Atomic-Layer-Deposited MoNx Thin Films on Three-Dimensional Ni

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Energy, Environmental, and Catalysis Applications

Atomic-Layer-Deposited MoNx Thin Films on Three-Dimensional Ni Foam as Efficient Catalysts for the Electrochemical Hydrogen Evolution Reaction Rahul Ramesh, Dip K. Nandi, Tae Hyun Kim, Taehoon Cheon, Jihun Oh, and Soo-Hyun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20437 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Materials & Interfaces

Atomic-Layer-Deposited MoNx Thin Films on Three-Dimensional Ni Foam as Efficient Catalysts for the Electrochemical Hydrogen Evolution Reaction Rahul Ramesh1, Dip K. Nandi1, Tae Hyun Kim1, Taehoon Cheon1,2, Jihun Oh3, and Soo-Hyun Kim1,*

1

School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Gyeongbuk

38541, Republic of Korea 2

Center for Core Research Facilities, Daegu Gyeongbuk Institute of Science & Technology,

Sang-ri, Hyeonpung-myeon, Dalseong-gun, Daegu, 711-873, Republic Korea 3

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), and Department

of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yoseong-gu, Daejeon 34141, Republic of Korea Keywords: Molybdenum nitride; atomic-layer-deposition; Ni foam, conformality; optimum thickness; hydrogen evolution reaction 1 ACS Paragon Plus Environment

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Abstract

Future realization of a hydrogen-based economy requires a high surface area, low-cost, and robust electrocatalyst for the hydrogen evolution reaction (HER). In this study, the MoNx thin layer is synthesized on to a high surface area three-dimensional (3D) nickel foam (NF) substrate using atomic-layer-deposition (ALD) for HER catalysis. MoNx is grown on NF by the sequential exposure of Mo(CO)6 and NH3 at 225 °C. The thickness of the thin film is controlled by varying the number of ALD cycles to maximize the HER performance of the MoNx-NF composite catalyst. The scanning electron microscopy and transmission electron microscopy (TEM) images of MoNx/NF highlight that ALD facilitates uniform and conformal coating. TEM analysis highlights that the MoNx film is predominantly amorphous with the nanocrystalline MoN grains (4 nm) dispersed throughout it. Moreover, the high-resolution (HR)-TEM analysis shows a rough surface of the MoNx film with an overall composition of Mo0.59N0.41. X-ray photoelectron spectroscopy depth-profile analysis reveals that oxygen contamination is concentrated at the surface because of surface oxidation of the MoNx film under ambient conditions. The HER activity of MoNx is evaluated under acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) conditions. In an acidic electrolyte, the sample prepared with 700 ALD cycles exhibits significant HER activity and a low overpotential (η) of 148 mV at 10 mA cm-2. Under an alkaline condition, it achieves 10 mA cm-2 with η of 125 mV for MoNx/NF (700 cycles). In both electrolytes, the MoNx thin film exhibits enhanced activity and stability because of the uniform and conformal coating on NF. Thus, this study facilitates the development of a large-area 3D free-standing catalyst for efficient electrochemical water splitting, which may have commercial applicability.

1. Introduction 2 ACS Paragon Plus Environment

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Producing hydrogen through sustainable and renewable sources is critical for the shift toward zero-emission-based economy. Hydrogen can be generated by various processes, such as catalytic reforming of natural gas and electrocatalytic/photoelectrocatalytic decomposition of water.1 Among such processes, electrocatalytic production of hydrogen via. water splitting is the most efficient, cost-effective, and eco-friendly method.1,2 However, the choice of material is limited to precious metals during acid electrolysis, which influences the overall cost.2,3 Moreover, the lower hydrogen generation rate and liquid electrolyte requirement are the major disadvantages of alkaline water electrolysis.2,3 For both acid and alkaline water electrolyzers, the unavailability of an active, stable, and cost-effective catalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is a crucial issue, which must be addressed for their further development.3–12 Currently, Pt is the state-of-art catalyst material for the HER in both acidic and alkaline electrolytes. However, transition metal-based catalysts, other than precious metals, are also efficient in catalyzing the HER under alkaline conditions.4,5 Recently, molybdenum (Mo)-based catalysts was found to exhibit excellent activity and stability in both alkaline and acid electrolytes.13,14 Accordingly, molybdenum sulfide (MoS2) is extensively studied because of its unique layered structure, which is similar to that of graphite.15–19 It is well-understood that unfortunately this material provides active catalyst sites primarily at its edge, while the basal planes are essentially inactive.20–22 Furthermore, most chalcogenides are either insulating or semiconducting in nature, which restricts their use in electrochemical systems, in which a high electronic conducting material is desirable.23–27 Therefore, researchers came up with alternative Mo-based binary compounds to MoS2, such as Mo2C, MoN/Mo2N, MoB, and MoP. 23–36 Among these compounds, nitrides have the distinct advantage of possessing high electrical conductivity and better corrosion resistance under acid and alkaline conditions. Most of these studies adopted 3 ACS Paragon Plus Environment


