Quantum Dot Based Heterostructures for Unassisted Photoelectrochemical Hydrogen Generation

TiO₂ hollow nanowires (HNWs) and nanoparticles (NPs) constitute promising architectures for QDs sensitized photoanodes for H₂ generation. We sensitize these structures with CdS/CdSe quantum dots by two different methods (chemical bath deposition, CBD and succesive ionic layer adsorption and reaction, SILAR) and evaluate the performance of these photoelectrodes. Remarkable photocurrents of 4 mA·cm and 8 mA·cm^?2 and hydrogen generation rates of 40 ml·cm^?2·day^?1 and 80 ml·cm^?2·day^?1 have been obtained in a three electrode configuration with sacrificial hole scavengers (Na₂S and Na₂SO₃), for HNWs and NPs respectively, which is confirmed through gas analysis. More importantly, autonomous generation of H₂ (20 ml·cm^?2·day^?1 corresponding to 2 mA·cm^?2 photocurrent) is obtained in a two electrode configuration at short circuit under 100 mW·cm^?2 illumination, clearly showing that these photoanodes can produce hydrogen without the assistance of any external bias. To the best of the authors' knowledge, this is the highest unbiased solar H₂ generation rate reported for these of QDs based heterostructures. Impedance spectroscopy measurements show similar electron density of trap states below the TiO₂ conduction band while the recombination resistance was higher for HNWs, consistently with the much lower surface area compared to NPs. However, the conductivity of both structures is similar, in spite of the one dimensional character of HNWs, which leaves some room for improvement of these nanowired structures. The effect of the QDs deposition method is also evaluated. Both structures show remarkable stability without any appreciable photocurrent loss after 0.5 hour of operation. The findings of this study constitute a relevant step towards the feasibility of hydrogen generation with wide bandgap semiconductors/quantum dots based heterostructures.     

Hybrid Heterojunction and Solid‐State Photoelectrochemical Solar Cells

A hybrid heterojunction and solid‐state photoelectrochemical solar cell based on graphene woven fabrics (GWFs) and silicon is designed and fabricated. The GWFs are transferred onto n‐Si to form a Schottky junction with an embedded polyvinyl alcohol based solid electrolyte. In the hybrid solar cell, solid electrolyte serves three purposes simutaneously; it is an anti‐reflection layer, a chemical modification carrier, and a photoelectrochemical channel. The open‐circuit voltage, short‐circuit current density, and fill factor are all significantly improved, achieving an impressive power conversion efficiency of 11%. Solar cell models are constructed to confirm the hybrid working mechanism, with the heterojunction junction and photoelectrochemical effect functioning synergistically.     

Tin Oxide as a Protective Heterojunction with Silicon for Efficient Photoelectrochemical Water Oxidation in Strongly Acidic or Alkaline Electrolytes

Photoelectrodes without a p–n junction are often limited in efficiency by charge recombination at semiconductor surfaces and slow charge transfer to electrocatalysts. This study reports that tin oxide (SnO_x) layers applied to n‐Si wafers after forming a thin chemically oxidized SiO_x layer can passivate the Si surface while producing ≈620 mV photovoltage under 100 mW cm^?2 of simulated sunlight. The SnO_x layer makes ohmic contacts to Ni, Ir, or Pt films that act as precatalysts for the oxygen‐evolution reaction (OER) in 1.0 m KOH(aq) or 1.0 m H₂SO₄(aq). Ideal regenerative solar‐to‐O₂(g) efficiencies of 4.1% and 3.7%, respectively, are obtained in 1.0 m KOH(aq) with Ni or in 1.0 m H₂SO₄(aq) with Pt/IrO_x layers as OER catalysts. Stable photocurrents for >100 h are obtained for electrodes with patterned catalyst layers in both 1.0 m KOH(aq) and 1.0 m H₂SO₄(aq).     

