Photocatalytic activity of twist-angle stacked 2D TaS2

The low-cost, efficient photoelectrosensitive electrodes as an alternative to expensive and complex rigid systems are yet in demand for advanced photoresponsive technology. Here, the light-induced efficiency of electrochemically exfoliated TaS2 nanosheets for hydrogen generation catalysis and photodetectors was demonstrated. Mutual twisting of the exfoliated 2H-TaS2 flakes leads to the redistribution of charge density induced by interlayer interaction of the individual nanosheets. External light irradiation on the TaS2 surface influences its conductivity making the material feasible for photoelectrocatalysis and photodetection. The TaS2-based photoelectrocatalyst demonstrates high hydrogen evolution reaction (HER) activity with the onset overpotential below 575 mV vs. reversible hydrogen electrode (RHE). The TaS2-integrated photodetector in the acidic medium represents its broadband response with the highest photoresponsivity (0.62 mA W−1) toward 420 nm light illumination. This finding will pave the way to a new realization of exfoliated twist-angle stacked TaS2 for photo-induced electrochemistry and sensing.


INTRODUCTION
With increasing demands in modern society, the energy crisis and environmental problems are currently in the spotlight. Hydrogen is the main sustainable source of renewable energy and is highly required for advanced energy conversion systems. Recently, photoelectrocatalytic and photoelectrochemical water-splitting methods are an efficient approach for the scalable generation of hydrogen. The concept of light-induced energy production through semiconductor catalysts attracted many research efforts aiming at the development of low-cost, efficient bifunctional materials for hydrogen production and environment-sensing response. The performance of these materials can be tuned while illuminating the light of various wavelengths. The occurred photoinduced electron transfer in the targeted nanosheets-based sensitive material will generate photoelectrochemical water splitting, e.g., faster hydrogen evolution reaction (HER) or broadband light-sensitive detector. These will pave new ways for emerging materials as a cheaper and more productive alternative to most common fossil fuels-based methods of hydrogen production as well as traditional photodetectors.
Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have been demonstrated as promising catalysts for water splitting, hydrogen reduction, and water oxidation 1,2 . The unique chemical and physical properties of 2D TMDs are demonstrated by their small size enriched by the number of edge active sites, band-edge position, quantum confinement effect and thus, exhibit photoinduced catalytic efficiency 3 . TMDs are considered the most promising cost-effective catalysts, and their properties are determined by the TMDs' polymorph type, namely hexagonal 2H or trigonal 1T. It has been reported that stable 2H-MoS 2 shows semiconducting properties; meanwhile, metastable 1T-MoS 2 4 or 1T-WS 2 5 forms demonstrate metallic properties. The metallic phase of a material is capable of a more efficient performance than the semiconducting 2H counterpart due to its enhanced electrical conductivity. The metallic character is associated with undercoordinated chalcogen atoms at the 1010 edge of 2D TMD, providing more efficient adsorption of hydrogen, which is a crucial step for electrochemical water splitting 6 . The MoS 2 and WS 2 exhibit superior catalytic activity and are used as alternatives to Ptbased catalysts [7][8][9] . The WSe 2 is demonstrated as an excellent output stability photodetector due to its high absorption coefficient in the visible and near-infrared regions 10,11 . Other known 2D materials, such as TiSe 2 12-14 , NiSe 2 15,16 , TaSe 2  17 , and  TaS 2 18-20 , are interesting due to their superconductivity, chargedensity wave, and metal-insulator transitions.
Among other 2D TMD materials, tantalum disulfide (TaS 2 ) has been subject to numerous studies due to the variety of the material's unique structural and electronic phases 21 . Its initial metallicity and, thus, electrical conductivity can lead to potential applications such as a light-responsive active electrode in the HER and photosensing. Albeit the study of TaS 2 is mainly focused on the superconductivity [22][23][24] , field emission 25,26 , photo-induced features of the material for HER and photosensing have not been explored yet. Aiming to enhance the photoresponse performance of the TaS 2 , the quality and dimensions of the material must be considered. For instance, bulk TaS 2 lacks exposed active sites that significantly affect the conversion efficiency of the material and narrows its application. Therefore, a thorough cleavage of the van der Waals structure by selecting the most appropriate method is essential to increase the conversion efficiency of TaS 2 .