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wet-chemical synthesis routes to prepare the bulk catalyst, typically in the form of a powder, which is then dispersed on a conducting substrate.23–34 However, the conventional methods for preparing electrocatalysts suffer from extra mass-loading and other associated ohmic issues, as well as poor adhesion of catalysts even though electrocatalysis being a surface-limited phenomenon requires only a thin layer of active compound for optimal performance. In this regard, atomic-layer-deposition (ALD) seems to be a promising alternative to grow an active electrocatalyst directly on a conducting substrate with a porous three-dimensional (3D) morphology.37–48 ALD is a gas-phase, two-stage, thin-film deposition technique, in which the precursor and reactant are input into the ALD reactor sequentially, with an intermediate purging by inert gas separated by a purging of an inert gas, such as Ar or N2. This sequential exposure of the precursors causes a self-limiting growth of the film, which makes ALD unique among the various deposition techniques. Under an ideal ALD condition, which guarantees self-limiting growth, the preparation of a uniform and conformal thin film of the electrocatalyst on any conducting 3D substrate is possible.44,45 In addition, there are other advantages of ALD in terms of preparing the electrocatalyst. First, it can significantly minimize the complications associated with the conventional electrode fabrication techniques, such as the use of polymer binders, which results in higher device resistance that degrades the overall performances.49 Second, ALD can coat various electrocatalyst materials on a 3D conducting substrate in the form of nitrides, carbides, oxides, sulfides as well as elements by rigorous selection of the precursors and reactants, while the direct electrodeposition of catalysts is mostly restricted to the coating of metals in elemental form or as metal alloys. In addition, the precise control on the thickness of the ALD-grown film facilitates depositing an optimal thin film of an active electrocatalyst, which is not possible by any other synthesis technique. Thus, it can help prevention of any extra mass loading of a respective 4 ACS Paragon Plus Environment

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catalyst. Furthermore, ALD exhibits high conformality; thus, it can be effectively used for the preparation of electrocatalysts that are coated on high surface area substrates, which are unstable in electrochemical conditions. The uniform coating of the substrate (core-shell type structure) prevents the direct contact of the substrate with the electrolyte, thereby enhancing its stability while retaining the high surface area. Despite the several advantages of ALD, very few successful attempts have been made so far in this regard.37–48 However, recently, few studies showed that the ALD-coated electrocatalysts on 3D structures could really boost the performance of the systems.38,40,44,45 In this study, ALD is used (for the first time in the literature) to coat a MoNx thin film onto a high surface-area 3D nickel foam (NF) substrate for electrochemical HER application. The thickness of the active catalyst layer is controlled by varying the ALD cycles. The HER performance of the MoNx/NF composite catalyst is evaluated in 0.5 M H2SO4 and 0.1 M KOH electrolytes. The highly controlled growth of MoNx can be achieved by ALD with precise thickness and excellent uniformity and conformity, which enhances the stability of the substrate electrode (NF) against dissolution in acid and alkaline electrolytes. The thickness of the MoNx thin film is optimized to be 25–30 nm (with 700 ALD cycles), which protects the substrate from leaching, while the activity can be tuned by Ni-MoNx interaction.

2. Experimental 2.1. Preparation of MoNx thin film on NF substrate The MoNx thin films were deposited on a 3D NF substrate in a showerhead-type ALD reactor (Lucida-M100, NCD Technology) at a deposition temperature of 225 °C. Before the deposition, 5 ACS Paragon Plus Environment


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the NF substrate was cleaned with de-ionized (DI) water and dried in air at room temperature. Molybdenum hexacarbonyl [Mo(CO)6, UP Chemical Inc., Republic of Korea] was used as the molybdenum source for the deposition, while ammonia gas (99.999 %) was the nitrogen source. The Mo precursor was kept at 40 °C, where its vapor pressure is ca. 0.8 Torr. The connecting pipelines and valves were maintained at 100 °C to prevent possible condensation of Mo(CO)6 before it is introduced into the chamber. High purity Ar (99.999%) was used as the carrier gas with a flow rate of 50 sccm (standard cubic centimeter per minute). The basic condition of this investigation was set as follows. A precursor pulsing of 10 s and a purge with 200 sccm of Ar for 10 s prior to a reactant pulsing were set. The NH3 gas at a flow rate of 50 sccm was used as the reactant. After reactant pulsing, another purge step was performed for 10 s prior to the follow-up precursor pulsing. This sequential process, i.e., the precursor pulsing, purging, reactant pulsing, and purging, formed up one ALD cycle. The thickness of the MoNx thin film on the NF substrate was controlled by varying the number of ALD cycles. In this study, samples were prepared with 300, 500, 700, and 1000 ALD cycles; and are abbreviated as MoNx/NF (300), MoNx/NF (500), MoNx/NF (700), and MoNx/NF (1000), respectively. For the purpose of characterization, the MoNx thin film is also deposited on a SiO2-Si substrate at 225 °C.

2.2. Physical characterizations X-ray diffractometry (XRD) analysis was performed to characterize the phase and crystallinity of the MoNx thin films on 3D NF [PANalytical XʹPert PRO MRD with Cu Kα radiation, wavelength (λ) 1.5406 Å]. Their surface morphology and uniformity were characterized by scanning electron microscopy (SEM, Hitachi, S–4800), equipped with energy-dispersive X-ray spectroscopy (EDS), and secondary ion mass spectroscopy (CAMECA nano-SIMS 50L with Cs+ 6 ACS Paragon Plus Environment

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Gun). Plan-view and cross-sectional view transmission electron microscopy (TEM, Hitachi, HF3300 equipped with a field emission gun of 300 kV accelerating voltage) were used to analyze the microstructures of ALD-MoNx thin films on 3D NF. EDS analysis was performed with the same in the scanning TEM mode to characterize the elemental distribution of MoNx in details. Further to confirm the Mo/N ratio Rutherford backscattering spectrometry (RBS, using He2+–ions (2MeV)) was performed on a MoNx thin film grown on SiO2-Si substrate at 225 °C, the atomic percent of Mo and N in the deposited film was determined using RUMP simulation. The X-ray photoelectron spectroscopy (XPS, VG Scientific ESCALAB 250 equipped with a monochromatic Al Kα source) analyses were carried out to investigate the elemental presence and their oxidation states in ALD-MoNx/NF. Survey scans were performed with a spot size of 400 µm, at 200 eV pass energy and with an energy step size of 1 eV. The HR individual spectra were obtained using a spot size of 400 µm, at 50 eV pass energy with an energy step size of 0.1 eV. The XPS spectra were charge-corrected to obtain the C 1s spectral component binding energy of 284.4 eV for all samples. Inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis was performed using ACTIVA, JY HORIBA spectrometer.

