Stability of Ag@SiO2 core–shell particles in conditions of photocatalytic overall water-splitting

Core–shell nanoparticles containing plasmonic metals（Ag or Au） have been frequently reported to enhance performance of photo-electrochemical（PEC） devices. However, the stability of these particles in water-splitting conditions is usually not addressed. In this study we demonstrate that Ag@SiOcore–shell particles are instable in the acidic conditions in which WO-based PEC cells typically operate, Ag in the core being prone to oxidation, even if the SiOshell has a thickness in the order of 10 nm. This is evident from in situ voltammetry studies of several anode composites. Similar to the results of the PEC experiments, the Ag@SiOcore–shell particles are instable in slurry-based, Pt/ZnO induced photocatalytic water-splitting. This was evidenced by in situ photodeposition of Ag nanoparticles on the Pt-loaded ZnO catalyst, observed in TEM micrographs obtained after reaction. We explain the instability of Ag@SiOby OH-radical induced oxidation of Ag, yielding dissolved Ag+. Our results imply that a decrease in shell permeability for OH-radicals is necessary to obtain stable, Ag-based plasmonic entities in photo-electrochemical and photocatalytic water splitting.

Tuning the photocatalytic water-splitting performance with the adjustment of diameter in an armchair WSSe nanotube

Due to the enigmatical electrostatic potential difference between the inside and outside layers,the relationship between the diameter and the photocatalytic property of the Janus transition metal dichalcogenides nanotube is still unclear.In this job,for the first time we calculate the electrostatic potential difference of the Janus WSSe armchair nanotubes with corresponding building block models through the first principles calculations.The electrostatic potential difference increases as the diameter increases.Then,it is observed that the WSSe armchair nanotubes with smaller diameter have stronger oxidation capacity,weaker reduction capacity,and higher solar-to-hydrogen conversion efficiency.Furthermore,the diminution of diameter could make the band gap drop,and even cause a direct-indirect transformation of band structure.The adjustment of diameter could also regulate the ability of adsorbing water molecules at the insider and outside layers.Moreover,the suitable band edge positions,wide optical absorbance region(to the near-infrared),outstanding solar-to-hydrogen efficiency(up to 28.99%),high carrier separation,adequate photoexcited carrier driving forces,as well as the energetic and thermal stability,render these nanotubes befitting the photocata lytic water-splitting application.Our study not only predicts a kind of ideal water-splitting photocatalyst,but also shows an effective way to improve their photocatalytic performances.

Enhanced Visible-Light-Sensitive Two-Step Overall Water-Splitting Based on Band Structure Controls of Titanium Dioxide and Strontium Titanate

Visible light-induced two-step overall water-splitting was achieved by combining two types of photocatalysts, which were prepared by introducing foreign elements into rutile titanium dioxide (TiO2) and strontium titanate (SrTiO3) with a controlled electronic band structure. Rutile TiO2 and SrTiO3 were doped with chromium and tantalum (Cr,Ta-TiO2) and with rhodium (Rh-SrTiO3), respectively, to introduce visible-light sensitivity. Under irradiation with only visible light from a 420-nm LED lamp, the simultaneous liberation of hydrogen and oxygen with a molar ratio of ~2:1 was achieved with these two types of photocatalysts in the presence of iodate ion/iodide ion as a redox mediator.

H2 Generation from Thermochemical Water-Splitting Using Sol-Gel Derived Ni-Ferrite