Several methods have been demonstrated to produce layered TaS 2, such as chemical vapor deposition (CVD) 27 , mechanical cleavages 21 , intercalation 28 , ultrasonication [29][30][31] , or liquid-phase exfoliation 22 . However, the CVD method remains to be improved before it can be used to produce large-domain homogeneous TaS 2 films. The mechanical cleavage is not scalable and undergoes a lack of control over product thickness and size. Alternatively, electrochemical exfoliation (one of the solution-based techniques) is the most convenient, controlled, and straightforward method that can be employed in ambient temperature for large-scale production of 2D TaS 2 . Recently the electrochemical approach has been successfully applied to exfoliate few-layer phosphorene 32 and arsenene 33 and has been pioneered for single-step production of platinoid-decorated phosphorene 34 .
Therefore, in this study, the photoresponse of electrochemically exfoliated TaS 2 nanosheets which have preliminarily been characterized by micro-and spectroscopic techniques (Supplementary Notes 1) was investigated in an effort to develop a lowcost, efficient photosensitive electrode that can replace noble metal-based catalysts 35 (as a photoelectrocatalyst-PEC) and CMOS-based photodetectors 36 (as a photodetector-PD). Pioneered low-potential electrochemical exfoliation of TaS 2 crystals was carried out in a nonaqueous electrolyte medium of tetrabutylammonium hexafluorophosphate (TBAPF 6 ) in N, Ndimethylformamide (DMF). The as-exfoliated 2D TaS 2 nanostructures consist of few-layer nanosheets and nanoparticles. Detailed characterization of bulk and exfoliated TaS 2 was performed through various microscopic and spectroscopic techniques. The photo-induced high HER electrocatalytic activity of the asexfoliated TaS 2 nanostructures with low overpotential (>575 mV and 320 mV) was reported. A room-temperature TaS 2 -based broadband photodetector in the acidic medium of 0.5 M H 2 SO 4 has been presented. The higher electrical conductivity of 2D TaS 2 compared to its bulk form can improve the efficiency of charge distribution in the catalytic/sensing systems and, thus, broaden the material's application area.

RESULTS
Synthesis and characterization of the materials Electrochemical exfoliation of bulk TaS 2 crystal [see scanning electron microscopy (SEM) images in Fig. 1a-c] was performed in anhydrous electrolyte of 0.01 M TBAPF 6 in DMF. Following three main steps of the procedure, namely wetting of the crystal, accumulation (or activation) of the tetrabutylammonium cations (TBA + ), and their intercalation between TaS 2 layers were carried out at −1, −2.5, and −3.8 V (vs. 0.1 M Ag/AgNO 3 ), respectively. The first two steps were completed in 2 min each, followed by 4 h of TBA + -assisted intercalation-delamination process at a potential of −3.8 V. It has to be noted that the duration of the process is defined by the initial size of the TaS 2 crystal and can be terminated when a sufficient amount of the exfoliated material is produced.
After the exfoliation of the TaS 2 crystal, flakes of various sizes and thicknesses were observed in the resulted dispersion based on STEM (Fig. 1d-f) and AFM analysis (Fig. 2). These include TaS 2 nanosheets of lateral size up to 6 μm with a thickness of 30 nm and 2.3 μm nanosheets with 7 nm thickness. (Fig. 2a, b, respectively). In addition, exfoliated material consists of TaS 2 nanoparticles of 100-200 nm lateral size and thickness in the range between 0.5 and 3 nm. (Fig. 2c). The successful exfoliation of the TaS 2 crystal into nanosheets and nanoparticles, as well as their size distribution, were confirmed by the statistical analysis (Supplementary Fig. 1) obtained from STEM images. The results are also in good agreement with high-resolution TEM images (Supplementary Fig. 2) of two types of exfoliated nanostructures with a lateral size of ten-hundreds of nanometers and several micrometers.
