Sarika Kelkar

Near‐Field Plasmonic Functionalization of Light Harvesting Oxide–Oxide Heterojunctions for Efficient Solar Photoelectrochemical Water Splitting: The AuNP/ZnFe2O4/ZnO System

have also shown moderate plasmonic enhancement under simulated 1 Sun conditions. In all these plasmonics based studies the work has been focused on a single junction of a semiconductor oxide with metal NPs serving as the photoelectrodes. In this work we demonstrate an entirely new scheme wherein a light harvesting suitably band-matched functional oxide-oxide heterojunction

Low temperature grown CuBi2O4 with flower morphology and its composite with CuO nanosheets for photoelectrochemical water splitting

In this work we highlight a peculiar synthesis protocol for the p-type ternary metal oxide system of copper bismuth oxide (CuBi2O4), which yields a highly crystalline spherulitic morphology at a low temperature of 78 °C. We associate this growth with the hydrogen bonding effects imparted by the ethanol–water co-solvent system used for the synthesis. We present a detailed growth mechanism by evaluating different synthesis conditions systematically. Furthermore we show that upon the use of the non-stoichiometric (excess copper) precursor mixture under the same experimental conditions the growth of spherulitic CuBi2O4 changes the size and type of the spherulites. Interestingly, careful optimization of the non-stoichiometric synthesis presents a complete impediment to the spherulitic growth and produces a composite of nanorods of CuBi2O4 and nanosheets of CuO. This anisotropic nanocomposite shows an order of magnitude higher surface area as compared to spherulitic CuBi2O4. Since both CuBi2O4 and CuO are visible light absorbing p-type semiconductors, when the synthesized nanocomposite materials are examined as photoelectrochemical (PEC) photocathodes for water splitting, they show a remarkable dependence on the morphology and phase constitution. Almost 13-fold stronger PEC response is observed as the morphology changes from spherulites to nanorods.

Nanostructured Cd2SnO4 as an energy harvesting photoanode for solar water splitting

We report the synthesis of pure phase (cubic as well as orthorhombic phase) nanoparticles of Cd2SnO4 (10–15 nm) by a one step solution combustion method and demonstrate their applicability for energy harvesting in photoelectrochemical devices. The criticality of the process parameters in obtaining pure phase nanocrystals is emphasized through the study of the structural, optical and electronic properties. Doctor bladed Cd2SnO4 films, when used as photoanodes for solar water splitting, show a maximum photocurrent of 80 μA cm−2 at 0.5 V for the orthorhombic phase and 250 μA cm−2 at 0.6 V for the cubic phase with respect to an Ag/AgCl reference electrode indicating the good promise of Cd2SnO4 for energy harvesting.

NiS1.97: A New Efficient Water Oxidation Catalyst for Photoelectrochemical Hydrogen Generation.

NiS1.97, a sulfur-deficient dichalcogenide, in nanoscale form, is shown to be a unique and efficient photoelectrochemical (PEC) catalyst for H2 generation by water splitting. Phase pure NiS1.97 nanomaterial is obtained by converting nickel oxide into sulfide by controlled sulfurization method, which is otherwise difficult to establish. The defect states (sulfur vacancies) in this material increase the carrier density and in turn lead to favorable band line-up with respect to redox potential of water, rendering it to be an effective photoelectrochemical catalyst. The material exhibits a remarkable PEC performance of 1.25 mA/cm(2) vs NHE at 0.68 V in neutral pH, which is almost 1000 times superior as compared with that of the stoichiometric phase of NiS2. The latter is well-known to be a cocatalyst but not as a primary PEC catalyst.

Quantum dot CdS coupled Cd2SnO4 photoanode with high photoelectrochemical water splitting efficiency

Quantum dot (QD) coupled wide band gap semiconductors such as TiO2 and ZnO have shown enhanced photoelectrochemical (in solar cells as well as water splitting) performance due to extended visible light absorption facilitated by QDs, compounded with favourable energetics for charge injection into the conduction band of the host semiconductor. In this work we investigate a new interesting system in this context, namely cadmium tin oxide (Cd2SnO4) coupled with CdS QDs. We find that the Cd2SnO4 photoanode, despite having a similar bandgap to that of CdS (2.2–2.5 eV), exhibits a very large (>40 fold) enhancement in the efficiency only when coupled to CdS QDs. By employing various microstructural, optoelectronic and photoelectrochemical characterization techniques we show that the favourable energetics and charge transport properties of Cd2SnO4 play the most crucial role in the enhancement of the photoelectrochemical performance. Our work suggests that it may be possible to design highly efficient photoelectrochemical systems by tailoring the constitution of nanocomposites based on the relatively less studied ternary oxide–sulphide heterosystems.

Photophysical, bandstructural, and textural properties of o-FeNbO4 in relation to its cocatalyst-assisted photoactivity for water oxidation

