Nickel telluride vertically aligned thin film by radio-frequency magnetron sputtering for hydrogen evolution reaction

In recent times, the largest component of worldwide energy resources has come from carbon-based fossil fuels, which contribute to climate change. In order to minimize this catastrophic phenomenon, it is essential to find alternative energy resources independent of carbon. One of the most promising clean and sustainable alternatives is hydrogen, as it has a high energy density of 120 MJ kg⁻¹–140 MJ kg⁻¹ and is also abundant in nature in various forms.¹ Water electrolysis is one method to extract highly pure hydrogen from water, which provides an efficient means to extract energy from renewable sources on an industrial scale.² The two significant reactions involved in water splitting are the hydrogen evolution reaction (HER) taking place at the cathode and the oxygen evolution reaction (OER) taking place at the anode. To fabricate an effective water electrolysis system, development of stable and active electrode materials for HER and OER is required. Various noble metals such as iridium, ruthenium, and their oxides are used for OER,³ and platinum-based compounds are known to be the best catalyst for HER.⁴ However, high cost and scarcity in nature reduce the possibilities for commercialization and device applications.⁵ 

In the search for a suitable substitute electrocatalyst for HER, MoS₂ has been highlighted.⁶ MoS₂ is one of the transition metal dichalcogenides (TMDCs) and is abundant in earth with adequate hydrogen binding energy,^5,7 which acts as an effective catalyst for HER applications. Although MoS₂ has shown outstanding performance as an HER catalyst,⁸ it has an active site on the edge site and hindering its inert basal plane limits maximized its activity.⁹ Extensive approaches have been suggested; one is activating the basal plane of MoS₂^10–12 and the other is substitution of the S atom in MoS₂ by another chalcogen element such as selenium or tellurium.^13,14 For example, 1T′-MoTe₂ has been shown to be metallic and a reasonably good electrochemical catalyst for HER;¹⁵ hence, 1T′-MoTe₂ is known to be Weyl semimetal.¹⁶ 

Moreover, there have been several approaches to find a substitute electrocatalyst for HER based on other metals which is more abundant than Mo and W, such as Co, Cu, Ni, and Fe.⁵ Out of them, Ni has the second highest abundance. Therefore, various Ni-based compounds are investigated for water electrolysis.^17,18 For example, Ni-based chalcogenide material groups (NiS₂, NiSe₂, and NiTe₂) are investigated as catalysts for HER and NiTe₂ showed electrochemical performance for HER.¹⁹ The majority of investigation used Ni foam as the substrate to synthesize nickel chalcogenide as an electrocatalyst, however, it was found that NiS showed better performance and stability when it is anchored by DNA with an ultralow quantity of NiS toward oxygen evolution reaction.²⁰ It is noteworthy that some Ni-based chalcogenide material groups show bi-functional catalytic activity. For example, Ni₃Se₄ nanoassemblies initiated by microwave catalyze hydrogen and oxygen in neutral and alkaline media.²¹ Nickel telluride is also applicable to be a bifunctional electrocatalyst for hydrogen evolution reaction in acidic conditions and oxygen evolution reaction in alkaline conditions.²² 

In addition, NiTe₂ has verified Weyl semimetal showing metallic properties like 1T′-MoTe₂.²³ As a result, tellurides of nickel in the form of NiTe, NiTe₂, and Ni₃Te₂ compounds have been reported to have high activity and stability.^22,24 However, there remains more opportunity for efficient investigation of the characteristics of nickel telluride in HER. Anantharaj et al. reported that NiTe₂ nanowires could be an alternative to platinum for HER applications, demonstrating lower overpotential than platinum foil in extreme pH conditions, viz., 0 and 14.²⁵ NiTe₂ is also widely applied to detect organic compounds,^26,27 supercapacitors,²⁸ and batteries,²⁹ based on its high electrical conductivity, fast electron transmission, and magnetic property. These results indicate that mass production and synthesis of large-scale NiTe₂ should be required for wider application. Despite the recent successful use of NiTe₂ in various fields, the majority of research has been based on the solvothermal method of synthesizing NiTe₂ nanostructures and they did not investigate the effect of alignment of NiTe₂ for HER electrolysis. As a result, the effect of vertical alignment of NiTe₂ thin films for HER was in veil. Moreover, synthesis of wafer-scale NiTe₂ has also not been widely explored even though it is essential for industrial applications.

