A Tantalum Nitride Photoanode Modified with a Hole‐Storage Layer for Highly Stable Solar Water Splitting

Photoelectrochemical (PEC) water splitting is an ideal approach for renewable solar fuel production. One of the major problems is that narrow bandgap semiconductors, such as tantalum nitride, though possessing desirable band alignment for water splitting, suffer from poor photostability for water oxidation. For the first time it is shown that the presence of a ferrihydrite layer permits sustainable water oxidation at the tantalum nitride photoanode for at least 6 h with a benchmark photocurrent over 5 mA cm^?2, whereas the bare photoanode rapidly degrades within minutes. The remarkably enhanced photostability stems from the ferrihydrite, which acts as a hole‐storage layer. Furthermore, this work demonstrates that it can be a general strategy for protecting narrow bandgap semiconductors against photocorrosion in solar water splitting.     

Efficient solar generation of hydrogen fuel – a fundamental analysis

Solar water splitting can provide clean, renewable sources of hydrogen fuel, although prior models had indicated only low conversion efficiencies would be attainable. A novel model is derived for electrochemical solar water splitting processes by semiconductors, which is the first derivation of band edge restricted thermal enhanced solar water splitting efficiencies. A theoretical basis is developed for solar energy conversion efficiencies in the 50% range as determined with contemporary thermodynamic values. The theory combines photodriven charge transfer, with excess sub-bandgap insolation to lower the water potential, providing a process of highly efficient elevated temperature solar electrolysis of water to hydrogen fuel.     

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.     

提高氧化铁光电催化分解水效率的策略进展

为了解决能源危机与环境污染问题,发展一种可再生的清洁能源至关重要.太阳能是一种取之不尽用之不竭的清洁能源,而氢气是一种良好的能源载体.利用太阳能光电催化水分解制氢,是一项有望能够解决能源与环境问题的技术,具有很大的应用前景.其中,氧化铁因为具有合适的能带位置与带隙、良好的稳定性与廉价无毒等优点,成为一种理想的光阳极材料.但是,在实际的测试中,氧化铁仅仅只能得到一个较低的光电转换效率,这可能是因为其较短的空穴扩散距离、较低的电导率以及极度缓慢的水氧化反应动力学所致.整个光电催化水氧化可分为三个过程,即光吸收过程、电荷分离过程以及表面空穴注入过程.这三个过程的效率共同决定了器件的太阳能转化效率.鉴于此,本文将从如何提高这三个效率的角度出发,总结近期研究报道中提高氧化铁光电催化分解水效率的一些策略.光吸收过程是指半导体中价带的电子在吸收具有一定能量的光子后发生跃迁,产生空穴-电子对的过程.其光子的损失主要来源于光的反射、透射以及半导体吸收边的限制.提高光吸收效率的主要策略包括制备具有特定纳米结构的氧化铁电极、利用表面等离子体共振效应以及组成双光吸收系统和掺杂等.电荷分离过程指的是受光激发产生的空穴电子对,在内建电场的作用下发生电荷分离,即光生空穴流向电极表面,光生电子流向半导体内部并从外电路导出.电荷分离效率的损失主要来源于光生载流子在迁移过程中的复合.因此,为了提高电荷分离效率,常见的策略是提高载流子在电荷分离过程中的复合时间τ₁和减少电荷迁移到表面(空穴)或者基底(电子)的时间τ₂.具体的策略包括制备特定的纳米结构(缩短体表相距离,减少τ₂)、构建异质结(增强能带弯曲,提高τ₁和减少τ₂)、掺杂(减少τ₂)和钝化复合中心(提高τ₁)等.表面空穴注入是指到达表面的光生空穴发生水氧化反应生成氧气的过程.除了空穴注入外,表面还可能存在复合与逆反应过程.因此,为了提高表面空穴注入效率,我们既可以提高水氧化反应动力学,具体的手段包括引入水氧化催化剂、F掺杂和碱处理等;也可以采用减少复合反应的策略,具体的方法包括引入钝化层、酸处理和高温热处理等;还可以采用减少逆反应的方法,最常见的手段就是在基底与氧化铁层之间引入电子阻挡层.上述三种途径都能提高表面空穴注入效率.最后,通过结合上述的一些策略,目前得到的最高性能的氧化铁电极在1.23 V(相对于可逆氢电极)能够达到6 mA cm^?2的光电催化分解水电流,但这个值依然明显低于氧化铁的理论值(12.6 mA cm^?2).这可能是由于体相复合所致.除此之外,氧化铁表面的水氧化机理现在依然不清晰,这些都是需要我们在未来解决的问题.     

