Te Vacancies Research Articles

Investigating the role of undercoordinated Pt sites at the surface of layered PtTe2 for methanol decomposition

Transition metal dichalcogenides, by virtue of their two-dimensional structures, could provide the largest active surface for reactions with minimal materials consumed, which has long been pursued in the design of ideal catalysts. Nevertheless, their structurally perfect basal planes are typically inert; their surface defects, such as under-coordinated atoms at the surfaces or edges, can instead serve as catalytically active centers. Here we show a reaction probability > 90 % for adsorbed methanol (CH3OH) on under-coordinated Pt sites at surface Te vacancies, produced with Ar+ bombardment, on layered PtTe2 — approximately 60 % of the methanol decompose to surface intermediates CHxO (x = 2, 3) and 35 % to CHx (x = 1, 2), and an ultimate production of gaseous molecular hydrogen, methane, water and formaldehyde. The characteristic reactivity is attributed to both the triangular positioning and varied degrees of oxidation of the under-coordinated Pt at Te vacancies.

SnTe/SnSe Heterojunction Based Ammonia Sensors with Excellent Withstand to Ambient Humidities.

Non-invasive breath testing has gained increasing importance for early disease screening, spurring research into cheap sensors for detecting trace biomarkers such as ammonia. However, real-life deployment of ammonia sensors remains hindered by susceptibility to humidity-induced interference. The SnTe/SnSe heterojunction-based chemiresistive-type sensor demonstrates an excellent response/recovery to different concentrations of ammonia from 0.1 to 100ppm at room temperature. The improved sensing properties of the heterojunctions-based sensors compared to single-phased SnTe or SnSe can be attributed to the stronger NH3 adsorptions, more Te vacancies, and hydrophobic surface induced by the formed SnTe/SnSe heterojunctions. The sensing mechanisms are investigated in detail by using in situ techniques such as diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), Kelvin probe, and a.c. impedance spectroscopy together with the Density-Function-Theory calculations. The formed heterojunctions boost the overall charge transfer efficiency between the ammonia and the sensing materials, thus leading to the desirable sensing features as well, with excellent resistance to ambient humidities.

Modifying Electronic Structure of Bismuth Telluride Through S Doping and Te Vacancy Engineering for Enhanced Zn-Ion Storage Ability in Aqueous Zn-proton Hybrid Ion Batteries.

Bismuth chalcogenides are used as cathode materials in Zn-proton hybrid ion batteries, which exhibit an ultraflat discharge plateau that is favorable for practical applications. Unfortunately, their capacity is not competitive, and their charge storage mechanisms are ambiguous, both of which hinder their further development. In this study, S-doped Bi2Te3- x (SBT) nanosheets are prepared by tellurizing a Bi2O2S precursor using a hydrothermal process. As revealed by density functional theory analyses, the S dopant and its induced Te vacancies can distinctly manipulate the electronic structure of SBT, resulting in decent electrical conductivity and more negative adsorption energy to Zn2+. These advantages boost the Zn2+ storage ability of SBT materials. Consequently, compared with defect-free Bi2Te3, the SBT cathodes have superior specific capacity, rate capability, and cycling stability.

Nonmonotonic Effects of Atomic Vacancy Defects on Friction.

The presence of defects such as vacancies has a significant impact on the frictional properties of 2D materials that are excellent solid lubricants. In this study, we demonstrate that the nonmonotonic effect of Te vacancy defects on the friction of MoTe2 is related to the change in the maximum sliding energy barrier due to the variation in tip position. The experimental results of atomic force microscopy suggest that the friction shows an overall increasing trend with the increase in Te vacancy density, but this variation is nonmonotonic. Molecular dynamics simulations show that the increase in friction force with defect density can be attributed to the large and more sliding energy barriers that the tip has to overcome. Furthermore, the nonmonotonic variation of friction with defect density is dominated by the change of the maximum sliding potential barrier caused by the variation of tip position perpendicular to the sliding direction during the sliding process. Additionally, the uneven charge distribution due to charge transfer occurring at the defect also contributes to the increase in friction. This work shows the mechanism of the effect of Te vacancy defects on the friction of MoTe2, which provides guidance for the modulation of the frictional properties of solid lubricants.

