Probing the Crystal Plane Effect of Co3O4 for Enhanced

Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST

Article

Probing the Crystal Plane Effect of Co3O4 for Enhanced Electrocatalytic Performance towards Efficient Overall Water Splitting Li Liu, Zhiqiang Jiang, Ling Fang, Hai-Tao Xu, Huijuan Zhang, Xiao Gu, and Yu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07793 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Probing the Crystal Plane Effect of Co3O4 for Enhanced Electrocatalytic Performance towards Efficient Overall Water Splitting Li Liua, Zhiqiang Jiangb, Ling Fanga, Haitao Xua, Huijuan Zhanga, Xiao Gub*, Yu Wanga* The State Key Laboratory of Mechanical Transmissions and the School of Chemistry and Chemical Engineering; bDepartment of Applied Physics. Chongqing University, 55 Daxuecheng South Road, Shapingba District, Chongqing City, P. R. China, 401331 E-mail: [email protected]; [email protected]

a

Keywords: Crystal plane effect, Density functional theory, Electrocatalysis, Overall water splitting, hydrogen evolution reaction, oxygen evolution reaction Abstract: Identifying effective methods to enhance the property of catalysts is urgent to broaden the scanty technologies so far. Herein, we synthesized four Co3O4 crystals with different crystal planes and explored the crystal planes effect on electrochemical water splitting through theoretical and experimental studies for the first time. The results illustrate the correlation of catalytic activity is established as {111}>{112}>{110}>{001}. Co3O4 crystals exposed with {111} facets emerge the highest OER and HER activities. When fabricated in an alkaline electrolyzer, bifunctional {111}||{111} couple manifests the highest catalytic activity and satisfying durability for overall water splitting. Density-functional theory (DFT) explains {111} facet owns the biggest dangling bond density, highest surface energy and smallest absolute value of ∆GH*, leading to the enhanced electrocatalytic performance. This work will broaden our vision to improve the activity of various electrocatalysts by selectively exposing the specific crystal planes. Introduction Hydrogen is deemed as an ideal alternate for fossil fuels due to the high energy content.1 However, molecular hydrogen (H2) cannot be directly acquired from nature, because hydrogen element usually exists in water.2 Electrochemical water splitting is an advisable method to produce hydrogen power, which involves in two synchronous half-reactions including hydrogen evolution reaction (HER) and sluggish oxygen evolution reaction (OER).3-4 Hence, it requires an efficient catalyst. However, noble metal catalysts are impracticable, for the high cost and rarity. Moreover, they cannot synchronously reveal the highly catalytic activity for both HER and OER.5-7 For industrial overall water splitting, HER and OER catalysts are assembled in an electrolyzer with same electrolyte.8 But this integration is incompatible and brings about a mediocre performance.9 Therefore, the development of a bifunctional catalyst for HER and OER is imperative and still keeps a challenge.10-12 Cobalt-based catalysts, especially Co3O4, enjoy high catalytic activity and stability, because the cobalt ions own some d-band electrons similar to precious metal,13-14 and the diversiform d-orbits grant more active sites for electrocatalytic water splitting.15 It’s speculated that Co3O4 has a potential as the catalyst for both HER and OER. For the interface application of heterogeneous catalysis, the properties of catalysts immensely depend on the atomic arrangement of exposed surface, which involves in atom steps, corner, edges, coordination status, dangling bonds and surface energy, etc.16 Moreover, the preferential exposure of some specific facets with high energy could lower the reaction gaps to expedite the reaction rates. However, most of catalysts consist of the crystals exposed with mixed facets revealing the 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

unsatisfactory catalytic activity. Thus, controlling the anisotropic structure of catalysts with specific crystal facets becomes considerably necessary.17 But most attention is devoted to the facet effects on methanol oxidation16, supercapacitors18 and electrochemical sensor19. Recently, Huang’s group reported the exposure of specific facets contributes to the enhanced electrocatalytic activity of platinum-cobalt catalysts for anodic fuel cell reactions.17, 18 Until now, little work has dealt with the facet effect on electrocatalysis for full water splitting. In addition, the comprehensive theoretical studies should be proposed to expound the facet effect, whereas few literatures could adequately combine the theoretical investigations with the experimental data.19, 20 Herein, we synthesized four Co3O4 crystals exposed with different crystal planes, including {001},{110},{111} and {112} facets. Combined with DFT calculations, we try to illuminate the facet-dependent electrocatalytic performance of Co3O4 catalyst. The results demonstrate the electocatalytic activity of overall water splitting is ranked in the order of {111}>{112}>{110}>{001}. It’s the first time to research the electrocatalytic performance of Co3O4 catalysts towards full water splitting in terms of crystal facets exposure through theoretical and experimental studies. Results and Discussion We successfully synthesized four morphological samples and tested the crystal phase and purity by X-ray powder diffraction (XRD). As revealed in Figure S1a, the precursors of nanobelt and nanosheet are identified as orthorhombic Co(CO3)0.5(OH)·0.11H2O (JCPDS card No.48-0083) without any evidence of impurities. After thermal treatment, the Co(CO3)0.5(OH)·0.11H2O nanobelt precursors and nanosheet precursors absolutely transformed into cubic spinel Co3O4 nanobelt and nanosheet (JCPDS card No. 42-1467). No diffraction peaks of precursors are found in the XRD patterns of products as shown in Figure S1c. It’s worth noting that the XRD patterns of nanocube precursors and nanooctahedron precursors are indexed to the standard card No. 42-1467 of Co3O4, meaning that Co3O4 nanocube and Co3O4 nanooctahedron are obtained by one-step hydrothermal method (Figure S1b). After calcinations, the crystallinity of Co3O4 nanocube and Co3O4 nanooctahedron is obviously enhanced (Figure S1c). Energy dispersive spectrometry (EDS) verifies that four elements exist in the four samples, including C, Si, Co, O (Figure S2). Thereinto, C and Si elements are derived from the external environment and substrate, respectively. The atomic ratio of Co/O in the four samples is close to 3:4, matching with the XRD data. Raman spectra in Figure S3 further corroborate that the features in the low-wavenumber region match well with the characteristic vibrational modes of Co3O4, in which the peaks located at 187, 512, 609 cm-1 are indexed to the F2g mode of Co3O4, and the peaks at 466 and 674 cm-1 are attributed to Eg and A1g modes, respectively. The results further testify the successful synthesis of the Co3O4 crystals. The morphologies of as-synthesized samples were identified by scanning electron microscopy (SEM). Figure S4a and S4b reveal the low- and high-magnification SEM images of the precursors with nanocube shape. It is clear that a large scale of nanocube precursors with regular shape is obtained. And the perfect sharp edges, corners and well-defined faces of the nanocube precursors can be observed. These nanocube precursors own a smooth surface and the edge length is about 300 nm. After thermal treatment, the nanocube precursors transform into Co3O4 nanocube by inheritance of morphology (Figure 1a and 1b). Likewise, nanobelt precursors are shown in Figure S4c and S4d, in which those nanobelt precursors are randomly scattered. The average length and diameter of the nanobelt precursors are approximately 10 µm and 200 nm, respectively. After calcination, Co3O4 nanobelt keeps the one-dimensional morphology without agglomeration 2