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.     

Carbon-based H2-production photocatalytic materials

Photocatalytic hydrogen production from water splitting is of promising potential to resolve the energy shortage and environmental concerns. During the past decade, carbon materials have shown great ability to enhance the photocatalytic hydrogen-production performance of semiconductor photocatalysts. This review provides a comprehensive overview of carbon materials such as CNTs, graphene, C₆₀, carbon quantum dots, carbon fibers, activated carbon, carbon black, etc. in enhancing the performance of semiconductor photocatalysts for H₂ production from photocatalytic water splitting. The roles of carbon materials including supporting material, increasing adsorption and active sites, electron acceptor and transport channel, cocatalyst, photosensitization, photocatalyst, band gap narrowing effect are explicated in detail. Also, strategies for improving the photocatalytic hydrogen-production efficiency of carbon-based photocatalytic materials are discussed in terms of surface chemical functionalization of the carbon materials, doping effect of the carbon materials and interface engineering between semiconductors and carbon materials. Finally, the concluding remarks and the current challenges are highlighted with some perspectives for the future development of carbon-based photocatalytic materials.     

半导体光解水制氢研究:现状、挑战及展望

通过半导体光催化分解水反应实现太阳能向清洁能源氢能的转化,是解决人类面临的能源和环境危机的终极途径之一。该过程的关键是开发宽光谱响应、高效的光催化剂,到目前为止,调控能带结构、制备活性晶面、构建异质结构、负载助催化剂等诸多方法被广泛应用于扩展半导体材料的吸光范围和提高其光催化活性。本文介绍了半导体光解水制氢的基本原理,并综述了该领域的研究进展,重点关注提高半导体光催化活性的方法及其所面临的挑战和瓶颈问题,并结合相关课题组的研究工作提出可能的应对策略。     

半导体光解水制氢研究:现状、挑战及展望

通过半导体光催化分解水反应实现太阳能向清洁能源氢能的转化,是解决人类面临的能源和环境危机的终极途径之一。该过程的关键是开发宽光谱响应、高效的光催化剂,到目前为止,调控能带结构、制备活性晶面、构建异质结构、负载助催化剂等诸多方法被广泛应用于扩展半导体材料的吸光范围和提高其光催化活性。本文介绍了半导体光解水制氢的基本原理,并综述了该领域的研究进展,重点关注提高半导体光催化活性的方法及其所面临的挑战和瓶颈问题,并结合相关课题组的研究工作提出可能的应对策略。     

Indolo[3,2-b]carbazole-based thin-film transistors with high mobility and stability

Proper functionalization of indolo[3,2-b]carbazole led to a new class of high-performance organic semiconductors suitable for organic thin-film transistor (OTFT) applications. While 5,11-diaryl-substituted indolo[3,2-b]carbazoles without long alkyl side chains provided amorphous thin films upon vacuum deposition, those with sufficiently long alkyl side chains such as 5,11-bis(4-octylphenyl)indolo[3,2-b]carbazole self-organized readily into highly crystalline layered structures under similar conditions. OTFTs using channel semiconductors of this nature exhibited excellent field-effect properties, with mobility up to 0.12 cm(2) V(-1) s(-1) and current on/off ratio to 10(7). As this class of organic semiconductors has relatively low HOMO levels and large band gaps, they also displayed good environmental stability even with prolonged exposure to amber light, an appealing characteristic for OTFT applications.     

