ZHANG Fei-Yan XIE Kui ②

a (College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, China) b (Key Laboratory of Design & Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

ABSTRACT Porous single crystals have the characteristics of long-range ordering structure and large specific surface area, which will significantly enhance their electrochemical performance. Here, we report a method different from the conventional porous single crystal growth method. This method is to directly convert single crystal precursors Co3O4 and Fe3O4 into Co2N and Fe2N, and then further reduces them to porous single crystals Co and Fe particles under H2/Ar atmosphere. The removal of O2- in the lattice channel at the pressure of 25～300 torr and the temperature of 300～600 °C will promote nitridation of the single-crystalline Co-O and Fe-O frames, and further remove N3- in H2/Ar atmosphere and recrystallize as Co and Fe. These porous single crystals exhibit enhanced electrochemical properties due to their structural coherence and highly active surface. We demonstrated that the aminobenzene yield was up to 91% and the selectivity reached 92% in the electrochemical reduction of nitrobenzene.

Keywords: porous single crystal, crystal growth, electrocatalysis;

1 INTRODUCTION

In the past decade, porous materials have received increasing attention in many fields, including electrocatalysis and photoelectrochemistry[1,2]. The large surface area of porous materials provides sufficient reactivity sites. In an electrochemical system, an electrode with porosity and a large surface will be well suited to support surface reaction requirements[3,4]. However, the separation, transmission and utilization of charges remain a fundamental challenge due to the presence of excessive grain boundaries in the electrodes[5]. Porous single crystal materials have no grain boundaries. Due to the inherent long-range electronic connectivity and overall structural coherence, energy loss at grain boundaries is redu- ced and the stability and activity are maintained.

Typical porous materials are mainly various metal-organic frameworks (MOFs), which are dominated by coordination chemistry between transition metal ions and organic ligands in nanometer/micron scale particles[6-8]. However, the durability and electronic transportation of MOFs remain challengeable for the materials themselves, which largely hinders their applications on electrodes[9]. Therefore, the growth of porous single crystal materials with high performance has been continuously studied. In particular, the growth along preferred surface and in the fixed direction largely limits the existence of block crystals with porous microstructures for crystal growth[10,11]. In recent years, we have grown large porous single crystals, which have shown excellent performance in photoelectric catalysis fields[12-14]. We try to grow porous single crystal particles and study their catalysis performance.

In this work, we grow porous metal nitride and metal mono-quality single crystal particles with a new strategy different from the ordinary concept. These materials combine the advantages of long-range ordered structure and three- dimensional interconnecting pores. The porous metal nitride single crystal particles will produce the metal-nitrogen active sites of unsaturated nitrogen coordination on the crystal surfa- ce. The warped surface has active sites with chemical components similar to those of a block crystal, which will be confined by the lattice and distributed over a long distance in an orderly manner on the surface, so that the kinetic capture of the active surface can be produced. The porous microstruc- ture will introduce surface stress on the distorted surface, which will cause all exposed surfaces to be in a high energy state to produce a surface with catalytic activity. The analysis of their microstructure and the excellent performance of selective reduction of nitrobenzene to aminobenzene indicate that they are ideal candidate for the electrode.

2 EXPERIMENTAL

2. 1 Syntheses

The precursors Co3O4cube and Fe3O4octahedron were obtained by a typical hydrothermal synthesis procedure[15,16]. The samples were placed in a horizontal alumina burner and introduced into a vacuum reaction chamber. The chamber was vacuumized to a basic pressure of 0 torrent for removing residuary oxygen, then a constant flow rate of NH3gas (100～600 sccm, 6 N purity) was aerated. Nitriding process main- tained the system pressure at 100～600 torrent for growing porous Co2N and Fe2N single crystal particles. Co3O4and Fe3O4precursors were then heated to 300 and 550 °C at a rate of 5 °C·min-1, respectively. The heating process is held for 1～10 hours and naturally cooled under nitrogen atmosphere (6 N purity). The porous Co2N and Fe2N single crystal particles obtained in the previous stage were further processed. Similarly, the residual gas in the vacuum reaction chamber was removed, and 5% H2/Ar (50～200 sccm) constant flow gas was introduced. The porous materials were heated at 300 and 400 °C for 1～5 hours to obtain porous Co and Fe single crystal particles.

2. 2 Characterization

The orientation and structure of samples were characterized using an X-ray diffractometer (XRD, Cu-Kα, Bruke D8 Advance). The morphologies of the single crystals were analyzed using field-emission scanning electron microscopy (FE-SEM, SU8010) at an accelerating voltage of 10 KV. Microstructure and orientation characterization of samples were expressed on a field-emission transmission electron microscope (TEM) and selected area electron diffraction (SAED) (Tecnai F30) at 200 KV. Nitrogen sorption isotherms were tested on Micromeritics analyzer (ASAP2020C+M). Electrochemical tests were monitored on the electrochemical workstation (IM6, Zahner).

