3D Brochosomes‐Like TiO2/WO3/BiVO4 Arrays as Photoanode for Photoelectrochemical Hydrogen Production

An ideal photoelectrochemical (PEC) anode should process effective light absorption, charge transport, and separation efficiency. Here, a novel 3D brochosomes‐like TiO₂/WO₃/BiVO₄ array as an efficient photoanode by combining a colloid polystyrene sphere template and electrochemical deposition routes for PEC hydrogen generation is reported. The as‐fabricated 3D TiO₂/WO₃/BiVO₄ brochosomes photoanode yields excellent PEC performance with photocurrent densities of ≈3.13 and ≈4.27 mA cm^?2 with FeOOH/NiOOH catalyst, respectively, measured in 0.5 m Na₂SO₄ solution with 0.1 m Na₂SO₃ at 1.23 V versus reversible hydrogen electrode (RHE) under simulated AM1.5 light illumination, which is ≈6 times the reference sample of a planar WO₃/BiVO₄ film electrode. The significantly improved performance could be benefited from the ordered hollow porous structure that provides enhanced light absorption and efficient charge transport as well as improved charge separation efficiency by WO₃/BiVO₄ “host–guest” heterojunctions.     

Designing a Transparent CdIn2S4/In2S3 Bulk-Heterojunction Photoanode Integrated with a Perovskite Solar Cell for Unbiased Water Splitting

The integration of photoelectrochemical photoanodes and solar cells to build an unbiased solar-to-hydrogen (STH) conversion system provides a promising way to solve the energy crisis. The key point is to develop highly transparent photoanodes, while its bulk separation efficiency (η_sep.) and surface injection efficiency are as high as possible. To resolve this contradiction, first a novel CdIn₂S₄/In₂S₃ bulk heterojunctions in the interior of nanosheets is designed as a photoanode with high transparency and an ultrahigh η_sep. up to 90%. Furthermore, decorating the ultrathin amorphous SnO₂ layer by atomic layer deposition, the surface oxygen-evolution kinetics of the photoanode are increased significantly. As a result, the onset potential of the photoanode shifts negatively to 0.02 V vs RHE, and the photocurrent density boosts to 2.98 mA cm⁻² at 1.23 V vs RHE, which is ten times higher than that of pristine CdIn₂S₄. Such a high-performance photoanode enables the integrated metal sulfide photoanode–perovskite solar cell system to deliver a STH conversion efficiency of 3.3%.     

Heterostructured TiO2 Nanorod@Nanobowl Arrays for Efficient Photoelectrochemical Water Splitting

Heterostructured TiO₂ nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO₂ nanorods grown on the inner surface of arrayed anatase TiO₂ nanobowls are designed and fabricated as a new type of photoanodes for photoelectrochemical (PEC) water splitting. The unique heterostructures with a hierarchical architecture are readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two‐dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO₂ NR@NB arrays provide multiple scattering centers and hence exhibit an enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated electron transfer. Accordingly, the PEC measurements of the TiO₂ NR@NB arrays on the fluoride‐doped tin oxide (FTO) substrate show significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the unmodified TiO₂ NR@NB array photoelectrode yields a photocurrent density of 1.24 mA cm^–2 at 1.23 V with respect to the reversible hydrogen electrode, which is almost two times higher than that of the TiO₂ nanorods grown directly on the FTO substrate.     

Dendritic TiO2/ln2S3/AgInS2 Trilaminar Core–Shell Branched Nanoarrays and the Enhanced Activity for Photoelectrochemical Water Splitting

Hierarchical TiO₂/ln₂S₃/AgInS₂ trilaminar core–shell branched nanorod arrays (T‐CS BNRs) have been fabricated directly on conducting glass substrates (FTO) via a facile, versatile and low‐cost hydrothermal and successive ionic layer adsorption and reaction (SILAR) for photoelectrochemical (PEC) water splitting. On the basis of optimal thickness of AgInS₂ shell, such TiO₂/ln₂S₃/AgInS₂ T‐CS BNRs exhibit a higher photocatalytic activity, the photocurrent density and efficiency for hydrogen generation are up to 22.13 mA·cm^?2 and 14.83%, which is, to the best of our knowledge, the highest value ever reported for similar nanostructures. The trilaminar architecture is able to suppress carrier recombination and increase electron collection efficiency via (i) increasing the photon absorption through the lager specific surface area of TiO₂ BNRs and a sensitizer layer (AgInS₂), (ii) a buffer layer (ln₂S₃), (iii) a better energy level alignment.     