Page 2 of 21

Page 3 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Figure 1c and 1d). Figure S4e displays the SEM image of the nanooctahedron precursors, in which a large number of nanooctahedron particles are obtained. According to the high-magnification SEM image (Figure S4f), the particle size of the nanooctahedron precursor is 100-200 nm. Moreover, Figure 1e and 1f demonstrate Co3O4 nanooctahedron remains the size and morphology of precursors without obvious change. In addition, Figure 1g and 1h demonstrate Co3O4 nanosheet inherits the length, width and thickness of 8-10 µm, 4-5 µm and 20-30 nm, respectively, from its precursors (Figure S4g and S4h). The microstructure was further identified by transmission electron microscopy (TEM) and high resolution-TEM (HRTEM). Obviously, thermal conversion didn’t destroy the morphology of samples (Figure S5). In the HRTEM image of Co3O4 nanocube (Figure 1i), there are two sets of crystal planes (0.28 nm) along with two mutually perpendicular directions, corresponding to (220) and (2-20) in cubic Co3O4. According to structural analysis, the normal direction perpendicular to the crystal plane is [001] zone axis. Accordingly, the preferentially exposed crystal facets are {001} facets. The corresponding fast-Fourier-transform (FFT) pattern displays square spot array, indicative of the [001] zone axis and the monocrystalline nature (inset). Along another zone axis [110], (004), (-220) and (2-2-2) crystal facets with interplanar spacings of 0.2 nm, 0.29 nm and 0.23 nm, respectively, are detected in Figure 1j, revealing Co3O4 nanobelt selectively exposes with {110} facets in accordance with the FFT pattern (inset). Besides, in Co3O4 nanooctehadron, (220) and (101) crystal facets with an included angle of 60° are found (Figure 1k), whose spacings are 0.29 nm and 0.14 nm, respectively. This crystal orientation relationship matches with Co3O4 [-111] direction, as exhibited in its FFT pattern (inset), indicating the exposure of {111} facets. In Figure 1l, the HRTEM image and the corresponding FFT pattern demonstrate two orthogonal crystal planes of (220) and (1-11) with 0.29 nm and 0.47 nm are indexed as [-112] crystal orientation, confirming the main exposed facets in Co3O4 nanosheet are {112} crystal plane. Proverbially, spinel Co3O4 includes octahedral coordinated Co3+ and tetrahedral coordinated Co2+. Previous literatures demonstrate Co3+ is active, whereas Co2+ is almost inactive.20 Therefore, the surface atomic configuration of the exposed crystal planes greatly affects the electrocatalytic performance of crystals. Given that, Figure 1m reveals the atomic arrangements of Co3O4 crystal along different lattice directions. Clearly, {111} facet solely exposes Co3+ in octahedral coordination, implying the plane with more undercoordinated atoms should own higher activity. Other than the {001}, {110} and {112} planes with mixed Co2+, Co3+ and O2- ions, {111} facet is composed of alternating layers of Co3+ and O2- ions creating an electrostatic dipole field perpendicular to the surface, which is beneficial to capturing charged groups from the solution. Bear this in mind, we speculate Co3O4 {111} facet should possess eminent catalytic activity for overall splitting water. The X-ray photoelectron spectroscopy (XPS) profiles of the as-obtained samples are exhibited in Figure 2. The binding energies of the four samples are calibrated strictly according to the standard C1s peak (284.6 eV). As depicted in Figure 2a, the Co2p spectra of the four samples reveal two similar XPS peaks, accompanying with two weaker satellite peaks. It's worth mentioning that the integral area of shake-up satellite peaks in Co3O4 nanooctahedron, Co3O4 nanosheet, Co3O4 nanobelt and Co3O4 nanocube gradually cuts down, demonstrating that the electron transition from 2p-orbit to 1s-orbit becomes more difficult in turn. The two peaks located in 780.2 eV and 795.4 eV are separated by 15.2 eV (spin orbit splitting), corresponding to Co2p1/2 and Co2p3/2, respectively, which is in agreement with the literatures about Co3O4.21 The principle peaks are fitted into four sub-peaks, wherein the fitting peaks at 779.5 eV and 794.7 eV 