Exploring the hidden catalyst from boron pnictide family for HER and OER

A systematic investigation of catalytic activity of boron phosphide nanowire (BP NW) towards over-all water-splitting reaction has been performed by evaluating the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities. Intended to the mentioned aim, we have utilized Kohn-Sham formulated extensively popular ab initio method based on density functional theory (DFT). The structural and electronic properties of the BP NW are computed and compared with its bulk phase. We observe dramatic indirect to direct bandgap transition with pronounced energy gap after introducing two-dimensional confinement that is akin to the other reported III-V NWs. The calculated partial density of states with van Hove singularity also confirms the same. Owing to its moderate bandgap value, the applicability of the BP NW as an HER/OER catalyst is assessed by computing the site dependent HER/OER activities. Our computation on Gibbs free energy for the case of hydrogen adsorption with −1.19 eV magnitude gives better results; whereas in case of OER, the results with higher magnitude of Gibbs energy implicate over binding of oxygen with adsorbent thus revealing non-feasible desorption of oxygen from adsorbent. Significant perturbation in electronic states of NW under hydrogen adsorption confirms high sensitivity of BP NW for hydrogen adsorption. Further, the effect of substitutional doping on HER and OER activities suggests that the doped NW shows poor HER activity in contrast to the site-dependent better OER activity in case of Ga doped BP NW. The present BP NW shows potential as an HER catalyst owing to its lower adsorption and Gibbs free energies (−1.07 and −0.84 eV), as compared to previously conventionally utilized III-V NWs. Henceforth, we believe that the present study would serve as a blueprint for the researchers to design and develop toxic and/or metal-free catalyst that can be utilized for efficient water-reduction.     

Catalytic activity and underlying atomic rearrangement in monolayer CoOOH towards HER and OER

For efficient hydrogen and oxygen production, design and synthesis of cost-effective, stable and active materials are inevitable. In this work, the catalytic activity of 2D CoOOH towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been investigated using first principles calculations based on density functional theory. The adatom induced structural rearrangement have been investigated from structural parameters as well as charge redistribution in 2D CoOOH. The preferred site for hydrogen and oxygen adsorption were found to be the top site of oxygen atom of 2D CoOOH. The catalytic activity of HER and OER towards 2D CoOOH was studied by calculating the Gibbs free energy. Our study revealed that the 2D CoOOH serve better as a catalyst for HER than OER with adsorption energy of −0.45 and −3.68 eV respectively suggesting its efficient use for hydrogen production. We further investigated the changes in electronic properties of 2D CoOOH on adsorption of hydrogen and oxygen atom.     

Hydrogen evolution reaction activity of III-V heterostructure nanowires

III-V materials have gained great interest in materials science community since a few decades due to their versatile properties. Experimenting with the crystal phase, dimensional confinement, chemical environment and external conditions open-up a window to finely tune the properties of material of interest at atomic scale. The density functional theory-based investigation of III-V semiconductors under heterostructure nanowire configuration is presented with specific focus on the electronic dispersion and band alignment. The combination of GaSb core having triangular(T)/circular(R) cross-sections with GaP and GaAs shells reveal formation of type-II heterostructure with moderate electronic band gap suggesting promising application in photocatalysis and photovoltaics. Motivated by these results, we have investigated the catalytic activity of the heterostructure nanowires towards hydrogen evolution reaction (HER). Interesting results showing reliable HER activity over different surface sites of the nanowires evidently approve their importance as an active HER catalyst. Furthermore, being within nano regime, the present systems suggest cost-effective production of HER catalyst as compared to the conventional novel metal-based catalysts.     

Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study

In the ever growing demand of future energy resources, hydrogen production reaction has attracted much attention among the scientific community. In this work, we have investigated the hydrogen evolution reaction (HER) activity on an open-shell polyaromatic hydrocarbon (PAH), graphene quantum dot “triangulene” using first principles based density functional theory (DFT) by means of adsorption mechanism and electronic density of states calculations. The free energy calculated from the adsorption energy for graphene quantum dot (GQD) later guides us to foresee the best suitable catalyst among quantum dots. Triangulene provides better HER with hydrogen placed at top site with the adsorption energy as −0.264 eV. Further, we have studied platinum decorated triangulene for hydrogen storage. Three different sites on triangulene were considered for platinum atom adsorption namely top site of carbon (C) atom, hollow site of the hexagon carbon ring near triangulene's unpaired electron and bridge site over C–C bond. It is found that the platinum atom is more stable on the hollow site than top and bridge site. We have calculated the density of states (DOS), highest occupied molecular orbitals (HOMO), lowest unoccupied molecular orbitals (LUMO) and HOMO-LUMO gap of hydrogen molecule adsorbed platinum decorated triangulene. Our results show that the hydrogen molecule (H₂) dissociates instinctively on all three considered sites of platinum decorated triangulene resulting in D-mode. The fundamental understanding of adsorption mechanism along with analyses of electronic properties will be important for further spillover mechanism and synthesis of high-performance GQD for H₂ storage applications.     

Investigating hydrogen evolution reaction properties of a new honeycomb 2D AlC

The hydrogen due to its high mass energy density is a new renewable, economically viable and clean resource. The most eco-friendly and economical approaches for the generation of hydrogen through hydrogen evolution is electrochemical water splitting. The two-dimensional (2D) nanomaterials have been recently found as potential candidates as non-noble metal catalyst for hydrogen evolution. In this work, we have systematically studied the structural and electronic properties of the newly predicted hexagonal-aluminium carbide monolayer (h-AlC ML) under the framework of dispersion-corrected density functional theory (DFT) calculations. The calculated electronic total density of states (TDOS) of h-AlC ML predict its metallic nature in contrast to other polar honeycomb 2D materials which are either semiconducting or semimetallic. The metallic behavior of h-AlC monolayer which motivates us to investigate its HER activity results due to the presence of delocalized charge density near Fermi level. Thus, we have investigated the HER activity of h-AlC ML by calculating hydrogen (H) adsorption energy (ΔE_H) and Gibbs free energy (ΔG_H) at three different sites of the 3 × 3 and 4 × 4 supercells of h-AlC ML; top of carbon atom (E_H-C), top of aluminium atom (E_H-Al) and hollow site (E_H-Hollow). Our results show that the hollow site is most catalytically active site in both supercells of h-AlC ML. We believe that our results will inspire experimentalists to fabricate this new 2D material for achieving the desired range of HER activity.     

Novel hydrogen-doped SrSnO3 perovskite with excellent optoelectronic properties as a potential photocatalyst for water splitting

Developing and designing novel electrodes for photocatalytic water splitting using computational analysis has become a crucial interest recently through bulk and surface calculations of the investigated materials. Doping wide band gap metal oxides has proven to be an efficient method for optical properties enhancement and band gap engineering. Herein, first-principles calculations were employed to investigate the possibility to engineer the optical and structural properties of SrSnO₃ perovskite as a potential catalyst for photo-driven hydrogen production. Specifically, the synergistic effect of hydrogen doping and oxygen vacancies (O_V) on the optoelectronic properties of SrSnO₃ was for the first time investigated and discussed in detail. The interstitial hydrogen defects (H_i) are energetically favorable compared with the substitutional hydrogen defects. Mono- and co-hydrogen occupied oxygen vacancies sites were further examined. Interstitial hydrogen doping was found to introduce a shallow defect state below the conduction band minimum (CBM) forming a band tailing and increasing the dielectric constant. Thus, it could be used in gate dielectric applications. The created defect states upon doping were found to depend directly on the defect site and the defect concentration. At high concentration of oxygen vacancies defect, the H_OV-O_V structural configuration showed localized and shallow defect states with a band gap of 1.3 eV below the CBM. It also considerably increased the dielectric constant with optical absorption enhancement, compared to the pristine SrSnO₃ counterpart. With optimum Gibbs free energy of hydrogen evolution reaction (HER) and theoretical band gap straddling of the oxygen and hydrogen evolution potentials, low exciton binding energy, and high permittivity, the H_OV-O_V structure is an ideal novel candidate catalyst for photocatalytic water splitting.     

