3R-TaS 2 as an Intercalation-Dependent Electrified Interface for Hydrogen Reduction and Oxidation Reactions

Hamid Ghorbani Shiraz, Zia Ullah Khan, Daniel Péré, Xianjie Liu, Yannick Coppel, Mats Fahlman, Magnus Berggren, Radoslaw Chmielowski, Myrtil Kahn, Mikhail Vagin, et al.

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To cite this version:

Hamid Ghorbani Shiraz, Zia Ullah Khan, Daniel Péré, Xianjie Liu, Yannick Coppel, et al. 3R-TaS 2 as an Intercalation-Dependent Electrified Interface for Hydrogen Reduction and Oxidation Reactions. Journal of Physical Chemistry C, 2022, 126 (40), pp.17056-17065. 10.1021/acs.jpcc.2c04290. hal-03877013

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Submitted on 29 Nov 2022

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3R-TaS₂ as Intercalation-dependent Electrified Interface for Hydrogen Reduction and
Oxidation Reactions

Hamid Ghorbani Shiraz*¹, Zia Ullah Khan¹, Daniel Péré², Xianjie Liu¹, Yannick Coppel³, Mats Fahlman¹, Magnus Berggren¹, Radoslaw Chmielowski², Myrtil L. Kahn³, Mikhail Vagin*¹,
Xavier Crispin¹

¹ Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, Norrköping, SE-60174, Sweden.

²Department of Advanced Materials, IMRA Europe S.A.S., 06904 Sophia Antipolis, France.

³LCC-CNRS, University of Toulouse, CNRS, Toulouse, France.

Corresponding Author:
Hamid Gh. Shiraz – Hamid.ghorbani.shiraz@liu.se
Mikhail Vagin - mikhail.vagin@liu.se

Abstract

The hydrogen technology, as a future breakthrough for energy industry, has been defined as an
environmentally friendly, renewable, and high-power energy carrier. The green production of
hydrogen, which majorly relays on electrocatalyst, is limited by the high cost and/or the
performance of the catalytic system. Recently, studies have been conducted in search of a Bi-
functional electrocatalysts accelerating both hydrogen evolution (HER) and hydrogen
oxidation (HOR). Herein, we report the investigation of the high efficiency bi-functional
electrocatalyst TaS₂ for both HER and HOR along with the asymmetric effect of inhibition by
organic intercalation. The linear organic agent, to boost the electron donor property and to ease
the process of intercalation, provides a higher interlayer gap in the tandem structure of utilized
nanosheets. XRD and XPS data reveals an increase of the interlayer distance by 22%. The HER and HOR were characterized in a Pt group metal-free electrochemical system. The pristine sample shows a low overpotential of ~0.016 V at onset. The intercalated sample demonstrates a large shift on its performance for HER. It is revealed that the intercalation is a potential key strategy for tuning the performance of this family of catalysts. The inhibition of HER by intercalation is considered as the increase of the operational window of water-based electrolyte on negative electrode, which is relevant to technologies of electrochemical energy storage.

**Introduction.** For many decades, hydrogen has been identified as a key energy carrier for a sustainable future society, as it offers a high energy density and principally no negative direct impact on the environment, with respect to CO₂ emissions. Hydrogen generation, as a first step towards a hydrogen economy, has been a matter of debate among scientists. Although many production methods, like photochemical or cracking reformation, or gasification of biomass, are being operated all over the world, there are several problems with current methods such as low yield or major environmental impacts.

