Faculty Publications
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Item Advanced two-dimensional heterojunction photocatalysts of stoichiometric and non-stoichiometric bismuth oxyhalides with graphitic carbon nitride for sustainable energy and environmental applications(MDPI, 2021) Sridharan, K.; Shenoy, S.; Girish Kumar, S.G.; Terashima, C.; Fujishima, A.; Pitchaimuthu, S.Semiconductor-based photocatalysis has been identified as an encouraging approach for solving the two main challenging problems, viz., remedying our polluted environment and the generation of sustainable chemical energy. Stoichiometric and non-stoichiometric bismuth oxyhalides (BiOX and BixOyXz where X = Cl, Br, and I) are a relatively new class of semiconductors that have attracted considerable interest for photocatalysis applications due to attributes, viz., high stability, suitable band structure, modifiable energy bandgap and two-dimensional layered structure capable of generating an internal electric field. Recently, the construction of heterojunction photocatalysts, especially 2D/2D systems, has convincingly drawn momentous attention practicably owing to the productive influence of having two dissimilar layered semiconductors in face-to-face contact with each other. This review has systematically summarized the recent progress on the 2D/2D heterojunction constructed between BiOX/BixOyXz with graphitic carbon nitride (g-C3N4). The band structure of individual components, various fabrication methods, different strategies developed for improving the photocatalytic performance and their applications in the degradation of various organic contaminants, hydrogen (H2) evolution, carbon dioxide (CO2) reduction, nitrogen (N2) fixation and the organic synthesis of clean chemicals are summarized. The perspectives and plausible opportunities for developing high performance BiOX/BixOyXz-g-C3N4 heterojunction photocatalysts are also discussed. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.Item Synthesis, characterization, and nonlinear optical properties of donor-acceptor conjugated polymers and polymer/Ag nanocomposites(2012) Murali, M.G.; Udayakumar, U.; Sridharan, K.Two new donor-acceptor (D-A) conjugated polymers P1 and P2 containing 3,4-didodecyloxythiophene and 1,3,4-oxadiazole units are synthesized via Wittig reaction methodology. Cyclic voltammetry studies reveal that the polymers are both p and n dopable, and possess low-lying LUMO energy levels (-3.34 eV for P1 and -3.46 eV for P2) and high-lying HOMO energy levels (-5.34 eV for P1 and -5.27 eV for P2). The optical band gap of the polymers is in the range of 2.25-2.29 eV, calculated from the onset absorption edge. The polymers emit orange to yellow light in the film state when irradiated with a UV light. The synthesized polymers are used to prepare polymer nanocomposites with different wt% of silver nanoparticles. The polymer nanocomposites are characterized by UV-Vis absorption spectroscopy, field emission scanning electron microscopy, and thermogravimetric analysis. Both polymers and polymer/ Ag nanocomposites show good thermal stability with onset decomposition temperature around 300 °C under nitrogen atmosphere. The nonlinear optical properties of polymers and polymer/Ag nanocomposites are measured by Z-scan technique. Both polymers and polymer nanocomposites show a good optical limiting behavior. Nearly five times enhancement in the nonlinear optical properties is observed for polymer/Ag nanocomposites. The value of effective two-photon absorption coefficient (?) is in the order of 10 -10-10 -11 m/W. These results indicate that the synthesized polymers (P1 and P2) and their Ag nanocomposites are expected to be good candidates for application in photonic devices. © Springer Science+Business Media, LLC 2012.Item Novel N-substituted-5-phenyl-1H-pyrazole-4-ethyl carboxylates as potential NLO materials(2013) Chandrakantha, B.; Isloor, A.M.; Sridharan, K.; Philip, R.; Shetty, P.; Padaki, M.In the present investigation we have synthesized a novel series of N-substituted-5-phenyl-1H-pyrazole-4-ethyl carboxylates, which are characterized by 1H NMR, UV-Vis and FT-IR spectroscopy methods. The optical nonlinearity of the compounds in chloroform solution has been studied at 532nm using 5ns laser pulses, employing the open-aperture z-scan technique. It is found that compound 3c having carboxylic acid group and ester substituent has maximum nonlinearity. From measurements we conclude that compounds 3c (4-[4-(ethoxycarbonyl)-5-phenyl-1H-pyrazol-1-yl]benzoic acid) and 3e (ethyl 1-(2-bromophenyl)-5-phenyl-1H-pyrazole-4-carboxylate) are potential candidates for optical limiting applications. © 2011.Item Superior Photostability and Photocatalytic Activity of ZnO Nanoparticles Coated with Ultrathin TiO2 Layers through Atomic-Layer Deposition(Wiley-VCH Verlag info@wiley-vch.de, 2015) Sridharan, K.; Jang, E.; Park, Y.M.; Park, T.J.Atomic-layer deposition (ALD) is a thin-film growth technology that allows for conformal growth of thin films with atomic-level control over their thickness. Although ALD is successful in the semiconductor manufacturing industry, its feasibility for nanoparticle coating