Rekha Dom

Synthesis of a hydrogen producing nanocrystalline ZnFe2O4 visible light photocatalyst using a rapid microwave irradiation method

A rapid microwave solid-state synthesis method is systematically investigated to achieve a H2 producing visible light active spinel photocatalyst. ZnFe2O4 nanocrystallites were obtained by microwave irradiation of precursor compacts under optimized conditions. This investigation led to a uniform sized nanocrystalline photocatalyst that yielded a quantum-yield of H2 evolution ∼3.8 times higher than that of conventionally synthesized ZnFe2O4. The synthesis parameters – microwave power, synthesis temperature, and time, were found to control the physico-chemical properties viz phase formation kinetics, phase purity, crystallinity, specific surface area and photochemical efficiency, of the synthesized photocatalyst. The study reveals that the threshold microwave power of ≥3 kW was necessary to obtain a spinel phase structure, while lower power (<3 kW) could not induce the crystallization even after prolonged low-power irradiation of 180 min. At the threshold power, a minimum of 10 min. synthesis time was enough to obtain uniform sized nanocrystallites, indicating that the synthesis method is ∼24 times faster than the solid state reaction method, which needs nearly 4 h. The particle morphology evolution with irradiation time from 10–150 min. exhibited de-crystallization phenomena. Longer irradiation displayed a morphological crystallization probably induced due to the simultaneous area and volumetric heating effect. The possible “formation mechanism” of these uniform nanocrystallites has been presented here for qualitative understanding. Thus synthesized photocatalysts generated hydrogen from a water–methanol mixture even without the co-catalyst loading. The ferrite photocatalyst was found to decolorize methylene blue dye with a maximum decay constant of 0.232 h−1, thereby demonstrating its capability in the pollutant decomposition applications, all under visible light photons.

Enhanced Solar Photoelectrochemical Conversion Efficiency of ZnO:Cu Electrodes for Water-Splitting Application

n-type ZnO:Cu photoanodes were fabricated by simple spray pyrolysis deposition technique. Influence of low concentration (range ~10−4–10−1%) of Cu doping in hexagonal ZnO lattice on its photoelectrochemical performance has been investigated. The doped photoanodes displayed 7-time enhanced conversion efficiencies with respect to their undoped counterpart, as estimated from the photocurrents generated under simulated solar radiation. This is the highest enhancement in the solar conversion efficiency reported so far for the Cu-doped ZnO. This performance is attributed to the red shift in the band gap of the Cu-doped films and is in accordance with the incident-photon-current-conversion efficiency (IPCE) measurements. Electrochemical studies reveal an n-type nature of these photoanodes. Thus, the study indicates a high potential of doped ZnO films for solar energy applications, in purview of the development of simple nanostructuring methodologies.

Eco-friendly ferrite nanocomposite photoelectrode for improved solar hydrogen generation

For the first time, a ferrite nanocomposite photoelectrode (FNCP) has been fabricated and investigated for photoelectrochemical hydrogen production from water under simulated solar light. The ZnFe2O4:Fe2O3 nanocomposite photoanode has been fabricated via a single step methodology using a novel approach involving the solution precursor plasma spraying technique. The FNCP is achieved by optimal phase formation during the deposition of the ferrite film (∼10 μm) over a stainless steel substrate. It exhibits an enhanced photoactivity 6 times higher compared to pure ZnFe2O4 (ZFO), under simulated-solar (AM1.5 G) illumination. Its Mott–Schottky characterization reveals an n-type semiconducting behaviour, indicating an order of magnitude higher donor density (Nd ∼ 1017 cm−3) than the pure phase ZnFe2O4 electrode. It also exhibits a low band gap of 1.94 eV demonstrating that it can more efficiently absorb visible light photons than other systems comprising of bare zinc ferrite or iron oxide. The FNCP yielded a solar-to-hydrogen conversion efficiency of 1.25% under simulated solar radiation (AM1.5 G) with a hydrogen evolution rate of 99 μmol h−1. Electrochemical impedance spectroscopy of the FNCP revealed a significantly improved charge transfer characteristic compared to ZnFe2O4. An enhanced photoactivity for the oxidation of water from the FNCP is attributed to its improved optical absorption and better charge transfer properties induced by the existence of Fe2O3 in ZnFe2O4.