CdS/TiO2纳米晶薄膜的原位法制备及光电化学性能研究

CdS/TiO₂异质结薄膜因其优异的可见光催化性能, 在光催化领域引起了广泛关注。然而, 目前传统方法制备的CdS/TiO₂薄膜可能存在交界面结合不紧密的问题, 不利于光生载流子在交界面处的传输。因此, 本研究基于原位转换的原理(TiO₂→CdTiO₃→CdS), 将TiO₂纳米晶表层原位转换成CdS, 制备了CdS/TiO₂纳米晶薄膜。采用X射线衍射(XRD)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)手段对样品薄膜的形貌和结构进行了表征。由表征结果可知, 在TiO₂纳米晶表面形成了CdS, 构成了交界面结合紧密的CdS/TiO₂异质结薄膜。光电化学性能研究表明, 与传统的连续离子层吸附反应法(SILAR)制备的薄膜相比, 原位法制备的CdS/TiO₂薄膜的光电流密度更高, 达到9.8 mA·cm^-2(V=0.4 V (vs. RHE)); 交流阻抗谱(EIS)结果表明, 原位法制备的CdS/TiO₂薄膜具有更小的电荷传输电阻, 说明原位法形成的CdS/TiO₂异质结结合更紧密, 能减小光生载流子在CdS/TiO₂界面处的传输阻力, 降低光生载流子在传输过程中的复合几率, 进而提高CdS/TiO₂薄膜的光电化学性能。     

Nanostructured WO3/BiVO4 Photoanodes for Efficient Photoelectrochemical Water Splitting

Nanostructured photoanodes based on well‐separated and vertically oriented WO₃ nanorods capped with extremely thin BiVO₄ absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO₃‐NRs/BiVO₄ photoanode modified with Co‐Pi oxygen evolution co‐catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm² at 1.23 V versus a reversible hydrogen electrode in a stable Na₂SO₄ electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO₄ layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO₃‐NRs/BiVO₄ photoanode effectively increases the optical thickness of the BiVO₄ layer and results in efficient absorption of the incident light.     

Polypyrrole Serving as Multifunctional Surface Modifier for Photoanode Enables Efficient Photoelectrochemical Water Oxidation

Conjugated polymer polypyrrole (PPy) with high electrical conductivity and excellent photothermal effect has been adopted as multifunctional surface modifier on ternary metal sulfide (CdIn₂S₄, CIS) photoanode for photoelectrochemical (PEC) water splitting for the first time. As a p-type conducting polymer, PPy forms p–n junction with n-type CIS to relieve the bulk carrier recombination. Besides, the incorporation of Ni ions into PPy matrix further enhances the surface charge carrier transfer at photoanode/electrolyte interfaces. Furthermore, the excellent photothermal effect of PPy produces heat under near-infrared (NIR) irradiation, which can elevate the temperature of CIS photoanode in situ and further enhance the PEC performance. As a result, the optimum CIS/Ni-PPy photoanode shows an obviously enhanced photocurrent density of 6.07 mA cm^?2 at 1.23 V versus reversible hydrogen electrode under the irradiation of AM 1.5G combined with NIR light, which is the highest among all the CIS based photoanodes reported to date. The synergetic effect of Ni-PPy significantly suppresses the bulk recombination, decreases the carrier transfer resistance, and accelerates the surface water oxidation dynamics, resulting in high carrier injection efficiency over 80% in the measured potential range. The universality of the multifunctional surface modifier strategy has also been confirmed on metal oxide photoanode.     