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with the satellite peak at 786.4 eV prove the chemical nature of Co3+, and the fitting peaks at 781.2 eV and 795.9 eV with the satellite peak at 804.0 eV are indexed to Co2+.22 Those results confirm that Co2+ ions and Co3+ ions co-exist in the four samples. More importantly, according to the integral areas in Table S2, a largest proportion of Co3+ is found in Co3O4 nanooctahedron, further suggesting {111} crystal plane owns the highest catalytic activity, which is in consistent with the result of atomic arrangements of Co3O4 {111} facet. Likewise, the O1s spectra (Figure 2b) of the four samples are also fitted into three components, including the stoichiometric O2- corresponding to metal-oxygen bond, non-stoichiometric oxygen species for deficiencies on the surface and weakly absorbed species.23 To clarify the facet effect on catalytic activity, the OER and HER performances of Co3O4 crystals with different facets were evaluated. Brunauer-Emmett-Teller (BET) method was adopted to normalize the catalytic activity so as to eliminate the effect of specific surface area (Figure S6). For comparison, the same measurements were also conducted for IrO2, Pt/C and Ni foam. As the OER catalysts, Figure 3a displays their linear sweep voltammetry (LSV) curves. Clearly, {111} facet reveals the earliest onset potential (1.48 V) with respect to {001}, {110} and {112} facets (1.56 V, 1.54 V and 1.52 V, respectively), except for IrO2 (1.47 V). Notably, Ni foam as substrate displays a negligible OER activity. Tafel slope is a crucial metric for OER catalysts to evaluate the OER increment rate. In Figure 3b, Tafel slope of {111} crystal plane is as low as 49 mV/dec, much smaller than that of {001},{110}, {112} crystal facets and even IrO2, suggesting more rapid OER rates. More importantly, the OER property of Co3O4 {111} facet is superior to most of cobalt-based catalysts reported in recent literatures, such as Co3O4 nanowire array (1.55 V, 70 mV/dec)24, Co3O4 nanowires (~1.56 V, 82 mV/dec)25, Co3O4 nanocubes (1.54 V, 76 mV/dec)26, etc (Table S3). Additionally, the overpotential (η10) at 10 mA/cm2 is also a metric relevant to catalytic performance. As depicted in Figure 3c, {111} crystal plane requires a smallest overpotential (η10) of only 285 mV to drive 10 mA/cm2. In stark contrast, the overpotential of 362, 341 and 312 mV for {001}, {110} and {112} facets, respectively, is demanded to reach the current density of 10 mA/cm2. The η10 of {111} crystal plane is also smaller than those of other analogous materials, such as Co3O4 nanocrystal (320 mV),14 mesoporous Co3O4 (380 mV),27 Co3O4 nanoparticles (320 mV),28 Co3O4 films (377 mV),29 Co3O4 nanowires (339 mV)25 and so on (see Table S3). Considering energy conversion systems, the stability toward the electrochemical reactions is another essential quality for a practical electrolyzer. Thus, the long-term durability of the four samples was conducted at the current density of 10 mA/cm2 by means of chronopotentiometric method. As depicted in Figure 3d, the water electrolysis potential for {001} crystal plane almost keeps unchanged after 25 h, revealing the eminent stability. However, the potential of {111} facet slightly increases. The inverse trend in stability indirectly illustrates higher activity of {111} crystal plane. What’s more, a multi-step chronopotentiometric curve for {111} facet in 1 M KOH is exhibited in Figure 3e. The current density successively increased 10 mA/cm2 in each time interval of 100 s. Obviously, the potential in every stage keeps stable further indicating the well-pleasing durability. Figure S7 displays a mild potential increase for {111} facet over 2000 cycles at 3 mV/s. The above results of stability tests confirm that {111} crystal plane emerges the strong activity maintenance over 25 h at 10 mA/cm2, even though the stability is slightly lower than that of {001} facet. To depict the effective active sites, electrochemical double layer capacitance (Cdl) and roughness factor (Rf) were measured (Figure 3f). The electrochemical capacitance was obtained by testing a small voltage range (1.05-1.15 V vs RHE) without obvious 4


