Dramatically Enhanced Stability of Silver Passivated Dicalcium Nitride

Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Dramatically Enhanced Stability of Silver Passivated Dicalcium Nitride Electride: Ag-Ca2N K. P. Faseela,† Ye Ji Kim,† Seong-Gon Kim,‡ Sung Wng Kim,*,†,§ and Seunghyun Baik*,§,∥ †

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea Department of Physics & Astronomy and Center for Computational Sciences, Mississippi State University, Mississippi State, Mississippi 39762, United States § Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Republic of Korea ∥ School of Mechanical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea Chem. Mater. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 11/10/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Electrides have received considerable attention due to their exotic properties. However, the high reactivity with oxygen and moisture immediately decomposed electrides in an ambient air environment. Here we passivated dicalcium nitride electride (Ca2N:e−) with silver via a wet chemical approach (Ag-Ca2N), significantly enhancing stability up to 17 min at room temperature in an ambient air environment. The Ca2N:e− was employed as a reducing agent due to the low work function (2.6 eV), high mobility, and high electron concentration facilitating electron transfer to Ag+ ion in an aprotic cosolvent. Moreover, the noble metal surface passivation (thickness: 55 nm) was achieved with negligible increase in work function of Ag-Ca2N (2.78 eV). The optimized molar ratio of AgNO3/Ca2N:e− was 0.5. The enhanced stability of Ag-Ca2N in organic reaction medium enabled successful aldol condensation reaction outside the glovebox, with a high α,βunsaturated ketone yield of 75.4%, without involving environmentally harmful strong acid or base. The enhanced stability and low work function may realize practical economic applications of Ag-Ca2N.

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INTRODUCTION

which needed to be removed by additional polishing or sputter etching.6,20 More recently, two-dimensional layer-structured electrides such as [Ca2N]+·e− and Y2C were discovered.21,22 [Ca2N]+·e− (hereafter referred to as Ca2N:e−) has a rhombohedral layered structure where anionic electrons are delocalized between positively charged atomic planes with an interlayer spacing of 0.4 nm.21 The open layered-structure provided anisotropic electrical properties (in-plane and out-of-plane work functions = 2.6 and 3.4 eV, Ne = ∼1.37 × 1022 cm−3) with superior electron accessibility (Hall mobility = 200 cm2 V−1 s−1) than C12A7:e− (4 cm2 V−1 s−1).6,21 However, Ca2N:e− was unstable in the ambient air atmosphere due to the high reactivity of the loosely bound excess electrons with oxygen and moisture.5,21,23 Organic reaction demonstrations involving single electron transfer or carbanion intermediate had to be carried out in an inert atmosphere inside a glovebox, undermining practical and economic benefits.24−26 First-principles calculations have been actively carried out screening new electride material

Electrides are well-defined crystalline ionic compounds that contain a finite density of electrons acting as anions.1−3 The early synthesis of electrides can be traced back to the 1960s,4 and electrides have received considerable attention after the discovery of inorganic electrides.5,6 Electrons are trapped in cavities and channels formed by close packing of large complexed cations.7 The intriguing electronic properties arising from the highly mobile non-nucleus-bound electrons prompted energy storage,8,9 optoelectronic,10−13 and catalytic applications.14−17 However, electrides are highly reactive with oxygen and water, impeding practical employment in an ambient air environment.1−15 In 2003, inorganic electride [Ca24Al28O64]4+·4e− (hereafter referred to as C12A7:e−) with enhanced thermal and chemical stability was reported.6 C12A7:e− has a three-dimensional cage-like crystal structure with a maximum theoretical electron density (Ne) of 2.3 × 1021 cm−3 and a low work function of 2.4 eV at room temperature.5,6,18 However, it was difficult to obtain a chemically pure C12A7:e− surface since electrons in the vicinity of the surface readily reacted with oxygen and water, resulting in an electron deficient layer.18,19 The contaminated surface layer was intrinsically formed during the synthesis © XXXX American Chemical Society

Received: July 28, 2018 Revised: October 17, 2018

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DOI: 10.1021/acs.chemmater.8b03202 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

Figure 1. Synthesis of Ag-Ca2N. The characterization was carried out for the Ag-Ca2N synthesized using 1 mmol of Ca2N:e− and 0.5 mmol of AgNO3 (30 min reaction time). (a) Schematic of the synthesis process. SEM images of bare Ca2N:e− and Ag-Ca2N are also provided. (b) Band diagram depiction of electron transfer from electride to Ag+ ion. (c) SIMS profile of Ag-Ca2N. (d) SEM and EDX analysis of Ag-Ca2N. (e) STEM and EDX analysis of Ag-Ca2N.