Crystal Growth and Physical Properties Evolution in Co‐Doped NiTe2 System

Transition metal tellurides have been widely studied for their diverse crystal structures and exotic physical properties such as large magnetoresistance, charge density waves, superconductivity, ferromagnetic and topological properties. NiTe2 has recently been found to be a Dirac semimetal, allowing it to achieve exotic properties by pressure and doping. A series of Ni1−xCoxTe2−δ single crystals are synthesized by the standard solid‐state reaction method. The energy‐dispersive X‐ray spectroscopy and X‐ray diffraction tests confirm Co is successfully doped into the crystal structure. The electrical transport measurements show typical metallic behaviors for all the samples. Magnetization measurements reveal that, in the doping range of x = 0.12–0.62, the samples exhibit a coexistence of antiferromagnetic and paramagnetic phases above the characteristic temperature Tt. By using the combined Curie–Weiss and spin‐wave model, the antiferromagnetic to ferromagnetic transition is found to occur as the temperature drops below Tt. The magnetic transition is attributed to the Te vacancies, which can be explained using the bound magnetic polaritons model. A magnetic phase diagram for Ni1−xCoxTe2−δ system is constructed.

Role of defects on carrier dynamics and transport mechanism in Bi2Te3 single crystals

Defects play an important role in determining the type of carriers as well as in tunning the physical properties of layered materials. In this study, we have demonstrated that by varying the growth kinetics one can control the defects and can achieve electrons or holes dominated Bi2Te3 single crystals using the modified Bridgman method. The correlation between structural defects and the type of dominant charge carriers in crystals is discussed using X-ray diffraction and Hall resistivity. Electrons are found to be originating from Te vacancy type defects, while holes are manifested from predominant structural defects viz. BiTe antisite defects or interstitial Te atoms. We observe that the alteration of charge carriers from electrons to holes has enhanced magnetoresistance from 103% to 224%. The enhancement in magnetoresistance emerges from the 2D multichannel quantum coherent conduction mechanism.

Attaining High Figure of Merit in the N-Type Bi2Te2.7Se0.3-Ag2Te Composite System via Comprehensive Regulation of Its Thermoelectric Properties.

n-Type Bi2Te2.7Se0.3 (BTS) is the state-of-the-art thermoelectric material near room temperature. However, the figure of merit ZT of commercial BTS ingots is still limited and further improvement is imperative for their wide applications. Here, the results show that through dispersion of the Ag2Te nanophase in BTS, one can not only elevate its power factor (PF) by as high as 14% (at 300 K) but also reduce its thermal conductivity κtot to as small as ∼29% (at 300 K). Experimental evidences show that the improved PF comes from both increased electron mobility via inhibited Te vacancies and enhanced thermopower due to energy filtering effect, while the reduction of κtot originates from the drop of both electronic thermal conductivity largely owing to the reduced number of vacancy VTe·· and intensified phonon scattering chiefly from the dispersed Ag2Te nanophase. Consequently, the largest ZTmax = 1.31 (at 350 K) and average ZTave = 1.16 (300-500 K) are achieved for the Bi2Te2.7Se0.3-0.3 wt % Ag2Te composite sample, leading to a projected conversion efficiency η = 8.3% (300-500 K). The present results demonstrate that incorporation of nanophase Ag2Te is an effective approach to boosting the thermoelectric performance of BTS.

