Hydrogen induced interface engineering in Fe2O3-TiO2 heterostructures for efficient charge separation for solar-driven water oxidation in photoelectrochemical cells.

Semiconductor heterostructure junctions are known to improve the water oxidation performance in photoelectrochemical (PEC) cells. Depending on the semiconductor materials involved, different kinds of junctions can appear, for instance, type II band alignment where the conduction and valence bands of the semiconductor materials are staggered with respect to each other. This band alignment allows for a charge separation of the photogenerated electron-hole pairs, where the holes will go from low-to-high valance band levels and vice versa for the electrons. For this reason, interface engineering has attracted intensive attention in recent years. In this work, a simplified model of the Fe2O3-TiO2 heterostructure was investigated via first-principles calculations. The results show that Fe2O3-TiO2 produces a type I band alignment in the heterojunction, which is detrimental to the water oxidation reaction. However, the results also show that interstitial hydrogens are energetically allowed in TiO2 and that they introduce states above the valance band, which can assist in the transfer of holes through the TiO2 layer. In response, well-defined planar Fe2O3-TiO2 heterostructures were manufactured, and measurements confirm the formation of a type I band alignment in the case of Fe2O3-TiO2, with very low photocurrent density as a result. However, once TiO2 was subjected to hydrogen treatment, there was a nine times higher photocurrent density at 1.50 V vs. the reversible hydrogen electrode under 1 sun illumination as compared to the original heterostructured photoanode. Via optical absorption, XPS analysis, and (photo)electrochemical measurements, it is clear that hydrogen treated TiO2 results in a type II band alignment in the Fe2O3-H:TiO2 heterostructure. This work is an example of how hydrogen doping in TiO2 can tailor the band alignment in TiO2-Fe2O3 heterostructures. As such, it provides valuable insights for the further development of similar material combinations.

Synergies of co-doping in ultra-thin hematite photoanodes for solar water oxidation: In and Ti as representative case.

Solar energy induced water splitting in photoelectrochemical (PEC) cells is one of the most sustainable ways of hydrogen production. The challenge is to develop corrosion resistant and chemically stable semiconductors that absorb sunlight in the visible region and, at the same time, have the band edges matching with the redox level of water. In this work, hematite (α-Fe2O3) thin films were prepared onto an indium-doped tin oxide (ITO; In:SnO2) substrate by e-beam evaporation of Fe, followed by air annealing at two different temperatures: 350 and 500 °C. The samples annealed at 500 °C show an in situ diffusion of indium from the ITO substrate to the surface of α-Fe2O3, where it acts as a dopant and enhances the photoelectrochemical properties of hematite. Structural, optical, chemical and photoelectrochemical analysis reveal that the diffusion of In at 500 °C enhances the optical absorption, increases the electrode-electrolyte contact area by changing the surface topology, improves the carrier concentration and shifts the flat band potential in the cathodic direction. Further enhancement in photocurrent density was observed by ex situ diffusion of Ti, deposited in the form of nanodisks, from the top surface to the bulk. The in situ In diffused α-Fe2O3 photoanode exhibits an improved photoelectrochemical performance, with a photocurrent density of 145 µA cm-2 at 1.23 VRHE, compared to 37 µA cm-2 for the photoanode prepared at 350 °C; it also decreases the photocurrent onset potential from 1.13 V to 1.09 V. However, the In/Ti co-doped sample exhibits an even higher photocurrent density of 290 µA cm-2 at 1.23 VRHE and the photocurrent onset potential decreases to 0.93 VRHE, which is attributed to the additional doping and to the surface becoming more favorable to charge separation.

In†Situ Solid-State Synthesis of a AgNi/g-C3 N4 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.

Visible-Light-Driven Photoelectrochemical and Photocatalytic Performance of NaNbO3 /Ag2 S 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.