candidates.27−32 Ambient-air-stable electrides such as Y5Si3, LaScSi, and LaCoSi have been recently synthesized.33−35 Here we dramatically enhanced stability of Ca2N:e− at room temperature in an ambient air environment and organic reaction medium by encapsulating it in a silver (Ag) surface layer. Ag was chosen as the passivating layer considering the stability, high conductivity, and atmospheric oxidation resistance. The large energy difference between the work function of Ca2N:e− (2.6 eV) and the highest occupied molecular orbital (HOMO) of Ag+ ion (6.4 eV)36 was utilized to form metallic Ag on the surface of Ca2N:e− in an aprotic solvent mixture of acetonitrile (CH3CN) and tetrahydrofuran (THF). Ca2N:e− was employed as a reducing agent. The type of solvents, concentration (1 mmol Ca2N:e−, 0.5 mmol AgNO3), and reaction time (30 min) were carefully optimized to obtain a uniform Ag passivation layer with a thickness of ∼55 nm. The low average work function (2.78 eV) of silvercoated Ca2N:e− (Ag-Ca2N) was well-maintained considering the anisotropic nature of Ca2N:e− (in-plane 2.6 eV and out-ofplane 3.4 eV). The stability of Ag-Ca2N at room temperature in an ambient air environment was dramatically enhanced up to 17 min in contrast to the immediate oxidation of bare Ca2N:e−. As an application demonstration, organic reaction with Ag-Ca2N was carried out in an ambient air environment.

The enhanced stability of Ag-Ca2N in an organic reaction medium enabled successful aldol condensation reaction with a high α,β-unsaturated ketone yield of 75.4%, without involving environmentally harmful strong acid or base. This is the first work of passivating Ca2N:e− with an inert noble metal via simple wet chemical reaction. The enhanced stability at room temperature in ambient air and organic reaction medium, while maintaining the low work function, makes Ag-Ca2N a more promising and economic candidate for future practical applications of electrides.

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EXPERIMENTAL SECTION

Synthesis of Ca2N:e−. Ca2N:e− was synthesized following a previously published protocol.21,24,25 The process involved the solid state reaction of calcium nitride (Ca3N2) powders and calcium metals. First, a pellet was made by pressing (30 MPa) mixtures of Ca3N2 powders and calcium chips at a molar ratio of 1:1. Second, the pellet was thoroughly covered with a molybdenum foil and annealed at 800 °C for 48 h under vacuum (∼10−3 Pa). The pellet was then quenched with water. The synthesized specimen was then ground into powders using an agate mortar in an Ar-filled glovebox. Finally, the specimen was reannealed under the same condition to improve homogeneity. Synthesis of Ag-Ca2N. The synthesis Ag-Ca2N was carried out in an Ar-filled glovebox. First, AgNO3 (0.25−1 mmol) was dissolved in CH3CN-THF (50:50 vol %) cosolvent (5 mL) by vigorous stirring B


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Figure 2. Air stability of Ag-Ca2N at room temperature. (a) The XRD patterns of bare Ca2N:e− before and after exposure to air. The Ca(OH)2 peak is marked by a red star. (b) The XRD pattern of Ag-Ca2N (1 mmol Ca2N:e−, 0.5 mmol AgNO3, 30 min reaction time) taken at ambient air atmosphere (exposure time = 5 min). The JCPDS data of Ca2N:e− and Ag are also provided. (c) The XRD patterns of Ag-Ca2N synthesized using different AgNO3 concentrations (Ar atmosphere). The Ca2N:e− concentration was 1 mmol. (d) The normalized intensity of the Ag(111) peak and the ratio of Ca2N:e− (003)/Ag(111) peaks are shown as a function of the molar ratio of AgNO3/Ca2N:e−. (e) UPS analysis of Ag-Ca2N (1 mmol Ca2N:e−, 0.5 mmol AgNO3, 30 min reaction time). The onset region is magnified in the inset. (f) The work function comparison of pure Ag, pure Ca2N:e−, and Ag-Ca2N synthesized using 4 different molar ratios of AgNO3/Ca2N:e−. The reaction time was 30 min. (15 min). Ca2N:e− (94 mg, 1 mmol) powder was then added and stirred for 2−90 min. The resulting black powder was filtered, rinsed with CH3CN for several times, and dried at 40 °C for 12 h. Aldol Condensation Reaction. The aldol condensation reaction was carried out in an ambient air environment. A solution of benzaldehyde in DMSO (106 mg, 1 mmol) was mixed with an acetophenone solution in DMSO (120 mg, 1 mmol). The resulting mixture was stirred at room temperature for 1 h. The Ag-Ca2N (4 mmol) was then added and stirred at 25 °C for 12 h. The stoichiometric ratio between acetophenone and Ag-Ca2N was 1:4. The progress of the reaction was monitored by thin layer chromatography. The precipitate (aldol condensation product) and Ag-Ca2N were filtered with diluted-ethanol (50%) rinsing and redissolved in ethyl acetate at 50 °C by stirring. Ag-Ca2N was removed by a second filtering. The aldol condensation product was recrystallized from ethyl acetate by decreasing the temperature to 4 °C. The reaction was also carried out at 50 °C for 8 h for comparison. As other control experiments, aldol condensation reaction was carried out using pure Ca2N:e− (2 mmol) or Ag nanoparticles (Ag NPs, 0.107 g). The synthesis with pure Ca2N:e− was carried out in an Arfilled glovebox. Measurements. The morphology was investigated by scanning electron microscopy (SEM, JEOL, JSM-7600F), scanning transmission electron microscopy (STEM, JEOL, JEM-ARM-200F), and transmission electron microscopy (TEM, JEOL, JEM-2100F). The thickness of the Ag passivation layer was measured by secondary ion mass spectrometry (SIMS, ION-TOF, TOF-SIMS-5) with a primary Cs ion beam. An X-ray diffractometer (Rigaku, Smart Lab, Cu Kα radiation at 1.5418 Å, 45 kV, and 200 mA) was also employed. The work function of Ag-Ca2N was determined by ultraviolet photoelectron spectroscopy (UPS, Thermo-Scientific, ESCALAB250, monochromatic He-I radiation at 21.2 eV) analysis. The aldol