Page 4 of 21

Page 5 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


faradaic process (Figure S8), in which the current is related to the double-layer charging. The current densities at 1.14 V under various scan rates draw up a line and its slope corresponds to Cdl. And Cdl is divided by the capacitance of ideal planar metal oxides (such as Co3O4 is normally taken to be 60 µF/cm2 24) with the smooth surface normally to get Rf. The Cdl of {111} crystal plane is 25.4 mF/cm2 higher than that of {001} crystal plane (5.2 mF/cm2), {110} crystal plane (9.8 mF/cm2) and {112} crystal plane (18.2 mF/cm2). For the reason that twice Cdl represents the corresponding electrochemical active surface area (ECSA)30, it can be concluded that {111} crystal plane possesses the highest ECSA (50.8 mF/cm2), compared with {001} crystal plane (10.4 mF/cm2), {110} crystal plane (19.6 mF/cm2) and {112} crystal plane (36.4 mF/cm2). And as expected, {111} crystal plane displays the biggest Rf value of 423, compared with {001} facet (Rf = 87), {110} facet (Rf = 163) and {112} facet (Rf = 303). Obviously, the preferentially exposed {111} crystal planes are more effective in amplifying the catalytically active surface area. As a result, sufficient exposure and improved employment of electroactive sites on the {111} facet of Co3O4 nanocrystals devote to the outstanding catalytic activity. The electrochemical impedance spectroscopy (EIS) was conducted to offer profound analysis towards the OER kinetics. Figure S9 displays the Nyquist plots of Co3O4 nanocrystals exposed with different crystal planes before and after electrochemical measurements. The four impedance spectra depict the similar form, which is consist of a semicircle at high-frequency range and a line at low-frequency range. The Nyquist plots were analyzed by circuit model fitting in the inset of Figure S9, which are made up of a series resistance (Rs) and a constant phase element (CPE) in series with one parallel branch involving in the charge transfer process (Rct-Zw). Thereinto, Rs relates to the intercept of real axis, Rct has a connection with the semicircle and Zw (Warburg impedance) has something to do with linear region. Rs and Rct values of the four samples are appeared in Table S4. Apparently, Rs values of the four crystal planes are ranged in the order of {111} < {112} < {110} < {001}. The lowest Rs value reveals that {111} crystal plane owns the smallest Ohmic loss in the electrolyte. Note that Rct value (0.228 Ω) of Co3O4 {111} facet is much lower than that of {001} facet (1.721 Ω), {110} facet (0.841 Ω) and {112} facet (0.383 Ω). For the reason that Rct is inversely proportional to the electron transfer rate, the smaller Rct value indicates the faster charge transport (electron-donating character) during OER process, which could assist the escape of the generated O2 gas. In addition, the resistance behavior of Zw is relative to the near linear in the low-frequency scope, reflecting the diffusion of ions in the electrolyte. The bigger the slope of line is, the smaller the Warburg impedance (Zw) is. It can be seen that {111} crystal plane displays the smallest Zw value, illustrating most facile diffusion of electrolyte. Compared with the Nyquist plots before and after OER tests, the radius of semicircle and intercept of Z’-axis for {111} facet are almost unchanged, while the increments of Rs and Rct are comparatively obvious for other three facets, demonstrating that the exposure of {111} crystal planes is advantageous to OER kinetics. The HER performance was also explored in 1M KOH. As observed in Figure 4a, Pt/C displays optimal activity, while Ni foam owns inferior catalytic activity. The LSV curve recorded for {111} facet presents the onset potential of -150 mV, superior to that of {001}, {110} and {112} facets (-248 mV,-217 mV, -196 mV, respectively). And the cathodic current rises rapidly with scanning toward negative potential. Figure 4b reveals HER Tafel slope of {111} facet (50 mV/dec) is competitive with Pt/C (47 mV/dec) and smaller than {001}(97 mV/dec), {110}(78 mV/dec) and {112}(59 mV/dec) facets, illustrating the most rapid rate of H2 evolution on the {111} facets. Additionally, η10 is arranged into the order of {111} (195 mV) < {112}(232 mV) < {110} (260 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mV) < {001}(284 mV) (Figure 4c). The HER property of Co3O4 {111} facet is also better than many reported Co-based materials, such as Co3O4 nanoplates (71 mV/dec, η10 = 523 mV)31, Co3O4 NC/carbon (116 mV/dec, η10 = 380 mV)14, CoP/CNT (54 mV/dec)7, etc (see Table S5).The smaller Tafel slope and η10 values testify the preferential exposure of {111} facet benefits HER activity. Durability measurements prove {111} facet displays slightly inferior stability compared with {001} facet (Figure 4d and Figure S10). Impedance measurements (Figure S11 and Table S6) further demonstrate {111} facet exhibits the lowest Rs and Rct values. Moreover, after HER tests, the variation in Rs and Rct for {111} facet is ignorable. The results indicate the exposure of {111} facet is beneficial to achieving the efficient conversion from H2O into H2 bubbles in alkaline electrolyte. Inspired by the results, we fabricated an alkaline electrolyzer by employing the bifunctional catalysts as both anode and cathode to explore practical electrolysis applications of this system. For argument’s sake, the benchmarking Pt/C||IrO2 couple was also assembled and exhibited the lowest onset potential (1.50 V) to drive water electrolysis (Figure 5a). Notably, its property is quickly surpassed by {111}||{111} couple though {111}||{111} couple displays higher onset potential (~1.58 V). Besides, {111}||{111} couple can deliver a current density of 10 mA/cm2 at 1.60 V (Figure 5a), which is lower than that of {001}||{001} (1.74 V), {110}||{110} (1.69 V), {112}||{112} (1.65 V) and other recently reported alkaline water electolyzers, for instance NiCo2O4|| Ni0.33Co0.67S2 (1.65 V)32, NiSe/NF||NiSe/NF NW(1.63 V)3 and Co0.13Ni0.87Se2|| Co0.13Ni0.87Se2 (1.62 V)33, etc (Table S7). Additionally, the current density can be kept at 10 mA/cm2 with a slight potential increment over 22 h for {111}||{111} couple, whereas the potential obviously increases for Pt/C||IrO2 couple (Figure 5b). To dissect the correlation between crystal facets and electrocatalytic performances, DFT calculated the optimized structures of the four crystal planes (Figure 6a-e). Generally, Co3+oct ions on the surface are coordinatively unsaturated and link with many dangling bonds, which could provide catalytically active sites. Thus, the facet with more dangling bonds owns more active sites. Clearly, Co3O4 {111} facet possesses the highest dangling bond density of Co3+ (0.32 Å-2), implying the most reactive sites (Table 1). Besides, the surface energy is arrayed as {111}>{112}> {110}>{001}. {111} facet with the highest surface energy (4.28 J/m2) unfolds the highest activity for adsorption of ionized oxygen species. Admittedly, electronic density of states (DOS) is a crucial indicator to probe the electrocatalytic property of catalysts. As depicted in Figure S12, the DOS near the Fermi level (taken as zero energy) is successive without any gap, uncovering the favorable electron mobility. More importantly, the DOS around Fermi level of {111} facet surges violently, demonstrating {111} facet possesses more charge carriers and higher intrinsic conductivity. Thus, the valence electrons in Co3O4 {111} facet are able to travel more feely, which aligns well with the EIS results. In addition, DFT was also applied to calculate the theoretical overpotential based on the free energy diagram of each OER path (Figure S13), and Gibbs free energy for H adsorption (∆GH*) on the surface of catalysts (Figure 6f) according to the optimal adsorption sites (Figure S14). As observed, {111} facet exhibits the smallest theoretical overpotential (0.72 V) among the four crystal planes, indicative of the fastest OER kinetics, verifying the experimental observations (Table S8-9). It’s worth noting that HER process in alkaline solution includes three states (the initial catalyst-H2O state, intermediate catalyst-H* state and ultimate catalyst-H2 state). An optimized HER catalyst should own the appropriate ability to absorb H* species and to release H2. Thus, a small absolute value of ∆GH* (close to 0 eV) is 6


