Research Group Publications

• Ashwini Verma, Nikhil Wilson And Kavita Joshi, Solid state hydrogen storage: decoding the path through machine learning, International Journal of Hydrogen Energy., 50, 1518 - 1528 (2024), DOI:https://doi.org/10.1016/j.ijhydene.2023.10.056.
  We present a machine learning (ML) framework HEART (HydrogEn storAge propeRty predicTor) for identifying suitable families of metal alloys for hydrogen storage under ambient conditions. Our framework includes two ML models that predict the hydrogen storage capacity (HYST) and the enthalpy of hydride formation (THOR) of multi-component metal alloys. We demonstrate that a chemically diverse set of features effectively describes the hydrogen storage properties of the alloys. In HYST, we use absorption temperature as a feature which improved H2wt% prediction significantly. For out-of-the-bag samples, HYST predicted H2wt% with R2 score of 0.81 and mean absolute error (MAE) of 0.45 wt% whereas R2 score is 0.89 and MAE is 4.53 kJ/molH2 for THOR. These models are further employed to predict H2wt% and H for 6.4 million multi-component metal alloys. We have identified 6480 compositions with superior storage properties (H2wt% 2.5 at room temperature and H 60 kJ/molH2). We have also discussed in detail the interesting trends picked up by these models like temperature dependent variation in the rate of hydrogenation and alloying effect on H2wt% and H in different families of alloys. Importantly certain elements like Al, Si, Sc, Cr, and Mn when mixed in small fractions with hydriding elements like Mg, Ti, V etc. systematically reduce H without significantly compromising the storage capacity. Further upon increasing the number of elements in the alloy i.e from binary to ternary to quaternary, the number of compositions with lower enthalpies also increases. From the 6.4 million compositions, we have reported new alloy families having potential for hydrogen storage at room temperature. Finally, we demonstrate that HEART has the potential to scan vast chemical spaces by narrowing down potential materials for hydrogen storage.
• S Mehta, M Kasabe, S Umbarkar And K Joshi, From Digital Blueprint to Chemical Reality: Methanol to Formaldehyde at Ambient Conditions, Applied Surface Science., 669, 160527 (2024), DOI:https://doi.org/10.1016/j.apsusc.2024.160527.
  Partial oxidation of methanol to value-added products presents an intriguing and challenging process. Among these products, formaldehyde is the simplest and one of the most vital aliphatic aldehydes, with extensive application across various domains. Industrially, silver and iron-molybdenum oxides are used as catalysts for converting methanol to formaldehyde at elevated temperatures (600? C and 250-400? C, respectively). However, in this computational and experimental study, we have demonstrated the efficacy of ZnO as a catalyst. Notably, in the presence of ZnO, methanol readily converts to formaldehyde even under ambient conditions. We employed periodic density functional theory (DFT) to explore the (1011) facet of ZnO to elucidate its interaction with methanol. Our comprehensive analysis identified the most active facet (1011) involved in the spontaneous conversion of methanol to formaldehyde. Subsequently, experimental validation supported our theoretical findings, demonstrating the conversion of methanol to formaldehyde with 100% selectivity at room temperature and atmospheric pressure in the presence of ZnO. This study exemplifies the pivotal role of theory in catalyst design.
• N Wilson, AD Verma, PR Maharana, AB Sahoo And K Joshi, HyStor: An Experimental Database of Hydrogen Storage Properties for Various Metal Alloy Classes, International Journal of Hydrogen Energy., , (2024), DOI:https://doi.org/10.26434/chemrxiv-2024-6.
  In this work, we introduce the HyStor database, consisting of 1280 metal alloys along with their hydrogen storage capacities (H2wt%) as a function of absorption temperature. Given the lack of updates in the existing open-access HydPark database since 2002, we sourced compositions from recent research articles and various patent documents, resulting in a total of 468 compositions. The addition is reflected in the data across all existing classes of alloy compositions, and low entropy alloys (LEA), medium entropy alloys (MEA), and high entropy alloys (HEA) have been included. This has broadened the scope of the database to encompass the latest materials of interest for hydrogen storage. HyStor contains a representation of 54 elements, with a temperature range of 200-800K and an H2wt% range of 0.1-7.19. To ensure data quality, we conducted thorough checks for duplicate entries, erroneous data, and conflicting compositions within the database. Furthermore, we conducted multiple tests to identify potential outlier compositions by benchmarking the database against the pre-trained HYST model on HydPark data. After eliminating these potential outliers, we successfully improved the error metrics of the HYST model, reducing the Mean Absolute Error (MAE) from 0.32 to 0.28 and increasing the R2 score from 0.78 to 0.82. We also tested individual classes and observed that the performance of the HYST model has increased for the majority of the classes.
• Kavita Thakkar And Kavita Joshi, Exploring the Catalytic Potential of Mg-Cu Alloys for Enhanced Activity toward CO$_2$ Hydrogenation, Molecular Catalysis., 556, 113839 (2024), DOI:https://doi.org/10.1016/j.mcat.2024.113839.
  CO$_2$, a well-known greenhouse gas, is a potential raw material that can produce various chemicals. Dissociation of CO$_2$ to CO or hydrogenation to formate (HCOO$^*$) or carboxyl (COOH$^*$) intermediate is crucial in determining the reaction pathway for CO$_2$ conversion. In this work, we demonstrate that alloys of Mg-Cu exhibit greater activity toward activation and hydrogenation of CO$_2$ than transition metal alloys reported so far. Two different compositions of Mg-Cu, namely Mg$_2$Cu and MgCu$_2$, have been studied using periodic Density Functional Theory (DFT). Our investigations reveal that CO$_2$ chemisorbs on both intermetallic alloys. Coadsorption of CO$_2$ with H$_2$O leads to the spontaneous formation of COOH$^*$ over Mg$_2$Cu(224), whereas a negligible barrier (0.04 eV) is observed for MgCu$_2$(311). HCOO$^*$ formation has a barrier of 0.34 eV and 0.42 eV on Mg$_2$Cu(224) and MgCu$_2$(311), respectively. Dissociation of CO$_2$ to CO is kinetically unfavourable on both compositions of Mg-Cu. We provide a rationale for the observed activity by analyzing the electronic structure. Notably, the spontaneous hydrogenation of CO$_2$ makes earth-abundant metals suitable candidates for alloying that await experimental verification.
• K Thakkar, A Bajpai, S Kumar And K Joshi, Boosting hydrogen production at room temperature by synergizing theory and experimentation, ChemRxiv., , (2024), DOI:https://doi.org/10.26434/chemrxiv-2024-hlhr5.
  Methane is a major constituent of natural gas and is widely used in hydrogen production. However, its high symmetry poses a challenge, as breaking the strong C-H bond requires substantial energy input. Hence, there is a pressing need to develop efficient catalysts for methane conversion. By synergizing theory and experimentation, the search for a better catalyst can be accelerated, potentially boosting methane conversion processes. In the present work, theoretical findings prompted the experiments, which revealed the spontaneous dissociation of CH4 on selected facets of ?-Ga2O3. Additionally, the activation barrier for ethane formation was merely 0.1 eV. NTP-assisted conversion of methane in the presence of ?-Ga2O3 confirmed these findings. The formation rate of hydrogen and ethane rises to 366 µmolg?1h?1 and 86.62 µmolg?1h?1, respectively, in the presence of ?-Ga2O3, in contrast to 281.4 µmolg?1h?1 and 66 µmolg?1h?1 without catalysts. For the CH4-H2O reaction in the presence of ?-Ga2O3, there is an increase of 74.42% in the CO formation rate compared to the reaction without the catalyst. An electronic structure analysis revealed that electrophilic oxygen species on the ?-Ga2O3 (-202) surface play a vital role in the decomposition of methane, facilitating C-H bond cleavage.
• Ashwini Verma And Kavita Joshi, PCTpro: A Machine learning model for rapid prediction of Pressure-Composition-Temperature (PCT) isotherms, ChemRxiv., , (2024), DOI:https://doi.org/10.26434/chemrxiv-2024-g33f9.