Efficient hydrogen generation over (100)-oriented ZnO nanostructured photoanodes under solar light

For the first time, ZnO photoanodes with the (100) preferential orientation have been shown to generate hydrogen under solar photon illumination. These films fabricated by the spray pyrolysis technique showed distinctly oriented and stacked layers with an average thickness of ~50 nm as observed in the cross-sectional studies. They exhibited a high texture coefficient (>2) for the (100) plane, mainly indicating an a-axis preferred oriented film. Implicitly, these films exhibited an improved absorption capability thereby yielding a high photocurrent density of ~149 μA and a solar-to-hydrogen efficiency of 0.12%. An evolved hydrogen of 260 μmoles h−1 under solar light was found to be maximum for the best film. The high donor density (1016 cm−3) and most suited flat band potential (−0.518 V vs. NHE) of the films as deduced from the electrochemical investigations seem to be a major factor responsible for such photoelectrochemical behavior of the films. The surface morphology of these films containing nanoprisms forming a “Y-shaped” network with uniform fibers of ~500 nm thickness seems to be additionally contributing to the displayed PEC performance. The photoanode was found to be 83.9% efficient at 360 nm wavelength as recorded from incident-photon-current conversion efficiency measurements. Unlike past reports on (002) oriented films, the present work demonstrates that even an (100)-oriented ZnO film shows a significantly high photocurrent generation, as well as a significant solar-to-hydrogen production yielding high evolved hydrogen over these films.

Design and Development of Ferrite Composite Film Electrode for Photoelectrochemical Energy Application

Development of efficient photoanodes for water splitting under solar light is desirable to surmount the possible fuel crisis in future. Ferrite systems, with their excellent visible light absorption capability, stability, non-toxicity, cost-effectiveness and abundance, are being preferred to titanates, niobates and sulfides. The present work briefly reviews the modified form of ferrites. Additionally, ZnFe2O4 an n-type semiconductor with the low band gap (~1.9eV) has been considered as special case of visible light PEC application. The work further emphasizes on the utilization of solution processed techniques to develop the ferrite photoanodes. The tuning of photoanode properties by virtue of electrode fabrication parameters say deposition parameters viz., precursor concentration, pH, stoichiometry has been reviewed and discussed.

Investigation of Solar Photoelectrochemical Hydrogen Generation Ability of Ferrites for Energy Production

Ferrites have been predicted to be potential material for photo catalytic and photo electrochemical (PEC) hydrogen generation under visible light photons. The article briefly reviews, as well as compares the supremacy of ferrites over titanates and sulfides with respect to their photo-electro catalytic hydrogen producing ability. It shows that though the band energetic of a material plays a vital role to induce the photo-splitting of water; but its optical structure, at the first instance is very important to utilize it to absorb the visible light photons. Among all the PEC materials, the low-band gap ferrites (Fe-d orbital) favor absorption of visible light photons; at the same time offer an advantage of being an eco-friendly material system. A specific focus is given to the single phase, nanostructure and composite forms of typical ZnFe2O4 system. Though a concise report, but also throws light on the importance and tunability of PEC properties.

Photocatalytic and Photoelectro-Chemical Study of Ferrites for Water Splitting Applications: A Comparative Study

This article presents a comparative study on the synthesis and characterization of the binary and ternary ferrites for photocatalytic and photoelectrochemical applications. The importance and role of ferrite photocatalysts is discussed in context to the visible-light active photocatalyst application viz. for hydrogen production via. water-splitting. It also demonstrates that computational-exploration of any material system is key to identify, and achieve visible-light active photocatalysts.