Strategic Atomic Layer Deposition and Electrospinning of Cobalt Sulfide/Nitride Composite as Efficient Bifunctional Electrocatalysts for Overall Water Splitting

Reported herein is comprehensive study of a highly active and stable cobalt catalyst for overall water splitting. This composite SFCNF/Co_1?xS@CoN, consisting of S‐doped flexible carbon nanofiber (SFCNF) matrix, Co_1?xS nanoparticles, and CoN coatings, is prepared by integration of electrospinning and atomic layer deposition (ALD) technique. Representative results include the following: 1) ultrathin CoN layer is deposited by ALD on the surface of flexible substrate without any sacrifice of SFCNF and Co_1?xS; 2) the composite exhibits strong electrocatalytic activity in both acidic and basic solutions. The overpotentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are 20 and 180 mV, respectively, at a current density of 10 mA cm^?2 in basic medium. A small Tafel slope of 54.4 mV dec^?1 is observed in 0.5 m H₂SO₄ electrolyte; 3) tested as overall water splitting electrode, the composite records a current density of 10 mA cm^?2 at a relative low cell voltage of 1.58 V and long‐term stability for 20 h at a current density of up to 50 mA cm^?2. The superior performance for overall water splitting is probably attributed to the synergistic effect of Co_1?xS and ALD CoN. Specifically, implementation of ALD can be extended to innovate nanostructured materials for overall water splitting and even other renewable energy aspects.     

Sb‐Doped SnO2 Nanorods Underlayer Effect to the α‐Fe2O3 Nanorods Sheathed with TiO2 for Enhanced Photoelectrochemical Water Splitting

Here, a Sb‐doped SnO₂ (ATO) nanorod underneath an α‐Fe₂O₃ nanorod sheathed with TiO₂ for photoelectrochemical (PEC) water splitting is reported. The experimental results, corroborated with theoretical analysis, demonstrate that the ATO nanorod underlayer effect on the α‐Fe₂O₃ nanorod sheathed with TiO₂ enhances the PEC water splitting performance. The growth of the well‐defined ATO nanorods is reported as a conductive underlayer to improve α‐Fe₂O₃ PEC water oxidation performance. The α‐Fe₂O₃ nanorods grown on the ATO nanorods exhibit improved performance for PEC water oxidation compared to α‐Fe₂O₃ grown on flat fluorine‐doped tin oxide glass. Furthermore, a simple and facile TiCl₄ chemical treatment further introduces TiO₂ passivation layer formation on the α‐Fe₂O₃ to reduce surface recombination. As a result, these unique nanostructures show dramatically improved photocurrent density (139% higher than that of the pure hematite nanorods).     

2D Porous TiO2 Single‐Crystalline Nanostructure Demonstrating High Photo‐Electrochemical Water Splitting Performance

Porous single crystals are promising candidates for solar fuel production owing to their long range charge diffusion length, structural coherence, and sufficient reactive sites. Here, a simple template‐free method of growing a selectively branched, 2D anatase TiO₂ porous single crystalline nanostructure (PSN) on fluorine‐doped tin oxide substrate is demonstrated. An innovative ion exchange–induced pore‐forming process is designed to successfully create high porosity in the single‐crystalline nanostructure with retention of excellent charge mobility and no detriment to crystal structure. PSN TiO₂ film delivers a photocurrent of 1.02 mA cm^?2 at a very low potential of 0.4 V versus reversible hydrogen electrode (RHE) for photo‐electrochemical water splitting, closing to the theoretical value of TiO₂ (1.12 mA cm^?2). Moreover, the current–potential curve featuring a small potential window from 0.1 to 0.4 V versus RHE under one‐sun illumination has a near‐ideal shape predicted by the Gartner Model, revealing that the charge separation and surface reaction on the PSN TiO₂ photoanode are very efficient. The photo‐electrochemical water splitting performance of the films indicates that the ion exchange–assisted synthesis strategy is effective in creating large surface area and single‐crystalline porous photoelectrodes for efficient solar energy conversion.     