Vacancy-induced spontaneous H2 evolution by overall water splitting on MoTe2/Ti2CO2: A two-dimensional direct Z scheme heterostructure

Photocatalysts with direct Z-scheme heterostructures hold great potential for hydrogen production by solar-driven overall water splitting. However, building highly efficient direct Z-scheme heterostructures is still a challenge. This work develops a novel MoTe2/Ti2CO2 heterostructure as a potential direct Z-scheme photocatalyst for overall water splitting. First-principles computations utilizing density functional theory (DFT) have been used in this study to carefully study several aspects of the heterostructure, including vdW binding energy, formation of the internal electric field, electronic structure, optical properties, and thermodynamic feasibility of water splitting. According to these calculations, the superior interlayer interaction in MoTe2/Ti2CO2 results in a substantial built-in electric field suitable for carrier separation. These findings indicate that the suggested vdW heterostructure is a direct Z-scheme water-splitting photocatalyst with suitable band alignments and a broad light absorption range. This enhanced interaction resulted in a significant increase of photocurrent compared to both the monolayers. Additionally, it is envisaged that the heterojunction will serve as photocatalyst for the oxygen evolution process (OER) and the hydrogen evolution process (HER). The results demonstrate that MoTe2/Ti2CO2 heterostructure with Te vacancy has a low Gibbs free energy of the HER, favoring spontaneous hydrogen evolution in both acidic and neutral conditions. The suggested heterostructure exhibits a commendable solar-to-hydrogen efficiency of 15.25%, making it a viable option for commercial implementation. This research offers a theoretical foundation for designing novel, two-dimensional overall water-splitting photocatalysts in an effort to produce clean energy.

Identification of native defects of 1T-HfTe2

Defects have significant impact on material properties. Identifying the atomic structure is the foundation for defect studies. Combining scanning tunneling microscopy/spectroscopy and density functional theory simulations, we have studied native defects in the transition-metal dichalcogenide 1T-HfTe2. We find that the dominant single-atom defects are the Te vacancy, Te substitution by residual iodine, and Te substitution by residual oxygen. The defect concentrations have been estimated from the scanning tunneling microscopy images and are explained using the defect formation energies. In addition to those single-atom defects, we have also identified two types of defect complexes, which are composed of Te vacancies. Our work clarifies the defect information of 1T-HfTe2 and thus paves the way for future studies.

Elucidating atomistic mechanisms of the formation of phase-controlled ultrathin MoTe2 films and lateral hetero-phase MoTe2 interfaces

Molybdenum ditelluride (MoTe2) is gaining great attention because of its phase tunability. However, the precise control of the phase of ultrathin MoTe2 films is quite challenging, and the atomistic mechanism has not been fully elucidated. Here, we establish phase-controlled growth of ultrathin MoTe2 films by independently adjusting the Te atomic flux. We employed a chemical vapor deposition method that uses Mo nanolayers as a precursor to synthesize ultrathin MoTe2 films with a thickness of 4 − 7 nm. We found that ultrathin 1T’ MoTe2 films were formed with low Te atomic flux, whereas ultrathin 2H MoTe2 films were obtained with high Te atomic flux. Lateral metal–semiconductor hetero-phase 1T’/2H MoTe2 ultrathin films with seamless interfaces were synthesized with medium Te atomic flux. Furthermore, we performed density functional theory calculations to elucidate the detailed mechanisms of the phase transition between 1T’ and 2H MoTe2 driven by Te vacancy as well as the property of the 1T’/2H MoTe2 interface. This work provides a full understanding of the formation of phase-controlled ultrathin MoTe2 films and lateral metal–semiconductor hetero-phase MoTe2 interfaces.

Boosting the Hydrogen Evolution Reaction Performance of P-Doped PtTe2 Nanocages via Spontaneous Defects Formation.

PtTe2 , a member of the noble metal dichalcogenides (NMDs), has aroused great interest in exploring its behavior in the hydrogen evolution reaction (HER) due to the unique type-II topological semimetallic nature. In this work, a simple template-free hydrothermal method to obtain the phosphorus-doped (P-doped) PtTe2 nanocages with abundant amorphous and crystalline interface (A/C-P-PtTe2 ) is developed. Revealed by density functional theory calculations, the atomic Te vacancies can spontaneously form on the basal planes of PtTe2 by the P doping, which results in the unsaturated Pt atoms exposed as the active sites in the amorphous layer for HER. Owing to the defective structure, the A/C-P-PtTe2 catalysts have the fast Tafel step determined kinetics in HER, which contributes to an ultralow overpotential (η=28mV at 10mAcm-2 ) and a small Tafel slope of 37mVdec-1 . More importantly, benefiting from the inner stable crystalline P-PtTe2 nanosheets, limited decay of the performance is observed after chronopotentiometry test. This work reveals the important role of the inherent relationship between structure and activity in PtTe2 for HER, which may bring another enlightenment for the design of efficient catalysts based on NMDs in the near future.