2.3. Electrochemical characterization The electrochemical characterization of bare NF and MoNx-coated NF were performed in a conventional three-electrode configuration using a potentiostat/galvanostat (Ivium-n-Stat, IVIUM Technologies, Netherlands). A platinum sheet with a surface area 1 cm2 and silver/silver chloride (Ag/AgCl, 3 M KCl) were used as the counter and reference electrodes, respectively. However, during the 6 h stability study a graphite rod counter electrode is used instead of platinum. Potentials are reported versus (vs.) reversible hydrogen electrode (RHE) in this study. The area of the 7 ACS Paragon Plus Environment


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working electrode (NF) in contact with the electrolyte was set to 2.48 cm2 (geometrical, area of both sides 1+1=2 cm2 and area with other three exposed surfaces 3 x 0.16 = 0.48 cm2, in total 2.48 cm2). The electrolyte used in this study was 0.5 M H2SO4 or 0.1 M KOH aqueous solution. Linear sweep voltammograms (LSVs) in the potential range of ˗0.4–0.1 V vs. RHE were obtained at a sweep rate of 5 mV s-1 for HER activity evaluation of the catalysts. The final measured current was normalized with the geometrical area of the electrode in contact with the electrolyte (2.48 cm2). The voltammograms were corrected for the iR drop using the typical feedback mode of correction while recording the LSVs by manually incorporating the resistance obtained from the impedance analysis. The electrochemical impedance spectroscopy (EIS) analysis was performed at a direct current (DC) bias of ˗0.3 V in a single sine mode. The spectra were also recorded with an alternating current (AC) amplitude of 5 mV, in the 10 KHz–0.05 Hz frequency range and with a data collection interval of 5 points per decade. The impedance spectra were fitted using the complex nonlinear least square (CNLS) method to a specific equivalent lumped circuit using the Ivium software. The stability of the catalysts during HER condition was studied by chronopotentiometry measurements under an applied current density of 10 mA cm-2(geometrical) in the same three-electrode electrochemical setup.

3. Results and discussion 3.1. Structural, morphological, and compositional analyses The MoNx thin film deposition is performed at a chamber temperature of 225 °C and with 10 s pulsing of both Mo(CO)6 and NH3 reactants. Molybdenum nitride thin film by ALD is wellestablished, and several reports on the phase, crystallinity, and composition of as-grown MoNx 8 ACS Paragon Plus Environment

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exists in the literature.50–57 However, most of the case the Mo-precursor used is either molybdenum pentachloride

or

molybdenum

alkylimido–alkylamido

complex

compound.

Recently,

molybdenum hexacarbonyl is also investigated as an ALD precursor for depositing variety of Mobased compounds.58–60 Generally, a narrow ALD temperature window of 170 to 210︒C is observed for Mo(CO)6 when compared to the other molybdenum-precursors. However, here a slightly higher deposition temperature of 225︒C is used, this is done to increase the growth rate to a respectful value of ~0.075 nm/cycle. By comparing the thickness of MoNx grown on Sisubstrate with that of Ni foam for a fixed number of ALD cycles (700 cycles), it is found that the thickness of the film grown in both case is similar (25–30 nm), even though the deposition temperature is 200 °C for SiO2-Si substrate and 225 °C for Ni foam. Hence, slightly higher temperature is required in case of Ni foam to maintain the same growth rate, and for both substrate an ALD-type growth is predominant. The MoNx thin film samples are prepared with 300, 500, 700, and 1000 ALD cycles to explore the thickness dependence of the MoNx/NF catalyst toward HER application. As discussed later in this article (Section 3.2), the sample with 700 ALD cycles exhibits the optimal HER performance among all the samples. Therefore, the structural, morphological, and compositional analyses of MoNx/NF (700) is conducted and discussed in details in this section. The crystallinity and phase of the MoNx thin film grown on NF by ALD technique are analyzed using XRD (Figure S1a). The XRD patterns of NF and MoNx/NF (700) are dominated by very sharp Ni peaks. Peaks corresponding to different phases of molybdenum nitride or of metallic molybdenum, or its oxides are not visible in the XRD patterns. This implies that the nature of the MoNx thin film grown on NF is likely to be amorphous. The deposition temperature of 225 °C might be inadequate for the formation of highly crystalline Mo2N or MoN films. To confirm the above-mentioned conjuncture, 9 ACS Paragon Plus Environment


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XRD (grazing incidence, GI) analysis of as-deposited and annealed (in H2 containing atmosphere) MoNx thin film on a SiO2-Si substrate is performed (Figure S1b). The as-deposited sample does not show any peak other than that from the silicon substrate. In contrast, after annealing in the hydrogen atmosphere, the peak from the cubic Mo2N phase is evident. The annealed sample also shows peaks because of the oxides of molybdenum. The formation of crystalline oxide phase might be from surface oxides of the as-deposited film because annealing is performed in a reducing atmosphere. The SEM (Figure 1) images confirm the uniform and conformal coating of MoNx on the 3D NF substrate. The as-deposited MoNx thin film inherited the morphology of the 3D cellular structure of NF, the film exhibits a rough surface with nanoporosity. In highly magnified SEM images, the bare NF shows a smooth surface, whereas the MoNx coating on NF appears distinct with rather rough appearance at the surface. The elemental composition of the samples is evaluated using EDS analysis and EDS elemental mapping (Figure S2). The presence of two constituent elements (Mo and N) is indicated by two different color representations, which show the uniform distribution of the elements throughout NF captured in the frame because of the uniform and conformal deposition by ALD. Noticeable amount of oxygen is detected in the EDS signals (not shown in the Figure S2) because of molybdenum nitride oxidation after sample preparation. Those transition metals in elemental state, nitrides, and carbides are prone to oxidation, and result in the formation of a thin layer of surface oxides if exposed to the atmosphere (refer to the XPS analysis results of MoNx/NF (700) and standard Mo samples shown in Figure S3).61 The EDS elemental mapping also confirms the uniform distribution of Mo and N throughout the sample. The EDS elemental mapping signals from Mo and Ni are very intense; whereas, for nitrogen, the signals are diffused because of its lower sensitivity. Hence, to confirm the formation of molybdenum nitride the RBS analysis of 10 ACS Paragon Plus Environment

