Nillohit Mukherjee

Palladium-silver-activated ZnO surface: highly selective methane sensor at reasonably low operating temperature.

Metal oxide semiconductors (MOS) are well known as reducing gas sensors. However, their selectivity and operating temperature have major limitations. Most of them show cross sensitivity and the operating temperatures are also relatively higher than the value reported here. To resolve these problems, here, we report the use of palladium-silver (70-30%) activated ZnO thin films as a highly selective methane sensor at low operating temperature (∼100 °C). Porous ZnO thin films were deposited on fluorine-doped tin oxide (FTO)-coated glass substrates by galvanic technique. X-ray diffraction showed polycrystalline nature of the films, whereas the morphological analyses (field emission scanning electron microscopy) showed flake like growth of the grains mainly on xy plane with high surface roughness (107 nm). Pd-Ag (70-30%) alloy was deposited on such ZnO films by e-beam evaporation technique with three different patterns, namely, random dots, ultrathin (∼1 nm) layer and thin (∼5 nm) layer as the activation layer. ZnO films with Pd-Ag dotted pattern were found show high selectivity towards methane (with respect to H2S and CO) and sensitivity (∼80%) at a comparatively low operating temperature of about 100°C. This type of sensor was found to have higher methane selectivity in comparison to other commercially available reducing gas sensor.

Mesoporous CuO–ZnO p–n heterojunction based nanocomposites with high specific surface area for enhanced photocatalysis and electrochemical sensing

We report the synthesis and characterization of p-CuO/n-ZnO heterojunction nanocomposites and their application as a broad spectrum photocatalyst. The different systems were also found to serve significantly as an electrode material for electrochemical (amperometric) sensing of H2O2 and NH3 (in μM or ppm level). To investigate their photochemical activities, three dyes, viz. Alizarin Red S, Methylene Blue and Rose Bengal, were chosen. The 60 hour milled composite gave better photocatalytic activity than the unmilled and 30 hour milled systems due to homogeneous mixing, formation of appropriate p–n junction, significant lowering of particle size and enhanced specific surface area. The defect level of ZnO, which also increases with increasing milling duration, again helps in better catalytic performance by trapping the photogenerated electrons and holes and also by raising the valance band edge and thus narrowing the band gap. The 60 hour milled sample took 55, 60 and 70 minutes to degrade the ARS, MB and RB dyes, respectively, which are better than many previous reports. The degradation rate was 0.01931, 0.04663 and 0.02318 min−1 for the ARS, MB and RB dyes, respectively.

Photocatalytic degradation of organic dye on porous iron sulfide film surface

Thin films of nanocrystalline and porous FeS(2) with marcasite phase have been deposited from a greenish-blue iron nitroprusside precursor film, which readily gives FeS(2) on reacting with an aqueous solution of sodium sulfide. High resolution X-ray diffraction (HRXRD) pattern indicated the formation of polycrystalline and orthorhombic (marcasite) phase of FeS(2), whereas the field emission scanning electron microscopy (FESEM) showed the morphology of the films to be consisted of grains of average 25 nm diameter with unevenly distributed numerous pores. Optical characterization (UV-Vis and photoluminescence) revealed significant amount of blueshift in the band gap energy of the deposited material, which is attributed to the strong quantum confinement effect exerted by the FeS(2) nanocrystals. The deposited FeS(2) films showed good photocatalytic activity toward the degradation of Rose Bengal dye and could be found efficient for wastewater treatment.

Electrochemically synthesized microcrystalline tin sulphide thin films: high dielectric stability with lower relaxation time and efficient photochemical and photoelectrochemical properties

A detailed study has been carried out on the structural, dielectric and impedance properties of polycrystalline p-type SnS thin films grown on transparent conducting oxide (TCO) coated glass substrates from an aqueous solution of tartaric acid, SnSO4 and Na2S2O3 by a modified electrochemical technique. The as-deposited films were found to be smooth, almost pinhole free and well adherent to the bottom substrate. X-ray diffraction studies revealed the formation of polycrystalline SnS films with an orthorhombic phase. Field emission scanning electron microscopy and atomic force microscopy revealed a moderately compact surface morphology with evenly distributed almost spherical grains. Optical measurements showed direct band gap energy of 1.5 eV. Detailed electrical (dc and ac) analyses showed the p-type nature of the deposited films with unique dielectric behavior. The band-gap energy, resistivity, dielectric constant and relaxation time make this material and ideal absorber layer, which is also reflected in the efficient photochemical and photoelectrochemical behavior.

α-Fe2O3 nanospheres: facile synthesis and highly efficient photo-degradation of organic dyes and surface activation by nano-Pt for enhanced methanol sensing

We report a simplified electrochemical route to synthesize thin films of nanosphere α-Fe2O3 from a suitable electrolytic solution. X-ray diffraction studies revealed the formation of pure hematite phase (hexagonal structure) α-Fe2O3 films. Field emission scanning electron microscopy revealed a highly compact surface morphology with evenly distributed almost spherical grains. Raman, electron paramagnetic resonance and Fourier transform infrared spectroscopic analyses confirmed the presence of α phase Fe2O3 (hematite). Optical analysis revealed a band gap energy of 2.15 eV; this is most suitable for visible light driven photocatalysis towards the degradation of Indigo Carmine (IC) and Rhodamine B (Rh B) dyes, which are widely used in the textile industry and were taken as model organic compounds. About 90% photodegradation was achieved at rates of 0.0188 min−1 for IC and 0.0133 min−1 for Rh B. The synthesized films were used as modified electrodes, and their catalytic activity towards methanol oxidation was investigated. A comparison was also made between Pt modified FTO/Fe2O3 and unmodified FTO/Fe2O3 electrodes towards dye degradation and methanol oxidation, and it was found that the Pt modified FTO/Fe2O3 electrode yielded superior results.

