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Vapor-Phase Atomic Layer Deposition of Nickel Sulfide and Its Application for Efficient Oxygen-Evolution Electrocatalysis Hao Li, Youdong Shao, Yantao Su, Yuanhong Gao, and Xinwei Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04645 • Publication Date (Web): 27 Jan 2016 Downloaded from http://pubs.acs.org on January 29, 2016

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Chemistry of Materials

Vapor-Phase Atomic Layer Deposition of Nickel Sulfide and Its Application for Efficient Oxygen-Evolution Electrocatalysis Hao Li, † Youdong Shao, † Yantao Su, Yuanhong Gao and Xinwei Wang* School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, China †

These authors contributed equally.

*Corresponding email: [email protected]

Abstract Vapor-phase

atomic

layer

deposition

(ALD)

of

nickel

sulfide

(NiSx)

is

comprehensively reported for the first time. The deposition process employs bis(N,N’-di-tert-butylacetamidinato)nickel(II) and H2S as the reactants, and is able to produce fairly smooth, pure, godlevskite-structured NiSx thin films following an ideal layer-by-layer ALD growth fashion for a relatively wide process temperature range from 90 to 200 °C. Excellent conformal coating is demonstrated for this ALD process, as the deposited NiSx films are able to uniformly and conformally cover deep narrow trenches with aspect ratio as high as 10:1, which highlights the general and broad applicability of this ALD process for fabricating complex 3D-structured nanodevices. Further, we demonstrate the applications of this ALD NiSx for oxygen-evolution reaction (OER) electrocatalysis. The ALD NiSx is found to convert to nickel (oxy)hydrate after 1

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electrochemical aging, and the aged product shows a remarkable electrocatalytic activity and long-term stability, which is among the best electrocatalytic performance reported for nonprecious OER catalysts. Considering that ALD can be easily scaled up and integrated with 3D nanostructures, we believe that this ALD NiSx process will be highly promising for a variety of applications in future energy devices.

TOC Figure

2


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Introduction Nickel sulfide (NiSx) has recently aroused great attention for its superb electrochemical/electrocatalytic activity,1,2 and, therefore, it has been widely applied in energy conversion and storage devices, such as supercapacitors,3,4 batteries,5,6 hydrogen evolution reaction,7,8 oxygen evolution reaction (OER),9 oxygen reduction reaction,10 and solar cells.11-13 Among these applications, much effort has been devoted to the miniaturization of the active NiSx materials to form various functional nanostructures (such as nanoparticles,14 nanosheets,8,15 nanotubes,16 nanoframes,3 and nano-hollow structures17). The nanostructuring has shown particular importance to greatly boost the electrochemical activity of NiSx,2 since the nanostructuring can, in general, minimize charge transport limitation, shorten the ion diffusion length through active material layer, and, more importantly, expose a considerably great amount of active surface sites to electrolyte.2 On the other hand, these merits can be also achieved by coating a uniform nanoscale NiSx thin film on a high-surface-area scaffold structure. A straight and effective approach to achieve this is to utilize vapor-phase atomic layer deposition (ALD), which is a well-known technique for nanofabrication.18,19 Vapor-phase ALD employs saturated, self-limiting surface chemistry reactions, and allows the film growth to proceed in a layer-by-layer fashion, so that, in theory, any complex 3D structures can be conformally and uniformly coated by ALD. Compared to alternative solution-phase electrochemical ALD,10,20 the vapor-phase ALD does not necessarily require the substrate to be 3



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conductive, and also the mass transfer is much faster in the vapor phase. The layer-by-layer growth fashion also allows for a precise control of film thickness at an atomic level, which is generally essential for many ALD applications in energy devices (e.g.

solar

cells,21-23

supercapacitors,24,25

batteries,26

electrocatalysis,27,28

and

photoelectrochemical catalysis29,30). Moreover, thin-film ALD can also enable various fundamental mechanism studies which require a uniform dispersion of active materials in layered format with a precise control of the layer thickness.31 Examples include the investigations of nanoscale charge transport limitations for poorly-conductive metal-oxide OER electrocatalysts.27,31 However, given many of the unique merits for ALD, as reviewed recently,32 there is, unfortunately, no currently existing ALD process for NiSx yet. Provided the important and broad applications of nickel sulfide, the development of new ALD process for nickel sulfide is, therefore, urgently in need.

