Recent advances in non-metal doped titania for solar-driven photocatalytic/photoelectrochemical water-splitting

Photocatalytic (PC) / Photoelectrochemical (PEC) water splitting under solar light irradiation is considered as a prospective technique to support the sustainable and renewable H_(2) economy and to reach the ultime goal of carbon neutral. TiO_(2) based photocatalysts with high chemical stability and excellent photocatalytic properties have great potential for solar-to-H_(2) conversion. To conquer the challenges of the large band-gap and rapid recombination of photo generated electron-holepairs in TiO_(2), non-metal doping turns out to be economic, facile, and effective on boosting the visible light activity. The localized defect states such as oxygen vacancy and Ti^(3+) generated by non-metal doping are located in the band-gap of TiO_(2), which result in the reduction of band-gap, thus a red-shift of the absorption edge. The hetero doping atoms such as B^(3+), I^(7+), S^(4+)/S^(6+), P^(5+) can also act as electron donors or trap sites which facilitate the charge carrier separation and suppress the recombination of electron-hole pairs. In this comprehensive review, we present the most recent advances on non-metal doped TiO_(2) photocatalysts in terms of fundamental aspects, origin of visible light activity and the PC / PEC behaviours for water splitting. In particular, the characteristics of different non-metal elements (N, C, B, S, P, Halogens) as dopants are discussed in details focusing on the synthesis approaches, characterization as well as the efficiency of PC and PEC water splitting. The present review aims at guiding the readers who want quick access to helpful information about how to efficiently improve the performance of photocatalysts by simple doping strategies and could stimulate new intuitive into the new doping strategies.

Insights on advanced substrates for controllable fabrication of photoanodes toward efficient and stable photoelectrochemical water splitting

Conversion of solar energy into H2 by photoelectrochemical(PEC)water splitting is recognized as an ideal way to address the growing energy crisis and environmental issues.In a typical PEC cell,the construction of photoanodes is crucial to guarantee the high efficiency and stability of PEC reactions,which fundamentally rely on rationally designed semiconductors(as the active materials)and substrates(as the current collectors).In this review work,we start with a brief introduction of the roles of substrates in the PEC process.Then,we provide a systematic overview of representative strategies for the controlled fabrication of photoanodes on rationally designed substrates,including conductive glass,metal,sapphire,silicon,silicon carbide,and flexible substrates.Finally,some prospects concerning the challenges and research directions in this area are proposed.

Defect engineering of ternary Cu-In-Se quantum dots for boosting photoelectrochemical hydrogen generation

Heavy-metal-free ternary Cu–In–Se quantum dots(CISe QDs)are promising for solar fuel production because of their low toxicity,tunable band gap,and high light absorption coefficient.Although defects significantly affect the photophysical properties of QDs,the influence on photoelectrochemical hydrogen production is not well understood.Herein,we present the defect engineering of CISe QDs for efficient solar-energy conversion.Lewis acid–base reactions between metal halide–oleylamine complexes and oleylammonium selenocarbamate are modulated to achieve CISe QDs with the controlled amount of Cu vacancies without changing their morphology.Among them,CISe QDs with In/Cu=1.55 show the most outstanding photoelectrochemical hydrogen generation with excellent photocurrent density of up to 10.7 mA cm-2(at 0.6 VRHE),attributed to the suitable electronic band structures and enhanced carrier concentrations/lifetimes of the QDs.The proposed method,which can effectively control the defects in heavy-metal-free ternary QDs,offers a deeper understanding of the effects of the defects and provides a practical approach to enhance photoelectrochemical hydrogen generation.