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MoNx thin film grown on SiO2 covered silicon substrate is performed. From the analysis the formation of MoNx is confirmed (Figure S4). Furthermore, to confirm the composition, nanoSIMS surface mapping of MoNx/NF (700) was also performed (Figure 2). Nano-SIMS gives a direct two-dimensional representation of the surface elemental distribution using very small-sized primary ions (50 nm in beam size). Figure 2a shows the MoN signals from a portion of MoNx/NF (700); the image confirms a homogeneous distribution of Mo and N on the surface. Furthermore, there is no distribution of Ni at the topmost surface, which confirms the conformal coating by ALD (samples are checked at multiple locations and rasters; however, for brevity those images are not included in this study). The nano-SIMS shows a fairly uniform distribution of molybdenum (Figure 2b) and nitrogen (Figure 2c), while oxygen exhibits a non-uniform distribution at the surface, which is possibly because of the oxidation of molybdenum in the ambient after deposition by ALD.

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(b)

(a)

10 µm

50 µm

(c)

(d) 500 nm

500 nm

200 nm

Figure 1(a–d). SEM images of ALD-MoNx/NF composite at different magnifications (bare Ni foam in insert to figure 1c). The MoNx thin film is deposited at 225 ℃ with 700 ALD cycles.


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(a)

(c)

MoNx @NF

14 N

(b)

98 Mo

(d)

16O

Figure 2. Nano-SIMS elemental distribution mapping of the ALD-MoNx/NF (700) surface; (a) total ions, (b) 98Mo, (c) 14N, and (d) 16O of MoNx.



(a)

(f)

(b)

Ni(111)

(e)

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(c)

d200=0.25 nm

2 nm

(d) d202=0.18 nm

2 nm

h-MoN200

Figure 3. (a, b, and c) Cross-sectional view TEM bright-field (BF) images of the MoNx/NF (700) composite at different magnifications, (d and e) HR images at location d and e of the Figure 3c, and (f) its selected-area electron diffraction (SED) patterns. The insets in Figs. 3d and 3e are the magnified HR images to show the nanocrystalline grains, and the inset in Figure 3f is the SED of the NF substrate. Figure 3 shows the cross-sectional-view TEM analysis results on ALD-MoNx/NF (700). The BF TEM images (Figs. 3a, 3b, and 3c) clearly show the uniform and conformal coating of NF with 25–30 nm thick MoNx film. The HR-TEM images are taken at two locations on the thin films, one from the location near to the interface with NF substrate (Figure 3d) and the other near to the surface of the film (Figure 3e). The image taken at the location near to the interface with NF represents amorphous characteristics (Figure 3d). However, the lattice fringes because of 14 ACS Paragon Plus Environment

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nanocrystallinity are evident in the HR images (inset to Figure 3d). The HR image that is taken at the outer portion of the MoNx film represents mostly amorphous characteristic, though it exhibits some degree of nanocrystallinity as well (Figure 3e). Small crystalline grains with c.a. 4 nm size are distributed inside the amorphous matrix. Near the surface, the film is rough as well as amorphous. The nanocrystalline grains near to the interface with NF exhibit the lattice fringe with the spacing of 0.18 nm (inset in Figure 3d), corresponding to the (202) plane of MoN, while the nanocrystalline grains near to the surface has (200) plane of MoN (inset in Figure 3e). The amorphous nature of MoNx is further confirmed using SED. The diffuse diffraction patterns without any ring or spot demonstrate the amorphous structure of the thin film (Figure 3f). However, highly diffuse ring from the MoN phase can be visible by a close examination of the SED pattern. The SED pattern on the NF substrate exhibits a well-defined pattern corresponding to the face-centered cubic structure of metallic Ni (inset in Figure 3f). The atomic composition of the thin film estimated from the TEM-EDS analysis is Mo0.59N0.41 (not shown in this study). The EDS also shows significant contamination of oxygen because of the oxidation of the surface molybdenum atoms or from the adsorbed water on the surface of the MoNx thin film. XPS analysis of the MoNx/NF composite catalysts is performed to identify the oxidation state of the elements on the surface and to determine the surface composition of the catalysts. The XPS spectra are obtained from an as-deposited sample before etching, and after etching the surface with an Ar ion beam for 480 s. The complete XPS survey spectra are displayed in Figure S3 in the supporting information. For comparison, XPS spectra for bare NF and Mo foil are also obtained (refer Figure S3).



Before Etching

Mo3d@MoNx

(a)

Mo(+) N1s

Mo(+5) Mo(+6)

Mo(+6)

After Etching

Mo3d@MoNx

(b)

Mo3p & N1s@MoNx Before Etching

(c)

Mo(+)

Mo(+4)

Intensity (a. u.)

