High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells.

Three-dimensional organic-inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley-Queisser limit for a single­junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden-Popper phases--layered two-dimensional perovskite films--have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.

Effect of Stacking Interactions on the Translation of Structurally Related Bis(thiosemicarbazonato)nickel(II) HER Catalysts to Modified Electrode Surfaces.

A series of crystalline nickel(II) complexes (1-3) based on inexpensive bis(thiosemicarbazone) ligands diacetylbis(4-methyl-3-thiosemicarbazone) (H2ATSM), diacetylbis(4,4-dimethyl-3-thiosemicarbazone) (H2ATSDM), and diacetylbis[4-(2,2,2-trifluoroethyl)-3-thiosemicarbazone] (H2ATSM-F6) were synthesized and characterized by single-crystal X-ray diffraction and NMR, UV-visible, and Fourier transform infrared spectroscopies. Modified electrodes GC-1-GC-3 were prepared with films of 1-3 deposited on glassy carbon and evaluated as potential hydrogen evolution reaction (HER) catalysts. HER studies in 0.5 M aqueous H2SO4 (10 mA cm-2) revealed dramatic shifts in the overpotential from 0.740 to 0.450 V after extended cycling for 1 and 2. The charge-transfer resistances for GC-1-GC-3 were determined to be 270, 160, and 630 Ω, respectively. Characterization of the modified surfaces for GC-1 and GC-2 by scanning electron microscopy and Raman spectroscopy revealed similar crystalline coatings before HER that changed to surface-modified crystallites after conditioning. The surface of GC-3 had an initial glasslike appearance before HER that delaminated after HER. The differences in the surface morphology and the effect of conditioning are correlated with crystal-packing effects. Complexes 1 and 2 pack as columns of interacting complexes in the crystallographic a direction with short interplanar spacings between 3.37 and 3.54 Å. Complex 3 packs as columns of isolated molecules in the crystallographic b direction with long-range interplanar spacings of 9.40 Å.

Intense pulsed light, a promising technique to develop molybdenum sulfide catalysts for hydrogen evolution.

We have demonstrated a simple and scalable fabrication process for defect-rich MoS2 directly from ammonium tetrathiomolybdate precursor using intense pulse light treatment in milliseconds durations. The formation of MoS2 from the precursor film after intense pulsed light exposure was confirmed with XPS, XRD, electron microscopy and Raman spectroscopy. The resulting material exhibited high activity for the hydrogen evolution reaction (HER) in acidic media, requiring merely 200 mV overpotential to reach a current density of 10 mA cm-2. Additionally, the catalyst remained highly active for HER over extended durability testing with the overpotential increasing by 28 mV following 1000 cycles. The roll-to-roll amenable fabrication of this highly-active material could be adapted for mass production of electrodes comprised of earth-abundant materials for water splitting applications.

The role of electronic coupling between substrate and 2D MoS2 nanosheets in electrocatalytic production of hydrogen.

The excellent catalytic activity of metallic MoS2 edges for the hydrogen evolution reaction (HER) has led to substantial efforts towards increasing the edge concentration. The 2H basal plane is less active for the HER because it is less conducting and therefore possesses less efficient charge transfer kinetics. Here we show that the activity of the 2H basal planes of monolayer MoS2 nanosheets can be made comparable to state-of-the-art catalytic properties of metallic edges and the 1T phase by improving the electrical coupling between the substrate and the catalyst so that electron injection from the electrode and transport to the catalyst active site is facilitated. Phase-engineered low-resistance contacts on monolayer 2H-phase MoS2 basal plane lead to higher efficiency of charge injection in the nanosheets so that its intrinsic activity towards the HER can be measured. We demonstrate that onset potentials and Tafel slopes of ∼-0.1 V and ∼50 mV per decade can be achieved from 2H-phase catalysts where only the basal plane is exposed. We show that efficient charge injection and the presence of naturally occurring sulfur vacancies are responsible for the observed increase in catalytic activity of the 2H basal plane. Our results provide new insights into the role of contact resistance and charge transport on the performance of two-dimensional MoS2 nanosheet catalysts for the HER.

