Iontronic and electrochemical investigations of 2D tellurene in aqueous electrolytes

The remarkable successes of graphene have sparked increasing interest in elemental two‐dimensional (2D) materials, also referred to as Xenes. Due to their chemical simplicity and appealing physiochemical properties, Xenes have shown particular potential for numerous (opto) electronic, iontronic, and energy applications. Among them, layered α‐phase tellurene has demonstrated the most promise, thanks to the recent successes in the chemical synthesis of highly crystalline 2D tellurene. However, the general electronic and electrochemical properties of tellurene in electrolyte systems remain ambiguous, hindering their further development. In this work, we studied the electrostatic gating, electrocatalysis, and electrochemical stability of tellurene in electrolyte systems. Our results show that tellurene obtained from both hydrothermal and chemical vapor deposition methods, two mainstream synthetic approaches for Xenes, demonstrates thickness‐dependent ambipolar transport with high hole mobility and stability in both aqueous electrolytes and ionic liquids. More importantly, the electrochemical properties of tellurene are investigated via the emerging on‐chip electrochemistry. Pristine tellurene demonstrates hydrogen evolution reaction with low Tafel slopes and remarkable electrochemical stability in acidic electrolytes over a large potential window. Our study provides a comprehensive understanding of the iontronic and electrochemical properties of tellurene, paving the way for the broad adoption of Xenes in sensors, synaptic devices, and electrocatalysis.

2][3][4][5][6][7] Among them, α-phase tellurene attracts intensive research interest due to its appealing properties, such as good stability, high hole mobility, and, more importantly, scalable preparation. 8In particular, the high hole mobility of tellurene is highly desirable due to the general scarcity of p-type 2D semiconductors. 9The evergrowing research enthusiasm in tellurene is primarily driven by synthetic successes in the last few years.4][15] Specifically, it shows intriguing properties as good photodetection capacity and stability in various solutions, like HCl and NaCl solutions. 168][19] In typical iontronic devices, the charge transport of the semiconductors is modulated via the ionic gate in the electrolyte solution, where a large capacitance can be achieved based on the electrical double layer capacitor (EDLC) formed at the semiconductor-electrolyte interface (SEI). 20,21Therefore, an ionic transistor (or EDLC transistor) enables a low-voltage operation and strong electrostatic modulation. 22The advances in iontronic studies promote diver applications such as synaptic and neuromorphic devices 23,24 and bioelectronic sensors. 18,19,25However, the iontronic properties of tellurene in electrolyte systems, or elemental 2D materials, in general, are not well understood, which requires further study and clarification.
Moreover, elemental 2D materials like tellurene are gaining increasing interest in energy applications such as electrocatalysis, 15,26,27 battery, 13,28 and photocatalysis. 14or instance, based on previous theoretical calculations, tellurene can transfer electrons isotopically at high mobility, assisted by the lone-pair electron delocalization, which can facilitate the charge transfer in the catalysis process and is beneficial for electrochemical kinetics. 29esides, it is easy for tellurene to form Te-metal interaction for site immobilization, which makes it suitable to composite with other noble metals and form a high-performance electrocatalyst. 26Despite some early research on the redox properties of bulk tellurium and its compounds, [30][31][32] the general electrochemical properties of tellurene nanomaterials, especially its stability, remain elusive due to the difficulty in forming efficient contact by physical stacking on conventional electrodes, such as glass carbon or carbon cloth.On the other hand, the recently developed on-chip electrochemistry has been a powerful platform for probing the electrochemical properties of single-entity nanostructures at the microscopic level. 33More importantly, the on-chip electrocatalytic microdevices (OCEMs) are particularly effective in characterizing the intrinsic electronic and electrochemical properties of SEI of 2D semiconductors by forming ideal electrical contact to eliminate the influence from the Schottky junction. 34,51n this study, we systematically investigated the iontronic and electrochemical properties of α-phase 2D tellurene synthesized by both hydrothermal and chemical vapor deposition (CVD) methods via the on-chip study of individual nanoflakes in aqueous electrolytes.Tellurene nanoflakes prepared by hydrothermal and CVD methods show good crystallinity, which is characterized by the high-resolution transmission electron microscope (HRTEM).All tellurene devices exhibit clear thickness-dependent ambipolar transport characteristics with high hole mobility and operation stability in aqueous electrolytes.Besides, tellurene has demonstrated promising performance in electrocatalytic hydrogen evolution reaction (HER).Tellurene from the hydrothermal method exhibits an overpotential of 435 mV and a Tafel slope of 53 mV/dec, while tellurene from CVD possesses a relatively inferior performance.More importantly, the electrochemical stability of tellurene in aqueous electrolytes is understood by using in-situ characterization in an OCEM setup.Our study paves the way for the broad adoption of tellurene in further electronic, iontronic, and energy applications.

