Arghya Narayan Banerjee

The design, fabrication, and photocatalytic utility of nanostructured semiconductors: focus on TiO2-based nanostructures.

Recent advances in basic fabrication techniques of TiO2-based nanomaterials such as nanoparticles, nanowires, nanoplatelets, and both physical- and solution-based techniques have been adopted by various research groups around the world. Our research focus has been mainly on various deposition parameters used for fabricating nanostructured materials, including TiO2-organic/inorganic nanocomposite materials. Technically, TiO2 shows relatively high reactivity under ultraviolet light, the energy of which exceeds the band gap of TiO2. The development of photocatalysts exhibiting high reactivity under visible light allows the main part of the solar spectrum to be used. Visible light-activated TiO2 could be prepared by doping or sensitizing. As far as doping of TiO2 is concerned, in obtaining tailored material with improved properties, metal and nonmetal doping has been performed in the context of improved photoactivity. Nonmetal doping seems to be more promising than metal doping. TiO2 represents an effective photocatalyst for water and air purification and for self-cleaning surfaces. Additionally, it can be used as an antibacterial agent because of its strong oxidation activity and superhydrophilicity. Therefore, applications of TiO2 in terms of photocatalytic activities are discussed here. The basic mechanisms of the photoactivities of TiO2 and nanostructures are considered alongside band structure engineering and surface modification in nanostructured TiO2 in the context of doping. The article reviews the basic structural, optical, and electrical properties of TiO2, followed by detailed fabrication techniques of 0-, 1-, and quasi-2-dimensional TiO2 nanomaterials. Applications and future directions of nanostructured TiO2 are considered in the context of various photoinduced phenomena such as hydrogen production, electricity generation via dye-sensitized solar cells, photokilling and self-cleaning effect, photo-oxidation of organic pollutant, wastewater management, and organic synthesis.

Size-dependent optical properties of sputter-deposited nanocrystalline p-type transparent CuAlO2 thin films

Nanocrystalline, p-type semiconducting, transparent CuAlO2 thin films were deposited by direct current sputtering of a prefabricated polycrystalline CuAlO2 target, with deposition time as a variable parameter. Transmission electron micrographs reveal the formation of CuAlO2 nanoparticles. For the films deposited in 3, 9, and 15min, the average particle sizes are determined to be around 10, 20, and 30nm, respectively. The interplaner spacings calculated from selected area electron-diffraction patterns obtained from transmission electron microscopy confirmed the proper phase formation of the material. X-ray diffraction measurements of the films deposited for 15 and 45min show some diffraction peaks, which depict the rhombohedral crystal structure of the material. The band-gap values obtained from the optical transmission and reflection data, for the films deposited in 3 and 9min, are 3.94 and 3.84eV, respectively, whereas for those films deposited in 15 and 45min, the band-gap values lie in the range of 3.7...

Preparation of p-type transparent conducting CuAlO2 thin films by reactive DC sputtering

Abstract P-type transparent conducting thin films of copper aluminate were prepared by reactive DC sputtering of a prefabricated target having 1:1 atomic ratio of Cu and Al. Films of CuAlO2 were deposited on Si (400) and glass substrates. The sputtering was performed in Ar+O2 (40 vol.%) atmosphere and the substrate temperature was 475 K. X-ray diffraction (XRD) spectra of the films showed the peaks which could be assigned with those of the crystalline CuAlO2. UV–Visible spectrophotometric measurement showed high transparency of the films in the visible region. Both direct and indirect band gaps were found to exist and their corresponding estimated values were 3.75 and 1.85 eV, respectively. The room temperature conductivity of the film was fairly high and was of the order of 0.22 S cm−1 while the activation energy was ∼0.25 eV. Seebeck coefficient at room temperature gave a value of +115 μV/K confirming the p-type conductivity. Room temperature Hall effect measurement also indicated positive value of Hall coefficient with a value RH=14.1 cm3/C.

Synthesis of crystalline carbon nitride thin films by electrolysis of methanol–urea solution

Abstract Polycrystalline carbon nitride films were deposited on Si (400) substrates by electrolysis of methanol–urea solution under high voltage, at atmospheric pressure and at temperature below 350 K. Fourier transform infrared spectroscopy (FTIR) measurements suggested the existence of both single and double carbon–nitrogen bonds in the film. X-ray diffraction (XRD) spectrum showed various peaks for different d values which could be assigned to different crystalline carbon nitride phases. Film morphology was studied by scanning electron microscopy (SEM) which indicated the existence of grains with average grain size of ∼2.5 μm.

