Size-dependent oxidation of supported FeO NSs

FeO NSs were prepared on Pt(111) and display typically the shapes of triangles or hexagons (Fig. 1a). The structure of FeO islands on Pt(111) has been well characterized as a polar FeO(111) bilayer, with the Fe layer in contact with Pt and the O layer exposed at the topmost plane27,28,29,30. Due to lattice mismatch, Fe atoms occupy successively the fcc, hcp and top positions on Pt(111), forming moiré domains on FeO (ref. 31). When exposed to O 2 at elevated temperatures, FeO NSs could be oxidized by oxygen penetration into the FeO/Pt interface to form FeO 2 (Fig. 1b), displaying an O–Fe–O trilayer structure26. DFT calculations suggested that the FeO 2 phase was thermodynamically more stable than FeO on Pt(111) (ref. 32). The structural difference between FeO 2 and FeO could be easily distinguished in STM from their apparent heights, moiré patterns and atomic structures (Supplementary Fig. 1).

Figure 1: The size-dependent oxidation kinetics of FeO nanostructures(NSs) on Pt(111). (a,b) STM images (50 nm × 50 nm) of the typical surface of as-prepared FeO NSs on Pt(111) (a) and the FeO/Pt(111) surface after the annealing in 1 × 10−5 mbar O 2 at 500 K for 10 min (b). Most FeO NSs were oxidized to form FeO 2 domains, except for FeO NSs with an equivalent diameter d<3.2 nm. Here, the FeO NS is treated as a circle and the equivalent diameter is defined as , where S is the NS surface area. (c,d) Atomic STM images of an oxidized FeO NS (d=6.2 nm) with the formation of FeO 2 domains and an FeO NS (d=2.1 nm) remaining the FeO phase, respectively. The structural models (side view) of FeO 2 and FeO on Pt(111) are shown in the insets. STM images were taken at 230 K and scanning parameters (sample bias: V s ; tunneling current: I t ) are (c) V s =+104 mV, I t =3.5 nA; (d) V s =+53 mV, I t =2.1 nA. Scale bars are 2 nm in c and 1 nm in d. The area of FeO 2 domains (S FeO2 ) on each NS, as well as the NS surface area (S), are measured over 247 FeO islands on the same surface. (e,f) plot the oxidation area (S FeO2 ) and the oxidation ratio (S FeO2 /S) of individual FeO NSs as a function of d, where the size range of FeO NSs and the s.d. of S FeO2 and S FeO2 /S are represented by error bars. Full size image

Interestingly, the oxidation kinetics of FeO NSs at 500 K or below were found to be strongly size-dependent (Fig. 1b). While most FeO islands were oxidized to form FeO 2 (Fig. 1c), small FeO islands remained the FeO phase (Fig. 1d). Figure 1e shows that the area of FeO 2 domains on an individual island increases with the increasing island size (d). No FeO 2 domains were observed on FeO islands with d<3.2 nm (Supplementary Fig. 2). In situ STM images (Supplementary Fig. 2) show that FeO 2 domains were developed by oxygen penetration from the step edges of FeO NSs, as generally proposed in the oxidation of NSs9. Assuming a uniform diffusion rate for oxygen insertion, the formation rate of FeO 2 domains should be proportional with the edge perimeters of FeO NSs and smaller FeO NSs should display a higher oxidation ratio (Supplementary Fig. 2c). In contrast, we found the oxidation ratio of FeO NSs went down drastically for islands with d<3.2 nm (Fig. 1f).