| Synthesis of tellurene via the hydrothermal method
Tellurene nanoflakes were synthesized via the hydrothermal method according to the previously reported procedure with slight modification. 35In a typical synthesis process, 3 g of PVP and 0.092 g of (92 mg) Na 2 TeO 3 were first dissolved in 32 mL of DI water at room temperature under magnetic stirring to form a homogeneous solution.Then, 3.32 mL of NH 3 •H 2 O and 1.676 mL of N 2 H 4 •H 2 O were added to the above clear solution.After being stirred for 10 min, the resulting solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 180°C for 10 h in an oven before cooling down to room temperature.The resulting gray products were then centrifugated at 3000 r/min for 5 min and washed three times with DI water to remove ions and PVP residual at the surface of the products.Note that the thickness of tellurene can be roughly reduced by decreasing the reaction time at the heating procedure and utilizing running water to cool down the autoclave to room temperature immediately.

| Synthesis of tellurene via the CVD method
One hundred milligram of TeO 2 powder was put on a quartz boat and then placed in the center of the heating zone of the tube furnace.The SiO 2 /Si substrate was located downstream and out of the heating zone.Before reaction, the tube was pumped to 10 −2 Torr to expel the air and repour with Ar/H 2 hybrid gas (Ar:H 2 is 9:1 by volume ratio) three times.During the reaction, a constant mixture of Ar (90 sccm) and H 2 (10 sccm) was used, and the pressure of the tube was kept at 1 atmosphere.The temperature gradually rose to 780°C with a ramp rate of 20°C/min and was maintained for 30 min to make the reaction proceed.After the reaction, the furnace was cooled down to room temperature naturally.

| Sample transfer and device fabrication
The washed tellurene nanoflakes via the hydrothermal method were suspended in the solvent consisting of water and ethanol.Then, tellurene nanoflakes were then deposited onto silicon wafers with 285 nm SiO 2 layers via drop-casting.Suitable individual tellurene nanoflakes were picked under the optical microscope (OM), and the target chip was spin-coated with poly(methyl methacrylate) (PMMA).The device was fabricated by the standard e-beam lithography (EBL) procedure.The chip bearing the tellurene samples is first coated with PMMA (A8, kayakuAM) resist and baked at 120°C for 15 min.EBL is carried out on a TESCAN VEGA3 LMH 119-0312 EBL with optimized beam current, dosage, and spacing.
Fifty-nanometer gold is deposited by thermal evaporation (PVD 75; Kurt.J. Lesker), followed by the lift-off process and drying in argon.To reduce the Schottky contact resistance and ensure good contact, high-work function gold was chosen as the contact metal.Besides, tellurene prepared via the CVD method was transferred onto SiO 2 /Si by the dry-transfer method with the assistance of polydimethylsiloxane (PDMS). 36The subsequent device fabrication is identical to that of h-Te.

| Materials characterization
The morphology of tellurene was observed in an OM (LV 150N).The Raman spectra were characterized at room temperature by Renishaw inViaTM with a laser of 532 nm wavelength.HRTEM imaging was performed using a Tecnai F20 TEM (TF20).Atomic force microscopy (AFM) tests were conducted using a Bruker's Dimension Icon in trapping mode.

