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

Engineering the surface structure at the atomic level can be used to precisely and effectively manipulate the reactivity and durability of catalysts. Here we report tuning of the atomic structure of one-dimensional single-crystal cobalt (II) oxide (CoO) nanorods by creating oxygen vacancies on pyramidal nanofacets. These CoO nanorods exhibit superior catalytic activity and durability 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 CoO nanorods are in the oxygen vacancies that can be readily created on the oxygen-terminated {111} nanofacets, which favourably affect the electronic structure of CoO, 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.


Supplementary Figures
Supplementary Figure 1. Experimental setup illustrating the cation exchange process in gas phase.
The carbon fiber paper (CFP) substrate loaded with ZnO nanorods (NRs) is placed in the center of the furnace tube and CoCl2 powder is placed upstream from the center of the tube. A complete cation exchange is accomplished by heating the tube at 600 °C for 30 min in Ar gas flow.

Supplementary
where Ik is the ORR kinetic current, Scatalyst and Sgeo are the real active surface area of catalyst and the geometrical area of the electrode, respectively. c Jspecific was calculated using specific , where I is the OER current. b Average data from inductively coupled plasma mass spectrometry (ICP-MS) measurements.

Supplementary Note 1. Estimation of Surface Area Ratio of {111} Facets
As shown in Supplementary Fig. 6, the surface area of an individual nanopyramid, consisting of two {100} and two {111} facets, can be expressed as follows: where a is the length of the square side shown in Supplementary Fig. 6. Therefore, the surface ratio of {111} facets is:

Supplementary Note 2. XPS Analysis
The information provided by XPS for inorganic materials is below 2-4 nm from the outermost surface 16 , which is about 17-33 alternate Co and O atomic planes for CoO from <111> direction.
As can be seen in Supplementary

Supplementary Note 3. Mott-Schottky Analysis
The acceptor density can be calculated from the slopes of the M-S plots ( Supplementary Fig. 15 Fig. 16 and Supplementary Fig. 17).
The intrinsic ORR catalytic activity of SC CoO NCs was first evaluated by CV. As can be seen in Supplementary Fig. 16a, SC CoO NCs exhibit an ORR onset potential of 0.92 VRHE.
Although this value is not as good as than that of commercial 20 % Pt/C (~1 VRHE), it is better than that of PC CoO NCs with larger ECSA (Supplementary Fig. 18 and Supplementary Table 3) and most of transition metal oxide catalysts (Supplementary Table 6). The RDE measurements were further carried out to reveal the ORR kinetics of SC CoO NCs (Supplementary Fig. 16c).
The electron transfer number (n) was calculated to be 4 at 0.52-0.72 VRHE from the slopes of Koutecky-Levich plots ( Supplementary Fig. 16c, inset), suggesting SC CoO NCs favour 4e oxygen reduction process, similar to that observed on a commercial Pt/C catalyst ( Supplementary Fig. 16b, inset). Moreover, as shown in Fig. 5a and 5b, and described in the main text, SC CoO NCs exhibit a kinetic current density (Jk) of 17.8 mA cm -2 and a specific kinetic current density (Jk, specific) of 0.144 mA cm -2 at 0.6 VRHE, which are 4.2 and 7.2 times larger than those obtained for PC CoO NCs, respectively. The excellent ORR activity of SC CoO NCs is further supported by the small Tafel slope of 66 mV decade -1 at low overpotentials, which is much smaller than that obtained for PC CoO NCs and approach the value determined for Pt/C (64 mV decade -1 ). This fact is fairly consistent with the results obtained for catalysts supported on CFP (Fig. 4b). Besides, SC NCs also exhibit better stability than PC NCs and Pt/C ( Supplementary Fig. 16f).
Moreover, the intrinsic OER catalytic activity of SC CoO NCs was also tested in comparison to PC CoO NCs. As shown in Supplementary Fig. 17, SC CoO NCs afford lower onset potential and higher OER oxidation current than the corresponding values obtained for PC CoO NCs.
Moreover, as shown in Fig. 5c and 5d, and described in the main text, the current density (J) and specific current density (Jspecific) at 1.65 VRHE of SC CoO NRs are about 1.5 and 2.6 times larger than those of PC CoO NCs, respectively.
These results unambiguously demonstrate that the specific pyramidal structure, and highly exposed vacancy-rich {111} facets play the key role in enhancing the intrinsic ORR/OER activity of SC CoO NCs.

Supplementary Note 5. Tuning the Concentration of O-vacancies in SC CoO NCs
The concentration of O-vacancies in SC CoO NRs can be controlled through tuning the cation exchange temperature (Supplementary Fig. 21

Experiment Section
Synthesis of ZnO NRs on CFP substrate. Prior to the synthesis, a hydrophilic commercial CFP was cleaned by rinsing with water and ethanol and dried. Then, ZnO NRs were grown on CFP under hydrothermal conditions 21 . The length of ZnO NRs was simply tuned by controlling the growth time. Specifically, growth for 1 h, 2 h and 3 h yielded the lengths of NRs of 50 nm, 500 nm and 1.6 μm, respectively ( Supplementary Fig. 4), and further prolongation of the growth time did not produce longer NRs. An optimum growth time of 3 h was determined and used in this study ( Supplementary Fig. 20). In ORR process, the electron transfer number, n, per O2 molecule can be calculated from the slope of Koutecky-Levich plot using the following equation 6 ,

Synthesis of PC CoO
where J is the measured current density on RDE, k J is the kinetic current density at a constant potential,  is the electrode rotating speed, and B is the reciprocal of the slope. B can be determined from the slope of Koutecky-Levich plot using the following expression: where n is the number of electrons transferred per oxygen molecule, F is the Faraday constant,  is the kinetic viscosity, 2 O C is the bulk concentration of O2, and O2 D is the diffusion coefficient of O2 in 1 M KOH.

Computation Section
Introduction. All density functional theory (DFT) calculations were performed using the Vienna Ab-initio Simulation Package (VASP) package 24,25 . An effective U parameter of 3.7 eV was applied for Co 3d states under the approximation introduced by Dudarev et al. 26 to describe well the electronic structure of CoO. The projector augmented wave (PAW) 27,28 pseudopotential and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional 29 were used in the calculations with a 400 eV plane-wave cut-off energy. All structures in the calculations were spin-polarized and relaxed until the convergence tolerance of force on each atom was smaller than 0.05 eV. The energy converge criteria was set to be 10 -5 eV for self-consistent calculations with a gamma-centre 2x2x1 k-mesh. All periodic slabs have a vacuum spacing of at least 15 Å.
where * refers to a given adsorption site in the specific facet. The free energy of ORR can be computed by the following equation: The value of E  was obtained by computation performed for specified geometrical structures, and the values of ZPE  and S  were determined by using the computed vibrational frequencies for {110} surface of CoO (Supplementary Table 4), and the reactants and products in the gas phase 34 . Moreover, an external bias U was imposed on each step by including a -eU term in the computation of the reaction free energy. It is noteworthy that the exploration of active sites on the specific facets was conducted and chosen according to the most energetically stable adsorption site. The calculated free energies of the adsorbed atoms/molecules on the active sites on different facets of CoO are listed in Supplementary Table 5.