Fe-MILs系可见光响应复合材料在光诱导反应中的应用研究进展

能源危机和环境问题已成为人类社会面临和亟待解决的两个重大问题。光催化技术被认为是解决能源危机和环境问题有效方法之一。来瓦希尔骨架材料（MILs）是一类著名的金属骨架有机材料（MOFs）。Fe-MILs是MILs系列材料中一个重要的分支,其带隙宽度约为2.39 eV ~ 2.79 eV,能被可见光激发。然而,Fe-MILs存在光的利用率低、导电性差、光生电子-空穴复合快、光腐蚀等缺点。近年来,研究者们采用多种方法对Fe-MILs进行改性,制备了很多Fe-MILs衍生的可见光响应复合材料,并将这些材料成功用于光诱导反应。本文主要综述了近年来Fe-MILs衍生的可见光响应复合材料在水的分解、CO2还原、有机物转化、光催化固氮等多个领域的应用研究,并对Fe-MILs光催化剂的发展提出了建议。     

Atomically Thin Arsenene and Antimonene: Semimetal–Semiconductor and Indirect–Direct Band‐Gap Transitions

The typical two‐dimensional (2D) semiconductors MoS₂, MoSe₂, WS₂, WSe₂ and black phosphorus have garnered tremendous interest for their unique electronic, optical, and chemical properties. However, all 2D semiconductors reported thus far feature band gaps that are smaller than 2.0 eV, which has greatly restricted their applications, especially in optoelectronic devices with photoresponse in the blue and UV range. Novel 2D mono‐elemental semiconductors, namely monolayered arsenene and antimonene, with wide band gaps and high stability were now developed based on first‐principles calculations. Interestingly, although As and Sb are typically semimetals in the bulk, they are transformed into indirect semiconductors with band gaps of 2.49 and 2.28 eV when thinned to one atomic layer. Significantly, under small biaxial strain, these materials were transformed from indirect into direct band‐gap semiconductors. Such dramatic changes in the electronic structure could pave the way for transistors with high on/off ratios, optoelectronic devices working under blue or UV light, and mechanical sensors based on new 2D crystals.     

Semiconducting Group 15 Monolayers: A Broad Range of Band Gaps and High Carrier Mobilities

Optoelectronic applications require materials both responsive to objective photons and able to transfer carriers, so new two‐dimensional (2D) semiconductors with appropriate band gaps and high mobilities are highly desired. A broad range of band gaps and high mobilities of a 2D semiconductor family, composed of monolayer of Group 15 elements (phosphorene, arsenene, antimonene, bismuthene) is presented. The calculated binding energies and phonon band dispersions of 2D Group 15 allotropes exhibit thermodynamic stability. The energy band gaps of 2D semiconducting Group 15 monolayers cover a wide range from 0.36 to 2.62 eV, which are crucial for broadband photoresponse. Significantly, phosphorene, arsenene, and bismuthene possess carrier mobilities as high as several thousand cm² V^?1 s^?1. Combining such broad band gaps and superior carrier mobilities, 2D Group 15 monolayers are promising candidates for nanoelectronics and optoelectronics.     

Iron Oxide Photoelectrode with Multidimensional Architecture for Highly Efficient Photoelectrochemical Water Splitting

Nanostructured metal oxide semiconductors have shown outstanding performances in photoelectrochemical (PEC) water splitting, but limitations in light harvesting and charge collection have necessitated further advances in photoelectrode design. Herein, we propose anodized Fe foams (AFFs) with multidimensional nano/micro-architectures as a highly efficient photoelectrode for PEC water splitting. Fe foams fabricated by freeze-casting and sintering were electrochemically anodized and directly used as photoanodes. We verified the superiority of our design concept by achieving an unprecedented photocurrent density in PEC water splitting over 5 mA cm⁻² before the dark current onset, which originated from the large surface area and low electrical resistance of the AFFs. A photocurrent of over 6.8 mA cm⁻² and an accordingly high incident photon-to-current efficiency of over 50 % at 400 nm were achieved with incorporation of Co oxygen evolution catalysts. In addition, research opportunities for further advances by structual and compositional modifications are discussed, which can resolve the low fill factoring behavior and improve the overall performance.     

Donor-acceptor type low band gap polymers: polysquaraines and related systems

In recent years, considerable effort has been directed towards the synthesis of conjugated polymers with low optical band gaps (Eg), since they show intrinsic electrical conductivity. One of the approaches towards the designing of such polymers is the use of strong donor and acceptor monomers at regular arrangements in the repeating units, which has limited success in many cases. An alternate strategy is the use of organic dyes, having inherently low HUMO-LUMO separation, as building blocks. Extension of conjugation in organic dyes is therefore expected to result in oligomers and polymers with near infrared absorption, which is a signature of low band gaps. Squaraine dyes are ideal candidates for this purpose due to their unique optical properties. This review highlights the recent developments in the area of donor-acceptor type low band gap polymers with special emphasis on polysquaraines.     