| Electrical, electrochemical tests, and in-situ optical characterization
Electrical measurements of tellurene transistors under back-gate voltage modulation involving current-voltage, transfer, and output curve characteristics were carried out inside a Lakeshore probe station with a B2902A source meter (keysight) under ambient conditions.The carrier mobility of the field effect transistor can be extracted from the transfer curves corresponding to the formula: where g m , L, W, and C g are the transconductance, channel length, channel width, and SiO 2 capacitance or EDL capacitance, respectively.The iontronic and electrochemical test was carried out on 2D tellurene devices.The tellurene devices were passivated with an ~1 μm thick PMMA film, followed by the opening of the reaction windows by EBL to expose the channel area/working electrode.For liquid-gate modulation and the electrochemical/in-situ transport measurement, a four-electrode microcell setup was adopted.Specifically, two electrodes were connected to the tellurene nanoflakes as the drain and source to collect the transport signals, while a graphite rod and an Ag/AgCl microreference electrode were used as the counter and reference electrodes, respectively.The 0.5 mol/L H 2 SO 4 solution or DI water, 0.1 mol/L KCl solution, PBS (10×, pH = 7.2), and ionic liquid (diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide) were dropped on the chip to serve as the electrolyte solution.Alternatively, for the sole electrochemical test, a three-electrode microcell setup was used, where only one electrode was connected to the tellurene nanoflakes, severing as the working electrode.The HER reaction current density was obtained by dividing the electrochemical current by the area of the exposing window.The electrochemical potential was referenced to the reversible hydrogen electrode (RHE), given by where 0.219 was obtained by measuring against a hydrogen reference electrode (eDAQ, ET070).
In-situ optical characterizations were carried out in a similar setup where the OM was used to characterize the morphology change of tellurene nanoflakes during the electrochemical measurement.

| Preparation and characterization of tellurene by hydrothermal and CVD methods
Hydrothermal synthesis and CVD, two mainstream methods, were used to prepare the α-phase layered tellurene nanoflakes.While the hydrothermal method produces high-quality suspensions of 2D tellurene nanosheets, the extensive usage of surfactants is particularly undesirable in device fabrication and applications.On the other hand, single-crystalline tellurene nanoflakes can be directly synthesized by CVD on dielectric substrates, such as SiO 2 11 and mica, 37 which is desirable for microdevice fabrication. 38However, the resulting tellurene nanoflakes are generally quite thick (>100 nm) and additional transfer steps are necessary for further processing.The ideal synthetic approaches of 2D tellurene remain an open question.Considering the advantages and disadvantages of hydrothermal and CVD methods, we here investigate how synthetic strategy affects tellurene's iontronic and electrochemical properties.In this regard, we first utilized tellurene nanoflakes from the hydrothermal method based on previously reported classic synthetic routes. 10,35The as-prepared tellurene nanoflakes suspended in solution, denoted as h-Te, can be readily deposited onto a Si substrate with a 285 nm SiO 2 dielectric layer, which also serves as the back gate to modulate the tellurene channel.The preparation procedure and the OM image of h-Te are shown in Figure 1A.The as-prepared tellurene nanoflakes possess a trapezoidal shape with a length of 10-50 μm and a width of a few micrometers.It is also noted that there are some tellurene nanowires forming, which are a typical byproduct of the hydrothermal process and can be effectively removed via the centrifugation procedure. 35During the hydrothermal process, tellurium atoms are first connected through covalent bonds to form helical tellurium chains, which are bundled together with adjacent parallel chains in the [0001] direction to form the layered structure (Figure 1B). Figure 1C,D shows a typical HRTEM image of the as-prepared h-Te flake.From the atomically resolved TEM image (Figure 1D), the helical chains can be clearly identified.The interlayer spacing is ~2.2 and 6 Å, corresponding to the (12 ̅ 10) and (0001) plane.The Raman spectrum of tellurene nanoflakes is shown in Figure 1E.Three distinct Raman peaks located at 93.4, 120.8, and 142.4 cm −1 correspond to the E 1 , A 1 , and E 2 modes of α-phase tellurene, respectively.The TEM and Raman characterizations are consistent with previous reports of solution-prepared tellurene, confirming the successful preparation of tellurene nanoflakes. 10,35,39The X-ray diffraction characterization of h-Te is demonstrated in Supporting Information: Figure S1.The discrete diffraction peaks prove the high crystallinity of tellurene nanoflakes.In addition, the as-prepared tellurene nanoflakes typically possess various thicknesses, ranging from 10 to 90 nm, as shown in Supporting Information: Figure S2.
On the other hand, the CVD synthesis of tellurene has recently been reported, showing good crystallinity with fewer surface defects and direct growth on the SiO 2 and mica. 11,40As illustrated in Supporting Information: Figure S3A, we also utilize the CVD method to prepare α-tellurene nanoflakes, denoted as c-Te.Despite a different synthetic route, c-Te exhibits identical morphologies (Supporting Information: Figure S3B), crystal structures (Supporting Information: Figure S3C,D), optical properties (Supporting Information: Figure S3E), and high crystallinity (Supporting Information: Figure S4) to h-Te.However, the thickness of c-Te nanoflakes is generally much thicker compared with h-Te, with thickness exceeding 100 nm (Supporting Information: Figure S5A,B), and requires additional transfer steps for device fabrications (see Section 2.4).