Electrodeposited copper oxides with a suppressed interfacial amorphous phase using mixed-crystalline ITO and their enhanced photoelectrochemical performances

The effectiveness of photoelectrochemical(PEC)water splitting is significantly restricted by insufficient light harvesting,rapid charge recombination,and slow water reduction kinetics.Since the presence of amorphous phases in the interfaces hinders the overcome of these inherent limitations,a photoelectrode must be built strategically.Herein,we artificially controlled the crystallographic orientation of indium tin oxide(ITO)to determine the orientation with the smallest lattice mismatch at the Cu_(2)O interface,thus significantly reducing the amorphous phase in the early stage of electrodeposition nucleation.The[222]/[400]mixed orientation ITO primarily exposed the{400}surface planes and accelerated charge transfer by forming an optimal interface with preferentially grown(111)oriented Cu_(2)O and minimized amorphous region.In addition,the ITO surface energy was calculated using density functional theory to theoretically verify which plane is more active for growing the photoactivation layer.The rationally designed ITO/Cu_(2)O/Al-dope Zn O/TiO_(2)/Rh-P device,with each layer serving a specific purpose,achieved a photocurrent density of 8.23 mA cm^(-2)at 0 VRHEunder AM 1.5 G illumination,providing a standard method for effective solar-to-hydrogen conversion photocathodes.

Sputtered Stainless Steel on Silicon Photoanode for Stable Seawater Splitting in Photoelectrochemical Flow Cell

Photoelectrochemical(PEC)seawater splitting is a promising method for the direct utilization of solar energy and abundant seawater resources for hydrogen production.Photoelectrodes are susceptible to various ions in seawater and complicated competitive reactions,resulting in the failure of photoelectrodes.This paper proposes the design and fabrication of diff erent sputtered stainless steel(SS)fi lms deposited on silicon photoanodes,completely isolating the electrolytes and semiconductor substrate.Upon coupling with the PEC flow cell,the back-illuminated photoanode coated with 316 SS cocatalyst achieves stable operation for 70 h in natural seawater with a highly alkaline KOH(30 wt.%,7.64 mol/L)electrolyte due to the remarkable protection eff ect of the substrate from stainless steel,while the PEC seawater splitting system achieves a record hydrogen production rate of 600μmol/(h·cm^(2)).An appropriate Ni/Fe ratio in the SS ensures remarkable oxygen evolution activity,while chromic oxide ensures the effective anticorrosion effect by adjusting the microenvironment of the photoanodes.Moreover,fabricating PEC flow cells with photoanodes coated with SS cocatalysts are a viable strategy for PEC seawater splitting.

Fabrication and research of bi-functional CuNi_(2)S_(4) nanosheets decorated TiO_(2)/CuNi_(2)S_(4) heterojunction photoanode for photoelectrochemical water splitting

As a traditional n-type semiconductor, TiO_(2)has good UV absorption ability and stable physical and chemical properties. However, its wide band gap and low oxygen evolution reaction(OER) activity limit its application in the field of photoelectrochemical(PEC) water splitting. In this work, a type-Ⅱ TiO2/CuNi2S4heterojunction photoanode is successfully constructed, which expanded the light absorption range to visible and enhanced the OER activity. Firstly, TiO2nanotubes(NTs) thin films are prepared on Ti substrates by two-step anodization, and then the bi-functional electrocatalytic material CuNi2S4is grown on TiO2NTs in the shape of nanosheets(NSs) in situ by solvothermal method. As a bi-functional electrocatalytic material, CuNi2S4has good visible light absorption property as well as OER catalytic activity. Compared with TiO_(2), the IPCE value of TiO_(2)/CuNi_(2)S_(4)is 2.59% at 635 nm, and that of TiO2is a mere 0.002%.The separation efficiency and injection efficiency increase from 2.49% and 31.52% to 3.61% and 87.77%, respectively. At 1.23 V vs. RHE, the maximum photocurrent density is 0.26 m A/cm^(2), which is 2.6 times than that of TiO2(0.11 m A/cm^(2)),and can be maintained at 0.25 m A/cm^(2)for at least 2 h under light illumination. Moreover, a hydrogen production rate of 4.21 μmol·cm^(-2)·h^(-1)is achieved within 2 h. This work provides a new idea for the application of TiO_(2)in the field of PEC water splitting and the construction of efficient and stable photoelectronic devices.