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.     

Computational Design of One‐Dimensional Ferromagnetic Semiconductors in Transition Metal Embedded Stannaspherene Nanowires

Developing low dimensional semiconductors with moderate band gaps, intrinsic ferromagnetism and large magnetic anisotropy energies (MAEs) is very desirable for high‐speed nano‐spintronic devices, which, however, still remains a big challenge. Here, via first principles calculations, a potential route to realize such materials is proposed based on a new class of one‐dimensional transition metal (TM) embedded stannaspherene (Sn₁₂^2–) nanowires [TM₂(Sn₁₂)]_∞ (TM = Ti‐Ni). Three semiconductors with robust ferromagnetism are achieved with TM = V, Cr and Fe, which all exhibit direct or quasi‐direct band gaps around 1.0 eV, rendering their great potentials for visible light optoelectronic applications. Interestingly, [Cr₂(Sn₁₂)]_∞ and [Fe₂(Sn₁₂)]_∞ are both identified as bipolar magnetic semiconductors (BMS) with valence and conduction band edges spin polarized in the opposite directions, which are promising for realizing switch of carriers’ spin orientation by electrical gating, while [V₂(Sn₁₂)]_∞ exhibits a half semiconductor (HSC) property with valence and conduction band edges spin polarized in the same direction and can be used for spin‐polarized carriers generation. Moreover, sizable MAEs are discovered in these nanowires, which are at least two orders of magnitude larger than those of Fe, Co and Ni bulks and also significantly larger than current ferromagnetic semiconductors.     

RuO2-loaded beta-Ge3N4 as a non-oxide photocatalyst for overall water splitting

Germanium nitride beta-Ge3N4 dispersed with RuO2 nanoparticles is presented as the first example of a non-oxide photocatalyst for the stoichiometric decomposition of H2O into H2 and O2. All of the successful photocatalysts developed for overall water splitting over the past 30 years have been based on oxides of metals. The discovery of a non-oxide photocatalyst, such as nitrides and oxynitrides, achieving the same function is therefore expected to stimulate research on non-oxide photocatalysts. New opportunities for progress in the development of visible light-driven photocatalysis can thus be expected, as the higher valence band positions of metal nitrides compared to the corresponding metal oxides provide narrower band gaps, which are suitable for visible light activity.     

二元化合物半导体的能带结构

本文用LCBOMO法计算了属于元素半导体和AB型、AB₂型化合物半导体的十种共价结构的能带。所得结果说明:它们的禁带宽度Eg都可统一地表为键能和邻键间共轭积分的线性函数,与共价结构的形式无关。给出许多文献用键性质归纳Eg数值规律的理论基础。     

Ternary chalcogenides XGaS2 (X = Ag or Cu) for photocatalytic hydrogen generation from water splitting under irradiation of visible light

Ternary chalcogenide silver gallium sulfide (AgGaS₂), which has an orthorhombic structure, was already synthesized. However, the feasibility of using the crystal for hydrogen production through photocatalytic water splitting has not been explored. Here, we systematically investigated the structural, electronic, optical, and transport properties of XGaS₂ (X = Ag or Cu) with orthorhombic structure by using the first principles calculations. The band alignments indicate that all calculated absolute potentials of the valence and conduction band edges met the requirement of photocatalytic water splitting reaction. The presence of 2.64 and 2.56 eV direct band energy gaps and obvious optical absorption within the visible light range imply that XGaS₂ can correspond to solar light. Moreover, the large electron mobility and the obvious differences between electron mobility and hole mobility were identified in XGaS₂ structures, which is beneficial to the photocatalytic performance of the water splitting reaction. The present findings can provide a helpful reference for developing novel photocatalytic materials with XGaS₂ for hydrogen generation from water splitting under irradiation of visible light.     

Electrodeposited Cu thin layers as low cost and effective underlayers for Cu2O photocathodes in photoelectrochemical water electrolysis

Journal of Solid State Electrochemistry - Cu2O is one of the most studied semiconductors for photocathodes in photoelectrochemical water splitting (PEC-WS). Its low stability is counterbalanced by...