In this study, a relationship between physicochemical, photophysical and photocatalytic properties of hydrothermally synthesized orthorhombic iron niobate (FeNbO4) is investigated. o-FeNbO4 displayed a multi-regime optical absorbance, which was ascribed to at least two distinct phenomena: (i) bandgap (∼3.4 eV) excitation giving rise to UV absorbance and (ii) energy transitions involving disorder-induced sub-bandgap donor or acceptor states leading to visible light absorbance. The preparation-dependent distortion in the crystal lattice and the existence of closely spaced inter-bandgap energy states were corroborated by powder X-ray diffraction, photoluminescence, thermoluminescence, and Raman spectroscopy studies. The first principles electronic structure elucidation and photoelectrochemical measurements supported a wide bandgap for FeNbO4, in contrast to the narrow bandgap reported previously. Correspondingly, a small photocurrent density was observed for FeNbO4 (∼2 to 3 μA cm−2) under 1 sun illumination, suggesting the availability of a smaller cross section of photogenerated charge pairs. Following these band characteristics, while no H2 evolution was observed, FeNbO4 gave rise to particle size-dependent O2 evolution during visible light irradiation of water in the presence of electron scavengers, the samples loaded with NiO as cocatalyst showing better activity. Further, the transmission electron microscopy examination revealed the dominant exposure of (011) facets of FeNbO4, besides a significant heterogeneity of inter-domain boundaries. Overall, our results confirm that the photoactivity of metal/oxide nanocomposites is governed by a combination of factors, such as: grain morphology, microstructure, surface adsorption states, and the localized inter-bandgap energy states. Our study also reveals that, in contrast to prevalent assumptions, the wavelength at the absorption edge may not represent the true band-to-band energy gap of metal oxide semiconductors, which is relevant to their photocatalytic activity.

Orthorhombic/cubic Cd2SnO4 nanojunctions: enhancing solar water splitting efficiency by the suppression of charge recombination

The low practical efficiency of binary metal oxide semiconductor-based photo-electrochemical (PEC) water splitting has prompted researchers to examine ternary and quaternary oxides, which provide more leverage for engineering the desired PEC properties via stoichiometry (valence) and phase control. One ternary system of interest in this context is cadmium tin oxide (Cd2SnO4), which supports the cubic and orthorhombic phases with optical, electronic and catalytic properties that are favourable for PEC water splitting. However, its practical PEC performance is limited by high surface recombination of the photogenerated charge carriers. In this work we circumvent this problem by engineering the constitution of Cd2SnO4 nanoparticles to a biphasic nanojunction form, comprising of a nanocomposite of cubic and orthorhombic phases. The favourable conduction band alignment between the cubic and orthorhombic phases leads to a dramatic reduction in the recombination of the photogenerated charges, leading to a 10-fold increase (from 250 μA cm2 to over 2 mA cm2) in the photocurrent vis-a-vis the single cubic or orthorhombic phase performance. We discuss the underlying mechanism for the observed dramatic enhancement in the water splitting efficiency.

Water Electrolysis with a Conducting Carbon Cloth: Subthreshold Hydrogen Generation and Superthreshold Carbon Quantum Dot Formation

A conducting carbon cloth, which has an interesting turbostratic microstructure and functional groups that are distinctly different from other ordered forms of carbon, such as graphite, graphene, and carbon nanotubes, was synthesized by a simple one-step pyrolysis of cellulose fabric. This turbostratic disorder and surface chemical functionalities had interesting consequences for water splitting and hydrogen generation when such a cloth was used as an electrode in the alkaline electrolysis process. Importantly, this work also gives a new twist to carbon-assisted electrolysis. During electrolysis, the active sites in the carbon cloth allow slow oxidation of its surface to transform the surface groups from COH to COOH and so forth at a voltage as low as 0.2 V in a two-electrode system, along with platinum as the cathode, instead of 1.23 V (plus overpotential), which is required for platinum, steel, or even graphite anodes. The quantity of subthreshold hydrogen evolved was 24 mL cm(-2)  h(-1) at 1 V. Interestingly, at a superthreshold potential (>1.23 V+overpotential), another remarkable phenomenon was found. At such voltages, along with the high rate and quantity of hydrogen evolution, rapid exfoliation of the tiny nanoscale (5-7 nm) units of carbon quantum dots (CQDs) are found in copious amounts due to an enhanced oxidation rate. These CQDs show bright-blue fluorescence under UV light.

Preparation-method-dependent morphological, band structural, microstructural, and photocatalytic properties of noble metal–GaNbO4 nanocomposites

We report the distinct physicochemical and photophysical properties of gallium niobate photocatalysts (bandgap: ∼3.1 eV), prepared by a solid-state (SS) reaction and sol–gel (SG) method and dispersed with a noble metal (∼0.5% of Pt, Au, or RuOx) cocatalyst. SG–GaNbO4 comprised smaller size particles (∼20–50 nm) and a larger surface area (∼160 m2 g−1) compared to SS–GaNbO4 (particle size ∼30–150 nm, surface area ∼27 m2 g−1). XRD patterns revealed a preparation-dependent variation in the relative intensity of prominent reflections. In TEM examination, SG samples exhibited small-range grain boundaries and heterogeneous metal/substrate interfacial contacts, while SS–GaNbO4 had long-range ordering. Laser-Raman and thermoluminescence investigations revealed that lattice distortion, defect-induced inter-bandgap charge trapping states, and the local environment around the metal/semiconductor interfaces may also depend on the preparation method. Metal–GaNbO4 nanocomposites showed no activity for the dissociation of pure water under UV (>250 nm) irradiation, despite the favourable conduction and valence band potentials. This was attributed to the sharp Ga and Nb d-levels in the narrow conduction band of GaNbO4, as confirmed by ab initio electronic structure calculation. These photocatalysts, however, showed good activity for semiconductor-mediated photo-dissociation of aqueous methanol to produce H2; a cocatalyst-dependent activity trend, Pt > RuOx > Au, was observed. Doping of S at ∼5% of the oxygen sites led to decreased photoactivity, ascribed to the presence of localized S 3p states just above the O 2p valence level. In conclusion, besides band characteristics, certain morphological and microstructural properties play a crucial role in the photoactivity of the metal/oxide nanocomposites.