Ti-Fe_(2)O_(3)/Ni(OH)_(x) as an efficient and durable photoanode for the photoelectrochemical catalysis of PET plastic to formic acid

Photoelectrochemical(PEC) technology provides a promising prospect for the transformation of polyethylene terephthalate(PET) plastic wastes to produce value-added chemicals.The PEC catalytic systems with high activity,selectivity and long-term durability are required for the future up-scaling industrial applications.Herein,we employed the interfacial modification strategy to develop an efficient and stable photoanode and evaluated its PEC activity for ethylene glycol(EG,derived from PET hydrolysate) oxidation to formic acid.The interfacial modification between Fe_(2)O_(3)semiconductor and Ni(OH)xcocatalyst with ultrathin TiO_(x) interlayer not only improved the photocurrent density by accelerating the kinetics of photogenerated charge carriers,but also kept the high Faradaic efficiency(over 95% in 30 h) towards the value-added formic acid product.This work proposes an effective method to promote the PEC activity and enhance the long-term stability of photoelectrodes for upcycling PET plastic wastes.

Photoactive Naphthalene Diimide Functionalized Titanium-Oxo Clusters with High Photoelectrochemical Responses

Photoactive functionalized titanium-oxo clusters(TOCs)are regarded as an important model compound for dye-sensitized titanium dioxide solar cells.However,the dyes used for sensitizing TOCs are still limited.Herein,two cyclic TOCs are reported,namely,[Ti_(6)(μ_(3)-O)_(2)(Oi-Pr)_(8))(LA)_(2)]·i-PrOH(S1)and[Ti_(6)(μ_(3)-O)2(Oi-Pr)_(8))(LV)_(2)]·i-PrOH(S2),which are functionalized by photoactive naphthalene diimide(NDI)chromophores.Their molecular structures and photophysical and photochemical properties were systematically studied.As shown by ultraviolet-visible(UV-vis)spectra and photocurrent study results,the band gap and the photocurrent response of S1 and S2 were derived from NDI ligands which extend the absorption edge of S1 and S2 approaching 500 nm and afford high photocurrent densities of 2.12μA/cm^(2)and 1.95μA/cm^(2)for S1 and S2,respectively,demonstrating the significance of the photoactive ligand in modulating photoresponse of TOCs.This work is expected to enrich the structural library of photoactive TOCs and provide insights into understanding the structure-property relationships of sensitized clusters.

Photothermal‐boosted polaron transport in Fe_(2)O_(3)photoanodes for efficient photoelectrochemical water splitting

Introduction of the photothermal effect into transition-metal oxide photoanodes has been proven to be an effective method to improve the photoelectrochemical(PEC)water-splitting performance.However,the precise role of the photothermal effect on the PEC performance of photoanodes is still not well understood.Herein,spinel-structured ZnFe_(2)O_(4)nanoparticles are deposited on the surface of hematite(Fe_(2)O_(3)),and the ZnFe_(2)O_(4)/Fe_(2)O_(3)photoanode achieves a high photocurrent density of 3.17 mA cm^(-2)at 1.23 V versus a reversible hydrogen electrode(VRHE)due to the photothermal effect of ZnFe_(2)O_(4).Considering that the hopping of electron small polarons induced by oxygen vacancies is thermally activated,we clarify that the main reason for the enhanced PEC performance via the photothermal effect is the promoted mobility of electron small polarons that are bound to positively charged oxygen vacancies.Under the synergistic effect of oxygen vacancies and the photothermal effect,the electron conductivity and PEC performance are significantly improved,which provide fundamental insights into the impact of the photothermal effect on the PEC performance of small polaron-type semiconductor photoanodes.

Recent advances in elaborate interface regulation of BiVO_(4)photoanode for photoelectrochemical water splitting

Bismuth vanadate(BiVO_(4))is an excellent photoanode material for photoelectrochemical(PEC)water splitting system,possessing high theoretical photoelectrocatalytic conversion efficiency.However,the actual PEC activity and stability of BiVO_(4)are faced with great challenges due to factors such as severe charge recombination and slow water oxidation kinetics at the interface.Therefore,various interface regulation strategies have been adopted to optimize the BiVO_(4)photoanode.This review provides an in-depth analysis for the mechanism of interface regulation strategies from the perspective of factors affecting the PEC performance of BiVO_(4)photoanodes.These interface regulation strategies improve the PEC performance of BiVO_(4)photoanode by promoting charge separation and transfer,accelerating interfacial reaction kinetics,and enhancing stability.The research on the interface regulation strategies of BiVO_(4)photoanode is of great significance for promoting the development of PEC water splitting technology.At the same time,it also has inspiration for providing new ideas and methods for designing and preparing efficient and stable catalytic materials.

