Kirtiman Deo Malviya

Systematic comparison of different dopants in thin film hematite (α-Fe2O3) photoanodes for solar water splitting

Numerous studies have shown that the addition of different impurities as dopants in hematite (α-Fe2O3) photoanodes improves water photo-oxidation. The improvements observed may have resulted from electronic and/or catalytic effects, but also from changes in the layer morphology and microstructure induced by different precursors. The latter could be quite substantial, especially in mesoporous layers produced by chemical routes, making it difficult to make a systematic comparison between different dopants. This work attempts to overcome this difficulty by comparing different dopants in thin films produced by pulsed laser deposition (PLD), a physical deposition method that produces highly reproducible films with no significant variations in the microstructure and morphology. This enables systematic comparison of the effect of different dopants without spurious side effects due to variations in the microstructure and morphology. Thus, we examine the effect of Sn, Nb, Si, Pt, Zr, Ti, Zn, Ni and Mn dopants on the photoelectrochemical properties of thin (∼50 nm) film hematite photoanodes deposited by PLD from Fe2O3 targets doped with ∼1 cation% of the respective dopants onto FTO coated glass substrates. The morphology and microstructure of the films were nearly the same, independent of the different dopants in the films. The Sn-doped hematite photoanode outperformed all the other photoanodes that were examined in this work in both the photocurrent and photovoltage, achieving the highest photocurrent (∼1 mA cm−2) and the lowest onset potential (∼1.1 VRHE). Based on a figure of merit that accounts for the maximum photocurrent × photovoltage product (i.e., power) as well as the potential at which the maximum power is achieved, our photoanodes ranked in the following order: Sn > Nb > Si > Pt > Zr > Ti > Zn > Ni > Mn. These observations are not always consistent with other reports on doped hematite photoanodes, suggesting that the photoelectrochemical properties and performance depend not only on the identity of the dopant but also on the dopant concentration, distribution and the morphology and microstructure of the photoanode in which the dopant is incorporated.

Heteroepitaxial hematite photoanodes as a model system for solar water splitting

Heteroepitaxial multilayer Pt(111)/Fe2O3(0001) films were deposited on sapphire c-plane (0001) substrates by RF magnetron sputtering and pulsed laser deposition, respectively. The films were highly crystalline, displaying an in-plane mosaic spread of less than 1° and a homogenous surface morphology with roughness of ∼3 A. Ellipsometry and UV-vis spectroscopy measurements were shown to be in excellent agreement with modelling, demonstrating that the optics of the system including absorption in the hematite layer are well described. For polycrystalline hematite photoanodes deposited on platinum, full characterization of the system is hampered by the inability to make measurements in alkaline electrolyte containing hydrogen peroxide (H2O2) due to spontaneous decomposition of H2O2 by the exposed platinum. The pin-hole free high quality of the heteroepitaxial films is demonstrated by the ability to make stable and reproducible measurements in H2O2 containing electrolyte allowing for accurate extraction of charge separation and injection efficiency. The combination of excellent crystalline quality in addition to the well characterized optics and electrochemical properties of the heteroepitaxial hematite photoanodes demonstrate that Al2O3(0001)/Pt(111)/Fe2O3(0001) is a powerful model system for systematic investigation into solar water splitting photoanodes.

Phase formation and stability of alloy phases in free nanoparticles: some insights

This paper explores phase formation and phase stability in free nanoparticles of binary alloys. A procedure for estimating the size and composition dependent free energies incorporating the contributions from the interfaces has been presented. Both single phase solid solution and two phase morphology containing interphase interfaces have been considered. A free energy scenario has been evaluated for two binary alloy systems Ag–Ni and Ag–Cu to predict the microstructure of the alloy nanoparticles at different size ranges as a function of composition. Both Ag–Cu and Ag–Ni systems exhibit wide bulk immiscibility. Ag–Ni nanoparticles were synthesized using the wet chemical synthesis technique whereas Ag–Cu nanoparticles were synthesized using laser ablation of a Ag–Cu target immersed in distilled water. Microstructural and compositional characterization of Ag–Ni and Ag–Cu nanoparticles on a single nanoparticle level was conducted using transmission electron microscopy. Nanoparticle microstructures observed from the microscopic investigation have been correlated with thermodynamic calculation results. It is shown that the observed two phase microstructure consisting of Ag–Ni solid solution in partial decomposed state coexisting with pure Ag phases in the case of Ag–Ni nanoparticles can be only be rationalized by invoking the tendency for phase separation of an initial solid solution with increase in nanoparticle size. Smaller sized Ag–Ni nanoparticles prefer a single phase solid solution microstructure. Due to an increase in particle size during the synthesis process the initial solid solution decomposes into an ultrafine scale phase separated microstructure. We have shown that it is necessary to invoke critical point phenomenon and wetting transition in systems showing a critical point that leads to phase separated Ag–Ni nanoparticles providing a catalytic substrate for the nucleation of equilibrium Ag over it. In the case of the Ag–Cu system, we report the experimental observation of a core shell structure at small sizes. This can be rationalized in terms of a metastable solid solution. It is argued that the nucleation barrier can prevent the formation of biphasic morphology with an internal interface. In such a situation, demixing of the solid solution can bring the system to a lower energy configuration. This has lead to the observed core–shell morphology in the Ag–Cu system during room temperature synthesis.

Different Roles of Fe1–xNixOOH Cocatalyst on Hematite (α-Fe2O3) Photoanodes with Different Dopants

Transparent Fe1–xNixOOH overlayers (∼2 nm thick) were deposited photoelectrochemically on (001) oriented heteroepitaxial Sn- and Zn-doped hematite (α-Fe2O3) thin film photoanodes. In both cases, the water photo-oxidation performance was improved by the cocatalyst overlayers. Intensity modulated photocurrent spectroscopy (IMPS) was applied to study the changes in the hole current and recombination current induced by the overlayers. For the Sn-doped hematite photoanode, the improvement in performance after deposition of the Fe1–xNixOOH overlayer was entirely due to reduction in the recombination current, leading to a cathodic shift in the onset potential. For the Zn-doped hematite photoanode, in addition to a reduction in recombination current, an increase in the hole current to the surface was also observed after the overlayer deposition, leading to a cathodic shift in the onset potential as well as an enhancement in the plateau photocurrent. These results demonstrate that Fe1–xNixOOH cocatalysts can play ...

Magnetic states at the surface of α−Fe2O3 thin films doped with Ti, Zn, or Sn

The spin states at the surface of epitaxial thin films of hematite, both undoped and doped with 1% Ti, Sn, or Zn, respectively, were probed with x-ray magnetic linear dichroism (XMLD) spectroscopy. Morin transitions were observed for the undoped

Wavelength Dependent Photocurrent of Hematite Photoanodes: Reassessing the Hole Collection Length

({T}_{M}\ensuremath{\approx}200

Film Flip and Transfer Process to Enhance Light Harvesting in Ultrathin Absorber Films on Specular Back‐Reflectors

K) and Sn-doped

Synthesis and Mechanism of Composition and Size Dependent Morphology Selection in Nanoparticles of Ag-Cu Alloys Processed by Laser Ablation Under Liquid Medium

({T}_{M}\ensuremath{\approx}300

High Solar Flux Concentration Water Splitting with Hematite (α-Fe2O3) Photoanodes

K) cases, while Zn- and Ti-doped samples were always in the high- and low-temperature phases, respectively. In contrast to what has been reported for bulk hematite doped with the tetravalent ions

Rigorous substrate cleaning process for reproducible thin film hematite (α-Fe 2 O 3 ) photoanodes

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