Engineering surface atomic structure of single-crystal cobalt (II) oxide nanorods for superior electrocatalysis — TEI-tagged view

Tao LingpersName, Dong-Yang YanpersName, Yan JiaopersName, Hui WangpersName, Yao ZhengpersName, Xueli ZhengpersName, Jing MaopersName, Xi-Wen DupersName, Zhenpeng HupersName, Mietek JaroniecpersName, Shi-Zhang QiaopersName (2016date). “Engineering surface atomic structure of single-crystal cobalt (II) oxide nanorods for superior electrocatalysis.” Nature Communicationstitle·j.

Object of study — single-crystal CoOterm nanorodsterm·key with oxygen vacanciesterm·key on pyramidal {111} nanofacets

Research question — how defects on chosen facetsterm·key act in electrocatalysisterm·key at the atomic level

Contribution — showing that surface atomic structure engineering yields efficient and durable electrocatalysts

Full text — tagged

Engineering surface atomic structure of single-crystal cobalt (II) oxideterm nanorodsterm·key for superior electrocatalysisterm·key Abstract Engineering the surface structure at the atomic level can be used to precisely and effectively manipulate the reactivity and durabilityterm·dom of catalysts. Here we report tuning of the atomic structure of one-dimensional single-crystal cobalt (II) oxideterm (CoOterm) nanorodsterm·key by creating oxygen vacanciesterm·key on pyramidal nanofacets. These CoOterm nanorodsterm·key exhibit superior catalytic activity and durabilityterm·dom towards oxygen reduction/evolution reactions. The combined experimental studies, microscopic and spectroscopic characterization, and density functional theory calculations reveal that the origins of the electrochemical activity of single-crystal CoOterm nanorodsterm·key are in the oxygen vacanciesterm·key that can be readily created on the oxygen-terminated {111} nanofacets, which favourably affect the electronic structureterm·dom of CoOterm, assuring a rapid charge transfer and optimal adsorption energies for intermediates of oxygen reduction/evolution reactions. These results show that the surface atomic structure engineering is important for the fabrication of efficient and durable electrocatalysts.

The growing concerns over climate change and energy security have stimulated a rapid development in the generation of clean energy 1 . Electrocatalysts play a key role in sustainable energy production, including fuel cells 2 , metal-air batteries 3 , 4 , 5 and water splitting 6 . Currently, noble metalsterm and their complexes are the most efficient catalysts 7 , 8 , 9 , but their high cost and scarcity greatly restricts commercial applications 10 . Transition metal oxidesterm·dom (TMOsterm·dom) are considered as the most probable alternatives to noble metal-based catalysts due to their low cost, nontoxicity and high stability 3 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 . Nevertheless, there is still an urgent need to further improve their activities and make them highly competitive in comparison with their noble-metal counterparts and useful for practical applications.

In the light of the above discussion, the surface structure engineering of TMOterm·dom catalysts involving better exposure of active sites to promote their electrocatalytic performance becomes of paramount significance. In the past decade, fundamental research has demonstrated that the rational design of active facetsterm·key with favourable atomic arrangement and coordination is the most promising route to control the atomic structure of noble metalsterm 7 , 8 and metal oxide 20 , 21 , 22 , 23 , 24 particulate catalysts, and achieve high catalytic activity. Further investigations suggest that the surface defectsterm·dom 25 , 26 , 27 , 28 can greatly influence the electronic structureterm·dom and thus the surface chemistry of faceted catalysts 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 . Hence, a proper manipulation of defects on the desired facetsterm·key of catalysts has received a considerable attention and brought some exciting breakthroughs. For instance, introduction of defects on {100} facetsterm·key has been effectively used to tailor the band structure of titanium dioxide (TiO 2 ) to make it suitable for photocatalytic hydrogen evolution under visible light illumination 30 , 33 . Engineering sulfur vacancies on the basal planes of molybdenum disulfide (MoS 2 ) nanosheets can be used to finely tune the adsorption free energy of hydrogen to achieve the highest activity of the aforementioned nanosheets towards hydrogen evolution reaction among various MoS 2 -based catalysts 31 . However, the application of faceted TMOterm·dom catalysts in electrocatalysisterm·key is in its infancy 37 . Also, engineering favourable defects on the desired facetsterm·key and understanding their role in electrocatalysisterm·key at the atomic level is still lacking.

