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Bilayered ZnO nanostar/TiO 2 nanotubes (TNTs) were developed by electrochemical anodization, followed by the hydrothermal method. A ZnO nanostar film composed of ZnO nanorods (NRs) was steadily changed depending on the hydrothermal time (1, 2, and 6 h). In the case of ZnO(1 h)/TNTs, the nanostar-shaped ZnO film with a length of 1.2–1.8 μm and a width of 40–60 nm consisting of conventional NRs was formed. On the other hand, with increasing hydrothermal time, a thicker pyramidal-shaped ZnO NRs film was formed with a length of 2.8–3.4 μm and a width of 50–80 nm resulting from a limited mass transport of the reacting precursor. Then, it was remarkable that ZnO NRs formed omnidirectional nanostar particles without any directionality. On the basis of this architecture, the photoelectrochemical properties were assessed using CdSe quantum dots (QDs) as photosensitizers. The ZnO/TNTs film showed enhanced light absorption due to the high deposition of CdSe QDs resulting from the increase of specific surface area. In particular, the extent of light absorption was steadily increased with the hydrothermal time of ZnO, indirectly demonstrating that longer hydrothermal time can make a thicker ZnO layer on the TNTs. Also, it was confirmed that the light scattering effect from micrometer-sized ZnO particles was achieved to improve light absorption in the long wavelength range (600–670 nm). This point was again confirm by the finding that the wavelength showing the maximum efficiency in the incident photon to current conversion efficiency (IPCE) was shifted from 555 nm to 585 nm (2 h synthesis) and 590 nm (6 h synthesis), which is indicative of the light scattering effect from ZnO particles on CdSe QD-sensitized solar cells (QDSSCs). Accordingly, the dual effects (specific surface area plus light scattering effect) gave rise to the abrupt increase of short-circuit photocurrent ( J sc ) in bilayered ZnO/TNTs based CdSe QDSSCs in the photocurrent–voltage ( J – V ) characteristics. In particular, the ZnO(2h)/TNTs electrode exhibited the best enhancement (0.94%), showing conversion efficiency with a J sc of 6.13 mA/cm 2 and a moderately increased open-circuit voltage ( V oc ) attributed from the upper ZnO layer in the total film (ZnO + TNTs), compared to that (0.49%) of TNTs-based QDSSCs. [ABSTRACT FROM AUTHOR]

Photoelectrochemical water splitting, which uses sunlight to produce clean hydrogen fuel, is gaining traction with rising concerns about fossil fuels and pollution. Silicon (Si) photocathodes are promising for this process due to their light absorption and favorable band alignment for hydrogen production. However, limitations such as reflectance, low photovoltage, and rapid photocorrosion hinder their overall performance and stability. An interfacial insulating layer such as SiO x ensures superior durability to p -Si photocathodes. However, the inherent heterogeneity and intrinsic defects pose challenges to effective surface passivation. This study investigates the challenges of photocorrosion and utilization of a thin titanium nitride (TiN) passivation layer deposited using direct-current magnetron sputtering to protect the native silicon oxide (SiO x ) layer. Additionally, a molybdenum oxynitride (Mo(O,N) x ) bifunctional layer is employed to enhance both the electrocatalytic activity and durability of the photocathode. The presence of oxygen and nitrogen within this cocatalyst layer modifies the surface chemistry of the p -Si photocathode, promoting favorable interfacial interactions with electrolyte species during PEC reactions. This modification facilitates efficient charge transfer processes and accelerates reaction kinetics, ultimately optimizing the performance and operational lifetime of the photocathode. The resulting Mo(O,N) x /TiN/ p -Si photocathode exhibited a remarkable onset potential of +0.46 V RHE in harsh acidic conditions under simulated sunlight (AM 1.5G illumination, 100 mW·cm -2 ). Notably, the photocathode demonstrated exceptional long-term stability exceeding 140 h, highlighting the combined TiN passivation and Mo(O,N) x cocatalyst as a protection strategy.