| Iontronic properties of h-Te and c-Te
The transport properties of the obtained h-Te nanoflakes are first investigated in a standard back-gate configuration on the SiO 2 /Si substrate, as shown in Figure 2A,B.Au (60 nm) is chosen as the contact metal due to its highwork function, which can reduce the contact barrier with the p-type semiconductor. 39Figure 2C shows the typical output curves of h-Te-based devices.Notably, the output curve exhibits linear behavior within relatively low drain-source bias (V ds ) under back-gate modulation ranging from −80 to 80 V, suggesting a close-to-ideal ohmic contact.We then investigated h-Te's transfer characteristics and its thickness dependence, which is an essential characteristic of 2D Te-based field-effect transistors (FETs). 8,17Notably, the thickness of h-Te nanoflakes can be controlled by the heating and cooling processes in hydrothermal synthesis.Thin h-Te nanoflakes with thicknesses of 12-20 nm can be obtained via short heating and rapid cooling processes, as shown in Figure S2A-C.The transfer curves of devices based on typical thin (14.3 nm) and thick (28.2 nm) h-Te nanoflakes are shown in Figure 2D.Both devices show typical unipolar p-type transport under back-gate modulation.Specifically, the device based on h-Te-28.2nm shows a low on/off ratio of ~10, which can be attributed to the thickness-dependent bandgap 11 as well as the thickness exceeding the maximum depletion width. 10The device based on h-Te-14.3nm possesses a higher on/off ratio exceeding 10 2 , which is comparable to the literature report of tellurene with similar thickness. 10,17Additionally, the field-effect hole mobility of h-Te-28.2nm is 768 cm 2 /(V•s), which is higher than most 2D semiconductors, such as black phosphorene, 41 transition metal dichalcogenides (TMDs), 42,43 and so on.By contrast, the thinner h-Te-14.3nm exhibits a smaller carrier mobility of 209 cm 2 /(V•s).The lower carrier mobility in thinner tellurene can be attributed to the reason that thinner tellurene is more susceptible to surface charge impurity and scattering. 10,17e further investigate the iontronic characteristics of h-Te in various aqueous electrolyte systems.Compared with back-gate modulation, ionic gating is based on the electrical double-layer capacitance at the SEI in electrolyte solution 44,45 or ion liquid. 46Due to the exceptional high field generated by the electrical double layer at the SEI, ionic gating can achieve ultrahigh electrostatic doping, much exceeding the conventional back gate modulation.As shown in Figure S6A, a reference is used to regulate the interfacial gate potential and avoid polarization at the SEI.The iontronic characteristics and thickness dependence of h-Te nanoflakes of various thicknesses (characterized by the atomic force microscope in Figure S2A-F) were tested in 0.5 mol/L H 2 SO 4 and are plotted in Figure 2E.The device performance can be categorized into two groups based on the thickness of Te nanoflakes.For both thick (28.2, 31.1, and 42.3 nm) and thin groups (14.3, 18.7, and 19.9 nm), clear ambipolar charge transport is observed in all h-Te nanoflakes with ionic gate modulation, in contrast to the unipolar p-type transport observed in back gate modulation tellurene (Figure 2D).Additionally, the low on/off ratio with high off-current is observed for all devices.However, the thick ones generally possess higher saturation current and relatively low on/off ratio, ~10 2 and ~10 for hole conduction and electron transport, respectively.In contrast, the thin ones possess a higher on/off ratio, exceeding 10 3 for hole conduction and 10 for electron transport.The discrepancy of saturated current as well as the on/off ratio between the two groups of h-Te can mainly be attributed to the thicknessdependent bandgap considering the effective modulation of ionic gating. 