Controlling the Chemical Bonding of Highly Dispersed Co Atoms

Jul 30, 2019 - DFT calculation, coupled with high-angle annular dark-field scanning ... The enhancement mechanism of chemical bonding provides guidanc...
0 downloads 0 Views 7MB Size
Download PDF
Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Controlling the Chemical Bonding of Highly Dispersed Co Atoms Anchored on an Ultrathin g‑C3N4@Carbon Sphere for Enhanced Electrocatalytic Activity of the Oxygen Evolution Reaction Qianqian Song, Junqi Li,* Lei Wang,* Lingyan Pang, and Hui Liu School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an 710021, P. R. China Downloaded via BUFFALO STATE on July 31, 2019 at 08:02:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Controlling the chemical bonding of an active atom and carbon support is an effective strategy for enhancing the electrocatalytic activity of a metal−nitrogen/carbon catalyst. Herein, highly dispersed Co atoms are successfully prepared by using an ultrathin g-C3N4@carbon sphere as the support, and subsequently the well-defined Co−N and Co−O bonds on the atomic level are controllably constructed by adjusting the calcination atmosphere. Results show that highly dispersed Co with Co−O and Co−N bonds exhibits excellent oxygen evolution reaction performance in alkaline media at low and high overpotentials, respectively, and outperform most single-atom catalysts reported to date. DFT calculation, coupled with high-angle annular dark-field scanning transmission electron microscopy and X-ray photoelectron spectrometry techniques, reveals that the high activities mainly originate from the precise O−Co−N and N−Co−N coordination in the ultrathin g-C3N4@carbon sphere support. The enhancement mechanism of chemical bonding provides guidance for the atomic exploration and design of electrocatalysts.

1. INTRODUCTION The oxygen evolution reaction (OER) is regarded as one key anode/cathode reaction process in electrochemical energy conversion.1,2 However, the OER process requires a high overpotential owing to the sluggish kinetics of complex multielectron transfer steps.3 Thus, the development of highly active and stable electrocatalysts is thought to be pivotal in enhancing the overall efficiency of electrochemical devices.4 Currently, noble-metal-containing catalysts, such as Ru and Ir, are mostly used in the OER process,5 but the scarcity, high cost, and poor stability have become a hindrance to large-scale application.6,7 As an alternative, transition-metal materials with excellent 3d electronic structure have been highly scrutinized.8 Although many studies have been devoted to the OER performance of transition-metal materials,8−11 the electrocatalytic activity and corresponding reaction kinetics still need to be further improved to promote the practical application of transition-metal catalysts. In catalytic reactions, the reaction kinetics can be enhanced by increasing the number of exposed active sites.12 A highly dispersed active atom catalyst (especially a single-atom catalyst (SAC)), featuring atomically dispersed metal atoms anchored on supports, presents the largest atom efficiency at the lowest limit size and exposes the most active sites in the catalyst,13,14 which has been considered to be a research frontier in the catalysis field.15 Unfortunately, it is still an enormous challenge to obtaining highly dispersed metal atoms owing to the easy © XXXX American Chemical Society

emergence of migration and agglomeration induced by the high surface energy of metal atoms in the synthesis process.16 Therefore, it is critical to develop suitable supports that can effectively anchor highly dispersed metal atoms.17,18 Currently, metal oxides19 and N-doped carbon-based materials20 have been proven to be good supports for stabilizing metal atoms, where the defects/voids and the coordination sites21 are very important in anchoring metal atoms and maintaining a high degree of isolation. Among these supports, graphitic carbon nitride (g-C3N4) has a high level of homogeneous nitrogen ligands, which could provide abundant electron lone pairs to capture metal ions and then form a stable, highly dispersed structure.22−24 In particular, thinning g-C3N4 with a high exposed atomic surface can offer not only richer and more controllable loading sites for metal atoms25−27 but also more precise information for the identification of catalytically active sites.28,29 Regrettably, ultrathin g-C3N4 is hard to obtain because it is prone to self-agglomeration under the effect of the high surface energy.30 Besides, the active sites have been proven to be due to the coupling of metal atoms and supports rather than a single metal atom,31 which makes the chemical bonding an essential factor in the enhancement of catalytic activity. The metal−N and metal−O bonds that are demonstrated to be very Received: April 14, 2019

A

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Co−C3N4@CS, the highly dispersed Co atoms anchored on ultrathin g-C3N4@carbon spheres with Co−O bonds (abbreviated as Co−O− C3N4@CS) were synthesized under a flow of Air. For comparison, on the basis of the synthesis procedure of Co−C3N4@CS, CoOx@CS was prepared by using CS instead of C3N4@CS. On the basis of the synthesis procedure of Co−C3N4@CS, Co−C3N4 was prepared in the absence of CS. 2.2. Structural Characterization. The microscopic structure was determined with a field-emission scanning electron microscope (FESEM, Hitachi S-4800). The microscopic, crystal, and composition structures were characterized on the basis of transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energydispersive spectrometer (EDS) mapping, and high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM), obtained with an FEI Tecnai G2 F20 S-TWIN field-emission electron microscope. The phase structure was analyzed with an X-ray diffractometer (XRD-D/max 2200pc, Japan) with Cu Kα radiation (λ = 1.54178 Å). The elemental composition, chemical state, and chemical bond were determined on the basis of Raman spectra detected with a Renishaw inVia, Fourier transform infrared (FT-IR) spectra were detected with a Bruker VECTOR-22 FT-IR spectrometer, and X-ray photoelectron spectra (XPS) detected with an ESCALAB MK II X-ray photoelectron spectrometer. The BET specific surface area was obtained by nitrogen adsorption−desorption isotherm measurements on an ASAP2460 specific surface area instrument. Elemental analyses were performed on an Elementar Vario EL III microanalyzer. The metal loadings were detected with a PE Optima 2100DV inductively coupled plasma−optical emission spectrometer (ICP−OES). 2.3. Electrochemical Measurements. All of the electrochemical tests were carried out in a three-electrode system on an electrochemical workstation (CHI660B). The glassy carbon (GC) electrode was utilized as a working electrode, the platinum foil was used as the counter electrode, and Hg/HgO was selected as the reference electrode. Typically, 8 mg of catalyst was dispersed in 0.3 mL of isopropanol solution by sonicating for at least 0.5 h to form homogeneous ink A. Isopropanol solution (0.7 mL) was mixed with 0.1 mL of Nafion solution and ultrasonicated for 0.5 h to form solution B. Then, 20 μL of A measured with a pipet was dropped onto a GC electrode. After drying in air, 10 μL of B was drop-cast on top to prevent the catalyst from dropping off during the measurements. Before OER testing, pure nitrogen gas was used to purge the 1 M KOH solution for 0.5 h to obtain the solution without air, and the working electrodes were activated several times until the signals became stabilized. The OER polarization curves were measured from 1.23 to 1.80 V (vs RHE) in 1 M KOH solution at a scan rate of 10 mV s−1. All of the potentials were calibrated to a reversible hydrogen electrode (RHE) according to the Nernst equation Evs RHE = Evs Hg/HgO + 0.095 + 0.059pH without the iR contribution. Electrochemical impedance spectroscopy (EIS) measurements were carried out by using an ac voltage with 5 mV amplitude in the frequency range from 100 kHz to 100 mHz. Cyclic voltammetry (CV) was performed at different scan rates from 5 to 100 mV s−1 and from 0 to 0.3 V (vs RHE). 2.4. Computational Section. All calculations in this work were carried out according to the plane-wave density functional theory (DFT) employing periodic boundary conditions as implemented in the CASTEP module.41,42 The Perdew−Burke−Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was employed to model electronic exchange-correlation energy.43 The interaction between valence electrons and the ionic core was described by an ultrasoft pseudopotential. The developed supercell for the free -energy calculation possessed 2 × 2 unit cells with a 15 Å separation between two layers in the z direction. To perform geometry optimization and electronic structure calculations, a kinetic energy cutoff of 350 eV and a Monkhorst−Pack 1 × 1 × 1 k-point grid were presented. The convergence criterion for the electronic structure iteration was selected to be 10−4 eV, and that for geometry optimization was selected to be 0.01 eV/Å on force.