One of the most efficient and clean processes to produce hydrogen is via electrochemical hydrogen evolution reaction (HER). There are many electrocatalysts for HER that possess high efficiencies, among which Pt, Rh, and Pd ¹,² are commonly considered and explored. Although numerous candidates like hybrid structures (core-shell, alloys, composites, etc.) ³,⁴, doped atom systems ⁵, metal organic frameworks ⁶, metal oxide arrays ⁷, etc. have shown proper performance characteristics, several major problems still lie ahead. Several studies are being conducted in the search for a cost-effective (especially targeting Platinum group metal-free catalysts), efficient, and scalable electrocatalyst. In that context, transition metal dichalcogenides (TMDs) constitute another class of two-dimensional materials where
the elemental periodic unit is composed of an atomic layer of a transition metal (Ti, Ta, Mo, W, etc.) in the center with a chalcogen atomic layer (S, Se) on either side. The geometry of chalcogen atoms with respect to transition metal results in various crystal structures (1T, 2H and 3R, etc.) where T, H and R represent overall structures (trigonal, hexagonal, and rhombohedral) and symmetry repetition is given with the number (1, 2, 3) of layers ⁸. TMDs like WS₂ display significant activity for HER ⁹. Also, MoS₂ is one of the most investigated TMDs towards HER ¹⁰⁻¹². Among TMDs, tantalum disulfide (TaS₂) exhibits the highest temperature-dependent electronic conductivity ¹³⁻¹⁵, which motivates the investigation of HER electrocatalytic activity of this 2D material for the use as (see Fig. 1a and b). It is demonstrated that the majority of 2D structures are catalytically, several times more active at the edge sites, as compared to the basal plane ¹⁶. To improve the (electro)catalytic activity, several modifications with the aim of activation of TaS₂ sheets are studied, such as plasma treatment ¹⁷, chemical exfoliation ¹⁸, atom-scale dispersion ¹⁹, ²⁰, interfacial engineering, etc. Yu et al. have been using theoretical calculations that shows that cracked eight-awn star TaS₂ nanostructure possesses significant electrocatalytic properties ²¹. They revealed that both basal and edge sites contribute to HER. Also, Hanlin et al. demonstrated that the electro-exfoliation of bulk TaS₂ into nanosheets can significantly enhance the electrocatalytic activity ²²; leading to low overpotential and a favorable Tafel slope of 197 mV (@10 mA.cm⁻²) and 100 mV.dec⁻¹, respectively, while compared to those of the parent material (overpotential and Tafel slope of 547 mV and 216 mV.dec⁻¹, respectively). This demonstrates that the exfoliation of bulk TaS₂ can improve the performance, which may be ascribed to the increase of the interlayer space. Hence, nano-organization of the material is crucial, as it impacts the diffusion of protons via accommodation of more protons at the edge sites where the hot reaction sites are located.

Interestingly, TaS₂ can be exfoliated into nano-flakes, which can host a variety of linear and ring-shaped organic molecules in the inter-layer gaps (Fig. 1c). To the best of our knowledge, we have not found any prior study reporting the role of intercalated organic
molecule on the catalytic efficiency of the HER. In this work, we studied the effect of the intercalation of hexylamine (HA) in TaS₂ nanosheets on the HER performance. The exfoliated TaS₂ nanoflakes were studied with scanning electron microscopy (SEM) and their intercalation with the hexylamine was studied by X-ray diffraction (XRD). The surface compositions were analyzed with X-ray and UV-photoelectron spectroscopies (XPS, UPS). Additional evidence of the structure of the interaction between the organic molecules and exfoliated-TaS₂ were obtained by solid-state NMR spectroscopy and, finally, HER was measured in a standard electrochemical cell.

<figure><img src="image_3.png" /><figcaption>**Fig. 1:** (a) configuration of sulfurs (yellow) and Metal (dark blue) for different crystal phases, adsorbed with H atom (light blue); (b) high-efficiency HER electrocatalysis on 3R TaS₂; (c) intercalation of the pristine (stacked) nanosheets and increasing of the interlayer space.</figcaption></figure>

**Methods**
**Material synthesis and ink formulation.** Tantalum (Ta) and Sulfur (S) powders were purchased from Goodfellow while hexylamine (HA) $C_6H_{15}N$ was bought from Sigma-Aldrich. Ta and S were mixed in stoichiometric ratio in an agate mortar and then transferred to quartz ampoule through a funnel. The ampoule was evacuated and flame-sealed. Later, it was placed in a vertical furnace for 48 hours at 900°C and then quenched in water at room temperature. The ampoule was opened inside a $N_2$ glovebox, and the powder was passed through a 125 µm mesh. The powder was heated to 650 °C under vacuum to produce the 3R-Ta$S_2$ crystal structure with an electrical conductivity ($\sigma$) of 500 S/cm. HA and Ta$S_2$ were then mixed in 20:1 molar ratio inside a sealed glass bottle and stirred at 50 °C for 5 days. To suppress the agglomerated particles, the inks of the electrocatalysts were subjected to ultrasonication just before the casting of the films.