Mixed-phase MoTe2 with exposed edges and rich defects for catalyzing hydrogen evolution reaction with noble-metal-like performance

Exploring low-cost electrocatalysts for the hydrogen evolution reaction (HER) is important in the development of renewable energy storage and conversion technology. Transition metal dichalcogenides, such as MoX2 (X = S, Se or Te), are a promising class of electrocatalysts for HER. However, MoX2 shows poorer catalytic performance than the commercially used platinum catalyst. In this study, we prepared MoTe2 with rich electroactive sites by morphology and defect engineering and investigated its electrocatalytic performance for the HER. The MoTe2 was grown on the surface of a Mo mesh and comprises the 2H and 1T’ phases of MoTe2 mixed evenly in the microscale. The morphology of MoTe2 depends on the cooling rate after its formation on the Mo mesh. A fast cooling process results in volcano-shaped protrusions and a large amount of defects in MoTe2, which facilitates the electrochemical reaction. The mixed-phase MoTe2 delivers better performance than does the commercial 5% Pt/C catalyst in an acidic medium, especially at high current densities. Density functional theory calculations reveal that the Gibbs free energy change for H atom adsorption is close to thermoneutral for both, the 2H and 1T’ phases of MoTe2. Their edges and Te vacancies can serve as active sites for catalyzing HER. This study provides a new strategy for developing inexpensive HER electrocatalysts that function optimally at high current densities.

Vacancy‐Defect Topological Insulators Bi2Te3−x Embedded in N and B Co‐Doped 1D Carbon Nanorods Using Ionic Liquid Dopants for Kinetics‐Enhanced Li–S Batteries

AbstractLithium–sulfur (Li–S) batteries are hindered by the shuttle effect and the sluggish redox kinetics of polysulfides. In this study, topological insulators (TIs) Bi2Te3−x with abundant Te vacancies embedded in N and B co‐doped carbon nanorods (Bi2Te3−x@NBCNs) are synthesized and used as sulfur host composites for high‐performance Li–S batteries. Bi2Te3−x@NBCNs effectively enhance the intrinsic conductivity, strengthened the chemical affinity, and accelerated the redox kinetics of polysulfides. 1D carbon nanorods with N and B co‐doped heteroatoms endowed with abundant polar sites improve the chemical affinity of polysulfides, while the embedded Bi2Te3−x nanoparticles further promote the nucleation and electrodeposition of Li2S2/Li2S. In situ Raman spectroscopy confirms that Bi2Te3−x@NBCNs effectively reduced cathode‐side accumulation of polysulfides and suppressed the shuttle effect. Owing to the extraordinary synergistic effects of rich heteroatom polar sites and conductive topological surface states, Bi2Te3−x@NBCN‐based cells exhibit a high initial specific capacity of 1264 mAh g−1 at 0.2 C and ultra‐long lifetime (>1000 cycles, with a degradation rate of 0.02% per cycle at 1.0 C). The fundamental insights offered by this work are likely to enable improvement of the electrochemical performance of Li–S batteries based on TI materials.

Experimental and Theoretical Studies of the Surface Oxidation Process of Rare‐Earth Tritellurides