• G P Kharabe, S Barik, S Kumar, A Nair, R Illath- valappil, A Yoyakki, K Joshi, C. P. Vinod And S Kurungot, Aluminum, Nitrogen-Dual-Doped Reduced Graphene Oxide Coexisting with Cobalt Encapsulated Graphitic Carbon Nanotube as an Activity Modulated Electrocatalyst for Oxygen Electrochemistry Applications, Small., , (2024), DOI:Just accepted.
• Aathira Nair And Kavita Joshi, What leads to direct epoxidation? An exhaustive DFT investigation of electrophilic oxygen-mediated epoxidation of ethylene on Ag(100), Computational Materials Science., 239, 112959 (2024), DOI:https://doi.org/10.1016/j.commatsci.2024.112959.
  Extensive research has contributed to a better understanding of the commercially important epoxidation reaction. Selectivity, a crucial aspect of this reaction, has received significant attention in both experimental and theoretical investigations. However, a consensus regarding the role of electrophilic oxygen in epoxidation is yet to be reached. The present study is a theoretical examination of the prerequisites necessary for direct epoxidation to occur on the Ag(100) surface, at varied monolayer concentrations. Additionally, the study investigates the characteristics of various oxygen species interacting with ethylene to promote the direct epoxidation pathway. Based on the effective charges and projected density of states (pDOS) analysis, three oxygen variants were identified on the Ag(100) surface: atomic oxygen, dissociatively adsorbed molecular oxygen, and O . The investigation reveals that all oxygen species, despite their physical and electronic differences, are electrophilic and undergo direct epoxidation. This work provides insights into the complex nature of epoxidation reaction and discusses electronic factors influencing the selective oxidation route on different Agsingle bondO complexes.
• Kavita Thakkar And Kavita Joshi, Single-atom alloys of Cu(211) with earth-abundant metals for enhanced activity towards CO dissociation, Journal of Molecular Graphics and Modelling., 126, 108656 (2024), DOI:https://doi.org/10.1016/j.jmgm.2023.108656.
  CO2, a byproduct from various industrial reactions, must not be released into the atmosphere and should be managed through capture, conversion, and utilization. The first step in converting CO2 into valuable products is to break the C–O bond. This work focuses on designing Single Atom Catalysts (SACs) by doping Cu(211) surface with 13 different s, p, and d block elements with an aim to minimize the activation barrier for C–O bond cleavage. Our work demonstrates that SACs of Mg/Al/Pt@Cu(211) favour CO2 chemisorption compared to Cu(211) where CO2 physisorbs. The barrier for CO2 dissociation is lowest for Mg@Cu(211) and it increases in the order Mg@Cu(211), Al@Cu(211), Pt@Cu(211), Zn@Cu(211) Ga@Cu(211) Cu@Cu(211) Pd@Cu(211). These findings suggest that doping Cu(211) with earth-abundant metals like Mg can potentially be a viable catalyst for CO2 conversion, providing a promising solution to reduce carbon footprint and mitigate climate change.
• Jeyavani Vijayakrishnan ; Deepali Kondhekar ; Meema Bhati ; Sahil Dev; Kavita Joshi; R. Nandini Devi; Shatabdi Porel Mukherjee, Remarkable SO2 and H2S Resistant Ability on CO Oxidation by Unique Pd/WO3 3D Hollow Sphere Nanocatalyst: Correlating Structure-Activity Relationships on SO2 Exposure, ACS Applied Energy Materials., 7, 1476 - 1487 (2024), DOI:https://doi.org/10.1021/acsaem.3c02664.
  We report a simple inorganic route for synthesizing a Pd/WO3 3D hollow sphere nanocatalyst, where Pd nanoparticles are encapsulated and well distributed on porous tungsten oxide nanospheres. The synthesis protocol has advantages, as it requires no surfactant or stabilizing agent, Pd loading is easily tuned, and the as-synthesized nanomaterials can be directly used as catalysts for the CO oxidation reaction. The synthesized nanocatalyst exhibited 100% CO to CO2 conversion efficiency at 260 °C. In addition, the nanocatalyst demonstrated remarkable SO2 (3 ppm) tolerance during the CO oxidation reaction for prolonged SO2 sulfation of 1–21 h at 260–400 °C. This represents the longest SO2 exposure time reported to date based on a single metal Pd/support-based nanocatalyst. No decrement in CO conversion efficiency was observed even after SO2 (3 ppm) treatment for 21 h for the first time based on a single metal Pd-based nanocatalyst. Moreover, the synthesized nanocatalyst shows H2S (4 ppm), even in situ H2S tolerance during the CO oxidation reaction at 260 °C for 1–3 h and exhibited less sensitivity to prolonged and stringent sulfur exposure, with the highest H2S concentration and maximum 100% CO to CO2 conversion efficiency obtained after H2S treatment for the first time based on a Pd-based nanocatalyst to the best of our knowledge. The composition and structure of the R-Pd/WO3 nanocatalyst were not much influenced, even after the prolonged SO2 and H2S exposure during the CO oxidation reaction, as verified from spent catalyst analysis. Finally, our DFT-based model provides insights into understanding the observed sulfur resistance on Pd/WO3 by analyzing the underlying electronic structure. Therefore, our strategic synthesis methodology will open up many opportunities to select Pd/metal oxide-based nanomaterials for designing highly efficient, stable, and SO2/H2S-resistant nanocomposite catalyst.
• Abhinav Bajpai, Shweta Mehta, Kavita Joshi And Sushant Kumar, Hydrogen from catalytic non-thermal plasma-assisted steam methane reforming reaction, International Journal of Hydrogen Energy., 48, 24328 - 24341 (2023), DOI:https://doi.org/10.1016/j.ijhydene.2023.03.281.
  Steam methane reforming reaction was carried out in a dielectric barrier plasma reactor. A systematic study is conducted to understand the influence of input power, flow rate, and water for the conversion, yield, and selectivity of the reaction over strategically designed catalysts. In particular, the production rate and selectivity of the products (H2, CO and C2 hydrocarbons) are monitored. CeO2 was used as packing material, mixed with oxides of manganese or copper and their combination. The optimum Cu/CeO2 catalyst illustrated the production rate of 248.7 ?molg?1h?1 and 11.25 ?molg?1h?1 for H2, and CO, respectively at specific energy input of 19.8 JL-1. DFT calculations exhibit apparent change in electronic structure of the CeO2 after inclusion of oxides of manganese and copper that enhance interaction with methane. Based on these findings, a plausible mechanism is elucidated which can help to design catalyst for other applications in non-thermal plasma atmosphere.
• Rohit Modee, Ashwini Verma, Kavita Joshi And U Deva Priyakumar, MeGen - generation of gallium metal clusters using reinforcement learning, Machine Learning: Science and Technology., 4, 025032-1 - 025032-9 (2023), DOI:10.1088/2632-2153/acdc03.
  The generation of low-energy 3D structures of metal clusters depends on the efficiency of the search algorithm and the accuracy of inter-atomic interaction description. In this work, we formulate the search algorithm as a reinforcement learning (RL) problem. Concisely, we propose a novel actor-critic architecture that generates low-lying isomers of metal clusters at a fraction of computational cost than conventional methods. Our RL-based search algorithm uses a previously developed DART model as a reward function to describe the inter-atomic interactions to validate predicted structures. Using the DART model as a reward function incentivizes the RL model to generate low-energy structures and helps generate valid structures. We demonstrate the advantages of our approach over conventional methods for scanning local minima on potential energy surface. Our approach not only generates isomer of gallium clusters at a minimal computational cost but also predicts isomer families that were not discovered through previous density-functional theory (DFT)-based approaches.
• Shweta Mehta And Kavita Joshi, Electronic fingerprints for diverse interactions of methanol with various Zn-based systems, Surface Science., 736, 122350 (2023), DOI:https://doi.org/10.1016/j.susc.2023.122350.
  We have investigated various Zn-based catalysts for their interaction with methanol (MeOH). MeOH is one of the most critical molecules being studied extensively, and Zn-based catalysts are widely used in many industrially relevant reactions involving MeOH. We note that the same element (Zn and O, in the present study) exhibits different catalytic activity in different environments. The changing environment is captured in the underlying electronic structure of the catalysts. In the present work, we compared the electronic structure of Zn-based systems, i.e., ZnAl2O4 and ZnO along with oxygen preadsorbed Zn (O-Zn) and metallic Zn. We demonstrate the one-to-one correlation between the pDOS of the bare facet and the outcome of that facet’s interaction (i.e. either adsorption or dissociation of MeOH) with MeOH. These findings would pave the way towards the in-silico design of catalysts.
