Doping‐Induced Amorphization,Vacancy, and Gradient Energy Band in SnS2 Nanosheet Arrays for Improved Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is a promising strategy to convert solar energy into hydrogen fuel. However, the poor bulk charge‐separation ability and slow surface oxygen evolution reaction (OER) dynamics of photoelectrodes impede the performance. We construct In‐ and Zn/In‐doped SnS₂ nanosheet arrays through a hydrothermal method. The doping induces the simultaneous formation of an amorphous layer, S vacancies, and a gradient energy band. This leads to elevated carrier concentrations, an increased number of surface‐reaction sites, accelerated surface‐OER kinetics, and an enhanced bulk‐carrier separation efficiency with a decreased recombination rate. This efficient doping strategy allows to manipulate the morphology, crystallinity, and band structure of photoelectrodes for an improved PEC performance.     

A Plasma‐Triggered O−S Bond and P−N Junction Near the Surface of a SnS2 Nanosheet Array to Enable Efficient Solar Water Oxidation

A photoelectrochemical (PEC) cell can split water into hydrogen and oxygen with the assistance of solar illumination. However, its application is still limited by excessive bulk carrier recombination and sluggish surface oxygen evolution reaction (OER) kinetics. Taking SnS₂ as an example, a promising layered optoelectronic semiconductor, Ar plasma treatment strategy was used to introduce a SnS/SnS₂ P?N heterojunction and O?S bond near the surface of a SnS₂ nanosheet array, simultaneously increasing the separation efficiency of photogenerated electron–hole pairs in the bulk and lowering the OER overpotential at the surface. The onset potential of the plasma‐treated SnS₂ nanosheet array shifts negatively to 0.16 V, and the photocurrent density at 1.23 V vs. RHE boosts to 2.15 mA cm^?2, which is 7 times that of pristine SnS₂. This work demonstrates a facile plasma treatment strategy to modulate the energy band structure and surface chemical states for improved PEC performance.     

Metal-Organic Framework Glass Catalysts from Melting Glass-Forming Cobalt-Based Zeolitic Imidazolate Framework for Boosting Photoelectrochemical Water Oxidation

Sluggish oxygen evolution kinetics and serious charge recombination restrict the development of photoelectrochemical (PEC) water splitting. The advancement of novel metal–organic frameworks (MOFs) catalysts bears practical significance for improving PEC water splitting performance. Herein, a MOF glass catalyst through melting glass-forming cobalt-based zeolitic imidazolate framework (Co-a_gZIF-62) was introduced on various metal oxide (MO: Fe₂O₃, WO₃ and BiVO₄) semiconductor substrates coupled with NiO hole transport layer, constructing the integrated Co-a_gZIF-62/NiO/MO photoanodes. Owing to the excellent conductivity, stability and open active sites of MOF glass, Co-a_gZIF-62/NiO/MO photoanodes exhibit a significantly enhanced photoelectrochemical water oxidation activity and stability in comparison to pristine MO photoanodes. From experimental analyses and density functional theory calculations, Co-a_gZIF-62 can effectively promote charge transfer and separation, improve carrier mobility, accelerate the kinetics of oxygen evolution reaction (OER), and thus improve PEC performance. This MOF glass not only serves as an excellent OER cocatalyst on tunable photoelectrodes, but also enables promising opportunities for PEC devices for solar energy conversion.     

A co-activation strategy for enhancing the performance of hematite in photoelectrochemical water oxidation

Hematite(α-Fe_2O_3) is a promising photoanode for photoelectrochemical(PEC) water splitting.However,the severe charge recombination and sluggish water oxidation kinetics extremely limit its use in photohydrogen conversion.Herein,a co-activation strategy is proposed,namely through phosphorus(P)doping and the loading of CoAl-layered double hydroxides(CoAl-LDHs) cocatalysts.Unexpectedly,the integrated system,CoAl-LDHs/P-Fe_2O_3 photoanode,exhibits an outstanding photocurrent density of 1.56 mA/cm~2 at 1.23 V(vs.reversible hydrogen electrode,RHE),under AM 1.5 G,which is 2.6 times of pureα-Fe_2O_3.Systematic studies reveal that the remarkable PEC performance is attributed to accelerated surface OER kinetics and enhanced carrier separation efficiency.This work provides a feasible strategy to enhance the PEC performance of hematite photoanodes.     