Very recently, one-dimensional (1D) nanoarraysterm·dom directly grown on the current collectors have attracted a lot of attention in electrocatalysisterm·key 16 , 38 , 39 , 40 , 41 , 42 because their 1D morphology assures adequate diffusion of reactants and rapid charge transport. Although a great progress has been achieved in electrocatalysisterm·key, much less has been done towards engineering the surface atomic structure of the aforementioned nanoarraysterm·dom to explore their full potential. Herein, we report the surface structure engineering of single-crystal (SC) CoOterm nanorodsterm·key (NRs) through creating desired facetsterm·key and defects ( Fig. 1 ). Our experimentsterm·mth, microscopic and spectroscopic characterization and the density functional theory (DFTterm·key) computation studies demonstrate that the O-vacanciesterm·dom present on the pyramidal nanofacets of CoOterm NRs can be effectively used to tailor the electronic structureterm·dom of NRs, which results in rapid charge transfer and favourable energetics for both oxygen reduction reactionterm·dom (ORRterm·dom) and oxygen evolution reactionterm·dom (OERterm·dom) as evidenced by excellent activity and durabilityterm·dom of CoOterm NRs towards both reactions. Significantly, their ORRterm·dom activity approaches that of platinumterm (Ptterm) catalysts and their OERterm·dom activity exceeds that of ruthenium dioxide (RuO 2 ) catalysts; their overall activity is comparable to that of the best bifunctional ORRterm·dom/OERterm·dom catalysts.

Schematic illustration of engineering the surface of SC CoOterm NRs. ( a ) SC CoOterm NRs fabricated directly on carbon fibre substrate. ( b ) Numerous nanopores present on the surface and across SC NRs. ( c ) The surface of SC CoOterm NRs covered with textured nanopyramids. ( d ) The dominant exposed facetsterm·key of nanopyramids are electrochemically active vacancy-rich O-terminated {111} facetsterm·key.

Results Synthesis and characterization of SC CoOterm NRs First, we report the synthesis of SC CoOterm NRs with textured pyramidal nanofacets immobilized directly on a carbon fibre paper (CFP) substrate ( Fig. 1a–c ) via a simple and well-controlled cation exchangeterm·mth method ( Supplementary Fig. 1 ). This synthesis was accomplished by converting SC zinc oxideterm (ZnOterm) NRs ( Supplementary Fig. 2 ) to CoOterm NRs ( Supplementary Fig. 3 ) via the aforementioned cation exchangeterm·mth reaction in gas phase 43 . A controlled fabrication of CoOterm NRs with tailorable length was achieved by precise tuning the length of ZnOterm NRs from dozen nanometres to several microns ( Supplementary Fig. 4 ).

The microstructure of as-synthesized CoOterm NRs was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As displayed in Fig. 2a , the entire surface of CFP is uniformly covered with CoOterm NRs, which are SCs ( Fig. 2c , inset). Interestingly, after cation exchangeterm·mth reaction, numerous nanopores with sizes of 5–20 nm are visible on the surface and across NRs ( Supplementary Fig. 5 ). Surprisingly, the surface of CoOterm NRs becomes rather rough as evidenced by tooth-like growths with sizes of about 5 nm ( Fig. 2b ). Atomic level high-angle annular dark field-scanning TEM (HADDF-STEM) image shows that these growths are sharply terminated with {111} nanofacets ( Fig. 2c ). Notably, the gradual contrast variation in these single tooth-like growths suggests a progressive variation in their thickness ( Fig. 2c ). Simulation of experimental image was performed to accurately determine the three-dimensional atomic arrangement in the aforementioned growths. A speculated nanopyramidal structure with exposed {100} and {111} facetsterm·key was constructed as shown in Fig. 2d,e . Figure 2f,g indicates a good agreement between the experimental and simulated images. Moreover, the intensity profile along the terminated {111} facetterm·key in the experimental image ( Fig. 2h ) closely resembles that in the simulated one ( Fig. 2i ). A good match between experimental and simulated HADDF-STEM images clearly demonstrates that the surface of SC CoOterm NRs is surrounded by nanopyramids and preferentially exposed {111} facetsterm·key. The surface area of exposed {111} facetsterm·key is estimated to be 46% of the total surface area of SC CoOterm NRs ( Supplementary Fig. 6 and Supplementary Note 1 ). Notably, for CoOterm the surface energy of {111} is much higher than that of other low-indexed facetsterm·key 44 . Such high percentage of {111} facetsterm·key without foreign stabilizer is rather difficult to achieve via thermodynamically controlled synthesis 44 . However, in our kinetics-governed cation exchangeterm·mth strategy, facetsterm·key with high surface energy and defects (discussed later) are forced to be exposed to facilitate the ion exchange process 45 , assuring the formation of a large amount of clean and defect-rich {111} facetsterm·key on the surface of SC CoOterm NRs, which is certainly highly preferable for catalysis. Furthermore, it should be noted that {111} facetsterm·key of the bulk CoOterm are polar, either terminated by oxygen (O) or Co-atomic layer 46 . A detailed X-ray photoelectron spectroscopy (XPS) analysis indicates that the exposed {111} facetsterm·key on CoOterm NRs should be O-terminated ( Supplementary Fig. 7 , Supplementary Table 1 and Supplementary Note 2 ).

