Nanotwinned materials possess a wide range of unprecedented mechanical and physical properties such as ideal maximum strength1-3, excellent ductility4, good fatigue resistance5, high electrical conductivity6, and exceptional thermal stability7 due to the existence of coherent twin boundaries (TBs). With the advent of nanotechnology, TBs have also been revealed to critically influence the catalytic reactivity8, electrochemical efficiency9, growth dynamics10, and oxidation rate11, 12 of metallic nanomaterials, which can be ascribed to the change of both surface property (e.g., coordination number, stress state and charge density) and internal lattice dynamics (e.g., diffusion) by TBs. Since the reaction dynamics at TBs plays an important role in subsequent structural evolution and functionality of metallic nanomaterials10, 12, 13, an atomic-scale understanding of this coherent defect contribution to chemical/electrochemical reactivity in nanomaterials is of general significance. However, in stark contrast to the well-established mechanistic insight into strengthening and softening mechanisms in nanotwinned materials3, 14, 15, the atomistic origins of TB-assisted reactivity in nanotwinned materials have been largely elusive.
Recent studies on nanotwinned metallic nanoparticles attributed their superior catalytic performances to low-coordinated atomic steps, large tension, and high density of negative charges on the surface, which are critically affected by TBs16-20. These studies suggest that coherent TBs may act as preferential sites for selective chemical/electrochemical reactions and as channels for fast atom transport19, 21. Likewise, experimental studies have found that TBs significantly reduce the energy barriers for growth13, mass transport9, and solid-state reaction22. On the contrary, the twin-modified surface structures were reported to enhance the resistance to corrosion and oxidation in nanotwinned metals, compared with their twin-free nanocrystalline counterparts12, 23. Therefore, the critical influence of TBs on the chemical response of metallic materials under different environments is still under debate. To date, however, systematic investigations of the defect-assisted oxidation dynamics at the atomic scale remain challenging in practice.
Here, we study the oxidation dynamics of nanotwinned metallic nanocrystals in atomistic detail by conducting integrated in situ transmission electron microscopy (TEM) characterizations and density-functional-theory (DFT) based ab initio calculations. The results expose a unique site-selective oxidation behaviour in nanotwinned silver (Ag) and palladium (Pd), where coherent crystal defects including TBs and stacking-faults (SFs) strongly favour surface oxygen binding and accelerate oxide nucleation and growth. Subsequent inward growth of the oxide is promoted by the fast diffusion of oxygen along TB and SF highways that intersect with the oxide/matrix interface, leading to a layer-by-layer oxide growth mechanism. Furthermore, we qualitatively validate this defect-assisted oxidation kinetics in air over largely different time scales spanning from few seconds to several days. These findings provide atomistic insights into the defect-assisted reaction dynamics of nanoscale metals, which has direct ramifications for the development of advanced nanomaterials through defect engineering.
Selective oxidation of nanotwinned Ag along isolated TBs and SFs
In contrast to the widely reported oxidation of nanosized FCC metals (e.g., Cu and Pd) from the {111} and {100} facets24-26, our in situ experiments unambiguously demonstrate preferential oxide nucleation at the TB-surface junction, even in the presence of numerous low energy (111) facets on the nearby surface. As shown in Fig. 1a, the as-fabricated Ag nanocrystal consists of a straight TB along the axial direction, which is confirmed by the fast Fourier transform (FFT) pattern in Fig. 1f. Upon oxidation, sequential images clearly show that an oxide embryo preferentially nucleates at the twinned tip (Fig. 1b), although the lattice structure of this as-formed oxide embryo cannot be quantitatively identified. After 88 s, recognizable lattices appear as the oxide grows continuously inward along the TB (Fig. 1c). Meanwhile, the oxide expands laterally to form an asymmetrical conical cap sitting on top of the nanocrystal (Fig. 1d). No oxidation was detected on {111} facets away from the TB24-26, suggesting a strong site selection upon oxidation of the nanotwinned tip. A close-up high-resolution TEM image is presented in Fig. 1e to show the atomistic structure of the Ag2O product above the Ag matrix at the tip. The interface is atomically sharp with one atomic (111) step. The corresponding FFT patterns (Fig. 1g) further confirm the as-formed oxide of Ag2O with a lattice constant of a = 4.718 Å (Space group: Pn3m), which bears a specific orientation relation with the Ag matrix as (1 0)Ag//(100)Ag2O and (11 )Ag//(010)Ag2O. It is further observed that oxidation along the TB always precedes that of the neighbouring lattices, as shown in Fig. 1e, which indicates evident TB-assisted oxidation. The selective oxidation mechanism is more conspicuous during the subsequent growth of this oxide layer, because the thickness of oxide (along axial direction) decreases gradually from ~4.3 nm at the tip down to almost zero at free surfaces on both sides (Fig.1d). It is noted that no other defect activity is observed throughout the oxidation process of this Ag twinned tip.