H2 generation from a thermochemical water-splitting reaction was performed using a sol-gel derived Ni-ferrite. The sol-gel synthesis involved addition of nickel chloride hexahydrate （NiCl2@6H2O） and ferrous chloride tetrahydrate （FeCl2·4H2O） in ethanol followed by gelation using propylene oxide. The gels were aged, dried and calcined at 900 ℃in air or N2 environment. The powders thus obtained were characterized using X-ray diffraction （XRD）. This analysis revealed a nominally phase pure Ni-ferrite （NiFe204） composition for the gels calcined in air, whereas those calcined in N2 environment exhibited primarily Ni04Fe2.604 composition mixed with metallic Ni. Particle size and specific surface area （SSA） of the ferrite powders were analyzed using scanning electron microscopy （SEM） and Brauner-Emmett-Teller （BET） surface area analyzer, respectively. The ferrites were placed in a packed bed reactor and water-splitting reaction was carried out at 700 ℃, 800 ℃, and 900 ℃. After water-splitting reaction, oxidized ferrites were regenerated at 900 ℃ for 2 h in N2 environment. Together water-splitting and regeneration steps designated as one thermochemical cycle. In four consecutive thermochemical cycles performed using NiFe204, an average of 40 mL of H2/g per cycle was generated at water-splitting temperature of 900 ℃, which was about five times higher than the average H2 produced at 700 ℃.

Metal-organic framework coated titanium dioxide nanorod array p-n heterojunction photoanode for solar water-splitting

This paper presents a p-n heterojunction photoanode based on a p-type porphyrin metal-organic framework (MOF) thin film and an n-type rutile titanium dioxide nanorod array for photoelectrochemical water splitting. The TiO2@MOF core-shell n anorod array is formed by coati ng an 8 nm thick MOF layer on a vertically aligned TiO2 nanorod array scaffold via a layer-by-layer self-assembly method. This vertically aligned core-shell nanorod array enables a long optical path length but a short path length for extraction of photogenerated minority charge carriers (holes) from TiO2 to the electrolyte. A p-n junction is formed between TiO2 and MOF, which improves the extraction of photogenerated electr ons and holes out of the TiO2 nano rods. In additi on, the MOF coati ng sign ificantly improves the efficie ncy of charge in jecti on at the photoanode/electrolyte interface. Introduction of Co(lll) into the MOF layer further enhances the charge extraction in the photoanode and improves the charge injection efficiency. As a result, the photoelectrochemical cell with the TiO2@Co-MOF nanorod array photoanode exhibits a photocurrent density of 2.93 mA/cm^2 at 1.23 V (vs. RHE), which is ~ 2.7 times the photocurrent achieved with bare T1O2 nanorod array under irradiation of an unfiltered 300 W Xe lamp with an output power density of 100 mW/cm^2.

3D urchin like V-doped CoP in situ grown on nickel foam as bifunctional electrocatalyst for efficient overall water-splitting

Cobalt phosphide (CoP) is considered to be a potential candidate in the field of electrocatalysis due to its low-cost, abundant resources and high electrochemical stability. However, there is a great space for further improvement of its electrocatalytic performance since its charge transfer rate and catalytic activity have not reached a satisfactory level. Herein, we design and fabricate a three dimensional urchins like V-doped CoP with different amounts of V-doping on nickel foam electrode. The V-doped CoP/NF electrode with optimized amounts of V-doping (10%) exhibits outstanding hydrogen evolution reaction (HER) performance under universal-pH conditions and preeminent oxygen evolution reaction (OER) performance in alkaline media. Notably, the assembled water-splitting cell displays a cell voltage of only 1.53 V at 10 mA·cm−2 and has excellent durability, much better than many reported related bifunctional catalysts. The experiment results and theoretical analysis revealed that vanadium atoms replace cobalt atoms in CoP lattice. Vanadium doping can not only raise the density of electronic states near the Fermi level enhancing the conductivity of the catalyst, but can also optimize the free energy of hydrogen and oxygen-containing intermediates adsorption over CoP, thus promoting its catalytic activity. Moreover, the unique nanostructure of the catalyst provides the various shortened channels for charge transfer and reactant/electrolyte diffusion, which accelerates the electrocatalytic process. Also, the in situ growth strategy can improve the conductivity and stability of the catalyst.