Synthesis and Characterization of Nano-Crystalline Fluorine-Doped Tin Oxide Thin Films by Sol-Gel Method

Thin films of fluorine-doped tin-oxide (FTO) were prepared by sol-gel dip-coating technique. Stannous chloride (SnCl2ċ2H2O) and hydrogen fluoride (HF) were mixed with isopropyl alcohol to serve as source solution. X-ray diffraction (XRD) spectrum showed all the peaks of the crystalline SnO2. Analysis of XRD spectrum showed the particle size to be nearly 6 nm, which indicated the nanocrystalline structure of the films. Strain calculation by integral breadth (IB) method from XRD data showed a value of 0.010. UV-Visible spectrophotometric measurement showed high transparency of the films in the visible region and the band gap was calculated to be 3.34 eV. The room temperature resistivity of the films were of the order of 1 Ωcm. Fluorine concentration in the films was determined from energy dispersive X-ray (EDX) study. Current-voltage (I-V) characteristics at high temperatures showed the Poole-Frenkel effect of thermionic emission. SEM study indicated the existence of fine grains in the film. FT-IR spectroscopy showed strong Sn—O and Sn—O—Sn bonding.

Oxygen Vacancy-Induced Structural, Optical, and Enhanced Supercapacitive Performance of Zinc Oxide Anchored Graphitic Carbon Nanofiber Hybrid Electrodes

Zinc oxide (ZnO) nanoparticles (NPs) anchored to carbon nanofiber (CNF) hybrids were synthesized using a facile coprecipitation method. This report demonstrates an effective strategy to intrinsically improve the conductivity and supercapacitive performance of the hybrids by inducing oxygen vacancies. Oxygen deficiency-related defect analyses were performed qualitatively as well as quantitatively using Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All of the analyses clearly indicate an increase in oxygen deficiencies in the hybrids with an increase in the vacuum-annealing temperature. The nonstoichiometric oxygen vacancy is mainly induced via the migration of the lattice oxygen into interstitial sites at elevated temperature (300 °C), followed by diffusion into the gaseous phase with further increase in the annealing temperature (600 °C) in an oxygen-deficient atmosphere. This induction of oxygen vacancy is corroborated by diffuse reflectance spectroscopy, which depicts the oxygen-vacancy-induced bandgap narrowing of the ZnO NPs within the hybrids. At a current density of 3 A g(-1), the hybrid electrode exhibited higher energy density (119.85 Wh kg(-1)) and power density (19.225 kW kg(-1)) compared to a control ZnO electrode (48.01 Wh kg(-1) and 17.687 kW kg(-1)). The enhanced supercapacitive performance is mainly ascribed to the good interfacial contact between CNF and ZnO, high oxygen deficiency, and fewer defects in the hybrid. Our results are expected to provide new insights into improving the electrochemical properties of various composites/hybrids.

Biofilm formation on a TiO₂ nanotube with controlled pore diameter and surface wettability.

Titania (TiO2) nanotube arrays (TNAs) with different pore diameters (140 - 20 nm) are fabricated via anodization using hydrofluoric acid (HF) containing ethylene glycol (EG) by changing the HF-to-EG volume ratio and the anodization voltage. To evaluate the effects of different pore diameters of TiO2 nanotubes on bacterial biofilm formation, Shewanella oneidensis (S. oneidensis) MR-1 cells and a crystal-violet biofilm assay are used. The surface roughness and wettability of the TNA surfaces as a function of pore diameter, measured via the contact angle and AFM techniques, are correlated with the controlled biofilm formation. Biofilm formation increases with the decreasing nanotube pore diameter, and a 20 nm TiO2 nanotube shows the maximum biofilm formation. The measurements revealed that 20 nm surfaces have the least hydrophilicity with the highest surface roughness of ∼17 nm and that they show almost a 90% increase in the effective surface area relative to the 140 nm TNAs, which stimulate the cells more effectively to produce the pili to attach to the surface for more biofilm formation. The results demonstrate that bacterial cell adhesion (and hence, biofilm formation) can effectively be controlled by tuning the roughness and wettability of TNAs via controlling the pore diameters of TNA surfaces. This biofilm formation as a function of the surface properties of TNAs can be a potential candidate for both medical applications and as electrodes in microbial fuel cells.

