Electronic Band Structure and Electrocatalytic Performance of Cu3N

Jun 20, 2018 - This heterostructured noble-metal-free catalyst served as a ... For proton exchange membrane fuel cells (PEMFCs), the ORR involving a f...
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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Electronic Band Structure and Electrocatalytic Performance of Cu3N Nanocrystals Li-Chen Wang,† Bo-Heng Liu,‡ Chung-Yi Su,† Wei-Szu Liu,† Chi-Chung Kei,‡ Kuan-Wen Wang,§ and Tsong-Pyng Perng*,† †

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan § Institute of Materials Science and Engineering, National Central University, Taoyuan 320, Taiwan Downloaded via 95.85.71.241 on July 8, 2018 at 23:58:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: High-density discrete Cu3N nanocrystals were deposited on XC-72 carbon black by plasma-enhanced atomic layer deposition (PEALD). This heterostructured noble-metal-free catalyst served as a high-performance electrocatalyst for enhanced oxygen reduction reaction (ORR). The electronic band structure of Cu3N was determined by ultraviolet photoelectron spectroscopy (UPS) and UV−vis spectrophotometry. The work function (Φ) of the Cu3N nanocrystals was calculated to be 5.04 eV, which is lower than that of Pt (∼5.60 eV). With lower energy barrier, Cu3N would exhibit stronger electron transfer to cause ORR than typical Pt catalyst. The UPS analysis also confirmed the synergistic coupling effect between the Cu3N nanocrystals and the carbon support. Coupled with the XC-72, the Cu3N200/C showed even smaller Φ (=4.34 eV) than pure Cu3N nanocrystals. Thus, the Cu3N200/C electrocatalyst prepared with 200 ALD cycles exhibited similar ORR catalytic activity, significantly improved mass activity, and potentially greater durability than its Pt/C counterpart in alkaline solution. The fabrication of Cu3N by PEALD and its good performance in ORR suggest a promising alternative of non-noble-metal electrocatalyst for application in fuel cells. KEYWORDS: copper nitride, atomic layer deposition, oxygen reduction reaction, electrocatalyst, fuel cell, band structure

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synthesizing Cu3N at low temperatures.12−16 For instance, Wu and Chen documented the fabrication of Cu3N nanocubes from copper(II) nitrate decomposition in a mixed solvent comprising octadecylamine and octadecene at 250 °C.13 Similarly, Xi et al. reported a solvothermal approach using 1octadecylamine and oleylamine as solvents to synthesize Cu3N nanocubes with high electrocatalytic reduction properties.14 Without employing high temperature or high pressure, Nakamura et al. used copper(II) decanoate and copper(II) acetate as precursors to prepare plate-like Cu3N nanoparticles and investigated the effect of long-chain alcohol and reaction temperature on the distribution and morphology of the prepared particles.15,16 Reichert et al. fabricated Cu3N nanocrystals by nitridation of Cu2O nanocrystals with either ammonia or urea as the nitrogen source.17 Besides, Cu3N films have also been successfully deposited by magnetron sputtering, molecular beam epitaxy, and CVD methods.18−20 However, few of these above-mentioned fabrication techniques are able to directly grow high-quality Cu3N nanocatalysts on support materials with fine control over the specific morphology, dimension, and dispersion. Thus, considerable attention has

INTRODUCTION Recently, transition metal nitrides have attracted immense attention for versatile applications, including utilization as photoactive layers for solar cells,1 components for nanoelectronic devices,2 functional hard coatings for cutting tools,3 and catalysts for energy conversion and storage.4−9 Among the wide variety of metal nitrides, copper nitride (Cu3N), which entirely comprises inexpensive and earth-abundant elements, is a promising material for widespread utilization because of its tunable optical qualities and unique electrical properties.1,10 Because of the relatively open structure of Cu3N, the cubic anti-ReO3 type with a large vacant site at the center of unit cell, (1/2, 1/2, 1/2), its properties could be altered by the chemical stoichiometry of Cu3N, and therefore Cu3N can be a p-type or n-type semiconductor depending on the fabrication condition.1 According to both theoretical and experimental studies, Cu3N has a narrow bandgap in the range 0.2−1.9 eV, exhibiting either metallic or semiconducting behavior.10 Although it has been documented that Cu3N is a new candidate for highly active electrocatalyst or efficient visible light driven photocatalyst,11 the low-temperature synthesis of Cu3N still remains challenging due to the low activity of nitrogen-containing precursors, i.e., N2 or NH3. Accordingly, since the first study of polycrystalline Cu3N by Juza and Hahn in 1939, much work has been focused on © XXXX American Chemical Society