2. 2 Cathode preparation

1 mg of catalyst and 4 μL of nafion solution (5 wt%) were dispersed in 98 μL of anhydrous ethanol and 98 μL of deionized water. Then ultrasonic treatment lasted 30 minutes to form a uniform ink-like solution. The prepared solution was supported on a carbon paper electrode with an area of 1 × 1 cm2and dried under an infrared lamp. A catalyst film with a loading capacity of 1 mg·cm-2was formed.

2. 3 Electrocatalytic reduction of nitrobenzene

The catalystic reaction of nitrobenzene was carried out in a two-chamber electrolytic cell which is separated by an anion exchange membrane (Nafion?212) at room temperature. The electrochemical experiment was performed on an electro- chemical workstation (Zahner?Ennium) by using a three- electrode system (working electrode was Co3O4, Fe3O4, porous single crystal Co2N, Fe2N, Co and Fe particles, counter electrode was Pt foil, and reference electrode was Hg2Cl2/Hg/saturated KCl). In this work, all potentials were calibrated and converted into reversible hydrogen electrodes (RHE). For nitrobenzene reduction experiments, the cathode chamber contained 10 mg nitrobenzene and 0.1 M Na2SO4solution (pH = 7.3) as the electrolyte, while the anode chamber only contained electrolyte. The solution was stirred evenly with a magnetic stirrer, and Ar gas was passed for 30 min to drive away the oxygen dissolved in the solution. The gas chromatography (Shimadzu GC-2014) and gas chromatography-mass (Agilent Technologies 7820A) were used to analyze the concentration of nitrobenzene and aminobenzene.

2. 4 Theory calculation

All density functional theory (DFT) calculations were performed in the Vienna ab initio Simulation Package (VASP) code[17]. Based on the projector augmented wave (PAW) approach, the plane-wave cutoff energy of 500 eV was used, which gives well converged relative energies for the system. The Perdew-Burke-Ernzerhof (PBE) functional was chosen here to describe the exchange correlation interactions.

The reduction progress is as follows[18]:

C6H5NO2* + H* → C6H5NOOH* + *

C6H5NOOH* + H* → C6H5N(OH)2* + *

C6H5N(OH)2* + * → C6H5NOH* + OH*

C6H5NOH* + H* → C6H5NHOH* + *

C6H5NHOH* + * → C6H5NH* + OH*

C6H5NH* + H* → C6H5NH2* + *

The total reaction equation is:

C6H5NO2* + 4H* → C6H5NH2* + 2OH* + 2*

Where C6H5NO2*, H*, C6H5NH2* and OH* mean C6H5NO2, H, C6H5NH and OH adsorbed on the crystal surface. And “*” provides active position.

We use Co and Co2N as examples to calculate Gibbs free energy of the total reaction equation.

The Gibbs free energy is calculated by:

ΔG = ΔE + ΔZPE - TΔS + ne(U - U0)

Where ΔE is the energy calculated by DFT, ΔZPE the zero-point energy, T the temperature, ΔS the entropy, (U - U0) the electrode potential change, n the electrode transfer number and e the elementary charge.

The lattice parameters of Co are a = b = c = 3.46 ? and the lattice parameters of Co2N are a = 4.53, b = 4.30, c = 2.84 ?. A p (2×2), superstructure for Co(111) surface with 4 layers and Co2N(111) surface with 6 layers, was used to simulate the periodic slab model. The bottom two layers of atoms are frozen for Co and atoms in bottom three layers are frozen for Co2N, with the others relaxed. A 25 ? vacuum region for Co2N slab model was inserted in the c direction to avoid the interactions between the slab and its repeating image. A 3×3×1 k-point grid was used for the two slab models to sample the Brillouin zone.

In this work, the adsorption energy was defined as:

Eads= E(Ad-surface)- E(surface)- E(Ad)

Where E(Ad-surface), E(surface)and E(Ad)are the total energies of the adsorbate absorbed on the crystal surface, clean crystal surface and free adsorbate in gas phase, respectively.