Page 6 of 21

Page 7 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


needed to overcome the reaction barriers in adsorption and desorption processes. Obviously, among the four crystal planes, ∆GH* of {111} facet (0.166 eV) is most close to zero, illustrating that the hydrogen atom adsorbed at hollow site of {111} has the lowest energy barrier, that is the lowest overpotential to drive the HER process (Table 2). The results explain the superior HER activity for {111} crystal plane. In brief, based on the theory and fact, we can conclude that facet effect is the key point contributed to the huge difference in the ability of water splitting for Co3O4 crystals. Conclusion In summary, we investigated the crystal plane effect of Co3O4 crystals on overall water splitting combined with theoretical and experimental studies. The correlation between the different Co3O4 crystal planes and the ability of full water splitting is established as {111}112}>{110}>{001}. When fabricated in an alkaline electrolyzer, bifunctional {111} ||{111} couple manifests the highest catalytic activity and satisfying durability for overall water splitting, even higher than benchmarking Pt/C||IrO2 couple. DFT offers the theoretical explanation that {111} facet owns more active Co3+, bigger dangling bond density, higher surface energy, smaller absolute value of ∆GH* and lower theoretical overpotential than {001}, {110} and {112} facets. It’s clear that selectively exposure of specific crystal planes opens up a new door to obtain the catalysts with high catalytic activity for overall water splitting.

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Experimental sections Material synthesis The Co3O4 nanocrystals with different crystal planes were synthesized by an ordinary hydrothermal strategy, followed by thermal decomposition. For the synthesis of Co3O4 nanosheets, 12.5 mL Solution A (concentrated NH3·H2O) was added into 15 mL Solution B (ethylene glycol) to form a transparent solution. After stirring of 2 min, 3.5 mL (1M) Na2CO3 was poured into the above mixture. Following that, 5 mL (1M) Co(NO3)2 was poured into the mixture, stirring for another 20 min. At this moment, the precursor solution turned into violet solution. When the resulting homogeneous solution was transformed to a 50 mL liner, a temperature-rise period was carried out at 170 oC for 17 h. Then, the deionized water and ethanol were employed to rinse the as-obtained samples after cooling process. Next, the sample was exsiccated at 60 oC. Finally, Co3O4 nanosheet was obtained by calcining the precursor in air at 300 oC for 5 h. For the synthesis of Co3O4 nanocube, nanooctahedron and nanoblet, the experimental procedures are similar to the Co3O4 nanosheet with some slight variations. The detailed experimental conditions are listed in Table S1. Structure characterization The powder X-ray diffractometer (Bruker D8 Advance, Cu Kα radiation (λ = 1.54184 Å)), the energy-dispersive X-ray spectrometer (Philips, Tecnai, F30), Raman spectrometer equipped with argon (532 nm) laser in the wavenumber of 100-1000 cm-1 (Horiba LabRAM HR Evolution) and X-ray photoelectron spectrometer (ESCALAB250) were used to identify the crystal structure, pure phase, component and valence states of the as-obtained samples. The scanning electron microscope (JEOL JSM-7800F) and transmission electron microsphere (Philips, Tecnai, F30) were performed to characterize the morphology and architectural features of the as-synthesized samples. Brunauer-Emmett-Teller surface area analyzer (BET, Quantachrome Autosorb-6B) was applied to test the surface area and pore size of the samples. Electrochemical Characterization The as-synthesized samples were dispersed a mixed solution with 10% Nafion (0.5 wt%) and 90% ethanol to form homogeneous slurry. Then the slurry was evenly painted on Ni foam with the coating area of 1 cm2 as working electrode. To eliminate the influence of specific surface area, BET method was adopted to normalize the catalytic activity of the four samples by controlling the loading mass. The loading mass of Co3O4 nanocube, Co3O4 nanobelt, Co3O4 nanooctahedron and Co3O4 nansheet is 1.23 mg, 1.41 mg, 1.00 mg and 0.59 mg, respectively. The electrochemical workstation (CHI660D, shanghai) was used to conduct the electrochemical measurements for OER and HER at room temperature using a typical three-electrode system with 1M KOH electrolyte. The above modified electrode, saturated calomel electrode (SCE) and graphite plate were acted as working electrode, reference electrode and counter electrode, respectively. According to the Nernst equation: E(RHE) = E(SCE) + 0.0592 pH + 0.241, all the voltage values were expressed by the reversible hydrogen electrode (RHE). At first, the cyclic voltammetry (CV) measurements were carried out after at least 10 cycles to reach a stable state. Then, linear sweep voltammetry (LSV) was conducted at 3 mV/s to obtain the polarization profiles and Tafel slopes. The voltage ranges are 1.1 V~1.75 V (vs RHE) for OER and 0.0 V~-0.4 V (vs RHE) for HER measurements. The stability evaluations of OER and HER were carried out by chronopotentiometric measurements at 10 mA/cm2 and -10 mA/cm2, respectively. Electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 105-10-2 8