In this work, we report, for the first time, an ALD process for nickel sulfide (NiSx). The process followed an ideal layer-by-layer ALD growth fashion, and was able to deposit high-quality NiSx films in a relatively wide temperature range. Excellent conformal coating was demonstrated by depositing NiSx into deep narrow trenches with a high aspect ratio of 10:1, which suggested a highly broad applicability of this ALD process for fabricating 3D-structured nanodevices in general. Further, as a demonstration for the energy device applications, we then investigated the use of ALD NiSx as an electrocatalyst for OER. Efficiently catalyzing the kinetically sluggish OER is 4


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considered33 as the pivot for a variety of energy devices (such as water electrolyzers, solar water-splitting devices, and rechargeable metal-air batteries34-38), and also, in order for large-scale applications, nonprecious catalysts are particularly needed. We demonstrated that the nonprecious NiSx prepared by our ALD process could be in-situ converted to nickel (oxy)hydrate, which was able to achieve excellent electrocatalytic activity with long-term stability. Considering that ALD can be easily scaled up and integrated with 3D nanostructures, we believe that this ALD NiSx process will be highly promising for applications in future energy devices.

Experimental Section Atomic Layer Deposition of nickel sulfide films.

NiSx films were deposited in a

home-built tubular ALD reactor, using bis(N,N’-di-tert-butylacetamidinato)nickel(II) (Ni(amd)2) as the nickel precursor and H2S as the sulfur source. The nickel precursor was kept in a glass bubbler in an oven, and was heated to 70 °C to afford saturated vapor pressure of ~140 mTorr. The nickel precursor was delivered into the deposition chamber with the assist of purified N2 carrier gas (through a Gatekeeper inert gas purifier). As for the H2S gas (3% diluted in N2), it was first delivered into a ~5 mL gas trap and then delivered into the deposition chamber for depositing films. The chamber base pressure during purging was ~0.3 Torr. The deposition temperature was varied from 90 to 300 °C. During the study of the saturation growth behavior, the exposures of Ni(amd)2 and H2S in each ALD cycle were varied by either changing the dose number in flow-through mode 5



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or changing the exposure time in closed-valve mode. Si wafer substrates were used for studying the deposition behavior. Prior to the deposition, the Si substrates were all treated with UV/ozone for 5 min, which formed a thin SiOx layer on Si surface.

Film characterizations. The film thickness was measured by X-ray reflectometry (Bruker, D8 Advance), and the film microstructure was examined by transmission electron microscopy (TEM) (Jeol, JEM-2100). Rutherford backscattering spectrometry (RBS) was employed to determine the chemical composition of the films deposited at various temperatures. The RBS measurements were performed in the Heavy Ion Institute at Peking University, using 2.022 MeV helium ions as the incident ions and collecting the backscattered signals at the scattering angle of 165°. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific, Escalab 250Xi) was also employed to analyze the film composition. Monochromated Al Kα radiation was normally used for XPS, but since the Ni Auger peaks overlap with the Fe 2p peaks, nonmonochromated Mg Kα radiation from a twin Mg/Al anode was used instead whenever necessary. atomic force microscopy (AFM) (Bruker, MultiMode 8) and scanning electron microscopy (SEM) (Zeiss, SUPRA55) were used to examine the film surface morphology, and the SEM was also used to evaluate the film conformality.

Electrochemical characterizations. The above ALD process was used to deposit NiSx films on 10×10×1 mm glassy carbon electrodes as the working electrode for characterizing the OER electrocatalytic performance of ALD NiSx. Before ALD, the 6