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

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Mo (+)

Mo3p & N 1s@MoNx After Etching

(d)

Mo (+5)

Mo (+4) Mo (+5)

238

Mo (+4)

Mo (+6)

236

Mo (+)

N 1s

Mo (+6)

240

Mo(+4) Mo(+)

234

232

230

228

Binding energy (eV)

226

405

400

395

390

385

Binding energy (eV)

Figure 4. Mo 3d XPS of (a) MoNx/NF (700) before etching and (b) MoNx/NF (700) after 480 s etching. Mo 3p-N1s XPS of (c) MoNx/NF (700) before etching and (d) MoNx/NF (700) after 480 s etching. Figure 4 shows the HR individual peaks of Mo 3d (Figs. 4a and 4b) and Mo 3p/N 1s (Figs. 4c and 4d) obtained before and after 480 s Ar ion etching for the MoNx/NF (700) sample. The Mo 3d spectrum is deconvoluted to estimate the distribution of various oxidation states of Mo in the MoNx sample. The deconvolution of the Mo 3d peak into 3d5/2–3d3/2 doublet is performed keeping the spin-orbital coupling constant as 3.13 eV, and maintaining a peak area ratio of ~3:2 for 3d5/2:3d3/2. The Mo 3d peak for the MoNx/NF (700) sample before etching can be deconvoluted to four 16 ACS Paragon Plus Environment

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doublets (Figure 4a).62,63 The Mo 3d5/2 peaks at 232.61 eV, 231.31 eV, and 230.018 eV match with the BE for +6, +5, and +4 oxidation state of molybdenum, respectively.62,63 The additional peak observed at 228.91 eV is assigned to the Mo+δ oxidation state from the MoNx phase.63 The slightly lower BE values observed for the MoNx (Mo+δ) peak when compared to those of the MoO2 peak (+4) might be because of the difference in the electronegativity of oxygen (3.44) and nitrogen (3.04). The binding energy values and the relative ratios of Mo+δ, Mo+4, Mo+5, and Mo+6 estimated from XPS analysis are tabulated in Tables S1 and S2. After 480 s of etching of the MoNx/NF (700) sample, a minor shift in the binding energy is observed for all peaks (Figure 4b). The surface composition estimated from XPS analysis is represented in Figure S5. For comparison, the composition of bare NF without any surface etching is also included in the figure. The MoNx/NF sample before Ar ion etching shows a noticeable amount of oxygen and carbon contamination. The carbon in the MoNx/NF sample before etching and after etching for 480 s indicates the presence of residual carbonyl (-CO) ligands during the deposition, and much higher O content could be explained by the external oxidation of the sample when exposed to the ambient. However, the surface of bare Ni foam is also contaminated with a significant amount of oxygen and carbon, and hence, it is difficult to precisely assign the carbon and oxygen content on MoN x thin film from XPS analysis. The percentage of the different oxidation state of Mo is tabulated in Table S2. The atomic percentage of Mo+δ in the sample is increased from 37.09 to 51.01, while that with the Mo+6 state is significantly reduced (Table S2). This indicates that only the surface of the MoNx thin film is contaminated with oxygen, while the interior portion is composed of amorphous and nanocrystalline MoNx phase. The Mo 3p peak is also deconvoluted to different components with Mo+δ, Mo+4, Mo+5, and Mo+6 oxidation states, and are tabulated in Table S3. The N 1s transition also appears at a similar binding energy and overlaps with the Mo 3d peaks. Thus, 17 ACS Paragon Plus Environment


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the presence of nitrogen is again confirmed and quantified from N KLL Auger transition (Figure S6). In summary, the XPS analysis confirms the formation of the MoNx thin film on the NF substrate. Further, the composition is estimated from the XPS match with that observed from EDS and nano-SIMS analyses. The surface of the MoNx thin film is rough and contaminated with oxygen; however, the inner portion of the MoNx thin film is composed of molybdenum and nitrogen, while minor oxygen presence is observed in the interface of the MoNx thin film and NF substrate. 3.2. Hydrogen evolution reaction The performance of the MoNx thin film deposited on the NF substrate as a catalyst for HER is evaluated. Before conducting the HER experiments on MoNx coated NF substrate, initially the HER activity (Figure S7) of MoNx coated (with a fixed ALD cycle number of 700 cycles) on TiNSi and carbon cloth are evaluated. From the Figure S7, it is clear that the highly conducting NF is a better substrate than other materials. The unique 3D structure of NF would provide efficient pores and channels that allow fast and easy access of the electrolyte and ions to the active sites of the electrode, while the high surface area brings more active sites for catalysis. Initially the HER activity of the MoNx/NF electrode prepared with different ALD cycles are evaluated in the 0.5 M H2SO4 electrolyte. The iR-corrected LSVs of MoNx/NF prepared with 300, 500, 700, and 1000 ALD cycles are shown in Figure S8a. The voltammograms without iR compensation are shown in Figure S8b. From the Figure S8a, it is clear that with the increase in MoNx film thickness (number of ALD cycles), a consistent decrease in the overpotential for HER is observed. This increase in activity with ALD cycle number continues till the MoNx/NF (700) sample (correspond to 20–30 nm of active catalyst layer). Any further increase in the number of ALD cycles causes reduction of HER activity (increase in η). This trend in the HER activity with an increase in the 18 ACS Paragon Plus Environment

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number of ALD cycles is further confirmed from Tafel analysis (Figure S9). Therefore, these preliminary set of experiments indicated a reasonably optimum thickness (20–30 nm) of this particular catalyst required for HER. Nevertheless, the optimum thickness may vary to some extent for other similar thin film catalysts for a specific application. Further, the LSVs for MoNx/NF (700) catalysts are conducted in acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) electrolytes (Figure 5), and the activities observed are compared with those of bare NF and standard Pt wire (area = 1 cm2). The onset potential for HER in aqueous 0.5 M H2SO4 solution (Figure 5a) on NF electrode is around c. a. −0.2 V. Further, a current density of 10 mA cm-2 (geometrical) is achieved at an overpotential (η) value of 0.24 V, thereby indicating its minor activity for HER. In case of MoNx-coated NF catalysts, the observed η to achieve 10 mA cm-2 is 148 mV. Moreover, the MoNx/NF (700) shows a very high hydrogen evolution capability (500 mA cm-2) at a relatively low η of 350 mV. These results indicate that the ALD technique can be possibly used for the fabrication of commercial HER catalysts. Figure 5b shows the HER voltammogram for MoNx/NF (700) recorded in 0.1 M KOH electrolyte, and the inset in this figure shows its enlarged view. The onset potential for HER on NF is −0.140 V, whereas η at a current density of 10 mA cm-2 (geometric) for NF electrode is 300 mV; and that for platinum wire is 78 mV. For comparison, the MoNx/NF (700) sample achieves a current density of 10 mA cm-2 at an operating overpotential of 124 mV. This indicates that the deposition of a small amount of MoNx (25–30 nm in thickness) on to the NF substrate alters the activity drastically. The η observed matches with the literature-reported values for molybdenum nitride catalysts. A comparison of η values from this study and those reported in the literature are shown in Table S4.25,27,64–71 It should be noted that in this study, the HER performance is evaluated on a free-standing metal foam electrode, while in most reports, a rotating disc electrode setup is used for electrochemical 19 ACS Paragon Plus Environment