Received: May 11, 2018 Accepted: June 20, 2018 Published: June 20, 2018 A

DOI: 10.1021/acsanm.8b00787 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Nano Materials recently been paid to the deposition of Cu3N thin film by ALD dry process at relatively low operating temperatures.21 The sequential and self-limiting gas−solid surface reactions of ALD process can not only precisely control the dimension and stoichiometry of coatings at atomic scale but also improve the dispersion of nanoparticles on catalyst supports.22−24 Hence, as one of the state-of-the-art deposition techniques, ALD process offers innovative design to fabricate Cu3N nanocatalyst. For proton exchange membrane fuel cells (PEMFCs), the ORR involving a four-electron transfer process is one of the most crucial factors to determine the performance. Over the past decades, extensive attempts have been made to develop ORR electrocatalysts with high activity, low cost, and environmental benignity to substitute for the expensive Pt catalyst.25−27 Lin et al. reported metal-free boron-doped onionlike carbons (B-OLC) as a highly active electrocatalyst with similar catalytic performance to that of commercial Pt/C.28 By using the UPS analysis, they summarized that the Φ and valence band edge of the B-OLC samples could be correlated to the ORR electrocatalytic activity. Besides, Yang et al. demonstrated that N-doped graphene nanoribbon networks with smaller Φ could have a lower energy barrier to donate electrons from the surface of the electrocatalyst, thus improving the catalytic performance.29 Nonetheless, for metal nitride electrocatalysts, the correlation of the ORR process with the electronic band structure is still limited to theoretical calculations. Lacking experimental investigation of the electronic properties may impede the elucidation of the ORR process and electrocatalytic performance. Furthermore, even though we have previously demonstrated that Cu3N coupled with carbon nanotubes (CNTs) exhibited a high ORR activity,30 the electronic properties and durability of Cu3N prepared by PEALD are still not clear. Accordingly, here we document how this advanced dry process can be employed to synthesize Cu3N nanocrystals directly on a widely used Vulcan XC-72 catalyst support at a relatively low temperature. Instead of theoretical calculations, the electronic band structure of the ALD-prepared Cu3N was experimentally determined by means of UPS and UV−vis absorbance spectroscopy. Furthermore, it was found that the hybrid Cu3N/C electrocatalyst exhibited fairly high performance in ORR under alkaline conditions. Compared to the commercial Pt/C catalyst, this hybrid catalyst showed similar ORR catalytic activity but significantly improved mass activity and potentially greater stability.

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Figure 1. Schematic representation of (a) preparation of Cu3N/C nanocomposite, (b) PEALD process, and (c) island growth mode. to the Cu3N deposition, the Cu(hfac)2-containing vertical bubbler was heated at a sublimation temperature of 85 °C for 1 h to ensure the equilibrium vapor pressure. Plasma-activated NH3 (99.999%) mixed with Ar (99.999%) was controlled at a pressure ratio of 1:1 to ensure ignition of plasma. The typical pulse durations in one ALD cycle were 2 s for Cu(hfac)2 under a 0.5 Torr carrier gas flow, 5 s for NH3 plasma, and 5 s of nitrogen purge at 1 Torr after each precursor pulse, as illustrated in Figure 1b. To name the samples, for instance, pure Cu3N nanoparticles prepared by 200 and 800 ALD cycles on Si substrate are named as Cu3N200 and Cu3N800, respectively. Similarly, the samples prepared with Cu3N nanoparticles deposited on XC-72 by 200 and 800 ALD cycles are named as Cu3N200/C and Cu3N800/C, respectively. Characterization of Cu3N/C Electrocatalysts. The surface morphology and crystal lattice of Cu3N were examined by scanning electron microscopy (SEM, Hitachi SU-8010) and transmission electron microscopy (TEM, JEOL JEM-ARM200FTH). The inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500ce) was employed to measure the amount of Cu3N loading. The crystal structure was investigated by glancing incidence X-ray diffraction (GIXRD, PANalytical X’Pert Pro MPD) with the angle of incidence kept at 0.3° for maximum intensity. The composition of Cu3N was analyzed by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI PHI5000 Versaprobe II). The Φ and band edges of Cu3N were determined by UPS which was performed on the same instrument as XPS using He I (21.22 eV) excitation source at an applied bias voltage of 5 V. The optical properties of Cu3N were measured by UV−vis spectrometry (Hitachi U3900). Electrochemical Measurement. Electrochemical measurement was performed in a conventional three-electrode electrochemical cell, including a reference electrode (Hg/HgO), a counter electrode (Pt wire), a glassy carbon rotating disk electrode (RDE, AFE3T050GC) which was controlled by an MSR rotator (Pine Instrument), and an electrochemical analyzer (CHI700a, CH Instrument). Although using Pt wire as a counter electrode has been proven to influence the result of electrochemical experiment in acidic solution, a recent study