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glassy carbon electrodes were carefully polished and washed to ensure that the surface was flat and clean, and then they were treated with UV/ozone for 5 min to remove any organic residual and form surface oxide for ALD nucleation. The ALD was performed at 200 °C for 500 cycles, which produced a 7.5 nm crystalline NiSx film uniformly covering the glassy carbon electrode. All the electrochemical measurements were carried out on a CHI 660E electrochemical workstation, using a standard three-electrode system with Pt wire and Hg/HgO electrode as the counter electrode and reference electrode, respectively. 1 M KOH (Fe impurity < 1 ppm) was used as the electrolyte, and the electrolyte was bubbled by pure N2 for 15 minutes prior to the measurements. 95% iR compensation was employed in all CV measurements, where the series resistance R was determined by current-interrupt method. All the potentials were iR drop corrected. The oxygen evolution overpotentials (η) were referenced to the reversible OER potential by η = EHg/HgO + 0.098 + 0.0591 × pH – 1.229 (V). Tafel plots were obtained by chronoamperometry measurements in steps of 10 mV, where a relaxation time of 60 s was used for each step to allow the current to achieve steady state. We also used the state-of-art catalyst of RuO2 for comparison. 12 mg RuO2 (99.9 wt %, Aladdin) was ultrasonically dispersed in 2 mL solution mixed from 0.12 mL Nafion solution (5 wt. %, DuPont), 0.80 mL water, and 1.08 mL ethanol. Then, 16 µL of this suspension was loaded on a glassy carbon disk electrode (0.196 cm2), which afforded a loading amount of 0.5 mg/cm2 for RuO2.39

Results and Discussion 7



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ALD was performed with the deposition temperature varying from 90 to 300 °C to investigate the film growth behavior. As will be shown later, we found that the film growth followed an ideal saturated, self-limiting ALD growth behavior at the deposition temperature ranging from 90 to 200 °C, however, partial decomposition of the nickel precursor was suggested when the deposition temperature was elevated above 250 °C. Figure 1a-b demonstrates the self-limiting growth behavior for two typical deposition temperatures of 120 °C and 200 °C, which were both within the above ALD temperature window. As shown in Figure 1a, by increasing the exposure for Ni(amd)2 while keeping fixed the exposure for H2S at ~0.40 Torr s, the film growth rate first increased, and then reached saturation as the Ni(amd)2 exposure exceeded ~0.12 Torr s. Similar trend was also shown in Figure 1b, where the film growth rate reached saturation when the exposure for H2S exceeded ~0.40 Torr s (in this set of experiments, the exposure for Ni(amd)2 was fixed at ~0.12 Torr s). These results clearly demonstrated that saturated film growth could be achieved with the minimum exposures of ~0.12 and ~0.40 Torr s for Ni(amd)2 and H2S, respectively, and therefore, without otherwise specified, we used this set of the minimum saturated exposures in the following experiments.

8


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Figure 1. Panels a and b show how the ALD growth rate of NiSx approaches saturated values (due to self-limiting surface reaction) as the precursor dosages increase. (a) Growth rate with respect to the exposure of Ni(amd)2 at fixed H2S exposure of ~0.40 Torr s. (b) Growth rate with respect to the exposure of H2S at fixed Ni(amd)2 exposure of ~0.12 Torr s. (c) Film thickness as a function of total ALD cycles. Data for two deposition temperatures of 120 and 200 °C are shown in a, b, and c. (d) Growth rate as a function of deposition temperature. Saturated exposures of ~0.12 and ~0.40 Torr s for Ni(amd)2 and H2S, respectively, were used for c and d.

Linear growth is an important feature for an ALD process, as good growth linearity could enable one to precisely control the thickness of the deposited films by digitally varying the total ALD cycles. As shown in Figure 1c, the thickness of the deposited films increased linearly with the total cycle number, which suggested a good linear growth behavior of this ALD process. Also, the linear fits gave out zero intercepts, which indicated that no appreciable nucleation delay occurred during the initial growth on SiOx. The film growth rate, which was extracted from the slope of each linear fit, was plotted in 9