**Structural Characterization.** The X-ray diffraction patterns (XRD) were collected at room temperature using a D8 Bruker diffractometer equipped with a copper anode and a point detector. The scanning electron microscopy (SEM) investigations were carried out with a Hitachi S-4700 equipped with a field emission gun. For both XRD (Bruker D8 Advance / copper / theta-2theta mode / 5° - 67° / step 0.04° / time : 10s per step) and SEM (5.00 kV and 12.5\*2.50k SE(M) and 10.0 kV and 13.2\*5.00k SE(M) for top view and cross section images, respectively) analyses, the Ta$S_2$ powders were drop-casted on glass substrates which were heated at 100°C for 30 minutes inside glovebox to evaporate ethanol used as a solvent.

XPS/UPS (X-ray, ultraviolet photoemission spectroscopy) measurements have been performed in Scienta ESCA 200 system under the base pressure of 2E-10 mbar with SES 200 electron analyzer, a monochromatic Al Ka x-ray source (h$\nu$=1486.6 eV) and a helium discharge lamp (h$\nu$=21.22 eV) for XPS and UPS, respectively. All spectra were collected at normal emission and at room temperature. The spectrometer was calibrated by a sputter-
cleaned Au film with the Fermi level at 0 eV and Au 4f7/2 peak at 84.0 eV with its full width at half maximum being 0.65 eV for XPS. The total energy resolution of UPS is about 0.08 eV estimated from the width of the Fermi edge of a clean Au film. The work function of sample was extracted from the edge of the secondary electron cutoff in UPS spectrum with a bias of -3V applied in sample.

NMR experiments were recorded on Bruker Avance 400 III HD spectrometer operating at magnetic fields of 9.4 T. Samples were packed into 2.5 mm zirconia rotors under argon inside a glove box. The rotors were spun between 10 and 15 kHz at 295 K. ¹H MAS were performed with the DEPTH pulse sequence and a recycle delay of 3 s. ¹³C MAS and CP-MAS were recorded with recycle delays of 5 s and 1.5 s, respectively and with a contact time of 2 ms for CP-MAS. Chemical shifts were externally referenced to liquid TMS. The NMR experiments had to be realized in small rotors (2.5 mm) because of the unfavorable electromagnetic properties of the samples.

**Electrochemical Characterization.** Electrochemical measurements were carried out using a three-electrode setup (BioLogic SP200 potentiostat) in 0.5 M H₂SO₄ solution. A graphite felt was used as a counter electrode to provide the highest possible surface area, and to avoid any metallic contamination ²³. The structure and performance of carbon felt characterized and proved during measurement. An Ag/AgCl and a rotating glassy carbon disk electrodes were used as a reference and working electrodes, respectively. A 5 wt% Nafion was used as a proton-conductive binder. The measurements were done in the electrolyte where the working electrode was rotating at 800 rpm and the system was purging using pure hydrogen gas. For analysis, all the potentials are converted to reversible hydrogen electrode using Nernst equation; in 0.5 M H₂SO₄ (pH=0) the equation is simplified as E(RHE) = E(SCE) + 0.242.

Linear sweep scan rate (LSV) curves were measured at the scan rate of 5 mV.sec⁻¹, in the range of 0.2 to -1 V versus RHE, all reported values are IR-compensated. The
electrochemical active surface area measurements were recorded at scan rates 10 to 500 mV.sec⁻¹ in a potential window of 0.14-0.24 (Vs. RHE).

