Chalcogen (S, Se, and Te) decorated few-layered Ti₃C₂T_x MXene hybrids: modulation of properties through covalent bonding

Abstract

2D carbides and nitrides of transition metals (MXenes) have shown great promise in a variety of energy storage and energy conversion applications. The extraordinary properties of MXenes are because of their excellent conductivity, large carrier concentration, vast specific surface area, superior hydrophilicity, high volumetric capacitance, and rich surface chemistry. However, it is still desired to synthesize MXenes with specific functional groups that deliver the required characteristics. This is due to the fact that a considerable amount of metal atoms is exposed on the surface of MXenes during their synthesis through an etching procedure; hence, other anions and cations are uncontrollably implanted on their surfaces. Because of this situation, the first invented Ti₃C₂T_x MXene suffers from low photoresponsivity and detectivity, large overpotential, and small sensitivity in photoelectrochemical (PEC) photodetectors, hydrogen evolution reaction (HER), and sensing applications. Therefore, surface modification of the MXene structure is required to develop the device's performance. On the other hand, there is still a lack of understanding of the MXene mechanism in such cutting-edge applications. Thus, the manipulations of MXenes are highly dependent on understanding the device mechanism, suitable modification elements, and modification methods. This study for the first time reveals the conjugation effect of pre-selected S, Se, and Te chalcogen elements on a few-layered Ti₃C₂T_x MXene to synthesize new composites for PEC photodetector, HER, and vapor sensor applications. Also, the mechanism of the chalcogen decorated few-layered Ti₃C₂T_x MXene composites for each application is discussed. The selection of a few-layered Ti₃C₂T_x MXene is due to its fascinating characteristics which make it capable to be considered as an appropriate substrate and incorporating chalcogen atoms. The Te-decorated few-layered Ti₃C₂T_x MXene composite provides better performances in PEC photodetector and vapor sensing applications. Although the potential value of the Se-decorated few-layered Ti₃C₂T_x composite is slightly lower than that of the Te-decorated sample in HER application, its overpotential is still greater than that of the Te-decorated sample. The acquired results show that the S-decorated few-layered Ti₃C₂T_x composite demonstrates the lowest performance in all three examined applications in comparison with the other two samples.

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1. Introduction

High-efficiency photodetectors, electrocatalysts, sensors, and many more other state-of-the-art devices, require robust nanostructures that overcome the current limitations of those instruments. Two-dimensional (2D) nanomaterials are capable of providing unique properties because of their attractive physical (i.e., electrical- and photo-conductivity), chemical (i.e., catalytic activity), and mechanical (i.e., high stability) properties.^1–6 These fascinating characteristics of 2D nanomaterials are due to their ultrathin planar structures which result in strong quantum confinement and their large surface area.^7–9 Among the different varieties of invented and synthesized 2D nanomaterials, transition-metal carbides and carbonitrides (MXenes) have demonstrated admirable performances because of their wide specific surface area, and tendency to be bonded covalently with guest atoms because of the inner metal vacancies which are generated during their synthesis.^10–12 In addition to these, the high electrical conductivity, tunable bandgap, and great mechanical stability of MXenes make them suitable candidates for energy storage and conversion without decomposition of the structure.^13–16

Four molecular structures of M₂X, M₃X₂, M₄X₃, and M₅C₄ MXene nanosheets (M and X stand for transition metal elements, and C or N elements, respectively) have been made accessible so far with single-, few- and multi-layer structures,¹⁷ and the sizes of a few micrometers to the recently achieved ultra-large flakes.¹⁸ Surface termination species (T_x) or intercalated compounds (ICs) are replaced with the “A” atoms as a result of the A-layer selective etching from the initial MAX phase. The resulting MXenes have a chemical formula of M_n+1X_nT_x or M_n+1X_nT_x-IC after the substitution of the end groups by the intercalation.¹⁹ T_x and ICs could be =O, –H, –OH, or –F, and NH⁴⁺ or NH₃ groups depending on the etchant employed and the etching process parameters (i.e., etching time, etching temperature, etc.).^20–22 In addition, the hydrophilicity of MXene nanosheets is discovered to be caused by these functional groups of T_x and ICs.²¹ Furthermore, the nature of MXenes is capable of being changed by its functional groups. For instance, the full functionalization of Ti₃C₂T_x MXenes with =O can turn the metallic nature of the MXenes into a semiconductor state with a significant Seebeck coefficient.²³

It has been investigated as a benchmark that the Ti₃C₂T_x MXene has shown promising performance in energy storage and conversion. However, despite its high electronic conductivity and excellent performance in comparable electrolytic systems, it does not function well in some applications such as self-powered photoelectrochemical (PEC) photodetectors, hydrogen evolution reaction (HER) electrocatalysis, and vapor sensing applications. For instance, terminated Ti₃C₂T_x MXenes have shown a large overpotential for the HER in aqueous electrolytes despite being predicted to be active in that application.^24,25 In this regard, significant efforts are required to first understand the underlying mechanisms behind the low performance of Ti₃C₂T_x MXenes, and then, attempt to develop the efficiency of the MXenes with appropriate approaches like promoting charge transfer, H⁺/e⁻ couplings, and surface catalytically active sites in such applications.