TEM and SAED microscopies demonstrate good quality of exfoliated TaS 2 nanosheets ( Fig. 3a- Fig. 3e, f). High-resolution TEM images in Supplementary Fig. 3 also show the formation of round-shaped TaS 2 nanoparticles localized on the surface of the larger flakes. This is consistent with the statistical analysis of particle size distribution ( Supplementary Fig. 1) that confirms the production of two types of TaS 2 nanostructures. Based on the fast Fourier transform (FFT) analysis of TEM images ( Supplementary Fig. 4), the crystal symmetry of TaS 2 is demonstrated as the fringes oriented in a three-fold rotation axis. The d-spacing of the symmetry of the TaS 2 crystal of about 2.9 Å and 2.93 Å defines its (100) planes' orientations. The electron diffraction patterns of a flat area of the TaS 2 nanosheet show this growing direction. Different magnification TEM images of the TaS 2 exhibit in-plane atoms are ordering with the lattice spacing of 0.305 nm. The morphology of the exfoliated material and thus sharply defined layered appearance are caused by the initial structure of the TaS 2 crystal.  (Fig. 4). The value of twist angle of as-exfoliated TaS 2 nanosheets is relatively regular for all analyzed samples (7-10 degrees) and increases between rows: 1 layer < 2 layers < 3 layers. The trigonal arrangement of Ta atoms in Fig. 3e, f corresponds to the standard stacking of S-Ta-S in the unit cell. This direct stacking of each layer via Ta atoms was theoretically proven in a few publications 37,38 ; however, experimentally was confirmed only for 1T-TaS 2 bilayer structure 39 . Based on the density of state calculations, the electronic properties of the 1T-TaS 2 bilayer depend on the twist angles. The increase of both formation energy and bandgap has been theoretically explained due to the interlayer interaction of twisted TaS 2 flakes and the redistribution of charge density induced by the in-plane distortion 38 . This feature can also lead to the increase of the electronic specific-heat coefficient of the randomly twisted TaS 2 nanosheets 22 , establishing the material as a promising candidate for photo-induced applications 40 .
The analysis of the crystal structure and optical characteristics of the bulk and exfoliated TaS 2 is demonstrated in Fig. 5. X-ray diffraction (XRD) was used to verify the successful synthesis of hexagonal 2H-TaS 2 (Supplementary Notes 2, 3). The XRD pattern in Fig. 5a confirmed the P-3m1 hexagonal structure of TaS 2 (PDF: 04-003-4190). No other phases were observed, indicating the purity of the starting material. The sharp peaks in the pattern  ascertaining the high crystallinity of the sample. Similar peaks were observed in the exfoliated material's XRD pattern, albeit broadened and with lower intensity caused by the disruption of initial crystallinity due to the exfoliation procedure. The broadening of the XRD peaks indicates a decrease in crystal size and lattice strain increase. Figure 5b shows the Raman spectra of bulk and as-exfoliated TaS 2 samples. Three main Raman features depicted at 239, 301, and 373 cm −1 from the bulk crystal were assigned to the E 2g , E 1g , and A 1g modes, respectively 28,41 . More prominent peaks of the same modes and their 4 cm −1 redshifts were observed for the exfoliated nanosheets. The shift is attributed to the decrease of the interlayer van der Waals forces and the material's thickness reduction. UV-VIS absorption spectrum of the as-exfoliated TaS 2 nanostructures recorded at room temperature is demonstrated in Fig. 5c. The TaS 2 represents low in-plain reflectivity at high energies (from 400-720 nm) and vice versa at low energies accordingly 42 .