**Results.** From the synthesis of TaS₂, we succeeded to obtain a powder of the 3R-TaS₂ crystal structure, which was supported by XRD studies. It was reported ²⁴ that this structurally-controlled electrocatalyst showed a highest HER activity in comparison with the other TaS₂ crystal structures illustrated by the lowest overpotential for HER (0.2 V, compared to 0.47 V for 2H-TaS₂ and 0.55 V for 1T-TaS₂ and 0.02 V for Pt) and the lowest Tafel slope of 85 mV/dec (compared to 100 mV/dec for 2H-TaS₂, 155 mV/dec for 1T-TaS₂, and 30 mV/dec for Pt/C). The SEM image of the powder shows platelets of TaS₂ of various sizes (Fig. 2a). A compressed layer of this powder displays an electrical conductivity of 500 S/cm suggesting that it has a metallic character. To intercalate HA into the crystal strucutre, the following simple recipe was used: HA and the TaS₂ powder were mixed at a 20:1 molar ratio inside a sealed glass bottle and stirred at 50 °C for 5 days. That step results in the full intercalation of HA into partially exfoliated Ta S₂ nano-flakes.

After exfoliation and intercalation, the suspension was casted on top of a glass substrate to form a thin layer. SEM cross-section images, given in Fig. 2b, clearly indicates that the thin layers are formed of re-stacked exfoliated structures oriented parallel to the substrate.

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2:** SEM images of the TaS₂ (a) top view of the bulk powder; (b) cross-section of the film with TaS₂ and HA. The scale bar is 10 µm long. (c) XRD patterns collected on a pristine TaS₂ powder indexed with the 01-089-2756 (in black) and TaS₂ intercalated with hexylamine.</figcaption></figure>

The X-ray diffraction patterns collected on pristine TaS₂ powder and TaS₂ intercalated with hexylamine are presented on figure 2c. The XRD pattern obtained from the drop-casted powder has the best match with the ICSD card 01-073-2201²⁵ that corresponds to 3R-TaS₂ (black pattern). The intercalation of hexylamine into the 3R-TaS₂ leads to a superlattice of TaS₂/HA characterized by a series of (00l) peaks where l = 1, 2 up to 7 (blue pattern). Beside strong diffraction from the superlative, some minor peaks are identified as impurity that could correspond to TaOₓ. The comparison of both patterns indicates that the 3R-TaS₂ structure is completely modified by the intercalation of hexylamine as the diffraction from the 3R-TaS₂ is entirely vanished on the pattern of the TaS₂/HA.
To better understand the intercalation phenomenon of HA in 3R-TaS₂, the samples were analyzed by X-ray photoelectron spectroscopy (XPS). To emphasize the change on the valency and different chemical environments, all XPS data were plotted with the binding energy relative to the vacuum level. The Ta 4f spectrum of the pristine 3R-TaS₂ clearly indicated that there is more than one type of Ta valency (left panel in Fig. 3a). The doublet around 27 and 29 eV are from 3R-TaS₂, where the doublet around 30.5 and 32.5 eV can be attributed a surface oxide (TaOₓ)²⁶ due to the exposure of 3R-TaS₂ to ambient air. Upon HA intercalation, the Ta4f doublet from the surface oxide TaO₅ₓ can be significantly decreased, which indicates that HA participates to the removal of the oxide. Also meanwhile, it can be clearly seen that the doublet from 3R-TaS₂ has a slight shift to the high binding energy, around 0.3 eV, compared to after HA intercalation. Both features indicate that there is electron transfer from HA to the surface oxide of 3R-TaS₂. Correspondingly, the spectral feature of S2p (right panel in Fig. 3b) from the pristine and HA-treated 3R-TaS₂ shows a decreasing intensity at the high binding energy contribution (ca. 168 eV) and a shift to the low binding energy (ca. 0.2 eV) after HA intercalation. The XPS data clearly indicates that there is a surface chemical reaction present; while the XRD proves that HA is fully intercalated.
<figure><img src="image_6.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_9.png" /></figure>
**Fig. 3:** Comparison of XPS spectra pristine and HA-intercalated samples; (a) Ta 4f and (b) S 2P. (c) and (d) are the second-electron cutoff and the UPS spectra of pristine and HA-intercalated TaS<sub>2</sub>.