Fe2O3–TiO2 Nano‐heterostructure Photoanodes for Highly Efficient Solar Water Oxidation

Harnessing solar energy for the production of clean hydrogen by photo­electrochemical water splitting represents a very attractive, but challenging approach for sustainable energy generation. In this regard, the fabrication of Fe₂O₃–TiO₂ photoanodes is reported, showing attractive performances [≈2.0 mA cm⁻² at 1.23 V vs. the reversible hydrogen electrode in 1 M NaOH] under simulated one‐sun illumination. This goal, corresponding to a tenfold photoactivity enhancement with respect to bare Fe₂O₃, is achieved by atomic layer deposition of TiO₂ over hematite (α‐Fe₂O₃) nanostructures fabricated by plasma enhanced‐chemical vapor deposition and final annealing at 650 °C. The adopted approach enables an intimate Fe₂O₃–TiO₂ coupling, resulting in an electronic interplay at the Fe₂O₃/TiO₂ interface. The reasons for the photocurrent enhancement determined by TiO₂ overlayers with increasing thickness are unraveled by a detailed chemico‐physical investigation, as well as by the study of photo­generated charge carrier dynamics. Transient absorption spectroscopy shows that the increased photoelectrochemical response of heterostructured photoanodes compared to bare hematite is due to an enhanced separation of photogenerated charge carriers and more favorable hole dynamics for water oxidation. The stable responses obtained even in simulated seawater provides a feasible route in view of the eventual large‐scale generation of renewable energy.     

Plasmon‐Enhanced Photoelectrochemical Water Splitting for Efficient Renewable Energy Storage

Photoelectrochemical (PEC) water splitting is a promising approach for producing hydrogen without greenhouse gas emissions. Despite decades of unceasing efforts, the efficiency of PEC devices based on earth‐abundant semiconductors is still limited by their low light absorption, low charge mobility, high charge‐carrier recombination, and reduced diffusion length. Plasmonics has recently emerged as an effective approach for overcoming these limitations, although a full understanding of the involved physical mechanisms remains elusive. Here, the reported plasmonic effects are outlined, such as resonant energy transfer, scattering, hot electron injection, guided modes, and photonic effects, as well as the less investigated catalytic and thermal effects used in PEC water splitting. In each section, the fundamentals are reviewed and the most representative examples are discussed, illustrating possible future developments for achieving improved efficiency of plasmonic photoelectrodes.     

Simultaneous Enhancement of Charge Separation and Hole Transportation in a TiO2–SrTiO3 Core–Shell Nanowire Photoelectrochemical System

Efficient charge separation and transportation are key factors that determine the photoelectrochemical (PEC) water‐splitting efficiency. Here, a simultaneous enhancement of charge separation and hole transportation on the basis of ferroelectric polarization in TiO₂–SrTiO₃ core–shell nanowires (NWs) is reported. The SrTiO₃ shell with controllable thicknesses generates a considerable spontaneous polarization, which effectively tunes the electrical band bending of TiO₂. Combined with its intrinsically high charge mobility, the ferroelectric SrTiO₃ thin shell significantly improves the charge‐separation efficiency (η_separation) with minimized influence on the hole‐migration property of TiO₂ photoelectrodes, leading to a drastically increased photocurrent density ( J _ph). Specifically, the 10 nm‐thick SrTiO₃ shell yields the highest J _ph and η_separation of 1.43 mA cm^?2 and 87.7% at 1.23 V versus reversible hydrogen electrode, respectively, corresponding to 83% and 79% improvements compared with those of pristine TiO₂ NWs. The PEC performance can be further manipulated by thermal treatment, and the control of SrTiO₃ film thicknesses and electric poling directions. This work suggests a material with combined ferroelectric and semiconducting features could be a promising solution for advancing PEC systems by concurrently promoting the charge‐separation and hole‐transportation properties.     

CuO纳米阵列结构光阴极的制备及其光电化学分解水的性能

用原位基体加热反应磁控溅射方法制备具有强捕光和电荷分离能力的CuO纳米阵列(CuO NAs)光阴极,并改变氧分压、基底温度、腔体压力以及溅射时间等参数调控其相组成、晶体形貌、晶体生长取向、晶面暴露、厚度以及电子结构。结果表明,结构优化的CuO NAs光阴极,其光电流密度可达2.4 mA·cm^-2。