significant in improving the catalytic activity of SAC have been constructed in previous reports.32−35 On the one hand, the metal−N bond can greatly facilitate the interfacial electron transfer for the hybrid electrocatalysts and then enhance the OER activity because of the high affinity of nitrogen for metal ions.36,37 On the other hand, the metal−O bond can promote electron transfer between the metal cation and oxygen adsorbates or facilitate the injection/extraction of electrons from oxygen, eventually leading to the acceleration of the OER rate.38 However, a comparative study of metal−N and metal− O bonds in the same complex system has been less reported. Herein, ultrathin g-C3N4 (ul-g-C3N4) is obtained by using the carbon spheres as the matrix, forming a hierarchical structure (C3N4@CS) of carbon spheres (CS) uniformly coated with ul-g-C3N4 nanosheets, which is used as the support for the catalyst. Taking transition-metal element cobalt as a research object, the chemical bonding between the C3N4@CS support and Co atoms is regulated, and the effect of chemical bond type on electrocatalytic performance is systematically investigated. When applied in OER catalysis, the material with Co−O (0.23 V at 10 mA cm−2) and Co−N (0.47 V at 50 mA cm−2) exhibits superior OER catalytic activity in alkaline media at low and high overpotentials, respectively. A combination of electrochemical investigations and density function theory (DFT) computation confirms that the high activities chiefly result from the precise O−Co−N and N−Co−N coordination in the C3N4@CS support.

2. EXPERIMENTAL WORK 2.1. Material Synthesis. 2.1.1. Synthesis of ul-g-C3N4 and CS. The ultrathin g-C3N4 nanosheets (abbreviated as ul-g-C3N4) were prepared by protonation of the as-prepared bulk g-C3N4, and the carbon spheres (abbreviated as CS) were synthesized by the Stöber method, which had been reported in our previous study.39,40 Dicyandiamide (2 g) was added to a crucible and then heated to 550 °C at a rate of 5 °C min−1 and maintained for 2 h in a muffle furnace. After naturally cooling to room temperature, yellow bulk gC3N4 was obtained. Subsequently, 1.2 g of bulk g-C3N4 and 20 mL of H2SO4 (98%) were mixed and stirred at 100 °C for 2 h until the appearance of a clear pale-yellow solution. Finally, the white ul-g-C3N4 nanosheets were separated by washing with H2O and subsequent drying at 40 °C for 24 h. 2.1.2. Synthesis of C3N4@CS. To homogeneously coat ul-g-C3N4 on the surface of CS, the suspension was first obtained by ultrasonically dispersing 100 mg of ul-g-C3N4 powder in 40 mL of deionized (DI) water and then centrifuging at a rotational speed of 3000 rpm to remove large particles. Second, 0.5 g of CS was introduced into the suspension, which was sequentially sonicated for 0.5 h. Third, the suspension containing CS was aged for 10 h and dried at 40 °C for 24 h, and thus the ultrathin g-C3N4@carbon spheres powder (abbreviated as C3N4@CS) was obtained. 2.1.3. Synthesis of Co−C3N4@CS and Co−O−C3N4@CS. To uniformly anchor the Co atoms on the surface of ul-g-C3N4 in C3N4@CS, 1 mmol of Co(NO3)2·6H2O powder was completely dissolved in 100 mL of DI water under continuous and vigorous stirring for 0.5 h. Afterward, 0.5 g of as-prepared C3N4@CS was added to the obtained Co(NO3)2·6H2O solution, sonicated for 0.5 h, and vigorously stirred for 4 h. After that, the precursor of C3N4@CS carried with Co2+ was collected via centrifuging and washing with DI water and ethanol, which was then dried in a vacuum for 12 h. Finally, 100 mg of the above precursor was added to a tube furnace, which was then heated to 400 °C at a rate of 1 °C/min under a flow of Ar and maintained for 3 h. After naturally cooling to room temperature, the highly dispersed Co atoms anchored on ultrathin g-C3N4@carbon spheres with Co−N bonds (abbreviated as Co−C3N4@CS) were obtained as a black powder. On the basis of the synthesis procedure of B

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Schematic illustration of the formation mechanism of the highly dispersed Co atoms anchored on ultrathin g-C3N4@carbon spheres with Co−N and Co−O bonds.

Figure 2. TEM, HRTEM, EDS, and HAADF-STEM images of (a−d) Co−C3N4@CS and (e−h) Co−O−C3N4@CS. Circles and shiny places indicate Co atoms.