In this study, we have synthesized a wafer-scale NiTe₂ thin film using radio-frequency (RF) magnetron sputtering to achieve wafer-scale synthesis of NiTe₂. The deposited film shows an aggregated columnar structure and its vertically grown region becomes dominant as the sputtering time increases. The vertical alignment is expected to achieve higher conductivity and more reactivity than the randomly aligned one that is synthesized via the solvothermal method. Herein, we suggest the possibility that the wafer-scale vertically grown NiTe₂ thin film on an arbitrary substrate shows a robust performance for hydrogen evolution reaction in extremely acidic conditions.

An intermetallic NiTe₂ target was utilized to fabricate NiTe₂ thin films by sputtering. The detailed experimental procedure has been published in a previous paper.³⁰ In brief, the sputtering target was composed of NiTe₂ powders fabricated by a solid-state reaction method. Ni (purity 99.9%) and Te pellets (purity 99.999%) were placed in a vacuum-sealed quartz tube. The quartz tube was placed in a furnace and heated at 1273 K for 24 h. After heating, it was cooled from 1273 K to 773 K at a rate of 100 K h⁻¹, and then further cooled to room temperature in the unheated furnace. Using the synthesized NiTe₂ powders, a sputtering target having a disk shape with a 2-in. diameter (see Fig. S1 in the supplementary material) was fabricated in a spark plasma sintering system (SPS, SPS-630lx, Fujimoto Electronic Industrial Co., Ltd., Saitama, Japan). In the SPS system, the NiTe₂ powders were pressed under 8.6 tons of pressure at 773 K in a vacuum chamber (5 × 10⁻² Torr) for 30 min. Using this sputtering target, thin films were deposited on a Ni foil substrate (Sigma-Aldrich, USA) using a radio-frequency (RF, 13.56 MHz) power of 90 W for various time: 60 s, 90 s, 300 s, 600 s, and 900 s. The sputtering pressure was 5 × 10⁻³ Torr (base pressure of 10⁻⁶ Torr), and the substrates were heated to 475 K during sputtering.

Scanning electron microscopy (SEM, Hitachi, Tokyo, Japan) at the MEMS Sensor Platform Center of SungKyunKwan University (SKKU) operated at 15.0 kV was used to analyze the surface morphology of the NiTe₂ thin film. Focused ion beam (FIB, Helios NanoLAB 600i, FEI company, Eindhoven, The Netherlands) was used to prepare TEM samples. Transmission electron microscopy (TEM, JEM-2100F, JEOL, Tokyo, Japan) and x-ray diffraction (XRD, Empyrean, PANalytical BV, Eindhoven, The Netherlands) were used to analyze the crystal structure of the NiTe₂ thin film. XRD was operated at 40 kV and 30 mA using Cu Kα radiation with a wavelength of 1.54 Å. Data were collected in reflection mode at 2θ values from 10° to 90° using stepwise scans of 0.01° with a scan speed of 0.02°/s. The relative area of (001) and (110) peaks in XRD profiles was measured to compare the relative volume ratio of two types of grains whose interslabs were parallel to and perpendicular to the substrate. X-ray photoelectron spectroscopy (XPS, K-Alpha+, Thermo Fisher Scientific, USA) was used to analyze the surface electronic states of the NiTe₂ thin film. The binding energy of each peak in core-level spectra was calibrated from the binding energy of the C1s core-level spectrum.

Electrochemical measurement was carried out to evaluate the hydrogen evolution reaction (HER) performance of each synthesized catalyst in a strongly acidic condition. A three-electrode standard cell system was used. Each catalyst, the NiTe₂ thin film, was in turn adopted as a working electrode and its exposed area was fixed to be the same area (0.9993 cm²) to minimize the potential effect of the exposed area. The Pt plate was used as a counter electrode and Ag/AgCl with 3M NaCl internal solution (ALS, Japan) was selected as a reference electrode. Every electrochemical test was carried out at room temperature in 0.5M H₂SO₄ (pH ≈ 0) solution and the result was analyzed and recorded by an Ivium-n-Stat (Ivium Technology, Eindhoven, The Netherlands). It is noteworthy that the counter electrode (Pt plate) and working electrode (NiTe₂ thin film) were separated by the proton exchange membrane to prevent the Pt transfer from the anode to the cathode, as it has been reported that Pt dissolves from the Pt plate and attaches onto the surface of a catalyst, thereby activating its inert surface. All the reported potentials in this work were iR compensated to minimize ohmic loss and corrected to a reversible hydrogen electrode (RHE) by adding a value of (0.209 + 0.059 × pH) V to all the measured potentials. The percentage of iR compensation was set to 99%. The catalytic activities for HER were determined by linear sweep voltammetry (LSV) with a scan rate of 5 mV s⁻¹ from 0 mV to −500 mV, in a cathodic scan direction. Tests for Ni foil were performed under the same conditions to compare the electrocatalytic performance of the NiTe₂ thin film to Ni foil. The overpotential value was defined as the applied potential vs RHE to acquire 10 mA cm⁻² of current density. Polarization curves were converted to a semi-log plot in the log scale of current density. The linear region of each result was selected to extract the Tafel slope value with the unit of mV dec⁻¹ and its validity was assured by confirming its R² value for linearity. For stability tests, both cyclic voltammetry and chronoamperometry were performed. In cyclic voltammetry, each cycle had a potential range of 0.0 V to −0.48 V vs RHE and the number of cycle was set to 1000 with a scan rate of 10 mV s⁻¹. In chronoamperometry, constant voltage to acquire 10 mA cm⁻² was applied for 90 000 s (25 h).