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Figure 1d with respect to the deposition temperature ranging from 90 to 300 °C. The growth rate was fairly constant at around 0.15 Å/cycle for the deposition temperature ranging from 120 to 200 °C, which demonstrated a fairly wide temperature window for this ALD process. Carefully examining the data further revealed that there was a very gentle deceasing trend of the growth rate with temperature (about 2% decrease from 120 to 200 °C), and the trend was more pronounced if the growth rate at an even lower temperature of 90 °C (i.e. 0.173 Å/cycle) was taken into account. Generally, the decreasing trend of the growth rate with temperature is often seen in ALD,40 because higher temperature usually produces denser films and therefore the film thickness is usually thinner. Similar explanation could also be applied to our case, since the measured density of the deposited films indeed followed an increasing trend with the deposition temperature (vide infra). On the other hand, the growth rate started to increase at 250 °C (i.e. 0.16 Å/cycle) and then increased sharply to 0.25 Å/cycle at 300 °C. As will be shown later, this increase was due to the partial decomposition of the nickel precursor at high temperature. Therefore, we focused the following material characterizations mainly for the films deposited at temperatures below or equal to 200 °C.

10


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Figure 2. TEM (a, c) images and (b, d) electron diffraction patterns of ~10 nm NiSx films deposited at (a, b) 120 and (c, d) 200 °C, respectively. The NiSx films were deposited on UV/ozone-treated SiNx TEM grids for imaging. (e) schematically illustrates the unit cell of Ni9S8 godlevskite structure. (f) shows the FFT image of the dash-boxed area in (c), and the Miller indices for three representative spots are indexed.

The microstructure of the deposited NiSx films was carefully examined by TEM. Figure 2a-d shows the TEM images and electron diffraction patterns for ~10 nm NiSx films deposited at 120 °C (panels a-b) and 200 °C (panels c-d), respectively. The films were both crystallized as shown in the images of Figure 2(a,c), and the film grain size was larger at the higher deposition temperature. The electron diffraction patterns (Figure 2b,d) also suggested the same trend for the grain size, as the diffraction rings were considerably sharper and showed more pronounced discrete dot-like features for the film deposited at the higher temperature. Careful analysis of the ring radii (see Figure S1 and 11



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Table S1 for details) revealed that both of the diffraction patterns corresponded to the orthorhombic godlevskite structure of Ni9S8 (a = 9.18, b = 11.263, c = 9.457 Å, PDF#22-1193). The Ni9S8 godlevskite structure, as schematically illustrated in Figure 2e, is based on a distorted cubic close-packed array of 32 S atoms per unit cell, with 20 Ni atoms in tetrahedral coordination and 16 Ni atoms in square-pyramidal coordination.41 To double-check the phase identification, we further analyzed the fast Fourier transform (FFT) of some representative TEM images for individual grains. As an example, the FFT pattern corresponding to the dash-boxed area in Figure 2c is shown in Figure 2f, and all the spots in the FFT pattern could be successfully indexed with the Ni9S8 godlevskite structure, which, therefore, confirmed our previous phase identification.

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Figure 3. (a) RBS spectrum (black) of an 8.5 nm NiSx film deposited on Si substrate at 200 °C, with simulated spectrum (red) plotted for comparison. (b) Ni/S atomic ratio and film density as functions of deposition temperature. (c) XPS survey spectrum for a ~10 nm NiSx film deposited at 200 °C, with the associated high-resolution scans for (d) Ni 2p, (e) S 2p, (f) C 1s, and (g) N 1s.

The composition of the deposited NiSx films was determined by RBS. Figure 3a shows a typical RBS experimental spectrum with a simulated pattern for an 8.5 nm NiSx film deposited on Si substrate at 200 °C. The RBS spectra for other deposition temperatures are provided in Figure S2. The atomic ratios of Ni/S for the films deposited at various temperatures were extracted from the corresponding RBS spectra, and were plotted in Figure 3b for comparison. The Ni/S ratio exhibited an increasing trend with the deposition temperature, from the ratio of 0.97 ± 0.03 for 90 °C to 1.13 ± 0.04 for 200 °C. Notice that the latter was in great agreement with the stoichiometric ratio of 1.125 for 13



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Ni9S8. Along with the previous TEM results, we, therefore, concluded that the nickel sulfide films deposited at 200 °C were stoichiometric godlevskite Ni9S8. However, the films deposited at lower temperatures contained appreciably more sulfur, although their crystal structures were the same, as identified by TEM. In addition, we could also calculate the film density from the atomic areal density from RBS and the film thickness from XRR. As also plotted in Figure 3b, the density followed a similar increasing trend with the deposition temperature, and reached, for the film deposited at 200 °C, the highest value of 5.25 ± 0.16 g/cm3, which was again in good agreement with the value for Ni9S8, whose bulk density is 5.331 g/cm3 (PDF#22-1193).