3. RESULTS AND DISCUSSION

O in the calcination environment of Ar and air atmospheres, which is abbreviated as Co−C3N4@CS and Co−O−C3N4@ CS, respectively. Notably, ultrathin g-C3N4 nanosheets are successfully exfoliated by the protonation of bulk g-C3N4, and the protonation has little influence on the valence bond structure and chemical composition (Figures S1−S3). To prevent the self-agglomeration of ultrathin g-C3N4 nanosheets during the final calcination process, the uniform carbon spheres are applied as the carrier, which is obtained by the polymerization process of resorcinol and formaldehyde. Furthermore, Figures S4−S6 show that ultrathin g-C3N4 is

3.1. Structural Characterization. For the formation of highly dispersed Co atoms on an ultrathin g-C3N4@carbon sphere, the C3N4@CS support plays a decisive role and is obtained by the electrostatic attraction of the oxygencontaining functional group on the surface of the carbon sphere and the H+ of protonated g-C3N4 (Figure 1). On the surface of the C3N4@CS support, the N with electron lone pairs captures the Co2+ in the reaction solution to form the ligands, eventually leading to the formation of Co−N and Co− C

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 3. (a) Illustration of the starting position for Co adsorption on the surface of the C3N4@CS support and (b) the corresponding calculated binding energy of Co−C3N4@CS and Co−O−C3N4@CS. High-resolution (c) N 1s, (d) Co 2p, (e) O 1s, and (f) C 1s XPS spectra of Co−C3N4@ CS and Co−O−C3N4@CS.

atomic content of Co slightly increases to 0.29 atom % for Co−O−C3N4@CS, indicating the successful introduction of O during calcination with the oxidizing of a small amount of carbon from CS in the air environment. More precise ICP and elemental analysis are employed to identify the contents of Co, O, N, and C in Co−C3N4@CS and Co−O−C3N4@CS (Table S2), whose results further show the similar variation trend in Co and O contents with EDS analysis. Besides, the XRD patterns of Co−C3N4@CS and Co−O−C3N4@CS in Figure S8a do not show any peaks assigned to cobalt metal or its compounds, indirectly suggesting that the cobalt species in the two samples are highly dispersed as single atoms. BET data in Figure S8b demonstrate that Co−C3N4@CS and Co−O− C3N4@CS exhibit surface areas of 506.07 and 664.09 m2/g, respectively, which contribute to providing more active sites for OER. To reveal the coupling form of the Co atom and support and identify the most stable site of Co in the support, DFT geometry optimization was conducted. As shown in Figures 3a and S9, these initial configurations result in four distinctively different positions of Co absorbed on the C3N4@CS support, which are based on the three nonequivalent nitrogen atoms (N1 (pyridinic nitrogen), N2 (tertiary nitrogen), and N3 (tertiary nitrogen)) and the void ring. As calculated in Figures 3b, S10, and S11 with eq S1, the most stable structure for Co− C3N4@CS is that Co atoms are embedded in the void ring of g-C3N4 and connected with two adjacent pyridinic-N atoms from two separate triazine units, eventually forming the CoN3C2 ring.31 The most stable structure for Co−O− C3N4@CS is that Co-atom-bonded O atoms are embedded in the void ring of g-C3N4, which is connected with two adjacent pyrimidine-N atoms from the same triazine units, eventually forming the CoON2C ring. To verify the coupling structures calculated by DFT geometry optimization, XPS analyses were carried out on

homogeneously coated on the surface of carbon spheres, forming a conductive network that can well connect every carbon sphere, which more controllable and uniform loading sites emerging for Co atoms. Next, the highly dispersed Co atoms on the ultrathin gC3N4@carbon sphere structures of Co−C3N4@CS and Co− O−C3N4@CS were characterized by electron microscopy techniques including SEM and TEM (Figure 2). Different magnification SEM images in Figure S7, TEM images in Figure 2a,e, and HRTEM images in Figure 2b,f show that Co− C3N4@CS and Co−O−C3N4@CS exhibit morphology and a coating structure similar to those of the C3N4@CS support, where no cobalt-containing nanoparticles and the corresponding lattice fringes can be detected, implying that the cobalt species may be highly dispersed in both Co−C3N4@CS and Co−O−C3N4@CS samples that are usually undetectable or invisible in SEM, TEM, and HRTEM images. To better obtain the existential form of cobalt species, the EDS and HAADFSTEM techniques were employed to inspect the Co−C3N4@ CS and Co−O−C3N4@CS samples. From Figure 2c,g, it is found that the signals of Co, N, C, and O are completely superimposed on each other, suggesting that g-C3N4 is homogeneously coated on the surface of carbon spheres and cobalt species are well dispersed over the whole carbon spheres. As seen from Figure 2d,h, a number of uniformly dispersed Co atoms are clearly observed on the surface of the C3N4@CS support, and no clusters or small particles exist in the vicinity of single atoms. These two points imply that cobalt species may exist in the form of single atoms, which are possible to bond with N, C, or O in Co−C3N4@CS and Co− O−C3N4@CS. Further quantitative analysis of the elemental mappings shows that the atomic contents of Co, O, N, and C are about 0.25, 1.37, 3.09, and 95.29 atom % in Co−C3N4@ CS, respectively (Table S1). Compared to that of Co−C3N4@ CS, the atomic content of O increases to 3.95 atom % and the D

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 4. (a) OER polarization curves and (b) corresponding Tafel plots for Co−C3N4@CS, Co−O−C3N4@CS, C3N4@CS, CoOx@CS, and Co− C3N4 in 1 M KOH solution. (c) Tafel slopes and current densities at overpotentials of 0.23 and 0.50 V. (d) Multistep chronoamperometric curves of OER in 1 M KOH solution (the inset shows the part between 0 and 0.4 V). (e) i−t curves obtained for OER at a current density of 10 mA cm−2. (f) OER polarization curves obtained before and after 3000 cycles for Co−C3N4@CS and Co−O−C3N4@CS.