Herein, we report a solid-state annealing strategy to investigate the decorating effect of the pre-selected chalcogen (S, Se, and Te) atoms on the performance of few-layered Ti₃C₂T_x MXenes in the aforementioned applications, as well as explain their mechanisms. This will reveal information about the active sites and how the distribution of electrons varies under reactional conditions. The PEC photodetector, HER, and vapor sensing working mechanisms of blank and chalcogen decorated few-layered Ti₃C₂T_x MXenes with S, Se, and Te elements are yet unclear and to the best of our knowledge, there are no published studies to explain those systems. This lack of studies can partially be attributed to the undesirable Ti₃C₂T_x MXene with low charge transfer and large overpotential. Thus, this work provides insight into modifications and conditions to enable improvement of the deficiencies through structural analyses. The chalcogen elements as a decoration agent have been chosen due to their high electronegativity, rich d-electron, large dipole atomic polarizability, high photoconductivity, and high responsivity characteristics.^26–28

2. Experimental methods

2.1. Sample preparation

2.1.1. Synthesis of few-layered Ti₃C₂T_x MXenes. The few-layered Ti₃C₂T_x MXene was synthesized based on a previously reported research study (Scheme 1a–c).²⁹ To do so, 10 g of as-received Ti₃AlC₂ MAX phase material (Jinzhou Haixin Metal Materials, China) was initially immersed in 500 ml of hydrofluoric acid (40% HF, Sigma-Aldrich) solution and continually stirred under ambient conditions for 7 days. Then, the sample was detached through repeated rounds of centrifugation and re-dispersion in water until getting a neutral 7 pH value for the residual water. Thereafter, the obtained sample was kept in a vacuum oven set at 50 °C for complete drying. In the next step, 5 g of Ti₃C₂T_x MXene was mixed with 250 mL of tetrabutylammonium hydroxide (40% TBAOH, Sigma-Aldrich) under ambient conditions for one hour. It was then ultrasonicated (∼30 minutes, 400 W) at room temperature and separated by suction filtration, followed by washing with distilled water. Finally, the delaminated MXene was dried overnight in a vacuum oven set at 50 °C.

Scheme 1 A schematic illustration of the (a–e) step-by-step procedure for synthesizing few-layered Ti₃C₂T_x and chalcogen decorated Ti₃C₂T_x MXenes via HF etching, TBAOH, and solid-state annealing technique, respectively, and (f) applications of few-layered chalcogen decorated Ti₃C₂T_x MXenes for PEC photodetector, HER, and gas sensors.

2.1.2. Synthesis of S-, Se- or Te-decorated few-layered Ti₃C₂T_x MXene composites. The synthesized few-layered Ti₃C₂T_x MXene (2 gr) and as-received chalcogen powders (S, Se, Te, 1 gr, 99.99% purity, Strem chemicals, Inc., USA) with the ratio of 2 MXene : 1 chalcogen were firstly weighed and inserted in a quartz glass ampule (Scheme 1d and e) and evacuated to 1 × 10⁻⁵ mbar with a diffusion pump and melt sealed. Following sealing with an oxygen–hydrogen torch, the quartz ampule was subsequently placed inside a muffle furnace and heated at 500 °C for 24 h using a 5 °C min⁻¹ heating and cooling rate. Finally, the chalcogen decorated few-layered Ti₃C₂T_x composite powders were manually collected from the broken quartz tube.

2.1.3. Preparation of working electrodes. To characterize the PEC photodetector, HER, and vapor sensing performances, indium tin oxide (ITO), glassy carbon (GC), and gold-interdigitated electrode (GIE) substrates were ultrasonically cleaned with acetone, ethanol, and deionized water and then dried by nitrogen flow. The surface of GC was also polished with a polishing kit including Al₂O₃ sol–gel suspension (Eposal, pH ∼ 8.0, 0.06 micron) and polishing cloth (zeta, Ø = 200 nm, short-nap, soft synthetic cloth) to remove any contaminations prior to performing the ultrasonic cleansing. Then, 30 mg of each Ti₃AlC₂, few-layered Ti₃C₂T_x, and chalcogen decorated few-layered Ti₃C₂T_x composite samples were dispersed in 1000 μl of N,N-dimethylformamide (DMF) using ice bath sonication for 15 min. Subsequently, 5 μl of each dispersed sample was uniformly drop-cast on ITO, GC, and GIE substrates (Scheme 1f) and dried in a vacuum oven at 65 °C for 12 h for further characterization.