XPS was used to evaluate the film purity. The XPS spectra were collected on the NiSx films with 5 s of 3 keV Ar+ sputtering to remove the adventitious carbon on surface. We found that the films deposited at temperature ≤200 °C were quite pure. A typical survey spectrum for a ~10 nm NiSx film deposited at 200 °C is shown in Figure 3c, where only peaks associated with nickel and sulfur were observed. High-resolution scans for nickel and sulfur were further performed. As the spectra shown in Figure 3d-e, the Ni 2p spectrum contained two main spin-orbit split peaks at 852.5 eV (2p3/2) and 869.8 eV (2p1/2) along with a broad shake-up satellite peak around 859.0 eV,42 and the S 2p spectrum contained characteristic spin-orbit doublet peaks at 161.6 eV (2p3/2) and 162.7 eV (2p1/2), which were all consistent with the reported values for nickel sulfide.42,43 Possible impurities of carbon and nitrogen were also carefully examined by 14


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high-resolution scans. As shown in Figure 3f-g, the carbon peak was just above the noise level, which corresponded to only ~1 at.% for the carbon impurity, and the nitrogen peak was not even detected, suggesting that the nitrogen impurity was below 1%. Additionally provided in Figure S3 were the XPS results for the films deposited at various other temperatures ranging from 90 to 300 °C. The spectra for the films deposited up to 200 °C showed generally the same observations as above, i.e. quite pure NiSx with only ~1 at.% carbon and no detectable nitrogen impurities. However, if the deposition temperature was further increased to 300 °C, the XPS spectra indicated considerable amounts of carbon and nitrogen residues in the film, which suggested that the Ni(amd)2 precursor decomposed at this temperature. The onset decomposition temperature was probably close to 250 °C, as the corresponding XPS spectra showed slightly higher contents of carbon and nitrogen comparing with the films deposited at lower temperatures (Figure S3d).

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Figure 4. (a, b) AFM and (c, d) SEM top-view images of ~10 nm NiSx films deposited at (a, c) 120 °C and (b, d) 200 °C, respectively. The corresponding rms roughnesses were (a) 1.05 nm and (b) 1.81 nm, respectively. (e, f) Cross-sectional SEM images showing conformal film deposition inside deep narrow trenches with an aspect ratio as high as 10:1.

Film surface morphology was examined by AFM and SEM. Figure 4a-d shows the AFM and SEM images for ~10 nm NiSx films deposited at 120 °C and 200 °C, respectively. The film deposited at 120 °C exhibited generally featureless top-view images (Figure 4a,c), suggesting that the film surface was quite smooth, and the rms roughness, which was extracted from the AFM data, was only 1.05 nm for a ~10 nm film. The film deposited at 200 °C was also smooth, although the rms roughness was slightly higher as 1.81 nm and the images showed some granular features (Figure 4b,d). Generally, the film morphology is closely related with the film crystallinity.40 The granular features observed here indicated that the deposited film was well crystallized, which was also 16


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observed by TEM. Metal sulfides generally tend to crystallize at relatively low temperature, and thus the film morphology, which associates with its crystallinity, highly depends on the deposition temperature.44 As additional AFM and SEM images shown in Figure S4, the film roughness showed a monotonously increasing trend with the deposition temperature from 0.62 to 2.56 nm for the films deposited at 90 and 300 °C, respectively.

Conformal coating is a particularly important feature for ALD,18 since the featured self-limiting surface reactions employed in an ideal ALD process should guarantee that the deposited film can uniformly cover all the exposed surface, provided that sufficient precursor exposures are supplied. Accordingly, the ideality of an ALD process is often evaluated by examining the film step coverage inside deep narrow trenches with high aspect ratios.45 In this work, we used 1.85 µm trenches with a high aspect ratio of 10:1 for this evaluation. Increased precursor exposures of ~0.35 and ~1.2 Torr s were used for Ni(amd)2 and H2S, respectively, in each ALD cycle.45 As the cross-sectional SEM images shown in Figure 4e-f, the films could be conformally deposited (at either 120 or 200 °C) inside these high-aspect-ratio trenches, and the film thickness was almost identical along each of the entire trenches. These results clearly demonstrated the excellent conformality of our NiSx ALD process over a wide temperature range.