Understanding the Roles of Oxygen Vacancies in Hematite‐Based Photoelectrochemical Processes

Oxygen vacancy (V_O) engineering is an effective method to tune the photoelectrochemical (PEC) performance, but the influence of V_O on photoelectrodes is not well understood. Using hematite as a prototype, we herein report that V_O functions in a more complicated way in PEC process than previously reported. Through a comprehensive analysis of the key charge transfer and surface reaction steps in PEC processes on a hematite photoanode, we clarify that V_O can facilitate surface electrocatalytic processes while leading to severe interfacial recombination at the semiconductor/electrolyte (S‐E) interface, in addition to the well‐reported improvements in bulk conductivity. The improved bulk conductivity and surface catalysis are beneficial for bulk charge transfer and surface charge consumption while interfacial charge transfer deteriorates because of recombination through V_O‐induced trap states at the S‐E interface.     

Identifying Copper Vacancies and Their Role in the CuO Based Photocathode for Water Splitting

Metal oxides are an important family of semiconductors for effective photoelectrodes in solar‐to‐chemical energy conversion. Defect engineering, such as modification of oxygen vacancy density, has been extensively applied in tailoring the optoelectric properties of photoelectrodes. Very limited attention has been paid to the influence of metal vacancies. Herein, we study metal vacancies in a typical CuO photocathode for photoelectrochemical (PEC) water splitting. The Cu vacancies can improve the charge carrier concentration, and facilitate the charge separation and transfer in the CuO photocathode. By changing the O₂ partial pressure, the density of Cu vacancies can be tuned, which leads to improved PEC performance. The CuO photocathode prepared in pure O₂ exhibits a 100 % photocurrent increase compared to that prepared in air. The promotion effect of Cu vacancies on the PEC is also observed in other Cu based photocathodes, showing the generic role of metal vacancies in efficient photocathodes.     

Atomic‐Layer‐Confined Doping for Atomic‐Level Insights into Visible‐Light Water Splitting

A model of doping confined in atomic layers is proposed for atomic‐level insights into the effect of doping on photocatalysis. Co doping confined in three atomic layers of In₂S₃ was implemented with a lamellar hybrid intermediate strategy. Density functional calculations reveal that the introduction of Co ions brings about several new energy levels and increased density of states at the conduction band minimum, leading to sharply increased visible‐light absorption and three times higher carrier concentration. Ultrafast transient absorption spectroscopy reveals that the electron transfer time of about 1.6 ps from the valence band to newly formed localized states is due to Co doping. The 25‐fold increase in average recovery lifetime is believed to be responsible for the increased of electron–hole separation. The synthesized Co‐doped In₂S₃ (three atomic layers) yield a photocurrent of 1.17 mA cm^?2 at 1.5 V vs. RHE, nearly 10 and 17 times higher than that of the perfect In₂S₃ (three atomic layers) and the bulk counterpart, respectively.     

Single‐Crystal Semiconductors with Narrow Band Gaps for Solar Water Splitting

Solar water splitting provides a clean and renewable approach to produce hydrogen energy. In recent years, single‐crystal semiconductors such as Si and InP with narrow band gaps have demonstrated excellent performance to drive the half reactions of water splitting through visible light due to their suitable band gaps and low bulk recombination. This Minireview describes recent research advances that successfully overcome the primary obstacles in using these semiconductors as photoelectrodes, including photocorrosion, sluggish reaction kinetics, low photovoltage, and unfavorable planar substrate surface. Surface modification strategies, such as surface protection, cocatalyst loading, surface energetics tuning, and surface texturization are highlighted as the solutions.     

The Role of Surface States in the Oxygen Evolution Reaction on Hematite

Hematite (α‐Fe₂O₃) is an extensively investigated semiconductor for photoelectrochemical (PEC) water splitting. The nature and role of surface states on the oxygen evolution reaction (OER) remain however elusive. First‐principles calculations were used to investigate surface states on hematite under photoelectrochemical conditions. The density of states for two relevant hematite terminations was calculated, and in both cases the presence and the role of surface states was rationalized. Calculations also predicted a Nerstian dependence on the OER onset potential on pH, which was to a very good extent confirmed by PEC measurements on hematite model photoanodes. Impedance spectroscopy characterization confirmed that the OER takes place via the same surface states irrespective of pH. These results provide a framework for a deeper understanding of the OER when it takes place via surface states.     