2.2. Characterization methods

2.2.1. Materials characterization. The crystal structure of the synthesized materials was determined by X-ray diffraction (XRD, Bruker D8 Advance, Germany) equipped with a Cu-Kα (λ = 1.5406 Å) source, with 2θ degree values from 5 to 90°, and a step size of 0.02°. The morphology, microstructure, and elemental mapping of the materials were analyzed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM, EFTEM Jeol 2200 FS microscope) equipped with an energy dispersive spectrometry (EDS) system with an acceleration voltage of 200 kV. SEM was also performed on Maia 3 and Lyra 3 (Tescan) in scanning transmission electron microscopy (STEM) mode for a better material contrast. X-ray photoelectron spectroscopy (XPS) analysis was performed via electron spectroscopy for chemical analysis with a Probe P spectrometer (Omicron Nanotechnology, Germany) using a monochromatized Al Kα X-ray source (1486.7 eV) to investigate the surface composition and bonding states. A LAMBDA 850 + UV/Vis Spectrophotometer (PerkinElmer, USA) inside the integration sphere with a scan range between 250 and 800 nm was used to carry out the UV-Vis spectroscopic characterization. To evaluate the specific surface area and pore size distribution of the synthesized materials, N₂ adsorption–desorption isotherms were analyzed at 77 K using a Quantachrome NOVAtouch LX². Before measuring gas sorption, the samples were activated at 150 °C for 10 hours under dynamic vacuum, with N₂ as the adsorbate.

2.2.2. Electrochemical characterization. PEC photodetection and HER electrochemical tests were carried out at room temperature in a three-electrode system with an Autolab PGSTAT 204 (Utrecht, Netherlands, NOVA Version 2.1.4). For the PEC photodetection and HER, drop-cast samples on ITO and GC substrates, respectively, were utilized as a working electrode, platinum (Pt) wire and graphite rod served as a counter electrode, and a Ag/AgCl/saturated KCl electrode served as a reference electrode. The CV scan range for the PEC photodetector was with a gate voltage from −0.5 to +0.5 V and a scan rate of 5 mV s⁻¹. However, that voltage range was changed from −1.5 to +2 V with the same scan rate for HER characterization. The HER and OER zones inside that particular characterization are denied due to the limited value of the scan range for the PEC photodetector. During the PEC photodetector characterization, we looked at illumination with several LED sources, ranging from 385 nm to 532 nm. Measurements with the PEC photodetector and the HER were carried out in 1 M KOH and 0.5 M H₂SO₄, respectively. An Auto lab (PGSTAT204) with the impedance module FRA32M was used to acquire and evaluate gas sensing in a two-electrode mode by impedance spectroscopy measurements at room temperature. The frequency range had a logarithmic scale of 10 points per decade and ranged from 0.01 Hz to 1 MHz.

3. Results and discussion

3.1. Crystal, morphological and chemical characterization

The crystal phases, structure, and morphology of the synthesized Ti₃C₂T_x and chalcogen (S, Se, and Te) decorated few-layered Ti₃C₂T_x MXenes (namely; Te/Ti₃C₂T_x, Se/Ti₃C₂T_x, and S/Ti₃C₂T_x) were characterized by XRD, SEM, and TEM/HRTEM. As depicted in the XRD characterization of Fig. 1, Aluminium (Al) layers of the Ti₃AlC₂ MAX phase were almost terminated after etching with 40% HF to gain the Ti₃C₂T_x MXene. The obtained Ti₃C₂T_x MXene also show a well-crystallized form. The critical heating temperature for the Ti₃C₂T_x MXene chemical and structural phase transitions has been estimated to be around 850 °C.³⁰ Thus, a benign annealing temperature of 500 °C was implemented to just modify the surface chemistry and thermally break down the functional groups. Subsequently, it has been observed from the XRD patterns of Te/Ti₃C₂T_x, Se/Ti₃C₂T_x, and S/Ti₃C₂T_x samples that the (002) and (004) peaks completely disappeared and were substituted by a group of sharp diffraction peaks that index mainly to Te and formed-Ti_0.95Te_0.05 (in Te/Ti₃C₂T_x), formed-TiSe₂ (in Se/Ti₃C₂T_x), formed-TiS₂ and TiS (in S/Ti₃C₂T_x), TiO₂ (in all samples), and some other compounds belonging to the Ti₃C₂ MXene and decorated chalcogen. It is hypothesized that a direct reaction between chalcogen elements and MXene layers which took place at 500 °C under vacuum conditions caused the majority formation of those chemical bonding and phases. The chemical reaction and bonding generally could be based on the fact that an atom from the surface functional groups (=O, –H, –OH, or –F) of the Ti₃C₂T_x structure leaves its initial position (mostly –OH and some parts of –F)³⁰ at temperatures below the annealing's critical point, and then a vacancy is created that can be filled with chalcogen atoms and create covalent bonding with the nearby atoms. As a result, the elimination of –OH and maybe some portion of –F causes the formation of Ti_0.95Te_0.05, TiSe₂, TiS₂, and TiS on the surface of the MXene. A small degree of chemical reaction could be noticed between the Te chalcogen and Ti₃C₂T_x MXene in the Te/Ti₃C₂T_x sample. This low reaction of Te with MXene might be attributed to the high boiling point of Te (∼988 °C) in comparison with the other S (∼445 °C) and Se (∼685 °C) chalcogen elements, where the annealing temperature applied at 500 °C is very less for a large chemical reaction for Te. The other formation phases of SeO₂ (in Se/Ti₃C₂T_x) and C₂S (in S/Ti₃C₂T_x) are also because of the existence of =O terminations on the MXene and the vacancy of Ti, respectively. The existence of the TiO₂ phase is also not astonishing because the annealed chalcogen decorated few-layered Ti₃C₂T_x MXenes contained oxygen-containing species and a portion of –F surface terminations that are known to stabilize the TiO₂ anatase phase.^31,32 Regularly, the etching treatment procedure is frequently recognized to crumble the hexagonal crystal structure and P63/mmc symmetry of the MAX phase by elimination of A-layers. Nonetheless, the surface terminations on the MXene by intercalation of termination ions hold a portion of the hexagonal symmetry components of the parent MAX material. The SEM, TEM, electron diffraction pattern and HRTEM images of the synthesized S/Ti₃C₂T_x, Se/Ti₃C₂T_x, and Te/Ti₃C₂T_x can confirm the structural stability of the Ti₃C₂T_x MXene after annealing under vacuum conditions in the presence of the chalcogen decoration agents and also the formation of the aforementioned formed phases (Fig. 1b–d). These results of characterization are consistent with the XRD results that demonstrate the various morphologies formed in the chalcogen decorated few-layered Ti₃C₂T_x MXene composite, without decomposition of the Ti₃C₂T_x MXene support after annealing.