Page 8 of 21

Page 9 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Hz in potentiostatic mode. And the Zview version 3.2c-software was applied to analyze the impedance data for OER and HER. The overall water splitting was conducted in a two-electrode system with 1 M KOH electrolyte at 3 mV/s, and as-prepared samples were used as anode and cathode, simultaneously. Computation Methods All the computations were performed by using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerh (PBE)34 within the DFT framework as performed in Vienna ab initio simulation package (VASP).35 The core-valence electron interaction was conducted by the project-augmented wave (PAW) method.36 The valence electronic states were expanded in plane wave basis sets within a cutoff energy of 500 eV. The Co3O4 surfaces were modeled periodic slabs separated by a vacuum layer 15Å wide. The bottom three layers were fixed, and all the other atoms were fully relaxed. For these surface slabs, a 3×3×1 k-point mesh was used. The force threshold for the optimization was set as 0.01 eV/Å. Acknowledgements This work was financially supported by the Thousand Young Talents Program of the Chinese Central Government (Grant No. 0220002102003), National Natural Science Foundation of China (NSFC, Grant No. 21373280, 21403019), the Fundamental Research Funds for the Central Universities (0301005202017), Beijing National Laboratory for Molecular Sciences (BNLMS), Hundred Talents Program at Chongqing University (Grant No. 0903005203205) and SKLMT-ZZKT-2017M11. Supporting information XRD patterns, EDS spectra Raman spectra, SEM images, TEM images, BET analysis, EIS data, DOS results, free energy diagrams of OER pathways, the configurations of different hydrogen adsorption sites are available free of charge via the Internet at http://pubs.acs.org.

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Reference 1. Kanan, M. W.; Nocera, D. G., In Situ Formation of An Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321 (5892), 1072-1075. 2. Vigil, J. A.; Lambert, T. N.; Christensen, B. T., Cobalt Phosphide-Based Nanoparticles as Bifunctional Electrocatalysts for Alkaline Water Splitting. J. Mater. Chem. A 2016, 4 (20), 7549-7554. 3. Tang, C.; Cheng, N. Y.; Pu, Z. H.; Xing, W.; Sun, X. P., NiSe Nanowire Film Supported on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting. Angew. Chem. Int. Ed. 2015, 54 (32), 9351-9355. 4. Yan, X. D.; Tian, L. H.; He, M.; Chen, X. B., Three-Dimensional Crystalline/Amorphous Co/Co3O4 Core/Shell Nanosheets as Efficient Electrocatalysts for the Hydrogen Evolution Reaction. Nano Lett. 2015, 15 (9), 6015-6021. 5. Subbaraman, R.; Tripkovic, D.; Strmcnik, D.; Chang, K. C.; Uchimura, M.; Paulikas, A. P.; Stamenkovic, V.; Markovic, N. M., Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)(2)-Pt Interfaces. Science 2011, 334 (6060), 1256-1260. 6. Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y., Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3 (3), 399-404. 7. Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X., Carbon Nanotubes Decorated with CoP Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution. Angew. Chem. Int. Ed. 2014, 53 (26), 6710-6714. 8. Yan, X. D.; Li, K. X.; Lyu, L.; Song, F.; He, J.; Niu, D. M.; Liu, L.; Hu, X. L.; Chen, X. B., From Water Oxidation to Reduction: Transformation from NixCo3-xO4 Nanowires to NiCo/NiCoOx Heterostructures. ACS Appl. Mater. Interfaces 2016, 8 (5), 3208-3214. 9. Jiao, L.; Zhou, Y. X.; Jiang, H. L., Metal-Organic Framework-Based CoP/Reduced Graphene Oxide: High-Performance Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Sci. 2016, 7 (3), 1690-1695. 10. Wang, Y.; Xie, C.; Liu, D.; Huang, X.; Huo, J.; Wang, S., Nanoparticle-Stacked Porous Nickel-Iron Nitride Nanosheet: A Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting. ACS Appl. Mater. Interfaces 2016, 8 (29), 18652-18657. 11. Yang, Y.; Fei, H. L.; Ruan, G. D.; Tour, J. M., Porous Cobalt-Based Thin Film as a Bifunctional Catalyst for Hydrogen Generation and Oxygen Generation. Adv. Mater. 2015, 27 (20), 3175-3180. 12. Yan, X. D.; Tian, L. H.; Li, K. X.; Atkins, S.; Zhao, H. F.; Murowchick, J.; Liu, L.; Chen, X. B., FeNi3/NiFeOx Nanohybrids as Highly Efficient Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Mater. Interfaces 2016, 3 (22), 1600368-1600375. 13. Wang, H. Y.; Hung, S. F.; Chen, H. Y.; Chan, T. S.; Chen, H. M.; Liu, B., In Operando Identification of Geometrical-Site-Dependent Water Oxidation Activity of Spinel Co3O4. J. Am. Chem. Soc. 2016, 138 (1), 36-39. 14. Du, S. C.; Ren, Z. Y.; Zhang, J.; Wu, J.; Xi, W.; Zhu, J. Q.; Fu, H. G., Co3O4 Nanocrystal Ink Printed on Carbon Fiber Paper as A Large-Area Electrode for Electrochemical Water Splitting. Chem. Commun. 2015, 51 (38), 8066-8069. 15. Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M.; Sun, X., Recent Progress in Cobalt-Based Heterogeneous Catalysts for Electrochemical Water Splitting. Adv. Mater. 2016, 28 (2), 215-230. 10