AbstractRecent studies have established Van der Waals (vdW) layered and 2D rare‐earth tritellurides (RTe3) as superconductors and near room‐temperature charge density wave (CDW) materials. Their environmental stability raises natural concern owing to aging/stability effects observed in other tellurium‐based layered crystals. Here, the results establish the stability and environmental aging characteristics of these RTe3 systems involving a variety of metals such as La, Nd, Sm, Gd, Dy, and Ho. The atomic force microscopy (AFM) and scanning electron microscopy (SEM) results show that all the RTe3 sheets oxidize to form thin TeOx layers that are primarily confined to the surface, edges, and grain boundaries. Time‐resolved in situ Raman spectroscopy measurements are used to understand the kinetics of the oxidization process for different lanthanide metal cations and establish their relative stability/resilience to oxidization. Overall results indicate that the vdW layers show higher air stability as the 4f electron number decreases going from Ho to La, resulting in the most stable LaTe3 compared to the least stable HoTe3. Comprehensive quantum mechanical simulations reveal that environmental degradation originates from a strong oxidizing reaction with O2 molecules, while humidity (H2O) plays a negligible role unless Te vacancies are present. Moreover, the simulations explain the effects of 4f electrons on the work function and Te vacancies formation, which directly impact the aging characteristics of RTe3 layers. Interestingly, optical and electrical measurements show that the CDW response is still observed in aged RTe3 layers owing to the presence of underlying pristine/nonoxidized RTe3 layers, except CDW transition temperatures increase due to the thickness effect. Overall results offer the first in‐depth environmental aging studies on these materials, which can be applied to engineer and design their chemical stability, surface properties, and overall CDW characteristics.

Toward Perfect Surfaces of Transition Metal Dichalcogenides with Ion Bombardment and Annealing Treatment.

Layered transition metal dichalcogenides (TMDs) are two-dimensional materials exhibiting a variety of unique features with great potential for electronic and optoelectronic applications. The performance of devices fabricated with mono or few-layer TMD materials, nevertheless, is significantly affected by surface defects in the TMD materials. Recent efforts have been focused on delicate control of growth conditions to reduce the defect density, whereas the preparation of a defect-free surface remains challenging. Here, we show a counterintuitive approach to decrease surface defects on layered TMDs: a two-step process including Ar ion bombardment and subsequent annealing. With this approach, the defects, mainly Te vacancies, on the as-cleaved PtTe2 and PdTe2 surfaces were decreased by more than 99%, giving a defect density <1.0 × 1010 cm-2, which cannot be achieved solely with annealing. We also attempt to propose a mechanism behind the processes.

Catalytic activity for hydrogen evolution reaction of Janus monolayer MoXTe (X=S, Se)

At present, the precious metal Pt is a common catalyst for large-scale hydrogen evolution reaction (HER) production of hydrogen, but due to its high price and scarcity, finding an innovative catalyst has become the key to electrocatalytic hydrogen evolution. Here, the HER electrocatalytic activity of Janus MoXTe (X = S, Se) monolayers was investigated through first-principles calculations. Mo vacancy, X vacancy and Te vacancy were introduced into 2H, 1T, and 1T’ phase respectively and their stability was studied. The results show that the introduction of vacancy can improve the electrocatalytic hydrogen evolution performance. Particularly, the Gibbs free energies (ΔGH) of Te vacancy of 2H phase MoSTe and MoSeTe are close to zero (ΔGH = 0.03, −0.05 eV, respectively), and has the highest exchange current density. We further find that the conductivity of 2H phase MoSTe and MoSeTe is enhanced after introducing Te vacancy. In details, H get 1.86 and 1.43 e on VTe in 2H phase MoSTe and MoSeTe. The bond between S and H is more stable, H is better adsorbed on the catalyst, and the performance is improved. Our research provides a strategy for designing MoXTe monolayer electrocatalysts, which are predicted to be employed in HER catalysts with low cost and high performance.

Hierarchical Architectural Structures Induce High Performance in n‐Type GeTe‐Based Thermoelectrics

AbstractCompatible p‐ and n‐type materials are necessary for high‐performance GeTe thermoelectric modules, where the n‐type counterparts are in urgent need. Here, it is reported that the p‐type GeTe can be tuned into n‐type by decreasing the formation energy of Te vacancies via AgBiTe2 alloying. AgBiTe2 alloying induces Ag2Te precipitates and tunes the carrier concentration close to the optimal level, leading to a high‐power factor of 6.2 µW cm−1 K−2 at 423 K. Particularly, the observed hierarchical architectural structures, including phase boundaries, nano‐precipitates, and point defects, contribute an ultralow lattice thermal conductivity of 0.39 W m−1 K−1 at 423 K. Correspondingly, an increased ZT of 0.5 at 423 K is observed in n‐type (GeTe)0.45(AgBiTe2)0.55. Furthermore, a single‐leg module demonstrates a maximum η of 6.6% at the temperature range from 300 to 500 K. This study indicates that AgBiTe2 alloying can successfully turn GeTe into n‐type with simultaneously optimized thermoelectric performance.