Fig. 1 Structural and morphological characterization of Ti₃C₂T_x, Te/Ti₃C₂T_x, Se/Ti₃C₂T_x, and S/Ti₃C₂T_x; (a) XRD, (b–d) typical SEM, TEM including elemental mappings, electron diffraction, and HRTEM of Te/Ti₃C₂T_x, Se/Ti₃C₂T_x, and S/Ti₃C₂T_x. (Note: the TEM and elemental mapping images of the synthesized Ti₃C₂T_x have been provided in Fig. S1.†)

To further analyze the surface chemistry, chemical states, and atomic bonding configurations, the as-received Ti₃AlC₂ MAX and synthesized Ti₃C₂T_x, Te/Ti₃C₂T_x, Se/Ti₃C₂T_x, and S/Ti₃C₂T_x MXenes were subjected to XPS characterization (Fig. S2† and Fig. 2a–c). The full XPS survey and high-resolution spectra could represent the distinct signals of Ti, C, O, F, S, Se, and Te elements. The samples’ XPS analysis reveals that after annealing them with pre-selected chalcogen elements, the amount of F species significantly decreased while the content of O increased slightly. Similar results have been observed in single Se atoms on Ti₃C₂T_x MXene nanosheets under CO₂ conditions,²⁶ as well as in our other recent work by the solid-state annealing method.³³ This variation would be due to the substitution in termination contents by chalcogen elements and oxygen-containing terminations (like Ti–O, C–O, S–O, and Se–O). High-resolution XPS spectra of the elements of the samples can further help evaluate the changes in chemical states and their occupancy after decoration of the few-layered Ti₃C₂T_x MXene with Te, Se, and S (Fig. 2a–c). The Ti 2p peaks of the samples are devoted to certain bonds belonging to Ti–O (bond in TiO₂) and Ti–C (bond in Ti₃C₂T_x).^34,35 The C 1s and O 1s peaks are assigned to C–Ti, C–C, Se–C, S–C, C–O, and Ti–O bonds. In the XPS spectra of the Se 3d, S 2p, and Te 3d, peaks are ascribed to the anchoring of chalcogen elements on Ti₃C₂T_x.

Fig. 2 XPS survey spectrum and high-resolution elemental spectra of (a) Te/Ti₃C₂T_x, (b) Se/Ti₃C₂T_x, and (c) S/Ti₃C₂T_x. (d) Schematic illustration of possible potential anchoring sites for the chalcogen decorated few-layered Ti₃C₂T_x MXene composite (side- and top-view).

There are three likely potential anchoring sites that could incorporate the chalcogen agent atoms to be bonded with the Ti₃C₂T_x MXene (Fig. 2d). These three possibilities are: (i) bonding with the terminal groups on the surface and producing chalcogen–oxygen species, (ii) bonding with Ti atoms like surface terminals on Ti₃C₂T_x, and (iii) occupying the Ti vacancies that already have been created during the etching process of Ti₃AlC₂ MAX. The XRD and XPS analyses indicate that at a 500 °C annealing temperature under 10⁻⁵ mbar vacuum conditions, chalcogen atoms are prone to bond with any of those three alternatives. Formation of the TiSe₂, TiS, and TiS₂ phases in considerable quantities, however, supports the chemical bonding arrangement of Se and S with Ti. The nature of this chemical bonding is perhaps due to the leaving of the functional group components of –OH and –F at this annealing temperature and providing low formation energy for the formation of Ti–Se, Ti–S, and Ti–Te bonds. This situation could be observed as insignificant in the case of the Te chalcogen with the Ti₃C₂T_x MXene maybe because of its high boiling temperature, and low thermal-annealing energy for the formation of chemical reaction bonding.