Co−C3N4@CS and Co−O−C3N4@CS samples. In the N 1s spectra (Figure 3c), the typical C3N4@CS peaks can be clearly observed and correspond to the pyridinic nitrogen (398.6 eV), pyrrolic nitrogen (399.7 eV), amino functional groups (400.9 eV), N-graphitic carbon (402.8 eV), and N−O (405.5 eV).44−47 Notably, a new peak assigned to the Co−N bond at 397.4 eV in these two samples can be also probed, indicating the formation of Co−N in Co−C3N4@CS and Co−O− C3N4@CS. For Co 2p spectra (Figure 3d), Co−C3N4@CS shows peaks at 780 and 796.2 eV that are respectively attributed to the Co 2p1/2 and Co 2p3/2 of Co2+, suggesting that Co in oxidized states is coordinated with two adjacent atoms, which further confirms the coupling form of Co−N. For

Co−O−C3N4@CS, a new peak at 782.2 eV belonging to Co− O is found, indicating the appearance of the Co−O bond in Co−O−C3N4@CS, which is again confirmed by the O 1s spectrum with a Co−O characteristic peak at 530.2 eV in Figure 3e. These results experimentally confirm the coupling form of the Co atom and support. In the C 1s spectra (Figure 3f), the C−NC assigned to g-C3N4 can be obviously detected.48 Besides, the quantitative analysis of survey spectra of Co−C3N4@CS and Co−O−C3N4@CS (Figure S12 and Table S3) also exhibits the same increasing trend in Co and O content with EDS analysis. 3.2. Electrocatalytic OER Performance. The OER activities of all as-prepared electrocatalysts are first evaluated E

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 5. (a) Nyquist plots obtained at an overpotential of 0.23 V, with the inset showing the equivalent circuit diagram. (b) Corresponding linear fits in the low-frequency region (ω = 2πf). (c) Corresponding charge-transfer resistance (Rct) and diffusion resistance (Zw) for Co−C3N4@CS and Co−O−C3N4@CS electrodes. CV curves at different sweep rates from 5 to 100 mVs−1 within the potential range of 0 to 0.3 V (vs RHE) of (d) Co−C3N4@CS and (e) Co−O−C3N4@CS. (f) Fitted lines and Δj (the difference in current density between the anodic and cathodic sweeps at a potential of 0.15 V) vs scan rate plots for Co−C3N4@CS and Co−O−C3N4@CS electrodes.

C3N4@CS catalyst exhibits a Tafel slope (48 mV dec−1) lower than those of Co−O−C3N4@CS (141 mV dec−1), C3N4@CS (128 mV dec−1), CoOx@CS (75 mV dec−1), Co−C3N4 (115 mV dec−1), ul-g-C3N4 (202 mV/dec−1) and CS (262 mV dec−1) catalysts, reflecting the fast mass transfer process of the Co−C3N4@CS catalyst at high overpotential. These suggest that Co−O−C3N4@CS and Co−C3N4@CS catalysts exhibit favorable OER kinetics at low and high overpotentials, respectively. To more explicitly illuminate the differences in OER activities, the Tafel slope and current density at low (0.23 V) and high (0.50 V) overpotentials are presented, as shown in Figure 4c. Obviously, at a low overpotential (0.23 V), the Co− O−C3N4@CS catalyst possesses a larger current density with a smaller Tafel slope, whereas the Co−C3N4@CS catalyst exhibits better OER activities at a high overpotential (0.50 V). Then, the OER stabilities of Co−O−C3N4@CS and Co− C3N4@CS catalysts are investigated. Figure 4d illustrates the multistep chronoamperometric (CA) curves of Co−O− C3N4@CS and Co−C3N4@CS at different overpotentials ranging from 0 to 0.6 V (an increment of 0.05 V every 500 s) without iR correction. Upon increasing the overpotential to 0.35 V, the current density of the Co−O−C3N4@CS electrode is significantly larger than that of the Co−C3N4@CS electrode. In sharp contrast, the Co−C3N4@CS electrode rises quickly with the continuous increase in the overpotential. These suggest that Co−O−C3N4@CS and Co−C3N4@CS electrodes exhibit outstanding mechanical robustness and a mass transfer property at low and high overpotentials, respectively, in accordance with the results of OER polarization curves and Tafel slopes. A continuous OER test at constant overpotential corresponding to the current density of 10 mA cm−2 was operated to evaluate the stability. As shown in Figure 4e, the i− t curves of Co−C3N4@CS and Co−O−C3N4@CS electrodes show small amounts of degradation of 4 and 8.7% even after a

by the linear scan sweep voltammetry (LSV) curves in N2satuated 1 M KOH medium. As presented in Figures 4 and S13, Co−O−C3N4@CS and Co−C3N4@CS catalysts exhibit higher OER performances than do C3N4@CS, CoOx@CS, Co−C3N4, ul-g-C3N4, and CS catalysts, which also outperform most SACs reported to date (Table S4). This indicates that the excellent OER performances of Co−O−C3N4@CS and Co− C3N4@CS catalysts determine the coupling of Co and C3N4@ CS rather than the effect of independent g-C3N4, CS, and transition-metal Co. Concretely, the onset potentials of OER on Co−O−C3N4@CS (1.25 V) and Co−C3N4@CS (1.46 V) catalysts are smaller than those on C3N4@CS (1.58 V), CoOx@CS (1.55 V), Co−C3N4 (1.74 V), ul-g-C3N4 (1.60 V), and CS (1.68 V) catalysts (Figures 4a and S13a). In particular, the current density of the Co−O−C3N4@CS catalyst has reached 10 mA cm−2 at the low overpotential of 0.23 V, and that of the Co−C3N4@CS catalyst has reached up to 50 mA cm−2 at an overpotential of 0.47 V. Notably, the current generated at low potential for the Co−O−C3N4@CS catalyst may be induced by the double-layer capacitance in the interface of the electrode/electrolyte resulting from the hard desorption of the adsorbed oxygen species on the surface of the catalyst. On the basis of the above results, the corresponding Tafel plots calculated from the polarization curves are investigated during different overpotential ranges to study their OER catalytic kinetics. As displayed in Figures 4b and S13b, at low overpotential, the Co−O−C3N4@CS catalyst has a lower Tafel slope (159 mV dec−1) than Co−C3N4@CS (211 mV dec−1), C3N4@CS (992 mV dec−1), CoOx@CS (675 mV dec−1), Co− C3N4 (559 mV dec−1), ul-g-C3N4 (590 mV/dec−1), and CS (866 mV dec−1) catalysts, reflecting the excellent charge transfer capacity of the Co−O−C3N4@CS catalyst at a low overpotential. On the contrary, at high overpotential, the Co− F