EXPERIMENTAL SECTION

Preparation of Cu3N Nanocrystals by PEALD. Figure 1a illustrates the synthesis procedure for depositing Cu3N nanocrystals on the XC-72 carbon black. First, 2.5 mg cm−2 of XC-72 powder was uniformly spread on the silicon substrate by drop coating, and the deposition of Cu3N by PEALD on the powder was conducted. For comparison, Cu3N nanocrystals were also directly grown on the silicon substrate by PEALD. The ALD of Cu3N was performed in a homemade vertical-flow type 8 in. ALD reactor with an inductively coupled plasma apparatus which was driven by a 13.56 MHz radio frequency power supply. The distance between the substrate of ALD reactor and the grid, which was the bottom electrode of the plasma apparatus, was maintained at 7 cm. The plasma power was controlled at 400 W, and the substrate temperature was kept at 150 °C. Copper(II) hexafluoroacetylacetonate (Cu(hfac)2) and ammonia plasma were utilized as the precursors of Cu and N, respectively. Ultrahigh purity nitrogen (99.999%) was used as a carrier gas for transporting Cu(hfac)2 and as a purge gas for the ALD reaction. Prior B

DOI: 10.1021/acsanm.8b00787 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials indicates that the reduction potential of oxidized Pt is below 0 V (vs Ag/AgCl) in 1 M KOH, which reveals that the electrochemical dissolution of Pt might not be that severe in alkaline media.31 Thus, in this work the 0.1 M KOH solution was used as the electrolyte. For preparation of the working electrode, an amount of 5 mg of catalyst was ultrasonically dispersed for 30 min in 1 mL of 2-propanol and 50 μL of 5 wt % Nafion to form a well-dispersed ink solution. Nafion perfluorinated resin solution was used to create a three-phase boundary among Cu3N, XC-72, and Nafion in KOH. The Nafion serves as a separator to transfer K+ from anolyte to catholyte.32 The catalyst ink, 20 μL, was transferred to a glassy carbon disk to form a working electrode (active area = 0.196 cm2) by a microliter pipet. For comparison, the commercially available Pt/C catalyst (JM, 20 wt % Pt) was also evaluated. Cyclic voltammograms (CV) and linear sweep voltammetry (LSV) at the scan rate of 100 and 5 mV s−1, respectively, were conducted in an N2- or O2-saturated solution of 0.1 M KOH to investigate the ORR performance of the catalysts. The rotation speed of LSV was set from 400 to 3600 rpm. Prior to each measurement, the electrolyte solution was saturated with high-purity oxygen (99.9%) for 20 min and then aerated with a moderate O2 gas flow during the entire electrochemical experiment. Besides, the electrode surface was electrochemically cleaned by cycling the potential between −0.22 and 1.38 V (vs RHE) for more than 20 cycles until a reproducible voltammogram was obtained. Furthermore, the accelerated degradation test (ADT) was operated by 1000-cycle potential cycling from 0.4 to 1.0 V (vs RHE) at a scan rate of 50 mV s−1 in O2-saturated 0.1 M KOH. The corresponding ORR polarization curves before and after the ADT were recorded at a rotation speed of 1600 rpm.

growth behavior during the PEALD process, which involves ALD growth mode, growth rate, preferred crystal orientation, etc.34 Moreover, recently it has been reported that the structural and electronic properties of Cu3N thin films prepared by dc reactive magnetron sputtering could be affected by the substrate.18 Thus, the study of substrate effect on crystal growth behavior for Cu3N by PEALD will be systematically studied in the future. Previously, we observed that there is an incubation period of forming Cu3N on CNTs in the first 50 cycles of ALD process.30 However, the TEM (Figure 2c,d) and SEM (Figure S2) observations in this work indicate that Cu3N nanocrystals prepared with 50 ALD cycles are uniformly distributed on the surface of XC-72, suggesting that incubation period is negligible at the initial growth stage of PEALD process.35 Besides, the PEALD is capable of precisely controlling the dimension of high-density unaggregated Cu3N nanoparticles on XC-72, as displayed in Figure S3. The particle size of Cu3N appears to increase linearly with the ALD cycle, as shown in Figure S4a. The average growth rate of Cu3N on XC-72 is close to the values reported in the literature and our previous work using the same copper precursor by ALD process.21,30 In addition, it is worth mentioning that the Cu3N loadings (Figure S4b) in the present study are all below 3.5 wt %, far lower than the Pt loading in commercial Pt/C electrocatalysts (20−50 wt %) for application in PEMFCs.36 The XRD pattern in Figure 3a evidences that all diffraction peaks of the as-deposited material on the silicon substrate can

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RESULTS AND DISCUSSION Characterization of the Cu3N Deposit. The SEM micrograph and the elemental mapping of the hybrid Cu3N200/C sample prepared with 200 ALD cycles are demonstrated in Figures 2a and 2b, respectively. The

