Regulation of the photogenerated carrier transfer process during photoelectrochemical water splitting:A review

Photoelectrochemical(PEC)water splitting is considered as an ideal technology to produce hydrogen.Photogenerated carrier migration is one of the most important roles in the whole process of PEC water splitting.It includes bulk transfer inside of the photoelectrode and the exchange at the solid-liquid interface.The energy barriers during the migration process lead to the dramatic recombination of photogenerated hot carrier and the reducing of their redox capacity.Thus,an applied bias voltage should be provided to overcome these energy barriers,which brings the additional loss of energy.Plentiful researches indicate that some methods for the regulation of photogenerated hot carrier,such as p-n junction,unique transfer nanochannel,tandem nanostructure and Z-Scheme transfer structure et al.,show great potential to achieve high-efficient PEC water overall splitting without any applied bias voltage.Up to now,many reviews have summarized and analyzed the methods to enhance the PEC or photocatalysis water splitting from the perspectives of materials,nanostructures and surface modification etc.However,few of them focus on the topic of photogenerated carrier transfer regulation,which is an important and urgent developing technique.For this reason,this review focuses on the regulation of photogenerated carriers generated by the photoelectrodes and summarizes different advanced methods for photogenerated carrier regulation developed in recent years.Some comments and outlooks are also provided at the end of this review.

Construction of charge transfer chain in Bi_(12)TiO_(20)-Bi_(4)Ti_(3)O_(12)/α-Bi_(2)O_(3)composites to accelerate photogenerated charge separation

Photogenerated charge separation and transfer is one of the bottleneck steps in photocatalysis,and efficient charge separation strategies are strongly desired.Here,mimicking the electron transport chain in natural photosynthesis,we report the design and fabrication of a charge transfer chain using bismuth-based semiconductor as a proof-of-concept.In view of the thermodynamic energy band positions and structural similarity based on the density functional theory(DFT)analysis,heterostructured combination ofα-Bi_(2)O_(3),perovskite-like Bi_(4)Ti_(3)O_(12),and sillenite Bi12TiO20 was designed for fabrication of charge transfer chain.By tuning the molar ratio of Bi and Ti precursors,the Bi_(4)Ti_(3)O_(12)and Bi12TiO20 particles were formed on the surface ofα-Bi_(2)O_(3)by an insitu transformation process,giving rise to Bi_(12)TiO_(20)-Bi_(4)Ti_(3)O_(12)/α-Bi_(2)O_(3)composites with charge transfer chain.We propose that the effective charge transfer is accomplished amongα-Bi_(2)O_(3),Bi12TiO20,and Bi_(4)Ti_(3)O_(12),which significantly improves the photogenerated charge separation and transfer,as indicated by photoluminescene,time-resolved photoluminescene,and electrochemical impedance spectra results.As expected,the Bi_(12)TiO_(20)-Bi_(4)Ti_(3)O_(12)/α-Bi_(2)O_(3)shows the superior photocatalytic activity for the degradation of environmental pollutants with high concentration.Even for the refractory pollutants like 4-chlorophenol,the optimal Bi_(12)TiO_(20)-Bi_(4)Ti_(3)O_(12)/α-Bi_(2)O_(3)composite shows 28 times higher than that ofα-Bi_(2)O_(3)for photocatalytic degradation,verifying the superiority of photogenerated charge transfer chain in photocatalysis.This work demonstrates the feasibility of the charge transfer chain strategy to boost the photogenerated charge separation,which is of great significance for designing energy and environmental-related materials in heterogonous photocatalysis.

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.