Large Spin Hall Conductivity and Excellent Hydrogen Evolution Reaction Activity in Unconventional PtTe1.75 Monolayer.

Two-dimensional (2D) materials have gained lots of attention due to the potential applications. In this work, we propose that based on first-principles calculations, the (2 × 2) patterned PtTe2 monolayer with kagome lattice formed by the well-ordered Te vacancy (PtTe1.75) hosts large and tunable spin Hall conductivity (SHC) and excellent hydrogen evolution reaction (HER) activity. The unconventional nature relies on the A1 @ 1b band representation of the highest valence band without spin-orbit coupling (SOC). The large SHC comes from the Rashba SOC in the noncentrosymmetric structure induced by the Te vacancy. Even though it has a metallic SOC band structure, the ℤ2 invariant is well defined because of the existence of the direct bandgap and is computed to be nontrivial. The calculated SHC is as large as 1.25 × 103 (Ω cm)-1 at the Fermi level (EF ). By tuning the chemical potential from EF - 0.3 to EF + 0.3 eV, it varies rapidly and monotonically from -1.2 × 103 to 3.1 . In addition, we also find that the Te vacancy in the patterned monolayer can induce excellent HER activity. Our results not only offer a new idea to search 2D materials with large SHC, i.e., by introducing inversion-symmetry breaking vacancies in large SOC systems, but also provide a feasible system with tunable SHC (by applying gate voltage) and excellent HER activity.

DFT study on the adsorption of CO, NO2, SO2 and NH3 by Te vacancy and metal atom doped MoTe2 monolayers

Based on the first-principles calculation, the adsorption structure and electronic characteristics of CO, NO 2 , SO 2 and NH 3 adsorbed on pure, defective and metal atom doped MoTe 2 monolayer were systematically studied. The original MoTe 2 monolayer and MoTe 2 monolayer containing Te defects can be used as room temperature gas sensors to detect NO 2 gas and SO 2 gas, and the adsorption effect can steadily increase in the simulated working environment. The adsorption behaviors of Ag, Pd and Rh atoms with different doping methods were studied. The calculation results show that the conductivity and chemical activity between metal atoms and gas is improved after doping, resulting in tremendous adsorption energy and charge transfer. Due to the changes in the energy band structure, the density of states, work function and recovery time after adsorption, different doped systems show the potential of high-performance gas sensors at room temperature and high temperature. The research results are significant to the gas sensing mechanism of the modified MoTe 2 monolayer and helpful in exploring the modified MoTe 2 monolayer as a sensing material and gas scavenger. • MoTe 2 containing defects can improve gas adsorption capacity, especially for SO 2 molecules. • Two methods of metal atom doping can improve the chemical activity of gas and substrate. • The adsorption structures under different systems were explored as gas sensors and scavengers.

Superconductivity in single-crystalline ZrTe3-x (x ≤ 0.5) nanoplates.

Superconductivity with an unusual filamented character below 2 K has been reported in bulk ZrTe3 crystals, a well-known charge density wave (CDW) material, but still lacks in its nanostructures. Here, we systemically investigated the transport properties of controllable chemical vapor transport synthesized ZrTe3-x nanoplates. Intriguingly, superconducting behavior is found at T c = 3.4 K and can be understood by the suppression of CDW due to the atomic disorder formed by Te vacancies. Magnetic field and angle dependent upper critical field revealed that the superconductivity in the nanoplates exhibits a large anisotropy and two-dimensional character. This two-dimensional nature of superconductivity was further satisfactorily described using the Berezinsky-Kosterlitz-Thouless transition. Our results not only demonstrate the critical role of Te vacancies for superconductivity in ZrTe3-x nanoplates, but also provide a promising platform to explore the exotic physics in the nanostructure devices.