Figure 2. (a) SEM image of Cu3N200/C. (b) Elemental mappings of Cu3N200/C. (c) and (d) are dark field TEM images in two magnifications of Cu3N50/C. Figure 3. (a) GIXRD pattern of Cu3N800 deposited on silicon substrate. (b) HRTEM micrograph of Cu3N200/C. (c) Magnified image from the square area in (b), and the inset is its diffraction pattern obtained by fast Fourier transform analysis. (d) Atomic arrangement of Cu3N lattice with viewing direction of [11̅0].

morphology of the high-density discrete Cu3N nanoparticles prepared by PEALD is different from that of a typical continuous film. It is also different from that of aggregated Cu3N nanocrystals on CNTs observed in our previous investigation.30 The island growth mode, or the Volmer− Weber growth as illustrated in Figure 1c, corresponds to the situation where the reactant molecules have a stronger tendency to couple with the molecules on the film than with the substrate during the ALD process.33 An island-like thin film of Cu3N was also observed on the silicon substrate, as presented in Figure S1. It has been documented that the substrate material could significantly influence the crystal

be well indexed to the Cu3N crystalline phase (JCPDS no. 04001-2547). The Cu3N nanocrystals grown by PEALD have high crystallinity and exhibit preferred growth direction along the [100] instead of the typical [111] orientation.37 The HRTEM image of a discrete Cu3N nanoparticle on XC-72 prepared with 200 ALD cycles is presented in Figure 3b. The C

DOI: 10.1021/acsanm.8b00787 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials lattice fringe combined with the corresponding fast Fourier transform (FFT) analysis in Figure 3c also evidence the high crystallinity of Cu3N. The interplanar spacings of the nanoparticle, 0.22 and 0.19 nm, can be indexed to the (111) and (002) planes, respectively, of the cubic anti-ReO3 structure of Cu3N (JCPDS no. 04-001-2547). The atomic arrangement of Cu3N along the [11̅0] zone axis was modeled by using CrystalMaker crystallography software, as shown in Figure 3d. The red circles which represent the Cu atoms fit well to the Cu atoms in the lattice fringe of Cu3N (Figure 3c). This result also indicates that the PEALD process could deposit highly pure Cu3N nanocrystals on the surface of XC-72. The XPS spectra of pure Cu3N200 nanocrystals and Cu3N200/C are demonstrated in Figures 4a and 4b for the

Figure 5. (a) UPS and (b) the corresponding UPS-VB spectra of Cu3N200 and Cu3N200/C. The inset of (a) is the enlarged region of the valence band. (c) UV−vis absorption spectrum of Cu3N200. (d) Electronic band structure of Cu3N200 derived from (a) and (c).

energy position of the valence band (VB) emissions in the UPS spectrum with the corresponding UPS-VB spectrum in Figure 5b, rendering the Fermi level located at 0.00 eV binding energy. The values of Φ were calculated using the equation Φ = hν − Ecut, where hν (21.22 eV) represents the incident photoenergy from He I excitation source and Ecut is the secondary electron cutoff level. Note that the peak near the cutoff level in the UPS spectrum is caused by the secondary electrons instead of the electrons emitted from the valence band.43,44 The Φ of Cu3N was estimated to be 5.04 eV, which is lower than the generally reported value of Pt (typical Φ = ∼5.60 eV).45 The Cu3N with lower energy barrier would exhibit stronger electron transfer to cause ORR than its Pt counterpart, resulting in better mass activity if on the basis of same amount of loading. Furthermore, the UPS analysis also confirms the synergistic coupling effect between the Cu3N electrocatalyst and the carbon support. Coupled with the XC72, the Cu3N/C shows even smaller Φ (=4.34 eV) than pure Cu3N nanocrystals, which reveals that the deposition of Cu3N on XC-72 has altered the electronic band structure of Cu3N, and Cu3N200/C could provide more electrons for ORR, leading to enhancement in electrochemical activity. Additionally, the distance between the valence band edge level (EV) and the Fermi energy level (EF) was determined by the linear intersection in the inset of Figure 5a as 0.42 eV for Cu3N200. The EV was then calculated to be −5.46 eV, and the conduction band level (EC) was estimated to be −3.54 eV by the equation EC = EV + Eg, where the Eg (1.92 eV) of the Cu3N was obtained from the UV−vis absorption spectrum (Figure 5c). Accordingly, the energy-band diagram of Cu3N prepared by PEALD could be proposed, as depicted in Figure 5d. The UPS and UV−vis absorbance analyses evidence that the Cu3N nanocrystals deposited by PEALD are a p-type semiconductor. Electrochemical Performance of Cu 3N/C Hybrid Catalyst. The electrochemical performance of Cu3N/C samples prepared with various PEALD cycles of Cu3N were