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characterization and HER performance evaluaiton.25,64,65,67,69–71 The literature reported that the η values for MoN/Mo2N are in the range of 80–300 mV. Thus, the advantage of using highly conducting NF as support for MoNx is evident because high current densities are obtained with lower η (Figure 5b).


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0

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Figure 5. LSVs (iR corrected) of MoNx/NF (deposited with 700 ALD cycles at 225︒C) and bare NF substrate at a potential sweep rate of 5 mV s-1 in (a) 0.5 M H2SO4 and (b) 0.1 M KOH 21 ACS Paragon Plus Environment


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electrolyte, inset in a and b are the enlarge version of the figure. Tafel plots derived from the voltammograms in (c) acidic and (d) alkaline electrolytes. Nyquist plots at an overpotential of 300 mV in (e) 0.5 M H2SO4 and (f) 0.1 M KOH electrolyte. Insets in e and f are the equivalent circuits used to fit the experimental data. To further evaluate the activity of MoNx/NF (700) catalysts, Tafel plots are derived from the volumetric data and are shown in Figs. 5c and 5d. In the acid electrolyte, a Tafel slope of 27 mV per decade is observed with platinum, which is close to the well-established value of 30 mV dec-1 reported in the literature.72,73 A Tafel slope of 27 mV dec-1 indicates that the rate-determining step is the reaction between the surface adsorbed hydrogen (Tafel reaction).72 The Tafel slope obtained for bare NF is 139 mV dec-1, which is closer to the theoretical Tafel slope of hydrogen adsorption to the active site, Volmer reaction (120 mV dec-1).72,73 The slightly higher value observed in this case might be because of the parallel Ni oxide reduction occurring during HER considering that the equilibrium potential for Ni2+ to Ni0 transition is -0.246 V vs. RHE. The Tafel slope for MoNx/NF is 114 mV dec-1 and is attributed to the Volmer-Heyrovsky step.25,64,65,72,74 Under the alkaline conditions (Figure 5d) the lowest Tafel slope is observed with Pt (55 mV);25,27,66 whereas for bare NF a high slope of 155 mV dec-1 is observed. A lower slope value indicates a better reaction kinetics and a more active catalyst. The MoNx/NF shows a value near to that observed via the theoretical Volmer-Heyrovsky reaction (121 mV dec-1).25,27,66,75,76 EIS analysis is performed to investigate the electrode-electrolyte interface and to resolve contributions from charge-transfer, ohmic, and diffusion toward HER. The Nyquist plot obtained with 300 mV η for HER in the acid electrolyte for both NF and MoNx/NF catalysts is composed of one semicircle (Figure 5e). However, EIS analysis of NF at different potential display two relaxation phenomenon in Nyquist and Bode plots (Figure S10). The presence of two time22 ACS Paragon Plus Environment

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constant is not that apparent in the Nyquist plot but is clearly evident in the Bode representation (Figure S10). The equivalent circuit (EC) model proposed by Armstrong and Henderson (inset to Figure 5e, and 5f, abbreviated as Model A) is used to fit the experimental EIS data.77–79 This EC is generally termed as a one-CPE model.78 The model produces one or two semicircles on the complex plane depending on the overpotentials, and both the high-frequency and low-frequency relaxations are potential dependent.78 Another EC generally adopted to model HER on various surface is the simplified Randles circuit (abbreviated as Model B), it is a modified version of the Armstrong model, where the Rp-Cp factor becomes negligible and omitted.75,

56, 80–82

The

simplified Randles circuit produce a single relaxation time constant, which is potential dependent.78, 80 Pictorial representation of the two EC (Model A and Model B) are shown in Figure S11. In case of the NF, the one-CPE EC model has much better fit compared to the simplified Randles model; a comparison of Bode representation of EIS fitted with both models are presented in Figure S12 in the supporting information, and the fitted parameters are tabulated in Table S5 for both models. The Model A clearly shows a better fit when compared to Model B, especially at lower overpotentials, however, with an increase in overpotential both models show similar results. In the case of MoNx/NF (700), both models gives satisfactory fit at all overpotentials (Figure S13 and Table S6). As mentioned before, the results from Model A is better for NF while for other electrodes both models give a good fit. The EIS data for MoNx/NF (700) fitted with Model A and Model B are presented in Figure S14 and Figure S15, respectively. At higher overpotential or electrode with a large surface area, the Rp→0, and 1/Cp→0, which leads to the simplification of Model A to Model B.83 This happens either due to the masking of one time constant by the other one or both the processes happens at a similar time-constant values and hence difficult to resolve. Similar results and conclusions have been reported by others for hydrogen evolution reaction on 23 ACS Paragon Plus Environment