Structural characterization of SC CoOterm NRs. ( a ) Top-view SEM image of SC CoOterm NRs fabricated directly on CFP. Scale bar, 10 μm. The inset in a shows morphology of SC CoOterm NRs. Scale bar, 1 μm. ( b ) High magnification TEM image of an individual SC CoOterm NR with saw-like edges. Scale bar, 20 nm. ( c ) High-resolution HAADF-STEM image taken from the outermost surface of a single SC CoOterm NR revealing the exposed {111} nanofacets (indicated by orange and green arrows), with inset showing the corresponding selected area electron diffraction pattern taken from [110] zone axis. Scale bar, 2 nm. ( d ) Atomic model of a nanopyramid enclosed with {100} and {111} facetsterm·key, and ( e ) the projection of this pyramidal structure along [110] zone axis. ( f , g ) Experimental and simulated HADDF-STEM images of the pyramidal structure, respectively. Scale bar in f , 1 nm. Note that inelastic and neutron scatterings were not considered in the simulation, which contribute to the background in h . ( h , i ) The intensity profiles taken from orange and grey lines in f and g , respectively.

Analysis of O-vacancy-rich pyramidal nanofacets To further probe the local chemical and electronic environment on the surface of SC CoOterm NRs, XPS and synchrotron-based X-ray absorption near edge fine structure (XANES) spectroscopy measurements were performed. The XPS O 1 s spectrum suggests an enrichment of O-vacanciesterm·dom on the surface of SC CoOterm NRs ( Supplementary Fig. 7a ). Further evidence comes from a close inspection of the fine structure of the O-K edge of XANES spectrum ( Fig. 3a ), in which the peak at ∼536.0 eV assigned to O deficiency 47 , 48 in SC CoOterm NRs is much higher than that of reference CoOterm. This is also consistent with the observation of a noticeable peak shift in Co-L 2,3 edge towards low photon energy and Co 2 p XPS spectrum towards low binding energy of SC CoOterm NRs ( Fig. 3b and Supplementary Fig. 8 ), which is an indicative of electron transfer from O-vacanciesterm·dom to Co d band. Thus, a combination of XANES and XPS results provides a crucial evidence for the presence of abundant O-vacanciesterm·dom on the surface of SC CoOterm NRs, the quantity of which is even higher than that on the surface of polycrystalline (PC) CoOterm NRs with threefold larger surface area ( Supplementary Table 1 ). Our DFTterm·key computations indeed reveal that the O-vacancy formation energy on the O-terminated {111} facetsterm·key (hereafter, referred to as ‘{111}-O facetterm·key') is by 3 eV lower than the corresponding values of {100} and {110} facetsterm·key ( Fig. 3c ). Clearly, such a significant reduction in the vacancy formation energyterm·dom results in larger concentration of the equilibrium O-vacanciesterm·dom on {111}-O facetsterm·key of SC CoOterm NRs. It indicates that the surface defectsterm·dom can be tuned and stabilized through facetterm·key engineering.