Size-dependent structural dynamics of supported FeO NSs in O 2

By examining the structure of FeO NSs and their interaction with O 2 , we found the enhanced oxidation resistance of FeO NSs with d<3.2 nm was caused by a dynamic size effect described below. As-prepared FeO NSs typically exhibit two types of step structures, exposing two-coordinated Fe or O atoms (Supplementary Figs 3 and 4). The steps terminated by Fe atoms, also known as coordinatively unsaturated ferrous (CUF) sites, are active sites for O 2 dissociation19,33, whereas the steps terminated by coordinatively unsaturated oxygen (CUO) atoms and the surface plane of FeO were found inert to O 2 (ref. 16). For simplification, the two types of steps were termed as the CUF step or the CUO step, whose atomic structures could be viewed directly in STM images (Supplementary Fig. 4). A detailed structural analysis showed that the structure of the CUF step is independent on the island size. As manifested in Fig. 2, three triangular FeO islands expose exclusively CUF steps, which display the same structure and lattice constant.

Figure 2: STM images and the corresponding structural models of equilateral-triangle-shape FeO NSs. In the structural models (d–f), the number of Fe atoms along each edge of the FeO triangle is denoted by n. The atomic compositions and structures of three FeO NSs are obtained from STM images (a–c). STM images were taken at 5 K and scanning parameters are (a) V s =+6 mV, I t =5.5 nA; (b) V s =+7 mV, I t =5.9 nA; and (c) V s =+7 mV, I t =4.5 nA. Scale bar, 2 nm. Full size image

Despite the same shape and structure, these triangular FeO islands exhibited a drastically different structural dynamics when exposed to O 2 at 270 K. Figure 3 shows O 2 dissociation at CUF sites of the Fe 78 O 66 NS led to the spontaneous and complete reconstruction of NS, which was also the case for smaller FeO NSs (Supplementary Fig. 5). Element-specific STM images (Supplementary Note 1) have allowed us to reveal not only the number of Fe and O atoms, but also their relative positions. Upon O 2 exposure, 23 O atoms were added to the edges of Fe 78 O 66 NS, accompanying a collective shift of all oxygen atoms to the adjacent 3-fold hollow sites of the Fe layer (Supplementary Note 2, Supplementary Figs 6 and 7). Consequently, all CUF sites at the edges could be saturated by two-coordinated oxygen atoms, forming the CUO steps rather than by dangling oxygen atoms as before reconstruction. The line profile of the CUO step appears inverted to that of the CUF step (Supplementary Fig. 7). DFT calculations show that, before reconstruction, dangling oxygen atoms bind weakly at the step edge, with the adsorption energy of 0.64 eV (Fig. 3g). The binding energy of O adatoms is increased to 1.26 eV by bonding with two neighbouring Fe atoms (Fig. 3h), though such a configuration is not stable and prefers to reconstruct. The reconstruction shifts surface O atoms to the adjacent three-fold hollow sites of Fe lattice and results in the strong binding of edge O atoms with the adsorption energy of 2.14 eV (Fig. 3i). Thus, all O adatoms were stabilized at the step edges by the spontaneous reconstruction of FeO NSs.

Figure 3: The structural dynamics of FeO NSs in O 2 . (a–f) In situ STM images and the corresponding structural models of an Fe 78 O 66 NS before (a–c) and after (d–f) the exposure of 1 × 10−9 mbar O 2 at 270 K. Element-specific STM images were used to identify the number of Fe and O atoms, as well as their relative positions. (a,b) are the Fe-mode and mix-mode STM images of the Fe 78 O 66 NS at 270 K, which underwent a complete reconstruction in O 2 and turned into an Fe 78 O 89 island. (d) Mix-mode STM image of the Fe 78 O 89 island, which is overlayed with the structural model in e. In the mix-mode images, O atoms are resolved as bright protrusions and the hollow sites of the Fe lattice, which are not filled by O atoms, are resolved as dark depressions. The positions of dark depressions with respect to O atoms have changed after O 2 exposure, which is illustrated in the structural models in c,f, with colour representations of: Fe—purple, O—orange and Pt—blue. STM images were taken at 270 K and scanning parameters are (a) V s =+16 mV, I t =4.3 nA; (b) V s =+34 mV, I t =1.9 nA; (d) and (e) V s =+80 mV, I t =1.8 nA. Scale bars are 1 nm for all STM images. (g–i) Calculated adsorption configurations of O atoms at the edges of an Fe 10 O 15 cluster on Pt(111). Among the three configurations, oxygen adatoms bind most weakly in g and most strongly in i. Compared with g, the configurations in h,i have energy gains of −2.10 and −2.79 eV, respectively. To reach i, oxygen atoms in h need be shifted, with the moving directions marked by green arrows. Full size image