Nickel telluride (NiTe₂) thin films were deposited by a radio-frequency (RF) magnetron sputtering process on a Ni foil substrate, as depicted in Fig. 1(a). A 2-in. NiTe₂ sputtering target was used (see Fig. S1 of the supplementary material) to deposit a NiTe₂ thin film on the substrate. The surface was quite flat with an aggregated columnar structure, which is generally known to be the microstructure of thin films deposited by RF magnetron sputtering. Any difference between before and after the deposition of NiTe₂ on Ni foil could not be observed by the naked eye as both Ni and NiTe₂ reflect light similarly, as shown in Fig. 1(b). A scanning electron microscope (SEM) image did not show the nanostructure on the surface of NiTe₂, as shown in Fig. 1(c), showing its flat morphology.

The thickness, crystallinity, and microstructure of NiTe₂ thin films gradually evolved as the sputtering time increased. Two representative samples of NiTe₂ thin films, sputtered for 60 s and 900 s, were analyzed using transmission electron microscopy (TEM). The thickness of the NiTe₂ thin film for each sample was confirmed by the TEM image. The film thickness increased from 30 nm to 140 nm for the longer sputtering time, as shown in Figs. 2(a) and 2(b). The inset of Figs. 2(a) and 2(b) shows diffraction spots in the Fourier transform image, which were seen for the 900 s sputter film, which revealed that the 900 s sputter film had an enhanced crystallinity over the 60 s sputter film. In addition, in Fig. 2(c), (001) lattice fringes of NiTe₂ (0.528 nm from ICDD Card No. 00-008-0004) were observed using high-resolution transmission electron microscopy (HRTEM), and its lattice fringes were correlated with the interlayer spacing of NiTe₂.³¹ The increase in crystallinity may be attributed to the heat generated during sputtering in addition to substrate heating. The columnar structure of NiTe₂ was demonstrated using TEM bright and dark field images taken for the same area, as shown in Figs. 2(b) and 2(d). From the dark field image, it was confirmed that the interslabs in the 2D structures were almost perpendicular to the substrate in a number of the grains. The interslabs of NiTe₂ were parallel to the (001) plane in the hexagonal structure. It was seen from the dark field image that the planes of white grains were 72° from the substrate, which was almost normal. For the dark field image, one of the (001) spots has been indicated by a^*, which is the reciprocal vector of the (001) plane, whereas b is the real space vector, parallel to the substrate, shown in the inset of Fig. 2(b). The contained angle between a^* and b vectors is 18°. The electrical properties of the thin film were influenced by the columnar structure and its crystallographic orientation. During this process, electrons flowed in between the Ni substrate and water without encountering any obstacles such as grain boundaries. The electrical conductivity along the a axis was higher than that of the c axis in 2D materials,³² and the electrical conductivity increased with the increase in the contained angle toward 90°.

In Fig. 3, the areas of (001) and (110) peaks of x-ray diffraction (XRD) profiles were compared to quantify the crystallographic orientation of the grains from the larger area of the NiTe₂ thin film with respect to the substrate, from its full XRD profiles, as shown in Fig. S2 of the supplementary material. With increasing sputtering time, the area of the (110) peak increased more compared to the area of the (001) peak, which indicated that the size of the grains of interslabs that are perpendicular to the substrate also increased. The (110) plane was normal to the interslabs in NiTe₂, and the incident angle of x-rays with respect to the substrate was only 5°. In these grains whose (110) planes satisfy Bragg’s law, the interslabs were almost perpendicular to the substrate. Thus, the efficiency of HER of the NiTe₂ thin film was improved by the evolution of the microstructure and crystal orientation as sputtering time increases. The relative area ratio of the two XRD peaks, (001) and (110), with respect to the sputtering time has been summarized in a table format. The table shows that the 900 s sputtered thin film had more (110) orientation than (001), whereas the 60 s sputtered thin film had no (110) orientation (see Table S1 of the supplementary material).