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 6. Free-energy diagram of the OER process on (a) Co−C3N4@CS and (b) Co−O−C3N4@CS surfaces. The red balls are O atoms, and the white balls are H atoms.

catalytic activities. To further evaluate the electrocatalytic performance, the Faradaic efficiencies (EFs) of Co−C3N4@CS and Co−O−C3N4@CS are calculated from the O2 gas production. The theoretical and experimental data of O2 are shown in Figure S17, where the Faraday efficiency is about 98.3 and 95.6% for Co−C3N4@CS and Co−O−C3N4@CS, respectively. In summary, such impressive electrochemical performances of Co−C3N4@CS and Co−O−C3N4@CS electrodes are mainly attributed to the highly dispersed Co atoms anchored to an ultrathin g-C3N4@carbon sphere. First, the strong bridging of N−Co−N and O−Co−N bonds between Co and the C3N4@CS support boosts the electron transfer ability and improve the activity of OER. Second, highly dispersed Co atoms equate to the active sites with high surface area exposure, which kinetically facilitates the formation of the active oxyhydroxide phase necessary for OER. Third, the ultrathin g-C3N4 conductive network, coupled with the Ngraphitic carbon bond between ul-g-C3N4 and CS, promotes the charge transfer and structural stability, which provide an efficient and stable environment for highly dispersed Co atoms. Fourth, the high surface areas of Co−C3N4@CS and Co−O− C3N4@CS increase the contact between the electrode and electrolyte and offer more catalytically active sites for OER. Fifth, the uneven charge distribution and positively charged C atoms in ultrathin g-C3N4 favors the capture of oxygen species. Notably, the electronegativity of N and O makes the Co atom electron-deficient, which favors the adsorption of oxygen species. When a low overpotential is applied, the O with higher electronegativity can further accelerate the adsorption of oxygen species, which leads to better OER performance of Co−O−C3N4@CS. However, the excessively strong adsorption ability of O−Co−N on oxygen species would affect the subsequent desorption process, resulting in a worse OER performance of Co−O−C3N4@CS at high overpotential. To gain deep insight into the influence of N−Co−N and O−Co−N bonds on the OER process on the atomic scale, multiple elementary reaction steps were further explored using the computational hydrogen electrode (CHE) model, which correspond to the energetics of the oxygenated adsorbates of *OH (step I, eq S2), *O (step II, eq S3), *OOH (step III, eq S4), and O2 (step IV, eq S5) on the catalyst surfaces. The computed structural models of every reaction step given in Figure S18 show that the active sites of all reaction steps are the N−Co−N and O−Co−N coordination in the ultrathin gC3N4@carbon sphere support for Co−C3N4@CS and Co−O−

long OER test (13 h), suggesting the excellent OER stabilities for both of them. As probed in Figure 4f, Co−O−C3N4@CS and Co−C3N4@CS electrodes show a polarization curve similar to the initially measured one, which exhibits very little deterioration after 3000 CV cycles, indicating the remarkable electrocatalytic stability for the OER. The catalysts after accelerated cycling were characterized by XPS spectra (Figures S14 and S15), which suggests that the cycling operation does not change the composition of the catalysts and the chemical states of Co and N. Electrochemical impedance spectrum (EIS) measurements were carried out at a constant overpotential of 0.23 V to probe the OER reaction kinetics of Co−C3N4@CS and Co−O− C3N4@CS electrodes. The Nyquist plots were modeled by the proposed equivalent circuit diagram (inset in the top left corner of Figure 5a), which consists of a semicircle at high frequency and a diagonal line at low frequency corresponding to charge transfer resistance (Rct) and diffusion resistance (Zw), respectively.49,50 Zw is obtained by calculating the slope of the fitted line in Figure 5b, which is decided by the oxygen species desorption process. As presented in Figures 5c and S16, Co− C3N4@CS and Co−O−C3N4@CS electrodes exhibit smaller Rct and Zw values than do the C3N4@CS, CoOx@CS, Co− C3N4, ul-g-C3N4, and CS electrodes, suggesting enhanced electrocatalytic kinetics, a faster reaction rate, and an O2 desorption process. Besides, the Rct of C3N4@CS is obviously smaller than that of CS, reflecting that the formation of an ultrathin g-C3N4 conductive network on the surface of the carbon spheres dramatically accelerates the charge transfer. Notably, the Zw of the Co−C3N4@CS electrode is much smaller than that of the Co−O−C3N4@CS electrode, implying the faster O2 desorption process of Co−N compared to that of Co−O, which leads to better OER performance of the Co− C3N4@CS electrode at a high overpotential compared to that of the Co−O−C3N4@CS electrode. To gain insight into the electrochemical active areas of Co−C3N4@CS and Co−O− C3N4@CS electrodes, CV tests were conducted at various scan rates from 5 to 100 mVs−1. As shown in Figure 5d,e, the CV curves are well preserved with the increase in the scan rate, suggesting that the two electrodes are capable of making a quick CV response to a fast potential scan, which further demonstrates the improved electrocatalytic kinetic and quick reaction rate. Figure 5f reveals that Co−O−C3N4@CS and Co−C3N4@CS electrodes have double-layer capacitances of 64 and 56 mF/cm2, respectively, reflecting that they have larger electrochemical active areas and explaining their superior G

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry C3N4@CS, respectively. The origin of the different reaction characteristics for these two configurations can be reflected in the different bonding strengths of the oxygenated adsorbates on the two active sites. The free energy (ΔG) of each step is used to evaluate the catalytic process at certain potentials (U) of 0, 1.23, and 1.68 V, as shown in Figure 6. Under U = 0 V, ΔG1, ΔG2, ΔG3, and ΔG4 are respectively calculated to be 0.55, 1.18, 1.65, and 1.54 eV for Co−C3N4@CS and 0.52, 0.94, −0.09, and 3.55 eV for Co−O−C3N4@CS. Obviously, all steps of the Co−C3N4@CS electrode move uphill, whereas step III of O* to OOH* for the Co−O−C3N4@CS electrode moves downhill. When the equilibrium potential of 1.23 V is applied, ΔG1, ΔG2, and ΔG3 of the Co−O−C3N4@CS electrode are decreased to −0.71, −0.30, and −1.315 eV, respectively, indicating that steps I−III are thermally spontaneous, whereas only steps I and II occur spontaneously for the Co−C3N4@CS electrode. This demonstrates that the O−Co−N bond contributes to transferring more electrons than N−Co−N at low potential, which is mainly induced by the high electronegativity of O in the O−Co−N bond. The high electronegativity of O effectively promotes the transformation of O* to OOH*, as proven by the large downhill Gibbs free energy difference in step III for the Co−O−C3N4@CS electrode. When the potential is further increased to 1.68 V, every step of the Co−C3N4@CS electrode goes downhill, whereas step IV of the Co−O−C3N4@CS electrode still exhibits a high potential barrier, demonstrating that the N−Co−N bond is capable of transferring more electrons than the O−Co−N bond at high potential. This main reason is that the excessive adsorption strength of O−Co−N to OOH* impedes the subsequent desorption process, resulting in worse OER performance of Co−O−C3N4@CS at high overpotentials. These results are consistent with the OER performance and the corresponding enhanced mechanism.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (L.W.). ORCID