To confirm chemical composition and binding states, we have measured the survey (Supplementary Fig. 6) and high-resolution X-ray photoelectron spectra (HR-XPS) of Ta 4 f and S 2p (Fig. 6). Survey spectra confirmed composition, which was close to TaS 2 for bulk and exfoliated samples as well. Since exfoliated samples were measured on a gold substrate, Au peaks can also be seen in its survey spectrum. HR-XPS Ta 4 f spectra were deconvoluted with two binding states Ta IV (TaS 2 ) and Ta V (Ta 2 O 5 ), with binding energies 22.9 and 26.1 eV, respectively 43 . Although O 2 s at 24.0 eV overlaps with the Ta 4 f, it was not used for deconvolution because its amount was less than 1% due to a low relative sensitivity factor   Table 1. Bulk crystal was covered by 20% of Ta 2 O 5, and after exfoliation, this amount increased to 30%. It has to be noted that TaS 2 is susceptible to oxidation, and this ability is even higher after the exfoliation 43 . Thus, operating the material in an oxygen-free environment and storing it in the dry solvents will increase the stability of asexfoliated TaS 2 .
Photoelectrocatalytic performance of as-exfoliated TaS 2 for HER Light-driven electrocatalysis using semiconducting materials (e.g., TaS 2 ) or their heterostructures is one of the most promising sustainable approaches for hydrogen generation by water splitting [44][45][46] . Even though a thermal treatment or catalyst support texturization of the catalytic films is used to increase the number of active centers of the TaS 2 flakes, these methods are timeconsuming and require a high-temperature and inert gas atmosphere 47 . The light-induced electrochemistry (photoelectrochemistry) can be a simple approach to enhance the HER activity of the TaS 2 nanosheets. To the best of our knowledge, the photoelectrocatalytic activity of TaS 2 has not been explored to date.
Herein, the photoelectrocatalytic performance of electrochemically exfoliated TaS 2 for HER was explored (see Supplementary Notes 4). The three-electrode linear sweep voltammetry (LSV) was performed in the 0.5 M H 2 SO 4 employing a few-layer TaS 2 deposited on a glassy carbon as a working electrode. The measurements have been repeated in the light mode for four different wavelengths (420, 532, 630, and 720 nm). In addition, the results were compared with the dark mode for as-exfoliated TaS 2 nanosheets of mixed composition. The mixed composition consists of nanosheets of various sizes and nanoparticles, as was confirmed by the AFM, STEM, and statistical analysis. Few-layer TaS 2 samples of a smaller lateral size (~30-40 nm, Supplementary  Fig. 7) were obtained as follows: first, the TaS 2 dispersion was centrifuged at 3500 rpm for 10 min; second, the collected supernatant was centrifuged at 10,000 rpm for 10 min, and the resulting precipitate was used for the test.
As shown in Fig. 7a, b, the HER for initial as-exfoliated TaS 2 starts at a potential of 575 mV (vs. RHE) under dark mode at a current density of 10 mA cm −2 . Under the illumination light of different wavelengths, the overpotential shifts towards lower potentials, specifically: 537, 549, 531, and 454 mV when blue, green, red, and far-red light-emitting diodes (LEDs) are implemented, respectively. This behavior is attributed to the carrier increment generated in the TaS 2 nanosheets with light irradiation enhanced by the intrinsic defect (wrinkles) and mutual twisting of the individual layers. The current density of the TaS 2 catalyst under dark decreases after 100 cycles ( Supplementary Fig. 8), demonstrating a slow degradation of the as-investigated catalyst. It has to be noted that the onset overpotential for the TaS 2 catalyst based on flakes of smaller size was significantly reduced to~320 mV (Fig. 7c). Considering the influence of the light on the as-exfoliated samples, the~100 mV decrement of the overpotential is expected that will make a fewlayer TaS 2 a superior HER photoelectrocatalyst. This study on bare TaS 2 catalysts can compete with recently reported results 48 , where the performance of TaS 2 was enhanced by the incorporation of conductive films 49 or thermal treatment in the H 2 -environment 47 .  Furthermore, thoroughly prepared catalysts under light illumination will outperform the HER activity of commercial Pt/C electrode (~40 mV), providing a cheaper alternative.

