Rate Law for Photoelectrochemical Water Splitting Over CuO

Photocatalytic splitting of water over p-type semiconductors is a promising strategy for production of hydrogen.However,the determination of rate law is rarely reported.To this purpose,copper oxide(CuO)is selected as a model photocathode in this study,and the photogenerated surface charge density,interfacial charge transfer rate constant and their relation to the water reduction rate(in terms of photocurrent)were investigated by a combination of(photo)electrochemical techniques.The results showed that the charge transfer rate constant is exponential-dependent on the surface charge density,and that the photocurrent equals to the product of the charge transfer rate constant and surface charge density.The reaction is first-order in terms of surface charge density.Such an unconventional rate law contrasts with the reports in literature.The charge density-dependent rate constant results from the Fermi level pinning(i.e.,Galvani potential is the main driving force for the reaction)due to accumulation of charge in the surface states and/or Frumkin behavior(i.e.,chemical potential is the main driving force).This study,therefore,may be helpful for further investigation on the mechanism of hydrogen evolution over a CuO photocathode and for designing more efficient CuO-based photocatalysts.

21世纪的制氢技术

世界能耗的急剧上升迫使人类努力开发化石燃料以外的新能源，其中氢能被认为是２１世纪最有发展前途的二次能源。本文介绍了近年来各国科学家正在研究的新的制氢技术，这些新方法将构成２１世纪制氢技术的主体。

Electrodeposition of Cu_2O/g-C_3N_4 heterojunction film on an FTO substrate for enhancing visible light photoelectrochemical water splitting

An immobilized Cu2O/g-C3N4 heterojunction film was successfully made on an FTO substrate by electrophoretic deposition of g-C3N4 on a Cu2O thin film.The photoelectrochemical（PEC） performance for water splitting by the Cu2O/g-C3N4 film was better than pure g-C3N4 and pure Cu2O film.Under-0.4 V external bias and visible light irradiation,the photocurrent density and PEC hydrogen evolution efficiency of the optimized Cu2O/g-C3N4 film was-1.38 mA/cm^2 and 0.48 mL h^-1 cm^-2,respectively.The enhanced PEC performance of Cu2O/g-C3N4 was attributed to the synergistic effect of light coupling and a matching energy band structure between g-C3N4 and Cu2O as well as the external bias.

Further revealing of the photo-dissociation of form aldehyde

Dr. YIN Hongming with the CAS Dalian Institute of Chemical Physics has discovered two main reaction paths of formaldehyde's photochemical decomposition. His latest research progress, which was published in the first issue of Chinese Science Bulletin in 2008, may shed new light on the reaction mechanism of its radical channel. Formaldehyde is a prominent carcinogenic pollutant in the atmosphere. Its degradation is mainly realized by photodissociation, a chemical reaction involving sunlight in which molecules are split into their constituent atoms. A deeper understanding of its photo-chemical reactions and evolutionary course is of special significance in the control of air pollution.

Conformal BiVO_(4)/WO_(3) nanobowl array photoanode for efficient photoelectrochemical water splitting

As one of the most promising photoanode candidates for photoelectrochemical(PEC)water splitting,the photocurrent density of BiVO_(4) still needs to be further improved in order to meet the practical application.In this work,a highly‐matched BiVO_(4)/WO_(3) nanobowl(NB)photoanode was constructed to enhance charge separation at the interface of the junction.Upon further modification of the BiVO_(4)/WO_(3)NB surface by NiOOH/FeOOH as an oxygen evolution cocatalyst(OEC)layer,a high photocurrent density of 3.05 mA cm^(−2) at 1.23 V vs.RHE has been achieved,which is about 5‐fold higher than pristine BiVO_(4) in neutral medium under AM 1.5 G illumination.5 times higher IPCE at 450 nm is also achieved compared with the BiVO_(4) photoanode,leading to about 95%faradaic efficiency for both H_(2) and O_(2) gas production.Systematic studies attribute the significantly enhanced PEC performance to the smaller BiVO_(4) particle size(<90 nm)than its hole diffusion length(~100 nm),the improved charge separation of BiVO_(4) by the single layer WO_(3) nanobowl array and the function of OEC layers.Such WO_(3)NB possesses much smaller interface resistance with the substrate FTO glass and larger contact area with BiVO_(4) nanoparticles.This approach provides new insights to design and fabricate BiVO_(4)‐based heterojunction photoanode for higher PEC water splitting performance.