Atomic-level snapshots in Fig.1i-j further emphasize the preferential oxide nucleation at the TB-surface junction. A zoomed-in image shows the atomic structure of the clean Ag nanocrystal, which exposes two conjugate (111) surfaces with multiple atomic kink steps very close to the TB-surface junction, while a high density of alternate (111) and (11 ) atomic facets dominate both sides of the surface away from the TB (Fig. 1h). The oxygen atoms are preferentially adsorbed to the surface layer of Ag near the TB (shown by the blurred profile of the top surface in Fig. 1i), which induces a slight lattice distortion visible in the sub-surface layers. In contrast, the faceted morphology of the surface away from the TB is generally retained, indicating significantly less oxygen adsorption on these low energy facets. Eventually, the adsorbed oxygen accumulating at the TB-surface junction incubates an oxide nucleus with an average height of 4.8 Å (Fig. 1j), which is favourably located on the (11 ) surface facet with the largest number of kink steps. With prolonged oxidation, the Ag2O layer simultaneously grows inward along the TB and expands laterally along conjugate (11 ) surface facets (Fig. 1k).
The above experiment demonstrates a TB-assisted site-specific oxidation behaviour in an Ag nanocrystal, which dominates over those widely reported low-energy surface facets for the nucleation and growth of oxides24-26. This unique behaviour is further validated in additional examples of Ag nanocrystals with an axial TB (see Supplementary Fig. 1), where the TB-surface junction serves as a universally preferential nucleation site for oxide embryos, accompanied by oxide growth along the TB. Similar oxidation behaviours are also observed in other FCC metallic nanocrystals, e.g. Pd, with significantly higher SF energy (Supplementary Figs. 2-4). This common site-selective nucleation and TB-assisted growth of oxide clearly indicate that an isolated TB can effectively promote the oxidation of FCC metals, which markedly differs from the suppressed chemical reactivity of nanotwinned Cu nanowires reported previously27.
Furthermore, this phenomenon was found to occur with isolated TBs positioned away from the Ag tip (see Supplementary Fig. 5), which suggests that the TB effect is intrinsic. DFT-based atomistic simulations were performed to quantitatively confirm our hypothesis by calculating the Ag-O surface binding energy (Eb), which decouple the intrinsic influence of TB from other atomic surface steps. We first constructed a twinned Ag tip (consistent with our experiments) with a junction between a vertically aligned coherent TB and two perfectly smooth (11 ) conjugate surfaces. These calculations reveal that the O interstitial site closest to the TB possesses the highest Ag-O surface binding energy Eb = 3.931 eV, compared with Eb = 3.807 eV at the equivalent interstitial site on the (11 ) surface of the single crystal farther away from TBs (Fig. 2a-b). Therefore, oxide nucleation should preferentially occur at the TB-surface junctions, which matches perfectly with our experimental observations. Furthermore, we find that the Ag-O binding energy at a kink step on the (111) surface is considerably increased from Eb = 3.897 eV (Fig. 2c) to Eb = 4.078 eV in the vicinity of a planar SF (Fig. 2d). These theoretical results are significant in two aspects. First, the DFT calculations quantitatively prove that both isolated TBs and SFs give rise to stronger surface binding of O atoms. Second, it has been reported that twinning creates low-coordinated sites to which dioxygen preferentially bonds at TB-surface junctions in nanotwinned metals28, suggesting that the O reactivity of surface atoms maybe directly relate to their coordination number (CN). However, our DFT-based calculations demonstrate that the coordinate number (CN) of atoms at the TB-surface junction (CN=8) is larger than that at the edge of a (111) kink step (CN=7), see black-coloured atoms in Fig. 2c and inset of Fig. 2e, respectively, clarifying the predominant role of binding energy in selective oxide nucleation at the TB-surface junction, in contrast to the CN-governed surface reactivity reported in literature28.
Oxidation efficiency as a function of defect configuration and spacing
Analogous to the strong twin-size dependence of mechanical behaviour in nanotwinned metals1-3, it is significant to study how the chemical reactivity and oxidation could vary with TB spacing, because smaller twins are frequently observed to reduce the reactivity via surface modification12. To this end, despite the technical difficulties in controlling TB spacing experimentally, DFT calculations are used to quantify the mean oxygen binding energy averaged over multiple O interstitial sites of different defect-modified surface configurations, as shown in Fig. 2e and Supplementary Figs. 6-9. Specifically, we compute the mean binding energy of different nanotwins (NTs) with a constant TB spacing varied from 4.8 Å to 28.8 Å, in comparison to that of two single-crystal configurations containing either an isolated SF or no imperfection (SGL). The surface intersecting either TB or SF is composed of zig-zag low-energy (111) and (001) facets (insets of Fig. 2e). For consistency, the perfect single-crystal configuration was constructed with the same type of zig-zag facets. Fig. 2e shows that the mean O binding energy (Eb) of NTs is noticeably higher than that of the perfect single crystal, which agrees with the experimental observations (Fig. 1). However, the binding energy tends to decrease as the TB spacing is reduced, which is in good agreement with the uniform and slower oxide growth observed in regions of copper nanowires containing highly dense and evenly spaced nanotwins12, 27. Even more remarkably, the average Eb at an isolated SF reaches 3.981 eV, which surpasses those of NTs with any size.