Hydrogen evolution reaction and electronic structure calculation of two dimensional bismuth and its alloys

We present a systematic ab initio study of atomic hydrogen and oxygen adsorption on bismuthene monolayer and its alloys with arsenic and antimony through electronic structure calculations based on density functional theory within generalized gradient approximation. We systematically investigated the preferable adsorption site for hydrogen and oxygen atom on 2D Bi, BiAs and BiSb. It was found that the hydrogen atom prefers top site of bismuth atom and oxygen atom prefers to reside in the hexagonal ring of these 2D bismuth alloys. The free energy calculated from the individual adsorption energy for each monolayer subsequently guides us to predict the best suitable catalyst among the considered 2D monolayers. The 2D BiSb serves better for hydrogen evolution reaction (HER) with hydrogen adsorption energy as ?1.384 eV while 2D BiAs is suitable for oxygen evolution reaction (OER) with oxygen adsorption energy as ?1.092 eV. We further investigated the effects of the adsorbate atom on the electronic properties of 2D Bi, BiAs and BiSb. The adsorption of oxygen on 2D BiAs and BiSb was shown to reduce the bulk band gap by 40.56 and 67.79% respectively which will be advantageous for the observation of Quantum Spin Hall effect at ambient conditions.     

Regulating surface wettability and electronic state of molybdenum carbide for improved hydrogen evolution reaction

Molybdenum carbide (Mo₂C) is a cost-effective transition metal carbides (TMCs) electrocatalyst for hydrogen evolution reaction (HER) due to its electronic structure similar to Pt-group noble metal. Herein, we report that an effective strategy of regulating the surface wettability and electronic state of Mo₂C by ammonia and hydrothermal co-treatment to enhance its HER activity. The electrochemical results demonstrate that Mo₂C undergoing ammonia and hydrothermal co-treatment (Mo₂C-wh) exhibits remarkably improved electrocatalytic HER activity as compared to the pristine Mo₂C (Mo₂C-p) catalyst. The activity-structure relationship studies manifest that ammonia and hydrothermal co-treatment increases the content of hydroxyl group and pyridinic-N, thus endowing Mo₂C-wh with lower charge transfer resistance, larger electrochemical active surface area and higher surface wettability. DFT calculations reveal that ammonia and hydrothermal co-treatment enhances the Mo 3d-band center and reduces the hydrogen adsorption free energy. These changes in electronic states of Mo sites and physical properties of Mo₂C positively contribute to the improvement of electrocatalytic HER activity.     

Reaction coordinate mapping of hydrogen evolution mechanism on Mg3N2 monolayer

In this work, we have envisaged the hydrogen evolution reaction (HER) mechanism on Mg₃N₂ monolayer based on electronic structure calculations within the framework of density functional theory (DFT) formalism. The semiconducting nature of Mg₃N₂ monolayer motivates us to investigate the HER mechanism on this sheet. We have constructed the reaction coordinate associated with HER mechanism after determining the hydrogen adsorption energy on Mg₃N₂ monolayer, while investigating all possible adsorption sites. After obtaining the adsorption energy, we subsequently obtain the adsorption free energy while adding zero point energy difference (ΔZPE) and entropic contribution (TΔS). We have not only confined our investigations to a single hydrogen, but have thoroughly observed the adsorption phenomena for increasing number of hydrogen atoms on the surface. We have determined the projected density of states (DOS) in order to find the elemental contribution in the valence band and conduction band regime for all the considered cases. We have also compared the work function value among all the cases, which quantifies the amount of energy required for taking an electron out of the surface. The charge transfer mechanism is also being investigated in order to correlate with the HER mechanism with amount of charge transfer. This is the first attempt on this material to the best of our knowledge, where theoretical investigation has been done to mapping the reaction coordinate of HER mechanism with the associated charge transfer process and the work function values, not only for single hydrogen adsorption, but also for increasing number of adsorbed hydrogen.     

Role of functionalized graphene quantum dots in hydrogen evolution reaction: A density functional theory study