Analysis of O-vacanciesterm·dom on the pyramidal nanofaceted surface of SC CoOterm NRs. ( a , b ) O-K edge and Co-L 2,3 edge XANES spectra of SC CoOterm NRs and reference CoOterm, respectively. In a the peak at ∼536 eV of SC CoOterm NRs is assigned to O deficiency. In b the peaks at 781.5 and ∼800 eV of SC CoOterm NRs shift towards low photon energy relative to the reference CoOterm, indicating the transfer of electrons from O-vacanciesterm·dom to Co d band. ( c ) O-vacancy formation energies on {100}, {110} and {111}-O facetsterm·key of CoOterm showing a significant reduction in the vacancy formation energyterm·dom on {111}-O facetsterm·key.

Activity and durabilityterm·dom of SC CoOterm NRs towards ORRterm·dom/OERterm·dom As-fabricated SC CoOterm NRs with length of 1.6 μm on CFP were directly used as the working electrodes for both ORRterm·dom and OERterm·dom ( Supplementary Fig. 9 ), and their performances were compared with analogous electrodes prepared by using PC CoOterm NRs ( Supplementary Fig. 10 ), the state-of-art Ptterm and RuO 2 catalysts supported on CFP. The polarization curves were recorded without iR correction. As regards ORRterm·dom ( Fig. 4a,b and Supplementary Fig. 11a ), PC CoOterm NRs exhibit low activity, while SC CoOterm NRs show an onset potential of 0.96 V versus reversible hydrogen electrode (V RHE ), a half-wave potential ( E 1/2 ) of 0.85 V RHE and a Tafel slope of 47 mV per decade, which approach the values measured for Ptterm catalysts. These values are better than those of the well-developed cobalt oxideterm·key nanocrystals (NCs) coupled with carbon materials ( Supplementary Table 2 ). Moreover, SC CoOterm NRs show high selectivity towards ORRterm·dom with strong methanol tolerance ( Fig. 4a , inset). Besides an extraordinary activity towards ORRterm·dom, SC CoOterm NRs also demonstrate an excellent durabilityterm·dom. As shown in Fig. 4c , SC CoOterm NRs retain 97% of the initial ORRterm·dom current after 10 h continuous testing, whereas Ptterm catalyst lost more than 26% of its initial current, confirming much better durabilityterm·dom of active reaction sites present on SC CoOterm NRs ( Supplementary Figs 12 and Supplementary Fig. 13). Moreover, even after 3,000 cycle catalytic tests with accelerated scan rate of 100 mV s −1 , SC CoOterm NRs still retained their structure ( Supplementary Fig. 14 ). This excellent durabilityterm·dom originates from direct growth of SC CoOterm NRs on the CFP substrate to avoid aggregating and detaching problems, which are usually encountered in other faceted catalysts 49 .

Bifunctional ORRterm·dom/OERterm·dom performance of SC CoOterm NRs. ( a ) ORRterm·dom linear-sweep voltammograms (LSVs) of SC, PC CoOterm NRs and Ptterm catalysts directly deposited on CFP in O 2 -saturated 1 M KOH solution at scan rate of 0.5 mV s −1 without iR correction, with ORRterm·dom chronoamperometric response to methanol addition shown in inset. ( b ) ORRterm·dom Tafel plots of SC, PC CoOterm NRs and Ptterm catalysts. ( c ) ORRterm·dom chronoamperometric response of SC CoOterm NRs and Ptterm catalysts at a constant voltage of 0.60 V RHE . ( d ) OERterm·dom LSVs of SC, PC CoOterm NRs and commercial RuO 2 catalysts directly deposited on CFP in O 2 -saturated 1 M KOH solution at scan rate of 0.5 mV s −1 without iR correction, with the corresponding OERterm·dom Tafel plots shown in inset.