Page 10 of 21

Page 11 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


16. Bu, L. Z.; Feng, Y. G.; Yao, J. L.; Guo, S. J.; Guo, J.; Huang, X. Q., Facet and Dimensionality Control of Pt Nanostructures for Efficient Oxygen Reduction and Methanol Oxidation Electrocatalysts. Nano Res. 2016, 9 (9), 2811-2821. 17. Hu, L. H.; Peng, Q.; Li, Y. D., Selective Synthesis of Co3O4 Nanocrystal with Different Shape and Crystal Plane Effect on Catalytic Property for Methane Combustion. J. Am. Chem. Soc. 2008, 130 (48), 16136-16137. 18. Wang, Y.; Zhong, Z. Y.; Chen, Y.; Ng, C. T.; Lin, J. Y., Controllable Synthesis of Co3O4 from Nanosize to Microsize with Large-Scale Exposure of Active Crystal Planes and Their Excellent Rate Capability in Supercapacitors Based on the Crystal Plane Effect. Nano Res. 2011, 4 (7), 695-704. 19. Yu, X. Y.; Meng, Q. Q.; Luo, T.; Jia, Y.; Sun, B.; Li, Q. X.; Liu, J. H.; Huang, X. J., Facet-Dependent Electrochemical Properties of Co3O4 Nanocrystals toward Heavy Metal Ions. Sci. Rep. 2013, 3, 2886-2892. 20. Petitto, S. C.; Marsh, E. M.; Carson, G. A.; Langell, M. A., Cobalt Oxide Surface Chemistry: The Interaction of CoO(100), Co3O4(110) and Co3O4(111) with Oxygen and Water. J. Mol. Catal. A: Chem 2008, 281 (1-2), 49-58. 21. Wang, Y. Y.; Lei, Y.; Li, J.; Gu, L.; Yuan, H. Y.; Xiao, D., Synthesis of 3D-Nanonet Hollow Structured Co3O4 for High Capacity Supercapacitor. ACS Appl. Mater. Interfaces 2014, 6 (9), 6739-6747. 22. Marco, J. F.; Gancedo, J. R.; Gracia, M.; Gautier, J. L.; Rios, E.; Berry, F. J., Characterization of the Nickel Cobaltite, NiCo2O4 Prepared by Several Methods: An XRD, XANES, EXAFS, and XPS Study. J. Solid State Chem. 2000, 153 (1), 74-81. 23. Chuang, T. J.; Brundle, C. R.; Rice, D. W., Interpretation of X-Ray Photoemission Spectra of Cobalt Oxides and Cobalt Oxide Surfaces. Surf. Sci. 1976, 59 (2), 413-429. 24. Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z., Metal-Organic Framework Derived Hybrid Co3O4-Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136 (39), 13925-13931. 25. Zhang, Y. Q.; Ouyang, B.; Xu, J.; Jia, G. C.; Chen, S.; Rawat, R. S.; Fan, H. J., Rapid Synthesis of Cobalt Nitride Nanowires: Highly Efficient and Low-Cost Catalysts for Oxygen Evolution. Angew. Chem. Int. Ed. 2016, 55 (30), 8670-8674. 26. Li, X.; Zhang, L.; Huang, M. R.; Wang, S. Y.; Li, X. M.; Zhu, H. W., Cobalt and Nickel Selenide Nanowalls Anchored on Graphene as Bifunctional Electrocatalysts for Overall Water Splitting. J. Mater. Chem. A 2016, 4 (38), 14789-14795. 27. Chen, S. Q.; Zhao, Y. F.; Sun, B.; Ao, Z. M.; Xie, X. Q.; Wei, Y. Y.; Wang, G. X., Microwave-Assisted Synthesis of Mesoporous Co3O4 Nanoflakes for Applications in Lithium Ion Batteries and Oxygen Evolution Reactions. ACS Appl. Mater. Interfaces 2015, 7 (5), 3306-3313. 28. Li, X. Z.; Fang, Y. Y.; Lin, X. Q.; Tian, M.; An, X. C.; Fu, Y.; Li, R.; Jin, J.; Ma, J. T., MOF Derived Co3O4 Nanoparticles Embedded in N-Doped Mesoporous Carbon Layer/MWCNT Hybrids: Extraordinary Bi-Functional Electrocatalysts for OER and ORR. J. Mater. Chem. A 2015, 3 (33), 17392-17402. 29. Jeon, H. S.; Jee, M. S.; Kim, H.; Ahn, S. J.; Hwang, Y. J.; Min, B. K., Simple Chemical Solution Deposition of Co3O4 Thin Film Electrocatalyst for Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2015, 7 (44), 24550-24555. 30. Fan, K.; Chen, H.; Ji, Y. F.; Huang, H.; Claesson, P. M.; Daniel, Q.; Philippe, B.; Rensmo, H.; 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Li, F. S.; Luo, Y.; Sun, L. C., Nickel-Vanadium Monolayer Double Hydroxide for Efficient Electrochemical Water Oxidation. Nat. Commun. 2016, 7, 11981-11989. 31. Zhou, X. M.; Xia, Z. M.; Tian, Z. M.; Ma, Y. Y.; Qu, Y. Q., Ultrathin Porous Co3O4 Nanoplates as Highly Efficient Oxygen Evolution Catalysts. J. Mater. Chem. A 2015, 3 (15), 8107-8114. 32. Peng, Z.; Jia, D. S.; Al-Enizi, A. M.; Elzatahry, A. A.; Zheng, G. F., From Water Oxidation to Reduction: Homologous Ni-Co Based Nanowires as Complementary Water Splitting Electrocatalysts. Adv. Energy Mater. 2015, 5 (9), 1402031-1402037. 33. Liu, T. T.; Asiri, A. M.; Sun, X. P., Electrodeposited Co-Doped NiSe2 Nanoparticles Film: A Good Electrocatalyst for Efficient Water Splitting. Nanoscale 2016, 8 (7), 3911-3915. 34. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77 (18), 3865-3868. 35. Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials to the pProjector Augmented-Wave Method. Phys. Rev. B 1999, 59 (3), 1758-1775. 36. Blochl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50 (24), 17953-17979.