has been less explored. Herein, the ALD coating of TiO2 layers on ZnO nanoparticles by employing a specialized rotary reactor is demonstrated. The photocatalytic activity and photostability of ZnO nanoparticles coated with TiO2 layers by ALD and chemical methods were examined by the photodegradation of Rhodamine B dye under UV irradiation. Even though the photocatalytic activity of the presynthesized ZnO nanoparticles is higher than that of commercial P25 TiO2 nanoparticles, their activity tends to decline due to severe photocorrosion. The chemically synthesized TiO2 coating layer on ZnO resulted in severely declined photoactivity despite the improved photostability. However, ultrathin and conformal ALD TiO2 coatings (?0.75-1.5 nm) on ZnO improved its photostability without degradation of photocatalytic activity. Surprisingly, the photostability is comparable to that of pure TiO2, and the photocatalytic activity to that of pure ZnO. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.Item Nonlinear transmittance and optical power limiting in magnesium ferrite nanoparticles: effects of laser pulsewidth and particle size(Royal Society of Chemistry, 2016) Perumbilavil, S.; Sridharan, K.; Abraham, A.R.; Janardhanan, H.P.; Kalarikkal, N.; Philip, R.We report comparative measurements of size dependent nonlinear transmission and optical power limiting in nanocrystalline magnesium ferrite (MgFe2O4) particles excited by short (nanosecond) and ultrashort (femtosecond) laser pulses. A standard sol-gel technique is employed to synthesize particles in the size range of 10-50 nm, using polyvinyl alcohol as the chelating agent. The structure and morphology of the samples are studied using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Growth of the particles in time is tracked through Fourier transform infrared spectroscopy. Nonlinear transmission measurements have been carried out using the open aperture Z-scan technique employing 532 nm, 5 nanosecond pulses and 800 nm, 100 femtosecond pulses, respectively. The measured optical nonlinearity is primarily of a reverse saturable absorption (RSA) nature, arising mostly from excited state absorption for nanosecond excitation, and two-photon absorption for femtosecond excitation. The optical limiting efficiency is found to increase with particle size for both cases. The calculated nonlinear parameters indicate that these materials are potential candidates for optical limiting applications. © The Royal Society of Chemistry.Item Memory type switching behavior of ternary Ge20Te80-xSnx (0 x 4) chalcogenide compounds(Institute of Physics Publishing michael.roberts@iop.org, 2016) Fernandes, B.J.; Sridharan, K.; Pumlianmunga, P.; Ramesh, K.; Udayashankar, N.K.Chalcogenide compounds have gained huge research interest recently owing to their capability to transform from an amorphous to a crystalline phase with varying electrical properties. Such materials can be applied in building a new class of memories, such as phase-change memory and programmable metallization cells. Here we report the memory type electrical switching behavior of a ternary chalcogenide compound synthesized by doping Tin (Sn) in a germanium-telluride (Ge20Te80) host matrix, which yielded a composition of Ge20Te80-xSnx (0 x 4). Results indicate a remarkable decrease in the threshold switching voltage (V T) from 140 to 61 V when the Sn concentration was increased stepwise, which is attributed to the domination of the metallicity factor leading to reduced amorphous network connectivity and rigidity. Variation in the threshold switching voltage (V T) was noticed even when the sample thickness and temperature were altered, confirming that the memory switching process is of thermal origin. Investigations using x-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed the formation of a crystalline channel that acts as the conduction path between the two electrodes in the switched region. Structural and morphological studies indicated that Sn metal remained as a micro inclusion in the matrix and hardly contributed to the rigid amorphous network formation in Ge20Te80-xSnx. Memory type electrical switching observed in these ternary chalcogenide compounds synthesized herein can be explored further for the fabrication of phase-change memory devices. © 2016 IOP Publishing Ltd.Item Eliminated Phototoxicity of TiO2Particles by an Atomic-Layer-Deposited Al2O3Coating Layer for UV-Protection Applications(Wiley-VCH Verlag, 2016) Jang, E.; Sridharan, K.; Park, Y.M.; Park, T.J.We demonstrate the conformal coating of an ultrathin Al2O3layer on TiO2nanoparticles through atomic layer deposition by using a specifically designed rotary reactor to eliminate the phototoxicity of the particles for cosmetic use. The ALD reactor is modified to improve the coating efficiency as well as the agitation of the particles for conformal coating. Elemental and microstructural analyses show that ultrathin Al2O3layers are conformally deposited on the TiO2nanoparticles with a controlled thickness. Rhodamine B dye molecules on Al2O3-coated TiO2exhibited a long life time under UV irradiation, that is, more than 2 h, compared to that on bare TiO2, that