To demonstrate the surface electronic states of deposited NiTe₂, x-ray photoelectron spectroscopy (XPS) of the NiTe₂ thin film sputtered for 900 s was analyzed and the Ni-2p core-level spectra and the Te-3d core-level spectra were deconvoluted to calculate the atomic composition and stoichiometry of the NiTe₂ thin film. In Fig. 4(a), the Ni-2p core level spectrum exhibited a clear doublet pattern with a spacing of 17.3 eV. The Ni 2p_3/2 state was deconvoluted into five different peaks: (1) Ni 2p_3/2 at 852.10 eV originated from the unreacted element of Ni, (2) Ni 2p_3/2 at 853.99 eV of NiTe₂ indicating nickel–chalcogen bonding, (3) Ni 2p_3/2 at 856.19 eV of NiO came from oxide, (4) Ni 2p_3/2 at 860.05 eV of Ni(OH)₂ reacted with moisture of surroundings, and (5) a satellite peak at 861.89 eV. Sometimes, Ni 2p_3/2 at 856.19 eV is illustrated as nickel–chalcogenide bond (NiTe) instead of oxide.^19,22 In Fig. 4(b), the Te 3d core-level spectrum also shows a clear doublet pattern with a spacing of 10.3 eV. Te 3d_5/2 was deconvoluted into three different peaks: (1) Te 3d_5/2 at 572.78 eV of NiTe₂, (2) Te 3d_5/2 at 574.02 eV originated from the unreacted element of Te, and (3) Te 3d_5/2 at 576.02 eV of TeO₂. Both Ni 2p and Te 3d core spectra indicate that surface oxidation of the NiTe₂ thin film occurred. This XPS analysis is consistent with the previously reported results.^22,24,25

To evaluate how much hydrogen gas was produced using NiTe₂, an electrochemical measurement was carried out in a standard three-electrode system as shown in Fig. 5(a). In this system, the NiTe₂ sample was adopted as a working electrode, an Ag/AgCl electrode was used as a reference electrode, and a Pt plate was used as a counter electrode. The proton exchange membrane was inserted in the middle of a bottleneck connecting two vessels and it was to prevent dissolution of Pt into the solution and its re-attachment onto the surface of the NiTe₂ thin film, as suspected on carbon-based cathodes for HER.³³ 

Cathodic polarization curves were recorded for the Ni foil substrate and NiTe₂ thin film deposited with various sputtering times from 60 s to 900 s. Before cathodic polarization curves were recorded, the electrochemical measurement system measured the ohmic resistance of the system and the value of ohmic resistance was used to iR compensation in the system. Increasing the sputtered time was shown to lead to more active catalytic performance for HER and was better than the pristine Ni substrate, as shown in Fig. 5(b). The HER performance of a catalyst was determined by the overpotential value necessary to acquire 10 mA cm⁻² of current density in cathodic polarization curves. The overpotential value of the NiTe₂ catalyst was 454 mV for the film sputtered for 60 s, decreasing to 416 mV for the film sputtered for 900 s. This result is consistent with the benefit of the charge transfer fabricated NiTe₂ thin film with a longer sputtering time.

To calculate the rate-determining step of the HER process in the NiTe₂ thin film, the Tafel plot was obtained by re-plotting the cathodic polarization curves with overpotential vs logarithm of current density. In acidic solutions, the principal reactions are, at first, the adsorption of protons on active sites of a catalyst (Volmer reaction). The next step occurs either by desorption of protons on a catalyst after combination of nearby protons in solutions (Heyrovsky reaction) or binding of two protons on a catalyst (Tafel reaction) with Tafel slope values of 120 mV dec⁻¹, 40 mV dec⁻¹, and 30 mV dec⁻¹, respectively.³⁴ Each Tafel plot of the NiTe₂ thin film was redrawn and the Tafel slope was calculated to be in the range from 63.79 mV dec⁻¹ to 86.61 mV dec⁻¹, which shows that the rate-determining step is the Heyrovsky reaction. The variation of the Tafel slope is likely to have come from the evolution of H₂ gas bubbles. Detailed information of overpotential values and Tafel slope values for fabricated NiTe₂ thin films with various sputtered times was summarized for quantitative analysis (see Table S2 of the supplementary material).

To ensure the long-term stability of the NiTe₂ thin film catalyst, both cyclic voltammetry and chronoamperometry were performed. In cyclic voltammetry, known as an accelerated-degradation test, the long-term stability was examined to compare the initial scan of cyclic voltammetry and the final scan of cyclic voltammetry after thousands of cycles. Fig. 6(a) shows that the performance of the NiTe₂ thin film was degraded after 1000 cycles. In chronoamperometry, as shown in Fig. 6(b), the current density at a given potential bias was decreased down to −5 mA cm⁻² from −10 mA cm⁻² in 50 000 s, in agreement with the result of cyclic voltammetry.