Enhanced Performance of a Monolayer MoS_2/WSe_2 Heterojunction as a Photoelectrochemical Cathode

Transition-metal dichalcogenide(TMD) semiconductors have attracted interest as photoelectrochemical(PEC) electrodes due to their novel band-gap structures,optoelectronic properties, and photocatalytic activities.However, the photo-harvesting efficiency still requires improvement. In this study, A TMD stacked heterojunction structure was adopted to further enhance the performance of the PEC cathode. A P-type WSe_2 and an N-type Mo S_2 monolayer were stacked layer-by-layer to build a ultrathin vertical heterojunction using a micro-fabrication method.In situ measurement was employed to characterize the intrinsic PEC performance on a single-sheet heterostructure.Benefitting from its built-in electric field and type II band alignment, the MoS_2/WSe_2 bilayer heterojunction exhibited exceptional photocatalytic activity and a high incident photo-to-current conversion efficiency(IPCE). Comparing with the monolayer WSe_2 cathode, the PEC current and the IPCE of the bilayer heterojunction increased by a factor of 5.6 and enhanced 50%, respectively. The intriguing performance renders the MoS_2/WSe_2 heterojunction attractive for application in high-performance PEC water splitting.

A review on photoelectrochemical cathodic protection semiconductor thin films for metals

Photoelectrochemical(PEC) cathodic protection is considered as an environment friendly method for metals anticorrosion. In this technology, a n-type semiconductor photoanode provides the photogenerated electrons for metal to achieve cathodic protection. Comparing with traditional PEC photoanode for water splitting, it requires the photoanode providing a suitable cathodic potential for the metal, instead of pursuit ultimate photon to electric conversion efficiency, thus it is a more possible PEC technology for engineering application. To date, great efforts have been devoted to developing novel n-type semiconductors and advanced modification method to improve the performance on PEC cathodic protection metals. Herein, recent progresses in this field are summarized. We highlight the fabrication process of PEC cathodic protection thin film, various nanostructure controlling, doping, compositing methods and their operation mechanism. Finally, the current challenges and future potential works on improving the PEC cathodic protection performance are discussed.

Recent Advances in Visible-Light-Driven Photoelectrochemical Water Splitting: Catalyst Nanostructures and Reaction Systems

Photoelectrochemical(PEC) water splitting using solar energy has attracted great attention for generation of renewable hydrogen with less carbon footprint, while there are enormous challenges that still remain for improving solar energy water splitting efficiency, due to limited light harvesting, energy loss associated to fast recombination of photogenerated charge carriers, as well as electrode degradation. This overview focuses on the recent development about catalyst nanomaterials and nanostructures in different PEC water splitting systems. As photoanode, Au nanoparticle-decorated TiO_2 nanowire electrodes exhibited enhanced photoactivity in both the UV and the visible regions due to surface plasmon resonance of Au and showed the largest photocurrent generation of up to 710 nm. Pt/Cd S/CGSe electrodes were developed as photocathode. With the role of p–n heterojunction, the photoelectrode showed high stability and evolved hydrogen continuously for more than 10 days. Further, in the Z-scheme system(Bi_2S_3/TNA as photoanode and Pt/Si PVC as photocathode at the same time), a self-bias(open-circuit voltage Voc= 0.766 V) was formed between two photoelectrodes, which could facilitate photogenerated charge transfers and enhance the photoelectrochemical performance, and which might provide new hints for PEC water splitting. Meanwhile, the existing problems and prospective solutions have also been reviewed.