Rapid advances in the field of catalysis require a microscopic understanding of the catalytic mechanisms. However, in recent times, experimental insights in this field have fallen short of expectations. Furthermore, experimental searches of novel catalytic materials are expensive and time-consuming, with no guarantees of success. As a result, density functional theory (DFT) can be quite advantageous in advancing this field because of the microscopic insights it provides and thus can guide experimental searches of novel catalysts. Several recent works have demonstrated that low-dimensional materials can be very efficient catalysts. Graphene quantum dots (GQDs) have gained much attention in past years due to their unique properties like low toxicity, chemical inertness, biocompatibility, crystallinity, etc. These properties of GQDs which are due to quantum confinement and edge effects facilitate their applications in various fields like sensing, photoelectronics, catalysis, and many more. Furthermore, the properties of GQDs can be enhanced by doping and functionalization. In order to understand the effects of functionalization by oxygen and boron based groups on the catalytic properties relevant to the hydrogen-evolution reaction (HER), we perform a systematic study of GQDs functionalized with the oxygen (O), borinic acid (BC₂O), and boronic acid (BCO₂). All calculations that included geometry optimization, electronic and adsorption mechanism, were carried out using the Gaussian16 package, employing the hybrid functional B3LYP, and the basis set 6-31G(d,p). With the variation in functionalization groups in GQDs, we observe significant changes in their electronic properties. The adsorption energy E_ads of hydrogen over O-GQD, BC₂O-GQD, and BCO₂-GQD is ?0.059 eV, ?0.031 eV and ?0.032 eV respectively. Accordingly, Gibbs free energy (ΔG) of hydrogen adsorption is extraordinarily near the ideal value (0 eV) for all the three types of functionalized GQDs. Thus, the present work suggests pathways for experimental realization of low-cost and multifunctional GQDs based catalysts for clean and renewable hydrogen energy production.     

Modulating oxygen electronic orbital occupancy of Cr-based MXenes via transition metal adsorbing for optimal HER activity

Modulating the surface electronic properties of the 2D MXenes is of significant importance to boost their hydrogen evolution reaction (HER) activity. Herein, a series of transition metal adatoms are employed to tune the surface electronic properties of Cr-based MXenes with oxygen function group for realizing impressive HER performance. The Results show that the charge of surface oxygen atoms, which is affected by both the host metal atoms and the adsorbed transition metal atoms, play critical role in the adsorption strength of hydrogen. The optimal performance is achieved by depositing Cr atom on the Cr₂TiC₂O₂ MXene, which results in the adsorption free energy of hydrogen very close to zero (0.03 eV). Systematic electronic structure analyses confirm that the charge transfer from the adsorbed transition metals to the neighboring surface oxygen atoms could tune the orbital occupancy of oxygen and their adsorption strength to hydrogen atom and therefore the HER activity. These findings and concepts may be useful for the design of advanced MXene-based HER catalysts.     

A review of heteroatomic doped two-dimensional materials as electrocatalysts for hydrogen evolution reaction

Emerging two-dimensional (2D) materials, such as graphene, transition metal disulfide compounds (TMDCs), MXenes, layer double hydroxides (LDHs), black phosphorus (BP) and hexagonal boron nitride (h-BN), play an important role in speeding up hydrogen evolution reaction (HER) due to its large specific surface area as well as function of loading and efficient support. However, as an electrocatalyst, pure 2D materials cannot meet HER needs caused by their monotonous performance. Therefore, some nanoparticles are used to load and tune the 2D materials to develop efficient and inexpensive catalysts. Herein, we conduct a thorough analysis for materials based on heteroatoms, especially transition metal atoms and non-metal atoms (N, P, S, etc.) doped with graphene, TMDCs, MXenes, LDHs, BP and h-BN. It can be found that doping or coupling between 2D materials will affect the electronic structure, energy band, active area, conductivity and stability of the catalyst, which will induct a huge change in the catalytic performance. This review reveals the relationship between active centers, H₂O adsorption and chemical reaction processes. It also analyzes and summarizes the design principles and performance improvement mechanisms of hybrid catalysts. These discussions can provide references for other researchers to develop derivatives of related catalysts.     

Efficient and stable electrocatalyst for hydrogen evolution reaction prepared by hybrid technique in surface engineering: Electrochemical and magnetron sputtering methods

Developing low-cost, stable, and robust electrocatalysts is significant for high effective hydrogen evolution reaction (HER). In this work, a coating system with Cu₂O/NiMoCu on stainless steel (SS) is employed as a highly active and stable catalyst for HER in acidic solutions. Electrochemical measurements for as-designed system on SS show a low onset overpotential, small Tafel slope of ～32 mV/decade and long-term durability over 7 days of HER operation. To further inspections of electrocatalytic behavior of as-prepared system in HER, the EIS measurements are performed at several overpotentials and temperatures. It is found that high hydrogen evolution activity and stability of Cu₂O/NiMoCu hybrid is likely due to special morphology of Cu₂O which result in large number of active sites for hydrogen adsorption, and a synergetic effect giving electronic structure suitable for the HER.     