Polaron Stabilization by Cooperative Lattice Distortion and Cation Rotations in Hybrid Perovskite Materials.

Solution-processed organometallic perovskites have rapidly developed into a top candidate for the active layer of photovoltaic devices. Despite the remarkable progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices, remain open. High on the list of unexplained phenomena are very modest mobilities despite low charge carrier effective masses. Moreover, experiments elucidate unique degradation of photocurrent affecting stable operation of perovskite solar cells. These puzzles suggest that, while ionic hybrid perovskite devices may have efficiencies on par with conventional Si and GaAs devices, they exhibit more complicated charge transport phenomena. Here we report the results from an in-depth computational study of small polaron formation, electronic structure, charge density, and reorganization energies using both periodic boundary conditions and isolated structures. Using the hybrid density functional theory, we found that volumetric strain in a CsPbI3 cluster creates a polaron with binding energy of around 300 and 900 meV for holes and electrons, respectively. In the MAPbI3 (MA = CH3NH3) cluster, both volumetric strain and MA reorientation effects lead to larger binding energies at around 600 and 1300 meV for holes and electrons, respectively. Such large reorganization energies suggest appearance of small polarons in organometallic perovskite materials. The fact that both volumetric lattice strain and MA molecular rotational degrees of freedom can cooperate to create and stabilize polarons indicates that in order to mitigate this problem, formamidinium (FA = HC(NH2)2) and cesium (Cs) based crystals and alloys, are potentially better materials for solar cell and other optoelectronic applications.

Phase-engineered low-resistance contacts for ultrathin MoS2 transistors.

Ultrathin molybdenum disulphide (MoS2) has emerged as an interesting layered semiconductor because of its finite energy bandgap and the absence of dangling bonds. However, metals deposited on the semiconducting 2H phase usually form high-resistance (0.7 kΩ µm-10 kΩ µm) contacts, leading to Schottky-limited transport. In this study, we demonstrate that the metallic 1T phase of MoS2 can be locally induced on semiconducting 2H phase nanosheets, thus decreasing contact resistances to 200-300 Ω µm at zero gate bias. Field-effect transistors (FETs) with 1T phase electrodes fabricated and tested in air exhibit mobility values of ~50 cm(2) V(-1) s(-1), subthreshold swing values below 100 mV per decade, on/off ratios of >10(7), drive currents approaching ~100 µA µm(-1), and excellent current saturation. The deposition of different metals has limited influence on the FET performance, suggesting that the 1T/2H interface controls carrier injection into the channel. An increased reproducibility of the electrical characteristics is also obtained with our strategy based on phase engineering of MoS2.

Assessment of groundwater quality: a fusion of geochemical and geophysical information via Bayesian neural networks.

Deplorable quality of groundwater arising from saltwater intrusion, natural leaching and anthropogenic activities is one of the major concerns for the society. Assessment of groundwater quality is, therefore, a primary objective of scientific research. Here, we propose an artificial neural network-based method set in a Bayesian neural network (BNN) framework and employ it to assess groundwater quality. The approach is based on analyzing 36 water samples and inverting up to 85 Schlumberger vertical electrical sounding data. We constructed a priori model by suitably parameterizing geochemical and geophysical data collected from the western part of India. The posterior model (post-inversion) was estimated using the BNN learning procedure and global hybrid Monte Carlo/Markov Chain Monte Carlo optimization scheme. By suitable parameterization of geochemical and geophysical parameters, we simulated 1,500 training samples, out of which 50 % samples were used for training and remaining 50 % were used for validation and testing. We show that the trained model is able to classify validation and test samples with 85 % and 80 % accuracy respectively. Based on cross-correlation analysis and Gibb's diagram of geochemical attributes, the groundwater qualities of the study area were classified into following three categories: "Very good", "Good", and "Unsuitable". The BNN model-based results suggest that groundwater quality falls mostly in the range of "Good" to "Very good" except for some places near the Arabian Sea. The new modeling results powered by uncertainty and statistical analyses would provide useful constrain, which could be utilized in monitoring and assessment of the groundwater quality.