product was characterized by Fourier transform infrared (FT-IR, Bruker, IFS-66/S, TENSOR27) spectroscopy.

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RESULTS AND DISCUSSION Synthesis of Ag-Ca2N. Figure 1a shows a schematic of the Ag-Ca2N synthesis process. The anionic electrons, sandwiched between positively charged atomic planes, are depicted as twodimensional electron gas (2DEG).21 The large difference between the work function of Ca2N:e− (in-plane 2.6 eV)21 and HOMO of Ag+ ion (6.4 eV)36 was utilized for the reduction of Ag+ into metallic Ag at the surface of Ca2N:e− (Figure 1b). The work function of metallic Ag is 4.7 eV.37 A theoretical investigation revealed that the HOMO of Ag+ is changed to 6.4 eV since the ns orbital is vacant in Ag+ ion.36 The charge transfer from Ca2N:e− to Ag+ ion was realized due to the significantly lower work function, high electron mobility, and high Ne of electrides. Ca2N:e− was synthesized following a previously published protocol for experimental verification.21,24,25 The average size of mechanically ground Ca2N:e− particles was 3.2 μm (see Figure S1 for the particle size distribution). Note that the aggregates of Ca2N:e− powders were excluded from this size analysis. The reaction medium was carefully optimized to realize the Ag+ reduction on Ca2N:e− via wet chemistry. Table S1 compares the physical properties of solvent candidates including water, methanol (CH3OH), CH3CN, THF, and dimethyl sulfoxide (DMSO). Electrides are relatively stable in nonpolar aprotic medium, and aprotic THF with a dielectric constant of 7.5 was the best candidate.24 The dielectric constant was taken as a measure of solvent polarity. In contrast, silver precursor (AgNO3) requires C


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XRD pattern after 5 min air exposure was identical to the JCPDS data of Ca(OH)2 (see Figure S4). In contrast, the peaks related to Ca(OH)2 could not be observed for Ag-Ca2N after 5 min exposure to ambient air, indicating excellent air stability (Figure 2b). The Ag-Ca2N powders (1 mmol Ca2N:e−, 0.5 mmol AgNO3, and 30 min reaction time) demonstrated peaks related to pristine Ca2N:e− (R3̅m space group, JCPDS no. 21-0837) and face centered cubic (fcc) Ag (JCPDS no. 65-2871). Figure 2c shows XRD data of Ag-Ca2N taken at an inert Ar atmosphere. The Ag-Ca2N was synthesized using different concentrations (0−1 mmol) of AgNO3 while the concentration of Ca2N:e− was fixed at 1 mmol. The normalized intensity of the Ag(111) peak and the ratio of Ca2N:e− (003)/ Ag(111) peaks are compared as a function of the molar ratio of AgNO3/Ca2N:e− (Figure 2d). The Ag peak increased as the concentration of AgNO3 increased. Interestingly, the ratio of Ca2N:e− (003)/Ag(111) peak decreased from infinity to zero as the molar ratio of AgNO3/Ca2N:e− increased from zero to one. However, there was no indication of Ca(OH)2 formation since the measurement was carried out at an Ar atmosphere. Ca2N:e− was employed as a reducing agent for the synthesis of Ag-Ca2N. Therefore, the electron transfer from Ca2N:e− to Ag+ decreased the electron concentration of core Ca2N:e− in the resultant Ag-Ca2N. One electron was consumed to reduce one Ag+ ion at the outer surface of Ag-Ca2N. The theoretically calculated electron concentrations of core Ca2N:e− were 1.37 × 1022, 1.03 × 1022, 0.69 × 1022, and 0.34 × 1022 cm−3 when the molar ratios of AgNO3/Ca2N:e− were 0, 0.25, 0.5, and 0.75, respectively. Note that the electron concentration of pristine Ca2N:e− was 1.37 × 1022.21 In the equimolar mixture of Ca2N:e− and AgNO3, the entire electrons of Ca2N:e− were consumed to reduce Ag+ completely, destroying the Ca2N:e− structure during the synthesis procedure. The optimized molar ratio of AgNO3/Ca2N:e− was found to be 0.5 as will be discussed later. The surface of Ca2N:e− was not completely covered by the reduced metallic Ag layer when the molar ratio of AgNO3/Ca2N:e− was 0.2 (Figure S5). One of the exciting features of Ca2N:e− is its low work function. The work function of Ag-Ca2N was investigated by UPS as shown in Figure 2e. The onset (Eonset) and cut off (Ecut‑off) energies are designated by arrows, and the onset region is magnified in the inset. The work function (Φ), which is the energy difference between the vacuum and Fermi level, was obtained by eq 1.38,39