Figure 4. XPS spectra of (a) Cu 2p and (b) N 1s for Cu3N200 and Cu3N200/C.

chemical binding states of Cu and N, respectively. As presented in Figure 4a, the doublet peaks with the symmetric binding energies at 932.4 eV (Cu 2p3/2) and 952.3 eV (Cu 2p1/2) manifest that only one chemical species is present. The positions are consistent with those of Cu(I), and the presence of Cu(II) can be excluded from the absence of satellite peaks at 933.6 eV (Cu 2p3/2) and 953.5 eV (Cu 2p1/2). The broad N 1s peak of Cu3N can be assigned to two peaks. The main peak centered at 397.5 eV is attributed to the Cu−N bond, and the other one with a lower intensity located at 398.7 eV originates from the free nitrogen atoms adhered to the deposit from NH3 plasma.38 On the other hand, the N 1s peak of Cu3N/C indicates the presence of pyridinic-N (398.4 eV), which is bonded to two carbon atoms and generally considered as the active sites during the ORR process.39 The XPS analysis confirms that the PEALD process could grow highly pure nanocrystals of Cu(I) nitride that is bonded to the surface of XC-72.40 Electronic Band Structure of Cu3N. The Φ could be one of the critical factors determining the electron transfer process since it represents the minimum energy for inner electrons to escape from the highest filled level in the Fermi distribution of a solid.28,41 A small value of Φ suggests that the electrons are easier to activate for reaction. Accordingly, the UPS and UV− vis absorbance spectra were employed to investigate the Φ, band edges, and electronic band structure of Cu3N200 and Cu3N200/C.42 This was an attempt on the basis of experimental studies rather than theoretical calculations. The UPS spectrum in Figure 5a was calibrated to match the binding D

DOI: 10.1021/acsanm.8b00787 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials measured by three-electrode LSV and CV experiments to estimate the ORR catalytic performance in alkaline media. For comparison, the commercially available Pt/C electrocatalyst (JM, 20 wt %), pure Vulcan XC-72 carbon black powder, and free-standing Cu3N nanoparticles were also evaluated to investigate the synergistic effect of XC-72 and Cu3N nanoparticles. The ORR polarization curves in 0.1 M KOH solution saturated with either N2 or O2 were recorded at a rotation speed of 1600 rpm, as displayed in Figure 6a and

Table 1. ORR Activities and Mass Activities at 0.7 V (vs RHE) of Various Electrocatalysts sample Cu3N800 XC-72 N-doped XC72 Cu3N50/C Cu3N200/C Pt/C

E1/2 (V)

Eonset (V)

j0.7a (mA cm−2)

0.383 0.603 0.607

0.648 0.720 0.735

0 0.13 0.24

0.609 0.684 0.814

0.756 0.817 0.993

0.54 2.24 5.03

MA (mA mg−1Cu3N or Pt) 1.3b 2.5b 900.1 1707.0 264.7

a

j0.7 is determined at a rotation speed of 1600 rpm. bThe MA of XC72 is normalized by the carbon mass.

lower electrochemical surface area, the ORR activities of Cu3N800/C and Cu3N400/C are poorer even though their Cu3N loadings are higher. In addition, Figure 6b compares characteristic reduction peak intensities of the CV tests in O2-saturated 0.1 M KOH solution for XC-72, Cu3N50/C, and Cu3N200/C. All of these samples exhibited a well-defined cathodic ORR current peak at 0.55−0.65 V. A remarkable positive shift is observed in the CV curve of Cu3N200/C. The more positive peak potential at 0.64 V (vs RHE) also confirms its higher performance. Therefore, both results of ORR polarization and CV curves manifest that the Cu3N200/C catalyst exhibits a higher electrocatalytic activity. The Cu3N200/C also reveals more favorable ORR catalytic activity than all of the Cu3N@CNTs samples in our previous work.30 There are several reasons for the better ORR activity of Cu3N200/C than that of [email protected] First, discrete Cu3N nanoparticles can be uniformly deposited on XC-72 by the PEALD process, but on CNTs Cu3N forms aggregates, which results in lower electrochemical surface area and consequently the ORR activity of Cu3N@CNTs is degraded.48 Second, the surface area of commercial XC-72 (230−250 m2 g−1)49 is approximately 2 times higher than that of commercial multiwalled CNTs (111−118 m2 g−1)50 we used previously. The Cu3N loading of Cu3N200/C (1.48 wt %) is accordingly approximately 2 times higher than that of Cu3N@CNTs (0.75 wt %). Generally, larger surface area and higher loading of electrocatalyst would enhance the ORR performance. Thus, the Cu3N200/C demonstrated higher ORR activity than Cu3N@CNTs. It once again confirms that the present low-cost and time-saving PEALD process provides high-efficiency Cu3N electrocatalysts. To gain a further insight into the electron transfer process of the Cu3N catalysts, a series of ORR polarization curves measured by rotating-disk voltammograms at various rotation speeds from 400 to 3600 rpm are displayed in Figure S6. The Koutecky−Levich equation shown in eq 1 was utilized to calculate the electron transfer numbers for various electrocatalysts: 1 1 1 1 1 = + = + j jk jd jk βω1/2 (1)