A visible-light-photocatalytic water-splitting strategy for sustainable hydrogenation/deuteration of aryl chlorides

Hydrogenation/deuteration of carbon chloride(C–Cl)bonds is of high significance but remains a remarkable challenge in synthetic chemistry,especially using safe and inexpensive hydrogen donors.In this article,a visible-light-photocatalytic watersplitting hydrogenation technology(WSHT)is proposed to in-situ generate active H-species(i.e.,Had)for controllable hydrogenation of aryl chlorides instead of using flammable H2.When applying heavy water-splitting systems,we could selectively install deuterium at the C–Cl position of aryl chlorides under mild conditions for the sustainable synthesis of high-valued added deuterated chemicals.Sub-micrometer Pd nanosheets(Pd NSs)decorated crystallined polymeric carbon nitrides(CPCN)is developed as the bifunctional photocatalyst,whereas Pd NSs not only serve as a cocatalyst of CPCN to generate and stabilize H(D)-species but also play a significant role in the sequential activation and hydrogenation/deuteration of C–Cl bonds.This article highlights a photocatalytic-WSHT for controllable hydrogenation/deuteration of low-cost aryl chlorides,providing a promising way for the photosynthesis of high-valued added chemicals instead of the hydrogen evolution.

Antarctic red algae in dye-sensitized photoelectrochemical cells for water splitting

Phycoerythrin extracted from Antarctic red seaweeds shows promising characteristics to be applied as an anode sensitizer in water-splitting photoelectrochemical cells.Under light irradiation and using an LED lamp,the red-colored protein shows an interesting ability to profit the incident light,as confirmed by the presence of oxygen bubbles next to the electrode surface without applying any external potential.Our results showed that the addition of iodide is helpful to allow the regeneration of the dye;nevertheless,oxygen evolution is not favored.Thermodynamics analysis of the involved semi-reactions is also helpful to understand the observed results.The exploration of Antarctic resources offers then an alternative for the development of green energies,with a particular focus on their use as sensitizers to profit from the sunlight in water-splitting as well as in photovoltaic devices.

Co-Ru alloy nanoparticles decorated onto two-dimensional nitrogen doped carbon nanosheets towards hydrogen/oxygen evolution reaction and oxygen reduction reaction

Constructing highly-efficient electrocatalysts toward hydrogen evolution reaction(HER)/oxygen evolution reaction(OER)/oxygen reduction reaction(ORR)with excellent stability is quite important for the development of renewable energy-related applications.Herein,Co-Ru based compounds supported on nitrogen doped two-dimensional(2D)carbon nanosheets(NCN)are developed via one step pyrolysis procedure(Co-Ru/NCN)for HER/ORR and following low-temperature oxidation process(Co-Ru@RuO_(x)/NCN)for OER.The specific 2D morphology guarantees abundant active sites exposure.Furthermore,the synergistic effects arising from the interaction between Co and Ru are crucial in enhancing the catalytic performance.Thus,the resulting Co-Ru/NCN shows remarkable electrocatalytic performance for HER(70 mV at 10 mA cm^(-2))in 1 M KOH and ORR(half-wave potential E_(1/2)=0.81 V)in 0.1 M KOH.Especially,the Co-Ru@RuO_(x)/NCN obtained by oxidation exhibits splendid OER performance in both acid(230 mV at 10 mA cm^(-2))and alkaline media(270 mV at 10 mA cm^(-2))coupled with excellent stability.Consequently,the fabricated two-electrode water-splitting device exhibits excellent performance in both acidic and alkaline environments.This research provides a promising avenue for the advancement of multifunctional nanomaterials.

Highly efficient mixed-metal spinel cobaltite electrocatalysts for the oxygen evolution reaction

Cation substitution in spinel cobaltites(e.g.,ACo2O4,in which A=Mn,Fe,Co,Ni,Cu,or Zn)is a promising strategy to precisely modulate their electronic structure/properties and thus improve the corresponding electrochemical performance for water splitting.However,the fundamental principles and mechanisms are not fully understood.This research aims to systematically investigate the effects of cation substitution in spinel cobaltites derived from mixed-metal-organic frameworks on the oxygen evolution reaction(OER).Among the obtained ACo2O4 catalysts,FeCo2O4 showed excellent OER performance with a current density of 10 mA·cm^-2 at an overpotential of 164 mV in alkaline media.Both theoretical calculations and experimental results demonstrate that the Fe substitution in the crystal lattice of ACo2O4 can significantly accelerate charge transfer,thereby achieving enhanced electrochemical properties.The crystal field of spinel ACo2O4,which determines the valence states of cations A,is identified as the key factor to dictate the OER performance of these spinel cobaltites.