Synergetic effect of graphene and Co(OH)2 as cocatalysts of TiO2 nanotubes for enhanced photogenerated cathodic protection

A layer of graphene(GR)particles was successfully deposited at the interface between Co(OH)2 nanoparticles and TiO2 nanotubes,aiming to improve the photoelectrochemical performance of the large-bandgap semiconductor TiO2.The obtained Co(OH)2/GR/TiO2 was extensively characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),UV–vis absorption spectra and photoluminescence(PL)emission spectra.Electrochemical impedance spectra,photogenerated potential-time(E-t),photocurrent density-time(i-t)and i-E curves and open circuit potential(OCP)curves were measured to investigate the photoelectrochemical activities and photogenerated cathodic protection properties.The results revealed that Co(OH)2/GR/TiO2 exhibits excellent photoelectrochemical and photogenerated cathodic performance due to synergistic effect between Co(OH)2 and graphene.Co(OH)2 and graphene co-modified TiO2 photoanode could provide an effective protection for 304 stainless steel(304 SS)in 3.5 wt%Na Cl solution for 12 h,which would be promising for future practical applications in the field of marine corrosion protection.

Significantly influenced photocatalytic performance for H_(2)O_(2)generation over ultrathin g-C_(3)N_(4)through regulating the migration orientation of photogenerated charge carriers

H_(2)O_(2)has been widely applied in the fields of chemical synthesis,medical sterilization,pollutant removal,etc.,due to its strong oxidizing property and the avoidable secondary pollution.Despite of the enhanced performance for H_(2)O_(2)generation over g-C_(3)N_(4)semiconductors through promoting the separation of photo-generated charge carriers,the effect of migration orientation of charge carriers is still ambiguous.For this emotion,surface modification of g-C_(3)N_(4)was employed to adjust the migration orientation of charge carriers,in order to investigate systematically its effect on the performance of H_(2)O_(2)generation.It was found that ultrathin g-C_(3)N_(4)(UCN)modified by boron nitride(BN),as an effective hole-attract agent,demonstrated a significantly enhanced performance.Particularly,for the optimum UCN/BN-40%catalyst,4.0-fold higher yield of H_(2)O_(2)was obtained in comparison with the pristine UCN.As comparison,UCN modified by carbon dust demonstrated a completely opposite tendency.The remarkably improved performance over UCN/BN was ascribed to the fact that more photo-generated electrons were remained inside of triazine structure of g-C_(3)N_(4),leading to the formation of larger amount of 1,4-endoxide.It is anticipated that our work could provide new insights for the design of photocatalyst with significantly improved performance for H_(2)O_(2)generation.

Impact of photogenerated charge behaviors on luminescence of Eu^(3+) -incorporated microporous titanosilicate ETS-10

A series of Eu3+ -incorporated ETS-10 samples were successfully prepared based on the traditional ion exchange method. The relationship between photogenerated charge behaviors and luminescent properties has been investigated in detail. It has been demonstrated that as a result of the charge transfer from the titanate quantum wires to Eu3+ crystal field states, the host matrix ETS-10 functions as the sensitizer of Eu3+ to enhance the red luminescence, while Eu3+ cations contribute to the recombination of photogenerated charges. The behavior of photogenerated charges has significant impact on the luminescent properties of Eu3+ -incorporated ETS-10 materials.

Co3O4 Decorated Ti/TiO_(2) Nanotubes for Photogenerated Cathodic Protection of 304 Stainless Steel

The Co_(3)O_(4) decorated TiO_(2) nanotube arrays(NTAs)coatings are fabricated by the combination of anodization and impregnating methods.It is found that the introduction of Co_(3)O_(4) can reduce the diffraction intensity of(101)plane of the TiO_(2) and accelerate the separation of photogenerated electron/hole pairs.In addition,the open circuit potential(OCP)and the corrosion potential of 304 stainless steel(304SS)with or without Co_(3)O_(4) decorated TiO_(2) NTAs were measured under visible light,which indicated the 304SS coupled with Co_(3)O_(4) decorated TiO_(2) NTAs had better anticorrosion performance than that of the 304SS or the 304SS coupled with pure TiO_(2) NTAs.The enhancement of the cathodic protection performance of the Co_(3)O_(4) decorated TiO_(2) NTAs can be ascribed to the matched energy levels and strong interaction between Co_(3)O_(4) and TiO_(2) NTAs,and the improvement of light absorption.