is, 8 min, indicating mitigation of photocatalytic activity by the coated layer. The effect of carbon impurities in the film resulting from various deposition temperatures and thicknesses of the Al2O3layer on the photocatalytic activity are also thoroughly investigated with controlled experimental condition by using dye molecules on the surface. Our results reveal that an increased carbon impurity resulting from a low processing temperature provides a charge conduction path and generates reactive oxygen species causing the degradation of dye molecule. A thin coated layer, that is, less than 3 nm, also induced the tunneling of electrons and holes to the surface, hence oxidizing dye molecules. Furthermore, the introduction of an Al2O3layer on TiO2improves the light trapping thus, enhances the UV absorption. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimItem Microwave assisted growth of stannous ferrite microcubes as electrodes for potentiometric nonenzymatic H2O2 sensor and supercapacitor applications(Elsevier Ltd, 2016) Bindu, K.; Sridharan, K.; Ajith, K.M.; Lim, H.N.; Nagaraja, H.S.Electrochemical sensors and supercapacitors are two noteworthy applications of electrochemistry. Herein, we report the synthesis of SnFe2O4 microcubes and Fe2O3 nanorods through a facile microwave assisted technique which are employed in fabricating the electrodes for nonenzymatic hydrogen peroxide (H2O2) sensor and supercapacitor applications. SnFe2O4 microcubes exhibited an enhanced specific capacitance of 172 Fg?1 at a scan rate of 5 mVs?1 in comparison to Fe2O3 nanorods (70 Fg?1). Furthermore, the H2O2 sensing performance of the fabricated SnFe2O4 electrodes through chronopotentiometry studies in 0.1 M PBS solution (at pH 7) with a wide linear range revealed a good sensitivity of 2.7 mV ?M?1 ?g?1 with a lowest detection limit of 41 nM at a signal-to-noise ratio of 3. These results indicate that SnFe2O4 microcubes are excellent materials for the cost effective design and development of efficient supercapacitors as well as nonenzymatic sensors. © 2016 Elsevier LtdItem Ultrafast and short pulse optical nonlinearity in isolated, sparingly sulfonated water soluble graphene(Elsevier Ltd, 2017) Perumbilavil, S.; Sridharan, K.; Koushik, D.; Sankar, P.; MahadevanPillai, V.P.M.; Philip, R.We report the nonlinear optical properties of graphene oxide (GO), reduced graphene oxide (rGO), sulfonated graphene oxide (S-rGO), and sparingly sulfonated water soluble graphene (WSG), measured under ultrafast (100 fs) and short pulse (300 ps, 50 ns) laser excitations at 800 nm, employing the open aperture Z-scan technique. The samples exhibit typical third order nonlinear optical behaviour including two-photon absorption (2 PA), reverse saturable absorption (RSA), and saturable absorption (SA). A significant variation in the nature of nonlinear absorption is seen when the laser pulse duration is changed. For example, RSA is prominent under nanosecond (ns) excitation, 2 PA is prevalent under femtosecond (fs) excitation, and SA is dominant under picosecond (ps) excitation. For comparison purposes the measurements are repeated using 5 ns laser pulses at 532 nm. RSA is enhanced in WSG in the nanosecond excitation domain, which can be attributed to the covalent bonding between p-phenyl-SO3H groups and sp2 graphitic planes. WSG exhibits a switching behaviour from SA to RSA upon increasing the excitation intensity under fs excitation. Results show that these graphene derivatives are potentially useful for saturable absorber and optical limiter device applications. © 2016 Elsevier LtdItem Crystallization kinetics of Sn doped Ge20Te80?xSnx (0 ? x ? 4) chalcogenide glassy alloys(Elsevier Ltd, 2017) Fernandes, B.J.; Naresh, N.; Ramesh, K.; Sridharan, K.; Udayashankar, N.K.Chalcogenide semiconductors have evolved as multifunctional materials due to their fascinating thermal, optical, electrical and mechanical properties. In this report, Ge20Te80?xSnx (0 ? x ? 4) glassy alloys are systematically studied in order to understand the effect of variation of Sn content on the thermal parameters such as glass transition (Tg) onset crystallization (Tc), peak crystallization (Tp), melting temperature (Tm), activation energy of glass transition (Eg), and crystallization (Ec). The values of Eg are calculated from the variation of Tg with the heating rate (?), according to Kissinger and Moynihan model, while the values of Ec are calculated from the variation of Tp with the heating rate (?), according to Kissinger, Takhor, Augis-Bennett and Ozawa model. Thermal stability and glass forming ability (GFA) are discussed for understanding the applicability of the synthesized materials in phase change memory (PCM) applications. Thermal parameters are correlated with the electrical switching studies to get an insight into the phase change mechanism. The results of the calculated thermal parameters reveal that the GFA of the synthesized Ge20Te80?xSnx (0 ? x ? 4) glassy alloys has a synchronous relationship with their thermal properties studied through differential scanning calorimetry, indicating their potential for phase-change memory device applications. © 2017 Elsevier B.V.