Deposition of zinc cobaltite nanoparticles onto bismuth vanadate for enhanced photoelectrochemical water splitting

Coastal aquatic pollutants degradation using ZnCo2O4 nanorods

Water pollution has caused problems in coastal areas, rivers, lakes, and other important water sources around the world as a result of inappropriate waste management. Meanwhile, these pollutants are harmful to humans and aquatic life. Textile dye effluent methyl orange (MO) was used in this work for dye degradation studies employing nanocomposites. As a result, the importance of synthesizing pure ZnO and Co3O4 nanoparticles with composites of ZnCo2O4 (zinc cobaltite) nanorods in three various proportions (90:10, 75:25, and 50:50) is emphasized in this study. Many advanced approaches were used to assess the various features of these materials, including size and shape. Fourier transform infrared (FT-IR) spectroscopy was used to determine the vibrational modes of the materials. The absorption measurements were then carried out using UV-vis spectroscopic techniques, and the photocatalytic breakdown of MO was done under visible light irradiation. The findings revealed that pure materials were inadequate for visible light activity, resulting in decreased degradation efficiencies. Spinel cobaltite structures have potential degradation efficiency under visible light, while ZnCo2O4 (50:50) catalyst has superior degradation efficiency of 59.8% over MO. The crystallite size, morphology, functional group, absorption wavelength, and band gap all play important roles in enhancing the material's photocatalytic activity under visible light. Meanwhile, ZnCo2O4 spinel structures are crucial for increasing charge carriers and reducing electron-hole recombination. As a result, zinc cobaltite minerals are widely used in industrial dye degradation applications.

Surface reconstruction in amorphous CoFe-based hydroxides/crystalline phosphide heterostructure for accelerated saline water electrolysis

Developing electrocatalysts with high activity and robust performance for large-scale seawater electrolysis to produce hydrogen holds immense significance. Herein, a highly active bifunctional electrode composed of amorphous cobalt-iron layered double hydroxides (CoFeLDH) and crystalline nickel phosphide (Ni2P) (denoted as CoFeLDH@Ni2P), is employed to boost hydrogen production through seawater electrolysis. The strong interface coupling effectively modifies the electronic structure at active sites, thereby accelerating the catalytic reaction kinetics. Impressively, in situ Raman and post-stability analyses demonstrate a unique reconstruction behavior on the CoFeLDH@Ni2P electrode. Bimetal co-incorporated NiOOH (CoFe-NiOOH) and Ni(OH)2 species are formed during the oxygen evolution reaction (OER), while CoFeLDH@Ni2P can transform into Ni(OH)2 species during the hydrogen evolution reaction (HER) process. Furthermore, the highly negatively charged surface selectively rejects Cl- ions by formed PO43-, endowing CoFeLDH@Ni2P with excellent tolerance and promising durability in saline electrolytes. Consequently, the CoFeLDH@Ni2P electrode exhibits an overpotential of 106 mV for HER at 10 mA cm-2 and 308 mV for OER to achieve 100 mA cm-2 in 1.0 M KOH solution. Additionally, the CoFeLDH@Ni2P(+,-) electrolyzer requires a low cell voltage of 1.56 V to deliver 10 mA cm-2 in 1.0 M KOH + Seasalt. This work presents an appealing strategy for the rational design of advanced electrocatalysts with amorphous-crystalline interfaces, which reveals the source of the activity of transition-metal phosphating compounds in saline water electrolysis.

Dual modification of BiVO4 photoanode for synergistically boosting photoelectrochemical water splitting

Bismuth vanadate (BiVO₄) is a promising nanomaterial for photoelectrochemical (PEC) water oxidation. However, the serious charge recombination and sluggish water oxidation kinetics limit its performance. Herein, an integrated photoanode was successfully constructed by modifying BiVO₄ (BV) with In₂O₃ (In) layer and further decorating amorphous FeNi hydroxides (FeNi). The BV/In/FeNi photoanode exhibited a remarkable photocurrent density of 4.0 mA cm^-2 at 1.23 V_RHE, which is approximately 3.6 times larger than that of pure BV. And the water oxidation reaction kinetics has an over 200% increased. This improvement was mainly because the formation of BV/In heterojunction inhibited charge recombination, and the decoration of cocatalyst FeNi facilitated the water oxidation reaction kinetics and accelerated hole transfer to electrolyte. Our work provides another possible route to develop high-efficiency photoanodes for practical applications in solar conversion.

Enhancement of charge separation and hole utilization in a Ni2P2O7-Nd-BiVO4 photoanode for efficient photoelectrochemical water oxidation

It is necessary for photoelectrochemical (PEC) water splitting to reduce the electron-hole recombination rate and enhance the water oxidation reaction kinetics. Here, we prepared Ni₂P₂O₇-Nd-BiVO₄ composite photoanodes by coupling Ni₂P₂O₇ co-catalysts to neodymium (Nd)-doped BiVO₄ surfaces through photo-assisted electrodeposition. The Ni₂P₂O₇-Nd-BiVO₄ photoanode exhibits a high photocurrent density of 3.6 mA cm^-2 at 1.23 V vs reversible hydrogen electrode (RHE), which is three times higher than that of the bare BiVO₄ (1.2 mA cm^-2). Detailed characterizations demonstrate that Nd doping reduces the band gap, significantly increases the carrier density and effectively reduces the charge transfer resistance. More importantly, the Ni₂P₂O₇ co-catalyst has multiple roles. Specifically, it can act as a hole extraction layer to accelerate hole migration and inhibit hole-electron recombination. At the same time, it significantly improves the water oxidation reaction kinetics. In addition, it also provides more water oxidation active sites. This work provides ideas for the design and study of efficient BiVO₄-based photoanodes.

