Accelerating the charge separation of ZnFe2O4 nanorods by Cu-Sn ions gradient doping for efficient photoelectrochemical water splitting

Graphene Quantum Dots as Hole Extraction and Transfer Layer Empowering Solar Water Splitting of Catalyst-Coupled Zinc Ferrite Nanorods

Despite the narrow band gap energy, the performance of zinc ferrite (ZnFe2O4) as a photoharvester for solar-driven water splitting is significantly hindered due to its sluggish charge transfer and severe charge recombination. This work reports the fabrication of a hybrid nanostructured hydrogenated ZnFe2O4 (ZFO) photoanode with enhanced photoelectrochemical water-oxidation activity through coupling N-doped graphene quantum dots (GQDs) as a hole transfer layer and Co-Pi as a catalyst. The GQDs not only reduce the surface-mediated nonradiative electron-hole pair recombination but also induce a built-in interfacial electric field leading to a favorable band alignment at the ZFO/GQDs interface, helping rapid photogenerated hole separation and serving as a conducting hole transfer highway, improve the hole transportation into the Co-Pi catalyst for enhanced water oxidation reaction kinetics. The optimized ZFO/GQD/Co-Pi hybrid photoanode delivers a 23-fold photocurrent enhancement at 1.23 V versus the reversible hydrogen electrode (RHE) and a significant 360 mV reduction in the onset potential, reaching 0.65 VRHE compared with the ZFO photoanode under 1 sun illumination in a neutral electrolytic environment. This investigation underscores the mechanism of synergistic interplay between the hole transport layer and cocatalyst in boosting the solar-illuminated water-splitting activity of the ZFO photoanode.

Interface designing of efficient Z-scheme Ti-ZnFe2O4/In2O3 photoanode toward boosting photoelectrochemical water oxidation

Ti-ZnFe₂O₄ photoanode has attracted extensive attention in photoelectrochemical (PEC) water oxidation due to its narrow band gap and good photostability. However, its low efficiency limits its development. Herein, we designed and constructed direct Z-scheme Ti-ZnFe₂O₄/In₂O₃ (Ti-ZFO/In₂O₃) photoanode. Under the interface electric field, photogenerated holes with stronger oxidation capacity on In₂O₃ are retained to participate in the water oxidation reaction, and the photocurrent density of Ti-ZFO/In₂O₃ is much higher than that of pure Ti-ZFO, reaching 2.2 mA/cm² at 1.23 V vs. RHE. Kelvin Probe, steady-state photovoltage spectroscopy (SPV), transient photovoltage spectroscopy (TPV) and in-situ double beam strategy were used to demonstrate the Z-scheme charge transfer mechanism of Ti-ZFO/In₂O₃ photoanode. Our work provides an effective scheme and technical means for further understanding the mechanism of interfacial charge transfer.

Boosting the Photoelectrochemical Performance of Au/ZnO Nanorods by Co-Occurring Gradient Doping and Surface Plasmon Modification

Band bending modification of metal/semiconductor hybrid nanostructures requires low-cost and effective designs in photoelectrochemical (PEC) water splitting. To this end, it is evinced that gradient doping of Au nanoparticles (NPs) inwards the ZnO nanorods (NRs) through thermal treatment facilitated faster transport of the photo-induced charge carriers. Systematic PEC measurements show that the resulting gradient Au-doped ZnO NRs yielded a photocurrent density of 0.009 mA/cm² at 1.1 V (vs. NHE), which is 2.5-fold and 8-fold improved compared to those of Au-sensitized ZnO and the as-prepared ZnO NRs, respectively. The IPCE and ABPE efficiency tests confirmed the boosted photoresponse of gradient Au-incorporated ZnO NRs, particularly in the visible spectrum due to the synergistic surface plasmonic effect of Au NPs. A gradient Au dopant profile promoted the separation and transfer of the photo-induced charge carriers at the electrolyte interface via more upward band bending according to the elaborated electrochemical impedance spectroscopy and Kelvin probe force microscopy analyses. Therefore, this research presents an economical and facile strategy for preparing gradient plasmonic noble NP-incorporated semiconductor NRs, which have excellent potential in energy conversion and storage technologies.

Magnetron Sputtered Al Co-Doped with Zr-Fe2O3 Photoanode with Fortuitous Al2O3 Passivation Layer to Lower the Onset Potential for Photoelectrochemical Solar Water Splitting

In this paper, we investigate the magnetron sputtering deposition of an Al-layer on Zr-doped FeOOH (Zr-FeOOH) samples to fabricate a Zr/Al co-doped Fe2O3 (Al-Zr/HT) photoanode. An Al-layer is deposited onto Zr-FeOOH through magnetron sputtering and the thickness of the Al deposition is regulated by differing the sputtering time. Electrochemical impedance spectroscopy, intensity-modulated photocurrent spectroscopy, Mott-Schottky and time-resolved photoluminescence spectra analyses were used to study, in depth, the correlations between sputtered Al-layer thicknesses and PEC characteristics. High-temperature quenching (800 °C) assists in diffusing the Al3+ in the bulk of the Zr-doped Fe2O3 photoanode, whilst an unintended Al2O3 passivation layer forms on the surface. The optimized Al-Zr/HT photoelectrode achieved 0.945 mA/cm2 at 1.0 VRHE, which is 3-fold higher than that of the bare Zr/HT photoanode. The Al2O3 passivation layer causes a 100 mV cathodic shift in the onset potential. Al co-doping improved the donor density, thus reducing the electron transit time. In addition, the passivation effect of the Al2O3 layer ameliorated the surface charge transfer kinetics. The Al2O3 passivation layer suppressed the surface charge transfer resistance, consequently expediting the hole migration from photoanode to electrolyte. We believe that the thickness-controlled Al-layer sputtering approach could be applicable for various metal oxide photoanodes to lower the onset potential.

