Enhanced photocurrent density for photoelectrochemical catalyzing water oxidation using novel W-doped BiVO4 and metal organic framework composites

State-of-the-Art, Insights, and Perspectives for MOFs-Nanocomposites and MOF-Derived (Nano)Materials

Composite structures created from metal‒organic framework (MOF) matrices are reviewed in this work. Depending on the nature of the second component apart from the MOF platform, several synergistic properties may arise; at the same time, the initial features of the single constituent materials are usually maintained, and individual shortcomings are mitigated. Currently, timely energy and environmental challenges necessitate the quest for more advanced materials and technologies. Significant developments in MOF-nanocomposites have enabled their application across a wide range of modern and traditional fields. This review demonstrates in an exhaustive and critical way a broad range of MOF-based nanocomposites, namely, MOF/perovskite nanoparticles (NPs), MOF/metal (non-iron) oxide NPs, MOF/Fe3O4 NPs, MOF/metal chalcogenide NPs, MOF/metal NPs, and MOF/carbon-based materials, as well as nanocomposites of MOFs with other semiconductor NPs. Key points related to the synthesis, characterization, and applications of these materials are provided. Depending on their configuration, the composites under discussion can be applied in domains such as photoelectrochemical sensing, antibiotic/dye degradation, optoelectronics, photovoltaics, catalysis, solar cells, supercapacitors, batteries, water remediation, and drug loading. Sometimes, MOFs can undergo certain processes (e.g. pyrolysis) and act as precursors for composite materials with appealing characteristics. Therefore, a special section in the manuscript is devoted to MOF-derived NP composites. Toward the end of the text, we conclude while also describing the challenges and possibilities for further investigations in the umbrella of material categories analyzed herein. Despite the progress achieved, key questions remain to be answered regarding the relationships among the morphology, properties, and polyvalent activity of these materials. The present work aims to shed light on most of their aspects and innovative prospects, facilitating a deeper comprehension of the underlying phenomena, functionality, and mechanistic insights governing their behavior.

Enhancing Photoelectrochemical Seawater Splitting Efficiency by a Dual-Strategy Approach of W Doping and CoOOH Layer Deposition on BiVO4 Photoanodes

Photoelectrochemical (PEC) seawater splitting offers a sustainable pathway for hydrogen production, yet its practical application is hindered by sluggish reaction kinetics and severe photocorrosion in chloride-rich environments. This study presents a dual-strategy modification of BiVO4 photoanodes through tungsten (W) doping and cobalt oxyhydroxide (CoOOH) nanolayer deposition to synergistically enhance the PEC performance and stability in natural seawater. W doping optimizes the electronic structure of BiVO4 by reducing the bandgap from 2.4 to 2.35 eV and increasing carrier concentration from 1.41 × 1021 to 3.31 × 1021 cm-3, while CoOOH acts as a dual-functional layer that suppresses surface recombination via oxygen vacancy formation and protects against chloride-induced corrosion. The optimized CoOOH/W-BVO photoanode achieves a photocurrent density of 3.77 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE) with 96 h stability in natural seawater, outperforming pristine BiVO4 by 150% and single-modified counterparts by 40-60%. Mechanistic analyses reveal that W6+ substitution elongates V-O bonds, thereby enhancing the bulk charge separation. Concurrently, CoOOH facilitates hole extraction through oxygen vacancies, with oxygen vacancy content increasing from 3.9% to 24.3%. The dual modification also reduces interfacial charge-transfer resistance to 94.44 Ω and shifts the flat-band potential negatively to 0.15 V vs RHE, improving light absorption and charge utilization efficiency (applied bias photocurrent efficiency (ABPE) of 0.95% at 0.77 V). This work provides a robust strategy for designing efficient and durable photoanodes, advancing marine-resource-utilized renewable energy technologies.

Reactive co-sputter deposition of Ta-doped tungsten oxide thin films for water splitting application

This study aimed to investigate the structural, optical, and electronic properties of WO3 thin films modified by Ta-doping, considering their potential application in photoelectrochemical (PEC) water splitting. Due to its unique physical and chemical properties, WO3 films have been commonly suggested as a promising photoanode for hydrogen production. However, the wide bandgap and unsuitable band edge positions of WO3 limit its PEC efficiency. Doping have been extensively applied as an effective strategy for bandgap engineering. Here, post-annealed WO3 films with different concentrations of Ta dopant were synthesized via reactive magnetron co-sputtering, while DC and RF sputtering powers were varied with the aim of achieving the desired properties. EDX analysis showed that Ta atoms were doped into WO3 in the range of 0-3.93 at%. As evident from SEM and AFM images, the surface morphology was significantly affected by increasing Ta doping, the formation of a granular structure with well-defined boundaries and increasing surface roughness (1.79-47.94 nm). XRD patterns confirmed that the incorporation of Ta atoms into a monoclinic WO3 improved the crystallinity, especially in the (002) direction. Most importantly, a decrease in the average transparency (92.82-74.27%), an increase in visible absorption, a red shift of the fundamental absorption edge corresponding to a favorable drop in the optical bandgap energy (3.07-2.61 eV) were found with increasing Ta concentration. Notably, the substitution of W6+ ions with Ta dopant (0-3.93 at%) led to an upward shift in the valence band maximum (3.62-3.31 eV) and a downward shift in the conduction band minimum (0.55-0.70 eV). The WO3 photoanode doped with 3.93 at% Ta exhibited the maximum photocurrent density of 0.65 mA/cm2 (at 1 V vs. Ag/AgCl) under simulated sunlight. Furthermore, WO3 photoanode doped with 3.93 at% Ta showed excellent photoresponsivity and slow electron-hole recombination. The obtained results predict the potential of Ta-doping coupled with post-annealing to optimize the structural and optoelectronic properties of sputtered WO3 thin films as photoanode for use in efficient PEC water splitting.