An optical tweezer-based microdroplet imaging technology

Microspheres can break the diffraction limit and magnify nano-structure imaging,and with its advantages of low cost and label-free operation,microsphere-assisted imaging has become an irreplaceable tool in the life sciences and for precision measurements.However,the tiny size and limited imaging field of traditional solid microspheres cause difficulties when imaging large sample areas.Alternatively,droplets have similar properties to those of microspheres,with large surface curvature and refractive-index difference from the surrounding environment,and they can also serve as lenses to focus light for observation and imaging.Previous work has shown that droplets with controllable size can be generated using an optical tweezer system and can be driven by optical traps to move precisely like solid microspheres.Here,a novel microdroplet-assisted imaging technology based on optical tweezers is proposed that better integrates the generation,manipulation,and utilization of droplets.

A p-n WO_(3)/SnSe_(2) Heterojunction for Efficient Photo-assisted Electrocatalysis of the Oxygen Evolution Reaction

Water splitting is important to the conversion and storage of renewable energy,but slow kinetics of the oxygen evolution reaction(OER)greatly limits its utility.Here,under visible light illumination,the p-n WO_(3)/SnSe_(2)(WS)heterojunction significantly activates OER catalysis of CoFe-layered double hydroxide(CF)/carbon nanotubes(CNTs).Specifically,the catalyst achieves an overpotential of 224 mV at 10 mA cm^(-2)and a small Tafel slope of 47 mV dec^(-1),superior to RuO_(2)and most previously reported transition metal-based OER catalysts.The p-n WS heterojunction shows strong light absorption to produce photogenerated carriers.The photogenerated holes are trapped by CF to suppresses the charge recombination and facilitate charge transfer,which accelerates OER kinetics and boost the activity for the OER.This work highlights the possibility of using heterojunctions to activate OER catalysis and advances the design of energy-efficient catalysts for water oxidation systems using solar energy.

Enhanced photocataly tic activity of TiO_2 by surface fluorination in degradation of organic cationic compound

Experiments were carried out to investigate the influence of TiO2 surface fluorination on the photodegradation of a representative organic cationic compound, Methylene Blue （MB）. The eleetropositive MB shows poor adsorption on TiO2 surface; its degradation performs a HO. radical-mediated mechanism. In the F-modified system, the kinetic reaction rate enlarged more than 2.5 fold that was attributed mainly to the accumulating adsorption of MB and the increased photogenerated hole available on the F-modified TiO2 surface.

In-situ fabrication SnO2/SnS2 heterostructure for boosting the photocatalytic degradation of pollutants

Heterostructure photocatalysts with a built-in electric field have become one of the most promising strategies to enhance photogenerated electron-hole pair separation. However, close contact between the two active components of heterogeneous photocatalysts remains a problem. Herein, the in-situ fabrication of an SnO2/SnS2 heterostructure photocatalyst was performed;the structure showed enhanced photocatalytic performance resulting from the tight-contact heterostructures. The results of photoelectrochemical measurements further verified that a tight-contact heterostructure improved the separation of photogenerated electron-hole pairs. The results of EIS Bode plots also demonstrated that such in-situ fabricated SnO2/SnS2 samples exhibited the longest carrier lifetime(41.6 μs) owing to the intimate interface of SnO2/SnS2 heterostructures.

A thioether-functionalized pyrene-based covalent organic framework anchoring ultrafine Au nanoparticles for efficient photocatalytic hydrogen generation

