Debabrata Sarkar

Three Dimensional Ag2O/TiO2 Type-II (p–n) Nanoheterojunctions for Superior Photocatalytic Activity

Type-II p-n junction three-dimensional Ag(2)O/TiO(2) microspheres have been fabricated by assembling p-type Ag(2)O nanoparticle on n-type TiO(2) 3D microsphere. Ag(2)O/TiO(2) microsphere nanoheterojunctions were obtained by hydrothermal synthesis of TiO(2) microspheres at 180 °C followed by photoreduction of AgNO(3). The samples were carefully characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM), and energy dispersive X-ray analysis (EDX). The photocatalytic activity toward degradation of methyl orange (MO) aqueous solution under UV light was investigated. The result showed that type-II p-n nanoheterojunctions Ag(2)O/TiO(2) significantly enhanced the photocatalytic degradation compared to n-type TiO(2) microsphere. It was found that the photocatalytic degradation followed the pseudo first-order reaction model. In particular, heterostructure with molar ratio of TiO(2) and AgNO(3) of 4:1 exhibited best photocatalytic activity and the corresponding apparent first-order rate constant of 0.138 min(-1) which is 4 times than that of pure n-type microsphere.

Morphology control of rutile TiO2 hierarchical architectures and their excellent field emission properties

Titanium dioxide nanoarchitectures with well-defined morphologies have been successfully synthesized by a hydrothermal process at temperature 180 °C for 4 h using titanium butoxide, oleic acid and hydrochloric acid as precursor materials. Different hierarchical morphologies such as cauliflower, 3D microsphere, densely-packed nanorod array, step edge faceted nanorod, branched structures of the rutile TiO2 nanostructures could be easily controlled by varying the volume fraction of the added hydrochloric acid. The structural, photoluminescence and field emission properties of the as-prepared nanoarchitectures were investigated. Among the different morphologies, dense nanorod arrays and multilevel branched architectures showed good field emission properties. Morphology, surface-related defects and oxygen vacancies were found to be the major responsible factors for the observed variations of field emission properties. Compared to any previously reported TiO2 based field emitter, lower turn-on field at 2.76 V μm−1 and higher field-enhancement factors 7.44 × 103 were observed for the hierarchically dense TiO2 nanorods array. Therefore, these nanoarchitectures can be used in vacuum microelectronic applications.

Branch Density-Controlled Synthesis of Hierarchical TiO2 Nanobelt and Tunable Three-Step Electron Transfer for Enhanced Photocatalytic Property

The successful adjustment of phase composition and morphology of hierarchical TiO2 nanobelts, which feature homoepitaxial nanobranches, has been developed via the hydrothermal method and chemical bath deposition technique. Effects of hydrothermal reaction time, titanium butoxide treatment in chemical bath deposition, and calcination temperature are systematically investigated. For the first time, three-step ultrafast electron transfers between the band edges of the engaged phases are realized through the enhanced photocatalytic activity results. Growth mechanism related to branch density control on nanobelt surface under such soft chemical process is discussed in detail on the basis of classical nucleation theory. The current work might provide new insights into the fabrication of one-dimensional homoepitaxial branched TiO2 nanostructures as high performance photocatalysts and facilitate their application in environmental cleanup.

Ag decorated topological surface state protected hierarchical Bi2Se3 nanoflakes for enhanced field emission properties

In this paper, we report an economical and low temperature synthesis route of Ag nanoparticle decorated hierarchical Bi2Se3 nanoflakes (NFs) over a large surface area of Si substrate in an open atmosphere. Detailed analysis of the field emission (FE) properties indicate that the hierarchical Bi2Se3 NFs shows reasonable FE properties due to their high aspect ratio and well-aligned uniform distribution. For improving the FE properties of as-synthesized Bi2Se3 NFs, silver (Ag) nanoparticles (NPs) have been attached to its surfaces. Ag NP attached Bi2Se3 NFs exhibit superior FE properties compared with pure Bi2Se3 NFs. The turn-on field is found to be reduced from 3.83 V μm−1 to 2.03 V μm−1 after attachment of the Ag NPs. Finite element analysis with ANSYS Maxwell simulation software also confirms that Ag decorated Bi2Se3 NFs are better field emitters than Bi2Se3 NFs. The enhanced FE properties of the Ag attached Bi2Se3 heterostructure is attributed to the electron injection from Ag nanoparticles to Bi2Se3 and also to the formation of extra emitting sites at the edge of the NFs. Finally, X-ray photo emission spectroscopy (XPS) confirms that unlike most of the previous reports, surface states are appreciably stable and strongly protected from any surface reactivity and oxidation, even if subjected to adverse atmosphere or high magnitude electric fields.

Electronic structure and optical properties of CuAlO2 under biaxial strain.

An ab initio calculation has been carried out to investigate the biaxial strain ( - 10.71% < ε < 9.13%) effect on elastic, electronic and optical properties of CuAlO(2). All the elastic constants (c(11), c(12), c(13), c(33)) except c(44) decrease (increase) during tensile (compressive) strain. The band gap is found to decrease in the presence of tensile as well as compressive strain. The relative decrease of the band gap is asymmetric with respect to the sign of the strain. Significant differences between the parallel and perpendicular components of the dielectric constant and the optical properties have been observed due to anisotropic crystal structure. It is further noticed that these properties are easily tunable by strain. Importantly, the collective oscillation of the valence electrons has been identified for light polarized perpendicular to the c-axis. From calculations, it is clear that the tensile strain can enhance the hole mobility as well as the transparency of CuAlO(2).