3.2. PEC photodetector

To discover the photoresponse performance of the chalcogen decorated few-layered Ti₃C₂T_x MXene, the photocurrent density of the samples was examined and compared to those of the blank ITO substrate, as-received Ti₃AlC₂ MAX phase, and synthesized few-layered Ti₃C₂T_x MXene. In this instance, a three-electrode setup was used to perform PEC characterization along with an appropriate sample dropped onto the conductive ITO wafer. Several LED light illumination of 420 nm, 460 nm, 385 nm, and 532 nm with a power of 800 mW were used to measure the dependence of the photocurrent on the wavelength. Fig. 3a depicts the representative potential–current density (V–I) curve for the synthesized few-layered Ti₃C₂T_x MXene that was obtained using linear sweep voltammetry in 1 M KOH at a scan rate of 10 mV s⁻¹. It shows a distinct response to the 420 nm LED illumination when it is turned on and off along with a steady increase in current density against the evolution of the applied voltage.

Fig. 3 (a) A representative anodic scan of a few-layered Ti₃C₂T_x sample in 1 M KOH under 420 nm LED illumination. Photocurrent density of (b) Te/Ti₃C₂T_x, (c) Se/Ti₃C₂T_x, and (d) S/Ti₃C₂T_x samples under the illumination of 420 nm, 460 nm, 385 nm, and 532 nm LED sources in 1 M KOH solution at 1.25 V vs. SCE.

The photocurrent density characterization of the ITO substrate, Ti₃AlC₂, and Ti₃C₂T_x samples revealed very low to negligible current density ranges with 420 nm, 460 nm, and 532 nm illumination (Fig. S3†). Among the Ti₃AlC₂ and Ti₃C₂T_x samples, the Ti₃C₂T_x MXene illustrates a higher current density (1.2 μA cm⁻²) than the Ti₃AlC₂ with only 0.3 μA cm⁻² under the 420 nm illumination. Nevertheless, this current density value is still away from an arbitrary state. Besides, the current density of the Ti₃C₂T_x MXene is also low under the other 460 nm (0.5 μA cm⁻²) and 532 nm (0.4 μA cm⁻²) illumination. Following that, the photocurrent activity of the chalcogen decorated Ti₃C₂T_x composites was also evaluated. The collected results not only show an extraordinary photocurrent density of 12 μA cm⁻² (Te/Ti₃C₂T_x with 420 nm illumination), but also some other current density values were collected under 460 nm, 385 nm, and 532 nm illumination (Fig. 3b–d) which are almost equal to the maximum value of the Ti₃C₂T_x sample under 420 nm. A steady stable state was observed in the current density of the chalcogen decorated few-layered Ti₃C₂T_x composite samples with passing time. However, it can be seen that the current density value shows fluctuation in the blank few-layered Ti₃C₂T_x sample with 460 nm and 532 nm LED lights. This achievement is probably due to the unstable condition of the sample on top of the ITO substrate. The value of response time, another important parameter in photodetectors, was determined on the representative Te/Ti₃C₂T_x sample. The obtained results show a swift response and recovery time of ∼1.0 s toward the illumination light (Fig. 4).

Fig. 4 A representative single-cycle response time, semi-cycle response, and recovery times of the PEC Te/Ti₃C₂T_x photodetector in 1 M KOH solution under the 420 nm LED light with a power of 800 mW.

In order to gain further insight into the efficiency of the PEC photodetector, the responsivity (R) values of the Ti₃C₂T_x, Te/Ti₃C₂T_x, Se/Ti₃C₂T_x, and S/Ti₃C₂T_x samples were assessed using , where ΔI is the difference in the photocurrent density with respect to the dark current, and P is the irradiance power intensity per unit area (Fig. S4a† and Fig. 4a–c).³³

