Somnath Chanda Roy

Current manuscript describes porous alumina (PA) template assisted electrodeposition of high aspect ratio nanowires and dense hierarchical structures of CdTe. We demonstrate here for the first time that simple structural engineering of a PA template can lead to electrochemical growth of diverse shapes of CdTe nanostructures. Facile and cost-effective modifications have been implemented for the fabrication of self-organized through-hole PA membrane and its transfer onto any rough substrate. These modifications have facilitated extended duration (30 minute to 1 hour) electrodeposition of CdTe nanostructures at high bath temperature of 60◦C without delaminating the PA membrane. High aspect ratio nanowires of 60 nm diameter and 2.8 μm length were growth through the self-ordered PA membrane without any underlying metal coating i.e. without altering its optical properties. An average of 56% optical absorption (within 350 nm - 1400 nm wavelength) and a moderate photoluminescence was observed for the CdTe nanowires. Minor variation in the anodization process resulted into a non-uniform/branched PA template that enabled the formation of dense 3D hierarchical structures of CdTe using similar electrodeposition conditions as that used for CdTe nanowires. The hierarchical CdTe nanostructures exhibited very high total optical absorption of ∼90% within 350 nm - 1400 nm wavelength and a strong photoluminescence was also demonstrated that was almost 10 fold more intense than the CdTe nanowires.

We report the direct growth of self-aligned single crystalline GaN nanorod array on flexible Ta metal foil using laser molecular beam epitaxy. Scanning electron microscopy reveals the vertically aligned nanorods on Ta surface having diameters in the range of 60–80 nm. The nanorods show well-defined hexagonal facets and are quite uniformly distributed across the metal foil. Transmission electron microscopy shows single crystalline nature of the individual rods having c-axis oriented growth with wurtzite structure. Room temperature photoluminescence study exhibits a sharp, intense band-to-band emission without any deep-level bands indicating the excellent optical quality of the GaN nanorod array. X-ray photoemission spectroscopy to elucidate the electronic structure of the nanorods confirms Ga–N bonding and the calculated chemical composition turns out to be slightly Ga rich. Location of valence band maxima also suggests the n-type character of GaN nanorods. The photoelectrochemical water-splitting behaviour of the self-aligned GaN nanorod arrays on Ta foil has been investigated using 1 M oxalic acid as the electrolyte with AM 1.5 G simulated solar radiation under 1 Sun (100 mW/cm2) conditions. The results demonstrate an effective way of fabricating well-aligned GaN nanorods on flexible metal foils for developing simple, relatively inexpensive, and flexible photo-electrodes for photocatalytic solar water-splitting applications.

In comparison with single-phase materials, heterostructures have been known for superior water splitting applications. In this study, Cu2O and NiFe2O4 are chosen to fabricate thin film heterostructures. Cu2O is electrodeposited at 60 °C for 5 min on ITO-coated glass substrates using three-electrode system. After deposition, the phase formation is confirmed using powder x-ray diffraction studies. The NiFe2O4 (NFO) thin films are deposited using RF sputtering method at room temperature for 2 h on Cu2O/ITO substrates to obtain NFO/Cu2O/ITO Type-II heterostructure. The scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) cross-sectional images show that the thickness of NFO layer is 120 nm and Cu2O layer is 1.5 µm. The photocurrent density of Cu2O on ITO is 0.08 ± 0.002 mA/cm2, and it increased to 0.12 ± 0.002 mA/cm2 after adding NFO layer on Cu2O film due to Type-II heterojunction formation.