11,47Additionally, the stability of tellurene nanoflakes in ionic gating modulation is investigated.The comparisons of iontronic characteristics of h-Te at the initial test and test after 30 cycles are demonstrated in Supporting Information: Figure S6B.Degradation in the electrical performance is negligible, suggesting good operational stability of the h-Te transistor in the aqueous electrolyte under a moderate potential window.Besides, there are no morphology changes in the OM after the test (Figure 2F), which is consistent with the identical Raman spectra before and after the test (Figure 2G).
To further study the chemical and iontronic stability of tellurene devices in electrolytes, we investigated the transport properties of h-Te in some widely used electrolyte systems, including DI water, 0.1 mol/L KCl solution, PBS (10X, pH = 7.2), and ionic liquid (diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide).The transfer curves of the device based on an h-Te-28.2nm nanoflake in the different electrolytes are shown in Supporting Information: Figure S6C,F.It can be found that the h-Te device exhibits similar stable ambipolar characteristics in all aqueous electrolytes and ionic liquids.In all aqueous electrolytes, h-Te possesses on/off ratios exceeding 10 2 and 10 for hole and electron transport, respectively.In contrast, h-Te shows a similar on/off ratio of 10 2 for hole transport but a negligible n-type response for electron transport in ionic liquid, which can be attributed to different capacitances between aqueous electrolytes and ionic liquid related to the size of positive ions. 48Besides, the h-Te devices remain chemically and operationally stable after multiple tests in neutral to acidic electrolyte systems and ionic liquids.However, in alkaline solutions (0.01 mol/L KOH or NaOH), h-Te devices degraded rapidly, which suggests an intrinsic electrochemical instability of tellurene in the alkaline solution. 49oreover, the stability of tellurene is investigated.We compared the electronic and iontronic properties of fresh h-Te and h-Te devices after 1 month and found no obvious performance degradation (Supporting Information: Figure S7).There is also no change in the Raman characterization (Supporting Information: Figure S8).
As a comparison, the transport properties of c-Te were investigated both in back gate modulation and electrolytes, as shown in Supporting Information: Figure S9.Compared with the relatively thin h-Te nanoflakes, c-Te nanoflakes exceed 100 nm in thickness due to synthetic limitations.c-Te exhibits a weak p-type dominated gate response with an on/off ratio smaller than 10 for hole transport in both back-gate modulation and ionic gating modulation in the 0.5 mol/L H 2 SO 4 electrolyte, as shown in Supporting Information: Figure S9B,C, respectively.Additionally, ambipolar transport with suppressed electron transport was observed in ionic gate modulation.Similar to the thick h-Te nanoflakes in Figure 2D, the weak gate modulation is attributed to the large thickness of c-Te nanoflakes.Above all, both h-Te and c-Te demonstrate consistent thickness-dependent ambipolar transport with good stability in acidic aqueous electrolytes, which is promising for synaptic devices and other electronic applications in liquid environments.