Figure 6. (a) ORR polarization curves of various electrocatalysts in 0.1 M KOH solution. (b) CV curves of XC-72, Cu3N50/C, and Cu3N200/C in N2 (dashed lines) and O2 (solid lines) saturated 0.1 M KOH solution. (c) Electron transfer numbers obtained from Koutecky−Levich equation. (d) Koutecky−Levich plots at 0.2 V for various electrocatalysts.

Figure S5. All plots were obtained with the same mass loading of electrocatalyst on the glassy carbon electrode and then normalized by the active area of the electrode (0.196 cm2). No noticeable feature of reduction reaction is observed in the N2saturated solution due to the absence of oxygen, while the Cu3N/C samples in O2-saturated solution possess similar ORR current curves, suggesting that these electrocatalysts are active for oxygen reduction. Commonly, the half-wave potential (E1/2) is defined as the potential where the wave current equals to half of the limiting current, and the onset potential (Eonset) is defined as the potential at which the catalyzed ORR polarization current starts to appear.46 As summarized in Table 1, the Cu3N/C catalysts exhibit higher ORR activities than free-standing Cu3N800 nanoparticles and pure XC-72 powder, both of which have poorer values of E1/2 and Eonset. The positively shifted ORR current curves of Cu3N/C hybrid electrocatalysts indicate that O2 molecules are efficiently adsorbed on and activated by the catalytic sites of Cu3N, promoting the subsequent oxygen reduction process.47 The enhanced ORR activity of Cu3N50/C evidence that Cu3N nanoparticles prepared with 50 ALD cycles have been successfully deposited on XC-72 without any incubation period at the initial stage of PEALD process. Figure S5 indicates that Cu3N200/C has a higher ORR performance than the other Cu3N/C samples, presumably because it possesses sufficient loading and optimal size of Cu3N. As the larger particle size of Cu3N results in

where j is the measured current density, jd and jk are the massdiffusion limiting and kinetic current densities, respectively, ω is the rotational speed (rpm), and β is the Levich slope given by β = 0.201nFν−1/6CO2DO2 2/3 E

(2) DOI: 10.1021/acsanm.8b00787 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials where n represents the number of transferred electrons per oxygen molecule, F is the Faraday constant (F = 96 485 C mol−1), ν is the kinetic viscosity of the electrolyte (0.01 cm2 s−1), and CO2 and DO2 are the dissolved concentration and diffusion coefficient of O2 in the electrolyte (1.2 × 106 mol cm−3 and 1.9 × 10−5 cm2 s−1 in 0.1 M KOH aqueous solution at 25 °C, respectively). Note that if angular velocity (rad/s) is used, the value of 0.201 should be replaced by 0.620 in eq 2.51,52 The electron transfer numbers at potentials from 0.1 to 0.4 V for the electrocatalysts are calculated by the Koutecky− Levich equation and summarized in Figure 6c. The Koutecky− Levich plots at 0.2 V (Figure 6d), which fit very well linearly to the data extracted from the ORR polarization curves, suggest a first-order reaction kinetics. The n values at 0.2 V for XC-72, Cu3N800, Cu3N50/C, and Cu3N200/C samples are approximately 2.0, 2.4, 3.1, and 3.6, respectively. Moreover, our previous study also shows that the highest n value of Cu3N@ CNTs is close to 3.5.30 These values indicate the synergistic coupling effect between Cu3N and carbon supports, and this study further elucidates that Cu3N indeed facilitates the electron transfer even when combined with commercial XC-72 (n = 2.0). Basically, the oxygen reduction may proceed by two possible routes in alkaline solutions: one is direct four-electron (4e−) pathway to produce OH− and the other one follows twoelectron (2e−) pathway generating hydroperoxyl radical (HO2−).53 Herein, it reveals that Cu3N200/C favors the high-efficiency 4e− reaction pathway rather than the less desirable 2e− reduction process, which could form more HO2− intermediate and then result in more malign degradation of the catalyst. Therefore, it can be inferred that Cu3N200/C exhibits almost completed oxygen reduction and produces less intermediate, which is very similar to ORR catalyzed by the high-efficiency commercial Pt/C catalyst. Previously, it was documented that nitrogen doping in carbon support may largely enhance the electrocatalytic activity of ORR.54 To check this possibility, the pristine XC72 was treated by various cycles of NH3 activated plasma in the PEALD process. These samples are named as N-doped XC-72. The XPS analysis was conducted, as illustrated in Figures 7a and 7b. The position of the C 1s peak at 284.3 eV for XC-72 without NH3 treatment is consistent with the value of pure sp2 C−C bonding in pristine pyrolytic graphite, confirming that C atoms are exclusive of sp2 hybridization. However, the peak positions of C 1s for all N-doped XC-72 samples are slightly shifted to 284.4 eV, which could be attributed to negligible structural disruption in the sp2 carbon framework after the N incorporation.55 The N 1s XPS spectra in Figure 7b were therefore obtained to further investigate the nitrogen coordination in XC-72. The N 1s peak can be deconvoluted into three peaks corresponding to pyridinic N (398.6 eV), pyrrolic N (400.5 eV), and quaternary-like N (401.2 eV).55 These nitrogen atoms within the graphitic structure, especially the pyridinic N next to C atoms, may create active sites for ORR.54 Although it can be seen that the intensity of N 1s signal increases with increasing the NH3-plasma cycle, the nitrogen concentration evaluated by the XPS analysis revealed that only approximately 2 at. % of N was doped into XC-72 even treated with 800 NH3−plasma cycles. It is also worth mentioning that there should be much less N doped into XC72 during the normal PEALD process due to the formation of Cu3N.