polar medium for dissolution. It could be dissolved in aprotic CH3CN with a dielectric constant of 37.5. The CH3CN-THF (50:50 vol %) cosolvent system was selected to synthesize AgCa2N considering both the stability (up to 26 h) and solubility. The reduction was carried out using 1 mmol of Ca2N:e− and 0.5 mmol of AgNO3 for 30 min in an argon (Ar)-filled glovebox (the optimized synthesis condition will be discussed later). The surface coating was initially expected to occur predominantly on the sides of electrides due to the lower inplane work function (2.6 eV) of Ca2N:e− than the out-of-plane work function (3.4 eV).21 However, a relatively uniform coating around the surface of electrides was observed from SEM images (Figure 1a). The out-of-plane work function (3.4 eV) of Ca2N:e− was still lower than the HOMO of Ag+ ion (6.4 eV).21,36 Besides, the coating could have been extended to the top and bottom surface from the sides through the conductive Ag path in the prolonged reduction process. The Ag passivation layer thickness of Ag-Ca2N was investigated by the SIMS depth profiling method (Figure 1c). The top silver layer was slowly sputter-etched using a primary cesium ion beam, and the thickness was found to be ∼55 nm. The coating thickness was also estimated from the component mass ratio and particle size information on AgCa2N (see Figures S2 and S3 for details). The mass and volume fractions of Ag in Ag-Ca2N were 38 wt % and 7 vol %. This corresponded to a Ag layer thickness of 39 nm for a spherical particle with a diameter of 3.2 μm. Note that the coating thickness would further increase in this theoretical analysis if the particle size is increased by aggregation. This rough theoretical estimation was in the similar order with the SIMS measurement. SEM and STEM images of Ag-Ca2N are shown in Figure 1d,e. The energy dispersive X-ray spectroscopy (EDX) analysis demonstrated a uniform Ag coating on the surface of Ca2N:e−. The noble metal surface passivation layer prevented direct contact between Ca2N:e− and oxygen/ water, significantly enhancing stability as will be discussed shortly. Enhanced Air Stability of Ag-Ca2N. The stability of Ca2N:e− in an ambient air environment was investigated by Xray diffraction (XRD) analysis (Figure 2a). In order to obtain the pristine Ca2N:e− data, XRD measurement was first carried out in an inert Ar atmosphere. The sharp characteristic (003) plane peak of pristine Ca2N:e− could be observed at 2θ = 13.8°. The measurement was also carried out in an ambient air environment. It took 2 min to scan from 2θ = 10° to 17.8° in the XRD measurement (step degree = 0.02, speed = 4°/min). Therefore, the total air exposure time consists of pre-exposure time and an additional 2 min measurement time (e.g., 3 min = 1 min pre-exposure time + 2 min). The loosely bound electrons render electrides highly reactive with oxygen and moisture in the ambient air environment.23 The adsorption of N2 caused stretching of the N−N bond and simultaneous charge transfer from Ca2N:e− to N2.23 Meanwhile, dissociative adsorption was observed for molecules more reactive than N2. O2 was cleaved into two atoms, and H2O was cleaved into H and OH.23 H entered the Ca2N:e− lattice binding to N atom, while OH was adsorbed at hollow sites, forming Ca(OH)2.23 Upon 3 min exposure to air, a large peak at 2θ = 17.8°, corresponding to the (001) plane of Ca(OH)2, was observed, indicating immediate formation of Ca(OH)2. The characteristic peak of Ca2N:e− (2θ = 13.8°) started to decrease upon exposure to air and completely disappeared after 5 min. The

Φ = hυ − (Ecut‐off − Eonset)