• Kranti N Salgaonkar, Himanshu Bajpai, Nitin B Mhamane, Naresh Nalajala, Inderjeet Chauhan, Kavita Thakkar, Kavita Joshi And Chinnakonda S Gopinath, A baby step in assembling and integrating the components of an artificial photosynthesis device with forced heterojunctions towards improved efficiency, Journal of Materials Chemistry A., 11, 15168 - 15182 (2023), DOI:10.1039/D3TA01850D.
  How to achieve unassisted, economical, scalable, and sustainable artificial photosynthesis for liquid fuels/products with improved solar-to-fuel efficiency (STFE) to address a carbon-neutral economy remains a big question. To a large degree, the extent of charge separation at heterojunction interfaces and charge utilization determine the STFE. Towards this, BiVO3 is assembled from ionic-precursors into TiO2 pores, and integrated structurally and electronically with TiO2 on calcination as BiVO4 quantum dots (BVQDs). BVQDs in TiO2 (BVT) pores lead to an all-inorganic system with a sub-quadrillion number of heterojunctions in a 1 cm2 device (contains ?25 ?g of BiVO4 (?2.5 wt%) in the nanopores of ?975 ?g of TiO2 (?97.5 wt%)) and facilitate artificial photosynthesis. We demonstrate 31–38% STFE with a photon to chemical conversion turn over frequency (ToFP2C) of 2.73 s?1 with a 1 cm2 wireless BiVO4–TiO2 artificial leaf (BVT-AL) device for HCHO and CH3OH. The sequential nature of CO2 reduction to HCHO and then to CH3OH is evident from the reaction results. 13CO2 isotopic labeling experiments confirm that the input CO2 is the source for product formation. A large increase in the photocurrent density and incident photon-to-current efficiency (IPCE) of BVT, over 100% for the BiVO4 photoanode in visible light, demonstrates and supports efficient visible light absorption, charge separation and migration to the redox sites. A device has been demonstrated to show sustainable activity in direct sunlight, and addresses scalability from 1 to 9 cm2. Assuming no change (50% decrease) in the STFE, a 6.74 m2 device is expected to convert 1 (0.5) kg h?1 CO2 into C1-oxygenates in sunlight. DFT calculations carried out with anatase TiO2 (101) and BiVO4 (121) interfaces support many of the experimental findings, including electron flow from the latter to the former, and interaction of the oxygen of TiO2 with BiVO4 and vice versa at the interface towards forced heterojunctions.
• Niwesh Ojha, Kavita Thakkar, Abhinav Bajpai, Kavita Joshi And Sushant Kumar, Photoinduced CO2 and N2 Reductions on Plasmonically Enabled Gallium Oxide, Journal of Colloid and Interface Science., 629, 654 - 666 (2023), DOI:https://doi.org/10.1016/j.jcis.2022.09.097.
  Ag-containing ZnO/?-Ga2O3 semiconductor, which exhibit reduced bandgap, increased light absorption, and hydrophilicity, have been found to be useful for photocatalytic CO2 reduction and N2 fixation by water. The charge-separation is facilitated by the new interfaces and inherent vacancies. The Ag@GaZn demonstrated the highest photocurrent response, about 20- and 2.27-folds that of the Ga and GaZn samples, respectively. CO, CH4, and H2 formed as products for photo-reduction of CO2. Ag@GaZn catalyst exhibited the highest AQY of 0.121% at 400nm (31.2W/m2). Also, Ag@GaZn generated 740 of NH4+ ions, which was about 18-folds higher than Ga sample. In situ DRIFTS for isotopic-labelled 13CO2 and 15N2 reaffirmed the photo-activity of as-synthesized catalysts. Density functional theory provided insight into the relative affinity of different planes of heterostructures towards H2O, CO2 and N2 molecules. The structure-photoactivity rationale behind the intriguing Ag@GaZn sample offers a fundamental insight into the role of plasmonic Ag and design principle of heterostructure with improved photoactivity and stability.
• Geeta Pandurang Kharabe, Narugopal Manna, Ayasha Nadeema, Santosh K Singh, Shweta Mehta, Aathira Nair, Kavita Joshi And Sreekumar Kurungot, A pseudo-boehmite AlOOH supported NGr composite-based air electrode for mechanically rechargeable Zn-air battery applications, Journal of Materials Chemistry A., 10, 10014 - 10025 (2022), DOI:https://doi.org/10.1039/D2TA00546H.
  Both mechanically and electrically rechargeable zinc-air batteries (ZABs) have received much interest due to their high energy density and suitability for mobile and stationary applications. However, their commercialization has been impeded by the lack of robust, low-cost and environmentally benign catalyst materials that can be easily scaled up. In this context, the present work introduces a new type of transition metal-free catalytic material (AlOOH/NGr) by anchoring the pseudo-boehmite phase of aluminium oxyhydroxide (AlOOH) nanosheets over nitrogen-doped graphene (NGr) via a single-step and straightforward hydrothermal process. Furthermore, density functional theory (DFT) based computation demonstrates that the nucleation of AlOOH starts from the N-sites and points towards the strong surface interaction between AlOOH and NGr via doped nitrogen. AlOOH/NGr consisting of thin layered pseudo-boehmite sheets uniformly distributed over NGr has displayed an oxygen reduction reaction onset potential of 0.83 V and a half-wave potential of 0.72 V, along with good catalytic durability in alkaline medium. With this, AlOOH/NGr, when used as an air electrode for fabricating a primary Zn-air battery, the system has exhibited an open circuit voltage of ?1.27 V with a flat discharge profile at a current rate of 10 mA cm?2. The fabricated system delivered a specific capacity of ?720 mA h g?1 and a high power density of 204 mW cm?2 and is comparable to the counterpart system based on the state-of-the-art Pt/C (20 wt% Pt) cathode. Additionally, the homemade battery was able to maintain its performance after 4 times of mechanical recharging of the battery, which lasted for more than 35 h at a discharge current density of 10 mA cm?2. Thus, we have uncovered the potential of an earth-abundant metal-based catalytic system for fabricating and demonstrating a robust mechanically rechargeable zinc-air battery.
• Siddharth Ghule, Soumya Ranjan Dash, Sayan Bagchi, Kavita Joshi And Kumar Vanka, Predicting the Redox Potentials of Phenazine Derivatives Using DFT-Assisted Machine Learning, ACS omega., 7, 11742 - 11755 (2022), DOI:https://doi.org/10.1021/acsomega.1c06856.
  This study investigates four machine-learning (ML) models to predict the redox potentials of phenazine derivatives in dimethoxyethane using density functional theory (DFT). A small data set of 151 phenazine derivatives having only one type of functional group per molecule (20 unique groups) was used for the training. Prediction accuracy was improved by a combined strategy of feature selection and hyperparameter optimization, using the external validation set. Models were evaluated on the external test set containing new functional groups and diverse molecular structures. High prediction accuracies of R2 > 0.74 were obtained on the external test set. Despite being trained on the molecules with a single type of functional group, models were able to predict the redox potentials of derivatives containing multiple and different types of functional groups with good accuracies (R2 > 0.7). This type of performance for predicting redox potential from such a small and simple data set of phenazine derivatives has never been reported before. Redox flow batteries (RFBs) are emerging as promising candidates for energy storage systems. However, new green and efficient materials are required for their widespread usage. We believe that the hybrid DFT-ML approach demonstrated in this report would help in accelerating the virtual screening of phenazine derivatives, thus saving computational and experimental costs. Using this approach, we have identified promising phenazine derivatives for green energy storage systems such as RFBs.
• Kavita Joshi And Shweta Mehta, From molecular adsorption to decomposition of methanol on various ZnO facets: A periodic DFT study, Applied Surface Science., 602, 154150 (2022), DOI:https://doi.org/10.1016/j.apsusc.2022.154150.