As regards the OERterm·dom activity ( Fig. 4d ), SC CoOterm NRs deliver a current density of 10.0 mA cm −2 ( E J =10 ) at 1.56 V RHE and a Tafel slope of 44 mV per decade. Such excellent OERterm·dom activity is better than that of the commercial RuO 2 catalyst. Importantly, SC CoOterm NRs exhibit an outstanding overall electrode activity as indicated by lower value (Δ E= 0.71 V) of the difference between the ORRterm·dom and OERterm·dom metrics (Δ E = E J =10 − E 1/2 ) 16 , 50 , outperforming the most of the reported highly active reversible oxygen catalysts ( Supplementary Table 2 ). This value is also comparable to the value obtained for Co 3 O 4 NCs deposited on N-doped graphene (Δ E =0.71 V) 11 , which is considered as the most efficient bifunctional catalyst.

Enhancement of electronic conductivity of SC CoOterm NRs High activity and durabilityterm·dom of SC CoOterm NRs clearly demonstrates that the as-synthesized SC CoOterm NRs are highly versatile and efficient electrocatalysts towards both ORRterm·dom and OERterm·dom. One prerequisite in the design of a highly efficient electrocatalyst is a rapid electron transfer 16 . It is acknowledged that the electronic conductivity of TMOsterm·dom is relatively poor 15 , greatly limiting their electrocatalytic activities. In the case of SC CoOterm NRs, a large quantity of O-vacanciesterm·dom localized on the {111}-O facetsterm·key, as well as the SC nature inherited from the ZnOterm NRs considerably enhance the carrier concentrationterm·dom in SC CoOterm NRs, which is one order higher than that in PC CoOterm NRs ( Supplementary Fig. 15 and Supplementary Note 3 ). Moreover, the nucleation and growth of SC CoOterm NRs directly on CFP also assures a rapid collection of charges. The aforementioned three important characteristics remarkably enhance the electronic conductivity of SC CoOterm NRs.

Intrinsic activity of SC CoOterm NCs towards ORRterm·dom/OERterm·dom To decouple the enhanced activity of SC CoOterm NRs from the contribution of advanced 1D nanoarrayterm·dom architecture, the intrinsic ORRterm·dom/OERterm·dom activity of SC CoOterm NCs with sizes of ∼50 nm ( Supplementary Method ) was evaluated in comparison to that of PC CoOterm NCs ( Supplementary Figs 16–18 and Supplementary Note 4 ) and other well-developed particulate cobalt oxideterm·key catalysts ( Supplementary Table 6 ). The ORRterm·dom kinetic current and OERterm·dom current are normalized by the electrochemically active surface area of catalyst to obtain the specific ORRterm·dom kinetic current density ( J k,specific ) and the specific OERterm·dom current density ( J specific ), respectively (see definition in Supplementary Table 3 ). As shown in Fig. 5a,b , for ORRterm·dom, J k and J k,specific of SC CoOterm NCs at 0.6 V RHE are about 4.2 and 7.2 times greater than those of PC CoOterm NCs, respectively. As regards OERterm·dom, J and J specific of SC CoOterm NCs at 1.65 V RHE are about 1.5 and 2.6 times larger than those of PC CoOterm NCs, respectively ( Fig. 5c,d ). These collective results clearly demonstrate that the intrinsic ORRterm·dom/OERterm·dom activity of CoOterm is strongly dependent on the surface structure.

Intrinsic ORRterm·dom/OERterm·dom activity of SC CoOterm NCs in comparison with the activity of PC CoOterm NCs. ( a , b ) J k and J k,specific for ORRterm·dom at 0.6 V RHE , respectively. ( c , d ) J and J specific for OERterm·dom at 1.65 V RHE , respectively.