We pursue the surface analysis by measuring the UV photoelectron spectra of the samples. A first observation is that the secondary electron cutoff of the UPS spectra (Fig. 3c) demonstrates that the work function of 3R-TaS<sub>2</sub> decreases from 4.3 eV, for the pristine air-exposed TaS<sub>2</sub> layer, to 3.9 eV after the intercalation of HA. The drop in work function by 0.4 eV indicates a significant change in the interfacial energetics. The most significant change of the UPS spectra upon HA intercalation is the appearance of the valence features of TaS<sub>2</sub> at 0.5 eV, 1.1 eV and 2.6 eV relative to the Fermi level (HA has a high valence band feature). On the contrary, there is no visible feature in the pristine 3R-TaS<sub>2</sub>, and the spectral intensity is very low and there is almost featureless close to the Fermi level, which is mostly due to the presence of the insulating oxide surface layer with a large energy gap. The surface oxide TaO<sub>x</sub> has the dominant contribution to UPS spectrum of the 3R-TaS<sub>2</sub> surface. Indeed, by zooming in on the energy region close to the Fermi level (inset of Fig. 3d), the Fermi edge of the metallic TaS<sub>2</sub> is visible together with the first band at 0.5 eV on the pristine TaS<sub>2</sub> sample. The removal of the surface oxide by the HA treatment leads to an enhancement of about 50 times of the valence bands in addition to clear valence band features.

Further insight on the structure of HA interacting with exfoliated TaS<sub>2</sub> were obtained from solid-state NMR spectroscopy. Similar results were obtained regardless of the HA-functionalized TaS<sub>2</sub> batch suggesting that the synthesis is reliable and reproducible. <sup>1</sup>H MAS spectrum of TaS<sub>2</sub> alone displays a weak and broad signal centered at 3 ppm that can be tentatively assigned to isolated hydroxyl groups and physisorbed water molecules (Fig 4a bottom). The <sup>1</sup>H MAS spectrum of HA-intercalated TaS<sub>2</sub> (Fig. 4a top) shows stronger signals with a characteristic alkyl chain peak at 1.24 ppm and a shoulder at 7.24 ppm attributable to
ammonium. This assignment is confirmed by signal deconvolution (Fig. S2), with a ratio between the 7.24 and 1.24 signals equal to 0.18 (±0.03), that corresponds to the expected value of 0.2 (3/15, $(NH_3^+)/C_6H_{15}$) for the ammonium species. The $^{13}$C CPMAS and MAS spectra (Fig. 4b) show broad peak characteristic of disordered structures. The alkylamines adopt several different conformations with varying coordination modes. The $^{13}$C CPMAS and MAS spectra are very similar with a good sensitivity of the CP MAS experiment indicating that the amines are rigid in the material $^{27}$. The position of the central $CH_2$ groups of the alkyl chain around 30 ppm indicates that the aliphatic chain of the amines is in the form of rigid pellets. Indeed, methylene C atoms with trans/gauche or gauche-gauche conformations exhibit signals at 29–31 ppm (γ-gauche effects) while for all-trans conformations, signals are observed at 33–34 ppm $^{28}$.

<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 4: a) ¹H MAS spectrum of HA-intercalated TaS₂ and pristine TaS₂; a closer view clearly expresses the fingerprint of the ammonium species, see the deconvoluted spectra (Fig. S2); b) ¹³C CP MAS and ¹³C MAS spectra of HA-intercalated TaS₂.</figcaption></figure>

Finally, we investigate the electrocatalytic performance of the TaS₂ with and without HA intercalation for both hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) in acidic medium. The electrode is prepared in the following way: the TaS₂ powder or the HA-TaS₂ nanoflakes are dispersed in an ethanol-water solution and mixed with 5wt% of Nafion as a binder, followed by a final deposition on a glassy carbon substrate. We have chosen the Nafion as the binder as it is not soluble in water and is also stable in acids. Moreover, it promotes the transport for protons inside the TaS₂ electrode layer, at humid/aqueous conditions. The linear sweep voltammetry visualizes a strong HER activity on the pristine 3R-TaS₂ (Fig. 5a). The potentials are referred to the zero-voltage corresponding to the thermodynamically dictated electronic energy threshold (reversible potential) of the reaction:

