 111 reflections of the metal and its oxide. The corresponding planes are slightly misoriented. They form an angle of 2° with respect to each other. The corresponding lattice parameters are 0.218 nm for Cu and 0.253 nm for cuprite (for the bulk materials they are 0.209 and 0.247 nm). Figure 27c and d shows the Fourier filtered image and the PS after filtering. The filtering was performed with both reflections e and f. Images e and f

2 nm

. . . ; " i -a a


r b

f d

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