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The conversion of solar energy into hydrogen fuel by splitting water into photoelectrochemical cells (PEC) is an appealing strategy to store energy and minimize the extensive use of fossil fuels. The key requirement for efficient water splitting is producing a large band bending (photovoltage) at the semiconductor to improve the separation of the photogenerated charge carriers. Therefore, an attractive method consists in creating internal electrical fields inside the PEC to render more favorable band bending for water splitting. Coupling ferroelectric materials exhibiting spontaneous polarization with visible light photoactive semiconductors can be a likely approach to getting higher photovoltage outputs. The spontaneous electric polarization tends to promote the desirable separation of photogenerated electron- hole pairs and can produce photovoltages higher than that obtained from a conventional p-n heterojunction. Herein, we demonstrate that a hole inversion layer induced by a ferroelectric Bi4V2O11 perovskite at the n-type BiVO4 interface creates a virtual p-n junction with high photovoltage, which is suitable for water splitting. The photovoltage output can be boosted by changing the polarization by doping the ferroelectric material with tungsten in order to produce the relatively large photovoltage of 1.39 V, decreasing the surface recombination and enhancing the photocurrent as much as 180%.

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Among the CoMoO4 polymorphs, the α-phase exhibits the highest performance for the oxygen evolution reaction.

Monoclinic BiVO 4 is recognized as a promising photoanode for water oxidation, but its relatively wide bandgap energy (E g %2.5 eV) and poor charge transport limit the light absorption (η abs ) and charge separation (η sep ) efficiencies, thus resulting in low photocurrents. To solve these drawbacks, here the η abs  η sep product has been decoupled by combining W-doped BiVO 4 and V 2 O 5 rods (E g %2.1 eV) for simultaneously increasing the light harvesting and the charge separation in photoanodes under back-side illumination. In this strategy, V 2 O 5 rods maximize the light absorption and hole transport throughout the W-BiVO 4 film, making more holes to achieve the V 2 O 5 /W-BiVO 4 /H 2 O interface to trigger the water oxidation reaction with photocurrents as high as 6.6 mA cm À2 at 1.23 V RHE after 2 h reaction. Notably, under back-side illumination, the W-BiVO 4 /V 2 O 5 photoanode exhibited η abs  η sep of 74.5 and 93.0% at 0.5 and 1.23 V RHE , respectively, the highest values reported up to date for BiVO 4 -based photoelectrodes. This simple strategy brings us closer to develop efficient photoanodes for photoelectrochemical water splitting devices.

The conventional chemical methods to produce graphene using strong oxidizing agents produce toxic gases during synthesis; therefore, these methods do not meet the principles of green chemistry. In this work, an alternative top-down method for the synthesis of a few layers of graphene sheets has been produced by a Fenton reaction- (a mixture of Fe2+/H2O2) assisted exfoliation process in water using graphite flakes as a starting material. Based on X-ray diffraction data and Fourier transform infrared (FTIR), Raman spectroscopy, and transmission electron microscopy measurements, it is proposed that the oxidation of graphite by Fenton chemistry facilitates the exfoliation of graphene sheets under mild sonication. Subsequent chemical reduction with ascorbic acid produced a few layers of reduced graphene oxide. Compared to Hummers’ method, the Fenton reagent has similar exfoliation efficiency, but due to the Fenton reagent’s preference to react with the edges of graphite, the chemical reduction can lead to the formation of less defective reduced graphene oxides. Moreover, since Fe and H2O2 are cheap and environmentally innocuous, their use in large-scale graphene production is environmentally friendlier than conventional methods that use toxic oxidizing agents.

Using dual-photoelectrode photoelectrochemical (PEC) devices based on earth-abundant metal oxides for unbiased water splitting is an attractive means of producing green H fuel, but is challenging, owing to low photovoltages generated by PEC cells. This problem can be solved by coupling n-type BiVO with n-type Bi V O to create a virtual p/n junction due to the formation of a hole-inversion layer at the semiconductor interface. Thus, photoelectrodes with high photovoltage outputs were synthesized. The photoelectrodes exhibited features of p- and n-type semiconductors when illuminated under an applied bias, suggesting their use as photoanode and photocathode in a dual-photoelectrode PEC cell. This concept was proved by connecting a 1 mol % W-doped BiVO /Bi V O photoanode with an undoped BiVO /Bi V O photocathode, which produced a high photovoltage of 1.54 V, sufficient to drive stand-alone water splitting with 0.95 % efficiency.

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