Strain tuned efficient heterostructure photoelectrodes

van der Waals（vd Ws） heterostructures based on two-dimensional（2D） materials have become a promising candidate for photoelectrochemical（PEC） catalyst not only because of the freedom in materials design that enable the band-offset construction and facilitate the charge separation. They also provide a platform for the study of various of interface effect in PEC. Here, we report a new kind of mixed-dimensional vd Ws heterostructure photoelectrode and investigate the strain enhanced PEC performance ...     

GaP/GaPN核壳纳米线阵列修饰的硅光阴极的光电化学制氢反应（英文）

能够大规模同时提升电极的催化效率和稳定性对光电化学分解水系统的开发具有重要意义.硅是一种地球储量丰富且成熟的工业材料,由于其合适的带隙(1.1 eV)和优异的导电性,已被广泛用于光电化学制氢反应.然而,缓慢的表面催化反应和在电解液中的不稳定性限制了其在太阳能制氢中的实际应用.III-IV族半导体材料也具有较高的载流子传输特性且被广泛用于光电器件.其中,GaP的直接带隙和间接带隙分别为2.78和2.26 eV,可与硅组成串联型光电极用于光电化学分解水.然而,GaP的光腐蚀电位位于禁带中,很容易在光电催化过程中发生光腐蚀而导致性能大幅下降.本文报道了一种新型的GaP/GaPN核/壳纳米线修饰的p型硅(p-Si)串联型光阴极,同未修饰的p-Si相比,其光电化学制氢性能更高.这可归因于以下几点:(1)p-Si和GaP纳米线之间形成的p-n结促进了电荷分离;(2)GaPN相对于GaP具有更低的导带边位置,进一步促进了光生电子向电极表面的转移;(3)纳米线结构既缩短了光生载流子的收集距离,又增加了比表面积,从而加快了表面反应动力学.此外,在GaP中引入氮元素还提高了体系的光吸收和稳定性.我们所提出的高效、简便的改进策略可应用于其他的太阳能转换体系.利用简单的化学气相沉积法制备GaP/GaPN核/壳纳米线修饰的p-Si光阴极.首先在p-Si衬底上利用Au纳米颗粒作为催化剂生长GaP纳米线;然后,去除Au催化剂,并在氨气中退火便形成了GaP/GaPN核壳纳米线.高分辨透射电子显微镜,拉曼光谱和X射线光电子谱的表征结果均证实了氨气退火使得GaP纳米线表面形成了GaPN的薄壳层,同时证明了GaP/GaPN核壳纳米线具有可调的核壳结构.在模拟太阳光下作为光阴极用于光解水制氢反应时,GaP/GaPN核壳纳米线修饰的p-Si光阴极的起始电位为~0.14 V,而未修饰的p-Si电极的起始电位大约在?0.77 V.而且,GaP/GaPN核/壳纳米线修饰的p-Si光阴极比未修饰的p-Si光阴极具有更高的光电流密度,在水的还原电位下,其光电流密度为?0.3 mA cm^-2,且饱和光电流密度在?0.76 V时达到了?8.8 mA cm^-2.此外,GaP/GaPN核/壳纳米线修饰的p-Si光阴极的光电化学活性在10 h内没有发生明显下降.由此可见GaP/GaPN核/壳纳米线可以规模化有效地提升Si光电极的催化效率和稳定性.     

Long‐Lived Charge Carriers in Mn‐Doped CdS Quantum Dots for Photoelectrochemical Cytosensing

Photoelectrochemical (PEC) biosensing with semiconductor quantum dots (QDs) has received great attention because it integrates the advantages of both photo‐excitation and electrochemical detection. During the photon‐to‐electricity conversion in PEC processes, electron–hole (charge) separation competes with electron–hole recombination, and the net effect essentially determines the performance of PEC biosensors. Herein, we propose a new approach for slowing down electron–hole recombination to increase charge separation efficiency for PEC biosensor development. Through doping with Mn²⁺, a pair of d bands (⁴T₁ and ⁶A₁) is inserted between the conduction and valence bands of CdS QDs, which alters the electron–hole separation and recombination dynamics, allowing the generation of long‐lived charge carriers with ms‐scale lifetime that decay about 10⁴–10⁵‐fold more slowly than in the case of undoped QDs. Photocurrent tests indicated that Mn²⁺ doping resulted in an approximately 80 % increase in photocurrent generation compared with undoped CdS QDs. For application, the Mn‐doped CdS QDs were coated on the surface of a glassy carbon electrode and functionalized with a cell surface carbohydrate‐specific ligand (3‐aminophenylboronic acid). In this way, a sensitive cytosensor for K562 leukemia cells was constructed. Moreover, the sugar‐specific binding property of 3‐aminophenylboronic acid allowed the electrode to serve as a switch for the capture and release of cells. This has been further explored with a view to developing a reusable PEC cytosensing platform.     