From the above characterizations, we have shown that our ALD process was able to produce pure, smooth, conformal NiSx thin films with precise controllability in film 17



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thickness, which are all desired merits for coating high-quality thin films on complex 3D-structured nanodevices. Therefore, we speculated that this NiSx ALD process would be of great benefit for fabricating nanostructured devices, and, in particular, for the energy-related devices, as considering the superior activity of NiSx for electrochemistry and electrocatalysis.1,2 To further demonstrate for this aspect, we employed our ALD process to deposit NiSx thin films on glassy carbon electrodes, and carefully evaluated the electrocatalytic performance of ALD NiSx towards OER.

The ALD was preformed at 200 °C with 500 ALD cycles, which produced a uniform 7.5 nm NiSx layer on a glassy carbon electrode. The composition of NiSx was verified by XPS, and no difference was found as compared to the films deposited on Si wafers. A standard three-electrode system with Pt counter electrode, Hg/HgO reference electrode, and 1 M KOH electrolyte was used to evaluate the OER electrocatalytic performance of the ALD NiSx films. The as-prepared NiSx-coated glassy carbon electrode was first measured by cyclic voltammetry (CV) with a scan range from 0 to 0.75 V vs. Hg/HgO at a slow scan speed of 2 mV/s. The curve for the first CV scan is plotted as the black line in Figure 5a, where the as-deposited NiSx showed a Ni(II)/Ni(III) redox couple with anodic (cathodic) peak at 0.450 V (0.393V) and an onset potential to evolve oxygen at ~0.60 V vs. Hg/HgO. To quantitatively evaluate the OER activity, the overpotential (η) for producing oxygen at a current density of 10 mA/cm2 was 408 mV. The value was reasonably good as benchmarked with other catalysts,46 especially considering that, in 18


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our case, the flat geometric area of the glassy carbon electrode was used to calculate the current density, and, in theory, nanostructures with large microscopically surface area could be created on the electrode. Thus, taking the particular advantage of ALD to conformally coat our catalyst on a nanostructured electrode, greatly enhanced per-geometric-area current density (given fixed overpotential) could be straightforwardly achieved. In addition, Tafel data were measured by the chronoamperometry method in steps of 10 mV. As shown in Figure 5b (black squares), a reasonably small Tafel slope of 56 mV/decade was also obtained for the as-deposited ALD NiSx. The electrocatalytic performance for the state-of-art catalyst of RuO2 was also measured for comparison (blue, Figure 5a-b).

Figure 5. (a) Cyclic voltammograms and (b) Tafel plots of the ALD NiSx film measured as deposited 19



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and after 10-hour chronoamperometric measurement. The corresponding data for the state-of-art catalyst of RuO2 were also plotted for comparison. (c) Chronoamperogram for the ALD NiSx-coated electrode biased under a constant overpotential of 372 mV. (d) Chronopotentiogram for the galvanostatic measurement with a constant current density of 10 mA/cm2.

We also noticed that there were quite a few previous works39,47-49 reported greatly enhanced activity for nickel-based OER catalysts upon electrochemical aging. Similar activity enhancement for our ALD NiSx was also suggested by the results of continuous CV scans (Figure S5), in which the overpotential continually lowered (given fixed current density). To further investigate this aging effect, we conducted a 10-hour chronoamperometric measurement on the NiSx-coated electrode biased at a constant overpotential of 372 mV. This overpotential was particularly chosen so that the initial current density was around 10 mA/cm2, which corresponds to the approximate current density expected for a 10 % efficient solar-to-fuels conversion device under 1 sun illumination.46 The resultant chronoamperogram is shown in Figure 5c, where the current density significantly increased in the first couple of hours, showing an enhanced OER activity, and then the current density gradually flattened out, achieving a steady value of ~15 mA/cm2 after ~3 hours. CV scans taken after the chronoamperometric measurement further confirmed this activity enhancement. As the CV curves compared in Figure 5a, the overpotential corresponding to the current density of 10 mA/cm2 was significantly reduced to only 353 mV, which was even smaller than the value (393 mV) for the state-of-art catalyst of RuO2. The Tafel slope was also significantly reduced to only 41 mV/decade (red squares in Figure 5b), which was also smaller than that for RuO2 (92 20