(1)

where hυ is the excitation energy (21.22 eV). The resulting work function of the Ag-Ca2N (1 mmol Ca2N:e−, 0.5 mmol AgNO3, and 30 min reaction time) was 2.78 eV. This reflects a direction-averaged work function of Ag-Ca2N since the particles were randomly oriented in the UPS measurement spot. The low work function of Ca2N:e− (in-plane 2.6 eV and out-of-plane 3.4 eV)21 was well-maintained even after the surface Ag layer passivation due to the high electrical conductivity of Ag. Figure 2f compares work functions of Ag-Ca2N powders synthesized using 4 different molar ratios of AgNO3/Ca2N:e− (0.3. 0.5, 0.7, and 1). The raw UPS data are provided in Figures 2e and S6. The Ag-Ca2N synthesized using an AgNO3/Ca2N:e− molar ratio of 0.3 had a work function (2.71 eV) closer to that of pristine Ca2N:e− (in-plane 2.6 eV).21 This indicated that the core Ca2N:e− lowered the work function of Ag-Ca2N by supplying electrons to the metallic Ag D


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Figure 3. Synthesis optimization of Ag-Ca2N for enhanced air stability. (a) The XRD patterns of Ag-Ca2N (1 mmol Ca2N:e−, 0.5 mmol AgNO3, 30 min reaction time) are provided with increasing air exposure time. The Ca(OH)2 peak is marked by a red star. (b) Optical images of Ca2N:e− and Ag-Ca2N before and after exposure to air. (c) Air stability of Ag-Ca2N (1 mmol Ca2N:e−, 30 min reaction time) as a function of the AgNO3 concentration. The solid and open symbols indicate stable and unstable specimens, respectively. (d) Air stability of Ag-Ca2N (1 mmol Ca2N:e−, 0.5 mmol AgNO3) as a function of the reaction time.

3b shows optical images of Ca2N:e− and Ag-Ca2N before and after the air exposure. The intrinsic color of Ag-Ca2N was wellmaintained after 17 min air exposure. In contrast, white spots immediately appeared in the pure Ca2N:e− powder upon exposure to air, and the entire powder became white after 90 s. Figure 3c shows the air stability of Ag-Ca2N synthesized using different molar ratios of AgNO3/Ca2N:e− (30 min reaction time). The concentration of AgNO3 was varied from 0.2 to 0.7 mmol, while that of Ca2N:e− was fixed at 1 mmol. The raw XRD data with 0.5 mmol of AgNO3 are shown in Figure 3a, and the rest of the data are provided in Figure S7. The filled circle and open diamond symbols denote stable and unstable specimens, respectively. The air stability increased as the molar ratio increased up to 0.5. As discussed in Figure S5, a smaller molar ratio of AgNO3/Ca2N:e− resulted in incomplete surface coverage of Ca2N:e− by the silver passivation layer. This decreased air stability. Surprisingly, the air stability also decreased when the molar ratio was greater than 0.5. This could be due to the depletion of electrons during the Ag-Ca2N synthesis process (Figure 2d). The air stability decreased when the remnant electron concentration in Ca2N:e− was small. Figure 3d shows the air stability of Ag-Ca2N synthesized with different reaction times. The molar ratio of AgNO3/ Ca2N:e− was 0.5. The raw XRD data with 30 min reaction time

surface layer. The work function of Ag-Ca2N increased as the AgNO3 concentration increased. This was consistent with the XRD analysis (Figure 2c,d). The Ca2N:e− peak decreased and the Ag peak increased as the AgNO3/Ca2N:e− molar ratio increased. The Ca2N:e− peak became negligible when the AgNO3/Ca2N:e− molar ratio was 1. This was because all the electrons in the electride were consumed for the Ag reduction process, and the work function of Ag-Ca2N was identical to the theoretical work function of pure Ag (4.7 eV).37 Optimization of Ag-Ca2N for Maximum Air Stability. The synthesis condition of Ag-Ca2N was optimized for maximum air stability, and the stability was investigated by XRD analysis at room temperature in an ambient air environment. Figure 3a shows XRD data of Ag-Ca2N (1 mmol Ca2N:e−, 0.5 mmol AgNO3, 30 min reaction time) as a function of the air exposure time. The Ca(OH)2 peak appeared after 20 min air exposure. However, the Ca2N:e− peak at 2θ = 13.8° was still large at 20 min. This indicated that the surface of Ag-Ca2N started to react with atmospheric oxygen and moisture, probably through the small holes or defects in the Ag passivation layer, although the core region still maintained the intrinsic structure of Ca2N:e−. The Ag-Ca2N was judged unstable when the Ca(OH)2 peak became distinctively large, and the maximum air stability was 17 min in Figure 3a. Figure E


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Figure 4. Aldol condensation reaction by Ag-Ca2N (molar ratio of AgNO3/Ca2N:e− = 0.5, reaction time = 30 min) at room temperature in ambient air environment. (a) Proposed mechanism of the aldol condensation reaction by Ag-Ca2N. (b) FT-IR analysis of the Ag-Ca2N promoted aldol condensation product (α,β-unsaturated ketone). (c) FT-IR analysis of the Ca2N:e− promoted aldol addition product. (d) The reaction yield comparison using different particles (Ag-Ca2N, Ca2N:e−, and Ag NPs).