Electronic structure modulation of CoSe2 nanowire arrays by tin doping toward efficient hydrogen evolution

The development of highly active, durable and earth-abundant electrocatalysts toward hydrogen evolution reaction (HER) is of great significance for promoting hydrogen energy. As one of the most potential substitutes for Pt-based materials, pyrite cobalt selenide (CoSe₂) still has shortcomings in terms of HER performance possibly due to its unfavorable hydrogen adsorption characteristics. Metal cation doping has been considered as one of the most available methods to modulate the electronic structure of electrocatalysts. Herein, non-transition metal tin (Sn) doped CoSe₂ nanowire arrays grown on carbon cloth have been constructed and fabricated via a simple gas-phase selenization treatment of hydroxide precursor. The successful doping of Sn element into CoSe₂ nanowires was confirmed by many experimental results. The as-prepared catalyst shows an obviously enhanced HER performance in alkaline media. Compared with pristine CoSe₂, the overpotential of Sn doped catalyst with optimal doping content decreases from 189 mV to 117 mV at 10 mA cm^?2 and the Tafel slope declines from 94 mV dec^?1 to 86 mV dec^?1, as well as shows long-term durability for 100 h. Experimental results and further density functional theory (DFT) calculations show that Sn doping can improve the ability of charge transfer and increase the electrochemical surface area, as well as optimize the hydrogen adsorption energy, all of which are instrumental in HER performance improvement. This work not only provides atomic-level insight into regulating the electronic structure of transition metal selenides by main group metal doping, but also broadens the avenue of developing high-efficiency and stable non-precious metal catalysts.     

Enhanced catalytic activities of Fe anchored on graphene substrates for water splitting and hydrogen evolution

Water splitting on single Fe atom catalyst anchored on defective graphene surfaces by using first-principles density functional theory. The structure and electronic features of isolated Fe atom anchored on three graphene surfaces with single vacancy (SV), double vacancy (DV) and Stone-Wales structure (SW) defect were systematically explored. The three structures prove to be high activity and high stability on catalytic. The adsorption and the energy barrier of water splitting as well as hydrogen adsorption free energy ΔG_H1 on single-atom Fe were also studied. The sequence of promoted splitting activity is found to be Fe@SW > Fe@DV > Fe@SV. Furthermore, by hydrogen adsorption free energy ΔG_H1 analysis, we predict that the HER catalytic activity of graphene nanosheet can be improved by anchoring Fe atom on SV and DV structures, which are comparable to or even better than noble metals. It is found that the catalytic activity of water splitting and HER can be changed with the shift in d-band center with respect to Fermi-level. Detailed investigations on electronic structure of Fe@graphene catalytic systems disclose an obvious orbital hybridization coupling and charge transfer between atom Fe on carbon surfaces and water molecule. These results provide us with new insight into design of high performer and low-cost catalysts and may inspire potential applications in the fields of clean and renewable energy.     

Hydrogen evolution reaction on in-plane platinum and palladium dichalcogenides via single-atom doping

Searching for the catalysts with excellent catalytic activity and high chemical stability is the key to achieve large-scale production of hydrogen (H₂) through hydrogen evolution reaction (HER). Two-dimensional (2D) platinum and palladium dichalcogenides with extraordinary electrical properties have emerged as the potential candidate for HER catalysts. Here, chemical stability, HER electrocatalytic activity, and the origin of improved HER performance of Pt/Pd-based dichalcogenides with single-atom doping (B, C, N, P, Au, Ag, Cu, Co, Fe, Ni, Zn) and vacancies are explored by first-principles calculations. The calculated defect formation energy reveals that most defective structures are thermodynamically stable. Hydrogen evolution performance on basal plane is obviously improved by single-atoms doping and vacancies. Particularly, Zn-doped and Te vacancy PtTe₂ have a ΔG_H value close to zero. Moreover, defect engineering displays a different performance on HER catalytic activity in sulfur group elements, in order of S < Te < Se in Pd-based chalcogenides, and S < Se < Te in Pt-based chalcogenides. The origin of improved hydrogen evolution performance is revealed by electronic structure and charge transfer. Our findings of the highly activating defective systems provide a theoretical basis for HER applications of platinum and palladium dichalcogenides.     