  Methanol is an interesting and important molecule to study because of its potential to replace existing fuels. It is also a prominent hydrogen source which can be used to generate hydrogen in-situ. ZnO is widely used as catalyst in synthesis of methanol from CO2 at industrial scale. In this work, we demonstrate that the same catalyst could be used for MeOH decomposition. We have investigated interaction of methanol with various flat and stepped facets of ZnO by employing Density Functional Theory (DFT). Two flat [(100) and (110)] and two stepped [(103) and (112)] facets are studied in detail for methanol adsorption. Chemisorption of MeOH with varying strength is common to all four facets. Most importantly spontaneous dissociation of O-H bond of methanol is observed on all facets except (110). Our DFT calculations reveal that molecular adsorption is favored on flat facets, while dissociation is favored on step facets. Also, (100) facet undergoes substantial reconstruction upon MeOH adsorption. Activation of C-H bond along with strengthening of C-O bond on ZnO facets suggest partial oxidation of methanol. With our DFT investigations, we dig deeper into the underlying electronic structure of various facets of ZnO and provide rationale for the observed facet dependent interaction of ZnO with MeOH.
• Meema Bhati, Jignesh Dhumal And Kavita Joshi, Lowering the C-H Bond Activation Barrier of Methane Using SAC@Cu(111): A Periodic DFT Investigations, The New Journal of Chemistry., 46, 70 - 74 (2022), DOI:https://doi.org/10.1039/D1NJ04525C.
  Methane has long captured the world's spotlight for being the simplest and yet one of the most notorious hydrocarbon. Exploring its potential to be converted into value added products has raised a compelling interest. In the present work, we have studied the efficiency of Single-Atom Catalysts (SACs) for methane activation employing Density Functional Theory (DFT). The Climbing Image-Nudged Elastic Bond (CI-NEB) method is used in tandem with the Improved Dimer (ID) method to determine the minimum energy pathway for the first C-H bond dissociation of methane. Our study reported that the transition-metal doped Cu(111) surfaces enhance adsorption, activate C-H bond, and reduce activation barrier for first C-H bond cleavage of methane. The results suggest Ru/Co/Rh doped Cu(111) as promising candidates for methane activation with minimal activation barrier and less endothermic reaction. For these SACs, the calculated activation barriers for first C-H bond cleavage are 0.17 eV, 0.24 eV, and 0.26 eV respectively, which is substantially lower than 1.13 eV, the activation barrier for Cu(111).
• Aathira Nair, Nivedita Kenge And Kavita Joshi, Role of facet in the competitive pathway of ethylene epoxidation, Surface Science., 716, 121954 (2022), DOI:https://doi.org/10.1016/j.susc.2021.121954.
  Ethylene epoxide (EtO) is used as raw material for a broad range of products from pharmaceuticals and plastics to paints and adhesives. Although the reaction of ethylene interacting with preadsorbed oxygen on Ag surface is known for decades, the underlying mechanism of EtO formation is not completely understood. Successful investigation of oxametallacycle (OMC) intermediate common to selective as well as non-selective path- ways has ensured at least 50% selectivity. The current study brings out the electronic signatures of distinct conformers of OMC stabilized on two different facets of Ag viz. (100) and (111). There are subtle differences between OMC conformers observed on these two facets with near-eclipsed on Ag(100) and near- staggered on Ag(111). A de- tailed analysis of Ag-O, C-O, C-C, and Ag-C interactions along with projected Density of States (pDOS) and projected Crystal Orbital Hamilton Population (pCOHP) imply towards ring closure on Ag(100) and hydrogen transfer on Ag(111). Finally, our un derstanding based on electronic and structural signatures are backed up by activation barriers computed through NEB calculations. Activation barrier for EtO is lower on ( 100) as compared to (111) facet. Thus, our study sheds light on how these differences between OMC affect the selectivity towards EtO.
• Sheena Agarwal and Kavita Joshi, Looking Beyond Adsorption Energies to Understand Interactions at Surface Using Machine Learning, ChemistrySelect., , (2022), DOI:10.26434/chemrxiv.14726184.v1.
  AbstractIdentifying factors that influence interactions at the surface is still an active area of research. In this study, we present the importance of analyzing bondlength activation, while interpreting Density Functional Theory (DFT) results, as yet another crucial indicator for catalytic activity. We studied theadsorption of small molecules, such as O 2 , N 2 , CO, and CO 2 , on seven face-centered cubic (fcc) transition metal surfaces (M = Ag, Au, Cu, Ir, Rh, Pt, and Pd) and their commonly studied facets (100, 110, and 111). Through our DFT investigations, we highlight the absence of linear correlation between adsorption energies (E ads ) and bondlength activation (BL act ). Our study indicates the importance of evaluating both to develop a better understanding of adsorption at surfaces. We also developed a Machine Learning (ML) model trained on simple periodic table properties to predict both, E ads and BL act . Our ML model gives an accuracy of Mean Absolute Error (MAE) ? 0.2 eV for E ads predictions and 0.02 Å for BL act predictions. The systematic study of the ML featuresthat affect E ads and BL act further reinforces the importance of looking beyond adsorption energies to get a full picture of surface interactions with DFT
• Shankar B Dalavi, Sheena Agarwal, Pooja Deshpande, Kavita Joshi* And Bhagavatula L. V. Prasad*, Disordered but Efficient: Understanding the Role of Structure and Composition of the Co–Pt Alloy on the Electrocatalytic Methanol Oxidation Reaction, J. Phys. Chem. C., , (2021), DOI:https://doi.org/10.1021/acs.jpcc.0c10165.
  A systematic investigation of the electrocatalytic Methanol Oxidation Reaction (MOR) was carried out using a model Co:Pt alloy system with different compositions and structural arrangements of the Co and Pt atoms. The structural variations with the same alloy composition included those with disordered arrangement of Co and Pt atoms in a face-centered cubic (fcc) lattice and ordered arrangements in face-centered tetragonal (fct) lattices. Our investigations clearly show that structures with disordered arrangements with Co:Pt atomic ratios near to 1:1 display better electrocatalytic efficiencies even when compared to pure Pt. These experimental findings were then rationalized by means of Density Functional Theory (DFT) calculations. Electronic level signatures in terms of charge transfer and relative shift in the peaks of the d band for surface metal atoms are proposed to be the reasons for the superior catalytic activity of a particular composition over the others. An increase in the number of inequivalent sites for methanol adsorption in disordered systems appears to result in better catalytic performance in comparison with ordered systems.
• Sachin Kochrekar, Ajit Kalekar, Shweta Mehta, Pia Damlin, Mikko Salomäki, Sari Granroth, Niko Meltola, Kavita Joshi And Carita Kvarnström, Copolymers of bipyridinium and metal (Zn & Ni) porphyrin derivatives; theoretical insights and electrochemical activity towards CO2, RSC Advances., 11, 19844 - 19855 (2021).
  This study reports the electropolymerization of novel keto functionalized octaethyl metal porphyrins (Zn2+ and Ni2+) in the presence of 4,4?-bipyridine (4,4?-bpy) as a bridging nucleophile. The polymer films were characterized by electrochemical, spectroscopic (UV-Vis, XPS, FT-IR and Raman spectroscopy) and imaging (AFM and SEM) techniques. The absorption and electronic spectra confirm the presence of both porphyrin and 4,4?-bipyridine units in the film. The surface morphology reveals homogeneous film deposition with average roughness values of approx. 8 nm. The theoretical studies performed offered insights into the interplay of different metal centres (Zn2+ and Ni2+) and the keto functionality of the porphyrin unit in the formation of copolymer films. The electrochemical interaction of polymer films with CO2 suggests a reversible trap and release of CO2 with low energy barriers for both the polymers.
• Rohit Modee, Sheena Agarwal, Ashwini Verma, Kavita Joshi And U. Deva Priyakumar , DART: Deep Learning Enabled Topological Interaction Model for Energy Prediction of Metal Clusters and its Application in Identifying Unique Low Energy Isomers, Phys. Chem. Chem. Phys., , (2021), DOI:DOI https://doi.org/10.1039/D1CP02956H.