Investigating BioCaRGOS, a Sol-Gel Matrix for the Stability of Heme Proteins under Enzymatic Degradation and Low pH.

There have been significant advances in the development of vaccines for the prevention of various infectious diseases in the last few decades. These vaccines are mainly composed of proteins and nucleic acids. Poor handling and storage, exposure to high temperatures that lead to enzymatic degradation, pH variation, and various other stresses can denature the proteins or nucleic acids present in any vaccine formulation. Therefore, it is necessary to maintain a proper environment to preserve the integrity of biospecimens. To overcome these challenges, we report a practical and user-friendly approach for sol-gels called "BioCaRGOS" that can stabilize heme proteins not only in the presence of degrading enzymes and acidic pH but simultaneously maintain stability at room temperature. Heme proteins, such as myoglobin and cytochrome c, have been used for this study.

Long-term stabilization of DNA at room temperature using a one-step microwave assisted process.

Long-term stabilization of DNA is needed for forensic, clinical, in-field operations and numerous other applications. Although freezing (<-20 °C) and dry storage are currently the preferential methods for long-term storage, a noticeable pre-analytical degradation of DNA over time, upfront capital investment and recurring costs have demonstrated a need for an alternative long-term room-temperature preservation method. Herein, we report a novel, fast (~5 min) silica sol-gel preparation method using a standard microwave-initiated polymerization reaction amenable to stabilization of DNA. The method involves use of one chemical, tetramethoxy silane (TMOS) and eliminates the use of alcohol as co-solvent and catalysts such as acids. In addition, the process involves minimal technical expertise, thus making it an ideal choice for resource-challenged countries and in-field applications. The sol-gel is capable to store and stabilize Escherichia coli DNA in ambient conditions for 210 days. DNA recovered from the sol-gel showed no significant nucleolytic and/or oxidative degradation, outperforming conventional storage conditions at -20 °C, and reported state-of-the-art technology. Supplementary Information: The online version contains supplementary material available at 10.1007/s42247-021-00208-3.

Enhancing the compatibility of BioCaRGOS silica sol-gel technology with ctDNA extraction and droplet digital PCR (ddPCR) analysis.

Previously, our group had demonstrated long term stabilization of protein biomarkers using BioCaRGOS, a silica sol-gel technology. Herein, we describe workflow modifications to allow for extraction of cell free DNA (cfDNA) from primary samples containing working concentrations of BioCaRGOS, as well as the compatibility of BioCaRGOS with droplet digital PCR (ddPCR) analysis for pancreatic cancer biomarkers i.e., KRAS circulating tumor DNA (ctDNA). Preliminary attempts to extract ctDNA from BioCaRGOS containing samples demonstrated interference in the extraction of primary samples and the interference with ddPCR analysis when BioCaRGOS was directly introduced to stabilize sample extracts. In our modified technique, we have minimized the interference caused by methanol with ddPCR by complete removal of methanol from the activated BioCaRGOS formulation prior to addition to the biospecimen or ctDNA extract. Interference of the silica matrix present in BioCaRGOS with ctDNA extraction was eliminated through the introduction of invert filtration of the sample prior to extraction. These modifications to the workflow of BioCaRGOS containing samples allow for use of BioCaRGOS for stabilization of trace quantities of nucleic acid biomarkers such as plasma ctDNA, while retaining the capability to extract the biomarker and quantify based on ddPCR.

Bio-CaRGOS: capture and release gels for optimized storage of hemoglobin.