12


Page 12 of 21

Page 13 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Low- and high-magnification SEM images of (a)-(b) Co3O4 nanocube, (c)-(d) Co3O4 nanobelt, (e)-(f) Co3O4 nanooctahedron and (g)-(h) Co3O4 nanosheet. HRTEM images and corresponding FFT patterns of (i) Co3O4 nanocube, (j) Co3O4 nanobelt, (k) Co3O4 nanooctahedron and (l) Co3O4 nanosheet. (m) The surface atomic configuration of Co3O4 in {001}, {110}, {111} and {112} facets.

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. XPS spectra of (a) Co2p and (b) O1s for Co3O4 nanocube, Co3O4 nanobelt, Co3O4 nanooctahedron and Co3O4 nanosheet.

14


Page 14 of 21

Page 15 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) OER polarization curves, (b) OER Tafel plots, (c) The required overpotential (η10) at the current density of 10 mA/cm2 and (d) Chronopotentiometric curves of Co3O4 {001}, {110}, {111}, {112} crystal planes and IrO2. (e) The multi-current process of {111} crystal plane in 1 M KOH with a current density increment of 10 mA/cm2 per 100 s. (f) Liner fitting of the current density vs the scan rate at 1.14 V vs RHE for Co3O4 {001}, {110}, {111} and {112} crystal planes.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. (a) HER polarization curves, (b) HER Tafel plots, (c) The required overpotential (η10) at -10 mA/cm2, (d) The Chronopotentiometric curves of Co3O4 {001}, {110}, {111}, {112} crystal planes and commercial Pt/C in 1.0 M KOH with the current density of -10 mA/cm2.

16


Page 16 of 21

Page 17 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a) Polarization curves of full water splitting for Pt/C||IrO2, {001}||{001}, {110}||{110}, {111}||{111} and {112}||{112} couples in a two-electrode configuration at a scan rate of 3 mV/s. (b) Chronopotentiometric curves of Pt/C||IrO2, {001}||{001}, {110}||{110}, {111}||{111} and {112}||{112} couples in 1.0 M KOH with a constant current density of 10 mA/cm2.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. (a)The structure of spinel Co3O4. The atom configuration of Co3O4 (b) {001}, (c) {110}, (d) {111} and (e) {112} crystal plane. (f) Free energy (∆GH*) diagram of Co3O4 {001},{110}, {111} and {112} crystal planes.

18


Page 18 of 21

Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Co3+ dangling bonds, relaxed surface areas and surface energies of {001}, {110}, {111} and {112} crystal planes.

Crystal planes

Relaxed surface area (Å2)

Co3+ dangling bonds(Å-2)

Co3+ dangling bonds density

Surface energy (J/m2)

{001} {110} {111} {112}

32.42 45.85 28.08 79.42

2 4 9 10

0.06 0.09 0.32 0.13

2.68 3.09 4.28 4.24

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

Table 2. The values of ∆E(H*) and ∆G(H*) of H adsorbed at Co3+site on different surface planes.

Crystal planes {001} {110} {111} {112}

Adsorption sites

∆E(H*) (eV)

∆G(H*) (eV)

Top Bridge Top Hollow Top bridge

0.179 0.185 0.050 -0.074 -0.014 -0.004

0.419 0.425 0.291 0.166 0.226 0.236

20


Page 21 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Content

21


Probing the Crystal Plane Effect of Co3O4 for Enhanced

Recommend Documents