  Recently, Machine Learning (ML) has proven to yield fast and accurate predictions of chemical properties to accelerate the discovery of novel molecules and materials. The majority of the work is on organic molecules, and much more work needs to be done for inorganic molecules, especially clusters. In the present work, we introduce a simple Topological Atomic Descriptor called TAD, which encodes chemical environment information of each atom in the cluster. TAD is a simple and interpretable descriptor where each value represents the atom count in three shells. We also introduce the DART, Deep Learning Enabled Topological Interaction model, which uses TAD as a feature vector to predict energies of metal clusters, in our case Gallium clusters with size ranging from 31 to 70 atoms. DART model is designed based on the principle that energy is a function of atomic interactions and allows us to model these complex atomic interactions to predict the energy. We further introduce a new dataset called GNC_31-70, which comprises structures and DFT optimized energies of Gallium clusters with sizes ranging from 31 to 70 atoms. We show how DART can be used to accelerate the identification of ground-state structures without geometry optimization. Albeit using topological descriptor, DART achieves MAE of 3.59 kcal/mol (0.15 eV) on testset. We also show that our model can distinguish core and surface atoms in the Ga-70 cluster, which the model has never encountered earlier. Finally, we demonstrate the transferability of DART model by predicting energies for about 6k unseen configurations picked up from Molecular Dynamics (MD) data for three cluster sizes (46, 57, and 60) within seconds. The DART model was able to reduce the load on DFT optimizations while identifying unique low energy structures from MD data.
• Sheena Agarwal, Shweta Mehta And Kavita Joshi*, Understanding the ML black box with simple descriptors to predict cluster–adsorbate interaction energy, New Journal of Chemistry., 44, 8545 - 8553 (2020), DOI:10.1039/D0NJ00633E.
  Density functional theory (DFT) is currently one of the most accurate and yet practical theories used to gain insight into the properties of materials. Although successful, the computational cost required is still the main hurdle even today. In recent years, there has been a trend of combining DFT with Machine Learning (ML) to reduce the computational cost without compromising accuracy. Finding the right set of descriptors that are simple to understand in terms of giving insights about the problem at hand, lies at the heart of any ML problem. In this work, we demonstrate the use of nearest neighbor (NN) distances as descriptors to predict the interaction energy between the cluster and an adsorbate. The model is trained over a size range of 5 to 75 atom clusters. When the training and testing is carried out on mutually exclusive cluster sizes, the mean absolute error (MAE) in predicting the interaction energy is ? 0.24 eV. MAE reduces to 0.1 eV when testing and training sets include information from the complete range. Furthermore, when the same set of descriptors are tested over individual sizes, the MAE further reduces to ?0.05 eV. We bring out the correlation between dispersion in the nearest neighbor distances and variation in MAE for individual sizes. Our detailed and extensive DFT calculations provide a rationale as to why nearest neighbor distances work so well. Finally, we also demonstrate the transferability of the ML model by applying the same recipe of descriptors to systems of different elements like (Na10), bimetallic systems (Al6Ga6, Li4Sn6, and Au40Cu40) and also different adsorbates (N2, O2, and CO).
• Pravin K. Dwivedi, Aathira Nair, Rupali S. Mehare, Vikash Chaturvedi, Kavita Joshi* And Manjusha V. Shelke*, Experimental and theoretical investigations of the effect of hetero-atom doped Carbon micro-sphere support on stability and storage capacity of nano-Co3O4 conversion anode for Lithium-ion batteries, Nanoscale Advances., 2, 2914 - 2924 (2020), DOI:10.1039/D0NA00261E.
  Conversion type anode materials are intensely being studied for Li-ion batteries (LIBs) for their potentially higher capacities over current graphite-based anodes. This work describes the development of high capacity and stable anode from a nanocomposite of N and S co-doped Carbon spheres (NSCS) with Co3O4 (NSCS-Co3O4). Hydrothermal reaction of saccharose with L-cysteine has been carried out followed by its carbonization. CSs, when used as support for conversion type materials provide efficient electron/ion transfer channels enhancing the overall electrochemical performance of the electrodes. Additionally, hetero atoms doped in carbon matrix alter the electronic properties, often increasing the reactivity of the carbon surface and reported to be effective for anchoring metal oxide nanoparticles. Consequently, the NSCS-Co3O4nanocomposites developed in this work exhibit enhanced and stable reversible specific capacity over the cycling. Stable cycling behavior is observed at 1 Ag-1 with 1285 mAhg-1 of specific capacity retained after 350 cycles alongwith more than 99% of coulombic efficiency. This material shows an excellent rate capability with specific capacity retained to 745 mAhg-1 even at a high current density of 5 Ag-1. Detailed DFT based calculations have revealed the role of doped support in controlling the volume expansion upon lithiatio
• Sheena Agarwal, Shweta Mehta, Nivedita Kenge, Siva Prasad Mekala, Vipul Patil, T. Raja And Kavita Joshi*, Mixed metal oxide: A new class of catalyst for methanol activation, Applied Surface Science., , (2020).
  In this work, we propose a mixed metal oxide as a catalyst and demonstrate it's ability to not only activate the MeOH molecule upon adsorption but also dissociate O-H and one of it's C-H bonds. MeOH activation is compared on two prominent facets of \znalo~ viz. (220) and (311). While spontaneous O-H bond dissociation is observed on both facets, C-H bond dissociates only on the (311) surface. Multiple factors like atomic arrangement and steps on the surface, coordination of surface atoms, and their effective charges have a combined effect on MeOH activation. The (311) surface offers higher catalytic activity in comparison with (220) surface. Having a stepped surface, availability of multiple sites, and variation in the charge distribution are some of the reasons for better catalytic performance of (311) facet. Effect of orientation of MeOH with respect to the surface adds both, information and complexity to the problem. Observations pertinent to understanding this effect are also reported. A detailed analysis of atomic arrangement on the two surfaces provides a rationale as to why MeOH gets dissociated spontaneously on the mixed metal oxide. The promising results reported here opens up a new class of catalyst for research.
• Nivedita Kenge, Sameer Pitale And Kavita Joshi, The Nature of Electrophilic Oxygen : Insights from Periodic Density Functional Theory Investigations, Surface Science., , (2018), DOI:https://doi.org/10.1016/j.susc.2018.09.009.
  Increasing demand of ethylene oxide and the cost of versatile chemical ethene has been a driving force for understanding mechanism of epoxidation to develop highly selective catalytic process. Direct epoxidation is a proposed mechanism which in theory provides 100% selectivity. A key aspect of this mechanism is an electrophilic oxygen (Oele) species forming on the Ag surface. In the past two and half decades, large number of theoretical and experimental investigations have tried to elucidate formation of Oele on Ag surface with little success. Equipped with this rich literature on Ag-O interactions, we investigate the same using periodic DFT calculations to further understand how silver surface and oxygen interact with each other from a chemical standpoint. Based on energetics, Löwdin charges, topologies and pdos data described in this study, we scrutinize the established notions of Oele. Our study provides no evidence in support of Oele being an atomic species nor a diatomic molecular species. In fact, a triatomic molecular species described in this work bears multiple signatures which are very convincing evidence for considering it as the most sought for electrophilic entity.
• A. Susan, V. Kaware And K. Joshi*, Multifaceted Thermodynamics of Pbn (n=16-24) Clusters: A Case Study, J. Phys. Chem. C., 119, 23698 - 23707 (2015), DOI:10.1021/acs.jpcc.5b05250.
  Thermodynamic response of small clusters is a challenging area of exploration, both experimentally, as well as theoretically. In this article, we study the thermodynamic behaviour of small Pb clusters (size 16--24) using Born Oppenheimer Molecular Dynamics. A new ground state structure is reported for Pb$_{20}$. Except Pb$_{21}$, all clusters fragment at temperatures above T$_{m[bulk]}$, and show no signs of melting. Characteristic behaviour like restricted diffusion and solid-solid transition is discussed in detail. Variation in the isomerization temperature of these clusters is explained using the bondlength analysis. Root mean square bondlength fluctuations (\deltarms) along with distribution of atoms about centre of mass of the cluster as a function of time and distance-energy plots are used to bring out the essential features of Pb cluster thermodynamics. Analysis carried out using these parameters, and their interpretation regarding state of the system, are discussed in detail. We highlight that it is not possible to define `liquid-state' for these small clusters, in the conventional frame of understanding.