Room temperature biospecimen storage for prolonged periods is essential to eliminate energy consumption by ultra-low freezing or refrigeration-based storage techniques. State of the art practices that sufficiently minimize the direct or hidden costs associated with cold-chain logistics include ambient temperature storage of biospecimens (i.e., DNA, RNA, proteins, lipids) in the dry state. However, the biospecimens are still well-exposed to the stress associated with drying and reconstitution cycles, which augments the pre-analytical degradation of biospecimens prior to their downstream processing. An aqueous storage solution that can eliminate these stresses which are correlated to several cycles of drying/rehydration or freezing of biospecimens, is yet to be achieved by any current technology. In our study, we have addressed this room temperature biospecimen-protection challenge using aqueous capture and release gels for optimized storage (Bio-CaRGOS) of biospecimens. Herein, we have demonstrated a single-step ∼95% recovery of a metalloprotein hemoglobin at room temperature using a cost-effective standard microwave-based aqueous formulation of Bio-CaRGOS. Although hemoglobin samples are currently stored at sub-zero or under refrigeration (4 °C) conditions to avoid loss of integrity and an unpredictable diagnosis during their downstream assays, our results have displayed an unprecedented room temperature integrity preservation of hemoglobin. Bio-CaRGOS formulations efficiently preserve hemoglobin in its native state, with single-step protein recovery of ∼95% at ambient conditions (1 month) and ∼96% (7 months) under refrigeration conditions. In contrast, two-thirds of the control samples degrade under ambient (1 month) and refrigeration (7 months) settings.

Long term storage of miRNA at room and elevated temperatures in a silica sol-gel matrix.

Storage of biospecimens in their near native environment at room temperature can have a transformative global impact, however, this remains an arduous challenge to date due to the rapid degradation of biospecimens over time. Currently, most isolated biospecimens are refrigerated for short-term storage and frozen (-20 °C, -80 °C, liquid nitrogen) for long-term storage. Recent advances in room temperature storage of purified biomolecules utilize anhydrobiosis. However, a near aqueous storage solution that can preserve the biospecimen nearly "as is" has not yet been achieved by any current technology. Here, we demonstrate an aqueous silica sol-gel matrix for optimized storage of biospecimens. Our technique is facile, reproducible, and has previously demonstrated stabilization of DNA and proteins, within a few minutes using a standard benchtop microwave. Herein, we demonstrate complete integrity of miRNA 21, a highly sensitive molecule at 4, 25, and 40 °C over a period of ∼3 months. In contrast, the control samples completely degrade in less than 1 week. We attribute excellent stability to entrapment of miRNA within silica-gel matrices.

Graphite Intercalation Compounds Derived by Green Chemistry as Oxygen Reduction Reaction Catalysts.

Precious group metal (PGM) catalysts such as Pt supported on carbon supports are expensive catalysts utilized for the oxygen reduction reaction (ORR) due to their unmatched catalytic activity and durability. As an alternative, PGM-free ORR electrocatalysts that offer respectable catalytic activity are being pursued. Most of the notable PGM-free catalysts are obtained either from a bottom-up approach synthesis utilizing nitrogen-rich polymers as building blocks, or from a top down approach, where nitrogen and metal moieties are incorporated to carbonaceous matrixes. The systematic understanding of the origin of catalytic activity for either case is speculative and currently employed synthesis techniques typically generate large amounts of hazardous waste such as acids, oxidizing agents, and solvents. Herein, for the first time, we investigate the catalytic activity of graphite-based materials obtained via intercalation strategies that minimally perturb the graphitic backbone. Our outlined approaches demonstrate initial efforts to not only elucidate the role of each element but also significantly reduce the use of hazardous chemicals, which remains a pressing challenge. Graphite intercalation compounds (GIC) were obtained using fewer steps and solvent-free processes. X-ray diffraction and Raman results confirm the successful intercalation of FeCl3 between graphite layers. Electrochemical data shows that the ORR performance of FeCl3-intercalated GIC displays slight improvement where the onset potential reaches 0.77 V vs RHE in alkaline environments. However, expansion of the graphite and solvent-free incorporation of iron and nitrogen moieties resulted in a significant increase in ORR activity with onset potential to 0.89 V vs RHE, a maximum half-wave of 0.72 V vs RHE, and a limiting current of about 2.5 mA cm-2. We anticipate that the use of near solvent-free processes that result in a high yield of catalysts along with the fundamental insight into the origin of electrochemical activity will tremendously impact the methodologies for developing next-generation ORR catalysts.

CVD for the facile synthesis of hybrid nanobiomaterials integrating functional supramolecular assemblies.

