Sipra Choudhury

Synthesis of surfactant encapsulated nickel hexacyanoferrate nanoparticles and deposition of their Langmuir-Blodgett film

Sodium hexametaphosphate (HMP) stabilized nickel hexacyanoferrate (NiHCF) nanoparticles were prepared in aqueous solution and were successfully extracted into an organic phase using cetyltrimethylammonium bromide (CTAB) as the surfactant. Dynamic Light Scattering (DLS) studies suggest that the average size of the nanoparticles is retained during the extraction process from the aqueous to the organic phase. X-Ray diffraction, cyclic voltammetry, IR spectroscopy and magnetic measurements carried on the organic phase shows specific signatures of the presence of the surfactant encapsulated NiHCF nanoparticles. Transmission Electron Microscopy (TEM) measurements show that the average size of these surfactant encapsulated nanoparticles in the organic phase is about 22 nm, and as has been suggested by DLS studies, it does not change with respect to repeated evaporation and re-extraction processes of the organic phase. Pressure–area isotherms of the organic phase in a Langmuir–Blodgett (LB) trough, with water as subphase, indicate stable monolayer formation of the surfactant-encapsulated NiHCF nanoparticles at the air–water interface. Multi-layered deposition of the surfactant-encapsulated nanoparticles onto an indium tin oxide (ITO) coated glass slide could also be carried out using the LB technique. Cyclic voltammetry studies on these LB multilayers confirm regular and systematic transfer of the NiHCF nanoparticles on the ITO substrate. The method described here is the first of its kind with respect to the synthesis of surfactant encapsulated molecular magnetic nanoparticles and subsequent deposition of their LB films.

Room temperature gas sensitivity of ultrathin SnO2 films prepared from Langmuir-Blodgett film precursors

The authors report the room temperature gas sensitivity of ultrathin SnO2 films prepared by thermal decomposition of multilayer Langmuir-Blodgett (LB) films. The SnO2 films, which are characterized by various spectroscopic techniques and electrical measurements, have been shown to sense ammonia gas at room temperature with fast response and recovery. The presence of a large number of surface states in the ultrathin SnO2 film is responsible for the room temperature gas sensitivity. Present results demonstrate the potential of LB technique to fabricate high quality ultrathin oxide films useful for sensor applications.

Synthesis of uniform gold nanoparticles using non-pathogenic bio-control agent: Evolution of morphology from nano-spheres to triangular nanoprisms

Green synthesis of gold nanospheres with uniform diameter and triangular nanoprisms with optically flat surface was carried out using a non-pathogenic bio-control agent Trichoderma asperellum for reduction of HAuCl(4). Kinetics of the reaction was monitored by UV-Vis absorption spectroscopy. No additional capping/complexing agent was used for stabilizing the gold nanoparticles. Evolution of morphology from pseudospherical nanoparticles to triangular nanoprisms was studied by transmission electron microscopy (TEM). It revealed that three or more pseudospheres fused to form nanoprisms of different shapes and sizes. Slow rate of reduction of HAuCl(4) by constituents of cell-free fungal extract was instrumental in producing such exotic morphologies. Isolation of gold nanotriangles from the reacting masses was achieved by differential centrifugation.

UV-shielding transparent PMMA/In2O3 nanocomposite films based on In2O3 nanoparticles

Self supporting poly(methyl methacrylate) (PMMA)/In2O3 with varying In2O3 content (1, 2, 5 and 10 wt% In2O3 loading) nanocomposite films have been prepared by solvent-casting and spin casting techniques. The nanocomposite films have been structurally characterized by X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy and the results confirm the incorporation of In2O3 nanoparticles in the PMMA matrix. The thermal properties of the nanocomposite films have been investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results show that the degradation of the polymer occurs at higher temperature in the presence of In2O3 nanoparticles and even a small amount of In2O3 nanoparticles (∼1 wt%) can greatly improve the thermal stability of PMMA. The UV-visible spectra of the nanocomposite films show that the films are UV-absorbing, and highly transparent in the visible region.

Langmuir–Blodgett self organized nanocrystalline tungsten oxide thin films for electrochromic performance

We explore a novel method to synthesize pebble-like nanocrystalline WO3 thin films for the first time by thermal decomposition of a multilayer Langmuir–Blodgett film of an octadecylamine–tungsten complex. The resulting film was thoroughly characterized by various characterization techniques. The electrochromic performance was evaluated in Li+ as a charge-balancing ion. The WO3 thin film displays a state-of-the-art performance with respect to optical modulation of 25.94% at λ630 nm with a very rapid coloration and bleaching time of 3.57 s and 3.14 s, respectively and a high coloration efficiency of 71.26 cm2 C−1. The excellent electrochromic performance can be attributed to the high size uniformity of WO3 nanoparticles, whose crystalline nature offer more active sites for Li+ diffusion and control the diffusion path length. Thus, the Langmuir–Blodgett WO3 film contributes to high energy conversion devices.