$$H^+ + e^- \leftrightarrow H^0 \qquad (1)$$
The zero voltage demarcates the potentials of HER and HOR manifested in reversible hydrogen electrode (RHE). The increase of electronic energy, which corresponds to the application of negative electric potential, beyond a threshold (so called overpotential) prioritizes the reaction (1) from left to right, *i.e.* the HER. The overpotential for our 3R-TaS₂/Nafion electrode is as low as 0.016 V at the onset. We speculate that the negatively charged Nafion might lead to small interfacial potential drop with respect to the electrolyte. So, the overpotential is very small and the HER-driven current rise is remarkable all through -70 mA cm⁻², at -0.08 mV, illustrating a diffusion-free behavior typical for fast proton transport. This impressive performance shows that the 3R-TaS₂/Nafion electrode is competitive with other state-of-the-art electrocatalysts for HER. In contrast, the HA-TaS₂/Nafion electrode with HA intercalation leads to a significantly higher overpotential to observe the onset of HER and a current level of -7 mA cm⁻² at -0.34 V, which is significantly smaller than with the TaS₂/Nafion electrode. Note that the blank current collector (glassy carbon) shows an overpotential of ca. 0.74 V, thus demonstrating that it has no impact on the current observed for the TaS₂-based electrodes deposited on glassy carbon.

The application-relevant steady-state measurements (Fig. 5b and 5c) confirm the increase HER rate on the TaS₂ electrode in comparison with HA intercalated electrode. Importantly, the steady-state measurements allow for the observation of HOR currents corresponding to the proceeding of the reaction (1) running from right to left. The data presentation in Tafel coordinates (Fig. 5c) enables us to investigate in detail the electrocatalytic phenomena of HER and HOR. In contrast to smooth glassy carbon electrode, both TaS₂-based electrodes showed non-monotonous dependence of current density on the overpotential of HER. Specifically, the region of current densities of 0.27 mA cm⁻² (ln(J) ~ -1.3, Fig. 5c) is characterized with the change of the slope. This might illustrate the effect of kinetic limitation by the mass transport on the highly porous electrocatalyst. The low current densities enable the full involvement of the electrocatalyst surface distributed within nanoscale pores, while the
higher current densities are characterized by a limitation of reagent or product transport inside the pores. All electrode systems showed a Tafel slope of ca. 120 mV dec⁻¹. This implies that the rate-determining step in HER on all electrodes of this study is a Volmer step²⁹:

$$H_3O^+ + e^- + M \leftrightarrow M \cdots H^0 + H_2O \quad (2).$$

where M denotes the surface empty site.

Coherently with linear sweep voltammetry measurements, HER showed a higher rate on pristine TaS₂ in comparison with the intercalated TaS₂. Interestingly, HOR current characteristics reveals an insensitivity to the effect of intercalation of HA in the TaS₂ electrodes. Note, however, that the HOR current is much higher than the vanishingly small current level of the glassy carbon collector. Both pristine and intercalated TaS₂-modified electrodes show a change of Tafel slopes from 40 mV dec⁻¹ to 120 mV dec⁻¹ as current densities increase. This corresponds to Volmer step as a rate-determining in HOR (reaction (2) proceeds from right to left)²⁹.