Covalent organic frameworks(COFs)have lately emerged as a blooming class of potential materials for photocatalytic water splitting because of their high crystallinity,huge surface areas,and structural versatility.However,the photocatalytic performance for most pure COFs face some limitations factors,such as the significant recombination of photogenerated carriers and slow charge transfer.Herein,a novel thioether-functionalized pyrene-based COF(S_(4)-COF)was effectively produced and chosen as a support for the immobilization of ultrafine gold nanoparticles(Au NPs).S_(4)-COF photocatalyst with Au as cocatalyst demonstrates remarkable photocatalytic activity with a H_(2) generation rate of 1377μmol g^(−1) h^(−1) under visible light(>420 nm),which is ca.4.5-fold increase comparing to that of pure S_(4)-COF(302μmol g^(−1) h^(−1)).Au NPs anchored on S_(4)-COF possess an ultrafine size distribution ranging from 1.75 to 6.25 nm with an average size centered at 3.8 nm,which benefits from the coordination interaction between thioether groups and Au.Meanwhile,the produced Au@S_(4)-COF can generate a stable photocatalytic H_(2) generation during the four recycles and preserve its crystallinity structure after the stability testing.The Au NPs anchored on the S_(4)-COF photocatalyst can greatly accelerate the separation of photogenerated carriers and increase charge transfer because of the combined function of Au NPs and thioether groups.Such a method can not only prevent the aggregation of Au NPs onto thioether-containing COFs to achieve long-term photostability but also allow uniform dispersion for an ordered structure of photocatalysts.This work provides a rational strategy for designing and preparing COF-based photocatalysts for solar-driven H_(2) production.

Preparation of semiconductor zinc telluride by photoelectrochemical deposition

With the continuous development of electronic industry, people’s demand for semiconductor materials is also increasing. How to prepare semiconductor materials with low cost, low energy consumption and high yield has become one of the hot spots of research. ZnTe is commonly used in the semiconductor industry due to its superior optoelectronic properties. Electrochemical deposition is one of the most frequently used methods to prepare ZnTe thin films. However,the traditional electrochemical deposition technology has many shortcomings, such as slow deposition rate and poor film quality. These hinder the large-scale promotion of zinc telluride electrochemical deposition technology. To solve the problems encountered in the preparation of semiconductor thin films by conventional electrochemical deposition, and based on the photoconductive properties of semiconductor materials themselves, the basic principles of photoelectrochemistry of semiconductor electrodes, and some characteristics of the electrochemical deposition process of semiconductor materials, the use of photoelectrochemical deposition method for the preparation of semiconductor materials was proposed. Firstly, the electrochemical behaviors(electrode reactions, nucleation growth and charge transport process) of the ZnTe electrodeposition under illumination and dark state conditions were studied. Then, the potentiostatic deposition of ZnTe was carried out under light and dark conditions. The phase structure, morphology and composition of the sediments were studied using X-ray diffractometer, scanning electron microscope and other testing methods. Finally, the photoelectrochemical deposition mechanisms were analyzed. Compared with conventional electrochemical deposition, photoelectrochemical deposition increases the current density during deposition and reduces the charge transfer impedance during ZnTe deposition process. In addition, since light illumination promotes the deposition of the difficult-to-deposit element Zn, the component ratio of ZnTe thin films prepared by photoelectrochemical deposition is closer to 1:1, making it a viable and reliable approach for ZnTe production.

Photo-induced self-formation of dual-cocatalysts on semiconductor surface

Cocatalyst plays key roles in photogenerated charge separation and surface catalytic reactions in photocatalysis.However,it is not clear if the chemical states of cocatalysts changed or remains unchanged under photocatalytic reaction conditions.Herein,taking NaTaO3 as an example,we systemically investigated the chemical states of nickel‐based cocatalysts during photocatalytic water splitting reaction.It was found that photo‐induced self‐formation of Ni and NiO cocatalyst species take place on the surface of NaTaO3 nanocrystals.The self‐formation of dual‐cocatalysts not only occurs on 26‐facet NaTaO3,but also takes place on a more general 6‐facet NaTaO3.Our work clarified that the chemical states of cocatalysts are changing and the redox dual‐cocatalysts are redistributed on the semiconductor surface owing to the reaction induced by photogenerated charges under the condition of photocatalytic reactions.