A promising strategy to boost electrocatalytic performance is via assembly of hetero-nanostructured electrocatalysts that delivers the essential specific surface area and also active sites by lowering the reaction barrier. However, the challenges associated with the intricate designs and mechanisms remain underexplored. Therefore, the present study constructs a p-n junction in a free-standing MnCo 2 O 4.5 @Ni 3 S 2 on Ni-Foam. The space-charge region's electrical characteristics is dramatically altered by the formed p-n junction, which enhances the electron transfer process for urea-assisted electrocatalytic water splitting (UOR). The optimal MnCo 2 O 4.5 @Ni 3 S 2 electrocatalyst results in greater oxygen evolution reactivity (OER) than pure systems, delivering an overpotential of only 240 mV. Remarkably, upon employing as UOR electrode the required potential decreases to 30 mV. The impressive performance of the designed catalyst is attributed to the enhanced electrical conductivity, greater number of electrochemical active sites, and improved redox activity due to the junction interface formed between p-MnCo 2 O 4.5 and n-Ni 3 S 2 . There are strong indications that the in situ formed extreme-surface NiOOH, starting from Ni 3 S 2, boosts the electrocatalytic activity, i.e., the electrochemical  surface reconstruction generates the active species. In conclusion, this work presents a high-performance p-n junction design for broad use, together with a viable and affordable UOR electrocatalyst., (© 2024 Wiley‐VCH GmbH.)

Microsphere-shaped cobalt selenide (Co 0.85 Se) structures were efficiently synthesized via a two-step hydrothermal process. Initially, cobalt hydroxide fluoride (Co(OH)F) microcrystals were prepared using a hydrothermal method. Subsequently, Co 0.85 Se microsphere-like structures were obtained through selenization. Compared to Co(OH)F, the microsphere-like Co 0.85 Se structure exhibited outstanding catalytic activity for the hydrogen evolution reaction (HER) in a 1.0 M KOH solution. Electrocatalytic experiments demonstrated an exceptional HER performance by the Co 0.85 Se microspheres, characterized by a low overpotential of 148 mV and a Tafel slope of 55.7 mV dec -1 . Furthermore, the Co 0.85 Se electrocatalyst displayed remarkable long-term stability, maintaining its activity for over 24 h. This remarkable performance is attributed to the excellent electrical conductivity of selenides and the highly electroactive sites present in the Co 0.85 Se structure compared to Co(OH)F, emphasizing its promise for advanced electrocatalytic applications.

Hydrogen is one of the most widely used essential chemicals worldwide, and it is also employed in the production of many other chemicals, especially carbon-free energy fuels produced via photoelectrochemical (PEC) water splitting. At present, gallium arsenide represents the most efficient photoanode material for PEC water oxidation, but it is known to either be anodically photocorroded or photopassivated by native metal oxides in the competitive reaction, limiting efficiency and stability. Here, we report chemically etched GaAs that is decorated with thin titanium dioxide (~30 nm-thick, crystalline) surface passivation layer along with nickel-phosphate (Ni-Pi) cocatalyst as a surface hole-sink layer. The integration of Ni-Pi bifunctional co-catalyst results in a highly efficient GaAs electrode with a ~ 100 mV cathodic shift of the onset potential. In this work, the electrode also has enhanced photostability under 110 h testing for PEC water oxidation at a steady current density J ph  > 25 mA·cm -2 . The Et-GaAs/TiO 2 /Ni-Pi║Ni-Pi tandem configuration results in the best unassisted bias-free water splitting device with the highest J ph (~7.6 mA·cm -2 ) and a stable solar-to-hydrogen conversion efficiency of 9.5%., (© 2023. Springer Nature Limited.)