The extrapolation of Tafel slopes (120 mV dec⁻¹) for both HOR and HER yields the interception point corresponding to the exchange current density of HOR/HER – overpotential-free rate of the process, where the rates of both HOR and HER are equal. In order to take into account the effect of the nanoscale roughness of porous electrocatalysts³⁰, we normalized the estimated exchange current densities to the capacitive current densities, which are assumed to represent the electrochemically available surface area (EASA) of the electrode. The HA intercalated TaS₂ electrode shows up to 20 times higher capacitive currents as compared to the pristine material of the same mass load. We identified this difference as due to the exfoliation process taking place upon HA treatment while producing the nanoflakes (Fig. 2b). The exchange current densities estimated from the regions of small overpotentials were 0.041 mA cm⁻² (ln(J₀) ~ -3.18) and 0.033 mA cm⁻² (ln(J₀) ~ -3.39) for pristine and intercalated TaS₂. The exchange current densities estimated from the high overpotentials, featured with porous mass transport limitations, were 0.025 mA cm⁻² (ln(J₀) ~ -3.68) and 0.021 mA cm⁻² (ln(J₀) ~ -3.87)
for pristine and intercalated TaS₂, respectively. The exchange current density for blank glassy carbon is significantly smaller (0.0015 mA cm⁻² (ln(J₀) ~ -6.44)). The kinetic characteristics obtained for TaS₂ are comparable with the rates of HER/HOR on gold (0.027 mA cm⁻² (ln(J₀) ~ -3.75)) and significantly smaller than on platinum (0.78 mA cm⁻² (ln(J₀) ~ -0.23)). The normalized exchanged current densities show a significant poisoning effect of the hexylamine intercalant (Table 1). The rate of HOR/HER on intercalated TaS₂ was more than 20 times smaller in comparison with pristine samples.
<figure><img src="image_12.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_14.png" /></figure>
**Fig. 5.** HER/HOR electrocatalysis on TaS₂. Linear sweep (a, 5 mV s⁻¹) and steady-state (b, waiting time of 5 minutes) voltammograms recorded on blank (■) and film-modified glassy carbon (pristine and intercalated TaS₂ as blue and red, respectively); (c) Tafel plot for the steady-state voltammetry data (filled and hollow symbols refer to HER and HOR, respectively; dashed lines – extrapolated Tafel regions); hydrogen-saturated 0.5 M H₂SO₄.

<table><caption>**Table 1.** Kinetic parameters of HER/HOR on TaS₂.</caption><thead><tr><th rowspan="2">Interface</th><th rowspan="2"></th><th colspan="2">J₀, mA cm⁻²</th><th colspan="2">J₀ / J<sub>capacitive</sub></th></tr><tr><th>Small overpotentials</th><th>High overpotentials</th><th>Small overpotentials</th><th>High overpotentials</th></tr></thead><tbody><tr><td rowspan="2">TaS₂</td><td>Pristine</td><td>0.041</td><td>0.025</td><td>0.041</td><td>0.025</td></tr><tr><td>Intercalated</td><td>0.033</td><td>0.021</td><td>0.0016</td><td>0.0010</td></tr><tr><td colspan="2">Glassy carbon<br>(Current collector)</td><td colspan="4">0.0015</td></tr><tr><td colspan="2">Gold <sup>31</sup></td><td colspan="4">0.027</td></tr><tr><td colspan="2">Platinum <sup>32</sup></td><td colspan="4">0.78</td></tr></tbody></table>
**Discussion.** To improve reaction rates, a critical phenomenon of interest is the removal/reduction/dissolution of the Ta₂O₅ surface oxide by the reaction with HA. Alkyl amines are known to serve as reducing agents for metal oxide nanoparticles ³³, hence we believe that HA reduces TaS₂ to functionalize its surface (Fig. 6a) but also to remove the surface oxide of TaS₂. Both XPS and UPS results are in agreement with this interpretation. Now, the XRD demonstrates the full intercalation of the HA in the TaS₂, hence HA does not only remove the oxide but truly intercalates in the Van der walls gaps of the TaS₂. Understanding the nature of the surface and edge of the TaS₂ nanoflakes are important to explore the electrocatalytic reactions. The UPS reveals that the metallic electronic structure of TaS₂ (high density of states at the Fermi level) is enhanced after HA treatment as the surface oxide is removed. Thus, one could imagine that the electrocatalytic activity would be boosted thanks to available metallic levels on the surface. However, the electrochemical data tells us the opposite. Indeed, a larger overpotential and lower current densities are obtained for the HER upon HA intercalation. It is thus reasonable to think that the HA molecules are also forming sub-monolayer on the surface of the TaS₂ nano-flakes and that its interaction with Ta atoms blocks electrocatalytic reactions.