In contrast, a partial reconstruction was observed for larger NSs, such as Fe 210 O 190 and Fe 378 O 351 . Supplementary Figure 8 shows only a portion of surface oxygen atoms was shifted to the adjacent 3-fold hollow sites of Fe lattice, resulting in a reconstructed oxygen domain and oxygen dislocation lines at the boundary between reconstructed/unreconstructed domains. At the dislocation, Fe atoms were over-saturated with 4-fold oxygen coordination and appeared as protrusion lines running parallel to the steps (Supplementary Fig. 8)34. The reconstructed domain was evidenced by the formation of CUO steps, while the edges of unreconstructed domain were terminated by either dangling or dislocated oxygen atoms19,34.

The size-dependent structural dynamics of FeO NSs could be driven via different channels. DFT calculations on supported FeO clusters show that, while the energy released from oxygen adsorption increases with the size increasing from Fe 10 O 6 to Fe 28 O 21 , the driving force (or thermodynamic preference) associated with the shift of oxygen atoms to achieve a complete reconstruction decreases more rapidly (Supplementary Fig. 9). Both energy release and the driving force will eventually level off with increasing size. These results suggest that a complete structural reconstruction likely occurs in small NSs, which is in agreement with STM study. For example, although the energy release from oxygen adsorption at Fe 10 O 6 /Pt(111) is the smallest among the FeO clusters being calculated, its reconstruction is thermodynamically most favourable. On the other hand, with size increasing, the increment in the energy released from oxygen adsorption cannot match the decrease in the driving force from reconstruction (Supplementary Fig. 9), and thus a complete reconstruction becomes less favourable. Indeed, STM study shows that the reconstruction of larger FeO NSs is only partial with the formation of surface dislocations (Supplementary Fig. 8).

The dynamic response of FeO NSs could be observed at even 15 K, upon the dissociative adsorption of oxygen at the CUF sites (Supplementary Fig. 10). The fact that an adsorption of several oxygen atoms at the edge of FeO NSs could induce locally the reconstruction of FeO NSs at such a low temperature suggests that the reconstruction could be initiated without thermal activation. However, DFT calculations show that, to initiate the reconstruction, diffusion of isolated oxygen atoms to adjacent 3-fold hollow sites of Fe needs to overcome a barrier (E d ) of ∼0.33 eV/O atom (Supplementary Fig. 11). Meanwhile, oxygen insertion into the FeO–Pt(111) interface, that is, the transition to FeO 2 , has an even higher energy barrier of 1.41 eV/O atom (Fig. 4a). Thus, the driving force for the reconstruction is unlikely to be limited to thermodynamics.

Figure 4: Dynamic size effect on the oxidation of FeO NSs. (a) Potential energy diagram depicting the mode of action for oxygen reconstruction. The diffusion pathways and barriers are displayed for oxygen penetration into the FeO–Pt(111) interface from the unreconstructed edge (1) and the reconstructed CUO edge (2). (b) A schematic illustration of the dynamic size effect in enhancing the oxidation resistance of active FeO NSs. The dynamic response of FeO NS enables FeO to reach an intermediate state, which has a lower total energy than that before the reconstruction and thus increases the barrier for further oxidation. Full size image

Note that we have not taken into account the partitioning of the energy released from oxygen adsorption, which should be in the form of hot electrons35 and local atomic displacements. The energy channelling model requires that these energies are effectively used for the reconstruction, rather than generating thermal losses36. It is believed that hot electrons typically decay on the timescale of picoseconds. Such a fast timescale could in principle inhibit the channelling to take place, or alternatively be the reason why the reconstruction of smaller FeO NSs is complete but not the larger ones. In any case, the concerted reconstruction of NSs is a complex process; their activation energies depend on the density of the CUF sites (that is, the number of adsorbed oxygen atoms) and the size of the NSs (Supplementary Fig. 11). As such, we cannot rule out the possibility of barrierless channels upon saturation adsorption of oxygen at the CUF sites. This size-dependent structural dynamics warrants further exploration both experimentally and theoretically using time-dependent approaches to gain in-depth understanding.