Galvanic deposition of nanocrystalline ZnO thin films from a ZnO–Zn(OH)2 mixed phase precursor on p-Si substrate

A galvanic technique for the deposition of ZnO thin films is reported. The depositions were carried out on p-type single-crystal silicon substrates at room temperature, from a solution of ZnSO(4), where the Zn rod acted as a sacrificing anode and p-Si was the cathode. The deposition of ZnO by this method is pH sensitive, and a pH between 4 and 5 is found to be optimum for film deposition. This deposition technique is simple, inexpensive and can be carried out at room temperature. X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies revealed the nanocrystalline structure of the films. The resistivity of the annealed ZnO films was determined by the Van der Pauw measurement technique.

Exfoliated Graphite Reinforced PMMA Composite: A Study on Nanoindentation and Scratch Behavior

The present work aims to compare the scratch hardness (Hs) with the indentation hardness (HIT) obtained from nanoindentation measurements of exfoliated graphite/poly (methyl methacrylate) (EG/PMMA) composites. EG/PMMA composites with thickness 50 μm each were produced by in situ melt mixing method. Scratch hardness was measured from the scratch width formed at low applied load during the course of the scratch measurements. The measured scratch hardness values were compared with the indentation hardness. There is a good correlation between the scratch hardness and the indentation hardness at low indentation depths, with a linear relationship between hardness and the reinforcement content of exfoliated graphite.

Electrochemical synthesis of p-CuO thin films and development of a p-CuO/n-ZnO heterojunction and its application as a selective gas sensor

p-CuO thin films have been synthesized on indium tin oxide (ITO)-coated glass substrates and on ZnO/ITO-coated glass substrates using a new, simple, cost-effective electrochemical technique (galvanic deposition) at room temperature. X-ray diffraction (XRD) studies of the films show a monoclinic phase of CuO, and UV-vis spectroscopy of the CuO/ITO film shows an indirect band-gap energy of about 1.85 eV. The surface morphology of CuO thin film consists of a c-axis-grown regular macroporous network structure with deep cavities surrounded by thin solid walls, which are suitable for gas trapping and sensing. Current–voltage characteristics of the formed p-CuO/n-ZnO film show good rectifying behavior. At 1 V reverse bias, the leakage current was as low as 2 × 10−9 A compared to a current of 1.2 × 10−7 A at the same forward bias, resulting in a forward-to-reverse current ratio of about 60. The ideality factor of the diode obtained was quite high, at about 9.5. The frequency dependence of the small signal AC response in both Rp–Cp and Rs–Cs mode of the fabricated heterojunction were measured at a reverse bias of 1.5 V. In the presence of 10 000 ppm gas exposure for gases, including CH4, H2S and CO, frequency-dependent changes in AC responses are different for different gases. The variation of the reactance of the fabricated device shows different behaviour with exposure to different types of gases. The minimum in reactance occurring at different frequencies for different gases indicates the selectivity of the device for gas sensing.

Experimental study on electron field emission, Raman scattering, and low temperature electrical properties of nanocrystalline lead selenide thin films

We report the electron field emission properties, Raman scattering, and low (77 K) and room temperature electrical properties of nanocrystalline PbSe thin films. Structural characterizations (high resolution x-ray diffraction, atomic force microscopy, and high resolution transmission electron microscopy) revealed the formation of cubic PbSe with an average crystallite diameter of ca. 8 nm. Raman analysis showed a strong peak at 136 cm−1. Due to the nanocrystalline nature, the threshold field (5.5 V/μm) and approximate work function values were high, making the films a very efficient field emitter.

Reusable iron sulfide nanospheres towards promoted photocatalytic and electrocatalytic activities

We report a facile electrochemical route for the synthesis of FeS nanospheres on fluorine doped tin oxide (FTO, SnO2:F) coated glass substrates. X-ray diffraction revealed the formation of the troilite phase of FeS, whereas field emission scanning electron microscopy and atomic force microscopy revealed highly compact surface morphology with evenly distributed almost spherical grains. Optical measurements showed a direct band gap energy of ∼1.95 eV, characteristic of FeS. The prepared FeS films showed significant photocatalytic degradation towards some widely used hazardous chemicals like Alizarin Red S, Methylene Blue, Rose Bengal and phenol. FeS modified FTO electrodes also showed excellent electrochemical properties towards hydrogen peroxide reduction and hence could be used as an efficient H2O2 sensor. The proposed sensor exhibited a good linear response in the range 10–700 μM with a detection limit of 2.699 μM with sensitivity 0.38717 μA μM−1 under optimal conditions.