For instance, it is known that HA is forming a monolayer on various 2D materials by a chemical reaction involving hydroxyl groups contaminating their surface in presence of air and humidity ³⁴. The mechanism proposed involves the basic character of the amine with a proton transfer from the surface hydroxide groups: Ta-OH + H₂N-R → (Ta-O⁻)(+NH₃-R) (See Fig. 6b). NMR spectroscopic measurements agree with this assumption. It is proved that the resulting formed monolayer can remove water molecules from the Van der walls gaps by expressing its hydrophobic character, which in turn suppresses the nanosheet catalytic activity and ion transport towards the HER. The structure of the aliphatic chain of HA in the form of rigid pellets also agreed with the hydrophobic character of the organic layer. The moderate water-blocking property is also demonstrated when similar architectures of graphene oxide is treated with different alkylamine ³⁵. Moreover, the contact angle of HA-treated TaS₂ nanoflakes is
twice of those measured on non-surface modified systems and further increases for longer alkyl chains of the alkylamine <sup>36</sup>.

Apart from that, surface charges will remain at the surface of the structure, which gives that the zeta potential of the intercalated TaS<sub>2</sub> likely decreases. This decrement in surface charges has a detrimental effect on the electron transfer between the surface and the protons <sup>36, 37</sup>; thus, negatively impress the HER activity of the electrocatalyst.

Moreover, the intercalation with HA increases the band gap, meaning that the higher overpotential is needed to drive the reaction of hydrogen reduction <sup>38</sup>. In principle, the HA intercalation will increase the interlayer gap between nanosheets.

<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /></figure>

**Fig. 6.** (a) reductant character of the hexylamine leading to electron transfer and chemical interaction with TaS₂. (b) Basic character of the hexylamine reacting with a hydroxyl surface group on TaS₂. (c) Proton diffusion is promoted in the hydrophilic Van der walls gaps of the TaS₂. (d) hexylamine intercalation of TaS₂ leads to hydrophobic Van der walls gaps that blocks the water diffusion and diminish the transport of proton to catalytic sites.

However, the effect of impurity was not inevitable in this study, as the impurity is detected by XRD spectrum (Fig. 2). We assume that we have some impurity during the synthesis of electrocatalyst, since the materials were transferred to ampoule and later passed through a mesh. These introduces impurities and possible defects to the film, which may impact the performance of electrocatalysts ³⁹, ⁴⁰. The optimized effect of defects might enhance the performance of electrocatalyst, though, as it is already reported in case of transitional metal dichalcogenide HER application ⁴¹. The reason is that the optimized level of defects can offer local higher charge density ⁴². Although there are some reports regarding the one-spot electrodeposition directly on electrode ⁴⁰, the side deposition of species is not inevitable.
**Conclusions.** In summary, 3R-TaS₂ nanosheets and the hexylamine-treated derivative were investigated for hydrogen evolution. Aliphatic intercalant can conduct a high electron donor process of intercalation. XPS and XRD spectroscopy measurements shows that the interlayer distance in nanosheets has been increased as a result of hexylamine intercalation. Polarization curves show us an excellent performance of the pristine 3R-TaS₂ sample, while the intercalated sample reveals a deteriorated performance. The suppressing effect of HA-TaS₂ could be arisen from the i) decrease of the surface charges associated to a decrease of their mobility, ii) water molecule-blocking, and iii) bandgap increment, as post-intercalation effects. Apart from that, the hydrogen oxidation measurements express that both catalysts are quite active for the reverse reaction when hydrogen is oxidized, converted to electricity. While the current density for catalyst are exponentially increases as potential increases, glassy carbon doesn't show any potential dependency and stays at current density of 0 V. Tafel slopes also show the Volmer reaction as the rate determining step for both evolution and oxidation of hydrogen. This work demonstrates TaS₂ nanosheets as versatile electrified interface, which can be used either as efficient HER/HOR electrocatalyst or as an inert current collector.

**Supporting Information**

Cyclic Voltammograms, ¹H MAS

**Acknowledgments**

This work was financially supported by the Swedish Research Council (VR 2016-05990), the Knut and Alice Wallenberg Foundation (KAW 2019.0604; 2021.0195), Karl Erik Önnesjös Foundation, and the Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009-00971).
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