the large Ag-Ca2N pellet was further improved, compared with smaller Ag-Ca2N powders, due to the increased thickness of the Ag passivation layer. The thicker Ag layer was easily achieved for the larger Ca2N:e− pellet. The temperature effect on the stability of Ag-Ca2N was also investigated. The Ag-Ca2N powders (1 mmol Ca2N:e−, 0.5 mmol AgNO3, 30 min reaction time) were placed in an airfilled oven at different temperatures (25, 50, 100, 200, and 300 °C) for 17 min. The XRD analysis clearly showed Ca2N peaks even after storing the Ag-Ca2N powders at 300 °C, demonstrating excellent stability of Ag-Ca2N at high temperatures (Figure S10). Electronic Structure. The thin metallic Ag layer (∼55 nm) was coated around the outer surface of the substantially larger core Ca2N:e− particle (∼3.2 μm) to construct the AgCa2N. As discussed in the XRD analysis (Figure 2c), there was negligible shift in the characteristic Ca2N:e− peak as the relative molar ratio of Ag/Ca2N:e− increased from 0 to 0.75. This indicated that the unit cell structure in the dominant core region of Ag-Ca2N was not significantly altered. Furthermore, the low work function of Ag-Ca2N was achieved, maintaining the characteristic feature of two-dimensional Ca 2 N:e − electrides (Figure 2f). However, anionic electrons of the core Ca2N:e− were partially consumed to reduce Ag+ in the outer shell of Ag-Ca2N, breaking the charge neutrality of Ca2N:e−. The effect of electron consumption on the electronic structure of Ca2N:e− was calculated because the characteristics of AgCa2N should be governed by the dominant core Ca2N:e−.

are shown in Figure 3a, and the rest of the data are provided in Figure S8. The air stability increased as the reaction time increased up to 30 min. However, a longer reaction time did not improve the air stability further. The reduction reaction of Ag+ was complete at 30 min when the molar ratio of AgNO3/ Ca2N:e− was 0.5. A shorter reaction time than 30 min decreased the air stability due to the incomplete reduction, decreasing surface coverage of Ca2N:e− by the Ag passivation layer. Overall, the optimized synthesis condition for maximum air stability of Ag-Ca2N was 30 min reaction time and 0.5 molar ratio of AgNO3/Ca2N:e− (1 mmol Ca2N:e− and 0.5 mmol AgNO3). The size effect of Ca2N:e− on the coating of the Ag surface layer was investigated (Figure S9). A large pellet (11 × 7 × 1 mm3) was made by pressing Ca2N:e− powders (20−30 MPa), and Ag coating was carried out on the pellet using the optimized condition (AgNO3/Ca2N:e− molar ratio = 0.5, 30 min reaction time). As shown in Figure S9a, the XRD measurement on the surface of the large Ag-Ca2N pellet showed Ag peaks only (3, 30, and 60 min air exposure) due to the large thickness of the Ag layer (∼5 μm). In order to investigate stability of the core Ca2N:e−, XRD measurement was carried out on a cross section of the Ag-Ca2N pellet (Figure S9b). The Ag-Ca2N pellet was first exposed to air for 60 min. The cutting and XRD measurement were then carried out under an Ar atmosphere to avoid oxidation during the measurement process. The Ca2N:e− peak was clearly observed even after 60 min air exposure. This indicated that stability of F