Additional experiments in samples with different planar defects further confirm the strong site-selective and defect-assisted surface oxidation in Ag and Pd nanocrystals, as shown in Fig. 3 and Supplementary Figs. 4-5. Specifically, Fig. 3a-d gives clear evidence for the SF-assisted oxidation in Ag nanocrystals with an isolated SF. A pre-existing SF laid almost perpendicular to the [111] axial direction of the Ag nanocrystal (Fig. 3a), intersecting the nanocrystal surfaces on both sides. A close-up image in the inset shows that the surface is oriented almost perfectly parallel to the (112) plane, albeit some (111) kink steps exist on the other side of the surface-SF junction. Upon initial oxidation, two oxide embryos nucleate simultaneously at the SF-surface junctions on both sides (Fig. 3b). This observation agrees well with the highest O binding energy at SF predicted by DFT. Moreover, the oxide embryo is followed by inward growth along the SF (Fig. 3c-d) similarly to our previous observations at isolated TBs, where the average oxidation rate (along the SF) is calculated to be ~ 0.07 nm s-1 (Supplementary Fig. 10).
To further confirm the DFT predictions that the O binding energy is higher at SFs than in TBs, in situ oxidation experiments were conducted on an Ag nanocrystal containing both pre-existing SF and multiple parallel TBs with uneven spacing, as shown in Fig. 3e-h. The surface structure was almost identical near the intersection sites with SF and TB (Fig. 3e), which makes the influence of interface facets negligible in this configuration. An oxide embryo preferentially nucleates at the SF-surface junction located between two TBs (TB3 and TB4 with a spacing of 1.88 nm), connecting a large twin to one of the smallest twins (Fig. 3f). It is worth noting, however, that no observable oxidation occurs at the nearby surface intersected by twins. Subsequent inward growth of this oxide island preferentially proceeds along the SF (Figs. 3g and 3h), suggesting that SF is a fast pathway for oxygen diffusion. Associated with the inward oxide growth is the lateral expansion of the oxide along the surface facets (consistent with that at the surface-TB junction), which leads to a spindle-shaped oxide. In summary, strong site-selection dictates the defect-assisted surface oxidation in metallic nanocrystals containing multiple TBs and SFs, and the general significance of coherent planar defects as fast pathways for oxygen diffusion is unambiguously demonstrated (see additional example in Supplementary Fig. 11).
Defect-assisted oxidation mechanism and kinetics
Figure 4a-d presents a series of TEM snapshots showing the atom-level dynamics of TB-assisted oxide growth during the twinned tip experiment in Fig. 1. Upon nucleation, the interface between Ag nanocrystal and as-formed Ag2O consists of six single-atom steps (Fig. 4a). For the next second, the pre-existing steps are observed to migrate laterally along the Ag/Ag2O interface, away from the TB, while the atoms at the TB tip react into Ag2O, forming a pair of new steps (7 and 8) in a sub-layer (Fig. 4b). This specific event suggests that interstitial O atoms are more rapidly diffused along the TB defect to bind with freshly exposed Ag atom columns, as will be discussed below. During the following seconds, each newly formed step moves away on the Ag/Ag2O interface (Fig. 4c) and gives space for the formation of new Ag2O lattices into sub-layers initiated from the TB (Fig. 4d). This layer-by-layer oxide growth is similar to the growth of CuO nanowires assisted by (002) TBs13, and eventually leads to a conical oxide cap on the Ag nanocrystal (Fig. 1d).
The layer-by-layer growth mechanism is schematically illustrated in Fig. 4e, where the TB atoms in the top Ag layer and atomic steps on the Ag/Ag2O interface naturally possess a lower CN than those in the perfect FCC lattice (CN = 12). In fact, our DFT calculations have indicated that the CN of FCC Ag atoms decreases to CN = 10 at an Ag/Ag2O interface (Supplementary Fig. 12). Therefore, these low-coordinated Ag atoms at the oxide interface favour oxygen binding and induce a ledge flow mechanism at the reaction front29, which is supported by experimental evidence (Fig. 4a-d). Furthermore, the surface oxide on the nanotwinned tip is observed to grow continuously and sequentially, suggesting that the coupled axial and lateral propagation of the Ag/Ag2O interface arises from fast diffusion of O atoms along the TB30. Similar TB-assisted diffusion has been reported in SnO2 nanowire for Li ions (during lithiation)9 and in α-Ti for O interstitials31. Also, the same evidence of TB-assisted layer-by-layer oxidation dynamics was found in our additional experiments of nanotwinned Pd, as exemplified in Supplementary Fig. 3.