Novel Bi2MoO6 Nanosheets/Vertical TiO2 Nanorods Arrays Heterojunction with Enhanced Photoelectrochemical Performance under Visible Light Irradiation

Novel Bi2MoO6/TiO2 heterojunction was fabricated by growing Bi2MoO6 nanosheets arrays on the vertically aligned TiO2 nanorods arrays via a two-step solvothermal method. The obtained Bi2MoO6/TiO2 hierarchical heterojunction showed excellent visible light photoelectrochemical performance. Compared with the pure TiO2 and Bi2MoO6, the photocurrent density of the heterojunction was increased 57 and 29 times, respectively. Furthermore, the hydrogen generation rate of the Bi2MoO6/TiO2 for photoelectrocatalytic water-splitting was about 6 times higher than that of the pure Bi2MoO6. The improved performance can be attributed to the synergistic effects of enhanced absorption of visible light, increase of migration rate and separation efficiency of photo-induced carriers.

Odd-membered cyclic hetero-polyoxotitanate nanoclusters with high stability and photocatalytic H_(2) evolution activity

We investigated the hydrolysis of TiⅣ along with naturally abundant AlⅢ ions and reported the formation of a stable and semiconducting nanocluster. Interestingly, this compound exhibits an unusual odd-membered ring structure and also represents the largest Al-containing polyoxotitanium cluster(PTC) observed thus far. The presence of a shell of organic ligands as well as the incorporation of hetero-AlⅢ ions endowed the nanocluster with high air, thermal, and pH stabilities. The present compound exhibited a record photocatalytic hydrogen evolution of 402.88 μmol g–1 h–1 among PTC materials. This work not only paves the way towards stable PTC materials but also provides new insights into the design of novel photocatalysts.

Applied bias photon-to-current conversion efficiency of ZnO enhanced by hybridization with reduced graphene oxide

The role of reduced graphene oxide（rGO） in the enhancement of photo-conversion efficiency of ZnO films for photoelectrochemical（PEC） water-splitting applications was analyzed. ZnO and rGO-hybridized ZnO（rGO/ZnO） films were prepared via a two-step electrochemical deposition method followed by annealing at 300 °C under argon gas flow. The physical, optical and electrochemical properties of the films were characterized to identify the effect of rGO-hybridization on the applied bias photon-to-current efficiency（ABPE） of ZnO. Scanning electron microscopy and X-ray diffraction indicated the formation of verticallyaligned, wurtzite-phase ZnO nanorods. Diffuse-reflectance UV–visible spectroscopy indicated that rGO-hybridization was able to increase the light absorption range of the rGO/ZnO film. UPS analysis showed that hybridization with rGO increased the band gap of ZnO（3.56 eV） to 3.63 eV for rGO/ZnO sample,which may be attributed to the Burstein–Moss effect. Photoluminescence（PL） spectra disclosed that rGOhybridization suppressed electron-hole recombination due to crystal defects. Linear sweep voltammetry of the prepared thin films showed photocurrent density of 1.0 and 1.8 m A/cm;for ZnO and rGO/ZnO at+0.7 V, which corresponded to an ABPE of 0.55% and 0.95%, respectively. Thus, this report highlighted the multi-faceted role of rGO-hybridization in the enhancement of ZnO photo-conversion efficiency.