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Raney nickel and porous nickel electrode in the alkaline electrolyte. Lasia et al. reported two types of spectra for Ni and NiMo composite electrode, porous Ni and Ni with Mo up to a composition of 0.233 displays one semicircle while NiMo (with Mo>0.233) exhibits two semicircles on the complex plane.78 Similarly, Okido et al. used the one-CPE model proposed by Armstrong and Henderson to fit the EIS spectra, and found that the time constant τp (from the Cp-Rp circuit) which characterizes the time dependence of the relaxation phenomenon of the surface coverage (θ) cannot be observed at overpotentials higher than that of 56 mV, the separation of the two semicircles is difficult beyond the overpotential value of 56 mV.83 The starting point of the semicircle (observed at a high frequency) is mainly decided by the ohmic contribution arising primarily from the solution resistance and because of the internal resistance of the electrode.73,75,76 The ohmic contribution in the system is modeled with a resistance (Rs) connected in series to the EC. In addition, a constant-phase-element (CPEdl) is used for the simulation instead of pure capacitance to account for the heterogeneity of double-layer capacitance arising from the rough nature and non-uniform composition of the surface.84–87 The first time constant observed at high frequency is attributed to the relaxation of surface coverage (θ), whereas, the second one observed at lower frequency is from the HER.83 The charge transfer resistance (Rct) derived from the semicircle of the Nyquist plots (Figs. 5e and 5f) is the measurement of the apparent rate of the reaction, and a lower value for Rct implies a more active catalyst.73,65,76,87 The Rct observed for bare NF in 0.5 M H2SO4 (Figure 5e) at 300 mV η is 13.12 Ω; whereas for MoNx/NF, a slightly lower value of 1.515 Ω is obtained. The circuit parameters obtained after fitting the data are included in Table S4 and S5 in the supporting information. The double-layer capacitance (Cdl) calculated from the CPEdl values at 300 mV η is ~0.61 mF cm-2 (geometrical) for NF substrate, whereas the capacitance value shows 24 ACS Paragon Plus Environment

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a two-order increase in magnitude for MoNx coated NF (~12.4 mF cm-2).87 This increase in the capacitance is attributed to the increase in surface area because of the surface roughness of the MoNx film, which is evident from TEM analysis illustrated in Figure 3. For HER in an alkaline electrolyte (0.1 M KOH), the impedance response (taken with 300 mV η) is shown in Figure 5f. Similar to that observed in an acid electrolyte, a lower Rct is obtained for MoNx/NF (Table S7). The Cdl calculated from the CPEdl shows values comparable with those obtained in an acid electrolyte. For the NF substrate with 300 mV η, the Cdl estimated is ~0.18 mF cm-2 (geometrical), while for the MoNx deposited NF, the Cdl observed is ~11.01 mF cm-2 (geometrical), this is because the apparent differences in the surface roughness of NF and MoNx surface. Further examining the EIS results it can be clearly observed that the frequency at which the Z imaginary maximum occurs at a higher value for NF than that for MoNx/NF (700). This is more clearly recognized from the Bode plot comparison in Figure S16. The relaxation peak in the Bode plot for NF happens at a higher frequency when compared to MoNx/NF (700). Hence, it can be interpreted that the intrinsic area specific activity of Ni foam and MoNx must be similar or the polycrystalline Ni has a better activity for the unit real area. The high current density observed for MoNx/NF (700) in LSVs and in Tafel analysis is mainly due to the increase in surface area (rough nature of MoNx surface). This increase in surface area is clearly reflected in the double-layer capacitance of the MoNx sample when compared to that of bare Ni foam substrate. Please note that in LSVs and Tafel analysis the current is normalized with the geometrical area and not with the real surface area of the electrode. The impedance is also not normalized with the real surface area. Similar observation is also seen with EIS analysis of NF and MoNx/NF (700) in alkaline electrolyte. The activity results for Ni and NiMo reported by others in the past also have similar



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conclusions.80, 82 However, all of those studies are done in an alkaline electrolyte.80, 82 Here we observe those trends in both alkaline and acid electrolytes. Further, the change in surface characteristic with number of ALD cycles is estimated from the impedance date shown in Figure S17. The obtained double-layer capacitance increases with increase in number of ALD cycles till 700 cycles (12.4 mF cm-2), and for 1000 cycles sample the capacitance drops to 10.31 mF cm-2. Thus, the decrease in HER activity for MoNx/NF (1000) might be because of the loss of active surface area. This difference in the surface roughness with number of ALD cycles is further reflected in the HR-SEM images (Figure S18). In addition, any thin film beyond a certain thickness will add unnecessary electrical resistance without participating in the catalytic processes; thus, it will deteriorate the overall charge transfer/transport associated with HER. As a result, with optimal 700 ALD cycles, both the capacitance (consequently the surface area) and HER activity shows the highest values. Further, the mass activity of the electrocatalysts is calculated using the composition estimated from the ICP-OES analysis. The estimated composition and the corresponding mass activities are tabulated in Table S8, here also the MoNx (700) shows maximum activity among all the catalysts. The stability of NF and MoNx-(700 ALD cycles) coated NF in acidic and alkaline electrolytes are assessed for 6 h. Figure 6a shows the chronopotentiometric stability analysis for bare NF and MoNx/NF (700) catalysts obtained at a current density of 10 mA cm-2 in the 0.5 M H2SO4 electrolyte. The NF substrate shows a gradual increase in the overpotential, and the low stability of Ni under the acid condition limits its use as an electrode material in acid electrolytes. Nickel is prone to dissolution under acidic conditions. Generally, Ni spontaneously reacts with mineral acid to form the corresponding salt and simultaneously generates hydrogen (Equation 1).73 2Ni + H2 SO4 ↔ Ni2 SO4 + H2

(1) 26 ACS Paragon Plus Environment

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Ni2+ + 2e− ↔ Ni0 (𝐸 0 = −0.246 𝑉 𝑣𝑠. 𝑅𝐻𝐸)

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Figure 6. Chronopotentiometric curve of MoNx/NF (700) and bare NF catalysts recorded at a current density of 10 mA cm-2 in the (a) 0.5 M H2SO4 and (b) 0.1 M KOH electrolytes. The Ni2+ ion will get reduced back to Ni0 under-potential below −0.246 V vs. RHE (as represented in the Equation 2). Therefore, if HER is conducted with η >246 mV, the dissolution of Ni into the solution can be minimized. The Ni2+ ion formed by the chemical reaction of Ni with H2SO4 is reelectrodeposited to Ni0. However, the potential required to maintain a current density of 10 mA cm-2 during the chronopotentiometric experiment is −300 mV, which is very close to the reversible potential of −0.246 V for Ni electrodeposition; thus, its takes place at a lower rate. The fast 27 ACS Paragon Plus Environment