• S. R. Suryawanshi, V. Kaware, D. Chakravarty, P. Walke, M. A. More, K. Joshi, C. S. Rout And D. J. Late, Pt-Nanoparticle Functionalized Carbon Nano-onions for the Ultra-High Energy Supercapacitors and Enhanced Field Emission, RSC Advances., 5, 80990 - 80997 (2015), DOI:10.1039/C5RA12364J.
  In the present work, we have investigated the charge storage capacitive response and field emission behaviour of platinum (Pt) nanoparticles decorated on carbon nano onions (CNOs) and compared them with those of pristine carbon nano onions. The specific capacitance observed for Pt–CNOs is 342.5 F g?1, about six times higher than that of pristine CNOs, at a scan rate of 100 mV s?1. The decoration with Pt nanoparticles, without any binder or polymer separator on the CNO, leading to enhanced supercapacitance is due to easy accessibility of Na2SO4 electrolyte in the active material. The Density Functional Theory (DFT) calculations of these systems reveal enhancement in the Density of States (DOS) near the Fermi energy (EF) on account of platinum decoration on the CNOs. Furthermore, the field emission current density of ?0.63 mA cm?2 has been achieved from the Pt-CNOs emitter at an applied electric field of ?4.5 V ?m?1 and from the pristine CNOs sample current density of ?0.4 mA cm?2 has been achieved at an applied electric field of ?6.6 V ?m?1. The observed enhanced field emission behavior has been attributed to the improved electrical conductivity and increased emitting sites of the Pt–CNO emitter. The field emission current stability of the Pt–CNO emitter over a longer duration is found to be good. The observed results imply multifunctional potential of Pt–CNOs, as supercapacitor material in various next generation hybrid energy storage devices, and field emitters for next generation vacuum nano/microelectronic devices
• J. Dash, S. Ray, K. Nallappan, V. Kaware, N. Basutkar, R. G. Gonnade, A. V Ambade, Kavita Joshi And B. Pesala, Terahertz Spectroscopy and Solid-State Density Functional Theory Calculations of Cyanobenzaldehyde Isomers, The Journal of Physical Chemistry A., 119, 7991 - 7999 (2015), DOI:10.1021/acs.jpca.5b01942.
  Spectral signatures in the terahertz (THz) frequency region are mainly due to bulk vibrations of the molecules. These resonances are highly sensitive to the relative position of atoms in a molecule as well as the crystal packing arrangement. To understand the variation of THz resonances, THz spectra (2–10 THz) of three structural isomers: 2-, 3-, and 4-cyanobenzaldehyde have been studied. THz spectra obtained from Fourier transform infrared (FTIR) spectrometry of these isomers show that the resonances are distinctly different especially below 5 THz. For understanding the intermolecular interactions due to hydrogen bonds, four molecule cluster simulations of each of the isomers have been carried out using the B3LYP density functional with the 6-31G(d,p) basis set in Gaussian09 software and the compliance constants are obtained. However, to understand the exact reason behind the observed resonances, simulation of each isomer considering the full crystal structure is essential. The crystal structure of each isomer has been determined using X-ray diffraction (XRD) analysis for carrying out crystal structure simulations. Density functional theory (DFT) simulations using CRYSTAL14 software, utilizing the hybrid density functional B3LYP, have been carried out to understand the vibrational modes. The bond lengths and bond angles from the optimized structures are compared with the XRD results in terms of root-mean-square-deviation (RMSD) values. Very low RMSD values confirm the overall accuracy of the results. The simulations are able to predict most of the spectral features exhibited by the isomers. The results show that low frequency modes (<3 THz) are mediated through hydrogen bonds and are dominated by intermolecular vibrations.
• Vaibhav Kaware And Kavita Joshi, Partial Ionic Bonding in Homogeneous Sodium Clusters, arXiv., , (2015), DOI:http://arxiv.org/abs/1504.03816.
  In this work, we report an interesting observation of partial ionic bonding due to charge transfer in homogeneous sodium clusters. The charge transfer causes the electronic charge to accumulate on the surface, and the resulting charges on atoms range between +0.4 to -1.0 |e^-|. We also demonstrate that this disparity among effective charges on atoms is geometry dependent, such that atoms experiencing similar surrounding, have equal effective charge. It is speculated that this phenomenon will occur among other homogeneous clusters as well, and its extent will be defined by the valence electron delocalization
• Vaibhav Kaware and Kavita Joshi*, Scaling up the shape: A novel growth pattern of gallium clusters, J. Chem. Phys., 141, 054308 (2014), DOI:http://dx.doi.org/10.1063/1.4891867.
  Putative global minima for Ga+N clusters with size “N” ranging from 49 to 70 are found by employing the Kohn-Sham formulation of the density functional theory, and their evolution is described and discussed in detail. We have discovered a unique growth pattern in these clusters, all of which are hollow core-shell structures. They evolve with size from one spherical core-shell to the next spherical core-shell structure mediated by prolate geometries, with an increase in overall diameter of the core, as well as the shell, without putting on new layers of atoms. We also present a complete picture of bonding in gallium clusters by critically analyzing the molecular orbitals, the electron localization function, and Bader charges. Bonding in these clusters is a mixture of metallic and covalent type that leans towards covalency, accompanied by marginal charge transfer from the surface to the core. Most molecular orbitals of Ga clusters are non-jellium type. Covalency of bonding is supported by a wide localization window of electron localization function, and joining of its basins along the bonds.
• S. Ray, J. Dash, K. Nallappan, V. Kaware, N. Basutkar, A. Ambade, Kavita Joshi And B. Pesala, Design and engineering of organic molecules for customizable Terahertz tags, PROC. of SPIE., 8985, 89850P1 - 89850P8 (2014).
  Cyanobenzaldehyde isomers have been investigated using THz spectroscopy demonstrating several distinct resonances from 2 to 21 THz DFT simulations have been carried out to understand the origin of resonances which show good agreement with experimental results. THz spectroscopy of isomers provides valuable information for designing molecules with customizable THz resonances.
• Anju Susan and Kavita Joshi*, Rationalizing role of the structural motif and the underlying electronic structure in the finite temperature behavior of atomic clusters, J. Chem. Phys., 140, 154307 (2014).
  Melting in finite size systems is an interesting but complex phenomenon. Many factors affect melting and owing to their interdependencies it is a challenging task to rationalize their roles in the phase transition. In this work, we demonstrate how structural motif of the ground state influences melting transition in small clusters. Here, we report a case with clusters of aluminum and gallium having same number of atoms, valence electrons, and similar structural motif of the ground state but drastically different melting temperatures. We have employed Born-Oppenheimer molecular dynamics to simulate the solid-like to liquid-like transition in these clusters. Our simulations have reproduced the experimental trends fairly well. Further, the detailed analysis of isomers has brought out the role of the ground state structure and underlying electronic structure in the finite temperature behavior of these clusters. For both clusters, isomers accessible before cluster melts have striking similarities and does have strong influence of the structural motif of the ground state. Further, the shape of the heat capacity curve is similar in both the cases but the transition is more spread over for Al 36 which is consistent with the observed isomerization pattern. Our simulations also suggest a way to characterize transition region on the basis of accessibility of the ground state at a specific temperature.
• A. Susan, A. Kibey, V. Kaware And K. Joshi*, Correlation between the variation in observed melting temperatures and structural motifs of the global minima of gallium clusters: An ab initio study, Journal of Chemical Physics., 138, 014303 (2013), DOI:http://dx.doi.org/10.1063/1.4772470.
  We have investigated the correlation between the variation in the melting temperature and the growth pattern of small positively charged gallium clusters. Significant shift in the melting temperatures was observed for a change of only few atoms in the size of the cluster. Clusters with size between 31?42 atoms melt between 500–600 K whereas those with 46?48 atoms melt around 800 K. Density functional theory based first principles simulations have been carried out on Ga+n clusters with n = 31,?…, 48. At least 150 geometry optimizations have been performed towards the search for the global minima for each size resulting in about 3000 geometry optimizations. For gallium clusters in this size range, the emergence of spherical structures as the ground state leads to higher melting temperature. The well-separated core and surface shells in these clusters delay isomerization, which results in the enhanced stability of these clusters at elevated temperatures. The observed variation in the melting temperature of these clusters therefore has a structural origin.