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mV/decade), indicating an excellent OER kinetics on this ALD catalyst. Notice that these particularly small overpotentials and Tafel slope were among the best values reported for nonprecious OER catalysts,46 which demonstrated a remarkable electrocatalytic activity for our ALD catalyst.

We then performed additional galvanostatic measurement to further examine the stability of the catalyst. The current density (j) was set at a constant value of 10 mA/cm2, and the resultant chronopotentiogram is shown in Figure 5d. The monitored overpotential remained fairly constant, and was only slightly increased from 350 to 369 mV after 20 hours of the galvanostatic measurement, which demonstrated a remarkable stability for our ALD electrocatalyst. The Faradaic efficiency for evolving O2 was also measured, and ~100% Faradaic efficiency was obtained (Figure S6). Considering that ALD is known for excellent conformal coating, we speculate that, in conjunction with a high-surface-area nanostructured electrode (e.g. black silicon) on which our ALD NiSx film can be uniformly and conformally coated, the resultant oxygen-evolving overpotential to achieve 10 mA/cm2 per geometric area can be further reduced to a great extent, which would therefore considerably reduce the energy loss for solar-to-fuels conversion.

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Figure 6. (a, c) Top-view SEM images and (b, d) the corresponding EDS spectra for (a, b) the as-deposited NiSx film and (c, d) the film after the OER measurements. (e, f) Comparatively showing the corresponding high-resolution XPS spectra for (e) Ni 2p and (f) S 2p,respectively. (g) CV curves for the as-deposited and after-OER films measured in non-Faradaic regime. The scan rate was 0.4 V/s.

To further understand the catalyst activity enhancement upon aging, we then carefully characterized the ALD NiSx films before and after the OER measurements, and the results are comparatively shown in Figure 6. We first examined the changes of the film morphology by SEM, as well as the accompanying compositional change by energy-dispersive X-ray spectroscopy (EDS). As the results suggested in Figure 6a-d, the NiSx film seemed to undergo a considerable change during the OER. The SEM images (Figure 6a,c) showed that the film morphology after OER was appreciably roughened, along with some porous nanostructures concurrently developed on the surface. Meanwhile, the EDS spectra (Figure 6b,d) revealed a pronounced change in film composition. Prior to OER, the as-deposited NiSx film showed an EDS spectrum (Figure 6b) that only contained nickel, sulfur, and oxygen peaks, where the oxygen peak was 22


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probably coming from the native surface oxide. However, after the OER, the film exhibited a rather different EDS spectrum (Figure 6d), where the sulfur peak completely disappeared while showing a new potassium peak along with a much intensified oxygen peak. These changes indicated that the ALD NiSx film was no longer sulfide after OER, and, very likely, the sulfide had already converted to nickel (oxy)hydrate (Ni(OH)2/NiOOH), in situ, under the strong oxidizing environment during the OER. This compositional conversion was also supported by high-resolution XPS (Figure 6e-f), where, for the film after OER, the Ni 2p spectrum did not show any sulfide-related peaks and also no sulfur was detected in the S 2p spectrum. Additionally, the presence of potassium in the above-mentioned EDS spectrum (Figure 6d) was also quite in consistence with the formation of nickel (oxy)hydrate, since the electrocatalytically active host material of γ-NiOOH actually has a layered structure which contains intercalated K+ ions and H2O molecules between the sheets of edge-sharing [NiO6] octahedra.50

In-situ metal sulfide-to-oxide conversion was also recently reported by Chen et al.,51 who showed that the cobalt, cobalt-iron, or cobalt-nickel-iron oxides in-situ electrochemically oxidized from their sulfide counterparts generally exhibited much enhanced OER activity and stability, as compared to the oxides directly prepared by electrochemical deposition, and the reasons for this enhancement were ascribed to the in-situ formed nanoporous structures which provided greatly increased surface area and 23