Near infrared-driven photoelectrochemical water splitting: Review and future prospects

Photoelectrochemical (PEC) water splitting supplies an environmentally friendly, sustainable approach to generating renewable hydrogen fuels. Oxides semiconductors, e.g. TiO₂, BiVO₄, and Fe₂O₃, have been widely developed as photoelectrodes to demonstrate the utility in PEC systems. Even though significant effort has been made to increase the PEC efficiency, these materials are still far from practical applications. The main issue of metal oxides is the wide bandgap energy that hinders effective photons harvesting from sunlight. In solar spectrum, over 40% of the energy is located in the near-infrared (NIR) region. Developing sophisticated PEC systems that can be driven by NIR illumination is therefore essential. This review gives a concise overview on PEC systems based on the use of NIR-driven photoelectrodes. Promising candidates as efficient yet practical NIR-responsive photoelectrodes are suggested and discussed. Future outlooks on the advancement of PEC water splitting are also proposed.     

Photoanodic and photocathodic behaviour of La5Ti2CuS5O7 electrodes in the water splitting reaction

The particulate semiconductor La₅Ti₂CuS₅O₇ (LTC) with a band gap energy of 1.9 eV functioned as either a photocathode or a photoanode when embedded onto Au or Ti metal layers, respectively. By applying an LTC/Au photocathode and LTC/Ti photoanode to, respectively, photoelectrochemical (PEC) water reduction and oxidation concurrently, zero-bias overall water splitting was accomplished under visible light irradiation. The band structures of LTC/Au and LTC/Ti calculated using a semiconductor device simulator (AFORS-HET) confirmed the critical role of the solid/solid junction of the metal back contact in the charge separation and PEC properties of LTC photoelectrodes. The prominently long lifetime of photoexcited charge carriers in LTC, confirmed by transient absorption spectroscopy, allowed the utilization of both photoexcited electrons and holes depending on the band structure at the solid/solid junction.     

Heterostructures Composed of N‐Doped Carbon Nanotubes Encapsulating Cobalt and β‐Mo2C Nanoparticles as Bifunctional Electrodes for Water Splitting

Herein, we demonstrate the use of heterostructures comprised of Co/β‐Mo₂C@N‐CNT hybrids for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline electrolyte. The Co can not only create a well‐defined heterointerface with β‐Mo₂C but also overcomes the poor OER activity of β‐Mo₂C, thus leading to enhanced electrocatalytic activity for HER and OER. DFT calculations further proved that cooperation between the N‐CNTs, Co, and β‐Mo₂C results in lower energy barriers of intermediates and thus greatly enhances the HER and OER performance. This study not only provides a simple strategy for the construction of heterostructures with nonprecious metals, but also provides in‐depth insight into the HER and OER mechanism in alkaline solution.     

Solar‐Light‐Driven Pure Water Splitting with Ultrathin BiOCl Nanosheets

A suitable photocatalyst for overall water splitting has been produced by overcoming the disadvantage of the band structure in bulk BiOCl by reducing the thickness to the quantum scale. The ultrathin BiOCl nanosheets with surface/subsurface defects realized the solar‐driven pure water splitting in the absence of any co‐catalysts or sacrificial agent. These surface defects cannot only shift both the valence band and conduction band upwards for band‐gap narrowing but also promote charge‐carrier separation. The amount of defects in the outer layer surface of BiOCl results in an enhancement of carrier density and faster charge transport. First‐principles calculations provide clear evidence that the formation of surface oxygen vacancies is easier for the ultrathin BiOCl nanosheets than for its thicker counterpart. These defects can serve as active sites to effectively adsorb and dissociate H₂O molecules, resulting in a significantly improved water‐splitting performance.     