3 RESULTS AND DISCUSSION

In this work, we synthesized Co3O4cubic and Fe3O4octahedral single crystals with sizes of 300 and 1 μm by conventional hydrothermal methods, the XRD patterns of our synthesized precursors are consistent with the standard XRD patterns, as shown in Fig. 1a and 1b, their morphologies are shown in Fig. 2a and 2d. These precursors were then nitrided and precisely grown into porous Co2N and Fe2N single crystal particles by adjusting the system temperature and the pressure of ammonia in a vacuum system. As shown in Fig. 2b and 2e, these products exhibit fine and uniform pore structures relative to the precursors. We further introduced a reducing atmosphere to transform porous Co2N and Fe2N single crystal particles into the porous Co and Fe ones. Fig. 2c and 2f show that the final products exhibit three-dimensional interconnec- ted porosity structures, but their dimensions have not changed undergoing the complicated transformation process. The specific surface areas reach 7～9 m2·g-1, and the average pore diameters are 70～90 nm, as shown in Fig. 1c and 1d.

Fig. 1. (a) XRD patterns of Co3O4 single crystal particles, porous Co2N and Co single crystal particles, (b) XRD patterns of Fe3O4 single crystal particles, porous Fe2N and Fe single crystal particles, (c) Surface specific area of porous Co2N, Fe2N, Co and Fe single crystal particles, (d) Average pore size of porous Co2N Fe2N, Co and Fe single crystal particles

Fig. 2. (a) SEM images of Co3O4 single crystal particles, (b) SEM images of porous Co2N single crystal particles, (c) SEM images of porous Co single crystal particles, (d) SEM images of Fe3O4 single crystal particles, (e) SEM images of porpus Fe2N single crystal particles, (f) SEM images of porous Fe single crystal particles, inset: the corresponding enlargements of the Co3O4, Co2N, Co, Fe3O4, Fe2N and Fe single crystal, respectively

In order to study the properties of porous metal nitride and metal mono-quality single crystal materials, we used transmission electron microscopy (TEM) to characterize the crystallinity. For single Co2N and Fe2N metal nitride particles, the crystal orientation at different positions is first analyzed by performing selective area electron diffraction (SAED), as shown in Fig. 3a～3e and Fig. 4a～4e. It is confirmed that a single nitride single crystal particle has the same orientation at different positions in a fixed diffraction direction. Therefore, we directly convert the precursors into porous Co2N and Fe2N single crystal particles in ammonia, and then further reduce them to porous Co and Fe single crystal particles at H2/Ar atmosphere under vacuum pressure. Fig. 3f～3j and Fig. 4f～4j perform similar analyses on the porous Co and Fe single crystal particles, respectively, and confirm that even though the grown metal nitride and metal mono-quality particles have three-dimensionally connected porosity, they are still in a single crystal state. Highly single crystallinity creates the atomically resolved active sites in a long-range orderly manner, while the porous structure will adapt to twisted surfaces with high energy states, resulting in electrochemi- cally active surfaces. In addition, we have further conducted the XPS tests of porous single crystals Co2N and Fe2N. As shown in Fig. 5a and 5b, the binding energies at 779.9 and 795.1 eV are the Co 2p3/2and Co 2p1/2orbitals of Co-N bonds, and the binding energy of 396.8 eV is N 1s orbital of Co-N bond. Then, Fig. 5c and 5d reveal the binding energies at 705.9 and 718.9 eV are Fe 2p3/2and Fe 2p1/2orbitals of Fe-N bonds, while that of 396.6 eV is ralated to the N 1s orbital of Fe-N bond[19-22].

We have studied the growth mechanism of porous metal nitride single crystal particles transformed from Co3O4and Fe3O4single crystal particles and their further conversion into porous metal mono-quality single crystal particles. As shown in Fig. 6a and 6d, the crystal structures of Co3O4and Fe3O4clearly illustrate the one-dimensional channel of intrinsic O2-ions in the framework. During nitriding process, O2-can be removed gradually through lattice channels, leaving the Co-O and Fe-O skeletons. During the progressive nitriding process, oxygen is replaced by nitrogen and porous Co2N and Fe2N single crystal particles recrystallize. Similarly, one-dimen- sional channels of intrinsic N3-ions also exist in the lattice structures of Co2N and Fe2N, as shown in Fig. 6b and 6e. Lattice reconstruction is also the main way to further remove N3-for growing metal single crystals, as shown in Fig. 6c and 6f.

