Aadesh P. Singh

Hydrogen treated anatase TiO2: a new experimental approach and further insights from theory

Hydrogenated TiO2 (H:TiO2) is intensively investigated due to its improvement in solar absorption, but there are major issues related to its structural, optical and electronic properties and therefore an easily compatible method of preparation is much needed. In order to clarify this issue we studied TiO2 nanocrystals under the partial pressure of hydrogen to modify the structural, optical and electrical properties and to significantly improve the photocatalytic and photoelectrochemical performance. The hydrogen treated TiO2 nanocrystals contained paramagnetic Ti3+ centers and exhibited a higher visible light absorption cross-section as was confirmed by electron paramagnetic resonance diffuse reflectance spectra measurements and X-ray photoelectron spectroscopy. The hydrogen annealed samples showed a noticeable improvement in photocatalytic activity under visible light (λ > 380 nm) which was demonstrated by the degradation of methylene blue dye and an improved photoelectrochemical response in terms of high photocurrent density. Ab initio simulations of TiO2 were performed in order to elucidate the conditions under which localized Ti3+ centres rather than delocalized shallow donor states are created upon the reduction of TiO2. Randomly distributed oxygen vacancies lead to localized deep donor states while the occupation of the oxygen vacancies by atomic hydrogen favours the delocalized shallow donor solution. Furthermore, it was found that localization is stabilized at high defect concentrations and destabilized under external pressures. In those cases where localized Ti3+ states are present, the DFT simulations showed a considerable enhancement of the visible light absorption as well as a pronounced broadening of the localized Ti3+ energy levels with increasing defect concentration.

In Situ Solid‐State Synthesis of a AgNi/g‐C3N4 Nanocomposite for Enhanced Photoelectrochemical and Photocatalytic Activity

A graphitic carbon nitride (g-C3 N4 ) polymer matrix was embedded with AgNi alloy nanoparticles using a simple and direct in situ solid-state heat treatment method to develop a novel AgNi/g-C3 N4 photocatalyst. The characterization confirms that the AgNi alloy particles are homogeneously distributed throughout the g-C3 N4 matrix. The catalyst shows excellent photoelectrochemical activity for water splitting with a maximum photocurrent density of 1.2 mA cm-2 , which is the highest reported for doped g-C3 N4 . Furthermore, a detailed experimental study of the photocatalytic degradation of Rhodamine B (RhB) dye using doped g-C3 N4 showed the highest reported degradation efficiency of approximately 95 % after 90 min. The electronic conductivity increased upon incorporation of AgNi alloy nanoparticles on g-C3 N4 and the material showed efficient charge carrier separation and transfer characteristics, which are responsible for the enhanced photoelectrochemical and photocatalytic performance under visible light.

Dominant {100} facet selectivity for enhanced photocatalytic activity of NaNbO3 in NaNbO3/CdS core/shell heterostructures

Design and engineering of crystalline advanced photocatalysts with specific facets is one of the most challenging tasks to enhance the photocatalytic performance. The surface energy of different facets is different in a crystal which leads to a corresponding change in their photocatalytic behaviour. The present study provides an experimental as well as theoretical understanding of the role of different facets of NaNbO3 in cubic and orthorhombic phases with crystals showing cubic and cuboctahedron morphologies in enhancing the photocatalytic activity of NaNbO3/CdS core/shell heterostructures. Herein, we discuss the importance of the approach of facet-selective synthesis and trace the origin of enhanced photoelectrochemical (PEC) water splitting and photocatalytic dye degradation activity for calculated surface energies of the {100} family of facets of the cubic phase and the (110) and (114) facets of the orthorhombic phase of NaNbO3. We propose that different mechanisms contribute to the enhancement of catalytic activity in these two phases. In the prepared core/shell heterostructures containing NaNbO3 as the core material, the presence of highly reactive facets of the cubic phase contributes to higher photocatalytic activity as compared to the orthorhombic phase which has a spatial charge separation assisted inter-facet charge transfer mechanism.

Enhanced photoelectrochemical performance of electrodeposited hematite films decorated with nanostructured NiMnOx

In the present work, we report a novel nickel-manganese oxide (NiMnOx) decorated hematite (α-Fe2O3) photoanode for efficient water splitting in a photoelectrochemical (PEC) cell. The photoanodes are prepared by a two step electrodeposition process. NiMnOx loading on the hematite surface is varied by changing the electrodeposition time. The NiMnOx loaded α-Fe2O3 photoanodes are characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, UV-Vis absorption and photoluminescence spectroscopy. The results demonstrate that the NiMnOx decorated α-Fe2O3 photoanode exhibits excellent photoelectrochemical activity. The α-Fe2O3/NiMnOx photoanode, where NiMnOx was coated for 200 s, shows a maximum photocurrent of 2.35 mA cm−2 at an applied potential of 0.23 V vs. the Ag/AgCl reference electrode. There is a large shift in the onset potential towards the cathodic region also observed. The photoconversion efficiency is calculated which is around 0.85% at 0.23 V vs. Ag/AgCl. The superior PEC performance of NiMnOx decorated α-Fe2O3 photoanodes can be explained by a combined effect of better water oxidation on the hematite surface and efficient separation of photogenerated electron–hole pairs on its surface due to NiMnOx modification.

Visible-Light-Driven Photoelectrochemical and Photocatalytic Performance of NaNbO3/Ag2S Core–Shell Heterostructures

Herein, we report the fabrication of visible-light-active NaNbO3 /Ag2 S staggered-gap core-shell semiconductor heterostructures with excellent photoelectrochemical activity toward water splitting, and the degradation of a model pollutant (methylene blue) was also monitored. The heterostructures show a pronounced photocurrent density of approximately 2.44 mA cm(-2) at 0.9 V versus Ag/AgCl in 0.5 m Na2 SO4 and exhibit a positive shift in onset potential by approximately 1.1 V. The high photoactivity is attributed to the efficient photoinduced interfacial charge transfer (IFCT). The core-shell design alleviates the challenges associated with the electron-hole paths across semiconductor junctions and at the electrolyte-semiconductor interface. These properties demonstrate that NaNbO3 /Ag2 S core-shell heterostructures show promising visible-light photoactivity and are also efficient, stable, and recyclable photocatalysts.

Metal Oxide Nano-architectures and Heterostructures for Chemical Sensors

Metal oxide nanostructures with hetero-contacts and phase boundaries offer a unique platform for designing materials architectures for sensing applications. Besides the size and surface effects, the modulation of electronic behaviour due to junction properties leads to modified surface states that promote selective detection of analytes. The growing possibilities of engineering nanostructures in various compositions (pure, doped, composites, heterostructures) and forms (particles, tubes, wires, films) has intensified the research on the integration of different functional material units in a single architecture to obtain new sensing materials. In addition, new concepts of enhancing charge transduction by surface functionalization and use of pre-concentrator systems are promising strategies to promote specific chemical interactions, however the challenge related to reproducible synthesis and device integration of nanomaterials persist.