• S. A. Blundell And Kavita Joshi, Precise correlation energies in small parabolic quantum dots from configuration interaction, Phys. Rev. B., 81, 115323 (2010), DOI:http://dx.doi.org/10.1103/PhysRevB.81.115323.
  We calculate precise correlation energies of ground and low-lying excited states in circular parabolic quantum dots containing N=2–20 electrons by means of a configuration interaction (CI) method with a numerical, mean-field basis set. All excitations are allowed for 2?N?7 (full CI), while up to hextuple excitations are included for N=8,9, and up to pentuple excitations for 10?N?20. The energies are extrapolated to the limit of basis-set completeness and the truncation error due to restricting the number of Slater determinants is monitored. For high electron densities (Wigner-Seitz radius rs?1.7a?0), the approach achieves errors of order 0.3?mHa? for N=3, a few mHa? for N=6–9, rising to about 100?mHa? for N=20. A comparison is made with recent quantum Monte Carlo calculations.
• P. Chandrachud, Kavita Joshi, S. Krishnamurty And D. G. Kanhere , Stability of gold cages (Au16 and Au17) at finite temperature, Pramana., 72, 845 (2009), DOI:10.1007/s12043-009-0076-x.
  We have employed ab initio molecular dynamics to investigate the stability of the smallest gold cages, namely Au16 and Au17, at finite temperatures. First, we obtain the ground state structure along with at least 50 distinct isomers for both the clusters. This is followed by the finite temperature simulations of these clusters. Each cluster is maintained at 12 different temperatures for a time period of at least 150 ps. Thus, the total simulation time is of the order of 2.4 ns for each cluster. We observe that the cages are stable at least up to 850 K. Although both clusters melt around the same temperature, i.e. around 900 K, Au17 shows a peak in the heat capacity curve in contrast to the broad peak seen for Au16.
• B. S. Pujari, Kavita Joshi, D. G. Kanhere And S. A. Blundell, Impurity effects on the electronic structure of square quantum dots: A full configuration-interaction study, Phys. Rev. B., 78, 125414 (2008), DOI:http://dx.doi.org/10.1103/PhysRevB.78.125414.
  We perform a full configuration-interaction study on a square quantum dot containing several electrons in the presence of an attractive impurity. The magnetic ordering in the dot is analyzed using appropriate pair-correlation functions. We find that a change in the size of the quantum dot can change the nature of the impurity from nonmagnetic to magnetic. In the low-density regime, the impurity traps one electron and the magnetic moment on the localized peaks outside the impurity fluctuates from negative to positive going through zero as a function of number of electrons. We also observe that the impurity changes the charge densities of excited states of two-electron quantum dot significantly, which in the absence of the impurity are almost similar. Our study also shows that in the strongly correlated regime the configuration-interaction approach yields ?20% more localization than density-functional theory. It has also been observed that only a small fraction of the total number of Slater determinants are required to produce ?99% of the converged charge density.
• Shahab Zorriasatein, Kavita Joshi And D. G. Kanhere, Electronic and structural investigations of gold clusters doped with copper: Aun?1Cu? (n=13–19), J. Chem. Phys., 128, 184314 (2008), DOI:http://dx.doi.org/10.1063/1.2913153.
  We have obtained the ground state and the equilibrium geometries of Aun? and Aun?1Cu? in the size range of n=13–19. We have used first principles density functional theory within plane wave and Gaussian basis set methods. For each of the cluster we have obtained at least 100 distinct isomers. The anions of gold clusters undergo two structural transformations, the first one from flat cage to hollow cage and the second one from hollow cage to pyramidal structure. The Cudoped clusters do not show any flat cage structures as the ground state. The copperdoped systems evolve from a general 3D structure to hollow cage with Cu trapped inside the cage at n=16 and then to pyramidal structure at n=19. The introduction of copper atom enhances the binding energy per atom as compared to gold cluster anions.
• P. Chandrachud, Kavita Joshi And and D. G. Kanhere, Thermodynamics of carbon-doped Al and Ga clusters: Ab initio molecular dynamics simulations, Phys. Rev. B., 76, 235423 (2007), DOI:http://dx.doi.org/10.1103/PhysRevB.76.235423.
  We have carried out extensive first principles thermodynamic simulations for Al13, Ga13, Al12C, and Ga12C. The results are based on the simulation time of 2.4ns for each of the clusters, and the heat capacity curves have been calculated using multiple-histogram technique. Both clusters Al13 and Ga13 show higher than bulk melting temperatures. Upon doping, there is a substantial reduction in the melting temperatures of the host clusters. In the case of Ga, the carbon atom changes the geometry from decahedral to icosahedral. This change in the geometry changes the heat capacity curve significantly, making the solidlike to liquidlike transition sharper. Our results bring out the fact that an impurity can be used to tune the finite temperature properties of small clusters.
• C. M. Neal, A. K. Starace, M. F. Jarrold, Kavita Joshi, S. Krishnamurty And D. G. Kanhere , Melting of Aluminum Cluster Cations with 31?48 Atoms:? Experiment and Theory, J. Phys. Chem. C., 111, 17788 (2007), DOI:10.1021/jp070952s.
  Heat capacities have been measured as a function of temperature for aluminum clusters with 31?48 atoms, complimenting previous measurements for larger clusters. Peaks in the heat capacities (due to the latent heat) indicate melting transitions. Large size-dependent fluctuations in the melting temperatures are found in the 31?48 atom size regime, with the lowest melting temperature differing from the highest by close to 400 K. There are also large variations in the latent heats; some clusters show prominent peaks in their heat capacities, whereas for others the peak is virtually absent. A first effort is made to explain the main features of these results by investigating the geometries of the clusters using first principles density functional methods. It appears that clusters that show strong first-order phase transitions have geometries with more uniform bonding (i.e., more similar bond energies and bond lengths) than clusters that lack a strong first-order phase transition. The variation in the melting temperature is associated with the core?surface connectivity and the average coordination of the atoms in the cluster.
• Bhalchandra Pujari, Kavita Joshi, D. G. Kanhere And S. A. Blundell, Electronic structure of many-electron square-well quantum dots with and without an attractive impurity: Spin-density-functional theory, Phys. Rev. B., 76, 085340 (2007), DOI:http://dx.doi.org/10.1103/PhysRevB.76.085340.
  The electronic structure of many-electron square-well quantum dots with and without an attractive impurity has been investigated within the spin-density-functional theory. We consider various sizes of quantum dots with the number of electrons varying from 2 to 20. We observe the emergence of localized Wigner-molecule-like behavior along with the wall-like feature in the charge density. However, unlike the parabolic quantum dots, we do not observe the analog of concentric rings. We also observe the typical broken symmetry configurations noted in earlier reports for quantum dots. The impurity induces the localized magnetic moment which, in many cases, generates the spin-polarized configurations with the antiferromagnetic coupling. An examination of the magnetic states indicates that the presence of impurity may change the ground state of quantum dot from magnetic to nonmagnetic and vice versa. We also observe that the localized charge at the center sharpens the walls.
• Sailaja Krishnamurty, Kavita Joshi, Shahab Zorriasatein And D. G. Kanhere, Density functional analysis of the structural evolution of Gan (n=30–55) clusters and its influence on the melting characteristics, J. Chem. Phys., 127, 054308 (2007), DOI:http://dx.doi.org/10.1063/1.2759215.
  Recent experimental results have reported surprising variations in the shapes of the heat capacity curves and melting temperatures of galliumclusters in the size range of 30–55 atoms [G. A. Breaux et al., J. Am. Chem. Soc.126, 8628 (2004)]. In the present work, we have carried out an extensive density functional investigation on ten selected clusters in the above mentioned size range. In particular, we have analyzed the ground state geometry and the nature of bonding in these clusters using electron localization function. We demonstrate that the existence or otherwise of a large island of atoms bonded with similar strength (i.e., the local order) in the ground state geometry is responsible for the variation in the shape of the heat capacity curve. We attribute the observed higher melting temperatures of some of the clusters (viz., Ga45–Ga48) to the presence of a distinct core and strong covalent bonds between the core and surface atoms. The present work clearly demonstrates that it is possible to understand the general trends observed in the heat capacity curves across the entire series on the basis of the analysis of their ground state.