Fig. 3. (a) TEM image of porous Co2N single crystal particle, (b～e) Corresponding SAED patterns of porous Co2N single crystal particle, (f) TEM image of porous Co single crystal particle, (g～j) Corresponding SAED patterns of porous Co single crystal particle

Fig. 4. (a) TEM image of porous Fe2N single crystal particle, (b～e) Corresponding SAED patterns of porous Fe2N single crystal particle, (f) TEM image of porous Fe single crystal particle, (g～j) Corresponding SAED patterns of porous Fe single crystal particle

Fig. 5. (a) XPS spectra in the Co 2p region of porous Co2N single crystal, (b) XPS spectra in the N 1s region of porous Co2N single crystal, (c) XPS spectra in the Fe 2p region of porous Fe2N single crystal, (d) XPS spectra in the N 1s region of porous Fe2N single crystal

Fig. 6. (a) Structure of Co3O4 single crystal, showing the 1D-channels of intrinsic O2- ions in the Co-O framework, (b) Structure of porous Co2N single crystal, showing the 3D Co-N framework, (c) Structure of porous Co single crystal, showing the 3D Co framework, (d) Structure of Fe3O4 single crystal, showing the 1D-channels of intrinsic O2- ions in the Fe-O framework, (e) Structure of porous Fe2N single crystal, showing the 3D Fe-N framework, (f) Structure of porous Fe single crystal, showing the 3D Fe framework

We investigate the selective reduction of nitrobenzene to aminobenzene with porous metal nitride and metal mono- quality single crystal particles. We summarize the correlation between current densities, aminobenzene yields and selec- tivities of these single crystal materials, and the data are shown in Fig. 7. It indicated from Fig. 7a and 6d that applying a higher bias would lead to a stronger reaction of hydrogen evolution. Fig. 7c and 7f show the relationship between yield and selectivity of aminobenzene for oxide precursors, nitrides, and metal single crystal particles. The best properties were observed using porous Co2N and Fe2N single crystal particles, and the yield of aminobenzene was 76～91% and the selec- tivity was 90 ～92%, indicating that the lower nitrogen coordination number was beneficial to the enhancement of electrochemical reduction performance compared with oxide and metal mono-quality single crystals. For porous Co2N and Fe2N single crystal particles, excellent durability was observed even after 15 hours at -0.6 V, as shown in Fig. 7b and 7e. The excellent stability can be attributed to the structural consistency and high surface activity of the porous single crystal. The theory calculations of C6H5NO2reduction on Co2N(111) and Co(111) surfaces are shown in Fig. 8a and 8b, and their catalysis processes of nitrobenzene to aminobenzene are different. The Gibbs free energy of the progress on Co2N surface is -1.35 eV and on Co surface is -1.17 eV. We can see clearly that Co2N reacts more easily under experimental conditions than Co.

Fig. 7. (a) LSV curves of Co3O4 single crystal particles, porous Co2N, Co single crystal particles and carbon paper in 0.1 M Na2SO4 solution with 10 mg nitrobenzene saturated with Ar. (b) The durability tests of Co3O4 single crystal particles, porous Co2N and Co single crystal particles at -0.6 V in the catalysis process of nitrobenzene amination to aminobenzene. (c) The aminobenzene yield and selectivity of Co3O4 single crystal partiales, porous Co2N and Co single crystal particles in the catalysis process of the selective nitrobenzene amination to aminobenzene at different electrode potentials. (d) LSV curves of Fe3O4 single crystal particles, porous Fe2N, Fe single crystal particles and carbon paper in 0.1 M Na2SO4 solution with 10 mg nitrobenzene saturated with Ar. (e) The durability tests of Fe3O4 single crystal, porous Fe2N and Fe single crystal particles at -0.6 V in nitrobenzene reduction reaction. (f) The aminobenzene yield and selectivity of Fe3O4 single crystal particles, porous Fe2N and Fe single crystal particles in the catalysis process of the selective nitrobenzene amination to aminobenzene at different electrode potentials

Fig. 8. (a) Top and side views of the four optimized structures of adsorbed C6H5NO2*, C6H5NH2*, H* and OH* on the Co2N(111) surface, together with the corresponding adsorption energies indicated, (b) Top and side views of the four optimized structures of adsorbed C6H5NO2*, C6H5NH2*, H* and OH* on the Co(111) surface, together with the corresponding adsorption energies indicated (cobalt in light blue, nitrogen in dark blue, carbon in gray, oxygen in red and hydrogen in white)

4 CONCLUSION

In this paper, we report a conceptually different growth method. The precursor Co3O4cube and Fe3O4octahedron are directly nitrided into porous Co2N and Fe2N single crystal particles, and then further converted into porous Co and Fe single crystal particles. Atoms diffuse in the lattice channels and finally chemically construct porous single crystals through a recrystallization process. The relatively large specific sur- face area and structural consistency of these porous single crystal materials enhance electrochemical performance. The current work shows a step forward in the growth of porous single crystal particles, which will open a new way for the development of various porous single crystal materials with enhanced functionality.

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