Photoelectrochemical study of MoO_3 assorted morphology films formed by thermal evaporation

Molybdenum oxide nanostructured thin films were grown on fluorine doped tin oxide（FTO）, indium doped tin oxide（ITO） and ordinary glass substrates by thermal evaporation process without vacuum and catalysts using molybdenum trioxide（MoO） powder as a source material and oxygen as a carrier gas.Various morphologies including nanobelts, disks and hexagonal rod-like nanostructures were obtained by changing the source and substrate temperatures during the growth of MoOthin films. Structural parameters, morphology, composition and surface features of the films were characterized by XRD, SEM, EDAX,XPS, AFM and Raman spectroscopy. The films were orthorhombic in structure with preferred orientation along（0 1 0） plane. Morphology analysis reveals randomly aligned nanobelts with 40 nm in thickness and a width of 800 nm and 3–12 mm in length. The disks have 1.5 μm diameters, 1 μm thickness and hexagonal rod-like nanostructures with a length, breath and width of 2 μm, 1 μm and 100 nm are formed. The samples were investigated under dark and photocurrent conditions in HSOaqueous solution as a function of applied potential. The photocurrent density of samples prepared on ITO and FTO substrate samples were compared and the results are discussed.

Efficient Photoelectrochemical Water Splitting by g-C_3N_4/TiO_2 Nanotube Array Heterostructures

Well-ordered TiO_2 nanotube arrays(TNTAs)decorated with graphitic carbon nitride(g-C_3N_4) were fabricated by anodic oxidization and calcination process.First, TNTAs were prepared via the anodic oxidation of Ti foil in glycerol solution containing fluorinion and 20%deionized water. Subsequently, g-C_3N_4 film was hydrothermally grown on TNTAs via the hydrogen-bonded cyanuric acid melamine supramolecular complex. The results showed that g-C_3N_4 was successfully decorated on the TNTAs and the g-C_3N_4/TNTAs served as an efficient and stable photoanode for photoelectrochemical water splitting. The facile deposition method enables the fabrication of efficient and low-cost photoanodes for renewable energy applications.

A Facile Self-assembly Synthesis of Hexagonal ZnO Nanosheet Films and Their Photoelectrochemical Properties

Here, large-scale and uniform hexagonal zinc oxide(ZnO) nanosheet films were deposited onto indium tin oxide(ITO)-coated transparent conducting glass substrates via a facile galvanic displacement deposition process. Compared with other commonly used solution methods, this process avoids high temperature and electric power as well as supporting agents to make it simple and cost-effective. The as-fabricated ZnO nanosheet films have uniform hexagonal wurtzite structure. The photoelectrochemical(PEC) cell based on ZnO nanosheet film/ITO photoelectrode was also fabricated and its performance was improved by optimizing the solution concentration. A higher photocurrent density of*500 l A cm^(-2)under AM 1.5 G simulated illumination of 100 m W cm^(-2)with zero bias potential(vs. Ag/AgCl electrode) was obtained, which may ascribe to the increased surface-to-volume ratio of disordered Zn O nanosheet arrays. Our developed method may be used to deposit other oxide semiconductors, and the Zn O nanosheet film/ITO PEC cell can be used to design low-cost optoelectronic and photoelectrochemical devices.

Highly Enhanced Visible-Light-Driven Photoelectrochemical Performance of ZnO-Modified In_2S_3 Nanosheet Arrays by Atomic Layer Deposition

Photoanodes based on In_2S_3/ZnO heterojunction nanosheet arrays(NSAs) have been fabricated by atomic layer deposition of ZnO over In_2S_3 NSAs, which were in situ grown on fluorine-doped tin oxide glasses via a facile solvothermal process. The as-prepared photoanodes show dramatically enhanced performance for photoelectrochemical(PEC) water splitting, compared to single semiconductor counterparts. The optical and PEC properties of In_2S_3/ZnO NSAs have been optimized by modulating the thickness of the Zn O overlayer. After pairing with ZnO, the NSAs exhibit a broadened absorption range and an increased light absorptance over a wide wavelength region of 250–850 nm. The optimized sample of In_2S_3/ZnO-50 NSAs shows a photocurrent density of 1.642 m A cm^(-2)(1.5 V vs. RHE) and an incident photonto-current efficiency of 27.64% at 380 nm(1.23 V vs.RHE), which are 70 and 116 times higher than those of the pristine In_2S_3 NSAs, respectively. A detailed energy band edge analysis reveals the type-II band alignment of the In_2S_3/ZnO heterojunction, which enables efficient separation and collection of photogenerated carriers,especially with the assistance of positive bias potential, and then results in the significantly increased PEC activity.