TaS 2 -integrated photodetector
Solution-based PDs offer unique advantages over standard photodetectors in the form of a simple and easy device manufacturing process. Recently 2D nanomaterials have demonstrated their feasibility for PDs ranging from visible to THz; however, this is not inherent to one material 42,50,51 . Therefore, a broadband photodetection capability of materials is highly demanded to its wide application possibility. The intrinsic parameters of the material's light sensitivity and response can be tuned by changing the applied voltage, analyte concentration, and power of the illuminated light. Herein, the photoelectrochemical response of TaS 2 in a 0.5 M H 2 SO 4 acidic medium was measured in a three-electrode system at ambient temperature. The TaS 2 -integrated PD was tested under four illumination wavelengths (420, 520, 630, and 720 nm) with a scanning speed of 10 mV·s −1 . The procedure of the electrode preparation and photoelectrochemical measurements are described in Supplementary Notes 5.
The results illustrating a steady response toward different illumination wavelengths are depicted in Fig. 8. As shown in Fig.  8a, the power-dependent photocurrent measurement was achieved by using chronoamperometry in 0.5 M H 2 SO 4 at 0.5 V against saturated calomel electrode (SCE) under the illumination of a 420 nm blue LED light source. The response of the TaS 2 -based electrode toward blue illumination was the highest in comparison with other LEDs. The response toward green illumination (Fig. 8b) was decreased and slightly increased after applying red (Fig. 8c) and far-red (Fig. 8d) light sources. The most prominent response of the electrochemically exfoliated few-layer TaS 2 is demonstrated under blue illumination and well consistent with the light absorbance of the material (Fig. 5d). The near-linear increment in current density with the increase of the illumination power from 100 to 1200 mW was illustrated ( Supplementary Fig. 9a) for all lights. Additionally, I-V measurements were performed using LSV with a scanning speed of 20 mV s −1 and under continuous blue light illumination with the power of 600 mW (Supplementary Fig.  9b). The results display an increment in photocurrent with the increase of applied voltage, demonstrating the maximum photoelectrochemical performance of the TaS 2 nanosheets in 0.5 M H 2 SO 4 at around −0.6 V vs. SCE. As the intensity increases, the current response jumps to a higher value corresponding to a higher conductance, indicating that the separation of electron-hall pairs is triggered.
The photodetection potential of the material has been assessed in more detail by calculating a few essential parameters. At first, the photoresponsivity (R ph ) of the TaS 2 -integrated PD has been calculated using the following equation: R ph = ΔI/P, where ΔI represents the difference in photocurrent density between the dark and photocurrent, and P is the irradiation power intensity per unit area. The calculated photoresponse demonstrated the highest responsivity of about 0.62 mA·W −1 under a 420 nm blue LED source illumination at 0.5 V applied potential (vs. SCE). Figure  9a highlighted the photoresponsivity of the devices concerning the power for different illumination wavelengths. Another important parameter for the evolution of photodetection capability is specific detectivity; it has been determined accordingly 52 In the above equation, A, B, and NEP stand for the active area, measured bandwidth, and noise equivalent power, respectively. NEP is defined as the input signal power resulting in a signal-tonoise ratio (S/R) of 1 in a 1 Hz output bandwidth. Furthermore, in the equation form, NEP can be presented as 52,53 Here, R ph and I N represent the photoresponsivity of the PD and the noise current, respectively. In addition, the noise, which is associated with the dark current (I D ) of the PD current, can be ascribed as I N 2 = 2eI D B, where e represents the electronic charge 42 . The calculated specific detectivity of our PD devices of about 1.28 × 10 9 cm·Hz 1/2 W −1 (Fig. 9b).
Prepared TaS 2 -based PD exhibited a broadband photodetection capability with the highest photoresponsivity (0.62 mA W −1 ) and specific detectivity (1.28 × 10 9 cm Hz 1/2 W −1 ) toward 420 nm light illumination, which is nearly double compared with 532, 630, and 720 nm illumination wavelengths. This photoresponsivity value is higher than the response of solution-based photodetectors integrated with other TMDs (Supplementary Table 2). Our results demonstrate the versatility of 2H-TaS 2 -based PD, which exhibits metal-like characteristics coupled with broadband light absorption and thus emission due to energy-separated bands. The original metallicity of the TaS 2 that is assisted by undercoordinated atoms at edge sites is enhanced by the random twisting between the TaS 2 monolayers, which form a rich homogeneous interface with enhanced conductivity 22 . This consequently increases the electron transfer in the "TaS 2 electrode/0.5 M H 2 SO 4 electrolyte" system and thus improves the catalytic performance of the material, establishing it favorable for electrocatalysis 1,2,43 .