The time-dependent oxidation kinetics along the TB is quantified in Fig. 4f. The TB-assisted oxidation of Ag nanocrystal (shown in Fig. 1) can be divided into three stages, i.e., fast nucleation, steady growth, and self-limiting equilibrium. During the fast nucleation (t ≤ 80 s), an oxide embryo nucleates by adsorption of rarefied O atoms from the ultrahigh vacuum chamber of TEM. Our DFT calculations have predicted intrinsic TB and SF effects that yield a substantial increase in the surface binding energy of oxygen in Ag nanocrystals. Quantitative analysis in inset 1 of Fig. 4f confirms this theory by revealing that the axial oxidation depth L (along TB) during the nucleation period is linear as a function of time. A linear rate of reaction is commonly associated with initial fast oxidation observed on a number of metals at low temperatures32. Subsequently, inward growth of the oxide proceeds layer-by-layer assisted by the TB (Fig. 4e), which is referred to as the steady growth stage (80 s < t < 200 s). Our quantitative study in inset 2 of Fig. 4f shows that the oxide growth kinetics follows a logarithmic law during steady growth. Direct logarithmic kinetics is usually derived from electron tunnelling and ionic O- flow in very thin oxide films32. However, the observation of spindle-shaped oxide islands (see Fig. 3 and Supplementary Fig. 2) indicates that oxygen diffusivity along TBs in the thin oxide layer is enhanced in comparison to the diffusivity on standard {111} FCC planes, which agrees well with recent DFT calculations31. Meanwhile, the high oxygen concentration radiates from the TB to propel the lateral flow of newly formed steps along the low-coordinated Ag/Ag2O reaction front. As the oxide thickness grows to ~ 10 nm, however, few O ions are pumped towards the oxidation front. Consequently, the reaction front tends to be short of O supply, and the oxide growth rate is gradually reduced. This process is self-limiting under our experimental condition, because oxygen is rare in the vacuum of the microscope. A similar oxidation trend is observed for an isolated SF as well (Supplementary Fig. 10).
It is established that pre-existing surface steps and lattice defects can facilitate diffusion and chemical reaction in defective metallic nanocrystals21, 25. When the defect density increases, the surface strain may change markedly16, 18-20, resulting in some unique properties, such as enhanced oxidation of metallic nanoparticles due to strain-mediated ion transport33. The presence of TBs can also change the properties of neighbouring surfaces27. In addition, twinning could induce large surface tension and highly negative charge density18, 19 that may also favour O adsorption. In contrast to nanotwinned Ag, however, we found that homogeneous oxidation initiates from multiple sites simultaneously in defect-free Ag single crystals (Supplementary Fig. 13); such process is mainly dominated by surface diffusion rather than defect-assisted diffusion, leading to significantly slower kinetics and reduced inward growth. This comparison points to the conclusion that in nanocrystals containing coherent planar defects, the chemical reactivity under different conditions is not only controlled by the nucleation kinetics, but also critically limited by the subsequent growth kinetics32.
Despite the exceptional chemical inertness of bulk Ag and Pd, nanosized noble metals are inherently vulnerable to oxidation, which probably originates from the increased specific surface area. Under electron beam irradiation, the binding of O with metallic atoms could be significantly facilitated at room temperature34, 35, even under low oxygen partial pressure in the ultrahigh vacuum TEM column (~2.5 10-7 Torr). Due to the high thermal conductivity of Ag (429 W mK-1) and Pd (72 W mK-1), temperature rise induced by the electron beam should be negligible in Ag and Pd nanocrystals36. However, the electron beam could also alter chemical bonds in the sample and induce ionization of gas molecules37, thereby catalysing chemical reactions (e.g., oxidation). To exclude any possible beam effects on oxidation, nanotwinned Ag (fabricated with the same method) with multiple pre-existing TB and SF, was exposed in air at room temperature for 48 hours. The TEM image captured after oxidation (Fig. 5) shows that the naturally oxidized nanotwinned Ag exhibits similar site-selective oxidation along pre-existing isolated SFs, where negligible oxide was formed at defect-free areas (Fig. 5). This experiment provides qualitative evidence for the insensitivity of site-selective oxidation to the electron beam irradiation and oxygen concentration, as well as to the time scale for oxidation.