• Shahab Zorriasatein, Kavita Joshi And D. G. Kanhere, Dopant-induced stabilization of silicon clusters at finite temperature, Phys. Rev. B., 75, 045117 (2007), DOI:http://dx.doi.org/10.1103/PhysRevB.75.045117.
  With the recent advances in miniaturization, understanding and controlling the properties of technologically significant materials such as silicon in the nano regime assumes considerable importance. The silicon clusters in the size range of 15–20 atoms are known to be unstable upon heating. For example, Si20 does not melt but fragments around 1250K, whereas Si15 has a liquidlike phase spread over a short temperature range and undergoes fragmentation at approximately 1800K. In this paper, we demonstrate that it is possible to suppress such a fragmentation process by introducing the appropriate dopant (in this case Ti). Specifically, by using the first-principles density functional simulations we show that Ti-doped Si16, having the Frank-Kasper polyhedral, remains stable till at least 2200K and fragments only above 2600K. Our calculations also indicate that the observed melting transition is a two-step process. The first step is initiated by the surface melting at approximately 600K. In the second step, the destruction of the cage takes place at approximately 2250K, giving rise to a peak in the heat capacity curve.
• Kavita Joshi, Sailaja Krishnamurty And D. G. Kanhere, “Magic Melters” Have Geometrical Origin, Phys. Rev. Lett., 96, 135703 (2006), DOI:http://dx.doi.org/10.1103/PhysRevLett.96.135703.
  Recent experimental reports bring out extreme size sensitivity in the heat capacities of gallium and aluminum clusters. In the present work we report results of our extensive ab initio molecular dynamical simulations on Ga30 and Ga31, the pair which has shown rather dramatic size sensitivity. We trace the origin of this size sensitive heat capacities to the relative order in their respective ground state geometries. Such an effect of nature of the ground state on the characteristics of heat capacity is also seen in case of small gallium and sodium clusters, indicating that the observed size sensitivity is a generic feature of small clusters.
• Sailaja Krishnamurty, Kavita Joshi, D. G. Kanhere And S. A. Blundell, Finite-temperature behavior of small silicon and tin clusters: An ab initio molecular dynamics study, Phys. Rev. B., 73, 045419 (2006), DOI:http://dx.doi.org/10.1103/PhysRevB.73.045419.
  The finite-temperature behavior of small silicon and tin clusters (Si10, Si15, Si20, Sn10, and Sn20) is studied using isokinetic Born-Oppenheimer molecular dynamics. We find that the low-lying structures for all the clusters are built upon a highly stable tricapped trigonal prism unit which is seen to play a crucial role in the finite-temperature behavior. The thermodynamics of small tin clusters is revisited in light of the recent experiments on tin clusters of sizes 18–21 [G. A. Breaux et al., Phys. Rev. B, 71, 073410 (2005)]. Our calculated heat capacities for Si10, Sn10, and Si15 show main peaks around 2300, 2200, and 1400K, respectively. The finite-temperature behavior of Si10 and Sn10 is dominated by isomerization and it is rather difficult to discern their melting temperatures. On the other hand, Si15 does show a liquidlike behavior over a short temperature range, which is followed by fragmentation observed around 1800K. The finite-temperature behavior of Si20 and Sn20 shows that these clusters do not melt but fragment around 1200 and 650K, respectively.
• Mal-Soon Lee, DG Kanhere And Kavita Joshi, Ab initio density-functional study of the equilibrium geometries and the electronic properties of Li 10? n Sn n (n= 0–10) clusters, Physical Review A., 72, 015201 (2005), DOI:http://dx.doi.org/10.1103/PhysRevA.72.015201.
  We have employed ab initio molecular dynamics to investigate the equilibrium geometries, energetics, and the nature of bonding in mixed Li-Sn clusters. Our studies reveal that a small percentage of Sn in Li-rich clusters introduces significant changes in the equilibrium geometries. It is also seen that the geometries of Sn-rich clusters are influenced by the Sn10 motif. Analysis of the nature of bonding shows that there are two competing interactions in the clusters: the polar bond between Li-Sn in the mixed clusters and Sn-Sn interaction leading to covalent bond in Sn-rich clusters.
• S. Chacko, Kavita Joshi, D. G. Kanhere And S. A. Blundell, Why Do Gallium Clusters Have a Higher Melting Point than the Bulk?, Phys. Rev. Lett., 92, 135506 (2004), DOI:http://dx.doi.org/10.1103/PhysRevLett.92.135506.
  Density functional molecular dynamical simulations have been performed on Ga17 and Ga13 clusters to understand the recently observed higher-than-bulk melting temperatures in small gallium clusters [G.?A. Breaux et al., Phys. Rev. Lett. 91, 215508 (2003)]. The specific-heat curve, calculated with the multiple-histogram technique, shows the melting temperature to be well above the bulk melting point of 303 K, viz., around 650 and 1400 K for Ga17 and Ga13, respectively. The higher-than-bulk melting temperatures are attributed mainly to the covalent bonding in these clusters, in contrast with the covalent-metallic bonding in the bulk.
• Kavita Joshi And D. G. Kanhere, Finite temperature behavior of impurity doped Lithium cluster, Li6Sn, J. Chem. Phys., 119, 12301 (2003), DOI:http://dx.doi.org/10.1063/1.1626538.
  We have carried out extensive isokinetic ab initio molecular-dynamic simulations to investigate the finite temperature properties of the impurity doped cluster Li6Sn and the host cluster Li7. The data obtained from about 20 temperatures and total simulation time of at least 3 ns is used to extract thermodynamical quantities like canonical specific heat. We observe that, first, Li6Sn becomes liquidlike around 250 K, at much lower temperature than that for Li7?(?425?K). Second, a weak shoulder around 50 K in the specific heat curve of Li6Sn is observed due to the weakening of Li–Li bonds. The peak in the specific heat of Li7 is very broad and the specific heat curve does not show any premelting features.
• Kavita Joshi, D. G. Kanhere And S. A. Blundell, Thermodynamics of tin clusters, Phys. Rev. B., 67, 235413 (2003), DOI:http://dx.doi.org/10.1103/PhysRevB.67.235413.
  We report the results of detailed thermodynamic investigations of the Sn20 cluster using density-functional molecular dynamics. These simulations have been performed over a temperature range of 150 to 3000 K, with a total simulation time of order 1 ns. The prolate ground state and low-lying isomers consist of two tricapped trigonal prism (TTP) units stacked end to end. The ionic specific heat, calculated via a multihistogram fit, shows a small peak around 500 K and a shoulder around 850 K. The main peak occurs around 1200 K, about 700 K higher than the bulk melting temperature, but significantly lower than that for Sn10. The main peak is accompanied by a sharp change in the prolate shape of the cluster due to the fusion of the two TTP units to form a compact, near spherical structure with a diffusive liquidlike ionic motion. The small peak at 500 K is associated with rearrangement processes within the TTP units, while the shoulder at 850 K corresponds to distortion of at least one TTP unit, preserving the overall prolate shape of the cluster. At all temperatures observed, the bonding remains covalent.
• Kavita Joshi, D. G. Kanhere And S. A. Blundell, Abnormally high melting temperature of the Sn$_{10}$ cluster, Physical Review B., 66, 155329 (2002), DOI:http://dx.doi.org/10.1103/PhysRevB.66.155329.
  We study the melting of the Sn10 cluster with ab initio isokinetic molecular dynamics. The electron localization function is used to probe the bonding character, which is found to be covalent. We use the multiple-histogram technique to calculate the ionic entropy and specific heat, from a total simulation time of more than 2 ns. The specific heat shows a shoulder around 500 K due to a permutational rearrangement of atoms that preserves the trigonal prism core of the ground state. Only at much higher temperatures T?1500K does this core distort and break up, yielding a peak in the specific heat around 2300 K. These findings suggest a natural explanation for the recently observed high melting temperatures of small Sn clusters [A. A. Shvartsburg and M. F. Jarrold, Phys. Rev. Lett. 85, 2530 (2000)].
• Kavita Joshi And D. G. Kanhere, Ab initio investigation of electronic structure, equilibrium geometries, and finite-temperature behavior of Sn-doped Lin clusters, Physical Review A., 65, 043203 (2002), DOI:http://dx.doi.org/10.1103/PhysRevA.65.043203.