Enhancement of Oxygen Evolution Activity of Ruddlesden-Popper-Type Strontium Ferrite by Stabilizing Fe4⁺

Development of active iron based water oxidation for designing an ideal artificial photosynthesis devices operating under benign neutral pH is highly demanded. We investigated the electrocatalytic activity of Ruddlesden-Pop-per-type strontium ferrite (Sr3Fe2O7) toward the oxygen evolution reaction (OER). Owing to the temperature-dependent efficiency of the charge disproportionation of Fe4+, the OER activity of Sr3Fe2O7 varied with the temperature, and the onset potential for the OER at a neutral pH underwent a negative shift of approximately 200 mV by increasing the temperature for the stabilization of Fe4+. When metal substitution was made to Sr3Fe2O7 for stabilizing Fe4+ at room temperature, the temperature dependence of the OER activity disappeared and the OER was driven at a small overpotential without increasing the temperature, indicating that the stabilization of Fe4+ is substantially important for achieving high OER activity.

Study on Hydrogen Production System by Coupling with DSSCs

The consumption of dye-sensitized solar cells (DSSCs) used to produce hydrogen, compared with the traditional water-splitting energy, is much less. First of all it is because of DSSCs’ low cost, easy fabrication process, high conversion efficiency and good stability;secondly it also solves the problem of serious corrosion of the electrode, for the entire solar system is in the air. We use three tandem dye-sensitized photovoltaic cells as a source of power;the open circuit voltage of photoelectric unit shows the feasibility of using dye-sensitized photovoltaic cell decomposition of water to produce hydrogen.

A functional hydrogenase mimic that catalyzes robust H_(2) evolution spontaneously in aqueous environment

Although great progress has been made in improving hydrogen production,highly efficient catalysts,which are able to produce hydrogen in a fast and steady way at ambient temperature and pressure,are still in large demand.Here,we report a[NiCo]-based hydrogenase mimic,NiCo_(2)O_(4) nanozyme,that can catalyze robust hydrogen evolution spontaneously in water without external energy input at room temperature.This hydrogenase nanozyme facilitates water splitting reaction by forming a three-center Ni-OH-Co bond analogous to the[NiFe]-hydrogenase reaction by using aluminum as electron donor,and realizes hydrogen evolution with a high production rate of 915 L·h^(-1) per gram of nanozymes,which is hundreds of times higher than most of the natural hydrogenase or hydrogenase mimics.Furthermore,the NiCo_(2)O_(4) nanozyme can robustly disrupt the adhesive oxidized layer of aluminum and enable the full consumption of electrons from aluminum.In contrast to the often-expensive synthetic catalysts that rely on rare elements and consume high energy,we envision that this NiCo_(2)O_(4) nanozyme can potentially provide an upgrade for current hydrogen evolution,accelerate the development of scale-up hydrogen production,and generate a clean energy future.

Heterobimetallic[NiCo]integration in a hydrogenase mimic for boosting light-driven hydrogen evolution in CaTiO_(3)

Light-drive hydrogen production using titanium-based perovskite is one sustainable way to reduce current reliance on fossil fuels,but its wide applications are still limited by high electron−hole recombination and sluggish surface reaction.Thus,the developments for low-cost and highly efficient co-catalysts remain urgent.Inspired by natural[NiFe]-hydrogenase active center structure,a hydrogenase-mimic,NiCo_(2)S_(4) nanozyme was synthesized,and subsequently decorated onto the CaTiO_(3) to catalyze the hydrogen evolution reaction(HER).Among the following test,CaTiO_(3)with a 15%loading of NiCo_(2)S_(4) nanozyme exhibited the highest HER rate of 307.76μmol·g^(–1)·h^(–1),which is 60 times higher than that of the CaTiO_(3) alone.The results reveal that NiCo_(2)S_(4) not only significantly increased the charge separation efficiency of the photogenerated carriers,but also substantively lowered the HER activation energy.Mechanism studies show that NiCo_(2)S_(4) readily splits H_(2)O by forming the Ni(OH)-Co intermediate and only Ni in the bimetallic center alters the oxidation state during the HER process in a manner analogous to the[NiFe]-hydrogenase.In contrast to the often-expensive synthetic catalysts that rely on rare elements such as ruthenium and platinum,this study shows a promising way to develop the nature-inspired cocatalysts to enhance the photocatalysts’HER performance.

