Oxidation properties of Au nanoparticles

To shed light on the nature of oxygen species formed by exposing various Au nanostructures to atomic oxygen atmosphere, XPS was used. When the nanostructured Au film (Fig. 5.4; sample C) was oxidized, increase of a single peak centered at 529 - 530 eV can be observed in the O 1s state in Fig. 5.5 c. A shoulder at higher binding energies can be observed in the Au 4f states (Fig. 5.7 c; square zone), which could not be observed before the oxidation. At the same time, the main Au 4f peak decreases in intensity upon oxidation. The appearance of the shoulders at higher binding energies in the Au 4f states upon oxidation has been widely interpreted as formation of Au2O3 (Au (III)).

Oxidation of metal generally results in the appearance of shoulders at higher binding energies, or shifts of the metal core levels to higher binding energies, which can be understood in terms of strong metal to oxygen charge transfer (initial state effects) in combination with final state effects. It is worth emphasizing that XPS is one of the most widely used experimental methods to identify metal oxide species in metal catalysts [26, 265]. Also for Au nanoparticles, Au (III) has been generally characterized using XPS, allowing us to attribute the shoulder of the Au 4f level to the Au (III) species [44]. The quantitative analysis of the Au (III) species in the Au 4f states and the O 1s signals also suggests a stoichiometry of ~ 2 : 3 for Au : O, in line with Au2O3

formation. For this quantitative analysis, the cross sections of Au4f and O1s in the photoemission process were assumed to be 3 : 10 [183].

Fig. 5.5 O1s XPS spectra of oxidized various Au nanostructures on HOPG.

(before subtraction of the O/HOPG signals (red zone) from O/Au/HOPG) a) O1s peak for sample A (~ 5 nm in diameter), b) sample B (~ 8 nm in diameter) and c) sample C (~ 23 nm in diameter)

For the Au nanoparticles with apparent lateral size of ~ 8 nm (sample B), exposures of the sample to atomic oxygen result in increase of three different states in the O 1s spectrum (Fig. 5.5 b). The O 1s peaks centered at 533 - 534 eV can be attributed to the O on/in HOPG (red zone in Fig. 5.5 b), since exposure of the sputtered bare HOPG to atomic oxygen yields a major O 1s peak at 533 - 534 eV and a shoulder at 531 eV (Fig. 4.12) [213, 266]. In Figs.

5.5 and 5.6, the O 1s signals are displayed before and after subtraction of the O/HOPG signals, respectively. Therefore one can say that the O signal in Fig.

5.6 (blue zone and green curve in Fig. 5.5) can be attributed to oxygen species bound to Au.

526 528 530 532 534 536

Binding Energy (eV)

In tens ity (arb. units )

a)

b)

c)

O1s

The O 1s state at 529 - 530 eV can be attributed to Au2O3 based on comparison with the results in Fig. 5.6 c. The oxygen species showing the O 1s state at 531 - 532 eV cannot be found for nanostructured Au films (sample C), implying that decrease of the particle size opens a formation route of a new oxygen species under the same experimental conditions. The nature of the oxygen species identified by the O 1s state at 531 - 532 eV is not clear yet. As it will be shown later, this O species does not react with CO to form CO2, and this oxygen species is not responsible for the Au 4f core level shifts. Since no Au 4f core level shifts can be associated to this oxygen species, this species can be assigned to either chemisorbed oxygen on Au surfaces or subsurface oxygen (or dissolved oxygen). However, we assign the O 1s state at 531 - 532 eV to subsurface oxygen species, since chemisorbed oxygen on Au surfaces should readily react with CO to CO2 according to previous studies [55, 61, 156, 267].

We can not exclude the possibility that there might be additional oxygen states of Au above 533 eV, which can be buried by the O/HOPG signals. Also, it is possible that the O 1s peak at 529 - 530 eV comes from more than two different oxygen species, which we cannot clearly be discriminated, i.e. besides Au-oxide, chemisorbed oxygen, which is electrically quite analogous to the lattice oxygen of Au-oxide, can exit and show an O1s feature at the same binding energy regime [156].

Fig. 5.6 Oxidation of various Au nanostructures studied using XPS (O 1s states) [214] a) O1s peak for sample A (~ 5 nm in diameter), b) sample B (~ 8 nm in diameter) and c) sample C (~ 23 nm in diameter)

526 528 530 532 534 536 538

5.0 x 10^-5, 40 min

526 528 530 532 534 536 538

5.0 x 10^-5, 30 min

526 528 530 532 534 536 538

8.0 x 10^-5, 60 min

Fig. 5.7 Oxidation of various Au nanostructures studied using XPS (Au 4f states) [214] a) Au4f peak for sample A (~ 5 nm in diameter), b) sample B (~ 8 nm in diameter) and c) sample C (~ 23 nm in diameter)

82 84 86 88 90 92 94

c)

Intensity (arb. units)

increasing O exposure

as prepared Au Au bulk

Au 4f

Binding Energy (eV)

82 84 86 88 90 92 94

b)

increasing O exposure

Au 4f

as prepared Au

Intensity (arb. units)

Binding Energy (eV)

82 84 86 88 90 92 94

increasing O exposure

as prepared Au

Binding Energy (eV)

Intensity (arb. units)

Au 4f

a)

In general, metal oxide formation results in a significant change of the metal–metal distance, since direct metal–metal bonds are broken and oxygen atoms are inserted into the metal–metal bonds upon oxidation. The metal oxide formation therefore leads to a different electronic structure with respect to the metallic counterpart, e.g. loss of the plasmon resonance and chemical shifts of the metal core levels can be observed. Metal oxides can show a band gap since there is no overlap of the metal–metal bond anymore, even though there are exceptional cases such as RuO2, which is metallic [193].

In contrast to the oxide formation, subsurface species or dissolved oxygen species are incorporated within the lattice of metallic substrates without significant change of the metal–metal bonds, even though small changes of the metal–metal distance due to the relaxation or reconstruction cannot be excluded.

The metallic characteristics of substrates can be preserved upon subsurface or dissolved oxygen formation. For example, no chemical shift of Ag is observed, when chemisorbed or dissolved oxygen exists, whereas oxidation of Ag results in significant Ag 3d shifts [235]. For even smaller Au nanoparticles (sample A), two different oxygen species are found, similar to the results of larger particles (Fig. 5.6). For the Au 4f levels (Fig. 5.7) also, shoulders appear at higher binding energies upon oxidation. We cannot obtain particles completely oxidized, implying that the relatively thin oxide layers prevent further oxidation of the deeper layers of Au nanoparticles.