Nanoscale assembly of BiVO4/CdS/CoOx core-shell heterojunction for enhanced photoelectrochemical water splitting

Porous BiVO4 electrodes were conformally decorated with CdS via a chemical bath deposition process. The highest photocurrent at 1.1 V vs. RHE was achieved for a BiVO4/CdS composite (4.54 mA cm(-2)), compared with CdS (1.19 mA cm(-2)) and bare BiVO4 (2.1 mA cm(-2)), under AM 1.5G illumination. This improvement in the photoefficiency can be ascribed to both the enhanced optical absorption properties and the charge separation due to the heterojunction formation between BiVO4 and CdS. Furthermore, the BiVO4/CdS photoanode was protected with a CoOx layer to substantially increase the photostability of the material. The new BiVO4/CdS/CoOx nanostructure exhibited a highly stable photocurrent density of similar to 5 mA cm(-2). The capability to produce O-2 was locally investigated by scanning photoelectrochemical microscope, which showed a good agreement between photocurrent and O-2 reduction current maps. This work develops an efficient route to improve the photo-electrochemical performance of BiVO4 and its long-term stability.Web of Science116art. no. 68

Magnetic polaron states in photoluminescent carbon dots enable hydrogen peroxide photoproduction

Photoactivation of aspartic acid-based carbon dots (Asp-CDs) induces the generation of spin-separated species, including electron/hole (e−/h+) polarons and spin-coupled triplet states, as uniquely confirmed by the light-induced electron paramagnetic resonance spectroscopy. The relative population of the e−/h+ pairs and triplet species depends on the solvent polarity, featuring a substantial stabilization of the triplet state in a non-polar environment (benzene). The electronic properties of the photoexcited Asp-CDs emerge from their spatial organization being interpreted as multi-layer assemblies containing a hydrophobic carbonaceous core and a hydrophilic oxygen and nitrogen functionalized surface. The system properties are dissected theoreti cally by density functional theory in combination with molecular dynamics simulations on quasi-spherical assemblies of size-variant flakelike model sys tems, revealing the importance of size dependence and interlayer effects. The formation of the spin-separated states in Asp-CDs enables the photoproduc tion of hydrogen peroxide (H2O2) from water and water/2-propanol mixture via a water oxidation reaction.Web of Science1932art. no. 220658

Radiative and Non-Radiative Recombination Pathways in Mixed-Phase TiO₂ Nanotubes for PEC Water-Splitting

Anatase and rutile mixed-phase TiO2 with an ideal ratio has been proven to significantly enhance photoelectrochemical (PEC) activity in water-splitting applications due to suppressing the electron⁻hole recombination. However, the mechanism of this improvement has not been satisfactory described yet. The PEC water oxidation (oxygen evolution) at the interface of TiO2 photoanode and electrolyte solution is determined by the fraction of the photogenerated holes that reach the solution and it is defined as the hole transfer efficiency. The surface and bulk recombination processes in semiconductor photoanodes majorly influence the hole transfer efficiency. In this work, we study the hole transfer process involved in mixed-phase TiO2 nanotube arrays/solution junction using intensity-modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS); then, we correlate the obtained hole transfer rate constants to (photo)electrochemical impedance spectroscopy (PEIS) measurements. The results suggest that the enhanced performance of the TiO2 mixed-phase is due to the improved hole transfer rate across the TiO2/liquid interface as well as to the decrease in the surface trap recombination of the holes

Ultrasound-driven defect engineering in TiO2–x nanotubes - Toward highly efficient platinum single atom-enhanced photocatalytic water splitting

Single-atom catalysts (SACs) have demonstrated superior catalytic activity and selectivity compared to nanoparticle catalysts due to their high reactivity and atom efficiency. However, stabilizing SACs within hosting substrates and their controllable loading preventing single atom clustering remain the key challenges in this field. Moreover, the direct comparison of (co-) catalytic effect of single atoms vs nanoparticles is still highly challenging. Here, we present a novel ultrasound-driven strategy for stabilizing Pt single-atomic sites over highly ordered TiO2 nanotubes. This controllable low-temperature defect engineering enables entrapment of platinum single atoms and controlling their content through the reaction time of consequent chemical impregnation. The novel methodology enables achieving nearly 50 times higher normalized hydrogen evolution compared to pristine titania nanotubes. Moreover, the developed procedure allows the decoration of titania also with ultrasmall nanoparticles through a longer impregnation time of the substrate in a very dilute hexachloroplatinic acid solution. The comparison shows a 10 times higher normalized hydrogen production of platinum single atoms compared to nanoparticles. The mechanistic study shows that the novel approach creates homogeneously distributed defects, such as oxygen vacancies and Ti3+ species, which effectively trap and stabilize Pt2+ and Pt4+ single atoms. The optimized platinum single-atom photocatalyst shows excellent performance of photocatalytic water splitting and hydrogen evolution under one sun solar-simulated light, with TOF values being one order of magnitude higher compared to those of traditional thermal reduction-based methods. The single-atom engineering based on the creation of ultrasound-triggered chemical traps provides a pathway for controllable assembling stable and highly active single-atomic site catalysts on metal oxide support layers.Web of Science1531379853797

Direct Observation of Photoinduced Higher Oxidation States at a Semiconductor/Electrocatalyst Junction

Photoelectrochemical (PEC) water splitting devices using semiconductors and electrocatalysts rely on heterogeneous interfaces that drive charge separation, thus determining potential gradients that dictate the reaction efficiency. The PEC potential of the electrocatalyst depends on the chemical oxidation state of forming elements, which may strongly vary under the photoinduced charge flow. However, element-sensitive, real-time measurements of the oxidation state of the electrocatalyst are not generally possible using conventional X-ray absorption techniques. Here, we show that fixed-energy X-ray absorption voltammetry and chronoamperometry, which measure the X-ray absorption coefficient variations along with photocurrent, can follow in real time the redox kinetics of electrocatalysts. To demonstrate the validity, we investigate hematite (α-Fe2O3) photoanodes covered with a nickel hydroxide electrocatalyst and show that it is fully oxidized by photogenerated holes to nickel oxyhydroxide with Ni reaching a higher oxidation state (NiIV) than that observed under electrocatalytic oxygen evolution in dark conditions. Highly oxidized Ni results from charge accumulation in the overlayer and can be observed only in the case of thick layers (with low PEC performance). On the other hand, the average oxidation state of Ni reaches lower values, under operative conditions, for very thin layers, resulting in high PEC activity. We complete our study by presenting PEC activity and impedance spectroscopy analysis using different thicknesses of the electrocatalyst, thus giving a detailed picture of the multiple and complex charge transfer processes occurring at a semiconductor/electrocatalyst junction