NiFe layered double-hydroxide nanoparticles for efficiently enhancing performance of BiVO_4 photoanode in photoelectrochemical water splitting

A bismuth vanadate(BiVO4)photoanode with a cocatalyst consisting of NiFe layered double‐hydroxide(NiFe‐LDH)nanoparticles was fabricated for photoelectrochemical(PEC)water splitting.NiFe‐LDH nanoparticles,which can improve light‐absorption capacities and facilitate efficient hole transfer to the surface,were deposited on the surface of the BiVO4 photoanode by a hydrothermal method.All the samples were characterized using X‐ray diffraction,scanning electron microscopy,and diffuse‐reflectance spectroscopy.Linear sweep voltammetry and current‐time plots were used to investigate the PEC activity.The photocurrent response of NiFe‐LDH/BiVO4 at 1.23 V vs the reversible hydrogen electrode was higher than those of Ni(OH)2/BiVO4,Fe(OH)2/BiVO4 and pure BiVO4 electrodes under visible‐light illumination.NiFe‐LDH/BiVO4 also gave a superior PEC hydrogen evolution performance.Furthermore,the stability of the NiFe‐LDH/BiVO4 photoanode was excellent compared with that of the bare BiVO4 photoanode,and offers a novel method for solar‐assisted water splitting.

Ni(OH)_(2)quantum dots as a stable cocatalyst modifiedα-Fe_(2)O_(3) for enhanced photoelectrochemical water-splitting

Depositing a cocatalyst has proven to be an important strategy for improving the photoelectrochemical(PEC)water-splitting efficiency of photoanodes.In this study,Ni(OH)2 quantum dots(Ni(OH)2 QDs)were deposited in situ onto anα-Fe_(2)O_(3)photoanode via a chelation-mediated hydrolysis method.The photocurrent density of the Ni(OH)2 QDs/α-Fe_(2)O_(3)photoanode reached 1.93 mA·cm^(−2)at 1.23 V vs.RHE,which is 3.5 times that ofα-Fe_(2)O_(3),and an onset potential with a negative shift of ca.100 mV was achieved.More importantly,the Ni(OH)2 QDs exhibited excellent stability in maintaining PEC water oxidation at a high current density,which is attributed to the ultra-small crystalline size,allowing for the rapid acceptance of holes fromα-Fe_(2)O_(3)to Ni(OH)_(2)QDs,formation of active sites for water oxidation,and hole transfer from the active sites to water molecules.Further(photo)electrochemical analysis suggests that Ni(OH)_(2)QDs not only provide maximal active sites for water oxidation but also suppress charge recombination by passivating the surface states ofα-Fe_(2)O_(3),thereby significantly enhancing the water oxidation kinetics over theα-Fe_(2)O_(3)surface.

Enhanced photoelectrochemical water splitting using a cobalt-sulfide-decorated BiVO_(4) photoanode

Solar-driven water splitting is considered as a promising method to mitigate the energy crisis and various environmental issues.Bismuth vanadate(BiVO_(4))is photoanode material with tremendous potential for photoelectrochemical(PEC)water splitting.However,its PEC performance is severely hindered owing to poor surface charge transfer,surface recombination at the photoanode/electrolyte junction,and sluggish oxygen evolution reaction(OER)kinetics.In this regard,a novel solution was developed in this study to address these issues by decorating the surface of BiVO_(4)with cobalt sulfide,whose attractive features such as low cost,high conductivity,and rapid charge-transfer ability assisted in improving the PEC activity of the BiVO_(4)photoanode.The fabricated photoanode exhibited a significantly enhanced photocurrent density of 3.2 m A cm^(-2)under illumination at 1.23 V vs.a reversible hydrogen electrode,which is more than 2.5 times greater than that of pristine BiVO_(4).Moreover,the Co S/BiVO_(4)photoanode also exhibited considerable improvements in the charge injection yield(75.8%vs.36.7%for the bare BiVO_(4)film)and charge separation efficiency(79.8%vs.66.8%for the pristine BiVO_(4)film).These dramatic enhancements were primarily ascribed to rapid charge-transport kinetics and efficient reduction of the anodic overpotential for oxygen evolution enabled by the surface modification of BiVO_(4)by Co S.This study provides valuable suggestions for designing efficient photocatalysts via surface modification to improve the PEC performance.