Optical-induced dielectric tunability properties of DAST crystal in THz range

The optical-induced dielectric tunability properties of DAST crystal in THz range were experimentally demonstrated.The DAST crystal was grown by the spontaneous nucleation method(SNM) and characterized by infrared spectrum. With the optimum wavelength of the exciting optical field, the transmission spectra of the DAST crystal excited by 532 nm laser under different power were measured by terahertz time-domain spectroscopy(THz-TDS) at room temperature. The transmitted THz intensity reduction of 26 % was obtained at 0.68 THz when the optical field was up to 80 m W. Meanwhile,the variation of refractive index showed an approximate quadratic behavior with the exciting optical field, which was related to the internal space charge field of photorefractive phenomenon in the DAST crystal caused by the photogenerated carrier.A significant enhancement of 13.7 % for THz absorption coefficient occurred at 0.68 THz due to the photogenerated carrier absorption effect in the DAST crystal.

Exploring the mechanism of Ta_(3)N_(5)/KTaO_(3) photocatalyst for overall water splitting by first-principles calculations

The rational fabrication of heterostructures is one of efficient strategies for improving photocatalytic performance of semiconductor photocatalysts.Very recently,Domen and co-workers found that Ta_(3)N_(5) single crystals grown on the surface of KTaO_(3) can accomplish photocatalytic overall water splitting for the first time.In order to comprehend the underlying mechanism of this photocatalytic system,we have performed a systematic study based on density functional theory first-principles calculations.Ta_(3)N_(5)(010)/KTaO_(3)(110)slab models have been built according to experimental observations by considering two common terminations of KTaO_(3)(110)surface,named as Ta_(3)N_(5)/O_(2) and Ta_(3)N_(5)/KTaO.The formations of interfacial bonds are thermodynamically stable,showing a covalent interaction between two components of a heterostructure.Ta_(3)N_(5)/O_(2) has a higher mobility of photogenerated charge carriers and lower recombination rate of charge carriers than Ta_(3)N_(5)/KTaO.The light absorption of heterostructures displays the feature of KTaO_(3) in the short wavelength region and the characteristic of Ta_(3)N_(5) in the long wavelength region.The calculated band offsets show that Ta_(3)N_(5)/O_(2) and Ta_(3)N_(5)/KTaO have distinct Type-II band alignments,with Ta_(3)N_(5) as the accumulator of photoinduced electrons in the former and the collector of photogenerated holes in the latter,respectively.The difference in charge density and electrostatic potential between two components acts as a driving force to promote the transfer of electrons and holes to different domains of the interface,which is beneficial to extend the lifetime of photoinduced carriers.Our results demonstrate that the function of Ta_(3)N_(5) in Ta_(3)N_(5)/KTaO_(3) photocatalytic system is determined by the termination property of KTaO_(3)(110)surface,which provides a likely reason of the observed photocatalytic activity of overall water splitting achieved by Ta_(3)N_(5) synthesized by using KTaO_(3) as a precursor for the nitridation reaction.

Edge assisted epitaxy of CsPbBr_(3)nanoplates on Bi_(2)O_(2)Se nanosheets for enhanced photoresponse

Bi_(2)O_(2)Se has been proved to be a promising candidate for electronic and optoelectronic devices due to their unique physical properties.However,it is still a great challenge to construct the heterostructures with direct epitaxy of hetero semiconductor materials on Bi_(2)O_(2)Se nanosheets.Here,a two-step chemical vapor deposition(CVD)route was used to directly grow the CsPbBr_(3)nanoplate-Bi_(2)O_(2)Se nanosheet hetero structures.The CsPbBr_(3)nanoplates were selectively grown on the Bi_(2)O_(2)Se nanosheet along the edges,where the dangling bonds provide the nucleation sites.The epitaxial relationships between CsPbBr3 and Bi_(2)O_(2)Se were determined as[200]_(Bi_(2)O_(2)Se)‖[110]_(CsPbBr_(3))and[110]_(Bi_(2)O_(2)Se)‖[200]_(CsPbBr_(3))by transmission electron microscopy characterization.The photoluminescence(PL)results reveal that the formation of heterostructures results in the remarkable PL quenching due to the type-Ⅰband arrangement at CsPbBr_(3)/Bi_(2)O_(2)Se interface,which was confirmed by ultraviolet photoelectron spectroscopy(UPS)and Kelvin probe measurements,and makes the photogenerated carriers transfer from CsPbBr_(3)to Bi_(2)O_(2)Se.Importantly,the photodetectors based on the heterostructures exhibit a 4-time increase in the responsivity compared to those based on the pristine Bi_(2)O_(2)Se sheets,and the fast rise and decay time in microsecond.These results indicate that the direct epitaxy of the CsPbBr_(3)plates on the Bi_(2)O_(2)Se sheet may improve the optoelectronic performance of Bi_(2)O_(2)Se based devices.