It is believed that the state at the surface of the NiTe₂ thin film was changed after the HER operation. To demonstrate this result, XPS characterization was performed for the NiTe₂ thin film after the long-term stability test was done. In both Ni-2p and Te-3d core level spectra, the oxide and/or hydroxide increased after the HER test (see Fig. S3 of the supplementary material). It is considered that the unreacted Ni and Te in the NiTe₂ thin film gradually react with oxygen and moisture of surroundings under water splitting. Considering this result, it is required not to leave the unreacted element of Ni and Te to enhance the long-term stability for further investigation, such as post-annealing.

It is noteworthy that the use of the overpotential at the current density normalized with the actual electrochemical surface area (ECSA), that is, η@10 mA cm⁻²_ECSA and turnover frequency (TOF) instead of the overpotential at the current density normalized with the geometric area (η at 10 mA cm⁻²_geo) and the Tafel slope when the intrinsic activity of the electrocatalyst for electrochemical water splitting is evaluated.^35,36 According to these reports, the latter two parameters, η at 10 mA cm⁻²_geo and the Tafel slope, are dependent on the catalyst’s mass. However, as they mentioned, the difficulty in measuring the exact ECSA of a catalyst hinders the unveiling of its intrinsic activity. Moreover, calculating TOF of a given catalyst requires the surface concentration or the number of active sites but distinguishing the number of active sites is the main challenge to calculate TOF, as reported in the MoS₂ catalyst.³⁷ Therefore, a new standard method should be formalized to compare the intrinsic activity of a given catalyst.

A vertically grown nickel telluride thin film was synthesized by RF magnetron sputtering, and its electrochemical performance for HER in a strongly acidic medium was evaluated. The sputtering time was varied to control the thickness, crystallinity, and vertically grown region of the NiTe₂ thin film. The evolution of the microstructure of the deposited film was also investigated by SEM, XRD, and TEM. The microstructure of the NiTe₂ thin film was found to be a columnar structure, and its vertically grown region became dominant and crystallinity was enhanced as the sputtering time increased. In extremely acidic conditions, the overpotential value was observed to be 416 mV, due to the vertically grown NiTe₂ columns enabling effective charge transfer from the substrate to the surface of NiTe₂. By this work, it was demonstrated that the RF magnetron sputtering method is another approach to synthesize the NiTe₂ thin film and its vertical growth facilitates enhancement of the electrochemical performance for water splitting. We expect that it is compatible with other conducting substrates such as carbon clothes and that it is scalable up to the wafer-scale, encouraging wider application of NiTe₂ for wafer-scale water electrolysis. However, there is an issue to solve the current methods for application into water electrolysis. NiTe₂ by the RF magnetron sputtering method forms a thin film instead of a bulk material, which makes it difficult to ensure its stability, requiring further investigation to enhance its catalytic stability under long-term operation and its intrinsic nature with much reliable methodology and evaluation parameters instead of generally misunderstood factors. Nonetheless, we believe that the RF magnetron sputtering method can be used to fabricate electrochemical catalysts made by other TMDC materials such as MoS₂, WSe₂, etc.

See the supplementary material for more detailed information on the preparation method of a 2-in. NiTe₂ sputtering target, XRD profiles of the NiTe₂ thin film fabricated by sputtering, the summary of qualitative analysis for XRD profiles and the catalytic test, and XPS core-level spectra of Ni-2p and Te-3d of the NiTe₂ thin film after the long-term stability test.

J.O. and H.J.P. contributed equally to this work.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

This research was supported, in part, by the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (Grant No. NRF-2018R1A2B2003558), a grant (No. 19CTAP-C152910-01) from the Technology Advancement Research Program (TARP) funded by the Ministry of Land, Infrastructure and Transport of the Korean government, the Korea Basic Science Institute (KBSI) National Research Facilities and Equipment Center (NFEC) grant funded by the Korea government (Ministry of Education) (Grant No. 2019R1A6C1010031), the Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science and ICT (MSIT), the Industrial Fundamental Technology Development Program (10076307, Development of additives to improve electrical conductance of screen-printed Ag electrodes) funded by the Ministry of Trade, Industry, and Energy (MOTIE) of Korea, and Creative Materials Discovery Program (NRF-2017M3D1A1039379) through the National Research Foundation of Korea. XRD analysis was performed at the Cooperative Equipment Center of the KOREATECH.