Enhancing photocatalytic CO2 reduction activity through Cobalt-Bismuth bimetallic Nanoparticle-Modified Nitrogen-Doped graphite carbon

Efficient charge transfer and effective separation of photo-generated charge carriers are crucial factors in photocatalysis. In this study, we present the design of a composite photocatalyst consisting of cobalt and bismuth (CoBi) bimetallic nanoparticles incorporated into a honeycomb nitrogen-doped graphitic carbon (N-GC) matrix. The ultra-thin porous N-GC matrix exhibits excellent electrical conductivity, a high number of active sites, and enables efficient absorption and multiple reflection of incident light. The CoBi bimetal-N-GC interface establishes a self-driven charge transport channel that effectively suppresses the backflow of photogenerated electrons, leading to prolonged separation of photo-generated carriers and a significant improvement in photocatalytic activity. The CoBi@N-GC catalyst showcases outstanding performance, producing CH4 and CO at rates of 36.07 μmol·g-1·h-1 and 44.09 μmol·g-1·h-1 respectively, confirming its superior photocatalytic capabilities.

Sandwich-structured continuous ZIF-8/Ti3C2 MXene/ZIF-8 for efficient sterilization: Enhanced photocatalytic activity, photothermal effect, and environmental safety

The development of effective water purification systems is crucial for controlling and remediating environmental pollution, especially in terms of sterilization. Herein, we demonstrate elaborately designed composite nanosheets with a sandwich structure, composed of two-dimensional (2D) Ti3C2 MXene nanosheet core and conformal ZIF-8 ultrathin outer layers, and their potential applications in photocatalytic sterilization. The study results indicate that the conformal ZIF-8-MXene nanosheet exhibits an expanded light absorption range (826 nm), improved photothermal conversion efficiency (6.2 °C s-1), and photocurrent response, thus boosting photocatalytic sterilization efficiency (6.63 log10 CFU mL-1) against Escherichia coli under simulated sunlight within 90 min. Interestingly, 2D ZIF-8 layers exhibit positive zeta potential (19 mV), good hydrophilicity (40.6°), and local photogenerated-hole accumulation, possessing efficient bacteria-trapping efficiency. Membrane filters fabricated from optimized composite nanosheets exhibit an outstanding bacteria-trapping and sterilization efficiency (almost 100 %) against Escherichia coli under simulated sunlight within 30 min of the flow photocatalytic experiments. This work not only presents a rational structural design of the conformal and ultrathin anchoring of ZIF-8 onto a 2D conductive material for bacteria-trapping and sterilization, but also opens new opportunities for using metal-organic frameworks in photocatalytic disinfection of drinking water.

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.

Enhanced Photoredox Activity of BiVO4/Prussian Blue Nanocomposites for Efficient Pollutant Removal from Aqueous Media under Low-Cost LEDs Illumination

Bismuth vanadate (BiVO4, BV) is a widely explored photocatalyst for photo(electro)chemical applications, but its full photocatalytic potential is hindered by the fast recombination and low mobility of photogenerated charge carriers. Herein, we propose the photodeposition of different amounts of Prussian blue (PB) cocatalysts on the surface of monoclinic BV to obtain BV-PB composite photocatalysts with increased photoactivity. The as-prepared BV and BV-PB composites were characterized by an array of analytic techniques such scanning eletron microscopy (SEM), transmission eletron microscopy (TEM), X-day diffraction (XRD), and spectroscopic techniques including Fourier-transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS), electrochemical impedance spectroscopy (EIS), photoluminescence (PL), and Raman spectroscopy. The addition of PB not only increases the absorption of visible light, as indicated by DRS, but also improves the charge carriers’ transfer across the photocatalysts/solution interface and hence reduces electron-hole (e−-h+) recombination, as confirmed by EIS and PL measurements. Resultantly, the BV-PB composite photocatalysts with optimum PB loading exhibited enhanced Cr(VI) photoreduction efficiency as compared to pristine BV under visible light illumination from low-power blue light-emitting diodes (LEDs), thanks to the cocatalyst role of PB which mediates the transfer of photoexcited conduction band (CB) electrons from BV to Cr(VI) species in solution. Moreover, as compared to pristine BV and BV + H2O2, a drastic increase in the methylene blue (MB) photo-oxidation efficiency was observed for BV-PB in the presence of a minute quantity of H2O2 due to a synergic effect between the photocatalytic and Fenton-like processes. While pure BV photodegraded around 70% of MB dye within 120 min, the BV-PB/H2O2 and BV/H2O2 system could degrade almost 100% of the dye within 20 min (kobs. = 0.375 min−1) and 40 min (kobs. = 0.055 min−1), respectively. The practical approach employed in this work may pioneer new prospects for synthesizing new BV-based photocatalytic systems with low production costs and high photoredox efficiencies.