Revealing the enhanced photoelectrochemical water oxidation activity of Fe-based metal-organic polymer-modified BiVO4 photoanode

Metal-organic polymers (MOPs) can enhance the photoelectrochemical (PEC) water oxidation performance of BiVO₄ photoanodes, but their PEC mechanisms have yet to be comprehended. In this work, we constructed an active and stable composite photoelectrode by overlaying a uniform MOP on the BiVO₄ surface using Fe²⁺ as the metal ions and 2,5-dihydroxyterephthalic acid (DHTA) as ligand. Such modification on the BiVO₄ surface yielded a core-shell structure that could effectively enhance the PEC water oxidation activity of the BiVO₄ photoanode. Our intensity-modulated photocurrent spectroscopy analysis revealed that the MOP overlayer could concurrently reduce the surface charge recombination rate constant (k_sr) and enhance the charge transfer rate constant (k_tr), thus accelerating water oxidation activity. These phenomena can be ascribed to the passivation of the surface that inhibits the recombination of the charge carrier and the MOP catalytic layer that improves the hole transfer. Our rate law analysis also demonstrated that the MOP coverage shifted the reaction order of the BiVO₄ photoanode from the third-order to the first-order, resulting in a more favorable rate-determining step where only one hole accumulation is required to overcome water oxidation. This work provides new insights into the reaction mechanism of MOP-modified semiconductor photoanodes.

Ni doped amorphous FeOOH layer as ultrafast hole transfer channel for enhanced PEC performance of BiVO4

Bismuth vanadate (BiVO₄), as the potential and prospective photocatalyst, has been limited by the issue of poor separation and transfer of charge carrier for photoelectrocatalytic (PEC) water oxidation. Here, a significant increase of surface injection efficiency for BiVO₄ is realized by the rationally designed Ni doped FeOOH (Ni:FeOOH) layer growing on BiVO₄ photoanode (Ni:FeOOH/BiVO₄), in which doped Ni²⁺ can induce partial-charge of FeOOH to serve as ultrafast transfer channel for hole transfer and transportation at the semiconductor/electrolyte interface. In addition, the Ni:FeOOH/BiVO₄ shows the η_surface value of 81.6 %, which is 3.28-fold and 1.47-fold of BiVO₄ and FeOOH/BiVO₄, respectively. The photocurrent density of Ni:FeOOH/BiVO₄ is 4.21 mA cm^-2 at 1.23 V vs. RHE, with the onset potential cathodically shifting 237 mV over BiVO₄ and a long-term stability for suppressing surface charge recombination. The UPS and UV-Vis spectra have confirmed the type-II band alignment between Ni:FeOOH and BiVO₄ for promoting carrier transfer. This facile and effective spin-coating method could deposit oxygen evolution catalysts (OECs) availably onto photoanodes with enhanced PEC water splitting.

Ni-Doped BiVO4 photoanode for efficient photoelectrochemical water splitting

BiVO₄ (BVO) based photoanode is one of the most mega-potential materials for solar water splitting while suffers from poor charge transfer and separation efficiency limit its practical application. Herein, FeOOH/Ni-BiVO₄ photoanode synthesized by the facile wet chemical method were investigated for improved charge transport and separation efficiency. The photoelectrochemical (PEC) measurements demonstrate that the water oxidation photocurrent density can reach as high as 3.02 mA cm^-2 at 1.23 V vs RHE, and the surface separation efficiency can be boosted to 73.3 %, which increases around 4 times comparing with that of pure sample. Further depth studies showed that the Ni doping can effectively promote hole transport/trapping and introduce more active sites for the oxidation of water, while FeOOH co-catalyst could passivate the Ni-BiVO₄ photoanode surface. This work provides a model for the design of BiVO₄-based photoanodes with combined thermodynamic and kinetic advantages.

Carbon hollow matrix anchored by isolated transition metal atoms serving as a single atom cocatalyst to facilitate the water oxidation kinetics of bismuth vanadate

Here, nitrogen doped carbon hollow matrix anchored by isolated transition metal atoms (M@NC, M = Fe, Co or Ni) are firstly utilized as new single atom cocatalysts (SACCs) to enhance the PEC performance of Mo, W ions co-doped BiVO₄ (Mo, W: BVO) through a simple spin-coating method. It is found that Mo, W: BVO modified with Fe@NC exhibits higher photocurrent density than the one decorated with Co@NC or Ni@NC due to the relatively low redox potential of Fe³⁺/Fe²⁺ (0.77 V vs SHE). During the photoelectrochemical (PEC) process, the Fe²⁺ ions are easier to accept the photogenerated holes of BVO and be oxidized to Fe³⁺ ions. Then, Fe³⁺ ions are reduced to Fe²⁺ again by accepting the electrons of water, and evolve oxygen simultaneously. Hence, Fe@NC could facilitate the water oxidation kinetics through the redox cycle of Fe ions and promote the charge separation efficiency by capturing the photogenerated holes. Theoretical calculations demonstrate that the deposition of Fe atoms make NC negatively charged, which is conducive to receiving the photogenerated holes. As a result, Mo, W: BVO/Fe@NC exhibits higher photocurrent density (3.2 mA/cm² vs RHE) than other BVO-based samples. This work opens up a new application field of SACCs serving as OER cocatalysts, and may provide a universal strategy to construct the efficient PEC photoelectrodes.