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allowed much more catalytically active sites to be exposed.51 Although their work51 did not investigate on pure nickel sulfide and their method to prepare sulfides were very different from us, we expect that the formation of high-surface-area nanoporous structure of nickel (oxy)hydrate should also be the reason for the excellent OER activity of our ALD NiSx. In fact, the SEM image in Figure 6c already showed the evidence of forming nanoporous feature after OER. To further quantify the change of surface area, we conducted CV experiments in non-Faradaic regime (i.e. from 0 to 0.1 V vs. Hg/HgO) to measure the equivalent electrochemical surface area for the NiSx-coated electrode before and after OER. As shown in Figure 6g, the CV scans showed nearly ideal rectangular-shaped curves for charging/discharging the electrical double layer capacitance of electrode/electrolyte interface, and the current indeed showed pronounced increase after OER. The corresponding electrical double layer capacitance was found to increase by almost 3 times from 0.145 to 0.415 mF after OER (Figure S7), suggesting a significant increase of the equivalent surface area for the in-situ oxidized NiSx film. This capacitance increase was also confirmed by CV measurements using tetrabutylammonium hexafluorophosphate (0.1 M in acetonitrile) as an aprotic electrolyte to avoid any possible pseudocapacitive charge transfer (Figure S8). However, only considering the surface roughness increase was not enough to fully account for the performance enhancement (Figure S9). On the other hand, although nickel (oxy)hydrate alone had once been considered as an active OER electrocatalyst, recent careful mechanism studies47,50 proved that its high electrocatalytic activity was, in fact, due to the Fe impurities which came 24


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from unintentionally purified electrolyte and formed electrocatalytically active Fe surface sites on the γ-NiOOH host structure. Accordingly, we also carefully examined the Fe content in the films by XPS, and, as expected, a noticeable amount of Fe impurity (~4 at. %) was indeed detected for the film after OER (Figure S10), since our KOH electrolyte was not intentionally purified. In fact, the previously shown CV curves (Figure 5a) displayed an anodic shift for the Ni(II)/Ni(III) redox couple after aging, which was also an indication for Fe incorporation.47 Nevertheless, these Fe impurities were expected to be highly active on the in-situ converted NiOOH host, so that an highly efficient OER activity was successfully achieved in our experiments.

Conclusions A new ALD process of NiSx, by using bis(N,N’-di-tert-butylacetamidinato)nickel(II) and H2S as the reactants, was comprehensively reported. The deposition process was shown to follow an ideal ALD layer-by-layer growth fashion, and was able to produce fairly smooth, pure, godlevskite-structured NiSx thin films for a relatively wide process temperature range from 90 to 200 °C. The process temperature was shown to have an appreciable effect on various film properties, such as crystallinity, composition, density, and morphology. Excellent deposition conformality was demonstrated for this process, as the ALD NiSx films were able to conformally coat into 10:1 high-aspect-ratio trenches, which highlighted the general and broad applicability of this ALD process for conformal NiSx coatings on various complex 3D nanostructures. We further demonstrated the 25



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applications of our ALD NiSx for OER electrocatalysis. The ALD NiSx was found to convert to nickel (oxy)hydrate after electrochemical aging, and the aged product showed remarkable electrocatalytic activity and long-term stability. The performance shown here was among the best electrocatalytic performance reported for nonprecious OER catalysts. Considering this excellent performance along with the easy integration of ALD with most 3D nanostructures in general, we believe that the ALD NiSx process reported in this work will have a variety of highly promising applications in future energy devices (e.g. batteries, supercapacitors, electrocatalysis, photoelectrocatalysis, etc.).

Acknowledgements This work was financially supported by NSFC (Grant No. 51302007), Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No. 2015A030306036), Guangdong Innovation Team Project (No. 2013N080), and Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ20140417144423201 and Peacock Plan KQCX20150327093155293).

Supporting Information Detailed analysis of electron diffraction, additional RBS, XPS, AFM, and SEM results for the NiSx films deposited at various temperatures, additional CV results, and Faradaic efficiency results.

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