Saraswati Kulkarni

Supercritical fluid processing has developed into a versatile and environmentally responsible approach for the controlled fabrication of nanomaterials, distinguished by low density, viscosity, high diffusivity, and negligible surface tension. These intrinsic features enhance nucleation, efficient mass transport, and accurate control over particle dimensions, morphology, and compositional uniformity. This review comprehensively examines the fundamental principles of supercritical fluids, encompassing phase behavior, thermophysical characteristics, and the mechanisms governing nanoparticle formation, along with important engineering aspects related to both batch and continuous‐flow reactor configurations. The preparation of metal, metal oxide, and composite nanostructures through supercritical techniques is critically evaluated, emphasizing their benefits compared with traditional solid‐state and solution‐based synthesis routes. Special attention is given to the evolution toward high‐entropy oxides, where supercritical conditions enable homogeneous multication distribution, enhanced phase development, and defect modulation under kinetically advantageous environments. By establishing clear relationships between processing conditions, structural characteristics, and electrochemical performance, this review highlights the promise of SCF‐enabled methodologies as scalable and sustainable pathways for designing advanced multicomponent oxide materials suited for energy storage and catalytic applications.

1. Agriculture is essential for guaranteeing food security and increasing the economic prosperity of countless individuals, especially in emerging nations. Nevertheless, traditional agricultural me...

Crop health monitoring is crucial for early detection of biotic stress, which can significantly impact crop yield and quality. This work reports detection of two volatile organic compounds (VOCs) - methyl salicylate and linalool which have promising utility in agriculture including indication of biotic stresses in crops. NiO nanoflowers were synthesized using a solvothermal method. The polycrystalline nature of the metal oxide was confirmed using X-ray diffractometer pattern and crystallite size was found to be 16.8 nm. The bandgap of the NiO nanoflowers were found to be 3.47 eV when using UV-Visible spectrometer. The morphology of the nanomaterial was confirmed using field emission scanning electron microscope. Brunauer–Emmett–Teller confirmed the surface area of NiO to be 39.6 m² g⁻¹. X-ray photoelectron spectroscopy confirms the successful synthesis of nickel oxide (NiO) with distinct Ni 2p peaks at 855.5 eV and 873.3 eV, along with O 1s lattice oxygen at 529.6 eV. Satellite peaks at 861 eV and 879 eV further validate the Ni(II) oxidation state, indicating a well-defined crystalline structure. The NiO nanoflower was seen to exhibit temperature modulated dual selectivity as it exhibited the highest response to linalool at 300 °C whereas for methyl salicylate the optimum sensing temperature was found to be 250 °C. The response of NiO was found to be varying from 5.2–40% towards 5–40 ppm of linalool at 300 °C and between 2.5–13.5% towards 10–40 ppm of methyl salicylate at 250 °C. The response times and recovery times of the sensor varied from 260 s to 160 s and 170 s to 650 s, respectively for 5 to 40 ppm of linalool. For 10 to 40 ppm methyl salicylate, the response times and recovery times ranged from 280 s to 230 s and 220 s to 1203 s, respectively. The sensor was found to be highly selective towards linalool at 300 °C but the response of NiO was found to be comparable for both methyl salicylate and linalool at 250 °C. A simple algorithm based on steady state responses of the sensor at 250 and 300 °C could be developed for accurate prediction of both the VOCs.

Analysis and understanding of the mixture of volatile organic compound (VOC) sensing are crucial for the development of sensors in conditions closer to real-life applications, such as health care, air quality monitoring, industrial safety, etc. In this study, we investigated the response dynamics of CuO-nanomaterial-based resistive sensors to 25–75 ppm of individual, binary, ternary, and quaternary mixtures of five VOCs—acetone, acetonitrile, isopropanol, methanol, and toluene at 300 °C. The CuO exhibited responses equal to the sum of its steady-state responses to individual VOCs for all the possible combinations of binary and ternary mixtures with 25 ppm of the constituent gases. A systematic study based on the recovery cycle was conducted by retracting the VOCs sequentially from the proximity of CuO surface after recording response cycle. Interestingly, the recovery time constant τ _rec was found to follow the order—isopropanol (96.93 – 435.45) ≥ methanol (111.82 – 313.21) > toluene (9.9 – 220.49) > acetonitrile (85.96 – 332.32) > acetone (could not be found) in all binary, ternary, and quaternary mixtures of VOCs, irrespective of the sequence of retraction of the VOCs from the mixture. Also, it was found that VOCs with –OH groups have higher adsorption capacity on the sensing layer as compared to –NH ₂ , – C = O, and aromatic VOCs.

Trace level and timely detection of multiple gases are desirous for any gas sensor that intends to facilitate proper air quality monitoring. In many cases, achieving these attributes simultaneously becomes very difficult with a single sensor. In this paper, MoS ₂ -ZnO sensing layer was synthesized by ultrasonicating MoS ₂ nanoflakes (synthesized using liquid exfoliation) and ZnO nanoflowers (synthesized using hydrothermal method) to detect NO ₂ and 2-Nitrotoluene (2-NT). The bandgaps of MoS ₂ and ZnO were found to be 1.71 and 3.12 eV, respectively, through UV-visible spectroscopy. While X-ray photoelectron spectroscopy indicated charge sharing between the materials, the electron microscopy images revealed co-existence of MoS ₂ nanoflakes and ZnO nanoflowers thereby, confirming the sensing material to be a composite. The intrinsic and the composite nanomaterials were tested for NO ₂ and 2-NT at different temperatures (25–300°C). The composite among all samples exhibited the highest response for NO ₂ and 2-NT but at two different temperatures, 200°C and 250°C, respectively. The response of the sensor varied from 31.5–544% and 71–193% for 5–15 ppm of NO ₂ and 2-NT, respectively, at the optimum temperatures. The limit of detection of the MoS ₂ -ZnO was found to be 8 and 62 ppb for NO ₂ and 2-NT, respectively. The response times of the composite were found to be varying from ~1000 to ~1500 s for NO ₂ and ~250 to 500 s for 2-NT, yet the prediction time of these gases could be reduced to 200 s with the help of transient analysis and cantor pairing function.