The oxidation of FeO NSs determined by dynamic size effect

The dynamic size effect governs the oxidation kinetics of FeO NSs by tuning the stability of O atoms at the step edges. In situ STM images (Supplementary Fig. 2) showed that the development of FeO 2 domains along step edges was indeed anisotropic and controlled by the step structures of FeO NSs. Once the oxidation has been initiated and oxygen entered the interface, we observed spontaneously the formation of FeO 2 domains, as modulated by the interface. Meanwhile, previous studies have also suggested that the oxidation of Fe-hcp domain26 or Fe-fcc domain37 to form FeO 2 is thermodynamically most favourable, which means the enhanced oxidation resistance of FeO NSs is not controlled by the different stability of FeO domains since FeO NSs with size below 5 nm consist of mostly Fe-hcp and Fe-fcc domains. In addition, DFT calculations suggested that the reconstruction to CUO-termination hindered significantly the diffusion of edge oxygen atoms to the FeO–Pt(111) interface (barrier of 2.37 eV/O atom, Fig. 4a) and consequently prevented the oxidation to form FeO 2 . In comparison, the diffusion barriers for oxygen at the unreconstructed edge (barrier of 1.41 eV/O atom, Fig. 4a) and surface oxygen dislocations (barrier of 0.50 eV/O atom, Supplementary Fig. 11) were much lower. Thus, FeO NSs with d<3.2 nm are likely passivated from oxidation by stabilizing all oxygen adatoms and forming CUO steps via the complete reconstruction in O 2 . In contrast, the partial reconstruction of larger FeO NS resulted in unreconstructed steps and the development of oxygen dislocation lines, both of which are vulnerable for oxygen penetration (Supplementary Fig. 11). Thermodynamically, the dynamic response of FeO NS enables FeO to reach an intermediate state, which has a lower total energy than that before the reconstruction and thus increases the barrier for further oxidation (Fig. 4b). The long-term stability of FeO NS in O 2 is thus not dependent on the structure of active sites, but rather determined by the dynamic size effect.

The generality and implications of dynamic size effect

The enhanced oxidation resistance was also found for CoO NSs with d<3 nm supported on Pt(111) and Au(111), whereas larger CoO NSs are susceptible for further oxidation (Supplementary Fig. 12). Thus, the enhanced oxidation resistance of smaller active NSs is not just a unique feature of the FeO/Pt(111) system, but could rather be observed in other supported NSs. We expect that the reconstruction mechanism discussed above could be transferred to supported active NSs with similar structural configuration. Indeed, a number of rocksalt-type oxides, such as FeO, CoO, NiO, MnO, VO and EuO, have been shown to exhibit similar structural configurations, when they were supported on different metal substrates, such as Pd, Rh, Pt, Au and Ag38,39,40. These supported oxide NSs are promising for a number of applications in catalysis, magnetic storage and material sciences21,24,39,40,41.

Oxides are usually considered as a rigid surface during the reaction at low temperatures42. We show that oxide NS can exhibit a rapid structural change at the elementary step. The triangular FeO NSs investigated above exhibit not only the same shape and structure, but also similar electronic properties and electrostatic potential (Supplementary Figs 9 and 13), which usually indicate their similar behaviour in oxidation. Instead, the dynamic size effect observed here manifests its dominant influence in the nanoscale chemistry. Although we have used a model system to illustrate the dynamic size effect, this effect should be somewhat general for active NS in exothermic reactions. Our results demonstrate how NS prevents the insertion of oxygen into the oxide–metal interface, which might be key to develop passivating coatings for metals.