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chemical dissolution and slow re-electrodeposition causes the gradual leaching of Ni and consequent decrease in the activity of the NF substrate. The deposition of the MoNx thin layer on NF enhances the stability of the electrode to a great extent. The SEM image taken after the chronopotentiometric experiments and the corresponding EDS elemental mapping are shown in Figure S19. The stability of MoNx thin film is clearly confirmed in the SEM image and the corresponding EDS results. Furthermore, the XPS analysis of MoNx/NF after the chronopotentiometric experiments shows features similar to that of the as-prepared MoNx/NF film (Figure S20). The XPS also confirms the purity of the sample from a trace of platinum; no Pt 4f peaks are detected in the spectrum. The Mo 3d and Mo3p-N1s spectra are deconvoluted, and the final composition is shown in Figure S13b. A slight increase in the Mo+6 state is observed from XPS analysis. The chronopotentiometric stability analysis in the 0.1 M KOH electrolyte (Figure 6b) also shows features similar to those in 0.5 M H2SO4. The NF is more stable under the alkaline condition than in the acidic solution because the dissolution of Ni into the solution is thermodynamically unfavorable. Under the alkaline conditions, and above the theoretical reversible potential of Ni electrodeposition, the formation of Ni hydroxide/oxy-hydroxide occurs, which is sparingly soluble in the alkaline water. The retention of lower η for MoNx/NF after 6 h operation confirms the superior stability of molybdenum nitride against leaching during the HER conditions.

4. Conclusions In this study, the molybdenum nitride (MoNx) thin film is successfully grown on a NF substrate using ALD by the sequential exposure of Mo(CO)6 and NH3 at 225 °C. The as-grown film exhibits an amorphous characteristic in XRD analysis; however, it displays nanocrystalline nature in HRTEM images and SED patterns. The SEM and TEM analyses confirm the conformal coating of 28 ACS Paragon Plus Environment

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MoNx with the thickness of 25–30 nm on 3D NF, and the composition of film from TEM-EDS is estimated as Mo0.59N0.41. The high-resolution SEM images also reveal that the ALD MoNx film on NF exhibits a high degree of surface roughness. The elemental depth-profiling by XPS and nanoSIMS confirm oxygen contamination of the MoNx thin film at the surface; however, the inner portion of the MoNx thin film is mainly composed of molybdenum and nitrogen. The thicknessdependent HER studies of these MoNx/NF catalysts help us to optimize the number of ALD cycles (700) to maximize the performance of the electrode. The fabricated ALD-MoNx/3D NF (700) composite catalyst shows the improved HER current density. In the acid electrolyte (0.5 M H2SO4), the observed overpotential (η) is 148 mV at 10 mA cm-2 for MoNx-coated NF catalyst, and a very high hydrogen evolution capability (500 mA cm-2) at a relatively low η of 350 mV is achieved. In the alkaline electrolyte (0.1 M KOH), the MoNx/NF (700) sample achieves a current density of 10 mA cm-2 at an operating potential of –124 mV. The high current density observed for MoNx/NF (700) when compared to bare NF is mainly due to the increase in surface area (rough nature of MoNx surface). Moreover, the deposition of an ultrathin (25–30 nm) layer of MoNx on NF enhances the stability of the electrode in both acidic and alkaline electrolytes. These results indicate that the ALD technique can be used for the fabrication of free-standing metal foam electrodes as HER catalysts in commercial electrolyzers.

Supporting Information Available: Detailed physical characterizations (XRD, SEM images, and XPS analysis) and electrocatalytic performances (electrochemical impedance analysis). Corresponding Author: *E-mail: [email protected] 29 ACS Paragon Plus Environment


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The work is funded by the Ministry of Science; and Ministry of Trade, Industry & Energy; Government of Korea under the R & D program NRF-2017M3D1A1040692 and Korea Basic Science Institute Project No. D38700. Conflicts of interest The authors of this study declare no conflicts ACKNOWLEDGMENT This work was financially supported by the Ministry of Trade, Industry & Energy (MOTIE; #10080651) and Korea Semiconductor Research Consortium (KSRC) support program for the development of the future semiconductor device; the Korea Basic Science Institute under the R&D program (Project No. D38700) supervised by the Ministry of Science and ICT; and Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2017M3D1A1040692). The precursor used in this study was provided by UP Chemical Co. Ltd., Korea. ABBREVIATIONS ALD, atomic-layer-deposition; HER, hydrogen evolution reaction; NF, Ni foam.

References 30 ACS Paragon Plus Environment

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Determination of Effective Capacitance and Film Thickness from Constant-phase-element Parameters. Electrochim. Acta 2010, 55, 6218–6227. 86. Jorcin, J.-B.; Orazem, M. E.; Pébère, N.; Tribollet, B. CPE Analysis by Local Electrochemical Impedance Spectroscopy. Electrochim. Acta 2006, 51, 1473–1479. 87. Singh, R. K.; Ramesh, R.; Devivaraprasad, R.; Chakraborty, A.; Neergat, M. Hydrogen Interaction (Electrosorption and Evolution) Characteristics of Pd and Pd3Co Alloy Nanoparticles: An In-situ Investigation with Electrochemical Impedance Spectroscopy. Electrochim. Acta 2016, 94, 199–210.


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For Table of Contents Only

Uniform and conformal coating of molybdenum nitride (MoNx) on three-dimensional freestanding porous Ni-foam by atomic layer deposition (ALD), and its use as an active and stable electrode for the production of hydrogen by electrochemical water splitting reaction.



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Atomic-Layer-Deposited MoNx Thin Films on Three-Dimensional Ni

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