| Electrochemical and in-situ transport measurements of h-Te and c-Te via OCEMs
2D tellurene has also attracted considerable interest as a potential nonnoble-metal electrocatalyst for electrocatalytic water splitting due to its many unique characteristics. 26,27,50However, the conventional electrochemical methods based on the macroscopic electrode may not be suitable for such investigation due to the notorious difficulty in forming efficient electrical contact with 2D semiconductors.3][54] Figure 3A shows the schematic image of the OCEM setup where graphite rod, Ag/AgCl, and Au-contacted tellurene nanoflake were used as the counter, reference, and working electrodes, respectively.A reaction window was selectively opened on the PMMA insulating layer where the tellurene nanoflake was exposed to the electrolytes (0.5 mol/L H 2 SO 4 ).The optical images of a typical OCEM device based on h-Te are shown in Figure 3B.The linear sweep voltammetry (LSV) curves are displayed in Figure 3C.
The HER onset potential of h-Te is 435 mV verse the RHE at the current density of 10 mA/cm 2 .As shown in Figure 3D, h-Te displays a Tafel slope of 53 mV/dec, which is superior to that of many pristine 2D catalysts, such as MoS 2 (127 mV/dec), MoP (68 mV/dec), 52 PtSe 2 (126 mV/dec), 55 WSe 2 (108 mV/dec), 33 and WTe 2 (145 mV/dec). 56Compared with h-Te, c-Te has an inferior electrocatalysis performance, which is demonstrated in both onset potential and Tafel slope: onset potential of 476 mV versus RHE and a Tafel slope of 66 mV/dec.The superior HER performance of h-Te is attributed to the abundant surface defects as a result of the involvement of surfactant (PVP) during the wetchemical growth, 10,57 which may serve as the active sites for HER reaction.In comparison, c-Te samples possess better crystallinity on the surface, thereby with fewer surface defects and active sites. 11urthermore, in-situ transport measurement was used to investigate the electrochemical stability of tellurene during the HER process.Typical HER characteristic can be observed with an onset potential at 435 mV versus RHE, as shown in the red polarization curve in Figure 4A.The in-situ transport curve (black curve in Figure 4B) shows clear n-type gate modulation under reductive electrochemical potential with the minimal conductance at 0.17 V versus RHE.Tellurene shows superior charge transport properties, operating at saturated conductance under the gate modulation from the entire reduction potential, making it a promising candidate or supporting material for electrocatalytic HER. 26 However, when the potential exceeds −0.58 V versus RHE, there is an abnormal fluctuation in the polarization curves along with an abrupt drop in the conductance toward zero, suggesting instability of the tellurene under high reductive potentials.It is noted that such potential exhibits small variation across multiple devices, ranging from −0.58 to −0.62 V. OM image and Raman characterization were used to investigate the origin of the disappearance of channel conductance (Figure 4C,D).As shown in Figure 4C, the tellurene exposed to the electrolyte degraded significantly after the test, with no morphology change in the area covered by PMMA.The Raman characterization confirms that the residue after degradation is still pure α-tellurene without additional solid substances on the substrate.Additionally, c-Te samples exhibit similar instability in high reductive potentials, as shown in Supporting Information: Figure S10.To further clarify the electrochemical degradation process of tellurene, an in-situ optical characterization was adopted to document the degradation process of tellurene nanoflakes during the HER test at high reductive potentials, as shown in Supporting Information: Figure S11.The h-Te flake showed no morphology change until reaching ~−0.6 V versus RHE, when the thinning effect started according to the contrast change in the optical images, which is consistent with the in-situ transport in Figure 4B.Raman characterization confirms that the residue after the thinning process is pure tellurene without other solid-state products.The thinning process of tellurene under highly reductive potentials is similar to the thinning effect observed in previous observations of tellurene degradation under light illumination. 582]59 Finally, we further investigate the electrochemical stability of tellurene in the oxidation potential range.As shown in Figure 4E, when the potential is sweeping toward the oxidation range, tellurene devices exhibit stable p-type modulation without any electrochemical signals.However, when the oxidation potential rises to ~+0.48 V (vs.RHE), the conductance of tellurene exhibits an abrupt drop toward zero, similar to the conductance drop at a high reduction potential (Figure 4B).Interestingly, two distinct oxidation peaks emerge in the electrochemical signals, suggesting rapid oxidization of tellurene at a relatively high oxidation potential (~+0.48 and ~+0.55 V vs. RHE), which is confirmed by the severe degradation observed in the OM (Figure 4F).The residues in Figure 4F are still tellurene based on the Raman characterization (Supporting Information: Figure S12).Due to the rapid oxidation process and the minimal chemical products in OCEMs, 53 the direct characterization of the oxidized substance is difficult.We attribute the two oxidation peaks to the oxidation of 2D tellurene to Te 2+ and Te 4+ , which occurs at +0.4 V (vs.SCE) and +0.548 V (vs.,60