In this letter, we present a simple one-step, versatile, scalable chemical vapor deposition (CVD)-based process for the encapsulation and stabilization of a host of single or multicomponent supramolecular assemblies (proteoliposomes, microbubbles, lipid bilayers, and photosynthetic antennae complexes and other biological materials) to form functional hybrid nanobiomaterials. In each case, it is possible (i) to form thin silica layers or gels controllably that enable the preservation of the supramolecular assembly over time and under adverse environmental conditions and (ii) to tune the structure of the silica gels so as to optimize solute accessibility while at the same time preserving functional dynamic properties of the encapsulated phospholipid assembly. The process allows precise temporal and spatial control of silica polymerization kinetics through the control of precursor delivery at room temperature and does not require or produce high concentrations of injurious chemicals that can compromise the function of biomolecular assemblies; it also does not require additives. This process differs from the conventional sol-gel process in that it does not involve the use of cosolvents (alcohols) and catalysts (acid or base).

Oral squamous cell carcinoma profile in North-Eastern regions of India from habits to histopathology: A hospital-based study.

BACKGROUND: Head and neck cancers constitute about 5%-8% of total body cancers in Europe, America, but in India, this figure is somewhat higher. The aim of this study is to evaluate the current burden of oral cancers in India, particularly North-East India. MATERIALS AND METHODS: A full-length study starting from patient counseling to clinical and histopathological examination and grading was planned. The study was conducted under the guidance of clinician, oral surgeon, oral pathologists, and statistician. RESULTS: In the 3 years study, all the patients with oral lesions are examined clinically, out of them suspected oral cancer patients were histopathologically confirmed as oral squamous cell carcinoma patient. The socioeconomic profile of oral cancer patients in relation to all examined patients was summarized, and results are drawn. CONCLUSION: The studied population is heavily indulgent tobacco consumption. Education for cancer prevention, early detection, and treatment is needed.

Dissolution kinetics of a Si-rich nanocomposite and its effect on osteoblast gene expression.

Silica-calcium phosphate nanocomposite (SCPC) has recently been proposed as a novel resorbable, bioactive, and mechanically compatible template for bone reconstruction. The effect of the physicochemical properties on the surface reactivity and dissolution kinetics of SCPC immersed in simulated body fluid (SBF) was investigated and compared to that of bioactive glass (BG). Moreover, the stimulatory effect on osteoblast gene expression of SCPC was determined using quantitative real-time polymerase chain reaction (qRT-PCR), and compared to that of hydroxyapatite (HA-200). Mercury porosimetry revealed that surface areas of SCPC particles containing 10 (SCPC10), 30 (SCPC30), and 50 (SCPC50) wt % Si-content were 14-, 18-, and 32-times higher than that of BG. Inductively coupled plasma analysis showed that after 192 h of immersion, Si-rich SCPC50 exhibited controlled bulk-dissolution and released 43.1 ppm Si, which was sixfold higher than that released from BG (7.7 ppm). Moreover, SCPC50 showed a rapid Ca-uptake from SBF and developed a surface apatite layer after only 2 h, whereas a similar layer was detected on BG after 8 days of immersion under the same experimental conditions. qRT-PCR revealed that osteopontin and osteocalcin mRNA expression by osteoblast-like cells attached to Si-rich SCPC50 was significantly higher than that on HA-200 or polystyrene after 2 days in culture. This suggested a role of dissolved Si in stimulating the differentiation and mineralization of osteoblast precursor cells. The favorable physiochemical and bioactivity properties of Si-rich SCPC nanocomposite indicate that SCPC can have wide applications as a synthetic bone graft for cell delivery applications in tissue engineering.

Enhancement of osteoblast gene expression by mechanically compatible porous Si-rich nanocomposite.