It can be observed that the responsivity of the Te/Ti₃C₂T_x sample (Fig. 5a) reaches 70 μA W⁻¹ which is almost 20 times higher than the responsivity of blank few-layered Ti₃C₂T_x (Fig. S4a†) and even possesses a higher value in comparison with the other Se/Ti₃C₂T_x (Fig. 5b) and S/Ti₃C₂T_x (Fig. 5c) samples. For further additional characterization, it is essential to evaluate the specific detectivity of a photodetector device which describes the minimum illumination power necessary for a photodetector to distinguish the signal from noise. Thus, the following formula is utilized to calculate the specific detectivity: , where A and B represent the active area and measured bandwidth, respectively, and NEP represents noise-equivalent power.^36,37 NEP is defined as the input signal power that produces a signal-to-noise ratio (S/R) of one in an output bandwidth of 1 Hz. It also can be expressed as follows: , where I_N is the noise current (dark current) and R_ph is the photodetector's photoresponsivity.³⁷Fig. 4d–f display the detectivity of Te/Ti₃C₂T_x, Se/Ti₃C₂T_x, and S/Ti₃C₂T_x samples, respectively. The maximum detectivity between the chalcogen decorated Ti₃C₂T_x composite samples is likely due to the high carrier concentration at low dark current densities for the Te/Ti₃C₂T_x sample with ∼4.5 × 10⁸ cm Hz^1/2 W⁻¹ (Fig. 5d). The detectivity value for Se/Ti₃C₂T_x and S/Ti₃C₂T_x samples is ∼3.0 × 10⁸ cm Hz^1/2 W⁻¹ and ∼1.5 × 10⁷ cm Hz^1/2 W⁻¹, respectively. On the other hand, the detectivity of the few-layered blank Ti₃C₂T_x is ∼1.2 × 10⁷ cm Hz^1/2 W⁻¹ (Fig. S4b†).

Fig. 5 The responsivity (a–c) and detectivity (d–f) of PEC Te/Ti₃C₂T_x, Se/Ti₃C₂T_x, and S/Ti₃C₂T_x photodetectors, respectively, in 1 M KOH solution as a function of power density upon 420 nm, 460 nm, and 532 nm illumination wavelengths.

The low photodetection characteristic of the Ti₃C₂T_x MXene could be attributed to the crystal defects which usually are formed on the Ti₃C₂T_x MXene during the etching process of the Ti₃AlC₂ MAX phase. Those defects would represent a trap and then capture the photogenerated electron–hole pairs. On the other hand, the optical bandgap of the blank few-layered Ti₃C₂T_x MXene has the highest value among the presented materials (Fig. 6a) of ∼3.7 eV. This high value of the optical bandgap is due to the concentration of defects on the Ti₃C₂T_x MXene, where the excess defect density shifts the Fermi level towards the conduction band, thereby giving rise to the Moss–Burstein effect and optical bandgap.³⁸ Hence, this high optical bandgap value of the Ti₃C₂T_x MXene could be another possible reason for the lower performance in its photodetector activities. In the case of the chalcogen decorated few-layered Ti₃C₂T_x MXene composite samples, those formation vacancies usually are occupied by the chalcogen elements. Therefore, photocurrent density could be developed in such decorated composite samples. Also, the optical bandgap of chalcogen decorated samples (Fig. 6a) declined to a lower value which has a beneficial effect on generation charge carriers and developing photocurrent density. The specific surface area of all samples also was analyzed using Brunauer–Emmett–Teller (BET) measurements. Fig. 6b shows that the surface area value of the Ti₃C₂T_x MXene is lower than those of the other samples. This results in a low active surface area and subsequently, low charge carriers and photocurrent density for that sample. Although the surface area of S/Ti₃C₂T_x is higher than those of the other two chalcogen decorated composite samples (Te/Ti₃C₂T_x and Se/Ti₃C₂T_x), the higher photocurrent density of Te/Ti₃C₂T_x is due to another physical phenomenon of ionization energy. One can speculate that an element with lower ionization energy can generate higher electron–hole pairs since the ionization energy is the minimum energy required to separate the most loosely bound electron. A comparison between the ionization energy of the chalcogen elements confirms that the ionization energy of Te is lower than those of the other two chalcogen elements (ionization energy: Te < Se < S).³⁹ This causes the enhancement of photocurrent density, photoresponsivity, and detectivity of Te/Ti₃C₂T_x in comparison with the other Se/Ti₃C₂T_x and S/Ti₃C₂T_x samples. The formation of transition metal dichalcogenides on top of Se- and S-decorated Ti₃C₂T_x MXenes in this work is the other possible reason for developing photodetection activities of the Ti₃C₂T_x due to the high charge carrier mobility of those shaped phases.⁴⁰

Fig. 6 (a) UV-Vis absorption spectra along with Tauc plots of the blank and chalcogen decorated Ti₃C₂T_x composite samples. (b) BET analysis of (i) blank Ti₃C₂T_x MXene, (ii) Te/Ti₃C₂T_x (210 pm), (iii) Se/Ti₃C₂T_x (190 pm), and (iv) S/Ti₃C₂T_x (180 pm) samples.