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Ca2N as a promoter without involving strong acid or base. Figure 4a shows the proposed mechanism in 3 steps.42,43,45 The proton abstraction is required to transform an acetophenone to a carbanion intermediate (keto form) as shown in Figure 4a-i. Ag-Ca2N initiates the proton abstraction by supplying an electron, eventually generating hydrogen bubbles. The carbanion intermediate is in equilibrium with its enol form. The enolate intermediate reacts with the benzaldehyde to form 3-oxo-1,3-diphenylpropan-1-olate (Figure 4a-ii).43 The 3-oxo-1,3-diphenylpropan-1-olate accepts a proton from the reaction medium, resulting in aldol addition product. The presence of the aldol addition product in the reaction mixture was confirmed by FT-IR analysis (Figure S12). The CO stretching, O-H stretching, O-H bending, and C-O stretching peaks were observed in the reaction mixture. Finally, proton abstraction occurs again with an electron supply from Ag-Ca2N, generating hydrogen bubbles (Figure 4a-iii). OH − is also removed ,yielding aldol condensation product (i.e., α,β-unsaturated ketone). In order to experimentally realize the proposed aldol condensation reaction by Ag-Ca2N, DMSO was selected as a solvent. Both Ca2N:e− and Ag-Ca2N were relatively stable in pure DMSO (Ca2N:e−: up to 20 h,46 Ag-Ca2N: more than 1 day). However, Ca2N:e− immediately generated hydrogen bubbles in acetophenone/benzaldehyde added DMSO and completely decomposed within 15 min (see Figure S13a). Owing to the rapid consumption of electrons at the initial stage, Ca2N:e− could not complete the aldol condensation reaction as will be discussed shortly. Moreover, the experiment with Ca2N:e− had to be carried out in an Ar-filled glovebox. In contrast, hydrogen bubble generation was observed for a significantly longer period when Ag-Ca2N was employed instead of Ca2N:e−. The experiment with Ag-Ca2N was carried out outside the glovebox. The core Ca2N:e− was still intact while Ca(OH)2 formed on the outer side after a prolonged reaction time. The complete dissociation of Ag-Ca2N took more than 10 h (see Figure S13b). The stoichiometric molar concentration of Ag-Ca2N:acetophenone:benzaldehyde was 4:1:1. Note that half of the electrons of Ca2N:e− were consumed to reduce Ag+ ion (molar ratio of AgNO3/Ca2N:e− = 0.5) during the synthesis of Ag-Ca2N. One electron was required for the proton abstraction at the initial stage (Figure 4a-i), and another electron was required for the proton abstraction at the last synthesis step (Figure 4a-iii). The progress of the reaction was monitored by thin layer chromatography and product precipitation. The precipitated α,β-unsaturated ketone was retrieved by filtering as mentioned in the Experimental Section. Although the Ag-Ca2N enabled the aldol condensation reaction, it could not be regenerated after the reaction. The electrons were consumed during the reaction process, and the Ca2N:e− peak slowly decreased over 12 h as shown in the XRD analysis (Figure S13b). The Ca2N:e− peak became negligible when the reaction was completed after 12 h. The Ag peak did not change during the reaction, and Ag worked as a conductive passivation layer for the electron transfer from Ca2N:e− to reactant. The reaction product was analyzed by FT-IR spectroscopy (Figure 4b). The FT-IR spectrum of Ag-Ca2N promoted product was identical to that of α,β-unsaturated ketone.47 The CO shifted to 1654 cm−1 due to the extended conjugation.48 As a control, the aldol condensation reaction was carried out using Ca2N:e− (Figure 4c). The CO stretching was found at 1680 cm−1 due to the absence of extended conjugation.

First-principles total-energy calculation and geometry optimization were performed within density functional theory using the Perdew−Burke−Ernzerhof generalized gradient approximation and the projected augmented plane-wave method.40 A primitive rhombohedral unit cell containing one chemical formula was used, and the electron wave function was expanded in a plane-wave basis set with a cutoff energy of 520 eV. The Brillouin zone was sampled using a 108 × 108 × 96 Monkhorst−Pack k-point set for electron localization function (ELF), partial charge density (PCD), and magnetization density calculations. An empty sphere with a Wigner−Seitz radius of 1.7 Å was used to obtain the projected density of states on the interstitial position of anionic electrons (“X” site). The electronic band structure, projected density of states, ELF, and PCD are shown in Figure S11. The electron consumption was denoted by [Ca2N]+·(1 − x)e−·x, where x was the vacancy of electrons. The isolated two-dimensional anionic electron structure was well-maintained for [Ca2N]+·0.5e−·0.5 which was the optimized synthesis condition of Ag-Ca2N (1 mmol Ca2N:e− and 0.5 mmol AgNO3). This was consistent with the crystal structure (Figure 2c) and work function (Figure 2f) analysis. Interestingly, the localization degree of anionic electrons at the central interlayer region changed, and the two-dimensional characteristic was further enhanced for [Ca2N]+·0.5e−·0.5. This could be due to the slightly decreased interlayer spacing with the depletion of electrons (Table S2), which could induce a strong localization as demonstrated in our recent study.41 The electronic structure completely changed for [Ca2N]+·1. This was also consistent with the XRD analysis since the Ca2N:e− peak disappeared when the molar ratio of Ag/Ca2N:e− was 1 (Figure 2c). The precise electronic structure at the interface between the core Ca2N:e− and shell Ag layer needs to be further investigated in the future. An extensive theoretical analysis needs to be carried out with various possible interfacial structures. Aldol Condensation Reaction Promoted by Ag-Ca2N. The enhanced stability of Ag-Ca2N was utilized to promote organic reaction. Ca2N:e− has been employed for organic reactions involving single electron transfer such as pinacol coupling reaction,24 hydrotrifluoromethylation of alkenes and alkynes,25 and chemoselective hydrodehalogenation of organic halides.26 However, these reactions had to be carried out in an inert gas atmosphere inside the glovebox due to the reactivity of Ca2N:e− with atmospheric oxygen and moisture. Besides, all the reported reactions involved protic solvents.24−26 The vigorous decomposition of Ca2N:e− in protic solvents limited reaction efficiency since the fast electron release did not match the electron transfer rate to the reactants.25 Some of the electrons released from electrides were quenched by protons in solvents.24,25 The aldol condensation is one of the most important organic reactions, capable of constructing new carbon−carbon bonds, and has attracted considerable attention in the area of bioorganic and medicinal chemistry.42 The reaction occurs between two carbonyl compounds, one serving as an electrophile and the other as a nucleophile. The aldol condensation reaction typically involves the use of strong acid or base as a catalyst, badly affecting the green chemical aspect of organic synthesis.43 Therefore, researchers have attempted to make it more atom-economical and environmentally benign.44,45 As shown in Figure 4, the aldol condensation reaction was successfully carried out in an ambient air atmosphere using AgG


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The yield was 75.4% when the stoichiometric amount of AgCa2N was used. The enhanced stability together with the low work function makes Ag-Ca2N a practical economic candidate for future electride applications.