CuO is a narrow band gap p-type semiconducting material having a wide range of applications. However, it is quite challenging to obtain phase pure CuO nanostructures grown directly on Cu substrate as most of the synthesis techniques like thermal oxidation results in the formation of additional Cu2O phase. In this work, we report the growth of CuO nanoflakes without the formation of Cu2O by a facile two-step synthesis process which consist of electrochemical anodization of Cu foil followed by low-temperature hydrothermal treatment at 100 °C. The phase purity of the sample is confirmed through XRD, XPS, and HRTEM. Further, photocurrent response of the sample is evaluated, and a rapid thermal treatment was used to improve the photo-response without altering the phase and morphology of the CuO nanoflakes. Such a process at 400 °C for 10 s resulted in a high photocurrent density of −4.6 mAcm−2 (at 0.05 V vs. RHE under AM 1.5G conditions). Electrochemical impedance spectroscopy and Mott Schottky analysis shows the direct role of rapid thermal treatment in increasing the charge carrier density of the sample.

Branched heterostructures play an important role in solar energy harvesting through an efficient separation of photogenerated charges as well as enhancement in catalytic reaction sites. Here we report the CuO/Cu2O nanoflake/nanowire heterostructure with the enhanced surface area directly grown on Cu foil through scalable process steps ensuring easy large-scale fabrication. A hydrothermal treatment on Cu2O nanowires, obtained through anodization of Cu foil and argon annealing, leads to a partial conversion of Cu2O into CuO nanoflakes resulting in a heterostructure with a highly enhanced surface area facilitating a higher semiconductor-electrolyte interface. Brief annealing at 250 °C for 5 min on the sample results in a photocurrent density of −1.9 mA/cm2 (at −0.3 V vs. Ag/AgCl under AM1.5G illumination), which is about an order of magnitude higher than the Cu2O nanowires. Further, CuO/Cu2O shell/core heterostructure is fabricated through direct annealing of Cu2O nanowires to understand the effect of highly enhanced surface area over the charge separation due to heterostructure. A comparative study on both heterostructures by photocurrent, electrochemical impedance, and Mott Schottky measurements reveals that although CuO/Cu2O heterojunction helps in charge separation, however, the enhanced surface area obtained through nanoflake morphology has a dominating effect in deciding high photocurrent density.

We report the fabrication of BaTiO3-Ni magnetoelectric nanocomposites comprising of BaTiO3 nanotubes surrounded by Ni matrix. BaTiO3 nanotubes obtained from the hydrothermal transformation of TiO2 have both inner and outer surfaces, which facilitates greater magnetoelectric coupling with the surrounding Ni matrix. The magnetoelectric coupling was studied by measuring the piezoelectric behavior in the presence of an in-plane direct magnetic field. A higher magnetoelectric voltage coefficient of 110 mV/cm·Oe was obtained, because of better coupling between Ni and BaTiO3 through the walls of the nanotubes. Such nanocomposite developed directly on Ti substrate may lead to efficient fabrication of magnetoelectric devices.

This work reports the growth of stable TiO2 nanotube arrays on flexible Kapton substrates by electrochemical anodization of a sputtered Ti (titanium) film. Although such nanotubes are conventionally fabricated on Ti foils, obtaining these on polymer-based flexible substrates remained a challenge because of higher annealing temperature not compatible with thermal stability of the substrates. Here, we demonstrate the fabrication of TiO2 nanotubes (1.5 μm long and 80 nm diameter) by anodization of the Ti film deposited using the RF sputtering technique at two different substrate temperatures (room temperature and 300 °C). Nanoindentation and nanoscratch techniques reveal better adhesion of the Ti film with an underlying Kapton substrate for 300 °C deposition temperature. Such investigations reveal a more than twofold enhancement of the "rear pileup"for the Ti film deposited at elevated temperature compared to that at room temperature. The amorphous TiO2 nanotubes are crystallized at 220 °C for 3 h using a solvothermal technique that allows crystallization at temperatures much lower than the annealing temperature. Application of these nanotubes for photo-electrochemical water splitting reveals a photocurrent density of 18 μA/cm2 under AM 1.5 G conditions. Furthermore, the charge density and flat band potential (VFB) are calculated from Mott-Schottky analysis, showing features comparable to the TiO2 nanotubes on the Ti foil crystallized through thermal annealing. The present work establishes a scalable approach for developing TiO2 nanotube arrays on the flexible substrate and its use for photo-electrochemical solar energy conversion.