Synthesis of a porous bioactive ceramic implant for load bearing applications is a challenging task in maxillofacial and orthopedic surgeries. A novel bioactive resorbable silica-calcium phosphate nanocomposite (SCPC) has recently been introduced as a potential bone graft. In the present study, we employed SCPC to develop a resorbable porous scaffold and analyzed the effects of composition and porosity on the mechanical properties. The ranges of compressive strength and modulus of elasticity of SCPC containing 32-56% porosity were 1.5-50 MPa and 0.14-2.1 GPa, respectively, which matched the corresponding values for trabecular bone. The compressive strength of dense SCPC was dependent on the Si content and acquired values (93-285 MPa) comparable to that of cortical bone. The superior mechanical properties of SCPC are attributed to the intricate interactions at the boundaries of the nanograins and to the homogenous distribution of hierarchical pore-structure throughout the material volume. X-ray computed tomography and mercury porosimetry analyses revealed high interconnectivity of the pores in the size range 3 nm to 650 microm. Quantitative real-time PCR analyses showed that neonatal rat calvarial osteoblasts attached to Si-rich SCPC expressed 5- and 26-fold higher osteocalcin mRNA levels compared to cells attached to ProOsteon hydroxyapatite disks and tissue culture polystyrene plates respectively, after four days in culture. Results of the present study strongly suggest that porous, bioactive resorbable SCPCs can serve as tissue engineering scaffolds for cell delivery to treat load-bearing bone defects in orthopedic and maxillofacial surgeries.

First Indian study to establish safety of immediate-spin crossmatch for red blood cell transfusion in antibody screen-negative recipients.

BACKGROUND AND OBJECTIVES: The US Food and Drug Administration and American Association of Blood Banks approved the type and screen approach in 1980s, long after antibody screen (AS) was introduced in 1950s. The present study omits conventional anti-human globulin (AHG) crossmatch and replaces it with immediate-spin (IS) crossmatch as part of pretransfusion testing in AS-negative patients to study the safety and effectiveness of IS crossmatch in recipients. MATERIALS AND METHODS: This prospective longitudinal study was conducted on over 5000 red cell units transfused to AS-negative patients admitted to the hospital. Pretransfusion testing comprised blood grouping and AS followed by IS crossmatch, at the time of issue of red cell unit. The patients were transfused IS compatible red cell units. AHG crossmatch was performed posttransfusion for all red cell units. Any incompatible AHG crossmatch was followed up as suspected transfusion reaction. RESULTS: A total of 5023 red cell units were transfused to 2402 patients with negative AS. 99.7% IS compatible red cell units were also compatible on posttransfusion AHG crossmatch. Anti-P1 alloantibody was identified in one patient who was transfused two IS crossmatch compatible units but later both units were incompatible on AHG crossmatch. There was no clinical or serological sign of hemolysis in the patient. CONCLUSION: In AS-negative patients, IS crossmatch is as safe as conventional AHG crossmatch and can, therefore, replace conventional AHG crossmatch protocol.

Experimental Determination of the Ionization Energies of MoSe2, WS2, and MoS2 on SiO2 Using Photoemission Electron Microscopy.

The values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe2, WS2, and MoS2) on SiO2 using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS2, to WS2, to MoSe2, in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron-hole separation in photovoltaics.

Unveiling Active Sites for the Hydrogen Evolution Reaction on Monolayer MoS2.

Here, the hydrogen evolution reaction (HER) activities at the edge and basal-plane sites of monolayer molybdenum disulfide (MoS2 ) synthesized by chemical vapor deposition (CVD) are studied using a local probe method enabled by selected-area lithography. Reaction windows are opened by e-beam lithography at sites of interest on poly(methyl methacrylate) (PMMA)-covered monolayer MoS2 triangles. The HER properties of MoS2 edge sites are obtained by subtraction of the activity of the basal-plane sites from results containing both basal-plane and edge sites. The catalytic performances in terms of turnover frequencies (TOFs) are calculated based on the estimated number of active sites on the selected areas. The TOFs follow a descending order of 3.8 ± 1.6, 1.6 ± 1.2, 0.008 ± 0.002, and 1.9 ± 0.8 × 10-4 s-1 , found for 1T'-, 2H-MoS2 edges, and 1T'-, 2H-MoS2 basal planes, respectively. Edge sites of both 2H- and 1T'-MoS2 are proved to have comparable activities to platinum (≈1-10 s-1 ). When fitted into the HER volcano plot, the MoS2 active sites follow a trend distinct from conventional metals, implying a possible difference in the reaction mechanism between transition-metal dichalcogenides (TMDs) and metal catalysts.