Multifarious function layers photoanode based on g-C_3N_4 for photoelectrochemical water splitting

We report on a novel g-C3N4/TiO 2/Co-Pi photoanode combining a TiO2 protection layer, Co-Pi hole capture layer, and g-C3 N4 light-absorption layer layer for photoelectrochemical（PEC） water splitting to generate hydrogen for the first time. This new photoanode with three function layers exhibits enhanced PEC performance with a photocurrent density of 0.346 mA ·cm–2 at 1.1 V（vs. RHE）,which is approximately 3.6 times that of pure g-C3N4 photoanode. The enhanced PEC performance of g-C3N4/TiO 2/Co-Pi photoanode benefits from the following：（1） excellent visible light absorption of g-C3N4;（2） stable protection of TiO2 to improve the durability of g-C3N4 film; and（3） photogenerated holes capture Co-Pi to separate photogenerated electron-hole pairs efficiently. This promising multifarious function layers structure provides a new perspective for PEC water splitting to generate hydrogen.

Dye-sensitized photoanode decorated with pyridine additives for efficient solar water oxidation

Splitting water into hydrogen and oxygen by dye-sensitized photoelectrochemical cell(DSPEC)is a promising approach to solar fuels production.In this study,a series of pyridine derivatives as surface additives were modified on a molecular chromophore and water oxidation catalyst co-loaded TiO_(2)photoanode,TiO_(2)|RuP,1(RuP=Ru(4,4′-(PO3H2)2-2,2′-bipyridine)(2,2′-bipyridine)2,1=Ru(bda)(L)2,(bda=2,2′-bipyridine-6,6′-dicarboxylate,L=10-(pyridin-4-yloxy)decyl)phosphonic acid).The addition of pyridine additives was found to result in up to 42%increase in photocurrent.Under simulated sun-light irradiation,TiO_(2)|RuP,1,P1(P1=4-Hydroxypyridine)produced a photocurrent density of 1 mA/cm2 at a bias of 0.4 V vs.NHE in acetate buffer.Moreover,the observed photocurrents are correlated with the electron-donating ability of the substituent groups on pyridine ring.Transient absorption measurements and electrochemical impedance spectroscopy revealed that surface-bound pyridine can effectively retard the back-electron transfer from the TiO_(2)conduction band to the oxidized dye,which is a major process responsible for energy loss in DSPECs.

Deliquescence and Efflorescence Processes of Aerosol Particles Studied by in situ FTIR and Raman Spectroscopy

Deliquescence and efflorescence are the two most important physicochemical processes of aerosol particles. In deliquescence and efflorescence cycles of aerosol particles, many fundamental problems need to be investigated in detail on the molecular level, including ion and molecule interactions in supersaturated aerosols, metastable solid phases that may be formed, and microscopic structures and deliquescence mechanisms of aerosol particles. This paper presents a summary of the progress made in recent investigations of deliquescence and efflorescence processes of aerosol particles by four common spectral techniques, which are known as Raman/electrodynamic balance, Fourier transform infrared/aerosol flow tube, Fourier transform infrared/attenuated total reftection, and confocal Raman on a quartz substrate.