Junqi Li: 0000-0002-6874-6795 Hui Liu: 0000-0002-5966-1191 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant nos. 51502165 and 51702193), the Natural Science Basic Research Plan in Shaanxi Province of China (grant no. 2017JQ5035), the Natural Science Foundation of Education Department of Shaanxi Provincial (grant no. 16JK1086), and the Scientific Research Fund of Shaanxi University of Science & Technology (grant nos. BJ16-20 and BJ16-21).

■

REFERENCES

(1) Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.; Wu, C.; Xie, Y. Atomically Dispersed IronNitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions. Angew. Chem., Int. Ed. 2017, 56 (2), 610−614. (2) Zhu, Y. P.; Ma, T. Y.; Jaroniec, M.; Qiao, S. Z. Self-Templating Synthesis of Hollow Co3O4 Microtube Arrays for Highly Efficient Water Electrolysis. Angew. Chem., Int. Ed. 2017, 56 (5), 1324−1328. (3) Fei, H.; Dong, J.; Feng, Y.; Allen, C. S.; Wan, C.; Volosskiy, B.; Li, M.; Zhao, Z.; Wang, Y.; Sun, H.; An, P.; Chen, W.; Guo, Z.; Lee, C.; Chen, D.; Shakir, I.; Liu, M.; Hu, T.; Li, Y.; Kirkland, A. I.; Duan, X.; Huang, Y. General synthesis and definitive structural identification of MN4C4 single-atom catalysts with tunable electrocatalytic activities. Nat. Catal. 2018, 1 (1), 63−72. (4) Tong, Y.; Chen, P.; Zhou, T.; Xu, K.; Chu, W.; Wu, C.; Xie, Y. A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Evolution: Cobalt Oxide Nanoparticles Strongly Coupled to B, NDecorated Graphene. Angew. Chem., Int. Ed. 2017, 56 (25), 7121− 7125. (5) Kong, X.; Xu, K.; Zhang, C.; Dai, J.; Norooz Oliaee, S.; Li, L.; Zeng, X.; Wu, C.; Peng, Z. Free-Standing Two-Dimensional Ru Nanosheets with High Activity toward Water Splitting. ACS Catal. 2016, 6 (3), 1487−1492. (6) Cui, C.; Heggen, M.; Zabka, W. D.; Cui, W.; Osterwalder, J.; Probst, B.; Alberto, R. Atomically dispersed hybrid nickel-iridium sites for photoelectrocatalysis. Nat. Commun. 2017, 8 (1), 1341. (7) Lyu, Y.; Wang, R.; Tao, L.; Zou, Y.; Zhou, H.; Liu, T.; Zhou, Y.; Huo, J.; Jiang, S. P.; Zheng, J.; Wang, S. In-situ evolution of active layers on commercial stainless steel for stable water splitting. Appl. Catal., B 2019, 248, 277−285. (8) Han, L.; Dong, S.; Wang, E. Transition-Metal (Co, Ni, and Fe)Based Electrocatalysts for the Water Oxidation Reaction. Adv. Mater. 2016, 28 (42), 9266−9291. (9) Zeng, Y.; Chen, L.; Chen, R.; Wang, Y.; Xie, C.; Tao, L.; Huang, L.; Wang, S. One-step, room temperature generation of porous and amorphous cobalt hydroxysulfides from layered double hydroxides for superior oxygen evolution reactions. J. Mater. Chem. A 2018, 6 (47), 24311−24316. (10) Huang, G.; Xiao, Z.; Chen, R.; Wang, S. Defect Engineering of Cobalt - Based Materials for Electrocatalytic Water Splitting. ACS Sustainable Chem. Eng. 2018, 6, 15954.

4. CONCLUSIONS Highly dispersed Co atoms anchored on an ultrathin g-C3N4@ carbon sphere with a Co−N or Co−O bond are developed, where ultrathin g-C3N4 is a key factor that presents controllable and uniform loading sites for Co atoms. The precise N−Co−N and O−Co−N coordination, combined with the fine conductive network of the ultrathin g-C3N4@ carbon sphere support, dramatically enhances the OER performance. Benefitting from the electronegativity difference of O and N, Co−O−C3N4@CS and Co−C3N4@CS catalysts exhibit favorable OER performances at low and high overpotentials, respectively. When applied as OER catalysts in alkaline media, Co−O−C3N4@CS and Co−C3N4@CS electrodes deliver overpotentials of 0.23 V at 10 mA cm−2 and 0.47 V at 50 mA cm−2, respectively, which are comparable to or even outperform other some reported SACs. Overall, this work demonstrates that the chemical bonding between the metal atom and support has a strong influence on the electrocatalytic activity, which offers guidance for the atomic exploration and design of SACs.