ORRterm·dom/OERterm·dom free energy diagram and electronic structureterm·dom A series of DFTterm·key computations was conducted to get a fundamental understanding of the correlation between the surface atomic structure of CoOterm and the ORRterm·dom/OERterm·dom activity. Fig. 6a clearly reveals that the atomic arrangement on different facetsterm·key significantly affects adsorption sites and configuration of the reactant, that is, O 2 , in ORRterm·dom. Moreover, the overall ORRterm·dom/OERterm·dom pathway was calculated, and the free energy diagram at the equilibrium potential ( U RHE 0 =1.23 V) are shown in Fig. 6b . As reported by Nørskov et al . 51 , 52 , 53 , both ORRterm·dom and OERterm·dom involve four elementary reaction steps, in which ORRterm·dom proceeds through the formation of HOO* from adsorbed O 2 , followed by its further reduction to O* and HO*, while OERterm·dom proceeds in the reverse direction. For both ORRterm·dom and OERterm·dom, the ideal thermodynamic free energy change of the intermediates should be (refs 51 , 52 ), indicating no energy would be wasted to activate the reactions. As illustrated in Fig. 6b , for ORRterm·dom a large on the surface of {100} and {111}-O facetsterm·key indicates that the first electron transfer step to reduce the adsorbed O 2 to OOH* is endothermic, which is consistent with observations for the well-developed metal and metal oxide catalysts 17 , 53 . Besides, the large negative and on {110} and {100} facetsterm·key indicate that the chemical adsorption of O* and OH*, respectively, is too strong, which is also unfavourable for the subsequent electrocatalytic reactions. However, when O-vacancy is created on the surface of {111}-O (hereafter, referred to as ‘{111}-O V facetterm·key'), the formation of OOH* is facilitated, and all , and exhibit the lowest values among the four facetsterm·key, suggesting the most favourable ORRterm·dom kinetics on the {111}-O V facetsterm·key. As regards OERterm·dom, a similar analysis of the diagram for the reverse reaction shows that the {111}-Ov surface outperforms the other three facetsterm·key. Overall, the {111}-Ov surface exhibits a mediated adsorption–desorption behaviour ( ), which is beneficial for the overall ORRterm·dom/OERterm·dom. Thus, the theory and experimentterm·mth are in an excellent agreement, suggesting that the ORRterm·dom/OERterm·dom activity is successfully enhanced through atomic structure engineering. Our results demonstrate that there is a strong correlation between activity and atomic structure of CoOterm; that is, the ORRterm·dom/OERterm·dom activity of CoOterm increases in the following order {100}<{110}<{111}-Ov. To our knowledge this atomic scale structure–function relationship has not been considered for any other TMOterm·dom surfaces in the analysis of electrocatalysts.

Origin of ORRterm·dom/OERterm·dom activity on various facetsterm·key of CoOterm. ( a ) Atomic configurations of O 2 molecules on {100}, {110}, {111}-O and {111}-O V facetsterm·key. Notably, on {110} and {111}-Ov facetsterm·key, the O–O bond of adsorbed O 2 is remarkably elongated (the O–O distance of O 2 is 1.23 Å), suggesting an effective activation of O 2 in ORRterm·dom. ( b ) The calculated ORRterm·dom/OERterm·dom free energy diagram at the equilibrium potential on different facetsterm·key. ( c ) The projected density of states (PDOS) on pristine CoOterm and ( d ) CoOterm with O-vacanciesterm·dom. The arrow in d points new electronic states, which appear near the Fermi level in CoOterm with O-vacanciesterm·dom, responsible for the adsorption of intermediates on the O-vacanciesterm·dom.

The nature of the O-vacanciesterm·dom on the {111}-O facetsterm·key is further revealed through investigation of their electronic structureterm·dom ( Fig. 6c,d ). As indicated by the arrow in Fig. 6d , when O-vacancy is created in CoOterm, some new electronic states are created by hybridization of O-2 p , Co-3 d and Co-3 s in the bandgap, which are directly responsible for stronger adsorption of intermediates on the O-vacanciesterm·dom and for higher electronic conductivity of CoOterm ( Supplementary Fig. 19 ). Therefore, optimization of the electrocatalytic activity of CoOterm through creation O-vacanciesterm·dom on the {111}-O facetsterm·key can be finally ascribed to the successful engineering of the electronic structureterm·dom of CoOterm.

Discussion In conclusion, SC CoOterm NRs with exposed vacancy-rich pyramidal nanofacets were successfully fabricated on the CFP support. The current experimental and theoretical study shows that the creation of O-vacanciesterm·dom on the {111}-O facetsterm·key favourably affects the electronic structureterm·dom of CoOterm, resulting in the enhanced charge transfer and optimal energetics for both ORRterm·dom and OERterm·dom, which boosts the overall electrode activity of SC CoOterm NRs. Our work indicates that the rational design of the atomic surface of nanoarrayterm·dom electrodes can pave a new avenue for the fabrication of efficient and durable TMO-based electrochemical devices.