Synthesis and control strategies of nanomaterials for photoelectrochemical water splitting

Photoelectrochemical water splitting to produce hydrogen using solar energy can capture and directly convert solar energy into chemical energy, which is an effective way to deal with the current energy and environmental problems. The conversion efficiency of solar energy depends on the performance of semiconductor photoelectrodes in photoelectrochemical water splitting. This article presents our recent advances in the design and performance control of high-efficiency photoelectrocatalytic materials, followed by the discussion of the strategies employed for improving the performances of photoelectrodes in terms of photon absorption, charge separation and migration, as well as surface chemical reactions.

Rational design of interface refining through Ti4+/Zr4+ diffusion/doping and TiO2/ZrO2 surface crowning of ZnFe2O4 nanocorals for photoelectrochemical water splitting

The interplay between diffusion/doping and surface passivation of TZF NCs exhibits a breakthrough photocurrent density of 0.73 mA cm⁻² (1.23 V vs. RHE) with 98% stability over 10 h in the TZF/Al₂O₃/CoO_x photoanode.

Zinc ferrite-based p-n homojunction with multi-effect for efficient photoelectrochemical water splitting

A novel one-dimensional core-shell zinc ferrite (ZnFe₂O₄) p-n homojunction is prepared by a facile two-step hydrothermal method. The core-shell homojunction is constructed by decorating p-type Ni-ZnFe₂O₄ (shell) onto n-type ZnFe₂O₄ (core). As expected, significant enhancement in the photocurrent density of the developed homojunction is realized compared to that of pristine ZnFe₂O₄ (6.64 times that of pristine ZnFe₂O₄). This improvement is ascribed to the fact that the ZnFe₂O₄ homojunction has multiple optimization effects, namely, a built-in electric field and active sites on Ni-ZnFe₂O₄, which are beneficial to carrier separation and transport. This study paves a promising pathway for the use of ion doping to design high-quality p-n homojunctions with multiple effects for enhancing charge separation in the photoelectrochemical water splitting configuration.

Screening metal-dicorrole-based dyes with excellent photoelectronic properties for dye-sensitized solar cells by density functional calculations

The dye molecules behaving as photosensitizers in dye-sensitized solar cells are the most critical factors to determine the power conversion efficiency. Therefore, ways to design dye molecules with excellent photoelectric properties has been the focus of dye-sensitized solar cells research. Here, we selected four representative different metal-corrole monomers to characterize their structures and photoelectronic properties. Then based on these metal-corrole monomers, six different architectures of metal-dicorroles were designed by varying the linking forms. The most stable architecture [Formula: see text] was screened out by binding energy calculations. A further two types of metal-dicorrole-based dyes were constructed by incorporating different bridge groups with the cyanoacry acceptor into the stable metal-dicorroles. A large number of density functional theory calculations and photoelectric properties analysis indicate that among these different metal-dicorrole-based dyes, Ga-dicorrole dyes have two strong and wide absorption bands in the visible region corresponding to Soret and Q bands, respectively, and have high charge separation efficiency under optical excitation. Especially for Ga-SN dye, by incorporating a [Formula: see text]-bridge-conjugated group, its Soret absorption band is greatly enhanced, broadened and red-shifted, resulting in its merging with the Q band into one absorption band. Moreover, its charge transfer efficiency is up to 76.86%, which will facilitate its coupling with semiconductor materials and transfer its electrons to the semiconductor materials.

A promising p-type Co-ZnFe2O4 nanorod film as a photocathode for photoelectrochemical water splitting

A p-type Co-ZnFe₂O₄ film with a one-dimensional (1D) rod-like morphology is fabricated for the first time on fluorine-doped tin oxide (FTO) through a hydrothermal reaction and sintering treatment. The p-type Co-ZnFe₂O₄ is obtained by doping Co ions into n-type ZnFe₂O₄, in which Zn sites are substituted by Co. Compared with the n-type ZnFe₂O₄, the light absorption edge of Co-ZnFe₂O₄ is clearly shifted from 589 to 624 nm, and the positions of the valence/conduction band of Co-ZnFe₂O₄ meet the thermodynamic requirements for water splitting. The photocurrent density of p-type Co-ZnFe₂O₄ is -0.22 mA cm^-2 at 0 V vs. the reversible hydrogen electrode (RHE), which is enhanced 7.33-times vs. that of n-type ZnFe₂O₄ (-0.03 mA cm^-2 at 0 V vs. RHE). This work provides useful insights into tuning the p-n character of semiconductors to realize efficient photoelectrochemical (PEC) water splitting.

2D elongated polyhedral-like YVO4 films: a novel photoanode for photoelectrochemical water splitting

YVO4 films, prepared on FTO substrates using a long-term hydrothermal method, possessing two-dimensional (2D) elongated polyhedral microcrystals and serving as a novel photoanode in the photoelectrochemical (PEC) field, are reported for the first time. The research indicates that YVO4 is a potential photoanode for PEC water splitting.