3.3. Hydrogen evolution reaction

The electrocatalytic efficiency of the chalcogen decorated Ti₃C₂T_x MXene composite samples for the HER was evaluated by conducting linear sweep voltammetry (LSV) in an acidic environment (0.5 H₂SO₄). The achieved results were then compared with those of blank few-layered Ti₃C₂T_x MXene, platinum–carbon (PtC, 1%), and GC substrate under the same experimental conditions to assess the origin of the developed HER performance of the chalcogen decorated influence on the Ti₃C₂T_x MXene (Fig. 7). According to the typical overpotential value at a −10 mA cm⁻² current density for the HER, the evaluated overpotentials for the chalcogen decorated few-layered Ti₃C₂T_x MXene composite samples illustrate an improvement in catalytic performance because of the new active sites as well as a newly formed phase by chalcogen decorated atoms. The Tafel analysis was also carried out to determine the electrochemical kinetics. Its slope identifies a logarithm of current density with overpotential and provides a simple way to recognize the rate-determining step, and hence, a possible electrochemical mechanism in the HER. It is observed from Fig. 7a that the Se/Ti₃C₂T_x sample is willing to generate bonding sites with hydrogen atoms easier than the other measured samples. Generally, the possible mechanisms for the HER in acidic media could be based on the Volmer–Heyrovsky or Volmer–Tafel mechanism. The Volmer process always occurs at the initial step, and it is followed by either the Heyrovsky or Tafel process. However, one of Heyrovsky's or Tafel's actions would be as a predominant mechanism. For the Volmer step to be the rate-determining step, it already has been proven that the magnitude of the Tafel slope has to be around 120 mV dec⁻¹.^41,42 This value for the corresponding mechanisms of the Heyrovsky and Tafel step is observed at around 40 and 30 mV dec⁻¹, respectively.^41,43 The Tafel slopes for each sample were obtained from the linear portion of the polarization curves (Fig. 7b). It can be seen that the slope values are as follows: PtC 1% ∼ 163 mV dec⁻¹, Se/Ti₃C₂T_x ∼ 223 mV dec⁻¹, Te/Ti₃C₂T_x ∼ 204 mV dec⁻¹, S/Ti₃C₂T_x ∼ 220 mV dec⁻¹, Ti₃C₂T_x ∼ 248 mV dec⁻¹, and GC ∼ 227 mV dec⁻¹. According to the above explanations for the HER mechanism in acidic media and the acquired distinct Tafel slope values of the samples, it can be proposed that another different mechanism would occur for both blank and chalcogen decorated Ti₃C₂T_x MXene composite samples that would be determined based on the following hypothesis. In the case of the blank Ti₃C₂T_x MXene, oxygen (=O) terminal groups could be protonated with the existing high concentration of H⁺ in the H₂SO₄ electrolyte resulting in Ti–OH bonding. This protonation could be followed by the adjacent Ti site and lead to the formation of H₂. Nonetheless, it is observed from Fig. 7b that the blank Ti₃C₂T_x MXene suffers from a 248 mV dec⁻¹ large overpotential value. This high overpotential value is maybe attributed to the lack of an ideal equilibrium between Ti and O sites in this sample.⁴⁴ In the case of the chalcogen decorated Ti₃C₂T_x MXene composite samples, it already was proven by XPS characterization (Fig. S2† and Fig. 2) that the =O content was enhanced slightly after decoration. Therefore, this variation in the =O content legitimized the evolution of more H₂ contents in the modified samples. Likewise, the arrangement of Ti_0.95Te_0.05, TiSe₂, TiS, and TiS₂ phases in the chalcogen decorated Ti₃C₂T_x composite samples is roughly unattainable to be stoichiometric, perhaps due to chalcogen vacancies.^45–47 The chalcogen vacancies and strain are also shown frequently to be the active sites in catalysts to maximize HER performance.^48–50 Also, the loss of chalcogen causes additional electrons to be redistributed among the nearby atoms.⁵¹ As a result, the chalcogen vacancies at the surface of the chalcogen decorated Ti₃C₂T_x composite sample may drive electron accumulation, which creates more active sites and speeds up the reaction rate of the H⁺/e⁻ coupling and the generation of additional H₂ with a lowering overpotential value.

Fig. 7 HER analysis of the blank Ti₃C₂T_x MXene, Te/Ti₃C₂T_x, Se/Ti₃C₂T_x, and S/Ti₃C₂T_x samples in 0.5 M H₂SO₄ medium and compared with GC and PtC (1%) samples. (a) Polarization curves and (b) Tafel plots of the corresponding overpotentials.