DISCUSSION
The light-induced efficiency of electrochemically exfoliated TaS 2 for photoelectrocatalysis and photodetector performance has been demonstrated. A well-controlled exfoliation process that is based on the TBA + -cation intercalation between the TaS 2 interlayers was performed in the nonaqueous DMF medium applying a potential of −3.8 V. Few-layer TaS 2 nanosheets of different lateral sizes, namely, 10-100 nm and 1-30 μm, were identified and characterized by numerous characterization and microscopy techniques. It was shown that exfoliated TaS 2 nanosheets possess a trigonal prismatic structure (2H type, hexagonal) and, thus, metallic character. The metallicity of the 2H-TaS 2 nanosheets was enhanced by the twisting of the individual flakes and their mutual interaction, causing chargedensity redistribution. This ability was further increased by the external illumination of the light of the following wavelengths: 420 nm (blue), 532 nm (green), 630 nm (red), and 720 nm (far-red). Based on the above, TaS 2 nanosheets were tested as PEC for HER and PD of visible light. The as-exfoliated TaS 2 -based catalyst, under light illumination, demonstrated high HER catalytic activity with the overpotential for -10 mA·cm −2 below 575 mV, which is 100 mV lower than during dark mode. Additionally, it was proven that the accurate preparation of the TaS 2 -based catalyst via sample centrifugation can significantly enhance the catalytic performance of the material. Solution-processed TaS 2 -enabled PD exhibited broadband light sensitivity in the visible range. The highest photoresponsivity (0.62 mA W −1 ) and specific detectivity (1.28 × 10 9 cm Hz 1/2 W −1 ) were observed under blue light illumination, nearly double compared to green, red, and far-red lights. These findings can potentially ignite multiple insights of 2H-TaS 2 in photo-induced electrochemistry and (nano)optoelectronics.

Synthesis and crystal growth
TaS 2 crystals were grown by the vapor transport method in quartz glass ampoule using iodine as a transport medium. For the synthesis were used tantalum (powder-200 mesh; 99.9%) and sulfur (99.9999%, granules) corresponding to 10 g of TaS 2 were placed together with 250 mg of iodine (granules, 99.999%) in quartz glass ampoule (35 × 180 mm) and melt sealed under high vacuum (1 × 10 −3 Pa). The ampoule was first heated at 600°C for 50 h, following 50 h of treatment at 800°C and 1000°C. The vapor transport was performed at 1000°C for the hot zone with the TaS 2 charge and 950°C for the cold crystal growth zone for seven days. Formed hexagonal crystals with sizes up to 20 mm were separated from the ampoule in the glovebox.

Electrochemical exfoliation
The electrochemical exfoliation of TaS 2 crystal was carried out in the organic-based electrolyte composed of 0.01 M TBAPF 6 in DMF. The procedure was conducted in the four-port electrochemical cell equipped with a reference (0.1 M Ag/AgNO 3 in acetonitrile), counter (platinum), and working (TaS 2 crystal) electrodes as well as argon purging. Three different potentials were applied to the working electrode to initiate its wetting at −1 V for 120 s, activate the tetrabutylammonium cations (TBA + ) at −2.5 V for 120 s, and finally stimulate cations' intercalation between TaS 2 interlayers at −3.8 V for several hours. As-exfoliated TaS 2 precipitate was collected, ultrasonicated for 30 min, and then washed in several polar organic solvents (acetonitrile, methanol, and DMF) using vacuum filtration. The exfoliated TaS 2 was filtered through the cellulose membrane, redispersed, and stored in the dried DMF solution.

Electrocatalytic, photoelectrocatalytic measurements, and characterization
More details on the exploration of electrocatalytic and photoelectrocatalytic measurements and material characterization methods are given in the Supplementary