Solar-to-chemical energy conversion is a potential alternative to fossil fuels. A promising approach is the electrochemical (EC) reduction of CO 2 to value-added chemicals, particularly hydrocarbons. Here, we report on the selective EC reduction of CO 2 to CO on a porous Au nanostructure (pAu) cathode in 0.1 M KHCO 3 . The pAu cathode anodized at 2.6 V exhibited maximum Faradaic efficiency (FE) for conversion of CO 2 to CO (up to 100% at -0.75 V vs reversible hydrogen electrode (RHE)). Furthermore, commercial Si photovoltaic cells were combined with EC systems (PV-EC) consisting of pAu cathodes and IrO 2 anodes. The triple-junction cell and EC system resulted in a solar-to-CO conversion efficiency (SCE) of 5.3% under 1 sun illumination and was operated for 100 h. This study provides a PV-EC CO 2 reduction system for CO production and indicates the potential of the PV-EC system for the EC reduction of CO 2 to value-added chemicals., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

A g-C 3 N 4 /CuO nanostructure featuring improved photoelectrochemical properties was successfully prepared using a facile and cost-effective method involving electrodeposition and thermal oxidation. The improved photoelectrochemical properties were mainly ascribed to the increased surface area and improved charge transportation of the g-C 3 N 4 /CuO photocathode. This photocathode can be used in novel strategies for resolving problems associated with low-efficiency CuO photocathodes., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Herein, we report bismuth niobate (BiNbO4), which is regarded as an emerging photoanode material for sustainable photoelectrochemical (PEC) solar energy conversion. BiNbO4 possesses a direct bandgap (Eg) of ∼2.6 eV, and shows an appropriate band alignment for the water oxidation/reduction reaction. In this study, a simple sol-gel route followed by a spin coating method was applied to develop BiNbO4 nanosheets under the optimum annealing conditions. It is known that the annealing temperatures of 500 and 550 °C influence the crystallinity and PEC properties of BiNbO4 films. In particular, the 550 °C annealed film exhibited sharply improved crystalline properties, and rapidly enhanced PEC performance, which were accompanied by a photocurrent density of 0.45 mA cm-2 at 1.23 V vs. the reversible hydrogen electrode (RHE) (briefly abbreviated as 1.23 VRHE) in a strong alkaline solution of 1 M NaOH, compared with 0.26 mA cm-2 at 1.23 VRHE of the 500 °C annealed film. This may be attributed to the main increase of the crystallinity, as well as the improvement of the electronic properties. In addition, the BiNbO4 (550 °C) film showed an incident photon-to-current efficiency of 20% at 425 nm, and produced a stable photoresponse under light illumination in a strong alkaline solution over 5 h, compared with a BiVO4 electrode.

Herein, bimetallic iron (Fe)-manganese (Mn) oxyhydroxide ((Fe 1- x , Mn x )OOH, FeMnOOH) nanosheets on fluorine-doped tin oxide conducting substrates and on semiconductor photoanodes are synthesized by a facile, room temperature, electroless deposition method as catalysts for both electrochemical and photo-electrochemical (PEC) water splitting, respectively. Surprisingly, Mn-doped FeOOH can significantly modulate the nanosheet morphology to increase the active surface area, boost more active sites, and augment the intrinsic activity by tuning the electronic structure of FeOOH. Due to the 2D nanosheet architecture, the optimized FeMnOOH exhibits superior electrochemical activity and outstanding durability for the oxygen evolution reaction with a low overpotential of 246 mV at 10 mA cm -2 and 414 mV at 100 mA cm -2 , and long-term stability for 40 h without decay, which is comparable to the best electrocatalysts for water oxidation reported in the literature. By integrating with semiconductor photoanodes (such as α-Fe 2 O 3 nanorod (NR) arrays), bimetallic FeMnOOH catalysts achieve solar-driven water splitting with a significantly enhanced PEC performance (3.36 mA cm -2 at 1.23 V vs reversible hydrogen electrode (RHE)) with outstanding long-term stability (≈8 h) compared to that of the bare Fe 2 O 3 NR (0.92 mA cm -2 at 1.23 V vs RHE)., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Ru and Pt-Ru (Pt:Ru = 1:1) nanoparticles supported on carbon black were prepared by the borohydride reduction method using oleylamine as a stabilizer in an anhydrous ethanol solvent. We investigated the effect of Pt segregation to the surface of alloy nanoparticles on the methanol oxidation reaction (MOR). As-prepared Pt(1)Ru(1)/C showed a narrow size distribution and a relatively uniform particle distribution on a carbon support. However, its electrocatalytic activity toward the MOR was poor due to the high surface concentration of Ru. As duration time of heat treatment at 200 degrees C was increased up to 2 h, the surface composition of Pt atoms was increased without significant particle growth due to thermally induced segregation of Pt atoms, which were revealed by TEM images, X-ray photoelectron spectroscopy (XPS) analysis, changes in the potentials of zero total charge (pztc), and increase in the oxidation charge of "reduced CO(2)". In particular, from the combination of CO adlayer oxidation and "reduced CO(2)" oxidation charges, the increased surface concentration of Pt of alloy catalysts was relatively quantified when compared to its as-prepared state. Cyclic voltammograms in 0.1 M HClO(4) solution with 0.5 M methanol showed that Pt(1)Ru(1)/C annealed for 2 h at 200 degrees C in a flow of mixture gas of Ar and H(2) (5 vol %) had a less positive onset potential for the MOR. These results demonstrate a definitive contribution from segregation of Pt atoms to the MOR activity.