Engineering Cu_(1.96)S/Co_(9)S_(8) with sulfur vacancy and heterostructure as an efficient bifunctional electrocatalyst for water splitting

Defect and interface engineering have been recognized as efficient strategies for developing high-performance electrocatalysts.However,it is still challenging to couple defect and interface engineering in transition metal sulfides and understand their dynamic evolution process during electrocatalysis.Herein,we developed one-step pyrolysis of bimetallic sulfide to construct S vacancy-rich Cu_(1.96)S/Co_(9)S_(8) heterostructure by controlling the critical decomposition temperature.The as-synthesized Cu_(1.96)S/Co_(9)S_(8) exhibits excellent bifunctional electrocatalytic performance,with a low overpotential of 99 and 200 mV at 10 mA cm−2 towards hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)in 1.0 mol/L KOH electrolyte,respectively.A symmetric two-electrode cell with Cu_(1.96)S/Co_(9)S_(8) delivered a current density of 10 mA cm^(−2) at a low voltage of 1.43 V and showed long-term stability for 200 h.A series of in/ex-situ techniques revealed that the electrochemical reconfiguration only appeared in the OER process,resulting in the CoOOH/CuO and SO42−species promoting OER performance.Meanwhile,the S vacancy and heterostructure interface in Cu_(1.96)S/Co_(9)S_(8) were proved to optimize the electronic structure and the adsorption of intermediates for HER by density function theory(DFT)simulations.This work provides a promising strategy to construct metal sulfides with rich defects and heterogeneous interfaces and understand their dynamic evolution process for electrochemical storage and conversion devices.

New Janus structure photocatalyst having widely tunable electronic and optical properties with strain engineering

Photoelectrochemical water splitting using solar energy,generating oxygen and hydrogen is one of the clean fuel production processes.Inspired by surface-dependent characteristics of Janus structures,a newly designed Janus monolayer Silicon Phosphorous Arsenide(SiPAs)was analyzed with Density Functional Theory(DFT)methods.Hybrid exchange-correlation functional(HSE06)combined with Wannier90-based analysis for electronic and optical properties of SiPAs reveals that it can act as a photocatalyst.SiPAs show an indirect bandgap of 1.88 eV,absorbing visible light range is 350 to 500 nm.The phonon spectrum confirms dynamic stability.The exciton binding energy is computed with GW/BSE methods.The electronic band edge positions are at-5.75 and-4.43 eV,perfectly straddling the water redox potentials.Interestingly the strain application modifies the bandgap and also non-homogenously widens the absorption band.A novel range of photocatalyst designs with Group IV-V elements with great promise for water-splitting,photovoltaic,and narrow bandgap semiconductor(optoelectronics)applications may be feasible.

Cobalt diselenide (001) surface with short-range Co-Co interaction triggering high-performance electrocatalytic oxygen evolution

Oxygen evolution reaction (OER) still suffers from the bottleneck in electrocatalytic water splitting. Herein, in virtue of volcano plots drawn by theoretical calculation, the (001) facet was screened as the superb facet of orthorhombic CoSe_(2) for OER. Afterwards, CoSe_(2)(001) nanosheets were synthesized and the exposure ratio of (001) facet is controllable with thermodynamics methods effectively. The single-facet CoSe2(001) delivered an overpotential as low as 240 mV at 10 mA·cm^(−2) in 1 M KOH, which outperformed the bulk (380 mV) as well as other CoSe_(2)-base OER catalysts reported before. Especially, a shorter Co-Co path was observed in CoSe_(2)(001) by X-ray absorption spectroscopy. Further density functional theory (DFT) studies revealed that the reversible compression on the shorter Co-Co path could regulate the electronic structure of active sites during the OER process, and thus the energy barrier of the rate-determining step was reduced by 0.15 eV. This work could inspire more insights on the modification of electronic structure for OER electrocatalysts.