■

Free-energy calculation, EDS and XPS analyses, OER performance comparison, and supporting figures (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01089. H

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

catalytic degradation of naproxen. Appl. Catal., B 2018, 221, 510− 520. (28) Zhao, J.; Chen, Z. Single Mo Atom Supported on Defective Boron Nitride Monolayer as an Efficient Electrocatalyst for Nitrogen Fixation: A Computational Study. J. Am. Chem. Soc. 2017, 139 (36), 12480−12487. (29) Vilé, G.; Albani, D.; Nachtegaal, M.; Chen, Z.; Dontsova, D.; Antonietti, M.; López, N.; Pérez-Ramírez, J. A Stable Single-Site Palladium Catalyst for Hydrogenations. Angew. Chem., Int. Ed. 2015, 54 (38), 11265−11269. (30) Kessler, F. K.; Zheng, Y.; Schwarz, D.; Merschjann, C.; Schnick, W.; Wang, X.; Bojdys, M. J. Functional carbon nitride materialsdesign strategies for electrochemical devices. Nat. Rev. Mater. 2017, 2 (6), 17030. (31) Zheng, Y.; Jiao, Y.; Zhu, Y.; Cai, Q.; Vasileff, A.; Li, L. H.; Han, Y.; Chen, Y.; Qiao, S. Z. Molecule-Level g-C3N4 Coordinated Transition Metals as a New Class of Electrocatalysts for Oxygen Electrode Reactions. J. Am. Chem. Soc. 2017, 139 (9), 3336−3339. (32) He, P.; Xu, J.; Terskikh, V. V.; Sutrisno, A.; Nie, H. Y.; Huang, Y. Identification of Nonequivalent Framework Oxygen Species in Metal-Organic Frameworks by 17O Solid-State NMR. J. Phys. Chem. C 2013, 117 (33), 16953−16960. (33) Wu, J.; Ren, Z.; Du, S.; Kong, L.; Liu, B.; Xi, W.; Zhu, J.; Fu, H. A highly active oxygen evolution electrocatalyst: Ultrathin CoNi double hydroxide/CoO nanosheets synthesized via interface-directed assembly. Nano Res. 2016, 9 (3), 713−725. (34) Fei, H.; Dong, J.; Arellano-Jimenez, M. J.; Ye, G.; Dong Kim, N.; Samuel, E. L.; Peng, Z.; Zhu, Z.; Qin, F.; Bao, J.; Yacaman, M. J.; Ajayan, P. M.; Chen, D.; Tour, J. M. Atomic cobalt on nitrogen-doped graphene for hydrogen generation. Nat. Commun. 2015, 6, 8668. (35) Zhang, Y.; Wang, Y.; Liu, L.; Wei, N.; Gao, M. L.; Zhao, D.; Han, Z. B. Robust Bifunctional Lanthanide Cluster Based MetalOrganic Frameworks (MOFs) for Tandem Deacetalization-Knoevenagel Reaction. Inorg. Chem. 2018, 57 (4), 2193−2198. (36) Wang, X. R.; Liu, J. Y.; Liu, Z. W.; Wang, W. C.; Luo, J.; Han, X. P.; Du, X. W.; Qiao, S. Z.; Yang, J. Identifying the Key Role of Pyridinic-N-Co Bonding in Synergistic Electrocatalysis for Reversible ORR/OER. Adv. Mater. 2018, 30 (23), No. 1800005. (37) Jiang, Y.; Deng, Y.-P.; Fu, J.; Lee, D. U.; Liang, R.; Cano, Z. P.; Liu, Y.; Bai, Z.; Hwang, S.; Yang, L.; Su, D.; Chu, W.; Chen, Z. Interpenetrating Triphase Cobalt-Based Nanocomposites as Efficient Bifunctional Oxygen Electrocatalysts for Long-Lasting Rechargeable Zn-Air Batteries. Adv. Energy Mater. 2018, 8 (15), 1702900. (38) Zhou, Y.; Sun, S.; Song, J.; Xi, S.; Chen, B.; Du, Y.; Fisher, A. C.; Cheng, F.; Wang, X.; Zhang, H.; Xu, Z. J. Enlarged CoO Covalency in Octahedral Sites Leading to Highly Efficient Spinel Oxides for Oxygen Evolution Reaction. Adv. Mater. 2018, 30 (32), No. 1802912. (39) Li, J.; Liu, Z.; Zhu, Z. Magnetically separable ternary hybrid of ZnFe2O4-Fe2O3-Bi2WO6 hollow nanospheres with enhanced visible photocatalytic property. Appl. Surf. Sci. 2014, 320, 146−153. (40) Song, Q.; Li, J.; Wang, L.; Qin, Y.; Pang, L.; Liu, H. Stable single-atom cobalt as a strong coupling bridge to promote electron transfer and separation in photoelectrocatalysis. J. Catal. 2019, 370, 176−185. (41) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169. (42) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15−50. (43) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (44) Hou, Y.; Wen, Z.; Cui, S.; Ci, S.; Mao, S.; Chen, J. An Advanced Nitrogen-Doped Graphene/Cobalt-Embedded Porous Carbon Polyhedron Hybrid for Efficient Catalysis of Oxygen Reduction and Water Splitting. Adv. Funct. Mater. 2015, 25 (6), 872−882.