| CONCLUSION
In this study, we have systematically investigated the iontronic and electrochemical properties of tellurene derived from both hydrothermal (h-Te) and CVD (c-Te) syntheses.In contrast to the typical p-type conduction in back-gate modulation, both h-Te and c-Te exhibit clear ambipolar characteristics in aqueous electrolytes and ionic liquid with high electrolyte stability.In addition, we utilized the OCEM to investigate the intrinsic electrochemical properties of tellurene by optimizing the charge transport with an ideal ohmic contact.Pristine tellurene exhibits stable electrocatalytic HER, with h-Te showing lower overpotential and Tafel slope, which is attributed to the abundant surface vacancy that may serve as the active sites.Additional in-situ measurement reveals that tellurene has a stable potential window from ~−0.6 to ~+0.48 V versus RHE.Our study concludes that tellurene could be a promising candidate for ionic applications such as catalysis, neuromorphic devices, and biosensors.However, its electrochemical stability in various electrolyte systems needs to be carefully evaluated before the broad applications.

1
Preparation and characterization of h-Te.(A) Schematic of the hydrothermal synthesis of h-Te (left) and its optical microscope image (right).(B) Schematic illustration of the crystal structure of h-Te.(C, D) High-resolution transmission electron microscope images of h-Te.The inset images of (C) and (D) are the diffraction image and atom arrangement of h-Te, respectively.(E) Raman spectrum of h-Te.PVP, poly(vinylpyrrolidone).
Electronic and iontronic properties of h-Te and c-Te transistors.(A) Schematic image of the Te device with the back-gate structure on the SiO 2 /Si substrate.(B) Optical image of a typical h-Te device.(C) Output curve of h-Te in (B) measured at room temperature.(D) Transfer curves of h-Te-14.3 nm and h-Te-28.2nm devices measured in ambient conditions.(E) Transfer curves of h-Te devices of various thicknesses with ionic-gate modulation in 0.5 mol/L H 2 SO 4 .(F) Optical microscope images of the h-Te device before and after the ionic gating test for 30 cycles in 0.5 mol/L H 2 SO 4 .(G) Raman spectra of the h-Te in (F) before and after the test.

F
I G U R E 3 Electrocatalytic hydrogen evolution reaction of h-Te and c-Te.(A) Schematic illustration of the on-chip electrocatalytic microdevice.(B) Optical microscope images of an h-Te device measuring the HER performance of the exposed area.(C) Polarization curves of h-Te and c-Te.(D) Tafel plots obtained from the polarization curves in (C).

F
I G U R E 4 Electrochemical stability of h-Te.(A) Schematic illustration of on-chip electrocatalytic microdevices for the in-situ transport measurement.(B) Polarization curve (red) and the in-situ transport profile (black) of h-Te during HER measurement in 0.5 mol/L H 2 SO 4 at V ds = 100 mV.(C) Optical microscope image and Raman mapping image of A 1 mode of the h-Te device in (B) after the test.(D) Raman characterization of the h-Te device in (B) before and after the test.(E) Electrochemical (red) and the in-situ transport (black) profiles in the oxidation potential zone.(F) Optical microscope images of the h-Te device in (E) before and after the test.