Nanostructured PdO Thin Film from Langmuir–Blodgett Precursor for Room-Temperature H2 Gas Sensing

Nanoparticulate thin films of PdO were prepared using the Langmuir-Blodgett (LB) technique by thermal decomposition of a multilayer film of octadecylamine (ODA)-chloropalladate complex. The stable complex formation of ODA with chloropalladate ions (present in subphase) at the air-water interface was confirmed by the surface pressure-area isotherm and Brewster angle microscopy. The formation of nanocrystalline PdO thin film after thermal decomposition of as-deposited LB film was confirmed by X-ray diffraction and Raman spectroscopy. Nanocrystalline PdO thin films were further characterized by using UV-vis and X-ray photoelectron spectroscopic (XPS) measurements. The XPS study revealed the presence of prominent Pd(2+) with a small quantity (18%) of reduced PdO (Pd(0)) in nanocrystalline PdO thin film. From the absorption spectroscopic measurement, the band gap energy of PdO was estimated to be 2 eV, which was very close to that obtained from specular reflectance measurements. Surface morphology studies of these films using atomic force microscopy and field-emission scanning electron microscopy indicated formation of nanoparticles of size 20-30 nm. These PdO film when employed as a chemiresistive sensor showed H2 sensitivity in the range of 30-4000 ppm at room temperature. In addition, PdO films showed photosensitivity with increase in current upon shining of visible light.

DNA-templated assemblies of nickel hexacyanoferrate crystals.

We report here the synthesis of nickel hexacyanoferrate (NiHCF) crystals using calf thymus DNA (CT-DNA) as a template. The double-stranded CT-DNA has been used as a template to self-assemble NiHCF crystals and to produce aggregates having different morphologies at different temperatures. The guided self-assembly behavior of DNA was studied at different temperatures by scanning electron microscopy. The cube-shaped crystals of NiHCF with an average diameter of 400 nm are observed along the DNA framework at room temperature; however, at higher temperatures, the morphology of NiHCF changed from open tubular to dendrimer. The intermediate temperatures show long chains (up to many micrometers) and spherical structures of NiHCF crystals. The micrometer long DNA template plays a key role in the formation of extended arrays of NiHCF crystals, suggesting that the templating action is retained even at the higher temperatures.

Studies on the formation of langmuir monolayer and Langmuir-Blodgett films of octadecyl amine-bromocresol purple dye complex

Abstract Properties of the monolayer of octadecyl amine (ODA) molecules on aqueous solution of bromocresol purple (Bcp) dye have been studied over a wide range of pH by measuring the surface pressure-area isotherms. A strong interaction of ODA molecule was observed with the dye molecule in the pH range of 4–9. Under the present study, the dye molecules were adsorbed on the ODA monolayer at the water surface and subsequently a Langmuir–Blodgett (LB) film of ODA–dye complex was transferred from the air–water interface onto a solid substrate. Alternatively, dye molecules were incorporated into LB film by immersing the predeposited LB film of pure ODA into the dye solution, i.e. by adsorption of dye molecule at the solid–liquid interface. The adsorption behavior of ODA–Bcp dye complexes obtained through these two different routes was characterised with the help of UV–Visible spectroscopy. The nature of the ODA–dye complex deposited from air–water interface using the LB technique was found to be quite different from that of the complex formed by the adsorption of the dye into the pure ODA LB film. The adsorption spectra of the ODA–dye complex obtained from the air–water interface did not change with pH, while those of the ODA–dye complex formed from the solid–liquid interface showed pH dependence.

Room Temperature Ammonia Gas Selectivity Studies On SnO2 Ultra Thin Film Prepared By Langmuir‐Blodgett Technique

Interaction of various reducing and oxidizing gases with ultrathin SnO2 film at room temperature has been studied to investigate the selectivity of SnO2 film towards ammonia. SnO2 thin films show no response with O2, H2, CO, ethanol and methane. In presence of H2S and SO2 gases conductivity of the film increases where as film conductivity decreases with ammonia and NO2. With ammonia gas, the response is fast and >90% recovery takes place within 30 min where as with NO2_recovery is partial. The impedance study shows distinctly different frequency dependent responses towards NH3 and NO2 indicating specificity to ammonia in presence of NO2.

Facile preparation of Silicon/ZnO thin film heterostructures and ultrasensitive toxic gas sensing at room temperature: Substrate dependence on specificity

Two types of silicon-Zinc oxide (ZnO) heterostructures were prepared simply by depositing (drop casting) chemically prepared ZnO nanoparticles onto single crystalline (p-type) silicon substrates (Si) as well as electrochemically prepared p-type porous silicon (PS). ZnO nanoparticles and PS/ZnO structures were characterized structurally by various techniques. By depositing in-plane gold contacts on the heterostructures, gas sensors were fabricated and characterized electrochemically by dc and ac impedance measurements. The PS/ZnO sensors showed specific response at room temperature for NO2 with increase in current and no significant response for other reducing and oxidizing gases. The sensor is sensitive to 200 ppb NO2 at 25 °C with 35% change in current and 50 s response time. Temperature dependent studies of sensor in the range of 25-100 °C have shown maximum sensitivity at 40 °C (50% change for 200 ppb) with decreasing sensitivity thereafter (23% change at 60 °C), indicating the suitability of the sensor till 60 °C. Alternatively Si/ZnO heterostructures showed maximum response with NO2, along with lesser specific responses for SO2 and NH3. Detailed multifrequency impedance studies with temperature suggested the role of space charge layers at various interfaces in the charge transport properties of PS/ZnO and Si/ZnO heterostructures resulting in their specific gas sensing properties.