Bio-silica coated with amorphous manganese oxide as an efficient catalyst for rapid degradation of organic pollutant

A novel rapid green one-step method is developed for the preparation of bio-silica coated with amorphous MnO2 nanoparticles by treating bio-silica with an acidic permanganate solution. The method developed has the advantage of selectively coating the surface of either one or both sides of the porous silica structure with a thin catalytic active amorphous MnO2 layer in a controlled way. The uncoated and MnO2 coated bio-silica are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The catalytic activity of amorphous MnO2-coated bio-silica is examined by degrading organic dye at ambient condition. The as-synthesized samples show highly efficient and rapid degradation of Rhodamine B. The simplicity and cost-effectiveness of the materials and method can be very useful for highly efficient degradation of organic pollutants for environmental remediation.

Photocatalytic degradation of organic dye by sol-gel-derived gallium-doped anatase titanium oxide nanoparticles for environmental remediation

Photocatalytic degradation of toxic organic chemicals is considered to be the most efficient green method for surface water treatment. We have reported the sol-gel synthesis of Gadoped anatase TiO2 nanoparticles and the photocatalytic oxidation of organic dye into nontoxic inorganic products under UV irradiation. Photodegradation experiments show very good photocatalytic activity of Ga-doped TiO2 nanoparticles with almost 90% degradation efficiency within 3 hrs of UV irradiation, which is faster than the undoped samples. Doping levels created within the bandgap of TiO2 act as trapping centers to suppress the photogenerated electron-hole recombination for proper and timely utilization of charge carriers for the generation of strong oxidizing radicals to degrade the organic dye. Photocatalytic degradation is found to follow the pseudo-first-order kinetics with the apparent 1st order rate constant around 1.3 × 10-2 min-1. The cost-effective, sol-gel-derived TiO2 :Ga nanoparticles can be used efficiently for light-assisted oxidation of toxic organic molecules in the surface water for environmental remediation.

Recent developments in TiO2 as n- and p-type transparent semiconductors: synthesis, modification, properties, and energy-related applications

TiO2-based thin films and nanomaterials have been fabricated via physical and solution-based techniques by various research groups around the globe. Generally, most applications of TiO2 involve photocatalytic activity for water and air purification, self-cleaning surfaces, antibacterial activity, and superhydrophilicity. As a wide-bandgap semiconductor, modified TiO2 belongs to a class of materials called transparent semiconducting oxides (TSOs), which are simultaneously optically transparent and electrically conductive. TSOs continue to be in high demand for a variety of applications ranging from transparent electronics and sensor devices to light detecting and emitting devices in telecommunications. However, reports on TiO2 applications as an effective TSO for transparent electronics applications have been limited. In general, TiO2 is intrinsically an n-type semiconductor but can be doped to have p-type semiconductivity. This provides a very important opportunity to fabricate all-transparent homojunction devices for light harvesting and energy storage. P-type TSOs have recently attracted tremendous interest in the field of active devices for emerging transparent electronics for potential use in ultra-violet light-based solar cells. Therefore, a detailed overview of the synthesis, band structure modification via doping, properties, and applications of modified TiO2 as n- and p-type TSOs is warranted. This article comprehensively reviews the latest developments. The discussion includes solution-based wet chemical techniques and vacuum-based dry physical techniques fabricating TiO2–TSOs. The synthesis of p-TiO2 in particular is discussed in detail as it may provide interesting breakthroughs in emerging transparent electronics applications. Also, the structural, optical, and electrical properties of TiO2 are discussed in the context of TSO applications, specifically the defect chemistry of TiO2 to obtain n- and p-type semiconductivity, which could provide interesting insights into the band structure engineering of TiO2 for conductivity reversal. Applications of both n- and p-type TiO2 have been reviewed in detail in relation to thin film transparent homo/heterojunction devices, dye-sensitized solar cells, electrochromic displays, and other energy-related applications.