Improved dielectric constant and breakdown strength of γ-phase dominant super toughened polyvinylidene fluoride/TiO2 nanocomposite film: an excellent material for energy storage applications and piezoelectric throughput.

Titanium dioxide (TiO2) nanoparticles (NPs) embedded γ-phase containing polyvinylidene fluoride (PVDF) nanocomposite (PNC) film turns to an excellent material for energy storage application due to an increased dielectric constant (32 at 1 kHz), enhanced electric breakdown strength (400 MV m-1). It also exhibits a high energy density of 4 J cm-3 which is 25 times higher than that of virgin PVDF. 98% of the electroactive γ-phase has been acheived by the incorporation of TiO2 NPs and the resulting PNC behaves like a super-toughened material due to a dramatic improvement (more than 80%) in the tensile strength. Owing to their electroactive nature and extraordinary mechanical properties, PNC films have a strong ability to fabricate the piezoelectric nanogenerators (PNGs) that have recently been an area of focus regarding mechanical energy harvesting. The feasibility of piezoelectric voltage generation from PNGs is demostrated under the rotating fan that also promises further utility such as rotational speed (RPM) determination.

Equibiaxial strain: tunable electronic structure and optical properties of bulk and monolayer MoSe2

Using ab initio calculations based on density functional theory, we have studied the equibiaxial strain effect on the electronic and optical properties of bulk and monolayer MoSe2. Very low value of the elastic constants of monolayer MoSe2 compared to bulk MoSe2 suggests that the tailoring of the electronic structure and optical properties of monolayer MoSe2 may easily be achieved by strain. We find that the band gap (Eg) of MoSe2 undergoes a transition from indirect to direct upon reducing the thickness of the monolayer which is associated with an enhancement of Eg and is further tunable by strain engineering. We noticed that the splitting of the bands that generally determines the luminescence property of MoSe2 can be modified by strain. Strain effect on bond ionicity, dielectric property (real and imaginary component) of bulk and monolayer MoSe2 has also been briefly discussed. We noticed that the plasma oscillation, found in bulk MoSe2, is absent in MoSe2 monolayer.

Electroactive β-crystalline phase inclusion and photoluminescence response of a heat-controlled spin-coated PVDF/TiO2 free-standing nanocomposite film for a nanogenerator and an active nanosensor

The electroactive β-phase is most desirable due to its highest piezo-, pyro- and ferroelectric properties in poly(vinylidene fluoride) (PVDF). Induction of the β-phase is successfully accomplished in titanium dioxide (TiO2) nanoparticles (NPs) doped spin-coated PVDF nanocomposite (PNC) films. The optimized yields of β-phase and homogeneous ultra-smooth free-standing PNC film is utilized in a mechanical-energy harvesting application by fabricating a nanogenerator (NG) where the typical electrical poling step is not undertaken. Under a repeated human finger touch and release process, it delivers an open-circuit voltage of 5 V. Moreover, the physical sensing capabilities of the NG are examined through harvesting mechanical energy from mouse clicking of a laptop and wrist pulse detection, which indicates that it can also be used as a nanosensor. The blue photoluminescence centred at 444 nm, which was also observed in PNC films, makes us anticipate a new type of photonic application where the design feasibility of hybrid sensors, i.e. electromechanical and photonic combination, is also possible.

Semiconductor–Metal Nanofloret Hybrid Structures by Self-Processing Synthesis

We present a synthetic strategy that takes advantage of the inherent asymmetry exhibited by semiconductor nanowires prepared by Au-catalyzed chemical vapor deposition (CVD). The metal-semiconductor junction is used for activating etch, deposition, and modification steps localized to the tip area using a wet-chemistry approach. The hybrid nanostructures obtained for the coinage metals Cu, Ag, and Au resemble the morphology of grass flowers, termed here Nanofloret hybrid nanostructures consisting of a high aspect ratio SiGe nanowire (NW) with a metallic nanoshell cap. The synthetic method is used to prepare hybrid nanostructures in one step by triggering a programmable cascade of events that is autonomously executed, termed self-processing synthesis. The synthesis progression was monitored by ex situ transmission electron microscopy (TEM), in situ scanning transmission electron microscopy (STEM) and inductively coupled plasma mass spectrometry (ICP-MS) analyses to study the mechanistic reaction details of the various processes taking place during the synthesis. Our results indicate that the synthesis involves distinct processing steps including localized oxide etch, metal deposition, and process termination. Control over the deposition and etching processes is demonstrated by several parameters: (i) etchant concentration (water), (ii) SiGe alloy composition, (iii) reducing agent, (iv) metal redox potential, and (v) addition of surfactants for controlling the deposited metal grain size. The NF structures exhibit broad plasmonic absorption that is utilized for demonstrating surface-enhanced Raman scattering (SERS) of thiophenol monolayer. The new type of nanostructures feature a metallic nanoshell directly coupled to the crystalline semiconductor NW showing broad plasmonic absorption.

Facile Synthesis of Large Scale Mesoporous TiO2 Microspheres by Template-free Hydrothermal Process: Their Photocatalysis Degradation

In this study, well-crystallized mesoporous anatase TiO2 microspheres were synthesized by simple hydrothermal reaction. These spheres are submicrometer-sized (the diameter size of about 1 to 3 μm) and consist of packed nanocrystallites with diameters of ~5 to 9nm. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and UV-vis spectrophotometer. The 3D TiO2 microspheres exhibited significant photocatalytic degradation and photocatalytic activity was evaluated via the photocatalytic oxidation of Rhodamine B (RhB) in air at room temperature. The result also shows that mesoporous microspheres followed Langmuir-Hinshelwood (L-H) kinetic model.