Furthermore, O-H stretching, O-H bending, and C-O stretching peaks were observed at 3638, 1395, and 1065 cm−1, respectively. This indicated formation of aldol addition product rather than α,β-unsaturated ketone.47 The proton abstraction in the last step (Figure 4a-iii) could not be realized since an electron could not be supplied due to the short lifetime of Ca2N:e−. The yield, defined as the ratio of experimental to theoretical mass of α,β-unsaturated ketone, was 75.4% at 25 °C when AgCa2N was used as a promoter (Figure 4d). The reaction completed in 12 h. The reaction time decreased to 8 h when the reaction temperature was increased to 50 °C. However, the reaction did not proceed when Ca2N:e− was employed at 25 °C as a control experiment. Figure S14 shows thin layer chromatography results carried out using Ag-Ca2N and Ca2N:e− after 9 h of reaction at 25 °C. Even at the elevated temperature of 50 °C, α,β-unsaturated ketone could not be obtained by Ca2N:e−. Only a trace amount of aldol addition product was obtained at 50 °C. Ag NPs were also tested as another control. There was no reaction at 25 and 50 °C up to 12 h (Figure 4d). This indicated that Ag itself did not work as a catalyst in the aldol condensation reaction. The aldol condensation reaction was also carried out at 25°C using Ag-Ca2N powders synthesized with different molar ratios of AgNO3/Ca2N:e− (0.4 and 0.6). The α,β-unsaturated ketone was successfully obtained in both cases (Figure S15). However, the reaction yield and sustained reaction time decreased compared with the optimized molar ratio of AgNO3/Ca2N:e− (0.5) as shown in Supporting Information Table S3. This could be due to the decreased stability of AgCa2N. As discussed in Figure 3c, the maximum lifetime was achieved when the molar ratio of AgNO3/Ca2N:e− was 0.5. The aldol condensation reaction performance of Ag-Ca2N was compared to the conventional strong basic or acid based catalysts in literature (Supporting Information Table S4). The reaction time, temperature, and yield typically varied between 4 and 96 h, room temperature (RT)−50 °C, and 60−90% in the literature. Ag-Ca2N provided a comparable reaction performance compared to the conventional catalysts in the literature. This work utilized the work function difference to passivate the electride surface with Ag. The same strategy can be applied to other types of electrides. This may enhance the stability and expand the application fields of various existing electrides.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b03202.

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Additional theoretical analysis, XRD, UPS, FT-IR, and thin layer chromatography data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.W.K.). *E-mail: [email protected] (S.B.). ORCID

Sung Wng Kim: 0000-0002-4802-5421 Seunghyun Baik: 0000-0001-8962-8045 Author Contributions

K.P.F, S.W.K, and S.B. conceived and designed the experiments, which were carried out by K.P.F and Y.J.K. S.-G.K. calculated the electronic structure. K.P.F, S.W.K, and S.B. wrote the paper. All authors contributed to data analysis and scientific discussion. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2017R1A2A1A17069289), IBSR011-D1, Fundamental Technology Research Program (2014M3A7B4052200) through the NRF grants funded by the Korean government (MSIP), and the NRF grant funded by the Korean government (Ministry of Science, ICT & Future Planning) (No. 2015M3D1A1070639).

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REFERENCES

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CONCLUSION In summary, we first employed Ca2N:e− as a reducing agent to achieve a Ag surface passivation layer (thickness: ∼55 nm) via a wet chemical approach. The low work function, high electron mobility, and high electron concentration of Ca 2 N:e − facilitated the electron transfer to Ag+ ion in CH3CN-THF (50:50 vol %) cosolvent. In contrast to the immediate decomposition of Ca2N:e− at room temperature in an ambient air environment, the noble metal surface passivation layer significantly enhanced air stability of Ag-Ca2N up to 17 min while maintaining the low work function of 2.78 eV. The optimized molar ratio of AgNO3/Ca2N:e− was 0.5 with 30 min reaction time. Ag-Ca2N also demonstrated enhanced stability in organic reaction medium. The prolonged electron supply from Ag-Ca2N enabled the successful proton abstraction and aldol condensation reaction. The α,β-unsaturated ketone was produced at an ambient air atmosphere outside the glovebox, without involving environmentally harmful strong acid or base. H


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