Photoelectrochemical Power,Chemical Energy and Catalytic Activity for Organic Evolution on Natural Pyrite Interfaces

Natural pyrite (FeS₂) has frequently been discussed as a material involved in CO₂ fixation in presence of H₂S and as a possible catalyst for the origin of life. A straightforward chemical fixation of carbon dioxide as proposed by Wächtershäuser could not be verified from thermo-chemical equilibrium calculations by minimizing Gibb's Free Energy in the system C, O, H, S, Fe and appears unlikely dueto the experimentally encountered large overpotentials involved in CO₂ fixation. However, the hypothesis, by W. R. Edwards, that pyrite in shallow coastal waters may have been involved, canbe sustained. In this case, daily available photoelectrochemical power from FeS₂/Fe^2+/3+ interfaces could have made thedifference in combination with electrochemical processes, such ashydrogen insertion, and the solubilization of pyrite by the aminoacid cysteine to yield dissolved chemical energy. Periodical changes in energy supply could also have entrained primitive self-organization processes for organic-biological evolution.Natural samples from thirteen ore deposits have been investigatedphotoelectrochemically. Efficient light-induced current generation has been found with several of these samples so thatphotoelectrochemical processes generated by pyrite have to be considered as naturally occurring phenomena, which could have been even more pronounced in oxygen deficient environments. Pyrite from the Murgul mine in Turkey of suboceanic volcanicorigin was closer examined as a model system to understand the morphology and chemistry of pyrite photoactivity.     

All‐Surface‐Atomic‐Metal Chalcogenide Sheets for High‐Efficiency Visible‐Light Photoelectrochemical Water Splitting

Artificial all‐surface‐atomic 2D sheets can trigger breakthroughs in tailoring the physical and chemical properties of advanced functional materials. Here, the conceptually new all‐surface‐atomic semiconductors of SnS and SnSe freestanding sheets are realized using a scalable strategy. As an example, all‐surface‐atomic SnS sheets undergo surface atomic elongation and structural disordering, which is revealed by X‐ray absorption fine structure spectroscopy and first‐principles calculations, endowing them with high structural stability and an increased density of states at the valence band edge. These exotic atomic and electronic structures make the all‐surface‐atomic SnS sheet‐based photoelectrode exhibit an incident photon‐to‐current conversion efficiency of 67.1% at 490 nm, much higher than the efficiencies of other visible‐light‐driven water splitting. A photocurrent density of 5.27 mA cm^‐2, which is two orders of magnitude higher than that of the bulk counterpart, is also achieved for the all‐surface‐atomic SnS sheets‐based photoelectrode. This will allow the manipulation of the basic properties of advanced materials on the atomic scale, thus paving the way for innovative applications.     

The TiO2–Catechol Complex: Coupling Type II Sensitization with Efficient Catalysis of Water Oxidation

Two main requirements must be fulfilled in order to construct an efficient TiO₂‐based photo‐electrochemical water splitting cell. One is the expansion of the cell's spectral response, usually by the attachment of a sensitizing dye monolayer on the surface of the TiO₂. The second involves the incorporation of a water oxidation catalyst that reduces the overpotential for the oxygen evolution reaction. These requirements are often achieved by the co‐adsorption of both the dye and the catalyst on the TiO₂, or by a covalent attachment of the catalyst to the dye molecule. Here, the possibility to use a single material that acts as a sensitizer and a catalyst is presented. The use of a catechol molecule to form a type II charge transfer complex with TiO₂ widens the absorption of the system into the visible region. The TiO₂‐catechol complex is highly catalytic toward the oxidation of water to oxygen, reducing the electrocatalytic reaction overpotential by 500 mV compared to bare TiO₂. A suggested catalytic mechanism for the water oxidation reaction is described. This methodology opens a new path for type II charge transfer complexes to be utilized as catalysts/light absorbers in water splitting systems based on TiO₂ or other metal oxides.     

Multifunctional Coatings from Scalable Single Source Precursor Chemistry in Tandem Photoelectrochemical Water Splitting

The straightforward and inexpensive fabrication of stabilized and activated photoelectrodes for application to tandem photoelectrochemical (PEC) water splitting is reported. Semiconductors such as Si, WO₃, and BiVO₄ can be coated with a composite layer formed upon hydrolytic decomposition of hetero­bimetallic single source precursors (SSPs) based on Ti and Ni, or Ti and Co in a simple single‐step process under ambient conditions. The resulting 3d‐transition metal oxide composite films are multifunctional, as they protect the semiconductor electrode from corrosion with an amorphous TiO₂ coating and act as bifunctional electrocatalysts for H₂ and O₂ evolution based on catalytic Ni or Co species. Thus, this approach enables the use of the same precursors for both photoelectrodes in tandem PEC water splitting, and SSP chemistry is thereby established as a highly versatile low‐cost approach to protect and activate photoelectrodes. In an optimized system, SSP coating of a Si photocathode and a BiVO₄ photoanode resulted in a benchmark noble metal‐free dual‐photoelectrode tandem PEC cell for overall solar water splitting with an applied bias solar‐to‐hydrogen conversion efficiency of 0.59% and a half‐life photostability of 5 h.