3.4. Vapor sensors

The improvement of cutting-edge gas sensor devices for tracking down a low-level density of volatile organic molecules is necessary. In this context, MXenes have been evaluated for gas sensors in view of their remarkable attributes, recently,^52–54 albeit the gas and vapor detection by MXenes has been viewed as more convoluted as opposed to the other 2D nanomaterials with surface adsorption and a regular charge movement.^55,56 This is maybe attributed to the layered accordion-like structure of MXenes, where the interlayers are able to swell and affect the gas response. To investigate the gas sensing efficiency of the chalcogen decorated few-layered Ti₃C₂T_x MXene composites and compare it with the pristine Ti₃C₂T_x, herein, we used the electrochemical impedance spectroscopy strategy and evaluated the samples on some polar organic vapors of methanol, ethanol, and acetone. The detection of the gases by MXenes generally occurs based on the sensitivity level of the impedance signals derived from the variety of the local carrier on the MXene layers. The Nyquist and Bode (impedance phase spectra) plots of the specimens are shown in Fig. 8. It can be observed from the Nyquist plots that all of the sample's behavior with the examined gases is like that of a series RC circuit. Also, it can be recognized that the resistance, combined capacitance, and inductance belonging to the real and imaginary sections of impedance (Z′ and Z′′) have been enhanced in the chalcogen decorated few-layered Ti₃C₂T_x composite, especially for the Te/Ti₃C₂T_x and Se/Ti₃C₂T_x samples, in the presence of all vapors (Fig. 8). This Z′ and Z′′ increment could probably be due to the increased oxygen content, and subsequently, the formation of TiO₂ with an electrical resistivity of 10¹² Ω cm⁻¹,⁵⁷ as well as the other formed phases during decoration of Ti₃C₂T_x with chalcogen elements. A comparison between the Bode phase plots of the pristine and chalcogen decorated few-layered Ti₃C₂T_x composite samples indicates an enhanced affinity in the sensitivity of the Te/Ti₃C₂T_x and Se/Ti₃C₂T_x MXene samples. Amongst the latest two samples, Te/Ti₃C₂T_x sensing performance seems to be even better maybe due to the less chemical reaction of Te with Ti₃C₂T_x. However, this trend is almost unchanged for the other S/Ti₃C₂T_x in comparison with the pristine Ti₃C₂T_x. The sensing mechanism of chalcogen decorated Ti₃C₂T_x composite samples with a positive variation could be discussed based on the following points of view: (i) the –F terminals with a detrimental effect on gas sensing have been reduced in chalcogen decorated Ti₃C₂T_x samples.⁵⁵ (ii) The high polarity of the applied organic gases (methanol, ethanol, and acetone) and the interaction of those polar molecules with the enhanced =O terminals on the surface of the samples lead to charge transfer through the formation of hydrogen bonds. (iii) More electrons are produced on the surface of the chalcogen decorated Ti₃C₂T_x composite samples as a result of the rich d-electrons of chalcogen elements and non-stoichiometric produced phases. This could increase the interaction of polar molecules with the sample and then enhance the extent of charge transfer. The suggested method for MXene anchoring components demonstrates that there are still areas that need to be thoroughly investigated while taking into account the manipulation of heterojunction interfaces and underlying gas-sensing processes.

Fig. 8 Impedance phase spectra and Nyquist plots of the (a) pristine few-layered Ti₃C₂T_x MXene, (b) Te/Ti₃C₂T_x, (c) Se/Ti₃C₂T_x, and (d) S/Ti₃C₂T_x vapor sensors with methanol (MeOH), ethanol (EtOH) and acetone organic vapors.

4. Conclusions

MXenes have great potential to be used in a range of energy storage and energy conversion devices since they provide remarkable promise as photo(electro)catalysts. However, their efficiencies are yet to be developed with a few additional possible components and routes. As a result, some potential chalcogen elements of Te, Se, and S were selected and successfully anchored on the few-layered Ti₃C₂T_x by a facile solid-state chemistry annealing technique. PEC photodetector, HER, and vapor sensor analyses revealed that the chalcogen decorated few-layered Ti₃C₂T_x composite has a greater impact on the efficiencies of such systems than the pristine few-layered Ti₃C₂T_x MXene. Occupation of pristine Ti₃C₂T_x MXene defects and optical bandgap reduction by the chalcogen element decoration are two reasons for the enhancement of the photodetector activity of the chalcogen decorated Ti₃C₂T_x MXene composite samples. Moreover, it was analyzed that the photocurrent density, photoresponsivity, and detectivity of Te/Ti₃C₂T_x are higher than those of the other Se/Ti₃C₂T_x and S/Ti₃C₂T_x samples. This variation was due to the lower ionization energy of the Te in comparison with the other Se and S elements. Thus, the Te/Ti₃C₂T_x sample could transfer the separated electrons to the electrode system in the PEC photodetector more quickly and efficiently. The evaluation of the HER in the chalcogen decorated few-layered Ti₃C₂T_x composite samples also showed an enhancement in the production of H₂ due to the large =O contents and non-stoichiometric formation phases on the surface of the Ti₃C₂T_x MXene after modification. These two attributes could generate active sites to raise the H⁺/e⁻ coupling reaction rate and form greater H₂. Furthermore, the reduced –F, higher =O, rich d-electron of chalcogen elements, and non-stoichiometric formed phases, all could boost electrons on the surface of the chalcogen decorated few-layered Ti₃C₂T_x MXene composite samples, and subsequently, interactions by the methanol, ethanol, and acetone polar molecules with result in a larger charge transfer and vapor sensing. This work indicates an effective synthesis strategy in developing efficient materials for state-of-the-art applications and can provide new visions into the advancement of the other MXene-based family members that can be employed in high-performance energy systems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

J. A. was supported by the European Structural and Investment Funds, OP RDE-funded project ‘CHEMFELLS IV’ (No. CZ.02.2.69/0.0/0.0/20_079/0017899). This project was supported by Czech Science Foundation (GACR No. 20-16124J).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr05730a

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