Ultrathin Silicon Oxide Film-Induced Enhancement of Charge Separation and Transport of Nanostructured Titanium(IV) Oxide Photoelectrode

The development of nanostructured semiconductor electrodes represented by a mesoporous TiO₂ nanocrystalline (mp-TiO₂) film is currently bringing great progresses in photoelectrochemical (PEC) devices for solar-to-electricity and solar-to-chemical conversion. Two serious losses can occur in PEC devices: 1) recombination between the conduction band (CB) electrons and valence band (VB) holes in the bulk and at the surface and 2) back reaction or electron trapping by oxidant in the electrolyte solution during transport to the electron-collecting electrode. Thus, the major challenge in common with the nanostructured semiconductor photoanodes is to achieve efficient charge separation and electron transport. In this study, an ultrathin SiO_x layer was formed on both the external and the internal surface of mp-TiO₂ using an original chemisorption-calcination technique employing 1,3,5,7-tetramethyltetrasiloxane as a starting material. The SiO_x surface modification of the mp-TiO₂ photoanode drastically prolongs the mean lifetime of CB-electrons in TiO₂ because of enhanced charge separation and electron transport by the negative charge applied in aqueous electrolyte solution. We have demonstrated that the performance of a one-compartment H₂O₂-photofuel cell using mp-TiO₂ as the photoanode is greatly boosted by the surface modification with the SiO_x layer. We anticipate that this methodology is widely applicable to nanostructured metal oxide semiconductor electrodes, contributing to the improvement in the performance of PEC devices.     

Research Progress of Ferrite Materials for Photoelectrochemical Water Splitting

Photoelectrochemical(PEC)water splitting is an effective strategy to convert solar energy into clean and renewable hydrogen energy.In order to carry out effective PEC conversion,researchers have conducted a lot of exploration and developed a variety of semiconductors suitable for PEC water splitting.Among them,metal oxides stand out due to their higher stability.Compared with traditional oxide semiconductors,ferrite-based photoelectrodes have the advantages of low cost,small band gap,and good stability.Interestingly,due to the unique characteristics of ferrite,most of them have various tunable features,which will be more conducive to the development of efficient PEC electrode.However,this complex metal oxide is also troubled by severe charge recombination and low carrier transport efficiency,resulting in lower conversion efficiency compared to theoretical value.Based on this,this article reviews the structure,preparation methods,characteristics and modification strategies of various common ferrites.In addition,we analyzed the future research direction of ferrite for PEC water splitting,and looked forward to the development of more efficient catalysts.     

Perovskite Oxide Based Electrodes for High‐Performance Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is an attractive strategy for the large‐scale production of renewable hydrogen from water. Developing cost‐effective, active and stable semiconducting photoelectrodes is extremely important for achieving PEC water splitting with high solar‐to‐hydrogen efficiency. Perovskite oxides as a large family of semiconducting metal oxides are extensively investigated as electrodes in PEC water splitting owing to their abundance, high (photo)electrochemical stability, compositional and structural flexibility allowing the achievement of high electrocatalytic activity, superior sunlight absorption capability and precise control and tuning of band gaps and band edges. In this review, the research progress in the design, development, and application of perovskite oxides in PEC water splitting is summarized, with a special emphasis placed on understanding the relationship between the composition/structure and (photo)electrochemical activity.     

Controlled Aqueous Growth of Hematite Nanoplate Arrays Directly on Transparent Conductive Substrates and Their Photoelectrochemical Properties

Two‐dimensional (2D) hematite nanoplate arrays were synthesized directly on fluorine‐doped tin oxide (FTO)‐coated glass by using a facile and novel hydrothermal method. High‐temperature annealing retained the morphology of the nanoplate arrays while simultaneously introducing porosity. The thickness and length of the nanoplates could be tailored by adjusting the precursor composition. Photoelectrochemical (PEC) measurements showed that the photocurrent generated with bare hematite nanoplate photoelectrode under backside illumination was about four times of that under frontside illumination in the entire bias range used, which suggested that slow electron transport was a limiting factor for its PEC performance. Upon Sn doping and Co‐Pi co‐catalyst addition, the photocurrent increased significantly owing to the enhancement of electron conductivity and oxidation kinetics. Electrochemical impedance spectroscopy (EIS) measurements were conducted to understand the surface properties of the nanoplate arrays. Since this strategy is simple, cost‐effective, and highly reproducible, it provides exciting opportunities for the large‐scale growth of porous 2D metal oxide photoelectrodes for a variety of photoelectrochemical and photocatalytic applications.