Highly ordered mesoporous Al(2)O(3)/TiO(2) was prepared by sol-gel reaction and evaporation-induced self-assembly (EISA) for use in dye-sensitized solar cells. The prepared materials had two-dimensional, hexagonal pore structures with anatase crystalline phases. The average pore size of mesoporous Al(2)O(3)/TiO(2) remained uniform and in the range of 6.33-6.58 nm while the Brunauer-Emmett-Teller (BET) surface area varied from 181 to 212 m(2)/g with increasing the content of Al(2)O(3). The incorporation of Al content retarded crystallite growth, thereby decreasing crystallite size while simultaneously improving the uniformity of pore size and volume. The thin Al(2)O(3) layer was located mostly on the mesopore surface, as confirmed by X-ray photoelectron spectroscopy (XPS). The Al(2)O(3) coating on the mesoporous TiO(2) film contributes to the essential energy barrier which blocks the charge recombination process in dye-sensitized solar cells. Mesoporous Al(2)O(3)/TiO(2) (1 mol % Al(2)O(3)) exhibited enhanced power conversion efficiency (V(oc) = 0.74 V, J(sc) = 15.31 mA/cm(2), fill factor = 57%, efficiency = 6.50%) compared to pure mesoporous TiO(2) (V(oc) = 0.72 V, J(sc) = 16.03 mA/cm(2), fill factor = 51%, efficiency = 5.88%). Therefore, the power conversion efficiency was improved by approximately 10.5%. In particular, the increase in V(oc) and fill factor resulted from the inhibition of charge recombination and the improvement of pore structure.

Anodically grown TiO(2) nanotubes (TONTs), approximately 13 microm thick, were prepared on an ethylene glycol-based electrolyte containing 0.25 wt% NH(4)F with extremely small amounts of water. A Ti substrate was pretreated electrochemically, which affected the TONT surface morphology. The TONT (abbreviated as two-step TONT) grown on the pretreated substrate showed a uniform surface morphology with an interconnected nanotubular structure, while the surface morphology of the TONT (abbreviated as one-step TONT) formed on the bare substrate was quite rough. The photocurrent (8.4 mA cm(-2)) of the two-step TONT-based dye-sensitized solar cell (DSSC) was improved by 14% compared to that (7.2 mA cm(-2)) of the one-step TONT-based DSSC. This improvement was attributed mainly to the increased light capturing efficiency, that is, light absorbance by a dye-sensitized TONT film. The discrepancy between the increasing light capturing yield (21%) and overall photocurrent (14%) was attributed to the slower electron transport rate as a result of the large surface area and lateral movement along the three-dimensional network. Therefore, the improved photocurrent of the two-step TONT-based DSSC led to an enhancement (12.5%) of the overall power conversion efficiency.

This paper reports the formation of a thin ZnO energy barrier between a CdSe quantum dot (Q dots) sensitizer and TiO2 nanotubes (TONTs) for improved current efficiency of Q dot-sensitized solar cells. The formation of a ZnO barrier between TONTs and the Q dot sensitizer increased the short-circuit current under illumination and also reduced the dark current in a dark environment. The power conversion efficiency of Q dot-sensitized TONT solar cells increased by 25.9% in the presence of the ZnO thin layer due to improved charge-collecting efficiency and reduced recombination.