CARRIER PHOTOGENERATION IN POLY(N-VINYL-CARBAZOLE)-BASED PHOTOCONDUCTIVE THIN-FILM DEVICES

In this paper, photoexcitation processes in the bilayer devices based on inorganic materials and poly（N-vinylcarbazole） （PVK） were investigated. In order to clarify the roles of inorganic materials in photoconductive properties of bilayer devices, TiO2 and ZnS were chosen to combine with PVK. A model for generation of photocurrent （Iph） in single layer device of PVK was obtained. It is deduced that the recombination rate constant （Pcomb） and the ionization rate constant （y） ofexcitons should be considered as the most important factors for Iph. For inorganic materials （TiO2 or ZnS）/PVK bilayer devices, in reverse bias of-4 V, the photocurrent of 115 mA/cm^2 in the TiO2/PVK device was observed, but the photocurrent in the ZnS/PVK device was only 10 mA/cma under the illumination light of 340 nm and the light intensity of 14.2 mW/cm^2. The weaker photocurrent is attributed to the absorption of ZnS within UV region and the energy offset at the interface between PVK and ZnS, which impedes the transport of charge carriers.

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells

The improvement of the performance of organic photovoltaic cells (OPVCs) and the photogeneration process in these devices may occur via multiple mechanisms depending on their structure and/or architecture. For this purpose we investigate how self-assembled monolayers of thiol molecules (C12H25SH and 3T(CH2)6SH) and benzoic acid molecules (ABA and NBA) affect the efficiency and the photogeneration of free carriers in a sexithiophene based photovoltaic cells. Firstly, we provide the results of absorption spectra for samples with SAM of thiol that show there effect on orientation of 6T molecules on these structures and the organization degree of the thiol molecules on ITO substrate. Afterward, we describe from current vs. applied voltage after illumination, the enhancement of the performance of these cells. In the second, we study the effect of SAM of benzoic acids molecules on the photovoltaic behavior. A theoretical model is used for quantitative description of the open circuit voltage as a function of carrier’s generation rates at the electrodes. The results of I-V characterization under illumination show that open circuit voltage as well as short circuit current is dramatically affected by the dipolar layer. The orientation and the magnitude of dipole moment of benzoic acid molecules are the crucial factors that affect the organic photovoltaic parameters.

Single-atom-mediated electron islands boost photocatalytic CO_(2)chemical fixation

Photocatalytic cycloadditions of CO_(2)with epoxides are emerging as a significant platform for the green synthesis of valuable carbonates,where efficient catalytic systems with spatially separated charge carriers are demanded.Herein,a single p-block metal atom is proposed to be a candidate for photogenerated electron localization on a metal oxide substrate.By taking the Bi single-atom supported on ZnO nanosheet(Bi1/ZnO)as an example,we show that the Bi atom plays the role of an electron island in the sea of delocalized holes within ZnO.Meanwhile,the as-formed electron island could readily promote the rate-determining ring-opening process of cycloaddition reactions.Benefiting from the unique spatially separated electrons and holes,the Bi1/ZnO achieves a high yield of cyclic carbonates with almost 100%selectivity.This study provides a pioneering strategy for enhancing the performances of photocatalytic CO_(2)chemical fixation.