Hierarchical NiMo alloy microtubes on nickel foam as an efficient electrocatalyst for hydrogen evolution reaction

Developing efficient, non-noble electrocatalysts for hydrogen evolution reaction (HER) is of high significance for future energy supplement, but challenging. NiMo alloy is a non-noble-metal-based efficient catalyst for HER due to its appropriate hydrogen binding energy and excellent alkali corrosion resistance. Herein, for the first time, we report the preparation of radially aligned NiMo alloy microtubes on Ni foam (NiMo MT/NF). The synthesized NiMo alloy catalyst was composed of the Ni₁₀Mo phase; notably, this hierarchically structured material possessed abundant active sites and a high surface area, and exhibited efficient electronic transport properties. The NiMo MT/NF electrode exhibited a low overpotential of 119 mV at 10 mA/cm² in a base solution, which was 50 mV less than that of NiMo alloy nanoparticles on NF (169 mV).     

Activating basal plane of Janus VSSe for efficient hydrogen evolution reaction by non-noble metal element doping: A first-principal study

Searching electrocatalysts with excellent hydrogen evolution reaction (HER) performance is very important for developing clean hydrogen energy. Two-dimensional (2D) materials have been widely studied as HER electrocatalysts, however, the basal planes of 2D materials, which dominate the surface area, are usually with poor activity. In this work, we theoretically studied the HER activity of Janus 2H–VSSe with or without non-noble metal element doping. Density functional theory (DFT) calculations suggest that doping As and Si atoms in the S or Se sites of VSSe and the C and Ge atoms in the Se site of VSSe greatly promote the HER performance of the basal plane of VSSe, resulting in hydrogen adsorption free energy close to zero (i.e. ?0.022, ?0.040, 0.066, 0.065, ?0.030, 0.058 eV, respectively), which are better than the Pt catalyst (?0.09 eV). The doped atoms strengthen the interaction between their pz-orbital and the hydrogen s-orbital, resulting in a lower bonding state in energy and higher bind strength for the hydrogen atom. This work opens up a new way to design highly efficient and low-cost catalysts for HER.     

Enhanced HER catalysis based on MXene/N-doped graphene heterostructures: A first-principles study

Based on the first-principles density functional theory, we investigated the structural and electronic properties and hydrogen evolution reaction (HER) activity of the heterostructures composed of different MXene and N-doped graphene (NDG). Our results show noticeable electron transfer occurring between the interfaces of the heterostructure, and the addition of MXene modifies the electronic structure of the NDG surface. Furthermore, it was observed that the heterostructure enhanced the adsorption of H on NDG surface and improved HER activity. The effects of heterostructure types and H coverage rate on HER activity were also investigated. This study suggests that appropriate design of MXene/NDG heterostructure can make it a potential HER catalyst.     

Role of V doping in core–shell heterostructured Bi2Te3/Sb2Te3 for hydrogen evolution reaction

Non-noble metal-based materials as low-cost hydrogen evolution reaction (HER) catalysts are key materials for sustainable hydrogen energy production. Bismuth and antimony chalcogenides are among the hopeful candidates to achieve this goal. In this work, a V-doped Sb₂Te₃ encapsulated Bi₂Te₃ core-shell electrocatalyst (Bi₂Te₃/V_x-Sb₂Te₃) has been synthesized by a two-step solvothermal method. V doping adjusts the electronic structure of catalyst, dramatically enhances electric double layer capacitance (C_dl) of the catalyst, decreases charge transfer resistance (R_ct) of the catalyst and increases carrier concentration of the catalyst. Therefore, the V doping method increases the active sites on the surface of the material, and promotes the charge transfer and electron transport in the HER process. In addition, V doping can also adjust the hydrophilicity of the material surface, promote the release of hydrogen, and quickly re-expose the active sites. Bi₂Te₃/V_x-Sb₂Te₃ electrocatalysts exhibit brilliant HER activity and high stability in both acidic and alkaline electrolytes. This study uses the strategy of V doping to control the electronic structure of materials, which will provide suggestions for the design and preparation for other high-activity catalysts.