Nanoforest-like CdS/TiO_2 heterostructure composites:Synthesis and photoelectrochemical application

In this study, YiO2 nanoforest films consisting of nanotubes have been synthesized by a simple hydrothermal method and a subsequent sintering technique. The hydrothermal reaction time is important for the controlling of the nanotube diameter and the specific surface area of holistic TiO2 films. When the hydrothermal process reaction time is up to 8 hours, the diameter of the nanotuhe is about 10 nm, and the specific surface area of TiO2 nanoforest films reaches the maximum. CdS nanoparticles are synthesized on TiO2 nanoforest films by the successive ionic layer adsorption and reaction （SILAR） technique. The transmission electron microscope （TEM） and energy dispersive x-ray spectroscopy （EDX） mapping results verify that TiOz/CdS heterostructures are realized. A significant red-shift of the absorption edge from 380 nm to 540 nm can be observed after the pure TiO2 film is sensitized by CdS nanoparticles. Under irradiation of light, the current density of the optimal TiO2/CdS photoanode is 2.30 mA.cm-2 at 0 V relative to the saturated calomel electrode （SCE）, which is 6 times stronger than that of the pure TiO2 photoanode. This study suggests that the TiO2 nanoforest consisting of interlinked pony-size nanotubes is a promising nanostructure for photoelectrochemical.

Ti, Zn co-doped hematite photoanode for solar driven photoelectrochemical water oxidation

Although there have been many reports of metal doping to ameliorate the drawbacks of hematite as the photoanode for water oxidation, most of them focused on monometallic doping, and only a few of them payed attention to bimetallic doping. What is worse, the synergetic mechanism between two metal dopants was not sufficiently studied, especially the density functional theory(DFT) calculation. In this work, the n-type hematite was synthesized by introducing Ti dopant into hematite through the hydrothermal method, and dipping-sintering treatment was employed to further introduce homogeneously dispersed Zn dopant into that, forming the Ti, Zn co-doped hematite. Under the optimal condition, Tidoped hematite photoanode reached approximately 2-times enhancement of the photocurrent density compared with the pristine one at 1.23 V vs. RHE, while Ti, Zn co-doped hematite anode obtained another25% elevation. UV–Vis spectroscopy, Mott–Schottky plots, EIS analysis, photo-oxidation of hole scavenger(H2O2), and DFT calculation were employed to understand the role of Ti, Zn dopant. Based on the obtained results, the synergetic mechanism of two dopants was discussed, i.e., the improvement of PEC performance of Ti, Zn co-doped hematite photoanode was possibly attributed to greater carrier density and improved charge separation efficiency at the surface of hematite. This work provides new strategy and understanding of the improvement of PEC performance of hematite by doping engineering.

Preparation of WO3 thin films by dip film-drawing for photoelectrochemical performance

Here we report the WO3 thin films on F-doped SnO2 conducting glass (FTO) substrates which were prepared by using dip film-drawing method. Dip film-drawing was a simple, convenient, economical method and in largescale to prepare photoanodes for future applications. The FTO substrates were dipped in tungstic acid solution then film-drawn included 3, 6, 9, 12 and 15 times for prepared different thicknesses of WO3 thin film photoanodes. Then the photoa no des were employed as the electrodes in photoelectrochemical property Keywords: WO3 thin films Dip film-drawing Photoelectrochemical Thicknesses Large-scale measurements, which include scan linear sweep, repeated on/off illumination cycles, electrochemical impedanee spectroscopy and incident phot on to current conversion efficiency, respectively. The results showed that the WO3 thin films dipped 9 times with 175 nm thicknesses had the best photoelectrochemical performance of 0.067 mA·cm^-2 at 1.23 V versus RHE.