Figure 7. (a) C 1s and (b) N 1s XPS spectra of N-doped XC-72 treated with various NH3−plasma cycles. (c) ORR polarization curves for N-doped XC-72 treated by 800 cycles of NH3 plasma in 0.1 M KOH. The inset is the corresponding Koutecky−Levich plot at 0.2 V (vs RHE).

In order to investigate whether the catalytic activity was affected by N doping, the ORR polarization curves of N-doped XC-72 treated by 800 cycles of NH3 plasma were recorded as a reference, as shown in Figure 7c. The E1/2 and Eonset (at 1600 rpm) are 0.607 and 0.735 V, respectively. These values are very similar to those of the pristine XC-72, confirming that the N doping did not significantly affect the ORR activities of Cu3N/ C. In addition, based on the corresponding Koutecky−Levich plot shown in the inset of Figure 7c, the n value is calculated to be 2.7. It has been reported that N-doped carbon materials (graphene and CNT) could promote a favorable 4e− ORR pathway.56,57 The low value of n for N-doped XC-72 implies that it follows mixed ORR pathway, and H2O2 is still generated during the ORR reaction. Hence, unlike our previous study on Cu3N@CNTs in which N-doped CNTs performed a favorable 4e− ORR pathway and largely enhanced the ORR performance,30 the N-doped XC-72 did not have much influence on the high catalytic activity of Cu3N/C. This once again confirms that the high catalytic performance is indeed contributed by Cu3N and the synergistic coupling effect between Cu3N and XC-72 rather than by the N doping. When evaluating the catalytic activities of various samples from the ORR polarization curves, it is needed to take the loading amount of catalyst into account. Accordingly, the activities were compared by using “mass activity (MA)” which is based on the mass of catalyst, rather than using “ORR activity” without considering the mass effect. The MAs for all samples are calculated by normalizing the current density (mA cm−2) at 0.7 V (vs RHE) to the actual weight of Pt or Cu3N, as summarized in Table 1. Although both Cu3N50/C and Cu3N200/C samples have more negative values of E1/2 and Eonset than Pt/C in the typical ORR polarization curves, their mass activities are actually much higher than that of commercial Pt/C. The Cu3N200/C electrocatalyst contains only 1.4 wt % Cu3N but has a mass activity of 1707.0 mA mg−1 F