(11) Lu, F.; Zhou, M.; Zhou, Y.; Zeng, X. First-Row Transition Metal Based Catalysts for the Oxygen Evolution Reaction under Alkaline Conditions: Basic Principles and Recent Advances. Small 2017, 13 (45), 1701931. (12) Liu, W.; Zhang, L.; Liu, X.; Liu, X.; Yang, X.; Miao, S.; Wang, W.; Wang, A.; Zhang, T. Discriminating Catalytically Active FeNx Species of Atomically Dispersed Fe-N-C Catalyst for Selective Oxidation of the C-H Bond. J. Am. Chem. Soc. 2017, 139 (31), 10790−10798. (13) Jones, J.; Xiong, H.; DeLaRiva, A. T.; Peterson, E. J.; Pham, H.; Challa, S. R.; Qi, G.; Oh, S.; Wiebenga, M. H.; Pereira Hernandez, X. I.; Wang, Y.; Datye, A. K. Thermally stable single-atom platinum-onceria catalysts via atom trapping. Science 2016, 353 (6295), 150−154. (14) Yang, X. F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. SingleAtom Catalysts A New Frontier in Heterogeneous Catalysis. Acc. Chem. Res. 2013, 46 (8), 1740−1748. (15) Cheng, N.; Stambula, S.; Wang, D.; Banis, M. N.; Liu, J.; Riese, A.; Xiao, B.; Li, R.; Sham, T. K.; Liu, L. M.; Botton, G. A.; Sun, X. Platinum single-atom and cluster catalysis of the hydrogen evolution reaction. Nat. Commun. 2016, 7, 13638. (16) Zhu, C.; Fu, S.; Shi, Q.; Du, D.; Lin, Y. Single-Atom Electrocatalysts. Angew. Chem., Int. Ed. 2017, 56 (45), 13944−13960. (17) Xing, J.; Chen, J. F.; Li, Y. H.; Yuan, W. T.; Zhou, Y.; Zheng, L. R.; Wang, H. F.; Hu, P.; Wang, Y.; Zhao, H. J.; Wang, Y.; Yang, H. G. Stable isolated metal atoms as active sites for photocatalytic hydrogen evolution. Chem. - Eur. J. 2014, 20 (8), 2138−44. (18) Xi, J.; Sun, H.; Wang, D.; Zhang, Z.; Duan, X.; Xiao, J.; Xiao, F.; Liu, L.; Wang, S. Confined-interface-directed synthesis of Palladium single-atom catalysts on graphene/amorphous carbon. Appl. Catal., B 2018, 225, 291−297. (19) Lin, J.; Wang, A.; Qiao, B.; Liu, X.; Yang, X.; Wang, X.; Liang, J.; Li, J.; Liu, J.; Zhang, T. Remarkable performance of Ir1/FeO(x) single-atom catalyst in water gas shift reaction. J. Am. Chem. Soc. 2013, 135 (41), 15314−15317. (20) Han, Y.; Wang, Y.-G.; Chen, W.; Xu, R.; Zheng, L.; Zhang, J.; Luo, J.; Shen, R.-A.; Zhu, Y.; Cheong, W.-C.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. Hollow N-doped carbon spheres with isolated cobalt single atomic sites: superior electrocatalysts for oxygen reduction. J. Am. Chem. Soc. 2017, 139 (48), 17269−17272. (21) Liu, W.; Zhang, L.; Yan, W.; Liu, X.; Yang, X.; Miao, S.; Wang, W.; Wang, A.; Zhang, T. Single-atom dispersed Co-N-C catalyst: structure identification and performance for hydrogenative coupling of nitroarenes. Chem. Sci. 2016, 7 (9), 5758−5764. (22) Chen, Z.; Mitchell, S.; Vorobyeva, E.; Leary, R. K.; Hauert, R.; Furnival, T.; Ramasse, Q. M.; Thomas, J. M.; Midgley, P. A.; Dontsova, D.; Antonietti, M.; Pogodin, S.; López, N.; Pérez-Ramírez, J. Stabilization of Single Metal Atoms on Graphitic Carbon Nitride. Adv. Funct. Mater. 2017, 27 (8), 1605785. (23) Deng, L.; Zhu, M. Metal-nitrogen (Co-g-C3N4) doping of surface-modified single-walled carbon nanohorns for use as an oxygen reduction electrocatalyst. RSC Adv. 2016, 6 (31), 25670−25677. (24) Hou, Y.; Wen, Z.; Cui, S.; Feng, X.; Chen, J. Strongly Coupled Ternary Hybrid Aerogels of N-deficient Porous Graphitic-C3N4 Nanosheets/N-Doped Graphene/NiFe-Layered Double Hydroxide for Solar-Driven Photoelectrochemical Water Oxidation. Nano Lett. 2016, 16 (4), 2268−77. (25) Wang, F.; Wang, Y.; Li, Y.; Cui, X.; Zhang, Q.; Xie, Z.; Liu, H.; Feng, Y.; Lv, W.; Liu, G. The facile synthesis of a single atomdispersed silver-modified ultrathin g-C3N4 hybrid for the enhanced visible-light photocatalytic degradation of sulfamethazine with peroxymonosulfate. Dalton Trans. 2018, 47 (20), 6924−6933. (26) Tong, T.; Zhu, B.; Jiang, C.; Cheng, B.; Yu, J. Mechanistic insight into the enhanced photocatalytic activity of single-atom Pt, Pd or Au-embedded g-C3N4. Appl. Surf. Sci. 2018, 433, 1175−1183. (27) Wang, F.; Wang, Y.; Feng, Y.; Zeng, Y.; Xie, Z.; Zhang, Q.; Su, Y.; Chen, P.; Liu, Y.; Yao, K.; Lv, W.; Liu, G. Novel ternary photocatalyst of single atom-dispersed silver and carbon quantum dots co-loaded with ultrathin g-C3N4 for broad spectrum photoI

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (45) Ning, H.; Li, G.; Chen, Y.; Zhang, K.; Gong, Z.; Nie, R.; Hu, W.; Xia, Q. Porous N-Doped Carbon-Encapsulated CoNi Alloy Nanoparticles Derived from MOFs as Efficient Bifunctional Oxygen Electrocatalysts. ACS Appl. Mater. Interfaces 2019, 11 (2), 1957− 1968. (46) Hou, Y.; Wen, Z.; Cui, S.; Guo, X.; Chen, J. Constructing 2D porous graphitic C3N4 nanosheets/nitrogen-doped graphene/layered MoS2 ternary nanojunction with enhanced photoelectrochemical activity. Adv. Mater. 2013, 25 (43), 6291−6297. (47) Guo, Y.; Yuan, P.; Zhang, J.; Hu, Y.; Amiinu, I. S.; Wang, X.; Zhou, J.; Xia, H.; Song, Z.; Xu, Q.; Mu, S. Carbon Nanosheets Containing Discrete Co-Nx-By-C Active Sites for Efficient Oxygen Electrocatalysis and Rechargeable Zn-Air Batteries. ACS Nano 2018, 12 (2), 1894−1901. (48) Li, L.; Cruz, D.; Savateev, A.; Zhang, G.; Antonietti, M.; Zhao, Y. Photocatalytic cyanation of carbon nitride scaffolds: Tuning band structure and enhancing the performance in green light driven C S bond formation. Appl. Catal., B 2018, 229, 249−253. (49) Liu, X.; Chao, D.; Su, D.; Liu, S.; Chen, L.; Chi, C.; Lin, J.; Shen, Z. X.; Zhao, J.; Mai, L.; Li, Y. Graphene nanowires anchored to 3D graphene foam via self-assembly for high performance Li and Na ion storage. Nano Energy 2017, 37, 108−117. (50) Zhang, X.; Liu, X.; Yang, C.; Li, N.; Ji, T.; Yan, K.; Zhu, B.; Yin, J.; Zhao, J.; Li, Y. A V2O5-nanosheets-coated hard carbon fiber fabric as high-performance anode for sodium ion battery. Surf. Coat. Technol. 2019, 358, 661−666.

J

DOI: 10.1021/acs.inorgchem.9b01089 Inorg. Chem. XXXX, XXX, XXX−XXX