Photodissociation Dynamics of Methanol and Ethanol at 157 nm

157 nm photodissociation of jet-cooled CH3OH and C2HsOH was studied using the high-n Rydberg atom time-of-flight （TOF） technique. TOF spectra of nascent H atom products were measured. Simulation of these spectra reveals three different atomic H loss processes： one from hydroxyl H elimination, one from methyl （ethyl） H elimination, and one from secondary dissociation of the methoxy （ethoxy） radical. The relative branching ratio indicates secondary dissociation of ethoxy is less important than that of methoxy. The average angular anisotropy parameter of methanol is negative （with β≈-0.3）, indicating the transition dipole moment is perpendicular to the C-O-H plane. The slightly more negative β value of ethanol （with β≈-0.4） implies that ethanol has a longer rotational period. These experimental results indicate that both systems undergo fast internal conversion to the 3s surface after it is excited to the 3px surface, and then dissociate on the 3s surface. The translational energy distribution of the CH3O＋H products reveals extensive CH3 rocking or CH3 umbrella excitation in the CH30 radical. However the vibrational structures are not resolved in the C2H5O radical.

Probing Molecular Dynamics with Ultrafast Electron Diffraction

Recent progress in ultrafast lasers,ultrafast X-rays and ultrafast electron beams has made it possible to watch the motion of atoms in real time through pumpprobe technique.In this review,we focus on how the molecular dynamics can be studied with ultrafast electron diffraction where the dynamics is initiated by a pumping laser and then probed by pulsed electron beams.This technique allows one to track the molecular dynamics with femtosecond time resolution and Angstr6m spatial resolution.We present the basic physics and latest development of this technique.Representative applications of ultrafast electron diffraction in studies of laser-induced molecular dynamics are also discussed.This table-top technique is complementary to X-ray free-electron laser and we expect it to have a strong impact in studies of chemical dynamics.

Electrochemical creation of surface charge transfer channels on photoanodes for efficient solar water splitting

Electrochemical treatment is a popular and efficient method for improving the photoelectrochemical performance of water‐splitting photoelectrodes.In our previous study,the electrochemical activation of Mo‐doped BiVO_(4) electrodes was ascribed to the removal of MoO_(x) segregations,which are considered to be surface recombination centers for photoinduced electrons and holes.However,this proposed mechanism cannot explain why activated Mo‐doped BiVO_(4) electrodes gradually lose their activity when exposed to air.In this study,based on various characterizations,it is suggested that electrochemical treatment not only removes partial MoO_(x) segregations but also initiates the formation of H_(y)MoO_(x) surface defects,which provide charge transfer channels for photogenerated holes.The charge separation of the Mo‐doped BiVO_(4) electrode was significantly enhanced by these charge transfer channels.This study offers a new insight into the electrochemical activation of Mo‐doped BiVO_(4) photoanodes,and the new concept of surface charge transfer channels,a long overlooked factor,will be valuable for the development of other(photo)electrocatalytic systems.

Enabling practical electrocatalyst-assisted photoelectro- chemical water splitting with earth abundant materials

Sustainable development and continued prosperity of humanity hinge on the availability of renewable energy sources on a terawatts scale. In the long run, solar energy is the only source that can meet this daunting demand. Widespread utilization of solar energy faces challenges as a result of its diffusive （hence low energy density） and intermittent nature. How to effectively harvest, concentrate, store and redistribute solar energy constitutes a fundamental challenge that the scientific community needs to address. Photoelectrochemical （PEC） water splitting is a process that can directly convert solar energy into chemical energy and store it in chemical bonds, by producing hydrogen as a clean fuel source. It has received significant research attention lately. Here we provide a concise review of the key issues encountered in carrying out PEC water splitting. Our focus is on the balance of considerations such as stability, earth abundance, and efficiency. Particular attention is paid to the combination of photoelectrodes with electrocatalysts, especially on the interfaces between different components.

Photo-electrochemical water splitting system with three-layer n-type organic semiconductor film as photoanode under visible irradiation

A three-layer structure of n-type organic semiconductors （PTCDA/PTCDA：PCBM/PCBM） is successfully identified as pho- toanode for photoelectrochemical water oxidation during the overall splitting of water into hydrogen/oxygen in a nearly stoi- chiometric ratio （H2：O2 = 2：1） under visible irradiation （2 〉 420 nm）. A possible charge separation mechanism under visible light illumination was also proposed.