Methods Synthesis of SC CoOterm NRs on CFP substrate ZnOterm NRs with tailorable length ( Supplementary Figs 4 and 20 ) were grown on CFP under hydrothermal conditions and finally converted into CoOterm NRs using a cation exchangeterm·mth process in gas phase ( Supplementary Fig. 1 ). Specifically, the CFP loaded with ZnOterm NRs was placed in the centre of a quartz tube and cobalt chloride (CoCl 2 ) powder was placed 2.5 cm upstream from the tube centre. After the quartz tube was outgassed under vacuum, argon (Ar) gas flow (50 s.c.c.m.) was introduced into the system. The furnace was heated to and kept at 600, 650 or 700 °C for 30 min, and then cooled down to room temperature. It is found that the cation exchangeterm·mth temperature can considerably affect the concentration of O-vacanciesterm·dom ( Supplementary Table 7 ), and thus the electrocatalytic performance of SC CoOterm NRs ( Supplementary Figs 21–23 and Supplementary Note 5 ). The optimal exchange temperature is 600 °C and used in this study unless specifically notified. The loading mass of as-synthesized SC CoOterm NRs was ∼0.19 mg cm −2 .

Characterization SEM and TEM images were taken on a Hitachi S-4800 SEM and a JOEL 2100 TEM, respectively. HAADF-STEM images were collected using a JEOL ARM200F microscope with STEM aberration corrector operated at 200 kV. HADDF-STEM image simulation was carried out using a software package MacTempasX. The convergent semiangle and collection angle were 21.5 and 200 mrad, respectively. The aberration coefficient ( C s ) used was equal to 1 μm. Inelastic and neutron scatterings were not considered in the simulation. The synchrotron-based XANES measurements were carried out using the soft X-ray spectroscopy beamline at the Canadian Synchrotron. XANES spectra were recorded in the surface sensitive total electron yield with use of specimen current. All samples were scanned from 750 to 820 eV and from 510 to 580 eV in 0.1 eV steps, which encompasses the Co-L 2,3 and O-K absorption edges, respectively.

Electrochemical characterization Electrochemical measurements were performed in a three-electrode electrochemical cell using an Hg/HgO electrode in saturated KCl solution as the reference electrode, Ptterm plate as the counter electrode and the CFP electrode as the working electrode ( Supplementary Fig. 9 ). A flow of O 2 was maintained over the electrolyte (1.0 M KOH) during measurements to ensure the O 2 /H 2 O equilibrium at 1.23 V RHE .

DFTterm·key calculations All DFTterm·key computations were performed using Vienna Ab-initio Simulation Package. An effective U value of 3.7 eV was applied for Co 3 d states. The projector augmented wave pseudopotential with the Perdew–Burke–Ernzerhof exchange-correlation functional was used in the computations. The relevant details, references and data are given in the Supplementary Methods section and in Supplementary Tables 4 and 5 .

Data availability The data that support the findings of this study are available from the corresponding author on request.

Additional information How to cite this article: LingpersName, T. et al . Engineering surface atomic structure of single-crystal cobalt (II) oxideterm nanorodsterm·key for superior electrocatalysisterm·key. Nat. Commun. 7:12876 doi: 10.1038/ncomms12876 (2016).

Supplementary Material Supplementary Figures 1-23, Supplementary Tables 1-7, Supplementary Notes 1-5, Supplementary Methods and Supplementary References Acknowledgments This work was supported by the National Basic Research Program of China (2014CB931703), the Natural Science Foundation of China (51571149, 51471115, 21576202 and 21203099), the Natural Science Foundation of Tianjin city (15JCYBJC18200 and 13JCYBJC36800) and the Australian Research Council (ARC) through the Discovery Project programme (DP130104459, DP140104062 and DP160104866). DFTterm·key calculations were undertaken on Special Program for Applied Research Super Computation of the NSFC-Guangdong Joint Fund (the second phase). We would like to thank Ning Lu for his kind help in this work.

Footnotes References Associated Data Supplementary Materials Supplementary Figures 1-23, Supplementary Tables 1-7, Supplementary Notes 1-5, Supplementary Methods and Supplementary References Data Availability Statement The data that support the findings of this study are available from the corresponding author on request.