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ACS Applied Nano Materials that is 544% higher than that of commercial Pt/C electrocatalyst, 264.7 mA mg−1. Note that the MA of our commercial Pt/C catalyst (20 wt %, Johnson Matthey) is a little higher than that of another commercial Pt/C catalyst (20 wt %, Sigma-Aldrich Co.) (∼229.7 mA mg−1) reported in the literature.58 Since the Cu3N800 nanoparticles show no current density at 0.7 V (Figure 6a), and the MAs (after normalization with the weight of carbon black) of pristine XC-72 and Ndoped XC-72 are only 1.3 and 2.5 mA mg−1, respectively, the excellent electrochemical performance of Cu3N200/C could be ascribed to the synergistic effect of coupling Cu3N with XC-72. It is believed that if the loading amount of Cu3N on XC-72 could be increased to 20 wt % while maintaining an optimal particle size, the ORR activity of Cu3N/C would be higher than that of Pt/C because the lower work function of Cu3N would reduce the activation energy and favor the ORR reaction. There are several possible reasons why Cu3N/C shows higher catalytic performance than commercial Pt/C. The potential charge transfer in transition metal nitrides is strongly related to the electronegativity difference between the transition metal and nitrogen. The acidic or basic sites created by the electronegativity difference may further activate the surface of Cu3N for O2 adsorption. Cu3N with a smaller Φ exhibits a lower energy barrier to donate electrons from its surface to the adsorbed O2, thus facilitating the formation of the OOH species, which in the ORR process is the ratedetermining step.28,29 In addition, the surface-exposed Cu+ cations could also serve as donor−acceptor reduction sites, aiding O2 chemisorption by their cationic d-orbitals.47 It is reminded that the exposed Cu3N facets could affect catalytic activity like the most active facets of (111) in zincblend CoN and (100) in rocksalt CoN.59 In the present study, by using PEALD only the Cu3N nanocrystals along the [100] growth direction could be formed on XC-72 rather than along the typical [111] orientation. Although it is difficult to establish the dependence of the ORR activity on the facet of Cu3N nanoparticles, our results still provide some insights for future study. An additional reason for the higher activity of Cu3N/C could be due to the intrinsic property of the nitrides since previous studies have shown that transition-metal nitrides such as VN and ZrN could provide higher electron density to enhance ORR catalytic activity. The N anions of Cu3N could be effective sites to supply more electrons and then to weaken the O−O bonding, facilitating the ORR.60,61 Not only the Cu3N nanocrystal itself but also the pyridinic-N in Cu3N/C, evidenced in the XPS analysis, may provide active sites for ORR; hence the catalytic efficiency is enhanced. Meanwhile, in this work the intrinsically high electrical conductivity, high surface area (230−250 m2 g−1),49 and porous structure of XC72 benefit itself to be a superior catalyst support for charge transfer, further enhancing the ORR performance when coupled with the Cu3N nanocrystals. All of the above explanations elucidate the reason why the Cu3N/C hybrid catalysts exhibit similar ORR catalytic activities and significantly improved mass activities with respect to the Pt/C catalyst. To evaluate the durability of Cu3N200/C, 1000 cycles of ADT was performed in 0.1 M O2-saturated KOH solution with the commercial Pt/C catalyst as a reference. The polarization curves before and after the ADT of Cu3N200/C and Pt/C are presented in Figure 8a for comparison. Some curves during the ADT test are shown in Figure S7 for reference. In the

Figure 8. (a) ORR polarization curves of Cu3N200/C and Pt/C catalysts before and after ADT in 0.1 M O2-saturated KOH. (b) Corresponding decay of MA at 0.7 V (vs RHE).

diffusion-limiting region, the decrease in the limiting current density at 0.2 V of Cu3N200/C is 5.5% (from −5.538 to −5.234 mA cm−2), while that of Pt/C is 24.3% (from −5.806 to −4.392 mA cm−2). In the kinetic region, the values of E1/2 of Cu3N200/C and Pt/C exhibit a negative shift of 15 and 24 mV, respectively. As depicted in Figure 8b, the MA at 0.7 V of Cu3N200/C is progressively degraded by 16.1% (from 1707 to 1433 mA mg−1) during the ADT, whereas the MA degradation of Pt/C is 31.7% (from 265 to 181 mA mg−1). Hence, the potentially greater stability of the Cu3N200/C catalyst can be assumed in both diffusion-limiting and kinetic regions when comparing its performance with commercial Pt/C under alkaline conditions.

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CONCLUSION In summary, we have demonstrated the fabrication of highdensity size-controllable Cu3N nanocrystals by PEALD as a high-performance catalyst for enhanced ORR. The uniform deposition of discrete Cu3N nanoparticles well-dispersed on XC-72 could be accredited to the island growth mechanism of PEALD. The Φ of Cu3N was evaluated to be 5.04 eV, which is lower than the generally reported value of Pt (∼5.60 eV). This implies that Cu3N would display stronger electron transfer to cause ORR than its Pt counterpart. The UPS analysis also confirmed the synergistic coupling effect between the Cu3N nanocrystals and the carbon support. Although Cu3N and XC72 individually exhibited poor ORR activity, their hybrid displayed similar ORR catalytic activity, significantly improved mass activity, and greater durability when compared to commercial Pt/C catalyst under alkaline conditions. Therefore, this non-noble-metal Cu3N catalyst fabricated by PEALD may minimize the cost and could be an alternative electrocatalyst for fuel cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00787. Figures S1−S7 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel (886)-3-5742634 (T.P.P.). ORCID

Li-Chen Wang: 0000-0003-0703-5030 Notes

The authors declare no competing financial interest. G

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ACKNOWLEDGMENTS The financial support of this work by the Ministry of Science and Technology of Taiwan under Contracts MOST 104-2119M-007-010 and 104-2221-E-007-110-MY3 is greatly appreciated. We also thank Wei-Lun Weng for helping the TEM characterization.

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